Polystyrene-Poly(Methyl Methacrylate) Silver Nanocomposites: Signiﬁcant Modiﬁcation of the Thermal and Electrical Properties by Microwave Irradiation

materials

Article Polystyrene-Poly(methyl methacrylate) Silver Nanocomposites: Signiﬁcant Modiﬁcation of the Thermal and Electrical Properties by Microwave Irradiation

Edreese H. Alsharaeh

College of Science and General Studies, Alfaisal University, P.O. Box 50927, Riyadh 11533, Saudi Arabia; [email protected]; Tel.: +966-1-215-7739

Academic Editor: Dinesh Agrawal Received: 4 March 2016; Accepted: 3 June 2016; Published: 13 June 2016

Abstract: This work compares the preparation of nanocomposites of polystyrene (PS), poly(methyl methacrylate) (PMMA), and PSMMA co-polymer containing silver nanoparticles (AgNPs) using in situ bulk polymerization with and without microwave irradiation (MWI). The AgNPs prepared were embedded within the polymer matrix. A modiﬁcation in the thermal stability of the PS/Ag, PMMA/Ag, and PSMMA/Ag nanocomposites using MWI and in situ was observed compared with that of neat PSMMA, PS, and PMMA. In particular, PS/Ag, and PSMMA/Ag nanocomposites used in situ showed better thermal stability than MWI, while PMMA/Ag nanocomposites showed improved thermal stability. The electrical conductivity of the PS/Ag, PMMA/Ag, and PSMMA/Ag composites prepared by MWI revealed a percolation behavior when 20% AgNPs were used as a ﬁller, and the conductivity of the nanocomposites increased to 103 S/cm, 33 S/cm, and 40 mS/cm, respectively. This enhancement might be due to the good dispersion of the AgNPs within the polymer matrix, which increased the interfacial interaction between the polymer and AgNPs. The polymer/Ag nanocomposites developed with tunable thermal and electrical properties could be used as conductive materials for electronic device applications.

Keywords: nanocomposites; silver nanoparticles; microwave irradiation; polymer; X-ray diffraction

1. Introduction Recently, nano- and thin-film technologies based on novel systems associating metal particles with a polymer matrix have achieved unique physical and mechanical properties that were not possible with the addition of micron-sized particles. These metal nanoparticles (NPs) embedded in host polymer matrices have become the focus of increasing attention because of their applications in many fields including optical fibers, sub-wavelength waveguides, nonlinear optical switches [1], superlenses [2], magnetooptic data storages, directional connectors, electronics as nanowires [3] for biomedical materials, NP barcode labels [4,5], sensitive materials for DNA screening [6], biosensors [7], and bio-films with anti-microbial effects of Ag [8] materials. Ag, Au, and Cu NPs are reported to exhibit a strong biocidal effect on more than 16 species of bacteria including Escherichia coli [9–11]. In addition, these polymers are attractive materials for application in biosensors because of the considerable flexibility in their chemical structures and their redox characteristics [12]. The extent of modification of the property depends on the base polymer; size, distribution, and dispersion of the NPs; and adhesion at the filler–matrix interface [13]. When the NPs are embedded or encapsulated in a polymer, the polymer acts as a surface capping agent. In addition, casting of film becomes easier, and the particle size is controlled well within the desired regime. However, the key problems in this area involve the synthesis and functionalization of the NPs and their dispersion in a polymer matrix. Ag and Au are favorite NP-coating materials because of their well-known property of exhibiting optical absorption Materials 2016, 9, 458; doi:10.3390/ma9060458 www.mdpi.com/journal/materials Materials 2016, 9, 458 2 of 17

(plasmons) in the visible region. Ag has been widely studied because it is more reactive than Au. Polymers are usually flexible, lightweight, and able to provide required immobilization of the NPs, avoiding their coalescence or segregation, thus protecting the novel size-dependent properties of the nanomaterials. However, depending on the polymer as well as the concentration of the NPs, the properties of nanocomposites may change. Among various polymers, poly(methyl methacrylate) (PMMA) is a highly transparent plastic with good mechanical strength and is used variously for optical and medical applications [14,15]. In addition, polystyrene (PS) exhibits many admirable properties such as biocompatibility, nontoxicity, high surface area, strong adsorption ability, and chemical inertness [16–18]. Additionally, the co-polymer of methyl methacrylate and styrene (PSMMA) is an important polymeric material that has numerous applications in medicine (e.g., as bone cement); dentistry (e.g., dentures); and the paper, paint, and automotive industries [19,20]. There have been many reports on AgNPs in polymers using different techniques [21,22] including solution mixing, melt blending, in situ polymerization, and in situ polymerization using microwave irradiation (MWI) [23,24]. However, it is extremely difficult to homogenously disperse NPs into the polymer matrix because of the easy agglomeration of the NPs and the high viscosity of the polymer. The MWI method offers a fast and easy way to synthesize polymer/AgNPs materials. In MWI, dielectric heating energy is transferred directly to the reactants. Energy is supplied to the molecules faster than they are able to relax, which creates high instantaneous temperatures and increases the yield and quality of the products [25–28]. Percolation concepts are used to describe an abrupt transition from one behavior to another caused by the formation of long-range networks and have been used to describe the concentration-dependent insulator-to-conductor transition in composites of conductive fillers in insulating matrices, particularly polymer nanocomposites. These composites are technologically useful because they combine the easy processability of the polymer matrix with the desirable electrical conductivity of the filler network. Such nanocomposites could find applications in static-discharge housing and packaging for electronics; electromagnetic interference shielding; and lightweight, flexible conductors for electrodes, circuits, displays, and sensors [29]. There have been numerous reports on the usage of AgNPs as conducting-filler-based polymer composites, where improved thermal, mechanical, and electrical properties of the composites were achieved [30–32]. White et al. [32] reported the electrical percolation behavior in Ag nanowire–PS composites. These researchers showed that the conductivity of the composites can be easily tuned by modifying the aspect ratio of nanowires, and Ag nanowires are superior to carbon nanotubes because they are straight, sufficiently large that minimal interparticle variation in electrical conductivity is achieved, and disperse easily into experimental solvents and polymers. In another report, Lee et al. [31] prepared PMMA/polyaniline (PANI)/Ag composites using an electroless coating of Ag on a PMMA sphere pre-coated with PANI using in situ chemical polymerization. These researchers reported that the resistivity of the PMMA/Ag and PMMA/PANI/Ag composites varied between 1014 and 10´1 Ω¨ cm and 108 and 10´4 Ω¨ cm, respectively. Despite the numerous reports on composites of PS with PMMA or AgNPs, very few studies have been performed on the combination of these three components. However, the development of polymer–AgNPs nanocomposites that can fully utilize conducting filler properties and achieve significantly enhanced electrical and thermal properties with low AgNPs loading remains necessary. In this paper, PSMMA/AgNPs nanocomposites were synthesized with a low concentration (20%) loading of AgNPs via in situ bulk polymerization using MWI. To the best of our knowledge, this report is the first on the enhancement of the thermal and electrical properties of PSMMA/Ag nanocomposites with percolation behavior using only 20% AgNPs filler, which exhibits a higher conductivity. Combining the advantages of PS, PMMA, and AgNPs, the nanocomposites exhibited many excellent properties, such as good solubility and dispersibility in water, satisfactory biocompatibility, and high electrical conductivity. The synthesized nanocomposites were characterized using Fourier-transform infrared spectroscopy (FTIR), ultraviolet/visible (UV/Vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), scanning electron Materials 2016, 9, 458 3 of 17 Materials 2016, 9, 458 3 of 16 microscopyscanning electron (SEM), microscopy high-resolution (SEM), transmission high-resolut electronion transmission microscopy electron (HRTEM), microscopy differential (HRTEM), scanning calorimetrydifferential (DSC),scanning thermal calorimetry gravimetric (DSC), analysis therma (TGA),l gravimetric and electrical analysis conductivity (TGA), measurementsand electrical toconductivity provide an measurements understanding to ofprovide the structure–property an understanding of relationships the structure–property as well as therelationships percolation as thresholdwell as the behavior. percolation threshold behavior.

