High Amorphous Vinyl Alcohol-Silica ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

High Amorphous Vinyl Alcohol-Silica Bionanocomposites: Tuning Interface Interactions with Ionic Liquids Katarzyna Z. Donato,†,‡ Marino Lavorgna,*,‡ Ricardo K. Donato,†,§ Maria G. Raucci,‡ Giovanna G. Buonocore,‡ Luigi Ambrosio,‡ Henri S. Schrekker,*,† and Raquel S. Mauler†

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†

Institute of Chemistry, Universidade Federal do Rio Grande do Sul (UFRGS), P.O. Box 15003, Av. Bento Gonçalves 9500, Bairro Agronomia, Porto Alegre, RS, CEP 91501-970, Brazil ‡ Institute of Polymers, Composites and Biomaterials (IPCB)-CNR c/o ENEA, P. le E. Fermi 1, Loc. Granatello, 80055 Portici, NA, Italy § MackGraphe (Graphene and Nano-Material Research Center), Mackenzie Presbyterian University, Rua da Consolaçaõ 896, São Paulo, SP, CEP 01302-907, Brazil S Supporting Information *

ABSTRACT: Herein, we demonstrate the effect of imidazolium ionic liquids (IL) applied as additives in the in situ formation of high amorphous vinyl alcohol (HAVOH)-silica bionanocomposites, using a simple sol−gel process approach. A complementary set of alkyl-, ether-, and carboxy-functionalized IL was used, allowing silica structure control and polymer−silica interphase tuning. Consequently, hybrids with diverse morphologies, as well as improved thermo-mechanical and barrier properties, were obtained. This diversity also highlighted the systems’ dependency on the IL’s molecular structure, where both the cation and anion influenced the hybrids’ final properties. This could be evidenced as the polar group functionalized-IL (ether- and carboxy-functionalized IL) allowed the formation of multiple hydrogen bonds at the organic−inorganic interphase, inducing a fine hybrid morphology with well-dispersed silica nanodomains. This significantly increased the storage (∼50%) and tensile moduli (∼20%), extensibility (up to 300%), and glass transition temperature (>20 °C) and decreased the water vapor permeability (∼50%), which are desirable characteristics for potential food and medical packaging. KEYWORDS: Imidazolium ionic liquid, Sol−gel silica, High amorphous vinyl alcohol, Bionanocomposite, Water vapor permeability, Packaging

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ical and barrier properties.3,17−19 On the other hand, PVOH processability enhancement has been obtained by chemical modifications.20 Recently, a new biodegradable polymer based on modified PVOH has been patented and commercialized with the trade name of G-Polymer (Nippon-Goshei, Japan). Novel high amorphous vinyl alcohol (HAVOH) is particularly interesting due to its excellent extrusion processability, ease of use in coating, and outstanding oxygen barrier properties. For this reason, it is used in applications where high barrier materials are needed, such as food or medical packaging. However, due to its low water barrier resistance, it is often used together with polyolefins in multilayer structures,14 decreasing the product’s biodegradability. Ionic liquids (IL) are biodegradable21 organic salts with ionic−covalent crystalline structures that keep the liquid state at 100 °C or below. These multifunctional ILs present high

INTRODUCTION Biodegradable and biocompatible polymers such as poly(vinyl alcohol) (PVOH) are strong competitors for conventional nondegradable polyolefins in high performance packaging for food,1 electronics,2 and pharmaceuticals.3,4 However, they suffer from three main drawbacks: (i) low water and oxygen barrier properties, (ii) poor thermo-mechanical properties, and (iii) limited processability through melt extrusion.5 In this context, both physical and chemical modifications of PVOH have been investigated to overcome these issues and obtain innovative functional materials with broader range of applications. Chemical cross-linking,6−8 filling,3,9,10 or grafting11 are some of the common techniques used for enhancing PVOH performance. In particular, its elevated hydrogen bonding (Hbond) capacity, high chain flexibility, and excellent water solubility open the way for incorporating various nanofillers9,12−16 and obtain nanocomposites with tunable morphology and interfacial interactions. The in situ formation of sol−gel silica also allowed for the successful preparation of PVOHbased hybrid nanocomposites with improved thermo-mechan© 2016 American Chemical Society

Received: October 2, 2016 Revised: November 11, 2016 Published: November 17, 2016 1094

DOI: 10.1021/acssuschemeng.6b02379 ACS Sustainable Chem. Eng. 2017, 5, 1094−1105

Research Article

ACS Sustainable Chemistry & Engineering

purchased from the Nippon Synthetic Chemical Industry Co., Ltd. (Japan). The chemical structure of this polymer consists of large PVOH units that alternate to modifying monomers, bringing hydrophilic branching units along the main chain of the polymer (Figure 1a). Three classes of ILs were used: alkyl-, ether-, and carboxyfunctionalized ILs (Figure 1b).

