Dynamics of Water Absorbed in Polyamides - ACS Publications

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Dynamics of Water Absorbed in Polyamides M. Laurati,*,†,‡,% P. Sotta,† D. R. Long,† L.-A. Fillot,† A. Arbe,‡ A. Alegrıa,̀ ‡,§ J. P. Embs,∥ T. Unruh,⊥ G. J. Schneider,# and J. Colmenero‡,§ †

Laboratoire Polymères et Matériaux Avancés (LPMA), UMR5268, CNRS and Rhodia, CRTL, 85 Rue des frères Perret, 69192 Saint-Fons, France ‡ Centro de Física de Materiales (CSIC, UPV/EHU) and Materials Physics Center (MPC), Paseo Manuel de Lardizabal 5, E-20018 San Sebastián, Spain § Departamento de Física de Materiales, University of the Basque Country (UPV/EHU), Paseo Manuel de Lardizabal 3, E-20018 San Sebastián, Spain ∥ Laboratory for Neutron Scattering, Paul Scherrer Institut, Villigen, Switzerland ⊥ Forschungsneutronenquelle Heinz Maier-Leibnitz, Lichtenbergstrasse 1, D-85747 Garching, Germany # Jülich Centre for Neutron Science at FRM II, Forschungszentrum Jülich GmbH, Institut für Festkörperforschung, Lichtenbergstrasse 1, D-85747 Garching, Germany ABSTRACT: We investigate the dynamics of water absorbed in amorphous and semicrystalline aromatic polyamide copolymers. The combination of dielectric spectroscopy and quasielastic neutron scattering experiments allows us to characterize the water dynamics over a wide range of temperatures (dielectric spectroscopy) and at microscopic length scales (neutron scattering). The dielectric investigation evidences two relaxations associated with water motions: a fast process corresponding to motions of loosely bonded water molecules and a slower process corresponding to motions of amide−water complexes. While the slower process presents the characteristic Arrhenius temperature dependence of a secondary local relaxation over the whole temperature range, the fast process shows a crossover from Arrhenius to Vogel−Fulcher−Tamman (VFT) behavior at T ≈ 225 K, characteristic of confined water dynamics. The microscopic investigation by neutron scattering shows than in the VFT regime of the fast process the dynamics present a diffusive nature similar to bulk water. A large distribution of diffusion coefficients indicates possible differences in the connectivity of the hydrogen bond network. Diffusive heterogeneous dynamics can arise from a nonuniform distribution of water. The confinement effect of the polymer matrix is detected as a considerable reduction of the diffusion coefficient of water with respect to bulk. The presence of a crystalline phase results in a slowing down of both the fast and slow processes involving water motions. This could give a hint to the presence of a rigid amorphous phase in the semicrystalline material.

I. INTRODUCTION Absorption of polar solvents has a strong effect on the physical properties of polyamides,1−3 a class of polymers intensively used in the production of engineering plastics. The principal effect of the interaction between water molecules and this hydrogen-bonded polymer is a decrease of the glass transition temperature Tg of polyamide, a phenomenon called plasticization, with the Tg shift becoming more pronounced with increasing amount of absorbed water. Such a plasticization effect has been documented by calorimetry,2 in dielectric spectroscopy experiments,4−8 dynamic mechanical analysis,9−11 and NMR measurements. 12 Plasticization is a general phenomenon not exclusive for polar solvents, but in the case of wet polyamides it is often explained in terms of a loosening of the hydrogen bond network of the dry material due to the intercalation of water which binds to the amide groups, replacing the existing amide−amide H-bonds. This picture is derived from the two-step absorption model of Starkweather and co-workers.10 Indeed, polyamide has hydrophilic absorption © 2012 American Chemical Society

sites corresponding to the amide groups, but at the same time the rest of the polymer is strongly hydrophobic. This intermediate nature of polyamide may result in a peculiar structural organization of water, with a tendency to aggregate, similar to water in cavities of hydrophobic materials and polymeric aqueous solutions.13,14 The interaction with the matrix is nevertheless significant. Simulations15 on the structure of water absorbed in amorphous polyamide effectively indicate that at saturation water tends to form small aggregates of a limited number of molecules inside the material, with only a fraction of water molecules directly bonded to the amide groups. These structural results are compatible with the few available experimental studies on the dynamics of absorbed water, obtained by thermally stimulated depolarization and NMR experiments, which indicate the presence of two Received: October 24, 2011 Revised: December 20, 2011 Published: January 26, 2012 1676

dx.doi.org/10.1021/ma202368x | Macromolecules 2012, 45, 1676−1687

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II. MATERIALS AND METHODS

populations of slow (tightly bonded) and fast (loosely bonded) water molecules.4,7,16,17 Such description is also supported by simulations.18 However, these studies do not address important questions like the nature of water dynamics in polyamides on the microscopic level, the possible confinement effect of the polymer matrix, the influence of a crystalline phase on water mobility, and the relation between dynamics and the microscopic distribution of water inside the material. Also, it is important to know on which time scale the two distinct water populations start to exchange. A better understanding of all these aspects will help the development of polyamide-based materials with improved barrier properties against polar solvents. In this study we investigate these issues by combining dielectric spectroscopywhich allows exploring a broad thermal range of the dynamicsand quasi-elastic neutron scattering, which (i) due to its spatial resolution enables us to access the microscopic details of the motions and (ii) with isotopic (H/D) labeling allows studying selectively each component. Combination of these techniques has already provided valuable results in the study of water dynamics in aqueous polymer solutions.19 From a different point of view, water absorbed in polyamide presents also an interesting case with respect to the dynamical behavior of water in confinement at supercooled temperatures. Dynamics of confined and interfacial water have important consequences for biological systems because they affect the properties of proteins and DNA.20−22 The confinement effect of nanocavities does not allow for a complete development of the ice structure, and therefore crystallization is not observed even below the homogeneous crystallization temperature (235 K). This phenomenon has also been exploited to investigate on a fundamental level the dynamics of water in the supercooled regime. Regarding this topic, there is a general agreement about the existence of a pronounced crossover from low-temperature Arrhenius to high-temperature Vogel−Fulcher−Tamman (VFT) behavior of the characteristic relaxation times obtained from broadband dielectric spectroscopy (BDS) or neutron scattering, in the temperature region between 215 and 228 K.23−27 Such crossover has been explained invoking different physical mechanisms. It has been attributed to a fragile-to-strong transition from the high density (HDL) to the low density phase (LDL) of supercooled water, based on neutron scattering experiments on water absorbed into hydrophilic and hydrophobic media and protein hydration water.23,24,26 Alternatively, also based on neutron scattering and BDS experiments on protein hydration water and water confined in MCM-41, it has been attributed to a crossover from hydrogen-bond dynamics of water at low temperatures to the α-relaxation of water at high temperatures.28−31 In addition, studies of aqueous polymer solutions showed that such a crossover happens in the temperature range where the mixture undergoes a glass transition.25,27 The crossover was therefore attributed to confinement effects of the frozen matrix on water dynamics. The environment that polyamide presents to water has interesting characteristics to complement these studies. Indeed, the glass transition temperature of polyamide, even in the wet state, is considerably higher than the Arrhenius to VFT crossover, allowing eventually for a clear distinction of the two phenomena. In addition, by comparing the results obtained with an amorphous and a semicrystalline copolymer, we studied the influence of the crystalline phase on the water motions.

