Langmuir 1991, 7, 1319-1324
1319
Articles Microheterogeneous Solutions of Amphiphilic Copolymers of N-Isopropylacrylamide. An Investigation via Fluorescence Methods Howard G. Schildt and David A. Tirrell’ Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003 Received July 19,1990. I n Final Form: November 27, 1990 Four copolymers of N-isopropylacrylamide (NIPAAM) with N-hexadecylacrylamide (HDAM) were prepared by radical copolymerization. Incorporation of more than 1.5 mol 5% of HDAM renders the copolymers insoluble in water at room temperature. The solution properties of NIPAAM copolymers containing 0.4-1.1 mol 5% HDAM were compared with those of the NIPAAM homopolymer (PNIPAAM). Cloud point and microcalorimetric measurements report lower critical solution temperatures (LCST) for the copolymers that are slightly depressed in comparison with the LCST of PNIPAAM. Fluorescence emission spectra were recorded for four probes (pyrene(11, 1-pyrenecarboxaldehyde(PyCHO,2), 8-anilino1-naphthalenesulfonicacid (ANS, 3), and 2-(N-dodecylamino)naphthalene-6-sulfonic acid (C~SNS, 4)) dissolved in aqueous solutions of PNIPAAM and of the NIPAAMIHDAM copolymers. Pyrene reports a decrease in polarity (inferred from an abrupt decrease in I1 13) at the LCST of PNIPAAM but reports either no change or a modest increase in polarity in the cop0 ymer solutions. Similar inferences may be drawn from the behavior of C12NS. In contrast, PyCHO reports large polarity losses at the LCST in each of the polymer solutions. The behavior of ANS is intermediate, in that small decreases in polarity are reported at the LCST of each of the copolymers. These results are interpreted in terms of a micellar model for the amphiphilic copolymers in which a relatively nonpolar HDAM core is segregated at room temperature from a hydrated NIPAAM corona. Collapse of the corona at the LCST is accompanied by increased mixing of NIPAAM and HDAM units. The consequencesof such mixing in terms of fluorescence emission spectra are strikingly dependent upon the micellar site of probe solubilization.
i
Introduction Microheterogeneous systems may be defined as mixtures in which the domains of the dispersed component(s) are large on the molecular scale yet small compared to the wavelengths of visible light. Such systems include, inter alia, micellar and vesicular dispersions, microemulsions, monolayers, bilayers, polymers, and inclusion complexes.’ The fascinating structural and dynamic properties of these preparations, coupled with their real and potential applications, have made the study of microheterogeneous systems one of the most active and exciting areas of investigation in the modern chemical sciences. Aqueous solutions of amphiphilic polymers constitute a class of microheterogeneous systems of particular technological interest since such solutions frequently exhibit unusual-and useful-rheological behavior. Schulz and co-workers,for example, have pointed out that copolymers of acrylamide that incorporate small amounts of N-alkylacrylamides characterized by enhanced viscosity combined with brine and/or shear stability.”2 Use of the fluorescene probe 8-anilino-1-naphthalenesulfonicacid (ANS) reveals the presence of hydrophobic sites (presumably aggregates akin to micelles) in aqueous solutions of such copolymers, even in solutions so dilute (100 ppm)
* To whom correspondence should be addressed.
t Present address: Polaroid Corp., 750 Main Street-5C, Cambridge, MA 02139. (1)Kalyanaeundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, FLJ987. Thomas, J. K. Chemistry of Excitation at Interlaces; American Chemical Society: Washington, DC, 1984. (2)Schulz, D.N.;Kaladas, J. J.; Maurer, J. J.; Bock, J.; Pace, S.J.; Schulz, W. W. Polymer 1987,!28,2110.
