Dynamics and Structure of Poly(ethylene oxide) Intercalated in the

Article pubs.acs.org/Macromolecules

Dynamics and Structure of Poly(ethylene oxide) Intercalated in the Nanopores of Resorcinol−Formaldehyde Resin Nanoparticles Fabienne Barroso-Bujans,*,†,‡,§ Silvina Cerveny,†,‡ Pablo Palomino,∥ Eduardo Enciso,∥ Svemir Rudić,% Felix Fernandez-Alonso,%,& Angel Alegria,†,⊥ and Juan Colmenero†,‡,⊥ †

Centro de Física de Materiales (CSIC-UPV/EHU), Paseo Manuel Lardizábal 5, 20018 San Sebastián, Spain Donostia International Physics Center, Paseo Manuel Lardizábal 4, 20018 San Sebastián, Spain § IKERBASQUE - Basque Foundation for Science, María Díaz de Haro 3, E-48013 Bilbao, Spain ∥ Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain ⊥ Departamento de Física de Materiales, Universidad del País Vasco (UPV/EHU), Apartado 1072, 20080 San Sebastián, Spain % ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom & Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom ‡

S Supporting Information *

ABSTRACT: The incorporation of high-molecular-weight poly(ethylene oxide) (PEO) in the nanopores of resorcinol− formaldehyde resin nanoparticles (RNPs) leads to the suppression of polymer crystallization, changes in the chain conformation, and a noticeable slowdown of the two dielectric relaxations that reflect the segmental and local PEO dynamics. Both relaxations are significantly slower than those corresponding to bulk PEO. These results are independent of the pore characteristics of the different RNP materials. The segmental relaxation shows a crossover at ca. 220 K in its temperature dependence from non-Arrhenius to Arrhenius-like behavior on cooling. These results suggest the occurrence of limited cooperativity at low temperatures due to the enhancement of long-living hydrogen bonding between PEO and RNP pore walls.

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INTRODUCTION The properties of molecules and macromolecules confined in reduced geometries play dominant roles in events of practical interest including adhesion, lubrication, catalysis, protein folding, and the fabrication of nanomaterials.1,2 Understanding the new physics that occurs due to finite-size and surfaceinduced effects is therefore relevant from both fundamental and practical perspectives. The questions of interest are concerned with how both effects may alter the structure and dynamics of the confined phase compared with its bulk counterpart. The structure and properties of a molecular phase confined in pores of reduced dimensionality are inevitably affected by the counterbalance between finite size and interactions at the surface, which sometimes leads to difficulties in interpretation. Despite a large body of theoretical and experimental works in this area, as well as a considerable and growing interest in the subject, no clear picture of the behavior of confined phases has been obtained to date.3−5 Confinement of polymers and soft matter is a broad topic as it encompasses confinement in 1-, 2-, and 3-dimensions (1-, 2-, and 3D); soft and hard confinement; and confinement with regular and irregular order.6 In the particular case of polymers confined in small geometries, it has been observed that the glass-transition © XXXX American Chemical Society

temperature (Tg) moves up, moves down, remains the same, or disappears altogether depending on the studied system. The absence of Tg has been reported in hard 2D confining systems such as the interlayer space of clays7,8 and graphite oxide (GO).9,10 Studies of the segmental relaxation have also accounted for the opposite behavior (slower11−13 and faster14,15) in the dynamics of confined polymers. A number of investigations have shown that the segmental relaxation of polymers under confinement appears at temperatures far below the Tg of the bulk polymer, exhibiting relaxation times much faster than those in the bulk.14,15 In some theories of the glass transition, it is assumed that the dynamics of glass-forming liquids is governed by the collective motions of molecules over a length scale ξ (so-called “cooperative length”), which increases with decreasing temperature.16,17 Under confinement, this length scale cannot exceed the confining length, therefore leading to limited cooperativity. Contrary to these findings, the slowing down of the dynamics has also been observed in polymers confined in pore systems. This effect has been Received: June 15, 2016 Revised: July 22, 2016

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DOI: 10.1021/acs.macromol.6b01285 Macromolecules XXXX, XXX, XXX−XXX

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chains by means of broadband dielectric spectroscopy (BDS). Such relaxation remains very distinct relative to that of bulk PEO, including an overall slower α-relaxation with the presence of a crossover in the temperature dependence of the relaxation time from non-Arrhenius to Arrhenius in going from higher to lower temperature. In addition, we found a significant slowing down of the β-relaxation of PEO upon intercalation. Highresolution inelastic neutron scattering (INS) experiments showed that the PEO chains intercalated in the pores of RNPs adopt a preferentially planar zigzag conformation, distinctly different from those characteristic of the 7/2 helical structure of the bulk crystal. This PEO conformation is nonetheless similar to that found for PEO intercalated in either the interlayer space of GO9 or in the pores of carbon nanoparticles.19

