Rubbers molecular entanglements

Such considerations appear to be very relevant to the deformation of polymethylmethacrylate (PMMA) in the glassy state. At first sight, the development of P200 with draw ratio appears to follow the pseudo-affine deformation scheme rather than the rubber network model. It is, however, not possible to reconcile this conclusion with the temperature dependence of the behaviour where the development of orientation reduces in absolute magnitude with increasing temperature of deformation. It was proposed by Raha and Bowden 25) that an alternative deformation scheme, which fits the data well, is to assume that the deformation is akin to a rubber network, where the number of cross-links systematically reduces as the draw ratio is increased. It is assumed that the reduction in the number of cross-links per unit volume N i.e. molecular entanglements is proportional to the degree of deformation. 

Several approaches to the description of molecular entanglements in polymers are available at present. A brief outline will be given here. The best known is the version of the binary hook [9,10] with some network features. At temperatures (T) exceeding the temperature of glass transition (Tg) for the polymer, the network density V(,i, is usually determined in the framework of the rubber-like elasticity, while for Tentanglement network is proven both theoretically and... 

Due to the dual filler and crosslinking nature of the hard domains in TPEs, the molecular deformation process is entirely different than the Gaussian network theories used in the description of conventional rubbers. Chain entanglements, which serve as effective crosslinks, play an important role in governing TPE behavior. The stress-strain results of most TPEs have been described by the empirical Mooney-Rivlin equation ... 

Meanwhile, developments in polymer science established that most long-chain linear polymers above their glass-transition temperatures can also exhibit rubberlike behavior whereby a network of molecular entanglements can serve the function of chemical cross links for deformation histories with oscillation periods shorter than the relaxation times of entanglement drift. It is this form of behavior of glassy polymers resembling that of rubbers which is a subject of principal concern and is discussed in Section 6.7. 

Rubber is considered as a network of molecular entropy springs (cf. p. 568) with junction points consisting of chemical cross links or molecular entanglements. The principal theoretical viewpoints dealing with the extension of such a network structure have been treated of in Chapter IV, p. 123. 

The factors inhibiting macro-flow in vulcanised rubber are the chemical cross links due to vulcanisation (and probably also molecular entanglements), considerably assisted by the phenomenon of crystallisation. In unvulcanised raw rubber macro-flow may occur to a certain extend. In cellulose crystalline junction points of a high degree of stability are responsible for the exclusion of macro-flow. In rubber the chains are very flexible and consist of a large number of statistical chain sections in cellulose the chains are stiffer and consist of a small number of chain elements. In the former case the intermolecular forces are weak, in the latter case they are strong. 

Fatty-acid based process aids can reduce the viscosity of a rubber containing them, without an actual chain scission and reduced molecular chain length being involved. The viscosity reduction is believed to be achieved by simple lubrication of the molecular entanglement, allowing individual chains to slip easily relative to their fellow chains and thus creating the illusion of the viscosity reduction normally associated with short-chainlength molecular movement. 

In contrast to common rubbers, the entanglement molecular weight. Me, of most acrylics is sufficiently high that without dilution they meet the Dahlquist criterion discussed earlier (G <3x 10 dynes/cm, 10 Pa) and exhibit tacL With little or no cross-finking the neat polymers form a pressure-sensitive adhesive with some cohesion. To obtain adequate cohesion it is common to use polar monomers which can form hydrogen bonds, and usually also some level of cross-linking. (Acrylics are also discussed in O Chap. 14.)... 

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooling they show a small increase in the shear modulus at temperatures well above T. This is the liquid-to-rubber transition. The values of shear modulus and viscosity for rubbery solids are considerably lower than those for glass forming liquids at an equivalent structural relaxation time. The local or microscopic viscosity relaxation time of the rubbery material, which is reflected in the 7], obeys a VTF equation with a pre-exponential factor equivalent to that for small-molecule liquids. Above the liquid-to-rubber transition, the VTF equation is also obeyed but the pre-exponential term for viscosity is much larger than is typical for small-molecule liquids and is dependent on the polymer molecular weight. 

It is clearly shown that chain entangling plays a major role in networks of 1,2-polybutadiene produced by cross-linking of long linear chains. The two-network method should provide critical tests for new molecular theories of rubber elasticity which take chain entangling into account. 

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