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

Abstract

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/cm 3 , 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.

Effect of a Bio-Based Dispersing Aid (Einar® 101) on PLA-Arbocel® Biocomposites: Evaluation of the Interfacial Shear Stress on the Final Mechanical Properties

“…From the plots of loss tangent (tan δ) ( Figure 4 ) as a function of temperature, it can be observed that the fibers addition slightly shifts the temperature position of the tan δ peak and also slightly lowers the magnitude of the peak that becomes broad. The variation of the tan δ peak is generally correlated to the interaction between the fillers and the matrix that reduces the polymer chains’ mobility [ 77 ]. With the tan δ variation not marked, also, the matrix–filler interaction is not intense, also confirming the results of the mechanical tests.…”

Promoting Interfacial Interactions with the Addition of Lignin in Poly(Lactic Acid) Hybrid Nanocomposites

“…This interphase is named the rigid amorphous fraction ( X RAF filler ) [ 35 ], in reference to the similar behavior commonly reported at the crystal/amorphous interface ( X RAF crystal ). It is known that X RAF crystal strongly affects the macroscopic properties, such as mechanical or barrier properties [ 36 , 37 ]. Klonos et al [ 38 ] showed that X RAF filler hinders thermal diffusivity, whereas X RAF crystal facilitates heat transport.…”

Temperature dependence of the rigid amorphous fraction of poly(butylene succinate)

“…[25][26][27] Recent studies have demonstrated that the rigid amorphous fraction inuences the performance of semi-crystalline polymers, because many physical properties of the RAF are different from those of the crystalline and the mobile amorphous fractions. [28][29][30][31][32][33][34][35] Experimental evidences and theoretical modelling have demonstrated that the elastic modulus of the RAF (E RA ) is between those of the crystalline (E C ) and mobile amorphous (E MA ) fractions, in the order E MA < E RA < E C . 29,31,32 On the other hand, the density of the RAF (r RA ) is lower than that of the MAF (r MA ), due to the higher RAF vitrication temperature, 32,33 so that the order of the densities turns out to be r RA <r MA <r C , where r C is the density of the crystalline phase.…”

“…[28][29][30][31][32][33][34][35] Experimental evidences and theoretical modelling have demonstrated that the elastic modulus of the RAF (E RA ) is between those of the crystalline (E C ) and mobile amorphous (E MA ) fractions, in the order E MA < E RA < E C . 29,31,32 On the other hand, the density of the RAF (r RA ) is lower than that of the MAF (r MA ), due to the higher RAF vitrication temperature, 32,33 so that the order of the densities turns out to be r RA <r MA <r C , where r C is the density of the crystalline phase.…”

“…If mechanical and barrier properties have to be ne-tuned, for example in case of lms for food packaging, it needs to be taken into account that the rigid amorphous and crystalline fractions have opposite effects on barrier properties, 34,35 whereas they together contribute to the material stiffness. [28][29][30][31][32] Thus, a proper balance between crystalline and rigid amorphous fractions is essential to develop a material with specic gas/vapor permeability and exibility.…”