Kinetics of Methyl Methacrylate Grafting Polymerizationonto Flaky Aluminum Powder

Hui Liu,1,2 Rui Sun,3 Jianhong Yi,2 Hongqi Ye,1 Xinde Tang,1 Youfeng Li1

1College of Chemistry and Chemical Engineering, Central South University, Changsha 410083,People’s Republic of China2State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083,People’s Republic of China3Lvcui Middle School, Haizhu District, Guangzhou 510300, People’s Republic of China

Received 28 February 2009; accepted 25 July 2009DOI 10.1002/app.31212Published online 30 October 2009 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: With ammonium persulfate (APS) as the ini-tiator, the kinetics of methyl methacrylate (MMA) graftingpolymerization onto flaky aluminum powder (Al) was stud-ied. It was found that the experimental apparent graftingpolymerization rate, Rg ¼ KC0:521

Al � C0:429APS � C0:978

MMA, was basi-cally consistent with the theoretical result based on thetheory of stable polymerization and equivalent activity, Rg

¼ KC0:5Al � C0:5

APS � CMMA. The activation energy of grafting,

homogenous, and total polymerization rate was calculatedas 65.1, 35.4, and 37.5 kJ mol�1, respectively. It could be vali-dated that the relationship among these activation energiesaccorded with the theoretical result of parallel reactions.VC 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 3040–3044, 2010

Key words: kinetics; methyl methacrylate; graftingpolymerization; flaky aluminum powder

INTRODUCTION

In our previous report,1 poly(methyl methacrylate)/flaky aluminum composite particle (called PMMA/Al) had been successfully prepared by in situ graft-ing polymerization in the presence of 3-methacrylox-ypropyltrimethoxysilane (MPS). The process was di-vided into two steps: flaky aluminum (Al) powderwas first coupled with MPS, and then encapsulatedwith PMMA by in situ emulsion grafting polymer-ization of methyl methacrylate (MMA). The mecha-nism of MPS coupling onto flaky Al was studied,and the effect of coupling percentage and emulsifierconcentration on in situ polymerization was alsoinvestigated. The resulting PMMA/Al compositeparticle had been characterized by means of Fouriertransformer (FTIR) spectroscopy, energy dispersivespectroscopic (EDS) analysis, and the evolved hydro-gen detection.

It was noticeable that the kinetics of MMA graft-ing polymerization onto flaky Al had not been stud-ied. Nevertheless, a tremendous research activityhad been devoted to the kinetics of the homogenouspolymerization of MMA. Murthy and coworkers2,3

had studied the kinetics of poly(styrene peroxide)initiated photopolymerization of MMA, and theresulting polymer was characterized by means ofultraviolet spectrum (UV), hydrogen nuclear mag-netic resonance (H-NMR), and thermogravimetricanalysis (TGA). Sebenik et al.4 investigated the effectof soft segment length on kinetics of seeded semi-batch-emulsion copolymerization of MMA and butylacrylate. Suzuki et al.5 studied the effect of mixingratio of anionic emulsifiers on the kinetic behavior ofMMA emulsion polymerization.In accordance, some research works about the

kinetics of organic monomer grafting polymerizationonto superfine particles had been graduallyreported. The influence of different surface func-tional groups of nano-SiO2 on nylon-6 in situ poly-merization was investigated, and the kinetics of thegrafting polymerization was systematically studied.6

The kinetics of in situ polymerization of isooctyl ac-rylate and butyl acrylate on cassava starch was alsoinvestigated.7 The kinetics of acrylic acid graftingpolymerization onto maize starch using ammoniumpersulfate-sodium bisulfate as initiator was studied.8

Chern et al.9 investigated the kinetics of atom trans-fer radical polymerization (ATRP) of styrene in thepresence of montmorillonite, and the effect of thepristine sodium montmorillonite on the styrene

Correspondence to: H. Liu (liuhui@mail.csu.edu.cn).Contract grant sponsor: China Postdoctoral Science

Foundation; contract grant number: 20080440987.Contract grant sponsor: Postdoctoral Science Foundation

of Central South University; contract grant number: 1332-74341015109.

Contract grant sponsor: Opening Foundation of StateKey Laboratory of Powder Metallurgy, Central SouthUniversity.

Journal ofAppliedPolymerScience,Vol. 115, 3040–3044 (2010)VC 2009 Wiley Periodicals, Inc.

polymerizations with different concentrations of so-dium dodecyl sulfate (SDS) was studied.

Based on the earlier discussions, it was well-known that there were few reports about the kineticsof MMA grafting polymerization onto flaky Al pow-der. Following our previous report,1 we studied thekinetics of MMA grafting polymerization onto flakyAl powder in this work. The goal of this work was,therefore, to gain a better understanding of the pro-cess for the formation of PMMA/flaky aluminumcomposite particles.