explained as arising from the interactions between the pore walls and polymer segments11−13 as well as by the decrease in the number of conformational transitions within the pore space.11 Chemical treatments of the pore surface (e.g., via silanization) can also lead to shifts of the Tg of the confined polymer phase compared to the nonfunctionalized surface.18 Concerning the crystallization behavior of the confined phase, the melting temperature (Tm), the degree of crystallinity,19 the lamellar orientation,20 and the kinetics of crystallization21−23 can be significantly altered due to finite-size and surfaceinduced-nucleation effects. A detailed discussion of the physical and theoretical aspects of the effects of confinement on the structure, dynamics, and phase transitions in soft-matter and polymer systems has been reviewed elsewhere.1,3−6,24,25 Tailorable nanoporous materials based on carbon,19,26 silica,24 silicate,14,27 alumina,22,28 and coordination polymers20 have been used as model templates for polymer confinement. These substrates can contain slit-shaped (clays, GO), cylindrical (anodized aluminum oxide, coordination polymers, MCM-41, SBA-15), or spherical pores (carbon aerogels). Following IUPAC recommendations, micro-, meso-, and macroporous materials are those characterized by pore diameters 50 nm, respectively. The intimate relationship between pore morphology, pore size, and pore chemistry will clearly dominate the properties of the confined phase under confinement. Therefore, studying these effects becomes pivotal for the understanding of molecular confinement in reduced and restricted geometries. In the above context, resorcinol−formaldehyde (hereafter, RF) aerogels are a class of tailorable nanoporous materials obtained by polycondensation of resorcinol and formaldehyde.29 RF aerogels (RFAs), also called RF resins30 or resin nanoparticles (RNPs),31 are predominantly composed of spherical nanoparticles of 10−100 nm diameter, which themselves are agglomerates of smaller spheres. Interstitials between nanoparticles constitute mesopores. Voids between the smaller particles form micropores. To achieve a welldefined RFA porosity, careful control over the steps of gelation and curing during the polycondensation reaction is critical.32 In a preliminary study, we used RNPs with mesopore diameter ⟨d⟩mes = 9.3 nm as a template to confine within its pore network poly(ethylene oxide) (PEO) chains with Mn = 94 kg/mol.31 In this particular case, we observed the absence of crystallization in the confined polymer phase as well as a very weak jump in the constant-pressure specific heat (Cp), corresponding to only ca. 9% of PEO segments contributing to the glass transition. Estimating a radius of gyration Rg = 11 nm for PEO (Mn= 94 kg/mol),33 this confinement case corresponds to a situation where ⟨d⟩mes/2 < Rg, whereby polymer−surface interactions can exert a greater influence on the properties of confined PEO relative to the hypothetical case where ⟨d⟩mes/2 > Rg. In addition to this small space for accommodating the PEO chains, polymer−surface interactions are relatively strong, hydrogen-bond-type interactions between the PEO ether oxygens and RNP hydroxyls. Motivated by these intriguing results, we have extended this study to explore how RNP pore size influences the dynamics and structure of PEO chains intercalated in the porous structure of this organic gel. To this end, we performed a complete study on this series of samples by using RNPs with ⟨d⟩mes from 4 to 31 nm and BET surface areas in the range 120−380 m2/g. For the first time in this family of materials, we have detected an α-relaxation process for intercalated PEO

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EXPERIMENTAL SECTION

Materials. The following compounds were used: resorcinol [(C6H4(OH)2), Aldrich, 99%], formaldehyde aqueous solution [(H2CO), Panreac, 37−38%], sodium hydroxide [(NaOH), Aldrich, >97%], deionized water (obtained from a Direct Q5Millipore system), resorcinol-d6 [(C6D4(OD)2), Aldrich, 98 atom % D, 98% purity], formaldehyde-d2 solution [(D2CO), Aldrich, ∼20 wt % in D2O, 98 atom % D], D2O (Aldrich), NaOD (Aldrich) 40 wt % in D2O, 99 atom % D, and PEO (Aldrich, Mn = 94 kg/mol and polydispersity index 1.08). Methods. RNPs were synthesized by polycondensation of resorcinol (R) and formaldehyde (F) in aqueous solution following the approach of Pekala et al.29 Deuterated RNPs (dRNPs) were obtained by polycondensation of resorcinol-d6 and formaldehyde-d2 in D2O. To obtain nanoparticles with different textural characteristics, the reactions were performed at different pH by addition of NaOH (Table 1) and by keeping the R-to-F molar ratio (R/F) at 0.5. The resulting

Table 1. Synthesis Conditions of RNPs RNPs

pH

R/Ca (mol/mol)

A B C D dRNPsb

7.2 7.0 6.7 6.3

100 150 250 500 246

a R/C: resorcinol-to-catalyst (NaOH) ratio. bResorcinol-d6 and NaOD were used.

solutions were placed in an oven at 85 °C for 3 days. The color of the solutions changed progressively from clear to orange, then to red, and finally to dark brown over the course of the reaction. After the curing process, the gels were dried at 85 °C at ambient pressure over the course of two additional days, leading to dry cross-linked polymer nanoparticles. PEO/RNPs hybrid samples were prepared from aqueous solutions consisting of 1 g of PEO and 1 g of RNPs codissolved in 40 mL of water. The mixture was stirred for 15 days to enable the filling of RNP galleries via the diffusion of PEO into the cavities. Excess PEO was removed by centrifugation and repeated aqueous washings. The resulting PEO/RNPs samples were dried at 80 °C under vacuum for 24 h and stored at room temperature in a vacuum oven until further use. Characterization. The texture and porosity of pristine and PEOfilled RNPs were analyzed by field-emission scanning electron microscopy (FESEM) and nitrogen adsorption−desorption isotherms.34 FESEM images were collected with a JEOL JSM-6700F instrument operating at 5−10 kV and 12 μA. The powder samples were supported on adhesive carbon tape and coated with a thin gold film. Nitrogen isotherms were obtained at 77 K using a Micromeritics ASAP 2020. RNPs and PEO-containing samples were outgassed at 110 B


