Bulk Phase and Surface Dynamics of PEG Microgel Particles

Article pubs.acs.org/Macromolecules

Bulk Phase and Surface Dynamics of PEG Microgel Particles Kornelia Gawlitza,†,§,∥ Oxana Ivanova,‡ Aurel Radulescu,‡ Olaf Holderer,‡ Regine von Klitzing,† and Stefan Wellert*,† †

Stranski-Laboratory for Physical and Theoretical Chemistry, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany ‡ Forschungszentrum Jülich Outstation at the Heinz Maier-Leibniz-Zentrum, Jülich Center for Neutron Science JCNS-MLZ , Lichtenbergstraße 1, 85747 Garching, Germany S Supporting Information *

ABSTRACT: The adsorption of ethylene glycol (EG) based microgel particles at silicon surfaces was studied. Monodisperse p-MeO2MA-co-OEGMA microgel particles were synthesized by precipitation polymerization. Translational and inner dynamics of p-ME3O5 and p-ME3O26 microgels in bulk were investigated by dynamic light scattering (DLS) and neutron spin echo spectroscopy in transmission geometry (NSE). Here, cooperative diffusion and Zimm type dynamics were observed. The inner dynamics of these microgel particles in adsorbed state was explored by neutron spin echo spectroscopy under grazing incidence (GINSES). The reported GINSES experiments demonstrate the feasibility of these surface sensitive measurements. It indicates a slowing down of the inner dynamics in the vicinity of the substrate. At increasing distance from the substrate, the relaxation is comparable to the bulk.

1. INTRODUCTION

Among other potential applications microgel layers at solid interfaces are discussed as new biomaterials and coatings.11 Research efforts aim to link protection from biofilm formation and microbial surface colonization with stimulus controlled drug release. Such coatings could support implant integration into its physiological environment by minimization of infections and inflammation. Unfortunately, for drug delivery systems inside the human body NIPAM containing microgels are of limited applicability.12,13 The NIPAM monomer is carcinogenic or teratogenic. This is an essential drawback in long-term applications. Because of its low cytotoxicity and immunogenicity, ethylene glycol (EG) is a potential alternative for the synthesis of biocompatible microgels. Microgel particles made of the monomer 2-(2methoxyethoxy)ethyl methacrylate and the comonomer poly(ethylene glycol)methyl ether methacrylate (p-MeO2MA-coOEGMA) have been successfully synthesized by precipitation polymerization.14−16 A temperature dependent swelling/ deswelling behavior was found. The LCST of the oligomers of the monomer MeO2MA in water is at ≈20 °C which is significantly below the body temperature. The LCST of the oligomers of the comonomer OEGMA at ≈90 °C is clearly above it. The combination of appropriate amounts of both

Microgels are colloidal particles with an inner polymer network structure based on chemical cross-linking. In a good solvent these particles are highly swollen. Poly(N-isopropylacrylamide) (PNIPAM) cross-linked with N,N′-methylenebis(acrylamide) (MBA) is the most intensively studied group of aqueous thermoresponsive microgels.1 They show a temperaturedependent swelling behavior. It originates from the lower critical solution temperature (LCST) of the oligomers of the monomer NIPAM. At a temperature of around 32 °C, the microgel particles exhibit a volume phase transition (VPT). The swelling/deswelling behavior, the particle structure, and the morphology of neutral and charged PNIPAM microgels have been investigated extensively.2−6 Beside bulk phase studies, the formation of stimuliresponsive surface structures using these microgel particles as colloidal building blocks is an active field of research. In this respect it is essential to achieve detailed knowledge about the properties of single adsorbed microgel particles and layers of adsorbed microgel particles and the control of the layer formation. In such studies usually AFM and ellipsometry measurements are performed. For example, the conservation of the thermoresponsivity in the adsorbed state was demonstrated as well as the influence of the microgel composition in terms of cross-linker content and comonomer addition on particle homogeneity, swelling ability, size and adsorption behavior.4,7−10 © XXXX American Chemical Society

Received: April 21, 2015 Revised: June 25, 2015

A

DOI: 10.1021/acs.macromol.5b00788 Macromolecules XXXX, XXX, XXX−XXX

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methods are available to study motions at interfaces. However, these methods often suffer from low signal intensities and require a large number of interfaces which makes highly curved interfaces preferred here.37 Surface anchored polymers have been investigated in NSE experiments in transmission mode. Surface sensitivity was achieved by using particles or pores as substrates for grafting the polymers.38−40 Neutron scattering methods are again required to access the inner dynamics of adsorbed microgels. A combination of grazing incidence geometry with neutron spin echo spectroscopy (GINSES) was recently established. This opens up new experimental routes for the investigation of the dynamics near solid surfaces in the time range between a few ns up to 100 ns. It enables experiments on shorter time and length scales compared to light scattering. For example, small angle neutron scattering under grazing incidence (GISANS), neutron reflectometry and GINSES were used to determine the near surface structure and dynamics of bicontinuous microemulsions near solid surfaces as a function of the neutron penetration depth.41 In this paper, we report experiments on the dynamics of pMeO2MA-co-OEGMA microgels in bulk and in the adsorbed state. We used dynamic light scattering and neutron spin echo spectroscopy measurements in transmission mode to characterize the translational and inner dynamics in the bulk. Neutron spin echo spectroscopy in reflection geometry under grazing incidence was performed to explore the inner dynamics of adsorbed PEG based microgel particles. The experimental challenge is to scan the dynamics profile within the microgels perpendicular to the substrate. This is of general interest in order to understand the changes in swelling/deswelling behavior of microgels after adsorption at a surface. Here, we report for the very first time about experimental results using this rather new technique to study the inner dynamics of adsorbed microgel particles.

