Effect of Compositions and Heat Treatments of Polypropylene/PP-g- MAH/CuO-TiO2 composites on Thermal, Crystallization and Antimicrobial Properties

doi:10.21203/rs.3.rs-3988183/v1

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Effect of Compositions and Heat Treatments of Polypropylene/PP-g- MAH/CuO-TiO2 composites on Thermal, Crystallization and Antimicrobial Properties

https://doi.org/10.21203/rs.3.rs-3988183/v1

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The leading cause of increased mortality rates is infections from implanted medical devices, with catheters accounting for more than 80% of these infections. Polypropylene (PP) composites with antimicrobial properties were developed by adding binary mixed oxide (CuO-TiO₂). The outcomes demonstrated that the spreading and encapsulation of CuO-TiO₂ particles in the PP matrix was much better with incorporation of PP-g-MAH compatibilizer. Matrix crystallinity is affected by addition of compatibilizers, the amount of CuO-TiO₂, and heat treatments. The synergy effect of CuO-TiO₂ as antimicrobial agents was analyzed. The antibacterial efficacy's reliance on matrix crystallinity is elucidated in relation with various heat treatments, PP-g-MAH compatibilizer, and amount of CuO-TiO₂. PP made of binary mixed oxides (e.g., CuO and TiO₂) and 3 wt% PP-g-MAH that was processed with a low degree of crystallinity increased the material’s capability of effectively rendering plausible antimicrobial species (e.g., •O²⁻, •OH⁻, and Cu²⁺) with excellent antimicrobial efficacy towards Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). This innovative composite, CuO-TiO₂-PP, offers new perspectives on managing bloodstream infections associated with catheter use.

PP-g-MAH compatibilizer

Copper oxide

Titanium dioxide

Polypropylene polymer

Heat treatments

Crystallinity

In the United States, infections from implanted health care devices are the primary reason of morbidity and death rates; catheters are responsible for over 80% of these infections. In common, Gram-positive, and Gram-negative cause catheter-related diseases within the circulation system. As such, preventive measures must be attempted. Catheters can exhibit antimicrobial properties in several ways, such as incorporating organic and inorganic biocides nanoparticles (NPs) into catheter polymers [1, 2], coating biocides NPs on the polymer [3], biocidal agents immobilization in polymer [4], and the employments of inherently microbial polymers [5]. Inorganic metal/metal oxide compounds have emerged in research due to their advantage as antimicrobial agents.

Inorganic biocides are strong disinfectants because of their excellent cytocompatibility, physicochemical stability, and remarkable inhibitory effect towards divergent strains (Table 1). The dominant factors affecting the antibacterial effectiveness of oxides are size, type, concentration or amount incorporated in the polypropylene (PP) matrix and the processing method of the composite. Selecting an appropriate method for processing metal/metal oxides is necessary to produce polymer composites that possess homogeneous dispersion and a low degree of crystallinity. These desired features are essential to attack various microbes. Reports have focused on single microbial agents and on the antimicrobial efficacy of the particle itself rather than that of the polymer composite. Previously, there has not been a single research work investigating the impact of heat treatment and crystallinity on the antimicrobial activity of PP derived from binary mixed oxides, such as CuO and TiO₂. Intrinsically, this work emphasizes the fabrication of CuO-TiO₂-PP composites and explores the biocidal activity of these composites. One possible killing mechanism for antimicrobial efficacy is the generation of metal oxide ions and species of oxygen radicals (ROS) to attack the microbes [17]. Therefore, the synergistic effect of CuO-TiO₂ embedded PP matrix was investigated using Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The dependence of the microbiological efficacy on the matrix crystallinity as well as the relationship with the incorporation of compatibilizer, heat treatments, and concentration of antimicrobial agents (CuO-TiO₂) are elucidated.