2.2. Results Results and and Discussion Discussion TheThe sizesize andand structurestructure ofof thethe AgNPs in the polymer matrix were investigated using XRD studies; therefore,therefore, XRDXRD patternspatterns ofof thethe polymer/Ag polymer/Ag nanocompositesnanocomposites were alsoalso obtained.obtained. FigureFigure1 1 presents presents XRDXRD patternspatterns ofof thethe AgNPsAgNPs andand polymer/Agpolymer/Ag nanocomposites.nanocomposites. As observed in Figure1 1a,a, thethe pure pure AgNPsAgNPs exhibitexhibit aa crystallinecrystalline naturenature withwith FCCFCC structurestructure withwith peaks corresponding toto (111), (200), (220), andand (311)(311) planes. planes. These These results results are ar consistente consistent with with the previousthe previous literature literature values values for AgNPs for AgNPs and JCPDS and No.JCPDS 00-003-0921. No. 00-003-0921. Figure1 Figureb–d presents 1b–d presents XRD patterns XRD pa oftterns the polymer/AgNPs of the polymer/AgNPs composites, composites, which exhibit which aexhibit two-phase a two-phase (crystalline (crystalline and amorphous) and amorphous) structure. structure. The polymer/AgNPs The polymer/AgNPs exhibit the exhibit broad the reﬂection broad andreflection typical and amorphous typical amorphous nature for the nature polymer, for the as polymer, expected, as and expected, the typical and pattern the typical of the pattern face FCC of Agthe crystallineface FCC Ag structure crystalline indicates structure the formationindicates the ofmetallic formation Ag. of metallic Ag.

FigureFigure 1. 1. XRDXRD patterns patterns of (a) ofAgNPs (a) AgNPsand (b) PS/AgNPs; and (b) PS/AgNPs; (c) PMMA/AgNPs; (c) PMMA/AgNPs; and (d) PSMMA/AgNPs and (d) PSMMA/AgNPsnanocomposites. nanocomposites.

Furthermore, the width of the (111) peak was employed to calculate the average crystallite size Furthermore, the width of the (111) peak was employed to calculate the average crystallite size using the Scherrer equation [33]: using the Scherrer equation [33]: DD“ = 0.90.9λλ{p/(B ˆ× cosθq) (1)(1) wherewhere λλis is thethe wavelengthwavelength ofof thethe incidentincident CuCu KKαα X-rayX-ray (1.514(1.514 Å),Å), BB isis thethe fullfull widthwidth atat halfhalf maximamaxima (FWHM)(FWHM) ofof thethe diffractiondiffraction peak,peak, andand θθis is thethe diffraction diffraction angle.angle. TheThe calculatedcalculated averageaverage sizessizes forfor thethe AgNPsAgNPs andand polymer/AgNPspolymer/AgNPs were observed to be ~46~46 nm andand 18,18, 16,16, andand 1313 nmnm forfor PS/AgNPs,PS/AgNPs, PMMA/AgNPs,PMMA/AgNPs, and and PSMMA/AgNPs, PSMMA/AgNPs, respectively. respectively. The The particle particle size size was was observed observed to be to besmaller smaller for forthe thePSMMA/AgNPs PSMMA/AgNPs nanocomposites nanocomposites than than for for the the AgNPs. AgNPs. However, However, for for the the different polymerpolymer composites,composites, thethe particleparticle sizesize diddid notnot varyvary greatly.greatly. TheThe decreasedecrease inin intensityintensity andand broadeningbroadening ofof peakspeaks inin thethe AgNPs/polymersAgNPs/polymers (Figure 11b–d)b–d) reflectsreflectsthe the decrease decrease in in particle particle size size of of the the polymer/AgNPs polymer/AgNPs comparedcompared withwith thatthat ofof thethe AgNPsAgNPs (Figure(Figure1 a).1a). TheThe particleparticle sizesize calculatedcalculated bybyXRD XRD was was further further confirmedconfirmed byby TEMTEM analysis.analysis. ToTo confirmconfirm the chemical chemical structure structure of of all all the the poly polymer/Agmer/Ag composites, composites, FTIR FTIR spectral spectral analysis analysis was wasperformed. performed. Figure Figure2 presents2 presents FTIR spectra FTIR of spectra the PS/Ag, of the PMMA/Ag, PS/Ag, PMMA/Ag,and PSMMA/Ag and nanocomposites. PSMMA/Ag nanocomposites.For the PS/Ag nanocomposites, For the PS/Ag nanocomposites,the spectrum shows the spectrum the presence shows of the presencecharacteristic of the bands characteristic of PS at bands3060, 2920, of PS atand 3060, 2840 2920, cm− and1, which 2840 cmcorrespond´1, which to correspond the aromatic to the ring aromatic and aliphatic ring and C–H aliphatic and –CH C–H2 −1 ´1 andstretching, –CH2 stretching,respectively. respectively. The aromatic The overtones aromatic are overtones observed are at observed1680–2000 at cm 1680–2000, and aromatic cm , C=C and aromaticstretching C=C is observed stretching at 1613 is observed cm−1. For at the 1613 PMMA/AgNPs, cm´1. For the the PMMA/AgNPs, spectrum shows the characteristic spectrum shows bands 2 −1 ´1 characteristicof the aliphatic bands C–H of and the aliphatic–CH at C–H2925 andand –CH28522 atcm 2925, respectively; and 2852 cm the ,bands respectively; were reduced the bands in

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were reducedMaterials 2016 in, 9 intensity, 458 and became broader compared with the neat PMMA (Figure S1). The4 of 16 bands at 1270–1000 cm´1 originated from the C–H deformations and C–O–C and C–O stretching. The band at 1747intensity cm´1 andis assigned became broader to the C=Ocomp stretchingared with the vibrations neat PMMA of the(Fig esterure S1). group The bands of the at PMMA. 1270–1000 It was cm−1 originated from the C–H deformations and C–O–C and C–O stretching. The band at 1747 cm−1 observed that this band shifted more in the nanocomposites than the peak previously reported for neat is assigned to the C=O stretching vibrations of the ester group of the PMMA. It was observed that PMMA at 1730 cm´1 [24]. For the PSMMA/Ag nanocomposites, the FTIR spectrum shows the typical this band shifted more in the nanocomposites than the peak previously reported for neat PMMA at ´1 characteristic1730 cm− bands1 [24]. atFor 2930 the and PSMMA/Ag 2860 and 1600nanocomposites and 1460 cm, the whichFTIR spectrum correspond shows to the the aliphatic typical C–H and –CHcharacteristic2 and aromatic bands at C=C 2930 stretching, and 2860 and respectively, 1600 and 1460 in cm the−1 PSwhich molecules. correspond In to contrast, the aliphatic characteristic C–H ´1 bandsand at 1745–CH2 andand 1160–1120aromatic C=C cm stretching,, which respectively, correspond in to the C=O PS stretchingmolecules. In vibrations contrast, ofcharacteristic ester carbonyl and C–O–Cbands at stretching 1745 and 1160–1120 vibrations, cm− respectively,1, which correspond appear to forC=O the stretching PMMA vibrations molecules of [ester17]. carbonyl Notably, the FTIR resultsand C–O–C of the stretching polymer/Ag vibrations, nanocomposites respectively, (Figureappear 2for) demonstrate the PMMA molecules that some [17]. characteristic Notably, the peaks in theFTIR aromatic results of of PS the and polymer/Ag ester of PMMA nanocomposites regions are (Figure shifted 2) to demonstrate much higher that wave some numbers characteristic compared with neatpeaks polymers in the aromatic (Figure of PS S1). and This ester ﬁnding of PMMA may regions suggest are that shifted this to shift much is higher due to waveπ–π stackingnumbers and compared with neat polymers (Figure S1). This finding may suggest that this shift is due to π–π acrylate interactions, which may be attributed to the contribution toward the stabilization of the AgNPs stacking and acrylate interactions, which may be attributed to the contribution toward the metal surface. These FTIR results may suggest that the π–π bonds of PS and acrylate of PMMA were stabilization of the AgNPs metal surface. These FTIR results may suggest that the π–π bonds of PS openedand by acrylate the MWI, of PMMA which were will opened induce by more the MWI, electron whic chainh will transferinduce more sites electron and will chain thus transfer promote sites more interactionsand will between thus promote polymers more intera and Ag.ctions between polymers and Ag.

