Phase Separation in Semidilute Aqueous Poly(N-isopropylacrylamide

Article pubs.acs.org/Langmuir

Phase Separation in Semidilute Aqueous Poly(Nisopropylacrylamide) Solutions Andreas Meier-Koll,† Vitaliy Pipich,‡ Peter Busch,‡ Christine M. Papadakis,† and Peter Müller-Buschbaum*,† †

Physik-Department, Lehrstuhl für Funktionelle Materialien/Physik Weicher Materie, Technische Universität München, James-Franck-Str.1, 85748 Garching, Germany ‡ Jülich Center for Neutron Science, Forschungszentrum Jülich GmbH, Outstation at FRM II, Lichtenbergstr. 1, 85747 Garching, Germany ABSTRACT: The phase separation mechanism in semidilute aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions is investigated with small-angle neutron scattering (SANS). The nature of the phase transition is probed in static SANS measurements and with time-dependent SANS measurements after a temperature jump. The observed critical exponents of the phase transition describing the temperature dependence of the Ornstein− Zernike amplitude and correlation length are smaller than values from meanfield theory. Time-dependent SANS measurements show that the specific surface decreases with increasing time after a temperature jump above the phase transition. Thus, the formation of additional hydrogen bonds in the collapsed state is a kinetic effect: A certain fraction of water remains as bound water in the system. Moreover, H−D exchange reactions observed in PNIPAM have to be taken into account.

1. INTRODUCTION Stimuli-sensitive polymers, which undergo a reversible volume phase transition, i.e. a sol−gel phase transition, in response to external physical or chemical stimuli show great potential in biomedical and biotechnological applications.1−11 In particular, external stimuli such as change of temperature, pH, ionic strength, light, electromagnetic radiation, and addition of biomolecules attract interest. Among the stimuli-sensitive polymers, poly(N-isopropylacrylamide) (PNIPAM) has attracted attention due to its sharp and reversible transition behavior in aqueous medium at 32−34 °C.12−22 PNIPAM exhibits a well-defined lower critical solution temperature (LCST) in water, with each individual chain undergoing a very sharp coil-to-globule transition23−27 when heated above the LCST. Thus, on a microscopic level, heating the aqueous solution above the LCST changes the conformation of a flexible, linear PNIPAM chain from a swollen coil to a collapsed globule. The change in chain conformation occurs because the solvent quality decreases abruptly. Small changes in the chemical composition of PNIPAM have important consequences on the water/PNIPAM phase diagram. The introduction of hydrophilic or hydrophobic comonomers increases or decreases the LCST of PNIPAM.28 Moreover, the phase transition temperature depends not only on the level of hydrophobe incorporation and on its chemical structure but also on its position on the chain.29,30 Especially with decreasing molecular weight of the PNIPAM main chain, the effect of end groups of PNIPAM homopolymers increases.31,32 © 2012 American Chemical Society

Upon cross-linking, PNIPAM forms a gel which undergoes an analogous collapse transition in aqueous solution.33−36 In contrast to the permanent networks formed by chemical crosslinking, stimuli-sensitive hydrogels from block copolymers with hydrophobic end blocks are transient (or reversible) physical networks.37−42 Such physical networks can be reversibly transformed into the sol state by varying the environmental conditions. In order to create hydrogels with faster response times with respect to the external stimulus, the size of the responsive units can be decreased. Microgels20,21,43 and colloidal particles with a cross-linked hydrophobic core and a cross-linked hydrophilic and responsive shell44−47 mark possible routes to achieve this in bulk systems. Despite the importance of PNIPAM hydrogels with respect to applications, PNIPAM solutions have attracted strong attention again because fundamental points of the phase transition of PNIPAM are still not completely understood. Very recently, the coil-to-globule transition in PNIPAM was revisited by several groups. Because the interchain aggregation hampers the observation of the coil-to-globule transition of individual chains, these investigations focused on dilute solutions.48−51 It is well agreed on that the sharp change in polymer conformation results from a balance between hydration (direct hydrogen bonds of water and the chain) and hydrophobic aggregation of the isopropyl groups. According to Zhou et al., Received: April 16, 2012 Revised: May 18, 2012 Published: May 18, 2012 8791

dx.doi.org/10.1021/la3015332 | Langmuir 2012, 28, 8791−8798

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dependent SANS, the time dependence of the structural changes caused by a temperature jump above the phase transition temperature is investigated. These kinetic measurements elucidate the morphologies above the LCST.60 This article has the following structure: The introduction is followed by an experimental section describing the sample preparation and the experimental techniques applied. The next sections show results and discussion on the phase separation in semidilute aqueous poly(N-isopropylacrylamide) solutions as probed with SANS and a temperature jump followed by timedependent SANS.