Table 1

Effective antimicrobial metal/metal oxides for selected microbes.
Microbe	Type of NPs in various substrates	Exposure time (h)	Efficacy	Refs
Escherichia coli	CuO	24	20 mm inhibition zone	[6]
	CuO in PP	4	> 99.9% (< 100 nm p.s)	[7]
	CuO in PVC	24	> 80% (1 wt% CuO)	[8]
	Cu in PP	4	> 90% (< 100 nm p.s.)	[7]
	Cu in PE	1	> 99.9% (3 wt% Cu)	[9]
	CuO/TiO₂ in PVAc	4	-	[10]
	TiO₂/Sn on glass substrate	5.3	> 99% (9.52 nm p.s.)	[11]
	TiO₂/Ag	24	> 99.9% (1.6 µg/ml)	[12]
	Cu₂O-TiO₂/rGO in PVA	24	8.75 mm inhibition zone	[13]
	ZnO/CuO in Cs	3	> 99.9% ((CFU/mL)	[14]
Staphylococcus aureus	CuO	4	> 99% (1000µg/ml)	[15]
	CuO	24	22 mm inhibition zone	[6]
	CuO/TiO₂ in PVAc	4	-	[10]
	Cu in PE	4	> 99.9% (3 wt% Cu)	[9]
	TiO₂/Sn on glass substrate	6	> 99% (9.62 nm p.s.)	[11]
	Cu₂O-TiO₂/rGO in PVA	24	10 mm inhibition zone	[13]
	TiO₂/Ag on Si catheter	1.5	> 90% (10 nm p.s.)	[16]
	Cu-Doped Anatase TiO₂ on glass	4	> 99.9% (CFU/mL)	[17]
	ZnO/CuO in Cs	24	> 99.9% (CFU/mL)	[14]
	Cu/N-doped TiO₂ on glass	24	-	[18]
	Copper-doped TiO₂ in CMC	-	-	[19]
	CuO/TiO₂ in HA	-	-	[20]

2.1. Materials and fabrication of PP composites

Mixtures of commercial binary metal oxide powders were prepared by grinding [1 wt% CuO powder (particle size, < 5 µm; Sigma Aldrich) + 0.1 wt% TiO₂ (particle size, 25 nm; Sigma Aldrich) + isopropanol] by using an agate mortar and subsequently dried at 100°C. PP (melt flow rate, 10 g/10 min at 230°C; density, 907 kg/m³) together with binary mixed oxides were blended in an internal compounding machine at 50 rpm for 10 min. Two different sequences were used for the integration of the PP-g-MAH compatibilizer (Fusabond 613; HH Saintifik Enterprise) and the processing temperature was set at 185°C. The first sequence was denoted as (m0-PP, m5-MO, m7-C) and the second sequence was denoted as (m0-PP + C, m5-MO). For the first sequence, the compatibilizer was added toward the final stage of mixing for 7 min, whereas the compatibilizer was added together with the PP for the second sequence. After 10 min, the mixture from the internal mixer was discharged. The CuO-TiO₂-PP composites were then crushed into small pieces. Rectangular specimens were created by compressing and molding the small CuO-TiO₂-PP composite pieces (dimensions,150 mm × 120 mm × 1 mm) via Go Tech compression molding equipment. The hot press event comprised preheating the charge of the binary mixed oxide PP (CuO-TiO₂-PP) composites for 6 min at the similar temperature as the melt mixing process, and then compressing the material for 3 min at 1000 psi. After 3 mins of flushing the press with cold water, the hot films of the CuO-TiO₂-PP composites were cooled at the same pressure. Next, using a Wallace die cutter, the molded samples were cut into dumbbell shapes. The compressed sheets were cut into circular shapes (diameter, 0.5 in) by using a puncher. The development of binary mixed oxide PP (CuO-TiO₂-PP) composites is depicted in the schematic diagram (Fig. 1). Different cooling rates were used to create four distinct levels of crystallinity in the sample. The processing temperature for this group of quenched samples was 175°C. Slow cooling overnight, cooling via water circulation in the press, quenching in warm water at 30°C, and quenching in liquid nitrogen are indicated by the symbols SCON, CP, QWW, and QLN, respectively. PP composites (0, 1, 3, 5, 7, and 10 wt% CuO-TiO₂) were prepared at 175°C for 10 min at a speed of 50 rpm to evaluate the antimicrobial efficacy against Gram-positive E. coli. In subsequent, a selected composition was used to temporally study the antimicrobial efficacy of CuO-TiO₂-PP towards E. coli and S. aureus for up to 4 days.

2.2. Characterization

2.2.1. Morphological examination

The microstructure of the commercial CuO-TiO₂ particles and CuO-TiO₂-PP composites were assessed using a field emission SEM (SUPRA 35VP ZEISS). The composite samples' cryogenic fracture surfaces were covered in gold before they underwent microstructure analysis. Semi-quantitative analysis of commercial CuO-TiO₂ particles was executed by energy-dispersive X-ray spectroscopy.

2.2.2. X-ray diffraction analysis

The XRD Bruker D8 powder diffractometer was utilized to investigate the crystal phases of the CuO-TiO₂-PP composites. This study was carried out at 40 kV and 30 mA under reflection mode of Cu Kα radiation at 1.5406 Å wavelengths. Using step size of 0.030°, the composite samples were examined at intervals of 2θ between 10° and 90°.