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°C for 1 and 6 h, respectively. The specific surface area (SBET) was determined from the linear part of the BET plot (P/P0 = 0.05−0.2).35 External surface areas (Sext) and micropore volumes (Vmic) were determined from the t-plots obtained via recourse to the Harkins−Jura equation.36 Average pore diameters (⟨d⟩BJH) and mesopore volumes (VBJH) were calculated with the Barrett−Joyner−Halenda (BJH) adsorption−desorption method37 assuming cylindrical pores in the Kelvin equation.38 The pore size distributions (PSDs) were obtained by applying the density functional theory (DFT) method to the isotherm data.34 PEO mass uptake in RNPs was determined by thermogravimetry (TGA) in a Q500 thermogravimetric analyzer from TA Instruments. Samples were heated from room temperature to 800 °C at a rate of 10 °C/min under a constant N2 flow of 60 mL/min. The fraction of PEO in PEO/RNPs (f PEO) was calculated as19

fPEO = (WPEO/RNPs − WRNPs)/(WPEO − WRNPs)

RESULTS AND DISCUSSION Pore Structure of RNPs and Uptake of PEO in the Pores. Nitrogen isotherms for all RNPs samples conform to type IV with a type H1 hysteresis loop.31 This loop is typically associated with capillary condensation in the mesopores.43 Figure 1 shows pore-structure parameters for four RNPs samples as obtained from nitrogen physisorption experiments.

(1)

where WPEO/RNPs, WRNPs, and WPEO are the weight losses of PEO/ RNPs, RNPs, and PEO at 800 °C, respectively. Differential scanning calorimetry (DSC) measurements were carried out on ∼10 mg specimens using a TA Instruments Q2000 by placing the samples in sealed aluminum pans. PEO/RNPs samples and bulk PEO were first heated to 373 K at the highest attainable heating rate. Then, samples were cooled to 125 at 10 K/min and heated back to 373 at 10 K/min. A helium flow rate of 25 mL/min was used all throughout. High-resolution INS data were collected on the TOSCA spectrometer39−41 located at the ISIS Facility, Rutherford Appleton Laboratory, U.K. TOSCA is an indirect geometry time-of-flight inelastic neutron spectrometer spanning an energy-transfer range up to 4000 cm−1 in neutron-energy loss with a spectral resolution of ∼1.25%. INS spectra were collected in both back- and forwardscattering geometries and then added together to obtain hydrogenprojected vibrational densities of states (VDOSs). Typical run times varied between 2 and 8 h depending on the hydrogen content of the sample. All samples were contained in flat aluminum cells of thickness 1−4 mm and cooled to ∼10 K. The INS response from the empty aluminum cell was first subtracted from all measured spectra. Then, the contribution from the RNP substrate was subtracted in order to isolate the INS response from PEO/RNPs. Finally, the resulting INS spectra were normalized to the amount of PEO content in the sample, as previously determined by TGA. INS data of bulk PEO were normalized to the sample mass. The dynamics of PEO in RNPs and semicrystalline bulk PEO was analyzed by broadband dielectric spectroscopy (BDS). To measure the complex dielectric permittivity, ε*(ω) = ε′(ω) − iε″(ω), we used a Novocontrol Alpha-N analyzer in the frequency range from 10−1 to 106 Hz. Parallel-plate capacitors of 0.3 mm thickness were prepared by placing the sample powder between gold-plated electrodes with a diameter of 20 mm. Samples were cooled down to 150 K and then reheated to 300 K while isothermal (±0.1 K) frequency scans were registered every 5 K. In addition, melted PEO was measured in the frequency range of 0.2−50 GHz using dielectric probe kit Agilent 85070E (bandwidth 200 MHz−50 GHz) with an open-ended coaxial probe connected to a network analyzer (VNA) Agilent E8361A at temperatures from 402 to 320 K. The dielectric response of the PEO/RNPs samples and bulk PEO can be described by using standard fit functions. We used the Cole− Cole function42 to describe each relaxation component ε*(ω) = ε∞ +

Δε 1 + (iωτ )α

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Figure 1. Textural data of RNPs specimens before PEO intercalation.

