Anomalous Transition in Aqueous Solutions of a Thermoresponsive

Aug 24, 2007 - An increase of the cloud point temperature with rising shear rate is reported, which is interpreted as being a ... Charged Star Diblock...
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J. Phys. Chem. B 2007, 111, 10862-10870

ARTICLES Anomalous Transition in Aqueous Solutions of a Thermoresponsive Amphiphilic Diblock Copolymer Kaizheng Zhu, Huiting Jin, Anna-Lena Kjøniksen, and Bo Nystro1 m* Department of Chemistry, UniVersity of Oslo, P. O. Box 1033, Blindern, N-0315 Oslo, Norway ReceiVed: May 30, 2007; In Final Form: July 13, 2007

The influence of shear flow on aggregation and disaggregation in aqueous solutions of the thermoresponsive methoxy-poly(ethylene glycol)-block-poly(N-isopropylacrylamide) (MPEG53-b-PNIPAAM113) copolymer that exhibits a lower critical solution temperature was investigated with the aid of turbidity, shear viscosity, and rheo small angle light scattering (rheo-SALS) methods. The turbidity results at quiescent conditions revealed a novel transition peak in the turbidity curve at intermediate temperatures, which reflects the delicate interplay between temperature-induced aggregation and shrinking of the species. A similar anomalous transition peak (located at the same temperature) was observed in the steady shear viscosity measurements at intermediate temperatures, and the amplitude of the peak was reduced with increasing shear rate as a consequence of breakup of interaggregate chains. At low temperatures (low sticking probability), enhanced shear rate generated interpolymer aggregates; whereas in the high-temperature domain (high sticking probability) association structures were broken up as the shear rate was increased. The rheo-SALS experiments disclosed growth of aggregates at low temperatures and destruction of association complexes at high temperatures. An increase of the cloud point temperature with rising shear rate is reported, which is interpreted as being a disruption of clusters under the influence of shear stresses.

Introduction In recent years, extensive attention has been paid to temperature-responsive block and graft amphiphilic copolymers for their capacity to form stable clusters with a core-shell structure in solution,1-12 and for their potential applications in targeted drug delivery, especially in delivering drugs to tumor sites13 and arthritis sites.14 These nanoparticles can undergo reversible structural transitions from a closed to an open state with the aid of external stimuli such as temperature, providing on-off switches for tuned drug delivery.15,16 Among those polymers that are sensitive to a temperature stimuli, poly(N-isopropylacrylamide) (PNIPAAM) has been widely studied because of its unique phase separation feature upon temperature changes. An aqueous solution of this polymer is characterized by a phase separation upon heating, and the system exhibits a lower critical solution temperature (LCST) at about 32 °C.17 To tune the temperature-induced-association power and physical properties of PNIPAAM in solution, several different block and graft amphiphilic copolymers containing PNIPAAM have been synthesized and characterized by various experimental techniques.3,10,14-16,18-22 To modulate the temperature-induced-association features of PNIPAAM, a diblock copolymer, containing PNIPAAM and a hydrophilic block of monomethoxy-capped poly(ethylene glycol) (MPEG), was synthesized in this work by using atom transfer radical polymerization (ATRP).23,24 The product ob* To whom correspondence should be addressed. Telephone: +47 22855522. Fax: +47 22855441. E-mail: [email protected].

tained from the synthesis is methoxy-poly(ethylene glycol)block-poly(N-isopropylacrylamide) with the following composition: MPEG53-b-PNIPAAM113. It will be shown that this amphiphilic copolymer, containing hydrophilic MPEG segments, in aqueous solution displays anomalous temperature-induced transitions monitored in turbidity and steady shear viscosity experiments. To the best of our knowledge, this type of transition and experimental registration has not been reported before. It will be demonstrated that the intensity of the interpolymer association can be modulated by the magnitude of the shear rate and polymer concentration. The intricate interplay between interpolymer aggregation and disruption of the association complexes under the influence of shear stresses will be elucidated. The structure of the association complexes at different temperatures and shear rates is probed by the rheo small angle light scattering (rheo-SALS) technique. Experimental Section Materials. Monomethoxy-capped poly(ethylene glycol) (MPEG, Mn ) 2000), 2-bromo-2-methylpropionic acid, N,N′dicyclohexylcarbodiimide, tris(2-aminoethyl)amine, formic acid (85%), copper(I) chloride, and CaH2 were purchased from Aldrich. The other reagents, 4-(dimethylamino)pyridine (DMAP) and formaldehyde (36%), were obtained from Fluka and used as received. N,N′-Dimethylformamide (DMF) was distilled under vacuum before use. N-Isopropylacrylamide (NIPAAM) was recrystallized from a toluene/n-hexane mixture solvent and dried under vacuum prior to use. The chloroform was dried by refluxing with CaH2 and distilled under N2. Copper(I) chloride

