Thermoresponsive hybrid microgel particles with intrinsic optical and magnetic anisotropy

Abstract

We describe the synthesis of thermoresponsive, magnetic, optically anisotropic and orientable colloidal particles based on poly(N-isopropylacrylamide) hybrid microgels (PNIPAMs) with an embedded ellipsoidal hematite (α-Fe₂O₃) core. Our ability to orient the particles with a magnetic field is demonstrated by small angle X-ray scattering and by optical polarization microscopy.

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Introduction

Stimuli responsive polymers have been extensively developed and studied in recent years, for their potential applications as drug carriers in targeted drug delivery applications,^1,2 for fundamental studies on self-assembling systems³ or on phase behaviour of hard and soft colloidal particles.^4,5 Among these stimuli responsive polymers, poly(N-isopropylacrylamide) (PNIPAM) linear polymers and microgel particles have raised a specific interest for their response to temperature changes.^2,4,6–8 Linear chains undergo a coil-to-globule transition when the temperature is increased to approximately 32 °C, and cross-linked PNIPAM particles show an analogous volume phase transition at the volume phase transition temperature T_VPT ≈ 32 °C. The swelling of the microgel particles can be used to tune the volume fraction of particles with an external trigger,⁸ avoiding the classical problems that arise during sample preparation when working with high volume fraction colloidal suspensions, such as high viscosity and aggregation. Moreover, NIPAM can be copolymerized with a variety of co-monomers bearing various functionalities such as acid groups, charges, fluorescent labels, ligands, groups with thermal response or different solubility, chemically reactive and photo-reactive groups, etc.,⁹ giving the possibility to further tailor the stimulus sensitivity of microgel particles. In addition core–shell structures with multiple sensitivities can be obtained.⁷ The inclusion of a latex core⁸ or one or several inorganic particles in PNIPAM microgels can also be achieved, leading to further possible applications including catalysis,⁸ colloidal crystals,¹⁰ magnetic carriers,¹¹etc.

Moreover, the study of rotational motion in colloidal systems as a probe of local dynamic properties, such as viscosity or elasticity, has gained attention over the last decade. A large number of studies of this type have been reported in the past using particle tracking on confocal microscopes or depolarized dynamic light scattering, on geometrically anisotropic systems such as rod-like particles,^12,13 and on intrinsically optically anisotropic particles, typically colloidal spheres of partially crystalline fluorinated particles.¹⁴ In soft particle systems, optical anisotropy can be used for example to probe rotational motion and shape fluctuations. Since classical PNIPAM microgel particles are optically isotropic, their rotational motion cannot easily be probed directly. A recent study showed that by including anisotropically distributed nanoparticles in PNIPAM some optical anisotropy of the microgel particles can be obtained.¹⁵ Here we propose a different approach. We include a spindle-shaped hematite core in the centre of the microgel beads, thus introducing anisotropy in the distribution of refractive index within the particle with the additional advantage that we can orient the particles by means of an external magnetic field. In principle this should allow one to externally control optical properties of microgel based system, although residual optical absorption will be a problem for very dense systems. Using a magnetic core also opens the possibility to study nonlinear rotational dynamics of spherical particles in a visco-elastic matrix.¹⁶ In addition, optically anisotropic particles are interesting for applications that can exploit externally tuneable properties for optical switching or birefringence.¹⁷

Here we describe a method developed to synthesize these inorganic–organic hybrid particles (Scheme 1), and demonstrate their potential for the study of rotational motion of particles at high effective volume fractions.

Scheme 1 Illustration of our procedure used to embed hematite spindle-shaped particles in PNIPAM microgels. We first add a layer of silica and then modify the surface with reactive silanes allowing the growth of the polymeric shell around the cores.

Synthesis

The synthesis of ellipsoidal α-Fe₂O₃ (hematite) particles with tuneable size and aspect ratio was reported by Matijevic et al.¹⁸ and further developed by Ocaña et al.¹⁹ to reach sub-micrometre scales. The latter method leads to ellipsoidal particles with the long axis corresponding to the c-axis of the crystalline lattice.¹⁹

The particles are coated with a layer of silica using the approach of Graf et al.²⁰ based on an initial adsorption of polyvinylpyrrolidone (PVP) on the particles to improve their colloidal stability and the subsequent addition of tetraethylorthosilicate (TEOS) as a precursor for the growth of the silica shell. This method has been found to be more suitable than the original method using bare particles,²¹ as it allows to reach about 6 times higher yields of coated hematite–silica particles when combined with dispersion by ultrasonic waves.¹²

