Polymerization Kinetics of n-Butyl Methacrylate in the Presence of

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Polymerization Kinetics of n‑Butyl Methacrylate in the Presence of Graphene Oxide Prepared by Two Different Oxidation Methods with or without Functionalization Ioannis S. Tsagkalias, Symela Papadopoulou, George D. Verros, and Dimitris S. Achilias* Laboratory of Polymer and Color Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece S Supporting Information *

ABSTRACT: Nanocomposite materials based on poly(butyl methacrylate) and either graphene oxide (GO) or functionalized graphene oxide (F-GO) were produced using the in situ bulk radical polymerization technique. It was found that the Hummers method results in a higher degree of oxidation, compared to the Staudenmaier, whereas F-GO was produced using a silane-modifying agent. Polymerization kinetics were studied both experimentally and theoretically, and it was found that the presence of hydroxyl groups in the surface of GO results in scavenging the primary initiator radicals, thus reducing the initiator efficiency and the reaction rate, whereas the number-average molecular weight of the polymer formed was increased. The presence of F-GO affected the polymerization kinetics in a different way resulting in partially grafted structures. The theoretical study included the addition of a phenomenological transfer to the polymer side-reaction to account for the polymerization occurring at the F-GO surface.

1. INTRODUCTION Graphene is a one-atom-thick, two-dimensional layer of sp2bonded carbon atoms arranged in a honeycomb lattice. It is believed to be the “thinnest and strongest material known so far” and has recently attracted significant research interest because of its outstanding mechanical (Young’s modulus near 1 TPa), thermal, optical, and electrical properties.1,2 Therefore, it has been used for the preparation of graphene-reinforced polymer nanocomposites. The raw material usually used is graphite, which is oxidized to graphite oxide using concentrated mineral acids in the presence of strong oxidants. Thus, in the basal planes and edges of graphite, many oxygen-containing groups, such as carboxyl, hydroxyl, and epoxy are formed.2,3 Such functional groups result in weakening of the van der Waals forces between layers and make graphite oxide hydrophilic. Thus, the graphite oxide exhibits an interlayer spacing of 6.0−10 Å, increased compared to the original 3.4 Å of graphite. This helps in exfoliation of layered graphite oxide into graphene oxide (GO) sheets via ultrasonication or mechanical stirring.4 Brodie in 1859 used KClO3 in concentrated fuming HNO3 for the oxidation of graphitic powder. This rather upsetting method was changed by Staudenmaier 40 years later by using a higher excess of KClO3 and concentrated H2SO4. Almost 100 years later, in 1958, Hummers, in order to significantly shorten the oxidation time, proposed the use of KMnO4 and NaNO3 as oxidants in concentrated H2SO4.5,6 © XXXX American Chemical Society

Among the techniques proposed for the preparation of polymer-based nanocomposites, the in situ polymerization is among the most promising, since it ensures good dispersion of the nanoadditive in the polymer matrix and usually improved final product properties.7−10 This technique has been used in the literature to produce noncovalent graphene-based nanocomposites of several polymers mainly focusing on poly(methyl methacrylate), PMMA.11−17 Moreover, nanocomposites with enhanced thermal properties were also obtained when using alkyl functionalized GO [15]. A recent review on the fabrication and properties of polymer/graphene nanocomposites has been published by Du and Cheng.10 Although the synthesis and study of properties of PMMA based nanocomposites with GO have been extensively studied in the literature, the publications on PBMA based GO nanocomposites are rather limited.18 Polymer reaction engineering is a very important research field with a lot of papers published during the last years.19−26 One of the most active areas is the polymerization under confinement.27−31 In particular, the in situ polymerization of methacrylates in the presence of nanoadditives, such as clays, attracted considerable interest due to the enhanced end-use properties of the final polymer matrix.31−42 Depending on the Received: Revised: Accepted: Published: A

September 12, 2017 January 16, 2018 January 26, 2018 January 26, 2018 DOI: 10.1021/acs.iecr.7b03781 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

deionized water was added, while temperature was kept below 20 °C, until pH neutralization. Thereafter, the final product was obtained via drying using the freeze-drying method For the preparation of the functionalized graphite oxide (FGO), initially, 1.2 g of graphite oxide was produced either by the modified Hummers or the Staudenmaier method, as mentioned above, and was then dispersed in 30 mL of dimethylformamide (DMF) solvent. Ultrasonication, for 1 h, was applied in order to achieve the desirable dispersion of the GO in DMF. Subsequently, an excess of (3-methacryloxypropyltrimethoxysilane-MPS), a silane-modifying agent, was added to the dispersed graphite oxide solution with DMF, and the whole system was stirred magnetically for a whole day. Thereafter, the solution was allowed to dry for a long time, about 1 week, in a vacuum oven at the temperature not exceeding 70 °C. Finally, washing it with chloroform was necessary in order to remove the excess silane, which was added during synthesis.44 A schematic illustration for the preparation of functionalized-GO from GO and MPS appears in Figure 1. 2.3. Preparation of the Monomer/GO Mixtures. The monomer (n-BMA) with graphite oxide or functionalized-GO

monomer type, nanoclay type, and amount, it was found that the reaction kinetics of the in situ polymerization could be slightly retarded or accelerated. In our previous work,18,39,40,43 it was shown that several nanoadditives and particularly GO possibly act as a primary radical scavenger resulting in a reduction of the reaction rate during polymerization thus leading to a retardation of the polymerization kinetics. The scope of this study was to verify the hypothesis that surface hydroxyl groups are mainly responsible for the reduction in the polymerization rate and to examine if graphene oxide prepared by different oxidation methods has a similar effect on the polymerization kinetics. Moreover, functionalized graphene oxide (F-GO) was produced by reacting the surface hydroxyl groups of GO with a silane having methacrylate groups and added into the polymerizing mixture. The challenge in this case was to investigate the in situ polymerization in the presence of F-GO both theoretically and experimentally. In particular, F-GO could further interact with the polymerization mixture due to the surface double bonds introduced by proper modification at the surface of the graphene oxide. Graphite was used, that was oxidized to graphite oxide and then exfoliated to GO using ultrasonication and subsequent polymerization. The effect of GO on the polymerization kinetics was studied by measuring the variation of monomer conversion with time measured gravimetrically. The oxidation and functionalization of graphite was investigated by X-ray diffraction and thermogravimetric analysis and the properties of the PBMA/GO nanohybrids were measured using gel permeation chromatography and differential scanning calorimetry. This research clearly shows that the method of GO preparation has an influence on the polymerization kinetics and materials properties.