10.1021/jp074163m CCC: $37.00 © 2007 American Chemical Society Published on Web 08/24/2007

Anomalous Transition in MPEG53-b-PNIPAAM113 from Aldrich was washed with glacial acetic acid, followed by washing with methanol and ethyl ether to remove impurities, and then dried under vacuum and kept under N2 atmosphere. N,N,N′,N′′,N′′′,N′′′-(Hexamethyltriethylenetetramine) (Me6TREN) was synthesized according to the method previously described.25 All water used in this study was purified with a Millipore Milli-Q system, and the resistivity was approximately 18 MΩ cm. Synthesis and Characterization of the Macroinitiator MPEG. The MPEG macroinitiator was prepared according to a procedure reported previously.26 In a 250 mL one-neck flask, methoxy-poly(ethylene glycol) (MPEG) (15 g, 7.5 mmol), 2-bromoisobutyric acid (2.5 g, 15 mmol), and 1,3-dicyclohexylcarbodiimide (DCC, 3.1 g, 15 mmol) were dissolved in 100 mL of dried chloroform, and the solution was cooled to 0 °C. 4-(Dimethylamino)pyridine (4-DMAP) (0.09 g) was added, and the reaction mixture was stirred overnight at room temperature. The precipitated N,N′-dicyclohexylurea was filtered off, and most of the chloroform was then removed by rotary evaporation. The macroinitiator was then precipitated twice in cold ether, followed by drying under vacuum. The yield was 13.7 g (84%). 1H NMR (300 MHz, D O) δ (1.84, 6H) and (3.81, 182 H) 2 indicated that the hydroxyl end groups of the MPEG were fully esterified. The molecular weight of the commercial MPEG sample was reported to be 2000 by the manufacturer, but our accurate asymmetric flow field-flow fractionation (AFFFF) experiments (the experimental technique is described below) showed that the molecular weight of the sample is 2330, which has been used to calculate the number (n ) 53) of ethylene glycol units in the copolymer. Copolymer Synthesis. The uncharged MPEG-b-PNIPAM diblock copolymer was synthesized via an ATRP system by using a water/DMF 40:60 (v/v) mixture as the solvent at 25 °C with MPEG-MI/CuCl/Me6TREN as the initiator/catalyst system and a molar feed ratio ([NIPAM] ) 2.0 M, [NIPAM]/[MPEGMI]/[CuCl]/[Me6TREN] ) 50:1:1:1:1). NIPAAM (17.0 g, 0.15 mol), MPEG macroinitiator (MPEG-MI) (3 mmol, 6.45 g), DMF (42 mL), and deionized water (26 mL) were added to a 100 mL Schlenk flask under magnetic stirring. After NIPAAM and MPEG-MI were completely dissolved, the mixture was degassed by three freeze-pump-thaw cycles. The flask was then filled with nitrogen and immersed in a water bath that was kept at about 25 °C. A Cu(I)-Me6TREN water stock solution was prepared by adding degassed water (8 mL) to CuCl (1.188 g, 12 mmol) and Me6TREN (3.3 mL, 12 mmol) under nitrogen flow. A 2 mL volume of the freshly prepared Cu(I)-Me6TREN stock solution was withdrawn via a syringe and quickly added to the above mixture under nitrogen flow, and the polymerization reaction was then initiated. The polymerization was allowed to proceed under vigorous continuous stirring at 25 °C. In such aqueous-based reaction solutions, exotherms with a temperature rise of 2-4 °C were typically observed, indicating the onset of the polymerization. The reaction mixture turned viscous, and when NIPAAM conversion reached approximately 95% (after approximately 1 h, 1H NMR analysis indicated that more than 95% of the NIPAAM had been polymerized (disappearance of the vinyl signals at δ 5.5-6.0 ppm)), diluting with THF terminated the reaction; the product was passed through an Al2O3 column (basic, activated) to remove Cu complexes. The polymer was further purified by dialyzing against distilled water for several days using a dialysis membrane of regenerated cellulose with a molecular weight cutoff of 3500. The white solid product was isolated by lyophilization (18.4 g). The synthesis route of MPEG-b-PNIPAM is shown in Scheme 1.

