Abstract
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).
Highlights
Active packaging systems are an innovative tool for food preservation consisting in packages that interact with the food and its environment playing a dynamic role to extend the food shelf life [1]
Antimicrobial active packaging systems release antimicrobial compounds, such as bacteriocins [2], [3], organic acids [4], [5], potassium sorbate [6], [7], or pimaricin [8], [9], that prevent the growth of spoilage microorganisms and food pathogens
In this work we study the physicochemical properties of PNIPA nanohydrogels with different hydrophilicity grades, synthesized by adding different concentrations of acrylic acid (AA) as comonomer
Summary
Active packaging systems are an innovative tool for food preservation consisting in packages that interact with the food and its environment playing a dynamic role to extend the food shelf life [1]. A proper delivery control is not possible and, for a particular system (food matrix + active packaging device + initial antimicrobial load in both packaging and food), the release rate will be a decay function of time. In these systems, preservatives are mainly released at the beginning of the storage period, which is the least opportune time, since food is still freshly packaged and the microbial load is low
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