2. EXPERIMENTAL SECTION 2.1. Materials. n-Butyl methacrylate (n-BMA) with purity ≥99% was the monomer used and purchased from Alfa Aesar. The 4-methoxyphenol inhibitor was removed by passing it, at least twice, through a disposable inhibitor-remover packed column (Aldrich). Benzoyl peroxide (BPO) was used as the free radical initiator (purity >97%), provided by Alfa Aesar and purified by fractional recrystallization twice from methanol (CHEM-LAB). Dichloromethane and methanol used in the dissolution and reprecipitation of the polymer were purchased from CHEM-LAB. All other chemicals used were of analytical grade and were used as received without further purification. 2.2. Preparation of Graphite Oxide (GO) and Functionalized Graphite Oxide (F-GO). Two different techniques were employed for the oxidation of graphite powder (from Sigma-Aldrich) to produce graphite oxide (GO), namely, the Hummers and the modified Staudenmaier methods. Details for the preparation of GO using the Hummers method can be found in Tsagkalias et al.43 The following procedure was employed for the preparation of GO according to the modified Staudenmaier method. A total of 10 g of commercial graphite powder (Sigma-Aldrich) was dispersed in 250 mL of sulfuric acid in a three-neck spherical bottle, at 0 °C under intense stirring with a magnetic stirrer for 1 h. Subsequently, 110 g of potassium chlorate (KClO3) was slowly added to the suspension to keep the temperature below 5 °C. Then, the mixture was removed from the ice bath and stirred with a magnetic stirrer at room temperature for 3 days. Subsequently, a 5 wt % solution of hydrochloric acid was added and stirring continuously for 30 min. Finally, 250 mL of

Figure 1. Schematic illustration of graphene oxide reaction with MPS to produce the functionalized graphene oxide, F-GO. B

DOI: 10.1021/acs.iecr.7b03781 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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universal calibration method with Mark−Houwink constants for PBMA, K = 5.03 × 10 −5 mL/g and α = 0.758. Differential Scanning Calorimetry (DSC). The DSCDiamond (from PerkinElmer) was used for the estimation of the glass transition temperature, Tg, of the materials prepared. Approximately 5−6 mg of each sample were weighed and the temperature program followed included, initially heating to 180 °C at 10 °C min−1 (to ensure complete polymerization of the residual monomer), cooling to 30 °C and heating again to 130 °C at 20 °C min−1. Tg was estimated from the second heating.

were positioned for ultrasonication (in Transsonic 460H ultrasonic bath from Elma) for 1 h, in order to have a satisfactory dispersion of graphite oxide to the solution, with accompanying exfoliation of graphite oxide to graphene oxide. In the final suspension, the initiator BPO at a concentration of 0.03 M was added and the mixture was degassed by passing nitrogen and immediately used. The initial initiator concentration was set equal to the value previously used by our group in the polymerization of n-BMA and MMA in order to have comparable results.35−40 2.4. Synthesis of PBMA/GO and PBMA/F-GO Nanocomposites by the in Situ Bulk Radical Polymerization Technique. Three different percentages of GO or F-GO relative to monomer were employed, namely, 0.1, 0.5, and 1.0 wt %. GO and F-GO prepared by both oxidation methods, i.e., Hummers and Staudenmaier were used and were given the code names GO Hum, GO St, F-GO Hum, and F-GO St, respectively. The amount of GO added was kept in relatively low amounts, since according to the literature, GO can significantly improve the properties of a nanocomposite material even at very low loadings.10 Neat polymer was also prepared under the same experimental conditions considered as reference material. The study of polymerization kinetics was similar to that followed in previous publications by our group and one could see experimental details reported there.38,39,43 Polymerization temperature was kept constant at 80 °C, whereas the product was isolated after dissolution in CH2Cl2 and reprecipitation in MeOH. In the final samples, when polymerization was almost completed and the product was a solid (and partially grafted in the PBMA/F-GO materials), the test-tubes were broken and the content was received as such. Conversion was estimated gravimetrically. Exactly the same procedure was repeated in the n-BMA/F-GO polymerization experiments. All experiments were repeated at least twice and the variation of the data was not more than 2%. 2.5. Measurements. X-ray Diffraction. The crystalline structure of graphite, GO Hum, GO St, F-GO Hum, and F-GO St was characterized using X-ray diffraction (XRD) in a Rigaku Miniflex II instrument equipped with CuKα generator (λ = 0.1540 nm). The XRD patterns were recorded at the range 2θ = 2−40° and scan speed of 2° min−1. Thermogravimetric Analysis (TGA). The thermal stability of GO Hum, GO St, F-GO Hum, and F-GO St was measured by thermogravimetric analysis. TGA was performed on a Pyris 1 TGA (PerkinElmer) thermal analyzer. Samples of about 5−6 mg were used. They were heated from ambient temperature to 600 °C at a heating rate of 20 °C min−1 under nitrogen flow. Gel Permeation Chromatography (GPC). GPC was used to measure the molecular weight distribution (MWD) and the average molecular weights of pristine polymer and all nanocomposites. The instrument employed was from Polymer Laboratories, model PL-GPC 50 Plus, equipped with a differential refractive index detector, and three PLgel 5 μ MIXED-C columns in series. Samples were dissolved in THF at a concentration of 1 mg mL−1, and the same solvent was used for the elution at a constant flow rate of 1 mL min−1. After filtration, 200 μL of each sample was injected into the chromatograph, which was kept at a constant temperature of 30 °C. Standard poly(methyl methacrylate) samples (Polymer Laboratories) were used for the calibration of GPC (molecular weights ranging from 690 to 1 944 000 g mol−1) using the

