Abstract
Bionanocomposite materials with enhanced photodegradability and bactericidal activity, as well as improved gas barrier properties, were manufactured by incorporating silicate nanolayers into the structure of low density polyethylene (LDPE) filled with nano titanium dioxide (TiO2) via melt compounding. Effects of interfacial compatibilization upon developed microstructure were studied by incorporating maleated LDPE into the nanocomposites formulation. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and rheo-mechanical spectroscopy (RMS) techniques were conducted to characterize the microstructure of the nanocomposites. Interfacially compatibilized TiO2/organoclay (OC) based nanocomposites exhibited shorter induction time for the onset of photodegradation, and an acceptable inactivation of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) microbe cells upon UV light irradiation, indicating enhanced photoactivity of the hybridized TiO2/OC nanosystem. TiO2/OC-based nanocomposites exhibited increased melt viscosity and pseudo solid like characteristics in melt linear viscoelastic behavior. Moreover, TiO2/OC-based nanocomposites presented improved barrier properties, which make such materials applicable for packaging applications.
1 Introduction
Polyolefins, such as polypropylene and polyethylene, have been known as the most widely used plastics in food packaging industries, due to their ease of processing, cost-effectiveness, transparency and excellent moisture barrier properties (1, 2), but have low resistance to the permeation of oxygen and hydrocarbons. Hence, polyolefin films with high barrier properties are either produced with multi-layer structures by co-extrusion processing, or the incorporation of inorganic fillers with high aspect ratios, such as clay silicate layers onto the polyolefin matrix (3). Moreover, products made from plastics can be contaminated or infected by microorganisms. The growth of microorganisms on the surface of polymeric films will inevitably increase the risk of transmitting infections (4). This has been shown to be due to the ability of microorganisms to survive on the surface of unmodified polymeric materials, making polymer surfaces a potential source for the growth of bacteria and fungi (5, 6). Hence, development of antibacterial polymer materials for applications in health and biomedical devices, food and personal hygiene industries has attracted considerable research interest (7, 8).
The most common method for the prevention of colonization of polymer surfaces from microorganisms and bacteria is to add an antimicrobial agent into the polymer matrix during the melt mixing process (9). Among the numerous antibacterial agents, titanium dioxide (TiO2) powder is a representative antimicrobial agent, as it possesses a strong oxidizing power for all types of organic materials and is capable of killing various bacteria when irradiated by ultraviolet light (λ<400 nm) (10–13). The possible mechanisms of the bactericidal activity of TiO2 towards many bacteria have been searched and reported, which provides an understanding of the mechanism for the generation of reactive species by UV light-activated TiO2 (14–16). TiO2 undergoes electronic excitation when subjected to UV light above its band gap (3–3.2 ev), leading to the formation of energy rich electron-hole pairs. Once at the surface of the material, such charge carriers would generate anion radicals (
In the present study, bionanocomposites with enhanced photocatalytic degradation, and antimicrobial activity, as well as improved barrier properties, have been prepared based on film grade low density polyethylene (LDPE) and a hybridized nanosystem composed of anatase type of nano TiO2 and organically modified montmorillonite (OC) via the melt mixing process. The activation effects of clay silicate nanolayers on the rate and induction time of photodegradation and also bactericidal activity of LDPE/TiO2 nanocomposites were investigated. The influence of maleated polyethylene (LDPE-g-MAH) on intensification of the TiO2/LDPE interface, and degree of clay intercalation/exfoliation, and eventually surface characteristics of nanocomposites has also been evaluated. All three classes of prepared nanocomposites based on OC, nano TiO2, and the TiO2/OC hybrid system exhibited significantly enhanced photodegradation compared with the unfilled LDPE.
2 Materials and methods
2.1 Materials
LDPE 0200 film grade, with a melt flow index of 2 g/10 min (190°C, 2.16 kg), was purchased from Bandar Imam Petrochemical Co. (Iran). Anatase type of nano TiO2 with mean particle size of 25 nm was provided by Degussa Chemical Co. (Germany). Maleated polyethylene (LDPE-g-MAH) type E-126, with a melt flow index of 4.5 (190°C, 2.16 kg) and graft level of 0.8–1.2% was delivered by Plüss Chemical Co. (India). Also, the used organo-modified clay was Cloisite 15A, a product of Southern Clay, TX, USA, with d-spacing of 31 Å and a bulk density of 0.172 g/ml. All materials were preheated in a vacuum oven at 70°C prior to being used for mixing process.
