Structural and Conformational Properties of Poly(ethylene oxide

Article pubs.acs.org/Macromolecules

Structural and Conformational Properties of Poly(ethylene oxide)/ Silica Nanocomposites: Effect of Confinement Anastassia N. Rissanou,†,‡ Hellen Papananou,∥,§ Vyron S. Petrakis,§ Manolis Doxastakis,⊥ Konstantinos S. Andrikopoulos,# George A. Voyiatzis,# Kiriaki Chrissopoulou,*,∥ Vagelis Harmandaris,*,†,% and Spiros H. Anastasiadis∥,§

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Department of Mathematics and Applied Mathematics, ‡QCN, Department of Physics, and §Department of Chemistry, University of Crete, Heraklion, Greece ∥ Institute of Electronic Structure and Laser, Foundation for Research and Technology - Hellas, P.O. Box 1527, 711 10 Heraklion, Crete, Greece ⊥ Department of Chemical Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA # Institute of Chemical Engineering Sciences, Foundation for Research and Technology - Hellas, P.O. Box 1414, 265 04 Patras, Greece % Institute of Applied and Computational Mathematics, Foundation for Research and Technology - Hellas, P.O. Box 1385, 711 10 Heraklion, Crete, Greece ABSTRACT: The conformations of polymer chains in poly(ethylene oxide)/silica nanoparticles, PEO/SiO2, nanohybrids have been investigated through a combined approach that involves molecular dynamics (MD) simulations and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements. Systems with different polymer molecular weights, nanoparticle radii, and concentrations have been employed to investigate the effect of the confinement on polymer conformations across a variety of different conditions. Qualitatively similar behavior between experimental and simulation results is observed since in both cases an increase of gauche population for the OCCO angle is attained, in comparison to the respective of the bulk. This increase becomes larger as the degree of confinement becomes higher. More specifically, both simulations and experiments indicate a corresponding progressive increase with the degree of confinement. On the contrary, the conformations of the C−O bond (COCC angle) seem to remain unaffected by the confinement, at least in the range of degrees of confinement covered computationally. In addition, chain dimensions in the nanocomposite are found to be slightly decreased compared to bulk, especially at low temperatures. This results in a reduced effective confinement that allows the polymer matrix to accommodate larger nanoparticle fractions.

I. INTRODUCTION Nanofillers are used in polymer matrices in order to improve the behavior of the corresponding bulk systems and to modify properties, such as electrical, mechanical, optical, thermal, etc.,1−5 leading to their utilization in a large variety of applications.6−10 In such nanocomposite systems the understanding of the relationship between the physical and chemical attributes of the nanofillers (e.g., nanoparticles, nanotubes, clays, graphene, etc.) and the host matrix is of great importance for the design of new materials with specific functionalities. Of particular importance is the role of the interface between the nanofiller and the polymer, whose properties are governed by parameters related to both the nanofiller and the polymer matrix, like the size ratio of nanoparticle to polymer (e.g., the ratio of the radius of a spherical nanoparticle over the radius of gyration of a polymer chain), the shape of the nanoparticles, the filler loading, the dispersion of the nanoparticles in the matrix, and the possible self-assemblies formed within the hybrid material as well as the polymer molecular weight and structure, © 2017 American Chemical Society

the nature of the interactions between the polymer and the nanoparticle, etc.11−14 Furthermore, the topological confinement of the polymer chains at the molecular level affects the chain structure and dynamics and, hence, their macroscopic properties, being the result of a complex competition between entropic and enthalpic driven forces.15−18 The effect of confinement has been investigated in both thin polymer films17 and polymer nanohybrids like polymer/spherical nanoparticles,19,20 polymer/nanorods,21 and polymer/layered nanoadditives.22−27 The study of the static and dynamic behavior of polymers close to interfaces and/or in very thin films has been a subject of great interest during the recent years since significant differences can emerge when the molecules are confined over distances comparable to their sizes.24,28−34 Depending on the Received: April 20, 2017 Revised: July 25, 2017 Published: August 8, 2017 6273

