Curing, rheological and morphological investigations of polyphenylene oxide blends with allylics and epoxy monomers
2017-02-14T00:32:28Z (GMT) by
There is considerable interest in blending thermoplastic with crosslinkable monomers. The original reason for studying these blends was to enhance the toughness of the majority phase thermoplastic. However one of the relatively new ways of improving the processibility of high performance thermoplastic such as polyphenylene oxide involves the use of a crosslinkable monomer as a reactive plasticizer where the plasticizer can reduce the viscosity during the early stages of processing but polymerize and phase separate into a dispersed phase during the final stages of processing. To be effectively used as reactive plasticizer, the polymer should not form a network at an early stage of cure and should react slowly enough so as to allow adequate time for mixing at the processing temperature of the blend. Since allylic and epoxy monomers possess low polymerization rates and high gel conversions at high temperatures, this study aims at investigating their effect on the cure, rheology, processing, thermal behaviour and morphology of their blends with PPO. An initial indication of PPO solubility and miscibility with a series of allylic and DGEBA epoxy monomers was obtained by theoretical calculation of the solubility parameters of the components in the blends. Experimental studies of the uncured blends by hot-stage microscopy and DMTA suggested that blends of PPO with either allylic and epoxy monomers can undergo phase separation, depending on the temperature, when the concentration of the monomers is more than 40wt%. Rheological studies on the miscible blends of PPO with the allylic and epoxy monomers showed a mark reduction of viscosity indicating their ability to dramatically improve the processibility of PPO. Curing studies of the allylic monomers with various organic peroxides revealed that the monomers exhibited similar cure kinetics when the same type of initiator was used while the heat of reaction depended on the initiator type. This data suggested that dicumyl peroxide, azo-tert butane and tert-butyl hydroperoxide are suitable initiators for the allylic monomers. Meanwhile, polymerization studies of pure DGEBA monomers and of DGEBA oligomer with various curing agents indicated that MCDEA has the lowest cure reaction thus the most suitable for high temperature processing and that the use of pure DGEBA only slightly reduces the reaction rate compared with the epoxy oligomer. The curing behaviour of epoxy-MCDEA in the presence of PPO indicated that the polymerization rate decreased in a monotonic pattern when the PPO concentration in the blend was raised due to a dilution effect on the reactants. A dilution effect was also observed in the cure of PPO/DAOP with DCP. However for blends PPO/DAOP with TBHP, a significant increase of polymerization rate was observed in the presence of PPO due to accelerated decomposition of TBHP to radicals by copper and cobalt impurities in the PPO. Further increases of PPO slightly reduced the rate of cure due to dilution effect but the rate was still faster than the system without PPO. Rheological properties of DGEBA-MCDEA and DAOP/DCP in the presence of PPO indicated a delayed gelation and vitrification with increasing PPO content. The variation of rheological properties during cure is affected by the PPO content and thus the morphology of the blends. The changes in the moduli/complex viscosity of PPO:DAOP/DCP is slightly different from that observed for PPO:DGEBA340/MCDEA. No significant inflection of the properties due to phase separation was observed in the former system and this might be due to different natures of polymerization, viz. chain growth polymerization versus step growth polymerization, respectively. The dynamic mechanical properties of the cured blends showed a slight reduction of Tg of PPO in the presence of reactive monomers but the Tgs were all above 170°C for DAOP/DCP and 230°C for DGEBA340/MCDEA respectively. SEM observations of the cured blends of indicated phase separated structure and that a bi-continuous morphology occurred at 40 wt% PPO for DAOP/DCP and 30 wt% PPO for DGEBA340/MCDEA, respectively. The significant reduction of viscosity in the presence of allylics or epoxy plasticizers would facilitate the dispersion and exfoliation of nanofillers in the blends matrix and would allow blending with techniques such as rotational moulding and powder coating.