as to preclude substantial interpolymeric overlapegDowling and Thomas have used fluorescence techniques to demonstrate the presence of similar hydrophobic sites in styrene-acrylamide copolymers prepared by emulsion copolymerization.4 We describe in the present paper the results of fluorescence probe studies of a related class of amphiphilic copolymers prepared from N-isopropylacrylamide (NIPAAM)and N-hexadecylacrylamide (HDAM)! Poly(N-isopropylacrylamide)(PNIPAAM) is soluble in water a t room temperature but precipitates above a lower critical solution temperature (LCST) of 32-34 “C.6 Modification of PNIPAAM by the attachment of a small number of hydrophobic hexadecyl chains might then be expected to lead to unusual micellar aggregates in which the polar surface undergoes a thermal phase transition independently of the hydrocarbon core. The temperature-dependent architectures of these aggregates form the subject of the present work. We have used the fluorescenceprobes 1-4 in combination with cloud point and microcalorimetric measurements to investigate the temperature-dependent solution properties of PNIPAAM and of a set of NIPAAMIHDAM copolymers in water. The results are interpreted in terms of a mi(3) Siano, D. B.; Bock, J.; Myer, P.; Valint, P. L. Polym. Mater. Sci. Eng. 1987,56,609. (4)Dowling, K. C.; Thomas, J. K. Macromolecules 1990,23,1059. (5) We havereported some of these results in preliminary form: Schild, H. G.; Tirrell, D. A. Am. Chem. SOC.,Diu. Polym. Chem., Prepr. 1989,30 (Z),342. Winnikand coworkerehavedescribedfluorescenceprobeetudies of similar amphiphilic NIPAAM copolymers: Ringsdorf, H.; Venzmer, J.: Winnik, F. M.Am. Chem. SOC.,Diu. Pol-ym. Chem.,. ReDr. 1990.31 -
(i), 568.
(6) Heskins, M.; Guillet, J. E.J.Macromol. Sci., Chem. 1968,A2,1441.
0743-7463/91/2407-1319$02.50/0 0 1991 American Chemical Society
1320 Langmuir, Vol. 7, No. 7,1991
Schild and Tirrell
ular weights of the copolymers estkmated by gel permeation chromatography (M,= 320 000; M,, = 120 OOO) were equal to one another and to those of the homopolymer' within experimental error. Sample Preparation. Aqueous stock solutions (4.0 mg/mL) of each polymer were prepared by stirring of the sample in distilled water with 0.1 % ' (w/v) sodium azide as bactericide for several days. Samples that failed to dissolve at room temperature were refrigerated (2' -0 "C). Aliquots (0.20 mL) of these stock solutions were diluted to 2.00 mL with distilled water for all measurements except those with ANS. For studies involving pyrene and PyCHO, microliters of stock solutions of the probe in cellar model that provides at least two-and perhaps acetone were first evaporated in vials after which the three-distinct sites for probe solubilization. diluted polymer stock solutions were added, yielding ca. 1pM concentrations of probe. An aqueous stock solution Experimental Section (1.80 mL) of ANS was added to 0.20 mL of polymer stock Materials. The sources and purifications of NIPAAM solution to yield 290 pM ANS. Techniques for C12NS and azobis(isobutyronitri1e) (AIBN) as well as the synthmis have been previously described in detail.* and characterization of the PNIPAAM sample used in Measurements. Infrared spectra were obtained on a this study are reported in detail el~ewhere.~ Acryloyl Perkin-Elmer 1320 spectrophotometer from films cast on chloride (98%),hexadecylamine (go%),tetrahydrofuran NaCl plates. NMR ('H and 13C)spectra were recorded on (THF, HPLC grade), triethylamine (>99%), methanol a Varian XL-300 spectrometer. Copolymer compositions (HPLC grade), benzene (spectrophotometricgrade), chlowere determined via quantitative l3C NMR spectrometry roform (HPLC grade), 1-pyrenecarboxaldehyde (PyCHO), under broad-band lH decoupling using a 90' pulse width and pyrene were used as received from Aldrich Chemical and a delay time of 5 s in deuterated chloroform a t 20 "C. Co. Acetone and hexane (HPLC grades) were obtained The longest relaxation time (0.8 s) was from Fisher Scientific Co. 8-Anilinonaphthalene-1-sul- measured by spin-lattice an inversion recovery experiment