3. THEORETICAL CALCULATIONS A typical kinetic mechanism of n-BMA free-radical homopolymerization includes the following elementary reactions: chain initiation, propagation, chain transfer to monomer, and termination either by combination or disproportionation.45 From the excellent fitting of the simulation model to the experimental data and polymerization taking place at rather low temperature, it was assumed that the depolymerization reaction was negligible.18,39 The same assumption was followed here. In order to account for the grafting reactions occurring on the FGO surface, due to the existence of double bonds from the reaction of GO with MPS, it was found convenient to include a phenomenological long chain branching reaction. The reaction mechanism used in this work is summarized in Table 1. Table 1. Kinetic Mechanism of n-BMA Free-Radical Polymerization in the Presence of a Nano-Additive Initiation kd

I → 2PR• kI

PR• + M → P1• Propagation kp

P•n + M → P•n + 1 Chain transfer to monomer k fM

P•n + M ⎯⎯⎯→ Dn + M• k pM

M• + M ⎯⎯⎯→ P1• Termination by combination or disproportionation ⎧ k tc ⎪ →Dn + m • • Pn + Pm⎨ k td ⎪ ⎯→ ⎩ ⎯ Dn + Dm Chain transfer to polymer mk fP

P•n + Dm ⎯⎯⎯⎯→ Dn + P•m where I denotes initiator, M monomer, PR• primary radicals, Pn• and Dn live radicals and polymer chains of length n, kd, kI, kp, kfM, ktc, ktd, kfP the rate constants for the initiator decomposition, chain initiation reaction, propagation, chain transfer to monomer, termination by combination, and disproportionation and chain transfer to polymer, respectively.

Following our previous work,18,39 the diffusion limitations on the propagation reaction were neglected and the diffusion limited reactions for n-BMA polymerization include termination (gel effect), residual termination, and initiation (cage effect). They were considered by combining the entropic model of Gam et al.46 with sound free volume principles47−50 according to our previous work.18,39,40 On the basis of the mechanism illustrated in Table 1, the equations describing the conservation of species in a batch isothermal reactor were derived51 (Table 2). C


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⎛k ⎞ 1 = rμk = (A′ − k tλ 0)λk + k tc ∑ ⎜ ⎟λiλk − i dt 2 i=0 ⎝ i ⎠ μ dV + k tdλk λ 0 + k fPλ 0μk + 1 − k ; k = 0, 1, 2 V dt

Table 2. Species Mass Balance Equations in an Isothermal Batch Reactor

d(μk )

Initiator

1 d(V I) = − kdI V dt Monomer−Fractional Monomer Conversion (X)

A′ = k fMM + k tλ 0 + k fPμ1

1 d(V M) = − (k p + k fM)MP0 V dt Macromolecular Species Balance

To break down the dependence on the k + 1 moment of the above equation due to transfer to polymer reaction, one has to resort to closure methods. In this work, the closure method of Lee and Marano53 was adopted. This closure method simply requires μn ≈ μn + λn or in other words μn ≫ λn which holds true. Thus, the rate equation for λ2, which depends on μ3 is not needed. By using this closure method the live radical and the dead polymer moment equations are written as

1 d(V PR•) = 2fkdI − kI PR•M = 0 V dt 1 d(V Pn) = (kI PR•M)δ(n − 1) + k p M(Pn − 1 − Pn) + k fPn DnP0 V dt ∞ ⎛ ⎞ − ⎜⎜k fM M + k tP0 + k fP ∑ r Dr ⎟⎟Pn ⎝ ⎠ r=0

⎡ k ⎛k ⎞ ⎤ d(λk ) • = rλk = kI PR M + k p M⎢∑ ⎜ ⎟λi − λk ⎥ − A′λk ⎢⎣ i = 0 ⎝ i ⎠ ⎥⎦ dt

=0 ∞ n−1 ⎞ 1 d(V Dn) ⎛⎜ 1 = ⎜k fM M + k fP ∑ r Dr ⎟⎟Pn + k tc ∑ Pr Pn − r + k tdP0Pn V dt 2 r=1 ⎝ ⎠ r=0

+ k fPλ 0μk + 1 −

− k fPn DnP0 where P0 is the total concentration of “live” polymer:

∑ Pr

∞

∑ nk Pn ; n=0

∑ nk Dn ;

X̅ n =

k = 0, 1, 2

n=0

X̅ w =

μ2 + λ 2 μ1 + λ1

≅

μ2 μ1 + λ1

X̅ w X̅ n

;

(8)

(9)

where ε is the volume contraction factor given as a function of the monomer, ρm, and polymer, ρp, density as ρp − ρm ε= ρp (10)

(3)

Live radical moment equations:

Then

⎡ ⎛k ⎞ ⎤ d(λk ) = rλk = kI PR•M + k p M⎢∑ ⎜ ⎟λi − λk ⎥ − A′λk ⎢⎣ i = 0 ⎝ i ⎠ ⎥⎦ dt

⎡ dX dV dε ⎤ dX = −V0⎢ε + X ⎥ = −V0ε ⎣ ⎦ dt dt dt dt −ε(dX /dt ) 1 dV = V dt 1 − εX

k

λk dV ; V dt

μ0 + λ 0

;

V = V0(1 − εX )

Fractional Monomer Conversion (X)

+ k fPλ 0μk + 1 −

(7)

Number and weight-average molecular weight of the final polymer are simply calculated by multiplying the number or weight-average chain length by the monomer molecular weight. To account for the volume contraction which is significant during bulk polymerization, the variation of the reaction volume with conversion was further considered:

(2)

d(X ) = (k p + k fM)(1 − X )λ 0 dt

μ1 + λ1

D=

(1)

On the basis of the species balance equations and the method of moments, one could derive the following set of differential equations for an isothermal batch reactor:51 Initiator d(I) I dV = −kdI − dt V dt

k = 0, 1, 2

From the above set of equations the variation with time of the initiator concentration, monomer conversion and averages of the MWD can be obtained. The number and the weightaverage degree of polymerization (X̅ n, X̅ w) as well as the polydispersity, D, of the MWD are given as follows:

∞

μk =

(6)

⎛k ⎞ 1 k tc ∑ ⎜ ⎟λiλk − i 2 i=0 ⎝ i ⎠

⎡ k ⎛k ⎞ ⎤ μ dV + k p M⎢∑ ⎜ ⎟λi − λk ⎥ − k ; ⎝ ⎠ ⎢⎣ i = 0 i V dt ⎦⎥

The next step is to recast the infinite number of macromolecular chain population balance equations into a finite set of modeling equations. This could be achieved by using the well-known method of moments of the chain length distribution (CLD) or molecular weight distribution (MWD). Moments of the CLD of the “live” radicals or “dead” polymer macromolecules are defined as52 λk =

k = 0, 1

= rμk = kI PR•M − k tcλ 0λk +

dt

r=0

and δ(n) is the Kronecker delta

λk dV ; V dt

k

d(μk )

∞

P0 =

(5)

k = 0, 1, 2

(4)

and

(11)

In order to calculate the complete molecular weight distribution, the Quasi Steady State Approximation (QSSA)

Dead polymer moment equations: D


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Industrial & Engineering Chemistry Research was applied to the “live” radical population balance equation and the equations summarized in Table 2 were numerically solved using a variable step 4th order Runge−Kutta algorithm. A complete description of this method is given in full detail elsewhere.54,55

with a higher degree of oxidation as it possesses the lower C/O ratio among the graphite oxides. This was further verified here by performing TGA measurements on the GO prepared by both methods and the data are illustrated in Figure 3. The first, almost 10% mass loss

4. RESULTS AND DISCUSSION 4.1. Characterization of GO and F-GO. In order to identify the degree of oxidation of graphite to GO obtained from the two methods as well as the functionalization of graphite oxide, XRD and TGA measurements were carried for the starting material, graphite, the two graphite oxides, GO Hum and GO St, and the two functionalized graphite oxides, FGO Hum and F-GO St. Oxygen embedded between graphene layers causes an increase in the interlayer distance from 0.34 nm in graphite to about 0.7 nm in graphite oxide. Consequently characteristic graphitic signal shift from 26° to 10−15° was observed in the literature.5,56 Figure 2 presents the XRD patterns for all these materials. Graphite shows a sharp peak at 26.5°, whereas in both graphite Figure 3. TGA thermograms of GO and F-GO prepared by the Hummers and Staudenmaier methods.

(approximately at 100 °C), is due to water solvent molecules absorbed into the GO bulk material. The following, almost 26%, mass loss, starting at 225 and ending at 269 °C, stands for the GO decarboxylation process, due to the thermal degradation of labile oxygen-containing functional groups, such as −COOH, −OH, etc. Therefore, the mass loss observed in this region is attributed to the elimination of labile functional groups from the surface of GO. This mass loss was smaller in the GO prepared by the Staudenmaier method due to the lower oxidation of GO. Further decomposition takes place above 550 °C. Furthermore, TGA scans of both functionalized F-GO do not show any initial mass loss near 100 °C, so that it seems that they do not have any absorbed water molecules. Moreover, the mass loss in the region 220 to 270 °C was negligible for F-GO St and very small (nearly 2%) for F-GO Hum. This means that during functionalization almost all oxygen-containing functional groups in the surface of GO have reacted with MPS. This was also verified from the XRD patterns, where F-GO St do not show any peak at 11°, whereas a small one appears at F-GO Hum shifted to 10.1°. In addition, a new peak appears at 5.6 and 5.9° for F-GO Hum and F-GO St, respectively, due to the functionalization of the GO surface. It seems that F-GO prepared using the Hummers method exhibits a larger distance between graphene layers at 1.55 nm followed by F-GO prepared by the Staudenmaier method where the interlayer distance is estimated at 1.47 nm. In conclusion, it seems that GO prepared by the Staudenmaier method results in a lesser oxidized GO surface though completely functionalized, whereas GO prepared by the Hummers method results in a higher degree of oxidation, though not completely functionalized with some −COOH or −OH groups still existing in the GO surface. When GO was included into the polymer matrix, the same spectrum was recorded for all nanocomposites without any clear peak at 11 or 5.6°. This ensures that graphite oxide has been exfoliated into graphene oxide during the reaction.

Figure 2. X-ray diffraction patterns of graphite, GO, and F-GO by the Hummers and Staudenmaier methods.

oxides this peak disappeared and a newer one located close to 11° appeared. Thus, the interlayer distance between graphene layers increases from 0.33 nm in graphite to almost 0.80 nm in GO. Similar distances ranging from 0.73 to 0.81 have been reported in the literature.5,56 In the XRD diffraction spectrum, the presence of the (002) diffraction line is an indication of the degree of oxidation of the graphite oxides where an absence of the (002) line at 26.5° implies a complete oxidation of graphite. As observed in Figure 2, the (002) diffraction line that was observed for graphite completely disappeared for GO-St and GO-Hum. This is an indication that graphite has completely oxidized to graphite oxide in both methods of oxidation employed. Using the Staudenmaier method, a much lower peak in the same degrees was observed. Using the Scherrer formula, it was calculated that the crystallites height (Lc) decreased after oxidation from 27 nm in graphite to 11 and 3.7 nm for GOHum and GO-St, respectively. These XRD observations are consistent with the corresponding literature data, where also from an elemental analysis it was found that the C/O ratio of GO prepared from either the Hummers or Staudenmaier methods were 1.12 and 2.52, respectively.56 This is an indication that the Hummers method produces graphite oxides E


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Figure 4. Conversion versus time curves of neat PBMA and PBMA/GO Hum (a), PBMA/GO St (b), PBMA/F-GO Hum (c), and F-GO St (d) nanocomposites with different relative amounts of GO.