DOI: 10.1021/acs.macromol.7b00811 Macromolecules 2017, 50, 6273−6284

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Macromolecules interactions of the polymer with the surface, either faster26,35,36 or slower dynamics37−40 has been observed. At the same time, for semicrystalline polymers, the hierarchy in the formation of polymer crystals is disrupted when the polymer chains are forced to crystallize in a restricted space.41 Polymers are found to exhibit crystallization enhancement in the presence of inorganic fillers,42 but, at the same time, their crystallization is hindered by confinement25 and/or affected by the addition of alkali cations.43−45 The crystallization and the rheological behavior seems to depend on the polymer molecular weight and the additive surface modification as well.46 Moreover, there is a controversy among various experimental findings concerning the perturbed polymer properties caused by the added nanoparticles.47−51 Expansion of polymer chains has been reported by neutron scattering for polystyrene/polystyrene nanoparticles system51 in agreement with a study of a poly(dimethylsiloxane)/silicate nanocomposite.47 On the contrary, in polystyrene/silica and poly(ethylene propylene)/silica nanocomposites, chains are found to be unperturbed.48−50 Recently, in a very comprehensive review Karatrantos et al.11 presented several computational studies of polymer nanocomposites containing different types of nanoparticles as fillers, including silica nanoparticles, in various polymer matrices.12,52−65 Simulations of silica nanoparticles embedded in polystyrene, PS,53,54,56 and polyethylene, PE, matrices52,58,66 revealed similar interfacial widths of about 1.5 nm. Nanocomposites, where silica nanoparticles are decorated with grafted polymer chains, have been also studied, examining the effect of the surface curvature and grafting density on the polymer organization.57,59,67,68 Poly(ethylene oxide), PEO, is a polymer with a broad range of technological applications,69−73 whereas its nanohybrids can be used as electrolytes constituting promising materials for applications in solid-state lithium batteries.74−79 Specific attractive interactions exist in nanocomposites of PEO and silica nanoparticles, mainly attributed to the formation of hydrogen bonds, which force polymer chains to adsorb onto the surface of the particle. As a basic result, adsorption causes the stabilization of the systems.80−82 Numerous pharmaceutical and industrial applications have been reported for such nanocomposite systems.83 Both experimental and simulation works have examined the adsorption of PEO on silica nanoparticles and the resulting configurational changes of polymer chains.80,82−88 From the experimental point of view, the effect of different end groups (hydroxyl and methoxy) of PEO in PEO/silica nanocomposites has been recently studied as well.89 The static characterization by small-angle neutron scattering does not reveal strong changes to the overall polymer structure; nevertheless, dynamic spin-echo measurements suggest that chains with hydroxyl ends assemble into micelle-like corona and extend away from the surface of the particle. When changing to methoxy end groups, chains adopt more flat conformations in the vicinity of the particles due to multiple adsorption sites per chain. These differences result in different dynamical behavior between the two types of PEO chains. The differentiation in the interactions between PEO and silica, due to different end groups, is addressed in an atomistic molecular dynamics study by Barbier et al. as well.66 In that work, two types of end groups for 9-mer PEO chains were considered (methyl, −CH3, and hydroxyl, −OH). The presence of the nanoparticle did not cause any significant effect on the average conformational properties of PEO in any of the two types. The polymer properties are

affected only very close to the interface (i.e., at 10−15 Å for the static and 20−30 Å for the segmental dynamics properties). The surface-induced changes in PEO dynamics have also been explored by Kim et al.90 as a function of polymer length, volume fraction of silica nanoparticles, and temperature using proton time-domain NMR. Three regions of different mobility have been distinguished: a strongly adsorbed phase, a phase with intermediate relaxation times, and a highly mobile phase. An older study of Cosgrove et al.80 based on small-angle neutron scattering experiments and molecular dynamics simulations illustrates the role of the relative size of the adsorbent and the adsorbate for a system with PEO and silica nanoparticles. On the other hand, the morphology, the chain conformation, and the dynamics of PEO were investigated under severe confinement in nanohybrids with sodium montmorillonite.25,26 In all cases, intercalated nanohybrids were obtained, with polymer chains severely confined within the 0.8 nm galleries of the clay whereas an abrupt transition from an almost ∼70% to zero crystallinity was observed at ∼70 wt % PEO. This indicates that it is only the excess polymer outside the galleries of the inorganic material that is able to crystallize and that the intercalated polymer chains as well as chains that are in close proximity to the outer surfaces of the inorganic particles remain purely amorphous. Moreover, a significant and systematic increase of the gauche vs trans conformations for both the C−C and the C−O bonds was found for the chains that were confined or adsorbed on the outer surfaces of the clay particles.25,91 At the same time, the segmental relaxation process (α-relaxation) of the confined PEO chains appears at temperatures far below the Tg of the bulk polymer with the relaxation times being much faster than those in the bulk and exhibiting an almost Arrhenius temperature dependence.26 In another work, the preservation of the PEO helical structure inside the clay galleries with the cations located in the center of the helix was proposed,74 whereas in a different case a highly distorted helical structure was reported to be a better description.92 A distorted helical conformation and a singlelayer arrangement was suggested as well, as a more accurate way to describe the conformation of the PEO chains within the clay galleries.93 More disordered PEO chains were reported in intercalated PEO/montmorillonite systems as well.94 PEO confined either in nanopores95 or between the layers of graphite oxide96 was found to exhibit a preferential planar zigzag conformation. Moreover, computer simulations have suggested that the intercalated PEO chains are in a liquid like state and that they are less ordered than the most disordered bulk system due to the strong confinement and the coordination of the ether oxygen with the alkali cations in the galleries.44,97,98 Besides all these studies, a detailed investigation of the polymer chains structure and conformations in polymer/silica nanocomposites at the molecular level for different molecular lengths and under various degrees of confinement and filler loadings is still missing. Moreover, the microscopic origin of the formed structures at the polymer/silica interfaces is poorly examined. Here we provide a systematic study of structural/ conformational behavior of PEO chains in PEO/silica nanocomposites, at multiple length scales, through molecular dynamics (MD) simulations and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements. Our main goals are (a) to examine the structural behavior of low and high molecular weight PEO/silica 6274


