Studies on Ultrasonic Initiated Copolymerization of Styrene and Acrylate Series

Polymer Journal, Vol. 32, No. 2, pp 103-106 (2000)

Jiang Lru, Keqiang CHEN, and Zhili Lr

State Key Lab. of Polymer Material & Engineering, Research Institute of Sichuan University, Chengdu 610065, People's Republic of China

(Received June 14, 1999)

ABSTRACT: Experiments with styrene (St) and acrylate monomer [methyl acrylate (MA), ethyl acrylate (EA) and butyl acrylate (BA)] prove that copolymerization can be initiated in two monomers with intense ultrasound. Within the experimental range, the yields of copolymer increase with rise of ultrasonic intensity, reaction time, temperature and the amount of ion surfactants (sodium dodecyl sulfate (SDS), hexadecyltrimethyl ammonium bromide (CTAB)), but de crease with increasing concentration of comonomers. The comonomer ratio and surfactant types have considerable effect on the yield of copolymer. The yield of copolymer is 38.8% by irradiating 10% BA/St (1/1, v/v) at 40°C with 600 w for 1 h. KEY WORDS Ultrasound/ Copolymerization /Monomer/

Ultrasound has been applied for polymer synthesis standard titanium horn to introduce ultrasound directly since 1980s.1 Compared with general chemical method, into the liquid. A compressed air cooled transducer was ultrasonic polymerization has characteristics as follows : connected to the horn. The copolymerization was per (1) accelerating chemical reactions or easing reaction formed in a 120-ml glass beaker surrounded by a circu conditions ; (2) lowering the requirement for reagent ; (3) lating water bath. The top of the beaker was covered initiating polymerization without the addition of an in with rubber lid. Nitrogen gas was bubbled through the itiator; (4) simplifying the procedures of synthesis; (5) solution by a metal frit. synthesizing polymers unable to be obtained through general chemical methods. Therefore, ultrasound pro Procedure vides energy to modify chemical reactivity, energy, The emulsion was prepared with 90.0-ml distilled which is different from the commonly used heat, light, water, surfactant and the monomers with different radiation etc. So far the vast majority of investigations monomer ratios (VMA/ V st, VEA I V st, V BA I V stl measured have been carried out in systems containing homopoly volumetrically. One variation was changed at a time mers. Since 1985, several papers concerning the ultra when other variations remained constant to gain data sonically initiated polymerization of vinyl monomers ap for optimizing the reaction. Variables include changing peared.2-4 However, study on copolymerization in mono the amount of surfactant (0.36 g, 0.72 g, 1.08 g, 1.8 g, mer-monomer system with ultrasound is little. This pa 2.16 g), monomer concentrations (10%, 15%, 20%, 25%, per examines the ultrasonically initiated emulsion co 35%), monomer ratio, bulk temperature (15-40°C), polymerization of styrene and acrylate monomers acoustic intensity (as a percent of maximum output: (methyl acrylate, ethyl acrylate and butyl acrylate). The 10%, 20%, 30%, 40%). effects of ultrasonic intensity, reaction time, bulk tem The emulsion was introduced into the flask and deoxy perature, ratio of comonomer, surfactant type, concen genated by bubbling with dry, oxygen free nitrogen for 5 tration of surfactant and comonomer on the yield of co min. The ultrasound was switched on and a nitrogen at polymer are systemically studied. mosphere was maintained during sonication. After soni cation, the beaker was removed and the emulsion was EXPERIMENTAL transferred to a 500-ml flask. The product was coagu lated by 200-ml ethanol. The polymer was dissolved in Material THF and precipitated by a large amount of distilled Methyl acrylate (MA), ethyl acrylate (EA), butyl acrylate (BA) and styrene (St) were vacuum-distilled to remove the hydroquinone inhibitor and refrigerated un til use. The sodium dodecyl sulfate (SDS) surfactant was purified through twice crystallization in ethanol. Hexa decyltrimethyl ammonium bromide (CTAB) and oc tylphenol polyethylene oxide-10 (OP-10) were of analyti o- Ultrasonic generator cal purity and used without further purification. The tet rahydrofuran (THF), acetone, ethanol, and ethyl acetate ~ooling water were of chemical purity grade.

