Preparation and Characterization of Poly(Ether-Block-Amide)/Polyethylene Glycol Composite Films with Temperature- Dependent Permeation

Article Preparation and Characterization of Poly(ether-block-amide)/Polyethylene Glycol Composite Films with Temperature- Dependent Permeation

Sarinthip Thanakkasaranee, Dowan Kim and Jongchul Seo *

Department of Packaging, Yonsei University, 1 Yonseidae-gil, Wonju-si, Gangwon-do 26493, Korea; [email protected] (S.T.); [email protected] (D.K.) * Correspondence: [email protected]; Tel.: +82-33-760-2697; Fax: +82-33-760-2954

Received: 29 January 2018; Accepted: 22 February 2018; Published: 24 February 2018

Abstract: A series of poly(ether-block-amide) (PEBAX)/polyethylene glycol (PEG) composite films (PBXPG) were prepared by solution casting technique. This study demonstrates how the incorporation of different molecular weight PEG into PEBAX can improve the as-prepared composite film performance in gas permeability as a function of temperature. Additionally, we investigated the effect of PEG with different molecular weights on gas transport properties, morphologies, thermal properties, and water sorption. The thermal stability of the composite films increased with increasing molecular weight of PEG, whereas the water sorption and total surface energy decreased. As the temperature increased from 10 to 80 ◦C, the low (L)-PBXPG and medium (M)-PBXPG films showed a trend similar to the pure PEBAX film. However, the high (H)-PBXPG film with relatively high molecular weight exhibited a distinct permeation jump in the phase change region of H-PEG, which is related to the temperature dependent changes in the morphology structure such as crystallinity and the chemical affinity between the polymer film and gas molecule. Based on these results, it can be expected that H-PBXPG composite films can be used as self-ventilating materials in microwave cooking.

Keywords: phase change materials (PCMs); poly(ether-block-amide) (PEBAX); polyethylene glycol (PEG); controllable permeability; permeation jump

1. Introduction The microwave market is expected to witness a remarkable growth fueled by consumer demands due to the easy preparation and portability of on-the-go eating. The important driving factors for this growth are not only improvements in packaging materials and design that permit food to be heated in the package itself and served instantly, but also the innovative technologies that overcome the limitations of microwave ovens like browning and crisping [1]. The safety issues of microwave ovens involve setting the correct combination of time and temperature when cooking a pre-packaged food product to prevent it from burning, and also using a proper package that can heat at high temperature and release steam [2]. However, most packaged foods are not automatically self-venting during microwave cooking, which can cause the package to explode due to an excessive internal pressure buildup. Therefore, the user has to partially lift the plastic sheet from the tray or puncture the plastic covering to provide an outlet for the steam that is generated by the heated food items. Nonetheless, such a problem can be solved by improving the design and quality of the packaging materials or using innovative technologies to obtain microwaveable packaging with self-ventilation during the cooking process. This idea would not only solve this issue, but also

Polymers 2018, 10, 225; doi:10.3390/polym10020225 www.mdpi.com/journal/polymers Polymers 2018, 10, 225 2 of 16 meet user demand by increasing convenience and safety, since users would not be required to manually prepare or enable the venting mechanisms thus eliminating the chances of cuts and burns [3]. Self-venting materials have been known to deal with issues like the ones described below: A weak heat seal that fractures from excess steam pressure during microwave steam generation; incorporation of shrink-film-covered vent valves that melt or otherwise fail due to steam pressure; or laser-scored or perforated film which fails from the internal steam-pressure and releases the steam in the microwave oven [4]. However, multiple processes are required to produce such microwavable packages, leading to relatively high production costs. Innovative technologies like the controllable permeability technology may be an alternative to solve these issues. Recently, controllable permeability technologies like developing polymers with temperature- dependent permeability are applied in many fields including membranes, drug delivery, and packaging (i.e., agricultural products, and medical devices) [5]. Therefore, studying the temperature dependence on gas or water vapor permeability of the polymer materials is crucial to food packaging in different or uncontrollable environmental conditions [6–8]. Generally, polymer films with temperature-dependent permeability are non-porous structures without pinholes, allowing water vapor or gas to pass through due to the morphological and chemical structure of the polymer material itself. Many polymers with temperature-dependent permeability have been studied, including thermoplastic polyurethanes, poly(N-isopropylacrylamide), and poly(ether-block-amide) (PEBAX). The application of these polymer films (non-porous structures) in food packaging can minimize the problems of quality loss and food deterioration usually originating from the porous structure of some polymer composite films under storage, distribution, and sales environments. The polymer films with temperature-dependent permeability can be positively modulated to achieve optimum qualities by controlling the gas or vapor permeability depending on the temperature changes of the environmental conditions [8,9]. PEBAX is a well-known block copolymer consisting of a polyamide block as a hard segment and a polyether block as a soft one. The crystalline amide block in PEBAX functions as an impermeable phase, whereas the ether block acts as the permeable phase due to its high chain mobility [10]. PEBAX has an excellent gas separation property for polar/nonpolar gas mixtures [11]. PEBAX also allows water vapor molecules to diffuse, while simultaneously blocks the transport of liquid water [12]. Thus, it can be used as a functional material in sportswear, food packaging, and medical packaging applications for gas sterilization [13]. The introduction of phase change materials (PCMs) into the polymer matrix is one feasible approach to control the gas or water vapor permeability responding to the desired temperature changes under the external atmosphere of packaging. PCMs are temperature-responsive substances that absorb and release large amounts of latent heat energy during temperature-driven phase changes [14]. Among various PCMs, polyethylene glycol (PEG) has attracted great interest due to its good characteristics such as various phase change temperatures depending on the molecular weight, congruent melting, non-toxicity, small or no volume changes during solid–liquid phase changes, and high thermal and chemical stability after long-term service. In addition, PEG can be directly incorporated into porous materials [15], it has a melting temperature around 3.2–68.7 ◦C, and a very high phase change enthalpy depending on its molecular weight [16]. Polyethylene glycol can possibly allow the development of composite films with optimal gas or water vapor permeability at different targeted temperature ranges. The composite films consisting of two different phases of polymer and PCM, and the transformation of this latter into a polymer matrix occurring near the PCM phase temperature can significantly affect the morphology of the entire polymer/PCM composite film, and effectively create passing channels for various penetrants in the polymer matrix. According to the above issues, the development of composite films with temperature-dependent permeability by introducing PCMs is a possible candidate for microwaveable packaging materials with controllable permeability properties that can be safe and easy to use. The unique property of the polymer/PCM composite films may create enough passing channels for the excessive internal gas or vapor buildup in packages during microwave cooking. In this work, we proposed to develop Polymers 2018, 10, 225 3 of 16 a polymer/PCM composite film packaging material with a temperature-dependent gas permeability, which could be used as a self-ventilating microwaveable material. This could be amenable to food packaging applications by preventing packaging damages and explosions during the cooking process. A series of (PEBAX)/(PEG) composite films (PBXPG) were prepared by the solution casting technique and their gas permeability, morphologies, thermal properties, and water sorption properties were investigated as a function of PEG with different molecular weights.

