Polyoxymethylene periodic

Tn the cationic polymerization and copolymerization of trioxane in the - melt or in solution, an induction period usually exists, during which no solid polymer is formed and the reaction medium remains clear. Nevertheless, reactions are known to occur during this period. By using BF3 or an ether ate as catalyst, in homopolymerization, Kern and Jaacks (I) reported the formation of formaldehyde via depolymerization of polyoxymethylene cations. 

Spectacular results have been obtained with two-stage drawing of polyoxymethylene by Clark and Scott (1974). In the first stage and polymer is drawn to the natural draw ratio of about 7. The second stage takes place at a very low velocity (50% elongation per minute) up to an ultimate draw ratio of about 20. The optimum temperature for the second step is about 30 °C below the standard melting point. A modulus of 35 GPa, about 70% of the theoretical value has been obtained in this way. The drawn material did not show folded chain periodicity anymore. 

The closely related monomer, sym-trioxan, is a crystalline trimer of formaldehyde, and again polymerizable by cationic means [151, 160]. However, considerable induction periods are observed in its solution polymerization, and the interpretation of kinetic data is additionally complicated by the formation of equilibrium amounts of monomeric formaldehyde as well as polyoxymethylene. 

We have recently shown that solid state polymerizations can also be carried out via plasma initiation ( ). In that work, 1, 3,5-trloxane and 1,3,5,7-tetraoxane were used as monomer crystals. Highly crystalline polyoxymethylene were obtained using either monomer. However, if the monomers were dissolved in an appropriate solvent, such as cyclohexane, then no polymer was formed with plasma initiation. These observations are the reverse of those for AM and MAM, where plasma initiated polymerizations in solution appear to proceed satisfactorily during homogeneous post-polymerization periods, but not in the bulk crystalline state. The unresolved question is then if water molecules may have dissociated in the plasma in highly active radical species, perhaps OH or H, to promote efficient initiation. 

Fig. 4.7. Decay curve of free radicals trapped in irradiated polyoxymethylene at 354 K after heating at room temperature for an extended time period , plot of raw data without any reduction O. plot after subtracting the amount of non-vanishing radicals obtained from Fig. 4-6 solid line indicates calculated decay curve with A = 5.0x 10 s" and B = 1.6x 10" s (Ref.

Acetal, Polyoxymethylene (Delrin/POM), is a robust polymer and an excellent choice for components that are to retain their mechanical strength over long periods (e.g. threaded connectors and adapters). Despite its mechanical strength, it cannot remain in contact with acid or alkaline solutions for a long time without degrading. Therefore, it should only be used with neutral aqueous solutions or organic solutions. 

The variation of lamellar thickness with crystallization temperature for polyoxymethylene crystallized from a variety of solvents is shown in Fig. 4.19(a). For a given solvent the lamellar thickness is found to increase with increasing crystallization temperature. This behaviour is typical of many crystalline polymers such as polyethylene or polystyrene for which the length of the fold period increases as the crystallization temperature is raised. The different behaviour in the various solvents displayed in Fig. 4.19(a) is again typical of solution crystallization and Fig. 4.19(b) is a plot of the lamellar thickness against the reciprocal of the supercooling AT(= T, — Tc) which gives a master curve of all the data in Fig. 4.19(a). This... 

An accurate evaluation of the energetics of different conformations in pol3nners is of basic importance for an understanding of their physical properties. We therefore investigated also other periodic conformations of polyethylene and polyoxymethylene including extensive geometry optimization. Results on the energies of (t), (g), and (tg) helices are collected in table 6,... 

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