the hysteresis observed in the coil-to-globule-to-coil transition of PNIPAM chains in water is related to the formation of additional hydrogen bonds in the collapsed state.49 Tanaka et al. improved the understanding of the high-temperature collapse and the temperature sensitivity of a PNIPAM chain on the basis of the concepts of cooperative hydration and dehydration.51 In this model, cooperativity in hydration is caused by a positive correlation between neighboring bound water molecules due to the presence of the large hydrophobic isopropyl side groups. Consecutive sequences of bound water molecules appear along the chain because one water molecule which succeeded in forming an H-bond with an amide group on the PNIPAM chain increases the probability for the second water molecule to form an H-bond. The first water molecule causes a displacement of the isopropyl group, thereby creating more accessible space for the next molecule. When temperature is increased above the LCST, each sequence is dehydrated as a whole, resulting in the sharp collapse of the chain.51 Polymer chains in dilute solution are isolated and interact with each other only seldomly. Increasing the PNIPAM concentration in solution increases the complexity due to the interpenetration of polymer molecules and the probability of forming additional, intermolecular hydrogen bonds. Balu et al. investigated PNIPAM solutions in the concentration rage of 1− 6 wt % and determined the spinodal temperature to be concentration dependent.27 Whereas dilute aqueous PNIPAM solutions show only one relaxation mode, in semidilute solutions, both a fast and a slow relaxation mode were observed by Yuan et al. by dynamic light scattering (DLS).52 At higher concentration, the presence of a single relaxation mode, the entanglement strand fluctuation mode,53 implies that the static and dynamic properties of polymers are determined just by one correlation length of concentration fluctuations. This correlation length ξ is often described as the mesh size of the system. It is predicted to be molecular weight independent and scales with the polymer concentration c as ξ ∼ c0.75.54−56 Following Yuan et al., the slow mode is related to long-range correlated concentration fluctuations, namely, the interaction among the segments in different blobs on chains.57 The divergence of the correlation length ξ in semidilute solutions approaching the phase transition was studied by Shibayama et al.58 The critical exponents of the corresponding phase transition were determined using small-angle neutron scattering (SANS). These relate to the temperature dependence of the correlation length ξ and of the forward intensity/ susceptibility, I(0). The latter was well described by the socalled Ornstein−Zernike equation54,59 in agreement with a diverging concentration fluctuation. Within the experimental error, both resulting critical exponents matched predictions of a simple mean-field approach.58 However, because of the limited number of experimental data points, the estimated exponents may not give precise values. Consequently, it is still not clear to what extent the special hydrogen interaction of PNIPAM causes deviations from the simple mean-field approach. In the present work, we revisit the phase separation in semidilute aqueous poly(N-isopropylacrylamide) solutions with respect to static and kinetic properties. In general, each phase transition belongs to a universality class with a set of unique critical exponents describing material properties in the vicinity of the phase transition. On the basis of SANS, we focus on the critical behavior approaching the LCST for one fixed PNIPAM concentration. The critical exponents are determined and are compared to mean-field theory. In addition, with time-

