Surface and Reducing End Modification of Nanocellulose to Tailor Miscibility and Mechanical Performance of Reinforced Elastomer Sustainable Composites

*Jinlong Zhang, Qinglin Wu and Weiguo Li*

#### **Abstract**

The development of sustainable nanocelluloses (CNCs) reinforced elastomer composites with high performance is of importance to address the plastic contamination issues, especially the ocean plastic pollution. However, achieving homogeneous dispersion and nanoscale reinforcement of CNCs in the hydrophobic elastomer matrix has been a primary challenging issue attributed to the hydrophilic feature of CNCs and agglomeration during processing. To tailor the mechanical performance of CNCs reinforced elastomer nanocomposites, surface modification of nanocellulose as the traditional way and its reducing end modification as an emerging novel method are ideal strategy to improve its overall performance. In this chapter, nanocellulose surface and reducing end modification to design high performance sustainable CNCs reinforced elastomer composites will be summarized.

**Keywords:** nanocellulose, sustainable composites, mechanical property, reducing end modification, surface modification

#### **1. Introduction**

Polymers commonly named as plastics have become so widely used as a consequence of safeness and low cost, and their production by 2050 is estimated to reach above 500 million tonnes. This ever- increasing production, combined with our desirability for disposability and poor mechanisms for recycling, has led to a plague of plastic in our environment as shown in **Figure 1**. Thus, plastic pollution is a big challenging issue in the world currently, especially ocean micro-plastics [2].

The research in terms of renewable resources derived sustainable composite materials is of importance to address the plastic pollution issues, especially the microplastics in the ocean [1, 3]. The utilization of renewable resources (e.g., natural rubbers and wood fibers) for the design of sustainable composites also has been attracted attentions due to the depletion of fossil resources [4]. According to the literature, the

unique traits of renewable resources based nanocellulose are modulus over 150 GPa and strength over 10 GPa, which is a widely used nanomaterial to design CNC highperformance composites and smart functional materials [5]. Polydimethylsiloxane (PDMS), polyurethane (PU) and polyacrylate as the commonly used elastomer materials have been widely used for surface coating, sealant, and medical cable materials. However, their low elastomer modulus is the primary drawback, which restrict their wide use in the industry fields. Therefore, CNCs are considered as an ideal renewable resource nanomaterial to tailor sustainable elastomer composite performance. These modified elastomer composites are potential for applications in biomedical devices, automotive parts, and stretchable electronics. Several elastomeric sustainable nanocomposite materials reinforced with CNCs were investigated for the last 10 years, such as PU/CNC composites [6], and these CNCs filled elastomer composites resulted in increased thermal and elastic modulus property. Despite of largely enhanced Young's modulus in the resulted CNC-reinforced nanocomposite materials, the tensile strength values were obviously decreased. The immiscibility in the CNC-reinforced elastomer sustainable composite system were the possible reasons.

Thus, to achieve the satisfied enhancement in term of mechanical property is hard attributed to the insufficient interactions at the molecular scale among CNCs and elastomer matrices [7]. The high polarity feature of CNCs resulted in its aggregation easily in the elastomeric matrices, thereby leading to the unsatisfactory compatibility [8]. Thus, it is still a challenging issue to enhance the miscibility and mechanical performance of CNC-reinforced elastomeric composite system. The use of a suitable compatibilizer is a direct way to tailor the compatibilization of CNC-reinforced elastomeric sustainable nanocomposites to establish interfacial interactions (e.g., electrostatic attractions and entanglement of molecular chains) [9]. In terms of basic principle, elastomer matrices and CNCs are compatibilized with the graft copolymer (A-g-B) or chain segments of block copolymer named as A-b-B as shown in **Figure 2**. For instance, the effect interfacial miscibility and dispersion of cellulose in the hydrophobic polyethylene (PE) matrix was achieved attributed to the chain segments in the copolymer compatibilizer interacted with both PE matrix and cellulose dispersion fillers [10]. For grafting or block copolymer noted as A-g-C/A-b-C working on the miscibility of

