Abstract
In the present work, crystallization of a soluble nucleator N, N′, N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (TMC-328) in a poly(l-lactic acid) (PLLA) matrix has been studied at different temperatures. Based on the change in solubility with temperature, different levels of supersaturation of TMC-328 in a PLLA matrix can be obtained. This nucleator presents a fibrous structure produced via self-assembling and develops into an interconnected network when the temperature is lowered. The TMC-328 crystal nuclei density is quantified via optical microscopy, using the average distance of the adjacent fibrillar structure, which shows a steady decrease with the decrease in temperature. The crystallization rates of TMC-328 were assessed through rheological measurements of network formation. Both fibrils’ density and crystallization kinetics display a power law dependence on supersaturation. For the first time, the solid–melt interfacial energy, the size of the critical nucleus, and the number of molecules making up the critical nucleus of the nucleator TMC-328 in the PLLA matrix have been determined by adopting the classical nucleation theory. The subsequent crystallization of PLLA induced by this nucleator was investigated as a function of the fibrils’ spatial density. The crystallization rate of PLLA is enhanced with the increase in the TMC-328 fibrils’ density because of the availability of a larger nucleating surface. The self-assembled fibril of TMC-328 can serve as shish to form a hybrid shish-kebab structure after the crystallization of PLLA, regardless of the number of nucleation sites.
Highlights
Nucleating agents (NAs) are typically used when crystallization is slow as in the case of poly(ethylene terephthalate) (PET) or to optimize property combinations for specific applications such as for isotactic polypropylene.[1−5] Apart from the two thermoplastics already mentioned, nucleation additives are mainly relevant for various polyamides (PA-6 and PA-66) and other polyesters such as poly(butylene terephthalate) (PBT) and poly(L-lactic acid) (PLLA)
When the sample is annealed at 240 °C for 3 min, TMC-328 is dissolved in the PLLA melt, and a homogeneous mixture is obtained above the melting temperature of PLLA because no obvious birefringence of TMC-328 crystals is observed within the resolution scale of polarized light optical microscopy (POM) (Figure 1a)
In the subsequent heating process (Figure 1d−f), the fibrils of TMC-328 formed during cooling crystallization dissolve gradually in the PLLA matrix, which is similar to a solute that dissolves in a solvent
Summary
Nucleating agents (NAs) are typically used when crystallization is slow as in the case of poly(ethylene terephthalate) (PET) or to optimize property combinations for specific applications such as for isotactic polypropylene (iPP).[1−5] Apart from the two thermoplastics already mentioned, nucleation additives are mainly relevant for various polyamides (PA-6 and PA-66) and other polyesters such as poly(butylene terephthalate) (PBT) and poly(L-lactic acid) (PLLA). Even if there are many nucleation sites provided by the fibrillar structure of TMC-328 after isothermal crystallization at temperatures below 185 °C, the continuous growth of PLLA crystals becomes the rate-determining step because of the early-stage collision of extremely close crystals formed on the surface of dense fibrils. Another speculative explanation might refer to the “consumption” of hydrogen bond sites because of the strong intermolecular association within the self-assembling nucleator at lower temperatures. The interpretation has the support of Fourier-transform infrared (FT-IR) data, while the idea of epitaxial growth is still lacking direct evidence so far
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