Oxymethylene copolymers

Fig. 4.170 Flexural-relaxation modulus of an unreinforced and a reinforced poly(oxymethylene) copolymer with 30 wt.-% of glass fibers at 23 °C and different strain levels [98Dom].

Fig. 4.179 Compression-relaxation modulus of a poly(oxymethylene) copolymer for various strain levels [12Els].

The term "acetal resins" commonly denotes the family of homopolymers and copolymers whose main chains are completely or essentially composed of repeating oxymethylene units (—CH2—O—). The polymers are derived chiefly from formaldehyde or methanal [50-00-00] either directly or through its cychc trimer, trioxane or 1,3,5-trioxacyclohexane [110-88-3]. 

Formaldehyde homopolymer is composed exclusively of repeating oxymethylene units and is described by the term poly oxymethylene (POM) [9002-81-7]. Commercially significant copolymers, for example [95327-43-8] have a minor fraction (typically less than 5 mol %) of alkyUdene or other units, derived from cycHc ethers or cycHc formals, distributed along the polymer chain. The occasional break in the oxymethylene sequences has significant ramifications for polymer stabilization. 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Chemical Structure and Properties. Homopolymer consists exclusively of repeating oxymethylene units. The copolymer contains alkyhdene units (eg, ethyUdene —CH2—CH2—) randomly distributed along the chain. A variety of end groups may be present in the polymers. Both homopolymer and copolymer may have alkoxy, especially methoxy (CH3 O—), or formate (HCOO—) end groups. Copolymer made with ethylene oxide has 2-hydroxyethoxy end groups. Homopolymer generally has acetate end groups. 

Random copolymers are similar to PEO but when the regular helical structure of the chains is demolished, the crystallinity is also destroyed. One of the simplest and most successful amorphous host polymers is an oxyethylene- oxymethylene structure in which medium length but statistically variable EO units are interspersed with methylene oxide groups. First described in 1990 [37], aPEO has the general structure... 

The industrial synthesis of polyformaldehyde [poly(oxymethylene)] occurs by anionic polymerization of formaldehyde in suspension. For this the purification and handling of monomeric formaldehyde is of special importance since it tends to form solid paraformaldehyde. After the polymerization the semiacetal end groups have to be protected in order to avoid thermal depolymerization (Example 5-13). This is achieved by esterfication with acetic anhydride (see Example 5-7). As in the case of trioxane copolymers (see Sect. 3.2.3.2) the homopolymers of formaldehyde find application as engineering plastics. 

Both end groups can be determined quantitatively. A second side reaction is the transacetalization. Here a poly(oxymethylene) cation attacks an oxygen of a poly(oxymethylene) chain with formation of an oxonium ion that decomposes. Through continued cleavage and recombination of poly(oxymethylene) chains one obtains polymers which are chemically and molecularly largely homogeneous. For the case of a trioxane/ethylene oxide copolymer the following reaction scheme can be formulated ... 

Reactions with monofunctional reagents are for example carried out in order to increase the thermal and/or chemical stability of the end groups (Poly-oxymethylenes, Example 5-7). Reactions with bifunctional reagents can be used to enlarge the degree of polymerization or to synthesize block copolymers (see Sect. 4.2.1). 

One method of reducing crystallinity in PEO-based systems is to synthesize polymers in which the lengths of the oxyethylene sequences are relatively short, such as through copolymerization. The most notable hnear copolymer of this type is oxymethylene-linked poly(oxyethylene), commonly called amorphous PEO, or aPEO for short. Other notable polymer electrolytes are based upon polysiloxanes and polyphosphazenes. Polymer blends have also been used for these applications, such as PEO and poly (methyl methacrylate), PMMA. The general performance characteristics of the polymer electrolytes are to have ionic conductivities in the range of cm) or (S/cm). 

It is clear, therefore, that polymers synthesized by polycondensation of sugars in acidic methyl sulfoxide are, actually, copolymers of sugars and formaldehyde in which some of the glycosyl residues are joined by glycosidic bonds, and others are linked by bridges of oxymethylene units. Nonetheless, because polycondensation of sugars can be effected in methyl sulfoxide, and because the methods developed for the polycondensation may be applicable to other solvent systems,10 the extensive studies reported by Micheel and his coworkers will be discussed. 