2. EXPERIMENTAL SECTION Sample Preparation. Poly(N-isopropylacrylamide) (PNIPAM) having a molecular weight of 25 000 g/mol and a polydispersity of 1.5 was purchased from Sigma-Aldrich. An aqueous solution with 13 wt % PNIPAM was obtained by the addition of deuterated water (D2O) (Sigma-Aldrich, purity 99.95%). After preparation, the samples were kept at room temperature for several days in order to equilibrate. At least 12 h before the measurements, the samples were transferred to the sample environment, which was set to the initial temperature of the experiment. For all SANS measurements, the PNIPAM solutions were kept in flat quartz cuvettes (Hellma Suprasil) of thickness 2 mm and width 2 cm. Small-Angle Neutron Scattering (SANS). Small-angle neutron scattering (SANS) measurements were performed using the KWS-2 instrument from JCNS at FRM II (Garching).61,62 The incident neutrons had a wavelength of λ = 0.7 nm (Δλ/λ = 20%) and were collimated over a distance of 8 m. A 6Li glass scintillation detector with an active area of 60 × 60 cm2 and 128 × 128 pixels was used. Two different sample−detector distances (SDDs) of 1.74 and 7.74 m were operated in order to cover a large range of scattering vectors (from 0.07 to 1.70 nm−1). The sample temperature was set with a thermocycle having a temperature stability of 0.01 K. For temperature-resolved measurements, the samples were equilibrated for at least 20 min at each temperature before measurement. Static SANS data were recorded in 8 min at an SDD of 1.74 m and in 12 min at an SDD of 7.74 m. For the time-dependent SANS studies, the samples were equilibrated at 20 °C before being transferred to the preheated sample environment at 50 °C. The time t = 0 of the kinetic investigation is defined by the time of the transfer. Temperatures above 33.5 °C were reached after ∼100 s. Measuring times were 15 s and download times 28 s. The kinetic investigations were carried out in the same way at both SDDs. Raw data were corrected for background, dark current, sensitivity dead time losses, and aberration effects using boron carbide, empty cell, empty beam, and plexiglass. In order to obtain absolute intensities, plexiglass was used as a secondary standard. Data reduction and radial averaging were performed using the software QtiKWS provided by JCNS. The obtained scattering cross sections were fitted with a superposition of contributions from Ornstein−Zernike and Porod laws and an incoherent background (see below).

3. THEORETICAL BACKGROUND In SANS, the elastically scattered intensity, I(q), on an absolute scale and the incoherently scattered intensity, Iinc, add up to the observed scattered intensity Iobs(q) for a given scattering vector, q: Iobs(q) = I(q) + Iinc

(1)

The elastically scattered intensity from semidilute polymer solutions is caused by concentration fluctuations and is well described by the Ornstein−Zernike function54,59 I1(q) = 8792

IOZ 1 + q 2ξ 2

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In theta solvents, these concentration fluctuations are characterized by a single correlation length, ξ. At length scales smaller than this correlation length, the system is inhomogeneous. The length scales of these inhomogeneities may be viewed as the average distance between polymer strands or as the range of spatial correlation of concentration fluctuations in the system. The Ornstein−Zernike (OZ) amplitude IOZ is related to the osmotic modulus of the system.54,59 The volume phase transition of PNIPAM is closely related to the divergence of the correlation length and the OZ amplitude. Above the phase transition, the network collapses and forms domains with interfaces between the water-rich and PNIPAMrich domains. In the SANS curves, these interfaces lead to strong forward scattering which is described by the Porod law63−65 I2(q) =

KPorod q4

(3) 2

with the Porod amplitude KPorod = 2π(Δρ) S/V, where S/V is the interface area per volume and Δρ is the excess neutron scattering length density between (protonated) PNIPAM and (deuterated) water. Δρ was calculated from the specific volumes of PNIPAM in water (0.892 mL/g)12 and D2O (0.903 mL/g) and the scattering length densities of the PNIPAM monomer (0.83 × 10−6 Å−2) and D2O (6.38 × 10−6 Å−2). Because the polymers are not cross-linked, no additional effects due to the cross-linker itself or to curvature effects need to be considered.58 Hence, we use a model Iobs(q) = I1(q) + I2(q) + Iinc

Figure 1. (a) SANS data shown in double-logarithmic representation for different temperatures: 15.6 °C (filled squares), 20.3 °C (open squares), 25.0 °C (filled diamonds), 32.5 °C (open diamonds), 32.9 °C (filled circles), 33.4 °C (open circles), 35.3 °C (filled triangles up), 36.2 °C (open triangles up), 43.7 °C (filled triangles down). Solid lines are fits to the data; see text. (b) Ornstein−Zernicke plot of the curves from (a) using the same symbols.

