silver nanocomposites prepared via in situ radical polymerization

European Polymer Journal 72 (2015) 256–269

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Synthesis, characterization and reaction kinetics of PMMA/silver nanocomposites prepared via in situ radical polymerization Mohammad Nahid Siddiqui a, Halim Hamid Redhwi b, Efthymia Vakalopoulou c, Ioannis Tsagkalias c, Maria D. Ioannidou c, Dimitris S. Achilias c,⇑ a b c

Chemistry Department and Center of Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Chemical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Laboratory of Organic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece

a r t i c l e

i n f o

Article history: Received 8 July 2015 Received in revised form 16 September 2015 Accepted 21 September 2015 Available online 25 September 2015 Keywords: Polymerization kinetics PMMA Silver nanoparticles In situpolymerization Nanocomposites

a b s t r a c t In this investigation, nanocomposites of poly(methyl methacrylate), PMMA, with silver nano-particles (Ag NPs) were produced using an in-situ radical polymerization technique, where the reduction of Ag+ precursors takes place during the reaction. The effect of silver NPs on the reaction kinetics is investigated by measuring the variation of conversion with time and the molecular weight distribution of the polymer formed. The properties of the PMMA/Ag nano-hybrids were measured using a variety of techniques including, X-ray diffraction, FTIR and UV–Vis spectrometry, thermogravimetric analysis and differential scanning calorimetry. FTIR data showed that the inclusion of Ag NPs in the polymer matrix was rather physical without a strong chemical bond. Silver nanoparticles with average diameter ranging from 37 to 47 nm were identified with broader size distribution observed at higher amounts of silver added. The in situ formation of Ag NPs from the reduction of Ag+ was explained based on a radical reaction mechanism and chain transfer reactions. It was found that the initiator efficiency is reduced by the presence of the Ag NPs, resulting in a reduction of the reaction rate and a slight increase in the number average molecular weight of the polymer formed. The polydispersity of the MWD as well as the glass transition temperature of the polymer was found to decrease with the amount of the NPs. Finally, from thermogravimetric analysis, it was verified that the presence of the NPs results in macromolecular chains with less defect structures. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction One of the major disadvantages of polymers when used in food-contact applications is that they are very susceptible to microbial attack. Incorporation of bioactive agents including antimicrobials into polymers has been commercially applied in drug and pesticide delivery, household goods, textiles, surgical implants and other biomedical devices. The number of recently published research articles and patents on the incorporation of antimicrobials into plastics for food applications has more than doubled in the past 5 years. Of all the antimicrobials, silver (Ag) is the most widely used as polymer additive

⇑ Corresponding author. E-mail address: [email protected] (D.S. Achilias). http://dx.doi.org/10.1016/j.eurpolymj.2015.09.019 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.

