Effect of chemical structure on the softening point of substituted polystyrenes and related materials

The concept of a limiting softening point, defined in relation to a partictilar standard test method (B. S. 1493) but otherwise only a function of the repeat unit structure of the polymer, is discussed. An approximate correlation of results obtained from different standard methods is given and its limitations analyzed. Limiting softening point data are given for polystyrene and 25 different substituted polystyrenes. Analysis of the results suggest the following: (1) the elevating effect of non‐polar nuelear substituents is greatest in the o‐position, and increases with the size of the substituent (2) in the p‐position, large rigid groups elevate the softening‐point whereas the reverse is true of flexible (articulated) groups; (3) effects of substituents are additive only where they produce the same hindrance to molecular motion, e.g., the effect of two Similar o‐substituents is additive; in other cases, the softening point elevation is determined by the biggest single energy barrier which has to be overcome. (4)Inchlorine‐substituted polystyrenes the L.S.P. can [in accordance with (3)] be determined by either polar or stenic factors, depending on the number and position of the substituents. It is also possible to classify the mechanisms by which substitution can elevate the L.S.P. of polystyrene, these being: (1) stiffening of the main‐chain backbone; (2) intramolecular steric hindrance (e.g., o‐substitution); (3) increased intermolecular steric hindrance; and (4) dipolar attraction. From a qualitative consideration of structural features in copolymers of styrene with substituted styrenes, it is deduced that many such copolymers should show a roughly linear variation of L.S.P. with composition.