A. Samples. The aromatic polyamide random copolymers PA6,6/ 6I and PA6,6/6T were provided by Rhodia. Figure 1 shows the

Figure 1. Chemical formulas of the copolymers PA6,6/6I (top) and PA6,6/6T (bottom). chemical formulas of the two copolymers. The polymers are formed by copolymerization of the PA6,6 and PA6I or PA6T monomers. The monomers are obtained by condensation of hexamethylenediamine with a diacid: adipic acid for the PA6,6 monomer and isophthalic (terephthalic) acid for the PA6I (PA6T) monomers. The average molecular weight of the copolymers is Mw = 10 000 g/mol, with polydispersity Mw/Mn = 2. The fraction of PA6,6, PA6I and PA6T monomers with respect to the total are 0.4 PA6,6 and 0.6 PA6I for PA6,6/6I, 0.65 PA6,6, and 0.35 PA6T for PA6,6/6T. The PA6,6/6I copolymer is amorphous, with a Tg = 363 K as determined by differential scanning calorimetry (DSC). The PA6,6/6T copolymer is semicrystalline, with a Tg = 349 K and a melting temperature Tm = 550 K, as determined by DSC. A crystalline fraction χc ≈ 0.31 was estimated from the DSC trace by using for the melting enthalpy of a 100% crystalline PA6,6/6T sample the literature value known for a 100% crystalline PA6,6 sample (ΔHm = 188.4 J/g).32 We investigated the six different samples listed in Table 1. Dry samples (named PA6,6/6I

Table 1. Water Mass Content (mw) Relative to Total (mTOT) and to the Mass of the Amorphous Fraction (mam), Corresponding Molar Content (mmol) relative to the Number of Amide groups (CONH) per Polymer Chain in the Amorphous Phase, and Tg of the Samples Investigated sample PA6,6/6I PA6,6/6I H2O PA6,6/6I D2O PA6,6/6T PA6,6/6T H2O PA6,6/6T D2O

mw/mTOT

mw/mam

mmol

Tg (K)

0.0 0.14 0.143 0.0 0.072 0.075

0.0 0.14 0.143 0.0 0.104 0.109

0.0 0.93 0.95 0.0 0.72 0.73

363 277 277 349 273 274

and PA6,6/6T) were annealed for 16 h at Tg + 20 K under vacuum before measurement. Fully wet samples (100% relative humidity) were obtained by immersing dry films in H2O (PA6,6/6I H2O, PA6,6/6T H2O) or D2O (PA6,6/6I D2O, PA6,6/6T D2O). The absorption of solvent was followed by measuring the total mass of the sample as a function of immersion time. Saturation of solvent is achieved after ∼10 h. In samples PA6,6/6I D2O and PA6,6/6T D2O the H atoms of the amide (CONH) groups of the amorphous phase have been substituted by D atoms following the procedure described in ref 33: First, the dry samples were immersed in an excess of D2O until fully saturated conditions were reached, and most of the NH groups of the amorphous phase were replaced by ND. Afterward, the samples were dried under vacuum (as described above) to fix the D atoms on the amide groups. After drying, samples were immersed again in D2O until fully saturated conditions were obtained. The mass fractions of water in the wet samples are reported in Table 1. The amorphous phase fraction of the semicrystalline samples amounts to 69%. Therefore, the 1677


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water content in the amorphous phase of these samples is finally similar to that in amorphous PA6,6/6I (see Table 1), assuming homogeneous water absorption within the amorphous phase. The Tg of the wet samples was measured by DSC and is reported in Table 1. B. Quasi-Elastic Neutron Scattering Experiments. In quasielastic neutron scattering experiments, the analysis of the distribution of energy changes (ℏω) of the neutrons scattered into a solid angle comprised between Ω and Ω + dΩ allows to retrieve information on the dynamics of the sample. The double-differential scattering cross section, or scattered intensity, is given by