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due to its properties and areas of use. Therefore, the introduction of silver nanoparticles (Ag NPs) into conventional polymers results in new materials with improved properties. The literature describing the preparation of Ag nanoparticles (Ag NPs) is particularly broad since the classical colloid methods are combined with modern nanotechnology leading to many procedures for surface modification, particle size control and particle preparation [1–10]. Although, a number of papers have been published on the preparation of silver nanoparticles with conventional and green methods [8–15], their incorporation into a polymeric matrix in order to form a nanocomposite polymeric material has only found limited research interest since now [16–21]. Particularly, Singh and Khanna [16] were among the first to synthesize silver nano-particles in poly(methyl methacrylate), PMMA using a one-step in-situ method. However, they initially prepared the polymer, PMMA and afterward a solution of AgNO3 in DMF was added to a solution of the polymer also in DMF in order to prepare the nanocomposites material. Better thermal stability was observed when using as high as 10-wt.% of silver over the polymer. A similar approach was used by Singho et al. [17], while the whole process was monitored using FTIR spectroscopy. In an analogous procedure, Su et al. [18] prepared nanocomposites materials of silver with poly(hydroxyethyl methacrylate), PHEMA this time. The dispersion polymerization technique was employed by Kim et al. [19] in order to synthesize monodisperse particles of PMMA with silver. The electrical resistivity of the hybrid materials was found to be enhanced. For the reduction of Ag+ to Ag0, butylated hydroxytoluene was used by Kassaee et al. [20], aiming to prepare composite materials of PMMA with silver nanoparticles. The only one who prepared silver/PMMA nanocomposites via a purely in-situ polymerization technique were Vodnik et al. [21]. NaBH4 was used as a reducing agent and the silver nanoparticles were added into the monomer, MMA and polymerized in situ. They found that the thermo-oxidative stability of PMMA was improved, while its glass transition temperature was lower compared to neat polymer. In a subsequent publication, [22] polymer dynamics of the same materials were also investigated. In our laboratory we have prepared in the past nanocomposites materials of PMMA with several nano-clays and studied the influence of the nano-filler on the reaction kinetics [23–29]. As a continuation of this work, here nanocomposite materials of PMMA with silver nano-particles are produced using an in-situ polymerization technique where the reduction of silver takes place during the reaction. The effect of silver on the reaction kinetics is investigated by measuring the variation of conversion with time, as well as the molecular weight distribution of the polymer formed. The properties of the PMMA/Ag nanoparticles hybrids were measured using a variety of techniques including, X-ray diffraction, FTIR and UV–Vis spectrometry, thermogravimetric analysis and differential scanning calorimetry. 2. Experimental 2.1. Materials The monomer used, i.e. methyl methacrylate (MMA) with a purity P 99% was purchased from Alfa Aesar and the hydroquinone inhibitor was removed by passing it, at least twice, through disposable inhibitor-remover packed columns, supplied from Aldrich, before any use. The free radical initiator, benzoyl peroxide (BPO) with a purity > 97% was provided by Alfa Aesar, and purified by fractional recrystallization twice from methanol, which was purchased from Chem-Lab. For the formation of silver nanoparticles, solid silver nitrate (AgNO3) was used from Mallinckrodt, which was dissolved in DMF (J.T. Baker). All other chemicals used were of analytical grade and were used as received without further purification. 2.2. Synthesis of PMMA/silver nanocomposites by in-situ bulk radical polymerization Initially, 0.1 M solution of AgNO3 in DMF was prepared. The appropriate amount of this solution was dispersed in 12 mL of the monomer MMA in a 100 mL conical flask, by adequate magnetic and supersonic agitation. Three different mixtures were prepared and studied with 2%, 6% and 10% v/v relative amounts of AgNO3/DMF compared to the monomer MMA. In the final suspension, the initiator, BPO 0.03 M was added and the mixture was degassed by passing nitrogen and immediately used. It should be noted here that, since this product could be used in commercial applications, the actual amount of Ag in the nanocomposites was rather low, i.e. 0.023, 0.069 and 0.114 wt.% corresponding to 2%, 6% and 10% v/v, in order to keep the final product cost at low levels. In order to study the reaction kinetics, free radical bulk polymerization was carried out in small test-tubes by heating the initial monomer–AgNO3–initiator mixture at 80 °C for a suitable time. According to this technique, 1 mL of the pre-weighted mixtures of monomer with the initiator and the appropriate amount of the AgNO3/DMF solution were placed into a series of 10 small test-tubes. After degassing with nitrogen they were sealed and placed into a pre-heated bath at 80 °C. Each test-tube was removed from the bath at pre-specified time intervals and was immediately frozen, after the addition of few drops of hydroquinone, in order to stop the reaction. The product was isolated after dissolution in CH2Cl2 and re-precipitation in MeOH. A different procedure for the nanocomposite isolation was followed in the last 1 or 2 samples of each experiment. Since the reaction was already finished and the polymer/nanofiller mixture was a solid, the test-tubes were broke and the products were obtained as such. In this way it was ensured that the filler was enclosed into the polymer matrix. Subsequently, all isolated materials were dried to constant weight in a vacuum oven at room temperature. All final samples were weighed and the degree of conversion was estimated gravimetrically. Experiments were repeated thrice and average values are reported. Throughout the polymerization, all solutions were kept in a dark place to avoid any photochemical reactions.

