Preparation and Properties of Nonleaching Antimicrobial Linear Low

Article pubs.acs.org/IECR

Preparation and Properties of Nonleaching Antimicrobial Linear Low-Density Polyethylene Films Hao Wang,† Dafu Wei,*,†,‡ Zainab Ziaee,† Huining Xiao,*,†,§ Anna Zheng,‡ and Yi Zhao§ †

Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada School of Materials Science and Engineering, Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai 200237, P. R. China § School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, P. R. China ‡

S Supporting Information *

ABSTRACT: Antimicrobial packaging films have been widely used in various applications. However, because of concerns regarding safe food and consumer demands for preservative-free products, the antimicrobial agents applied to packaging have to be highly effective and the leaching effect is not allowed. In the present work, the antimicrobial agent polyhexamethylene guanidine hydrochloride was compounded with starch to prepare antimicrobial thermoplastic starch (ATPS). ATPS was then melt-blended with linear low-density polyethylene (LLDPE) to prepare the novel antimicrobial packaging film. The resultant LLDPE/ATPS films exhibited excellent antimicrobial properties against Escherichia coli in the shaking-flask test. The ringdiffusion test further confirmed the nonleaching effect of the films. However, the addition of ATPS decreased the crystallinity of LLDPE. Both rheological analysis and scanning electron microscopy images indicated that a high ATPS content might result in poor compatibility even in the presence of a compatibilizer. mechanical properties and high draw-down ratio.18 These advantages made LLDPE an ideal matrix for flexible packaging material. The integration of PHGH with LLDPE in the presence of appropriate carrier and compatibilizer is able to prepare a novel packaging material with inherent antimicrobial properties at a relatively low cost. In this work, PHGH was blended with starch through a twinscrew extruder to prepare antimicrobial thermoplastic starch (ATPS). Then, ATPS and LLDPE were compounded in the presence of a compatibilizer, polyethylene-grafted maleic anhydride, to prepare the nonleaching antimicrobial LLDPE films. The antimicrobial properties and nonleaching effect of the films were examined by the shaking-flask method and the ring-diffusion method, respectively. In addition, the thermal, mechanical, and rheological properties as well as cross-sectional morphologies were also investigated.

1. INTRODUCTION In recent years, the booming demands on safe food and personal care products have given rise to the emergence of active packaging materials. As an alternative to traditional packaging, active packaging is usually considered to be a novel material that performed desired functions other than merely providing a barrier between the product and external environment.1−4 Antimicrobial packaging is one of the most typical active packaging materials, which is not only beneficial to the consumers but also to the food and hygiene industries, because the innovative packaging is able to extend the shelf life of products through deactivating or reducing the growth rate of various microorganisms.5−8 However, because of a potential concentration-dependent toxicity, the active concentration of antimicrobial agents or preservatives applied to food packaging and medical devices must be kept as low as possible.9,10 Such a market and regulatory requirement suggested that the development of a flexible polymeric packaging material with highly effective antimicrobial properties appeared to be necessary. Meanwhile, the leaching effect of the incorporated antimicrobial agent has to be well controlled.11 Common cationic polymers, especially guanidine-based cationic polymers, have presented excellent growth inhibition against bacteria,12 fungi,13 and viruses.14 As one of the most representative guanidine polymers, polyhexamethylene guanidine hydrochloride (PHGH) has exhibited great potential in the development of materials with sterile surfaces, which can be readily prepared.15,16 More importantly, bacteria can rarely develop drug resistance as a consequence of the unique physical action of guanidine polymers.17 In the engineering plastic field, linear low-density polyethylene (LLDPE) has a broad range of applications in film production and injection molding because of its excellent © 2015 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Linear low-density polyethylene (LLDPE, a product of NOVA Chemicals, film grade; comonomer, octene; density 0.920 g/cm3; MFI = 1.0g/10 min at 190 °C; 2.16 kg load) and potato starch (food grade) was kindly provided by Al Pack Co. (Moncton, New Brunswick, Canada). Hexamethylene diamine, guanidine hydrochloride, glycerol, and polyethylenegraf t-maleic anhydride (PE-g-MAH; saponification value, 32− 36 mg of KOH/g; viscosity, 1700−4500 cP) were purchased from Sigma-Aldrich without any further purification. Received: Revised: Accepted: Published: 1824

