Synthesis,Chain Structure And Crystallization Of Biobased Poly(Ethylene Furan-dicarboxylate)

Poly(ethylene furandicarboxylate)(PEF)is a promising bio-based polyester synthesized from 2,5-furandicarboxylic acid(2,5-FDCA)or its diester(2,5-DMFD)and ethylene glycol(EG).PEF is deemed as a good counterpart of poly(ethylene terephthalate)(PET),especially for high performance and packaging applications because of its comparable or even better thermal,mechanical and much higher gas barrier properties.Moreover,PEF production can reduce the nonrenewable energy use and the greenhouse gas emission by 40-50%and 45-55%,respectively.However,the synthesis of PEF has many problems,for example,the low molecular weight,severe etherification side reaction and so on.In addition,PEF in general crystallizes even more slowly than PET which is well known for its slow crystallization.Aiming at these problems,the following issues including optimization of synthesis conditions,generation mechanism of diethylene glycol furandicarboxylate(DEGF)units and the catalyst and chain structure-dependences of crystallization have been investigated in this research.Finally,PEF with moderate molecular weight and low DEGF content or high molecular weight was synthesized.Firstly,the effects of catalyst quantity,diol/diester ratio,transesterificatian and polycondensation time were investigated using DBU catalyst,stannous oxalate/ethylene glycol antimony compound catalyst(Sn/Sb)and self-made modified titanium catalyst,respectively.It is found that terminal hydroxyl ethylene glycol(THEG)ester content and transesterification rate are important factors to tune the chain structure of PEF,namely DEGF content and molecular weight.Dark brown PEF with low DEGF content and low molecular weight(DEGF 1.71%,[η]0.541 dL/g)was synthesized with DBU;Yellow PEF with low DEGF content and high molecular weight(DEGF 1.79%,[η]0.80 dL/g)was synthesized with the Sn/Sb catalyst;Faint yellow PEF with low DEGF and moderate molecular weight(DEGF 1.22%,[η]0.663 dL/g)or with high mocular weight([η]>1.0 dL/g)was synthesized with modified titanium catalyst.Secondly,the 1H NMR spectra of transesterification products were tracked to try to make clear the detailed generation procedure of DEGF units in PEF via etherification side reaction.With the help of’H NMR,it is found that a trace of terminal hydroxyl diethylene glycol(THDEG)ester but no DEGF unit was generated in transesterification stage.So it’s supposed that most of DEGF units were generated from the dehydration reaction between THEG units and a little of DEGF units were from the THDEG units.So the THEG content at the end of transesterification stage is an important factor to determine the final DEGF content.Thirdly,the effect of thermal history on PEF crystallization was studied.It was found there is a self-nucleation(SN)domain in regular DSC scan at isothermal temperature ranging 230-260℃.The thermal history was eliminated at isothermal temperature above 260℃ for 5 min.More importantly,it was found that PEF crystallized extremely slowly from melt or even could not melt crystallize after completely erasing its heat history,suggesting that it was very difficult to nucleate from PEF melt and PEF crystal growth was also very slow.Considering the unique crystallization behavior,a so-called "standard" DSC temperature program partially erasing PEF heat history was recommened and used in this study:(a)heating PEF sample at 10℃/min from room temperature to 250℃;(b)keeping at 250℃ for 5 min.Finally,the crystallization of PEF with different DEGF content and molecular weight synthesized with different catalysts was explored.It is found that the crystallization rate of PEF became more slowly along with DEGF content and mocular weight increasing;It is interesting to find that Sn/Sb-catalyed PEF has better crystallizability than Ti-catalyzed PEF;DBU-catalyzed PEF had low DEGF content and relatively low molecular weight,so it exhibited rapid melt crystallzation.A minimum of melt crystallization half time,1.6 min,appeared at 160 ℃.To our knowledge,it is the shortest melt crystallization half time of PEF ever reported up to now.In conclusion,based on catalyst screening and understanding of etherification mechanism,the chain structure,namely,DEGF content and intrinsic viscosity of PEF can be tuned by means of catalyst and reaction conditions,and PEF with low DEGF content or high molecular weight PEF was successfully synthesized.What’s more,the effect of catalyst,chain structure and heat history on the crystallization of PEF was explored.These results have laid a good foundation and therefore will be very helpful for further research of PEF.