Ultrasonic measurements of the elastic moduli of ultradrawn polyoxymethylene

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.     

Sorption and diffusion of toluene in isotropic and oriented linear polyethylene

Sorption and diffusion of toluene vapor in linear polyethylene with mass-fraction crystallinity between 0.48 and 0.82 and draw ratios λ up to 10 have been studied at 30°C. The sorbed concentration in the amorphous phase C_a is little affected by crystallinity, indicating that the free-volume fraction is roughly the same for all isotropic samples. However, the diffusion path becomes more tortuous with increasing crystalline content, thereby leading to a sixfold drop in the zero-concentration diffusion coefficient D₀. Drawing has more drastic effects, reducing C_a and D₀ by factors of 4 and 60, respectively, as λ increases to 10. These large changes result from the transformation of the initially spherulitic material into a fibrous structure, which is composed of aligned microfibrils with taut tie molecules lying on the outer boundaries. The effects of crystallinity and orientation on the concentration dependence of the diffusion coefficient are also discussed.     

Orientation of polypropylene fibre

The modulus of elasticity and the degree of orientation of the amorphous phase of a polypropylene fibre drawn at 130 °C in the range of drawing ratios between 2 and 7.1 was determined by the sonic method and by birefringence. The degrees of orientation of the amorphous phase determined by the two methods differ from each other. This has been explained by a model concept wherein the intrafibrillar taut tie molecules affect the mechanical, but not the optical properties of the system; the lamellar structure becomes fibrillar already at the drawing ratioλ=2 and the orientation of the fibrils is almost completed at the drawing ratioλ=4. Further drawing leads to the strain of interfibrillar tie molecules; atλ=6.5 the fibrils slide past each other, and interfibrillar tie molecules become strained.     

Mechanical anisotropy in rolled nylon 66

The nine independent stiffness constants C_pq for rolled nylon 66 at thickness reduction ratios λt = 2.4, 4.9, 7.6 have been measured from ?40 to 50°C by an ultrasonic method at 10 MHz. Analysis of x-ray pole figures indicates that at high λt the material has a uniplanar-axial texture, with hydrogen-bonded (010) planes parallel to the roll plane and the molecular chains along the roll direction. The mechanical behavior is determined not only by the alignment of molecular chains and hydrogen bonds but also by the microfibrillar morphology. On the basis of this idea the magnitudes of the tensile moduli along the three principal axes can be understood in terms of the Takayanagi model. The effects of water absorption have also been investigated.     

Structure of iodide ion arrays in iodinated nylon 6 and the chain orientation induced by iodine in nylon 6 films

Two species of iodide ions (I₃^? and I₅^?) are found in iodine—nylon 6 complexes. Orientation of I₅^? arrays (most likely I₂/I₃^? complex) along the polymer chain and I₃^? ions perpendicular to the chain axis in uniaxially drawn films and in films with planar orientation suggests that there is and intrinsic relation between the direction of iodide ion arrays and nylon 6 chains. When an unoriented film of nylon 6 in the amorphous or the α crystalline form is treated with an aqueous solution of iodine—potassium iodide, the I₃^? species in the resulting iodine—nylon complex lie in planes parallel to the surface of the film, and I₂/I₃^? units are oriented normal to the surface of the film. The γ form obtained by desorbing the iodine from this complex shows considerable uniaxial rientation with the nylon chains oriented perpendicular to the plane of the film; this orientation is maintained during the γ to α transition. It is proposed that the iodine-induced orientation of the nylon 6 chains is due to the nucleating effects of the iodide ion species as the iodine diffuses unidirectionally into the film.     

Relationship between piezoelectricity and superstructure in highly oriented poly(vinylidene fluoride) film prepared by zone drawing

The oriented superstructure of poly(vinylidene fluoride) is controlled by using a forced-quenching type of zone drawing apparatus. Systematic variation of the weight fraction χ(I) of form-I crystals and the orientation function f_a of amorphous chains shows that the piezoelectricity increases with increasing χ(I) and f_a. A change in the state of molecular aggregation during poling is also effective in increasing the piezoelectricity and the orientation of the crystal b axis along the poling direction. Equations relating piezoelectricity to the form-I crystallinity, the orientation of amorphous chains, and the orientation of the crystal b axis along the poling direction are derived. These are based on a mechanical model having regions of taut tie molecules in parallel with composite regions consisting of crystalline and amorphous blocks in series.     

