Cross-linking network characteristic

Phenol-formaldehyde adhesives are produced by a condensation polymerization reaction between phenol and formaldehyde. The phenolics used for exterior particleboard are made at a formalde-hyde/phenol ratio greater than 1.0 i.e., they are classified as resoles and additional formaldehyde is not required to complete the curing reaction to a highly cross-linked network structure. Many characteristics can be incorporated into the adhesives by changes in the F/P ratio, condensation pH, and condensation time. The reactive solids content is normally between kO and 50 percent since the stability and viscosity are adversely affected at higher solids. 

Experimental observations show that bituminous coals satisfy at least one important macroscopic characteristic of a cross-linked network they swell in numerous solvents without being dissolved by them even at high temperatures Cl, 6 9), unless thermal degradation or reaction occurs In the development of mathematical models to describe network behavior, crosslinks are assumed to be points (usually carbon atoms) or short bfudgeA (usually of molecular weight much smaller than Mc) whence three or more chains are initiated. 

Figure 6 shows transmitted x-ray diffraction patterns for the POD films with progressive HTT. The characteristic (101) and (112) reflections are absent for HTT s below 2200 C and suddenly appear at 2500 C. This result implies that the three-dimensional order of the graphite lattice is not established at lower HTT s, namely the condensed aromatic layers exist as individuals and have a highly two-dimensional character. This may have come from the absence of cross-linked network structures at lower HTT s and may have facilitated the the recrystallization of graphite at such low temperatures and without pressure. 

Hybrid versions of silicone-thermoplastic semi-IPNs have been developed (19). A hybrid interpenetrating network is one in which the cross-linked network is formed by the reaction of two polymers with structurally distinct backbones. Hydride-functionalized siloxanes can be reacted with organic polymers with pendant unsaturated groups such as polybutadienes (5) in the presence of platinum catalysts. Compared with the polysiloxane semi-IPNs discussed earlier, the hydride IPNs tend to maintain mechanical and morphologically derived properties, whereas properties associated with siloxanes are diminished. The probable importance of this technology is in cost-effective ways to induce thermoset characteristics in thermoplastic elastomers. 

Another vivid example of the exceptional role of network topology is the unexpectedly high deformation abUity of hypercrosslinked polystyrenes under loading, which is usuaUy characteristic of conventional slightly cross-linked networks or linear polymers in the rubber elasticity state. Hypercrosslinked polymers, however, differ from the latter in that they retain their mobUity even at very low temperatures. In fact, hypercrosslinked materials do not exhibit typical features of polymeric glasses, nor are they typical elastomers. Their physical state thus cannot be described in terms of generaUy accepted notions. More likely, the hypercrosslinked networks demonstrate distinctly different, unique deformation and relaxation properties. 

Polymeric betaines are usually insoluble in pure water and have gel characteristics but are soluble in salt-containing solutions. The loss of water solubihty and gel-like structure that adopts polybetaines are probably due to the formation of intra- and interchain ion contacts which result in the appearance of cross-linked networks. The intrinsic viscosity [t]], second virial coefficient A2, exponent a in the MKH equation, the radius of hydration Rg and the hydrodynamic radius % increase with the increase in salt concentration [132] (Table 16). 

Such small and intercatenary cross-linked macromolecules are just like branched macromolecules in generally being soluble in some solvent or other. Thus, they characteristically differ from actual cross-linked networks, which possess many intercatenary cross-links. Such cross-linked molecules, or networks, are infinitely large in comparison with the usual branched or unbranched macromolecules. They are not soluble in any solvent. On the other hand, not all insoluble polymers are cross-linked, of course. 

The sol-gel blend or pre-crosslinked grades of polychloroprene such as the T Type polychloroprenes available from DuPont Dow contain a highly cross-linked network, which that acts as an internal processing aid (94,95). The hlend has many of the same characteristics as the sol component, hut different compoimd rheology properties. This composition results in faster extrusion with extrudates having smoother surfaces. Extrudates also retain their shape better with good definition until vulcanized. 

An important application relates to oxygen diffusion through soft contact lenses (75). Soft contact lenses are made of poly(2-hydroxyethyl methacrylate) and its copolymers, in the form of cross-linked networks. These are swollen to thermodynamic equilibrium in water or saline solution. The hydroxyl group provides the hydrophilic characteristic and is also important for oxygen permeability. Oxygen permeability is important because of the physiological requirements of the eye. Thus the polymer is highly swollen with water and also serves as a semipermeable material. 

Unked membrane. Furthermore, the storage modulus after cross-linking is not decreased after 300 °C but it reaches a plateau and this behavior is characteristic of cross-linked networks. The cross-linked membranes were subjected to treatment with the Fenton s reagent in order to evaluate their oxidative stability. In all cases they preserved their mechanical properties showing thus the high oxidative stability of the cross-linked structures. 

If the chemical structure of a polymer is altered during a viscoelastic experiment—in particular, if a cross-linked network is subjected to a reaction which increases or decreases the number of network strands while it is being investigated in the rubbery zone of viscoelastic behavior—the apparent mechanical properties will be profoundly influenced. For example, scission of the network strands will cause stress relaxation at constant strain" (Fig. 14-12) or creep under constant stress. Formally, if a single first-order chemical reaction is responsible, the relaxation may be described by a single relaxation time which is the reciprocal of the chemical rate constant, instead of the broad spectra which are characteristic of the usual mechanical processes. 

The thermal degradation mechanism of the cross-linking networks is investigated by monitoring the change of characteristic IR absorptions with the treatment temperature (Figure 8.11) [35]. For the cured Epo-P2, after treatment at 260°C for 2 min, the absorption intensity of phosphate band apparently decreases, and this trend becomes... 

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