Shear-Induced Transformation of Polymer-Rich Lamellar Phases to

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Shear-induced transformation of polymerrich lamellar phases to micron sized vesicles Sören Großkopf, Brigitte Tiersch, Joachim Koetz, Andreas Mix, and Thomas Hellweg Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02786 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Shear-induced transformation of polymer-rich lamellar phases to micron sized vesicles Sören Großkopf,† Brigitte Tiersch,‡ Joachim Koetz,‡ Andreas Mix,¶ and Thomas Hellweg∗,† †Physical and Biophysical Chemistry, Department of Chemistry, Bielefeld University, Universitätstraße 25, Bielefeld, Germany ‡Colloid Chemistry, Department of Chemistry, Potsdam University,Karl-Liebknecht-Straße 24-25, Golm, Germany ¶Inorganic and Structural Chemistry, Department of Chemistry, Bielefeld University, Universitätstraße 25, Bielefeld, Germany E-mail: [email protected] Phone: +49(0)521 1062055. Fax: +49(0)521 1062981 Abstract In the present work we study the shear-induced transformation of polymer-rich lamellar phases into vesicles. The evolution of vesicle size is studied by different scattering techniques, rheology and microscopy methods. The lamellar phase found in the system D2 O / o-xylene / Pluronic PE9400 /C8 TAB can be fully transformed to multilamellar vesicles (MLV) applying shear. The size of the MLVs is proportional to the inverse square-root of the shear-rate. Hence, the polymer based quaternary system behaves similar as lamellar phases based on small surfactant molecules. Additionally, we found a growth effect leading to a size increase of the vesicles after shearing was stopped.

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Introduction In the past 40 to 50 years complex fluids like surfactant based systems 1–10 or microgels 11 were studied intensively. In this study our focus lays on surfactant based mixtures, socalled microemulsions. This includes studies of binary, tertiary and quaternary and even more involved mixtures. These systems show a rich phase behavior with structures varying from droplets, 12 via wormlike assemblies 13 to bicontinuous 14 and lamellar 15 phases. Beside normal short chain surfactants (e.g. Ci Ej or SDS) especially amphiphilic copolymers were in the focus of different studies during the past decades. 16–23 Due to their amphiphilic nature they adsorb at the interface and change the elastic properties of the amphiphilic film if they are used as co-surfactant. Even binary and ternary systems with only a tri- or diblockcopolymer have been investigated. 24–26 All those systems exhibit lyotropic mesophases like hexagonal, cubic or lamellar, which are built from micelles or stacked bilayers. 25 In this contribution our main focus lays on the lamellar phase and its corresponding spherical structure, the multi-lamellar vesicle or so called ’onion’ state. In the early 90s Diat et al. reported the transformation of a lamellar phase containing water, sodiumdodecylsulfate, pentanol and dodecane to multi-lamellar vesicles (MLVs) of defined sizes by applying shear stress. 27–30 The mechanism for the ’onion’ phase formation is still partly unclear. However, Roux et al. proposed a mechanism which involves a buckling instability during shearing caused by a non-uniform gap between the two surfaces. 31 The non-uniform gap causes then a dilative (or compressive) strain perpendicular to the lamellar phase which then causes the buckling instability. Zilman and Granek proposed another mechanism which involves the membrane excess area caused by the short wavelength undulations. 32 By applying flow the short wavelength undulations of the bilayers are suppressed. This supression acts as a lateral pressure on the layers which induces buckling of the whole lamellar phase above a critical shear rate leading finally to the formation of the ’onion’ phase. The transition process from the lamellar phase to the MLVs or defective lamellar phase, 33,34 respectively, is continuous. 35 Depending on the composition and the repeating distance of the lamellar 2


