Dynamics of molten polymers on the sub-molecular scale. Application of small-angle neutron scattering to transient relaxation

Abstract

The application of small-angle neutron scattering (SANS) to transient viscoelastic relaxation is advantageous in that it can, for example, give the form factor of a single chain in a polystyrene melt undergoing transient relaxation, namely restoration (stress relaxation) at constant shape after a sudden deformation. The deformation is typically a uniaxial extension of ratio λ= 3, and the form factor is measured instantaneously on samples quenched from deformation temperature at different times t after the extension.Complete data are given here for one partly reported experiment, [F. Boueet al., J. Phys. (Paris), 1982, 43, 137; J. Phys. (Paris) Lett., 1982, 43, L585/L591 and L593/L600] covering an increased range of scattering vector q and time t. These data permit comparison of calculated form factors for the Rouse model and the tube model (reptation), confirming the following. (a) At small times the data agree both with the Rouse model and with the tube model, which assumes a three-dimensional Rouse motion over lengths smaller than the tube diameter. At larger times the data disagree with the Rouse model. (b) A previously reported discrepancy with the tube model, concerning the process of contraction of the deformed chain in its ‘tube’, persists [F. Boueet al., J. Phys. (Paris), 1982, 43, 137: M. Doi and S. F. Edwards, J. Chem. Soc., Faraday Trans. 2, 1978, 74, 1789, 1802 and 1818]. (c) At larger times, data are not in disagreement with the tube model (disengagement process) if one accepts that contraction does not occur but that the chain has already relaxed by this stage up to the scale of tube diameter.Supportive results from other experiments which we have carried out are briefly presented.