4.2. Polymerization Kinetics. The effect of adding GO prepared by the Hummers or Staudenmaier methods as well as of the functionalized F-GO Hum and F-GO St on the PBMA polymerization kinetics is illustrated in Figure 4a−d for neat PBMA and its nanocomposites with 0.1, 0.5, and 1.0 wt % GO. Conversion−time curves follow the typical behavior of radical polymerization kinetics affected by diffusion-controlled phenomena, which have been extensively studied by our group in former publications.55,57 From Figure 4a it is observed that the addition of GO prepared according to the Hummers method results in reduced polymerization rates in correspondence to the amount of GO added. This has been also observed in the literature in other nanoadditives added to PMMA as well as in our previous publication during PBMA polymerization.28,18 This reduced reaction rate has been attributed to the reaction of primary radicals formed from the fragmentation of the initiator with the surface functional groups of GO and mainly the hydroxyl groups, −OH. A similar effect, though not so intense, was also observed when GO prepared with the Staudenmaier method was added (Figure 4b). This can be attributed to the lower oxidation of GO prepared using this method and therefore of the lower number of surface functional groups. It should be noted here that according to TGA thermograms shown in Figure 3, almost 10% of water seems to be adsorbed to the GO surface at the polymerization temperature (i.e., 80 °C). This amount of water is unlikely to be responsible for the reduction in the reaction rate observed for the following reasons: A reduction in the reaction rate was observed in the function-

alized-GO experiments also, whereas no water was adsorbed in the surface of F-GO (Figure 3). Furthermore, from TGA scans of the final polymer nanocomposites (not shown here) reduction of the material mass at such low temperatures was not observed. Finally, all GOs and F-GOs were left in an oven at 100 °C for 30 min before their use in the polymerization experiments. Furthermore, when the functionalized F-GO was used (Figure 4c), a strange phenomenon was observed, where the reaction rate initially significantly decreased, though as the amount of F-GO was increased it also increased and tended to be similar to neat PBMA. This phenomenon is explained in the next section using model simulations. When F-GO St was used (Figure 4d), results were not greatly affected by the amount of GO which was added. Regarding the experimental data presented in our previous work18 it is shown in this work that even small alterations in the experimental procedure have a moderate effect on polymerization kinetics. The average molecular weights of all nanocomposites prepared were measured with GPC and presented next in Table 3. In addition, the full MWD of neat PBMA and all nanocomposites are included in Figure S1. It was observed that the number-average molecular weight, Mn, increases with increased amounts of GO added prepared by either method. This was reflected to a shift of the MWD to higher values. Moreover, when using the functionalized GO it was found that the average molecular weights of the polymer were only slightly affected and particularly decreased compared to neat polymer. Again, as it can be seen in Figure S1, the full MWD shifted to lower values with an increasing amount of F-GO. This could be F


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Industrial & Engineering Chemistry Research Table 3. Averages of the MWD (M̅ n, M̅ w, and Polydispersity) and Glass Transition Temperature (Tg) of neat PBMA and Its Nanocomposites with Various Amounts of GO and F-GO Prepared by the Hummers and Staudenmaier Methods sample PBMA PBMA PBMA PBMA PBMA PBMA PBMA PBMA PBMA PBMA PBMA PBMA PBMA

pure + 0.1% + 0.5% + 1.0% + 0.1% + 0.5% + 1.0% + 0.1% + 0.5% + 1.0% + 0.1% + 0.5% + 1.0%

GO Hum GO Hum GO Hum GO Staud GO Staud GO Staud F-GO Hum F-GO Hum F-GO Hum F-GO Staud F-GO Staud F-GO Staud

M̅ n

M̅ w

PD

Tg (°C)

280540 326450 381560 478960 311350 315300 450400 262200 249430 226480 259860 235370 214260

620910 579410 698480 962450 604930 719730 936340 523720 521450 496390 615590 503040 435930

2.21 1.77 1.83 2.01 1.94 2.28 2.08 2.00 2.09 2.19 2.37 2.13 2.03

32.3 33.2 37.0 37.6 34.6 37.6 38.8 36.0 40.7 45.5 36.1 42.8 47.8

values, such as 0.03364 or 0.07670 L/mol/s, were reported in the literature. The bulk in situ polymerization of butyl methacrylate in the presence of GO nanoadditive was studied in our previous work.18 This work focuses on the theoretical study of the butyl methacrylate radical polymerization in the presence of functionalized GO (F-GO). The model equations for the diffusion controlled reactions are summarized below in eqs 14 − 18. Diffusion Controlled Limitations for Termination Reactions A. Gel Effect: 1 1 M̅ 2 ; = + k te k t0 Dp00 exp( −b/Vf )

(14)

B. Residual Termination: k t = k te + k t,reac ;

k t,reac = Ak pM

(15)

Diffusion Controlled Limitations for Initiation Reaction (Cage Effect) 1 1 C 1 C = + = + f f0 DI f0 DI0 exp( −b/Vf )

attributed to the methacrylate groups incorporated into the GO surface through the functionalization process and the particular modifying agent used (MPS). Thus, it seems that some of the monomer reacts with these groups, resulting in decreased available molecules to find a macroradical and produce macromolecular chains. Moreover, these results can be attributed on the basis that, using the functionalized GO, with more than one surface vinyl groups, it may act as a surface for grafting of macromolecules and result to partially grafted materials. This phenomenon does not directly affect the conversion vs time experimental data, since, as it was reported in the experimental part, at high degrees of conversion, the tubes were broken and the material was received as it was. However, it is expected that it may slightly affect the average molecular weight data, since solubilization of the material in THF, in order to perform the GPC experiments, result in measuring the MWD of only the soluble part and not of the partially grafted. Indeed some insoluble material was visual in the samples prepared for GPC measurements, when the amount of F-GO added was high. 4.3. Simulation Results. n-BMA polymerization kinetics is extensively studied in the literature.58−66 The value for initiator decomposition kinetic rate constant was adopted from a very interesting and comprehensive work carried out some years ago by Stickler:65 18330 ± 140 ln[kd (s−1)] = 39.90 ± 0.38 − T (K) (12)

(16)

Free Volume Parameters Vf = 0.025 + 0.00048(T − Tg)

