Comparison of Solvent and Probe Reorientation in Polyacrylamide Gels

J. Phys. Chem. 1996, 100, 1705-1710

1705

Comparison of Solvent and Probe Reorientation in Polyacrylamide Gels Uta M. Biermann, Wolfgang Mikosch, Thomas Dorfmu1 ller,* and Wolfgang Eimer* Department of Chemistry, UniVersity of Bielefeld, D-33615 Bielefeld, Germany ReceiVed: March 1, 1995; In Final Form: October 19, 1995X

Raman spectroscopy and time-resolved fluorescence anisotropy decay were used to measure the rotational relaxation dynamics of acetonitrile and the dianion polyphenyl 2 (PP2) in polyacrylamide (PAA) gels in a water-acetonitrile solvent mixture. In the neat solvent mixture the acetonitrile molecule rotates, underlying a reduced friction in comparison to its dynamics in pure acetonitrile. The rotational behavior of PP2 in the mixture is well described by the Stokes-Einstein-Debye equation. In the PAA gels the rotational relaxation of acetonitrile is only weakly influenced by the gel network even at high polymer concentrations. In contrast, the larger probe molecule PP2 reflects a significant retardation of the reorientational motion with increasing PAA concentration. We suggest that this behavior results from the different environments experienced by the probe and solvent molecules.

Introduction

Experimental Section

Understanding the transport properties of small molecules in polymer solutions and gels requires an understanding of the role of the solvent. It has been shown that the presence of polymer chains alters the solvent dynamics.1-7 Besides probing the solvent itself, studies of the rotational diffusion of probe molecules provided evidence that the frictional characteristic of the solvent is modified in the presence of a polymer.7-9 Gisser et al.7 have compared the rotational and translational dynamics of probe and solvent molecules of different size in polystyrene/tetrahydrofuran solutions. The change in microscopic friction as manifested by the decrease of the diffusion coefficients with increasing polymer concentration was comparable for all large probe molecules, while the smaller solvent molecules were moving significantly faster. The authors suggested that the size and possibly the shape of the molecules are important factors in determining microscopic friction. In addition to size and shape effects it was observed in organic polymer solutions that the influence of the polymer on solvent dynamics depends also on the specific chemical composition and interactions of the polymer/solvent system.1,3,4 The aim of the present study was to compare the influence of a polymer on the rotational dynamics of two very different molecules in a system with highly interacting components. We have chosen the cross-linked hydrogel polyacrylamide (PAA) as well as PAA solutions in a water-acetonitrile mixture. In the first part we present a Raman study to investigate the rotational motion of the acetonitrile solvent molecules. These data are compared with the results of a time-resolved fluorescence anisotropy experiment following the reorientational relaxation of the larger probe molecule polyphenyl 2 (PP2). Both molecules differ in their size, their interactions with the solvent environment, and their characteristic time scale for reorientational motion. Thus, we might expect to probe the influence of the PAA matrix on the effective friction that the molecules experience on different length scales. While acetonitrile is small enough to provide a closer look into the local solvent structure, the larger PP2 should reflect the properties of the solvent environment as a continuum fluid.

