Behaviour of stereoblock poly(N-isopropyl acrylamide) in acetone–water mixtures

The purpose of this study was to investigate the usability of poly( N -isopropyl acrylamide) (PNIPAM) as an additive for crystallization of a model drug, nitrofurantoin (NF), and utilizing the thermoresponsivity of the polymer to enable control of the viscosity during the crystallization process. Crystallization of NF in the presence of PNIPAM resulted in dendritic crystal growth and the originally observed growth of needle-shaped crystals was prohibited. The effect of acetone, a cosolvent used for the crystallization, on thermosensitivity of PNIPAM was studied and the properties of atactic PNIPAM and triblock PNIPAM polymers containing atactic and isotactic rich blocks were compared. The investigated PNIPAMs were all soluble in the acetone–water mixtures leading to phase separation at lower temperatures with increasing acetone content, up to approximately 50 vol% (0.20 mol fraction). At higher acetone contents no phase separation was observed on heating. The presence of acetone altered the viscosity of the solutions prior to the phase separation depending on the polymer architecture. The PNIPAM polymers induced smaller and potentially more easily processable crystals, however, the viscosity increase in the presence of acetone occurred only 1–3 °C prior to phase separation complicating the practical use of this approach.