Polymer reaction crosslinking

The presence of the unsaturated substituent along this polyester backbone gives this polymer crosslinking possibilities through a secondary reaction of the double bond. These polymers are used in paints, varnishes, and lacquers, where the ultimate cross-linked product results from the oxidation of the double bond as the coating cures. A cross-linked polyester could also result from reaction (5.J) without the unsaturated carboxylic acid, but the latter would produce a gel in which the entire reaction mass solidified and is not as well suited to coatings applications as the polymer that crosslinks upon drying. ... 

The theories of Miller and Macosko are used to derive expressions for pre-gel and post-gel properties of a crosslinking mixture when two crosslinking reactions occur. The mixture consists of a polymer and a crosslinker, each with reactive functional groups. Both the polymer and crosslinker can be either collections of oligomeric species or random copolymers with arbitrary ratios of M /Mj. The two independent crosslinking reactions are the condensation of a functional group on the polymer with one on the crosslinker, and the self-condensation of functional groups on the crosslinker. 

Chemical reactions are used to modify existing polymers, often for specialty applications. Although of considerable importance for plastics, very few polymer reactions (aside from crosslinking) are important for elastomers. Chlorination and bromination of Butyl rubber to the extent of about one halogen atom per isoprene unit yields elastomers which are more easily crosslinked than Butyl rubber. Substitution occurs with rearrangement to yield an allylic halide structure... 

A second method of immunotoxin preparation by reductive amination involves the use a polysaccharide spacer. Soluble dextran may be oxidized with periodate to form a multifunctional crosslinking polymer. Reaction with antibodies and cytotoxic molecules in the presence of a reducing agent forms multivalent immunotoxin conjugates. The following sections discuss these options. 

Chain-growth polymerization. A 1,2-polybutadlene polymer is crosslinked with t-butylstyrene, utilizing a free radical initiator. Reaction rates include... 

These reactions form polymer melamine crosslinks (Ml and MIO), melamine-melamine crosslinks (M2, M3, M4, M5, M8, M9, and Mil) or Interconvert functional groups (M6 and M7). The Importance of the different reactions depends on the catalyst level and type, the bake conditions, and most Importantly on the structure of the melamine resin. Reaction Mil occurs only under basic conditions (used In the preparation of melamine-formaldehyde crosslinkers) and can be Ignored In coatings where acid catalysts are used. Reaction MIO Is slow compared to reaction Ml (5). The reactions Involving water probably make at most a minor contribution under normal bake conditions. The most Important reactions appear to be Ml for fully alkylated melamines and Ml and M9 for partially alkylated melamines. Reaction M4... 

Differences in Network Structure. Network formation depends on the kinetics of the various crosslinking reactions and on the number of functional groups on the polymer and crosslinker (32). Polymers and crosslinkers with low functionality are less efficient at building network structure than those with high functionality. Miller and Macosko (32) have derived a network structure theory which has been adapted to calculate "elastically effective" crosslink densities (4-6.8.9). This parameter has been found to correlate well with physical measures of cure < 6.8). There is a range of crosslink densities for which acceptable physical properties are obtained. The range of bake conditions which yield crosslink densities within this range define a cure window (8. 9). 

The calculation requires measurements of the extent of reaction and of the distribution of functionality on the polymer and crosslinker. The functionality of the isocyanate crosslinker is formally three although it must be recognized that all of these materials have a distribution of functionality. Fully alkylated melamines have a functionality of 6. Coatings are formulated with an excess of alkoxy to hydroxy in order to achieve rapid cure (2). Thus, not all of the melamine alkoxy groups will react (ignoring side reactions) and the effective functionality will be less than six. 

High crosslink densities may severely depress polymer reactivity as a result of large decreases in swelling and diffusion rate within the polymer. Diffusion control in a polymer reaction can be detected by the inverse dependence of rate on polymer particle size (radius for spherical particle, thickness for film or sheet) [Imre et al., 1976 Sherrington, 1988]. 

The concentration of modifier will tend to diminish during the reaction, and the more reactive the modifier, the more serious is the effect likely to be. An extension of the Flory treatment of crosslinking to take into account the presence of modifier demonstrates the point. When branches arise by transfer to polymer and crosslinks by the subsequent coupling of branched radicals, it is easy enough to show that the rate of formation of crosslinks, X, as a function of conversion is ... 

Fig. 19. Dynamical thermal stability of electric field poling-induced electro-optic activity for two samples. The data were obtained as described in [121] by slowly increasing temperature while monitoring second harmonic generation. The chromophore is a DEC chromophore described in [138]. Uncrosslinked refers to the precursor polymer where only one end of the DEC chromophore is attached to the polymer lattice. Crosslinked refers to the situation where both ends of the DEC chromophore have been reacted to achieve covalent coupling to the polymer lattice. The ends of this DEC chromophore are asymmetrically functionalized so that attachment reactions can be carried out independently...

We now well appreciate, of course, that polymers are virtually everywhere. Some of them occur naturally, and we continue to better understand their compositions, structures, and properties. Many of these materials have been used since the dawn of human existence, for food, obviously. Cellulose alone has been essential for clothing, fire, shelter, tools, weapons, writing, and art. Leather is probably the result of the first synthetic polymer reaction, essentially the crosslinking of protein (elastin). How we progressed over time to the Polymer Age is a fascinating series of stories, some of which are well worth recounting here. 

The number of two-polymer, multipolymer, and multimonomer systems reported in the scientific and patent literature continues to rise without an adequate nomenclature to describe the several materials. This chapter is divided into three parts. (1) A proposed nomenclature system which uses a short list of elements (polymers or polymer reaction products). These elements are reacted together in specific ways by binary operations which join the two polymers to form blends, grafts, blocks, crosslinked systems, or more complex combinations. (2) The relationship between the proposed nomenclature and the mathematics of ring theory (a form of the new math9 ) is discussed. (3) A few experimental examples now in the literature are mentioned to show how the new nomenclature scheme already has been used to discover new multipolymer systems. 

Similarly, the crosslinking operation C may be broadened by also considering that it represents the addition of a monomer capable of being crosslinked in a later reaction. For example, when one of the polymers is crosslinked (semi-IPN s), the picture becomes interesting. Assuming first that G12 is simultaneous with P2, as in Equation 7, a number of... 

The four materials are (1) polymer 1 linear and polymer 2 crosslinked (2) polymer 1 crosslinked and polymer 2 linear (3) and (4) are obtained by interchanging polymers 1 and 2. In each case the second polymerized material is grafted to the first polymerized material. Equations 9-14 and the related discussion result in 72 possibilities. The difference lies in the counting or omission of the time sequence of events. The reaction network scheme does not consider the importance of the time-order of events. Thus Equation 31 yields the minimum number of distinguishable materials. 

As is well known, two main routes can be adopted for obtaining crosslinked derivatives, as schematically depicted in Fig. 159 these include u.se of polyfunctional oligomers or monomers (route a) and reaction of high-molecular-weight linear polymers with crosslinking reagents (route b). 

The objective of this study is to record both network structure (curemeter curves) and chemical conversion simultaneously. The influence of the chemical reaction on the network formation is determined. The kinetics with respect to different polymers and crosslinkers will be also described. 

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