RNPs exhibit BET surface areas (SBET) in the range 120−380 m2/g. Samples A−C are characterized by the larger presence of mesopores (Sext ∼ 300 m2/g) compared to micropores (Smic = 1−46 m2/g), whereas sample D is characterized by the presence of both mesopores and micropores, with Sext = 150 m2/g and Smic = 64 m2/g, respectively. In going from sample A to D, average mesopore diameters ⟨d⟩mes increase from 4 to 31 nm with a concurrent increase of mesopore volumes (Vmes) from 0.25 to 1.2 cm3/g. Micropore volumes are very low in all the samples. RNPs exhibit a globular morphology with characteristic diameters of several nanometers, a length scale that depends on the synthesis protocol. In general, we observe that the larger the RNP diameter, the larger the cavities between particles. Figure 2a shows a SEM image of RNPs-D characterized by nanoparticles of ∼100 nm diameter. Intercalation of PEO occurs in the interstices between RNPs (Figure 2b). According to the pore-size distribution for RNPs and their respective PEO-filled RNPs (Figure S1 of the Supporting Information), PEO chains are likely to be adsorbed on the pore walls, giving rise to a new distribution of pore sizes. Adsorption of PEO onto RNP walls occurs by hydrogen bonds between PEO ether oxygens and RNP hydroxyls (Figure 2c). This type of confinement conforms to a case of hard confinement as the host is formed by a cross-linked RF polycondensate that does not exhibit any thermal transition (Tg and Tm) over the temperature range investigated in this work (see below). Figure 3 shows the TGA data for a representative PEO/ RNPs specimen. The sample exhibits a maximum decomposition temperature at 358 °C. At this temperature, the thermal decomposition of confined PEO chains occurs, as inferred from the single decomposition step of bulk PEO in the same temperature range. It is interesting to note that PEO chains in PEO/RNPs-C decompose 24 °C lower than bulk PEO, a result also observed for PEO chains adsorbed on

(2)

where ω = 2πf is the angular frequency, Δε is the dielectric strength (Δε = εs − ε∞), ε∞ and εs are the unrelaxed and relaxed values of the dielectric constant, respectively, τ is the relaxation time, and α is a symmetric broadening parameter (0 < α ≤ 1). At low frequencies/high temperatures, a power law term was also added to account for conductivity-related contributions. C


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Figure 2. (a) SEM image of pristine RNPs-D. (b) Cartoon of the RNPs showing the interstitials between nanoparticles where the PEO chains are confined by adsorption on the pore walls. (c) Schematic showing the hydrogen bonds formed between PEO ether oxygens and RNP free hydroxyls. The structure of resorcinol−formaldehyde resins has been taken from ref 30.

n=

VPEO mPEO = Sext,RNPslPEO dPEOSext,RNPlPEO

(3)

where VPEO is the volume occupied by the PEO chains confined in RNPs, Sext,RNPs is the external surface of RNPs, and lPEO is the thickness of a PEO monolayer (l = 3.4 Å).9,45 VPEO can be estimated from the ratio mPEO/dPEO, where mPEO is the mass of confined PEO per gram of substrate and dPEO is the density of confined PEO. As an approximation, we estimated that dPEO is equal to the density of bulk PEO, namely, 1.14 g/cm3,46 although it is known that the density of confined phases can be slightly lower than that of the bulk.9,47 Figure 4 shows the Figure 3. TGA data for PEO/RNPs-C and its pristine materials, RNPs-C and PEO, using a heating rate of 10 °C/min in a N2 atmosphere. Samples were carefully dried at 80 °C in the vacuum oven before these measurements.

graphene sheets,44 and interpreted as to be primarily driven by surface-assisted decomposition. TGA data of sample RNPs-C shows continuous weight loss associated with the loss of water and volatiles from the beginning of the experiment. Then, decomposition is triggered at ∼300 °C, leading to an ∼40 wt % loss at 800 °C. In contrast, PEO/RNPs-C shows a far-less-pronounced weight loss at low temperatures, a result which could be attributed to pore blockage by the adsorbed polymer chains. These differences were taken into account in the calculation of PEO fractions by scaling the TGA curves of RNPs and PEO/RNPs in such a way that 100 wt % corresponds to the data at 200 °C. As a result, RNPs showed uptake values from 9 to 22 wt % of PEO per total mass of sample (cf. Table 2). These figures can also be translated into the corresponding amounts of PEO layers adsorbed on the pore walls (n) by using the equation

Figure 4. Number of PEO layers adsorbed on the mesopore walls.

obtained n values as a function of RNP mesopore diameter. The data show increasing n values, from 0.8 to 4.1 layers, with increasing dmes, suggesting a model of polymer confinement as that illustrated in Figure 2b. Nitrogen isotherm data of RNPs after PEO uptake showed decreasing values of all the textural parameters relative to those of neat RNPs, corroborating the occurrence of polymer confinement in RNP pores. Values of Sext and Vmes for PEO/RNPs and their relative decrease relative to neat RNPs are reported in Table 2. The data show a partial decrease of both textural parameters, suggesting partial occupancy of pore volume (Figure 2b). Differential Scanning Calorimetry. Figure 5 shows DSC data of representative samples of RNPs and PEO/RNPs. To aid comparison, data for bulk PEO are also included. The data show very distinct thermal behavior of confined PEO chains compared to bulk PEO. While the latter crystallizes at 316 K during cooling, and melts at 331 K during heating, the intercalated PEO does not exhibit crystallization and melting. This thermal behavior is similar for all the studied PEO/RNPs

Table 2. PEO Mass-Uptake Data for Different RNP Specimens; Textural Data (Sext and Vmes) of PEO/RNPs and Their Relative Decrease (D) Compared to Neat RNPs RNPs

PEO mass uptake (wt %)

Sext,PEO/RNPs (m2/g)

