Langmuir 1993,9, 2907-2913
2907
Micellization of Amphiphilic Star Polymers with Poly(ethylene oxide) Arms in Aqueous Solutions Guang-bin Zhou and Johannes Smid' Polymer Research Institute, Chemistry Department, College of Environmental Science & Forestry, State University of New York, Syracuse, New York 13210 Received May 3,1993. In Final Form: July 2 8 , 1 9 9 9
Well-definedthree- and four-armedamphiphilicstar polymerswith a large hydrophobic core were made by the reaction of methoxypoly(ethy1eneglyco1)s(MPEG)and tri- or tetrafunctionalisocyanatesabbreviated as T3TMI and D4TMI. The latter are the hydrosilylation produds of m-isopropenyl-ru,a-dimethylbenzyl isocyanate (m-TMI) with methyltris(dimethylsiloxy)silane and 2,4,6,8-tetramethy1cyclotetrasiloxane, respectively. In aqueous media the stars aggregate, the critical micelle concentrations (cmc) being in the order of 1V M, with In cmc being proportional to the hydrophile-lipophile balance (HLB) of the stars. Cloud points (T,)were measured in the absence and presence of salts. For the salting-out electrolytes NaF and NaH2P04, T is proportional to the molal salt concentration. The proportionality constant, K,, is found to be inverseiy proportional to the length of the MPEG arm of the star.
Introduction
We recently synthesized well-defined aliphatic tri-, tetra-, and pentaisocyanates21and used them as precursors for PEO stars with a large hydrophobic core. T w o of the compounds, abbreviated as T3TMI and D4TMI, are shown in Chart I. They are hydrosilylation products of m-isopropenyl-a,.-dimethylbenzyl isocyanate (m-TMI),a commercial product from American Cyanamid.22 Most commercial multifunctional isocyanates are mixtures of compounds with different numbers of NCO gr0ups.23 However, T3TMI and DITMI, on reacting with methyield starswith exactly oxypoly(ethy1eneglyco1)s (MPEG), three or four PEO arms. In a preliminary report17 we presented some results on the solution properties of these stars including molecules in which the PEO arms were end-capped with hydrophobic groups. The latter polymers exhibit associative properties leading to interesting phase separation In this paper we give a more detailed account of the aqueous solution properties of the PEO star homopolymers such as their micellization, the relationship between critical micelle concentration and star structure, and the effect of salts on the cloud points of the stars.
Water-soluble amphiphilic are currently of interest in such applications as polymeric surfactants, rheology modifiers, drug carriers, polymer blend compatibilizers, and phase transfer catalysts. In a number of these polymers the hydrophilic component consists of segments of ethylene oxide units. Examples are block copolymers of ethylene oxide and styrene: graft or comblike copolymers with poly(ethy1ene oxide) (PEO) branches anchored to a hydrophobic backbone,%12and star-shaped macromolecules with PEO arms attached to a hydrophobic Rempp et al. showed that by anionic polymerization techniques using cross-linked divinylbenzene as the hydrophobic core amphiphilic star molecules with up to 250 PEO arms can be obtained although their molecular weight distribution is rather broad.14 e Abstractpubliahedin Advance ACSAbstracts, October 15,1993.
