Microstructure and mechanical properties of hot‐air drawn poly(ethylene terephthalate) fibers

Hot‐air drawing method has been applied to poly(ethylene terephthalate) (PET) fibers in order to investigate the effect of strain rate on their microstructure and mechanical properties and produce high‐performance PET fibers. The hot‐air drawing was carried out by blowing hot air controlled at a constant temperature against an as‐spun PET fiber connected to a weight. As the hot air blew against the fibers weighted variously at a flow rate of about 90 ℓ/min, the fibers elongated instantaneously at a strain rate in the range of 2.3–18.7 s⁻¹. The strain rate in the hot‐air drawing increased with increasing drawing temperature and applied tension. When the hot‐air drawing was carried out at a drawing temperature of 220°C under an applied tension of 27.6 MPa, the strain rate was the highest value of 18.7 s⁻¹. A draw ratio, birefringence, crystallite orientation factor, and mechanical properties increased as the strain rate increased. The fiber drawn at the highest stain rate had a birefringence of 0.231, degree of crystallinity of 44%, tensile modulus of 18 GPa, and dynamic storage modulus of 19 GPa at 25°C. The mechanical properties of fiber obtained had almost the same values as those of the zone‐annealed PET fiber reported previously. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1703–1713, 1999     

Strain‐rate dependence of the microstructure and mechanical properties for hot‐air‐drawn nylon 6 fibers

A hot‐air (HA) drawing method was applied to nylon 6 fibers to improve their mechanical properties and to study the effect of the strain rate in the HA drawing on their mechanical properties and microstructure. The HA drawing was carried out by the HA, controlled at a constant temperature, being blown against an original nylon 6 fiber connected to a weight. As the HA blew against the fiber at a flow rate of 90 liter/min, the fiber elongated instantaneously at strain rates ranging from 9.1 to 17.4 s⁻¹. The strain rate in the HA drawing increased with increasing drawing temperature and applied tension. When the HA drawing was carried out at a drawing temperature of 240 °C under an applied tension of 34.6 MPa, the strain rate was at its highest value, 17.4 s⁻¹. The draw ratio, birefringence, crystallite orientation factor, and mechanical properties increased as the strain rate increased. The fiber drawn at the highest strain rate had a birefringence of 0.063, a degree of crystallinity of 47%, and a dynamic storage modulus of 20 GPa at 25 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1137–1145, 2000     

Drawing of nylon 6 fibers (bristles)

By means of electron microscopy of surface replicas and both small-angle and wide-angle x-ray scattering, nylon 6 fibers were investigated in the as-spun state, after drawing at 180°C to a draw ratio up to 4.95, and after subsequent annealing. As spun, the fiber exhibits a small fraction of row-nucleated cylindrites and a great many spherulites (with an average diameter of a few microns) side by side. Drawing deforms the spherulites into spindle-shaped structures (λ = 2) and subsequently produces well-aligned microfibrils. Small-angle x-ray scattering yields a two-point diagram at small λ and a fourpoint diagram at high λ. The long period seems to decrease slightly with draw ratio. Annealing at temperatures above the temperature of drawing increases the long period to a greater extent with samples of lower λ. The crystal lattice orientation is nearly complete at λ = 4.95.     

Effect of drawing on the α relaxation of poly(vinyl alcohol)

The α relaxation of isotropic and drawn poly(vinyl alcohol) dried gel films was studied using dynamic mechanical analysis. The temperature of the relaxation T_α increased from 160°C in the isotropic gel to 220°C in a fiber drawn 19 ×. The relaxation, which is associated with the crystalline regions of the material, also decreased continuously in magnitude as drawing proceeded, although crystallinity increased. At draw ratios over 12 ×, the relaxation became difficult to resolve, and no relaxation was observed in fibers drawn over 19 ×. The melting points of the fibers increased with draw ratio, but not enough to account for the large change in T_α. Crystal thickness in the fiber direction also increased with draw ratio. An analogy is drawn to the case of polyethylene where crystal thickness has been found to control T_α. The absence of a resolvable α relaxation is one reason why it is difficult to draw poly(vinyl alcohol) gels to extremely high ratios.     

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.     

