Annealed Fiber

For polymers, deformation is followed by either stress relaxation or strain recovery to some thermodynamically stable state. The relaxation or recovery process occurs because polymers are viscoelastic materials. In melts, the process occurs rapidly because the time constants are relatively small. In cooled polymers, relaxation or recovery occurs much more slowly, because the time constants are much larger. The process can be greatly hastened by the application of swelling agents or heat. 

Drawing introduces stresses, structural defects, and voids within fibers. Annealing the fibers quickly relaxes the stress, heals the voids and structural defects, and leads to increases in degree of crystallinity, and to perfection and size of crystallites. If heated with free ends, annealing causes the fiber to shrink. Annealing can lead to an improvement in mechanical 

FIGURE 3.48 Tenacities of fibers and films as a function of amorphous orientation (O) film, draw temperature 135°C (x) film, draw temperature 110°C ( ) fiber draw temperature 90°C (A) fiber, heat-set. (From Samuels, R.J. Structured Polymer Properties, John Wiley Sons, New York, 1974. With permission.) 

Balta-Calleja and Peterlin [210] investigated annealing phenomena of drawn polypropylene. They found that the relaxation of tie molecules, shrinkage, and disorientation proceeded relatively fast, compared with the long period growth and the increase in density, which continued as a linear function of log time through 1000 min. 

Nadella et al. [204] found that cold-drawing of polypropylene caused voids and a decrease of crystallite size and greatly enhanced lattice strains and defects. Annealing the fibers at 140°C restored a well-formed monoclinic structure and healed the voids. 

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Fig. 13. Elongation to break as a function of birefringence for undrawn, hot-drawn, and cold-drawn annealed fibers (6) , undrawn , cold-drawn,...

The value of Ea in this case is a positive value of the root of the above equation. In the case of the lamellar substructure, i.e., for A = 1, typical of stretched and then annealed fibers, the equation has the form ... 

Lamellar, single crystals of cellulose triacetate, precipitated from nitromethane with butyl alcohol, were studied by X-ray and electron diffraction. Only the crystals containing the mother liquor, or moistened with nitromethane, showed rich diffraction details. From stretched and annealed fibers, it was found that the unit cell is tetragonal, with a = fe = 21.15A (2.115 nm), and c = 41.36 A (4.136 nm). 

Conversion of copolymers to fibers and pertinent tensile date. Copolymers I-III described in Table I were melt extruded, and the extrudates were oriented by drawing and then annealed. The tensile properties of the unannealed and annealed fibers are summarized in Table II and III, respectively. 

Isotropic solutions (12-15% by weight) of XVI in m-cresol were dry-jet wet spun into a coagulation bath of water/methanol. The as-spun fibers were drawn at temperatures above 380 °C to give fibers having a tensile strength of about 3.2 GPa and a modulus of 130 GPa. The annealed fibers displayed distinct wide-angle X-ray patterns from which a monoclinic unit cell was determined. 

Figure 3a. Surface contours of normalized WAXS intensity versus Bragg angle 29 and azimuthal angle in polar coordinate form. Only the upper right hand quadrant is shown. Expermental data on annealed fiber.

Densities of the hot-drawn and cold-drawn and annealed fibers were slightly higher than those of the spun fibers. Crystalline fractions of around 61% were observed. 

The mechanical properties of one-step-drawn and annealed fibers of P(3HB-co-8%-3HV) with or without isothermal crystallization are summarized in Table 3. The tensile strength of as-spun fibers was ca. 30 MPa independent of isothermal crystallization. The tensile strength of 10 times drawn fiber without isothermal crystallization was 90 MPa. However, 10 times one-step-drawn P(3HB-co-3HV) fiber with isothermal crystallization had a tensile strength of 1.06 GPa, elongation to break of 40%, and Young s modulus of 8.0 GPa.Table 3 summarizes the mechanical properties of P(3HB) and P(3HB-co-3HV) fibers. 

Table 2 Mechanical properties of cold-drawn two-step-drawn and annealed fibers of...

Type I Annealed fiber (high order structure in older literature) Orthorhombic Helical Isochiral 8 14.5 5.6 7.4 C222i 0.93 0.90 (5, 9,17-19)... 

This form is obtained by cold-drawing the polymer quenched from the melt. IR spectroscopy and WAXD techniques indicate that in this modification sPP adopts the alTtrans conformation and the crystallographic density is 0.945 g/cm. Upon annealing fibers of this form at about 100°C for a few hours, the more stable helical form results, without losing the preferred chain orientation along the stretching direction. 

Un-annealed fibers drawn at room temperature contain the planar zigzag low-temperature orthorhombic form characterized by a 0.505 nm c axis, obviously corresponding to the layer line spacing. The strongest equatorial reflections of this form are the 020,110 and 130 at 0.558, 0.473 and 0.303 nm respectively (with Cu-Ka radiation, 20 = 15.9, 18.8 and... 

Figure 2. DSC curves of the as-spun CPE-1 fibers (iisp=4.4) and annealed CPE-1 fibers recorded at a heating rate of 40 C/min. DSC curves 1 and 2 correspond to the as-spun fibers on first heating and to the samples that were preliminarily heated to 230 C at a heating rate of 10 C/min, respectively. Curve 3 corresponds to the annealed fibers that were preUminarily heated at a heating rate of 10 C/min to 230 C. Curves 4-7 correspond to cooling of the (4, 6) as-spun and (5, 7) annealed fibers from (4) 280, (5) 320, and (6, 7) 380 C at a cooling rate of 40 C/min. Curve 8 corresponds to the as-spun and atmealed fibers that were preliminarily heated to 380°C.

Figure 5. (1, 2) DSC traces obtained on heating and (3, 4) DSC traces obtained on cooling of CPE-2 fibers fi om, respectively, 350 and 250°C. Heating and cooling rates were 40 K/min. (1) As-spun fiber (2) annealed fiber.

Figure 13. (a, b) Equatorial and (c, d) meridional X-ray diffraction patterns registered at room temperature (a, c) as-spun CPE-2 fiber (b, d) annealed fiber. The dashed curves indicate the diffuse maxima corresponding to scattering from non-crystalline part of the material. 

The effect of heat treatment on structure, fracture strength and chain scission has been very extensively investigated by Statton, Park and DeVries [25-27] and by Lloyd [5]. With respect to our understanding of chain breakages in annealed fibers the following morphological changes seem to be particularly noteworthy. Relaxed heat treatments result in [25] ... 

The cross section of the annealed fiber was sputter-coated with gold before imaging. Scanning electron micrographs were obtained with Hitachi S-800 SEM. 

Figure 2 SEM micrographs showing non-annealed fiber cross-sections with a) 50% PCL and b) 30% PCL.

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