The Use of Mechanical Property Measurements to Characterize Gels and Gelation Processes
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THE USE OF MECHANICAL PROPERTY MEASUREMENTS TO CHARACTERIZE GELS AND GELATION PROCESSES Y. YANG,* N. ICHISE,* Z. LI,* Q. YUAN,*J. E. MARK,* E. K. M. CHAN,** R. G. ALAMO,** AND L. MANDELKERN* *Department of Chemistry and the Polymer Research Center, The University of Cincinnati, Cincinnati, OH 45221-0172 **Department of Chemistry, Florida State University, Tallahassee, 32306
FL
ABSTRACT There are a variety of gels (highly swollen solids) that are of considerable interest to polymer scientists, materials scientists, and ceramists. One type consists of typical organic polymers such as polyethylene or polystyrene, in networks which are formed by means of physical cross links, such as crystallites or physical aggregates. Such gels are thermoreversible in that liquefaction occurs upon heating. Another type consists of chain-like structures permanently bonded into covalent networks. These permanently branched and cross-linked chains can be either organic (phenol-formaldehyde resins, epoxies, etc.), or inorganic [silica (SiO2), titania (TiO2), zirconia (ZrO2), etc.] Both the organic and inorganic covalent types have been used to prepare aerogels, and the inorganic ones are now much used to prepare high-tech ceramics by the new sol-gel route. In the case of the thermoreversible, organic polymer gels, moduli can be measured as a function of concentration, temperature, and structural characteristics of the polymer (molecular weight, molecular weight distribution, and nature and degree of any chain branching). Such equilibrium results give information on the nature of the gels, including the influence of morphology, and the presence of dangling-chain irregularities. Measurements carried out as a function of time, for example, on polyethylene homopolymers and copolymers, can give information about their gelation kinetics. In the case of the ceramic materials, the evolution of the shear modulus with time is very useful in establishing induction times, rates of gelation, and aging effects. Correlation of such information with results of scattering studies can give much insight into the nature of the sol-gel process. INTRODUCTION One of the most promising strategies for the preparation of highperformance materials is to separate their constituents, frequently only temporarily. Important examples are polymer chains or aggregates of colloidal ceramic particles, separated by large amounts of low molecular Mat. Res. Soc. Symp. Proc. Vol. 248. @1992Materials Research Society
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weight liquids in gel-like structures. 1- 24 These gels can be quite dilute and this has a number of advantages, including (i) minimization of chain entangling, (ii) control of alignment in the case of rigid-rod polymers, (iii) preparation of aerogels and foams of remarkably low densities, and (iv) facilitation of chemical reactions which are dependent on the viscosity of the surrounding medium. Gels are solids but, because of their highly swollen nature, are generally very weak. If the linkages between the units which give rise to the required network str
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