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Rubbery Polymer-Inorganic Nanocomposite Membranes: Free Volume Characteristics on Separation Property Ben Li, Dan Xu, Xiongfei Zhang, Zhongyi Jiang,* Yu Wang, Jing Ma, Xiao Dong, and Hong Wu Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China, and China State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China
The rational design of polymer-inorganic nanocomposite membranes relies heavily on the precise insight and elaborate control of the interface. Presently, the direct exploration of the hierarchical structure of nanocomposite membranes still remains elusive. In the present study, we propose a facile and generic methodology to quantitatively probe the interfacial structure by complementary positron annihilation lifetime spectroscopy (PALS) and molecular dynamics simulation (MDS) techniques. MDS is used to acquire the molecular level information such as the polymer-inorganic interface interaction energy, chain mobility within the nanocomposite membranes, whereas PALS is used to acquire the free volume characteristics of the nanocomposite membranes. As proof-of-principle, we choose anisotropic inorganic nanotube embedded rubbery polymer membrane as a model, which generates the interface between soft polymer and rigid inorganic. PALS reveals that incorporation of titanate nanotubes (TNTs) narrows the free volume pore radius distribution of the membranes. MDS indicates that the segmental chain mobility in the vicinity of the polymer-inorganic interface is substantially restrained, which creates numerous nanosized voids for molecular transport, and dramatically enhances the fractional free volume (FFV) of the membranes. Quite interestingly, it was found that the rubbery membranes can also exhibit simultaneously increased permeability and membrane selectivity, and this unusual phenomenon was tentatively elucidated by relating the separation properties to the free volume characteristics of the membranes. Introduction Since its inception, nanocomposite (hybrid or mixed matrix) materials pave a facile, versatile, and efficient way to endow the pristine polymers with notably enhanced properties.1-5 The so-called nanocomposite materials are of hierarchical structures usually comprising nanosized inorganic “inserts” embedded within a polymer matrix, which ingeniously combines the advantages of each moiety, rendering superior performance to each individual phase, e.g., increased permeability, desirable thermal and mechanical properties, specific electrical and optical properties, and economical processability. Nanocomposite materials can be further catalogued as glassy polymer-based and rubbery polymer-based nanocomposite materials. It is found that permeability of certain glassy polymer membranes with nanoinclusions increases with the particle volume fraction, and they are not subject to the permeability/ selectivity trade-off limitation.6 Theoretical explanations of such phenomenon are ascribed to the polymer chains repelled from the inclusion during membrane casting.7,8 To the best of our knowledge, quite few works set foot on the rubbery polymerbased polymer-inorganic interface structure presently. In particular, no successful report has been found to solve the “trade-off” bottleneck between permeability and selectivity. However, nobody will deny that these issues are of great scientific and technological perspective: First and foremost, the elastomeric and thermoplastic rubbery polymers endow the nanocomposite materials flexible interfacial morphology, which can effectively decrease the nonselective voids. Second, most rubbery polymeric materials have little or no active functional groups and are quite difficult to chemically modify via cross-linking9,10 or grafting/copolymerization.11-13 Physical * To whom correspondence should be addressed. E-mail: zhyjiang@ tju.edu.cn.