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phase different shear-rates have to be applied to induce the transformation from the lamellar phase to the ’onion’ structure. 31 Many different works concerning the shear-induced transformation of a lamellar phase were already published. 36–42 Most investigated systems were binary with non-ionic 35,41,43–45 or ionic 27,46,47 surfactants. A few pseudo-ternary systems with SDS were also reported 28,31 as well as systems with a PEO-PPO-PEO triblock copolymer as co-surfactant added to the non ionic, binary system. 48–50 Even pseudo-binary systems containing 23 % of the triblock copolymer Pluronic P123 and butanol showed the presence of multi-lamellar vesicles. 51 But with an increased content of 30 % triblock-copolymer the system exhibits only oriented lamellar phases when applying shear stress but no multi-lamellar vesicles. Hao et al. showed in 2016 another approach for the preparation of MLVs. They showed a spontaneous formation of multilamellar vesicles in a system contaning C12 E4 by addition of di-(2-ethylhexyl)phosphoric acid. This is caused by a supression of the bilayer undulations. 52 Another approach is the use of specifically taylored diblock copolymers in an aqueous solution where both blocks have the correct geometry, which is needed for the formation of MLVs. 53,54 In 2017 Kawabata et al. studied the interlamellar interactions by adding salts with high charge density and bulky salts. 55 Recently Kuczera et al. found that the transition of a lamellar phase containing C10 E3 and water is also possible using large amplitude oscillating shear. 56 Investigations of the last 25 years show that almost any lamellar phase can be transformed into MLV phases. The size of the MLVs is defined by the applied shear rate. But the systems and the structures are still metastable and could only be used for liquid systems like fabric softener 57 or cosmetics. 58 Due to the nature of ternary, surfactant containing systems every substance which is either hydrophilic, hydrophobic or amphiphilic can be integrated in such a system. Moreover, by using polymerizable amphiphiles and by adding a crosslinker it should be possible to fix the resulting structures which can then be used for other purposes e.g., drug delivery. In the present work we want to use Pluronics as a model system to establish the shear induced transformation to vesicles in case of a mainly polymer-based lamellar phase in a quaternary system. For the mentioned purpose

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the polymer could be exchanged to a polymerizable one (e.g. triblock-terpolymer) or the oil could be replaced by a hydrophobic monomer. The resulting structures could either be used, if they are single MLVs, as drug delivery systems, liposomes or as ingredients in cosmetics. In presence of a network with interconnected vesicles the application as a thermal isolator foam becomes possible. 59 Different methods can be used to distinguish the ’onion’-like vesicle from the planar lamellar phase. Especially combinations of rheology with other methods like small angle scattering (neutrons, 7,41,43–45,48,50,60–62 X-rays 44,61 or light 27,31,41,43,44,48,63 ) or 2 H NMR 35,45 are useful to scrutinize the transition from the lamellar phase to the MLV. Due to their large size the vesicles can also be analyzed by light microscopy. However, due to the low optical contrast, contrast enhancing methods like polarized light (PL), 31,48,50,60,63 difference interference contrast (DIC) 46,64 or phase contrast (PhC) 27 microscopy need to be applied. Another method to obtain real space pictures from the vesicles is either cryo-scanning electron microscopy (cryo-SEM 65–70 ), cryo-transmission electron microscopy (cryo-TEM) 71 or freeze fracture TEM (FF-TEM). 7,62 Hence, to gain detailed insight into the formation of polymer-rich lamellar phases we used an inverse and polymer-rich quaternary system (D2 O / o-xylene / PE9400 / C8 TAB) with the amphiphilic triblock copolymer Pluronic PE9400 which consists of two ethyleneoxide (EO) and one propyleneoxide (PO) block (EO21 − PO47 − EO21 ). To the best of our knowledge this is the first study of such rather complex mixture in the context of ’onion’ phase formation. The system was sheared in a region where it exhibits a lamellar phase and in situ investigated with small-angle light scattering (SALS) to measure the size of the resulting vesicles. Furthermore, we studied the size evolution after stopping the shear and found an increase of size of the vesicles.

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Materials and Methods Deuterium oxide (D2 O > 99 Atom D %) , Carl Roth, Germany) and o-xylene (C8 H10 > 98 %, Sigma-Aldrich) was used. The triblock-copolymer PE9400 (EO21 − PO47 − EO21 ) was a gift by BASF (Ludwigshafen, Germany) and is technical grade. The water, o-xylene and the triblock-copolymer were used without further purification. The cationic co-surfactant noctyl-trimethylammoniumbromide (C8 TAB, >97 %, Sigma Aldrich, Munich, Germany) was dried over night in vacuo and stored afterwards under nitrogen atmosphere. The polymer was dissolved in o-xylene (50 wt %) and then all components were weighed, mixed and stored at 60 ◦ C for three hours. The highly viscous samples were then stored for at least seven days at 20 ◦ C to assure that the phases are in equilibrium. All measurements were performed at a constant oil mass ratio of α = 0.875. The mass ratio of the oil to water and oil is defined as:

α=

mo-xylene . mD2 O + mo-xylene

(1)

The mass ratios are defined as follows: mi wi = P mi

(2)

Here, mi are the masses of the component i.

