Segmental Motions of Poly(ethylene glycol) Chains Adsorbed on

Article pubs.acs.org/Langmuir

Segmental Motions of Poly(ethylene glycol) Chains Adsorbed on Laponite Platelets in Clay-Based Hydrogels: A NMR Investigation Cédric Lorthioir,* Mouhamad Khalil,† Véronique Wintgens, and Catherine Amiel Equipe “Systèmes Polymères Complexes”, Institut de Chimie et des Matériaux Paris-Est (UMR 7182 CNRS/Université Paris-Est Créteil), 2-8 rue Henri Dunant, 94320 Thiais, France S Supporting Information *

ABSTRACT: The segmental dynamics of poly(ethylene glycol) (PEG) chains adsorbed on the clay platelets within nanocomposite PEG/Laponite hydrogels was investigated over the tens of microseconds time scale, using combined solution and solid-state NMR approaches. In a first step, the time evolution of the molecular mobility displayed by the PEG chains following the addition to a Laponite aqueous dispersion was monitored during the aggregation of the clay disks and the hydrogel formation, by means of 1H solution-state NMR. Part of the PEG repeat units were found to get strongly constrained during the gelation process. Comparisons between this time evolution of the PEG local dynamics in the PEG/Laponite/water systems and the increase of the macroscopic storage shear modulus, mainly governed by the assembling of the Laponite disks, indicate that the slowing down of the segmental motions arises from adsorbed PEG repeat units or chain portions strongly constrained between aggregated clay layers. In a second step, after completion of the gelation process, the molecular motions of the adsorbed PEG chains were probed by 1H solid-state NMR spectroscopy. 1H double-quantum experiments indicate that the adsorbed PEG repeat units, though reported to be frozen over a few tens of nanoseconds, still display significant reorientational motions over the tens of microseconds time scale. Using a comparison with a model system of amorphized PEG chains, the characteristic frequency of these segmental motions was found to range between 78.0 kHz and 100.7 MHz at 300 K. Interestingly, at this temperature, the level of reorientational motions detected for these adsorbed PEG chain portions was found to be as restricted as the one of bulk amorphous PEG chains, cooled at a slightly lower temperature (about 290 K). dynamics7 of the polymer chains near the filler surfaces may significantly differ from the behavior of the same polymer component, considered in the bulk state, due to the vicinity of a solid surface on the one hand and to the possible interactions between the filler surface groups and the polymer chains on the other hand. As soon as the size of the reinforcing objects ranges in the nanometer length scale and a homogeneous dispersion of these particles within the matrix is achieved, the interfacial regions and their contribution to the mechanical/rheological properties cannot be neglected any longer. Therefore, both local conformation and segmental dynamics of the interfacial polymer chains are important issues to be investigated. The dynamical behavior displayed by the interfacial chains within polymer-based (nano)composites has been and is still thoroughly investigated in the literature. In this respect, nanocomposites characterized by various polymer matrices, different natures, shapes and sizes of the filler particles, as well as distinct process conditions and filler dispersions within the chains were considered. The comparison between systems with or without covalent bonds between the polymer chains and the filler particles was also performed. 1H solid-state NMR proved

1. INTRODUCTION The incorporation of rigid nanoparticles within molten polymers is a common way to increase their Young modulus, if the neat polymer matrices are cross-linked,1,2 or their viscosity, for uncross-linked polymer chains.3 Such a mechanical/rheological reinforcement may arise from the combination of several physical features. First, the hydrodynamic effects partly account for the observed reinforcing effects and depend, at a molecular length scale, on the filler content as well as the size and the shape of the nanoparticles.1,2,4 A second feature that should also be considered to rationalize the abovementioned mechanical/rheological reinforcement is the organization of the filler particles within the polymer matrix. In this respect, the formation of percolating networks composed of single or aggregated filler nanoparticles may also significantly contribute to the enhancement of the Young modulus or viscosity of the polymer matrix.2,5 At the molecular level, the possibility to form such a percolating filler network depends on the filler volume fraction, the aspect ratio of the reinforcing particles, and the extent of interactions between the nanoparticles, compared to the polymer chain/nanoparticle interactions.2,5 Lastly, the contact area between the polymer chains and the filler nanoparticles also plays a key role in the enhancement of the mechanical/rheological behavior of the matrix.2 Indeed, the local structure6 and the segmental © 2012 American Chemical Society

Received: March 13, 2012 Revised: April 17, 2012 Published: April 18, 2012 7859

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tensile modulus was found to be an increasing function of the clay content. However, the molecular origin of the reinforcement induced by adding clay layers within the hydrogel structure was not so extensively investigated as in the case of nanocomposites based on bulk polymer matrices and clay platelets. In the hydrogels, the filler particles were considered to act as multifunctional cross-links, but the respective contributions from the hydrodynamic effects, the precise extent of the clay dispersion, and the interactions between the polymer chains and the clay surfaces to the reinforcement were not considered until now. To the best of our knowledge, only one recent work, dedicated to poly(N,N-dimethylacrylamide) (PDMA)-based hydrogels filled by silica nanospheres, evidenced that the hydrodynamic effects were not the only ones to be considered in order to account for the variation of the Young’s modulus with the filler content.21,22 In this contribution, swelling measurements performed on these nanocomposite hydrogels indicated the occurrence of significant attractive interactions between the silica particles and the cross-linked PDMA chains. Using an approach proposed by Lequeux et al.,23 the composition dependence of the hydrogel swelling led to estimate a characteristic thickness of adsorbed PDMA chains at the filler surfaces, ranging between 1 and 2 nm. However, the segmental dynamics of these interfacial PDMA chains was not probed directly, at the molecular length scale. Nanocomposite hydrogels composed of poly(ethylene oxide) (PEO) and clay platelets have also attracted a lot of attention in the past decade.16 In particular, the influence of PEO chains on the aggregation and gelation of Laponite platelets in aqueous solutions was deeply investigated, by means of light scattering24 and rheology25,26 experiments. In this respect, the PEO molecular weight was found to play a crucial role in the aggregation kinetics. In the low molecular weight regime, the adsorption of PEO chains at the Laponite surfaces results in a slowing down of the aggregation rate of the clay layers, due to steric hindrance. For a Laponite aqueous dispersion at a concentration of 1 g·L−1, with 10 mM NaCl, this effect was found to be more and more pronounced as the PEO chain molecular weight Mw is increased up to about 1000 g·mol−1.25 Above this critical value, the aggregation rate of the Laponite platelets gets faster and faster as the PEO Mw is raised up, due to bridging phenomena. The mechanisms involved in bridging flocculation were studied at the molecular level in the case of hydrogels consisting in PEO and butylammonium vermiculite.27,28 Beyond the aggregation and gelation kinetics, the organization of polymer chains and clay platelets in the resulting PEO/Laponite hydrogels was characterized at various length scales, ranging from 1 nm up to a few thousands of nanometers, using the combination of transmission (TEM) and scanning (SEM) electron microscopy as well as light, X-ray, and neutron scattering experiments.24,26,29,30 One of the interesting features that emerged from these works concerns the local structure of the adsorbed layers formed around the Laponite platelets. From their small-angle neutron scattering (SANS) data, Cosgrove et al. evidenced the adsorption of PEO chains on both faces and edges of the clay particles, with a higher characteristic thickness te for the adsorbed layer on the edges than on the faces, tf.29 Surprisingly, in the range of PEO molecular weight Mw investigated (20 000−965 000 g·mol−1), both te and tf are much weaker than the undisturbed radius of gyration of the PEO chains, R0g. Besides, the adsorbed layer thickness tf remains constant, close to 15 Å, independently of

