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PACKAGING TECHNOLOGY AND SCIENCE Packag. Technol. Sci. 2016; 29: 415–427 Published online 26 May 2016 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/pts.2222

Synthesis, Characterization and O2 Permeability of Shape Memory Polyurethane Films for Fresh Produce Packaging By Deniz Turan,1 Gurbuz Gunes1* and F. Seniha Güner2 1

Department of Food Engineering, Faculty of Chemical and Metallurgical Engineering, Istanbul Technical University, Maslak, Istanbul 34469, Turkey 2 Department of Chemical Engineering, Faculty of Chemical and Metallurgical Engineering, Istanbul Technical University, Maslak, Istanbul 34469, Turkey

The aim of this work was to investigate the effect of film formulations in an effort to obtain a fresh produce packaging film with increased temperature sensitivity for gas permeability. Series of shape-memory polyurethane (SMPU) were synthesized using poly(ethylene glycol) (PEG), 1,6-hexamethylene diisocyanate, 1,4-butanediol and castor oil (CO) and casted into films. The changes in thermal, viscoelastic, shapememory properties and oxygen permeability of the films were studied. The polyurethane films with 1500 g mol1 PEG showed a phase transition temperature (switching temperature) between 20 and 27°C. The SMPU consisting of 50/50 CO/PEG had a log E′ value of 8.32 Pa and showed good elasticity as low density polyethylene. SMPU prepared from 1500 g mol1 PEG with 50/50 CO/PEG and 40/60 butanediol/(PEG + CO) ratios showed excellent shape-memory properties with shape recovery ratio >85% and shape fixity ratio >90%. This film had higher oxygen permeability and showed up to 67% increase in Q10 value for oxygen permeability compared with commercial packaging films like low density polyethylene. This film can be used to develop smart packaging with increased thermally responsive gas permeability to similar levels observed in respiration rates of fresh produce. Copyright © 2016 John Wiley & Sons, Ltd.

Received 2 July 2015; Revised 22 February 2016; Accepted 23 April 2016 KEY WORDS: polyurethane; food packaging; gas permeability; smart film; temperature sensitive film

INTRODUCTION Fresh fruits and vegetables (whole or fresh-cut) are metabolically active, respiring products consuming O2 and producing CO2. Modified atmosphere packaging (MAP) is used to maintain the quality of fresh produce. The MAP of fresh produce involves packaging of the product in an optimum atmosphere with reduced O2 and elevated CO2 levels compared with air. Exposure of the fresh produce to O2 and CO2 beyond their threshold O2 and CO2 levels results in physiological damages, causing spoilage of the product. The reliability of the MAP depends on strict maintenance of refrigerated temperature during storage, transportation and marketing of fresh produce.1 However, modified atmosphere-packaged fresh produce are usually exposed to ambient temperature (temperature abused) during transportation, storage and marketing for short periods because of broken cold chain for various reasons. Because the temperature dependence of respiration rate (expressed by Q10 value) is higher than the temperature dependence of permeability of the packaging films, respiration rate increases at a faster rate than the gas transmission rate through the package, resulting in too low O2 or anaerobic condition and too high CO2 * Correspondence to: Gurbuz Gunes, Department of Food Engineering, Faculty of Chemical and Metallurgical Engineering, Istanbul Technical University, Maslak 34469, Istanbul, Turkey. E-mail: [email protected] † Supporting information may be found in the online version of this article. Copyright © 2016 John Wiley & Sons, Ltd.

