Melt-state polymer chain dimensions as a function of temperature

The unperturbed chain dimensions (<R-2>(o)/M) of cis/trans-1,4-polyisoprene, a near-atactic poly(methyl methacrylate), and atactic polyolefins were measured as a function of temperature in the melt state via small-angle neutron scattering (SANS). The polyolefinic materials were derived from polydienes or polystyrene via hydrogenation or deuteration and represent structures not encountered commercially. The parent polymers were prepared via lithium-based anionic polymerizations in cyclohexane with, in some cases, a polymer microstructure modifier present. The polyolefins retained the near-monodisperse molecular weight distributions exhibited by the precursor materials. The melt SANS-based chain dimension data allowed the evaluation of the temperature coefficients [dln <R-2>(o)/dT(kappa)] for these polymers. The evaluated polymers obeyed the packing length (p)-based expressions of the plateau modulus, G(N)(o) = kT/n(t)(2)p(3) (MPa), and the entanglement molecular weight, M-e = rhoN(a)n(t)(2)p(3) (g mol(-1)), where n(t) denotes the number (similar to21) of entanglement strands in a cube with the dimensions of the reptation tube diameter (d(t)) and p is the chain density. The product n(t)(2)p(3) is the displaced volume (V-e) of an entanglement that is also expressible as pd(t)(2) or kT/G(N)(o). (C) 2002 Wiley Periodicals, Inc.