Model comb-like poly(butyl methacrylate) : synthesis, characterization and structure - property correlations

Abstract

This study has the ultimate goal of illuminating structure-property relationships for model comb-like polymers. Comb-like polymers of poly(n-butyl methacrylate) were prepared using an activated ester-type comonomer (N-acryloxysuccinimide, NAS) to generate branch points. The conventional solution free-radical copolymerization kinetics of n­butyl methacrylate (BMA) and NAS were first investigated by following individual monomer consumption rates by lH-NMR spectrometry, and reactivity ratios ofBMA and NAS determined using the terminal model. The reactivity ratios so obtained are both close to 0.2; the joint confidence interval is also determined. Reversible addition­fragmentation chain transfer (RAFT) was then used to grow polymers with controlled backbone and branch chain length. Both reactivity ratios have similar values, which implies that the copolymer will have a random distribution of NAS and hence of branch points. RAFT-mediated polymerization was first used to synthesize linear poly(BMA-co­NAS) chains. The amount of NAS in the starting BMA/NAS composition was varied without adversely affecting the uniformity of the NAS distribution along the resulting linear poly(BMA-co-NAS) backbone. Primary hydroxy-functionalized RAFT agents were then immobilized on this linear poly(BMA-co-NAS) through nucleophilic substitution on the activated ester units of the NAS. From these immobilized RAFT agents, branches were grown upon addition of a further aliquot of monomer (BMA) and initiator (AIBN). This results in branched polymers whose molecular weight, branching density, and degree of polymerization of branches, are all relatively narrow and controlled. SEC-MALLS was used to characterize linear and comb polymers. Branches were cleaved from the main backbone chains using a transesterification procedure, which enabled direct detection and analysis by using SEC-MALLS. Individual architectural parameters (branch length, branch number and molecular weight distribution of the comb backbone) were systematically varied in order to study the impact of each structural parameter on the thermal and rheological properties of the resulting comb polymers. Thermal properties were investigated using differential scanning calorimetry (DSC). DSC thermograms showed lower glass transition temperatures for the comb polymers as compared to the original linear backbones. The difference in glass transition temperature between comb polymers containing branches of different lengths, was negligible. The same negligible difference in glass transition temperature was observed between comb polymers containing backbones of narrow and broad molecular weight distributions respectively. Slight differences in glass transition temperatures were observed between comb polymers with different amounts of branching. Viscoelastic properties of the comb polymers were investigated using dynamic mechanical analysis. Time-temperature superposition (TTS) was used to extend the rheological data over a wide frequency range. ITS enabled observation of characteristic changes in modulus of the comb polymers that were attributed to Rouse­like relaxations and branch retraction. The changes in moduli were completely dominated by branch retractions.