(1) Landoll, L. M. J. Polym. Sci., Polym. Chem. Ed. 1982,20,443. (2) McCormick,C. L.;Block, J.; Schulz,D. N. Encyclopedia ofPolymer Science and Engineering; John Wdey & Sona: New York, 1989; Vol. 17, pp 730-784. (3) Glass,J.E.,Ed.PolymersinAqueousMediu;AdvanceainChemistry Series; American Chemical Society: Washington, DC, 1989. (4) Xu, R.; Winnik, M. A.; Hallett, F. R.; Ries, G.;Croucher, M. D. Macromolecules 1991, 24, 87. (5) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G.0. Macromolecules 1992,25, 1304. (6) Ito, K.; Tomi, Y.; Kawaguchi, S. Macromolecules 1992,25,1534. (7) Yokoyama, M.; Miyauchi, M.; Yamada, N.; Okana, T.; Sakurai, Y.; Kataoka. K.: Inoue. S. J. Controlled Release 1990,II. 269. (8) D d e h , A. J.; Steiner, C. A. Macromolecules 1991,24, 112. (9) Khan, I. M.; Yuan, Y.; Fish, D.; Wu, E.; Smid, J. Macromolecules 1988,21,2684. (10) Bo, G.;Wesalen, B.; Wesslen, K. B. J. Polym. Sci., Polym. Chem. Ed. 1992,30,1799. (11) Berlinova, I. V.; Panavotov, . I. M. Makromol. Chem. 1989, 190, 1515. (12) Maltesh, C.; Xu, Q.; Somasundaran, P.; Benton, W. J.; Nguyen, H. Langmuir 1992,8,1511. (13) Lutz,P.; Rempp, P. Makromol. Chem. 1988, 189, 1061. (14) Gnanou, Y.; Lutz,P.; Rempp, P. Makromol. Chem. 1988, 189, 2885. (15) Brown, R. G.; Glass,J. E.Proc. Polym. Mater. Sci. Eng. 1987,57, 709. (16) GBczy, I. Acta Chim. Hung.1987,124, 547. (17) Zhou, G.;Smid, J. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1991, 32, (2), 613. (18) Xie, H.; Xm,J. Makromol. Chem. 1987, 188, 2543. (19)Saunders, R. S.; Cohen, R. E.; Wong, S. J.; Schrock, R. R. Macromolecules 1992,25, 2055. (20) Kanaoka, S.; Sawamoto, M.; Higashimura, T. Macromolecules 1991,24,5741.
0743-7463/93/2409-2907$04.00/0
Experimental Section Materials. The colorless liquids T3TMI and D4TMI (Chart I) were synthesized as reported.21 The methoxy(polyethy1ene glyco1)s (see Table I) were obtained from Aldrich. The low molecular weight liquids were dried azeotropicallywith benzene while the solid glycols were freeze dried from benzene. More precise moleeular weights were obtained by 'H NMR (Table I). Dibutyltin dilaurate (DBTDL) and N,N,Nt,Nt-tetramethylethylenediamine (TMEDA), both from Aldrich, were used as catalysts. (Nonylphenoxy)poly(ethylene glyco1)s (Igepale)used in cloud point measurements were obtained from Aldrich. Solvents were distilled from calcium hydride. Solutions of the stare were made in doubly distilled water. Saltswere used without further purification. Synthesisof Star Polymers. StarmoleculeswithPEOarms were Synthesized by reacting T3TMI or D4TMI in toluene with (21) Zhou, G.; Smid, J. J. Polym. Sci., Polym. Chem. Ed. 1991, 29, 1097. (22) Dexter, R. W.; Saxon, R.; Fori, D. E. J. Coat. Technol. 1986,58, 43. (23) Gnanou, Y.; Hild, G.;Rempp, P. Macromolecules 1984,17,945. (24) Zhou, G.;Smid, J. Polym. Prepr. (Am. Chem. SOC.Diu. Polym. Chem.) 1993, 34 (l), 822. (25) Zhou, G.Ph.D. Thesis, State University of New York at Syracuse, 1992. (26) Zhou, G.;Smid, J. Polymer, in press. Q
1993 American Chemical Society
Zhou and Smid
2908 Langmuir, Vol. 9,No. 11, 1993
Chart I 0
T3TMI
r H3C-Si
N
-CH3
I
II
f
0
y3
F-D4TMI
N U C U 0
pz
"3
H3C--(il
