Segmental motion of polymer

When the VFT equation was applied every system gave a linear fit (Fig. 9). This implies that ion transport was assisted by segmental motion of polymer chains in a... 

The absorption of vapor by the surface layer of a polymer film will necessitate some rearrangement of the polymer molecules, and it is reasonable to consider that the more active the segmental motion of polymer chains becomes, the more rapidly the surface layer takes up penetrant to the equilibrium concentration. This implies that the surface concentration gradually approaches an equilibrium value at a finite rate which may depend upon the rate of relaxation motions of the polymer molecules. Crank and Park (1951) expressed this situation by the equation ... 

As is clearly seen from this equation, spin relaxation parameters are expressed in terms of two kinds of correlation times and the order parameter for the inner motion. However, two motions are not enough to describe fully the segmental motion of polymers as demonstrated later. 

Segmental mobility of polymer chains is one of the important properties of polymer materials. It has intimate relations to several physical properties of polymer materials. NMR relaxation measurements are wellknown as powerful techniques for elucidating the segmental motions of polymer materials not only in solution but also in the solid state. 

In many fluid systems, the rotational diffusion proceeds at the timescales comparable with the fluorescence decay and can be used for studying the viscosity of the microenvironment, local geometrical constraints, segmental motion of polymer chains, etc. In non-viscous solvents, it decays usually faster than fluorescence, but in very viscous systems a full randomization occurs at times much longer than the fluorescence lifetime. The extrapolation tot yields the residual anisotropy, r . 

The characterization of the segmental motions of polymers is difficult, especially in concentrated solutions. One technique that is well suited to study polymer segmental motions is NMR relaxation measurement.(i, Studies have mostly focussed on the carbon or proton relaxation behavior at lower concentrations. Dipolar interactions among protons, or protons and tie the relaxation phenomena to local motions of polymer segments. Proton and techniques have been of limited use in more concentrated solutions, which to some extent is the most important regime for the development of many polymer properties. In more concentrated solutions, the overlap of spectral features and/or the complexity of the interactions make extracting motional information difficult, even if the relaxation measurements can be made. 

As the temperature is lowered further, a point is reached where many physical properties of the polymer, such as elastic moduli, viscosity, and specific volume, fall dramatically over a narrow temperature range. This is the glass transition temperature, Tg, which was discussed in Chapter 6. At this temperature, segmental motions of polymer molecules practically cease. It appears that the temperature of around -20°C, at which crumb firming rate becomes negligible, may correspond closely to the Tg of amy-lopectin at the water content of bread. 

The thermal stability of both systems decreases with the addition of epoxidised oil, although the coefficient of thermal expansion increased in a Unear manner. This may enhance the degree of freedom in the segmental motion of polymer chains in the network structure. 

The molecular motions responsible for the relaxation include (1) overall translational or rotational motions, (2) local motions such as internal rotations around C—C bonds or molecular axis of symmetry or segmental motions of polymer chains, and (5) exchange between two different distinct chemical sites within the same molecule or different molecules. 

Certain factors would be expected to restrict the segmental motions of polymer chains in the solid state. Obvious among these factors are (a) the presence of crystallinity, (b) stereoregularity, (c) copolymer composition, (d) molecular weight effects, (e) crosslinking, and (f) mechanical stress. We shall give brief examples of these influences. 

This chapter has considered three different physical techiuques, all sensitive primarily to local and segmental motions of polymers. Different methods reflect different aspects of segmental motion with different sensitivities, but there is a unity of findings about chain motion and observed relaxation times. As a summary of the above results, one notes ... 

Figure 12.17 shows the temperature dependence of Na" -ionic conductivity for 10%-neutralized PAA/POE400 with various POE incorporation. Each temperature dependence shows WLF type behavior. It indicates that ion-conduction significantly relates to the segmental motion of polymers. An increase in ionic conductivity ((Tj) with POE incorporation is due to the lower glass transition temperature of the film. The ionic conductivity of 4.9 x 10" (S/cm) was obtained at room temperature by incorporating four times the amount of POE4oo ... 

The second model is called the free volume model, where dye diffusion takes place in the void volume in the non-crystalline regions of the fibres. This void volume is formed by the segmental motion of polymer chains, a process that commences at the glass transition temperature of the fibre. The shift in the free volume in the non-ciystalline regions promotes the diffusion of molecules that are sufficiently small. ... 

V at 150 °C, respectively. The observed OCV values are much lower than the theoretical value of 1.23 V. The reason for the low OCV might be related to gas cross-over, poor contact between electrode and membrane due to somewhat poor mechanical strength of the membrane, or poor membrane electrode assembly (MEA) formation. It is necessary to improve and optimize these factors in order to attain better cell performance. Anyway, it was demonstrated that proton transport coupled with segmental motion of polymer under non-humidified conditions is possible by fuel cell test using the CL-HBP-SA membrane. 

The correlation between the ionic migration and the local segmental motion of polymer chains may be subjected to closer examination. According to the free volume theory, the free volume in amorphous polymers that occurs above 7g increases in accordance with the temperature difference T — [23]. Because of the enlarged free volume above T, the polymer chain becomes locally mobile. The mode of the micro-Brownian motion is considered to be cooperative, involving several repeat units of the polymer chains. The dielectric relaxation time for the backbone motion of PPO and... 

P-transition temperature. This brittle-to-tough P-transition is believed to be associated with the local segmental motion of polymer molecules. The proposed association between the macroscopic mechanical properties and molecular motion still remains a conjecture. An independent verification, especially by using molecular spectroscopy as a probe to elucidate the dynamics of segmental motion of polymers under dynamic deformation, is welcome once again. 

---