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J. Phys. Chem. B 2009, 113, 10600–10606
Thermo-Induced Aggregation Behavior of Poly(ethylene oxide)-b-poly(N-isopropylacrylamide) Block Copolymers in the Presence of Cationic Surfactants Junpeng Zhao,†,‡ Guangzhao Zhang,† and Stergios Pispas*,‡ Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui, China, and Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou AVenue, 11635 Athens, Greece ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: June 1, 2009
Low-molecular-weight cationic surfactants, dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB), were introduced to dilute aqueous solutions of thermosensitive poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) block copolymers at concentrations (Cs) either lower or higher than the critical micelle concentrations (cmc) of the surfactants. The copolymer/surfactant mixtures were investigated by dynamic and static light scattering at different temperatures. At temperature lower than the aggregation temperature (Tagg), the disaggregation of the copolymers from the loose associations was observed upon addition of the surfactants. The thermo-induced aggregation behavior was found to be profoundly influenced with the cooperation of cationic surfactants in terms of Tagg and the structural characteristics of the aggregates formed at high temperature. In general, Tagg was increased together with the decrease in the size and molecular weight of the aggregates. These were attributed to the copolymer/surfactant interactions and the electrostatic repulsion coming from the ionic head groups of the surfactants within the mixed aggregates. These changes were much more pronounced at higher Cs. CTAB, which has a longer hydrophobic tail, displayed higher influences compared to DTAB. The formation of vesicles, by one of the copolymers, was suppressed in the presence of CTAB. At the higher CTAB concentration, only small mixed aggregates with very low mass were observed even at the highest temperature investigated. Introduction The interaction of amphiphilic copolymers with low-molecular-weight surfactants has received considerable attention,1,2 as the addition of surfactants may induce the aggregation of the copolymer or change the aggregation state of the pure polymeric aggregates.3 Such systems are also present in a variety of industrial colloidal formulations. The interaction of stimuliresponsive double hydrophilic block copolymers with ionic surfactants has also attracted academic interest.4-11 However, most of the latter studies focus on the interaction of copolymers containing an ionic and a nonionic water-soluble component with oppositely charged ionic surfactants.4-7 Few works have been reported on the interaction of double hydrophilic copolymers containing two nonionic components with ionic surfactants.8-11 Poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEOb-PNIPAM) has been studied widely as a family of thermosensitive double hydrophilic block copolymers. Above the lower critical solution temperature (LCST) of PNIPAM (∼32 °C),12,13 the aggregation behavior of PEO-b-PNIPAM has been found to be greatly dependent on the copolymer composition, copolymer concentration, and additives,8,9,14-18 in terms of aggregation temperature and the structural characteristics of the aggregates. However, the effect of low-molecular-weight surfactants on the aggregation behavior of PEO-b-PNIPAM has been investigated scarcely.8,9 * Corresponding author. Tel.: +30210-7273824. Fax: +30210-7273794. E-mail address:
[email protected]. † University of Science and Technology of China. ‡ National Hellenic Research Foundation.
Ionic surfactants, especially anionic surfactants such as sodium dodecyl sulfate (SDS), have been known to have a great ability in modifying the thermosensitive behavior of PNIPAM19-22 and PNIPAM-based microgels23-25 due to the intensive polymer/ surfactant interactions and the formation of charged polymer/ surfactant complexes. Cationic surfactants, especially DTAB and CTAB, are often introduced as an oppositely charged surfactant to form complexes with PNIPAM-based copolymers5-7 or nanoparticles26,27 containing anionic polyelectrolyte components, to modify the solubility and the thermosensitivity of the copolymer systems. However, studies focusing on the interaction of nonionic PNIPAM-based polymers with cationic surfactants are limited.10,11,28,29 This is most probably because cationic surfactants interact more weakly with polar polymers30,31 including PNIPAM,10,11 compared to anionic surfactants. Nevertheless, the interaction of PNIPAM with cationic surfactants, especially with CTAB which has a longer hydrophobic tail, at temperatures lower than LCST has been found to be worth noting as demonstrated in the isothermal titration calorimetry study by Loh and co-workers.29 Schryver et al. studied the selfassociation of PNIPAM and N-n-octadecylacrylamide (PNIPAMC18) copolymers in the presence of cationic surfactants including CTAB.11 The influence of SDS, an anionic surfactant, on the association and micellization of PEO-b-PNIPAM was studied by Nystro¨m et al.8,9 However, no work has been presented on the influence of cationic surfactants on the aggregation behavior of PNIPAM-based nonionic block copolymers such as PEO-b-PNIPAM. Since the interaction of cationic surfactants with PEO has been reported to be very weak,11,32-34 the effect of cationic surfactants on the aggregation
10.1021/jp9038896 CCC: $40.75 2009 American Chemical Society Published on Web 07/14/2009
PEO-b-PNIPAM Block Copolymers
J. Phys. Chem. B, Vol. 113, No. 31, 2009 10601
TABLE 1: Molecular Characteristics of PEO-b-PNIPAM Block Copolymers
SCHEME 1: Molecular Structures of PEO-b-PNIPAM Block Copolymer and the Cationic Surfactants
sample Mwa × 10-4 (g/mol) Mw/Mnb NPEOc NPNIPAMc wt % PEOa EON-1 EON-2
4.94 2.65
1.18 1.12
44 44
419 216
4.0 7.5
a Calculated by 1H NMR data as described in the Experimental Section. b By GPC in THF at 30 °C (PS calibration). c Number of monomeric units in each block.