to be one of the powerful tools available to probe the molecular mobility of polymers at the interfaces from an experimental point of view.8−12 These NMR investigations mainly rely on the analysis of the 1H transverse relaxation signal (T2(1H) relaxation), by means of different pulse sequences such as solid echo,8,10 magic-sandwich solid echo,11 and Hahn and Carr− Purcell−Meiboom−Gill (CPMG) experiments.8,10 This experimental approach, based on the T2(1H) relaxation, allows one to detect the occurrence of immobilized protons (if any), to characterize the extent of segmental reorientations of the polymer chain portions, and to propose a quantification of different phases related to different molecular mobilities. At this stage, it is important to mention that the characteristic frequencies these experiments are sensitive to amount to a few tens of kilohertz. In other words, if polymer chains are detected as “immobilized” through these NMR experiments, it does not discard the possibility that they display molecular motions occurring at lower frequencies. It is outside the scope of this contribution to review the results obtained on various nanocomposites using NMR T2(1H) measurements. We will just indicate that some of these investigations report the occurrence of glassy polymer layers surrounding the filler particles. This was in particular the case of model poly(ethyl acrylate) matrices filled by silica particles, for which a fraction of immobilized protons was detected.10,11 The occurrence of such a glassy population should be a significant feature to account for the mechanical reinforcement of nanocomposites. Along this line, the formation of glassy bridges between the filler nanoparticles was recently proposed to be an efficient reinforcing mechanism.13 Interestingly, inorganic nanofiller particles have also been incorporated within polymeric hydrogels, another active field of investigation in polymer science.14−16 In contrast to bulk polymer matrices reinforced by nanosized fillers, the preparation of such nanocomposite hydrogels is quite recent since the first contribution along this line was reported by Haraguchi et al. in 2002.17,18 Free-radical polymerization of water-soluble monomers, such as N-isopropylacrylamide17,18 or N,Ndimethylacrylamide19 for instance, was performed within an aqueous clay dispersion. One of the key points in this protocol is the use of a radical initiator that may adsorb on the clay platelet surfaces. This clay component was initially incorporated within the hydrogels in order to improve the mechanical properties of more conventional, chemically cross-linked hydrogels. Their mechanical weakness and their brittle nature limit their use for applications requiring resistance to external stresses or strains. At this stage, it is worth remarking that the strategy proposed by Haraguchi et al. was to prepare polymeric hydrogels characterized by an average cross-link density, ν, and an average molecular weight between cross-links, Mc, that can be varied independently.17 Indeed, in conventional chemically cross-linked hydrogels, ν is inversely proportional to Mc, and at high cross-linker concentrations, short and widely distributed values of Mc occur within the network, a feature which is thought to be responsible for the weak mechanical strength of these hydrogels.20 In this context, the approach developed by Haraguchi et al. offers an avenue to get significantly higher and less distributed Mc values for the chain portions adsorbed on clay platelets at both extremities. Physically cross-linked hydrogels of poly(N-isopropylacrylamide)18 and poly(N,Ndimethylacrylamide),19 for instance, with an excellent mechanical toughness were thus obtained and displayed an elastic behavior up to a deformation ratio of about 10. Besides, the 7860

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Mw, whereas te displays a very weak dependence on the molecular weight that may described by the power law te ∝ Mw0.13, te being close to 30 Å for Mw = 78 000 g·mol−1. Therefore, all these experimental features demonstrate that the PEO chains adsorbed on the Laponite disks display a very oblate average conformation. The structural characterization of the PEO adsorbed layer within Laponite-based hydrogels was further extended to the oligomeric regime (Mw between 200 and 1500 g·mol−1) by De Lisi et al.26 Assuming a similar value for te and tf in the fit of their SANS data, a continuous increase of the adsorbed layer thickness with Mw was found, but again, the corresponding values were much weaker than R0g, which indicates a rather flat and compact conformation at the Laponite layer surfaces. In this context, it would be of a great interest to investigate the dynamical behavior displayed by the adsorbed PEO chains in these hydrogels. Indeed, the conformational behavior of the PEO chains in the adsorbed layer may suggest that the corresponding chain segments are submitted to strong local constraints and may undergo a significant slowing down of their segmental motions. Having in mind the different concepts (hydrodynamic effects, organization of the filler nanoparticles and polymer/filler interactions) extensively discussed for bulk polymer matrices reinforced by nanosized fillers, it is clear that a significant reduction of the chain mobility at the interfaces with the clay particles, if any, should be taken into account in the detailed description of the rheological properties of such materials. To the best of our knowledge, only one recent work by Frielinghaus et al., based on neutron spin−echo experiments, was dedicated to the dynamics of PEO chains within PEO/ Laponite hydrogels.31 A remarkable result obtained through this work is the detection of an immobile fraction of repeat units among the adsorbed chains, with a characteristic thickness estimated to 9 Å at room temperature. The dynamical window covered by such experiments ranges between a few picoseconds to hundreds of nanoseconds. In this contribution, our aim is to investigate the segmental dynamics of poly(ethylene glycol) (PEG) chains adsorbed on Laponite platelets, over a longer time scale than the one available through neutron spin−echo measurements and to determine whether part of these adsorbed chains are still immobile or not over this longer time scale. The formation of the PEG/Laponite hydrogel will be first monitored through real-time 1H NMR experiments, giving insight into the variation of the PEG segmental dynamics during both aggregation of the Laponite platelets and gelation. After completion of the gelation process, 1H/13C NMR experiments will be used to investigate the molecular motions of the PEG chains adsorbed on the clay platelet surfaces, over a time scale given by a few tens of microseconds.

Table 1. Molecular Characteristics of the PEG Samples As Determined by SEC, Characteristic Gelation Time τNMR, and Parameter (1 − A∞) for the PEG/Laponite/D2O Samples, Obtained through 1H NMR Experiments Performed at 298 K PEG-300 PEG-400 PEG-600 PEG-1000 PEG-2000 PEG-3000 PEG-4000 PEG-h4 PEG-d4

Mw (g·mol−1)

Ip

τNMR (h)

1 − A∞

310 420 620 940 1915 2920 3780 18 300 18 300

1.11 1.09 1.06 1.05 1.03 1.03 1.03 1.02 1.12

6.71 6.48 4.48 0.64 3.52 2.79 3.91

0.03 0.04 0.08 0.14 0.27 0.28 0.28

Top and sodium iodide, by Acros Organics. All compounds were used as received, without further purification. Before being used, the PEG samples were dried at 353 K under vacuum for 8 h. 2.2. Sample Preparation. 2.2.1. PEG/Laponite/D2O Hydrogels. A 3 wt % dispersion of Laponite in D2O was stirred vigorously for 1 h. Another solution of PEG and NaCl in D2O was then added, with a salt concentration in the resulting solution fixed to 0.29 g·L−1, i.e., 5 mM, and a PEG (Laponite) concentration of 0.63 wt % (2 wt %). The PEG/Laponite/D2O mixture was vigorously stirred for 3 additional minutes. At this stage, it is worth remarking that the state of dispersion of the Laponite platelets within the aqueous solution just before the addition of the aqueous solution of PEG and NaCl strongly depends on the stirring conditions. The ones used to prepare all of the hydrogels investigated in this study are reported in the Supporting Information. 2.2.2. PEG/NaI. NaI was dried overnight at 373 K under vacuum and then dissolved in acetonitrile, with a salt concentration of 76 g·L−1. A separate solution of PEG-2000 in acetonitrile at 193 g·L−1 was added to the NaI solution in acetonitrile, the mixed volumes being adjusted to get an ether oxygen to sodium ion ratio of 9. The resulting solution was stirred for 24 h and cast on a Teflon plate at room temperature. The PEG-2000/NaI system was further dried overnight at 353 K under vacuum. 2.3. Solution and Solid-State NMR Spectroscopy. 1H solutionstate NMR measurements were performed on a Bruker Avance 300 NMR spectrometer operating at a 1H Larmor frequency of 300.1 MHz. A 5 mm inverse 1H/13C selective probe was used. 1H one-pulse experiments were performed with a 90°(1H) pulse length of 7.5 μs and a recycle delay of 2 s, adjusted according to the T1(1H) relaxation time. 1 H and 13C solid-state NMR experiments were carried out on a Bruker Avance III NMR spectrometer (1H and 13C Larmor frequencies of 400.4 and 100.7 MHz, respectively), with a doubleresonance MAS probe (4 mm rotors). For all the 1H NMR experiments, the sample volume was centered and restricted within the coil, ensuring a good homogeneity of the 1H radio frequency (RF) field. The 90°(1H) pulse length was 2.5 μs and the recycle delay was varied between 8 and 60 s, depending on the T1(1H) relaxation behavior of the sample. The 1H double-quantum (DQ) experiments were performed under static conditions by means of the five-pulse sequence [90° x−t DQ−90°x−t1−90° y−tDQ−90°y −t z−90°x−acquisition].32 The phase cycling scheme used to measure the even-order multiple-quantum coherences is described in ref 32. The evolution time t1 and the z-filter delay tz were fixed to 4 μs. 1H DQ build-up curves were obtained by monitoring the amplitude of the 1H DQ signal as a function of the DQ excitation time tDQ between 5 μs an 1.6 ms. 13C direct polarization (DP) NMR spectra were recorded under magic-angle spinning (MAS) of the samples at a spinning frequency of 3 kHz. The 90°(13C) pulse length was 4.0 μs. Two-pulse phase modulation (TPPM) was applied for 1H−13C heteronuclear dipolar decoupling during the acquisition of the 13C NMR signal. The corresponding 1H RF field was 78 kHz. For these DP/MAS