A. Materials and Sample Preparation. The PAA gels were prepared by radical copolymerization of acrylamide (AA, analytical grade) and N,N′-methylenebisacrylamide (BIS, research grade). Ammonium persulfate (APS, analytical grade) was used as starter and N,N,N′,N′-tetramethylethylenediamine (TEMED, research grade) as accelerator. All reagents were purchased from Serva Feinbiochemica, FRG. The initiator and the accelerator concentrations were kept constant in all samples ([APS] ) 0.7 mg/mL, [TEMED] ) 1 µL/mL). All stock solutions were prepared in the water-acetonitrile mixture. After the solution was filtered (0.22 µm filters) into a quartz cuvette (Hellma, FRG) the polymerization was initiated at room temperature by adding APS. Measurements were performed after a minimum of 48 h to ensure complete gelation. For the fluorescence anisotropy decay measurements the probe molecule polyphenyl 2 (p-quaterphenyl-4,4′-disulfonic acid potassium salt, laser grade from Lambda Physik, FRG) was added before polymerization. The concentration of polyphenyl 2 was kept constant ([PP2] ) 3 × 10-5 M). The gels are characterized by the total monomer concentration C ) [AA] + [BIS] and for cross-linked PAA gels by the crosslink ratio fBIS ) [BIS] / ([AA] + [BIS]), where [AA] and [BIS] represent the corresponding monomer concentration in grams/ milliliter. For all cross-linked PAA gels in this work the ratio fBIS was 2%. The total monomer concentration range was 0.025 e C e 0.2 g/mL. The solvent mixture was prepared gravimetrically containing 90 wt % triple-distilled water and 10 wt % acetonitrile (spectroscopic grade, Merck). Viscosities were measured with an Ubbelohde capillary viscosimeter, and the corresponding densities were determined gravimetrically. B. Raman Experiment. The spectra were recorded using a conventional Raman spectrometer.10 The excitation source was an argon-ion laser (Coherent, Innova 305) operating in a single longitudinal mode at 488 nm. The required polarization of the incident and scattered light was selected by a GlanThompson polarizer with an extinction ratio better than 1 × 10-7. The scattered light was collected by a two-lens system and analyzed by a double-grating monochromator (SPEX 1403) with a spectral slit width of 0.5 cm-1 for the polarized spectra (VV geometry) and 1 cm-1 for the depolarized spectra (VH

* Authors to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, January 1, 1996.

0022-3654/96/20100-1705$12.00/0

© 1996 American Chemical Society

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Biermann et al.

geometry). The instrumental profile was periodically checked by recording the spectrum of a latex suspension. The detection system consists of a cooled (-30 °C) photomultiplier tube (RCA C31034 A) and a photon-counting system (Ortec Brookdeal 5C1). All spectra were recorded in the wavelength range between 2150 and 2350 cm-1 with a monochromator step width of 0.02 cm-1. Depolarized spectra were accumulated 10 times to improve the signal to noise ratio. The temperature of the sample cell was controlled with an accuracy of (0.5 °C. The isotropic and anisotropic Raman spectra are related to the experimental spectra in VV or VH scattering geometry according to

Iiso(ν) ) IVV(ν) - 4/3IVH(ν)

(1)

Ianiso(ν) ) IVH(ν)

(2)

For the strongly polarized CtN stretch vibration the second term of eq 1 can be neglected. The recorded spectra were a convolution of the polarized or depolarized spectra, represented by a sum of Lorentzians, with the instrumental function

F(ν) ) A(ν) X ∑Lor(ν,pi)

(3)

i

According to eq 4 each Lorentzian is characterized by the peak intensity, I(ν), the position of the peak maximum, ν(0), and the half-width at half peak height (hwhh), Γ.

[ [

I(ν) ) Imax 1 +

]]

ν - ν(0) Γ

2 -1

(4)

A nonlinear least-squares fitting procedure based on the Levenberg-Marquardt algorithm was used to evaluate the parameters.11 For a Lorentzian band shape, the value of Γ is related to the relaxation time τ,

τ ) (2πc′Γ)-1

(5)

where c′ is the velocity of the light. Thus, a fit of the polarized spectra directly provides the vibrational relaxation time τv. The depolarized spectrum is a convolution of the isotropic and anisotropic spectra, which can be converted to a simple product of the reorientational and the vibrational relaxation function by Fourier transformation. F

Ianiso ) Iiso X Ireo 98 Caniso ) CisoCreo

(6)

The reorientational relaxation time τr can be calculated by subtracting the isotropic contribution from the half-width Γaniso, as obtained from the depolarized spectrum.

Γreo ) Γaniso - Γiso

(7)

C. Fluorescence Anisotropy Decay. Time-resolved fluorescence anisotropy was measured by a time-correlated singlephoton-counting apparatus from Edinburgh Instruments, using a hydrogen-filled flash lamp as excitation source. The experimental instrumentation and the data analysis method used here were described in detail elsewhere.12 After exciting the probe molecule PP2 by a linear polarized excitation pulse (308 nm) the single-particle reorientation time was obtained from the anisotropy r(t),

r(t) )

IVV(t) - IVH(t) IVV(t) + 2IVH(t)

