Dynamics of water in a molecular sieve by quasielastic neutron scattering

We have investigated the dynamics of water confined in a molecular sieve, with a cylindrical pore diameter of 10 A, by means of quasielastic neutron scattering (QENS). Both the incoherent and coherent intermediate scattering functions I(Q,t) were determined by time-of-flight QENS and the neutron spin-echo technique, respectively. The results show that I(Q,t) is considerably more stretched in time with a slightly larger average relaxation time in the case of coherent scattering. From the Q dependence of I(Q,t) it is clear that the observed dynamics is almost of an ordinary translational nature. A comparison with previous dielectric measurements suggests a possible merging of the alpha and beta relaxations of the confined water at T=185 K, although the alpha relaxation cannot be directly observed at lower temperatures due to the severe confinement. The present results are discussed in relation to previous results for water confined in a Na-vermiculite clay, where the average relaxation time from spin-echo measurements was found to be slower than in the present system (particularly at low temperatures).     

Starlike dendrimers in solutions: structural properties and internal dynamics

We measured the shape and the internal dynamics of starlike dendrimers under good solvent conditions with small-angle neutron scattering and neutron spin-echo (NSE) spectroscopy, respectively. Architectural parameters such as the spacer length and generation were varied in a systematic manner. Structural changes occurring in the dendrimers as a function of these parameters are discussed, i.e., in terms of the fractal dimension and deviations of the radius of gyration from the Gaussian value. A first cumulant evaluation of the NSE spectra for each scattering vector q separately yields the length scale dependent relaxation rates. We observe a local minimum in the normalized relaxation rates Omega(q)q(3) on length scales corresponding to the overall dendrimer dimension. The dynamics is discussed within a Rouse-Zimm approach generalized to the case of starlike dendrimers of arbitrary geometry. The model allows an identification of the modes contributing to the relaxation of the dendrimer in the q and time range of the NSE experiment. The local minimum is due to collective breathing motions of (parts of) the dendrons relative to each other. Shape fluctuations are not observed.     

Diffusion of water in clays on the microscopic scale: modeling and experiment

Diffusion of water in montmorillonite clays at low hydration has been studied on the microscopic scale by two quasi-elastic neutron scattering techniques, neutron spin-echo (NSE) and time-of-flight (TOF), and by classical microscopic simulation. Experiment and simulation are compared both directly on the level of intermediate scattering functions, I(Q, t), and indirectly on the level of relaxation times after a model of atomic motion is applied. Regarding the dynamics of water in Na- and Cs-monohydrated montmorillonite samples, the simulation and NSE results show a very good agreement, both indicating diffusion coefficients of the order of (1-3) x 10(-10) m(2) s(-1). The TOF technique significantly underestimates water relaxation times (therefore overestimates water dynamics), by a factor of up to 3 and 7 in the two systems, respectively, primarily due to insufficiently long correlation times being probed. In the case of the Na-bihydrated system, the TOF results are in closer agreement with the other two techniques (the techniques differ by a factor of 2-3 at most), giving diffusion coefficients of (5-10) x 10(-10) m(2) s(-1). Attention has been also paid to the elastic incoherent structure factor, EISF(Q). Simulation has played a key role in understanding the various contributions to EISF(Q) in clay systems and in clearly distinguishing the signatures of "apparent" and true confinement. Indirectly, simulation highlights the difficulty in interpreting the EISF(Q) signal from powder clay samples used in experiments.     

EFFECT OF PLASTICIZER ON THE MICROSTRUCTURE OF PS/PVC BLENDS

In this work, controlling of the particle size of PVC in PS/PVC blends was studied. Itis shown that viscosity ratio and particle size can be changed by adding a third composition,such as plasticizers, and the distribution of the third composition in two phases plays avery important role in controlling viscosity ratio and particle size. When DOP was used asthe plasticizer of PVC in PS/PVC blends, the particle size of PVC could not be reduceddue to the transference of DOP into PS phase. When polycaprolactone (PCL) was usedas the plasticizer of PVC in the same blends, the particle size of PVC could be descreasedobviously because PCL does not migrate to PS phase.     

