Colloqium: Laws controlling crystallization and melting in bulk polymers

Abstract

After the fundamental structure of semicrystalline polymers - plate-like crystallites with thicknesses in the nanometer range being embedded in a liquid matrix - had been discovered in the late 1950s, attention turned to the mechanism of formation. After intense, controversial discussions an approach put forward by Hoffman and Lauritzen prevailed and was broadly accepted. The picture envisaged by the treatment - plate-like crystallites with atomically smooth side faces and a surface occupied by chain folds, growing side-ways layer by layer with a secondary nucleation as rate determining step - was easy to grasp and yielded simple relationships. The main control parameter is the supercooling below the equilibrium melting point of a macroscopic crystal, T_f^∞, which determines both the thickness of the crystallites and their lateral growth rate. The impression of many in the community that the mechanism of polymer crystallization is principally understood and the issue essentially settled however was wrong. Experiments carried out during the last decade on various polymer systems provided surprising new insights which are now completely changing the understanding. They revealed a number of laws which control polymer crystallization and melting in bulk, showing in particular that the crystal thickness is inversely proportional to the distance to a temperature T_c^∞ which is located above the equilibrium melting point and that crystal growth stops already at a temperature T_zg which is below T_f^∞. The observations indicate that the pathway followed in the growth of polymer crystallites includes an intermediate metastable phase. In a model proposed by us a thin layer with mesomorphic inner structure forms between the lateral crystal face and the melt. The first step in the growth process is an attachment of the coiled chain sequences of the melt onto the mesomorphic layer which subsequently is transformed into the crystalline state. The transitions between melt, mesomorphic layers and lamellar crystallites can be described with the aid of a temperature-thickness phase diagram. T_c^∞ and T_zg are identified with the temperatures of the (hidden) transitions between the mesomorphic and the crystalline phase, and between the liquid and the mesomorphic phase, respectively. Comparison of the predictions of the model theory with experimental results from small angle X-ray scattering, optical microscopy and calorimetry yields in addition to the three equilibrium transition temperatures latent heats of transition and surface free energies.