Negatively Charged Poly(acrylic acid) Side Chains Grafted onto Poly(3-hexylthiophene): Impact on the Structural Order and Molecular Dynamics

Conducting polymer poly(3-hexylthiophene) (P3HT) and a graft copolymer of P3HT with poly(acrylic acid) (P3HT-g-PAA) were synthesized. The structure and molecular dynamics of P3HT and P3HT-g-PAA were investigated by solid-state NMR (SSNMR), Fourier transform infrared (FTIR), Raman, UV–vis, and fluorescence spectroscopies and X-ray diffraction (XRD) technique. The spectroscopic data confirmed the grafting of the PAA chains from the P3HT backbone at the molar ratio of ≈3:1 and significant changes in molecular dynamics in the grafted sample. XRD results suggested that overall grafting introduced the structural order in P3HT-g-PAA due to steric effects probably caused by charged polyanionic side chains. Solution NMR indicates the prevalence of the head-to-tail, HT-HT sequence in P3HT-g-PAA. The spin–lattice relaxation time constant, T1(13C), for the thiophene rings in P3HT-g-PAA (13.5 s) is approximately an order of magnitude longer than T1(13C) for the thiophene rings in P3HT (1.8 s). The rotating-frame relaxation time constants T1ρ(1H) for the thiophene ring carbons in P3HT were 2.2 and 3.2 (0.5) and 26.1 (0.5) ms for P3HT-g-PAA s, respectively. The longer component of the cross-polarization time TCH in P3HT (5.3 ms) becomes ca. 5 times shorter for P3HT-g-PAA (1.1 ms). The carbon rotating-frame relaxation time constants T1ρ(13C) were 13.3 and 29.1 ms for the thiophene ring carbons in P3HT and P3HT-g-PAA, respectively. The T1ρ(1H), T1(13C), and T1ρ(13C) for the side chains in P3HT and P3HT-g-PAA were measured in the aliphatic spectral region. T1ρ(1H) constants were 2.9 and 27.0 ms for P3HT and P3HT-g-PAA, respectively. T1(13C) values were 0.4 for P3HT and 0.5 (0.65) and 5.6 (0.35) ms for P3HT-g-PAA. T1ρ(13C) values were 7.0 (0.45) and 1.1 (0.55) ms for P3HT and 19.0 (0.75) and 0.9 (0.25) ms for P3HT-g-PAA. The data imply significant changes in structural order and molecular dynamics after grafting the PAA polyelectrolyte side chains.