Open AccessArticle

A Study on the Effects of Vacuum, Nitrogen, and Air Heat Treatments on Single-Chain Cellulose Based on a Molecular Dynamics Simulation

Youna Hua

Wei Wang

^*,

Jingying Gao

Ning Li

and

Zening Qu

College of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150040, China

Author to whom correspondence should be addressed.

Forests 2024, 15(9), 1613; https://doi.org/10.3390/f15091613 (registering DOI)

Submission received: 14 July 2024 / Revised: 22 August 2024 / Accepted: 11 September 2024 / Published: 13 September 2024

(This article belongs to the Special Issue Physical and Mechanical Properties of Wood- and Bamboo-Based Materials)

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Abstract

Employing molecular dynamics software, three models—vacuum–cellulose, nitrogen–cellulose, and air–cellulose—were built to clarify, via a microscopic perspective, the macroscopic changes in single-chain cellulose undergoing vacuum, nitrogen, and air heat treatments. Kinetic simulations were run following model equilibrium within the NPT system of 423, 443, 463, 483, and 503 K. The energy variations, cell parameters, densities, mean square displacements, hydrogen bonding numbers, and mechanical parameters were analyzed for the three models. The findings demonstrate that as the temperature climbed, the cellular characteristics among two models—the nitrogen and vacuum models—decreased and subsequently increased. The nitrogen model reached its lowest value at 443 K. In contrast, the vacuum model reached its minimum value at 463 K. The vacuum heat treatment may enhance the structural stability of the single-chain cellulose more effectively than the nitrogen and air treatments because it increases the number of hydrogen bonds within the cellulose chain and stabilizes the mean square displacement. Furthermore, the temperature has an impact on the mechanical characteristics of the cellulose amorphous zone; the maximum values of E and G for the vacuum and nitrogen models are found at 463 and 443 K, respectively. The Young’s modulus and shear modulus were consistently more significant for the vacuum model at either temperature, and the Poisson’s ratio was the opposite. Therefore, the vacuum heat treatment can better maintain wood stiffness and deformation resistance, thus improving wood utilization. These findings provide an essential theoretical basis for wood processing and modification, which can help optimize the heat treatment and enhance wood’s utilization and added value.

Keywords:

heat treatment; cellulose; molecular dynamics; mechanical properties

1. Introduction

Cellulose is one of the most abundant organic polymer materials in nature, and its unique physical and chemical properties make it play a crucial role in the fields of wood, paper, textiles, and biodegradable materials [1]. The structure of cellulose is complex, as many glucose groups are closely connected to form a long and thin molecular chain. These molecular chains are divided into highly ordered crystal regions and relatively loose amorphous regions, and this composite structure gives cellulose materials diversified performance characteristics [2]. Among them, the amorphous region is not as regular as the crystal region due to its irregular structure and high degree of freedom, and it is particularly sensitive to changes in external environmental factors such as temperature, pressure, and gas atmosphere. Therefore, studying the properties of the amorphous region of cellulose is of great significance to gain an in-depth understanding of the overall properties of cellulose materials.

There is a close and profound relationship between the cellulose chain and wood. The properties of wood, as the main natural carrier of cellulose, are largely determined by the arrangement of cellulose chains, how they bind, and how they interact with other components in the wood. As the growth years of trees increase, the lignin content gradually decreases, while the cellulose content shows a steady rising trend [3]. Cellulose chains are interwoven into the cell walls of wood, providing the necessary mechanical strength and determining key properties such as hygroscopicity, thermal stability, and processability. Therefore, the study of the behavior of cellulose chains in wood and the influence of external conditions such as heat treatment is the key to understanding the changes in wood properties and optimizing wood processing and utilization strategies.

The heat treatment is an advanced process that uses water vapor, inert gasses, air, hot oil, or water as a heat transfer medium to treat materials at high temperatures ranging from 423 to 513 K. The heat treatment process leads to significant changes in the cell wall structure, which can realize the structural control and performance optimization of cellulosic materials. This method is not only an effective way to optimize the physical and chemical properties of wood, but also significantly improves the durability of wood [4]. The internal structure and chemical makeup of heat-treated wood change for the better. This is demonstrated by the wood expanding and contracting less, becoming significantly less hygroscopic, and having more water in it at equilibrium, all of which contribute to the wood’s improved stability in terms of dimension. Furthermore, applying a heat treatment strengthens the wood’s resistance to corrosion and insects, giving a more reliable assurance for a variety of wood uses [5].