*Surface and Reducing End Modification of Nanocellulose to Tailor Miscibility and Mechanical… DOI: http://dx.doi.org/10.5772/intechopen.114105*

**Figure 2.**

*Compatibilization strategy of CNCs reinforced elastomer composites.*

CNC-reinforced elastomer composite system as another strategy, the basic principle is that one segment of compatibilizer miscible with CNCs, and another segment is identical with elastomer matrix. For instance, poly(L-lactic acid) (PLA)/cellulose composite system was compatibilized with PLA grafted xyloglucan [11]. Despite of the improved interfacial miscibility in CNC reinforced elastomer sustainable composite system via the compatibilizer approach, its mechanical performance is still not satisfied with industry requirements and has a large space for further improvement. Therefore, two primary strategies as the primary theme in this chapter are summarized to further tailor the interfacial miscibility and mechanical performance of sustainable CNC-reinforced elastomer composites, namely, CNC reducing end and surface modifications.

#### **2. Surface modification of CNCs for tailoring miscibility and mechanical performance of its sustainable elastomer nanocomposites**

#### **2.1 Surface modification of CNCs**

CNC surface modification strategies are primarily divided into two categories. The first approach is to construct the effective linkage among functional compounds and CNCs through the covalent or non-covalent bonds (hydrogen bond and electrostatic interaction). The second approach is to transform CNC surface properties (e.g., hydrophilicity) with the purpose of enhancing its miscibility with the elastomeric matrices. Polymer grafting (grafting from and grafting to) [12], TEMPO-mediated oxidation [13] and adsorption of surfactants [14] as well as esterification [15] as the common strategies have been systemically explored to tailor the surface property of CNCs. However, a relatively weak durability in term of a modifier absorpted onto the surface of CNCs is a big problem resulting in the surface modifications in low efficiency. The grafting copolymerization via the covalent bond linkages was employed for addressing these

drawbacks. According to the literature, it is worth noting that surface grafting polymer chains, surface group derivatization and surface bonding with functional molecules are the primary theme in terms of surface modification of CNCs for the last 20 years. Please see the representative review articles in terms of CNC surface modifications [16–18].

#### **2.2 Surface modification of CNCs on miscibility and mechanical performance of its reinforced elastomer nanocomposites**

Taking CNC-reinforced polyacrylate elastomer composites as a typical example to track the technique progress of CNC surface modification for tailoring miscibility and mechanical performance of CNC-reinforced elastomer sustainable composites. The CNC-reinforced polyacrylate copolymer latex in terms of varying CNC contents was reported about 20 years ago [19, 20], while the obvious aggregation of CNCs in the polyacrylate elastomer matrix was observed as the high loadings of CNCs were added. Subsequent study was done to address the interfacial miscibility issues for the bacterial cellulose and polyacrylate latex. However, the mechanical property was obviously decreased despite of the enhancement of compatibility in the bacterial cellulosereinforced polyacrylate latex composite system [21]. In order to address the compatibilization of CNCs and polyacrylate elastomer materials, the novel approaches named as in situ polymerization or surface grafting modification were explored [22]. The micro-phase separation and mechanical performance of CNC-reinforced polyacrylate elastomer composites via the surface grafting modification avenue were further investigated as well. For instance, to reduce the chance of hydrophilic CNC aggregation in the elastomer matrix named as polyacrylate copolymers, poly(methyl methacrylate) were chemically grafted onto the surface of rod-like CNC surface to impart its hydrophobic character [23]. The obviously enhanced mechanical performance of resulted CNCs/ polyacrylate nanocomposite system were resulted from the micro-phase separation structure and good interfacial miscibility [24]. Thus, it is a feasible method to tailor the mechanical property and compatibilization of CNCs and elastomer matrix via the surface grafting modification approach. However, this conventional surface modification strategy needs to consume CNC surface hydroxyl groups, resulting in the disruption of percolation network, which was constructed via the interactions of hydrogen bonding formed from CNC surface hydroxyl groups. Thus, how to homogeneously disperse CNCs and construct strong interfacial miscibility while maintaining percolating networks of CNCs is a challenging issue. A novel approach named CNC end modification working on the aldehyde groups at the end of CNCs has been attracted attentions a few years ago. The CNC end modification as a topochemical and local modification strategy is obviously different from previous surface grafting modification. The CNC end modification is still in its infancy and demands further study compared to the hydroxyl group surface modification strategies already reported in the literature.

