Cationic Temperature-Responsive Poly(N-isopropyl acrylamide

Five copolymers were synthesized using aqueous atom transfer radical ... exhibited temperature-triggered formation of nanometer-sized aggregates at th...
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Langmuir 2008, 24, 7099-7106

7099

Cationic Temperature-Responsive Poly(N-isopropyl acrylamide) Graft Copolymers: from Triggered Association to Gelation R. Liu,†,‡ P. De Leonardis,§ F. Cellesi,§ N. Tirelli,§ and B. R. Saunders*,† Polymer Science and Technology Group, The School of Materials, The UniVersity of Manchester, GrosVenor Street, M1 7HS, U.K., School of Material and Chemical Engineering, Zhengzhou UniVersity of Light Industry, Zhengzhou, 450002, P.R. China, and Laboratory of Polymers and Biomaterials, School of Pharmacy, The UniVersity of Manchester, Oxford Road, M13 9PT, U.K. ReceiVed January 27, 2008. In Final Form: April 23, 2008. ReVised Manuscript ReceiVed April 18, 2008 In this work temperature-triggered association and gel formation within aqueous solutions of a new family of cationic poly(N-isopropyl acrylamide) (PNIPAm) graft copolymers have been investigated. Five copolymers were synthesized using aqueous atom transfer radical polymerization (ATRP) involving a macroinitiator based on quaternarized N,N-dimethylaminoethyl methacrylate units (DMA+). The PDMA+x-g-(PNIPAmn)y copolymers have x and y values that originate from the macroinitiator; values for n correspond to the PNIPAm arm length. The copolymer solutions exhibited temperature-triggered formation of nanometer-sized aggregates at the cloud point temperature, which was 33-34 °C. The aggregates were investigated using variable-temperature turbidity, hydrodynamic diameter, and electrophoretic mobility measurements. The aggregates were clearly evident using SEM and flowerlike or spherical morphologies were observed. Variable-temperature electrophoretic mobility measurements revealed that the zeta potentials of the aggregates increased with DMA+ content. A study of the effect of added NaNO3 showed that electrostatic interactions controlled the size of the aggregates. The concentrated graft copolymer solutions showed temperature-triggered gelation when the copolymer concentrations exceeded 5 wt %, Fluid-to-gel phase diagrams were constructed. It was found that electrostatic interactions also controlled the gelation temperature. A correlation was found between aggregate size and the minimum copolymer concentration needed to form a gel. A mechanism for the temperature-triggered structural changes leading to the formation of aggregates (in dilute solution) or gels (in concentrated solutions) is proposed.

Introduction Poly(N-isopropylacrylamide) (PNIPAm) is the most widely studied temperature-responsive polymer. It has a lower critical solution temperature (LCST) of about 32-34 °C for linear chains in aqueous solution. PNIPAm precipitates from water upon heating above the LCST due to its coil-to-globule transition.1,2 This effect has been exploited in a number of areas, including preparation of PNIPAm-based temperature-responsive hydrogels for biomedical applications3–5 and surface modification of substrates.6,7 PNIPAm-based transient networks have also been reported.8–10 In earlier work from the Saunders group7 it was demonstrated that temperature-triggered capture of polymer particles could be achieved using PNIPAm chains grafted from electrodeposited laponite particles using atom transfer radical polymerization (ATRP). That work led us to prepare a new, related, family of cationic PNIPAm graft copolymers which have potential for improved adsorption to anionic surfaces. In order to control their responsive behavior when adsorbed to surfaces * To whom correspondence should be addressed. E-mail: brian.saunders@ manchester.ac.uk. † The School of Materials, The University of Manchester. ‡ Zhengzhou University of Light Industry. § School of Pharmacy, The University of Manchester.

(1) Heskins, M.; Guilet, J. J. Macromol. Sci. Chem. 1968, A2, 1441. (2) Wu, C.; Wang, X. Phys. ReV. Lett. 1998, 80, 4092. (3) Qiu, Y.; Park, K. AdV. Drug DeliVery ReV. 2001, 53, 321. (4) Canavan, H. E.; Cheng, X.; Graham, D. J.; Ratner, B. D.; Castner, D. G. Langmuir 2005, 21, 1949. (5) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Sakurai, Y.; Okano, T. Nature 1995, 374, 240. (6) de las Alarcon, C.; Farhan, T.; Osborne, V. L.; Huck, W. T. S.; Alexander, C. J. Mater. Chem. 2005, 15, 2089. (7) Saunders, J.; Saunders, B. R. Chem. Commun 2005, 3538. (8) Durand, A.; Hourdet, D. Polymer 1999, 40, 4941. (9) Durand, A.; Hourdet, D. Polymer 2000, 41, 545. (10) Durand, A.; Hourdet, D.; Lafuma, F. J. Phys. Chem. B 2000, 104, 9371.

