Search results for "Self-diffusion"
showing 10 items of 34 documents
Computer simulation of the glass transition of polymer melts
2007
Bond fluctuation models on square and simple cubic lattices at melt densities are simulated, using potentials depending on the length of the (effective) bond (and also on the bond angle, in d=3 dimensions). Various relaxation functions have the Kohlrausch-Williams-Watts (KWW) form; the associated relaxation time diverges as exp (const/T 2) in d=2 and as exp [const/T−T 0)] in d=3. For d=3 the self-diffusion constant also follows the Vogel-Fulcher law, with T 0=250 K for chain lengths N=20 and potentials adapted to bisphenol-A-polycarbonate [BPA-PC].
Heptane adsorption in silicalite-1 : molecular dynamics simulation.
2009
Molecular dynamics (MD) simulations have been used to study the adsorption process of n-heptane molecules in silicalite-1 at 300 K. MD simulated results were compared to experimental neutron diffraction (ND) and experimental self-diffusion coefficients. The analysis of MD data indicated a packing of the adsorbed molecules around 4 mol./u.c., which is not the consequence of an enthalpic effect but of an entropic effect. The role of the n-heptane chain flexibility (cis–trans conformation) in relation with the silicalite-1 channel type (straight versus sinusoidal) was outlined and enabled to understand the mobility change arising at 4 mol./u.c., according to previous experimental results. The …
Dynamics of Copolymer Micelles in a Homopolymer Melt: Influence of the Matrix Molecular Weight
2000
We have studied the dynamics of styrene−isoprene (SI) block copolymer micelles in a matrix of linear entangled polyisoprene (PI) chains, using forced Rayleigh scattering (FRS) and dynamic mechanica...
Statics and Dynamics of Bidisperse Polymer Melts: A Monte Carlo Study of the Bond-Fluctuation Model
1998
As a first step toward the computer simulation of polydisperse polymeric melts, a lattice model containing two types of chains with lengths N1 = 20 − x and N2 = 20 + 4x (0 ≤ x ≤ 10 ) is studied. This variation of x, together with the fixed composition of 80% of short and 20% of long chains, leads to a polydispersity of 1 ≤ Nw/Nn ≤ 2 (Nw, Nn: weight-, number-average chain lengths). To represent dense melts, the bond-fluctuation model at a volume fraction, φ = 1/2, of occupied lattice sites is used. The simulation treats both the athermal case (chain connectivity and excluded volume interaction only) and a thermal case, where additionally a choice for the bond length and bond angle potential…
1,3,5-triisopropylbenzene diffusion in polystyrene solutions
1996
Self-diffusion coefficients of 1,3,5-triisopropylbenzene (TIB) in binary solutions with polystyrene and ternary mixtures comprised of TIB, toluene, and polystyrene have been measured by static gradient nuclear magnetic resonance (SG-NMR) techniques. These data, as well as mutual-diffusion coefficient data measured by capillary column inverse gas chromatography (CCIGC), have been analyzed here with the Vrentas-Duda free-volume diffusion model. Although both binary and ternary diffusion coefficient data can be accurately correlated with the model, the results contradict those of an earlier investigation, which suggested that TIB diffuses as a single unit in polystyrene solutions. The new data…
Analysis of a parabolic cross-diffusion population model without self-diffusion
2006
Abstract The global existence of non-negative weak solutions to a strongly coupled parabolic system arising in population dynamics is shown. The cross-diffusion terms are allowed to be arbitrarily large, whereas the self-diffusion terms are assumed to disappear. The last assumption complicates the analysis since these terms usually provide H 1 estimates of the solutions. The existence proof is based on a positivity-preserving backward Euler–Galerkin approximation, discrete entropy estimates, and L 1 weak compactness arguments. Furthermore, employing the entropy–entropy production method, we show for special stationary solutions that the transient solution converges exponentially fast to its…
Development of core-shell colloids to study self-diffusion in highly concentrated dispersions
2007
To study single particle motion in highly concentrated colloidal dispersions, a host-tracer colloid system was developed, consisting of crosslinked polymer micronetwork spheres placed in a good solvent. The host colloid is made invisible to the experimental probe by matching its refractive index to that of the solvent. For the tracer particles a core-shell structure was chosen to ensure the interaction potential to be identical to that of the host particles. Therefore the shell was made of the same polymer as the host. The core differs in refractive index from the solvent and is therefore visible due to scattered light.
Toluene self-diffusion in solutions of linear and crosslinked polystyrene
1995
Toluene self-diffusion coefficients in solutions of linear and crosslinked polystyrene (PS) at concentrations up to 30 wt% polymer have been determined over the temperature range 143–230 K using the1H n.m.r. static gradient stimulated echo method. At similar polymer concentrations, the diffusion coefficient values of toluene were higher for mixtures with crosslinked PS than for solutions containing linear PS. On cooling and approaching the solution glass transition, the amplitude decays in diffusion and spin-relaxation measurements become non-exponential and are interpreted in terms of a two-component decomposition. The estimated fraction of the toluene motionally bound to polymer increases…
Self-diffusion in polymer solutions using the bond-fluctuation MC-algorithm
1991
Abstract A lattice Monte Carlo study of the self-diffusion of polymer chains in an athermal solution of equal chains is presented. The examined chain lengths, N (= 20–200), and volume fractions, φ (= 0.025-0.5), cover the range from dilute solution to concentrated solution, respectively. The dynamics show a gradual crossover from Rouse to reptation-like behaviour. Analysing the data according to a scaling theory and taking into account the density dependence of the microscopic length and time-scales, an almost perfect scaling of the self-diffusion coefficient is achieved. The high statistical accuracy of the data (103–104 chains per parameter combination) was obtainable by using a transpute…
NMR Matrix Technique for Detecting Self Diffusion in Polymers
1979
A technique is discussed where monodisperse small spheres or cylindrical fibers of a polymer are embedded in a matrix of the corresponding fully deuterated polymer. On annealing the sample at a temperature above the glass transition the protonated macromolecules diffuse into the deuterated matrix and vice versa. Since the NMR spectrum depends upon the intermolecular proton proton dipolar coupling, the decrease of this coupling can be used to monitor the diffusion process. For particle diameters d ∼ 10−4 cm and annealing times t ∼ 105 s, extremely small diffusion constants D < d2/t are attainable. The NMR analysis of prospective diffusion experiments with polystyrene is discussed in some det…