tunes the VPT temperature (VPTT) close to body temperature (≈37 °C).17 Microgel particle suspensions served as model systems for the dynamics of soft colloids. Mainly optical methods and light scattering were used in these studies. For example, by tuning the pH the interaction between fluorescently labeled PNIPAMco-AAc was tuned from attractive to repulsive. The dynamics in these suspensions was studied by 3D confocal microscopy.18 In a wide concentration range of such suspensions many phases in the fluid, crystalline and glassy state were observed.19−21 It was found that swelling/deswelling of ionic microgels is not only temperature dependent but also a function of the particle concentration.22 Because of the colloidal size of the microgel particles, light scattering only detects the translational diffusion but does not provide direct access to the inner dynamics. To this end, quasi- and inelastic neutron scattering such as neutron spin echo spectroscopy (NSE) are well-suited. Thermally excited cooperative diffusion of the polymer network and Zimm-type dynamics (on length scales smaller than the mesh size) contribute to the observed inner dynamics. Several studies investigated the effect of the microgel composition in terms of copolymer content and cross-linker concentration. For example, a decrease of the cooperative diffusion coefficient Dc with increasing cross-linker concentration was found.23 A comparison of chemically identical PNIPAM micro- and macrogels revealed that the cooperative diffusion and the accociated dynamic correlation length are very similar.24 Recent studies explored the influence of the cononsolvency on the dynamics.25,26 Beside the cooperative diffusion, also Zimm-type dynamics at higher values of the momentum transfer Q were reported. Beside NSE also inelastic neutron scattering was used to access the dynamics of PNIPAM microgels. Inelastic neutron scattering revealed a Debye−Waller type of motion and a translational motion with a diffusion coefficient decreasing above the LCST. PFG-NMR suggested the presence of two different diffusion components assigned to regions with high and low cross-linking densities.27 Experiments addressing the corresponding inner dynamics in the adsorbed state are challenging. Many experimental techniques provide only indirect access to the dynamics in the vicinity of a solid interface. The analysis of the thermal noise spectrum of a thermally activated scanning-forcemicroscope cantilever upon approach provides information about the dynamics of a polymer film at a solid surface. Here, excitation energies of a few kBT generate excitation amplitudes of a few Å. This method was used to study neutral polymer and polyelectrolyte brushes in response to variations of the environment.28,29 So far, mainly the dynamics of polymer brushes or surface attached gel layers was investigated by dynamic light scattering (DLS) in rare experimental studies, e.g.30,31 Here, evanescent wave dynamic light scattering techniques provide access to the near surface dynamics in the micro- and millisecond time range. This allows the investigation of relaxation modes of concentration fluctuations in polymer brushes, e.g.32,33 By means of fluorescence correlation spectroscopy the dynamics in hydrogel layers was investigated by monitoring the translational mobility of molecular tracers.34 In such experiments, diffusion times in the microsecond to millisecond range were observed. X-ray photon correlation spectroscopy was used to study the height fluctuations of linear35 and branched36 polystyrene chains attached to solid surfaces. Additionally, IR spectroscopic

2. EXPERIMENTAL SECTION 2.1. Materials. 2-(2-Methoxyethoxy)ethyl methacrylate (95%) (MeO2MA), poly(ethylene glycol) methyl ether methacrylate (average Mn = 500 g/mol) (OEGMA), ethylene glycol dimethacrylate (≥99%) (EGDMA), and potassium peroxodisulfate (≥99%) (KPS) were purchased from Sigma-Aldrich (Munich, Germany). The sugar surfactant Glucopon 220 was a gift from Henkel. All chemicals were used as received. A three-stage Millipore Milli-Q Plus 185 purification system was used for water purification. 2.2. Microgel Synthesis. Microgel particles based on the monomer 2-(2-methoxyethoxy)ethyl methacrylate (MeO2MA), the comonomer poly(ethylene glycol) methyl ether methacrylate (OEGMA), and the cross-linker ethylene glycol dimethacrylate (EGDMA) were synthesized by precipitation polymerization (pMExOy, where x is mol % of cross-linker and y is mol % of comonomer with respect to the amount of monomer).16 The sugar surfactant Glucopon 220 was used to obtain higher monodispersity. While keeping the amount of cross-linker (3 mol %) and surfactant (0.9 mol %) constant the amount of comonomer was changed from 5 to 26 mol %. Briefly, 1.205 g of MeO2MA (6.00 mmol), 0.038 g of EGDMA (0.19 mmol, 3 mol %), 0.020 g of Glucopon 220 (0.05 mmol, 0.9 mol %), and the desired amount of OEGMA were dissolved in 100 mL of water in a three-neck flask. The temperature of the solution was increased to 70 °C and degassed for 30 min. Afterward, 1 mL of an aqueous solution of KPS (0.74 mM) was added to the mixture while stirring continuously. After 4 h of reaction time the temperature was decreased to room temperature and the mixture was stirred overnight B


