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Temperature-sensitive poly(N-isopropylacrylamide) (PNIPA) nanohydrogels were synthesized by nanoemulsion polymerization in water-in-oil systems. Several cross-linking degrees and the incorporation of acrylic acid as comonomer at different concentrations were tested to produce nanohydrogels with a wide range of properties. The physicochemical properties of PNIPA nanohydrogels, and their relationship with the swelling-collapse behaviour, were studied to evaluate the suitability of PNIPA nanoparticles as smart delivery systems (for active packaging). The swelling-collapse transition was analyzed by the change in the optical properties of PNIPA nanohydrogels using ultraviolet-visible spectroscopy. The thermodynamic parameters associated with the nanohydrogels collapse were calculated using a mathematical approach based on the van't Hoff analysis, assuming a two-state equilibrium (swollen to collapsed). A mathematical model is proposed to predict both the thermally induced collapse, and the collapse induced by the simultaneous action of two factors (temperature and pH, or temperature and organic solvent concentration). Finally, van't Hoff analysis was compared with differential scanning calorimetry. The results obtained allow us to solve the problem of determining the molecular weight of the structural repeating unit in cross-linked NIPA polymers, which, as we show, can be estimated from the ratio of the molar heat capacity (obtained from the van't Hoff analysis) to the specific heat capacity (obtained from calorimetric measurements).

This paper reports on the interpolymer complex formation and polymer blends between poly(monomethyl itaconate) (PMMI) and two polymer acceptors poly(N,N-dimethylacrylamide) (PDMA) and poly(ethyloxazoline) (PEOX). We have found that the formation or inhibition of interpolymer complexes for these systems strongly depends upon the solvent medium. The stoichiometry of the complexes prepared from methanol solutions has been calculated from elemental analysis, resulting in a stoichiometry near a 1:1 ratio. Specific interactions of PMMI/PDMA and PMMI/PEOX complexes and blends have been characterized by FTIR. Strong hydrogen bonding for complexes and blends has been found. Finally, a calorimetric and thermogravimetric study of the complexes and blends has been performed in a wide temperature range.