Antimicrobial Performance of PBAT Nanocomposites and Composites Incorporating Copper, Cu/Cu₂O Nanoparticles, and Copper Sulfate

Introduction

Traditional plastics, derived from fossil fuels, are largely non‑degradable and pose significant environmental challenges. In response, the scientific community has intensified efforts to develop biodegradable polymers that can replace petroleum‑based materials while offering enhanced functional properties through nanotechnology. Bionanocomposites—organic matrices dispersed with inorganic nanomaterials—exhibit superior optical, thermal, mechanical, magnetic, and optoelectronic characteristics due to the high surface area and reactivity of the nanoscale fillers.

Poly(butylene adipate‑co‑terephthalate) (PBAT) is a linear aliphatic polyester that closely resembles low‑density polyethylene in flexibility and processability but suffers from limited mechanical strength. Incorporating nanoscale fillers can address this weakness and endow PBAT with multifunctional attributes, including improved thermomechanical performance.

Controlling microbial colonization is a critical requirement for biomedical devices such as catheters and endotracheal tubes. Embedding antibacterial nanoparticles within a polymer matrix can synergistically enhance the material’s intrinsic antimicrobial properties, potentially preventing device‑associated infections. Despite PBAT’s promising mechanical and biodegradability profiles, its application in medical devices remains underexplored.

Metal nanoparticles, notably copper, exhibit intrinsic antibacterial activity that depends on particle size, morphology, and surface chemistry. Copper’s efficacy against a broad spectrum of pathogens—including drug‑resistant strains—has been demonstrated, yet systematic studies comparing Cu, Cu/Cu₂O, and CuSO₄ in PBAT are scarce. This study evaluates the antimicrobial performance of PBAT nanocomposites containing Cu‑NPs, Cu|Cu₂O‑NPs, and a CuSO₄‑based composite, along with comprehensive analyses of their structural, mechanical, and thermal properties.

Materials and Methods

Materials

PBAT (Ecoflex, BASF, Ludwigshafen, Germany) served as the polymer matrix. Commercial Cu‑NPs (99.99 % purity, 100–200 nm, Sigma‑Aldrich, St. Louis, MO) were used directly. Cu|Cu₂O‑NPs were synthesized from CuSO₄·5H₂O, ascorbic acid, and NaOH via a chemical reduction protocol. CuSO₄ (Sigma‑Aldrich) was also used to prepare the composite material.

Synthesis of Nanoparticles by Chemical Reduction

Following Khan et al., 0.1 M CuSO₄·5H₂O was dissolved in water (120 mL). The solution was heated to 80 °C in a propylene glycol bath while 50 mL of 10 mM ascorbic acid was added. After 30 min of stirring at ~390 rpm, 30 mL of 1 M NaOH was introduced dropwise. The mixture was stirred for an additional 2 h, then cooled, centrifuged, washed with water and ethanol, and sonicated. The final product was dried at 60 °C overnight.

Nanocomposite Synthesis

PBAT was melted at 140 °C, and 1, 3, or 5 wt % of Cu‑NPs, Cu|Cu₂O‑NPs, or CuSO₄ was added. The blend was homogenized in a torque rheometer (model 835205, Brabender) at 60 rpm for 7 min. Loads above 5 wt % led to fluorescence artefacts in Raman spectra, so this limit was imposed.

Characterization

Structural, mechanical, thermal, and antibacterial properties were examined. Cu‑NPs and Cu|Cu₂O‑NPs were analyzed by XRD and TEM. PBAT nanocomposites and the CuSO₄ composite were evaluated via TGA, DSC, SEM, FTIR, XRD, tensile testing, and agar‑diffusion antibacterial assays. Sample plates (100 × 100 × 1 mm) were produced by compression molding (160 °C, 110 bar, 5 min) in a Labtech hydraulic press.

Morphological and Structural Properties

Cu|Cu₂O‑NPs displayed a mixture of spherical Cu (≈ 40 nm) and polyhedral Cu₂O (≈ 150 nm) particles. XRD confirmed Cu (111, 200, 220) and Cu₂O (111, 200, 220) reflections. Cu‑NPs were spherical (100–200 nm) with XRD peaks matching fcc Cu (JCPDS 85‑1326). The CuSO₄ composite did not alter the PBAT diffraction pattern, though low‑level Cu and Cu₂O peaks appeared at higher loadings.

Mechanical Properties (Tensile Test)

ASTM D638 tensile tests (50 mm/min, 1 kN load cell) were performed on five specimens per formulation. Results indicate that Cu‑NPs slightly increased tensile strength and decreased elongation at higher loadings, while Cu|Cu₂O‑NPs and CuSO₄ composites showed minimal changes, preserving PBAT’s inherent ductility.

