Macroscopic Alignment of Micellar Crystals with Magnetic Micro

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

Macroscopic Alignment of Micellar Crystals with Magnetic Micro-Shearing APURVE SAINI, and Max Wolff Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03701 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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Macroscopic Alignment of Micellar Crystals with Magnetic Micro-Shearing Apurve Saini,* and Max Wolff Department of Physics and Astronomy, Uppsala University, Uppsala 75120, Sweden ABSTRACT The effect of small quantities of magnetic polymer nanocomposite (formed by surfactant Pluronic F127 @ Fe O nanoparticles of 10 nm and 30 nm diameters) on the crystallization 3

4

behavior of Pluronic F127 micelles solvated by 20 % in water was investigated in the vicinity of hydrophilic and hydrophobic interfaces. Introducing magnetic nanoparticle at core imparts magnetic properties to the polymeric micelle and increases its hydrodynamic diameter. These magnetic polymer nanocomposites act as defects in the Pluronic crystal and hinder crystallization in comparison to pure Pluronic F127 micelles behavior. Magnetic field results in a motion of the magnetic micelles and a micro-shear effect. This micro-shearing assists self-organization of the crystal. Addition of magnetic micelles formed using 30 nm magnetite particles show similar crystallization behavior, however, with an overall reduced crystallinity due to their significantly larger size compared to the lattice parameter and the dimension of the interstitial cavity for a fcc structure.

ACS Paragon Plus Environment

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INTRODUCTION Hybrid hydrogels/ block-copolymer mesophases (physical or chemical network of water-soluble polymers), formed via incorporation of small amounts of inorganic nanoparticles as additives into the gel matrix, have unlocked hydrogels with tailored mechanical properties. Magnetic 1,2

polymer nanocomposites as additives give additional tunability via the ability to manipulate the properties of the hydrogels by applying a magnetic field. Magnetic polymer nanocomposites are of significant interest due to the combination of excellent magnetic properties, stability and biocompatibility. In this regard, combining block polymers, like e.g. pluronics, with magnetic 3,4

nanoparticles seems highly attractive given that they offer a model system for the study of gelation, percolation, crystallization, or the glass transition and a colloidal solution of magnetic 5-9

nanoparticles forms the ideal system for the study of magnetic field induced self-assembly.

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Pluronics exhibit an amphiphilic behavior in aqueous solution and micellisation is found for elevated temperature and/or copolymer concentration. The core of the micelle is composed of 11

PPO (polypropylene oxide) blocks and an outer shell consisting of PEO (polyethylene oxide) end blocks, which have a better solubility in water and are highly hydrated. As a critical volume fraction of micelles is reached, they crystallize. Iron oxide nanoparticles synthesized using thermal decomposition (organometallic route) are highly uniform in all respects (size, shape, composition and crystallography), making them ideal candidates for many bio applications such as magnetic separation, drug delivery and cancer hyperthermia.

12-14

Unfortunately, the surfactant coating (typically oleic acid) on the as-synthesized

nanoparticles render the nanoparticles insoluble in aqueous solutions. The triblock co-polymer Pluronic-F127 (PF127) is a good phase transfer agent for this purpose. The hydrophobic PPO chains assemble onto the hydrocarbon tails of the oleic acid layer via hydrophobic interactions,

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providing a high yield of nanoparticles coated with PF127. PF127 coatings render these magnetic nanoparticles biocompatible. After phase transfer, the nanoparticles remain superparamagnetic 4

with saturation magnetization ~ 96% of the maximum theoretical value.

3

While the interaction between spherical polymer micelles can be varied continuously by changing the temperature, concentration or pressure, the presence of magnetic nanoparticles at the micellar cores gives additional magnetic tunability. These magnetic micelles orient themselves along the field lines. Previous studies have shown that the surface crystallization 10

behavior of pluronics depend on the properties of an adjacent solid substrate.

