Cross-link density dependences

During the vulcanization, the volatile species formed are by-products of the peroxide. Typical cure cycles are 3—8 min at 115—170°C, depending on the choice of peroxide. With most fluorosihcones (as well as other fluoroelastomers), a postcure of 4—24 h at 150—200°C is recommended to maximize long-term aging properties. This post-cure completes reactions of the side groups and results in an increased tensile strength, a higher cross-link density, and much lower compression set. 

Long-term compression set resistance is described in Figure 4. Lower set values are achievable by use of higher viscosity gumstock at comparable cross-link densities. Compression set resistance is also very dependent on the cure system chosen. The bisphenol cure system offers the best compression set resistance available today, as shown in Table 5. 

The conventionally covalently cross-linked rubbers and plastics cannot dissolve without chemical change. They will, however, swell in solvents of similar solubility parameter, the degree of swelling decreasing with increase in cross-link density. The solution properties of the thermoelastomers which are two-phase materials are much more complex, depending on whether or not the rubber phase and the resin domains are dissolved by the solvent. 

Whereas polymers of sufficiently high molecular weight may be soluble in the common solvents with some difficulty, network polymers do not dissolve, even at elevated temperature. They usually swell depending on the nature and cross-link density. Marcus [10] described the swelling of polystyrene cross-linked by divinylbenzene. 

The effect of hydrophobicity of the polymer on the permeability of poly(2-hydroxyethyl methacrylate (HEMA)-co-methacrylic acid (MAAc) hydrogels was studied [12], The hydrophobicity was controlled by copolymerization with butyl methacrylate (BMA). The dependence of permeability on pH increased as the hydrophobicity increased even though the rate of diffusion decreased. Cross-link density of the hydrogel also contributed to pH-dependent permeability. 

A unified approach to the glass transition, viscoelastic response and yield behavior of crosslinking systems is presented by extending our statistical mechanical theory of physical aging. We have (1) explained the transition of a WLF dependence to an Arrhenius temperature dependence of the relaxation time in the vicinity of Tg, (2) derived the empirical Nielson equation for Tg, and (3) determined the Chasset and Thirion exponent (m) as a function of cross-link density instead of as a constant reported by others. In addition, the effect of crosslinks on yield stress is analyzed and compared with other kinetic effects — physical aging and strain rate. 

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. 

St and divinylbenzene (DVB) were polymerized in a dispersion of acryl-amide-methacrylic acid-methylenebisacrylamide terpolymer particles (25). Fine polystyrene particles were formed in/on each seed terpolymer particle. The former was smaller by about one-twentieth than the latter. The distribution of polystyrene particles depended on the cross-link density. Different amounts of St and DVB were charged in the seeded polymerization, and the resulting composite particles were used for protein adsorption measurement to assess the hydrophobicity of the particle surface. The adsorbed amount was almost proportional to the amount of St and DVB charged. In contrast, cells were less stimulated by the 5% St-containing particle than by the 0% St-containing one, that is, the seed particle. This phenomenon is attributed to selective protein adsorption on the 5% St-containing particle (26). 

Radiation cross-linking of polyethylene requires considerably less overall energy and less space, and is faster, more efficient, and environmentally more acceptable. Chemically cross-linked PE contains chemicals, which are by-products of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable. The disadvantage of electron beam cross-linking is a more or less nonuniform dose distribution. This can happen particularly in thicker objects due to intrinsic dose-depth profiles of electron beams. Another problem can be a nonuniformity of rotation of cylindrical objects as they traverse a scanned electron beam. However, the mechanical properties often depend on the mean cross-link density. ... 

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-... 

Finally, it must be pointed out that the close to first-order kinetic law observed in this study is by no means specific to polymerizations induced by intense laser irradiation a similar kinetic law was obtained by exposing these multiacrylic photoresists to conventional UV light sources that were operated at much lower light-intensities (27,34). This indicates that the unimolecular termination process does not depend so much on the rate and type of initiation used but rather on the monomer functionality and on the cross-link density which appear as the decisive factors. 

This depends on the cross link density. With a tighter cure, permanent sets are low. Some high saturation polymers such as butyl rubbers have characteristically high permanent set. 

Since the cross linking density of the gel is low enough, there are many contact points between polymer chains. The structure of the gel, therefore, is analogous to that of the semi-dilute polymer solution suggesting that the concentration dependence of the correlation length is explained by the following power law relationship [6]... 

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