2.2.3. Thermal analysis

Perkin Elmer DSC 6 was employed to examine the composite's melting and crystallization behavior. Each CuO-TiO₂-PP sample, which weighed roughly 10–15 mg, was heated at a rate of 20°C per min to 200°C in an inert N₂ atmosphere with flow rate of 50 ml/min. For the composite, the enthalpy of fusion (∆H_f) and the melting temperature (Tm) were measured. Based on ∆H, the relative crystallinity was computed, and 209 J/g was the parameter value for 100% crystalline PP [21]. Eq. 1 [22] was utilized to compute the relative crystallinity.

\(\left(1-\lambda \right)\%= \frac{\varDelta {H}_{f}}{\varDelta {H}_{f100\% }^{^\circ }.w}x 100\) Eq. 1

w = weight fraction of the filler or matrix in blends

∆H _f = apparent enthalpy of melting of the filler or matrix

∆H ^o _f100%= extrapolated value of the enthalpy corresponding to the melting of 100% crystalline sample (209 J/g for PP)

2.2.4. Microbiological testing

The biological activities of CuO-TiO₂-PP composites were determined using the strain of S. aureus (ATCC 25923) and E. coli (ATCC 25922); the control sample for this experiment was PP alone. Mueller-Hinton broth was employed to cultivate the bacteria, and they were kept at 37°C and spinning at 100 rpm for 18 h in shaking incubator. To evaluate the bactericidal effect of CuO-TiO₂-PP, circular test discs (diameter, 0.5 in) were carefully set in sterilized 24-well plates at aseptic conditions. Next, each test disc in the well-plate was pipetted with 1.5 ml of the bacteria solution, which had been adjusted to 1.5×10⁸ CFU/ml. In an orbital shaking incubator, plates containing the test disc containing the inoculated strains were incubated for 24, 36, 72, and 96 h at 37°C and 100 rpm. After the respective incubations, the circular test disks from 24-well plates were taken using sterile forceps and rinsed thrice with PBS. To get rid of bacteria’s cells that do not adhere to surfaces, the circular test discs were aseptically inserted into freshly sterilized 24-well plates. After that, 1.5 ml of PBS was pipetted into each test disc sample in the well plate. For 40 min, plates with the circular test disc and PBS were shaken in an incubator at 400 rpm and 37°C. Following the incubation period, 100 µl of the solution was extracted at every test sampling, and then plated onto Mueller Hinton-prepared agar to recover unharmed microbes. For every sample, three duplicate plates were utilized. After being incubated for 24 h at 37°C, the quantity of colony-forming units (CFU/ml) was ascertained.

3.1. Morphological exploration

3.1.1. SEM morphology of CuO and TiO₂ particles

Figure 2a and 2b shows the commercial titanium dioxide powder and copper (II) oxide particles. Figure 2c shows the binary mixed oxide CuO-TiO₂ particles which is used to prepare CuO-TiO₂-PP composite. The FESEM image shows the CuO particles are flaky in shape with size of 5 µm. In contrast, TiO₂ powders are very fine and look like spherical in shape with agglomerated morphology. The size is about 25 nm. Upon mixing, the possibility of dispersing both particles homogeneously within the polymer matrix is a crucial task. The EDS profile of mixed oxide CuO-TiO₂ particles (Fig. 2d) indicates that the sample contains copper, titanium, and oxygen elements.

3.1.2. SEM morphology of CuO-TiO₂-PP composite with compatibilizer

Figure 3a displays the representative SEM microstructural images of the CuO-TiO₂-PP composite that was prepared with ~ 1 wt% CuO-TiO₂ without compatibilizer. The temperature of mixing was fixed to 185^oC and rotor speed was 50 rpm. Fine dispersion of CuO-TiO₂ was observed in specific regions of the PP matrix; other sites were still covered with agglomerated CuO-TiO₂. CuO-TiO₂ appeared as discrete particles with poor interfacial adhesion within the PP matrix [21, 23, 24]. Particle detachment was noticeable on the fractured surface. The number of free voids or pores increased on tensile fractured surfaces of CuO-TiO₂-PP composites. The tiny pores within the PP matrix permit the entry of water molecules into CuO-TiO₂-PP composites [23, 25, 26], which enhances the migration of ions and increased the material’s antibacterial activity. However, the voids within the PP matrix may result in poor mechanical properties of CuO-TiO₂-PP composites. These voids also imply poor interfacial bonding/connection between the binary CuO-TiO₂ and the semi crystalline PP. The hydrophilic nature of binary mixed oxide particles and their poor compatibility with the hydrophobic PP matrix are the contributing factors for the weak interfacial bonding.

