Surface Treatment of Bacterial Cellulose in Mild, Eco-Friendly Conditions
Bacterial cellulose (BC) with increased hydrophobicity is required for several applications including packaging. Surface functionalization of BC may provide good resistance to moisture, increased barrier properties or improved compatibility to polymer matrices. For this purpose, chemical grafting of BC in mild, eco-friendly conditions was carried out using different agents. BC membranes were surface functionalized with vinyl-triethoxy silane (VS) or 3-aminopropyl triethoxysilane (APS), by
... ion and acrylation. The efficiency of the surface treatments was highlighted by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, by contact angle measurements and by dynamic mechanical analysis. The morphological investigation by atomic force microscopy and scanning electron microscopy revealed an increased compactness for surface functionalized BC, which correlated well with the different increase of the contact angle. BC treated with APS and VS showed more than a twofold increase in contact angle value. Similarly, the crystallinity degree was reduced to 69.6% and 72.9% after APS and VS treatments as compared with 84.1% for untreated BC, confirming the grafting reaction and the decrease in hydrogen bonding. All the applied treatments delayed the degradation of BC. However, the highest increase in thermal stability was observed for silanes treated membranes. Effective, eco-friendly methods for improving the surface hydrophobicity of bacterial cellulose for food packaging were proposed in this study. shown that surface modified BC can also be a valuable material for other applications, such as food packaging    , ultrafiltration  or heavy metal ion removal  . Surface functionalization of BC membranes for increased hydrophobicity is needed for these applications, for example for moisture resistant packaging films with good barrier against water penetration  . Several attempts to increase BC surface hydrophobicity have been reported [5,       . Dried BC membranes were immersed in the colloidal suspension of flavonoid silymarin-zein nanoparticles and showed increased hydrophobicity and antimicrobial activity, being studied for food packaging  . Dried BC membranes were also functionalized by 3-aminopropyl triethoxysilane (APS) in hexane, and showed enhanced cell attachment and proliferation, being studied for wound dressing application  . Wet BC membranes were modified by APS in ethanol at room temperature, and it showed effective antibacterial and antifungal activities against Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Candida albican  . An extra-curing step at 110 • C was proposed for APS functionalized BC membranes to ensure the chemical grafting of bioactive moieties onto the surface of BC and to avoid the leaching of the biocidal agent  . BC was also surface functionalized for increasing the compatibility between hydrophilic BC and hydrophobic polymers, for the manufacture of bioplastics     . Thus, BC nanoribbons, released from the membrane by a wet mechanical process, were surface modified by organic acids with different length (acetic, hexanoic and dodecanoic acids)  or by acetylation using a solvent-free process  . Pristine BC membranes were also acetylated using a solvent-free process  or in toluene by solvent exchange  . Acetylation and grafting of aminoalkyl silanes are the most studied methods for compatibilization in polymer-BC composites or for improving the antimicrobial activity of bacterial cellulose. These treatments might be the choice in the case of BC membranes for food packaging too. BC is highly hydrophilic, similar to the cellulose fibers from plants and the functionalization of its surface is required for a better resistance to moisture, improved barrier properties and the preservation of these properties in contact with food. Although the physical and chemical modification of cellulosic films was carried out for distinct applications, the surface functionalization of BC for food packaging is still not studied in detail. The purpose of our work was to select a suitable treatment to increase the hydrophobicity of BC membranes for food packaging. To this aim, chemical grafting of BC under environmentally friendly conditions was carried out using different agents. Water or ethanol as "green" reaction medium and room or slightly higher temperature was chosen as eco-friendly conditions for these treatments. BC membranes were functionalized with two silanes, APS and vinyl-triethoxy silane (VS), by acylation and acrylation in water or ethanol/water mixture. Untreated and surface treated BC membranes were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and contact angle to emphasize the surface functionalization. X-ray diffraction (XRD) was used to detect the possible changes in crystallinity or structure and thermo-gravimetric analysis (TGA) to assess the influence of these treatments on the thermal stability of BC. A thorough study was carried out to highlight the morphological changes using atomic force microscopy (AFM), Peak force-Quantitative nanomechanical mapping (PFQNM) and scanning electron microscopy (SEM). Materials and Methods Materials BC membranes were used as received from the National Institute for Chemical Pharmaceutical Research and Development (Bucharest, Romania). Reagent grade 3-aminopropyl triethoxy silane (99%), vinyl-triethoxy silane (≥97%) and acrylic acid (99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Glacial acetic acid, acetic anhydride (≥98%), sulfuric acid (99%), and ethanol were procured from Chimreactiv (Bucharest, Romania). Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer (PHBV) with 2 wt.% hydroxyvalerate was purchased from Goodfellow (Cambridge Limited, Cambridge, UK). All the reagents were used without further purification.