Swelling equilibrium data

ENTANGLH IENT NETV/ORKS OP 1,2-POLYBUTADIENE CROSS-LINKED IN STATES OF STRAIN XV. SIMPLE EXTENSION SPECIAL CASE COMPARISON OP TEIE CKP THEORY TO SWELLING EQUILIBRIUM DATA... 

The modern theory of real networks now permits a more accurate determination of network structures through use of equations (84),(87), and (95) (187-204). Stress-strain measurements can he analyzed as shown in Figure 17. The phantom modulus thus determined leads to v and Me through equations (75) and (76) (189). Swelling equilibrium data are similarly analyzed through equation (94), with the parameter k given hy equation (87) (189). 

In eomparison with all methods of determination of solvent aetivities from swelling equilibrium of network polymers, the gravimetrie vapor sorption/pressure measurement is the easiest applieable proeedure, gives the most reliable data, and should be preferred. 

A typical apparatus used for thermoelastic studies of networks in elongation and at thermodynamic equilibrium is shown in Figure 11. Equations (83), (86) and (88) were derived to interpret thermoelastic data obtained at constant volume and length, constant length and pressure, and constant pressure and deformation, respectively. Equation (83) was obtained without recourse to a model of rubberlike elasticity. However, it is necessary to impose a sizeable hydrostatic pressure to keep the specimen volume constant with change in temperature. This constancy of volume may be achieved also by the study of the network at swelling equilibrium with a diluent such that the change in polymer-solvent interactions with temperature exactly offsets the effect of thermal expansion. 

Unfortunately, reliable data on the equilibrium swelling and long-term stability of hydrogels obtained by this method are not available. It should be noted that crosslinking of polymers with metal ions is one of the few reactions which can be carried out directly in the soil medium, which is efficiently used, e.g., in oil recovery [65, 66]. 

B was also obtained from equilibrium-swelling data of selected block copolymers the results showed excellent agreement with the other methods (Equations 7.1 and 7.2)7... 

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). 

The two network precursors and solvent (if present) were combined with 20 ppm catalyst and reacted under argon at 75°C to produce the desired networks. The sol fractions, ws, and equilibrium swelling ratio In benzene, V2m, of these networks were determined according to established procedures ( 1, 4. Equilibrium tensile stress-strain Isotherms were obtained at 25 C on dumbbell shaped specimens according to procedures described elsewhere (1, 4). The data were well correlated by linear regression to the empirical Mooney-Rivlin (6 ) relationship. The tensile behavior of the networks formed In solution was measured both on networks with the solvent present and on networks from which the oligomeric PEMS had been extracted. 

Equilibrium swelling measurements have been done on B2 and on But 21-27 in benzene. No swelling data is reported for the PCP2 system for the reasons stated later in this paper. 

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