Odd–Even Effect on Rotational Dynamics of Spin-Labeled Polyacid

Mar 12, 2019 - A nitroxide spin label has been covalently attached to the weak polyelectrolyte poly(ethylene-alt-maleic acid) (P(E-alt-MA)) to study t...
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Odd−Even Effect on Rotational Dynamics of Spin-Labeled Polyacid Chain Segments in Polyelectrolyte Multilayers Uwe Lappan,* Cindy Rau,† Carolin Naas, and Ulrich Scheler Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Straße 6, 01069 Dresden, Germany

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S Supporting Information *

ABSTRACT: A nitroxide spin label has been covalently attached to the weak polyelectrolyte poly(ethylene-alt-maleic acid) (P(E-alt-MA)) to study the rotational dynamics of the polyacid backbone in swollen polyelectrolyte multilayers (PEMs) formed by P(E-alt-MA) and the oppositely charged weak polycation poly(allylamine hydrochloide) (PAH) by continuous wave electron paramagnetic resonance (EPR) spectroscopy. The growth of the PEM film with increasing number of layers has been monitored by quantitative EPR using the spin-labeled polyanion (SL-P(E-alt-MA)) for the preparation of every polyanion layer. A parabolic growth has been found for PEMs with up to 16 layers. In further experiments the SL-P(E-alt-MA) has been placed in selected layers, and the line shape has been analyzed. A pronounced odd−even effect has been observed, i.e., the rotational dynamics of the P(E-alt-MA) backbone in the PEMs is influenced by the chemical nature of the polyelectrolyte in the terminating layer.

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incorporation of different charged compounds.3 Reviews have summarized the state of the art of PEMs.3,5,7−9 Properties like sensitivity to the environment or stability are highly correlated with the internal structure of the PEMs. Therefore, understanding and control of the internal structure are essential for tuning their physicochemical properties.8 In the area of fundamental science, there is still much to learn about the PEM materials.10 It has repeatedly been pointed out that so far only a few studies on the dynamics of polyelectrolytes in multilayers have been published.3,8,10 Schönhoff stated that a crucial issue for refining models of PEMs is dynamic studies, for example clarifying the question whether PEMs should be considered as flexible networks, as glasses, or as solids.3 However, the experimental possibilities are limited and so far only few data are available.3 Less attention has been paid to dynamical parameters such as the polyelectrolyte translational diffusion coefficients within the PEM films as noticed by von Klitzing.8 Schlenoff stated that whereas the structure of LbL films is well

INTRODUCTION Polyelectrolytes are a fascinating class of polymers that exhibit various interesting phenomena due to their dual character of highly charged electrolytes and macromolecular chain molecules.1 Polyelectrolyte multilayers (PEMs) composed of oppositely charged polyelectrolytes can be prepared on planar surfaces or colloidal templates by the layer-by-layer technique (LbL) introduced by Decher et al.2 As applications of planar PEMs, for example, sensor materials, functional coatings, and selective membranes are discussed and explored.3 The LbL technique has proven to be a versatile and simple method for the fabrication of very thin polyelectrolyte multilayers making it highly suitable for the preparation of separation membranes.4 The coating of colloidal particles leads to the formation of very stable, hollow polymeric shell structures after the removal of the core. Such thin-walled microcapsules have attracted particular interest from the viewpoint of applications in encapsulation, for example as drug carrier systems, or microreactors.5 Most significant advantages of PEM capsules are multifunctionality and availability of various stimuli to affect and control their properties.6 Since the electrostatic interaction is a very general principle, the LbL technique is very versatile with respect to the © XXXX American Chemical Society

Received: January 15, 2019 Revised: February 27, 2019

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DOI: 10.1021/acs.macromol.9b00101 Macromolecules XXXX, XXX, XXX−XXX

