Elastomers cross-linked

At still longer times a more or less pronounced plateau is encountered. The value of the plateau modulus is on the order of 10 N m", comparable to the effect predicted for cross-linked elastomers in Sec. 3.4. This region is called the rubbery plateau and the sample appears elastic when observed in this time frame. 

In the rubbery plateau, a new impediment to movement must be overcome entanglements along the polymer chain. In discussing the effects of entanglements in Chap. 2, we compared them to crosslinks. Is it any surprise, then, that rubbery behavior similar to that shown by cross-linked elastomers characterizes this region ... 

Following this work, the y -12F-diol was used for the direct reaction with hexamethylene-1,6-diisocyanate in the presence of dibutyltin dilaurate to produce a cross-linked elastomer or a reactive prepolymer which was terminated with either isocyanate or hydroxyl groups, depending on which reactant was in excess (142,143). 

With a typical of 25 000-30000 the molecular size is low compared wjth most conventional covalently cross-linked elastomers. With such rubbers values of about 100000 are desirable so that the effects of a significant amount of non-load-bearing chain ends do not occur. Such a problem does not arise in block copolymers terminated by hard segments. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

FIGURE 1.12 Master curve of tear energy Gc versus rate R of tear propagation at Tg for three cross-linked elastomers polybutadiene (BR, Tg — —96°C) ethylene-propylene copolymer (EPR, Tg — —60°C) a high-styrene-styrene-butadiene rubber copolymer (HS-SBR, Tg — —30°C). (From Gent, A.N. and Lai, S.-M., J. Polymer Sci., Part B Polymer Phys., 32, 1543, 1994. With permission.)... 

FIGURE 5.18 Comparison of different thermoplastic elastomer (TPE) classes with corresponding cross-linked elastomers. (From Rader, C.P., Kunstst. Ger. Plast., 83, 111, 1993.)... 

Urry DW, Trapane TL, McMichens RB et al (1986) N-15 nmr relaxation study of inverse temperature transitions in elastin pol)q)entapeptide and its cross-linked elastomer. Biopolymers 25 5209-5228... 

G Rossi, KA Mazich. Kinetics of swelling for a cross-linked elastomer or gel in the presence of a good solvent. Phys Rev A 44 r4793-r4796, 1991. 

Recent Two-Network Results on the Effect of Chain Entangling in Cross-linked Elastomers... 

The role of chain entangling in cross-linked elastomers is an old issue which has not yet been settled. The success of Flory s new rubber elasticity theory 0-5) in describing some of the departures from the simple Gaussian theory has acted as a strong catalyst for new work in this area. 

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. 

In spite of these important results, the two-network method has had little impact on the discussion of the role of chain entangling in cross-linked elastomers. It was therefore decided to make a more detailed examination of the method and to try to develop a simpler method which would require fewer assumptions. The present paper is a discussion of recently published and unpublished work. 

Thus, the simplified Two-Network experiment shows by a direct comparison of forces at constant length that the trapped entangled structure of a well cross-linked elastomer contributes to the equilibrium modulus by an amount that is approximately equal to the rubber plateau modulus. The modulus contribution from the trapped entangled structure will be less for lower molecular weights and especially at low degrees of cross-linking (14). 

The process that makes the chemistry, properties, and applications of elastomers so different from other polymers is cross-linking with sulfur, commonly called vulcanization. The modem method of cross-linking elastomers involves using a mixture of sulfur and some vulcanization accelerator. Those derived from benzothiazole account for a large part of the market today. Temperatures of 100-160°C are typical. 

While polymer melts and non-cross-linked elastomers flow readily when stress is applied, structural plastics must resist irreversible deformation and behave as elastic solids when relatively small stresses are applied. These plastics are called ideal or Bingham plastics with their behavior described mathematically by... 

Gheneim R, Perez-Beramen C, Gandini A. Diels-Alder reactions with novel polymeric dienes and dienophiles synthesis of reversibly cross-linked elastomers. Macromolecules 2002 35 7246-7253. 

Methods for detecting whether peroxy compound have been used for cross-linking elastomers have been reviewed. An important application of dialkyl peroxides is as initiators of cross-linking and graft polymerization processes. The success of both processes depends on the ability of the peroxide to produce free radicals and the ability of the free radicals for H-abstraction from a relevant donor substrate. A method for evaluating this ability consists of inducing thermal decomposition of the peroxide dissolved in a mixture of cyclohexane and MSD (225). The free radical X" derived from the... 

Only when chemical bonds between neighboring molecules are introduced is a raw elastomer converted into a rubber vulcanizate, which is essentially a three-dimensional network structure (see Figure 5.3). The process is referred to as vulcanizahon or curing, or more accurately, as cross-linking. A cross-linked elastomer, or rubber vulcanizate, is capable of large reversible deformations within a broad temperature range and does not dissolve, but only swells in solvents and other liquids. 

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). 

Physical Properties of Radiation Cross-Linked Elastomers... 

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

Another application would be to minimize the swelling of a cross-linked elastomer in contact with a solvent. In this case, of course, one would be looking for a polymer giving the largest mismatch with the solubility parameters of the solvent to which the elastomer was to be exposed. 

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