α-Titanium Phosphate

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Facile Synthesis of Poly(vinyl alcohol)/α-Titanium Phosphate Nanocomposite with Markedly Enhanced Properties Chenlu Bao,† Yuqiang Guo,† Lei Song,† Hongdian Lu,‡ Bihe Yuan,† and Yuan Hu*,†,§,|| State Key Laboratory of Fire Science and §National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, 166 Ren'ai Road, Suzhou, P. R. China ‡ Key Laboratory of Powder and Energy Materials, Department of Chemistry and Materials Engineering, Hefei University, Hefei, Anhui 230022, P. R. China

)

†

ABSTRACT: Nanocomposites based on poly(vinyl alcohol) and α-titanium phosphate were prepared by the solvent-cast method. The α-titanium phosphate nanosheets were homogeneously dispersed in the poly(vinyl alcohol) matrix, and marked property enhancements were obtained. In thermogravimetric analysis, the maximum decomposition temperature of the nanocomposites was increased by 80 °C, and the value of maximum decomposition was reduced by 60%. The storage modulus and tensile strength were improved by 64% and 84%, respectively. According to microscale combustion calorimetry, the ignition temperature (89 °C) and the temperature for peak heat release rate (192 °C) were significantly increased, whereas the peak heat release rate was sharply reduced (79%). The property enhancements are mainly due to strong hydrogen bonds between the poly(vinyl alcohol) and α-titanium phosphate nanolayers, as well as the molecule-movement-restricting and physical-barrier effects due to the α-titanium phosphate nanolayers. In addition, the effects of the water content and α-titanium phosphate amount on the electrical conductivity of poly(vinyl alcohol) nanocomposites were investigated.

1. INTRODUCTION Poly(vinyl alcohol) (PVA), a hydrophilic and biodegradable polymer with abundant hydroxyl groups, is gaining increasing attention in various applications such as proton-exchange membranes and polymer electrolyte fuel cells,1,2 separation applications,3,4 permeability membranes,5,6 drug delivery,7 and packaging materials.8,9 These applications have stimulated interest in improving the properties of PVA. Many efforts have been made to prepare high-performance PVA. According to the open literature, the nanocomposite technique based on twodimensional (2-D) layered materials is one of the most attractive because the combination of polymer and 2-D layered materials results in obvious enhancements in various properties such as mechanical properties, thermal stability, fire resistance, and gas permeability.10 12 The history of PVA/2-D layered material nanocomposites (PLMNs) can be traced back to 1990, when Suzuki synthesized alumina-pillared fluorohectorite in the presence of PVA, although he focused on fluorohectorite instead of PVA.13 In 1997, Kawakage reported an increased storage modulus in PVA/clay blends.14 Since then, PLMNs have been widely studied and reported. For example, montmorillonite (MMT) tripled the Young’s modulus and caused a 60% reduction in the water permeability;10 graphene markedly improved the tensile strength (150%) and Young’s modulus (10 times) at only a 1.8 vol % loading;15 and reduced graphite oxide increased the maximum degradation temperature by more than 100 °C and caused a 13-order increase in electrical conductivity (0.1 S cm 1).16 The interactions between PVA and the nanoadditives, mainly through hydrogen bonding, which allows efficient load transfer, are responsible for the r 2011 American Chemical Society

marked increase in mechanical properties, and the conducting graphite network is the reason for the increased electrical conductivity.16 18 The most reported 2-D layered materials include layered silicate, layered double hydroxides (LDHs), graphene, and graphite oxide. As is well-known, the dispersion and interface interaction of nanoadditives in a polymer matrix are two key factors for property enhancements.19 In the case of PVA, dispersion and interface interactions are mainly related to the hydroxyl groups in the PVA and the nanoadditives.16,17,19 If there are abundant hydroxide groups in the nanoadditives, good dispersion and strong interactions in the PVA matrix can be easily obtained, and significant property improvements are available. Based on this idea, we chose a layered compound with abundant hydroxyl groups, namely, α-titanium phosphate [αTiP, Ti(HPO4)2 3 H2O], to improve the properties of PVA. As a 2-D layered tetra-valent metal phosphate, α-TiP is known as an inorganic ion exchanger with a fairly large cation-exchange capacity [CEC, ∼775 mmol (100 g) 1]. The α-TiP layer is constructed with titanium oxygen octahedra, phosphate oxygen tetrahedra, and exchangeable protons. The hydroxyl groups in hydrophosphate are attached to the surfaces of α-TiP nanolayers (Figure 1c), which make α-TiP naturally compatible in aqueous media and PVA matrix because of hydrogen bonding. In addition to hydrogen bonds, the 2-D layered structure of α-TiP Received: April 4, 2011 Accepted: August 21, 2011 Revised: July 26, 2011 Published: August 21, 2011 11109