thermal and chemical resistance, stability in air and moisture, insignificant flammability and volatility,22 as well as promising antimicrobial (in solution and into materials surfaces)23,24 and antidermatophytic activity.25 Additionally, feasible structural alterations of hydrophobic and hydrophilic regions allow taskspecific IL synthesis and optimization of the intermolecular forces for each modification. Specific structural changes, e.g., anion exchange or differences in alkyl imidazolium side-chain length or its functionalization, cause variations in viscosity, ionic conductivity, and solubility, as well as shifts in degradation and glass transition temperatures.22,26 The ILs are being explored as additives in polymers due to their potential as plasticizers, lubricants, and reinforcing or interfacial agents, improving, e.g., barrier, mechanical, and thermal properties.27 Our group has been studying the effects of imidazolium IL application as “morphology drivers” on the sol−gel silica formation process28,29 and their role as compatibilizers and properties enhancers in various polymeric systems.30−32 Neat imidazolium ILs self-assemble into supramolecular structures due to the H-bond-co-π−π stacking mechanism.33 Moreover, the C−H units within the imidazolium ring and, depending on the alkyl imidazolium side-chain length, methylene and/or methyl groups of the alkyl chains are H-bond donors. Further cation functionalization with carboxylic or ether groups increases the possibility of multiple H-bonding among IL molecules.34 Thus, when applied into the sol−gel process, these drive silica evolution processes and allow structure control.28 It is known that size, geometry, polarity, and interaction forces between ionic parts have definitive contributions to the final silica particle size, compactness, and morphology. Furthermore, the presence of IL from the first moments of sol−gel silica formation speeds up the gelation time;28 hence tailor-made silica/IL hybrid synthesis becomes more suitable for in situ applications in water-soluble polymers. It has been found that in situ silica/IL hybrid formation in diverse polymeric matrices improves the nanofillers’ dispersion and provides an increase in the nanocomposites’ thermo-mechanical properties.30−32 To the best of our knowledge, ILs have been applied to PVOH matrices mainly for improving their electrical, electrochemical, or thermal properties.35−37 Within this context, the present work has brought attention for the first time to the use of ILs as multifunctional agents in nanocomposite systems based on HAVOH to improve its water barrier and mechanical properties. The proposed approach is based on the development of hybrid nanocomposites by combining HAVOH and silica that has been synthesized and modified in situ with IL. This allowed tuning of the interphase interactions and enhanced the polymer’s thermo-mechanical and water barrier properties. We show that the addition of small amounts of in situ-formed homogeneously dispersed silica/IL hybrid nanofillers effectively improves the pristine properties of HAVOH. The in situ formation of modified and nonmodified silica allowed observation of both the silica effect in the presence of the matrix and the IL effect in the polymer− filler interphase. Samples were prepared by solvent casting, and two postdrying times were applied to evaluate how the water content affects the nanocomposites properties. The systems were investigated for their structural, morphological, thermomechanical, and barrier properties, as well as biocompatibility.

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Figure 1. (a) Chemical structure of HAVOH and (b) alkyl-IL: [C4MIm][NTf2] and [C4MIm][Cl]. carboxy-IL: [CH2CO2HMIm][NTf2] and [CH2CO2HMIm][Cl]. Ether-IL [C7O3MIm][MeS] and [C7O3MIm][NTf2]; applied in this work. Alkyl-IL: 1-n-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4MIm][NTf2] and 1-n-butyl-3-methylimidazolium chloride [C4MIm][Cl] were purchased from SigmaAldrich. A procedure reported previously in the literature38 was used for the synthesis of the ether-IL 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate [C7O3MIm][MeS], which was used as a precursor for the preparation of the ether-IL 1triethylene glycol monomethyl ether-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C7O3MIm][NTf2]. This anion exchange was performed by applying [C7O3MIm][MeS] and lithium bis(trifluoromethylsulfonyl)amide (LiNTf2) in 1:1 molar ratio. The salts were stirred for 2 days in dichloromethane. Formed LiMeS was removed by filtration, and [C7O3MIm][NTf2] was obtained after solvent removal under a vacuum. The purity was evaluated by 1H NMR, where spectral data of [C7O3MIm][MeS] were in accordance with those reported in the literature (Figure S1, Supporting Information).38 Also, a previously published method was applied for the synthesis of the carboxy-IL 1-methyl-3-methylcarboxylic acid imidazolium bis(trifluoromethylsulfonyl)imide [CH2CO2HMIm][NTf2] and 1-methyl-3-methylcarboxylic acid imidazolium chloride [CH2CO2HMIm][Cl].39 Before application, ILs were dried for 3 h at 60 °C under a vacuum. The silica precursor tetraethoxysilane (TEOS) was purchased from Sigma-Aldrich. Hydrochloric acid (HCl) and ́ ethanol (EtOH) were purchased from VETEC Quimica Fina LTDA and used without further purification. Deionized water was purchased from Best-Chemical s.r.l. The in Situ preparation of HAVOH-based Films with 3.0 wt % of Silica. An adapted literature procedure28 was used for preparing the prehydrolyzed TEOS/IL solution: the molar ratios of nEtOH/nTEOS, nH2O/nTEOS, and nIL/nTEOS were fixed at 5.0, 3.0, and 0.03, respectively. An aqueous acid solution (0.01 M) was prepared by dilution of 0.82 mL of concentrated HCl in water (1000 mL; HClaq, pH = 2). One of the ILs (Figure 1b, Table 1) and EtOH (310 mg) were sonicated until homogeneous (∼5 min). Next, TEOS (250 mg) and subsequently HClaq (64 mg) were added. The reaction mixture was mechanically stirred for 10 min and then left for 1 h to prehydrolyze. The 10 wt % solution of polymer was prepared by adding HAVOH (2.40 g) into deionized water (21.60 g) and stirring until homogeneous (∼3 h at 60 °C). The HAVOH solution was cooled to room temperature, and the prehydrolyzed sol was added dropwise. After stirring for 5 min, the mixture was poured into a Petri dish. All samples were left to cast at

EXPERIMENTAL SECTION

Materials. Highly amorphous polyvinyl alcohol (G-Polymer, grade OKS-8049 with a viscosity of 4.6 mPa.s (4%aq at 20 °C)) was 1095


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Table 1. Mass of IL Applied in the for Nanocomposites Preparation and the DSC Thermal Properties Determined after Two Post-Drying Procedures: 15 min at 120 °C and 1 h at 120 °C (Bold) sample

mILa [mg]

HAVOH HAVOH-Silica

a

HAVOH-C4MImCl

12.6

HAVOH-C4MImNTf2

30.2

HAVOH-CH2CO2HMImCl

12.8

HAVOH-CH2CO2HMImNTf2

30.4

HAVOH-C7O3MImMeS

23.4

HAVOH-C7O3MImNTf2

36.6

TgDSC [°C]

Tm [°C]

Tc [°C]

ΔHm [J/g]

ΔHc [J/g]

Xc [%]

66.1 73.4 74.0 76.2 73.8 73.8 74.3 74.1 75.3 75.6 75.2 76.0 74.6 73.8 74.2 73.8

184.3 186.3 181.8 184.1 186.2 186.5 184.8 185.8 183.4 184.4 184.5 184.9 184.9 186.2 185.2 185.2

156.4 163.1 143.5 147.7 160.2 154.3 154.3 150.3 150.7 152.3 153.3 153.5 153.9 151.8 155.2 149.2

