Fluid uptake behavior of multifunctional epoxy blends

Profound changes in network architecture from blending trifunctional (m-triglycidylaminophenol, mTGAP) or tetrafunctional (tetraglycidyldiaminodiphenylmethane, TGDDM) epoxides with diglycidyl ether of bisphenol-A (DGEBA) and a curative amine (3,3′-diaminodiphenylsulfone, 3,3′-DDS) were observed using PVT, DMA, and PALS analyses. Increasing multifunctional content, which increased the crosslink density (with the expected increase in Tg), produced a decrease in the average free volume hole size (Vh) accompanied by a counterintuitive increase in fractional free volume (FFV). This unusual inverse relationship between FFV and Vh allowed clear resolution of their respective roles in equilibrium moisture uptake vs. the rate of uptake (diffusivity). Equilibrium water uptake increased with increasing multifunctional content, concomitant with the increase in FFV. Water diffusivity, on the other hand, decreased with increasing multifunctional content, concomitant with the decrease in Vh. The decreasing Vh in the epoxy blends also had interesting consequences for organic solvent sensitivity. MEK ingress was substantial in the binary DGEBA/DDS epoxy and completely inhibited for most of the blends, implying hole size selectivity was responsible for the MEK uptake inhibition. MEK uptake was precluded in epoxies whose Vh was below a critical threshold value of ∼68 Å3. A small amount of mTGAP or TGDDM was sufficient to reduce the Vh of DGEBA/DDS epoxy below the threshold and prevent MEK uptake.