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Table 1. Formulation of PVA Nanocomposites and MEA-TiP sample

Figure 1. (a) Preparation route of PVA/α-TiP nanocomposite, (b) TEM image of α-TiP, and (c) sketch of α-TiP structure. The green statistical histogram in panel b shows the sizes of the α-TiP particles.

can provide advantages simlarly to layered silicate, LDH, graphene, and graphite oxide.15,18,20 23 Moreover, the exchangeable protons in α-TiP can result in improvements in the electrical properties of PVA nanocomposites. Based on the above analysis, α-TiP should have natural advantages for reinforcing PVA because of its abundant hydroxyl groups, 2-D layered structure, and large amounts of exchangeable protons. In earlier works, the investigation of α-TiP was mainly focused on intercalation behaviors and cation-exchange properties. Polymer/α-TiP composites were first reported when ethylaminemodified α-TiP was incorporated into flame-retardant polypropylene to improve its thermal stability, flame retardancy, and mechanical properties.24 In this work, PVA/α-TiP nanocomposites were prepared by the solvent blending and casting method; the thermal stability, mechanical properties, fire resistance, transparency, and electrical conductivity of the resulting materials were investigated. This work presents a facile and efficient route to prepare high-performance PVA with potential applications in academic research and industrial fields.

2. EXPERIMENTAL SECTION 2.1. Materials. Titanium tetrachloride (CP), phosphoric acid (H3PO4, 85 wt %), monoethanolamine (MEA, CP), and PVA (polymerization degree 1750 ( 50, hydrolysis 94 95%, CP) were all obtained from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). All materials were used as received. 2.2. Synthesis of α-TiP. In earlier works, the refluxing method was the main strategy used to synthesis α-TiP. The refluxing method is usually carried out at 50 °C for several days in a flask equipped with a reflux condensing tube. Recently, a simple and fast hydrothermal method to synthesis α-TiP was developed in our group.24 In a typical procedure, TiCl4 is dripped into deionized water with strong mechanical stirring until the suspension turns into a transparent colloid, and H3PO4 is added to the colloid to form a viscous mud. The TiCl4/H2O/H3PO4 volume ratio is 1:5:4. The mud is sealed in a Teflon-lined stainless steel autoclave and reacted at 180 °C for 12 h under autogenous pressure. After centrifugation at a speed of 5000 rpm and washing with deionized water until a pH of 5 is reached, humid α-TiP is obtained. To calculate an accurate content of α-TiP in the humid samples, 10.00 g of the humid product was dried in a vacuum oven at 60 °C for 48 h, and the obtained white product was 4.55 g, giving an α-TiP weight ratio of 45.5 wt %.

component

PVA0

PVA

PVA1

PVA/α-TiP (1 wt %)

PVA2

PVA/α-TiP (3 wt %)

PVA3

PVA/α-TiP (5 wt %)

PVA4

PVA/α-TiP (10 wt %)

