Effects of composition on the solution properties of styrene–acrylonitrile copolymers

Abstract

AbstractShort‐range interactions between chain units of random copolymers in solution may be influenced by the composition or precisely by the distribution of sequence lengths of the same monomer units. Steric factors were derived for random copolymers of styrene and acrylonitrile with different compositions from the relation between the limiting viscosity number and the molecular weight. Mark‐Houwink relations were obtained in methyl ethyl ketone (MEK) or in N,N′‐dimethylformamide (DMF) at 30°C. for random copolymers containing 0.383 (Co‐1) and 0.626 (Co‐2) mole fraction of acrylonitrile, the expressions are: [η] = 3.6 X 10−4 Mw0.62, for Co‐1 in MEK; [η] = 5.3 X 10−4 Mw0.61, for Co‐2 in MEK; [η] = 1.2 × 10−4Mw0.77 for Co‐2 in DMF. With the Stockmayer‐Fixman expression, these correlations become, respectively: [η]/M1/2 = 1.24 × 10−3 + 8.0 × 10−7 M1/2; and [η]/M1/2 = 1.70 × 10−3 + 6.3 × 10−7 M1/2; and [η]/M1/2 = 1.68 × 10−3 + 31.3 × 10−7 M1/2. From the unperturbed mean‐square end‐to‐end distances, 〈L2〉0, determined from the first terms of the latter expressions, together with 〈L2〉0f calculated by assuming the completely free rotation, gives the steric factor σ = (〈L2〉0/〈L2〉0f)1/2 as 2.25 ± 0.05 for Co‐1, and 2.31 ± 0.10 for Co‐2. These values of σ are close to those for polystyrene (σ = 2.22 ± 0.05) and for polyacrylonitrile (σ = 2.20 ± 0.05). Therefore, it is concluded that the dimensions of random copolymers of styrene and acrylonitrile in solution are not significantly influenced by the composition. In other words, the unperturbed dimensions are not affected by a change in the alternation tendency between styrene units with phenyl side groups having a large molar volume and acrylonitrile units with nitrile groups responsible for the electrostatic interactions. On the other hand, the long‐range interactions reflect the effect of sequence length. The Huggins constant and the second virial coefficient obtained from the light‐scattering measurements have optimum values at about 0.5 mole fraction of acrylonitrile, where the greatest tendency for alternation seems to exist.