Synthesis,Cocrystallization And Properties Of Biobased Aliphatic Copolyesters

The fossil resource depletion and environmental pollution have become the two primary challenges.Biobased degradable polyesters are materials with the required ecological advantages to offer alternative solutions for such challenges,which have witnessed increasing interest in society,academia and economy fields.A large number of small molecules such as aliphatic dicarboxylic acids and diols can be produced directly from agricultural and forestry products or by microbial fermentation.Moreover,these biobased small molecules can be precisely designed at the molecular level to synthesize materials with properties similar to those of petroleum-based polymers.Therefore,biobased aliphatic polyesters are considered as the most promising alternative to petroleum-based materials,which would become an integral part of human society.In practice,aliphatic polyesters are usually prepared into random copolyesters by copolymerization,which can not only combine the properties of the two homopolyesters,but also control the properties of copolyesters by adjusting composition.Random copolymers,with two or more different types of chemical units joined together at random,exhibit a complex phase behavior.Up to date,however,the thermodynamic properties,crystallization properties and phase transition laws of random copolyesters are still poorly understood.In this work,a variety of saturated aliphatic copolyesters and unsaturated aliphatic copolyesters with controllable chemical structures and excellent properties were synthesized from the biobased aliphatic dicarboxylic acids and diols.The influences of dicarboxylic acid or diol chain length,odd-even effect,and conformational geometric effect of unsaturated double bond on the physical properties of copolyesters were systematically studied,furthermore new theoretical models were proposed to give an insight into the mechanism of how does the chemical structure factor regulates the phase behavior of copolymer.The main results were summarized as follows:(1)Four series of biobased even-even copolyesters were designed and synthesized by 1,6-hexanediol,succinic acid and another comonomeric diacid with gradual increases in chain length.Isodimorphism was confirmed for all the copolyesters by means of several characterization and analysis methods.The hexamethylene succinate(HS)composition of crystal type transition point for the four copolyesters was different depending on the comonomeric diacid chain length.The comonomer diacid with longer chain length had stronger competitive ability in controlling the cocrystallization even when they were present in a minor proportion.Moreover,the mutual insertion content of comonomers was quantificatively investigated by the defect Gibbs energy calculated from Wendling-Suter method.It indicated that the chain length of comonomer could exert a decisive influence on the degree of mutual insertion of comonomers.Moreover,the chain length inclusion-exclusion model on the basis of the Wendling-Suter theory was proposed to give an insight into the mechanism of how does the chain length of comonomers regulate the isodimorphism behavior.(2)Five series of biobased odd-even copolyesters varied with chain length of dicarboxylates and diols were designed and prepared,and the cocrystallization behavior of these five odd-even copolyesters were studied systematically.A new strict isomorphism copolyester was revealed,which was a very rare crystallization behavior for copolymer.In addition,the chain length of comonomeric diols can also exert a considerable influence on the isodimorphic behavior of copolyesters.This part in combination with the even-even copolyester study,we constructed the concept of "competition" and "miscibility" for isomorphism and isodimorphism in order to shed some insight on the relationships between the chemical structure and cocrystallization.Furthermore,a close dependence between the mechanical properties with"competition" and "miscibility" in such copolyesters was observed and discussed.Hence,the present study regarding the effect of comonomeric chain length on the "competition" and"miscibility" provided a guidance in the selection of comonomers for preparation of aliphatic copolyesters with desirable performances.(3)Biobased copolyesters poly(butylene succinate-co-cis-butene succinate)(PBSeBS)with various compositions were synthesized using cis-2-butene-1,4-diol,1,4-butanediol and succinic acid for the first time by optimized methods,achieving linear random copolyesters without isomerization or cross-linking side reactions.Isodimorphism was confirmed for PBScBS by means of several characterization and analysis methods.The tensile properties of PBScBS were assessed,suggesting all the copolyesters exhibited high elongations at break and outstanding toughness.More importantly,significant differences of "competition" and"miscibility" between comonomer units existed in PBScBS,which resulted from the conformational geometric effect of cis-double bonds in cis-2-butene-1,4-diol.The "competition,,and "miscibility" directly affect the band spherulites of PBScBS,especially to the band spacings.Meanwhile,in unsaturated copolyesters,the laws of dependence of mechanical properties with"competition" and "miscibility" still apply.(4)Fully biobased poly(butylene hydromuconoate-co-butylene fumarate)(PBHBF)unsaturated copolyesters were successfully synthesized from trans-?-hydromnconic acid,fumaric acid,and 1,4-butanediol by a melt polycondensation.Polymerization conditions(catalyst amount,reaction temperature and reaction time)were changed to improve PBHBF and molecular weight.The optimized strategy was considered to be applicable for preparing high molecular mass unsaturated polyesters without isomerization or cross-linking side reactions.On the basis of studies for thermal and crystallization properties,PBHBF showed an isodimorphism character,and in sum,the degree of crystallization and glass transition,melting,and crystallization temperatures could be effectively adjusted though the control of comonomer composition.PBHBF showed a good thermal stability and a broad processing window.In all cases,PBHBF exhibited excellent mechanical performances at room temperature similar to,or even better than,those of low density polyethylene and the most popular aliphatic polyesters.Due to the potential biodegradability and excellent thermomechanical properties,these fully renewable unsaturated polyesters are expected to be alternatives to the fossil fuel-derived polyesters and polyethylene for practical applications.(5)A universal method for the facile synthesis of stereochemistry-controllable poly(butylene maleate-co-butylene fumarate)(PBMF)copolyesters was presented.The microstrUcture and physcial properties of PBMF were systematically investigated to illuminate the respective conformational geometric effect of cis and trans isomers on the physical properties of copolyesters.These stereochemistry-controllable copolyesters transform from rigid plastics into flexible elastomers with the variation of the stereochemistry composition.The variation of the stereochemistry composition of PBMF allowed the crystallization,thermal and mechanical properties to be tailored over a broad range.Finally,the prepared unsaturated copolyesters were further exploited as a chemoselective polymer platform to incorporate various functional groups through a green and mild Michael addition reaction onto the backbone,showing the potential of PBMF for a wide variety of applications.