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Exactly the same experiment was repeated using only the monomer MMA and the initiator, without adding any amount of AgNO3. 2.3. Measurements X-ray diffraction. The crystalline structure of the prepared PMMA/Ag NPs materials was characterized using X-ray diffraction (XRD) in a Rigaku Miniflex II instrument equipped with Cu Ka generator (k = 0.1540 nm). The XRD patterns were recorded at the range 2h = 5–85° and scan speed of 2° min
1. Ultraviolet–Visible (UV–Vis) spectroscopy. The optical properties of the PMMA/Ag NPs were examined by measuring their absorption spectra using UV–vis spectroscopy. The instrument employed was a Shimadzu Pharmaspec UV-1700 UV–visible spectrophotometer and the spectra were recorded over the range of 300–700 nm. Fourier-Transform Infra-Red (FTIR). The chemical structure of the neat PMMA and PMMA/Ag NPs was confirmed by recording their IR spectra. The instrument used was the Spectrum 1 spectrophotometer from Perkin Elmer with an attenuated total reflectance (ATR) device. ATR was necessary since the samples with high amounts of AgNPs were not transparent. Measurements were carried out using thin films prepared in a hot hydraulic press and spectra recorded over the range from 4000 to 600 cm
1 at a resolution of 2 cm
1 and 32 scans were averaged to reduce noise. The instrument’s software was used to identify several peaks Differential Scanning Calorimetry (DSC). In order to estimate the glass transition temperature of every material prepared, the DSC-Diamond (Perkin–Elmer) was used. Approximately 5–6 mg of each sample were weighed, put into the standard Perkin–Elmer sample pan, sealed and placed into the appropriate position of the instrument. Subsequently, they were initially heated to 180 °C at a rate of 10 °C min
1 to ensure complete polymerization of the residual monomer. Following, the samples were cooled to 30 °C and their glass transition temperature was measured by heating again to 180 °C at a rate of 20 °C min
1. All results are from the second heating. Gel Permeation Chromatography (GPC). The molecular weight distribution (MWD) and the average molecular weights of pristine copolymers and all nanocomposites were determined by GPC. The instrument used was from Polymer Laboratories, model PL-GPC 50 Plus, and included an isocratic pump, a differential refractive index detector, and three PLgel 5l MIXED-C columns in series. All samples were dissolved in THF at a constant concentration of 1 mg mL
1. After filtration, 200 lL of each sample was injected into the chromatograph. The elution solvent was THF at a constant flow rate of 1 mL min
1, and the entire system was kept at a constant temperature of 30 °C. Calibration of GPC was carried out with standard poly(methyl methacrylate) samples (Polymer Laboratories) with peak molecular weight ranging from 690 to 1,944,000 and the universal calibration method with appropriate Mark–Houwink constants. Thermogravimetric Analysis (TGA). The thermal stability of the samples was measured by thermogravimetric analysis. TGA was performed on a Pyris 1 TGA (Perkin–Elmer) thermal analyzer. Samples of about 5–8 mg were used. They were heated from ambient temperature to 600 °C at a heating rate of 20 °C min
1 under nitrogen flow. 3. Results and discussion 3.1. Formation of silver nanoparticles During synthesis of silver nanoparticles from AgNO3, it is necessary to prepare stable particles from the reduction of Ag+ to Ag0. In order to meet this demand a number of reducing agents have been employed. In this investigation, besides the small amount of DMF which was used as a solvent for AgNO3 no other compound added to the reaction mixture. Then, it was really interesting to see if reduction of Ag cations really happened. The photograph in Fig. 1 shows the final polymer obtained after the reaction of either neat MMA with BPO at 80 °C (a) or the nano-hybrid obtained after adding 2% and 6% of AgNO3/DMF solution (b and c). From the photograph it was clear that while neat PMMA produced was almost transparent and colorless, the other two polymers had a color from golden yellow to pale brown. The color observed is characteristic of the reduction of silver ions into zero valence Ag nanoparticles. The question then arises on how this was achieved. In order to provide an explanation we turn back into the free radical polymerization of MMA. As it is well known, the main reaction steps are those of initiation, propagation and termination. Propagation, meaning the reaction of a macro-radical with a monomer molecule in order to increase the length of the radical by the addition of the monomer, appears in Scheme 1 (reaction 1). A reaction, competitive to propagation, which takes place during MMA polymerization, is that of chain transfer to monomer (reaction 2 in Scheme 1). Accordingly, the reactants are exactly the same, while the products are different. In CTM usually transfer of a b-hydrogen takes place from the macro-radical to the monomer molecule. Thus, the radical stops, forming a macromolecule with a terminal double bond, while a new radical starts at the monomer molecule. Thus, from this reaction we can assume that instantaneously a Hydrogen radical is formed (reaction 3). Concerning silver, the Ag+ cations formed from the dissociation of AgNO3, disperse in PMMA and coordinate to the oxygen atoms (reaction 4). This is a reasonable suggestion and has been reported previously in literature for the acrylate in PMMA [20]. Then, the reduction of Ag+ to atomic silver can be obtained by the hydrogen radicals formed from the CTM reaction (reaction 5). Finally, silver produced aggregates to form Ag nanoparticles in PMMA (reaction 6). The PMMA constituent prevents precipitation and further aggregation of Ag NPs while stabilizes and protects them through its carboxylate functional groups.

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(a)

(b)

259

(c)

Fig. 1. Photograph of the polymer obtained after polymerization of MMA with BPO at 80 °C without any additive (a) and after adding 2% and 6% of AgNO3/DMF solution.

3.2. Characterization of the PMMA/Ag NPs Possible physicochemical interactions between the Ag NPs and the PMMA matrix were tester using FTIR–ATR measurements. The FTIR spectra of the material prepared appear in Fig. 2. The spectrum of pure PMMA and all the nanocomposites show a sharp peak at 1724 cm
1 which correspond to the carbonyl bond, C@O stretching vibrations. Two small peaks at 3000/2940 cm
1 are attributed to methyl ester CAH stretching vibrations. An additional small peak at 2855 cm
1 is due to ACH3 stretching vibrations. The peaks at 1436/1482 cm
1 correspond to CAH deformations. The peak at 1365 cm
1 correspond to ACH3 symmetrical deformation. Finally, the peaks at 1271/1233/1143/985 cm
1 are attributes to CAO stretching. Similar reflectance bands have been observed in FTIR–ATR of pure PMMA in literature [30]. From Fig. 2, the spectra of pure PMMA and all nanocomposites appear similar, indicating that the inclusion of Ag NPs in the polymer matrix is rather physical without a strong chemical bond. An analogous observation has been reported in literature [20,31]. These results indicate that Ag/PMMA nanocomposites resemble solid solution with weak interaction between the polymer matrix and nanofiller particles. However, it should be noted that, the highest concentration of silver used in nanocomposites was only 0.114 wt.%, and potential bond formation between surface silver atoms and polymer chains would be difficult to observe. Chemical bonding between Ag NPs and different polymer matrices has been reported in literature [16,17]. Furthermore, the XRD patterns of neat PMMA and PMMA/Ag NPs materials were recorded in the angle range of 2h (5° < 2h < 85°) and are shown in Fig. 3. It is sheen that in pure PMMA three very broad peaks appear at 16°, 32° and 43°, respectively denoting the amorphous structure of the polymer. When AgNPs are formed in the polymer matrix, 5 additional sharp peaks are present indicating the formation of the silver crystalline structure. XRD diffraction peaks were recorded at 2h values of 38.2°, 44.4°, 64.7°, 77.7° and 82.0°. These peaks correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystalline planes of the face-centered cubic silver, respectively [20]. The average particle size of silver nanoparticles can be calculated using Debye–Scherrer equation:

