RESEARCH ARTICLE StructureâProperty of Thermoplastic ...
RESEARCH ARTICLE Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America
Advanced Science Letters Vol. 4, 1–9, 2011
Structure–Property of Thermoplastic Polyurethane–Clay Nanocomposite Based on Covalent and Dual-Modified Laponite Ananta K. Mishra1 , P. R. Rajamohanan2 , Golok B. Nando1 ∗ , and Santanu Chattopadhyay1 ∗ 1
Rubber Technology Center, Indian Institute of Technology Kharagpur, Kharagpur 721302, India 2 Central NMR Facility, National Chemical Laboratory, Pune 411008, India
Modification of surface of clay platelets by ionic and covalent modification techniques renders it to be easily dispersed in polymers like, Thermoplastic Polyurethane (TPU). Only ionic or covalent modification techniques in isolation are not sufficient to achieve uniform nanoscale dispersion of Laponite (synthetic hectorite nanoclay) in TPU. Hence, the dual modification of Laponite (both ionic and covalent) is performed and the effects of the modification on the morphology, thermal and rheological behaviors of the TPU-modified clay nanocomposites have been studied. The degree of exfoliation of clay platelet in TPU matrix is found to be higher for dual functionalized nanoclays compared to their singly modified counterparts. Interestingly, dual modified Laponite clays prepared by using two different techniques (ionic followed by covalent and covalent followed by ionic) exhibit different morphology and properties. The dual modified clays significantly alter the equilibrium morphology of TPU. The storage modulus of the dual modified Laponite-TPU nanocomposite in the glassy region (at −60 C) and in the rubbery region +98 C) is improved by 172.8% and 85%, respectively as compared to the neat TPU. Similarly, the onset of degradation is found to be enhanced by 28.7 C as compared to the neat TPU.
1. INTRODUCTION Polymer–clay nanocomposites with a high degree of exfoliated morphology of nanoclays have gained increasing interest in recent years.1 These composites offer improved mechanical properties,2 higher heat distortion temperature,2 better barrier properties,3 lower water absorption,4 increased thermal stability and reduced flammability.5 In this regard TPU is of popular choice in contrast to other polymers because of its wider applicability many engineering works. Several grades of clays are available (natural and synthetic) which can be used as fillers in polymers (Table I). Sheng et al.6 have observed that matrix stiffening effect (commonly encountered in case of clay with greater size and aspect ratio) can be greatly reduced with nanoclay possessing lower aspect ratio. Among the nanoclays listed in Table I, Laponite RD possesses the advantage of lower size (25–30 nm), which nearly matches with the size scale of the phase separated hard domains present in TPU. It is also chemically pure as it is derived synthetically. It has an empirical molecular formula of Na07 (Si8 Mg55 Li03 O20 (OH)4 . It is insoluble in water but forms a clear and colorless colloidal dispersion. However, to make it compatible with polymer, the surface of the nanoclay should be made hydrophobic. ∗
Authors to whom correspondence should be addressed.
Adv. Sci. Lett. Vol. 4, No. 1, 2011
This is possible by the organic modification of the surface and can be done in two major ways. The Na+ ions present in the intergallery spacing of the nanoclay can be ion exchanged with long chain alkyl ammonium ions. Alternately, the –OH group present on the edge of the nanoclay can also be covalently bonded with alkoxy silane groups. It is well known that optimum property can be achieved by increasing the degree of exfoliation of the clay platelets. In an earlier communications7 8 the authors have reported about the difficulties in dispersing Laponite RD in an organic solvent, even if it is modified with either dodecylammonium chloride or cetyl trimethyl ammonium bromide through simple ion exchange process. Based on the previous experience, the present work has been undertaken for dual modification of Laponite RD. Both ionic modification9–14 and covalent modification (silylation)15–18 techniques of clay is well established in the literature. However, the dual modification (simultaneous covalent and ionic modification) of clay has been scarcely reported.19–21 This paper embodies a novel approach for improving the dispersion of Laponite clay in the TPU matrix to uplift the property spectrum of the nanocomposites. The modified clays are characterized by FTIR, solid state NMR, XRD and the extent of grafting of the modifiers are quantified by using TGA. This manuscript describes a close comparison
1936-6612/2011/4/001/009
doi:10.1166/asl.2011.1174
1
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
Table I. Important clay minerals used to prepare polymer clay nanocomposites. Type 1:1 2:1
2:2
Name of clay∗ bN
Kaolinite HalloysitebN MontmoriloniteaN HectoriteaN SaponiteaN LaponiteaS FluorohectoriteaS FluoromicaaS SapioliteS AttapulgiteS
General formula Al2 Si2 F5 (OH)4 Al2 Si2 F5 (OH)4 · nH2 O Mx (Al2−x Mgx Si4 O10 (OH)2 · n(H2 O) Mx (Mg3−x Lix Si4 O10 (OH)2 · n(H2 O) Mx Mg3 (Si4−x Alx O10 (OH)2 · n(H2 O) Mx (Mg3−x Lix Si4 O10 (OH)2 · n(H2 O) Mx (Mg3−x Lix Si4 O10 F2 · n(H2 O) Na Mg25 Si4 O10 F2 (Mg789 Al008 Fe3+ 004 (Si1180 Al020 Ca001 O30 (OH)4 (OH2 4 Mg5 Si8 O20 (HO)2 (OH2 4 · 4H2 O
∗ “a”
Represents clays with negative surface charges; “b” Represents clays with positive charge; N-Natural; S-Synthetic.
between the TPU-modified Laponite RD clay nanocomposites. Three varieties of modified Laponite have been used for this study. (a) Laponite RD modified covalently (with octyl trimethoxy silane), (b) ionic followed by covalent functionalization and (c) covalent followed by ionic modification. The structure property correlation for such novel nanocomposites have been reported with the help of XRD, TEM, DMA, TGA and dynamic rheological studies.
2. EXPERIMENTAL DETAILS 2.1. Materials Desmopan KU2 8600E (with a specific gravity of 1.11, which is a polyether based TPU prepared from 4,4 -Methylene bis(phenylisocyanate) and Polytetramethylene glycol chain extended with 1,4-butanediol) used in this work. The TPU was kindly supplied by the M/S Bayer Materials Science Ltd, Chennai, India. Laponite RD was supplied by the Rockwood additives Ltd. UK. Cetyl trimethyl ammonium bromide (CTAB) and octyl trimethoxy silane were used to modify Laponite RD purchased from the Sigma-Aldrich, USA and used without further purification. Tetrahydrofuran (THF, Reagent grade) was received from the Merck, Germany and used without any further purification. 2.2. Modification of Clay 2.2.1. Ionic Modification Laponite RD (L) was modified with required amount of cetyl trimethyl ammonium bromide (c) by using the standard ion exchange process7 reported earlier by the authors. The modified clay was purified by repeated washing with hot deionised water so as to remove the resulting sodium halide ion and the excess CTAB. The following equation is used for calculating the amount of surfactant (CTAB) required for the cation exchange: Amount of Surfactant =
28 × 12 ×W ×M 31 × 100
(1)
where, 2.8 is the percentage of Na2 O present in L, 1.2 is the number of equivalents of surfactant taken with respect to exchangeable Na+ , 31 is the equivalent weight of the replaceable sodium ion present, W is the amount of clay in grams to be modified and M is the molecular weight of the surfactant. Clay modified with ‘c’ is designated as ‘cL’ and will be referred in this form now onwards in the text. 2
2.2.2. Covalent Modification 2 gm of Laponite RD (dried in vacuum oven at 70 C) was dispersed in 50 ml of dry toluene (dried over pressed sodium metal) in a two neck round bottomed flask under nitrogen atmosphere. 2 ml of octyl trimethoxy silane (OS) was added to it and the colloidal suspension was refluxed for 6 hours with stirring. The solvent was dried and then the excess amount of silane was extracted by a soxhlet extractor for 12 hours. The clay modified by this procedure is designated as ‘OSL’ and will be referred in this subsequently. 2.2.3. Ionic Modification Followed by Covalent Modification Ionically modified clay, ‘cL’ was further covalently modified by using the similar procedure as described in Section 2.2.2. The clay modified by this process is designated as ‘cOSL’. 2.2.4. Covalent Modification Followed by Ionic Modification In another batch ‘OSL’ (already covalently modified clay) was further ionically modified by using the procedure as described in Section 2.2.1 except the fact that in this case the medium was 1:1 solution of deionised water and acetone in place of only deionised water. The clay modified by this process is designated as ‘OScL’. 2.3. Preparation of Nanocomposites A 20% solution of TPU was first prepared in THF solvent. Calculated amount of clay was mixed with THF in another container and left under ultrasonic vibration for 30 min, then added slowly to the TPU solution, maintaining gentle stirring. Stirring was continued for additional 15 minutes. The mixture was then sonicated for 15 minutes for making a homogeneous dispersion of clay in the TPU solution. It was then casted on a petridis and allowed the solvent to evaporate slowly at room temperature. Sample was kept in a vacuum oven at 70 C till constant weight was achieved, after complete removal of the solvent at room temperature. Parallely, a control TPU sample was prepared by following the same procedure excluding the addition of clay (control sample) to accomplish a better comparison. The nomenclature of the clay is presented in Table II.
Table II. Nomenclature of modified clay along with percentage of modification and designation of TPUCN.
Designation cL OSL OScL cOSL
Details Laponite RD modified by cetyl trimethyl ammonium bromide Laponite RD modified by octyl trimethoxy silane Laponite RD modified by octyl trimethoxy silane followed by cetyl trimethyl ammonium bromide Laponite RD modified by cetyl trimethyl ammonium bromide followed by octyl trimethoxy silane
Degree of modification 15.6 11.2 17.2 22.0
Weight % of clay (1.3%) ⏐ Thermoplastic polyurethane ←Sxy→ Types of clay (cL, OSL, cOSL or OScL) Example: S0 = TPU with 0% clayS1OSL = TPU with 1% of OSLS3cOSL = TPU with 3% of cOSL.
