Thermoresponsive Hydrogels from Symmetrical Triblock Copolymers

Article pubs.acs.org/Langmuir

Thermoresponsive Hydrogels from Symmetrical Triblock Copolymers Poly(styrene-block-(methoxy diethylene glycol acrylate)block-styrene) Anna Miasnikova,† André Laschewsky,*,†,‡ Gabriele De Paoli,§ Christine M. Papadakis,§ Peter Müller-Buschbaum,§ and Sergio S. Funari∥ †

Department of Chemistry, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany Fraunhofer Institute for Applied Polymer Research, Geiselberg-Str. 69, 14476 Potsdam-Golm, Germany § Technische Universität München, Physikdepartment, Lehrstuhl für Funktionelle Materialien/Physik Weicher Materie, James Franck-Straße 1, 85747 Garching, Germany ∥ HASYLAB at DESY, Notkestr. 85, 22607 Hamburg, Germany ‡

S Supporting Information *

ABSTRACT: A series of symmetrical, thermo-responsive triblock copolymers was prepared by reversible addition− fragmentation chain transfer (RAFT) polymerization, and studied in aqueous solution with respect to their ability to form hydrogels. Triblock copolymers were composed of two identical, permanently hydrophobic outer blocks, made of low molar mass polystyrene, and of a hydrophilic inner block of variable length, consisting of poly(methoxy diethylene glycol acrylate) PMDEGA. The polymers exhibited a LCSTtype phase transition in the range of 20−40 °C, which markedly depended on molar mass and concentration. Accordingly, the triblock copolymers behaved as amphiphiles at low temperatures, but became water-insoluble at high temperatures. The temperature dependent self-assembly of the amphiphilic block copolymers in aqueous solution was studied by turbidimetry and rheology at concentrations up to 30 wt %, to elucidate the impact of the inner thermoresponsive block on the gel properties. Additionally, small-angle X-ray scattering (SAXS) was performed to access the structural changes in the gel with temperature. For all polymers a gel phase was obtained at low temperatures, which underwent a gel−sol transition at intermediate temperatures, well below the cloud point where phase separation occurred. With increasing length of the PMDEGA inner block, the gel−sol transition shifts to markedly lower concentrations, as well as to higher transition temperatures. For the longest PMDEGA block studied (DPn about 450), gels had already formed at 3.5 wt % at low temperatures. The gel−sol transition of the hydrogels and the LCST-type phase transition of the hydrophilic inner block were found to be independent of each other.

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INTRODUCTION Amphiphilic block copolymers can form physical hydrogels under appropriate conditions.1 Among them, symmetrical triblock copolymers BAB, with hydrophobic outer blocks B and a hydrophilic inner block A, are of particular interest in this context, for academic research as well as for industrial applications.2,3 The specific BAB architecture with two hydrophobic “stickers” at both ends of the polymer induces two possibilities of chain arrangement in micelles. On one hand, the inner block can form loops with the two hydrophobic extremities placed in the same micelle core, which at high concentration leads to gels of densely packed micelles, similar to gels of amphiphilic diblock copolymers.4 On the other hand, the outer blocks can be placed in two different micelle cores, while the inner block adopts a bridge conformation. Thus, above a critical concentration, which is much lower than in the case of diblock copolymers, a gel of bridged micelles is formed. Whereas both types of organization coexist in a polymer © 2012 American Chemical Society

solution, the mechanical properties result mainly from the proportion of active bridges.2 This proportion depends on several molecular parameters such as the lengths of the inner and outer blocks,5−7 and their respective chemical compositions.4 However, little is known about the influence of the latter parameter, and this adds to the difficulties foreseeing how a given polymer will behave in solution. The development of reversible deactivation radical polymerization RDRP (formerly often referred to as controlled/living radical polymerization)8 has given a major push to this field, by offering the synthetic possibility to form well-defined block copolymer architectures from a wide variety of monomers. Reports on new amphiphilic symmetrical triblock copolymers synthesized by these polymerization techniques are increasingly Received: November 26, 2011 Revised: January 31, 2012 Published: February 22, 2012 4479

dx.doi.org/10.1021/la204665q | Langmuir 2012, 28, 4479−4490

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evolving.9−12 Among them, two classes of amphiphilic polymers can be distinguished, namely, the permanent ones and the (mostly thermo-) responsive ones. As opposed to permanent systems, where the character of the blocks A and B remains unchanged, in the thermo-responsive systems the character of block A or B can be switched from hydrophilic to hydrophobic by temperature.3,13−15 Most of the research on the latter systems is focused on polymers with a permanently hydrophilic inner block A and thermo-responsive outer blocks B exhibiting a lower critical solution temperature (LCST). At low temperatures, these polymers are fully hydrophilic and dissolved in water as single molecules. When the temperature crosses the phase transition temperature of the B block, the outer blocks become hydrophobic and, thus, trigger the formation of micelles or gels. The association of such BAB block copolymers has been investigated employing various thermo-responsive B blocks, such as poly(2-hydroxypropyl methacrylate),16 poly(N-isopropyl acrylamide) PNIPAM,17−21 poly(ethoxy diethylene glycol acrylate),19 or related copolymers with acrylic acid.22 Studies on BAB triblock with permanently hydrophobic outer blocks B and switchable hydrophilic inner block A are, however, rare. Some reports can be found on the presumably related associating telechelics made of a switchable A block functionalized with low molecular weight hydrophobic endcaps, as in the work of F. Winnik et al. using PNIPAM.23,24 For analogous BAB triblock copolymers, however, most reports are confined to the micellization of such polymers in dilute solutions.25−27 Reports on the gelation behavior of thermoresponsive BAB block copolymers with switchable inner A block have been generally restricted to poly(ethylene oxide)28−32 and PNIPAM.33,34 The latter A-block was, for instance, end-capped with permanently hydrophobic polystyrene,34−36 poly(2-ethylhexyl acrylate),34 or poly(octadecyl acrylate)34 outer B blocks. In this study, we describe a series of BAB symmetrical triblock copolymers, namely, polystyrene-b-poly(methoxy diethylene glycol acrylate)-b-polystyrene (PS-PMDEGA-PS), and their temperature dependent gelation behavior. These polymers are thermo-responsive by virtue of their switchable PMDEGA inner block. Though being easily accessible and disposing of advantageous features, PMDEGA has been virtually overlooked so far. Within the class of poly((oligoethylene glycol) [meth]acrylate)s,26,37−39 PMDEGA has the shortest possible side chain to achieve water solubility. Thus, PMDEGA is sterically much closer to the group of thermo-responsive poly(N-alkylacrylamide)s than the up-tonow mostly used, rather comb- or brush-like polymers derived from PEG macromonomers. Further, the volume phase transition temperature of PMDEGA in aqueous solution for medium to high molar mass homopolymers is around 35−45 °C,40−42 i.e., in the most interesting physiological window. Different from other poly((oligoethylene glycol) [meth]acrylate)s and the frequently used PNIPAM (LCST 32 °C),43,44 this can make an adjustment of the transition temperature by copolymerization39,38,45 for biomedical applications, and others, unnecessary. Moreover, different from PNIPAM, PMDEGA cannot form self-consistent inter- and intramolecular H-bonds. This changes the thermo-responsive behavior of the PMDEGA type polymers and presumably leads, among others, to transitions with virtually no hysteresis between the cooling and the heating cycles.38,39

