The emulsion polymerization induced self-assembly of a thermoresponsive polymer poly(: N -vinylcaprolactam)

Abstract

A thermoresponsive polymer, poly(N-vinylcaprolactam) (PNVCL), was synthesized in an emulsion above its thermal transition temperature to produce particles via polymerization induced self-assembly (PISA). Two amphiphilic poly(ethylene glycol) (PEG) based xanthates were compared as macro-chain transfer agents (mCTAs) for RAFT/MADIX polymerization; thus the products were PEG-PNVCL block copolymers. Only the mCTA with a higher PEG degree of polymerization (DP) was able to stabilize the particles during the polymerization. The morphologies of the particles ranged from spherical with an inner lumen (vesicle) to spherical with a denser core and looser shell upon increasing the DP of PNVCL. This article provides proof of the polymerization induced self-assembly (PISA) of NVCL in an emulsion, producing higher morphologies (vesicles, i.e. spherical particles with an inner lumen) than the most commonly found spherical core-shell particles. Molecular weight analysis via size exclusion chromatography (SEC) revealed that the polymers had an M n value close to the theoretical one. However, some PEG chains were not incorporated into the polymer but were observed as separated populations. This problem was resolved by reaction parameter optimization. Increasing the initiator concentration led to a decrease in polymer dispersity (D) from 1.5 to 1.2 and to all PEG being incorporated into the formed polymer. The optimized reaction parameters were used to synthesize high molecular weight PNVCL, with an M w value of 1.27 × 10 6 g ml -1 (M n : 810000 g mol -1 ), which had a PEG (5000 g mol -1 ) end-group. The size of the high molecular weight polymer particles was in the micrometer range and thus too big for proper analysis with light scattering. The polymerization produced particles were stable at the polymerization temperature (50 °C) but disassembled upon cooling to room temperature (22 °C) due to the polymer becoming soluble. To prevent the dissolution of the polymer, the particles were stabilized through hydrogen bonding through adding salicylic acid.