We have investigated the fracture behaviors of tetra-arm polyethylene glycol (Tetra-PEG) gels with controlled network structures. Tetra-PEG gels were prepared by AB-type crosslink-coupling of mutually reactive tetra-arm prepolymers with different concentrations and molecular weights. This series of controlled network structures, for the first time, enabled us to quantitatively examine the LakeThomas model, which is the most popular model predicting fracture energies of elastomers. The experimental data showed good agreement with the Lake-Thomas model, and indicated a new molecular interpretation for the displacement length (L), the area around a crack tip within which the network strands are fully stretched. L corresponded to the three times of end-to-end distance of network strands, regardless of all parameters examined. We conclude that the Lake-Thomas model can quantitatively predict the fracture energy of polymer network without trapped entanglements, with the enhancement factor being near 3.

Tetra-PEG gel, which is a polymer gel with a well-controlled network structure, was utilized to examine the model predicting the ultimate elongation ratio of elastomeric materials, i.e., the Kuhn model. The effect of polymer fraction, degree of polymerization of network strands, and connectivity of the Tetra-PEG gel was discussed. The ultimate elongation ratio was estimated from the stress-elongation curves using the extended Gent model. The ultimate elongation ratio of the Tetra-PEG gels with different structural parameters did not obey the Kuhn model at all. Instead, we proposed a new semi-empirical model predicting the ultimate elongation ratio of Tetra-PEG gel.

Photoionization spectra of the titanium and zirconium metallocarbohedrene clusters (Met-Cars) of the stoichiometry Ti8 - x Zr x C12 (x = 0−4, 8) have been investigated near threshold. Study of the photoionization efficiencies of these species at different photon energies led to a determination that the ionization potentials (IP's) for the titanium and zirconium Met-Cars, Ti8C12 and Zr8C12, are 4.40 ± 0.02 and 3.95 ± 0.02 eV, respectively. The IP's for the binary metal Met-Cars, Ti8 - x Zr x C12 (x = 1−4), were also determined, and it was found that the IP decreases continuously from that of the pure titanium Met-Car toward that of the pure zirconium Met-Car as the number of substituting zirconium atoms increases.

The understanding of the physical properties of conventional hydrogels has been controversial because hydrogels inherently have a substantial amount of various heterogeneities in their structures. In this study, we focused on one of the simplest heterogeneities, heterogeneous distribution of strand length, and investigated its influence on physical properties. We prepared tetra-PEG gels with bimodal distribution in strand length (tetra-PEG bimodal gels) by combining tetra-PEG prepolymers with different molecular weights and measured the physical properties. The physical properties of tetra-PEG bimodal gels formed above the overlapping concentration of prepolymers were well described by the models for conventional tetra-PEG gels with the average polymerization degrees between cross-links. We conclude that the mechanical properties of hydrogels that have heterogeneous distribution in strand length can be predicted from those of hydrogels with the average strand length in the range tested in this study.

Titanium metallocarbohedrenes (Met-Cars) are observed to adsorb methane molecules at low temperatures. The observed formation of the methane-Ti8C12 cluster complexes provides further direct evidence establishing that stable neutral titanium Met-Cars are present in copious amounts in the cluster beam. At sufficiently low temperatures, a dominant peak is observed for Ti8C12(CH4)4 along with less intense peaks of other methane-Ti8C12 clusters, which suggests the geometry of titanium Met-Cars may be the tetrahedral cage structure with Td symmetry. By studying the photoionization efficiency of the methane-Ti8C12 complexes near ionization threshold, their ionization potentials are found to be lower than that of Ti8C12.