Polyoxymethylene polymers

When formaldehyde is heated with concentrated sulphuric acid, it forms the polymer polyoxymethylene, which contains at least 100 monomers. Polyoxymethylene is linear like paraformaldehyde. The main difference between the two of them is the number of monomers in the polymer. When heated, it also decays into formaldehyde. It is widely used in the textile industry. 

The amount of transmitted energy was measured with a fixed thermocouple. Output was recorded with a computer-controlled data acquisition system operating with a sampling rate of 10 Hz. The thermocouple was positioned on a block of white acetal polymer (polyoxymethylene) to avoid any charring in the region of the sensor since this can have a significant effect on temperature readings. 

Less work has been reported on other polymers. Polyoxymethylene and polypropylene have been described as potential high modulus products. Several... 

Stress-strain isotherms have also been calculated with this approach. Examples are unimodal networks of polyethylene and POMS, " polymeric sulfur and seleniirm, short n-alkane chains, natural rubber, several polyoxides, and elastin, and bimodal networks of PDMS. It is possible to include excluded volume effects, in such simulations. In the case of the partially helical polymer polyoxymethylene, the simulations were used to resolve the overall distributions into contributions from imbroken rods, once-broken rods, twice-broken rods, and so on. It was also shown how applying stresses to the ends of chains of this typ>e can be used to bias the distributions in the direction of increased helical content and increased average end-to-end distances. In this sense, imposition of a stress has the same effect on the helix-coil equilibriirm as a decrease in temperature. ... 

In a second paper, Franzen et al. (2008b) extended the scope of their investigation to include four additional polymers polyoxymethylene (POM), polyethylene (PE),... 

Paraformaldehyde [30525-89-4] is a mixture of polyoxymethylene glycols, H0(CH20) H, with n from 8 to as much as 100. It is commercially available as a powder (95%) and as flake (91%). The remainder is a mixture of water and methanol. Paraformaldehyde is an unstable polymer that easily regenerates formaldehyde in solution. Under alkaline conditions, the chains depolymerize from the ends, whereas in acid solution the chains are randomly cleaved (17). Paraformaldehyde is often used when the presence of a large amount of water should be avoided as in the preparation of alkylated amino resins for coatings. Formaldehyde may also exist in the form of the cycHc trimer trioxane [110-88-3]. This is a fairly stable compound that does not easily release formaldehyde, hence it is not used as a source of formaldehyde for making amino resins. 

Acetals. Acetal resins (qv) are polymers of formaldehyde and are usually called polyoxymethylene [9002-81-7]. Acetal homopolymer was developed at Du Pont (8). The commercial development of acetal resins required a pure monomer. The monomer is rigorously purified to remove water, formic acid, metals, and methanol, which act as chain-transfer or reaction-terminating agents. The purified formaldehyde is polymerized to form the acetal homopolymer the polymer end groups are stabilized by reaction with acetic anhydride to form acetate end groups (9). 

Polymers produced by methods as described above have thermal stabilities many times greater than those obtained by the earlier bulk and solution methods of Staudinger. Staudinger had, however, shown that the diacetates of low molecular weight polyoxymethylenes (I) (polyformaldehydes) were more stable than the simple polyoxymethylene glycols (II) (Figure 19.2). 

Staudinger also found that diacetates of polyoxymethylenes with a degree of polymerisation of about 50 were less stable. Truly high molecular weight polyoxymethylenes (degree of polymerisation -1000) were not esterified by Staudinger this was effected by the Du Pont research team and was found to improve the thermal stability of the polymer substantially. 

Chemical Designations - Synonyms Fonnaldehyde polymer Polyformaldehyde Polyfooxymethylene Polyoxymethylene glycol Chemical Formula HO(CHjO) H. 

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

Staudinger relentlessly championed the molecular, or primary valence, viewpoint in the years which followed. He supported his original contentions with the observation that hydrogenation of rubber, as well as its conversion to other derivatives, does not destroy its colloidal properties. In contrast to association colloids, high polymers (or macromolecules as he chose to call them ) exhibit colloidal properties in all solvents in which they dissolve. Polyoxymethylenes were ex-... 

Engineering polymers are often used as a replacement for wood and metals. Examples include polyamides (PA), often called nylons, polyesters (saturated and unsaturated), aromatic polycarbonates (PCs), polyoxymethylenes (POMs), polyacrylates, polyphenylene oxide (PPO), styrene copolymers, e.g., styrene/ acrylonitrile (SAN) and acrylonitrile/butadiene/styrene (ABS). Many of these polymers are produced as copolymers or used as blends and are each manufactured worldwide on the 1 million tonne scale. 

This depolymerisation is inherent in the polymer structure and can be prevented by either making a copolymer (such that when un-zipping reaches the co-monomer moiety it is stopped from going any further (e.g., POM (polyoxymethylene) in which a few percent of ethylene oxide has been incorporated), or by using free radical traps (see anti-oxidants). 

The shape of macromolecules within a folded lamella is not the same for all polymers. In crystalline polyethylene, for example, the chains assume a planar zigzag conformation, but in some other polymers like polypropylene and polyoxymethylene the chains prefer a helical shape, as in proteins. The helix might have three, four, or five monomer units per turn, i.e., the helices are three-, four-, or five-fold (Fig. 1.12)... 

Hydrolytic degradation is especially important in polymers with hydrolyzable links between the CRUs. Thus, polyesters can be saponified to yield the starting materials from which they were formed. Acetal links in synthetic polymers such as polyoxymethylene, or in natural polymers such as cellulose, can be hydrolyzed with acids. However, the resistance to hydrolysis depends very much on the structure of the polymer for example, polyesters of terephthalic acid are very difficult to hydrolyze while aliphatic polyesters are generally easily hydro-... 

Polyacetals. Simionescu and coworkers (28) have extended to polyoxy-methylene the process of grafting vinyl polymers (acrylonitrile and methylmethacrylate). They performed the synthesis using a virbomill at room temperature under vacuum (10.1 Torr). The initial monomer-polymer ratio was 1 -5/1.0 and the degree of vibromill packing 0.44. Before milling the polyoxymethylene granules were dissolved in dimethyl formamide and repredpitated with the aim of stabilizer removal and for reduction of polymer particle size (from 2-2.5 mm to 0.05-0.10 mm). Full details of the reaction have been described (40). 

These compounds are polyoxymethylene blycols. Walker (Ref 1, p 65 Ref 3, p 142) gives a table describing the properties of various formaldehyde polymers... 

All of these polymers are linear with the exception of sym-trioxane [frtrioxymethylene) (HCHO)3] and tetraoxymelhylene HCHO)4, both of which are true polyoxymethylenes, although they are not prepd by the polymerization of monomeric formaldehyde... 

The chains in the polymers discussed so far consist of isochains of catenated carbon atoms. However, many polymers such as the polyacetal— polyoxymethylene (POM), Delrin—have other atoms in addition to the carbon atoms in the polymer chain. As shown by the abbreviated segmental formula for the polyacetals... 

The Tg is related to chain stiffness and the geometry of the polymer chain. Flexible polymers with methylene and oxygen atoms in the chain, such as polyethylene, polyoxymethylene, and polysiloxane (silicone), have relatively low Tg values. The Tg of polyoxymethylene is somewhat higher than would be anticipated because of the dipole character of the C—O—C group, which increases the intermolecular forces and restricts segmental motion. 

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