J. Phys. Chem. B, Vol. 111, No. 37, 2007 10863 SCHEME 1: Synthetic Route for MPEG-b-PNIPAM Diblock Copolymer via ATRP

Characterization of the Copolymer. The monomer conversion was determined by 1H NMR with a Bruker AVANCE DPX 300 NMR spectrometer (Bruker Biospin, Fa¨llanden, Switzerland), operating at 300.13 MHz at 25.0 °C by using heavy water (D2O) as solvent. A small amount of the sample was withdrawn from the mixture and diluted with D2O; the monomer conversion was calculated by the disappearance of the monomer double bond (vinyl) signals at δ ) 5.5-6.0 ppm and the appearance of the polymer backbone at δ ) 1.5-2.0 ppm. The 1H chemical shift in D2O is referred to the residual DHO proton (δ ) 4.70 ppm) in D2O. The chemical structure of the block copolymer were ascertained by its 1H NMR spectrum (see Figure 1). The numberaverage molecular weight and the composition of the diblock copolymer MPEGm-b-PNIPAAMn, where m and n denote the numbers of ethylene glycol (EG) units and NIPAAM units, respectively, were evaluated by comparing the area of the methylene proton peak (2) of PEG (δ 3.63 ppm) and the methyne proton peak (6) of PNIPAAM (δ ) 3.85 ppm) obtained from its 1H NMR spectrum. By using the number-average molecular weight of MPEG (MMPEG ) 2330), obtained from the AFFFF experiment, a value of m ) 53 is obtained. From the NMR results, the number of repeating NIPAAM units in the PNIPAAM block is calculated to be 74 (n ) 74). However, due to the presence of small amounts of unreacted macroinitiator in the sample (see discussion below) this number is too low, and a more accurate value of n ) 113 is calculated from AFFFF. Asymmetric Flow Field-Flow Fractionation. The AFFFF experiments were conducted on an AF2000 FOCUS system (Postnova Analytics, Landsberg, Germany) equipped with a refractive index (RI) detector (PN3140, Postnova) and a multiangle (seven detectors in the range 35-145°) light scattering detector (PN3070, λ ) 635 nm, Postnova). The diblock copolymer sample dissolved in aqueous medium (0.5 wt % in 0.01 M NaCl) was measured using a 350 µm spacer, a regenerated cellulose membrane with a cutoff of 5000 (Z-MEMAQU-426N, Postnova), and an injection volume of 20 µL. The sample was investigated using a constant detector flow rate of 1.0 mL/min. The focusing time was 7 min at a cross-flow of 2

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Figure 1. 1H NMR spectrum of MPEG53-b-PNIPAAM113 in D2O (300 MHz, 25.0 °C).

Figure 2. Illustration of the molecular weight distribution of MPEG53b-PNIPAAM113 in aqueous solution (0.01 M NaCl) with the aid of AFFFF.

mL/min. Thereafter, the cross-flow was reduced exponentially (exponent value of 0.7) to 0.9 mL/min during a 5 min period. The cross-flow was then linearly reduced to zero during a period of 5 min. Processing of the measured data was accomplished by the Postnova software (AF2000 Control, version 1.1.0.11). A weight-average molecular weight of Mw ) 17 200 and a rootmean-square radius of gyration of Rg ) 19 nm for the sample in the dilute concentration regime were obtained using this software with a Zimm fit. The instrument yielded a refractive index increment (dn/dc) of 0.129 (determined by using the RI detector at 32 °C). The diblock copolymer has a fairly narrow molecular weight distribution (see Figure 2) with a polydispersity index Mw/Mn ) 1.1. As can be seen from Figure 2, there is a very small peak with a molecular weight of about 2000. This peak indicates that there is a small amount of the MPEG macroinitiator left in the

sample even after the dialysis of the polymer. However, since the MPEG macroinitiator is a hydrophilic polymer which does not show any significant thermoresponsive properties in water, we do not expect this tiny amount of macroinitiator to affect our results reported below. However, since some unreacted macroinitiator is left in the sample, the number of NIPAAM units in the PNIPAAM block calculated from the NMR spectrum is too low. We have therefore used the Mn values from AFFFF to calculate the size of the PNIPAAM block (n ) 113). Turbidimetry. Turbidities of aqueous solutions of the MPEG53-b-PNIPAAM113 copolymer were determined with the aid of an NK60-CPA cloud point analyzer from Phase Technology, Richmond, BC, Canada. The specifications of this instrument and the procedure for the determination of turbidities have been reported previously.12 The measured signal S from the cloud point analyzer can empirically be related to the turbidity τ, which was determined from measurements of the transmittance on a standard spectrophotometer in a 1 cm cuvette, using the expression τ ) (-1/L) ln(It/I0), where L is the light path length of the cuvette, It is the transmitted light intensity, and I0 is the incident light intensity. A direct empirical relationship between the calculated turbidity from the spectrophotometer experiments and S from the cloud point analyzer is found12 to be τ ) (9.0 × 10-9)S3.751. All data from the cloud point analyzer will be presented in terms of turbidity. The very accurate temperature control (array of Peltier elements) of the sample in this instrument and the registration of the diffuse scattered light from the surface of the plate make this a powerful apparatus to monitor even small temperature-induced turbidity changes. In this study, the heating rate in most of the experiments was set to 0.2 °C/min, and no effect of the heating rate on the signal was observed at low heating rates. However, at high heating rates the profile of the turbidity curve is significantly changed (cf. the discussion below). The temperature at which the first deviation of the scattered intensity from the baseline occurred was taken as the cloud point (CP) of the considered sample.