in 300 mL of water, and unreacted MPEG removed by repeated ultrafiitration through an Amicon YM2 membrane (MW cutoff 1OOO). The water was rotaevaporated, and the last traces were removed azeotropically with benzene. D4TMI-MPEG550 was obtained as a viscous, nearly colorless liquid, while other stars ranged from viscous liquids to white powdery solids (Table I). Data for DITMI-WEGSSO 'H NMR (CDCL) 6 (ppm)-0.16 (e, SiCHa), 0.82 (d, SiCH2) 1.17 (d, CHCHa), 1.53 (8, CCHs), 2.90 (m,CCH), 3.26 (8, OCHa), 3.34-4.02 (m, OCHz), 5.26 (8, CNH), 6.96,7,10 (m, phenyl); 13CNlMR (CDCb) 6 (ppm) 1.42 (SiCHs), 27.93 (SiCHz),26.10 (CHCHa), 29.83 (CCHs), 35.80 (CCH), 55.32 (CHaC), 59.00 (OCHs), 63.25 (COOCH21, 69.49, 70.37, 71.70 (CH20), 121.14, 121.87, 123.59, 127.00, 145.43, 147.91 (phenyl), 152.68 (NCO). Data for TSTMI-MPEGSSO The 'H NMR spectrum ie nearly identical to that of D4TMI-MPEGSSO except there are two SiCH3 peaks (-0.05 and -0.15 ppm) and SiCHa ie a doublet NMR spectrum is ale0 nearly the aame, at 0.91 ppm. The except the SiCHs peaks are at -0.50,1.76, and 2.40, and SiCH2 is at 29.03 ppm. Other stars derived from TITMI and D4TMI have chemical shift values similar to those of the two compounde listed above. Measurements. Siliconanalysiswae done with an ICP FMA03 emission spectrometer. lH and '9c NMR spectra were recorded on a Bruker AMX-300 spectrometer, TMS and CDCb being the respective internal standards. A Perkin-Elmer 1310 wae used for IR meaeurementa, and a Perkin-Elmer DSC4 (heating rate 20 "C/min) for thermal analysis of the polymers. Gel permeation chromatogramswere obtained in tetrahydrofuran with a Waters GPClA system and 100- to 106-A Ultraetyragel columns. Vicosities were measured with an Ubbelohde viscometer at 30.0 f 0.1 OC. Surfacetension data were acquired by means of a DuNoUly tansion meter calibrated with deionized water and benzene. Critical micelle concentratione (cmc)were obtained ae inflection pointa of surface tension (7)versus log concentrationplots. Cloud points (Tp)were measured with a calibrated thermometer placed in a 5-mL 3 wt % aqueous solution of the star polymer. The Tp values are reproducible within 0.2 OC.
Results and Discussion
As stated in the Introduction, moat commercially Table I. Properties of Star Polymers with PEO Arms and a TlTMI or D4TMI Core polymer MW f Tm,OC T.,OC MPEG550 530 20 MPEG750 700 30 1940 55 MPEG1900 MPEG5000 4800 63 TITMI-MPEG350 1900 3.0 -49 T3TMI-MPEG550 2480 3.0 T3TMI-MPEG75O 2980 3.0 34 -51 6460 2.9 57 T3TMI-MPEG1900 T3TMI-MPEG5000 15300 3.0 63 D4TMI-MPEG350 2360 3.8 -41 D4TMI-MPEG550 3150 4.0 13 -49 D4TMI-MPEG750 3750 3.9 36 -50 D4TMI-MPEG1900 8320 3.8 59 Molecular weights of methoxypoly(ethy1ene glyco1)s (MPEG) and stars were determined by lH NMR. For the calculation of the number of arms, f, see text. MPEG, using as catalyst either DBTDL or TMEDA. Equivalent quantities of OH and NCO groups were used in reactions with MPEG16O and MPEG350. The higher molecular weight (MW) MPEG compounds required excess glycol to minimize side reactions. In a typical synthesis, 1g (0.96 mmol) of D4TMI and 6.5 g (11.8 "01) of dry MPEG550 were added to 10 mL of toluene to form a homogeneous mixture. After addition of a few drops of DBTDL (or TMEDA), the mixture was kept for 3 h at 25 OC and then heated at 70 "C until no NCO peak (2260 cm-l) could be detected in the IR. Catalyst and toluene were removed by phase-separating the star and excess MPEG from cold hexane. After decanting the hexane and drying, the residue was dissolved
