Supramolecular assembly of a thermoresponsive ...

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Cite this: Phys. Chem. Chem. Phys., 2017, 19, 1504

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Supramolecular assembly of a thermoresponsive steroidal surfactant with an oppositely charged thermoresponsive block copolymer† M. C. di Gregorio,a M. Gubitosi,a L. Travaglini,a N. V. Pavel,a A. Jover,b F. Meijide,b d a a J. Va´zquez Tato,b S. Sennato,c K. Schille ń, F. Tranchini, S. De Santis, a G. Masci and L. Galantini* Supramolecular rearrangements are crucial in determining the response of stimuli sensitive soft matter systems such as those formed by mixtures of oppositely charged amphiphiles. Here mixtures of this kind were prepared by mixing the cationic block copolymer pAMPTMA30-b-pNIPAAM120 and an anionic surfactant obtained by the modification of the bile salt sodium cholate. As pure components, the two compounds presented a thermoresponsive self-assembly at around 30–35 1C; a micelle formation in the case of the copolymer and a transition from fibers to tubes in the case of the bile salt derivative. When both were present in the same solution they associated into mixed aggregates that showed complex thermoresponsive features. At room temperature, the core of the aggregate was comprised of a supramolecular twisted ribbon of the bile salt derivative. The block copolymers were anchored on the surface of this ribbon

Received 15th August 2016, Accepted 24th November 2016 DOI: 10.1039/c6cp05665b

through electrostatic interactions between their charged blocks and the oppositely charged heads of the bile salt molecules. The whole structure was stabilized by a corona of the uncharged blocks that protruded into the surrounding solvent. By increasing the temperature to 30–34 1C the mixed aggregates transformed into rods with smooth edges that associated into bundles and clusters, which in turn induced clouding of the solution. Circular dichroism allowed us to follow progressive rearrangements of the supramolecular

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organization within the complex, occurring in the range of temperature of 20–70 1C.

1. Introduction Mixtures of oppositely charged amphiphilic molecules are among the most used for application-oriented goals in many fields.1–4 a

Department of Chemistry, ‘‘Sapienza’’ University of Rome, P.le A. Moro 5, 00185 Rome, Italy. E-mail: [email protected] b Departamento de Quı´mica Fı´sica, Facultad de Ciencias, Universidad de Santiago de Compostela, Avda. Alfonso X El Sabio s/n, 27002 Lugo, Spain c Department of Physics and CNR-IPCF UOS Roma, ‘‘Sapienza’’ University of Rome, P.le A. Moro 5, 00185 Rome, Italy d Division of Physical Chemistry, Department of Chemistry, Lund University, SE-221 00 Lund, Sweden † Electronic supplementary information (ESI) available: Pictures of Na-tbuPhC samples at temperature below and above CTT; intensity-weighted distributions of apparent hydrodynamic diameters, UV and CD spectra of pNIPAAM120-bpAMPTMA30 solutions as a function of temperature; pictures of the mixture sample at x = 0.5 at temperatures below and above CP; average scattered light intensity and average reduced relaxation time obtained from DLS measurements on the catanionic mixtures; CD spectra of mixtures at x = 0.33 and 0.66 as a function of temperature and CD evolution description; CD spectra of pNIPAAM50/ Na-tbuPhC and pNIPAAM96-b-pAMPS36/Na-tbuPhC mixtures as a function of temperature; length distribution of the mixed aggregates inferred from TEM image analysis; SAXS curves of a mixture with x = 0.50 as a function of temperature and the IFT fit of the curve at 20 1C. See DOI: 10.1039/c6cp05665b

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These systems show peculiar phase behavior and properties in water, mainly due to the electrostatic interactions between the oppositely charged parts.4 By controlling the molar ratio of the anionic and cationic components, the morphology of the mixed structures that form can be varied5,6 along with the superficial charge,7 giving rise to structures suitable for interaction, storage and carriage of charged molecules.1,8–10 In the last few decades, a great deal of interest has been focused on mixtures of polyelectrolytes and oppositely charged amphiphiles, due to their importance in fundamental polymer physics/biophysics research as well as in biological and industrial applications.11–15 Polyelectrolytes and oppositely charged surfactants often form polyion–surfactant complexes where both electrostatic interactions between the charged components and hydrophobic interactions between the polymer backbone and the alkyl chains are involved. The association is entropically favorable due to the gain in entropy of mixing as the counterions are released into the solution in a uniform distribution. The reinforced intermolecular interaction can then result in the formation of highly ordered structures under concentrated16,17 or dilute conditions, where the structures sometimes are stabilized by a third nonionic component.18–20

