Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol On the effects of hydroxyl substitution degree and molecular weight on mechanical and water barrier properties of hydroxypropyl methylcellulose films Caio G. Otonia,b,⁎, Marcos V. Lorevicea,c, Márcia R. de Mourad, Luiz H.C. Mattosoa aNanotechnology National Laboratory for Agriculture (LNNA), Embrapa Instrumentation – Rua XV de Novembro, 1452, São Carlos, SP, 13560-970, Brazil b PPG-CEM, Department of Materials Engineering, Federal University of São Carlos – Rodovia Washington Luís, km 235, São Carlos, SP, 13565-905, Brazil c PPGQ, Department of Chemistry, Federal University of São Carlos – Rodovia Washington Luís, km 235, São Carlos, SP, 13565-905, Brazil d Department of Physics and Chemistry, FEIS, São Paulo State University – Av. Brasil, 56, Ilha Solteira, SP, 15385-000, Brazil A R T I C L E I N F O Chemical compounds studied in this article: Hydroxypropyl methylcellulose (PubChem CID: 57503849) Keywords: Biopolymer Cellulose derivative Cellulose ether Hypromellose Food packaging Edible film A B S T R A C T In line with the increasing demand for sustainable packaging materials, this contribution aimed to investigate the film-forming properties of hydroxypropyl methylcellulose (HPMC) to correlate its chemical structure with film properties. The roles played by substitution degree (SD) and molecular weight (Mw) on the mechanical and water barrier properties of HPMC films were elucidated. Rheological, thermal, and structural experiments supported such correlations. SD was shown to markedly affect film affinity and barrier to moisture, glass transition, resistance, and extensibility, as hydroxyl substitution lessens the occurrence of polar groups. Mw affected mostly the rheological and mechanical properties of HPMC-based materials. Methocel® E4M led to films featuring the greatest tensile strength (ca., 67MPa), stiffness (ca., 1.8 GPa), and extensibility (ca., 17%) and the lowest permeability to water vapor (ca., 0.9 g mm kPa−1 h−1 m−2). These properties, which arise from its longer and less polar chains, are desirable for food packaging materials. 1. Introduction Recently, there has been an increasing trend towards the use of biopolymers as film-forming materials (e.g., for food packaging appli- cations) in an effort to reduce the environmental problems arising from the unrestricted exploitation of fossil fuels and the inadequate disposal of non-biodegradable materials (Azeredo & Waldron, 2016; Garavand, Rouhi, Razavi, Cacciotti, & Mohammadi, 2017). Cellulose is a widely available homopolysaccharide. It is made up of β-D-glucopyranoside units, linked by 1,4-glycosidic bonds, and arranged as long, linear, unbranched chains. These aspects, in addition to high occurrence of hydroxyl groups – three per anhydroglucose ring – provide cellulose with extremely strong intermolecular interactions, which in turn result in fibrous aspect and high stiffness as well as in infusibility and in- solubility in water and most organic solvents (Zhang, Zhang, Tian, Zhou, & Lu, 2013), characteristics that are undesirable from the polymer processing standpoint. In order to increase the processability of cellulose as a film-forming matrix, cellulose derivatives have been produced by the partial sub- stitution of hydroxyl groups by bulkier, less reactive groups. Different cellulose ethers have been demonstrated to be suitable film-forming matrices for food packaging applications, including methylcellulose (Bertolino et al., 2016), hydroxypropyl cellulose (Cavallaro, Donato, Lazzara, & Milioto, 2011; Cavallaro, Lazzara, Konnova, Fakhrullin, & Lvov, 2014), and hydroxypropyl methylcellulose (HPMC) (Alzate, Miramont, Flores, & Gerschenson, 2017; Moghimi, Aliahmadi, & Rafati, 2017). HPMC stands out because it is also water soluble, odorless, tasteless (Burdock, 2007), generally recognized as safe (GRAS) by the United States Food and Drug Administration (US FDA) (GRAS Notice No. GRN 000213, 2007), and allowed for direct (e.g., as an additive) and indirect (e.g., as a food-contacting packaging material) food ap- plications by US FDA (21 CFR 172.874, 2011) and European Union https://doi.org/10.1016/j.carbpol.2018.01.016 Received 10 November 2017; Received in revised form 3 January 2018; Accepted 5 January 2018 ⁎ Corresponding author at: Nanotechnology National Laboratory for Agriculture (LNNA), Embrapa Instrumentation – Rua XV de Novembro, 1452, São Carlos, SP, 13560-970, Brazil. E-mail addresses: cgotoni@gmail.com (C.G. Otoni), marcos.lorevice@gmail.com (M.V. Lorevice), marcia@dfq.feis.unesp.br (M.R.d. Moura), luiz.mattoso@embrapa.br (L.H.C. Mattoso). Abbreviations: BET, Brunauer-Emmett-Teller; DSC, differential scanning calorimetry; DTG, derivative thermogravimetric; DVS, dynamic vapor sorption; FFS, film-forming solutions; FT- IR, Fourier-transform infrared spectroscopy; GAB, Guggenheim-Anderson-de Boer; GRAS, generally recognized as safe; HP, hydroxypropyl content; HPC, hydroxypropyl cellulose; HPMC, hydroxypropyl methylcellulose; HP-SEC, high-performance size exclusion chromatography; M, methoxyl content; Mn, number average molecular weight; MS, molar substitution; Mw, weight average molecular weight; Mw/Mn, polydispersity index; PMMA, poly(methyl methacrylate); RH, relative humidity; SD, substitution degree; TG, thermogravimetric; US FDA, United States Food and Drug Administration; WVP, water vapor permeability Carbohydrate Polymers 185 (2018) 105–111 Available online 06 January 2018 0144-8617/ © 2018 Elsevier Ltd. All rights reserved. T http://www.sciencedirect.com/science/journal/01448617 https://www.elsevier.com/locate/carbpol https://doi.org/10.1016/j.carbpol.2018.01.016 https://doi.org/10.1016/j.carbpol.2018.01.016 mailto:cgotoni@gmail.com mailto:marcos.lorevice@gmail.com mailto:marcia@dfq.feis.unesp.br mailto:luiz.mattoso@embrapa.br https://doi.org/10.1016/j.carbpol.2018.01.016 http://crossmark.crossref.org/dialog/?doi=10.1016/j.carbpol.2018.01.016&domain=pdf (EPCD No. 95/2/EC, 1995). These characteristics