Exploration of Optimal Reaction Conditions for Constructing Hydrophobic Polymers with Low Deformation to Facilitate the Dimensional Stability of Laminated Bamboo Lumber

The environmental moisture changes would result in the deformation and cracking of laminated bamboo lumber (LBL) easily due to the unreleased internal stress, leading to poor durability. In this study, a hydrophobic cross-linking polymer with low deformation was successfully fabricated and introduced in the LBL by polymerization and esterification to improve its dimensional stability. In an aqueous solution, the 2-hydroxyethyl methacrylate (HEMA) and Maleic anhydride (MAh) were employed as the base compounds for synthesizing the copolymer of 2-hydroxyethyl methacrylate and maleic acid (PHM). The hydrophobicity and swelling performance of the PHM was adjusted by controlling the reaction temperatures. PHM-modified LBL’s hydrophobicity as indicated by the contact angle, increased from 58.5° to 115.2°. The anti-swelling efficiency was also improved. Moreover, multiple characterizations were applied to clarify the structure of PHM and its bonding linkages in LBL. This study demonstrates an efficient avenue to facilitate the dimensional stability of LBL by PHM modification and sheds new light on the efficient utilization of LBL using a hydrophobic polymer with low deformation.


Introduction
Laminated bamboo lumber (LBL) has high strength, uniform structure, and flexible size characteristics, making it a popular material in furniture and construction. The green building material engineered bamboo has become increasingly important to modern construction due to its eco-friendliness and recyclable qualities [1,2]. LBL has been used to develop bamboo engineering products, compensating for bamboo's inherent drawbacks [3]. However, a material derived from bamboo contains hydrophilic components and nutrients, which can cause cracking, deformation, and mildew when placed in alternately wet and dry environments [4,5]. These defects restrict bamboo's development, which dramatically limits its outdoor application. Developing solid durable products would be LBL's future focus [6]. However, LBL has not yet been extensively studied for durability, unlike raw bamboo and bamboo scrimber. Therefore, the damage caused by moisture changes to LBL must be prevented or limited [7].
Bamboo's durability can be enhanced by increasing its hydrophobicity [8,9]. Bam-boo materials have been produced with hydrophobic or super-hydrophobic surfaces using polydimethylsiloxane (PDMS) lithography or micro-nano structure mimicking the lotus leaf structure [10]. However, internal modification is necessary for LBL outdoor applications due to the hydrophobic surface's proneness to wear and weathering. Round bamboo was impregnated with polyethylene glycol and heated with paraffin, significantly improving its dimensional stability and hydrophobicity [11]. To improve bamboo's dimensional stability,

Synthesis of PHM
HEMA and MAh monomers at a molar ratio of 3:2 were dissolved in deionized water to prepare a 40 wt. % monomer solution. 0.6 wt % APS and 0.4 wt % MBA of the total monomer amount were added into the solution, followed by sufficiently stirring until a homogeneous reaction solution was obtained. Subsequently, 30 g solution was weighed into each glass container and reacted at 100, 120, 140, 160, and 180 • C, respectively, for 2 h. The resulting polymers were correspondingly designated as PHM-100, PHM-120, PHM-140, PHM-160, and PHM-180.

Characterization
The PHMs for characterization were firstly extracted with a 2:1 (volume ratio) mixture of ethyl alcohol and deionized water to remove the unreacted monomers, then dried at 80 • C to constant weight before characterization. The extracted polymer samples were fractured in liquid nitrogen to obtain smooth sections. Then homogeneous gold layers were sputter-coated on their sections. The microstructure of PHM was observed using the TM3030 (HITACHI, Tokyo, Japan) scanning electron microscope (SEM) at a voltage of 1.5 kV.
Fourier transform infrared (FT-IR) spectra were recorded using the IR Prestige-21 Spectrophotometer (Shimadzu, Kyoto, Japan) in the range of 500-4000 cm −1 with a 4 cm −1 resolution and 32 scans. The copolymer samples were prepared as spectroscopic quality KBr pellets with a sample/KBr ratio of 1/100. 13 C-NMR analysis was performed at 400 MHz (Bruker, Bremen, Germany) using solid powder samples. The glass transition temperatures (T g ) of PHM were determined at differential scanning calorimetry (DSC) measured with a Q2000 thermal analyzer (TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere. Each sample was heated from −70 • C to 150 • C at 10 • C/min.

