Confinement Effects on the Local Motion in Nanocomposites

The local segmental dynamics of polymers confined within the 15-20 1 interlayer spacing of nanocomposites consisting of poly(methyl phenyl siloxane) intercalated within organically modified silicates, has been investigated utilizing Dielectric Relaxation Spectroscopy. The effect of confinement on the local reorientational dynamics is evident by the observation of a relaxation mode, which is much faster than the segmental α-relaxation of the bulk polymer and exhibits much weaker temperature dependence. This is attributed to the restrictions placed by the interlayer spacing on the cooperative volume required for the α-relaxation. INTRODUCTION One promising way to synthesize polymer nanocomposites is by intercalating polymers in layered inorganic hosts. Layered-silicate based polymer nanocomposites have become an attractive set of organic-inorganic hybrid materials because of their obvious potential as technological materials [1-6]. Beyond the conventional phase separated polymer/silicate nanocomposites, for which the polymer and the inorganic host remain immiscible, two types of hybrids are possible: (i) intercalated, in which a single extended polymer chain is intercalated between the host layers resulting in a well ordered multilayer with alternating polymer/inorganic layers and a repeat distance of a few nm; (ii) exfoliated or delaminated, in which the silicate layers (1nm thick) are exfoliated and dispersed in a continuous polymer matrix. The intercalated polymer/silicate nanocomposites offer a unique avenue for studying the static and dynamic behavior of small molecules and macromolecules in nanoscopic confinements. Actually, in these model systems, one can utilize conventional analytical techniques (like thermal analysis, NMR, dielectric spectroscopy, inelastic neutron scattering, rheology) on macroscopic samples and, nevertheless, study the properties of 2-3 nm thick polymer films [7]. The dynamics in thin polymer films and of small molecules and macromolecules in porous media has recently attracted the interest of the scientific community. For thin polymer films, reports of both enhanced and reduced mobility relative to the bulk have been reported. It appears that the effect of a free surface is to decrease the glass transition temperature (Tg ) and, thus, to enhance the mobility [8,9], due a decrease in density [10]. However, the effect of an attractive wall is to increase the Tg thus reducing the mobility [11,12,13]. Computer simulations showed that the effect of confinement even with neutral walls is to reduce the mobility due to an † Also at University of Crete, Physics Department, 710 03 Heraklion Crete, Greece ý Present Address: University of Leeds, Department of Physics and Astronomy, Leeds LS2 9JT, U. K. # Present Address: Pennsylvania State University, Department of Materials Science and Engineering, State College, PA 16802, U.S.A. increase in density [14]. For free standing thin films, Tg was found to decrease linearly with thickness [15]. Moreover, finite size effects were favorable for the depression of the 1st-order phase transitions, leading to a considerable shift of the freezing point to far below the appropriate solidification temperature of bulk liquids [16]. In the case of confinement in porous media, the spin-lattice NMR relaxation times of small organic molecules are significantly shorter than for the bulk liquids with the effect depending on the organic/silica interactions [17,18]. The issue of a characteristic length scale of cooperativity, which determines the dynamic glass transition, has been raised following studies by solvation dynamics [19] and dielectric spectroscopy [20-23], with the data showing slightly slower [19,21,23] or slightly faster [20,24] dynamics for the fluid in the pores than in the bulk. The effects of coating the pore surface on the dynamics was also investigated [21,22]. In this manuscript, a dielectric relaxation spectroscopy investigation is presented attempting to probe the role of confinement on local dynamics utilizing a series of intercalated polymer/silicate nanocomposites. The organically-modified layered silicates were mixed with low-molecular weight poly(methyl phenyl siloxane), PMPS, whose segmental motion is dielectrically active. PMPS is confined within the 15-20 1 interlayer spacing of the nanocomposites. A relaxation mode, which is much faster than the segmental α-relaxation of the bulk polymer, is consistently observed with much weaker temperature dependence. This should be attributed to the restrictions placed by the interlayer spacing on the cooperative volume required for the α-relaxation. EXPERIMENTAL SECTION Materials Organically modified layered silicates (a hectorite and a bentonite) were synthesized by a cation exchange reaction between the layered silicate hosts and excess dioctadecyl-dimethylammonium bromide, as outlined previously [25], in order to render the originally hydrophilic silicate surface organophilic. Hybrids were prepared by mixing dry organosilicate and poly(methyl phenyl siloxane), PMPS, (Mw=2600, Mw/Mn=1.20). Various percentages of PMPS