Apparatus Cooling water _____::,,. As shown in Figure 1, a Sonics and materials ultra sonic generator (20 KHz Model VC-1500) was used with Figure 1. Ultrasonic reactor. 103 J. Liu, K. CHE.~, and Z. L, water to remove surfactants. The reaction product was extracted with acetone and ethyl acetate for 48 h to re 90-: move unreacted monomers and homopolymers. The final 80 - for further testing. copolymer was weighed and collected 70 -

Identification of Copolymer 60 - The purified polymer was analyzed by IR (Nicolet-FT 50 - Infrared Spectrometer). The glass transition tempera 40- was determined with a Differen ture (Tg) of copolymer 30 - tial Scanning Calorimeter (DSC 2 c, Perkin Elmer), at a heating rate of20°C min - 1. 20 -

4000 3000 2000 1000 1 Wavenumbers {cm- ) RESULTS AND DISCUSSION Figure 2. IR spectrum of PS-co-PEA copolymer. Figure 2 is the IR spectrum of final purified copolymer. W avenumbers and corresponding structures are shown in Table I , which indicates that the IR spectrum of the Table I. IR assignment of PS-co-PEA copolymer copolymer includes characteristic absorption of both Wavenumbers / cm - 1 Group Vibration mode polystyrene and polyethyl acrylate. DSC curve of the co 3027.03 yc-H polymer (Figure 3) suggests the structure of copolymer, 0 which shows only one Tg (49.54 'C), lower than Tg of PS 2924.61 -CH,- yc-H 2852.92 ye H (100°C) and higher than Tg of PEA (-24°C). /0 Figure 4 represents the effect of change in ultrasonic -CH-c-o- intensity, which shows an increase in the percent yield 1731.44 /0 YC=O as a function of ultrasonic intensity. The reason is that -c-o- at a certain frequency, increase of ultrasonic intensity 1603.41,1495.87, 0 YC=C determines the magnitude of the cavitation area. Cavita 1449.79 tion bubbles cannot be created until the ultrasonic inten 1449.19 -CH, Sas

initiated polymerization. The longer the reaction time, (1) the higher the yield of the copolymer is. However, in crease in yield slows with time, suggesting that copolym (2) erization of monomers and degradation of polymers com petes with each other during the sonication. At the be P =P [P (y-1)/P]Y1Cy-u (3) max gm v ginning, copolymerization dominates because the con centration of monomer is much higher than that of poly Where Pm is the pressure generated on collapse, y ra mer. The degradation starts with increase of polymer. tio of specific heat capacities of the solvent vapor, and Pv During the degradation, although the number of free vapor pressure of the solvent at temperature, p liquid radicals in the system remains, the dispersing rate of density, Rm maximum cavitation bubble radius, Pg in free radicals falls with increase of viscosity and decrease itial pressure in cavitation bubbles and Tbulk ambient of concentration of monomers. temperature. Cavitational collapse creates drastic condi The relationship between bulk temperature and yield tions inside the medium : temperatures of 2000-5000 K of copolymer is illustrated in Figure 6. Two tempera and pressures up to 1800 atm within the collapsing cav tures exist in the system of ultrasonic copolymerization : ity.1 For example, in the water (25 °C) system keeping T max (the maximum temperature) and Tbulk (bulk tem with nitrogen atmosphere, the estimated values of P max, perature). The most widely accepted treatment assumes 7 adi T max, and 't are respectively 9.80 X 10 Pa, 4290 K and very high temperatures and pressures during an 1 µs .5 abatic bubble collapse. On simplified treatment, T max is Equations 1-35 mean that the increase of Pm is help given by eq 2. Clearly, this is sufficiently high to dissoci ful to obtain shorter 't, higher T max and P max, respectively, ate solvent vapor entering the bubble, leading to bond which intensify the extent of cavitation collapse and fa breakage and radical formation. When Tbulk is relatively cilitate the dispersion of monomers and free radicals. low, the content of solvent vapor is low, resulting in a Therefore the yield of copolymer increases with sonic in low value of Pv and a higher T max· At this time, free radi tensity. cals with high energy tend to combine with each other to Figure 5 shows the time dependence of ultrasonically form oligomer, which results in decrease of number of

