Cross-linking activation energy

A unique process for chemical stabili2ation of a ceUular elastomer upon extmsion has been shown for ethylene—propylene mbber the expanded mbber obtained by extmsion is exposed to high energy radiation to cross-link or vulcani2e the mbber and give dimensional stabUity (9). EPDM is also made continuously through extmsion and a combination of hot air and microwaves or radio frequency waves which both activate the blow and accelerate the cure. 

Pure PHEMA gel is sufficiently physically cross-linked by entanglements that it swells in water without dissolving, even without covalent cross-links. Its water sorption kinetics are Fickian over a broad temperature range. As the temperature increases, the diffusion coefficient of the sorption process rises from a value of 3.2 X 10 8 cm2/s at 4°C to 5.6 x 10 7 cm2/s at 88°C according to an Arrhenius rate law with an activation energy of 6.1 kcal/mol. At 5°C, the sample becomes completely rubbery at 60% of the equilibrium solvent uptake (q = 1.67). This transition drops steadily as Tg is approached ( 90°C), so that at 88°C the sample becomes entirely rubbery with less than 30% of the equilibrium uptake (q = 1.51) (data cited here are from Ref. 138). 

If the cross-links or polymers are severed, then some elastic energy is released and the system will adopt a new (larger) equilibrium volume where greater distortion is conferred upon a meshwork that has fewer cross-links. Thus, upon increases in Ca2+, gelsolin activity leads to an increase in the volume of the actin-filament network. The additional influence of myosin on such a meshwork is similar to that proposed in the Stossel model. Thus, three-dimensional Ca2+ gradients (between a localised region of the cell surface and an external structure) can result in complex shape changes. 

Actually, crosslinks control the molecular packing and indeed significantly affect the elastic modulus of the material. As the intermolecular energy of kink formation is also determined by elastic modulus, the yield stress will definitely vary with modulus and thus the cross -linking density. In other words, crosslinks may not seriously affect the activation segment configuration in the molecular chain but will indirectly control the yield stress. 

In fact, the polymer is quite stable with respect to precipitation. Once isolated it can be kept in aqueous solution indefinitely (37). This stability is presumably kinetic in origin. Since all evidence points to a different internal structure for the polymer from all crystalline ferric oxide or hydroxide phases, the reorganization required for precipitation would be expected to have a high activation energy. Addition of base to pol5maer solutions does produce an immediate precipitate, presumably by cross-linking the polymer particles. In hydrolyzed ferric nitrate solutions with less than 2.5 base equivalent per mole of iron the eventual precipitates observed are probably formed directly from low molecular weight components. The low rate of dissociation would then be another factor in polymer stability. 

The presence of cross-linked phosphates may be recognized by their ready hydrolysis, which leads to a rapid drop in the viscosity of the solution and a parallel decrease in its pH. Aqueous solutions of all cross-linked phosphates are hydrolyzed after twenty hours. In contrast to the hydrolysis of normal P—O—P bonds in meta- and polyphosphates, that of the cross-linking sites is practically independent of concentration, pH, ionic strength and the nature and concentration of added salts. It does, however, follow a first-order law, as for normal P—O—P bonds, and is strongly temperature dependent. Activation energies of 18.9 and 15.4 kcal/mole have been... 

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