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accumulation in the package causing spoilage of the product.2 New packaging materials with increased temperature sensitivity (Q10 values) for gas permeabilities compared with the commercial packaging materials are required to solve this problem. The Q10 values for gas permeability of these new materials should be close to the Q10 values for respiration rate of fresh produce for better performance. Polymeric materials are widely used in packaging applications because of their desired properties. Among these, shape-memory polymers (SMPs) are able to change their structure from a temporary shape to the original shape back and forth upon an external stimulus such as temperature, light, electrical field, etc.3 Polyurethane (PU) copolymers are an important member of the group of thermoplastic elastomers. It is possible to obtain very brittle or soft materials, tacky and viscous materials, depending on the properties of raw materials used in their synthesis.4 PUs have been widely used in many areas such as medical, textile, automotive and chemical industry.5 Previously, the side-chain crystallizable polymers were designed, and polyurethane packaging films comprising a block copolyether ester or a block copolyether amide were formulated for respiring and patented products.6–8 It has been reported that segmented PUs showed a shape-memory characteristic with good mechanical properties and a substantial increase in gas permeability with temperature by forming functional gates to control permeation.9 Shape-memory polyurethanes (SMPUs) are block copolymers composed of alternating soft segments (polyols) and hard segments (chain extender and diisocyanates). Polyethylene glycol is an important polyol within the polyether group. They feature typically low melting temperatures, which makes them promising materials for the switching block of SMPUs. They also have non-toxic degradation products, biocompatibility and shows good mechanical strength.5 The crystalline part of polyurethane occurs in soft segment which forms a reversible phase, and it determines the shape recovery temperature and controls gas permeability.10,11 The hard segments function as both physical cross links and reinforcing fillers dispersed in the soft segment matrix.12,13 These structures determine their unique physical, mechanical and thermal properties as well as gas permeabilities of polyurethane films.14,15 SMPUs possess flexibility, high shape-recovery ratio, ease of manufacture, strong resistance to organic solvents and aqueous solutions, excellent and consistent elastic properties and biocompatibility.16 These unique properties are promising for food packaging applications as well. Therefore, SMPU can be a good choice to obtain a new packaging material with increased temperature sensitivity for gas permeability. It was important to prepare SMPU films for tailored transition temperature close to room temperature in terms of fresh produce packaging. Thus, poly(ethylene glycol) (PEG) can be applied as soft block in SMPU. The overall objective of this study was to develop a new packaging material from thermoplastic polyurethane with increased temperature sensitivity for gas permeability compared with the commercial films used for fresh produce products. Effect of chemical composition such as chain extender 1,4 butanediol (BDO) content, molecular weight of the polyol, PEG and castor oil (CO) content on thermal, viscoelastic, shape-memory properties and O2 permeability along with its temperature sensitivity were studied.

MATERIALS AND METHOD Materials Castor oil (functionality of 2.67) with a hydroxyl number of 161.01 mg KOH g1 and an acid number 0.99 mg KOH g1 was purchased from Aldrich (Aldrich, Milwaukee, USA). PEG with a numberaverage molecular weight of 1500, 3000 g mol1 and 1, 6-hexamethylene diisocyanate were obtained from Fluka (Fluka, Seelze, Germany). Technical grade 1, 4-BDO was purchased from Sigma (Sigma, Seelze, Germany). Synthesis and casting of polyurethane films Shape-memory polyurethane films were prepared by one-step bulk polymerization technique according to Akkas et al.17 Prior to the synthesis, PEG was dried on a rotary evaporator (RV 10 IKA, Satufen, Germany) for 6 h at 90–95°C, and CO was dried at 80°C under vacuum for 24 h. BDO was dried overnight at 50°C in a vacuum oven (Vacucell MMM, Graefelfing, Germany). Copyright © 2016 John Wiley & Sons, Ltd.

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Poly(ethylene glycol) and/or CO was added into a flask and mixed at rotary evaporator with 80 rpm. Then, BDO was added into the flask and mixed until a homogeneous mixture was obtained. Neither catalyst nor solvent was used to get a material suitable for food contact. The mixture was poured into a three-necked reaction flask equipped with a mechanical stirrer, dropping funnel and nitrogen inlet and outlet. The temperature was then increased to 50°C. Hexamethylene diisocyanate was added to the mixture during stirring at 300 rpm, and the reaction was continued for 150 s. Polyurethane films were prepared by casting–evaporation technique. The reaction mixture was poured into glass petri dishes and placed in an oven at 80°C for 24 h. All the diisocyanates were consumed in the reaction at the end of 24 h, which was assessed by the disappearance of the absorption peak at 2250 cm1 assigned to the N = C = O group using a Fourier transform infrared spectrophotometer (Perkin-Elmer, Spektrum 100, Massachusetts, USA). Films were prepared at two different molecular weights of PEG (1500 and 3000 g mol1), at two different weight ratios of CO/PEG (50/50 and 70/30) and at three different weight ratios of BDO/(PEG + CO) (40/60, 50/50 and 60/40). They are named using the abbreviation PU a–b–c, where a indicates the molecular weight of PEG, b indicates the weight ratio of CO to PEG (CO/PEG), and c indicates the weight ratio of BDO to PEG and CO [BDO/(PEG + CO)], respectively. Each film production is replicated three times. The thickness range of the casted SMPU films was 700–1100 μm. Hard segment content of the films was calculated using the equation in the succeeding texts, where W is the weight of the monomers used in polymerization 18: Hard segment content ð%Þ ¼

