| dc.description.abstract | The large values of ‘a’, the M.H. exponent, in good solvents for both St–M and MMA–AN suggest that these molecules are highly extended in solution. Application of the different theories to calculate the unperturbed dimensions leads to very low values of ?r²?? (i.e., low steric factor) and high values of ?, suggesting that these molecules are highly extended mainly due to unusual polymer–solvent interactions. The observed decrease of [?] with rise in temperature in all cases is also in agreement with this trend.
However, if the high extension of these molecules in these solvents is mainly due to unusual solvent effects, then one can expect high A? and low ? values. But the A? values obtained by light scattering for these systems are low, and the ? values very high—opposite to what is expected in good solvents.
Cellulose derivatives also exhibit high values of ‘a’, very low values of A?, and a decrease of [?] with temperature, which have been interpreted as typical properties of stiff?chain polymers. Kurata and Stockmayer, based on the large values of ?? obtained by the Flory equation (Equation 7, page 16), concluded that these molecules are highly extended due to unusually favourable polymer–solvent interactions. Recent theoretical studies suggest that cellulose chains assume large extensions mainly due to unusual short?range interactions.
All the experimental data obtained in the present investigation—viz., large ‘a’ values, low A? values, high ? values, and a decrease of [?] with temperature—suggest that the incorporation of AS into the MMA chain must have introduced some stiffness to the chain. Comparison of the properties of MMA–AS and St–AS copolymers shows that the extent of stiffness in the copolymer chain is greater for the former than for the latter. The values of A? are very low for the former and decrease with increase in AS content, and the values of ‘a’ are very high in DMSO and MeCN.
The specific solvent interaction seems to be higher for MMA–AS than for St–AS copolymers, as shown by the K values in different solvents, particularly for MA? and MA?. This is in agreement with expectations, since in MMA–AS copolymers, both monomers being highly polar, one would expect greater deviation from normal behaviour.
It is observed that for none of the copolymer systems in good solvents do the S–F and K–S methods give the unperturbed dimension values obtained in ??solvents, showing that these equations are not sufficient to eliminate the perturbation effects caused by the solvent.
All these experimental findings suggest that the theory for [?] is not yet complete, since it fails to describe certain observed features of the dependence of [?] on M?. It is hoped that these results may prove useful in the development of improved theoretical estimates of this dependence. | |
