Development of nanofibrillar morphologies in poly(l-lactide)/poly(amide) blends: role of the matrix elasticity and identification of the critical shear rate for the nodular/fibrillar transition

Abstract

Bio-based poly(L-lactide)/poly(amide-11) blends (PLA/PA11, 80/20 w/w) and poly(L-lactide)/poly(amide-6) blends (PLA/PA6, 80/20 w/w) are processed by twin-screw extrusion followed by injection-moulding and key rheological parameters controlling their morphologie are investigated. The same work is done using the same PLA modified by a multi-step reactive extrusion route with an epoxy-based chain extender to obtain modified poly(lactide)/poly(amide-11) (PLA-j/PA11 80/20 w/w) blends. The morphologies of the extruded materials and of the injection moulded parts are characterized by SEM and their formation is deeply discussed via rheological investigation to highlight the contribution of viscosity, elasticity and interfacial tension. The existence of a critical shear rate related to the transition from nodular to fibrillar morphology is highlighted and the results are in good agreement with the condition of fibrillation Ca/Ca (crit) $ 4. Interestingly, with the exception of PLA/PA6 specimens, all blends obviously display uniform thin-thread fibrillar morphologies after injection-moulding. Compared with pure PLA, a drastic increase of the ductility was observed in the blends exhibiting a fiberlike structure without meanwhile sacrificing the stiffness. This study confirms that, through the appropriate choice of blend components (viscosity and elasticity ratio, flow conditions, interfacial tensions) the in situ fibrillation concept provides access, at a reasonable cost, to new materials with improved thermomechanical performances, without sacrificing weight and ability to be recycled.

PLA/PA Bio-Blends: Induced Morphology by Extrusion

“…Yousfi et al [16], worked on PLA/PA11 and PLA/PA6 blends, estimated k' from the shear oscillator rheometry measurements. Assuming that both fluids behave like Maxwell fluids and applying the Cox-Merz rule, k' can be determined from to the storage (G') and the loss (G") modulus of each blend component as a function of the angular frequency (ω) according to the following expression (Equation (4)):…”

“…Among the strategies considered, melt blending of PLA with other polymers has been a practical and economic way for improving the properties of PLA [6,8,10,12,13]. Recently, blends with bio-sourced polyamides (PAs) have gained interest, regardless of their limited miscibility [14][15][16][17][18]. The control of the resulting morphology of these blends has proven to tailor their properties, such as mechanical, crystallization kinetics, and processability.…”

“…This type of induced morphology is referred to as in situ microfibrillated composites (MFCs). Some successful examples of these systems are blends using polyethylene terephthalate (PET) acting as the dispersed phase in fibrillar form [19][20][21], and recently PLA/PA blends [14,16,22]. In all of these cases, an increase in properties, such as fracture toughness, particularly during crack propagation, melt strength, and even enhancing the crystallization of the matrix has been reported [20,21,23,24].…”

“…Until now, few studies have been developed from a rheological point of view. Some of the works worth mentioning are Ding et al [27] with PLA/PCL blends and Yousfi et al [16] with PLA/PAs blends. Both studies confirm that through the appropriate choice of blend components in terms of viscosity and elasticity ratio, flow conditions, and interfacial tensions, the production of in situ fibrillation morphology allows an improvement in the thermomechanical performance at a reasonable cost without sacrificing weight and ability to be recycled.…”

“…Both studies confirm that through the appropriate choice of blend components in terms of viscosity and elasticity ratio, flow conditions, and interfacial tensions, the production of in situ fibrillation morphology allows an improvement in the thermomechanical performance at a reasonable cost without sacrificing weight and ability to be recycled. Thus, a rheological analysis of parameters, such as viscosity ratio (p) and elasticity ratio (k') could be a way to estimate a morphology based on the behavior of the pristine polymers, even to try to control the final behavior obtained to produce a desired morphology like MFC's [16,28].…”