2. Materials and Methods

2.1. Materials PEBAX MH 1657, consisting of 60 wt % poly(ethylene oxide) (PEO) and 40 wt % polyamide 6 (PA-6) was purchased from Arkema Co. Ltd. (Paris, France). Three grades of PEG, PEG950–1050 (average Mn 950–1050), PEG3350 (average Mn 3,350), and PEG35000 (average Mn 35,000), were purchased from Sigma-Aldrich Korea Ltd. (Yongin, Korea). Ethyl alcohol (94 wt %) was purchased from SK Chemicals Co., Ltd. (Gyeonggi, Korea). All materials in this study were used as received.

2.2. Preparation of PEBAX, L-PBXPE, M-PBXPE, and H-PBXPE Composite Films First, 10 g of PEBAX was dissolved at a 10 wt % concentration in a mixture of ethanol/water (70/30 wt %) under reflux at 80 ◦C and stirring for 2 h. An equal amount of PEG was added to the PEBAX solution and stirred for 1 h. The obtained PEBAX and PEG mixture was casted onto a glass substrate using a bar-type automatic film coating apparatus (KIPAE E&T Co. Ltd., Hwasung, Korea). The films were dried in a vacuum oven at 30 ◦C for 24 h and then at 70 ◦C for 12 h. All films were maintained at a 100 ± 3 µm thickness to aid in evaluating their physical properties. Depending on the molecular weight of PEG, the composite films were designated as follows; L-PBXPG for PEG950–1050, M-PBXPG for PEG3350, and H-PBXPG for PEG35000, respectively.

2.3. Characterization

2.3.1. Fourier Transform Infrared (FT-IR) To characterize the chemical structure of pure PEBAX and composite films, FT-IR spectra were recorded on a Spectrum 65 FT-IR spectrometer (PerkinElmer Co., Ltd., Waltham, MA, USA) from 4000 to 400 cm−1 in an attenuated total reflection (ATR) mode with diamond/ZnSe (1 reflection) crystal.

2.3.2. Scanning Electron Microscopy (SEM) To investigate the surface morphology of the pure PEBAX and composite ﬁlms, we obtained SEM images for the top and fractured surfaces using a Quanta FEG250 scanning electron microscope (FEI Co., Ltd., Hillsboro, OR, USA) at acceleration voltage of 30 kV and working distance of 10 mm. The fractured surfaces of the ﬁlms were analyzed using specimens obtained after a tensile test. Prior to examination, all samples were coated with a thin layer of Pt/Pd.

2.3.3. Thermogravimetry (TGA) and Differential Scanning Calorimetry (DSC) We measured the thermal properties for different PEGs, pure PEBAX and the composite ﬁlms using a Q10 differential scanning calorimeter (TA Instrument Co. Ltd., New Castle, DE, USA). Samples of approximately 5 mg were heated from −80 to 220 ◦C at a heating rate of 10 ◦C/min in a nitrogen atmosphere (20 mL·min−1). We also tested the thermal stability of the composite ﬁlms using a TGA 4000 thermogravimetric analyzer (Perkin Elmer Co. Ltd., Waltham, MA, USA). The measurements were performed in a temperature range of 30 to 800 ◦C at a heating rate of 10 ◦C/min in a nitrogen atmosphere (20 mL·min−1). The mass of the samples was about 10 mg. Polymers 2018, 10, 225 4 of 16

2.3.4. Universal Testing Machine (UTM) We measured the mechanical properties of the composite films using a universal testing machine QM 100T (Qmesys Co. Ltd., Kwangmyeong, Korea) and compared them with the pure PEBAX film. The measurements were performed at a test speed of 500 mm/min for specimens that were 8 cm × 1 cm. We analyzed the resulting profiles using a MC tester version 12.6 (Qmesys Co., Ltd., Kwangmyeong, Korea).

2.3.5. Dynamic Vapor Sorption (DVS) We performed water sorption of the composite ﬁlms using a DVS Intrinsic (Surface Measurement Systems, London, U.K.) equipped with SMS UltraBalanceTM with a mass resolution of ± 0.1 µg. Typically, we placed 5 mg samples into the sample pan and dried them under a ﬂow of dry nitrogen at 25 ◦C and 0% relative humidity (RH) for 5 h. After that, we tested the samples at 25 ◦C and 95 % RH for 24 h. We obtained the water uptake by measuring the change in mass with respect to the dry mass.