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During the past few years, a series of studies have reported on core–shell nanoparticles (named block ionomer complexes,21,22 polyion complex micelles,23,24 or complex coacervate core micelles25–28) formed by the electrostatically driven self-assembly of charged–neutral block copolymers with oppositely charged surfactants in water.29 Sometimes nanoparticles with a liquid crystalline internal structure were obtained, which attracted a large scientific interest. The internal order provides a large surface area, which enables the particles to solubilize quite high concentrations of hydrophobic molecules, proteins, or peptides. This ability makes them useful in a wide range of practical applications, for instance in the field of medicine.30,31 Nanoparticles that show temperature-induced transitions of their internal crystalline structure have also been reported.19,20 Thermoresponsiveness is a desirable feature in the selfassembly of soft matter because temperature can be generally easily controlled in applicative environments and also varied in a wide range without altering the integrity of the surrounding systems. It is often pursued to obtain smart aggregates for inclusion, carrying and release of active molecules. Block copolymers with hydrophilic and hydrophobic moieties can self-assemble in aqueous solution forming micelles or vesicles with a hydrophobic core and a hydrophilic shell (corona). By using blocks with a temperature-sensitive solubility these copolymers can provide systems with a versatile thermoresponsive aggregation that have received particular attention in recent years.32–40 Starting from conditions where the blocks are soluble in water, a variation of temperature (often an increase) can trigger a self-association as a fraction of the blocks becomes water-insoluble. The positively charged block copolymer pNIPAAM120-bpAMPTMA30 formed by 120 units of N-isopropylacrylamide (NIPAAM) and 30 units of (3-acrylamidopropyl)-trimethylammonium chloride (AMPTMA) represents an example of a thermoresponsive double-hydrophilic diblock copolymer (Fig. 1). It is water-soluble at room temperature and self-assembles into micelles with a pNIPAAM core at temperature above 32–34 1C,41,42 which corresponds to the lower critical solution temperature (LCST), above which pNIPAAM is insoluble.41,43–45 Recently an interesting thermo sensitivity has also been observed in the aggregates of steroidal surfactants derived by bile salts46,47 such as [3b,5b,7a,12a]-3-(4-t-butylbenzoilamine)7,12-dihydroxycholan-24-oic acid sodium salt (Na-tbutPhC) obtained from sodium cholate by substituting with a tert-butylphenyl amide group an original OH (Fig. 1).48,49 In water, this compound forms gels or viscous solutions at room temperature that turn into a dispersion of tubules at a critical temperature of 30–36 1C.48 In this work mixtures of Na-tbutPhC and pNIPAAM120-bpAMPTMA30 in water were prepared and studied in order to obtain oppositely charged block copolymer-surfactant mixed aggregates with emphasized thermosensitive properties. The pure components and Na-tbutPhC/pNIPAAM120-b-pAMPTMA30 mixed solutions at different charge molar fractions were investigated as a function of temperature. The supramolecular structures and their morphological variation have been analyzed by means of UV absorption, circular dichroism (CD), dynamic light


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Fig. 1 Molecular structures of the bile salt derivative [3b,5b,7a,12a]-3(4-t-butylbenzoilamine)-7,12-dihydroxycholan-24-oic acid sodium salt (Na-tbutPhC) and the block copolymer pNIPAAM120-b-pAMPTMA30.

scattering (DLS), small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), cryogenic scanning electron microscopy (cryo-SEM), atomic force microscopy (AFM), differential scanning calorimetry (DSC) and electrophoretic mobility measurements.

2. Materials and methods 2.1

Materials

Na-tbuPhC was obtained by reacting p-tert-butylbenzoyl chloride with the 3b-amino derivative of cholic acid.46 The pNIPAAM120-bpAMPTMA30 block copolymer (Mn,GPC = 20 kDa, Mw/Mn = 1.21) was prepared by Atom Transfer Radical Polymerization (ATRP) as previously reported.41,50 All the investigated samples were prepared in carbonate/bicarbonate buffer pH = 10.0 containing an equimolar concentration (1.5 10 2 M) of sodium carbonate and bicarbonate from Carlo Erba Reagents. The mixture composition is reported as a negative charge molar fraction, defined as x = n /ntot, where n and ntot are the number of moles of the bile salt derivative and the number of moles of the total charge (charge of the bile salt derivative + charge of the block copolymer), respectively. The concentration of charges for the block copolymer was calculated as n+ = 30m/(MwV), where m is the grams of the block copolymer, Mw its molecular weight and V the volume of the solution. Solutions of the pure components (x = 0 and 1) and mixtures were investigated at a total charge molar concentration in the range of 1.0–5.0 10 3 M. The mixtures were prepared by mixing stock solutions of the pure components. Prior to the DLS measurements the stock solutions and the pure samples were filtered at room temperature through polycarbonate membranes with a pore size of 200 nm to eliminate dust. 2.2

UV and CD measurements

CD spectra were recorded on a JASCO model 715 and reported in molar ellipticity [y] and millidegrees, mdeg, for pure Na-tbutPhC

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and mixture samples, respectively. The UV spectra were recorded on a Cary/1E and reported in molar extinction coefficient e and absorbance abs for pure molecules and mixture solutions, respectively. The spectra were recorded in the range of wavelength (l) 190–350 nm and the samples were equilibrated for at least 20 min at a measuring temperature within 0.1 1C using a Peltier device before the spectrum collection. No significant changes in the CD profiles were observed after rotation of the samples under all the investigated conditions. 2.3

DLS measurements

Measurements were carried out in a Brookhaven instrument using a BI2030AT digital correlator with 136 channels and a BI200SM goniometer. The light source was a He–Ne laser operating at l = 632.8 nm. The temperature was controlled within 0.5 1C by a circulating water bath. In the DLS measurements, the temporal fluctuations of the scattered intensity were analyzed at specific scattering vectors q = (4pn sin y)/l, where n is the refractive index of the solvent and y is half of the scattering angle. The normalized temporal autocorrelation function of the scattered intensity was first estimated, and it allowed for the determination of the normalized field autocorrelation function g1(q,t) through the Siegert relation.51 The CONTIN analysis of g1(q,t) was performed to obtain the distribution of the relaxation times (G(t)). An average value of the relaxation time tav was estimated by a cumulant analysis of g1(q,t).52 When strictly related to the translational diffusion motion of the particles (i.e., when 1/tav was a linear function of q2), the apparent hydrodynamic diameters were directly reported as a result of the DLS measurements. 2.4