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scattering geometry.41,45 This method requires a well-defined angle of incidence αin to provide a defined depth of neutron penetration Λi into the sample. Therefore, the entrance aperture is reduced to 2 mm. A neutron wavelength of 8 Å was used. The sample was a single crystal silicon block of 50 mm × 80 mm × 15 mm covered with adsorbed PEG microgel particles. The sample cell consists of a Teflon trough, filled with D2O and closed with the single crystal silicon block in a thermostated aluminum housing. This cell was placed vertically in the collimated neutron beam. Both front ends of the cell were covered by cadmium shielding to prevent bulk phase scattering. Figure 1 illustrates

under N2-atmosphere. The crude microgel particles were purified by filtering over glass wool, dialysing for 2 weeks with daily water exchange and finally freeze-drying the particles at −85 °C and 1 × 10−3 bar for 48 h. 2.3. Scattering Measurements. 2.3.1. Dynamic Light Scattering (DLS). The translational diffusion properties of the PEG microgel particles were determined by dynamic light scattering (DLS). The intensity autocorrelation functions were recorded at several scattering angles using an ALV/CGS-3 compact goniometer system equipped with an ALV/LSE-5004 correlator and a He−Ne laser (λ = 632.8 nm, 35 mW). Data analysis was done using the inverse Laplace transformation algorithm (CONTIN42). The measurements were carried out at 15 °C using a Huber thermostat. 2.3.2. Small Angle Neutron Scattering (SANS). SANS measurements were carried out at the KWS-2 instrument of the JCNS at the FRM-II research reactor at the Heinz Maier-Leibnitz Zentrum (Garching, Germany). Three sample-to-detector distances (2.05 m, 8 m, and 20 m) were combined to cover a Q-range from Q = 0.003 Å−1 to 0.3 Å−1. Here, Q is the momentum transfer given by Q = 4π sin(θ/2) with θ beeing the scattering angle. The neutron beam (λ = 5 Å) collimation was adapted to the sample−detector distances. Samples of the microgel systems p-ME3O5 and, p-ME3O26 were dispersed in D2O and adjusted to a mass concentration of 2 wt %. The samples were measured in quartz cells with 2 mm neutron path way. Details of raw data treatment and further analysis using the SasView 2.2.1 software can be found in our previous work.43 In Table 1 the bulk structure data obtained from DLS and SANS are summarized.

Figure 1. Left: Scheme of the grazing incidence neutron spin echo experiment. A neutron beam impinges onto a silicon substrate covered with a sample layer under an angle of incidence αin. Below the critical angle of total reflection αc an evanescent wave is generated, penetrates the sample to a depth Λi and scattered from the sample layer. The scattered intensity I(Q,τNSE,z) is detected at αdet. Right: Number of neutron counts per second arriving at the detector plane, here a function of αin. Mainly, coherent intensity is detected.

Table 1. Summary of the Results Obtained by DLS and SANS Measurements on Samples of p-ME3O5 and p-ME3O26 at 15° Ca sample

RH [nm]

R [nm]

σF [nm]

ξ1 [nm]

ξ2 [nm]

p-ME3O5 p-ME3O26

145 218

35.0 100.4

67.4 35.5

1.6 1.1

11.2 9.9

the GINSES scattering geometry (left side). The neutron beam enters the sample from the silicon block and is reflected at the interface. The critical angle of total reflection αc depends on the chosen neutron wavelength λ and the scattering length density contrast Δρ. It is given by αc = λ Δρ /π . At an angle of incidence αin < αc an evanescent neutron wave of intensity Iev(z) penetrates into the sample layer. The depth of penetration Λi is given by

a The hydrodynamic radii RH were obtained from DLS. From SANS the radius R of the core, the fuzzyness σF and the correlation lengths ξ1 and ξ2 were determined for bulk samples (for further details see ref 43).

Λi =

2.3.3. Neutron Spin Echo Spectroscopy in Transmission Geometry (NSE). The NSE measurements on the bulk phase samples were carried out using the J-NSE instrument in transmission scattering geometry FRM-II research reactor at the Heinz Maier-Leibnitz Zentrum (Garching, Germany).44 A neutron wavelength of 8 Å was used. The neutron path way in the Hellma quartz cells was 2 mm. All samples were measured at 15 °C to ensure the swollen state. The dry PEG microgel particles were dispersed in D2O at a concentration of 12 wt % to suppress center of mass diffusion during the NSE measurements. The measured intermediate scattering functions S(Q, τNSE)/S(Q, 0) consist of the coherently scattered neutrons with spin up Icoh(Q, τNSE) and the incoherently scattered fraction Iinc(Q, τNSE)

