Langmuir 1994,lOI 418-422
418
Interaction of Surfactants with Poly(N-isopropylacrylamide) Microgel Latexes K. C. Tam,?S. Ragaram,$and R. H. Pelton*p* School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 2263, Republic of Singapore, and McMaster Centre for Pulp & Paper Research, Department of Chemical Engineering, McMaster University, Hamilton] Ontario, Canada U S 4L7 Received June 16, 1993. I n Final Form: October 13, 199P The effects of surfactant type and concentration on the particle size and electrophoretic mobility of cross-linkedpoly (N-isopropylacrylamide) (polyNIPAM)microgel latexes were studied. Without surfactant the particle diameter of the polyNIPAM decreased by a fador of 2 when the temperature was increased above 31 “C, the cloud point temperature (CPT) for polyNIPAM homopolymer in water. Anionic sodium dodecyl sulfate (SDS) in the concentration range 0.001-0.008M caused the particle diameter to increase and the CPT to shift to higher temperatures. Higher SDS concentrations caused further swelling of the latex and the diameters became nearly independent of temperature in the range 10-60 “C. Nonionic Triton X-100had no effect on the particle diameterhemperature behavior indicating no surfactant binding. Below the CPT cationic dodecyltrimethylammonium bromide had no effect on swelling, whereas the cationic surfactant induced latex coagulation at the CPT. Surfactant binding was also reflected in the electrophoreticmobilities. Low levels of SDSbinding gave higher negativemobilitieswhich were temperature sensitive. High SDS concentrations removed the temperature swelling transition so mobility was temperature insensitive. Cationic surfactant binding increased the mobility of positively charged polyNIPAM latex; however, in contrast to SDS, the mobility was temperature sensitive at high surfactant concentrations.
Introduction Poly(N4sopropylacrylamide) (polyNIPAM) is a watersoluble polymer which possesses a cloud point temperature (CPT) of about 31 “C.l The unusual solubility behavior in water has led to many fundamental studies and novel applications of polyNIPAM which are summarized in a recent review by Schild.2 Many of the potential applications of polyNIPAM involve macroscopic three-dimensional gels which are obtained by copolymerization with N,”-methylenebis(acrylamide).3 Water-swollen polyNIPAM gels shrink when temperature is increased with most of the shrinkage occurring at the volume phase transition temperature (i.e. the temperature corresponding to the greatest change in volume), T,, is in the range 31-35 OC. In 1986,Pelton and Chibante4reported the preparation of polyNIPAM microgel latex by the surfactant-free polymerization of NIPAM monomer with N,”-methylenebis(acry1amide) using potassium persulfate initiator. The particle formation mechanism was felt to be analogous to the surfactant free polymerization of more hydrophobic monomers such as p~lystyrene.~ That is the polyNIPAM particles formed by homogeneous nucleation. Below T, the microgel latex particles are colloidally stable because of electrostatic and steric forces combined with low van der Waals attractive forces for the highly swollen particles. Above T,the particles are only stabilized by electrostatic stabilization caused by charged groups originating from the potassium persulfate initiator. As with surfactant-
* To whom correspondence should be addressed. t Nanyang Technological University. 8 McMaster University.
Abstract published in Advance ACS Abstracts, January 1,1994. (1)Heskin, M.; Guillet, J. E. J. Macromol. Sci.-Chem. 1968, A2 (8), 1441. (2) Schild, H. G. h o g . Polym. Sci. 1992, 17, 163. (3)Osada, Y.;Ross-Murphy, S. B. Sci. Am. 1993, 268, 82. (4) Pelton, R. H.; Chibante, P. Colloids Surf. 1986,20, 247. ( 5 ) Goodwin, J. M.;Ottewill,R. H.; Pelton, R. H.; Vianello, G.;Yates, D. E.Br. Polym. 1978, 10, 173.