16,17

In the present

study, we analyze the effect of small amounts of magnetic nanoparticles on the crystallization behavior of pluronic micelles from bulk solution close to a flat solid interface with different surface energies. We combine PF127 with trace amounts of Pluronic F127 @ Fe O nanoparticles 3

4

of 10 nm and 30 nm diameters and probe them using neutron reflectometry with and without externally applied magnetic field. It turns out that the crystallinity of a micellar solution which is dependent on the interface properties, is widely affected by the presence of magnetic particles as 18

well as external field. EXPERIMENTAL SECTION Materials. The sample under investigation is a mixture of 2.3 ml of 18.5 wt. % solution of triblock copolymer PF127 in D O and 0.5 ml of 0.83 vol. % of Pluronic F127 @ magnetite (Fe O ) 2

3

4

nanoparticles (approximately 44.27 mg/ml). This corresponds to a volume ratio of PF127 : F127@Fe O ≈ 23:1. PF127, purchased from Sigma Aldrich, was used without further 3

4

purification. The bulk properties are known in great detail. PF127 is a tri-block copolymer with 19

a central part of 65 propylene oxide units terminated by two end groups of 99 ethylene oxide units (EO99-PO65-EO99), see Figure 1A. For increased temperature/ copolymer concentration,


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it forms micelles in aqueous solution. The resulting aggregates have a hydrophobic core and a 11

hydrophilic shell, as illustrated in Figure 1(B, C). At a critical micelle volume fraction (CMV) and above the critical gelation temperature (CGT), the micelles crystallize into cubic close packed structures,

17-20

illustrated in Figure 1D. Magnetite nanoparticles with 10 nm diameter were

synthesized and particles with 30 nm diameter were obtained commercially from Ocean Nanotech (USA). Both particles were prepared by thermal decomposition with oleic acid as surfactant. The particles were phase transferred to water from hexane through coating them with PF127. PF127 plus F127@Fe O samples prepared using 10 nm and 30 nm diameter Fe O 3

4

3

4

particles are denoted as S1 and S2, respectively.


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Figure 1. Illustration of (A) PF127 chemical structure, (B) PF127 unimers combining to form a micelle with a hydrophobic core and a hydrophilic shell, (C) with an increase in temperature/concentration, micellization is achieved in aqueous solvent, (D) above CGT/CMV micelles crystallize in close packed structures.

Oleic acid, iron(III) acetylacetonate (Fe(acac) ), 1,2-hexadecanediol (1,2 HDD), benzyl ether, 3

oleylamine, PF127 (12000 g mol ), phosphate-buffered saline (PBS, pH 7.4) were obtained from -1

Sigma-Aldrich Ch. Co., Inc., USA and were used as received. Heavy water (D O) was used as 2

aqueous solvent. Sample preparation. Synthesis of Fe O nanoparticles. Fe O nanoparticles were synthesized 3

4

3

4

under argon atmosphere through thermal decomposition of Fe(acac) in benzyl ether in the 3

presence of oleic acid and oleylamine. In this procedure, 0.5 mmol Fe(acac) and 4 mmol 1,23

HDD were mixed in 70 ml of benzyl ether as a solvent. Oleic acid and oleylamine (both 0.5 mmol) were used as surfactants in order to obtain well dispersed nanoparticles. After stirring for an hour, the solution was kept at 180° C for 3 days in a Teflon lined autoclave which was subsequently cooled to room temperature. A black colored solution was obtained indicating the complete reduction and hence formation of Fe O nanoparticles. Ethanol was added to the 3

4

solution to precipitate the 10 nm Fe O nanoparticles and then separated by centrifugation at 3

4

12000 rpm for 15 min and finally dispersed in hexane. Synthesis of F127@Fe O nanoparticles. In order to prepare PF127 coated water-soluble iron 3

4

oxide nanoparticles, 5 ml of a hexane dispersion of as-synthesized nanoparticles (10 mg/ml) was mixed with 10 ml of 500 µM solution of PF127 in 10Χ phosphate buffered saline (the concentration is higher than critical micelle concentration (CMC); CMC of PF127 is 0.007 g/cm

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at room temperature). The mixture was vigorously stirred together at 6°C for 1 hour. The 21

obtained solution was dried under argon gas flow overnight, resulting in a fine powder of pluronic coated magnetite nanoparticles, which is readily dispersible in water. Unbound PF127 polymers were then removed by dialyzing the solution of nanoparticles against 1 L D O with a 2

cellulose membrane (Mw. cut-off: 25 kDa) for 48 hours. The particles were recovered by ultracentrifugation at 25000 rpm for 30 minutes at 10 °C. The supernatant was discarded, and the sediment was redispersed in 10 ml D O by sonification. The suspension was centrifuged as 2

above, and the sediment was washed three times with D O. We expect less than 10 % of unbound 2

pluronic left after this cleaning procedure. Agglomerated particles if any were removed by filtration. Characterization. Figure 2 shows the result of a macroscopic test to confirm the phase transfer of the nanoparticles. As synthesized nanoparticles are only soluble in nonpolar organic solvents, represented by black/brown coloring of the hexane layer. After phase transfer, the nanoparticles are in the aqueous phase.


