Shear-induced crystallization in blends of model linear and long-chain branched hydrogenated polybutadienes

Quiescent and shear-induced crystallizations were performed on several well-defined linear monodisperse hydrogenated polybutadiene blends with a high-molecular-weight long-chain branched (LCB) comb-shaped additive. The connection between microscopic molecular motion and crystallization kinetics has been quantitatively studied with respect to the formation of either isotropic or oriented shish kebab crystal morphology using time-resolved X-ray scattering techniques. Using a constant preshear rate, the addition of small amounts of LCB combs to the sample blends, at concentrations below and just above the overlap concentration c*, has significantly increased the crystallization rates compared to quiescent conditions. However, only one blend showed the formation of an oriented shish kebab morphology. Also, for these model blends, the transition between isotropic and oriented crystals occurs quite sharply between 5 and 10%, which is around c*. We explain these data by using a shish formation mechanism in which, to form shish kebabs, the combs must be mutually overlapping and the comb Weissenberg number must be in the strong stretch regime.