(4)

4. RESULTS AND DISCUSSION Critical Exponents of the Phase Transition. Temperature-resolved SANS measurements have been performed at 21 temperatures to investigate the critical exponents of the phase transition for an aqueous solution with 13 wt % PNIPAM: 13 temperatures have been selected below the LCST and 8 temperatures above with fine temperature steps around the phase transition temperature. Figure 1a shows a representative selection of SANS curves both below and above the LCST. Curves from both sample−detector distances are merged on the absolute intensity scale and give the full range of scattering vectors q. At temperatures significantly below the phase transition temperature (15.6−25.0 °C), the scattering curves are similar and show an intensity plateau (0.1−0.3 nm−1) for q < 0.4 nm−1. For larger q values, the intensity decays. In this range of temperatures, all SANS curves are well described by the sum of the Ornstein−Zernike contribution I1(q) (eq 2) and an incoherent background Iinc. No Porod contribution is needed, which shows that no internal interfaces are present. Two parameters, namely the correlation length ξ and the Ornstein−Zernike amplitude IOZ, are sufficient to fully characterize the system in the investigated q range. The fact that the Ornstein−Zernike function fits the curve shape in the decay region indicates that D2O is a theta solvent in this temperature range. As temperature is increased above 25.0 °C, the scattering becomes more intense but can still be satisfactorily described by I1(q) + Iinc. At 32.5 °C, some forward scattering becomes visible, which affects only a few data points at very low q values and thus is not included in the model fit. Only above the LCST, i.e. above 32.9 °C, an additional strong forward scattering is present at q < 0.2 nm−1 which goes beyond the Ornstein−

Zernike contribution. This strong forward scattering follows the Porod law (eq 3) with a q−4 dependence; hence, eq 4 was used for fitting. Such behavior indicates that interfaces between polymer-rich and water-rich domains have been formed, which are created due to phase separation and which are relatively sharp.65 For modeling with more sophisticated approaches which include the shape of the PNIPAM aggregates the resolution toward small q values is not high enough. SANS curves measured at higher temperatures (35.3 °C and higher) still show Porod-like forward scattering, but with a reduced intensity. This decrease in intensity, which is also observed at larger q values, cannot be understood from the static SANS data and is revisited in the time-dependent SANS measurements after a temperature jump. In Figure 1b, the SANS curves are replotted in the Ornstein− Zernike representation (1/IOZ vs q2). For this presentation, the incoherent background Iinc has been subtracted. The data below the LCST follow straight lines, reflecting the validity of our approach. The lines intersect the abscissa. The intersection point with the ordinate gives 1/IOZ and the intersection with the abscissa the quantity ξ2/IOZ. At temperatures above the LCST, deviations from the linear behavior occur at higher q values, which are due to domain formation. The values of the correlation length ξ and the Ornstein− Zernike amplitude IOZ obtained from the fits to the SANS curves are shown in Figure 2. One clearly observes a singularity for both quantities at the phase transition temperature TS = 8793


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Figure 2. (a) Ornstein−Zernike amplitude and (b) correlation length plotted versus temperature. Solid lines are power law fits to the data; see text. The dashed vertical lines mark the phase separation temperature, TS. Filled and open symbols mark data for temperatures below and above the LCST, respectively.

Figure 3. Double-logarithmic representation of the temperaturedependent (a) Ornstein−Zernike amplitude and (b) correlation length below TS.

33.1 ± 0.1 °C which is marked with dashed lines in Figure 2. TS corresponds to the spinodal temperature of PNIPAM and thus to the LCST, as spinodal temperature and LCST deviate only at low PNIPAM concentrations. The determined value of TS matches to the trend of increasing spinodal temperatures observed for an increase in PNIPAM concentration of the aqueous solution (TS = 31.5 °C at 0.98 vol % and TS = 32.3 °C at 6.14 vol %) as reported by Balu et al.27 Below TS, the data are successfully described by power laws: ξ ∝ |TS − T |−ν

tional to the Ginzburg number, determined by the Ginzburg criterion.67 Typically, in high molecular weight systems, the Ginzburg number is significantly reduced; thus, mean-field behavior is observed unless very close to the critical point.54 As a consequence, it is generally accepted that polymer solutions and blends are well described by the Flory−Huggins mean-field theory.54 In reality, however, the renormalized ranges appear to be significant larger.68 In polymer blends, the larger 3D Ising critical range has been attributed to the effect of compressibility.69 Thus, polymer solutions behave more mean-field-like than polymer blends. In case of poly(acrylic acid), in a binary mixture of water and 2,6-lutidine, To and Choi observed a scaling of the correlation length with ν = 0.44 ± 0.03 when approaching the critical temperature.70 In contrast the pure 2,6-lutidine/water solution without poly(acrylic acid) showed a critical exponent of ν = 0.6. Thus, in case of PNIPAM solutions, the tendency to form hydrogen bonds plays an important role in the critical behavior. Following classical semidilute solution theory, the motions of segments between different blobs are screened and therefore not correlated. Such an assumption may break down when there is interaction like hydrogen bonding in water or the strong attraction between polymers in poor solvents. Longrange concentration fluctuations will result, which may be at the origin of the slow dynamics observed in semidilute PNIPAM solutions.52 Thus, the observed deviation of the critical exponents from the standard mean-field behavior can be attributed to a deviation from the simple Flory−Huggins meanfield theory approach for PNIPAM solutions.

