Enhanced crystallization kinetics of bacterially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanate) with structural optimization of oxalamide compounds as nucleators

doi:10.1016/j.polymdegradstab.2018.06.001

Polymer Degradation and Stability

Volume 154, August 2018, Pages 170-176

https://doi.org/10.1016/j.polymdegradstab.2018.06.001 Get rights and content

Highlights

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Crystallization kinetics of PHBH was greatly enhanced.
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The t_1/2 of PHBH was decreased by 93% after adding OXA_n.
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PHBH/OXA_n with superior and steady mechanical performance was achieved.
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A scientific approach to optimal structure of nucleators was provided.

Abstract

Bacterially synthesized poly (3-hydroxybutyrate-co-3-hydroxyhexanate) (PHBH) has attracted much attention as biodegradable plastics and potential biomaterials. However, the crystallization rate of PHBH is very low, which limits its application in plastic field. Herein, oxalamide compounds (OXA_n) with a formula of C₆H₅NHCOCONH(CH₂)_nNHCOCONHC₆H₅ (n = 2, 4, 8, 12) were tailor-made as nucleators to improve the crystallization behaviors of PHBH. The effect of aliphatic spacer length of OXA_n, i.e., -(CH₂)_n-on the nucleation behavior was investigated. The OXA_n would assemble into shish-like superstructures and PHBH crystals preferred to grow on the surfaces of the shish. The crystallization rate of PHBH/OXA_n blends increased with the spacer length of OXA_n up to n = 8 and then leveled off, which trend was consistent with the nucleation activity (Ψ) and the nucleation constant (K_g) of the OXA_n. Meanwhile, the OXA_n increased the nuclei density of the PHBH while inhibited its secondary crystallization, resulting in a superior and steady mechanical performance of PHBH materials. This work provides a scientific approach to design the optimal structure of OXA_n as nucleators for PHBH materials, which may broaden the application range of PHBH materials.

Introduction

Amounts of petroleum-based polymer materials have been used for modern life leading to increasing environmental pollution [1,2]. Therefore, researchers have focused on the development of eco-friendly polymers. Biodegradable polyhydroxylalkanoates (PHAs) have attracted special interests due to bacterially synthesized and fully degradable characters [[3], [4], [5], [6], [7]]. Poly (3-hydroxybutyrate) (PHB) and poly (3-hydroxybutyrate-co-3-hydroxyhexanate) (PHBH) are the most studied polyesters in the family of PHAs. Compared with PHB homopolymer, PHBH copolymer has better toughness, faster degradation rate and much wider thermal processing window [4,6]. However, PHBH still has some drawbacks, especially for the crystallization rate, which is very slow because of the irregularity of chain configuration. Meanwhile, large spherulite size and secondary crystallization lead to poor mechanical properties, as a consequence limit the application of PHBH [6,7].

Adding nucleators has been the most efficient and straightforward approach to improve crystallization behaviors of polymer [8,9]. Up to now, some chemicals have been studied as nucleators for PHAs, such as boron nitride (BN) [10,11], uracil [12], orotic acid [13,14], poly (vinyl alcohol) particle [15], and so on. BN is regarded as the most effective and wide-used nucleator for PHA materials [10]. Ohura [16] and Yamamoto [17] have attained the PHA fibers with high crystallinity and strength by adding BN. Although, these chemicals as nucleators are found to be effective to increase crystallization rate, the aggregation of nucleators usually occurs because of their poor miscibility between nucleators and PHA matrix and the nucleation efficiency is still inadequate to satisfy the requirements for application. Hence, it is necessary to develop more high-efficiency and miscible nucleators for PHAs.

Previously, our group revealed that oxalamide compounds (OXA) can serve as effective soluble-type nucleators for polyester [[18], [19], [20], [21]]. The configuration of the OXA is schematically shown in Fig. 1, where the terminal structures and the aliphatic spacer length (CH₂)_n are crucial to the nucleation efficiency. In a previous study, we revealed the effect of terminal structures (n-hexane, cyclohexyl and phenyl) of the OXA on the crystallization behavior of PHAs [18]. However, the effect of the aliphatic spacer length on the crystallization of PHAs matrix has been researched yet. Therefore, it will be systematically studied in this work and the nucleation activity (Ψ) and the nucleation constant (K_g) of the OXA with different spacer lengths will be discussed by the kinetics of crystallization of PHBH. The aim of this work is to provide a scientific approach to design the optimal structure of oxalamide compounds as nucleators for PHA materials, and to broaden the applied range of PHA materials.

Section snippets

Materials

Poly (3-hydroxybutyrate-co-3-hydroxyhexanate) (PHBH, containing ∼6 mol% of 3-hydroxyhexanate) was purchased from Kaneka Corporation, Japan. The oxalamide compounds with different spacer length between the oxalamide moieties (C₆H₅NHCOCONH(CH₂)_nNHCOCONHC₆H₅, n = 2, 4, 8, 12) were synthesized according to the reported methods [21] and the chemical structures were shown in Fig. 1 (abbreviated as OXA_n).