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Figure 2. Phase transfer of nanoparticles after coating with PF127. Before phase transfer assynthesized nanoparticles are soluble in hexane. After coating with PF127 the nanoparticles are transferred to the water phase.

The structure, morphology and size of the crystallites were investigated by a Tecnai T30 Transmission Electron Microscope (TEM). A carbon coated 200 mesh copper specimen grid (Agar Scientific Ltd. Essex, UK) was glow-discharged for 1.5 minutes. One drop of diluted sample solution was deposited on the grid and left to stand for 2 minutes. After, excess fluid was removed with a filter paper. The grids were then air-dried at room temperature and examined with an electron microscope. Images (Figure 3) of the nanoparticles before and after phase transfer show that the particles are monodisperse both in hexane and water. There was no significant change in the size/ shape of the particles and no sign of agglomeration as a result of the phase transfer.


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Figure 3. TEM images before and after phase transfer for samples S1 and S2.

The hydrodynamic diameter and the organic coating were studied before and after phase transfer by Dynamic Light Scattering (DLS). The measurement was carried out in a Zetasizer Nano-ZS (Malvern Instruments Corp.) at 27°C in polystyrene cuvettes with a path length of 10 mm. The autocorrelation functions of the scattered intensity were fitted using cumulants analysis to exchange the average translational diffusion coefficient. The hydrodynamic diameters were determined by the Stokes-Einstein relation . For all the measurements, the samples were diluted 22

in de-ionized water to ~ 0.01 mg/ml and sonicated using a water bath sonicator for 10 minutes.


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The particle sizes as determined from TEM and their average hydrodynamic diameter as measured from DLS before and after phase transfer, are tabulated in Table 1.

Table 1. Size of nanoparticles as determined through TEM and DLS. Sample

Fe O ↦ F127@Fe O (S1) 3

4

3

Fe O ↦ F127@Fe O (S2) 3

Particle size from TEM (nm)

Pluronic shell diameter (nm)

Before phase transfer

After phase transfer

10 ± 0.8

13 ± 0.61

36 ± 0.91

23 ± 0.3

30 ± 1.5

37 ± 2.1

71 ± 1.4

34 ± 1.3

4

4

3

Hydrodynamic diameter (nm)

4

DLS results show the average hydrodynamic diameters of Fe O nanoparticles in hexane are 13 3

4

± 0.61 nm (PDI = 0.21) and 37 ± 2.1 nm (PDI = 0.22) while the particle diameter as determined from the TEM images are 10 ± 0.8 nm and 30 ± 1.5 nm, respectively. This discrepancy in particle size measured by DLS and by TEM is frequently observed in the case of a hydrophilic polymer layer coated on a nanoparticle surface causing an increase in the average hydrodynamic diameter extracted from DLS measurements.

23,24

After phase transfer, the hydrodynamic diameter

increased to 36 ± 0.91 nm (PDI = 0.26) for S1. The diameter of PF127 micelles is ~23 nm,

25

therefore the increase in hydrodynamic diameter is consistent with single nanoparticles coated with a layer of PF127. For S2 the hydrodynamic diameter measured by DLS is 71 ± 1.4 nm (PDI = 0.32).


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A spectroscopic analysis of the coatings, was carried out on a PerkinElmer instrument, Spectrum One AT-FTIR. The infrared vibration spectra for PF127 and F127@Fe O are shown in 3

4

Figure 4. In both samples, the Pluronics show very intense peaks close to 1107 cm . These peaks -1

are related to the CH rocking and C-O-C stretch vibrations of Pluronic. The peaks observed at 3

2

2841 cm and 2893 cm are related to symmetric (C-H) and asymmetric (C-H) stretching from -1

-1

CH chain assigned to oleic acid, respectively. A peak corresponding to the C=0 asymmetric 2

26

stretch of the ester of oleic acid was observed at 1720 cm . The peaks from both oleic acid and -1 26

PF127 were clearly identified in the spectrum of F127@Fe O , indicating the presence of both 3

4

molecules on the particle surface.


