The structure transformation of pre‐oriented polylactic acid film during uniaxial stretching at room temperature

Abstract

In this article, the pre‐oriented polylactide (PLA) films were prepared by extrusion casting under melt‐stretching flow and cast roll temperature field. Shish‐kebab like structure was induced and the elongation at break under room temperature of PLA film prepared with melt‐draw ratio of 40 was high to 270%. To clarify the origin of this ductile behavior, the pre‐oriented PLA structure evolution during stretching at room temperature was followed. It was found that melt‐stretching makes the chains fully extended to form amount of rigid amorphous fraction (RAF). The low chain entanglement induces weak connection strength between RAF and mobile amorphous fraction. The craze is easily formed at the interface between these two phases. The initial ductile deformation during stretching of pre‐oriented PLA films comes from the rigid extended chain slip and free amorphous chain network deformation. When the strain is higher than 40%, the RAF region appears periodic ordered structure. With the strain increasing, the RAF gradually transforms to mesophase. The molecular chains are stretched to extend in the crazes. The strain‐hardening behavior comes from the crystal destroy and chain extension. The room temperature stretching of pre‐oriented PLA film can only increase chain's orientation and package tightness, but could not induce the formation of highly perfect crystals.

Amorphous rigidification and cooperativity drop in semi−crystalline plasticized polylactide

“…In PLA different approaches have been proposed to follow the evolution of the RAF by MTÀ DSC, from isothermal and nonÀ isothermal crystallization conditions [72], but none has referred to this event. However, in several studies regarding oriented PLA [73,74], it has been suggested that the organization of RAF could lead to mesophase, which is observable from calorimetric techniques. Although the macromolecules are not oriented in the scÀ PLAs studied there, the endothermic thermal event might be the signature of a fraction exhibiting intermediate degree of ordering between RAF and mesophase.…”

Effects of the Three-Phase Crystallization Behavior on the Hydrolysis of Amorphous and Semicrystalline Poly(lactic acid)s

“…The restriction of the MAF was related to the intraoriented structure. 54 Additionally, the stretching could increase the tightness of RAF chains. The derivative of the complex heat capacity (C p ″) peaks of PLA films in the hydrolysis at 0 and 24 h is shown in Figure S5, Supporting Information, which became flat and broader due to the increase of crystallinity and degrading of amorphous chains after hydrolysis.…”

“…This diffraction peak could refer to the other disordered forms of α, previously reported as α′′ form by Marubayashi et al 52 and as α′ (δ) form by Takasaki et al, 53 which has loose chain packing and lower crystal density compared with the α form. Xu et al 54 also reported that the broad dispersing diffraction peak could be obtained from the small and dispersed crystals in the film structures. These small crystals were crystallized through a transformation of the mesophase and confined in the chain network during the melt-stretching crystallization.…”

“…On the other hand, the value of PLAL–NC and PLAH–NC was 1.8 nm at the higher T α of approximately 68.5 °C, with slightly broad δT of 6.1 and 5.1 °C, respectively. The restriction of the MAF was related to the intraoriented structure . Additionally, the stretching could increase the tightness of RAF chains.…”

Supertoughened Polylactide via the Addition of Low Content Poly(ε-caprolactone) and Tensile Deformation above the Glass Transition Temperature

“…The improvements of tensile strength and elongation at break for pre-stretched PLA films have been proved. 23,24,37 In this work, the impact strength of PLA materials is particularly focused. Figure 1B demonstrates the change of notched Izod impact strength with pre-strain for PLA/PCL 95/5 blend.…”