2.3.6. Contact Angle and Surface Free Energies We measured the contact angles and surface free energies of the composite films using a Phoenix 300 contact angle goniometer (SEO Co. Ltd., Suwon, Korea). We carried out the measurements using deionized water and methylene iodide on samples at 25 ◦C, and we observed the changes via an eyepiece by applying the θ/2 method. We maintained the volume of the sessile drop as 5 µL in all cases by using a micro syringe. We averaged the contact angles for five drops and calculated the surface free energies applying the harmonic-mean equation [17] using the contact angles for water and methylene iodide on the composite films.

2.3.7. Oxygen Transmission Rate (OTR) We measured the OTR of the composite ﬁlms using a BT-1 gas transmission rate tester (Toyo Seiki Seisaku-sho, Ltd., Tokyo, Japan) at different temperatures ranging from 10 to 80 ◦C and a 0% RH in the chamber.

3. Results and Discussion

3.1. Preparation of Composite Films We studied the incorporation of different molecular weight PEG into the PEBAX matrix via FT-IR, as illustrated in Figure1. The pure PEBAX film exhibited several strong bands: 1093 cm −1 corresponding to the –C–O–C– group in the PEO segment, 1637, 1730, and 3306 cm−1 indicating −1 the –HNCO–, O–C=O, and –NH– group in the PA-6 segment, 2866 cm representing –CH2– in the soft and hard segment, and 3500 cm−1 for the terminal –OH group in PEBAX, respectively [18]. Although the composite films showed almost the same bands as pure PEBAX, there were some shifts in the characteristic peaks depending on the PEG introduction. The –NH– peak at 3306 cm−1 for pure PEBAX apparently shifted to lower wavenumbers: 3299 cm−1 for L-PBXPG, 3298 cm−1 for M-PBXPG, and 3298 cm−1 for the H-PBXPG composite films, whereas the –C–O–C– peak at 1093 cm−1 for pure PEBAX evidently moved to higher wavenumbers: 1095 cm−1 for L-PBXPG, 1099 cm−1 for M-PBXPG, and 1098 cm−1 for the H-PBXPG composite films, respectively. These changes happen due to the disruption of the existing interchain hydrogen bonding in PEBAX [19] and the formation of new hydrogen bonds between PEG and PEBAX [20]. PEG provides a relatively high population of –OH groups, which could interact with the –OH, –NH–, and –HNCO– groups and reduce the attractive intermolecular forces (hydrogen bonding) in PEBAX. It is expected that the composite films show a relatively good dispersion of PEG in the PEBAX matrix. Besides, a decrease in the intensity of the –OH peak with increasing molecular weight of the PEG in the composite films is likely to cause a decrease in the hydrophilicity of the composite films. Polymers 2018, 10, 225 5 of 16

Polymers 2018, 10, 225 5 of 16

Figure 1.Figure Fourier 1. Fourier Transform Transform Infrared Infrared (FT (FT-IR)‐IR) spectra of of the the pure pure poly(ether-block-amide) poly(ether‐block‐amide) (PEBAX) (PEBAX) and compositeand composite films. ﬁlms.

3.2. Morphology 3.2. Morphology SEM analysis is frequently performed to determine the dispersion and miscibility in the SEMcomposites, analysis which is frequently generally affect performed their thermal to determine stability, mechanical the dispersion properties, and and permeationmiscibility in the composites,properties which [21 –generally23]. As shown affect in Figuretheir 2thermal, the pure stability, PEBAX film mechanical had a relatively properties, rough appearance and permeation propertiesand [21–23]. a long ribbon As shown form resembling in Figure a nanofibril2, the pure on thePEBAX top surface, film had which a relatively might be related rough to appearance the and a longtwo ribbon separated form microphases resembling of PEBAX. a nanofibril The nanofibril on the morphology top surface, might which originate might from be the related oriented to the two polymer lamella and crystalline nature of PA-6 [24–26]. The L-PBXPG and M-PBXPG composite films separated microphases of PEBAX. The nanofibril morphology might originate from the oriented showed relatively smooth surfaces, whereas the H-PBXPG composite film exhibited a relatively rough polymersurface. lamella These and occur crystalline due to thenature low molecularof PA‐6 [24–26]. weight PEGs The withL‐PBXPG high chain and mobility, M‐PBXPG which composite act as films showedplasticizers relatively and smooth increase surfaces, the chain mobility whereas of PEBAX. the H This‐PBXPG results incomposite a smooth surface film inexhibited the composite a relatively rough surface.films with These low molecular occur due weight to PEGsthe low [27]. molecular High molecular weight weight PEGs PEGs withwith low high chain chain mobility mobility, seem which act as plasticizersto inhibit the and rearrangement increase the of the chain PEBAX mobility chain, resulting of PEBAX. in a relatively This results rough surface.in a smooth In the tensilesurface in the fracture surface SEM images, the pure PEBAX film showed a uniform and regular failure surface composite films with low molecular weight PEGs [27]. High molecular weight PEGs with low chain with an embossed line, which could be attributed to the plastic deformation of the pure PEBAX [28]. mobilityMorphology seem to inhibit changes arethe more rearrangement distinguishable forof thethe composite PEBAX filmschain, with resulting higher molecular in a weightrelatively of rough surface. PEG.In the The tensile H-PBXPG fracture film exhibited surface the SEM most irregular images, and the rough pure failure PEBAX surface film of all showed those composite a uniform and regular films.failure This surface might with be related an embossed to the chemical line, interaction which could and difference be attributed in flowability to the betweenplastic PEGdeformation of the pureand PEBAX.PEBAX The [28]. composite Morphology films show changes good interactionare more betweendistinguishable PEG and PEBAX,for the ascomposite described infilms with FTIR. However, the high molecular weight PEG has difficulty flowing in the PEBAX matrix, resulting higher molecular weight of PEG. The H‐PBXPG film exhibited the most irregular and rough failure in a very rough surface and noticeable plastic deformation under tensile stress. This result can be surface ofexpected all those to cause composite a decrease films. in mechanical This might properties. be related to the chemical interaction and difference in flowability between PEG and PEBAX. The composite films show good interaction between PEG and PEBAX, as described in FTIR. However, the high molecular weight PEG has difficulty flowing in the PEBAX matrix, resulting in a very rough surface and noticeable plastic deformation under tensile stress. This result can be expected to cause a decrease in mechanical properties.