using fonic acid ammonium salt (ANS) was obtained as puriss. broad-band decoupling, a 90' pulse width of 16.5 FS, and biochem (>99%) grade from Fluka Chemical Corp. Moan array of ten delay times varying from 0.008 to 4.000 s lecular Probes, Inc., was the source of the sodium salt of under identical conditions. Calculations of copolymer 2-(N-dodecylamino)naphthalene-6-sulfonicacid (C12NS). composition were based on the ratio of the integrated Synthesis and Characterization. Hexadecylacrylaintensity of the main signal of the n-hexadecyl chain (11 mide (HDAM) was prepared by the slow addition of a CH2 at 29.5 f 1.5 ppm) to that at ca. 23 ppm (2 CH3 of THF (18 mL) solution of acryloyl chloride (6.0 mL, 0.070 NIPAAM, 1 CH2 of HDAM). mol) to an ice cold, stirred mixture of hexadecylamine Gel permeation chromatography (GPC) was performed (13.4 g, 0.056 mol), triethylamine (9.8 mL, 0.070 mol), and with a Waters M45 solvent pump coupled to an R410 THF (400 mL). Triethylamine hydrochloride was filtered differential refractometer, four Microstyragel columns off after stirring the reaction mixture under nitrogen for (lo6, lo5, lo4, and lo3 A), and a Hewlett-Packard 3380A 23 h at room temperature. The filtrate was evaporated recorder. Degassed THF was eluted at 1.1 mL/min. and the product recrystallized from methanol (225 mL) Polystyrene standards (Polysciences) were used for caland dried to give 9.8 g (59%) of HDAM as a white polyibration, and molecular weights were estimated as those crystalline powder (mp 63-65 "C). Anal. Calcd for C19H37of polystyrenes of equivalent elution volume. PNIPAAM NO: C, 77.2; H, 12.6; N, 4.7. Found: C, 77.2; H, 12.6; N, samples were injected at 5 mg/mL and data analyzed with 4.7. TLC (methanol, 254 nm): single spot, Rf= 0.74. IR BASIC programs on a Macintosh SE computer. (CHCl3cast film) cm-l: 3300,3050,2975,2955,2920,1650, The techniques for determination of cloud point and 1620,1540,1470,1410,1380,1310,1240,995,955. 'H NMR microcalorimetric transitions and for temperature control (300 MHz, CDC13) 6: 6.0-6.4 (2 H, m), 5.5-5.8 (1H, m; 1 were identical with those reported elsewhere in detaiL7 NH, br), 3.3 (2 H, l),1.0-2.0 (28 H, complex), 0.9 (3 H, Emission spectra were obtained on a Perkin-Elmer MPFt). '3C NMR (75.4 MHz, CDCl3) 6: 165.4, 131.0, 125.9, 66 spectrophotometer with excitation at 303 nm (CI~NS), 39.0, 32.0, 29.7, 29.4, 27.1, 22.8, 14.2. 337 (pyrene), 365.5 (PyCHO), and 377 (ANS) nm using The copolymers were synthesized by following the slit widths of 5,3,5, and 10 nm, respectively. Errors are procedure used for preparation of the h~mopolymer.~ f0.05 for 11/13 (see below) and f0.8 nm for wavelength Various monomer feeds of HDAM (Table I) were copomaxima. Conditions used for observations of pyrene lymerized with recrystallized NIPAAM in benzene (100 emission were consistent with the recommendations of mL) initiated by AIBN (ca. 1mol % , recrystallized from Streeter and A ~ r e e . ~ methanol) at ca. 50 "C for ca. 24 h under nitrogen. The solvent was stripped, and after further vacuum drying, Results and Discussion the polymer was dissolved in chloroform (ca. 100 mL), Copolymer Samples. Five NIPAAM polymers were precipitated in hexane (ca. 900 mL), and dried to constant prepared for use in this work. In each case, polymerizaweight (yields, 70-85 7% ). Absent from the IR spectra of tion was run in benzene a t 50 "C for 24 h with AIBN as the copolymers were the 1620 (C-C), 1410 ( C H y ) , and initiator. The feed concentration of the hydrophobic 955 and 995 cm-1 (C-H vinyl out of plane bending) comonomer HDAM varied over a range from 0 to 10 mol vibrations observed in the spectrum of the monomer. % (Table I) and copolymer compositions determined by Copolymer compositions obtained from the NMR analyses 13C NMR spectrometry were in all cases similar to the described below are summarized in Table I. The molecfeed compositions. Gel permeation chromatography re(7) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352. The sample of PNIPAAM used in the present work is designated as sample A1 in ref. 6.
