Council for Innovative Research

ISSN 2321-807X POLYELECTROLYTE NANOCOMPOSITE MEMBRANES BASED ON PVAHA-HAP FOR FUEL CELL APPLICATIONS: SYNTHESIS AND APPLICATION Alaa Fahmya, Mouhamed A. Abu-Saiedb, Elbadawy A. Kamounb*, Hazem F. Khalila, M. Elsayed Youssefc , Attia M. Attiaa and Farag A. Esmaila a

Applied Chemistry Department, Faculty of Science, Al-Azhar University, Cairo, Egypt

b

Polymeric Materials Research Department, Advanced Technology and New Materials Research Institute (ATNMRI), City of Scientific Research & Technological Applications (SRTA -City), New Borg Al-Arab City, 21934 Alexandria, Egypt c

Computer Based Engineering Applications Dep., Informatics Research Institute, City of Scientific Research & Technological Applications (SRTA-City), New Borg Al-Arab City, 21934 Alexandria, Egypt

Corresponding author E-Mail: [email protected] ABSTRACT Novel composite membranes prepared by blending hyaluronic acid (HA) with modified polyvin yl alcohol (PVA) and hydroxyapatite as nano-filler (HAP), followed by ex-situ crosslinking with epichlorohydrin (EPI) to achieve the desired chemical and mechanical stability, are reported for use as nanocomposite polyelectrolyte membranes (PEM) in direct methanol fuel cell (DMFC). In this work, PVA-H A-H AP membranes are synthesized by solution-casting method using EPI as chemical crosslinker. PVA is first modified using orthophosphoric acid (OPA) for creating the ion conducting property. Different concentrations of HA, HAP and OPA modifier agent are used. Some physicochemical properties e.g. water uptake, gel fraction, mechanical and thermal properties were determined as function of varied membrane components. In addition, PVA-HA-H AP membranes molecular structure is verified by FTIR, while morphological changes due to addition of HAP is investigated by SEM. Results revealed that addition HAP and modification of PVA with OPA in different contents affected sharply on physicochemical and electro-chemical properties of obtained PVA-HA-H AP membranes. It was noticed that addition of HAP decreased the swelling ability and imp roved the thermal and mechanical properties of PVA-H A-HAP, as compared to pristine PVA-H A membranes. Whereas, both the modification of PVA with OPA and HAP nano-fillers incorporation created ionic conduction, which result in electrochemical properties impr ovement such as, ion exchange capacity (IEC) values and ionic conductivity.

Keywords: PVA; Polyelectrolyte nanocomposite membrane; Hyaluronic acid; Hydroxyapatite; Nano-fillers; Fuel cell applications.

Council for Innovative Research Peer Review Research Publishing System

Journal: Journal of Advances in Chemistry Vol. 11, No. 3 [email protected] www.cirjac.com 3426 | P a g e

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ISSN 2321-807X 1. INTRODUCTION Organic-inorganic composite materials-membranes have attracted much attention and they have been extensively studied for long time. Organic-inorganic nano-fillers are usually organic polymer composite with inorganic nanoscales building blocks 1. These blocks collect the advantages of the organic part of polymer (e.g. flexibility, chemical stability and processability) and the inorganic part of nano-fillers (e.g. thermal, mechanical stabilities and conductivity) [1-3]. Polyelectrolyte membranes (PEM) or organic-nano-filler composite membranes can be fabricated by diverse synthetic ways, first, the organic component (i.e. polymer) has been incorporated as: (a) precursor (monomer or oligomers), (b) linear polymer (molted, dissolved polymer, or in em ulsion state) or (c) a polymer network either physically (e.g. crystalline or semi-crystalline linear polymer) or chemically crosslinked (e.g. thermosets, elastomers, chemical crosslinker). Second, the inorganic part can be included for PEM as , (a) precursor, or (b) nano-sized particles. The chemical crosslinking was preferred to crosslink PEM, due to its operating in harsh conditions (e.g. high operating temperature ~ 60 -120 oC and pressures) which limit the use of physical crosslinking for crosslinking of PEM [1]. PVA is a water soluble synthetic polymer and it was chosen as an organic part in PEM [4]. Additionally, it was previously used in different studies as polymer membranes. For example [4], used crosslinked nanocomposite membranes of Nafion and PVA. The y found that the Nafion/PVA membrane reduced the methanol permeability by ~50% comparing to the native Nafion membranes in PEM for fuel cell applications. However, it was found that the PVA membranes possessed a poor proton conductivity as compared with native Nafion membranes, because the PVA itself does not have any polyatomic anions like negative charged ions, such as –COO- or SO42- groups [4, 5]. Thus, several studies showed the chemically modified PVA polymer membranes with the sulfonated, phosphoric, or sulfuric acids to reduce the methanol permeability and to create the ionic conductivity at PVA for being a suitable PEM for fuel cell applications. Accordingly, the surface modification of PVA is an urgent step before its use for PEM for creating ionic surface conductivity [4, 5]. Hyaluronic acid (HA) is a natural linear di-polysaccharides consisting of β-(1, 4)-linked D-glucuronic acid and β-(1,3) Nacetyl-D-glucosamine units, and it is the only non-sulfated glycosaminoglycans skin (GAG) in the extracellular matrix (ECM) of all higher animals. HA is a poly anionic polymer that owing diacritical properties and distinctive biological functions [6-8]. Here, the use of HA with PVA simultaneously as blend polymer in the organic part of PEM, is reported for the first time in the literature. Likewise, sodium alginate (SA) was blended with PVA based proton exchange electrolyte membranes for direct methanol fuel cell (DMFC) applications [9]. They demonstrated that addition of SA into PVA improved the efficiency of DMFC such as, selective water sorption a nd methanol barrier properties. Hydroxyapatite or calcium phosphate (Ca 10(PO4)6 (OH)2, H AP has been frequently employed formerly as scaffold or implant materials in biomedical applications, due to its excellent biocompatibility, bioactivity, and mechani cal/thermal properties [10]. The addition of HAP as inorganic ceramic fillers into PVA polymer matrix is allow to reduce the glass transition temperature (Tg ), the crystallinity, and increase the amorphous phase of the PVA polymer matrix, which then increase the ionic conductivity and the mechanical strength of obtained PEM. Dispersion of nano-fillers with high aspect ratio in organic polymer matrices can enhance widely the material properties such as mechanical [11-13] and thermal [13], fire retardency properties [14] and barrier properties [15]. Varieties of layered nano-particles are suited intensively to increase gas barrier properties, among them semectite clays, like montmorillonite (MMT), which is potentially used for designing a polymer nanocomposite. Additionally, nano-sized MMT cla ys were previously used with PVA as filler not only to improve the thermal and mechanical properties of PVA-MMT membranes, but also to enhance the ionic conductivity of PVA-MMT nanocomposite membranes for direct methanol fuel cells [2]. Recently, HAP nano-particles are used for the same aforementioned purpose instead of MMT nano-clay, with PVA composite polymer membranes for alkaline direct methanol fuel cell applications [11]. According to all these contributions, the PVA membranes could be used as PEM for fuel cell applications if the negative ions are incorporated within its structure. PVA-H A-H AP is synthesized by premature modification of PVA with OPA, and then followed by crosslinking the mixed-polymer matrix using EPI as a chemical crosslinker. The introductions of negative charged ions have been achieved by both chemical crosslinking and the PVA modification with OPA. Eventually, the objective of this study was to examine the influence of the modification of PVA with OPA and the addition of both HA and HAP in different portions on the physicochemical properties e.g. water uptake, gel fraction, thermal and chemical properties of obtained PVA-H A-H AP membranes, and to evaluate its potential as a feasible PEM for the fuel cell applications in the near future.

