Structure and Properties of High Performance Gels Made by Module Assembling Method
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Structure and Properties of High Performance Gels Made by Module Assembling Method Mitsuhiro Shibayama,1 Hanako Asai,1 Kenta Fujii,1 Yuki Akagi, 2 and Takamasa Sakai 2 1 Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan. 2 Department of Bioengineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan. ABSTRACT High performance polymer network gels consisting of tetra-arm poly(ethyleneglycol) (TetraPEG) gels were fabricated via a module assembling method and their mechanical properties and structure were investigated by stretching and compression measurements, dynamic mechanical measurements, and small-angle neutron scattering (SANS). It was found that Tetra-PEG gels are nearly-ideal polymer network with negligible fractions of defects and entanglements. SANS intensity functions indicated that the network structure was uniform free from spatial inhomogeneities. It is deduced that this uniform structure is ascribed to its unique preparation method, i.e., module assembling method (cross-end-coupling of tetra-functional macromers with complementary functional groups). Characteristic properties originated from the near-ideality as polymer networks are demonstrated, including its application to ion gels, i.e., polymer network in ionic liquid. INTRODUCTION Recently, we developed a new class of biocompatible high-strength hydrogels, consisting of four-arm poly(ethylene glycol) (PEG) networks (hereafter we call Tetra-PEG gels) [1]. The Tetra-PEG gels are made by “cross-end-coupling” of two kinds of four-arm PEG having different functional groups at chain ends. Since the functional groups are amine group (TAPEG) and N-hydroxysuccinimidyl ester group (TNPEG), the coupling reaction occurs exclusively between TAPEG and TNPEG. Thus, a network with a functionality of four is prepared. Figure 1 shows a comparison of cross-linking of telechelic polymer chains and cross-end-coupling of symmetric multi-functional polymer chains with complementary functional groups. The former cannot exclude formation of self-biting and missing of cross-linking due to large asymmetry of the sizes of the reactants, i.e. long polymer chains vs small cross-linkers. As a result, there is a high probability of inhomogeneous distribution of cross-linkers and trapped entanglement. On the other hand, cross-end-coupling occurs exclusively to a partner chain and the symmetry of the macromers guarantees more uniform structure than the former. As a matter of fact, it is reported that the mechanical properties of Tetra-PEG gel are much superior to those of conventional gels and the compressive strength of resulting gel was in a MPa range which was much superior to those of agarose gels or acrylamide gels having the same network concentrations [1]. Structure characterization of Tetra-PEG gels by small-angle neutron scattering (SANS) revealed no characteristic upturn in the scattering intensity indicating spatial frozen-inhogeneities and the scattering function can be simply described by Ornstein-Zernike type function [2,3]. This means
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