Laser Surface Treatment of a Biodegradable Polymer at Varying Fluences

For biodegradable polymers such as poly (Llactic acid) (PLLA), material crystallinity has a considerable effect on degradation and physical properties. In this work, the effects of laser fluence of a 355 nm UV Q-switched Nd-YAG laser on the crystallinity and conformation at the surface of PLLA films and the factors affecting the transformation are investigated. Electron microscopy, X-ray diffraction, and infrared spectroscopy are used to study the effects at varying fluences. The melting and crystallization kinetics of PLLA are examined to understand the important parameters that determine the overall crystallinity. Potentially, laser surface treatment at varying fluences can be used to spatially control the generation of a polymer surface with an altered degree of crystallinity. Such a structure may have applications including timereleased drug delivery. INTRODUCTION Biodegradable polymers, due to their extensive applications and potential have recently generated significant interest. Poly ( hydroxy acid) polymers, especially poly (lactic acid) (PLA) and its copolymers are particularly important because they are U.S. FDA approved, have desirable properties, and degrade, primarily hydrolytically, into bioresorbable products. Their applications include drug delivery devices, fixation plates, tissue engineering, packaging, and agriculture. In biomedical applications, it is important to control polymer surface properties, especially crystallinity. Typical drug delivery devices consist of a drug-infused biodegradable polymer matrix that gradually releases the drug as the matrix degrades hydrolytically, leading to erosion. Drug-release profiles are hence affected by degradation rates, which in turn are a strong function of material crystallinity. Tsuji and Ikada (1998) studied the effect of crystallinity on the PLA degradation and concluded that hydrolysis of PLA chains initiates in the amorphous regions within and between the spherulites. Also, hydrolytic bulk degradation of polyesters shows an initial incubation period, which is disadvantageous in applications Transactions of NAMRI/SME 25 Volume 36, 2008 requiring rapid initial drug release. Hence, surface amorphous regions can potentially be used to initiate surface erosion during incubation and hence alter drug-release profiles. Although methods like plasma treatment can affect surface properties, structural modifications have not been attempted, as they are inherently difficult to control spatially. Laser processing of biodegradable polymers has primarily focused on micro and nano scale fabrication, as studied by Kancharla et al. (2001). Laser treatment as a means of surface modification is attractive due to spatial control and the ability to control laser properties to control the affected material depth. Aguilar et al. (2005) have studied the effects of laser patterning on poly (glycolic acid) (PGA). They observed chemical changes and initial degradation increase without affecting overall degradation time but did not study the reasons for the affected changes. Lazare and Benet (1993) used a single excimer pulse to induce periodic roughness on Mylar (PET) film surface and with ellipsometric measurements showed formation of a surface layer with different optical properties, but the structural changes were not studied. Dunn and Ouderkirk (1990) have also studied excimer laser texturing of crystalline PET and, due to observed increase in refractive index and IR spectroscopy data, suggested that the anisotropy of the surface amorphous layer causes texturing. However, they did not investigate details of the crystal structure changes or factors affecting it. The focus of this paper is to utilize laser processing to reduce the crystallinity at the surface of PLA films, with the potential to allow for faster surface degradation in the modified regions compared with bulk. Effects of varying fluence on the affected depth are studied. Scanning electron microscopy (SEM), wide angle X-ray diffraction (WAXD), and Fourier transform infrared spectroscopy (FTIR) are used to analyze the morphology and crystallinity changes at the surface. Factors affecting the laser melting and crystallization of PLA are investigated to study the important parameters and reasons for modified crystallinity post-laser irradiation. MELTING AND CRYSTALLIZATION IN POLY (L-LACTIDE) To consider the effect of laser processing on PLA, it is important to understand the melting and crystallization characteristics of polymers in general and PLA in particular. High molecular weight PLA is a thermoplastic with a melting temperature of 170–210°C, which makes it suitable for thermal processing. PLA can exist as isomers poly-L-lactide (PLLA) or poly-Dlactide (PDLA), of which PLLA is semicrystalline and can be crystallized by cooling from melt, annealing, and under strain. It is known to crystallize by chain folding, forming lamellae perpendicular to the chain axis. Polymers show a melting transition over a temperature range, with the equilibrium melting temperature (GibbsThompson Equation) varying with the distribution of the lamellar thickness. Nanosecond laser irradiation can cause rapid heating with cooling rates on the order of 10 K/s, which are much higher than traditional thermal rates of 10–100 K/s. The total amount of the crystallinity in the material is a function of nucleation and growth, which are a strong function of the cooling rate. In general, in polymers, melting is a fast process completed with low superheating, versus crystallization that is a relatively slower process requiring high supercooling. Crystallization studies of PLLA have shown that the kinetics of melt crystallization of PLLA are relatively slow and hence the possibility of affecting its crystallinity using laser irradiation. While Wunderlich (1980) has discussed crystallization in detail, nucleation rate is a function of the free energy of nucleation barrier ( G), which is a function of undercooling ( T= Tm–Tc; Tm is equilibrium melting temperature and Tc is crystallization temperature) and the free energy of activation ( G ). Nucleation rate increases rapidly close to glass transition reaching a maximum at Tc = Tc*, which is on the order of 105–110°C in PLLA. During laser processing, large deviations from Tm occur and hence the final development of crystallinity is expected not to be limited by the nucleation barrier, but by growth kinetics. According to the widely accepted Lauritzen-Hoffman Theory (1976), the growth rate, G, of the crystal can be