The selection of heat treatment media profoundly influences the shaping of wood properties, and each medium shows its unique advantages and potential limitations in the treatment process. In their investigation into using different gas and oil media in the heat processing of bamboo, Yang et al. [6] methodically examined the impacts of various media on the mechanical, appearance, dimensional stability, and thermodynamic characteristics of the material. A thorough analysis was conducted to determine how multiple media affected the mechanical, dimensional stability, quality of the exterior, and thermal and physical features of bamboo. They discovered that the density of bamboo treated with linseed oil as a heat treatment medium increased significantly. Additionally, they found that the bamboo’s moisture removal and shrinkage resistance significantly improved via a gradual rise in the treatment temperature, giving the bamboo more excellent dimensional stability. The thermal alteration from black pine wood was investigated by Bal et al. [7] in three distinct atmospheres: nitrogen, air, and vacuum. The experiment encompassed three temperature points: 453 K, 473 K, and 493 K. According to the research results, the vacuum environment has the least harmful effect on the mechanical properties of wood, which indicates that vacuum has a promising application in wood heat treatment. In addition, the study by Candelier et al. [8] focused on beech wood and observed the changes in its physicochemical properties in vacuum and nitrogen heat treatment environments. According to the experimental data, beech wood treated in a vacuum environment demonstrated a more moderate decrease in important mechanical property indexes, such as the modulus of rupture (MOR) and modulus of elasticity (MOE) along with the Brinell hardness (HB), when compared to the heat treatment with nitrogen. This further supports the vacuum treatment’s superiority in maintaining timber’s mechanical characteristics.

Molecular simulation technology provides a new idea for the study of wood heat treatments, and this method provides a good theoretical basis for existing macroscopic experiments. Molecular dynamics, as an advanced computer simulation technique, aims at accurately calculating and predicting the structure and properties of a system by modeling the trajectories of molecular motions. Alder and Wainwright [9] first proposed this concept in the late 1950s, and it was successfully applied in the exploration of hard-sphere interaction mechanisms. Not only does it close the gap between theory and experiment, allowing us to test the precision of the model through contrasting empirical findings, but it also more intuitively reproduces the subtle change processes that are challenging to capture in macroscopic experiments in a computer environment [10]. Molecular dynamics simulation has demonstrated excellent ability in material property prediction [11]. Tsai et al. [12] used this technique to deeply analyze the mechanical properties of graphite. They revealed the significant advantages of monolayer graphene in mechanical properties by comparing the key parameters of monolayer graphene, such as the Young’s modulus, shear modulus, and Poisson’s ratio, with those of multilayer graphene. With an eye toward their possible uses, Mao et al. [13] investigated the changes in the characteristics of carbon nanotubes using molecular dynamics simulations. They examined the dispersion process in tiny molecules like methane inside them. Cao et al. [14] created a carbon dioxide–cellulose model using the Materials Studio 2020 software. They carefully examined the thermal and mechanical features of the cellulose composite structure at different pressures. This microscale numerical simulation technique not only reveals the microscopic-level information that is difficult to capture by traditional experimental means, but also dramatically simplifies the complexity of the experimental operation and effectively circumvents the various limitations and unfavorable conditions that may be encountered in conventional experiments.

Therefore, based on molecular dynamics simulation technology, an in-depth study of the effects of vacuum, nitrogen, and air heat treatments on the properties of a cellulose amorphous region not only helps to reveal the microscopic mechanism of cellulose materials during heat treatment, but also provides an important theoretical basis and technical guidance for the processing and utilization of cellulose materials. The choice of heat treatment media is the key to optimizing the process parameters, improving the materials’ properties, reducing costs, and achieving environmental friendliness. Using the Materials Studio software, this study simulated the effects of vacuum, nitrogen, and air thermal treatments on single-chain cellulose at five different temperatures through a microscopic perspective. The models were analyzed based on their variations in energy, mechanical parameters, mean square displacements, number of hydrogen bonds, cell parameters, and densities. The changes in the macroscopic properties of the heat treatment process under different temperatures and media are explained from the microscopic level, which provides more theoretical support for wood heat treatment and the optimization of the wood heat treatment process.