TEM measurements

TEM images were obtained using a JEOL JEM-1011, operating at 80 kV, equipped with a Mega View III camera. Prior to the measurements, the samples were equilibrated for one hour at the desired temperature and then a drop of the solution was deposited and dried onto carbon-coated copper grids. 2.5

Cryo-SEM

Cryo-SEM photographs were recorded using a JEOL JSM-6360 LV-GATAN ALTO 2100, operating at 30 kV. Samples were equilibrated for one hour at the desired temperature. Therefore, a drop of the solution was deposited and frozen with liquid nitrogen. The frozen drop was cut and then freeze-dried. 2.6

AFM measurements

AFM experiments were performed using a Dimension Icon (Bruker AXS) in the Scan Asystt mode in air. This imaging mode allows for the application of lower forces than the standard Tapping mode by employing an algorithm operating in Peak Force Tappingt, which uses a precisely controlled force at each pixel and optimizes the feedback signal. This permits us to apply lower imaging forces than those obtained with the standard tapping mode thus protecting soft samples from damage without compromising the image resolution. We used an ultra-sharp silicon tip (a nominal radius of curvature 2 nm) mounted on a triangular silicon nitride cantilever, with a very


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low spring constant (0.4 N m 1) and a nominal resonant frequency of 70 kHz, which allowed for the required high level force control on soft samples in air. The sample for the AFM measurements was prepared by depositing aliquots of 10–20 mL of each sample, previously equilibrated at an appropriate temperature, onto the freshly cleaved mica surface and after 5 minutes were gently washed with 200 mL of Milli-Q water. The mica surface with the adsorbed sample was then flushed with a stream of nitrogen for drying and analyzed after 30 min. Images were analyzed using the NanoScope Analysis software v1.40 (Brucker) and presented as raw data, except for Flattening. 2.7

SAXS measurements

SAXS measurements were performed at the MAX II SAXS beamline I911-4 at MAX IV Laboratory in Lund, Sweden.53 Calibration measurements were carried out using a LaB6 sample. The solutions were injected into thermostated quartz capillary sample holders and equilibrated for at least 20 min before measurement. The scattered intensity was recorded at l = 0.91 Å on a 165 mm diameter MarCCD detector. The two-dimensional (2D) SAXS patterns were processed using the Fit2D software.54 Scattering curves were recorded within the range of 0.1 o q o 4.0 nm 1, and were corrected for solvent and capillary contributions. The Indirect Fourier Transform method,55 developed in the ATSAS program,56,57 was used for interpreting the curves of the block copolymer micelle and mixed aggregate solutions. With this method, the pair distribution functions of the single scattering particle p(r) or of the particle cross section pc(r) (in the case of rod-like particles) are extracted by indirect Fourier transform of the scattered intensity I(q) or of the qI(q) profile. The pair distribution functions p(r) go to zero at distances greater than the maximum size of the particle Dm and permit the determination of its electronic gyration radius Rg. Similarly, the maximum size Dmc and the gyration radius Rc of the cross section can be estimated from the pair distribution function pc(r) of rods.58 2.8

Electrophoretic mobility measurements

Electrophoretic mobility measurements on mixtures at total charge molar concentrations of 1.0 and 2.0 10 3 M were performed using a Malvern NanoZetasizer apparatus equipped with a 4 mW HeNe laser source (632.8 nm), at 25 1C. The sample was placed in a disposable folded capillary cell, DTS1070 (Malvern Instruments Ltd, Worcestershire, UK), and was equilibrated for 15 minutes before the measurements. 2.9

DSC measurements

High sensitivity DSC measurements were performed on solutions of the pure components and the mixtures at x = 0.5 by using a VP-DSC high-sensitivity differential calorimeter (GE Healthcare Life Sciences, Northhampton, MA). The volume of both the sample and reference cells was 0.5065 mL. For all samples, two consecutive up-scans were carried out in the 5–80 1C temperature range with a scanning rate of 1 1C min 1. The cold solution was injected into the sample cell to be equilibrated at 5 1C for


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20 minutes before the scan was started. After the first scan, the temperature was decreased and the sample was again left to equilibrate at 5 1C for at least 5 hours before the second scan was performed. For all of the samples, the result of the second scan was identical to that of the first, hence only first scans were presented in this work.


3. Results and discussion 3.1

Pure Na-tbutPhC

The Na-tbutPhC samples were viscous at room temperatures while they became non-viscous and turbid upon heating (Fig. S1, ESI†). The DLS data of a pure Na-tbutPhC solution are reported in Fig. 2 in terms of average scattered intensity (I) and average reduced relaxation time defined as tavT/Z, where T is the temperature and Z the solvent viscosity (here approximated to that of water). Both I and tavT/Z were essentially constant at low temperatures and they drastically increased by increasing the temperature above a critical threshold, suggesting an abrupt structural variation of the aggregates. Data previously reported show that interconnected fibrils form a viscous solution at a low temperature and that tubular aggregates are formed via a self-assembly of fibrils around the transition temperature.48 Static light scattering measurements reported in the previous paper48 showed a typical intensity vs. q profile confirming the tubular structure of the aggregates.

Fig. 2 Average scattered light intensity (I) at a 901 scattering angle (a) and average reduced relaxation time (tavT/Z) obtained from DLS measurements (b) on a Na-tbutPhC 1.0 10 3 M sample as a function of temperature.


