Correlation between Mechanical Properties and Structure in Polymer Gels with Controlled Network Structure
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Correlation between Mechanical Properties and Structure in Polymer Gels with Controlled Network Structure Takamasa Sakai, Yuki Akagi, Ung-il Chung University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, JAPAN ABSTRACT Elastmeric materials are of great importance in both academic and industrial field due to the soft and highly stretchable properties. Thus, many theories and models are proposed to correlate the physical properties and structural parameters. However, in general, it is difficult to validate these models experimentally. Thus, to this day, we do not know the requirement conditions for each model or even the validity of each model. The validation of these models has been inhibited by the inherent heterogeneity of polymer networks. Recently, we, for the first time, succeeded in fabricating polymer network with extremely suppressed heterogeneity with a novel molecular design of prepolymers. The homogeneous polymer network, called Tetra-PEG gel, is prepared by AB-type crosslink-coupling of mutually reactive tetra-arm prepolymers. In this study, we examined the models of elastic modulus and fracture energy using Tetra-PEG gel as a model system. We controlled the structural parameters with tuning the molecular weight and concentration of prepolymers, and reaction conversion of the reaction. This series of controlled network structures, for the first time, enabled us to quantitatively examine these models. We performed the stretching and tearing measurements for these polymer gels. As for the elastic modulus, we observed the shift of the models from the phantom to affine network models around the overlapping concentration of prepolymers. As for the fracture energy, we confirmed the validity of the Lake-Thomas model, which is the most popular model predicting fracture energies of elastomers. INTRODUCTION The mechanical properties of rubbery materials have great importance in both scientific and industrial field. The elastic modulus and the fracture energy are the important parameters characterizing the mechanical properties of elastomeric materials. Although many models predicting these properties are proposed, 1 in general the validation of these models is difficult. This difficulty is caused by the heterogeneous nature of polymer networks. Because the transient heterogeneity is fixed at the gelation threshold, the polymer networks have inherent heterogeneity.2 Thus, at this point, we do not know the requirement conditions for each model or even the validity of each model.
Figure 1. Schematic image of Tetra-PEG gel
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Recently, we, for the first time, succeeded in fabricating polymer network with extremely suppressed heterogeneity with a novel molecular design of prepolymers (Fig 1). 3 The homogeneous polymer network, called Tetra-PEG gel, is prepared by AB-type crosslinkcoupling of mutually reactive tetra-arm prepolymers. We confirmed the homogeneity of TetraPEG gels with small angle neutron scattering (SANS) and NMR measurements. 4-6 No excess scattering originated from the boundary structure was observed in SA
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