2. MATERIALS AND METHODS Materials PVA (typically average Mw = 72,000 g/mol, 98 % hydrolyzed) was obtained from Merck, Germany. Hyaluronic acid (HA) was purchased from Shanghai Jiaoyuan Industry Co., Ltd., China. Hydroxyapatite, (HAP) (nano p owder, OD> 200 nm, purity 99.9 %) obtained from Nano Inglobal, China. Orthophosphoric acid, (OPA) (M= 98 g/mole , 85% purity, 1.691~1.721 g/ml density) was obtained from Poch, Poland. Epichlorohydrin, (EPI) (99.5%) was obtained from Fluka Chemie, Germany.

Preparation of PVA-HA-HAP Nanocomposite Membranes PVA-HA-HAP nanocomposite membranes were prepared by solution -casting method. Typically, 30 g of PVA powder is dissolved in 270 ml distilled water at 80 oC for 2 h, to get 10 wt. % polymer solutions. Then, the solution is kept at room temperature according to the reported procedure of Liang et al. [16]. One gram of HA powder is dissolved in 49 ml of

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ISSN 2321-807X distilled water under continuous stirring at room temperature for one hour to obtain 2 wt. % of HA solution. Aqueous solution containing 10 wt. % of PVA is allowed to react with orthophosphoric acid, for getting a modified PVA. OPA is o added to PVA solution step-wise (during a half hour) with continuous stirring at 40 C for one hour, followed by addition of different concentrations of HA (0.5, 1, 1.5, and 2 wt. %) with aforementioned conditions, where the ratio of PVA: HA is 4 : 1. Different amounts of HAP (0.5, 1, 2, 3, 4, 6, and 8 wt. %), is dispersed into the bulk solution under harsh stirring for 4 h. The polymer-nanofiller bulk solution is sonicated for one hour to ensure the homogeneity among compositions. Epichlorohydrin with different ratios (0.1, 0.25, 0.5, 1, 2, 4, and 5, v/v, %) is added for crosslinking the two polymers. Th e bulk solution is kept under stirring for one hour until the solution mixture became a homogenous viscous appearance at 60 o C, followed by sonication to remove any formed air bubbles. The PVA-HA-HAP solution was poured gently in plastic Petri-dish (diameter was 14 cm). The membrane thickness is adjusted by using 20 ml of the poured bulk solution. The plastic dish is remained at room temperature for 24 h, and then carefully incubated in humidity-chamber to adjust the solvent evaporation. The thickness of the wet composite membranes was between 200 µm and 300 µm. Finally, composite membranes are then vacuum -dried at room temperature for 4 h. The resultant PVA-HA-HAP hybrid composite membranes are gently washed twice with a hot water for further purification and removing any unreacted membrane compositions, such as OPA or EPI. The thickness of resultant dried composite membranes was adjusted in the range of 150-200 µm. The dried PVA-HA-HAP membranes are kept in plastic bags to avoid the moisture exposure for storage till use.

Figure (1) Schematic reaction route for PVA-HA-HAP polyelectrolyte nanocomposite membranes preparation (a) and crosslinking reaction mechanism of both HA and PVA with EPI (b) and (c), respectively.

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ISSN 2321-807X Methods Determination of gel fraction The obtained PVA-HA-HAP composite membranes are vacuum -dried at room temperature to avoid any surface shrinking for 12 hours and weighted (W0), then soaked in distilled water for further 24 h up to an equilibrium swelling weight (Ws), for removing the leachable or soluble HA parts from membrane. The composite membrane s are then vacuum -dried and weighted again (We). The gel fraction (GF %) was carried out according to the method reported by Yang et al. [17], and calculated by the following equation (1). Gel fraction (GF %) = (We / W0) ×100.

