Fourier Transform Infrared Studies of pH- and Temperature-Dependent Conformational Changes of Solid Poly(Methacrylic Acid)

Abstract

Fourier transform infrared (FT-IR) spectral measurements have been undertaken to investigate the conformational changes of poly(methacrylic acid) (PMA) cast from salt-free aqueous solutions. pH values, which markedly affect the overall conformation of the polymer, i.e., contracted coil at low pH (<4) and expanded coil at higher pH (>6), have a significant influence on the hydrogen bonding structure of the COOH side group. The COOH groups with different hydrogen bonding strengths and associated forms are ionized successively in the pH range of 3∼8, in accord with the development of conformational states. Ionization of the bonded COOH in the interior can be regarded as the onset of the overall conformational change (pH ∼ 5). The temperature-variable FT-IR spectra facilitate discrimination of the local conformational changes that are also brought about by pH changes. The interaction between the methyl groups is weakened by both the local conformational changes and the overall expansion of the chain, whereas the interaction between the COO− anions occurs significantly only during the transition. The present study shows how the hydrogen bonding force as a key player stabilizes the coiled conformation of PMA by using different forms of COOH groups. The importance of hydrophobic interaction between methyl groups in the conformational transition in the aqueous solutions is also elucidated, as continuing from the solid to solution state, the addition of the hydrophobic interaction gives rise to a sharper transition of the chain state.