We were interested in the effects that the PMDEGA inner block will exert on the hydrogel mechanical properties. For better comparison, we choose polystyrene as hydrophobic block, since BAB triblock copolymers with polystyrene outer blocks and poly(ethylene oxide),31,32 poly(N-isopropylacrylamide),34−36 or poly(acrylic acid)46 inner blocks have been investigated. In particular, we were focusing on the influence of the inner block length on the gel formation. Moreover, we wanted to correlate the changes of the mechanical properties to the structural changes with temperature. For this purpose, we carried out temperature-resolved small-angle X-ray scattering experiments exemplarily on one of the block copolymers.

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EXPERIMENTAL SECTION

Materials. Hexane (min. 99.5%; J. T. Baker), methanol (puriss.; Chemie-Vertieb Magdeburg) and benzene (spectrophotometric grade, >99%; Sigma-Aldrich) were used as received. Styrene (99%, SigmaAldrich) was purified by distillation. Azoisobutyronitrile (AIBN; Wako) was recrystallized twice from methanol. The synthesis of monomer methoxy diethylene glycol acrylate (MDEGA), chain transfer agent 1,2 bis (4-(t-butoxycarbonyl)benzyl sulfanylthiocarbonyl sulfanyl) ethane (CTA), of the model compounds butyl-1-phenylethyltrithiocarbonate and 2-(n-butylsulfanylthiocarbonylsulfanyl)-propionic acid 2-(2-methoxy-ethoxy)ethyl ester used for UV−vis end-group analysis and of polystyrene macroRAFT was reported before.47 Synthesis of PS8-PMDEGA180-PS8. In a typical procedure, polystyrene macroRAFT agent PS16 (0.3024 g, 0.133 mmol), MDEGA (6.89 g, 39 mmol), and AIBN (0.0036 g, 0.022 mmol) were mixed with benzene (20 mL) and purged with N2 for 15 min at ambient temperature. After 5 h at 70 °C, the polymerization was stopped by immersing the flask into liquid nitrogen. The yellow liquid was precipitated three times into hexane and dried in vacuo at ambient temperature for 3 d. Yield: 4.5 g (63%) highly viscous yellow oil. 1 H-NMR (300 MHz in acetone d6, δ in ppm): δ = 1.57 (br, 18H, -(CH3)3), 3.33 (br s, 3H × DPn, −O−CH3), 4.23 (br s, 2H × DPn, -C(O)O−CH2−CH2−O−), 4.89 (br s, 2H, -S-CH-C(O)O−), 6.40−7.42 (br, 4H aromatic R group and 5H × DPn polystyrene), 7.75−7.93 (br, 4H, aromatic R group). Analytical Methods. 1H-NMR spectra were taken with an apparatus Bruker Avance 300 (300 MHz) in acetone d6. UV−vis spectra were recorded on a spectrophotometer Cary-1 (Varian) equipped with temperature controller (Julabo F-10). Quartz cuvettes (Suprasil, Hellma, Germany) with an optical path length of 10 mm were used. Number average molar masses (Mn) of the polymers were calculated by comparing the absorbance coefficient of the thiocarbonyl band of a given polymer with the coefficient ε of the model RAFT agent determined in dichloromethane,47 namely, for polystyrene the ε = 16 400 L·mol−1·cm−1 determined with the model compound butyl-1-phenylethyltrithiocarbonate, and for the BAB blocks copolymers with the coefficient ε = 13 900 L·mol−1·cm−1 determined with the model compound 2-(n-butylsulfanylthiocarbonylsulfanyl)-propionic acid 2-(2-methoxy-ethoxy)ethyl ester.47 Size-exclusion chromatography (SEC) was run at 50 °C in DMF (flow rate 1 mL min−1) using a Spectra Physics Instruments apparatus equipped with a UV-detector SEC-3010 and a refractive index detector SEC 3010 from WGE Dr. Bures (Columns: Guard (7.5 × 75 mm), PolarGel-M (7.5 × 300 mm)), calibration with linear polystyrene standards (PSS, Germany). The theoretically expected molar masses Mntheo were calculated from gravimetrically obtained yields, as described previously.34 Sample Preparation. The copolymers were directly soluble in water, due to the small polystyrene blocks, at any concentrations and without the need of organic cosolvents. Polymers and “Millipore” water were mixed in the desired proportions and left to equilibrate at 10 °C in a tightly screw-capped vial for 1 to 4 d depending on the concentration. The procedure yielded clear, homogeneous samples. 4480


