silver nanocomposite: Effect of thermal treatment temperature on the morphology, oxygen and water transport properties

Accepted Manuscript Title: Starch/silver nanocomposite: Effect of thermal treatment temperature on the morphology, oxygen and water transport properties. Author: Perrine Cheviron Fabrice Gouanv´e Eliane Espuche PII: DOI: Reference:

S0144-8617(15)00696-7 http://dx.doi.org/doi:10.1016/j.carbpol.2015.07.067 CARP 10169

To appear in: Received date: Revised date: Accepted date:

14-5-2015 17-7-2015 20-7-2015

Please cite this article as: Cheviron, Perrine., Gouanv´e, Fabrice., & Espuche, Eliane., Starch/silver nanocomposite: Effect of thermal treatment temperature on the morphology, oxygen and water transport properties.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2015.07.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Starch/silver nanocomposite: Effect of thermal treatment temperature on the morphology, oxygen and water transport properties.

Perrine Cheviron, Fabrice Gouanvé*, Eliane Espuche* Université de Lyon, F-69003, Lyon, France, Université Lyon 1, F-69622 Villeurbanne, France, CNRS, UMR5223, Ingénierie des Matériaux Polymères.

Corresponding author: Fabrice Gouanvé Université de Lyon, F-69003, Lyon, France, Université Lyon 1, F-69622 Villeurbanne, France, CNRS, UMR5223, Ingénierie des Matériaux Polymères. Tel.: +33 (0) 4 72 43 12 10 E-mail address: [email protected]. Eliane Espuche Université de Lyon, F-69003, Lyon, France, Université Lyon 1, F-69622 Villeurbanne, France, CNRS, UMR5223, Ingénierie des Matériaux Polymères. Tel.: +33 (0) 4 72 43 27 01 E-mail address: [email protected]. Abstract The present work reports a strategy involving the preparation of nanostructured starch based film containing silver nanoparticles (AgNPs) using a completely green chemistry process. The nanocomposite films were prepared by solution cast process. The AgNPs were in situ generated inside the polymer film by thermal treatment at different temperatures (25, 40 and 85 °C). The influence of the presence and the amount of reducing agent (glucose) were also investigated. For all nanocomposite films, the AgNPs were spherical with a diameter less than 15 nm. Contrary to the presence of glucose, thermal treatment condition was a key factor for the AgNPs structure. Crystalline AgNPs were obtained only after thermal treatment at 85 °C. Improvements of water and oxygen barrier properties near to one decade were observed in

this last case and were explained by the formation of crystalline AgNPs associated to the establishment of strong interactions between AgNPs and starch polymer matrix.

Keywords: Silver nanoparticle, starch, nanocomposite, oxygen permeability, water permeability

Highlights − Silver nanocomposites were prepared by an in situ generation route − Amorphous nanoparticles were obtained after treatment at low temperature − Crystalline nanoparticles were obtained after treatment at high temperature − The presence of crystalline particles led to an improvement of barrier properties

1. Introduction Polymer nanocomposites have attracted great attention in the two last decades and became key materials in many nanotechnology applications. This interest arises as a result of remarkable enhancements in various properties like mechanical, barrier and thermal properties, even at very low volume fraction loading (Das et al., 2010; Kim & Cha, 2014; Sadegh-Hassani & Mohammadi Nafchi, 2014). In the family of polymer nanocomposites, polymer reinforced by metal nanoparticles are advanced functional materials which have attracted considerable attention (Clémenson, Léonard, Sage, David, & Espuche, 2008; Nesher, Marom, & Avnir, 2008; Nimrodh Ananth, Umapathy, Sophia, Mathavan, & Mangalaraj, 2011). Among the numerous nanoparticles that have been used as polymer functionalizing agent, silver nanoparticles (AgNPs) represent one of the most sought-after. This is mainly due to their optical, catalytic, magnetic, electronic and antimicrobial properties which are extensively investigated mainly in colloidal systems (Kuila, Garai, & Nandi, 2007; Panáček et al., 2006; Severin, Kirstein, Sokolov, & Rabe, 2009). However, numerous applications of AgNPs require their entrapment in various solid substrates (Akamatsu et al.,

2000; Mbhele et al., 2003; Zeng, Rong, Zhang, Liang, & Zeng, 2002). As a result, polymers are one of the materials of choice because of their specific morphology, chemical and structural nature with their long chains allowing easy nanoparticle incorporation. In order to fully improve the properties of the silver nanocomposites, the selection of the host polymer as well as the controlled distribution of the uniformly shaped and sized nanoparticles is required (Burda, Chen, Narayanan, & El-Sayed, 2005; Lahav, Gabriel, Shipway, & Willner, 1999; F.K. Liu, Hsieh, Ko, Chu, & Dai, 2003). Improved properties are generally reached when small dispersed nanodomains are obtained (H. Huang & Yang, 2004). AgNPs in various synthetic polymers