available multifunctional isocyanates are mixtures of compounds each with a different number of isocyanate groups. On the other hand, the aliphatic isocyanates T3TMI and D4TM1, obtained by the quantitative hydrosilylation of m-TMI with the commerical products methyltris(dimethylsiloxy)silane and 2,4,6,8-tetramethylcyclotetrasiloxane,21 are pure compounds with functionalities of three and four, respectively. Hence, they are effective in making star polymers with three and four PEO arms (DSTMI made with D5H and m-TMI can be used to make five-armed stars). Moreover, the isopropyl group adjacent to the NCO moiety (or urethane linkage in the star) renders formation of an allophanate structure less likely. This side reaction is more probable with higher MW MPEG's since the lower concentration of functional groups requires higher temperatures and longer reaction times for completing the reaction. For this reason excess MPEG was used in those systems. Removal of unreacted MPEG by ultrafiltration is facilitated by the formation of star polymer aggregates in aqueoussolutions while MPEG molecules do not associate (vide infra). GPC tracings for D4TMI-MPEG650 before and after ultrafiltration (Figure 1)show that the removal of MPEG is effective and that t no higher molecular weight species are present. The E spectrum did not show any allophanate absorption centered at 1650 cm-'. The 300-MHz 'H NMR spectrum of T3TMI-MPEG550 is shown in Figure 2 and ita lX! NMR spectrum in Figure 3. Chemical shift values are given in the Experimental Section, including those for D4TMIMPEG550. The OSi(CH2)(CH& methyl protons of the T3TMI star appear as two singlets of equal intensity at
Micellization of Amphiphilic Star Polymers B
Elution volume
Elution volume
Figure 1. GPC tracings of D4TMI-MPEG550 in tetrahydrofuran before (1) and after (2) ultrafiltration: A, star peak, B, excess MPEG550. CH3 (1)
(~3c-+-+si-cH3
().
FHp(b)
-
COO ( C H s C H 2 0 ) l 2 C H 9
cn
Langmuir, Vol. 9, No. 11, 1993 2909
arms even at 3 wt % star. However, the two-phase separation found with PEO stars containing hydrophobic arm does not occur with the homopolymer stars. The amphiphilic nature of our PEO stars with their large hydrophobic cores is expected to cause aggregation. Surfacetension (y)measurements prove that micellization occurs. Plots of y (dyn/cm) versus the log of the star polymer concentration are shown in Figure 4. Micelle formation is indicated by the distinct break in the plots. Cmc values for the D4TMI stars are listed in Table 11. Consistent with the findings for linear nonionic PEO ~urfactants,2~ the cmc increases with EO unit content. Linearity between In cmc and the EO number (Le., the number of EO units in the PEO arm) has been claimed for some nonionic surfactants,28but such a plot for our star polymers is strongly curved. Other researchers reported for ethoxylated alkylphenol-formaldehyde surfactants (EO number 5-9) a linear correlation between In cmc and the hydrophile-lipophile balance (HLB).n' The where latter can be defined as HLB = 20MH/(MH + ML), M H and ML are the respective formula weights of the hydrophilic and lipophilic segments of the molecule.27 HLB values for our four D4TMI-MPEG stars (their EO numbers vary from 8 to 43 per arm) are listed in Table 11,and a plot of In cmc versus HLB is shown in Figure 6. We also took cmc data reported by Schick30 for (nonylphenoxy)poly(ethylene glyco1)s or Igepals (EO number lo-!%), calculated their HLB values, and made a similar plot (Figure5). The two plots exhibit reasonable linearity. For the stars the correlation In cmc = -11.6
r
h
-
-
In cmc = -13.4
Chemical shift S (ppm)
Figure 2. 'H NMR spectrum (300 MHz) of T3TMI-MPEG55O in CDC13.
