An optical investigation of high-tenacity polyester (H-T PET) fibers annealed at different temperatures.

INTRODUCTION

Poly (ethylene terephthalate) PET is a semicrystalline polymer, of considerable commercial importance, having a wide variety of end uses in fibers, films, or bottle form. PET can be readily fabricated into fibers, whose physical properties can be modified by simple thermal treatments (1).

The main factor that controls the properties of polymer fibers is the chain conformation in the solid state. Which depends strongly on the crystallization and therefore on the thermomechanical history of the polymer (2). Annealing is a readily available and simple technique for changing the physical and structural properties of such fibers. For example, it is known that annealing a polymeric fiber at different conditions, i.e., temperature and duration leads to changes in optical parameters as well as in density and crystallinity, and thereby its viscoelastic properties (3-5).

Suzuki et al. (4) and El-Mohager and Heymans (6) have applied annealing treatment to improve the mechanical and structural properties of polymer fibers. It is also reported that it may be used to increase the crystallinity of polyester fibers (7). The method employed was previously described by Huang et al. (8), and they emphasized the importance of the trans conformer in the annealing induced crystallization of PET. The growth in crystallization, as a function of annealing temperature and the trans conformer, was found to increase continuously with increasing annealing temperature.

Hamza et al. (9), (10) used interferometric techniques to study the optical and mechanical properties of annealed polymer fibers. They also reported that annealing has a significant effect on the optical and structural properties of polypropylene fibers. Recently, they concluded that with increasing annealing temperature the necking deformation along a cold-drawn pp fiber decreases. Therefore, it was suggested that the necking deformation of pp fibers could be eliminated by annealing at temperatures within the range of 90-120[degrees]C.

The work reported in this article is an interferometric study carried out to evaluate the effect of annealing on the optical and structural parameters of high-tenacity poly(ethylene terephthalate) H-T PET. An Interphako microscope was used to observe the changes in the optical retardation of light through H-T PET fibers treated at different annealing temperatures. The variation in refractive indices, birefringence, orientation function, and crystallinity of H-T PET at different annealing temperatures were noted the effect of annealing temperature on fiber shrinkage, onset temperature (DSC), and orientation angle were also determined.

MATERIALS AND METHODS

A 1000 denier H-T polyester yarn was used. The yarn consists of 192 filaments and regular cross-section filament that has an average diameter of 23 [micro]m. According to the manufacturer's information (Brilen Company, Spain), the yarn had a tenacity of 8.9 g/den, a molecular weight of 60,000 g/mol, and 7.5% shrinkage in hot air at 180[degrees]C for 2 min.

Six sample lengths of the H-T PET yarn were successively annealed without being tensioned (free ends) for 1 hat respective temperatures of 100, 120, 140, 160, 180, and 200[degrees]C. From each sample length, a single fiber was withdrawn, mounted on a microscope slide and, after adding appropriate immersion liquid, studied with an Interphako microscope. Two different liquids of light refractive indices 1.705 (parallel) and 1.566 (perpendicular) for a wavelength of 550 nm were used.

The thermal shrinkage (S) ratio of the PET fibers was measured at different annealing temperatures. The percent shrinkage of the annealed fibers was calculated using the following equation (11).

S = L - [L.sub.0]/[L.sub.0] x 100 (1)

Where [L.sub.o] is the initial fiber length (it was 50 cm) and L is the fiber length after treatment.

INTERFEROMETRIC TECHNIQUE

The Interphako polarizing interference microscope (12) was supplied by Carl Zeiss JENA, Germany. A schematic diagram showing the optical arrangement of this microscope is shown in Fig. 1. A common path operation throughout the imaging optics is achieved with a MachZehnder interferometer fitted between the objective and the eyepiece. This produces excellent stability and phase uniformity over the field of view of 0.01 fringes with maximum shear. The condenser aperture is a slit whose width and length can be adjusted to control spatial coherence (required for interference between sheared images) and the maximum numerical aperture (N.A) of the condenser, respectively. A half-shade plate, located at an intermediate image plane, produces a phase shift between adjacent areas of the image and provides a very sensitive method for making visual settings. In one arm of the interferometer sliding, wedges act as a variable phase compensator and a pair of counter-rotating wedges allow variable shear to be introduced between the images formed by the eyepiece. This microscope gives a direct measurement of fiber refractive indices ([n.sup.[parallel]] and [n.sup.[perpendicular to]]), also overcomes the overlapping problem with the Pluta microscope (13) in case of thick fibers and gives high contrast interferogram better than that of the Pluta one.

