Elastomeric, characteristics cross-links

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

The HMW subunits therefore display the characteristics required of an elastomeric material. These are an extensive central repetitive domain that is unconstrained by covalent cross-links and can undergo structural changes on deformation and shorter N- and C-terminal domains in which cross-linking can occur. 

Hot-melt thermoplastic elastomer systems (23. 24) are also effective coating materials. These materials are generally based on copolymers that are comprised of hard (crystalline or glassy) and rubbery (amorphous) segments contained in separate phases. The hard-phase regions form physical cross-links below their crystallization or vitrification temperature, and the system therefore has elastomeric properties. The moduli and low-temperature characteristics of these materials can be tailored to compare reasonably well with silicone rubbers at -40 C. However, they are limited in high-temperature applicability because of enhanced creep or flow due to softening. 

The elastomeric polypropylene materials studied in this chapter are from a class of thermoplastic elastomers since they possess the physical properties of elastomers along with the processing characteristics of thermoplastics. These materials are characterized by a low degree of crystallinity (23-26), where the crystalline regions dispersed in the amorphous matrix essentially provide physical cross-links to the amorphous elastomeric segments of the chain (19, 20). The size and distribution of these crystalline regions in the amorphous matrix thus have important influences on the mechanical properties. 

The last characteristic cited is required in order to obtain the elastomeric recoverability. It is obtained by joining together or cross-linking pairs of segments, approximately one out of a hundred, thereby preventing stretched polymer chains... 

Silicone elastomers are based on linear polymers which are analogous to the fluids but which have higher molecular weight. As with other elastomeric materials, it is necessary to cross-link the linear polymers in order to obtain characteristic elastic properties. General purpose elastomers are based on polydimethylsiloxanes but special purpose ihaterials which contain a small proportion of groups other than methyl are also available. These various products are described below. 

Not represented in Figure 2.10 is the deformation behavior of cross-linked networks of highly flexible elastomeric chains. The characteristic tensile deformation of elastomers is not based on energy elasticity but rather on the change of entropy accompanying the deformation and orientation of randomly coiled chain molecules... 

The efficient utilization of any polymeric material requires a detailed molecular understanding of its unique properties. In its most useful form, such information consists of quantitative relationships between the physical properties of interest and the structural characteristics of the material that determines them. In the case of elastomeric materials, the molecular feature of surpassing importance is the interlinking or cross-linking of the polymer chains into a macroscopic, three-dimensional network structure ". Such networks can not be molecularly dispersed in a solvent, and the usual solution characterization techniques can not be applied to obtain the required structural information. For this reason, it has been exceedingly difficult to obtain reliable structure-property relationships for elastomeric materials ... 

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