Evaluating the Performance of PHM under Water Immersion
The percentage of water absorption (P w ) and volume swelling (P v ) of PHM were determined through three cycles of water immersion-oven dry (W-D) procedure. Samples were not extracted before the test. In each process, samples were soaked in deionized water at room temperature for 24 h, then dried at 80 • C to a constant weight. The mass and volume variation was recorded after soaking or drying. P w and P v were calculated from Equation (1) and Equation (2), respectively. Additionally, the average volumetric gain per unit water absorption (VG) based on three cycles test was computed to evaluate the dimensional stability of the polymer upon contacting water.
In the equations, M d and V d represent the mass and volume of the specimens after drying, and M w and V w are data after water absorption.
In addition, the average mass loss ratio (ML) of the copolymers was calculated based on the three cycles swelling experiment to show their leaching resistance when exposed to water immersion. The specific formula is as follows: In the above equations, M 1 and M 2 represent the samples' dry weight before and after soaking.

Construction of PHM in LBL and Its Dimensional Stability
The reaction solution (40 wt. %) prepared as described in Section 2.2 was impregnated into LBL blocks under a vacuum of 0.09 MPa for 15 min and pressurized at 0.8 MPa for 30 min. The impregnated LBL was then heated at designated temperatures (100, 120, 140, 160, 180 • C) for 2 h to complete the polymerization. Then, LBLs were extracted with a 2:1 (volume ratio) mixture of ethyl alcohol and deionized water to remove the unreacted monomers and dried at 80 • C to constant weight before tests.
The contact angle characterizes the hydrophobicity of modified LBL conducted using a video optical contact angle tester (*OCA50AF, Dataphysics, Filderstadt, Germany). The deionized water was dropped on the sample surface at 9 different sites. A fixed droplet volume of 5 µL deionized water was used, and the test duration was set at 3 s. The contact angle was measured and averaged to obtain the contact angle of LBL.
LBLs were soaked in water for 72 h to simulate the moisture circumstances, allowing full saturation at the moment of measurement in the soaked state. The dimensional stability is characterized by the swelling ratio (S w ) and ASE after water absorption. 8 samples were used for the test.
V d and V w represent the volume of the test block before and after water absorption, respectively. S is the swelling ratio of modified LBL, and S 0 is the data of raw LBL.

The Water Absorption and Swelling Behaviors of PHM
To obtain an optimal crosslinked polymer with excellent hydrophobic properties for LBL modification, the reaction temperature that initiated both the polymerization and esterification of HEMA and MAc was paid special attention. Figure 1 presents polymer samples' water absorption and swelling behaviors during the three water immersion-oven dry (W-D) procedure cycles, reflecting the hydrophobicity and deformation resistance. Overall, polymers' P w and P v decreased gradually in three cycles. During the first two cycles, PHM showed more than 12% water absorption capacity, causing a volume swelling rate exceeding 10%. The hydrophilic functional groups can generate strong interactions with the swelling medium to cause water absorption and volume expansion [31]. Conversely, the initially stable crosslinking network presented hydrophobicity after two cycles of water-dry procedures, with P w dropping significantly, in the range of~2.1 to~6.7%, and correspondingly, P v diminishing below 5.1%. The decreased water absorption and volume swelling might be ascribed to the leaching of unreacted monomers and low molecular non-crosslinked polymers. The formed three-dimensional crosslinking network could improve PHM's cohesive strength and decrease the swelling rate. The effect of reaction temperature on the P w and P v was not as evident as expected, especially for the P v . Except for PHM-180, the P v remained almost the same in the 3rd cycle. No volume swelling occurred to PHM-180 on the third water immersion cycle, indicating a stable dimension for PHM synthesized at 180 • C.
