Polymers transport

The hydrodynamic radius reflects the effect of coil size on polymer transport properties and can be determined from the sedimentation or diffusion coefficients at infinite dilution from the relation Rh = kBT/6itri5D (D = translational diffusion coefficient extrapolated to zero concentration, kB = Boltzmann constant, T = absolute temperature and r s = solvent viscosity). 

The dynamical properties of polymer molecules in solution have been investigated using MPC dynamics [75-77]. Polymer transport properties are strongly influenced by hydrodynamic interactions. These effects manifest themselves in both the center-of-mass diffusion coefficients and the dynamic structure factors of polymer molecules in solution. For example, if hydrodynamic interactions are neglected, the diffusion coefficient scales with the number of monomers as D Dq /Nb, where Do is the diffusion coefficient of a polymer bead and N), is the number of beads in the polymer. If hydrodynamic interactions are included, the diffusion coefficient adopts a Stokes-Einstein formD kltT/cnr NlJ2, where c is a factor that depends on the polymer chain model. This scaling has been confirmed in MPC simulations of the polymer dynamics [75]. 

Baas, P. W. and Brown, A. Slow axonal transport the polymer transport model. Trends Cell Biol. 7 380-384,1997. 

Polymer transport in ternary systems including an analysis of the cross diffusion coefficients and component distribution within the systems. 

Detailed discussion of measurement of and factors involved in rapid polymer transport in multicomponent systems. 

Identification of structured flows associated with rapid polymer transport and some simple mechanistic interpretations. 

Rapid polymer transport and model biological systems. 

Experiments on Rapid Polymer Transport Carried out on a Standard... 

Identification of Structured Flows Associated with Rapid Polymer Transport 134... 

Whereas polymer transport in dilute solution has been widely studied, investigations of polymer transport in concentrated solutions have been reported only infrequently. When solutions of high molecular weight polymers are concentrated in a way that intermolecular interactions between individual molecules occur, it becomes increasingly evident that both the static and dynamic properties of the macromolecules may be markedly altered compared to their behaviour in dilute solution l). 

Polymer Transport in Binary Systems 2.1 Basic Theoretical Considerations... 

In view of the highly unusual nature of these results and the lack of a routine method for transport measurements unambiguously establishing that rapid transport was indeed a real manifestation of the system, our studies on rapid polymer transport remained unreported in detail. However, in a recent article 46> we have demonstrated that rapid polymer transport actually occurs in these systems due to the formation of ordered macroscopic structures which move rapidly. This rapid transport has been shown to be not the result of bulk convection since normal diffusional kinetics was observed for solvent markers such as [l4C]sorbitol. The striking feature of this new type of transport process is that it is accompanied by ordered structured flows in the... 

To study the nature of this rapid polymer transport in detail, this section will be concerned with a series of experimental measurements on one particular system, namely a solution of dextran T10 (N5W 10 ) with a uniform concentration of 135 kg m-3 and an imposed gradient of a linear, flexible polyvinylpyrrolidone) (NTn 3 x 10s) (PVP 360). This gradient initially extended from 5 kg m3 to zero concentration. The choice of using the polymers at this concentration was based upon our earlier work441 in which it was shown that nearly maximal transport rates of PVP 360 occur in such a system. This system will be referred to as the standard system. The phase diagram of this PVP 360/dextran T10 mixture clearly demonstrates that the transport experiments were performed within the one-phase region 47). 

Limited studies would suggest that, by decreasing the diameter of the initial boundary to 1 mm or less, the rate of polymer transport decreases. On the other hand, increasing the length of the cylindrical transport cell had no effect51 K... 

In accordance with theoretical predictions of the dynamic properties of networks, the critical concentration of dextran appears to be independent of the molecular weight of the flexible polymeric diffusant although some differences are noted when the behaviour of the flexible polymers used is compared e.g. the critical dextran concentrations are lower for PEG than for PVP and PVA. For ternary polymer systems, as studied here, the requirement of a critical concentration that corresponds to the molecular dimensions of the dextran matrix is an experimental feature which appears to be critical for the onset of rapid polymer transport. It is noteworthy that an unambiguous experimental identification of a critical concentration associated with the transport of these types of polymers in solution in relation to the onset of polymer network formation has not been reported so far. Indeed, our studies on the diffusion of dextran in binary (polymer/solvent) systems demonstrated that both its mutual and intradiffusion coefficients vary continuously with increasing concentration 2. ... 

The inclusion of low molecular weight solutes in the lower compartment of the standard system to supplement density stabilization does generally not affect the rapid transport of [3H]PVP. However, this has not been studied systematically. When using 2 mol dm-3 NaCl or 10 kg m 3 sorbitol in the lower solution, no effect on polymer transport is observed 50-52). Similarly, the use of 100% DzO in the lower solution does not effect transport46,50). 

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