Influence of tacticity on the structure formation of poly(methacrylic acid) in Langmuir/Langmuir-Blodgett and thin films

Abstract

A perfect isotactic poly(methacrylic acid) (it-PMAA) is synthesized and characterized by NMR spectroscopy. Surface pressure versus mean area per repeat unit (π−A) isotherms are recorded and compared with atactic poly(methacrylic acid) (at-PMAA). it-PMAA exhibits a π−A isotherm, indicating the formation of a stable Langmuir film. The isotherm has a characteristic pseudo-plateau at π of ≈11–14 mN m−1 attributed to the creation of worm-like entities confirmed by atomic force microscopy (AFM) in Langmuir–Blodgett (LB) films. These structures transform into spherical nanoparticles of ≈40 to 80 nm seen in LB films transferred at the end of the pseudo-plateau of the isotherm. Grazing incidence wide-angle X-ray scattering (GI-WAXS) shows a broad scattering signal (halo-like) at a q position of ≈11.12 nm−1, revealing the amorphous nature of the nanoparticles. An ordered morphology, however, is observed in thin films prepared by precipitating it-PMAA from dimethylformamide (DMF) solution under vigorous stirring and coating them on a solid support. Optical microscopy (OM), AFM, and GI-WAXS reveal more details about the structure. Finally, various structural modifications of PMAAs are explained based on their tacticity and the subsequent hydrogen bonding effects between the carboxylic acid groups.