Polymer cooperative dynamics

Instabilities in UTR polymer films are manifested in two main ways, namely, (i) defects resulting from the coating process, substrate nonuniformities, and conjoining pressure, and (ii) discontinuihes in the thermophysical properties of the hlms due to interfacial effects and polymer cooperative and surface dynamics. 

Instabilities in UTR films can also be manifested as discontinuities in the thermophysical properties of the films due to interfacial effects and polymer cooperative and surface dynamics. Polymer surfaces are regions of enhanced molecular mobility as compared to the bulk, given the decreased constraints on macromolecules at a free surface. The orientation of the surface groups is affected by the nature of the interfacing environment. This implies that polymeric surfaces can restrucmre (in terms of orientation of surface functionalities, concentration of surface groups, etc.) in response to a change in the interfacial phase, in order to adjust their surface properties to the properties of the interfacial medium. °... 

J.A. Forrest and J. Mattson, Reductions of the glass transition temperature in thin polymer films Probing the length scale of cooperative dynamics, Phys. Rev. E. 61, R53 (1999). 

In the previous section, it was supposed that the anomalous cooperative dynamics in thin films compared to that of bulk polymer is related to the singular glass transition behavior in thin films. In order to clarify the above idea, it is better to investigate the cooperative dynamics or CRR in thin films directly however, concrete theoretical and experimental approaches have not been presented up to now. Evaluation of the distribution of Tg in thin films would be a good starting point for accomplishing such a difficult task. 

The reason why Aoop is given by Eq. 4.43 is as follows. In Section 3.2.7, we learned that the hydrodynamic radius of a linear chain polymer is given as the reciprocal of the average of where r is the distance between two monomers on the chain. We used the definition to estimate Re for a chain with a Gaussian chain conformation. We can use the same formula to calculate D oop for the cooperative dynamic mode of the blob. It is given by... 

Similarly, a strong influence of hydrodynamic interactions has been found on the polymer translocation dynamics through a small hole in a wall [125] or in polymer packing in a virus capsid [126,127]. Cooperative backflow effects lead to a rather sharp distribution of translocation times with a peak at relatively short times. The fluid flow field, which is created as a monomer moves through the hole, guides following monomers to move in the same direction. 

According to the coupling model, for neat polymers at the times appropriate for most experimental measurements, the slowing down of segmental relaxation gives rise to a correlation function having the form of equation (1). The stretch exponent is a measure of the strength of the intermolecular constraints on the relaxation. These constraints depend on molecular structure because the chemical structure determines the intermolecular interactions. However, the complexity of cooperative dynamics in dense liquids and polymers precludes direct calculation of P it is invariably deduced from experiment. An assumption fundamental to the model is that the time at which intermolecular cooperativity effects become manifest is independent of temperature. 

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

We have identified three diffusion coefficients. These are the self-translational diffusion coefficient D, cooperative diffusion coefficient Dc, and the coupled diffussion coefficient fly. fl is the cooperative diffusion coefficient in the absence of any electrostatic coupling between polyelectrolyte and other ions in the system, fly is the cooperative diffusion coefficient accounting for the coupling between various ions. For neutral polymers, fly and Dc are identical. Furthermore, we identify fly as the fast diffusion coefficient as measured in dynamic light scattering experiments. The fourth diffusion coefficient is the slow diffusion coefficient fl discussed in the Introduction. A satisfactory theory of flj is not yet available. 

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