Elsevier

Polymer

Volume 267, 13 February 2023, 125673
Polymer

Effect of gamma irradiation on the physical properties of poly(butylene succinate) (PBS) and poly(butylene succinate-co-adipate) (PBSA)

https://doi.org/10.1016/j.polymer.2023.125673Get rights and content

Highlights

  • Chain scission predominates over crosslinking.

  • Chain scission takes place preferentially at –COO–CH2- and –CO–O- bonds.

  • Crystallization temperature is very sensitive to irradiation.

  • Mechanical properties decay catastrophically at low doses.

Abstract

PBS and PBSA were irradiated with gamma up to a 400 kGy dose. Gel content appeared at approximately 100 kGy dose with a higher gel content for PBSA. Charlesby-Pinner analysis indicated that chains scission predominated over crosslinking. Dispersity was increased by branching, that increased molecular weight until gelation, and by chain scission that decreased molecular weight. Careful analysis of the chemical structure by NMR showed that new species appeared with irradiation and some species increased whereas other decreased with the increase in dose. The same species were present in PBS and PBSA and the same variation in species took place in both polymers. From NMR results a mechanism for PBS and PBSA chain scission was proposed. Thermal properties were affected by irradiation, especially crystallization temperature, that decreased significantly with the increase in dose. Mechanical properties were strongly decreased by irradiation and both polymers became fragile at relatively low doses.

Introduction

Poly(butylene succinate) (PBS) and its copolymers with adipic acid or poly(butylene succinate-co-adipate)s (PBSA), are proven biodegradable polymers obtained from sustainable sources that have been proposed as alternatives to polymers based on fossil sources. The introduction of a co-monomer in the PBS chemical structure modifies and tune the PBS properties such as thermal and mechanical behaviour or biodegradability rate.

Adipic acid, 1,4-butanediol and succinic acid can be obtained by fermentation. In 2004, the USA Department of Energy declared the bio-based succinic acid as a chemical platform of high potential for the synthesis of a wide range of chemicals [1], and currently there are several companies producing bio-succinic acid at industrial scale [2]. Because of the difficulty to synthesize high molecular weight PBS, it was not commercialized until the work of Takiyama et al. [3]. In 1993, Showa High Polymers commercialized the first PBS resin under the brand Bionolle™ [2], and in 2003 Mitsubishi Chemical introduced its PBS and PBS range of polymers [1].

The glass transition temperature of these polymers is well below room temperature, therefore they possess a broad workability range which allows its processing through extrusion, injection molding and thermoforming. Its commercialisation has been mainly devoted to biodegradable packaging, because of the similar mechanical properties of PBS and PBSA to polyolefins, but the use of these polymers in the biomedical field has recently attracted considerable attention because of their biodegradability and lack of toxicity [2].

In both applications, these materials are susceptible to be sterilized with, among other methods, ionizing radiation (e-beam, gamma), that has the advantage over steam and heat sterilization methods (at temperatures too high for these polymers to maintain their dimensional stability) that can be carried out at ambient temperature, and over ethylene oxide that no residual chemicals are left in the material.

Ionizing radiation leads to the formation of very reactive intermediates that include excited states, ions and free radicals resulting in the rearrangement and in the formation of new bonds [4,5]. The ultimate effects of these reactions are the formation of oxides products, grafts, crosslinking and scissioning of the main side chains [6].

In literature, only a few works can be found dealing with the effect of ionizing radiation on PBS and PBSA, most of them using e-beam [[7], [8], [9], [10], [11], [12], [13], [14]] and more rarely using gamma [[15], [16], [17]]. In all these works PBS was irradiated and in approximately half of them also PBSA [7,[9], [10], [11],15].

The present work aims to investigate in detail the effect of gamma irradiation on the chemical structure of PBS and PBSA to elucidate the mechanism of interaction of the radiation with these materials, and to correlate the changes induced, to the variations in the physical properties, in particular, in the mechanical properties.

Section snippets

Materials and characterization techniques

Poly(butylene succinate) (PBS) and poly(butylene succinate-co-adipate) (PBSA), with commercial name BioPBS™ grades FZ91PM and FD92PM respectively, were provided by MCPP Germany GmbH (Düsseldorf, Germany).

Molecular weights were determined by size exclusion chromatography (SEC), with chloroform as solvent, using an Agilent gel permeation chromatograph equipped with an Agilent degasser, an isocratic HPLC pump (flow rate = 1 mL min−1), an Agilent autosampler (loop volume = 100 μL, solution

PBS and PBSA characterization

Before irradiation, the materials were characterized for its chemical structure and physical properties.

In Fig. 1A the proton NMR spectrum of PBS is shown. The signals confirm the chemical structure of a poly(butylene succinate) (PBS) with multiplets at 4.11 (a) and 1.70 ppm (c) related to esterified 1,4-butanediol (–COO–CH2-CH2-CH2-CH2-OOC- and –COO–CH2-CH2-CH2-CH2-OOC-, respectively) and a singlet at 2.62 ppm (b) related to the succinate (–OOC–CH2-CH2-COO-), as already described in literature

Conclusions

Gamma irradiation of pure PBS and PBSA with 26% adipate content of high molecular weight as determined by proton NMR and tensile measurements produced a crosslinked material at doses above 70 kGy with a higher crosslinking for PBSA respect to PBS due to its lower crystallinity. Crosslinking density was low, with a maximum gel content of 29% at 400 kGy for PBSA.

Charlesby-Pinner analysis for PBSA demonstrated that both crosslinking and chain scission took place, with a significant predominance of

CRediT authorship contribution statement

Carlos Pérez-Valdez: Investigation, Data curation.

Guillermina Burillo:: Data curation, Formal analysis, Investigation, Resources, Supervision, Validation, Writing – review & editing.

Rodrigo Navarro: Data curation, Formal analysis, Investigation, Validation, Visualization, Writing – review & editing.

Ángel Marcos-Fernández: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgments

This work was supported by projects PID2020-119047RB-I00 and RTI2018-096636-J100, financed by spanish Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033/).

It is acknowledged the technical support provided by F. García Flores from Instituto de Ciencias Nucleares-UNAM in the irradiation of the samples, and by Dr. Valentina Sessini from Universidad de Alcalá in the SEC measurements.

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