Molecular motion in polymers

The lower cycle represents the chemical changes occurring during polymerization and relates them to the free volume of the system. In general, free volume of a polymer system is the total volume minus the volume occupied by the atoms and molecules. The occupied volume might be a calculated van der Waals excluded volume [139] or the fluctuation volume swept by the center of gravity of the molecules as a result of thermal motion [140,141]. Despite the obscurity in an exact definition for the occupied volume, many of the molecular motions in polymer systems, such as diffusion and volume relaxation, can be related to the free volume in the polymer, and therefore many free volume based models are used in predicting polymerization behavior [117,126,138]. 

Vol. 1, "Molecular Motion in Polymers by ESR" Harwood Academic Publ. GmbH Chur, Switzerland, 1979. 

Royer R, Keinath S, editors. Molecular motion in polymers by ESR. Michigan MMI press 1979. 

The molecular dynamic methods can also be very useful in the study of the molecular motions in polymer chains with bulky side groups. 

Solid state NMR offers powerful tools for probing miscibility, phase separated structure and molecular motion in polymer blends and which may be beyond the resolution limits of conventional microscopic or thermal analysis. A large number of NMR works have been published and some of them were reviewed. In this review, therefore, we introduce recent research works on polymer blends by solid state NMR and focus on the miscibility and phase separation of polymer blends that are responsible for the improvement in their physical properties. 

One of the important advantages of neutrons over x-rays, as mentioned in Section 1.1, is that measurements of inelastic neutron scattering yield information on atomic and molecular motions in polymers and other materials. This arises because neutrons whose wavelengths are of the order of atomic or molecular dimensions possess energy that is comparable to the thermal energies of atomic motions. Both the (kinetic) energy E and the wavelength k of a neutron are related to its velocity v by... 

Techniques which are more specific to the various morphological states, especially the amorphous domain, are needed. NMR and ESR are two such molecular probes. By monitoring the mobilities of protons as a function of temperature, Bergmann has defined the onset of molecular motion in various polymers (14). The applicability of NMR as a measure of molecular motion in polymer solids has been the subject of several reviews 15,16,17). ESR monitors the rotational and translational properties of stable radicals, usually nitroxides, and relates their mobilities to polymeric transitions. As stated in several works (18,19), the radical s sensitivity to freedom of motion of the polymer chain is infiuenced by its size, shape, and polarity. The above probes are both high frequency in nature, 10 -10 Hz. Measurement at high frequency has decreased resolving power for the various transitions in contrast to low frequency or static experiments, such a dilatometry with an effective frequency of 10 Hz (20). 

Boyer RF, Keinath SE (eds) (1978) Molecular motions in polymers by ESR, Harwood Acad. Chur... 

Molecular motion in polymer solutions can have significant effects on the physical properties of the systems formed from these solutions. For example, the rates of drying polymer films can determine the film properties. We have shown that the drying of a polystyrene film from toluene solutions could be predicted with the knowledge of thermodynamic parameters, plus solvent diffusion data.(i) The ability of polymers to respond to changes in conditions is determined by die ability of the polymer and/or its segments to reorient. Solvent diffusion is also correlated to the segmental motions of the polymer chains.(2) The reason for this correlation appears to be that both molecules are coupled to the same fractional free volume. 

For the characterization of these processes there have been developed several methods and they are shown in Figure 2. They can be divided into two large groups. One is related to the structure of polymer systems and the other is related to the molecular motion in polymer systems. Information on the degree of molecular motion is essential for the understanding of dynamical properties of polymer systems. 

Odier bulk properties whose temperature coefficients undergo marked changes at Tg include heat content, refractive index, stiffness, and hardness. Experiments that are sensitive to the onset of molecular motion in polymer chains may also be used to... 

R. F. Boyer and S. E. Keineth, eds.. Molecular Motions in Polymers by E.S.R., Ssrnipo-sium Series Vol. 1, MMI Press Harwood, Chur, 1980. 

One method of investigating molecular motion in polymer physics is the observation of the temperature dependence of the line width of broad-line NMR spectra. However, since UPEC is composed of polyethylene and urea molecules, the protons in urea molecules must be replaced by deuterons in order to observe the behavior of the polyethylene chain by proton magnetic resonance. For this purpose, deuterated urea molecules were used in the preparation of UPEC (d-UPEC). In the preparation of d-UPEC, deuterated methanol has been used as a solvent in order to prevent proton exchange. In order to compare the new data with the data of bulk polymers, solution-grown polyethylene and extended-chain crystals of polyethylene were also used in the NMR study. 

It can be seen that the rotating frame technique in conjunction with more conventional Ti and T2 measurements can give a great deal of information about molecular motions in polymers, and in spite of certain experimental and theoretical difficulties, useful data concerning their crystalline content. More recently it seems that worthwhile studies of the anisotropy of oriented polymers can be made using Tip and T2 measurements, and this appears to be one of the most promising directions of future research in this field. 

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