Critical concentrations of polymers in solutions according to measurements of the viscosity and specific surface area of aerogels resulting after sublimation of the solvent

Conclusion

Measurement of the viscosities of polystyrene and cyclolinear polyphenylsiloxane depending on their concentrations has revealed critical concentrations of these polymers to exist corresponding to the formation of a fluctuating continuous supermolecular network in the solutions. It has been shown on the example of polystyrene solutions that changing from a good to a poor solvent has little effect on the value of the critical concentration. Calculation of the critical concentration with the use of the parameters employed in the free volume theory after the Pezzin method shows good agreement with the critical concentrations determined from the shape of the viscosity vs. concentration dependences of polystyrene solutions.

A method is described for producing polymer aerogels by sublimation of the frozen solvent from polymer solutions. It is shown that to obtain aerogels with very high specific surface areas from crystallized solutions is necessary to fulfil at least two conditions: 1. to freeze them quickly; 2. to sublimate the solvent in vacuo at a sufficiently low temperature. Special attention is drawn to the fact that the effectivity of sublimation drying should be estimated by the specific surface area of the preparations obtained as a result of drying.

Mesurement of the specific surface areas of aerogels obtained under appropriate conditions from polymer solutions of different concentrations shows that with a poor solvent the aerogels have specific surface areas one decimal order lower. Hence it is concluded that solutions of polymers in poor solvents should yield stronger and less permeable polymeric systems. In all the cases studied the dependence of the specific surface area of the aerogel on the concentration of the polymer in the solution has a distinct maximum which corresponds to the critical concentration determined viscometrically. This is evidence of the correspondence between the structure of the aerogels and the structure of the initial solutions, at least, at concentrations in the region of their critical and above-critical values. Sublimation of the solvent from frozen polymer solutions results in contraction of the samples, which is the most considerable at polymer concentrations below critical. This is also connected with the relatively low specific surface areas of aerogels obtained from solutions of low concentrations.