Structural and Dynamic Heterogeneity in the Amorphous Phase of Poly(l,l-lactide) Confined at the Nanoscale by the Coextrusion Process

Generation of Tough, Stiff Polylactide Nanocomposites through the In Situ Nanofibrillation of Thermoplastic Elastomer

Polylactide (PLA) resins are among the most desirable biopolymers due to their biobased and compostable nature, excellent stiffness, and tensile strength. However, the widespread application of PLA has long been hindered by its inherent brittleness. While multiple routes have been successfully developed for the toughening of PLA, this toughening has always come at the cost of compromising the stiffness and strength of the matrix. In this work, we report a robust and scalable method for the development of PLA nanocomposites with an unprecedented combination of stiffness and toughness. Using the in situ nanofibrillation technique, we generated PLA composites containing nanofibrils of thermoplastic polyester elastomer (TPEE). Due to the high aspect ratio of these nanofibrils, they form physically percolated networks at low weight fractions (∼2.8 wt %) which dramatically change the mechanical behavior of the material. We found that, upon network formation, the material transitions from brittle to ductile behavior, dramatically increasing its toughness with only a marginal decrease in Young's modulus. We investigate the peculiar rheological behavior and crystallization kinetics of these blends, and propose an extension of the critical ligament thickness mechanism, wherein intrinsic toughening arises at the fiber-matrix interface in the presence of entangled elastomer networks.

Constrained Amorphous Interphase in Poly(l-lactic acid): Estimation of the Tensile Elastic Modulus

The mechanical properties of semicrystalline PLLA containing exclusively α'- or α-crystals have been investigated. The connection between experimental elastic moduli and phase composition has been analyzed as a function of the polymorphic crystalline form. For a complete interpretation of the mechanical properties, the contribution of the crystalline regions and the constrained amorphous interphase or rigid amorphous fraction (RAF) has been quantified by a three-phase mechanical model. The mathematical approach allowed the simultaneous quantification of the elastic moduli of (i) the α'- and α-phases (11.2 and 14.8 GPa, respectively, in excellent agreement with experimental and theoretical data reported in the literature) and (ii) the rigid amorphous fractions linked to the α'- and α-forms (5.4 and 6.1 GPa, respectively). In parallel, the densities of the RAF connected with α'- and α-crystals have been measured (1.17 and 1.11 g/cm³, respectively). The slightly higher value of the elastic modulus of the RAF connected to the α-crystals and its lower density have been associated to a stronger chain coupling at the amorphous/crystal interface. Thus, the elastic moduli at T _room of the crystalline (E _C), mobile amorphous (E _MAF), and rigid amorphous (E _RAF) fractions of PLLA turned out to be quantitatively in the order of E _MAF < E _RAF < E _C, with the experimental E _MAF value equal to 3.6 GPa. These findings can allow a better tailoring of the properties of PLLA materials in relation to specific applications.

Distinct dynamics of structural relaxation in the amorphous phase of poly(l-lactic acid) revealed by quiescent crystallization

Fast scanning calorimetry (FSC) experiments were performed to investigate physical aging in amorphous and semi-crystalline poly(l-lactic acid)s (PLLAs) that were thermally crystallized under conditions leading to the α'- or α-crystalline form, and either favouring or inhibiting the development of a rigid amorphous fraction (RAF). The enthalpy of recovery was calculated after two procedures of rescaling to the content of the whole amorphous phase and also to the only content of the mobile amorphous fraction (MAF), which helped in clarifying the contribution of the RAF. From the dependence of the structural relaxation rate on the aging temperature, two regimes were evidenced for all samples. In the aging temperature domain situated close to the glass transition, the structural relaxation occurs significantly faster in the MAF. Its rate is independent of the aging temperature and is not influenced by the microstructure. However, the distance to equilibrium is higher in samples for which the coupling is strong between crystal and amorphous, implying that the time to reach equilibrium is also higher. In contrast, at low aging temperatures, for which the whole amorphous phase can be considered as solid, MAF and RAF exhibit the same structrural relaxation rate. This convergence in the relaxation kinetics by decreasing the temperature of physical aging was interpreted as the evolution of relaxation dynamics in the MAF from segmental to local. This change is highlighted by the comparison between MAF and RAF relaxation kinetics, but it occurs similarly in a pure amorphous system.

Impact of Nanoconfinement on Polylactide Crystallization and Gas Barrier Properties

The barrier properties of poly(l-lactide) (PLLA) were investigated in multinanolayer systems, probing the effect of confinement, the compatibility between the confining and the confined polymer, crystal orientation, and amorphous phase properties. The multilayer coextrusion process was used to confine PLLA between two amorphous polymers (polystyrene, PS; and polycarbonate, PC), which have different chemical affinities with PLLA. Confined PLLA layers of approximately 20 nm thickness were obtained. The multinanolayer materials were annealed at different temperatures to obtain PLLA crystallites with distinct polymorphs. PLLA annealed in PC/PLLA films at 120 °C afforded a crystallinity degree up to 65%, and PLLA annealed in PC/PLLA or PS/PLLA films at 85 °C had a crystallinity degree of 45%. WAXS measurements evidenced that the PLLA lamellas between PS layers had a mixed in-plane and on-edge orientation. PLLA lamellas between PC layers were uniquely oriented in-plane. DMA results evidenced a shift of the PC glass transition toward lower temperature, suggesting the possible presence of an interphase. The development of the rigid amorphous fraction (RAF) in the amorphous phase during annealing was impacted by the confiner polymer. The RAF content of semicrystalline PLLA was about 15% in PC/PLLA, whereas it was neglectable in PS/PLLA. The oxygen barrier properties appeared to be governed by RAF content, and no impact of the PLLA polymorph or the crystalline orientation was observed. This study shows that the confinement of PLLA on itself does not impact barrier properties but that the proper choice of the confiner polymer can lead to decrease the phase coupling which creates the RAF. It is the prevention of RAF that decreases permeability.

Nanorheology with a Conventional Rheometer: Probing the Interfacial Properties in Compatibilized Multinanolayer Polymer Films

Measuring the viscoelastic behavior of polymers in the vicinity of a surface or under confinement is an experimental challenge. Simple rheological tests of nanolayered films of polyethylene/polyamide 6 compatibilized in situ during the coextrusion process enabled the probing of these interfacial properties. Taking advantage of the different melting points and of the multiplication of the number of interfaces, a drastic increase of dynamic moduli was reported when increasing the interphase volume fraction in the films. A solid-like behavior for the interphase was identified. The complex viscosity of nanolayered films as a function of angular frequency was quantitatively captured for all samples using a weighted mixing law of bulk and interphase viscosities, without additional adjusting parameters, highlighting the interfacial synergy developed in nanolayered polymer films.