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
3. CHARACTERIZATION TECHNIQUES
Table III.
Fourier transform infrared (FTIR) spectroscopy studies were performed on a Perkin Elmer FTIR spectrophotometer ata resolution of 4 cm−1 in the range from 4000–400 cm−1 . The clay samples were ground with KBr salt (FTIR grade) and made in the form of a disk under pressure and used for the analysis. 13 C and 29 Si solid state Nuclear magnetic resonance spectroscopy was conducted on a Bruker instrument AV 300 spectrophotometer operating at 75.46 and 59.6 MHz, respectively in the magic angle crosspolarization mode. Wide angle XRD (WAXRD) in the lower angular range (2 value 2 to 10 was performed using a Phillips Pan alytical X-ray diffractometer (model: XPert Pro, the Netherlands) equipped with Cu target (Cu K and Ni filter operating at a voltage of 40 kV and with a beam current of 30 mA (within an error limit of ± 0.2 . Transmission Electron Microscopy (TEM) was performed in a high resolution TEM of JEOL JEM 2100 make, Japan, operating at a voltage of 200 kV after cutting thin section from the bulk of the samples (∼50 nm section) using LEICA ULTRACUT UCT (Austria) microtome, equipped with a diamond knife. DMA 2980 V1.7B and TGA Q50 V6.1 of TA instruments make, USA were used for Dynamic Mechanical Analysis and Thermogravimetric Analysis, respectively. For DMA analysis, the samples were subjected to a sinusoidal displacement of 0.1% strain in tension mode at a frequency of 1 Hz from −75 to 100 C and following a heating rate of 5 C/min. TGA was performed in N2 environment in the temperature range from ∼50–600 C, at a heating rate 20 C/min. Each time, approximately 10 mg samples were taken for TGA experiment. Dynamic rheological behavior of the samples was studied on a rubber process analyzer, RPA-2000 of Alpha Technologies, USA. Frequency sweep was carried out at 140 and 170 C using 1% dynamic strain. The percent error associated with the frequency sweep experiment was within ±1%.
Wave number (cm−1
4. RESULTS AND DISCUSSION 4.1. Fourier Transform Infrared Spectroscopy (FTIR) Table III presents the FTIR bands corresponding to different stretching and bending vibrations for the unmodified Laponite and modified Laponites clays. The appearance of new peaks corresponding to 3685, 2928, 2852, 1469 and 980 cm−1 in case of ‘cL’ as compared to the unmodified Laponite shows that the surface of Laponite is modified with cetyltrimethyl ammonium ion by cation exchange process. The appearance of new peaks at 2928, 2852, 1469 and 796 cm−1 in case of ‘OSL’ as compared to the unmodified Laponite evidences that the surface of Laponite is modified with trimethoxy octyl silane. Similarly, appearance of the peaks 3685, 3429, 2928, 2852, 1636, 1469, 1042, 980, 796, 652 and 518 cm−1 in case of ‘cOSL’ and ‘OScL’ confirms the successful dual modification of Laponite with silane and CTAB, respectively.7 22 4.2. Solid State Nuclear Magnetic Resonance Figures 1(a and b) displays the solid state 13 C and 29 Si NMR spectra of the unmodified and modified nanoclays. 13 C NMR spectrum of unmodified Laponite does not register any peak due to the absence of any form of carbon in its structure (not shown). On the other hand, ‘OSL’ shows peaks at 14.2 ppm (due to C8 and C1), 23.2 ppm (due to C2 and C7), 29.9 ppm (due to C4 and C5) and 32.5 ppm (due to C3 and C6).23 Similarly, cL shows
3685 3429 2928 2852 1636 1469 1042 980 796 652 518 a “—”
FTIR bands of the modified and unmodified clays.22
L
cL
OSL
cOSL
OScL
Peak assignment
—a P — — P — P — — P P
Pa P P P P P P P — P P
— P P P P P P — P P P
P P P P P P P P P P P
P P P P P P P P P P P
N–H str O–H str C–H str (asymmetric) C–H str (symmetric) O–H def C–H def Si–O–Si str C–N str Si–C str Si–O def Si–O–Si def
Indicates absence of the peak, “P” Indicates the presence of the peak.
peaks at 14.6 ppm (due to C16’), 23.3 ppm (due to C3’ and C15’), 30.7 ppm (due to C2’ and carbon from C4’ to C13’), 32.6 ppm (due to C14’), 53.8 ppm (due to N–CH3 and 67.2 ppm (due to N–CH2 or C1’).24 13 C NMR spectra of both the dual modified clays ‘OScL’ and ‘cOSL’ are almost similar with a slight variation in relative peak positions (both contain the combined signature of ‘OSL’ and ‘cL’). The absence of –OMe (present in OS) peak at 50 ppm for both varieties of modified clays indicates the complete silylation reaction of the Si–OMe group of OS during the modification. 29 Si NMR spectrum of unmodified Laponite registers two peaks at −947 and −851 ppm due to Q3 [Si∗ (OMg)(OSi)3 ] and Q2 [Si∗ (OMg)(OSi)2 (OH)] structure, respectively.20 Both the peaks are retained in the similar position with similar intensities in case of ‘cL’. However, reduction in the intensity of the peak at −852 ppm and development of new peaks at −670 and −584 ppm due to T3 [Si∗ (OSi)3 R] and T2 [Si∗ (OSi)2 (OR’)R] structure confirms the reaction of ‘OS’ with the Si–OH present only on the edge of the clay (to produce ‘cOSL’). In case of OSL the peaks at −949 −843 −658 and −554 ppm due to Q3 , Q2 , T3 and T2 confirms the modification of Laponite by OS. OScL shows similar type of peaks corresponding to −953 −852 −680 and −588 ppm due to Q3 , Q2 , T3 and T2 structures. This indicates that structurally there is not much difference between the two types of dual modified clays, namely, OScL and cOSL. The absence of T1 [Si∗ (OSi)(OR’)2 R] and T0 [Si∗ (OR’)3 R] structure confirms the oligomerization of ‘OS’ and absence of unreacted silanes, respectively. Appearance of T2 structure indicates the presence of either –OMe or –OH group on the clay surface after modification. However, absence of any peak corresponding to 50 ppm in 13 C NMR spectrum of ‘OScL’ and ‘cOSL’ confirms the presence of –OH group (as indicated earlier from the FTIR spectrum as well). Based on these observations a scheme has been proposed and approximate structure of modified clay is illustrated in Scheme 1. 4.3. Evaluation of Degree Organic Modification Using TGA Figure 2 shows the TGA thermogram of the unmodified and modified nanoclays. Weight loss from 130 to 600 C correspond mainly the degradation of the alkyl groups. It is observed that within this temperature range unmodified Laponite RD also displays 4% weight loss. Hence, by deducting 4% weight loss from the weight loss of the modified clays, weight loss due to modifiers is calculated. The weight loss due to such hydrophobic 3
RESEARCH ARTICLE (a) 5
7 8 13′
15′ 16′
14′
12′
8′
10′
3′ X
6′
Q3
T2
OS
Si
2 5′
7′
9′
X
Q2
T3
(b)
X
1
4
6 11′
3
Adv. Sci. Lett. 4, 1–9, 2011
1′
2′
4′
CH3 N+
CH3
CTA+
CH3 N-CH3 cOSL –100 cOSL
D
D
OScL C OScL
C
–50
–100
C2, C7
C4, C5 C3, C6
C1, C8 OSL OSL OSL
B
–50
B
–100
C2′, C4′-C13′
C14′
C3′, C15′ N-CH3
C1′
C16′
cL
cL
A
A
–100 70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
ppm
Chemical shift δ ppm
Fig. 1.
(a)
13
C NMR spectra of cL, OSL, OScL and cOSL, (b)
–50
29
–150
ppm
Si NMR spectra of L, cL, OSL, OScL and cOSL.
modifiers is presented in Table II. Higher extent of surface modification is observed in case of ‘cOSL’. This is possibly because initial ionic modification does not interfere with the subsequent covalent modification. However, after initial covalent modification, the silane modifiers restrict the entrance of alkyl ammonium group to enter inside the clay gallery due to the formation of siloxne oligomer and crosslinked structure.14–16 20 4.4. Wide Angle X-ray Diffraction Study Figure 3(a) and (b) show the WAXRD diffractograms in the angular range of 2–10 (2 of the modified clays and the PUCN, respectively. WAXRD patterns reveal no distinct peaks for unmodified Laponite RD.8 18 However, modification increases the sharpness of the diffraction pattern. This is because of the increase in the ordering of the diskettes.8 However, it is very less prominent in ‘OSL’. A broad band in the form of a shoulder (from 2 value 4.0–7.0 is observed in case of OSL and OScL. However, relatively sharp peak centered at 6.0 2 value is found in case of cL. Upon silane modification of ‘cL’, the peak corresponding to 6.0 2 value is shifted towards lower angle with a peak centred at 5.8 2 value. Hence, from the diffractograms it is clear that the features of the initial modification persists in both ‘OScL’ and ‘cOSL’ (e.g., broadness in case of ‘OScL’ similar to that 4
–100
Chemical shift δ ppm
present in ‘OSL’ and sharper peak in case of ‘cOSL’ similar to that present in ‘cL’). The increased broadness of the peak is possibly due to the occurrence of complex modified structure of the Laponite25 as also depicted in Scheme-1. It is observed from the diffractograms of TPU-clay nanocomposites (Fig. 1(b)) that broadness of the diffraction pattern still persists with SOSL. Increase in clay amount possibly leads to an increase in aggregation tendency (as evidenced by the increased intensity and shifting of the broad band towards higher angular range). However, a similar type of feature is not present in PUCN based on dual modified clays. Although few hallows are observed but possibly those fall within the error limit of the measurements. 4.5. Transmission Electron Microscopy 4.5.1. Morphology of Solution Casted Films Figure 4 shows the representative TEM photomicrographs of the nanocomposites containing various modified clays at constant clay content of 3%. In the photomicrographs, the dark coloured spots and linings indicate clay platelets. The grey spots possibly indicate the hard domains whereas, lighter spots indicates the soft domains. It is observed that S3OSL shows an aggregated morphology with a small amount of intercalation (Fig. 4(a)). Mostly intercalated with slight exfoliated morphology persists with S3OScL (Fig. 4(b)), whereas, for S3cOSL, along with a few
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
O OR Si
O
OS
Si Na+
OH OH
OH OH Na+ Na
+ Na
Na
Na+
O
R′O
O
Si R′O + Na+ O Na O O Si
OR
OR
OH
O
+
Si
Si
Si
O
O
O
O
Na+ Si Na+
R′O
Na+
O
O
Na+
O
O Si
OH
O II
OH
OH
I
+ OH + NR3 NR3
+ X– NR3
NR3 OH + + NR3
c
NR3 NR 3 + +
OH
OH
III
II OH
+ X– NR3
O
Si O + NR3
O + + NR3
OH
NR + 3
Si O
+ NR3
+ NR3
O Si
NR3 OH NR OH NR3 + + 3 O
O
O
+ + NR3
Si O
NR3 +
+ + NR3
Si
NR3 + O
NR NR3 + 3 Si
OH
O
O III
OR Si
T3 OR T2
OR
IV
Scheme 1.