free polystyrene latex, cationic polyNIPAM latex was obtained by polymerizing with azobis(isobutyramidine)hydrochloride, a positively charged initiator. PolyNIPAM microgel latexes display fascinating temperature-sensitive swellingproperties. Below T,, the latex consists of a water-swollen matrix of polyNIPAM chains held together by methylenebis(acry1amide) cross-links. Above the T,the particles exist as colloidally stable latex particles. In a subsequent publication, particle sizes and electrophoretic mobilities of polyNIPAM latexes were determined as a function of temperature.6 As the temperature was raised above the T,, the particle size decreased by about a factor of 2 whereas the electrophoretic mobility increased by a factor of 10. The particles contained a fixed number of covalently bonded electrically charged groups. Therefore when the temperature was raised, the diameter decreased giving increased surface charge density. Recently McPhee7 et al. refined the preparation technique of Pelton and Chibante by polymerizing NIPAM monomer in the presence of sodium dodecyl sulfate. The particle size, stability, and polydispersity were better controlled using surfactant. Kinetics of polyNIPAM microgel latex polymerization has recently been described? The extremely temperature sensitive solubility of polyNIPAM and the corresponding temperature sensitive swellingof polyNIPAM microgel latexes and macroscopic gels suggest many potential applications in fields such as biotechnology, pharmacology, and robotics. However, a serious potential difficulty is that aqueous polyNIPAM, like many other polymers? strongly associates with surfactants which in turn modify or completely eliminate
@
(6)Pelton, R.H.; Pelton, H. M.; Morfesis, A.; Rowell, R. L. Langmuir 1989, 5, 816.
(7) McPhee, W.; Tam, K. C.; Pelton, R. H. J. Colloid Interface Sci.
1993, 156, 24. (8) Wu, X.; Pelton, R. H.; Hamielec, A. E.; Woods,D. R.; McPhee, W.
Colloid Polym. Sci., in press. (9) Goddard, E. D. Colloids Surf. 1986,19, 255.
0743-7463/94/2410-0418$04.50/00 1994 American Chemical Society
Interaction of Surfactants with Microgel Latexes the temperature sensitivity. In early work Eliass&o reported that 1 wt % sodium dodecyl sulfate (SDS) increased the intrinsic viscosity and cloud point of polyNIPAM solution. The effects of chain lengths of n-alkyl sulfates were studied by Schild and Tirrell using microcalorimetry' and fluorescence probe" techniques. We have described conductivity12 and rheology'3 measurements which were used to probe the interactions of SDS with polyNIPAM homopolymers. Recently Kokufuta et al. reported the effects of SDS, DTAB, and a nonionic surfactant on the swelling of macroscopic polyNIPAM gel.l4 The main conclusion of this growing body of work is that ionic surfactants bind polyNIPAM homopolymer and gels. SDS levels raise the CPT of linear polyNIPAM and T,of gels to a maximum near 90 "C. The extent of gel swelling also substantially increases in the presence of SDS. Interestingly, cationic surfactant binds but has a much smaller effect on swelling and T,. Unresolved issues include the detailed structure of the polymer/surfactant complex. Presumably, polymerbound micelles exist; however, there is no direct information about size or shape in the polyNIPAM/SDS system. Surfactants also bind to polyNIPAM microgel latex. This paper describes the results of an investigation of the effects of surfactant on the swelling behavior of aqueous polyNIPAM latex. This work has 2-fold significance.First, the influence of surfactant on the properties of polyNIPAM microgel latexes may influence potential applications of the latexes. Second, the colloidal nature of the microgels permits electrophoresis measurements which give new information of SDS/polyNIPAM interactions.
Experimental Section Materials. Sodium dodecyl sulfate (BDH, specially pure), dodecyltrimethylammonium bromide (Aldrich), tetradecyltrimethylammonium bromide (Aldrich), and octylphenoxypoly(ethoxyethanol) (Triton X-100, Aldrich) were used as supplied. All solutions were made with water obtained from Milli-Q treatment of distilled water. Latex Preparation. T w o polyNIPAM microgellatexes were used in this work; they were anionic latex 05-75L and cationic latex 31-136. The preparation, purification,and characterization of anionic polyNIPAM latex 05-75L was described in a previous publication.' Latex 05-75Lwas cross-linkedwith 10w t % ,based on total monomer,of NJV'-methylenebis(acry1amide). Thelatex was polymerizedwith potassiumpersulfateinitiator givinganionic charge groupson the gel particles. McPhee' estimated the charge content of the latex to be 0.36 C/g of dry latex based on conductometric and potentiometric titration. The cationic latex was prepared at 60 OC by adding azobis(isobutyramidine)hydrochloride,a cationicfree radical initiator, to a solution of 7 g/L NIPAM, 0.7 g/L N,hJ'-methylenebis(acrylamide), and 4 g/L dodecyltrimethylammonium bromide (DTAB). Theinitiator concentrationwas 0.5g/L. Theapparatus and purification by serum replacement have been described.' Particle Sizing. The particle size of polyNIPAM latex as a function of temperature was determined by dynamic light scattering using a fixed angle NICOMP 370 submicrometer particle sizer (PacificScientific). Adiluted samplewas prepared by mixing 1part of cleaned latex to 10 parts of the appropriate surfactant solutions. The diluted latex was loaded into the measuring cell and further dilution carried out automatically by the instrument to yield an optimum signal output. The sample (10) Eliassaf, J. J. Appl. Polym. Sci. 1978,22, 873. (11) Schild, H. G.; Tirrell, D. A.Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1989, 30 (2), 350. (12) Wu, X. Y.;Pelton, R. H.; Tam, K. C.; Woods, D. R.; Hamielec, A. E.J . Polym. Sei., Part A Polym. Chem. 1993,31,957. (13)Tam, K. C.; Wu, X.Y.;Pelton, R. H. J . Polym. Sci., Part A Polym. Chem. 1993,31,963. (14) Kokufuta,E.; Zhang,Y.;Tanaka,T.;Mamada,A.Macromolecules 1993,26,1053.