4π Δρ(1 − αi 2/αc 2)

(2)

The calculation of αc and Λi uses the scattering length densities (SLDs) of the substrate and the sample layer in the vicinity of the substrate. In microgel particles a gradient in polymer density from the particle core region to the outer shell occurs which is pronounced vertical to the substrate. Close to the substrate a compression of the network can occur which increases the polymer density at the interface. A particle SLD is estimated by averaging the SLDs of the polymer components (ρMeO2MA ≈ 0.7 × 10−6 Å−2 and ρOEGMA ≈ 1.1 × 10−6 Å−2). The coverage of the substrate by adsorbed microgel particles was determined from AFM measurements as discussed in section 3.2.1. Up to αc the intensity of the evanescent wave increases with increasing αin. The evanescent neutrons are scattered at the sample interface. The scattered intensity I(Q,τNSE,z) is detected at αdet. The right side shows the scattered spin coherent and incoherent elastic intensity at Q = 0.08 Å−1 as a function of αin. In the spin-up configuration (all polarization flippers off) no spin manipulation occurs and mainly the coherent contribution is measured. Contrary, the spin-down configuration (180°-polarization flip at the sample position) is dominated by incoherent scattering. At increasing angle of incidence αin the scattered intensities increase until αc is reached. 2.4. Atomic Force Microscopy Measurements (AFM). Aqueous microgel suspensions with concentrations of 5 wt % were deposited onto pure silicon wafers without additional supporting polyelectrolyte layer. The substrates were cleaned using RCA-1 solutions (NH3, H2O2, and H2O in the ratio 1:1:5) at 70 °C for 20 min. After rinsing the wafers with ultrapure Milli-Q water and drying under nitrogen flow the microgel particles were spin coated onto the

1

Icoh(Q , τNSE) − 3 Iinc(Q , τNSE) S(Q , τNSE) = 1 S(Q , 0) Icoh(Q , τNSE ≈ 0) − 3 Iinc(Q , τNSE ≈ 0)

1

(1)

The bulk samples were measured at three scattering angles αdet corresponding to Q values of Q = 0.08 Å−1, Q = 0.11 Å−1, and Q = 0.15 Å−1. If the scattered intensity is determined over the whole detector area only these three Q-values were taken into account. Additionally, at sufficiently high scattered intensities, the detector can be subdivided into five zones. This provides an increased Q-resolution due to additional Q-values and intermediate scattering functions. This method was used for the analysis of the bulk sample data only. Because of the lower scattering intensity in grazing incidence scattering geometry, it was not applied to the GINSES data. 2.3.4. Neutron Spin Echo Spectroscopy under Grazing Incidence (GINSES). Neutron spin echo spectroscopy under grazing incidence combines the neutron spin echo technique and the grazing incidence C


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Macromolecules substrates. The rotation speed was adjusted to 5000 rpm. Two cycles of spin coating were applied to the samples. After deposition of 200 μL of microgel suspension, the rotation was immediately started. The deposition duration was 300 s for all samples. AFM images under ambient conditions were recorded with an Asylum Cypher at room temperature using Igor Pro 6.22A for the analysis of the recorded images. Imaging was done in intermittent contact mode using silicon cantilevers (AC160TS-W2) with a spring constant of 42 N/m. Scan sizes of 20 × 20 μm2 were chosen. AFM measurements in liquid were recorded in a ECCell electrochemistry cell from JPK using the JPK Nanowizard II in intermittent contact mode. Here, silicon cantilevers from Micromash (CSC17) with a spring constant of 45 N/m were used. After deposition of the microgels on the silicon wafers (2 × 2 cm2) the dried sample was fixed in the liquid cell. The chamber was filled with water and left overnight for complete particle swelling. The samples were scanned at 20, 60, and 20 °C again to investigate the swelling behavior. Equilibration time at each temperature was 30 min and the scan size was 5 × 5 μm2. The obtained images were analyzed by the Software JPK Data Processing 4.0.13.

suspensions of the microgels p-ME3O5 and p-ME3O26 was characterized by DLS. The plot on the left side of Figure 3

Figure 3. Left: Intensity weighted relaxation rate distribution measured at θ = 60° for samples (a) p-ME3O5 and (b) p-ME3O26. Right: Relaxation rates ΓDLS as a function of Q2. All measurements were done at 15 °C.

3. RESULTS AND DISCUSSION 3.1. Structure and Dynamics in the Bulk Phase. 3.1.1. Structural Properties. Figure 2 shows two SANS curves

shows the intensity weighted distributions of the relaxation rate ΓDLS for both types of microgel at 15 °C at a scattering angle of 60°. Both distributions illustrate the low polydispersity of these microgels. On the right-hand side the mean relaxation rate ΓDLS is plotted as a function of Q2. In both cases ΓDLS(Q2) shows linear behavior and the best fit to the data is a line fit through the origin. This confirms the observation of pure translational diffusion related to ΓDLS = DtransQ 2

