General structure sensitive polymers

Temperature-sensitive polymers, depending on polymer structure and polymer-polymer interactions, generally exhibit two behaviors, lower critical solution temperature (LCST) [31] and upper critical solution temperature (UCST). Phase diagrams for these behaviors are presented in Figure 9. 

Law et al. (24) described a recording layer composed of a carbocyanine dye in a poly(vinyl acetate) matrix. A 1050-A film containing 20 wt % dye produced micrometer-sized pits on exposure to 5-ns pulses from a dye laser (835 nm). Patent references disclose dye-polymer films obtained using dyes with general structures shown in Chart 7.1, and high performance sensitivities to diode laser recording are claimed (25-28). 

Also shown in Table lO-l is an (alkylcyclohexylaryloxy)-substituted polyacetylene [77]. Polymers of this general structure have been found to display liquid-crystalline behavior. In contrast to vinyl-based liquid-crystalline polymers, the geometric isomerism of the main-chain double bonds plays a role in determining the type of phase that is found. Advincula et al. have examined Langmuir films of polyacetylenes at the air-water interface [78]. Polyacetylene derivatives are unusual in that the polymer backbone itself acts as a chromophore therefore, in studies such as these, UV-visible spectroscopy can be a sensitive probe of polymer conformation. 

ESC occurs, in general, in amorphous polymers such as PC, PMMA, PS, PVC, SAN, and ABS as well as in semicrystalline thermoplastics like PE, PP, PA, and PB (Wright 1996). Amorphous polymers exhibit a higher tendency for this type of failure because their loose structure facilitates fluid permeation into the polymer. Amorphous polymers show enhanced sensitivity to ESC at temperatures close to... 

Polymer crystals are always semicrystalline, and thus, they are far from perfect. Limited crystal sizes and complicated aggregates make the basic understanding of the structure-property relationships very difficult. To simplify, two types of properties in general exist for crystals of small molecules structure-insensitive and structure-sensitive properties. The structure-insensitive properties include... 

A polymer gel is a network of flexible chains, with the general structure shown in Fig. V.l. Structures of this type can be obtained by chemical or physical processes. Since the final gel properties are sensitive... 

The discussion so far is a general one for all materials. It assumes that the structure extends through the sample, and takes no account of radiation sensitivity. Polymers can have very small crystals and can be very radiation sensitive. Simulation of a small crystal embedded in disordered material indicates that the surroundings will reduce contrast and add noise, but not otherwise affect the periodic image. As described... 

The advantages of C-NMR in measurements of polymer stereochemical configuration arise primarily from a useful chemical shift range which is approximately 20 times that obtained by proton NMR. Structural sensitivity is enhanced through well-separated resonances for different types of carbon atoms. Overlap is generally not a limiting problem. The low natural abundance (-1%) of C nuclei is another favourable... 

In the previous chapter, it was pointed out how the chemical structure of polymers influences their mechanical properties. The glass transition temperature and the rate of cooling from the melt determine whether the polymer will be a hard, stiff material, or a soft flexible material. These qualitative differences can be quantifled by measuring standard mechanical properties. The student, generally, is introduced to the subject of mechanical behavior through the study of linear elastic metals that exhibit solid behavior at normal operating temperatures and conditions. However, this study of polymers reveals that polymers exhibit fluid as well as solid behavior and are viscoelastic and viscoplastic at room temperature. The mechanical properties of polymers are strain rate sensitive and highly temperature dependent. 

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