allowed HPMC to be used as matrix for edible films (Otoni et al., 2017), tablets (Zhang et al., 2017), and oral-disintegrating sheets (Borges, Silva, Coelho, & Simões, 2015). It is produced upon etherification reactions among alkaline cellulose and methyl chloride and propylene oxide, leading to the re- placement of hydroxyl groups by methoxyl (–OCH3) and hydroxypropyl (–OCH2CH(OH)CH3) ones, respectively (Burdock, 2007; Keary, 2001). The average number of hydroxyl groups replaced by methoxyl groups in an anhydroglucose unit is known as substitution degree (SD) whereas molar substitution (MS) indicates the number of propylene oxide moles reacted in each anhydroglucose unit. It has been demonstrated that HPMC chemical structure, markedly substitution pattern, plays a role in drug release mechanisms (Caccavo et al., 2017), hydration capacity (Arai & Shikata, 2017; Caccavo et al., 2017), and emulsion-stabilizing ability (Shimada, Fonseca, & Petri, 2017). We therefore hypothesized that substitution degree and mole- cular weight also affect the physical-mechanical properties of free- standing thin films. Larsson, Viridén, Stading, and Larsson (2010) have shown that substitution pattern affects glass transition temperature and water plasticization of HPMC-based films, while Espinoza-Herrera, Pedroza-Islas, San Martín-Martinez, Cruz-Orea, and Tomás (2011) stu- died the thermal, mechanical, and microstructural properties of dif- ferent cellulose derivative films, including HPMC. However, to the best of our knowledge, no previous studies set out to investigate the effect of HPMC SD and molecular weight on the most important technical as- pects of films intended for food packaging applications, i.e., mechanical and water barrier properties. In this context, this contribution aimed at using HPMC grades of different SDs and molecular weights to produce films as well as to study aspects related to physical-mechanical prop- erties and barrier to moisture in an effort to correlate the chemical structure with the properties and performance of the resulting mate- rials. Rheological, thermal, spectroscopic, and water sorption experi- ments were also carried out to further support these correlations. 2. Experimental 2.1. Materials Three HPMC (CAS No. 9004-65-3; EU No. E464; INS No. 464; NAS No. 0534) grades were kindly donated by The Dow Chemical Company (São Paulo, Brazil): Methocel® E15 (average methoxyl content/hydro- xypropyl content (M/HP): 3.05), Methocel® K4M (average M/HP: 2.26), and Methocel® E4M (average M/HP: 3.05). The SD, MS, and methoxyl and hydroxypropyl contents of the HPMC grades are compiled in Table 1. Ultrapure water (ρ=18.2MΩ cm), deionized on a Milli-Q system (Barnstead Nanopure Diamond, USA), was used in all experi- ments. Hydroxypropyl cellulose (HPC) samples having weight average molecular weights of 12,000; 20,500; 50,500; 86,300; 153,500; 205,600; 388,000; 637,000; and 865,000 gmol−1 were purchased from American Polymer Standards Co. (Mentor, OH, USA). 2.2. Molecular weight The molecular weights of the different HPMC grades were de- termined through high-performance size exclusion chromatography (HP-SEC) on a liquid chromatograph (model SCL-10A, Shimadzu Co., Japan) equipped with differential refractive index detector (model RID- 20A, Shimadzu Co.) and UV–vis spectrophotometric detector (model SPD-10AV, Shimadzu Co.). The mixture NaNO3 0.1M/ethylene glycol 0.1% was used as eluent. A 50mm×6mm (10 μm) pre-column (Shodex OHpak KB-G) as well as two 8mm ID×300mm (13 μm) columns (Shodex OHpak KB–806M) were associated in series, filled with poly(hydroxy methacrylate) gel, and used in HP-SEC runs. Standard HPC, cellobiose, glucose, and ethylene glycol samples were used to build a standard curve. Runs were performed with injection volume of 20 μL, temperature of 35 °C, and flow equal to 1.0 mLmin−1. Data acquisition and treatment were carried out in CLASS-LC10 soft- ware (version 1.21). 2.3. Rheological measurements Aqueous HPMC solutions at 1, 2 or 3% (wt.) were analyzed on a rotational rheometer (model MCR 301, Anton Paar GmbH, Austria) operating with concentric cylinder geometry (DG26.7) and in steady shear rates ranging from 0.01 to 10,000 s−1, at 20 °C. 2.4. Film casting HPMC powders were solubilized in distilled water under magnetic stirring for 12 h to form 2% (wt.) film-forming solutions (FFS). The solutions were degassed under vacuum and spread with a controlled thickness over level poly(ethylene terephthalate) supports, where they were allowed to dry at 25 ± 2 °C for 24 h. Dried films were equili- brated at 50% RH for at least 48 h prior to testing. 2.5. Fourier-transform infrared spectroscopy (FT-IR) The infrared spectra of HPMC films were obtained on a FT-IR spectrometer (model VERTEX 70, Bruker Optik GmbH, Germany) equipped with an ATR module and operating in reflectance mode. The samples were screened within the spectral region from 4000 to 600 cm−1 with a resolution of 2 cm−1. 2.6. Thermogravimetry Film samples (5–6mg) were accurately weighed in a platinum pan and heated from 25 to 600 °C at a rate of 10 °Cmin−1, within an at- mosphere comprising synthetic air (21% O2) flowing at 40mLmin−1. Sample weight was monitored by a high-precision balance within an atmosphere comprising nitrogen flowing at 60mLmin−1 as a function of temperature on a TA Q500 (TA Instruments, Inc., New Castle, USA) equipment in order to obtain thermogravimetric (TG) and derivative TG (DTG) curves. 