MODEL NETWORKS BASED ON ENDLINKING PROCESSES - SYNTHESIS, STRUCTURE AND PROPERTIES

The present review comprises five main sections. The first section is devoted to the synthesis by 'endlinking' processes of homogeneous mode l networks of well-defined architectures, which can be characterized b y their structural parameters. In most of the methods developed in our Institute over the past two decades, linear polymer 'precursor' chain s, fitted at both chain ends with reactive groups, are first synthesiz ed. Under selected experimental conditions, the addition of an appropr iate compound to this alpha, omega-functional polymer, allows the form ation of model networks, provided no 'syneresis' has occurred. In such a way, various polymer 'precursors' of known molar mass and narrow po lymolecularity have been used, such as 'living' poly(styrene), poly(is oprene), 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 sc attering experiments, as is described in the second section of the rev iew. 'Labeled' poly(styrene) and poly(dimethylsiloxane) have been used to characterize the spacial distributions of the crosslinks and the c onformations 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 aris ing from rubber elasticity theories and from swelling equations. Alter nately, interpretation of the experimental data based on 'scaling' con cepts is developed using the analogy between semidilute solutions and permanent model networks swollen to equilibrium in a 'good' solvent. I n the fourth section, it is shown that the use of branching theories a llows one to evaluate the number of effective crosslinks, as well as t he extent of the reaction, knowing experimentally the amount of the so l-fraction in the network. The theories of the equilibrium elastic res ponse of a collection of nu network chains have been tested using equi librium swelling and elastic modulus data obtained from poly(ethylene oxide) model networks. Systematic departures from the predictions of t he classical theories of rubber elasticity are observed. However, sati sfactory agreement between theory and experiments is found by the intr oduction of two additional contributions to the elastic free energy, n amely the limitation of free fluctuations of the junctions and the ent anglement effect. In the last section, a new approach based on rheolog y, kinetics and branching theory is developed. In these studies, poly( urethane) model networks are used. The rheological behavior of the rea ction medium is evaluated by measuring the storage and the loss moduli at various stages of the process, over a wide range of oscillatory sh ear frequencies. For stoichiometric systems, at a given conversion (de termined 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 determi ne precisely the time at which crosslinking occurs (gel point). This e xperimental value is in excellent agreement with that arising from bra nching theory.