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23 mL of anhydrous pyridine and stirred at 50 °C under argon for 3 h. 4-Amino-TEMPO (0.5 mmol) was dissolved in 2 mL anhydrous pyridine and added dropwise to the solution of P(E-alt-MAn). The mixture was stirred overnight at room temperature under argon. The polymer was isolated by precipitation into n-hexane (2 L) and dried in vacuum for 48 h at room temperature. The product was dissolved in 50 mL anhydrous tetrahydrofuran (THF), filtered, and again isolated by precipitation into n-Hexane. Then the precipitate was dried for 48 h in vacuum at room temperature. The concentration of the SL determined by quantitative EPR spectroscopy is approximately one SL per 40 repeat units in the polymer chain. Preparation of Polyelectrolyte Multilayers (PEMs). Aqueous stock solutions of 10 mmol/L PAH, 5 mmol/L P(E-alt-MA), as well as 5 mmol/L SL-P(E-alt-MA) in 0.2 mol/L acetate buffer adjusted to pH 4.0 and 5.0, respectively, were used to prepare the PEMs. The concentrations of the polymer solutions mentioned above are quoted with respect to the repeat units. Note that P(E-alt-MA) has two carboxylic acid groups per repeat unit. The stock solutions additionally contain 0.1 mol/L NaCl to increase the ionic strength. The pH of the stock solutions was adjusted by acetate buffer rather than by simple acids in order to keep the pH during the preparation of the PEMs constant. Without buffer the pH decreases, because protons are released from the weak polyacid into the solution as a result of the interaction of the carboxylic acid groups with the polycation. The counterion release is the driving force for the complexation. The PEMs were prepared on the inner surface of glass capillaries (Blaubrand, intraMark, 50 μL) via the layer-by-layer technique introduced by Decher et al.2 At first, the capillary was filled with a stock solution of PAH (pH 4) using capillary forces. After an adsorption time of 10 min, the solution was removed using a tissue which was pressed on the lower end of the capillary. The outer surface of the capillary was wiped clean using a tissue. Afterward, the capillary was rinsed by capillary forces two times with 0.2 mol/L acetate buffer (pH 4). Then, the capillary was filled with a stock solution of P(E-altMA) or SL-P(E-alt-MA) (pH 4), respectively. The adsorption and rinsing processes were repeated until reaching the desired number of layers. Samples with up to 17 layers were prepared. Finally, the capillaries were filled with an acetate buffer solution of pH 4 and sealed with wax, i.e., the multilayers were swollen in buffer solutions before recording the EPR spectra. A similar procedure was used to prepare PEMs at pH 5. EPR Spectroscopy. X-band CW EPR spectra (microwave frequency ≈ 9.4 GHz) were recorded on an EMX-plus spectrometer (Bruker BioSpin), equipped with the resonator ER 4119 HS-W1, and the variable temperature unit ER4141VT. Acquisition parameters for the spectra of PEMs prepared in glass capillaries had a sweep width of 150 G, a microwave power of 10 mW, a modulation frequency of 100 kHz, a modulation amplitude of 4 G, a time constant of 10.24 ms, a conversion time of 40.96 ms, 64 scans, and 1024 data points. All spectra of PEMs were measured at room temperature. Relatively large power and modulation amplitude were used to improve the signal-tonoise ratio. It has been verified that there was no broadening or distortion of the signal due to power saturation. Spectra of the buffer solutions were subtracted from the spectra of the PEMs to eliminate weak background signals. Solutions of the polymers were loaded into glass capillaries (Blaubrand, intraMark, 50 μL) and were recorded with the parameters mentioned above but with microwave power of 2 mW, modulation amplitude of 1 G, and 16 scans. Rigid-limit spectra were obtained by recording spectra of frozen solutions in quartz tubes with i.d. = 3 mm at 123 K, and were measured with parameters mentioned for the liquid solutions but with microwave power of 0.1 mW. Simulation of CW EPR Spectra. The spectra were simulated and fitted using the MATLAB-based software package EasySpin.40−42 The g and 14N hyperfine tensors of SL-P(E-alt-MA) were determined by the simulation of the rigid-limit spectra of the solution measured at 123 K using the EasySpin function pepper. The slow-motion spectra of the SL-P(E-alt-MA) selectively placed in PEMs were calculated using the EasySpin function chili. The values of the g and 14N hyperfine tensor components were fixed throughout the analysis of the slow-

