Effect of melt shearing on D-mannitol crystal twisting in the presence of small molecule and macromolecular additives

Many molecular crystals grow as twisted lamellae from the melt when the driving force is high, but the mechanisms governing the spontaneous formation of helicoidal crystallites are varied and those operative in particular cases are difficult to identify. Polyvinylpyrrolidone (PVP) induces the crystallization of D-mannitol as banded spherulites comprised of twisted fibrils emanating radially from spherulite nucleation centers. The molecular weight of the PVP phase affects the twisting pitch, P, (rotation of the crystallite by 180°), in D-mannitol thin films crystallized from the melt. At a relatively high crystallization temperature (T_c = 130 °C), P was sensitive to the PVP molecular weight, ranging from 390 ± 20 μm for D-mannitol films incorporating 15 wt% 10 kDa PVP to 20 ± 3 μm for the same weight of 1300 kDa PVP. Magnitudes of complex viscosities, η^∗, of D-mannitol/PVP melts measured via small-amplitude oscillatory shearing were strongly dependent on the PVP molecular weight, but were not correlated to P(T). Instead, P was sensitive to the dynamic PVP chain conformation during D-mannitol crystallization. Under steady torsional shear, P decreased from ∼ 30 μm to ∼ 8 μm with increasing shear rates from 0.01 to 100 s⁻¹ for D-mannitol films crystallized at 130 °C in the presence of 15 wt% 10 or 1300 kDa PVP. Shear forces decrease the entanglement density of polymer chains while orienting the chains along the stream lines of the viscometric flow, indicating that the conformations of macromolecular additives can affect the pitch of banded spherulites. By contrast, P in D-mannitol twisted by D-sorbitol was independent of shear rate.