Wall slip of polyisobutylenes: effect of molecular characteristics

The rheology and wall slip behavior of several polyisobutylene (PIB) melts with different molecular weights were studied using parallel-plate and capillary rheometry. All PIBs studied were found to slip even at very low wall shear rates. The transition from weak to strong slip (nearly plug flow) was found to occur at a wall shear stress of approximately 0.09 MPa. The slip model developed by Ebrahimi et al. (2015) based on double reptation was used in order to relate the slip velocity, V _s, to the molecular weight distribution of the polymers. It was found for the monodisperse polymers (extracted from the general model) that the slip velocity scales with wall shear stress, σ _w, and the number average molecular weight as, M _n as \( {V}_{\mathrm{s}}\propto {M}_{\mathrm{n}}^{-2}{\sigma}_{\mathrm{w}}^{1/n} \) with n equal to the local slope of the flow curve of the corresponding polymer, \( n\equiv d\left( \log \left({\sigma}_{\mathrm{w}}\right)\right)/d\left( \log \left({\dot{\gamma}}_{\mathrm{w}}\right)\right) \). The exponent −2 in the molecular weight dependence for PIBs was found to be the same with those reported for high-density polyethylenes (HDPEs) and polybutadienes (PBDs), suggesting that it is universal for linear polymers.