&
V
Schemetic depiction of various structural modification of Laponite. I = L, II = OSL, III = cL, IV = OScL, V = cOSL.
signatures of intercalated morphology, exfoliated structures are predominant (Fig. 4(c)). The existence of more exfoliated morphology in case of ‘cOSL’ as compared to ‘OScL’ is due to the higher extent of modification in case of ‘cOSL’ (Table II). Initial silane modification of Laponite with trimethoxy silane leads to the formation of crosslinked structure derived from the oligomeric siloxane chains
(Scheme 1). This crosslinked structure partly restricts the entrance of the cetyltrimethyl ammonium group to effectively utilize all the exchangeable Na+ ions present inside the clay gallery. However, initial ionic exchange by the CTAB can effectively utilize the exchangeable Na+ ions present inside the clay gallery. Hence, extent of modification (Table II) and degree of exfoliation is higher in case of ‘cOSL’ as compared to ‘OScL’. 5
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
3000
100 L OSL cL OScL cOSL
95
2500
Intensity (a.u.)
90
Weight %
L cL OSL OScL cOSL
85
80
75
2000
1500
1000
500
70
0 100
200
300
400
500
600
700
800
2
4
Temperature (°C)
4.5.2. Morphology Development Upon Annealing In our earlier work,8 it has been observed that Laponite modified by dodecylammonium ion act as a hard domain marker. Finnigan et al.26 have observed that clay neither can impart any observable effect on the microphase separation process nor it can affect the morphological changes of TPU to deformation during solution casting and annealing, regardless of the platelet size of the nanoclay used. Hence, in order to look into these aspects, annealed morphology of the solution casted films of the neat TPU and TPUCN containing covalent and dual modified Laponite clays were studied. For this study, the samples were annealed at 140 C for 12 hrs in nitrogen environment and then slowly cooled it down to room temperature within a time span of 8 hrs, to achieve equilibrium hard domain ordering. Figure 5 shows the TEM photomicrographs of the annealed samples. The grey features indicate the hard domain, bright region indicates the soft domain whereas, very dark regions correspond to the clay inside the hard domain. Annealed morphology of the neat TPU exhibits spherical pattern due to the molecular arrangement in the hard domain. It is observed that in all the nanocomposites (S3OSL, S3OScL and S3cOSL), clay particles are found not to be distributed in the soft domain region. Interestingly, S3OSL clay retains the spherical pattern (as observed in the neat TPU), but both S3OScL and S3cOSL, give rise to tubular morphology (Fig. 5). This type of novel morphology of TPU-clay nanocomposites are not yet reported in the literature. The formation of this tubular morphology can be explained based on the formation of arrays of H-bonding between the hard segment of TPU and the –OH groups present on the siloxane oligomer (covalently attached to clay platelets) in combination with the hindrance provided by the cetyltrimethyl ammonium ions (CTA+ ion) against the clay platelets to come closer. Presence of –OH group in siloxane oligomer is confirmed earlier from FTIR and NMR study. Absence of the CTA+ ion may be responsible for the formation of spherical pattern in S3OSL. Modification of Laponite by simple cation exchange of reaction also leads to the formation of spherical hard domains with clay platelets (unpublished result). Thus, from these results it can be inferred that singly modified clay does not possess the capability to modify the phase separated morphology in TPU which is in agreement with the results reported by Finnigan et al.26 . To prove 6
8
TGA thermograms of unmodified and modified clays.
10
S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
600
Intensity (a.u.)
Fig. 2.
6
2θ (degree)
400
200
0 2
4
6
8
10
2θ (degree) Fig. 3. (a) WAXRD of unmodified and modified clays, (b) WAXRD of PUCN with different modified clays (1 and 3 wt%).
this fact of transformation (of the morphology of TPU with dual modified Laponite clay) exclusively, further study is in progress in our laboratory. 4.6. Dynamic Mechanical Analysis (DMA) Figure 6 displays the storage modulus versus temperature plot of the TPU nanocomposites. Table IV represents the magnitude of storage modulus at different temperatures, the glass transition temperature (Tg and the tan max values. It is observed that in the glassy region, highest storage modulus (E’) is imparted by S3OScL. But at −20 and +20 C, S3OSL exhibit maximum E’ and at higher temperatures +80 and +98 C) the storage modulus value is highest for S3cOSL. Hence, it can be inferred that the dual modified clay with a combined intercalated and exfoliated (OScL) structure provides better retention of modulus at lower temperature (glassy region). But the nanocomposite possessing highest degree of exfoliation (ScOSL), retains the modulus at higher temperatures. It is interesting to observe from Figure 6 (inset) that at a very high temperature +98 C) all the nanocomposites tend to register similar storage modulus values comparable to that of the neat TPU. However, S3cOSL exhibits exceptionally high storage modulus value at this temperature.
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
In case of SOSL, S1OSL registers higher storage modulus value as compared to S3OSL. This is because of the increased aggregation tendency with the increase in clay content from 1 to 3%. This is also reflected from the WAXRD diffractogram (Fig. 3(b)). In case of SOScL, 3% clay content is giving rise to the maximum storage modulus both in the rubbery and glassy
S0
(a)
(b)
S3OSL
Spherical pattern
Spherical Pattern (a) Aggregated
(c)
S3OScL
(d)
S3cOSL Tubular morphology
Tubular morphology
Fig. 5. TEM photomicrographs of annealed samples, S0 (a) S3OSL (b), S3OScL (c) and S3cOSL (d).
Intercalated
(b)
Exfoliated
state. But in case of ScOSL, nanocomposite containing 1% clay shows maximum storage modulus value below Tg whereas, above Tg, higher storage modulus value can be achieved with nanocomposite containing 3% clay. The Tg of all the nanocomposites are found to decrease with the addition of clay and maximum decrease is observed with nanocomposites containing dual modified clays. This is possibly because of the plasticizing effect of the modifiers. Tan max is found to decrease with the addition of clay in the TPU matrix. This is due to the effect of reinforcement and due to the obvious decrease in the fraction of TPU in the nanocomposite with an increase in clay content. 4.7. Dynamic Rheological Analysis Dynamic rheological analysis (frequency sweep) was performed with the neat TPU and PUCN with 3% clay content. Figure 7(a and b) display the complex viscosity vs frequency plot of PUCN
Exfoliated Storage modulus (MPa)
(c)
Storage modulus (MPa)
4000
3000
40
S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
30 20 10 0
20
40
60
80
100
Temperature (°C)
2000
S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
1000
0 –60
–40
–20
0
20
40
60
80
100
Temperature (°C) Fig. 4. TEM photomicrographs of solution casted TPUCN, S3OSL (a), S3OScL (b) and S3cOSL (c).
Fig. 6.
DMA thermogram of PUCN.
7
RESEARCH ARTICLE Table IV.
Adv. Sci. Lett. 4, 1–9, 2011
Dynamic mechanical properties of TPU-clay nanocomposite. 0
Sample ID
Storage modulus (MPa)a
S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
−60 C 1586.0 4241.3 (167.4) 3778.4 (138.2) 3709.3 (133.9) 4327.3 (172.8) 3790.3 (140.0) 3254.2 (105.1)
−20 C 99.4 132.6 (33.4) 155.5 (56.4) 117.2 (17.9) 141.2 (42.1) 150.1 (51.0) 142.1 (42.9)
+20 C 13.0 23.2 (78.5) 24.0 (84.6) 22.7 (74.6) 23.0 (76.9) 19.0 (46.1) 21.0 (61.5)
+80 C 7.7 12.1 (57.1) 11.8 (53.2) 12.5 (62.3) 13.5 (75.3) 9.9 (28.6) 13.6 (76.6)
+100 C 6.0 7.7 (28.3) 7.4 (53.2) 8.2 (36.7) 8.2 (36.7) 8.2 (36.7) 11.1 (85.0)
Tg ( C) by DMA
Tan max
−220 −235
0.59 0.58
−247
0.56
−271
0.58
−238
0.57
−239
0.59
−260
0.55
a Values given inside parenthesis indicate the percentage increase in storage modulus of the nanocomposites as compared to S0 (within an error limit of ±05%.
at 140 C and 170 C, respectively. The changes in dynamic storage modulii and complex viscosity values with angular frequency were monitored. A steady decrease in complex viscosity (∗ and a monotonous increase in dynamic storage modulus is (a) 3.0×104
Complex viscosity (Pa-s)
2.5×10
S0 S3OSL S3OScL S3cOSL
4
2.0×104
1.5×104
1.0×104
5.0×103
5
10
15
20
25
30
Frequency (Hz) (b) 1600 S0 S3OSL S3OScL S3cOSL
Complex viscosity (Pa-s)
1400 1200 1000 800 600 400
5
10
15
20
25
30
Frequency (Hz) Fig. 7. (a) Complex viscosity versus frequency plot of PUCN at 140 C, (b) Complex viscosity versus frequency plot of PUCN at 170 C.