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Macromolecules nanocomposites, in particular near the interfaces, and (b) to correlate the chain conformational properties to the degree of conf inement for PEO oligomers, low molecular weight (MW) chains, and high MW chains. The degree of confinement is a crucial parameter for the arrangement of polymers close to the interface, and it has to be examined as a potential factor to enhance or suppress crystallization within a specific temperature range. To achieve the above and to study systems with varying PEO chains molecular length, from oligomers up to high MW, allatom MD simulations and FTIR spectroscopy are used as complementary tools. Indeed, on one hand, detailed atomistic simulations are capable to provide direct quantitative prediction for the density and conformation of well-characterized monodisperse polymer chains at the PEO/silica interface for PEO oligomers or short, Rouse-like (unentangled) chains, which are not easy to be investigated experimentally due to their low viscosity, under relatively weak up to moderate confinement. On the other hand, FTIR measurements can provide information about the conformation of high MW wellentangled PEO chains in PEO/silica nanocomposites from moderate up to severe confinement, which nevertheless cannot be studied by atomistic MD simulations due to the very long equilibration times required.

Table 1. System Details hybrids

wSiO2a (wt)

φSiO2b (vol)

dc (nm)

d/Rgd

PEO2270/NP7 PEO2270/NP7 PEO2270/NP7 PEO2270/NP7 PEO2270/NP7 PEO2270/NP7 PEO10/NP2 PEO48/NP2 PEO100/NP2 PEO48/NP2

0.03 0.11 0.25 0.48 0.72 0.82 0.05 0.33 0.39 0.57

0.015 0.06 0.14 0.31 0.56 0.69 0.02 0.19 0.23 0.39

13.3 4.92 2.156 0656−4.704 0.611−1.363 0.32 8.24 1.67 1.31 0.42

0.875 0.324 0.142 0.043−0.309 0.04−0.09 0.021 12.68 0.979 0.558 0.247

wSiO2: weight fraction of SiO2 nanoparticles. bφSiO2: volume fraction of SiO2 nanoparticles. cd: nearest interparticle distance. dd/Rg: degree of chains confinement (ratio of d over the radius of gyration of the bulk polymer chains). a

fraction, and φmax is the maximum possible nanoparticle concentration according to the specific crystal structure. For the system with φNP = 0.56 (φPEO = 0.44) and φNP = 0.31 (φPEO = 0.69), values for d assuming the simple cubic (SC, φmax = 0.52) and the hexagonal closepacked (HCP, φmax = 0.74) arrangements of NPs were used, as limiting cases of packing. The above relation was also used for the simulation model systems, since in these systems the NPs are arranged in a simple cubic like arrangement imposed by the periodic boundary conditions. Experimentally, the bulk PEO chain size was measured utilizing dynamic light scattering as RH = 12 nm and Rg = 15.2 nm, assuming Rg/RH = 1.27 at Θ-solvent conditions. For the simulated model systems the radii of gyration were calculated as Rg,PEO10 = 0.65 nm, Rg,PEO48 = 1.71 nm, and Rg,PEO100 = 2.35 nm. Note that the ratio between the nearest interparticle distance d and the bulk (unperturbed) radius of gyration of the polymer chain, Rg, shown in the last column of Table 1, is anticipated to provide a measure of the degree of confinement of the polymer chains. Experimental Techniques. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). The conformation of the PEO chains in the bulk and in the hybrid materials was investigated at ambient conditions as well as at high temperatures by ATR-FTIR. The IR spectra of the different samples were recorded by an Equinox 55 Bruker spectrometer using a DTGS detector (spectral resolution was set at 3 cm−1). For the high temperature measurements, a single reflection diamond ATR accessory of Golden Gate was properly attached to the FTIR instrument. Temperature was controlled with accuracy better than 1 °C. In order to maintain inert atmosphere during the high temperature experiments, the sample compartment was properly flushed with nitrogen for 30 min before every measurement. For background correction purposes, corresponding spectra were recorded for each temperature. Special care was given so that no bands associated with CO2 or H2O were present after the background correction. Simulation Methodology. Atomistic molecular dynamics simulations were performed for PEO/silica nanocomposites (PEO/SiO2) using the GROMACS simulation package.101 The polymeric matrix is composed of PEO chains of 10, 48, and 100 monomeric units, terminated with methyl groups and a silica nanoparticle, with almost spherical shape, of diameter ∼4.2 nm, as referred above as well. PEO was represented by a united atom model, and interactions were described by a slightly modified TraPPE force field.102,103 A full atom representation was used for the SiO2 nanoparticle.58 The particle-mesh Ewald (PME) method was applied for the calculation of the electrostatic interactions. Simulation runs were performed in the NPT statistical ensemble, where the pressure was kept constant with the use of Parrinello−Rahman barostat. The temperature was kept constant using the Nosé−Hoover thermostat, and a series of runs in a range of temperatures T = 300−400 K were made. Initial equilibration