Dynamics of small-molecule glass formers confined in nanopores

We report a comparative neutron scattering study of the molecular mobility and nonexponential relaxation of three structurally similar glass-forming liquids, isopropanol, propylene glycol, and glycerol, both in bulk and confined in porous Vycor glass. Confinement reduces molecular mobility in all three liquids, and suppresses crystallization in isopropanol. High-resolution quasielastic neutron scattering spectra were fit to Fourier transformed Kohlrausch functions exp[-(t∕τ)(β)], describing the α-relaxation processes in these liquids. The stretching parameter β is roughly constant with wavevector Q and over the temperature range explored in bulk glycerol and propylene glycol, but varies both with Q and temperature in confinement. Average relaxation times <τ(Q)> are longer at lower temperatures and in confinement. They obey a power law <τ(Q)> ∝ Q(-γ), where the exponent γ is modified by confinement. Comparison of the bulk and confined liquids lends support to the idea that structural and∕or dynamical heterogeneity underlies the nonexponential relaxation of glass formers, as widely hypothesized in the literature.     

Discrimination of poly(vinyl chloride) samples with different plasticizers and prediction of plasticizer contents in poly(vinyl chloride) using near-infrared spectroscopy and neural-network analysis.

In the recycling of poly(vinyl chloride) (PVC), it is required to discriminate every plasticizer for quality control. For this purpose, the near-infrared spectra were measured for 41 kinds of PVC samples with different plasticizers (DINP, DOP, DOA, TOTM and Polyester) and different plasticizer contents (0-49%). A neural-network analysis was applied to the near-infrared spectra pretreated by second-derivative processing. They were discriminated from one another. The neural-network analysis also allowed us to propose a calibration model which predicts the contents of plasticizers in PVC. The correlation coefficient (R) and the root-mean-square error of prediction (RMSEP) for the DINP calibration model were found to be 0.999 and 0.41 wt%, respectively. In comparison, a partial least-squares regression analysis was carried out. The R and RMSEP of the DINP calibration model were calculated to be 0.993 and 1.27 wt%, respectively. It is found that a near-infrared spectra measurement combined with a neural-network analysis is useful for plastic recycling.     

Preparation and performance of plasticizers for poly(vinyl chloride) from products of thermal decomposition of waste polystyrene

2,4-Diphenylbutyl-2,4-diphenylbutyrate (DPBDPB) and 2,4,6-triphenylhexyl-2,4,6-triphenylhexoate (TPHTPH), plasticizers for poly(vinyl chloride), were synthesized from the products of thermal decomposition of waste polystyrene. Their heat stabilities were studied by thermogravimetric analysis and differential thermal analysis, and compared with those of typical plasticizers for PVC such as dibutyl phthalate (DBP), dihexyl phthalate (DHP) and bis(2-ethylhexyl) phthalate (DOP). DPBDPB and TPHTPH showed much higher heat resistance than DOP. PVC was plasticized with a mixed system consisting of DOP as the primary plasticizer and DPBDPB as the secondary. It became clear that DPBDPB is an excellent heat-resistant plasticizer which does not affect the compatibility of PVC with DOP.     

Water dynamics in silica nanopores: the self-intermediate scattering functions

The dynamics of water molecules confined in approximately cylindrical silica nanopores is investigated using molecular simulation. The model systems are pores of diameter varying between 20 and 40 ? containing water at room temperature and at full hydration, prepared using grand canonical Monte Carlo simulation. Water dynamics in these systems is studied via molecular dynamics simulation. The results of the basic characterization of these systems have been reported in A. A. Milischuk and B. M. Ladanyi [J. Chem. Phys. 135, 174709 (2011)]. The main focus of the present study is the self-intermediate scattering function (ISF), F(S)(Q, t), of water hydrogens, the observable in quasi-elastic neutron scattering experiments. We investigate how F(S)(Q, t) depends on the pore diameter, the direction and magnitude of the momentum transfer Q, and the proximity of water molecules to the silica surface. We also study the contributions to F(S)(Q, t) from rotational and translational motions of water molecules and the extent of rotation-translation coupling present in F(S)(Q, t). We find that F(S)(Q, t) depends strongly on the pore diameter and that this dependence is due mainly to the contributions to the ISF from water translational motion and can be attributed to the decreased mobility of water molecules near the silica surface. The relaxation rate depends on the direction of Q and is faster for Q in the axial than in the radial direction. As the magnitude of Q increases, this difference diminishes but does not disappear. We find that its source is mainly the anisotropy in translational diffusion at low Q and in molecular reorientation at higher Q values.     

