Study On Crystallization Behavior Of Poly (L-Lactic Acid) Thin Films

In this paper, the crystallization behavior of poly (l-lactic acid) ultrathin films was investigated by in-situ reflection absorption infrared spectroscopy (RAIR) and atomic force microscopy (AFM). The influence of crystallization temperature, thickness of films and substrate on the crystallization behavior of PLLA ultrathin films has been discussed. Meanwhile, the crystallization behaviors of PLLA ultrathin films induced by different solvents at room temperature were discussed in this paper.It is found that the crystallization temperature has an important effect on the crystallization behavior of PLLA ultrathin films. The molecular chain orientation of PLLA changes with the crystallization temperature, when the crystallization temperature is low (70 and 80oC), the intensity of the characteristic bands of 10/3 helix chain conformation and some perpendicular bands increase; while the temperature is high (90 and 100oC), the intensity of the characteristic bands of 10/3 helix chain conformation and perpendicular bands decrease, meanwhile, the intensity of parallel bands increase. And the morphology of PLLA ultrathin films were observed by AFM measurement, it is found that the edge-on lamellar crystals of PLLA thin films are mainly formed at low temperature (70 and 80oC), whereas the flat-on lamellar crystals occur when the crystallization temperature reaches to 100oC.There are also many differences crystallization behaviors of PLLA thin films at various thicknesses. When the thickness of PLLA thin films is thin (50nm), the flat-on lamellar crystals are mainly formed, but when the thickness of PLLA thin films is thicker (80 and 150nm), the edge-on lamellar crystals are mainly formed because of the orientation of PLLA thin films. When the temperature rises to 90oC and 110oC respectively, there is less difference among the crystallization of PLLA thin films with three thicknesses, it is shown that when the crystallization temperature is high enough, the thickness of PLLA thin films have no influence on the crystallization of PLLA thin films. From the morphologies of the crystallization of PLLA thin films, we can see that it mainly form the flat-on lamellar crystals.Solvent polarity and solubility parameter have important effect on crystallization behavior of the PLLA ultrathin films. PLLA ultrathin films which are induced by acetone and dichloromethane formαcrystal. The crystallinity of PLLA ultrathin films induced by acetone is the largest, while the crystallization rate of PLLA ultrathin films induced by chloroform is the fastest. This is because the intensity of solvents induction and solvent effect is different among different solvents.The crystallinity and crystallization rate of PLLA ultrathin films, whose substrates have been modified by mercaptoethanol and mercaptoacetic acid are improved obviously. And the later has relatively higher crystallinity. It is proved that the density of grafting is higher in the substrate modified by mercaptoacetic acid than mercaptoethanol, and the chemical graft modification on the substrate can act as nucleation agent.