Nevertheless, high-temperature products showed significant advantages in resistance against deformation, especially PHM-160 and PHM-180. The appearance of the samples can also indicate that the higher reaction temperature causes lower water absorption during soaking and reduced cracking after drying the samples (Figure 1c). PHM-180 samples exhibited a darker color due to a significant increase in the number of ester bonds and the possible oxidation reaction.   Table 1 also provides information about polymers' volumetric gain per unit wa absorption (VG) and mass loss ratio (ML) during the swelling test. The values of VG around 1.0 except for PHM-100 and PHM-140, indicating a synchronized gain in wa absorption and volume swelling, which is essential in stabilizing the dimension of L Moreover, the overall trend of VG declined as the temperature increased, indicating t the volume expansion of polymers synthesized at lower temperatures was more p nounced when absorbing the same amount of water. The relatively loose structure of th polymers was more easily destroyed by water. Polymers with an excellent performa by the higher crosslinking density will not yield noticeable volume variation even appl to a water immersion or highly humid environment, which lays a foundation for mod ing LBL. The low VG could be attributed to the stable and firm structure forming fr crosslinking and esterification reactions, also probably caused by the mass loss-indu shrinking. The ML values displayed little relevance to the temperature, which might attributed to the coexistence of polymerization and hydrolysis in the aqueous react system. However, no correspondence between VG and ML could be concluded. Comp atively, PHM synthesized at 160 °C is lower in both VG and ML than PHM obtained different temperatures, very suitable for LBL modification. PHM-160's excellent per mances benefit from its hydrophobic crosslinking network.  Table 1 also provides information about polymers' volumetric gain per unit water absorption (VG) and mass loss ratio (ML) during the swelling test. The values of VG are around 1.0 except for PHM-100 and PHM-140, indicating a synchronized gain in water absorption and volume swelling, which is essential in stabilizing the dimension of LBL. Moreover, the overall trend of VG declined as the temperature increased, indicating that the volume expansion of polymers synthesized at lower temperatures was more pronounced when absorbing the same amount of water. The relatively loose structure of these polymers was more easily destroyed by water. Polymers with an excellent performance by the higher crosslinking density will not yield noticeable volume variation even applied to a water immersion or highly humid environment, which lays a foundation for modifying LBL. The low VG could be attributed to the stable and firm structure forming from crosslinking and esterification reactions, also probably caused by the mass loss-induced shrinking. The ML values displayed little relevance to the temperature, which might be attributed to the coexistence of polymerization and hydrolysis in the aqueous reaction system. However, no correspondence between VG and ML could be concluded. Comparatively, PHM synthesized at 160 • C is lower in both VG and ML than PHM obtained at different temperatures, very suitable for LBL modification. PHM-160's excellent performances benefit from its hydrophobic crosslinking network. As a water-soluble monomer, HEMA can homopolymerize into PHEMA and copolymerize with another monomer, such as MAc [32]. PHEMA is a hydrophilic polymer, with P w as high as 25% and P v 30% [33]. MAc can copolymerize with HEMA to form PHM. PHEMA and PHM can form a crosslinked network structure in the presence of MBA, and the plentiful hydroxyl groups and carboxyl groups can react with each other to produce hydrophobic ester bonds under high temperatures and the continuous removal of water. A higher reaction temperature contributed to the esterification efficiency, as water is apt to discharge [27,34]. Meanwhile, MAc would be converted into MAh (see Supplementary Materials), which promoted the esterification between HEMA and MAh, and further improved the crosslinking density. The results in Figure 1 and Table 1 presented an optimal reaction temperature at 160 • C for the fabrication of an overall dimension stable PHM, which will be applied in the modification of LBL.

Characterization of PHM
The properties of the material are closely related to its composition and microstructure. Therefore, it is vital to understand the microstructure of obtained polymers and clarify the internal relationship between structure and properties. Figure 2 shows a homogeneous but rough morphology for the HEMA homopolymer (PHEMA) was observed. The wrinkled surface was probably related to its brittleness during the drying procedure [35]. In contrast, HEMA/MAc copolymer (PHM) showed a smooth and uniform appearance without apparent phase separation, which indicates good compatibility between HEMA and MAc before and after the reaction. As both monomers dissolved well in an aqueous solution, they would react to form the copolymer. Besides, esterification could occur between the hydroxyl group on HEMA and the carboxy group on MAc. Consequently, a crosslinked structure with better continuity and homogeneity was formed through esterification and copolymerization, providing enhanced toughness. As a water-soluble monomer, HEMA can homopolymerize into PHEMA and copolymerize with another monomer, such as MAc [32]. PHEMA is a hydrophilic polymer, with Pw as high as 25% and Pv 30% [33]. MAc can copolymerize with HEMA to form PHM. PHEMA and PHM can form a crosslinked network structure in the presence of MBA, and the plentiful hydroxyl groups and carboxyl groups can react with each other to produce hydrophobic ester bonds under high temperatures and the continuous removal of water. A higher reaction temperature contributed to the esterification efficiency, as water is apt to discharge [27,34]. Meanwhile, MAc would be converted into MAh (see Supplementary Materials), which promoted the esterification between HEMA and MAh, and further improved the crosslinking density. The results in Figure 1 and Table 1 presented an optimal reaction temperature at 160 °C for the fabrication of an overall dimension stable PHM, which will be applied in the modification of LBL.