we must understand their structure-property relationships. The present study is the first report for this family of responsive copolymers to our knowledge and aims to identify structureproperty relationships in the contexts of temperature-triggered association and aggregation in solution. In recent years, stimuli-responsive water-soluble associative polymers, which are sensitive to various environmental conditions such as temperature have attracted considerable interest.11–13 One commonly used method for preparing stimuli-responsive water-soluble associative polymers involves grafting the responsive polymers onto a hydrophilic backbone. Concentrated solutions can form gels when triggered using temperature9 and form associative networks. Like traditional associating copolymers,14,15 the association between hydrophobic segments forms a transient physical network, which is responsible for substantial viscosity increases of the medium. The water-soluble associative copolymers that have received most attention to date consist of a hydrophilic backbone bearing a number of hydrophobic (side) chains.8–10 The inverse of this approach (stimulus responsive backbone and hydrophilic side chains) has been shown to confer temperature-responsiveness to emulsions and latexes.11,16 An example of the former approach is PAA-g-PNIPAm, which consists of PNIPAm side chains grafted onto a poly(acrylic acid) (PAA) backbone.8,10,17 In aqueous solutions temperature-triggered phase separation of the PNIPAm side chains is confined to a local scale due to the highly (11) Alava, C.; Saunders, B. R. Langmuir 2004, 20, 3107. (12) Daly, E.; Saunders, B. R. Langmuir 2000, 16, 5546. (13) Liu, B.; Perrier, S. J. Polym. Sci., Part A 2005, 43, 3643. (14) Berret, J.-F.; Calvet, D.; Collet, A.; Viguier, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 296. (15) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424. (16) Koh, A.; Saunders, B. R. Langmuir 2005, 21, 6734. (17) Chen, G.; Hoffman, A. S. Nature 1995, 373, 49.

10.1021/la8002756 CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

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Scheme 1. Synthesis of Cationic MI and Cationic Graft Copolymers

hydrophilic, deprotonated, PAA backbone.8,10 The hydrophobic aggregates that form result in a significant viscosity increase due to intermolecular associations of hydrophobic microdomains that act as physical cross-links between the hydrophilic backbones. Other backbones such as poly(vinyl alcohol),18 polyacrylamide,19 and carboxymethylcellulose20 have been used to graft PNIPAm side chains. In the present work we use a cationic poly(N,Ndimethylaminoethyl methacrylate) (PDMA+) backbone. ATRP has provided a versatile tool for preparing low polydispersity polymers21–23 and the approach allows initiation from surface groups.24 A range of responsive copolymers have been prepared by this method,25 including triblock copolymers containing26 DMA. A particularly useful example of ATRP in the context of the present work is represented by the use of macroinitiator-derived water-soluble copolymers developed by Chen and Armes.27 ATRP of NIPAm has been conducted from macroscopic surfaces6,7 and also from dispersed particles. PNIPAm copolymers prepared by ATRP have also been reported that are both temperature- and pH-responsive.28 The majority of copolymers based on PNIPAm that have been reported contain nonionic or anionic comonomers. Cationic PNIPAm polymers have been less frequently reported. This is probably due in part to the inherent difficulty in determining the molar mass of cationic polymers by GPC. There have been a number of reports involving (18) Nanaka, T.; Ogata, T.; Kurihara, S. J. Appl. Polym. Sci. 1994, 52, 951. (19) Sudor, J.; Barbier, V.; Thirot, S.; Godfin, D.; Hourdet, D.; Millequant, M.; Blanchard, J.; Voivy, J.-L. Electrophoresis 2001, 22, 720. (20) Bokias, G.; Mylonas, Y.; Staikos, G.; Bumbu, G. G.; Vasile, C. Macromolecules 2001, 34, 4958. (21) Braunecker, W. A.; Matyjaszewski, K. Prog. Polym. Sci. 2007, 32, 93. (22) Martter, T. D.; Foster, M. D.; Ohno, K.; Haddleton, D. M. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 1704. (23) von Natzmer, P.; Bontempo, D.; Tirelli, N. Chem. Commun. 2003, 13, 1600. (24) Bontempo, D.; Tirelli, N.; Feldman, K.; Masci, G.; Crescenzi, V.; Hubbell, J. A. AdV. Mater. 2002, 14, 1239. (25) Pietrasik, J.; Sumerlin, B. S.; Lee, R. Y.; Matyjaszewski, K. Macromol. Chem. Phys. 2007, 208, 30. (26) Munoz-Bonilla, A.; Fernandez-Garcia, M.; Haddleton, D. M. Soft Matter 2007, 3, 725. (27) Chen, X. Y.; Armes, S. P. AdV. Mater. 2003, 15, 1558. (28) Ge, Z.; Cai, Y.; Yin, J.; Zhu, Z.; Rao, J.; Liu, S. Langmuir 2007, 23, 1114.

