Infusible polymers

The reaction conditions can be varied so that only one of those monomers is formed. 1-Hydroxy-methylurea and l,3-bis(hydroxymethyl)urea condense in the presence of an acid catalyst to produce urea formaldehyde resins. A wide variety of resins can be obtained by careful selection of the pH, reaction temperature, reactant ratio, amino monomer, and degree of polymerization. If the reaction is carried far enough, an infusible polymer network is produced. 

Polymers of diethylene glycol bis(a)lyl carbonate) (CR-39) are optically clear infusible polymers which are used for lenses. 

Polynapthalenes are essentially insoluble, infusible polymers, resistant to oxidation. 

Copolymerization can be employed in a similar fashion to modify the properties of the homopolymer of /7-hydroxybenzoic add (5-6). Poly(/ -hydroxybenzoic acid) is an infusible polymer which can be shaped only by compression sintering. A melt processable variation of this high modulus, thermally stable material can be made, however, by copolymerizing an ester of 5-6 with equimolar quantities of terephthalic acid (5-7) and biphenol (5-8) to produce an aromatic polyester which can be fabricated at temperatures near400 C but still retain many useful properties at 300°C. 

Tris transaminates readily with ammonia or primary amines when catalyzed by carbon dioxide or strong organic acids. The polysilazane products range from discrete solid disilazanes, to liquid distillable oligomers, and to highly cross-linked infusible polymers. Some of these polysilazanes can be pyro-lyzed to amorphous silicon nitride or mixtures of silicon nitride and silicon carbide below 1550 or to crystalline ceramics above that temperature. 

On the other hand, 4-hydroxybenzoic acid and syringic acid give infusible polymers exhibiting high temperature transitions at 330 and 320 °C, respectively. The polymer obtained from 3,5-dichloro-4-hydroxybenzoic acid exhibits neither a melting point nor a transition below 400 °C. 

We will discuss in this section the various ways that can be used to improve the thermal stability of polymers. The synthesis and thermal behaviour of some typical heat-resistant polymers (sometimes commercially available) will then be given. The volatilization of these materials has very seldom been thoroughly studied orders of reaction, activation energies and pre-exponential factors have generally not been determined. Therefore the thermal stability of the polymers will be characterized in an arbitrary way for the purpose of comparison. It must be stressed, however, that the physical properties of a polymer are at least as important for use at high temperature as the volatilization characteristics an infusible polymer is very difficult to process, and a heat resistant polymer with a low softening temperature is often useless. The softening temperature corresponds to the loss of mechanical properties. It can be measured by the standard heat deflection test. 

Polymeric phthalocyanines (Chap. 5.2) indude a great variety of properties. The construction of electrical devices or catalysts for spedal use is most hopeful. But all these applications depend on the reproduribility of well defined structural uniform polymers. Preparative work must help to standardize synthetic procedures and to investigate structures. The well reproducible in situ synthesis of thin layers of pure polymeric phthalocyanines from the gas phase opens a way for electrocatalysis and visible light energy changing devices. Another new kind of preparation goes via prepolymers which may be converted to mechanically and thermally stable, infusible polymers. 

Polyimides are insoluble, infusible polymers that cannot be used as adhesives. Which of the following two compounds would you copolymerize with the starting monomers to produce a usefLil adhesive Comment on the temperature resistance of the resulting adhesive. 

Conventional aliphatic polyamides, like PA-66, melt at 250 °C. By inserting aromatic rings between the amide groups, near-infusible polymers are formed. Aromatic polyamides (aramides) are known mainly in the form of fibres. Two grades of DuPont are widely used. Nomex is made from m-phenylene diamine and isophthalic acid (anhydride). In Kevlar, phenylene groups are para-substituted by amide groups,... 

Then, the prepolymer (as an immersion lacquer for laminates or when intended as a filler) is converted to an insoluble and infusible polymer with sulfuryl chloride (SO2CI2). Powdered poly(phenylenes) are compressed into isotactic molded components at 400°C and high pressure in a kind of sinter process. 