(5)

and

IOZ ∝ |TS − T |−γ

(6)

The solid lines in Figure 2 are fits of eqs 5 and 6. The power law behavior is demonstrated in Figure 3. In contrast to the work of Shibayama et al.,58 we use 13 values instead of 6 values (and they cover a wider temperature range) for the determination of the critical exponents, which increases the accuracy of the extracted values. Critical exponents ν = 0.44 ± 0.01 and γ = 0.81 ± 0.01 are obtained. Considering the uncertainties, both values (ν and γ) are smaller than expected from simple mean-field theory (ν = 0.5 and γ = 1.0).66,67 Composition fluctuations in homogeneous polymer solutions and blends are generally described by the universality classes of mean-field approximation or three-dimensional Ising behavior, if the system is very far and very near the critical point of phase decomposition.66,67 The characteristic crossover temperature which separates mean-field and 3D Ising behavior is propor8794


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Behavior above LCST. Above the LCST, the solution is phase separated. Whereas the SANS curves below the LCST do not exhibit a Porod contribution, above, the Porod contribution has to be included into the model fit (Figure 1a). In this way, we account for the interfaces caused by phase separation into water-rich and polymer-rich domains. The specific surface S/V obtained from the Porod term is shown in Figure 4. It

between protonated (PNIPAM) and deuterated (D2O) components is required to give a SANS contribution. From the fact that the Ornstein−Zernike amplitude is positive above TS, we conclude that even the collapsed PNIPAM chains include D2O molecules. To get a deeper insight into the structures observed with SANS above the LCST, time-dependent SANS measurements have been performed for an aqueous solution with 13 wt % PNIPAM (see Figure 5a). After a fast temperature jump from

Figure 4. Static results of the specific surface S/V for temperatures below (filled symbols) and above (open symbols) the LCST, which is marked by a dashed line.

characterizes the phase-separated structure. For T < TS, the specific surface S/V is zero as expected for a homogeneous solution, but above TS, one observes a sudden increase of S/V. Thus, at the phase transition, internal interfaces are created due to the formation of water-rich and polymer-rich domains. For the temperature right above the spinodal temperature TS (33.4 °C), a value of S/V = 39 mm−1 is determined, assuming pure PNIPAM and D2O domains. Thus, no mesoglobules of hundred nanometer size as reported by Balu et al.27 were found. The presence of larger aggregates might result from a higher concentration and a higher mobility of the PNIPAM (due to lower molecular weight of PNIPAM) used in the present investigation as compared to the work of Balu et al.27 For temperatures above TS, the specific surface of the PNIPAM domains in water decreases rapidly, following S/V ∝ (T − TS)−α with α = 0.67 ± 0.15. The same holds for the parameters of the Ornstein−Zernike contribution (ξ and IOZ), as seen in Figure 2. In a simplified view, the decrease of S/V matches the expectation for a phase-separated structure with domains which grow with increasing temperature. The decrease in correlation length ξ and Ornstein−Zernike amplitude means that the mesh size in the PNIPAM-rich domains decreases and that the osmotic modulus increases; i.e., the PNIPAM-rich domains become more compact. An increase of the domain size with temperature was also observed in diblock copolymer solutions of P(S-b-NIPAM) above the LCST of PNIPAM.39 However, the scaling of the specific surface of the PNIPAM-rich domains observed here (S/ V ∝ (T − TS)−α with α = 0.67 ± 0.15) misses a thermodynamic explanation. Moreover, there is no straightforward explanation for the temperature dependence of the correlation length above TS (Figure 2b) which exhibits a maximum value at TS. The overall decrease of the correlation length above LCST may be due to the collapse of the PNIPAM chains, i.e., a decrease of the mesh size. One needs to keep in mind that contrast

