Composite Films Based on Linear Polyethyleneimine Polymer and Starch or Polysaccharides from DDGS: Synthesis, Characterization, and Antimicrobial Studies

Abstract

Different films were synthesized from starch or polysaccharides extracted from distillers dried grains with soluble (DDGS) in combination with different percentages of linear polyethyleneimine (PEI) hydrochloride polymer to assess the mechanical and antimicrobial properties of the resulting composites. Moreover, a simple method for the extraction of the polysaccharide content from DDGS is reported. The materials obtained were characterized by ATR-FTIR, NMR, and XPS spectroscopy, swelling capacity, and by organic elemental analysis. In particular, the stability of the film prepared with only DDGS in copper ion solutions was improved by the incorporation of PEI. ¹³C HRMAS NMR studies evidenced the incorporation of the PEI polymer in the new films. Moreover, the release of PEI molecules from the films was studied by ¹H NMR experiments in D₂O to explain the antimicrobial properties of the PEI-based films against Staphylococcus aureus, with the DDGS-10% PEI films being the most active surface. Furthermore, the incorporation of copper ions into the different films enhanced their antimicrobial activity. Additionally, the starch-10% PEI film exhibited good swelling capacity in deionized water (~1500%), which decreased with the addition of salts (~250%). Instead, the DDGS-10% PEI film showed low swelling capacity in deionized water (~80%), with this capacity increasing with the addition of salts (~250%). The mechanical properties of the films improved considerably when 3% PEI was used.

Keywords: DDGS; NMR; XPS; antimicrobial materials; films; mechanical properties; polyethyleneimine; starch.

Figure 1

¹³C CP-MAS spectra (MAS rate: 15 kHz) for cow horn keratin (A), starch (B), native DDGS (C), DDGS-W (D), and DDGS-P samples (E).

Figure 2

ATR-FTIR spectra for the films: starch (A), starch–3% PEI (B), starch–10% PEI (C), pristine linear PEI.HCl polymer (D), glycerol (E), DDGS (F), DDGS–3% PEI (G), and DDGS–10% PEI (H). The strong hydrogen bond interactions in the PEI.HCl polymer and the films are highlighted (2900–2300 cm⁻¹).

Figure 3

¹³C HRMAS NMR spectra (MAS rate: 4 kHz) for the different films obtained from starch (A), starch–3% PEI (B) or starch–10% PEI (C), DDGS–3% PEI (E) and DDGS–10% PEI swollen in D₂O (F). The ¹³C NMR spectrum for PEI.HCl dissolved in D₂O (D) is shown for comparison with those of the materials. The ¹³C HRMAS NMR experiments were acquired with direct-polarization techniques with ¹H high-power decoupling during acquisition.

Figure 4

High resolution C 1s (A,B), O 1s (C,D), and N 1s (E,F) core level spectra for the indicated films and materials.

Figure 5

Mechanical behavior for starch, starch–PEI, and DDGS–PEI films with different PEI percentages.

Figure 6

SEM images for DDGS–3% PEI (A) or DDGS–10% PEI (B), starch (C), and starch–3% PEI (D) or starch–10% PEI (E) films.

Figure 7

Swelling studies for the starch–3% PEI (A), starch–10% PEI (B), DDGS–3% PEI (C), and DDGS–10% PEI films (D). Error bars in the figures indicate the experimental standard error ± standard deviation.

Figure 8

Inhibition halo in a solid medium against S. aureus for the indicated films. The ilms were based on DDGS (left) or starch (right). The materials indicated with an asterisk induced contact inhibition. Error bars in the figures indicate the experimental standard error ± standard deviation.

Figure 9

¹H NMR spectra of the D₂O solution obtained after the incubation of the starch–3% PEI (A), starch–10% PEI (B), DDGS–3% PEI (C), and DDGS–10% PEI (D) films after contact with 600 μL of D₂O.