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Correlation between microhardness and optical anisotropies and crystallinity in lamellar regenerated cellulose.

INTRODUCTION

Many physical and mechanical properties of polymers are profoundly influenced by crystallinity (1). This work represents a continuation of research about properties of regenerated cellulose tubular films depending on crystallinity (2, 3).

Some plastics are naturally birefringent because of their crystalline structure, and amorphous plastics are not birefringent unless deformed as a result of stress (4). For a polymer, molecular orientation will cause the material to be birefringent, and the magnitude of the birefringence depends on the degree of molecular orientation (5). In similar terms, the anisotropy of microhardness indentations in polymers depends on the degree of molecular orientation, but this one is a technique more scarcely studied (6, 7).

In the present paper, we describe an experimental study of the relationship between birefringence, microhardness anisotropy and crystallinity of a series of such polymer. These two techniques of anisotropy measurements (microhardness and birefringence) are expected to be convenient tools for evaluating the crystallinity or state of order of that polymer with a significant commercial value because of its wide use in food packaging.

In order to complete this study, five samples of regenerated cellulose tubular films were prepared following the viscose method by varying manufacture conditions so that the crystallinity values varied from 0.53 to 0.66, covering the range of crystallinities of interest in the commercial applications of these films.

EXPERIMENTAL PROCEDURE

Films Preparation

The samples studied in this investigation were five commercially available regenerated cellulose tubular films of about 28 [[micro]meter] thickness, prepared according to the viscose process (3, 8, 9), by varying the manufacture conditions as referred previously (3) in order to obtain materials with different degrees of crystallinity. These kinds of films are used chiefly in food packaging. The composition of every film was analogous, namely: 75 wt% of cellulose, 14 wt% of glycerine and 11 wt% of water. The produced films are microscopically oriented because the viscose solution was extruded in every case through an annular die before being regenerated. But speed and temperature of drawing during the extrusion were similar for all cases.

Measurement of Crystallinity

The crystallinity of these films was determined experimentally using an X-ray diffraction method. X-ray diffractograms were recorded with a Philips Geiger Counter X-ray diffractometer provided with a PW 1710 automatic data processor and a PW 1820 goniometer. The radiation used was Cu[K.sub.[Alpha]] of wavelength 0.1542 nm and the scanning speed was 2 [degrees] (2[Theta]) [min.sup.-1]. The X-ray unit was operated at 40 kW and 40 mA.

The crystalline fraction or degree of crystallinity, X, was obtained through:

X = [I.sub.c]/([I.sub.c] + [I.sub.a]) (1)

where [I.sub.c] is the intensity contribution from crystalline regions to X-ray scattering and [1.sub.a] is the intensity contribution from amorphous or noncrystalline regions.

The separation into crystalline and noncrystalline (amorphous) scattering in cellulose has always been a somewhat arbitrary procedure, as explained several authors (10, 11). Thus, the values of crystallinity can only be considered as a relative parameter, which depends strongly on the methods of separation in the contribution of each phase to the total intensity, as has been reviewed by Tripp (12). We have proceeded following the calculation method explained in previous works (2, 3).

Measurement of Birefringence

Birefringences of the films were measured by using an Ehringhaus compensator attached to an Amplival Pol polarizing microscope. The measurements were carried out with white light at room temperature.

Measurement of Microhardness

Microhardness values of the samples were measured using a Vickers indentor attached to a Leitz microhardness tester. A loading time of 3 s and a load of 5 g were used. All the measurements were performed at 23 [degrees] C. Microhardness values, MH, were calculated according to the expression:

MH = 2 [multiplied by] (sin 68 [degrees]) [multiplied by] P/[d.sup.2] (in MPa) (2)

where P is the contact load (in N) and d is the diagonal of the indentation base (in mm).

Results and Discussion

The compensator technique measures directly the birefringence in the plane of the films, [Delta][n.sub.zy], i.e., the difference in refractive indices [n.sub.z] and [n.sub.y], corresponding to the two mutual perpendicular directions, along the extrusion direction and along the width of the film, respectively.

Figure 1 shows the birefringence of samples against the degree of crystallinity. The optical anisotropy, measured by means of [Delta][n.sub.zy], increases linearly with X (correlation coefficient R = 0.993) in the studied range. Obviously, as expected, an increase in crystallinity originates a higher difference between [n.sub.y] and [n.sub.z].