∂ 2σ = I(Q , ω) ∂Ω ∂ω 1 kf [σcohScoh(Q , ω) + σincSinc(Q , ω)] = 4π k i

(PA6,6/6I) and 0.25 mm (PA6,6/6T) were put into flat aluminum slabs. For samples containing water the slabs were sealed using indium wire in order to avoid solvent loss. The water content of the samples was checked before and after measurements by weighing. The transmission of the samples was close to 90%; therefore, possible multiple scattering contributions were neglected. We corrected raw data for the detector efficiency, the scattering of the sample container, and absorption. In case of TOF measurements the time-of-flight was converted to energy transfer using standard programs available at PSI34 and FRM-II. For the TOF measurements on FOCUS we used λ = 5 Å, leading to a resolution with half-width at half-maximum (hwhm) δE ≈ 100 μeV. The range of scattering angles explored goes from 10° to 130°. The energy window goes from −2 to some hundreds of meV. After applying a standard interpolation program we obtained Iexp(Q,ω) in the elastic Q-range 0.6 ≤ Q ≤ 1.8 Å−1. Temperatures in the range 20 K < T < 225 K were investigated. On TOF-TOF we used λ = 6 Å, leading to δE ≈ 65 μeV. The range of scattering angles explored goes from 8° to 140° in an energy window similar to FOCUS. We obtained Iexp(Q,ω) in an elastic Q-range 0.4 ≤ Q ≤ 1.9 Å−1 and in a T-range 225 K < T < 325 K. The resolution spectrum was measured at 5 K. For the BS measurements on SPHERES (λ = 6.27 Å) the covered Q-range was 0.4 ≤ Q ≤ 1.8 Å−1. An energy window −30 μeV ≤ E ≤ +30 μeV with δE ≈ 0.325 μeV was explored. Temperatures in the range 225 K < T < 325 K were investigated. The resolution spectrum was measured at 4 K. C. Dielectric Spectroscopy. A broadband high-resolution dielectric spectrometer (BDS), Novocontrol Alpha-N, was used to measure the complex dielectric function ϵ*(ω) = ϵ′(ω) − iϵ″(ω) (ω = 2πf) in the frequency range of 10−2−107 Hz; an Agilent rf impedance analyzer 4192B covered the frequency range of 106−109 Hz. Parallel gold-plated electrodes were used, with a diameter of 40 and 10 mm for low- and high-frequency measurements, respectively. The sample thickness was 0.17 mm for PA6,6/6I and 0.25 mm for PA6,6/6T. For dry samples PA6,6/6I and PA6,6/6T the isothermal frequency scans were performed every 10 K over the temperature range of 150−170 K and every 5 K from 170 K up to 300 K. For wet samples PA6,6/6I H2O and PA6,6/6T H2O isothermal scans were performed every 5 K over the temperature range of 150−300 K. The maximum temperature was limited to 300 K in order to avoid significant water evaporation from samples. Water content was checked before and after measurement by weighing the sample. The sample temperature was controlled with a stability better than 0.1 K.

(1)

where kf and ki are the momentum of the neutron after and before scattering, respectively. Q = (4π/λ) sin(θ/2) is the modulus of the momentum transfer, with λ the wavelength of the incoming neutrons and θ the scattering angle. It determines the length scale at which the dynamics of the samples are observed in the experiment. σcoh and σinc are the total coherent and incoherent scattering cross sections, respectively. Scoh(Q ,ω) and Sinc(Q ,ω) are the coherent and incoherent dynamic structure factors. Sinc(Q ,ω) is related by Fourier transformation to the intermediate incoherent scattering function, Ss(Q ,t). The Fourier transform of this function is the self-part of the Van Hove correlation function Gs(r,t). In the classical limit, Gs(r,t) is the probability of a given nucleus to be at a distance r from the position where it was located at a time t before. This means that incoherent scattering gives information about the single particle dynamics of the sample. The subunits forming the random copolymers investigated in this study contain respectively a number of hydrogen H atoms equal to 22 (PA6,6) and 18 (PA6I and PA6T) per repeat unit. Given that the H = 80.27 barn/atom, is incoherent scattering cross section of H, σinc much larger than all the other cross sections involved, the scattering from these polymers is largely dominated by the incoherent contributions of the hydrogens. In the experiments we therefore probe the single particle dynamics of the hydrogens in the system. In order to measure the dynamics of the water absorbed into the copolymers, for both PA6,6/6I and PA6,6/6T we performed measurements on samples fully saturated either with H2O or D2O. For the sample containing D2O the contribution of water to the scattering is essentially negligible; therefore, one follows the dynamics of the polymer in presence of water at full saturation, i.e., I(Q ,ω) ≈ pol pol Sinc (Q,ω). On the other hand, for the sample (1/4π)(kf/ki)σinc containing H2O the contribution of water, due to its hydrogens, is significant, and the double differential cross section can be expressed as

I(Q , ω) ≈

1 k f pol pol wat wat [σ S (Q , ω) + σinc Sinc (Q , ω)] 4π k i inc inc

III. RESULTS AND DISCUSSION A. Dielectric Spectroscopy. Results of isothermal frequency scans (BDS data) of samples in the dry (PA6,6/6I, PA6,6/6T) and fully wet states (PA6,6/6I H2O, PA6,6/6T H2O) are shown in Figure 2 for 180 K < T < 300 K. This is the regime where water dynamics can be observed in the frequency window accessed by BDS. In the dry state (Figure 2a,b) two relatively broad temperature-dependent relaxation processes can be distinguished, which can be identified with the γ (higher frequency) and β (lower frequency) relaxations usually observed in polyamides.35,36 These relaxations are generally attributed to local motions of the methylene groups (γ) and rotations of the amide groups (β). The segmental α-relaxation, on the other hand, is not visible in this temperature range, as expected from the Tg values obtained by DSC for the dry copolymers. The intensity and temperature dependence of the γ and β relaxations in the amorphous PA6,6/6I (Figure 2a) and semicrystalline PA6,6/6T (Figure 2b) copolymers are comparable. In the wet state (Figure 2c,d) the overall intensity of the dielectric spectra is considerably higher due to the presence of water. A relatively broad relaxation process is seen in the temperature range where the γ relaxation is observed in the dry state. The temperature dependence of this process, which we

(2)

The incoherent dynamic structure factor corresponding to water contributions can therefore be extracted by subtracting the results of the measurements of polymer + D2O from those of polymer + H2O. This procedure requires good statistics of both sets of data, which is the case of data obtained from time-of-flight (TOF) measurements. Because of the lower flux of neutrons at the sample, backscattering (BS) measurements show poorer statistics, leading to an excessively large uncertainty on the results obtained with the subtraction procedure. In this case the measurements of polymer + D2O were used to model the polymer contribution to the dynamics. The result of modeling was then used to describe the polymer contribution in the wat (Q,ω) could be extracted as an polymer + H2O measurements, and Sinc additional model intensity added to the polymer contribution. Dynamics Measurements. The measurements were carried out in two dynamic windows (≈2 × 10−12−3 × 10−10 s and ≈8 × 10−10−4 × 10−8 s) by means of TOF (FOCUS, Paul-Scherrer Institut, Switzerland, and TOF-TOF, FRM-II, Germany) and BS (SPHERES, JCNS, Germany) spectrometers. Samples of thickness 0.17 mm 1678


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Figure 2. Imaginary part of the complex dielectric function obtained by isothermal frequency scans (ϵ″ vs frequency), measured every 10 K in the Trange from 180 K up to 300 K (from bottom to top curves). (a) PA6,6/6I in the dry state, (b) PA6,6/6T in the dry state, (c) PA6,6/6I H2O, i.e., the fully wet state, (d) PA6,6/6T H2O, i.e., the fully wet state. Symbols: experimental data. Full lines: best fits. Trends of the different relaxation processes contributing to ϵ″ are indicated by arrows. Error bars are smaller than symbols.

