Homopolymers, crystallization kinetic nucleation rate

The isothermal crystallization of PEO in a PEO-PMMA diblock was monitored by observation of the increase in radius of spherulites or the enthalpy of fusion as a function of time by Richardson etal. (1995). Comparative experiments were also made on blends of the two homopolymers. The block copolymer was observed to have a lower melting point and lower spherulitic growth rate compared to the blend with the same composition. The growth rates extracted from optical microscopy were interpreted in terms of the kinetic nucleation theory of Hoffman and co-workers (Hoffman and Miller 1989 Lauritzen and Hoffman 1960) (Section 5.3.3). The fold surface free energy obtained using this model (ere 2.5-3 kJ mol"1) was close to that obtained for PEO/PPO copolymers by Booth and co-workers (Ashman and Booth 1975 Ashman et al. 1975) using the Flory-Vrij theory. 

In a similar fashion, DSC isothermal scans were recorded in order to study the crystallization kinetics of the PPDX homopolymer after melting the samples for 3 min at 150 °C and quenching them (at 80 °C/min) to the desired crystallization temperature (7(.). After the crystallization was complete, the inverse of the half -crystallization time, (i.e., the time needed for 50% relative conversion to the crystalline state [31,60]), was taken as a measure of the overall crystallization (nucleation and crystal growth) rate and its dependence on the crystallization temperature was analyzed. 

Taking into account all of the results presented above, we can conclude that in order to be sure that homogenous nucleation is indeed present (even when first-order crystallization kinetics is encountered), the crystallization rate must exhibit a dependence on the volume of the droplets or on the cube of the particle diameter. Additionally, even in extremely small droplets comparable to only a few chains in size, the nucleation still occurs within the interior of the droplets. Furthermore, the homogeneous nucleation event is independent of the molecular weight and of the molecular architecture (at least when comparing homopolymers and diblock copolymers). The homogeneous nucleation temperature is a function of the particle size. In certain cases, when the droplets size is nanometric, modifications of the crystal structure of the polymer as compared with that usually observed in the bulk have been reported. The effects of superficial nucleation are important and should be taken into consideration. 

Approaches used for crystallization in homopolymers may be used to calculate the change in melting temperature due to finite crystal thickness (Thompson-Gibbs equation), lamellar crystal surface energies (Flory-Vrij theory), and growth rates (kinetic nucleation theory). Details can be obtained from [1]. 

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