Phase determination Microemulsion systems form complex structures when they are mixed. 72 The determination of the system-specific phase behavior is thus essential to understand the structure formation and to estimate the efficiency and the role of the used components. 73 The phase behavior of the quaternary system was studied in two different ways. First of all by optical inspection by the eye where the existence of a birefringent phase or an isotropic phase can be determined.

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The sample was placed between two crossed polarizers to detect birefringence. Since observation by eye can be very challenging, a more precise determination of the birefringence and phase state can be done by means of a polarized-light microscope. Second, small angle X-ray scattering (SAXS) was applied to determine the structures precisely. SAXS experiments will be described in the next sub-section.

SAXS SAXS experiments were performed at 20 ◦ C on an inhouse SAXS/WAXS system (XEUSS, Xenocs, Sassenage, France) with a CuKα source (λ = 1.541 Å, GeniX Ultra low divergence, Xenocs, Sassenage, France). Detection of the scattered intensity was done employing a Pilatus 300K hybrid pixel detector (Dectris, Baden Deattwil, Switzerland). The covered q-values range from 0.2 to 1.6 nm−1 . The data were analyzed using the Foxtrot (Version 3.3.4, G. Viguier, R. Girardot) software. The samples were normalized to incident intensity, sample thickness, measuring time, transmission and background. The scattered intensity was brought to absolute scale using glassy carbon (type 2, sample P11 74 ). In small-angle X-ray scattering experiments, the scattered intensity

I(q) = N · I0 · ∆ρ2 · V 2 · P (q) · S(q)

(3)

is dependent on the number of particles N, the incident intensity I0 , the scattering volume of the sample V, the electron density difference ∆ρ = ρparticle − ρsolvent , on the particle form factor P (q), and the structure factor S (q). The scattering vector is defined as

q=

4π · sin(θ) λ

(4)

at a scattering angle of 2θ. The experimental data was treated using the software GIFT of the PCG software package provided by O. Glatter . 75,76

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Generalized, indirect Fourier transformation The generalized indirect Fourier transformation (GIFT) method is based on the IFT (indirect Fourier transformation) and is able to compute the pair distance distribution function (p(r )), the structure, and form factor simultaneously. 75 By applying the Fourier transform to the scattering curve the pair distance distribution function, which gives information about the sample in real space, can be calculated. p(r ) is the Fourier transform of the angle dependent scattered intensity I (q) and thus the convolution square (spatial correlation function) of the excess scattering length density distribution ∆ρ. 77 The GIFT method is a good tool for analyzing scattering data in concentrated systems, which show a prominent structure factor contribution. In the present case the structure factor model is given by the modified Caillé theory with polydispersity. 78,79 This model includes four parameters which are the numbers of coherently scattering bilayers (N ), the interlamellar spacing (d ), the Caillé parameter (η) and the diffuse scattering term (D). The Caillé parameter describing the rigidity of the bilayer

η=

π · kB · T √ 2d2 B · κ

(5)

consists of the Boltzmann constant kB , the absolute temperature T, the interlamellar spacing d, the bulk modulus of compression B related to the membrane-membrane interactions 80 and the bilayer bending rigidity κ. η gives information about the elastic properties, but elastic constants can’t be extracted easily from the value of η. In systems where B and κ change in an anti-proportional way no modification of η might occur despite of changes in κ. 81 The modified Caillé theory can also be applied to a concentrated phase of multi-lamellar vesicles since on the local scale analyzed by SAXS it cannot be distinguished from a planar lamellar phase.

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Viscosity The viscosity of a pure and perfectly ordered lyotropic, lamellar phase is in the range of 0.030.07 Pa. 34 If now introducing defects (MLVs) into the lamellar phase the viscosity increases. When the lamellar phase has been fully transformed to the defective phase in other words the volume fraction of MLVs is unity then the viscosity reaches a maximum with one magnitude larger as the defect free lamellar phase. The reason for the increased viscosity can be found in the flow behavior. Whereas in the case of the MLVs the system flows by moving the defects which might collide 30 it is different in the case of the defect free lamellar phase. Here, the layers can glide with less force. Furthermore, the viscosity is also dependent on the size of the defects and larger defects exhibit a larger viscosity.

