Shear-Induced Oriented Crystallization for Isotactic Poly(1-butene) and Its Copolymer with Ethylene

Shear-induced crystallization was examined for poly(1-butene) (PB) and its random copolymer with 1.5 mol % ethylene (PBE) upon shearing at a temperature slightly above the nominal melting temperature. When the shear rate approached the reciprocal of the Rouse time of a small fraction of the longest chains (∼7%), shear thickening was observed in both shear stress and normal stress after the temporary steady state had been achieved. The thickening appeared to arise from gel-like percolation of precursors of the fibrillar crystals, yielding a birefringent region seen in polarized optical microscopy after the crystallization. Critical work W_crit was evaluated as (1) the applied work before the onset of thickening and (2) the localized work at the boundary of the birefringent region (shown in the figure at right). For both cases, W_crit decreases as the shear rate is increased, while saturation of W_crit is only seen for the onset of thickening. W_crit of PBE is consistently ∼twice higher than that of PB, possibly owing to the fewer stereoregular chains that need more applied work to form the fibrillar bundles. The precursors lead to a slow relaxation process detected in the storage modulus, further lending support to its gel-like structure.