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The processing–structure–property relationship using poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET) is explored. Specifically, both pre‐extension and preshear of amorphous PLA and PET above their glass transition temperaturesT_g, carried out in the affine deformation limit, can induce a specific type of cold crystallization during annealing, i.e., nanoconfined crystallization (NCC) where crystal sizes are limited to a nanoscopic scale in all dimensions so as to render the processed PLA and PET optically transparent. The new polymer structure after premelt deformation can show considerably enhanced mechanical properties. For example, premelt stretching produces geometric condensation of the chain network. This structural alternation can profoundly change the mechanical characteristics, e.g., turning brittle PLA ductile. In contrast, after preshear of amorphous PLA aboveT_g, the NCC containing PLA remains brittle, showing the importance to have geometric condensation from processing. Both AFM imaging and SAXS measurements are performed to verify that premelt deformation of PLA and PET indeed results in NCC from annealing that permits the strain‐induced cold crystallization to take place on the length scale of the mesh size of the deformed chain network.

Different semicrystalline polymers including poly(l‐lactic acid), poly(ethylene terephthalate), syndiotactic polystyrene, and polyamide 12 are studied in terms of their mechanical response to uniaxial compression deformation. Apparent decoupling of yielding of amorphous and crystalline phases is identified as separate peaks in the stress–strain curve in the vicinity of the glass transition temperature. The same feature is also observed for the uniaxial extension of predrawn semicrystalline poly(ethylene terephthalate). It is indicated that in absence of a strong amorphous phase a semicrystalline polymer is unable to yield and undergo plastic deformation and it fails in a brittle manner in the uniaxial compression. Treating a semicrystalline polymer as a composite of amorphous and crystalline phases, putting emphasis on the crucial role of amorphous phase in acting as connectors between crystalline domains and indicating that the yielding of amorphous phase is a prerequisite for yielding of crystalline phase, work toward a better understanding of the mechanical properties of semicrystalline polymers at the molecular level is done.