such

as

polyvinylalcohol,

polyvinylpyrolidone,

polystyrene

or

polymethylmethacrylate have been extensively reported (Balan, Malval, Schneider, Le Nouen, & Lougnot, 2010; Singh & Khanna, 2007; Valmikanathan, Ostroverkhova, Mulla, Vijayamohanan, & Atre, 2008; Zheng, Gu, Jin, & Jin, 2001). However, in most of the silver nanocomposite preparations, organics solvents and non-environmentally friendly components are used. In order to minimize or eliminate pollution to the environment, development of green chemistry processes are required. Utilization of nontoxic chemicals, environmentally benign solvents, and renewable materials are some of the key issues that deserve important consideration in a green chemistry strategy. In the last decade, efforts were made in the incorporation of beforehand preformed AgNPs into natural polymers for their potential application in biotechnology (Cheviron, Gouanvé, & Espuche, 2014; Kouvaris et al., 2012; Raveendran, Fu, & Wallen, 2003, 2006). Polysaccharide polymers, such as chitosan, alginate and starch are used as host matrices (Brayner, Vaulay, Fiévet, & Coradin, 2007; Djoković et al., 2009; Vigneshwaran, Nachane, Balasubramanya, & Varadarajan, 2006; Vimala et al., 2010; Wei, Sun, Qian, Ye, & Ma, 2009). In this ex situ method, the nanocomposite films are often prepared by using the solvent (often water based) cast process (Akamatsu et al., 2000; Dirix, Bastiaansen, Caseri, & Smith, 1999; Radheshkumar & Münstedt, 2005; Zeng et al.,

2002). However, the main disadvantages of this technique are the formation of AgNPs clusters and the non-homogenous repartition of these particles within the polymer matrix. Because of these issues, the interest for the in situ method has become predominant these last years. In the in situ method, the AgNPs are directly formed in the polymer matrix. The methodology generally consists in the dissolution of a silver precursor in a polymer solution. The solution is then cast to obtain a precursor film after solvent evaporation. Depending on the polymer and on the silver precursor, chemical, physical or thermal treatments can be used to obtain the silver ion reduction (Clémenson, Espuche, David, & Léonard, 2010; Clémenson et al., 2008; Compton, Kranbuehl, Martin, Espuche, & David, 2007; Ramesh, Porel, & Radhakrishnan, 2009; Simon et al., 2012). In the literature, the simplest and most used procedure to convert the silver precursor to AgNPs inside the polymer film is the thermal treatment of the precursor film. In this context, the ability of some polymers acting simultaneously as the reducing agent for the silver ions and the stabilizer for the resulting nanoparticles is quite attractive. However, in some cases, the presence of a specific reducing agent is required especially when the polymer matrix is sensitive to high thermal treatment temperature (Božanić et al., 2011). Reducing agents such as hydrazine, ethylene diamine tetra acetic acid and above all sodium borohydride are generally used (Bright, Musick, & Natan, 1998; Evanoff & Chumanov, 2004; Merga, Wilson, Lynn, Milosavljevic, & Meisel, 2007; Wang, Efrima, & Regev, 1998). Most of these reducing agents are considered as nonenvironmentally friendly component. In a green chemistry approach, saccharide molecules such as aldehydes can be used as alternative reducing agents (Mehta, Chaudhary, & Gradzielski, 2010; Panáček et al., 2006). The use of glucose as green reducing agent has been extensively reported in the literature. (Anastas & Warner, 1998; H. Huang & Yang, 2004; Raveendran et al., 2003). Patakfavi et al. have shown that in presence of glucose, large number of nuclei was formed and the obtained silver nanoparticles were smaller and

monodisperse (Patakfalvi, Viranyi, & Dekany, 2004). However, among all the studies related with AgNPs, no detailed studies have been concerned with the determination of the effects of the temperature of the thermal treatment and the influence of the amount of reducing agent on the structure and properties of the silver nanocomposite films stemming from a natural polymer. Therefore, the present work aims to develop a nanostructured starch based film containing silver nanoparticles using a completely green chemistry process. The nanocomposite films were prepared by solution cast process. The AgNPs were in situ generated inside the polymer film by thermal treatment at different temperatures (25, 40 and 85°C). The influence of the thermal treatment temperature, the presence and the amount of reducing agent were investigated. The formation of the AgNPs was confirmed by Ultravioletvisible absorption spectroscopy (UV-vis). The size, size distribution and the dispersion of the AgNPs within the starch polymer matrix were determined by transmission electron microscopy (TEM). The structure of the AgNPs was characterized by X-ray diffraction (XRD). Water sorption and permeability to water vapour and oxygen at different water activities were performed on the different nanocomposite films. 2. Experimental 2.1. Materials AgNO3 (ACS reagent > 99.0%) was purchased from Aldrich and used as silver precursor. D(+)-Glucose from Merck was used as reducing agent and was supplied from Aldrich. Native potato starch with a weight ratio of amylopectin to amylose equal to 77/23 was purchased from Sigma and glycerol (99% purity-supplied from Aldrich) was used as plasticizer. Deionised water was used as solvent. 2.3. Preparation of the nanocomposite films The nanocomposite film preparation consisted firstly in the dissolution of potato starch (containing 6 wt.% of water) with glycerol (weight ratio 85:15) in deionised water at a