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Figure 2. Scanning Electron Microscopy (SEM) images of the pure PEBAX and composite films. Figure 2. Scanning Electron Microscopy (SEM) images of the pure PEBAX and composite ﬁlms.

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3.3. Thermal Properties PolymersThe crystalline2018, 10, 225 phase of a polymer composite plays a significant role in determining its7 of physical 16 properties [29], and the degree of crystallinity can be generally estimated using the heat of fusion in a DSC3.3. Thermalthermogram Properties [30]. Figure 3 shows DSC curves of prepared composites and the values associatedThe with crystalline the thermal phase ofphenomena a polymer composite are reported plays in a Table significant 1. Regardless role in determining of the molecular its physical weight, all PEGsproperties showed [29], one and melting the degree temperature of crystallinity (Tm1 can): 40 be °C generally for L‐PEG, estimated 58 °C for using M‐PEG, the heat and of 64 fusion °C for H‐ PEG,in respectively. a DSC thermogram However, [30]. the Figure pure3 PEBAXshows DSC film curvesdisplayed of prepared two melting composites temperatures and the at values 16 and 199 °C correspondingassociated with theto thermalthe fusion phenomena of the crystalline are reported fraction in Table of1. Regardlessthe blocks of PEO the molecular and polyamide weight, (PA), ◦ ◦ ◦ respectivelyall PEGs showed [24]. All one the melting composite temperature films also (Tm1 showed): 40 C two for L-PEG,melting 58 temperatures,C for M-PEG, similar and 64 toC the for pure PEBAXH-PEG, film. respectively. The Tm1 and However, heat of thefusion pure (∆ PEBAXHm1) of film the displayed L‐PBXPG, two M‐ meltingPBXPG, temperatures and H‐PBXPG at 16 composite and ◦ films199 increasedC corresponding from 16 to the32, fusion51, and of 57 the °C, crystalline and 33 to fraction 68, 87, of and the 95 blocks J/g, respectively. PEO and polyamide Apparently, (PA), the respectively [24]. All the composite films also showed two melting temperatures, similar to the pure lower Tm1 and ∆Hm1 for the composite films increased with increasing molecular weight of PEG. This PEBAX film. The T and heat of fusion (∆H ) of the L-PBXPG, M-PBXPG, and H-PBXPG composite result indicates that m1there is an increased tendencym1 for higher molecular weight PEG to form the films increased from 16 to 32, 51, and 57 ◦C, and 33 to 68, 87, and 95 J/g, respectively. Apparently, crystalline phase due to the lower segmental mobility and more convenient geometrical alignments the lower Tm1 and ∆Hm1 for the composite films increased with increasing molecular weight of PEG. [31].This The result higher indicates melting that temperature there is an increased (Tm2) of tendencythe pure for PEBAX higher and molecular composite weight films PEG occurred to form the around 200 crystalline°C due to phase the melting due to the of lower the PA segmental domains mobility [32]. Comparing and more convenient Tm1 and geometrical ∆Hm1, with alignments Tm2 and ∆ [H31m2]., they showedThe higher different melting dependencies temperature on (theTm2 molecular) of the pure weight PEBAX of andPEGs. composite The incorporation films occurred of PEGs around into the ◦ PEBAX200 filmsC due did to the not melting influence of the Tm2 PA, but domains affected [32 ].the Comparing ∆Hm2 of theTm1 PAand domains∆Hm1, with by reducingTm2 and ∆ itH m2from, 35 J/g forthey pure showed PEBAX different to 5 dependenciesJ/g for L‐PBXPG, on the 6 J/g molecular for M‐PBXPG, weight of and PEGs. 9 J/g The for incorporation the H‐PBXPG of PEGscomposite films,into respectively. the PEBAX films The did reduction not influence of ∆HTm2m2 ,indicates but affected that the the∆H m2crystallinityof the PA domains of the composite by reducing films it is highlyfrom deteriorated, 35 J/g for pure and PEBAX that is to related 5 J/g for to L-PBXPG, the disruption 6 J/g for of M-PBXPG, the interchain and 9 hydrogen J/g for the bonding H-PBXPG in the PA compositedomain itself films, [19], respectively. as explained The reduction by the FT of‐IR∆H results.m2 indicates that the crystallinity of the composite films is highly deteriorated, and that is related to the disruption of the interchain hydrogen bonding in the PA domain itself [19], as explained by the FT-IR results.

FigureFigure 3. Differential 3. Differential Scanning Scanning Calorimetry Calorimetry (DSC) (DSC) curves curves of of the the pure pure PEBAX PEBAX and and composite composite ﬁlms. films.