2. Materials and Methods

2.1. Model Establishment

Cellulose is a linear polymer connected by β-1,4-glycosidic linkages [15]. Scientists have identified four crystalline forms of cellulose: cellulose I, II, III, and IV. In nature, natural sources of cellulose such as bacterial cellulose, algae, and higher plants fall under the category of cellulose type I. The most prevalent kind of cellulose is called type I cellulose. When further subdivided, natural cellulose type I exhibits two crystalline forms: cellulose Iα and cellulose Iβ [16]. A molecular chain is a long polymer chain that consists of many repeating units connected by covalent bonds. The structure of cellulose molecular chains consists of a series of crystalline regions alternating with amorphous regions. These two types of areas exhibit significant differences in the characteristic expression of the molecular chains, the arrangement patterns, the binding forces between molecules, and the materials’ mechanical characteristics. Specifically, the molecular chains in the crystalline region are more compact and ordered, and the intermolecular bonding is tight, giving it excellent mechanical stability and resistance to the external environment. On the contrary, the amorphous region presents a looser molecular arrangement and weaker binding ability, and outside factors influence its mechanical characteristics. The amorphous area of cellulose Iβ was chosen as the paper’s focal point in light of these variations.

In this study, simulations using the Materials Studio program (2020, San Diego, CA, USA) were conducted to characterize the effects of different temperatures and media on the properties of single-chain cellulose from a microscopic point of view. The degree of polymerization refers to the number of repeating units in the polymer molecule. It is a measure of the length of the polymer chain. It has been found that the simulation results are consistent with the actual situation when the polymerization degree of cellulose is greater than 10, and the different polymerization degrees of cellulose have little effect on the simulation results of the material properties. Although the actual chain length of cellulose is quite large, the increase in chain length during the simulation will increase the simulation time and complexity. Therefore, cellulose chains with a polymerization degree of 20 were selected in this study [17]. As a result, this study established a cellulose chain with a degree of polymerization of 20 [18]. Theodorou’s [19] technique and Wang et al.’s [20] simulation model scale parameters were utilized in the construction of the amorphous zone polymers. Three simulation models were created, all with density settings of 1.5 g/cm³. The three simulation models are vacuum–cellulose, nitrogen–cellulose, and air–cellulose models, as shown in Figure 1. In the vacuum–cellulose model, a chain of cellulose molecules with a degree of polymerization (DP) of 20 was added, and the pressure parameter was set to 20 kPa [8]. In the nitrogen–cellulose model, 20 nitrogen molecules and a chain of cellulose molecules with a degree of polymerization 20 were added. Sixteen nitrogen molecules, four oxygen molecules, and a cellulose molecular chain with a degree of polymerization of 20 were added to the air–cellulose model. The cellulose Iβ molecular chain is shown as a stick model, the nitrogen molecules as a blue ball-and-stick model, and the oxygen molecules as a red ball-and-stick model.

2.2. Dynamic Simulation

After the models were constructed, they were subjected to an energy balance treatment. In this paper, geometric optimization and relaxation were performed on the three models, and the subsequent operations were carried out through the Forcite module. Firstly, geometry optimization was carried out using the Forcite Geometry Optimization module, and the intelligent algorithm was chosen to run for 5000 steps to minimize the model energy and achieve initial structural relaxation. Secondly, the Forcite Dynamics module was used to complete the dynamics relaxation, with the regular system molecular dynamics (NVT) simulations being selected for the system. The NVT ensemble can keep the number of particles (N), volume (V), and temperature (T) of the system constant. Through the NVT ensemble, the thermal motion of the system at a normal temperature can be simulated, and the system can reach the thermal equilibrium state. The temperature was selected to be 300 K at room temperature, the initial speed was set to random, the step size was set to 1 fs, the total step size was set to 1 ns, and a frame was output every 5000 steps [21]. Finally, the system energy of the three models was minimized, the local unreasonable structure was eliminated, and the equilibrium state was reached.

Molecular dynamics simulations were conducted following the energy balance treatment for all three models. The isothermal isobaric system molecular dynamics (NPT) simulations system was set, and the kinetic simulations were carried out at 423, 443, 463, 483, and 503 K, respectively. The NPT ensemble not only keeps the particle population and temperature of the system constant, but also allows the volume of the system to change. This is essential for studying properties such as the phase transition, thermal expansion, and compressibility of materials at different temperatures and pressures. The impacts of the vacuum and nitrogen heat treatments on the macroscopic characteristics of cellulose were confirmed by experimental investigations by Bal [7] and Candelier et al. [8], whose studies offer a solid theoretical foundation for the modeling parameters used in this work. Temperature control in the kinetic simulation was carried out using Andersen’s method [22], pressure control was carried out using Berendsen’s method [23], electrostatic interactions were carried out using Ewald’s method [24], van der Waals interactions were carried out using an atom-based method [25], and the force field was selected from PCFF, which is appropriate for the computation of organic matter [26].