Thermodynamic, structural, and dynamic properties of supercooled water confined in mesoporous MCM-41 studied with calorimetric, neutron diffraction, and neutron spin echo measurements

Thermodynamic, structural, and dynamic properties of heavy water (D(2)O) confined in mesoporous silica glass MCM-41 C10, C12, and C14 were investigated by differential scanning calorimetry, neutron diffraction, and neutron spin echo (NSE) measurements, respectively. The DSC data showed that no crystallization of D(2)O confined in C10 occurs in a temperature range between 298 and 180 K, and that crystalline ice is formed at 204 and 221 K for C12 and C14, respectively. For C10, the neutron radial distribution functions of confined D(2)O suggested a structural change in the supercooled state between 223 and 173 K. For C10 sample, it has been found that the tetrahedral-like water structure is partially enhanced in the central part of pores at 173 K. For all the samples, the intermediate scattering functions from the NSE measurements are fitted by the Kohlrausch-Williams-Watts stretched exponential function which implies that confined supercooled D(2)O exhibits a wide distribution of relaxation times. For C10, C12, and C14 samples, between 298 and 240 K, the relaxation times of supercooled D(2)O follow remarkably well the Vogel-Fulcher-Tamman equation; for C10 sample, below 240 K, the relaxation times of nonfreezing D(2)O show an Arrhenius type behavior. From the present experimental results on calorimetric, structural, and dynamic properties, it has been concluded that supercooled D(2)O confined in MCM-41 C10 experiences a transition from high-density to low-density hydrogen-bonded structure at around 229 K.     

邻苯二甲酸二辛酯增塑聚氯乙烯膜的富氧分离性能

本文研究了DOP含量为0—70%(重量)的增塑PVC膜的透氧性能,指出DOP含量为50%左右的PVC膜要比纯PVC膜的气体透过率提高二个数量级,约为1.7×10^-9cm³。cm/cm²·s·cmHg,氧氮分离系数为4。DOP含量为20%的PVC膜有较高的氧氮分离系数,约为6.9。     

An investigation of water dynamics in binary mixtures of water and dimethyl sulfoxide

The motion of water molecules in mixtures of water and d6-dimethyl sulfoxide (DMSO) has been explored through molecular dynamics (MD) simulations using the SPC/E water model (J. Chem. Phys. 1987, 91, 6269) and the P2 DMSO model (J. Chem. Phys. 1993, 98, 8160). We evaluate the self-intermediate scattering functions, FS(Q,t), which are related by a Fourier transform to the incoherent structure factors, S(Q,omega), measured in quasielastic neutron scattering (QNS) experiments. We compare our results to recent QNS experiments on these mixtures reported by Bordallo et al. (J. Chem. Phys. 2004, 121, 12457). In addition to comparing the MD data to the experimental signals, which correspond to a convolution of S(Q,omega) with a resolution function, we examine the rotational and translational components of FS(Q,t) and investigate to what extent simulation results for the single-molecule dynamics follow the dynamical models that are used in the analysis of the experimental data. We find that the agreement between the experimental signal and the MD data is quite good and that the portion of FS(Q,t) due to translational dynamics is well represented by the jump-diffusion model. The model parameters and their composition dependence are in reasonable agreement with experiment, exhibiting similar trends in water mobility with composition. Specifically, we find that water motion is less hindered in water-rich and water-poor mixtures than it is near equimolar composition. We find that the extent of coupling between rotational and translational motion contributing to FS(Q,t) increases as the equimolar composition of the mixture is approached. Thus, the decoupling approximation, which is used to extract information on rotational relaxation from QNS spectra at higher momentum transfer (Q) values, becomes less accurate than that in water-rich or DMSO-rich mixtures. We also find that rotational relaxation deviates quite strongly from the isotropic rotational diffusion model. We explore this issue further by investigating the behavior of orientational time correlations for different unit vectors and corresponding to Legendre polynomials of orders 1-4. We find that the rotational time correlations of water molecules behave in a way that is more consistent with the extended jump rotation model recently proposed by Laage and Hynes (Science 2006, 311, 832).     