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(ethylene glycol) (PEG) of various molecular weights ranging from 300 to 4000 g·mol−1 were purchased from Sigma-Aldrich (BioUltra). Their weight-average molecular weight Mw and polydispersity index Ip, determined by SEC, are reported in Table 1. A perdeuterated poly(ethylene glycol) monomethyl ether (Polymer Source; Mw = 18 300 g·mol−1, corresponding to a degree of polymerization DP of 338; Ip = 1.12), PEG-d4, was also considered, and NMR data obtained with this sample were compared to the ones determined with protonated PEG chains displaying a similar degree of polymerization (Polymer Laboratories; Mw = 18 300 g·mol−1, i.e., DP = 415; Ip = 1.02), PEG-h4. Laponite RD was kindly provided by Rockwood Additives Ltd. D2O (99.97% D) was supplied by Euriso7861

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experiments, the recycle delay, adjusted according to the T1(13C) relaxation time, was between 1 and 25 s over the temperature range 243−333 K. The 1H and 13C chemical shifts were referenced with respect to tetramethylsilane (TMS), using external reference standards: the 1H NMR peak of the ethanol methyl protons at δ = 1.11 ppm and the 13C NMR peak related to the α-glycine carbonyl carbon at δ = 176.03 ppm. For both solution and solid-state NMR studies, the sample temperature was regulated within an accuracy of ±0.1 K. During the variable-temperature measurements, a waiting time of 20 min, after stabilization at the target temperature, ensured thermal equilibration of the samples. Prior to these acquisitions, temperature calibration of the solution-state (solid-state) NMR probes was performed by monitoring the 1H (207Pb) chemical shift of methanol33 (lead nitrate34) under static (MAS) conditions. 2.4. Rheological Measurements. Oscillatory shear experiments were performed in the linear viscoelastic regime, with an AR1000 rheometer (TA Instruments), equipped with a Peltier plate for temperature control and a cone−plate geometry (60 mm cone diameter, 2° cone angle and 53 μm truncation gap). A solvent trap was used to limit evaporation of water during the measurements.

shift of the hydroxyl protons observed on neat Hectorite (0.35 ppm), through 1H solid-state NMR.12 Though the 1H NMR spectrum of Figure 1 was recorded just after the sample preparation, a broadening of the PEG line is observed, in comparison to the 1H NMR line shape obtained on the solution of PEG-2000 and NaCl in D2O, at the same concentration (see the Supporting Information, Figure S1). In particular, an increase of the PEG half-height line width δν1/2 from 2 to 13 Hz occurs. Besides, spectral wings located at the basis of the PEG 1H NMR peak, extending over about 450 Hz, are clearly detected in Figure 1 and were not present in Figure S1. The PEG line broadening together with the occurrence of spectral wings are significant enough to prevent the contribution from the terminal PEG repeat units as well as the 13C satellites (Figure S1) to be observed. Such spectral features may result from several effects at a molecular length scale. First, the presence of the Laponite platelets should increase the local viscosity experienced by the PEG chains and restrict their reorientational motions, resulting in a decrease of the 1H transverse relaxation time T2(1H) of the PEG protons. Besides, due to adsorption phenomena, part of the PEG chains may already interact with the Laponite disk surfaces, thus also contributing to reduce the segmental mobility and, thus, the T2(1H) value of the corresponding PEG chains. Lastly, the difference in the diamagnetic susceptibility between the inorganic clay layers and the aqueous PEG solution may induce distortions of the NMR magnetic field within the interfacial regions35 and, consequently, a distribution of the 1H chemical shift values experienced by the PEG protons. One way to separate the dynamic contributions (T2(1H) relaxation time) from the static ones (1H chemical shift dispersions) respectively involved in the observed PEG line broadening would be to refocus the chemical shift inhomogeneities and to determine the 1H transverse relaxation signal of the initial PEG/Laponite/ D2O mixture, using the CPMG experiment. However, an evolution of the PEG/Laponite/D2O sample toward a hydrogel structure will occur over the time scale of such CPMG experiments, thus preventing a reliable analysis of the CPMG measurements. The gelation process occurring within the PEG-2000/ Laponite/D2O system at 298 K, following the mixing of both PEG-2000 and Laponite components, is then investigated by monitoring the variation of the 1H one-pulse NMR spectrum recorded on the same sample, every 5 min. The acquisition time of each spectrum is limited to 30 s, so that the time evolution of the sample may be considered as negligible over this duration, in a first approach. Figure 2 shows the 1H NMR peak related to the PEG-2000 protons after 1.5 and 15 h. For the sake of comparison, the PEG line shape corresponding to the mixture in the initial state is recalled. As illustrated in Figure 2, a significant decrease of both the height of the PEG 1H NMR peak and the amplitude of its spectral wings occurs over 15 h, suggesting that the PEG-2000 chains become, on average, more and more constrained during the hydrogel formation. During the gelation process, the PEG-2000 chains within PEG-2000/ Laponite/D2O display an overall slowing down of their segmental motions. The corresponding decrease of the molecular mobility and, thus, of the T2(1H) relaxation time is so important for part of the PEG repeat units that their contribution to the 1H NMR spectrum is broader than the considered spectral window, i.e., 4.5 kHz in the present case, and is not observed any longer.

3. RESULTS AND DISCUSSION 3.1. Evolution of the PEG Segmental Mobility during Gelation. Figure 1 depicts the 1H solution NMR spectrum

Figure 1. 1H solution NMR spectrum of the aqueous solution of PEG2000 and NaCl, after addition of the Laponite dispersion in D2O. The mixture was stirred for 3 min before the beginning of the 1H one-pulse NMR experiment.

obtained on the aqueous solution of PEG-2000 and Laponite after the mixing of both component solutions, described in the Experimental Section. From a macroscopic point of view, the mixture is still rather fluid despite the presence of NaCl, which speeds up the hydrogel formation. The duration of the stirring step after mixing is thus negligible compared to the characteristic gelation time, as will be confirmed in the following. Therefore, both PEG-2000 chains and water molecules display a high level of reorientational motions, resulting in rather narrow peaks on the 1H NMR spectrum of Figure 1. The peak at 3.6 ppm is assigned to the PEG-2000 −O−CH2− protons, whereas the line at 4.7 ppm is related to the residual protons from heavy water. A broad peak of weak amplitude is also observed at about 0.2 ppm and is not detected on the 1H NMR spectrum of the aqueous solution of PEG2000 before mixing to the Laponite dispersion in D2O. This contribution may be assigned to Laponite silanol protons, as suggested by the similar value determined for the 1H chemical 7862

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significant slowing down of their reorientational motions during the gelation process that they cannot be detected anymore through our 1H solution-state NMR spectrometer. In the present case, the fitting parameters, A∞ and τNMR, were found to amount to 0.73 and 3.52 h, respectively. Let us first focus on the characteristic gelation time. The formation of the Laponite gel in the presence of PEG-2000 and NaCl, with the same concentration for each component than for the 1H NMR studies described above, was monitored through the time evolution of the shear storage modulus G′, measured at a frequency of 1 Hz and at the same temperature (298 K). This variation is reported in Figure 4. The conditions

Figure 2. Time evolution of the line shape of the 1H NMR peak related to the PEG-2000 chains within the PEG/Laponite/D2O system 1.5 h (dashed line) and 15 h (solid line) after the acquisition of the first 1H NMR spectrum. This latter was recorded after addition of the aqueous PEG/NaCl solution to the Laponite dispersion in D2O, followed by 3 min under stirring. The PEG 1H NMR line shape of this initial spectrum (t = 0 h) was also included (dotted line), for the sake of comparison.