November 5, 2014 January 14, 2015 January 26, 2015 January 26, 2015 DOI: 10.1021/ie504393t Ind. Eng. Chem. Res. 2015, 54, 1824−1831

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Industrial & Engineering Chemistry Research

was 170/180/190/205/190/180 °C from feed throat to die with a melt temperature of 195 °C. The screw speed was 25 rpm, while the melt pressure was 13.8−16.2 MPa, and the nip drive was 1.5 m/min. The thickness of the resultant films was 40−60 μm. 2.3. Antimicrobial Property Tests. There were two types of tests conducted in this work to investigate the antimicrobial properties of both ATPS and LLDPE/ATPS film products, i.e., the shaking-flask and ring-diffusion methods. The Gramnegative bacterium Escherichia coli (ATCC 11229) was chosen as the representative of bacteria. The shaking-flask method was a quantitative approach to evaluate the antimicrobial properties of the samples. The procedure was as follows: 0.10 g of test sample scraps and 5 mL of bacterial culture (105 CFU/mL) were mixed and shaken at 200 rpm at 37 °C in an incubator for a certain duration, for instance, 1 h. After shaking, a series of dilutions (10−1, 10−2, 10−3, and 10−4) were prepared by the successive addition of 0.5 mL of culture into 4.5 mL of PBS solution. Then, 0.1 mL of the culture was seeded on an agar plate. The plates were kept in an incubator at 37 °C for 24 h. The number of the colonies was counted, and the process was repeated three times to have an average colony count for each sample. The inhibition of cell growth was calculated based on eq 1:

Polyhexamethylene guanidine hydrochloride (PHGH) with the number-average molecular weight 720 Da (tested by electrospray ionization time-of-flight mass spectrometry, ESITOF-MS) was synthesized according to the procedures described in Wei’s previous work,19 and its structure is shown in Figure 1.

Figure 1. Chemical structure of PHGH.

Luria−Bertani (LB) broth, LB agar, and phosphate-buffered saline (PBS; pH = 7.4) were used for the cultivation of bacteria. Deionized water was used in all experiments. 2.2. Preparation of Thermoplastic Starch (TPS), Antimicrobial Thermoplastic Starch (ATPS), LLDPE Composite Film, and Antimicrobial LLDPE Film. The plasticization of starch was conducted in a ZSK 18 MEGAlab twin-screw extruder (screw outer diameter, 18 mm; ratio of length to diameter, 40:1; Coperion, Ramsey, NJ). Potato starch, glycerol, and deionized water were homogenized and extruded for the TPS pellets. The temperature profile used was 85/110/ 135/155/170/160/150 °C from feed throat to die with a melt temperature of 165 °C. The screw speed was 150 rpm, The preparation of ATPS pellets was similar to that of TPS. The difference was the use of a PHGH aqueous solution instead of water. The final content of PHGH within ATPS was 15.7 wt %. The preparation of LLDPE/TPS and antimicrobial LLDPE/ ATPS pellets was to mix LLDPE pellets with previously obtained TPS or ATPS pellets and the compatibilizer, PE-gMAH. Then, the mixtures were extruded using a twin-screw extruder to produce LLDPE/TPS or antimicrobial LLDPE/ ATPS composite pellets. The temperature profile used was 100/125/150/175/200/205/190 °C from feed throat to die with a melt temperature of 200 °C. The screw speed was 180 rpm. The recipes are shown in Table 1.

growth inhibition of cell (%) = (A − B)/ A × 100%

where A and B are the number of colonies counted from the control and corresponding film sample, respectively. The ring-diffusion method used in this work is a qualitative test to evaluate the antimicrobial properties and to confirm the nonleaching effect of the film samples. The following specific procedures were used: 0.1 mL of bacterial culture (106 CFU/ mL) was spread on the LB agar plate. Then, a round piece of sample (diameter: 25−30 mm) was plated on the surface of the agar. The plates were kept in an incubator at 37 °C for 24 h. The nonleaching effect was evaluated by examining whether there was any inhibition zone (ring) around the film sample. 2.4. Differential Scanning Calorimetry (DSC). A differential scanning calorimeter (model DSC Q20, TA Instruments, New Castle, DE) was used to determine the melting behavior and crystallinity of the film samples. A set of heating/cooling/ heating ramps were carried out following a three-step procedure. Approximately 10 mg of sample was first heated from room temperature to 190 °C at 10 °C/min and held in the melting state for 2 min to erase the thermal history. Then, a subsequent cooling was performed from 190 to 0 °C at 10 °C/ min. In the second run, the sample was heated from 0 to 190 °C at 10 °C/min again to investigate the melting behavior and to measure the crystallinity of LLDPE. The DSC cell was constantly purged with nitrogen at a flow rate of 50 mL/min. The relative crystallinity (Xc) was calculated by eq 2:

