Dependence on cross-link

Some properties are not significantly dependent on cross-linking and remain nearly iavariant as cure progresses. These iaclude thermal conductivity, electrical properties, and low temperature britdeness. 

The cured resins are stable at 150-200°C depending on cross-linking agents used. They are resistant to chemical attack and are flexible and strongly adhesive. They are used as surface coatings and yield an excellent enamel after esterification which is used for floors, walls, tanks, domestic equipment, etc. 

Differing from the previous studies (5-7) where the parameters Goo, m, loay have been treated as constants, we find that they depend on cross-link density which is consistent with the measurements of Dickie and Ferry (4). Figure 5 shows the dependence of the viscoelastic relaxation on cross-link density. The solid curves are calculated from Equations 17, 19 and 20 by using a value of xq = 2.5 x 102 hrs at T = 25°C. Figure 5 resembles the corresponding figure in ref. 5. 

Regardless of the method of cross-linking, mechanical properties of a cross-linked elastomer depend on cross-link density. Modulus and hardness increase monotonically with cross-link density, and at the same time, the network becomes more elastic. Fracture properties, i.e., tensile and tear strength, pass through a maximum as the cross-link density increases (see Figure 5.4). 

Regardless of the method of cross-linking, mechanical properties of a cross-linked elastomer depend on cross-link density. Modulus and hardness increase mono-... 

From a fit of Equation (10) to spatially resolved relaxation curves, images of the parameters A, B, T2, q M2 have been obtained [3- - 32]. Here A/(A + B) can be interpreted as the concentration of cross-links and B/(A + B) as the concentration of dangling chains. In addition to A/(A + B) also q M2 is related to the cross-link density in this model. In practice also T2 has been found to depend on cross-link density and subsequently strain, an effect which has been exploited in calibration of the image in Figure 7.6. Interestingly, carbon-black as an active filler has little effect on the relaxation times, but silicate filler has. Consequently the chemical cross-link density of carbon-black filled elastomers can be determined by NMR. The apparent insensitivity of NMR to the interaction of the network chains with carbon black filler particles is explained with paramagnetic impurities of carbon black, which lead to rapid relaxation of the NMR signal in the vicinity of the filler particles. 

Fig. 18 Gel structures resulting from irradiation of temperature-sensitive PVME, showing the dependence on cross-linking parameters (polymer concentration and temperature during irradiation). (Reprinted from [40], copyright 2009, with permission of Elsevier)...

Despite the size of the protein subunits, their integrity does not depend on cross-linking via disulfide bonds (63) and no disulfide bridges have been identified within the partially completed amino acid sequence (41, 6Sa). Nor is there any evidence that association of subunits depends on covalent bonding rather, it appears to involve mainly hydrophobic interactions (50). Of particular interest in this context is the observation that some forms of acatalasemia are attributable to the formation of a catalase variant, of approximately normal specific activity, but with a tendency to dissociate into subunits (64). 

Hahn echo and solid echo produce different contrast in imaging, and the corresponding transverse relaxation times Tj and T2l- depend on cross-link density in a different fashion. 

Unlike the situation when the total monomer content is varied, no dependence on cross-linking agent content is observed above 5 X 10 mol... 

Thermal Expansion. A more pronounced dependence on cross-linking was found for the thermal expansion. As is shown in Fig. 5, the thermal expansion coefficient at 15 K differs by a factor of three for the same resins and segment lengths. 

It should be noted that, for all the resins considered, the specific heat does not depend on cross-linking or the chemical structure below 100 K. This can be explained by the Debye theory, which states that the specific heat is a function only of the oscillator density, N, and 0/T. 6 is the Debye temperature, which can be determined by elastic parameters, such as Young s modulus, E, N is approximately equal for all resins, since they have nearly equal densities. At low temperatures, roughly the same value of E is asymptotically reached by the epoxy resins. 

Property Dependence on cross-link distance, S Dependence on temperature... 

A typical entropy-elastic material is cross-linked natural rubber, ds-poly(l-methyl-1-butenylene) or cts-l,4-polyisoprene, as summarized in Fig. 5.166 (see also Fig. 1.15). Its extensibility is 500 to 1,000%, in contrast to the 1% of typical energy-elastic sohds. Natural rubber has a molar mass of perhaps 350,000 Da (about 5,000 isoprene monomers or 20,000 carbon backbone bonds) and is then vulcanized to have about 1% cross-links (see Fig. 3.50). A rubber with a Young s modulus of 10 Pa (depending on cross-link density) must be compared to its bulk modulus (= 1/p,... 

Within the limit of experimental errors scaled dynamic order parameters measured from the MQ experiments show a linear dependence on cross-link density (cf Fig. 11). 

In this case, one expects (cf equation 32) that (M2) has a polynomial dependence on cross-link density or shear modulus. 

The glass transition temperature is also dependent on cross-link density. According to Di Marzio (1964) ... 

Theories of rubber elasticity [119], such as the affine network theory [120] or the phantom network theory [121], provide expressions for the network pressure, depending on cross-link functionality and network topology. For a perfect tetrafunctiOTial network without trapped entanglements, the elastic network pressure is given by [120] ... 

Extrapolated to zero cross-linking from measurements on lightly cross-linked samples for which the dependence on cross-linking was very slight. 

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