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Development and analysis of responsive polymers as sensing elements for storage conditions and freshness of food
- Author / Creator
- Lopera Valle, Adrian
As the global population growth continues to increase the needs for higher production of food, governments, organizations, and individuals are faced with the task to implement strategies to mitigate the impact that food waste has in food security, the economy and the environment. One of the sources of this waste is directly linked to the lack of accurate information that retailers and end-consumers (individuals, hotels, restaurants, caterers, etc.) have about the storage conditions history and freshness food. In order to reduce this source of waste, tools such as active, intelligent, and smart packaging. Intelligent packaging can monitor food storage conditions and freshness, as one potential solution to mitigating waste.
Recently, responsive polymers are strong candidates to fabricate intelligent packaging given that they can show measurable changes in properties that can be used to monitor changes in storage conditions and freshness. Other advantages of responsive polymers include versatility, low weight, and inexpensive price, which make them attractive in intelligent packaging applications.
This thesis studies two particular responsive polymer systems that present potential to be used in packaging systems. These can provide information about humidity, as factor of correct packaging and handling of food, and about the concentration of amines, by-product of produce decomposition, as indicator of freshness.
The first polymer system is a conductive polymer composite (CPC), comprised of commercial silver (Ag) micro-particles that provides high conductivity to the films, and of a hygroscopic polymer matrix (poly(hydroxyethyl methacrylate)-co-poly(ethylene glycol diacrylate) (HEMA-co-PEGDA)) that responds to humidity by undergoing an increase in volume (through swelling). The electrical resistivity and swelling behavior of the CPCs are characterized, thin films or patterned lines are fabricated to form sensors, and the effects of relative humidity on the electrical resistivity of the composite films for different compositions and geometries are investigated. To characterize the material responsive behaviour, a data acquisition system was used to measure changes in resistance while the relative humidity was controlled in a humidity chamber. When the humidity was increased from 25% to 95% RH, the resistance of the films increased up to 650%. Composites with a higher fraction of HEMA in the matrix were found to have both lower recovery times and less hysteresis than composites in which the matrix was pure PEGDA. Such materials can form the basis of inexpensive, printable humidity sensors.
The second polymer system is a functional graft-polymer of poly(lactic acid) (PLA) and succinic anhydride (SAh) to engineer colorimetric indicators that respond to the presence of amine vapours. In this graft-polymer, PLA behaves as backbone for SAh, which responds to amines by undergoing a ring opening amidation reaction. This reaction can be reported through the inclusion of a pH responsive dye. Firstly, the grafting reaction was carried by means of free radical polymerization of PLA and maleic anhydride (MAh) in ratios of PLA:MAh from 90:10 wt% to 25:75 wt%, using AIBN as free radical initiator. The characterization of the grafting reaction and of the chemical-response to amines was carried out on solvent cast polymer films that were exposed to amine vapours. The response of the PLA-g-SAh polymers with different compositions to amines was characterized by studying their thermal properties by DSC before and after exposure to the vapours from a 400 ppm methylamine solution. To engineer colorimetric sensors, the PLA-g-SAh polymers with varying composition were mixed in solution with a pH-responsive dye and air-sprayed onto paper. Colorimetric indicators were exposed to concentrations of methylamine ranging from 50 to 600 ppm at temperatures from 5ºC to 22ºC.Visible changes in color were observed to be affected by both solution temperature and amine concentration. From this data, the kinetics of the color change reaction were studied, and rate constants were estimated. This work presents potential opportunities for the development of real-time amine sensors under different conditions of temperature and analyte concentration.
The development of responsive materials like those studied in this thesis have great potential in applications of intelligent packaging. Their further development can contribute to mitigate some of the sources of food waste that are linked to producers and consumers lacking knowledge of the storage conditions and freshness of food.
- Graduation date
- Spring 2020
- Type of Item
- Doctor of Philosophy
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