(1)

Where, (W0) and (We) are the weights of dried membrane samples before and after soaking, respectively.

Determination of Water Uptake Water uptake of the PVA-H A-HAP membranes are usually defined in weight percent with respect to the weight of the dried membrane. For measuring the swelling ability of PVA-AH-HAP composite membranes, membrane samples were cut into 3.5 cm × 3.5 cm pieces and vacuum -dried for 12 h, the dried sample weight is determined (W dry). The dried samples were soaked in distilled water at room temperature, then weighted (W wet) at specific interval times. The water uptake of PVA-HA- HAP membranes was given as equation (2) [18]. Water uptake (%) = [(W wet – Wdr y) / W dry] × 100.

(2)

Determination of Ion-Exchange Capacity (IEC) IEC of membrane was determined by the classical acid -base titration method using phenolphthalein indicator. To determine IEC of membrane, PVA-H A-H AP nanocomposite membrane was first soaked in 100 mL of 0.01 NaOH solution for 6 h at RT, then 10 mL of NaOH solution is titrated versus 0.01 HCl. After finish the titration, the soaked membrane was washed gently with deionized water twice to remove any remained acids then weighted. The IEC is defined as the number of milli-equivalents of ions in 1 g of the membrane. IEC was estimated using equation 3 as given here [19]. IEC = [(B – S) × 0.01 × 10] / W m

(mmol/g).

(3)

Where, B is amount of HCl in mL used to neutralize blank sample, S is amount of HCl in mL used to neutralize membrane sample, 0.01 normality of HCl, 10 is the factor of the initial used amount ratio of NaOH solution, and W m is dried membrane weight.

Determination of Ionic Conductivity Proton conductivity measurements of PVA-HA-H AP composite membranes were performed using the two probe cell- AC impedance method (using impedance gain-phase analyzer, model 1260A, Solatron Analytical, UK). The membrane is cut into this dimension (2.0 cm wide × 6.0 cm long), the membrane is soaked in de -ionized water for 12 h before the test, then vacuum-dried and the thickness is digitally determined. The measurements are conducted in 100% humidity b y leaving the deionized water at the bottom of the glass container reactor, at 25 -50 oC, AC impedance spectra of tested samples are monitored in frequency between 1-10 MHz and 10 mV amplitude. The ionic conductivity of membranes can be calculated using the given equation 4. σ = l (RA)

-1

S/cm.

(4)

Where, σ is the ionic conductivity of membrane, l is the thickness of membrane in cm., R is the resistance of the membrane was determined at high frequency intercept the impedance wit nth real axis, and A is the cross section area of membrane in cm 2 (Mohanapriya et al. 2010).

Membrane Characterizations 

FT-IR

Vacuumed and dried samples of crosslinked PVA, PVA-HA, PVA-HA-HAP xerogels were analyzed by FT-IR on an EQUINOX 55 instrument (BRUKER, Germany). KBr-discs were prepared by grinding the dried sample with infrared grade KBr, followed by pressing. The FTIR spectrums were obtained by recording 64 scans between 4000 –400 cm −1 with a resolution of 2 cm −1. All samples were pressed by applying a force 105 N into transparent disk (maximum disk weight = 145 mg, with a diameter 13 mm). KBr-sample discs were measured in the absorbance mode. 

Mechanical Property Measurements

The maximum tensile strength and the elongation-at-break of PVA-HA-HAP membranes have been conducted using a tensile test machine (model: AG-I/ 50N-10KN, Japan). PVA-HA-HAP membranes were cut into specific a dog-bone shape (6 cm long, 2 cm wide at the ends, and 1 cm at the middle). The measurements were carried out at stretching rate 10 mm/min with pre-load of 0.5 N to determine load for each sample. The thickness of membrane samples were measured with a digimatic caliper before examination.

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ISSN 2321-807X 

SEM

The surface of the PVA-H A-H AP membrane samples were investigated by SEM (type: JEOL, JSM-6360LA, Japan) with 5 kV. The membranes were first coated with Au using an ion sputter coater in (model: 11430, USA, combined with vacuum base unit or SPi module control, model: 11425, USA).



Thermal Properties

The thermal characterization of vacuumed-dried PVA-HA-HAP membranes, have been accomplished using thermo gravimetric analysis (TGA) and differential scanning calo rimeter (DSC) thermograms. TGA was applied on a 204 Phoenix TGA instrument (NETZSCH, Germany) from 50–600 °C at a heating rate 5 °C/min. The onset temperature (T onset) was determined by TGA thermograms. Tonset is defined as the temperature at the intersection of the baseline mass and tangent drawn to the mass curve at the inflection point or point of greatest rate of mass loss%. Glassy transition temperatures (T g) of dried PVA-HA-HAP membranes were determined using DSC (model: 204 Phoenix DSC instrument), ( NETSCH, Germany). Tg values were determined as mid-point in the thermograms, as measured from the extensions of the pre-and post-transition baselines. DSC measurements were done at a heating rate of 10 °C/min from 25 –400 °C under N 2 gas.

