Cross-linked polymer, flexibility

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 ciystallisation in both the unstressed and the stressed state. 

The softening behaviour of a thermoplastic material depends to a large extent on the flexibility of the chain and the ability to crystallise. Significant cross-linking of a reasonably stiff-chained polymer will lead to material that is unlikely to soften below its decomposition temperature. Intermediate to the linear and cross-linked polymers are various ladder polymers in which the polymer molecule consists of a pair of more-or-less parallel chains bridged in a manner analogous to the rungs of a ladder. 

The same aplies to polymer brushes. The use of SAMs as initiator systems for surface-initiated polymerization results in defined polymer brushes of known composition and morphology. The different polymerization techniques, from free radical to living ionic polymerizations and especially the recently developed controlled radical polymerization allows reproducible synthesis of strictly linear, hy-perbranched, dentritic or cross-linked polymer layer structures on solids. The added flexibility and functionality results in robust grafted supports with higher capacity and improved accessibility of surface functions. The collective and fast response of such layers could be used for the design of polymer-bonded catalytic systems with controllable activity. 

Hydrogels Hydrogels consist of a cross-linked polymer system or a meshlike structure that is flexible and can swell in the presence of water, producing a hydrophilic surface and masking the components below. The most widely used hydrogels consist of poly(hydroxyethyl methacrylate) (PHEMA), poly(ethylene glycol) (PEG), or poly(vinyl alcohol) (PVA) and tend to have reduced interaction with tissue.22 31 44 53 55... 

Figure 2 Pass Ale types ofpolymers based upon catenane architecures A Linear mechanically-linked polymers B mechanically cross-linked polymers C stars with a flexible, potentially switchable, catenane core D combs with catenarw linkages E main chain catenanes with switchable surface properties F highly mechani-cally-crosslinked networks G poly- or [n]catenanes. Only examples of types A and F have thus far been prepared...

The cross-linked polymers obtained from the addition-cure approach are often a complex arrangement of atoms bonded in heterocyclic and carbocyclic rings. However, the objective in the preparation of these systems is not simplicity. It is to obtain systems with the desirable properties of phenolic resins retained and the undesirable properties improved or removed. Voids are an undesirable result in the synthesis of both resols and novolacs. Hence, addition-cure phenolic resins are designed to avoid this result. Ease and flexibility of processing are also sought in the addition-cure systems. 

In the glassy state, the polymer backbone is considered to be almost completely immobile, causing the material to be brittle. Between Tg and Tm the system is not to be considered as a highly viscous liquid but as an elastic material the transition near Tg is called a glass-rubber transition. The crystallites act as cross-links between flexible stretches of the polymer chains, causing the material to have a rubbery consistency, with an appreciable elastic modulus. Above Tm a viscous liquid is formed. Figure 16.3b illustrates the rheological relations. 

Unlike U/F and M/F resins, acrylic nitrogen resins need not be blended with another component to obtain flexibility this can be built into the acrylic copolymer by appropriate choice of monomers (see examples on p. 171). Thus resins with ester linkages in their main polymer chains (e.g. alkyds, polyesters) can be avoided. A cross-linked polymer network can be built up which is resistant to hydrolysis by alkalis in detergents. Coatings can therefore be formulated which are suitable for domestic appliances, being additionally hard but flexible. 

An ideal robber consists of flexible cross-linked polymer chains undergoing violent liquid-like motions. No matter what the type of cross-linking, physical or chemical, the elastomers all have this in common the macromolecules between cross-links undergo extremely rapid molecular movement. 

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