Thermal properties polymer matrices

The rich bibliography on polymer-clay nanocomposites shows that the effect of montmorillonite on thermal properties of matrix is complex and many factors contribute to the enhancement of the glass transition, such as the OMMT dispersion, interfacial strength, type of polymer matrix, preparation method, possible catalytic effects induced by organomodifier and/or montmorillonite itself, and so on [10]. Also, in the case of CNTs, many studies report the importance of thermal properties of nanocomposites containing CNTs, particularly with thermogravimet-ric analysis (TGA), since a significant enhancement in thermal stability of the... 

Polyimides (PI) were among the eadiest candidates in the field of thermally stable polymers. In addition to high temperature property retention, these materials also exhibit chemical resistance and relative ease of synthesis and use. This has led to numerous innovations in the chemistry of synthesis and cure mechanisms, stmcture variations, and ultimately products and appHcations. Polyimides (qv) are available as films, fibers, enamels or varnishes, adhesives, matrix resins for composites, and mol ding powders. They are used in numerous commercial and military aircraft as stmctural composites, eg, over a ton of polyimide film is presently used on the NASA shuttle orbiter. Work continues on these materials, including the more recent electronic apphcations. 

All VGCF was graphitized prior to composite consolidation. Composites were molded in steel molds lined with fiberglass reinforced, non-porous Teflon release sheets. The finished composite panels were trimmed of resin flash and weighed to determine the fiber fraction. Thermal conductivity and thermal expansion measurements of the various polymer matrix composites are given in Table 6. Table 7 gives results from mechanical property measurements. 

Table 6. Thermal properties of VGCF polymer matrix composites...

Almost any known polymer or polymer mixture can be used in the capacity of a polymer matrix various additives may be introduced in the matrix to reduce melt viscosity, increase thermal stability of the composition or its plasticity, etc. A choice of a matrix is determined mainly by the operating conditions of a material and the desired physical-mechanical properties of a composite. One may state rather confidently that, other things being equal, the value of the CPCM conductivity does not depend on a choice of a polymer matrix [3]. 

Chemicals like polyorthoaminophenol, diphenylamine in small amounts have been found to decrease the yield of cross-linking [388]. The tensile strength of the carbon black-filled polychloroprene compounds has been found to be comparable to the conventional thermally cured one. The physical properties [389] have been observed to improve on adding cross-linking promoters like A,A -hexamethylene-bis-methacrylamide into the polymer matrix. 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

Estimates of oCbiend using a rule-of-mixtures relationship are 3.0 X 102 and 7.2 X 103 cm lor 0.2 and 5.0% polyimide, respectively. This dependence of the optimum absorption coefficient (in terms of ablation rate), OVx on fluence is consistent with the observations of Chuang et al.6% for ablation of several UV-transparent (at 308 nm) polymers sensitized with low-molecular-weight dopants, e.g., PMMA doped with pyrene. For the pyrene-PMMA system, Chuang et al.6S reported maximum etch rates for 1.2 J/cm2 at a = 7 X 102 cm 1. It should not be expected that different dopant-matrix systems would yield the same optimum absorption coefficient for a given fluence level since the thermal properties for different polymers may vary significantly. 

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

The purpose of the second dwell is to allow crosslinking of the matrix to take place. It is during the second dwell when the strength and related mechanical properties of the composite are developed. To characterize the exothermic crosslinking reaction of a thermosetting polymer matrix, a thermal cure monitor technique such as Differential Scanning Calorimetry... 

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