8
Table V.
Temperature corresponding to 5, 50 and 80% degradation.
Sample ID S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
T5
T50
T80
300.0 317.1 (17.1)a 315.6 (15.6) 319.4 (19.4) 308.1 (8.1) 328.7 (28.7) 325.5 (25.5)
378.5 388.3 (9.8) 387.6 (9.1) 388.9 (10.4) 389.2 (10.7) 389.3 (10.8) 392.4 (13.4)
400.2 412.4 (12.2) 414.6 (14.4) 414.3 (14.1) 415.9 (15.7) 412.2 (12.0) 418.2 (18.0)
a Values presented in the parenthesis indicate the increase in thermal stability as compared to the neat TPU (within an error limit of ±1%.
observed with the increase in frequency, independent of the temperatures studied here (140 and 170 C). In general, two different sorts of behaviors are encountered at two different temperatures. At 140 C, G > G whereas, at 170 C, G > G , for all the composites (independent of the types of clay).8 It is observed that at 140 C, the complex viscosity (∗ of the all the nanocomposites are lower than the neat TPU within the frequency range studied here (0.033 to 30 Hz). This is due to the contribution of the shear thinning events due to the change in molecular relaxation.27 However, among the nanocomposites, ∗ values grossly follows the order: S3cOSL > S3OScL > S3OSL. This is ascribed to the degree of dispersion of the clay platelets in TPU matrix. This is possibly because of the increase in the degree of dispersion leading to increased polymer-filler interaction thereby increasing the ∗ values. In all the cases a pseudoplastic behavior is observed at this temperature. Interestingly, at 170 C, the complex viscosity (∗ of the all the nanocomposites are higher than that of the neat TPU within the frequency range studied here (0.033 to 30 Hz). This may be due to softening of the hard domain (long range ordering) present in the neat TPU at this temperature. Hence, major contributions towards the ∗ values are attributed to the presence of fillers. The trend in complex viscosity remains similar to that observed at 140 C (S3cOSL > S3OScL > S3OSL). This is again due to the greater degree of dispersion in cOSL as compared to other varieties of modified nanoclays. 4.8. Thermogravimetric Analysis TGA thermograms of the neat TPU and nanocomposites with 1% clay content are shown in Figure 8. It is observed that maximum onset degradation (T5 is observed with 1% clay content in all the nanocomposites. Improvement in thermal stability of 17.1, 19.4 and 28.7 C with S1OSL, S1OScL and S1cOSL, respectively is observed as compared to the neat TPU (Table IV). From the TEM photomicrographs (Section 4.4.1), it is observed that the extent of dispersion follows the order ScOSL > SOScL > SOSL. Thus, it may be inferred that thermal stability of the nanocomposite is directly proportional to the extent of dispersion. As compared to the ionically modified clay,8 silane modified and dual modified clays offer enhanced thermal stability. This is because of the formation of a stronger siloxane oligomer. However, increase in the amount of clay renders the decrement in thermal stability of the nanocomposites. This is because of the increased contribution from the decomposition of the excess modifiers present inside the clay galleries.8 9 However, at higher levels of degradation (e.g., 50% and 80%) nanocomposites containing 3 wt% clay offers higher thermal stability.
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
100 S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
60
100
References and Notes 90
40
20
Weight %
Weight %
80
providing financial support during this work. The authors are highly thankful to Mr T. K. Mallik of M/S Bayer Materials Science Pvt Ltd, Chennai for supplying TPU to carry out this work. The helps provided by Mr. S. Praveen, DRDO, Mumbai is highly acknowledged.
S0 S1OSL
80
S3OSL S1OScL
70
S3OScL S1cOSL S3cOSL
60 275
0
300
325
350
Temperature (°C)
300
400
Temperature (°C) Fig. 8. TGA thermograms of PUCN. (Inset represents the magnified view from 275 to 350 C).
5. CONCLUSIONS Laponite RD is successfully modified by covalent and dual modification techniques. Highest extent of modification is observed with initial ionic modification followed by covalent modification technique (cOSL). Simple covalently modified clay exhibits an aggregated morphology in TPU matrix. Initial covalent modification followed by ionic modification (OScL) results in a combined aggregated and exfoliated morphology in TPU matrix, whereas, a very high degree of exfoliation persist with cOSL. Dual modified clays possess the potential to change the phase separated morphology of TPU owing to the sites for secondary interaction. The storage modulus value in the glassy region (at −60 C) and in the rubbery region +98 C) are improved by 172.8% (with S3OScL) and 85% (with S3cOSL), respectively as compared to that of the neat TPU. Complex viscosities of the nanocomposites are directly proportional to the degree of dispersion of the modified nanoclays, independent of the temperatures studied here. The onset of degradation is found to increase by 28.7 C as compared to that of the neat TPU in case nanocomposite containing merely 1% cOSL. Thus the dual modification of Laponite offers the avenue to highly disperse the clay platelet in the TPU matrix and this can significantly improve the technical properties and thermal stability of the TPU matrix.
Acknowledgment: The authors are thankful to the Indian Space Research Organization (ISRO) and IIT Kharagpur for
1. M. Alexandre and P. Dubois, Mater. Sci. Eng. (R) 28, 1 (2000). 2. Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, and O. Kamigaito, J. Mater. Res. 8, 1185 (1993). 3. P. B. Messersmith and E. P. Giannelis, J. Polym. Sci. A., Polym. Chem. 33, 1047 (1995). 4. A. Okada, Y. Fukushima, M. Kawasumi, S. Inagaki, A. Usuki, S. Sugiyama, T. Kurauchi, and O. Kamigaito, US Patent 4,739,007 (1988). 5. J. W. Gilman, C. L. Jackson, A. B. Morgan, R. H. Harris, E. Manias, E. P. Giannelis, M. Wuthenow, D. Hilton, and S. Phillips, Chem. Mater. 12, 1866 (2000). 6. N. Sheng, M. C. Boyce, D. M. Parks, G. C. Rutledge, J. I. Abes, and R. E. Cohen, Polymer 45, 487 (2004). 7. A. K. Mishra, S. Chattopadhyay, G. B. Nando, and E. Devadoss, Designed Monom. Polym. 11, 395 (2008). 8. A. K. Mishra, G. B. Nando, and S. Chattopadhyay, J. Polym. Sci. Part B: Polym. Phys. 46, 2341 (2008). 9. A. K. Mishra, S. Chattopadhyay, and G. B. Nando, J. Appl. Polym. Sci. 115, 558 (2010). 10. Y. I. Tien and K. H. Wei, Macromolecules 34, 9045 (2001). 11. Y. I. Tien and K. H. Wei, J. Appl. Polym. Sci. 86, 1741 (2002). 12. P. Ni, J. Li, J. Suo, and S. Li, J. Appl. Polym. Sci. 94, 534 (2004). 13. P. Ni, Q. Wang, J. Li, J. Suo, and S. Li, J. Appl. Polym. Sci. 99, 6 (2006). 14. N. N. Herrera, J.-L. Putaux, and E. Bourgeat-Lami, Prog. Solid State Chem. 34, 121 (2006). 15. N. N. Herrera, J.-M. Letoffe, J.-L. Putaux, L. David, and E. Bourgeat-Lami, Langmuir 20, 1564 (2004). 16. N. N. Herrera, J.-M. Letoffe, J.-P. Reymond, and E. Bourgeat-Lami, J. Mater. Chem. 15, 863 (2005). 17. M. Park, I.-K. Shim, E.-Y. Jung, and J.-H. Choy, J. Phys. Chem. Solid 65, 499 (2004). 18. P. A. Wheeler, J. Wang, J. Baker, and L. J. Mathias, Chem. Mater. 17, 3012 (2005). 19. P. A. Wheeler, J. Wang, and L. J. Mathias, Chem. Mater. 18, 3937 (2006). 20. J, Wang, P. A. Wheeler, W. L. Jarrett, and L. J. Mathias, J. Appl. Polym. Sci. 106, 1496 (2007). 21. S. Borsacchi, M. Geppi, L. Ricci, G. Ruggeri, and C. A. Veracini, Langmuir 23, 3953 2007. 22. G. Socrates, Infrared Characteristic Group Frequencies, John Wiley and Sons, Wiley-Interscience Publication, New York (1980). 23. R. Gadzała-Kopciuch, M. Kluska, M. Wełniak, and B. Buszewski, Mater. Chem. Phys. 89, 228 (2005). 24. N. V. Venkataraman and S. Vasudevan, J. Phys. Chem. B 104, 11179 (2000). 25. J. L. Sormana, S. Chattopadhyay, and J. C. Meredith, J. Nanomater. 869354, 9 (2008). 26. B. Finnigan, K. Jack, K. Campbell, P. Halley, R. Truss, P. Casey, D. Cookson, S. King, and D. Martin, Macromolecules 38, 7386 (2005). 27. A. K. Mishra, S. Mushtaq, G. B. Nando, and S. Chattopadhyay, Rheol. Acta. (2010).
Received: 29 April 10. Accepted: 28 May 10.