W CO þ W BDO þ W HDI 100 W PEG þ W CO þ W BDO þ W HDI

Differential scanning calorimetry Melting temperature (Tm), enthalpy changes (ΔH) (J/g) and crystallinity of polymer films were determined using a differential scanning calorimeter (DSC) (Perkin Elmer Diamond DSC, Massachusetts, USA). First, the samples were heated from 70 to 150°C at 20°C min1 and held at that temperature for 1 min to eliminate the thermal history and subsequently were cooled to 70°C. At 70°C, the samples were held for 40 min. Then, second thermal heating was operated from 70 to 150°C, with a heating rate of 20°C min1. Each measurement is replicated two times. The sample weights employed were around 10 mg. The degree of crystallinity (Xc) of a polymer sample can be determined by DSC by analyzing the measured latent heat of fusion ΔHm, which is the area under the curve of the melting peak above the baseline. Xc is calculated by dividing the heat of fusion for the polymer sample by the heat of fusion for a 100% crystalline polymer ΔH0m, i.e. by the equation X c ð %Þ ¼

ΔH m 100 ΔH m o

where ΔH is the heat of fusion of the sample which is caused by semicrystalline PEG in the synthesized SMPU films determined from the thermograms in J/g, and ΔHmo is the enthalpy value of 197 J/g for the fusion of 100% crystalline PEG given by literature.19–21 Although there are potentially urethane/urea linkages from the hard segments dissolving in the soft segment region, we presume that the crystalline phase contains only PEG polyether structures because of the well-organized packing of molecules in crystals. Previous studies also supported the assignment of the crystalline region to be composed of PEG crystals.15 Thermal gravimetric analysis Thermal stability of the polymers was performed using the thermal gravimetric analysis (TGA) (Perkin Elmer Diamond TGA, Massachusetts, USA). The degradation temperature of the polymers has been evaluated by heating from room temperature to 550°C with a heating rate of 20°C min1 under nitrogen atmosphere. Each measurement is replicated two times. Copyright © 2016 John Wiley & Sons, Ltd.

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Dynamic mechanical analysis The glass transition temperature (Tg) and the viscoelastic properties of the polymer were determined over temperatures ranging from 70 to 160°C at a heating rate of 3°C min1 and a frequency of 1 Hz using a dynamic mechanical analyser (DMA) (Perkin Elmer Diamond, Massachusetts, USA) operated in tensile mode. The dimension of the sample was 5 × 1 × 1 mm. Storage modulus (E′) and damping coefficient (tan δ) were determined from the data. Each measurement is replicated two times. Shape-memory properties The shape memory was examined in order to investigate the temperature-stimulating behaviour of the polymer using the bending test.22 Transition temperature was chosen according to the melting temperature of crystals in the soft segment of the films. A specimen (50 mm length, 15 mm width and 1 mm thickness) was heated to transition temperature, 35°C, for 5 min in a drying oven (BINDER FD 23, New York, USA) and folded to an angle (θmax) 180° under 3-kg force. The deformed sample was cooled to 5°C for 5 min in a refrigerator, and the force was released before the angle was re-measured and recorded as θfixed. The sample was then heated again to 35°C for 5 min, and the final angle (θfinal) was measured. The shape fixity ratio (Rf) was defined as (θfixed / θmax) * 100. The shape recovery ratio (Rr) was defined as [(θfixed  θfinal) / θfixed] * 100. The studies were performed with three replicated samples and each measured two times. O2 permeability coefficient of the films O2 permeabilities of the SMPU films were measured according to ASTM D-3985 at 5 and 23°C, 0% relative humidity by oxygen permeation analyser (Systech 8001, Johnsburg, IL, USA).23 Permeability of commercial low density polyethylene (LDPE) (80 μm), high-density polyethylene (HDPE) (80 μm) and oriented polypropylene (30 μm) were also measured to be used as reference. Temperature sensitivity of the permeability was determined by calculating the Q10 value: Q10 ¼ ðP2 = P1 Þ½10= ðT2T1Þ where P1 and P2 are the O2 permeabilities of the films at temperatures T1 and T2, respectively, and expressed in Barrer [1 Barrer = 1010 cm3 (STP) cm/(cm2 s cmHg)].24 Colour measurements Colour values, L*, a* and b* were measured for films at 1-mm thickness using a colourimeter (Minolta Chroma Meter 400, Ramsey, New Jersey, USA). A standard white tile (L = 96.82, a = +0.02 and b = +2.08 as reference) is used for calibration of the instrument. Each measurement is replicated three times. Statistical analysis Statistical analysis was performed using Minitab 16.0 (Minitab Inc., State College, PA, USA). Data were obtained from a full factorial design. There were two levels for molecular weight of PEG, two levels for CO/PEG weight ratio and three levels for BDO/ (PEG + CO) weight ratio. The data were subjected to analysis of variance to determine main effects and interactions, and Tukey method to evaluate the differences between the treatments’ means.