2.7. Differential scanning calorimetry (DSC) Film samples (3–4mg) were precisely weighed in aluminum pans and heated from −80 to 240 °C at a rate of 10 °Cmin−1, in an atmo- sphere with nitrogen flowing at 50mLmin−1. Heat flow was monitored as a function of temperature on a DSC Q100 (TA Instruments, Inc.) calorimeter. Table 1 Substitution pattern and HP-SEC. Substitution degree (SD), methoxyl content (M), molar substitution (MS), hydroxypropyl content (HP), weight (Mw) and number (Mn) average molecular weights, and polydispersity indexes (Mw/Mn) of different hydroxypropyl methylcellulose (HPMC) grades. HPMC SD M (%) MS HP (%) Mw (gmol−1) Mn (g mol−1) Mw/Mn Methocel® E15 1.9 28–30 0.23 7–12 51,097 15,481 3.30 Methocel® E4M 1.9 28–30 0.23 7–12 351,490 80,890 4.35 Methocel® K4M 1.4 19–24 0.21 7–12 331,893 75,211 4.41 C.G. Otoni et al. Carbohydrate Polymers 185 (2018) 105–111 106 2.8. Mechanical properties Films were shaped in at least six specimens per treatment according with ASTM D882–12 (ASTM, 2012) and submitted to uniaxial tensile test. Films were stretched at 10mmmin−1 by flat grips initially sepa- rated by 100mm (L0) on a DL3000 universal testing machine (EMIC Equipamentos e Sistemas de Ensaio Ltda., São José dos Pinhais, Brazil) equipped with a 10-kgf load cell. The mechanical attributes engineering tensile strength (σT), percent elongation at break (εR), and Young’s modulus (E) were determined by Eqs. (1)–(3), respectively, wherein F, L, and A0 are the maximum load, the ultimate specimen extension (at break), the initial specimen cross-sectional area (i.e., width * thickness). Thickness was taken as the average of at least three random measure- ments throughout sample gauge length, measured to the nearest 0.001mm with a digital micrometer (Mitutoyo Corp., Kanogawa, Japan). = F Aσ /T 0 (1) = − ⋅L L Lε [( )/ ] 100R 0 0 (2) = → E li Lmσ/ L 0 (3) Dynamic mechanical thermal analyses have been performed to provide further insights on the mechanical and thermal behaviors of HPMC films. Rectangular (12.0–13.0mm in length, 6.4–6.8 mm in width, and 0.023–0.033mm in thickness) specimens were stretched in oscillatory mode at amplitude of 0.1% and frequency of 1 Hz on a DMA Q800 (TA Instruments, Inc.) operating at tension mode with tempera- ture ramping at 2 °Cmin−1 from −60 to 250 °C. 2.9. Water vapor permeability (WVP) WVP was determined in accordance with the gravimetric modified cup method based on ASTM E96-92 (McHugh, Avena-Bustillos, & Krochta, 1993). Films were shaped into circles and sealed with silicone grease onto poly(methyl methacrylate) (PMMA) cups having 19.6-cm2 openings that were then topped with symmetrically screwed, open PMMA rings. Test cups were filled with 6mL of distilled water and placed in cabinets with controlled RH (lower than 30%, maintained with silica) and temperature (30 ± 1 °C). After steady state of water vapor transmission rate was reached, cups were periodically weighed within 24 h in 2-h intervals. At least four replicates of each film were used for WVP determination. 2.10. Dynamic vapor sorption (DVS) Adsorption/desorption isotherms in/from HPMC films were ob- tained at 25 °C on a DVS-1 system (Surface Measurement Systems Ltd., USA). Films were previously dehydrated in desiccators at 25 °C and the weights of ca. 5-mg samples were monitored while RH was varied from 0 to 98% and then from 98 to 0% in 7% intervals. 2.11. Statistical treatment of data Quantitative data were submitted to analysis of variance followed by Tukey’s mean comparison test, both at 5% of significance. 3. Results and discussion 3.1. Molecular weight and rheological aspects The size and size distribution of HPMC chains were determined by HP-SEC. The obtained standard curve was MW=5.322·10−4 (10− t)3+ 2.295·10−3 (x− 10)2− 03644 (x− 10)+ 7.3559; R2=0.9983, wherein MW is the logarithm of the molecular weight – in g – and t is retention time – in min. The obtained chromatograms are presented in Supplementary Fig. S1, whereas molecular weight data are compiled in Table 1. It can be observed that HPMC Methocel® E4M and Methocel® K4M have remarkably longer chains than HPMC Methocel® E15. Indeed, the designations 15 and 4M are related to the viscosities of 2% (w v−1) aqueous HPMC solutions at 20 °C, as disclosed by the manufacturers: 12–18 and 3000–5600mPa s. This is in accordance with the rheological behaviors of the HPMC solutions produced here (Fig. 1), which in turn are direct consequences of their molecular weights. As expected, solutions comprising higher HPMC contents presented greater steady shear viscosity, regardless of the HPMC grade. SD af- fected the steady shear viscosity, which can be attributed to branching and intermolecular interactions that have been demonstrated to affect the flow behavior of cellulose derivatives (Borges et al., 2015). Mole- cular weight, in turn, had a pronounced effect on the steady shear viscosity of HPMC solutions, particularly at low shear rates. This is indicated by the steady shear viscosity values of solutions comprising longer HPMC chains, which are ca. two orders of magnitude greater than their shorter analogues (Fig. 1). Longer chains experience greater entanglement levels, being capable of offering resistance to flow and, therefore, leading to increased viscosity. It is also noteworthy the shear thinning-to-Newtonian transition in HPMC Methocel® E15. At low shear rates, the highly entangled chains of polymer solutions offer high re- sistance to flow, resulting in high viscosity. As shear rates increase, the macromolecules are gradually unraveled, bringing about shear thinning behavior. At sufficient shear levels, entanglements are scarce and chains are aligned towards flow direction, situation in which polymer solutions may present Newtonian behavior. Provided that it is easier to disentangle shorter chains, this transition was exclusively observed in HPMC Methocel® E15 because of its lower molecular weight. 