Fig. 3 CryoSEM (a) and TEM (b) micrographs of 1.0 10 3 M Na-tbutPhC samples. Samples were kept at 32 1C for one hour before the measurements. As the transition was not yet complete, both tubules (white arrows) and few residual fibers (black arrows) were visible.

Here, the transition temperature was identified as a critical tubular temperature (CTT) for which a value of 30 1C was estimated at the onset of intensity and reduced relaxation time increase shown in Fig. 2. TEM and cryo-SEM images collected around CTT confirmed this transition (Fig. 3). The average values of 0.45 mm for the cross section diameter and 3 mm for the length were measured in the TEM images. After the first increase, further heating resulted in a monotonic decrease of the average scattered intensity and reduced relaxation time (Fig. 2). This could be ascribed to a progressive breaking of the tubules into micellar aggregates. It is known that for optically isotropic rods with length L the normalized electric field correlation function may be expanded to a weighted sum of two or more exponential decays,59,60 where the first term is the purely translational part of the correlation function and the other exponential terms are related to the coupled rotational and translational diffusion. This equation was successfully used to reproduce DLS relaxation time distribution of rodlike micelles of the block copolymer by assuming the expressions of Broersma for the diffusion coefficients.59,61,62 However, it failed to interpret the DLS relaxation time distribution measured on tubule dispersions of Na-tbutPhC as several of the conditions implied in the expansion, such as length and thickness limits at the measurement q values, were not fulfilled


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Fig. 4 UV spectra (a) at 20 (dotted line) and 40 1C (full line) and CD spectra (b and c) at 20 (black), 25 (red), 30 (green), 32 (blue), 34 (orange), 36 (pink), 38 (brown), 40 (cyan) and 50 1C (grey) of a 1.0 10 3 M Na-tbutPhC solution. CD curves are reported in different panels (b) and (c) for clarity.

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(Fig. 4c). In parallel only a slight lowering of the UV bands took place (Fig. 4a). The CD spectral variations indicated that the aggregates underwent an abrupt change in the chromophore packing at the CTT. Above CTT, a further increase in temperature dictated a lowering of the CD bands until they disappeared at around 50 1C. This effect took place in parallel with the progressive lowering of the scattered light intensity and the decrease in the relaxation time (Fig. 2). This suggested that the gradual breakage of the tubules gave rise to small micelles in which no significant interaction and/or preferential chiral arrangement of the chromophores occurred. From the DLS data in Fig. 2 and the CD spectra we concluded that the highest temperature at which tubules still existed was 50 1C. This was identified as the upper critical tubular temperature (UCTT). Bile salts and their derivatives, such as the one reported in this work, have an amphiphilic structure that is quite different from that of the conventional head–tail surfactants. In fact, besides the negatively charged head, bile salts exhibit a rigid steroidal structure with a variable number of hydroxyl groups in specific positions. Because of this structure the aggregate morphologies of these compounds cannot be explained on the basis of the conventional geometric rules of surfactant packing and, even in the simple case of natural bile salts, the definition of aggregate models is still an open question.46 The selfassembly of the reported derivative was related to a delicate balance between hydrophobic forces involving mainly the nonpolar moieties (t-butyl phenyl groups and steroid hydrophobic regions) and hydrogen bonds involving the amide and the hydroxyl groups. By increasing the temperature, a progressive breaking of the intermolecular hydrogen bonds and an increase of the hydrophobic interactions were expected to be crucial for the transitions. The resulting aggregates were quite complex and the data collected so far did not allow us to outline unique models for their arrangements and transition at a molecular level. 3.2

under our experimental conditions (Fig. S2, ESI† and the corresponding description). The reformulated expressions of Broersma’s relations by Tirado et al.63 were also explored, without any significant improvement. The CD spectra of the 1.0 10 3 M Na-tbutPhC sample collected at room temperatures presented a bisignate conservative Cotton effect at 210 nm along with a less intense positive band at 250 nm; a typical spectrum is shown in Fig. 4b. The CD signals were related to the reciprocal orientation of the electric dipole moments of p - p* transitions of the phenylamide residues that generated two absorption bands in the UV spectra in the same wavelength range as the CD measurements (Fig. 4a). By increasing the temperature, the overall intensity of the CD spectrum slightly decreased while the shape was preserved. However, when the CTT was overtaken, at about 32 1C, drastic variations in the CD profile occurred: (i) the band at 250 nm increased its intensity, (ii) the bisignate Cotton effect turned into a less intense positive band and two negative peaks


Pure pNIPAAM120-b-pAMPTMA30

The thermoresponsive self-assembly of pNIPAAM120-b-pAMPTMA30 in buffer was highlighted by a typical scattered light intensity pattern (Fig. 5). At room temperature, the low intensity demonstrated that the block copolymer was present as single chains (unimers) in solution. By increasing the temperature a steep growth of the intensity was observed because of the formation of micelles starting from a critical micellar temperature (CMT) of 34 1C.64 A further increase in temperature resulted in a plateau at around 50 1C. At room temperature the distributions of hydrodynamic diameters inferred by DLS showed a bimodal behavior where the peak corresponding to the smallest value (around 10 nm) was ascribed to the unimers of pNIPAAM120-b-pAMPTMA30 (Fig. 5 inset). A bimodal size distribution has previously been reported for the same type of diblock copolymer with shorter blocks (pNIPAAM24-b-pAMPTMA9).33 In that work, it was suggested that the second peak was related to either multi-chain aggregates that form due to the amphiphilic properties of pNIPAAM or to the


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Fig. 5 Average scattered light intensity (I) as a function of temperature at a scattering angle of 901 of a 1.0 wt% pNIPAAM120-b-pAMPTMA30 solution. Intensity-weighted distributions of apparent hydrodynamic diameters obtained from CONTIN analysis of DLS data at 25 (a) and 54 1C (b) are reported in the inset.