Complementary information is obtained by considering the time evolution of the area under the PEG 1H NMR peak, reported in Figure 3. More precisely, the integration was

Figure 4. Variation of the storage shear modulus G′, measured at a frequency of 1 Hz, as a function of time, for a PEG-2000/Laponite/ H2O system characterized by the same composition as the one used for the NMR measurements reported in Figure 3, i.e., 0.63 wt % PEG2000, 2 wt % Laponite, and 5 mM NaCl. As for the NMR experiments, the temperature was fixed at 298 K.

of the hydrogel formation involved in the NMR and the rheology experiments are somehow different and may affect the details of the gelation kinetics. However, it is worth remarking that the significant increase of G′ occurs over the first 4 h, i.e., a time scale that is of the same order of magnitude than the characteristic NMR time τNMR (3.52 h) determined previously. Another experimental feature supporting the correlation between τNMR, which essentially describes the kinetics of the significant slowing down experienced by the PEG chains, and the gelation time, deduced from rheology measurements, is provided by considering the influence of the PEG molecular weight on the hydrogel formation. Similar 1H NMR experiments to the ones reported for PEG-2000 were carried out for the other PEG samples of Table 1. In all these cases, the time evolution of the area under the PEG 1H NMR peak, A(t), could also be described using eq 1 and the corresponding fitting parameters were reported in Table 1. The dependence of τNMR with the PEG molecular weight is plotted in Figure 5a. τNMR is found to decrease until a PEG molecular weight Mw of 1000 g·mol−1 and above this value, it increases and finally tends to a plateau value above 2000 g·mol−1. It is worth noting that the difference between τNMR obtained for Mw = 1000 g·mol−1 and the NMR gelation time measured for 600 g·mol−1 and 2000 g·mol−1 stands beyond the experimental accuracy. The variation of τNMR with Mw and, in particular, the minimum of τNMR detected around 1000 g·mol−1 are in very good qualitative agreement with the evolution of the gelation time τg as a function of the PEG molecular weight reported by De Lisi et al.,26 which evidenced a minimum of τg around 900 g·mol−1. In this reference, the gelation time values were deduced from viscosity measurements on PEG/Laponite/H2O systems with 3

Figure 3. Time dependence of the area A under the 1H NMR peak of the PEG-2000 protons, during the PEG/Laponite/D2O hydrogel formation. The data were normalized by A0, the area measured on the PEG 1H NMR peak of the spectrum recorded after mixing the different component (PEG-2000/NaCl and Laponite) solutions and stirring for 3 min. The solid line represents the fit of the experimental data by eq 1.

performed from 1.0 to 4.3 ppm, the upper limit being selected to minimize the effect of the overlap with the narrow peak arising from the residual protons of heavy water. Figure 3 clearly shows that the amplitude A of the contribution of the PEG-2000 protons to the 1H one-pulse NMR spectrum displays a significant decrease over 15 h and then, tends to a plateau value A∞: as far as the PEG chain dynamics probed through these NMR measurements is concerned, a time independent state of PEG-2000/Laponite/D2O is achieved after 15 h. More quantitatively, the experimental data A(t) may be satisfactorily fitted by the phenomenological equation A(t ) = A∞ + (1 − A∞)exp[− (t /τ0)β ]

(1)

as illustrated in Figure 3. In eq 1, τNMR = (τ0/β)Γ(1/β) may be viewed as a characteristic gelation time constant and (1 − A∞) represents the fraction of PEG repeat units that undergo a so 7863

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PEG repeat units. Several mechanisms may account for this observed dynamical slowing down of the polymer chains. First, in the early stage of the gelation, PEG chains adsorb on the Laponite disk surfaces, thus inducing a significant reduction of the PEG segmental mobility. However, these adsorption phenomena take place over a typical time scale of 5 min37,38 after the mixing of both component solutions and therefore, are not the only ones to be involved in the dynamical slowing down of the PEG chain portions occurring over τNMR. Another feature that could be responsible for this decrease of the polymer chain mobility inferred from Figure 3 is related to bridging phenomena.27,28,39 Indeed, during the aggregation of the Laponite layers, the average interplatelet distance d decreases, and as a result, a PEG chain partly adsorbed at the surface of a clay disk may subsequently adsorb on another Laponite particle. Additional trains should thus be formed at the surface of the second clay platelet and the contribution of the corresponding PEG repeat units to the 1H one-pulse NMR spectrum should not be detected any longer within the considered spectral window. Moreover, the reorientational motions of the PEG chain portion bridging both clay surfaces should also experience a strong slowing down and this local constraint is expected to be stronger and stronger as d decreases. At least a fraction of these chain segments could also be involved in the decrease of A(t) over τNMR, depicted in Figure 3. This interpretation at the molecular level rationalizes the fact that τNMR, accounting for the strong decrease of the segmental mobility for part of the PEG repeat units over gelation, is strongly correlated to the rheological gelation time, governed by the aggregation of the Laponite disks. Along this view, the parameter (1 − A∞) in eq 1 corresponds to the PEG repeat units related to chain portions which get strongly constrained between Laponite layers following their aggregation. It may be worth reminding that this population does not include the trains that are directly formed due to adsorption at the clay surfaces, a few minutes after the mixing of both Laponite and PEG aqueous solutions. Considering both PEG-2000 and clay concentrations in the hydrogel (0.63 and 2 wt %, respectively), the maximum amount of adsorbed PEG per gram of Laponite should be equal to 315 mg/g, i.e., well below the saturation value of 580 mg/g reported for PEG chains of the same molecular weight.25 Therefore, one may consider that this situation corresponds to the high-affinity part of the adsorption isotherm25 and that most of the PEG chains present in the hydrogel are adsorbed. Along this line, the parameter (1 − A∞) corresponds to the PEG repeat units experiencing additional constraints applied to the adsorbed chains upon clay disk aggregation. An estimate of the fraction of PEG chain segments involved in trains before gelation may be deduced from a NMR study of PEO adsorption on silica nanoparticles of similar size as the Laponite platelet diameter (25 nm).40 This fraction was found to represent about 55% of the PEG units. Assuming a similar value in the case of the PEG-2000/ Laponite/D2O hydrogel, one may realize that the parameter (1 − A∞) should only correspond to about 12% of the total amount of PEG units. Lastly, the variation of the parameter (1 − A∞) with the PEG molecular weight is depicted in Figure 5b. The amount of PEG units that get strongly constrained as a consequence of the Laponite disk aggregation continuously increases until 2000 g·mol−1 and then, tends to a plateau value for higher PEG molecular weights. These results can be interpreted in terms of conformational changes of the PEG chains upon early

Figure 5. Dependence on the PEG molecular weight, Mw, of (a) the characteristic gelation time, τNMR, and (b) the parameter (1 − A∞), representing the fraction of PEG repeat units strongly slowed down during the gelation process (see text for details), as determined on PEG/Laponite/D2O mixtures through 1H solution-state NMR experiments.