For less time-consuming and more accurate measurements, the Interphako microscope was modified by replacing the eyepiece with a CCD camera connected to a PC for image capture and subsequent analysis. Examples of the captured microinterferogram (images) of the treated H-T PET fibers are given for illustration.

[FIGURE 1 OMITTED]

THEORETICAL CONSIDERATIONS

Refractive Index and Birefringence of Fibers

The captured images of the fibers were used to determine the refractive indices, parallel ([n.sup.[parallel]]), and perpendicular ([n.sup.[perpendicular to]]) to the fiber axis. The following expression gives the refractive indices[n.sup.[parallel]])and [n.sup.[perpendicular to]]) of a fiber (14):

[n.sup.i] = [n.sub.L] + [Z.sup.i] [lambda]/bt (2)

where "i" is the state of polarization of the monochromatic light used (parallel [parallel] or perpendicular [perpendicular to] to the fiber axis), "[n.sub.L]" is the refractive index of the immersion liquid, "Z" is the fringe shift, "[lambda]" is the wavelength of monochromatic light used, "t" is the thickness of the fiber, and "b" is the interfringe spacing.

The difference between the measured values of [n.sup.[parallel]] and [n.sup.[perpendicular to]] gives the fiber birefringence [DELTA]n (i.e., [DELTA] = [n.sup.[parallel]]--[n.sup.[perpendicular to]]). The birefringence of a fiber provides information on the overall degree of the molecular orientation within the fiber, and a measurement of the fiber orientation would indicate the degree of change in the fiber properties by and annealing treatment. The degree of orientation with respect to a particular direction ([n.sup.[parallel]], [n.sup.[perpendicular to]]) would express the extent of anisotropy (birefringence) produced by the annealing process.

Orientation Function and Angle of Orientation

The Hermans's orientation function or factor (F) is probably the quantity most frequently used to characterize the molecular orientation of polymers. Hermans in 1946 (15) derived this orientation function, which relates optical birefringence [DELTA]n to segmental orientation factor as follows:

F = [DELTA]n/[DELTA][n.sub.0] (3)

where [DELTA][n.sub.0] is the maximum (intrinsic) birefringence of polymer fibers, which for PET fibers the value of [DELTA][n.sub.0] is equal 0.240 (16). In addition, the molecular orientation can be assessed via the changes in the molecular orientation angle ([theta]), and it can be obtained by the following relationship (16):

[theta] = [sin.sup. -1]([square root of 2/3](1 - F)) (4)

To determined the orientation function (F) and the angle of orientation ([theta]), the calculated birefringence. [DELTA]n, at the different annealing temperatures values were substituted into Eqs. 3 and 4.

Volume Fraction Crystallinity

Crystallinity or semicrystalhne polymers is one of the important thermodynamic parameters affecting the density ([rho]), the mechanical, chemical, and thermal properties of a fiber as well as its refractive indices (17).

The density ([rho]) is one of the useful physical parameter that reflects the thermal effect on materials. It is related to refractive index, of PET fibers by the following equation (16);

[rho] = 4.0486([n.sup.2] - 1/[n.sup.2] + 2) (5)

where "n" is the isotropic (average) refractive index, n = 1/3 (n.sup.[parallel]] + 2[n.sup.[perpendicular to]]).

Most polymer fibers are of a semicrystalline form, comprising both crystalline and amorphous phases. Depending on the density-refractive index relation [Eq, 5), the volume-fraction crystallinity ([x.sub.v]) for PET fibers can be calculated using the following equation: (18):

[[chi].sub.v] = [rho] - [[rho].sub.a]/[[rho].sub.c] - [[rho].sub.a] x 100 (6)

where "[[rho].sub.c]" and "[[rho].sub.a]" are the density of 100% crystalline sample and totally amorphous .sample, respectively. The values of [[rho].sub.c] and [[rho].sub.a] for PET are 1.455 and 1.335 g/[cm.sup.3] respectively (11). To study the influence of annealing on the structural properties of H-T PET fibers, the volume fraction crystallinity (x.sub.v]) was calculated at different annealing temperatures using the Eqs. 5 and 6.