Rheo-SALS Samples were sheared with a shear stress controlled rheometer (MCR 101,Anton-Paar, Graz, Austria) with a quartz plate-plate geometry (d = 42.93 mm) with a 1 mm gap to obtain the shear viscosity of the sample. The rheometer was equipped with a small angle light scattering module including a 10 mW 658 nm continuous wave laser (Schäfter +Kirchhoff, Hamburg, Germany) a polarizer and an analyzer (perpendicular). The depolarized scattered light was recorded with a CCD camera (Lumunera, Florida, USA) below a screen on which the scattered light was directed to. The recorded images were evaluated with SALSSoftware (Version 1.1 alpha, K. U. Leuven, Belgium). The maximal obtained q was 1.5 µm−1

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Pulsed field gradient-NMR (PFG-NMR) The NMR measurements were performed on a Bruker Avance 600 FT NMR spectrometer (Bruker, Billerica, USA) operating at a proton resonance frequency of 600.13 MHz. The instrument was equipped with a z -gradient broadband observe probe(BBO-Smart, Bruker ) exhibiting a maximum gradient strength of 50 G · cm−1 . The sample temperature was set to 298 K using a gas flow of 800 L · h−1 . The pulse sequence ledbpgp2s delivered with the Bruker Topspin TM software was used for the diffusion experiments. The diffusion delay (∆) was set to 100 ms. For the gradient duration (δ) a value of 7 ms has been used. Diffusion coefficients (D) of the components were calculated by analyzing the intensities of the resonances according to equation 6 using MestReNova 10 software (Mestrelab, Santiago de Compostela, Spain). 82,83 δ 2 2 2 I(g) = I(0) · exp − D · γ · g · δ ∆ − 3

(6)

Here, I(g) is the observed intensity, I(0) intensity in the absence of gradient pulses, γ the gyromagnetic ratio and g the gradient strength. Normalizing the diffusivities of the single species to the bulk diffusivities allows, with the help of other techniques (e.g. SAXS, SANS, conductivity measurements), to exactly determine the investigated phase. 84,85

Conductivity measurements The conductivity measurements have been done with a CyberScan PC 510 (Thermo Scientific Eutech, Waltham, USA) measurement tool and an ECCONSEN91W electrode.

Microscopy a) optical microscopy Due to the low optical contrast of the samples it is necessary to enhance the contrast by different methods. The phase contrast (PC) microscopy images were obtained by an Olympus IX51 microscope (Olympus, Tokio, Japan). By the addition of two polarizing filters the 9


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polarized light images could be obtained as well with that microscope. The difference interference contrast (DIC) microscope images were obtained at an Olympus IX71 microscope with 200 µm working distance. This method generates the contrast through the differences of the refractive index in different regions of the sample resulting in different optical path lengths. This difference can be visualized by a special setup of prisms and polarizing filters within the microscope. 86

b) cryo-SEM To obtain high magnification pictures of the resulting structure a field emission cryo-scanning electron microscope was used (S-4800, Hitachi, Tokio, Japan) with a resolution of 1.5 nm at 1kV. The sample was plunged into near solid, liquid nitrogen to prevent water crystallization. The frozen sample was then transferred to a cryo preparation unit, heated to -100 ◦ C to remove condensed water and then cooled to -120 ◦ C. The frozen sample was fractured with a cooled scalpel and then sputtered with platinum before it was transferred to the SEM.

2D-Fast Fourier Transform To analyze the PL-microscope images 2D-Fast Fourier Transform (2D-FFT) was applied. The 2D-FFT were performed with Fourier Painter (JCrystalSoft, Ver. 1.0.1.0).

Results and Discussion Phase behavior The phase behavior of the studied system is quite complex due to the history dependent structure formation of the lamellar phase. Four components increase the possible number of phases and especially the high polymer amount makes the investigation quite challenging 10


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because of the high viscosity. Due to this the equilibration time which is needed by the system is very long. Therefore, it is sometimes not obvious that the system has reached its equilibrium state because even in the one phase state the system exhibits a slight turbidity. Additionally, the used polymer is technical grade which can perturb the phase behavior as well. Every phase which is denoted in the following as lamellar or Lα could be a normal lamellar phase, a phase consisting of densely packed multi-lamellar vesicles or a mixture of both, because both phases can transform to the other phase by shear stress and it is hardly possible to distinguish both phases either by the eye or by SAXS.