(3) −8

The resulting diffusion coefficients are Dtrans = 1.3 × 10 cm2/s for p-ME3O5 and Dtrans = 8.4 × 10−9 cm2/s for p-ME3O26. The experimental conditions satisfy to use the Stokes−Einstein relation Dtrans = Figure 2. SANS curves of the PEG microgels p-ME3O5 and p-ME3O26 measured at 15 °C. Solid lines are fits to the model combining a form factor of an inhomogeneous fuzzy sphere with the contribution of thermally excited network fluctuations.43 Resulting values are summarized in Table 1. The Q-range of the NSE experiment is marked.

kBT 6πηRH

(4)

to determine the hydrodynamic radius RH from the diffusion coefficients. For p-ME3O5 the RH was determined as RH = (145 ± 5) nm and as RH = (218 ± 4) nm in case of p-ME3O26. Under the experimental conditions the mean square displacement ⟨x2⟩ = 6tDtrans of the microgel particles diffusing over a distance related to ⟨x2⟩ = RH2 can be used to estimate the time t required to move this distance. Rearranging eq 4 gives t = RH3πη/kBT and at a temperature of T = 15 °C such particles need 3 (p-ME3O5) to 10 ms (p-ME3O26) to travel a distance corresponding to their size. In neutron spin echo spectroscopy a time range of a few 10 ns up to a few 100 ns is probed. Within this time range only a short distance compared to the size is explored by translational diffusion. Hence, its contribution to the detected signals is rather small. However, high volume fractions were used for the NSE measurements to suppress translational diffusion and its contribution to the measured intermediate scattering functions.23 Moreover, the scattered intensity is increased. Such high volume fractions can lead to the formation of colloidal crystals. This phenomenon is out of the scope of our experiments. Collective and Single Chain Dynamics. Neutron spin echo spectroscopy in transmission geometry (NSE) uses a large neutron beam cross section of a few square centimeters and illuminates nearly the whole sample cell area of similar size. This experimental condition ensures the measurement of the

of samples from the PEG microgel systems p-ME3O5 and pME3O26 measured at 15 °C. The solid lines are fits to a model which combines the form factor of an inhomogeneous sphere and the contribution of thermally excited network fluctuations. A detailled discussion of these SANS measurements can be found in the literature.43 Bulk structure properties are summarized in Table 1. At the higher comonomer content the formation of a distinct dense core surrounded by a fuzzy shell was reported. At the lower comonomer content the particle structure is largely inhomogeneous and fuzzy. The analysis of the thermally excited fluctuations in the microgel network reveals the existence of two correlation lengths corresponding to fluctuations in the fuzzy shell and the core region with higher polymer density. In Figure 2, the Q-region of our NSE experiments is marked. Depending on the particle size, in this region typically collective network and single-chain dynamics can be observed. 3.1.2. Dynamics in the Bulk Phase. Translational Diffusion. The translational diffusion in the aqueous D


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Figure 4 shows a set of representative normalized intermediate scattering functions S(Q,τNSE)/S(Q,0) measured

ensemble averaged normalized intermediate scattering function S(Q,τNSE)/S(Q,0). We can assume the system to behave pseudoergodically. The validity of this assumption was already demonstrated in NSE experiments on PNIPAM microgels.23,24 As discussed above, the chosen high volume fraction of microgel particles allows to neglect the contribution of translational diffusion. Hence, the measured normalized intermediate scattering functions should completely decay to zero and S(Q,τNSE)/S(Q,0) can be fitted by S(Q , τNSE) = A 0 exp( −ΓτNSE)β S(Q , 0)

(5)

This approach provides information about the inner dynamics. Here, A0 is the amplitude accounting for very fast incoherent solvent contributions from eq 1. Γ is the relaxation rate of the observed dynamics and τNSE accounts for the Fourier time parameter of the NSE measurement. The stretching exponent β depends on the dynamics probed in the given Fourier time window at the respective Q value. In case of β = 1, a single exponential decay is observed. Then Γ = Γc and cooperative network dynamics is observed. These motions are often described as breathing modes of the polymer network excited by interactions with the surrounding medium. The relaxation rate 2 Γc = DQ = c

kBT 2 Q 6πηeff ξ

Figure 4. Sets of normalized intermediate scattering functions S(Q,τNSE)/S(Q,0) of bulk samples (a) p-ME3O5 and (b) p-ME3O26 Q = 0.08 Å−1, Q = 0.11 Å−1, and Q = 0.15 Å−1 measured in transmission geometry. Solid lines are fits to eqs 5 and 8 (at Q = 0.15 Å−1).