strengths of the γ and β relaxation processes present a similar, almost T-independent behavior in the amorphous and semicrystalline polymers. In the wet state process A shows an intensity considerably larger than the γ relaxation process in the dry state (∼0.45 for both wet polymers compared to 0.12 and 0.15 for the amorphous and semicrystalline dry polymers, respectively). This suggests that such process does not correspond to the γ relaxation of the dry polymer but is rather associated with water motions: The large dipole moment of water explains the larger strength of the process. However, we cannot exclude the occurrence of a γ relaxation of the polymer that could not be resolved in the dielectric data due to its low strength. In the amorphous polymer (Figure 4a) the strength of process A shows no T dependence for T < 240 K and a moderate increase for higher T. For the semicrystalline polymer (Figure 4b) the strength of process A remains approximately constant over the whole T range. The intensity of process B shows, for both polymers, an increase with temperature for T < 260 K, considerably more pronounced for the semicrystalline material. For higher temperatures ΔϵB reaches a constant value comparable to that of process A and slightly larger than that of the β relaxation in the dry state. The strength of process C increases with temperature for both samples. The increase becomes particularly sharp in the vicinity of Tgwet. The exponents βCi are shown in Tables 2 and 3. For processes γ, β, B and C the exponents were found to be approximately T-independent, while βCA shows a pronounced T dependence which will be discussed later. All processes show a

call process A, is more pronounced than that of the γ relaxation though. Moving toward lower frequencies, a process which we call process B is observed, together with another process, process C, located roughly in the same region as the β relaxation in the dry state. The different model contributions used to describe the experimental dielectric spectra are shown in Figure 3 for copolymers in the dry (a, b) and fully wet states (c, d) at T = 240 K. A frequency range from 1 to 109 Hz is shown, where local polymer motions and water motions are observed. At lower frequencies additional processes related to larger scale polymer dynamics also contribute to the dielectric signal. These processes are not discussed in this work. The relaxation processes were described by Cole−Cole functions:

⎛ βi π ⎞ sin⎜ C ⎟ ⎝ 2 ⎠ ϵ″i (ω) = Δϵi ⎛ βi π ⎞ i i i βC i 2βC 1 + 2(ωτC ) cos⎜ C ⎟ + (ωτC ) ⎝ 2 ⎠

(3)

where Δϵi is the dielectric strength and τCi the characteristic relaxation time of process i (i = γ and β (A, B, and C) in the dry (wet) state). The exponent βCi governs the width of the peak and can assume values between 0 and 1. Also, a conductivity contribution ϵcond″(ω) = σ0/(ϵ0ω), where σ0 is the dc conductivity of the sample, was included above T = 280 K (240 K) in the dry (wet) state. Figure 4 shows the values of Δϵi obtained by fitting for the different processes described in Figure 3. In the dry state the 1679


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Figure 3. Results of isothermal frequency scans (ϵ″ vs frequency) at T = 240 K for samples: (a) PA6,6/6I in the dry state, (b) PA6,6/6T in the dry state, (c) PA6,6/6I H2O, i.e., the fully wet state, (d) PA6,6/6T H2O, i.e., the fully wet state. Full symbols: data obtained by BDS. Open symbols: data obtained using a high-frequency impedance analyzer. Full lines represent the total fit; the different contributions (dashed lines) are indicated by the annotations in the plots. Error bars are smaller than symbols.

Figure 4. Dielectric strength Δϵ, obtained from the model fits as a function of T for the different relaxation processes described in Figure 3 and distinguished by annotations in the plots. Plot a shows results for PA6,6/6I and plot b for PA6,6/6T. The glass transition temperature of the polymers in the wet state is indicated by arrows. Filled symbols: dry samples. Open symbols: wet samples. Error bars are smaller than symbols.

large broadening reflected in the small values of βCi , except perhaps for process B. In the dry sample the distributions in the amorphous material are narrower than in the semicrystalline material while the opposite is observed in the wet state. Figure 5 shows Arrhenius plots for the amorphous PA6,6/6I copolymer (a) and for the semicrystalline PA6,6/6T copolymer (c). The processes considered here are the ones described in Figure 3. In the dry state both γ and β processes show an Arrhenius dependence on T, τCi = τ0i exp(Ei/kBT), where i = γ, β. The obtained activation energies Ei and the logarithm of the time prefactors τ0i are reported in Table 2. The values of Eγ, Eβ, and log τ0β compare well with values obtained from measurements on similar polyamide materials.4,5,7,35,37 Values of log τ0γ are more difficult to compare since

significantly different values have been reported in the literature for different kinds of polyamides.5,7,35,37 We note that the β-process in the amorphous sample is faster than in the semicrystalline one, in particular in the low-T range. The slightly higher value of Eβ in the semicrystalline material may result from the presence of a rigid amorphous phase, as suggested for example in refs 38 and 39. A rigid amorphous phase corresponds to the amorphous fraction contained within the lamellae of the crystalline phase, its mobility being reduced by the interaction with the crystallites.38,39 In the wet state process A shows a particularly interesting temperature dependence, comparable for the amorphous and semicrystalline material. While τCA presents similar values to τCγ around 220 K, their T dependence is markedly different. At low temperatures (1000/T > 4.4, T ≲ 225 K) the T dependence of 1680


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Table 2. Samples PA6,6/6I and PA6,6/6T (Dry State): Cole−Cole Exponent βCi , Prefactors log τ0i , and Activation Energies Ei for the γ and β Relaxation Processesa γ relaxation (200 K < T < 300 K)

β relaxation (150 K < T < 290 K)

sample

βCγ

log[τ0γ (s)]

Eγ (eV)

βCβ

log[τ0β (s)]

Eβ (eV)

PA6,6/6I PA6,6/6T

0.25 ± 0.06 0.38 ± 0.05

−12.7 ± 0.2 −12.5 ± 0.1

0.39 ± 0.01 0.39 ± 0.01

0.32 ± 0.03 0.40 ± 0.03

−16.2 ± 0.2 −16.5 ± 0.3

0.66 ± 0.03 0.67 ± 0.04

a

The T range considered is specified for each process.