Table 1. Recipes of LLDPE/TPS and LLDPE/ATPS Pellets entry

LLDPE/ phr

LLDPE C1 C1M L1M L2 L2M L3 L3M L4M L5M L6 L6M L7M

100 93.6 93.6 100 100 100 93.6 93.6 93.6 93.6 87.2 87.2 80.8

TPS/ phr

ATPS/ phr

6.4 6.4 2.5 5.0 5.0 6.4 6.4 6.4 6.4 12.8 12.8 19.2

PE-g-MAH/ phr

1.5 1.5 1.5 1.5 3.0 5.0 5.0 5.0

(1)

PHGH content/ wt %

0.38 0.75 0.74 1.00 0.99 0.98 0.96 2.00 1.91 2.87

Xc = ΔHm/f ΔHm* × 100%

(2)

where ΔHm is the melting enthalpy detected in the experiment, while f is the weight fraction of LLDPE in the blend. ΔHm* represents the perfect enthalpy of 100% crystalline LLDPE, which is calculated as 293 J/g.20 2.5. Mechanical Properties. The mechanical properties were measured using a universal testing machine (model 4465, Instron, Norwood, MA) at room temperature (23 °C). The tensile strength and elongation at break were recorded in the machine direction with a cross-head speed at 500 mm/min, according to ASTM D882. At least 10 dumbbell-shaped

The films were produced using a Saturn Laboratory Blown Film Extrusion System (Future Design Inc., Mississauga, Canada). The single-screw outer diameter and the ratio of length to diameter of the extrusion system were 25 mm and 30:1, respectively. The die diameter of the blown film system was 6.3 cm with a gap of 1 mm. The temperature profile used 1825

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Industrial & Engineering Chemistry Research samples (115.0 mm × 6.0 mm) were tested for each film, and the average thickness was recorded using a micrometer caliper prior to the test. 2.6. Rheological Properties. Rheological studies were carried out with a rotational rheometer (model MCR302, Anton Paar, Graz, Austria) in the oscillatory shear mode. The diameter of the parallel plates was 25 mm, and a gap spacing of 1 mm was set in all measurements. The range of the frequency sweeps was from 0.01 to 100 rad/s, and the strain was 0.1%. The rheological data were recorded at 180 °C under a nitrogen atmosphere. The phase angle δ is defined in eq 3: δ = a tan G″ /G′

Table 2. Antimicrobial Test Results of LLDPE/ATPS Films antimicrobial test results ring-diffusion method

(3)

where G″ and G′ are the loss and storage moduli, respectively. 2.7. Scanning Electron Microscopy (SEM). A scanning electron microscope (model JSM6400, JEOL, Peabody, MA) was used to examine the cross-sectional morphologies of the prepared film samples. The top surfaces of the samples were coated with a thin layer of carbon. An accelerated voltage of 10 kV and a vacuum pressure of approximately 10−6 Pa were used as the operating conditions.

entry

shaking-flask method (1 h shaking, %)

inhibition zone (ring)

