Molecular architecture and linear viscoelasticity of homogeneous ethylene/styrene copolymers

We have studied correlations between molecular features and the linear rheology of two series of carefully synthesized ethylene/styrene copolymers. The materials were polymerized using two single-site catalyst systems; a commercial constrained geometry catalyst and an ansa-metallocene single-site catalyst designed on purpose. The copolymers obtained are characterized by a narrow molecular weight distribution and a broad range of different styrene content. The thermorheological properties and the shape of the viscoelastic fingerprint of the samples obtained from the ansa-metallocene catalyst are those featured by linear polymers. The flow activation energy in these materials depends on the amount of comonomer, but the values are much higher than those corresponding to aliphatic copolymers with the same amount of comonomer, likely due to the hindrance effect of the styrene units in flow behavior. The results obtained for the samples polymerized by the constrained geometry catalyst indicate a complex molecular architecture. In fact, the samples show characteristic features like thermorheological complexity, increased values of the flow activation energy, and a bimodal linear viscoelastic fingerprint in the terminal zone. From these results it can be argued that these materials are actually blends of linear and branched species, these later with the highest relaxation times and flow activation energy values.