#### **3. Improved miscibility and mechanical performance of sustainable CNC elastomer nanocomposites via its reducing end modification**

#### **3.1 Reducing end modification of CNCs**

In comparison with hydroxyl group modification on CNC surface, it is a new perspective for cellulose chain end modification named as a regioselective reaction

#### *Surface and Reducing End Modification of Nanocellulose to Tailor Miscibility and Mechanical… DOI: http://dx.doi.org/10.5772/intechopen.114105*

on multiple areas of nanocellulose particle. Thus, the active aldehyde groups on CNC end provides an avenue for the modification of CNCs. Surface hydroxyl group modification approach is obviously different from aldehyde group modification in terms of reactivity or quantity of surface hydroxyl and end aldehyde active groups. The hydroxyl functionality on the backbone of CNCs with varied reactivities (C3- OH/C2-OH < C6-OH) are primarily used for the surface modification, while aldehyde groups with high reactivity are mainly provided for the reducing end modification, but steric hindrance influence as an important factor should be considered [25]. The content of aldehyde groups on CNC end is much less than that of surface hydroxyl groups considering the available group contents for the CNC chemical modification. However, some unique functions or features enable to be introduced on CNC end through tailoring the aldehyde groups directly on one end of CNCs [26].

The CNC reducing end modifications are mainly divided into three categories according to its reaction mechanism [28]. The direct and common strategy is transforming aldehyde groups (−CHO) to carboxyl groups (−COOH) followed by the carboxyamine condensation with amino compounds by covalent bonding. For instance, the biotin functionality was selectively introduced at CNC end based on carbodiimide reaction, which includes the carbonyl group oxidation followed by the nucleophilic reaction with biotin compounds containing amine groups as shown in **Figure 3** [27].

Another feasible strategy is using aldimine condensation reaction between aldehyde group at the CNC reducing end and amino compounds. For instance, a twostep chemical reaction, namely the hydrazone reaction and subsequent amidation reaction, has been studied to selectively introduce alkyl long chains with 18-carbon on the end of CNCs as shown in **Figure 4**. The resulted oil/water Pickering emulsions with the alkyl chains (18 chains) grafted CNCs as the stabilizers showed a high emulsifying performance, and the completely cleaved 18-carbon alkyl chains enabled to be achieved with a proper acid condition, so the resulting Pickering emulsions had a pH responsive property [29]. The Knoevenagel condensation between diketone and aldehyde on the reducing end as another strategy has been studied as well. For example, the of Knoevenagel condensation has been employed to chemically decorate the acetyl groups on CNC reducing end by its reaction with beta-diketone, which

#### **Figure 3.**

*Schematic illustration of carboxyamine condensation reaction at nanocellulose reducing end published in 2018 [27].*

**Figure 4.**

*Synthesis route of CNC-C18 via the hydrazone and amidation reaction published in 2017 [29].*

pave a new way to endow unique characters or functional species on reducing end of cellulose based nanomaterials [30].

