Variation of physical and mechanical properties in the bicomponent hydrogels of melamine with positional isomers of hydroxybenzoic acid

The positional isomers of hydroxybenzoic acid have a significant influence on the physical and mechanical properties of their bicomponent hydrogels with melamine (M) produced at 1 : 1 molar composition. In the bicomponent hydrogels of salicylic acid (s), meta-hydroxy benzoic acid (m) and para-hdroxy benzoic acid (p), a gradation of morphology of the xerogels from tape to fibre occurs. WAXS study suggests different structures of the gels. FTIR spectra indicate that H-bonded complexes are produced in the gels. The aromatic protons in the gels become upfielded suggesting π–π stacking that follows the order Mp > Ms ≈ Mm. The UV-vis spectrum indicates better H-type aggregate formation in Mp than those in Ms and Mm systems. The photoluminescence (PL) intensity of the gels has increased by 200, 25 and 350 times those of pure acids for Ms, Mm and Mp gels, respectively. With the increase of temperature the PL intensity decreases and at 30 °C the highest PL intensity is observed at pH 6.8 but at pH 4 and 9.2 PL the intensity is drastically reduced due to breaking of the gel network. The melting point values suggest that the thermal stability of the gels follow the order Mp > Ms > Mm and both storage modulus (G′) and critical strain also follow the same order. In G′ vs. temperature plots, the Mp system exhibits a linear variation whereas the Ms and Mm systems exhibit peaks at 82 and 63 °C, respectively. This is attributed to the disaggregation of the gel macromolecule at high temperature forming a microgel and shear assisted aggregates.