Effect of Film Formation Method and Annealing on Morphology and Crystal Structure of Poly(L-Lactic Acid) Films

The poly(L-lactic acid) (PLLA) has potential medical usage such as drug delivery since it can degrade into bioabsorbable products in physiological environments, while the degradation is affected by its crystallites. In this paper, the effects of film formation method and annealing on the crystallites formed in PLLA films are investigated. The films are made through solvent casting and spin coating, and subsequent annealing is conducted. The resulting morphology, molecular order, conformation, and intermolecular interaction are examined using optical microscopy, wide-angle X-ray diffraction, and Fourier transform infrared spectroscopy. It is observed that solvent casting produces category 1 spherulites while annealing the spin coated films leads to spherulites of category 2. The crystal structure of the two kinds of films also shows distinct features. The results enable better understanding of the crystallites in PLLA, which is essential for its medical application. [DOI: 10.1115/1.4025909] There is a significant interest in use of biodegradable polymers due to their biocompatibility and biodegradability. Poly lactic acid (PLA) is a biodegradable polymer and can be obtained from renewable sources such as corn starch. It has a wide range of applications in food packaging and tissue engineering. It is also preferred in drug delivery because it can degrade into bioabsorbable products in physiological environments. Its degradation mainly comes from the cleavage of its ester groups, and is sensitive to chemical hydrolysis. In this application, drug molecules are encapsulated in the polymer by dispersing or dissolving in the polymeric solution, formed through melting the polymer or dissolving it in a solvent. During the process polymer may crystallize, which influences its degradation [1]. Tsuji and Ikada [2] proposed that the degradation of PLA begins in the amorphous region between the lamellae, followed by the disorientation of the lamellae and disappearance of the spherulitic structure. Namely, crystallites affect PLA degradation, and thus play a significant role when the material is used in the drug delivery system. Efforts have been done to investigate the crystallization behavior and resulting crystal structures of PLA films. Among them, solvent casting [3,4] and spin coating [5] are the two commonly used film formation methods. Solvent casting is a simple film formation technique in which quiescent polymer solution is deposited on a substrate, while evaporation of solvent leaves the solid polymer film. This process generally takes hours. During the casting process crystallization could occur in solution if the evaporation rate is low enough for polymer molecules to diffuse to the crystal growth front and overcome the energy barrier of deposition, so that crystals can grow. Category 1 spherulites, in which lamellae grow in all directions and the spherical symmetry extends to the central region as defined by Norton and Keller [6], can be commonly observed in the as cast films [3,7]. Spin coating generates polymer films with thinner and more uniform thickness. It involves rapid rotation of polymer solution, with the centrifugal force pushing it to flow radially outward, decreasing its thickness. The solvent evaporates simultaneously. This process typically finishes within several seconds or minutes. Due to rapid evaporation, it is less likely for crystallites to develop in the as coated films. To increase crystallinity in as cast/coated PLA films, further annealing has been attempted. For the as cast films fully covered by spherulites formed during casting, subsequent annealing increases the degree of crystallinity, but does not alter the mor