Steric hindrance affects interactions of poly(styrene–alt–DMHPMI) copolymer with strongly hydrogen-bond-accepting homopolymers

In this study, we synthesized the monomer N-(2,6-dimethylhydroxyphenyl)maleimide (DMHPMI) in three steps from 2,6-dimethylphenol and then reacted it with styrene through free radical copolymerization to prepare the alternating copolymer poly(styrene–alt–dimethylhydroxyphenylmaleimide) [poly(S–alt–DMHPMI)]. We used Fourier transform infrared (FTIR) spectroscopy, 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and mass-analyzed laser desorption ionization/time-of-flight (MALDI-TOF) mass spectrometry to confirm its structure. Differential scanning calorimetry revealed that blends of the alternating copolymers poly(S–alt–DMHPMI) and poly(styrene–alt–(para–hydroxyphenylmaleimide)) [poly(S–alt–pHPMI)] with the Strongly Hydrogen-Bond-Accepting homopolymers poly(4-vinylpyridine) (P4VP) and polyvinylpyrrolidone (PVP) each exhibited single-value glass transition temperatures (Tg) over the entire compositional range, indicative of full miscibility. Nevertheless, a negative deviation from the linear rule occurred for the Tg behavior of the PS–alt–PHPMI/P4VP blends, due to the weak acidity of the HPMI units, while the intermolecular interactions of the PS–alt–PHPMI coplymers were also inhibited because of strong self-association of the PHPMI units. FTIR spectral analyses of the ratio of hydrogen-bonded OH and pyridyl groups confirmed this behavior. In contrast, slightly positive deviations from the linear rule occurred for the Tg behavior when blending poly(S–alt–DMHPMI) or poly(S–alt–pHPMI) with the PVP homopolymer; here, the intermolecular interactions of the poly(S–alt–DMHPMI)/PVP blends were weaker than those of the poly(S–alt–pHPMI)/PVP blends, due to the steric hindrance of the DMHPMI units being greater than that of the pHPMI units.