Table 3. Samples PA6,6/6I H2O and PA6,6/6T H2O (Wet State): Cole−Cole Exponent βCi , Prefactors log τ0i , and Activation Energies Ei for Processes A, B, and Ca process A (200 K < T < 300 K) sample

βCA

log[τ0A (s)]

EA (eV)

PA6,6/6I H2O PA6,6/6T H2O

0.3 ± 0.12 0.25 ± 0.1

−16.9 ± 0.2 −17.1 ± 0.1

0.58 ± 0.01 0.60 ± 0.01

a

process B (160 K < T < 290 K) βCB 0.64 ± 0.04 0.47 ± 0.04

process C (150 K < T < 225 K)

log[τ0β (s)]

EB (eV)

⟨βCC⟩

log[τ0C (s)]

EC (eV)

−17.3 ± 0.3 −16.3 ± 0.1

0.57 ± 0.02 0.59 ± 0.02

0.31 ± 0.04 0.43 ± 0.05

−15.5 ± 0.2 −17.1 ± 0.4

0.59 ± 0.04 0.59 ± 0.05

The T range considered is specified for each process.

The prefactors log τ0A are also very small, much smaller than typical vibrational times. Note that the obtained values of EA and log τ0A well correspond to those found for water confined in a variety of systems.27,40,41 This observation, together with the previously discussed considerably larger dielectric strength of process A compared to the γ relaxation, supports the interpretation of process A as a relaxation associated with water motions. At high temperatures (T > 225 K) the T dependence of τCA changes from Arrhenius to VFT (τ = τ0 exp(B/(T − T0)) type, which is characteristic of the α relaxation in glass-forming systems. We note that this occurs in the T-range where the γ process crosses the A process. We can exclude that the change in the T dependence of the A process is an artifact due to the presence of the γ process in the wet samples given that the dielectric strength of this is negligible as compared to process A. As commented in the Introduction, such a crossover from Arrhenius to VFT behavior has been observed, in a narrow temperature range around T = 225 K, for water segregated in different media23,24,27 and has been attributed either to confinement effects,27 a transition from H-bond (β) to glassy (α) dynamics31 or to a strong-to-fragile transition of water.24,23 Figure 5b,d shows the T dependence of the inverse of βCA that is a measure of the width of the spectrum. Both polymers display a crossover from T-dependent width to constant width at temperatures corresponding to the Arrhenius to VFT crossover of the relaxation times. Process B shows, in both PA6,6/6I and PA6,6/6T, an Arrhenius T dependence, with EB comparable to EA and Eβ (see Tables 2 and 3). The values of τCB are intermediate between those of τCA , associated with water motions, and those of τCβ . Since ΔϵB ≈ Δϵβ, we may speculate that process B could be associated with movements of amide groups associated with water molecules by hydrogen bonding. The motions of the amide−water complex would be faster than those of the amide groups due to the faster dynamics of water. This is also supported by the values of the prefactors, which are rather comparable to those of process A. The τCC values almost overlap with those of τCβ . This suggests that process C can be associated with the β relaxation of the amide groups which are not forming any bond with water. Note that around 250 K the T dependence of τCC shows a small kink. At the same temperature, ΔϵC starts to increase sharply. Both findings could be related to the influence of the α relaxation in the wet state, as can be seen in Figure 6. On top of this figure

Figure 5. (a, c) Arrhenius plots of relaxation times τCi vs inverse temperature for PA6,6/6I (a) and PA6,6/6T (c). Results for both the dry (full symbols) and wet (open symbols) state of the polymer are shown. Letters in the plot distinguish the different processes. Lines represent fits to Arrhenius laws. (b, d) Inverse of the Cole−Cole exponent of process A for polymers PA6,6/6I (b) and PA6,6/6T (d). Error bars are shown when bigger than symbols.

τCA is Arrhenius-like, with an activation energy EA (Table 3) considerably larger than Eγ (and rather comparable to Eβ, Table 2). 1681


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molecule in the semicrystalline material, which translates into a larger relaxation time. Note that τCA in the VFT regime is also different for the amorphous and semicrystalline polymer, but the difference is considerably smaller than in the Arrhenius regime. One can speculate that in the VFT regime most of the H-bonded structure of water is disrupted due to the glass transition of water. Therefore, the relaxation times of the amorphous and semicrystalline material in this regime tend to approach each other, with some residual difference which could be arising from surface interactions with the polymer. Note that no significant peak in dCp/dT is observed in the region of the Arrhenius to VFT crossover of process A (Figure 6, top), suggesting that in this case such crossover can hardly be associated with confinement effects imposed by the freezing of the structural relaxation of the polymer matrix. Nevertheless, we observe that the crossover takes place in the T-range where the A process crosses the γ process if the dry polymer. This could suggest that the local mobility of the polymer could play an important role in the origin of this crossover. B. Neutron Scattering. Figure 7 presents incoherent dynamic structure factors measured by TOF. Plot a shows S(Q ,ω) (we omit the suffix inc in the following) for sample PA6,6/6I H2O, which reflect the dynamics of both the polymer and water molecules absorbed into it. Up to T = 250 K the dynamic structure factors show almost no quasi-elastic contribution and a Boson peak42 at EBP ≈ 2.5 meV whose position does not change with T. This means that for T < 250 K we essentially observe localized vibrational motions. The results on the semicrystalline sample PA6,6/6T H2O are similar (Figure 7b), with nonvibrational dynamics starting to be observed at T = 225 K. Figures 7c (PA6,6/6I) and 7d (PA6,6/6T) show a comparison between the spectra measured for the dry polymer, the polymer saturated in D2O and in H2O in the range 20 K ≤ T ≤ 225 K. In the amorphous sample (PA6,6/6I) containing D2O the intensity of the Boson peak is reduced compared to the dry polymer, and its position is shifted to slightly higher energy. Since in the sample containing D2O we observe the selfmotions of the polymer in presence of water, the vibrational dynamics of the polymer are reduced upon addition of water. Similar results concerning the shift of the Boson peak to higher energy in relation to hydration have been found for poly(vinyl methyl ether) (PVME)19 and for proteins in neutron scattering experiments and molecular dynamics simulations.43−47 One possible explanation for this shift is to assume a distribution of elastic constants48 in the wet sample. This interpretation is in agreement with the formation of water−polymer H-bonds upon hydration which are stronger than polymer−polymer Hbonds.49 In the H2O-saturated sample the water contribution adds an additional intensity to that of the polymer and the Boson peak shifts to slightly higher energies in the same way as with D2O. Note that this reduced vibrational dynamics of the hydrated polymer must be clearly distinguished from the largescale motions of the polymer, which is accelerated, thus resulting in the observed drop of Tg in the presence of water. In the semicrystalline material the Boson peak is less discernible, and its shift in the presence of water is less clear than for the amorphous sample. Nevertheless, within the uncertainties we can state that the effects of hydration are qualitatively the same in both samples. Figure 8a,b shows dynamic structure factors of water as obtained by subtraction of the measurements with protonated and deuterated water (see Materials and Methods). Graph a