Figure 1: Gibbs’ phase triangle of the system D2 O - o-xylene / PE9400 / C8 TAB with α = 0.875. Between a polymer amount of 0.2 - 0.4 a significant lamellar region exists. At higher polymer amounts a low viscous inverse phase (L2 ) arises. At very low co-surfactant amounts an inverse hexagonal phase has been determined (H2 ) and at higher co-surfactant amounts the lamellar phase is adjacent to an isotropic phase. The triangle marks the used composition for rheo-SALS and the points mark the compositions for which additional SAXS and PFG-NMR data can be found in the supporting information. For the point in the H2 area only SAXS experiments have been done. The phase behavior has been investigated by the previously discussed methods and is summarized in a Gibbs’ phase triangle in Figure 1. The phase triangle shows a large lamellar region (Lα ) where the lamellar phase coexists with a phase of multi-lamellar vesicles. The Phase boundaries only include the one-phase lamellar region which has an intermediate 11


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Table 1: Compositions, normalized diffusivities (to bulk diffusivity D0,x , whereas x is the component), phase type and electric conductivity values for investigated samples. α is for all samples 0.875. Sample

wwater+oil

wpolymer

wC8TAB

phase

A B C D E

0.72 0.65 0.52 0.41 0.59

0.24 0.23 0.42 0.49 0.19

0.04 0.12 0.06 0.1 0.22

H2 Lα L2 L2 network

Dwater D0,water

0.05 0.06 0.10

Do-xylene D0,o-xylene

0.25 0.14 0.17

κ / µScm−1 0 350 40 70 1400

electric conductivity (values can be found in table 1). At very low co-surfactant amount the system shows a low viscous inverse hexagonal phase (H2 , SAXS data can be found in the supporting information) with a non measurable conductivity. By increasing the polymer amount the system exhibits a phase of very low viscosity which consists of inverse structures (L2 in samples C,D) with a very low conductivity. PFG-NMR measurements showed that the diffusion coefficient of water slightly decreases with increasing polymer amount. Also, the decrease of oil diffusivity with increasing polymer amount might be attributed to a growing amount of polymer which is dissolved in the oil phase. (PFG-NMR and SAXS data can be found in the supporting information, the normalized diffusivities can be found in table 1). Hence, these structures may be inverse micelles or cylinders. Starting at a polymer amount of 0.10 and at a C8 TAB amount of 0.15 the system exhibits a bicontinuous-like phase which is adjacent to the lamellar phase at large co-surfactant amounts. In fact a classic bicontinuous phase (with a sponge structure) can only be found in microemulsions with almost equal volumes of water and oil. Additionally, the SAXS pattern does not exhibit the typical classic bicontinuous like shape. Hence, it is more likely a water network 87 within an oil phase. The network phase was on the one hand determined by its large electric conductivity and on the other hand via PFG-NMR diffusion measurements with similar diffusivities of water and o-xylene (SAXS and PFG-NMR data shown in supporting information). The Lα region of the phase diagram was investigated more precisely by SAXS. A typical scattering curve is shown in Figure 2. The data reveal the typical Bragg peaks present in 12


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scattering curves from lamellar structures. The interlayer spacing is calculated from the first order peak by the GIFT analysis and is 9.65 ± 0.20 nm. 76,79

Figure 2: SAXS Intensities as a function of q for composition B. Scattering data (points) was fitted with GIFT using the modified Caillé theory with polydispersity. The parameters are d = 96.5 Å (Interlayer spacing), N = 19 (Number of correlated bilayers), η = 0.181 (Caillé parameter).