Figure 2. FTIR spectra of polymer/AgNPs nanocomposites. Figure 2. FTIR spectra of polymer/AgNPs nanocomposites. Further study on the formation of AgNPs on the surface of nanocomposites was performed Furtherusing XPS, study and on the the results formation are presented of AgNPs in Figure on the 3. surface XPS is ofa nanocompositespowerful and reliable was technique performed for using XPS, andexploring the results the interaction are presented of Ag inNPs Figure and polymers.3. XPS is aThe powerful XPS survey and spectra reliable of technique the polymer/AgNPs for exploring the interaction(not shown of here) Ag NPsindicated and polymers.that not only The Ag, XPS O, an surveyd C were spectra present. of theDetailed polymer/AgNPs scans of Ag 3d, (not C 1s, shown here)and indicated O 1s are that presented not only in Ag,Figure O, and3a–c, Crespectively. were present. In Figure Detailed 3a, the scans peak of observed Ag 3d, C in 1s, the and energy O 1s are presentedregion in ofFigure the Ag3 3da–c, transition respectively. is symmetrical, In Figure an3da, two the characteristic peak observed binding in theenergy energy peaks region for Ag of3d the for metallic Ag at 374.33 and 368.33 eV are observed, corresponding to doublets of Ag 3d3/2 and Ag Ag 3d transition is symmetrical, and two characteristic binding energy peaks for Ag 3d for metallic 3d5/2 [34], respectively. These results indicate the metallic nature of Ag, and no evidence for the Ag atexistence 374.33 and of Ag 368.33+ was eVobtained. are observed, Therefore, corresponding the XPS study toconfirmed doublets the of success Ag 3d of3/2 theand formation Ag 3d5/2 of [34], + respectively.metallic TheseAgNPs results within indicatethe PS/Ag, the PMMA/Ag, metallic nature and PSMMA/Ag of Ag, and nanocomposites. no evidence for the existence of Ag was obtained.Notably, Therefore, for the PSMMA/Ag the XPS study nanocomposites conﬁrmed the(Fig successure 3), the of theextent formation of two binding of metallic energies AgNPs withincorresponding the PS/Ag, PMMA/Ag,to doublet Ag and 3d3/2 PSMMA/Ag and Ag 3d5/2 nanocomposites.were shifted to the lower side compared with the Notably,characteristic for thepeaks PSMMA/Ag of metallic Ag. nanocomposites This lower side shift (Figure (~0.053), eV) the is extent possibly of twodue to binding the stronger energies interaction between Ag and O in C=O groups promoted using MWI by enhancing electron transfer corresponding to doublet Ag 3d3/2 and Ag 3d5/2 were shifted to the lower side compared with the characteristicfrom the electron-rich peaks of metallic acrylate Ag.s of PMMA This lower groups side to AgNPs. shift (~0.05 eV) is possibly due to the stronger interaction between Ag and O in C=O groups promoted using MWI by enhancing electron transfer from the electron-rich acrylates of PMMA groups to AgNPs. Materials 2016, 9, 458 5 of 17 Materials 2016, 9, 458 5 of 16

(a) (b)

(c)

Figure 3. 3. (a) Ag(a) 3d Ag XPS 3d spectra XPS spectra of AgNPs; of ( AgNPs;b) C 1s XPS; (b) Cand 1s (c) XPS; O 1s andspectra (c) of O polymer/AgNPs 1s spectra of polymer/AgNPsnanocomposites. nanocomposites.

Figure 3b,c presents the C 1s and O 1s spectra of the Ag/polymer system. The peaks are wide Figure3b,c presents the C 1s and O 1s spectra of the Ag/polymer system. The peaks are wide and and symmetrical. For PS/AgNPs, three different carbon functionalities are considered: (1) symmetrical. For PS/AgNPs, three different carbon functionalities are considered: (1) hydrocarbon hydrocarbon (C–H/C–C) at 284.6 eV; (2) alcohol or ether (C–OH/C–O–C) at 285.9 eV; and (3) ester (C–H/C–C) at 284.6 eV; (2) alcohol or ether (C–OH/C–O–C) at 285.9 eV; and (3) ester (O–C=O) at (O–C=O) at 288.6 eV [35,36], mainly due to the presence of the carbon atoms in C12 H25 SH 288.6 eV [35,36], mainly due to the presence of the carbon atoms in C12 H25 SH introduced onto introduced onto the AgNPs. For the PMMA/AgNPs and PSMMA/AgNPs, the profile of the C 1s line the AgNPs. For the PMMA/AgNPs and PSMMA/AgNPs, the profile of the C 1s line was altered, was altered, indicating that there was a change in the intensity of its three components, as observed indicating that there was a change in the intensity of its three components, as observed in Figure3b,c. in Figure 3b,c. In addition, the binding energy was observed to shift toward the lower side for all In addition, the binding energy was observed to shift toward the lower side for all three components three components of the carbon peaks. This lower side shift in the binding energy occurred because of the carbon peaks. This lower side shift in the binding energy occurred because Ag tends to lose Ag tends to lose unpaired valence electrons; therefore, a strong A–C interaction would result in a unpaired valence electrons; therefore, a strong A–C interaction would result in a shift of the C 1s shift of the C 1s peak toward low binding energy. This claim was further confirmed by considering peak toward low binding energy. This claim was further confirmed by considering the O 1s spectra the O 1s spectra (Figure 3c). The O 1s core-level peaks of the polymer/AgNPs are presented in Figure (Figure3c). The O 1s core-level peaks of the polymer/AgNPs are presented in Figure3c. As resolved by 3c. As resolved by deconvolution, the O 1s spectrum consisted of a BE peak at 532.6 eV related to the deconvolution, the O 1s spectrum consisted of a BE peak at 532.6 eV related to the O–C=O group [37,38]. O–C=O group [37,38]. This result implies interaction between Ag and O, especially between Ag and This result implies interaction between Ag and O, especially between Ag and O–C=O groups. A shift O–C=O groups. A shift in the binding energy to the lower side was observed for the PMMA/AgNPs in the binding energy to the lower side was observed for the PMMA/AgNPs and PSMMA/AgNPs and PSMMA/AgNPs nanocomposites, and the electron transfer from the Ag to the oxygen led to a nanocomposites, and the electron transfer from the Ag to the oxygen led to a shifting in the O 1s binding shifting in the O 1s binding energy. The reaction between Ag and O and even polymers should be energy. The reaction between Ag and O and even polymers should be responsible for the variation. responsible for the variation. The formation of AgNPs in the polymer matrix was also confirmed by UV-Vis spectra of the The formation of AgNPs in the polymer matrix was also confirmed by UV-Vis spectra of the PS/Ag, PMMA/Ag, and PSMMA/Ag nanocomposites (Figure4). The weak absorption band of the PS/Ag, PMMA/Ag, and PSMMA/Ag nanocomposites (Figure 4). The weak absorption band of the AgNPs at approximately 410–420 nm corresponds to the characteristic peak of metallic silver [39–41]. AgNPs at approximately 410–420 nm corresponds to the characteristic peak of metallic silver [39–41]. The spectra clearly demonstrate that the absorption peaks of PSMMA/AgNPs nanocomposites were The spectra clearly demonstrate that the absorption peaks of PSMMA/AgNPs nanocomposites were red-shifted to higher wavelength compared with the PMMA/AgNPs and PS/AgNPs nanocomposites, red-shifted to higher wavelength compared with the PMMA/AgNPs and PS/AgNPs nanocomposites, respectively. This finding indicates the restoration of the electronic conjugation within the polymer respectively. This finding indicates the restoration of the electronic conjugation within the polymer matrix and the formation of AgNPs [31]. A very weak peak characterized the absorption peak of the matrix and the formation of AgNPs [31]. A very weak peak characterized the absorption peak of the AgNPs dispersion into the polymer matrix, which might be attributed to a non-aggregated dispersion of NPs [42].