n¼

Kk b cos h

ð1Þ

where K is the Scherrer constant with value from 0.9 to 1 (shape factor), k is the X-ray wavelength (1.5418 Å), b1/2 is the width of the XRD peak at half height and h is the Bragg angle. Using the above Scherrer’s equation and considering the dominant peak at 38.2° the average particle size of silver nanoparticles was calculated to be around 41–42 nm in all three compositions. It seems that the average particle size does not vary with the amount of silver. These values are somehow higher than corresponding values reported in literature (around 30 nm [20]) but this can be explained since no additional compound was added into the mixture for the reduction of silver and the formation of the nanoparticles. Furthermore, in order to provide a more accurate estimation of the Ag NPs average size as well as to investigate the optical properties of the nanocomposites, their UV absorption spectra were recorded and appear in Fig. 4. Characteristic peaks around 420 nm were observed for all nanocomposites, whereas no peak appears for neat PMMA. The size and shape of metal nanoparticles are typically measured by analytical techniques such as Transmission electron microscopy (TEM) or atomic force microscopy (AFM). However, due to the unique optical properties of silver nanoparticles, a great deal of information about the physical state of the nanoparticles can be obtained by analyzing their UV spectra at a specific wavelength range.

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Scheme 1. Proposed mechanism for the formation of Ag NPs during the in situ polymerization of MMA.

A peak position of the absorption band from 400 to 500 nm represents the dipole component of the Surface Plasmon Resonance (SPR) of silver nanoparticles. As the particle diameter increases, the peak plasmon resonance shifts to longer wavelengths and broadens. The peak wavelength, the peak width, and the effect of secondary resonances yield a unique spectral fingerprint for a plasmonic nanoparticle with a specific size and shape. Additionally, UV–Visible spectroscopy provides a mechanism to monitor how the nanoparticles change over time. When silver nanoparticles aggregate, the metal particles become electronically coupled and this coupled system has a different SPR than the individual particles. For the case of a multi-nanoparticle aggregate, the plasmon resonance will be red-shifted to a longer wavelength than the resonance of an

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261

Reflectance (a.u)

a b c d

1724

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Fig. 2. FTIR spectra of neat PMMA (a) and PMMA with 2% (b), 6% (c) and 10% (d) AgNO3.

individual nanoparticle, and aggregation is observable as an intensity increase in the red/infrared region of the spectrum. Carefully monitoring the UV–Visible spectrum of the silver nanoparticles with time is a sensitive technique used in determining if any nanoparticle aggregation has occurred [32]. From Fig. 4 it can be seen that the PMMA/Ag nanocomposites exhibits a broad surface Plasmon absorption band peaking at approximately 420 nm. This result is in agreement with the optical absorption spectra of Ag nanoparticles embedded in different polymer matrixes, such as poly(vinyl alcohol) [33] and polystyrene [34]. The shift to the longer wavelengths and broadening of the surface plasmon absorption band upon incorporation of Ag nanoparticles into the polymer can be induced by agglomeration of the Ag nanoparticles and/or change of the dielectric properties of the surrounding environment [33]. However, Vodnik et al. [31] from comparison of the absorption spectrum of dissolved PMMA/Ag nanocomposite in chloroform to that of Ag NPs in chloroform after phase transfer from water concluded that incorporation of silver nanoparticles into PMMA did not induce their agglomeration or growth. The average size of the Ag NPs was obtained from a correlation of the particle diameter with peak plasmon resonance. A lot of such data have been reported in literature so far [35–38]. Malynych [35] collected a number of such data and provided an empirical expression. Fig. 5 depicts various experimental data of the dipole Plasmon resonance as a function of particle diameter. One can see a clear shift of the average particle size to higher values with longer wavelengths. In the same figure a fitting line is included according to the empirical equation provided by Malynych [35]. From fitting to the data this is expressed as: 2

½dðnmÞ
5 ¼
2063 þ 94:5  ðkmax
385Þ

ð2Þ

However, from Fig. 5, it seems that this expression does not fit very well the whole different set of experimental data (R2 = 0.879). Hence, we preferred to fit all the experimental data with a simple polynomial equation of 2nd order,

dðnmÞ ¼ A þ Bkmax þ Ck2max

ð3Þ

PMMA PMMA+ 2% AgNO3

60000

PMMA+ 6% AgNO3

Intensity (counts)

50000

PMMA+10% AgNO3

(111) (200)

40000

(311) (222)

30000

(220)

20000 10000 0

10

20

30

40

50

60

70

80

90

2θ (degrees) Fig. 3. X-ray diffraction patterns of neat PMMA and PMMA/Ag NPs.