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Characterization in Dilute Aqueous Solution. For dilute solutions (3.0 g L−1), cloud points were determined using a temperature controlled turbidimeter (model TP1, E. Tepper, Germany) with heating and cooling rates of 1.0 °C min−1, respectively. Temperatures were precise within 0.5 °C. The association of the polymers in aqueous solutions was studied by dynamic light scattering (DLS) using a high-performance particle sizer (HPPS-ET, Malvern Instruments, UK) equipped with a He−Ne laser (λ = 633 nm) and a thermoelectric Peltier temperature controller. The measurements were made at the scattering angle of θ = 173° (“backscattering detection”) and the autocorrelation functions were analyzed with the CONTIN method. Characterization in Concentrated Aqueous Solution. For concentrated solutions (5 wt % to 30 wt %), cloud points were determined by visual observation, as the turbidimetry setup could not be used because of the high viscosity. The samples were placed in a thermostatted bath, and the temperature was raised by 1 °C·min−1 until the samples became turbid. Concentration dependent sol−gel transitions were evaluated by the tube inversion test. We distinguished, on the one hand, between liquid and viscous liquid samples, able, respectively, to flow freely or slowly upon tube inversion, and on the other hand between soft and hard gels. Samples considered as gels were not flowing upon tube inversion for at least 30 min; however, soft gels presented a visible surface deformation, whereas hard gel did not. Rheological experiments were performed with an ARG2 rheometer (TA Instruments) equipped with cone−plate geometry and a Peltier plate for temperature control. The cone diameter was 40 mm, cone angle 1°, and the truncation gap 30 μm. A water-filled solvent trap was used to minimize solvent evaporation during the measurements. For each measurement, the sample was first heated to temperatures about 2° to 3 °C below its transition temperature and left to equilibrate under gentle oscillations for at least 20 min. The temperature was then set to the desired value for the measurement, and the sample left to equilibrate for 5 more min. For frequency sweeps, the samples were subjected to frequencies ranging from 0.002 to 100 Hz and oscillatory stress of 5 Pa, at both 10 and 20 °C. Temperature-dependent experiments were conducted in the temperature range between 5 and 45 °C with a heating/cooling rate of 1 °C·min−1, applying an oscillatory stress of 5 Pa at a frequency of 1 Hz. Based on the ratio of the values for the storage modulus G′ and the loss modulus G″, a system was considered as viscous liquid for G′ < G″, as soft-gel for G′ equal to G″, and as hard gel for G′ > G″.48 Small-Angle X-ray Scattering (SAXS). SAXS experiments were performed at beamline A2, at HASYLAB (DESY) in Hamburg, Germany. An X-ray beam with a wavelength λ = 0.15 nm was used. A 2D MarCCD detector was mounted at 3.24 m distance behind the sample. As beamstop, a piece of lead including a photodiode was used. With the setup described, the accessible q-range was 0.097−1.5 nm−1. A custom-made sample holder allowed heating from 17 to 44 °C. After equilibration at the set temperature for 10 min, scattering curves were measured for 10 min. Increments of 3 °C were adopted to span the above-mentioned temperature interval. The q-calibration was performed using dry collagen. Aqueous polymer solutions were mounted between two pieces of Kapton foil resulting in a sample thickness of 1 mm. The background from the Kapton foil was subtracted. To gain detailed structural information, the SAXS curves were modeled using the NIST SANS package in Igor Pro.49

Figure 1. Chemical formulas of the RAFT agent (CTA), and the monomers methoxy diethylene glycol acrylate (MDEGA) and styrene (S).

moieties and two benzylic R groups. Similar RAFT agents were found suitable for the polymerization of styrene, acrylamides, and acrylates.51 The RAFT agent bears specific groups to facilitate polymer characterization by 1H NMR spectroscopy, as due to the overlapping of the 1H signals, simple benzylic R-groups are not suited for end group analysis of polystyrenes. Appropriate substitution of the aryl moieties is thus necessary to induce a strong signal shift of aromatic protons and resolve the R group from the polystyrene signals. We choose the tert-butyl ester substituent, which shifts the signals of 2 aromatic protons of the RAFT agent to 7.84 ppm (Figure 2). Moreover, this group can

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RESULTS AND DISCUSSION Synthesis and Characterization of PS-PMDEGA-PS. Symmetrical BAB triblock copolymers were synthesized by a two steps RAFT polymerization sequence50 using a bifunctional RAFT agent with the R groups placed at the extremities and the Z groups in the middle of the molecule. Our approach resulted in block copolymers with an identical hydrophobic polystyrene block and various number average polymerization degrees (DPn) of the hydrophilic PMDEGA block. We employed a special difunctional CTA (Figure 1), with two trithiocarbonate

Figure 2. 1H NMR spectra of (a) PS16 and (b) PS8-PMDEGA53-PS8 in acetone-d6. Signals marked by “ × ” originate from the solvent.

be selectively deprotected in a postpolymerization reaction to yield carboxylic acid end-groups suited for further polymer modification. 4481


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Table 1. Analytical Data of the Polystyrene MacroRAFT Agent and the BAB Triblock Copolymers PS-PMDEGA-PS Prepared molar mass [g mol−1] theoretically expected polymer

a

PS16 PS8-PMDEGA17-PS8 PS8-PMDEGA53-PS8 PS8-PMDEGA93-PS8 PS8-PMDEGA180-PS8 PS8-PMDEGA452-PS8

b

conversion [%] 66 73 67 63 74

Mntheob 5200 12300 18700 34900 79900

by end-group analysis Mn,NMR

c

2200 5200 11500 18500 33600 81000

by SECe

Mn,UV

d

h

2150 4000 8700 16100 28400 62400

Mnapp j

2270 4300 8900 15600 23900 40900

characterization in dilute aqueous solution

PDI 1.09 1.25 1.15 1.17 1.17 1.30

j

RHf [nm] k

CPg [°C] k

k

k

5 7 10 14

22 26 30 35

a Numbers indicate the number average degree of polymerization DPn according to (c). bConversion by gravimetry. cBy 1H NMR analysis in acetone d6, using the integrals of aromatic end-block signal and of the CH3O- signal of the constitutional repeat unit. dBy UV−vis analysis in CH2Cl2 using the absorption coefficient ε = 13 900 L·mol−1·cm−1 of 2-(n-butylsulfanylthiocarbonylsulfanyl)-propionic acid 2-(2-methoxy-ethoxy)-ethyl ester as model RAFT agent.47 eIn DMF, RI detection, calibrated with polystyrene standards. fHydrodynamic radius in water, determined by DLS at concentration of 3 g L−1. gCloud point in water determined by turbidimetry at concentration of 3 g L−1. hUsing the absorption coefficient ε = 16 400 L·mol−1·cm−1 of butyl-1-phenylethyltrithiocarbonate as model RAFT agent for polystyrene.47 jSEC in THF using polystyrene standards. kSamples not soluble in water.