at Q = 0.08 Å−1, Q = 0.11 Å−1, and Q = 0.15 Å −1 of bulk samples of both systems (a) p-ME3O5 and (b) p-ME3O26. With increasing Q-value, the curves decay faster. All curves are assumed to decay down to zero at Fourier times τNSE > 40 ns. The underlying dynamics were identified from a qualitative inspection of the data using a double logarithmic representation ln(−ln(S(Q,τNSE)/S(Q,0))) vs ln(τNSE) (see Figure S1 in the Supporting Information).46 From that qualitative analysis shown in Figure S1 for sample p-ME3O5 the presence of a collective and Zimm dynamics at lower Q-values and Zimm dynamics only at larger Q values was found. The transition between both regimes occurs at Q ≈ 0.13 Å−1 which corresponds to a length of ≈5 nm. For sample pME3O26, only stretched exponential behavior with a stretching exponent β ≈ 0.67 could be deduced for all measured Q values. This suggests that in the chosen Q-range no cooperative dynamics is detectable. Hence, in sample p-ME3O5 the transition region from collective to Zimm-type dynamics is observed which is not present for sample p-ME3O26. At the chosen bulk contrast in the NSE measurements all polymers are protonated while only the solvent is deuterated. This condition prevents a distinction between the contribution from the two fluctuations regimes. In this case a mean inner dynamics is observed. Figure 5 shows the obtained relaxation rates Γ as a function of Q3 applying eq 5 and β = 0.67. For both samples the resulting relaxation rates yield a linear dependence on Q3. From the slopes of (56 ± 3) Å3/ns for p-ME3O26 and (17 ± 1) Å3/ns for p-ME3O5 the apparent inner viscosities of 2.78 and 9.2 mPa s have been estimated. Compared to the viscosity of D2O, both samples show an increase in viscosity by a factor of 2, respectively 6.6. This indicates a stronger interaction between the polymers segments in case of p-ME3O5 than in p-ME3O26. This observation corresponds to results of NSE measurements on microgels given in the literature.25,26 Here, a similar trend of the Zimm dynamics was observed indicating that the polymer segments experience a higher effective viscosity due to the

(6)

is proportional to Q2 and both are linked by the diffusion coefficient Dc of these density fluctuations. For a further analysis a relation similar to the Stokes−Einstein equation can be applied. It provides the dynamic correlation length ξ, which represents the length scale of this collective fluctuations. For β < 1 and sufficiently large Q-values Zimm type dynamics of the polymer segments might be observable. Here, ΓZimm ∝ Q3 is given by ΓZimm = AZimmQ 3 = 0.7386 ×

kBT 3 Q 6πηeff

(7)

In this case, β has values of 2/3 for large τNSE and in an approximation taking into account the Q and time range of NSE, β = 0.85. The relaxation rate ΓZimm is determined by the hydrodynamic interaction of the polymer segments with the surrounding medium excerting some drag on the segments. The proportionality factor AZimm provides a measure of the viscosity ηeff of the surrounding medium. This effective viscosity depends on the particular interaction between the locally probed polymer segment and the environment formed by neighboring segments and the solvent and can deviate from the bulk value of the solvent viscosity. Here, we assume, that this effective viscosity contributes in eq 6 and eq 7. In particular, at low and intermediate Q values, several contributions to the dynamics of microgels may be observed.25,26 In our experiments, a superposition of collective and single chain dynamics described by S(Q , τNSE) = A exp( −ΓcτNSE) + (A 0 − A) S(Q , 0) exp( −ΓZimmτNSE)β

(8)

is possible. A and (A0 − A) are the partial amplitudes of the observed dynamics. E


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the inner core than in the fuzzy shell. This would lead to a stiffer core and a comparatively soft outer region. A strong gradient in the mechanical properties between both regions could prevent a coupling of the cooperative dynamics from the outer to the core region. Additionally, this would lead to widely separated and eventually, rare cross-linking points. Hence, either no cooperative diffusion would occur or would not be visible within the Q-range of the performed NSE experiments since the cross-linking points could not or hardly couple by cooperative diffusion. However, the proposed relation between inner structure and dynamics should be tested in future using contrast variation experiments and measurements of the elastic modulus by AFM. 3.2. Structure and Dynamics in the Adsorbed State. 3.2.1. Structure of the Adsorbed Microgels. Figure 6 summarizes the results of AFM measurements on the samples of the investigated PEG microgel systems adsorbed on single crystal silicon blocks prior to the neutron scattering measurements. In this Figure, the surface topography of p-ME3O5 (a) and p-ME3O26 (b) is shown. Scans of 50 × 50 μm (scale bar corresponds to 10 μm) were done in the dry state. A loose packing was observed for both samples. The analysis of the AFM scans results in mean interparticle distances in the order of the particle size. Hence, contributions to the scattered signal due to lateral particle−particle interaction will be neglected. A mean particle coverage of about 50% at the surface can be estimated taking into account the swelling in aqueous environment. This allows us to estimate the mean scattering length density of the adsorbed PEG microgel samples. This is used to calculate the angle of incidence αin and the penetration depth Λi of the neutrons in the GINSES experiments. Figure 6c) compares the averaged cross sections of five individual particles of parts a and b in an aqueous environment in the swollen state at 20 °C. These cross sections define the lateral dimension of the particle footprints at the substrate of about 400 nm (p-ME3O5) and about 300 nm (p-ME3O26). The footprints are in good agreement with the diameter in the bulk solution, obtained from DLS experiments (Table 1). The corresponding heights are 120 and 140 nm. In a previous work

Figure 5. Relaxation rates Γ as a function of Q3 for samples p-ME3O5 and p-ME3O26. The inset represents the contribution of cooperative diffusion in case of sample p-ME3O5.