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Figure 4 shows TGA curves of the pure PEBAX and composite films. All the PEGs and PEBAX showed one‐step decomposition. The decomposition temperature is the temperature at which the substancePolymers chemically2018, 10, 225 decomposes [33]. The decomposition temperature at 1%, 3%, and 85% of 16 mass loss in PEG, pure PEBAX, and composite films are reported in Table 1. The decomposition temperature increased withFigure increasing4 shows TGA molecular curves of weight the pure of PEBAX PEG. and However, composite the films. composite All the PEGs films and showed PEBAX a strong dependenceshowed of one-step molecular decomposition. weight on The the decomposition thermal stability. temperature The is thermal the temperature degradation at which of the PEG and PEBAX substanceis related chemically to the random decomposes chain [33 scission]. The decomposition mechanism temperature of the main at 1%, chain 3%, and [34,35]. 5% mass The loss composite films showedin PEG, one pure‐step PEBAX, decomposition, and composite similar films are to reported the pure in Table PEBAX,1. The indicating decomposition that temperature the presence of the increased with increasing molecular weight of PEG. However, the composite films showed a strong PEG does not significantly influence the thermal degradation pattern in the composite films. dependence of molecular weight on the thermal stability. The thermal degradation of PEG and PEBAX However,is related the composite to the random films chain showed scission higher mechanism thermal of stability the main chainfor higher [34,35]. molecular The composite weight films PEG. This result is showedrelated one-step to the thermal decomposition, stability similar of PEGs to the depending pure PEBAX, on indicating their molecular that the presence weight. of theHigh PEG molecular weight doesPEGs not with significantly relatively influence high the thermal thermal stability degradation delay pattern the in initial the composite thermal films. degradation However, in the thermallythe unstable composite PEO films blocks. showed higher thermal stability for higher molecular weight PEG. This result is related to the thermal stability of PEGs depending on their molecular weight. High molecular weight PEGs with relatively high thermal stability delay the initial thermal degradation in the thermally unstable PEO blocks.

FigureFigure 4. Thermogravimetry 4. Thermogravimetry (TGA) (TGA) curves curves of of the the pure pure PEBAX PEBAX and compositeand composite ﬁlms. films.

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Table 1. Thermal properties of the pure PEBAX and composite ﬁlms.

DSC TGA Code Mn Tm1 ∆Hm1 Tm2 ∆Hm2 Td1% Td3% Td10% Residual (mol wt) a (◦C) b (J/g) d (◦C) c (J/g) e (◦C) f (◦C) f (◦C) f content (%) g L-PEG 950–1050 40 135 - - 210 274 342 0 M-PEG 3350 58 141 - - 254 284 342 1 H-PEG 35000 64 180 - - 333 361 386 2 Pure PEBAX - 16 33 199 35 323 360 388 2 L-PBXPG - 32 68 199 5 245 324 376 3 M-PBXPG - 51 87 202 6 312 356 383 4 H-PBXPG - 57 95 197 9 337 364 385 4 a Number average molecular weight of L-PEG, M-PEG, and H-PEG. b,c Melting temperature of PEG, pure PEBAX, and composite films, respectively. d,e Melting enthalpy of PEG, pure PEBAX, and composite films, respectively. f 1 %, 3 %, and 5 % decomposition temperature of PEG, pure PEBAX, and composite films. g Residues of remained at 700 ◦C. Polymers 2018, 10, 225 10 of 16

3.4. Mechanical Properties PEBAX has flexible PEO and rigid PA‐6 segments. The crystalline PA‐6 domains provide mechanicalPolymers strength2018, 10, 225 and act as intermediate spacers between the PEO domains hindering10 of 16 their crystallization and offering greater chain mobility by the ether linkage [32]. As shown in Figure 5, the introduction3.4. Mechanical of PEGs Propertiesinto PEBAX induced an increase in both tensile strength and elongation at break in the compositePEBAX films. has flexibleAs described PEO and by rigid FT‐IR, PA-6 PEGs segments. have good The crystalline interaction PA-6 with domains PEBAX provide by forming hydrogenmechanical bonds with strength the PEBAX and act molecule as intermediate and disrupting spacers between the interchain the PEO domains hydrogen hindering bonding their between the PA‐6crystallization domains, resulting and offering in a greater reduction chain of mobility crystallinity by the ether in PA linkage‐6. As [32 a]. consequence As shown in Figure the composite5, films losethe their introduction mechanical of PEGs strength into PEBAX and induced elasticity an increase [36,37]. in However, both tensile the strength composite and elongation films atshowed a break in the composite films. As described by FT-IR, PEGs have good interaction with PEBAX by strong dependenceforming hydrogen of the bonds mechanical with the PEBAXproperties molecule on molecular and disrupting weight. the interchain In general, hydrogen amorphous bonding regions provide betweencertain elasticity, the PA-6 domains, whereas resulting the mechanical in a reduction strength of crystallinity of the materials in PA-6. As depends a consequence on the the degree of crystallinity.composite Higher films crystallinity lose their mechanical results strength in a andharder elasticity material [36,37 ].with However, more the brittle composite properties. films As expected,showed the composite a strong dependence films showed of the mechanical lower elongation properties on at molecular break, weight.but higher In general, tensile amorphous strength with higher molecularregions provide weight certain PEG. elasticity, This result whereas is therelated mechanical to the strength crystallinity of the materials of PEGs depends depending on the on their degree of crystallinity. Higher crystallinity results in a harder material with more brittle properties. molecularAs weight. expected, theHigher composite molecular films showed weight lower PEGs elongation with atrelatively break, but higherhigher tensile crystallinity strength with effectively enhancedhigher the crystallinity molecular weight in the PEG. PEO This segment result is related of PEBAX, to the crystallinityas reflected of by PEGs the depending ∆Hm1, and on shown their in the DSC results.molecular The increased weight. Higher crystallinity molecular of weight the composite PEGs with films relatively caused higher more crystallinity brittleness, effectively which brings about anenhanced increase the in crystallinitymechanical in strength the PEO segment and a decrease of PEBAX, in as elongation reflected by the at break∆Hm1, andin the shown composite in the films. DSC results. The increased crystallinity of the composite films caused more brittleness, which brings about an increase in mechanical strength and a decrease in elongation at break in the composite films.

