Investigation of the Melting Behavior of the Alternating PEO/PCL Multiblock Copolymer by SAXS and WAXS
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Investigation of the Melting Behavior of the Alternating PEO/PCL Multiblock Copolymer by SAXS and WAXS Tae Chul Kim, Jong Ho Kim, Hyun Hwi Lee, You Han Bae, Do Young Noh* Department of Materials Science and Engineering, Kwangju Institute of Science and Technology, Kwangju 500-712, Korea ABSTRACT The melting of the semicrystalline poly(ethylene oxide)(PEO)/ poly( ε -caprolactone) (PCL) multiblock copolymer was investigated by small and wide angle x-ray scattering. The two-stage melting was found in which the PEO block and the PCL block melt independently. The crystalline PEO Bragg peaks disappear at 30.5 oC while the block spacing increases due to the chain relaxation. The PEO crystalline domain size decreases continuously in proportion to the amount of the crystalline part. Above 40 oC, the PCL lamellae domains started to melt and the integrated intensity of the PCL Bragg peak decreases with increasing temperature. In the case of the PCL, however, the crystal domain size decreases only slightly. This indicates that the number of the PCL crystalline domains decreases during the melting. INTRODUCTION Multiblock copolymers composed of biocompatible hydrophilic polymers and biodegradable hydrophobic polymers can play potentially important roles in medical implant, biosensors and matrices for drug delivery [1]. To design applicable multiblock copolymers as thermosenstive hydrogels, understanding their phase behavior and the morphological change with varying temperature is an essential step. In this paper, we report the melting behavior of the amphiphilic poly(ethylene oxide)(PEO)/poly( ε -caprolactone) (PCL) multiblock copolymer by small and wide angle x-ray scattering. The multiblock copolymer is composed of an alternating block sequences with hydrophilic PEO(soft segments) and hydrophobic PCL(hard segments) that crystallize into monoclinic and orthorhombic structures respectively [2-3]. The PEO/PCL multiblock copolymer received much attention due to the biocompatibility of PEO and the biodegradability of PCL. Around room temperature, it stays in a semicrystalline state where parts of the PCL form nanocrystals that act as cross-linking parts in an amorphous matrix. Wide angle x-ray scattering (WAXS) measurements reveal that the PEO/PCL multiblock copolymer melts in two stages in which the PEO block and the PCL block melt independently. The overall morphological change FF4.10.1
during this two stage melting is investigated by small angle x-ray scattering (SAXS). EXPERIMENTAL DETAILS The PEO/PCL multiblock copolymer was synthesized by dissolving dicarboxy-terminated PEO (Mw=2000g/mol) and PCL(Mw=2000g/mol) purchased from Aldrich Chemical Co. in a dry methylene chloride. N,N-Dimethyl aminopyridine (DMAP) and dicyclohexyl carbodiimide(DCC) were added and stirred overnight at room temperature under dry nitrogen [4]. The polymer solution (10 wt%) was poured on a casting mold made of a flat Teflon plate. The average molecular weight and the polydispersity of the (PEO/PCL)n multiblock copolymer measured by gel permeation chro
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