2.2 Preparation of nanocomposites
All nanocomposites were fabricated via the melt mixing technique using a laboratory size Brabender internal mixer (Germany), with a capacity of 60 ml and a rotor speed of 80 rpm at 175°C. Nano TiO2 powder was incorporated in the form of a melt mixed masterbatch with LDPE-g-MAH as a compatibilizer, in order to prevent agglomeration of TiO2 particles and increase their uniform dispersion in the nonpolar LDPE matrix. For this purpose, LDPE-g-MAH was compounded with an appropriate amount of nano TiO2 at 175°C, with a rotor speed of 60 rpm and a mixing time of 10 min, so that the final composition was 50:50 (w:w%). The prepared masterbatch was used to fabricate the LDPE/TiO2/LDPE-g-MAH nanocomposites composed of 1.0%, 1.5%, 3.0% and 5.0 wt% of nano TiO2. The mixing chamber was filled with LDPE, and after 1 min the required amount of masterbatch was fed, and mixing was continued for 15 min at a rotor speed of 80 rpm. The blended composite was then ground in the form of pellets. The LDPE/clay/TiO2 nanocomposites were fabricated by melt compounding of interfacially compatibilized LDPE/TiO2 composites with pre-dried OC for 10 min at 175°C. The LDPE/LDPE-g-MAH/clay nanocomposites were also synthesized by direct melt mixing at 175°C and a total mixing time of 15 min. Pelletized composites were molded between two plates of the hydraulic hot press at 160°C for 3 min under 14 MPa, and subsequent cooling for 10 min. The obtained 2 mm thick films were used for further experiments. Moreover, prepared nanocomposites were also subjected to a film blowing process to study processability, antimicrobial activity and as oxygen permeability. The compositions of the prepared nanocomposites are illustrated in Table 1. Samples have been coded as LD/CX/TY/OCZ, in which C, T, OC denotes compatibilizer, nano TiO2, OC, and X, Y, Z, represent their weight percent in corresponding LDPE nanocomposites.
Formulation of prepared samples.
| Sample code | LDPE matrix (wt%) | LDPE-g-MAH (wt%) | Nano TiO2 (wt%) | Cloisite 15A (wt%) |
|---|---|---|---|---|
| Neat LDPE | 100 | – | – | – |
| LD/C1.5 | 98.5 | 1.5 | – | – |
| LD/C5 | 95.0 | 5.0 | – | – |
| LD/C1/T1 | 98 | 1.0 | 1.0 | – |
| LD/C1.5/T1.5 | 97 | 1.5 | 1.5 | – |
| LD/C3/T3 | 94 | 3.0 | 3.0 | – |
| LD/C5/T5 | 90 | 5.0 | 5.0 | – |
| LD/C5/OC5 | 90 | 5.0 | – | 5.0 |
| LD/C5/T1.5/OC5 | 88.5 | 5.0 | 1.5 | 5.0 |
| LD/C5/T3/OC5 | 87 | 5.0 | 3.0 | 5.0 |
LDPE, low density polyethylene; LDPE-g-MAH, maleated polyethylene.
2.3 Structural analysis
An X-ray diffraction (XRD) spectrum of the used nano TiO2 was obtained using a Philips X-ray diffractometer (Netherlands), operating in reflection mode with Ni-filtered Cu Kα radiation (λ=0.154056 nm) at a generator voltage of 30 kV and a current of 30 mA. The morphology, size and dispersion state of TiO2 nanoparticles on the surface of nanocomposites, and the degree of clay intercalation were characterized by performing field emission scanning electron microscopy (FE-SEM) with model Hitachi S-4160 (Japan), transmission electron microscopy (TEM) with model Philips Em208S, operating at 100 kV, small angle X-ray sacttering (SAXS) with model Hecus S3micro (Austria), with Cukα radiation (λ=0.154 nm) and a tube voltage of 40 kV, with a current of 40 mA. Braggs law, defined as λ=2d sin θ, was used to compute the crystallographic spacing for OC and nanocomposites. The samples were scanned with a rate of 0.02 s-1 between 2θ=1–10°. To further analyze the bulk microstructure, melt linear viscoelastic measurement was also carried out using a rheo-mechanical spectroscopy (RMS), model Paar physical USD200 (Austria). For this purpose, a dried pelletized sample was put between the two parallel plates with a diameter of 25 mm and a gap setting of 1 mm at 190°C. The melt was subjected to oscillation shearing within the frequency range of 0.1 to 1000 rad/s and a strain amplitude of 1%, which was within the linear viscoelastic range of the samples.