Figure 3b-c displays the representative morphological SEM images of CuO-TiO₂-PP composites produced with 3 wt% PP-g-MAH by using two different sequences. For the first sequence, a compatibilizer was added towards the final mixing stage (7 min); the sequence was denoted as (m0-PP, m5-MO, m7-C). For the 2nd sequence, the compatibilizer was added into the mixing chamber along with the PP; the sequence was denoted as (m0-PP + C, m5-MO). CuO-TiO₂ was uniformly distributed without large agglomerations. Not many voids were discovered on the fractured surface of CuO-TiO₂-PP composites containing the compatibilizer. CuO-TiO₂ also appeared in discrete form because of the short mixing time between PP and binary mixed oxide and PP-g-MAH.

Improved dispersion, encapsulation (inset in Fig. 3c), and interfacial adhesion of CuO-TiO₂ within the PP matrix were obtained when the compatibilizer was introduced in the early mixing stage (0 min). The compatibilizer, which encouraged a favorable chemical reaction between polar and non-polar compounds during the melt mixing process in the internal mixer, was the main reason for this improvement. Figure 4 clarifies a plausible reaction mechanism that occurs during the modification of PP polymer with the integration of PP-g-MAH and metal oxide NPs (CuO-TiO₂) via melt blending technique. Initially, the PP polymer chain is bonded with the chemically reactive maleic anhydride ring, which enhances the interfacial adhesion of the PP polymer. Thereafter, the metal oxide particles (i.e., CuO and TiO₂) are further interacting with the maleic anhydride ring in long PP chain to form an improved antibacterial Polypropylene/PP-g-MAH/CuO-TiO₂ composites for biomedical application [27]. Strong interaction and surface adhesion was thus induced between polar binary mixed oxide and non-polar PP matrix.

3.1.3. SEM morphology of different amount of CuO-TiO₂ powder in PP matrix

Figure 5a-f displays representative SEM morphology images of binary mixed oxide PP (CuO-TiO₂-PP) composites which produced at different content of binary mixed oxide CuO-TiO₂ antimicrobial agent (0, 1, 3, 5, 7 and 10 wt%) with mixing temperature of 175^oC. The concentration of PP-g-MAH compatibilizer is fixed into 3 wt%. It is described that the bactericidal qualities of the PP composite were influenced by the concentration of antimicrobial agent in PP matrix. Besides, the stronger and faster rate of antimicrobial properties of PP composites could be achieved at higher NPs content [7]. However, the mechanical properties of such PP composite were deteriorated at higher antimicrobial agent loadings [7]. This might be attributed to the facts of bactericidal agent were highly agglomerated; bactericidal agent was poorly encapsulated within the interior structure of the polymer matrix; formation of higher voids or cavities around agglomerated morphology of bactericidal agent; the composite's weak structure because of the polymer's debonding from the bactericidal agent's surface and bactericidal agent were dispersed non-uniformly within polymer matrix. It is very interesting to note such phenomena were able to be solved in this work. The processing parameter and the compatibilizer used in this work could result in homogeneous microstructure (Fig. 5) up to 10 wt% loading. Based on the SEM photographs of the binary mixed oxide PP (CuO-TiO₂-PP) composites which were processed with 1, 3 & 5 wt% CuO-TiO₂ (Figs. 5b-d), were well encapsulated deeply into PP matrix and distributed uniformly. The PP composites which produced at higher binary mixed oxide CuO-TiO₂ antimicrobial agent (7–10 wt%) showed little agglomeration at certain region only (Figs. 5e-f). Overall, the entire sample showed encapsulated nature of the binary mixed oxide CuO-TiO₂ antimicrobial agent with good bonding within binary mixed oxide antimicrobial agent surface and polymer matrix.

3.2. Thermal and crystallization behavior

3.2.1 Thermal and crystallization behavior of compatibilized and non-compatibilized CuO-TiO₂-PP composites

The DSC results of the compatibilized and non-compatibilized CuO-TiO₂-PP composites samples are captured in Table 2. Obviously, the addition of a small amount of 1 wt % of binary mixed oxide CuO-TiO₂ antimicrobial agent in PP matrix leads to slight changes on T_m and increase the total crystallinity of pure PP. The increased crystallinity for the PP composites with 1 wt % binary mixed oxide CuO-TiO₂ antimicrobial agent are attributed to the role of CuO-TiO₂ antimicrobial agent acting as the site of heterogeneous nucleation [27]. The process of compatibilization is crucial to optimize surface tension and facilitate interfacial adhesion between the hydrophilic binary mixed oxide (CuO-TiO₂) and the hydrophobic PP matrix. Moreover, crystallinity influences water absorption, which in turn affects antimicrobial activity. Consequently, the importance of controlling the degree of crystallinity was examined in the following section.