29.5 28.6 23.6 21.5 27.4 21.2 24.9 18.9 23.4 21.2 23.5 21.5 25.1 27.5 25.5 19.0

36.2 29.4 24.0 23.2 28.4 28.5 25.4 28.0 25.2 24.5 25.9 24.3 26.9 26.2 25.8 26.4

21 21 18 16 20 16 19 14 17 16 17 16 19 20 19 14

Mass of the corresponding IL applied for the nanocomposite formation. Transmission Electron Microscopy (TEM). The dispersion and size of nanofillers were examined by TEM using FEI Tecnai G12 Spirit Twin equipment operating at an accelerating voltage of 120 kV. Ultrathin sections of nanocomposites (70 nm) were prepared at room temperature and retrieved on 300 mesh Cu grids directly from the diamond knife without any liquid medium. Scanning Electron Microscopy (SEM). The surfaces of the residues obtained from TGA (oxidative atmosphere) were analyzed with a FEI Quanta 200F microscope operating at 30 kV. The specimen was placed on a sample holder covered with a Carbon Tab and sputtered with platinum to increase the electric conductivity. Small and Wide Angle X-ray Scattering (SWAXS). The analyses were performed using an Anton Paar SAXSess camera equipped with a 2D imaging plate detector. Cu Kα 1.5418 Å wavelength X-rays were generated by a Philips PW3830 sealed tube source (40 kV, 50 mA) and slit-collimated. Thin rectangular films were placed in the mold before obtaining the spectra. The time of exposition was 15 min. All scattering data were corrected for the background and normalized for the primary beam intensity. In order to remove the inelastic scattering from the data, the SAXS profiles were additionally corrected for both the Porod constant and desmearing effect. Water Vapor Barrier Properties. The measurements of water vapor permeability were performed using the infrared sensor technique by means of a Permatran W3/31 (Mocon, Germany). Samples with a surface area of 5 cm2 were tested at 23 °C and relative humidity on the upstream side of ∼80% until a stable permeability value was obtained (48 h). Only samples submitted to shorter postdrying (15 min at 120 °C) were analyzed. Dynamic Mechanical Analyses (DMA). A TA model QA 800 instrument was used at a fixed frequency of 1 Hz and strain amplitude of 0.05%. DMA analyses were performed in the tensile mode, and the rectangular specimens were heated from 30 to 140 °C at a rate of 3 °C/min. Before analysis, the samples were conditioned at 25 °C for 24 h. The glass transition temperature (TgDMA) was obtained from the tan delta curve’s maximum. The DMA measurements were performed for the samples submitted to both postdrying procedures (15 min and 1 h at 120 °C). Tensile Tests. The tensile strength and modulus were determined with an Instron 5565 (Instron, Canton, MA) instrument at a crosshead speed of 20 mm/min. The tensile specimens were cut out (21 × 5 mm) with a CEAST cutting tool and submitted to the shorter postdrying protocol (15 min at 120 °C). An extensometer was used to directly monitor the strain variation and to determine the modulus. The values reported are averages of five measurements. In Vitro Elution Materials and AlamarBlue Assay. The HAVOH systems (0.1 g) were dissolved into 2.5 mL of Dulbecco’s Modified

room temperature for 48 h and then dried under a vacuum (8 h at 60 °C). Before analysis, in order to evaluate how the water content affects the systems’ properties, casted films were submitted to two additional procedures of drying under a vacuum; 15 min at 120 °C or 1 h at 120 °C. For the reader’s clearer understanding, shorthand notations were used for the HAVOH nanocomposites. The abbreviation HAVOH-IL (e.g., HAVOH-C4MImNTf2) was used for samples modified with the corresponding silica/IL hybrid and the abbreviation HAVOH-Silica was used for the IL-free one. Characterization Methods. Thermogravimetric Analyses (TGA). A TA Instruments QA-5000 instrument was used, and the analyses were carried out at a heating rate of 10 °C/min from 40 to 800 °C. All experiments were conducted under an air or nitrogen atmosphere with a gas flow rate of 25 mL/min. The samples (∼10.0 mg) were placed in a platinum crucible, and an empty platinum pan was used as a reference for all measurements. The inorganic content (residual mass left at 800 °C) and temperature of highest degradation rate (Td) as well as temperatures at 5%, 10%, and 50% of weight loss (T5%, T10%, and T50%) were determined. The Td relates to the temperature at maximum weight loss rate of the first TGA derivative curve (DTG) in either an inert (TN2) or oxidative atmosphere (TO2). Differential Scanning Calorimetry (DSC). A TA Instruments DSC Q1000 instrument calibrated with indium standards was used for analyses. The entire thermal scan was conducted under a nitrogen atmosphere with a gas flow rate of 50 mL/min. Each sample had an initial weight of ∼10 mg and was placed in a closed aluminum specimen holder before placing it in the oven. The thermal history of the samples was erased by a preliminary heating run, which was applied from 0 to 240 °C (isotherm at 240 °C for 5 min) at a rate of 20 °C/min. The results were obtained from the first cooling and from the second heating scan at a rate of 10 °C/min between 0 and 240 °C. The melting (ΔHm) and crystallization (ΔHc) enthalpies were obtained by integrating the areas of the endothermic and exothermic peaks, respectively. A rough estimation of the HAVOH’s crystallinity degree (Xc) was calculated from eq 1.

Xc =

ΔHm × 100% ΔHm° × f

(1)

where ΔH°m is the reference melting enthalpy value of 138.60 J/g40 for 100% crystalline PVOH and f is the polymer fraction. Melting (Tm) and crystallization (Tc) temperatures were taken from the maxima of the endothermic and exothermic peaks, respectively. The glass transition temperatures (TgDSC) were obtained from the melting curves at the transition’s midpoint. 1096


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Figure 2. DSC crystallization (a, b) and melting (c) curves for (1) HAVOH; (2) HAVOH-Silica; (3) HAVOH-C4MImCl; (4) HAVOHC4MImNTf2; (5) HAVOH-CH2CO2HMImCl; (6) HAVOH-CH2CO2HMImNTf2; (7) HAVOH-C7O3MImMeS; and (8) HAVOHC7O3MImNTf2. Samples numbered with ′ (e.g., 1′, 2′) correspond to the ones submitted to longer postdrying (1 h at 120 °C).