Elastic moduli of ultradrawn polyethylene

The five independent elastic moduli C₁₁, C₁₂, C₁₃, C₃₃, and C₄₄ of oriented high-density polyethylene with draw ratio λ from 1 to 27 have been determined from ?60 to 100°C by an ultrasonic method at 10 MHz. At low temperature the sharp rise in the axial extensional modulus C₃₃ with increasing λ and the slight changes in the other moduli result from chain alignment and the increase in the number of intercrystalline bridges connecting the crystalline blocks. At high temperature (say, 100°C) the transverse extensional modulus C₁₁, as well as the axial (C₄₄) and transverse (C₆₆) shear moduli, also show substantial increases, reflecting the prominent reinforcing effect of stiff crystalline bridges in this temperature region where the amorphous matrix is rubbery. If the crystalline bridges are regarded as the fiber phase, the mechanical behavior can be understood in terms of the Halpin–Tsai equation for aligned short-fiber composites.     

Mechanical and transport properties of highly drawn isotactic polypropylene

Quenched films of isotactic polypropylene were drawn at 110°C up to draw ratio λ = 18. The axial elastic modulus was measured as function of λ up to the highest achieved λ. The sorption and diffusion of CH₂Cl₂ at 25°C in the undrawn and drawn samples were studied. Exclusively transparent samples were used for the measurement of the density and transport properties. This reduces the maximum usable draw ratio to 15. The drawing process is inhomogeneous with neck propagation. In the neck the draw ratio increases by about 6. As a consequence of the increasing fraction of taut tie molecules the axial elastic modulus increases faster than the draw ratio. The transport parameters D, S, and λ indicate that the original lamellar morphology is completely transformed into the microfibrillar structure.     

Dynamic mechanical properties of random ethylene/1-octene copolymers prepared by rapid cooling

The dynamic mechanical properties of a well-characterized series of homogeneous ethylene/1-octene copolymers with different random hexyl branch contents and prepared using different cooling conditions have been examined using dynamic mechanical analysis (DMA). It was confirmed that the relaxation behavior of copolymers varied continuously with the branch content: the magnitude of the β relaxation increased with branch content while the intensity of the α relaxation decreased with the branch content; both relaxation temperatures decreasing with increasing branch level in the copolymers. Copolymers prepared at different cooling conditions were further examined and strikingly continuous changes were found for the first time. The β relaxation was shown to correlate to the amorphous region, while the α₁ and α₂ relaxations can be clearly differentiated for some samples and are assumed to be associated with the interlamellar slip and intra-crystalline c-shear processes respectively. With increasing cooling rate, the relative intensity of α₁ relaxation to α₂ relaxation was found to decrease while the β relaxation did not change. The most informative data is determined from deconvolution of tan δ spectra. In higher crystallinity polymers the α₁ and α₂ relaxations are closely related in activation energy but have different temperature locations. For lower crystallinity systems, where the α₁ relaxation cannot be observed, the α₂ and β relaxations are closely linked, with activation energies approaching one another. These results show very clearly that, although the observed relaxation data can be separated through deconvolution into three separate peaks, the behaviors are closely linked. Presumably, this a clear reflection of the role of tie molecules in binding phases together and in influencing dynamic mechanical behavior. A clear change of behavior has also been observed in the β relaxation when a distinct amorphous phase exists outside of the spherulites, confirming the general belief that the crystalline phase influences the amorphous phase when it is confined within a spherulite. Again, this behavior is reflecting the role of tie molecules in binding together the nanocomposite structure of a spherulite.     

Orientation behavior of high-modulus polyoxymethylene produced by microwave heating drawing

The orientation behavior of high-modulus polyoxymethylene tapes produced by tensile drawing with microwave heating has been investigated over the draw ratio range 10–29. Young's modulus E increases monotonically with draw ratio λ and reaches 55 GPa. The volume fraction of taut tie molecules (TTMs) in the amorphous phase has been estimated by using a Takayanagi model for oriented tapes. The increase in E at draw ratios of less than 10 is mainly due to the increase in crystalline orientation (crystalline orientation function, 0.00 → 0.99). The increase in E at draw ratios of more than 10 is due to the increase both in crystallinity (volume-fraction crystallinity, 0.84 → 0.95) and in TTM (TTM fraction, 0.14 → 0.40). The maximum Young's modulus obtainable by this method of drawing is estimated to be ca. 72 GPa from the relation between 1/E and 1/λ².     