wt % of Laponite and 2.2 wt % of PEG. This 3 wt % Laponite fraction was selected to lead the gelation process into a timewindow well-suited for the detection through rheology experiments. In our case, a lower clay content (2 wt %) was used but 5 mM NaCl was added to speed up the gelation process. In summary, τNMR stands in the same range as the time related to the strong increase of the storage shear modulus G′ measured on PEG-2000/Laponite/H2O hydrogels of identical composition and its dependence on the PEG molecular weight is in qualitative agreement with the one displayed by the gelation time deduced from viscosimetry on PEG/Laponite/ H2O hydrogels26 with a rather close composition. The gel formation is generally assumed to arise from the aggregation of Laponite platelets, due to electrostatic interactions between the negatively charged faces of the clay disks and their positively charged rims.25,36 The progressive growth of such aggregates finally leads to a connected structure, the so-called “house of cards” structure. From a rheological point of view, the strong increase of the shear modulus G′, shown in Figure 4, may be assigned to the percolation of the platelet aggregates. At this stage, it is worth reminding that τNMR describes the typical time over which the PEG chain segments undergo a slowing down of their segmental motions that is so significant for part of them that their contribution to the 1H one-pulse NMR spectrum (Figure 2) cannot be detected any longer with our solution-state NMR spectrometer. Thus, τNMR essentially probes the PEG segmental dynamics, at a molecular length scale. The correlation between τNMR and the gelation time deduced from rheological approaches implies that the assembling process of the clay platelets resulting in the hydrogel formation occurs simultaneously with a pronounced slowing down of the reorientational motions for part of the 7864

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adsorption at the Laponite surfaces, as a function of Mw. An increasing fraction of loops and tails should then be formed as Mw is raised up. The corresponding chain segments are more inclined to get adsorbed or strongly constrained during the aggregation of the clay disks, thus tending to increase the (1 − A∞) value with the PEG molecular weight. As mentioned earlier, at the polymer concentration used to prepare the hydrogels (0.63 wt %), the PEG chains are essentially adsorbed at the Laponite surfaces. These conditions allow the dynamics of these interfacial chains to be probed selectively, as will be shown in the following. Though of interest, the influence of the PEG concentration as well as the effect of temperature on the variation of the PEG segmental dynamics during the gelation phenomena are beyond the scope of this contribution and will be presented in another report. 3.2. Molecular Mobility of Adsorbed PEG Chain Segments. As mentioned above, after the gelation process, part of the PEG chains become so constrained that their spectral contribution to the 1H one-pulse NMR spectrum cannot be detected on a solution-state NMR spectrometer. In order to further investigate the dynamical behavior of such PEG chain segments, 1H double-quantum (DQ) NMR spectra, obtained through the five-pulse sequence,32,41 were recorded for various excitation times tDQ, varying between 10 μs and 1 ms. In the PEG-2000/Laponite/D2O hydrogel, a nonzero signal is clearly detected for tDQ values close to a few tens of microseconds. More precisely, the amplitude of this 1H DQ signal was monitored as a function of tDQ, leading to the buildup curve depicted in Figure 6. The maximum of this curve is

Figure 7. Schematic representation of the different proton pairs that may potentially contribute to the 1H DQ signal detected within the PEG/Laponite/D2O hydrogels.

least one proton of water molecules first; pairs formed by neighboring Laponite −OH protons, in a second step and last, pairs between two PEG protons or between one PEG proton and a silanol proton. In the hydrogel, water molecules are expected to display fast exchange between bulk water and water molecules interacting with the Laponite disk surfaces. Water molecules at the clay surfaces display anisotropic reorientations and are therefore related to residual 1H dipolar couplings, Dsurface. Due to the fast exchange between the bulk and the interacting states, the resulting 1H dipolar coupling is given by psDsurface, ps standing for the probability for a water molecule to interact with the clay layers. 2H NMR investigations performed on solutions of Montmorillonite clays in D2O evidenced a splitting of the resonance related to D2O, estimated to 9 Hz at a concentration of 20 g·L−1, in a NMR magnetic field of 8.465 T.42 In this reference, the maximum of the Montmorillonite platelet size distribution ranges between 50 and 75 nm and the splitting of the D2O resonance was found to increase with the lateral size of the clay layers. In our case, the Laponite disks display a smaller size, with a diameter of 25 nm,29 while our measurements were performed at a slightly higher NMR magnetic field (9.4 T). Both differences should lead to opposite trends on the D2O splitting and in a first approach, one may consider that its order of magnitude will remain nearly unchanged in our conditions, i.e., about 9 Hz. From this value of the 2H residual quadrupolar coupling, one may estimate43 the value of the 1H dipolar coupling between both protons of H2O molecules in an aqueous solution of Laponite at 20 g·L−1 to be near to 16 Hz. Besides, in contrast to Montmorillonite or Hectorite,42 no or undetectable orientation of the Laponite disks was observed at 20 g·L−1 in a magnetic field of 9.4 T, indicating an isotropic or nearly isotropic orientation of the Laponite layers within the hydrogel. The interactions of H2O molecules with distinct platelets characterized by different orientations with respect to the NMR magnetic field should lead to a further averaging of the intramolecular 1H dipolar couplings for water and thus, to a value much weaker than 16 Hz. Moreover, the fraction of fully protoned water molecules in the hydrogel is weaker than 0.03%. For both reasons, the 1H dipolar couplings associated to both H2O protons are too weak to account for the 1H DQ build-up curve shown in Figure 6.

Figure 6. 1H DQ build-up curve obtained on the hydrogel PEG-2000/ Laponite/D2O (0.63 wt % PEG-2000, 2 wt % Laponite, and 5 mM NaCl), using the five-pulse sequence [90°x−tDQ−90°x−t1−90°y−tDQ− 90°y−tz−90°x−acquisition]. These measurements were performed after completion of the gelation process. Both evolution time t1 and z-filter delay tz were set to 4 μs and the amplitude of the 1H DQ signal was normalized by the proton magnetization at full equilibrium, determined by means of a 1H one-pulse NMR experiment. The solid line stands for the fit of the experimental data by eq 2.

reached for tDQ = 90 μs. These results show that nonzero 1H homonuclear dipolar couplings occur within this hydrogel. Though the 1H double quantum spectra are centered on the PEG 1H chemical shift value, the nature of the protons involved in the spin pairs giving rise to the DQ signal observed on PEG2000/Laponite/D2O cannot be straightforwardly deduced from the five-pulse experiments, performed under static conditions, due to the lack of spectral resolution. The respective contributions from the different kinds of proton pairs that may be considered in the hydrogel, schematically depicted in Figure 7, will be analyzed in the following: pairs involving at 7865