II. SYSTEMS AND METHODS Materials. The poly(ethylene oxide), PEO, homopolymer that was utilized in this work was purchased from Aldrich. Its molecular weight is 100 000 g/mol (PEO2270), and its polydispersity index is Mw/Mn = 2.4, as determined by size exclusion chromatography utilizing polystyrene standards. The polymer possesses hydroxyl chain ends. It exhibits a glass transition temperature Tg = 206 K and a melting temperature Tm = 338 K. The silica nanoparticles, SiO2, used were purchased from Aldrich (Ludox LS) in aqueous dispersion. Their size (average radius) was measured R = 6 and 7 nm (NP7) through transmission electron microscopy (TEM) and dynamic light scattering (DLS), respectively. Both techniques within their resolution indicate that the nanoparticles are fairly monodisperse and that there is no aggregation. The nanoparticles possess hydroxyl groups whereas their surface area is ∼220 m2/g as given by the vendor. PEO/SiO2 nanocomposites were prepared via solution mixing over a broad range of compositions. First, a PEO/water solution was prepared, and then, an aqueous dispersion of the nanoparticles was added. The systems were left to mix for at least 24 h, and the final dispersions were dried in a Petri dish under vacuum. Following solvent evaporation, the nanocomposites were annealed at 373 K for 1 h, and the room temperature was reached at a rate of 10 K/min to ensure equilibration. For the simulations, the polymeric matrix composed of poly(ethylene oxide) chains of 10, 48, and 100 monomeric units, terminated with methyl groups. The silica nanoparticle used had almost spherical shape, of radius ∼ 2.1 nm (NP2). Note that model silica NPs possess hydroxyl groups at the surface, which can form hydrogen bonds with the PEO chains. All nanohybrid systems investigated in the current work either through simulations (model systems) or in the experiments are presented in Table 1. The nearest interparticle distance is calculated according to a procedure proposed by Torquato99 based on analytical expressions of nearest-neighbor probability functions for random isotropic packing of hard spheres. The latter is in excellent agreement with Monte Carlo simulations of 3-D hard spheres and is valid up to the random closepacking fraction, i.e., up to volume fraction of the nanoparticles (NPs), φNP = 0.64 (φPEO = 0.36). For the experimental samples with larger volume fractions of NPs, a hexagonal close-packed (HCP) arrangement of NPs is assumed. In that case the nearest interparticle distance is calculated according to the equation100 d = D[(ϕmax/ϕNP)1/3 − 1], where D is the nanoparticle diameter, φNP is the nanoparticle volume 6275


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Figure 1. (a) Characteristic snapshot of a model system PEO48/NP2 with 33 wt % SiO2 content and at T = 330 K. (b) Mass monomer density profiles of PEO as a function of distance from the center of mass of the silica nanoparticle, r, for the PEO10/NP2 with 5 wt % silica at various temperatures (T = 285−400 K) and (c) for the PEO48/NP2 with 33 and 57 wt % silica content at T = 400 K. Horizontal lines represent the bulk values for the respective temperatures. runs were followed by long production runs for times between 0.1 and 0.5 μs depending on the specific system and the temperature, with a time step of 2 fs. Periodic boundary conditions were applied in all three dimensions, and in the initial configuration SiO2 nanoparticle is located at the center of a cubic simulation box. In our model nanocomposites, an ideal dispersion of NP’s in the PEO matrix is assumed, imposed by the periodic boundary conditions and the cubic simulation box. We quantify the confinement through the ratio of the distance between two nanoparticles, given by the formula referred above,100 over the bulk radius of gyration of the polymer chain (d/Rg). A characteristic snapshot of our model system is presented in Figure 1a. Simulations of the corresponding bulk systems, at the same conditions, were also performed for comparison reasons.

First, data about the density of polymer chains are presented. Simulation results are analyzed as a function of radial distances from the silica nanoparticle center of mass, through a binning of 0.05 nm. Figures 1b and 1c show the monomer density profiles of PEO chains as a function of distance from the silica nanoparticle center of mass, for two concentrations of the PEO10/NP2 and the PEO48/NP2 nanohybrids, respectively. In more detail, results for PEO10/NP2 with 5 wt % silica at various temperatures are shown in Figure 1b. Regimes of high and low monomeric densities are observed for small distances due to the PEO/SiO2 attraction, induced by the dispersion (van der Waals) and electrostatic interactions, whereas for longer distances the bulk density is attained. The latter is characteristic for a polymer nanocomposite with low nanofiller loading (weak confinement of polymer chains), for which a bulk-like regime is expected. The main high density peak is observed at distances of about 0.4 nm from the surface of the silica NP. Three additional well-ordered peaks of gradually decreased height are observed at distances of about 0.8, 1.2, and 1.6 nm; for longer distances the density reaches its bulk value. Horizontal lines represent the bulk values for each temperature. Furthermore, the effect of temperature is evident, indicating lower densities at higher temperatures, as expected. An interfacial regime, based

III. RESULTS AND DISCUSSION Oligomers and Low MW PEO/Silica Nanocomposites. Results from all-atom MD simulation concerning the structural properties of PEO oligomers and short chains in PEO/SiO2 nanocomposites with various nanoparticle concentrations (i.e., various degrees of confinement), as shown in Table 1, are presented in the following. More specifically, data are presented for silica nanoparticles in a polymer matrix consisting of 48-mer and 100-mer PEO chains as well as in a matrix of PEO oligomers (10-mer)/SiO2 systems. 6276


