Crosslinked structures branched polymers

Dendrimers are a relatively new class of macromolecules different from the conventional linear, crosslinked, or branched polymers. Dendrimers are particularly interesting because of their nanoscopic dimensions and their regular, well-defined, and highly branched three-dimensional architecture. In contrast to polymers, these new types of macromolecules can be viewed as an ordered ensemble of monomeric building blocks. Their tree-like, monodispersed structures lead to a number of interesting characteristics and features globular, void-containing shapes, and unusual physical properties [107-111]. 

In polymer science and technology, linear, branched and crosslinked structures are usually distinguished. For crosslinked polymers, insolubility and lack of fusibility are considered as characteristic properties. However, insoluble polymers are not necessarily covalently crosslinked because insolubility and infusibility may be also caused by extremely high molecular masses, strong inter-molecular interaction via secondary valency forces or by the lack of suitable solvents. For a long time, insolubility was the major obstacle for characterization of crosslinked polymers because it excluded analytical methods applicable to linear and branched macromolecules. In particular, the most important structural characteristic of crosslinked polymers, the crosslink density, could mostly be determined by indirect metho ds only [ 1 ], or was expressed relatively by the fraction of crosslinking monomers used in the synthesis. 

Polymerization of the A—B plus Ay system (with/ > 2) in the presence of B—B will lead not only to branching but also to a crosslinked polymer structure. Branches from one polymer molecule will be capable of reacting with those of another polymer molecule because of the presence of the B—B reactant. Crosslinking can be pictured as leading to structure XVII, in... 

Branched polymers have side chains, or branches, of significant length which are bonded to the main chain at branch points (also known as jimction points), and are characterised in terms of the number and size of the branches. Network polymers have three-dimensional structures in which each chain is connected to all others by a sequence of junction points and other chains. Such polymers are said to be crosslinked and are characterised by their crosslink density, or degree of crosslinking, which is related directly to the number of junction points per unit volume. 

As a consequence, together with linear chains, branched and crosslinked structures are also formed. They strongly affect molecular masses, MMD, and solution properties. Moreover, these non-crystal 1izable units cause a decrease of both the polymer melting temperature and the crystallization rate, as well as a poorer thermo-oxidative stability (16). 

Another important feature controlling the properties of polymeric systems is polymer architecture. Types of polymer architectures include linear, ring, star-branched, H-branched, comb, ladder, dendrimer, or randomly branched as sketched in Fig. 1.5. Random branching that leads to structures like Fig. 1.5(h) has particular industrial importance, for example in bottles and film for packaging. A high degree of crosslinking can lead to a macroscopic molecule, called a polymer network, sketched in Fig. 1.6. Randomly branched polymers and th formation of network polymers will be discussed in Chapter 6. The properties of networks that make them useful as soft solids (erasers, tires) will be discussed in Chapter 7. 

Branched polymer molecules of this type cannot react with each other since the reactive group at the end of each branch is A. Thus, the formation of network or crosslinked structures cannot take place. For A—-B + A/ with / = 2, the polymer formed will be linear, and for all higher values of / only branched polymers will be formed. 

A polymer network can be envisioned as a polymer molecule with an infinite molecular weight [1]. Crosslinking is a physical or chemical route by which polymers with branched or crosslinked structures are produced. The chemical route may imply a polymerization or postpolymerization stage [2-4]. Crosslinking is especially important from the commercial point of view. Polymer networks may... 

Lignin is synthesised in plants from monomeric molecnles, whose functionality varies from two to four. Thus, both branched chain and the crosslinked structure may be formed. In plant tissue, the polymer chains of lignin are snrronnded by macromolecules of noncellulosic polysaccharides, with which they form an amorphous lignocarbohydrate matrix. The experimental methods that allow the stndy of the complex topology of macromolecules in a multicomponent solid composite are very limited. Therefore, most of the data are interpreted using theoretical methods developed from polymer chemistry. 

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