-0.05 and 0.15 as a result of the chiral CH group (see Chart I). This was also found for T3TMLZ1The average number of PEO arms per star molecule cf, was calculated from the expression f = (MW of the star - MW of the core)/(MW of the PEO arms). The MWs of the stars and of MPEG, listed in Table I, were obtained from their respective lH NMR spectra, and further confiimed from the ICP silicon analysis data. The silicon content of the stars was found to be within a few percent of the calculatedvalue. Melting points (T,)and some glass transition temperatures (T,) are also given in Table I. The T, values of stars made with MPEG of MW > 750 are all slightly above those of their MPEG precursors. The f values, combined with GPC and NMR data, indicate that the stars are essentially single molecules with either three or four PEO arms. Solubility and Micellization. The star homopolymers are soluble in organicsolventssuch as chloroform,toluene, tetrahydrofuran, and ethanol. The solutions are transparent over a wide concentration range. The water solubility is more complex. T3TMI-MPEG350 is not soluble in water, but T3TMI-MPEG550 and D4TMIMPEG350 and -550 give transparent solutions up to at least 10 wt % star. Above a PEO arm length of 550, the star solutionsbecome progressively more cloudy for longer
(1)
+ 0.28(HLB)
(2)
is found, and for the Igepals
h
I
+ O.BO(HLB)
For the same HLB number the cmc values of the PEO stars exceed those of the Igepals. This could be caused by steric hindrance in the aggregation of star molecules. Future experimentswill have to include the determination of the aggregation number of the micelles, since this is an important variable in understanding the aqueous solution behavior of these systems. The decrease in y with In C for the stars below their cmc (Figure 4) appears to be close to linear and similar to what is observed for most nonionic surfactants. Recently, a more complex pattern was reported for comblike amphiphilic polymers with short PEO branches.12 It showed discontinuities in the low concentration region. We have recently found the same for PEO star polymers made with atris(4isocyanatophenyl)methane core. The hydrophobic core of these stars is much smaller than that of the D4TMI stars, and consequently their cmc's are an order of M. However, a magnitude higher, around (1-5) X second distinct transition is found around a star concentration of 1V M. Preliminary studies with D4TMIMPEG1900 show a similar pattern but at much lower star concentration (-1V M). We are currently checking whether this behavior is typical for starlike polymeric surfactants. The observation may well be related to the recently reported behavior of amphiphilic block copolymers at water-air interfaces, showing conformational (27) Rosen, M.J.Surfactants andlnterfacialPhenomena;John Wiley & Sone: New York, 1978. (28) Heiao, L.;Dunning, H.N.; Lorenz, P.B.J. Phys. Chem. 1966,60, 657 --.
.
(29) Gendy,T.S.;Barakat,Y.;Mohammad,A.I.;Youssef,A.M.Polym. Znt. 1991,24,235. (30) Schick, M. J. J . Colloid Sci. 1962, 17, 801.
Zhou and Smid
2910 Langmuir, Vol. 9,No. 11,1993
2
.
PP.
I
I
140
L
h
i
160
h
-
I '
1
120
I
I
100
80
I
I
I
I
60
40
20
0
Chemical Shift S (ppm)
Figure 3. 13CNMR spectrum (75.6 MHz) of T3TMI-MPEG550 in CDCls.
40
35
* .-
100
10
[Polymer] x 10
4
1000
,M
Figure 4. Surface tension versus log polymer concentration for (0)D4TMI-MPEG350, (A) D4TMI-MPEG650, (+) D4TMIMPEG760, and (B) D4TMI-MPEG1900. Table 11. Critical Micelle Concentrations of DITMI-MPEG Stars in Water at 26 OC Star
EOnumber (perarm)
cmcx 1V(M)
D4TMI-MPEG350 D4TMI-MPEGSSO D4TMI-MPEG750 D4TMI-MPEG1900
7.5 11.7 15.5 43.0
0.9 1.3 1.6 3.0
YUIl.2
HLB (dyncm-'I 11.5 13.4 14.5 17.6
42.6 45.6 47.0 51.0
changes in the polymer aggregates that accumulate as mono- and multilayers at the i n t e r f a ~ e . ~ ~ ~ ~ ~ Viscosities. Figure 6 depicts Huggins viscosity plots for aqueous solutions of D4TMI-MPEG1900, Igepal990 (31) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992,25, 6556.
(32) Cha, X.; Yin, R.; Zhang, X.; Shen, J. Macromolecules 1991, 24, 4985.
t
1 ~
- A I
10
12
14
16
18
20
HLB
Figure 5. In cmc versus HLB number for ( 0 )D4TMI-MPEG star polymers and.(.) Igepala (the latter taken from ref 30).
