Cross-linked structure

Much of the success of the poly(ethylene terephthalate) bottle has arisen from the control of the biaxial orientation that occurs during manufacture to give a product both strong and of low gas permeability. 

The liquid crystal polymers consist of rod-like molecules which, during shear, tend to orient in the direction of shear. Because of the molecular order the molecules flow past each other with comparative ease and the melts have a low viscosity. When the melt is cooled the molecules retain their orientation, giving self-reinforcing materials that are extremely strong in the direction of orientation. 

The homopolymers of p-hydroxybenzoic acid have such a high value for the Tm that they are somewhat intractable. Useful materials may, however, be made by copolymerising with a view to introducing some molecular flexibility or reducing chain packing or introducing some non-linetu links. Commercially important liquid crystal polyesters are discussed in Chapter 25. 

A cross-linked polymer can generally be placed into one of two groups  

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of crystallisation in both the unstressed and the stressed state. 

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The novolak resins themselves contain no reactive methylol groups and do not form cross-linked structures on heating. If, however, they are mixed with compounds capable of forming methylene bridges, e.g. hexamethylenetetramine or paraformaldehyde, they cross-link on heating to form infusible, thermoset structures. 

As these condensation reactions can occur at the two ortho and the para positions in phenol, w-cresol and 3,5-xylenol, cross-linked structures will be formed. It has been pointed out by Megson that because of steric hindrance the amount of cross-linking that can take place is much less than would involve the three reactive groups of all the phenolic molecules. It is now generally considered that the amount of cross-linking that actually takes place is less than was at one time believed to be the case. 

Since this reaction may occur at both ends of the molecule (in case of glycidyl ether resins) a cross-linked structure will be built up. 

Many polymers containing boron in the main chain have been prepared but most of them have either been of low molecular weight or intractable cross-linked structures. Some interest has been shown in the tri-3-aminoborazoles, which polymerise on heating (Figure 29.15f. ... 

Liquid crystal polymers Cross-linked Structures Polyblends... 

In previous works [18-20,23,102] water-soluble polymers such as polyacrylamide (pAM), polysodium acrylate (pAA Na), poly(acrylamide-sodium acrylate) (pAM-AA Na), poly(acrylamide-diallyethylamine-hy-drochloride) (pAM-DAEA-HCl), and poly(acrylamide-sodium acrylate-diallyethylamine-hydrochloride) (pAM-AANa-DAEA-HCl) were used in the recovery of cations and some radioactive isotopes from aqueous solutions. It was found that the floe is formed between the added polymer and ions of the solution in the flocculation process with the formation of a crosslinked structure. The formed cross-linked structure is characterized by [103-105] ... 

Cross-linked structure of pAM CuS0 5H20 complenes... 

Cross-linked structure of pDAA-HCl-CuS0 5H20... 

A new polymer type which emerged as important materials for circuit hoards are polycyanurates. The simplest monomer is the dicyanate ester of hisphenol A. When polymerized, it forms three-dimensional, densly cross-linked structures through three-way cyanuric acid (2,4,6-triazinetriol) ... 

Our laboratory has planned the theoretical approach to those systems and their technological applications from the point of view that as electrochemical systems they have to follow electrochemical theories, but as polymeric materials they have to respond to the models of polymer science. The solution has been to integrate electrochemistry and polymer science.178 This task required the inclusion of the electrode structure inside electrochemical models. Apparently the task would be easier if regular and crystallographic structures were involved, but most of the electrogenerated conducting polymers have an amorphous and cross-linked structure. 

Cross-linked structure of polypyrrole, 311 Cross-linking, 330... 

Divinyl monomers such as divinylbenzene, divinyl adipate, and ethylene dimethacrylate, in which the reaction of one double bond does not alter greatly the reactivity of the other, polymerize to highly cross-linked structures. 

Thermosetting materials, which have a rigid, cross-linked structure for example, the polyester and epoxy resins. 

Ceramic materials have a cross-linked structure and are therefore brittle. 

Figure 39 The enantioselective polymer-supported catalysts (61) of chiral oxazaborolidinone with cross-linking structures for use in the Diels-Alder reaction of methacrolein with cyclopentadiene. (Adapted from ref. 85.)...

Generally, during hydroboration polymerization of dicyano compounds, the formation of the borazine structures that have a six-membered boron-nitrogen ring (scheme 19a) and dihydroborated end groups (scheme 19b) as a structural defect is unavoidable. The borazine cross-linked structures often cause the gelation, and dehydroboration causes a decrease in molecular weight. 

The whole of a multi-cellular organism is contained by outer cell layers, which are described in biology texts, and maintained by connective tissue. Connective tissue is a novel, external biopolymer structure of multi-cellular organisms found within their new extracellular, circulating fluid compartments (see Section 8.9). As mentioned there, the main connective tissues, covalently cross-linked structures, are (1) those of plants, celluloses (polysaccharides), often cross-linked by lignin (2) those of lower animals and insects, mixed cross-linked polysaccharides and... 

Dynamic theological tests were used to monitor the evolving cross-linking structure during UV curing. In dynamic tests, a sinusoidal strain, y, deformation... 

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