2.4 Film blowing process
To evaluate the processing behavior, and also to measure the barrier properties and antimicrobial activity of the prepared nanocomposites, a film blowing process was carried out using a Brabender (Germany) single screw extrusion film blowing machine model plasticorder, with an L/D ratio of 50/2. The temperature profiles for the heating sections and die were set at 160°C, 175°C, 195°C, and 210°C, and the gap thickness of the mandrel die was set at 18 mm. The blown films were stretched biaxially by drawing and air blowing. The film processing conditions and parameters are summarized in Table 2.
Conditions of film blowing process.
| Temperature profile (°C) | Screw speed (rpm) | Extension ratio (ms-1) | Mean blow up ratio |
|---|---|---|---|
| 160–210 | 40–45 | 2.0 | 3.15 |
2.5 Antimicrobial measurements
The bactericidal activity of the prepared nanocomposites was assessed by measuring the killing rate (colony survival rate) of two types of bacteria including Escherichia coli (E. coli) as a Gram-negative and Staphylococcus aureus (S. aureus) as a Gram-positive. The test was carried out in accordance with the agar diffusion plate ASTM E2149-01 and viable cell counting method. For this purpose, 20×20 mm square specimens cut from the nanocomposite blown films, were first disinfected with ethanol solution, and then a suspension solution of each bacteria with a cell concentration of 105 colony formation units (CFU/ml) was prepared. The bacterial culture (0.2 ml) was then put on the surface of the films and irradiated by a UV black lamp (Philips) with an intensity of 1 mW/cm2. For comparison, culture dishes containing sterilized samples of neat LDPE were also subjected to the same irradiation condition. Irradiation was performed for 10 min, 20 min, 30 min, and 40 min, and the surface of each sample was flashed with physiological saline, and several dilutions were made after each stated time. The surviving bacteria in each diluted solution were counted using a spread plate method. The measurement of antimicrobial effect and CFU/ml was carried out according to the standard (QBT2591-2003, ASTM G21-1996). A total of 0.2 ml of microbial cells was dropped on the surface of a negative control sample (a), a blank control sample (b), and an antimicrobial nanocomposite sample (c). They were cultured under the UV light for the determined period of time. The cultured cells were washed by culture medium three times, the colony formed was counted after 48 h. The antimicrobial rate was calculated by the following equation:
where: R is antibacterial rate (%), b=average number of cell colony of blank control sample (CFU/piece), c=average number of cell colony of antibacterial sample (CFU/piece). If R≥99% the material has a strong antibacterial effect, if R≥90% the material has an antibacterial effect and if R<90% the material has no antibacterial effect.
2.6 Photocatalytic degradation test
For this purpose, the prepared films with a mean thickness of 2.0 mm were placed in a cabinet equipped with a UV lamp (Philips TL8W/08F8T5/BLB) with a primary wave length of 375 nm. The light intensity was controlled by adjusting the distance between the samples and the lamp that was set at 1 mW/cm2. The onset and rate of photodegradation of the samples were followed by measuring the carbonyl index at various irradiation times using the Fourier transform infrared spectroscopy model Bomem Hartmann & Braun, SPG800G (Canada), operating within the wave number range of 4000–250 cm-1. The carbonyl index was calculated using the following equation in which the 1740 cm-1 and 1470 cm-1 absorptions are assigned to carbonyl and methylene groups, respectively.
2.7 Permeability measurement
The oxygen barrier property of the film blown nanocomposites was evaluated according to the standard ASTM 3985, using the oxygen transmission rate tester model GDP-X (Germany), at ambient temperature (25°C), relative humidity of 50% and 100 cm³/min oxygen flow rate.