9
Advanced Science Letters Vol. 4, 1–9, 2011
Structure–Property of Thermoplastic Polyurethane–Clay Nanocomposite Based on Covalent and Dual-Modified Laponite Ananta K. Mishra1 , P. R. Rajamohanan2 , Golok B. Nando1 ∗ , and Santanu Chattopadhyay1 ∗ 1
Rubber Technology Center, Indian Institute of Technology Kharagpur, Kharagpur 721302, India 2 Central NMR Facility, National Chemical Laboratory, Pune 411008, India
Modification of surface of clay platelets by ionic and covalent modification techniques renders it to be easily dispersed in polymers like, Thermoplastic Polyurethane (TPU). Only ionic or covalent modification techniques in isolation are not sufficient to achieve uniform nanoscale dispersion of Laponite (synthetic hectorite nanoclay) in TPU. Hence, the dual modification of Laponite (both ionic and covalent) is performed and the effects of the modification on the morphology, thermal and rheological behaviors of the TPU-modified clay nanocomposites have been studied. The degree of exfoliation of clay platelet in TPU matrix is found to be higher for dual functionalized nanoclays compared to their singly modified counterparts. Interestingly, dual modified Laponite clays prepared by using two different techniques (ionic followed by covalent and covalent followed by ionic) exhibit different morphology and properties. The dual modified clays significantly alter the equilibrium morphology of TPU. The storage modulus of the dual modified Laponite-TPU nanocomposite in the glassy region (at −60 C) and in the rubbery region +98 C) is improved by 172.8% and 85%, respectively as compared to the neat TPU. Similarly, the onset of degradation is found to be enhanced by 28.7 C as compared to the neat TPU.
1. INTRODUCTION Polymer–clay nanocomposites with a high degree of exfoliated morphology of nanoclays have gained increasing interest in recent years.1 These composites offer improved mechanical properties,2 higher heat distortion temperature,2 better barrier properties,3 lower water absorption,4 increased thermal stability and reduced flammability.5 In this regard TPU is of popular choice in contrast to other polymers because of its wider applicability many engineering works. Several grades of clays are available (natural and synthetic) which can be used as fillers in polymers (Table I). Sheng et al.6 have observed that matrix stiffening effect (commonly encountered in case of clay with greater size and aspect ratio) can be greatly reduced with nanoclay possessing lower aspect ratio. Among the nanoclays listed in Table I, Laponite RD possesses the advantage of lower size (25–30 nm), which nearly matches with the size scale of the phase separated hard domains present in TPU. It is also chemically pure as it is derived synthetically. It has an empirical molecular formula of Na07 (Si8 Mg55 Li03 O20 (OH)4 . It is insoluble in water but forms a clear and colorless colloidal dispersion. However, to make it compatible with polymer, the surface of the nanoclay should be made hydrophobic. ∗
Authors to whom correspondence should be addressed.
Adv. Sci. Lett. Vol. 4, No. 1, 2011
This is possible by the organic modification of the surface and can be done in two major ways. The Na+ ions present in the intergallery spacing of the nanoclay can be ion exchanged with long chain alkyl ammonium ions. Alternately, the –OH group present on the edge of the nanoclay can also be covalently bonded with alkoxy silane groups. It is well known that optimum property can be achieved by increasing the degree of exfoliation of the clay platelets. In an earlier communications7 8 the authors have reported about the difficulties in dispersing Laponite RD in an organic solvent, even if it is modified with either dodecylammonium chloride or cetyl trimethyl ammonium bromide through simple ion exchange process. Based on the previous experience, the present work has been undertaken for dual modification of Laponite RD. Both ionic modification9–14 and covalent modification (silylation)15–18 techniques of clay is well established in the literature. However, the dual modification (simultaneous covalent and ionic modification) of clay has been scarcely reported.19–21 This paper embodies a novel approach for improving the dispersion of Laponite clay in the TPU matrix to uplift the property spectrum of the nanocomposites. The modified clays are characterized by FTIR, solid state NMR, XRD and the extent of grafting of the modifiers are quantified by using TGA. This manuscript describes a close comparison
1936-6612/2011/4/001/009
doi:10.1166/asl.2011.1174
1
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
Table I. Important clay minerals used to prepare polymer clay nanocomposites. Type 1:1 2:1
2:2
Name of clay∗ bN
Kaolinite HalloysitebN MontmoriloniteaN HectoriteaN SaponiteaN LaponiteaS FluorohectoriteaS FluoromicaaS SapioliteS AttapulgiteS
General formula Al2 Si2 F5 (OH)4 Al2 Si2 F5 (OH)4 · nH2 O Mx (Al2−x Mgx Si4 O10 (OH)2 · n(H2 O) Mx (Mg3−x Lix Si4 O10 (OH)2 · n(H2 O) Mx Mg3 (Si4−x Alx O10 (OH)2 · n(H2 O) Mx (Mg3−x Lix Si4 O10 (OH)2 · n(H2 O) Mx (Mg3−x Lix Si4 O10 F2 · n(H2 O) Na Mg25 Si4 O10 F2 (Mg789 Al008 Fe3+ 004 (Si1180 Al020 Ca001 O30 (OH)4 (OH2 4 Mg5 Si8 O20 (HO)2 (OH2 4 · 4H2 O
∗ “a”
Represents clays with negative surface charges; “b” Represents clays with positive charge; N-Natural; S-Synthetic.
between the TPU-modified Laponite RD clay nanocomposites. Three varieties of modified Laponite have been used for this study. (a) Laponite RD modified covalently (with octyl trimethoxy silane), (b) ionic followed by covalent functionalization and (c) covalent followed by ionic modification. The structure property correlation for such novel nanocomposites have been reported with the help of XRD, TEM, DMA, TGA and dynamic rheological studies.
2. EXPERIMENTAL DETAILS 2.1. Materials Desmopan KU2 8600E (with a specific gravity of 1.11, which is a polyether based TPU prepared from 4,4 -Methylene bis(phenylisocyanate) and Polytetramethylene glycol chain extended with 1,4-butanediol) used in this work. The TPU was kindly supplied by the M/S Bayer Materials Science Ltd, Chennai, India. Laponite RD was supplied by the Rockwood additives Ltd. UK. Cetyl trimethyl ammonium bromide (CTAB) and octyl trimethoxy silane were used to modify Laponite RD purchased from the Sigma-Aldrich, USA and used without further purification. Tetrahydrofuran (THF, Reagent grade) was received from the Merck, Germany and used without any further purification. 2.2. Modification of Clay 2.2.1. Ionic Modification Laponite RD (L) was modified with required amount of cetyl trimethyl ammonium bromide (c) by using the standard ion exchange process7 reported earlier by the authors. The modified clay was purified by repeated washing with hot deionised water so as to remove the resulting sodium halide ion and the excess CTAB. The following equation is used for calculating the amount of surfactant (CTAB) required for the cation exchange: Amount of Surfactant =
28 × 12 ×W ×M 31 × 100
(1)
where, 2.8 is the percentage of Na2 O present in L, 1.2 is the number of equivalents of surfactant taken with respect to exchangeable Na+ , 31 is the equivalent weight of the replaceable sodium ion present, W is the amount of clay in grams to be modified and M is the molecular weight of the surfactant. Clay modified with ‘c’ is designated as ‘cL’ and will be referred in this form now onwards in the text. 2
2.2.2. Covalent Modification 2 gm of Laponite RD (dried in vacuum oven at 70 C) was dispersed in 50 ml of dry toluene (dried over pressed sodium metal) in a two neck round bottomed flask under nitrogen atmosphere. 2 ml of octyl trimethoxy silane (OS) was added to it and the colloidal suspension was refluxed for 6 hours with stirring. The solvent was dried and then the excess amount of silane was extracted by a soxhlet extractor for 12 hours. The clay modified by this procedure is designated as ‘OSL’ and will be referred in this subsequently. 2.2.3. Ionic Modification Followed by Covalent Modification Ionically modified clay, ‘cL’ was further covalently modified by using the similar procedure as described in Section 2.2.2. The clay modified by this process is designated as ‘cOSL’. 2.2.4. Covalent Modification Followed by Ionic Modification In another batch ‘OSL’ (already covalently modified clay) was further ionically modified by using the procedure as described in Section 2.2.1 except the fact that in this case the medium was 1:1 solution of deionised water and acetone in place of only deionised water. The clay modified by this process is designated as ‘OScL’. 2.3. Preparation of Nanocomposites A 20% solution of TPU was first prepared in THF solvent. Calculated amount of clay was mixed with THF in another container and left under ultrasonic vibration for 30 min, then added slowly to the TPU solution, maintaining gentle stirring. Stirring was continued for additional 15 minutes. The mixture was then sonicated for 15 minutes for making a homogeneous dispersion of clay in the TPU solution. It was then casted on a petridis and allowed the solvent to evaporate slowly at room temperature. Sample was kept in a vacuum oven at 70 C till constant weight was achieved, after complete removal of the solvent at room temperature. Parallely, a control TPU sample was prepared by following the same procedure excluding the addition of clay (control sample) to accomplish a better comparison. The nomenclature of the clay is presented in Table II.
Table II. Nomenclature of modified clay along with percentage of modification and designation of TPUCN.
Designation cL OSL OScL cOSL
Details Laponite RD modified by cetyl trimethyl ammonium bromide Laponite RD modified by octyl trimethoxy silane Laponite RD modified by octyl trimethoxy silane followed by cetyl trimethyl ammonium bromide Laponite RD modified by cetyl trimethyl ammonium bromide followed by octyl trimethoxy silane
Degree of modification 15.6 11.2 17.2 22.0
Weight % of clay (1.3%) ⏐ Thermoplastic polyurethane ←Sxy→ Types of clay (cL, OSL, cOSL or OScL) Example: S0 = TPU with 0% clayS1OSL = TPU with 1% of OSLS3cOSL = TPU with 3% of cOSL.