According to the literature, the motivation of CNC end modification is mainly focusing on the aldehyde group transformation to introduce new functional groups and polymer grafting wit functional species on the end of CNCs as shown in **Tables 1** and **2**. For the functional group derivazation by the carboxyamine condensation, aldimine condensation and hydrazone reaction as well as knoevenagel condensation reactions, acetyl group (−COCH3), thiol group (−SH) and amino group (−NH2) are introduced on the CNC reducing end, and the purpose of these derived functional groups are primary for the improvement of redispersion of CNCs, preparation of Pickering emulsions, template for nanomaterial preparation and conventional free radical polymerization according to **Table 1**. The radiolabeled molecule, fluorescent molecule and biotin derivatives are also grafted on the CNC reducing end to impart the CNCs with biological features for the applications in the biomedical fields, such as antimicrobial properties as shown in **Table 1**. For instance, the chlorotriazine reactive dye conjugated with CNCs produced an effective emulsifying effect of Pickering emulsions. The end of CNC chemically decorated with a hydrophobic dye species while maintaining hydroxyl functionality on CNC surface endowed it with amphiphilic property, thereby increasing its capacity for adsorption on the oil/water interface [45]. Except for the functional group derivazation on the CNC reducing end, polymer grafting reactions on the CNC end were also reported primary via atom transfer radical polymerization, carboxyamine condensation and aldimine condensation reactions to endow CNCs with stimuli-responsive features and apply it for stabilizers in Pickering emulsions. According to the report in the literature, the main grafting polymers are polystyrene, poly(N-isopropyl acrylamide, poly(sodium 4-vinylbenzene sulfonate) and poly(amidoamine) dendrimer as shown in **Table 2**. Compared to the conventional CNC surface grafting reaction, the well-preserved original structure of CNCs can be achieved, thereby offering an intact surface chemistry of resulted CNCs. For instance, temperature responsive polyetheramine polymers were grafted on the end of CNCs via a carboxyamine condensation reaction, and resulted polyetheramine grafted CNCs showed the temperature responsive property, which opens a new approach to design drug delivery carrier materials in biomedical applications.

*Surface and Reducing End Modification of Nanocellulose to Tailor Miscibility and Mechanical… DOI: http://dx.doi.org/10.5772/intechopen.114105*


#### **Table 1.**

*Strategy of functional group transformations on the CNC end.*


#### **Table 2.**

*Strategy of grafted polymers on the CNC end.*

#### **3.2 Reducing end modification of CNCs on miscibility and mechanical performance of its nanocomposites**

The application of CNC reducing end modification is still new according to the report in the literature. It is a promising approach to tailor the miscibility and mechanical performance of CNC-reinforced elastomer sustainable composite system as the aldehyde groups on CNC end are reacted with functional groups to transform its property, especially hydrophilic or hydrophobic character, while the well-preserved surface hydroxyl groups can be achieved, and its stable percolation networks enabled to be well-maintained, resulting in the composite performance enhancement. Taking recent published work from Prof. Lin and Alain's group in term of CNC end modification reinforced elastomer nanocomposites as a typical example, the excellent interfacial interaction among modified CNCs and styrene-butadiene-styrene (SBS) copolymer elastomer matrix and well-dispersion of reduced end modified CNCs were achieved combined with the UV light-initiated thiol-ene click reaction [46]. The CNC surface property and its stable percolation networks were well preserved in this novel strategy resulting in property improvement in term of mechanical performance compared to the previous reported CNC surface modification approach. In particular, the elongation-at-break values are almost no chance compared to the raw rubber materials. The similar work in term of reducing end modified CNC-reinforced natural rubber elastomer composite system was subsequent reported in the same group. The thiol group was first decorated on CNC end via the group transformation strategy named as aldimine condensation in **Table 1**, and the resulted thiol group modified CNCs was further reacted with natural rubber matrix via the UV light-initiated thiolene click reaction for construction of cross-linking network structures. Attributing to the cross-linking at the interface and reinforcement effect as well as the retaining stable percolating network in the CNC-natural rubber elastomer nanocomposites, the excellent interfacial miscibility and mechanical properties were achieved [47].