Characterization of PHM.
The properties of the material are closely related to its composition and microstructure. Therefore, it is vital to understand the microstructure of obtained polymers and clarify the internal relationship between structure and properties. Figure 2 shows a homogeneous but rough morphology for the HEMA homopolymer (PHEMA) was observed. The wrinkled surface was probably related to its brittleness during the drying procedure [35]. In contrast, HEMA/MAc copolymer (PHM) showed a smooth and uniform appearance without apparent phase separation, which indicates good compatibility between HEMA and MAc before and after the reaction. As both monomers dissolved well in an aqueous solution, they would react to form the copolymer. Besides, esterification could occur between the hydroxyl group on HEMA and the carboxy group on MAc. Consequently, a crosslinked structure with better continuity and homogeneity was formed through esterification and copolymerization, providing enhanced toughness. Compared with PHM reacted at 100 °C (PHM-100), a more rough and porous structure was observed on PHM at a reaction temperature of 160 °C (PHM-160), which could be ascribed to the violent reaction at high temperature, particularly in an aqueous system. Once the water evaporated from the reaction solution, the system became viscous. The copolymerization of HEMA and MAc further increased the system's viscosity, MAc tended to convert to MAh. At that moment, a reaction temperature higher than 100 °C accelerated the esterification reaction, resulting in water vapor. A more porous structure Compared with PHM reacted at 100 • C (PHM-100), a more rough and porous structure was observed on PHM at a reaction temperature of 160 • C (PHM-160), which could be ascribed to the violent reaction at high temperature, particularly in an aqueous system. Once the water evaporated from the reaction solution, the system became viscous. The copolymerization of HEMA and MAc further increased the system's viscosity, MAc tended to convert to MAh. At that moment, a reaction temperature higher than 100 • C accelerated the esterification reaction, resulting in water vapor. A more porous structure would thus be formed. Moreover, the grey phase shown as tiny dots in PHM-160 might be the esterification of HEMA and MAh, which induced slight phase separation [36].
In order to determine the chemical reaction between monomers HEMA and MAc, FT-IR analyses were performed on both monomers and the reaction product PHM shown in Figure 3a,b. The vinyl vibration peak appearing at 3059 cm −1 and 1636 cm −1 decreased in the spectrum of PHM due to polymerization [37]. The N-H stretching vibration peak at 3257 cm −1 and the C−N absorption peak at 1411 cm −1 appeared owing to the groups on the segment of MBA crosslinking sites [38,39]. Figure 3a,b. The vinyl vibration peak appearing at 3059 cm −1 and 1636 cm −1 decreased in the spectrum of PHM due to polymerization [37]. The N-H stretching vibration peak at 3257 cm −1 and the C−N absorption peak at 1411 cm −1 appeared owing to the groups on the segment of MBA crosslinking sites [38,39].
The esterification of HEMA and MAc in PHM could also be detected from the specific FT-IR bands. The decreased band of hydroxyl groups around 3600-3200 cm −1 compared with the monomers probably resulted from the consumption of -OH during esterification [40]. At 1714 cm −1 and 1709 cm −1 , a C=O stretching vibration is observed for HEMA and MAc, whereas 1728 cm −1 is for PHM. The significantly shifted peak of C=O also suggested the formation of an ester bond. Additionally, the absorbances at 1261 cm −1 and 1161 cm −1 characterize the ester C-O bond enhanced in PHM, further confirming the esterification reaction between HEMA and MAc [41,42]. These bands were found to be intensified when the reaction was conducted at a higher temperature, especially 160 °C and 180 °C (Figure 3b), which related to the discharge of water and the high reactivity of MAh. The increase in ester bonds in the polymer established that elevated temperature facilitated the esterification reaction. The result of 13 C-NMR became an essential corroboration of the characterization of the copolymer combined with the FT-IR analysis. The spectral features of PHEMA and copolymer PHM are presented in Figure 4.