104 Polym. J., Vol. 32, No. 2, 2000 Ultrasonic Initiated Copolymerization of Styrene/Acrylate

20 ,------,

- a 99-1-)# (99073) 45 •-BA 40 +-MA Tgfrom 41 78 T-EA to 5395 -.J. 35 Onset=4760 ---... 15 J/g•dag = 18 0 30 ... . E Tg =49 54 >. I 25 #·--7· ....u 20 ~. 0 -0 -;:; 15 :;: 10 :?'.-----,/ 5 ,, 0 30 40 50 60 70 80 90 40 60 80 l00 Time/ min 0 Temperature, C Figure 5. Effects of irradiation time on yield of copolymer at 25 Figure 3. DSC spectrum of PS-co-PEA copolymer. (sonic intensity : 600 w ; cone. of comonomer : 10% ; cone. of SDS : 2.5% ; VA I V8,=l).

35 ,------~

50 30 ~•-BA 45 -;;R, ~+-MA 25 0 40 ~T-EA ------0... • 1 35 /---.~... §. 30 ... t.... :: u 25 ------0 ..... 0 -0 10 ----:/ ------+ -;:; -0 20 :;: -;:; :;: 15 b ~· 10 ,,y· 5 + ----· 300 350 400 450 500 550 600 Ultrasonic Intensity/ w 0 15 20 25 30 35 40 Figure 4. Effects of ultrasonic intensity on yield of PS-co-PEA at l8°C (cone. of comonomer: 10%, cone. of SDS: 2.5%, irradiation Temperature I °C time : 1 h; VEA I Vs,=1). Figure 6. Effects of bulk temperature on yield of copolymer (sonic intensity : 600 w ; cone. of SDS : 2.5% ; cone. of comonomer : 10%; irradiation time : 1 h ; VA I V8,=1). radicals able to disperse out of the cavitation bubble to initiate polymerization. When Tbulk increases, Pv in creases quickly which facilitates the formation of cavita the SDS oil / water system, -SO4 Na, which extrudes tion bubbles. Higher Pv leads to the fall of T max, but into water phase from the phase interface, is electro enough to generate radicals. The generated radicals lyzed into [-SO4 -l and [Na +l. The phase interface is sur with a medium energy prefer to disperse out of the cavi rounded by a layer of [-SO4 -],which forms a double tation bubbles and initiate copolymerization. Therefore electron layer with closely orientated arranging [Na +l. the yield of copolymer increases with bulk temperature. This special structure accelerates the electrophilic free 2 Kruus Riesz reports that St, EA, and H 2O can form radicals to disperse into phase interface and react with free radicals under the cavitating of ultrasound. How the monomer existing in phase interface to copolymerize. ever, the yields of the copolymerization in systems re Therefore, the yield of copolymer increases with amount spectively containing only pure monomers (St/EA), ofSDS. Cation surfactant acts in the same way. Acrylate monomer/surfactant, monomer/water are almost nau monomer is split into free radicals (CH3 ·, ·CH2R, and ght. This suggests that whether free radicals can initiate RO·). The nucleophilic CH3 ·, and · CH2R are more ac polymerization is dependent on their longevity and reac tive than the electrophilic free radical (RO·) generated tivities. Only those free radicals which exist long enough in the system and more easily to combine with each to disperse out of the cavitation bubbles can initiate the other than to disperse out of the cavitation bubble. This copolymerization. results in lower number of free radicals able to disperse Experiments prove that the addition of anion surfac out of the cavitation bubbles. Therefore, cation surfac tant SDS and cation surfactant CTAB facilitates the dis tant has less contribution to the copolymerization than persion of free radicals from cavitation bubbles to the anion surfactant. Non-ion surfactant without double monomer solution. But non-ion surfactant OP-10 has no electron layer has no effect on the dispersion of free radi such effect, as shown in Figure 7 and Table II . The rea cals. son is that ion surfactant is able to form micelles con Figure 8 shows that as the concentration of comono taining double-electron layers in monomer solution. In mer increases the yield of copolymer decreases. This

