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Mechanical properties of polymer gels with controlled network structure

Takamasa Sakai Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo

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.


Fig. 1. Schematic picture of Tetra-PEG gel

Recently, we, for the first time, succeeded in fabricating polymer network with extremely suppressed heterogeneity with a novel molecular design of prepolymers (Figure 1). 3 The homogeneous polymer network, called Tetra-PEG gel, is prepared by AB-type crosslink-coupling of mutually reactive tetra-arm prepolymers. We confirmed the homogeneity of Tetra-PEG gels with small angle neutron scattering (SANS) and NMR measurements. No excess scattering originated from the boundary structure was observed in SANS measurement. Furthermore, Tetra-PEG gel has high compression mechanical strength comparable to that of a native articular cartilage, stemming from the homogeneous network structure. In this study, we investigated the connectivity defects and trapped entanglements by infrared spectroscopy and tearing measurement, respectively. After confirming the homogeneity of Tetra-PEG gel, we examined the models of elastic modulus using Tetra-PEG gel as a model system. We controlled the structural parameters with tuning the molecular weight and the concentration of tetraarmed prepolymers and the reaction conversion. This series of controlled network structures, for the first time, enabled us to quantitatively examine these models. We performed the mechanical tests for these polymer gels and compared the experimental results with those predicted by the theories.

References

1. M. Rubinstein and R. H. Colby, Polymer Physics. (Oxford University Press, New York,2003).
2. M. Shibayama, Macromol Chem Physic 199, 1 (1998).
3. T. Sakai, T. Matsunaga, Y. Yamamoto, C. Ito, R. Yoshida, S. Suzuki, N. Sasaki, M.
Shibayama and U. I. Chung, Macromolecules 41, 5379 (2008).