‘‘slow mode’’, an effect commonly observed in polyelectrolyte solutions. Upon increasing the temperature to 35–40 1C, the large radii related peak disappeared and the distribution became monomodal with a peak positioned at about 40 nm (Fig. 5 inset). This was attributed to the micelles of pNIPAAM120-b-pAMPTMA30. As reported in the literature, the temperature responsiveness of the pNIPAAM120-b-pAMPTMA30 self-assembly is dictated by a change in the hydrophobicity of the pNIPAAM block. Below the CMT, the pNIPAAM block is hydrophilic, thus allowing a free dispersion of the single polymer chains in solution. When the CMT (i.e., 34 1C) was reached, pNIPAAM lost its affinity for aqueous medium and the block copolymer self-assembled into micelles with a pNIPAAM core. A further heating to higher temperatures led to a progressive dehydration of the micelles, which was reflected in a slow decrease of the hydrodynamic radii. SAXS curves of the pure pNIPAAM120-b-pAMPTMA30 solution at 20 and 60 1C were also reported and analyzed to infer information on the structure of the unimers and the micelles (Fig. S3, ESI† and the corresponding description). The polymer presented a single UV band at around 200 nm that was not affected by the micelle formation above 34 1C. No CD signals were observed for the polymer solutions at any of the studied temperatures (Fig. S4, ESI†). 3.3 Oppositely charged block copolymer/bile salt derivative mixtures At room temperature, the mixtures of pNIPAAM120-b-pAMPTMA30 and Na-tbutPhC were stable, slightly turbid dispersions. A temperature increase led to clouding and over time to flocculation (Fig. S5, ESI†). 3.3.1 UV-vis spectroscopy and circular dichroism. At room temperature, the UV spectra of the mixtures were very similar to that of pure Na-tbutPhC solution except for a higher intensity of the band at low wavelengths due to the contribution of the polymer (Fig. 4a and 6). By increasing the temperature,


Fig. 6 UV spectra of pNIPAAM120-b-pAMPTMA30/Na-tbutPhC mixtures with a total charge concentration of 2.0 10 3 M and x = 0.66 (a), 0.50 (b) and 0.33 (c) at temperatures of 20 (black line), 30 (red line), 32 (green line), 34 (blue line), 40 (pink line), 50 (cyan line), and 70 1C (orange line). Turbidity (d) expressed as absorbance at 300 nm as a function of temperature of mixtures at x = 0.66 (black circles), 0.50 (red circles) and 0.33 (blue circles).

a complex evolution of the curves occurred implying a variation of the absorption and the appearance of a scattering background (Fig. 6). This transformation was particularly significant for the mixture with x = 0.50, where a lowering of the main band and the appearance of a tail at high wavelengths due to scattering were observed beyond a critical temperature (32–34 1C) (Fig. 6b). The smooth profile at the highest temperature was very different compared to that of the pure components where none (polymer) or very slight (surfactant) UV variations were detected with temperature. Based on the UV profiles, the turbidities (expressed as absorbance on the tail of the UV curve at 300 nm) for the different mixed solutions were calculated. They are displayed as a function of temperature in Fig. 6d. The different mixtures showed a steep increase of the turbidity at


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Fig. 7 CD spectra of pNIPAAM120-b-pAMPTMA30/Na-tbutPhC mixtures with a total charge concentration of 2.0 10 3 M and x = 0.50 at 20 (full black), 30 (full red), 32 (full yellow), 34 (full blue), 36 (full pink), 40 (full cyan), 46 (full green), 52 (full brown), 56 (full orange), 60 (dashed green), 64 (dashed red) and 70 1C (dashed black). Curves are reported in different panels (a), (b) and (c) for clarity. Arrows indicate the evolution induced by increasing temperature.

temperatures in the range 30–34 1C, which we indicated as cloud points (CPs) of the mixtures and observed to be slightly dependent on the mixture composition. At the cloud points there were also abrupt changes in the average scattered light intensity and reduced relaxation time obtained by DLS. Irregular patterns were observed in this case above the CP of the mixtures due to flocculation effects (Fig. S6, ESI†). A complex variation of the CD profiles occurred in parallel with the changes in the UV spectra. For the mixture at x = 0.50 three stages were recognized in the transformation of the CD profiles (Fig. 7). In a first stage, up to 32 1C, the CD curves were constant upon a temperature increase, presenting three minima at 195, 207 and 260 nm, and two positive bands at 240 and 200 nm (Fig. 7a). Reaching 32–34 1C (i.e. the cloud point according to the turbidity measurements) an abrupt profile change occurred.


