This paper concerns with the changes of structure and some mechanical properties of the poly-(ethylene terephthalate) (PET) films in the biaxial stretching.
Unoriented, low crystalline T-die extruded PET film with a thickness of 140μ were used. In a circulating air bath at 90°C, the films were stretched at a constant rate of 300%/min by various methods using a film-stretcher, (i.e., uniaxially stretched under free width or constant width, two-way succesively, and simultaneously biaxially stretched), and then heat setting was carried out at 110°C for 15min.
Mechanical properties (tensile strength, elongation, Young's modulus and F₅ value) of the variously stretched films were measured at 20°C, 65% R. H. using Autograph tensile tester. The crystalline orientation and degree of crystallization were investigated by X-ray diffraction and density measurement.
The following results were obtained:
(1) The stress-strain curves reflect the differences of orientation between machine and transverse directions quite distinctly.
(2) Irrespective of stretch ratio the anisotropy of mechanical properties for the films uniaxially stretched under free width is greater than that stretched under constant width at the same stretch ratio of machine direction.
The extent of the anisotropy of mechanical properties for the two-way successively biaxially stretched films is larger in the stretching direction (MD) than in the transverse direction (TD) for the TD-stretch ratio before “the balance point” of mechanical properties is attained. Then, the anisotropy is reversed after the balance point. The difference of the balance point between four mechanical properties in the stretched PET films is not so large as in polypropylene (PP) films.
The mechanical properties of the simultaneously biaxially stretched films, irrespective of stretch ratio, are equal in both the machine and the transverse directions as seen for polyvinylchloride and PP films. That is, these properties are isotropic within the plane.
(3) The degree of crystallinity for the uniaxially and simultaneously biaxially stretched films increases with increasing stretch ratio. For the two-way successively biaxially stretched films, however, it decreases with stretch ratio before the “balance point” is attained, and then it increases with stretch ratio.
(4) The simultaneously biaxially stretched films (MD×TD=√14×√14) have higher tensile strength and lower Young's modulus, compared with the two-way successively biaxially stretched ones at the same stretch ratio (MD×TD=4.0×3.5) and with the balanced mechanical properties.
(5) The difference between the uniaxial stretchings under free and constant width, can be observed not only in the mechanical properties but also in the changes of structure of transgauche rotational isomers.
In the two-way successively biaxially stretched films at the stretch ratio near “balance point”, the change of orientation of the chain segments and crystallites occurs rather than the transformation of isomers from gauche to trans form.
(6) In the simultaneously biaxially stretched films at higher stretch ratios, the trans contents and the F₅ values are large, suggesting that the chain segments are extended and tautened in all directions within the plane.

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In the previous report (part 10) in this series, the changes in structure and mechanical properties of the biaxially hot stretched poly (ethylene terephthalate) films were reported. In this paper the orientation behavior of crystalline films stretched biaxially is discussed in relation to the mechanical properties.
The samples and stretching conditions were the same as those described in the previous paper. The orientation of crystallites was studied by means of X-ray diffractometry.
The following results were obtained:
(1) In the case of uniaxial stretching under free width, the c-axis orients preferentially parallel to the stretching direction with cylindrical symmetry around the stretching axis.
(2) When the films were stretched under a constant width the crystallites show a uniplanar-axial orientation where the c-axis orients parallel to the stretching direction and the (100) plane parallel to the film surface as in a rolled film.
(3) The crystalline orientation in the two-way successively biaxial stretching is characterized as follows; In the successive stretching of film to the transverse direction, the degree of planar-orientation always increases, whereas the c-axis orientation decreases with the successive stretching ratio to pass through the state of random orientation and then increases to approach the uniplanar-axial orientation along the transverse direction.
(4) The crystalline orientation of simultaneously biaxially stretched films shows a uniplanar orientation at any stretch ratio; i.e. the (100) plane which is parallel to the phenyl ring orients nearly parallel to the film plane and the c-axis orients randomly within the plane.

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Fine structure of polyester fibers prepared by the necking drawing of various undrawn fibers was investigated.
The results obtained are as follows;
1. The birefringence of the necking region and the necking draw ratio of polyester fibers are strongly influenced by preorientation of undrawn fibers. For an isotropic undrawn polyester fibers, the maximum value of birefringence of the necking region and necking region and necking draw ratio were about 0.200 and 6 times, respectively.
2. The birefringence of the necking region (Δn_neck) vs necking draw ratio curves at low necking draw ratio for various undrawn fibers can be interpreted by Kuhn and Grun's theory for rubber photo-elasticity, and at high necking draw ratios, it agrees with Kratky's ‘first borderline case’.
3. The experimental coefficients of Kuhn and Grun's theory, and K₂=NkT increase with increasing pre-orientation of undrawn fibers.
4. The crystallization induced by necking is dependent only on the necking draw ratio of the samples.
5. The highest density obtained by means of necking drawing method was 1.369g/cm³ and the crystal structure of this sample is smectic at high orientation.

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The orientation behavior of crystallites during uniaxial stretching and its releasing for spherulite PVA are interpreted in terms of a spherulite deformation mechanism modified from that proposed by Sasaguri, Yamada and Stein (Model 1) and that proposed by Oda (Model 2).
Five deformation parameters characterizing the spherulite deformation mechanism, i.e., fraction of the folded-chain type crysta within crystal lamella at undeformed state, easiness of crystal transition from folded chain type to fringed-micellar type, easiness of lamella rotating and lamella sliping to [010] and [001] direction, showed reasonable changes with the stretching of the bulk polymers.
For the cold drawn spherulite PVA, the deformation mechanism of its spherulite is interpreted in terms of the lamella rotating around the a axis crystal, but for the hot drawn one, it is interpreted in terms of the lamella sliping to [010] direction of (100) plane.

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In order to improve the tensile properties of polyethylene oxide (PEO) fiber, two different drawing methods were employed; one was a dry process which was carried out in an air oven, and another a wet process using liquid paraffin as a medium which was a non-solvent of PEO.
In the hot air, PEO could be drawn more than in the liquid paraffine as shown in Figure 1.
The change in the molecular orientation in the fibers was examined by means of X-ray analizer and a polarizing microscope. In the case of dry process, the degree of orientation tended to decrease in the earlier stage of drawing and showed a minimum at draw ratio of 1.5, On the other hand, in the wet drawing, it was monotonically decreased with increasing draw ratio. Even in this case, however, the increase of the drawing rate resulted the relation of draw ratio to birefringence similar to that in the hot air.
From these results, it is evident that the orientation of drawn fiber is governed not only by the draw ratio but also by the heat capacity and the heat conductivity of a drawing medium.
Regardness of the anomolous change in the optical anisotropy, the drawing method does not affect the tensile properties of the drawn fibers, and hence, the tenacity and the residual elongation of PEO fibers are determined only by its molecular weight and the draw ratio.

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