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

Laura Aliotta,Elisa Passaglia,Norma Mallegni,Andrea Lazzeri,Maria Cristina Righetti,Massimo Gazzano,Patrizia Cinelli

doi:10.3389/fchem.2019.00790

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.

Highlights

Poly(3-hydroxybutyrate) (PHB) is the most widespread and best characterized homopolymer of the polyhydroxyalkanoate (PHA) family
The HV co-units, which are characterized by a higher mobility with respect to the HB units (Laycock et al, 2014), are partially excluded from the crystals, and accrue at the crystal/amorphous interphase
The arrangement of the polymeric segments into ordered crystal structures is prevented by the limited chain mobility, which leads to large connection between the crystalline and amorphous areas, with a high number of chain segments subjected to geometrical restrictions

Summary

Introduction

Poly(3-hydroxybutyrate) (PHB) is the most widespread and best characterized homopolymer of the polyhydroxyalkanoate (PHA) family. PHAs are a group of biocompatible and completely biodegradable polyesters, synthesized by many bacteria as intracellular carbon and energy reserve (Sudesh et al, 2000). Owing to their biodegradability in compost, soil and marine water, they can be utilized for many different applications, in particular for single use packaging and in agriculture (Bugnicourt et al, 2014). Its elongation at break is noticeably lower, which makes PHB a material much more brittle than polypropylene This poor mechanical property was attributed to the large spherulites that grow as a consequence of a low nucleation density (Barham, 1984). The further and progressive crystallization was found to seriously constrain the amorphous phase, further promoting the PHB embrittlement (de Koning and Lemstra, 1993)

Methods

Results

Conclusion