Prepolymer illustration

Copper-based thermal stabilizers are also effective photostabilizers for nylon. They can be added before polymerization, or the soluble salts (eg, CuSO can be appHed to fibers as part of the finish or to fabrics as post-treatments. The effectiveness of the copper salt—alkah haUde system added to prepolymer in retarding phototendering and photoyeUowing of the resulting spun yam is illustrated in Figure 5. 

In this paper detailed methods for the determination of placement and assay of silicon hydride (Si-H), silicon hydroxide (Si-OH) and silicon phenyl (Si-0) functional groups in molecular weight components of silicones of the Sylgard (Dow-Corning Co.) type will be described. The methods are illustrated with the analysis of Sylgard addition prepolymers and of model polydimethylsiloxanes (PDMS). 

The classical synthetic pathway to prepare polyimides consists of a two-step scheme in which the first step involves polymerization of a soluble and thus processable poly(amic acid) intermediate, followed by a second dehydration step of this prepolymer to yield the final polyimide. This preparative pathway is representative of most of the early aromatic polyimide work and remains the most practical and widely utilized method of polyimide preparation to date. As illustrated in Scheme 4, this approach is based on the reaction of a suitable diamine with a dianhydride in a polar, aprotic solvent such as dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), dimethylformamide (DMF), or AT-methylpyrrolidone (NMP), generally at ambient temperature, to yield a poly(amic acid). The poly(amic acid) is then cyclized either thermally or chemically in a subsequent step to produce the desired polyimide. This second step will be discussed in more detail in the imidization characteristics section. More specifically, step 1 in the classical two-step synthesis of polyimides... 

The last chemical characteristic is the attachment of ligands. Unlike the other properties (reservoir capacity and biocompatibility) that could be incorporated into a one-shot process, this aspect is most conveniently practiced at the prepolymer level. The philosophy is to use some of the isocyanate functionality of the prepolymer to attach active side chains. This is illustrated in two examples that we will discuss again. 

This approach can be illustrated by unsaturated polyesters based on an almost equimolar combination of maleate and phthalate of propylene glycol, crosslinked by styrene (45 wt%) (Mortaigne el al., 1992). Six samples differing by the prepolymer molar mass were analyzed. The chain-ends concentration, b, was determined by volumetric analysis of alcohols and acids in the initial reactive mixture. Then, the system was cured, elastic measurements were made in the rubbery state at Tg + 30° C, and the shear modulus G was plotted against chain-ends concentration (Fig. 14.7). The following relationship was obtained ... 

The production by Loeb and Sourirajan of the first successful anisotropic membranes spawned numerous other techniques in which a microporous membrane is used as a support for a thin, dense separating layer. One of the most important of these was interfacial polymerization, an entirely new method of making anisotropic membranes developed by John Cadotte, then at North Star Research. Reverse osmosis membranes produced by this technique had dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Souri-rajan process. Almost all reverse osmosis membranes are now made by the interfacial polymerization process, illustrated in Figure 3.20. In this method, an aqueous solution of a reactive prepolymer, such as a polyamine, is first deposited in the pores of a microporous support membrane, typically a polysul-fone ultrafiltration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, such as a diacid chloride in hexane. The amine and acid chloride react at the interface of the two immiscible... 

The chemical reaction proceeds with the terminal isocyanate of the initial prepolymer reacting with the available hydroxyl groups of the diol. This is illustrated in Figure 2.26. 

Both the ratios in the prepolymer production and the curing ratios will affect the final properties of the polyurethane. In the initial prepolymer production, the properties vary according to the molar ratios of the prepolymer. This is illustrated by the graphs compiled in Saunders and Frisch (Saunders, 1962). 

The urethane linkage is the fundamental group in polyurethane chemistry. The initial step in the preparation of a cast prepolymer is to react a suitable linear diol with a difunctional isocyanate, as illustrated in Figure 2.2. 

Performance Flexibility. Figure IV illustrates the effect of the 0H/NC0 ratio of 180° peel strength and rolling ball tack for both the Hycar 2103-Prepolymer C and Hycar 2106-Prepolymer C adhesive systems. Peel strength values of 493 N/m (45 oz./in.) or lower are possible for the Hycar 2103 system simply by altering the 0H/NC0 ratio. Likewise for the Hycar 2106 system, peel strengths of 657 N/m (60 oz./in.) or lower are possible. 

Soft blocks are composed of linear, dihydroxy poly ethers or polyesters with molecular weights between 600 and 3000. In a typical polymerization of a thermoplastic polyurethane elastomer, the macroglycol is end capped with the full amount of aromatic diisocyanate required in the final composition. Subsequently, the end-capped prepolymer and excess diisocyanate mixture reacts further with the required stoichiometric amount of monomeric diol to complete the reaction. The diol links the prepolymer segments together while excess diol and diisocyanate form short hard-block sements, leading to the (AB)n structure illustrated in Figure 1. Block lengths in (AB)n polymers are frequently much shorter than those in anionically synthesized ABA block copolymers. 

These examples barely touch the wide variety of epoxy polymer structures and curing reactions. They illustrate the point that the latent functionality of the prepolymers and the chemical and mechanical properties of the linal polymeric structures will vary with the choice of ingredients and reaction conditions. 

The previous discussion illustrates the difficulties in crystal engineering as an approach to the development of new nonlinear materials. With polymers, the situation is, conceptually at least, straightforward, and the problems that must be overcome are subject to analysis. The approach is to apply an external alignment field to polymer (or prepolymer) in a softened state and to fix the orientation with one of a variety of processing approaches. This approach is illustrated schematically in Figure 6.5, which shows how... 

In any case, the prepolymer synthesis results in somewhat better defined domains, characterized by the relative translucency of the prepolymer route material relative to the one-shot route, and as illustrated in the transition behavior. Figure 3. Better defined domains mean a purer rubber phase, which in turn may yield greater toughness to this form of the material. Important questions as to the development of intermediate Tg s with extensive interfacial boundary material (48) or on complete thermodynamic miscibility (49) remain for the future. 

The formation of step copolymers that are forming blocks is best illustrated through considering polyurethanes that are formed by prepolymers. This was discussed earlier in outline for the polyurethane co-polyureas. In the following example, a polyester-urethane is formed by reacting an —OH-terminated polyester with an -NCO-terminated polyurethane. Each of these is formed by the appropriate stepwise polymerization and then reacted to give a larger -NCO-terminated prepolymer ... 

Similar to their approach and extended from Hsleh s previous paper, (3), a simple but much Improved route was devised. Stepwise addition of an alkylalumlnum halide, which follows that of an alkylene oxide to the alkylllthlum-lnltlated "living" prepolymer, provides the desired oxyl-alumlnum chain end as a modified ringopening site for cyclic esters. The Improved process Is Illustrated In Figure 1. 

In the transesterification process, the first step is the removal of phenol to produce a prepolymer. The reaction is illustrated in Eq. (52) ... 

Using only one of these cyclic configurations, for purposes of illustration an approximate structure of a diallyl o-phthalate prepolymer near the gel point is given in Fig. 5. 

The control of the molecular weight of the prepolymer of diallyl o-phthalate has led to several processes which may be considered solution polymerizations in which the solvent serves also as a chain-transfer agent. Procedure 8-4 is an adaptation from a patented process given here to illustrate the process. In this... 

Unsatuiated polyesters represent an excellent illustration of a polymeric system with a latent abihty to cross-link. Unsaturated polyesters are, as the name implies, unsaturated polyester prepolymers mixed with... 

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