Points cross-linking

FIGURE 27 14 A section of polystyrene showing one of the benzene rings modified by chloromethylation Indi vidual polystyrene chains in the resin used in solid phase peptide synthesis are con nected to one another at various points (cross linked) by adding a small amount of p divinylbenzene to the styrene monomer The chloromethylation step is carried out under conditions such that only about 10% of the benzene rings bear —CH2CI groups... 

The cross-hnking efhciency of EP copolymer has been investigated by some workers [368,371] and the yield is found to increase with increasing ethylene content. The value lies closer to PP homopolymer. The amounts of cross-hnking as well as scission have been observed to increase with increasing diene content, the labile point. Cross-linking enhancers like chlorobenzene, nitrous oxide, allylacrylate, and A-phenylmaleimide are reported to promote cross-linking [85,370,371]. 

During the course of the reaction, the divinylbenzene will act as a cross-linking agent, and in the structure of the polymer obtained we will find cross-linking points (cross-links) of the type... 

All thermoplastics flow at room temperature under load stresses far below the yield point, cross-linked plastics at higher temperatures. This clearly demonstrates why it is not permissible to use tensile and breaking strength or the tensile, bending. 

Figure 9.28 Types of thermoreversible cross-links (a) point cross-links, b) junction zones, and (c) fringed micelles.

Natural rubbers crystallize to a greater extent than most other rubbers, and crystallization is limited by a number of factors, including molecular imperfections (branch points, cross links, chain ends) and chain entanglements. Butadiene copolymers such as GR-5, Hycar, or Buna 5 are considered as non-crystallizing due to the likelihood of a cis-trans isomer mixture in butadiene and the irregularities associated with the presence of a secondary constituent [21]. 

Fig. 4-1). This explains why some polypeptides may fall outside calibration curves prepared with standard globular proteins. Theoretically, the steepest selectivity curve and thus the highest separation power is obtained with the idealized gel medium in which all the pores are of identical size, giving a separation range per pore volume of approximately one decade [7]. Such gels do not exist but are approached by gels made of point cross-linked linear poljmiers such as dextran and polyacrylamide. 

Other techniques that work well on small computers are based on the molecules topology or indices from graph theory. These fields of mathematics classify and quantify systems of interconnected points, which correspond well to atoms and bonds between them. Indices can be defined to quantify whether the system is linear or has many cyclic groups or cross links. Properties can be empirically fitted to these indices. Topological and group theory indices are also combined with group additivity techniques or used as QSPR descriptors. 

Irregularities such as branch points, comonomer units, and cross-links lead to amorphous polymers. They do not have true melting points but instead have glass transition temperatures at which the rigid and glasslike material becomes a viscous liquid as the temperature is raised. 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

If the concentration of junction points is high enough, even branches will contain branches. Eventually a point is reached at which the amount of branching is so extensive that the polymer molecule becomes a giant three-dimensional network. When this condition is achieved, the molecule is said to be cross-linked. In this case, an entire macroscopic object may be considered to consist of essentially one molecule. The forces which give cohesiveness to such a body are covalent bonds, not intermolecular forces. Accordingly, the mechanical behavior of cross-linked bodies is much different from those without cross-linking. 

It is not particularly difficult to introduce thermodynamic concepts into a discussion of elasticity. We shall not explore all of the implications of this development, but shall proceed only to the point of establishing the connection between elasticity and entropy. Then we shall go from phenomenological thermodynamics to statistical thermodynamics in pursuit of a molecular model to describe the elastic response of cross-linked networks. 

Figure 3.3 Comparison of experiment (points) and theory [Eq. (3.39)] for the entropy elasticity of a sample of cross-linked natural rubber. [From L. R. G. Treloar, Trans. Faraday Soc. 40 59 (1944).]...

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

The point at which two polymeric chains are joined together by a cross-linker such as divinylbenzene, or sites where tertiary hydrogens are located in the stmcture, are other locations for oxidative attack. In both cation- and anion-exchange resins, oxidative attack results in the removal of cross-linking. 

During the press operation, which is actually a form of compression mol ding, the resin-treated laminate pHes are heated under pressure and the resins cured. The initial heating phases cause the resin to melt and flow into voids in the reinforcing ply and bond the individual pHes together. The appHed heat simultaneously causes the resin to polymerize and eventually to cross-link or gel. Therefore, resin viscosity reaches a minimum during the press cycle. This is the point at which the curing process becomes dominant over the melt flow process. Dynamic mechanical and dielectric analyses (11) are excellent tools for study of this behavior. 

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