Conformationally disordered crystals and their influence on material properties: The cases of isotactic polypropylene, isotactic poly(1-butene), and poly(L-lactic acid)

his article provides a comprehensive review of the physical properties of the conformationally disordered (condis) structures of isotactic polypropylene (iPP), isotactic poly(1-butene) (iPB-1) and poly(l-lactic acid) (PLLA), in comparison with the respective more stable crystalline forms. The aim of this review is to define the influence of the condis modifications on the thermal and mechanical properties of these materials. The condis structures of the three polymers are metastable and spontaneously transform into the more stable crystalline structures upon annealing above a critical temperature. The transition from the mesophase to the more stable crystalline structure becomes possible when the chains have sufficient mobility to allow rearrangements of chain conformations. A rigid amorphous fraction develops during solidification of iPP, iPB-1 and PLLA. Crystallization of iPB-1 and PLLA into the more stable forms leads to a larger coupling of the amorphous and crystalline chain segments, compared to the conformationally disordered arrangements, which results in a higher fraction of rigid amorphous chain segments. The difference in chain packing, together with the varied mobility of the coupled amorphous chain portions, affects both the initial resistance to the tensile strain and the large strain properties. All the three stable crystalline forms have a higher Young's modulus compared to the condis mesophases, and can sustain lower deformation under mechanical stimuli.