RESULTS AND DISCUSSION Thermal properties The influence of chemical composition on the thermal properties of SMPUs was investigated by the DSC method. The DSC curve of the PU 1500-50-40 and PU 3000-50-40 coded polyurethane films were shown in Figure 1 as an illustration. There was only one endothermic peak for all SMPUs, and Copyright © 2016 John Wiley & Sons, Ltd.

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Figure 1. Differential scanning calorimeter thermogram of PU 1500-50-40 and PU 3000-50-40 coded shape-memory polyurethane film [poly(ethylene glycol) (PEG) molecular weight 1500 and 3000 g mol1; castor oil (CO)/PEG weight ratio 50/50; butanediol (BDO)/(PEG + CO) weight ratio 40/60]. the melting temperature was in the range of 18–41°C (Table 1). The melting temperature suggests that the crystalline phase is constituted by PEG blocks. This range of temperature was a result of melting of the PEG crystal in the soft segment of the polymer.25 The melting temperatures (Tm) of the polyol soft segments (reversible phase) are actually the transitional temperatures (Ttr) of the corresponding SMPUs.26 Tm was the temperatures at which the gas permeability of polyurethane films increased as a result of the change in the crystal structure of polymer. Therefore, Tm is an important parameter in

Table 1. Thermal and physical properties of the shape-memory polyurethane films as affected by the film formulations. Film formulations BDO/ Tm PEG molecular CO/PEG weight (g mol1) ratio (PEG + CO) (°C)* (w/w) ratio (w/w) 1500

50

1500

70

3000

50

3000

70

40 50 60 40 50 60 40 50 60 40 50 60

24.8de 24.5de 24.1de 26.9cd 19.6e 20.8e 40.3a 36.9ab 40.1a 32.6bc 29.4cd 29.7cd

Tg (°C)* 36.1e 34.7e 35.6e 22.4e 34.7de 24.2de 32.2bc 30.6e 30.9cd 16.2ab 13.8a 17.2abc

Shape Crystallinity Hard Shape Xc (%)* segment fixity ratio recovery ratio (Rr)*** (%)** (Rf)*** 9.0abc 6.5abc 5.2abc 7.0abc 3.1bc 1.5c 23.9a 15.9abc 14.8abc 19.5a 17.4ab 13.8abc

64.1 65.5 66.7 68.6 69.7 70.6 62.4 63.7 64.9 68.4 69.2 70.1

92.4a 75.5b 57.7de 65.2c 51.7f 48.0g 95.6a 77.0b 76.7b 65.3c 61.4d 61.0d

85.8b 76.4c 69.0d 65.1de 48.9g 36.5h 96.5a 87.5b 69.6d 56.6f 48.8g 58.8ef

*The data presented are the average of two independent samples. **Data calculated from the equation given in the Materials and Method section, thus no statistics presented. ***The data presented are the average of six independent samples. Values with different letters (a-d) are significantly different (p < 0.05). Copyright © 2016 John Wiley & Sons, Ltd.

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determining the right SMPU film composition to have switched gas permeability at a target temperature. The crystallinity of the polymer obtained from their DSC curve is also given in Table 1. There is a 70% linear correlation between Tm and % crystallinity of the films. Crystallinity and Tm increased with an increase in molecular weight of PEG (p < 0.001). As the CO/PEG ratio increased, the crystallinity and the Tm decreased (p = 0.016 and p < 0.001, respectively). Because the alkyl groups in the structure of CO would cause chain mobility in the polymer, Tm appears at lower temperatures.27 Increasing the content of BDO in the polymer mixture decreased the crystallinity and Tm (p = 0.007 and p = 0.017, respectively). A decrease in the crystallinity of the polymer with increased CO/PEG and BDO/(PEG + CO) ratios is expected as CO and BDO contribute to the hard segment length.28 Statistical analysis also showed that Tm decreased to a greater extend with increased CO/PEG in the presence of PEG3000 compared with PEG1500 (p = 0.002). According to Chen, Hu, Liu, Liem, Zhu and Liu, increased phase separation among the segments and short urethane groups inside long soft region could be the reasons for a decrease in Tm.29 Some of the synthesized SMPUs have Tm around room temperature which was a target temperature for a polymer film intended to be used in packaging of fresh fruits and vegetables. Because Tm has a direct influence on the gas permeability of the SMPU films which is discussed in the further parts, it might be a useful parameter in development of thermo-sensitive smart packaging films. Previous studies were conducted on segmented polyurethane with crystalline melting transition soft segment and suggested that polyurethane can be used in intelligent food packaging.9,30,31 The hard-segment content, the nature of the polyol and the composition of the polymers have prominent effects on the thermal and crystallization properties. According to our results, the SMPU films synthesized from PEG 1500 showed Tm between 20 and 27°C, and thus, it would be suitable for the development of thermo-sensitive smart packaging film for fresh produce. The Tm can be decreased down to 20°C (which can be an advantage) using 70% CO and 50 or 60% BDO in the formulation (Table 1). Thermal stability of SMPU films was studied by evaluating the weight reduction with temperature using TGA (Figure 2). Because the TGA thermogram of PU 1500-50-40 coded film is overlapped with others, only one TGA thermogram was presented in Figure 2 to show the trend. All the samples decomposed in two steps because of the soft and hard segments in the polyurethane, and weight loss

