The structure changes of the industrial yarn of poly(ethylene terephthalate)(PET)of 1455dTex／ 110f under the tension of 0.3N and at the temperature range of 375～494K were characterized by WAXD

density gradient method

sonic velocity measurements

and DSC The shrinkage could be divided into two different processes characterized by different relaxation times and final shrinkage amounts The molecular mechanism for the first process

the fast and dominant one

mainly was the disorientation of the extended segments in the amorphous region

which can be proved by the change of orientation function of the amorphous region(fam)with shrinkage time at different temperatures．More meaningful thing is that the shape and change tendencies of fam and relative length(lt)with shrinkage time are very similar The second slow one was resulted from the restructuring of the crystalline region

which included the rapid growth of the erystallites

the formation of new imperfect crystals and the reorganization of existed crystals during the whole shriknage process．The changes of crystalline regions became fast with the temperature rising and consequently their effect of restriction of the disorientation of the amorphous region became more obvious．However

the orientation function of crystalline regions keeps unchanged almost during the thermal shrinkage process It should be indicated that the structure changes of amorphous and crystalline regions are not independently．Each process proceeds and competes each other during the thermal shriknage The contribution of both processes to the shrinkage mainly depends on the temperature and time The study of structure change of PET yarn during thermal shrinkage provides an important idea of developing more perfect crystalline regions to limit the thermal shrinkage due to the disorientation in amorphous regions PET yarns with strength of 7cN／dTex

modulus of 4．4cN／dTex at 5％elongation and thermal shrinkage of 3.5％have been successfully obtained.