Controlled crystallization of small molecule organic compounds using polymers

Abstract

Crystallization of organic compounds is of great industrial and academic importance. It affects many properties of functional materials such as the dissolution kinetics, self-assembly, purity, and electronic and optical properties, to name a few. Therefore, understanding and establishing control over the thermodynamic states of such organic compounds is imperative, especially for pharmaceutical and electronic applications. The proposed research investigates the multi-faceted phase behaviors of crystallizable small molecule organic compounds with a library of polymers by extensive characterizations using differential scanning calorimetry (DSC), X-ray scattering, and fluorescence emission spectroscopy. In the first part of this research, the thermal miscibility between the polymers and the small molecule organic compounds was studied from the viewpoint of the melting temperatures of the organic crystals in the mixtures. The second part involved developing a consistent method for the measurement of the phase separated compositions and understanding the effect of different heat treatment procedures on the phase states of the mixtures. Finally, the thermodynamic interactions between the polymers and the organic crystals were probed from the standpoint of crystallization of the organic compounds.

The crystallization temperatures of pyrene in the polymers were found to be invariant to the cooling rates of the mixtures and showed upper critical solution temperature profiles, suggesting that the crystallization of pyrene was assisted by a preceding liquid-liquid phase separation by spinodal decomposition. These spinodal decomposition assisted crystallizations governed different heat treatment sequences that controlled the phase states of pyrene. Assuming that the crystallization happened instantaneously after the spinodal decomposition, the effective interaction parameters were extracted and compared to those obtained from the melting temperatures of pyrene crystals. While polymers with enthalpically favorable interactions showed a greater depression in the melting temperature of pyrene crystals, the crystallization temperatures, on the other hand, did not show a similar correlation. This was because the liquid-liquid phase separation showed a strong dependence not only on the strength of the attractive interactions between pyrene and the polymers but also on the self-solvating tendency of the polymer segments due to the local dipole moments of the polymer. This suggested that while the melting temperatures of pyrene depend strongly on polymer and pyrene interactions, the spinodal decomposition assisted crystallization involves additional factors such as the strength of the permanent local dipoles of the polymer and the tendency for the polymer coils to collapse under these local dipoles.

The phase behavior of pyrene and the model polymers: polystyrene, poly(2-vinylpyridine), poly(3-vinylanisole), poly(1,4-isoprene) and poly(ethylene-alt-propylene), was investigated using the differential scanning calorimetry technique. This involved determination of the effective interaction parameter, χ, using the melting temperatures of pyrene crystals in the mixtures. While χ was found to be less than 0.5 for pyrene mixed with polystyrene, poly(2-vinylpyridine) and poly(3-vinylanisole), it was greater than 0.5 for mixtures of pyrene with poly(1,4-isoprene) and poly(ethylene-alt-propylene). This suggested that polymers with aromatic side groups showed favorable interactions with pyrene, which were of enthalpic origin, originating likely from the polarities of the local motifs of these polymers.