1. Influence of Thermal Prehistory on Crystal Nucleation and Growth in Polymers

Abstract

Observations regarding the effect of thermal history of crystallizing polymer melts onto the outcome of crystal nucleation and growth processes are investigated experimentally. Some results can be at least on a qualitative basis explained by classical nucleation theory (CNT) while others are not easy to understand in the framework of CNT. The origin of the respective problems and possible extensions of CNT to overcome them are briefly discussed. We chose polymers as model systems because they allow one a separate investigation of nucleation and growth processes in a wide temperature and time range. Furthermore they are well suited to be analyzed experimentally by the recently developed fast scanning calorimetry. In particular, by applying fast scanning calorimetry we are able to investigate processes in bulk samples and do not need to use droplets to study homogeneous nucleation kinetics. Fast scanning calorimetry enables us to observe homogeneous nucleation in materials crystallizing relatively fast like most of the industrial relevant semicrystalline polymers. Quantitative analysis allowed us to judge the nucleation efficiency of additives in the whole range of temperatures, where polymers crystallize. Besides the observed nucleation and growth below the glass transition information about the nucleation activity of small crystals grown at different temperatures relative to the cold-crystallization temperature range were obtained. Finally we show how relaxation of the under-cooled melt (glass) influences homogeneous nucleation. These data may serve as input for a more general description of the interplay of nucleation-growth and relaxation processes within the framework of a structural order-parameter model.

Temperature Dependency of Nucleation Efficiency of Carbon Nanotubes in PET and PBT

“…All higher cooling rates are supercritical and can be applied without introducing morphological changes on cooling. Nevertheless, also after supercritical cooling there may be different amounts of homogenously formed nuclei …”

“…Nevertheless, also after supercritical cooling there may be different amounts of homogenously formed nuclei. [9,32,34,61] In the second step, the crystallization behavior can be determined by cooling the sample from the melt to the temperature of interest and annealing it isothermally for a given time (see Figure 4). From the subsequent heating after annealing the enthalpy change during melting can be determined too; this reflects the enthalpy change during crystallization on annealing.…”

Kinetic-freezing and unfreezing of local-region fluctuations in a glass structure observed by heat capacity hysteresis

“…It was also found that occurrence of several other phenomenon is affected by local-region fluctuations. For example, Dzuba and Tsvetkov 10 reported the effects of these fluctuations on chemical reactions, and in a study of nucleation and crystallization by fast calorimetry (10 5 K/s), Schick and coworkers 20,21 found that nucleation and crystallization in the glassy state of some fast crystallizing polymers occur in the local-regions of molecular motions, i.e., in the regions where molecular motions are observed as the JG relaxation.…”

On the solubility advantage of a pharmaceutical’s glassy state over the crystal state, and of its crystal polymorphs

“…When that occurs, its specific volume, entropy, enthalpy, and G decrease with time and the rate of decrease becomes slower as the amount of the crystalline phase formed increases. Nucleation occurs in randomly dispersed local regions in the bulk of an amorph [61][62][63][64][65][66], and the growth of the nuclei to crystals is slow and gradual. Crystal growth in a glass also causes local contraction, and the consequent internal (tensile) strains often cause the sample to develop micro-cracks or even fracture it.…”