Model networks based on `endlinking' processes: synthesis, structure and properties

Abstract

The present review comprises five main sections. The first section is devoted to the synthesis by `endlinking' processes of homogeneous model networks of well-defined architectures, which can be characterized by their structural parameters. In most of the methods developed in our Institute over the past two decades, linear polymer `precursor' chains, fitted at both chain ends with reactive groups, are first synthesized. Under selected experimental conditions, the addition of an appropriate compound to this α, ω-functional polymer, allows the formation of model networks, provided no `syneresis' has occurred. In such a way, various polymer `precursors' of known molar mass and narrow polymolecularity have been used, such as `living' poly(styrene), poly(isoprene), poly(alkyl methacrylates), poly(2-vinylpyridine), as well as poly(dimethylsiloxane), poly(ethylene oxide) and poly(1,3-dioxolane), which become the elastic chains of the model networks. Among all these materials, `labeled' model networks with deuterated molecules are of great interest, since they allow one to perform small-angle neutron scattering experiments, as is described in the second section of the review. `Labeled' poly(styrene) and poly(dimethylsiloxane) have been used to characterize the spacial distributions of the crosslinks and the conformations of elastic chains in the dry, the equilibrium swollen and uniaxially stretched states. As shown in the third section, uniaxial deformation and equilibrium swelling data are compared with those arising from rubber elasticity theories and from swelling equations. Alternately, interpretation of the experimental data based on `scaling' concepts is developed using the analogy between semidilute solutions and permanent model networks swollen to equilibrium in a `good' solvent. In the fourth section, it is shown that the use of branching theories allows one to evaluate the number of effective crosslinks, as well as the extent of the reaction, knowing experimentally the amount of the sol-fraction in the network. The theories of the equilibrium elastic response of a collection of ν network chains have been tested using equilibrium swelling and elastic modulus data obtained from poly(ethylene oxide) model networks. Systematic departures from the predictions of the classical theories of rubber elasticity are observed. However, satisfactory agreement between theory and experiments is found by the introduction of two additional contributions to the elastic free energy, namely the limitation of free fluctuations of the junctions and the entanglement effect. In the last section, a new approach based on rheology, kinetics and branching theory is developed. In these studies, poly(urethane) model networks are used. The rheological behavior of the reaction medium is evaluated by measuring the storage and the loss moduli at various stages of the process, over a wide range of oscillatory shear frequencies. For stoichiometric systems, at a given conversion (determined by Fourier transform infra-red spectroscopy), the two moduli become congruent and proportional to the root square of the frequency over the entire frequency range. The method used allows one to determine precisely the time at which crosslinking occurs (gel point). This experimental value is in excellent agreement with that arising from branching theory.