Glass transition temperatures networks from different

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Polymer networks such as epoxies play an increasing role as adhesives in industry. Two properties are of special importance for their application (a) a strong adhesive bond is required between the solidified adhesive and the bonded object, which is often a metal (b) the mechanical stiffness of the adhesive has to be adapted to the desired level. As a consequence, the adhesive has to be selected according to its adhesion properties as well as its mechanical properties. Several studies have shown that both properties are linked as soon as the epoxy polymer layer is sufficiently thin the contact of the polymer with the substrate may induce in the polymer a broad interphase where the morphology is different from the bulk. Roche et al. indirectly deduced such interphases, for example from the dependence of the glass transition temperature on the thickness of the polymer bonded to a metal substrate [1]. Moreover, secondary-ion mass spectroscopy or Auger spectroscopy provided depth profiles of interphases in terms of chemical composition, which showed chemical variations at up to 1 pm distance from the substrate. 

Differences here include branching, network formation, and polymers derived from isomeric monomers, for example, polyfethylraie oxide), 1, poly(vinyl alcohol), 11, and polyacetaldehyde. 111, in which the chemical composition of the monomer units is the same, but the atomic arrangement is different in each case. This makes a considerable difference to the physical properties of the polymers, e.g., the glass transition temperature Tg of structure I is 206 K, for 11 = 358 K, and for 111 = 243 K. 

Diol-modified epoxy resins were prepared from bisphenol A diglycidyl ether modified with alpha,omega-diols of different chain lengths and at different molar ratios, the molecular orientation of the epoxy networks was investigated using rheooptical FTIR spectroscopy and uniaxial deformation was carried out above and below the glass transition temperature. The effects of diol chain length and molar ratio on the mechanical properties and orientation parameter were discussed. 17 refs. 

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