Anomalous Transition in MPEG53-b-PNIPAAM113 Rheology. Viscosity measurements were performed in a PaarPhysica MCR 300 rheometer using a cone-and-plate geometry, with a cone angle of 1° and a diameter of 75 mm. This rheometer operates effectively with this geometry even on dilute polymer solutions, and the viscosity of water can easily be measured over an extended shear rate domain. The samples were introduced onto the plate, and to prevent evaporation of the solvent, the free surface of the sample was always covered with a thin layer of low-viscosity silicone oil (the viscoelastic response of the samples are not observed to be affected by this layer). The measuring unit is equipped with a temperature unit (Peltier plate) that provides a rapid change of the temperature and gives an accurate temperature control ((0.05 °C) over an extended time for all the temperatures considered in this work. Several viscosity measurements were carried out at a fixed shear rate under the influence of a temperature gradient of 0.2 °C/ min (this heating rate was used in the turbidity and the rheoSALS experiments) over an extended temperature interval. Rheo Small Angle Light Scattering (Rheo-SALS). Combined rheological and small angle light scattering experiments during shear flow were performed using the Paar-Physica MCR 300 rheometer, equipped with a specially designed parallel plate-plate configuration (the diameter of the plate is 43 mm) in glass. The instrumentation for the rheo-SALS experiments was purchased from Physica-Anton Paar. In all measurements a 10 mW diode laser operating at a wavelength of 658 nm was used as the light source, and a polarizer was placed in front of the laser and an analyzer below the sample, making both polarized (polarizer and analyzer parallel) and depolarized (polarizer and analyzer perpendicular) experiments possible. Utilizing a prism, the laser beam was deflected and passed through the sample placed between the transparent parallel plates. The sample was applied onto the lower plate by filtering the solution through a 0.2 µm Millipore filter. The distance between the plates is small (1.0 mm), so the effect of multiple scattering was reduced when the sample became turbid at elevated temperatures. The light propagated along the velocity gradient direction, thus probing the structure in the plane of flow and vorticity. The forward scattered light at small angles was collected on a flat translucent screen below the sample (distance between sample and screen was 12.3 cm). A schematic diagram of the rheo-SALS setup utilized in this work is shown in Figure 3. The two-dimensional scattering patterns formed on the screen were captured using a CCD camera (driver LuCam V. 3.8), whose plane was parallel to that of the screen. A Lumenera (VGA) CCD camera (Lumenera Corporation, Ottawa, Canada) with a Pentax lens was utilized, and the scattered images were stored on a computer using the StreamPix (NorPix, Montreal, Quebec, Canada) application software (version 3.18.5), which enables a real-time digitalization of the images. The images were acquired via the CCD camera with an exposure time of 200 ms. Subsequently, the pictures were analyzed using the SALS software program (version 1.1) developed by the Laboratory of Applied Rheology and Polymer Processing, Department of Chemical Engineering, Katholieke Universiteit Leuven, Leuven, Belgium. The scattering functions were recorded continuously during the temperature up-ramp scan (0.2 °C/min). The approximate accessible scattering wave vector (q) range is between q ) 4 × 10-4 nm-1 and q ) 2 × 10-3 nm-1; here q is defined as q ≡ |q| ) (4πn/λ) sin(θ/2), where λ is the wavelength of the incident beam and θ is the scattering angle. The refractive index was measured at different temperatures with an automatic refractometer (Model PTR 46) purchased from

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Figure 3. Experimental setup for the rheo-SALS studies.

Index Instruments Ltd., England. The temperature of the instrument was controlled electronically to a high stability by using a Peltier cell. The refractive indexes for temperatures above 40 °C (which is the limit of the instrument) were extrapolated from the values at lower temperatures. Results and Discussion Copolymer Synthesis. The MPEG53-b-PNIPAAM113 diblock copolymer was prepared via an atom transfer radical polymerization in aqueous media according to the method shown in Scheme 1. This method was adopted from a recent procedure to produce a well-controlled ATRP system of N-isopropylacrylamide (NIPAAM), using ethyl 2-chloropropionate (ECP)/CuCl/ CuCl2/Me6TREN as the initiator/catalyst system in a DMF/water mixture at room temperature.27 DMF is an interesting solvent component as it is miscible with water and is able to solubilize a wide range of hydrophilic and hydrophobic polymers, and water has been demonstrated to strongly increase the rate of ATRP. In our experiments, we chose methoxy-poly(ethylene glycol) macroinitiator as the initiator of the ATRP system and as one of the blocks in the block copolymer, because the MPEG derivative shows good solubility in most organic and aqueous media and MPEG-based ATRP initiators can be used for many water-insoluble as well as water-soluble systems. The 1H NMR spectrum (Figure 1) of MPEG53-b-PNIPAAM113 in heavy water displayed two well-separated peaks at δ ) 3.63 ppm and δ ) 3.85 ppm, which were assigned to the methylene proton peak (2) of PEG and the methylene proton peak (6) of PNIPAAM, respectively. The NMR spectrum indicates that the target copolymer has been successfully synthesized. Turbidimetry. Figure 4 shows the temperature dependence of the turbidity at different concentrations of MPEG53-bPNIPAAM113 at a heating rate of 0.2 °C/min and also at a heating rate of 5 °C/min (asterisks) for the 1 wt % sample. The novel and conspicuous feature is the distinct peaks observed at intermediate temperatures. The height of the peak increases with increasing polymer concentration, and its location is shifted toward lower temperature as the polymer concentration increases. The temperature at which the first deviation of the