concentration of 3 wt.%. The starch/glycerol (SG) aqueous solution was heated to the gelatinization temperature (85 ± 0.5 °C) and continuously stirred at this temperature for 3 h. The resulting SG aqueous solution was cooled down at room temperature. Then, a silver nitrate aqueous solution and a glucose aqueous solution were added to the SG aqueous solution and stirred at room temperature during 5 min. The resulting mixture was poured into polystyrene petri dish and water evaporation was carried out at ambient temperature away from light during at least four days. After this evaporation, the precursor films were removed from the petri dish. Then, thermal treatment conditions were applied. The first one consisted in a storage at ambient temperature (25 °C) during several weeks, and the two others one corresponded to a thermal treatment in an oven at 40 °C and 80 °C during 15, 48, and 72 h. In all cases, the nanocomposite films were kept in dark place. The nanocomposite films stored at 25 °C and thermal treated are denoted Ag-SG x:y(aW-bC) and Ag-SG x:y(cH-bC), respectively. x:y are the molar ratios of AgNO3/glucose, a the weeks number of storage, b and c are the thermal treatment temperature and the duration in hours of thermal treatment, respectively. The molar ratios of AgNO3/glucose used were 1:0, 1:1.5 and 1:3. The silver content in the films was 2 wt.% and was expressed with respect to the total matter content including glycerol. Neat matrices were also prepared as references using the same experimental conditions and are denoted SG x:y(aW-bC) and SG x:y(cH-bC) where x is equal to 0. In all cases, the films thickness was approximately 50 µm ± 5 µm. 2.4. Characterization methods 2.4.1. Ultraviolet-visible absorption spectroscopy (UV-Vis) Ultraviolet-visible (UV-Vis) absorption studies were performed on the different films with a Perkin Elmer Lambda 750 spectrophotometer in the wavelength range of 200-700 nm by step of 1 nm. The absorption values were normalized to a film thickness of 50 µm using a cross multiplication taking into account the real thickness of the films. Maximal absorbance (Amax)

was determined using Excel software. Duplicate experiments for each sample were carried out. 2.4.2. Transmission electron microscopy (TEM) Size analysis of silver nanoparticles was carried out with a Philips CM120 electron microscope with an accelerating voltage of 120 kV. Samples were cut with a cryoultramicrotome at −90 °C with a Reicher Ultracut S instrument equipped with a diamond knife to obtain ultrathin sections of about 60 nm thicknesses. The samples were placed on Formwar coated grids. For each analysis, low electron beam intensity was used and short time of exposure was performed to avoid any evolution of the samples during their exposure to the electron beam. The average diameter and size distribution of the AgNPs were determined by the ImageJ Software based on the data of an average of 1 000 nanoparticles in the nanocomposite films. The histogram of the size distribution was established by Excel software. 2.4.3. X-ray diffraction (XRD) X-ray diffractometry (XRD) analyses were carried out on the nanocomposites films using a Cu tube and a Bruker D8 Advance diffractometer, where the Kβ line was removed with a nickel filter. The diffraction patterns were obtained at room temperature in the range of 2θ between 1° and 50° by step of 0.02°. The films were deposited on neutral monosubstrates with a thin transfer adhesive on sides with low scattering response. 2.4.4. Dynamic vapor sorption (DVS) Dynamic vapor sorption analyzer, DVS Advantage, was used to determine water sorption isotherms of the different samples. The vapor partial pressure was controlled by mixing dry and saturated nitrogen, using electronic mass flow controllers. The experiments were carried out at 25 °C. The initial weight of the sample was approximately 30 mg. The sample was predried in the DVS Advantage by exposure to dry nitrogen until the equilibrated dry mass of

the sample was obtained (m0). A partial pressure of vapor (p) was then established within the apparatus and the mass of the sample (mt) was followed as a function of time (t). The mass of the sample at equilibrium (meq) was considered to be reached when changes in mass with time (dm/dt) were lower than 2.10-4 mg min-1 for at least five consecutive minutes. Then, vapor pressure was increased in suitable activity up to 0.9 by step of 0.1. The value of the concentration of water at equilibrium in the material (C) for each water activity (aw) allowed to plot the water sorption isotherm for each sample. C is expressed in cm3STP per g of material according to the following equation: C=

meq − m0 22414 . m0 18

(Eq. 1)

The sorption rate was also estimated at each water activity by applying the Fick’s diffusion law. Taking into account the film thickness (L), the water diffusion coefficient (D) was calculated for the short time (mwater t/mwater eq < 0.5) according the following equation:

mwater t mwater eq

=

4 L

D.t

π

(Eq. 2)

mwater t is the mass of water sorbed as a function of the time and mwater eq is the mass of water sorbed at equilibrium for a water activity. Measurements were duplicated and the precision on the values of the water concentration at equilibrium and the values of the diffusion coefficient was estimated to be better than 5% 2.4.5. Water permeability Water permeability measurements were performed on a Mocon Permatran W 3/33 (Minneapolis, USA) equipped with an infrared sensor. Nitrogen was used as the carrier gas. The film was placed on an aluminum mask with an open testing area of 5 cm2. The test cell was composed of two chambers separated by the film. Prior to testing, specimens were conditioned in nitrogen inside the unit for at least 12 h to remove traces of atmospheric water.