Table 2

DSC findings of PP and CuO-TiO₂-PP composites with compatibilizer and pure PP without compatibilizer.
No.	Material	T_m (°C)	∆H_f (J/g)	Crystallinity (%)
1	Pure PP without compatibilizer	163.2	69.55	33.28
2	Pure PP with compatibilizer	164.8	68.51	32.78
3	CuO-TiO₂-PP with compatibilizer	160.64	79.33	40.00
Note: The material contains 3wt% compatibilizer and 1wt% CuO-TiO₂ that was added according to sequence 2 (m0-PP + C, m5-MO). At 0 min of mixing, at 5 min of mixing, Compatibilizer, and Metal Oxide (CuO-TiO₂) are denoted by the symbols m0, m5, C, and MO, respectively.

3.2.2 Thermal and crystallization behavior of different-concentrated CuO-TiO₂-PP composites

The DSC analysis was performed on PP composites prepared with different concentrations of binary mixed oxide CuO-TiO₂ antimicrobial agent and the findings are presented in Table 3. Based on the DSC results, incorporation of binary mixed oxide CuO-TiO₂ antimicrobial agent with different content did not yield a notable change in the T_m of PP composite. However, the percentage of crystallinity dropped with increasing amount of antimicrobial agent content in PP matrix up to 10 wt%, accounting for the lower mobility of the PP chain segments at high antimicrobial agent contents. Some agglomeration was found in certain region at higher weight percentage (> 5wt%) of antimicrobial agent addition. This would lower the available heterogeneous nucleation sites for PP crystallization at the interfaces, and sometimes even retard the crystallization rate as well. This phenomenon occurs due to the several competing factors such as prevention of heterogeneous nucleation effect of antimicrobial agent, obstruction on three-dimensional growth of crystal domain or decreased chain diffusion rate.

Table 3

DSC analysis results of binary mixed oxide PP (CuO-TiO₂-PP) composites with different content of binary mixed oxide CuO-TiO₂ antimicrobial agent.
No.	Material PP composite	Wt% of CuO/TiO₂ in PP	Tm (°C)	∆H_f (J/g)	Crystallinity (%)
1	CuO/TiO₂-1	1	164.88	56.55	28.43
2	CuO/TiO₂-3	3	164.65	52.87	27.06
3	CuO/TiO₂-5	5	165.62	46.19	24.11
4	CuO/TiO₂-7	7	164.20	42.33	22.48
5	CuO/TiO₂-10	10	164.07	37.33	20.37

The XRD spectrum of the PP composites with distinct quantity of CuO-TiO₂ are presented in Fig. 6. Dominant diffraction peaks associated with PP and minor peaks of copper oxide at 2θ = 35.25 and 38.430 were observed for entire composites. However, the nano TiO₂ peaks were not observed in the entire samples because the nanoparticles were completely dispersed throughout PP matrix. Most prominently, it was noticed that the peak intensity of pure PP drastically dropped with increasing amount of CuO-TiO₂. This peak intensity reduction indicates a drastic drop in degree of crystallinity.

3.2.3 Thermal and crystallization behavior of various heat treated CuO-TiO₂-PP composites

To optimize the CuO-TiO₂-PP composites' antimicrobial capabilities, they underwent several heat treatment processes such as quenching in warm water (QWW) at 30°C, slow cooling overnight (SCON), cooling via water circulation in the press (CP) and quenching in liquid nitrogen (QLN). The processing temperature for this batch of quenched samples was 175°C. Table 4 provides the DSC data for the 4 types of quenched samples to assess the correlation between matrix crystallinity and antibacterial activity. As cooling rate increased, both ∆H_f and the degree of crystallinity decreased; with nitrogen cooling, ∆H_f and melting temperature significantly decreased, resulting in the observation of highly amorphous regions. The results were attributed to the prevention of 3D crystallite growth at fast cooling rates. Apparently, rapid cooling rates caused a delay in crystallization as there is not enough time for the development of three-dimensional crystalline growth of CuO-TiO₂-PP composite.