Sample preparation

PHBH and OXA_n powders were dried at 60 °C under vacuum oven for 12 h before use. The PHBH/OXA_n

Non-isothermal crystallization

In this work, effects of spacer length of the OXA_n on the non-isothermal crystallization behaviors of neat PHBH and PHBH/OXAn blends are studied via DSC. The DSC cooling and subsequent melting traces are displayed in Fig. 2 and detailed thermal parameters are shown in Table 1. Neat PHBH hardly crystallized upon cooling (Fig. 2a) and a pronounced cold crystallization peak (T_cc = 68.6 °C) was detected in the subsequent heating process (Fig. 2b). This result showed a poor crystallization ability

Conclusions

In this work, the oxalamide compounds (OXA_n) with different spacer length between the oxalamide moieties (C₆H₅NHCOCONH(CH₂)_nNHCOCONHC₆H₅, n = 2, 4, 8, 12) were used to improve the crystallization behaviors of semi-crystalline PHBH. The effect of the spacer length, i.e., n values, on their nucleation efficiency and mechanical properties was investigated systematically. The OXA_n could assemble into shish-like superstructures in the PHBH matrix and the PHBH crystals prefer to grow on the surfaces

Acknowledgments

This work is supported by the National Natural Science Foundation of China (51573074), the Excellent Youth Natural Science Foundation of Jiangsu Province (BK20170053), the Fundamental Research Funds for the Central Universities (JUSRP51624A) and Postgraduate Research & Practice Innovation Program of Jiangsu Provence (KYCX17_1429).

References (39)

T.G. Volova et al.
Biodegradation of polyhydroxyalkanoates (PHAs) in tropical coastal waters and identification of PHA-degrading bacteria
Polym. Degrad. Stabil.
(2010)
J.X. Chen et al.
Insights into the nucleation role of cellulose crystals during crystallization of poly(β-hydroxybutyrate)
Carbohydr. Polym.
(2015)
J. Mifune et al.
Engineering of pha, operon on cupriavidus necator, chromosome for efficient biosynthesis of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) from vegetable oil
Polym. Degrad. Stabil.
(2010)
K. Okada et al.
Role of epitaxy of nucleating agent (NA) in nucleation mechanism of polymers
Polymer
(2007)
R.E. Withey et al.
The effect of seeding on the crystallisation of poly (hydroxybutyrate), and co-poly (hydroxybutyrate-co-valerate)
Polymer
(1999)
T. Ohura et al.
Biodegradation of poly (3-hydroxyalkanoic acids) fibers and isolation of poly (3-hydroxybutyric acid)-degrading microorganisms under aquatic environments
Polym. Degrad. Stabil.
(1999)
P. Xu et al.
Crystallization behaviours of bacterially synthesized poly(hydroxyalkanoate)s in the presence of oxalamide compounds with different configurations
Int. J. Biol. Macromol.
(2017)
A. Dobreva et al.
Activity of substrates in the catalyzed nucleation of glass-forming melts. II. Experimental evidence
J. Non-Cryst. Solids
(1993)
J. Chen et al.
Insights into the nucleation role of cellulose crystals during crystallization of poly (β-hydroxybutyrate)
Carbohydr. Polym.
(2015)
M. Yokouchi et al.
Structural studies of polyesters: 5. Molecular and crystal structures of optically active and racemic poly (β-hydroxybutyrate)
Polymer
(1973)

J. Cobntbekt et al.

Physical properties of poly-β-hydroxybutyrate: IV. Conformational analysis and crystalline structure

J. Mol. Biol.

(1972)

T. Iwata et al.

Processing of a strong biodegradable poly [(r)-3-hydroxybutyrate] fiber and a new fiber structure revealed by micro-beam x-ray diffraction with synchrotron radiation

Macromol. Rapid Commun.

(2004)

S. Safari et al.

Effect of crystallization conditions on the physical properties of a two-layer glassine paper/polyhydroxybutyrate structure

J. Mater. Sci.

(2015)

J. Asrar et al.

Biosynthesis and properties of poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) polymers

Biomacromolecules

(2002)

H. Sato et al.

Thermal behavior and molecular interaction of poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) studied by wide-angle X-ray diffraction

Macromolecules

(2004)

D. Libster et al.

Advanced nucleating agents for polypropylene

Polym. Adv. Technol.

(2007)

J. Qian et al.

Comparison of different nucleating agents on crystallization of poly (3-hydroxybutyrate-co-3-hydroxyvalerates)

J. Polym. Sci., Polym. Phys. Ed.

(2007)

F. Yu et al.

Nucleation Effect of layered metal phosphonate on crystallization of bacterial poly [(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)]

Macromol. Mater. Eng.

(2011)

N. Jacquel et al.

Effect of orotic acid as a nucleating agent on the crystallization of bacterial poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymers

J. Appl. Polym. Sci.

(2009)

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Enhanced crystallization kinetics of bacterially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanate) with structural optimization of oxalamide compounds as nucleators

Highlights

Abstract

Introduction

Section snippets

Materials

Sample preparation

Non-isothermal crystallization

Conclusions

Acknowledgments

Polym. Degrad. Stabil.

Carbohydr. Polym.

Polym. Degrad. Stabil.

Polymer

Polymer

Polym. Degrad. Stabil.

Int. J. Biol. Macromol.

J. Non-Cryst. Solids

Carbohydr. Polym.

Polymer

J. Mol. Biol.

Processing of a strong biodegradable poly [(r)-3-hydroxybutyrate] fiber and a new fiber structure revealed by micro-beam x-ray diffraction with synchrotron radiation

Macromol. Rapid Commun.

Effect of crystallization conditions on the physical properties of a two-layer glassine paper/polyhydroxybutyrate structure

J. Mater. Sci.

Biosynthesis and properties of poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) polymers

Biomacromolecules

Thermal behavior and molecular interaction of poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) studied by wide-angle X-ray diffraction

Macromolecules

Advanced nucleating agents for polypropylene

Polym. Adv. Technol.

Comparison of different nucleating agents on crystallization of poly (3-hydroxybutyrate-co-3-hydroxyvalerates)

J. Polym. Sci., Polym. Phys. Ed.

Nucleation Effect of layered metal phosphonate on crystallization of bacterial poly [(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)]

Macromol. Mater. Eng.

Effect of orotic acid as a nucleating agent on the crystallization of bacterial poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymers

J. Appl. Polym. Sci.