Crystal Morphology Control by Melt Phase Separation in Biodegradable Polymer Blends
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Crystal Morphology Control by Melt Phase Separation in Biodegradable Polymer Blends Y. A. Akpalu1, J. C. Meredith2 and E. J. Amis3 Polymers Division, National Institute of Standards & Technology, Gaithersburg, MD 20899 1 Current address: New York State Center for Polymer Synthesis, Department of Chemistry, Rensselaer Polytechnic Institute, Troy NY 12180. E-mail: [email protected]. 2 Current address: School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 36332-0100. E-mail: [email protected]. 3 Email: [email protected] ABSTRACT The effect of lower critical solution temperature (LCST) phase separation on the crystallization of SRO\ -caprolactone) PCL in PCL/poly(D,L-lactide) (PDLA) blends is studied by simultaneous small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS). Phase separation is induced by controlled temperature jumps into the LCST (two-phase) region, which is above the melting temperature (60 °C) of PCL and the glass transition temperature (50 °C) of PDLA. We have followed the nanoscale structural changes (< 100 nm) during subsequent crystallization at 45 oC of critical (0.36 PCL) and off-critical (0.50 PCL) blend compositions in both one-phase and two-phase melts. The spherulite morphology (1- P LV examined with optical microscopy. When crystallization follows LCST phase separation, the shape, size and distribution of the spherulites depends on the extent of melt phase separation. In our x-ray measurements, the WAXS crystallinity of PCL is less than 40 % for the temperature range of interest. We perform a correlation function and intensity model analysis of our SAXS data to obtain morphological variables that characterize the intraspherulitic morphology. These morphological variables are relatively constant during crystallization and are also independent of melt phase separation. On the other hand, the ultimate crystallinity and the crystallization rate depend on the extent of melt phase separation. INTRODUCTION Although few biological tissues are homogeneous, one-component materials, tissue scaffolds are often made from single-component biodegradable aliphatic polyesters [1]. Typically poly(lactic acid), poly(glycolic acid), poly(lactic acid-co-glycolic acid) and other copolymers of lactic acid are used [2,3]. These scaffolds are brittle polymer foams usually prepared by techniques where water-soluble salts are incorporated into a polymer matrix followed by leaching to obtain the polymer foams [4,5]3RO\ -caprolactone) (PCL) is a biodegradable polymer that has also received increasing interest from the scientific and medical communities [1,6]. Blending PCL (an elastomer) with amorphous poly(D,L-lactic acid) (PDLA, a brittle polymer) may offer a simple and cost-effective method of achieving a range mechanical properties that is similar to many load bearing tissues. The advantage of blending is that melt phase separation can be used to create a rich range of morphologies if the polymer sample is quenched from a one-phase state to a two-phase s
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