Structural Design And Enhance Properties Of Bio-based Polyester Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Bio-based polyester is a new kind of biodegradable polyesters synthesized bybiosynthesis or combined with chemosynthesis using renewable resources as rawmaterials, which have the advantages with eco-friendly, environmental and renewable.It will be very useful for environmental protection and sustainable development. Overthe past years, PHAs as the only bio-based polyester completely synthesized bybiological means have been developed into applications in fiber, bioplastics, implantbiomaterials, drug delivery carriers, biofuels and package materials. PHAs suffer fromits high stereo-tacticity, low glass transition temperature, slow crystallization rate,larger spherulite size and secondary recrystallization behaviors which led to not onlyfiber strand phenomena but also existing viscoelasticity-brittleness transition process,resulting in its brittle products, which have limited the engineering applications ofPHAs.Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV) is a commercializationof typical PHAs products. In this thesis, a series of studies are presented in an attemptto overcome above-mentioned limitations of PHBV and extend its application. Firstly,the thermal decomposition mechanism of PHBV was studied to get the structureinformation leading to degradation. Then, natural polyphenol/PHBV composites withenhanced tensile strength and fracture toughness were prepared by solution casting toavoid structural damage by melt processing. Meanwhile, the ABA type tri-blockcopolymers and branched/crosslinked copolymers were constucted by atom transferradical polymerization (ATRP) and reactive processing, respectively. This part focuseson the influence of molecules structure to achieve better properties in thermal stability,high nucleation temperature, high crystallization rate, high tensile strength andtoughness for PHBV copolymers. The main contents and results are summarized asfollows:1. The thermal decomposition process and mechanism of PHBV were examined by using Thermal gravity analysis(TGA), Gel permeation chromatography(GPC), Elemental analyzer, Pyrolysis-gas chromatography-mass spectrometry(PyGC-MS) and1H nuclear magnetic resonance (1H NMR), etc. The degradationactivation energy was calculated via the dependance of residual mass on isothermaltemperature.1H NMR and PyGC-MS were carried out to investigate the chemicalstructure and component proportion of volatile gases and degradation residues, whichwere produced in thermal decomposition, to infer the process of macro-molecular chain scission. Besides, the influence of factors such as outfield atmosphere,residual metal ions on the degradation behaviors of PHBV were also examined.Finally, the PHBV thermal decomposition mechanism was speculated based on thedegradation behaviors of molecular and chemical structure. A detailed description ofthe changes in the molecular chain structure during the thermal degradation processprovides the theory basis for bio-based polyesters modification. According PHBVthermal decomposition mechanism, on the one hand, the intermediate transition states,a six-membered ring formed by ester oxygen bond and a hydrogen atom adjacent βposition induce random breakage of molecular chains in the early degradation. On theother hand, the terminal carboxyl existed in the low molecular weight PHBV(LMW-PHBV) which formed in thermal decomposition, as well as other factors suchas the residual impurity elements, have accelerated thermal decomposition process,resulting in the decrease in molecular weight and product performance.2. Structural characteristics and enhanced mechanical and thermal properties offull biodegradable natural polyphenol/PHBVcomposite films were investigated. Inthis part, low molecular weight tea polyphenol (TP) and tannic acid (TA) were chosenas tougheners to prepare green PHBV composite films by co-solvent solution casting.This method can not only avoid structural damage and performance loss caused byPHBV melt processing, but also regulate crystallization behavior and productproperties of composite films through intermolecular interactions of naturalpolyphenol and PHBV. The chemical structure and intermolecular interaction ofnatural polyphenol/PHBV composite films were characterized by FTIR measurementswith an attenuated total reflection (ATR). The formation of intermolecular interactioncan be obtained effectively by the analysis of the infrared spectrum of the stretchingvibrations of hydroxyl (υO-H) and carbonyl (υC=O) groups. The effects of temperatureon intermolecular interaction of samples were carried out by FTIR with heatingaccessory. The uniaxial tensile mechanical testing was performed using a computercontrolled Instron5969machine to evaluate the mechanical properties of natural polyphenol/PHBV composite films. When the TA mass fraction was up to10wt%,the stress reached to23.56MPa with an increasing ratio of139%, while TA contentwas up to20wt.%, the strain reached to17.98%, which was15times more than thatof neat PHBV.3. A series of crystalline/amorphous triblock copolymers based on biodegradablePHBV as crystalline component and polymethyl methacrylate (PMMA) asamorphous component were prepared via atom transfer radical polymerization(ATRP). The chemical structure, molecular weight and distribution weresystematically characterized by FT-IR,1H NMR and GPC. From the perspective ofregulating crystallinity and changing the environment of PHBV six-membered esterbond, narrow molecular weight distribution PHBV macromere were prepared byalcoholysis method. Then, the amorphous component PMMA was introduced intothe main chain of PHBV. In this way, carboxyl end group structure is eliminated andthe glass transition temperature of the copolymer increases, which leads to thereduction of secondary crystallization PHBV component and the improvementthermal stability of triblock copolymer. The TG analysis revealed that T0、T5%、Tmaxof triblock copolymer with the molar ratios of PMMA and PHBV between2:1to8:1were raised to25oC,15oC and40oC, and the degradation activation energy oftriblock copolymers decreased from428.25kJ·mol-1to244.67kJ·mol-1.4. A series of branched/crosslinked PHBV copolymers (BC-PHBV) wereobtained by reactive processing. In order to solve the drawbacks of PHBV, dicumylperoxide (DCP) was used as the initiator to achieve molecular chain self-branched orself-crosslinked. And octavinyl polyhedral oligomeric silsesquioxane (OV-POSS)was used as the co-crosslinking agent to prepare a series of crosslinked PHBVcopolymers. The effects of branched/crosslinked structure on the crystallizationkinetics and mechanical properties of PHBV copolymers were investigated DSC andDynamic mechanical analyzer(DMA). The integrated performance of better thermalstability, high nucleation temperature, high crystallization rate, high tensile strengthand toughness for PHBV copolymers were assembled in this branched/crosslinkedPHBV copolymers. On the one hand, branched molecular chain and crosslinkingpoints increase the free volume of the molecular chain and the moving space ofmolecular chain, thus increasing the toughness of BC-PHBV copolymers. On theother hand, branching/crosslinking points, and the formation of molecular chainsbundle around them play an enhanced role in free molecular chains, thus increasingthe uniaxial tensile strength of copolymers. Take a kind of BC-PHBV copolymer which was initiated by DCP cooperated with OV-POSS for example, the fracturestrength increased by154%, reaching33.80MPa and fracture work increased by697%, reaching320.7mJ. DMA testing results showed that the motion ability of themolecular chain of BC-PHBV copolymers would not be affect by adding smallamounts of initiator or co-crosslinking agent. Besides, the product performance canbe achieved by controlling the process, and its practical applications can be widelyachieved.