Figure 3. (a) Hydrodynamic radii (RH) as a function of temperature, as determined by DLS at concentration of 3.0 g L−1, for (●) PS8-PMDEGA452PS8, (△) PS8−PMDEGA180-PS8, (▼) PS8-PMDEGA93-PS8, and (□) PS8-PMDEGA53-PS8. Cloud point temperatures of aqueous solutions of PS8PMDEGAn-PS8 block copolymers (b) as a function of DPn of the inner PMDEGA block, at concentration of 3.0 g L−1, and (c) as a function of polymer concentration. Lines are guides to the eye.

copy.47 This cross-check is extremely informative, as it allows us to detect a possible loss of the central trithiocarbonates,52 which is difficult by other methods. In fact, the disappearance of the absorption signal may indicate a cleavage of the triblock copolymer into two diblock copolymers of half the original size with presumably different mechanical properties. Instead we noted that the Mn values calculated by UV−vis end-group analysis were systematically 10−20% lower than found by 1H NMR end-group analysis. However, any loss of trithiocarbonyl during polymerization would pretend a higher molar mass. Therefore, our findings cannot be explained by a loss of trithiocarbonate Z groups. Keeping in mind that the absorption coefficient ε of the thiocarbonyl group is known to vary notably with the environment,47 it is possible that the specific triblock architecture somehow influences the absorption coefficient of the polymer. Another possible reason might be a low molar mass chromophore impurity or side product in the sample. However, neither SEC nor 1H NMR analysis gave any support to this hypothesis. All the triblock copolymers were soluble in various organic solvents such as acetone, dichloromethane, acetonitrile, benzene, toluene, dimethylformamide, and tetrahydrofuran. Additionally, water was a solvent at temperatures below the respective cloud points (CP) of the polymers, with the

Five symmetrical triblock copolymers were thus synthesized with identical polystyrene end-blocks with a number average degree of polymerization DPn = 8, and an inner block of PMDEGA varying from 17 to 452 DPn. Their molecular characteristics are summarized in Table 1. Analysis by SEC in the nonselective eluent dimethylformamide showed for all five polymers monomodal molar mass distributions and low polymer dispersities (PDI) between 1.1 and 1.3. To determine the number average molar mass (Mn), end-group analyses using 1H NMR and UV−vis spectroscopies were also conducted. The 1H NMR spectra of the triblock copolymers, as exemplarily illustrated in Figure 2, exhibit in addition to the characteristic signals of the MDEGA repeating units (protons 6 to 10), signals from the polystyrene repeating units (aryl protons) and specific, resolved signals of the end groups incorporated via the CTA (protons 1). Using characteristic signals of the CTA (protons 1) and of the styrene repeat unit (aryl protons), or of the MDEGA repeat units (protons 10), respectively, the number average molar mass Mn was derived. An excellent agreement was found with the molar mass theoretically expected according to the conversion. Additionally, the strong thiocarbonyl chromophore of the central Z groups allows the performance of alternative endgroup determination of the molar mass via UV−vis spectros4482


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Table 2. Gelation Behavior of Aqueous Solutions of Triblock Copolymers PS-PMDEGA-PS behavior at 5 °Ca

behavior at 21 °Cb copolymer concentration (wt %)

copolymer

5

10

20

30

PS8-PMDEGA53-PS8 PS8-PMDEGA93-PS8 PS8-PMDEGA180-PS8 PS8-PMDEGA452-PS8

L L L gel

L gel gel gel

L gel gel gel

gel gel gel gel

5 L L VL

10

20

← biphasic system → L VL VL soft gel soft gel hard gel

30 soft gel hard gel hard gel

a

From rheological measurements f = 1 Hz: L = liquid (G″> G′). bVisually according to tube inversion test: L = freely flowing liquid, VL = viscous liquid (cf. Experimental Section for definitions).

However, it cannot be excluded that a minimum occurs at a low concentration, which was not covered in this study. Similar phase diagrams were observed or theoretically expected in other polymer solutions.57−59 However, our results contrast with the very flat phase diagram of PNIPAM44 or the monotonous increase of the cloud points in PEO copolymers.30,60 Possibly, the particular triblock architecture influences the evolution of the cloud point observed by us. On one hand, the overall sensitivity of the cloud points to the length of the inner block is interesting, because different transition temperatures may be targeted via this molecular parameter. On the other hand, this behavior makes it more difficult to compare the mechanical properties of the polymers as a function of temperature, since for example, at 25 °C, not all polymers of this series are soluble in water. Gel Formation. To elucidate the influence of the length of the inner block on the gelation properties of the triblock copolymers, solutions of concentrations varying from 5 to 30 wt % were prepared for each copolymer. Visual observation was possible at 21 °C for all samples with the exception of the samples PS8-PMDEGA17-PS8 and PS8-PMDEGA53-PS8. Depending on the length of the inner block, different gelation concentrations were observed (Table 2). In the case of the triblock copolymer with the longest PMDEGA block, the onset of gelation (indicated by a viscous behavior) was found at 5 wt %, whereas for polymers with inner block half the size and shorter, viscous behavior was first observed at 10 wt %. The samples gained strength with concentration, and at 30 wt %, all soluble polymers formed a gel. To overcome the transition temperature boundaries and compare all the triblock copolymers, gelation behaviors at 5 °C are also described. The data were derived from rheological experiments and are discussed below in more details. In summary, the mechanical properties for all the viscous samples improved at 5 °C compared to observations at 21 °C. Table 2 shows that polymers characterized as viscous liquid at 21 °C by the visual test were found to form true gels at 5 °C by rheological experiments. At all temperatures, polymers with long inner blocks yielded gels at lower concentrations. This observation is in good agreement with theoretical expectation: loop formation of the inner block, leading to flower-like micelles and competing with the bridge formation, is known to be associated with an entropic penalty. This penalty increases with the length of the inner block.4 Long inner blocks are thus more hindered to fold back to the same micelle core and have a greater probability to bridge two different micelles. To further look into the gel characteristics, we conducted frequency sweeps at 20 °C and at a concentration of 20 wt % (see Figure 4; frequency sweep data at 10 °C for all polymers at

