Reversible Crystallization and Dissolution of Poly(γ-benzyl L-glutamate) in Thin Film Solutions via Addition and Removal of a Nonsolvent

Positional and orientational order of molecules has an important effect on the physical properties of the bulk materials. Therefore ordering of polymers in nano and microscale structures has been a hot topic in different fields of polymer science due to their application in organic electronics, bio inspired devices, etc. An effective and important way to achieve high order of polymer molecules on a large scale is polymer crystallization. Crystallization can be done either from polymer melts or polymer solutions. Crystallization of polymers from solution can lead to large microscale structures by efficiently controlling the parameters which promote and prevent nucleation, which is the first step in the crystallization process. The ability to have a control on these parameters provides a possibility for a more fundamental understanding of phenomena like ordering and growth, affected by competing intermolecular, molecule-solvent and molecule-nonsolvent interactions.

In this work we have investigated nucleation, growth and dissolution of poly(γ -benzyl L-glutamate) (PBLG) objects from semi-dilute thin film solutions of chloroform by condensation of methanol from the vapor phase onto the liquid film. The experimental approach which we adopted here, allows us to vary the quality of the solvent by condensation and evaporation of methanol in a controlled way by adjusting the saturation of methanol in the vapor phase and the temperature of the thin film solution. Adding and removing different amounts of methanol allowed us to reversibly control nucleation, growth and dissolution of objects possessing a hexagonal columnar liquid crystalline internal structure. Adding methanol to the isotropic polymer solution promoted nucleation and growth even at very low concentrated polymer solutions, i.e. significantly decreased the solubility limit (equilibrium volume fraction). Additionally, the variation of the number density of nuclei with the supersaturation ratio for various equilibrium concentrations was found to fit well the predictions of the classical nucleation theory. Based on our data and concentration regime that we have worked in, we conclude that at a specific supersaturation ratio the number of nucleated objects will be lower for lower equilibrium concentrations.

After drying extraction of solvent and nonsolvent from thin film solution by fast evaporation, PBLG objects transformed to dry crystals. Each PBLG crystal possessed an internal domain structure exhibiting a zig-zag pattern consisting of parallel stripes of alternating orientations between domains. X-ray scattering and electron diffraction revealed a pseudo-hexagonal packing of the PBLG a-helices within these crystals with their axis oriented parallel to the plane of the substrate. Based on optical anisotropy studies, it could be shown that the orientation of the helix axis was parallel to the stripes. While forming in solution, the objects are assumed to consist of a hexagonal columnar liquid crystalline phase. Upon drying, lateral packing density of the helices increased and resulted in a net dilative strain perpendicular to the columns, which is supposed to cause the formation of zig-zag patterns.