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Polymer Density (polymer + density)

Distribution by Scientific Domains

Polymers and Materials Science	100%

Selected Abstracts

End-Anchored Polymers: Compression by Different Mechanisms and Interpenetration of Apposing Layers

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 2 2005
Mark D. Whitmore
Abstract Summary: This paper presents a systematic study of the compression of end-anchored polymer layers by a variety of mechanisms. We treat layers in both good and , solvents, and in the range of polymer densities that is normally encountered in experiments. Our primary technique is numerical self-consistent field (NSCF) theory. We compare the NSCF results for the different mechanisms with each other, and with those of the analytic SCF theory. For each mechanism, we calculate the density profiles, layer thicknesses, and free energies, all as functions of the degree of polymerization and surface coverage. The free energy and the deformation of each layer depend on the compression mechanism, and they can be very different from the ASCF theory. For example, the energy of compression can be as much as three times greater than the analytical SCF (ASCF) prediction, and it does not reduce to simple, universal functions of the reduced distance between the surfaces. The overall physical picture simplifies if the free energy is expressed in terms of the layer deformation, rather than the reduced surface separation. We also examine and quantify the interpenetration of layers, discuss why ASCF theory applies better to some compression mechanisms than others, and end with comments on the difficulties in extracting quantitative information from surface-forces experiments. Comparisons of forces of compression in a good solvent for the three different systems, as functions of D/nb. The lower three curves are for ,*,=,3, and the upper three are for ,*,=,23. [source]

Tunable Hydrogels for External Manipulation of Cellular Microenvironments through Controlled Photodegradation

ADVANCED MATERIALS, Issue 1 2010
April M. Kloxin
A photocleavable, poly(ethylene-glycol)-based hydrogel is presented in which predictable, user-defined gradients in the network's structure can be fabricated in real time under cytocompatible conditions. This platform provides new opportunities to investigate how material structure influences cell function. Here, cell morphology is directed spatially by degradation-induced gradients in the local polymer density (see figure, scale bars: 50 ,m). [source]

Continuous Soluble Ziegler-Natta Ethylene Polymerizations in Reactor Trains, 3 , Influence of Operating Conditions upon Process Performance

MACROMOLECULAR REACTION ENGINEERING, Issue 2 2008
Marcelo Embiruçu
Abstract The behavior of continuous solution ethylene/but-1-ene polymerizations through Ziegler-Natta catalysts is analyzed, based on a previously developed mathematical model. In order to do that, dynamic simulations are carried out and process responses are analyzed as functions of process operating policies and flowsheet configuration, at conditions that resemble the actual operation of industrial sites. It is shown that system responses are highly nonlinear and very sensitive to disturbances of the operating conditions and that catalyst decay is of fundamental importance for proper understanding of process behavior. Results indicate that mixing conditions inside the reactor vessels exert a significant impact upon the final polymer quality and can be manipulated for in-line control of final resin properties. Finally, it is shown that the development of feed policies, based on the use of lateral feed streams, allows the simultaneous control of melt flow index, stress exponent and polymer density of the final polymer resin. [source]

Isotactic polypropylene solidification under pressure and high cooling rates.

POLYMER ENGINEERING & SCIENCE, Issue 11 2000
A master curve approach
Solidification in industrial processes very often involves flow fields, high thermal gradients and high pressures: the development of a model able to describe the polymer behavior becomes complex. Recently a new equipment has been developed and improved to study the crystallization of polymers when quenched under pressure. An experimental apparatus based on a modified, special injection moulding machine has been employed. Polymer samples can be cooled at a known cooling rate up to 100°C/s and under a constant pressure up to 40 MPa. Density, Micro Hardness (MH), Wide angle X-ray diffraction (WAXD), and annealing measurements were then used to characterize the obtained sample morphology. Results on one iPP sample display a lower density and a lower density dependence on cooling rate for increasing pressure. Micro hardness confirms the same trend. A deconvolution technique of WAXD patterns is used to evaluate the final phase content of samples and to assess a crystallization kinetics behavior. A master curve approach to explain iPP behavior under pressure and high cooling rates was successfully applied on density results. On the basis of this simple model it is possible to predict the final polymer density by superposition of the effect of cooling rate and the effect of pressure in a wide range of experimental conditions. [source]