| The thermodynamics, polymer crystallization kinetics, sequence of crystallization events and microstructure of the eutectic class of thermally-induced phase separation (TIPS) were studied as a function of melt composition, W{dollar}sb{lcub}rm p{rcub}{dollar}, and isothermal crystallization temperature, T{dollar}sb{lcub}rm c{rcub}{dollar} using the model system isotactic polypropylene (iPP) and hexamethylbenzene (HMB). Differential scanning calorimetry (DSC) was used to test iPP crystallization kinetics via the crystallization theories of Avrami and Hoffman-Lauritzen, to determine the sequence of phase transitions during crystallization, and combined with a modified Flory lattice model to generate an equilibrium phase diagram. Electron microscopy combined with solvent extraction and permanganic etching permitted characterization of iPP crystal structure and voids formed by removal of HMB. Lamellar structures coincided with pure iPP lamellae. Spherulitic structures were deformed for compositions to the left of the eutectic as a result of primary diluent crystals. Diluent crystallization prior to polymer crystallization was predicted by DSC studies and resulted in a random distribution of rectangular shaped voids. Both the dimensions of these voids and iPP spherulite radii decreased as T{dollar}sb{lcub}rm c{rcub}{dollar} was decreased. Below 383 K, a secondary void microstructure appeared in between the larger voids. This structure consisted of tiny needle-like voids and was attributed to diluent crystallizing simultaneously with polymer. Above 383 K, all the diluent was consumed prior to polymer crystallization below 383 K, polymer crystallization rate and the viscosity of the supercooled melt both increase, as predicted by the kinetic theories, reducing the transport rate of diluent remaining in the eutectic fraction, and preventing random diluent crystal distribution within the eutectic. To the right of the eutectic, primary iPP spherulites dominated structure, restricting voids to interspherulite regions. Spherulite size decreased as T{dollar}sb{lcub}rm c{rcub}{dollar} was decreased.; Both solid and void size, shape and relative distribution were controlled by T{dollar}sb{lcub}rm c{rcub}{dollar} and W{dollar}sb{lcub}rm p{rcub}{dollar}. Trends were interpreted in terms of the mechanism, sequence and type of phase transitions in the supercooled melt. |