established, the dynamics of their polymer or inorganic constituents is not.10 The internal structure of PEMs has been widely examined, but little work, apart from some NMR studies of supported films11−13 and hollow capsules,14 has been done to correlate the macroscopic properties of the PEMs with the underlying chain dynamics as noticed by Fortier-McGill et al.15 Friess et al.16 have studied the mobility of polymer chains in free-standing polyelectrolyte multilayers by solid-state NMR. The combination of 1H T1rho as a measure of molecular mobility with 13C chemical shift resolution for the assignment to chemical structure provided a molecular insight. In recent years, mainly two techniques have been employed to investigate diffusion of polyelectrolytes within multilayers. Neutron reflectometry has been used to measure and quantify the vertical diffusion of deuterated polymer chains and the layer intermixing in PEMs,17−28 and fluorescence recovery after photobleaching has been applied to study the lateral diffusion of fluorescently labeled polyelectrolytes within PEMs.20,21,25,29−31 Moreover, radiolabeling has been used to provide sensitive and unambiguous tracking of diffusing species within PEMs.28,32−34 Interestingly, it was discovered that in PEMs of poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) extrinsic compensation sites diffuse much faster than the polyelectrolytes themselves.33 The explanation given by the authors is that site diffusion requires only local rearrangements of polyelectrolyte repeat units. Nevertheless, it has been stated that in spite of an increasing number of studies on PEM films, the dynamics of these films are still poorly understood, especially in films of nonlinear growth.27 Recently, we have shown that chain dynamics in polyelectrolyte multilayers (PEMs) and polyelectrolyte complexes (PECs) can be studied using the electron paramagnetic resonance (EPR) spin-label (SL) technique.35−39 The technique employs stable nitroxide radicals as SLs which are covalently linked to macromolecules. Rotational dynamics of such a nitroxide SL on time scales between 10 ps and 1 μs can be characterized by continuous wave (CW) EPR spectroscopy, simulating the line shape. We have found that the dynamics in PECs of a weak polyanion and a strong polycation is strongly influenced by the degree of dissociation of the weak polyacid.37,38 The recent study has shown that the nature of the ion pairs formed between the oppositely charged polyelectrolytes in the PECs has a strong impact on the dynamics of the polyanion.39 We have also demonstrated that the spin-label EPR can be used to investigate the formation and stability of PEMs.35 The present work deals with the study of the effect of the outermost layer on rotational dynamics in PEMs composed of the weak polyanion poly(ethylene-altmaleic acid) (P(E-alt-MA)) and the weak polycation poly(allylamine hydrochloride (PAH) using spin-labeled P(E-altMA) as a reporter molecule.

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EXPERIMENTAL SECTION

Materials. Poly(ethylene-alt-maleic anhydride) (P(E-alt-MAn)) with a molecular weight of 100−500 kg/mol, poly(allylamine hydrochloride) (PAH) with an average molecular weight of 450 kg/ mol, and 4-amino-2,2,6,6-tetramethylpiperidine 1-oxide (4-aminoTEMPO) were purchased from Sigma-Aldrich Corp. (St. Louis, MO, U.S.A.) and were used without further purification. Spin-Labeling Reaction. The spin-labeled poly(ethylene-altmaleic anhydride) (SL-P(E-alt-MAn)) was synthesized by the reaction of P(E-alt-MAn) with 4-amino-TEMPO in pyridine. More precisely, P(E-alt-MAn) (10 mmol) was introduced into a flask with B

