Microstructure Regulation Of Thermoplastic Polyurethane Elastomersand Its Effect On Dielectric Properties

Dielectric elastomer materials are considered the most promising "artificial muscle" materials.Among them,thermoplastic polyurethane elastomers have received extensive attention because of their microphase separation structure and excellent structure controllability,processability,mechanical properties,and electrical property controllability.However,the high modulus and low dielectric constant of polyurethane dielectric elastomers limit their practical application in dielectric elastomer devices In this thesis,the dielectric and mechanical properties of PUEs were adjusted by controlling the microphase separation and crystallization of PUEs via adjusting the synthesis method,chain extender,hard segment content and blending with polar small molecules.The effects of synthesis method,chain extender,hard segment content and types of small molecule on the microphase separation and crystallization behavior of PUEs were investigated with various characterization methods.The experimental results showed as following:(1)In the PUEs with polycaprolactone diol(PCL)as soft segment(SS),hexamethylene diisocyanate(HDI)and butanediol(BDO)as hard segment,the crystallinity of the hard segment(HS),degree of microphase separation,and modulus were significantly increased with the increase of HS content.And,the the dielectric constant of PUEs is significantly affected by the crystallization of SS.Among them,the PUE with 20% of HS had highest dielectric constant(6.0 at 100 Hz)and the PUE with 15% of HS had highest electromechanical sensitivity factor(EMSF)(β=2.0).(2)The sequent length and distribution of HSs were controlled by changing the feeding sequence in the polymerization process.Compared with PUE synthesized by traditional two-step method,PUE synthesized by modified new two-step method had longer sequent length of HS and had better microphase separation,crystallization of HS and higher modulus.Whereas,the dielectric constant and the EMSF showed lower.(3)In PUEs with different chain extender structures,the stereoisomeric structure of chain extender significantly affected the microphase separation and crystallization behavior of SS and HS.the PUEs with mixed chain extenders of cis-and trans-isomers showed lower modulus and higher dielectric constant than that with corresponding single component of isomer.Incoporation of cis and trans mixed isomer extender into PUEs can influence the HS packing,resulting in reducing the degree of hydrogen bonding between HSs and microphase separation of HS and SS.Among the PUEs with chain extender of mixture isomer 1,4-cyclohexane dimethyl alcohol,cis-1,4-cyclohexane dimethyl methanol,mixed isomer 1,4-cyclohexane dimethyl alcohol(MCHDM)and trans-1,4-cyclohexane methanol(T-CHDM),the PUE with M-CHDM had the lowest modulus and EMSF(0.65)at 100 Hz.(4)In PUE/organic small molecule blend system,the interacted organic small molecule can strongly influence the microphase separation structure of PUE,resulting in influence on the mechanical properties and dielectric properties.The methyl cyclohexylenediamine(HTDA),4,5-dichlorophenylene diamine(4,5-DOPD),sorbitol and urea were used to modify the PUE.Among them,the dielectric constant of PUE/4,5-DOPD had highest dielectric constant which was about 231% higher than pure PUE.When adding 15 wt% of 4,5-DOPD in PUE,the EMSF was reached about 2.5,which was about 250% higher than pure PUE.