Dielectric relaxation in the odd-numbered polyamides: Nylon 7-7 and nylon 11

Odd-numbered polyamides have dipoles with components that point in the same direction in the planar zigzag conformation (rather than alternating as in the even-numbered polyamides). An investigation of the dielectric properties of two odd nylons was undertaken here to see if dipole correlation resulting from this effect could be detected. It was found that the dielectric constant and relaxation strength associated with the α loss process (amorphous phase glass–rubber relaxation) when normalized for dipole concentration was indeed markedly higher in the odd nylons. The effect of uniaxial orientation induced by solid-state extrusion on the isotropy of the dielectric properties was also studied. A previous study on nylon 6–10 found a reduction in relaxation strength of the α process in both the parallel and perpendicular directions but with the perpendicular direction being much stronger than the parallel. A similar result was found here for the odd polyamides. Thus the reduction in intensity appears to be due to effective reduction in the amount of amorphous material and not due to any change in dipole correlation induced by orientation. The anisotropy in relaxation strength can be explained by a composite material effect resulting from parallel stacking of crystal lamellas and interlamellar amorphous material and without invoking any anisotropy in the amorphous-phase dielectric strength. Thus there is no reason to conclude that the amorphous phase undergoes orientation as a result of the crystal-phase orientation.     

Mechanical and transport properties of drawn crosslinked low-density polyethylene

The values of drawing dependence of the density ρ, axial elastic modulus E, and maximum draw ratio λ of crosslinked low-density polyethylene (CLPE) rather similar to those obtained with un-crosslinked branched material of similarly low density. Very much the same applies to the equilibrium concentration of sorbed methylene chloride in the amorphous component and the zero-concentration diffusion coefficient D₀. The exponential concentration coefficient γD , however, even at the maximum draw ratio, shows no indication of the rapid increase so characteristic of the completed transformation from the lamellar to the fibrous structure. On the basis of this finding, one can understand the small deviations in the dependence of the mechanical properties between the crosslinked and uncrosslinked branched material. The segments between the crosslinks, much shorter than the free molecules, favor the formation of the interfibrillar tie molecules that limit the drawability of the sample. But since they cannot be extended to the same length as the free molecules, they contribute less to the total fraction of tie molecules per amorphous layer and hence yield a smaller axial elastic modulus.     

Pulsed NMR observation of ultradrawn polypropylene

Proton spin-spin relaxation times have been measured as a function of temperature for ultradrawn polypropylene with draw ratios λ up to 24. The three relaxation times T_2a (the longest), T_2i (intermediate), and T_2c (the shortest), observed for all the samples, have been ascribed to the relaxations of the amorphous, constrained amorphous, and crystalline components, respectively. T_2i and T_2a, which reflect the changes in structure and mobility in the noncrystalline regions, decrease with increasing λ; T_2i becomes saturated at λ > 9, whereas T_2a shows a substantial decrease up to λ = 24. The continued decrease in T_2a indicates that the constraint on the amorphous segments keeps increasing up to the highest λ. The associated mass fractions F_a, F_i, and F_c also change with λ. At λ < 9, the increasc in F_i with increasing λ is accompanied by a decrease in F_a, with F_c remaining unchanged. At higher λ, however, F_a is almost constant, and stepwise rises in F_c at about λ = 12 and 24 are accompanied by corresponding drops in F_i. It seems that, in this high draw ratio range, some of the taut molecules are fully extended and are in sufficiently good lateral register to transform into crystalline bridges. This conjecture is supported by the similarity in the λ dependence of F_c and the mass-fraction crystallinity obtained from the heat of fusion.     

The fatigue process in highly oriented nylon 6 fibers

The micromechanism of the fatigue process in highly oriented nylon 6 fibers is discussed on the basis of changes in mechanical and structural properties during fatiguing. Experimental results show that the fatigue process can be divided into two stages. The characteristic features in the initial period are increases in breaking strength, long period, and molecular orientation, and a reduction in dye penetration. In the second period, after about 500 cycles, breaking strength and orientation decrease slightly, and the long period, permanent strain, and dye penetration increase with duration of fatiguing. It is demonstrated that the structural changes mainly occur in the amorphous regions of the fiber structure. The structural and mechanical changes in the initial period lead to the conclusion that the initial cyclic strain causes strain hardening caused by extended tie chains which do not rupture. A combination of load bearing by tie chains and sliding motion of the fibrillar elements can explain the progressive degradation of the fiber during the second stage of fatiguing.     