3. Results and Discussion

3.1. Energy Balance

The temperature and energy trends accumulated during the molecular dynamics simulations could be utilized to assess whether the system attained equilibrium [27]. Figure 2 displays the system’s time-varying energy and temperature changes following the vacuum–cellulose model’s kinetic simulation, which lasted 1 ns. The system is considered to have reached equilibrium when the changes in temperature and energy are within the range of 5% to 10%. In the meantime, whether or not the system’s energy is in equilibrium is ascertained using the convergence parameter [28]. Its formula is as follows:

δ E = \frac{1}{N} \sum_{i = 1}^{N} |\frac{E_{i} - E_{0}}{E_{0}}|

(1)

Equation (1) uses the notation N to represent the total number of simulation steps, E₀ to represent the starting energy value, and E_i to represent the energy value following the stage I of the simulation. The system tends to equilibrate, and the simulation findings are trustworthy while 0.001 < δE < 0.003. It is found that the energy convergence parameters of the vacuum, nitrogen, and air models are 0.0023, 0.0028, and 0.0026 at the last 200 ps, respectively, which indicates that the simulation results are reliable.

Figure 2b displays the trend plot of the vacuum–cellulose model’s temperature over time. At the beginning of the simulation, the system usually starts from a specific initial state, which can result in uneven energy and temperature distribution. Therefore, in the early stages of the simulation, significant fluctuations in energy and temperature can be observed, which are manifestations of the transition of the system from a non-equilibrium state to an equilibrium state. During the simulation, if the system gradually reaches an equilibrium state, then the fluctuations in energy and temperature will stabilize and fluctuate around some average value. The model’s temperature changes during the last 200 ps are within ±25 K, indicating that kinetic relaxation brought the model to equilibrium. Both the nitrogen and air systems attained equilibrium, and their temperature–time charts resemble those of the systems.

By analyzing the energy and temperature variations in the three models, it can be concluded that the systems of all three models were stabilized after the initial geometrical optimization and kinetic relaxation, proving the present study’s reliability and allowing subsequent kinetic simulations to be carried out.

3.2. Lattice Parameters and Density

Cell characteristics and model densities may also be utilized to describe the degree of densification of cellulose chains and to analyze the mechanical characteristics of cellulose [29]. A cell is the smallest unit of a parallelepiped that can fully reflect the chemical and structural characteristics of atoms or ions in a crystal in three dimensions. It is important to note that the cell structures of the vacuum and nitrogen or air models both exhibit a cubic appearance. In light of this, we decided to quantitatively characterize the cell size by documenting these model cells’ lengths, breadths, and heights in three dimensions. Table 1 lists each model’s precise temperature-dependent variations in cell characteristics and density.

According to the data presented in Table 1, we can observe that the cell sizes of both models, vacuum–cellulose and nitrogen–cellulose, show a decreasing trend and then an increasing trend with the increasing temperature, and the density changes in the opposite trend. In particular, when the temperature hits 463 K, the vacuum–cellulose model’s cell size decreases to the lowest value of 20.13, and its density rises to the highest value of 1.386 g/cm³. When the temperature hits 443 K, the density increases to the highest point of 1.341 g/cm³, while the nitrogen–cellulose model simultaneously achieves a minimum density of 20.89. The cellulose chain’s molecular mobility intensifies with the temperature, resulting in changes in the intermolecular contact forces (such as hydrogen bonding and van der Waals forces). Initially, the temperature rise could have encouraged a tighter intermolecular arrangement, which would have led to a reduction in cell size and a rise in density. However, when the temperature rises to a certain threshold, the thermal motion between molecules becomes too intense, leading to an increase in intermolecular distances and a subsequent increase in cell size, leading to a decrease in density. Despite the temperature change, the overall arrangement of the cellulose chains did not fundamentally change, resulting in a relatively small range of density changes. At the same time, different media affect cellulose chains differently, leading to various temperatures at which cell size minima are reached for the vacuum and nitrogen models. In contrast, the air environment contains more types of gas molecules, which interact with the cellulose chains more complexly, leading to different cell sizes and density trends with temperature than the vacuum and nitrogen environments. As the temperature rises, the air–cellulose model’s cell size grows steadily while the density continuously falls, as Table 1 illustrates.

