Supramolecular morphology of two-step, melt-spun poly(lactic acid) fibers

Fibers of poly(lactic acid) produced by twostep melt spinning have been studied. The morphology is elucidated with respect to the thermal and mechanical properties of fibers produced at cold-draw ratios of 1-8. With atomic force microscopy and small-angle X-ray scattering, a fibrillar morphology is found, with microfibril diameters ranging from 30 to 60 nm. Shrinkage properties indicate that, with increasing draw ratio, the fibers undergo a transition from class 2 to class 1 within the classification proposed by Keller. A supramolecular model for the morphology of the fibers is presented that entails a highly oriented skin with a core consisting of microfibrils. The orientation of the crystalline blocks within the microfibrils is similar to what has been reported for nylon fibers.