Magnetic PNIPAM microgel particles are traditionally obtained in two steps. Bare microgel particles are first synthesized, and numerous magnetic magnetite or maghemite nanoparticles are subsequently deposited on the surface^22,23 or synthesized directly in the microgels.¹¹ Our approach, however, consists in growing the microgel around the previously prepared magnetic particle. We use the method reported by Zha et al. to grow a PNIPAM microgel around a silica particle.²⁴ An analogous method has been shown to be efficient to embed spherical inorganic particles covered with a layer of silica.¹⁰ To ensure chemical bonding between the inorganic core and the polymeric matrix, the surface of the silica-coated hematite particles is grafted with 3-(trimethoxysilyl)propyl methacrylate (TPM). N-Isopropylacrylamide (NIPAM) monomers and a cross-linker, N,N′-methylene bisacrylamide (BIS) are then polymerized by precipitation polymerization in water in the presence of the surface-modified inorganic particles (Scheme 1).

Results

Synthesis

The bare hematite particles used as cores have geometrical dimensions given by the respective average length of the long axis, short axis and aspect ratio—defined as the length of the long axis divided by the length of the short axis—of 300 ± 40 nm, 56 ± 6 nm and 5.5 ± 0.6, as measured by transmission electron microscopy. Once adding the thermoresponsive shell, we measure 570 ± 50 nm, 470 ± 40 nm and 1.2 ± 0.2 for the long axis, the short axis and the aspect ratio respectively. We therefore notice that the aspect ratio is considerably reduced and that the particles are almost spherical in the dried state. When swollen in suspension, we expect this ratio to be even smaller, assuming a homothetic deformation of the microgel shell. We use this assumption in particular when determining the hydrodynamic radius by dynamic light scattering.

Small angle X-ray scattering

Over the range of temperatures used in our experiments, hematite particles are weak canted antiferromagnets with a magnetic moment normal to the long axis (c-axis).²⁵ However, for the particles shown in Fig. 1 the cross-linked polymer layer around the hematite spindles acts as a steric barrier. This results in a sufficiently large distance between the magnetic cores where interparticle dipole–dipole interactions become negligible at not too high values of an applied magnetic field. With respect to the effective interaction potential between the particles, the microgels thus behave like isotropic hard spheres and the orientation of each particle is therefore decoupled from the orientation of its neighboring particles.

Fig. 1 TEM image of hematite–PNIPAM core–shell particles, a picture of a higher magnification is presented in the inset (scale bar: 200 nm) showing a microgel with a size of about 550 nm in diameter (dried state). The ellipsoidal hematite core and the silica shell are clearly visible due to their different electron contrast. The red dotted circle in the main panel illustrates the estimated size.

On the other hand we can use their magnetic response to an external magnetic field in order to orient the anisotropic cores along a preferred direction. This alignment is confirmed by the anisotropic scattering patterns obtained by small angle X-ray scattering in a liquid suspension of PNIPAM–hematite hybrid microgels (Fig. 2A). Without magnetic field the scattering pattern is isotropic, confirming the random orientation of the cores. However, when the suspension is subjected to a magnetic field we observe an elliptical scattering pattern. This clearly indicates a preferred orientation of the particles, with the long axis perpendicular to the magnetic field B. Moreover after removing the magnetic field the scattering pattern relaxes to its initial isotropic shape. The absence of memory effect shows that magnetic interactions between the cores are indeed negligible.

Fig. 2 SAXS scattering pattern measured at 24 °C without magnetic field and with a 725 mT magnetic field for (A) freely rotating particles (effective volume fraction φ_eff = 0.27) and (B) dynamically arrested particles (effective volume fraction φ_eff = 0.8). The samples were both prepared initially in the absence of a magnetic field. Dashed lines are guides to the eye. Corresponding orientation of particles is schematically represented next to each pattern. Red arrows show the direction of the magnetic field. Effective volume fractions φ_eff are estimated from number densities n_conf measured on a confocal microscope and hydrodynamic radii r_h measured by dynamic light scattering using φ_eff = 4/3πn_confr_h³.

It has previously been demonstrated that microgels can undergo a hard sphere glass transition around a volume fraction Φ_g corresponding to random close packing (Φ_rcp ≈ 0.64) for temperatures at or below T_VPT. We thus perform the same experiment using a more concentrated dispersion such that the increase of the effective volume fraction at lower temperatures results in an arrested solid phase that corresponds to a so-called squeezed state.^4,6,26 In this case the application of a magnetic field does not affect the isotropy of the scattering pattern (Fig. 2B), which shows that the rotation of particles is inhibited at this high effective volume fraction.

