Solution properties of polyoxymethylene

Light scattering and viscosity have been measured at 25°C. for dilute solutions of six unfractionated polyoxymethylene samples in the mixed solvent hexafluoroacetone–water (mole ratio 1/1.7) slightly buffered with triethylamine. Dialysis equilibrium through porous Vycor glass thimbles indicates that the polymer is strongly solvated by the hydrate (CF₃)₂C(OH)₂, and this must be taken into account in evaluating weight-average molecular weights from the light-scattering data. Over the molecular weight range 23,000–185,000, the intrinsic viscosities (in deciliter per gram) follow the relation The corresponding unperturbed dimensions are σ = 2.3 ± 0.2 or r₀²/nl² = 10.5 ± 1.5.     

Thermal expansion of polyoxymethylene

The thermal expensivities of polyoxymethylene crystals in the direction parallel (α_|^c) and perpendicular (α_‖^c) to the chain axis have been measured from 160 to 400 K using wide-angle x-ray diffraction. Although polyoxymethylene has a helical chain structure, it exhibits a thermal expansion behavior similar to that of polymer crystals with planar zigzag chains, namely that α_‖^c is negative while α_|^c is positive and larger by an order of magnitude. The negative α_‖^c arises from the shortening along the chain axis caused by the torsional and bending motions of the chain, whereas the large and positive α_|^c reflects the weak interaction across the chains. Combining the crystal data with dilatometric measurements on semicrystalline samples, the thermal expansivity is found to vary linearly with crystallinity, thus allowing the expansivity of the amorphous phase to be derived by extrapolation. With the thermal expansivities of the crystalline and smorphous phases known, the draw ratio dependence can be calculated in terms of existing models and is found to agree reasonably with experimental data.     

Hydrostatic extrusion of polyoxymethylene

The hydrostatic extrusion behavior of polyoxymethylene (POM) is described. Extrusions were performed at 164°C for a range of different molecular weight grades. Excellent unflawed lengths of extrudate were obtained with axial Young's moduli, measured at room temperature, reaching values as high as 24 GPa. The extrusion characteristics are discussed in terms of the very strong dependence of the flow stress of POM on strain and strain rate and pressure. In addition to measurements of Young's modulus over a wide temperature range, data for shear modulus and transvers modulus are also presented. A limited amount of other structural measurements is presented.     

Segmental motion in polyoxymethylene

The proton spin-lattice relaxation times (T₁) of melt-crystallized, solution-crystallized, and solid-state-polymerized polyoxymethylene (POM) were measured between ?60 and +150°C. The three types of samples each have a pronounced T₁ minimum near room temperature which is a high-frequency manifestation of the γ process. From the quantitative dependence of the relaxation intensity on crystallinity as well as from the absolute magnitude of the relaxation times, it is concluded that the γ process in POM arises from hindered rotation of noncrystalline chain segments. The relation between the relaxation times and the long period indicates that these noncrystalline segments constitute disordered lamellar surface layers, the thickness of which depends on thermal history of the material. The temperature dependence of the motion of the relatively thin surface layers of solution crystallized POM is quite straightforward. The γ process in the bulk-crystallized material involves cooperative motion, however, leading to temperature-dependent kinetic parameters.     

Slip-step on polyoxymethylene crystals

Crystals of polyoxymethylene have been grown from solution in bromobenzene. In many growth features, which are similar to those for monomeric crystals, the movement of dislocation can be traced by a slip-step raised in the growth surface. Such slip-steps are associated with single spirals, two spirals of opposite senses and two spirals of the same sense. Anomalies in the growth patterns of crystals have also been observed and attributed to the movement of screw dislocation long before the cessation of growth.     