(17)

⎛ 1 1 1 1 ⎞⎟ = + X ⎜⎜ − Tg Tgm Tgm ⎟⎠ ⎝ Tgp

(18)

In the above equations, the termination rate constant, kt is taken as the sum of the diffusion-controlled termination rate constant, kte, and the reaction-diffusion controlled term, kt,reac. A is a constant. The later term, kt,reac also known as residual termination, accounts for the implicit movement of the macroradicals in space through the propagation reaction even when these macroradicals are “frozen” and cannot move through the center-of-mass diffusion. kt0 is the kinetic rate constant for termination in the absence of diffusional limitations, and the physical quantity M̅ stands for the number-average degree of polymerization of the macroradicals as defined in terms of the leading moments of the chain length distribution of free radicals, Dp00 is an overall adjustable parameter for the termination rate constant, respectively. Tgm and Tgp are the glass transition temperatures of the monomer and polymer set equal to 150 and 303 K, respectively.27−29,36 Finally, the parameter, b, was set equal to b0X with b0 equal to 1, according to the literature.27−29 X stands for monomer conversion. The value for residual termination parameter A was set equal to 6.19 from ref 18, and the cage effect parameter (Di0/C) was set equal to 3.45 × 105 based on ref 18. To account for the formation of graft structures due to polymerization at surface double bonds introduced by the modification of the GO surface, two different strategies (models I and II) were applied. The adjustable parameters of the model I include the initial initiator efficiency (f 0), the gel effect parameter (Dp00), and the transfer to monomer kinetic rate constant (kfm). In this case the transfer to polymer kinetic rate was set equal to zero. It should be pointed here that the selection of these three parameters reflects the effect of the chemical reactions taking place (primary radical termination, f 0), the diffusion phenomena (Dp00), and the existence of additional vinyl groups in the

The IUPAC benchmark data set for the propagation rate constant of n-BMA radical polymerization was adopted:58 kp = 106.58 exp( −22.9 kJ mol−1/RT ) dm 3 mol−1 s−1

k t0 = k tc + k td

(13)

18

Following our previous work, the termination kinetic rate constant in the absence of diffusional limitations, kt0, was set equal to 1.2 × 109 × e−1770/T dm3 mol−1 s−1. It should be stressed here that all purely kinetically controlled parameters (i.e., kp0, kt0, and kd) should be evaluated by experimental data at low conversion values.67−69 The thermophysical properties as well as all the other kinetic rate constants used in this work were adopted from the work of Hutchinson et al.62 However, it was found convenient to introduce the transfer to monomer kinetic rate constant as an adjustable parameter since different G


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Industrial & Engineering Chemistry Research Table 4. Estimated Parameters (Polymerization Temperature, 80 °C; [I0] = 0.03 M) parameter

a

0.1% (w/w) F-GO

0.5% (w/w) F-GO

F-GO Staudenmaier (Model I) f0 Dp00 × 10−15a kfma

1.0% (w/w) F-GO

0.52 ± 0.04 0.565 ± 0.071 0.74 ± 0.06

0.7 ± 0.06 1.095 ± 0.11 0.78 ± 0.05

0.8 ± 0.05 1.24 ± 0.12 0.86 ± 0.06

F-GO Staudenmaier (Model II) f0 Dp00 × 10−15a a kfm kfpa

0.35 ± 0.04 0.857 ± 0.081 0.76 ± 0.05 0.131 ± 0.01

0.39 ± 0.04 0.5 ± 0.06 0.85 ± 0.06 0.04 ± 0.004

0.6 ± 0.05 0.53 ± 0.05 0.9 ± 0.07 1.1 × 10−4 ± 0.021 × 10−4

F-GO Hummers (Model I) f0 Dp00 × 10−15a kfma

0.345 ± 0.05 0.672 ± 0.045 0.745 ± 0.03

0.39 ± 0.06 0.527 ± 0.034 0.788 ± 0.05

0.6 ± 0.05 0.582 ± 0.04 0.86 ± 0.06

In dm3 mol−1 s−1.

reacting mixture from the functionalization of GO (kfm). The fitted data for model I includes the monomer conversion data and the number-average molecular weight at the end of polymerization. Standard methods of nonlinear regression analysis were used.71 The estimated values for adjustable parameters are given in Table 4. A fairly good fitting for conversion data is illustrated in Figures 5 and 6. The predicted values for the number-average

Figure 6. Comparison of the model I results to the experimental data for nanocomposites with various amounts of F-GO (Hummers method). Polymerization at 80 °C with [I]0 = 0.03 M.

satisfactory agreement with theoretical predictions (model I). However, the application of model I leads to a discrepancy between the experimental and the predicted weight-average molecular weight data of the final matrix as prepared by the Staudenmaier method. This discrepancy is directly corrected by adding as an adjustable parameter the transfer to the polymer kinetic rate constant and by further including the weightaverage molecular weight in the fitted data (model II). The NAMW and WAMW thus obtained for the nanocomposites with 0.1, 0.5, and 1.0 wt % nanofiller were 2.57 × 105, 2.34 × 105, and 2.14 × 105 and 6.29 × 105, 5.11 × 105, and 4.3 × 105, respectively. A slightly better fitting than the one depicted in Figure 6 for conversion was obtained (not shown). The introduction of kfp as a fitting parameter was mainly to account for the grafting reactions taking place on the vinyl groups in the F-GO surface. The values estimated for kfm here were larger compared to the literature values (i.e., 0.076 in ref 70 or 0.033 in ref 64). The reason could be that this parameter cannot be directly measured independently and is usually estimated from fitting to experimental data.