(8)

where IVV(t) and IVH(t) are the components of fluorescence decay, polarized parallel and perpendicular to the excitation polarization, respectively. The emission wavelength was 381 nm. In the case of Brownian diffusion, the anisotropy r(t) is directly related to the second-order orientation correlation function. In general, the expression for the time evolution of r(t) consists of five exponentials.13,14 We have shown that the shape of PP2 can be approximated by a prolate ellipsoid with the transition dipole parallel to the main symmetry axis.12 Hence, the anisotropy is given by a single-exponential function:

r(t) ) (2/5)P2(cos λ) exp(-t/τr)

(9)

where P2(cos λ) is the second-order Legendre polynomial, with λ describing the angle between the absorption and emission dipole moment. The rotational correlation time for the reorientation about an axis perpendicular to the long axis is related to the corresponding diffusion coefficient by

D ) (6τr)-1

(10)

Fluorescence lifetime and reorientation times were extracted from the sum curve S(t) and the difference curve D(t) defined as

S(t) ) IVV(t) + 2GIVH(t)

(11)

D(t) ) IVV(t) - GIVH(t) They were least-squares fitted to single exponentials using the Grinvald algorithm with time-shift and base-line parameters and proper weighting factors.15-17 G () ∫IHV/∫IHH) is a correction factor accounting for the anisotropic sensitivity of the detection system. In each case S(t) and D(t) were well represented by a single exponential with the time constants τS and τD. The reproducibility of the rotational correlation times τr, as calculated from

1/τr ) 1/τD - 1/τS

(12)

was better than 10%. The fluorescence lifetime of PP2 was in all samples constant (τf ) 0.97 ( 0.02 ns). The presence of molecular oxygen does not change the fluorescence lifetimes and the rotational relaxation times of PP2. Results and Discussion Dynamics of the Solvent Molecule. Within the observed temperature range (5-40 °C) the VV spectrum of acetonitrile in pure acetonitrile could be fitted to the sum of three Lorentzians: (i) 2249 cm-1, ν2h ) ν2 + ν8 - ν8 (“hot band” transition of the first excited state of the C-CN bending mode (ν8)); (ii) 2253 cm-1, ν2f (total symmetric CtN stretch); (iii) 2293 cm-1, ν3 + ν4 (combination mode of the near forbidden CH3 deformation (ν3) and the C-C stretch (ν4)). As the Raman spectrum of acetonitrile is well described in the literature,18-23 only the most important results for the further handling of the spectra shall be pointed out. (i) The “hot band” ν2h appears as an asymmetric shoulder at the low-frequency side of the fundamental. In the depolarized spectrum as well as for low acetonitrile concentrations (solvent mixture) we could not resolve the “hot band” transition. Due to the strong overlap with the total symmetric CtN stretching, band fitting with two Lorentzians led to unstable fits; hence, the CtN stretch was fitted to a single Lorentzian. Typical fits are shown in Figure 1.

Solvent and Probe Reorientation in PAA Gels

J. Phys. Chem., Vol. 100, No. 5, 1996 1707

Figure 2. Rotational relaxation times of acetonitrile as a function of ηs/T in neat acetonitrile (O) and in water-acetonitrile (90:10 wt %) (b).

TABLE 1: Comparison of the Activation Enthalpy of the Viscous Flow ∆Hηq, the Reorientational Motion ∆Hτq, and the Hydrodynamic Volume Vh of Acetonitrile and the Water-Acetonitrile Solvent Mixture solvent

∆Hηqa (kJ/mol)

∆Hτqb (kJ/mol)

Vh/kBc (ps K/cP)

τ0d (ps)

acetonitrile 6.9 ( 0.2 5.4 ( 1.1 721 ( 20 0.15 ( 0.03 water-acetonitrile 18.9 ( 0.6 9.9 ( 1.3 328 ( 35 0.96 ( 0.13 (90:10 wt %) a The activation enthalpy of the viscous flow ∆H q was determined η from the Arrhenius equation. b The activation enthalpy of the reorientational motion ∆Hτq was determined from eq 15. c We obtained the slopes from a linear regression fit. d τ0 is the zero viscosity limit of τr.