DOI: 10.1021/acs.macromol.9b00101 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules motion spectra. The simulated spectra were fitted to experimental spectra by the EasySpin function esf it using the Levenberg− Marquardt minimization algorithm. The relatively large modulation amplitude of 4 G is explicitly taken into account in the simulations. For each experimental spectrum, 24 fits were carried out with different sets of starting parameters, which partly result in different local minima. Average values of the fitted parameter were calculated including fits with a root-mean-square-deviation (rmsd) according to “rmsdFit < rmsdBestFit + 0.001”. For the simulation of the slow-motion spectra of the SL-P(E-alt-MA) in the PEMs the microscopic order/ macroscopic disorder (MOMD) model of restricted rotational diffusion was applied.43,44 The rotational diffusion of the SL attached to polymer chain segments was approximated by superposition of the isotropic rotational diffusion of the polymer chain segment with the rotational diffusion coefficient RS, and the internal rotation of the SL with the rotational diffusion coefficient RI. It is assumed that RS = Rprp and RI = Rpll − Rprp, because the SL in SL-P(E-alt-MA) is attached to the polymer chain via a short tether. Rpll and Rprp are the parallel and perpendicular rotational diffusion coefficients in the model of Brownian diffusion with an axially symmetric rotational diffusion tensor. The axis of internal rotation zR is tilted relative to the zm axis in the xmzm plane of the magnetic axis system of the SL and defined by the angle βD. The restricted motion of the SL attached to the polymer backbone is approximated by the orienting potential coefficients c20, c22, and c40.

confirms the formation of PEM films on the inner surface of the glass capillaries. The signal amplitude increases with increasing number of layers N indicating that the amount of SL-P(E-alt-MA) goes up with the growing of the PEM. The spectra are dominated by a slow-motion component, which is caused by the complexation of the spin-labeled polyanion with the oppositely charged polycation in the PEMs as shown before.35 Evidently, the complex formation leads to a restricted rotational mobility of the SL. A minor, fast-motion component becomes visible in the spectra measured after 6 d as indicated by arrows in Figure 1b. This fast-motion component is hardly detectable in the spectra measured 1 d after preparation, i.e., their concentration increases with increasing storage time. Thus, this fast-motion component is assumed to arise from the cleavage of the SL from the SL-P(E-alt-MA) due to a partial hydrolysis of the amide bonds because of the low pH. The simulations mentioned below resulted in very low concentrations of less than 2 mol% free SL with respect to the covalently linked SL. In addition to the spectra of PEMs with even number of layers the spectrum of 17 layers was also recorded. This PEM-17 is terminated with PAH in contrast to the PEMs with even number of layers, which have SL-P(E-altMA) in the outermost layer. Figure 1 shows that the chemical nature of the polyelectrolyte in the terminating layer has an influence on the line shape of the slow-motion component. The outer extrema separation 2Azz′ of the spectral line shape, i.e., the difference of the resonance fields of the low-field maximum to the high-field minimum of the spectrum, is smaller for the termination with PAH than with SL-P(E-altMA). This interesting result is investigated in detail in the second part of the study using selectively placed SL-P(E-altMA) as described below. P(E-alt-MA) is a weak polyacid with two dissociation constants of pK01 = 4.0 and pK02 = 6.3,45 i.e., the degree of dissociation αD is about 0.25 at pH 4 and about 0.48 at pH 5. Thus, further PEMs of PAH/SL-P(E-alt-MA) were prepared at pH 5, where the concentration of carboxylate ions is about twice as much compared to pH 4. The spectra are plotted in Figure 2. At first view, these spectra are similar to the spectra of the PEMs prepared at pH 4. However, the signal amplitude of the thickest layer is higher. The integrated signal intensity Iint calculated by double integration of the spectra plotted in Figures 1 and 2 increases with increasing number of layers N in a nonlinear manner as shown in Figure 3. The intensity Iint is proportional to the amount of SL-P(E-alt-MA) in the PEMs, i.e., only the growth of the even layers is monitored by EPR. Multilayers prepared at pH 5 show a higher growth for N ≥ 12 than PEMs prepared at pH 4. A more detailed analysis of the data revealed that the integrated signal intensity Iint increases parabolically for 4 ≤ N ≤ 16 as shown in Figure 3. Beside exponential and linear growth regimes parabolic growth has been already described in literature for PEMs of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrenesulfonate) (PSS).26,46 The delayed onset of the parabolic growth is assumed to arise from the fact that isolated islands are formed at the beginning according to the roughness model, i.e., the substrate is not completely covered by the PEM. The islands grow with increasing number of layers until the islands begin to coagulate.47 The differences between the intensities measured after 1 and 6 d for both series are within the limits of the experimental uncertainty of the EPR measurements. The EPR line shapes of PEM-16 and PEM-17 are different for both pH

