Effect of chemical clusters on photoelastic behaviour and small-angle X-ray scattering of epoxide networks based on poly(oxypropylene)diamines

Viscoelastic and equilibrium photoelastic behaviour and small-angle X-ray scattering of epoxide networks based on bisphenol A diglycidyl ether (DGEBA) and two poly(oxypropylene)diamines (Jeffamine® D-400 and D-2000) were investigated. Networks were prepared with the stoichiometric ratio of amine hydrogens and epoxy (E) groups (rH=2[NH2]/[E]=1) with various compositions of (D-400 + D-2000)/DGEBA; seven networks with the weight ratios of D-400/D-2000=1/0, 0.83/0.17, 0.67/0.33, 0.5/0.5, 0.33/0.67, 0.17/0.83 and 0/1 were prepared. Time-temperature superposition was applied to photoelastic data. With increasing content of long D-2000 diamine, the superimposed photoelastic functions at the reference temperature Tr=0°C are shifted to shorter reduced times t/a(T, Tr) (a(T, Tr) is the horizontal shift factor) by ∼18 decades. Although mechanical and optical shift factors, log a(T, Tr), of two-component networks are virtually the same and their temperature dependences can be described by the WLF equation, these factors for three-component networks are different and cannot be described by the WLF equation. As expected, the equilibrium modulus decreases with increasing contents of D-2000 diamine in networks. On the contrary, the equilibrium stress-optical coefficient Ce passes through a maximum at the weight fraction of DGEBA in networks, wE≈0.50. SAXS experiments show a non-homogeneous distribution of DGEBA segments in three-component networks; the deviations from homogeneous distribution reach a maximum for networks with wE≈0.50. All data suggest that in three-component networks, chemical clusters formed by sequences with non-random participation of short and long diamines exist and that optical functions are more sensitive to these chemical heterogeneities than mechanical modulus. In addition, a modification of photoelastic apparatus for automatic measurement of the strain-birefringence was suggested.