Plastic deformation of polypropylene. IV. Wide-angle x-ray scattering in the neck region

Wide-angle x-ray scattering (WAXS) patterns of two polypropylene samples, a quenched sample drawn at 21°C and an annealed sample drawn at 100°C, were investigated in a range of values of draw ratio λ very closely spaced through the neck region. In both cases, a range of small λ where deformation occurred by spherulite deformation was followed by one of higher λ where microfibrils were formed. The contribution to the WAXS pattern of microfibrils could be clearly distinguished from that of deformed spherulites because of the better orientation parallel to the draw direction of the former as compared to the latter. Additionally, for a drawing temperature of 21°C, microfibrils crystallize in the “smectic” phase as compared to the monoclinic phase for the initial sample and deformed spherulites. At this temperature, plastic deformation proceeds through the spherulite deformation mechanism up to λ = 1.4 accompanied by an increase in chain orientation with increasing λ. For λ > 1.4 plastic deformation appears to occur exclusively through microfibril formation. For drawing at 100°C, spherulite deformation is accompanied by very little change in chain orientation up to λ = 2, where microfibril formation begins. For λ > 2 (T_d = 100°C) plastic deformation is accompanied by both microfibril formation and some spherulite deformation as reflected by changes in both orientation and crystallite size. At this temperature the lateral crystallite size in the microfibrils is related to the long period according to the “equilibrium crystallite shape” previously found for annealed polypropylene.     

Transport properties of methylene chloride in drawn polyethylene as a function of the draw ratio

The sorption and diffusion constants of CH₂Cl₂ at room temperature in quenched polyethylene film drawn at 60°C to different draw ratios λ between 6 and 25 drop drastically between λ = 8 and λ = 9 and then remain nearly constant, dropping only slightly up to λ = 25. Also, the exponential dependence of diffusion constant on concentration of sorbent increases abruptly in the same draw interval and then remains constant at the higher draw ratios. The data may be explained well by a composite madel: a low-permeability fiber structure embedded in a high-permeability spherulitic matrix. As the draw ratio is increased, the initially spherulitic film is gradually transformed into the fiber structure with the transformation being completed between λ = 8 and λ = 9. During subsequent drawing to λ = 25 the mutual arrangement of microfibrils, the basic elements of the fiber structure, changes by longitudinal sliding. However, their transport properties remain nearly constant. The diffusion constant drops a little as a consequence of the increased fraction of tie molecules which reduces the number of unperturbed sorption sites.     

Effects of the drawing form and draw ratio on the fiber structure and mechanical properties of CO2-laser-heated-drawn poly(ethylene terephthalate) fibers

The structure, mechanical properties, and thermomechanical properties of poly(ethylene terephthalate) (PET) fibers obtained by laser-heated drawing were investigated in terms of their dependence on the draw ratio and feed speed and the differences between neck-drawn fibers and flow-drawn fibers. The long period at a draw ratio of 6.0 reached 19.0 nm, notably larger than at lower ratios, whereas the tilting angle of the laminar structure was constant at about 60°, regardless of the draw ratio. A maximum value of 15.0 GPa was attained for the initial modulus, and 1.07 GPa was attained for the tensile strength. A higher tensile strength orientation-induced crystallized fiber at the same initial modulus was obtained from higher molecular weight PET. The relationship between the compliance and molecular orientation of the amorphous phase was studied with a series model of crystalline and amorphous phases. The results revealed that, in the high-draw-ratio fibers, the compliance of the amorphous phase decreased with the draw ratio at a higher rate than indicated by extrapolation to intrinsic values. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 79–90, 2004     

Effects of posttreatment on the properties of modified PLLA/PDLA fibers

Poly(L ‐lactic acid)/poly(D ‐lactic acid) (PLLA/PDLA) blended with plasticizer poly(ethylene glycol) and nucleation agent TMC‐306 as‐spun fibers were prepared by melt spinning. The posttreatment was applied by hot drawing at 70°C and then heat‐treating at different temperatures for 30 minutes. In the process of hot drawing, orientation induced the further formation of the sc crystals and increased the degree of crystallinity of drawn fibers. When the hot drawing ratio reached 3 times, the properties of the fibers were relatively better. The highly oriented fibers containing pure sc crystals with high crystallinity were obtained by heat‐treating at a temperature above the melting point of α crystals. The posttreated PLLA/PDLA fibers with poly(ethylene glycol) and TMC‐306 (LDTP) obtained by hot drawing to 3 times at 70°C and then annealing at 170°C for 30 minutes exhibited the best antioxidative degradation and heat resistance properties. The initial decomposition temperature (T_5%) and heat resistance of posttreated LDTP fiber were about 94°C and 20°C higher than those of the commercial PLLA fiber, respectively.     