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AgNPs dispersion into the polymer matrix, which might be attributed to a non-aggregated dispersion of NPsMaterials [42]. 2016, 9, 458 6 of 16

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Figure 4. UV spectra of polymer/AgNPs nanocomposites. Figure 4. UV spectra of polymer/AgNPs nanocomposites.

The compositions of the polymer/AgNPs nanocomposites were further examined using Raman Figure 4. UV spectra of polymer/AgNPs nanocomposites. Thespectroscopy compositions (Figure of 5). the Raman polymer/AgNPs spectroscopy is nanocomposites a powerful tool that were provides further essential examined information using Raman for evaluating the covalent modification of composites. The Raman spectra of the neat polymers spectroscopyThe compositions (Figure5). Raman of the spectroscopypolymer/AgNPs is ananocomposites powerful tool were that providesfurther examined essential using information Raman for (Figure S2) reveal the major scattering peaks of PS and PMMA. The PS spectrum contains peaks at evaluatingspectroscopy the covalent (Figure modiﬁcation 5). Raman spectroscopy of composites. is a po Thewerful Raman tool spectra that provides of the neatessential polymers information (Figure S2) 1602 and 1585 cm−1 due to stretching of benzene rings. The PMMA spectrum (Figure S2) shows revealfor the evaluating major scattering the covalent peaks modification of PS and of PMMA.composites. The The PS Raman spectrum spectra contains of the peaksneat polymers at 1602 and characteristic peaks at 600 and 812 cm−1 due to stretching of C–C–O and C–COO as well as C–O–C, 1585(Figure cm´1 due S2) toreveal stretching the major of scattering benzene rings.peaks of The PS PMMA and PMMA. spectrum The PS (Figure spectrum S2) contains shows characteristicpeaks at respectively, at 1450 cm−1 due to in-plane bending of C–H and at 1728 cm−1 due to stretching of C=O peaks1602 at 600 and and 1585 812 cm cm−1 ´due1 due to stretching to stretching of benzene of C–C–O rings. and The C–COO PMMA as spectrum well as C–O–C, (Figure respectively,S2) shows at [43]. The most prominent peak appearing at 2951 cm−1 is due to the C–H stretching vibration. Notably, characteristic´1 peaks at 600 and 812 cm−1 due to stretching of´ 1C–C–O and C–COO as well as C–O–C, 1450 cmthe locationsdue to of in-plane the characteristics bending ofpeaks C–H of and all the at polymer/Ag 1728 cm duenanocomposites to stretching (Figure of C=O 5) were [43]. blue- The most respectively, at 1450 cm−1 due to in-plane´1 bending of C–H and at 1728 cm−1 due to stretching of C=O prominentshifted, peak which appearing might indicate at 2951 interfacial cm is intera duection to the between C–H stretching the AgNPs vibration. and the polymer Notably, matrix. the locations [43]. The most prominent peak appearing at 2951 cm−1 is due to the C–H stretching vibration. Notably, of the characteristics peaks of all the polymer/Ag nanocomposites (Figure5) were blue-shifted, which the locations of the characteristics peaks of all the polymer/Ag nanocomposites (Figure 5) were blue- might indicate interfacial interaction between the AgNPs and the polymer matrix. shifted, which might indicate interfacial interaction between the AgNPs and the polymer matrix.

Figure 5. Raman spectra of polymer/AgNPs nanocomposites.

The morphology of the polymer/Ag nanocomposites was studied using SEM and HRTEM, and Figure 5. Raman spectra of polymer/AgNPs nanocomposites. the results are presentedFigure in 5. FiguresRaman spectra6–8. Various of polymer/AgNPs reports have shown nanocomposites. that the incorporation of NPs into polymer resins can improve the mechanical and rheological properties or even introduce novel The morphology of the polymer/Ag nanocomposites was studied using SEM and HRTEM, and functionalities [44–48]. The important factors involved in improving these properties are the type, Thethe results morphology are presented of the in polymer/Ag Figures 6–8. nanocompositesVarious reports have was shown studied that using the incorporation SEM and HRTEM, of NPs and size, shape, and aspect ratio of the NPs; particle dispersion in polymer resins; and interfacial the resultsinto polymer are presented resins can in improve Figures the6– 8mechanical. Various reportsand rheological have shown properties that or the even incorporation introduce novel of NPs interaction between organic polymer resins and inorganic NPs. Figure 6a presents an SEM image of functionalities [44–48]. The important factors involved in improving these properties are the type, into polymerAgNPs of resins polyhedral can improve shape with the agglomerated mechanical andmorphology, rheological indicating properties the high or evensurface introduce energy of novel size, shape, and aspect ratio of the NPs; particle dispersion in polymer resins; and interfacial functionalitiesthese particles. [44– 48The]. SEM The importantmicrograph factors of PS/AgNPs involved (Figure in improving 6b) clearly thesereveals properties that very small are the sizes type, of size, interaction between organic polymer resins and inorganic NPs. Figure 6a presents an SEM image of shape,Ag and are aspectdispersed ratio at ofthe the surface NPs; and particle embedded dispersion within inthe polymer PS matrix. resins; The SEM and interfacialmicrograph interactionof the AgNPs of polyhedral shape with agglomerated morphology, indicating the high surface energy of betweenthese organic particles. polymer The SEM resinsmicrograph and inorganicof PS/AgNPs NPs. (Figure Figure 6b) clearly6a presents reveals an that SEM very image small sizes of AgNPs of of polyhedralAg are dispersed shape at with the agglomeratedsurface and embedded morphology, within the indicating PS matrix. the The high SEM surface micrograph energy of ofthethese

