Constrained Amorphous Interphase and Mechanical Properties of Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate)

Abstract

In the present study, for the first time the evolution of tensile mechanical properties of different poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers (PHBV8 and PHBV12, with 8 mol% and 12 mol% of HV co-units, respectively) as a function of the storage time at room temperature has been investigated in parallel with the quantification of the crystalline, mobile amorphous, and rigid amorphous fractions. A comparison with the evolution of the crystalline and amorphous fractions in the homopolymer poly(3-hydroxybutyrate) (PHB) was also performed. For all the samples, the crystallinity was found to slightly increase during storage. In parallel, the mobile amorphous fraction (MAF) decreased markedly, with the result that a relevant increase in the rigid amorphous fraction (RAF) was detected. The RAF content in the copolymers was lower than that of PHB. For all the samples, the RAF formation during aging was ascribed to the growth of secondary crystals in geometrically restricted areas. It was demonstrated that the storage at Troom leads in PHB, PHBV8, and PHBV12 to a progressive increase in the total solid fraction (crystal phase + rigid amorphous fraction) and to a simultaneous physical aging of the rigid amorphous fraction. The two different processes cannot be separated and distinguished, so that only the resulting effect on the mechanical properties was considered. The experimental elastic modulus of both PHBV8 and PHBV12 was found to increase regularly with the total solid fraction, as well as the tensile strength. Conversely, the elongation at break turned out to be an increasing function of the mobile amorphous fraction. The elastic moduli of the crystalline, mobile amorphous, and rigid amorphous fractions of PHBV8 and PHBV12 were estimated by means of a three-phase modified Takayanagi's model, to take into account also the contribution of the rigid amorphous fraction. The calculated values were found in agreement with theoretical expectations.

Plasticization of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with an Oligomeric Polyester: Miscibility and Effect of the Microstructure and Plasticizer Distribution on Thermal and Mechanical Properties

“…The PHB microstructure is reported to be composed of a crystalline phase and two different amorphous phases: the mobile phase, which has the same properties and glass transition temperature ( T g ) as the bulk amorphous phase of PHB, and the rigid phase, located adjacent to the crystalline phase, which has reduced chain mobility and, as a consequence, higher T g . − The T g of the PHB amorphous phase is close to room temperature. This is responsible for the polymer chains’ mobility and for the embrittlement with time, a phenomenon called aging.…”

“…The aging could occur through two independent processes: secondary crystallization and physical aging of the amorphous phase. Both phenomena increase the fraction of the crystalline and rigid amorphous phases, resulting in a decrease in ductility and an increase in the stiffness and brittleness of the PHB. ,,, …”

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.…”