colonies found underneath the film

LLDPE C1 C1M L1M L2 L2M L3 L3M L4M L5M L6 L6M L7M

0 0 0 36.40 74.28 72.76 >99.99 >99.99 >99.99 >99.99 >99.99 >99.99 >99.99

n/a n/a n/a none none none none none none none none none none

yes yes yes yes yes yes yes yes yes yes no no no

and above, the corresponding 1 h shaking-flask growth inhibition was over 99.99%. The high bacteria deactivating efficiency suggested that a high PHGH bulk content (around 1.0 wt %) was essential for a better antimicrobial activity of the prepared film products. As a typical hydrophobic polymeric material, LLDPE is hardly compatible with the highly hydrophilic starch and PHGH. A simple blending of LLDPE with ATPS was more likely to generate ATPS agglomerations rather than an even distribution within the matrix of LLDPE, which might have a negative effect on the antimicrobial and mechanical properties. The compatibilizer, PE-g-MAH was able to react with both starch and PHGH.22,23 The introduction of PE-g-MAH was supposed to improve the compatibility between ATPS and LLDPE as well as to enhance the retention of PHGH, so that PHGH could have a better distribution on the surface of the film. However, the 1 h shaking-flask test results between L2 and L2M did not demonstrate any significant difference (74.28% vs 72.76%). A similar situation happened among L3, L3M, L4M, and L5M, and the growth inhibition values were all over 99.99%. It seemed that the introduction of PE-g-MAH has barely affected the test results. The antimicrobial properties of the film were more likely PHGH bulk content dependent, which was in accordance with the previous discussion. The possible explanation was that, although PE-gMAH may have anchored PHGH on the surface of the film, the compatibility has been improved less than expected. Besides the shaking-flask method, the ring-diffusion method was applied to evaluate the nonleaching effect and antimicrobial properties of the film as well. The antimicrobial agents with migration ability in certain hygiene materials are usually able to form an inhibition zone in the ring-diffusion test because of the leaching effect (see Figure 2a, from our previous work24). The antimicrobial properties of such materials can be evaluated by measuring the diameter of the inhibition zones.25,26 With regard to antimicrobial materials without the leaching effect, although they held the same ability to deactivate bacteria, there will not be any inhibition zone around the test sample. From the results in Figure 2, none of the films prepared in this work have shown any inhibition zone, which confirmed the nonleaching effect. Nonetheless, it was interesting to notice that there were colonies found underneath most of the film samples, except L6, L6M, and L7M. This phenomenon was simply ascribed to the

3. RESULTS AND DISCUSSION 3.1. Antimicrobial Properties of ATPS and LLDPE/ ATPS Films. The antimicrobial properties of ATPS were investigated by the shaking-flask method, and the results are presented in Table S1 (Supporting Information). ATPS has achieved excellent growth inhibition (>99.99%) even when the shaking time was only 15 s, which was mainly attributed to the incorporated antimicrobial agent PHGH. As one of the most representative guanidine-containing polymers, PHGH is a broad-spectrum antimicrobial substance with a typical minimum inhibitory concentration of 4−16 ppm.21 The positively charged PHGH was highly affinitive to the negatively charged cell walls and membranes of bacteria, such as E. coli.17 The contact between bacteria and PHGH would first result in damages to membranes, then leaking of the cytoplasm, and finally the death of bacteria. Moreover, the high concentration of PHGH within ATPS would accelerate the bacterial deactivating process. ATPS prepared through melt blending in this work could be further compounded with various thermoplastic polymers, such as polypropylene and polyethylene terephthalate, to prepare novel antimicrobial materials through a facile approach. The antimicrobial properties of LLDPE/ATPS films are presented in Table 2. Apparently, the coupling agent PE-gMAH in the control sample C1M did not show any growth inhibition in the shaking-flask test, suggesting that PE-g-MAH had no capability of deactivating E. coli. However, films containing ATPS have shown a different inhibition efficiency based on the shaking-flask method, which might be associated with the concentration and distribution of PHGH on the surface of the film. By compounding with starch and LLDPE, PHGH was distributed through the matrix of the film. Once bacteria had contacted with PHGH on the surface of the film, they would be killed instantly. It was believed that the more PHGH was located and evenly distributed on the surface of the film, the better bacteria growth inhibition would be reached by the antimicrobial film. The PHGH contents of L1M, L2, and L2M were 0.38, 0.75, and 0.74 wt %, respectively, corresponding to the growth inhibition of 36.40%, 74.28%, and 72.76% in the 1 h shaking test, respectively. However, when the PHGH content was increased to 0.96 wt % (L5M) 1826

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Figure 3. DSC melting traces of LLDPE/ATPS films.