Table 2. TGA Determined Thermal Properties after Two Post-Drying Procedures: 15 min at 120 °C and 1 h at 120 °C (Bold) sample HAVOH HAVOH-Silica HAVOH-C4MImCl HAVOH-C4MImNTf2 HAVOH-CH2CO2HMImCl HAVOH-CH2CO2HMImNTf2 HAVOH-C7O3MImMeS HAVOH-C7O3MImNTf2

T5%a [°C]

T10%a [°C]

T50%a [°C]

TN2b [°C]

T50%c [°C]

TO2d [°C]

Res.e [%]

285.7 283.7 278.9 183.5 276.9 270.1 288.0 280.4 298.6 273.9 290.4 276.3 285.9 262.5 282.5 276.8

306.4 305.2 292.8 283.8 291.7 303.6 310.4 296.9 320.7 289.9 305.6 301.1 308.3 296.7 306.5 300.9

360.6 363.0 362.9 364.2 363.0 378.0 381.6 369.1 381.5 363.6 370.8 368.4 377.9 373.0 379.1 371.6

363.9 366.3 358.1 355.3 363.6 374.2 374.6 369.7 371.5 362.1 367.2 365.3 372.5 371.7 375.2 369.0

364.9 376.7 379.6 400.0 394.2 394.4 382.2 404.3 391.6 403.3 382.6 399.3 383.8 403.3 378.9 403.7

352.8 368.0 354.7 359.6 373.6 369.1 357.4 365.7 355.1 364.9 355.0 366.6 353.6 368.2 353.0 366.9

0 0 3.2 3.2 2.9 2.5 3.1 3.2 3.4 3.0 3.1 3.2 3.0 3.0 3.0 3.0

a

From TGA measurements under nitrogen. bMaximum degradation temperature from DTG curve under nitrogen. cFrom TGA measurements under air. dMaximum degradation temperature from DTG curve under air. eAt 800 °C under air. Surface Wettability. The contact angle was evaluated with an OCA 20 Dataphysics instrument in the sessile drop mode using 1 μL of water. Samples were measured seven times at room temperature, and the contact angles were expressed as average values.

Eagle’s Medium (DMEM) according to the ISO 10993-5 guidelines. Next, 500 μL of each solution was pipetted into sterile 48-well cell culture plates (Falcon USA) previously seeded with 5000 L929 cell line cells. The negative (nontoxic) control was a tissue culture plastic in DMEM without a HAVOH solution. The plates were further incubated (37 °C, 5% CO2, 95% humidity) for 24 and 48 h of exposure. AlamarBlue was used as an assay to evaluate the biocompatibility. This assay quantified the redox indicator, which changed to a fluorescent product in response to the chemical reduction by mitochondrial enzymes such as flavin mononucleotide dehydrogenase, flavin adenine dinucleotide dehydrogenase, and nicotinamide adenine dinucleotide dehydrogenase. This redox phenomenon allowed the quantification of the cell viability (living and metabolic active cells). At selected time points of 24 and 48 h, the medium was removed from the wells, and an aliquot of 500 μL of AlamarBlue diluted 1:10 in phenol red-free medium was added to each well and incubated for a further 4 h at 37 °C (5% CO2). Afterward, 100 μL each solution was transferred to a 96-well plate for colorimetric analysis (n = 6). Wells without any cells were used to correct any background interference from the redox indicator. The absorbance was measured at 570 nm, and the subtracting background absorbance was determined at 600 nm.

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RESULTS AND DISCUSSION Thermal Behavior. DSC and TGA were used to study the effect of the in situ formation of silica/IL hybrids on the thermal transitions as well as the thermal degradation. Figure 2 shows polymer characteristic curves with both exothermic and endothermic events obtained by DSC for the samples submitted to two postdrying times. In comparison to the Tc of neat HAVOH, the incorporation of siloxane phase caused a significant T c shift to the lower temperatures, which corresponds to the crystallization deceleration (Figure 2a and b; curves 1 and 2 and 1′ and 2′).41 All six ILs increase the Tc up to 15 °C in comparison to HAVOH-Silica (Figure 2a and b, Table 1), indicating the HAVOH crystallization induction caused by the IL was confined on the silica surface.30 The HAVOH-C4MImCl presented the maximum increase of the Tc only for the longer 1097


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Figure 3. SEM images of microstructures obtained after calcination under an oxidative atmosphere at 800 °C: (a) HAVOH-Silica, (b) HAVOHCH2CO2HMImCl, and (c) HAVOH-C4MImCl.

presented a higher T50% (up to 20 °C under nitrogen and 30 °C under air); however only the samples HAVOHCH2CO2HMImCl, HAVOH-CH2CO2HMImNTf2, HAVOHC7O3MImMeS, and HAVOH-C4MImNTf2 presented better thermal resistances in all evaluated weight loss steps (T5%, T10%, and T50% under nitrogen; Table 2). The addition of any of the six presented ILs resulted in a Td increase in comparison to the neat HAVOH. The most significant TN2 (maximum degradation temperature in nitrogen) improvement (∼10 °C) was observed for the systems containing the thermally more stable [NTf2] anion-based IL with either alkyl or polar cation side chain. As for the oxidative degradation curves, the maximum increment of degradation temperature (TO2) was observed for HAVOHC4MImCl (∼30 °C), and this could be related to the formation of weaker bounded silica particles, which would enable their migration to the surface and act as a thermal insulation layer.42 The TGA ashes obtained in an oxidative atmosphere at 800 °C were investigated by SEM to evaluate the influence of the ILs on the in situ silica orientation. Despite the polymer matrix used being biodegradable, the evaluation of the nanocomposites’ calcination process could also indicate their recyclability. It was noted that the addition of any of the six presented ILs induced formation of ordered microstructures with different morphologies. Figure 3 represents the three most contrasting structures observed. Calcinated HAVOH-Silica (Figure 3a) displayed a porous surface, which is typical for amorphous materials. The residue from HAVOH-CH2CO2HMImCl presented needle-like morphology (Figure 3b), also observed for HAVOH-C 7 O 3 MImMeS (Figure S3a) and HAVOHC7O3MImNTf 2 (Figure S3b), while HAVOH-C 4MImCl formed much thicker bead-like structures (Figure 3c). The pristine silica glass first transformation from amorphous silica into β-cristobalite occurs only at temperature of ∼1000 °C. Thus, as samples herein were thermally treated only up to 800 °C, the applied ILs were most likely responsible for catalyzing/ inducing crystallite growth. This phenomenon was exclusive for samples submitted to the postdrying procedure. When the same calcination process was applied to non-post-dried samples, HAVOH-CH 2 CO 2 MImNTf 2 (Figure S3c) or HAVOH-C4MImNTf2 (Figure S3d), no organized silica− carbon species could be observed (similar to HAVOH-Silica, Figure 3a). The postdrying process seems to cause a reproducible organization of the film due to the increase of internal pressures. Apparently, this initial organization caused by the volume decrease (water loss) facilitated further organization at higher temperatures. Additionally, by changing