Figure 5. Tensile strength and elongation at break of the pure PEBAX and composite films. Figure 5. Tensile strength and elongation at break of the pure PEBAX and composite films. 3.5. Water Sorption and Surface Free Energy 3.5. Water Sorption and Surface Free Energy As shown in Figure6, the pure PEBAX film showed lower water uptake than the composite As shownfilms. With in Figure increasing 6, molecularthe pure weightPEBAX of film PEG, showed the water lower uptake water and the uptake time to than get saturated the composite films. Withdecreased, increasing indicating molecular that the waterweight sorption of PEG, behavior the water strongly uptake depends and on thethe molecular time to weightget saturated of PEG. Usually, the water sorption behaviors of polymer films are correlated with the chemical decreased, indicating that the water sorption behavior strongly depends on the molecular weight of and/or morphological structure [38,39]. The water uptake relies on the chemical affinity to water PEG. Usually, the water sorption behaviors of polymer films are correlated with the chemical and/or morphological structure [38,39]. The water uptake relies on the chemical affinity to water (hydrophilicity). As expected, the composite films with higher molecular weight PEG showed lower water uptake because of a lower hydroxyl group’s population, as explained in the FT‐IR spectra. Besides, the rate of water uptake depends on the morphological structure, which relates to crystallinity and/or free volume. The composite films with higher molecular weight PEG showed a

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(hydrophilicity). As expected, the composite films with higher molecular weight PEG showed lower lower diffusion rate, i.e., longer time to get saturated due to higher crystallinity. H‐PBXPG has the water uptake because of a lower hydroxyl group’s population, as explained in the FT-IR spectra. highestBesides, crystallinity the rate of wateras reflected uptake by depends the heat on the of morphologicalfusion of 95 J/g structure, for the whichPEO segment relates to crystallinityand 9 J/g for the PA‐and/or6 segment, free volume. as depicted The composite in Table films1. with higher molecular weight PEG showed a lower diffusion rate,As i.e.,shown longer in timeFigure to get7, the saturated water duecontact to higher angle crystallinity. of the composite H-PBXPG films has showed the highest strong crystallinity dependence on theas reflected introduction by the of heat PEG. of However, fusion of 95 the J/g incorporation for the PEO of segment PEG into and the 9 J/g PEBAX for the matrix PA-6 did segment, not affects the asdiiodomethane depicted in Table contact1. angle. The contact angle is mostly affected by the chemical structure (polar and nonpolarAs shown groups) in Figure and7, morphology the water contact (surface angle roughness) of the composite of the films polymer showed surface strong dependence[40]. PEBAX and PEGon are the hydrophilic introduction polymers, of PEG. However, and the the polar incorporation groups easily of PEG interact into the with PEBAX water matrix molecules, did not affects hence, that obviouslythe diiodomethane influences the contact water angle. contact The angle. contact With angle increasing is mostly affected molecular by the weight chemical of PEG, structure the water contact(polar angle and nonpolar of the groups)composite and morphologyfilms increased, (surface indicating roughness) that of the the polymer composite surface films [40]. PEBAXbecome less and PEG are hydrophilic polymers, and the polar groups easily interact with water molecules, hence, hydrophilic. The total surface energies of the composite films were determined by the Owens–Wendt that obviously influences the water contact angle. With increasing molecular weight of PEG, the water methodcontact (geometric angle of themean composite combining films rule), increased, which utilizes indicating the that contact the composite angle measurement films become of deionized less waterhydrophilic. and diiodomethane The total surface [41]. energies In general, of the the composite total surface films were energy determined decreases by thewhen Owens–Wendt hydrophobicity increasesmethod [42]. (geometric In this meanwork, combining the total surface rule), which energy utilizes of the the composite contact angle films measurement decreased of with deionized increasing molecularwater and weight diiodomethane of PEG, showing [41]. In general, that the the hydrophilicity total surface energy of the decreases composite when films hydrophobicity also decreased. Higherincreases molecular [42]. In weight this work, of PEG the total has surfaceless polar energy (–OH) of the groups, composite leading films to decreased a lower total with surface increasing energy. Themolecular results of weight contact of angle PEG, showingand water that sorption the hydrophilicity confirm that of the the composite hydrophilicity films also of composite decreased. films decreasesHigher with molecular increasing weight molecular of PEG has weight less polar of (–OH) PEG. groups, leading to a lower total surface energy. The results of contact angle and water sorption confirm that the hydrophilicity of composite films decreases with increasing molecular weight of PEG.

FigureFigure 6. Water 6. Water sorption sorption isotherms isotherms of the purepure PEBAX PEBAX and and composite composite ﬁlms films measured measured at 95 at % relative95 % relative humidityhumidity (RH) (RH) and and 25 25 °C.◦C.

Polymers 2018, 10, 225 12 of 16 Polymers 2018, 10, 225 12 of 16

Figure 7. Contact angle and surface energy of the pure PEBAX and composite ﬁlms. Figure 7. Contact angle and surface energy of the pure PEBAX and composite films.