Figure 1. (a) VV spectrum of neat acetonitrile fitted with three Lorentzians at 2249 cm-1 (ν2h), 2253 cm-1 (ν2f), and 2293 cm-1 (ν3 + ν4). (b) VH spectrum of neat acetonitrile fitted with two Lorentzians (ν2; ν3 + ν4). The “hot band” transition (ν2h) is included in the fundamental CN stretch vibration. (c) VV spectrum of the wateracetonitrile (90:10 wt %) solvent mixture. The “hot band” overlaps with the ν2 fundamental, which is shifted to 2259 cm-1.

(ii) The CtN stretching mode (ν2f) is in Fermi resonance with the combination mode ν3 + ν4. As the calculated Fermi coefficient W ) (2x2)-1K234 contains information about the anharmonic force constant K234 of acetonitrile, it was calculated from the VV spectra to obtain information about possible interactions between acetonitrile and water and between acetonitrile and the polymer matrix, respectively. We found a Fermi coefficient for acetonitrile of W ) 12.6 ( 0.5 cm-1, as calculated from the polarized spectra. In the spectrum of acetonitrile in water the CtN stretching band (ν2) was shifted by 7 cm-1 to higher frequencies. This can be explained by hydrogen bonding between the “lone pair” at the nitrogen atom of acetonitrile and the surrounding water molecules, as already suggested by Besnard et al.24 for methanol-acetonitrile. The increase of the Fermi coefficient, calculated from the polarized spectra for the water-acetonitrile mixture (W ) 13.0 ( 0.5 cm-1), indicates an increasing anharmonic force constant, supporting the proposed association between water and acetonitrile molecules. To investigate the reorientational dynamics of neat acetonitrile

in comparison to the water-acetonitrile solvent mixture, we measured temperature dependent Raman spectra and calculated the rotational relaxation times according to eq 5. The rotational relaxation times τr of neat acetonitrile as well as of acetonitrile in water show a linear dependence on ηs/T (Figure 2). From this we can assume that it is appropriate to describe the rotational behavior of acetonitrile in terms of the modified StokesEinstein-Debye (SED) equation:25,26

τr )

Vh η s + τ0 kBT

(13)

where kB is the Boltzmann constant, and ηs is the solvent viscosity. The hydrodynamic volume

Vh ) Vm f z

(14)

is related to the molecular volume Vm of the solute, the shape factor f, and the factor z, reflecting the hydrodynamic boundary conditions. The additive term in eq 13 was introduced to account for the sometimes experimentally observed nonzero intercept in the zero-viscosity and infinite-temperature limit. For small molecules τ0 is often related to the relaxation time of the free rotor, but a stringent physical interpretation is still lacking.27 From the slopes in the SED plot (Figure 2) we obtained the hydrodynamic volumes of neat acetonitrile and acetonitrile in water (Table 1). To compare the experimental data with predictions from hydrodynamic theories, we modeled the acetonitrile molecule as a biaxial ellipsoid of revolution.13,28,29 The volume of the rotating ellipsoid was computed as the sum of the van der Waals increments for acetonitrile.30 The bond lengths were taken from an ab-initio calculation for the molecule (GAMESS).31 We calculated the friction coefficients for hydrodynamic slip

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Biermann et al.

boundary conditions according to Hu and Zwanzig.32 The resulting theoretical value of Vh/kB ) 728 ps K/cP for neat acetonitrile is in good agreement with the experimental value of Vh/kB ) 721 ( 20 ps K/cP. Fourier transform infrared measurements of neat acetonitrile performed by Hashimoto gave comparable results.18 For acetonitrile in water the slope in the SED plot decreases to about half of its value as obtained for neat acetonitrile. In addition, an extrapolation to ηs/T ) 0 gives an intercept significantly different from zero (Table 1). Although for the temperature range studied Figure 2 reveals a linear dependence of τr on ηs/T, the nonzero intercept indicates that in this specific solvent system the assumptions of the SED equation are not strictly valid. It seems that a hydrodynamic description according to eq 13 cannot properly account for the specific structural and dynamic features of the water-acetonitrile mixture, unless we allow for an unreasonably large value of τ0. The decreased friction experienced by the rotating molecule might reflect the existence of specific solvent structures in the binary solvent mixture.33 Although the detailed molecular nature of the reorientational motion cannot be unambigiously characterized, the data form a good basis to compare the dynamics of acetonitrile in the pure solvent mixture with the behavior in the gels. We determined the activation enthalpy ∆Hτq for the reorientational motion from the temperature dependence of the rotational relaxation times of neat acetonitrile and acetonitrile in water by fitting the data to an Arrhenius-type equation: q

ln(τrT) )