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The change was particularly remarkable in the wavelength range 220–290 nm, where the signal was roughly inverted due to the formation of a negative and a positive band at 230 and 260 nm, respectively. A temperature increase from 34 to 56 1C (defined as the second stage, Fig. 7b) induced a gradual change in the profile. A blue shift of the positive and negative bands at 260 and 230 nm was observed along with a decrease (at 260 nm) and an increase (at 230 nm) of their absolute intensity values. It also involved the signal at 207 nm that showed a pronounced lowering in the intensity (Fig. 7b). In the third stage, beyond a second mixture critical temperature of 56–58 1C and up to the highest investigated temperature of 70 1C, an inversion of the intensity evolution of the band at 230 nm occurred (Fig. 7c). In summary, the results from the CD measurements indicated that a sudden molecular packing re-organization took place around the CP of the mixture (32–34 1C) and that a further gradual re-organization occurred in two stages, one in the range 34–56 1C and the other at temperatures higher than 56 1C. The comparison of the thermo-sensitivity of the mixtures and those of the solutions of a pure surfactant and a pure polymer allowed us to outline some interesting correlations. Reasonably, the first transition of the mixture was related to the abrupt variations of the properties of both the polymer and surfactant that were responsible for their transitions in the pure samples (unimers-to-micelles and fibrils-to-tubules). Indeed, this happened at the CP of the mixture that was close to the pure block copolymer CMT and the bile salt derivative CTT. The second stage in the thermo-response of the mixture occurred at an interval of temperature, where the pure surfactant system had a tubuleto-micelle transition (CTT o T o UCTT) and where the pure block copolymer micelles showed a gradual dehydration of their pNIPAAM cores. In the third temperature regime of the mixed sample there was no further transformation observed in the pure surfactant sample (T 4 UCCT) and only the polymer micellar dehydration proceeded in the pure polymer solution. A reasonable temperature-induced disorder and dehydration of the supramolecular packing constituted the driving forces of the mixture thermo-sensitivity. In the presence of a larger fraction of the polymer, at x = 0.33, the evolution was very similar to that of the mixture at x = 0.5, except for some very small differences (Fig. S7, ESI† and the corresponding description). On the other hand, the CD spectra of the mixtures at x = 0.66 (which was richer in the surfactant compared to the charge equimolar sample) were very similar to those of pure Na-tbutPhC (Fig. S8, ESI† and the corresponding description). It is important to remark that all the temperature-induced changes in the UV and CD spectra were reversible and could be reproduced even after a long storage (up to 3 months) of the sample under room conditions. 3.3.2 Microscopy imaging and DLS. TEM and AFM imaging were performed on the mixture with x = 0.50 at room temperature (Fig. 8a, c and e). The images showed the presence of elongated structures. Sometimes, details mainly highlighted by TEM showed that they present architectures of twisted or helically wrapped ribbons (Fig. 8c). The structures were different from those of fibers formed at room temperature by the pure


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Fig. 8 TEM (a–d) and AFM (e and f) images of pNIPAAM120-b-pAMPTMA30/Na-tbutPhC mixtures with x = 0.50 at 25 (a, c and e) and 38 1C (b, d and f). Images (c) and (d) are magnifications of parts indicated by arrows in the images (a) and (b). The lateral size and the false-color map are the same in both AFM images. Scheme describing the transition at CP (g).

surfactant (see Fig. 3 black arrows). The difference in the structures was further confirmed by comparing the CD signals at room temperature of the mixtures (Fig. 7a) and the pure surfactant (Fig. 4b) which turned out to be significantly different. The formation of twisted ribbons is typical in bile salt and bile salt derivative self-assembly.46 This observation suggests that in our system at temperatures below the LCST of pNIPAAM, the elongated aggregates of the bile salt derivative constituted the core of the aggregates. The polymer could thus arrange at the surface of the Na-tbutPhC core due to the electrostatic interaction between pAMPTMA blocks and the negatively charged heads of Na-tbutPhC facing the solvent. At temperatures


below LCST, the pNIPAAM blocks were hydrophilic and extended toward the solvent forming a protective corona, which prevented the aggregation (Fig. 8g left). The proposed core-corona model was in agreement with the typical complex coacervate core structures proposed for other co-assemblies of block copolymers with one charged block and oppositely charged molecules.25–29 The presence of a core of bile salt derivative molecules was also in agreement with the UV and CD data that showed a strong interaction of the derivative chromophores in all the investigated mixtures. To demonstrate the crucial role of the electrostatic interactions in the mixed aggregates, we analyzed also mixtures of Na-tbutPhC and the homopolymer pNIPAAM or the diblock copolymer


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composed of a pNIPAAM block and an anionic block of poly(2acrylamido-2-methyl-1-propanesulfonic acid) (pNIPAAM96-bpAMPS36). At room temperature, these mixed solutions were transparent at all analyzed molar charge ratios and the CD spectra as a function of temperature showed a profile evolution similar to that of the pure Na-tbutPhC solutions, namely dominated by the pure Na-tbutPhC aggregates rather than by new mixed polymer-surfactant ones (Fig. S9 and S10, ESI†). The distribution of the lengths of the aggregates inferred from the TEM images was in the range 450–1350 nm (Fig. S11, ESI†). The profiles of the aggregates were irregular both in TEM micrograph and AFM images shown in Fig. 8a and e. This suggested that the pNIPAAM corona collapsed into a nonhomogeneous shell of dehydrated chains when the particles were dried on the TEM and AFM supports giving a wide distribution of irregular thicknesses for the aggregates. In particular, thickness values spreading in the 5–60 nm range were estimated by TEM. It was proposed that the maximum value of this distribution corresponded to particles with extended chains in the corona and therefore it was similar to the thickness of the particles in solution. Based on this, we tried an interpretation of the DLS data by assuming the sample as formed by rods with the distribution of the length imaged by TEM and a cross section diameter of 60 nm. Again the expansion to a weighted sum of exponential decays59,60 was used (eqn (S1)–(S3) in the ESI†) with Broersma’s expressions for the rotational and translational diffusion coefficients.61,62 At a scattering angle of 401 used in the measurements, contributions up to the third term were considered. The calculated values were compared with the experimental ones in Fig. 9. They fell within the wide peak of the experimental relaxation time distribution. However they were unable to reproduce the whole distribution. Again some conditions non rigidly fulfilled, such as the length of some rods of the distribution beyond the maximum accessible length at the experimental q value or the presence of interparticle interactions despite the low concentration of the sample, could explain the disagreement. After the transition, rods with smooth edges and more homogeneous cross section diameters were observed by TEM, often associated in bundles (Fig. 8b and d). An average value of 38 nm was estimated by the analysis of the diameter distribution inferred by TEM images