Then, water vapor was introduced in the upstream compartment of the test cell. The water molecules transferred through the film were conducted by the carrier gas to the infrared sensor. The water permeability coefficient ( PH 2O ) was calculated considering the following equation:

PH 2O =

J stH O .L

(Eq. 3)

2

∆p

where L is the thickness of the film, J stH O the water stationary flux and ∆p the difference of 2

pressure between the upstream and the downstream compartments. PH 2O can be expressed in barrer (1 barrer = 10-10 cmSTP3 cm cm-2 s-1 cmHg-1 = 3.36 x10-16 mol m m-2 s-1 Pa-1 in SI). Measurements were performed at controlled temperature (T = 25 °C) for a water activity range from 0.45 to 0.8. Measurements were duplicated and the precision on the values of the permeability coefficient was estimated to be better than 5%.

2.4.6. Oxygen permeability Oxygen permeability measurements were performed on a Mocon Oxtran 2/21 (Minneapolis, USA) equipped with a Coulox sensor. The film was placed on an aluminum mask with an open testing area of 5 cm2. The test cell was composed of two chambers separated by the film. Nitrogen containing 2% of hydrogen was used as the carrier gas and pure oxygen was used as the test gas. The water activity of the two gases was controlled by a humidifier. Prior to testing, specimens were conditioned in nitrogen/hydrogen (N2/H2) inside the unit for at least 24 h to remove traces of atmospheric oxygen. Subsequently, oxygen was introduced in the upstream compartment of the test cell. Oxygen transferred through the film was conducted by the carrier N2/H2 gas to the coulometric sensor. The oxygen permeability coefficient PO2 was calculated considering the following equation:

PO2 =

J stO2 .L

(Eq. 4)

∆p

where L is the thickness of the film, J stO the oxygen stationary flux and ∆p the difference of 2

pressure between the upstream and the downstream compartments. PO2 can be expressed in barrer (1 barrer = 10-10 cmSTP3 cm cm-2 s-1 cmHg-1 = 3.36 x10-16 mol m m-2 s-1 Pa-1). Measurements were performed at controlled temperature (T = 25 °C) and for a water activity range from 0.45 to 0.8. Measurements were doubled and the precision on the values of the permeability coefficient was estimated to be better than 5%.

3. Results and discussion The films obtained just after unsticking from the polystyrene petri dishes were handleable and not brittle. The colour was orange indicating a partial reduction of silver nitrate during the film process formation. Moreover, the coloration was darker for the samples elaborated in presence of glucose. UV-vis analysis was performed and the obtained absorption spectra are shown in Fig. 1a.

(a)

(b)

(c)

(d)

Figure 1: UV-Visible absorption spectra of: (a) the neat matrix and silver nanocomposite films just after water evaporation. (b) the silver nanocomposite films without glucose stored at 25 °C during several weeks. (c) the neat matrix and silver nanocomposite films without glucose treated at 40 °C until 72 h. (d) the neat matrix and silver nanocomposite films without glucose treated at 85 °C until 72 h. The absorption spectra of the nanocomposite film without glucose showed a very small surface plasmon resonance (SPR) peak. The maximal absorbance (Amax) of the SPR peak was significantly higher when the film contained glucose. However, the increase of glucose amount did not have an influence on the SPR peak intensity. Amax was equal to 0.98 and 0.97 for AgNO3/glucose molar ratio of 1:3 and 1:1.5, respectively. The presence of this peak confirmed that the silver nitrate reduction already started during the film formation. As explained by Gao et al., the aldehyde terminal of starch polymer chains can reduce silver nitrate in absence of reducing agent (Gao, Wei, Yan, & Xu, 2011). However, the reduction rate of the silver ions was higher in presence of reducing agent. In order to investigate the silver nitrate reduction within the starch matrix, the nanocomposite films were stored in dark place at 25 °C during several weeks. The obtained UV–vis absorption spectra for the film in

absence of glucose are shown as example in Fig. 1b. The intensity of the SPR peak increased with the increasing of the storage time indicating a continued reduction of the silver ions. The same trend was observed when the film contained glucose. A saturation effect in the absorbance SPR peak was achieved after 12 weeks and 20 weeks for the nanocomposite films elaborated in presence of glucose and in absence of glucose, respectively. The time dependence of maximal absorbance (Amax) for the nanocomposite films was determined before the saturation occurred. The obtained curves of maximal absorbance time dependence were fitted by a first order rate equation according the following relationship: Amaxt = Amax∞ (1 − exp (−k .t ))

(Eq. 5)

where Amaxt is the maximal absorbance at time t, Amax∞ the maximal absorbance at very long time and k the first order-rate constant.

