Study On The Ring Opening Polymerization Of Cyclic Butylene Terephthalate And Its Application

Thermoplastic resins offer a lot of advantages over thermosetting ones such as short processing cycles,post-processability,better recyclability,reusability and low influence on the environment.However,thermoplastics are difficult to be processed due to their high melt viscosity,which results in serious problems in wetting fillers and filling the mould.The invention of cyclic butylene terephthalate(CBT)has overcome the above shortcomings.CBT has low melt viscosity and high fluidity,its melt could rapidly infiltrate many types of fillers or reinforcements.The ring opening polymerizing(ROP)process of CBT has no reaction heat and small molecules release,the polymerization reaction could be controlled to complete in a few tens of seconds to tens of minutes and finally form the corresponding linear poly(cyclic butylenes terephthalate)(p CBT).Therefore,CBT is expected to become an ideal substrate for the preparation of nanocomposites or fiber reinforced composites.The typical thermosetting resin processing technology could be used to process CBT composites and its market prospect is promising.In this thesis,the polymerization kinetics of CBT under different polymerization conditions and its effect on the structure and properties of resultant p CBT were studied.The p CBT/multiwalled carbon nanotubes(MWCNT)nanocomposites and p CBT-poly(ethylene glycol)(PEG)block copolymer were prepared via in situ polymerization of CBT,and their structure and properties were analyzed.The details of research are as follows:(1)The rotary rheometer enabled online monitoring of the polymerization process under different polymerization conditions,the polymerization kinetic equations were established to simulate the ROP process of CBT using the chemical rheology theory of thermosetting resins for reference.The crystallization process of p CBT and the changes in the molecular weight of resultant p CBT were also investigated.The rheological results showed that the ROP started earlier and became faster with the increase of catalyst or polymerization temperature,the crystallization also started earlier and the molecular weight of resultant p CBT decreased,the value of Mn decreased from 7.49×104g/mol to 1.81×104g/mol.When the isothermal polymerization was carried out below the melting temperature of p CBT,there was viscoelastic transition gel point in the system.However,there were no gel points when CBT polymerized over the melting temperature of p CBT.Differential scanning calorimeter(DSC)and microscopic observation showed that the crystallization during the polymerization process of CBT was a heterogeneous nucleation process,which was different from melt crystallization process of p CBT.With the increase of the catalyst content,the crystallization process of resultant p CBT became faster,the crystal size became smaller,the melting temperature of p CBT increased and the degree of crystallization also became higher.(2)The performances of p CBT with different molecular weight prepared under different polymerization conditions were investigated.The rheological results showed that higher molecular weight of p CBT had higher melt viscosity and higher storage and loss modulus,the highest melt viscosity was 9×103 Pa·s.In addition,with the increase of molecular weight of p CBT,the behavior of shear thinning of its melt became more obvious.The linear fitted equation was found by analyzing the relationship between the molecular weight of p CBT and the content of added catalyst.The DSC analysis revealed that the final melting and crystallizing temperature,the degree of crystallization and re-crystallization of p CBT all increased with the decrease of its molecular weight.X-ray diffraction(XRD)results showed that degree of crystallization of p CBT increased with the decrease of its molecular weight,and the crystal sizes reduced.The degree of crystallization increased from 31.0% to56.8%.The thermostability of higher molecular weight of p CBT was better than that of the lower ones,illustrated by thermal gravimetric analyzer(TGA),the Tonset of p CBT-0.1-190 was 4 ?higher than p CBT-1.0-190.The mechanical properties of p CBT depended on the molecular weight,the higher molecular weight resulting in better toughness of p CBT,the maximum value of impact strength was 6.1KJ/m2.To the contrary,the samples became brittle when the molecular weight decreased.The resultant p CBT-0.5-230 had the best comprehensive mechanical properties.