e-Polymers 2005, no. 025. ISSN 1618-7229

http://www.e-polymers.org

Synthesis and characterization of poly(methyl methacrylate) obtained by ultrasonic irradiation Cristina Parra 1, Gema González 1, Carmen Albano 2 *

1 Laboratorio de Materiales, Instituto Venezolano de Investigaciones Científicas, apdo. 21827, Caracas 1020-A, Venezuela; gemagonz@ivic.ve 2 Laboratorio de Polímeros, Centro de Química, Instituto Venezolano de Investigaciones Científicas, apdo. 21827, Caracas 1020-A, Venezuela; Fax +58 212 5041418; calbano@ivic.ve (Received: November 14, 2004; published: April 10, 2005)

This work has been presented at the 12th Annual POLYCHAR World Forum on Advanced Materials, January 6-9, 2004, in Guimaraes, Portugal

Abstract: We have studied the influence of surfactant nature and concentration, and monomer concentration on the synthesis of poly(methyl methacrylate) (PMMA) using high-frequency ultrasound. Polymerization was carried out via free radicals from aqueous solutions with several concentrations of an anionic (sodium lauryl sulfate, SLS) or a cationic surfactant (cetyltrimethylammonium bromide, CTAB) and different concentrations of the insoluble monomer methyl methacrylate (MMA) as the dispersed phase. The polymer particles obtained were characterized by FTIR, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The IR spectra show the presence of the characteristic absorption bands for PMMA when SLS was used. When CTAB was employed, for all surfactant concentrations and high monomer concentration (14% v/v), PMMA was obtained. However, bands of the surfactant were present for lower monomer concentrations. The glass transition temperature measured by DSC was in the range 106 - 126°C characteristic of PMMA. Maximum conversion was obtained when the anionic surfactant was employed. SEM studies show the formation of sub-micrometric PMMA latex spheres with average particle size from 65 to 100 nm when the anionic surfactant was used and 65 nm - 0.3 µm for the cationic surfactant.

1. Introduction Poly(methyl methacrylate) (PMMA) is a very versatile polymer due to its excellent properties such as resistance to strong alkaline or acid solutions and its stability to heat and light. The polymer behaves quite satisfactory in the presence of very aggressive environments. Therefore, its applications have extended to many areas including medicine [1], e.g., in osseous cements [2], dental applications [3], intra-ocular lenses [4] and as facial implants and a number of other applications [5]. PMMA can be obtained by conventional methods by polymerization in emulsion resulting in particles in the range of 1 to 20 µm. Recently, other methods have been used for the synthesis of this polymer, among them sonochemistry [6] using ultra-sound waves as the source of energy. 1

The last method is based on the fact that when a sound wave travels through a liquid medium an elevated number of bubbles are formed which grow and collapse in a few microseconds. This phenomenon is called cavitation [7]. Several theoretical studies on sonochemistry [8] have shown that ultrasonic cavitation can generate temper-atures of the order of 5000 K and local pressures of 500 atm, heating and cooling the system at rates as high as 109 K/s. The environment created by the dramatic collapse of bubbles has enough energy to produce bond rupture, forming free radicals (from the monomer, surfactant and solvent), which play the role of initiators in the polymerization. Furthermore, this is a relatively simple method (beginning only with a monomer and the aqueous solution of an emulsifier), the product obtained has a high degree of purity and due to a very efficient dispersion of the medium it has been possible to synthesize nanometric polymers [9]. The work of Suslick and Price [10] has been a pioneer in the first attempts of ultra-sound application to the synthesis of materials. For instance, the sonochemical decomposition of organometallic precursors in low volatile solvents produces nano-structure materials with high catalytic activities. Also metals, alloys, oxides, carbides, sulfides and nanometric colloids can be obtained by this new route. Another impor-tant application has been the preparation of biomaterials based on microspheres of proteins and polymers. Many syntheses of various polymers and copolymers have been carried out by ultra-sonic irradiation [11-13]. Also detailed studies have been reported on the mecha-nisms of degradation suffered by macromolecules due to the effect of ultrasonic waves [14]. An important factor considered has been the increase in the medium viscosity as the polymer is synthesized affecting the cavitation process and, there-fore, the reaction velocity [15]. PMMA polymerization based on the irradiation of pure monomer following the effect of a number of parameters on the process has been reported by Price [16] and Kruus [17]. They obtained polymers with controlled molec-ular weight, polydispersity and tacticity although low conversions were achieved. Recently, Yongqin et al. [18] synthesized PMMA through the polymerization in emulsion of the anionic surfactant sodium dodecyl sulfonate. A high molecular weight polymer with a conversion of 67% was obtained after 30 min of reaction. A new scope to the synthesis of polymeric materials by the use of ultrasound energy has been opened and important contributions are still to come. The object of the present work is to study the influence of the type and concentration of surfactant and of monomer concentration in the synthesis of PMMA by sonochemical methods. 2. Experimental part Methyl methacrylate (MMA) analytical grade 99.8% (density: 0.943 g/cm3) was employed. This monomer was thoroughly washed in 10% NaOH solution to remove the inhibitor agent and several times in distilled water, dried with anhydrous Na2SO4 and finally distilled in vacuum. The surfactants used were sodium lauryl sulfate (SLS) and cetyltrimethylammonium bromide, both analytical grade, without any additional treatment. An ultrasonic generator (Fisher PG100 MSE, Mod. 150W) operating at a frequency of 20 kHz was employed. Fig. 1 shows a schematic diagram of the equipment used - similar to the schematic diagram presented in ref. [11]. This is formed by a double jacket glass vessel and a 13 mm Ti sonic wave emission probe controlled by a standard power source.