hybridization seems a “shortcut” to upgrade the properties of these inert rubbery polymers. It is well-recognized that the specific properties of nanocomposite materials are mainly attributed to the interfacial interactions between the inorganic filler and the polymer matrix.14,15 Therefore, the rational design of nanocomposite materials strongly depends on penetrating analysis and characterization of the polymer-inorganic interface structure at microscopic scale or molecular level.16-24 Despite some classical theory of diffusion in nanocomposites having been proposed and having captured most of the qualitative physics, these models mainly focus on the glassy-polymer-based nanocomposites. In addition, it utilizes density profiles based on theories for polymer solutions near flat surfaces whose applicability for polymer melts near highly curved surfaces (nanocomposites) is elusive.7,8 Positron annihilation lifetime spectroscopy (PALS), which is able to quantitatively measure the nanosized voids within the matrix, launches new discernment into the microscopic structure of rubbery nanocomposite materials.6,20 PALS, however, could only acquire static free volume characteristics of the materials. In comparison, molecular dynamics simulation (MDS) can potentially quantify the polymer-inorganic interface and provide dynamic information including molecular interactions, chain dynamics, and diffusion behavior of small molecules at the molecular level.25-27 Rational synergy between PALS and MDS will enable detailed insight into the nanoscale interface structure. In the present study, we selected rubbery polymer poly(dimethylsiloxane) (PDMS) as continuous phase and titanate nanotubes (TNTs) as dispersed phase. We extensively probed the structure, morphology, and dynamics of the PDMS-TNTs interface region by PALS and MDS. The nanocomposite membranes were found to crossover the trade-off hurdle between permeability and membrane selectivity. Although a few studies have investigated PDMS-based hybrid materials by PALS,28,29
10.1021/ie101142b 2010 American Chemical Society Published on Web 11/08/2010
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the present study mainly features the following: (1) detailed insight into the nanoscale interface structure was gained by complementary information from both PALS and MDS; (2) the unusual separation results were reasonably reconciled by correlating the apparent properties to the microscopic structures of the membranes. Experimental Section Materials. Rutile-type titanium dioxide powders were purchased from Shanghai Zhuerna High-Tech Powder Material Co., Ltd., and used without further purification. PDMS oligomer (the viscosity was 5000 mPa · s, and the corresponding average molecular weight was about 40 000) ethyl orthosilicate and dibutyltin dilaurate were obtained from Beijing Chemical Co., Beijing, China. Asymmetric polysulfone (PS) ultrafiltration membranes were ordered from Shanghai MegaVision Membrane Engineering and Technology Co., Ltd. N-Octane, thiophene, and n-heptane were supplied by GuangFu Fine Chemical Research Institute, Tianjin, China. Deionized water was used in all experiments. Synthesis of TNTs. Titanate nanotubes were synthesized by a hydrothermal method as described by Geng et al.30 Preparation of PDMS-TNTs Nanocomposite Membranes. PDMS, cross-linking agent ethyl orthosilicate, and catalyst dibutyltin dilaurate were dissolved into n-heptane to form a homogeneous solution at room temperature. TNTs were dispersed in heptane under ultrasound. The PS membrane support (the area of 10 cm ×10 cm) was soaked in deionized water for 24 h to remove glycerin and then fully dried to be employed as the support layer of the composite membrane. The PDMS membranes were prepared by solution-casting mixtures of the fillers and PDMS/heptane solutions. The mixture was vigorously stirred to obtain a pseudohomogenous solution. After degassing, the solution was cast onto the PS support at room temperature. The membrane was first dried in air for 24 h and then placed in a 75 °C oven to complete cross-linking and evaporate the residual solvent. All membrane samples were stored in a dust free and dry environment before being used in the pervaporation experiments. Characterizations. Fourier transform infrared (FT-IR) spectra were measured using a Nicolet, Magna-IR 560 spectrometer equipped with horizontal attenuated transmission accessories. Pore volume, size distribution, and Brunauer-Emmett-Teller (BET) surface area measurement of the TNTs were performed using an ASAP 2020 BET system. Scanning electron micrograph (SEM) images were acquired on a Philips XL-30 M SEM instrument. The morphology of TNTs was observed with the transmission electron microscopy (TEM), JEM-100CX II. PALS experiment was conducted by using an ORTEC fast-fast coincidence system at room temperature. The integral statistics for each spectrum was more than 1 × 107 coincidences. The spectra were evaluated using “the maximum entropy for lifetime analysis” (MELT) program.31 The MELT program automatically inverted the lifetime spectrum into a continuous lifetime distribution using a quantified maximum entropy method. Compared with numerical Laplace inversion method (CONTIN program),32 it was found that results of MELT were more accurate.33 One simply needed to enter the value of the entropy weight E, time resolution, cutoff values, and the time zero channel of the spectrum. And, then, the MELT program would automatically calculate the results. E was set between 4 × 10-8 and 5 × 10-9; the cutoff value was 5 × 10-3; the time resolution was 202 ps; the t0 shift (channels) was 0.690. Three or four well-separated peaks could be observed; their characteristic
Figure 1. Positron annihilation lifetime spectra in PDMS control and PDMSTNTs nanocomposite membranes.
Figure 2. Distribution of free volume cavity radius in PDMS control and PDMS-TNTs nanocomposite membranes.