Table 4

DSC results of various heat-treated CuO-TiO₂-PP composites.
No.	Sample CuO-TiO₂-PP	Tm (°C)	∆H_f (J/g)	Crystallinity (%)
1	Slow Cooling Overnight (SCON)	165.21	65.03	32.68
2	Cold Press (CP)	164.88	56.55	28.43
3	Quenching in Warm Water (QWW)	164.00	56.16	27.23
4	Quenching in Liquid Nitrogen (QLN)	151.66	11.94	6.00
Note: The material contains 1wt% CuO-TiO₂ particles

Figure 7 depicts the crystal structure of different types of heat-treated CuO-TiO₂-PP composites. The degree of crystallization for different treatments followed the order QLN < QWW < CP < SCON. The XRD findings were in agreement with the thermal analysis data, thereby indicating that the respective percentages of crystallinity for QLN, QWW, CP, and SCON were 6.00%, 27.23%, 28.43%, and 32.68%. The intensities of the PP peaks drastically decreased when the hot-pressed samples were subjected to rapid cooling rates, in which highly amorphous regions were observed. Intense diffraction peaks associated with PP and few peaks for copper oxide at 2θ = 34.70° and 37.95° were observed for the composites. However, the peak intensities of composites quenched in liquid nitrogen after molding were lower than those to room temperature by SCON. The quick cooling rate caused a sharp decrease in crystallinity in the QLN specimens.

3.3. Antimicrobial performance of optimized CuO-TiO₂-PP composites

3.3.1 Antimicrobial properties of various heat-treated CuO-TiO₂-PP composites

Supplementary Fig. 1 illustrates the antimicrobial efficacies of various heat-treated CuO-TiO₂-PP composites towards E. coli. Table 4 and supplementary Fig. 1 reveal that crystallinity exhibits a vital function in releasing antimicrobial ions. The sample with the highest crystallinity (SCON) was prone to microbial attacks, whereas the sample with the lowest crystallinity (QLN) was resistant to these attacks. The findings suggested that the processing parameters, which impacted the polymer matrix's crystallinity, had a significant impact on the antimicrobial ions' release. A high degree of crystallinity reduced the release of antimicrobial ions. A semi-crystalline polymer of polypropylene (PP) exhibits a preferential interaction (hydrogen bonding) between absorbed water and the functional groups in the zone of amorphous due to the inability of water to pass through the zone of crystalline. The water penetration barrier substantially reduces the intensity of copper metal ion production and migration from the SCON sample, resulting in low antimicrobial efficacy. On the other hand, samples with more amorphous regions (QLN) allowed the water to permeate the matrix of polymers and reach the surface of CuO NPs, carrying absorbed oxygen molecules. Copper ions are released from the polymer matrix more quickly owing to the diffusion, which effectively targets E. coli. However, for the subsequent section, (CP) treatment was adopted instead of QLN because QLN treated samples were found to be very brittle.

3.3.2. Antimicrobial activity of PP incorporated with various amount of CuO-TiO₂

The antimicrobial efficacies were evaluated for PP composites containing different contents of CuO-TiO₂ particles. Supplementary Fig. 2 illustrates the antimicrobial efficacy of the CuO-TiO₂-PP composites against E. coli after 24 h. The participation of CuO-TiO₂ markedly reduced the survival of bacteria. In comparison to other samples, the PP composite containing 10 wt% CuO-TiO₂ particles effectively decreased the number of bacteria. The reduction was attributed to the increase in released ions with increasing content of CuO-TiO₂ particles. Furthermore, water uptake in the composites was enhanced by their amorphous nature, which had a high CuO-TiO₂ content (Table 3).

3.3.3. Time-dependent antimicrobial action against E. coli

E. coli are human pathogenic Gram-negative bacteria that are widely involved in blood stream infections [28]. To evaluate the antimicrobial efficacy of CuO-TiO₂-PP, the rate of survival of E. coli (% viability) was determined. In this section, 3 wt% CuO-TiO₂ was selected due to the translucent appearance of the PP. Supplementary Fig. 3 displays the findings of the antimicrobial characteristics. For the control pellets, the attachment or propagation of E. coli temporally increased and reached its maximum after 4 days. The increase in colony counts in the control pellets was attributed to the PP matrix, which favored the growth of E. coli. PP is characterized as a hydrophobic matrix; E. coli prefers hydrophobic conditions for propagation. CuO-TiO₂-PP was effective against the bacteria with increasing incubation time especially after 2 days. The low activity in less than two days indicated that water diffusion and the ensuing release of Cu²⁺ and free radicals (i.e., •O²⁻ and •OH⁻) were needed to achieve effectiveness. In addition, the E. coli outer cell walls were discovered to be connected to the attacking delay. Research indicates that Gram-negative bacteria demonstrate greater resistance/tolerance to NPs than their Gram-positive counterparts, likely due to the lipopolysaccharide present in their outer membrane [29]. Gram-negative bacteria are known to possess the cytoplasmic membrane, which is essential for preserving cellular viability. Therefore, free radicals and Cu²⁺ do not readily harm Gram-negative bacteria. Therefore, longer times and more concentrated ions are needed to break down the bacteria's cell membrane.

