Curing Kinetics Of PMI Copolymer And Preparation And Properties Of Modified Foam

Polymethacrylimide?PMI?foam is a thermosetting material with excellent properties,which is suitable for the high load and high strength fields.The curing process of materials is closely related to their properties.It is necessary to understand the curing process of PMI and develop PMI foam with higher performance.In this paper,PMI copolymers were prepared with acrylonitrile?AN?/ methylacrylonitrile?MAN?and methacrylic acid?MAA?as monomers,azodiisobutyronitrile?AIBN?as initiator and acrylamide?AM?as cross-linking agent by two-step method.The isothermal curing kinetics of PMI/nano-SiO₂ composites were investigated by DMA.The isothermal imide kinetics of AN/MAA/AMS copolymer was studied by in-situ FTIR.Besides,MAA/MAN/St copolymer and the foaming materials were prepared with styrene?St?as the third comonomer.AN/MAA/SiO₂ copolymer was prepared with AN,MAA,nano-SiO₂ ,etc.The relative conversion was defined by the storage modulus.Avrami model-fitting method,Friedman method and integral method were applied to analyze the curing kinetics.The storage modulus and loss modulus increased appreciably with spanning three orders of magnitude throughout the curing.The frequency correlation of relative conversion was noticeable at180?.Avrami model gave good fits for the experimental data.The activation energy calculated by Avrami equation changed from 65.46 k J/mol to 25.28 k J/mol at 180 ? 190 oC.While at 190 ? 200?,the activation energy changed from 107.14 k J/mol to 63.82 k J/mol.The activation energy increased from 104.3 k J/mol to 130.6 k J/mol by Friedman method when the relative conversion ranged between 0.4 ? 0.8.Similarly,the activation energy calculated by integral method increased from 71.5 k J/mol to 103.4 k J/mol.When the relative conversion exceeded 0.8,the activation energy decreased gradually.The mobility of the reactive groups was hindered and the crosslinking density of composite was much higher.The curing kinetics became diffusion controlled.The activation energy of PMI/SiO₂ composite was greater than PMI,which could be attributed to the hindrance effect caused by nano-SiO₂ .AN/MAA/AMS copolymers were prepared with AN,MAA,methyl styrene?AMS?,etc.The isothermal imidation of AN/MAA/AMS was investigated by in-situ FTIR between 180? 200 oC.The bending vibration absorption peak of hydrogen on benzene ring in AMS at700 cm-1 was selected as the internal standard.The extent of imidation was defined by thearea ratio of characteristic absorption peak of the nitrile groups at 2243 cm-1to the internal standard.The imidation process was analyzed by Friedman method and Avrami equation.The activation energy at imidation extent between 0 ? 0.2 was 60.4 ? 65.1 k J/mol,which was ascribed to the reaction of forming imide ring structures.The increase of activation energy from 65.1 k J/mol to 92.3 k J/mol at imidation extent between 0.2 ? 0.4 may be ascribed to the reaction of forming polyimine cyclic structures.When the imidation extent exceeded 0.4,the activation energy decreased from 92.3 k J/mol to 52.1 k J/mol and frequency factor?ln A?fell from 20.5 min-1 to 12.6 min-1.At this stage,the reaction was controlled by diffusion.Moreover,the Avrami curves were in good agreement with the experimental data of imidation,except for the late stage.The decrease of kinetic constant from 2.14×104 s-1 to 0.92 s-1 and activation energy from 54.05 k J/mol to 20.39 k J/mol further indicated that the imidation mechanism of AN/MAA/AMS copolymer changed from kinetically controlled at prophase to diffusion controlled at anaphase.The MAN/MAA/St copolymer was prepared with MAN,MAA and different content St as the third comonomer,etc.The MAN/MAA/St copolymer was prepared by free foaming at 180 oC for 8 hours.The results show that the density and mechanical properties of foam are positively correlated with St content.The density of MAN/MAA copolymer foam without St was 45.516 kg/m³,the bending strength was 0.461 MPa,and the compression strength was 1.305 MPa.The density of MAN/MAA/St with 7 wt% St terpolymer foam reached 68.159 kg/m³,the flexural strength was 1.01 MPa,and the compressive strength was 1.69 MPa,which respectively increased by 119.08% and 29.65%.The increase in density is accompanied by a decrease in foam pore size and a thickening of the ribs.The pore size of the MAN / MAA copolymer foam is 300 um,while the pore size of the 7% St MAN/MAA/St foam is within 100 um.In the infrared spectrum,the reduction of carboxyl group and nitrile group and the formation of amide bond and acid anhydride bond were observed.TG comparison showed that the addition of St improves the heat resistance of PMI to a certain extent.