On the spectrum of PHM, carbon signals on leading chains appeared at 55.1 ppm (c,k) and 45.2 ppm (b,l), and -CH3 carbon was located at 16.5 ppm. The signals at 178.3 ppm and 166.2 ppm were attributed to Carbonyl (C=O) of HEMA and MAc groups, respectively. There are both HEMA segments and MAc segments in PHM. In addition, absorption at 67.3 ppm and 60.2 ppm were attributed to -CH2 (e,f) of HEMA groups. Due to the esterification of the -OH group, this signal appeared at 63.6 ppm on the PHM spectrum. Unsaturated carbons of C=C in MAc were observed at 127.1 ppm and 134.5 ppm [43][44][45].
Glass transition temperatures based on differential scanning calorimetry measurements were used to investigate the effect of reaction temperature on the cross-linkage of PHM. As shown in Figure 5, Tg significantly increased with the reaction temperature raised to 160 °C (Tg = 71.39 °C). The increase in Tg was attributed to the restrained mobility of polymer chains [46], which resulted from the improved degree of esterification. The -COOH groups on MAc with the -OH groups on HEMA could form ester bonds under The esterification of HEMA and MAc in PHM could also be detected from the specific FT-IR bands. The decreased band of hydroxyl groups around 3600-3200 cm −1 compared with the monomers probably resulted from the consumption of -OH during esterification [40]. At 1714 cm −1 and 1709 cm −1 , a C=O stretching vibration is observed for HEMA and MAc, whereas 1728 cm −1 is for PHM. The significantly shifted peak of C=O also suggested the formation of an ester bond. Additionally, the absorbances at 1261 cm −1 and 1161 cm −1 characterize the ester C-O bond enhanced in PHM, further confirming the esterification reaction between HEMA and MAc [41,42]. These bands were found to be intensified when the reaction was conducted at a higher temperature, especially 160 • C and 180 • C (Figure 3b), which related to the discharge of water and the high reactivity of MAh. The increase in ester bonds in the polymer established that elevated temperature facilitated the esterification reaction.
The result of 13 C-NMR became an essential corroboration of the characterization of the copolymer combined with the FT-IR analysis. The spectral features of PHEMA and copolymer PHM are presented in Figure 4.
On the spectrum of PHM, carbon signals on leading chains appeared at 55.1 ppm (c,k) and 45.2 ppm (b,l), and -CH 3 carbon was located at 16.5 ppm. The signals at 178.3 ppm and 166.2 ppm were attributed to Carbonyl (C=O) of HEMA and MAc groups, respectively. There are both HEMA segments and MAc segments in PHM. In addition, absorption at 67.3 ppm and 60.2 ppm were attributed to -CH 2 (e,f) of HEMA groups. Due to the esterification of the -OH group, this signal appeared at 63.6 ppm on the PHM spectrum. Unsaturated carbons of C=C in MAc were observed at 127.1 ppm and 134.5 ppm [43][44][45].
Glass transition temperatures based on differential scanning calorimetry measurements were used to investigate the effect of reaction temperature on the cross-linkage of PHM. As shown in Figure 5, T g significantly increased with the reaction temperature raised to 160 • C (T g = 71.39 • C). The increase in T g was attributed to the restrained mobility of polymer chains [46], which resulted from the improved degree of esterification. The -COOH groups on MAc with the -OH groups on HEMA could form ester bonds under heating conditions, and the rise of temperature was beneficial to water discharge to generate more ester bonds. Furthermore, MAc can easily be converted to MAh above 140 • C (see Supplementary Materials), providing a higher reactivity. The number of free hydroxyl and carboxyl groups significantly reduced, leading to decreased fluidity in the molecular chains and reduced free volume in the copolymer network. A study of DSC analysis showed that the elevated temperature facilitates the cross-linkage of PHM in agreement with the above FT-IR analysis. heating conditions, and the rise of temperature was beneficial to water discharge to generate more ester bonds. Furthermore, MAc can easily be converted to MAh above 140 °C (see Supplementary Materials), providing a higher reactivity. The number of free hydroxyl and carboxyl groups significantly reduced, leading to decreased fluidity in the molecular chains and reduced free volume in the copolymer network. A study of DSC analysis showed that the elevated temperature facilitates the cross-linkage of PHM in agreement with the above FT-IR analysis.  heating conditions, and the rise of temperature was beneficial to water discharge to generate more ester bonds. Furthermore, MAc can easily be converted to MAh above 140 °C (see Supplementary Materials), providing a higher reactivity. The number of free hydroxyl and carboxyl groups significantly reduced, leading to decreased fluidity in the molecular chains and reduced free volume in the copolymer network. A study of DSC analysis showed that the elevated temperature facilitates the cross-linkage of PHM in agreement with the above FT-IR analysis.  Based on the comprehensive analysis above, the elevated temperature is more propitious to obtain a polymer with higher hydrophobicity and dimensional stability, and 160 • C is considered the optimized reaction temperature. The crosslinking reactions are shown in Figure 6. Based on the comprehensive analysis above, the elevated temperature is more propitious to obtain a polymer with higher hydrophobicity and dimensional stability, and 160 °C is considered the optimized reaction temperature. The crosslinking reactions are shown in Figure 6.