DMA copolymers25,26,29 prepared using ATRP. Adsorption of PDMA copolymers onto silica substrates was reported by Sakai et al.29 However, we are not aware of any reports for copolymers of DMA+ and NIPAm prepared by ATRP. Here, we identify structure-property relationships for a new family temperatureresponsive cationic graft copolymers containing both of these units.

Experimental Section Materials. NIPAm (Aldrich, 97%) was purified by crystallization from hexane and stored at -20 °C. 2-(N,N-Dimethylamino)ethyl methacrylate (DMA, Aldrich, 98%) and 2-hydroxyethyl methacrylate (HEMA, Aldrich, 99%) were distilled in vacuum and stored under at -20 °C. Triethylamine (TEA, Aldrich, 99.5%) was dried using CaH2 and freshly distilled at atmospheric pressure. N,N′,N′′,N′′Pentamethyldiethylenetriamine (PMDETA; 99%), 2,2-bipyridine (bpy, 99%), CuBr (99.99%), 2-bromoisobutyryl bromide (BiBB, 98%), iodomethane (CH3I, 99.5%), and ethyl R-bromoisobutyrate (EBiB, 98%) were purchased from Aldrich and used as received. All other reagents were purchased from Aldrich and used without further purification. Water was of Milli-Q quality. Synthesis of the Cationic Macroinitiators. The PNIPAm graft copolymers were synthesized by ATRP using the grafting from technique (see Scheme 1). The synthesis of cationic macroinitiator (MI) was conducted using the method of Chen et al.30 using a threestep synthesis (Scheme 1). The copolymers composition is represented as PDMA+x-g-(PNIPAmn)y, where x, y, and n are, respectively, the number of DMA, esterified HEMA, and NIPAm units in the copolymer. We use abbreviations of the type MI-PNIPAm20k to simplify identification of the copolymers (see Table 1). (For the example given “20k” indicates the target molar mass for the PNIPAm arms.) First, a statistical copolymer of DMA and HEMA was synthesized (I) in methanol at 20 °C using EBiB as ATRP initiator, CuBr as catalyst and bpy ligand. Then, esterification of the hydroxyl groups of the poly(DMA-s-HEMA) copolymer (0.01 mol) was conducted to give (II). This was achieved by adding excess BiBB (29) Sakai, K.; Smith, E. G.; Webber, G. B.; Baker, M.; Wanless, E. J.; Butun, V.; Armes, S. P.; Biggs, S. J. Colloid Interface Sci. 2007, 314, 381. (30) Chen, X. Y.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 2004, 20, 587.

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Table 1. Preparation Conditions and Compositions of the Copolymersa abbreviation

compositionb

x/y

Mnc (PD)

[M]o/[MI]od

ne

Mnf /g mol-1

MI1-PNIPAm20k MI2-PNIPAm20k MI3-PNIPAm20k MI2-PNIPAm2k MI2-PNIPAm50k

PDMA+23-g-(PNIPAm195)23 PDMA+30-g-(PNIPAm210)14 PDMA+37-g-(PNIPAm195)12 PDMA+30-g-(PNIPAm21)14 PDMA+30-g-(PNIPAm570)14

1.0 1.9 3.0 1.9 1.9

6580 (1.44) 6480 (1.41) 7440 (1.34) 6480 (1.41) 6480 (1.41)

200 200 200 20 500

195 210 195 21 570

515 000 348 000 280 000 46 000 918 000

a The meanings for x, y, and n are depicted in structure IV of Scheme 1. b Composition is PDMA+x-g-(PNIPAmn)y based on the x, y, and n values. c Data for the respective poly(DMA-s-HEMA) from the GPC data. d Note that this value corresponds to the theoretical value for n. e These values were calculated from 1H NMR data obtained using alkaline hydrolysis in the presence of 0.5 M NaOH (see text). f Calculated using the x, y, and n (from 1H NMR data) values.

Figure 1. 1H NMR spectra in D2O for (a) poly(DMA-s-HEMA), (b) esterification product from (I), (c) MI2, and (d) MI2-PNIPAm20k.