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Figure 4. Fourier transform infrared spectroscopy of PF127 and PF127-coated nanoparticles.


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Neutron reflectivity (NR) measurements. Neutrons are a unique probe for studying solidliquid interfaces. Their spin and low energy make them sensitive to the magnetic induction in solids and suitable to investigate lattice vibrations and diffusive processes. Since, they interact with the nuclei via the weak interaction, they are characterized by a weak absorption, resulting in a high penetration power, for many engineering materials like, e.g. silicon, aluminum or sapphire. As a result, the density profiles of liquids close to solid surfaces become accessible. Neutron reflectivity, R(Q), is defined as the ratio between the intensity of the reflected beam with respect to the intensity of the incident beam, at the specular condition (angle of the incident beam = angle of the outgoing beam). Reflectivity in the specular condition provides information such as the thickness, composition, and roughness of layers parallel to the interface and is typically plotted versus the momentum transfer: &'

𝑄 = % ( ) sin (𝜃0 )

(1)

where λ is the wavelength of the neutrons and θ is the incident beam angle. The reflectivity can i

be calculated from the transmission and reflection coefficients of each layer with the refractive index calculated from the scattering length density (SLD), ρ: ρ = ∑ n4 b 4

(2)

where n is the number density for atoms of element i and b is the bound coherent scattering i

i

length. The sum is taken over all isotopes in a layer. The bulk SLD values of the components of the sample under study are, Fe O = 6.91 x 10 nm , D O = 6.33 x 10 nm and PF127 = 0.48 x 10 3

-4

4

-2

2

-4

-2

-4

nm .

-2 10,11

Neutrons are sensitive to different isotopes of the same element, therefore, our sample with 18.5 % (in weight) of PF127 was prepared with deuterated water for an increased scattering contrast (molar ratio equivalent to 20 % F127 in H O) at a low temperature of 6 °C under constant stirring 2


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until a homogeneous solution was obtained. Two functionalized single-crystalline silicon (100) substrates (50 X 50 X 10 mm, optically polished, obtained from CrysTec, Germany) were used for the experiments. In order to provide high surface energy, one of the two wafers was chemically cleaned in freshly prepared piranha solution [50/50 (v/v) H SO (concentrated) and 2

4

H O (30% aqueous)], resulting in a wetting surface with a surface energy of approximately γsl = 2

2

70 mJ m . The second silicon wafer was cleaned by the same method before chemically grafting -2 27

a monolayer of octadecyl trichlorosilane (OTS) onto it, resulting in a surface energy of γsl = 19 mJ m .

-2 28

The sample was then filled into a pre-cooled sample cell in the liquid state (T < 14 °C) to prevent shearing of the crystalline phase. The liquid cell confined the sample with the surface 18

treated silicon on one side and teflon well (with inlet/outlet) on the other side. Both the teflon well and the silicon wafer were stacked together by aluminum plates on either side, providing temperature control from a chiller. Sample cell volume is 2.8 ml. The design of the cell is described in reference. The total liquid thickness was 1 mm, however, due to scattering 29

geometry the neutron beam penetrates only few tenth of a micrometer from the silicon surface into the liquid. All measurements were done at a temperature of 27 °C, slightly above the critical temperature for crystallization. At this temperature relatively large but randomly oriented crystallites are present and the sample is highly sensitive to shear alignment. 35

Measurements were carried out at the reflectometer MAGIK (vertical geometry reflectometer) at the NIST Center for Neutron Research (USA) with a neutron wavelength of 0.5 nm ± 1%.

30

Figure 5(a) shows the sketch of the experimental setup along with the liquid cell. The sample was subjected to an in-plane magnetic field of 120 mT applied using two neodymium magnets (50 X 50 X 25 mm) mounted at the top and bottom of the liquid cell, also shown in Figure 5(a).


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The temperature was controlled within ±1 °C by circulating ethylene glycol. Figure 5(b) shows the scattering geometry, schematically. The incident neutron beam penetrates a silicon wafer through the narrow side, gets reflected at the solid-liquid interface, is scattered at the micellar lattice and subsequently registers on a 2D detector. The arrows in the figure symbolize specular reflectivity (incident beam angle equal to the exit beam angle) with information about the density profile along the surface normal.