∆Hτ + cτ RT

(15)

For neat acetonitrile the results are given in Table 1. The activation enthalpy ∆Hτq is very similar to the activation enthalpy of the macroscopic shear viscosity ∆Hηq. From this we can conclude that the reorientational motion of the acetonitrile molecule is correlated with the viscous flow. For the water-acetonitrile mixture, however, we observed a significant increase of the activation enthalpy for the viscous flow, while the activation enthalpy for the rotational motion increases only slightly. This is a further indication that the reorientation of acetonitrile in the binary mixture is not determined by the macroscopic viscosity of the solvent, as would be expected from the SED equation. The increase of the activation enthalpy for the viscous flow is probably determined by long-range interactions. On the other hand, the reorientation of acetonitrile is governed by the local environment of the molecules, and hence, in the presence of water ∆Hτq is not that much affected compared to pure acetonitrile. Influence of the Polymer Matrix. We have investigated the reorientational motion of the acetonitrile molecule as a function of the polymer concentration in PAA gels in wateracetonitrile (90:10 wt %) as solvent. Our aim was to compare the reorientational dynamics of the acetonitrile molecule in the pure water-acetonitrile mixture with its behavior in the gel state. As is well-known, in solution the reduced variable ηs/T in the Stokes-Einstein-Debye equation is useful to describe the rotational diffusion of molecules. But in gels the macroscopic viscosity is infinity and obviously not an appropriate variable to characterize changes in the diffusional motion of probe molecules within the polymer network. Therefore, we will use the macroscopic viscosity of the solvent, ηs, as a reference frame to monitor the influence of the polymer component on the local friction as experienced by the rotating probe molecules in comparison to their behavior in pure solvent. At this point one

Figure 3. SED plot for acetonitrile in the presence of PAA with wateracetonitrile (90:10 wt %) as solvent. The polymer concentrations are (g/mL) 0.05 ([), 0.1 (0), and 0.2 (b, O). The filled symbols represent gels with fBIS ) 2%, while the open symbols mark samples without cross-linker. The straight lines represent the best fit to the SED equation. For comparison, the dashed line corresponds to the rotational relaxation behavior in the solvent mixture in the absence of polymer.

TABLE 2: Variation of the Slope in the SED Plots with Polymer Concentration for Acetonitrile and PP2 in PAA Gels Containing Water-Acetonitrile (90:10 wt %) slope of τr vs ηs/T C (g/mL)

fBIS

acetonitrile (ps K/cP)

PP2 (ns K/cP)

0 0.025 0.05 0.1 0.1 0.2 0.2

0 0.02 0.02 0 0.02 0 0.02

328 ( 35

117 ( 12 372 ( 15 499 ( 21 690 ( 48 714 ( 34 1167 ( 64

359 ( 39 361 ( 48 475 ( 43 465 ( 45

should keep in mind that the solvent viscosity is a macroscopically well-defined quantity and not necessarily suitable to describe hydrodynamic effects on a microscopic or, at the most, mesoscopic length scale. Hence, in the following we will define the macroscopic viscosity in a formal way when we discuss the rotational motion of solvent and probe molecules in gels. Figure 3 shows the ηs/T dependence of the rotational relaxation times for different PAA gels (0.05 e C e 0.2 g/mL, 0 e fBIS e 0.02) as obtained from temperature dependent measurements. We observed only a slight increase of the slopes in the SED plot with increasing polymer concentration (Table 2). This implies that the influence of the polymer on the reorientational dynamics of the acetonitrile molecule is rather weak. Furthermore, a comparison between cross-linked gels and polymer solutions of the same polymer concentration revealed no effect of cross-linking, although the macroscopic properties of the gels strongly depend on the BIS concentration.34 The activation enthalpy for the rotational motion of the acetonitrile molecules is within experimental error, independent of the polymer concentration and cross-link ratio. We found a good aggreement of ∆Hτq ) 10.7 ( 1.3 kJ/mol determined in the gels with ∆Hτq ) 9.9 ( 1.3 kJ/mol in the solution without polymer. Therefore, we can conclude that specific forces between the polymer chains and the acetonitrile molecule can be neglected. This is further supported by the observation that the frequency shift of 7 cm-1 in the spectra of acetonitrile is not affected by the addition of polymer, indicating that the interactions between the acetonitrile molecule and the solvent environment due to hydrogen bonds are not changed by the presence of polyacrylamide. In addition, Figure 3 clearly demonstrates that the nonzero intercept is still retained, suggesting that the mechanism for the reorientational motion of

