High-performance green electronic substrate employing flexible and transparent cellulose films

Highlights

• A strategy combining chemical and physical crosslinking networks was reported.

• The film had high tensile strength (120.56 MPa) and an improved elongation of 263%.

• Thermal stability was superior to RF and CCF due to its more stable structure.

• The properties of the film offered its potential as an electronic substrate.

Abstract

Today's widely used and rapidly updated electronic substrates are composed of petroleum-based polymers, but the resulting electronic waste (such as Dioxin, oxole, PCBs, etc.) will cause massive harm to the environment and human body. Therefore, we report an effective approach for fabricating recyclable and high-performance cellulose films as green electronic substrates by calendering. The crosslinking between CH and CHCH in cellulose modified by maleic anhydride led to the in-situ formation of a chemical crosslinking network, and hydrogen bonds acted as a sacrificial physical crosslinking network. The dual crosslinked cellulose film exhibits high strength (120.56 MPa), improved elongation (increased by 263%), and outstanding thermal stability (thermal decomposition temperature is 311 °C). Further, the film has been successfully used as a substrate for biomass sensor and realized apparent responses to changes. The scientific strategy paves the way for the large-scale fabrication of high-performance cellulose films and simultaneously promotes green electronic substrates' industrialization.

Introduction

With the vigorous development of 5G and server industries, there is an increasing demand for cost-efficient and environmentally friendly electronics (Li et al., 2020). Among them, the substrates of electronics, in particular, are attracting keen commercial and research interests (Fu et al., 2020; Gan et al., 2019; Sun, Wang, Li, Huang, & Zhou, 2020). However, the large-scale application of traditional plastic substrates will also produce more electronic waste (such as Dioxin, oxole, PCBs, etc.) that endanger the environment and human safety. This predicament cannot be ignored in the development of the information industry at present, but it also provides new opportunities and challenges for the industrial application of bio-friendly polymers.

Cellulose, the most abundant natural polymer, demonstrates the exceptional performance of renewability (Cheng et al., 2018; Varma, 2019), thermal stability (George et al., 2011), light-weight (NagarajaGanesh et al., 2019), and high specific strength (Kalali et al., 2019), making it the most promising candidate to substitute for petrochemical-based polymers in electronic substrates (Gan et al., 2021; Jung et al., 2015; Nie et al., 2020; Zhao et al., 2020). However, it exhibits enhancement in strength and stiffness but worsening in elongation due to the complicated and rigid hydrogen-bond network and excessive crystallization regions. Further, strength and elongation are always opposite and mutually exclusive conflicts, and almost all remarkable improvements in strength and stiffness come at the expense of elongation (Gao et al., 2020; Guan et al., 2020), which is still a significant challenge limiting cellulose industrialization. Recently, many scientists have devoted considerable efforts to exploring how to resolve the current dilemma. In past decades, fabrication approaches (such as wet-drawing (Wang et al., 2018), mechanical pressing (Song et al., 2018), chemical treatment (Deng et al., 2016; Yeo et al., 2017), polymer blending (Gao et al., 2020; Zhou et al., 2019), etc.) have been witnessed, including the formation of highly aligned fibers, tightly packed structural materials, surface modification of nanofibers, as well as embedding fibers into a soft polymer matrix or combining with functional materials, etc. Nonetheless, these fabrications suffer from high energy consumption, adverse environmental impacts, and difficulty in meeting the requirements of large-scale fabrication, which limits the scalable application of cellulose in electronic substrates to a large extent. Therefore, an innovative approach suitable for green and large-scale manufacturing of high-performance flexible cellulose materials with high strength and improved elongation remains an urgent demand for practical applications.

Herein, we report an effective approach that cellulose films with both improved strength and elongation were successfully fabricated via calendaring and dual crosslinking. Encouraged by the plasticizing effect of ionic liquids (ILs) on cellulose, transparent cellulose films were prepared under the combined effects of shear force and heat provided by the two-roll mill of the calender, and selective surface dissolution of cellulose by ILs (Li, Zhang, et al., 2020; Qiao et al., 2020). Subsequently, compression molding at 160 °C, during which in-situ chemical crosslinking occurred. Simultaneously, hydrogen bonds derived from a large proportion of the surface hydroxyl groups in cellulose (Ling, Kaplan, & Buehler, 2018; Sun et al., 2020), as physical crosslinking points, were combined to form double crosslinked films (DCFs). Investigation of properties showed that DCF exhibited excellent strength of 120.56 ± 2.61 MPa, improved elongation of 1.56 ± 0.16%, and thermal decomposition temperature of 311 °C. In addition, it also had better dimensional stability, with thermal expansion and wet expansion coefficients of 30 ppm/k and 33.7 ppm, respectively, which preceded some typical plastic substrates (Huang et al., 2013; Pan et al., 2020). Combined with the above performance, a DCF-based biomass sensor was designed, highlighting its potential as a next-generation biomass electronic substrate. This work aims to provide new insights into the large-scale fabrication of high-performance flexible cellulose films with dual crosslinking networks, hoping that our scientific strategy might pave the way to enabling the industrialization of cellulose towards electronic substrates.