Hydrophobicity and Dimensional Stability of the PHM-Modified LBL
Based on the analyses of PHM synthesized at different temperatures, the reaction solution was immersed and in situ reacted in LBL. Hydrophobicity was assessed by measuring the water contact angle (WCA) of the modified LBL generated by polymerization, crosslinking, and esterification of HEMA and MAc (as shown in Figure 7). Since heat modification is also an effective method of reducing the hygroscopicity of bamboo [47,48], contact angles were measured for both heat-treated and PHM-treated blocks, with the former as the control. Due to numerous hydroxyl groups, the raw bamboo had a low water contact angle, which was 58.5°. However, since the number of hydrophilic groups reduced due to the thermal modification, the surface wettability significantly decreased, and the higher temperature resulted in higher contact angles (Figure 7a). The hemicellulose was hydrolyzed after the heat treatment, and the higher the temperature, the more intense the hydrolysis reaction. Degradation of the hemicellulose reduced available free polar adsorption sites, which was the main reason for the increasing hydrophobicity [49]. The heattreated blocks at 100 °C showed a lower contact angle than untreated ones, this might be caused by the rising infiltration due to cracks on the bamboo surface. Compared to the heat-treated method, the hydrophobicity of PHM-treated materials was sharply enhanced. PHM had a crucial effect on the hydrophobicity modification of LBL. The WCAs achieved a peak value (115.2°) when the temperature reached 140 °C. When the temperature further increased, hydrophobicity changed inconspicuously. The reason might be that the hydroxyl esterification of bamboo by the synergistic effect of high temperature and polymer had reached saturation. Moreover, it can be discovered that the color of both heat-treated bamboo and PHM-treated ones became dark in comparison with the control, and the latter was more profound than the former due to the combination of heat treatment and resulted in polymers formed in bamboo, as well as esterification of MAc/MAh and hydroxyl groups in bamboo [46].

Hydrophobicity and Dimensional Stability of the PHM-Modified LBL
Based on the analyses of PHM synthesized at different temperatures, the reaction solution was immersed and in situ reacted in LBL. Hydrophobicity was assessed by measuring the water contact angle (WCA) of the modified LBL generated by polymerization, crosslinking, and esterification of HEMA and MAc (as shown in Figure 7). Since heat modification is also an effective method of reducing the hygroscopicity of bamboo [47,48], contact angles were measured for both heat-treated and PHM-treated blocks, with the former as the control. Due to numerous hydroxyl groups, the raw bamboo had a low water contact angle, which was 58.5 • . However, since the number of hydrophilic groups reduced due to the thermal modification, the surface wettability significantly decreased, and the higher temperature resulted in higher contact angles (Figure 7a). The hemicellulose was hydrolyzed after the heat treatment, and the higher the temperature, the more intense the hydrolysis reaction. Degradation of the hemicellulose reduced available free polar adsorption sites, which was the main reason for the increasing hydrophobicity [49]. The heat-treated blocks at 100 • C showed a lower contact angle than untreated ones, this might be caused by the rising infiltration due to cracks on the bamboo surface. Compared to the heat-treated method, the hydrophobicity of PHM-treated materials was sharply enhanced. PHM had a crucial effect on the hydrophobicity modification of LBL. The WCAs achieved a peak value (115.2 • ) when the temperature reached 140 • C. When the temperature further increased, hydrophobicity changed inconspicuously. The reason might be that the hydroxyl esterification of bamboo by the synergistic effect of high temperature and polymer had reached saturation. Moreover, it can be discovered that the color of both heat-treated bamboo and PHM-treated ones became dark in comparison with the control, and the latter was more profound than the former due to the combination of heat treatment and resulted in polymers formed in bamboo, as well as esterification of MAc/MAh and hydroxyl groups in bamboo [46]. As a porous and hydrophilic material, water invasion does not exclusively occur on the bamboo's surface. When LBL is applied to a humid environment, the hygroscopic components of the cell wall in the bamboo will absorb moisture to