Synthesis and characterization of diblock copolymers of styrene and N-tert-butylacrylamide and gelation evidenced by rheology

Abstract

The self-assembly of block copolymers in melt and in solution states is of current interest for the design of hierarchical materials. In this contribution, we report the synthesis of amphiphilic block copolymers of styrene and N-tert-butylacrylamide (S_mNtBAM_n) with narrow molecular weight distributions by reversible addition–fragmentation chain transfer (RAFT) technique. Block copolymers were characterized by gel permeation chromatography (for distribution width), ¹H NMR spectroscopy (for molar mass and composition), thermal analysis and transmission electron microscopy (morphology investigation). These diblock copolymers in 1-octanol, a selective solvent for the N-tert-butylacrylamide block, self assemble to form micelles. At higher concentrations, these diblock copolymers in 1-octanol form glass-clear gels. Dynamic viscoelastic properties of a diblock copolymer, S₂₇₈NtBAM₅₁₇, in 1-octanol have been studied as a function of copolymer concentration. Scaling relations for the micelles and gels have been established in the terminal frequency range. Critical gelation concentration (c_g) was determined by frequency independence of loss tangent in the vicinity of gel point. The c_g, viscoelastic exponent, n and front factor, S were determined to be 11.3 wt%, 0.677 and 0.55 Pa-s^0.677, respectively.