MEA-TiP

MEA-modified α-TiP

2.3. Preparation of PVA/α-TiP Nanocomposites. PVA/αTiP nanocomposites were prepared by the ultrasonic solvent blending and casting technique. Figure 1a shows the synthesis strategy, and Table 1 lists the formulations of the PVA/α-TiP nanocomposites. The hydroxyl groups in α-TiP make it compatible with the PVA matrix; however, intercalation or exfoliation of α-TiP nanolayers is still not possible because the original d spacing of α-TiP (0.76 nm) is too small and the interlayer space is too narrow to be intercalated by PVA molecule chains. Therefore, α-TiP was modified and swelled with MEA before being incorporated into the PVA, so as to increase the d spacing of α-TiP. As a type of alkylamine, MEA has an amine group and a hydroxyl group in each molecule. The amine groups can react with the P OH groups in α-TiP, forming protonized ammonium, swelling the α-TiP platelets, and thereby increasing the d spacing.25,26 The hydroxyl groups preserve the physical and chemical properties of α-TiP nanolayers. In our strategy, the required portion of humid α-TiP was added to MEA aqueous solution with mechanical stirring to obtain homogeneous blending (α-TiP/MEA molar ratio of 1:3). After about 0.5 h of ultrasonication (KS-900, Ningbo Kesheng Instrument, Ningbo, China), the mixture became a translucent colloid. The colloid was dripped into PVA aqueous solution (0.05 g mL 1) with mechanical stirring; the mixture was stirred at 90 °C for 10 h and treated ultrasonically for 0.5 h. The obtained blend was cooled to room temperature and casted in molds. The molds were placed in an oven at 40 °C for 2 days to obtain humid membranes, which were then peeled off and further annealed in a vacuum oven at 80 °C for 1 day to remove the remnant solvent. The thickness of the membranes obtained by the employed technique is dependent on the the weight of raw PVA and the dimensions of the mold, and it was 400 ((25) μm in this work. The obtained membranes were cut into pieces for tests. To investigate the microstructure and morphology of the nanocomposites, a sample of MEA/α-TiP colloid was dried at 80 °C in a vacuum oven for 2 days and pulverized to powder (denoted MEA-TiP). 2.4. Characterization. X-ray diffraction (XRD) was performed using a Japan Rigaku D/Max-Ra rotating-anode X-ray diffractometer equipped with a Cu Kα tube and a Ni filter (λ = 0.1542 nm). The scanning rate was 4° min 1. Titanium phosphates (TiPs, including α-TiP and MEA-TiP) were analyzed as powders, whereas PVA nanocomposites were analyzed as membranes. The scanning range was 3 65° for α-TiP and 1 10° for PVA nanocomposites and MEA-TiP. The surface area of α-TiP was measured with a TriStar II 3020 V1.03 instrument (Micromeritics Instrument Corporation) using N2 as the analysis adsorptive. The analysis bath temperature was 195.85 °C. Transmission electron microscopy (TEM, JEM-2100F, Japan Electron Optics Laboratory Co., Ltd.) was employed to investigate 11110

