Search results for "dynamics."
showing 10 items of 9637 documents
Molecular Mobility in Oriented and Unoriented Membranes Based on Poly[2-(Aziridin-1-yl)ethanol]
2021
Unoriented and oriented membranes based on dendronized polymers and copolymers obtained by chemical modification of poly[2-(aziridin-1-yl) ethanol] (PAZE) with the dendron 3,4,5-tris[4-(n-dodecan-1-yloxy)benzyloxy]benzoate were considered. DSC, XRD, CP-MAS NMR and DETA, contribute to characterize the tendency to crystallize, the molecular mobility of the benzyloxy substituent, the dendritic liquid crystalline group and the clearing transition. The orientation of the mesogenic chain somewhat hindered this molecular motion, especially in the full substituted PAZE. The fragility, free volume and thermal expansion coefficients of these membranes near the glass transition are related to the orie…
Topology-Sensitive Microfluidic Filter for Polymers of Varying Stiffness
2017
The separation of polymers based on their size, rigidity, and topology is an essential but also highly challenging task for nanoscience and engineering. Using hybrid molecular dynamics simulations that correctly take into account hydrodynamics, we have designed microfluidic channels for separating linear from ring polymers in dilute solutions. We establish that the transport velocity of the polymers is independent of their topology and rigidity when the channel walls are smooth and repulsive. However, when the walls are decorated with attractive spots arranged on lines parallel to the flow, ring polymers exhibit an order of magnitude higher transport velocity compared to linear chains. The …
The glass transition in polymer melts
1994
This paper presents some results of a Monte Carlo simulation for the glass transition in two- and three-dimensional polymer melts. The melt was simulated by the bond-fluctuation model on a d-dimensional cubic lattice which was combined with a two-level hamiltonian favouring long bonds in order to generate a competition between the energetic and topological constraints in the system. This competition prevents crystallization and makes the melt freeze in an amorphous structure as soon as the internal relaxation times match the observation time of the simulation set by the cooling rate. The freezing point of the melt, i.e the glass transition temperature Tg, thus depends upon the cooling rate …
Excess viscosity and glass transition
2001
Abstract Literature data on the viscosity of solutions of poly(butyl methacrylate) (PBMA) and poly(methyl methacrylate) (PMMA) in diethyl phthalate (DEP) for different temperatures, including the range around and below T g , the glass transition temperatures of the pure polymers, were evaluated by means of an approach that uses surface fractions as composition variables. The discussion of these results together with information on solutions of the isomeric polymers, poly(vinyl acetate) (PVAc) and poly(methyl acrylate) (PM(A)), in the same solvent testifies that the previously published relations remain valid for T T g . They enable the determination of viscosities of the pure polymers below…
Crystallization kinetics in relation to polymer processing
1993
Phase distribution of quenched samples has been determined by a deconvolution procedure of WAXS spectra in a wide range of cooling rates. The informations collected together with isothermal and DSC results provide a very wide set of data on the crystallization kinetics of polymers relevant which covers conditions encountered in most polymer processing operations. They have been compared with predictions of a non-isothermal crystallization model assuming two independent and parallel crystallization processes competing during solidification.
Interpolymer complexes and polymer compatibility.
2012
A reliable method to decide whether two polymers A and B are miscible or incompatible would be very helpful in many ways. In this contribution we demonstrate why traditional procedures cannot work. We propose to use the intrinsic viscosities [η] of the polymer blends instead of the composition dependence of the viscosities as a criterion for polymer miscibility. Two macromolecules A and B are miscible because of sufficiently favorable interactions between the two types of polymer segments. For solutions of these polymers in a joint solvent, this Gibbs energetic preference of dissimilar intersegmental contacts should prevail upon dilution and lead to the formation of interpolymer complexes, …
Dynamics of phase separation and critical phenomena in polymer mixtures
1987
The phenomenological mean-field theory for statics and dynamics of polymer mixtures is described, generalizing the approaches of Flory-Huggins, Cahn-Hilliard and de Gennes. Predictions are made for critical behavior, spinodal decomposition and homogeneous nucleation. The validity of the mean-field approximations is discussed with Ginzburg criteria. The results of the theory are compared to computer simulations and recent experiments.
Influence of the cross-linking density on the main dielectric relaxation of poly(methyl acrylate) networks.
2005
A series of polymer networks of varying cross-linking density was prepared by copolymerization of methyl acrylate and ethyleneglycol dimethacrylate. The aim of this work is to study the influence of cross-linking on the conformational mobility of the polymer chains using dielectric relaxation spectroscopy (DRS) in the temperature range of the main dielectric relaxation. As expected, the temperature range in which glass transition takes place became wider with increasing crosslinking density. DRS results were analyzed using the Havriliak-Negami equation. Master Cole-Cole arcs could be drawn for all the networks. The arcs become more symmetric as cross-linking density increases, as a conseque…
Rheological behavior of a semirigid liquid crystal polymer
1993
The rheological behavior of liquid crystal polymers is still far from completely clarified from both experimental and theoretical points of view. In this paper, the shear flow and the non-isothermal elongation flow behavior of a semi-rigid liquid crystal copolyester is discussed. The viscosity strongly decreases when the test temperature is increased above the crystal-nematic transition temperature; below this temperature the viscosity is very high. The thermal history strongly affects the shear viscosity. The elongational flow behavior depends also on temperature. Above the crystal-nematic transition temperature and the mesophase is easily spinnable, whereas below this temperature, the spi…
Dielectric properties of a filled epoxy resin: Effect of thermal treatments
1982
Dielectric measurements have been performed at several frequencies on samples of a cycloaliphatic epoxy resin filled with aluminum hydroxide. The samples were thermally treated at three different temperatures for times up to 2,000 hours. At low aging temperatures and times an improvement of the characteristics is observed due perhaps to an assessment of the bulk of the polymer and more precisely to a change in the free volume. At high temperatures and times a thermo-oxidation involving layers deeper and deeper becomes the predominant mechanism responsible of the decrease in the properties with respect to the virgin material.