#### **3.3 Economic analysis of CNCs reinforced elastomer nanocomposites**

To give a comprehensive evaluation of competitive advantages by using CNC nanomaterials for reinforced elastomer composites, a simple economic analysis is conducted by comparative study of different types of fillers in reinforced elastomer composite materials. Carbon fibers, carbon nanotubes, and CNCs as representative fillers are comparatively studied attributed to their excellent elastic modulus. For instance, the elastic modulus of carbon fibers ranges from 150 to 500 GPa, while CNCs has elastic modulus between 110 and 220 GPa [48]. According to the literature, the cost of carbon fiber is \$20/kg, which is much lower than carbon nanotube at \$100/ kg [49]. However, the prices of these carbon materials are much high for reinforced composite materials compared to the relatively less expensive CNC nanomaterial. Its price ranges from \$4 to 10/kg and expects to reach at \$5/kg or even less as the increasingly technique updates in terms of CNC preparation [50], so its price is around 4-times less than carbon fibers. Therefore, the CNCs has competitive advantages compared to carbon fibers and carbon nanotubes as it is important to choose reinforced fillers with excellent elastic modulus but also decent price for reinforced elastomer composites.

#### **4. Conclusions**

The progress in terms of CNC reducing end and surface modification to tailor miscibility and mechanical property of CNC-reinforced elastomer sustainable composites are summarized in this chapter. Compared to the traditional surface modification of CNCs, its unique traits of reducing end modification to establish the *Surface and Reducing End Modification of Nanocellulose to Tailor Miscibility and Mechanical… DOI: http://dx.doi.org/10.5772/intechopen.114105*

percolating network of CNC-reinforced elastomer composites still demands in-depth study. Reducing end modifications of CNCs tailoring PU, PDMS, polyacrylate and other types of rubber elastomeric materials are still no studied up-to-now except for reinforced styrene-butadiene-styrene copolymer elastomer and nature rubber composite system. As the original structure of CNC can be well-preserved for this novel end modification approach, it has wide applications in antibacterial, selfhealing, recycling vitrimer, Pickering emulsions and chiral photonic smart material design in near future. The vitrimer as a new type of dynamic covalent cross-linked network polymers enables to achieve cross-linked CNC elastomer composites with recycling and reprocessing [51]. Due to its unique chiral liquid crystal structure, CNCs/poly(deep eutectic solvents) photonic elastomer composites displayed dynamic structural color and self-healing ability [52, 53].

#### **Acknowledgements**

We appreciate the financial support from the Natural Science Foundation of Hebei Province (Grant No. E2020203063) and Department of Education Foundation of Hebei Province (Grant No. QN2020104).

#### **Author details**

Jinlong Zhang1 \*, Qinglin Wu<sup>2</sup> \* and Weiguo Li3

1 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ, USA

2 School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA, USA

3 College of Aerospace Engineering, Chongqing University, Chongqing, China

\*Address all correspondence to: jzhan620@asu.edu and wuqing@lsu.edu

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### *Edited by Md. Salim Newaz Kazi*

This book introduces the fundamentals of nanocelluloses. Nanocellulose is a cellulosic material with one dimension in the nanometer range and is derived from natural cellulosic materials, animals, and bacterial sources. This book discusses the mechanical, optical, ecofriendly, and thermal properties of nanocelluloses as well as their availability, recyclability, and renewability. It also highlights manufacturing methods and environmentally friendly extraction methods involving enzymatic processes. Moreover, the book explores advancements in nanocellulose preparation and the wide variety of applications of nanocellulose, such as in reinforcement, medical applications, material surface alterations, pest control, and more. Finally, the book examines the various stages of nanocellulose production, ranging from isolation methods to strategies for functionalization. With its thorough examination, this volume serves as a valuable resource for researchers and professionals, equipping them with the knowledge and techniques needed to fully exploit the capabilities of this remarkable material.

### *Jung Huang, Nanotechnology and Nanomaterials Series Editor*

Published in London, UK © 2024 IntechOpen © fotoliaxrender / Fotolia

Nanocellulose - Sources, Preparations, and Applications

IntechOpen Series

Nanotechnology and Nanomaterials, Volume 3

Nanocellulose

Sources, Preparations, and Applications

*Edited by Md. Salim Newaz Kazi*