Field-induced dipole reorientation and piezoelectricity in heavily plasticized nylon 11 films

Nylon 11 films with very low initial crystallinity were made by dissolving the nylon 11 in 2-ethyl-1,3-hexanediol at 150°C. Films were cast from the solution and excessive plasticizer was removed in a vacuum oven. Films were then melt pressed and quenched to yield heavily plasticized nylon 11 films containing ca. 30% by weight of the plasticizer. These films were poled under vacuum to allow the plasticizer to evaporate in the presence of an electric field. A high piezoelectric response (d₃₁ = 7.1 pC/N) was observed for the films subjected to the maximum electric field (E_p = 350 kV/cm) while the sample contained a large fraction of plasticizer. Significant development of crystallinity was observed without apparent indication of orientation of the crystallites. These studies suggest that the observed piezoelectric response originates primarily from oriented hydrogen bonds in the amorphous regions of nylon 11.     

Interfacial dilational properties of anionic gemini surfactants with polymethylene spacers

The dilational properties of anionic gemini surfactants alkanediyl-α,ω-bis(m-octylphenoxy sulfonate) (C₈C_mC₈) with polymethylene spacers at the water–air and water–decane interfaces were investigated by oscillating barriers and interfacial tension relaxation methods. The influences of oscillating frequency and bulk concentration on the dilational properties were explored. The experimental results show that the linking spacer plays an important role in the interfacial dilational properties. The moduli pass through one maximum for all three gemini surfactants at both water–air and water–decane interfaces. However, the values of moduli at the water–air interface are obviously higher than those at the water–decane interface because the sublayer formed by spacer chains will be destroyed by the insertion of oil molecules. Moreover, with the increase of spacer length, the surface adsorption film becomes more viscous at high concentration, which can be attributed to the process involving the formation of the sublayer. On the other hand, the spacers of the adsorbed C₈C₆C₈ molecules will extend into the oil phase when the interface is compressed. As a result, the interfacial film becomes more elastic with the increase of spacer length at high concentration. The experimental results obtained by the interfacial tension relaxation measurements are in accord with those obtained by the oscillating barriers method.     

Effects of orientation on the mechanical and dielectric relaxation behavior of cycloaliphatic polyester networks

The mechanical and dielectric relaxation behavior of strained and unstrained networks, prepared from hydroxyl-terminated poly(diethylene glycol-trans-1,4-cyclohexane dicarboxylate) (PDGC), is studied over a wide interval of frequencies and temperatures. The mechanical relaxation spectrum exhibits a glass-rubber absorption, designated β, located in the vicinity of 0°C at 0.1 Hz, followed by a β relaxation which appears to be the result of two overlapping peaks centered at ?80°C (β₁) and ?110°C(β₂). These two peaks coalesce into a single peak in the case of strained networks. The dielectric relaxation spectrum also exhibits an α absorption followed by a subglass β relaxation whose width decreases as the elongation ratio λ increases. The activation energy associated with the mechanical β₁ appears to increase as λ increases. However, the activation energy of the dielectric β process does not show a clear dependence on the elongation ratio. The analysis of the conformational characteristics of PDGC chains indicates that rotational transitions through the C_cy? C* bonds of the acid residue would give rise to high dielectric activity. Conformational changes about the CH₂? CH₂ bonds of the glycol residue would produce significant mechanical activity but, comparatively, low dielectric activity. The glass-rubber absorption is slightly displaced toward the high-temperature side as the elongation ration increases, suggesting that the entropic effects overcome the volume effects. The glass-rubber transition is interpreted in terms of the free volume theory.     

Microstructural investigation of high‐speed melt‐spun nylon 6 fibers produced with variable spinning speeds

The structural details of high‐speed melt‐spun nylon 6 fibers at spinning speeds ranging from 4500 to 6100 m/min were investigated by solid‐state proton nuclear magnetic resonance (¹H NMR) spectroscopy, density and birefringence measurements, differential scanning calorimetry (DSC), and X‐ray diffraction (XRD). The analyses of the proton spin‐lattice relaxation times in the rotating frame and correlation times confirmed the existence of three different phases, the immobile crystalline, intermediate rigid amorphous, and mobile amorphous regions, in the fiber sample. At spinning speeds lower than 5200 m/min, the portion of the crystalline phase increased at the expense of the rigid amorphous region and then reached a plateau afterward, from which the mobile amorphous portion increased. Combined analyses of density and birefringence measurements, DSC, and XRD in conjunction with NMR results indicated that the formation of the γ crystal became predominant compared to that of the α crystal. The orientation factor of the crystalline phase increased slightly with increasing spinning speed, whereas the amorphous orientation factor decreased because of the increase of the purely amorphous region. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1285–1293, 2000