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Figure 4. Temperature dependence of the turbidity during a heating rate of 0.2 °C/min at the MPEG53-b-PNIPAAM113 concentrations indicated. The solid curve represents the subsequent cooling (0.2 °C/ min) of the 1 wt % solution. The data points (asterisks) for the 1 wt % sample at a heating rate of 5 °C/min are also included. The inset shows the effect of polymer concentration on the cloud point.

scattered intensity from the baseline occurred was taken as the CP, and it is evident from the inset that the value of the CP drops with rising temperature. The solid curve illustrates the down-scan behavior at a slow cooling rate (0.2 °C/min) for the 1 wt % solution. The hysteresis in the heating and cooling cycle indicates that the structural rearrangements of the solution are slow and depend on the history of the sample. In the cooling cycle the sharp transition at intermediate temperatures is not observed, and this may be due to a slow structural reorganization of the large aggregates (flocs) formed at elevated temperature. As will be discussed below, hysteresis effects also appear in the viscosity data. The height of the peak (Figure 4) reflects the intensity of the interpolymer aggregation, which becomes more pronounced as the polymer concentration increases because the collision frequency of particles increases and this generates larger flocs. A close inspection of the data reveals that even for the lowest polymer concentration a transition peak is visible. The initial rise of the turbidity with increasing temperature is ascribed to an augmented hydrophobicity and enhanced stickiness of the entities, leading to intensive aggregation of the molecules and formation of flocs. Flocs are usually characterized by a loose structure, which can be destroyed by the application of shear stresses or by the Brownian motion of the entities. The subsequent falloff of the turbidity after the maximum is to a large extent attributed to contraction (cf. the discussion below) of the polymer species, but a certain fragmentation of the association complexes under the action of Brownian motion cannot be excluded. The peak may be rationalized in the following scenario. As the temperature and polymer concentration rise, individual copolymer chains have a greater probability to associate with each other before they collapse. As a result, the sizes of the particles increase and core-shell structures are formed. At high polymer concentration, the number density of PNIPAAM chains inside the core is high and this promotes an enhanced temperature-induced contraction of the particles. Temperature-induced shrinkage of clusters formed in aqueous solutions of amphiphilic copolymers has been reported previously.3,10,12,28 It is likely that the aggregation behavior continues during the contraction stage, but the effect of shrinking dominates over

Figure 5. Effects of temperature, polymer concentration, and shear rate on the relative viscosity for solutions of MPEG53-b-PNIPAAM113 at the conditions indicated and at a heating rate of 0.2 °C/min. The solid curves indicate down ramps at a cooling rate of 0.2 °C/min.

interpolymer association over a restricted temperature interval as illustrated by the drop of the turbidity. In addition, some disaggregation may occur due to perturbations generated by Brownian motion. At still higher temperatures (the upturns of the turbidity curves at high temperatures), the compression of the particles has ceased and the sticky collapsed species will associate to form large interpolymer associations, and at sufficiently high temperatures and concentration macroscopic phase separation will occur. We notice that a higher concentration gives rise to a more marked upturn, and only a modest upturn can be traced for the lowest polymer concentration, which supports the idea that aggregates are more easily formed at higher concentration due to a more frequent collision of the particles. The subsequent cooling cycle (0.2 °C/min) for the 1 wt % sample reveals a hysteresis effect; i.e., the heating and cooling curves do not collapse onto each other but the amplitude of the transition peak is much lower for the cooling cycle (solid curve in Figure 4). The reason for this is probably that the restructuring of the aggregates during the cooling cycle is a very slow process. It is obvious that, in order to detect the transition peak, the heating rate must be sufficiently low. At a heating rate of 5 °C/min no peak is observed (see the asterisks in Figure 4), but a direct steplike crossover is found that is reminiscent of that reported for aqueous solutions of many other amphiphilic copolymers.3,10,12 This stresses the importance of using a slow heating or cooling rate in probing temperature-induced turbidity changes of a system. Rheology. In solutions of “sticky” particles, it is established29-32 that shear flows tend to bring molecules together in the form of aggregates more rapidly than Brownian motion (at quiescent state) does, thus affecting the aggregation kinetics by speeding up the process. This kind of aggregation in the presence of shear forces is known as orthokinetic aggregation. It is known that shear flows enhance the rate of formation of aggregates if the Pe´clet number Pe (Pe ) γ˘ a2/D, where γ˘ is the shear rate, a is the particle radius, and D is the diffusion coefficient) is high.

