Microparticles of Functionalized Polylactide Copolymers
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Mat. Res. Soc. Symp. Proc. Vol. 501 © 1998 Materials Research Society
were 6%, 5.5%, respectively [10]. The molecular weights of the polymers are listed in Table 1. Poly(vinyl alcohol) (average molecular weight = 3000, 75% hydrolysis from the acetate) was from Polysciences Inc. All other chemicals were purchased from Acros and were used as received. Characterization Regular FTIR spectra of the microparticles were obtained on a Nicolet 60SX FTIR spectrometer. The samples were mixed with KBr and then pressed into a thin KBr pellet. Surface FTIR spectra were recorded on a Perkin-Elmer Spectrum 2000 spectrometer equipped with a MCT cryodetector. The samples were analyzed in the 4000-450 cm-1 range with a 4 cm-1 resolution. A thin disk of the sample was prepared by pressing the pure powder onto a stainless steel grid for a homogeneous temperature distribution. The molecular weights and molecular weight distributions of the polymers were determined using gel permeation chromatography (GPC). The GPC analysis was conducted in THF with a Waters 150-C ALC/GPC equipped with m-Styragel HT columns of 105, 105, 103 and 103 Apore size at 35 'C and a flow rate of 1 ml/min. Narrow molecular weight polystyrene standards were used for calibration. Particle size and morphology were observed by scanning electron microscope (SEM, AMR 1200B). Dynamic light scattering (DLS) (wavelength=632.8 nm) was utilized to determine the mean particle size with the help of a Nicomp 370 Submicro Particle Sizer from Particle Sizing System Inc. A Nicomp size-distribution analysis was used. Synthesis of Polylactide and its Copolymer Microparticles The microparticles were prepared by the optimum condition used by Scholes et al [11]. In 20 ml of 10% PVA aqueous solution, 10 ml of 1% CH2C12 solution of polymer was added dropwise under mechanical stirring (7500 rpm). After addition, the emulsion formed was stirred for another 10 min, and then put in a sonicator for 5 min. The emulsion was placed in a hood overnight under magnetic stirring to evaporate CH2CI2. After that, the particles were precipitated by ultracentrifugation and washed three times with deionized and distilled water. The particles were then dried under vacuum overnight and stored for further analysis. RESULTS The mean particle size of each type of microparticles was analyzed by DLS and is listed in Table 1. The mean particle size is in the range of 200 nm. The morphology of microparticles was observed by SEM. The three polymer particles have similar morphology, which is shown by the SEM of PLac (Figure 1). The particles are spherical and separated due to the presence of the surfactant, which stabilizes the particles, as depicted in Scheme 1. The surface of the particles is shown in Scheme 1. This was further characterized by surface FTIR spectroscopy. The major peak assignments are listed in Table 2. An absorption band at 3507 cm- 1 was observed for all the three types of particles (Figure 2). It is assigned to the v(OH) stretching vibration of the hydroxy groups from PVA surfactant. This is
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