Materials 2016, 9, 458 7 of 17 particles. The SEM micrograph of PS/AgNPs (Figure6b) clearly reveals that very small sizes of Ag are dispersed at the surface and embedded within the PS matrix. The SEM micrograph of the PMMA/Ag nanocompositesMaterials 2016, 9 (Figure, 458 6c) shows that the polyhedral shaped AgNPs are dispersed within the7 of PMMA 16 matrix and are well separated from each other. For the PS–PMMA/Ag nanocomposites (Figure6d), PMMA/Ag nanocomposites (Figure 6c) shows that the polyhedral shaped AgNPs are dispersed the AgNPswithin the (white PMMA spots) matrix are and anchoredare well separated and dispersed from each other. within For the the PS–PMMA/Ag PSMMA matrix. nanocomposites According to the MARTINI(Figure 6d), force the AgNPs field, different(white spots) values are ofanch theored strength and dispersed of interaction within oftheε PSMMArepresent matrix. levels of hydrophilicity/hydrophobicityAccording to the MARTINI force [49]. field, A larger different value values of ε provides of the strength a more of attractiveinteraction interactionof ε represent among highlylevels polar of groups,hydrophilicity/hydrophobicity whereas a smaller one [49]. reflects A larger a lowervalue of degree ε provides of hydrophobic a more attractive repulsion interaction between polaramong and nonpolar highly polar phases. groups, The whereas size and a smaller shape one of reflects the NPs a lower can play degree an of important hydrophobic role repulsion in changing the propertiesbetween polar of nanocomposites. and nonpolar phases. It has The been size experimentally and shape of the determined NPs can play that an nanocomposites important role in with smaller-sizedchanging NPsthe exhibitproperties better of performancenanocomposites. than It those has containingbeen experimentally larger-sized determined NPs [50–52 that]. In the presentnanocomposites work, the NPs with in thesmaller-siz PSMMA/AgNPsed NPs exhibit nanocomposites better performance were than smaller those compared containing with larger- those of sized NPs [50–52]. In the present work, the NPs in the PSMMA/AgNPs nanocomposites were smaller the other polymer composites, thus increasing the density at the polymer–NP interface. These results compared with those of the other polymer composites, thus increasing the density at the polymer– clearlyNP suggest interface. that These the AgNPsresults wereclearly successfully suggest that embedded the AgNPs in were the polymersuccessfully matrix embedded and modified in the the functionalpolymer properties matrix and of modified the polymer. the functional properties of the polymer.

Figure 6. SEM micrographs of (a) AgNPs and (b) PS/AgNPs; (c) PMMA/AgNPs; and (d) PSMMA/AgNPs Figure 6. SEM micrographs of (a) AgNPs and (b) PS/AgNPs; (c) PMMA/AgNPs; and (d) nanocomposites. PSMMA/AgNPs nanocomposites. The morphology of the nanocomposites and formation of AgNPs were further studied using TheHRTEM, morphology and the results of the are nanocomposites displayed in Figure and formation 7. The HRTEM of AgNPs images were of all further the polymer/Ag studied using HRTEM,nanocomposites and the results (Figure are 7b–d) displayed reveal good in Figuredispersi7.on Theof AgNPs HRTEM within images the polymer of all thematrices, polymer/Ag with nanocompositesbetter homogeneity (Figure for7 b–d)PSMMA/Ag reveal (Figure good dispersion7d) compared of with AgNPs the other within nanocomposites the polymer (Figure matrices, 8b,c). with betterThe homogeneity good dispersion for PSMMA/Ag and poor aggregation (Figure7d) resulted compared from with the van the der other Waals nanocomposites attraction between (Figure the8 b,c). particles, indicating the stabilization of AgNPs, which may be due to the electron transfer interaction The good dispersion and poor aggregation resulted from the van der Waals attraction between the between polymers and AgNPs. This finding is consistent with the XPS results and our previous work particles,[17]. indicating the stabilization of AgNPs, which may be due to the electron transfer interaction

Materials 2016, 9, 458 8 of 17 between polymers and AgNPs. This ﬁnding is consistent with the XPS results and our previous Materials 2016, 9, 458 8 of 16 workMaterials [17]. 2016, 9, 458 8 of 16

Figure 7. HRTEM micrographs of (a) AgNPs and (b) PS/AgNPs; (c) PMMA/AgNPs; and (d) Figure 7. HRTEM micrographs of (a) AgNPs and (b) PS/AgNPs; (c) PMMA/AgNPs; and (d) FigurePSMMA/AgNPs 7. HRTEM nanocomposites. micrographs of (a) AgNPs and (b) PS/AgNPs; (c) PMMA/AgNPs; and (d) PSMMA/AgNPs nanocomposites. PSMMA/AgNPs nanocomposites.

Figure 8. SEM and HRTEM micrographs of (a,d) PS–AgNPs; (b,e) PMMA–AgNPs; and (c,f) PS– PMMA–AgNPs nanocomposites (in situ). Figure 8. SEM and HRTEM micrographs of (a,d) PS–AgNPs; (b,e) PMMA–AgNPs; and (c,f) PS– Figure 8. SEM and HRTEM micrographs of (a,d) PS–AgNPs; (b,e) PMMA–AgNPs; and PMMA–AgNPsTGA as used nanocomposites to investigate the (in thermalsitu). stability and interfacial interaction between the AgNPs (c,f) PS–PMMA–AgNPs nanocomposites (in situ). and polymer matrices. Figure 9a present the TGA curves for the polymer/AgNPs composites using MWITGA and as in used situ reduction,to investigate respectively; the thermal with stabilit the inclusioy andn interfacialof AgNPs, onsetinteraction degradation between temperature the AgNPs andTGAmodification polymer as used matrices. was to investigateobserved. Figure The9a the present temperature thermal the stability TGA at wh curvesich and 5% for interfacial of themass polymer/AgNPs loss interaction has occurred between composites was used the usingfor AgNPs andMWI polymermeasuring and in matrices. situ the reduction, degradation Figure respectively; 9temperature.a present with the TGA TGA the re inclusiosults curves shown for of thatAgNPs, the PS/AgNPs polymer/AgNPs onset deandgradation PS-PMMA/AgNPs composites temperature using modification was observed. The temperature at which 5% of mass loss has occurred was used for measuring the degradation temperature. TGA results show that PS/AgNPs and PS-PMMA/AgNPs

