Development of conductive protein-based film reinforced by cellulose nanofibril template-directed hyperbranched copolymer
Graphical abstract
Introduction
With increasing concerns for environmental deterioration and human health, the development of functional materials has shifted from petrochemical feedstock to renewable and sustainable natural resources (Chen, Xu, Wang, Qian, & Sun, 2015). Owing to their advantages of low cost, biocompatibility, biodegradability, and easy processability, biopolymer-based conductive materials have aroused significant interest from researchers in both academic and industrial fields. They have emerged as promising candidates for the production of multifunctional electroconductive components for different advanced applications such as strain sensors, wearable devices, and portable electronic equipment (Ardyani et al., 2020; Lin et al., 2019). Unfortunately, conventional biomaterials typically exhibit relatively poor water resistance and low mechanical performance because of their inherent hydrophilicity and lack of enhanced interfacial adhesion. This severely limits their large-scale industrial production and practical application (Jin, Li, Xia, & Li, 2019). To address these issues, there is a challenging yet necessary demand to develop a strategy for the fabrication of high-performance materials with both excellent conductive properties and mechanical strength.
The design and synthesis of hyperbranched polymers, a class of synthetic dendritic macromolecules with dense branched structures, have emerged as among the most efficient strategies to tailor the surface of functional materials with enhanced interfacial performance (Cui et al., 2019). Compared to linear polymers, hyperbranched polymers display the advantages of low bulk viscosity, low chain entanglement, and high solubility, which enhance molecular mobility and endow the biomaterials with greater strength and flexibility (Demircan & Zhang, 2017). Furthermore, their highly branched structure with dense terminal functional groups can facilitate easier processing, reduced shrinkage, and enhanced the toughness and thermal stability of the multifunctional cross-linking agents in different industrial applications (Lee, Chung, & Kwak, 2016). Several methods have been reported to prepare hyperbranched polymers as effective biopolymer reinforcers with favorable mechanical behavior. Gu et al. (2019) prepared a hyperbranched polyester using maleic anhydride and glycerol as monomers to fabricate a soy protein (SP)-based composite with enhanced tensile strength and UV-light barrier capacity. Zhang et al. (2018) developed a strategy for preparing a soybean meal-based adhesive with favorable shear strength and water resistance via oxidation of soybean polysaccharide grafted with a hyperbranched polyamide. However, traditional synthetic procedures are relatively complex and specific reagents are toxic. Therefore, it is vital to develop the ability to fabricate the necessary biopolymer-based conductive materials using a facile and clean approach.
As an effective conductive polymer, pyrrole, a heterocyclic conductive polymer with a Ļ-conjugated structure, has driven ever-increasing research interests owing to its remarkable in vitro/vivo cytocompatibility, favorable ion-exchange performance, high thermal stability, and controllable conductivity (Liang et al., 2018). Recently, different approaches have been designed to develop functionalized biopolymer materials using the pyrrole group (Raghunathan, Narayanan, Poulose, & Joseph, 2017; Wan, Jiao, & Li, 2017). However, the delocalized Ļ-electron system on the conjugated polymer skeleton frequently causes rigid chains of polypyrrole. After the polymerization of the pyrrole monomers in the biopolymer composites, the polypyrrole is difficult to disperse and easily forms irreversible aggregations because of its rigidity, inaccessibility, and dense layers.
Herein, a hyperbranched poly(amino ester) (HPA) with vinyl end group is prepared, and pyrrole monomers are subsequently introduced to obtain pyrrole-terminated hyperbranched poly(amino ester)-pyrrole (HPPy) polymers through a two-step Michael addition. HPPy with a hydrophobic backbone is not sufficiently compatible with a polymer matrix, whereas renewable biomass-derived TEMPO-oxidized cellulose nanofiber (CNF) has excellent biocompatibility. Our previous research confirmed that CNF can be used as a template to improve the dispersion of carbon-based materials in the matrix (Jin, Li, & Li, 2018). Ding et al. (2018) also demonstrated the biotemplate role of CNF to synthesize conductive complexes, which were further well-dispersed into the polymer matrix to prepare a homogeneous conductive hydrogel. Therefore, an HPPy polymer is in situ polymerized onto the bio template of a CNF surface by Fe3+ oxidation to prepare conductive HPPy@CNF nanohybrids. SP is a ubiquitous and sustainable plant protein resource with a low production cost and acceptable film-forming property. However, it has numerous hydrophilic groups and a weak crosslinking network that deteriorate its comprehensive performance (Li, Jin, Chen, & Li, 2019). It is assumed that HPPy@CNF nanohybrids can effectively enhance an SP matrix via multiple interfacial mechanisms. In this study, we explore this possibility through a novel approach to fabricate a flexible conductive HPPy@CNF/SP composite film. The formed conductive HPPy nanohybrids are expected to serve as a scaffold for multivalent anchoring and crosslinking to increase the interfacial strength of the polymer network and endow the SP-based films with a superior conductive performance. CNF as a biotemplate can promote the uniform dispersion of the reinforcement phase in the SP matrix and the formation of a hyperbranched cross-linking network. The regulating effects of HPPy@CNF on the mechanical properties, conductivity, and water resistance of the prepared composites are investigated in detail. This strategy can provide workable guidelines for the development of biomass material research and can be extended to the reinforcement of other bio-based materials.
Section snippets
Materials
SP (95 % protein) was obtained from Yuwang Ecological Food Industry Co., Ltd. (Shandong, China). The CNF aqueous suspension (1.2āÆwt %), with a length and diameter of approximately 300ā500 and 2ā10āÆnm, respectively, was supplied by Tianjin Woodelf Biotechnology Co., Ltd. (Tianjin, China). The surface charge of the CNF was ā39.8āÆmV, according to the Zeta-potential measurement. Poly(ethylene glycol) diacrylate (98 % purity), pentaerythritol triacrylate (99 % purity), and pyrrole (99 % purity) were
Design and synthesis of HPPy@CNF/SP composites
The facile synthesis process for the HPPy@CNF/SP composites is schematically illustrated in Scheme 1. An HPA polymer was first constructed using poly(ethylene glycol) diacrylate, pentaerythritol triacrylate, and dopamine molecules through a Michael addition. The pyrrole groups were then end-capped to the double bond in the terminal groups of the above polymer to obtain the HPPy precursor. CNF was used as the biotemplate to deposit HPPy because of its excellent dispersibility and nanoscale
Conclusion
A novel strategy was designed to fabricate a hyperbranched biopolymer composite based on natural protein through the in situ polymerization of HPPy@CNF nanohybrids triggered by Fe3+. CNF segments served as a biological template to guide the HPPy precursor growth into a stable and compact HPPy@CNF complex with favorable dispersibility to overcome the water solubility and phase separation problems of HPPy polymers formed directly by the polymerization of free pyrrole monomers. Moreover, the
CRediT authorship contribution statement
Shicun Jin: Conceptualization, Methodology, Writing - original draft. Kuang Li: Visualization, Investigation, Software. Qiang Gao: Formal analysis, Investigation. Wei Zhang: Validation, Formal analysis. Hui Chen: Validation, Formal analysis. Jianzhang Li: Supervision, Writing - review & editing.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 21703009), Nation Key Research and Development Program of China (2017YFD0601205), and Beijing Forestry University Outstanding Young Talent Cultivation Project (2019JQ03004).
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