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Results of changes in the structure and the functional groups of rice straw are shown in Table 3. Bands have been found in all of the samples in the range of 450-3,600 cm-1. band between 3600 and 3100 cm-1 is related to OH stretching vibrations present in the cellulose, hemicellulose, and lignin that caused by the presence of alcoholic and phenolic hydroxyl groups involved in hydrogen bonds. (Rahnama et al., 2013). The intensity of band 3415 cm-1 was decreased, as a result after the steam explosion pretreatment in the high-pressure cycles 10 bar for 5-10 min and 15 bar for 1-5 min with blank moisture, 15 bar for 10-15 min with moisture 35% and 10 bar, 10 min and 15 bar for 5-10 min with moisture 70%. Therefore it indicates that the partial hydrogen bond in cellulose was destroyed. This is a positive step toward enhancing the accessibility of cellulose for enzymes and microorganisms (He et al., 2008). The absorption bands at 2920 and 2861 cm-1 were assigned to C-H stretching vibrations. The intensity of both peaks indicates the distinguished features of cellulose (methyl and methylene) and hemicellulose. The intensity of bands was reduced after the pretreatment pressure increased (Kazeem et al., 2017). Structural changes in lignin and loss of aromatic units were shown by changes of intensity in the 1,649, cm-1 band (Isroi et al., 2012). Moreover, the peak at 1516 cm-1 corresponded to C=C stretching of the aromatic ring of lignin. The intensity of both peaks decreased with increase in pretreatment pressure in the cycle 10 bar for 5-10 min and 15 bar for 1-5 min with blank moisture, 10 bar and 10 min and 15 bar for 10-15 min with moisture 35% and 10 bar for 5-10 min and 15 bar for 5-10 min with moisture 70%. This is an indication of lignin structure changes.
The intensity of band obtained at 1064 cm-1 is usually attributed to the structural characteristics of cellulose and hemicelluloses (Rahnama et al., 2013). It was clear that the intensity of the band at 1064 cm-1 was lower compared to the unpretreated rice straw. The drop observed in this band is related to decreased hemicellulose content after steam explosion pretreatment in the cycles 10 bar for 5-10 min and 15 bar for 1-5 min with blank moisture, 10 bar, 10-15 min and 15 bar for 10-15 min with moisture 35% and 10 bar for 10-15 min and 15 bar for 1-5-10 min with moisture 70%. The increase in the intensity of this band might be dependent to the dissolution of non-cellulose components that causes the increase of cellulose content in the rice straw (Ang et al., 2012). The absorption band 1430 cm-1 is related to amorphous cellulose (Kazeem et al., 2017). The absorption band by 1430 cm-1 has reduced after the steam explosion pretreatment and it means the degraded of the amorphous part structure, especially the lignin structure changes has occurred. Lignin structure was degraded in the cycles 10 bar for 5-10 min and 15 bar for 1-5 min with blank moisture and 15 bar for 10-15 min with moisture 35% and 15 bar for 5-10 min with moisture 70%.
3-2-1 effect of pretreatment on crystallinity index
Crystallinity index of the lignocelluloses is determined by two parts amorphous and crystalline. The amorphous part consists of hemicelluloses and lignin, while the crystalline constituent part includes cellulose (Sakdaronnarong & Jonglertjunya, 2012). The crystallinity index CrI (%) unpretreated and pretreated rice straws are shown in Table 4. Two peaks were observed at 2? of between 18°-19° and between 22°-23°, that relating amorphous and crystalline regions of the rice straw. According to results, there was an increase in the crystallinity in pretreated rice straw than to the unpretreated rice straw. The highest crystallinities of pretreated rice straw were 28.8%, 28.58% and 29.3% in the cycles 10 bar and 10 min with blank moisture, 15 bar and 15 min with moisture 35% and 15 bar and 10 min with moisture 70%, respectively, than to the unpretreated rice straw 22.9%. In all pretreated samples, the region of crystalline cellulose was higher than to the unpretreated rice straw (Table 4). During the steam explosion pretreatment, Hemicellulose and lignin are hydrolyzed with the increase of pressure, which mainly constitutes amorphous regions of rice straw. This led to the Increase of crystallinity index after pretreatment.
Previous research has also suggested that the crystallinity index of rice straw could enhance by other thermal pretreatments, and this increase in the CrI could not have negative effect on enzymatic hydrolysis (Kshirsagar et al., 2015). Increase in crystallinity index due to the effect of pretreatment was more in the amorphous region than to the crystalline region. (Johar et al., 2012), reported an increase in the CrI from 46.8% in the unpretreated RH to 50.2%, 56.5%, and 59.0% in the alkali, bleached, and acid pretreated RH respectively.

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