Materials 2016, 9, 458 9 of 17

MWI and in situ reduction, respectively; with the inclusion of AgNPs, onset degradation temperature modiﬁcation was observed. The temperature at which 5% of mass loss has occurred was used for measuring the degradation temperature. TGA results show that PS/AgNPs and PS-PMMA/AgNPs nanocomposites using in-situ method showed good thermal stability than that of MWI technique. Materials 2016, 9, 458 9 of 16 Only in the case of PMMA/AgNPs composites, those prepared using MWI have slightly better thermal stabilitynanocomposites than those using prepared in-situin method situ. Weshowed attributed good thermal this result stability to thethan presence that of MWI and technique. dispersion of AgNPsOnly within in the the case polymer of PMMA/AgNPs matrix. In composites, MWI, dielectric those heatingprepared energy using MWI is transferred have slightly directly better to the reactants,thermal and stability the energy than those is supplied prepared to in the situ molecules. We attributed faster this than result they to the are presence able to relax,and dispersion creating high instantaneousof AgNPs within temperatures the polymer that matrix. increase In the MWI, yield dielectric and quality heating of energy the product, is transferred which directly is consistent to the with the resultsreactants, from and our the study. energy Among is supplied all theto the polymer/AgNPs molecules faster than nanocomposites, they are able to PS/AgNPs relax, creating prepared high by instantaneous temperatures that increase the yield and quality of the product, which is consistent the in situ method showed the highest thermal stability. The values of degradation are summarized in with the results from our study. Among all the polymer/AgNPs nanocomposites, PS/AgNPs Tableprepared1. On the by basis the in of situ these method results, showed one disadvantage the highest thermal of MWI stab methodility. The is values that the of degradation composites are usually havesummarized lower thermal in Table stability. 1. On the basis of these results, one disadvantage of MWI method is that the composites usually have lower thermal stability. Table 1. Thermal behavior data of polymer/AgNP nanocomposites obtained from TGA and DSCTable measurements. 1. Thermal behavior data of polymer/AgNP nanocomposites obtained from TGA and DSC measurements. a ˝ a ˝ b ˝ Sample Tdeg (MWI, C) Tdeg (in situ, C) Tg (MWI, C) Sample Tdeg a (MWI, °C) Tdeg a (in situ, °C) Tg b (MWI, °C) PS/AgNPsPS/AgNPs 191191 334 334 107 107 PMMA/AgNPs 232 202 130 PS-PMMA/AgNPsPMMA/AgNPs 159232 202 198 130 110 PS-PMMA/AgNPs a 159 b 198 110 : Tdeg from TGA; : Tg from DSC. a: Tdeg from TGA; b: Tg from DSC. Figure9b shows the TGA plot of the neat polymers. The degradation of the neat polymers started at over 333,Figure 176, 9b and shows 300 the for TGA PS, PMMA, plot of the PSMMA, neat polymers. respectively The degradation (see Table of S1). the However,neat polymers the started PS/AgNPs at over 333, 176, and 300 for PS, PMMA, PSMMA, respectively (see Table S1). However, the (MWI) composite is less stable than pure PS; also, the composite PS-PMMA/AgNPs is less stable than PS/AgNPs (MWI) composite is less stable than pure PS; also, the composite PS-PMMA/AgNPs is less pure PS-PMMA. Only in the case of PMMA/AgNPs (MWI) are the nanocomposites more stable than stable than pure PS-PMMA. Only in the case of PMMA/AgNPs (MWI) are the nanocomposites more purestable PMMA, than and pure the PMMA, thermal and stability the thermal has stability been improved. has been improved.

(a)

Figure 9. Cont.

Materials 2016, 9, 458 10 of 17 Materials 2016, 9, 458 10 of 16

(b)

(c)

Figure 9. (a) TGA thermograms of polymer/AgNPs nanocomposites using MWI and in situ method; Figure 9. (a) TGA thermograms of polymer/AgNPs nanocomposites using MWI and in situ method; (b) TGA thermograms of neat polymers and polymer/AgNPs nanocomposites using MWI; (c) DSC (b) TGA thermograms of neat polymers and polymer/AgNPs nanocomposites using MWI; (c) DSC thermograms of polymer/AgNPs nanocomposites and neat polymers. Bottom right: enlarged image thermogramsof area marked of polymer/AgNPs by a box. nanocomposites and neat polymers. Bottom right: enlarged image of area marked by a box. Therefore, our approach is promising for the development of a new class of polymer/metal NP Therefore,composites. ourThis approachresult demonstrates is promising that the for ther themal development stability of the of aPMMA new class is improved of polymer/metal because of NP composites.the presence This of result AgNPs demonstrates using the MWI that method, the thermal which is stability consistent of with the PMMA the results is improvedobtained by because [53]. of To understand the effect of MWI on the thermal behavior and dispersion of AgNPs within the the presence of AgNPs using the MWI method, which is consistent with the results obtained by [53]. polymer matrix, DSC of the polymer/Ag composites was performed, and the results are presented in ToFigure understand 9c and summarized the effect in of Table MWI 1. on In order the thermal to get a behaviorclear picture and of dispersionthe degradation of AgNPs and the withineffect the polymerof AgNPs matrix, on DSC the ofdegradation the polymer/Ag temperature, composites DSC of was the performed,neat polymers and was the also results performed are presented and in Figurecompared,9c and summarized as shown in inFigure Table 9c.1. For In orderthe neat to poly get amers, clear the picture Tg values of the are degradation 118 °C, 127 °C, and and the 79 effect °C of AgNPsfor onPS, the PMMA, degradation and PS-PMMA, temperature, respectively DSC of (see the neatTable polymers S1). For wasthe PS/A alsog performed nanocomposites, and compared, the ˝ ˝ ˝ as shownthermogram in Figure shows9c. Forthat thethe neatTg value polymers, of the thenanocomposites Tg valuesare (Tg 118= 107 C,°C) 127 decreasedC, and by 79 11C °C for PS, PMMA,compared and PS-PMMA, with that ofrespectively neat PS (Tg = 118 (see °C), Table as indicated S1). For thein Table PS/Ag S1 also nanocomposites, [54]. For the PMMA/AgNPs the thermogram composite, the curve shows an improved thermal stability,˝ with a Tg of 130 °C,˝ which is 3 °C higher shows that the Tg value of the nanocomposites (Tg = 107 C) decreased by 11 C compared with that of ˝ neat PS (Tg = 118 C), as indicated in Table S1 also [54]. For the PMMA/AgNPs composite, the curve ˝ ˝ shows an improved thermal stability, with a Tg of 130 C, which is 3 C higher than that of the neat ˝ PMMA (Tg = 127 C). For the PSMMA/AgNP composite, the curve shows a signiﬁcantly improved Materials 2016, 9, 458 11 of 17

Materials 2016, 9, 458 11 of 16 ˝ ˝ ˝ thermal stability, with a Tg of 110 C, which is 31 C higher than that of the neat PSMMA (Tg = 79 C). This resultthan that suggests of the a neat very PMMA strong ( interactionTg = 127 °C). between For the thePSMMA/AgNP PSMMA chains composite, and AgNPs. the curve Previous shows work a has demonstratedsignificantly improved that the interfacialthermal stability, strength with between a Tg of 110 nanofillers °C, which and is 31 polymers °C higher and, than consequently, that of the the thermalneat PSMMA properties (Tg = of 79 nanocomposites °C). This result suggests can be altered a very st byrong varying interaction the sample between preparation the PSMMA method. chains In and AgNPs. Previous work has demonstrated that the interfacial strength between nanofillers and this work, the Tg shift may be attributed to the presence of so-called “interphase” polymer networking, polymers and, consequently, the thermal properties of nanocomposites can be altered by varying the which arises because of the interaction of the chains with the AgNPs surface, which may restrict the sample preparation method. In this work, the Tg shift may be attributed to the presence of so-called mobility,“interphase” creating anpolymer enormous networking, volume which of matrix arises polymer. because Percolationof the interaction of this of network the chains of interphasewith the polymerAgNPs could surface, then manifestwhich may as restrict a large theTg shiftmobility, of the creating polymer an composite.enormous volume Therefore, of matrix good polymer. dispersion withoutPercolation agglomeration of this ofnetwork AgNPs of may interphase result from polymer the fastcould thermal then manifest reduction as process a large thatTg shift is offered of the by MWIpolymer for PMMA/AgNPs composite. Therefore, and PSMMA/AgNPs good dispersion nanocomposites, without agglomeration while theof AgNPsin situ methodmay result produced from morethe stable fastnanocomposites. thermal reduction process that is offered by MWI for PMMA/AgNPs and PSMMA/AgNPs Purenanocomposites, Ag is a good while conductor; the in situ it is method metallic, produced and its conductivitymore stable nanocomposites. on the Pauling scale is 15.87 nΩ¨ m at 25 ˝C[54Pure] and Ag inis a SI good units conductor; is ~6 ˆ 10 it 5isS metallic, cm´1. Asand shown its conductivity in Figure on 10 thea, thePauling Ps/Ag scale nanocomposites is 15.87 nΩ·m film exhibitsat 25 °C a[54] resistance and in SI ofunits ~1.45997 is ~6 × Ω105and S cm a−1 conductivity. As shown in ofFigure ~103 10a, S/cm, the Ps/Ag which nanocomposites is significantly film lower than thatexhibits of pure a resistance Ag (~6 ôf 10~1.459975 S cm ´Ω1 ).and In a addition, conductivity the Ps/Agof ~103 nanocompositesS/cm, which is significantly film exhibits lower an than Ohmic 5 −1 behavior,that of which pureis Ag clearly (~6 × observed10 S cm in). In the addition, plot. By the comparing Ps/Ag nanocomp our resultsosites with film the exhibits reported an Ohmic work on behavior, which is clearly observed in the plot. By comparing our results with the reported work on Ps/Ag nanowires (see Table2), we observed that the conductivity values in our work are far superior Ps/Ag nanowires (see Table 2), we observed that the conductivity values in our work are far superior to the reported ones. White et al. [55] reported that the conductivity of PS/Ag nanowire composites to the reported ones. White et al. [55] reported that the conductivity of PS/Ag nanowire composites was ~0.01was ~0.01 S/cm S/cm for afor nanowire a nanowire aspect aspect ratio ratio of of 8, 8, and and the the conductivity conductivity of Ps/Ag nanowire nanowire composites composites with anwith aspect an aspect ratio ratio of 16 of was 16 was ~100 ~100 S/cm. S/cm. Further Further increaseincrease of the aspect aspect ratio ratio of of (31) (31) resulted resulted in ina a conductivityconductivity of ~0.001 of ~0.001 S/cm S/cm for for PS/Ag PS/Ag nanowire nanowire composites.composites.