Fig. 9 Experimental intensity weighted relaxation time distribution obtained by DLS (green) of a pNIPAAM120-b-pAMPTMA30/Na-tbutPhC mixture with x = 0.50 and at total charge molar concentration of 2.0 10 3 M and calculated distribution for rods with a length range of 450–1350 nm and a cross-diameter of 60 nm obtained from TEM (red). The temperature was 20 1C.


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for well-separated rods. According to AFM microscopy images, elongated structures with a height of 5 nm and a lateral dimension of 15 nm were observed at 25 1C. These values are smaller than the thicknesses inferred from the TEM images, meaning that the pNIPAAM block was probably completely collapsed on the ribbon surface and did not spread on the mica support when imaged by AFM. An increase of the lateral diameter to an average value of about 30 nm was observed after the transition (Fig. 8f). The morphological variations of the aggregates observed in the TEM images were dictated by the loss in hydrophilicity of the pNIPAAM block. Probably, this forced the polymer to enter into the core of the supramolecular structure. This phenomenon, together with variations in the aggregation features of Na-tbutPhC, which occurred in the same range of temperature, caused changes in the molecular packing as highlighted in the CD spectra. Meanwhile, the loss of the protecting ability of the corona promoted the aggregation into bundles, thus causing clouding and flocculation. According to TEM measurements, the morphology of the aggregates did not show any appreciable variation by increasing the temperature up to 70 1C, thus indicating that it is roughly unaffected by the second and third stages of packing rearrangement highlighted by CD. TEM images collected on the samples at x 0.33 and 0.66 before and after the transitions did not allow us to infer significant differences in the structure of the aggregates formed at different mixture composition (Fig. S12, ESI†). 3.3.3 Small angle X-ray scattering. SAXS curves of the mixture at x = 0.50 were collected as a function of temperature. All the curves showed a steep increase of the scattered intensity by approaching the low q values in agreement with the presence of large particles (Fig. S13, ESI†). The maximum sizes of the scattering particles at all temperature were beyond those related to the low q limit of the SAXS data, therefore these data did not allow us to infer neither the size nor the shape of the particles. However, according to the shape inferred by TEM and AFM, the IFT method for elongated particles was used to interpret the curve at 20 1C. The pair distribution function of the cross section pc(r) was inferred from the indirect Fourier transform of the qI(q) profile. It was observed that a Dmax value of at least 65 nm was needed to fit the SAXS data in agreement with the cross section diameter value of roughly 60 nm estimated from the TEM images. With this value, pc(r) was consistent with a gyration radius of the cross section of 14 1 nm. When increasing the temperature above CP, the ascendant part of the curve moved to lower q values and became steeper at very low q indicating the formation of larger aggregates like bundles with a thicker cross section. 3.3.4 Electrophoretic mobility measurements. The electrophoretic mobility (m) was measured for the mixtures as a function of the composition (Fig. 10). For the samples at an equimolar charge concentration or with a minority surfactant charge (x r 0.50) no mobility or only slightly positive values were collected, suggesting that neutral or only slightly positively charged aggregates were formed. This means that exceeding the block copolymer in terms of charges was not involved in the formation of the mixed aggregate. As the block copolymer was


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Fig. 10 Electrophoretic mobility (m) as a function of the composition for mixtures at total charge concentrations 2.0 10 3 M (open circles) and 1.0 10 3 M (full circles).

completely soluble at the measurement temperature it was involved in the formation of the aggregates up to the neutralization of the aggregate charges, whereas the excess was dispersed in solution in the form of unimers and not detected by the measurements. Conversely, for samples rich in surfactant (x Z 0.6), negative values were found, which decreased up to remarkably negative ones at the highest x fraction of 0.8. This indicated the formation of aggregates containing a significant excess of surfactant negative charges, which increased progressively by increasing the surfactant fraction in the sample. Reasonably this happens because the surfactant in excess, unlike the block copolymer, is included in the mixed aggregate by hydrophobic interactions due to its amphiphilic structure. This inclusion is expected to become significant beyond a concentration threshold of the exceeding surfactant. This explains why the x fraction at which the electrophoretic mobility starts to decrease and becomes negative moving to a higher value as the total concentration is decreased (Fig. 10). 3.3.5 Differential scanning calorimetry. Differential scanning calorimetry measurements were performed on mixed solutions at x = 0.5 and on the solutions of the pure components for comparison. Samples in water and in carbonate/bicarbonate buffer at the same concentration of the mixtures were investigated for the pure block copolymer. The DSC thermograms, expressed as apparent molar heat capacity of the sample (Capp) vs temperature, displayed one main transition peak for all the solutions (Fig. 11). For each of them, Tonset values and enthalpies of transition, DH, were calculated from the intersection of the linear extrapolation of the baseline and the peak ascent and the area under the peak, respectively (Table 1). The peaks for the pure components were related to the formation of micelles for the block copolymer and of tubules for the surfactant. The Tonset values estimated for Na-tbutPhC and pNIPAAM120-b-pAMPTMA30 in buffer were in good agreement with the CTT and CMT values estimated by light scattering. Tonset for the mixture was much lower than that of pNIPAAM120-b-pAMPTMA30 in buffer and very similar to the one of the pure surfactant. The neutralization of the charges of the pAMPTMA block and proximity of the PNIPAAM chains in the mixed aggregates could explain this result, as these conditions are expected to favor the thermal transition of


