Solid-state structure and dynamics of lactide copolymers and blends

by 1972- Kanchanasopa, Mantana

This dissertation focuses on polymers synthesized from L-lactide and its stereoisomers, i.e., poly L-lactide (L-PLA), L-lactide/meso-lactide random copolymers and L-PLA/D-PLA blends. Height and phase images from tapping mode AFM experiments, acquired at various tapping forces, reveal that the morphology and lamellar thickness of the L-lactide/meso-lactide copolymers are strongly influenced by meso-lactide content and crystallization conditions. Good agreement was observed between mean lamellar thicknesses derived from AFM and previous small-angle X-ray scattering experiments. The considerable decrease in lamellar thickness with increasing comonomer content suggests that meso-lactide units are substantially excluded from the L-lactide crystal lattice, at least under the isothermal crystallization conditions used here. A dielectric study of amorphous and crystalline samples of the L-lactide/mesolactide copolymers demonstrates that the relaxation strength and mean relaxation time (and its distribution) of the segmental process are modified by the presence of the crystalline phase. The mean segmental relaxation time is longer and its distribution is broader in the samples with higher degrees of crystallinity. The significant difference in the fraction of mobile segments (determined from the relaxation strength) and the amorphous fraction derived from DSC) indicates the existence of the so-called “rigid amorphous phase”. This conclusion is also supported by the observation of a difference in temperature dependent behavior of the relaxation strength of amorphous and crystalline samples. Differences in the details of the relaxation processes, as well as the fraction of rigid amorphous phase of iv these copolymers, suggest that the chain dynamics of amorphous segments are influence by both degree of crystallinity and lamellar organization. For D-PLA/L-PLAs blends, stereocomplex crystallites are observed along with homopolymer crystals. The content of each crystal type is found to depend strongly on the composition and molecular weight of the component polymers. These parameters have only a small influence on the melting temperature of the stereocomplex but significantly affect those of homopolymer crystallites. It is suggested by the results of real-time dielectric crystallization experiments, along with WAXD and DSC measurements after crystallization, that crystallization of the stereocomplex is faster than homopolymer crystallites. Consequently, the homopolymer crystals developed under these conditions are influenced significantly by constraints imposed by existing racemic crystallites and, therefore, have lower melting temperatures than those of the neat polymers. v