Structure property relationships in rolled poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) film was processed by four paths. In Path I, PET was rolled from the amorphous glassy state. In Path II, the samples were crystallized at 150°C for 12 hours and then rolled. For Path III, amorphous PET was rolled and then annealed at 150°C for two hours. In Path IV, initially amorphous films were rolled at an elevated temperature (above T_g) in an attempt to simultaneously orient and crystallize the samples. However, unsatisfactory results were obtained by Path IV so most of the research involved Paths I, II, and III.

WAXD patterns and pole figures indicate the chains in Path I samples become preferentially oriented parallel to the RD during rolling. As the chains become more oriented, the morphology changes from the initial amorphous structure to a somewhat more ordered paracrystalline structure which increases in degree of order with increases in roll reduction. Small crystallites are believed to form at the highest reductions. Density results indicate an increase in crystallinity of about 10%.

In Path II, the samples are highly crystalline and spherulitic before rolling. During rolling, the spherulites are crushed and the lamellae become tilted toward the TD. At the highest reductions, the lamellae themselves are crushed and the chains become preferentially aligned parallel to the RD. The crystallinity remains almost constant during rolling, but the density decreases.

Path III is more complicated. In this case some line nuclei are believed to form in the samples during rolling. These are a part of the strain-induced changes in structure noted for Path I samples. Upon annealing, these line nuclei give rise to cylindrites. At high reductions, these cylindrites grow in a staggered conformation which gives rise to a four-point SAXS pattern. Both the density and degree of crystallinity are relatively independent of roll reduction.

The alignment of the chains produced a significant increase in the modulus and tensile strength in the RD for Paths I, II, and III. Tensile properties in the TD were relatively unaffected by the rolling treatment. In general, Path III samples were stronger than Path II samples which were stronger than Path I samples. Path I samples were more ductile and usually had higher elongations to break than Path II or III samples.

Rolling PET by Path I and Path II allowed direct comparisons to be made between rolling of crystalline and amorphous polymers.