(0.05 mol) in 10 mL of dried THF dropwise into a reaction mixture of copolymer, triethylamine (0.05 mol) and 30 mL dried THF at 0 °C. The reaction temperature was then allowed to rise slowly to ambient temperature and the reaction continued for a further 19 h. 1H NMR analysis (Figure 1) indicated a degree of esterification of greater than 90% with respect to the HEMA residues. Finally, after the copolymer solution pH was adjusted to 8.8, the DMA residues (0.01 mol) were fully quaternarized by addition of excess CH3I (0.05 mol) for 24 h at room temperature to give the macroinitiator (III in Scheme 1). 1H NMR analysis indicated a degree of quaternarization of ca. 100%. Synthesis of Cationic PNIPAm Graft Copolymers. All of the cationic PNIPAm graft copolymers (Scheme 1) were prepared by aqueous ATRP. We used aqueous solution conditions in this work because it offered a straightforward approach for the preparation of our graft copolymers (which involved the use of quarternarized macroinitiators) and a simplified purification procedure. Other nonaqueous ATRP methods31 are known to give low polydispersities for linear PNIPAm. Typically, CuBr/PMDETA (1:1) and 0.25 mL of water were placed in a 100 mL two-neck reaction flask, which was then sealed with a rubber septum. The flask was repeatedly evacuated and filled with Ar. Different ratios of NIPAm to MI were used (Table 1) as 6% (w/v) monomer solutions in water. The polymerizations were conducted for at least 40 min, then opened to the air and acetone immediately added to stop the reaction. After concentrating the solution by rotary-evaporation the graft copolymers were purified by dialysis to remove Cu catalyst/ligand and any unreacted monomer. The purified copolymers were isolated by freezedrying. We investigated the copolymer compositions using 1H NMR. A representative spectrum is also shown in Figure 1d. Synthesis of Linear Poly(N-isopropylacrylamide). Linear PNIPAm was synthesized as a reference polymer using a similar method to that of the cationic PNIPAm graft copolymers (above) (31) Xia, Y.; Yin, X.; Burke, N. A. D.; Stover, H. D. H. Macromolecules 2005, 38, 5937.

without the use of a macroinitiator. The [NIPAm]0/[EBiB]0/[CuBr]0/ [PMDETA]0 ratio was in this case equal to 200:1:1:1 and the polymerization was conducted at room temperature using 6% (w/v) NIPAm aqueous solution. Physical Measurements. 1H NMR spectra were measured using a Bruker 300 or 400 MHz NMR spectrometer using D2O or CDCl3 as the solvent and tetramethylsilane as the internal reference. Alkaline hydrolysis of the copolymers was also investigated (discussed below) and was performed using NaOH in a mixed THF/MeOH solution at 29 °C for 24 h. The product was isolated and redispersed in D2O. The determination of the cloud point temperature (Tcp) of the copolymer solutions was conducted with a Hitachi U-1800 spectrophotometer using a wavelength of 400 nm and thermostatic control. Molar mass and polydispersity for the poly(DMA-s-HEMA) copolymers were determined by GPC. A PL-GPC 50 system (Polymer Laboratories) was equipped with an Ultrahydrogel linear column and a refractive index detector. Variable-temperature particle size and zeta potential measurements were conducted using a Malvern Zetasizer. SEM measurements were obtained using a Philips FEGSEM instrument.

Results and Discussion Copolymer Compositions. The composition of the MIs and their relevant graft copolymers are summarized in Table 1. Three different MIs containing x/y ratios (Scheme 1) of about 1, 2, and 3 were used to prepare the (PDMA)+x-g-(PNIPAmn)y copolymers. (n is the number of NIPAM repeat units in the PNIPAm arms.) Representative 1H NMR for species I-IV are shown in Figure 1. The spectra for species I (Figure 1a) enabled the values for x/y ratio (the molar ratio between DMA and HEMA units) for poly(DMA-s-HEMA) to be determined unambiguously. Their molar masses were also measured by GPC (Table 1). We assume that the x/y and degrees of polymerization for the copolymer backbones did not change as a consequence of preparing the

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Figure 2. Variation of turbidity with temperature for various graft copolymer solutions. Data for linear MI2-PNIPAm2k are shown for comparison. The copolymer concentrations were 0.2 wt %.

respective MIs and graft copolymers. Unfortunately, it was not possible to determine the molar mass of the MIs or the graft copolymers using GPC because of their cationic nature. The values of n for the (PDMA)+x-g-(PNIPAmn)y graft copolymers (Scheme 1) were investigated using 1H NMR (Figure 1d). Spectra were initially measured using CDCl3 or D2O. The values for n were calculated by taking the ratios of the areas for the protons from the -CH groups of NIPAm (labeled j in Figure 1d) to those of the -N+(CH3)3 groups of DMA+ (labeled i). The following equation was used.