exception of PS8-PMDEGA17-PS8, which was insoluble in water down to near 0 °C. Collapse Transition of PS-PMDEGA-PS. The thermoresponsive and associative behavior of the triblock copolymers was first investigated in dilute aqueous solution of 3.0 g L−1. This concentration lies well above the critical micelle concentration for this type of polymer1 (if existent at all). Small micelles with a monomodal distribution were observed by DLS. The volume average hydrodynamic radii of the micelles at ambient temperature increased from 5 to 14 nm with increasing length of the inner block (Table 1). These values are in the typical range found for hairy micelles of block copolymers with long hydrophilic and short hydrophobic blocks.53 Temperature dependent DLS measurements revealed the sudden formation of large aggregates (RH > 400 nm, volume average) above the respective transition temperatures (Figure 3a). Turbidimetry presented a sharp transition at the same temperature as the one observed by DLS, with a small hysteresis between the cooling and the heating cycle. Noteworthy, the length of the inner block influences the cloud points (Figure 3b): for example, the thermal transition was observed at 35 °C for PS8-MDEGA452-PS8 and at 22 °C for PS8-PMDEGA53-PS8. This effect, though opposite the classical molar mass dependence of a LCST, can be explained by the decreasing contribution of the hydrophobic end blocks and the resulting increase in the overall polymer hydrophilicity with an increase of the hydrophilic inner block. Similar results were reported with PMDEGA homopolymers with other hydrophobic end-groups40 and with poly(ethylene oxide) BAB block copolymers with hydrophobic poly(butylene oxide) endblocks.30,54,55 In the latter case, increasing the inner block length from 76 to 260 units resulted, for instance, in an increase of the cloud point from 12 to 25 °C.54 Moreover, even when bearing hydrophilic end-groups PMDEGA seems to exhibit an unusual increase of the cloud points with increasing molar mass.56 We further noted that an increase in concentration from 0.3 to 30 wt % induced a variation of cloud points, not necessarily in a monotonous way (see Figure 3c). For PS8-PMDEGA53PS8, cloud point values decreased from 22 to 18 °C as the concentration increased from 0.3 to 20 wt %, but increased at higher concentrations to reach 20 °C at 30 wt %. PS8PMDEGA93-PS8 showed similar concentration dependence, but the minimum of the transition temperature was shifted to lower concentrations: the cloud point decreased first from 26 °C at 0.3 wt % to 21 °C at 5 wt %, above which concentration it continuously increased. In the cases of the copolymers with longer hydrophilic blocks, i.e., PMDEGA180 and PMDEGA452, the cloud points steadily increased with concentration. 4483


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PS8-PMDEGA93-PS8, PS8-PMDEGA180-PS8, and PS8-PMDEGA452-PS8. Since the lifetime of the junctions determines the stability of a physical gel,2 the influence of the inner blocks’ length on the gel formation is evident. Presumably, these results reflect the enhanced possibility of polymers with long inner block to form intermicellar bridges. A greater number of bridges increases the average lifetime of the network and lowers the frequency required to observe the network breakdown.61 However, the frequency sweeps also revealed that all the gels studied are very dynamic, with a network average lifetime of about 1 s at best. Short average lifetimes (≈ ms) are usually observed for BAB-like structures made of a hydrophilic inner block and low molecular mass hydrophobic end-caps, such as C8F17 end-capped poly(ethylene oxide),63 C18 end-capped polyNIPAM,23 or C12 end-capped poly(N,N-dimethylacrylamide).64 On the one hand, the average lifetime τ determined by us was 3 orders of magnitude higher than those of polymers with classical low molar mass hydrophobic end-groups. On the other hand, the measured τ value was at least 3 orders of magnitude lower than the values observed for associating block polymers of the BAB type terminated with longer polystyrene end-blocks. For example, the group of Lodge examined the rheological behavior of poly(ethylene oxide) end-capped with short polystyrene block, PS16-PEO-PS16.31,32 For a polymer concentration of 10 wt % in an ionic liquid, no frequency dependence of G′ and G″ was observed up to 40 °C, i.e., the lifetime of the junctions was too long to be determined experimentally. Only above 60 °C, the rheological profile presented a crossover point and looked very similar to the one observed by us. Similar results were presented by Tsitsilianis and Iliopoulos for a 1 wt % aqueous solution of PS23poly(acrylic acid)1134-PS23.46 In the case of the PS8-PMDEGAn-PS8 studied here, the polystyrene blocks have DPn = 8, and are therefore 2 to 3 times shorter, than the ones in the examples cited above. The association strength of such short polystyrenes is probably weaker, thus enabling an enhanced mobility of the physical cross-links and eventually leading to bridge disruption with increasing temperature. This could be the reason why the various gels analyzed by us undergo a gel to sol transition even at relatively high concentration (20 wt %) and low temperature (20 °C). Carrying on the thermo-responsive behavior found in dilute aqueous solution, dynamic shear moduli of the different polymers were measured during a temperature sweep from 5 °C to a temperature above the respective cloud point of the polymer solution, in a heating to cooling cycle (Figure 5). The rheological measurements were consistent with the previously conducted visual tests. At 20 °C and 20 wt %, the gels identified as hard by the tube inversion test presented G′ > G″, while for soft gels we found G′ ≈ G″, and for viscous liquids G′ < G″. The shortest polymer, PS8-PMDEGA53-PS8, exhibited a characteristic liquid-like behavior at all temperatures investigated. At 5 °C, all gelling triblock copolymers had a similarly high storage modulus of around 104 Pa, but G′ and G″ decreased significantly upon heating. Only when crossing the cloud point was a very small increase of both moduli G′ and G″ noted. Qualitatively, such an increase was also observed for gels made from analogous PS-PNIPAM-PS triblock copolymers. However, the effect of passing the cloud point was much more pronounced for the PS-PNIPAM-PS triblock systems, and eventually resulted in reentrant gelling.34 This striking difference is

the same concentration are given in the Supporting Information).