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because of steric interactions between adjoining methylene groups, whereas the reverse holds for the C−C bond, where a lower energy for the gauche rotational state is attributed to a favorable dispersion interactions between the oxygen atoms.105,106 Moreover, a slight temperature effect on the distribution of both dihedral angles can be observed as well; the trans peak for COCC increases with decreasing temperature with a consequent reduction of the gauche ones, while for OCCO the effect is reversean increase of the gauche peak with decreasing temperature is observed. We further analyze the above data by presenting the fraction of “trans”, wtrans, and “gauche”, wgauche, dihedral angles as a function of temperature for the different nanocomposites with varying silica concentrations. Results for all hybrids together with the results for the corresponding bulk systems are presented in Figures 3a and 3b for the OCCO and COCC dihedral angles, respectively. Interesting observations are deduced from this figure. First, a small but clear increase of “gauche” population is observed for the OCCO angle in the polymer nanocomposite system as compared to the corresponding bulk system, which becomes more pronounced at higher particle concentrations (i.e., under stronger confinement); on the contrary such a concentration dependence for the COCC angle is not observed. The inset of Figure 3a shows the gauche fraction of the various systems at the same temperature (T = 400 K). A rough quantification of the differences between the confined and the corresponding bulk system reveals larger differences for stronger confinement. Second, the temperature dependence for both angles is apparent; however, different trends are observed. Specifically, an enhancement of the gauche population for the C−C bond with a simultaneous reduction of the same population for the O−C and C−O bonds with decreasing temperature is observed, as was also discussed above. An Arrhenius fit (A0e−ΔE/kBT) of wgauche/wtrans as a function of temperature can provide the activation energy of the system, ΔE (figure is not presented here). Activation energies for the bulk PEO10, PEO48, and the PEO48/NP2 at wSiO2 = 0.33 are −6.19, −7.24, and −6.27 kJ/mol, respectively, calculated from the OCCO angle temperature dependence. The corresponding values from COCC are 5.52 kJ/mol for the bulk 48-mer, 5.10 kJ/mol for the bulk 10-mer, and 4.63 kJ/mol for the PEO48/ NP2 at wSiO2 = 0.33. The modified conformations of PEO chains at the segmental level (dihedral angles) due to the presence of the silica NP’s could also lead to different chain dimensions compared to the bulk ones. In Figure 3c, data are shown on the temperature dependence of the radius of gyration (Rg) of PEO chains in PEO48/NP2 hybrids with 33 and 57 wt % silica NP’s. Indeed, a slight decrease in Rg values of the polymer chains in the nanocomposite compared to the corresponding bulk values at various temperatures is observed. The decrease is more pronounced at low temperatures, whereas the ratio Rg/Rg,bulk tends to almost 1 at the highest temperature value studied here (T = 400 K). However, we should stress the rather high statistical uncertainty of these values, especially at low temperatures. This is more pronounced for the system with 57 wt % silica due to the small number of model polymer chains. High MW PEO/Silica Nanocomposites. The chain conformations in PEO/SiO2 were probed experimentally utilizing Fourier transform infrared spectroscopy. In the present

on the density profile, can be defined from the data shown in Figure 1b, which is of the order of 0.25−2 nm, from the periphery of the NP, for the systems studied here. A more detailed analysis of the density profile of the model PEO/SiO2 configurations will be given elsewhere.104 Figure 1c presents the density profiles for the model PEO/ SiO2 nanocomposite systems with longer PEO chains and larger nanoparticle loading. More specifically, these data correspond to 48-mer PEO chains at hybrids with 33 and 57 wt % silica nanoparticles at T = 400 K. A high monomeric density peak is observed at small distances from the surface for both systems. However, the bulk regime of polymer density is not attained, which indicates that there is a high degree of confinement for the PEO chains in these systems. More specifically, for the system with 33 wt % silica two peaks are observed, whereas for the systems with 57 wt % silica NP’s even the first density peak is roughly approached. In other words, for the strongly confined systems the monomeric density of polymer chains, analyzed as a function of radial distance from NP’s center of mass, does not attain a constant value, in contrast to the less confined systems, with lower concentration of silica NPs. Overall, as expected, the larger the concentration of NP’s, the more inhomogeneous, with respect to the density, the hybrid material becomes. Next, the conformational properties of the polymer chains in the model PEO/SiO2 nanocomposites were analyzed based on the calculation of the distribution of the torsional angles, Pdih, for the two dihedrals, which are defined on the PEO chain, i.e., OCCO and COCC. Results for the PEO chains of the PEO48/ NP2 hybrid with 57 wt % silica nanoparticles are depicted in Figures 2a and 2b for a range of temperatures T = 330−400 K.

Figure 2. Torsional angles distribution at various temperatures for the system of PEO48/NP2 chains with 57 wt % SiO2: (a) OCCO and (b) COCC angle.

