Study On The Well-Controlled Synthesis And Properties Of Azobenzene-Containing Liquid Crystal Networks

Azobenzene-containing liquid crystal networks(Azo-LCNs)combining the LC anisotropic property,elasticity of soft rubber,and the unique photoisomerization of azobenzene,exhibit macroscopic motion by changing their size or shape under the external stimulation of light or temperature because of their excellent molecular cooperative effect and multi-scale cooperative motion,which have important applications in the fields of aerospace,life medicine,and so on.As a typical soft material,the properties of Azo-LCNs are usually limited to the mechanical strength of materials.Moreover,the structure of Azo-LCNs is not well-controlled,and the influence mechanism of structure on properties is not clear.Side chain liquid crystal polymers(SCLCPs)show the characteristics of structural designability,which could be used to tailor advanced materials with specific functions through the rational design of polymeric backbones and mesogens with different functions.The properties of SCLCPs are comprehensively affected by the flexibility of backbones,spacers,mesogens and grafting density,and so on,developing Azo-LCNs with expected structure and property is still challenging.To address above problems,this paper starts from the molecular structure design.A series of well-controlled polymeric backbones are prepared by living anionic polymerization(LAP).Azobenzene(Azo)mesogenic moieties with different structures are designed and synthesized.Then,well-controlled SCLCPs are produced in combination with highly efficient hydrosilylation to reveal the influence mechanism of structure on property.Furthermore,AzoLCNs with expected structure and property are synthesized through the rational design of modular functions.(1)Well-controlled Si-H and vinyl-functionalized polystyrenes,i.e.,PVPDMS(PS-SiH)and PVSt(PS-Vinyl),with well-controlled degree of polymerizations were prepared by LAP,and were modified with vinyl-and Si-H-terminated Azo mesogens,i.e.,Azo-Vinyl,Azo-SiH,AzoB-Vinyl,and AzoB-SiH,respectively,for the fabrication of PS-SCLCPs with tunable spacers and degree of polymerizations,i.e.,PS-[SiC]-Azo,PS-[SiOSi]-Azo,PS-[SiC]-AzoB and PS-[SiOSi]-AzoB.The influences of structures on the property were systematically explored.Based on degree of polymerizations and spacers,the temperature ranges of mesomorphic formation(ΔT):PS-[SiOSi]-Azo(ΔT=34-49℃)<PS-[SiC]-Azo(ΔT=49-60℃)<PS-[SiOSi]-AzoB(ΔT=154-164℃)<PS-[SiC]-AzoB(AT=162-193℃).The result indicated that degree of polymerizations showed slight influence on the thermal properties while the phase transitions could be significantly tuned by the spacers.Then,two crosslinking agents were prepared and well-controlled PS-LCNs were further designed.The resulting PS-LCNs showed expected room temperature glass transition temperatures(Tg),controlled AT,and good mechanical properties with a storage modulus of 120 MPa.PS-LCNs films with selective crosslinking can exhibit directional bending upon irradiation with UV light at 50℃,and this was shown to be a photomechanical deformation.(2)Si-H and Vinyl bi-functionalized styrene-butadiene block copolymers,i.e.,PVPDMS-bPB and PS-b-PB,were synthesized by LAP.Based on the difference in reactivity between alkynyl and vinyl with Si-H during hydrosilylation,alkynyl and Si-H-terminated Azo mesogens,i.e.,Azo-Alkynyl(LC1)and AzoB-SiH(LC2)were attached onto PVPDMS-b-PB and PS-b-PB to obtain liquid crystal block copolymers(LCBCPs),i.e.,(PVPDMS-g-LCi)-b-PB,PS-b-(PBg-LC2)and(PVPDMS-g-LCi)-b-(PB-g-LC2).The independent and cooperative effect of different functions was confirmed by mixing PVPDMS3k-g-LCi and PB6k-g-LC2 in different mole ratios in comparison with LCBCPs.The influences of structures on the property were systematically explored.It was demonstrated that the modular functionalization showed independence and cooperative contributions to bi-functionalized LCBCPs in terms of mesomorphic performances and microphase separation.Based on the principle of modular functionalization,self-crosslinked liquid crystal networks,i.e.,[PVPDMS-b-PB]-LCNs with well-controlled grafting density and self-crosslinking density were synthesized,which showed room temperature Tg and tunable clearing temperatures(Ti)(Ti=78～102℃).(3)Random styrene-butadiene rubber(r-SBR)with well-controlled molecular compositions and microstructures was synthesized by LAP.r-SBR contained approximately 75 wt.%butadiene units that improve molecular mobility,in which styrene units could improve the mechanical strength.Microstructural 1,2-olefins of 51.5-64.0 mol%were designed to attach 50 mol%mesogenic moieties Azo-SiH(M)along Bd units using hydrosilylation to yield r-SBRg-M.Based on the epoxidation and ring-opening reaction of 1,4-olefins,hexamethylene diisocyanate(HMDI)as the irreversible covalent bonds and 2-amino-4-hydroxy-6methylpyrimidine(UPy-NCO)as reversible H-bonds agents in various mole ratios were grafted onto 1,4-olefins to obtain dynamic liquid crystal networks[r-SBR]-LCNs,i.e.,r-SBR-g[M·HMDI·UPy].The effects of molecular weight(MW),HMDI and UPy contents on the liquid crystal textures,thermal properties,recyclability and reprocessability,self-healing/welding behaviors and shape memory properties were systematically studied.The manipulation mechanism of molecular control on the mechanical performance and chain mobility of the resulting[r-SBR]-LCNs was revealed.Because of well-designed 50 mol%orderly LC packing,[r-SBR]-LCNs showed obvious morphological textures,presented low Tg(6-14℃)and Ti(4253℃)values,which was beneficial for both the mechanical performance and chain mobility,providing basis for the self-healing/welding effect and shape memory properties of materials.In addition,UPy could improve the breaking strain and sacrifice the mechanical strength,which was beneficial for the recyclability and reprocessing,self-healing/welding ability and temporary shape fixity ratio(Rf),and decreased the shape recovery ratio(Rrec).While HMDI showed the contrary effect,as HMDI could effectively enhance the mechanical strength and reduce the chain mobility.It was evidenced that r-SBR45k-g-[M-0%,2%](Rf=92.6%,Rrec=1%)showed good shape fixity ability while displayed no shape recovery ability.With the increase of HMDI contents,the Rrec of r-SBR45k-g-[M-10%,2%](Rf=46.2%,Rrec=100%)greatly increased to 100%but Rf decreased to 46.2%.All[r-SBR]-LCNs showed good recyclability and a higher than 82%self-healing/welding efficiency,while r-SBR-g-[M-10%,2%]with relative high HMDI showed poor self-healing/welding and recyclability.With the increase of MW,all LCNs presented good shape fixity ability and could not completely recover their original shapes.The Rf values of[r-SBR]-LCNs were 96.6%,93.0%and 94.5%,respectively.The Rrec values were 48.2%,73.5%and 54.0%,respectively,which indicated that MW showed no significant effect on the shape memory property.