Abstract

The purpose of this research was to explore the possibility of studying crystallization behavior and morphology of thin solution cast polylactide films using Atomic Force Microscopy. The crystallization of L-/D-polylactide blends was studied and precise measurements of the spherulite growth rates were performed. The changes of crystalline morphology over crystallization temperature was observed and analyzed from the point of view of modern crystallization theory. Also a variety of different experimental techniques was used for the polylactide blends characterization including DSC and FT-IR. Since the crystallization rate of polylactide is very high, a custom superfast cyclic heating and cooling technique was developed to ensure isothermal annealing conditions. The validity of this technique was successfully verified. DSC analysis showed that L-/D-polylactide blends form a triclinic stereocomplex during casting with a high melting point which suppresses cold crystallization. In this study, a custom technique for the measurement of the isothermal glass transition temperature using elongational measurements was performed on a miniature uniaxial stretching device. It was found that small amounts of residual solvent can significantly decrease the glass transition temperature of polylactide. Significantly higher nucleation in the solution cast polylactide as compared to bulk polymer was also observed. Crystallization behavior and morphology of solution cast polylactides containing different amounts of D-polylactide content were studied. It was experimentally proven that while blends of L-/D-polylactide copolymer with D-polylactide demonstrate typical "bell" shaped crystallization temperature dependence, blends of pure L-polylactide with the same D-polylactide have unusually high spherulite growth rates at high temperatures. For all blends, addition of D-polylactide significantly decreased the spherulite growth rate. An extensive kinetic analysis was performed on the experimental data, crystallization regimes were identified and critical nuclei formation energies were determined for the different blends. Moreover, despite having a higher molecular weight, the pure poly(L-lactide) demonstrates almost twice higher spherulite growth rate than copolymer probably caused by the absence of D-polylactide units in chain thus absence of the steric difficulties for the crystallization. This, in turn, leads to the significantly lower Kg values for the Purasorb PL and its blends with Purasorb PD as compared to those of the copolymer in regimes II and III and, respectively, to higher spherulite growth rates. The addition of the D-polylactide not only suppresses the overall crystallization of the blend due to stereocomplexation but also shifts the crystallization maximum to the lower temperature region. From the kinetics analysis, it was found that this is caused by the lowering of the regime II transition temperature