Systems that employed HREELS for interfacial-composition determinations included poly(ethylene oxide)-polystyrene diblock copolymer on Si wafers formaldehyde poly(oxymethylene) films on Cu(lOO) and Lang-muir-Blodgett films of 4,4 -oxydianiline-pyromellitic dianhydride polyimide on Au and on highly ordered pyrolytic graphite. ... 

See Figure 17.7 for two simple examples of the utilization of the shortcut described above. Polyoxytrimethylene can be treated formally as the alternating copolymer of ethylene and oxymethylene. Similarly, poly(p-xylylene) can be treated formally as the alternating copolymer... 

Other types of modifications tried include preparation of ethylene oxide-propylene oxide copolymers [129] as well as polymers containing regular sequences of oxyethylene and oxymethylene units [130]. 

Opitz,35>. Studying the NMR spectra of polymers and copolymers of TXN, DXL and 1,3,5-trioxocane, modeling the various distributions of oxymethylene (M) and oxyethylene (E) units in the resulting polymers, these authors were able to determine the concentrations of MMM, MME and EME triads. The respective chemical shifts are given in Table 7.14. Thus, the chemical shifts of M in triads is not constant, and depends on the neighbouring units. The difference, however, between MMM and EMM triads is only 0.05 ppm 6 for the same TXN-DXL copolymer, which is insufficient to distinguish between these triads with a lower resolution apparatus. 

This results in the random distribution of C-C bonds in the polymer chain. The depolymerization of ethylene oxide units is much more difficult than that of oxymethylene units. Thus copolymerization confers thermal stability on the acetal copolymer. The copolymer exhibits good property retention when exposed to hot air at temperatures up to 220°F or water as hot as 180°F for long periods of time. For intermittent use, higher temperatures can be tolerated. 

PREPARATIVE TECHNIQUES The homopolymer is prepared by anionic polymerization of purified formaldehyde with the addition of an initiator such as an amine, phosphine, or metal alcohol. The copolymers are manufactured commercially by copolymerization of trioxane, the cyclic trimer of formaldehyde, with small amounts of a comonomer. T5qjically, acetal copolymer reisns have 95% or more oxymethylene units. 

Acetal, (Polyacetal) Poly-oxymethylene (POM) Acetal is a polymer obtained through an addition reaction of formaldehyde — (CH2—0) . It excels in mechanical performance and is regarded as a prominent engineering polymer. It appeared in 1959 with the commercial name Delrin . A short time later a useful copolymer was also developed with a cyclic ether like ethylene oxide. The monomer formaldehyde is a gas produced mostly by oxidizing methanol, and it is very useful in thermoset polymers like phenol, urea and melamine-formaldehydes. For high purity it is initially converted to trioxane or paraformaldehyde. The polymerization is carried out by ionic mechanism, wherein the monomer is dispersed in an inert liquid (heptane). The molecular weights reach 20,000 to 110,000. 

Copolymers from a single monomer can be obtained by copolymerization of a preformed unit or by partial isomerization before or during polymerization. Polymerization of preformed units can occur with a ringopening polymerization or with a cyclopolymerization. For example, the ring-opening polymerization of 1,3-dioxolan leads to a copolymer of alternating oxymethylene and oxyethylene units ... 

Poly(oxymethylene), with the monomeric unit -(rCUiO, can be produced from formaldehyde, HCHO, or its cyclic trimer, trioxane (1,3,5-trioxacyclohexane). The commerical polymer obtained from formaldehyde is also known as polyacetal homopolymer, and that from trioxane is known as polyacetal copolymer. 

The simultaneously occurring transacetalization then provides for a random distribution of ethylene oxide residues in the copolymers. The ethylene oxide groups terminate depolymerization of the oxymethylene chains if the depolymerizations are adventitiously started by chain scission. 

---