EXPERIMENTAL

Materials

Flaky Al powder (Shanghai Weiye Co., China, D50 ¼15 lm, aspect ratio is about 100–200) was dried in vac-uum at 100�C for 24 h to remove the volatile organiccompounds before use. MMA (Guangzhou Chemicalfactory, China) was washed with dilute alkali solutionand distilled water, dried over calcium chloride, anddistilled under reduced pressure. MPS (Wuhan Uni-versity Silicone New Material Co., Hubei, China) as acoupling agent, cetytrimethylammonium bromide(CTAB, Shanghai Chemical Reagent Plant, China)as an emulsifier, and ammonium persulfate (APS,Tianjin Chemicals Co., China) as an initiator, were allanalytical grade reagents and used as received.

Determination of grafting polymerization rate

A certain amount of CTAB, 100 mL distilled water,6 g MMA, and 0.5 mL acrylic acid were added into

a four-necked round-bottomed flask equipped witha mechanical stirrer, a thermometer, a reflux con-denser, and a guttate funnel. After complete emulsi-fication through stirring for 30 min, 10 g MPS-coupled flaky Al was charged into the flask. Whenthe mixture was heated to 65�C with 25 mL/minnitrogen flowing, 0.6 g APS aqueous solution wasslowly added into flask by guttate funnel. Then, themixture was heated to 85�C (To obtain the kineticparameters, the experiments were carried out in 65,75, 85, and 95�C, respectively). At the scheduled re-active time, diphenylpicrylhydrazyl (DPPH), a uni-versal retarder, was added into the polymerizingsystem to terminate the reaction. The mixture wasprecipitated with methanol, demulsified with potas-sium chloride, and filtered. The resulting cake wasdispersed in acetone with ultrasonic vibration andthe nongrafted PMMA was dissolved in acetone. Af-ter centrifugation (10,000 rpm) for 1 h, nongraftedPMMA was obtained by precipitating the filtratewith methanol. The cake was dried in vacuum, andPMMA-grafted Al was obtained. Grafted PMMA inPMMA-grafted Al can be calculated from the con-tent of coupled MPS through Si elemental analysesand the content of Al through evolved hydrogendetection in a reaction of aqua regia described in lit-erature.10 Total PMMA was the sum of graftedPMMA and nongrafted PMMA. So the grafting poly-merization rate (Rg), homogenous grafting polymer-ization rate (Rh), and total grafting polymerizationrate (Rt) were calculated according to the followingrelationships:

Rg ¼grafted PMMA g

� �

molecular weight of MMA g=mol� �� reactive time minð Þ � reactive volume Lð Þ mol � L�1 �min�1

� �

Rh¼nongrafted PMMA g

� �

molecular weight of MMA g=mol� �� reactive time minð Þ � reactive volume Lð Þ mol � L�1 �min�1

� �

Rt ¼ Rg þ Rh mol � L�1 �min�1� �

Therefore, the rate constants of grafting, homoge-nous, and total polymerization could be calculatedaccording to the Rg, Rh, and Rt, respectively.

RESULTS AND DISCUSSION

The experimental kinetics of graftingpolymerization

The existence of solid particles was the essential dif-ference between grafting and homogenous polymer-ization.11,12 Figure 1 shows the logarithmic relation-ship between the grafting polymerization rate (Rg)and flaky Al concentration. As can be seen, Ln(Rg)

increased linearly with the increasing Ln(CAl), whichcould be attributed to the augment of the probabilityof MMA in situ polymerization onto flaky Al. It wasalso found that slope of the simulated line was0.521. So the relationship between grafting polymer-ization rate at the beginning and flaky Al concentra-tion could be described as Rg 1 C0:521

Al .The effect of initiator concentration on the grafting

polymerization rate was demonstrated in Figure 2.As was clearly observed, an approximate linear rela-tionship was found between Ln(Rg) and Ln(CAPS).The number of radicals increased with the increasinginitiator concentration, which led to the increase ofgrafting polymerization rate at the beginning.13,14 Aswas apparent, Rg 1 C0:429

APS could be used to depict

MMA GRAFTING POLYMERIZATION ONTO FLAKY AL POWDER 3041

Journal of Applied Polymer Science DOI 10.1002/app

the relationship between the grafting polymerizationrate and initiator concentration.

Analogously, the influence of MMA concentrationon the grafting polymerization rate was studied, andthe result was given in Figure 3. A good linear rela-tionship was observed between Ln(Rg) andLn(CMMA). According to the principle of chemicalreaction dynamics, grafting polymerization ratewould increase with the reactant concentration. Theslope of the line was 0.978, so the relationshipbetween grafting polymerization rate and MMA con-centration could be simulated as Rg 1 C0:978

MMA.According to the earlier investigations, the experi-

mental apparent grafting polymerization rate ontoflaky Al powder could be described as Rg ¼ KC0:521

Al

� C0:429APS � C0:978

MMA.