Vmes,PEO/RNPs (cm3/g)

DSexta (%)

DVmesb (%)

A B C D

9 18 22 19

201 207 165 111

0.18 0.28 0.47 1

36.6 31.9 50.9 25.5

28 24 31 16

DSext = 100(Sext,RNPs − Sext,PEO/RNPs)/Sext,RNPs. bDVmes = 100(Vmes,RNPs − Vmes,PEO/RNPs)/Vmes,RNPs. a

D


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modes are allowed, INS spectral assignments can be performed on the basis of previous Raman and infrared studies. Figure 6a shows the mass-normalized INS spectra of hydrogenated and deuterated RNPs. Upon deuteration, the data show a decrease in intensity for dRNPs owing to its inherently lower scattering cross section and the clear disappearance of the stretching vibrations of aliphatic C−H (νC−H) and aromatic C−H at around 2990 and 2500 cm−1, respectively. The data also show the reduction in intensity of the bands at energies lower than 2000 cm−1. In hydrogenated RNPs the weak band around 1650 cm−1 is associated with aromatic ring stretching vibrations while the band spanning the wavenumber region between 1500 and 1150 cm−1, in order of decreasing wavenumber, corresponds to CH2 scissor and wagging vibrations, COH in-plane bending vibrations, and CH2 twisting vibrations of the CH2OH and CH2OCH2 moieties. The peak around 900 cm−1 is associated with CH2 rocking motion and aromatic ring CH out-of-plane bending. Finally, the bands between 600 and 300 cm−1 are broadly associated with aromatic ring torsions and OH out-of-plane bending within the hydroxymethyl moiety (see Supporting Information). Figure 6b shows mass-normalized INS spectra of PEO/ dRNPs and of its pristine precursors, PEO and dRNPs. First of all, the data show the dominance of PEO vibrational bands in PEO/dRNPs relative to that of dRNPs. Second, the data of PEO intercalated in dRNP pores of display significant differences in the INS response compared to bulk PEO at energy transfers below 1000 cm−1, in agreement with previous results of PEO intercalated in the interlayer space of GO.9,45 The bands in the region 800−1000 cm−1 are very sensitive to macromolecular conformation. They arise from combinations of chain-backbone modes, in particular the C−O−C stretching vibrations, and ethylene rocking modes [r(CH2)]. The INS band at 948 cm−1 can be assigned to both modes and the band at 846 cm−1 can be assigned to r(CH2) in the PEO crystal.51 The latter has been assigned to trans (COOC), gauche (OCCO), and trans (COCC) (tgt) conformers along the chain in a 7/2 helix. In agreement with previous data for PEO intercalated in pores of carbon nanoparticles,19 and in the interlayer space of GO,45 the bands at 948 and 846 cm−1 for PEO confined in RNPs undergo a shift to lower energies at 911 and 820 cm−1. The literature on the conformational changes of PEO bonds upon melting,52−55 complexation with salts56,57 and

Figure 5. DSC data for PEO/RNPs, RNPs, and bulk PEO obtained at cooling and heating rates of 10 K/min.

samples and is therefore independent of the RNP pore diameter. These results are in agreement with previous studies of two-dimensional confinement of PEO in clays48,49 and in GO.9 Concerning the glass transition, semicrystalline bulk PEO (∼60% crystallinity) shows onset Tgs at 223 and 213 K upon cooling and heating, respectively. On the contrary, PEO intercalated in RNPs does not exhibit any clear thermal event associated with Tg at 10 K/min or at higher cooling/heating rates.31 High-Resolution Inelastic Neutron Scattering. To track possible changes to PEO macromolecular conformation of the confined PEO phase, high-resolution INS measurements were performed at 10 K on the TOSCA spectrometer at the ISIS Facility, Rutherford Appleton Laboratory, UK.39−41 INS spectroscopy is highly sensitive to vibrational modes involving hydrogen. This particular feature allows direct access to vibrational spectra of hydrogenated polymers confined in non-hydrogenous host matrices.50 To obtain a clean and unambiguous INS response from PEO in PEO/RNPs, we used deuterated RNPs (dRNPs) as a host matrix in this study. The synthesis of dRNPs was done using deuterated reagents and following a similar synthetic protocol as that used for the synthesis of RNPs (see textural data of dRNPs in the Supporting Information). Making use of one of the primary advantages of INS where there are no selection rules and all

Figure 6. Mass-normalized INS spectra measured on the TOSCA spectrometer at 10 K. (a) Comparison between hydrogenated and deuterated RNPs. (b) Spectrum of PEO in PEO/dRNPs. To aid comparison, spectra of dRNPs, bulk PEO, and PEO in PEO/GO are also included. E