postdrying procedure (Figure 2b; curve 3′), which, most likely, was promoted by partial phase separation and/or formation of larger nucleation centers. This can be evidenced when correlating the SAXS determined scattering invariant (INV) with the Tc, where particle size and Tc seem to follow a defined pattern. The silica/IL influence on crystallization is explained in more detail in the structural investigation section. The Xc decreased for all hybrid systems when compared to neat HAVOH, and this decrease was more pronounced (up to 5%) for the systems submitted to a shorter postdrying procedure. In comparison with the neat HAVOH matrix, the TgDSC of all silica-filled samples increased independent of postdrying time, confirming the direct interaction of in situ formed hybrids with the polymer matrix. The TgDSC positive shifts cannot be attributed to the water absorption from the ambient atmosphere, since the postdrying process was performed under a vacuum and samples were further stored in a desiccator. This TgDSC shift is a consequence of HAVOH-silica interactions, which would work as “physical crosslinks,” decreasing the molecular mobility of polymer chains and organizing the system. Samples HAVOH-CH2CO2HMImCl and HAVOH-CH2CO2HMImNTf2 (Figure 2c; curves 5′ and 6′) showed the most significant TgDSC increase (almost 10 °C, when compared to HAVOH), which is an indication of more intense multiple H-bonding formation among HAVOH, silica, and carboxy-IL. The Tm slightly increased (max 2 °C) for most of the HAVOH-IL systems, while it decreased for the HAVOHSilica (∼2 °C), which could also indicate multiple H-bonding for HAVOH-IL. Altogether, the DSC results show that the IL applied in HAVOH-silica hybrids work as compatibilizers rather than plasticizers. The measurements obtained by TGA revealed an important role of the in situ silica/IL hybrids formation in the nanocomposites’ thermal resistance. Figure S2 shows representative TGA/DTG curves for samples degrading under both inert and oxidative atmospheres. In general, all the samples presented two or three broad degradation peaks when analyzed under nitrogen (Figure S2a and c) or air (Figure S2b and d), respectively. As both HAVOH and IL28 do not completely degrade under a nitrogen atmosphere, an oxidative atmosphere was used to determine the inorganic filler contents in the nanocomposites (Table 2; 3 ± 0.4 wt %). The general small standard deviation suggests that the synthetic process itself induces the formation of nanocomposites with very well dispersed silica. Longer postdrying produced samples with higher thermal resistance, and these were examined in more detail. All HAVOH-IL nanocomposites 1098


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Figure 4. TEM images of (a) HAVOH-Silica, (b) HAVOH-C4MImCl, and (c) HAVOH-CH2CO2HMImCl.

Scheme 1. Representation of Possible Interactions in Hybrid Systems between HAVOH; Silica; and (a) alkyl-IL, (b) ether-IL, and (c) carboxy-IL

amorphous continuous phase (Figure 5a; curve 1′). With the addition of a siloxane phase, the scattering profile of HAVOH completely changed (Figure 5a). The diffraction spectra for all hybrids show the characteristic knee diffraction feature, which is ascribed to the presence of small particles (likely spherical) and is a common hierarchical structure of the siloxane phase aggregate that results in these secondary structures.44 The hybrids’ Porod corrected SAXS plots showed differences in profiles and scattering intensities depending on the IL applied. It was possible to obtain slope values in the linear fractal region, as well as scatter vectors at the transition from the Guinier region to the fractal region (qG) and at the transition from Porod to the fractal region (qP; Table S1).45 The average characteristic dimensions of both a long amorphous−crystalline period in neat HAVOH and silica particles in hybrids (long period, δ) was roughly estimated based on eq 2:

the IL applied, it is also possible to control the microstructure morphology. The calcinated products could be considered as recycled fillers in new materials synthesis. Nevertheless, a more detailed investigation is necessary to discover the exact compositions and structures of formed species. Morphology and Structural Organization. All obtained films were homogeneous and transparent, suggesting the absence of large clusters and good silica dispersion. This was also observed in TEM images (Figure 4). In the cases of HAVOH-Silica (Figure 4a) and HAVOH-C4MImCl (Figure 4b), it is rather difficult to determine the particle size, since these form co-continuous networks. Independent of this, it is possible to notice that the alkyl-IL favors a more pronounced silica agglomeration, which could be ascribed to the alkyl-IL’s tail−tail interaction (Scheme 1a). On the other hand, the HAVOH-CH2CO2HMImCl (Figure 4c) promoted excellent silica dispersion, where particles (2−4 nm) were evenly spread across the sample. The high compatibility of all components of this system, as after TEOS hydrolysis practically all the components are able to form H-bonding, makes it more difficult to segregate into isolated domains up to a later stage of condensation, not allowing considerable coalescence into big silica domains. Additionally, the acid character of this IL could promote a lower pH of the system retarding the condensation process (cluster−cluster mechanism).28,43 This effect of IL as silica morphology and agglomeration controllers has also been observed in different in situ silica based nanocomposites.32,43 The structural organization of the hybrid films was investigated by SWAXS analysis, and the obtained plots are gathered in Figure 5. On the basis of the SAXS profile, neat HAVOH presents structural organization where the layer-like lamellar crystallites form spherical domains embedded in an

δ = 2π /qmax

(2)

where qmax is the scattering vector modulus at the maximum of the q vs q2I(q) plot (Figure 5b). The systems showed variations of qmax and invariant values (INV, eq 3): 3

INV =

∫0.2 q2I(q) dq

(3)

which depend on the electron density difference between the siloxane domains and the surrounding polymer. This is an estimation of the scattering phase densification extent (mainly as primary particles), which depends on the hybrid formulation. The δ for neat HAVOH, ascribed only to the presence of lamellar crystalline domains, slightly increased when submitted 1099