The theoretical kinetics of grafting polymerization

To clarify the mechanism of MMA grafting polymer-ization onto flaky Al powder, the theory of stable

polymerization and equivalent activity could beused to simplify the reactive process. So the mecha-nism of grafting polymerization rate might bedescribes as follows:

1. The generation of primary radicals:

S2O2�8 APSð Þ ! 2SO�

4 R�ð Þ

2. The formation of flaky Al skeleton radicals:

Al�OHþ R� �!kd Al�O � þRH

3. The initiating step:

Al�O � þ MMA �!ki Al�OMMA:

4. The propagating step:

Al�OMMAn � þ MMA �!kp Al�OMMAnþ1:

5. The terminating step: Combination would bedominant terminating way at low-initiator con-centration, and then,

Al�OMMAn � þ Al�OMMAn� �!ktAl�OMMA2nO�Al

According to the theory of stable polymerizationand equivalent activity, the following relationshipcould be established:

Figure 1 The relationship between Ln(Rg) and Ln(CAl).

Figure 2 The relationship between Ln(Rg) and Ln(CAPS).

Figure 3 The relationship between Ln(Rg) and Ln(CMMA).

3042 LIU ET AL.

Journal of Applied Polymer Science DOI 10.1002/app

dCAl�O�dt

¼ kdCAlCAPS � kiCAl�O�CMMA ¼ 0

Namely, kdCAlCAPS ¼ kiCAl�O�CMMA

dCAl�OM�dt

¼ kiCAl�O�CMMA � ktC2Al�OMMAn� ¼ 0

That is to say, kiCAl�O � CMMA ¼ ktC2Al�OMMAn�

So, CAl�OMMAn� ¼ kd=ktð Þ12C12

AlC12

APS

Consequently, Rg ¼ kpCAl�OMMAn�CMMA ¼ kp kd=ktð Þ12C

12

AlC12

APSCMMA.For sake of simplification, kpðkd=ktÞ

12 ¼ K, the theo-

retical apparent grafting polymerization rate ontoflaky Al could be described as: Rg ¼ KC0:5

Al � C0:5APS �

CMMA.As was clearly observed, a basic agreement was

obtained between experimental and theoretical appa-rent MMA grafting polymerization rate onto flakyAl powder. So the radical polymerization could beused to describe the mechanism. Of course, therewere some small differences between experimentaland theoretical grafting polymerization rate, and thereason could be possibly explained as follows.Firstly, a small quantity of primary radicals mighttake part in the terminating step. Secondly, gel effectwould take place at high monomer conversion.Finally, there was discrepancy between the theory ofstable polymerization (or equivalent activity) andactual experiments.

The activation energy of grafting, homogenous,and total polymerization

It was generally admitted that reactive temperatureplayed an important role in the polymerization rate.As shown in Figure 4, a good linear relationshipwas observed between Ln(kg) and (1/T), and theslope of the line was simulated as �7834.2. Accord-ing to the Arrhenius equation,15–17 the activationenergy of MMA grafting polymerization (Eag) ontoflaky Al was calculated as:

Eag ¼ �8:314� �7834:2ð Þ ¼ 65:1 kJ mol�1:

As we know, the molecular structure was nottaken into account in the Collisions Theory, so theactivation energy of MMA grafting polymerizationcould possibly not be explained according the Colli-sions Theory.

Similarly, the activation energy of homogenousand total polymerization rate could be also calcu-lated as 35.4 and 37.5 kJ mol�1, respectively. It couldbe validated that the relationship among the activa-tion energy of grafting, homogenous, and total poly-merization accorded with the following equation:

Eat ¼kgEag þ khEah

kg þ kh

where, Eat was the activation energy of totalpolymerization;

Eag was the activation energy of graftingpolymerization;

Eah was the activation energy of homogenouspolymerization;

kg was the rate constant of grafting polymeriza-tion; and

kh was the rate constant of homogenouspolymerization.

In fact, grafting and homogenous polymerizationwere a pair of parallel reactions. So the equationabout the activation energy was exactly consistentwith the theoretical result of parallel reactions.18

CONCLUSION

With APS as the initiator, the kinetics of MMA graft-ing polymerization onto flaky Al was studied. It wasfound that the experimental apparent polymeriza-tion rate equation, Rg ¼ KC0:521

Al � C0:429APS � C0:978

MMA, wasbasically consistent with the theoretical result basedon the theory of stable polymerization and equiva-lent activity, Rg ¼ KC0:5

Al � C0:5APS CMMA. The activation

energy of grafting, homogenous, and total polymer-ization rate was calculated as 65.1, 35.4, and 37.5 kJmol�1, respectively. It could be validated that therelationship among these activation energiesaccorded with the theoretical result of parallel reac-tions: Eat ¼ kgEagþkhEah

kgþkh.

The authors thank Zilu Chen and Yingchao Zhang for theskillful experimental works.

Figure 4 The relationship between Ln(kg) and 1/T.

MMA GRAFTING POLYMERIZATION ONTO FLAKY AL POWDER 3043

Journal of Applied Polymer Science DOI 10.1002/app

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3044 LIU ET AL.