Figure 2. Thermal gravimetric analysis thermogram of PU 1500-50-40 coded shape-memory polyurethane film (PEG molecular weight 1500 g mol1; CO/PEG weight ratio 50/50; BDO/(PEG + CO) weight ratio 40/60). Copyright © 2016 John Wiley & Sons, Ltd.

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as a result of polymer decomposition was observed at about 250–350°C. Because there is no catalyst or additive used during the polymer synthesis, weight loss as a result of their decomposition was not observed between 100 and 250°C. The first and second degradation steps occurred between 300–400°C and 400–500°C, respectively. Weight losses for all the films at 550°C were between 98 and 100%. Two different slopes of reduction were observed between 300 and 450°C. These two different slopes show that there are two different weight reductions in prepared polymers. The first one is related to the breakage in urethane bonds and CO, while the second step is related to the thermal decomposition of PEG polyol.32 The decomposition temperatures of all the samples in various weight losses are given in Table 2. The decomposition temperature was decreased with increased PEG molecular weight, but it increased with increased CO and BDO contents at 10 and 90% degradations (p = 0.001). The increased thermal stability has been associated with high stability of ester groups in CO and increasing amount of the hard segment.28,33 The Tg values were measured by DMA because it is reported that compared with DSC, DMA can be 10 to 100 times more sensitive to the changes occurring at the Tg by measuring the response of the material to temperature under constant oscillatory stress.34 The glass transition temperatures (Tg) of the SMPUs determined by DMA measurements (Table 1) were all below 14°C. Above this Tg value, the polymer becomes soft and elastic and shows a rubbery behaviour at room temperature. DMA technique was also applied for the evaluation of viscoelastic behaviours of the SMPU films. The changes in storage modulus (E′) and damping coefficient (tan δ) of SMPU as a function of temperature are shown in Figure 3. The storage modulus (E′) measures the stored energy, representing elastic properties of the films, and the loss modulus (E″) measures the energy dissipated as heat, representing the viscous portion. Tan δ (E″/E′) represents the ratio of the viscous to elastic response of materials. Peaks in tan δ can be associated with the glass transition, which corresponds to the ability of segmental movement of chains. Therefore, both high E′ values and low tan δ values show high elasticity of the films. It is found that as CO/PEG ratio decreased, E′ increased (p < 0.001), and tan δ decreased (p < 0.001) at a temperature range between 5 and 25°C. This result could be associated with the reduced plasticizing effect of alkyl groups in the CO.4,35–37 Also, there was a significant interaction between PEG molecular weight and CO content. The reduction in tan δ value by decreased PEG molecular weight was significant at 70/30 CO/PEG ratio (p < 0.001) but not significant at 50/50 CO/PEG ratio (p > 0.05). The main effect of BDO content on E′ and tan δ value was not significant, but its interaction with CO content was significant. While SMPU prepared at 50/50 CO/PEG ratio showed an increasing trend in the E′ value with increasing BDO content, the E′ value decreased with BDO for the SMPUs prepared at 70/30 CO/PEG ratio (p = 0.003). Also, the reduction in tan δ by the decreased CO content was significant at 50/50 and Table 2. Thermal decomposition temperatures (°C) at various levels of weight loss as affected by film formulations. Film formulations PEG

CO %

BDO %

Mw

(w/w)

(w/w)

1500

50

1500

70

3000

50

3000

70

40 50 60 40 50 60 40 50 60 40 50 60

Weight loss 10%

50%

90%

c

cd

436bc 454ab 462a 455ab 454ab 463a 430c 445b 440b 455ab 453ab 445b

313 318bc 329a 320bc 322ab 327a 310c 329a 315bc 328a 330a 317bc

391 407b 421a 398c 399c 405b 385d 409b 393cd 402bc 400bc 387d

Values with different letters (a-d) are significantly different (p < 0.05). Copyright © 2016 John Wiley & Sons, Ltd.