p-
Langmuir, Val. 10, No. 2, 1994 419
600 SO0
E
omo . + A
400
d 300 to
E 200
0, 0.001
0-
SDS in Anionic PoiyNlPAM Latex
100
0
20
40
60
Temperature (C) Figure 1. Intensity averaged volume of polyNIPAh4 microgel latex as a function of temperature in SDS solutions. The SDS molarities are given on the graph. Note that data for 0 M SDS (0)and 0.001 M SDS ( 0 )fell on the same curve. was then allowed to equilibrate until the desired temperature was reached before data were sampled. Intensity averaged particle sizes are reported based on Gaussian analysis using Version 3.70 of the NICOMP software. Electrophoretic mobilities were measured with the Coulter DELSA. Experimental procedures have recently been described in detail.ls
Results (A) Anionic polyNIPAM Latex. polyNIPAM microgel latexes polymerized with potassium persulfate initiator are negatively charged due to sulfate and carboxyl groups originating from the initiator.6 Although the charge contents of polyNIPAM microgel particles are typically 1/10 of surfactant-free polystyrene latex: the charges influence the properties of the polyNIPAM gel. The influence of surfactants on the swellingand electrophoretic behavior of anionic and cationic polyNIPAM microgel latexes was investigated. Figure 1shows the effect of SDS concentrations on the particle size of polyNIPAM latex over a temperature range of 10 to 60 "C. Without surfactant the diameter-temperature relationship of the latex in deionized water was similar to previously reported behavior of polyNIPAM latex? At 10 "C, the latex particles were swollen with water. As the temperature was increased, the particle diameter decreased from an initial size of 330 nm to approximately 160 nm at 60 "C. Over this temperature range, the average particle diameter decreased by about 2 times which represents an effective volume change of a factor of 8. The greatest change in diameter occurred a t the volume phase transition temperature, T,,which was in the temperature range of 30-35 "C. The absolute water content of the polyNIPAM was not determined. However, Dong and Hoffman recently reported that water content of macroscopicpolyNIPAM gels in the temperature range 40-50 "C was about 25 wt %, which corresponds to about two water molecules per NIPAM repeat unit.16 The SDS concentration at which the surfactant first interacts with linear polyNIPAM was reported to be 7.9 X 10-4 and 6.9 X 1V M by Schild and Tirrell" and by Wu et al.,12 respectively. However, 1 X 10-3 M SDS did not influence particle diameter. Thus, insufficient binding occurred to increase particle swelling. In the SDS concentration range 0.0025-0.005 M the diameter versus temperature curves showed two trends. (15) Pelton, Robert; Miller, Peter; McPhee, Wayne; Rajaram, Sridhar. Colloids Surf. 1993, 80, 181. (16) Dong, L. C.; Hoffman, A. S. As reported by Osada, Y.;RossMurphy, S. Sci. Am. 1993, May, 86.
Tam et al.
