RESEARCH MOTIVATION

• UHMWPE fibers widely used in ballistic applications due to their exceptionally high specific strength and stiffness
• Understanding of structure /property relationships is vital because current strengths and stiffness only approach 50% of theoretical values
• The major goal of our efforts is to understand the macroscale energy dissipation mechanisms of gel spun UHMWPE fibers by first understanding the load pathways at the meso/nano-scale arising from complex structure/property relationships
• In order to fully understand the structural hierarchy within these fibers, we must be able to visualize and quantify the length scales of each constituent

STRUCTURAL HIERARCHY IN UHMWPE FIBER

IMAGING FIBRILLAR MORPHOLOGY

• Atomic force microscopy (AFM) used to image surface of fibers
• Fiber morphology is complex with features oriented parallel and perpendicular to the fiber axis
• 3 fiber types being studied:
-Precursor fiber and post-draw fiber from the same spinning line
-Highly drawn S130 fiber
•Microfibril identified as fundamental structural unit
•Distribution of microfibril widths measured with AFM

EPITAXIAL FEATURES / ANNEALING EXPERIMENTS

• UHMWPE fibers composed primarily of extended chain crystals oriented axially
• AFM images reveal features oriented in circumferential direction 10’s of nm thick
• Features are larger in the circumferential direction than expected for typical shish-kebab structures, and they span multiple extended chain crystal domains
• Fibers annealed at 130°C (below equilibrium melting temp. of extended chain crystals) for 15 minutes to explore the nature of these structures

CONCLUSIONS

• Microfibril width distribution significantly narrows with increasing draw
-Likely due to melting of smaller crystals during the heating and drawing stage of production as well as consolidation of larger fibrils
• Epitaxial structures coarsen when annealed at temperatures in the range of lamellar crystal melting temperature
-Epitaxial structures only noticed in highly drawn fibers suggesting they form during the elevated drawing in the gel spinning process

ACKNOWLEDGEMENTS

Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-12-2-0022. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.