Anomalous Transition in MPEG53-b-PNIPAAM113

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Figure 7. Relative shear viscosity is plotted versus shear rate for the polymer concentrations and temperatures indicated. The inset shows the polymer concentration dependence of the power law exponent m, describing shear rate dependence of the relative viscosity (ηrel ∼ γ˘ -m).

Figure 6. Effects of polymer concentration and temperature on the shear rate dependence of the relative viscosity for aqueous solutions of MPEG53-b-PNIPAAM113 (Z38).

When the Pe´clet number is large, particles and clusters follow the flow streamlines, whereas for low values of Pe species undergo Brownian motion (turbidity measurements) and perikinetic aggregation may occur. When the clusters are sufficiently large, they break up under the influence of shear stresses, and as a result the aggregation rate levels off or decreases. The breakup process is usually considered to consist of two steps: one is an instantaneous breakup due solely to fluid stresses, and the other is a kinetic breakup generated by shear-induced collisions between interaggregate chains. In Figure 5, effects of temperature, polymer concentration, and shear rate on the relative viscosity ηrel (ηrel ≡ η/ηwater, with ηwater the solvent viscosity) are depicted. At low shear rate, the results show a pronounced transition peak in the relative viscosity at intermediate temperatures and a concentrationdependent upturn of the relative viscosity at high temperatures. The profiles of the viscosity curves are reminiscent of those observed from the turbidity experiments, and the intermediate transition peak is located at the same temperature as that for the corresponding turbidity curve. Although the growth of the clusters and the succeeding contraction of the aggregates also play a role in the appearance of the viscosity peak, a more complex picture emerges in this case. In contrast to the turbidity results, the viscosity peak is more dominant at the lowest polymer concentration. In the interpretation of this finding, we should bear in mind that shear flows can activate two competing processes, namely shear-induced aggregation and breakup of large association complexes. At a low polymer concentration, a shear flow favors the formation of aggregates and this leads to the prominent transition peak in the viscosity curve. At a high concentration, large flocs are evolved at moderate temperatures and the predominant process is the disruption of these structures under the influence of shear stresses. Hence the rate of breaking up flocs dominates over the rate of formation of aggregates, and this leads to the weak transition peak in the viscosity at the highest polymer concentration. The drop of the relative viscosity, after passing the maximum, can probably be

attributed to both shear-induced aggregate breakup and shrinking of the species as observed in the turbidity experiments. This is an intricate interplay of phenomena, and it is not possible at the present state to separate the contribution from each process. It should be mentioned that since the molecular weight of the copolymer and the polymer concentrations are low, effects of network formation and entanglements can be ignored. The upturn of the relative viscosity at high temperatures becomes more pronounced with increasing concentration (Figure 5) at a given shear rate, and this phenomenon is ascribed to the formation of association complexes of species of loose and compact structures. By employing different shear rates, it is interesting to examine the impact of shear flow on the formation of association structures. It is obvious that an increasing shear rate reduces the tendency of forming aggregates; at the highest shear rate the transition peak has virtually disappeared and the upturn of the relative viscosity at high temperatures is suppressed substantially. Our surmise is that strong shear flow perturbations will repress the ability of forming flocs or aggregates. This issue is further elucidated below in connection with the analysis of the rheo-SALS results. Studies29-32 on the effect of shear flow on aggregating colloids frequently reveal that the initial shear flow induces formation of aggregates, and when the flocs are sufficiently large they are broken up by the shear stresses. The tendency to break up clusters is accentuated at high shear rates. In a recent rheological study33 on chemical cross-linking of dilute hydroxyethyl cellulose solutions, it was shown that shear forces could tone down interpolymer cross-linking. The solid viscosity curves (Figure 5), representing down scans at a cooling rate of 0.2 °C/min, disclose hysteresis effects around the transition peaks at intermediate temperatures. The hysteresis feature is strong at low shear rates and becomes less pronounced at higher shear rates because at this stage the large association structures are disintegrated by shear forces and the structural rearrangements are easier to reestablish. In view of the rheological findings above and the surmise that association complexes are formed at elevated temperatures, it is instructive to examine the influence of shear rate on the viscosity. The effect of shear rate on the relative viscosity for solutions of MPEG53-b-PNIPAAM113 of different concentrations and at some interesting temperatures is displayed in Figure 6. At 25 °C, where the stickiness of the molecules is low, a

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Figure 8. Scattered intensity patterns at the shear rates and temperatures indicated for a 0.5 wt % MPEG53-b-PNIPAAM113 solution.