3 Results and discussion
3.1 Characterization
Figure 1 illustrates the XRD pattern of the used TiO2 powder (Degussa P25). The main characteristic diffraction peaks of TiO2 crystals appeared at the diffraction angles of 2θ=29.43, 43.16 and 56.34, which are consistent with the XRD pattern of anatase type TiO2. The SAXS patterns of the pristine OC (Cloisite 15A) and corresponding prepared nanocomposite samples are displayed in Figure 2. As can be seen, the d001 reflection of the used OC appeared at 2θ=2.95, corresponding to an interlayer distance of 29.5 Å. However, this peak was shifted to the lower angle (2θ=2.63) in the internal structure of both nanocomposites based on OC/LDPE-g-MAH and OC/TiO2/LDPE-g-MAH, indicating an increase in clay d001 gallery spacing. In other words, an intercalated microstructure was developed in the structure of both nanocomposites. Nevertheless, to further obtain qualitative understanding of the internal structure of nanocomposites through direct observation, TEM examination was performed on the surface of ultra-thin sections prepared from both groups of nanocomposites. Typical TEM photomicrographs of LDPE/OC nanocomposites (LD/C5/OC5) comprising 5 wt% OC and nanocomposite based on nano TiO2/OC hybrid (LD/C5/T1.5/OC5) are displayed in Figures 3A and B, respectively. In these micrographs, the dark lines and dark spots account for the clay nanolayers and TiO2 particles or aggregates, respectively, and gray bases are relevant to the LDPE matrix.
X-ray diffraction (XRD) pattern of nano TiO2 (Degussa P-25).
SAXS patterns of: (A) Cloisite 15A; (B) LD/C5/OC; (C) LD/C5/T1.5/OC5.
Transmission electron microscopy (TEM) photomicrographs of: (A) nanocomposites; (B) LD/C5/OC5; and (C) LD/C5/T1.5/OC5.
Intercalated OC tactoids with varying thickness (average≈7 nm) were observed in the internal structure of both interfacially compatibilized LDPE/OC and LDPE/OC/TiO2 nanocomposites, which is consistent with the SAXD results illustrated in Figure 2. Nanosize spherical TiO2 clusters and particles are also dispersed in between the OC nanolayers. From TEM micrographs (Figure 3), it is evident that intercalation and delamination of OC nanolayers occurred in the structure of both nanocomposite systems. The OC nanolayers in the structure of the OC/TiO2-based nanocomposite might behave as a barrier for the flocculation of TiO2 fragments, leading to better dispersion throughout the LDPE matrix. Moreover, higher melt viscosity of the LDPE/OC/TiO2 compound, and hence more intensified shearing imposed during the melt mixing process, is also expected to enhance the dispersion state of both TiO2 aggregates and clay nanolayers. Nevertheless, TiO2 aggregates were still present in the structure of LDPE/TiO2/OC nanocomposites, which is due to the high tendency of TiO2 particles to form physical networks.
Investigation of TiO2 content and its morphology on the surface of LDPE/TiO2-based nanocomposites is essential, as antimicrobial and all other surface activities are governed mainly by these two factors. Therefore, FE-SEM examination was performed on the surface of nanocomposite specimens. The FE-SEM photograph of the interfacially compatibilized LDPE/TiO2 nanocomposite (LD/C1.5/T1.5) is demonstrated and compared with the micrograph of the used pristine nano TiO2 powder in Figure 4. It is seen that the size of TiO2 aggregates was reduced to a few nanometers, with very good dispersion throughout the LDPE matrix. Interfacial interactions between LDPE and TiO2 agglomerates reinforced by LDPE-g-MAH as a compatibilizer leads to the improved dispersion state of nano TiO2 throughout the LDPE matrix. Moreover, Figure 5 shows the energy dispersive X-ray analysis (EDXA) images of Ti, Si and AL elements on the surface of nanocomposite film samples comprising the nano TiO2 /OC hybrid. The white dots in each image represent the elements of the corresponding nanoparticle. It is seen that both TiO2 nanoparticles and silicate nanolayers are randomly dispersed on the surface of the hybrid-based samples, which are expected to impart surface antimicrobial and photocatalytic activity.
