INFLUENCE OF CROSS-LINKING ON SURFACE HARDNESS OF POLY(METHYL METHACRYLATE)

The influence of crosslinking on the surface hardness of poly(methyl m ethacrylate) sheets examined by means of damping of standard pendulum oscillations as well as on the nonisothermal mass loss is discussed in this paper. Crosslinked poly(methyl methacrylate)s with different cro sslink densities were prepared by copolymerization of methyl methacryl ate with polyfunctional comonomers of the allyl and vinyl type and by additional crosslinking of poly(methyl methacrylate) by transamination with aliphatic alpha,omega-diamines. The highest increase in surface hardness, up to 52% of its value for a silicate plate glass standard, was observed for (PMMA-co-DAIP) sheets with 17 to 25 wt% of comonomer. For commercial noncrosslinked PMMA cast sheets, this value is only 27 % of the above standard. Comonomers of the allylic type are more suita ble than the methacrylic multifunctional crosslinkers. A large portion of the double bonds remains unreacted if a high concentration of mult ifunctional agent is copolymerized with MMA. A similar effect on PMMA surface hardness was achieved using substantially lower concentrations of alpha,omega-diamines as transamination crosslinking agents. Diamin e crosslinked PMMA is also a thermally more stable material compared t o copolymer networks and noncrosslinked PMMA. Volatilization of the ma in portion (> 80 wt%) of PMMA crosslinked with 5 wt% of 1,3-DAP is shi fted to a higher temperature compared to the standard PMMA by 200 K. T he shape of the TG curves and thermal stability of MMA-co-multimethacr ylates copolymers differ only slightly from the noncrosslinked PMMA.