Effect of cross-linking time

3 Effect of cross-linking time (Aging time) 

---

Figure 2. Effect of cross-linking time on ultimate tensile strength (UTS). Data for 0.2% glutaraldehyde solution.

Table 11.4 The effect of cross-linking time on the percentage of loading content, loading efficienq of BSA for grafted, and mixed beads...

The third molecular variable to be considered is the effect of cross-linking. Less data are available on the effects of this variable, especially for fatigue properties, and there are significant differences in results obtained by different investigators or even for the same investigators in results acquired at different times. Test results obtained on several different polymer systems will be presented and discussed and possible reasons for the reported differences in results will be reviewed. 

To study the effect of cross-linking reaction on relaxation prcx ess, the transition rates are modified. We note that t of many glass-forming liquids has an identical form as that of the viscosity r. The r of polymer system increases exponentially with time t near the gel point, i.e., r c exp[cXpt] with cCp being the cross-linking reaction rate. Therefore we assume that x exp[(Xp t], as a result, can be modified as... 

The general effect of cross-link density on the elastic modulus of an elastomer is indicated by Eq. 2.3. In their paper Landel and Fedors [189] consider the influence of a time-dependent cross-link density on the shape of the stress-strain curves of silicon, butyl, natural, and fluorinated rubbers. Introducing an additional shift-factor a related to the cross-link density, they were able to represent reduced breaking stresses as a function of reduced time in one common master curve. 

A potentially very useful apphcation of UTDR is to design membranes that offer improved resistance to compaction. Kelley et al. (2002) smdied the effect of cross-linking on the compaction resistance of cellulose-acetate membranes. Figure 33.6 shows the compressive strain as a function of time for pure water permeation through a cellulose-acetate membrane at 4.1 MHz that has been exposed for different periods of time at 23°C to a titanium-isopropoxide cross-hnking agent. Sufficient cross-linking time can reduce the compressive strain by 65% and nearly totally eliminate the elastic compaction. 

Figure 4 (a)-(c). Effect of cross-linking reaction time on the swelling ratio of genlpin-fixed CM, and CMsps/CDsps (with 0.7 wt% genlpin) at (a) pH 1.2 (b) pH 7.0 and (c) pH 12.0. Data represent the mean standard deviation, n=i. 

In summary, none of the individual components necessary for recombinant resilin curing were cytotoxic, and there were no leachables from the cured resilin that caused cell death. The cured resilin polymer was not a good surface for cell adhesion, but cells can survive and proliferate in the resilin on a gelatine bead. The curing of recombinant resilin in the presence of cells on beads has no effect on the cells ability to migrate and proliferate with new tissue formation. The resilin is seen to degrade with time, but it is believed that this could be controlled by the type and extent of cross-linking. 

The method of mathematical simulation has many advantages, and is very close to the physical experiment. However the further development of this approach /a consideration of volume effects, reversible reactions and so onj can be rather difficult because it will reau.ire too much computer time, therefore it is expedient to search some simple analytical or semianalytical approximate approaches to the calculation of cross-linking kinetics and conformational properties of cross-linked macromolecules. The results obtained bv the Fonte Carlo calculation can serve as criteria of the accuracy of such approximation. 

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. 

---