Cross-linking network structure, polymers from

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

The combined results demonstrate the complexity of the system. Cross-linking must include kinetic contributions to the lateral resistance that are similar to those observed in the networks, but a combination of structural and dynamic factors is likely responsible for the signihcant but opposite effects from kinetically dissimilar cross-links. Stimulus-responsive polymer brush layers hold great potential (Minko et al. 2000 Motornov et al. 2003 Granville et al. 2004 Kaholek et al. 2004a,... 

The viscosity method for soluble polymers and the swelling method for cross-linked network polymers yield quite unambiguous values for polymer solubility parameters, so long as one is confined to a series of structurally similar solvents. For example, the data in Figure 6-1 apply to aliphatic hydrocarbons as well as to long-chain esters and ketones. Cycloaliphatic hydrocarbons and short-chain esters such as ethyl acetate deviate significantly from the curves shown. 

These eompounds are formed into a prepolymer, which in turn is converted into a series of high-moleeular-weight eompounds when reacted with a suitable euring agent. Polymers featuring the epoxide group as a part the chemieal struetures will reaet with other chemical species to form a three-dimensional cross-linked network. One of the most popular forms of epoxide resin is derived from bisphenol A, and the associated epoxide resin has the following chemical structure ... 

If the network structure is such that the crystallization of the cross-linked units is not restricted, AH and A5 can be taken to be independent of the fraction of units cross-linked. Under these conditions, A5 is identified with the entropy of fusion of the pure non-cross-linked polymer, and the ratio of A5 to AH is identified with the equilibrium melting temperature of the pure polymer. If, however, steric requirements are such that cross-linked units are excluded from the crystalline regions, an alteration will occur in these quantities. The presence of cross-linked units in the molten phase and not in the crystalline phase results in an increase in A 5 (when compared with the non-cross-linked polymer) of an amount Rp per mole of chain units. The melting temperature must accordingly be depressed for this reason, as long as AH is unaffected by the presence of cross-links. 

Cross-linked Structures n (Crosslinked and Cross Linked) Theoretically, branched molecules are soluble in some solvent, and are thus distinguishable from cross-linked networks. Conversely, however, not all insoluble polymers are cross-linked networks. Irregular cross-linked networks are the result of either certain uncontrolled, nonstereospecific reactions, or they are produced by the subsequent cross-linking of linear or... 

At the University of Wisconsin since 19 6, studies of viscoelasticity have evolved from concentrated polymer solutions to undiluted amorphous polymers, dilute solutions, lightly cross-linked rubbers, glassy polymers, blends of different molecular weights, copolymers, cross-linked rubbers with controlled network structures, and so forth. It became evident that each type of system required a different approach. Moreover, in amorphous polymers, the terminal, plateau, and transition zones had to be described separately. Both dynamic (sinusoidal) and transient measurements such as creep and stress relaxation have been utilized. The inderlying theme of this work is the relation of macromolecTilar dynamics—modes of motion of polymer molecules— to mechanical and other physical properties. 

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