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
3. CHARACTERIZATION TECHNIQUES
Table III.
Fourier transform infrared (FTIR) spectroscopy studies were performed on a Perkin Elmer FTIR spectrophotometer ata resolution of 4 cm−1 in the range from 4000–400 cm−1 . The clay samples were ground with KBr salt (FTIR grade) and made in the form of a disk under pressure and used for the analysis. 13 C and 29 Si solid state Nuclear magnetic resonance spectroscopy was conducted on a Bruker instrument AV 300 spectrophotometer operating at 75.46 and 59.6 MHz, respectively in the magic angle crosspolarization mode. Wide angle XRD (WAXRD) in the lower angular range (2 value 2 to 10 was performed using a Phillips Pan alytical X-ray diffractometer (model: XPert Pro, the Netherlands) equipped with Cu target (Cu K and Ni filter operating at a voltage of 40 kV and with a beam current of 30 mA (within an error limit of ± 0.2 . Transmission Electron Microscopy (TEM) was performed in a high resolution TEM of JEOL JEM 2100 make, Japan, operating at a voltage of 200 kV after cutting thin section from the bulk of the samples (∼50 nm section) using LEICA ULTRACUT UCT (Austria) microtome, equipped with a diamond knife. DMA 2980 V1.7B and TGA Q50 V6.1 of TA instruments make, USA were used for Dynamic Mechanical Analysis and Thermogravimetric Analysis, respectively. For DMA analysis, the samples were subjected to a sinusoidal displacement of 0.1% strain in tension mode at a frequency of 1 Hz from −75 to 100 C and following a heating rate of 5 C/min. TGA was performed in N2 environment in the temperature range from ∼50–600 C, at a heating rate 20 C/min. Each time, approximately 10 mg samples were taken for TGA experiment. Dynamic rheological behavior of the samples was studied on a rubber process analyzer, RPA-2000 of Alpha Technologies, USA. Frequency sweep was carried out at 140 and 170 C using 1% dynamic strain. The percent error associated with the frequency sweep experiment was within ±1%.
Wave number (cm−1
4. RESULTS AND DISCUSSION 4.1. Fourier Transform Infrared Spectroscopy (FTIR) Table III presents the FTIR bands corresponding to different stretching and bending vibrations for the unmodified Laponite and modified Laponites clays. The appearance of new peaks corresponding to 3685, 2928, 2852, 1469 and 980 cm−1 in case of ‘cL’ as compared to the unmodified Laponite shows that the surface of Laponite is modified with cetyltrimethyl ammonium ion by cation exchange process. The appearance of new peaks at 2928, 2852, 1469 and 796 cm−1 in case of ‘OSL’ as compared to the unmodified Laponite evidences that the surface of Laponite is modified with trimethoxy octyl silane. Similarly, appearance of the peaks 3685, 3429, 2928, 2852, 1636, 1469, 1042, 980, 796, 652 and 518 cm−1 in case of ‘cOSL’ and ‘OScL’ confirms the successful dual modification of Laponite with silane and CTAB, respectively.7 22 4.2. Solid State Nuclear Magnetic Resonance Figures 1(a and b) displays the solid state 13 C and 29 Si NMR spectra of the unmodified and modified nanoclays. 13 C NMR spectrum of unmodified Laponite does not register any peak due to the absence of any form of carbon in its structure (not shown). On the other hand, ‘OSL’ shows peaks at 14.2 ppm (due to C8 and C1), 23.2 ppm (due to C2 and C7), 29.9 ppm (due to C4 and C5) and 32.5 ppm (due to C3 and C6).23 Similarly, cL shows
3685 3429 2928 2852 1636 1469 1042 980 796 652 518 a “—”
FTIR bands of the modified and unmodified clays.22
L
cL
OSL
cOSL
OScL
Peak assignment
—a P — — P — P — — P P
Pa P P P P P P P — P P
— P P P P P P — P P P
P P P P P P P P P P P
P P P P P P P P P P P
N–H str O–H str C–H str (asymmetric) C–H str (symmetric) O–H def C–H def Si–O–Si str C–N str Si–C str Si–O def Si–O–Si def
Indicates absence of the peak, “P” Indicates the presence of the peak.
peaks at 14.6 ppm (due to C16’), 23.3 ppm (due to C3’ and C15’), 30.7 ppm (due to C2’ and carbon from C4’ to C13’), 32.6 ppm (due to C14’), 53.8 ppm (due to N–CH3 and 67.2 ppm (due to N–CH2 or C1’).24 13 C NMR spectra of both the dual modified clays ‘OScL’ and ‘cOSL’ are almost similar with a slight variation in relative peak positions (both contain the combined signature of ‘OSL’ and ‘cL’). The absence of –OMe (present in OS) peak at 50 ppm for both varieties of modified clays indicates the complete silylation reaction of the Si–OMe group of OS during the modification. 29 Si NMR spectrum of unmodified Laponite registers two peaks at −947 and −851 ppm due to Q3 [Si∗ (OMg)(OSi)3 ] and Q2 [Si∗ (OMg)(OSi)2 (OH)] structure, respectively.20 Both the peaks are retained in the similar position with similar intensities in case of ‘cL’. However, reduction in the intensity of the peak at −852 ppm and development of new peaks at −670 and −584 ppm due to T3 [Si∗ (OSi)3 R] and T2 [Si∗ (OSi)2 (OR’)R] structure confirms the reaction of ‘OS’ with the Si–OH present only on the edge of the clay (to produce ‘cOSL’). In case of OSL the peaks at −949 −843 −658 and −554 ppm due to Q3 , Q2 , T3 and T2 confirms the modification of Laponite by OS. OScL shows similar type of peaks corresponding to −953 −852 −680 and −588 ppm due to Q3 , Q2 , T3 and T2 structures. This indicates that structurally there is not much difference between the two types of dual modified clays, namely, OScL and cOSL. The absence of T1 [Si∗ (OSi)(OR’)2 R] and T0 [Si∗ (OR’)3 R] structure confirms the oligomerization of ‘OS’ and absence of unreacted silanes, respectively. Appearance of T2 structure indicates the presence of either –OMe or –OH group on the clay surface after modification. However, absence of any peak corresponding to 50 ppm in 13 C NMR spectrum of ‘OScL’ and ‘cOSL’ confirms the presence of –OH group (as indicated earlier from the FTIR spectrum as well). Based on these observations a scheme has been proposed and approximate structure of modified clay is illustrated in Scheme 1. 4.3. Evaluation of Degree Organic Modification Using TGA Figure 2 shows the TGA thermogram of the unmodified and modified nanoclays. Weight loss from 130 to 600 C correspond mainly the degradation of the alkyl groups. It is observed that within this temperature range unmodified Laponite RD also displays 4% weight loss. Hence, by deducting 4% weight loss from the weight loss of the modified clays, weight loss due to modifiers is calculated. The weight loss due to such hydrophobic 3
RESEARCH ARTICLE (a) 5
7 8 13′
15′ 16′
14′
12′
8′
10′
3′ X
6′
Q3
T2
OS
Si
2 5′
7′
9′
X
Q2
T3
(b)
X
1
4
6 11′
3
Adv. Sci. Lett. 4, 1–9, 2011
1′
2′
4′
CH3 N+
CH3
CTA+
CH3 N-CH3 cOSL –100 cOSL
D
D
OScL C OScL
C
–50
–100
C2, C7
C4, C5 C3, C6
C1, C8 OSL OSL OSL
B
–50
B
–100
C2′, C4′-C13′
C14′
C3′, C15′ N-CH3
C1′
C16′
cL
cL
A
A
–100 70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
ppm
Chemical shift δ ppm
Fig. 1.
(a)
13
C NMR spectra of cL, OSL, OScL and cOSL, (b)
–50
29
–150
ppm
Si NMR spectra of L, cL, OSL, OScL and cOSL.
modifiers is presented in Table II. Higher extent of surface modification is observed in case of ‘cOSL’. This is possibly because initial ionic modification does not interfere with the subsequent covalent modification. However, after initial covalent modification, the silane modifiers restrict the entrance of alkyl ammonium group to enter inside the clay gallery due to the formation of siloxne oligomer and crosslinked structure.14–16 20 4.4. Wide Angle X-ray Diffraction Study Figure 3(a) and (b) show the WAXRD diffractograms in the angular range of 2–10 (2 of the modified clays and the PUCN, respectively. WAXRD patterns reveal no distinct peaks for unmodified Laponite RD.8 18 However, modification increases the sharpness of the diffraction pattern. This is because of the increase in the ordering of the diskettes.8 However, it is very less prominent in ‘OSL’. A broad band in the form of a shoulder (from 2 value 4.0–7.0 is observed in case of OSL and OScL. However, relatively sharp peak centered at 6.0 2 value is found in case of cL. Upon silane modification of ‘cL’, the peak corresponding to 6.0 2 value is shifted towards lower angle with a peak centred at 5.8 2 value. Hence, from the diffractograms it is clear that the features of the initial modification persists in both ‘OScL’ and ‘cOSL’ (e.g., broadness in case of ‘OScL’ similar to that 4
–100
Chemical shift δ ppm
present in ‘OSL’ and sharper peak in case of ‘cOSL’ similar to that present in ‘cL’). The increased broadness of the peak is possibly due to the occurrence of complex modified structure of the Laponite25 as also depicted in Scheme-1. It is observed from the diffractograms of TPU-clay nanocomposites (Fig. 1(b)) that broadness of the diffraction pattern still persists with SOSL. Increase in clay amount possibly leads to an increase in aggregation tendency (as evidenced by the increased intensity and shifting of the broad band towards higher angular range). However, a similar type of feature is not present in PUCN based on dual modified clays. Although few hallows are observed but possibly those fall within the error limit of the measurements. 4.5. Transmission Electron Microscopy 4.5.1. Morphology of Solution Casted Films Figure 4 shows the representative TEM photomicrographs of the nanocomposites containing various modified clays at constant clay content of 3%. In the photomicrographs, the dark coloured spots and linings indicate clay platelets. The grey spots possibly indicate the hard domains whereas, lighter spots indicates the soft domains. It is observed that S3OSL shows an aggregated morphology with a small amount of intercalation (Fig. 4(a)). Mostly intercalated with slight exfoliated morphology persists with S3OScL (Fig. 4(b)), whereas, for S3cOSL, along with a few
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
O OR Si
O
OS
Si Na+
OH OH
OH OH Na+ Na
+ Na
Na
Na+
O
R′O
O
Si R′O + Na+ O Na O O Si
OR
OR
OH
O
+
Si
Si
Si
O
O
O
O
Na+ Si Na+
R′O
Na+
O
O
Na+
O
O Si
OH
O II
OH
OH
I
+ OH + NR3 NR3
+ X– NR3
NR3 OH + + NR3
c
NR3 NR 3 + +
OH
OH
III
II OH
+ X– NR3
O
Si O + NR3
O + + NR3
OH
NR + 3
Si O
+ NR3
+ NR3
O Si
NR3 OH NR OH NR3 + + 3 O
O
O
+ + NR3
Si O
NR3 +
+ + NR3
Si
NR3 + O
NR NR3 + 3 Si
OH
O
O III
OR Si
T3 OR T2
OR
IV
Scheme 1.
