A study of the relation between molecular weight distribution and mechanical properties of polyvinyl alcohol

Abstract

A study of the relation between mechanical properties and molecular weight distribution of polyvinyl alcohol was carried out.

The polyvinyl alcohol was fractionated with respect to degree of polymerization (D.P.) by precipitation of the fractions from 2% solution in water with a n-propyl alcohol-water nonsolvent. The initial small scale procedure was modified so as to accumulate fractions in sufficient quantity for the testing of mechanical properties and for the preparation of blends. The fractions were refractionated until further refractionation gave no further change in D.P.

The homogeneous fractions were cast into films using water as the solvent. These films were then used in determining mechanical properties.

A series of three blends, all with normal distributions and the same D.P. at the maximum of the differential distribution curve but with variable heights at the maximum D.P., were prepared and the mechanical properties were determined.

A second series of six blends, all with constant height and constant D.P. at the maximum, but all being skewed from the normal distribution, were prepared and their mechanical properties were determined.

A correlation of tensile strength with the shape of the distribution curve was found to have the form: T = (8200-l0^{6 .87-.00476P_m}) + (2.786 x 10⁶H^4.767-1000) + (1450-10.4B), Where T is tensile strength; P_m is D.P. at maximum of the differential distribution curve; H is height at the maximum; B is area skew. The second term in brackets is dropped when H is greater than 0.190 and the entire last term is dropped when B = 0.

A graphical correlation of mechanical properties with total percentage of low D.P. material gives fairly good results for polyvinyl alcohol and, using the data of other investigators, for cellulose nitrate, cellulose acetate, and polyvinyl acetate.