Elsevier

Polymer

Volume 145, 6 June 2018, Pages 11-20
Polymer

Tailoring poly(butylene 2,5-thiophenedicarboxylate) features by the introduction of adipic acid co-units: Biobased and biodegradable aliphatic/aromatic polyesters

https://doi.org/10.1016/j.polymer.2018.04.063Get rights and content

Highlights

  • A class of copolymers of poly(butylene 2,5-thiophenedicarboxylate) is presented.

  • Aliphatic/aromatic copolyesters containing adipate subunits have been syntesized.

  • Copolymer characteristics can be tailored by acting on molar composition.

  • Adipate subunits impart flexibility and increase biodegradation rate.

  • Thiophenedicarboxylate subunits play a major role in improving gas barrier properties.

Abstract

A class of eco-friendly copolyesters based on poly(butylene 2,5-thiophenedicarboxylate) (PBTF) is presented. The main goal of the study was to enhance the biodegradation rate of PBTF homopolymer. To this purpose, aliphatic sequences based on adipic acid have been introduced in the macromolecular backbone. The synthesized polymers have been characterized from thermal and structural point of view and their properties have been compared to those of the two homopolymers, i.e. PBTF and poly(butylene adipate). The peculiar microstructure of PBTF, due to the presence of an uninduced 2D-ordered phase, i.e. meso-phase, impacted also the copolymers' behavior, e.g. causing an increase of the materials' stiffness.

Biodegradation rate in compost increased with the increase of mobile aliphatic sequences that allowed for an easier access to the ester bonds by the enzymes responsible for chain depolymerization. Lastly, gas permeability resulted strictly dependent on copolymer composition. Copolymers rich in thiophenedicarboxylate co-units displayed a gas barrier behavior comparable to poly(ethylene furanoate), while with the increase of adipate content the permeability increased, remaining however competitive with respect to that of polyolefines and polylactic acid.

Introduction

The growing concern about aquatic and environmental pollution, together with the depletion of fossil fuel resources, is pulling academic and industrial research toward the design of high performance eco-friendly plastic materials capable of substituting the traditional plastics, especially for the realization of short life-span or disposable items. In this view, bioplastic production is increasing at a considerable rate, as it is expected to grow from the actual 4.2 to 6.1 Mtons/year in 2021 [1]. Among the studied bioplastics, polyesters, whose discovery dates back to the work of Carothers at the beginning of 20th century [2], are probably the most interesting class. Depending on the monomers used, polyesters can be divided into two main categories: aliphatic and aromatic. By coupling aromatic and aliphatic moieties, aromatic/aliphatic copolymers can be also prepared. Copolymerization is indeed one of the best tools to boost the performances of a polymer by coupling the positive characteristics of each comonomeric unit.

In particular, as regards aromatic/aliphatic polyesters, the aromatic fraction confers physical and mechanical stability, while the aliphatic one guarantees the material biodegradability. Moreover, copolymerization itself is a means of enhancing polyester biodegradability, generally owing to a reduced degree of crystallinity of the final materials with respect to the parent homopolymer [[3], [4], [5]].

So far, the most successful example of aliphatic/aromatic copolyester is Ecoflex, developed and commercialized by BASF since the late 90s [6]. Ecoflex is a partially biobased, biodegradable plastic of poly(butylene adipate/terephthate) random copolymers.

Due to its very interesting properties, many research groups have focused on the synthesis of Ecoflex-like polyesters having a fully-biobased character. In particular, in the last years, with the aim of substituting the terephthalic subunit, polymers based on 2,5-furandicarboxylic acid (FDCA) have received considerable attention, also because of their low gas permeability [7] that rendered poly(ethylene furanoate) (PEF) so far the most credible biobased alternative to PET [8]. As a consequence, also aliphatic/aromatic copolymers of FDCA have been synthesized [[9], [10], [11], [12], [13], [14], [15], [16], [17]].

Recently, we have reported on the synthesis of an aromatic polyester, i.e. poly(butylene 2,5-thiophene dicarboxylate) (PBTF), as a possible alter ego of furanoate-based polyesters, given its exceptionally high gas barrier properties [18]. With the purpose of enhancing its biodegradability, in this work we propose the synthesis and characterization of Ecoflex-like copolyesters based on 2,5-thiophenedicarboxylic acid (TFDCA) and adipic acid (AA), namely poly(butylene adipate/2,5-thiophenedicarboxylate)s (P(BAxBTFy).

The combination of the above mentioned dicarboxylic acids with 1,4-butanediol (BD) imparts to the here proposed copolymers a 100% biobased character, as all the raw materials can be obtained from renewable resources. BD can be prepared by hydrogenation of succinic acid [19], while bio-AA is derived from glucaric or muconic acid [20]. Most interestingly, TFDCA is a product of the reaction of adipic acid with thionyl chloride [21,22].

P(BAxBTFy) random copolymers have been prepared by melt polycondensation and characterized from the molecular and thermo/mechanical point of view. Compostability and gas barrier properties have been also considered to provide a full insight into the behaviour of this new class of TFDCA-based polymers.

Section snippets

Materials

2,5-thiophenedicarboxylic acid (TFDCA) and adipic acid (AA) were purchased from TCI (Tokyo, Japan). BD, chloroform, methanol and titanium tetrabutoxide (TBT) were obtained from Sigma Aldrich (Saint Louis, MO, USA). TBT was distilled before use.

Polymer synthesis and film preparation

The synthesis of poly(butylene 2,5-thiophene dicarboxylate) has been reported elsewhere [18].

Poly(butylene adipate) (PBA) and poly(butylene adipate/2,5-thiophene dicarboxylate) copolymers (P(BAxBTFy)) have been synthesized through melt polycondensation,

Synthesis and molecular characterization

After the purification process, the synthesized polymers appeared as white to light yellow powders. All data regarding their molecular characterization are contained in Table 1.

The two homopolymers and the P(BAxBTFy) copolymers are characterized by a comparable and relatively high molecular weight. Moreover, the 1H NMR analysis confirmed the awaited structures (Fig. 1). The copolymer composition has been calculated from the relative areas of the 1H NMR resonance peak of the d aliphatic protons

Conclusions

Poly(butylene adipate/2,5-thiophene dicarboxylate) random copolymers have been successfully synthesized throughout the composition range by melt polycondensation.

The presence of uninduced meso-phase has been for the first time demonstrated in random copolyesters, due to the peculiar behavior of BTF sequences capable of organizing themselves both in 2D as well as in 3D ordered domains.

Meso-phase has some beneficial repercussion on the barrier properties of the copolyesters, as it hampers the gas

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