The same generic dry spinning process can be used to fabricate the precursor fiber for a carbon fiber from an infusible polymer, such as polyacrylonitrile, for a polycrystalline aluminate fiber from a sol-gel or for a polycrystalline alumina fiber from a slurry. Again, a high temperature curing step is required to convert the as-spun, amorphous precursor fiber into the final functional fiber. In these cases, however, an amorphous polycrylonitrile precursor fiber changes into a carbon fiber, and an amorphous aluminate precursor fiber into a crystalline aluminate fiber. The final functional fiber is therefore directly derived from a solid and amorphous precursor fiber and only indirectly from a liquid phase, i.e., a melt or sol-gel, respectively. 

To convert this essentially fiquid/semi-solid epoxy into an infusible solid, the adhesive will also contain a hardener (curing agent), which, under the correct conditions, will chemically react with the epoxy to produce a cross-linked, infusible polymer. 

Chemical characteristics. Both resins will react with formaldehyde to initiate and form monomeric addition products. Six molecules of formaldehyde added to one single molecule of melamine will form hexamethylol melamine, whereas a single molecule of urea will combine with two molecules of formaldehyde to form dimethylolurea. If carried on further, these condensation reactions produce an infusible polymer network. The urea/formaldehyde and melamine/formalde-hyde reactions are illustrated in Fig. 2.2. 

The second stage of the process takes place by heating the composition up to 120°-150°C during 3 hours. Under such conditions, insoluble and infusible polymers are obtained by the interaction of the functional groups and the formation of new cross-links due to the reaction of the secondary hydroxyl groups. 

Modifications of novolacs has to take place at the phenolic hydroxyl. Dannels and Shepard report that novolacs of high ortho-ortho substitution can be esterified with relatively large amounts of inorganic acid to yield soluble, fusible products. Regular novolacs under the same conditions yield crosslinked insoluble, infusible polymers, undergoing intermolecular esterification. Cyclic esters from ortho-ortho novolacs are described by Prochaska to form cyclic carbonates at low temperatures, which resinify at elevated temperatures.Structure 30 is typical of the type of products obtained from these reactions. 

Infusible polymers cannot be produced by these techniques. Hence, unmodified PTFE is frequently used as a sintered coating, or as an additive in special bearing compositions. Polyimide (PI) is even more difficult to process, and is normally supplied direct from the polymer manufacturer in the form of precision-sintered mouldings. 

The reaction of curing is not clear. It is known that under controlled conditions phenol and hexamethylenetetramine form a crystalline salt of the stiochiometric composition C6H12N4 SCeHsOH [30] which, when heated, evolves ammonia with the formation of an insoluble, infusible polymer [31]. In the presence of water, hexamethylenetetramine hydrolyzes with the formation of two moles of dimethylolamine DMA, one mole of formaldehyde and two moles of ammonia. Water is ubiquitious in novolacs and therefore under basic reaction conditions in the presence of tert and sec amines and also ammonia as shown in the chart, methylene bridges are formed by entering formaldehyde into the reaction. With increasing amounts of hexamethylenetetramine, the benzylamine type bridges become predominant. 

When heated to about 450 C, hexaphenylcyclotrisilazane forms an infusible polymer (Eq. 4.5) ... 

A dark-brown, insoluble, and infusible polymer was obtained by a similar polycondensation (1-3) (32). Solvents useful for this type of reaction included benzyl alcohol (44), absolute ethanol (50), mixtures of alcohol and Cellosolve (50), and even water (32). The use of piperidine as a basic catalyst was also reported (44). Temperatures ranging from room temperature (32) to 90°C (44) were employed. Reaction times of a few hours are required. 

An insoluble and infusible polymer that possessed semiconducting properties was obtained by a similar polycondensation of diacetylferrocene (38). 

The zinc chloride-catalyzed condensation of polycyclic aromatic hydrocarbons with pyromellitic dianhydride at temperatures of 250°-300°C yielded dark, insoluble, infusible polymers. Their structure was not unequivocally determined. Judging from what is known in the literature, they could possess either a quinone [39] or a lactone [40] type structure, with the former predominating (22, 30). 

For example. Fig. 4.4 shows the result of topical SEM analysis of a drug-infused polymer coating on... 

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