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Figure 3. The changes in storage modulus (E′) and damping coefficient (tan δ) of shape-memory polyurethane films as a function of temperature. The legend is coded as PU a–b–c, where a is the molecular weight of PEG (g mol1), b is the CO/PEG weight ratio, and c is the BDO/(PEG + CO) weight ratio. 60/40 BDO/(CO + PEG) ratio (p < 0.001). Based on our findings, the log E′ value of PU 3000-50-60 at 25°C was the highest (8.32 Pa). However, its tan δ value was also high (0.071). PU 1500-50-40 showed the lowest tan δ (0.061) at 25°C, and its log E′ value was 8.14 Pa. Commercial LDPE and HDPE films have log E′ values of 8.39 and 8.84 Pa at 25°C, and tan δ values of 0.125 and 0.066, respectively.38 Thus, the synthesized SMPUs showed similar viscoelastic properties to these commercial packaging films. Shape-memory properties The shape-memory effect is not an intrinsic property of polymeric material, but it is an important feature for developing smart materials. This feature is associated with the temperature-sensitive characteristic of SMPU film in terms of its gas permeability. The shape-memory behaviours of SMPU were investigated using bending test. Shape recovery ratio (Rr) and shape fixity ratio (Rf) of SMPUs are shown in Table 1. Shape recovery (Rr) is one of the most important parameters used to assess the quality of a shape-memory polymer. The shape fixity (Rf) is another key parameter to assess the ability of shape-memory polymers for maintaining a temporary shape after the applied load is removed.26 High Rr and Rf values are the indicator of the temperature-stimulating behaviour of the polymer. These parameters show the relationship between shape-memory behaviour and temperature-sensitive oxygen permeability for smart food packaging applications. As mentioned earlier, permeability packaging material needs to compensate the change in the respiration rate of modified atmosphere-packaged fresh produce when they are exposed to ambient temperature during storage even for short periods as a result of broken cold chain. Shape-memory property is important in terms of fresh produce packaging to show how well the film preserves its temperature sensitivity in case of multiple breaks in cold chain. It is found that as the molecular weight of PEG increased from 1500 to 3000 g mol1, the Rr and Rf Copyright © 2016 John Wiley & Sons, Ltd.

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values increased (p < 0.001). On the other hand, the Rr and Rf decreased with increased CO/PEG ratio and BDO content (p < 0.001). The effect of the PEG molecular weight was not significant on the shape-memory properties (Rr and Rf) at 40% BDO content, while the shape-memory properties decreased with PEG1500 at 60% BDO (p < 0.001). Moreover, as BDO content increased, Rr and Rf showed a rapid decrease at 50/50 CO/PEG ratio (p < 0.001). It was clear to see that when the hard segment content increased in the polymer structure, the Rr and Rf values of the polymer decreased. Because SMPU synthesized with PEG 3000 would have a higher soft segment in structure, PU 3000-50-40 coded sample showed higher Rr and Rf among other SMPUs (Table 1). This excellent shape recovery of the polymers may be attributed to the increased stored energy of the system because of uniform distribution among hard segments. Moreover, higher urethane linkages may cause more physical cross linking, and thus, more deformation energy is stored in the system.39,40 Lower hard segment content (HSC) samples ( 85% and Rf > 90%. O2 permeability coefficients of the films O2 permeability coefficients (Po2) of the films were measured at 5 and 23°C to evaluate temperature coefficient values (Q10) (Table 3). It is found that Po2 decreased as the molecular weight of PEG increased (p < 0.001), or CO content decreased (p < 0.001) at both 5 and 23°C. There was a significant interaction between the PEG molecular weight and the CO content at 23°C. The reduction in Po2 with increased molecular weight was significant at 50/50 CO/PEG weight ratio (p = 0.002) but not significant at 70/30 CO/PEG weight ratio at 23°C. The PEG interaction with CO was not significant at 5° C. The main effect of BDO content on Po2 was not significant, but its interaction with PEG was significant. At 40/60 BDO/(PEG + CO) ratio, the Po2 of the PEG 3000-based film was higher than the PEG 1500-based films. However, at 60/40 BDO/(PEG + CO) ratio, the effect of the PEG molecular Table 3. O2 permeability coefficients (Po2) and the corresponding temperature coefficients (Q10) of shapememory polyurethane films as affected by the film formulation and temperature. Film formulations CO/PEG ratio (w/w)

