Crystal cross-linking

Fig. 17 Schematic illustrations of the proposed structures of PVA/y-CD dried films upper) and swollen gels lower) produced by F-T cycling. 1 channel structure of PVA-y-CD-IC, 2 hydrogen-bonded interactions/crystals/cross-links between PVA chains, and 3 free y-CD molecules...

Protein crystals cross-linked by compounds with pyridine and/or succinimide fragments as novel catalytic materials 01AG(E)2204. 

FIGURE 1.23 Three basic principles—crystallization, cross-linking, and chain stiffening—for making polymers strong and temperature resistant are represented at the three comers of the triangle. The sides and the center of the triangle indicate various combinations of the principles. (After Mark, H. F. 1967. Set. Am., 217, 3, Sept. 19, 148, see also p. 156.)... 

As an alternative to native protein crystals, protein crystals cross-linked by glutaraldehyde provide attractive solid reaction vessels for preparing novel nanomaterial-in-crystal hybrids with potential application in catalysis. Au nanoparticles were S5mthesized within the solvent channels of cross-linked lysoz5mie crystals in situ without the introduction of extra chemical reagents or physical treatments (Liang et al., 2013). 

No polymer is ever 100% crystalline at best, patches of crystallinity are present in an otherwise amorphous matrix. In some ways, the presence of these domains of crystallinity is equivalent to cross-links, since different chains loop in and out of the same crystal. Although there are similarities in the mechanical behavior of chemically cross-linked and partially crystalline polymers, a significant difference is that the former are irreversibly bonded while the latter are reversible through changes of temperature. Materials in which chemical cross-linking is responsible for the mechanical properties are called thermosetting those in which this kind of physical cross-linking operates, thermoplastic. 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

If a modest number of cross-links between the polymer backbone are introduced, the polymer Hquid crystal takes on elastomeric properties. The useflilness of these materials probably Hes in the coupling of mechanical and optical effects. 

Carbonless Copy Paper. In carbonless copy paper, also referred to as pressure-sensitive record sheet, an acid-sensitive dye precursor, such as crystal violet lactone or /V-hen2oy11eucomethy1ene blue, is microencapsulated with a high boiling solvent or oil within a cross-linked gelatin (76,83,84) or in synthetic mononuclear microcapsules. Microcapsules that have a starch binder are coated onto the back of the top sheet. This is referred to as a coated-back (CB) sheet. The sheet intended to receive the image is treated on the front (coated-front (CF)) with an acid. When the top sheet is mechanically impacted, the dye capsules mpture and the dye solution is transferred to the receiving sheet where the acid developer activates the dye. 

In general, however, the vulcanizates suffer from poor low temperature crystallization performance compared to a conventional sulfur cure, and also have inferior tensile and tear properties. Urethane cross-linking systems (37), eg, Novor 950 (see Table 3) are also extremely heat resistant, but exhibit inferior tensile and dynamic properties compared to conventional sulfur-cured vulcanizates. One added virtue is that they can be used in conjunction with sulfur systems to produce an exceUent compromise according to the ratios used (38). 

There is Htfle crystal growth during carbonization, which always occurs in the soHd phase. The soHd cross-linking that occurs at this time does not lend itself to crystal growth. The glassy carbons are composed of random crystaUites of the order of 5.0 nm across and are not significantly altered by ordinary graphitization heat treatment to 2800°C. 

The heHcoidal stmcture of such Hquid crystals can be carried to the soHd state by cross-linking (119,120) or by careful evaporation of solvent (121,122). Underivatized ceUulose can also form ordered mesophases (123,124), and gel films precipitated from lithium chloride—dimethyl acetamide retain some mesophase stmcture (122). 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

Advanced Thermoplastics Materials. Thermoplastics and linear plastics of finite molecular weight that can be fabricated into very complex stmctures by hot melt or injection mol ding are different from the thermoset materials that require cross-linking to build up infinite molecular weight to form network (cross-link) stmctures. Advances in thermoplastic engineering materials include amorphous thermoplastics, crystalline thermoplastics, Hquid crystal thermoplastics, and fluorinated thermoplastics (see Engineering plastics). 

---