3.6.3.6. Oxygen Oxygen Permeability Permeability 3 2 ◦ AsAs shown in Figure 88,, thethe OTR OTR of of the the pure pure PEBAX PEBAX film film was was 521 521 cm cm3/(m/(m2∙·24h24h∙atm)·atm) at at 10 10 °C,C, 3 2 3 2 whereaswhereas the the composite composite films films showed showed 34.4 34.4 cm cm3/(m/(m2∙24h·24h∙atm)·atm) for for L‐PBXPG, L-PBXPG, 254 254 cm cm3/(m/(m2∙24h·∙24hatm)·atm) for 3 2 Mfor‐PBXPG, M-PBXPG, and and 201 201 cm cm3/(m/(m2∙24h·∙24hatm)·atm) for forH‐PBXPG, H-PBXPG, respectively. respectively. Obviously, Obviously, the the composite composite films films ◦ showedshowed lower lower values values than than the the pure pure PEBAX PEBAX film. film. With With increasing increasing temperature temperature from from 10 to 10 80 to °C, 80 theC, 3 2 purethe pure PEBAX PEBAX film film showed showed a linear a linear increase increase from from 521 521 to 5190 to 5190 cm cm3/(m/(m2∙24h·∙24hatm).·atm). The TheL‐PBXPG L-PBXPG and andM‐ 3 2 PBXPGM-PBXPG films films also also showed showed a linear a linear increase increase in OTR in OTR from from 39.4 39.4 to 3970 to 3970 cm cm3/(m/(m2∙24h·∙atm),24h·atm), and andfrom from 254 3 2 to254 7980 to 7980 cm3/(m cm2∙24h/(m∙atm),·24h ·respectively.atm), respectively. However, However, the H‐ thePBXPG H-PBXPG film showed film showed an abnormally an abnormally large 3 2 increaselarge increase from 201 from to 201 24,900 to 24,900 cm3/(m cm2∙24h/(m∙atm).·24h ·Theatm). OTR The of OTR H‐PBXPG of H-PBXPG film slowly film slowly increased increased with ◦ increasingwith increasing temperature temperature from from 10 to 10 60 to °C 60 andC and distinctly distinctly jumped jumped when when the the temperature temperature reached reached ◦ approximatelyapproximately 65 65 °C.C. This This demonstrates demonstrates that that the the oxygen oxygen permeability permeability of of the the composite composite films films may may dependdepend onon thethe molecular molecular weight weight of of PEG. PEG. The The permeation permeation of gases of gases through through polymer polymer films can films be usuallycan be usuallyexplained explained in terms in of terms a “solution-diffusion” of a ‘‘solution‐diffusion’’ mechanism mechanism [43] which [43] consists which of consists the following of the following steps [44]: steps(1) solution [44]: (1) (absorption) solution (absorption) of small molecules of small molecules into the film into at the the film side at of the the side higher of the potential higher (pressure,potential (pressure,concentration, concentration, etc.); (2) molecular etc.); (2) molecular diffusion ofdiffusion the molecules of the molecules in and through in and the through film; and the (3) film; release and (3)(desorption) release (desorption) of the diffused of the moleculesdiffused molecules from the from solution the solution at the opposite at the opposite side into side the into liquid the liquid or gas orphase gas atphase lower at potential.lower potential. The gas The permeability gas permeability of the polymer of the polymer relies on relies many on factors many like factors the degree like the of degreecrystallinity, of crystallinity, molecular molecular weight, and weight, chemical and chemical affinity [ 45affinity]. As described[45]. As described in the introduction in the introduction section, section,the PEO the segment PEO segment acts as a acts permeable as a permeable phase due phase to its due high to its chain high mobility chain mobility and PEG and is a PEG very is ordered a very orderedstructure, structure, which is which easy is to easy crystallize. to crystallize. This will This create will create a semicrystalline a semicrystalline soft phasesoft phase and and reduce reduce the themass mass transport transport properties properties in in the the film film or or membrane membrane [24 [24].]. As As expected, expected, the the compositecomposite films films showed aa lower lower OTR OTR at at temperatures temperatures below below Tm1T thanm1 than the pure the pure PEBAX PEBAX film. film.The composite The composite films have films higher have crystallinityhigher crystallinity (lower free (lower volume) free volume) than the than pure the PEBAX pure PEBAX film, therefore film, therefore gas molecules gas molecules cannot cannoteasily diffuseeasily diffuse through through them. The them. molecular The molecular weight weight of the polymer of the polymer is related is related to the tonumber the number of chain of ends, chain whichends, whichalso influences also influences the gas the permeability. gas permeability. The chain The chainends represent ends represent discontinuities discontinuities and may and form may sitesform for sites the for gas the molecules gas molecules to be absorbed, to be absorbed, diffused, diffused, and desorbed and desorbed through through the polymer the polymer film. As film. the molecularAs the molecular weight weightof the polymer of the polymer increases, increases, the number the number of chain of chainends also ends decreases, also decreases, resulting resulting in a