262

M.N. Siddiqui et al. / European Polymer Journal 72 (2015) 256–269 0,7 PMMA+ 2% AgNO3 PMMA+ 6% AgNO3

Absorbance (a.u)

0,6

PMMA+10% AgNO3 PMMA

0,5 0,4 0,3 0,2 0,1 0,0

300

400

500

600

700

Wavelength (nm) Fig. 4. UV–vis absorption spectra of neat PMMA and PMMA/Ag NPs.

The best-fit parameters obtained were: A =
653.71, B = 2.375 and C =
0.00174 and the correlation coefficient was, R2 = 0.912. The maximum of the SPR wavelength obtained from the experimental data of Fig. 4 appear in Table 1. Employing these values in Eq. (3), the average Ag NPs diameter was estimated and values are included in Table 1. It can be commented that the original rough estimate (i.e. 41–42 nm) from XRD measurements, was verified. Average particle diameters of the silver nanoparticles range from 37 to 47 nm, with the smaller size obtained with 6% initial AgNO3 solution. According to the Mie theory, SPR of non-interacting silver nanoparticles in PMMA is positioned at 402 nm (refractive index of PMMA is 1.49) [21]. The difference of 20–30 nm between this value and the experimentally measured (Table 1) indicates that the Mie theory may not be applicable in this case. Therefore, according to Ref. [21] the effective medium Maxwell–Garnett (M–G) theory can be used to explain optical properties of PMMA/Ag nanocomposites. In the M–G theory, the position of the absorption maximum, kmax, can be estimated using the following equation:

kmax ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2þf e þ e xp 1 1
f m hc

ð4Þ

Peak Plasmon Resonance Wavelength (nm)

where h is the Planck constant (h = 6.626  10
34 J s), c is the speed of light (c = 2.998  108 m/s), xp = 9.5 eV (9.5  1.602  10
19 J) is the Plasmon frequency of silver, em = (nPMMA)2 = (1.49)2 = 2.22, e1 = 5 and f is the filling factor that represents the occurrence of the inorganic phase in the nanocomposite. From Eq. (4) the filling factors estimated for the different nanocomposites based on their kmax appear in Table 1. Higher f value was estimated when 10% AgNO3 was used. Another important characteristic of the silver nanoparticles is the dispersity of their particle sizes. It has been reported that the absorption spectra of silver nanoparticles in the region of the SPR wavelength can be simulated by a Lorentzian distribution [39]. Such fit was also carried out in this investigation and indicative plots appear in Fig. 6. It can be seen that the experimental data are simulated very well with correlation coefficient R2 = 0.998. Thus, the peak plasmon resonance wavelength can be estimated together with the full width at half maximum (FWHM) of the corresponding peaks. These results are

550

500

450

400

350 0

20

40

60

80

100

120

140

Particle diameter (nm) Fig. 5. Dependence of peak SPR wavelength on the particle diameter. Experimental data: Evanoff and Chumanov [36] (j), Mock et al. [37] (d), Agnihotri et al. [38] (}), Oldenburg [32] (H), fitting lines: according to an empirical Eq. (2) proposed by Malynych [35] (  ), 2nd order polynomial (continuous line).

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Table 1 Peak plasmon resonance wavelength from experimental data and estimated from Lorentzian fit, estimated average particle diameter from Eq. (3), filling factor, f, from Eq. (4) and width at half maximum. Sample

Peak plasmon resonance wavelength from experimental data (nm)

Estimated average particle diameter from Eq. (3) (nm)

Filling factor, f

Peak plasmon resonance wavelength (nm) from Lorentzian fit

Width at half maximum from Lorentzian fit (nm)

PMMA + 2% AgNO3 PMMA + 6% AgNO3 PMMA + 10% AgNO3

430 420 432

45.8 36.8 47.6

0.175 0.121 0.185

436 417 452

150 175 169

0,20 PMMA+ 2% AgNO3

0,18

Lorentz Fit of B

Absorbance (a.u)

0,16 0,14 0,12 0,10

y = y0 + (2*A/PI)*(w/(4*(x-xc)2 + w2))

Equation Adj. R-Square

0,99833 Value

0,08 0,06

Standard Error

B

y0

0,05632

B

xc

435,86725

B

w

150,27744

2,11094

B

A

30,32208

0,50951

B

300

H

350

7,87688E-4 0,4302

0,18477

400

450 500 550 Wavelength (nm)

600

650

700

Fig. 6. Absorption spectra of PMMA + 2% AgNO3 and best fit using a Lorentzian model.

Absorbance (a.u)

0,15

0,10

a to d 0,05

0,00

300

400

500

600

700

Wavelength (nm) Fig. 7. Effect of the reaction time on the formation of silver nanoparticles from UV measurements. PMMA + 2% AgNO3 at (a) 22, (b) 32, (c) 37 and (d) 57 min.