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Other possible contributions to the 1H DQ signal detected in the PEG-2000/Laponite/D2O hydrogel may arise from 1H dipolar couplings between Laponite −OH protons. The 1H DQ build-up curve obtained on a Laponite hydrogel, prepared under the same conditions than the system PEG-2000/ Laponite/D 2 O, is reported in Figure S2 (Supporting Information). The maximum of the 1H DQ signal occurs at a max similar excitation time tmax DQ for both Laponite/D2O (tDQ = 80 μs) and PEG-2000/Laponite/D2O hydrogels (tmax = 90 μs). DQ However, for a given tDQ value, the amplitude of the 1H DQ signal measured on the Laponite hydrogel is found to be much weaker than the one determined on the PEG-2000/Laponite/ D2O system. For tDQ = 90 μs, the former corresponds to 4% of the latter, after normalization of the 1H DQ signal amplitude by the one obtained through a 1H one-pulse NMR experiment, performed on the same sample, and considering the contribution of the PEG-2000 protons to the one-pulse spectrum of PEG-2000/Laponite/D2O. Such a normalization procedure is aimed at taking into account the difference between the quantities of each of the two hydrogels used for these experiments. From this result, one may conclude that despite a weak contribution arising from pairs of Laponite −OH protons, the 1H DQ signal detected on the PEG-2000/ Laponite/D2O hydrogel mainly arises from 1H dipolar couplings among PEG-2000 −O−CH2− protons and, additionally, from potential couplings between one PEG-2000 proton and one Laponite −OH proton. Lastly, before going into a physical interpretation of the 1H DQ NMR experiments described in Figure 6, we performed a last control experiment to prove unambiguously that a major contribution of the 1H DQ signal recorded on the PEG/ Laponite/D2O hydrogels under static conditions is induced by 1 H dipolar couplings involving at least one PEG proton. For this purpose, two hydrogels were prepared under the same experimental conditions, i.e. a Laponite concentration of 2 wt % and a NaCl molar concentration of 5 mM. In one sample, protonated PEG chains (PEG-h4) were introduced at a concentration of 0.63 wt % while for the other hydrogel, perdeuterated PEG (PEG-d4) was considered. The amount of PEG-d4 was adjusted to obtain the same molar concentration of PEG repeat units in both hydrogels. 1H DQ measurements, by means of the five-pulse sequence, were performed on both PEG/Laponite/D2O systems. The 1H DQ spectra obtained with an excitation time tDQ of 90 μs, i.e. the maximum of the 1H DQ build-up curve measured on the hydrogel prepared with PEG-h4 chains, are depicted in Figure 8. Despite the local organization between clay layers, polymer chains and water molecules within both hydrogels is essentially the same, the amplitude of the 1H DQ signal in the deuterated PEG-based sample corresponds to 8.0% of the one determined on the protonated PEG-based one. This result clearly evidences that in PEG-h4/Laponite/D2O, the 1H DQ signal should be assigned to the occurrence of nonzero 1H dipolar couplings between two close PEG protons and to a less extent, one Laponite −OH proton and a proton of a PEG chain. In other words, the contribution from the 1H homonuclear dipolar interactions between nearest Laponite −OH protons to the 1H DQ buildup curve determined on the hydrogels (Figure 6) may be neglected. Therefore, in the following, the build-up curve will be interpreted in terms of PEG chain dynamics within the PEG-2000/Laponite/D2O hydrogel. At this stage, it may be worth reminding that in an aqueous solution of short PEG chains, such as PEG-2000, at a rather low

Figure 8. 1H DQ edited spectra, obtained for a 1H DQ excitation time tDQ of 90 μs, on PEG/Laponite/D2O hydrogels (0.63 wt % PEG, 2 wt % Laponite, and 5 mM NaCl) prepared under identical conditions, with (a) a perdeuterated PEG (DP = 338) and (b) a protonated PEG, with similar molecular characteristics (DP = 415). The evolution time t1 as well as the z-filter delay tz were equal to 4 μs. For the sake of comparison, the level of magnification is the same for both 1H DQfiltered spectra.

concentration (0.63 wt %), the 1H dipolar couplings are fully averaged to zero by the isotropic reorientations of the chain segments. The residual 1H dipolar couplings involving PEG protons within the PEG-2000/Laponite/D2O hydrogel should thus result from the adsorption of PEG chains on Laponite platelets. Indeed, the repeat units of both trains and loops displayed by the adsorbed chains should undergo anisotropic reorientational motions and as a consequence, the corresponding 1H dipolar interactions, the one between both protons of the −O−CH2− groups for instance, are not averaged to zero any longer. Besides, the protons of the PEG chain portions bridging two clay disks27,28,39 after completion of the gelation process will also contribute to the 1H DQ signal detected in the hydrogel. The maximum of the 1H DQ build-up curve, shown in Figure 6, occurs for a tDQ value of 90 μs. This 1H DQ excitation time is higher than 15 μs, the value that would be typically measured for fully immobilized polymer chains.44,45 Interestingly, the chain dynamics of PEO adsorbed on Laponite layers was recently investigated using neutron spin−echo (NSE) experiments.31 In this reference, the intermediate scattering function S(Q,t), Q standing for the scattering vector, was recorded for Q values ranging between 0.08 and 0.15 Å−1 and was found to be not fully relaxed after 30−40 ns. Such a behavior was interpreted as resulting from the occurrence of a fraction of immobilized PEO repeat units among the adsorbed chains. The characteristic length scale of such a frozen-like layer was estimated to extend over 9 Å from the Laponite clay surfaces. In our case, the 1H DQ experiments indicate that though displaying slow and anisotropic reorientational motions the adsorbed PEG-2000 chains are not fully immobilized. The differences between the NSE results derived by Frielinghaus et al.31 and our NMR data are not contradictory and should result from the intrinsic dynamical windows related to both kinds of experiments: NSE allows probing the molecular motions over a typical time scale lying between a few picoseconds to hundreds of nanoseconds while the 1H double-quantum approach used in this work is related to a characteristic time scale of several tens of microseconds. Therefore, in these PEG (PEO)/Laponite/ D2O hydrogels, the segmental motions of the adsorbed polymer chains are frozen over several tens of nanoseconds, but still display significant segmental reorientations over the microsecond time scale. 7866

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3.3. Extent of the Reorientational Motions of Adsorbed PEG Chain Segments. In the following, we aim at precising both amplitude and characteristic frequency of the anisotropic reorientational motions displayed by part of the PEG chains in the PEG/Laponite/D2O hydrogels. Part of these informations may ideally be derived from the comparison of the 1 H residual dipolar coupling Dres between both protons of the PEG −O−CH2− groups with its static value, which amounts to about 34 kHz.43 However, the five-pulse sequence, though well suited to detect strong 1H dipolar couplings since the 1H DQ excitation scheme enables short DQ excitation times to be reached, may lead to difficulties as far as a quantitative determination of the 1H dipolar couplings is concerned.46 An alternate approach, based on the excitation scheme of 1H DQ coherences initially proposed by Baum and Pines,47 was recently developed by Saalwächter et al.46,48 and successfully applied for the characterization of the gelation phenomena involved during the cross-linking of linear polymer chains.49 This experiment offers the possibility to get an accurate determination of the distribution of residual 1H homonuclear dipolar couplings within molten polymers under nonspinning conditions. Attempts to use this approach in order to monitor the 1H DQ build-up within the PEG-2000/Laponite/D2O hydrogel were limited. Indeed, with the pulse sequence described in ref 48, the typical value of the shortest 1H DQ excitation time tDQ that may be obtained with the NMR equipment used in this work amounts to 160 μs. In the considered hydrogels, the decrease of the segmental mobility displayed by the PEG chains adsorbed on the Laponite disks is so significant that the initial growth of the 1H DQ signal with the excitation time cannot be easily captured through the approach described in refs 46 and 48, due to its intrinsic time resolution. Therefore, in order to describe the extent of the restriction of the segmental mobility undergone by the adsorbed PEG chain portions, we chose to compare the results obtained on the hydrogels using the five-pulse sequence with the ones determined on a model system of amorphous PEG chains, with the same NMR experiment. The underlying idea is to determine the temperature decrease that is required to detect a 1 H DQ signal related to these amorphous PEG chains and to match the maximum of the corresponding 1H DQ build-up curve to a tDQ value of 90 μs. At this stage, it may be worth reminding that this value was obtained, at room temperature, on the PEG-2000/Laponite/D2O hydrogel. Such an approach could be applied to a solution of PEG in D2O, at a PEG volume fraction of 0.37, i.e. a value similar to the one determined within the adsorbed PEG layer of a PEG-1500/Laponite/D2O hydrogel, with PEG and Laponite concentrations of 1.0 wt %, using neutron scattering experiments.26,29 However, in that case, the temperature variation will be somehow limited by the solvent crystallization, which would induce a significant contribution from the protons of the residual H2O molecules involved in the crystallites to the 1H DQ signal. In order to circumvent this problem, we considered a model system of bulk PEG chains in the amorphous state. As for water molecules, PEG crystallization should be avoided, and for this aim, a sodium salt, NaI, was introduced. The complexation of the PEG oxygen atoms by the sodium cations prevents PEG crystallization, provided the ratio oxygen/sodium is high enough (9/1 for an oxymethylene-linked PEO).50 A PEG2000/NaI blend, characterized by the above-mentioned composition, was prepared as described in the Experimental