3.2 Melt rheological behavior
While an amount of solid inorganic particles with high surface hydrophilicity are incorporated into a hydrophobic polymer matrix, the resulting hybrid can be supposed as particle aggregates dispersed in a polymer matrix, especially if the filler particle size approaches the mean radius gyration of the host polymer. Interaction between individual particles or aggregates and also between polymer chains and particles surfaces, modify both solid-state and melt rheological characteristics of the host polymer as motion of the chains is retarded. However, two main parameters would affect the rheological behavior of these nanocomposite systems. The increase of the composite free volume by the dispersed solid particles would result in decrease of the melt apparent viscosity compared to that of the neat polymer matrix. Similar results were reported for high impact polystyrene (HIPS)/TiO2 nanocomposite systems by Wang et al. (34). The second factor is the volume effect induced by the filler, which can increase the melt viscosity if the melt of the nanocomposite is regarded as a suspending system. Hence, in some nanocomposites such as polymers based on nano TiO2, the apparent viscosity might be lower than that of the unfilled LDPE matrix, especially at low TiO2 content and high shearing process (35). The melt linear viscoelastic behavior of various prepared nanocomposites generated by nano TiO2 and nano TiO2/OC hybrid was assessed and compared with the compatibilized LDPE sample. For this purpose, a strain sweep test was first performed on various samples to determine the linear strain region. In Figure 6, the typical strain sweep rheograph of the nanocomposite sample based on 3 wt% of nano TiO2 is presented. For all samples, the strain amplitude of 1% was found to be within the linear viscoelastic behavior, therefore all frequency sweep tests were conducted at an amplitude of 1%. In Figure 7, variation of complex viscosity and melt storage modulus vs. angular frequency for various nanocomposite samples are demonstrated.
Field emission scanning electron microscopy (FE-SEM) photomicrographs of: (A) pristine nano TiO2; (B) LD/C1.5/T1.5 nanocomposite.
EDXA images: elemental illustration of nanocomposite sample based on nano TiO2/OC hybrid (LD/C5/T1.5/OC5).
The melt strain sweep rheograph of LD/C3/T3 nanocomposite at frequency of 1 s-1.
Linear melt viscoelastic behavior of samples: (A) complex viscosity; (B) storage modulus.
As can be seen in this figure, all nanocomposites generated by various amounts of nano TiO2 showed insignificant changes in viscoelastic characteristics compared to the compatibilized LDPE sample. This is due to the increase in the composite free volume caused by the dispersed TiO2 particles, leading to the decrease in the melt viscosity of the composites, even if the physical networks are formed throughout the LDPE matrix. The effects of increase in free volume of composites upon the melt viscosity could be attributed to the high tendency of TiO2 particles to form agglomerated structures, which can result in increasing the free volume of the molten composite due to the enhanced repulsive interactions between the TiO2 aggregates. However, nanocomposite samples based on TiO2/OC hybrid systems showed higher melt elasticity and viscosity, with pseudo solid like characteristics within the low frequency region. These evidenced the formation of large physical networks by the TiO2 particles and clay nanolayers throughout the LDPE matrix, which retarded the dynamic motions by the LDPE segments, accompanied by increased relaxation time.
3.3 Photoinduced degradation
Figure 8 displays the variation of carbonyl index as a function of UV irradiation time for the LDPE and corresponding interfacially compatibilized nanocomposites comprised of various amounts of nano TiO2. As can be observed, all nanocomposites exhibited photodegradation induced by TiO2 particles without any induction time, with a much higher rate compared to that of neat the LDPE sample. All nanocomposites showed a significant increase in carbonyl index with exposure time, indicating catalytic photodegradation induced by the TiO2 surfaces as a result of electron-hole (e–/h+) formation by irradiated TiO2 nanoparticles. The rate of increase in carbonyl index was shown to be higher from the early stage of irradiation for the composites comprising high levels of nano TiO2 (3.0 wt%). However, above 300 h irradiation, the photoactivity of nanocomposites composed of low concentrations of TiO2 tends to be comparable with those containing high nano TiO2 content. This could be attributed to the coupling rather than formation of electron-holes (e–/h+) in the structure of the high concentrated TiO2 nanocomposites, due to the shorter inter particle or inter aggregate distances, and hence higher possibility of recombination phenomena by the TiO2 particles carrying e-/h+.
Carbonyl index vs. irradiation time of neat low density polyethylene (LDPE), and TiO2-based nanocomposites.