( )( )

n)9

Aj x Ai y

(1)

Where x and y are the values determined above for the respective MI. It was found that for all of the cationic graft copolymers prepared in this work that the area ratio Aj/Ai was about a factor of 3.3 higher than expected. It follows from eq 1 that the calculated values for n would be 3.3 times those expected for samples both in D2O or CDCl3. It is well known that restricted mobility within star copolymers reduces the intensity of NMR peaks.32 The structure of our graft copolymers, compared to star architectures, have an even larger number of branch points (Scheme 1). Therefore, we propose that erroneously low signal intensities (small Ai values) for the -N+(CH3)3 groups in (PDMA)+x-g-(PNIPAmn)y occur due to restricted mobility of the PDMA+ backbone caused by the relatively long and numerous grafted PNIPAm arms. We applied an alkaline hydrolysis method in an attempt to determine the molar masses of the copolymers. Unfortunately, accurate molar mass determination was not possible due to excessive fragmentation of the copolymer. However, values for n were estimated using this method (see discussion and Figures S1 and S2 of Supporting Information). These values are shown in Table 1 and all agree reasonably well to the theoretical values (from [M]0/[MI]0). This is an indication that our approach has good consistency. Because there is no independent measure for n or molar mass available the calculated values for n and the respective calculated molar masses in Table 1 for the graft copolymers are relative values. Temperature-Triggered Association in Dilute Cationic Graft Copolymer Solutions. The effect of temperature on the turbidity of (PDMA)+x-g-(PNIPAmn)y solutions was investigated using identical solution concentrations (Figure 2). It was observed (32) Plummer, R.; Hill, D. J.; Whittaker, A. K. Macromolecules 2006, 39, 8379.

that at temperatures above Tcp all of the graft copolymers (except MI2-PNIPAm50k) had lower turbidity (with a light-blue color) than linear PNIPAm. At temperatures greater than the LCST the PNIPAm segments within (PDMA)+x-g-(PNIPAmn)y become hydrophobic, while the cationic backbone will remain hydrophilic. Temperature-triggered association of these amphiphilic copolymers results in formation of nanometer-sized aggregates. The data for MI2-PNIPAm50k (Figure 2) show a small kink at higher temperatures which may be due to secondary aggregation. The cloud point temperatures (Tcp) were determined from the points of inflection of the data shown in Figure 2 (see Table 2). All of the Tcp values were close to that for linear PNIPAm. Salt effects on the LCST for linear PNIPAm polymers and microgels have been reported and related to the Hofmeister series.12,33 At low concentration (below 0.1 M) the effect of NaNO3 on the LCST (deduced from Tcp) was weak (below). For our systems, however, the effect of NaNO3 on the turbidity of MI2PNIPAm20k is pronounced in that major increases occur even for concentrations of 0.005 M (see Figure 3a). The data suggest that in this system electrostatic interactions control the turbidity above Tcp to a large extent. Colloidally stable aggregates are formed at NaNO3 concentrations less than or equal to 0.001 M. The increase in turbidity for NaNO3 concentrations less than or equal to 0.001 M is probably due to an increase in the extent of aggregation as a result of decreased electrostatic interactions. At NaNO3 concentrations of 0.005 M or more, aggregation increases uncontrollably and macroscopic aggregates form, which sediment. Under those circumstances electrostatic repulsion is unable to resist the attractive interactions (van der Waals) between the dispersed nanosized aggregates. Thus, 0.005 M NaNO3 can be considered as the critical coagulation concentration (CCC) for this dispersion. Horne et al.34 showed many years ago that for an aqueous polymer solution added electrolyte may increase or decrease the cloud point depending on its nature. Electrolytes like NaNO3 decrease the cloud point. Figure 3b shows that the Tcp for MI2PNIPAm20k decreases significantly with increasing NaNO3 concentration. Data for linear PNIPAm are shown for comparison. It can be seen that the decrease in Tcp for this copolymer is significantly lower than for linear PNIPAm. This can be attributed to a greater segment density of the PNIPAm chains within MI2PNIPAm20k compared to the linear polymer, which is what one would expect for a graft copolymer containing closely spaced PNIPAm arms. It is well known that the LCST decreases with increasing segment density for water-soluble temperatureresponsive polymers.35 The variation with temperature of the hydrodynamic diameters of the aggregates that are formed above Tcp was investigated (Figure 4a). The larger diameters were observed at temperature below 42 °C, whereas smaller diameters that were not dependent on temperature were observed at temperature above 42 °C. This general trend is similar to that reported by Pietrasik et al.25 for their temperature-responsive graft copolymers. In the following discussion we use the x/yn to compare the copolymers. This ratio is the calculated number fraction of nonbranched (cationic) units within the copolymer (Table 2). It can be seen from the data shown in Table 2 that for the MI1-PNIPAm20k, MI2PNIPAm20k, and MI3-PNIPAm20k series the d50 value generally increases with x/yn. For the MI2-PNIPAm50k, MI2-PNIPAm20k, (33) Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. J. Am. Chem. Soc. 2005, 127, 14505. (34) Horne, R. A.; Almeida, J. P.; Day, A. F.; Yu, N-T. J. Colloid Interface Sci. 1971, 35, 77. (35) Zhu, P. W.; Napper, D. H. J. Colloid Interface Sci. 1994, 168, 380.

