Abstract
Significant research has been directed toward the incorporation of bioactive plant extracts or essential oils (EOs) into polymers to endow the latter with antimicrobial functionality. EOs offer a unique combination of having broad antimicrobial activity from a natural source, generally recognized as safe (GRAS) recognition in the US, and a volatile nature. However, their volatility also presents a major challenge in their incorporation into polymers by conventional high-temperature-processing techniques. Herein, antimicrobial polypropylene (PP) cast films were produced by incorporating carvacrol (a model EO) or carvacrol, loaded into halloysite nanotubes (HNTs), via melt compounding. We studied the composition-structure-property relationships in these systems, focusing on the effect of carvacrol on the composition of the films, the PP crystalline phase and its morphology and the films’ mechanical and antimicrobial properties. For the first time, molecular dynamics simulations were applied to reveal the complex interactions between the components of these carvacrol-containing systems. We show that strong molecular interactions between PP and carvacrol minimize the loss of this highly-volatile EO during high-temperature polymer processing, enabling semi-industrial scale production. The resulting films exhibit outstanding antimicrobial properties against model microorganisms (Escherichia coli and Alternaria alternata). The PP/(HNTs-carvacrol) nanocomposite films, containing the carvacrol-loaded HNTs, display a higher level of crystalline order, superior mechanical properties and prolonged release of carvacrol, in comparison to PP/carvacrol blends. These properties are ascribed to the role of HNTs in these nanocomposites and their effect on the PP matrix and retained carvacrol content.
Highlights
Colonization of microorganisms on polymeric surfaces is a major concern, as it may lead to severe contamination and biofilm formation
Micrographs of PP/carvacrol and PP/Halloysite nanotubes (HNTs) films are shown in Figure S1
The latter is further supported in the chapter, where negative energy of mixing for PP/HNTs is calculated by molecular dynamic simulations, implying good solubility of HNTs in the PP
Summary
Colonization of microorganisms on polymeric surfaces is a major concern, as it may lead to severe contamination and biofilm formation. Essential oils (EOs), which are natural and highly-effective antimicrobials against both bacteria and fungi, and are categorized as GRAS (generally recognized as safe) by the Food and Drug Administration (FDA) [8,9,10,11]. Their incorporation into polymers offers significant advantages, as they can be released as a vapor, exerting an antimicrobial effect, both on the polymer surface and in its proximity [12,13,14]. The incorporation of such low-molecular weight additives, which act as plasticizers, may profoundly affect the physico-mechanical properties of polymers [6]
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