Morphological Study on Thermal Shrinkage and Dimensional Stability Associated with Oriented Poly(lactic acid)

Abstract

Morphological features governing dimensional stability of variously processed poly(lactic acid) have been investigated. The exotherm observed near the glass transition temperature (Tg) is one of the most sensitive indicators reflecting structural changes as a function of temperature and time. A combination of spectroscopic and calorimetric techniques was used to analyze the amorphous chain conformation of deformed PLA samples. We found that nucleation-enhanced crystallization is the major contributor to the exotherm, not shrinkage. Our analysis suggests the existence of two amorphous regions. One is characteristic of the deformed structure. The other is consistent with a relaxed isotropic conformation distribution. Results showed that the existence of deformed amorphous chains is necessary for any fiber shrinkage. But above a critical crystallinity of 40%, a rigid crystalline network prevents shrinkage, even for highly deformed chains. In addition, the previously reported value of tg′t fraction in the amorphous region (f a ) 0.80) was validated in the current study.

Structure–property relationship for poly(lactic acid) (PLA) filaments: physical, thermomechanical and shape memory characterization

“…The first heating scan of the T sample exhibited an intense exothermal peak right after T g due to chain rearrangement in the deformed amorphous regions. The post-T g exothermic peak was attributed to the cold crystallization and/or mechanical shrinkage of oriented non-crystalline domains [26,32]. The rearrangement of oriented amorphous chains into the crystalline phase increases the rubbery plateau modulus dramatically [41].…”

“…Differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) have been used to ascertain the processing/crystallization effects on the fiber thermal [6,8,11,12,25,26] and thermomechanical properties [8,14,25]. Detailed thermomechanical analysis of the PLA fibers revealed storage modulus, E′, loss modulus, E″ and tan δ curves sensitivity to fiber structure.…”

Crystallization behavior and spherulitic morphology of poly(lactic acid) films induced by casting process

“…And this main transition is suggested to be the origins of shrinkage in PLA fiber samples according to Aou and al. [2]. Concerning the heat of melting, studies in pure PLA containing less than 6% D [18] did not show a heat of crystallization on reheating process indicating that most of the crystallization process was completed during the quenching procedure.…”

“…Some authors [17] found that the higher T g at higher L lactide contents could be explained by intrinsically greater chain stiffness. Others [2] declared that the exothermal observed near the glass transition temperature (T g ) is one of the most sensitive indicators reflecting structural changes as a function of temperature and time. And this main transition is suggested to be the origins of shrinkage in PLA fiber samples according to Aou and al.…”

“…This involves complex crystallization behavior similar to those random copolymers. Hence, varying the D/L ratio, the stereoisomer content in the backbone, has proven useful in controlling crystallization kinetics and maximum degree of crystallinity [2]. Besides, thermal behavior of PLA is different from other semi-crystalline polymers and it involves multiphase transitions during heating, such as glass transition, chain relaxation, cold crystallization, and melting of crystals [2].…”