Polymers 2018, 10, 225 13 of 16 Polymers 2018, 10, 225 13 of 16 decrease in gas permeability. Generally, gas permeability decreases when the crystallinity and inmolecular a decrease weight in gas of permeability.the polymer increase. Generally, However, gas permeability our results decreases show a contrary when the trend: crystallinity the H‐PBXPG and molecularfilm with a weight higher of molecular the polymer weight increase. of PEG However, and crystallinity our results has show a higher a contrary OTR trend: than the M H-PBXPG‐PBXPG filmand withL‐PBXPG a higher films. molecular This phenomenon weight of is PEG related and to crystallinity the chemical has affinity a higher (functional OTR than group) the M-PBXPG between andthe polymer L-PBXPG films films. and This gas phenomenon molecules. The is related hydroxyl to the terminal chemical groups affinity (polar (functional groups) group) in PEG between have a theweak polymer affinity films to oxygen and gas molecules molecules. (non The‐polar hydroxyl gas), resulting terminal in groups a lower (polar solubility groups) and inpermeability PEG have aof weak oxygen. affinity As the to oxygen molecular molecules weight (non-polar of PEG increases, gas), resulting the number in a lower of hydroxyl solubility terminal and permeability groups of the of oxygen.films decreases As the too, molecular leading weight to a higher of PEG solubility increases, of oxygen. the number Therefore, of hydroxyl the H‐PBXPG terminal film groups with higher of the filmsmolecular decreases weight too, PEG leading showed to a highera higher solubility OTR than of the oxygen. M‐PBXPG Therefore, and L the‐PBXPG H-PBXPG films. film with higher molecularWhen weight increasing PEG showedthe temperature a higher above OTR than Tm1 theup M-PBXPGto 80 °C, the and crystalline L-PBXPG films.structure of the PEO ◦ segmentWhen starts increasing to convert the to temperature an amorphous above structure.Tm1 up Also, to 80the intramolecularC, the crystalline interaction structure collapses of the PEOduring segment the phase starts change to convert process to of an PEG amorphous in the vicinity structure. of the Also, phase the transition intramolecular temperature. interaction These collapsesoccurrences during contribute the phase to an change increase process in the of chain PEG mobility in the vicinity and free of volume. the phase As transition a result, the temperature. composite Thesefilms seem occurrences to open contribute the diffusion to an path increase for oxygen in the molecules chain mobility [46–48], and which free leads volume. to a Aspermeation a result, thejump. composite The L‐PBXPG films seemand M to‐PBXPG open the films diffusion showed path a steady for oxygen increase molecules in OTR similar [46–48 to], the which pure leads PEBAX to afilm. permeation However, jump. the H The‐PBXPG L-PBXPG film apparently and M-PBXPG exhibited films a showeddistinct permeation a steady increase jump in in the OTR vicinity similar of tothe the phase pure PEBAXchange film.of H However,‐PEG. The the H‐ H-PBXPGPBXPG film film contains apparently H‐PEG exhibited (Mn a~ distinct 35,000), permeation which is jumpapproximately in the vicinity 10 to of 30 the times phase higher change than of M H-PEG.‐PEG ( TheMn H-PBXPG~ 3350) and film L‐PEG contains (~950–1050), H-PEG ( Mrespectively.n ~35,000), whichThis dissimilarity is approximately can affect 10 to the 30 timeschemical higher affinity than for M-PEG oxygen (Mn molecules, ~3350) and which L-PEG influences (~950–1050), the respectively.solubility of oxygen, This dissimilarity as mentioned can affectabove. the Based chemical on these affinity factors, for the oxygen H‐PBXPG molecules, film with which high influences oxygen thesolubility solubility exhibited of oxygen, a big as permeation mentioned jump above. in the Based vicinity on these of the factors, phase the transition H-PBXPG temperature film with of high H‐ oxygenPEG. The solubility unique exhibited properties a big like permeation a big permeation jump in thejump vicinity and gas of the controllable phase transition permeability temperature with oftemperature H-PEG. The dependence unique properties can be potentially like a big permeation as self‐ventilation jump and for gasmicrowavable controllable food permeability products. with temperatureIn this study, dependence the composite can be potentially film showed as self-ventilation a low OTR value for microwavable at the low temperature. food products. The low permeationIn this study,properties the compositeof the composite film showed films can a be low maintained OTR value the at quality the low and temperature. shelf life of prepared The low permeationfood under properties storage, distribution, of the composite and filmssale canenvironment be maintained condition. the quality Some and food shelf products life of prepared such as foodseafood under generate storage, internal distribution, steam around and sale 62 environment°C during microwave condition. heating Some [49]. food The products H‐PBXPG such film as seafoodshowed generatea big permeation internal steamjump at around the vicinity 62 ◦C duringof 65 °C. microwave It is highly heating anticipated [49]. Thethat H-PBXPGthis composite film showedfilm can a be big used permeation as a potential jump at packaging the vicinity material of 65 ◦C. with It is highlyself‐ventilation anticipated to that release this compositeinternal steam film canduring be used microwave as a potential cooking. packaging material with self-ventilation to release internal steam during microwave cooking.

FigureFigure 8.8. Oxygen transmission raterate ofof thethe purepure PEBAXPEBAX andand compositecomposite ﬁlms.films.

Polymers 2018, 10, 225 14 of 16

4. Conclusions In this study, we prepared a series of PBXPG composite films with different molecular weight of PEG for microwave packaging applications. Apparently, incorporation of PEG into PEBAX film lead to changes in morphology and chemical structure in the composite film, which affected thermal stability, mechanical properties, water sorption, surface properties, as well as permeability, depending on the molecular weight of PEG. As the molecular weight of PEG increased, thermal stability and mechanical strength increased. Further, the water vapor uptake and rate of water vapor diffusion in the composite films decreased, whereas the gas permeability increased. This demonstrates that the gas permeability and water sorption properties of composite films can be controlled by introducing different molecular weight of PEG. Among composite films, the H-PBXPG composite film with a relatively high molecular weight apparently showed a big permeation jump near the phase transition temperature of H-PEG (~65 ◦C). This temperature is very near the internal steam temperature (~62 ◦C) in some food products. Such unique properties like a big permeation jump and gas controllable permeability with temperature dependence achieved by incorporating H-PEG into the PEBAX film lead us to conclude that these films can be potentially used as packaging materials with self-ventilation by releasing internal steam for microwavable food products.

Acknowledgments: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4011234). Author Contributions: Jongchul Seo and Dowan Kim designed the experiments; Sarinthip Thanakkasaranee performed the experiment; Sarinthip Thanakkasaranee, Dowan Kim, and Jongchul Seo analyzed the data; Sarinthip Thanakkasaranee wrote and Jongchul Seo finalized the manuscript; all authors have read and approved the final version of this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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