Segmental motions, influence

In these complexes, the cations coordinate with the oxygen atoms of the backbone and, under the influence of an electrical potential, they are transferred from an oxygen atom to another through the amorphous region of the polymer assisted by the segmental motion of the polymer backbone. 

The variation of Tg with ac (or Mc) is a reflection of the influence of junction-point density on the freedom of segmental motion. The maximum range of Tg values shown, 301 to 312K, possibly reflects the maximum influence for these MDI/POP triol systems. 

These conclusions are further generalized by the more extensive data presented in Fig. 7 for polyethylene oxide and poly-trimethylene oxide. The continuous nature of the Ti function for both these polymers over a large temperature range is quite definite and is emphasized by the detailed data in the vicinity of the respective melting temperatures. This is true even for the polyethylene oxide samples where discontinuities in the linewidth are clearly indicated in Fig. 7. Obviously, the type of segmental motions which contribute to the two different relaxation pareim-eters are influenced quite differently by the presence of crystallinity. 

The results that have been obtained indicate that the major influence of the crystalline regions on segmental motions, and hence to the structure of the non-crystalline regions, is in the linewidth and T2. The different morphologies are reflected in different values of T2- The segmental motions in long chain molecules which exert major influence on the spin-lattice relaxation times and the nuclear Overhauser enhancements are not in general the same motions which determine the resonant linewidth. 

For all the cases cited above, which represent those data for which a comparison can be presently made, there is a direct connection between the critical molecular weight representing the influence of entanglements on the bulk viscosity and other properties, and the NMR linewidths, or spin-spin relaxation parameters of the amorphous polymers. Thus the entanglements must modulate the segmental motions so that even in the amorphous state they are a major reason for the incomplete motional narrowing, as has been postulated by Schaefer. ( ) This effect would then be further accentuated with crystallization. 

Carbon-13 spin-lattice relaxation times TL (Section 3.3) are relatively insensitive to the chain length of polymers [531]. The influence of local segmental motions predominates, as shown for low-density polyethylenes in which Tx values are one to two seconds for the main chain but up to seven seconds for peripheral side-chain carbon nuclei at 120 C [532] due to segmental mobility (Section 3.3.3.4). To conclude, quantitative evaluation of polymer carbon-13 spectra as necessary for side-chain determination requires the knowledge of spin-lattice relaxation times. 

The questions to be considered here are, how overall and segmental motion are correlated to each other, whether certain segments of a chain behave like rigid subparts or whether each carbon atom undergoes individual reorientation, the behaviour of the end groups, the determination of temperature and thereby the influence of macroviscosity on the various parts of the molecular motion, and how branching of the chains or of some attached substituents influences the relaxation. For this reason chain-like molecules were very early objects of relaxation measurements. After some earlier theoretical papers (Levine et al., 1974) many experimental studies have been published recently. The general... 

Azobenzenes have been utilized to measure the free volume in polymers and the speed of polymeric segmental motion [42, 43], Azobenzenes that are covalently bonded to a polymer backbone may influence various properties of the macromolecule. Photoisomerization of such substances will cause changes in wettability [44], viscosity [45], solubility [46], membrane properties [47], and swelling properties [48]. 

Interesting in this context is the finding that oxides such as A1203, Ti02 etc. can be used as fillers that enhance the ion conductivity.116 Even though effects such as impact on crystallinity or on segmental motion are expected to have a great influence, the picture developed in Section V.2. should also be relevant In a covalent matrix most of... 

Thermal conductivity and expansion are important properties of adhesives used in electronics. Both properties influence the performance of computer chips. Generally, the chip has a protective cover which is attached by an adhesive. The adhesive bond must be maintained during thermally induced movement in the chip. The chip is bonded to its base with an adhesive which must also take thermal movement and, in addition, transfer heat from the chip. Two epoxy adhesives were used in the study silica filled epoxy (65 and 75 wt% SiO2 epoxy) and epoxy containing 70 wt% Ag. Figure 15.6 shows their thermal conductivities. The behavior of both adhesives is completely different. The silver filled adhesive had a maximum conductivity at about 6()"C whereas the maximum for SiOz filled adhesive was 120"C. The Tg of both adhesives was 50 and 160 C, respectively. Below its Tg, the thermal conductivity of the adhesive increases at the expense of increased segmental motions in the chain molecules. Above the Tg the velocity of photons rapidly decreases with increasing temperature and the thermal conductivity also decreases rapidly. 

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