A further analysis shows that the cell volume of the vacuum model is the smallest compared to the other models at identical temperatures, and this property enables the vacuum model to exhibit a higher compression tightness at the same temperature. To be precise, in the vacuum environment, the arrangement of cellulose chains is mainly controlled by the internal solid intermolecular interaction forces, as the pressure interference from external gas molecules is wholly excluded. This undisturbed internal force field encourages a tighter and more organized arrangement of cellulose chains, which immediately results in a considerable decrease in cell volume and a rise in density. In addition, the vacuum conditions also limit the activity of heat transfer and molecular collisions, and the range and amplitude of thermal motion of the cellulose chain molecules in the vacuum conditions are more limited at the same temperature than in the environment with a gaseous medium, which further consolidates their tightly packed state.

3.3. Mean Square Displacement

Thermal stability is a parameter that indicates the retention of a material’s properties in a thermal environment. The intensity of cellulose chain movement is closely related to the structural stability of the cellulose; the more intensely it moves, the less stable it is [30]. Thus, the thermal stability of materials at the macroscopic level can be characterized by the intensity of movement of the cellulose chains at the microscopic level. The mean square displacement (MSD), which may be used to accurately describe the migration routes and behaviors of molecules in a system, is the mean square summation of the increments of a molecule’s position vectors after a simulated period. The MSD value in the following equation represents the deviation in the particle’s location with respect to a reference point over time.

M S D = \sum_{i = 1}^{n} 〈{|\vec{r_{i}} (t) - \vec{r_{i}} (0)|}^{2}〉

(2)

In Equation (2), n represents the number of small molecules, <> indicates the mean value, and

{|\vec{r_{i}} (t) - \vec{r_{i}} (0)|}^{2}

symbolizes the center-of-mass displacement of a small molecule i between an instant, t, and the beginning moment.

Figure 3 displays the mean square displacement curves for the three models at various temperatures.

The graphs of the mean square displacements for the air, nitrogen, and vacuum models at various temperatures are displayed in Figure 3. Taking 503 K as an example, the mean square displacement of the vacuum–cellulose model increases from 0 to 8.9, the mean square displacement of the nitrogen–cellulose model increases from 0 to 12.8, and the mean square displacement of the air–cellulose model increases from 0 to 13.4. It can be seen that the mean square displacement of the cellulose chain in the vacuum model fluctuates gently, while that in the air model fluctuates sharply. The fluctuation in MSD values of cellulose chains in the vacuum model is smoother mainly because its environment is simple and stable, and cellulose chains are primarily affected by intermolecular interaction forces. On the other hand, the cellulose chains in the air model show more drastic fluctuations in their MSD curves due to the interference of multiple gas molecules. Meanwhile, the mean square displacement curves of the vacuum model show a smoother state than those of the nitrogen and air models at any temperature. At 463 K, the mean square displacement of the vacuum model increases from 0 to 3.9, and the mean square displacement curves of the nitrogen model fluctuates most gently at 443 K. One central element influencing molecular motion is temperature. As the temperature increases, the molecules’ thermal motion intensifies, and the vibration and displacement of the cellulose chains increase. However, at specific temperatures (e.g., 463 K and 443 K), the cellulose chains in the vacuum and nitrogen models may have reached some dynamic equilibrium, making the fluctuations in their MSD curves the flattest. This may be because, at these temperatures, the internal forces of the cellulose chains are optimally matched to the interaction with the external environment, reducing unwanted fluctuations.

3.4. Hydrogen Bonding

As a remarkable non-covalent interaction force, the essence of hydrogen bonding lies in forming a stable link between a hydrogen atom covalently bonded to an electronegative atom and another more electronegative atom [31]. This force is vital in polymer systems, where it constructs a network of hydrogen bonds that significantly enhances the robustness of the molecular structure and has a profound effect on the overall conformation of the molecule. The glycosidic bonds in cellulose chains and multiple hydroxyl groups in its smallest constituent unit, glucose, produce strong hydrogen bonding in cellulose [32].

Hydrogen bonding in cellulose composite models is divided into intra-chain and inter-chain bonding. Intra-chain hydrogen bonding refers to the bonding formed within the cellulose chain, enhancing thermal stability. In contrast, inter-chain hydrogen bonding spans different molecules and promotes interactions between cellulose chains, contributing significantly to the material’s mechanical properties and overall strength [33].

