Crystallization Characterization Polymer Crystallization Hybrid Particle ...
Graduate Category: Engineering and Technology Degree Seeking: Mechanical Engineering, Materials Science Abstract ID# 1797
Hierarchically Reinforced Polypropylene-Alumina Discontinuous Composites Jessica L. Faust, Marilyn L. Minus, Randall M. Erb
Polymer Crystallization
Motivation There is a large industrial push towards using composites which offer high specific strengths, flexibility, and flaw tolerance. Most high performance applications use continuous fiber composites due to their superior strength and modulus properties. However, discontinuous fiber composites (DFCs) can be implemented in applications requiring non standard geometries and applied to bulk manufacturing techniques like injection molding or tape casting. The reduced strength in DFCs is due to (1) the loss of directionality in alignment of filler-fiber; (2) stress concentration between neighboring filler-fibers; and (3) regions of high stress and strain that arise at the edge of filler-fibers1. If these factors can be addressed then the performance of DFCs will be enhanced leading to increased usage of lightweight strong materials offering cost and energy savings across many industries.
Our Approach – Graded Reinforcement
Crystal growth is induced using polymer solvent crystallization techniques. A solvent/nonsolvent process is used to nucleate crystal growth on the surface of hybrid particles at the interface of the immiscible solvents. Introduction of HP’s to the dilute polypropylene solution was conducted just above the crystallization temperature in order to promote localized crystallization at the surface. Isothermal Solution Processing to Create Local Crystallization Polymer crystallization during droplet addition Syringe HP’s dispersed in DMF
Polypropylene/ xylene solution
Solvent/nonsolvent process at T = 72°C
Reinforcing the fibers Concentrated regions of stress at the fiber edge cause premature failure. In this work, we instead suggest forming a graded strength architecture around the fibers to reduce these high stress regions and improve the overall toughness of the material. Graded reinforcement architecture supports the area surrounding the fiber and allows the matrix to better transfer the load to the fiber. Graded architectures are abundant in nature and have been show in synthetic systems to greatly increase material toughness. Discontinuous Fiber Composite (DFC) Interaction between the fiber-matrix interface plays significant role in mechanical properties
Hot plate
The total crystallization time is varied by adjusting the volume and height of the polymer/xylene solution. The excess solvent is removed and the concentrated polymer/particle solution is dried and cast into a mold before characterization of the sample.
Crystallization Characterization Successful Crystallization on Particle Surface Polypropylene crystals readily form on the surface of the CNT-coated hybrid particles creating a graded architecture in the final composite.
Traditional DFC’s under load Poor interaction with polymer matrix Crystalline Region
Particle
Micro-crack formation
3 um
Carbon Nanotube
DFC with stiffness gradient under load
30.0 nm
Ductile polymer matrix
2.0 μm
Polymer shish-kabab nucleated by CNT
Local stiffness gradient
Differential Scanning Calorimetry and Wide Angle X-Ray Diffraction Improvements in the fiber-matrix interface improves mechanical properties
Stiffness gradient improves load transfer and reduces micro-crack formation
Using a Crystalline Strength Gradient
DSC results show an increase in crystallization temperature for the hybrid particle films. WAXD diffraction peaks show the presence of the gamma phase indicating the trans-crystalline lamella structure surrounding the hybrid particles, seen only in the presence of CNT nucleated crystals.2
To form this graded architecture, our approach is to grow crystalline regions of the polymer matrix on the surface of the fiber. Carbon nanotubes (CNT’s) have been found to be an excellent nucleating agent for crystal growth2 and are used as a step between the atomically flat fiber to the crystal region of the matrix. Amorphous Region Crystalline Region Carbon Nanotube
Particle Carbon Nanotube
Alumina Platelet
18
120.8
14
WAXD Results – Transcrystalline Peaks
5wt% Hybrid Particles 5wt% Alumina Polypropylene Control
16
Heat Flow, W/g
Crystallized Polymer Chain
Change in Crystallization Temperature
126.4 12 10 120.9
8 6 4 2 0
Hybrid Particle Assembly
105
Homogeneous Hybrid Particle Assembly Dispersed Single-walled Carbon Nanotubes
115
120
CNT Saturation Limit
Hybrid Particles
Above: The surface of two randomly selected platelets showing CNT’s uniform coating on the surface
130
135
140
Polymer Crystallization Growth To further understand the crystallization growth process, a single crystal alumina (7.5mm dia., 400μm thick) was decorated with CNT’s and submerged in a dilute PP-xylene solution at 72°C. The results of these studies showed that uniform coating was accomplished after 5 minutes. The experimental procedure is easily transferrable to the hybrid particles resulting in a uniform polymer coating. Figures A and B show polymer growth on surface of alumina connects to form uniform coating. These isothermal conditions were applied to hybrid particles and lead to a uniform surface coating shown in Figure C.
