Control Of Crystallization Behavior And Properties Of Polyhydroxyalkanoates

The increasing public awareness regarding the serious environmental problems aroused by traditional polymers imminently requires academic/industrial community to promote the development of sustainable biopolymers to partially replace the traditional polymers.Polyhydroxyalkanoates?PHAs?obtained by the fermentation of the microorganisms are considered as promising biopolymers in biomedical and packaging applications due to their preferable properties such as full biodegradability,biocompatibility,non-cytotoxicity and composting.However,the poor mechanical properties of the PHAs induced by unsatisfactory crystallization ability such as low nucleus density and crystal growth rate limit their practical applications.Therefore,it is of great significance to study the crystallization kinetics and improve the crystallization ability of PHAs.In this work,the crystallization kinetics and crystallization mechanism of neat poly?3-hydroxybutyrate-co-hydroxyhexanoate??PHBH?ultra-thin films were firstly studied.Then,various methods,i.e.,adding oxalic acid amide nucleating agents and modified bio-based fillers,applying self-seeding and external shear force were utilized to improve the crystallization behaviors of PHBH,in which the crystallization kinetics associated with the relationship between crystal morphology and performance were discussed in detail.The primary objectives of this work were to enrich the crystallization theory of PHBH and expand its application fields.The main contents are summarized as follows:?1?The crystal morphology evolution with temperature of PHBH ultra-thin film was studied and Hoffman-Lauritzen theory was used to explain the relationship between crystal morphology and crystallization mechanism of PHBH.Results indicated that the crystal morphology of PHBH would convert from non-band spherulites?corresponding to crystallization mechanism II?to lath-like single crystals?corresponding to crystallization mechanism??with increasing the crystallization temperature.Meanwhile,the self-seeding induced crystallization mechanism of PHBH were studied as well.Afterwards,high-melting poly?3-hydroxybutyrate??PHB?was used as a nucleating agent to improve the crystallization rate of PHBH and the crystallization kinetics and mechanical properties of the PHBH/PHB blends were systematically investigated.The results showed that PHB could serve as a high-efficiency nucleating agent for PHBH,even better than the traditional inorganic nucleating agents Talc.?2?Various oxalamide nucleating agents?C₆H₅NHCOCONH?CH₂?_nNHCOCONHC₆H₅,n=2,4,8,12,OXA_n?were synthesized as nucleating agents to systematically investigate the effect of the chemical structure of OXA_n on the crystallization kinetics of PHBH.The results exhibited that all OXA_n can be"dissolved"in PHBH matrix at high temperature,indicating a good compatibility between OXA_n and PHBH.The crystallization rate of PHBH/OXA_n increased gradually with increase of the aliphatic spacer length until reaching the maximum when n=8,while the crystallization half-life time(t_1/2)of PHBH at 105 ? decreased tremendously from 44 min to 2.25 min at only 0.75 wt%of OXA₈ content.It should be mentioned that the incorporation of OXA_n did not change the crystal structure of PHBH.Therefore,the crystallization rate of PHBH could be improved by controlling the chemical structure of OXA_n.?3?The crystallization kinetics of PHBH under different shear conditions were investigated by rheometers.Firstly,the flow activation energy of PHBH was successfully obtained by Arrhenius equation.Then the stretch relaxation time and corresponding critical shear rate at different temperatures for the flow regime transition were obtained from the discrete Maxwell relaxation time spectra.Results showed that the crystallization rate of PHBH can be improved with increasing the shear rate or the shear time until reaching a maximum value.Specially,the crystallization process of PHBH incorporated with OXA₈ induced by shear force was further studied and results showed that the t_1/2/2 of PHBH/OXA₈ could be reduced by33%.Surprisedly,the external shear force and OXA₈ showed a synergetic effect on promoting the crystallization rate of PHBH.?4?Nanocellulose?NCC?was used as bio-nucleating agents to improve the crystallization and mechanical properties of PHBH.Firstly,PHBH was successfully grafted onto the surface of NCC to obtain NCC-g-PHBH nanohybrids with a 25 wt%grafting degree,which was used to improve the compatibility between NCC and PHBH.Then PHBH/NCC-g-PHBH nanocomposites were prepared via a facile solution casting.The NCC-g-PHBH was found to uniformly disperse in PHBH matrix,as a consequence of improving the crystallization and mechanical properties.For instance,the Young'modulus and tensile strength of the PHBH/NCC-g-PHBH composite were increased by 15%and 28%,respectively,with only 1.0wt%of NCC-g-PHBH addition,which is in agreement with the Halpin-Tsai model.Furthermore,the presence of NCC-g-PHBH nanohybrids not only reduced the surface contact angles by 9°,but also increased the crystallinity of PHBH from 0%to 33.2%without changing the crystal structures.?5?Graphene oxide?GO?was used as both nucleating agents and functional fillers to blended with PHBH to prepare high performance packaging nanocomposites.Firstly,N-?2-?methacryloyloxy?ethyl?-N,N-dimethyldodecan-1-aminium bromide?LAQ?was grafted onto the surface of GO via a coupling reaction and a free radical polymerization process to prepare GO-g-LAQ nanohybrids.Then PHBH/GO-g-LAQ blends were prepared via solution casting.The GO-g-LAQ could disperse in the PHBH matrix uniformly,showing an improving interfacial adhesion between GO and PHBH.After incorporation of a small amount of GO-g-LAQ,the crystallization rate and the nucleation density of PHBH were increased.The tensile strength and storage modulus of PHBH materials were increased by 60%and 140%,respectively,while the oxygen permeability of PHBH was reduced by 86%.Meanwhile,the heat resistant temperature of PHBH increased by 20 ?.Moreover,GO-g-LAQ,as a laminar antibacterial agent,improved the antibacterial ability of PHBH and the PHBH/GO-g-LAQ nanocomposites showed the antibacterial rate of 99.9%against Gram-negative and Gram-positive bacteria without any leaching.Therefore,the well-designed PHBH/GO-g-LAQ nanocomposites could be ideal candidates for packaging applications.