In the performed analysis, all torsional angles with values between −60° and +60° are defined as trans, whereas the ones outside this interval are considered as gauche+ [from +60° to +180°] or gauche− [from −180° to −60°]. It is clear that the preferable state for the OCCO angle is gauche+ and gauche−, whereas for the COCC is the trans. This is in qualitative agreement with what is generally accepted for PEO according to which the energy for the gauche rotational states about the O−C and C−O bonds exceeds that of the trans state primarily 6277


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Figure 4. TEM image of a cryomicrotomed PEO2270/NP7 hybrid with 25 wt % SiO2 content.

necessary for the specific materials since the glass transition temperature of PEO is Tg = 206 K, and at room temperature the material is too soft to be sliced in uniform films. The very good dispersion of the nanoparticles within the PEO matrix is evident throughout the whole film. The situation is the same for hybrids with either lower of higher loading in nanoparticles, and no aggregation is observed. Moreover, the dispersion seems to become even better with increasing silica content.107 Figure 5 shows ATR-FTIR measurements of bulk PEO2270 (Figure 5a) and of a PEO2270/NP7 nanocomposite with 72 wt % SiO2 (Figure 5b) in the spectral region where the CH2 bending, wagging, and twisting vibrational modes appear. It is noted that the use of FTIR-ATR was chosen instead of the more traditional FTIR, which is widely used in similar studies, to avoid the use of KBr, the presence and the hydrophilicity of which might seriously disturb the PEO crystalline structure as well as affect the evaluation of chain conformation by broadening the absorption bands. All measurements are shown for temperatures higher than 340 K, i.e., higher than the melting temperature of PEO which has been measured by DSC to be Tm = 341 K.107 The intensities of the characteristic PEO bands strongly depend on the conformational characteristics of the polymer chains as indicated by a number of works on PEO melts and solutions.108−111 All assignments attribute the bands at 1285 and 1325 cm−1 generally to trans conformations of the C−O and C−C bonds, respectively, whereas the bands at 1300 and 1350 cm−1 are assigned basically to the respective gauche conformations. The spectra of the hybrid with 72 wt % SiO2 show important differences from those of neat PEO2270 especially in the 1315−1375 cm−1 spectral region: PEO chains residing between silica nanoparticles seem to preferably adopt gauche conformations since the peak at 1325 cm−1 decreases in intensity. It is noted that the spectral region of the 1285−1300 cm−1 peak is dominated by the absorption of silica nanoparticles and will not be analyzed further on. To be more quantitative, fittings of the infrared spectra of the pure PEO and of the nanohybrids were performed utilizing a number of Gaussian functions, and the ratios of the integrated intensities of the peaks attributed to trans and gauche conformations were calculated. Figure 6 demonstrates the fitting procedure in the case of the pure polymer. Six main peaks together with bands for fitting the ends have been utilized to satisfactory fit the spectra of the pure polymer. In the case of the nanohybrid a similar analysis is prohibited by the strong contribution of the SiO2 absorbance (below 1300 cm−1). Thus, in order to minimize the error in the obtained

Figure 3. Fraction of “gauche” population as a function of temperature for (a) OCCO and (b) COCC dihedral angles for all systems. Inset: values of the “gauche” fraction for all systems at 400 K. Error bars are of the order of 5−10% of the actual values. (c) Ratio of the radius of gyration of PEO chains in the polymer nanocomposite over the corresponding bulk values as a function of temperature for the same systems, with the corresponding error bars.

case, ATR-FTIR was used to measure the infrared absorption spectra of pure PEO2270 and of nanohybrids with varying nanoparticle content 25, 72, and 82 wt % SiO2 or volume fraction φ = 0.14−0.69 up to almost the highest possible value, which according to the hexagonal close-packed model is φmax = 0.74. In all cases the dispersion of the nanoparticles was verified by transmission electron microscopy measurements. Figure 4 shows a TEM image of a PEO2270/NP7 nanohybrid with 25 wt % SiO2. The films were cryomicrotomed at T = 120 K in slices of 50 nm thickness. The use of a cryomicrotome was 6278


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found to be the least intervening to the real data and the following analysis. From such analysis, the ratio of the Igauche/Itotal for the C−C bonds can be calculated to provide information about the effect of the inorganic nanoparticles on the conformations of the PEO chains. Figure 7 shows the ratios of the integrated intensities of

Figure 7. Temperature dependence of the intensity ratios I1350/(I1325 + I1350), signifying the fraction of the gauche conformation of the C−C bonds, for pure PEO2270 (squares) and PEO2270/NP7 nanohybrids with 97 wt % PEO2270/3 wt % NP7 (circles), 89 wt % PEO2270/11 wt % NP7 (triangles up), 75 wt % PEO2270/25 wt % NP7 (triangles down), 52 wt % PEO2270/48 wt % NP7 (diamonds), 27 wt % PEO2270/73 wt % NP7 (triangles left), and 18 wt % PEO2270/82 wt % NP7 (stars).