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RESULTS AND DISCUSSION At first, PEMs of PAH and SL-P(E-alt-MA) composed of 2 to 17 layers were prepared applying stock solutions of both polyelectrolytes in acetate buffer of pH 4. After preparation of the desired number of layers N, the PEMs were swollen in acetate buffer of pH 4. The EPR spectra of the swollen PEMs recorded 1 and 6 d after preparation are shown in Figure 1a,b, respectively. The sequence of the spectra up to 16 layers

Figure 1. EPR spectra of PAH/SL-P(E-alt-MA) multilayers prepared and swollen in buffer solution of pH 4 and measured (a) 1 day and (b) 6 days after preparation at room temperature. The number of layers N in the PEMs is indicated in the legend. C

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and segmental motions of the polymer backbone at the point of the attachment of the SL.48 Multilayers of PAH/P(E-altMA) with 16 and 17 layers, respectively, were prepared where the SL-P(E-alt-MA) was selectively placed in a single layer, which is referred to as NSL. PEM-16 in combination with NSL = 10 means, for instance, that a structure of glass/(PAH/P(E-altMA))4/PAH/(SL-P(E-alt-MA)/(PAH/P(E-alt-MA))3 was built. Figure 4 shows the spectra of such PEMs prepared at

Figure 2. EPR spectra of PAH/SL-P(E-alt-MA) multilayers prepared and swollen in buffer solution of pH 5 and measured (a) 1 day and (b) 6 days after preparation at room temperature.

Figure 4. EPR spectra of PAH/P(E-alt-MA) multilayers with SL-P(Ealt-MA) placed in a selected layer (NSL), prepared and swollen in buffer solution of pH 4, and measured 1 day after preparation at room temperature. The PEMs consist of (a) 16 layers and (b) 17 layers.

pH 4 and measured after 1 d. In Figure 4a,b, the signal amplitude increases with increasing NSL because of the nonlinear growth of the PEMs as mentioned above. This nonlinear growth is also the reason for the decision, that PEMs with NSL = 2, 4, 6, or 8 were not prepared because the signal intensity and the S/N ratio was expected to be low. Figure 4 shows, that the spectra of PEM-16 and PEM-17 differ in the line shape indicating an influence of the terminating layer on the dynamics of the spin-labeled polyanion both on the surface and in the bulk of the multilayer assemblies. Spectra of PEMs with selectively placed SL-P(E-alt-MA) prepared at pH 5 are plotted in Figure 5. Consistent with the results for pH 4, the signal amplitude increases with increasing NSL in both series and the line shapes are different for PEM-16 and PEM-17. The spectra of the PEMs prepared at pH 4 and pH 5, respectively, were also recorded 6 d after preparation (Supporting Information, SI, Figures S1 and S2). Differences compared to the spectra measured 1 d after the preparations only occur in the amount of free SL as a result of acid-catalyzed hydrolysis. The integrated signal intensity Iint of the PEMs with selectively placed SL-P(E-alt-MA) was also calculated by