3.3.4. Time-dependent antimicrobial action against S. aureus

Bloodstream infections caused by S. aureus increase morbidity, mortality, and medical costs; these infections are often avoidable [30, 31]. Therefore, CuO-TiO₂-PP composites were tested for S. aureus. Control pellets (PP matrix) are suitable platform for the attachment of the bacteria (Supplementary Fig. 4). S. aureus are preferably grown in hydrophobic environments. Additionally, the bacteria grow more readily in stationary-phase than in exponential-phase [32]. The Gram-positive microbes were attached and grown on pellets with common growth patterns, which include exponential growth, stationary, and logarithmic decline phases [33]. The exponential growth phase was observed from 1 to 2 days, whereas stationary phases were observed from 2 to 3 days (Supplementary Fig. 4); the logarithmic declining phase was observed after 4 days. The CuO-TiO₂-PP composites were more effective against S. aureus compared with the control samples. The composites showed signs of antimicrobial efficacy starting on day 3. Specifically, the CuO-TiO₂-PP composites demonstrated significant control over S. aureus (p < 0.05). The significant reduction in S. aureus from 2 to 3 days indicates that Gram-positive microbes are easily attacked by •O²⁻, •OH^-, or Cu²⁺ compared with Gram-negative microbes [34].

3.4. Postulated Killing mechanism of CuO-TiO₂-PP composites

Copper is already in the maximal stage of oxidation in CuO; hence, microbes can only be attacked by releasing Cu²⁺. However, the increased release of Cu²⁺ in binary mixed oxides as well as the presence of free radicals •O²⁻ and •OH⁻ were postulated to be advantageous in the CuO-TiO₂-PP composite (Fig. 8). Two possible reaction pathways (reactions 1 and 2; Fig. 8) for Cu²⁺ release was proposed. In reaction 1, water that has taken up oxygen molecules diffuses to the surface of CuO particles via the polymer matrix. Diffusion accelerates the release of Cu²⁺ to assault the microorganisms from the polymer matrix. TiO₂ in the PP composites yield the following photocatalytic reactions: TiO₂ + hν → e⁻ + h⁺; e⁻ + O₂ → •O²⁻ ; h⁺ + H₂O → •OH⁻ + H⁺. Bacterial deaths results from the interactions between the organic substances of the pathogens and the derived •O²⁻ and •OH⁻ [17]. Moreover, the free radicals from the photocatalytic reaction interact with CuO to initiate reaction 2. In this reaction, the considerably increased release of Cu²⁺ in the presence of TiO₂ is possible. Additional Cu²⁺ released further enhances the antimicrobial efficacy of CuO-TiO₂-PP composites [17].

CuO-TiO₂ distribution in the polymer matrix, crystallinity, and water uptake greatly affected the release of antimicrobial species. The incorporation of 3 wt% of compatibilizer (PP-g-MAH) in the fabrication of PP nanocomposites greatly improved the dispersion of CuO-TiO₂ particles. The composites that underwent QLN showed a low crystallinity, which enhanced their ability to absorb water. Additionally, these composites demonstrated a greater ability to decrease the quantity of CFU/ml of S. aureus and E. coli. CuO-TiO₂-PP composites might exhibit a high abundance of antimicrobial species, including •O²⁻, •OH⁻, and Cu²⁺, which improved its bactericidal effectiveness against both tested pathogens. To fully understand the underlying killing mechanisms, more molecular analysis is needed. CuO-TiO₂-PP composite biocide materials may improve catheter antimicrobial efficacies to prevent infections associated with catheter use.

Authors Contributions

G Ambarasan Govindasamy: Conceptualization; Data curation; Investigation; Methodology; Writing - Original draft.

Srimala Sreekantan: Funding acquisition; Resources; Administration; Supervision; Validation; Writing - Review & Editing.

Khairul Arifah Saharudin: Investigation.

Ming Thong Ong: Investigation.

Priscilla Jayanthi Thavamany: Investigation.

Geethaa Sahgal: Investigation.

Tan Aik Aun: Project administration; Funding acquisition.

Acknowledgements

In their acknowledgement, the authors express their gratitude to all technical staff from Universiti Sains Malaysia. Also, this work was supported by the B. Braun Medical Industries Sdn Bhd and Malaysian Ministry of Higher Education.