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Industrial & Engineering Chemistry Research the micromorphology and structure of TiP and the PVA nanocomposites. The accelerating voltage was 200 kV. Before observation, α-TiP was dispersed in alcohol with 20 min of ultrasonication, dripped onto a copper grid, and then dried with an infrared lamp; PVA nanocomposites were cut into ultrathin sections using a CM1900 microtome (Leica, Wetzlar, Germany). The ultrathin sections were transferred from liquid nitrogen to carbon-coated copper grids and then observed by TEM. Fourier transform infrared (FTIR) spectra were obtained with a Nicolet 6700 spectrometer (Nicolet Instrument Corporation, Madison, WI). α-TiP, MEA-TiP, and the PVA/α-TiP nanocomposites were mixed with KBr powders and pressed into tablets for characterization. Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., New Castle, DE) under a nitrogen flow of 60 mL min 1. The temperature was increased from 30 to 700 °C at a linear heating rate of 10 °C min 1. The reproducibility was (0.1% in mass and (1 °C in temperature. Two parallel runs were performed with each sample. TGA and differential thermogravimetric (DTG) curves were plotted. The fire resistance of PVA and the nanocomposites was evaluated with an MCC-2 microscale combustion calorimeter (Govmark Organization, Inc., Farmingdale, NY). The test sample was put in a ceramic crucible in air and heated at 80 600 °C at a linear heating rate of 60 °C min 1. The heat release data were calculated with the operating software of the MCC-2 instrument. Differential scanning calorimetry (DSC) was performed using a Q2000 DSC instrument (TA Instruments Inc.). Samples (2 4 mg) were heated from 0 to 190 °C at a linear heating rate of 10 °C min 1; the temperature was kept at 190 °C for 10 min and then decreased from 190 to 0 °C at a linear rate of 10 °C min 1. This heating cooling cycle was repeated, and the data obtained from the second heating section were plotted. The tensile strength and elongation at break were measured according to the Chinese standard method (GB 13022-91) with a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co. Ltd., Changchun City, China) at a crosshead speed of 50 mm min 1. Nanocomposite membranes were cut into special pieces, and 10 parallel runs were performed for each sample to obtain averages. Dynamic mechanical analysis (DMA) was performed using a DMA Q800 apparatus (TA Instruments Inc.) at a fixed frequency of 10 Hz in the temperature range from 30 to 100 °C at a linear heating rate of 5 °C min 1. The dynamic storage modulus and tan δ curves were plotted. The transparency of the PVA and nanocomposite membranes was evaluated with a Shimadzu Solidspec-3700 UV vis NIR spectrophotometer in the scanning wavelength range of 300 900 nm. The electrical conductivity of the PVA and nanocomposite membranes at 23 °C was measured with a ZC36 high-resistance meter (Cany Precision Instruments Co., Ltd., Shanghai, China). The effects of water content and added amount of α-TiP were investigated. First, samples were equilibrated in an environmant of 23 °C with 50% dampness (PYX-250 Q-B, Guangdong Shaoguan Keli Experiment Instrument Co., Ltd., Shaoguan City, China) until a constant weight was reached. The humid samples were then heated at 80 °C in an oven to remove the absorbed water. The weight, thickness and volume electrical conductivity of the samples were measured every 1 5 h in a room with constant

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Figure 2. TEM image of PVA3 ultrathin section.

Figure 3. XRD patterns of (a) α-TiP and (b) MEA-TiP and PVA/αTiP nanocomposites.

temperature (23 °C) and humidity (50%), until the samples reached constant weight again. Every time electrical resistance was measured, five parallel runs were done for each sample to get average values.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure. The morphology of α-TiP was observed by TEM, as shown in Figure 1b. The average particle size was about 0.7 μm (Figure 1c). The BET surface area of α-TiP was found to be 9.0 m2 g 1, and the Langmuir surface area was 12.4 m2 g 1. Dispersion of nanofillers in a polymer matrix is very important to obtain property enhancements. Because of their hydroxyl groups, TiP nanosheets can be well dispersed in PVA matrix, as shown in Figure 2, the TEM image of ultrathin sections obtained from sample PVA3. The dark lines in the visual field are the side views of TiP nanosheets. As can be seen, the TiP nanosheets were homogenously dispersed, and most of them were exfoliated. The interlayer d spacing of α-TiP is increased when intercalated by agents such as monoethanolamine.25 27 XRD was performed to investigate the crystal structure and d spacing of TiP (Figure 3). The [002] reflection of α-TiP was located at 11111

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Figure 4. IR spectra of titanium phosphates, MEA, and PVA nanocomposites.