Fig. 11 DSC curves expressed as apparent heat capacity Capp versus temperature of 1.0 wt% pNIPAAM120-b-pAMPTMA30 in water (a) and in 1.5 10 3 M buffer (b) in the lower panel. The curves of 2.0 10 3 M NaTbutPhC (c), 6.0 10 5 M pNIPAAM120-b-pAMPTMA30 (d) and mixture at x = 0.5 (e), (1.8 10 3 M NaTbutPhC and 6.0 10 5 M pNIPAAM120-bpAMPTMA30) in buffer are reported in the upper panel. Curves are arbitrarily shifted.

Table 1 DSC data obtained for the solutions of the block copolymer (BC), surfactant (S) and mixture at x = 0.5 (M)

Sample

BC (104 M)

S (103 M)

Tonset (1C)

DH (kJ mol 1)

S/buffer BC/buffer BC/buffer BC/water M/buffer

— 0.6 5.0 5.0 0.6

2.0 — — — 1.8

29.5 37.8 35.0 40.0 30.6

38.0 261 431 133 209a

a

Value expressed per mole block copolymer.

PNIPAAM and the lowering of its LCST. In addition we could have that the surfactant thermoresponse triggered the transition for the mixture. Also in this case Tonset was roughly consistent with the CP highlighted by turbidity-based analysis. The derived endothermic micellization enthalpy for pNIPAAM120-b-pAMPTMA30 in buffer corresponded to 3.6 kJ per mole of the NIPAAM unit. A comparison with the data reported in water indicated that the buffer determined a relevant decrease of Tonset and an increase of the micellization enthalpy of the polymer. The enthalpy change obtained for the polymer in water (1.1 kJ mol 1 NIPAAM) was lower than that of a PNIPAAM homopolymer of similar length,65 which is probably a consequence of the repulsive electrostatic interaction between the charged parts that hindered a complete dehydration of the PNIPAAM block in the copolymer. Interestingly an endothermic transition enthalpy was also estimated for the tubule formation of pure Na-tbutPhC. The reason for this is not yet understood. The value obtained for the mixed sample at x = 0.50 was lower than that of the pure block copolymer at the same concentration as in the mixture (209 kJ mol 1 compared to 261 kJ mol 1). This small


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difference suggests that a partial dehydration of pNIPAAM blocks, as a result of the intermolecular interaction between the two species, occurred before the transition. Both the effects on enthalpy and Tonset induced by the presence of the surfactant confirmed that the transition in the mixture occurred by a mechanism involving a supramolecular rearrangement within the whole complex.


4. Conclusions We demonstrated that a mixture of an oppositely charged block copolymer and a surfactant that were both thermoresponsive could provide systems with a thermosensitivity of high complexity. The mixture was formed by an anionic surfactant, obtained by chemical modification of sodium cholate, and a block copolymer that has a thermosensitive and a cationic block. Experiments on the pure components showed thermoresponsive properties, with thermally induced transitions from unimers to micelles in the case of the block copolymer and from fibers to tubes in the case of the surfactant. Regarding the mixed samples, stable dispersions of twisted ribbons were formed at room temperature that transformed into rod-like structures around 30–34 1C, which is the range of the transition temperatures of the surfactant (30 1C) and the polymer (34 1C). Despite the relatively low morphological transformation, remarkable rearrangements occurred in the re-organization of the aggregates at a molecular level. The most complex behavior took place in the sample with an equimolar charge fraction. Mixed aggregates were readily formed at room temperature with a core of surfactant molecules where the cationic block of the copolymer acted as an anchor block interacting with the surface via electrostatic interactions. The aggregates had therefore a corona of nonionic blocks that prevented their association by steric stabilization. At the LCST, the corona chains became hydrophobic and an abrupt supramolecular rearrangement occurred due to the copolymer chains buried in the hydrophobic core of surfactant molecules. The ability of the corona to prevent aggregation was lost under this condition and clouding and flocculation occurred. Spectroscopic data showed that a further increase in temperature dictated rearrangements in the molecular packing of the aggregates, which took place in two stages strictly correlated with the temperature-induced evolutions highlighted in the pure components.

Acknowledgements MAX-IV is acknowledged for beam time and EU for financial support via 7th Framework Program CALIPSO (ID20120367). This work benefited from SasView software, originally developed by the DANSE project under NSF award DMR-0520547.

Notes and references 1 Y.-C. Liu, A.-L. Le Ny, J. Schmidt, Y. Talmon, B. F. Chmelka and C. T. Lee Jr, Langmuir, 2009, 25, 5713–5724.


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