Figure 5(a). Sketch of the experimental setup showing the solid-liquid cell along with the permanent magnets mounted at the sides. The neutron beam is indicated by the yellow arrow. The silicon crystal is shown in green. Figure is modified from reference. (b) Schematics for the 29

scattering geometry depicting the incident and reflected neutron beam (k , k ) and the sample i

f

assembly. Qz is the wave vector momentum transfer perpendicular to the interface.


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RESULTS AND DISCUSSION Figure 6. shows the reflectivity of the 18.5% solution of PF127 in deuterated water taken on MAGIK at the hydrophilic (pirhana treated) and hydrophobic (OTS covered) substrate. The data were corrected for the footprint and normalized to the incident beam intensity using the standard data processing software tool Reductus. In addition to the region of total external reflection 31

visible at small Q values (below 0.1 nm ), two Bragg reflections are visible for the data taken at -1

27°C. The pronounced reflection at Qz ≈ 0.38 nm (d spacing of 16 nm) is the first-order Bragg -1

reflection corresponding to the (111) reflection in the fcc structure (cubic close packing), and 17

consistent with the findings on bulk samples The unit cell size (a) and the hard sphere diameter 20

(D , the distance between adjacent micelle centers) can be estimated from the (111) reflection as HS

7'

7'

𝑎 = 8

999

√ℎ7 + 𝑘 7 + 𝑙 7 = 8

999

√3

'√C

𝐷AB = 8

(3) (4)

999

From Eqs. (1) and (2), we estimate a = 28.5 nm and D = 20.25 nm respectively. These values HS

are in line with literature.

28,32

At Q ≈ 0.38 nm , a huge difference in the intensity of the (111) z

-1

Bragg reflection is clearly visible. The intensity is increased for the hydrophobic interface, showing layers of adsorbed micelles.


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Figure 6. Neutron reflectivity of the sample measured in the crystalline phase. Apart from the significant difference in the (111) and (222) Bragg reflection, the reflectivity curves look identical and are dominated by small angle scattering. Therefore, we will focus on 33

the discussion of the Bragg reflections only in the remaining part of the manuscript. For the first order specular (111) reflection at Q = 0.38 nm , subsequent rocking scans were -1

z

taken with (i) PF127 and (ii) PF127 + Fe O @F127 particles solutions (S1), without and with an 3

4

externally applied magnetic field of 120 mT, against the hydrophilic and hydrophobic silicon wafer. For S2, the data were only measured for the hydrophobic surface. Figure 7 shows the


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result for (i) Pluronic F127 (black squares) and S1 (without and with applied magnetic field, blue and red squares respectively) at the hydrophobic surface.

Figure 7. Exemplary rocking curves of the (111) Bragg reflection at Q = 0.38 nm for PF127 z

-1

and F127 + PF127@Fe O (S1) samples at hydrophobic substrate at 27°C. Lines represents fits to 3

4

the data.

The rocking curve consists of two components reflecting different length scales. In the Born approximation with a cutoff length , the in-plane pair correlation function is represented as the 34

combination of two parts: 𝐶(𝑅) = 〈𝜌(𝑂)𝜌(𝑅)〉 − 〈𝜌(𝑂)〉7

(5)


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where R symbolizes the in-plane distance, C is the height-height correlation function, and ρ is the scattering length density. Accordingly, the in-plane scattering function separates into two parts as well: 𝑆LML (𝑸) = 〈𝜌(0)〉7 ∫ 𝑒 0𝑸𝑹 𝑑 T 𝑅 + ∫ 𝐶(𝑹)𝑒 0𝑸𝑹 𝑑 T 𝑅

(6)

The scattering function becomes the sum of specular reflection (first summand) and the diffuse scattering (second summand). The specular reflection is related to scattering from the mean potential, averaged along the in-plane direction and over the coherence volume of the neutron beam. It reveals long range orientational correlations (LROC). The diffuse scattering reflects the structural correlations resulting from scattering length density fluctuations parallel to the interface on length scales smaller than the coherence length of the beam, which are on a length scale in the micrometer range providing information on the crystallite size . 34