Funding

This work was supported by the Malaysian Ministry of Higher Education (FRGS/1/2021/TK0/USM/01/1).

Data Availability

Not applicable.

Code Availability

Not applicable.

Ethical Approval

Not applicable.

Consent for Publication

As the manuscript's authors, we hereby consent to the publication in this journal of any identifiable details related to the manuscript.

Conflict of Interest

The authors declare no competing interests.

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No competing interests reported.

SupplementaryfileJournalAdvancedCompositesandHybridMaterials.docx

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Effect of Compositions and Heat Treatments of Polypropylene/PP-g- MAH/CuO-TiO2 composites on Thermal, Crystallization and Antimicrobial Properties

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental methodology

2.1. Materials and fabrication of PP composites

2.2. Characterization

2.2.1. Morphological examination

2.2.2. X-ray diffraction analysis

2.2.3. Thermal analysis

2.2.4. Microbiological testing

3 Results and discussion

3.1. Morphological exploration

3.1.1. SEM morphology of CuO and TiO₂ particles

3.1.2. SEM morphology of CuO-TiO₂-PP composite with compatibilizer

3.1.3. SEM morphology of different amount of CuO-TiO₂ powder in PP matrix

3.2. Thermal and crystallization behavior

3.2.1 Thermal and crystallization behavior of compatibilized and non-compatibilized CuO-TiO₂-PP composites

3.2.2 Thermal and crystallization behavior of different-concentrated CuO-TiO₂-PP composites

3.2.3 Thermal and crystallization behavior of various heat treated CuO-TiO₂-PP composites

3.3. Antimicrobial performance of optimized CuO-TiO₂-PP composites

3.3.1 Antimicrobial properties of various heat-treated CuO-TiO₂-PP composites

3.3.2. Antimicrobial activity of PP incorporated with various amount of CuO-TiO₂

3.3.3. Time-dependent antimicrobial action against E. coli

3.3.4. Time-dependent antimicrobial action against S. aureus

3.4. Postulated Killing mechanism of CuO-TiO₂-PP composites

4 Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Effect of Compositions and Heat Treatments of Polypropylene/PP-g- MAH/CuO-TiO2 composites on Thermal, Crystallization and Antimicrobial Properties

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental methodology

2.1. Materials and fabrication of PP composites

2.2. Characterization

2.2.1. Morphological examination

2.2.2. X-ray diffraction analysis

2.2.3. Thermal analysis

2.2.4. Microbiological testing

3 Results and discussion

3.1. Morphological exploration

3.1.1. SEM morphology of CuO and TiO2 particles

3.1.2. SEM morphology of CuO-TiO2-PP composite with compatibilizer

3.1.3. SEM morphology of different amount of CuO-TiO2 powder in PP matrix

3.2. Thermal and crystallization behavior

3.2.1 Thermal and crystallization behavior of compatibilized and non-compatibilized CuO-TiO2-PP composites

3.2.2 Thermal and crystallization behavior of different-concentrated CuO-TiO2-PP composites

3.2.3 Thermal and crystallization behavior of various heat treated CuO-TiO2-PP composites

3.3. Antimicrobial performance of optimized CuO-TiO2-PP composites

3.3.1 Antimicrobial properties of various heat-treated CuO-TiO2-PP composites

3.3.2. Antimicrobial activity of PP incorporated with various amount of CuO-TiO2

3.3.3. Time-dependent antimicrobial action against E. coli

3.3.4. Time-dependent antimicrobial action against S. aureus

3.4. Postulated Killing mechanism of CuO-TiO2-PP composites

4 Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

3.1.1. SEM morphology of CuO and TiO₂ particles

3.1.2. SEM morphology of CuO-TiO₂-PP composite with compatibilizer

3.1.3. SEM morphology of different amount of CuO-TiO₂ powder in PP matrix

3.2.1 Thermal and crystallization behavior of compatibilized and non-compatibilized CuO-TiO₂-PP composites

3.2.2 Thermal and crystallization behavior of different-concentrated CuO-TiO₂-PP composites

3.2.3 Thermal and crystallization behavior of various heat treated CuO-TiO₂-PP composites

3.3. Antimicrobial performance of optimized CuO-TiO₂-PP composites

3.3.1 Antimicrobial properties of various heat-treated CuO-TiO₂-PP composites

3.3.2. Antimicrobial activity of PP incorporated with various amount of CuO-TiO₂

3.4. Postulated Killing mechanism of CuO-TiO₂-PP composites