11.8°, giving a d spacing of 0.76 nm. The particle size of α-TiP was about 0.7 μm, so the aspect ratio of α-TiP was about 900. When α-TiP was modified with MEA, the [002] reflection shifted to a lower angle, indicating expansion of the lattice from 0.76 to 1.35 nm. In the traces of PVA1, PVA2, and PVA3, the absence of visible peaks implies the loss of long-range order in TiP. PVA4 had a weak and broad diffraction at 3 8° with a peak at about 5.2°, giving a d spacing of 1.70 nm, 0.35 nm larger than that of MEA-TiP, which is evidence for intercalation. Considering the XRD and TEM results together, it is clear that the TiP nanosheets were well dispersed in the PVA matrix, and most of them achieved exfoliation. Figure 4 shows the IR spectra of TiP and PVA nanocomposites. In the spectrum of α-TiP, there are two sharp peaks at 3550 and 3480 cm 1, which are the symmetrical P OH stretching vibration. The broad absorption at 2800 3300 cm 1 and the peak at 1620 cm 1 are attributed to lattice water. The peaks at 900 1300 cm 1 and below 650 cm 1 are assigned to the symmetrical stretching vibration of PO3. The peak at 970 cm 1 is assignable to the stretching modes of P O, and that at about 750 cm 1 corresponds to nonbridging oxygen Ti O bond vibrations.28,29 In the spectrum of MEA-TiP, the peaks at 3550 and 3480 cm 1 corresponding to P OH are no longer visible, replaced by a broad band at 2200 3700 cm 1 that might be due to the overlapped absorptions of ammonium, CH2, OH, and hydrogen bonds, indicating that P OH became P O + NH3 C2H5OH.25,26,30,31 In the spectrum of PVA, there is a broad and strong absorption at 3000 3700 cm 1, peaking at 3377 cm 1, which is due to the symmetrical stretching vibration of hydroxyl groups in PVA. The peaks at 2800 3000 cm 1 are due to CH2, and that at 1645 cm 1 can be assumed to be the hydroxyl bending vibration. The bands at 1000 1200 cm 1 are attributed to the stretching vibration of C O in the C O H groups. In the spectrum of PVA4, the stretching vibration of C O at 920 cm 1 is increased, and the frequency of the symmetrical stretching vibration of hydroxyl moves to 3373 cm 1, which is lower than that in PVA0, indicating slightly reduced hydrogen bonding among the PVA chains.16,32,33 There are two kinds of hydrogen bonds in PVA/α-TiP nanocomposites: (a) hydrogen bonds among PVA molecules and (b) hydrogen bonds between TiP nanosheets and the PVA matrix (see discussion of DSC results

Figure 5. (a) TGA and (b) DTG curves of TiP and PVA nanocomposites as a function of temperature. (c) Detail of the decomposition at 200 300 °C.

and Figure 7b, below). When TiP is dispersed in the PVA matrix, it forms strong hydrogen bonds of type b because of the hydroxyl groups in both PVA and α-TiP; however, it reduces the hydrogen bonds of type a because the α-TiP nanosheets cut off the hydrogen bonding among PVA chains. However, although there are many TiP nanosheets in PVA4, the change in the hydroxyl stretching vibration (from 3377 to 3373 cm 1) is fairly small, implying that the TiP nanosheets must have formed strong hydrogen bonds of type b with the PVA matrix. 3.2. Thermal Behavior and Fire Resistance. Incorporation of 2-D layered materials usually improves the thermal stability, glass transition temperature (Tg). and fire resistance of polymer nanocomposites, because 2-D layered materials reduce the heat conduction and limit the motions of polymer chains.34 37 The initial decomposition temperatures of PVA/clay nanocomposites were increased by 7 15 °C;20 the Tg value of a PVA/LDH nanocomposite was increased by 16 °C because the hydrogen bonds between LDH and PVA molecules tend to hold the molecules in a regular structure.23 Compared with clay and LDH, α-TiP has many more hydroxyl groups that directly influence the interface interactions, so it might have an advantage in improving the thermal stability. Figure 5 shows the TGA (Figure 5a) and DTG (Figure 5b) curves of TiP and PVA nanocomposites. The TGA data are also 11112

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Table 2. TGA Parameters for Evaluating the Thermal Stability of PVA and PVA/α-TiP Nanocomposites peaka

residueb

Table 3. MCC Combustion Data of PVA and PVA/α-TiP Nanocomposites sample

Tignition (°C)

Tpeak (°C)

PHRR (w g 1)

THR (k J g 1)

Tonset (°C)

Tmax (°C)

(% °C )

[(wt %) ]

PVA0

261

261

400

14.4

PVA0 PVA1

238 260

260 340

1.4 1.1

9.6 5.3

PVA2 PVA3

344 348

344 453

126 102

14.3 12.4

PVA2

263

336

0.8

6.0

PVA4

350

438

85

10.3

PVA3

253

321

0.7

9.2

PVA4

268

315

0.6

sample

1

MEA-TiP

1

11.1 54c

Peak value in the DTG curve. b Residue at 700 °C excluding the residue from MEA-TiP. c Residue amount of MEA-TiP at 700 °C. a

Figure 6. Heat release curves obtained from MCC.