Thermal Properties

TGA (20 °C–600 °C, 10 °C/min, N₂) revealed that Cu‑NPs raised the initial decomposition temperature (T_di) by 3–5 °C and slightly lowered the final decomposition temperature (T_df) compared to neat PBAT. Cu|Cu₂O‑NPs and CuSO₄ composites produced similar trends but with marginally lower T_df. DSC (25–200 °C, 10 °C/min, N₂) showed a modest increase in crystallization temperature and a slight decrease in melting temperature, reflecting the influence of nanofillers on crystallite nucleation.

Antimicrobial Activity Assays of the NCs and MC Using Agar Diffusion

Four bacterial strains were tested: Enterococcus faecalis (oral isolate), Streptococcus mutans (ATCC 25175), Staphylococcus aureus (ATCC), and Acinetobacter baumannii (clinical isolate ABA 538). A qualitative diffusion test identified 3 wt % as the optimal loading for further quantitative analysis. Quantitative assays were performed in a biosafety cabinet, measuring colony‑forming units (CFU/mL) at 2, 4, 6, and 8 h. Results demonstrated significant bacterial inhibition by CuSO₄ composites, especially against the resistant A. baumannii (complete eradication by 8 h). Cu|Cu₂O‑NPs effectively suppressed E. faecalis and S. mutans, while Cu‑NPs were most potent against S. aureus.

Results and Discussion

Rheometric analysis confirmed that 7 min of melt mixing achieved equilibrium torque, indicating complete dispersion of the nanofillers. The presence of Cu‑NPs reduced the torque by ~10 %, suggesting improved processability. Cu|Cu₂O‑NPs and CuSO₄ loads had negligible impact on torque, likely due to their larger particle size.

XRD patterns of the nanocomposites show that the Cu and Cu₂O peaks grow in intensity with increasing filler content, while PBAT’s crystalline peaks diminish slightly, indicating a modest reduction in overall crystallinity. The calculated crystallinity index (X_c = I_c/(I_c + I_a)) increased for Cu‑NPs and Cu|Cu₂O‑NPs but decreased for CuSO₄ composites, consistent with the observed thermal behavior.

FTIR spectra revealed unchanged functional‑group frequencies across all formulations, confirming that no chemical interactions occurred between PBAT chains and the inorganic fillers.

Tensile testing revealed that Cu‑NPs raised ultimate tensile strength from 19.2 MPa (neat PBAT) to 20.4 MPa (5 wt %) while reducing elongation at break from 35 % to 28 %. Cu|Cu₂O‑NPs and CuSO₄ composites maintained tensile strengths within ±2 % of neat PBAT, indicating that the mechanical integrity of PBAT is preserved even at 5 wt % loading.

TGA results demonstrate a slight enhancement in thermal stability (ΔT_di ≈ +4 °C) for all composites, with Cu‑NPs yielding the highest improvement. Cu|Cu₂O‑NPs and CuSO₄ composites showed similar but less pronounced effects. These observations are consistent with the barrier role of inorganic particles, which retard mass loss by hindering heat transfer.

The antibacterial assays underscore the superiority of CuSO₄ composites, achieving complete inhibition of A. baumannii, E. faecalis, and S. mutans within 6 h and S. aureus within 4 h. Cu|Cu₂O‑NPs were highly effective against S. aureus and E. faecalis, while Cu‑NPs excelled against S. aureus. These differences reflect the varied ion‑release profiles and surface chemistries of the three copper‑based additives.

Conclusions

The study demonstrates that PBAT can be functionalized with copper‑based nanoparticles or copper sulfate to produce biocompatible, antibacterial composites without compromising mechanical or thermal performance. CuSO₄ composites exhibited the most potent antimicrobial activity, including against resistant strains, making them attractive candidates for biomedical device coatings. Future work should focus on in‑vivo testing, long‑term biocompatibility, and scaling up production for commercial applications.

Abbreviations

Cu|Cu₂O‑NPs:

Copper/cuprous‑oxide nanoparticles

Cu‑NPs:

Copper nanoparticles

CuSO₄:

Copper sulfate

DSC:

Differential scanning calorimetry

FTIR:

Fourier transform infrared spectroscopy

MC:

Composite material

MCs‑PBAT/CuSO₄:

Composite materials of poly(butylene adipate‑co‑terephthalate) with copper sulfate

NCs:

Nanocomposites

NCs‑PBAT/Cu:

Nanocomposites of poly(butylene adipate‑co‑terephthalate) with copper nanoparticles

NCs‑PBAT/Cu|Cu₂O:

Nanocomposites of poly(butylene adipate‑co‑terephthalate) with copper/cuprous‑oxide nanoparticles

PBAT:

Poly(butylene adipate‑co‑terephthalate)

TEM:

Transmission electron microscopy

TGA:

Thermogravimetric analysis

XRD:

X‑ray diffraction