3.2. Fourier-transform infrared spectroscopy (FT-IR) Fig. 2 shows the FT-IR spectra of the HPMC films. All spectra pre- sented bands close to 2900 cm−1 (at 2972, 2902, and 2836 cm−1, more specifically) attributed to the axial deformation of the CeH bonds in aliphatic chains (Sakata & Yamaguchi, 2011), that is, to eCH3 arising from the substitution of hydroxyl groups by methoxyl and hydro- xypropyl ones. Absorptions related to the axial deformation of CeOeC bonds, typical in cellulose ethers, may be observed from 900 to 1300 cm−1 (Anuar, Wui, Ghodgaonkar, & Taib, 2007; Zaccaron, Oliveira, Guiotoku, Pires, & Soldi, 2005). Bands at wavenumbers ran- ging from 1250 to 1460 cm−1 (herein observed at 1315, 1374, 1410, and 1452 cm−1) are assigned to the angular deformation of CeH bonds 0,1 1 10 100 1000 1E-3 0,01 0,1 1 10 0.01 0.1 0.1 Fig. 1. Rheological aspects. Steady shear viscosity of aqueous solutions comprising 1 (■), 2 (●) or 3% (►) (m v−1) of hydroxypropyl methylcellulose Methocel® E15 (black), HPMC Methocel® E4M (red) or HPMC Methocel® K4M (blue) – shear stress versus shear rate curves are presented in Supplementary Fig. S2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) C.G. Otoni et al. Carbohydrate Polymers 185 (2018) 105–111 107 within e(CH2)ne chains (Sakata & Yamaguchi, 2011). The band close to 1645 (Zaccaron et al., 2005) and 1650 cm−1 (Anuar et al., 2007), herein observed at 1637 cm−1, is related with the axial deformation of carbonyl groups present in the glucose unit of cellulose. Although similar, these spectra have differences, such as the band at 3420 cm−1 corresponding to the axial deformation of OeH bonds in HPMC Methocel® K4M (Zaccaron et al., 2005). The shift of this band towards higher wavenumbers – ca. 3460 cm−1 – in the other samples suggests the weakening of the hydrogen bond interaction network (Banks, Sammon, Melia, & Timmins, 2005), provided that the former has a higher occurrence of hydroxyl groups. This is supported by the broader band presented by the less substituted grade, i.e., HPMC Methocel® K4M (Sakata & Yamaguchi, 2011). 3.3. Thermal properties The TG and DTG curves of the produced films show three well de- fined weight loss stages (Fig. 3). The first stage took place between 30 and 100 °C, with maximum weight loss rates at 51–60 °C. It is attributed to intermolecular dehy- dration, i.e., physical desorption of free moisture within the hygro- scopic matrix (Ford, 1999; Li, Huang, & Bai, 1999). Films based on Methocel® E15 and Methocel® EM grades presented weight losses not higher than 1% at this stage, whereas those based on HPMC Methocel® K4M lost 4.7% of their original masses (the DTG peak temperatures and sample weights after each weight loss stage are presented in Supple- mentary Table S1). This reflects the higher equilibrium moisture of the less substituted sample, which therefore comprise a higher hydroxyl content. Feller and Wilt (1990) stated that, for films based on cellulose ethers, the lower the SD, the greater the equilibrium moisture. This was corroborated by determinations of the moisture contents of HPMC- based films at 105 °C in oven drying until constant weight was achieved: Methocel® K4M (20.6 ± 1.8%) was higher (p < 0.05) than Methocel® E4M (7.0 ± 0.2%) and Methocel® E15 (9.3 ± 2.4%), the latter not differing among themselves (p > 0.05). The films were thermally stable up to ca. 200 °C, when the second weight loss stage began, although the weight loss rates were maximum at much higher temperatures: 330–345 °C. This stage may be assigned to the oxidative decomposition of cellulose ethers, involving simulta- neous processes of intramolecular dehydration and demethylation (Li et al., 1999; Yin, Luo, Chen, & Khutoryanskiy, 2006). Finally, at tem- peratures higher than 400 °C, the compounds resulting from the thermal cleavage or scission at the previous stage (which implied in ca. 83–87% weight loss) underwent thermal oxidation and ignition (Li et al., 1999). As shown in Fig. 3, three endothermal events were identified when HPMC films were heated. The first thermal event at ca. −18 °C may be related to either i) the melting of water that is weakly linked to polymer chain (type II water) and that presents remarkable supercooling, therefore freezing and thawing at low temperatures (Ford, 1999) or ii) secondary thermal transitions, i.e., the onset of conformational changes (rotation, particularly) in small segments of the main chain as well as in side groups (Gómez-Carracedo, Alvarez-Lorenzo, Gómez-Amoza, & Concheiro, 2003). The ductile behaviors of the films (Fig. 4) assayed at temperatures lower than their glass transitions temperatures support the second hypothesis. The subsequent thermal event showed maximum heat flows be- tween 80 and 100 °C and is attributed to the evaporation of moisture adsorbed to the highly hydrophilic films. This has been previously re- ported for HPMC samples (Ford, 1999). The peak temperatures (Tmax) as well as the areas of the endothermic peaks (ΔH), presented in Sup- plementary Table S1, correlate well with equilibrium moisture, corro- borating this hypothesis. Considering water enthalpy of vaporization (2257 J g−1), sample weight, and the area of the endothermic peaks, the amount of water evaporated can be calculated: HPMC Methocel® K4M: 15.8%; HPMC Methocel® E4M: 10.3%; HPMC Methocel® E15: 9.5%. These data correlate well with those obtained in oven drying at 105 °C, follow the same trend of those obtained in TG, and are again a consequence of the higher occurrence of hydroxyl groups in HPMC Methocel® K4M, leading to greater water holding capacity. 4000 3200 2400 1600 800 ® ® 1460-1250 cm-1 1637 cm-1 2902 cm-1 3420 cm-1 3460 cm-1 3460 cm-1 Methocel E15 Methocel E4M 1300-900 cm-1 Fig. 2. Fourier-transform infrared spectroscopy (FT-IR). FT-IR absorption spectra of films based on different hydroxypropyl methylcellulose grades. 