expand, resulting in changes in the size of the cell wall or even the entire material, especially in the transverse directions, leading to a series of problems. During the water immersion procedure, modified LBL resisted wet expansion, especially when the reaction temperature reached 140 °C. The untreated LBL exhibited a significant expansion due to its hydrophilicity, and Sw is 10.5% (Figure 8). The high swelling ratio and low ASE of LBL treated under 100 °C were related to the poor crosslinking and the high VG of PHM. Furthermore, the esterification between maleic acid and bamboo was limited, the unreacted MAc had a negative effect on bamboo, resulting in increased hydrophilicity. Unlike surface hydrophobicity, LBL's dimensional stability improved when the reaction temperature increased to more than 140 °C. The swelling ratio was as low as 4.5%, and ASE achieved 46.0% when the temperature reached 180 °C. The high temperature may lead to a reversion of MAc back to MAh state to permit a better level of cross-esterification between the MAh and the HEMA or the MAh and the bamboo. The improved dimensional stability reflected the importance of internal modification. As a porous and hydrophilic material, water invasion does not exclusively occur on the bamboo's surface. When LBL is applied to a humid environment, the hygroscopic components of the cell wall in the bamboo will absorb moisture to expand, resulting in changes in the size of the cell wall or even the entire material, especially in the transverse directions, leading to a series of problems. During the water immersion procedure, modified LBL resisted wet expansion, especially when the reaction temperature reached 140 • C. The untreated LBL exhibited a significant expansion due to its hydrophilicity, and S w is 10.5% (Figure 8). The high swelling ratio and low ASE of LBL treated under 100 • C were related to the poor crosslinking and the high VG of PHM. Furthermore, the esterification between maleic acid and bamboo was limited, the unreacted MAc had a negative effect on bamboo, resulting in increased hydrophilicity. Unlike surface hydrophobicity, LBL's dimensional stability improved when the reaction temperature increased to more than 140 • C. The swelling ratio was as low as 4.5%, and ASE achieved 46.0% when the temperature reached 180 • C. The high temperature may lead to a reversion of MAc back to MAh state to permit a better level of cross-esterification between the MAh and the HEMA or the MAh and the bamboo. The improved dimensional stability reflected the importance of internal modification.

Mechanism of Action of PHM on LBL's Hydrophobicity and Dimensional Stability
The reaction monomers were diffused into the interior of bamboo by vacuum-pressure impregnation. PHM could consume the free hydroxyl groups and increase its surface hydrophobicity when constructed in the material. When the monomer solution diffused into the bamboo cell wall, for one thing, MA (MAc or MAh) chemically bonded to the hydroxyl groups of bamboo components, reducing the number of free hydroxyl groups and weakening the diffusion of water molecules into the bamboo cell wall [27], many reports indicate that MAh has been used to modify lignocellulosic materials via esterification reactions and achieve good results [50].
The FT-IR spectra of bamboo powder untreated or treated in various ways were recorded in Figure 9. In the case of treated samples, the absorption of C=O stretching vibrations at 1604 cm −1 decreased due to the degradation of the xylan and glucomannan backbone during the thermal modification [51]. Meanwhile, the increase in C-H in-plane deformation of lignin at 1043 cm −1 was observed in the spectra of all treated materials. Bamboo has a higher relative lignin content with the thermal degradation of hemicellulose after heat treatment [52]. For MAc and PHM-treated samples, peaks at 2903 cm −1 and 1732 cm −1 associated with the methylene stretching and carbonyl stretching became more intense and discernible, indicating the occurred esterification between bamboo and MAc or PHM [53]. In addition, the enhanced absorption band at 1378 cm −1 related to C-CH3 of the HEMA units of PHM was observed in the spectrum of PHM-treated bamboo, representing the successful grafting of polymer chains onto the bamboo. Degradation of the hemicellulose and the increasing relative content of lignin on behalf of PHM modification's effect were considered the main reason for the enhanced hydrophobicity. The result was in good agreement with the dimensional stability results mentioned above.