The microhardness values are also assigned by the inferior letters z and y, corresponding to the same directions referred before for birefringence. The plastic deformation when the load is applied during the Vickers indentation and the elastic recovery after releasing the indentor are different at parallel and normal directions in the plane of the film. The material, strongly oriented because of the characteristic manufacture processing, is more readily indented in the normal direction to the extrusion direction than in the parallel direction. The consequence of this mechanical anisotropy is a rhombic shape of the base of Vickers indentations, as explained in other work related to drawn samples of poly(ethylene terephthalate) (6). A schematic representation of the shape of obtained Vickers indentations is represented in Fig. 2. If one of the diagonals of the base of the Vickers square pyramidal diamond is parallel to the extrusion direction, two microhardness values are obtained:

[Mathematical Expression Omitted] (3)

The microhardness anisotropy, [Delta][MH.sub.zy], can be obtained by means of the expression:

[Delta][MH.sub.zy] = 1 - ([MH.sub.y]/[MH.sub.z]) (4)

The difference between longitudinal and transverse microhardness is due to the degree of molecular orientation. Both [MH.sub.z] and [MH.sub.y] increases when the crystallinity increases, as it can be observed in Fig. 3. Two linear relationships were obtained, with correlation coefficients of 0.991 and 0.974 respectively. Figure 4 shows that [Delta][MH.sub.zy] rises when crystallinity increases, following also a linear relationship (with R = 0.993).

CONCLUSION

The most important conclusion that can be drawn from the results of this work is that the crystallinity in regenerated cellulose is closely related to both microhardness and optical anisotropies. Especially microhardness measurement of polymers, scarcely used but simple and rapid technique, is an optimum procedure for providing information on the molecular orientation. This is due to its easy calculation and nondestructive character. Furthermore, this technique is alternative, for similar samples, to the more complicated X-ray scattering technique, all the more ff we take into account that, as referred before, the values of crystallinity calculated starting from X-ray diffraction can only be considered as relative parameters.

NOMENCLATURE

d = Diagonal of the indentation base for microhardness measurement.

[I.sub.a] = Intensity contributions from amorphous regions to X-ray scattering.

[I.sub.c] = Intensity contributions from crystalline regions to X-ray scattering.

[MH.sub.y] = Transversal microhardness (perpendicular to the extrusion direction in the manufacture processing).

[MH.sub.z] = Longitudinal microhardness (parallel to the extrusion direction).

[n.sub.y] = Transversal refractive index (perpendicular to the extrusion direction).

[n.sub.z] = Longitudinal refractive index (perpendicular to the extrusion direction).

P = Contact load for microhardness measurement.

R = Correlation coefficient.

X = Degree of crystallinity.

y = Perpendicular direction to the extrusion direction in the plane of the film.

z = Direction of extrusion in the manufacture processing.

[Delta][MH.sub.zy] = Microhardness anisotropy.

[Delta][n.sub.zy] = Birefringence.

[Theta] = X-ray diffraction angle.

REFERENCES

1. G. R. Moore and D. E. Kline, in Properties and Processing of Polymers for Engineers, Prentice-Hall, Englewood Cliffs, N. J. (1984).

2. A. Larena, M. C. Pena and G. Pinto, J. Mater. Sci. Lett., 12, 1745 (1993).

3. A. Larena and G. Pinto, Polym. Eng. Sci., 35, 1155 (1995).

4. D. L. Keyes, in Engineering Materials Handbook, Vol. 2, Engineering Plastics, p. 597, ASM International, Metals Park, Ohio (1988).

5. D. Campbell and J. R. White, in Polymer Characterization, Physical Techniques, p. 278, Chapman and Hall, London (1989).

6. V. Lorenzo and J. M. Perena, J. Appl Polym. Sci., 39, 1467 (1990).

7. F. J. Balta-Calleja, Adv. Polym. Sci., 66, 117 (1985).

8. J. W. Shappel and G. C. Bockno, in Cellulose and Cellulose Derivatives, Vol. 5, Part 5, N. M. Bikales and L. Segal, eds., Wiley, New York (1971).

9. D. J. Bridgeford, U. S. Patent No. 4,590,107 (1986).

10. N. R. Bertoniere and S. H. Zeronian, in The Structures of Cellulose, p. 257, R. H. Atalla, ed., American Chemical Society, Washington, D. C. (1987).

11. G. A. F. Roberts, in Paper Chemistry, p. 9, J. C. Roberts, ed., Blackie Academic & Professional, London (1991).

12. V. W. Tripp, in Cellulose and Cellulose Derivatives, Vol. 5, Part 4, N. M. Bikales and L. Segal, eds., Wiley, New York (1971).
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Author:Pinto, Gabriel; Lorenzo, Vicente
Publication:Polymer Engineering and Science
Date:Mar 1, 1998
Words:1480
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