Whatever the system, a same value of k around 4.0 ± 0.1 ×10-5 min-1 was obtained, indicating a similar kinetic behaviour of the reactions. At this stage, the rate of silver ions reduction was not dependent on the presence and the amount of glucose molecules. It can be concluded, that after the water evaporation process, the viscosity medium increased and the accessibility of the silver ions by the glucose molecules decreased. The wavelength at the maximum of the absorbance (λmax) as a function of the reaction time was also determined for the different AgNO3/glucose molar ratios. Whatever the AgNO3/glucose molar ratio, λmax was constant (412 ± 2 nm) and did not change as a function of the reaction time. Therefore, it can be firstly concluded that AgNPs of almost same size were formed and secondly that the nanoparticles size did not drastically change during the storage at 25 °C. The dispersion of the AgNPs in the nanocomposite films just after the water evaporation process was further analysed using Transmission Electron Microscopy. Examples

of the obtained images for the nanocomposite films in absence of glucose are shown in Fig. 2a. Whatever the AgNO3/glucose molar ratios, the AgNPs dispersion within the matrix was homogenous. The nanoparticles were spherical, very small and the size distribution seemed to be narrow. More than 98% of the nanoparticles were smaller than 10 nm. The average size

( d ) and the standard deviation (σ) were equal to 6 nm and 2 nm respectively. The influence of the temperature on the silver nanoparticle formation was then studied. UVvisible absorption spectra obtained after treatment at 40 and 85 °C, respectively are shown in Fig. 1c and d.

(a)

(b)

(c) Figure 2: Transmission electron micrographs and size distribution of the silver nanocomposite films without glucose (a) just after water evaporation, (b) thermal treated at 40 °C during 48 h and (c) thermal treated at 85 °C during 48 h.

Whatever the AgNO3/glucose molar ratio, the intensity of the SPR peak increased as the treatment time at 40 °C increased. All SPR peaks were characterized by a symmetrical shape centred at 412 nm. As explained by Huang et al., the absence of widening at longer wavelengths of the SPR peak is synonym of the absence of nanoparticle clusters (H. H. Huang et al., 1996). It can be also noticed that the maximal absorbance (Amax) of the SPR peak of the Ag-SG 1:0(72H-40C) nanocomposite film was equivalent to that obtained for the Ag-SG 1:0(6W-25C). So, as expected an increase of the annealing temperature led to an increase of the reduction kinetics of the silver ions. At 85 °C, a saturation effect in the absorbance SPR peak was observed whatever the AgNO3/glucose molar ratio as soon as the thermal treatment duration exceeded 15 h. The morphology of the nanocomposite films treated at 40 °C and 85 °C for 15 h, 48 h and 72 h was further analysed in terms of nanoparticles size and dispersion. Fig. 2b and c show examples of the typical morphology obtained after 48 h of thermal treatment. The same kind of morphology was observed whatever the AgNO3/glucose molar ratio and treatment duration. In a qualitative point of view, it can be remarked that the AgNPs were spherical and were also well distributed in the starch matrix. However, the quantitative analysis of the

( )

morphological data thanks to the ImageJ Software showed that the mean size d and the size distribution (σ) both increased as the treatment time and the temperature increased. The evolutions of d as a function of the reaction are represented in Fig. 3.

( )

Figure 3: Evolution of the average size d

of the AgNPs as a function of the duration of

thermal treatment.

An effect of the treatment temperature was underlined. Modifications were more pronounced for nanocomposite films treated at 85 °C. After 72 h of thermal treatment, d was equal 10 nm and 15 nm for 40 °C and 85 °C, respectively. It can be also noticed that the smallest nanoparticles in size were obtained for the films elaborated without glucose whatever the treatment temperature. The XRD patterns of the nanocomposite films and the associated neat starch/glycerol film obtained just after the water evaporation process are presented in Fig. 4a.

(a)

(b)

(c) Figure 4 : XRD patterns of starch neat matrix and different nanocomposite films (a) just after water evaporation (b) thermal treated at 40 °C and (c) thermal treated at 40 °C.