Figure 5. Comparison of the simulation model results to the experimental data for nanocomposites with various amounts of GO (F-GO, Staudenmaier method). Polymerization at 80 °C with [I]0 = 0.03 M, model I.

molecular weight (NAMW) and the weight-average molecular weight (WAMW) using model I for the PBMA/F-GO nanocomposites prepared by the Staudenmaier’s method with 0.1, 0.5, and 1.0 wt % nanofiller were 2.6 × 105, 2.35 × 105, 2.14 × 105 and 5.21 × 105, 4.73 × 105, 4.32 × 105, respectively. The corresponding values using model I for the PBMA/F-GO nanocomposites prepared by the Hummers method with 0.1, 0.5, and 1.0 wt % nanofiller were 2.63 × 105, 2.49 × 105, 2.25 × 105 and 5.29 × 105, 5.01 × 105, 4.94 × 105, respectively. Although the experimental weight-average molecular weight data for the F-GO prepared according to Hummers method was not used in the parameter estimation procedure, it is in H


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Industrial & Engineering Chemistry Research In Table 4, the estimated values for the adjustable parameters of model II are also summarized. The estimated values for initiator efficiency (f 0), gel effect parameter (Dp00), and transfer to monomer kinetic rate constant (kfm) by using model II and fitting experimental data obtained by the Staudenmaier method are very close to the estimated values by using model I and fitting experimental data obtained by the Hummer method. Moreover, as shown in Table 4, the estimated values for initiator efficiency (f 0) are lower than unity in all cases studied, meaning that F-GO still plays the role of radical scavenger. Interestingly enough, the best-fit value at 1 wt % F-GO (i.e., 0.6) was similar to that estimated at 1 wt % GO (i.e., 0.63) which was presented in our previous publication.18 However, the trend in the values estimated with an increasing amount of graphene oxide was different, meaning that f 0 was decreased in GO, whereas it increases in F-GO. This could be probably associated with F-GO acting as a promoter of the polymerization reaction due to the introduction of functional groups containing double bonds at its surface. There is also in Table 4 a clear tendency for the estimated value of transfer to the polymer kinetic rate constant to decrease as the concentration of nanoadditive increases. This behavior could be attributed to the introduced steric hindrance effects introduced by increasing the nanoadditive concentration and thus leading to the enhancement of polymerization at the nanoadditive surface. Moreover, increasing the F-GO amount results in more vinyl groups available for grafting reactions and therefore to a more insoluble polymer fraction. Concerning the variation of the diffusivity coefficient, Dp00, which seems to decrease with an increasing amount of F-GO, the physical justification is that the presence of larger amounts of F-GO and possibly grafted structures results in slower diffusion of the macroradicals to find one another and to react. Finally, the incorporation of kfm as an adjustable parameter and its increase with the amount of F-GO added is justified if we follow the chain transfer to monomer reaction. Accordingly, usually a hydrogen atom is abstracted from the monomer molecule, which reacts with the radical-end in the macroradical, resulting in deactivated polymer molecules and a new radical starting from the monomer molecule. In our case, addition of F-GO in the system results in adding more vinyl groups in the reacting mixture, which potentially could lose more Hydrogen atoms in chain transfer reactions, resulting also in synthesis of soluble macromolecules having lower molecular weights (verification of the predictions of GPC). Therefore, chain transfer to monomer is expected to increase with the amount of F-GO added. A simple question arises for model II: is this model able to capture the entire molecular weight distribution (MWD) of the produced matrix? The answer to this question is given in Figure 7. In this figure an excellent agreement is depicted between experimental data and theoretical predictions by using model II. In this Figure, the theoretical MWD is shown for both branched polymer (transfer to polymer reaction included) and linear polymer (no transfer to polymer). It can be seen that inclusion of chain transfer to polymer reactions shifts the right part of the curve to higher molecular weights. Please notice that there is a strong effect of the preparation method on both conversion data and final product average molecular weights throughout this work. This effect could be attributed to the alteration due to a different method of preparation of the functional group number leading not only to

Figure 7. Polymer molecular weight distribution (model II) compared with experimental data. Polymerization at 80 °C with [I]0 = 0.03 M, 0.1% (w/w) F-GO Staudenmaier. Simulation results by using the chain transfer to polymer reaction are also included.

different dispersion of the nanoadditive, but also to different polymerization conditions at its surface. The results shown in this section further validate the applied procedures and the assumptions made in this work. 4.4. Glass Transition Temperature of the Nanocomposites. Finally, the Tg of neat PBMA and the nanocomposites was determined using DSC, according to the half Cp extrapolation method. All the Tg values are given in Table 3. The value measured for pristine PBMA (i.e., near 32 ◦C) is close to that reported in the literature, usually in the range of 30−40 °C.39 Compared to neat PBMA, nanocomposites with 0.5 or 1.0 wt % graphene oxide presented increased Tg, with higher values measured at increasing GO content. Furthermore, the addition of functionalized GO resulted in even higher Tg values. Tg is a macroscopic indication of the segmental relaxation behavior of nanocomposite systems, strongly dependent on embedded nanoparticles. The interaction of polymer chains and nanoparticles surface results in enhancement of the Tg of the nanocomposites, attributed to the restriction in chain mobility due to the confinement effect of the 2D-layered graphene incorporated into the matrix and the strong nanofiller−polymer interactions, as has also been observed and explained in ref 43.

5. CONCLUSIONS In this work, the in situ bulk radical polymerization of n-BMA in the presence of GO and functionalized graphene oxide produced by two different oxidation methods, namely, Hummers and Staudenmaier was studied both experimentally and theoretically. XRD and TGA data verified the functionalization of GO and showed that a complete oxidation of graphite to graphite oxide occurred during both methods of preparation. Moreover, the Hummers method produces graphite oxides with a higher degree of oxidation, whereas GO prepared by the Staudenmaier method results in less oxidation. During functionalization, almost all oxygen-containing functional groups in the surface of GO reacted with MPS. From polymerization kinetics measurements, it was found that the presence of hydroxyl groups in the surface of GO results in scavenging the primary radicals formed by the fragmentation of the initiator, thus reducing the initiator I