The primary goal of this research is to compare how well the three models perform in different settings. The vacuum model is a single-chain model, and its only role is intra-chain hydrogen bonding. Therefore, the study on hydrogen bonding presented in this paper focuses on analyzing the effect of intra-chain hydrogen bonding on the thermal stability of cellulose chains, and the variation in the number of intra-chain hydrogen bonds at different temperatures for the models is shown in Figure 4.

The stability of cellulose’s structure and characteristics are primarily dependent on intra-chain hydrogen bonding. The number of intra-chain hydrogen bonds in the vacuum model is much larger than that of the nitrogen and air models at any temperature, as Figure 4 illustrates. With the increase in temperature, the number of hydrogen bonds in both the vacuum and nitrogen models tended to increase and decrease. This could be because, in the interval of a lower heat treatment temperature, the cellulose chain has thermal movement with an increasing temperature, which produces more hydrogen bonds while reducing molecular spacing, thus enhancing molecular bonding and making the molecular structure more stable. With the increase in temperature, the speed and frequency of molecular movement will increase, resulting in more frequent intermolecular collisions and changes in the distance and relative positions between molecules, and the destruction or even fracture of hydrogen bonds within the chain occurs, resulting in a decrease in the overall number of hydrogen bonds. The above analysis corresponds to the mean orientation shift curve of the cellulose chain. Consequently, the cellulose chain’s structure may become unstable at overly high temperatures, which may impact the structure’s overall thermal stability.

According to a further data analysis, the total amount of intra-chain hydrogen bonds in cellulose in the nitrogen and vacuum models peaked at 443 K and 463 K. This indicates that the formation and stabilization of intra-chain hydrogen bonding are optimized in this temperature interval, which significantly enhances the cellulose’s intermolecular bonding. The intermolecular contacts increased as the amount of intrachain hydrogen bonds increased, limiting the amplitude of the thermal movement of cellulose chains and slowing down the changes in molecular structure brought about by thermal movement. This improved molecular bonding strengthened the stability of the cellulose chain itself and raised the structure’s overall heat resistance. In particular, under vacuum conditions, the lack of interference and collisions with air molecules allows cellulose chains to form and maintain more intra-chain hydrogen bonds in a purer and more stable environment. As a result, cellulose chains under vacuum conditions exhibit stronger molecular bonding and superior thermal stability compared to other environments such as nitrogen or air. This property makes the vacuum heat treatment an effective means to better maintain and enhance heat-treated wood’s structural strength and durability.

3.5. Mechanical Properties

The mechanical properties reveal the material’s susceptibility to damage and distortion due to outside forces. The mechanical properties can be characterized in molecular dynamics simulations by many mechanical property parameters. Since amorphous cellulose is isotropic [34], it can be calculated using Formula (3).

[C_{i j}] = [\begin{matrix} λ + 2 μ & λ & λ & 0 & 0 & 0 \\ λ & λ + 2 μ & λ & 0 & 0 & 0 \\ λ & λ & λ + 2 μ & 0 & 0 & 0 \\ 0 & 0 & 0 & μ & 0 & 0 \\ 0 & 0 & 0 & 0 & μ & 0 \\ 0 & 0 & 0 & 0 & 0 & μ \end{matrix}]

(3)

where λ and μ are known as Lamey’s constants and are employed in the calculation of the Poisson’s ratio (η), Young’s modulus (E), shear modulus (G), and other values, and the calculation formula is as follows:

E = \frac{μ (3 λ + 2 μ)}{λ + μ}

(4)

G = μ

(5)

K = λ + \frac{2}{3 μ}

(6)

γ = \frac{λ}{2 (λ + μ)}

(7)

Each mechanical parameter of the vacuum, nitrogen, and air models is calculated with the help of the above equation, as shown in Table 2.

The modulus of elasticity may be thought of as a gauge for how difficult it is to cause elastic deformation in a material; the higher its value, the higher the tension required to cause elastic deformation depending on the stress, the Young’s modulus (E), shear modulus (G), and bulk modulus (K). It was calculated that the K of the cellulose chain produces a small change with an increasing temperature, while E and G can first show a clear increasing trend and then a decreasing one. Therefore, to highlight the comparative effect of the three models, we choose E and G as the primary mechanical parameters for a comparative analysis.