BDO/(PEG + CO) 3 ratio (w/w)

1500

50

1500

70

3000

50

3000

70

40 50 60 40 50 60 40 50 60 40 50 60

PEG molecular weight (g mol1)

LDPE* HDPE* OPP*

Po2 (Barrer) 5°C 0 % RH

Po2 (Barrer) 23°C 0 % RH

Q10 (5–23°C)

1.24gh 1.73ef 1.99cde 1.87de 2.42bc 3.03a 1.31fgh 1.52efg 0.99h 2.26bcd 2.70ab 1.56efg 1.28fgh 0.85i 0.32j

5.17cdef 5.14cdef 6.61bc 5.27cdef 6.39bcd 7.99ab 4.86defg 4.11fg 3.40g 8.24a 6.06cde 4.69efg 2.32h 1.40i 0.53j

2.21a 1.83cde 1.95bcd 1.78de 1.71ef 1.71ef 2.07ab 1.73ef 1.98bc 2.05ab 1.57f 1.85cde 1.40g 1.32gh 1.34gh

1 Barrer = 1 × 1010 [cm3 (STP) cm/cm2 s cmHg]. *Measured independently using the same system as the experimental film samples. Values with different letters (a-j) are significantly different (p < 0.05). Copyright © 2016 John Wiley & Sons, Ltd.

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weight was the opposite (p < 0.001). It is also found that Po2 decreased as Tm and crystallinity of the films increased (p = 0.001 and p = 0.018, respectively). Crystalline domains in the semi-crystalline films act as impermeable barriers to gases, forcing the penetrants to travel a longer route in the crystalline–amorphous interface than in the amorphous region, thus decreasing the permeability.14,15 Moreover, the decreasing trends of Po2 with increased PEG molecular weight can be explained with the increased Tm and Tg of the films and the increased crystallinity of soft segment. Permeability is correlated to diffusivity and solubility of gases into polymer. The diffusion of small molecules in rubbery polymers is a thermally activated process. Rubbery polymers are characterized by segmental mobility. A low Tg implies large segmental mobility, resulting in an increase of free volume with temperature and high diffusivity. Also, free volume depends on the number of polymer chain ends present in the system. Because the PEG1500 films have lower Tm values (close to room temperature) than the PEG3000 films, larger free volume at room temperature as a result of PEG crystal’s melting is expected, because the Po2 of the films was measured at a temperature of 23°C. A polymer sample with long chain lengths (high molecular weights) will have fewer chain ends per total units and less free volume than a polymer sample consisting of short chains.43 Moreover, BDO content in the polymer structure increased the hard segment content, and this may prevent the crystallinity related with PEG and increase the free volume. Therefore, this may contribute to the increased Po2 of PEG1500-containing films which indeed, have larger free volume. Moreover, the sorption and diffusion take place only in the amorphous regions. The crystalline regions have two effects; they increase the effective path length of diffusion, and they reduce the mobility of the polymer chains. This results in higher activation energy of diffusion. Because the PEG 3000-based films have higher crystallinity, they have lower diffusivity which results in reduced Po2. Overall, the Po2 of the SMPU films were mainly affected by transitional temperatures of the synthesized polymers. The Q10 values of the SMPU films were not affected by PEG molecular weight (p > 0.05). Increased BDO content decreased the Q10 values (p < 0.001). The decreasing effect of BDO on the Q10 was more in the films with PEG3000 than those with PEG1500 (p = 0.002). As CO content decreased, the Q10 values of the films decreased (p < 0.001). The decreasing effect of CO on the Q10 was more in the films with PEG1500 than those with PEG3000 (p = 0.004). Overall, it was seen that the Q10 values showed a strong correlation with the shape-memory characteristics of the film (p = 0.001), and an increase of both CO and BDO content decreased the shapememory properties of the films. Moreover, CO and BDO contents increased hard segment content and reduced the crystallinity of the soft segment and the cross-link density. Previous studies showed that the crystallinity of the soft segments and the formation of a physical cross-linking structure or the crystallinity of the hard segments in the PUs are the necessary conditions for good shape-memory behaviour.25 It is known that the respiration rates of fruits and vegetables increase more with temperature than do gas permeability of the packaging films.44 The degree of this inconsistency in activation energies of produce respiration and film permeability can be best described by Q10, which shows the change in rate of a biological or chemical reaction by a rise of 10°C. We found that the Q10 values were 1.40 for LDPE and 1.32 for HDPE over the range of 5 and 23°C (Table 3). These temperatures were chosen to represent refrigerator and room temperatures, respectively, and to compare the differences in the permeability and the Q10 value of the synthesized SMPU and the commercial films. In literature, the Q10 values for the Po2 of LDPE and HDPE between 0 and 10°C were given as 1.96 and 1.73, respectively.45 The reason of difference between our reported values and the literature may be a result of the differences in the measurement temperature range and the sample to sample variations. LDPE is one of the most commonly used packaging materials for fresh produce with low to medium respiration rates. However, LDPE may not be convenient for products exhibiting very high rates of respiration such as strawberries, mushrooms, cauliflowers, brussel sprouts, cabbages, celeries, leeks and many fresh-cut produce. Accordingly, there has been many commercial interests to develop smart films with high gas transmission rates.45,46 The polymers used in forming the food packaging are smart in that they have permeabilities which may be changed by relatively small changes in temperature. According to our results, the PU 1500-50-40 coded SMPU film revealed the highest Q10 value by up to 67% increase compared with other commercial films and more than twofold higher Po2 compared with LDPE. Therefore, it may be used as a new packaging film for fresh products with high respiration rates where Copyright © 2016 John Wiley & Sons, Ltd.

