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We present experimental observations at comparatively low supercooling of morphology transitions from dendritic to faceted structures in polymer crystals growing in thin films of a poly-2-vinylpyridine-block-polyethyleneoxid copolymer. Our results are compared with theoretical concepts describing morphological instabilities of single crystals. Although these concepts originally were not developed for polymers, they allow to describe and interpret our experimental results quite well. In particular, the measured temperature dependence of the width W and frequency of dendritic side branches and the radius of curvature p of the growth tips of the crystals follow these concepts. We present preliminary evidence for the influence of polymer attachment kinetics and reorganisation processes behind the growth front. Polymer thin films provide valuable model systems for studying general concepts of crystallisation and allow to distinguish at which point the connectivity of the crystallising units within chain-like molecules starts to play a measurable role.

We present an experimental study of structure formation in a polypeptide hetero-arm star block copolymer solution, obtained by swelling thin films in chloroform solvent vapor to variable poly(g-benzyl-L-glutamate) (PBLGlu) concentrations (c p ). Direct observation by optical microscopy allowed us to follow in real time nucleation and growth of ordered three-dimensional structures of ellipsoidal shape. At low c p , growth stopped when c p decreased below the solubility limit (c critical ) but additional structures were formed when c p was rapidly increased to a higher value. Although water is not a solvent for this polymer, we demonstrate that water, even in trace amounts, is nonetheless considerably affecting solubility and consequently the process of structure formation. We have varied systematically the amount of water present in the environment. c critical changed from about 0.53 (dry, i.e. desiccated surrounding vapour phase) via c critical z 0.16-0.25 for 30-50% humidity of the vapour phase to c critical z 0.03 for a vapour phase at 100% humidity. We attribute this change in solubility to complexation of water molecules with PBLGlu a-helix, which increases the interfacial tension between the polymer and the solvent. We have tested our hypothesis by replacing water with other non-solvents for the polymer. Only protic non-solvents changed the solubility of PBLGlu in chloroform.