420 Langmuir, Vol. 10, No. 2, 1994
loooo
I
FDTAB in Anionic polyNlPAM
n
Molarity
Y
Apwrsgm-
Q,
E
Anionic PolyNlPAM Latex
-3 15
25
0
100
35
Temperature (C) Figure 2. Electrophoretic mobility versus temperature for anionic polyNIPAM latex in the presence of surfactant. A constant total cation concentration0.0083 M was maintained by adding KC1. 400
Triton X-100 in Anionic
60
Temperature (C)
Figure 4. Influence of DTAB on anionic polyNWAM latex. Samples in the shaded area were colloidally unstable: .,0.016 M DTAB; A, 0.0048 M DTAB; 0, water. 1300
Cationic PolyNlPAM
E
MTAB Molam
L
s Q,
ti c,
5 200 A
50
40
Y
5E 3 0 0 :
6
30
n
n
:
20
10
0
45
\
5
ii
A0.00024 g
0.01
A
500 -
300
a
15 0
10
20
30
40
50
60
Temperature (" C)
Figure 3. Average diameter of anionic polyNIPAM latex as a function of temperature and concentration of Triton X-100.
First, at low temperatures the size of the latex increased with SDS concentration. For example at 20 "C the diameter was 400nm in 0.0082M SDS compared with 570 nm in 0.2 M surfactant. Second, T,,the temperature correspondingto the inflection point in the curves, shifted to higher temperatures with increasing the surfactant concentrations. This shift in Tvagrees with the results of published studies of the effect of surfactant on the CPT of linear polyNIPAM solutionsll and with recent results for macroscopic polyNIPAM geld4 At higher SDS concentrations the average particle diameter was nearly independent of temperature giving no indication of a T,up to 60 "C, the temperature limit of the instrument. The experimental data followthe same trend as that reported by EliassGl for linear polyNIPAM solutions in 1 wt % SDS. Figure 2 shows the electrophoretic mobility of the microgel latex particles as a function of temperature. KC1 was used to maintain a constant ionic strength. With no surfactant the electrophoretic mobility of the particles increased dramatically when the temperature was raised above the CPT. In the presence of 8 X 10-4 M SDS the mobility data were more negative. The electrophoretic mobility of the polyNIPAM latex was significantly increased with decreased temperature sensitivity in the presence of 0.0069 M SDS. The obvious interpretation was that bound SDS increased the surface charge density. It is noteworthy that the shape of the 0.007 M curve in Figure 2 (electrophoretic mobility) was similar in shape to the 0.0083 M curve in Figure 1 (particle diameter). The influence of nonionic Triton X-100on the diameter/ temperature behavior of the polyNIPAM microgel latex is summarized in Figure 3. The nonionic surfactant did not alter either the extent of swelling or the transition temperature range. Note, the higher surfactant concentration, 0.01M, is far greater than the cmc of Triton X-100. Similar results were obtained by Kokufuta et al. with
0 0.01 0 0.05
700 -
*
.
25
*
35
*
45
8
.
55
65
Temperature (C)
Figure 5. Influence of MTAB molarity on the swelling/ temperature behavior of cationic polyNIPAM microgel latex.
macroscopic gels.14 Schild showed that cloud point temperature of polyNIPAM was lowered about 3 "C in 0.5 M Triton X-100,whereas in 0.01 M the decrease in cloud point was insignificant." The influence of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) on diameter/temperature behavior of the polyNIPAM is illustrated in Figure 4. Over the temperature range 10-33 "C DTAB did not influence the average diameter. Above 33 "C the latex suspensions were colloidally unstable as evidenced by large and time-dependent particle diameters. Previous work has shown that polyNIPAM microgel latex particles are colloidally stable above the CPT because of electrostatic stabili~ation.~,~ Therefore, the colloidal instability shown in Figure 4 is believed to be due to the adsorbed DTAB lowering the surface potential of the particles. Similar destabilization of anionic polystyrene latex by cationic surfactant has been reported.18 (B) Cationic polyNIPAM Latex. The influence of tetradecyltrimethylammoniumbromide (MTAB) cationic surfactant on the diameter of cationic polyNIPAM Latex 31-136is summarized in Figure 5. Without surfactant, the cationic latex showed behavior similar to the anionic latex. With surfactant, the diameter decreased at every temperature. However, unlike the SDS results in anionic latex (seeFigure l), intermediateconcentrations of MTAB did not raise the Tv very much. Also, high MTAB concentrations (0.2 M) did not remove the volume/ temperature transition. Kokufuta et al. reported the effects of dodecyltrimethylammonium chloride on the swelling of macroscopic polyNIPAM geld4 They found that 0.5 M surfactant gave a Tvof about 40 "C which was rather insensitive to surfactant concentration. It is not (17) Schild, H. G. Conformational Transitione of Poly(N-isopropylacrybide) in Aqueous Solution. Ph.D. Thesis, University of Mamachusetta, 1990. (18) Connor, P.; Ottewill, R. H. J. Colloid Interface Sci. 1971,37,642.
