Abstract

Poly(lactic acid) is one of the most promising “green” polymers, having huge potential for use as a packaging and construction material. The lack of knowledge about polylactide-specific crystallization behaviour is one of the barriers to widespread industrial use.
In the present thesis, different aspects of one of the most important phenomena in polylactide crystallization were studied. In the series of articles included in this thesis, parameters of quiescent and field-induced crystallization were investigated. Significant attention was paid to the spherulitic morphology, growth rates, and stereocomplexation phenomenon that are intrinsic to polylactide, due its optical activity. A novel technique for growth rate measurements based on ultrafast heating and cooling of cast films, was suggested and validated.
Melt crystallization of polylactide was studied using rheological methods. A new advanced model allowing the relation of complex viscosity to melt crystallinity was proposed and verified. The physical cross-linking phenomenon was introduced and quantified in the frame of the proposed model. Also, a new technique allowing the determination of the induction time of crystallization was presented and successfully validated. A proposed technique allowed dramatically improved precision of the measurement of induction time for both quiescent and field-induced crystallization.
The phenomena of surface crystallization that had previously gone unexamined by the research community were investigated in detail. A theoretical model was suggested and its predictions were supported by extensive computer simulations. The simulation program used for research was developed during this study. It was found that surface crystallization can have an immense effect on crystallization kinetics and dramatically distort the observed thermal analysis results. Custom-designed experiments allowed the application of these results to polylactide crystallization.
Extensive studies of shear-induced crystallization were done in this work. The novel techniques developed in the research allowed for a better understanding of the field-induced phenomenon. New techniques allowing the implementation of rheological measurements to discover and quantitatively assess electric field-induced polylactide crystallization were investigated in this study as well. It was shown that a DC electric field significantly promotes homogeneous nucleation of polylactide and enhances crystallization. An explanation to this observation based on the dipole moments of repeat units in the polylactide chain was offered and validated.