100 200 300 400 500 600 0 20 40 60 80 100 W eight variation (% °C -1) 0.0 0.7 1.4 2.1 -80 0 80 160 240 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 He at fl ow (m W g -1 ) EXO -0.5 -2.0 -1.5 -1.0 -2.5 0.0 Fig. 3. Thermal aspects. Thermogravimetric (TG) and derivative TG (left) and differential scanning calorimetry (right) curves of films based on hydroxypropyl methylcellulose Methocel® E15 (black), HPMC Methocel® E4M (red) or HPMC Methocel® K4M (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) C.G. Otoni et al. Carbohydrate Polymers 185 (2018) 105–111 108 The third thermal event was assigned to the glass transition of the samples. The glass transition temperature (Tg), taken as the average among the onset and offset temperatures, was higher in films based on HPMC Methocel® K4M (Tg=207.9 °C) than in films based on HPMC Methocel® E4M (Tg=176.5 °C) and HPMC Methocel® E15 (Tg=172.4 °C). These values are similar to those previously reported in the literature: 172–175 °C for HPMC Methocel® E15 (Masilungan & Lordi 1984); 191–196 °C for HPMC Methocel® K4M (Gómez-Carracedo et al., 2003; Nyamweya & Hoag, 2000); and 162–184 °C for HPMC Methocel® E4M (Gómez-Carracedo et al., 2003; McPhillips, Craig, Royall, & Hill, 1999). The slightly higher Tg of films based HPMC Methocel® E4M when compared to those made up of HPMC Methocel® E15 is a consequence of the longer chains of the former (Table 1), which feature lower free volume available for conformational changes and thus requiring higher energy input for chains to acquire mobility. The remarkably higher Tg of films based on HPMC Methocel® K4M are at- tributed to the lower SD of such grade, as the higher hydroxyl content leads to increased intermolecular interaction through hydrogen bonds (Gómez-Carracedo et al., 2003). 3.4. Mechanical properties The mechanical attributes of the studied HPMC films are summar- ized in Table 2, whereas their typical mechanical profiles upon testing are presented in Fig. 4. Films based on HPMC Methocel® E4M were remarkably more re- sistant and extensible (p < 0.05) than those made up of HPMC Methocel® E15. This is a consequence of the longer chains of the former (Table 1), implying more molecules among the few crystalline domains and leading to a stronger anchoring effect on the aggregate state. As a result, the resistance, extensibility, and toughness of the material are increased. The greater level of physical entanglement arising from the longer chains of HPMC Methocel® E4M also contribute to the improved mechanical resistance of its films when compared to those based on grades of lower molecular weights. HPMC Methocel® E4M-based films also presented higher tensile strength (p < 0.05) than those made up of HPMC Methocel® K4M. This is attributable to the steric effect that methoxyl groups provide HPMC chains with, once they are bulkier than the original hydroxyl groups. This anchor-like action required a higher input of mechanical energy to break the films during a tensile assay, explaining the increased tensile strength. Young’s modulus was not influenced by SD once this mechanical property was equal (p > 0.05) for films made up of HPMC Methocel® E4M and Methocel® K4M. Molecular weight, on the other hand, affected Young’s modulus, as suggested by the stiffer (p < 0.05) films based on HPMC Methocel® E4M in comparison to those made up of Methocel® E15. Again, longer polymer chains tend to experience decreased free volume and, as a consequence, limited mobility. In this sense, de- formation is hampered, especially at the predominantly elastic region of the viscoelastic regime, leading to increased Young’s modulus. Oscillatory tests have been carried out to provide further insight on the mechanical and thermal properties of HPMC-based films (Supplementary Fig. S3). 3.5. Water barrier properties The driving force for water vapor diffusion is the RH gradient from the interior of the test capsules towards chamber atmosphere. Because all capsules were held within the same chamber, the final RH was equal for all specimens. Therefore, to allow proper comparison, the RH values inside the capsules must also be the same, condition which was achieved here (Table 2). Molecular weight did not affect the WVP of HPMC films, whereas SD had a pronounced effect on this variable. Films made up of HPMC Methocel® K4M presented higher (p < 0.05) WVP values than those based on the more substituted grades. This is a straightforward con- sequence of the higher occurrence of hydroxyl groups in the former, leading to a higher capacity of interaction with water molecules and, therefore, providing films with increased hydrophilicity. This outcome is in line with the higher affinity to moisture of films based on HPMC Methocel® K4M, corroborating the results obtained through oven drying, DSC, and TG. DVS was carried out to further elucidate the hygroscopicity of the HPMC films. The adsorption and desorption isotherms are presented in Fig. 5. All films presented affinity to water molecules, as indicated by the higher masses in desorption cycles than in their adsorption analogues for a given RH, leading to hysteresis. This phenomenon is believed to arise from the adsorption of water molecules to hydroxyl groups as well as to different availabilities of these polar groups at different RH values (Salmén & Larsson, 2018). At low RH, hydrogen bonding is the main interaction involved in moisture adsorption (Enrione, Hill & Mitchell, 2007). In this sense, the initial portion of the adsorption curve corre- sponds to the attachment of water molecules to the hydrophilic groups of the HPMC, notably the hydroxyls. Indeed, in this region, the slopes of the curves assigned to the more substituted HPMC grades (i.e., Meth- ocel® E4M and Methocel® E15) are similar and lower than that of the less substituted grade (i.e., Methocel® K4M). This finding is also in line with WVP data as well as moisture contents determined through oven drying, DCS, and TG. After a certain RH, the isotherms of the E grades 0 5 10 15 20 25 0 5 10 15 20 25 30 35 40 ® ® Methocel E15 Methocel K4M x x x ® Fig. 4. Uniaxial tensile test. Typical mechanical behaviors of films based on different hydroxypropyl methylcellulose grades evidencing their ductile behaviors upon stretching. Table 2 Mechanical and barrier properties. Thickness, engineering tensile strength (σT), percent elongation at break (εB), Young’s modulus (E), water vapor permeability (WVP), and relative humidity (RH) of films based on different hydroxypropyl methylcellulose (HPMC) grades. HPMC Methocel® Thickness (μm) σT (MPa) εB (%) E(GPa) WVP (g mm kPa−1 h−1 m−2) RH (%) E15 25.0 ± 3.8a 30.83 ± 6.43a 6.06 ± 1.56a 1.45 ± 0.15a 0.754 ± 0.285a 72.7 ± 3.4a K4M 43.5 ± 3.3b 52.13 ± 3.33b 11.89 ± 1.98b 1.74 ± 0.08b 1.532 ± 0.164b 76.1 ± 1.6a E4M 30.1 ± 2.6a 67.28 ± 8.39c 17.37 ± 3.32c 1.76 ± 0.16b 0.923 ± 0.151a 76.3 ± 1.8a a–c Mean values ± standard deviations followed by different letters within the same column are significantly different (p < 0.05). C.G. Otoni et al. Carbohydrate Polymers 185 (2018) 105–111 109 diverge because moisture adsorption is no longer driven by the occur- rence of hydrophilic groups, but by the swelling capacity of the polymer (Fringant et al., 1996). The models that best fitted the experimental data were Brunauer- Emmett-Teller (BET), for water activity (aw) values ranging from 0.07 to 0.35 (Eq. (4)), and Guggenheim-Anderson-de Boer (GAB), for aw=0.07–0.84 (Eq. (5)). The correlation coefficients (R2) as well as the parameters adjusted to each model are presented in Table 3. = − − + X C a a a C a Δ m (1 )(1 ) m BET w w w BET w (4) = − − + X C Ka Ka Ka C Ka Δ m (1 )(1 ) m GAB w w w GAB w (5) In the aforementioned equation, Δm is weight variation (%) at a given aw, Xm is the value of the monolayer (%, dry basis), CBET is a constant dependent on the temperature, CGAB is a constant that is re- lated with the binding energy among water molecules and the mono- layers, and K is a constant that depends upon the temperature and is related with the sorption heat of the monolayer (Imran, El-Fahmy, Revol-Junelles, & Desobry, 2010). The Xm values, which are related to the water strongly adsorbed to specific hydrophilic sites (Imran et al., 2010) and were found to be greater for films based on HPMC Methocel® K4M regardless of the model, corroborate the above discussion concerning the initial slope of the moisture adsorption isotherms. Another indicative of the stronger interaction of moisture with this HPMC grade is the greater CBET value when compared to the other grades, suggesting higher bonding energy (Imran et al., 2010). 4. Conclusions In summary, we confirmed our hypothesis that both chain length and backbone pendant group affected the mechanical and water barrier properties of HPMC films. Thermal and rheological properties also showed influence of HPMC chemical structure. SD had a pronounced effect on the affinity and barrier to moisture, glass transition, tensile resistance, and extensibility of HPMC films. This effect has been at- tributed to the reduced polarity provided by methoxyl substitution. Molecular weight, in turn, affected mostly the rheological behavior of HPMC solutions as well as the mechanical properties of its films. This outcome arises from the higher level of physical entanglement and re- duced free volume of longer chains. Considering food packaging ap- plications, the trend towards more mechanically resistant and less permeable films guide the choice of HPMC Methocel® E4M as the op- timum film-forming matrix among the studied HPMC grades. Further studies are suggested to investigate the role played by other substitution degrees as well as hydroxypropyl substitution on the physical-me- chanical properties of HPMC-based films. Funding This work was supported by São Paulo Research Foundation (FAPESP) [grant numbers 2013/14366-7 and 2014/23098-9]; and National Council for Scientific and Technological Development (CNPq) [grant number 402287/2013-4]. Acknowledgements The authors are thankful to the financial support given by FAPESP (grants #2013/14366-7 and 2014/23098-9), CNPq, SISNANO/MCTI, FINEP, CAPES (grant #33001014005D-6), and Embrapa AgroNano research network. They are also thankful to the experimental support by Beatriz Lodi. Prof. Elisabete Frollini (IQSC/USP) is acknowledged for HP-SEC experiments. Dr. Roberto Avena-Bustillos and Dr. Tara McHugh (WRRC/ARS/USDA) are also acknowledged for DVS runs. Finally, the authors thank Paulo Martins at The Dow Chemical Company for kindly providing Methocel® samples. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.carbpol.2018.01.016. References ASTM D882−12 (2012). Standard test method for tensile properties of thin plastic sheeting. West Conshohocken, PA: ASTM International12. http://dx.doi.org/10.1520/ D0882-12. Alzate, P., Miramont, S., Flores, S., & Gerschenson, L. N. (2017). Effect of the potassium sorbate and carvacrol addition on the properties and antimicrobial activity of tapioca starch hydroxypropyl methylcellulose edible films. Starch – Stärke, 69(5–-6), 1600261. Anuar, N. K., Wui, W. T., Ghodgaonkar, D. K., & Taib, M. N. (2007). Characterization of hydroxypropylmethylcellulose films using microwave non-destructive testing tech- nique. Journal of Pharmaceutical and Biomedical Analysis, 43(2), 549–557. Arai, K., & Shikata, T. (2017). Hydration/dehydration behavior of cellulose ethers in aqueous solution. Macromolecules, 50(15), 5920–5928. Azeredo, H. M. C., & Waldron, K. W. (2016). Crosslinking in polysaccharide and protein films and coatings for food contact – A review. Trends in Food Science & Technology, 52, 109–122. Banks, S. R., Sammon, C., Melia, C. D., & Timmins, P. (2005). Monitoring the thermal gelation of cellulose ethers in situ using attenuated total reflectance Fourier trans- form infrared spectroscopy. Applied Spectroscopy, 59(4), 452–459. Bertolino, V., Cavallaro, G., Lazzara, G., Merli, M., Milioto, S., Parisi, F., & Sciascia, L. (2016). Effect of the biopolymer charge and the nanoclay morphology on nano- composite materials. Industrial & Engineering Chemistry Research, 55(27), 7373–7380. Borges, A. F., Silva, C., Coelho, J. F. J., & Simões, S. (2015). Oral films: current status and future perspectives: I – Galenical development and quality attributes. Journal of Controlled Release, 206, 1–19. Burdock, G. A. (2007). Safety assessment of hydroxypropyl methylcellulose as a food 0 20 40 60 80 0 3 6 9 12 Fig. 5. Dynamic vapor sorption. Adsorption (●) and desorption (○) of water vapor in/ from films based on hydroxypropyl methylcellulose Methocel® E15 (black), HPMC Methocel® E4M (red) or HPMC Methocel® K4M (blue). (For interpretation of the refer- ences to colour in this figure legend, the reader is referred to the web version of this article.) Table 3 Dynamic vapor sorption data. Parameters of the Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) models fitted to films based on different hydro- xypropyl methylcellulose (HPMC) grades. HPMC BET GAB R2 Xm (mol g−1) CBET R2 Xm (mol g−1) CGAB K E15 0.986 0.002 4.765 0.946 0.004 0.513 4.76 K4M 0.994 0.003 5.683 0.986 0.005 0.512 6.20 E4M 0.987 0.002 3.799 0.913 0.004 0.489 3.70 C.G. Otoni et al. Carbohydrate Polymers 185 (2018) 105–111 110 https://doi.org/10.1016/j.carbpol.2018.01.016 http://dx.doi.org/10.1520/D0882-12 http://dx.doi.org/10.1520/D0882-12 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0010 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0010 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0010 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0010 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0015 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0015 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0015 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0020 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0020 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0025 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0025 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0025 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0030 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0030 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0030 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0035 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0035 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0035 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0040 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0040 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0040 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0045 ingredient. Food and Chemical Toxicology, 45(12), 2341–2351. Caccavo, D., Lamberti, G., Barba, A. A., Abrahmsén-Alami, S., Viridén, A., & Larsson, A. (2017). Effects of HPMC substituent pattern on water up-take, polymer and drug release: An experimental and modelling study. International Journal of Pharmaceutics, 528(1), 705–713. Cavallaro, G., Donato, D. I., Lazzara, G., & Milioto, S. (2011). Films of halloysite nano- tubes sandwiched between two layers of biopolymer: From the morphology to the dielectric, thermal, transparency, and wettability properties. The Journal of Physical Chemistry C, 115(42), 20491–20498. Cavallaro, G., Lazzara, G., Konnova, S., Fakhrullin, R., & Lvov, Y. (2014). Composite films of natural clay nanotubes with cellulose and chitosan. Green Materials, 2(4), 232–242. Enrione, J. I., Hill, S. E., & Mitchell, J. R. (2007). Sorption behavior of mixtures of glycerol and starch. Journal of Agricultural and Food Chemistry, 55(8), 2956–2963. Espinoza-Herrera, N., Pedroza-Islas, R., San Martín-Martinez, E., Cruz-Orea, A., & Tomás, S. A. (2011). Thermal, mechanical and microstructures properties of cellulose deri- vatives films: A comparative study. Food Biophysics, 6(1), 106–114. Feller, R. L., & Wilt, M. (1990). Evaluation of cellulose ethers for conservation. Research in conservation. Los Angeles: The Getty Conservation Institute. Ford, J. L. (1999). Thermal analysis of hydroxypropylmethylcellulose and methylcellu- lose: Powders, gels and matrix tablets. International Journal of Pharmaceutics, 179(2), 209–228. Fringant, C., Desbrières, J., Milas, M., Rinaudo, M., Joly, C., & Escoubes, M. (1996). Characterisation of sorbed water molecules on neutral and ionic polysaccharides. International Journal of Biological Macromolecules, 18(4), 281–286. Gómez-Carracedo, A., Alvarez-Lorenzo, C., Gómez-Amoza, J. L., & Concheiro, A. (2003). Chemical structure and glass transition temperature of non-ionic cellulose ethers. Journal of Thermal Analysis and Calorimetry, 73(2), 587–596. Garavand, F., Rouhi, M., Razavi, S. H., Cacciotti, I., & Mohammadi, R. (2017). Improving the integrity of natural biopolymer films used in food packaging by crosslinking approach: A review. International Journal of Biological Macromolecules, 104, 687–707. Imran, M., El-Fahmy, S., Revol-Junelles, A.-M., & Desobry, S. (2010). Cellulose derivative based active coatings: Effects of nisin and plasticizer on physico-chemical and anti- microbial properties of hydroxypropyl methylcellulose films. Carbohydrate Polymers, 81(2), 219–225. Keary, C. M. (2001). Characterization of METHOCEL cellulose ethers by aqueous SEC with multiple detectors. Carbohydrate Polymers, 45(3), 293–303. Larsson, M., Viridén, A., Stading, M., & Larsson, A. (2010). The influence of HPMC substitution pattern on solid-state properties. Carbohydrate Polymers, 82(4), 1074–1081. Li, X.-G., Huang, M.-R., & Bai, H. (1999). Thermal decomposition of cellulose ethers. Journal of Applied Polymer Science, 73(14), 2927–2936. Masilungan, F. C., & Lordi, N. G. (1984). Evaluation of film coating compositions by thermomechanical analysis. I. Penetration mode. International Journal of Pharmaceutics, 20(3), 295–305. McHugh, T. H., Avena-Bustillos, R., & Krochta, J. M. (1993). Hydrophilic edible films: Modified procedure for water vapor permeability and explanation of thickness ef- fects. Journal of Food Science, 58(4), 899–903. McPhillips, H., Craig, D. Q. M., Royall, P. G., & Hill, V. L. (1999). Characterisation of the glass transition of HPMC using modulated temperature differential scanning calori- metry. International Journal of Pharmaceutics, 180(1), 83–90. Moghimi, R., Aliahmadi, A., & Rafati, H. (2017). Antibacterial hydroxypropyl methyl cellulose edible films containing nanoemulsions of Thymus daenensis essential oil for food packaging. Carbohydrate Polymers, 175, 241–248. Nyamweya, N., & Hoag, S. W. (2000). Assessment of polymer-polymer interactions in blends of HPMC and film forming polymers by modulated temperature differential scanning calorimetry. Pharmaceutical Research, 17(5), 625–631. Otoni, C. G., Avena-Bustillos, R. J., Azeredo, H. M. C., Lorevice, M. V., Moura, M. R., Mattoso, L. H. C., & McHugh, T. H. (2017). Recent advances on edible films based on fruits and vegetables – A review. Comprehensive Reviews in Food Science and Food Safety, 16(5), 1151–1169. Sakata, Y., & Yamaguchi, H. (2011). Effects of calcium salts on thermal characteristics of hydroxypropyl methylcellulose films. Journal of Non-Crystalline Solids, 357(4), 1279–1284. Salmén, L., & Larsson, P. A. (2018). On the origin of sorption hysteresis in cellulosic materials. Carbohydrate Polymers, 182, 15–20. Shimada, R. T., Fonseca, M. S., & Petri, D. F. S. (2017). The role of hydroxypropyl me- thylcellulose structural parameters on the stability of emulsions containing Spirulina biomass. