Investigation of chain mobility in different crystal polymorphs of poly(lactic acid), PLA

The NMR-study of the chain mobility of a’-crystals of PLA is considered as a continuation of our research efforts for understanding the structure, properties, and stability of that mesophase. Among others, prior work included the analysis of the effects of the molar mass and d-isomer content in PLA on the characteristics of the a’-phase, in particular the thermal stability with respect to the a’-phase. It has been found that a’-crystal may reorganize on heating in the solid state, that is, below their melting point, however, with the mechanism of this particular reorganization process not known yet (1-2).

The aim of the project is the characterization of phase structure and of the chain mobility in the crystals of PLLA. Based on earlier results from calorimetric experiments (E7-E9), we plan to apply different crystallization regimes to samples of PLLA of different chain architecture. The sample program covers different length of the polymers (i.e. different molar masses), as well as a different content of the D-enantiomere. The latter is also important because it may result in either a substantial share of stereocomplexes or a substantially modified crystalline structure which in turn will affect both the molecular mobility as well as the mechanical properties (3). The NMR investigations will be complemented by caloric and mechanical experiments. The latter are necessary for the proper sample preparation and characterization on one hand, as well as to link the microscopic dynamic pictures to the macroscopic mechanical properties on the other hand. The more scientific emphasis is the effect of conformationally disorder on molecular dynamics and material properties, the more practical final goal is to adjust the manufacturing conditions for an optimum relation of material performance and cost.