MOLECULAR-CHANGES OF PET YARNS DURING STRETCHING MEASURED WITH RHEOOPTICAL INFRARED-SPECTROSCOPY AND OTHER TECHNIQUES

A method has been developed for measuring infrared spectra during the mechanical deformation of yarns. This rheooptical technique was applie d to study the molecular processes that take place along the stress-st rain curve of PET yarns. The results were combined with data obtained from size exclusion chromatography (SEC) and tensile measurements at e levated temperatures. The results indicate that the first modulus maxi mum marks the breakdown of the amorphous entanglement network and the start of molecular uncoiling by gauche --> trans transitions. In addit ion, stress develops on the crystals and particularly on tie molecules with a short contour length in the amorphous domains. Ultimately, mol ecular fracture of taut-tie molecules causes the modulus to pass throu gh a second maximum. The chain ends of broken molecules recoil by tran s --> gauche transitions. Local stress accumulation will lead eventual ly to yarn rupture. (C) 1993 John Wiley & Sons, Inc.