Principles of preparation of nanocellulose freeze-drying method

Lyophilization is a technique for preparing porous nanocellulosic materials This method can effectively retain the high specific surface area and porous structure of nanocellulose, and is widely used in biomedical, food packaging, environmental protection and other fields. The following is a detailed discussion of the scientific and technological connotation of nanocellulose freeze-drying method from the perspectives of principles, process parameters and materials science.by freezing and vacuum sublimation to remove solvents .

1. The basic principles of freeze-drying

The freeze-drying process is divided into three stages: freezing 、primary drying (sublimation) and secondary drying (desorption). Physical and chemical changes at each stage have an important influence on the structure and properties of the final nanocellulose.

Freezing stage

Principle : The nanocellulose suspension is rapidly frozen to a low temperature (usually below its glass transition temperature), allowing the solvent (usually water) to crystallize into ice crystals, while the nanocellulose is fixed in the voids between the ice crystals.

Ice crystal formation mechanism : The size and distribution of ice crystals depend on the freezing rate. Quickly freeze (such as liquid nitrogen freezing) to form small ice crystals, and slow freezing (such as refrigerator freezing) to form large ice crystals. The size of the ice crystals directly affects the pore structure of the final material.

Fixation of nanocellulose : During the freezing process, nanocellulose self-assembles at the ice crystal interface through hydrogen bonding and van der Waals forces to form a network structure.

Primary drying stage (sublimation)

Principle : Under vacuum conditions, the ice crystals are sublimated directly from the solid to the gaseous state by reducing the pressure (usually 0.1-10 Pa) and appropriately heating (usually -50°C to -20°C), bypassing the liquid stage (i.e. sublimation).

Thermodynamic control : The sublimation process requires providing sufficient heat to overcome the sublimation enthalpy of the ice (approximately 2.8 kJ/g) while keeping the system low to avoid thermal degradation of nanocellulose.

Mass transfer and heat transfer : After the ice crystals sublimate, water vapor escapes through the porous structure, and the nanocellulose network is retained. The sublimation rate is affected by ice crystal size, vacuum degree and temperature gradient.

Secondary drying stage (desorption)

Principle : After primary drying, part of the bound water is still adsorbed on the nanocellulose surface. These bound water are removed by further heating (typically 20°C to 30°C) and maintaining a vacuum.

Adsorption water removal mechanism : The bound water interacts with the hydroxyl group on the surface of nanocellulose through hydrogen bonds. The desorption process requires overcoming hydrogen bond energy (about 20-40 kJ/mol).

2. Key scientific issues in freeze-drying of nanocellulose

Ice crystal template effect
Ice crystal acts as a template during the freezing process, which determines the pore structure of the final material. The relationship between ice crystal size and freezing rate can be described by ice crystal growth kinetic model :

r = k⋅tn r = k ⋅tn

Where r r is the ice crystal radius, k k is the growth rate constant, t t is the time, and n n is the growth index. By controlling the freezing rate, the ice crystal size can be regulated, thereby designing the multi-stage pore structure of nanocellulose.

Self-assembly behavior of nanocellulose
During the freezing process, nanocellulose self-assembles at the ice crystal interface through hydrogen bonds, van der Waals forces and electrostatic interactions to form a three-dimensional network structure. This self-assembly behavior is affected by the surface chemistry of the nanocellulose (such as hydroxyl, carboxyl content) and dispersion state.

Structural stability during drying During
sublimation and desorption, the nanocellulose network may collapse due to capillary force or thermal stress. Structural stability can be improved by adding cryoprotective agents (such as sucrose, polyvinyl alcohol) or controlling drying conditions (such as heating rate, vacuum).

3. Effect of process parameters on material properties

Freezing rate

Rapid freezing: form small ice crystals to obtain nanocellulose materials with small pore size and high specific surface area.

Slow freezing: form large ice crystals to obtain nanocellulose materials with large pore size and low specific surface area.

Solvent selection

Water: Environmentally friendly, low cost, but ice crystals grow fast and are prone to large pores.

Organic solvents (such as tert-butanol): ice crystals grow slowly and can form a more uniform pore structure.

Vacuum degree and temperature

Vacuum degree: affects the sublimation rate, too high may lead to structure collapse, and too low will prolong the drying time.

Temperature: The kinetic process that affects ice crystal sublimation and desorption needs to be balanced between the thermal stability of the material and the drying efficiency.

4. Performance and application of nanocellulose freeze-dried materials

Structural characteristics

High specific surface area (up to 200-400 m²/g).

Multi-stage pore structure (micropores, mesopores, macropores).

Good mechanical properties (elastic modulus up to 10-20 GPa).

Application areas

Biomedicine : Tissue-engineered scaffolds (pore structure promotes cell growth), drug carriers (high specific surface area increases drug loading).

Food packaging : Degradable, highly barrier materials.

Environmental protection : Highly efficient adsorbent materials (such as heavy metal ions, organic pollutants).

5. Future development direction

Multi-scale structural design : By regulating the freeze-drying parameters, the multi-scale structure (such as gradient pores, directional pores) design of nanocellulose materials is realized.

Functional modification : Introduce functional components (such as nanoparticles, polymers) during freeze-drying to impart special properties such as conductivity and magnetic properties to the material.

Green process development : Explore low-energy consumption and low-cost freeze-drying processes to promote the industrial application of nanocellulose materials.

Summarize

Nanocellulose freeze-drying method is an advanced material preparation technology based on ice crystal template effect and self-assembly behavior. By deeply understanding the physical and chemical principles of freeze-drying and optimizing process parameters, nanocellulose materials with high specific surface area, multi-stage pore structure and excellent performance can be prepared, providing innovative solutions for biomedical, environmental protection and other fields.