Figure 3. Integrated signal intensity Iint of PAH/SL-P(E-alt-MA) multilayers prepared and swollen in buffer solutions of pH 4 and pH 5, respectively, and measured 1 day and 6 days after preparation at room temperature as a function of the number of layers N.

values as mentioned above. Nevertheless, Figure 3 shows that the integrated signal intensities Iint are equal within the experimental uncertainty as expected. In the first series of experiments we have shown that the growth of PEMs on the inner surface of glass capillaries, which are normally used for EPR measurements of aqueous solutions, can be monitored by quantitative EPR using the SL-P(E-altMA) for the preparation of every even layer as already shown earlier for PEMs of SL-P(E-alt-MA) and PDADMAC.35 In the second series the strength of the spin-label technique was exploited, i.e., the SL-P(E-alt-MA) is used as a reporter molecule for the study of the dynamics in the assembly. The SL reports about the dynamics of the spin-labeled macromolecule because the rotational mobility of the SL is influenced by the motion of the side chain bearing the SL D

DOI: 10.1021/acs.macromol.9b00101 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. Integrated signal intensity Iint of PAH/P(E-alt-MA) multilayers with SL-P(E-alt-MA) placed in a selected layer (NSL), measured 1 day and 6 days after preparation at room temperature as a function of the place NSL of the SL-P(E-alt-MA). The PEMs were prepared and swollen in buffer solutions of (a) pH 4 and (b) pH 5. In addition, the increment of the integrated signal intensity ΔIint for PAH/SL-P(E-alt-MA) multilayers prepared and swollen in buffer solutions of (a) pH 4 and (b) pH 5 is shown.

Figure 5. EPR spectra of PAH/P(E-alt-MA) multilayers with SL-P(Ealt-MA) placed in a selected layer (NSL), prepared and swollen in buffer solution of pH 5, and measured 1 day after preparation at room temperature. The PEMs consist of (a) 16 layers and (b) 17 layers.

double integration of the spectra and plotted in Figure 6. In addition, the increment of the integrated signal intensity ΔIint for PEMs of PAH/SL-P(E-alt-MA) measured in the first series of experiments is shown in Figure 6. The increment ΔIint is calculated according to eq 1: ΔIint(NSL) = Iint(N ) − Iint(N − 2)

(1)

For both pH values, the intensity Iint increases with increasing NSL because of the nonlinear growth, and the intensities Iint are equal for PEM-16 and PEM-17 within the uncertainty. Furthermore, the intensity Iint coincides with increment ΔIint within the experimental uncertainty and its propagation. The partial substitution of SL-P(E-alt-MA) by P(E-alt-MA) has no influence on the growth of the multilayer assembly. It is concluded that identical multilayer assemblies were built with and without SL. It is well-known that the EPR line shape is influenced by the rotational mobility of the SL.49 Figure 7 shows the spectra of SL-P(E-alt-MA) in buffer solution of pH 4 measured at two different temperatures. At a temperature of 123 K, the SL is immobile and the tumbling is much slower than the ESR time scale. A powder pattern spectrum is observed. The hyperfine coupling tensor A gives rise to three broad lines with an outer extrema separation 2Azz′ of 75.0 G. At higher temperatures, when the rotational motion of the SL becomes faster, the anisotropy of the hyperfine interactions is partly averaged out, giving rise to a narrowing of the spectrum, reflected in a decrease of the outer extrema separation. At a temperature of 295 K the spectrum of SL-P(E-alt-MA) in aqueous solution has an outer extrema separation 2Azz′ of 36.7 G. Indeed, the rotational motion of the SL covalently attached to the

Figure 7. EPR spectra of SL-P(E-alt-MA) in buffer solution of pH 4 measured at a temperature T of 295 K and of 123 K.

