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Particle-Matrix Interaction in Cross-Linked PAAmHydrogels Analyzed by Mössbauer Spectroscopy Joachim Landers, Lisa Roeder, Soma Salamon, Annette M. Schmidt, and Heiko Wende J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03697 • Publication Date (Web): 17 Aug 2015 Downloaded from http://pubs.acs.org on August 24, 2015

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The Journal of Physical Chemistry

Particle-Matrix Interaction in Cross-Linked PAAmHydrogels Analyzed by Mössbauer Spectroscopy

Joachim Landers 1, Lisa Roeder 2, Soma Salamon 1, Annette M. Schmidt 2, and Heiko Wende 1* 1

University of Duisburg-Essen, Faculty of Physics and Center for Nanointegration Duisburg-

Essen (CENIDE), 47048 Duisburg, Germany 2

Department Chemie, Institut für Physikalische Chemie, Universität zu Köln, Luxemburger Str.

116, 50939 Köln, Germany

Keywords: Brownian motion, rheology, ferrohydrogels, anisotropic nanoparticles, Mössbauer spectroscopy

ABSTRACT

The constrained motion of spindle-shaped hematite nanoparticles of about 400 nm in PAAmhydrogels with different degrees of cross-link density is measured utilizing the line broadening observable in Mössbauer spectra at 265 K – 293 K. A slight decrease of nanoparticle mobility is

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observed upon increasing cross-link density. Mössbauer spectra of the same nanoparticles in 60 wt% sucrose solution, used as reference material, display line broadening of the same magnitude as the hydrogel samples, indicating a similar degree of motion at the atomic scale and the time scale of the Mössbauer experiment. AC (alternating current) susceptibility data indicate that the magnetic relaxation of the nanoparticles in sucrose solution mainly occurs by Brownian motion, while the absence of magnetic loss within the investigated frequency range observed in measurements on hydrogel samples reveals very limited particle mobility. This apparent contradiction between results on particle dynamics in hydrogels by Mössbauer spectroscopy and AC susceptibility measurements is explained in terms of constrained particle mobility at atomic scales.

INTRODUCTION In ferrohydrogels, magnetic nanoparticles are incorporated into a hydrogel matrix to create tunable magnetoresponsive behavior. The transfer mechanism between particles reacting to an external magnetic field and the macroscopic response of a hydrogel or elastomeric matrix is determined by the type and strength of the particle-matrix interaction. This makes ferrohydrogels and ferroelastomers a versatile and promising system for applications as e.g. microscale actuators, sensor devices and medical application.1-4 Despite widespread interest in ferrohydrogels over the last years, it is not completely understood in which way and to which degree the hydrogel matrix influences the nanoparticular motion on different time and length scales.5-9 While the macroscopic viscosity η of a system can be directly determined by e.g. shear or flow viscosimetry, several experiments indicate the existence of a micro- or nanoviscosity, which becomes relevant when the size of the moving particle is comparable to that of the


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structural matrix units. When constrained in their movement by the local surrounding, the particles display relatively free motion on short time scales, attainable only by ‘fast’ measurement methods with high time resolution. On longer time scales, the constriction of particle movement becomes visible as an increased effective viscosity.10 The main challenge is to obtain detailed information on motion in these small length and time scales. Frequency-dependent measurements of the complex AC susceptibility χ* are a well proven technique to determine the viscosity of e.g. ferrofluidic systems, while Mössbauer spectroscopy stands out due to its extremely short characteristic time window and its ability to probe material properties on an atomic scale. This makes it suitable for the observation and quantification of restricted Brownian motion. Mössbauer spectroscopy has previously been used for studies on molecular dynamics11,12 and diffusive motion of iron ions in solids13,14 as well as of nanoparticles in fluids.15-17 The authors of the latter publications were able to demonstrate, for particles dissolved in several fluids, the direct correlation between dynamics of Brownian motion and linewidths observed by Mössbauer spectroscopy, as described in the following by eq. (2-3) 15,16

, and to quantify the decrease of the diffusion coefficient in case of sufficiently high particle

volume concentration in the fluid.

17

In spite of the considerable experimental effort involved,

there is a limited number of reports on Mössbauer spectroscopy applied to composite hydrogel systems.18,19 In this work, we applied Mössbauer spectroscopy and AC susceptometry to several crosslinked ferrohydrogel samples to study nanoparticle motion. Nanoparticles from the same batch in sucrose solution were used as a reference sample, in which particle motion is dominated by (Newton-like) viscosity. Mössbauer and AC susceptibility data of both types of samples


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(hydrogels and sucrose solution) were compared to determine the influence of the polymer network with different degrees of cross-link density on diffusive particle movement.

EXPERIMENTAL SECTION Chemicals Acrylamide (AAm) and sucrose were purchased from Merck in p. a. quality. Sodium dihydrogen phosphate monohydrate (NaH2PO4⋅H2O, p.a.) was obtained from AppliChem. N,N´Methylene bis(acrylamide) (MBA, p.a.) and tetramethylammonium hydroxide (TMAOH, 25 % in water) were received from Sigma Aldrich. Ammonium peroxodisulfate (APDS) and N,N,N´,N´-tetramethylethylene diamine (TEMED) were obtained from Acros Organics in p. a. quality. Iron(III) chloride hexahydrate (98 %, ABCR GmbH) and citric acid monohydrate (p.a. quality, Jungbunzlauer GmbH) were used without further purification. The synthetic water used for sample preparation was obtained from Acros Organics in pure, demineralized quality. Instrumentation Scanning electron microscopy (SEM) images were taken from samples prepared on a silicon wafer on a Zeiss Neon 40 ESB CrossBeam FIB-SEM. AC susceptibility measurements were carried out using a Quantum Design MPMS-5S SQUID magnetometer with an implemented AC option using an applied magnetic AC amplitude of 4 Oe at temperatures within 270 K - 320 K at frequencies between 0.01 Hz and 1500 Hz. For these measurements the samples were sealed within an airtight 0.2 ml PCR-tube. Mössbauer spectroscopy was performed in transmission geometry using a self-constructed sample holder with an integrated Peltier element, as displayed in Fig. 1. For reliable temperature recordings, a thermo sensor was placed in proximity to the sample on a copper block, which


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connects the gel/fluid sample with the cold side of the Peltier cooling/heating element. Thermal insulation was provided by a polystyrene box enclosing this part of the experimental setup. This setup allows precise temperature control down to 240 K with a temperature instability

Particle–Matrix Interaction in Cross-Linked ... - ACS Publications

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