Microscopy Shear is used to generate vesicles and the large size of the formed onion-like vesicles allows to investigate their overall size and shape by a set of different techniques. 27,31,46,48,64 A general problem of all microscopy methods is the high shear sensitivity of the vesicles. The sample had to be transferred from the rheometer to the microscopy slides and even this transport puts some shear stress on the sample. In addition, the use of cover glasses was not possible due to the same reason. Lamellar phases and MLVs become visible in a polarized-light (PL) microscope because of their optical anisotropy and resulting birefringence. 88 Lamellar phases can be seen as elongated structures (oily streaks 89 ) and multi-lamellar vesicles (also known as focal conic domains) 88 give rise to a maltese cross pattern. 27 Figure 3 shows a typical image obained by polarized light microscopy in sample with composition B. The presence of the cross-shaped

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structures confirms the presence of lamellar structures in all regions of the sample. The shape of the maltese crosses depends on the local anisotropy of the scattering particles. If the MLVs are densely packed, single vesicles cannot be distinguished from the almost isotropic background. In figure 3 only the MLVs in the foreground can be seen which are on top of the sample. Because of this a large number of vesicles can only be seen at the edges of the sample. Here, the vesicles release from the densely packed state and and can now due to an increased contrast (of the vesicle to the less denser background) be seen individually. This phenomenon of ’onion jets’ was already found by Buchanan et al. for a simple surfactant system in contact with a solvent. 90 We did this experiment with a droplet of the lamellar phase in contact with air. The flow of the MLVs is eventually caused by local heating by the microscope light followed by a demixing of the components. Also, evaporation effects might play a role in the demixing of components. Then, there might be an excess solvent phase next to the lamellar phase in which the vesicles can flow. In the supporting information a video of the vesicles similar to Buchanan’s ’onion jets’ can be found.

Figure 3: Polarized-light microscopy picture of of the unsheared sample B. Different Maltese crosses can be seen with different sizes on almost all positions. The picture has been taken without a cover glass, because of the high shear sensitivity of the MLVs. Only at the edges such a large number of vesicles is found.

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Figure 4 shows the same giant MLV in a polarized-light (PL) and phase-contrast (PC) microscopic image in sample with composition B without shear history. The diameter of this vesicle is about 150 µm. Such large vesicles are extremely shear sensitive and are found seldomly.

Figure 4: Giant multi-lamellar vesicle as seen in the polarized-light microscope (PL, left) and in the phase-contrast microscope (PC, right) in sample with composition B without shear history. One can see the Maltese cross shape even at this large vesicle at the PL image.

Figure 5: PL-microscope image of the sheared sample with composition B which has been transferred to a microscope slide and then analyzed via 2D-FFT. The sample was sheared for 300 s at 100 s−1 . The FFT image shows a long range order of the MLVs. Figure 5 shows the appearance of the sheared sample B as seen by polarized light microscopy. The sample exhibits a homogeneous structure with no single vesicles visible. Now 15


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by applying 2D-FFT one is doing a virtual depolarized SALS experiment and obtains the typical four-clover leaf pattern which is typical for MLVs. Additionally, the 2D-FFT image shows a long range order. Hence, after shearing the system is a stable, solid like MLV gel which can be transferred with only little perturbance to a microscope slide. DIC microscopy is a good method for the visualization of the onion-like structure of the vesicles. Figure 6 shows a DIC microscopic image of the unsheared sample B on the microscope slide. With DIC microscopy contrast variations in perpendicular orientation to the optical axis can be visualized. Because of this the vesicles can only be seen as crosssections. The contrast of the DIC images is still very poor because of the surrounding medium of the vesicles, consisting of a mixture of lamellar phase and MLVs, which have nearly the same refractive index as the vesicles.

Figure 6: DIC microscopic image of the unsheared sample B in the lamellar region. Despite of the low contrast the image shows the ’onion’ structure of the MLVs. On almost every position a vesicle can be seen. However, the polydispersity is quite high. The radii of the vesicles were determined with ImageJ 91 and were found to range from 2-10 µm. Obviously, vesicles with different sizes are formed all over the sample during the shearing process. Since this method is only sensitive to µm-sized structures, vesicles having a size below the resolution limit of the used technique cannot the detected by this method. One has to consider that the transfer process probably has destroyed larger vesicles. Hence, the increased polydispersity is most likely a sample preparation artifact. 16


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Cryogenic scanning electron microscopy was applied additionally to investigate the formed structures which are partly below the resolution limit of the used optical microscopy. Moreover, with cryo-SEM much more details can be seen in comparison with DIC microscopy due to a higher resolution. Figure 7 shows a typical cryo-SEM image of the unsheared MLV phase with composition B.