3. RESULTS AND DISCUSSION FT-IR Fig. 2 presents the FT-IR spectra of the pure PVA, PVA-HA, and crosslinked PVA-H A-H AP xerogels membranes crosslinked by EPI. The recorded IR spectra present the dominant CH stretching vibrations of the polymer backbone as the CH (as/sym) stretches were observed in the spectral range 2916–2844 cm -1 [20]. As reported, the absorption bands of pristine PVA appeared at 3734 cm -1 for –OH free groups (unreacted and non-bonded groups), while intramolecular and intermolecular hydrogen bonds among –OH groups in PVA chains due to high hydrophilicity stretching bands were observed at υ 3389 cm-1 respectively[18]. It clearly reveals the intermolecular hydrogen bonding interactions between the produced polymers with terminal hydroxyl moieties owing to a red shift and the broadening of –OH stretch in the range of 2500–3600 cm -1 in case of PVA and PVA-HA and in presence of HAP. Therefore, the CH stretch expected at 2900 -3150 cm -1) was barely observed around 2924 cm -1, however the broad shoulder nearby 2600 cm -1 is attributed to a combination -1 band (C=O stretch and –OH out-of-plane deformation). A broad C–H stretching band appeared at 2924 cm , and bending -1 –CH 2 bands appeared at 1429 cm . Stretching bands for C=O groups remained from unconverted polyvin yl acetate into PVA appeared at υ 1678 cm -1 and 1560 cm -1. Moreover, high intensity absorption band appeared at 1097 cm -1 indicates the crystallization or entanglement degree of PVA [18]. For more details, in case of PVA-HA, the wide band observed at 3410 cm -1 which can be attributed to the –OH stretch -1 -1 mode of HA and the band at 2912 cm was the asymmetric stretching of C–H groups [21]. In addition bands at 3271 cm -1 is due to both –NH 2 group in glucosamine unite and bands at 1676 cm corresponding to C=O (–COOH group) in guluronic acid residues unit of HA. Moreover, the absorption band at 1568 cm -1 indicates the presence of C=O related to (–COOH groups) of orthophosphoric acid at modified PVA. The intensity values of O–H out of plan motion were reduced significantly, owing to further hydrogen bonding reaction occurred between PVA and HA. From aforementioned results, these spectral changes of –OH stretch mode are evidence also the covalent bind between –OH of HA and epichlrohydrine. Furthermore, bands at 1329, 1419 cm -1 might be related to C–C groups of PVA or H A.

Figure (2) FTIR spectra for (a) PVA, (b) PVA-HA membranes, and (c) PVA-HA-HAP nanocomposite membranes.

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ISSN 2321-807X SEM SEM pictures showing the surface morphology of PVA-H A membrane and PVA-H A-H AP nanocomposite membrane are presented respectively in Fig.3. The smooth surface morphology and glossy-like surface structure of PVA-H A membrane (Fig. 3, left) refers the homogeneity and the good compatibility between two polymer matrix components ( i.e. PVA and HA). Whereas, distinctive mixed-polymer matrices with nano filler was detected for PVA-HA-H AP nanocomposite membrane (Fig. 3, right), refers to no micro-holes formed due to the good dispersion of HAP nano-fillers which appear as nano-platelets onto PVA membrane surface, in addition HAP nano-particles surface aggregation was observed too.

3.1. Effect of HA Addition on Water Sorption Ability and Mechanical Stability of Membranes Fig.4 displays the total water content in the PVA-HA-HAP membranes as function of the added HA contents. The HA contents are varied in membranes as follows 0, 0.5, 1.0, 1.5, and 2.0 wt. %. The total water co ntent in PVA-HA-HAP increased progressively until ~ 460% when the HA content reached to the maximum loading content i.e. 2.0 wt. %, however the lowest water content was detected with the virgin PVA-HAP membranes (0 wt. % HA). While, the three HA concentrations (0.5, 1.0, 1.5 wt. %) very slight differences in their water uptake were observed, indicating that increase the crosslinking in the PVA-HA-HAP membranes might result in more rigid and compacted polymer structure with (low HA contents) and more flexible and high swellable polymer structure with high HA contents. This speculation is based to also the high hydrophilicity nature of HA added to PVA-HAP membranes which results in high water imbibing and high sorption capacity [6, 7], which might produce a less crosslinked membrane compared to pristine PVA membranes. This result is consistent with the obtained data by Yokoyama et al. [22] In addition, Hassan and Peppas [23] have proved related results, they found that less crosslinked hydrogels, exhibited a higher water uptake because high crosslinked hydroge ls could not sustain much imbibing water within their structure. The depicted data in Table 1 show the mechanical properties e.g. tensile strength and elongation -at- break (%) of PVAHA-HAP membranes against varied HA contents added. The tensile strength of PVA-HA-HAP membranes increased parentally with increasing the HA content till the highest value at 1.5% of HA, and then it decreased sharply at 2% HA. Likely, the maximum tensile strength possessed the same results pattern of elongation -at-break of hydrogel membranes as showed in Table 1.

Figure (3) Scanning electron micrographs for PVA-HA membrane (left) and PVA-HA-HAP nanocomposite membrane (right).

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Figure (4) Effect of HA (wt. %) addition on water uptake values of PVA-HA-HAP membranes. “16 ml of PVA aqueous solution (10 wt.%) is modified with 400 µl of OPA for one hour, 4 ml of HA (0, 0.5, 1, 1.5 and 2 wt.%) is then added to last mixture solution, the aforementioned solution is kept under stirring for 2h at ambient temperature. 0.4 g (2 wt.%) of HAP is added to PVA-HA solution under continuous stirring for 4h at 40 oC, then 200 µl of EPI was added and kept under stirring for one hour. PVA-HA-HAP viscous solution is allowed to cold at RT and it was then poured in Petri-dish, then was relieved for 24 h at RT and then incubated in humidity chamb er for drying at 35 oC for 6h”. Table (1) Mechanical properties of PVA-HA-HAP membranes in dependence on different HA contents.

HA contents (wt. %)

Tensile strength

Elongation-at-break

(N/mm)

(%)

0

15.95

267.87

0.5

18.49

280.54

1.0

19.56

318.05

1.5

21.14

379.18

2.0

11.91

225.69

These results are ascribed to addition of HA into PVA hydrogels might improve the mechanical properties till certain extend, however the continuous addition of HA to PVA resulted in mechanical deterioration of PVA-HA-HAP membranes, because of the introduction of the polymeric matrix can restrict the chain segmental mobility and increase the membrane strength. This explanation is opposite with the fact that the increase o f HA content between PVA polymer chains decreases the interaction between polymer chains resulting in lower crosslinking densities which lead to weak hydrogel membrane structure. These results are coincided with results of Rosiak et al. [24] and Jeon et al. [25]. They have explored that the maxim um tensile strength of PVA hydrogel decreased with increasing blend materials due to decreased crosslinking density.