Polym. J., Vol. 32, No. 2, 2000 105 J. Lil,, K. CHE~. and Z. L1

45 35 • 40 0 --- 30 35 ______..II) .. E ---..II) ;,-.. E 30 25 ;,-.. //·-· 8. 25 0 ·\\ 0 u C. .... 20 0u 0 .... 20 32 0 II) 15 \·~. 15 .:;-0 :;: ·-:~~~/' 10 :;: 10 ..~+ ·~:~.~. 5 + 5 +-~------+ • 0 + 0.5 1.0 1.5 2.0 2.5 3.0 10 15 20 25 30 35 Cone. of SOS / % Cone. of eomonomer I % on yield of copolymer Figure 7. Effects of amount of surfactant on yield of copolymer Figure 8. Effects of cone. of comonomer irradiation time : 1 h ; (sonic intensity : 600 w ; cone. of comonomer : 10% ; irradiation time : (sonic intensity: 600 w; cone. of SDS : 2.5%' 25°C; VAi V ,=1). 1 h; 25°C; VAIV8,=1). 8

Table II. Effects of surfactant on yield of PS-co-PBA Surfactant 40 Reaction condition------SDS CTAB OP-10 25°C, 60 min 35.4% 18.0% 0 ---:;; 30 35°C, 60 min 34.7% 25.2% 0 [ 40°C, 60 min 38.8% 27.8% 0 25°C, 45 min 20.8% 10.8% 8 20 25°C, 90 min 31.2% 15.3% 0 'o -0 Polymerization condition : sonic intensity : 600 w ; cone. of .:i 10 comonomer : 10% ; cone. of SDS : 2.5% ; VBA I Vs,=1. :;: may result from (1). high monomer concentration lowers phase, leading to decrease of relative volume of water 0 4 6 number of micelles able to disperse out of cavitation bub Ratio of eomonomer I V e/V si' V /VSt' V M/Vst bles (2). with increase of viscosity of the system, the high 8 concentration of co monomer results in fast rise of viscos Figure 9. Effects of comonomer ratio on yield of copolymer (sonic ity, which weakens the effect of cavitation, and lowers intensity : 600 w ; cone. of comonomer : 10% ; cone. of SDS : 2.5%, 1 h; 25°C). the dispersing rate of free radicals. irradiation time: The ratio of comonomer also influences the yield of co polymer considerably. As Figure 9 shows, increasing the amount of acrylate monomer is helpful to improve the acrylate monomers). The yield of copolymer increased bulk yield of copolymer. When VA!Vst is 1-1.7, the yield with the rise of ultrasound intensity, reaction time, but de reaches maximum. During this period, the number of temperature and the amount of ion surfactant, electrophilic free radical generated by cavitation in creases with the increase of concentration of comonomer. creases, which facilitates the oil/water emulsion co An optimum ratio of comonomer (VA IVst= 1-1. 7) exists ef eolymerization leading to the rise of y-ield. That CHr in the system. The type of surfactant has significant Anion surfactant has more CHCOOCH R is more active than Q-CH-CH2 also con fect on the yield of copolymer. 2 surfac tributes to the rise of yield. But when more acrylate contribution to copolymerization than the cation is ob monomer is added into the system, the yield drops. This tant. In non-ion surfactant system, no copolymer is dependent on the reactivities of monomers. Because tained. acrylate monomer is far less active than St, further in crease in their amount influences the equilibrium of the REFERENCES reaction and then lower the yield of copolymerization. 1605 (1996). Therefore, an optimum ratio of comonomer exists in the 1. D. Peter, J. Mater. Chem., 6, 2. P. Kruus and T. J. Patraboy, J. Phys. Chem., 89, 3379 (1985). system. 3. G. J. Price, M. R. Daw, N. J. Newcombe, and P. F. Smith, Br. Polym. J ., 23, 63 (1990). CONCLUSIONS 4. G. J. Price, D. J. Norris, and P. J. West, Macromolecules, 25, 6447 (1992). An• Copolymers (PS-co-PMA, PS-co-PEA, and PS-co-PBA) 5. R. Feng and H. Li, "Sonochemistry and Its Application", Press of Science & Technology, China, 1992, pp 74-88. are prepared by the sonication of monomers. (St and hui

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