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Water in

Water out

N2 inlet

N2 outlet

Reaction Vessel

Titanium Probe

UltrasonicHorn

Fig. 1. Schematic diagram of our high-frequency equipment The preparation of the dispersion for the reaction was the following: 3 - 7 ml of MMA were introduced in the reaction vessel and an appropriate volume of surfactant to complete a total volume to 50 ml. Tab. 1 shows a summary of the experimental variables. Tab. 1. Experimental variables

MMA concentration in % (v/v)

Surfactant concentration in % (w/v)

Surfactant

14 0.5; 1.0; 1.5; 2.0 SLS, CTAB 10 0.5; 1.0; 1.5; 2.0 SLS, CTAB 8 0.5; 1.0; 1.5; 2.0 SLS, CTAB 6 0.5; 1.0; 1.5; 2.0 SLS, CTAB

Once both phases were introduced in the reaction vessel, a constant flow of oxygen-free N2 was bubbled through the solution until the end of the process. Ultrasonic irra-diation was carried out for 1 h for all the different conditions of reaction. Cool water was circulated to maintain the temperature at 28°C. For polymer precipitation the emulsion produced was poured into semi-frozen water-methanol (1:1) mixture. The precipitated solid was separated from the mother liquor by centrifugation at 30 000 rpm, and three consecutive washes of 20 min each with cold water. Finally, the weight of the polymer obtained was determined and the conversion calculated. The morphology of the synthesized polymer was determined in a scanning electron microscope (SEM) Hitachi Field Emission S-4500 operating at 8 kV. The particle size was determined from SEM micrographs. The samples to be observed in the SEM were prepared by suspension in an ethanol-water mixture (75/25) and Pt-C coated in a Balzers BAE 300 evaporator.

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All synthesized polymers were analyzed in a Nicolet FTIR spectrophotometer Mod. 500. Thermal analysis was also carried out for all the samples in a Mettler Toledo differential scanning calorimeter, Mod. DSC821e, from 25 to 230°C at a heating rate of 20°C/min. The efficiency of the reaction towards the formation of polymer was determined by the conversion: Conversion in % = Mp/Mm · 100 where Mp is mass of polymer obtained from the reaction, and Mm the mass of mono-mer employed in the reaction.

500 1000 1500 2000 2500 3000 3500Wavenumbers (cm-1)

a b c d

Fig. 2. FTIR spectra of PMMA synthesized with: (a) 0.5% SLS - 14% MMA; (b) 1.0% SLS - 10% MMA; (c) 1.5% SLS - 8% MMA; (d) 2.0% SLS - 6% MMA 3. Results and discussion Polymerization using SLS surfactant always resulted in PMMA as the synthesis product for all different concentrations of surfactant and monomer. Fig. 2 shows some of the FTIR spectra obtained for different conditions. The presence of the characteristic bands of PMMA is observed at exactly the same vibration frequencies for all samples. All the bands correspond to those of PMMA without any other contribution present. This is indicative of the purity of the polymer obtained. The bands at 3000 and 2900 cm-1 correspond to the C-H stretching of the methyl group (CH3). The intense band located at 1730 cm-1 corresponds to the C=O bond of the carbonyl group. Also the 1300 and 1450 cm-1 bands are observed asso-ciated to C-H symmetric and asymmetric stretching, respectively. The 1240 cm-1 band is assigned to torsion of the methylene group (CH2) and the 1150 cm-1 band corresponds to vibrations of the ester group C-O. The 1000 and 800 cm-1 bands are assigned to C-C stretching. In the synthesis with CTAB, for all different surfactant concentrations but only for high monomer concentrations, PMMA was obtained with