Section, and no PEG crystallites were detected, either by DSC or WAXS experiments (see Figures S4 and S5, Supporting Information). At room temperature, a nonzero 1H DQ signal is detected on PEG-2000/NaI, evidencing the occurrence of residual 1H dipolar couplings related to the PEG chains. Indeed, due their attractive interactions with the PEG-2000 oxygens, Na+ cations act as temporary cross-links which constrain polymer chain portions, thus inducing anisotropic reorientational motions for the chain segments between two consecutive interacting oxygen atoms along the chains. The fact that 1H dipolar couplings are observed indicates that the dynamical exchange of the PEG2000 oxygens interacting with the Na+ cations is slower than a few tens of microseconds typically. The amplitude of the 1H DQ signal was monitored as a function of the 1H DQ excitation time tDQ, at 298 K, and the corresponding evolution is depicted in Figure 9. The maximum of the build-up curve is reached for

Figure 9. 1H DQ build-up curve measured on PEG-2000/NaI, at 298 K, using the five-pulse sequence. A value of 4 μs was selected for both evolution time t1 and z-filter delay tz. The amplitude of the 1H DQ signal, IDQ, was monitored as a function of the 1H DQ excitation time tDQ and normalized by the proton magnetization at full equilibrium, obtained through a 1H one-pulse NMR experiment.

tmax DQ = 150 μs, suggesting a weaker Dres value for the PEG-2000 chains within the blend PEG-2000/NaI than the one related to the adsorbed PEG-2000 chains within the hydrogel PEG-2000/ Laponite/D2O. In other words, the reorientational motions of the PEG repeat units at the surfaces of the Laponite disks are more anisotropic than the ones in the NaI-based complex, implying that the corresponding chain portions undergo a higher local constraint. Such a conclusion concerning the comparison of the Dres value between both systems assumes that the apparent relaxation time T2* involved in the 1H DQ build-up curve of these samples is not too different. In order to check whether this assumption is correct or not, the variation IDQ(tDQ) obtained on the hydrogel was fitted using the expression IDQ (t DQ ) ∝ [1 − exp( −9Dres 2t DQ 2/10)]exp( −2t DQ /T2*) (2)

which assumes a monomodal distribution of the Dres value as well as a single apparent relaxation time T2*.46 This equation allows a good description of the 1H DQ build-up curve obtained on the hydrogel, as shown in Figure 6, with resulting fitting parameter values amounting to Dres = 17.25 kHz and T2* = 0.25 ms. Keeping Dres constant but replacing T2* by the value (T2* = 0.55 ms) deduced from the fitting of the PEG-2000/NaI build-up curve (Figure 9), in the high excitation time regime (tDQ > 400 μs), by a single exponential exp(−2tDQ/T2*), leads 7867

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to a maximum of the IDQ(tDQ) variation at 102 μs. This value is significantly weaker than the experimental maximum determined on the 1H DQ build-up curve of the blend PEG-2000/ NaI (tmax DQ = 150 μs), thus confirming that on average, at room temperature, stronger residual 1H dipolar couplings are detected in the hydrogel PEG-2000/Laponite/D2O than in the model system PEG-2000/NaI. The 1H DQ build-up curve displayed by PEG-2000/NaI was monitored from room temperature until 273 K and the corresponding evolution of the maximum of the IDQ(tDQ) dependence, tmax DQ , is reported in Figure 10. A continuous

one may conclude that the level of segmental mobility of the adsorbed PEG chains in the hydrogel at 300 K is similar to the one observed on bulk amorphous PEG chains in the complex PEG-2000/NaI at 291.5 K. In order to picture the extent of molecular mobility of the PEG-2000 chains adsorbed at the Laponite surfaces within the hydrogel on the basis of the comparison with the model PEG2000/NaI system, a more precise characterization of the segmental dynamics of the PEG chains in PEG-2000/NaI, at 291.5 K, is required. For this purpose, two different kinds of 13 C NMR measurements, apparent T2(13C) and T1(13C), were performed on PEG-2000/NaI. First, the 13C DP/MAS/DD NMR spectrum of PEG-2000/NaI was monitored from 243 up to 333 K and the temperature dependence of the half-height line width, δν1/2, of the PEG 13C NMR peak is shown in Figure 11. Starting from a plateau value at low temperatures, δν1/2

Figure 10. Temperature dependence of the excitation time tmax DQ , corresponding to the maximum of the 1H DQ build-up curve IDQ(tDQ), determined on the complex PEG-2000/NaI. The measurements were performed with 4 μs for the evolution time t1 and the zfilter delay tz. Figure 11. Variation of the 13C half-height line width, δν1/2, of the PEG main chain carbon peak, measured on the fully relaxed 13C DP/ MAS/DD spectrum of PEG-2000/NaI, as a function of temperature. The MAS spinning frequency was set to 3 kHz and the intensity of the 1 H dipolar decoupling field applied during the 13C acquisition is equal to 78.0 kHz.

decrease of tmax DQ , from 150 ± 5 μs at 298 K down to 15 ± 5 μs at 283 K, is observed and below 283 K, a plateau value is achieved, which corresponds to the situation where PEG-2000 chains are immobilized over a few tens of microseconds. A similar value was obtained, for instance, for glassy polystyrene within polystyrene-poly(ethylene oxide)44 or polystyrene-poly(methyl methacrylate)45 block copolymers. Focusing on the maximum of the 1H DQ build-up curve, a max tDQ value of 90 μs can be obtained at a temperature of 291.5 K for the bulk amorphous PEG chains within PEG-2000/NaI, a similar value as the one measured on the hydrogel PEG-2000/ Laponite/D2O at 300 K. In order to compare the tmax DQ value between PEG-2000/Laponite/D2O at 300 K and PEG-2000/ NaI at 291.5 K, one should check whether or not a similar relaxation behavior occurs for both systems, at these temperatures. Therefore, the IDQ(tDQ) variations recorded on PEG2000/NaI between 298 and 273 K were fitted, in the high tDQ regime, by a single exponential in order to determine the apparent relaxation time T2* (see Table S3, Supporting Information). A similar value as the one determined on the PEG-2000/Laponite/D2O hydrogel (T2* = 0.25 ms) at room temperature is achieved, for PEG-2000/NaI, at 292 K, i.e., close to 291.5 K. Moreover, using the Dres value (Dres = 17.25 kHz) previously determined at room temperature on the hydrogel PEG-2000/Laponite/D2O, the maximum of its 1H build-up curve is shifted from 90 to 88 μs if its apparent T2* relaxation time is replaced by the value estimated on PEG-2000/NaI at 291.5 K. The deviation between both tmax DQ values is weaker than the experimental accuracy on the determination of tmax DQ derived from Figure 6. A reliable comparison of the excitation time corresponding to the maximum of the 1H DQ signal amplitude between the PEG-based hydrogel at 300 K and PEG-2000/NaI at 291.5 K may thus be performed and from this equivalence,

increases from 585 Hz at 253 K up to 891 Hz at 278 K. At 278 K, a maximum line-broadening is detected. As soon as the temperature is increased until 318 K, a continuous reduction of the line width δν1/2 is observed and above this value, a plateau around 8 Hz is achieved. The overall decrease of δν1/2 between 243 and 333 K is induced by the increase of the PEG-2000 segmental mobility from the glassy to the molten state, i.e., the α-relaxation process related to glass transition. The line broadening observed in Figure 11, with a maximum effect occurring at 278 K, results from interference effects between the modulation of the PEG-2000 1H−13C dipolar couplings, induced by the segmental reorientations, and the coherent modulation related to the 1H decoupling field applied during the 13C NMR signal acquisition.51−53 This dynamic line broadening mechanism is expected to be maximum as the characteristic frequency of the PEG-2000 segmental motions matches the frequency characterizing the 1H decoupling field intensity (78.0 kHz). Therefore, the 13C NMR data of Figure 11 indicates that at 278 K, the reorientational motions displayed by the PEG chains in PEG-2000/NaI achieve a characteristic frequency of 78.0 kHz. Another motional line broadening mechanism, involving the 13C chemical shift tensor anisotropy of the PEG carbons,51,54 could have accounted for the experimental δν1/2(T) variation measured on PEG-2000/ NaI. However, in the case of the PEG carbons, the modulation of the 1H−13C dipolar interactions was found to be the main mechanism to be considered in order to rationalize the 13C 7868