(8) Schild, H. G.; Tirrell, D. A. Langmuir 1990,6, 1676. (9) Streeter, K. W.; Acree, W. E. Analyst 1986, 111, 1197.
Langmuir, Vol. 7, No. 7, 1991 1321
Microheterogeneous Solutions Table I. Characterizationof PNIPAAM/HDAM Copolymers
monomer feed HDAM, sample A
B C
D E
mol % 0 0.29 1.14 1.90 9.98
polymer composition HDAM,mol % 0
0.4 i 0.1 1.1 i 0.3 1.7 i 0.2 9.9 f 0.1
2.00 1
i
1.75
microcalorimetry transition O C width,OC 32.4 0.9 30.8/32.0 2.5 30.0 3.3 insoluble insoluble
x
T-,
1.50 1.25
8z
1.00
0.75
0.50 0.25
0.00
vealed no significant differences in the molecular weighta of these samples. Incorporation of even very small quantities HDAM into NIPAAM copolymershas a striking effect on the solubility of the polymers in water. Samples D and E in Table I, which carry 1.7 and 9.9 mol 96 HDAM, respectively, were insoluble in water at all temperatures. The experiments described below were thus restricted to samples A-C. At the polymer concentration (0.40mg/mL) used in this work, samples B and C contribute 14 and 39 pM hexadecyl chains, respectively, to their aqueous solutions. The critical micelle concentration of the analogous single-chain surfactant sodium n-hexadecyl sulfate (SHS)is 550 pM above a Krafft temperature of 31 OC,l0 but aggregation of SHS in 0.40 mg/mL PNIPAAM solutions is observed at surfactant concentrations well below 100pM." Furthermore, critical aggregation concentrations for double-chain surfactants with C16 tails are on the order of nanomolar.12 If we assume a degree of polymerization of ca. lo00 for each of the polymer samples used in this work, samples B and C are estimated to carry ca. 4 and 12 hexadecyl groups per chain, respectively. Thus it appears likely that samples B and C would form microheterogeneous solutions in aqueous media through aggregation of their nonpolar HDAM units. Cloud Point and Microcalorimetric Measurements. The LCST of PNIPAAM is readily detected by cloud point and microcalorimetric method~.~J Figure 1shows typical cloud point data for samples A-C in water. Incoporation of 0.4 mol 96 HDAM depresses the cloud point by ca. 0.5 "C without any apparent change in the shape of the transition. Increasing the HDAM content to 1.1 mol 7% depresses the LCST further and causes a marked broadening of the transition. Figure 2 shows the corresponding calorimetric results. The homopolymer affords a sharp calorimetric endotherm centered at 32.4 "C, as reported previ~usly.~ Sample B yields a double-peaked endotherm, which may result from compositional heterogeneity, given the small average HDAM content of this sample. Sample C exhibits a broad endotherm at about 30 OC, with no evidence of multiple maxima. Table I summarizes the calorimetric data, which are in generally good agreement with the cloud point results. Fluorescence Spectra. Samples A-C were examined by fluorescenceprobe experiments using each of the probes 1-4. By using four different probes and two classes of chromophores (pyrene and naphthalene), we hoped to be able to map the microenvironmental polarity of the copolymer micelle with some measure of confidence. In the discussion that follows, each probe is described in turn before we conclude with a model of the temperature-de(10) (a) Schwuger, M. J. In Structure/Perjormance Relationships in Surjactantr;h n , M. J.,Ed.;American ChemicalSociety: Washington, DC,1984. (b) Lan e, V. H.; Schwuger, M.J. Kolloid 2.1968,2!?3,146. (11) Schild, H. Ph.D. Dissertation, University of Massachusetts, 1990. (12) Tanford, C. The HydrophobicEffect;Wiley: New York, 1980, p 112.
