Tensile drawing of glassy PET fibers via environmental crazing is studied. Direct microscopic observations show that this process includes the development of macroscopic porosity due to the initiation and growth of multiple crazes with their specific fibrillar-porous structure. Environmental crazing of fibers is characterized by the early collapse of the thermodynamically unstable structure of crazes. This process commences at the stage of craze widening and proceeds until the crazes lose their porosity and become fully monolithic. The collapse is provided by the coagulation of the flexible nanoscale craze fibrils via their interaction by side surfaces. The collapse is markedly intensified when the active liquid is removed from the volume of crazes and, as a result, the crazed fibers acquire a specific surface relief with alternating thick and thin regions corresponding to the bulk unoriented polymer and the collapsed crazes. Practical advantages of the stage of the collapse of the crazed nanoporous structure of crazes for the creation of the fiber-based nanocomposite materials are discussed. (C) 2014 Elsevier Ltd. All rights reserved.