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ISSN 2321-807X 3.2. Effect of HAP Addition on Water Sorption Ability, Mechanical and Thermal Stability of Membranes The influence of HAP nanoparticles incorporation on water uptake % of PVA-H A-H AP nanocomposite membranes was presented in Fig.5. The swelling results exhibit a significant swelling behavior change after HAP nanoparticles addition. As seen, the swelling of PVA-H A-H AP nanocomposite membranes increase with the increase of HAP contents in hydrogel membranes, due to the addition of HAP might decrease and restrict the crosslinking density of membranes which lead to increase of swelling ability of nanocomposite membranes. This assumption is consis tent with obtained results of Kamoun and Menzel [26]. They found that a considerable increase in swelling ability of h ydroxyeth yl starch modified hydro xyethyl methacrylate (HES-HEMA) hydrogel due to addition of montmorillonite clay nanoparticles in dramatic portions. On the other hand, addition of HAP nanoparticles (2.0 wt.% HAP, Fig. 5) decrease the high water content behavior of PVA-H AHAP membranes as compared with the same sample condition in Fig.4 (0.5 wt.% of HA), where water uptake was decreased from ~385 % to 200 %. With the knowledge that high water content of nanocomposite membranes can be regarded as a big restriction for using as PEM for D MFC applications as aimed.

Figure (5) Effect of HAP (wt. %) nanoparticles addition on water uptake values of PVA-HA-HAP membranes, these samples were synthesized according to the same preparation conditions as showed in Fig.4, but with different HAP contents (0, 1, 2, 3, 4, and 6 wt.%) and 05. wt.% of HA). Table (2) Mechanical and thermal properties of PVA-HA-HAP membranes with respect to different contents of HAP nano-fillers. Mechanical properties HAP contents (wt. %)

Thermal stability

Tensile strength

Elongationat-break

DSC results

(N/mm)

(%)

Tg (o C)

TGA results Td1 (oC)

Td2 (oC)

Td at 50 % mass loss, o ( C)

Tonset (oC)

0

26

550

88

40-163

164-241

415

161

0.5

18

320

82

45-179

180-252

431

176

1.0

17.5

394

80

40-171

172-246

443

170

2.0

16.4

385

79

38-176

177-248

452

174

3.0

14.47

311

77

40-220

221-263

468

210

4.0

14

303

75

40-200

201-257

499

198

6.0

12

140

74

40-253

254-479

502

251

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ISSN 2321-807X The mechanical properties of all membranes represented in their tensile strength and the elongation -at-break percentage were determined and the collected data are presented in Table 2. Generally, it was noticed that the HAP fillers are introduced into the PVA-H A membranes. It exhibits more accessible free volumes, more defect sites, deteriorated structure especially with high HAP contents, and more voids onto membranes surface. Thus, the mechanical properties of PVA-H A-HAP nanocomposite membranes were clearly reduced by addition of HAP nano fillers. However, the only benefit of HAP nano fillers addition remains by the fact that to improve and create ionic conduction shells which will be verified by obtained results of electrochemical measurements. The thermal properties of PVA-H A and PVA-H A-H AP with different HAP contents (0, 0.5, 1, 2, 3, 4, and 6 wt. %) have been conducted. The TGA and DSC thermograms data have been depicted in Table 2. Inspection TGA results, it is apparent that the pristine PVA-H A membranes lost their weights in three thermal degradation stages. The first degradation stage varies from 40-163 °C and shows about 10% weight loss, which corresponds to the loss of residual solvent, vapor or absorbed moisture. However, the first degradation stage (T d1) of PVA-H A-H AP nanocomposite membranes ranges with high temperature at ~ 40-253 °C and weight loss reach to low weight loss ~ 3-7 %, due to presence of inorganic nonvolatilized ash residues. This thermal behavior reveals that incorporation of inorganic nanoparticles can resistant to moisture absorption owing to introduction of bulky and compacted nanocomposi te structure, compared to pristine PVA-H A membrane. The second degradation stage (Td2) referred to as a pyrolysis stage starts at 164 °C and continues up to 241 °C with weight loss 28%, for pure PVA-H A, which corresponds to degrade the macromolecular polym er chains. Whereas, in case PVA-H A-H AP with different HAP contents. Second pyrolysis stage starts with relative higher temperature at 180 °C up to 263 °C (3.0 wt.% of HAP) with a verage mass loss ~27% which corresponds to degrade most remained crosslinked chains of organic polymer parts. The third degradation stage of both PVA-H A (0 wt.% of H AP) and PVA-HA-H AP (0.5-6 wt. % of H AP) starts at 240-400 °C and average value 250-468 °C with the highest weight loss % due to remaining inorganic parts of HA without thermal volatilization, respectively. In this degradation stage, the remained polymer mass is fully degraded as a result of carbonization or volatilization. On the contrary, the T ons et values significantly enhanced with HAP incorporation. It is also evidence from TGA data that the degradation temperature at 50 % weight loss of PVA-H A-HAP was improved from 415 oC o (pure PVA-H A) to 431-502 C of PVA-H A-HAP nanocomposite membranes due to HAP nanoparticles addition. In the overall, the presented thermogravimetric data of TGA illustrate that the thermal stability of PVA-H A-HAP nanocomposite membranes has improved by the HA nanoparticles addition process. In addition, it can be concluded that the improved thermal stability of PVA-HA-H AP nanocomposite membranes is probably due to the additive effect of the HAP nanoparticles filler and the chemical crosslinking reaction between PVA-H A and EPI. Finally, the Tg values show an opposite trend as compared to thermal stability behavior which extracted from TGA results, wher e they appeared clear reduction in the thermal transition behavior, particularly after introduction of HAP nanoparticles, (Table 2). As shown, T g values are reduced from 88 oC to 82-74 oC of pure PVA-H A and PVA-HA-H AP nanocomposite membranes, respectively due to addition of HAP nanoparticles. This behavior is due to the addition of filler reduces T g values and crystallinity phase of PVA and increases the amorphous phase, then increases the ionic conductivity as aimed for the final application. These results are fully compatible with DSC results of Yang et al. [8, 11]. They also explored the addition of HAP into PVA which reduced both Tg values and crystallinity and then increase both amorphous phase and ionic conductivity of D MFC membranes.

