Molecular parameters, conformation and rigidity of polyamic acid chains in dimethyl formamide investigated by gel permeation chromatography coupled with multiple detectors

Chain conformation and molecular parameters of three different polyamic acids (PAA), including (6FDA − DMB) PAA, (6FDA − TFDB) PAA and (Ultem 1000) PAA were investigated by gel permeation chromatography coupled with multi − angle laser light scattering detector, viscosity detector, and differential refractive index detector (GPC − MALLS − VIS − RI) using 0.02 M LiBr/0.20 M HAc/DMF as mobile phase. The weight-average molecular weights of the PAAs are 220.2–293.9 kg/mol. The scaling exponents ν obtained by the scaling relationship between the root-mean-square radius of gyration (R_g) and molecular weight (M) are 0.59, 0.59, and 0.57 and the scaling exponents α obtained by the Mark − Houwink − Kuhn − Sakurada (MHKS) relations are 0.78, 0.78, and 0.74 for (6FDA − DMB) PAA, (6FDA − TFDB) PAA and (Ultem 1000) PAA, respectively. These scaling relationships indicate that all three polyamic acids with different structures have a random coil conformation in the mobile phase. The persistence lengths l_p estimated based on R_g through a Kratky-Porod wormlike model are 2.3, 2.1, and 1.6 nm, and estimated from intrinsic viscosity through a Bohdanecky approach are 1.9, 1.8, and 1.7 nm for (6FDA − DMB) PAA, (6FDA − TFDB) PAA and (Ultem 1000) PAA, respectively. These results imply that the three PAA show a characteristic of a flexible chain with local rigidity in the mobile phase. The (Ultem 1000) PAA always has the smallest l_p among the three PAA samples, which may be attributed to a flexible ether linkage in the backbone.