Epoxy network glass transition temperature

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. 

Fig. 45. Glass transition temperature of solvent-modified epoxy networks used for SENB testing and prepared via CIPS with various amounts of cyclohexane...

Figure 45 shows the glass transition temperature of solvent-modified networks prepared with various amounts of cyclohexane. It is seen, that Tg is independent of, or varies only slightly with, the initial amount of cyclohexane after the heat treatment. The T -values of solvent-modified epoxy networks are lower than for the fully crosslinked network, which is a result of the cyclohexane dissolved in the matrix, with a concentration given by the binodal curve and therefore is independent of the initial amount of cyclohexane. 

The weight loss after the sorption/desorption cycle is 1.3% for the specimens irradiated to 10,000 Mrads. The low weight loss indicates limited degradation. The glass transition temperature should have returned to the value of the unirradiated epoxy (280°C) if degradation products are the only species plasticizing the network. 

The time and temperature dependent properties of crosslinked polymers including epoxy resins (1-3) and rubber networks (4-7) have been studied in the past. Crosslinking has a strong effect on the glass transition temperature (Tg), on viscoelastic response, and on plastic deformation. Although experimental observations and empirical expressions have been made and proposed, respectively, progress has been slow in understanding the nonequilibrium mechanisms responsible for the time dependent behavior. 

The epoxy/siloxane/PACM-20 mixture was poured into a hot (120 °C) RTV-silicone mold of the precise shapes to be used for solid-state testing. The mixture was cured at 160 °C for 2.5 hours. The curing time and temperature chosen were considered to provide enough mobility for network formation. This conclusion was partially based on earlier studies which found a glass transition temperature of 150 °C for Epon 828/PACM-20 3S). 

TaUe-2. Glass Transition Temperatures of Epoxy Networks after Modification with Siloxane Oligomers... 

The dynamic mechanical properties of the siloxane-modified epoxy networks were also investigated. The DMTA curves for the control epoxy network exhibit the two major relaxations observed in most epoxy polymers 39 40,41>. A high temperature or a transition at 150 °C corresponds to the major glass transition temperature of the network above which large chain motion takes place. The low temperature or (5 transition is a broad peak extending from —90° to 0 °C with a center near —40 °C. It has been attributed predominantly to the motion of the CH2—CH(OH)—CH2—O (hydroxyether) group of the epoxy 39-40 2 ... 

The addition of microspheres lowers the glass transition temperature of the epoxy binder (Fig. 13). This seems to be because the filler causes defects in the matrix network. Equal diffusion coefficients of filled and unfilled epoxy binder indicates, therefore, that the diffusion processes are insensitive to binder changes. The sorption of water by epoxy resins is in fact known to depend mainly on their polarity and only slightly on the three-dimensional compactness of the network. 

The dynamic mechanical results and the solid-state 13C NMR measurements lead to a deeper insight of the motions occurring below the glass transition temperature in the considered pure aryl-aliphatic epoxy networks, in particular those involved in the p transition of these systems, and the nature of their cooperativity. 

In addition, from thermal and thermomechanical measurements, it is found that typical epoxy-amine networks exhibit one glass transition temperature, Tg, and one sharp well-defined relaxation peak. The same techniques were used for crosslinked polyurethanes based on triol and diisocyanate or diol and triisocyanate (Andrady and Sefcik, 1983). Similar conclusions to those found for epoxy-amine networks were attained. 

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