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Table 2. Properties of the Copolymers in Aqueous Solution abbreviation

x/(yn)a × 103

Tcpb/ °C

τ49c/ m-1

d50d/ nm

ζ50e/ mV

Cp,crit(35) f/ wt.%

MI1-PNIPAm20k MI2-PNIPAm20k MI3-PNIPAm20k MI2-PNIPAm2k MI2-PNIPAm50k PNIPAm

5.18 10.1 15.7 102 3.75

33.3 33.6 33.6 33.3 33.3 33.4

117 129 165 33.4 538 410

69 73 120 137 101 -

23 32 39 49 23 -

6.4 6.4 10.0 no gel g 9.0 no gel

a The x, y, and n (from 1H NMR data) values are given in Table 1. b Cloud point temperature. c Turbidity data measured at 49 °C. d Hydrodynamic diameter measured at 50 °C. e Zeta potential measured at 50 °C. f Critical concentration for gel formation at 35 °C. g No gel formed at Cp values of less than or equal to 40 wt %.

Figure 3. (a) Variation of turbidity with temperature for MI2-PNIPAm2k in the presence of different NaNO3 concentrations and (b) variation of cloud point temperature with NaNO3 concentration for MI2-PNIPAm2k and linear PNIPAm.

and MI2-PNIPAm2k series the d50 value appears to reach a minimum for MI2-PNIPAm20k. It is useful at this point to consider briefly the implications of the calculated composition for the graft copolymers (Table 1). We select MI2-PNIPAm50k for this purpose. A fully collapsed copolymer chain with a molar mass of 918 000 g mol-1 would have a calculated diameter of about 14 nm. This value is consistent with the d50 values (Table 2) which are interpreted in terms of copolymer aggregates. The effect of added NaNO3 on the hydrodynamic diameter for MI2-PNIPAm20k was also investigated (Figure 4b). It can be seen that addition of the salt (at less than the CCC) caused a substantial increase in the nanoparticle size. This is consistent with the turbidity data shown in Figure 3a and is attributed to an increase in the extent of aggregation as a consequence of decreased electrostatic repulsion. In order to investigate the morphology of the aggregates, an SEM investigation was undertaken. The samples were prepared by heating to 50 °C and maintaining the temperature during water evaporation. The SEM image for MI2-PNIPAm20k prepared using a copolymer concentration of 0.002 wt % showed nanoparticles, i.e., spherical aggregates (Figure S3). The aggregates have a broad size distribution with an average diameter of 45 nm. The coefficient of variation was

Figure 4. (a) Variation of hydrodynamic diameter with temperature for the cationic graft copolymer dispersions. The data shown in (b) illustrate the effect of 0.001 M NaNO3. The copolymer concentration used was 0.2 wt %. The error bars ((4%) represent the uncertainty of the data.

30%. A similar morphology occurred using 0.002 wt % of MI3PNP20k. Interestingly, SEM images obtained using 0.001 wt % MI3-PNIPAm20k showed micrometer-sized flowerlike aggregates (Figure 5.). This is the first report of flowerlike aggregates for PNIPAm copolymers to our knowledge. Clearly, copolymer concentration affects aggregate morphology. Flowerlike aggregates have been reported for polystyrene-b-PAA copolymers.36 We propose that these aggregates occur as a consequence of the expanded structure of MI3-PNIPAm20k and form as a result of the drying process. An extensive investigation of the origin of the flowerlike aggregate morphology will be conducted in future work. From consideration of the structure of the (PDMA)+x-g(PNIPAmn)y copolymers it was expected that increasing the temperature to above Tcp would cause a major rearrangement of the backbone and side chains. This was probed using variabletemperature electrophoretic mobility measurements (Figure 6). These measurements are sensitive to the outer periphery of the aggregates. Because these nanoparticles are not fully collapsed (36) Shi, L.; Zhang, W.; Yin, F.; An, Y.; Wang, H.; Gao, L.; He, B. New. J. Chem. 2004, 28, 1038.