Drawing behavior of linear polyethylene. II. Effect of draw temperature and molecular weight on draw ratio and modulus

The drawing behavior of linear polyethylene homopolymers with weight-average molecular weights (M?_w) from 101,450 to ca. 3,500,000 has been studied over the temperature range 75°C to the melting point. In all cases 1-cm gauge length samples were drawn in an Instron tensile testing machine at a constant cross-head speed of 10 cm/min. With the exception of the lowest molecular weight polymer, it was found that increasing the draw temperature led to substantial increases in the maximum draw ratio which could be achieved, and that this increased monotonically with increasing draw temperature. Measurements of the Young's modulus of the drawn materials showed, however, that the unique relationship between modulus and draw ratio previously established for drawing at 75°C was not maintained to the highest draw temperatures. The highest draw temperature at which this relation held was found to be strongly molecular weight dependent, increasing from ca. 80 to ca. 125°C when M?_w increased from 101,450 to 800,000. In all cases conditions could be found for drawing samples to draw ratios of 20 or more with correspondingly large values of the Young's modulus.     

Melt spinning and laser‐heated drawing of a new semiaromatic polyamide,PA9‐T fiber

Fibers of PA9‐T, a new semiaromatic polyamide containing a long aliphatic chain, were prepared by melt spinning. As‐spun fibers were subsequently drawn with a CO₂ laser‐heated drawing system at different draw ratios and various drawing velocities. On‐line observations of drawing points deciphered two drawing states; namely, flow drawing and neck drawing, over the entire range of drawing. Drawing stress revealed that flow drawing is induced by slight drawing stress under a low draw ratio up to 3, and neck drawing is induced by relatively high drawing stress under a higher draw ratio. The effect of drawing stress and drawing velocity on the development of the structure and properties has been characterized through analysis of birefringence, density, WAXD patterns, and tensile, thermal, and dynamic viscoelastic properties. For the neck‐drawn fibers, almost proportional enhancements of crystallinity and molecular orientation with drawing stress were observed. The flow‐drawn fibers have an essentially amorphous structure, and birefringence and density do not always have a linear relation with properties. The fibers drawn at high drawing speed exhibit improved fiber structure and superior mechanical properties. The maximum tensile strength and Young's modulus of PA9‐T drawn fibers were found to be 652 MPa and 5.3 GPa, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 433–444, 2004     

New technique of processing highly oriented poly(vinylidene fluoride) films exclusively in the β phase

Oriented β‐phase films were obtained by utilizing two different techniques: conventional uniaxial drawing at 80 °C of predominantly α‐phase films, and by drawing almost exclusively β‐phase films obtained by crystallization at 60 °C from dimethylformamide (DMF) solution with subsequent pressing. Wide angle X‐ray diffraction (WAXD) and pole figure plots showed that with the conventional drawing technique films oriented at a ratio (R) of 5 still contained about 20% of phase α, a crystallinity degree of 40% and β‐phase crystallographic c ‐axis orientation factor of 0.655. Drawing at 90 °C and with R = 4 of originally β‐phase films results in exclusively β‐phase films with crystallinity degree of 45% and orientation factor of 0.885. Crystalline phase, crystallinity degree, and crystallographic c‐axis orientation factor of both phases were also determined for α‐phase oriented films obtained by drawing α‐phase films at 140 °C. For films drawn at 140 °C the α to β phase transition drops to about 22%. Reduction in crystallinity degree with increasing R is more pronounced at draw temperature of 140 °C compared with 80 °C. Moreover, for both phases the c ‐axis orientation parallel to the draw direction is higher at draw temperature of 140 °C than at 80 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2793–2801, 2007     

Fuming nitric acid treatment of polyethylene. IV. Morphology of drawn polyethylene

Drawn PE of different draw ratios (ranging from 1 to 25) and thermal treatment (annealing temperature 80, 100, 110, 120, 127°C.) was treated with fuming nitric acid at 80°C. Weight loss, molecular weight, elastic modulus, and thermograms were measured for annealed and unannealed samples as a function of the treatment time and draw ratio. As a consequence of the preferential oxidation of the noncrystalline portions, there occurs initially a high rate of weight loss and a steep drop in molecular weight, followed by a lower rate of weight loss at nearly constant molecular weight. The elastic modulus stays practically constant up to the moment where the brittleness of the sample prevents further measurement. During the later period the thermograms exhibit one melting peak during the first melting. The remelt of the same sample, however, has two melting peaks with a relative intensity independent of the treatment time. That the two melting peaks are caused by two components of different molecular weights present in the sample is substantiated by fractionation. At very high annealing temperature (127°C.), two peaks appear, not only in the first melting curve of the etched sample, but also in the melting curve of the unetched material. Such an effect is the consequence of partial melting during annealing followed by new crystallization during cooling the sample to room temperature. The findings are related to the morphology of the drawn material under the assumption of preferential scission of chain loops in the amorphous-crystalline sandwich layer model.     

