Process And Mechanism Of Nanocellulose On Toughening Modification Of PHBV

This study aims to evaluate potentials of rice straw nanocellulose used as toughening materials of nanocellulose/Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV) composites, and to investigate the effects of morphologies and addition of nanocellulose on toughness of PHBV. Based on this, the effect of addition of PEG on toughness of CNCs/PHBV composites was also discussed by improving the interfacial compatibility between CNCs and PHBV. In this paper, the conclusions were summarized as follows:(1) Rice straw cellulose nanocrystals(CNCs) were isolated successfully using 64 wt% acid hydrolysis combined with 800 bar homogenization process and rice straw cellulose nanofibrils(CNFs) were isolated successfully using 1500 r/min five-time grinding treatment. In the process of preparing CNCs/PHBV composites and CNFs/PHBV composites, conditions were: addition of CNCs(1 wt%~7 wt%), addition of CNFs(1 wt%~7 wt%), temperature of melt compounding(176 °C), speed of rotating(24 r/min), temperature of mold pressing(183 °C), time of mold pressing(4 min). The average diameter and length of rod-like rice straw CNCs were 9.1 nm and 145.8 nm, respectively, the average diameter of rice straw CNFs was 13.4 nm, the sizes of rice straw CNCs and CNFs were all smaller than those of poplar. FTIR showed that more free hydroxyl groups were exposed on rice straw because of sulfuric acid hydrolyzation and high-strength mechanical shearing. Besides, esterification reaction was generated by sulfuric acids on the surface of rice straw CNCs. Sulfuric acid hydrolyzation and high-strength mechanical shearing could also improve the degree of crystallinity(Cr I) of cellulose, the order of CrI was: poplar CNCs(83.0%) > rice straw CNCs(73.8%) > poplar CNFs(68.1%) > rice straw CNFs(64.3%). Besides, a part of rice straw celllose I turned into cellulose II because of sulfuric acid hydrolyzation. In the aspect of thermal stability, the order of which was: rice straw ? cellulose(activation energy E=112.0 k J/mol) > rice straw CNFs(E=94.9 k J/mol), the thermal stability of rice straw nanocellulose was a little worse than that of poplar nanocelluose.(2) In the process of preparing CNCs/PHBV/PEG composites,conditions were: addition of PEG(5 wt%~25 wt%), temperature of melt compounding(176 °C), speed of rotating(24 r/min), temperature of mold pressing(183 °C), time of mold pressing(4 min). It was found that when the addition of CNCs was 1 wt%, the composite exhibited most comprehensive mechanical properties. Young's modulus, elongation at break and impact strength of the composites were 595.7 MPa, 16.4 %, and 26.6 k J/m2, respectively, which increased by 8.9 %, 1.4 % and 3.6 % compared with neat PHBV. When at the same addition of CNCs and CNFs(1 wt%~7 wt%), CNCs exhibited stronger nucleating ability, which was more beneficial to the toughening modification of PHBV. Tmc of CNCs/PHBV composite was lower than that of CNFs/PHBV composite, the number of CNCs/PHBV spherulite was more than that of CNFs/PHBV, correspondingly, the size of CNCs/PHBV spherulite was smaller than that of CNFs/PHBV. In the respect of thermal stability, when CNCs and CNFs were added into PHBV, the thermal stability of composite was improved, CNFs exhibited more brilliant. When the addition of CNCs and CNFs increased from 2 wt% to 6 wt%, Tmax of CNCs/PHBV composite increased from 278.6 °C to 284.3 °C, Tmax of CNFs/PHBV composites increased from 278.6 °C to 287.6 °C. When the addion of CNCs was 1 wt%, toughness of PHBV was improved due to the following two reasons: in the aspect of crystallization behaviors, CNCs could act as nucleating agents in PHBV matrix, enhancing crystallization nucleation density, speeding crystalline rate and reducing crystalline size; in the ascpect of fracture mechanisms, when the composite was under shock, CNCs would leave little holes on the surface of composite, which were beneficial to absorb energy.(3) When PEG was added from 5 wt% to 25 wt%, elongation at break of CNCs/PHBV/PEG composites increased from 16.4% to 31.8%, increasing by 93.9%; and impact strength increased from 26.6 kJ/m2 to 40.2 k J/m2, increasing by 51.1% compared with PHBV. While Young's modulus of CNCs/PHBV/PEG composite decreased from 593.1 MPa to 424.0 MPa and tensile strength decreased from 37.3 MPa to 28.1 MPa. In the aspect of crystallization behavior, there emerged defects on the spherulite of CNCs/PHBVcomposite, Tm decreased from 167.8 °C to 165.1 °C, one melting peak turned into two during melting process. Besides, due to small molecule PEG inserted into PHBV molecular chain, which weakened the attraction of PHBV molecular chain, enhancing the mobility of PHBV molecular chain, thus the crystallization of PHBV became more difficult. Tmc of composite decreased from 99.8 °C to 73.5 °C,Xc decreased from 54.3% to 50.2%. The number of CNCs/PHBV/PEG spherulite decreased, correspondingly, the size increased. However, under the synergistic effect of CNCs and PEG, even at the content of PEG was 25 wt%, size of CNCs/PHBV/PEG spherulite was smaller than that of PHBV. Thermal degradation process of CNCs/PHBV composite turned from one-step process into two-step, T0 of the first stage increased from 247.6 °C to 252.4 °C, Tmax1 increased from 281.5 °C to 285.3 °C, Tmax2 of second stage increased from 371.5 °C to 394.3 °C?The toughness of CNCs/PHBV/PEG composite was enhanced due to the following reasons: in the aspect of interface compatibility, the interface compatibility among nanocellulose, PHBV and PEG was improved, which was conformed in ESEM pictures; in the aspect of fracture mechanism, when the composite was under shock, CNCs would leave little holes on the surface of composite, which were beneficial to absorb energy, in the aspect of crystallization behavior, under the synergistic effect of CNCs and PEG, the crystalline size and the degree of crystallinity of PHBV was decrased, and new structure defects of spherulite emerged; lastly in the aspect of structure of molecular, flexible PEG molecules may be inserted into the main chain PHBV, inducing that the plasticity of CNCs/PHBV/PEG composite incrased. Finanlly, the brittleness of PHBV was ultimately improved.