As shown in Figure 5, the E and G values of the vacuum model are significantly larger than those of the nitrogen and air models at any temperature. The maximum values of E and G for the vacuum and nitrogen models are reached at 463 K and 443 K, respectively. The conclusions of the above data on mean square displacement, hydrogen bonding, and densification degree are generally consistent with this. Cellulose chains are used in many heat treatment experiments for wood, and the analysis of the mechanical properties of cellulose chains is helpful for the relevant heat treatment experiments. The elastic modulus and density of heat-treated wood were found to correlate positively, as shown in the study conducted by Bao et al. [35]. A greater degree of hydrogen bonding among cellulose chains reinforces the van der Waals forces in the interchain contacts, strengthening the cellulose system as a whole and increasing the Young’s modulus and shear modulus as well as timber’s ability to withstand deformation [36]. The vacuum environment contributes to the rapid discharge of water inside the wood during the heat treatment process, thus promoting the densification of the wood. With less moisture, the fiber structure of the wood becomes tighter, reducing pores and defects, which helps to improve the overall mechanical properties of the wood. As a result, heat-treating wood in a vacuum preserves its strength and stiffness better than heat-treating it in a nitrogen environment, and the results are superior at 463 K.

The ratio of the axial positive strain to transverse positive strain in a material under unidirectional tension or compression is known as Poisson’s ratio (γ). It is an elastic constant that represents the material’s transverse deformation and is positively correlated with the material’s plasticity; the greater the value, the more plastic the material is. The data in Table 2 show that the γ value of the air model is more significant than that of the vacuum and nitrogen models at any temperature. The oxygen in the air can promote the oxidation reaction on the surface of the wood and form an oxide layer, which can maintain the moisture and humidity inside the wood to a certain extent, and it is conducive to maintaining the plasticity of the wood. Therefore, air conditions can maintain wood plasticity better than vacuum and nitrogen conditions for heat treatments.

Wood is a widely used cellulose-based material; its properties can be changed by heat treatments in different environments so as to meet diversified application needs. In decoration and construction scenarios, the strength and stability of wood are of paramount importance. Compared with heat treatments in nitrogen or air, heat treatments in a vacuum environment can significantly improve the strength and deformation resistance of wood. When wood is used in art design, sculpture, and other fields that need to show its unique texture and shape change, heat treatments in an air environment may be more appropriate. Although the improvement in the strength of wood through a heat treatment in air may not be as significant as that in a vacuum environment, this treatment can better maintain the toughness and plasticity of wood. Therefore, when selecting a heat treatment environment, it is necessary to take into account the specific application scenarios and required characteristics of wood in order to maximize the potential of wood and meet the diverse needs of cellulose-based materials in different fields.

4. Conclusions

In this paper, three models, namely vacuum–cellulose, nitrogen–cellulose, and air–cellulose, were constructed using the Materials Studio software, and five temperature gradients were set for a molecular dynamics simulation. Following a detailed analysis of the models’ energy balance, mechanical characteristics, intra-chain hydrogen bonding, mean square displacements, cell parameters, and densities, the following results were drawn:

The cell parameters and densities of the vacuum, nitrogen, and air models were compared at five different temperatures. Taking 463 K as an example, the cell size of the vacuum–cellulose model was 20.13, that of the nitrogen–cellulose model was 20.93, and that of the air–cellulose model was 21.53. In the vacuum model, the cell volume was smaller, and the average density was larger, which indicates that the cellulose cells treated by a vacuum at the same temperature were more compact. Therefore, a vacuum heat treatment can better improve the density of the material.
In the vacuum and nitrogen models, the MSD of cellulose chains first fell and subsequently increased as the temperature rose. In contrast, due to the interference of various gas molecules, the MSD of the cellulose chain in the air model increased from 0 to 13.4 at 503 K, and its MSD curve fluctuated more violently. Overly high temperatures weaken the structural integrity of cellulose chains by upsetting their internal structure. This also concerns how many hydrogen bonds are in the cellulose chain; more intra-chain hydrogen bonds bolster intermolecular links and solidify the cellulose chain’s structural integrity.
At all temperatures, the vacuum model consistently exhibited bigger values for both the Young’s modulus and shear modulus when compared to the other two models. This indicates that the cellulose chains treated with the vacuum heat treatment were stiffer and more deformation resistant. On the contrary, comparing the Poisson’s ratios of the three models, the Poisson’s ratio of the air model was always more considerable, which indicates that the plasticity of the air heat-treated cellulose chains was better. Thus, to maximize the use of wood, appropriate heat treatment temperatures and media may be chosen based on real-world requirements.

