Synthesis of Bio-Based Nanocomposites for Controlled Release of Antimicrobial Agents in Food Packaging

Open Access
Author:
Liu, Min
Graduate Program:
Food Science
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 04, 2014
Committee Members:
  • Gregory Ray Ziegler, Dissertation Advisor
  • John Floros, Dissertation Advisor
  • Evangelos Manias, Committee Member
  • Ramaswamy C Anantheswaran, Committee Member
  • John Coupland, Committee Member
Keywords:
  • polyhydroxyalkanoate
  • bio-based polymer
  • layered double hydroxide
  • nanocomposites
  • antimicrobial packaging
  • release study
Abstract:
The utilization of bio-based polymers as packaging materials has attracted great attention in both scientific and industrial areas due to the non-renewable and non-degradable nature of synthetic plastic packaging. Polyhydroxyalkanoate (PHA) is a bio-based polymer with excellent film-forming and coating properties, but exhibits brittleness, insufficient gas barrier properties, and poor thermal stability. Recently, reinforcement of such biopolymer matrices with nanoparticles was proven a promising option for improving their properties. Furthermore, modifying the nanoparticles with specific organic compounds may increase the compatibility between the bio-based polymer and nanoparticles resulting in better polymer/film properties. Antimicrobial agents (AM) incorporated into packaging materials could be used to prevent the growth of microorganisms on food surfaces and thus lead to a shelf life extension and better safety of foods. However, some properties of the AM packaging, such as mechanical, or barrier properties, may be sacrificed, because of the incorporation of antimicrobial agents and the migration and the release of AM cannot be easily predicted or controlled. The overall goal of the project was to develop the polyhydroxyalkanoate-based bio-nanocomposite films modified by antimicrobial agents with improved mechanical and gas barrier properties, along with a controlled release rate of antimicrobial agents for the inhibition of foodborne pathogens and fungi in food. The specific objectives were: a) to develop layered double hydroxide (LDH) nanoparticles modified and functionalized by antimicrobial agents (sodium benzoate, sodium gallate and potassium sorbate); b) to develop effective PHA-based LDH composite films with enhanced mechanical and barrier properties; c) to analyze and model the diffusion of antimicrobial agents through the PHA/LDH nanocomposites. The ability for antimicrobial agents to intercalate into layered double hydroxides depended on the nature of the antimicrobial agents, such as size, spatial structure, and polarity, etc. Benzoate and gallate anions were successfully intercalated into LDH in the present study and different amounts of benzoate anion were loaded into LDH under different reaction conditions. Incorporation of nanoparticles showed no significant effect on mechanical properties of polyhydroxybutyrate (PHB) films, however, significantly increased the tensile strength and elongation at break of polyhydroxybutyrate-co-valerate (PHBV) films. The effects of type and concentration of LDH nanoparticles (unmodified LDH and LDH modified by sodium benzoate and sodium gallate) on structure and properties of PHBV films were then studied. The arrangement of LDH in the bio-nanocomposite matrices ranged from exfoliated to phase-separated depending on the type and concentration of LDH nanoparticles. Intercalated or partially exfoliated structures were obtained using modified LDH, however, only phase-separated structures were formed using unmodified LDH. The mechanical (tensile strength and elongation at break) and thermo-mechanical (storage modulus) properties were significantly improved with low concentrations of nanoparticles incorporated into the polymer. The incorporation of LDH modified by sodium benzoate further improved the mechanical properties in comparison with unmodified LDH, which may be due to the increased compatibility between PHBV and nanoparticles and the larger basal distance between nanolayers after modification. The concentration of benzoate anions in LDH nanoparticles was another factor which affected the properties of PHBV composite films. The PHBV film with 2% modified LDH with 20.9 % w/w of benzoate anions in LDH had the best mechanical and thermo-mechanical properties. Apparent glass transition temperature increased with the addition of modified LDH but did not change with the addition of unmodified LDH. Moreover, the effect of nanoparticles on thermal properties as well as crystallization of PHBV composites was dependent on the type of nanoparticles. Unmodified LDH and LDH modified by sodium gallate had limited influence on thermal properties. However, LDH nanoparticles modified by sodium benzoate increased the crystallization temperature and enhanced recrystallization during melting, which may due to the nucleation effect by nanoparticles. The temperature at 10 % weight loss was decreased with the addition of nanoparticles, which indicated the reduction of thermal stability, but the residue left from the polymer at 300 °C was increased and the mass loss rate was decreased. Water vapor permeability was reduced with the increase of unmodified LDH due to the barrier effect of nanolayers providing a torturous pathway for small molecules to diffuse out of the film. Due to the hydrophilic nature of the modifiers, the water vapor permeability was increased with the addition of modified LDH. A comparison of mechanical properties and release kinetics of antimicrobial agents directly dispersed in PHBV and modified in LDH and then dispersed in PHBV was made. The results indicated that mechanical properties increased and release rate decreased in the latter case. The release of benzoate and gallate into DI water from PHBV composite films with LDH modified by benzoate and gallate followed pseudo-Fickian behavior fitted with a power law model. The release of benzoate from PHBV composite films with LDH modified by benzoate was also fitted with a Weibull model indicating Fickian behavior in fractal substrate morphologically similar to the percolation cluster. The concentration of modified LDH and the loading of benzoate in modified LDH showed a significant effect on the release kinetics of benzoate. The diffusivities of benzoate at 21 °C ranged from 3.41 to14.97 × 10-16 m2/s. The slowest release rate was achieved by the PHBV film containing 5 % w/w of modified LDH with medium loading of benzoate (21 % w/w of benzoate) in nanoparticles. The release of gallate from PHBV was much faster than that of benzoate. The effective diffusivity of benzoate increased with increase of temperature and the activation energy Ea for benzoate diffusion was calculated as 66.4 kJ/mol. It will be thus possible to design biodegradable polymeric nanocomposites with a tunable release of active molecules for various applications. All in all, the results obtained in this work showed that mechanical and barrier properties of PHBV were increased with very low concentration (2%) of unmodified LDH and LDH modified by antimicrobial agents, which may extend the application of this bio-based polymer. Moreover, these PHBV/modified-LDH nanocomposite films containing antimicrobial agents can release different amounts of antimicrobial agents at different release rates when parameters such as incorporation methods, the nature of the interaction between the intercalated antimicrobial agents and the LDH layers, the amount of the filler in the polymer, and the amount of antimicrobial agents in the LDH are taken into consideration.