Figure 7: cryo-SEM image of unsheared sample with composition B. The onion-like structure can clearly be seen. The size of the vesicles ranges from 80 nm to 2 µm. The MLVs are densely packed and deformed due to the packing. The typical ’onion’ structure of the MLVs and a close packing of differently sized vesicles can be seen. The diameter ranges from 80 nm to 2 µm and has quite a large polydispersity which is attributed to the transfer process. In comparison to the DIC images much smaller structures are resolved due to much smaller wavelength of the electrons compared to light.

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Rheo-SALS The influence of the shear-rate on the vesicle size was investigated during simultaneous SALS experiments during sample shearing. Figure 8 shows thus SALS intensities at different times during shearing. As it can be seen, the measured scattering angles increase with increasing time. The numbers refer to samples marked in figure 9. The radius of the vesicles can be calculated from the images by equation 7: 92

R=

1.025 · λ . π · sin θmax 2

(7)

Here, λ is the wavelength of light in the medium, which is assumed to correspond to the wavelength of the laser, and θmax is the scattering angle where the intensity reaches its maximum. At constant shear stress, the size of the vesicles is determined from θmax . The overall intensity decreases from the beginning to the end by a factor of 100, because smaller particles scatter less intense than larger ones. 93 This has been compensated with an increase of the exposure time. Vesicles were prepared at different shear rates γ˙ and shear times. The results are shown in Figure 9. The analysis has shown that the radius of the MLVs in the steady state is dependent on the shear rate γ, ˙ the interlayer spacing d, the bending modulus κ, the Gaussian elastic modulus, which is also referred to as the saddle splay modulus κ ¯ and the viscosity at the used shear-rate η 27 s R=

4π(2κ + κ ¯) . η · d · γ˙

(8)

Figure 9 shows furthermore a more detailed study of the shear-rate time depedence. The viscosity of a phase with multi-lamellar vesicles is always higher than the viscosity of a pure lamellar phase, because to flow the system needs to move the MLVs. 30 At the beginning the viscosity increases very strongly because new vesicles are created and the less viscous lamellar phase is being transformed into them. It has to be mentioned that the lamellar 18


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Figure 8: Rheo-SALS scattering intensities at different times for multi-lamellar vesicles. The numbers refer to the shear stress vs. time plot in figure 9. The size has been calculated by equation 7. All images show the typical clover leaf pattern. 42,92 The ring in some images in the center is an artifact.

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Figure 9: Shear viscosity as a function of the time and shear-rate. The shear rate was increased after 600 s of shearing at one shear rate. The system is shear-thinning. The numbers refer to Figure 8.

Figure 10: Steady state radius of MLVs as a function of the inverse square root of the applied shear rate. The size of the vesicles was extracted via SALS. The experimental data was fitted 1 linearly and the slope is 25.7 µm · s− 2

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phase does not reach its steady state in which the lamellar phase is fully transformed. To reach this state a longer time of shearing is necessary. After an increase of the shear rate the viscosity decreases because smaller vesicles are created. In the steady state the viscosity as well as the size of the vesicles decreases with increasing shear-rate. The slope of the linear function in Figure 10 is then: s m=

4π(2κ + κ ¯) η·d

(9)

It is a measure for the stability and shear sensitivity of the multi-lamellar vesicles. A larger slope means that vesicles of the same size are more stable at larger shear stress. In figure 10 the steady state radius of the MLVs during shearing can be seen. The size 1

follows nicely the predicted 27 dependence R ∼ γ˙ − 2 .

Figure 11: The size of the vesicles obtained by SALS analysis after shearing the sample with composition B at 120 s−1 for 5 min. The size of the vesicles increases again after the vesiclesize has been decreased by shear stress. The evolution has been fitted with a exponential growth function R(t) = R(0) + A · exp( τt ) with τ = 15.8 min with A being the amplitude Additionally, it has been found an apparent spontaneous formation of MLVs during SALS measurements. Upon shearing the sample at very high shear rates (γ˙ = 500 s−1 ) for 300 s 21