Newtonian feature of the viscosity appears over the considered shear rate range for all concentrations. This suggests that at this stage the growth of the aggregates is modest. At 40 °C, corresponding to the approximate maximum in the turbidity curves at intermediate temperatures, shear thinning is observed for all concentrations and this behavior is attributed to shearinduced disintegration of large flocs. At 50 °C, where the turbidity results support the formation of large association structures, a salient shear-thinning effect is found for the highest polymer concentration. This finding reflects the shear-induced disaggregation of flocs. The shear-thinning features support our conjecture that interpolymer associations are responsible for the effects emerging at intermediate and high temperatures for the polymer samples. The increase in ηrel at high shear rates is probably due to turbulence. It has been reported29 for aggregating colloids that the shearinduced disaggregation can be described in terms of a power law of the steady-state viscosity versus the shear rate: ηrel ∼ γ˘ -m, where values of the power law exponent m in the range 0.5-0.6 have been observed. Figure 7 shows a log-log plot of the relative viscosity against shear rate for different concentrations of MPEG53-b-PNIPAAM113 at various temperatures. The values of m are depicted in the inset plot. At the lowest temperature (25 °C), the values of m are negative and this effect seems to become stronger as the polymer concentration is increased. Since the shear rates are below the region where turbulence is observed in Figure 6, this trend indicates that the shear flow induces the formation of aggregates, and that this effect is strengthened at higher polymer concentration. At this temperature, the stickiness of the species is low and small complexes are evolved in this shear-enhanced aggregation regime. At higher concentration, the number of collisions rises under the influence of shear rate and this facilitates the building up of interchain aggregates. It is obvious that, at the intermediate temperature (40 °C), shear-induced fragmentation of the flocs takes place at all concentrations. This effect is most accentuated at the lowest polymer concentration because, as mentioned above, the building up of flocs is the dominant process under this condition. At the highest temperature (50 °C), where the stickiness of the particles is highest, larger association complexes are formed as the polymer concentration increases, and this generates a more intensive shear-induced breakup of interaggregate chains. The increase of m with increasing polymer concentration suggests the evolution of rather weak association structures. These results clearly show that the aggregation dynamics is fundamentally different, depending on the stickiness of the aggregating species.

Figure 9. Scattering intensity profiles along the flow direction for a 0.5 wt % solution at the temperatures and shear rates indicated.

Rheo-SALS. In this type of experiment, SALS during flow is used to gain insight into structural changes of association complexes on a global dimensional scale. The SALS scattering patterns in the vorticity plane were isotropic at all the studied shear rates, and the circle at the center of each pattern is the beam stop (Figure 8). The scattering patterns have been recorded at a constant shear rate of 50 s-1 for a 0.5 wt % MPEG53-bPNIPAAM113 solution at various temperatures. It is obvious from Figure 8 that the overall scattered intensity increases with increasing temperature and the scattered patterns are isotropic for all shear rates considered. Figure 9 shows the profiles of the scattering intensities under steady-state shear flow for a 0.5 wt % solution at different shear rates and temperatures. At a high temperature (55 °C), the shift of the scattering curves toward lower q values as the shear rate rises suggests a shear-induced breakup of interaggregate chains. At the lower temperatures, the picture is more complex. For the samples submitted to shear, the shift of the scattering curve toward higher q values and the higher intensity at low q values as the shear rate rises signal the evolution of shear-induced multichain aggregates. However, the zero-shear-rate curve deviates from this trend by being shifted toward higher q values than the sample submitted to a shear rate of 50 s-1. This indicates that a shear rate of 50 s-1 breaks up the aggregates formed at zero-shear conditions, while at higher shear rates, shear-induced aggregation caused by the alignment of the polymer chains is observed.

Anomalous Transition in MPEG53-b-PNIPAAM113

J. Phys. Chem. B, Vol. 111, No. 37, 2007 10869 under the influence of shear stresses. At higher concentrations, in the entangled regime, an opposite trend with shear-induced demixing was observed. In light of the above findings, the direction at which the cloud point temperature is altered under the influence of shear stresses is governed by factors such as the stickiness of the particles and the size and strength of the formed association complexes. Conclusions

Figure 10. Temperature dependences of the scattered intensity (at a fixed q value of 0.27 µm-1) for 0.5 wt % MPEG53-b-PNIPAAM113 solutions at the shear rates indicated. The inset shows the shear rate dependence of the cloud point.

