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Preparation and Swelling Properties of Poly (NIPAM)

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Preparation and Swelling Properties of Poly (NIPAM)
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  Journal of Colloid and Interface Science  221,  268–272 (2000)doi:10.1006/jcis.1999.6593, available online at http://www.idealibrary.com on Preparation and Swelling Properties of Poly(NIPAM) “Minigel” ParticlesPrepared by Inverse Suspension Polymerization Peter John Dowding, Brian Vincent, 1 and Elizabeth Williams School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom Received May 20, 1999; accepted October 14, 1999 The characterization of temperature- and pH-sensitive poly-  N  -isopropylacrylamide (poly-NIPAM) microgel particles, producedby surfactant-free emulsion polymerization, has been extensivelyreported. In the work described here poly(NIPAM) gel particles,cross-linked with  N  -  N   -methylenebisacrylamide (BA), have beenproduced using inverse suspension polymerization. These parti-cles have been termed “minigels” here since they are somewhatlarger than conventional microgels. Results suggest that minigelparticles are formed as a dilute suspension, within the aqueous dis-persed (droplet) phase. The hydrodynamic diameter of the minigelparticles produced in this work is  ≤ 2.5  µ m, at 25 ◦ C. The effectsof temperature and pH changes, variation in cross-linker concen-tration, and incorporation of a charged comonomer (methacrylicacid, MAA) have been investigated. Both poly(NIPAM-BA) andpoly(NIPAM-BA-MAA)minigelparticlesaretemperaturesensitivewith swellingbehaviorconsistent with comparablemicrogels. Vari-ationsinpH werefound toeffectthesizeofminigelscontainingion-izable groups (such as a carboxylate) by a mechanism of increasedelectrostatic repulsionof charged groupswith increasingpH. Over-all,theproductionoftemperature-and/orpH-sensitivepolymersbyinversesuspensionpolymerizationresultsinparticleswith swellingcharacteristics similar to those produced by emulsion polymeriza-tion, albeit with differing particle sizes.  C  2000 Academic Press Key Words:   suspension polymerization; poly-  N  -isopropylacry-lamide; microgel.INTRODUCTION Amicrogelisacross-linkedlatexparticle,withaporous,openstructure in the swollen state, resulting from a relatively lowconcentration of cross-linking monomer (typically up to ≈ 10%of the total monomer concentration). This open structure allowsfor changes in particle size (swelling and deswelling) whichmay be induced by changes in temperature, pH, solvent type,etc. The study of such particles is an area of growing interest ina number of applications, in particular for rheological controlfor high-solid formulations used in the coating industry (1), andfor selective uptake of heavy metal ions (2, 3). 1 To whom correspondence should be addressed. Fax: +44 (0)117 9250612.E-mail: brian.vincent@bris.ac.uk. Of particular interest are thermally sensitive microgels,which upon heating above a lower critical solution temperature(LCST)becomelesssolvatedbywater,resultinginacollapseof the particles (4). The preparation of colloidally stable poly-  N  -isopropylacrylamide (NIPAM) microgel particles, cross-linkedwith  N  -  N   -methylenebisacrylamide (BA), was first reportedby Pelton and Chibante (5), using an emulsifier-free, emulsionpolymerization process (with a solids content ≤ 2.5% wt). Theresultant particles were temperature sensitive, with a LCST of 31 ◦ C, and particle diameters (in the swollen state) in the range200–700 nm.Poly(NIPAM) microgel particles which are sensitive to pHmay be prepared by copolymerizing acid (or basic) groups intothepolymerstructure(6).SuitablevariationsinthepHwillresultin ionization of the acidic/basic groups in the polymer network and a corresponding change in particle size.Previous work regarding NIPAM-BA