The three reversible crystallization and melting processes of semicrystalline macromolecules

Linear, flexible macromolecules have long been recognized as being distributed between different metastable phases limited to micrometer and nanometer dimensions with their strong, covalent bonds crossing the phase boundaries. This structure leads to special properties and a multitude of changes are caused by differing thermal and mechanical histories. Temperature-modulated differential scanning calorimetry (TMDSC) and related thermal analysis techniques which can separate equilibrium and non-equilibrium responses within a sample are ideal for the analysis of such systems. A surprising observation in the last 5 years has been that there are some local, reversible ordering and disordering processes within this overall metastable aggregate. Three different reversible processes with latent heats have been identified and will be discussed using the example of polyethylene crystals: (1) The formation of larger concentrations of defects within the crystals. (2) The melting of small, paraffin-like crystals existing within the metastable arrangement. (3) The reversible, partial melting of chains at high-temperature which avoids molecular nucleation.