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Figure 5. SEM images for flowerlike aggregates of MI3-PNIPAm20k at different magnifications.

over the entire temperature range the data are shown in terms of the electrophoretic mobility. (Zeta potentials at 50 °C, ζ50, are shown in Table 2.) The interpretation of these data is rather complex because there is a temperature-triggered transition from

soft (swollen) particles to hard (collapsed) particles. The former may be described by the equations of Ohshima et al.,37 while the latter by the Smoluchowski equation.38 (The Smoluchowski equation was used to calculate ζ50 values based on the assumption

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Figure 6. Variation of electrophoretic mobility with temperature for cationic graft copolymer dispersions. The polymer concentration was 0.2 wt %. The error bars are the uncertainty ((9%).

that the aggregates are collapsed at that temperature.) Generally, an increase in temperature will result in a change in the mobility from being dominated by volume charge density to surface charge density. Because ζ values are generally correlated with surface charge density, MI2-PNIPAm2k, should have the highest surface charge density at 50 °C (Table 2). It can be seen from Figure 6 (and Table 2) that for the MI1-PNIPAm20k, MI2-PNIPAm20k, and MI3-PNIPAm20k series the ζ50 values increase with x/yn. For MI2-PNIPAm50k, MI2-PNIPAm20k, and MI2-PNIPAm2k the ζ50 values also increase with x/yn. This is what one would expect if positively charged backbone chains were present at the surface of the aggregates. The data shown in Figure 6 are consistent with an evolution of the structure from one which had a PDMA+ core (at lower temperatures) to another which has a PDMA+-rich shell (at higher temperatures). Temperature-Triggered Gelation of Concentrated Cationic Graft Copolymer Solutions. We investigated the effect of temperature on the behavior of concentrated solutions in order to probe triggered intermolecular association in more detail. Tubeinversion experiments were used to define the immobile-gel region of the phase diagram (Figure 7). Immobility in these experiments is the point where the gel can endure vigorous shaking of the sample tube by hand. The figure shows that gel formation occurs over a wide range of temperature (from Tcp to at least 50 °C) and Cp values. With the exception of linear PNIPAm or MI2PNIPAm2k, all the solutions exhibited temperature-triggered gelation. The general shape of the curves shown in Figure 7 is similar to those reported earlier for PNIPAm copolymer stabilized emulsions.11 For the MI2-PNIPAm20k systems a gel can be formed at Cp values as low as 5 wt %. A transient network forming a gel occurs for (PDMA)+x-g-(PNIPAmn)y above a critical polymer concentration (Cp,crit) and temperature (Tgel), through an association mechanism between hydrophobic side chains. Electrostatic interactions were considered likely to be important in this process. Therefore, the effect of added NaNO3 was investigated (Figure 8) for a 5 wt % solution of MI2-PNIPAm20k. It was found that Tgel decreased significantly. This shows that electrostatic interactions oppose gel formation. In this context the behavior for MI2-PNIPAm2k is important because it did not form a gel at the highest Cp investigated (40 wt %) at temperatures less than or equal to 50 °C. This is attributed to strong electrostatic repulsion between the aggregates. This copolymer had the highest ζ50 value (Table 2). (37) Ohshima, H.; Makino, K.; Kato, T.; Fujimoto, K.; Kondo, T.; Kawaguchi, H. J. Colloid Interface Sci. 1993, 159, 512. (38) Shaw, D. J., Colloid and Surface Chemistry, 4th ed.; ButterworthHeinemann: Oxford, 1992; pp 200-201.

Figure 7. Fluid-to-gel phase diagrams for aqueous cationic graft copolymer solutions. Note that the values for MI1-PNIPAm20k and MI2-PNIPAM20k are coincident in (a) when Cp is greater than 5 wt %.

Figure 8. Variation of gelation temperature with salt concentration for a 5 wt % MI2-PNIPAm20k solution.

Copolymer solutions which form space-filling structures with a high number density of associative cross-links form gels at low Cp,crit values. The strength of the associative cross-links is also important as this provides the structural support necessary to prevent flow under shear. It can be concluded from the data presented in Figure 7 that the MI1-PNIPAm20k and MI2PNIPAm20k systems have the most suitable x/yn values for efficient gel formation. However, there is clearly a minimum x/yn value for efficient gel formation because linear PNIPAm (x/yn ) 0) does not form a gel, rather it precipitates. It must be the hydrophilic, positively charged backbone that maintains the extended region between hydrophobic microdomains and is essential in extended network formation. A charge-density that is too high (large x/yn) prevents efficient hydrophobic domain formation and opposes gelation. In order to comprehensively compare the behaviours of the copolymers at temperatures greater than Tcp the hydrodynamic diameter and mobility (at 50 °C), Cp,crit(35) and τ49 have all been

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Liu et al.

