Vasily A. Topolkaraev

Crystallization and phase separation in blends of high stereoregular poly(lactide) with poly(ethylene glycol)

The effect of cooling rate on crystallization and subsequent aging of high stereoregular poly(lactide) (PLA) blended with poly(ethylene glycol) (PEG) was studied by thermal analysis and by direct observation of the solid state structure with atomic force microscopy (AFM). Blending with PEG accelerated crystallization of PLA. When a PLA/PEG 70/30 (wt/wt) blend was slowly cooled from the melt, PLA crystallized first as large spherulites followed by crystallization of PEG. The extent of PLA crystallization depended on the cooling rate, however, for a given blend composition the PEG crystallinity was proportional to PLA crystallinity. The partially crystallized blend obtained with a cooling rate of 30 °C min−1 consisted of large spherulites dispersed in a homogeneous matrix. The blend was not stable at ambient temperature. With time, epitaxial crystallization of PEG on the edges of the spherulites depleted the surrounding region of PEG, which created a vitrified region surrounding the spherulites. Further from the spherulites, the homogeneous amorphous phase underwent phase separation with formation of a more rigid PLA-rich phase and a less-rigid PEG-rich phase. Decreasing the amount of PEG in the blend decreased the crystallization rate of PLA and increased the nucleation density. The amount of PLA crystallinity did not depend on blend composition, however, PEG crystallinity decreased to the extent that PEG did not crystallize in a PLA/PEG 90/10 (wt/wt) blend.

Breathable polymer films produced by the microlayer coextrusion process

Fabrication of a breathable film by the microlayer coextrusion process is described. Poly(ethylene oxide) (PEO) was microlayered with a filled polyolefin, either CaCO3-filled polyethylene or CaCO3-filled polypropylene. The thickness of individual layers was varied by increasing the total number of layers in the microlayered film from 8 to 4096. The water vapor transport rate (WVTR) was measured for microlayer films that varied in composition and number of layers. Especially with the PP(CaCO3)/PEO system, systematic variation in composition and number of layers made it possible to obtain large changes in the WVTR. The results were related to the tortuousity of the pathway through the microlayer. The filled polyolefins acted as a barrier to water vapor transport through the hydrophilic PEO. As the individual layers were made thinner by increasing the total number of layers, the polyolefin layers changed from continuous to discontinuous. Tortuousity concepts were used to correlate the increase in WVTR with an effective aspect ratio of the discontinuous polyolefin layers. In addition to high WVTR values, the breathable films produced by microlayering PEO with a filled polyolefin exhibited excellent mechanical properties.