behavior of PEO-b-PNIPAM can be ascribed mostly to the polymer/surfactant interaction with PNIPAM. In this paper, the aggregation behavior of PEO-b-PNIPAM block copolymers in the presence of cationic surfactants, DTAB and CTAB, is reported based on dynamic and static light scattering measurements. The influence of the cationic surfactants at different surfactant concentrations on the aggregation temperature and the structural characteristics of the aggregates will be discussed taking into account the copolymer/surfactant interactions. Additionally, by employing different types of cationic surfactants, we are able to look into the effect of the length of the hydrophobic tail of the surfactant on the formation of copolymer/surfactant complexes and on the aggregation behavior as a function of temperature. Experimental Section Materials. Poly(ethylene glycol) monomethyl ether (PEO-OH, Mn ) 2000 g/mol, Mw/Mn ) 1.04) was purchased from Fluka and used after drying in vacuum for 48 h. N-Isopropylacylamide (NIPAM) was obtained from Aldrich and recrystallized twice from benzene/n-hexane (1:4). 4,4′-Azobis(isobutyronitrile) (AIBN) from Fluka was purified by recrystallization from ethanol. 4-Cyanopentanoic acid dithiobenzoate (CPAD) was synthesized according to a reported procedure.35 Dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB) were purchased from Aldrich and Fluka, respectively, and used without further purification. Other reagents were used as received. The double hydrophilic poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) block copolymers were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization, using a PEO-based chain transfer agent (PEO-CTA), resulting from the reaction of PEO-OH with CPAD.36 The molecular structures of the copolymers were designed to be asymmetric with a short PEO block and long PNIPAM blocks. The molecular weights of the copolymers were calculated based on 1H NMR spectra, which were recorded on a Bruker DMX-300 NMR spectrometer, using chloroform-d (CDCl3) as the solvent and tetramethylsilane (TMS) as the internal standard. Gel permeation chromatography (GPC) on a Waters system (Waters 1515 pump, Waters 2414 differential refractive index detector, three µ-styragel columns), calibrated by using a series of monodisperse linear polystyrene standards, was used to determine the molecular weight distribution (Mw/Mn) of the polymers with THF as the eluent at a flow rate of 1.0 mL/min. The molecular weights of PNIPAM blocks were calculated using the known molecular weight of PEO and the 1 H NMR data of the copolymers. The molecular characteristics of the copolymers are given in Table 1. The molecular structures of PEO-b-PNIPAM and of the surfactants utilized in this study are shown in Scheme 1. Solution Preparation. Stock solutions of the block copolymers and the surfactants were made by dissolving a known amount of the sample directly in distilled water (conductivity
1
In the heating processes, each aqueous solution of EON with or without surfactant was heated from 25 to 50 °C (or 55 °C in some cases) by 2-4 °C intervals, and LLS measurements were performed 30 min after the stabilization of temperature so that equilibrium can be reached. It has to be noted that for each of the solutions SLS measurements were performed only after the temperature was high enough for the aggregation to take place, which was judged by the increase of 〈I90〉 and 〈Rh,90〉 and the appearance of narrow unimodal f(Rh,90) in DLS measurements. This was because at low temperatures, 〈Rg〉 values of the single EON copolymer chains and/or copolymer/surfactant complexes were not large enough for accurate SLS measurements. Moreover, in some cases, bimodal distributions of Rh,90 could be observed before the aggregation, which indicated the existence of more than one species in the solution and made it more difficult to utilize SLS data.