Figure 5. (a) Selected SANS curves as a function of time after a temperature jump from 20 to 50 °C in double-logarithmic representation: 60 s (filled squares); 100 s (open squares); 230 s (filled diamonds); 780 s (open diamonds); 2390 s (filled circles); 5160 s (open circles). Solid lines are fits to the data; see text. (b) Resulting temporal evolution of the specific surface S/V. The dashed line at 600 s separates the two regimes observed.

the one-phase regime at 20 °C into the two-phase region at 50 °C, we have followed the time-dependent changes. The SANS curves are again modeled with Ornstein−Zernike and Porod contributions (eq 4 and Figure 5a), and the resulting specific surface S/V is plotted as a function of time in Figure 5b. Because of the domain formation, the Porod contribution dominates, and the Ornstein−Zernike contribution is negligible except for the very first curves (60 and 100 s). After TS is crossed (i.e., after ∼100 s), the specific surface decreases with time, and the temporal evolution of the two-phase morphology comprises two stages. Both coarsening processes follow a power law behavior S/V ∝ t‑β, however, with different exponents β: Up to 600 s (marked by a vertical dashed line in Figure 5b), β = 1.03 ± 0.02, whereas after 600 s, the exponent is higher: β = 1.87 ± 0.02. Both values can neither be explained by Ostwald ripening (β = 1/3)71 nor by a diffusioncontrolled aggregation process (β = 0.57).72 Moreover, 8795


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Notes

reaction-limited aggregation, which would result in an exponential growth of the aggregate radii, can be ruled out as well.73,74 Possibly, several mechanisms overlap. We conclude that aggregates are formed very rapidly (within 1 min after the phase transition temperature is crossed) and that they keep growing during the duration of our measurement (∼5000 s). These kinetic measurements show that the structures above TS are kinetically controlled and depend on the details of the quench depth and the time after having passed the phase transition. Therefore, the structures observed above the LCST at different temperatures as plotted in Figures 2 and 4 are not in thermodynamic equilibrium even though equilibration times of 20 min were applied after each temperature change. The decrease of the overall intensity in the static SANS measurements at temperatures above TS may be due to sedimentation of the polymer-rich domains which are very large after having been kept for long times above the LCST. We conclude that below TS the semidilute polymer solution is macroscopically homogeneous with concentration fluctuations on the nanometer length scale, whereas above, it phaseseparates into polymer-rich and water-rich domains.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Golosova and J. Adelsberger for help during the SANS experiment. Financial support by Deutsche Forschungsgemeinschaft (DFG) in the priority program SPP 1259 (MU1487/8 and PA771/4) is gratefully acknowledged.

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5. SUMMARY We revisit the phase separation mechanism in semidilute aqueous PNIPAM solutions with respect to static properties. The nature of the phase transition is probed with static and time-dependent measurements. A series of SANS measurements at different temperatures in the vicinity of the phase transition give an improved database as compared to existing work by Shibayama et al.58 For the concentration fluctuations below the LCST, our data are close to a mean-field behavior, but the critical exponents are slightly smaller than those predicted by mean-field theory. We attribute this behavior to the nature of the hydrogen bonds involved in the phase separation of PNIPAM and water. In time-dependent SANS measurements, we probe the behavior of the system following a temperature jump from a start temperature below the LCST to a target temperature far above. PNIPAM-rich aggregates form rapidly. Their specific surface decreases with increasing time after the temperature jump and follows power laws. An early and a late stage are discriminated. Thus, the formation of additional hydrogen bonds in the collapsed state, which is assumed to be at the origin of hysteresis effects observed in the coil-to-globule-tocoil transition of PNIPAM chains in water46 and the subsequent squeezing out of water is a kinetic effect. From the presence of contrast in the SANS experiment, we can conclude that not all water is repelled, and some water remains as bound water to the system. H−D exchange reactions observed in PNIPAM75 have to be taken into account. However, the aggregate growth is less complex than the one encountered in solutions of triblock copolymers having a comparable PNIPAM middle block and two short polystyrene end blocks.60 Complementary SAXS experiments appear interesting and may allow for a separation of the different aggregation regimes. However, the unfavorable contrast conditions for aqueous PNIPAM solutions make SAXS experiments challenging.

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*Phone +49 89 289 12451; Fax +49 89 28912473; e-mail [email protected]. 8796


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