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LDPE is not sufficient without microperforation. Gas permeability of the film at higher relative humidities (usually exist in practice) is required in package design. Thus, the change in the gas permeability of the film with relative humidity is a subject of another study in our laboratory. Colour measurements The L*, a* and b* values of obtained SMPU films were presented in Table 4. Higher positive a* value indicates more red, and higher positive b* value indicates more yellow colour.47 No difference in the L* and a* values of the films with different formulations was detected (Table 4). However, the yellowness (b* values) of the films with 1-mm thickness increased with the PEG molecular weight and the CO content. Higher yellowness (b* values) in the films with 70/30 CO/PEG ratio was a result of a darker colour of CO used in the film formulations. At 70/30 CO/PEG ratio, the b* values of the films with PEG3000 were higher than those with PEG1500 (p < 0.001). On the other hand, all the films were quite transparent (visually) at 100 μm thickness as shown in Figure 4.

Table 4. Colour values of the shape-memory polyurethane films as affected by film formulation. Film formulation

Colour values

CO/PEG ratio (w/w)

BDO/(PEG + CO) ratio (w/w)

L*

a*

b*

1500

50

1500

70

3000

50

3000

70

40 50 60 40 50 60 40 50 60 40 50 60

80.06a 84.79a 83.67a 83.71a 76.68a 78.78a 79.60a 80.15a 83.71a 80.36a 80.91a 80.23a

1.99a 1.76a 1.78a 1.78a 2.43a 2.11a 1.39a 1.63a 1.99a 1.88a 2.13a 1.90a

8.07b 8.80b 8.17b 8.78b 8.03b 9.97ab 10.24ab 9.43ab 8.97ab 10.70a 11.61a 10.76a

PEG molecular weight (g mol1)

The data presented are the average of three independent samples. Values with different letters (a-b) in the same column are significantly different (p < 0.05).

Figure 4. Images of the casted polyurethane films. The legend is coded as PU a–b–c, where a is the molecular weight of PEG (g mol1), b is the CO/PEG weight ratio, and c is the BDO/(PEG + CO) weight ratio. Copyright © 2016 John Wiley & Sons, Ltd.

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D. TURAN, G. GUNES AND F. SENIHA GÜNER

CONCLUSIONS PEG 1500-based films showed a higher (up to 67%) temperature sensitivity (Q10) for oxygen permeability and had at least a twofold increase in Po2 compared with the commercial films, including LDPE, HDPE and oriented polypropylene. A melting temperature (Tm) in the range of 20–27°C as gas permeability switch point was obtained with PEG 1500-based film formulations. Tm can further be modified by changing the CO and BDO compositions of the film with PEG 1500. The polymer films were thermally stable up to 300°C, which enables its processability by extrusion without thermal degradation. PU 1500-50-40 coded sample showed excellent shape-memory behaviour (Rr and Rf > 85%). The results showed that the SMPU with the given compositions can have a significant potential for the development of temperature-sensitive packaging materials for fresh produce. Further studies are needed to investigate film formation for commercial production, tests on fresh produce items and migration studies to show suitability of food-contact materials. ACKNOWLEDGEMENT This work is supported by the Istanbul Technical University research fund and The Scientific and Technical Research Council (TUBITAK, project 115O559) of Turkey.

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Packag. Technol. Sci. 2016; 29: 415–427 DOI: 10.1002/pts