Hydrolytic degradation of polylactide films deformed by the environmental crazing mechanism

Abstract

Hydrolytic degradation of porous polylactide (PLA) films with various porosity and morphology has been thoroughly investigated. It has been found that the structural and mechanical modification of PLA films by the environmental crazing mechanism results in the formation of a porous structure, is accompanied by the change in molecular-weight characteristics of the polymer and affects the localization and rate of its hydrolytic degradation. After the films were exposed in phosphate-buffered saline solution at 37 °C for 6 weeks, their average molecular weights MW and MN decreased by 1.5–2 and 3–5 times respectively, while their dispersity ĐM increased to 4–5. For PLA films containing alternating porous crazes and nonporous bulk parts, the hydrolysis leads to the appearance of bimodal molecular-weight distribution (MWD) curves. The degradation of the polymer material localizes predominantly at the craze–bulk polymer interfaces, thereby leading to a rather rapid and significant decrease in the deformation and strength properties of the films, which eventually become brittle. For PLA films, that have completely passed to the fibrillar-porous structure during the hydrolytic degradation, the character of the MWD curves remains preserved, but they gradually widen and shift toward lower molecular weights. The destruction of similar samples occurs uniformly throughout the volume, and the changes in their structural and mechanical parameters are not so abrupt, and even after the hydrolysis for 6 weeks their strength remains at a level of 90 MPa. When the porous matrix of PLA is filled with calcium phosphate (up to 30 wt%), the degradation of the polymer material occurs predominantly in the crazes and hardly proceeds in the bulk parts. The approaches proposed using PLA as an example enable one to control the degradation processes in polymers by varying their structural and morphological characteristics and incorporating inorganic fillers, thus opening new ways to the creation of bioactive and biodegradable materials with predictable degradation times.