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only a very small amount of surfactant remaining. For the other monomer concen-trations either a mixture of surfactant and polymer or other products were obtained. Fig. 3 shows the IR spectra for different samples obtained with CTAB at different surfactant and monomer concentrations, as an example of the products obtained with this surfactant. a

b

c Fig. 3. FTIR spectra of PMMA synthesized with: CTAB 1.5% - MMA 8%; (b) CTAB 0.5% - MMA 14%; (c) CTAB 1.5% - 14%MMA

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Fig. 3a corresponds to 1.5% CTAB - 8% MMA; the presence of all the characteristic PMMA bands can be observed except the band at 2849 cm-1, which corresponds to the surfactant indicating that some is remaining in the structure. Fig. 3b corresponds to 0.5% CTAB - 14% MMA; the bands in this spectrum correspond very nearly to PMMA, indicating that only a very small amount of surfactant is present, and Fig. 3c corresponds to 1% CTAB - 14% MMA, in which the formation of a different product is observed. This is an evidence of the strong influence of surfactant and monomer concentration on the product obtained.

Solvent (water)

H 2 O H• + OH•

Surfactant (SLS)

C 12 H 25 OSO 3 Na C12H25 • + NaSO3O•

O

CH 2 C C O CH3

CH 3

• CH3 + CH2 = C− CH = CH 2

.

CH2 = C − CH3

+ • CH = CH 2 .

Fig. 4. Free radicals formed by ultrasonic irradiation during MMA polymerization The vibration bands observed in the range 3600 - 3300 cm-1 correspond to OH-

groups, which could be indicative of the H• and OH• radicals formed from the decom-position of water under ultrasonic radiation (see Fig. 4). Some radicals will react to form H2O, H2, H2O2 and O2 [20-21]. Tabs. 2 and 3 show a summary of the glass transition temperatures, Tg, obtained from the DSC analysis of a representative group of the synthesized polymers. The Tg values correspond to that expected for PMMA. NMR experiments (not shown here) confirm the presence of atactic PMMA (Tg = 106 - 114°C) or syndiotactic PMMA (Tg = 120 - 126°C) observed for different synthesis conditions, which are in agreement with the reported temperatures for these configurations [19]. The general tendency ob-served seems to show that high monomer concentrations (14%) using different concentrations of SLS result in the formation of atactic PMMA and low monomer (8%) concentrations for the same surfactant seem to produce syndiotactic PMMA. However, when the surfactant is changed to CTAB, for high monomer concentrations PMMA was obtained with the presence of some remaining surfactant. For low mono-mer concentration no formation of PMMA was observed in some cases; in other cases a mixture of surfactant and polymer was obtained. These results indicate that the stereochemistry of the polymer obtained by ultrasonic irradiation varies depend-ing on the reaction conditions. It seems that the principal factors affecting the PMMA configuration are the nature of the surfactant and the surfactant concentration. This could be explained considering that ester substituents are oriented in alternate order (indicating syndiotacticity) or in a completely random way (indicating atacticity). Given the complexity of the synthesis environment, it can be understood that the growth of the polymer chains depends on the size of the radical species, what will increase or decrease the steric effect and, therefore, will limit the space region where the bonding to a new monomer molecule will take place and, thus, will contribute to form one of the polymer configurations. Fig. 5 shows the apparent conversion vs. surfactant concentration for four different monomer concentrations. In general, as the SLS surfactant concentration increases, the conversion increases. Maximum conversion is obtained with 1.5% surfactant concentration, except for the highest monomer concentration (14%) where con-version increases monotonically with surfactant concentration.

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Tab. 2. Tg values obtained for MMA 14% (v/v)

Surfactant Surfactant conc. in % (w/v) Tg ±2°C

SLS 0.5 117 SLS 1.0 120 SLS 1.5 116 SLS 2.0 106

CTAB 0.5 111 CTAB 1.0 120 CTAB 1.5 117 CTAB 2.0 123

Tab. 3. Tg values obtained for MMA 8% (v/v)

Surfactant Surfactant conc. in % (w/v) Tg ±2°C

SLS 0.5 126 SLS 1.0 115 SLS 1.5 126 SLS 2.0 120

CTAB 0.5 120 CTAB 1.0 110 CTAB 1.5 110 CTAB 2.0 111

0

10

20

30

40

50

60

0 0,5 1 1,5 2 2,5 Surfactant Concentration (%P/V)

MMA (6% V/V) MMA (8% V/V) MMA (10% V/V) MMA (14% V/V)

% C

onve

rsio

n

Fig. 5. Apparent conversion vs. SLS concentration

7

0

5

10

15

20

25

0 0,5 1 1,5 2 2,5 Surfactant Concentration (%P/V)

% C

onve

rsio

n

MMA (14% V/V) MMA (10% V/V) MMA (8% V/V) MMA (6% V/V)