presence and eventually larger fluctuation amplitudes of the neighboring polymer segments in the network. The inset in Figure 5 represents the contribution of cooperative diffusion in case of sample p-ME3O5. According to eq 6 the diffusion coefficient of the cooperative mode is Dc = (1.3 ± 0.1) Å2/ns and the dynamic correlation length ξ of 12 nm was obtained using the pure solvent viscosity. The amplitude of this contribution is only a few percent of the total amplitude. Only few results from NSE experiments on microgels have been published. The cooperative diffusion coefficient of pure PNIPAM microgels of a few 10−11 cm2/s decreases with increasing cross-linker concentration.23 These values are on the same order of magnitude as the cooperative diffusion coefficient of the p-ME3O5 microgel. Our result corresponds to that of PNIPAM microgels with a rather high cross-linker content. There are two possible reasons for the absence of cooperative dynamics in the NSE data of sample p-ME3O26. In our previous SANS measurements we found that a core is surrounded by a fuzzy shell in system p-ME3O26. This core is about 30% larger than for sample p-ME3O5. Additionally, the fuzzy outer region is larger for sample p-ME3O26. Because of the synthesis, the comonomer could be more concentrated in

Figure 6. AFM images of the PEG microgels (a) p-ME3O5 and (b) p-ME3O26 adsorbed on the surface of a silicon block. Scan size is 50 × 50 μm, the scale bar corresponds to 10 μm. The samples were prepared as loosely packed particle layers to minimize particle−particle interactions. Image c compares the averaged cross sections of five individual particles of parts a and b in aqueous environment in the swollen state at 20 and 60 °C and after cooling down to 20 °C (for further details see ref 47). F


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Table 2. Volume of p-ME3O5 and p-ME3O26 in Bulk and at the Surface Calculated from DLS and AFM Measurements at 20, 60, and 20 °C Again and the Corresponding Deswelling Ratios (αDLS, αAFM) at 60 and 20 °Ca VDLS [107 nm3]

a

VAFM [107 nm3]

sample

20 °C

60 °C

20 °C

20 °C

60 °C

20 °C

αDLS

αAFM

p-ME3O5 p-ME3O26

1.05 4.34

0.31 1.17

1.01 4.46

0.54 0.61

0.31 0.41

0.45 0.52

0.29 0.27

0.57 0.67

The relative experimental errors ΔVDLS/VDLS are 5−10%, ΔVAFM/VAFM are 10−20% and ΔαAFM,DLS/α are 1−5%.47

side column of Figure 7 show the corresponding results for pME3O26 under the same conditions. The solid lines are fits to eq 5 using a fixed value of 0.67 of the stretching exponent β which was obtained from the bulk measurements. As a first approximation the analysis of the data presented in Figure 7 assumes a contribution of Zimm-type dynamics only. The resulting relaxation rates are summarized in Table 3.

we studied the behavior of adsorbed PEG microgel particles from measurements of the particle cross sections in the swollen (20 °C) and deswollen state (60 °C).47 Table 2 compares the calculated particle volumes. A reduction of the volume in the adsorbed state in comparison to the corresponding bulk phase was found. Compression of the polymer network due to the adsorption and a flattening of the particle morphology, which induces the strong interaction of the fuzzy particle shell with the substrate may lead to the observed reduction in volume after adsorption. 3.2.2. Dynamics Near the Solid Interface. Figure 7 summarizes the normalized intermediate scattering functions

Table 3. Relaxation Rates Γ for Samples p-ME3O5 and pME3O26 in Bulk (NSE) and at the Surface (GINSES)a Γ [ns−1] (GINSES) sample

Γ [ns−1] (NSE)

Λi = 35 nm

Λi → ∞

p-ME3O5 p-ME3O26

0.010 0.012

0.0036 0.0032

0.010 0.013

a The data are shown in Figure 7. All measurements were done at a Q value of 0.08 Å−1 at a temperature of 15° C.

The fits of parts a and b were restricted to Fourier times up to 20 ns. On these assumptions we obtain relaxation rates of Γ = 0.010 ns−1 for p-ME3O5 (Figure 7a)) and Γ = 0.012 ns−1 for p-ME3O26 (Figure 7b)) in bulk. At a neutron penetration depth Λi of 35 nm the neutrons are scattered from a region in the sample very close to the substrate. Here, for parts a1 and b1, values of Γ = 0.0036 ns−1 and Γ = 0.0032 ns−1 were found. At infinite penetration depth the full height of the adsorbed particles is explored. The resulting relaxation rates are Γ = 0.010 ns−1 and Γ = 0.013 ns−1. Compared to the bulk results the relaxation rates in parts a1 and b1 are slowed down by a factor of 3−4. In the framework of Zimm dynamics, this result corresponds to an apparent increase of the intrinsic solvent viscosity. We attribute this apparent increase to the presence of the solid substrate. In previous work we found experimental evidence for a strong interaction of the PEG based microgels with silicon substrates.47 This adhesion and the compression of the network to the substrate could be responsible for a region with inner dynamics that differs from the bulk behavior. Here, the thermal fluctuations seem to be damped or suppressed. The high elastic background would also be in line with the presence of static inhomogeneities in the surface region which might be related to the network compression and confinement. At larger distances to the substrate its influence seems to vanish and network fluctuations are again the dominant contribution to the dynamics which is reflected by the observed relaxation rates in Figure 7, parts a2 and b2. The relaxation rates are approximately the same as in the bulk measurement. This suggests, that the confinement effect on the inner dynamics reduces with increasing distance from the substrate restoring a freely fluctuating network as observed in the bulk. The rather poor statistics of the GINSES data results from the adsorbed highly swollen microgel particles serving as sample layer. The contrast between PEG and D2O is rather low

