Relationships between cure kinetics, network architecture, and fluid sensitivity in glassy epoxies
Relationships between chemical structure, cure kinetics, network morphology and free volume have been correlated with fluid ingress for glassy epoxy network blends. Polymers synthesized from diglycidyl ether of bisphenol-A (DGEBA) and diglycidyl ether of bisphenol-F (DGEBF) were blended with varying amounts of triglycidyl- m- aminophenol (TGAP), tetraglycidyl-4,4-diaminodiphenylmethane (TGMP), napthylamine (NA), adamantylamine (AA), and aminopropylisobutyl polyhedral oliogmericsilsesquioxane (AI-POSS) and cured with 3,3'- and 4,4'- diaminodiphenylsulfone (DDS) to control fractional free volume, average hole size and morphology. Varying curing profiles introduced morphological changes resulting in differences in network architectures. Epoxy with 10% NA had a smaller V h (71 Å 3 ) than with 10% AA (74 Å 3 ); the decrease was due to pi-pi stacking and growth kinetics of the 10% NA network. Architecture was a key determinant of moisture and solvent ingress in blends and off-stoichiometry epoxies. Hole size decreased with increasing crosslink density, from 75 Å 3 (DGEBA-33DDS) to 48 Å 3 (m-TGAP-33DDS). Fractional free volume increased with increasing crosslink density. Equilibrium water uptake increased with FFV, from 2.9% to 7.3% (DGEBA-33DDS and m-TGAP, respectively). Solvent uptake was almost completely inhibited in the epoxy blends when the V h of the epoxies decreased below the size of the solvent molecule. In networks formulated with excess epoxy, the importance of chain packing on solvent ingress was clarified. The excess-epoxy networks had lower crosslink densities than the on-stoichiometry benchmarks; however, they exhibited lower hole sizes. Equilibrium water uptake decreased from 2.9% to 2.0% and MEK uptake rate decreased from 3.3 x 10 -3 to 2.1 x 10 -3 weight percent h -1 between DGEBA-33DDS and DGEBA XS- 33DDS. The improved resistance to fluid was attributed to improved packing by the longer chain segments in the off-stoichiometry networks. Dispersion of pendant POSS was improved by pre-reacting amine-functionalized POSS with an excess of epoxy. In later experiments, using an improved POSS prereaction product, two separate morphologies were identified for unmodified and prereacted POSS at loading levels of 0-2.5 weight percent. Unmodified POSS exhibited crystallites in a neat epoxy matrix, whereas pre-reacted POSS exhibited a weakly crystalline POSS-rich phase and an epoxy-rich phase. Fluid ingress in the epoxies was not affected by POSS loading.