Thermal expansion of irradiated polyoxymethylene

The thermal expansion coefficient of gamma irradiated polyoxymethylene has been measured in the temperature range 80–340 K by using a three terminal capacitance technique. The radiation induced changes are measured by recording the i.r. spectra of the irradiated samples. The change in crystallinity caused by irradiation is measured by an X-ray technique. The thermal expansion coefficient increases with radiation dose below 170 K due to the predominant effect of degradation. Above 170 K, this trend reverses and the expansion coefficient decreases with radiation dose due to the increased crystallinity caused by irradiation.     

Extra meridional reflections of drawn polyoxymethylene

Extra meridional reflections (00l: l = 3, 5, 13, 15, 21) were observed in drawn polyoxymethylene (POM). The 003, 0015, and 0021 x-ray diffraction intensities increase on annealing, while the 005 and 0013 intensities decrease. It is concluded that crystalline regions are of two kinds: in the first, which generates the 003, 0015, and 0021 reflections, the molecular conformation is slightly distorted from the uniform helix by intermolecular steric hindrance between the CH₂ groups; and in the second, which generates 005 and 0013 reflections, the molecular conformation is distorted in another way. The second form is converted into the first by annealing. The displacements in the c direction of the CH₂ groups and the oxygen atoms from the uniform helix in the first form are about 1.0% and 0.75% of the interval between a CH₂ group and an oxygen atom along helix axis of the POM crystal; thus the correct conformation in the stable state is known. The nature of distortion in the second region is not clarified but several characteristics are offered.     

Vibrational spectra of hexagonal crystalline polyoxymethylene

Infrared and Raman spectra of two hexagonal crystalline forms of polyoxymethylene were measured, and the characteristic infrared bands have been determined. The Raman spectra of these two forms do not differ. The vibrational spectra of both forms can be explained by the occurrence of defects in the helical structure of one of them.     

Thermal dimensional stability of isotropic polyoxymethylene

A parametric mapping analysis of the effect of various structural and morphological parameters on the thermal dimensional stability of polyoxymethylene is highlighted. The molecular composites model is found to describe adequately the expansional behavior of isotropic polyoxymethylene. The thermal expansivity is highly sensitive to the level of crystallinity followed by the transverse dimension of the crystallites. The longitudinal dimension of the crystallites, however, is not effective in altering the expansion behavior of isotropic polyoxymethylene. When comparing the model calculations with experimentally published data, an agreement of better than 15% is observed.     

Least-squares refinement of molecular structure of polyoxymethylene

Least-squares refinement of the molecular structure of polyoxymethylene was carried out by using intensity data measured with an automatic four-circle diffractometer. The discrepancy factor was improved to 4.1% by using the anisotropic temperature factor. Physically significant values were obtained for the bond length, bond angles, and internal rotation angle: C? O = 1.429 ± 0.008 Å, ∠COC = 112.9 ± 0.6°, ∠OCO = 110.4 ± 0.8°, and τ(C? O) = 77.0 ± 0.3°. Samples for x-ray diffraction measurements were prepared by solid state polymerization of a tetroxocane single crystal.     

DSC non-isothermal crystallization curves in polyoxymethylene

Non-isothermal crystallization curves by differential scanning calorimetry for two-dimensional spherulite growth in polyoxymethylene have been modeled using the Avrami equation, taking into account heat dissipation in the sample holder during the phase transition. Good agreement is found for scanning rates up to 40 Kmin^–1 between experimental curves and predictions based on a knowledge of the isothermal crystallization kinetics of the same samples.     

The rheological and crystallization behavior of polyoxymethylene

In the polymer melt processing, the solidification has a huge importance on the properties of the resulting part. For a semi-crystalline resin, this phenomenon involves a complex interplay between crystallization and the material rheology. In this work, an investigation is carried out on the influence of thermal conditions on crystallization kinetics and rheology of two commercial polyoxymethylene (POM) copolymers. In particular, isothermal crystallization experiments using differential scanning calorimetry (DSC) and rotational rheometry to measure the dynamic viscosity are performed. The evolution of the relative crystallinity and Normalized Rheological Function (NRF) are correlated by a recent technique which allows simultaneous analysis of several measurements, even if they are not carried out at same temperatures. On this basis, a relationship between the crystallinity and the hardening, i.e. the sharp increase in the viscosity, is obtained.     