In Figure 9, variation of carbonyl index vs. UV irradiation time for nanocomposites based on TiO2/OC hybrid (1.5/5, w:w%) has been demonstrated and compared with their corresponding nanocomposites composed of each individual nanofiller. It is clearly seen that the two samples generated by 1.5 wt% of nano TiO2, and 1.5/5 (w:w%) of nano TiO2/OC hybrid exhibited photocatalytic degradation from the early time of irradiation, whereas the nanocomposite generated by only 5.0 wt% of OC (LD/C5/OC5) showed an induction time of 300 h. However, based on the obtained results, it was concluded that the rate in increase of carbonyl index for the nanocomposite based on the TiO2/OC 1.5/5 (w:w%) hybrid was slower than the sample based on 1.5 wt% nano TiO2. This could be attributed to the barrier effect of clay nanolayers, which could reduce the accessibility of activated TiO2 particles to the LDPE chains, leading to the lower rate of photooxidation reactions.
Carbonyl index vs. irradiation time of TiO2/OC-based nanocomposites.
SEM photomicrographs taken from the surface of the unfilled compatibilized LDPE film (LD/C5) and nanocomposite originated from a hybrid of 1.5 wt% nano TiO2 and 5.0 wt% of OC (LD/C5/T1.5/OC5) exposed to UV irradiation for 700 h are presented in Figures 10A and B. It is observed that the surface of the nanocomposite sample has degraded and surface cracks and cavities have appeared, whereas the surface of the reference sample has remained almost intact. This evidences the occurrence of severe degradation induced by the nano TiO2/clay hybrid system. The photodegradation is initiated in the interface of LDPE and the activated TiO2 surface, hence the formation of cracks and cavities is expected to appear in proximity of the TiO2 particles. Similar results have been reported by Zan et al. for LDPE/TiO2 nanocomposites (36). Two mechanisms of LDPE are expected to be involved in the enhancement of photodegradation of LDPE matrix by TiO2. Activation of TiO2 upon UV light absorption at λ<400 nm leads to the formation of electron-hole pairs which would generate reactive anion radicals,
Scanning electron microscopy (SEM) photomicrographs taken from the surface of: (A) unfilled LD/LD-g-MAH film; (B) hybrid nanocomposite (LD/C5/T1.5/OC5), subjected to 700 h UV irradiation.
3.4 Antimicrobial properties
To evaluate the bactericidal activity of the prepared nanocomposites, the neat LDPE as a control sample, LD/C1.5/T1.5 and LD/C5/T1.5/OC5 nanocomposites comprising 1.5 wt% of TiO2 and TiO2/OC nanohybrid (1.5/5 w:w), respectively, were tested. The UV alone effect upon the used microorganisms survival was also measured for the studied time period, and no significant inactivation was observed. Figure 11 displays the antimicrobial effect and CFUs vs. UV irradiation time for E. coli and S. aureus microbe cells cultivated on the surface of the control and the two nanocomposite films. Both nanocomposites exhibited excellent cell inactivation behavior, especially above irradiation times of 20 min, indicating enhanced bactericidal effects for the nanocomposites induced by photoactivated TiO2 nanoparticles. However, antimicrobial activity against S. aureus was found to be higher than E. coli for both nanocomposites. This can be attributed to the different membrane structures for the two cells, especially the lack of outer membrane in the structure of S. aureus. The hybrid nanocomposite sample comprised of the TiO2/OC nanosystem, showed more bactericidal potential than the counterpart sample comprising only 1.5 wt% nanoTiO2. This implies the potential of our prepared nanocomposites based on the TiO2/OC hybrid nanosystem for activity against pathogens involved in spoilage of packed foods. E. coli and S. aureus concentrations were reduced significantly by the nanocomposite film composed of the TiO2 hybrid.
Antimicrobial activities of neat low density polyethylene (LDPE) and LDPE/C1.5/T1.5, LDPE/C5/T1.5/OC5, film samples towards: (A) Escherichia coli (E. coli); (B) Staphylococcus aureus (S. aureus), as a function of UV irradiation time.