(4) Results and Discussion
The values of dn/dC of the block copolymers were calculated using the following equation40
(dn/dC)EON ) WPEO(dn/dC)PEO + WPNIPAM(dn/dC)PNIPAM (5) For the aggregates formed in the copolymer/surfactant complex systems, dn/dC were evaluated as
(dn/dC)complex ) WEON(dn/dC)EON + Wsurfactant(dn/dC)surfactant
Figure 1. Hydrodynamic radius distributions (f(Rh,90)) of EON-1 (upper figure) and EON-2 (lower figure) with DTAB at 25 °C, where Cp is constant at 1.0 × 10-3 g/mL and Cs are 2.0 × 10-3 g/mL (cmc).
(6)
W denotes the weight fraction in both eq 5 and eq 6. The values of dn/dC of the surfactants (DTAB and CTAB) are used as found elsewhere.41,42 At high temperatures, the apparent diffusion coefficient of the aggregates, Dapp, was obtained by extrapolation to zero angle, which leads to the apparent average hydrodynamic radius of the aggregates, 〈Rh〉, via the StokesEinstein equation. In all cases, 〈Rg〉/〈Rh〉 and density values reported here were calculated by using zero angle extrapolated values for the radii.
Effect of DTAB on the Aggregation Behavior of EONs. To clarify the effect of the cationic surfactants on the aggregation behavior of PEO-b-PNIPAM block copolymers, LLS experiments with pure copolymer solutions were also carried out, for direct comparison purposes. Figure 1 shows the hydrodynamic radius distributions (f(Rh,90)) of EONs with DTAB at 25 °C. When Cs is lower than cmc (2.0 × 10-3 g/mL), bimodal distributions of Rh,90 can be observed. The peaks at lower Rh,90 are assigned to the single copolymer chains, while the peaks located at the high size range can be ascribed to the association of the copolymer chains into loose micelle-like structures,9,43 which is also observed in the pure copolymer solutions in our experiments (not shown). The interaction of cationic surfactants with PNIPAM is well-known to be much lower than that with SDS.10,29 Apparently, at Cs ) 2.0 × 10-3 g/mL and at low temperature, the copolymer/surfactant interactions are not high enough to cause the disaggregation of the copolymer chains from loose associations. However, at higher levels of DTAB addition (Cs ) 8.0 × 10-3 g/mL), unimodal f(Rh,90) was observed for both EON-1 and EON-2. The peaks of the large species are not present indicating the disintegration of the loose copolymer associations. This observation indicates that at this Cs the surfactant molecules can interact more strongly with the copolymer chains in the form of micelles, or the copolymer
PEO-b-PNIPAM Block Copolymers
Figure 2. Temperature dependence of average scattering intensity at 90° (〈I90〉) of EON-1 with and without DTAB, where Cp is constant at 1.0 × 10-3 g/mL and Cs are 2.0 × 10-3 g/mL (cmc).
Figure 3. Temperature dependence of average hydrodynamic radius at 90° (〈Rh,90〉) of EON-1 with and without DTAB, where Cp is constant at 1.0 × 10-3 g/mL and Cs are 2.0 × 10-3 g/mL (cmc).