35

To extract information on the alignment of the micellar crystals with and without magnetic nanoparticles at the micellar core and on the relaxation of the long-range orientational correlations (or layering) after applying a magnetic field, the integrated intensity of the (111) reflection can be defined as order parameter. One single Gaussian line with fixed position, width (resolution limited), and background was fitted to the narrow component (fits shown in Figure 7). The broad component was fitted by a Gaussian with variable width. The results are tabulated in Tables 2 and 3. Table 2. Intensities of the narrow and the diffused components for PF127 against hydrophobic and hydrophilic surface, along with FWHM values for the diffuse components. Specular Intensity (10 )

Diffuse

-2

Intensity (10 ) -2

F127 against

6.91 ± 1.19

2.20 ± 0.2

FWHM (10 nm ) -4

-1

2.12± 0.32


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hydrophobic surface F127 against hydrophilic surface

2.21 ± 0.23

0.34 ± 0.25

2.19 ± 0.12

Table 3. Specular and diffuse intensities (along with FWHM values) against hydrophobic (for S1 and S2) and hydrophilic surface (for S1) under applied field (120 mT) and no field condition. Without Field Specular

With Field (120 mT)

Diffuse

Specular

(10 )

Diffuse

(10 )

-2

-2

Intensity (10 ) -2

FWHM (10 nm ) -4

Intensity (10 )

FWHM (10 nm )

6.80 ± 0.57

3.35 ± 0.35

1.87 ± 0.12

-1

ABSENT

-2

-4

-1

S1 against hydrophobic surface

4.71 ± 0.75

ABSENT

S1 against hydrophilic surface

1.85 ± 0.10

0.31± 0.06 1.64 ± 0.40

7.01 ± 0.72

2.56 ± 0.26

2.33 ± 0.27

S2 against hydrophobic surface

1.38 ± 0.35

ABSENT

6.68 ± 0.78

2.54 ± 0.25

2.00 ± 0.27

ABSENT

The intensity of the (111) reflection is higher for PF127 sample at the hydrophobic interface (Table 2). The micelles self-assemble better at the hydrophobic interface compared to hydrophilic one, which is in line with literature.

17,36,37

This is surprising since the micellar corona is

more hydrophilic than the core. Yet, the result can be explained by the fact that the micellar shell is still less polar than the water itself.


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The diameter of the largest spherical entity fitting into an interstitial cavity of an FCC crystal can be calculated geometrically (from the lattice parameter) to 41.4% and 22.5% of the size of the micelles for the octahedral and the tetrahedral sites, respectively. Given that the magnetic micelles with their hydrodynamic diameters (36 nm and 71 nm) are significantly larger than the lattice parameter and the dimension of the interstitial cavities, they push the smaller nonmagnetic micelles resulting in a significant interdigitation between the segments of the nearby coronas and the formation of defects. With the size of the magnetic micelles even bigger, they change the lateral correlations further and the structure becomes richer in defects. Thus, the crystal structure is perturbed/distorted. Resultantly, the presence of magnetic micelles decreases the amplitude of the narrow component (without field column in Table 3) for both surface potentials as compared to the intensities for PF127. In the present case, the number of magnetic micelles is low as compared to non-magnetic micelles. It can be seen that the specular intensity for S1 is significantly larger for the hydrophobic surface as compared to hydrophilic surface. This is consistent with the findings of the samples without magnetic nano-particles. However, for the hydrophobic surfaces, the specular intensity is even further reduced for S2 incorporating significantly bigger magnetic micelles. The decrease in intensity can be attributed to a more distorted crystal structure as explained above. Given that the SLD of Fe O is close to D O and far from that of PF127, a larger 3

4

2

decrease in amplitude of the narrow component for S2 against the hydrophobic surface as compared to S1 might be attributed to the presence of the larger Fe O content in the core of the 3

4

pluronic micelle owing to its larger nanoparticle size as compare the nanoparticle incorporated in S1, however, in the present case, the number of magnetic micelles is low (less than 5 % of the micelles are magnetic). The structure factor stems predominantly from the non-magnetic


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micelles with the magnetic ones acting as defects. For S1 and S2, the intensity scattered diffusely into the (111) reflection also decreases or is even absent with the presence of magnetic micelles as compared to the diffuse intensities for PF127 further supporting the discussion presented above. The FWHM values representing the lateral correlations parallel to the interface also reduces for S1 and S2. Building onto the studies of Clark and Ackerson in the early 1980s,