microgel particles inthis laboratory has focused on the effect of   n -alcohols (7) andthepresenceoffreepolymer(8)onparticlesize.Themajorityof systems studied to date have been prepared using an emulsifier-free, emulsion polymerization process where the particle size iscontrolled by the reaction temperature: an increase in tempera-ture results in the formation of smaller particles) (9).Particlesizesaretypicallylessthan1 µ mindiameter.Smallerparticles have been synthesized using microemulsion polymer-ization (10–14) including poly(NIPAM) particles (15, 16).Todate,littleattentionhasbeenpaidtotheformationoflargerNIPAM-BAmicrogelparticles.Fortheproductionoflargerpar-ticles, either a multistep swelling/emulsion polymerization pro-cess or a suspension polymerization must be employed. Fora successful suspension polymerization the dispersed dropletphase must have a low solubility in the dispersion medium, tomaintain the discrete emulsion droplets. NIPAM has a signifi-cant solubility in water ( ∼ 22.5%, w/w), and so water cannot beused as the dispersion medium. This necessitated the use of aninverse suspension polymerization reaction, with NIPAM andBA dissolved in water droplets (together with a water-solubleinitiator), dispersed in a suitable organic medium (heptane wasused in this work, due to the low solubility of NIPAM and BAin this solvent).Initiation of an inverse suspension polymerization reaction isgenerally achieved thermally using a free-radical mechanism. 2680021-9797/00 $35.00 Copyright  C  2000 by Academic PressAll rights of reproduction in any form reserved.  PREPARATION OF MINIGEL PARTICLES  269The initiator is usually dispersed in the dispersed (aqueous)phase such that polymerization should occur entirely withinthe droplets, by a solution polymerization mechanism. The useof continuous phase (oil-soluble) initiators has been reported(17). A limited amount of work has been published regard-ing the production of large microgel (or “minigel”) particlesby inverse suspension polymerization. Large poly(NIPAM-BA)minigel particles have been produced by Tanaka  et al.  (18). Theemulsion droplets were stabilized using nonionic “Span” sur-factants, with hexane as the continuous phase, at a fixed cross-linker concentration of 1.6% (w/w) BA. The resultant parti-cles were temperature sensitive, with a transition temperature of 34 ◦ C. The effects of the incorporation of a charged comonomer(  N  -(acryloxy)succinimide) in various concentrations were alsoinvestigated.Large poly(NIPAM-BA) minigel particles with diameters inthe range 0.25–2.8 mm, stabilized by a nonionic polymeric sur-factant, have been produced with paraffin oil as the continuousphase (19, 20). However, studies of the resultant particles wererestricted to the swollen state (i.e., below 31 ◦ C) (19). Askari et al.  prepared temperature-sensitive acrylic acid minigel parti-cles, using toluene as the continuous phase (21).Intheworkdescribedhere,NIPAM-BAminigelparticleshavebeen prepared using an inverse suspension polymerizationmethod using an ABA block copolymer where A = poly(hy-droxy stearic acid) and B = poly(ethylene oxide) (PEO) as apolymeric stabilizer (22). The cross-linker concentration hasbeen varied, and a charged comonomer (methacrylic acid) alsoincorporated into the NIPAM-BA minigel particle. A solidcontent of  ∼ 2.5% (w/w) was obtained from the polymerizationreaction.The effects of temperature and pH variation have been inves-tigated. EXPERIMENTAL  Materials NIPAM(97%),BA(99%),methacrylicacid(99%),potassiumpersulfate (99 + %), and heptane (99 + %) were all supplied byAldrich and used without further purification. The ABA block copolymer of polyhydroxy stearic acid with poly(ethylene ox-ide) (“Hypermer B246”) was supplied by ICI. Purite “Milli-Q”grade water was used throughout.  