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at 20 ◦ C the SALS pattern vanishes completely. Hence, the vesicles are too small to give rise to a scattering signal or a lamellar phase has been formed. The re-transformation from the vesicle phase to the lamellar phase at high shear rates was already found by Roux et al. 31 After 10 minutes with no applied shear stress the clover-leaf pattern re-appears and the SALS intensity increases with time. The resulting final MLV size seems to be independent of the magnitude of the previously applied shear rate. There are two possibilities why large MLVs (visible by SALS) have been formed. How the MLVs are formed depends on the state which the sample has reached when shearing at high shear rates. If there are still vesicles present which are just too small to give rise to the SALS scattering signal, then a fusion of small vesicles too larger ones is likely. If the MLVs have been transformed to a well ordered lamellar phase, then a real spontaneous transformation of a pure lamellar phase to MLVs is more likely. In order to investigate if a fusion of small vesicles to larger ones takes place we sheared the sample at a shear rate at which the SALS pattern doesn’t disappear. Subsequently, we measured the size evolution of the vesicles via SALS with no further shear stress applied. In figure 11 the size evolution of the vesicles, after stopping applied shear stress, can be seen. The vesicles have a radius of approximately 4.5 µm directly after shearing at 120 s−1 for 120 s at 20 ◦ C. One can see an increase in size of the vesicles which could be attributed to a vesicle fusion. Because the maximum detectable size via SALS is reached at 12 µm we cannot say if the vesicles grow to even larger sizes. One has to keep in mind that in light scattering the intensity is heavily weighed to larger particles 93 (I ∼ r6 see 92 ) and so smaller vesicles cannot be seen anymore if larger vesicles are present. Hence, the SALS analysis of the growth effect just gives qualitative hints but no very exact quantitative information about the size distribution. Additionally, quantitative analysis of the data is quite challenging due low image quality in non-shear mode, because in the shear mode every picture represents a mean values from many positions of the sample and is hence, smoother and more easy to evaluate. The growth upon stopping the shear also explains why samples which were prepared a longer

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time ago exhibit a stronger scattering. During the time at rest the vesicles could age and fuse to larger vesicles which give rise to a stronger scattering signal than smaller vesicles. Hence, in the older samples the size-distribution is shifted to larger sizes. Therefore, the spontaneous formation of MLVs mentioned before (shearing at 500 s−1 ) is less likely because we already saw a fusion and size evolution process. However, the spontaneous formation is less likely but not impossible as other groups have already shown. The spontaneous transformation was already found by Harris et al. in an aqueous solution of ethylene oxide - butylene oxide diblock copolymers. 53 The formation of the MLVs is driven by the architecture of the polymer which governs the local curvature of the amphiphilic film. Also, Wang et al. showed the spontaneous formation of MLVs in a system with a non-ionic surfactant in water just by the addition of an organic phosphoric acid. 52 Here, the negative charge of the phosphoric acid headgroup suppresses the bending undulations of the bilayer and causes the formation of MLVs. Hence, the positive charge of the C8 TA⊕ might suppress the bilayer undulations as well and cause a spontaneous formation of MLVs.

Conclusion We have studied quaternary system D2 O / o-xylene / Pluronic PE9400 / C8 TAB with respect to its phase behavior. An extended lamellar phase could be detected. This lamellar phase is exposed to shear and shear-induced formation of multi-lamellar vesicles is revealed by different microscopy techniques and by SALS. Hence, this work shows that Pluronic rich phases can easily be transformed to multi-lamellar vesicles with defined average diameter. 1

The vesicle radius nicely follows the predicted linear dependence on γ˙ − 2 . 28,31 Thus, the Pluronic triblock copolymer behaves similar to previously studied systems. We will extend this approach to photo cross-linkable block-copolymers in the future. Additionally, we found a size-increase effect after shear-stress was applied and then stopped. The smaller vesicles might subsequently fuse to larger vesicles. A spontaneous transformation of the lamellar

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phase to a vesicle phase might also be possible but has not been proven yet. The growth effect upon shearing and allowing the sample to rest is not yet exactly understood. We will study this effect in future using FF-TEM/SEM and rheo-SANS.

Acknowledgments We like to acknowledge Prof. Dr. Thomas Huser from the Biomolecular Photonics Group at Bielefeld University for the access to the DIC microscope and Dr. Wolfgang Hübner for the introduction to it.

Supporting Information Available The following files are available free of charge. • Grosskopf_et_al-Supporting_Information_final.docx: SAXS and PFG-NMR spectra • 2017_02_22_5870_xvid.avi : Video of MLVs in the polarized light microscope This material is available free of charge via the Internet at http://pubs.acs.org/.

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Shear-Induced Transformation of Polymer-Rich Lamellar Phases to

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