and secondary crystallization behavior resulting from the comonomers.28−30 The LLDPE used in this work was a copolymer of ethylene and 1-octene. Like larger comonomer molecules, octene was mostly located on the shorter side chains. The spherulites generated from both the main and side chains with two different lamellae distributions were the major reason for the two melting peaks.31 The onset and final melting temperatures (∼86 and ∼128 °C, respectively) as well as both melting peak temperatures (∼112 and ∼123 °C, respectively) were quite close among all of the film samples, suggesting that there were no new crystalline forms generated due to the addition of ATPS, while the existing spherulites did not grow, maintaining the same size. The crystallinity decreased with increasing ATPS content, indicating that ATPS was not a nucleating agent for LLDPE. The difference among L3, L3M, L4M, and L5M was the content of the compatibilizer, PE-gMAH, in the recipes. However, neither the crystallinity nor the thermal transition temperatures among L3, L3M, L4M, and L5M have shown any significant changes. Although the thermal transition temperatures between L6 and L6M were quite close as well, the crystallinities of L6 and L6M (∼34%) both decreased compared to those of L3, L3M, L4M, and L5M (∼36%). Apparently, the addition of ATPS partially hindered the chain movement and affected the crystallization ability of LLDPE, which should be responsible for the decrease of crystallinity. In the meantime, PE-g-MAH barely improved the compatibility of the blends from a calorimetric point of view. A similar situation was observed in the starch/LLDPE system, in which succinylated starch was prepared as the compatibilizer.18 3.3. Mechanical Properties. The mechanical properties of pure LLDPE and LLDPE/starch films are shown in Figure 4. The tensile strength and elongation at break of pure LLDPE were 50.4 MPa and 612%, respectively. After starch was added, the tensile strength of the films decreased dramatically. The TPS content of both C1 and C1M was 6.4 phr, but the tensile strength of C1M (35.1 MPa) was higher than that of C1 (31.8 MPa) because of the presence of PE-g-MAH. L3 and C1 had the same starch content (6.4 phr ATPS and TPS, respectively), and the tensile strengths were quite close (31.5 vs 31.8 MPa), which suggested that the addition of ATPS and TPS had an

Figure 2. Ring-diffusion test to confirm the nonleaching effect: (a) demonstration of the leaching effect; (b) LLDPE; (c) L1M (nonleaching); (d) L2M (nonleaching); (e) L3M (nonleaching); (f) L6M (nonleaching); (g) L7M (nonleaching).

higher E. coli culture concentration (106 CFU/mL) used in the ring-diffusion test, while the shaking-flask method was 1 magnitude (105 CFU/mL) fewer. The shaking-flask method is a dynamic approach to examine the antimicrobial properties through different time intervals, while the ring-diffusion method was more focused on evaluating the nonleaching effect of the material in this work. Nevertheless, the results gave an indication that the higher PHGH bulk content did play a major role in enhancing the antimicrobial properties. In order to further investigate the rapid bacterial deactivating efficiency of LLDPE/ATPS films, L6M and L7M were chosen for the shaking-flask test within shorter time intervals. The test results are summarized in Table 3. Both film samples were able to reach over 99.99% growth inhibition within 10 min. L7M exhibited an even better performance because its growth inhibition efficiency was over 90% within only 15 s. As previously indicated, the higher PHGH bulk content was the key reason that more PHGH was capable of capturing and deactivating E. coli instantly. This kind of material has a great potential to be prepared as food packaging or other applications where a rapid bacterial deactivating efficiency is needed. Moreover, another advantage of incorporating PHGH into the matrix of the film rather than applying a thin coating layer on the surface of the material is that the antimicrobial properties would last through the lifetime of the product upon regular usage.27 3.2. DSC Analysis. The DSC melting traces and thermal transition data of LLDPE/ATPS films for which the growth inhibition values were over 99.99% are presented in Figure 3 and Table S2 (Supporting Information), respectively. There were two melting peaks observed in the DSC melting traces, which were placed at around 112 and 123 °C, respectively. Conventional copolymers, such as ethylene− vinyl acetate (EVA), usually exhibited more than one melting peak because of the presence of different lamellae thicknesses

Table 3. Antimicrobial Properties of LLDPE/ATPS Films within Shorter Shaking Time Intervals (%) entry

