Elsevier

Composites Communications

Volume 22, December 2020, 100497
Composites Communications

Biodegradable poly (lactic acid)-poly (ε-caprolactone)-nanolignin composite films with excellent flexibility and UV barrier performance

https://doi.org/10.1016/j.coco.2020.100497Get rights and content

Highlights

  • Lignin nanoparticles grafting poly(lactide-ε-caprolactone) copolymer was synthesized.

  • LNP-g-P(LA-CL) copolymer contributed to 4 folds of toughness of PLA/PCL blend in comparison with PLA.

  • The crystallinity of PLA/PCL-12L were enhanced from 29.4% to 41.3% due to the improved mobility.

  • The cavitation and fibrillation effects could well explain the toughing mechanism.

  • Incorporation of LNP-g-P(LA-CL) copolymer make PLA/PCL film shield 100% of UV-B/C irradiation.

Abstract

In this work, poly(lactide-ε-caprolactone) copolymer grafted lignin nanoparticles (LNP-g-P(LA-CL) were synthesized, through a ring-open polymerization (ROP) of L-lactide and ε-caprolactone, initiated from the hydroxyl groups on LNP surface. Then PLA, PCL and various amounts of the copolymer were blended via a facile solvent casting approach. Mechanical tests showed that the addition of 6 wt% LNP-g-P(LA-CL) copolymer made a significant contribution to the toughness of PLA/PCL blend, which was enhanced by 4 folds in comparison with neat PLA. Meanwhile, the crystallization ability of both PLA and PCL were improved due to the better interfacial adhesion and mobility of PLA and PCL chains. Well-distributed elongated-fibrils and crazes, observed after tensile measurements for the nanocomposites, were responsible for absorbed energy, improved toughness and flexibility of PLA. Furthermore, UV–Vis spectroscopy measurements showed that the incorporation of 12 wt% of LNP-g-P(LA-CL) copolymer make PLA/PCL film able to shield almost 100 wt% of UV-B and UV-C irradiation. PLA/PCL-LNP nanocomposites can be used as impact resistance materials and in UV-resistance fields, such as sunshade and food packaging areas.

Introduction

Poly(lactic acid) (PLA) is frequently reported as a high potential bio-based polymer, owing to its excellent mechanical properties, biodegradability, high transparency and commercial availability. Nonetheless, fragility and lack of functionality are the strong drawbacks for industrial application in some special fields [1,2].

Numbers of approaches have been developed to overcome the intrinsic brittleness of PLA, involving blend modification, plasticization and copolymerization. Among them, the most efficient method is blending it with soft, ductile polymers, for example, poly(ε-caprolactone) (PCL), which it is also a biodegradable semicrystalline polyester [3]. However, the miscibility between PLA and PCL is unsatisfactory. Therefore, improvement of compatibility of PLA/PCL blends is critical to enhance the toughness for neat PLA. PLA-PCL block copolymers represent an efficient route to be followed for the improved compatibilization of PLA/PCL blends. Xiang et al. [4] investigated the compatibilization efficiency of PLA-b-PCL block copolymers with various compositions and molecular weights for PLA/PCL blends, and they found that 5 wt% of the copolymer reduced the PCL particle size and substantially enhanced elongation at break. PLLA-b-PCL copolymers with large molecular weight showed higher efficiency. Similar results have been also obtained by Finotti et al [5].

Lignin, as the most abundant natural polyphenol, has drawn much attention due to the multi-functionalities, including mechanical reinforcements, UV protection and anti-oxidation activity [6,7]. Our previous studies [8,9] have showed that nanolignin possessed high potential applications in various biodegradable polymers. Hence, we attempted to use lignin nanoparticles (LNP) to synthesize the ternary LNP-g-P(LA-CL) copolymer through a ring-open polymerization (ROP) method of L-lactide and ε-caprolactone. Then neat PLA, PCL and various amount of ternary copolymer were blended via a facile solvent casting approach for the sake of improving the PLA toughness. Then microstructure, mechanical and optical behaviour of the produced formulated films were characterized.

Section snippets

Materials

Poly(lactic acid) (PLA 4032D, Mw 100000 Da) was supplied by NatureWorks LLC, USA. L-lactide (99% purity) was kindly provided from Corbion Purac, Netherlands. Poly(ε-caprolactone) (PCL, Mw 80000 Da) and ε-caprolactone (99% purity) were purchased from Aladdin Chemical Reagent Co., Ltd. Pristine alkali lignin was supplied by Shandong Longlive Co., Ltd. Lignin nanoparticles (LNP) were extracted from micro lignin through a simple acidolysis process [7]. Chemicals were supplied by Sinopharm Chemical

LNP-g-P(LA-CL) copolymer characterization

FTIR spectra of L-lactide, ε-caprolactone, LNP and purified LNP-g-P(LA-CL) copolymer are reported in Fig. 1a. The multiple peaks at 1510 cm−1 and 1600 cm−1 are related to Cdouble bondC stretching in aromatic ring of lignin [12], while the band at 1030 cm−1 in LNP represents the aromatic C–H in-plane deformations. In the meanwhile, the characteristic band at 1750 cm−1 was assigned to –Cdouble bondO of L-lactide or PLA chain [13]. For ε-caprolactone, the band at 1725 cm−1 was associated to the carbonyl (-Cdouble bondO) [14],

Conclusions

In this study, poly(lactide-ε-caprolactone) (LNP-g-P(LA-CL) copolymer grafted lignin nanoparticles were synthesized through a ring-open polymerization (ROP) method of L-lactide and ε-caprolactone, initiated from the hydroxyl groups on LNP surface. Then PLA, PCL and various amount of copolymer were blended through a facile solvent casting method. Enhancement of toughness and crystallization properties demonstrated the satisfactory compatibility between PCL and PLA, due to the presence of

CRediT authorship contribution statement

Weijun Yang, Piming Ma: Data curation, Writing - original draft, Funding acquisition. Pengwu Xu, Weifu Dong: Resources, Writing - review & editing. Piming Ma: Project administration. Weijun Yang, Guochuang Qi, Hui Ding, Xiangmiao Zhu, Ting Zheng: Methodology, Formal analysis. Guochuang Qi, Hui Ding: Conceptualization, Validation, Formal analysis.

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.

Acknowledgement

This work is financially supported by National Science Foundation of China (51903106, 51873082), Fundamental Research Funds for the Central Universities (JUSRP11928) and the MOE & SAFEA 111 Project (B13025).

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