Search

The properties of ultra-oriented polyoxymethylene tubes produced by drawing under microwave heating have been assessed by mechanical testing, optical microscopy, scanning electron microscopy, X-ray analysis, birefringence and differential scanning calorimetry. The highest Young's modulus of 58 GPa was obtained at room temperature (77 GPa at −150°C) at a draw ratio of 33. The maximum tensile strength was 1.7 GPa at a draw ratio of 26. The nonuniformity of Young's modulus in a radial direction has been compared with the nonuniformity of the birefringence and heat of fusion.

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.

The drawing behaviour of a series of poly(oxymethylene) homopolymers with number-average molecular weight ( M n ) ranging from 38 000 to 66 000 has been studied at a feed speed of 0.06 m min−1 under microwave heating. Tubes with outer and inner diameter of 4 mm and 1 mm, respectively, were used for drawing experiments. The optimum tensile load and ambient temperature for high modulus increased with M n . The highest tensile modulus (54 GPa) and strength (1.6 GPa) were obtained for higher M n samples, although the attainable draw ratio decreased from 34 to 28 with increase of M n .

The influence of tensile load and precursor size on the drawing behaviour of polyoxymethylene tubes, with outer diameters of 3, 4 and 6 mm, has been studied at a feed speed of 6 cm min −1 under microwave heating. As the temperatures of sample portions are difficult to measure, tensile load is used as a corresponding parameter. It is shown that there is an optimum tensile load for a given tube diameter to achieve high modulus and high strength over all draw ratios. The optimum tensile load gives rise to uniform heating which is confirmed by differential scanning calorimetry. The attainable dynamic modulus decreased from 59 GPa to 51 GPa with increase in the precursor tube diameter from 3 to 6 mm.

We have employed an ultrasonic method to measure from −40 to 60°C the five independent elastic moduli C11, C13, C33, C44, and C66 of polyoxymethylene with draw ratio λ from 1 to 26 prepared by continuous drawing under microwave heating. The elastic moduli are controlled by three major factors: molecular orientation in the crystalline regions, fraction of noncrystalline taut tie molecules, and void content. The steep rise in the axial extensional modulus C33 and axial Young's modulus E0 with increasing draw ratio results from the alignment of chains in the crystalline blocks and an increase in the number of disordered taut tie molecules. Below the γ relaxation (located at 0°C at our measurement frequency of 10 MHz), these two factors also give rise to a slight decrease in the transverse extensional modulus C11, Young's modulus E90 and shear modulus C66. At high temperature where the amorphous regions have very low modulus, the stiffening effect of taut tie molecules becomes dominant, leading to an increase in all moduli as λ increases from 1 to 10. At higher λ the void fraction increases appreciably, causing small decreases in E90, C11, and C66 at all temperatures.

Dynamic mechanical measurements on highly oriented poly(oxymethylene) produced by microwave heat-drawing have been undertaken at four frequencies (3.5, 11, 35 and 110 Hz) over the temperature range from −150°C to the melting point. The dynamic modulus at 110 Hz reached a value of 85 GPa at −150°C for the most highly-oriented sample (draw ratio, 41). The α and γ absorptions are clearly discernible, even at the highest draw ratios. The γ absorption can be divided into two absorptions, i.e. a low-temperature component due to crystal defects and a high-temperature one due to amorphous regions.

A tensile drawing process under dielectric heating has been developed for polyoxymethylene. The influence of ambient temperature, electric field strength, and strain rate on the maximum draw ratio and the tensile modulus has been examined. Tubes possessing tensile moduli up to 63GPa were produced by the new drawing technique. It is speculated that the achievement of such ultrahigh moduli is due to the fact that the stress by drawing is used effectively to orient the molecular chains in the noncrystalline regions and at defect regions within the crystal lamellae. This is because these regions are heated to higher temperatures than the crystalline regions during dielectric heating.