results of DSC measurements are presented in the form of the derivative with respect to T of the specific heat Cp vs 1000/T, to be compared to the Arrhenius plot at the bottom of the figure. The calorimetric glass transition of the wet polymers is evidenced by a peak in dCp/dT. The bending of τCC toward faster times appears to be correlated with the occurrence of the glass transition. Figure 6 also shows τCA and τCB for the amorphous and semicrystalline copolymers. In the Arrhenius regime the relaxation

Figure 6. (bottom) Temperature dependence of the relaxation time τC of processes A, B, and C for the fully wet amorphous and semicrystalline copolymers observed by BDS (open symbols), compared with the corresponding temperature variation of the inverse of the diffusion coefficients obtained by quasi-elastic neutron scattering (full symbols, see section III.B). (top) dCp/dT as a function of inverse temperature, obtained from DSC measurements for PA6,6/6I (green) and PA6,6/6T (red).

times measured for the semicrystalline copolymer are slower than for the amorphous. This could be attributed to the absorption of part of the water into the rigid amorphous phase38,39 of the semicrystalline material, whose dynamics are slower than those of the mobile amorphous phase38,39 (the bulk amorphous phase). Note that there are experimental evidences that water does not penetrate the crystalline phase (see ref 39 and references therein). Slower dynamics of the polymer clearly imply larger relaxation times of process B, in agreement with the interpretation of this process as the result of the movements of the water bonded to amide groups. It is more difficult to explain the slowing down of the water dynamics of process A. We would expect that τCA would be determined by the strength and number of H-bonds that water molecules form with other water molecules. Therefore, one could think that in the semicrystalline material the structure of water nonbonded to the polymer could be different from that in the amorphous material due to changes in available free volume. A different water structure may result in a different (higher) degree of H-bonding per 1682


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Figure 7. Dynamic structure factors measured by TOF (on FOCUS and TOFTOF, as indicated in legend) at Q = 1.6 Å−1. (a) Measurements at different temperatures (indicated in legend on the right-hand side) for sample PA6,6/6I H2O. (b) Same as in (a) for sample PA6,6/6T H2O. (c) Comparison of the dynamic structure factors of samples PA6,6/6I (lines), PA6,6/6I H2O (empty symbols), and PA6,6/6I D2O (full symbols) for T = 100 K (red), 175 K (blue), 225 K (green), and 20 K (black) (FOCUS data). (d) Same as in (c) for samples PA6,6/6T, PA6,6/6T H2O and PA6,6/6T D2O. Error bars were omitted for clarity.

from exponential behavior. It can take values between 0 and 1. As in other works, in our case, eq 4 with βwH2O ≈ 0.5 was found to properly fit the data at different Q values. Representative fits for water absorbed in the amorphous copolymer PA6,6/6I at T = 300 K are shown in Figure 9. Backscattering results allowed us studying water dynamics down to T = 225 K. First, the data on the samples with D2O give access to the polymer contribution Swet PA(Q ,ω). Figure 10a presents results for sample PA6,6/6I D2O. At fixed Q the quasi-elastic component of the spectra moderately broadens with increasing temperature, indicating the acceleration of the polymer motions due to thermal activation. The S(Q ,ω) corresponding to polymer dynamics was modeled as the Fourier transform of a KWW function (eq 4) with fixed shape parameter βwwet PA = 0.5. Fits for data of Figure 10a, obtained by convoluting the fit function with the experimental resolution, are shown as solid lines. Such a procedure was adopted for all fits. Figure 10b presents BS results on the amorphous sample (PA6,6/6I) with H2O at the same Q value and temperatures as in Figure 10a. Also in this case the spectra present an increasingly broadened quasi-elastic component with increasing T, now due to the thermal activation of both polymer and water dynamics. Consistently with the TOF data analysis (see above), in the BS window we also assumed a KWW (eq 4) with βwH2O = 0.5 to describe the water contribution, SH2O(Q,t). The total intermediate incoherent scattering function was therefore modeled as

displays data obtained for water absorbed into the PA6,6/6I copolymer and graph b data obtained for water absorbed into the PA6,6/6T copolymer. In the amorphous material (a) the quasi-elastic contributions start to be significant above T = 250 K. In the semicrystalline material they are smaller, indicating that water dynamics are slower. Figure 8c presents S(Q ,t) obtained by Fourier transforming the dynamic structure factors of plots a and b and deconvoluted from resolution by dividing through S(Q ,t,T=0). A significant decay of S(Q ,t) is only observed for T > 250 K in both copolymers, indicating that on the TOF instruments the time scales characteristic of water dynamics are only accessible for such temperatures. The decay of S(Q ,t) of absorbed water is considerably more pronounced in the amorphous than in the semicrystalline polymer, confirming a higher mobility of water into the amorphous material. For T > 250 K the time dependence of S(Q ,t) of water cannot be described with a single-exponential decay. Using the same approach as followed in other works on confined water dynamics as accessed by neutron scattering,19,50,51 we used Kohlrausch−Williams−Watts (KWW) or stretched exponential functions:

⎡ ⎛ ⎞βw ⎤ t S(Q , t ) = A exp⎢ − ⎜ ⎟ ⎥ ⎢ ⎝ τw (Q , T ) ⎠ ⎥ ⎣ ⎦

(4)

where A is a prefactor which accounts for the contribution of the fast dynamics, τw(Q ,T) is the relaxation time of the dynamics, and βw is the stretching exponent accounting for deviations

SPA/H2O(Q , t ) = fH O S H2O(Q , t ) + fPA S wet PA(Q , t ) 2 1683

(5)


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Figure 10. Normalized dynamic structure factors (symbols) measured by BS (SPHERES) at Q = 1.55 Å−1 and temperatures reported in legends, for samples: (a) PA6,6/6I D2O and (b) PA6,6/6I H2O. Solid lines are fits obtained by (a) describing the polymer dynamics using the FT of KWW functions and (b) describing the polymer dynamics using the results of (a) and using in addition the FT of KWW functions to model the water contributions. Dashed line is the measured instrument resolution.