Hydrogen motions in the alpha-relaxation regime of poly(vinyl ethylene): a molecular dynamics simulation and neutron scattering study

The hydrogen motion in poly(vinyl ethylene) (1,2-polybutadiene) in the alpha-relaxation regime has been studied by combining neutron spin echo (NSE) measurements on a fully protonated sample and fully atomistic molecular dynamics simulations. The almost perfect agreement between experiment and simulation results validates the simulated cell. A crossover from Gaussian to non-Gaussian behavior is observed for the intermediate scattering function obtained from both NSE measurements and simulations. This crossover takes place at unusually low Q values, well below the first maximum of the static structure factor. Such anomalous deviation from Gaussian behavior can be explained by the intrinsic dynamic heterogeneity arising from the differences in the dynamics of the different protons in this system. Side group hydrogens show a markedly higher mobility than main chain protons. Taking advantage of the simulations we have investigated the dynamic features of all different types of hydrogens in the sample. Considering each kind of proton in an isolated way, deviations from Gaussian behavior are also found. These can be rationalized in the framework of a simple picture based on the existence of a distribution of discrete jumps underlying the atomic motions in the alpha process.     

Dynamics of propylene glycol and its oligomers confined to a single molecular layer

The dynamics of propylene glycol (PG) and its oligomers 7-PG and poly-propylene glycol (PPG), with M(w) = 4000 (approximately 70 monomers), confined in a Na-vermiculite clay have been investigated by quasielastic neutron scattering. The liquids are confined to single molecular layers between clay platelets, giving a true two-dimensional liquid. Data from three different spectrometers of different resolutions were Fourier transformed to S(Q,t) and combined to give an extended dynamical time range of 0.3-2000 ps. An attempt was made to distinguish the diffusive motion from the methyl group rotation and a fast local motion of hydrogen in the polymer backbone. The results show that the average relaxation time tau(d) of this diffusive process is, as expected, larger than the relaxation time tau averaged over all dynamical processes observed in the experimental time window. More interesting, it is evident that the severe confinement has a relatively small effect on tau(d) at T = 300 K, this holds particularly for the longest oligomer, PPG. The most significant difference is that the chain-length dependence of tau(d) is weaker for the confined liquids, although the slowing down in bulk PG due to the formation of a three-dimensional network of OH-bonded end groups reduces this difference. The estimated average relaxation time tau at Q = 0.92 Angstroms(-1) for all the observed processes is in excellent agreement with the previously reported dielectric alpha relaxation time in the studied temperature range of 260-380 K. The average relaxation time tau (as well as the dielectric alpha relaxation time) is also almost unaffected by the confinement to a single molecular layer, suggesting that the interaction with the clay surfaces is weak and that the reduced dimensionality has only a weak influence on the time scale of all the dynamical processes observed in this study.     

Interchain coupled chain dynamics of poly(ethylene oxide) in blends with poly(methyl methacrylate): coupling model analysis

Quasielastic neutron scattering and molecular dynamics simulation data from poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) blends found that for short times the self-dynamics of PEO chain follows the Rouse model, but at longer times past t(c) = 1-2 ns it becomes slower and departs from the Rouse model in dependences on time, momentum transfer, and temperature. To explain the anomalies, others had proposed the random Rouse model (RRM) in which each monomer has different mobility taken from a broad log-normal distribution. Despite the success of the RRM, Diddens et al. [Eur. Phys. Lett. 95, 56003 (2011)] extracted the distribution of friction coefficients from the MD simulations of a PEO/PMMA blend and found that the distribution is much narrower than expected from the RRM. We propose a simpler alternative explanation of the data by utilizing alone the observed crossover of PEO chain dynamics at t(c). The present problem is just a special case of a general property of relaxation in interacting systems, which is the crossover from independent relaxation to coupled many-body relaxation at some t(c) determined by the interaction potential and intermolecular coupling/constraints. The generality is brought out vividly by pointing out that the crossover also had been observed by neutron scattering from entangled chains relaxation in monodisperse homopolymers, and from the segmental α-relaxation of PEO in blends with PMMA. The properties of all the relaxation processes in connection with the crossover are similar, despite the length scales of the relaxation in these systems are widely different.     