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Figure 5. SWAXS profiles: (a) Porod plot after subtraction of the Porod constant; (b) the q vs q2I(q) plots representing the transition of qmax values with an increased drying time; and (c) WAXS region for (1) HAVOH, (2) HAVOH-Silica, (3) HAVOH-C4MImCl, (4) HAVOH-C4MImNTf2, (5) HAVOH-CH2CO2HMImCl, (6) HAVOH-CH2CO2HMImNTf2, (7) HAVOH-C7O3MImMeS, (8) HAVOH-C7O3MImNTf2. Curves with ′ (e.g., 1′, 2′) correspond to the samples with longer postdrying (1 h at 120 °C).

to the longer postdrying (Figure 5b; curves 1 and 1′). In the case of the hybrids, the silica primary particles were estimated to have an average size in the range of ∼3−8 nm (Table S1). In agreement with the literature,45 for the samples with larger silica primary particle sizes (HAVOH-C4MImCl and HAVOHC4MImNTf2), the scattering intensity at low wave vectors decreased, while this scattering intensity increased for the samples with smaller silica primary particle sizes (HAVOHCH2CO2HMImCl and HAVOH-CH2CO2HMImNTf2; Figure 5a). However, the differences in silica porous structures and the spatial arrangements of the nanoparticles can also contribute to the total scattering intensity and have to be taken into account. The scattering properties values are presented in Table S1. More detailed structural investigations were performed only for the samples submitted to longer postdrying, and an interesting trend between INV and Tc has been found (Figure 6). In the case of HAVOH-Silica, denser and smaller particles at ∼5 nm (high INV value) and very strong HAVOH-silica polar interactions significantly shifted the Tc to lower temperatures. This behavior was “softened” by IL addition into the system, shifting Tc to the higher temperatures, as previously mentioned in the DSC description. Simultaneously, the type of IL applied induced the formation of particles with different sizes. Samples modified with carboxy-IL showed the highest value of INV (denser and smaller particles, ∼ 3 nm) and presented the lowest Tc values, in opposition to the alkyl-IL modified ones presenting the lowest INV (looser and bigger particles, ∼ 7 nm) and highest Tc (negligible interaction with the polymer).

Figure 6. Correlation between invariant values (INV = ∫ 0.23q2 I(q) dq) obtained from SAXS analysis and Tc obtained from DSC measurements.

In comparison to the alkyl- and ether-IL, a more effective HAVOH-silica-IL multiple H-bonding for the samples with carboxy-IL caused the formation of smaller primary particles. Those presented a broader filler−matrix interphase, due to higher surface area, delaying the crystallization process. In the case of HAVOH-Silica, the crystallization occurs with more significant delay (−10 °C) than in the systems with IL. This can be explained by the existence of an almost twice bigger 1100


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Table 3. DMA and Tensile Tests Results for HAVOH Samples after Two Post-Drying Procedures: 15 min at 120 °C and 1 h at 120 °C (Bold) sample HAVOH HAVOH-Silica HAVOH-C4MImCl HAVOH-C4MImNTf2 HAVOH-CH2CO2HMImCl HAVOH-CH2CO2HMImNTf2 HAVOH-C7O3MImMeS HAVOH-C7O3MImNTf2 a

E′a [GPa]

TgDMAb [°C]

modulusc [GPa]

tensile strength [MPa]

toughness [J/m−3]

5.0 4.6 5.9 5.4 5.3 5.2 5.7 5.8 6.4 6.8 5.7 6.2 5.3 5.9 5.5 6.5

94.1 62.4 90.9 79.9 88.7 72.5 90.4 76.1 92.4 67.6 91.6 83.9 89.1 77.8 92.5 73.9

4.0 ± 0.2

87 ± 12

405

3.5 ± 0.2

73 ± 12

504

3.3 ± 0.5

74 ± 8

590

3.7 ± 0.2

79 ± 4

1212

4.1 ± 0.1

82 ± 4

777

4.6 ± 0.1

82 ± 2

1614

4.7 ± 0.3

83 ± 7

716

4.1 ± 0.3

88 ± 6

1176

Storage modulus at 30 °C obtained from DMA. bTan delta peak maximum. cYoung modulus at 25 °C.

Figure 7. E′ (a,b) and tan delta (c,d) DMA curves of (1) HAVOH; (2) HAVOH-Silica; (3) HAVOH-C4MImCl; (4) HAVOH-C4MImNTf2; (5) HAVOH-CH2CO2HMImCl; (6) HAVOH-CH2CO2HMImNTf2; (7) HAVOH-C7O3MImMeS; (8) HAVOH-C7O3MImNTf2. Curves with ′ (e.g., 1′, 2′) correspond to the samples submitted to longer postdrying (1 h at 120 °C). In b, only curves 1′ and 5′ were numbered due to overlapping.

primary particle than for HAVOH-CH2CO2HMImCl, which would reduce the active surface and HAVOH-silica interactions. Moreover the presence of alkyl-IL on the silica surface decreased the HAVOH-silica affinity, causing partial phase separation and allowing the formation of larger nucleation centers. As a result, the crystallization was favored and occurred at higher temperatures. The polarity of anion−cation

combinations also played an important role in the strength of the HAVOH-silica-IL interactions. It was noted that ILs of imidazolium cations substituted with polar functional groups (carboxy-IL or ether-IL) with more coordinative [Cl] or [MeS] anions increased the strength of HAVOH-silica-IL interactions, in opposition to the less coordinative [NTf2] anion. On the other hand, when the alkyl-IL was paired with the [Cl] anion, 1101


