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Chain Collapse (chain + collapse)

Distribution by Scientific Domains
Polymers and Materials Science100%
Distribution within Polymers and Materials Science
Polymer Science & Technology General75%
General & Introductory Materials Science25%

Selected Abstracts

Polymer Chain Collapse in Supercritical Fluids.

MACROMOLECULAR SYMPOSIA, Issue 1 2009

Abstract A few years ago we reported the first observation, by computer simulations, of polymer chain collapse near the lower critical solution temperature (LCST).1 In the present work, we extended the above study to understand the underlying physics of a single polymer chain collapse near LCST and its relationship to phase boundaries in the T-x plane. Effects of solvent and monomer sizes, and solvent and monomer energetic interactions are studied. Using Monte Carlo simulations, the mean end-to-end distance (R) and gyration radius (Rg) are calculated for a single chain in a supercritical fluid solvent over a broad range of densities, pressures and temperatures. In general, the chain collapses as temperature increases at constant pressure. Upon a further temperature increase, the chain expands again to approach the athermal limit provided that the temperature is sufficiently high. The collapse is related to an LCST phase boundary while the expansion represents the signature of an upper-critical solution temperature (UCST) suggesting the existence of a closed-immiscibility loop. By manipulating the strength of the energetic interactions as well as the solvent-to-monomer size ratio, the size of the size of the immiscibility loop can be fine-tuned. The relationship among size and the segment-solvent energetic interaction are correlated by a conformational parameter (,) for the first time. By monitoring the , behavior, it is possible to predict solution's phase behavior, transition zone from LCST-UCST in a closed-loop miscibility behavior. The above relationship between chain conformation to phase boundaries may be useful in understanding phase stability in compressible polymer-solvent mixtures. [source]

Polymer Chain Collapse in Supercritical Fluids.

MACROMOLECULAR SYMPOSIA, Issue 1 2009

Abstract The phase behavior of a polymer in a supercritical solvent at the LCST equilibrium limits is described in this work, in the proximity of , point, proposing the use of a conformational parameter, ,. The results obtained by molecular simulation in an NVT ensemble have been correlated by extensive, varied experimental information. The relationship between polymer/solvent solubility parameters has shown that the behavior of these systems is a function of the energetic structure-interaction relationship between the polymer chain and the solvent. , results in a generalized parameter indicative of the phase stability of the solution. At greater magnitudes, the solution becomes unstable, requiring elevated pressure to stabilize. However, stable solutions are found at lower pressures when , approaches 1. The experimental evidence, together with the determination of the solubility parameter with the Sanchez-Lacombe equation (also obtained from the literature) strengthens this observation. The analysis of the polar contribution on the Hansen Parameter (HSP) enables their effect to be studied in systems where high polar interactions between the polymer and solvent (as in the case of biopolymers) are expected. [source]

The Influence of Alkyl-Chain Length on Beta-Phase Formation in Polyfluorenes

ADVANCED FUNCTIONAL MATERIALS, Issue 1 2009
Daniel W. Bright
Abstract Di- n -alkyl substituted polyfluorenes with alkyl chain lengths of 6, 7, 8, 9, and 10 carbon atoms (PF6, PF7, PF8, PF9, and PF10) are studied in dilute solution in MCH using optical spectroscopy. Beta-phase is formed upon cooling in solutions (, 7,µg mL,1) of PF7, PF8, and PF9 only, which is observed as an equilibrium absorption peak at , 437,nm and strong changes in the emission spectra. Beta-phase is formed upon thermal cycling to low temperature in solutions (,7,µg mL,1) of PF7, PF8, and PF9, which is observed as an equilibrium absorption peak at , 437,nm and strong changes in the emission spectra. Beta phase is found to occur more favorably in PF8 than in PF7 or PF9, which is attributed to a balance between two factors. The first is the dimer/aggregate formation efficiency, which is poorer for longer (more disordered) alkyl chain lengths, and the second is the Van der Waals bond energy available to overcome the steric repulsion and planarize the conjugated backbone, which is insufficient in the PF6 with a shorter alkyl chain. Beta phase formation is shown to be a result of aggregation, not a precursor to it. A tentative value of the energy required to planarize the fluorene backbone of (15.6,±,2.5) kJ mol,1 monomer is suggested. Excitation spectra of PF6, PF7, PF8, and PF9 in extremely dilute (, 10,ng mL,1) solution show that beta phase can form reversibly in dilute solutions of PF7, PF8 and PF9, which is believed to be a result of chain collapse or well dispersed aggregates being present in solution from dilution of more concentrated solutions. PF7, PF8, and PF9 also form beta phase in thermally cycled solid films spin-cast from MCH. However, in the films the PF7 formed a larger fraction of beta phase than the PF9, in contrast to the case in solutions, because it is less likely that the close-packed chains in the solid state will allow the formation of planarized chains with the longer PF9 side chains. [source]

Biodegradable Thermoresponsive Microparticle Dispersions for Injectable Cell Delivery Prepared Using a Single-Step Process

ADVANCED MATERIALS, Issue 18 2009
Wenxin Wang
Surface-engineered microparticles with a biodegradable polymer core and a programmable thermoresponsive biocompatible copolymer corona are produced. The particles form free-flowing dispersions below 37,°C, but form porous space-filling gels above this temperature, as a result of chain collapse of the copolymer corona. When particles are mixed with biological materials, they form encapsulating gels that can support cell growth. [source]

Polymer Chain Collapse in Supercritical Fluids.