Radical and void formation in ultradrawn polyoxymethylene

In the production of ultrahigh-modulus (45–60 GPa) polyoxymethylene (POM) by microwave heating drawing, ultradrawing to draw ratios over 20 causes formation of numerous voids and radicals. The volume fraction of the internal voids is proportional to the drawing-induced radical concentration. On the basis of the idea that the rupture of taut tie-molecules (TTM) is responsible for void and radical formation, the relationship between the fraction of tiemolecules that are taut and the fraction that are ruptured is discussed by assuming both a Takayanagi structure model and the normal distribution of the tie-molecule lengths. The results indicate that longer tie-molecules are successively converted to TTM's followed by ruptures of shorter TTM's in the ultradrawing process; a small variance in the tie-molecule length distribution is effective for increasing TTM's without void formation. The maximum value of the volume fraction of taut tie-molecules for highly oriented POM tubes is estimated to be about 0.5.     

聚甲氧基二甲醚的合成及其物理化学性质表征

聚甲氧基二甲醚（H₃CO（CH₂O）_nCH₃, PODE_n或DMM_n, n ≥ 2）具有独特的物理化学性质;作为一种柴油添加剂,可以有效提高油品燃烧效率并达到节能减排的目的。首先合成了一系列聚合度n为2、3、4和5单一组分的聚甲氧基二甲醚,采用NMR、FT-IR、Raman和DFT计算等手段对每个聚甲氧基二甲醚单体的化学结构进行表征,并对其在298.15-323.15K温度的密度和黏度进行了测试。结果表明,聚甲氧基二甲醚的密度和黏度随着温度的升高而逐渐降低,随着聚合度的增加而逐渐升高。同时,聚甲氧基二甲醚PODE_n（n =2-5）的闪点和倾点以及溶解热和凝固热均随着聚合度的增加而提高。     

分子筛催化剂催化合成聚甲氧基二甲醚

以甲醇、甲缩醛和三聚甲醛为原料,考察了分子筛孔道结构、晶粒粒径、酸性及反应体系对合成聚甲氧基二甲醚反应性能的影响,并筛选出最佳反应条件。结果表明,HMCM-22分子筛具有良好的催化性能。在最佳反应条件下,以甲醇与三聚甲醛为原料,DMM2~8的选择性为65.1%;而以甲缩醛和三聚甲醛为原料,DMM2~8的选择性则达到90.6%。NH3-TPD结果表明,弱酸位有利于三聚甲醛解聚为甲醛,中强酸位促进长链分子生成,而强酸位导致DMM为主要产物。     

Radiation aging and chemi-crystallization processes in polyoxymethylene

The radiochemical degradation of a polyoxymethylene homopolymer (POM) was used to study the effects of molar mass changes in the crystalline structure. The dose rate was 20 kGy h⁻¹ with doses of up to 30 kGy used. Both WAXS and SAXS were used to analyse the structures. Results showed that, under irradiation, the polymer undergoes random chain scission. The radiochemical yield was found to be G = 1.6 chain scission events per 100 eV. It was found that no crosslinking occurs and that only one chain scission mechanism, leading to the formation of formate groups, operates. Proof for the existence of chemi-crystallization is evidenced by (i) an increase in the crystallinity ratio as well as (ii) a decrease in the amorphous layer thickness. Simple models, derived from Rault’s theory, are used to predict both (i) and (ii) from molar mass values.     

A source of screw dislocation in polyoxymethylene crystals

Polyoxymethylene crystallizes as hexagonal lamellar crystals accompanied by spiral growth from dilute bromobenzene solution. Spirals of this polymer are formed by several mechanisms. There is discussion of various origins of screw dislocation due to interlocking of two independently growing crystals with crevices. After entanglement, various growth stages of crystals have been observed.