chains can be incorporated in the surfactant micelles because of the hydrophobic interaction.21 The copolymer chains in the large loose associations disaggregate because of the enhanced intermolecular electrostatic repulsion coming from the ionic head groups of the surfactants bound on PNIPAM.9,19-21 This observation primarily presents the fact that cationic surfactants also have the ability to interact with PEO-b-PNIPAM in aqueous solutions; however, high Cs is needed to observe this effect. It has to be noted that, although the peaks of the single copolymer chains are merged with the peak of DTAB micelles (see Figure S1, Supporting Information), it can be assumed that the number of free surfactant micelles in the solution is still large at low temperatures, as discussed for SDS.21 Here, we are not going to discuss much on the critical aggregation concentration (cac) in the copolymer/surfactant systems, which has already been studied by other methods,29 and the two different concentrations we used here are already high enough to give the first insight into the effect of Cs on the aggregation behavior of EONs. Figures 2 and 3, respectively, show the temperature dependence of average light scattering intensity (〈I90〉) and average hydrodynamic radius (〈Rh,90〉) of EON-1 with and without DTAB. The interaction of DTAB with PNIPAM is known to increase with the rise of temperature even below LCST.29 However, in our experiments with the lower Cs, the increase of the copolymer/surfactant interaction seems to be not high enough to break the loose associations, since no decrease in 〈Rh,90〉 before LCST (Figure 3) is observed and the peaks located at high Rh,90 in f(Rh,90) (not shown) are still present under these conditions. The aggregation temperature (Tagg) is not changed at this Cs, compared with that of pure EON-1, as indicated by the increase of both 〈I90〉 and 〈Rh,90〉 at ca. 32 °C. However, at the higher Cs, Tagg is observed at 34 °C, indicating much stronger intermolecular electrostatic repulsion which effectively prevents the aggregation of PNIPAM chains as was observed for SDS.8,9,19-21
J. Phys. Chem. B, Vol. 113, No. 31, 2009 10603 When the temperature is higher than Tagg, aggregation takes place abruptly in all cases. The levels of 〈I90〉 and 〈Rh,90〉 both decrease in the presence of DTAB, showing that the copolymer/ surfactant interaction is strong enough (when the temperature is higher than LCST of PNIPAM) to affect the aggregation behavior, even at the lower Cs used, giving rise to the mixed aggregates. Due to the effect of copolymer-bound surfactants,9 mixed aggregates with lower size and mass are formed. It has to be noted that scattering intensity instead of molecular weight of the aggregates is shown in the figures. This is because: (a) some errors exist in the evaluation of dn/dC for the copolymer/ surfactant systems, which causes an error in the determination of the molecular weight of the aggregates; (b) the mass of the aggregates may be underestimated since it is probable that not all the surfactant molecules are bound to the aggregates, especially at relatively lower temperatures. The value of 〈Rg〉/〈Rh〉 (Table 2, Figure S2 in Supporting Information) of pure EON-1 solution at 50 °C is 0.76, close to the value of a uniform sphere,40 probably implying the formation of “crew-cut” micelles.44,45 At the lower Cs, 〈Rg〉/〈Rh〉 has a higher value (0.85), indicating a nonuniform mass distribution. As discussed above, at lower Cs, intensive copolymer/surfactant interaction begins only at high temperature (>Tagg), and therefore the surfactants are bound mostly to the outer parts of the aggregates resulting in the nonuniformity of the overall mass distribution. However, at the higher Cs, DTAB micelles interact with the copolymer at lower temperature ( cmc) (Figure 11, Figure S4 in Supporting Information). The higher stiffness of the shorter PNIPAM chain in EON-2 may be the reason: it is more difficult for the shorter PNIPAM chains to wrap around the surfactant micelles.21 The weaker copolymer/surfactant interaction can also be the reason for the higher 〈Rh,90〉 (Figure 11) and higher Mw,agg and Nagg,p (Table 2) at high temperatures, compared to EON-1/CTAB mixed aggregates at the same Cs. Conclusions In the presence of cationic surfactants, the thermo-induced aggregation behavior of PEO-b-PNIPAM block copolymers is remarkably influenced in terms of aggregation temperature, size, and mass of the aggregates formed at higher temperatures. Moreover, disintegration of the loose copolymer associations, existing at low temperature, is also observed upon addition of surfactants. These observations are attributed to the copolymer/ surfactant hydrophobic interactions and the electrostatic repulsion caused by the surfactants bound on the copolymer chains and the mixed aggregates. Higher surfactant concentration and higher hydrophobicity of the surfactant are found to cause more pronounced changes. Additionally, the presence of copolymer also affects the structure of surfactant micelles; i.e., the breakdown of wormlike CTAB micelles by the copolymer having the longer PNIPAM chain is observed. The morphologies of the copolymer/surfactant mixed aggregates also seem to be influenced compared with the pure copolymer aggregates. Acknowledgment. J.Z. thanks the China Scholarship Council for offering the scholarship to work in TPCI-NHRF, Greece. The financial support of National Natural Science Foundation (NNSF) of China (20474060) and Ministry of Science and Technology of China (2007CB936401) is also acknowledged. Supporting Information Available: Hydrodynamic radius distribution, temperature dependence of 〈Rg〉/〈Rh〉, and Γ vs q2 data of EON/CTAB systems. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sastry, N. V.; Hoffmann, H. Colloids Surf., A 2004, 250, 247. (2) Jonsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solutions; J. Wiley & Sons: Chichester, U.K., 1998.
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