Thermal and viscoelastic structure–property relationships of model comb-like poly(n-butyl methacrylate)

A range of comb polymers of poly(n-butyl methacrylate), where the degree of polymerization (DP) of both the backbone and branches was controlled using RAFT-mediated free-radical polymerization, was synthesized using the method of Vosloo et al. Macromolecules 2004;37: 2371]. Individual architectural parameters (branch length, branch number and DP of the comb backbone) of these relatively monodisperse samples were systematically varied in order to study the impact of each structural parameter on the thermal and rheological properties of the resulting comb polymers. Differential scanning calorimetry showed lower glass transition temperatures for the comb polymers compared to the original linear backbones. There were negligible differences in glass transition temperatures between comb polymers containing branches of different lengths, and between comb polymers containing backbones of narrow and of broad molecular weight distributions. These observations suggest that because the comb polymers are very tightly spaced, the branches act in the same way as long chain polyBMA. Viscoelastic properties of the comb polymers were investigated using dynamic mechanical analysis, using time–temperature superposition to extend the rheological data over a wide frequency range. Major differences in the viscoelastic responses of the original linear backbones and the comb polymers were observed, which were explained in terms of arm retraction/relaxation leading to tube dilation. All comb polymers showed viscoelastic responses that are characteristic of combs, but differences in responses due to changes in branch length and branch number were difficult to detect. This was mainly due to the relatively high number of branches, whose retractions occurred over a broad frequency range, and thus dominated the observed changes in moduli, thereby possibly masking subtle differences in responses.