I1350/(I1325 + I1350), i.e., the ratio of Igauche/Itotal, for the C−C bond, for PEO2270, and for all PEO2270/NP7 nanohybrids; this ratio signifies the wgauche fraction calculated by the molecular dynamics simulations. For pure PEO2270, it seems that the numbers of the gauche and trans conformations are more or less similar; the values of the ratio I1350/(I1325 + I1350) is only slightly higher than 0.5 and shows no evidence of a temperature dependence. The results of the analysis indicate a similar situation for the PEO2270/NP7 nanocomposites with the high polymer content (i.e., up to 25 wt % SiO2) with the ratios of the intensities being very similar to those of pure PEO2270. Nevertheless, the situation changes in the case of the nanocomposites with 48 wt % and more with 72 and 82 wt % NP7, where a significant increase of the gauche conformation is observed. However, in the latter systems, the polymer chains exist in severe confinement between the nanoparticles. The ratio Igauche/Itotal increases with increasing nanoparticle content with the fraction of gauche conformations going from almost one-half for the neat PEO2270 to 3 or 4 times higher for the 72 and 82 wt % SiO2 hybrids, respectively. However, there appears to be no significant temperature dependence. The energy difference between the gauche and the trans state is ΔG = Ggauche − Gtrans = −0.4 ± 0.07 kJ/mol for the pure PEO2270, and it becomes ΔG = −0.35 ± 0.08 kJ/mol for the PEO2270/ NP7 with 25 wt % SiO2, ΔG = −3.28 ± 0.04 kJ/mol for the PEO2270/NP7 with 72 wt % SiO2, and ΔG = −4.53 ± 0.10 kJ/ mol for the PEO2270/NP7 with 82 wt % SiO2. In a previous work, nanohybrids composed of the same PEO and a hydrophilic sodium montmorillonite were investigated in which intercalated structures were attained. For nanohybrids with 5−40 wt % polymer, i.e., when all chains were confined

Figure 5. ATR-FTIR spectra of (a) pure PEO2270 and (b) a PEO2270/ NP7 nanohybrid with 72 wt % SiO2 content at various temperatures above the melting transition. The inset in (b) shows the spectrum of silica nanoparticles at T = 393 K. Spectra are shifted for clarity.

Figure 6. Fitting procedure utilizing Gaussian functions for the pure PEO2270.

results, each spectrum was brought to zero at 1375 cm−1, and it was, then, multiplied by a constant so that the peak at 1350 cm−1 matches the peak intensity of the respective spectrum of the bulk PEO2270. This type of background subtraction was 6279


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Macromolecules within the ∼1 nm intergallery spacing of the inorganic material, the conformations of the polymer chains were found to change with a systematic increase of the gauche conformations observed with increasing montmorillonite content, which approach almost 100% gauche at the highest loading; in that case the peak attributed to the trans conformation at 1325 cm−1 was completely suppressed. It should be emphasized, however, that in that work and for all hybrids the chains were under a very strong confinement.25,91 Discussion. In the following, we provide a critical discussion of our findings concerning the chain conformations obtained computationally and experimentally for the series of PEO/SiO2 nanohybrids with different polymer molecular weight, nanoparticle size, and nanofiller content. As discussed before, simulation and experimental methods have been used in order to provide complementary results and deeper understanding on the parameters that define the behavior; simulation provides information for nanocomposites with oligomers up to medium MW (unentangled) PEO chains (i.e., in the Rouse like regime) under a rather broad range of degrees of confinement from weak to moderate, whereas experimental measurements concern high MW chains giving the opportunity to cover the regime from moderate to severe confinement. A similar behavior between experimental and simulation results has been observed qualitatively, since in both cases an increase of the “gauche” conformation concerning the C−C bond is detected in the nanocomposites compared to the corresponding bulk systems. However, quantitative differences are observed concerning the temperature dependence of the fraction of the gauche population for each material as well as the magnitude of the increase of the gauche vs trans conformation, where experimentally a much stronger increase is identified. Nevertheless, the stronger confinement in these systems should be kept in mind. In this case, detailed calculations based on a random packing of SiO2 NPs at low and moderate values of NPs volume fraction and various crystal packing arrangements predict that the mean nearest distance between the particles and, thus, the “size” of the polymer domain is on the average ∼2 nm, and it reaches distances of less than ∼0.5 nm in the case of the low polymer content hybrids. These volume fractions lead to a relative degree of confinement, quantified through the d/Rg ratio, ranging from d/Rg = 0.875 to d/Rg = 0.021. Different is the case of the simulation model systems for which the d/Rg ranges from 12.68 to 0.247. Another difference between the PEO chains used in the experiments and the simulated PEO model systems was that in the former the chains had −OH end groups whereas in the latter the chains were CH3 terminated. In order to examine the effect of the different end groups on the conformational properties of PEO chains in the presence of the silica NP’s, which has been reported in previous simulation and experimental studies,66,89 we have further performed two identical simulation runs where PEO chains were terminated with methyl and hydroxyl groups. No effect was observed on the distribution of torsional angles (see also Figure 8), whereas a small decrease of the dimensions (Rg) of the hydroxylterminated chains was obtained. This is consistent with the reported studies. Simulations based on low MW oligomers revealed hardly any structural differences, and on top of that these are expected to be totally eliminated for longer chains. In experimental studies, differences have been reported only on dynamical properties.112 A further investigation of this issue will be presented in a forthcoming publication.