8.
20
22
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32
34
36
Temperature ("C)
Figure 1. Optical density (at 500 nm) for aqueous polymer solutions (0.40mg/mL); ( 0 )PNIPAAM; (0)PNPAAM/HDAM (0.4 mol % 1; ( 0 )PNPAAM/HDAM (1.1mol % 1. Samples were heated at ca. 0.4 "C/min. m
0 1.2-c
"P
n
T
Figure 2. Microcalorimetric endotherms for aqueous polymer solutions (0.40 mg/mL) heated at 15 "C/h. The samples are described and temperatures of peak maxima are given in Table I. The temperature scale is in the upper left portion of the figure.
pendent architecture of the copolymer aggregate. It is important to note that none of these probes, at the concentrations used herein, perturbs the cloud point or microcalorimeteric results for any of the samples of interest. Pyrene. The fluorescenceemission spectrum of pyrene is strikingly sensitive to the time-averaged environment of the c h r o m ~ p h o r e and ~ ~ Jhas ~ been used widely in studies of microheterogeneous solutions.lJ6 Although several characteristic features of the emission are useful in such studies, we have examined only the variation in Zl/Z3, the ratio of emission intensities at 373 and 384 nm, which is a sensitive reporter of local polarity. Figure 3 shows the temperature dependence of Z1/Z3 for aqueous solutions of polymers A-C. In PNIPAAM solutions at room temperature, 13/13 is identical with the value (1.87) measured in pure water. As the temperature is raised through the LCST, 11/13 decreases abruptly to 1.60, signaling transfer of the probe into the precipitated polymer phase. Pyrene at 1pM perturbs neither the cloud point nor the microcalorimetric behavior of PNIPAAM, as noted above, and the LCST determined fluorometrically is in good agreement with that measured by the other techniques. The results for sample B are significantly different, in that 11/13 is insensitive to the LCST reported by the cloud point and microcalorimetric experiments and remains essentially constant at 1.57 over the temperature range of (13) Kalyanaeundaram, K.;Thomas, J. K. J.Am. Chem. Soc. 1977,99, 2039. (14) Dong, D.;Winnik, M.A. Photochem. Photobiol. 1982,96,17. (15) Ananthapadmanabhan,K. P.; Goddard, E. D.; Turro,N. J.; Kuo, P.L. Langmuir 1986, I , 352.
Schild and Tirrell
1322 Langmuir, Vol. 7, No. 7, 1991 2.00 1.90
".i
0
0
i
1.70
24
26
28
30
32
34
36
38
40
Temperature ("C) Figure 3. Zl Za of yrene (1 rM) in aqueous polymer solutions (0.40 mg mL{: ( 0 )$NIP AAM; (0)PNIPAAM/HDAM (0.4 mol %); ( 0 ) NIPAAM/HDAM (1.1 mol %). ZI/ZS = 1.86 f 0.03in water, independent of temperature.
4
24
26
28
30
32
34
36
38
40
Temperature ("C) Figure 4. Emission maxima for PyCHO (1 rM) in aqueous polymer solutions (0.40 mg/mL): ( 0 ) PNIPAAM; ( 0 ) PNIPAAM/HDAM (0.4 mol %); ( 0 )PNIPAAM/HDAM (1.1 mol 5%). A, = 477.6 f 0.2 nm in water, independent of temperat UTe.
