Supercooling-dependent morphology evolution of an organic nucleating agent in poly(l-lactide)/poly(d-lactide) blends

Abstract

The morphology evolution of an organic nucleating agent (NA) in poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blends was investigated under a series of multi-step isothermal and nonisothermal processes at identical NA solubility achieved via the same NA concentration and final molten temperature of heating (T_f). It was found that the morphology of NA frameworks depended on the supercooling determined by the recrystallization temperature of the NA in the polymer melt, except for NA solubility. When the NA partly dissolved in the matrix, the residuum of the original flat-plate NA acting as a nucleus induced recrystallization at a low degree of supercooling, and small sheet-like NA particles formed in the melt. When the NA dissolved in the matrix absolutely, NA aggregation increased in the melt due to the low nucleation density of the NA itself. The shape of the NA aggregation depended on the recrystallization temperature. With the increase in the supercooling, the large aggregated sheet-like NA transformed into a dendritic-like framework accompanied by a decrease in the stem size of the NA frameworks. Moreover, the stereocomplex efficiency of the blend was influenced by the effective nucleation surface induced by the shape of the NA aggregation. For PLLA/PDLA blends with 1.0 wt% NA, the crystallization half time (t_1/2) of stereocomplex (sc) crystallites at the crystallization temperature of 180 °C is as long as 79.8 min for the sample with a large aggregated sheet-like NA, with 1 order of magnitude reduction for that with a dendritic-like NA framework, leading to a t_1/2 of 7.9 min. The morphological control of NA frameworks is of great significance to guide the formation of various crystal morphologies of sc crystallites. The established relationship between the self-assembled morphology of NA and NA concentration, NA solubility and supercooling affords guidance in practical processing to control the crystal morphology and polymorphic crystalline morphology of PLLA/PDLA blends via tuning the crystal morphology of NA in the polymer matrix.