Figure 8. Dynamic structure factors of water extracted from TOF measurements (FOCUS and TOFTOF) at Q = 1.6 Å−1 for polymers (a) PA6,6/6I and (b) PA6,6/6T. Temperatures are indicated in legends. (c) Intermediate scattering functions obtained by Fourier transforming data of plots a and b. Solid symbols represent data of water absorbed in PA6,6/6I; open symbols represent data of water absorbed in PA6,6/6T.

Figure 11. Dynamic structure factor of sample PA6,6/6I H2O at T = 275 K and Q = 1.55 Å−1 measured by BS (symbols). The solid line represents the total fit. Dotted line: modeled water contribution. Dashed-dotted line: polymer contribution as obtained by fitting data measured for sample PA6,6/6I D2O at the same T and Q. Dashed line: measured instrument resolution.

Figure 9. Intermediate scattering functions of water extracted from TOF measurements (TOFTOF), for sample PA6,6/6I at T = 300 K and different Q values (listed in legend). Lines are fits to KWW functions with βwH2O ≈ 0.5.

PA PA/H2O and f PA = σinc /σinc = 0.852. The polymer scattering function was obtained from the D2O samples results. Figure 11 shows the so-obtained total fit and the separate contributions of the

where the weights f H2O and f PA were calculated on the basis of H2O PA/H2O /σinc = 0.148 the corresponding cross sections: f H2O = σinc 1684


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polymer and water, for sample PA6,6/6I H2O at T = 275 K and Q = 1.55 Å−1. Water contributes with a fast component affecting the spectra in the whole BS window. Figure 12a presents the Q dependence of S(Q ,ω) for sample PA6,6/6I H2O at T = 325 K. For the total dynamic structure

Figure 13. Average relaxation time of water, obtained from fitting TOF (T = 300 K) and BS data (all remaining temperatures), vs wave vector Q, for different temperatures (in legend). Solid symbols: water absorbed in the amorphous copolymer PA6,6/6I; open symbols: water absorbed in the semicrystalline copolymer PA6,6/6T. Lines (solid for PA6,6/6I, dashed for PA6,6/6T): power law dependencies having exponents comprised between −1.9 and −2.3.

procedures used in each case (spectra subtraction and FT vs modeling of two components and convolution with resolution). The Q dependence of ⟨τ⟩ can be described using power laws ⟨τ⟩ ∝ Q−n having exponents n in the range 1.9 < n < 2.3, i.e., values compatible with a Q−2 dependence. This tells us that on average the dynamics of water absorbed in PA can be depicted as diffusive (let us remind that in simple diffusion the characteristic time τ is related with the self-diffusion coefficient D as τ = D−1Q−2). However, the scattering function is not described by a single exponential, as expected for simple diffusion. Note also at this point that the Q range investigated corresponds to length scales at which we observe both incluster and intercluster motions of water. Indeed, on the basis of simulations results,15 one can estimate a radius of 1 nm for spherical water clusters and on the basis of volumetric arguments an average distance of 0.7 nm between neighbor amide groups. We will discuss this point in the following section. It is also clear that for T > 250 K the relaxation times of absorbed water become considerably shorter in the amorphous than in the semicrystalline material. An average diffusion coefficient ⟨D⟩ can be obtained by fitting the Q dependence of ⟨τ⟩ in Figure 13 to ⟨τ⟩ = ⟨D⟩−1Q−2. The results are presented in Figure 14 as a function of temperature T for both copolymers. The values of ⟨D⟩ of water absorbed in the PA copolymers are ∼2 orders of magnitude smaller than those of bulk supercooled water, which are shown as a solid line for comparison. The interaction of water with the polymer matrix considerably reduces the average mobility of the water molecules. Similar effects have been reported for water confined in different media.24 This is usually attributed to the fact that water in layers close to the water/confining medium interface presents a different hydrogen bond structure due to bonding to the host material.53 This different structure results in a reduction of mobility close to the interface even for molecules not directly bonded to the matrix. One can think therefore that the slower mobility of water molecules close to the interface will influence the average value of the diffusion coefficient inducing a reduction with respect to bulk water. The strong reduction observed in our case suggests that the hydrogen bond structure is significantly modified

Figure 12. (a) Normalized dynamic structure factor of sample PA6,6/ 6I H2O at T = 325 K and different Q values (listed in legends). Solid line: total fit. Dashed line: measured instrument resolution. (b) Single contributions of water (solid lines) and polymer (dashed lines) to the total fits of (a), illustrating the strongly different Q dependence of water and polymer dynamics.

factor, this dependence appears to be rather weak. In order to distinguish the Q dependence of the polymer and water dynamics, the relative contributions to S(Q,ω) are plotted in Figure 12b. The decomposition clearly shows that while the polymer dynamics, whose contribution dominates, are relatively insensitive to Q, the water dynamics present a pronounced Q dependence. The weak Q and T dependences (average activation energy around 0.37 eV for both amorphous and semicrystalline) of Swet PA(Q,t) suggest that the motions observed could be related to the γ relaxation. The relaxation times found are also in the same range of values. We recall that this process is detected by BDS in the dry sample and is associated with methylene groups but cannot be resolved in the wet sample. The Q dependence of the characteristic relaxation times of water dynamics is illustrated in Figure 13. We have represented the average value ⟨τ⟩ of the associated distribution of single relaxation times, calculated as ⟨τ⟩ = τwΓ(1/β)/β for KWW functions. It is noteworthy the consistency of the results obtained from the two different instruments and analysis 1685


