Effects of Crosslinking

Between T j, and Tg, depending on the regularity of the polymer and on the experimental conditions, this domain may be anything from almost 100% crystalline to 100% amorphous. The amorphous fraction, whatever its abundance, behaves like a supercooled liquid in this region. The presence of a certain degree of crystallinity mimics the effect of crosslinking with respect to the mechanical behavior of a sample. 

Table 4.1. Examples for the empirical factor , that quantifies the effect of crosslinks on Tg...

A conclusion as to the effect of crosslinking on thermal expansion is not possible. Clearly, the polymers with many crosslinks and with short strands expand less than the uncrosslinked materials when heated. However, this effect cannot exclusively be attributed to the presence of crosslinks. It may just as well originate from the increased density of the crosslinked materials which was shown to be responsible for the increase in the moduli. 

The onset of plastic deformation in a material under load is called yielding [60]. In contrast to the experiments described in the previous sections, yielding causes a permanent deformation, i.e. a deformation that remains after the load is removed. The effects of crosslinks on the yield behaviour of polymers are demonstrated by three experiments ... 

Broutman and McGarry [98] examined the effects of crosslinking on toughness as early as 1965. Bell [99] observed a threefold increase in notched impact strength as the molecular mass between crosslinks was increased. Schmid et al. [100] and Lohse et al. [101] pointed out the dominating effect of molecular strand length on the ultimate properties and the toughness of crosslinked polymers. Later, Batzer et al. [46], Schmid [44], and Fischer et al. [45] compared the behavior of various networks composed of epoxy resins. 

Fig. 8.1. Effects of crosslinking on various properties (X) of the polymer. The deviation [X — XcoJ/Xa, from the thermoplastic property (X ) is plotted against Me1, the inverse molecular mass between crosslinks.

Table 8.1. Effect of crosslinks on small strain behavior...

GP Andrews. The effect of crosslinking on extractions using polyacrylamide gel. MS Thesis, University of Minnesota, 1985. 

The Effect of Crosslinker Concentration on the Rate of Polymerization. Ethylene glycol dimethacrylate is used most frequently as the crosslinker for HEMA formulations useful in contact lens manufacturing. To demonstrate the effect of crosslinker concentration on the curing rate, formulations derived from HEMA/Glycerine/BME at 85/15/0.17, while varying EGDMA (from 0.34 to 0.68), the peak times were about the same (3.73 and 3.61 minutes respectively). This is reasonable due to the similarity in molecular structure of the crosslinker and the monomer, and the low amount of crosslinker used. The possible presence of other crosslinker, such as the dimerization product of HEMA, is even less a factor to be considered in polymerization kinetics, due to low concentration (normally much less than 0.1 %, in-house information). 

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. 

Figure 2. Effect of crosslinking, chain termination, and plasticization on effective strain. Numbers on points refer to percent plasticizer, moles of chains X 10A/cc. or moles of chains X 10A /cc. dangling, respectively...

Table VI. Effect of Crosslinking on Brittleness of Carbon Black-Filled Polymer"...

Fig. 11. Effect of crosslinking in binding ability of DBQP resins for Cu(II) Ions43)...

The difference in the effects of crosslinking in liquid and vapour phase processes, and especially the different behaviour of standard and macro-... 

Decreasing the degree of crosslinking will increase the water uptake for a mass of dry gel, though compromises in the efficiency will result. The effect of crosslinks on the separation of vitamin B-12, a nonionic solute of molecular weight 1355, is shown in Fig. 4 [16]. As the crosslink density decreases, the polymer chain length between crosslinks increases, yielding a looser structure which vitamin B-12 can more easily penetrate. The behavior fits well with the prediction from Flory excluded volume theory [16] ... 

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