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Macromolecules molecules,58 in solution,59 and in confinement60 is extensive. Depending on the studied system and the (experimental and theoretical) methods used for analysis, conformers with preferential tgt, tgg, ggg, or ttt conformations are found, sometimes leading to contradictory results. In previous studies of PEO confinement,13,14 we have interpreted our results as arising from an increase of the population of trans−trans−trans (ttt) conformers. Specifically, in the case of PEO intercalated in the interlayer space of GO, where PEO chains can be accommodated in ∼3.4 Å monolayers (extreme confinement), we observed a clear INS band at 814 cm−1. Considering that the external diameter of the 7/2 helix is 4.9 Å,61 it is unlikely that in such a reduced lateral space within the interlayer space of GO the polymer chains can twist and display a high population of gauche conformers. Therefore, also based on previous Raman studies of molten PEO,54,55 we interpret our current results in terms of the formation of a confined PEO phase within the RNPs pores with preferential ttt conformations. Broadband Dielectric Spectroscopy. To probe the dynamics of intercalated PEO, BDS experiments were carried out on both PEO/RNPs and RNPs samples. Figure 7a shows

temperature increases; this behavior is common to all PEO/ RNPs samples. Figure 7b shows the comparison of the relaxation behavior obtained for different PEO/RNPs at T = 210 K, where the two relaxation components (hereafter named fast and slow relaxations) are well visible in the frequency window. Independently of the type of RNP host, the observed dynamics is quite similar. The dielectric response of PEO intercalated in RNPs was described by using two Cole−Cole functions to account for both relaxations components. In Figure 7, solid lines through the data points represent the obtained fits of the experimental data. In addition, an example of the two PEO components resulting from such fitting is shown in Figure 7b for PEO/RNPs-D. The α-parameter in the Cole−Cole equation (eq 2) describing the slow relaxation component is about 0.4, whereas that describing the fast component gradually increases with temperature taking a value of 0.23 at 200 K. The dielectric relaxation strength (Δε) of the slow and fast components resulted to be nearly temperature independent for all the samples. Unfortunately, we could not accurately determine the changes in Δε as a function of the textural characteristics of RNPs mainly due to the relatively large uncertainties in the filling fraction of the capacitor. The characteristic time scales, τ, obtained from the fitting of the dielectric relaxation data of PEO/RNPs are shown in Figure 8a. This plot clearly reflects that both dynamics are nearly unaffected by the different pore texture of RNPs, although small differences are observed in the slow component as described below. The relaxation time for the faster dynamics follows an Arrhenius temperature dependence, τ = τ0 exp(Ea/ kT), with an average activation energy, Ea = 38 kJ/mol (see

Figure 7. (a) Imaginary part of the complex dielectric permittivity of PEO/RNPs-A at different temperatures. Solid lines through the data points represent the fits to the experimental data (see text). Data of neat RNPs-A are included for comparison. (b) Imaginary part of the complex dielectric permittivity and their corresponding fitting curves at 210 K for all the PEO/RNPs samples. The data have been vertically shifted to better observation. Slow and fast processes of PEO/RNPs-D are shown by dash and dash-dotted lines, respectively. Figure 8. (a) Arrhenius plot of the relaxation times of PEO/RNPs corresponding to the relaxation processes observed in Figure 7. The bars at 210 K represent a typical uncertainty in the relaxation time values. Solid straight lines correspond to Arrhenius fits to the relaxation times. (b) Temperature dependence of the relaxation times of PEO/RNPs-D, bulk PEO melt, and semicrystalline bulk PEO. Solid and dashed-dotted lines represent the VFT and Arrhenius behavior describing the data of PEO/RNPs-D and bulk PEO, respectively. Pink circles correspond to TSDC data. For comparison, data of PEO/GO are also included as empty stars.

the imaginary part (ε″) of the dielectric permittivity of dry neat RNPs as empty circles. In this frequency window, RNPs show low and almost flat permittivity losses, which are nearly independent of temperature. In the same figure, we show the dielectric permittivity of PEO/RNPs-A at temperatures from 180 to 250 K. When PEO is incorporated into RNPs, two relaxation processes emerge that shift to higher frequency as the F


