Effects of conformational change of polyacrylonitrile on aging behavior of solutions in N,N-dimethyl formamide

The analysis of polyacrylonitrile solution properties is still an outstanding topic since dipole-dipole interactions between nitrile groups initiate anomalous phase characters counting on dissolution condition, solution concentration, and solvent type. Although polyacrylonitrile gives a transparent solution when dissolved in a polar solvent, chemical attraction between the polar nitrile groups gives rise to the internal structure. The resulting solution heterogeneity is responsible for the time dependent change of the physical attributes, which is physical aging. Physical aging is responsible for the deterioration of the stability of the polyacrylonitrile solutions during manufacturing.

For this reason, many researchers have focused their attention of the impact of physical aging on the solution properties in a bid to optimize the manufacturing conditions of the polyacrylonitrile-based products. However, before understanding the aging behavior of the polyacrylonitrile solutions, clarifying the mode as well as the degree of the molecular interactions between the polar particles based on physical conditions and solvent systems would be helpful. The strength as well as interaction modes between polar polymer molecules in polar solvents are significantly influenced by chain conformation. Therefore, conformational analysis of polyacrylonitrile in polar solvents based on storage temperature along with time would be paramount in the analysis of the aging behavior of the polyacrylonitrile solutions.

Professor Byoung Chul Kim from Hanyang University in Republic of Korea led a team of researchers in exploring the aging mechanism of polyacrylonitrile solutions in N,N-dimethyl formamide by tracking its conformational change. They implemented the temperature dependent FTIR spectra and its 2-dimentional correlation spectroscopy in analyzing the thermal-induced transformation of the chain conformation as well as dipole-dipole interactions. Their work is published in Polymer.

The authors purged all solutions with nitrogen in order to prevent oxidative degradation. The solutions were then heat treated between 30-100°C. The researches recorded the UV-vis spectra, the hydrodynamic diameter and the inherent viscosity of polyacrylonitrile solution in N,N-dimethyl formamide. The inherent viscosity measurement was done at a given temperature between 30-80°C and was repeated to trace the deviation from the initial viscosity value.

In a bid to verify the effect of aging on the physical properties of the molecules, the authors evaluated the dielectric constant of the polyacrylonitrile solution in N,N-dimethyl formamide.

The authors observed that the inherent viscosity increased slightly with time below 60°C but decreased above this temperature. However, the decrease was noticeable at 70-80°C and resulted from hydrodynamic volume reduction of the polyacrylonitrile coil. Aging at a high temperature produced a smaller hydrodynamic diameter of the coils. This means that aging was accompanied by a drop in the polymer coil dimension. Therefore, this coil shrinkage corresponded to the conformational change of the polyacrylonitrile chain.

The team found that polyacrylonitrile consisted two forms of chain conformation dictated by nitrile group orientation. The two chain conformation types included extended planar zigzag and contracted helix structures with parallel and anti-parallel oriented nitrile groups respectively. This was a clear indicator that the drop in hydrodynamic volume resulted from conformational change from a planar zigzag to helix structure.

Helix structure conformational change was observed to induce nitrile groups reorientation, which reduced their repulsion, and this was accompanied by dipole-dipole pairs development. This phenomenon was the principle cause of the solution aging. Therefore, the individual polyacrylonitrile coils developed stable conformations and equilibrium states in aged solutions than in fresh solutions. The authors therefore recommend analyzing conformational change in order to characterize phase change of polymer solutions in polar solvents.