Why Is Crystalline Poly(lactic acid) Brittle at Room Temperature?

Abstract

This work shows that the environmental-friendly biorenewable and biodegradable poly(lactic acid) (PLLA) can be made to have superior mechanical and thermal characteristics and thus show promise to replace conventional petroleum-based polymers such as polyethylene terephthalate. Using time-resolved polarized optical microscopy (POM), we have investigated how and why conventional crystallization tends to cause deterioration of ductility in semicrystalline PLLA. Specifically, the POM study based on partially crystallized PLLA samples promotes the idea that the spherulitic crystals are mechanically weaker than glassy noncrystalline domains whose cohesive strength stems from the chain networking because of intermolecular uncrossability. By removing the large spherulitic crystal formation and inducing nanocrystal formation through melt-stretching of PLLA in its amorphous state, we identified a completely transparent crystalline state of PLLA that is extremely tough and resistant against heat.

Long-Term Physical Aging Tracked by Advanced Thermal Analysis of Poly(N-Isopropylacrylamide): A Smart Polymer for Drug Delivery System

“…This process applies to natural and synthetic materials. The structural recovery can be monitored by the volume recovery or the enthalpy recovery [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. In literature, the physical aging term is also used, but it applies to changes in mechanical properties.…”

“…In literature, the physical aging term is also used, but it applies to changes in mechanical properties. The result of the physical aging process is the change of material properties, such as in length, hardness, and brittleness [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ]. On the other hand, the physical properties of amorphous and semicrystalline material at the non-equilibrium state are changed versus time and temperature.…”

Double Yielding in Deformation of Semicrystalline Polymers

“…The explanation of why semicrystalline PLLA is brittle below T g in extension, was presented in detail in our previous publication. [ 19 ] Also the reason behind why the same semicrystalline polymer which is brittle in extension could turns ductile in compression is the subject of our future study. [ 20 ] Here we only focus on importance of understanding semicrystalline state as a composite structure and the crucial role of amorphous phase in exerting deformation on crystalline phase.…”

“…Interestingly as predicted, near T g the two yielding appear as two individual peaks in the stress versus strain curves. Because of higher segmental mobility of amorphous phase and amplification effect in deformation of this phase, [ 19 ] the earlier peak has to associate with the yielding of amorphous phase and the second one should be related to the yielding of crystalline phase. In other words, in order to crystalline phase to be able to yield it requires that initially the amorphous phase to yield and develop enough chain tension, so the developed high chain tension can act on the crystalline regions and result in yielding of crystalline phase.…”

“…Amorphous portions of the chains were selectively degraded by maintaining a semicrystalline compression piece of PLLAc at high temperature ≈140–150 °C for 100 h. Molecular weight M w measurements based on gel permeation chromatography GPC indicated that after this treatment the overall molecular weight changes from 108 to 47 kg mol −1 . The molecular weight M w after degradation (47 kg mol −1 ) is still far above entanglement molecular weight M e of 3.24 kg mol −1 ; [ 19 ] however, the response of semicrystalline polymer to the uniaxial compression changed from ductile to a brittle one ( Figure ) upon this thermal treatment. This indicates that degradation mostly took place in the amorphous phase of the semicrystalline sample.…”