Nanoscale Confinement Effects on the Relaxation Dynamics in Networks of Diglycidyl Ether of Bisphenol-A and Low-Molecular-Weight Poly(ethylene oxide)

Abstract

Thermoplastic poly(ethylene oxide) (PEO) (M w(PEO) ≈ 4000) has been used to prepare thermosetting nanocomposites incorporating diglycidyl ether of bisphenol A (DGEBA) epoxy oligomer. Blends with various PEO/DGEBA weight ratios were cured using stoichiometric portions of 4,4′-diaminodiphenylmethane. The resulting semi-interpenetrating polymer networks were studied by several techniques. Nanoscale confinement effects, thermal (glass transition, melting and crystallization temperatures) and structural features of our materials are similar to those for networks with much higher M w(PEO) and different curing agents; however, the polyether crystallization onset occurs in our case at a lower PEO concentration; shorter PEO chains organize themselves more easily into crystalline domains. Very low estimates of the k parameter of the Gordon-Taylor equation, used to fit the compositional dependences of the dielectric and calorimetric glass transition temperatures, and a strong plasticization of the motion of the glyceryl segments ( -relaxation) in the epoxy resin were observed. These illustrate an intensified weakening in the strength of the intermolecular interactions in the modified networks, as compared to the high strength of the self-association of hydroxyls in the neat resin. The significance of hydrogen-bonding interactions between the components for obtaining structurally homogeneous thermoseti-thermoplastic networks is discussed.

Mechanical Properties and Morphology Studies of Thermosets from a Liquid‐Crystalline Epoxy Resin with Biphenol and Aromatic Ester Groups

“…An analogous result was found by some authors for the epoxy system, for which two peaks in the dynamic mechanical spectrum were observed 8, 19, 27. Since thermoset networks are believed to arise from a competition among kinetic and thermodynamic factors,26, 28 their mobility has been discussed in the presence of hydrogen bonding, ion‐dipole forces and electron donor‐acceptor interactions among the components 29. Due to the above disturbing effect of three‐dimensional cross‐links on DGEBA blends, no uniform degree of molecular transition could be obtained during network formation and then two peaks were appeared in the tan δ curve.…”

“…The contribution of the second peak tended to prevail at the higher frequencies and finally became predominant with respect to the former one. The glass transition signal peaks reflecting the onset of mobility of major segments of the thermosetting component (i.e., the so‐called α relaxation) were close to the calorimetric T g = 162 °C for aromatic amine cured DGEBA 26. In correspondence with the peaks in the tan δ curve, E was slightly reduced with increasing of temperatures and dropped further in Figure 1 when going through the glass transition.…”

“…The basic interaction force between the constituent chain segments appears to be hydrogen bonding between group pairs, such as the hydroxyl (–OH) of epoxy and the ether group (–O–) of liquid crystalline epoxy, which act antagonistically to the self‐association of hydroxyl groups in pure DGEBA 26. As the hydrogen bond is a directional, attractive interaction between electron‐deficient hydrogen and a region of high electron density, its strength is directly related to all the elements affecting the acidity of the proton donor, the basicity of the proton acceptor, the accessibility of the donor and acceptor, and the chemical and stereo structures of the donors and acceptors 26. Furthermore, the concentration of unreacted epoxy groups decreases, as it should, with increasing cross‐linked density 32.…”

“…Specific intermolecular interactions in such systems were much more complicated than in linear polymer blends. Moreover, curing program, the kinetic factors, and the topological structures of the blends were closely coupled with the thermodynamics properties of thermosets 26…”

Dielectric Analysis (DEA)

“…Dielectric analysis can be used to monitor changes in characteristics of local relaxations and more importantly to study the variation of the glass transition signal and σ DC at different loadings of the modifi er . 6.32 b) (Kalogeras et al 2005a(Kalogeras et al , 2006b(Kalogeras et al , 2007b. 6.32 a).…”