Monte Carlo Simulation of Short Chain Branched Polyolefins: Structure and Properties

The effect of higher alpha-olefin comonomer on physical properties of short chain branched (SCB) polyethylene (PE) melts at 450 K has been studied using connectivity altering Monte Carlo simulations. The calculated chain dimensions per molecular mass scale with backbone weight fraction, phi, as < S-2 >/M similar to phi(1.27 +/- 0.03) for the radius of gyration S and < R-2 >/M similar to phi(1.27 +/- 0.03) for the end-to-end distance R, in very good agreement with the experiment-based result < R-2 >/M similar to phi(1.30). The observed dependence is consistent with the decrease in the fraction of trans states along the backbone. The entanglement tube diameter, a(pp), computed for SCB melts scales < a(pp)> similar to phi(0.46 +/- 0.01), ales as < a(pp)> which is close to the result for model concentrated (< R-2 > = const) PE solutions created by deleting randomly chosen chains from equilibrated melt configurations of linear PE (< a(pp)> similar to phi(-0.41 +/- 0.01)). The latter result agrees very well with the scaling based on rheological experiments on concentrated hydrogenated polybutadiene (HPB)/C24H50 solutions at 413 K (< a(pp)> similar to phi(-0.45)). The tube diameter in model athermal PE solutions scales as < a(pp)> similar to phi(-0.6 +/- 0.03), in excellent agreement with the scaling based on the neutron spin-echo experiments on athermal HPB/C19D40 solutions at 509 K (< a(pp)> similar to phi(-0.6)). The computed scaling relationships for both SCB melts and model PE solutions are close to the binary contact model (a(pp) similar to phi(-0.5)) and disagree with the packing model (a(pp) phi(-1.27)). The solubility parameters calculated for poly(ethylene-co-1-butene) (PEB) melts are in excellent agreement with relative solubility parameters based on SANS analysis of appropriate SCB blends, which scale as delta similar to phi(0.18). The SANS derived relative solubility parameters for poly(ethylene-co-1-hexene) (PEH) and poly(ethylene-1-octene) (PEO) systems scale more weakly (delta similar to phi(0.1)) and suggest breakdown of a universal correlation. This pattern is followed by simulated relative solubility parameters.