Figure 4. Frequency dependence of storage modulus G′ (closed red symbol) and loss modulus G″ (open blue symbol) for 20 wt % aqueous solution of copolymers at 20 °C: (●, ○) PS8-PMDEGA452-PS8; (▲, △) PS8-PMDEGA180-PS8; and (▼, ▽) PS8-PMDEGA93-PS8.

The dynamic oscillatory experiments provide a quantification of the elastic and the viscous components of the gel, namely, of the storage modulus G′, and of the loss modulus G″.61,62 The slope of moduli values as a function of frequency allows differentiation among liquid solutions, weak and strong gels, and in the case of a gel, determination of the lifetime of the network junctions. For example, polymeric liquids are characterized by G′ < G″ and scale as ω and ω2, respectively. Strong physically or chemically cross-linked gels are characterized by both G′ > G″ and an almost infinite lifetime of their network. As a result, the frequency sweep exhibits an almost flat profile, with G′ and G″ independent of the frequency.62 Finally, for the intermediate case of weak gels, G′ > G″ in the high frequency region, where the lifetime of the network junctions is superior to the measurement time, but this relation reverses in the low frequency region, i.e., for long measurement times. The average relaxation time τ is then a key parameter to determine the lifetime of a network and can be taken as the inverse of the radial frequency (ω = 2πf) at ωc for which G′ and G″ cross.62 τ = 1/ωc(s)

(1)

For all PS8-PMDEGAn-PS8 polymers at all concentrations, G′ and G″ were found to be frequency dependent. In the low frequency region, G″ was larger than G′, both exhibiting a power law dependence of ω. G″ was scaling with ω1 and G′ with ω1.8, i.e., the samples behaved in a liquid-like manner. At higher frequencies, G′ became larger than G″ with a much weaker frequency dependence, i.e., the samples showed a characteristic gel behavior (Figure 4). Via eq 1, we estimated the relaxation time τ for the different copolymers. With increased inner block length, τ strongly increased from 0.03 s, to 0.25 s and to 1.45 s for, respectively, 4484


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change in water−PMDEGA interactions with temperature. It is well-known for poly(ethylene oxide) block copolymers that with increasing temperature a progressive dehydration of the swollen polymers takes place, with a resulting contraction of the corona.30,66 Considering the chemical similarity between poly(ethylene oxide) and the poly(oligo ethylene glycol acrylate), it is reasonable to expect a similar temperature dependence. The fast drop of storage modulus recorded at low temperatures for the polymers with shorter inner blocks, precisely those with the lowest LCST, might be a consequence of the progressive dehydration of their micelle shell. Figure 6

Figure 6. Gel−sol boundaries (closed symbols) for the polymers as a function of the concentration and temperature, as determined from dynamic oscillatory experiments. Sol−dispersion boundaries (open symbols) were determined from visual test: (a) (●) PS8-PMDEGA452PS8, (▲) PS8-PMDEGA180-PS8; (b) (▼) PS8-PMDEGA93-PS8, and (■) PS8-PMDEGA53-PS8. Lines are guides to the eye.

summarizes the phase boundaries for all four triblock copolymers. Noteworthy, the transition temperatures from gel to sol and from sol to dispersion all increased with the length of the inner block. It was thus interesting to investigate further the correlation between these two critical temperatures. Structural Characterization of the Association As Studied on PS8-PMDEGA452-PS8. To learn more about the structural changes of the gels when passing the gel and the cloud points, we focused on the triblock copolymer with the longest inner block, PS8-PMDEGA452-PS8. Its structure was investigated using SAXS in the temperature range 17−44 °C at three different concentrations, namely, 5, 10, and 20 wt %. Accordingly to the rheological studies, the polymer formed a gel for concentrations as low as 3.5 wt % in the temperature range 5−10 °C (see Supporting Information). Both the elastic and storage moduli increased with concentration by 1 order of magnitude, as the concentration doubled from 5 to 10 wt % and from 10 to 20 wt % (Figure 7). This evolution is attributed to the decreasing intermicellar distances in concentrated solutions, a fact that favors the probability of bridges between the micelles. Additionally, the gel−sol transition temperature increased with the concentration, namely, from 16 to 25 °C and to 29 °C at 5, 10, and 20 wt %, respectively. In our previous study of analogously designed PS-PNIPAMPS triblock copolymers, a clear correlation between mechanical properties and mesoscopic structure was observed.34,67 At the cloud point, important changes in the structure of the micellar network were found, with a strong and sharp decrease of the shell thickness and the micellar distance. Thus, we wondered whether analogous structural changes occur for the PMDEGA based BAB triblock copolymer systems at the gel point, and to which extent the overall behavior resembles that of the frequently studied model systems based on PNIPAM.

Figure 5. Temperature dependence of storage modulus G′ (closed red symbol) and loss modulus G″ (open blue symbol) at 1 Hz frequency, for 20 wt % aqueous solution of copolymers: (■, □) PS8-PMDEGA53PS8; other symbols as in Figure 4.

