Thermodynamic spreading values

Table II. Thermodynamic Spreading Values of P/3BA from Pyridine-Free Spreading Solutions ...

The lower the value of the more likely it is that S is positive indicating a thermodynamic tendency for the process to occur. Longitudinal wave theory has been appHed to the defoamer spreading process as in equation 5 where P is the penetration depth of a spreading droplet of initial radius R, viscosity Tj, and density p. 

The value of is very small for low energy surfaces, but it cannot be neglected for fillers, on the contrary can be used for the calculation of the thermodynamic characteristics of their surface. The spreading pressure can be determined from the adsorption isotherm in the following way [74] ... 

These agents may operate via a number of mechanisms, but the most common ones appear to he those of entry and/or spreading. The defoamer must first of all he insoluble in the foaming liquid for these mechanisms to function. Second, the surface tension of the defoamer must be as low as possible. The interfacial tension between defoamer and foamer should be low. but not so low that emulsification of the defoamer may occur. Third, the defoamer should be dispersible in the foaming liquid. It was first shown in I fM8 that thermodynamically the entry of the defuamcr droplet into a bubble surface occurs when the entering coefficient has a positive value. The physics of bubbles is described in entry on Foam. 

We also calculated the thermodynamic spreading functions (Table IV) for five surface area values. In this case, the polymer-interface... 

A somewhat different dependence is observed for RS substances (see Fig. 2.21, curve 1). For small quantities of surfactant this dependence for RS substances coincides with the analogous dependence for IS substances at greater quantities of surfactant this dependence does not indicate any drop in the adhesion strength in that it becomes independent of the surfactant content and the change in the adhesion strength shows the same dependence as the polymer surface tension. This correlation is not random insofar as the adhesive spreads well on the steel surface, the thermodynamic work of adhesion of the cured adhesive to a solid surface is essentially twice the value of the surface tension, and consequently the dependence of the adhesion strength on the surfactant content must coincide with the change in the siuTace... 

Scaling laws have been derived for homopolymer monolayers [19,20] however they can be applied to spread copolymer films to provide a first approximation of the thermodynamic behaviour of each copolymer. The validity of the results obtained should be treated with caution due to the additional interactions between separate entities of the copolymer, and each copolymer with the subphase but they at least allow a qualitative comparison of the materials. Extrapolation of the low concentration linear portions of the surface pressure isotherms for the graft copolymers to zero surface concentration gives their limiting areas. It is evident from the values calculated (4.4 x 10 g for nl5, 4.6 x 10 g for n25... 

The scaling law exponents for the relation between surface pressure and surface concentration, i.e., n = where y = 2v/(2v - 1) and v is the excluded volume exponent, the value of which reflects the nature of the thermodynamic interaction between polymer and subphase. The values of v obtained for the copolymers, from the linear region of the isotherm, 0.62, 0.64 and 0.68 for /i75, 25 and n50 respectively, are all very close to the value of 0.75 for spread films of PEO on water [18], indicative of thermodynamically favourable conditions. As the PEO content of the copolymer increases, v increases suggesting that the graft copolymer-water interactions become more favourable and perhaps the grafts become less coiled as the percentage of PEO in the copolymer increases. 

---