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 529, 137–145. Yin, J., Luo, K., Chen, X., & Khutoryanskiy, V. V. (2006). Miscibility studies of the blends of chitosan with some cellulose ethers. Carbohydrate Polymers, 63(2), 238–244. Zaccaron, C. M., Oliveira, R. V. B., Guiotoku, M., Pires, A. T. N., & Soldi, V. (2005). Blends of hydroxypropyl methylcellulose and poly(1-vinylpyrrolidone-co-vinyl acetate): Miscibility and thermal stability. Polymer Degradation and Stability, 90(1), 21–27. Zhang, X. X., Zhang, W., Tian, D., Zhou, Z. H., & Lu, C. H. (2013). A new application of ionic liquids for heterogeneously catalyzed acetylation of cellulose under solvent-free conditions. RSC Advances, 3(21), 7722–7725. Zhang, J., Yang, W., Vo, A. Q., Feng, X., Ye, X., Kim, D. W., et al. (2017). Hydroxypropyl methylcellulose-based controlled release dosage by melt extrusion and 3D printing: Structure and drug release correlation. Carbohydrate Polymers, 177, 49–57. C.G. Otoni et al. Carbohydrate Polymers 185 (2018) 105–111 111 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0045 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0050 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0050 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0050 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0050 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0055 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0055 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0055 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0055 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0060 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0060 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0065 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0065 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0070 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0070 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0070 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0075 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0075 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0080 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0080 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0080 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0085 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0085 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0085 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0090 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0090 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0090 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0095 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0095 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0095 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0100 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0100 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0100 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0100 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0105 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0105 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0110 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0110 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0110 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0115 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0115 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0120 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0120 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0120 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0125 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0125 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0125 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0130 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0130 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0130 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0135 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0135 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0135 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0140 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0140 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0140 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0145 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0145 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0145 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0145 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0150 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0150 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0150 http://refhub.elsevier.com/S0144-8617(18)30016-X/sbref0155 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molecular weight on mechanical and water barrier properties of hydroxypropyl methylcellulose films Introduction Experimental Materials Molecular weight Rheological measurements Film casting Fourier-transform infrared spectroscopy (FT-IR) Thermogravimetry Differential scanning calorimetry (DSC) Mechanical properties Water vapor permeability (WVP) Dynamic vapor sorption (DVS) Statistical treatment of data Results and discussion Molecular weight and rheological aspects Fourier-transform infrared spectroscopy (FT-IR) Thermal properties Mechanical properties Water barrier properties Conclusions Funding Acknowledgements Supplementary data References