(EO number 841, and MPEG5000. A distinct break in the plot of the star polymer can be seen at 0.25 g/dL or 2.9 x lV Myclose to the cmc of the star. Discontinuitiesof this type were ale0 reported for linear nonionic surfactanta,33 but in our work with IG990 no measurements of qsp/cwere carried out below the cmc. Intrinsic viscosities were found to be 0.17 for MPEG5000, 0.13 for IG990, and 0.11 for the star,the respectiveHuggins constants, k,being 0.48,2.48, and 1.85. The [ql values for IG990 and the star refer of course to those of their micelles which may well resemble compact ''fuzzy spheres".M The linearity of the plots impliesthat no significant breakdown of the micelles occurs. It has been S~OWIP that for uncharged solid spheres the Huggins constant k equals 2.0 and approaches 0.35 for flexible polymers. Hence, our (33) Laurence, M. K.; Willard, D. A. J. Phys. Chem. 1964,68,1163. (34)Bauer, B. J.; Fetters,L. J.; G r a d y , W. W.; Hadjichristidie, N.; Quack, G. F. Macromolecules 1989,22, 2337. (35) Tanford, C. Physical Chemiatry of Macromolecules; John Wiley & Sons,Inc.: New York, London, 1961.
Langmuir, Vol. 9, No. 11,1993 2911
Micellization of Amphiphilic Star Polymers
20
10
0
30
~ @ / M W MP E G 0.0
1.0
2.0
3.0
4.0
[CI, g/dl
Figure 7. Cloud point versus the inverse of the molecular weight of the MPEG arm for (A) TITMI-MPEG stars, (+) D4TMIMPEG stars, ( 0 )Igepals, and (m) MPEG.
Figure 6. Reduced viscosity versus polymer concentration for ).( D4TMI-MPEG1900, ( 0 )Igepal990, and (A) MPEG5000. Table 111. TDoand K, Values for Star and Linear Polymers polymer T,","C salt K. (dedm) D4TMI-MPEG350
35.9
D4TMI-MPEG550
69.4
D4TMI-MPEG750
85.5
D4TMI-MPEG1900 T3TMI-MPEG350 T3TMI-MPEG550
101 21.5 64.9
NdzPOi NaF NaSCN NdzPOi NaF NdzPOr NaF NdzPOi NaF
-41.4 -29.6 23.2 -51.9 -38.4 -55.7 -43.1 -62.0 -55.1
NdzP04 NaSCN
-49.1 52
T3TMI-MPEG750 T3TMI-MPEG1900 MPEG750 MPEG1900 MPEG5000 PEO (MW 4 X 106) IG520 IG720 IG890 IG990
k values appear reasonable. Bauer et aLU have argued that in a good solvent the [VI of a multiarmed star polymer approaches that of a linear polymer with a MW twice that of the star arm and can go below that value in a poor solvent. Twice the arm length of our D4TMI-MPEG1900 star resembles a MPEG of MW 3840. For PEO in water at 30 "C,[ q ] = 0.0125 M0.78.36Since for MPEG5000 [VI = 0.17, we compute an [VI of 0.14 dL/g for MPEG3840. This compares with [J= 0.11 for the star micelle D4TMIMPEG1900 and 0.13 for the IG990 micelle. Both these values, therefore, appear reasonable. Note that on breaking down the star micelle to the single star molecule the [VI decreases to around 0.08 dL/g (Figure 6). CloudPointsand Salt Effscts. Lower critical solution temperatures (LCST) or cloud points (Tp)are frequently encountered in aqueous solutions of polymers and nonionic surfactants containing segments of EO units. Our star polymers are no exception. Since cloud points were found to have constant values (within 0.2 OC) in solutions exceeding 1w t % polymer, all our Tpmeasurements were carried out in 2-3 wt % star solutions. Values of Tpfor the three- and four-armed stars are collected in Table I11 and denoted as Tpo(vide infra). Included are Tp data
10
12
14
16
18
20
HLB
Figure 8. Cloud point Venus HLB number for (A)T3TMIMPEG stars, (m) D4TMI-MPEG stars, and ( 0 )Igepals.