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PEO, it is found that the fast component detected for PEO intercalated in RNPs can be directly related to the β-relaxation of bulk PEO. The main effect of confinement on the βrelaxation is exhibited as a significant slowing down of the local polymer dynamics and a small reduction of the apparent activation energy. The local dynamics of PEO in all the RNPs is quite similar to that observed in PEO intercalated in the interlayer space of GO (PEO/GO, see Figure 8b),9 a host also leading to strong polymer−host interactions. However, this result is at odd with the common expectation for confinement effects on the local dynamics. Usually, β-relaxation undergoes a broadening due to confinement but the peak position is not significantly affected.14,23,65,66 The peculiar behavior found for PEO incorporated in GO and RNPs highlights the relevance of hydrogen bond interactions between polymer and substrate in the local dynamics. In PEO/GO we determined that the ratio of PEO oxygens to GO hydroxyls reached unity at maximal polymer uptake. This result was interpreted as the occurrence of maximized polymer−host interactions via hydrogen bonding where PEO/GO acts as hydrogen acceptor/donor. In this context, the amount of hydrogen bonds in PEO/RNPs seems also to be rather large, which is independent of the RNPs texture. By comparing the slow process of PEO intercalated in RNPs with the α-relaxation of molten PEO, it is clear that the difference in time scale at high temperatures is extremely large; i.e., no hint of a PEO dynamics in RNPs similar to that of PEO bulk is detectable. To characterize the slower PEO dynamics in RNPs-D at high temperatures, we also used the VFT equation with D = 9.5, T0 = 147 K, and τ0 = 3 × 10−10 s as resulting fitting parameters. The result is shown as a solid line in Figure 8b. From these data, we note that this slow process extrapolates to a time scale of about 100 s at T = 200 K, which is quite similar to the Tg of bulk PEO. However, the time scale does not merge at high temperatures with that of bulk PEO. PEO in RNPs-D has a higher value of τ0 and the slow relaxation of PEO in RNPs-D is nearly 3 decades slower than the α-relaxation of bulk PEO at around room temperature (see Figure 8b). These differences would be caused by the strong attractive interactions between PEO and the pore walls, which on the one hand impede PEO crystallization and on the other hand prevent the PEO bulklike dynamics to occur when incorporated in RNPs. The fact that this finding applies also to RNPs with larger pore sizes evidence that PEO chains are not filling the pores but mainly wetting the pore walls likely by forming an adsorbed layer of few nanometer thickness. By decreasing temperature, some cooperativity is developed as indicated by the VFT behavior. However, the value of the parameter D for intercalated PEO is much larger than that for bulk PEO, indicating a markedly “stronger” glass character for PEO in RNPs. The increasing cooperativity seems to be limited at about 220 K, causing the crossover to an Arrhenius-type temperature dependence. This crossover occurs at similar temperatures for all the PEO/RNPs samples. The activation energies characterizing this low-temperature range are also nearly the same for all the PEO/RNPs samples. According to these results, the arrangement of PEO chains is likely to be very similar in the different RNPs hosts. To further confirm the Arrhenius temperature dependence of the relaxation times below the crossover temperature, we performed thermally stimulated depolarization current (TSDC) experiments on PEO/RNPs-D (see Supporting Information for details). This technique overcomes the difficulties found in

Table 3). This energy would represent the mean potential barrier for the activated jumps of dipolar entities. In this Table 3. Activation Energy, Ea, and Preexponential Factor, log(τ0), Corresponding to the β-Relaxation of Bulk PEO and the Fast Relaxation of PEO/RNPs As Obtained from the Arrhenius Fit of the Data in Figure 8 sample

Ea (kJ/mol)

log(τ0/s)a

PEO (bulk) PEO/RNPs-A PEO/RNPs-B PEO/RNPs-C PEO/RNPs-D

40 38 36 40 37

−16.6 −15.2 −14.8 −15.5 −14.7

a

Note that these extremely low values indicate that activation entropic terms are relevant.63

context, the values found for the Cole−Cole parameter corresponds to a Gaussian distribution of potential barriers with 26% standard deviation. The relaxation time of the slow process exhibits a more intricate temperature dependence. It also shows an Arrhenius-like behavior below ca. 220 K, with a higher activation energy (about 55 kJ/mol). However, at higher temperatures a crossover to faster temperature dependence is evident. This finding is analogous to that observed in systems under confinement such as in poly(methylphenylsiloxane) confined in layered silicates4 or liquid-crystalline poly(methacrylate) confined between smectic layers,62 where the relaxation times of the process related to the α-relaxation of the confined polymer also showed a crossover in the temperature dependence. In those cases, the presence of the crossover was explained as the result of geometrical restrictions imposed by the confinement walls to the increase of the characteristic length, ξ, of the segmental relaxation during cooling from T ≫ Tg to Tg. At T ≫ Tg, ξ would be smaller than the confinement size, and no difference between the dynamics of the bulk polymer and the confined polymer phase is expected. By decreasing the temperature toward Tg, ξ cannot further increase when reaching the confinement size, thus causing that the time scale dependence deviates from the typical super-Arrhenius behavior of viscous liquids to an Arrhenius-like temperature dependence. As discussed below, this scenario hardly applies for PEO/RNPs. In Figure 8b, we compare the relaxation time of PEO/RNPsD with that of bulk PEO of similar molecular weight (αrelaxation of molten PEO denoted with filled circles and βrelaxation of semicrystalline PEO denoted with crosses). The temperature dependence of the β-relaxation of semicrystalline PEO follows an Arrhenius law with Ea = 40 kJ/mol, which is similar to that of the fast relaxation of PEO intercalated in RNPs, whereas the relaxation times at a given temperature for bulk PEO are about 10 times smaller than those in PEO/RNPs. The α-relaxation of molten bulk PEO follows the VFT equation (τ = τ0 exp[DT0/(T − T0)]) with D = 3.5, T0 = 186.1 K, and τ0 = 4 × 10−13 s (see Figure 8b). These VFT parameters where obtained by imposing that the relaxation time of the α-process of molten PEO reaches a value of 100 s at T = 206 K, which is the reported calorimetric Tg of fully amorphous bulk PEO.64 This Tg value is slightly lower than those determined by DSC for semicrystalline bulk PEO during either cooling or heating (see Figure 6). Discussion of the Dynamic Results. By comparing the data obtained for PEO intercalated in RNPs with those for bulk G


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Macromolecules

As already commented, a common structural feature in all the samples is the presence of interactions between PEO and host surface via hydrogen bonding, which are expected to be enhanced with decreasing temperature, and therefore it could be relevant for the crossover. In this sense, the kinetic rate of formation and rupture of hydrogen bonds slows down by decreasing temperature, which would give rise to long-living hydrogen bonds on approaching the crossover temperature. Below the crossover temperature the frozen hydrogen-bond structure would limit the increasing cooperativity of the dipole reorientation, leading to an Arrhenius temperature dependence.

