Search results for "Phase Transition"

Processing and adjusting the hydrophilicity of poly(oxymethylene) (co)polymers: nanoparticle preparation and film formation

2016

Handling the insoluble POM: the preparation of nanoparticles based on hyperbranched-linear-hyperbranched ABA triblock copolymers with variable hydrophilicity and composed of short hyperbranched polyglycerol (hbPG) as the A-blocks and linear poly(oxymethylene) (POM) as a B-block is described. The POM-hbPG-nanoparticles with diameters in the range of 190 to 250 nm were generated in a convenient process, combining the solvent evaporation process with the miniemulsion technique, a water borne handling for POM-copolymers. Furthermore, the film formation properties of the nanoparticles were investigated by deposition on silicon and subsequent sintering, which leads to films with a thickness in th…

Phase transitions in polymeric systems: A challenge for Monte Carlo simulation

1995

Polymers are more difficult to simulate than small molecule systems, due to the large size of random polymer coils (and their slow relaxation, that is observed when dynamic simulation algorithms are used). However, variation of the chain length N of a flexible polymer chain provides a very useful additional control parameter, allowing stringent tests of theories, and new physical phenomena may emerge. As an example of these concepts, critical phenomena in polymer mixtures are described. It is shown that unmixing of symmetrical mixtures ( N A = N B = N ) is described by an equation for the critical temperature T c ( N ) = aN + b rather than T c ∝ N as claimed by some theories. While for fini…

On the phase transition in CsCN

1983

X-ray diffraction and dielectric measurements have been performed on the molecular crystal CsCN as a function of temperature. The order parameter of the cubic to rhombohedral phase transition (Tc=186 K) was determined and interpreted by a coupling of the (CN)− orientations to the shear strain. At lower temperatures the dielectric response is dominated by thermally activated relaxations processes rather than by electrical ordering of the CN dipoles.

Collapse of Cylindrical Brushes with 2-Isopropyloxazoline Side Chains Close to the Phase Boundary

2013

A high-molar-mass cylindrical brush polymer with a main chain degree of polymerization of Pw = 1047 is synthesized by free-radical polymerization of a poly-2-isopropyloxazoline macromonomer with Pn = 28. The polymerization is conducted above the lower phase transition temperature of the macromonomer, i.e., in the phase-separated regime, which provides a sufficiently concentrated macromonomer phase mandatory to obtain high-molar-mass cylindrical brushes. Upon heating to the phase transition temperature, the hydrodynamic radius is observed to shrink from 34 to 27 nm. Further increase in temperature resulted in aggregated chains which were observed to coexist with single chains until eventuall…

Phase Transition of SBN: Ce Studied by Anisotropic Holographic Scattering

1991

Light scattering phenomena in (Sr0.61Ba0.39) Nb2O6:Ce are investigated from room temperature to the phase transition temperature. Temperature dependence and sign of the birefringence are determined from anisotropic holographic scattering at a wavelength of 514.5 nm. The majority charge carriers are electrons. Zwischen Zimmertemperatur und der Phasenubergangstemperatur werden Lichtstreuphanomene in (Sr0,61Ba0,39)Nb2O6:Ce untersucht. Aus der anisotropen holographischen Streuung bei einer Wellenlange von 514,5 nm werden Temperaturabhangigkeit und Vorzeichen der Doppelbrechung bestimmt. Die Majoritatsladungstrager sind Elektronen.

1984

Influence de la structure de chaine sur le type de phase et relation entre la masse moleculaire et les temperatures de transition

Molecular motion, thermal expansion, and phase transitions in paraffins: A model for polymers

2007

Owing to their simple chemical structure paraffins are generally regarded as representing a system which is especially suitable for a model study of the thermal behavior of oligomeric and polymeric solids. Extensive investigations on the properties of n-tritriacontane (C33H68) provide an example. For this system an identification of several types of motional mechanisms become possible. Their presence leads to peculiarities in the thermal expansion and to the occurrence of solid-solid phase transitions. Paraffins and polyethylene exhibit a marked anisotropy in the lateral thermal expansion. An analysis of the molecular background can be based on a modified Gruneisen theory which connects exp…

Effect of the crystallization on the phase transitions IV ⇐ III and IV ⇐ II of ammonium nitrate

1993

Abstract The solid state phase transition path of ammonium nitrate is influenced by the manner of crystallization. Slow crystallization from water gives ideal crystals, which behaved differently from crystals prepared by rapid crystallization from the melt.

Freely-Suspended and Transferred Freely-Suspended Films of Polymeric Liquid Crystals

1993

Freely-suspended (FS) liquid-crystal (LC) films of a polymethylsiloxane homo- and a copolymer with different mesogenic side groups have been prepared and investigated. For both substances the preparation of the films succeeded only in the isotropic phase but the higher ordered phases were reached after cooling. The films of the homopolymer have only been obtained with thicknesses estimated to be in the pm-range and with an unusual “Book-Shelf” orientation of the smectic layers. In contrast, the films of the copolymer were thinner and showed a homeotropic orientation of the mesogens in the smectic A phase. Interestingly, this material exhibited a different layer spacing in film and bulk, alt…

Phase Separation of Colloid Polymer Mixtures Under Confinement

2013

Colloid polymer mixtures exhibit vapor-liquid like and liquid-solid like phase transitions in bulk suspensions, and are well-suited model systems to explore confinement effects on these phase transitions. Static aspects of these phenomena are studied by large-scale Monte Carlo simulations, including novel “ensemble switch” methods to estimate excess free energies due to confining walls. The kinetics of phase separation is investigated by a Molecular Dynamics method, where hydrodynamic effects due to the solvent are included via the multiparticle collision dynamics method.