possibly due to the ability of PNIPAM for interpolymer Hbonding, inducing close to the thermal transition strong new physical cross-links,33 which cannot be realized by PMDEGA. Well below the cloud points, the continuous decrease of both moduli with heating led eventually to cross points of G′ and G″ for all the gel samples. These cross temperatures were taken as the gel−sol critical temperatures, namely, 13 °C for PS8PMDEGA93-PS8, 21 °C for PS8-PMDEGA180-PS8, and 29 °C for PS8-PMDEGA452-PS8. For thermo-responsive hydrogels made of BAB triblock copolymers with switchable hydrophobic blocks, temperature induces a sharp transition from liquid to gel for LCST polymers20 or from gel to liquid for UCST polymers,32 as the bridges get suddenly created or disrupted, respectively, upon heating. In our case, i.e., with a switchable hydrophilic block, one might intuitively expect a sudden transition from a gel to a freely flowing dispersion upon the collapse of the hydrophilic inner block. Instead, a smooth transition from gel to liquid developed, followed by a sudden change from a homogeneous liquid to dispersion at the cloud point. The transition from gel to sol might arise from two effects: first, the bridges between individual polymeric micelles disrupt progressively upon heating. This phenomenon has been described for polymer solutions with permanently hydrophilic inner blocks.65 The second effect might arise from the particular thermo-responsive nature of the PMDEGA inner block and lies in the gradual 4485


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Figure 7. Temperature dependence of storage modulus G′ (closed red symbol) and loss modulus G″ (open blue symbol) at 1 Hz frequency, for aqueous solutions of PS8-PMDEGA452-PS8 at (●, ○) 20 wt %, (▲, △) 10 wt %, (■, □) 5 wt %.

Representative scattering curves of PS8-PMDEGA452-PS8 are displayed in Figure 8. At 5 wt %, a broad peak at 0.21 nm−1 (at 17 °C) is present in the otherwise smoothly decaying curve (Figure 8a). We attribute this peak to the correlation between the positions of the micelles. It becomes weaker upon heating to the cloud point, and, at the same time, the forward scattering increases. At 41 and 44 °C, the peak is shifted to higher q-values (∼0.35 nm−1), indicating the collapse of the micellar network. The decrease of the overall intensity above the cloud point is due to sedimentation of large clusters formed by collapsed micelles. Similar overall behavior is observed at 10 and 20 wt % (Figure 8b,c) with the peaks being more pronounced the higher the polymer concentration as well as shifted to higher q-values (at 17 °C, it is located at ∼0.24 nm−1 and 0.30 nm−1 at 10 and 20 wt %, respectively). This means the correlation between the micelles becomes stronger with increasing concentration, and their distance decreases, as expected. Following our previous approach on 5−20 wt % PSPNIPAM-PS solutions,67 we first modeled the small-angle Xray scattering (SAXS) curves by spherical core−shell micelles with PS and PMDEGA forming the core and the shell of the micelles. We attributed the peak to the correlation between the PS cores, which is liquid-like with a close to hard-sphere interaction; thus, a Percus−Yevick structure factor is sufficient to model the correlation.68 The forward scattering, which unexpectedly was present even below the cloud point, was modeled using a Porod law describing large, homogeneous, and smooth clusters formed by water-insoluble and bridged micelles.69 Using these models, good fits could be obtained throughout the entire temperature and concentration range (not shown). However, the fitting parameters obtained were not fully reasonable. Whereas the hard-sphere radius, deduced from the structure factor, i.e., the peak, seemed reasonable, with values between 8 and 13 nm, the core radius took fit values of 5−11 nm and was in some cases only 2 nm lower than the micelle radius. This is in conflict with the large ratio of PMDEGA to PS block lengths. The reason is that the scattering signal in PS-PMDEGA-PS is dominated by the scattering from the PMDEGA blocks: PS and H2O have nearly the same X-ray scattering length density (9.56 × 1010 cm−2 and 9.34 × 1010 cm−2 for PS and H2O, respectively, and 11.0 × 1010 cm−2 for PMDEGA). Thus, the scattering from the PMDEGA corona dominates over the one from the small PS cores.

Figure 8. Representative temperature dependent SAXS scattering curves (symbols) together with model fits (lines) for aqueous solutions of PS8-PMDEGA452-PS8 at (a) 5 wt %, (b) 10 wt %, (c) 20 wt %: ■, 17 °C; ▲, 26 °C; ▼, 35 °C; □, 38 °C; △, 41 °C; ▽, 44 °C. For clarity, only every second data point is shown.

Since the scattering curves are very smooth at high q-values (beyond the peak), it is likely not possible to resolve the core and the shell. The reasons are that the X-ray scattering length density of PS is only 1% higher than the one of H2O and that the PS core is very smallthe contour length of PS8 is estimated at ∼2 nm only. Moreover, the PMDEGA shell is highly swollen below the cloud point and thus diluted with H2O. We therefore chose a different approach and modeled the scattering using the following expression: I(q) = Pmic(q)SHS(q) + Sfluct(q) + bkg

(2)

in which the form factor Pmic(q) describes the entire spherical micelles. A simplified sphere form factor is used.70 Pcl(q) =

K 1 + 0.22(qrmic)4

(3)

It contains the amplitude, K, proportional to the number of micelles, and the average micelle radius, rmic. It does not display the oscillations of the correct form factor of homogeneous spheres with a smooth surface, but has the right limiting 4486


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Figure 9. Temperature-dependent results from SAXS for 5 wt % (a,b), 10 wt % (c,d), and 20 wt % (e,f). (a,c,e): □, correlation length ξ; △, rmic; ○, rHS. (b,d,f): volume fraction of correlated micelles, ϕ. The dashed and dash-dotted lines mark the cloud point and the gel point from Figure 6.

behavior at low and high q-values. Since the observed scattering curves do not show oscillations, it seems appropriate to use this simplified form. The liquid-like correlation of the micelles in solution was modeled by a hard-sphere structure factor, SHS(q),68 which includes the hard sphere radius rHS and the volume fraction ϕ, describing the average half-distance between the micelles in solution as well as the fraction of micelles which are correlated with each other. The scattering intensity due to the inner structure of the micelle was modeled by the Ornstein−Zernike structure factor which constitutes the last contribution in eq 2