3.3. Effect of PVA Modification Using OP on Water Sorption Ability, and Mechanical Properties of Membranes Pure PVA does not carry any protonic conductivity compared to the used commercial membranes like Nafion, because PVA itself does not possess any negative charged ions. However, the PVA modification is necessary step for use as polyelectrolyte membranes particularly for fuel cell applications should have an ionic conductivity property [4, 5]. It is possible to induce protonic conductivity into PVA b y sulphonating agents such as, concentrated sulfuric acid [5], sulfosuccinic acid [4], sulfophthalic acid [27] and chlorosulfonic acid. Additionally some these agents can induce negative charge ions e.g. –COOH functional groups or phosphate polyatomic ions e.g. o -phosphoric acid for a dual role as secondary crosslinking agent. From this point, PVA was modified before crosslinking with different concentrations of OPA (0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 v/v, %). The modified PVA was further crosslinked by EPI and HA simultaneously according to the optimized synthesis conditions as shown in the caption of Fi g 4. Fig.6 shows the water sorption and swelling behavior of PVA-HA-H AP nanocomposite membranes as function of OPA different concentrations for PVA. It is noteworthy that swelling behavior or water sorption of PVA-H A-HAP nanocomposite membranes are reduced with the increase of modifier agent concentrations (i.e. OPA), while gel fraction of PVA-HA-H AP is slightly increased. The swelling reduction and increasing the gel fraction can be attributed to the crosslinking degree improvement occurred which is advantageous for the application in fuel cells. This explanation is further evidenced by the swelling ability of membranes decreased, perhaps due to OPA was applied as a secondary chemical crosslinker or as a catalyst for crosslinking reaction adjacent to EPI [28, 29]. The swelling results are fully consistent with obtained results by Maiti et al. [5], they found that a significant swelling reduction for membranes due to the increase of modifier agent concentration of PVA and the crosslinking reaction as well. The effect of OPA different concentrations as a modifier agent for PVA on the mechanical properties of PVA-HA-H AP nanocomposite membranes is listed in Table 3. As show when the used OPA concentration increases, the mechanical properties of membranes increase for certain extend, and then decreased again. This might owing to increment the amount of modifier acid might result in somewhat decreasing in the imbibed solvent as proved in Fig.6 and OPA can attack the main polymer chains leading to decomposition reaction followed by increased brittleness due to excessive crosslinking degrees and mixed-matrix membranes decrease due to the increased membrane rigidity. The mechanical

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ISSN 2321-807X properties results are consistent with those results obtained by Maiti et al. [5]. They found that the used acid for modification of PVA improved the mechanical properties of membranes, due to that an excessive crosslinking degrees occurred by the used modifier acid.

Figure (6) Swelling ability and obtained membrane-gel fraction of PVA-HA-HAP nanocomposite membranes with different concentrations of OPA (as modifier agent for PVA).

Table (3) Effect of OP concentrations as modifier agent for PVA on the mechanical properties of PVA-HA-HAP nanocomposite membranes.

OP

Tensile strength

Elongation-at-break

contents (v/v, %)

(N/mm)

(%)

0

14

320

0.5

29

330

1.0

25

348

1.5

25

346

2.0

19

380

2.5

18

385

3.0

15

429

3.4. Ion Exchange Capacity (IEC) of Membranes Ion exchange capacity of membranes were estimated as an indicator for the ionic groups contents present in the nanocomposite membranes responsible for the proton conduction which refer to an indirect approximated indication for the proton conductivity [3, 4, 11, 30]. As shown in Fig.7, the IEC values of PVA-H A membranes increased with the used amount of OPA as PVA modifier agent (0, 1, 2, and 3 v/v,%) due to OPA contains a phosphate negative charge anion PO43- (Fig. 7a). The IEC values increased patently from 0 -1.5 mmol/g owing to that PVA was modified by OPA amounts

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ISSN 2321-807X ranged 0 – 3 (v/v,%). Thus the modification of PVA was proposed because the proton conductivity of PVA-HA due to OPA modification should increase proportionally with the amount of used OPA. This expectation is consisten t with obtained 4 results by Rhim et al. . They reported that IEC of PVA modified by sulfosuccinic acid (SSA) increased according to the increase of the used amounts of SSA. In addition PVA has previously modified before use as PEM in various ways [5]. Interestingly, the HAP nano-filler (2.0 w/v, %) was incorporated into the same samples, a significant improvement in the IEC values was detected. The IEC values of PVA-H A-H AP nanocomposite membranes resulting from HAP addition were increased form 0-1.5 mmol/g to 0.75-2 mmol/g (Fig. 7a). These results proved that addition of HAP nano -fillers reduced Tg values as presented in Table 3 and shifted the nanocomposite thermal phase from crystalline to amorphous phase, which then increased IEC values and subsequently protonic conductivity increases.