SEM Images of Uniform CNT Coating on Al2O3 Platelets Cellulose nanocrystals offer low cost alternative to CNT’s
2.0 μm
125
Temperature, °C
CNT’s are attached to the surface of the particles by colloidal forces. Attachment seems irreversible due to the presence of Van der Waals forces. Aggregates drastically stunt crystal growth, therefore we have conducted studies towards homogeneity. The CNT’s are thoroughly sonicated and filtered before attaching to the platelets to ensure a uniform surface coverage.
Alumina Reinforcing Micro-Platelet
110
1.0 μm
4.0 μm
Contact Information: Jessica Faust, [email protected] or Prof. Randall M. Erb, [email protected] 334 Snell Engineering Center, 360 Huntington Avenue, Boston, MA 02115, USA
A
C
20.0 μm
B
20.0 μm
5.0 μm
Conclusion Here we suggest a method for graded reinforcement for discontinuous fiber composites in a polymer matrix. The graded reinforcements are formed by decorating the reinforcement particles with carbon nanotube’s and growing crystalline regions along the fiber surface. By improving the regions of high stress and strain that arise at the edges of the filler-fibers, the performance of discontinuous fiber composites is enhanced. We hope to increase the performance of DFCs and increase the use of these lightweight, strong materials and offer both cost and energy savings across many industries.
References: [1] Zhang S., Minus M., Zhu L., Wong C., and Kumar S., Polymer, 2008, 49(5) 1356.
Hierarchically Reinforced Polypropylene-Alumina Discontinuous Composites Jessica L. Faust, Marilyn L. Minus, Randall M. Erb
Polymer Crystallization
Motivation There is a large industrial push towards using composites which offer high specific strengths, flexibility, and flaw tolerance. Most high performance applications use continuous fiber composites due to their superior strength and modulus properties. However, discontinuous fiber composites (DFCs) can be implemented in applications requiring non standard geometries and applied to bulk manufacturing techniques like injection molding or tape casting. The reduced strength in DFCs is due to (1) the loss of directionality in alignment of filler-fiber; (2) stress concentration between neighboring filler-fibers; and (3) regions of high stress and strain that arise at the edge of filler-fibers1. If these factors can be addressed then the performance of DFCs will be enhanced leading to increased usage of lightweight strong materials offering cost and energy savings across many industries.
Our Approach – Graded Reinforcement
Crystal growth is induced using polymer solvent crystallization techniques. A solvent/nonsolvent process is used to nucleate crystal growth on the surface of hybrid particles at the interface of the immiscible solvents. Introduction of HP’s to the dilute polypropylene solution was conducted just above the crystallization temperature in order to promote localized crystallization at the surface. Isothermal Solution Processing to Create Local Crystallization Polymer crystallization during droplet addition Syringe HP’s dispersed in DMF
Polypropylene/ xylene solution
Solvent/nonsolvent process at T = 72°C
Reinforcing the fibers Concentrated regions of stress at the fiber edge cause premature failure. In this work, we instead suggest forming a graded strength architecture around the fibers to reduce these high stress regions and improve the overall toughness of the material. Graded reinforcement architecture supports the area surrounding the fiber and allows the matrix to better transfer the load to the fiber. Graded architectures are abundant in nature and have been show in synthetic systems to greatly increase material toughness. Discontinuous Fiber Composite (DFC) Interaction between the fiber-matrix interface plays significant role in mechanical properties
Hot plate
The total crystallization time is varied by adjusting the volume and height of the polymer/xylene solution. The excess solvent is removed and the concentrated polymer/particle solution is dried and cast into a mold before characterization of the sample.
