Arm Relaxation in Deformed H-Polymers in Elongational Flow by SANS

Abstract

We present a small-angle neutron scattering (SANS) investigation of the relaxation process of branched polymer melts under deformation. The selected model polymer is an H-shaped polyisoprene having deuterium-labeled blocks at the dangling tips of the arms. The melt is step strained in a strain rig especially built for that purpose, where the sample temperature and its deformation conditions are precisely controlled. Uniaxial extensions to values of 2 and 3 are performed at a temperature above the polymer glass transition temperature T g. The sample is then allowed to relax during a given time before being quenched to below its Tg in order to freeze the chains conformation to perform the SANS experiments. Very large anisotropies in S(q) develop on time scales corresponding to the relaxation of the dangling arms. The experimental structure function is then compared to theoretical ones obtained by the random phase approximation applied to the tube model. While theory describes well the scattering for short times, it seems not to be able to describe the increase of intensity in the direction parallel to the strain at higher times, even when highly nonaffine processes such as arm retraction and branch point withdrawal are taken into account.

Tube theory of entangled polymer dynamics

“…The prediction of an extension of the`random phase approximation' to non-equilibrium states [337] is that very strong scattering should appear in the strain direction as the arm relaxation proceeds following the step strain. Qualitatively this is exactly what is seen [50,338], but with about twice the magnitude predicted! These observations remain a challenge for the future, and underline the importance of supplementing rheological observations with direct structural probes such as SANS.…”

“…Data sets on clean materials of this kind are still woefully few, and there is clearly a role for future experiments of this type (see, e.g. the recent results of reference [338]). …”

“…At intermediate levels of entanglement, some molecules will be well into a BPW transition while others will not yet have reached it. It is possible that in the rather smeared response of rheology, this distinction is not severe, but the approximation is not likely to be benign for calculations of scattering functions [338,345]. In any case, the examination of pre-averaging assumtions in entangled rheology is likely to be an important topic in the near future [346].…”

“…Tube deformation It is of interest to think about relaxing the assumption that the local tube con®ning ®eld is unchanged even in nonlinear¯ow, although hard experimental tests for speci®c assumptions of the deformation of the tube constraint itself can never be con®ned to rheology alone, but will involve at least careful analysis of neutronscattering experiments [311,338]. Not only might the tube diameter depend on the local strain, but the localizing ®eld described by the tube may well take on an anisotropy consistent with the symmetry of the bulk strain.…”

“…The form of the tensor is modi®ed quite strongly in these models. Simulation [313] and carefully-chosen scattering experiments [338] in consort with optical rheology will be needed to advance these questions of tube and entanglement deformation.…”