Temperature dependences of the scattered intensity (at a fixed q value of 0.27 µm-1) at various shear rates for 0.5 wt % MPEG53-b-PNIPAAM113 solutions are illustrated in Figure 10. The general trend is that the intensity exhibits a rather steep rise over a narrow temperature interval and the curve gradually levels out at elevated temperatures. It is interesting to note the impact of shear rate on the intensity at low and high temperatures. In the low-temperature range, an increase in shear rate gives rise to enhanced scattered intensity, which supports the hypothesis that shear stresses in solutions containing species of low sticking probability promote the evolution of interchain complexes. In the high-temperature stage, on the other hand, the scattered intensity drops with increasing shear rate, and this feature is consistent with the surmise that the large flocs formed at elevated temperatures are progressively broken up as the shear rate is increased. The inset plot in Figure 10 shows the effect of shear rate on the value of the cloud point temperature for 0.5 wt % polymer solutions. It is evident that CP is shifted toward a higher temperature with increasing shear rate. This result may be rationalized in the following scenario. At the cloud point for this concentration, some flocs are formed that gives rise to the turbidity observed at quiescent conditions. When shear stresses are applied, some of the flocs are disrupted and a higher temperature is required to reestablish the cloudiness matching the cloud point. The intensity of the breakup process becomes stronger as the shear rate increases, and therefore the shift of CP is more pronounced at a high shear rate. A more intricate picture concerning the influence of shear rate on the cloud point temperature has emerged from previous rheo-SALS studies on associating systems. By conducting shear viscosity, SALS, and turbidity measurements on aggregating colloidal stearyl silica particles, dispersed in benzene, a decrease of CP with rising shear rate for γ g0.3 s-1 was reported,34 whereas for very small shear rates (γ < 0.3 s-1) the value of CP was found to increase. In another investigation35 on aqueous solutions of hydrophobically modified ethyl hydroxyethyl cellulose, which exhibits LCST behavior, the cloud point was shifted toward higher values with increasing shear rate. This was ascribed to a shear-induced destruction of intersegmental clusters, which were formed via the hydrophobic side chains. In recent rheo-SALS work36 on semidilute aqueous methyl hydroxypropyl cellulose solutions (the solutions display LCST features), CP was shifted toward higher values with increasing shear rate. This effect was interpreted as being a disruption of slightly entangled clusters

The influence of shear flow on temperature-induced associations in aqueous solutions of the amphiphilic copolymer MPEG53-b-PNIPAAM113 was investigated with the aid of turbidity, shear viscosity, and rheo-SALS experiments. At quiescent conditions, the turbidity results disclose a novel transition peak at intermediate temperatures. This peak grows with increasing polymer concentration, and it originates from interplay between interpolymer aggregation, shrinking of flocs, and a possible fragmentation due to Brownian motion. At high temperatures, the sticky species form clusters and the intensity of this association process rises with increasing polymer concentration. The cloud point temperature falls off with increasing concentration, and hysteresis effects are detected. The steady-shear viscosity measurements also reveal a transition peak in the relative viscosity at intermediate temperatures, but in this case the peak is most prominent at the lowest polymer concentration, where the shear-induced interchain aggregation reigns over the breakup of interaggregate chains. The results clearly show that a high shear rate suppresses both the growth of the intermediate peak and the upturn of the relative viscosity at elevated temperatures. This is a direct confirmation that augmented shear stresses break up the flocs. Hysteresis effects are also found in the viscosity results, and this feature becomes stronger at low shear rate where large association structures are evolved at elevated temperatures. Significant shear-thinning behaviors are found under conditions where huge interchain complexes are formed, suggesting the breakup of flocs. The shear rate dependence of the relative viscosity for the polymer solutions at some chosen temperatures could be described by a power law (ηrel ∼ γ˘ -m). The analysis of the results indicates that at weak sticking conditions (low temperature) shear-induced aggregation prevails, whereas in the high sticking regimes (elevated temperatures) shear-induced disaggregation is the dominant process. The rheo-SALS data divulge isotropic scattered intensity patterns at all studied conditions of temperature and shear rate. The scattered intensity features at different shear rates demonstrate that at low temperatures shear-induced growth of association complexes is promoted, whereas at elevated temperatures shear stresses generate destruction of flocs. In agreement with previous studies on polymer solutions exhibiting LCST behavior, the cloud point is found to be shifted toward higher temperatures with increasing shear rate, a feature that is attributed to shearinduced disruption of association complexes. Acknowledgment. A.-L.K., B.N., and K.Z. gratefully acknowledge support from the Norwegian Research Council through Project No. 177665/V30. References and Notes (1) Macromolecular Complexes in Chemistry and Biology; Dubin, P., Bock, J., Davies, R. M., Schulz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994. (2) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Science 1997, 277, 1225.

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