Dynamics of deuterated polystyrene-protonated butadiene diblock copolymer micelles by neutron spin echo

We report on neutron spin-echo (NSE) measurements on deuterated styrene-protonated butadiene diblock copolymer micelles in deuterated n-decane to investigate the dynamics of butadiene blocks in the corona. Before the NSE measurements, we performed small-angle neutron-scattering (SANS) measurements on the micelles to evaluate the structure to give a basis for the discussion of the dynamics. In the SANS study, we have estimated the form factor P(Q) in terms of a hard-core-shell model from the direct evaluation without curve-fitting procedure while a more flexible core-shell model with the structure factor S(Q) gives a better fit to the observed data. The observed normalized intermediate scattering function I(Q,t)/I(Q,0) by NSE does not show the collective motions corresponding to the so-called breathing mode but rather single chain motion (Zimm modes) for both the 2 and 20 wt % micelle solutions. The Zimm decay rate Gamma(z) in the micelle solution is slow compared with that in the homopolymer solution. This slowing down is assigned to the effective high concentration in the corona. The differences in Gamma(z) between concentrated solutions and the 20% micellar solution are attributed to end-tethering effect of the corona chains on the core surface. The possible reasons why the breathing mode was not observed in the present micelle system are discussed on the basis of chain density in the corona.     

Dynamic mechanical behavior of the dioctyl phthalate plasticized polyvinyl chloride-epoxidized soya bean oil

The mixing of polyvinyl chloride (PVC) with dioctyl phthalate (DOP) shows two stages of gelation and fusion, but the homogeneity of each stage is influenced by the thermal stability of PVC and its rheological behavior. A torque rheometer has been used to gather almost all critical data related to the plasticized PVC in the epoxidized soya bean oil (ESBO). This study shows that, rheological data reflects the effects of DOP and epoxidization levels of SBO, in a DOP plasticized PVC-ESBO. The DOP plasticizer forms a thermodynamically miscible solution with ESBO; that reduces the rate of fusion and torque at balance of PVC. The storage modulus and tanδ of the plasticized PVC-ESBO have been used to show the extent of the homogeneity; but the dynamic mechanical behavior of PVC-ESBO is strongly influenced by DOP and the epoxidization level of SBO. The glass transition temperatures and dynamic properties of DOP plasticized PVC-ESBO are also reported and discussed in terms of the thermal stability and homogeneity of PVC.     

Binary blends based on poly(vinyl chloride) and multi-block copolymers containing poly(ϵ-caprolactone) and poly(ethylene glycol) segments

Binary blends based on poly(vinyl chloride) (PVC) were prepared both by casting from tetrahydrofuran (THF) and by mixing in the melt form, in a discontinuous mixer, PVC and multi-block copolymers containing poly(ϵ-caprolactone) (PCDT) and poly(ethylene glycol) (PEG) segments. PCDT-PEG copolymers were synthesized using a polycondensation reaction where the α,ω-bis-chloroformate of an oligomeric poly(ϵ-caprolactone) diol terminated (PCDT) and oligomeric PEG were employed as macromonomers. For comparison purposes, blends PVC with starting oligomers as well as with mixtures containing a typical low molecular plasticizer, dioctylphthalate (DOP), were also prepared. The copolymer miscibility was studied by differential scanning calorimetry (DSC) and FT-IR spectroscopy. The blend morphology was investigated by polarized light microscopy (PLM). A higher miscibility with PVC was observed for copolymers compared to PEG.     