Crystallization Characterization Successful Crystallization on Particle Surface Polypropylene crystals readily form on the surface of the CNT-coated hybrid particles creating a graded architecture in the final composite.
Traditional DFC’s under load Poor interaction with polymer matrix Crystalline Region
Particle
Micro-crack formation
3 um
Carbon Nanotube
DFC with stiffness gradient under load
30.0 nm
Ductile polymer matrix
2.0 μm
Polymer shish-kabab nucleated by CNT
Local stiffness gradient
Differential Scanning Calorimetry and Wide Angle X-Ray Diffraction Improvements in the fiber-matrix interface improves mechanical properties
Stiffness gradient improves load transfer and reduces micro-crack formation
Using a Crystalline Strength Gradient
DSC results show an increase in crystallization temperature for the hybrid particle films. WAXD diffraction peaks show the presence of the gamma phase indicating the trans-crystalline lamella structure surrounding the hybrid particles, seen only in the presence of CNT nucleated crystals.2
To form this graded architecture, our approach is to grow crystalline regions of the polymer matrix on the surface of the fiber. Carbon nanotubes (CNT’s) have been found to be an excellent nucleating agent for crystal growth2 and are used as a step between the atomically flat fiber to the crystal region of the matrix. Amorphous Region Crystalline Region Carbon Nanotube
Particle Carbon Nanotube
Alumina Platelet
18
120.8
14
WAXD Results – Transcrystalline Peaks
5wt% Hybrid Particles 5wt% Alumina Polypropylene Control
16
Heat Flow, W/g
Crystallized Polymer Chain
Change in Crystallization Temperature
126.4 12 10 120.9
8 6 4 2 0
Hybrid Particle Assembly
105
Homogeneous Hybrid Particle Assembly Dispersed Single-walled Carbon Nanotubes
115
120
CNT Saturation Limit
Hybrid Particles
Above: The surface of two randomly selected platelets showing CNT’s uniform coating on the surface
130
135
140
Polymer Crystallization Growth To further understand the crystallization growth process, a single crystal alumina (7.5mm dia., 400μm thick) was decorated with CNT’s and submerged in a dilute PP-xylene solution at 72°C. The results of these studies showed that uniform coating was accomplished after 5 minutes. The experimental procedure is easily transferrable to the hybrid particles resulting in a uniform polymer coating. Figures A and B show polymer growth on surface of alumina connects to form uniform coating. These isothermal conditions were applied to hybrid particles and lead to a uniform surface coating shown in Figure C.
SEM Images of Uniform CNT Coating on Al2O3 Platelets Cellulose nanocrystals offer low cost alternative to CNT’s
2.0 μm
125
Temperature, °C
CNT’s are attached to the surface of the particles by colloidal forces. Attachment seems irreversible due to the presence of Van der Waals forces. Aggregates drastically stunt crystal growth, therefore we have conducted studies towards homogeneity. The CNT’s are thoroughly sonicated and filtered before attaching to the platelets to ensure a uniform surface coverage.
Alumina Reinforcing Micro-Platelet
110
1.0 μm
4.0 μm
Contact Information: Jessica Faust, [email protected] or Prof. Randall M. Erb, [email protected] 334 Snell Engineering Center, 360 Huntington Avenue, Boston, MA 02115, USA
A
C
20.0 μm
B
20.0 μm
5.0 μm
Conclusion Here we suggest a method for graded reinforcement for discontinuous fiber composites in a polymer matrix. The graded reinforcements are formed by decorating the reinforcement particles with carbon nanotube’s and growing crystalline regions along the fiber surface. By improving the regions of high stress and strain that arise at the edges of the filler-fibers, the performance of discontinuous fiber composites is enhanced. We hope to increase the performance of DFCs and increase the use of these lightweight, strong materials and offer both cost and energy savings across many industries.
References: [1] Zhang S., Minus M., Zhu L., Wong C., and Kumar S., Polymer, 2008, 49(5) 1356.