15 s

30 s

1 min

3 min

5 min

10 min

20 min

L6M L7M

0 93.53

0 91.79

0 97.72

14.43 >99.99

99.71 >99.99

>99.99 >99.99

>99.99 >99.99

1827

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3.4. Rheological Properties. The rheological properties of LLDPE/ATPS films, for which the growth inhibition values were over 99.99%, are presented in Figure 5a−e. The dependence of the complex viscosity (η*) on the angular frequencies (ω) at 180 °C in Figure 5a exhibited a drastic decrease in viscosity with an increase in frequency for all of the samples, which can be described as a typical pseudoplastic liquid behavior, yet none of the samples showed any Newtonian region. It was worth noting that the complex viscosities of both L3 and L6 were close to that of pure LLDPE, while the other samples containing PE-g-MAH were slightly lower. The dynamic storage modulus (G′) and loss modulus (G″) as a function of the angular frequency (ω) are illustrated in parts b and c of Figure 5, respectively. The storage modulus was related to the elastic properties of the blends or the energy stored during deformation, while the loss modulus reflected the viscous properties of the blends or the energy dissipated in the flow.32 The storage modulus of L7M was the largest among all of the samples at the lower-frequency region but started to become smaller than that of other samples, such as LLDPE and L3, at the higher-frequency region, which was mainly attributed to the high ATPS content of L7M. The loss modulus displayed a similar pattern, but the difference at the lower-frequency region was insignificant. Both the storage and loss moduli did not completely converge over the tested angular frequency region, which indicated the existence of ATPS-concentrationdependent morphological changes in the blends. The transition of a polymer blend from liquidlike (viscous) to solidlike (elastic) is due to the restricted molecular flexibility or mobility. Figure 5d shows the viscoelastic behavior of LLDPE/ATPS blends via the plot of tan δ versus the angular frequency (ω). When tan δ is 1, G″ equals G′. With regard to pure LLDPE, G′ was only larger than G″ when the angular frequency reached 100 rad/s, which indicated the dominance of the viscous component over the elastic part. Other LLDPE/ ATPS blends containing 6.4 phr ATPS exhibited a similar pattern because G′ was not beyond G″ until the angular frequency was close to 100 rad/s. For blends with higher ATPS content, such as L6, L6M, and L7M, tan δ was equal to 1 at a rather lower angular frequency of around 72 rad/s. The elasticity of a material is a restoring force, which tends to return the material to its original dimensions.33 The massive amount of hydroxyl groups within starch molecules usually generates an interconnected gel network structure resulting from the interand intramolecular hydrogen bonding between adjacent starch macromolecules.34,35 In this work, the physical cross-linking network existing in ATPS improved the elastic property of the blends, especially those with higher ATPS content. Thus, tan δ of L6, L6M, and L7M was equal to 1 at a lower-frequency region. However, the introduction of PE-g-MAH might impair the network by consuming part of the hydroxyl groups. Therefore, among the blends with similar recipes, the sample without PE-g-MAH exhibited higher elasticity or a smaller tan δ at the same angular frequency, such as a comparison between L6 and L6M. The compatibility of LLDPE/ATPS blends was evaluated using the criteria proposed by Han and Chuang36−38 in the form of G′ versus G″ in the logarithmic scale, as shown in Figure 5e. As previously discussed, G′ and G″ can be illustrated as the energy stored and dissipated in the molecules, respectively, during the shearing deformation. Therefore, the ratio of the energy stored to the energy dissipated while the test

Figure 4. Mechanical properties of pure LLDPE and LLDPE/starch films: (a) tensile strength; (b) elongation at break.

effect similar to that of LLDPE. The introduction of 1.5 phr PEg-MAH raised the tensile strength of L3M to 36.8 MPa, a 16.8% increment compared to that of L3. The further addition of PE-g-MAH from 1.5 phr to 3.0 phr (L4M) and 5.0 phr (L5M) was still able to increase the tensile strength but within a limited amount, which was probably attributed to the low ATPS content. When the ATPS content was increased to 12.8 phr, the tensile strength of L6 dropped to 28.4 MPa. However, the addition of 5.0 phr PE-g-MAH had a positive effect on L6M, the tensile strength of which was 27% higher (36.2 MPa) than that of L6. Moreover, the decrease of the crystallinity might be another reason for the compromised tensile strength. As a semicrystalline polymer, the mechanical strength of LLDPE is highly dependent on the crystallinity as well. From DSC analysis, the addition of ATPS not only impaired the matrix of LLDPE but also hindered the chain movement and decreased the crystallinity of LLDPE. The excessive amount of ATPS in L7M (19.2 phr) resulted in the low crystallinity of 30.8% compared to 38.8% of pure LLDPE. The 5 phr PE-gMAH in L7M was not able to fully compensate for the compromised strength due to the addition of ATPS. Therefore, the tensile strength of L7M (26.1 MPa) was only over half of pure LLDPE. In fact, for a commercial process, the preparation of three-layer films via a coextrusion film blowing process using LLDPE as the central layer and ATPS/LLDPE as the internal/ external layers could readily address the issues related to mechanical strength, while maintaining the high antimicrobial activities for various applications. Therefore, the strength concern is not critical. The least elongation of break occurred for L7M (483%), which was 21% less than that of pure LLDPE (612%). Considering that the tensile strength of L7M was just over 50% of pure LLDPE, it can be concluded that the elongation of break of LLDPE/starch films was less sensitive to the content of starch compared to the tensile strength. 1828

