Bulk thermomechanical analysis

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Two examples of the use of localised thermal analysis are provided in order to illustrate the generic applications of this approach. Figure 8 shows localised thermomechanical analysis of the surface of the multi-layer film in Figure 4. Measurements were made at points within this image describing the bulk polymer, the central gas-barrier layer and the thin tie-layer between this and the bulk film. The melting transition temperatures are consistent with high density polyethylene, poly(ethylene-co-vinyl alcohol) and medium density polyethylene for the bulk, gas barrier and tie layers, respectively [101],... 

In the description of the basics of thermomechanical analysis in Figs. 2.16 and 2.17, the mechanical properties were assumed to result from perfect elasticity, i.e., the stress is directly proportional to the strain and independent of the rate of strain. Hooke s law expresses this relationship via a constant, the modulus, which relates stress to strain, as shown in Fig. 6.20. Three moduli are commonly distinguished the shear modulus, G, where the shear strain, y, is expressed as the tangent of the deformation angle the tensile or Young s modulus, E and the bulk modulus, B. The latter two moduli are defined in Fig. 2.16. 

Three carbon fibre-reinforced polyimides were exposed to UV radiation at 177C, at three different intensities for three different times, so that the product of intensity and time was a constant. Intensities of 1,2 and 3 suns, where one sun is the power in space at one earth-sun distance, were used, for a time periods of 500, 250 and 167 h. The samples were characterised by X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, thermomechanical analysis and dynamic mechanical analysis. Measurement of bulk properties showed no difference between samples exposed to heat and UV radiation, and control samples. Surface analysis by XPS showed an apparent decrease in carbonyl concentration on the surface of some exposed samples. This was correlated to surface contamination by a silicone-containing material. 3 refs. 

PAS-8 were greater than those on SILASTIC 500-1, PAS-41, and 71. Although the surfaces of PASs and SILASTIC 500-1 were hydrophobic, the contact angle for PAS-8 was slightly lower than those for SILASTIC 500-1, PAS-41, and 71 (see Table 9). It was thought that the PAS-8 surface might have an influence on the aramid block. The phenomenon of the cell adhesion onto PAS-41 and 71 could not be explained by hydrophobicity alone. In former sections, the results of the gas-permeability measurement and the dynamic thermomechanical analysis of PAS implied the presence of a microphase-separated structure between PDMS and aramid phases in the PASs containing over 26 wt% of PDMS [14]. Moreover, a TEM study indicated that PAS films possessed microdomain structures in their bulk phases. That is, not only the hydrophobicity of the surfaces, but also the presence of a microphase-separated structure in the PASs may influence cell adhesion. 

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