A Study of the Structure and Molecular Dynamics of Oriented, Semi-crystalline Poly(lactic Acid)

Open Access
Author:
Ellis, Annie Ruth
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
September 21, 2012
Committee Members:
  • James Patrick Runt, Thesis Advisor
Keywords:
  • Poly(Lactic Acid)
  • crystallinity
  • dielectric relaxation spectroscopy
Abstract:
Poly(lactic acid), a semi-crystalline polymer, is used in a wide range of applications because it is biodegradable, biocompatible, versatile, and easy to process. Some of these applications include oriented fibers and films. In this research, poly(lactic acid), [PLA], was uniaxially deformed at 70°C, which is above its glass transition temperature. Samples of different magnitudes of deformation (25%, 70%, and 110%) were selected as oriented samples. For use as controls, amorphous and crystalline unoriented PLA sample were produced under similar temperature and pressure conditions. Differential scanning calorimetry was used to confirm a constant glass transition temperature of 63°C and a constant melting temperature 149°C independent of orientation. Differential scanning calorimetry confirmed that the degree of crystallinity increased with orientation. Wide angle x-ray diffraction demonstrated that the α crystal structure was formed during initial crystallization and amorphous chains orient parallel to the deformation direction and convert to crystalline regions during orientation. Small angle x-ray scattering verified that orientation did not affect the long period spacing. Finally, dielectric relaxation spectroscopy was able to confirm that orientation lead to the formation of additional rigid amorphous regions in between crystalline regions and at the crystalline and amorphous interface. Uniaxial orientation lead to a decrease in dielectric strength and a broader segmental relaxation, but the mean relaxation time and temperature were unaffected.