Figure 10. Cont. Materials 2016, 9, 458 12 of 17 Materials 2016, 9, 458 12 of 16

FigureFigure 10. I–V 10. I–Vcurves curves of (ofa) ( PS/AgNP;a) PS/AgNP; ( b(b)) PMMA/Ag; PMMA/Ag; and and (c ()c PSMMA/Ag) PSMMA/Ag nanocomposites. nanocomposites.

When a polymer matrix is filled with a conducting filler, the composite gains a conductivity When a polymer matrix is filled with a conducting filler, the composite gains a conductivity value value of σ. When the loading of the filler is increased such that the volume filler fraction φ reaches a of σ. When the loading of the filler is increased such that the volume filler fraction ϕ reaches a critical critical value φc, an infinite cluster is formed, and the composite becomes conducting [54]. Because of ϕ valuethe presencec, an infinite of a conduction cluster is formed,or percolation and thepath composite across the entire becomes sample, conducting a change [from54]. an Because insulator of the presenceto a semiconductor of a conduction occurs. or percolation As the filler pathconcentration across the increases entire sample,to the filling a change limit F from, the value an insulator of r to aincreases semiconductor rapidly occurs.over several As the orders filler of concentrationmagnitude from increases the value to rc theat the filling percolation limit F threshold, the value to of r increasesthe maximal rapidly value over severalσm. Above orders the percolation of magnitude threshold, from the the value electricalrc at theconductivity percolation is related threshold to the to the maximalcontent value of conductingσm. Above filler. the percolation To produce threshold, a conducting the electricalAgNPs/PMMA conductivity nanocomposites, is related to~16 the wt content % of conductingloading of AgNPs filler. To would produce be needed, a conducting as per the AgNPs/PMMA requirement of nanocomposites,the percolation threshold. ~16 wt Therefore, % loading of AgNPsin our would case befor needed, PMMA/Ag as per nanocomposites, the requirement as illustrated of the percolation in Figure threshold. 10b, we used Therefore, 20 wt % inof ourAgNPs, case for PMMA/Agwhich produced nanocomposites, conducting as composites. illustrated inFor Figure the perc 10b,olation we used to occur, 20 wt the % ofvolume AgNPs, occupied which by produced the conductingconducting composites. phase in For the the composite percolation is tomost occur, important, the volume which occupied explains by thewhy conducting the PMMA/Ag phase in nanocomposites produced were conducting because of the percolation threshold. the composite is most important, which explains why the PMMA/Ag nanocomposites produced were conducting because ofTable the 2. percolation Resistance and threshold. conductivity of polymer/AgNP nanocomposites.

NanocompositesTable 2. Resistance Resistance and conductivity (Ohm) of polymer/AgNP Conductivity (S/cm) nanocomposites. References Ag 10−5−5 6 × 105 [52] 2 7 NanocompositesPS Resistance10 –10 (Ohm) Conductivity10−2 to 10−7 (S/cm)[53] References PMMA 1014–1016 10−14 to 10−16 [53] ´5 5 PS/AgAg nanowires 10´5 6 ˆ 10 [52] 102 2 7 1 × 10−2 [53] (A.RPS = 8) 10 –10 10´2 to 10´7 [53] 14 16 ´14 ´16 PS/AgPMMA nanowires 10 –10 10 to 10 [53] 103 1 × 10−3 ´ [53] PS/Ag nanowires(A.R = 31) (A.R = 8) 102 1 ˆ 10 2 [53] PS/Ag nanowiresPS/Ag (A.R = 31) 10−110 3 1.4 ×1 10ˆ−110 ´3 [54] [53] PMMA/AgPS/Ag 1014–1010´7 1 10−141.4–10ˆ−7 10´1 [55] [54] PANI-PMMA/AgPMMA/Ag 10910–1014−–104 7 10−109–10´144 –10´7 [55] [55] PANI-PMMA/AgPS/Ag 1.46109–10 ´4 1.03 10× 10´92 –104 Our Work[55 ] PMMA/AgPS/Ag 10−1 1.46 3.3 1.03× 10 ˆ 102 OurOur Work Work PMMA/AgPS-PMMA/Ag 73.98 ×10 10´31 40 × 3.310−3 ˆ 10Our OurWork Work PS-PMMA/Ag 73.98 ˆ 103 40 ˆ 10´3 Our Work For the PSMMA/Ag nanocomposites, we obtained a resistance of ~73.98347 kΩ and a conductivity of ~40 mS/cm with non-ohmic behavior, as shown in Figure 10c. These resistance and For the PSMMA/Ag nanocomposites, we obtained a resistance of ~73.98347 kΩ and a conductivity conductivity values indicate that the electrical properties of the nanocomposites were enhanced of ~40 mS/cm with non-ohmic behavior, as shown in Figure 10c. These resistance and conductivity compared with that of pure PS and PMMA (see Table 1). The resistance values for pure PS and PMMA values indicate that the electrical properties of the nanocomposites were enhanced compared with that are 102–107 Ω and 1014–1016 Ω, respectively. To the best of our knowledge, this paper is the first to 2 7 of purereport PS the and electrical PMMA properties (see Table 1of). PSMMA/Ag The resistance nanocomposites. values for pure Lee PS et andal. [31] PMMA reported are on 10 the–10 effectΩ and 1014–1016 Ω, respectively. To the best of our knowledge, this paper is the ﬁrst to report the electrical properties of PSMMA/Ag nanocomposites. Lee et al. [31] reported on the effect of polyaniline on the conductivity of a PMMA/Ag hybrid composite. These researchers demonstrated that the resistivity of Materials 2016, 9, 458 13 of 17

PMMA/Ag varied from 1014 to 107 Ω-cm, whereas that of PMMA/PANI/Ag composites varied from 109 to 10´4 Ω-cm. Thus, the resistivity of the PMMA/PANI/Ag composites was much lower than that of the PMMA/Ag composites, indicating that PANI strongly enhanced the conductivity. However, in our case, the resistivity is much lower than that of this work, resulting in the higher conductivity value.

