Linear and nonlinear rheological behavior and crystallization of semicrystalline poly(styrene)-poly(L-lactide) block copolymers
The rheological behavior of a poly(styrene)-poly(l-lactide) (PS-PLLA) block copolymer was investigated in the viscoelastic linear and nonlinear regime. Large amplitude oscillatory shear (LAOS) experiments in the nonlinear regime led to an exponential decay of the G moduli and the nonlinearity parameter I3/1. During LAOS, the lamellar microstructure of a low molecular weight PS-PLLA copolymer was orientated parallel to the shear field to achieve a macroscopically ordered material. The degree of orientation was analyzed via small angle X-ray scattering (SAXS) measurements. The crystallization kinetics could be accurately described by the Avrami equation as determined by DSC and rheology. These experiments revealed that the crystallization process was slower for the PS-PLLA copolymers than for the PLLA homopolymers. SAXS was also used to monitor the crystallization of the PS-PLLA copolymers. When the temperature of the crystallization experiments, Tc, was lower than the gl ass transition temperature of the amorphous PS block, Tg a (hard confinement), the lamellar microstructure of the low molecular weight PS-PLLA was maintained apart from a small increase in the domain spacing. When Tc was higher than Tg a of PS (soft confinement), the block copolymer also retained its lamellar structure, but the increase in the domain spacing was unexpectedly increased compared to what occurred for crystallization under hard confinement conditions. In addition, two-dimensional (2-D) SAXS diffractograms indicated a loss of the structure periodicity during crystallization under soft confinement.