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the molecular mobility displayed by the PEG chains was monitored during the aggregation of the clay disks and the gelation process. An overall slowing down of the segmental dynamics is observed, with part of the PEG chain segments becoming so constrained that their contribution to the 1H solution-state NMR spectrum could not be detected any longer. Such chain segments should correspond to the repeat units adsorbed on the Laponite surfaces and/or confined, strongly constrained between clay disks, after aggregation of the Laponite platelets. The time dependence of the amount of PEG chain segments observable through 1H one-pulse solutionstate NMR measurements was used to probe the gelation kinetics, taking a point of view based on the dynamical response of the polymer component in the forming hydrogel. The characteristic time scale τNMR of the gelation process, as determined by NMR, was found to be similar to the one obtained macroscopically, through rheological measurements, and which is mainly governed by the assembling of the clay platelets. Such a correlation should result from the fact that the significant dynamical slowing down displayed by a fraction of the PEG repeat units over τNMR essentially results from a strong local chain constraint induced by the aggregation of the Laponite layers so that following the PEG molecular mobility within the forming hydrogel is an indirect way to probe the formation of the “house-of-cards” structure, usually captured by rheological measurements. In a second step, the dynamics of the PEG repeat units at the interface with the clay layers was investigated using 1H solidstate NMR. 1H double-quantum NMR experiments evidenced that, though displaying restricted reorientational motions, the adsorbed PEG chain portions are not frozen over the tens of microseconds time scale. On the basis of a comparison with the NMR data obtained on a model system consisting of bulk amorphous PEG chains, the motional frequency of the adsorbed PEG portions in the hydrogels was found to range between 78.0 kHz and 100.7 MHz, at 300 K. This result is somehow complementary to the recent report by Friehlinghaus et al.31 on hydrogels composed of PEO and Laponite, in which the adsorbed PEO repeat units were found to be frozen over 30−40 ns at room temperature, implying that their segmental motions, if any, should occur over characteristic frequencies lower than 25 MHz. As far as the degree of anisotropy of the reorientational motions displayed by the adsorbed PEG chains is concerned, a similar behavior was observed for these chains, though surrounded by water molecules, than for the bulk amorphous PEG chains within PEG/NaI, considered 10 K below. In addition to the influence of the surrounding PEG chains (interchain effects) on the segmental dynamics of a given PEG repeat unit, intrinsic to the bulk state, an additional constraint arises from the fact that on average, there are only nine repeat units between two consecutive PEG oxygens interacting with sodium cations. This topological constraint, added to the high cooperativity of the segmental dynamics in PEG/NaI as well as the lower temperature value found for the equivalence between PEG-2000/Laponite/D2O and PEG-2000/NaI suggest a significant anisotropy of the reorientational motions of the PEG chains at the Laponite surfaces. In the case of the hydrogel, a quantitative estimation of this degree of anisotropy cannot be reliably extracted from the NMR data presented in this contribution. Such a description of the dynamical properties of polymer chains at the surfaces of inorganic filler nanoparticles, here

NMR line broadening induced by the thermal activation of the α-relaxation process related to amorphous PEG chains in the kilohertz regime.53,55 Therefore, at 278 K, most of the PEG2000 chain portions undergo segmental motions occurring at a frequency of 78.0 kHz, so that at 291.5 K, the corresponding motions should be faster. From another point of view, the 13C spin−lattice relaxation signal was recorded on PEG-2000/NaI between 243 and 333 K. For any temperature within this range, this relaxation signal could be satisfactorily described using a single exponential component. The corresponding relaxation time value, T1(13C), was reported as a function of inverse temperature in Figure 12.

Figure 12. 100.7 MHz 13C spin−lattice relaxation time, T1(13C), related to the PEG main chain carbons within PEG-2000/NaI, versus inverse temperature.

The minimum of this T1(13C) variation is obtained at 323 K, implying that at this temperature, the characteristic frequency of the segmental motions displayed by the PEG chains within PEG-2000/NaI approximately matches the 13C Larmor frequency, i.e., 100.7 MHz.53,56 Combining both 13C T2 and T1 data, one may conclude that the reorientational motions of the PEG chains in PEG-2000/NaI occurs within a dynamical window ranging between 78.0 kHz and 100.7 MHz, at the intermediate temperature of 291.5 K. Reminding the equivalence of the extent of PEG segmental mobility between PEG-2000/Laponite/D2O at 300 K and PEG-2000/NaI at 291.5 K, the characteristic frequency of the PEG chains adsorbed at the Laponite surfaces in the hydrogel should thus lie in the 78.0 kHz to 100.7 MHz frequency range. Though of interest to describe the motional frequencies displayed by the PEG chains, these 13C NMR experiments could not be performed directly on the PEG/Laponite/D2O hydrogels because of the relatively low PEG concentration (0.63 wt %) on the one hand and the weak natural abundance of carbon 13 on the other hand. In this context, a hydrogel composed of a 13C-labeled PEG would be useful. Besides, complementary solid-state 13C NMR experiments on such a sample should greatly help probing the geometry of the PEG reorientational motions related to the chains adsorbed at the clay surfaces.

4. CONCLUDING REMARKS This contribution aimed at probing the segmental dynamics of PEG chains adsorbed on the clay platelets of PEG/Laponite/ D2O hydrogels, over a characteristic time scale of several tens of microseconds. PEGs with a rather low molecular weight were mainly considered. In a first step, the time-resolved evolution of 7869

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obtained for PEG/Laponite/water systems, could be derived, using a similar experimental approach, on other hybrid hydrogels characterized by higher storage shear moduli after completion of the gelation process. This approach would constitute an instructive pathway to correlate the local chain dynamics at the filler interfaces with the mechanical reinforcement of the Young modulus induced by the incorporation of filler particles in the hydrogel. While currently attracting a lot of attention for bulk polymer-based nanocomposites in the solid state, this issue, poorly addressed in the context of hydrogels, is a key feature and should help modeling the mechanical behavior of nanocomposite hydrogels.

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ASSOCIATED CONTENT

S Supporting Information *

1

H solution-state NMR spectrum of PEG-2000 in D2O with NaCl, 1H DQ build-up curve of the Laponite/D2O hydrogel obtained through the five-pulse sequence, temperature dependence of the apparent relaxation time T2* derived from the 1H DQ build-up curves of PEG-2000/NaI, and DSC thermogram and WAXS diffractogram of the complex PEG-2000/NaI. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +33 1 49 78 13 08. Fax: +33 1 49 78 12 08. E-mail: [email protected]. Present Address †

Ingénierie des Matériaux Polymères (UMR 5223), Université Jean Monnet, 23 rue du Docteur Paul Michelon, 42023 Saint Etienne Cedex 2, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.K. is grateful for funding from the University Paris-Est Créteil Val-de-Marne (UPEC). C.L. acknowledges Région Ilede-France for the purchase of the solid-state NMR spectrometer. The authors warmly thank Stéphane Longeville (Laboratoire Léon Brillouin, CEA-CNRS, CEA Saclay, Gif-surYvette, France) for providing the perdeuterated poly(ethylene oxide) sample.

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REFERENCES

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