Figure 9. Variation of d50 and ζ50 or Cp,crit(35) and τ49 with x/yn. The values for x/yn are taken from Table 2. The inset shows the variation of d50 with Cp,crit(35) on a linear scale.

plotted as a function of x/yn in Figure 9. These data indicate that d50 and the ability to form a gel (via Cp,crit(35)) are related, i.e., efficient gelation occurs in systems that have smaller aggregates at high temperature. This implies that the balance between high cross-link density and good spatial extension via the hydrophilic backbones is nearly optimal when x/yn ) 0.005-0.010. At higher values for x/yn the electrostatic repulsion is too high and crosslink efficiency falls to the point where gelation cannot occur. These data also show clearly that the τ49 values for MI2PNIPAm50k and MI2-PNIPAm2k cannot be attributed to particle size differences. Rather, they can be explained by, respectively, high and low polymer volume fractions (φp) in the aggregates. This is what would be expected for aggregates comprised of NIPAm-rich (MI2-PNIPAm50k) or poor (MI2-PNIPAm2k) cationic graft copolymers. Proposed Model for Temperature Triggered Aggregation and Gel Formation. We can now propose a mechanism for particle and gel formation based on the results obtained above. At temperatures above Tcp the hydrophobic side chains collapse which results in interchain association and aggregate formation. The aggregation number (p) of the nanometer-sized aggregates that form under these conditions is governed by electrostatic interactions and tends to be smallest when the charge density of the aggregates is highest. The nanometer-sized aggregates can be envisaged as having a core-shell structure (Figure 10) and must also contain a significant fraction of internal positive charged groups which prevent complete collapse when the backbone charge density is high. Thus, the average φp should generally decrease with increasing x/yn. These particles can be considered as a type of microgel particle39 where intraparticle cross-linking occurs due to reversible hydrophobic association. A microgel particle is a cross-linked polymer particle that swells in a good solvent39 or when there is a pH change that causes electrostatic repulsion between segments of the network that comprises the particle. The difference is that the network size for the aggregates is not fixed and depends on temperature and electrostatic repulsion. In concentrated solutions at temperatures greater than Tgel the polymer chains will collapse as described above. However, due to the close proximity of neighboring chains, the hydrophobic PNIPAm side chains have a higher probability of forming an intermolecular associative cross-link. This leads to extensive network formation (Figure 10). The gel formation process is also governed by electrostatics, and a higher interparticle cross-link density occurs when the electrostatic repulsion is decreased. Pietrasik et al.25 also noted that their temperature-responsive graft copolymers exhibited aggregation at higher polymer solution concentrations on heating. For the present study an aggregate

Figure 10. Depiction of temperature-triggered association of cationic graft copolymer chains resulting in formation of colloidally stable aggregates (at low concentration) or gels (at high concentration).

morphology of connected nanoparticles that emerge from a central point was evident from the SEM data shown in Figure 5. This could provide a glimpse of the interconnected aggregate structure that exists in the gelled state.

Conclusion In this work the properties of cationic PNIPAm graft copolymers with a range of compositions have been investigated. It was proposed that the molecular architecture of the copolymers enabled them to form reversible aggregates with peripheries rich in positive charges at temperatures greater than Tcp. The aggregate size and surface charge appears to be mainly controlled by the ratio of positive charge density on the backbone to PNIPAm side chain length. SEM data revealed evidence of flowerlike aggregate morphologies. It was found that for concentrated solution conditions gels formed at temperatures greater than Tgel and this value depended on copolymer composition and concentration. The gel formation is controlled by a balance between electrostatic repulsion (due to the cationic backbone groups) and attraction (due to PNIPAm rich microdomains). Because this new family of cationic comb copolymers have tunable temperature responses they will be employed in our future work involving modification of anionic surfaces for temperature-triggered capture of dispersed particles.7 Given their gel-forming tendencies, these copolymers may also have wider application as rheological control agents. Acknowledgment. BRS gratefully acknowledges the ESPRC for funding this work. We thank Mr. Dave Hui for technical assistance with SEM. Supporting Information Available: A discussion of the hydrolysis of the copolymers, 1H NMR spectra for M3-PNIPAm20k measured before and after hydrolysis (Figure S1), variation of Aj/Ai with NaOH concentration (Figure S2), and an SEM micrograph for MI2PNIPAm20k aggregates (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. LA8002756 (39) Saunders, B. R.; Vincent, B. AdV. Colloid Interface Sci. 1999, 80, 1.