End-linked elastomers

The first SANS experiments on end-linked elastomers with a well-defined functionality were carried out by Hinkley et al, (22). Hydroxy-terminated polybutadiene was crosslinked by a trifunctional isocyanate, and the resultant polymer was uniaxially stretched. 

Because of their known structures, such end-linked elastomers are now the preferred materials for the quantitative characterization of rubber-like elasticity. In addition, bimodal networks prepared by these end-linking techniques have very good ultimate properties, and there is currently much interest in preparing and characterizing such networks (13,22,25,29-33) and developing theoretical interpretations for their properties (34r-37). Both subjects are discussed below. 

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. 

Figure 6. Sketch of expected variation of the initial (c0) and residual (cr) concentration of functional groups, reaction conversion , and the ratio ve/Te in end-linked networks as a function of molecular weight of elastomer component M. For discussion see text.

The results indicated that the ease with which a diluent could be removed from a network decreased with increase in Md and decrease in Mc, as expected. High molecular-weight diluents are extremely hard to remove at values of Mc of interest in the preparation of model elastomers [3, 18, 20, 157]. Estimating the extents of reaction in end-linking processes from sol fraction data could be complicated by this difficulty. 

In the simplest study of this type, Al-ghamdi and Mark [138] studied reinforcement of PDMS by two zeolites of different pore sizes. The zeolites were a zeolite 3A (pore diameter 3 A) and a zeolite 13X (pore diameter 10 A), both with a cubic crystalline structure. They were simply blended into hydroxyl-terminated chains of PDMS which were subsequently end-linked with tetraethoxysilane to form an elastomeric network. These elastomers were studied by equilibrium stress-strain measurements in elongation at 25°C. Both zeolites increased the modulus and related mechanical properties of the elastomer, but the effect was larger for the zeolite with the larger pore size. 

These are silicones in liquid or paste form that can be transformed into cross-linked elastomers at or near room temperature. There are two basic types, one component and two component. The one-component room temperature vulcanizing (RTV) elastomers cure upon exposure to air. A typical material of this type is synthesized from a dihydroxy-terminated siloxane oligomer and methyltriacetoxysilane (Scheme 2). The resulting tetraacetoxy compound will flow as a paste and so can be molded into shape. Hydrolysis of the acetoxy groups takes place over a period of hours. Condensation of the resulting silanols gives a highly cross-linked end product. 

FIGURE 5.14 The logarithm of the reduced retardation spectrum, Lp, shown as function of the logarithm of the reduced retardation time, A./uj , for the three urethane-end linked polybutadiene elastomers, (o) TB-1, ( ) TB-2, ( )TB-3. The response has been reduced to corresponding state temperatures of 74°C, 0°C, and 17°C, respectively, for the primary softening transition. 

FIGURE 18 Logarithm of the reduced shear creep compliance curves / (t) (in Pa ) for the three urethane-end linked polybutadiene elastomers displayed as a function of the logarithm of the reduced time t/a- (in seconds). The reference temperatures of reduction are chosen so that superposition is achieved at short times in the primary softening dispersion. (O) TB-1,74°C, ( ) TB-2,0°C, (0) TB-3,17°C. 

SYNTHESIS TECHNIQUES (a) Stokes thermally polymerized hexachlorocyclotriphosphazene via a ring-opening process to provide a cross-linked elastomer, but it was not until 1965 that a high molecular poly(dichlorophosphazene) was isolated and subsequently transformed, via nucleophilic substitution, into thermally stable semicrystailline homopolymers. This procedure was used widely to synthesize a variety until a few years ago, but it suffered from relatively low conversions (<70% so as to avoid cross-linking), unknown chain-end groups and lack of molecular weight control of the product. These difficulties obstructed its commercialization. 

NMR Line Shape of End-Linked PDMS Elastomers. Macromolecules 2009, 42,8882-8888. 

Bimodal elastomers prepared by end linking have very good ultimate properties, which underlies recent interest in such materials. ... 

There is evidence of large-scale supramolecular structures in end-linked PDMS elastomers, particularly in the case of bimodal distribu-tions. ° Small-angle neutron scattering on bimodal networks of poly(tetrahydrofuran) suggests segregation of short and long chains. ... 

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