Molecular entanglements

Solution Properties. Typically, if a polymer is soluble ia a solvent, it is soluble ia all proportions. As solvent evaporates from the solution, no phase separation or precipitation occurs. The solution viscosity iacreases continually until a coherent film is formed. The film is held together by molecular entanglements and secondary bonding forces. The solubiUty of the acrylate polymers is affected by the nature of the side group. Polymers that contain short side chaias are relatively polar and are soluble ia polar solvents such as ketones, esters, or ether alcohols. As the side chaia iacreases ia length the polymers are less polar and dissolve ia relatively nonpolar solvents, such as aromatic or aUphatic hydrocarbons. 

When polymer melts are deformed, polymer molecules not only slide past each other, but they also tend to uncoil—or at least they are deformed from their random coiled-up configuration. On release of the deforming stresses these molecules tend to revert to random coiled-up forms. Since molecular entanglements cause the molecules to act in a co-operative manner some recovery of shape corresponding to the re-coiling occurs. In phenomenological terms we say that the melt shows elasticity. 

Cast material is stated to have a number average molecular weight of about 10. Whilst the Tg is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx. 170°C). There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. For injection moulding and extrusion much lower molecular weight materials are employed. Such polymers have a reasonable melt viscosity but marginally lower heat distortion temperatures and mechanical properties. 

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. 

Fig. 2.1. Interface bonds formed (a) by molecular entanglement (b) by electrostatic attraction (c) by interdiffusion of elements (d) by chemical reaction between groups A on one surface and groups B on the other surface (e) by chemical reaction following forming of a new compound(s), particularly in MMCs (0 by mechanical interlocking. After Hull (1981) and Naslain (1993).

Molecular Entanglement Theories of Linear Viscoelastic Behavior... 

Graessley.W.M. Molecular entanglement theory of flow behavior in amorphous polymers. J. Chem. Phys. 43, 2696-2703 (1965). 

The mass fractions of these three phases are shown in Table 14. the crystalline fraction is relatively small as 0.43 or 0.44. This low level of crystallinity may arise from relatively strong molecular entanglement due to the network structure. 

Hubin, T.J. and Busch, D.H. (2000) Template routes to interlocked molecular structures and orderly molecular entanglements, Coord. Chem. Rev. 200-202, 5-52. 

Sensitivity to shear would also be a function of the extent of molecular entanglements in the swollen granules. Svegmark and Hermansson150,437 have found that warm cereal starch pastes are less sensitive to shearing than similarly treated potato starch pastes (Figure 8.14) the complex shear modulus of the sheared (HS) 10%... 

Kulicke and Nottelmann (1989) divided gels into three generic classes, viz., physical, ionotropic, and covalent. Physical gels are held together by hydrogen bonds and molecular entanglements they expand when hydrated and contract when dehydrated to Silberberg (1989) they are swollen me-... 

And so we arrive at the basis for the strength of many polymers, especially plastics—molecular entanglement Picture hitting a glass windowpane with a hammer. The same blow to an equivalently shaped piece of Lexan (polycarbonate) or Ludte poly[methyl methacrylate]), both transparent plastics, does not break the polymer. In fact, the plastic glass is somewhat resilient, causing the hammer to bounce off of it. 

For over a year Carothers and his group made polyesters in their laboratory, each time obtaining product with a molar mass of 3000 to 4000. As noted in the equations above, the esterification reaction is reversible. Ester groups can be hydrolyzed (react with water), to reform carboxylic acid and alcohol. When they worked out a procedure to remove the last traces of water, Carothers and his team obtained polyester with a molar mass of approximately 12,000. The value was far above that needed for molecular entanglement, a significant achievement. They were elated. 

In this polymerization, the biofunctional component (enzyme) can be concentrated in an interfacial area between the frozen ice crystal and the supercooled monomer phase, and immobilized by molecular entanglement between the enzyme and polymer molecules. This is a different procedure for fixation from the usual entrapping method with a crosslinked structure in a gel. Therefore, we may call this procedure the adhesion-method to distinguish it from the usual entrapping. This term was extended to cover the use of the usual synthetic polymers including hydrophobic polymers as the supports. One of the characteristic properties of products obtained in this way was that there is a maximum activity at a certain monomer concentration. The maximum activity is observed when the increased inner surface area is balanced by the increased leakage of enzyme and these occur with a decrease of monomer concentration. Immobilization by physical entrapping was also studied by Rosiak [26], Carenza [27] and Ha [28]. 

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