were used (Table I) in order to span the range from starved to just over full galleries. The materials were allowed sufficient time ~60 úC under ultrasonication for the PMPS chains to diffuse into the organically modified silicates (PMPS is liquid at room temperature). The hybrids as well as the silicates themselves were characterized by x-ray diffraction (Scintag Inc. θ-θ diffractometer equipped with a Germanium detector using Cu Kα radiation) in order to determine the interlayer spacing before and after intercalation. The data are shown in Table I. Code Silicate Polymer wt% Polymer d100 spacing B34 B34 23 1 B38 B38 25 1 15% B34 PMPS 15 33 1 (*) 20% B34 PMPS 20 32 1 25% B38 PMPS 25 36 1 30% B34 PMPS 30 34 1 * A shoulder is present at 23 1 spacing indicating the existence of non-intercalated silicates, i.e., a starved system Dielectric Spectroscopy Dielectric relaxation spectroscopy, DS, was used to investigate the collective segmental dynamics of PMPS as a function of temperature. The complex dielectric permitivity ε (ω ) = ε' (ω) − iε"(ω ) of a macroscopic system is given by the one-sided Fourier transform of the time derivative of the normalized response function C(t) ε (ω ) − ε∞ = −∆ε dC(t) / dt [ ]exp −iωt ( )d 0 ∞ ∫ t (1) where i = −1, ∆ε = ε0 − ε∞ is the relaxation strength with ε0 and ε∞ being the low and high frequency limiting values of ε ' for the process under investigation. The quantity C( t) is the normalized dipole-dipole correlation function. For non-zero dipole moment perpendicular to the chain contour, the response function C( t) is sensitive to segmental motion. A Solatron-Schlumberger frequency response analyzer FRA 1260 supplemented by using a high-impedance preamplifier of variable gain was used for the dielectric measurements covering the frequency range 10-2 to 106 Hz. The sample was pressed in form of a pellet and was residing between two gold-plated stainless steel electrodes (diameter 25 mm). It was kept in a cryostat with its temperature controlled via a high-pressure nitrogen gas jet heating system with a Novocontrol Quatro controller allowing a stability of the sample temperature in margins of ±0.1 úC in a broad temperature range of -160 úC to +300 úC. Note that the absolute values of the loss part of the dielectric permitivity, ε", depend on the accuracy of the sample thickness provided to the software. For quantitative analysis, the generalized relaxation function according to HavriliakNegami [26] is traditionally being used; the absence of a clear physical interpretation of the adjustable parameters involved and the fitting uncertainties restrict its use solely as a phenomenological way to account for the features of the underlying processes. Besides, an additional conductivity contribution at low-frequencies and high temperatures due to free charges has to be accounted for. For multiple relaxation processes, use of more than one empirical Havriliak-Negami functions is required; this, however, leads to a large number of adjustable parameters in the fitting procedure (4 parameters are needed for each Havriliak-Negami function). Recently, we have presented a method [27] based on a modification of the widely used CONTIN routine of analysis of photon correlation spectroscopy data for the inversion of the experimental correlation function in order to obtain the distribution of relaxation times F(ln τ) with no a priori assumption of the form of the relaxation function. The method performs an inversion of the experimental ε"(ω) spectrum in order to determine F(ln τ) assuming a superposition of Debye processes, i.e., ε"(ω ) / ∆ε = F(ln τ) ωτ / 1+ (ωτ) [ ] { }d(ln τ) −∞ ∞ ∫ (2a) where F(ln τ) is normalized. Alternatively, eq. 2a may be written as ε"(ω ) = ̃ F (ln τ ) ωτ / 1+ (ωτ) [ ] { }d(ln τ) −∞ ∞ ∫ (2b) where ̃ F (ln τ) = ∆εF(ln τ ); integration of the resulting ̃ F (ln τ) spectrum yields ∆ε, since the F(ln τ) distribution is normalized. RESULTS AND DISCUSSION Figure 1 shows a semilog plot of the loss part of the dielectric permitivity, ε"(ω), versus frequency for the bulk PMPS sample in the frequency range 10-2 to 3×106 Hz for temperatures 223 K to 253 K. In this range the segmental motion of the PMPS homopolymer (Tg =223 K) is observed. This process shows strong temperature dependence in this range (see Figure 4 below), which can be analyzed with a Vogel-Fulcher-Tamman (VFT) equation. Figure 2 shows the dielectric spectra for the organically-modified silicate B34 in the temperature range of 173 to 223 K. The dielectric spectra of the silicate (both of B34 and B38) are broad and quite complicated. Inversion of the dielectric data using eq. 2 results in two individual relaxation processes for each silicate (see Figure 4 below); the faster one observed in the range of (e.g., Fig. 2) should be related to orientational motions of the alkylammonium surfactant used to modify the silicate surface whereas the slower one observed in the temperature range 275 to 335 K (not shown) is most probably related to the Maxwell-Wagner polarization due to the presence of interfaces in the system. 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 0.000 0.005 0.010 0.015 0.020