38-40

it was shown by Eiser et

al. that when colloidal crystals of pluronic polymer are sheared, they undergo a structural transition such that the close packed direction aligns parallel to the flow direction and the planes of highest density stack parallel to the shear plane. This results in macroscopic alignment of the 41

micellar cubic crystal similar to the one close to a solid substrate . Under an applied magnetic 42

field, the magnetic micelles move along the field lines and self-assemble due to the magnetic dipole interaction. As a result, they slide past the non-magnetic PF127 micelles as their spatial arrangements are controlled by steric repulsion and not by covalent or other strong bonds. This motion of the big magnetic micelles under the application of a magnetic field generates a microshear effect. In other words, under magnetic field the magnetic micelles may phase separate or even segregate due to magnetic dipolar interaction. This results in a macroscopic motion of micelles and gives rise to micro-shear effect. This, causes the non-magnetic micelles to selfassemble and demonstrate improved crystallinity. Simple shear has been shown to anneal lattice defects and to align the colloidal crystals so that the bulk of the particles are part of the same crystallites and have the same macroscopic orientation.

43,44

Similarly, the micro-shearing effect caused by the motion of magnetic micelles

leads to an improved crystallinity of the pluronic micelles. A strong influence on the (111) reflection is seen after application of a magnetic field of 120 mT (see Table 3) for samples S1


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and S2. The amplitudes of the narrow components become predominant denoting an improved layering under magnetic field. The maximum specular intensity for S1 is nearly similar for both the surface potentials. This indicates that the micro-shear effect is predominant, similar to macroscopic shear at higher shear rates , and the influence of the interface becomes negligible. 28

Both the samples showed higher diffusely scattered intensity with the applied magnetic field. This indicates that the size of the crystallites grows together with the formation of better layering . 35

Figure 8 is a model illustration of how the crystallization in the polymeric crystal incorporating small amounts of magnetic micelles proceeds under magnetic field. At first, PF127 crystallites and crystallites incorporating magnetic nanoparticles are distributed at the solid interface, illustrated in Figure 8(A). Under magnetic field, F127@Fe O crystallites move past the non3

4

magnetic micelles following the field lines eventually leading to a micro-shear induced longrange order structure transition similar to macroscopic shear, Figure 8(B).


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Figure 8. Model illustration for the crystallization of F127@ Fe O crystals under magnetic 3

4

field. (a) PF127 and F127 @ Fe O crystallites in D O in the absence of magnetic field, (b) under 3

4

2

in-plane magnetic field of 120 mT, F127@Fe O crystallites move following the field lines and 3

4

possibly phase-separates thereby generating a micro-shear effect which helps in tuning the selforganization of crystal.

CONCLUSIONS In summary, we have demonstrated the effect of introduction of small amounts of magnetic polymer nanocomposites (Pluronic F127 @ Fe O nanoparticles) on the crystallization behavior 3

4

of PF127. Introducing large magnetic particles does suppress the crystallization due to distortions and the formation of defects in the cubic lattice but the application of a magnetic field assists the self-assembly due to a micro-shear effect. Thus, the organization of crystals can be tuned by applying magnetic field. This microscopic shear orientation caused by magnetic field is expected to provide a valuable tool in the design of novel nanostructured materials with controllable properties. We expect research following along the presented line to contribute to a deeper understanding of crystallization and magnetic field induced self-assembly. Additionally, our work will have an impact on optimization in templating nanostructures. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Apurve Saini: 0000-0001-6066-4616


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Max Wolff: 0000-0002-7517-8204 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by the grants STINT (contract No. IG-2011-2067) and Swedish research council (contract No. A0505501). The authors also acknowledge NCNR, NIST for the beam time itself as well as the support during the measurement. The authors thank Julie Borchers and Brian Maranville for their support with the instrumentation, software and fruitful discussion. Authors would also like to thanks University Science Instrumentation Centre (USIC), University of Delhi, for help with the electron microscopy and light scattering measurements. REFERENCES 1

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Cover Art caption Small quantity of magnetic micelles formed by pluronic F127 surfactant @ Fe O nanoparticles 3

4

generate a micro-shear effect under an applied magnetic field, thereby, resulting in an improved structural ordering of pluronic F127 micelles at Si interface.


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Macroscopic Alignment of Micellar Crystals with Magnetic Micro

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