On the nucleation of polylactide by melt-soluble oxalamide based organic compounds

Abstract

In this work, a series of oxalamide based organic compounds (OBOC) are synthesized and their capability to enhance the nucleation of polylactide is reported. The OBOCs are soluble in the polylactide melt and crystallize upon cooling, providing surface for heterogeneous nucleation to the polylactide matrix. One interesting observation is that the nucleation efficiency of the OBOCs increases at high cooling rates, making the use of OBOCs as a nucleating agent attractive for industrial processing conditions. The paper addresses the mechanism involved in the cooling rate dependence on the nucleation efficiency of the OBOC: The nucleation efficiency of polylactide is significantly enhanced in the presence of OBOC crystals, as a transcrystalline PLA morphology grows from the OBOC crystal surface (at relatively low supercooling from the equilibrium melting temperature of PLA). However, such crystallization of PLA occurs only when the OBOC crystals are formed at or below 145 °C while cooling at 10 °C/min. In contrast, when the OBOC crystals are formed above 145 °C (i.e. from a lower supersaturated state), the OBOC crystals display a significantly reduced capability for PLA nucleation. These findings override the possibility of both epitaxy and soft-epitaxy as a plausible nucleation mechanism. Supported by polarized optical microscopy, differential scanning calorimetry, plate-plate rheology and molecular modelling we evaluate the possibility of nucleation resulting from local stresses imposed on the polylactide melt invoking stress-enhanced nucleation. The imposed local shear rates, facilitated by the rapid growth of the OBOC crystals, are found high enough to facilitate contour orientation of the high molecular weight PLA chains next to the growing OBOC crystals, confirming the possibility for stress-enhanced nucleation. In addition, we identify surface roughness of OBOC crystals as a second parameter that influences the PLA nucleation process; the OBOC crystallization at high supersaturation is expected to yield smaller/defected OBOC crystals, which are presumed to provide high surface area and surface roughness. In contrast, when the OBOC crystals are grown at lowered supersaturation (>150 °C), they are bound to anneal during crystallization, providing a smoother surface and lower surface area – retrospectively exhibiting a decreased capability to promote the PLA nucleation, irrespective of the PLA supercooling. Interestingly, both the surface roughness of OBOC crystals and the local stresses they impose on the PLA melt increase when the OBOC crystal growth proceeds from a highly supersaturated state, providing an explanation to the cause of the favored crystallization of PLA at the high cooling rates in the presence of the chosen OBOCs.