interest. The most reasonable interpretation of this result is that pyrene is associated with the hydrophobic HDAM units of the copolymer in such a way that the probe environment is unperturbed by collapse of the NIPAAM backbone at the LCST. The probe environment appears to be only slightly less polar than that provided by the collapsed chains of the homopolymer (Le., sample A) above the LCST, so the location of the probe at high temperature, or the possibility of mixing of NIPAAM and HDAM units above the LCST, is difficult to judge from this experiment. The behavior of sample C suggests that mixing of NIPAAM and HDAM units does increase above the LCST. Solubilization of pyrene in this copolymer at room temperature shifts 11/13 to 1.29, a value similar to that characteristic of micellar solutions of the nonionic surfactant Triton X-100.13 Increasing the temperature of the solution raises 11/13 to 1.45, with the rise broadened somewhat in comparison to the behavior of the homopolymer. The increase in the breadth of the transition is consistent with the cloud point and microcalorimetric results cited earlier, and the rise in 11/13 can be rationalized on the basis of penetration of the NIPAAM backbone units into the HDAM "core". Because the collapsed PNIPAAM chain creates an environment of higher polarity than that reported by pyrene solubilized in copolymer C a t room temperature, mixing of surface and core regions would be expected to increase 11/13 as observed. An alternative explanation for the rise in 11/13, i.e., that backbone collapse at the LCST expels pyrene from the nonpolar interior of the aggregate, seems less likely, since no driving force for probe expulsion is apparent. 1-Pyrenecarboxaldehyde. The solvent-dependent fluorescence of 1-pyrenecarboxaldehyde (PyCHO) was reported by Brederick and co-workers in 1960l6and used first in studies of microheterogeneous solutions by Kalyanasundaram and Thomas.17Js The latter workers demonstrated that the position of the PyCHO emission maximum (A& varies linearly with the dielectric constant of the solvent, a t least in solvents of dielectric constant greater than 10, and that the red shift with increasing
solvent polarity is accompanied by an increase in the fluorescence quantum yield. Ananthapadmanabhan and co-workers found that plots of ,A, vs surfactant concentration showed sharp beaks at concentrations very close to the criticle micelle concentration (cmc's) determined by surface tension measurements for both ionic and nonionic ~urfactants.'~ Figure 4 shows the dependence of A, on temperature in aqueous solutions of NIPAAM polymers A-C. In to be polymer-free solutions of PyCHO we find A, essentially constant at 478 nm, independent of temperature over the range investigated in this work. In contrast, in any of the three polymer solutions, we observea marked blue shift at the LCST, suggesting a decrease in the polarity of the microenvironment of the probe. These results are consistent with those obtained with pyrene only for the homopolymer. Whereas pyrene is insensitive to the LCST of sample B, PyCHO reports nearly identical polarity changes for this copolymer and for the homopolymer as the NIPAAM backbone units collapse. We suggest that PyCHO is solubilized near the surface of the copolymer aggregate at room temperature, such that the local environment of the probe is virtually that of the NIPAAM homopolymer. This interpretation is consistent with the suggestions of Kalyanasundaram and Thomas17and Turro and Kuo'~that PyCHO is solubilized at the surfaces of anionic17 and nonioniclg surfactant micelles. The results obtained with the more hydrophobic copolymer C can be rationalized within this same qualitative framework. Mixing of HDAM and NIPAAM units a t the LCST creates a micellar surface of lower polarity than that characteristic of copolymer B, and the blue shift is correspondingly larger. The surface probe, PyCHO, reports a decrease in polarity at the LCST, while the more hydrophobic pyrene probe reports a polarity increase as a fraction of its HDAM neighbors are replaced by more polar NIPAAM units (and perhaps by associated water). 8-Anilinonaphthalene-1-sulfonate.(Ary1amino)naphthalenesulfonates fluoresce weakly in polar media and strongly in nonpolar solvents. For example, the fluorescence quantum yield of 8-anilinonaphthalene-l(16) Bredereck, K.; Forster, T.; Oesterlin, H. G. In Luminescence of Organic and Inorganic Materials; Kallman, H. P., Spruch, G. M., as., sulfonate (ANS) is reported to be 0.003 in water and 0.57 Wiley: New York, 1962; p 161. in dioxane.20 Transfer of the probe from polar to non(17) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977,81, polar media is also accompanied by a pronounced blue 2176. shift, such that the position of the ANS fluorescence (18) The compound used in this work is sold as l-pyrenecarboxaldeemission maximum can be correlated with the empirical hyde by Aldrich Chemical Co., and is identical in structure to the probe referredto as pyrene-3-carboxaldehydein refs. 13,15,and 17. We believe solvent polarity scale (theZ scale) introduced by Kosower.21
the designation 1-pyrenecarboxaldehyde to be consistent with IUPAC nomenclature (cf. Handbook of Chemistry and Physics, 54th ed.; CRC Press: Cleveland, OH, 1973; p C-15). We also believe this compound to be misdrawn on p 42 of ref 1.
