Spray drying formulation of amorphous solid dispersions.

The role of metastability in polymer phase transitions

This article reviews the various theories of polymer crystallization which have been proposed over the last thirty years. Their background in thermodynamics and crystal growth theories of simple substances is laid out in some detail. This provides the basis for a discussion of the application of surface nucleation and surface roughening concepts to polymer crystallization. The widely used nucleation model of Lauritzen and Hoffman is examined in depth with emphasis on its consistency with experimental evidence and including its recent modifications and extensions. Alternative approaches are also analyzed, in particular Point's ‘multipath’ model and the ‘roughness/pinning’ model by Sadler and Gilmer, but also ‘molecular nucleation’ and ‘chain sliding’. The aim of this article is to lead to a better understanding of the basic hypotheses of the various models and to give guidance to the theorist in improving the models as well as to the experimentalist in interpreting his/her data. It will hopefully make the subject more accessible and stimulate discussion and further research directed at resolving the controversial issues.

Polymer crystallization theories

Mechanism of increased dissolution of diazepam and temazepam from polyethylene glycol 6000 solid dispersions

Learning polymer crystallization with the aid of linear, branched and cyclic model compounds.

Two available theories of melting in extended chain polymer crystals are critically examined and compared with calorimetric data. Melting points, T m , and enthalpy of fusion, ΔH, have been measured for extended-chain crystals of poly(ethylene-oxide) fractions, covering a wide range of molecular weights. It appears that the theory of Flory and Vrij is the most appropriate to describe the present results and literature data. Application of this theory to experiments yields T m (∞) = (68.9 ± 0.4) °C and suggests that the value of the surface free energy, σ e , lies within 0.34 and 0.81 Kcal/mole, its enthalpic part (ΔH e /2) being (4.18 ± 0.22) Kcal/mole. Such a low value of σ e may be attributed to hydrogen bonding of OH end-groups located in the disordered surface layer of closely stacked crystalline lamellae.

A. J. Kovacs

Papers

Melting behaviour of low molecular weight poly (ethylene-oxide) fractions