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ACS Sustainable Chemistry & Engineering the HAVOH-silica-IL interactions decreased, while this interaction increased with the [NTf2] anion. This suggests a completely different interphase interaction mechanism when using an IL with either a polar or a nonpolar cation side chain, where alkyl-IL interacts through hydrophobic tail−tail interactions (Scheme 1a). In contrast, carboxy- and ether-IL produce interphases mainly based on H bonds (Scheme 1b and c).43 Thus, by changing the IL structure, it was possible to control both primary particle size as well as the strength of the interphase interactions. On the basis of the WAXS patterns (Figure 5c), the crystalline structure of HAVOH changes when the siloxane phase is present in the system. It seems that the intensity of the crystalline peaks of all modified samples decreased; however, with the multiple overlapping peaks in the scattering pattern it is difficult to quantify with accuracy the crystallinity reduction.46 Nevertheless, this observation is consistent with the DSC results (Table 1) where Xc decreased significantly for the hybrid systems, which indicates the formation of more spread HAVOH crystals as a result of the added inorganic nanosized filler. Mechanical Properties. The DMA and tensile measurements were performed to study the reinforcement of the HAVOH matrix with the addition of the silica/IL filler. Table 3 summarizes the storage (E′) and tensile (Young modulus) moduli as well as TgDMA and tensile strength. Figure 7 presents E′ and tan delta curves of the hybrids. All modified samples presented higher E′ than the neat HAVOH independent of postdrying time (Figure 7a and b). The HAVOHCH2CO2HMImCl showed the largest E′ increase of 2.2 and 1.5 GPa for shorter and longer postdrying times, respectively (Figure 7a and b; curves 5 and 5′). Also HAVOHC7O3MImNTf2 and HAVOH-CH2CO2HMImNTf2 presented significant improvements of 1.9 and 1.6 GPa for shorter postdrying. It was noted that longer postdrying caused a decrease in the E′ values for most of the IL-modified samples, while TgDMA strongly increased in all cases (up to 30 °C), when compared to shorter postdrying. In comparison with neat HAVOH (curve 1 or 1′), shorter postdrying of modified samples caused a TgDMA increase (up to 22 °C; Figure 7c), while it slightly decreased for the longer one (Figure 7d). Comparing the Tg results from DMA and DSC, it is possible to notice significant differences especially for samples with longerpost drying. Since the thermal memory was erased in the first DSC heating scan, the TgDSC values were much lower than TgDMA. Additionally, the longer-post drying resulted in free volume reduction, due to improved packing, which caused internal stresses resulting in samples with higher TgDMA. Since E′ values for nanocomposites submitted to shorter postdrying were mostly higher than for those submitted to the longer postdrying, the tensile tests were carried out only for this setup. Moreover, longer postdrying led to too brittle samples for the use in the toughness dependent measurements. The results from tensile tests are gathered in Table 3, and representative stress−strain curves are presented in Figure 8. Rigid fillers, like silica, with a density higher than the polymer matrix, can enhance not only a polymer’s rigidity, hardness, and modulus but also toughness. The increase in toughness is directly proportional to the filler content, its dispersion, and the area and quality of polymer−filler interfacial interactions. Samples modified with polar-IL showed increased Young moduli, and such behavior is generally a result of reduced chain mobility and increased rigidity, which normally leads to a

Figure 8. Stress−strain curves of samples submitted to the shorter postdrying procedure: (1) HAVOH, (2) HAVOH-Silica, (3) HAVOH-C4 MImCl, (4) HAVOH-C 4 MImNTf2 , (5) HAVOHCH 2 CO 2 HMImCl, (6) HAVOH-CH 2 CO 2 HMImNTf 2 , (7) HAVOH-C7O3MImMeS, (8) HAVOH-C7O3MImNTf2.

decrease in toughness due to low extensibility.47 However, all the IL-modified nanocomposites presented a toughness increase, where some polar-IL-modified samples had more than a 3 times increase in comparison to HAVOH-Silica. Since the filler content and dispersion for all IL-based samples were very similar, the significant toughness differences indicate that the interfacial interaction plays a very relevant role in the final nanocomposites’ properties. At the same time, this increase in filler−matrix interphase interaction resulted in a reduced mobility of the matrix and, as a consequence, a decrease in the tensile strength when compared to neat HAVOH. Systems containing IL with the more hydrophobic [NTf2] anion tend to present higher elongation, indicating lubrication of the interphase (Figure 8; curves 4, 6, and 8). Toughness, calculated from the area under the curve, increased for all modified samples, especially for the ones with the [NTf2] anion. Corroborating with previous results,31,43 systems with the IL of imidazolium cations substituted with polar functional groups, especially carboxy-IL, showed the best balance of mechanical properties. The mechanical performance suggests higher organization of HAVOH chains, good dispersion of the silica/IL domains, and strong interfacial interaction due to multiple H-bonding. Barrier Properties. The water vapor barrier property was measured via a water vapor permeation test. HAVOH polymer presents very good oxygen barrier properties but is particularly sensitive to water, thus a reduction of water permeability is highly desirable. On the basis of possible applications, the barrier properties were tested only for the samples submitted to shorter postdrying, since this procedure produced samples with better mechanical properties. All the hybrids presented lower water vapor permeation values at different extents than neat HAVOH (Figure 9). Since both SWAXS and DSC confirmed that the crystallinity of the hybrid systems decreased, the observed reduction in permeation can be attributed to the presence of silica and/or IL. In particular, the hybrids HAVOH-CH2CO2HMImCl and HAVOH-CH2CO2HMImNTf2 presented the most significant decrease in permeation (∼50%), indicating that the carboxy-IL allows more efficient silica dispersions and the formation of more stable HAVOH-silica-IL physical cross-links, which 1102


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∼36° clearly indicates higher surface wettability for this nanocomposite. Increased water adhesion can be directly connected with the number of available interactions (Hbond) in the silica/CH2CO2HMImCl hybrid. The relatively small standard deviation, comparable to the one obtained for neat polymer, also indicates good filler dispersion and film homogeneity. The same system also presented a strong interaction with the polymer matrix, confirmed by combined SAXS, thermo-mechanical, and barrier results, once more emphasizing the high capacity of multiple H-bonding. Furthermore, surface wettability is an important feature for evaluating the fungal biofilm adhesion at the polymer surface. Higher wettability is directly related to better antifouling ability where a tightly bound water layer hinders the surface adsorption of fungi and bacteria.54 The decreased contact angle indicates this system as a potential material for medical and food packaging. Biocompatibility. Toxicity of cross-linkers51 and fillers55 is an important issue when newly developed material has its application in the food or medical field. Thus, an evaluation of nanocomposites’ cytotoxicity was performed using the L929 cell line. The cell metabolic function and proliferation assessment was used as a quantitative method to determine the biocompatibility of the nanocomposites. The results after 24 and 48 h of cells’ exposure to the nanocomposites are shown in Figure 11. A general trend of increasing cell response with

