J. CHEM. SOC. FARADAY TRANS., 1994, 90(13), 1999-2000 FARADAY COMMUNICATIONS Microwave Synthesis of the Colloidal Poly(N4sopropylacrylamide) Microgel System Mary Murray, David Charlesworth, Linda Swires, Phillip Riby, Janice Cook, Babur 2. Chowdhry, Martin J. Snowden" School of Biological and Chemical Sciences, University of Greenwich, Woolwich, London, UK SE 18 6PF The preparation of monodisperse colloidal microgel particles of poly(N-isopropylacrylamide) by the use of microwave radiation is reported. The total synthesis time has been reduced from 6 h (conventional method) to less than 1 h (microwave method). The physicochemical characteristics of the microwave synthesized microgel are similar to those of conventionally prepared samples. This is the first report of the synthesis of a particulate colloid using microwave energy. Microwave radiation has been used for the initiation of a variety of different chemical reactions including S,2 type synthesis* and polymerisation processes e.g.for preparation of polyurethanes2 and p~lyamides.~This communication reports the first successful microwave synthesis of a colloidal microgel. The physical properties of the poly(N-isopro-pylacrylamide) (NIPAM) microgel system have received con- siderable attention in the recent literature. Studies reported include the reversible flocculation of the dispersion4,' and uptake and release characteristics, particularly with regard to polymers6 and heavy-metal ions.7 We have prepared poly(N1PAM) aqueous microgels using two different methods : (a)The conventional method by the free radical emul- sion polymerisation of NIPAM in water at 70°C, in the presence of N,N'-methylenebisacrylamide [(CH, = CH-CO-NH),CH,; from BDH Chemicals] as a cross-linking agent, following the procedure described by Pelton and Chibante.* Briefly the monomer was dissolved to a final concentration of 5 g 1-' with the cross-linker 0.5 g 1-1 and the initiator, ammonium persulfate, 0.5 g 1-'.The prep- aration took 6 h; the reaction mixture was thermostatted at 70 "C and stirred under an N, atmosphere. (b)A new method using microwave radiation for the syn- thesis of the microgel in a CEM MDS 2100 microwave cavity operating at 2450 MHz with a maximum power output of 1000 W. The cavity was equipped with a pressure transducer to allow continuous pressure monitoring within the CEM- lined digestion reaction vessel and a fibre-optic temperature probe to allow temperature control.These cavities are tradi- tionally used for digestion of solid samples for analytical pur- pose~.~Unlike most traditional synthetic routes the magnetron, or heat source, can be cycled in a feedback loop with reference to either the pressure, since the vessels are sealed, or the temperature within the vessel. Pressures can be varied from atmospheric pressure up to 200 psi? while tem- peratures of up to 200°C can be achieved. Heating within microwave fields is normally caused by either dipole rotation or ion migration. Since water is the continuous medium here, this mechanism is extremely effi- cient.The magnetron feedback was based on a pressure of 1.01 x lo5 Pa i.e. the magnetron cycled on and off so as not 7 1 psi = 6894.76 Pa. to exceed this limit. These conditions were chosen as they most closely resemble those used in traditional thermal- heating procedures. The reaction was carried out at 70 "C for 1 h using 400 W microwave power. The total volume of the reaction mixture was 75 ml and the stoichiometry of monomer to cross-linking agent identical to that for the free radical method (a). Transmission electron microscopy (TEM) showed the microgels prepared by both methods to be monodisperse spheres with mean diameter 380 & 28 nm for the convention- ally synthesized and 360 f25 nm for the microwave-prepared samples.The temperature dependence of the particle diameter of poly(N1PAM) microgel was determined turbidimetrically using a Perkin-Elmer Lambda 2 spectro- photometer connected to a programmable temperature- scanning water bath at a scan rate of 1 K min-'. The temperature in the measuring cell was monitored using a platinum thermocouple temperature probe. The turbidity of the dispersions was measured against distilled water in the range 20-50°C. Fig. 1 shows the change in turbidity at 547 nm as a function of temperature, for both heating and cooling scans. On cooling, the microgels re-expand to their original size. There is a good correlation between the tran- sition temperature obtained from the maximum of the first derivative of the turbidity measurements (34.9"C) and the value obtained from high-sensitivity differential scanning calorimetry (HSDSC) (35.6"C) for the microwave-synthesized microgel.This shows that the process(es) at the phase tran- sition are similar. The results from TEM, DSC and turbidity measurements suggest indeed that the structures of the microgels produced by methods (a) and (b) are similar. For both samples, the decrease in hydrodynamic diameter with increasing temperature is a consequence of the increase in the Flory interaction parameter, 2, for poly(N1PAM) in water. This facilitates more polymer-polymer contacts, hence the particles contract, forcing out solvent from the interstitial spaces.HSDSC measurements11,12 on the poly(N1PAM) microgel were carried out using a MicroCal MC-2D ultrasensitive DSC, with the DA2 software package for data acquisition and analysis supplied by the instrument manufacturer. Doubly deionized water provided the reference for all the measurements. The endothermic phase transitions exhibited by poly(N1PAM) using both synthetic methods are shown in 2000 om 01 I 0 10 20 30 40 50 60 TlOC -m .... 