included in Table 1. The FWHM determines dispersity of the nanoparticles, where a large FWHM is attributed to peak broadening and thus polydispersity. The FWHM values of SPR absorption band obtained here are rather large (i.e. from 150 to 175) compared to corresponding reported in literature for narrow distribution Ag NPs (around 50–60 nm). This is an indication of a broad size distribution of silver nanoparticles mainly resulted from their synthesis in situ during polymerization and not before polymerization. Finally, the UV–vis spectra were recorded for samples taken during the reaction and the effect of the reaction time on the absorbance appears in Fig. 7. The UV–vis spectra show that with increasing the reaction time the intensity of the absorbance increases due to the formation of more Ag NPs during the reaction. In addition a slight shift of the curve is observed to higher values possibly related with the slight agglomeration of the Ag NPs formed. In addition, thermal stability of pure PMMA and PMMA/Ag nanocomposites was examined by thermogravimetric analysis in nitrogen atmosphere. Results on the variation of mass and differential change of mass with temperature appear in Fig. 8a and b. The thermal degradation of radically prepared PMMA has been a subject of numerous studies and usually involves multiple steps assigned to: presence of weak head-to-head linkages, scission of unsaturated terminal groups and

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PMMA PMMA+ 2% AgNO3

100

PMMA+ 6% AgNO3 PMMA+10% AgNO3

Mass (%)

80

60

40

20

0 100

200

300

400

500

400

500

Temperature (°C) 0

-5

dTGA

-10

-15

-20

-25

PMMA PMMA+ 2% AgNO3 PMMA+ 6% AgNO3 PMMA+10% AgNO3

100

200

300

Temperature (°C) Fig. 8. (a) Mass loss and (b) derivative TGA curves of neat PMMA and PMMA/Ag nanocomposites with various amounts of added AgNO3.

random scission of the carbon–carbon main chain. It is generally considered that most PMMA thermally degrade through depolymerisation, therefore, the kinetics of mass loss are determined by the mode of degradation initiation. Degradation starts at almost 280 °C for neat PMMA and PMMA + 2% AgNO3 while at lower temperatures, near 220 °C for the other two nanocomposites. The lower starting degradation temperature for these compositions is attributed to the existence of some amount of unreacted monomer (as it was observed from kinetic measurements) which may be evaporated at these temperatures. Two distinct peaks were observed in all materials investigated. The first which is attributed to the degradation of weak links inside the macromolecular chain or at its end appears at 320 °C. The second peak which is due to random chain scission appears at 365 °C shifted to 385 °C for the nanocomposites. Therefore, incorporation of the silver nanoparticles shifts the main degradation step to higher temperatures resulting in protection of the material from thermal degradation. Another interesting point is that the relative area of the first step is decreased as we are going from neat PMMA to PMMA + 10% AgNO3, while the inverse holds for the second peak. This means that the incorporation of the Ag NPs in the polymer matrix results in a decrease of the defect structures and production of macromolecules with more uniform chain characteristics. This is in accordance with the results found from GPC that the polydispersity of the MWD decrease with the addition of the nanoparticles. 3.3. Polymerization kinetics The evolution of conversion with time measured gravimetrically for neat PMMA and the materials with the Ag NPs appears in Fig. 9. From an inspection of the curves, it is clear that all polymerizations exhibit a behavior typical of poly (methyl methacrylate) with a strong gel-effect starting at low conversions (i.e. near 30%). The key-points of the phenomena taking place are briefly discussed next. In the first stage of polymerization (low conversions), the conversion vs time curve, follows the ‘classical’ free-radical kinetics. An almost linear dependence of conversion, X, or better of
ln(1
X) vs time, appears denoting purely chemical control of the polymerization [40]. After a certain point in the region of 20–30% conversion, an increase in the reaction rate takes place followed by an increase in the conversion values. This is the well-known auto-acceleration or gel-effect and is attributed to the effect of diffusion-controlled phenomena on the termination reaction and the reduced mobility of live macro-radicals in order to find one another and react. Therefore, their concentration increases locally, leading in increased reaction rates [41,42]. Afterward, the reaction rate falls significantly and the curvature

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1,0

PMMA PMMA+ 2% AgNO3

0,8

PMMA+10% AgNO3

265

Monomer conversion

PMMA+ 6% AgNO3

0,6

0,4

0,2

0,0 0

10

20

30

40

50

60

Time (min) Fig. 9. Conversion versus time curves of neat PMMA and PMMA/Ag nanocomposites with different relative amounts of silver obtained from in situ bulk polymerization at 80 °C with 0.03 mol/L initial initiator concentration.

of the conversion versus time changes. At this conversion interval from approximately 50% to 80%, the observed decrease in the termination reaction rate is not so abrupt but only gradual. At this stage, the center-of-mass motion of radical chains becomes very slow and any movement of the growing radical site is attributed to the addition of monomer molecules at the chain end. This additional diffusion mechanism is so-called ‘reaction diffusion’. The higher the propagation reaction rate value the more likely is reaction–diffusion to be rate determining. Finally, at very high conversions, beyond 90%, the reaction rate tends asymptotically to zero and the reaction almost stops before the full consumption of the monomer, thus a glassy state appears and it corresponds to the well known glass-effect. This is attributed to the effect of diffusion-controlled phenomena on the propagation reaction and the reduced mobility of monomer molecules to find a macro-radical and react [40–42]. It has been reported, [43] that the presence of nano-particle may influence polymerization kinetics especially in monomers exhibiting strong effect of diffusion phenomena on the reaction kinetics. These results were attributed to the decreased free-volume of the reacting mixture as well as to the restriction imposed in the diffusion of macro-radicals in space due to the existence of the organic modifiers in the MMT platelets. Therefore, the OMMT platelets with the large chemical structure of the modifiers, add an extra hindrance in the movement of the macro-radicals in space in order to find one another and react (terminate), resulting in locally increased radical concentrations. According to these experimental data it seems that the presence or the formation of silver NPs during the reaction imposes two effects in the polymerization kinetics. Initially, it seems that the initial rate (slope of the conversion vs time curve) is slightly reduced as the amount of AgNO3 increases. Afterward, a clear reduction of the conversion values in the region of the gel-effect appears. The first has to do with pure reaction chemical kinetics, while the second with the effect on the diffusion-controlled phenomena. In order to explain the effect of Ag NPs on the initial polymerization kinetics, some basics of radical polymerization are presented. Thus, the polymerization rate, denoted by the variation of monomer conversion, X with time, t is expressed by the following equation, assuming the steady-state approximation for the total radical population, which has been proven to hold at low conversion values [40].