Sfluct(q) =

in the clusters becomes denser as the PMDEGA chains collapse, as expected. Interestingly, no discontinuity is visible at the cloud point, in contrast to the PS-b-PNIPAM-b-PS system.67,71 For PS-b-PMDEGA-b-PS, the transition seems thus to be more smooth on the mesoscopic level. The correlation of the micelles within the clusters is stronger than below the cloud point and increases above the cloud point as the temperature is increased (Figure 9b). The 10 wt % solution behaves similarly (Figure 9c,d). Below the cloud point, rHS decreases slightly as temperature is increased. The micelle radius is smaller than at 5 wt %, increasing from 8.1 nm at 17 °C to 11.7 nm at 35 °C (Figure 9c). Further, the volume fraction of correlated micelles is 0.29− 0.20, thus higher than in the 5 wt % sample, as could be expected, and also decreases slightly (Figure 9d). This may be due to the fact that single micelles are released from loose aggregates of micelles. No change is observed at the gel point. Above the cloud point, rHS continues to decrease, whereas ϕ increases steadily. In the 20 wt % sample, rHS is with 10.4−9.9 nm (17−35 °C) significantly smaller than at 5 and 10 wt % (Figure 9e). In the same temperature range, rmic amounts only to 4.4−7.3 nm, and is thus much smaller than at lower concentrations. The correlation between the micelles is again stronger than at 5 and 10 wt %, as evident from the high values of ϕ (0.32−0.28, Figure 9f) and decreases only above 29 °C. A portion of the micelles forms large clusters at 17−26 °C, as evident from the forward scattering (below q = 0.15 nm−1) in this temperature range. Again, no structural change is observed at the gel point. We conclude that the SAXS curves of PS8-PMDEGA452-PS8 can be described by a model including spherical micelles with liquid-like correlation and a loose corona consisting of a solvent-swollen polymer matrix. Interestingly, discontinuities are observed neither at the gel points nor at the cloud points. The distance between the micelles keeps decreasing smoothly. Only the correlation between micelles decreases slightly below the cloud point, while it increases above (cf. Figure 9b,d,f). This makes an important difference in the thermo-responsive BAB triblock copolymer systems containing PMDEGA as switchable inner A block compared to the analogous system based on

IOZ 1 + (qξ)2

(4)

Here, ξ denotes the average correlation length in the PMDEGA corona. The background bkg were used as fitting parameters. With this model, good fits were obtained for all three concentrations and at all temperatures investigated (Figure 8). The most important results are given in Figure 9. The 5 wt % sample is in the sol state above 17 °C with the cloud point at 37 °C. As the temperature is increased from 17 to 35 °C, the micelle radius rmic increases from 11.3 to 13.0 nm (Figure 9a). The hard-sphere radius rHS decreases from 12.8 to 10.1 nm in the same temperature range, i.e., the micelle distance decreases as the cloud point is approached. At the same time, the volume fraction ϕ, indicative of the fraction of micelles which are correlated, decreases from 0.225 at 17 °C to 0.172 at 35 °C (Figure 9b); the correlation becomes weaker. The average monomer correlation length, ξ, is 1.67 ± 0.05 nm. Hence, below the cloud point, the solution structure is prone to changes, namely a decrease of the micelle distance with a simultaneous decay of the micelle correlation. In contrast, the average monomer correlation length in the corona is unchanged. Above the cloud point, the micelle radius is scattered and cannot be determined reliably. The monomer correlation length does not change and stays at 1.7−2.0 nm. rHS decreases further down to 6.8 nm and the volume fraction assumes higher values (0.18−0.23), which shows that the network of micelles 4487


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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Frequency dependent sweep data at 10 °C and 20 wt % polymer concentration, temperature dependent sweep data for all polymers at all concentrations, and details of macroCTA synthesis. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We thank S. Jaksch and D. Magerl (Technische Universität München) for the help with SAXS measurements; C. Wieland (Universität Potsdam) and E. Wischerhoff (Fraunhofer Institute for Applied Polymer Research, Potsdam-Golm) for SEC measurements; and M. Heydenreich and A. Krtitschka (Universität Potsdam) for NMR measurements. Financial Support was given by the Deutsche Forschungsgemeinschaft DFG, priority program SPP1259 “Intelligente Hydrogele” (grants LA611/7, PA771/4, MU1487/8).

CONCLUSIONS We investigated a series of new amphiphilic symmetrical triblock copolymers BAB, based on the thermo-responsive PMDEGA as inner block A of varying length, and bearing two short hydrophobic polystyrene outer blocks B, which still allow direct dissolution in water. Optical and rheological studies of dilute and concentrated aqueous solutions allowed us to study the LCST-type phase separation, hydrogel formation, and mechanical properties as function of temperature. Clearly, the behavior of the triblock copolymers is dominated by the length of the thermo-responsive inner block, and cloud points increase from 20 to 35 °C with increasing length of the PMDEGA block. At temperatures below 10 °C, the critical gelation concentration of the polymer with the longest inner block, PS8PMDEGA452-PS8, was as low as 3.5 wt %, whereas a concentration of 30 wt % was necessary to obtain a gel from PS8-PMDEGA53-PS8. Noteworthy, the mechanical properties monotonously declined for all the gels with rising temperatures, leading eventually to two thermal transitions, namely, from gel to liquid at temperatures well below the cloud point, and from sol to phase separated liquid at the cloud point. This makes a marked difference from the behavior of analogous triblock copolymers containing PNIPAM as thermo-responsive inner block, for which gelling can be induced by heating close to the phase transition temperature. This exemplifies the importance of the chemical nature of the thermo-responsive block for controlling not only the phase transition temperature, but also the polymer−polymer interactions in the amphiphilic copolymers. SAXS data of a selected polymer were modeled successfully as micellar gel and allowed it to relate the volume fraction of correlated micelles with the rheological profile. While the correlation between micelles becomes higher with concentration, it decreases below the collapse temperature, thus indicating structural changes. As the SAXS measurements did not provide evidence for progressive dehydration and shrinkage of the individual micelles with temperature, the unusual temperature dependence of the hydrogels seems to be due to a decreasing number of micelle bridging polymers. Though polystyrene is commonly believed to build kinetically frozen glassy micelle cores, the short PS8 blocks studied seem to represent a good compromise for already providing sufficient hydrophobicity while still maintaining a certain mobility in aqueous self-organization.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 49 331 977 5036; Tel: 49 331 977 5225; E-mail: [email protected]. Notes

The authors declare no competing financial interest. 4488


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Thermoresponsive Hydrogels from Symmetrical Triblock Copolymers

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