Thermal behavior of drawn acrylic fibers

The effect of stretching on the thermal behavior of acrylic fibers was investigated with differential scanning calorimetry (DSC), thermogravimetric analysis, and Fourier transform infrared spectroscopy (FTIR). In air atmosphere, the peak temperature of the dynamic DSC thermogram was significantly lowered from 289 to 273 °C when the gel fibers (undrawn) were drawn to a draw ratio of 11.2. However, the initiation temperature was unchanged at 202 °C. The shoulder in the region of 310–380 °C was gradually converted to a sharp peak during the drawing process. However, the dynamic DSC in nitrogen atmosphere did not change in all cases. In air atmosphere the total heat liberated, ΔH, for gel fiber was 851 J g^?1. However, upon drawing to 11.2, ΔH increased to 1580 J g^?1 showing an increase in the total chemical changes. An intimate relationship of chemical changes during the heating process was observed with FTIR of heated samples at various temperatures. The initiation of a DSC exotherm in air begins with nitrile cyclization, and subsequently dehydrogenation was initiated between 220 and 260 °C. An increase in the X‐ray orientation factor and sonic modulus gave a correlation between the stretching draw ratio and crystalline/overall molecular orientation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2949–2958, 2003     

Application of continuous zone-drawing/zone-annealing method to poly(ethylene terephthalate) fibers

A continuous zone-drawing/zone-annealing method was applied to poly(ethylene terephthalate) fibers in order to improve their mechanical properties. Apparatus used for this treatment was assembled in our laboratory. The continuous zone-drawing treatment was carried out at a drawing temperature of 103°C under an applied tension of 6.6 MPa to fully orient amorphous chains in the drawing direction without inducing thermal crystallization. The continuous zone-annealing treatment was carried out twice at an annealing temperature of 160°C under 102.2 MPa and at 183°C under 161.1 MPa to crystallize the highly oriented amorphous chains. The fiber was continuously drawn and annealed at a rate of 420 mm/min. The fiber obtained had a birefringence of 0.260, a degree of crystallinity of 55%, a tensile modulus of 18 GPa, and a storage modulus of 21 GPa at 25°C. Despite the large difference in the treating speed between the continuous zone-annealing and zone-annealing, their values are approximately equal to those of the zone-annealed PET fiber that was reported previously. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 473–481, 1998     

Drawing by solid-state coextrusion of blends of atactic and isotactic polystyrene with poly(2,6-dimethyl-1,4-phenylene oxide)

Atactic polystyrene (aPS)/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and isotactic polystyrene (iPS)/PPO compatible blends of varied composition were subjected to solid-state coextrusion. The efficiency of drawing, orientation, and crystallinity development were studied as a function of composition and draw ratio. The efficiency of drawing, as measured by elastic recovery, is high for coextrusion at temperatures ?40°C above the glass transition temperature of the particular blend. The maximum attainable draw ratio for the blends decreased with increasing PPO concentration; the highest blend draw ratio attained was 6.5 for 25 wt % PPO. The orientation on drawing, as measured by birefringence, increased with draw but decreased with increasing PPO component at the same draw ratio. When PPO was <50% in iPS/PPO blends, iPS crystallized on draw. The morphology of drawn blends was studied by electron microscopy and wide-angle x-ray scattering.     

Drawing of ultrahigh-molecular-weight polyethylene single-crystal mats: The crystallinity

Single-crystal mats of ultrahigh-molecular-weight polyethylene can be drawn uniformly to high draw ratios, more than 20χ at the highest, after the necking process is completed. The dynamic mechanical modulus of the drawn mats increases markedly during the uniform drawing stage. The structural changes induced by the uniform drawing at 100°C have been followed by wide-angle and small-angle x-ray scattering, infrared absorption, differential scanning calorimetry, and birefringence. The crystallinity is estimated from the x-ray amorphous scattering intensity, the IR absorbance of gauche bands, the heat of fusion from DSC, and the density. The estimated crystallinities of the drawn mats are all very high and increase slightly and monotonically with increased drawing after necking, though the values of the crystallinity depend on the method of estimation. IR gauche bands and the SAXS peak due to the long period disappear at a draw ratio of about 80χ. All the results suggest that the uniform drawing after necking destroys the two-phase structure made up of alternately stacked crystalline and amorphous regions and then reorganizes it into a single-phase crystalline structure.     