Solvent and Probe Reorientation in PAA Gels

Figure 4. Rotational relaxation times of PP2 versus ηs/T in the pure solvent (×) and as a function of the PAA concentration in wateracetonitrile (90:10 wt %). The polymer concentrations are (g/mL) 0.025 (2), 0.05 ([), 0.1 (9, 0), and 0.2 (O). The filled symbols represent gels with fBIS ) 2%, while the open symbols mark samples without cross-linker. The straight lines represent the best fit to the SED equation.

acetonitrile in the pure solvent mixture does not change in the gels. Dynamics of the Probe Molecule. In a next step, our aim was to compare the influence of the polymer matrix on the rotational motion of the small acetonitrile molecule with the effect on the reorientational relaxation of a larger probe molecule. It has been shown that the dianion polyphenyl 2 is a suitable probe molecule to monitor modifications of the frictional characteristics of the solvent in the presence of a polymer matrix.9 Thus, in addition to the Raman measurements we have investigated the rotational motion of PP2 in PAA gels containing the water-acetonitrile mixture as solvent by timeresolved fluorescence depolarization spectroscopy over a similar concentration and temperature range. In Figure 4, the ηs/T dependence of the rotational relaxation times of PP2 is presented for different PAA gel concentrations (0.025 e C e 0.2 g/mL, 0 e fBIS e 0.02). For comparison, we have included the rotational relaxation times of PP2 in the pure water-acetonitrile mixture. For all gel concentrations and under pure solvent conditions τr reveals a linear dependence on ηs/T, in agreement with the SED equation. In contrast to the behavior of the acetonitrile molecule the rotational motion of PP2 is significantly slowed down in the gels, as demonstrated by a much stronger increase of the slopes in the SED plot with PAA concentration (Table 2). In addition, we have investigated the effect of cross-linking density on the rotational motion of PP2 in a PAA gel with C ) 0.1 g/mL with and without cross-linker. A comparison of the temperature dependent rotational relaxation times is given in Figure 4. As similarly observed for acetonitrile, the rotational relaxation times of PP2 are not affected by the cross-linker. The hydrodynamic volume of PP2 (Vh/kB ) 117 ( 12 ns K/cP) in pure water-acetonitrile, which we have obtained from the slope in the SED plot, is in good agreement with the value (Vh/kB ) 119 ( 11 ns K/cP) determined for water, watermethanol, and water-glycerol mixtures.12 Our computed rotational relaxation times of PP2 based on the Perrin model for an ellipsoid of revolution and a modified version of the bead model, respectively, indicated that the reorientational motion of PP2 is consistent with hydrodynamic stick boundary conditions.12 Furthermore, in a previous study9 we have found that a change in the hydrodynamic boundary conditions and specific interactions between PP2 and the polymer can be neglected, and we have concluded that the polymer component alters the effective frictional force the probe molecule experiences under

J. Phys. Chem., Vol. 100, No. 5, 1996 1709

Figure 5. Concentration dependence of the effective friction coefficient ζ(C) for the rotational relaxation of PP2 in PAA gels in water (×), water-acetonitrile (90:10 wt %) (]), and water-methanol (75:25 vol %) (+) and for the rotational relaxation of acetonitrile in PAA gels in water-acetonitrile (90:10 wt %) ([). The lines, as obtained by a polynomial fit, should provide a guide to the eye, but have no further justification.

rotation. This polymer-induced modification of the frictional forces on the solvent can be phenomenologically defined as an effective “microviscosity”:

ηeff ) ηsζ(C)