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solution,19 as can be seen in Figure 14. In that case, however, the water concentration was significantly higher (30 wt %). The Arrhenius to VFT crossover at T ≈ 225 K for process A could only be observed in the BDS measurement due to the limited dynamic window accessible in the QENS experiments. It is interesting to note that this crossover occurs at a temperature considerably lower than the glass transition temperature of the hydrated polyamide, as shown in Figure 6, where the calorimetric glass transition is reported on top of the dielectric results. Note also that the calorimetric transition is relatively narrow, indicating a clear separation of the two phenomena. This suggests that confinement effects due to the frozen dynamic state of the polymer matrix cannot alone explain the Arrhenius to VFT crossover in this case. At this point it is worth noting that in the water mixtures where the Arrhenius to VFT crossover was reported to occur close to glass transition,27 the water concentration was always above ≈30%. We can speculate that the most-likely scenario to explain the observed crossover in the case of water absorbed in polyamide is that for T < 225 K we are observing local hydrogen bond motions while for T ≥ 225 K the water α process is dominating the relaxations in the BDS measurements.28,31 In the corresponding temperature range, process B has a much slower dynamics and is only accessed in BDS measurements. As already commented, the close similarity to the T dependence of the β relaxation of the dry polymer suggests that this process might be associated with the movements of the amide groups bonded to water molecules.

Figure 14. Average diffusion coefficients of water absorbed in the amorphous (PA6,6/6I) and semicrystalline (PA6,6/6T) copolymers vs temperature, extracted from the fitted τ vs Q dependence of Figure 13. Solid line: bulk supercooled water.52 Green symbols: water absorbed in PVME.19 Error bars are smaller than symbols.

as a result of the small size of water clusters. Note also that the diffusion coefficients of water absorbed in PA6,6/6I and PA6,6/6T separate for T ≥ 250 K. C. Discussion of the Results in a Unified Picture. Results obtained with dielectric spectroscopy (BDS) and quasielastic neutron scattering (QENS) can be discussed in their ensemble in order to construct a consistent picture of the dynamics of water absorbed in polyamide. BDS results indicated the presence of two well-separated dynamical processes associated with absorbed water molecules, namely A and B, which may be related to two distinct populations of water, as noted by several authors.4,7,16,17 Process A was interpreted in terms of motions of water molecules not directly (tightly) bonded to the polymer or confined into cavities of the polymer (at the molecular or nanometric scale). In Figure 6, the temperature dependence of the diffusion coefficient ⟨D⟩ obtained by QENS for absorbed water is compared to that of the relaxation times τCi obtained from BDS, in both the amorphous (PA6,6/6I) and semicrystalline (PA6,6/6T) copolymers. The T dependence of ⟨D⟩ (from QENS) is in very good agreement with that of process A, confirming the fact that process A can be assigned to water motions (which are selectively studied in QENS experiments). In the common region of temperatures investigated by QENS and BDS (above 225 K typically) the T dependence of τCA and ⟨D⟩ is of VFT type and could be interpreted as related to the α relaxation of confined water, which would be consistent with the Q dependence of the characteristic times indicative for diffusive-like motions obtained from the QENS experiments. Note though that in this framework the description of Swater(Q ,t) in terms of a KWW function with β = 0.5 clearly indicates a broad distribution of diffusion coefficients of water molecules, which origin would be the existence of markedly heterogeneous environments of the water molecules. This can be qualitatively understood in terms of a nonuniform degree of bonding of water molecules associated with the nonuniform distribution of water and the nonfully achieved tetrahedral structure. We comment that in both polyamides the water dynamics is slower than in the recently investigated PVME aqueous

IV. CONCLUSIONS The dynamics of water absorbed in amorphous and semicrystalline polyamide was characterized by neutron scattering experiments and broadband dielectric spectroscopy. A fast water process (A) could be observed with both techniques. The neutron scattering analysis allows to characterize such process by diffusive water motions having a broad distribution of mobilities. The diffusive nature of the motions on one hand points at similarities with bulk dynamics. The large distribution of diffusivities, on the other hand, indicates possible differences in the connectivity of the hydrogen bond network. These two features become coherent if we interpret our observations in terms of a nonuniform distribution of water molecules in terms of bonding to amide groups, with a fraction of water molecules only loosely bonded to amide groups (as reflected in process A). Diffusive-like motions of water with a similar distribution of mobilities have also been observed for concentrated aqueous solution of PVME.19 The average diffusion coefficients associated with these dynamics are considerably smaller than for bulk water. This clearly indicates that the interaction with the polymer matrix induces structural modifications of water. The fast water process presents an Arrhenius to VFT crossover at T ≈ 225 K observed by BDS. Such a crossover is well decoupled from the α relaxation of the polyamide + water system and the process presents characteristics of a local relaxation for T < 225 K. This suggests that the crossover could be associated with the observation in the BDS measurements of hydrogen bond dynamics for T < 225 K and the α relaxation of supercooled water for T > 225 K at least in this case. A second slower process (B) associated with water was observed in the BDS measurements. Such process was associated with motions of hydrated amide groups. A third process at higher temperatures (C), which coincides with the 1686


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β relaxation of the dry polymer, indicates that not all amide groups are bonded to water. The presence of a crystalline phase slows down both the fast and the slow processes associated with water motions. This result might be related to the presence of some degree of motional constraint in the amorphous phase in semicrystalline material,38,39,54 which would slow down the water dynamics in the semicrystalline copolymer. Nevertheless, it would need further investigation to be fully clarified. Finally, the absorption of water modifies the vibrational density of polyamide deep in the glassy state. We suggest that this effect, also observed for aqueous solutions of PVME, is associated with the fact that water−amide bonds are stronger than amide−amide bonds in the dry state.

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ACKNOWLEDGMENTS This work is partially based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. We thank support by the DIPC, Rhodia, the European Commission NoE SoftComp, Contract NMP3CT-2004-502235, the projects MAT2007-63681 (Spanish Ministerio de Educacion y Ciencia) and IT-436-07 (GV) (Basque Government). M.L. acknowledges support from CNRS. We also thank Véronique Bossennec and Thierry Badel (Rhodia) for synthesizing the copolymers and for fruitful discussions. We acknowledge the funding of the Broadband Dielectric equipment in LPMA by the GRAND LYON metropolitan council.

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AUTHOR INFORMATION

Present Address %

Condensed Matter Physics Laboratory, Heinrich-Heine University, Universitätsstr. 1, 40225 Düsseldorf, Germany.

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REFERENCES

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