Crystallization of polylactide films: An atomic force microscopy study of the effects of temperature and blending

Surface crystallinity on films of poly(l-lactide), poly(l/d-lactide) and their blends with poly(d-lactide) was studied. The isothermal spherulitic growth rate and its dependence on temperature were studied using tapping mode atomic force microscopy and ex situ isothermal crystallization. Using this technique, it is possible to extend spherulitic growth rate measurements to the region of significantly higher supercooling where nucleation concentration makes the use of in situ hot stage optical microscopy impossible. It was confirmed that while a poly(l/d-lactide) copolymer exhibits the typical “bell” shaped crystallization rate-temperature dependence, poly(l-lactide) exhibits a nonsymmetrical behavior having two crystallization rate maxima at 105 °C and 130 °C. As expected, the spherulitic growth rate of poly(l-lactide) was significantly higher than that of poly(l/d-lactide). The different types of crystalline formations exhibited at the surface of polylactide films are shown and discussed. The crystalline long spacing of poly(l-lactide) was also measured directly using tapping mode AFM and was found to be 19 nm at 165-170 °C. At low supercooling, several different scenarios of individual crystal formation were observed: purely flat-on stacks, purely edge-on stacks and scenarios where edge-on crystals flip to flat-on crystals and vice versa, where flat-on crystals yield edge-on sprouts. The preferred direction of growth of lamellae of both poly(l-lactide) and poly(d-lactide) was found to be counter-clockwise relative to the free surface.Finally, the crystallization kinetics of blends of poly(l-lactide) and poly(l/d-lactide) with poly(d-lactide) were studied. In such blends a triclinic stereocomplex crystalline structure forms between chains of opposite chirality and a pseudo-orthorhombic α-crystal structure forms between chains of like chirality. The presence of the stereocomplex crystals affects both the nucleation and the growth of the α-crystals. In fact depending on the stereocomplex content and the crystallization temperature the α-crystallization can either be enhanced or be inhibited. Interestingly it was found that the presence of the stereocomplex had a much stronger effect on the α-crystallization of poly(l/d-lactide) than on the α-crystallization of poly(l-lactide).