(16)

where ηs is the viscosity of the pure solvent. A thorough physical understanding would require a microscopic description of the solvent viscosity inside the gel matrix. The frictional coefficient ζ(C) that characterizes the change in effective friction upon addition of polymer was obtained from the slopes in the SED plot:

ζ(C) )

τr(C,ηs/T) τr(C)0,ηs/T)

(17)

where τr(C,ηs/T) is the slope at a specific polymer concentration C. τr(C)0,ηs/T) refers to the slope in pure solvent without added polymer. Figure 5 displays the gel concentration dependence of the frictional coefficient. For comparison we have included the coefficient ζ(C) determined from the rotational relaxation times of PP2 in gels containing water and water-methanol as solvent.9 While water is known as a good solvent for PAA gels, the methanol mixture is a theta solvent. In our previous study9 we have suggested that a strong polymer-solvent interaction, as expected for water as a good solvent (with strong hydrogen bonding and dipolar interactions), results in an extensive modification of the local frictional forces on the probe molecules and, hence, to a large value for ζ(C). The effective frictional coefficients for PP2 in gels containing water-acetonitrile as solvent fall between those of both reference solvents, indicating that the addition of 10 wt % acetonitrile to water significantly decreases the solvent quality, accompanied by a less pronounced polymer-solvent interaction. In contrast, the calculated frictional coefficient, ζ(C), for the rotational motion of acetonitrile in this mixture is very small, reflecting a weak influence of the polymer on the rotational motion of the solvent molecules. From the temperature dependence of the rotational relaxation times of PP2 we obtained the activation enthalpy ∆Hτq for the reorientational motion. In all gel samples ∆Hτq ) 20.6 ( 3.3 kJ/mol was independent of the polymer concentration. Furthermore, we found a good aggreement with the activation enthalpy determined in the pure solvent (∆Hτq ) 18.4 ( 1.8 kJ/mol). In contrast to the behavior of acetonitrile (see Table

1710 J. Phys. Chem., Vol. 100, No. 5, 1996 1) these activation enthalpies are very similar to those for the macroscopic shear viscosity of the solvent (∆Hηq ) 18.9 ( 0.6 kJ/mol), indicating that the reorientational motion of PP2 is related to the solvent acting as a continuous viscous fluid.

Biermann et al. gratefully acknowledge financial support from the Fonds der Chemischen Industrie. W.E. thanks the Deutsche Forschungsgemeinschaft for an award of a Heisenberg fellowship. References and Notes

Summary Our investigations on the rotational motion of acetonitrile in a binary mixture with water have shown that the dynamics of the solvent molecule acetonitrile is only weakly influenced by the presence of a polymer network. The reorientation slows down by about a factor of 1.45 in the presence of a PAA polymer matrix up to a concentration of C ) 0.2 g/mL. Experiments on the translational motion of water in PAA gels indicate a similar and small effect of the polymer matrix on the diffusion of water.35 In comparison, the retardation of the probe molecule PP2 was about 10 times stronger, as specified by the effective frictional coefficient. One way to understand this very different behavior is to take into account the size of the probe and solvent molecules and hence the different length scales on which the dynamic processes occur. Comparing the reorientation of neat acetonitrile and in the water-acetonitrile mixture indicates that the dynamics are governed by specific local interactions between water and acetonitrile molecules. According to our Raman measurements, these local interactions persist in the presence of the polymer matrix. On the contrary, PP2 probably does not experience the detailed molecular features of the solvent mixture during the reorientational motion. The probe molecule seems to reflect the bulk viscoelastic properties of the solvent that are strongly modified by the polymer component. Similar effects have been observed by Gisser et al.7, who studied the rotational and translational diffusion of probe molecules in polystyrene/ tetrahydrofuran solutions. They observed a much stronger retardation of the larger probe molecules than for the solvent THF itself and suggested that the THF molecule is too small to feel the same friction as larger probe molecules. Although there are no corresponding data on the translational diffusion of water in the presence of acetonitrile and PAA available, we conclude that the effect of the polymer network on the transport coefficients of the solvent molecules is rather limited. These different experimental results provide strong evidence that molecules of different size monitor frictional forces in polymer solutions on different length scales. Acknowledgments. W.M. is grateful for a fellowship from the Studienstiftung des deutschen Volkes. Th.D. and W.E.

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