macromolecule is not sufficiently fast to average out the anisotropy completely, which would be indicated by three lines with equal line widths. The outer extrema separation 2Azz′ in the spectra of the different PEMs with selectively placed SL-P(E-alt-MA) is shown in Figure 8. The values of 2Azz′ are in the range of 56 to 70 G, i.e., compared to the immobilized SL-P(E-alt-MA) in frozen solution the SLs in the PEMs have a rotational mobility which is detectable by X-band EPR. However, compared to the SL-P(E-alt-MA) in aqueous solution at room temperature the E

DOI: 10.1021/acs.macromol.9b00101 Macromolecules XXXX, XXX, XXX−XXX

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Figure 8. Outer extrema separation 2Azz′ in the EPR spectra of PAH/ P(E-alt-MA) multilayers with SL-P(E-alt-MA) placed in a selected layer, prepared, and swollen in buffer solution of pH 4 and pH 5, respectively, and measured 1 day and 6 days after preparation at room temperature as a function of the place NSL of the SL-P(E-alt-MA).

Figure 9. Experimental and simulated EPR spectra of PAH/P(E-altMA) multilayers with SL-P(E-alt-MA) placed in a selected layer (NSL), prepared and swollen in buffer solution of pH 4, and measured 1 day after preparation at room temperature. The number of layers N in the PEMs is indicated in brackets. The spectra are normalized to the maximum height of the middle-field line.

rotational motion of the SLs in the swollen PEMs is much lower than in solution. The origin is the complexation of the spin-labeled polyanion with the polycation as mentioned above. The recording of the spectra 1 and 6 d after preparation results in nearly identical values for 2Azz′ indicating that no reorganization and no loss of material occur during this time. The extrema separation 2Azz′ is lower for PEM-17 compared to PEM-16, which means that the SLs have higher rotational mobility in the PEMs terminated with PAH compared to PEMs terminated with P(E-alt-MA). Such effects are termed odd−even effects, because the polycation-terminated PEMs consist of an odd number of layers and the polyanionterminated PEMs of an even number. Figure 8 shows, that 2Azz′ of the PEM-17 samples is lower for pH 5 than for pH 4. Furthermore, 2Azz′ decreases with increasing NSL for both PEM-17 series, indicating that the rotational mobility of the SL depends on the distance of the layer with the SL-P(E-alt-MA) from the outermost surface of the PEM. In order to obtain a detailed picture of the rotational dynamics of the polyacid chain segments the spectra of the PEMs with selectively placed SL-P(E-alt-MA) were simulated and fitted on the basis of two spectral components: a main component for the SL-P(E-alt-MA) in the PEM and a minor component for the free SL. Models used for the simulations are the MOMD model for the main, slow-motion component, and the simple model of isotropic rotational diffusion for the minor, fast-motion component. The experimental and the simulated spectra are shown in Figure 9 for PEMs prepared at pH 4 and measured 1 d after preparation and in the SI for multilayers prepared at pH 4 and measured 6 d after preparation (Figure S3) as well as for PEMs prepared at pH 5 after 1 d (Figure S4) and after 6 d (Figure S5). These figures show that all of the experimental spectra are well reproduced by the simulations. For the main component, the simulation and fitting result in the rotational diffusion coefficients RI and RS, the diffusion tilt angle βD, and the orienting potential coefficients c20, c22, and c40 as shown in SI Figures S6−S9. The results for the parameters RI and RS of all PEMs are summarized in Figure 10. The coefficient RI characterizing the internal rotation of the SL is at least 2 orders of magnitude larger than the coefficient RS characterizing the rotational mobility of the polyacid backbone. There is no difference in RI for the different PEMs in view of

Figure 10. Rotational diffusion coefficients RI and RS of the main component in PAH/P(E-alt-MA) multilayers with SL-P(E-alt-MA) placed in a selected layer, prepared and swollen in buffer solution of pH 4 and pH 5, respectively, and measured 1 day and 6 days after preparation at room temperature as a function of the place NSL of the SL-P(E-alt-MA). The isotropic limit (correlation time τc < 10 ps, log(R/s−1)>10.2) and the rigid limit (τc > 1 μs, log(R/s−1)