Bohumil Meissner

Cage-like structure formation during sol-gel polymerization of glycidyloxypropyltrimethoxysilane

Abstract The sol–gel polymerization of 3-glycidyloxypropyltrimethoxysilane (GTMS) was followed by size exclusion chromatography and 29Si NMR. Extensive non-random cyclization under formation of polyhedral cycles – cubic cages – predominates at the beginning of the reaction. Structure growth of polysilsesquioxanes proceeds by combining the incompletely condensed cage frameworks. The extent of the cage formation increases with dilution and the amount of water and depends appreciably on a catalyst. The cage fraction was isolated from a reaction mixture using preparative size exclusion chromatography and identified by 29Si NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). High content of polyhedral cages prevents gelation of the trifunctional GTMS monomer. Reaction of pendant epoxy groups is much slower; however, at a late reaction stage the epoxy hydrolysis can be significant. Under some conditions, like base catalysis, polysilsesquioxane clusters are crosslinked by intermolecular condensation of SiOH with hydrolyzed epoxy groups and the system gels. The cage with epoxide functionalities may serve as a rigid precursor of crosslinking.

Tensile stress-strain behaviour of rubberlike networks up to break. Theory and experimental comparison

Abstract A satisfactory description of the tensile stress–strain dependences (SSDs) of lightly crosslinked single-phase networks is not obtained with use of the existing theories (Langevin, van der Waals, slip-link). A combination of the Langevin theory-based James–Guth equation with the phenomenological C2 term of the Mooney–Rivlin equation (JGC2 equation) is shown to represent the SSDs of a number of networks (bimodal polysiloxane, pre-strained SBR) very well. Hysteresis-based deviations of some networks from the JGC2 equation in the high elongation region can be quantitatively taken into account as an increase in the finite extensibility parameter with extension ratio. The accuracy of the SSD description up to break is generally better than 3–4%.

Description of the tensile stress–strain behaviour of filler-reinforced rubber-like networks using a Langevin-theory-based approach. Part II

Abstract A combination of the Langevin-theory-based James–Guth equation with the phenomenological C 2 term of the Mooney–Rivlin equation (modified by introducing an additional empirical parameter) is shown to represent the tensile stress–strain dependencies obtained on retraction of a number of carbon-black- and silica-reinforced butadiene–styrene networks. The stress–strain behavior at increasing strain of both pre-strained and virgin specimens is more complex but it can be satisfactorily described using the concept of a strain-dependent finite extensibility parameter (introduced previously for unfilled networks). The accuracy of data description is better than ca. 4%. Similarly to unfilled networks, the increase in the finite extensibility parameter with increasing strain is ascribed to strain-induced changes in network topology (increase in network mesh size). On retraction, such changes probably take place to a much lesser degree if at all.

Comparison of recent rubber-elasticity theories with biaxial stress–strain data: the slip-link theory of Edwards and Vilgis

Abstract The Edwards–Vilgis (EV) slip-link theory (1986) derives the elastic free energy of a rubber-like network model containing stable and sliding network junctions (crosslinks and slip-links) and predicts both low-strain softening and high-strain hardening. The four-parameter stress–strain relations calculated by the theory for geometrically different deformation modes up to high strains were tested experimentally using published biaxial stress–strain data on simple covalently crosslinked networks. For networks with low degrees of strain softening and low extensibilities, the experimental dependencies could be described rather well but, generally, a simultaneous satisfactory fit to uniaxial, pure shear and equibiaxial data was not obtained. Systematic experiment–theory deviations exceeding 10% were observed and some of the parameters had a tendency to assume values lying outside the reasonably expected range. The prediction of a pronounced maximum in the strain dependence of stress supported by slip-links seems to be a reason for the discrepancy. Also, modeling of the high-strain singularity in entropy is done in the EV theory using a rather simple approximation. As a result, the finite extensibility contribution to the stress of a slip-link-free network model becomes improbably high and significantly exceeds that following, at a given modulus and locking stretch, from the rigorously derived Langevin-statistics-based eight-chain-network elasticity theory of Arruda and Boyce.

Langevin-elasticity-theory-based description of the tensile properties of double network rubbers

Abstract A previously proposed and successfully tested constitutive equation denoted by the ABGIL code (a combination of the Arruda–Boyce equation based on the Langevin elasticity theory and a constraint term based on tube theories; strain-induced increase in the finite extensibility parameter is assumed) has been found to provide a good basis for quantitative interpretation of the stress–strain data recently obtained by Mott and Roland on double networks of natural rubber, prepared by introducing additional crosslinks (second network) into a first network stretched to various extents. Experimental information on properties of the first and second networks has been used to obtain their ABGIL parameters and to calculate, under the common assumption of additivity of contributions, the stress–strain properties and residual stretch of the resulting double networks. The predictive ability of the ABGIL equation has been found to be very good. Effects of the finite extensibility of network chains appear to be significant in double networks while the possible role of orientational crystallization cannot be excluded.

Potential of recent rubber‐elasticity theories for describing the tensile stress‐strain dependences of two‐phase polymer networks

The potential of recent rubber-elasticity theories for giving a well-founded, molecularly based and as-precise-as-possible description of experimental tensile stress-strain dependences of two-phase polymer networks (such as block polymers of the polyurethane and styrenebutadiene-styrene type) up to the break has been explored. Theoretical arguments lead to the conclusion that the rigorous theory of the high-strain hardening effect based on Langevin statistics (Arruda and Boyce, 1993) is to be preferred to the slip-link and extended-tube theories in view of the approximations involved in their derivation. None of the existing theoretical treatments of the low-strain softening was found suitable for the purpose of the study. Therefore, a semiempirical solution had to be chosen by combining the Langevin-theory-based term with an empirically modified C 2 term of the Mooney-Rivlin equation, similarly to the treatment used previously for unfilled and filler-reinforced networks (Meissner and Matějka, 2000, 2001). A very good data description is obtained for networks with different contents of hard blocks, up to 50 %, provided that yield does not appear. The parameter values of the proposed equation have been determined for a number of two-phase networks and their relations to structural parameters are discussed. The experimental high-strain behavior strongly suggests the presence (absence) of topological changes on increasing (decreasing) elongation, of the same kind as observed previously for unfilled and filler-reinforced networks. Thus, a unified picture of the tensile stress-strain behavior of rubbery networks of all kinds emerges.