Synthesis of an efficient bio-based plasticizer derived from waste cooking oil and its performance testing in PVC

In order to develop an efficient and sustainable plasticizer, the waste cooking oil and malic acid were used as the main raw materials in this study to synthesize a bio-based plasticizer (acetylated-fatty acid methyl ester-malic acid ester, AC-FAME-MAE) by environment-friendly methods, and the structure was characterized by FTIR and ¹H NMR. The properties of the poly (vinyl chloride) (PVC) with AC-FAME-MAE were tested and compared with those of the PVC plasticized with DOP (di-2-ethylhexyl phthalate) and EFAME (epoxy fatty acid methyl ester), respectively. The results of tensile test, TGA and leaching test showed that the mechanical properties, thermal stability and overall solvent resistance of PVC films with AC-FAME-MAE were significantly better than those of PVC films plasticized by DOP or EFAME. From the results of DMA, the plasticized efficiency of AC-FAME-MAE was as good as DOP. The application of AC-FAME-MAE has higher safety in the food industry based on the results of food simulation fluids experiment.     

The dynamics of glass-forming polymers in the microscopic-mesoscopic time scale. A quasielastic neutron scattering phenomenological approach

The dynamics of three glass-forming polymers, PVC, PB and PI, has been investigated by time of flight (TOF) neutron scattering in a time scale from 10⁻¹³s to 10¹¹s looking for the crossover from microscopic dynamics to segmental dynamics (α relaxation). A new analysis procedure has been applied to TOF data in order to separate harmonic vibrational and relaxational contributions. Due to the involved assumptions, this procedure can be considered only as a first approximation adequate for the case of “fragile” systems (in the Angell's meaning) like the polymers here investigated. The behaviour obtained was the same for the three polymers studied. The intermediate scattering function corresponding to the relaxational dynamics, I_o(Q,t), shows two different dynamical regimes separated by a crossover time t_c (≈ 2 ps), which hardly depends on Q (momentum transfer) and temperature. At t < t<_c, I_o(Q,t) displays a Debye-like behaviour (exponential decay). The activation energy found for the relaxation time corresponding to this regime was in the range of 2–5 Kcal/mol, i.e., in the range of the activation energy for local conformational transitions in isolated macromolecular chains. At t > t_c and, at least at high temperature, I_o(Q,t) shows a Kohlrausch-Williams-Watts (KWW) behaviour similar to the obtained one by means of backscattering neutron techniques in the mesoscopic time scale (10⁻¹¹s to 10⁻⁷s) and dielectric measurements in the macroscopic time scale (10⁻⁷s to 10°s). This KWW regime can be associated to the segmental dynamics involved in the α relaxation. A phenomenological interpretation is outlined. In this framework, the Debye-like regime is interpreted to be the segmental dynamics free from intermolecular hindrances. Therefore, t_c should be the time at which intermolecular interactions start to play a significant role concerning to the segmental dynamics. This interpretation recalls some of the basic ideas of the so called “Coupling-Model” proposed a long time ago by Ngai.     

Phase behavior and local dynamics of concentrated triblock copolymer micelles

We report a neutron-scattering study to characterize the ordering and local dynamics of spherical micelles formed by the triblock copolymer polyethylene oxide (PEO)--polypropylene oxide (PPO)--polyethylene oxide (Pluronic) in aqueous solution. The study focuses on two Pluronic species, F68 and F108, that have the same weight fraction of PEO but that differ in chain length by approximately a factor of 2. At sufficiently high concentration, both species undergo a sequence of phase changes with increasing temperature from dissolved chains to micelles with liquid-like order to a cubic crystal phase and finally back to a micelle liquid phase. A comparison of the phase diagrams constructed from small-angle neutron scattering indicates that crystallization is suppressed for shorter chain micelles due to fluctuation effects. The intermediate scattering function I(Q,t)I(Q,0) determined by neutron spin echo displays a line shape with two distinct relaxations. Comparisons between I(Q,t)I(Q,0) for fully hydrogenated F68 chains in D2O and for F68 with deuterated PEO blocks reveal that the slower relaxation corresponds to Rouse modes of the PPO segments in the concentrated micelle cores. The faster relaxation is identified with longitudinal diffusive modes in the PEO corona characteristic of a polymer brush.