(729b) Fabrication of Biodegradable Corn Zein Films with Varying Hydrophobic/Hydrophilic Balance Using Different Contact Surfaces and Treatment with SF6 Plasma

Zein, a corn prolamin protein, is vastly available as a byproduct of the ethanol industry and is used with plasticizers to create strong and flexible polymers for use in the packaging industry, coatings, as well as in the creation of biodegradable sensor platforms. Zein proteins are compromised with alpha helices formed by hydrophobic amino acids and are connected with glutamine beta turns that are hydrophilic[1]. Zein’s amphiphilic ability allows its surface chemistry to be specifically tailored and designed when protein solutions are cast on either hydrophilic or hydrophobic material, as the protein uses different parts of its molecule to interact with these surfaces. The films exhibit then different wettability properties due to the differences in chemistry. The surface chemistry of zein films can be modified using plasma treatment of the films, specifically oxygen plasma onto the material zein will be cast on. Zein-plasticizer solutions cast on oxygen plasma treated surfaces led to more hydrophilic films due to the orientation of the hydrophilic plasticizer to the surface of the zein composite film.

The objectives of this study were to 1) cast zein solutions on a variety of contact surfaces with different levels of hydrophilic/hydrophobic balance to analyze the ability of zein to mimic the wettability of the material it was in contact with. In addition to water contact angle measurements the distribution between dispersive and polar surface energies of zein films were analyzed using three different liquids’ contact angles with different levels of hydrophobicity/hydrophilicity on zein films with different levels of plasticizer and crosslinker in conjunction with the Van Oss method to estimate dispersive and polar surface energies. Objective 2) of this study explores the use of SF₆ plasma to fabricate super hydrophobic oleic acid plasticized zein films. The ability to fabricate protein polymers, from the same materials, with different surface properties expands the applicability of zein films.

Zein solutions were prepared using different amounts of plasticizer, oleic acid, and crosslinking agent, glutaraldehyde and were cast on polystyrene (PS), polydimethylsiloxane (PDMS), and polytetrafluoroethylene (PTFE) petri dishes as surfaces with different degrees of hydrophobicity. Films were cast and dried at controlled relative humidity in a desiccator. FTIR was used to study the chemistry of the bonds present in the zein films and an optical tensiometer (Biolin scientific) was used to measure contact angles of the three different liquids to calculate the surface energy of the zein films. Separately, zein formulations cast on PS were then treated with a plasma system (Harrick Plasma) using SF₆ plasma for 0, 1, 3, 5, 10, and 30 minutes. An Olympus optical microscope was used to evaluate the macrotopography of the films while atomic force microscopy (Asylum Cypher) evaluated nano-sized topography.

The chemistry and surface free energy of zein formulas, regardless of oleic acid or glutaraldehyde concentrations, was found to be significantly affected by the contact surface (PDMS; PS; PTFE) zein solutions were cast on. Zein films cast on PDMS had higher amide I band intensities and had higher total surface free energy values compared to those cast on PS and PTFE. The dispersive components of all films regardless of the material cast on were not significantly affected, however the polar component of the films cast on PDMS were significantly higher than those cast on PS and PTFE. This increase in the polar component results in the total surface free energy values of the films to increase. The amide I band (C=O of amino acid) increase in concentration for those films cast on PDMS could explain the polar component and the total surface energy increase. The glutamine rich (hydrophilic) residues seem to align themselves toward the surface of PDMS, more so than when cast on PS and PTFE.

When zein films were treated with SF₆ gas plasma system, the water contact angles increased in magnitude after 3 minutes of treatment and reached over 150° after 10 minutes of treatment. Extending plasma treatment to 30 minutes increased the contact angle to 160°. FTIR results showed that as plasma treatment increased, the amide bands decreased sharply while 1207cm^-1 increased and a new peak appeared at 1036cm^-1. The plasma treatment causes damage to the physical surface of the zein films and allows for the migration of oleic acid to the surface of the films, where then the oleic acid is both hydrogenated and esterified by the plasma creating a coating on the zein film surface. Esterification of the lipid is supported by the very intense 1207cm^-1 peak (ester bond vibrations). AFM showed that films roughness was much higher with microscale structures formed after 10 minutes and at 30 minutes. Since perfectly smooth hydrophobic surfaces can only reach a water contact angle of about 120°, the increase in roughness explains how the zein films reach contact angles of 160°[2]. Like the lotus leaf, the super-hydrophobic zein surfaces use a combination of chemical hydrophobicity through the lipid based film as well as surface roughness which causes possibly air to be trapped between the structures and under the droplet as in the case of the lotus leaf.

In conclusion, the surface energy and hydrophilic/hydrophobic balance of zein films is significantly affected by the contact surface the solutions are cast on, and can be used to modify at will the chemistry and wettability of the films to meet different applications' needs. This study also showed that plasma treatment of zein films helped create a super-hydrophobic surface. The two techniques form a toolbox to modify zein films in different ways to create the necessary hydrophilic/hydrophobic balance.