Abstract
A series of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAAm-co-PAA) random copolymers were synthesized through free radical copolymerization in MeOH. The incorporation of the acrylic acid units into PNIPAAm tended to enhance the glass transition temperature (Tg), due to strong intermolecular hydrogen bonding between the amide groups of PNIPAAm and the carboxyl groups of PAA, as observed using 1H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopic analyses. The lower critical solution temperature (LCST) increased upon increasing the pH of the aqueous solution containing PNIPAAm-co-PAA because the COOH groups of the PAA segment dissociated into COO− groups, enhancing the solubility of the copolymer. In addition, high-pressure differential scanning calorimetry revealed that the LCSTs of all the aqueous solutions of the copolymers decreased upon increasing the pressure of CO2, suggesting that CO2 molecules had displaced H2O molecules around the polar CONH and COOH groups in PNIPAAm-co-PAA, thereby promoting the hydrophobicity of the copolymers in the aqueous solution. In addition, the values of Tg of a film sample increased upon treatment with supercritical CO2, implying that intermolecular interactions in the copolymer had been enhanced after such treatment.
Highlights
Increasing the glass transition temperatures (Tg ) of polymeric materials is interesting in polymer science due to the strong economic incentives arising from their potential industrial applications [1,2,3,4].In general, values of Tg are strongly dependent on the chemical and physical nature of polymeric materials, including their molecular weights, numbers of bulky groups, degrees of branching, degrees of crosslinking, and strength of intermolecular interactions [5,6,7,8,9]
In previous studies [10,11,12,13], we found that copolymerization of two monomers possessing strong hydrogen bonding donor or acceptor units can lead to significant enhancements in glass transition temperatures, due to so-called “compositional heterogeneity” [14]
We synthesized a series of PNIPAAm-co-Poly(acrylic acid) (PAA) random copolymers through free radical
Summary
Increasing the glass transition temperatures (Tg ) of polymeric materials is interesting in polymer science due to the strong economic incentives arising from their potential industrial applications [1,2,3,4]. Combinations of PNIPAAm and PAA in random copolymers result in dual stimuli-responsive behavior (temperature and pH) in aqueous solution [20,21,22] and significant increases in values of Tg in response to strong intermolecular hydrogen bonding in the bulk state. PNIAAm-co-PAA random copolymers have been studied previously as superhydrophobic surfaces, hydrogels, nanoparticles, and microgels for drug delivery [25,26,27,28], to the best of our knowledge their thermal properties, hydrogen bonding, and LCST behavior under CO2 atmosphere or after CO2 treatment have not been investigated. The copolymer compositions, hydrogen bonding interactions, thermal properties, and LCST behavior of PNIAAm-co-PAA random copolymers were characterized in this study using 1 H nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and high-pressure differential scanning calorimetry (HP-DSC) analyses
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