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containing graphene sheets using microwave irradiation. Molecules 2013, 18, 3152. (8) Yuan, X. Y.; Zou, L. L.; Liao, C. C.; Dai, J. W. Improved properties of chemically modified graphene/poly(methyl methacrylate) nanocomposites via a facile in-situ bulk polymerization. eXPRESS Polym. Lett. 2012, 6, 847. (9) Kuila, T.; Bose, S.; Hong, C. E.; Uddin, M. E.; Khanra, P.; Kim, N. H.; Lee, J. H. Preparation of functionalized graphene/linear low density polyethylene composites by a solution mixing method. Carbon 2011, 49, 1033−1037. (10) Du, J.; Cheng, H.-M. The Fabrication, Properties, and Uses of Graphene/Polymer Composites. Macromol. Chem. Phys. 2012, 213, 1060−1077. (11) Potts, J. R.; Lee, S. H.; Alam, T. M.; An, J.; Stoller, M. D.; Piner, R. D.; Ruoff, R. S. Thermomechanical properties of chemically modified graphene/poly(methyl methacrylate) composites made by in situ polymerization. Carbon 2011, 49, 2615−2623. (12) Heo, S.; Cho, S. Y.; Kim, D. H.; Choi, Y.; Park, H. H.; Jin, H.-J. Improved thermal properties of graphene oxide-incorporated poly(methyl methacrylate) microspheres. J. Nanosci. Nanotechnol. 2012, 12, 5990−4. (13) Tripathi, S. N.; Saini, P.; Gupta, D.; Choudhary, V. Electrical and mechanical properties of PMMA/reduced graphene oxide nanocomposites prepared via in situ polymerization. J. Mater. Sci. 2013, 48, 6223. (14) Wang, J.; Hu, H.; Wang, X.; Xu, C.; Zhang, M.; Shang, X. Preparation and mechanical and electrical properties of graphenenanosheets-poly(methyl methacrylate) nanocomposites via in situ suspension polymerization. J. Appl. Polym. Sci. 2011, 122, 1866−1871. (15) Wang, Y.; Liao, X.; Li, S.; Luo, Y.; Yang, Q.; Li, G. Poly(methyl methacrylate) nanocomposites based on graphene oxide: a comparative investigation of the effects of surface chemistry on properties and foaming behavior. Polym. Int. 2016, 65, 1195−1203. (16) Feng, L.; Guan, G.; Li, C.; Zhang, D.; Xiao, Y.; Zheng, L.; Zhu, W. In situ synthesis of poly(methyl methacrylate)/graphene oxide nanocomposites using thermal-initiated and graphene oxide initiated polymerization. J. Macromol. Sci., Part A: Pure Appl.Chem. 2013, 50, 720−727. (17) Morimune, S.; Nishino, T.; Goto, T. Ecological approach to graphene oxide reinforced poly(methyl methacrylate) nanocomposites. ACS Appl. Mater. Interfaces 2012, 4, 3596−3601. (18) Michailidis, M.; Verros, G. D.; Deliyanni, E. A.; Andriotis, E. G.; Achilias, D. S. An experimental and theoretical study of butyl methacrylate in situ radical polymerization kinetics in the presence of graphene oxide nanoadditive. J. Polym. Sci., Part A: Polym. Chem. 2017, 55 (8), 1433. (19) Delgadillo-Velázquez, O.; Vivaldo-Lima, E.; Quintero-Ortega, I. A.; Zhu, S. Effects of diffusion-controlled reactions on atom-transfer radical polymerization. AIChE J. 2002, 48, 2597. (20) D’hooge, D. R.; Reyniers, M.-F.; Marin, G. B. Methodology for kinetic modeling of atom transfer radical polymerization. Macromol. React. Eng. 2009, 3, 185. (21) D’hooge, D. R.; Reyniers, M.-F.; Marin, G. B. The crucial role of diffusional limitations in controlled radical polymerization. Macromol. React. Eng. 2013, 7, 362. (22) Wang, D.; Li, X.; Wang, W.-J.; Gong, X.; Li, B.-G.; Zhu, S. Kinetics and modeling of semi-batch RAFT copolymerization with hyperbranching. Macromolecules 2012, 45, 28−38. (23) D’hooge, D. R.; Van Steenberge, P. H. M.; Reyniers, M.-F.; Marin, G. B. The Strength of Multi-Scale Modeling to Unveil the Complexity of Radical Polymerization. Prog. Polym. Sci. 2016, 58, 59− 89. (24) Van Steenberge, P. H. M.; D’hooge, D. R.; Reyniers, M.-F.; Marin, G. B. Improved kinetic Monte Carlo simulation of chemical composition-chain length distributions in polymerization processes. Chem. Eng. Sci. 2014, 110, 185−199. (25) Van Steenberge, P. H. M.; Vandenbergh, J.; D’hooge, D. R.; Reyniers, M.-F.; Adriaensens, P. J.; Lutsen, L.; Vanderzande, D. J. M.; Marin, G. B. Kinetic Monte Carlo Modeling of the Sulfinyl Precursor

efficiency and as a result the reaction rate. The NAMW and the polydispersity of the MWD of the polymer formed increased. This observation was more pronounced in the GO Hum compared to GO St and was attributed to its higher degree of oxidation. The presence of F-GO affected the polymerization kinetics in a different way, since most of surface hydroxyl groups in GO reacted with the silane coupling agent used to functionalize the GO. The theoretical study includes the addition of a phenomenological transfer to polymer reaction to account for the polymerization reaction occurring at the nanoadditive surface. It was shown that under the proper modification (F-GO Hum), graphene oxide can additionally act as a promoter of the polymerization reaction. Finally, nanocomposite products with enhanced thermal properties, such as stability and glass transition temperature, were produced for F-GO prepared by both methods. The model predictions including MWD data were found in acceptable agreement with the corresponding experimental data.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03781. Full molecular weight distribution of neat PBMA and its nanocomposites with 0.1, 0.5, and 1.0 wt % GO prepared by the Hummers method (a), GO prepared by the Staudenmaier method (b), F-GO prepared by the Hummers method (c), and F-GO prepared by the Staudenmaier method (d) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dimitris S. Achilias: 0000-0003-2872-8426 Author Contributions

The manuscript was written through contributions of all authors. All authors have given their approval for the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Ms. Terry MacCallum for her help in the preparation of the manuscript REFERENCES

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