The diffraction diagram of the neat matrix corresponded to a typical XRD pattern of potato starch (B-type). The observed diffraction peaks for native potato starch at 2θ = 5.6, 15.2, 17.2, 19.8, and 22.5°, were related to the complex structures characteristic of starch tubers, such as B-type crystals (Masclaux, Gouanvé, & Espuche, 2010; Zobel, 1988). After the formation of AgNPs into the polymer matrix, no noteworthy difference was observed in the 2θ range from 5 to 50° whatever the molar fraction of AgNO3/glucose. The presence of AgNPs did not modify drastically the matrix crystalline morphology. However, the XRD patterns did not exhibit any characteristic diffraction peak for the AgNPs. As explained by Liu et al. and Salkar et al., the absence of the silver diffraction peak could be due by the presence of amorphous nanoparticles (S. Liu, Huang, Chen, Avivi, & Gedanken, 2001; Salkar, Jeevanandam, Aruna, Koltypin, & Gedanken, 1999). The XRD analyses of the nanocomposite films after thermal treatment at 40 °C and 85 °C were performed. Examples of the XRD

patterns of the nanocomposite films in absence of glucose are presented in Fig. 4b and c respectively. In a general point of view, for all systems, the crystallinity structure of the potato starch matrix did not drastically change as function of the thermal treatment temperature and the duration time. For the nanocomposite films treated at 40 °C, even after 72 h, none characteristic diffraction peak for the AgNPs was observed indicating that AgNPs were probably amorphous. However, for all nanocomposite films treated at 85 °C, a diffraction peak at 38° was observed right from 15 h of treatment. This diffraction peak was related to the (111) crystal plan of the face-centred cubic (fcc) silver (Johnson, Thielemans, & Walsh, 2010). The presence of this peak demonstrated that crystalline AgNPs were formed. For longer time, the silver crystalline peak area increased but seemed to be identical after 48 h and 72 h of treatment. Likewise, the increase of the amount of glucose did not have also a significant influence on the crystalline peak area for a same treatment time. Then, it was possible to obtain amorphous or crystalline AgNPs by adjusting the thermal treatment temperature, keeping small AgNPs. In the following part of this manuscript, the influence of the in situ formation of the silver nanoparticles on the transport properties was investigated for the same thermal treatment duration (48 h) but for different temperatures, namely 40 and 85 °C.

Water sorption properties

(a)

(b) Figure 5: Evolution of the water concentration at equilibrium versus the water activity for the nanocomposite films and neat matrix elaborated: (a) without glucose and (b) in presence of glucose (AgNO3/glucose molar ratio 1:3)

The sorption isotherms curve of the untreated and treated films at 40 and 85 °C during 48 h are represented in Fig. 5. The experimental data defined a sigmoidal shape corresponding to BET II in the classification of Brunauer–Emmett–Teller (Brunauer, Deming, Deming, & Teller, 1940). The BET II model, which is a combination of dual-mode and clustering contribution, is a typical model of water sorption in hydrophilic materials (Gocho, 2000; Gouanve et al., 2006; Hellman, Boesch, & Melvin, 1952). The first part of the isotherm presents a concave form, which was usually analyzed as the sorption step corresponding to the formation of the primary hydration sphere of the hydroxyl groups which act as Langmuir sites. The second part was linear and corresponds to a Henry sorption mode. The last part of the isotherm presented a convex form, which can be explained by the formation of water clustering (Chenlo, Moreira, Prieto, & Torres, 2011). For the neat matrices, the comparison of the curves obtained for SG 0:0 (0H) and SG 0:3 (0H) in the range of water activity 0 < aw < 0.6, shows that the amount of water sorbed at equilibrium was lower for the film containing glucose. This could be related to a decrease of the available hydroxyl groups of starch in presence of glucose molecules. Indeed, hydroxyl groups of glucose could form hydrogen bonds with hydroxyl groups of starch, which led to decrease the number of available sorption sites. At higher water activity (aw > 0.6), the water sorbed molecules weakened the glucose starch hydrogen bonds, leading to a same amount of water at equilibrium for the different films (Gaudin, Lourdin, Le Botlan, Ilari, & Colonna, 1999; Talja, Helén, Roos, & Jouppila, 2007; Talja et al., 2007). Whatever the amount of glucose, it can be then noticed that the thermal treatment at 85 °C did not have a significant effect on the water sorption capacity of the neat matrices. A single curve was thus obtained showing a superimposition of the sorption isotherms in the whole range of water activity. As a result, it was expected that the possible change in water uptake for the nanocomposite films will not be attributed to the thermal treatment performed at 40 or 85 °C but to the in situ formation of the AgNPs.