Author Contributions

Conceptualization, Y.H. and J.G.; methodology, Y.H. and Z.Q.; software, Y.H. and N.L.; validation, Y.H., W.W., N.L. and Z.Q.; formal analysis, Y.H.; investigation, Y.H.; resources, Y.H.; data curation, Y.H.; writing—original draft preparation, Y.H.; writing—review and editing, Y.H. and W.W.; visualization, Y.H.; supervision, W.W., Z.Q. and J.G.; project administration, W.W.; funding acquisition, W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Natural Scientific Foundation of Heilongjiang Province, grant number LC201407.

Data Availability Statement

The data are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Diagrams of the three models: (a) the vacuum–cellulose model; (b) the nitrogen–cellulose model; and (c) the air–cellulose model.

Figure 2. Energy versus temperature fluctuation plots for vacuum–cellulose model: (a) energy–time variation; and (b) temperature–time variation.

Figure 3. The mean square displacement curves for the three models: (a) the vacuum–cellulose model; (b) the nitrogen–cellulose model; and (c) the air–cellulose model.

Figure 4. The number of hydrogen bonds at different temperatures for the three models.

Figure 5. (a) Young’s modulus and (b) shear modulus of three models at different temperatures.

Table 1. Cell parameters and densities of three models at different temperatures.

Temperature (K)	Medium	Cell Parameters (Å)			Density (g/cm³)
Temperature (K)	Medium	The Length	The Width	The Height	Density (g/cm³)
423	Vacuum	20.24	20.24	20.24	1.363
	Nitrogen	20.99	20.99	20.99	1.322
	Air	21.42	21.42	21.42	1.313
443	Vacuum	20.15	20.15	20.15	1.382
	Nitrogen	20.89	20.89	20.89	1.341
	Air	21.45	21.45	21.45	1.306
463	Vacuum	20.13	20.13	20.13	1.386
	Nitrogen	20.93	20.93	20.93	1.334
	Air	21.53	21.53	21.53	1.289
483	Vacuum	20.20	20.20	20.20	1.372
	Nitrogen	21.05	21.05	21.05	1.311
	Air	21.57	21.57	21.57	1.276
503	Vacuum	20.21	20.21	20.21	1.369
	Nitrogen	21.11	21.11	21.11	1.300
	Air	21.69	21.69	21.69	1.271

Table 2. Each mechanical parameter of the three models at different temperatures.

Temperature (K)	Medium	$λ$	$μ$	E	G	$γ$
423	Vacuum	6.14	4.16	10.8	4.16	0.30
	Nitrogen	6.94	2.12	5.86	2.12	0.38
	Air	6.82	2.03	5.62	2.03	0.39
443	Vacuum	7.15	4.75	12.35	4.75	0.30
	Nitrogen	5.66	2.26	6.14	2.26	0.36
	Air	6.33	1.98	5.47	1.98	0.38
463	Vacuum	6.97	6.05	15.34	6.05	0.27
	Nitrogen	3.18	2.14	5.56	2.14	0.30
	Air	6.17	1.89	5.23	1.89	0.38
483	Vacuum	4.1	4.82	11.86	4.82	0.23
	Nitrogen	5.77	1.48	4.14	1.48	0.40
	Air	6.08	1.33	3.75	1.33	0.41
503	Vacuum	3.63	3.83	9.52	3.83	0.24
	Nitrogen	6.39	1.02	2.92	1.02	0.43
	Air	6.01	0.81	2.33	0.81	0.44

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Hua, Y.; Wang, W.; Gao, J.; Li, N.; Qu, Z. A Study on the Effects of Vacuum, Nitrogen, and Air Heat Treatments on Single-Chain Cellulose Based on a Molecular Dynamics Simulation. Forests 2024, 15, 1613. https://doi.org/10.3390/f15091613

AMA Style

Hua Y, Wang W, Gao J, Li N, Qu Z. A Study on the Effects of Vacuum, Nitrogen, and Air Heat Treatments on Single-Chain Cellulose Based on a Molecular Dynamics Simulation. Forests. 2024; 15(9):1613. https://doi.org/10.3390/f15091613

Chicago/Turabian Style

Hua, Youna, Wei Wang, Jingying Gao, Ning Li, and Zening Qu. 2024. "A Study on the Effects of Vacuum, Nitrogen, and Air Heat Treatments on Single-Chain Cellulose Based on a Molecular Dynamics Simulation" Forests 15, no. 9: 1613. https://doi.org/10.3390/f15091613

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