Figure 3. Free volume parameters of the PDMS control and PDMS-TNTs nanocomposite membranes.
lifetimes τi and intensities Ii were calculated as the mass center of and the area below the peaks. τ3 was attributed to orthopositronium (o-Ps) pick-off annihilation in the present study, which directly reflected the free volume properties of the membranes. The free volume is assumed as a spherical potential well-surrounded by an electron layer of thickness ∆r, and the following expressions were employed to relate lifetime τ3 and radius of free volume holes, r3 34,35 τ)
2πr 1 1 r + sin 12 r + ∆r 2π r + ∆r
[
( ) (
-1
)]
(1)
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Figure 4. Models of (a) PDMS-bulk and (b) PDMS-TNT interface.
VF,i )
4πri3 3
Table 1. Interaction Energies between PDMS and TNTs, kcal/mol
(2)
where ∆r was the electron layer thickness with an estimated value 0.1656 nm. Thus, fractional free volume (FFV) of the membranes could be represented using the values of VF,3I3 in the present study. Moreover, it should be noted that positron annihilation measurement had strong dependence on the thickness of the samples. The conventional positron annihilation, due to the positron energy spectrum being continuous, reflected the comprehensive/average information from the surface to the substrate of sample materials. For polymeric membranes or thin films, only part of the information was directly related to the bulk matrix of the samples. Positron annihilation inevitably would occur on the material surface and the substrate, which would have influence on the results to some extent. In the present study, to ensure enough positron annihilation within the bulk matrix of the membranes, PDMS control membrane and nanocomposite membranes were prepared as films (without substrate) with thickness of around 1 mm for the PALS measurement. Molecular Dynamics Simulation. Molecular dynamics simulations in this study were carried out using “Forcite” and “Amorphous Cell” module of “Materials Studio”, a powerful workstation developed by Accelrys Software Inc. “Universal” force field was employed. For dynamics, the Andersen thermostat and Berendsen barostat methods were employed to maintain a constant temperature and pressure, respectively. The nonbond cutoff distance was set as 12.5 Å (with a spline width of 1.0 Å and a buffer width of 0.5 Å). The time step was set as 1.0 fs for all dynamics runs. For pure PDMS, the initial atactic polymer chain consisted of 20 repeat units. The packing model with a density of 0.97 g/cm3 containing five PDMS chains was constructed by Amorphous Cell module. For PDMS-TiO2 interface, two confined layers of PDMS and TiO2 were contacted and contained in a single layer. The resulting structures were subsequently optimized by the following procedure. A 5000step energy minimization (using geometry optimization method) was adopted. And then a 1000 ps MD equilibration run was performed in the NPT (T ) 298 K, P ) 1.01 × 105 Pa) ensemble to obtain the equilibrium density. An additional 500 ps NVT (T ) 298 K) dynamics was implemented on the end point of the NPT run to obtain the equilibrium molecular structures, and the atomic trajectory of every picosecond was recorded for later analysis. Cross-Link Density Measurement. The cross-link density was measured by equilibrium swelling method. The membranes (active layer) were weighed carefully before being immersed in the feed solution at 25 °C. The swollen membrane samples
PDMS TNTs PDMS-TNTs ∆E
Etotal
Epotential
Ekinetic
-8488.0 -36806.5 -45981.9 -687.4
-9383.1 -37476.9 -47551.1 -691.2
895.1 670.4 1569.2 3.7
pressure (MPa)
temperature (K)
0.098 0.086 0.099
298.2 298.4 298.6
were taken out from the feed mixture after 24 h and wiped with tissue paper to remove the residual liquid before being weighed. A common simplified method to obtain relative cross-link density was the reciprocal of the swelling index: υe ) 1/SI
(3)
SI ) m1 /m0
(4)
where υe was the cross-link density, SI was the swelling index, and m0 and m1 were the weights of the dried and swollen membranes, respectively. Pervaporation Experiments. The scheme of the pervaporation setup and the configurations of the membrane module were reported in our previous literature.36 Feed solution containing 500 µg/g (in terms of sulfur) was pumped into the membrane cell with the flow rate of 40 L/h. The temperature of the membrane room was controlled at 30 °C. The pressure in the downstream side was maintained at
