Elastomers behavior

Elastomer behavior is depicted by the bottom curve in Figure 3.3. Here the modulus (ratio of stress to strain, as of strength to elongation measure of polymer stiffness) is low, but elongations to several hundred percent are possible before failure. 

Case,L.C., Wargin,R.V. Elastomer behavior. IV. The loop structure of elastomer networks. Makromol. Chem. 77,172-184 (1964). 

The characterization of surface activity of fillers is obtained by use of several analytical techniques [1]. Examples of them are inverse gas chromatography [1, 2], the adsorption of a low molecular weight analog of elastomers [3], the adsorption of elastomer chains fi om dilute solutions [4], the wettability, viscosity of PDMS fluids in the boundary layer at the surface of solids [5], the determination of the specific surface area, and the analysis of surface groups [1]. It should, however, be mentioned that the results obtained by these methods do not provide direct information on the elastomer behavior at the interface, due to the use of small probe molecules or the presence of a solvent in the systems studied. 

A variation of the sequential monomer addition technique described in Section 9.2.6(i) is used to make styrene-diene-styrene iriblock thermoplastic rubbers. Styrene is polymerized first, using butyl lithium initiator in a nonpolar solvent. Then, a mixture of styrene and the diene is added to the living polystyryl macroanion. The diene will polymerize first, because styrene anions initiate diene polymerization much faster than the reverse process. After the diene monomer is consumed, polystyrene forms the third block. The combination of Li initiation and a nonpolar solvent produces a high cis-1,4 content in the central polydiene block, as required for thermoplastic elastomer behavior. 

Commercial BC s are prepared from monomers that upon polymerization yield immiscible macromolecular blocks, one rigid and the other flexible, that separate into a two-phase system with rigid and soft domains. The concentration and molecular weights provide control of the size of the separated domains, thus morphology and the interconnection between the domains. The existence of a dispersed rigid phase in an elastomeric matrix is responsible for its thermoplastic elastomer behavior. For symmetric block copolymers, Leibler [1980] showed that a sufficient condition for microphase separation is (%abN) = 10.5, where binary thermody-... 

It is important to note that any molecular architecture that provides a thermoplastic block chemically bonded to an elastomeric block, which is in turn bonded to another thermoplastic segment, should exhibit the properties of a thermoplastic elastomer. For example, grafting thermoplastic branches onto an elastomeric backbone produces thermoplastic elastomer behavior [285, 298]. Other examples are the segmented-type polymers—[AB] — with alternating hard and soft segments thus, a variety of segmented polyesters and polyurethanes with polyether or polyester soft segments exhibit properties of thermoplastic elastomers [263,298,299]. 

These networks exhibit a nematic mesophase. Clearly, the type of response to the mechanical field is extremely structure dependent. The effect of spacer length on elastomer behavior was examined [27]. The reported work showed that the coupling between the mechanical stress and polymer backbone can greatly influence the mesomorphic order. This effect can be enhanced by coupling the mesogenic group more tightly to the polymer backbone. 

Many types of polymers, both glassy and rubbery, are cross-linked to improve elastomer behavior or to control swelling. 

In order to explain thoroughly elastomers behavior under oscillatory deformation, let be the longitudinal displacement in an uniaxial deformation from which the nonlinear Lagrangian strain follows as ... 

Samples, such as D., which have short Isotactic sequences and high comonomer content, display ideal elastomer behavior, in... 

In the case of the elastomers, formulations that use diisocyanates (O Fig. 14.16) and mixtures of long diols (oligomers of polybutadiene, polycaprolactone, or polaxyethylene) and short diols lead to thermoplastic elastomer behavior, with physical cross-links (rigid sequences) finding themselves associated, dispersed in micro-doinains, with a continuous elastomer phase (flexible sequences). 

Samples, such as D., which have short Isotactic sequences and high comonomer content, display ideal elastomer behavior, in that the ratio of birefringence to stress is almost independent of temperature and the stress at constant elongation is proportional to absolute temperature. All other samples have some critical temperature (e.g., 50 C for Sample Aj ) above which the vulcanizates behave as an ideal elastomer. This suggests that Sample D2 does not crystallize in either the stressed or unstressed state. 

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