Interaction of Surfactants with Microgel Latexes 3
ICationic P~I~NIPAM ktex
1
.
p
I
9
1
35
45
0.0089MMTAB
/
0.0083 M KCI
0 15
25
Temperature (C)
Figure6. Influence of the MTAB on the electrophoretic mobility versus temperaturebehavior of cationicpolyNIPAM latex. Total electrolyte concentration was maintained at 0.0083 M by KCl
addition.
.5
1
Cationlc PolyNIPAM La3x
I
-2.5 15
25
35
45
Temperature (C)
Figure 7. Influence of SDS on the electrophoreticproperties of cationic polyNIPAM latex.
possible to get accurate transition temperatures from the data in Figure 5; however, the trends seem the same for MTAB. The corresponding electrophoretic mobility versus temperature data are shown in Figure 6. The total ionic strength was maintained at 0.008 M by KC1 addition. The electrophoretic behavior of the positive latex in the presence of positive surfactant showed the same general features observed with SDS and anionic polyNIPAM latex (comparewith Figure 2). Without surfactant the mobility values were low indicating low numbers of cationic groups distributed in large swollen particles. With increasing temperature, the particles appeared to have a higher surface charge density. The electrophoretic mobility was a steep linear function of temperature for the 0.0069 M curve. This is unusual behavior for colloidal systems. Figure 7 shows the influence of SDS on the mobility of cationic latex. The anionic surfactant caused charge reversal with the highest surfactant concentration giving the largest effect. No flocculation was observed with the two SDS concentrations studied. Possibly a lower SDS concentration could have exactly balanced the cationic groups to give coagulation.
Discussion The literature shows that there are many parallels between the behavior of polyNIPAM homopolymer in SDS and polyNIPAM macrogel in SDS. For example, the influence of SDS on the CPT of linear polyNIPAM parallels the effects of T,on polyNIPAM gels. Similarly, binding of SDSincreases chain dimension of homopolymer as it does swelling of gels. The present work shows that these relations extend to polyNIPAM microgel particles. SDS caused increased particle swelling and, at high concentrations, rendered the particle diameters insensitive to temperature. Followingthe conclusionsfrom published
Langmuir, Vol. 10, No. 2, 1994 421
studies of the interactions SDS with linear polyNIPAM, we postulate that SDS binds with the polyNIPAM particles which increases the concentration of sulfate charge groups in the particles. Increased swelling of the particles with could therefore be an electrostatic effect. At high surfactant concentrations, electrostatic effects dominate and prevent the collapse of the polyNIPAM gel. The interaction of SDS with anionic polyNIPAM latex or MTAB with cationic latex increased the magnitude of the latex electrophoretic mobility. At high surfactant concentrations the latex diameters and mobilities were not strong functions of temperature. By contrast with low or no surfactant the electrophoretic mobility of polyNIPAM latex was a strong function of temperature. The binding of SDS with polyNIPAM influences electrophoretic mobility in two ways. First, the effective surface charge density of the particle increases which gives a higher mobility. Second, bound surfactant can give increased particle swelling which tends to lower the effective surface charge density. Comparison of the SDS curves in Figure 2 illustrates these effects. With the higher SDS concentration the particles remained swollen and the mobility was insensitive to temperature. By contrast, the low temperature mobility data were lowest for the low SDS concentration curve but with increasing temperature the curves crossed because the particles shrank at high temperature giving a higher effective surface charge density. The fo€lowingmodel expresses these concepts quantitatively. Previously, we derived the following equation for the dependence of the electrophoretic mobility on particle radius where p is electrophoretic mobility, N is the total number of charges per particle, e is the electronic charge, q is the viscosity, K is the Debye kappa value, and r is the particle radius? p=- -Ne (1) 4?rt/ltr2
The assumptions for this simple model included the following: all charges were located on the hydrodynamic surface of the particle; the surface charge density was related to the surface potential by the Helmholtz condenser formula; the surface potential equaled the zeta potential; and, the zeta potential was given by the Smoluchowski equation. N can be thought of as number charges on a sphere of equivalent hydrodynamic diameter required to give the same electrophoretic mobility as the polyNIPAM latex. This model was originally derived for surfactant free latex where the N was independent of temperature. By application of this equation to the electrophoretic behavior of polyNIPAMlatex in the presence of surfactant, it is assumed that the amount of bound surfactant (and hence N) is independent of temperature. Note, assuming all the charged groups are located on the exterior surface of the particle is an extreme assumption. In our previous work we also considered the case of charges uniformly distributed throughout the particle volume; the predicted trends were similar? Figure 8 showsthe mobility data for anionic polyNIPAM latex (Figure 2) in 0 and 0.0008 M SDS as a function of particle diameter (i.e. the 0 and 0.001 data from Figure 1). Note the size data from Figure 1 was obtained without KC1 addition so that for the 0 M SDS case the particles would have been slightly more swollen during dynamic light scattering than during the electrophoresis experiments. The solid lines in Figure 8 were computed using eq 1and the N values shown in the figure. According to the model the number of bound charges per particle increased from about 500 to 800 due to SDS sorption.
