Thermoplastics branching

The first commercial grades were introduced by Phillips Petroleum in 1968 under the trade name Ryton. These were of two types, a thermoplastic branched polymer of very high viscosity which was processed by PTFE-type processes and an initially linear polymer which could be processed by compression moulding, including laminating with glass fibre, and which was subsequently oxidatively cross-linked. 

Definition Thermoplastic branched chain polyethylene Formuia [CH2CH2]x... 

It is important to note that any molecular architecture that provides a thermoplastic block chemically bonded to an elastomeric block, which is in turn bonded to another thermoplastic segment, should exhibit the properties of a thermoplastic elastomer. For example, grafting thermoplastic branches onto an elastomeric backbone produces thermoplastic elastomer behavior [285, 298]. Other examples are the segmented-type polymers—[AB] — with alternating hard and soft segments thus, a variety of segmented polyesters and polyurethanes with polyether or polyester soft segments exhibit properties of thermoplastic elastomers [263,298,299]. 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

Both polymers are linear with a flexible chain backbone and are thus both thermoplastic. Both the structures shown Figure 19.4) are regular and since there is no question of tacticity arising both polymers are capable of crystallisation. In the case of both materials polymerisation conditions may lead to structures which slightly impede crystallisation with the polyethylenes this is due to a branching mechanism, whilst with the polyacetals this may be due to copolymerisation. 

The thermoplastic polyurethane (TPU) adhesives must, of necessity, contain low gel content because they must be processable in an extruder. Most adhesives are relatively linear, with a functionality of 2.0, although small amounts of branching may be introduced, usually at the expense of a lower melt flow. Good physical properties of TPU s are obtained when the thermoplastic urethanes have molecular weights of 100,000 or higher (see p. 56 in [63]). Most TPU adhesives are based on symmetrical polyesters with a fast crystallizing backbone or a backbone slightly modified to increase the open time. 

Potential advantages of both thermoplastics and cellu-losic materials combined with the economic and environmental viewpoint have lead to a promising utilization of both these materials in various forms of composites. Although various branches of cellulosic-thermoplastic composites industries are booming in recent years, their growth rate is very slow. In order to achieve the full potential of such valuable materials as various engineering materials and commodity products more incentives from academic, industrial, and governmental authorities are needed. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

There are a number of methods of classifying polymers. One is to adopt the approach of using their response to thermal treatment and to divide them into thermoplastics and thermosets. Thermoplastics are polymers which melt when heated and resolidify when cooled, while thermosets are those which do not melt when heated but, at sufficiently high temperatures, decompose irreversibly. This system has the benefit that there is a useful chemical distinction between the two groups. Thermoplastics comprise essentially linear or lightly branched polymer molecules, while thermosets are substantially crosslinked materials, consisting of an extensive three-dimensional network of covalent chemical bonding. 

Markel E.J., Weng W., Peacock A.J., and Dekmezian A.H. Metallocene-based branched-block thermoplastic elastomers. Macromolecules, 33, 8541, 2000. 

Puskas, J.E., Pattern, W.E., Wetmore, P.M., and Krukonis, A. Multiarm-star polyisobutylene-polystyrene thermoplastic elastomers from a novel multifunctional initiator, Polym. Mater. Set Eng., 82,42 3, 1999. Brister, L.B., Puskas, J.E., and Tzaras, E. Star-branched PIB/poly(p-t-bu-Styrene) block copolymers from a novel epoxide initiator, Polym. Prepr., 40, 141-142, 1999. 

Polyethylene, a thermoplastic, is the largest selling plastic material. LDPE is a branched polyethylene whose branches prevent close packing and gives low density. HDPE is polyethylene that has essentially no branching, so the molecules pack very well, which leads to high density and high crystallinity. LLDPE is actually a copolymer prepared at low temperature and low pressure from a mixture of ethylene and about 10% of a C4-C8 olefin. 

We can further characterize polymers into thermoplastics and thermosets . Thermoplastics consist of linear or lightly branched chains that can slide past one another under the influence of temperature and pressure. These polymers flow at high temperatures which facilitates their molding into useful products. Thermosets consist of a network of interconnected chains whose positions are fixed relative to their neighbors. Such polymers do not flow when heated. 

The terms thermoplastic and thermoset refer to the processability of a particular polymer and the properties of the finished article. Thermoplastic polymers are mostly a linear or branched linkage of monomers containing many thousands of repeat units. All the commodity polymers and most of the engineering polymers are thermoplastic. 

III.) BRANCHED Thermoplastic Divalent Branch Cell Monomers V 0 "AiQrr 0 Z J n... 

IV.) DENDRITIC Thermoplastic Polyvalent Branch Cell Monomers ( l) Z /vZ ( L)... 

Figure 1.23 Intermediary of (III) branched and (IV) dendritic architecture in the conversion of (I) linear thermoplastics to (II) crosslinked thermoset polymers. Intermediary of (IVb) dendrigrafts and (IVc) dendrimers in the formation of megamers...

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