Small‐angle and wide‐angle X‐ray investigations on poly‐m‐bromostyrene and poly‐o‐bromostyrene in benzene solution

Abstract

AbstractVarious fractions of poly‐m‐bromostyrene and poly‐o‐bromostyrene in benzene were studied by the small‐angle and wide‐angle x‐ray methods. The combination of the two methods makes it possible to supply information on short‐range order and long‐range order in the same molecule. Four fractions of poly‐m‐bromostyrene of the approximate ratio of the degrees of polymerization (1:2:3:4) were studied. The molecular weight was determined from the absolute intensity of the small‐angle scattering at zero angle and the radius of gyration from the slope in the Guinier plot. The concentration dependence of both figures is discussed. The determination of the persistence length is not directly possible but results after elimination of the cross‐section factor (a = 17.3 Å). It is on the average about 20% smaller than the value calculated from the hydrodynamic length L and the radius of gyration Rexp supposing a Gauss coil, which can be explained by polydispersity and by the fact that benzene does not represent a Θ‐solvent but is better than this. The scattering curves of poly‐m‐bromostyrene solution and of a solution of m‐bromotoluene containing the same number of bromine atoms show no difference in the exactly measured wide‐angle field, which indicates that with the polymer product the distances between the bromine atoms are irregular. With poly‐o‐bromostyrene two possibilities of subtraction of a blank solution were compared. The small‐angle investigations of poly‐o‐bromostyrene permitted the determination of the molecular weight, radius of gyration, persistence length, radius of gyration of the cross section, mass per unit length, and hydrodynamic length. The high mass covering per unit length (Mq ≃ 90–100/Å), the large radius of gyration of the cross‐section (Rq ≃ 4.4 Å), and the large radius of the cross‐section, obtained in three independent ways (rq ≃ 6 Å) afford indications for the appearance of helical ranges in solution. A further support for this is the discovery of a definite interbromine spacing (l = 4.6 Å) by wide‐angle studies. This can only be explained by order of the bromine atoms over a certain range. On the other hand the presence of large coil ranges is shown by the direct determinability of the persistence length (a = 16.5 Å) from the transition point in the plot of I (2θ)2 against (2θ) as well by the not too great deviation from the Gaussian statistic of the coil. The most plausible explanation of all findings with poly‐o‐bromostyrene is that helical and coil ranges coexist in solution. The helical zones will, however, not extend over any too great range, so that the general impression of a wormlike chain remains. The verification of the procedure of interferometric measurement of defined spacings of heavy atoms in molecules is made with the example of the 2,5‐dibromotoluene by remeasurement of the known interbromine spacing.