Control of Crystal Morphology in Poly(<scp>l</scp>-lactide) by Adding Nucleating Agent

In the past few years, poly(L-lactide) (PLLA) has attracted increasing attention due to its excellent biocompatibility and processability. As an eco-friendly thermoplastic polyester, PLLA can be produced completely from renewable sources, such as corn, and degrade into carbon dioxide and water in soil or a composting environment. This sustainability makes PLLA a suitable alternative to traditional petrochemical-based polymers where their recycling still remains a challenge. PLLA has good mechanical properties at room temperature, such as high tensile strength and elastic modulus, but its article prepared by practical processing methods (e.g., injection molding) exhibits poor mechanical performance above its glass transition temperature (around 60 C) since it remains almost amorphous after processing due to its slow crystallization rate.Moreover, PLLA is brittle and its long-term behavior is poor because of a pronounced creep. The crystallization can restrict molecular mobility and then improve the long-term performance. Obviously, PLLA article with suitable crystallinity is required in many commercial applications. On the other hand, the mechanical, thermal properties, and even biodegradability of such semicrystalline polymer have been demonstrated to be strongly dependent on the crystal morphology and structure. Therefore, the realization of effective control on the crystal superstructure consisting of crystalline lamella, such as spherulites and shish-kebab structure, is of great scientific and technical significance because it might provide a route to prepare PLLA products with excellent macroscopic performance. In most cases, isotropic spherulite-like crystal morphology is observed for semicrystalline polymer including PLLA, while shish-like fibril crystals are often obtained from the sheared melt. Many researchers reported that adding nucleating agent is a useful method for controlling the crystal superstructure of polymer, such as polyolefins. For example, the exclusive formation of bundle-like superstructure for β-phase and the radiating, spherulitic superstructure for R-phase in polypropylene (PP) can be induced by adding highly active β-form and R-form nucleating agent, respectively. Moreover, the assembled structure could vary with its concentration and the thermal conditions during melting and crystallization if the nucleating agent is partially or completely dissolved in the polymer melt and recrystallize during cooling. As a result, different superstructures of PP could be induced. For instance, sorbitol-based nucleating agents, such as well-known 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS), can self-organize into nanofibils on cooling, and then lateral growth of PP lamellae occurs orthogonally to the fibrils, forming a typical shish-kebab structure. Thus, adding nucleating agent with the ability of self-organizing in polymer melt is believed to be an important method to manipulate crystal morphology as well as properties of polymer articles. Similar to DMDBS, 1,3,5-benzenetricarboxylamide derivatives, another family of nucleating agents for PP, are also found to be capable of self-organizing in PPmelt through intermolecular hydrogen bonding of the amides in the form of a three-dimensional fibrillar network. Very recently, the derivatives have been successfully used to enhance the crystallization of PLLA. However, no attention has been paid to the self-organization of the derivatives in PLLA melt and its subsequent effect on the crystal morphology of PLLA though it is very important for the macroscopic performance. Therefore, in this Communication, we use one of the above derivatives, i.e., N,N0,N00-tricyclohexyl-1,3,5-benzenetricarboxylamide (TMC-328), as a model to tailor the crystal superstructure of PLLA, and three crystal morphologies including cone-like, shish-kebab, and needle-like structures have been successfully obtained for the first time using melt crystallization. Furthermore, the evolution of crystal morphology during crystallization is investigated using in-situ POM and rheological measurement.