Figure (7) The change of IEC values in the PVA-HA membranes and PVA-HA-HAP nanocomposite membranes depending on the amounts of OPA as PVA modifier agent (a) and the used amounts of HAP nano-filler added (b).

For further evidence, various amounts of HAP nano-fillers (0, 1, 1.5, 2, 3, 4, and 6 w/v, %) were incorporated into treated PVA-H A nanocomposite membranes and their IEC values were presented in Fig.7b. According to the above results, IEC values of treated PVA-H A-HAP polyelectrolyte nanocomposite membranes increase proportionally between 0.75 mmol/g and 2.5 mmol/g, when the used amount of HAP nano-fillers added to mixed-matrix polymer increases. However, IEC value returns to reduction with the highest HAP contents in membranes, due to the mechanical properties deterioration. The increase of IEC according to accounted addition of HAP nano-fillers was expecting and proved through presented thermal data in Table 3 and Figs. 7a and 7b.

3.5. Ionic Conductivity of Membranes The ionic or proton conductivity measurements of PVA-HA-H AP composite membranes as function of both different amounts of HAP nano-filler (0, 0.5, 1.0, 2.0, 3.0, 4.0 and 6.0 w/v, %) and different concentrations of OPA (0, 0.5, 1.0, 1.5, 2, 2.5 and 3.0 v/v, %), were presented in Fig. 8. As seen, the proton conductivity values range progressively from 0.015 to 0.03 S/cm, owing to the incorporation increase of HAP nano -filler from 0-6 (w/v, %), (Fig. 8a). Likewise, the proton conductivity values increase dramatically in the range from 0 to 0.058 S/cm, when the PVA is modified by varied OPA concentrations from 0 to 3.0 (v/v, %), respectively (Fig. 8b). It was suggested that, PVA modification with OPA is regarded the primary factor to generate the proton conductivities. This might be due to the fact that, the OPA is the donor and the carrier of phosphoric acid groups PO43- (as charged negative ions), which are basically responsible for proton conductivity creation. However, the introduction of HAP nano-fillers might stimulate the proton conductivity improvement, as obtained in Fig. 8b. Evidently, when compare the obtained results of Figs. 7 and 8, the IEC values increase dramatically from the zero-value to high values. The obtained results of proton conductivi ty are typically consistent with reported results of Rhim et al. [4], they e xplored that the proton conductivities of PVA modified membranes with sulfosuccinic acid improved significantly, due to increase the number of charged negative ions which responsible to protonic conductivity generation (i.e.–SO3-H+ groups). Each of these directories concludes that, IEC or proton conductivity values increase with increasing

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ISSN 2321-807X the introduced phosphoric acid group numbers within a polymer matrix, while the nano -filler incorporation can also stimulate the same behavior.

Figure (8) The ionic conductivity of PVA-HA-HAP com posite membranes, as function of different contents of HAP nano-filler (a), and different concentrations of OPA modifier agent (b), all membranes were measured at 50 oC.

4. CONCLUSIONS The foregoing results show that, the polyelectrolyte nanocomposite membrane based on crosslinked PVA-HA-H AP containing phosphate groups was synthesized and obtained by a solution -casting method. The obtained membranes were evaluated in terms of physicochemical and some of electrochemical properties as a potential polymer electrolyte membrane for fuel cell applications. In hybrid nanocomposite membranes, the flexibility and the stability are derived from the organic and inorganic components in the polymer structure, respectively. Notable, the PVA modification using OPA has achieved the purpose of which used, where OPA has induced phosphate groups PO 43- which converted PVA as poor proton conductor into a good proton conductor. It was thus noted that water-sorption behavior have profound influence on PVA-H A-HAP membranes conductivity, accordingly high water sorption facilitates proton transfer through the membranes, showing quicker proton conduction as aimed. In addition, the incorporation of HAP nano-fillers and the crosslinking step using EPI for the mixed-polymer matrix improved significantly the thermal stability (TGA results) and IEC values range from 0–2.5 mmol/g, referring to the similar result trend of protonic conducti vity. Also, HAP nano-fillers incorporation and high crosslinked membranes limited the water sorption of membrane and reduced T g thermal values, which induced protonic conductivity via amorphous phase structure formation. Interestingly, both PVA modificatio n with OPA and HAP incorporation improved and stimulated the ionic conductivity of PEM, respectively. It was demonstrated that PVA-HA-H AP membranes possess acceptable electrochemical performance e.g. IEC values at ambient temperature and ionic conductivity. Finally, the PVA-H A-H AP polyelectrolyte nanocomposite membranes show a high potential for further direct methanol fuel cell applications, and it is desirable to enhance their proton conductivity which in turn increase their electrochemical properties performance in fuel cell applications.

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ISSN 2321-807X ACKNOWLEDGEMENTS E. Kamoun is thankful for Prof. Henning Menzel, Institute of Technical Chemistry, Technical University of Braunschweig, Germany, for approval of instrumental measurements and laboratories use, also E. A. Kamoun and A. Fahmy gratefully thank the Higher Education Ministry of Eg ypt, Sector of Cultural Affairs & Missions for financial support in part and Post doctoral granting.

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Maiti, J., Kakati, N., Lee, S. H., Jee, S. H., Viswanathan, B., and Yoon, Y. S. 2012. Where do poly(vin yl alcohol) based membranes stand in relation to Nafion for direct methanol fuel cell applications? J. Power Sources 216, 48-66.

