Crosslinks nature

Hill, O.F., Ward, A.J., and Clement, C.C. "Austin Chalk Fracturing Design Using a Crosslinked Natural Polymer as a Diverting Agent," J. Pet. Technol.. 1978, 30(12), 1795 804. 

Friedrichson, T., and Kurzchalia, T.V. (1998) Microdomains of GPI-anchored proteins in living cells revealed by crosslinking. Nature 394(6695), 802-805. 

The highly crosslinked nature of the polymer formed in the discharge is immediately apparent from the C,j spectrum which exhibits approximately equal quantities of the four major carbon environments CFj, CF, CF and carbon with four nearest carbon neighbors. The overall stoichiometry determined from a comparison of the F,5 and Ci peak intensities is reported to be CiFi.4. The amount of oxygen functionalities found in these polymers, due to reaction of free radical sites in the polymer with atmospheric oxidants is usually less than 0.1 atomic % and is observed in the ESCA spectrum as a small broad signal in the 0, region . ... 

Figure 5.71 Dependence of shear modulus on temperature for three common engineering polymers crosslinked natural rubber, amorphous polyvinyl chloride (PVC), and crystalline Nylon 6. The typical use temperatures are indicated by dotted lines. Reprinted, by permission, from N. G. McCrum, C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, 2nd ed., p. 154. Copyright 1997 by Oxford University Press.

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

A similar experiment has been performed [68] but under MAS conditions. For a series of crosslinked natural rubber samples (A - FI), 13C edited 1H spinning sidebands have been extracted from the 2D spectrum. These sideband pattern are encoded by the residual dipolar couplings of the corresponding functional groups and are presented in Figure 14.12. 

Figure 14.12 Slices from a 2D experiment corresponding to the pulse sequence of Figure 14.11 performed on a series of crosslinked natural rubber samples A- FI (defined in [68]) under MAS. The slices reflecting proton sideband pattern for the different functional groups are encoded by the residual dipolar couplings. The distinct features of dipolar slices prove that the different functional groups may be considered as relatively isolated groups of spins on the time scale of the evolution and...

There are at least two reasons why these relationships have not yet been elucidated the crosslinked nature of cured epoxies and their glassy state. The structure-properties relationship for the glassy state of matter, including polymers, is poorly understood and our knowledge of crosslinked glasses is even more insufficient. 

The possibility of crosslinking natural macromoleculcs, such as cellulose and proteic materials, via the Mannich reaction has also been inve.stigatcd. Cellulose derivatives, however, are mostly subjected to the analogous amidomcthylation reaction, " usually employing bis-methylol amides (urea, oxalyl, adipoyl derivativas, etc.) capable of reacting with the cellulose hydroxy groups. 

The microparticles prepared in inverse emulsion form can be dispersed into aqueous media with agitation and the aid of inverting surfactants. Because of their highly crosslinked nature, the size of the kernel microparticles changes very little in solutions of different salinity. Consequently, the rheological properties of the carrier fluid are not affected by the salinity change encountered in... 

Chapter 19 considers the effects of polymer-penetrant interactions on the sorption of aromatic penetrants into a polyurethane thermoplastic elastomer(91). A direct liquid immersion approach was used, so the heat transfer problems noted above should not be important. Nevertheless, non Fickian phenomena are still observed. Unlike simple elastomers, thermoplastic elastomers achieve their crosslinked natures by formation of microdomains of either crystalline or glassy hard segments. The anomalous sorption behavior presumably reflects interactions of the solvents with these microdomains (93-94). 

Figure 8. Dj for heat corrected data in unfilled, crosslinked, natural rubber sample, versus volume fraction of solvent, compared with empirical extrapolation (dashed line) to the self-diffusion constant for toluene.

Some hydrolysis of the trimesoyl chloride takes place during membrane fabrication. ESCA studies indicated that approximately one-sixth of the carboxyl groups ere present as ionic carboxylate and five-sixths of the carboxyl groups are present as amides, leading to the above structure. The FT-30 barrier layer is insoluble in sulfuric acid and in all organic solvents, in agreement with the crosslinked nature indicated above. Its chemical structure is somewhat similar to the composition of the duPont Permasep B-9 hollow fiber polyamide, believed to be approximately as follows ... 

Although the dynamic mechanical properties and the stress-strain behavior iV of block copolymers have been studied extensively, very little creep data are available on these materials (1-17). A number of block copolymers are now commercially available as thermoplastic elastomers to replace crosslinked rubber formulations and other plastics (16). For applications in which the finished object must bear loads for extended periods of time, it is important to know how these new materials compare with conventional crosslinked rubbers and more rigid plastics in dimensional stability or creep behavior. The creep of five commercial block polymers was measured as a function of temperature and molding conditions. Four of the polymers had crystalline hard blocks, and one had a glassy polystyrene hard block. The soft blocks were various kinds of elastomeric materials. The creep of the block polymers was also compared with that of a normal, crosslinked natural rubber and crystalline poly(tetra-methylene terephthalate) (PTMT). 

Hytrel 4055 was molded under the same conditions except that the molding temperature was 260°C, and the slow-cooled specimens were cooled to 180°C in 10 min. The initial molding temperature for PTMT was 255°C. TPR-19 was molded similarly except that the initial molding temperature was 200°C. Kraton 1101 was molded at 135°C. Most of the specimens measured 4X3/8 X 0.030 in. except for the crosslinked natural rubber specimens which were cut from large rubber bands. 

Hydrogels are physically or chemically cross-linked polymer networks swollen with large amounts of water. Due to their crosslinked nature, these gels do not dissolve in aqueous media but contain an enormous amount of solvated water molecules within the entangled polymer chain matrix. Hydrogel properties are reviewed elsewhere in this book. This chapter is dedicated to a unique hydrogel family that responds to... 

Due to the dual filler and crosslinking nature of the hard domains in TPEs, the molecular deformation process is entirely different than the Gaussian network theories used in the description of conventional rubbers. Chain entanglements, which serve as effective crosslinks, play an important role in governing TPE behavior. The stress-strain results of most TPEs have been described by the empirical Mooney-Rivlin equation ... 

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