Plastic deformation of amorphous Poly(L/DL-lactide): Structure evolution and physical properties

Plastic deformation of amorphous, thermally noncrystallizable poly(L/DL-lactide) 70/30 (P(L/DL)LA) was induced by a plane-strain compression in a channel-die at different temperatures, above the glass transition (T-g) from 60 to 90 degrees C. Samples undeformed (reference) and deformed to different compression ratios, from 4.6 to 23.0, were studied by X-ray diffraction, thermally modulated differential scanning calorimetry, light microscopy, and mechanical methodsviscoelastic and tensile tests. The effects of the compression ratios and deformation temperatures on the final structure and properties of the P(L/DL)LA were evaluated. It was revealed that plastic deformation transformed an amorphous P(L/DL)LA (thermally noncrystallizable) to a crystalline fibrillar texture oriented in the flow direction. Fibrillar texture was formed in spite of the tendency of the plane-strain compression to form single-crystal-like texture. The crystallite size in the transverse direction was small, up to 90 A at the highest compression ratio. No evidence of lamellar organization and features of supermolecular structure were detected by small-angle X-ray scattering and light microscopy, respectively. The oriented samples exhibited a low crystallinity degree at the level of 6-9% at the highest compression ratio. The main transformation mechanism was shear and orientation-induced crystallization. The crystalline phase was in the alpha crystallographic modification of poly(lactide) typically formed in more stereoregular poly(lactide) by thermal treatment. The glass transition increased with the increase of compression ratio reflecting the increase of orientation of the polymer chains. The tensile strength of deformed samples was improved considerably in comparison to that of the reference sample.