Polymer Journal, Vol. 32, No. 2, pp 103-106 (2000) Studies on Ultrasonic Initiated Copolymerization of Styrene and Acrylate Series 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<c-H1 sity reaches a certain point known as cavitation thresh­ 1333.21 -CH, &cH,1 1265.43, 1163.02 yas1c-o-c1 old. After the cavitation threshold, with increase of sonic ,f -c-o-c-'° intensity (I), pressure and amplitude of ultrasound (Pa) in the medium increase, resulting in higher pressure in /0 ys<c-o c1 1034.99 -c-o-c- liquids (Pm, P m=Ph +Pa, Ph represents static pressure of fluid). The relationships between Pm and maximum tem­ 708.71,702.13 0 &mout side) perature (Tmax) and maximum pressure (Pmax) and col­ lapsing time of cavitation bubbles (t) are shown as, 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).