synopsisThe two e n d o t h e m found during DSC analysis of annealed or drawn poly(ethy1ene terephthalate), PET, are discussed in greater detail. Earlier workers proposed that the endothenns were the result of separate morphologies, i.e., extended-chain and foldedchain crystals, but more recently Roberts and others have presented data on the effect of DSC heating rate on annealed PET endothenn areas which indicate that the higher temperature endothenn is the result of recrystallization in the DSC. The present work explains the reasons for recrystallization, and presents data showing that samples cooled at various rates from the melt also exhibit recrystallization in the DSC, in much the same manner as samples annealed for different lengths of time. Further, by prolonged annealing before analysis, part of the recrystallization exotherm can be observed in the DSC scan. Drawn nylon 66 also exhibits recrystallization in the DSC, in a manner similar to annealed or slowly crystallized PET. The amount of material that recrystallizes is determined by the time and supercooling available between first melting and the ultimate recrystallization temperature, i.e., a temperature a t which there is too little time and temperature driving force for further recrystallization to occur. Infrared absorption data show an increase in "regular" fold content during prolonged annealing of PET, while dynamic mechanical data show a marked decrease in a dispersion that is likely associated with the looser fold crystal morphologies. Annealed PET does superheat in the DSC, leaving unanswered the question as to whether any partially extended material is present along with the regular-fold material. For cold-drawn PET, the infrared data indicate disappearance of regular folds and the dynamic mechanical data indicate disappearance of the looser folds.These data indicate a likelihood of at least partially extended morphologies in cold-drawn PET; these observations do not apply to PET drawn at high temperatures or to polyethylene.Cold-drawn PET also super!eats. 1273

Poly(ethylene terephthalate) (PET), after certain thermal and mechanical histories, exhibits stress cracking when exposed to 40% aqueous methylamine. This reagent has also been used for selective degradation of PET films. Stress cracking is shown to occur during degradation only when a specimen supports an internal or externally applied stress, above a critical level. The cracking density in a filament is shown by the present work to increase as the draw ratio is increased or when the fiber is highly annealed. This increased cracking is associated with an increase in the magnitude of the internal residual stress resulting from molecular orientation developed during these processes. Because of this, crack density and fiber birefringence were found to correlate well. In addition, the geometry of the stress‐cracking pattern along a filament is affected by internal residual stress since the propagation of spiral and helical cracks results from the effect of a biaxial stress field remaining at the filament surface after processing.

Several primary amines have been examined as selective degradative etchants for the investigation of poly(ethylene terephthalate) (PET) morphology. The objective is to remove less ordered regions, leaving crystals intact. The amines include 40% and 20% aqueous methylamine, 70%–40% aqueous ethylamine and pure and 40% aqueous n‐propylamine. Weight‐loss and x‐ray diffraction data show that certain concentration of aqueous amine solution simultaneously degrade and crystallize PET. This observation indicates the hazard of using some of these amine reagents to characterize PET morphology since the crystalline structure found after etching is likely to be a result of solvent‐induced crystallization during degradation. Data for 40% aqueous methylamine used at room temperature shows that crystallization does not occur during etching, and in light of earlier research indicates the favorable nature of this reagent as a selective degradative medium for PET. Application of this reagent disclosed that in oriented PET fibers chemical stress cracking occurs, causing the degradative reagent to lose its selectivity.

Diffusion rates of an acid dye into commercially drawn Nylon 6 yarns were measured and are discussed in relation to fiber structure. The yarn samples investigated were drawn at two different draw rates and various draw ratios up to 3times;. A four-fold decrease in dye diffusion was found as the draw ratio increased from 1X to 3×; this decrease was not monotonic, however. Maxima and inflections occurred in plots of the diffusion constant vs. draw ratio. The dynamic loss modulus E″, measured under the dyeing conditions, exhibited similar maxima; E″ is indicative of the mobility of the amorphous chain segments. Taken with birefringence measurements, the data are interpreted as indicating a structural breakup during drawing of the undrawn yarn, resulting in an overall less stable microstructure in draw-ratio regions where maxima or inflections occur in the diffusion rate. This instability is markedly affected by drawing rate.

The yield behavior during cold drawing of commercially spun poly(ethylene terephthalate) (PET) filament yarn was investigated. Microscopic examination revealed the presence of inherent flaws within the spun filaments; these act as points for localized stress concentration. These inhomogeneities appear to be either internal cracks or crazes developed during the fiber melt spinning process. During elongation, stress magnification at these flaws results in shear band formation, indicating the onset of inhomogeneous yielding. At the yield bend in the load‐elongation curve a circumferential crack propagates within these shear band regions. This yield crack develops into the classical neck geometry which further localizes additional plastic deformation within the sample at the neck.