Mechanism of Action of PHM on LBL's Hydrophobicity and Dimensional Stability
The reaction monomers were diffused into the interior of bamboo by vacuum-pressure impregnation. PHM could consume the free hydroxyl groups and increase its surface hydrophobicity when constructed in the material. When the monomer solution diffused into the bamboo cell wall, for one thing, MA (MAc or MAh) chemically bonded to the hydroxyl groups of bamboo components, reducing the number of free hydroxyl groups and weakening the diffusion of water molecules into the bamboo cell wall [27], many reports indicate that MAh has been used to modify lignocellulosic materials via esterification reactions and achieve good results [50].
The FT-IR spectra of bamboo powder untreated or treated in various ways were recorded in Figure 9. In the case of treated samples, the absorption of C=O stretching vibrations at 1604 cm −1 decreased due to the degradation of the xylan and glucomannan backbone during the thermal modification [51]. Meanwhile, the increase in C-H in-plane deformation of lignin at 1043 cm −1 was observed in the spectra of all treated materials. Bamboo has a higher relative lignin content with the thermal degradation of hemicellulose after heat treatment [52]. For MAc and PHM-treated samples, peaks at 2903 cm −1 and 1732 cm −1 associated with the methylene stretching and carbonyl stretching became more intense and discernible, indicating the occurred esterification between bamboo and MAc or PHM [53]. In addition, the enhanced absorption band at 1378 cm −1 related to C-CH 3 of the HEMA units of PHM was observed in the spectrum of PHM-treated bamboo, representing the successful grafting of polymer chains onto the bamboo. Degradation of the hemicellulose and the increasing relative content of lignin on behalf of PHM modification's effect were considered the main reason for the enhanced hydrophobicity. The result was in good agreement with the dimensional stability results mentioned above.
On the other hand, a hydrophobic polymer structure formed by the reaction of monomers physically filled in bamboo cells, hindering moisture migration inside the material. The cross-section of LBL was observed under a scanning electron microscope. Obviously, polymers formed and filled in modified LBL cells compared to the raw LBL ( Figure 10). Monomers could enter the cell cavity of bamboo during the impregnation process and copolymerize to form PHM, which blocks water migration inside the material. This reaction behavior might be the main reason for the improved dimensional stability. On the other hand, a hydrophobic polymer structure formed by the reaction of monomers physically filled in bamboo cells, hindering moisture migration inside the material. The cross-section of LBL was observed under a scanning electron microscope. Obviously, polymers formed and filled in modified LBL cells compared to the raw LBL ( Figure 10). Monomers could enter the cell cavity of bamboo during the impregnation process and copolymerize to form PHM, which blocks water migration inside the material. This reaction behavior might be the main reason for the improved dimensional stability. Dense pits were found on the cell wall when observing the longitudinal section of untreated LBL (Figure 10c). These pits provided channels for small molecule agents to enter the adjacent cells. Besides, the existence of PHM could be observed from the longitudinal section of the modified material, polymers formed and blocked the pits (Figure 10f). The modifier could enter the cell lumen, diffuse to the surrounding cells through the pits, and complete the polymerization in both the cell cavities and cell walls to achieve the filling effect. Dense pits were found on the cell wall when observing the longitudinal section of untreated LBL (Figure 10c). These pits provided channels for small molecule agents to enter the adjacent cells. Besides, the existence of PHM could be observed from the longitudinal section of the modified material, polymers formed and blocked the pits (Figure 10f). The modifier could enter the cell lumen, diffuse to the surrounding cells through the pits, and complete the polymerization in both the cell cavities and cell walls to achieve the filling effect.

Conclusions
This study established a dual modification method that uses a hydrophobic polymer with low deformation to physically fill bamboo cells with the elimination of hydroxyl groups via esterification to improve the dimensional stability of LBL. A crosslinked copolymer PHM was successfully synthesized in an aqueous solution. FT-IR and 13 C-NMR identified their morphology and chemical structure. DSC analysis and swelling tests revealed that elevated temperatures facilitate PHM's crosslinking, which results in ideal dimensional stability. Using PHM on LBL improved its hydrophobicity; anti-swelling properties decreased hydrophilic substances; and filled bamboo cells contributed to this sufficient improvement. The ASE of the modified LBL was up to 46.0%. PHM modification of LBL provided a novel approach to effectively improving its hydrophobicity and dimensional stability. LBL's anti-mold performance and the efficient utilization of MAh will be researched in forthcoming studies.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/polym15122637/s1, Figure S1: FT-IR spectra of MAh aqueous solution heated at different temperatures; Table S1: Surface free energy of untreated and treated bamboo surfaces; Table S2: Water contact angle hysteresis on bamboo surface; Table S3