reported in Table 2. Calculating from the residual amounts of αTiP and MEA-TiP, the molecular formula of MEA-TiP can be assumed to be Ti(HPO4)2 3 2.2C2H5NH2 3 H2O, and the final residue of α-TiP and MEA-TiP at 700 °C should be titanium pyrophosphate.26,27 The decomposition of PVA occurs in three steps. In the first stage, PVA loses about 7 wt % before 240 °C through primary decomposition and the release of absorbed water because PVA is a hydrophilic polymer. The mass loss at 220 320 °C (about 50 wt %) can be assumed to be the degradation of PVA chains and the heating rearrangement of the polyalkene structure to a polyaromatic form.38 The third stage occurs from 360 to 500 °C and is due to dehydration in the PVA chains. The final residual amount of PVA was found to be 9.6 wt %. When TiP was introduced, the thermal stability was found to be markedly enhanced according to three criteria. First, the degradation onset temperature (Tonset), taken as the temperature at 10 wt % mass loss, increased by 15 30 °C. Second, the temperatures for the maximum decomposition in the DTG curves (Tmax) increased by 55 80 °C. Third, the peak values in the DTG curves decreased by 21 60%. At 700 °C, the PVA nanocomposites had smaller residual amounts than neat PVA. qAs seen from the TGA and DTG curves, nanocomposites PVA3 and PVA4 exhibited similar degradation behaviors, indicating that 5 wt % α-TiP is enough to reinforce the thermal stability. The significantly enhanced thermal stability can be assumed to be due to the strong interactions between α-TiP and PVA and

the physical barrier effect of the TiP nanolayers.34 37 The strong hydrogen bonding suppresses the chain-transfer reactions, slows the degradation process, and limits the motions of polymer chains;34 37 the layered structure of TiP can efficiently slow the diffusion of heat and decomposition products. Compared with clay, LDH, graphene, and graphite oxide, α-TiP is much more efficient in increasing the thermal stability of PVA, which can be attributed to the strong hydrogen bonds between α-TiP and the PVA matrix caused by the abundant hydroxyl groups in both α-TiP and PVA. Such strong hydrogen bonding affects not only the thermal stability, but also the char yield, because it interrupts the dehydration step in the PVA degradation process (300 450 °C). As a result, samples PVA1 PVA3 had smaller residual amounts. In addition to the hydrogen bonding, the physical barrier effect also affects char formation because it slows the diffusion of pyrolysis products, hence providing more time to form char. The hydrogen bonding reduces the char yield, whereas the physical barrier effect increases the char yield, so the residual amounts of PVA1 and PVA2 are much smaller than that of neat PVA, PVA3 has a similar residual amount, and PVA4 has a larger residual amount. Microscale combustion calorimetry (MCC) is a new technique for investigating the fire resistance properties of materials. MCC can measure the heat release process when very small samples (∼5 mg) are heated in a combustor. The heat release rate (HRR) is a very important parameter because it expresses the intensity of a fire and can be used to predict the material combustion behavior in a real fire. The MCC results for the PVA nanocomposites are shown in Figure 6 and Table 3. Compared with neat PVA, the PVA nanocomposites achieved significant improvements in flame resistance. First, the peak heat release rates (PHRRs; i.e., the peak values in the HRR curves) were sharply reduced by 69 79%. Second, the temperatures for PHRR were all increased. Third, the total heat release (THR) values were reduced. PVA4 obtains an 89 °C increase in ignition temperature, a 79% reduction in PHRR, a 25% decrease in THR, and a 192 °C increase in the temperature for PHRR. Thus, α-TiP is effective in improving the fire resistance of PVA, which can also be mainly attributed to the strong interactions and physical barrier effect, as discussed with respect to TGA. DSC was carried out to investigate the glass transition process of the PVA nanocomposites. When TiP was incorporated, Tg was found to be decreased. This finding can be attributed to the 2-D layered structure of TiP, which cuts off the hydrogen bonds of type a (see the discussion of the FTIR results and Figure 7b) between PVA molecules. The FTIR results show reduced hydrogen bonds of type a, although TiP forms hydrogen bonds with PVA molecules (hydrogen bonds of type b; see the discussion of the FTIR results and Figure 7b), because the hydroxyl group density in PVA is much larger than that in TiP. The layered structure of TiP restricts the motion of the PVA chains, and the strong hydrogen bonds of type b limit the 11113

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Figure 8. DMA curves of PVA nanocomposites.