 1=2 dX fkd ½I ¼ ðkp þ ktrM Þ ð1
XÞ dt kt

ð5Þ

where kp, ktrM, kt and kd denote the kinetic rate constants of the propagation, chain transfer to monomer, termination and initiator decomposition reactions, f is the initiator efficiency and [I] the initiator concentration. Assuming that the initiator concentration remains almost constant at small reaction times and all kinetic rate constants independent of conversion, Eq. (5) can be integrated to give:


lnð1
XÞ ¼ keff t

ð6Þ

    fkd ½I0 1=2 keff ¼ kp þ ktrM kt

ð7Þ

with

By plotting the left-hand side of Eq. (6),
ln(1
X) versus time, t at low conversions, straight lines should appear having a slope equal to the effective rate constant, keff. Such results for neat PMMA and PMMA with different relative amounts of the Ag NPs appear in Fig. 10. Experimental data in the range 1–24% conversion were used. From the slope of these lines, the following values of the effective rate constant were obtained: 112.6  10
4 ± 6.9  10
4, 107.1  10
4 ± 3.1  10
4,

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0,30 PMMA PMMA+ 2% AgNO3

0,25

PMMA+ 6% AgNO3 PMMA+10% AgNO3

-ln (1-X)

0,20

liner fit

0,15 0,10 0,05 0,00 0

5

10

15

20

25

30

Time, t (min) Fig. 10. Estimation of the effective rate constant for the bulk polymerization of MMA and its nanocomposites with different amounts of silver nanoparticles at 80 °C and 0.03 M initial BPO concentration. Experimental data and linear fitting curves.

95  10
4 ± 7.9  10
4, 86.1  10
4 ± 5.3  10
4 min
1 for neat PMMA and PMMA/AgNO3-2%, 6% and 10%, respectively. The corresponding correlation coefficients, R2 were 0.9853, 0.9967, 0.9729 and 0.9812. It was observed that keff reduces when adding silver nanoparticles and increases continuously as the amount of AgNO3 added increases. The effective rate constant of PMMA can be evaluated from available literature data on the kinetic rate constant at low conversions. A number of different values have been proposed. In the following we decided to make use those reported in a very recent paper by Zoller et al. [44]. These authors propose the following values: kp = 2.67  106 ⁄ exp(
22,360/RT), kt = 1.984  108 ⁄ exp(
5890/RT) L/mol/s, kd = 5  1016 ⁄ exp(
143,000/RT) s
1, ktrM = 5  10
5 ⁄ kp while for f different values have proposed in literature. We used the value f = 0.5. The values of the above kinetic rate constants at 80 °C, where the experiments of this study were carried out, are: kp = 1314 L/mol/s, kt = 2.668  107 L/mol/s, kd = 3.5  10
5 s
1, ktrM = 0.066 L/mol/s. Using these values and for [I]0 = 0.03 mol/L, the theoretical value of the effective rate constant becomes, keff = 1.84  10
4 s
1 or 110.6  10
4 min
1. This value is very close to the experimentally observed 112.6  10
4 min
1, which confirms our experimental data. The next step was to identify which kinetic parameter is affected by the addition of AgNO3. From Eq. (5) it is unlikely that kp, ktrM, kt, or kd would change with the existence of the Ag NPs. Then, it seems that the initiator efficiency, f could be affected. In order to test this assumption, Eq. (5) was used again, setting f = 0.5 only for neat PMMA and estimating other f values for the nano-hybrids based on the experimentally measured values of the overall rate constant, keff. The values of f thus obtained are plotted as a function of the amount of initially added AgNO3 in Fig. 11. A very good straight line dependence was observed with R2 = 0.9981. Then, it is confirmed that the presence of Ag+ interacts with some of the primary radicals formed from the decomposition of the initiator, which results in a decreasing amount of effective primary radicals capable of reacting with monomer molecules to initiate polymerization. Concerning the gel-effect region of the conversion vs time profile, it was observed that the auto-acceleration starts at higher reaction times as the amount of Ag+ added increases. This is a direct result of the lower initial slope of the conversion versus time profile explained previously (meaning that since the abrupt increase in conversion starts at approximately 30% conversion, this value is obtained at higher reaction times as the amount of silver increases). In addition, it is observed that the final conversion values are slightly lower compared to neat PMMA. This could be attributed to the hindered movement of small molecules in the reaction mixture (i.e. monomer molecules or primary initiator radicals) to find a macro-radical and react caused by the existence of the Ag NPs as shown in Eq. (6) of Scheme 1. This results in larger amounts of unreacted monomer molecules. In order to better explain these results the average molecular weights of all nano-hybrids prepared and their full molecular weight distribution was measured with GPC. Results appear in Fig. 12 and Table 2. From the results on the number and weight average molecular weight of the polymer measured, it is observed that the number average molecular weight, Mn, increases with increased amounts of Ag+ added, while the inverse holds for the weight average molecular weight, Mw. From Fig. 12 it seems that polymers with slightly higher average molecular weight are produced, while they have a narrower MWD distribution. In order to explain these observations we returned again to classical free radical polymerization kinetics. Accordingly, the number average molecular weight of a polymer is given by its average degree of polymerization which in turn is calculated from the average kinetic chain length, m. This is given by the following equation:

1

m

¼

kt ½P  ktrM ½M ðfkd ½Ikt Þ þ ¼ kp ½M kp ½M kp ½M

1=2

þ CM

ð8Þ

From this equation, it is clear that a decreased initiator efficiency, f, results in increased m and as a result, of higher number average molecular weight. A lower number of primary initiator radicals results in macro-radicals with higher chain length. Moreover, the lower final conversion caused by the presence of higher amounts of Ag NPs results in not letting the polymer to increase its high molecular weight tail to higher values. This reduces the polydispersity of the MWD.

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267

0,50

0,45

f

0,40

0,35

0,30

0,25 0

2

4

6

8

10

AgNO3 (vol%) Fig. 11. Estimated initiator efficiency, f, at different amounts of AgNO3.

1,2 PMMA PMMA+ 2% AgNO3

1,0

PMMA+ 6% AgNO3 PMMA+10% AgNO3

dW/dlogM

0,8 0,6 0,4 0,2 0,0 3,5

4,0

4,5

5,0 5,5 log (MW)

6,0

6,5

7,0

Fig. 12. Full Molecular weight distribution of neat PMMA and PMMA/Ag nanocomposites.

4,0 o

Heat Flow Endo Up (mW)

3,5

Tg=80 C

d

3,0 o

Tg=82 C

2,5

c

2,0 b

1,5 1,0

o

Tg=95 C

a

0,5 0,0

o

Tg=98 C

-0,5 60

80

100

120

140

160

o

Temperature ( C) Fig. 13. DSC traces of PMMA and all PMMA/Ag nanocomposites, to estimate their glass transition temperatures.

Furthermore, the glass transition temperature, Tg of neat PMMA and the nano-hybrids was determined using DSC, according to the procedure described in the experimental section. Indicative results of the amount of heat flow versus temperature obtained are shown in Fig. 13. Tg was estimated using the half Cp extrapolation method [25]. Using the procedure described in the experimental section no DSC artifacts are observed and the results are reproducible. All the Tg values are given in

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Table 2 Number-average, weight-average and z-average molecular weights (M n , M w , M z ), polydispersity of the MWD (PD) and glass transition temperature (Tg) of neat PMMA and its nanocomposites with various amounts of AgNO3. Sample

Mn

Mw

Mz

PD

Tg

PMMA PMMA + 2% AgNO3 PMMA + 6% AgNO3 PMMA + 10% AgNO3

114,570 120,035 130,695 164,820

427,250 279,810 288,830 370,620

1,121,690 528,740 572,750 734,560

3.73 2.33 2.21 2.25

98 95 82 80

Table 2. The value measured for pristine PMMA (i.e. 98 °C) is close to that reported in the literature, usually near 100 °C [25]. A tendency of decrease of the Tg values by increasing silver content was observed for all Ag/PMMA nanocomposites. The same trend has been also reported in literature [21]. This may be due to the decreased restricted segmental chain mobility of the PMMA anchored to the particles surface. The interaction of polymer chains and nano-particles surface can alter the chain kinetics by either decreasing or increasing the glass transition temperature of the polymer [21,23–27]. The decrease of the Tg values can be explained in terms of the thin film model. When the inter-particles distance is small enough then the polymer between two particles can be considered as a thin film. Assuming that small or no interfacial interaction between the filler and matrix exists, the Tg decreases as the film thickness, i.e. inter-particles distance decreases [21]. 4. Conclusions Nanocomposite materials of poly(methyl methacrylate) with silver nanoparticles were produced using a purely in situ bulk radical polymerization technique from AgNO3, MMA monomer and benzoyl peroxide, initiator. The amount of Ag in the nanocomposites was kept low in order to keep the final product cost at low levels. The formation of silver nanoparticles with average diameter around 37–47 nm was identified using XRD and UV–Vis measurements. Somehow broader size distribution of silver nanoparticles was observed with higher amount of silver added. FTIR data showed that the inclusion of Ag NPs in the polymer matrix is rather physical without a strong chemical bond. The formation of Ag NPs during the reaction was explained based on the radical mechanism and the chain transfer reactions taking place. The latter result in the formation of small radicals which may contribute in the reduction of Ag+ to atomic silver. 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