(19) Turro, N. J.; Kuo, P. L. Langmuir 1985,1, 170. (20) Turner, D. C.; Brand, L. Biochemistry 1968, 7, 3381.
Langmuir, Vol. 7, No. 7, 1991 1323
Microheterogeneous Solutions
C
.-0 v1
.zw
480 470 4
16
4504...0 . 24 26
0 .
.
I
.
28
.
.
I
b .
30
.
.
I
.
32
" .
.
Y
-
"
Y
,...,...,...I 34
36
38
40
Temperature ("C) Figure 5. Emission maxima for ANS (290 rrM) in aaueous poiymer solutions (0.40 mg/mL): ( 0 ) PNIPAAM; (e) PNIPAAM/HDAM (0.4 mol 5%); (0)PNIPAAM/HDAM (1.1 mol %). A, = 521 f 1nm in water, independent of temper-
22
24
26
28
30
32
34
36
38
Temperature ("C) Figure 6. Emission maxima of ClzNS (0.4 pM) in aqueous polymer solutions (0.40 mg/mL): (0)PNIPAAM; (e) PNIPAAM/HDAM 0.4 mol %; ( 0 )PNIPAAM/HDAM 1.1%. X, = 429 k 1 nm in water, independent of temperature.
ature.
Gitler22and Mast and Haynes23 have used ANS and/or related compounds in studies of surfactant aggregation, and Shinohara and co-workers have reported similar experiments on block copolymer micelles.24 Figure 5 summarizesthe fluorescence emission behavior of ANS in aqueous solutions of polymers A-C. As expected, the LCST of PNIPAAM is reported by a striking changes from 521 to 467 nm over a temblue shift; ,A, perature range of ca. 1"C. In contrast, the ANS emission is nearly insensitive to the conformational transitions that occur a t the LCST of copolymers B and C. In each case there is a small but measurable blue shift in the range of temperature corresponding to the LCST reported by PyCHO or by the calorimetric and cloud point methods described earlier. The behavior of the ANS probe is generally consistent with that of pyrene, although there are important differences in detail to be discussed below. 2-(N-Dodecylamino)naphthalene-6-sulfonate. We have recently reported the use of 2-(N-dodecylamino)naphthalene-6-sulfonate (C12NS)as an amphiphilic probe of polymer-surfactant interaction.* The emission maximum for Cl2NS shifts from 430 nm in water to 408 nm in 1-butanol,and a marked blue shift, signalingaggregation of the probe, is observed as the concentration of CnNS is raised to ca. 20 pM in dilute solutions of PNIPAAM. Because the amphiphilic structure of Cl2NS would be expected to pin the naphthalene chromophore near the interface between the core and surface regions of micellar aggregates, this probe offers special advantages in studies of microheterogeneous solutions. Figure 6 shows the temperature-dependent variation of the emission maximum for 0.4pM ClzNS in solutions of polymers A-C. The polarity changes reported by this probe are remarkably consistent with those inferred from the behavior of the neutral probe pyrene (cf. Figure 2), i.e., a decrease in polarity at the LCST of PNIPAAM, no change in solutions of the 0.4% HDAM copolymer, and a small increase in polarity for the more hydrophobic 1.1% HDAM copolymer. A Micellar Model. We suggest that all of the observations discussed above can be rationalized on the basis of the structural model shown schematically in Figure 7. The figure is intended to represent the behavior of copolymers B and C but makes no distinction between (21) Koeower, E. M. J. Am. Chem. SOC. 1968,80,3253. (22) Cordes, E. H.; Gitler, C. In Progress in Bioorganic Chem&ry; Kaiser, E. T., Kezdy, F. J., Eds.; Wiley: New York, 1973; Vol. 2, p 1. (23) Mast, R. C.; Haynes, L. V. J. Colloid Interface Sci 1975,53, 35.