Figure 9. Water vapor permeability for (1) HAVOH, (2) HAVOHSilica, (3) HAVOH-C4MImCl, (4) HAVOH-C4MImNTf2, (5) HAVOH-CH2CO2HMImCl, (6) HAVOH-CH2CO2HMImNTf2, (7) HAVOH-C7O3MImMeS, and (8) HAVOH-C7O3MImNTf2.

decreased the chain mobility and created a more tortuous path for water diffusion. This improvement of water barrier properties has been achieved despite the possible sorption of water molecules by IL, which was already reported even for more hydrophobic IL.48,49 The obtained result, in terms of permeability reduction, is especially interesting since it is higher than that obtained for PVOH films loaded with cellulose nanocrystals isolated from banana pseudostems fibers (28%)50 or with micro- and nanocellulose cross-linked with tertbutyl acrylate-co-2-hydroxyethyl methacrylate (23%).51 Similar value reductions, to those obtained in our study (∼45%), have been reported by Aloui et al.52 and by Spoljaric et al.53 However, the presented decrease in permeability is related to PVOH samples loaded with much higher amounts of filler than that used in this work (3 wt %), i.e., 10 wt % as a total amount of halloysite and nanocellulose and 50 wt % of commercial nanofibrillated cellulose, respectively. Higher permeability reduction (∼65%) has been obtained only by loading 5 wt % of quaternized cellulose.47 In this case, despite the enhancement of water barrier properties, the produced film was not transparent, and a significant reduction (84%) of elongation at break has been observed, whereas our results show, mainly for the HAVOHCH2CO2HMImCl sample, a 50% permeability reduction and a simultaneous increase (∼60%) of elongation at break. On the basis of this, the developed material can certainly be considered a good candidate as a single-layer material exhibiting a simultaneously enhanced water barrier, as well as good mechanical properties, overcoming the previously described drawbacks related to the use of vinyl alcohol-based materials. Surface Wettability. The intermolecular interactions between material surfaces and water were studied by the contact angle method. Figure 10 shows differences in the water

Figure 11. AlamarBlue assay to determine the proliferation of L929 cells after 24 and 48 h of cell culture on control (CTR), (1) HAVOH, (2) HAVOH-Silica, (3) HAVOH-C 4 MImCl, (4) HAVOHC 4 MImNTf 2 , (5) HAVOH-CH 2 CO 2 HMImCl, (6) HAVOHCH 2 CO 2 HMImNTf 2 , (7) HAVOH-C 7 O 3 MImMeS, and (8) HAVOH-C7O3MImNTf2.

increased exposure time to materials was observed for the negative control (CTR) and tested nanocomposites. This indicates that L929 cells were able to express increased metabolic activity through either increased population density or cellular function. Neat HAVOH (1) was found to be a biocompatible material independent of exposure time when compared to CTR. The same behavior was observed for HAVOH-CH2CO2HMImCl (5) and HAVOH-C7O3MImMeS (7), which presented an increased growth at both exposure times. This indicates more favorable conditions for the cell growth, which was most likely due to the materials’ higher hydrophilicity (Figure 10). The samples HAVOH-Silica (2), HAVOH-C4MImNTf2 (4), and HAVOH-C7O3MImNTf2 (8) demonstrated better results after 48 h of incubation time, suggesting slower cell growth rates. The strong HAVOH-silica interaction and/or more hydrophobic character of the [NTf2] anion containing IL could retard this process. Results for HAVOH-C4MImCl (hydrophobic cation, 3) and HAVOH-

Figure 10. Contact angle measurements for (a) neat HAVOH and (b) HAVOH-CH2CO2HMImCl.

droplet shape when deposited over neat HAVOH (Figure 10a) or HAVOH-CH2CO2HMImCl (Figure 10b) surfaces. The contact angle measurements were also performed for the HAVOH-Silica (Figure S4); however no significant difference in wettability was noticed when compared to neat HAVOH. Contact angles of 91° ± 2.9 for HAVOH and 55.2° ± 2.4 for HAVOH-CH2CO2HMImCl were obtained. The difference of 1103



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CH2CO2HMImNTf2 (hydrophobic anion, 6) indicate the formation of a more hostile environment for the cells. Despite that, no severe inflammatory response was observed in any of the demonstrated cases, and all tested samples can be considered as biocompatible.

CONCLUSIONS The in situ sol−gel technique in the presence of six different ILs was used to prepare biocompatible HAVOH hybrid films. Very efficient filler hybrid dispersions were achieved, causing a positive reflex on the thermal, mechanical, and water vapor barrier properties. By changing the IL structure and, as a consequence, its characteristics, it was possible to control the primary silica particle size and strength of interfacial interactions, which reduced polymer mobility and crystallinity near the organic−inorganic interphase. The carboxy- and etherIL exerted a stronger influence on both silica dispersion and nanocomposite properties, indicating the importance of the polar functionalized IL in the formation of a multiple Hbonding network throughout HAVOH-silica-IL. The system HAVOH-CH2CO2HMImCl presented the best balance of thermo-mechanical and barrier properties versus the other materials. Altogether, the presented sol−gel strategy provided a promising approach to prepare reinforced silica/IL-based single-layer polymeric nanocomposites in aqueous solutions for biomedical and food packaging applications. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02379. 1 H NMR spectra of the IL’s anion exchange process from [C7O3MIm][MeS] to [C7O3MIm][NTf2], TGA/DTG curves, SEM images of ashes obtained from TGA, comparison of contact angle images of neat HAVOH and HAVOH-Silica, and table with SAXS scattering properties of nanocomposites (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Phone: + 39 0817758838. E-mail: [email protected]. *Phone: +55 51 33086302. E-mail: [email protected]. ORCID

Henri S. Schrekker: 0000-0002-8173-3841 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Brazilian funding agencies CNPq (Science without Borders Special Visiting Scientist project 400531/2013-5), CAPES, and FAPERGS for financial support, as well as CNR-Italy and AVCR-Czech Republic for funding the cooperation project “New sustainable approaches in the synthesis of epoxy-silica hybrids with tunable properties (20132015).” R.K.D. is thankful to FAPERGS-CAPES for the DOCFIX postdoctoral fellowship and to the Short Term Mobility Program 2015 funded by CNR (Prot. N. 26996). The authors are thankful to Ewa Pavlova (IMC Prague) for the TEM images. 1104


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