01 I 0 10 20 30 40 50 60 TrC Fig. 1 Turbidity of a 0.1% dispersion of poly(N1PAM) at 547 nm as a function of temperature; (m) heating and (0)cooling at 60 and -60 K h-', respectively, for (a) conventionally produced and (b) microwave-produced microgel Fig. 2 and the associated thermodynamic parameters are listed in Table 1.Clearly there are differences in the thermo- grams and this is reflected in the results derived from the mea- surements given in Table l. It should be noted that repeated heating and cooling scans were superimposable suggesting that the transitions are genuinely thermo-reversible. The ther- modynamic parameters for both samples are similar in terms of T, and AH,,, values. However the AT,,, of the conven- tionally produced microgel is ca. 60% greater than that of the E c I Y r b, 2.5 7 cp -20 30 40 50 T/"C Fig. 2 Temperature dependence of the partial excess specific heat capacity of the colloidal poly(N1PAM) microgel system in doubly deionised water at a concentration of 0.5 wt.%.The HSDSC record- ings shows the heating scans for (a)conventionally prepared and (b) microwave-synthesized microgel at a scan rate of 60 K h-'. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Microcalorimetrically recorded thermodynamic parameters for poly(N1PAM) microgels at a scan rate of 60 K h-thermodynamic conventionally microwave-parameter produced microgel produd microgel TZC 34.8 35.6 AHcaJJ g-' 24 23 AHvH/cal mol- ' 97400 167OOO ATI,,/oC 6.0 3.7 Cp,max/J g-' K-' 3.1 5.2 T, is the temperature at which the excess specific heat is a maximum (CpVmax);AHca,is the calorimetric enthalpy; AHvH the van't Hoff enthalpy and ATl,* the half-width of the transition at +Cp,max. corresponding value for the microwave-synthesized material.In addition the base molar unit (AHv,.JAHca,,using corre- spondingly appropriate units) has a significantly greater value in the case of the microwave-produced gel (1800 u) compared to the conventionally synthesized microgel (ca. lo00 u). These differences together with the differences in Cp, values appear to suggest that, in fact, the phase transition for the microwave-synthesized material has a higher cooperatively than the conventionally produced microgel. The T, values for both microgels were independent of scan rate (10, 30 and 60 K h-'). The values for the thermodynamic parameters had a standard deviation of 5% for microgels produced at different times using the two methods except for the T, which was always the same to within fO.l "C.Note that, because of the nature of the microgel, samples prepared even by the conven- tional method, at different times, will have slightly different thermodynamic parameters.More recent microwave results indicate synthesis times of as little as 12 min and the production of novel microgels at elevated pressure. Further reports of these findings and an outline of how the mechanism of the polymerisation process is effected by microwaves will be reported in a subsequent paper. Financial support for a PhD Studentship (M.M.) from the University of Greenwich postgraduate student bursary is gratefully acknowledged. This paper is released by the Bio- polymer section of the SMPG group of the University of Greenwich. References 1 R.Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge and J. Rousell, Tetrahedron Lett., 1986,27,279. 2 H. Jullien and H. Valot, Polymer, 1985,26, 506. 3 S. Watanabe, K. Hayama, K. H. Park, M-A. Kakimoto and Y. Imai, Makromol. Chem., Rapid Commun., 1993, 14,481. 4 M. J. Snowden and B. Vincent, J. Chem. SOC., Chem. Commun., 1992,16,1103. 5 M. J. Snowden, N. Marston and B. Vincent, Colloids Surf., 1994, in the press. 6 M. J. Snowden, J. Chem. SOC., Chem. Commun., 1992,11,803. 7 M. J. Snowden, D. Thomas and B. Vincent, Analyst (London), 1993,118,1367. 8 R. H. Pelton and P. Chibante, Colloids Surf, 1986,20,247. 9 Introduction to Microwave Sample Preparation, ed. H. M. King- ston and L. B. Jassie, American Chemical Society, Washington, DC, 1988. 10 P. J. Flory, Principles of Polymer Chemistry, Cornell University Press, 1953. 11 J. M. Sturtevant, Annu. Rev. Phys. Chem., 1987,38,463. 12 S. C. Cole and B. Z. Chowdhry, TIBTECH., 1989,7,11. Communication 4/016985; Received 22nd March, 1994