Epoxy network free volume

The intrinsic moisture sensitivity of the epoxy resins is traceable directly to the molecular structure. The presence of polar and hydrogen bonding groups, such as hydroxyls, amines, sulfones and tertiary nitrogen provides the chemical basis for moisture sensitivity, while the available free volume and nodular network structure represent its physical aspect. 

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. 

The principal physical structural parameters that control the modes of deformation and failure and mechanical response of epoxies are (1) macroscopic inhomogenieties such as microvoids or concentrations of unreacted monomer, (2) the glassy-state free volume and (3) the crosslinked network structure characteristics. 

The flexibility and extensibility of a crosslinked epoxy network are determined by the available glassy-state free volume. If the free volume is insufficient to allow network segmental extensibility via rotational isomeric changes then the brittle mechanical response of the epoxy glass is not controlled by the network structure but rather by macroscopic defects such as microvoids. For epoxies with sufficient free volume that allows plastic network deformation the mechanical response is controlled by the network structure. 

In principle, these relationships open the way to a determination of fg which is found to decrease with crosslink density as well in ideal epoxy networks (Gerard et al., 1991), as in nonideal polyesters (Shibayama and Suzuki, 1965). However, it must be recognized that, in both series of data, it is impossible to have consistent values of, Cf, Cf, a, and fg except if BD varies with the structure, which can be considered as a serious argument against the free volume interpretation of WLF parameters. 

A linear decrease of KIc with an increase in crosslink density was reported for model PU based on triisocyanate and diols of various molar masses (Bos and Nusselder, 1994), and for epoxy networks (Lemay et al., 1984). It was suggested that the dilational stress field at the crack tip may induce an increase in free volume and a devitrification of the material. A linear relationship between GIc and M XJ2 was verified for these systems, although other empiric equations were found in other cases (Urbaczewski-Espuche et al., 1991). 

Apparently, there is less free volume in the aged polymer network. Any water coming into the aged network would tend to swell the polymer because there are simply less vacant sites in the distribution of free volume. In short, there is more polymer-solvent interactions as water diffuses into an aged epoxy network. 

Fig. 37. Swelling efficiency and free volume estimation in well-cured Fi-berite 934 network epoxies, as affected by sub-Tf annealing. Percent swelling is plotted on the ordinate axis while percent of moisture uptake is plotted on the abscissa...

Whereas Tq-ps is in the range 1710 10 ps inside the unaged epoxy films on Al and Au, it is only 1695 10 ps on the Cu substrate. As described below, these films on Cu contain fewer residual oxirane groups than on Al and Au. This could be the reason for the lower size of the free-volume voids in the epoxy network on Cu. 

For the bulk FP1-PC40, the aging peak first decreases for short TA and then increases again (Fig. 30.6). The decrease is explained in the following way. Tg and hence (Tg-TJ increase at a high rate due to intensified epoxy consumption in the early stages of TA with Ti=constant. Now, the amount of relaxed free volume decreases in a network with growing difference (Tg-Ta). Conse-... 

The properties of a cured epoxy network depend primarily on two factors nature of the resin and nature of the curing agent. The functionality of an epoxy resin plays an important part in determining the thermomechanical properties. The properties of epoxy resins of various functionalities cured with DETDA are presented in Table 3.4. Multifunctional epoxies exhibit a higher T compared with the difunctional epoxy when cured with the same hardener. This is due to the increase in crosslink density as a result of an increase in epoxy functionality and the formation of a tighter network. This significantly reduces the free volume of the network, leading to an increase in the T. ... 

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