New Insights into Polymer Crystallization by Fast Scanning Chip Calorimetry

Abstract

A summary of recent research about the kinetics of polymer crystallization and crystal nucleation using fast scanning chip calorimetry (FSC) is given. In the first parts, information about polymer crystallization and advantages of FSC are provided. The latter include the determination of critical cooling rates to suppress crystallization and nucleation, the determination of the (bimodal) temperature-dependence of crystallization rates, the analysis of homogeneous crystal nucleation using Tammann’s two-stage nuclei development method, the detection of changes of the structure of polymer glasses, or the analysis of the kinetics of crystal reorganization and melting. Main part of this chapter is the thorough description of analysis of the kinetics of homogenous crystal nucleation. Annealing of amorphous polymer samples below the glass transition temperature allows formation of homogeneous nuclei and ordered structures with latent heat. Disordering of the ordered domains occurs on devitrification of the glass, which then may be followed by cold-crystallization, which, in amount, is proportional to the fraction of priorly disordered structures. It seems proven that the rate of homogeneous nuclei formation is fastest slightly above the glass transition temperature, however, does not start immediately after reaching an annealing temperature in the glassy state. For all polymers studied, it was observed that first enthalpy relaxation (densification) towards the supercooled liquid state and only then homogeneous nucleation occurs. The nucleation and growth of small ordered particles in the glass of most polymers suggests that noncooperative local mobility of chain segments is sufficient to form ordered structures. By comparison of crystallization and nucleation half-times it has been found that crystal growth and nucleation cannot be fitted with a single viscosity-related term which slows the process in parallel to the bulk glass-transition kinetics; nucleation needs much faster, local transport terms. For this reason, the data of the present work strongly suggest that the classical nucleation and crystallization theories for polymers need modification. Further observations discussed in this chapter include the hindrance of slow, long-range diffusion-controlled crystal growth by the formation of a rigid amorphous fraction, and the quantitative analysis of the nucleation efficiency of carbon nanotubes. Though highly nucleated samples show two orders of magnitude faster crystallization in the region where heterogeneous nucleation dominates (low supercooling, high temperatures), homogeneous nucleation dominates at low temperatures making crystallization in this region independent of purposely added nucleating agents. Finally, the stability of homogenous nuclei has been assessed by Tammann's development method. Fast scanning calorimetry allows transfer of nuclei formed at the nucleation temperature to the development temperature in a wide range of heating rates covering at least seven orders of magnitude, ultimately allowing an estimation of the cluster (nuclei) size distribution present after the nucleation stage.