&
V
Schemetic depiction of various structural modification of Laponite. I = L, II = OSL, III = cL, IV = OScL, V = cOSL.
signatures of intercalated morphology, exfoliated structures are predominant (Fig. 4(c)). The existence of more exfoliated morphology in case of ‘cOSL’ as compared to ‘OScL’ is due to the higher extent of modification in case of ‘cOSL’ (Table II). Initial silane modification of Laponite with trimethoxy silane leads to the formation of crosslinked structure derived from the oligomeric siloxane chains
(Scheme 1). This crosslinked structure partly restricts the entrance of the cetyltrimethyl ammonium group to effectively utilize all the exchangeable Na+ ions present inside the clay gallery. However, initial ionic exchange by the CTAB can effectively utilize the exchangeable Na+ ions present inside the clay gallery. Hence, extent of modification (Table II) and degree of exfoliation is higher in case of ‘cOSL’ as compared to ‘OScL’. 5
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
3000
100 L OSL cL OScL cOSL
95
2500
Intensity (a.u.)
90
Weight %
L cL OSL OScL cOSL
85
80
75
2000
1500
1000
500
70
0 100
200
300
400
500
600
700
800
2
4
Temperature (°C)
4.5.2. Morphology Development Upon Annealing In our earlier work,8 it has been observed that Laponite modified by dodecylammonium ion act as a hard domain marker. Finnigan et al.26 have observed that clay neither can impart any observable effect on the microphase separation process nor it can affect the morphological changes of TPU to deformation during solution casting and annealing, regardless of the platelet size of the nanoclay used. Hence, in order to look into these aspects, annealed morphology of the solution casted films of the neat TPU and TPUCN containing covalent and dual modified Laponite clays were studied. For this study, the samples were annealed at 140 C for 12 hrs in nitrogen environment and then slowly cooled it down to room temperature within a time span of 8 hrs, to achieve equilibrium hard domain ordering. Figure 5 shows the TEM photomicrographs of the annealed samples. The grey features indicate the hard domain, bright region indicates the soft domain whereas, very dark regions correspond to the clay inside the hard domain. Annealed morphology of the neat TPU exhibits spherical pattern due to the molecular arrangement in the hard domain. It is observed that in all the nanocomposites (S3OSL, S3OScL and S3cOSL), clay particles are found not to be distributed in the soft domain region. Interestingly, S3OSL clay retains the spherical pattern (as observed in the neat TPU), but both S3OScL and S3cOSL, give rise to tubular morphology (Fig. 5). This type of novel morphology of TPU-clay nanocomposites are not yet reported in the literature. The formation of this tubular morphology can be explained based on the formation of arrays of H-bonding between the hard segment of TPU and the –OH groups present on the siloxane oligomer (covalently attached to clay platelets) in combination with the hindrance provided by the cetyltrimethyl ammonium ions (CTA+ ion) against the clay platelets to come closer. Presence of –OH group in siloxane oligomer is confirmed earlier from FTIR and NMR study. Absence of the CTA+ ion may be responsible for the formation of spherical pattern in S3OSL. Modification of Laponite by simple cation exchange of reaction also leads to the formation of spherical hard domains with clay platelets (unpublished result). Thus, from these results it can be inferred that singly modified clay does not possess the capability to modify the phase separated morphology in TPU which is in agreement with the results reported by Finnigan et al.26 . To prove 6
8
TGA thermograms of unmodified and modified clays.
10
S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
600
Intensity (a.u.)
Fig. 2.
6
2θ (degree)
400
200
0 2
4
6
8
10
2θ (degree) Fig. 3. (a) WAXRD of unmodified and modified clays, (b) WAXRD of PUCN with different modified clays (1 and 3 wt%).
this fact of transformation (of the morphology of TPU with dual modified Laponite clay) exclusively, further study is in progress in our laboratory. 4.6. Dynamic Mechanical Analysis (DMA) Figure 6 displays the storage modulus versus temperature plot of the TPU nanocomposites. Table IV represents the magnitude of storage modulus at different temperatures, the glass transition temperature (Tg and the tan max values. It is observed that in the glassy region, highest storage modulus (E’) is imparted by S3OScL. But at −20 and +20 C, S3OSL exhibit maximum E’ and at higher temperatures +80 and +98 C) the storage modulus value is highest for S3cOSL. Hence, it can be inferred that the dual modified clay with a combined intercalated and exfoliated (OScL) structure provides better retention of modulus at lower temperature (glassy region). But the nanocomposite possessing highest degree of exfoliation (ScOSL), retains the modulus at higher temperatures. It is interesting to observe from Figure 6 (inset) that at a very high temperature +98 C) all the nanocomposites tend to register similar storage modulus values comparable to that of the neat TPU. However, S3cOSL exhibits exceptionally high storage modulus value at this temperature.
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
In case of SOSL, S1OSL registers higher storage modulus value as compared to S3OSL. This is because of the increased aggregation tendency with the increase in clay content from 1 to 3%. This is also reflected from the WAXRD diffractogram (Fig. 3(b)). In case of SOScL, 3% clay content is giving rise to the maximum storage modulus both in the rubbery and glassy
S0
(a)
(b)
S3OSL
Spherical pattern
Spherical Pattern (a) Aggregated
(c)
S3OScL
(d)
S3cOSL Tubular morphology
Tubular morphology
Fig. 5. TEM photomicrographs of annealed samples, S0 (a) S3OSL (b), S3OScL (c) and S3cOSL (d).
Intercalated
(b)
Exfoliated
state. But in case of ScOSL, nanocomposite containing 1% clay shows maximum storage modulus value below Tg whereas, above Tg, higher storage modulus value can be achieved with nanocomposite containing 3% clay. The Tg of all the nanocomposites are found to decrease with the addition of clay and maximum decrease is observed with nanocomposites containing dual modified clays. This is possibly because of the plasticizing effect of the modifiers. Tan max is found to decrease with the addition of clay in the TPU matrix. This is due to the effect of reinforcement and due to the obvious decrease in the fraction of TPU in the nanocomposite with an increase in clay content. 4.7. Dynamic Rheological Analysis Dynamic rheological analysis (frequency sweep) was performed with the neat TPU and PUCN with 3% clay content. Figure 7(a and b) display the complex viscosity vs frequency plot of PUCN
Exfoliated Storage modulus (MPa)
(c)
Storage modulus (MPa)
4000
3000
40
S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
30 20 10 0
20
40
60
80
100
Temperature (°C)
2000
S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
1000
0 –60
–40
–20
0
20
40
60
80
100
Temperature (°C) Fig. 4. TEM photomicrographs of solution casted TPUCN, S3OSL (a), S3OScL (b) and S3cOSL (c).
Fig. 6.
DMA thermogram of PUCN.
7
RESEARCH ARTICLE Table IV.
Adv. Sci. Lett. 4, 1–9, 2011
Dynamic mechanical properties of TPU-clay nanocomposite. 0
Sample ID
Storage modulus (MPa)a
S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
−60 C 1586.0 4241.3 (167.4) 3778.4 (138.2) 3709.3 (133.9) 4327.3 (172.8) 3790.3 (140.0) 3254.2 (105.1)
−20 C 99.4 132.6 (33.4) 155.5 (56.4) 117.2 (17.9) 141.2 (42.1) 150.1 (51.0) 142.1 (42.9)
+20 C 13.0 23.2 (78.5) 24.0 (84.6) 22.7 (74.6) 23.0 (76.9) 19.0 (46.1) 21.0 (61.5)
+80 C 7.7 12.1 (57.1) 11.8 (53.2) 12.5 (62.3) 13.5 (75.3) 9.9 (28.6) 13.6 (76.6)
+100 C 6.0 7.7 (28.3) 7.4 (53.2) 8.2 (36.7) 8.2 (36.7) 8.2 (36.7) 11.1 (85.0)
Tg ( C) by DMA
Tan max
−220 −235
0.59 0.58
−247
0.56
−271
0.58
−238
0.57
−239
0.59
−260
0.55
a Values given inside parenthesis indicate the percentage increase in storage modulus of the nanocomposites as compared to S0 (within an error limit of ±05%.
at 140 C and 170 C, respectively. The changes in dynamic storage modulii and complex viscosity values with angular frequency were monitored. A steady decrease in complex viscosity (∗ and a monotonous increase in dynamic storage modulus is (a) 3.0×104
Complex viscosity (Pa-s)
2.5×10
S0 S3OSL S3OScL S3cOSL
4
2.0×104
1.5×104
1.0×104
5.0×103
5
10
15
20
25
30
Frequency (Hz) (b) 1600 S0 S3OSL S3OScL S3cOSL
Complex viscosity (Pa-s)
1400 1200 1000 800 600 400
5
10
15
20
25
30
Frequency (Hz) Fig. 7. (a) Complex viscosity versus frequency plot of PUCN at 140 C, (b) Complex viscosity versus frequency plot of PUCN at 170 C.
