Water transport into epoxy resins and composites
Abstract
The processing-property relationships were established for the epoxy system of tetraglycidyl 4,4$\sp\prime$-diaminodiphenyl methane (TGDDM) cured with diaminodiphenyl sulfone (DDS). The TGDDM-DDS epoxy system was selected for analysis as the ensuing polymer matrix is most common in high-performance fiber-reinforced epoxy composites. The cured TGDDM-DDS epoxy resins with varying DDS concentrations were characterized using infrared spectroscopy (IR), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA). It was shown that the hydroxyl-epoxide or etherification reaction occurred in the TGDDM-DDS epoxy system. Intramolecular hydrogen bonds exist in this epoxy network and arise primarily from hydroxyl groups auto-association. In addition, vitrification during cure was observed which prevented epoxy resins from relaxing to their quasi-equilibrium glassy state. Experiments on water transport in epoxy resins with varying compositions were performed and a relaxation-coupled transport behavior was observed in these epoxy resins. By post-curing vitrified epoxy resins, the additional free volume usually measured in them was removed and maximum water uptake was reduced. Since epoxy resins were in a quasi-equilibrium glassy state after the post-cure, Fick's law with a constant diffusion coefficient could adequately describe the water sorption behavior. A network formation model based on the branching theory was developed, taking into account the difference in reactivities of primary and secondary amines and the etherification reaction. Using this network formation model, water uptake in post-cured epoxy resins was found to be proportional to tertiary amine concentration. It was also found that although the etherification reaction raises the concentration of elastically active network chains, it consumes the hydrogen-bond-forming hydroxyl groups and generates ether linkages. Reducing the hydroxyl concentration lowers the crosslinking density since the hydrogen bonds can be considered as chemical crosslinks at low extension. The ether linkages are also undesirable since they increase the flexibility of macromolecular chains and diminish the glass transition temperature. Therefore, a stage curing process is required to suppress the possibility of etherification reaction in epoxy resins.
Degree
Ph.D.
Advisors
Peppas, Purdue University.
Subject Area
Chemical engineering
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