In Figure 12, the colony morphology of the control film sample including neat LDPE was compared with that of tested nanocomposite films based on the TiO2/OC hybrid after 40 min of UV irradiation. The percent antimicrobial effect (R) measured for both control and nanocomposite samples is also illustrated in Table 3. As can be observed, the nanocomposite samples showed significant antimicrobial effects against E. coli, as the number of grown bacterial colonies strongly decreased. The bacterial killing potential of TiO2 irradiated by UV light has been reported by many researchers (38–40). When a photon with energy of hν exceeds the energy of the band gap, an electron (e-) is promoted from the valence band to the conduction band leaving a hole (h+) behind. In electrically conductive materials, i.e., metals, the produced charge-carriers are immediately recombined. In semiconductors, a portion of photo-excited electron hole pairs diffuse to the surface of the catalytic particle (electron-hole pairs are trapped at the surface) and take part in the chemical reaction with the adsorbed donor (D) or acceptor (A) molecules. The holes can oxidize donor molecules, whereas the conduction band electrons can reduce appropriate electron acceptor molecules.
The colony morphology of Escherichia coli (E. coli) incubated on agar plates obtained from cultivated suspensions with control and hybrid nanocomposite films.
% Antibacterial effect after 40 min irradiation.
| Sample | Escherichia coli | Staphylococcus aureus |
|---|---|---|
| LDPE control film | 9 | 10 |
| LD/C5/T1.5/OC5 hybrid film | 92.5 | 95 |
LDPE, low density polyethylene.
A characteristic feature of semiconducting metal oxides is the strong oxidation power of their holes h+, as they can react in a one-electron oxidation step with water to produce the highly reactive hydroxyl radical (‧OH). Both the holes and the hydroxy radicals are very powerful oxidants, which can be used to destroy most organic contaminants. The active radical species generated upon UV excitation of TiO2 photocatalyst are known to damage cell membranes by lipid peroxidation, which results in the elimination of the cell protection wall. Photocatalytic action progressively increases the cell permeability, and subsequently, free efflux of intracellular content that leads to the eventual death of the cell (41, 42).
3.5 Oxygen barrier properties
The oxygen permeation coefficients of the neat LDPE film and corresponding nanocomposites measured at oxygen gas pressure of 10 MPa, are given in Table 4. The results accounted for the fact that inclusion of OC into the composition of composites led to a substantial decrease of oxygen permeability, which is consistent with the SAXD and TEM findings that intercalated and somehow exfoliated microstructures exist in the internal structure of organoclay-based nanocomposites. However, the nanocomposites generated by the TiO2/OC hybrid showed lower permeability than LDPE/OC nanocomposites, indicating retarded diffusivity of oxygen molecules into the LDPE caused by the coexistence of TiO2 and clay nanolayers throughout the LDPE matrix. Incorporation of clay silicate layers onto the polymer matrix has been shown to decrease the permeability of molecules through the polymer membrane via formation of a torturous path by the dispersed nanolayers (43).
Oxygen permeability coefficient of neat low density polyethylene (LDPE) and nanocomposites.
| Sample code | Permeability (cm3/m2d bar)×103 |
|---|---|
| Neat LDPE | 1.36 |
| LD/C1.5/T1.5 | 1.14 |
| LD/C5/OC5 | 0.67 |
| LD/C5/T1.5/OC5 | 0.55 |
| LD/C5/T3/OC5 | 0.59 |
LDPE, low density polyethylene.
4 Conclusion
Nanocomposites based on photosensitive nano TiO2 and organically-modified clay as a hybrid nanosystem and interfacially compatibilized LDPE, were fabricated the by melt mixing process. Microstructure examinations performed on the surface of the nanocomposites revealed that intercalated/exfoliated clay co-existed with nanosize TiO2 particles aggregated in LDPE matrix. Nanocomposites originated from the nano TiO2/OC hybrid exhibited significant photocatalytic degradation without induction time. Moreover, nano TiO2/OC-based nanocomposites showed enhanced antimicrobial activity compared with nanocomposites based on each individual nanofillers. LDPE/TiO2 nanocomposites with different levels of nano TiO2 showed shear thinning characteristics in dynamic melt viscoelastic measurements, and melt viscosity comparable to that of the neat LDPE. However, nanocomposites generated by the TiO2/OC hybrid exhibited higher melt elasticity and viscosity as a result of retarded segmental motions by the LDPE segments trapped within the physical networks of TiO2/OC nanoparticles. Thus, improved oxygen barrier properties with enhanced antimicrobial activity, and also significant photodegradability, suggest our prepared nanocomposite films as good candidates for food packaging applications.
The authors would like to give their sincere thanks to the microbiology group and laboratory, in the faculty of science, Tehran University, who collaborated with antimicrobial measurement of the prepared samples.
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