8
Table V.
Temperature corresponding to 5, 50 and 80% degradation.
Sample ID S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
T5
T50
T80
300.0 317.1 (17.1)a 315.6 (15.6) 319.4 (19.4) 308.1 (8.1) 328.7 (28.7) 325.5 (25.5)
378.5 388.3 (9.8) 387.6 (9.1) 388.9 (10.4) 389.2 (10.7) 389.3 (10.8) 392.4 (13.4)
400.2 412.4 (12.2) 414.6 (14.4) 414.3 (14.1) 415.9 (15.7) 412.2 (12.0) 418.2 (18.0)
a Values presented in the parenthesis indicate the increase in thermal stability as compared to the neat TPU (within an error limit of ±1%.
observed with the increase in frequency, independent of the temperatures studied here (140 and 170 C). In general, two different sorts of behaviors are encountered at two different temperatures. At 140 C, G > G whereas, at 170 C, G > G , for all the composites (independent of the types of clay).8 It is observed that at 140 C, the complex viscosity (∗ of the all the nanocomposites are lower than the neat TPU within the frequency range studied here (0.033 to 30 Hz). This is due to the contribution of the shear thinning events due to the change in molecular relaxation.27 However, among the nanocomposites, ∗ values grossly follows the order: S3cOSL > S3OScL > S3OSL. This is ascribed to the degree of dispersion of the clay platelets in TPU matrix. This is possibly because of the increase in the degree of dispersion leading to increased polymer-filler interaction thereby increasing the ∗ values. In all the cases a pseudoplastic behavior is observed at this temperature. Interestingly, at 170 C, the complex viscosity (∗ of the all the nanocomposites are higher than that of the neat TPU within the frequency range studied here (0.033 to 30 Hz). This may be due to softening of the hard domain (long range ordering) present in the neat TPU at this temperature. Hence, major contributions towards the ∗ values are attributed to the presence of fillers. The trend in complex viscosity remains similar to that observed at 140 C (S3cOSL > S3OScL > S3OSL). This is again due to the greater degree of dispersion in cOSL as compared to other varieties of modified nanoclays. 4.8. Thermogravimetric Analysis TGA thermograms of the neat TPU and nanocomposites with 1% clay content are shown in Figure 8. It is observed that maximum onset degradation (T5 is observed with 1% clay content in all the nanocomposites. Improvement in thermal stability of 17.1, 19.4 and 28.7 C with S1OSL, S1OScL and S1cOSL, respectively is observed as compared to the neat TPU (Table IV). From the TEM photomicrographs (Section 4.4.1), it is observed that the extent of dispersion follows the order ScOSL > SOScL > SOSL. Thus, it may be inferred that thermal stability of the nanocomposite is directly proportional to the extent of dispersion. As compared to the ionically modified clay,8 silane modified and dual modified clays offer enhanced thermal stability. This is because of the formation of a stronger siloxane oligomer. However, increase in the amount of clay renders the decrement in thermal stability of the nanocomposites. This is because of the increased contribution from the decomposition of the excess modifiers present inside the clay galleries.8 9 However, at higher levels of degradation (e.g., 50% and 80%) nanocomposites containing 3 wt% clay offers higher thermal stability.
RESEARCH ARTICLE
Adv. Sci. Lett. 4, 1–9, 2011
100 S0 S1OSL S3OSL S1OScL S3OScL S1cOSL S3cOSL
60
100
References and Notes 90
40
20
Weight %
Weight %
80
providing financial support during this work. The authors are highly thankful to Mr T. K. Mallik of M/S Bayer Materials Science Pvt Ltd, Chennai for supplying TPU to carry out this work. The helps provided by Mr. S. Praveen, DRDO, Mumbai is highly acknowledged.
S0 S1OSL
80
S3OSL S1OScL
70
S3OScL S1cOSL S3cOSL
60 275
0
300
325
350
Temperature (°C)
300
400
Temperature (°C) Fig. 8. TGA thermograms of PUCN. (Inset represents the magnified view from 275 to 350 C).
5. CONCLUSIONS Laponite RD is successfully modified by covalent and dual modification techniques. Highest extent of modification is observed with initial ionic modification followed by covalent modification technique (cOSL). Simple covalently modified clay exhibits an aggregated morphology in TPU matrix. Initial covalent modification followed by ionic modification (OScL) results in a combined aggregated and exfoliated morphology in TPU matrix, whereas, a very high degree of exfoliation persist with cOSL. Dual modified clays possess the potential to change the phase separated morphology of TPU owing to the sites for secondary interaction. The storage modulus value in the glassy region (at −60 C) and in the rubbery region +98 C) are improved by 172.8% (with S3OScL) and 85% (with S3cOSL), respectively as compared to that of the neat TPU. Complex viscosities of the nanocomposites are directly proportional to the degree of dispersion of the modified nanoclays, independent of the temperatures studied here. The onset of degradation is found to increase by 28.7 C as compared to that of the neat TPU in case nanocomposite containing merely 1% cOSL. Thus the dual modification of Laponite offers the avenue to highly disperse the clay platelet in the TPU matrix and this can significantly improve the technical properties and thermal stability of the TPU matrix.
Acknowledgment: The authors are thankful to the Indian Space Research Organization (ISRO) and IIT Kharagpur for
1. M. Alexandre and P. Dubois, Mater. Sci. Eng. (R) 28, 1 (2000). 2. Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, and O. Kamigaito, J. Mater. Res. 8, 1185 (1993). 3. P. B. Messersmith and E. P. Giannelis, J. Polym. Sci. A., Polym. Chem. 33, 1047 (1995). 4. A. Okada, Y. Fukushima, M. Kawasumi, S. Inagaki, A. Usuki, S. Sugiyama, T. Kurauchi, and O. Kamigaito, US Patent 4,739,007 (1988). 5. J. W. Gilman, C. L. Jackson, A. B. Morgan, R. H. Harris, E. Manias, E. P. Giannelis, M. Wuthenow, D. Hilton, and S. Phillips, Chem. Mater. 12, 1866 (2000). 6. N. Sheng, M. C. Boyce, D. M. Parks, G. C. Rutledge, J. I. Abes, and R. E. Cohen, Polymer 45, 487 (2004). 7. A. K. Mishra, S. Chattopadhyay, G. B. Nando, and E. Devadoss, Designed Monom. Polym. 11, 395 (2008). 8. A. K. Mishra, G. B. Nando, and S. Chattopadhyay, J. Polym. Sci. Part B: Polym. Phys. 46, 2341 (2008). 9. A. K. Mishra, S. Chattopadhyay, and G. B. Nando, J. Appl. Polym. Sci. 115, 558 (2010). 10. Y. I. Tien and K. H. Wei, Macromolecules 34, 9045 (2001). 11. Y. I. Tien and K. H. Wei, J. Appl. Polym. Sci. 86, 1741 (2002). 12. P. Ni, J. Li, J. Suo, and S. Li, J. Appl. Polym. Sci. 94, 534 (2004). 13. P. Ni, Q. Wang, J. Li, J. Suo, and S. Li, J. Appl. Polym. Sci. 99, 6 (2006). 14. N. N. Herrera, J.-L. Putaux, and E. Bourgeat-Lami, Prog. Solid State Chem. 34, 121 (2006). 15. N. N. Herrera, J.-M. Letoffe, J.-L. Putaux, L. David, and E. Bourgeat-Lami, Langmuir 20, 1564 (2004). 16. N. N. Herrera, J.-M. Letoffe, J.-P. Reymond, and E. Bourgeat-Lami, J. Mater. Chem. 15, 863 (2005). 17. M. Park, I.-K. Shim, E.-Y. Jung, and J.-H. Choy, J. Phys. Chem. Solid 65, 499 (2004). 18. P. A. Wheeler, J. Wang, J. Baker, and L. J. Mathias, Chem. Mater. 17, 3012 (2005). 19. P. A. Wheeler, J. Wang, and L. J. Mathias, Chem. Mater. 18, 3937 (2006). 20. J, Wang, P. A. Wheeler, W. L. Jarrett, and L. J. Mathias, J. Appl. Polym. Sci. 106, 1496 (2007). 21. S. Borsacchi, M. Geppi, L. Ricci, G. Ruggeri, and C. A. Veracini, Langmuir 23, 3953 2007. 22. G. Socrates, Infrared Characteristic Group Frequencies, John Wiley and Sons, Wiley-Interscience Publication, New York (1980). 23. R. Gadzała-Kopciuch, M. Kluska, M. Wełniak, and B. Buszewski, Mater. Chem. Phys. 89, 228 (2005). 24. N. V. Venkataraman and S. Vasudevan, J. Phys. Chem. B 104, 11179 (2000). 25. J. L. Sormana, S. Chattopadhyay, and J. C. Meredith, J. Nanomater. 869354, 9 (2008). 26. B. Finnigan, K. Jack, K. Campbell, P. Halley, R. Truss, P. Casey, D. Cookson, S. King, and D. Martin, Macromolecules 38, 7386 (2005). 27. A. K. Mishra, S. Mushtaq, G. B. Nando, and S. Chattopadhyay, Rheol. Acta. (2010).
Received: 29 April 10. Accepted: 28 May 10.
9