which we obtained for Igepals and MPEGs (the value Tpo = 237 "C for MPEG750 was obtained by extrapolation of the linear Tp plot of this compound with the molal salt concentration in solutions of NaH2P04 (vide infra)). Figure 7 shows plots of Tpas a function of the inverse of the MW of the MPEG star arm. This plot is nearly linear for the MPEG series. However, for the stars a shorter arm length also increases their hydrophobic content, causing the downward trend in Tp (Figure 7). Not surprisingly, the Tp values in all series converge on the value Tp= 101 OC, close to the value of 99 "C reportad for high MW PE0.37 The hydrophobicity effect on Tpshould disappear by plotting the cloud point versus the HLB number of the stars or the Igepals. Figure 8 shows that for the same HLB number the cloud points of three- and four-armed star polymers do not differ much, but they do differ from the Igepals. Below HLB = 14 the stars have higher cloud points while above this number the Igepals are more water soluble in terms of their Tpvalues. It should be stressed that all Tpmeasurements were carried out on solutions in which the respective compounds were in their micellar form. For Igepals the micellar aggregation number N rapidly increases with decreasing EO number.30i38 This could result in a more rapid decline in the Tp of Igepals with decreasing HLB number than for the stars aseuming their N is less susceptible to the HLB. Although their N values are not known, extensive aggregation of star (37) Sacki, S.;Kuwahara, N.; Nakata, M.; Kaneko, M. Polymer 1976, 17,686.
(36) Brandrup,J.; Immergut, E. H. Polymer Handbook, 2nd ed.;John Wiley & Sons: New York, 1975; p IV-23.
(38) Tanford, C.; Nozaki, Y.; Rohde, M. F. J. Phys. Chem. 1977,81, 1555.
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2912 Langmuir, Vol. 9,No. 11, 1993
t
0.0
NaSCN
0.2
0.4
0.6
0.8
1.0
I
1.2
[salt], m
0.0
Figure 9. Cbud point of D4TMI-MPEG550 versus the molal concentration of ( 0 )NaSCN, (A)NaF, and (W) NdzPOr.
molecules may well be hindered by the MPEG arms. Also, the exchange rate between micelle and single star molecule could be slow relative to that reported for Igepals which is in the order of le7s.39 This in turn could facilitate increased crystallization in micelles of stars with long MPEG arms, resulting in a lower solubility of these stars. Interestingly, these stars (i.e., with MPEG750 and 1900 arms) also exhibit haziness in aqueous solution in spite of their increased hydrophilicity. Electrolytes are known to affect the solubility and cloud point of nonionic surfactants, PEO, and other watersoluble polymer^.^^>^^^ We only looked at three salts, namely, NaF, NaH2P04, and NaSCN. The first two are typical "salting-out" electrolytes while NaSCN increases the cloud point. Figures 9 and 10 reveal that for NaF and Na&P04 the cloud points of D4TMI-MPEG stars decrease linearly with the molal salt concentration up to at least 1m. The results parallel our recent findings for PEO h y d r ~ g e l sand ~ ~ for comblike polymers with oligo(oxyethylene) side chains.42 In the latter work the Tpdependence on the molal salt concentration, C,,was expressed as
Tp = TpO+ K,C,
(3)
where TpOis the cloud point in the absence of salt. K,is a measure of the salt's effectiveness in modifying the stability of the polymer solution. Values for K,derived from the slopes of the respective plots are collected in Table I11 for several star and linear polymers, together with TpOdata. K , values for NaSCN were taken as the difference Tp- TpOa t 1m NaSCN. Since the change in Tp with the nature of the salt is primarily an anion effect,3O~~~ we only looked at the sodium salts. More extensive electrolyte effect studies on water-soluble polymers can be found in ref 42 and in work by other researchers (e.g., refs 30, 40, and 41). Table I11 clearly demonstrates that the salt effect as expressed by K,is more pronounced for stars with longer MPEG arms. Reasonably linear plots are found when K, is plotted versu the inverse of the degree of polymerization (DP) of the MPEG star arm. A similar relationship is found for comb poly[methoxypoly(ethylene glycol) meth(39) Ottewill,R.H. In Surfactants;Tadros, Th. F., Ed.;AcademicPress, Inc.: New York, 1984; p 1. (40) Bailey, F. E., Jr.; Callard, R. W . J. Appl. Polym. Sci. 1959,1,56. (41) Florin, E.; Kjellander, R.; Erikson, J. C. J . Chem. SOC.,Faraday Trans. 1 1985,80,2889. (42) Nwankwo, I.; Xia, D. W.; Smid, J. J. Polym. Sci., Polym. Phys. Ed. 1988,26, 581. (43) Xia, D. W.; Smid, J. Polym. Prepr. (Am. Chem. SOC., Diu. Polym. Chem.) 1991, 31 (I), 168.