Fig. 6. Apparent conversion vs. CTAB concentration For CTAB (Fig. 6) conversion increases with monomer concentration. However, as was established above for these syntheses, there is always some surfactant remaining. Comparing both graphs it can be observed that the highest conversions were obtained in the presence of SLS and in both cases a strong dependence between yield, concentration and surfactant nature was found. These results suggest that the increase in the conversion is due to the surfactants acting as a strong source of free radicals, which are initiators of the polymerization. The fact that CTAB is a much larger and bulky molecule than SLS could explain the lower conversions observed for it. It is clear that in the presence of a surfactant of larger size (more carbon atoms and larger hydrophobic part) an increment in the medium viscosity is produced, which will cause a decrease in the reaction velocity due to the negative effect in the cavitation phenomena [22]. This effect was also observed on experi-ments of styrene polymerization [23]. For SLS the contribution to the viscosity of the medium is lower due to its smaller molecular size and since its principal role is not only that of an emulsifier; it can be easily degraded to form free radicals and, there-fore, its presence increases the velocity of the reaction in a significant way resulting in an increasing conversion. The average particle size of PMMA was determined by scanning electron micro-scopy. Figs. 7 to 14 correspond to electron micrographs of a representative group of polymer samples prepared under different reaction conditions. In all the samples the formation of small spherical PMMA particles formed in the polymerization process is observed. Figs. 7 to 10 show the formation of latex particles from 10% (v/v) MMA and four different SLS concentrations (0.5, 1.0, 1.5 and 2.0% w/v). The average particle size is in the range of 65 - 100 nm. Figs. 11 to 14 show micrographs corresponding to polymers obtained from the synthesis carried out with CTAB. It is clear that the surfactant and monomer concen-tration have a strong influence on the polymer formation. As the surfactant concen-tration increases, the particle size increases. The average particle size is 65 nm for low surfactant concentration and 0.3 µm for high surfactant concentration. 8

Fig. 7. (left) PMMA synthesized with 0.5% SLS and 10% MMA Fig. 8. (right) PMMA synthesized with 1.0% SLS and 10% MMA

Fig. 9. (left) PMMA synthesized with 1.5% SLS and 10% MMA Fig. 10. (right) PMMA synthesized with 2.0% SLS and 10% MMA

Fig. 11. (left) PMMA synthesized with 0.5% CTAB and 10% MMA Fig. 12. (right) PMMA synthesized with 1.0% CTAB and 10% MMA

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Fig. 13. (left) PMMA synthesized with 1.5% CTAB and 10% MMA Fig. 14. (right) PMMA synthesized with 2.0% CTAB and 10% de MMA It is also observed that the PMMA particles are embedded in a matrix that can be associated to the surfactant structure. Most probably during the polymerization process some molecules of CTAB and SLS have remained occluded between the polymer chains. The polymer particle size is influenced directly by the concentration and the type of surfactant; smaller particles were obtained for the anionic surfactant. From the results obtained in the present work it is concluded that ultrasonic irradi-ation is a very effective and simple method for the synthesis of PMMA. The surfactant nature and concentration and the monomer concentration strongly influence the syn-thesis of PMMA. Its yield is affected by the surfactant’s nature and concentration, SLS being the surfactant that showed the highest conversion. Also particle size is affected by surfactant nature and concentration; smaller particle sizes were obtained with SLS.

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[12] Fujiwara, H.; Kimura, T.; Masafuni, S.; Nakatuka, T.; Nakamura, H.; Polym. Bull. 1992, 28, 189. [13] Fujiwara, H.; Kikyu, T.; Nanbu, H.; Honda, T.; Polym. Bull. 1994, 33, 317. [14] Price, G.; Smith, P.; Polym. Int. 1991, 24, 159. [15] Price, G.; Ultras. Sonochem. 1996, 3, 159. [16] Price, G.; Norris, D.; West, P.; Macromolecules 1992, 25, 6447. [17] Kruus, P.; Patraboy, T.; J. Phys. Chem. 1985, 89, 3379. [18] Yongqin, L.; Wang, Q.; Xia, H.; Xu, X.; Baxter, S.; Slone, E.; Wu, S.; Swift, G.; Westmoreland, D.; J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3356. [19] Stevens, M.; “Polymer Chemistry”, 2nd edition, Oxford University Press, New York 1990, p. 619. [20] Kruus, P.; Ultrasonics 1983, 21, 201. [21] Henglein, A.; Ultrasonics 1987, 25, 6. [22] Yim, B.; Okuno, H.; Nagat, Y.; Maeda, Y.; Ultras. Sonochem. 2002, 9, 209. [23] Keat, S.; Biggs, S.; Ultras. Sonochem. 2000, 7, 125.

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