Addition crosslinking

Stretching a polymer sample tends to orient chain segments and thereby facilitate crystallization. The incorporation of different polymer chains into small patches of crystallinity is equivalent to additional crosslinking and changes the modulus accordingly. Likewise, the presence of finely subdivided solid particles, such as carbon black in rubber, reinforces the polymer in a way that imitates the effect of crystallites. Spontaneous crystal formation and reinforcement... 

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

The above results proved the potential viability of the adsorbed hydrophilic macromolecules as bonded phases in chromatography of biopolymers but it must be admitted that additional crosslinking of previously adsorbed macromolecules is usually needed in order to obtain stable composites. The cross-linked bonded polymeric phases, however, may suffer from the restricted flexibility of the chain segment and their steric repellency may be diminished. Moreover, the conformational adaptivity of cross-linked chains for binding with solutes is poorer than that of grafted or chemically bound macromolecules. 

Modified PAN fibres have been obtained from copolymers containing up to 15% or ISP units using the wet spinning process30. Some properties of modified fibres are presented in Table 1. For comparison are also given the properties of fibres obtained from copolymers additionally crosslinked with conventional crosslinking agents used in the vulcanization of nitrile rubbers. 

This reasoning does not account for segments that are not properly attached to the network nor for additional crosslinks and any deviation from the outlined mechanism of the phenol curing reaction. Side reactions are not considered. 

Networks obtained by anionic end-linking processes are not necessarily free of defects 106). There are always some dangling chains — which do not contribute to the elasticity of the network — and the formation of loops and of double connections cannot be excluded either. The probability of occurrence, of such defects decreases as the concentration of the reaction medium increases. Conversely, when the concentration is very high the network may contain entrapped entanglements which act as additional crosslinks. It remains that, upon reaction, the linear precursor chains (which are characterized independently) become elastically effective network chains, even though their number may be slightly lower than expected because of the defects. 

Thermosets and thermoplastics behave differently from each other in fires. Thermosets do not melt when heated but may well undergo further crosslinking. The presence of such additional crosslinks hinders movement of any volatile degradation products through the polymer matrix. Hence the combustion zone tends to be starved of fuel and for this reason thermosets tend to be relatively non-flammable. 

To calculate the additional crosslinking density, one can formulate the following balance eguation. 

Although hydrogen abstraction is not expected to be an important factor in 0(3P) reactions with polybutadienes, additional crosslinking could result from abstraction of the tertiary hydrogen in the vinyl unit. The resulting, resonating radical ... 

Polyethylene is, by far, the most commonly used resin in rotational molding processes. Low density, high density, and linear low density polyethylene are all used extensively. Additionally, crosslinked polyethylene is used for parts that require high chemical or heat resistance or enhanced impact resistance. 

There is one more report on the synthesis of a library of phosphorus ligands on solid phase. Waldmann et al. prepared a library of phosphoramidites on beads (Fig. 36.5), but these were only applied in enantioselective C-C-bond formation. In fact, as two ligands need to be bound to the catalyst, the use of an immobilized monodentate ligands should most likely be avoided unless the proximity between the ligands is sufficiently close. In addition, crosslinking by the metal may have a negative impact on the permeability of the polymer for the substrate. 

Chain scission is the predominant process at lower dose levels (<5000 Mrads), yielding a decrease in elastic modulus at ambient temperature. Additional crosslinking at high doses (>5000 Mrads) results in an increase in elastic modulus at ambient temperature and in the rubbery region above Tg. 

A new class of functional comonomers exemplified by acrylamidobutyraldehyde dialkyl acetals 1 and their Interconvertible cyclic hemlamidal derivatives 2 were prepared and their chemistry was Investigated for use In polymers requiring post-crosslInking capability. These monomers do not possess volatile or extractable aldehyde components and exhibit additional crosslinking modes not found with conventional am1de/forma1dehyde condensates, eg, loss of ROH to form enamides 9 or TO and facile thermodynamically favored reaction with diols to form cyclic acetals. 

Crystallization of the polymer when the propellant formulation is subjected to low temperatures can be annoying (12). Formation of additional periodic attractions between molecules has the same effect as additional crosslinking. Upon crystallization, the propellant becomes hard and brittle with low strain capability. If the effect is caused by crystallization of the polymer, the original physical properties are obtained when the propellant is heated above the melting point of the polymer. These effects are time-temperature dependent and can have a significant effect on the selection of operating and storage temperatures... 

Load Sharing of Filler Particles. Comparison of ultimate strength of a propellant and its unfilled binder matrix almost always shows that the propellant has up to several times the tensile strength of the matrix. This filler reinforcement is presently thought to stem from additional crosslinks formed between filler particles and the network chains of the binder matrix (5, 8, 9, 34). Effective network chains are defined as the chain segments between crosslinks. From the classical theory of elasticity, the strength and/or modulus of an elastomer is proportional to the number of effective network chains per unit volume, N, or... 

A second mechanism in the. aging of CTPB propellants also exists and proceeds concurrently with the reactions proposed above. It consists of an attack at the reactive points of unsaturation in the backbone polymer, which causes additional crosslinking and hence an increase in propellant modulus, particularly at the surface. The exposed surface of CTPB propellants changes, as indicated by an increase in hardness. Heavy metal ions are particularly harmful, and it was found that an increase from 10 to 80 p.p.m. of iron caused a significant increase in surface hardening by catalytic attack on the double bonds. Antioxidants in general provide sufficient protection for polymer storage. In CTPB propellants the antioxidant selected to protect the double bond is very important. Amine-type antioxidants have provided better surface stability than phenolic compounds. 

Additional crosslinked polyvinyl alcohol copolymers, (I), were prepared by Schulte et al. (1) and used as components in adhesives, emulsifiers, and detergents. 

Crosslinkable monomers including 1,2-ethanediol di(meth)acrylate, 1,12-dode-canediol di(meth)acrylate, and 1,4 butanediol di(meth)acrylate were reacted with step 2 monomers of the current application by Moore et al. (3) to create biological amine-reactive materials useful in medical diagnostics. Additional crosslinked amine-reactive materials were prepared by Leir et al. (4) and are discussed. 

The (oxiran-2-yl)-methyl acrylate phenolic derivative, (II), was polymerized to the corresponding polydithiocarbonate, (III), with trifhioromethanesulfonateby Ohta et al. (1) and used in high oxygen-permeable contact lenses. Additional crosslinkable analogs are described by Kamata et al. (2) and Honda et al. (3). 

Crosslinked products exhibit superior optical transparency, heat resistance, and adhesion, as well as improved dimensional stability, solvent resistance, and chemical resistance. In addition, crosslinking prevents the occurrence of cracks in films. 

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