3. Materials and Methods

3.1. Materials Styrene (S) and methyl methacrylate (MMA) monomers (99%, Acros Chemical Co., One Reagent Lane, Fair Lawn, NJ, USA) were stored in a refrigerator and used as received. Benzoyl peroxide (BP) (BDH Chemicals Ltd., Dammam, Saudi Arabia) was used as an initiator, hydrazine hydrate (HH, 80%) was obtained from LobaChemi. Pvt. Ltd., Mumbai, India and silver nitrate (AgNO3) was obtained from Merck., Kenilworth, NJ, USA. The other solvents and chemicals were of analytical grade and used without further puriﬁcation.

3.2. MWI Preparation of PS/Ag, PMMA/Ag, and PS–PMMA/Ag Nanocomposites

A mixture of 2.0 g (S-MMA) monomers, 80 mg AgNO3, and 0.1 g BP was sonicated for 1 h, and then, the mixture were maintained at 60 ˝C for 20 h to promote in situ free radical bulk polymerization. After the polymerization was completed, the product was poured into an excess of methanol, stirred for 15 min, and washed with methanol and hot water several times before being ﬁltered and dried in an ˝ oven at 80 C overnight. Then, a mixture of 0.40 g (PS-PMMA) polymer/AgNO3 composites dissolved into a solvent, 40 µL of HH, was sonicated for 1 h followed by reduction using MWI. The same procedure was performed with PS and PMMA.

3.3. In Situ Preparation of PS/Ag, PMMA/Ag, and PS–PMMA/Ag Nanocomposites

A mixture of 2.0 g (S-MMA) monomers, 80 mg AgNPs (prepared via MWI reduction of AgNO3), and 0.1 g BP was sonicated for 1 h, and then the mixture was maintained at 60 ˝C for 20 h to promote in situ free radical bulk polymerization. After the polymerization was completed, the product was poured into an excess of methanol, stirred for 15 min, and washed with methanol and hot water several times before being ﬁltered and dried in an oven at 80 ˝C overnight. The same procedure was performed with PS and PMMA. The neat PS, PMMA, and PS-PMMA were prepared for comparison using the same procedure but without the addition of AgNPs.

3.4. Characterization The FTIR spectra (Thermo Scientiﬁc, Waltham, MA, USA, Nicolet-iS10) of the nanocomposites were recorded in the range of 4000–500 cm´1. The UV-Vis spectra (Perkin-Elmer Lambda 35, Waltham, MA, USA) of the nanocomposites were recorded in the range of 200–800 nm. XRD analysis (Philips–Holland, Amsterdam, The Netherlands, PW 1729) of the nanocomposites was performed using Cu radiation (30 kV, 40 mA, Kα radiation (λ = 1.54430 Å)) between 2θ of 5˝ and 100˝. The XPS measurements were performed using a SPECS GmbH X-ray photoelectron spectrometer. Before analysis, the samples were degassed under a vacuum inside the load lock for 16 h. The Raman spectra of nanocomposites were measured using a Bruker Equinox 55 FT-IR spectrometer equipped with an FRA106/S FT-Raman module and a liquid N2-cooled Ge detector using the 1064 nm line of a Nd:yttrium aluminum garnet laser with an output laser power of 200 mW. SEM (FEI Quanta 200, FEI, Hillsboro, OR, USA) was employed to examine the morphology of the nanocomposites after they were mounted on the nanocomposite slabs and coated with Au via a sputtering system (Polaron E6100, Bio-Rad, Herts HP2 7DX, Hemel Hempstead, UK). HRTEM (JEOL JSM-2100F, Tokyo, Japan) was performed at 200 kV. A drop of the composite dispersed in ethanol was placed on copper grids and dried for studies. TGA of the nanocomposites was performed under an N2 atmosphere at a heating rate of 10 ˝C per minute from 25 ˝C to 800 ˝C using a NETZCH 209 F1 thermogravimetric analyzer. Materials 2016, 9, 458 14 of 17

DSC (NETZCH 204 F1) measurements were employed to estimate the glass-transition temperature ˝ ˝ (Tg) of each nanocomposite. The nanocomposites were heated from –25 C to 100 C at a heating rate of 10 ˝C per min. Then, a double run was performed after cooling at a heating rate of 2 ˝C per ˝ ˝ min from 25 C to 350 C. The Tg was taken as the midpoint of the transition. The resistances of the nanocomposites were calculated using two point probe method in a two-electrode (Cu) configuration by using a Keithley 4200 SCS-four-probe electrical current-voltage (I–V) measurements system. The conductivities of the samples were calculated by fitting their I–V characteristics (10 cycles). After the measurements were taken, the mass of the film consisting of each composite was measured (for PS/AgNPs = 1 mg, PMMA/AgNPs = 2 mg, PS-PMMA/AgNPs = 4 mg), and the effective thickness of each film was calculated (for PS/AgNPs = 0.1 mm, PMMA/AgNPs = 0.2 mm, PS-PMMA/AgNPs = 0.4 mm)). This procedure yielded a value of the film conductivity from the measured resistance.

4. Conclusions In conclusion, the incorporation of AgNPs within PS, PMMA, and PSMMA co-polymer matrices using in situ bulk polymerization and MWI was achieved, and the resulting functional properties were compared. FTIR and XPS studies confirmed the formation of metallic AgNPs within the polymer matrix. UV-Vis spectra of the PS/Ag and PMMA/Ag nanocomposites revealed a red shift with respect to the PSMMA/Ag nanocomposites, confirming the size effect of AgNPs. Raman spectra indicated that the characteristics peaks of all the polymer/Ag nanocomposites were blue-shifted with respect to the neat polymer, which indicates interfacial interactions between the AgNPs and polymer matrix. SEM and HRTEM micrographs of the polymer/AgNPs revealed that the AgNPs dispersed at the surface and embedded within the polymer matrix. TGA and DSC results revealed a modification in the thermal stability using AgNPs within the polymer matrix. These results indicate that the nanocomposites obtained using in situ technique exhibited better thermal stability than MWI—except PMMA/AgNPs (MWI), which showed better thermal stability than the in situ method. The electrical conductivity of the nanocomposites significantly improved, and the PS/Ag nanocomposites exhibited the highest conductivity, which is governed by the percolation model. These nanocomposites may prove particularly effective for the design of fuel cell electrodes, which are often made with conductive nanocomposites, or simply mats of conductive particles.

Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/9/6/458/s1. Figure S1: FTIR Spectra of Neat Polymers Polystyrene (PS), Poly methyl methacrylate (PMMA), and Polystyrene-Poly(methyl methacrylate) (PS/PMMA); Figure S2: Raman spectra of polymers Polystyrene (PS), Poly methyl methacrylate (PMMA), and Polystyrene-Poly(methyl methacrylate) (PS/PMMA); Table S1: Summary of the thermal behavior data obtained from TGA and DSC measurements. Acknowledgments: This study is a part of the research Project No. 405010901101. The author gratefully acknowledges the continued support of Alfaisal University and its Office of Research. Conflicts of Interest: The author declares no conflict of interest.

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