For the thermal treated nanocomposite films, whatever the used temperature, no noteworthy difference of water concentration at equilibrium (C) was observed compared to the associated neat matrix in the whole range of water activity. A slight decrease of C was observed for the nanocomposite film treated at 85 °C, in absence of glucose, compared to the associated neat matrix. So, in a general point of view, it can be concluded that the presence of AgNPs did not drastically modify the water sorption capacity of the starch polymer matrix. The effect of the presence of AgNPs on the water diffusion was also investigated. The evolutions of the water diffusion coefficient (D) as a function of the water concentration at equilibrium in a semi-logarithmic scale for the different films are represented in Fig. 6. D was not constant and depended on the amount of water molecules sorbed by the materials. Whatever the films, two domains can be clearly distinguished. D increased and then decreased. These variations were in agreement with the water sorption mechanism complying with the shape of the BET II curve. In the range of low water activity, typically below 0.5 (C < 120 cm3STP g-1), the increase of D can be explained by the dual-mode sorption contribution (Rouse, 1947). Water molecules were predominately sorbed on the specific hydroxyl groups in which they were partially immobilized. Then, Henry’s type sorption was dominant and the diffusion coefficient increased with the contribution of mobility of the water molecules. For higher water content (C > 120 cm3STP g-1), the decrease of D was attributed to the water clustering phenomenon which can be assigned to the increase of the cluster size of the diffusing water molecules which became fewer movable (Masclaux et al., 2010). Concerning the effect of the thermal treatment on the starch polymer neat matrices, none modification of D was observed compared to the untreated neat matrix. So a modification of the evolution of D for the nanocomposite films should not be due to a direct effect of the thermal treatment.

(a)

(b) Figure 6: Evolution of the diffusion coefficient versus the water concentration at equilibrium for the nanocomposite films and neat matrix elaborated: (a) without glucose and (b) in presence of glucose (AgNO3/glucose molar ratio 1:3)

For the nanocomposite films treated at 40 °C, whatever the molar ratio of AgNO3/glucose, no noteworthy difference was observed compared to the neat matrices in the whole range of water concentration. On the other hand, for the films treated at 85 °C, for water concentration below 120 cmSTP3 g-1, the obtained values of D were lower compared to those obtained for the neat matrices. This difference can be explained by the presence of crystalline AgNPs which could limit in a higher extent the diffusion of the water molecules in the starch chains matrix. The increase of water diffusion as a function of the water concentration in the material is generally related to a plasticization phenomenon. This phenomenon could be described by the following relationship: D = D0 .exp (γ C ) where D0 is the limit diffusion coefficient defined at nil concentration and can give information on the microstructure of the materials, γ is the plasticization coefficient. D0, and γ were deduced from a linear regression of the curve at low water concentration. Taking into account the uncertainty of the determination of γ , it can be concluded that for the neat matrices and the different nanocomposite films, the values of γ were closed to each other and were equal to 4.3 ± 0.3 × 10-2 g cmSTP-3. This is indicated the major role of the starch polymer matrix on the plasticization phenomenon. For D0, the same values were obtained for the neat matrices (untreated and thermal treated) and for the nanocomposite films treated at 40 °C (D0 = 6.1 ± 0.3 × 10-11 cm2 s-1). However, a halving decrease of D0 was obtained for the nanocomposite films treated at 85 °C (D0 = 3.0 ± 0.2 × 10-11 cm2 s-1). This high decrease of D0, which represents the diffusion of the first water molecules in the materials in absence of concentration dependency could be explained by the presence of the crystalline AgNPs which were considered as obstacle for water diffusion and also by the presence of strong interaction between the AgNPs and the starch polymer matrix. Water permeability

The evolutions of water permeability coefficient PH 2O as a function of the water activity (aw) for the neat matrices was shown in a semi log scale in Fig. 7a. The obtained value for the neat matrix in absence of glucose was in a good agreement with the one which obtained by Tang et al. (Tang, Alavi, & Herald, 2008). For both matrices, the permeability coefficient increased as the water activity increased. This variation could be related to the general water sorption mechanism as seen previously. The sorbed water molecules by the starch polymer matrix increased the polymer plasticization, resulting in the increase of the water permeability. Similar behaviors have been reported in the literature for polysaccharides polymers like starch based films (Talja et al., 2007). The presence of glucose did not modify the water permeability coefficient of the potato starch films. To discuss more specially the effect of the thermal treatment and the presence of AgNPs, the values of the relative water permeability ( Prwater ) which is defined as the ratio of the permeability coefficient of the sample on the permeability coefficient of its associated matrix were calculated. The evolutions of Prwater as a function of the water activity are shown in Fig. 7b and c. The effect of the thermal treatment at 40 and 85 °C on the water transport properties of the neat matrices was investigated at first. Taking into account the uncertainty of measurements, the calculated relative water permeability was closed to unity in the range of tested water activity for the neat matrices treated at 40 °C indicating none influence of the thermal treatment on the starch polymer. For the matrices treated at 85 °C, a significant decrease of Prwater was observed whatever the AgNO3/glucose molar ratio. As example, at aw = 0.5, Prwater was equal to 0.61 for the neat matrix that contains glucose. For the nanocomposite films, the evolutions of the relative permeability are also represented in Fig. 7b and c. The values were close to unity in the whole range of water activity for all the silver nanocomposite films treated at 40 °C. So, the presence of the non-crystalline AgNPs

