Uncross-linked

The Hst price of 35 kg/m polyethylene foam boards is about 0.58/BE ( 6.90/kg) and that of cross-linked polyethylene foam is about 0.33 to 0.88/kg more expensive than uncross-linked polyethylene foam depending on the density and thickness of the foam. 

This material finds a limited application in films and coatings which require good abrasion and flexing resistance. Some typical properties of cross-linked and uncross-linked polymers are given in Table 18.9. 

Property Uncross-linked Cross-linked (conditions as in text)... 

FIGURE 1.8 In-phase (jx ) and out-of-phase (pf ) components of the complex shear modulus of uncross-linked namral ruhher versus log (frequency) at various temperatures. (From Payne, A.R. and Scott, J.R., Engineering Design with Rubber, Interscience Pubhshers, New York, 1960.)... 

All the data provided in the figure are on uncross-linked mbbers. This clearly shows the tremendous impact of the finer distribution of the silica particles on the performance of the composites from the tensUe strength point of view. 

FIGURE 3.17 Tensile stress-strain plots for uncross-linked acrylic rubber (ACM)-siUca and epoxidized natural rubber (ENR)-silica hybrid composites synthesized from various pH. Letter p in the legends indicates pH and the numbers following the letter are indicative of the pH ranges e.g., 1 means pH range of 1.0-2.0, similarly 3 means 3.0-4.0. (From Bandyopadhyay, A., De Sarkar, M., and Bhowmick, A.K., J. Mater. Sci., 41, 5981, 2006. Courtesy of Springer.)... 

Amino acid analysis, by reverse-phase HPLC, of acid-hydrolyzed uncross-linked recombinant resilin and cross-linked recombinant resilin clearly shows the presence of dityrosine in the cross-linked sample (Figure 9.3c). Further evidence of the presence of dityrosine was obtained by UV irradiation (Xmax,ex 315 nm Xmax,em 409 nm). Dityrosine endows natural resilin with pH-dependent blue fluorescence [38] on UV irradiation. The cross-linked recombinant resilin material was similarly fluorescent, strongly suggesting dityrosine cross-links. 

That is, in the model Figure 18.19 (i.e., in the new interface model), there are two points of the opposite contribution. The uncross-linked SH layer promotes the sliding of molecules passing... 

As known, the free energy of an uncross-linked polymer melt can be calculated by the usual Gibbs formula,... 

FIGURE 22.2 Flocculation behavior of the smaU-strain modulus at 160°C of uncross-linked solution-based styrene-butadiene rubber (S-SBR) composites of various molar mass with 50 phr N234, as indicated (left) and strain dependence of the annealed samples after 60 min (right). (From Kliippel, M. and Heinrich, G., Kautschuk, Gummi, Kunststoffe, 58, 217, 2005. With permission.)... 

Simple physical entanglements can be sufficient to produce a structurally stable gel if the polymer has a sufficiently great molecular weight and if the polymer is of only modest hydrophilicity. In this case, the polymer will swell in water without dissolving, even in the absence of covalent cross-links. Poly(2-hydroxyethyl methacrylate) (PHEMA) is a prominent example of this type of hydrogel when uncross-linked, it will dissolve in 1,2-propanediol but only swell in water. 

The mesh size was determined from an equivalent network, prepared from a mixture of protonated and deuterated precursors. The radius of gyration of the mesh strands was found to be 25 A, which is identical to the radius of gyration of the uncross-linked precursors. 

Table III. Half-time of crystallization (t]/2) at —20°C of uncross-linked cis-1,4 BR, cis contents were determined by IR spectroscopy.

The distribution of the thi monomer in molecular chains or in the whole polymer should affect the perfection of the vulcanizate network, free chain ends or the uncross-linked parts in the polymer making no contribution to the tensile strength but acting as a plasticizer of like structure as the polymer. 

Since the excellent work of Moore and Watson (6, who cross-linked natural rubber with t-butylperoxide, most workers have assumed that physical cross-links contribute to the equilibrium elastic properties of cross-linked elastomers. This idea seems to be fully confirmed in work by Graessley and co-workers who used the Langley method on radiation cross-linked polybutadiene (.7) and ethylene-propylene copolymer (8) to study trapped entanglements. Two-network results on 1,2-polybutadiene (9.10) also indicate that the equilibrium elastic contribution from chain entangling at high degrees of cross-linking is quantitatively equal to the pseudoequilibrium rubber plateau modulus (1 1.) of the uncross-linked polymer. 

A8. The Helmholtz elastic free energy relation of the composite network contains a separate term for each of the two networks as in eq. 5. However, the precise mathematical form of the strain dependence is not critical at small deformations. Although all the assumptions seem to be reasonably fulfilled, a simpler method, which would require fewer assumptions, would obviously be desirable. A simpler method can be used if we just want to compare the equilibrium contribution from chain engangling in the cross-linked polymer to the stress-relaxation modulus of the uncross-linked polymer. The new method is described in Part 3. 

A new stress-relaxation two-network method is used for a more direct measurement of the equilibrium elastic contribution of chain entangling in highly cross-linked 1,2-polybutadiene. The new method shows clearly, without the need of any theory, that the equilibrium contribution is equal to the non-equilibrium stress-relaxation modulus of the uncross-linked polymer immediately prior to cross-linking. The new method also directly confirms six of the eight assumptions required for the original two-network method. 

Figure 9. Inverse square root of the difference of the Rayleigh factors of cross-linked [R(l)] and uncross-linked [R(0)] samples as a function of square of the...

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