Figure 7. (a) DSC curves of PVA nanocomposites and (b) illustration of the “hydrogen bond a reduction” effect.

Table 4. Mechanical Properties of PVA Nanocomposites samplea

a

storage modulus

elongation

tensile strength

Tg

at 30 °C (MPa)

(%)

(MPa)

(°C)

PVA0

3270

252 ((7)

81 ((3)

52

PVA1

4076

249 ((7)

101 ((2)

59

PVA2 PVA3

5348 4817

200 ((10) 75 ((6)

149 ((2) 105 ((2)

58 58

PVA4 was too crumbly to be measured.

motions of the PVA chains, which might increase Tg. However, the effect of the reduction in hydrogen bonds of type a is so significant that it exceeds those effects, and as a combination result, Tg was reduced. 3.3. Mechanical Properties. Because of the strong hydrogen bonds, actually hydrogen bonds of type b, efficient load transfer in the interface occurs, which directly increases the mechanical properties.17,18 The incorporation of α-TiP increases the tensile strength of PVA nanocomposites with low addition (e5 wt %). The largest increase in tensile strength was 84% in PVA2 (Table 4). DMA was carried out to evaluate the mechanical properties and phase-change behavior. The storage modulus and loss tangent curves are shown in Figure 8. The storage modulus is a measure of the stiffness, and it increased in all of the PVA/α-TiP nanocomposites. Sample PVA2 had the largest increase (64%) compared with PVA0 at 30 °C. The homogeneous dispersion of the TiP nanolayers and the strong interactions between nanolayers and PVA matrix explain the observed increase. 3.4. Electrical Conductivity. Usually, conductive additives such as metal particles, carbon nanotubes, graphite, and graphene are used to improve the electrical conductivity of polymers through their natural conductivity. In this work, we assumed

Figure 9. Plot of the electrical conductivity of PVA/α-TiP nanocomposites as a function of α-TiP content.

that α-TiP could increase the electrical conductivity of PVA nanocomposite membranes because of the exchangeable cations in TiP. If the cations formed a sufficient and continuous charge pathway, this would increase the conductivity of the PVA nanocomposites.39 42 Moreover, as a hydrophilic polymer, PVA can absorb water, which can also influence the electrical conductivity. To clarify the mechanism for electrical conductivity in PVA/α-TiP nanocomposites, the electrical conductivities of PVA nanocomposite membranes with different water contents and α-TiP amounts were measured. Figure 9 shows the volume electrical conductivities for several nanocomposite samples as a function of water content. When only absorbed water was present (PVA0), the electrical conductivity increased with increasing water content. When TiP was incorporated, the electrical conductivity increased further by 1 3 orders, compared with that of neat PVA. Thus, the increased electrical conductivity must be the combined effect of TiP and water.43 However, comparing PVA4 with neat PVA, the further increased electrical conductivity is about 3 orders of magnitude with low water content (about 2%) and about 1 order with high water content (nearly 20%). This indicates that α-TiP can increase the electrical conductivity without the help of water. 11114

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resistance, electrical conductivity, visible light transparence, and antiultraviolet function.

Figure 10. Transmittance properties of PVA nanocomposites in the wavelength range of 300 900 nm.

This is probably because α-TiP has many exchangeable cations that act as “proton vehicles”.42 3.5. Transparence. PVA is widely used in paper coating and packing materials, which require high transparence in practical applications. Thus, a transparence measurement based on UV IR transmittance analysis was performed. The transmittance of PVA is about 76% at the wavelength of 300 nm, and it remains at about 88% at 400 900 nm (Figure 10). When TiP was incorporated, the transmittance at 300 nm was sharply reduced to 0%, and it was increased to 70 85% at about 400 nm in the case of PVA1 PVA3. Thus, when the amount of TiP added was small (