Radiation cross-linked networks

Fig. 1. (a) Interaction of high energy electromagnetic radiation with a preformed thermoplastic polymer to develop (b) cross-linked network polymer... 

SiHcone mbber has a three-dimensional network stmcture caused by cross-linking of polydimethyl siloxane chains. Three reaction types are predominantiy employed for the formation of siHcone networks (155) peroxide-induced free-radical processes, hydrosdylation addition cure, and condensation cure. SiHcones have also been cross-linked using radiation to produce free radicals or to induce photoinitiated reactions. 

It has already been mentioned in Sect. 2 that the simplest assumption, affine deformation of the tubes d = dgk, yields the Mooney-Rivlin equation (1). The value V = 1/2 was obtained by a microscopic model which is briefly discussed in Sect. 2. It is suitable for the description of moderately but almost completely cross-linked networks (e.g. sulphur-, peroxide-, or radiation-crosslinked NR, PB and PDMS chains of very high degree of polymerization). 

Elastomers are classically blended to improve the physical and mechanical properties of the final material. For example, Dubey et aiP incorporated suitable amounts of EPDM in the SBR matrix because it was expected to impart significant heat and ozone resistance to the SBR matrix. The irradiation-induced vulcanization of SBR/EPDM blends was proved to combine desired properties and high mechanical strength. For example, the formation of a cross-linked network led to a decrease in elongation at break. Cross-linking restricts the movement of the polymer chain against the applied force. Moreover, to decrease the radiation dose (and hence, the cost of radiation), multifunctional acrylates (MFAs) and allylic reactive molecules were used. 

Tang (134) studied the effect of the interaction between carbon black and the polymer on electrical behavior. The absorption of polymer on the carbon black surface may be physical or chemical. The latter is caused by free-radical reaction between the polymer and carbon black, and it can occur during radiation or the preparation of the composite. In carbon-black-lilled high density polyethylene, the cross-linked network of the pol5uner restricts the freedom of movement of carbon black. The free-radical reaction enhances the binding force between the polymer and carbon black. 

In a cross-linked polymer the chains are joined chemically at tie points. There are many ways of preparing a cross-linked network. For polyethylene the simplest way is to irradiate the molten polymer with ionizing radiation, such as P- or y-rays. The chains then cross-link so that the entire specimen is one giant molecular network (see Fig. 1.2). In the liquid state the cross-links inhibit flow they cause the polymer to be form stable and to possess properties typical of rubbers. Once the specimen is cross-linked its natural shape is fixed it can be deformed under stress when heated and forced into a different shape, which can be frozen in by cooling to a sufficiently low temperature but if the specimen is re-heated it will revert to the shape it had when the cross-links were... 

Although this technique of fitting data linearly has now been thoroughly discredited it did provide a means of examining earlier work. The R values for peroxide (.35 +. 15) and radiation (1.69 +. 2) cross-linked networks are compared with that of the sulfur (1.30 +. 2) cross-linked networks. The low value of R for peroxide cross-linked networks is comprehensible if junctions are considered to be of high functionality. 

The higher value of R for radiation cross-linked networks is explicable... 

As the length and frequency of branches increase, they may ultimately reach from chain to chain. If all the chains are coimected together, a cross-linked or network polymer is formed. Cross-links may be built in during the polymerisation reaction by incorporation of sufficient tri- or higher functional monomers, or may be created chemically or by radiation between previously formed linear or branched molecules (curing or vulcanisation). Eor example, a Hquid epoxy (Table 1) oligomer (low molecular weight polymer) with a 6-8 is cured to a cross-linked soHd by reaction of the hydroxyl and... 

Certain polymeric stmctures can also be blended with other coreactive polymers or multifunctional reactive oligomers that affect curing reactions when exposed to ir radiation. These coreactive polymers and cross-linking oligomers undergo condensation or addition reactions, which cause the formation of network stmctures (Table 9) (4,5,47). 

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