Drawing of ultrahigh molecular weight polyethylene fibers in the presence of supercritical carbon dioxide

The drawing behavior of ultrahigh molecular weight polyethylene fibers in supercritical carbon dioxide (scCO₂) is compared to that in air at different temperatures. The temperature substantially influences the drawing properties in air, whereas in scCO₂, a constant draw stress and tensile strength are observed. Differential scanning calorimetry shows an apparent development of a hexagonal phase along with a significant increase in the crystallinity of air‐drawn samples with increasing temperature. The existence of this phase is not confirmed by wide‐angle X‐ray scattering, which instead shows that air‐drawn samples crystallize in an internally constrained manner. In contrast, scCO₂ allows crystals to grow without constraints through a possible crystal–crystal transformation, increasing the processing temperature to 110 °C. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1375–1383, 2003     

Melt drawing of ultra‐high molecular weight polyethylene: Comparison of Ziegler‐ and metallocene‐catalyzed reactor powders

The drawing behavior of the ultra‐high molecular weight polyethylene (UHMW‐PE) melts has been studied by comparing the stress/strain curves for two types of samples as polymerized using conventional Ziegler and newer metallocene catalyst systems. Two UHMW‐PE samples, having the same viscosity average molecular weight of 3.3 × 10⁶, but different molecular weight distribution, have been drawn from melt at special conditions. The sample films for drawing were prepared by compression molding of reactor powders at 180°C in the melt. Differences in the structural changes during drawing and resultant properties, ascribable to their broad or narrow molecular weight distribution, were estimated from tensile tests, SEM observations, X‐ray measurements and thermal analyses. The metallocene‐catalyzed sample having narrower molecular weight distribution, could be effectively drawn from the melt up to a maximum draw ratio (DR) of 20, significantly lower than that obtained for the Ziegler‐catalyzed sample, ∼ 50. The stress/strain curves on drawing were remarkably influenced by draw conditions, including draw temperature and rate. However, the most effective draw for both was achieved at 150°C and a strain rate of 5 min⁻¹, independent of sample molecular weight distribution. The efficiency of drawing, as evaluated by the resultant tensile properties as a function of DR, was higher for the metallocene‐catalyzed sample having narrower molecular weight distribution. Nevertheless, the maximum achieved tensile modulus and strength for the Ziegler sample, 50–55 and 0.90 GPa, respectively, were significantly higher than those for the metallocene sample, 20 and 0.65 GPa, respectively, reflecting the markedly higher drawability for the former than the latter. The stress/strain behavior indicated that the origin of differences during drawing from the melt could be attributed to the ease of chain relaxation for the lower molecular weight chains in the melt. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1921–1930, 1999     

Structure development and physical properties achieved in the drawing and/or annealing of PEN fibers

As‐spun poly(ethylene‐2,6‐naphthalate) (PEN) fibers (i.e., precursors) prepared from high molecular weight polymer were drawn and/or annealed under various conditions. Structure and property variations taking place during the treatment process were followed via wide‐angle X‐ray scattering (WAXS), small‐angle X‐ray scattering, differential scanning calorimetry (DSC), and mechanical testing. Both the WAXS and DSC measurements of the cold‐drawn samples stretched from a low‐speed‐spun amorphous fiber indicate that strain‐induced crystallization can occur at a temperature below the glass‐transition temperature and that the resultant crystal is in the α‐form modification. In contrast, when the same precursor was subjected to constrained annealing, its amorphous characteristics remained unchanged even though the annealing was performed at 200 °C. These results may imply that the application of stretching stress is more important than elevated temperatures in producing α‐form crystallization. The crystalline structure of the hot‐drawn samples depends significantly on the morphology of the precursor fibers. When the precursor was wound at a very low speed and in a predominantly amorphous state, hot drawing induced the formation of crystals that were apparently pure α‐form modification. For the β‐form crystallized precursors wound at higher speeds, a partial crystalline transition from the β form to the α form was observed during the hot drawing. In contrast with the mechanical properties of the as‐spun fibers, those of the hot‐drawn products are not improved remarkably because the draw ratio is extremely limited for most as‐spun fibers in which an oriented crystalline structure has already formed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1424–1435, 2000