Figure 7. Intermediate scattering functions S(Q,τNSE)/S(Q,0) measured from samples p-ME3O5 (a−a2) and p-ME3O26 (b−b2). All measurements were done at the same scattering vector, Q = 0.08 Å. Plots a and b are results of bulk measurements in transmission geometry. Plots a1−b2 were measured under grazing incidence in GINSES mode. (a1 and b1) Angle of incidence αin was 0.35° (αc ≈ 0.5°) and the penetration depth about 35 nm. (a2 and b2) At αin = 1.0°, the condition αin > αc was used to obtain scattering over the whole particle height. Solid lines are fits to the Zimm-dynamics, eq 5.

measured at Q = 0.08 Å −1 in reflection geometry under grazing incidence (GINSES). As a reference, graphs a and b again show the data measured in transmission geometry for p-ME3O5 and p-ME3O26. Parts a1 and a2 of Figure 7 show intermediate scattering functions recorded at constant Q value and two angles of incidence below and above the critical angle αc. A critical angle of αc = 0.5° was estimated for samples a and b taking into account the mean coverage of the silicon substrate of 50% (section 3.2.1) and the high D2O content of the swollen particles. In part a1, αin was 0.35°. According to eq 2 a neutron penetration depth Λi of 35 nm was estimated for that configuration. At αin = 1.0°, a penetration depth of virtually infinity was achieved (αin > αc). Graphs b1 and b2 on the right G


Article

Macromolecules and a relatively low amount of sample material is used in such measurements. In the literature, NSE and light scattering experiments on dynamics of polymers confined to solid surfaces or inside pores report a strong influence of the confinement, e.g., refs 31, 38, and 39 Although results from non-cross-linked anchored polymer chains cannot be applied directly to adsorbed microgels some similarities occur. For example, for poly(ethylene oxide) chains adsorbed to dispersed clay platelets an immobile fraction of polymers tightly bound to the platelet surfaces and a mobile fraction due to loops at larger distance from the surface were observed.38 Speculatively, the significantly modified dynamics in the vicinity of a solid substrates indicates a stiffening of the microgel network. This effect might influence the drug release properties. In summary, the reported experiment demonstrates the feasibility of neutron spin echo spectroscopy measurement under grazing incidence on microgel particles adsorbed to a solid substrate. It indicates a slowing down of the inner dynamics in the vicinity of the substrate for both investigated samples. At increasing distance from the substrate the relaxation is comparable to the bulk.

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AUTHOR INFORMATION

Corresponding Author

*(S.W.) E-mail: (e-mail:[email protected]). Present Addresses ∥

BAM Federal Institute for Materials Research and Testing Division 1.9 Chemical and Optical Sensing, BAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Straße 11, 12489 Berlin, Germany §

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by JCNS to perform the neutron scattering measurements. The authors thank the German Research Council (DFG) for financial support via the programm G8.

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REFERENCES

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4. SUMMARY AND CONCLUSION In this work we report on the investigation of the dynamics of ethylene glycol (EG) based microgel particles in bulk and adsorbed at silicon surfaces. Monodisperse p-MeO2MA-coOEGMA microgel particles were synthesized by precipitation polymerization. Translational and inner dynamics of (a) pME3O5 and (b) p-ME3O26 microgels in bulk were investigated by dynamic light scattering (DLS) and neutron spin echo spectroscopy in transmission geometry (NSE). Here, cooperative diffusion and Zimm type dynamics were observed in case of p-ME3O5, while sample p-ME3O26 shows only Zimm-type dynamics in the investigated Q-range. Additionally, for the first time, the inner dynamics of microgel particles in adsorbed state was explored by neutron spin echo spectroscopy under grazing incidence condition. These experiments demonstrate the feasibility of the recently developed surface sensitive technique. It indicates a slowing down of the inner dynamics of the adsorbed PEG microgels in the vicinity of the substrate. At increasing distance from the substrate the relaxation is comparable to the bulk measurements. The results of this study are interesting with respect to a technical and a scientific aspect. Technically it is shown that the dynamic profile within colloidal objects of less than 200 nm height can be scanned perpendicular to the substrate surface. The scientific impact is that the slowing down of the dynamics toward the substrate might have a strong effect on the swelling/ deswelling ratio, the swelling kinetics and eventually even on the VPTT. This is of fundamental interest with respect to application of adsorbed gels, e.g., in catalysis, for sensors or for implants.

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Double logarithmic representation of scattering functions (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00788. H


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Bulk Phase and Surface Dynamics of PEG Microgel Particles

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