did not have a significant impact on the water barrier properties. One the other hand, a huge decrease of Prwater was observed for the nanocomposite films treated at 85 °C. Indeed, at aw = 0.5, Prwater were equal to 0.36 and 0.2 for the nanocomposite films elaborated in the absence and in the presence of glucose molecules, respectively. This important decrease of permeability cannot be explained only by the thermal treatment itself but by also the presence of crystalline AgNPs within the polymer matrix. However, the tortuosity effect induced by the presence of small amount of AgNPs and the spherical shape of these impermeable fillers was not sufficient to explain the improvement of barrier properties. Indeed, the permeability decrease was much higher than the theoretical decrease calculated by Maxwell law (Frisch, 1970). Compton et al. and Simon et al. have already observed a decrease of permeability higher than the theoretical decrease for the in situ generation of palladium and palladiumsilver alloy nanoparticles in a polyimide matrix (Compton et al., 2006; Simon, Alcouffe, & Espuche, 2014). They attributed this decrease by the presence of crystalline nanoparticles and the formation of strong interface between the particles and the polymer matrix. However, the relative permeability increases as the water activity increases. This result can be explained by a plasticization effect of the polymer matrix due to the water molecules sorbed as the water activity increased which tended to minimize the contribution of the strong interface between the nanoparticles and the polymer matrix presented at lower water activity.

(a)

(b)

(c) Figure 7: (a) Evolution of the water permeability coefficient versus the water activity for the neat matrices (b) Evolution of the relative permeability of the films elaborated without glucose versus the water activity (c) Evolution of the relative permeability of the films elaborated in presence of glucose (AgNO3/glucose molar ratio 1:3) versus the water activity.

Oxygen permeability The evolution of oxygen coefficient permeability PO2 as a function of the water activity (aw) for the neat matrices is shown in a semi log scale in Fig. 8a.

(a)

(b) Figure 8: (a) Evolution of the oxygen permeability versus the water activity for the neat matrices (b) Evolution of the relative permeability of the films elaborated without glucose versus the water activity The permeability values were in a good agreement with those reported in the literature by Gaudin et al. (Gaudin, Lourdin, Forssell, & Colonna, 2000). For both matrices, the same trend

was observed, namely PO2 tended to increase as the water activity increased. Similar behaviors have been reported in the literature for polysaccharides polymers like starch (Gaudin et al., 2000). This result can be explained by the plasticization of the polymer matrix due to the presence of water sorbed molecules by the polymer which tended to decrease the cohesive density energy of the films. As in case of water permeability, the presence of glucose did not modify the oxygen permeability coefficient of the potato starch films. The barrier oxygen properties were determined for the neat matrix and nanocomposite films elaborated in absence of glucose molecules. The values of the relative oxygen permeability coefficient ( Proxygen ) were calculated and their evolution as a function of the water activity is represented in Fig. 8b. For the nanocomposite film treated at 40 °C, an improvement of the oxygen barrier properties was observed. However, this improvement was only due to the effect of the thermal treatment and not to the presence of the non-crystalline AgNPs. Indeed, the obtained values of Proxygen for the nanocomposite film were in the same range of order than that calculated for the associated neat matrix. Here again, the presence of the non-crystalline AgNPs did not lead to significant variation of the oxygen barrier properties. For the nanocomposite film treated at 85 °C, an improvement of the barrier oxygen permeability near to one decade was observed at aw = 0.5. This can be partially explained by the effect of the thermal treatment at 85 °C but also by the presence of crystalline AgNPs within the polymer matrix and the strong cohesive interface between the AgNPs and the polymer matrix as seen previously. However, an increase of Proxygen was also observed as the activity increased, but in spite of the plasticization effect of the polymer matrix by the sorbed water molecules, the value of Proxygen at high water activity remained low ( Proxygen = 0.44 for aw = 0.85). Conclusion

The in situ method developed in this work showed that the presence of glucose was not necessary to reduce the silver ions when they were entrapped in the starch polymer film. Starch polymer played the role at once stabilizer and reducing agent. After storage at 25 °C, it was shown that amorphous AgNPs were obtained. The dispersion of spherical AgNPs was homogenous and the mean average nanoparticle size was around 6 nm. After thermal treatment at 40 °C, none significant change of size, size distribution and dispersion of the AgNPs were observed and the nanoparticles remained amorphous. After thermal treatment at 85 °C, the nanoparticle size (around 15 nm) and the size distribution increased and crystalline AgNPs were formed. The water sorption, water and oxygen barrier properties of the films exhibited a strong dependence on the thermal treatment temperature used. Transport properties for the nanocomposite films treated at 40 °C were equivalent to those obtained for the neat matrices. A huge decrease of water and oxygen permeability, near to one decade, was obtained for the nanocomposite films treated at 85 °C compared to the neat matrix. It was pointed out that the presence of crystalline AgNPs and the presence of cohesive interface between the crystalline AgNPs and the starch matrix were at the origin of the improvement in barrier properties.

Acknowledgements The authors gratefully acknowledge Ruben Vera and the “Centre de Diffractométrie Henri Longchambon” of University of Lyon 1 for the reflection XRD experiments, and the “Centre Technologique des Microstructures” of University of Lyon 1 for TEM photomicrographs.

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