Minigel Synthesis Suspension polymerization reactions were performed in a500-ml cylindrical round-bottom glass flask, fitted with a re-fluxcondenser.ThecontinuousphasecomprisedtheABAblock copolymer (as polymeric stabilizer) (0.92 g), dissolved in hep-tane (200 ml). For NIPAM-BA microgels, the dispersed phasecomprised NIPAM (5 g, 44.2 mmol), BA in the range 2–9 wt%BA, based on the total mass of monomers added (0.1–0.5 g,0.65–3.24 mmol), and potassium persulfate (water-soluble ini-tiator) (0.35 g, 1.29 mmol), dispersed in water (40 ml). ForNIPAM-MAA-BA minigels, NIPAM (4.5 g, 39.8 mmol), MAA(0.5 g, 5.81 mmol), BA in the range 2–9 wt% (0.1–0.5 g, 0.65–3.24 mmol), and potassium persulfate (0.35 g, 1.29 mmol) weredissolved in water (40 ml). The pH of the aqueous monomer-containingdispersedphaseis ∼ 4.0forNIPAM-BAand ∼ 2.7forNIPAM-MAA-BA. The polymerization reaction is performedunbuffered.Emulsification was achieved by shearing the organic andaqueous phases, under an atmosphere of nitrogen for 1 h, us-ing a toothed-disc stirrer rotating at 600 rpm with the stirrerblade kept at a constant distance of 1 cm from the bottom of the flask. The agitator speed was reduced to 400 rpm duringpolymerization, which was performed under reflux at 70 ◦ C, us-ing a thermostatted water bath. Polymerization was allowed toproceed to completion overnight.After the polymerization was completed, water (200 ml) wasadded to the sample, and the heptane removed by rotary evapo-ration (resulting in phase inversion). The minigel particles werepurified by centrifugation (10,000 rpm, at 15 ◦ C for 45 min) andredispersedinwater.Thispurificationwasperformedat15 ◦ Csothat the minigel particles would be in a swollen state, allowingmore efficient purification. Characterization of Particle Size during Emulsificationand Polymerization In situ  droplet size analysis was carried out during the com-plete preparation procedure for the poly(NIPAM/2.9% BA)minigel using a “Lasentec” M200L instrument. The measure-mentsweremadethroughoutemulsificationandpolymerization.The probe tip was placed in the reaction flask, to the side of theagitator, approximately 1 cm above the toothed disk.The Lasentec instrument uses the Focused Beam ReflectanceMeasurementtechnique(FBRM),inwhichparticlespassingthesapphire window of the probe reflect a laser beam. The time of reflectance is directly dependent on particle size. The mannerin which the laser beam strikes the particles will vary depend-ing upon which part of the particle the beam strikes (since thewavelength of the laser is much smaller than the particle size)and hence a “chord length” is measured. Characterization of Minigel Particles Particle size measurements were obtained by photo cor-relation spectroscopy (PCS) using a Zeta Plus Analyser(Brookhaven Instruments Corporation, New York) operating ata wavelength of 632.8 nm and at a fixed scattering angle of 90 ◦ .Typically, five measurements were made and an average valueof the hydrodynamic radius was calculated.The effect of temperature changes on the particle size wasdetermined using PCS experiments in the range 15 and 50 ◦ C.Samples were left to equilibrate for at least an hour prior tomeasurement. After heating to 50 ◦ C, particle sizes were rede-termined on cooling back to 15 ◦ C, to check the reversibility of the swelling/deswelling.  270  DOWDING, VINCENT, AND WILLIAMS The effect of changes in pH on particle size (at 25 ◦ C) in boththeNIPAM-BAandtheNIPAM-MAA-BAminigelsampleswasalso investigated using PCS. Changes in pH were made usingstandard solutions of hydrochloric acid and sodium hydroxide.All samples were left overnight to equilibrate prior to measure-ment. The quoted pH is that measured immediately prior to thePCSmeasurements.Allthemeasurementsweremadeataback-ground electrolyte (sodium chloride) concentration of 0.1 moldm − 3 to allow for the effects of changes in ionic strength result-ing in pH change. RESULTS AND DISCUSSION Characterization of