Tam et al.
422 Langmuir, Vol. 10, No. 2, 1994
.
='
2
0 MA*
o,oo, 100
~
8
~
5
SDS in Anionic PoiyNlPAMLatex
M; 200
300
Diameter (nm)
Figure 8. Electrophoretic mobility of anionic polyNIPAM microgel as a function of particle diameter measured from 20 to 40 O C . Data for 0 and 0.001 M SDS are shown along with
theoreticalcurves calculated from eq 1. N , the number of charges per particle, was the only adjustable parameter.
McPhee et al. measured the charge content of a ~ total polyNIPAM latex made by the same r e ~ i p e .The charge content of the cleaned latex (no bound SDS) was found to be 3.8 pequivfg of dried latex. The diameter of the anionic polyNIPAM latex was 165nm a t 40 "C. It was assumed that the water content of the polyNIPAM particles at 40 "C was two water molecules per NIPAM unit based on Hoffman's data.16 Furthermore, it was assumed that the specific gravity of the particles was 1. Based on these assumptions the molecular weight of polyNIPAM particles was estimated to be 1 X logg/mol of particles. Combiningthe particle molecular weight with McPhee's titration data gives a charge content, N , from titration of 5400 which is an order of magnitude greater than the N values estimated from eq 1. In summary, the simple theory predicted the main features of the experimental data. That is, the mobility was a strong function of temperature (diameter) and that SDS binding increased the particle charge. However, the absolute charge content of surfactant-free latex predicted from the electrophoresis model was an order of magnitude less than values given by the titration. It is interesting to compare the influence of MTAB on cationic polyNIPAM latex with behavior of anionic polyNIPAM latex in the presence of SDS. Both surfactants caused increased swelling at low temperature. Similarly, the magnitude of the electrophoretic mobility was higher with ei$her surfactant. The major difference was that SDS binding significantly shifted T,to higher
temperatures whereas MTAB caused only a small increase. The origin of this difference is not obvious. It is possible that the detailed structure of the bound surfactant was different in the two cases. For example, if the MTAB formed a few relatively large polymer bound micelles, the collapse of most of the polyNlPAM chains with increasing temperature would not be affected. If, on the other hand, the SDS formed many small polymer-bound micelles, the polyNIPAM chain collapse would be repressed by electrostatic interactions between neighboring polyNIPAM chains. Clearly, information about the detailed distribution of polymer-bound surfactant in the particles is required. It is remarkable that nonionic Triton X-100had no measurable influence on the swelling or mobility of polyNIPAM latex. An obvious explanation is that the surfactant did not complex with the particles. Goddard supported this view when he stated "There is little indication of reactivity of polyoxyethylated nonionic surfactants with water-soluble polymer^."^ Alternatively, Kokufuta et al.14 postulated that nonionic surfactants do bind to polyNIPAM; however, there was no effect on swelling because of the absence of electrostatic effects. In support of binding, Schild reported that 0.4 M Triton X-100lowered the CPT of linear polyNIPAM by 3 O C . 1 7 Conclusions The main conclusions of this work are as follows: 1. The influence of anionic, cationic, and nonionic surfactant on the swelling behavior of polyNIPAM microgel particles is similar to reported surfactant effects for macroscopic polyNIPAM gels and polyNIPAM homopolymer. 2. Binding of ionic surfactants was reflected in the electrophoretic mobility of gel particles and the trends with temperature were predicted with a simple model. 3. The presence of nonionic Triton X-100caused no measurable changes in the properties of the microgel latex. Acknowledgment. The authors wish to thank Dr.X.
Y.Wu for assisting in the preparation of the latex. This work was supported by the Mechanical and Chemimechanical Woodpulps Network of the Canadian Network of Centers of Excellence program.