RESULTS AND DISCUSSION

Figure 2 shows the increase in the shrinkage values against the annealing temperature for the H-T PET fibers. There appears to be a constant shrinkage of 1% at the higher temperatures (180 and 200[degrees]C), which is due to the high tenacity of these fibers.

[FIGURE 2 OMITTED]

Figure 3a and b shows the microinterferograms of fibers for the different temperatures with regard to the light vibrating parallel and perpendicularly to the fiber axis, respectively, and Fig. 4 depicts the calculated refractive indices graphically.

[FIGURE 3 OMITTED]

It would appear from Fig. 3 that the changes in the fringe shifts inside the liber with increasing temperature arc insignificant. However, Fig. 4 indicates that the parallel refractive index ([n.sup.[parallel]]) of the annealed H-T PET fiber remains almost constant as the annealing temperature increases, whereas the perpendicular refractive index ([n.sup.[perpendicular to]]) increases with annealing temperature. The negligible changes in [n.sup.[parallel]] may be due to the high orientation of the molecules within the H-T PET fiber, which was been fully drawn, whereas the trend of [n.sup.[perpendicular to]] with increased annealing temperature could be attributed to the onset of thermal degradation of the polymer (19). In addition, the refractive indices depend on a combination of the degree of orientation of the chains with respect to the fiber axis and the density. In [n.sup.[parallel]]", the effects of these oppose each other; hence, there is little change with annealing temperature. In [n.sup.[perpendicular to]], the effects are in the same direction, leading to a significant increase in the refractive index with temperature.

[FIGURE 4 OMITTED]

Using the measured values of the refractive indices [n.sup.[parallel]] and [n.sup.[perpendicular to]], the birefringence [DELTA]n was calculated. The relationship between birefringence and annealing temperature is shown in Fig. 5. As it can be seen as the annealing temperature is increased, the birefringence of H-T PET fibers decreases.

[FIGURE 5 OMITTED]

Figure 6 shows the variation in the orientation parameters with the annealing temperature, in which the orientation factor (F) has a similar trend as the birefringence is apparent, while the angle of orientation ([theta]) increases with temperature.

[FIGURE 6 OMITTED]

The crystallinity of the H-T PET fibers was found to increase with the annealing temperature as illustrated in Fig. 7. Kuleznev and Shershnev (20) attributes this to the high mobility of the segments that easily break away from the crystal lattice resulting in mass redistribution amongst the polymer chains and changes in the fiber density. Durand and coworkers (21) found that increasing the annealing temperature increasing the transition temperature for PET fibers which must be related to the total increase in fiber crystallinity. The characteristics of assessed onset temperature of the annealed fibers, which have been determined by using DSC as shown in Fig. 8, show an improvement of crystallinity for H-T PET fibers as well as the refractive index.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

CONCLUSIONS

An Interphako interference microscope was used to investigate the effect of annealing of the optical properties of H-T PET fibers.

From the results obtained, it may be concluded that the annealing temperature has no significant effect on the refractive index parallel to the fiber axis ([n.sup.[parallel]]), but a quite significant increase of the refractive index perpendicular to the fiber axis ([n.sup.[perpendicular]]), which may be attributable to the shrinkage caused by the annealing.

Although the orientation of H-T PET fiber decreased as the annealing temperature increased, the % crystallinity was found to increase.

In general, the interferometry and DSC measurements showed an improvement in the crystallinity of H-T PET fibers when annealing temperature increases.

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H.M. EI-Dessouky (1), (3), M.R. Mahmoudi (1), K.M. Yassien (2), C.A. Lawrence (1)

(1) Centre for Technical Textiles, School of Design, University of Leeds, LS2 9JT, UK

(2) Physics Department, Faculty of Science, south valley University, Aswan, Eqypt

(3) Physics Department, Faculty of science, Mansoura University, 35116, Eqypt

Correspondence to: Hassan EI-Dessouky; e-mail: [e-mail] or [e-mail]

H.M. EI-Dessouky is currently at Physics Department, Faculty of Science, Mansoura University, 35116, Egypt.

DOI 10.1002/pen.21184

Published online in Wiley InterScience (www.interscience.wiley.com).