Particle Size during Emulsificationand Polymerization An indication of the variation in average particle size of apoly(NIPAM/3% BA) minigel particles during emulsificationand polymerization is shown in Fig. 1. The droplet diameteris determined as a “mean chord length.” During emulsificationthe average chord length is reduced, reflecting the productionof smaller droplets as the component phases are sheared. Dur-ing the initial stages of polymerization the average chord lengthis 6–7.5  µ m. However, after  ∼ 2.5 h of reaction the averagechord length increases to 9–11.5  µ m for the remainder of thepolymerization. This increase in average chord length duringpolymerization may be due to aggregation of partially polymer-ized “sticky” particles. The average chord length of the finalminigel-containing droplets is 11  µ m, which is much greaterthantheparticlediameteroftheminigels(2.4 µ mintheswollenstate, shown in Fig. 2a). This implies that after polymeriza-tion, the minigel particles are in the form of a dilute aqueousdispersion, within the larger water phase droplets, dispersed inheptane.  Effect of Temperature The effect of variation in temperature on the hydrodynamicradius, as a function of cross-linker concentration, is shownfor poly(NIPAM-BA) minigels in Fig. 2a and for poly(NIPAM-BA-MAA)minigels(atpH3.5)inFig.2b.Thedatashowthatall FIG. 1.  Average mean chord length of poly(NIPAM/2% BA) minigels dur-ing emulsification and polymerization, measured as a function of time. The startof polymerization is shown as the broken line. FIG. 2.  (a) The effect of temperature on the hydrodynamic diameterof poly(NIPAM-BA) minigels as a function of cross-linker concentration(  , 2.9%;  , 4.9%;   , 6.5%; and  , 9.1% BA). (b) The effect of temperatureon the hydrodynamic diameter of poly(NIPAM-BA-MAA) minigels ([MAA] = 5.81 mmol, 1.96% (w/w)) at pH 3.5, as a function of cross-linker concentration(  , 1.9%;  , 4.9%; × , 5.7%; and  ,9.1% BA). the minigel particles deswell with an increase in temperature, inagreement with Tanaka’s results (18). The effect of variation intemperature on the hydrodynamic radius of minigels was stud-ied both heating to 50 ◦ C and cooling back to 25 ◦ C and suggeststhat the swelling/deswelling process is fully reversible. How-ever,reswelling(oncoolingto25 ◦ C)wasfoundtobearelativelyslow process, taking up to 15 min for the minigels to reach theswollen state.The extent of particle collapse is dependent upon the concen-tration of cross-linker, with a decrease in cross-linker concen-tration resulting in an increased degree of collapse. This trendis in agreement with poly(NIPAM-BA) microgel particles pro-duced by dispersion polymerization (7, 23, 24). The deswellingprocess may be described in terms of the deswelling ratio, de-fined as ( d  / d  0 ) 3 , where  d   is the hydrodynamic diameter and  d  0 is the hydrodynamic diameter at 25 ◦ C. In the fully collapsedstate, minigels with the lowest cross-linker density (2%) showa deswelling ratio of   ∼ 0.01. As the cross-linker concentrationis increased (to 10%) the reduction in the degree of collapseis shown by an increase in the deswelling ratio to ∼ 0.2. Thesedeswelling ratios are of similar magnitude to those obtainedpreviously for poly(NIPAM-BA) microgel particles as a func-tion of cross-linker concentration (7), implying roughly similarswelling properties for microgels and minigels.  PREPARATION OF MINIGEL PARTICLES  271Previous studies by Pelton and Chilbante (5) concern-ing poly(NIPAM) microgels suggest a transition tempera-ture of 31 ◦ C. However, Crowther  et al.  (7) reported that forpoly(NIPAM-BA) microgels, only particles with a low cross-linker concentration (1 and 2% BA) show a narrow transitionregion near this temperature. An increased concentration of cross-linker results in a broadening of the temperature rangeover which deswelling occurs.The presence of carboxylic acid groups in the poly(NIPAM-BA-MAA) minigel particles reduces the temperature depen-dence of the hydrodynamic radius of the minigel particles com-pared with poly(NIPAM-BA) minigels (Fig. 2b). Morris  et al. (2)studiedtheeffectofacrylicacidpresentinpoly(NIPAM-BA-acrylic acid) microgel particles, and reported similar findings.However,thepresenceofthecarboxylicacidgroupdoesnotsig-nificantly affect the deswelling transition. Tanaka reported thatfor poly(NIPAM-BA-  N  -(acryloxy)succinimide) minigel parti-cles, an increase in  N  -(acryloxy)succinimide (ionizable group)concentration resulted in a decreased particle diameter (18).  Effect of pH  The hydrodynamic diameters of poly(NIPAM-BA-MAA)minigels as a function of pH are given in Fig. 3 for a tempera-ture of 25 ◦ C and an ionic strength of 0.1 mol dm − 3 NaCl. Thepoly(NIPAM-BA) minigels (i.e., no MAA present) were foundto display no change in hydrodynamic diameter with variationinpH,asexpected.Forminigelparticlescontainingmethacrylicacid, the hydrodynamic diameter shows the expected increasewith increasing pH. This type of behavior is typical of conven-tionalweakpolyelectrolytes.AspHisincreased,carboxylicacidgroups present in the NIPAM-BA-MAA minigel will undergodeprotonation. Thiswill result in acharging of theminigel. Thiscan result in electrostatic repulsion and may provide a possibleexplanationfortheobservedincreaseinhydrodynamicdiameter(above the p K  a  of MAA).The actual polymer architecture with respect to the distri-bution of ionizable MAA groups (whether distributed evenly FIG. 3.  The effect of pH on the hydrodynamic diameter of poly(NIPAM-BA-MAA) minigels as a function of cross-linker concentration (  , 1.9%;  , 4.9%; × , 5.7%;  ,7.4%; and  , 9.1% BA) measured against a backgroundelectrolyte concentration of 0.1 mol dm − 3 NaCl. throughout the particle or as a localized region) is not known.However, Sawai  et al.  (25) suggest that a gradual swelling pro-cess may be attributed to an inhomogeneity in the distributionof ionizable groups. CONCLUSIONS Poly(NIPAM)gelparticles,cross-linkedwithBA,canbepro-ducedusinginversesuspensionpolymerization;thesehavebeentermed“minigels”heresincetheyaresomewhatlargerthancon-ventionalmicrogelparticles.Theaveragedropletdiameteroftheemulsion droplets is  ∼ 7  µ m, which rises to  ∼ 10  µ m duringpolymerization. The hydrodynamic diameter of the resultantminigel particles is  < 2 . 5  µ m, at 25 ◦ C. This suggests that theminigel particles are formed as a dilute suspension, within theaqueous dispersed (droplet) phase.Bothpoly(NIPAM-BA)andpoly(NIPAM-BA-MAA)minigelparticles are temperature sensitive. Heating above the LCSTcollapses the particle, the swelling behavior being consistentwith comparable microgels produced by dispersion polymer-ization. The amount of cross-linker used in the polymerizationaffects the swelling properties of the minigel formed. A greaterdegree of collapse is observed for particles with the lowest con-centration of cross-linker (BA).VariationsinpHwerefoundtoeffectthesizeofminigelscon-tainingionizablegroups(suchasacarboxylate)byamechanismof increased electrostatic repulsion of charged groups with in-creasing pH. Variations in pH were found to have little effect onthe hydrodynamic radius of minigels without ionizable groups.Again this observation is in agreement with trends observed forcomparable microgel particles.Overall, the production of temperature- and/or pH-sensitivepolymers by suspension polymerization results in particles withswelling characteristics similar to those produced by dispersionpolymerization, albeit with differing particle sizes. REFERENCES 1. Bradna,P.,Stern,P.,Quadrat,O.,andSnuparek,J., ColloidPolym.Sci. 273, 324 (1995).2. Morris, G. E., Vincent, B., and Snowden, M. J.,  J. Colloid Interface Sci. 190,  198 (1997).3. Snowden, M. 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