MACROMOLECULAR SYMPOSIA, Issue 1 2009

Abstract A few years ago we reported the first observation, by computer simulations, of polymer chain collapse near the lower critical solution temperature (LCST).1 In the present work, we extended the above study to understand the underlying physics of a single polymer chain collapse near LCST and its relationship to phase boundaries in the T-x plane. Effects of solvent and monomer sizes, and solvent and monomer energetic interactions are studied. Using Monte Carlo simulations, the mean end-to-end distance (R) and gyration radius (Rg) are calculated for a single chain in a supercritical fluid solvent over a broad range of densities, pressures and temperatures. In general, the chain collapses as temperature increases at constant pressure. Upon a further temperature increase, the chain expands again to approach the athermal limit provided that the temperature is sufficiently high. The collapse is related to an LCST phase boundary while the expansion represents the signature of an upper-critical solution temperature (UCST) suggesting the existence of a closed-immiscibility loop. By manipulating the strength of the energetic interactions as well as the solvent-to-monomer size ratio, the size of the size of the immiscibility loop can be fine-tuned. The relationship among size and the segment-solvent energetic interaction are correlated by a conformational parameter (,) for the first time. By monitoring the , behavior, it is possible to predict solution's phase behavior, transition zone from LCST-UCST in a closed-loop miscibility behavior. The above relationship between chain conformation to phase boundaries may be useful in understanding phase stability in compressible polymer-solvent mixtures. [source]

AB-Block Copolymer with Moving B Blocks as a Model for Interpolymer Complexes

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 5 2010
Olga S. Pevnaya
Abstract The conformational behavior of a single AB block copolymer is studied by Monte Carlo simulation. The A-A and A-B interactions have the character of excluded volume interactions while the B units attract each other; the attractive B blocks can move along the chain. The collapse transition of the chain with increasing attraction between the B units is analyzed. Intrachain separation of the A and B units takes place in the course of the chain collapse with the formation of "globule with a tail" conformations. The globule is formed by the attractive moving B blocks while the tail consists of the swollen A segments. The model of AB block copolymer with moving B blocks can describe the behavior of interpolymer complexes between a long macromolecule and shorter polymer chains. [source]

Polymer Chain Collapse in Supercritical Fluids.

MACROMOLECULAR SYMPOSIA, Issue 1 2009

Abstract A few years ago we reported the first observation, by computer simulations, of polymer chain collapse near the lower critical solution temperature (LCST).1 In the present work, we extended the above study to understand the underlying physics of a single polymer chain collapse near LCST and its relationship to phase boundaries in the T-x plane. Effects of solvent and monomer sizes, and solvent and monomer energetic interactions are studied. Using Monte Carlo simulations, the mean end-to-end distance (R) and gyration radius (Rg) are calculated for a single chain in a supercritical fluid solvent over a broad range of densities, pressures and temperatures. In general, the chain collapses as temperature increases at constant pressure. Upon a further temperature increase, the chain expands again to approach the athermal limit provided that the temperature is sufficiently high. The collapse is related to an LCST phase boundary while the expansion represents the signature of an upper-critical solution temperature (UCST) suggesting the existence of a closed-immiscibility loop. By manipulating the strength of the energetic interactions as well as the solvent-to-monomer size ratio, the size of the size of the immiscibility loop can be fine-tuned. The relationship among size and the segment-solvent energetic interaction are correlated by a conformational parameter (,) for the first time. By monitoring the , behavior, it is possible to predict solution's phase behavior, transition zone from LCST-UCST in a closed-loop miscibility behavior. The above relationship between chain conformation to phase boundaries may be useful in understanding phase stability in compressible polymer-solvent mixtures. [source]

Nanometer-Scaled Hollow Spherical Micelles with Hydrophilic Channels and the Controlled Release of Ibuprofen

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 23 2008
De'an Xiong
Abstract PS- b -PAA spherical micelles with a liquid core and a PAA shell are prepared with the assistance of 1,2-dichloroethane. During the process of adding a mixture of PNIPAM- b -P4VP and PEG- b -P4VP, multi-layered micelles with a mixed corona that consists of both PNIPAM and PEG chains are constructed through the electrostatic interaction and hydrogen bonding between the PAA block and the P4VP block. When heating above the LCST, the PNIPAM chains collapse onto the PAA/P4VP complex layer while the PEG chains still stretch into the solution through the collapsed PNIPAM layer, which leads to the formation of hydrophilic channels around the PEG chains. The ibuprofen encapsulated in the hollow space can diffuse through the channels and its release rate can be controlled by changing the ratio of PEG chains to PNIPAM chains in the corona. [source]