Configuration of the Polyisobutylene Chain according to Neutron and X-ray Scattering

Abstract

The dimensions and configurations of polyisobutylene (PIB) chains in the bulk and in solution have been investigated by neutron and X-ray scattering at small and at intermediate angles. Measurements in bulk were carried out by neutron scattering on protonated PIB (PIB-H) dispersed in matrices consisting either of fully deuterated PIB-d8 or of the hexadeuterio polymer, PIB-d6, in which only the methyl groups are deuterated. Neutron scattering from PIB-H dissolved in benzene-d6 at 26.4 °C provided results in a 9-solvent, and X-ray scattering measured in n-heptane allowed observations on the configurations as represented by carbon scattering centers. The z-average radii of gyration, (s2)z1/2, were calculated from Zimm plots in the Guinier region. Values of (s2)z1/2 measured in the bulk (75 ± 5 A) and in the 0-solvent (77 ± 5 Á) are in good agreement. Results of scattering measurements at intermediate angles were compared by use of Kratky plots on an absolute scale, i.e., Fx(μ) = (x + 1)μ2P(μ) vs. μ, where μ is the magnitude of the scattering vector, x + 1 the number of monomer units per chain, and P(μ) the normalized scattering function. The scattering curves for PIB-H observed in the PIB-d8 matrix and in the 0-solvent are in excellent accord in the range of 0 < μ < 0.6 A″1. These results demonstrate that the configuration in the bulk is virtually the same as in a θ-solvent. The experimental data were compared also with theoretical scattering functions calculated on the basis of the conformational analysis of random PIB chains presented in the preceding paper. Five absolute Kratky functions Fx(μ) were calculated for scattering points consisting of (i) all eight protons, (ii) six methyl protons, (iii) two methylene protons, (iv) all four carbon atoms, and (v) only the substituted carbons in the monomer unit. These five functions differ appreciably for larger values of μ, indicating the importance of a proper choice of scattering points when calculations are compared with experimental curves. The first function (i) was compared with the experimental neutron scattering curves for PIB-H in the PIB-d8 matrix and in the 9-solvent, the second (ii) with that observed in the PIB-d6 matrix, and the fourth (iv) with the X-ray scattering curve observed in solution. Close agreement throughout the range of μ covered by the experiments confirms the conformational analysis of PIB and demonstrates randomness of configurations in the bulk down to distances of ca. 10 Å. © 1983, American Chemical Society. All rights reserved.