0.2
0.4
0.6
0.8
10
1.2
[NaFl, m
Figure 10. Cloud point versus the molal concentrationof NaF for D4TMI-MPEG stars with MPEG arms of MW (m) 350,(X) 660,(A)750, and ( 0 )1900.
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
1IDPam
Figure 11. K,versus the inverse of the DP of the MPEG star arm for (0)TBTMI-MPEG stars + NaF,).( D4TMI-MPEG stars NaHzPOI, and (A) D4TMI-MPEG stars NaF.
+
+
Table IV. Values for the Constants a and b of Equation 4 star salt a (dedm) b X (dedm) NdSOr -67 2.0 D4TMI-MPEG D4TMI-MPEG NaF -59 2.4 T3TMI-MPEG NdzPOr -85 4.3 PEO (MW 4 X 106) N d z P O 4 -89 NaF -77 Igeph NdSOr -89 4.9
acrylatels in Na2S04 solutions when K,is plotted versus the inverse of the DP of the MPEG side chains (MW 200, 500, and 900).42 The relationship between K, and star arm MW can be written in the form
K,= a + b/DP,
(4) where a and b are constants which depend on the structure of the star polymer and the nature of the salt. Combining eqs 3 and 4 yields
+
Tp- Tpo= (a b/DP,)C, (5) Empirical equations of this type are useful in evaluating the effect of the number of arms and their molecular weight on the cloud point of salt-containing solutions of star polymers. Values of a and b obtained from the plots of Figure 11 are listed in Table IV. If in Figure 11the extrapolation to high MW arm length is permitted, then the constant a can be defined as the K,of a star with high MW MPEG arms. For such stars the hydrophobic core should have
Micellization of Amphiphilic Star Polymers
little effect on K,. Hence, differences in the constant a for PEO star polymers would be chiefly the result of differences in the number of MPEG arms. The constant b measures the sensitivity of K,(i.e., the change in cloud point when the star is transferred to a molal salt solution) to a change in the arm length. Note that K,changes more rapidly for the three star than for the four star. Also, for the former, a is lower than that of the four star by 18O m are nearly the although their Tpovalues for DP, same, and close to that of linear PEO (Figure 7). To compare the behavior of the PEO stars with the linear Igepals and with PEO itself, we measured cloud points of PEO (MW 4 X lo6) in solutions of NaF and NaHzP04, and of Igepal720,890,and 990 in aqueous NaH2PO4. Plots of Tpversus the molal salt concentration again are linear, and the respective K. values are listed in Table 111. For the Igepals a plot of K, vs 1/DP of the PEO segments also is linear. The a and b values for this system are given in Table IV. Not surprisingly, the a’s for the Igepals and PEO in NaHzP04 have the same value of -89 deg/m. As the MW O J , the properties of the Igepals should become indistinguishable from that of high MW PEO, also because micellization disappears. For example, at the 2 w t 5% concentration of the Tpmeasurements, an Igepal of MW lo5 has a molar concentration of 2 X 10-4
-
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Langmuir, Vol. 9, No. 11, 1993 2913
M. This is even below the cmc of the more hydrophobic IG990 (MW 4600) which from eq 2 is found to be 4 X le
M, although likely to be somewhat lower in 1 M NaH2Pod. It should be stressed that the Tpvalues are those of the star micelles. They resemble a star molecule with multiple arms. A high b value for the three-armed star may imply a more rapid increase in the aggregation number of their micelles than that for the four-armed star as the MW of the MPEG arm decreases. A highNmeans a high density of EO unit segments close to the hydrophobic core. This in turn reduces the K,value as these EO units may not be exposed to the salt solution. Higher a values for the stars (Table IV)relative to PEO or Igepals also can be traced to reduced exposure of EO unit segments to the salt solutions. A knowledge of the aggregation number of the star micelle as a function of arm length is critical to confiim some of our conclusions.
Acknowledgment. The authors gratefully acknowledge the financialsupport of this research by the Polymers Program of the National Science Foundation Grant No. DMR 8722245 and by the Petroleum Research Fund, administered by the American Chemical Society.
