Research on the surface modification and functionalization of cellulose nanocrystals

Research on surface modification and functionalization of cellulose nanocrystals (CNCs) is an important direction in the current field of nanomaterials Since the surface of . cellulose nanocrystals is rich in hydroxyl groups, its chemical activity is high, it can be modified and functionalized through various methods, thereby improving its dispersibility, hydrophilicity, interface compatibility and other properties, making it in many fields The application potential has been greatly improved.

1. The principle of surface modification of cellulose nanocrystals

The surface of CNCs has a large number of hydroxyl groups (–OH), which are key sites for surface modification. Introducing new functional groups through chemical reactions can impart specific physical and chemical characteristics to CNC, including hydrophobicity, conductivity, optical activity, etc. The goals of surface modification include:

1. Improve dispersion: especially in non-polar organic solvents or polymer matrix.

2. Enhance interfacial compatibility: improve bond strength with the matrix in composite materials.

3. Introduce functionality: confers CNC electroactive, magnetic or antibacterial properties.

2. Surface modification method

1. Chemical modification

Chemical modification is the main method of introducing new functional groups on the surface of CNC through chemical reactions.

Esterification reaction

React CNC with organic acid or anhydride to introduce ester groups (–COOR).

Functionalization effect: Improve hydrophobicity, used in polymer composite materials.

Commonly used reagents: acetic anhydride, capric acid, and stearic acid.

Etherification reaction

CNC reacts with halogenated hydrocarbons or epoxides to introduce ether groups (–OR').

Functionalization effect: Enhance the solubility and chemical stability of CNC.

Commonly used reagents: propylene oxide, chloromethyl compounds.

Silanization

A silane coupling agent is used to react with the hydroxyl group on the surface of CNC to form a silicon oxygen bond.

Functionalization effect: Improve compatibility with hydrophobic substrates, such as in coatings.

Commonly used reagents: 3-aminopropyltriethoxysilane (APTES).

Oxidation modification

The hydroxyl group on the surface of CNC was oxidized with TEMPO (2,2,6,6-tetramethylpiperidine oxide) as carboxylic groups (–COOH).

Functionalization effect: Improve water solubility and charge density, used in biomedicine and catalysis.

2. Physical adsorption modification

Functional molecules or nanoparticles are introduced into the CNC surface through physical adsorption.

Polymer adsorption: Adsorbing polymers (such as polyvinyl alcohol, chitosan) through electrostatic interaction or hydrogen bonds.

Nanoparticle adsorption: Immobilize metal nanoparticles (such as Ag, Au) on the surface of CNC to impart conductivity or antibacteriality.

3. Polymer grafting

The polymer chain is grafted on the surface of the CNC by the 'graft to' or 'graft from' method.

Grafting: Preformed polymer chains are bound to the CNC surface by chemical bonding.

Grafting from: polymerization is initiated directly on the CNC surface by atom-transfer radical polymerization (ATRP) or ring-opening polymerization (ROP).

Functionalization effect: significantly improves the dispersion and stability of CNC in the matrix and is used in polymer composite materials.

3. Functional effect of cellulose nanocrystals

1. Improve dispersion

Enhanced hydrophilicity: The introduction of carboxy groups through oxidation improves stability in aqueous solution.

Enhanced hydrophobicity: Modification by esterification or silanization makes it more uniformly dispersed in non-polar solvents.

2. Give electrical performance

Conductivity: CNC can be used in conductive films or sensors by adsorbing conductive polymers (such as polyaniline) or metal nanoparticles (such as Ag, Au).

3. Improve biological activity

Antibacteriality: By introducing silver nanoparticles or quaternary ammonium salt modifications, CNCs exhibit good antibacterial properties and can be used in packaging and medical materials.

Drug loading: The oxidatively modified CNC surface introduces carboxyl groups to facilitate loading of drug molecules for drug delivery.

4. Enhance mechanical properties

Surface modification improves the interface compatibility between CNC and polymer matrix, allowing it to transfer load more efficiently in composite materials, significantly improving the strength and modulus of composite materials.

IV. Application of functionalization of cellulose nanocrystals

Composite materials: Functionalized CNC can be used as a reinforcement material and is widely used in lightweight and high-strength polymer composite materials.

Application areas: Automobile, Aerospace, Construction.

Pharmaceutical and Biomaterials: CNC surface modification enables the development of drug delivery, tissue engineering stents and biosensors.

Functional Coating: Antibacterial, waterproof, and antifouling coatings can be prepared by silanized or modified CNCs.

Adsorption and Catalysis: Oxidation modified CNC has excellent adsorption properties and can be used in heavy metal ion removal, dye adsorption and catalyst support.

V. Current research status and development prospects

Current research status

At present, the research on surface modification and functionalization of CNC has become a hot topic, mainly focusing on the following directions:

1. Green modification: Use environmentally friendly solvents and gentle modification methods, such as enzyme catalytic modification.

2. Multifunctionalization: realizes multiple functions of CNC through one-step modification, such as having both antibacterial and electrical conductivity.

3. Efficient preparation: optimize the modification process to improve the yield and performance stability of functionalized CNCs.

Development prospects

1. Mass production: Develop cost-effective modification technologies to meet industrial needs.

2. High-end application expansion: Functional CNC has broad prospects in the fields of high-performance materials, medicine, energy and environmental governance.

3. Compound technology development: combine other nanomaterials (such as graphene and silica) to achieve performance superposition to meet complex application needs

Due to its unique structural and surface characteristics, cellulose nanocrystals can significantly improve their physical, chemical and biological properties through surface modification and functionalization. In-depth research of this technology not only promotes the widespread application of CNC in composite materials, biomedical and environmental fields, but also provides the possibility for the development of new high-performance, green and environmentally friendly materials.