Optical polarization microscopy

As previously demonstrated we can explore the thermal response of the PNIPAM shell surrounding the magnetic core in order to tune the effective volume fraction and reversibly cycle from a dynamically arrested state to a liquid-like state where free rotational motion is restored. Moreover, when combining this with an externally applied magnetic field we can align the magnetic cores in the liquid state prior to cooling the sample. Given the intrinsic optical anisotropy of our hybrid particles, the distribution of refractive indices in the sample thus becomes macroscopically anisotropic and the sample will be birefringent. Quite analogous to suspensions of anisotropic colloidal particles where the alignment of the particles in a liquid crystalline phase induces birefringence we thus expect that we can investigate the macroscopic orientation of the ellipsoidal magnetic cores using optical polarization microscopy.²⁷

We use a polarizing microscope equipped with a heating stage to study the evolution of the alignment of the particles as a function of temperature between 10 °C and 34 °C. In a pre-treatment step we align the particles with an external magnetic field at 35 °C, where the sample exhibits liquid-like behavior, and subsequently quench the sample for ten minutes in this aligned state to a low temperature, 10 °C, where the sample is dynamically arrested. After removing the magnetic field, we increase the temperature in steps of three minutes each and record the transmitted intensity I_VH for crossed polarizers. We average the value over the last minute. The same procedure is repeated without prior magnetic alignment of the particles, and these measurements serve as a baseline I₀ for our analysis. These reference measurements are crucial as the high density of our particle dispersion combined with the temperature-dependent scattering contrast of the microgel shell results in a variable amount of multiple scattering that causes some degree of depolarization even for randomly orientated particles.

We subtract this baseline contribution and obtain a direct experimental quantification ΔI = I_VH − I₀ for the degree of alignment of the magnetic cores. As we increase the temperature ΔI decreases (Fig. 3), indicating that the alignment is progressively lost when the samples go from a jammed arrested state to a liquid-like state.

Fig. 3 (A) Black filled squares: hydrodynamic radius of the hematite–PNIPAM core–shell particles measured by dynamic light scattering (DLS). Red open circles: effective volume fraction φ_eff deduced from DLS data and confocal microscopy. (B) Intensity difference ΔI = I_VH − I₀ as a function of effective volume fraction determined by optical polarization microscopy as a measure of the optical anisotropy of the sample. As a support, corresponding temperatures (in degree centigrade) have been indicated on the upper X-axis.

At the solid to liquid transition ΔI drops to zero, indicating that the random orientation of the particle cores is recovered. This behavior again confirms that there is no magnetic memory effect, supporting the assumption that magnetic dipole–dipole interactions are negligible. The transition occurs around 27 °C, which corresponds to an effective volume fraction of approximately 0.6. Free rotational motion is thus restored at an effective volume fraction that seems to qualitatively coincide with the previously reported glass transition where long-range translational motion becomes arrested. Given the previously postulated subtle differences between the dynamics of classical hard sphere colloids and microgels at and beyond the arrest transition,²⁶ it will be interesting to quantitatively compare the arrest transition for translational and rotational diffusion using our particles. This will allow us to critically re-examine the importance of the classical cage picture for the arrest transition in particles with a partially soft and interpenetrable shell.

Conclusion

In summary we presented for the first time a route to obtain orientable magnetic and thermoresponsive spherical particles with intrinsic anisotropy. These hybrid core–shell particles combine the temperature sensitivity of PNIPAM microgels with the anisotropic and magnetic features of hematite ellipsoids. The possibility to orient particles in the liquid state and the absence of rotational motion at high volume fractions could be demonstrated by small angle X-ray scattering and optical polarization microscopy. Considering the major improvement in terms of control and characterization of rotational motion that the addition of ellipsoidal hematite cores to PNIPAM microgels has led to, this new hybrid system is an ideal candidate for further fundamental studies on model colloids. Combined with the possibility to add functionalities by copolymerization of NIPAM with various monomers, it also has a remarkable potential for different applications involving its magnetic and thermoresponsive properties, for example as controllable tracer particles, in the domain of targeted drug delivery, or as building blocks for stimuli responsive optically anisotropic materials.

Acknowledgements

The small angle X-ray scattering patterns were measured at the cSAXS beam line of the Swiss Light Source at the Paul Scherrer Institute (Villigen, Switzerland), the authors thank in particular A. Menzel for his assistance during the measurements and M. Reufer for his help with the initial data treatment. J.-F. Dechezelles is thanked for his help with the design of the cover picture. We also would like to acknowledge the financial support for the project provided by the Adolphe Merkle Foundation and the Swiss National Science Foundation (project No. 200020-117762/126772/117755).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis details, integrated SAXS data, hydrodynamic radius measurements by dynamic light scattering and details on optical polarization microscopy. See DOI: 10.1039/c000305k

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