Microstructure
The microstructure of the cross-section of the treated inner and outer bamboo is shown in Fig. 2 and Additional file 1: Fig. S1. The change of microstructure in outer bamboo and inner bamboo was similar. There was no significant change in the microstructure of fibers and parenchyma cells with different moisture contents after freeze–thaw treatment. The parenchyma cells in untreated bamboo had a round cell cavity and massive starches. After freeze–thaw treatment, the shape of parenchyma cells did not change, but the number of starches reduced dramatically. It can be seen from T-2 and T-3 that there were only a few parenchyma cells containing a large amount of starch. In addition, there was almost no starch in bamboo treated with T-1. The number of starches reduced more pronouncedly and even disappeared when treated at – 40 \(\mathrm{^\circ{\rm C} }\), especially in the F-1and F-2. A previous study showed that the content of amylose in noodles decreased significantly during 10-day frozen storage (− 18 \(\mathrm{^\circ{\rm C} }\)), which was probably attributed to the growth and diffusion of ice crystals, resulting in the starch granules being damaged mechanically. Another reason was that the freeze process resulted in the increase of α-amylase activity, which led to the rapid degradation of starch [16]. Meziani et.al revealed that lower temperature, i.e., – 40 \(\mathrm{^\circ{\rm C} }\), would lead to phase change and breakdown of starch [17]. In addition, moisture content was another crucial factor for starch retrogradation. The cross-linking entanglement and crystal rearrangement of starch molecules depend on the moisture content [18], the hydrogen bonds between starch molecules and water molecules were easy to break, and stable hydrogen bonds could be formed between starch molecules. When the moisture content of starch is at a high level, the chances of cross-linking entanglement and polymerization of starch molecules decreased, which hindered the crystal rearrangement of starch molecules [19].
Chemical compositions
The FT-IR spectra of freeze–thaw bamboo are displayed in Fig. 3. The band at 1730 cm−1 was attributed to the C=O vibration of hemicellulose in bamboo [20]. The characteristic bands of lignin were at 1594 cm−1, 1509 cm−1 and 1460 cm−1, respectively (aromatic skeleton vibration) [21]. The absorption band did not change significantly in both inner and outer bamboo regardless of the difference in their moisture content or freeze–thaw treatment, indicating that the freeze–thaw treatment had little impact on the chemical composition of bamboo.
Figure 4 shows the XRD patterns and crystallinity index of cellulose in freeze–thaw-treated bamboo. The peaks at 2θ around 16°, 22° and 34° in the XRD patterns of untreated bamboo were ascribed to (10_), (1200) and (040) reflection of the crystalline structure of typical cellulose Ι [22]. Similar XRD patterns of all treated bamboo were observed as the untreated samples, suggesting the similar cellulose Ι. The crystallinity index of outer and inner bamboo is presented in Fig. 4c and the ANOVA analysis is shown in Additional file 1: Tables S1–S4. The CrI of cellulose of the water-saturated bamboo with freeze–thaw treatment was the highest. Moreover, the ANOVA analysis showed the moisture content (Pinner bamboo = 0.026 < 0.05) had a significant effect on the CrI of inner layer bamboo, but the temperature affected little on both inner and outer layer bamboo. It indicated that the higher the moisture content was, the higher the CrI of inner bamboo. The outer layer of bamboo contained more fibers and fewer parenchyma cells compared with the inner layer of bamboo (Fig. 4d). To get more insight into the difference in crystallinity, the fibers and parenchyma cells were separated mechanically from bamboo and treated with the same methods. The CrI of the freeze–thaw-treated bamboo fibers and parenchyma cells with different moisture content is shown in Fig. 4e. The CrI of fibers was much higher than that of parenchyma cells, which may account for that the CrI of outer bamboo was higher than that of inner bamboo. After freeze–thaw treatment, the CrI of outer bamboo slightly increased, the extent of increase varied depending on the moisture content.
