Synthesis And Properties Of Novel Block Copolyethers Used As Solid Propellant Binders

Binder is an important component of solid propellants,and its performance affects the solid propellants'mechanical,low-temperature,aging and processing properties directly.Therefore,the research and development of new binders are an important aspect to promote the development of solid propellants.With the development of polymer synthesis technology,the emergence of controlled living polymerization and precise,efficient reactions provides technical support for the synthesis of new binders.Thus,in this dissertation we focus on the design,synthesis and properties of new solid propellants binders.In the chapters 2,3 and 4 of this dissertation,hydroxyl-terminated polybutadiene(HTPB)with only 1,4-addition structure and P(THF-co-PO)-b-PB-b-P(THF-co-PO)triblock copolymers with two different number-average molecular weight(M_n)were synthesized by ring-opening metathesis polymerization(ROMP)and cationic ring-opening polymerization(C ROP),respectively based on research of traditional HTPB binder.Chapters 5 and 6 describe the synthesis of a series of alkynyl terminated poly(ethylene glycol)-b-polytetrahydrofuran(PEG-b-PTHF)multiblockcopolyethersandpoly(2-phenyl-5-ethyl-5-acryloxymethyl-1,3-dioxacy clohexane)-b-poly(ethylene glycol)(PDAEP-b-PEG)amphiphilic diblock copolymers via the combination of various polymerization techniques and efficient reactions,such as CROP,anionic ring-opening polymerization(AROP),reversible addition fragmentation chain transfer polymerization(RAFT),Glaser coupling reaction and click chemistry.Specific excerpts of the research results are summarized as follows.(1)HTPB with only 1,4-addition microstructure,controllable M_n,narrow molecular weight distribution(M_w/M_n)and accurate microstructure was synthesized by ROMP using cyclooctadiene as monomer.The results of polymerization process study showed that the feed ratio of the monomer to chain transfer agent is the main factor to affect the M_n of HTPB.The microstructures and properties of HTPB synthesized by free radical polymerization,anionic polymerization and ROMP were compared.And the results showed that the molecular weight distributions of HTPB synthesized by anionic polymerization and ROMP were narrower than that of HTPB synthesized by free radical polymerization.The backbones of HTPB synthesized by anionic polymerization and free radical polymerization were composed by cis-1,4,trans-1,4 and 1,2 butadiene units,while in the case of HTPB synthesized by ROMP,the backbone contained only cis-1,4 and trans-1,4 butadiene units and no 1,2 butadiene units.As for the structure of hydroxy,there are three different hydroxy end groups in HTPB synthesized by free radical polymerization,that is,hydroxy groups bonded to methylene of the cis-1,4,trans-1,4 and 1,2 butadiene terminal units.There are two different hydroxy end groups in HTPB synthesized by ROMP,which are hydroxyl groups bonded to methylene of the cis-1,4 and trans-1,4 butadiene terminal units.However,there is only one kind of hydroxy end group in the structure of HTPB synthesized by anionic polymerization,which is aliphatic hydroxyl.The results also showed that the HTPBs synthesized by free radical polymerization and anionic polymerization are amorphous polymers,and HTPB synthesized by ROMP is semicrystalline polymer.With the increase of the content of 1,2 structure,the viscosities of three HTPBs increased.The three kinds of HTPBs have nearly the same thermal stabilities.(2)Triblockcopolymerbinderpoly(tetrahydrofuran-co-propylene oxide)-b-polybutadiene-b-poly(tetrahydrofuran-co-propylene oxide)[P(THF-co-PO)-b-PB-b-P(THF-co-PO)]was synthesized via the cationic ring-opening copolymerization of tetrahydrofuran(THF)and propylene oxide(PO)in the presence of HTPB(M_n=2200 g/mol).The copolymerization mechanism was studied using fourier transform infrared spectroscopy(FT-IR),nuclear magnetic resonance(NMR),and size-exclusion chromatography coupled with multi-angle laser light scattering(SEC-MALLS).The results showed both active chain end(ACE)and activated monomer(AM)mec hanisms contribute to the chain propagation process to different extents,and the copolyether segments in triblock copolymer had a gradient microstructure accordingly.The results of polymerization process study showed that the M_n of triblock copolymer can be tailored by changing the feed ratio of THF to HTPB.Differential scanning calorimetry(DSC),thermogravimetric analysis(TGA),and viscosity testing results showed that the triblock copolymer had lower T_g than HTPB,low crystallization tendency and viscosity,and excellent thermal stability.To explore its potential applications,the triblock copolymer(M_n=4500 g/mol)was used to prepare elastomer by thermosetting process.Dynamic thermomechanical analysis(DMA)and tensile test results indicated that the triblock copolymer based elastomer had lower T_g,as well as dramatically higher tan?value and elongation at break,than those of HTPB based elastomer because of the introduction of copolyether segments.(3)P(THF-co-PO)-b-PB-b-P(THF-co-PO)(M_n=8000 g/mol)was also synthesized by cationic ring-opening copolymerization of THF and PO in the presence of HTPB(M_n=6000g/mol).The triblock copolymer showed excellent thermal and storage stability.The triblock copolymer is amorphous polymer,and the T_g value is-84.28°C.The effect of NCO/OH on mechanical properties of triblock copolymer elastomer was studied systematically.The mechanical properties and low-temperature properties of the elastomers based on triblock copolymer are superior to HTPB elastomers with the same crosslink density.(4)PEG-b-PTHF-b-PEG triblock copolyether binder was synthesized by combining CROP and AROP,and then the hydroxyl end groups in PEG-b-PTHF-b-PEG triblock copolyether were modified to alkynyl groups.Two kinds of alkynyl terminated(PEG-b-PTHF)_n multiblock copolyethers with different M_n were synthesized by Glaser coupling reaction between alkynyl terminated PEG-b-PTHF-b-PEG triblock copolyethers.The thermal stabilities of alkynyl terminated(PEG-b-PTHF)_n multiblock copolyethers are similar to those of PEG-b-PTHF-b-PEG triblock copolyether.However,the crystallization and melting temperatures of multiblock copolyethers are lower than those of triblock copolyether.(5)A novel aromatic acetal based monomer 2-phenyl-5-ethyl-5-acryloxymethyl1,3-dioxacyclohexane(HEDPA)was synthesized,and then alkynyl terminated poly(2-phenyl-5-ethyl-5-acryloxymethyl-1,3-dioxacyclohexane)(PDAEP-Alk)was obtained by RAFT polymerization of HEDPA using alkynyl functional chain transfer agent(CTA-Alk).A series of amphiphilic diblock copolymers(PDAEP-b-PEG)binder composed of fixed PDAEP segments and various lengths of mPEG segments were prepared through click reaction between alkynyl terminated PDAEP and azido terminated mPEG.Due to the amphiphilic structure feature,the diblock copolymers could self-assemble into spherical micelles in aqueous solution,and the sizes of micelles decreased with the increase of the mPEG segments lengths.The research also found that with the acetal groups located at the side chain of PDAEP blocks hydrolyzed gradually in acidic environment,the PDAEP-b-PEG micelles swelled.When the acetals'hydrolysis was carried out for a sufficiently long time,the micelles disassembled and were converted to water soluble unimers.The assembly/disassembly transition behaviors of the diblock copolymers could be adjusted by changing the lengths of the mPEG sengments.