Figure 8. Normalized percentage of gauche OCCO angles as a function of the degree of confinement, quantified through the ratio d/ Rg for the different systems studied here by both experiments and simulations.

In order to bring all the nanohybrids systems composed of polymers of different molecular weight as well as of nanoparticles of different sizes together, a critical parameter expressing the confining length of all systems should be defined; here we have used the average nearest interparticle distance divided with the (unperturbed) chain radius of gyration, d/Rg (see also Table 1).19,113 Figure 8 shows a comparative plot with the results of the calculations, measurements, and analysis expressed as the ratio of the fraction of the gauche population with respect to the corresponding bulk as a function of the ratio d/Rg for all systems studied through experiments and simulations at 400 K. For completeness, and in order to check the effect of different end groups of PEO chains, we also include data from a model system (PEO48/ NP2), where PEO chains are terminated with −OH groups. Figure 8 demonstrates that indeed confinement is a crucial parameter for the conformational behavior of polymer chains close to the nanoparticle and the stronger the confinement the larger the change in the conformations. Moreover, a confinement more severe than the chain dimensions is necessary in order to clearly observe this change. In the case of high volume fraction of nanoparticles the increase of gauche fraction of OCCO angles is expected to decrease chain dimensions (see also Figure 3c). Note that if we consider the d/Rg as a measure of “the effective confinement”, then this ratio increases with the decrease of Rg, which means that the confinement is reduced. The latter could be an effective way of polymer matrix to accommodate large NP fractions. We should also state here that alternative to d/Rg parameters could be used in order to present the conformational and dynamical changes of polymer chains for the different concentrations of NPs.114−117 For example, the ratio of the NP’s diameter to the Rg could be also is considered. However, for studying the influence of the NPs to the polymer chain conformations d/Rg (or d/2Rg) would be expected to be more appropriate, since from our knowledge on the structural properties of polymer chains confined between two parallel solid surfaces,17,18 it is recognized that the chain dimensions are affected when the distance between the two substrates becomes of the order of Rg. Data reported in the present work that correspond to a different geometry support such a change of the chain conformations, but for much smaller values of d/Rg. A 6280


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Macromolecules

Research and Technology, Ministry of Education, Greece and the European Regional Development Fund (Sectoral Operational Programme: Competitiveness and Entrepreneurship, NSRF 2007-2013)/ European Commission and was cofinanced by the European Union’s Seventh Framework Programme (FP7-REGPOT- 2012-2013-1) under Grant agreement No. 316165. The hospitality of Prof. A. Avgeropoulos for obtaining the TEM images is greatly acknowledged.

more detailed examination of this issue will be the subject of a future work.

IV. CONCLUSIONS Atomistic molecular dynamics simulations and attenuated total reflection−Fourier transform infrared spectroscopy have been utilized to investigate the chain conformations in poly(ethylene oxide)/silica, PEO/SiO2, nanohybrids. Polymers with different molecular weights, nanoparticles with different sizes, and hybrid compositions that ranged from dilute systems to the highest possible loaded (both computationally and experimentally) have been used to reveal the effect of confinement on the polymer conformational and structural properties. This is a combined study with complementary results, where simulations provide information for nanocomposites with oligomers up to medium MW of PEO chains under a rather broad range of degrees of confinement. Experimental results concern much higher MW chains thus inducing stronger confinement. The homogeneity of the hybrids for all compositions has been verified by transmission electron microscopy. Qualitative similar behavior between experimental and simulation results is observed since in both cases an increase of gauche population for the OCCO angle is observed. This increase becomes larger as the degree of confinement becomes higher in both simulations and experiments. On the contrary, the conformations of the C−O bond seem to remain unaffected by the confinement, at least in the range covered computationally whereas the respective experimental information for higher degrees of confinement is limited since this regime is screened by the strong absorption of the silica nanoparticles. Furthermore, simulation results reveal a systematic temperature dependence of the percentage of gauche, which decreases with temperature for OCCO and increase for COCC dihedral angles. In addition, chain dimensions in the nanocomposite are slightly modified compared to bulk; a small decrease in Rg is observed that is more pronounced at low temperatures. Overall, it is clear that for PEO/silica nanocomposites confinement is a crucial parameter for the conformational behavior of polymer chains. The stronger the confinement, the larger the increase of gauche fraction in the polymer conformations. Such disturbances become stronger under very high nanoparticle volume fractions that correspond to severe confinement of polymer chains. Under such conditions, the increase of gauche fraction of OCCO angles is expected to decrease chain dimensions and consequently reduce the effective confinement, in order for the polymer matrix to accommodate large NP fractions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (V.H.). *E-mail [email protected] (K.C.). ORCID

Vagelis Harmandaris: 0000-0002-9613-7639 Spiros H. Anastasiadis: 0000-0003-0936-1614 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed in the framework of PROENYL research project, Action KRIPIS, project MIS-448305 (2013SE01380034), funded by the General Secretariat for 6281


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Structural and Conformational Properties of Poly(ethylene oxide

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