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Figure 5. Rheological analysis of pure LLDPE and LLDPE/ATPS films: (a) complex viscosity; (b) storage modulus; (c) loss modulus; (d) tan δ; (e) Han−Chuang plot.

was performed is expected to be independent of the blend composition as long as the blend system is fully miscible and the molecular structure is kept the same. In general, for homogeneous blend systems, the Han−Huang plot usually exhibits a composition-independent correlation, whereas the incompatible systems display a composition-dependent correlation.39,40 In the LLDPE/ATPS blends, a concentrationdependent correlation was observed at the lower angular frequency for samples with higher ATPS content, such as L6 and L7M. L6M was found to have shown a better compatibility, of which the correlation was similar to those with less ATPS content blends. 3.5. SEM Images. The cross-sectional morphologies of pure LLDPE and LLDPE/ATPS films are shown in Figure 6. Apparently, the morphology of the pure LLDPE film was rather smooth, while the other films exhibited vacant sites and embedded ATPS particles in the matrix. Although the ATPS contents among L3, L3M, L4M, and L5M were the same (6.4 phr), the existence of PE-g-MAH in various amounts resulted in

different morphologies. The size of the vacancies and the aggregated ATPS particles in L3 were reduced in L3M, L4M, and L5M because the compatibilizer improved the dispersion and adhesion between the interphase, which was in accordance with the mechanical properties. The vacancies within L6 were even interconnected in certain regions, which could also explain the lower tensile strength compared to that of L6M. The larger sized particles within L7M had less interaction with the LLDPE matrix, yet strongly interacted with each other through hydrogen bonding, which eventually resulted in the poor mechanical properties even in the existence of 5 phr PE-gMAH.

4. CONCLUSIONS A LLDPE-based novel antimicrobial packaging film has been prepared through melt compounding. The antimicrobial properties and nonleaching effect were examined by the shaking-flask and ring-diffusion methods, respectively. The test results suggested that the antimicrobial property was 1829

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related to the content of PHGH. A relatively high PHGH content (around 1.0 wt % within the film) was essential for a high antimicrobial activity. The dynamic antimicrobial test demonstrated a rapid bacterial deactivating efficiency (within 3 min) of the film at a PHGH content of less than 3 wt %. The ring-diffusion test confirmed the nonleaching effect of the prepared LLDPE/ATPS film. The addition of ATPS decreased the crystallinity of LLDPE, while the existing spherulites did not grow but maintained the same size. However, the tensile strength of the films has been compromised because of the addition of ATPS. The compatibilizer PE-g-MAH was able to improve the mechanical properties, yet a high content of ATPS might still result in a poor compatibility between LLDPE and ATPS, which has been further confirmed by rheological analysis and SEM observations.

ASSOCIATED CONTENT

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AUTHOR INFORMATION

REFERENCES

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Figure 6. SEM images of a film cross section.

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Article

S Supporting Information *

Antimicrobial properties of ATPS and LLDPE/ATPS films (Table S1) and DSC melting results of LLDPE/ATPS films (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail: [email protected]. Tel.: 1 (506) 453-3532. Fax: 1 (506) 453-3591. *E-mail: [email protected]. Tel.: 86 (21) 6425-3343. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the NSERC Sentinel-Bioactive Paper Network (Canada), NSERC Green Wood Fiber Network (Canada), AIF/Al-Pack Co. (Canada), and NSF China (Grant 51379077) for funding this work. 1830

DOI: 10.1021/ie504393t Ind. Eng. Chem. Res. 2015, 54, 1824−1831

Article

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DOI: 10.1021/ie504393t Ind. Eng. Chem. Res. 2015, 54, 1824−1831