Cross-linked polymer networks insolubility

Fabrication of solution-processed multilayered organic light-emitting diodes (OLEDs) from oxetane-ftinctionalized precursors, which are commonly photochemically cross-linked by UV radiation in the presence of a photoinitiator, has been reported recently. The reaction proceeds via CROP. Insoluble cross-linked polymer networks were obtained using precursors that contain two or more oxetane units per molecule. This novel approach simplifies OLED fabrication, and is generally compatible with various deposition/printing techniques. 

It was suggested that the irradiation causes homolytic cleavage of the benzylic Sn—C bonds in 96 giving rise to formation of a cross-linked organic network. This network is insoluble and tin species are thought to be trapped in it. Upon heating, the cross-linked polymer film functions as a matrix for the organotin precursors of SnC>2 and is completely removed after pyrolysis (Scheme 49). 

The use of insoluble, highly cross-linked anisotropic networks created by the polymerisation of photoreactive monomers, eliminates the problem of crystallisation, at least for organic materials, since polymer networks are macromole-cular structures incapable of crystallising, see Chapter 6. Furthermore, the fabrication of multilayer devices would be facilitated by the use of a cross-linked stable HTL next to the anode on the solid substrate surface, onto which subsequent layers can be deposited by vapour deposition. Multilayer OLEDs are intrinsically more stable than monolayer devices due to a better balance of charge-carriers and concentration of the charged species away from the electrodes. The synthesis and cross-linking of a suitable aromatic triarylamine derivative with a polymerisable oxetane group at each end of the molecule for use as a HTL has been reported recently, ... 

The PL method permits the investigation of polymers of any chemical structure linear or branched soluble polymers and even cross-linked insoluble polymers The only exceptions are polymers extingui iing luminescence. However, this property permits the development of new approaches based on this property For cross-linked polymers it was found sufficient to disintegrate the polymer into particles of a uniform size not exceeding 0.5 /Li. This approach is widely used in the investigation of polyelectrolytic networks ... 

A polymer is composed of repeating units (i.e., monomers) that are linked together into long chains that can be linear, branched, or cross-linked. If a polymer contains two different types of monomers, it is a copolymer. A linear polymer is a thermoplastic. At elevated temperatures it melts and flows as a liquid. In a cross-linked polymer, the repeat units are actually linked into a three-dimensional network of macroscopic size. It is a thermoset. Once the polymerization is completed, the cross-linked polymer cannot be softened or melted. It is hard, infusible, and insoluble. Hence a thermoset adhesive is the most durable but is also difficult to characterize as compared to a thermoplastic one. 

Polymers are frequently classified in terms of bonding in one dimension versus bonding in two or three dimensions. Bonding in one dimension results in linear polymers with single-strand chains. Bonding in two or three dimensions results in cross-linked polymers having infinite sheets or three-dimensional networks. Linear polymers are produced by addition polymerization if the reactant has only one double bond or by condensation polymerization if the reactant or reactants each have two reactive sites. Such polymers are usually soluble in suitable solvents. Since they also tend to soften when heated, they are called thermoplastic polymers. Cross-linked polymers may be produced by addition polymerization if the reactant has more than one double bond, or by condensation polymerization if the reactant or reactants each have more than two reactive sites. Such network polymers are usually insoluble and Infusible and are called thermosetting polymers. 

Coal has been proposed to have a macromolecular network structure to which concepts of cross-linked polymers can be applied. These concepts have been employed to understand and model such properties of coal as (1) the insolubility, (2) the equilibrium swelling and penetration of solvents, (3) the viscoelastic properties, (4) similarities between the parent coal and products of hydroge-nolysis or mild oxidation, (5) cross-linking during char formation, and (6) the formation of coal tar in pyrolysis. 

Should branching become excessive, infinite networks can form. The products become cross-linked, insoluble, and infusible. Such materials are called popcorn polymers. This phenomenon is more common in bulk polymerizations. The cross-linked polymers form nodules that occupy much more volume than the monomers from which they formed and often clog up the polymerization equipment, sometimes even rupturing it. 

Individual polymer chains in an ensemble can also be covalently joined to other chains around it at discrete points along it. This yields a 3D network of chains (or open-tree structures of chains or a mix of both). Cross-linking is desirable where insolubility and high mechanical strength are demanded of aplastic. Ideally, each and every chain will be linked to each other so that the entire ensemble of chains is a single giant molecule (this actually does occur in natural rubber when vulcanized or cross-linked.) An automobile or aircraft tire is an example of a fully cross-linked polymer. On heating, cross-linked polymers do not convert into a viscous liquid melt as the molecules are chemically linked to one another and cannot flow independently. 

The mean features of polymer networks are their insolubility and infusibility. In a covalent network (or irreversible cross-linked polymer), the network chains are connected by chemical bonds. The network can be formed by physical interactions. The so-called physical network (reversible cross-linking) can be destroyed by changing the interactions, e.g. by heating (thermoplastic elastomers). Reversible cross-linked polymers of technical interest are thermoplastic elastomers (TPE). Thermoplastic elastomers show proper-... 

There are two main classes of hydrogels (Figure 2.1), those composed of three-dimensional networks of cross-linked polymer chain strucmres that are insoluble in water chemical hydrogels) and those produced by the self-assembly of (macro) molecules to form noncovalent structures physical hydrogels). 

Cation exchange resins manufactured by addition polymerization are generally co-polymerized with divinylbenzene in order to produce a cross-linked and water-insoluble structure. The cross-linking has some important effects on catalyst structure and behaviour, mainly by altering the size of the pores and the flexibility of the polymer network. It is generally not known whether such cross-linking takes the form of block polymers or of random co-polymers. 

On heating with a peroxide, DAP therefore polymerizes and eventually cross-links, forming an insoluble network polymer. However, it is possible to heat the DAP monomer under carefully controlled conditions, to give a soluble and stable partial polymer in the form of a white powder. The powder may then be blended with peroxide catalysts, fillers, and other ingredients to form a molding powder in the same manner as polyester alkyds. Similar products can be obtained fi-om diallyl isophthalate (DAIP). 

Bulk Polymerization. The bulk polymerization of acryUc monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to form by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acryUc monomers is of limited importance. 

---