Preparation And Characterization Of Poly (Aryl Ether)s Microcellular Materials

To overcome the deficiencies of the conventional foams, a processingapproach for the preparation of 'microcellular foams' was developed by Suhand his co-workers in the early 1980's. With much unique properties,microcellular foams have encouraged a number of innovative applications inthe fields such as packaging materials, insulation, filtration membrane, sportsequipment, automobile and aircraft parts. The main goals of the presentinvestigations are the preparation, characterization, and evaluation ofpolymer-based foams of sufficient toughness and high temperature stability tobe useful as structural materials in Air Force applications.Several techniques are developed since then to prepare microcellular foamsusing gases in their supercritical and nonsupercritical state as physicalblowing agents. In some cases, the influence of processing condition on themicrocellular morphologies are interpreted in terms of classical nucleationtheories.In this work, the poly(aryl ether)s were selected as the polymer matrixbecause they were a class of high performance engineering thermoplasticsknown for their excellent combination of chemical, physical and mechanicalproperties.At first, the interaction of the supercritical carbon dioxide (sc-CO2) with theamorphous poly (ether ether ketone)s was systematically investigated bydesorpted measurement. The PEEK films were treated with CO2 at differentpressure, temperature and soaking time. The gravimetric desorption data werekinetically and thermodynamically evaluated assuming Fickian diffusion. Thebehavior of desorpted diffusion deviated from Fickian diffusion, when thepressure increased and ranged between 3 MPa and 28 MPa. Swelling anddiffusion of CO2 in relation to system temperature and pressure werediscussed. Double novel extrapolation methods were introduced here to obtainthe C0 and they could be considered that they expressed the kinetics ofdiffusion. Comparison to the gas contents in these film samples containingdifferent side group, the effects of side groups on the gas diffusion andsolubility were discussed. The data of gas permeability could be cited tovalidate this result. All these experiments could be used to direct the nextexperiments.The second, a high-performance microcellular closed-cell foams wereprepared by a two-stage batch foaming process from fluorinated poly(etherether ketone) and characterized by scanning electronic microscopy (SEM),tensile and dynamic mechanical analysis (DMA), respectively. The effects ofsaturation pressure and temperature on the cell size, cell density and bulkdensity of porous materials had been discussed. The resulting materials hadaverage cell diameters in the range 3 – 17 μm, and cell densities (Nf) in theorder of 0.6×109 – 1.39×1010 cells/cm3. The porosity (Vf) was in the range 0.2– 0.85. The result could be predicted in terms of classical nucleation theories.The existing models based on classical nucleation theory are not able toexplain satisfactorily the nucleation phenomenon of microcellular foams onthermoplastics. Here we extend the analysis of heterogeneous nucleation inour system. The model depends on various process parameters such assaturation pressure, foaming temperature and polymers with different pendant.In contrast, experimental values of Young's moduli were in goodagreement with theoretically predicated values, but the relative strengths weresomewhat lower than predicated. The relaxation mechanism of microcellularwas systemically investigated by DMA. The dynamic mechanicalspectrometry showed that the storage modulus curve at high temperatureregion appeared a peak and the loss modulus was lower as compared to theirsolid counterparts.The third, based on the study of foaming amorphous poly(aryl ether)s, someporous material with different polymer structures were studied and prepared.The porous morphology can be made with cellular voids between 30 and500nm and all porous materials have dielectric constants in the ultralow-krange. The relation between porous morphology and polymer structures werediscussed detailedly. The influences of gas content on the dielectric constantswere predicted by four equations.The fourth, for the semi-crystalline systems, the effects of processingcondition on porous morphology were more complicated. Compressed CO2dissolves to a considerable extent in polymers, causing a depression in Tg byseveral tens of degrees. This also is accompanied by a decrease in thecrystallization temperature (Tc). This can lead to considerable swelling in thecase of amorphous polymers and to induction of crystallinty or growth of thealready nucleated crystalline phase in the case of crystallizable orsemi-crystalline polymers. In this work, supercritical CO2 provided amoderate condition to make the amorphous CO2/PEEK mixtures at 30 MPaand 40 °C. The crystal is obtained directly after treating CO2/PEEK mixturefrom 70 °C to 240 °C. The crystallization behavior of CO2/PEEK mixturesbefore and after treatment is investigated in detail by using DSC, DMA andWAXD. DSC curves of CO2/PEEK samples showed the double coldcrystallization peaks. The lower cold crystallization peak moves to highertemperature with the content of CO2 decreasing and the higher coldcrystallization peak keeps their temperature at about 172 °C without aremarkable change. The dynamic mechanical spectrometry was alsointroduced to explain the relaxation behavior of glass transition and thecrystalline process.At last, the microcellular processing of poly(ether ether ketone) in theamorphous and semicrystalline states was studied in order to quantify theprocessing differences. Particular emphasis was given to the cell nucleationand growth stage of microcellular processing comparing the processingcharacteristics of semicrystalline and amorphous PEEK. Based on the resultsof this study, a number of critical process parameters were identified. Abimodal cell size distribution was observed due to combined effects of heattransfer and molecular orientation.