Polymer mobility, dynamics

Photophysical Studies of Molecular Mobility in Polymer Films and Bulk Polymers. 3. Dynamic Excimer Formation of Pyrene in Bulk PDMS, Macromolecules 23, 2217-2222. 

For the investigation of the molecular dynamics in polymers, deuteron solid-state nuclear magnetic resonance (2D-NMR) spectroscopy has been shown to be a powerful method [1]. In the field of viscoelastic polymers, segmental dynamics of poly(urethanes) has been studied intensively by 2D-NMR [78, 79]. In addition to ID NMR spectroscopy, 2D NMR exchange spectroscopy was used to extend the time scale of molecular dynamics up to the order of milliseconds or even seconds. In combination with line-shape simulation, this technique allows one to obtain correlation times and correlation-time distributions of the molecular mobility as well as detailed information about the geometry of the motional process [1]. 

The electrochemical quartz crystal microbalance is a versatile technique for studying several aspects of electroactive polymer film dynamics. For rigid films, it is a sensitive probe of mobile species (ion and solvent) population changes within the film in response to redox switching. For non-rigid films, it can be used to determine film shear moduli. In the former case, one simply follows changes in crystal resonant frequency. In the latter case, the frequency dependence of resonator admittance in the... 

Akcasu et al. [74] attempted to identify the fast and slow modes with the two modes observed in dynamic scattering experiments from ternary polymer solutions. They defined the vacancies as the third component in a mixture of A and B polymers and concluded that the slow mode was obtained when vacancies were gradually removed, resulting in an incompressible binary mixture of A and B. The fast mode was obtained in the opposite limit of high vacancy concentration or a matrix with very high mobility. Since the polymer mobility and the vacancy concentration are small below, and high above, Tg, this suggested that the slow and fast-mode theories described interdiffusion below and above Tg, respectively. 

In addition to its structure, the dynamics of the interphase are extremely important. Studies on poly(styrene) have shown not only that there exists significant polymer mobility below Tg, but that these nanocomposites display a large range of mobilities both above and below Tg, indicating a large range of distinctly different polymer environments. This, in turn, points to fundamentally different matrix mobility near the silicate surface. Similar results have also been found in poly(methylphenylsiloxane) nanocompos-ites, while a number of other studies on a variety of systems have reached similar conclusions. ... 

For the tyrosine-derived polycarbonates tested, the enthalpy relaxation process was not sensitive to the length of the pendent chain. This obser ation suggests that structural relaxation in these polymers is limited by backbone flexibility, and that the fraction of free volume in these polymers is not the limiting factor for polymer mobility. Furthermore, since the enthalpy relaxation time is short at aging temperatures of Tg-15°C, a few hours of storage at that temperature will be sufficient to bring the physical aging process to completion. The results obtained by dynamic... 

Another example of the type of information that can be extracted from electrokinetic data is the hydro-dynamic thickness of ion-penetrable surface layers, e.g. surface-bound, neutral, hydrophilic polymers such as polyethers and polysaccharides (11). Surface-bound, neutral, hydrophilic polymers are known to dramatically reduce protein adsorption. The passivity of these surfaces has been attributed to steric repulsion, bound water, high polymer mobility, and excluded volume effects, all of which render adsorption unfavourable. Consequently, these polymer-modified surfaces have proven useful as biomaterials. Specific applications include artificial implants, intraocular and contact lenses, and catheters. Additionally, the inherent non-denaturing properties of these compounds has led to their use as effective tethers for affinity ligands, surface-bound biochemical assays and biosensors. 

In previous chapters, a range of experiments on aspects of polymer solution dynamics, from electrophoretic mobility to single-chain diffusion to linear viscoelasticity, has been treated(l). The previous chapter described results that were found with each method. What do these types of measurement tell us about how polymer molecules move through solution The answers to this question come in a substantial number of parts and pieces, best treated separately before being assembled into final conclusions. There are undoubtedly other parts and pieces that might have been discussed, such as the consequences of changing the relative size of matrix and probe polymers, or the consequences of polymer topology. This chapter stays with answers most central to our purpose. 

Since most of the polymer properties significant to technological applications must depend on molecular motions present in the material, detailed studies of molecular dynamics of the polymer chains are inevitably very important. In order to understand the molecular dynamics of polymer chains in bulk, one customarily begins with estimation of the intrinsic flexibility of the chain. This can be done best when the intramolecular effects are isolated from the interchain forces and effects. For this purpose, studies are performed on polymers dissolved in carefully chosen solvents. Apart from solvation effects, this offers the best available approximation of the free chain. Knowing the intramolecular dynamics of the isolated chain, one can move to investigate polymer mobility in a bulk sample, in particular to clarify interchain phenomena. 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

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