(3)The investigations on the isothermal and non-isothermal crystallization kinetics of the p CBT has been carried out by DSC,with considering to achieving different properties by controlling the crystalline structure of p CBT.The crystallization kinetics under isothermal conditions was described by Avrami equation.The crystallization activation energy of p CBT decreased from 637.27KJ/mol to 439.05KJ/mol with the decrease of its molecular weight,but the crystallization rate increased.As the crystallization temperature increased,the p CBT had higher crystallization rate and bigger crystal sizes.When the non-isothermal crystallization process of p CBT was analyzed by Avrami equation and Jeziorny equation,it was found that the value of Avrami index n increased to above 3 when the molecular weight of p CBT decreased to a certain extent.This phenomenon indicated that the crystal nucleation and growth mechanism of non-isothermal crystallization had changed.The crystallization rate constant K of p CBT increased with the decrease of the cooling rate or with the decrease of the molecular weight of p CBT.Furthermore,the non-isothermal crystallization process couldn't be well described by Ozawa theory,however,it could be well described by Mo rule.According to the result of equation fitting,F(T)decreased along with the decrease of molecular weight of p CBT when reaching the same degree of crystallinity.The non-isothermal crystallization activation energy calculated with Kissinger rule decreased to 176.32KJ/mol with the decrease of molecular weight of p CBT.XRD results showed that the crystal sizes got bigger with the increase of the cooling rate or the increase of the molecular weight of p CBT.(4)The p CBT/MWCNT nanocomposites were prepared by in situ polymerization of CBT.The effect of MWCNT content on the polymerization kinetics of CBT and the structure and properties of resultant p CBT/MWCNT nanocomposites were studied.The results showed that oxidized MWCNT could postpone the polymerization process of the CBT.The addition of moderate MWCNT made the polymerization time of p CBT lengthen by 8-33 minutes and broadened the processing window to prepare p CBT/MWCNT nanocomposites.The molecular weight and degree of conversion of p CBT in nanocomposites both decreased with the increase of MWCNT content,the value of Mn decreased from 3.44g/mol to 1.19g/mol.But the melting temperature,degree of crystallization and crystallization temperature increased.The crystal sizes decreased slightly.With the increase of MWCNT content,the bending properties and tensile properties of the nanocomposites increased firstly and then decreased,while its impact strength,tensile breakage elongation and fracture energy decreased all the time.The nanocomposites gradually changed from plastic deformation to brittle fracture.When the MWCNT content was 0.1~0.5wt%,the p CBT/MWCNT nanocomposites had the best comprehensive properties.(5)A serial of p CBT–PEG block copolymers with different content of soft PEG segment were synthesized by ring opening polymerization of CBT with PEG as macromonomer using a one pot,one step melting polymerization process.The FT-IR and 1H NMR confirmed that the copolymerization had happened.The composition of the p CBT–PEG copolymers obtained on the basis of 1H NMR analysis showed that the content of soft and hard segments in the copolymers was close to their feed ratio,and the average sequence length of p CBT decreased with the increase of the PEG content,while the average sequence length of PEG remained constant.When the dosage of PEG was 70wt% the average sequence length of p CBT became 2.3.The intrinsic viscosity reflected that the polymerization speed increased with the increase of catalyst content and the intrinsic viscosity of the copolymers decreased with the increase of fed content of CBT.DSC results showed that the melting and crystallization temperature and the degree of crystallinity of hard p CBT segments decreased with increasing soft PEG segments content,and vice versa.XRD results indicated that the addition of soft PEG segments destroyed the original crystallization process of p CBT and therefore the degree of crystallinity of copolymers decreased.TGA analysis revealed that the initial degradation temperature of copolymers were higher than that of pure PEG by more than 120?,which was close to that of pure p CBT.With the increase of the soft PEG segments,the tensile modulus and tensile strength of the p CBT–PEG copolymers decreased,while the breakage elongation and fracture energy increased obviously,the value reached 1860% and 26.8J.Which indicated that PEG copolymerization had a better toughening effect on p CBT.Therefore the mechanical properties of p CBT–PEG copolymers could be adjusted by varying the feed ratio of CBT and PEG.