(24) Ikemi, M.; Odagiri, N.; Tanaka, S.; Shinohara, I.; Chiba, A. Macromolecules 1982,15, 281.
T > LCST
Figure 7. Model for PNIPAAM HDAM micelles in aqueous solution, below and above the L S T pyrene and CllNS ( 0 ) ; ANS (A). PyCHO (0);
d
intrapolymeric and interpolymeric micelles.2s Below the LCST, aggregation of HDAM units leads to segregation of a hydrophobic core region from a hydrated surface zone, or corona, comprised of the relatively highly expanded NIPAAM backbone. PyCHO is proposed to sample the expanded corona, since its emission behavior is identical-below the LCST-in solutions of the copolymers and the homopolymer. On the other hand, the remaining probes (pyrene, ANS, and C12NS) report reduced polarity in the copolymer solutions as compared to those of the homopolymer. This seems to require some association of these probes with the HDAM core. Given the hydrophobic nature of pyrene and the amphiphilic structure of ClZNS, such association is to be expected. The similarity in the behavior of these probes suggests similar solubilization sites, and from the structure of C12NS we infer that these sites lie near the interface between the core and corona regions. We also place ANS in an interfacial site, but for reasons to be discussed below, this site is proposed to be richer in NIPAAM than those sampled by pyrene or C12NS. The LCST signals the collapse of the NIPAAM corona. PyCHO, in the cgrona, reports identical polarity changes in the homopolymer and in the more dilute HDAM copolymer, but the larger blue shift in the 1.1% copolymer suggests that segregation of the core and corona is reduced a t the LCST. Collapse of the corona is thus accompanied by an increase in mixing of NIPAAM and HDAM units and a reduction in the polarity of the PyCHO binding site. This interpretation is consistent with the behavior of pyrene and ClzNS as well, since these probes appear to be transferred a t the LCST from an HDAM-rich site to a site of higher polarity. The behavior of ANS is intermediate, (25) All of the measurements discussed in this paper were done at a polymer concentration of 0.4 mg/mL. Above the LCST, there is viaible interpolymeric aggregation, ae reported by the cloud point, but below the LCST the extent of interpolymeric association is unknown. We do not believe that any of our conclusions are affected by this ambiguity.
1324 Langmuir, Vol. 7, No. 7, 1991
in that only small polarity changes are reported at the LCST of either of the copolymers. The charged nature of the probe would appear to preclude solubilization a t a site deep in the HDAM core, if indeed such site exist in these solutions. Instead we propose an interfacial site, at which the mixing of core and corona changes the NIPAAM/HDAM concentration gradient without large changes in the average composition or polarity. Since ANS does not report an increase in polarity a t the LCST, it seems logical to propose a solubilization site richer in NIPAAM than those sampled by pyrene or ClzNS, but less polar than the corona sites probed by PyCHO. Figure 7 shows in crude fashion our proposed micellar model as well as the probe solubilization sites inferred from the fluorescence data.
Conclusions Amphiphilic copolymersof NIPAAM and HDAM form microheterogeneous solutions a t polymer concentrations
Schild and Tirrell
of 0.40 mg/mL. Association of HDAM units leads to segregation of a hydrophobic core region from a hydrated NIPAAM corona. The probes 1-4 report a t least twoand perhaps three-distinct solubilization sites: that of PyCHO in the corona, that of pyrene and CtzNS near the interface between core and corona, and that of ANS at an interfacial site relatively rich in NIPAAM. Collapse of the NIPAAM corona a t the LCST is reported most strikingly by PyCHO, but all four probes provide evidence of increased mixing of HDAM and NIPAAM units in the precipitated polymer phase at elevated temperature.
Rsgistry NO. 1,129-00-0;2,3029-19-4;3,82-76-8; 4,12998562-2;(HDAM)(NIPAAM) (copolymer), 125300-01-8;acryloyl chloride, 814-68-6; hexadecylamine,143-27-1; N-hexadecylacrylamide, 21216-80-8.