Mechanical properties
Figure 5 shows the bending strength and elastic modulus of bamboo after freeze–thaw treatment with different moisture contents. Both the bending strength and elastic modulus increased after freeze–thaw treatment, and the lower the treatment temperature was used, the higher bending strength was achieved. Also, the ANOVA analysis is presented in Additional file 1: Tables S5–S7. It showed that the temperature (P = 0.003 < 0.05) had a significant effect on the bending strength. The previous research indicated that the molecule of cellulose and lignin approached each other much tighter and the connection became stronger when the temperature decreased, which led to the increase of strength [9]. Therefore, the bamboo slivers treated at the lowest temperature have the highest strength. However, the elastic modulus of bamboo strips with freeze–thaw treatment at − 20 \(\mathrm{^\circ{\rm C} }\) was higher than that treated at − 40 \(\mathrm{^\circ{\rm C} }\). In addition, the moisture content had a significant influence on the mechanical properties of bamboo strips with freeze–thaw treatment. The ANOVA analysis in Additional file 1: Tables S8, S9 shows both the moisture content (P = 0.041 < 0.05) and the temperature (P = 0.001 < 0.05) have a significant effect on the elastic modulus, but the temperature plays a more critical role. However, there was no significant change between − 20 and − 40 \(\mathrm{^\circ{\rm C} }\) (P = 0.173 > 0.05) (Additional file 1: Table S10). The elastic modulus of bamboo strips was greatly influenced by low temperature, but it was not that the lower the temperature, the greater the elastic modulus. The mechanical properties of bamboo strips with less moisture content increased more significantly after freeze–thaw treatment. The bending strength and elastic modulus of water-saturated bamboo strips (T-1 and F-1) were the lowest in comparison with those air-dried and oven-dried ones. It might be possibly ascribed to that a large number of water molecules diffused into the cell wall, the water molecules not only formed hydrogen bonds between cellulose molecules, but continued to form hydrogen bonds with the molecules that had already formed hydrogen bonds. Moreover, it led to the increase of weak bonding among water and water in cellulose molecule chains, which would weaken the intermolecular forces and eventually result in a decreased elastic modulus [23].
Thermal conductivity
The thermal conductivity of the outer and inner layer of bamboo after freeze–thaw treatment is shown in Fig. 6. The thermal conductivity of untreated outer and inner bamboo was almost the same. This suggested that bamboo material has good thermal insulation, which is similar to wood [24]. The thermal conductivity of all samples increased except T-1 of outer bamboo and T-2 of inner bamboo. The thermal conductivity of oven-dried bamboo strips (T-3 and F-3) was higher than that of water-saturated (T-1 and F-1) and air-dried samples (F-2) except the T-2. The results of ANOVA analysis are presented in Additional file 1: Tables S11–S14. It indicated that the moisture content (Pouter bamboo = 0.000 < 0.05; Pinner bamboo = 0.035 < 0.05) has significant effects on thermal conductivity, but the temperature was not related. The density of bamboo after freeze–thaw treatment is presented in Fig. 6b. The results showed that the density of outer bamboo did not change significantly, while the density of inner bamboo decreased slightly. For outer bamboo, the density of T-1 increased a little, T-3 remained unchanged and the rest of the samples decreased slightly but was in the error bar range of the C0 except for F-1. For inner bamboo, T-1and F-1 decreased more obviously than other bamboo specimens.
The thermal conductivity was influenced by many factors including crystallization area, moisture content and density [25, 26]. The thermal conductivity would increase when the crystalline area increase as the transport of heat in all non-metals (no free electrons) was by the flow of lattice vibrational energy [27, 28]. The CrI of cellulose in water-saturated bamboo strips (T-1 and F-1) with freeze–thaw treatment increased, which was possibly one of the reasons why the thermal conductivity of water-saturated bamboo strips increased. The final moisture content of treated bamboo in our study was from 5 to 8%, and the moisture content of untreated bamboo was about 10%. It was reported that the thermal conductivity of bamboo plywood reduced as the moisture content decreased gradually from 100 to 0% [29]. While the thermal conductivity of treated bamboo with lower, final moisture content was higher than that of untreated samples. The material with lower density had lower thermal conductivity [30], but the treated bamboo with the lowest density did not have the lowest thermal conductivity in our study. It indicated that the thermal conductivity was the result of the combined influence of many factors.