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[10] Sun, F., Zhou, H., and Lee, J. 2011. Various preparation methods of highly porous hydroxyapatite/polymer nanoscale biocomposites for bone regeneration. Acta Biomaterialia 7, (11), 3813–3828. [11] Yang, C.-C., Lin, C.-T., and Chiu, S.-J. 2008. Preparation of the PVA/HAP composite polymer membrane for alkaline DMFC application. Desalination 233, 137-146. [12] Pavlidou, S., and Papaspyrides, C. D. 2008. A review on polymer-layered silicate nanocomposites. Progress Polym. Sci. 33, 1119-1198. [13] Manias, E., Touny, A., Wu, L., Strawhecker, K., Lu, B., and Chung, T. C. 2011. Polypropylene/ montmorillonite nanocomposites. review of the synthetic routes and materials properties. Chem. Mater. 13, 3516-3523. [14] Samyn, F., Bourbigot, S., Jama, C., Bellayer, S., Nazare, S., Hull, R., Fina, A., Castrovinci, A., and Camino, G. 2008. Characterization of the dispersion in polymer flame retarded nanocomposites. Europ. Polym. J. 44, 1631-1641. [15] Xu, B., Zheng, Q., Song, Y., and Shangguan, Y. 2006. Calculating barrier properties of polymer/ clay nanocomposites: Effects of clay la yers. Polymer 47, 2904-2910. [16] Liang, S., Liu, L., Huang, Q., and Yam, K. L. 2009. Preparation of single or double -network chitosan/poly(vin yl alcohol) gel films through selectively cross -linking method. Carbohyd. Polym . 77, (4), 718-724. [17] Yang, X., Liu, Q., Chen, X., Yu, F., and Zhu, Z. 2008. Investigation of PVA/ws -chitosan hydrogels prepared by combined gama-irradiation and freeze–thawing. Carbohyd. Polym. 73, 401-408. [18] Kenawy, E., Kamoun, E. A., Moh y Eldin, M. S., and El-Meligy, A. 2014. Physically crosslinked poly(vinyl alcohol)hydroxyeth yl starch blend hydrogel membranes: Synthesis and characterization for bi omedical applications. Arab. J. Chem. 7 (3), 372–380. [19] Bhat, S. D., Sahu, A. K., George, C., Pitchumani, S., Sridhar, P., Chandrakumar, N., Singh, K. K., Krishna, N., and Shukla, A. K. 2009. Mordenite-incorporated PVA–PSSA membranes as electrolytes for DMFCs. J. Membrane Sci. 340, 73-83. [20] Fahmy, A., Mi x, R., Schonhals, A., and Friedrich, J. F. 2011. Structure of plasma -deposited poly(acrylic acid) films. Plasma Process Polym. 8, 147-159.

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ISSN 2321-807X [21] Donghtd, F., Beibei, W., Zheng, X., and Qisheng, G. 2006. Determination of hyaluronan by spectroscopic methods. J. Wuhan University of Technology - Mater. Sci. Ed. 21, (3), 32-34. [22] Yokoyama, F., Masada, I., Shimamura, K., Ikawa, T., and Monobe, K. 1986. Morphology and structure of highly elastic poly(vin yl alcohol) hydrogel prepared by repeated freezing-and-melting. Colloid. Polym. Sci. 264, 595–601. [23] Hassan, C. M., and Peppas, N. A. 2000. Structure and Morphology of Freeze/Thawed PVA H ydrogels. Macromolecules 33, (7), 2472–2479. [24] Rosiak, M. T., Darmawan, D., and Zainuddin, S. 2001. Irradiation of polyvinyl alcohol and polyvinyl p yrrolidone mixed hydrogel for wound dressing. Radiat. Phys. Chem. 62, 107–113. [25] Jeon, O., Song, S. J., Lee, K. J., Park, M. H., Lee, S. H., Hahn, S. K., Kim, S., and Kim, B. S. 2007. Mechanical properties and degradation behaviors of hyaluronic acid hydrogels cross -linked at various cross -linking densities. Carbohyd. Polym. 70, 251–257. [26] Kamoun, E. A., and Menzel, H. 2012. HES-HEMA nanocomposite polymer hydrogels: Swelling behavior and characterization. J. Polym. Res. 19, 9851-9864. [27] Chanthad, C., and Wootthikanokkhan, J. 2006. Effect of crosslinking time and amount of sulfoopthalic acid on properties of the sulfonated poly(vinyl alcoho) membrane. J. Appl. Polym. Sci. 101, 1931-1936. [28] Zhu, A., Cai, A., Huawei, J. Z., and Wang, J. J. 2008. PMMA-grafted-silica/PVC nanocomposites: mechanical performance and barrier properties. J. Appl. Polym. Sci. 108, 2189-2196. [29] Ebina, T., and Mizukami, F. 2007. Flexible transparent clay films with heat-resistant and high gas -barrier properties. Ad v. Mater. 19, 2450-2453. [30] Yang, C.-C., Li, Y. J., and Liou, T. H. 2011. Preparation of novel poly(vinyl alcohol)/SiO 2 nanocomposite membranes by a sol–gel process and their application on alkaline DMFCs. Desalination 276, 366–372.

Coresponding author Dr. Elbadawy A. Kamoun

Elbadawy Kamoun was born and educated in Kafr El-Sheikh, Egypt. He obtained his B.Sc. in 2001 and his M.Sc. in 2003 in Applied Chemistry from Faculty of Science, Al-Azhar University, Cairo, Egypt. Dr. Kamoun was obtained his PhD. (Polymer Chemistry) in 2011 from Institute for Technical Chemistry, Technical University of Braunschweig (TU-BS), Germany. Dr. Kamoun was granted three Post doctoral followships in 2012, 2013 and 2014 at Fudan University, Shanghai, China and Technical University of Braunschweig, Germany. Currently, Dr. Kamoun is working as Assistant Professor at Polymeric Materials Research Dep., ATN MRI, City of Scientific Research and Technological Applications (SRTA-City), New Borg AlArab City 21934, Alexandria, Egypt.

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