BDS experiments at very low temperatures due to the relatively low signal and correspondingly higher uncertainties. The analysis of the TSDC curves was performed by recording the peak temperature (Tmax) of curves obtained at different heating rates (q = 0.2−5 K/min) after polarization between 190 and 150 K during cooling at 5 K/min. In this way, the relaxation times were evaluated as67 τ(Tmax ) =

kB(Tmax )2 qEa

(4)

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where Ea is the activation energy of the relaxation process under investigation (Ea = 55 kJ/mol as obtained above by BDS). The obtained τ values are included in Figure 8b, showing a good correspondence with those expected by extrapolating the Arrhenius equation using the BDS data. It is noteworthy that the behavior found for the slow relaxation of PEO intercalated in RNPs has not been observed in other confinement systems for PEO such as in GO,9,10 clays,14 or anodized aluminum oxide.23 In the case of lowmolecular-weight PEO intercalated in the interlayer space of GO, we detected a slow relaxation that was explained to be originating from interfacial polarization phenomena promoted by the relatively high sample conductivity.10 This interfacial relaxation showed a dependence on molecular weight, indicating that hydroxyl end-groups of PEO chains participate in the ionic conductivity of the sample thus favoring ionic migration. Noticeably, we were unable to detect such relaxation in the case of a PEO with Mn = 94 kg/mol (the same as that used in the current study). Moreover, the temperature dependence of this slow interfacial relaxation for the highest molecular weight PEO (Mn = 46 kg/mol) where the interfacial relaxation was detectable does not match with that of PEO intercalated in RNPs. Furthermore, the data for PEO/RNPs exhibit relatively low conductivity with not much of an effect at low frequencies (see Figure 7b), suggesting that dipole reorientation is at the origin of the slow relaxation. This is better evidenced in the data measured at different temperatures, as for example those shown in Figure 7a. For all these reasons, we attribute the slow relaxation in PEO/RNPs to the segmental mobility of intercalated PEO chains. Small differences in the slow-relaxation component across different samples are mainly observed above the crossover temperature. We found that the relaxation times increase in going from sample C to D, a result which correlates well with the increasing number of PEO layers (cf. Figure 4). However, this result is of difficult interpretation because we would expect that the thinner the layer, the stronger the effect of the substrate is and therefore the slower the relaxation would be. Consequently, our results suggest a different picture than that of PEO layers uniformly arranged on the pore walls, possibly involving a heterogeneous distribution of polymer chains inside the pores. The similarities of the dynamics among the different PEO/RNPs samples suggest a weak dependence of PEO arrangement on pore properties. A last point that needs to be discussed is the molecular origin of the crossover in the temperature dependence of the slow relaxation process. In the context of the glass-forming dynamics, such a crossover is expected to occur on confinement when the characteristic length of the segmental dynamics reaches the size of confinement. However, as shown above we observed a similar crossover phenomenon in all PEO/RNPs irrespective of the geometrical characteristics of the RNP hosts.

CONCLUSIONS High-molecular-weight PEO is efficiently incorporated into the pores of RNPs, with uptakes as high as 22 wt %. INS experiments have been used to show that PEO incorporated in RNPs exhibits a preferentially planar, zigzag conformation, similar to that found for PEO monolayers intercalated in GO of thickness ∼3.4 Å and in stark contrast with the characteristic 7/ 2 helical structure of the bulk crystal. DSC data show that polymer intercalation in RNPs pores leads to the complete suppression of PEO crystallization and to hardly detectable glass-transition phenomena. We have identified a dielectric relaxation originating from the segmental dynamics of PEO intercalated in RNPs, which has not been detected in any other confining system for highmolecular-weight PEO. This relaxation shows a cooperative character associated with a non-Arrhenius temperature dependence of the relaxation time at high temperature and is much slower than that of molten PEO. Moreover, at low temperature the segmental relaxation of intercalated PEO shows an Arrhenius-like temperature dependence with a crossover temperature at ca. 220 K. The latter is interpreted as due to limited cooperativity caused by the enhancement of long-living hydrogen bonding between PEO and RNP pore walls. This interpretation would explain why this behavior is similar for all the investigated pore textures. We have also identified a faster relaxation related to the βrelaxation of bulk PEO, although significantly slower, with similar characteristics to those found in other confining systems for high-molecular-weight PEO. This result suggests the importance of polymer−surface interactions on the local polymer dynamics.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01285. Nitrogen physisorption data, TSDC experiments, and computational methods (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +34 94301 8803; Fax +34 94301 5800 (F.B.-B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the Spanish Ministry “Ministerio de Economı ́a y Competitividad”, code: H


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MAT2015-63704-P (MINECO/FEDER, UE), the Basque Government (IT-654-13), and the UK Science and Technology Facilities Council for the provision of beam time on the TOSCA spectrometer. P.P. acknowledges a PhD research contract from UCM (BE45/10). F.F.A. and S.R. acknowledge financial support from the UK Science and Technology Facilities Council.

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Dynamics and Structure of Poly(ethylene oxide) Intercalated in the

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