Investigation On The Phase Separation Morphology And Properties Of Conjugated Polymer-Elastomer Blends

Stretchable electronics are at the forefront of plastic electronics,with promising applications in biomedical,implantable and wearable devices,and portable power supplies.Among various electroactive materials that can be used in stretchable electronic devices,conjugated polymers have attracted wide attention due to their advantages of solution-processable,light weight and adjustable mechanical properties.However,most high mobility polymers are extrinsically stretchable.Conjugated polymer-elastomer blending strategy is a simple and efficient method for highperformance stretchable polymer films.The phase-separated morphology has a vital effect on the electrical and mechanical properties of blend films.The ideal phase separation morphology is that the two components are separated vertically,the polymer forms a continuous nanofibril network at film suefaces to ensure good charge transport,while their interfacing with the deformable elastomer prevents crack propagation.The phase separation of polymer and elastomer during film formation process is determined by various physicochemical factors,such as solvent evaporation rate,polymer crystallinity,as well as the interactions among conjugated polymer,elastomer,solvent and substrate,which makes it a great challenge to precisely regulate the phase separation of two components.Herein,the P(NDI20D-T2)(N2200)and polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene(SEBS)blend was selected as the model system.Through controlling polymer molecular weight,the polymer solubility and miscibility of the two components were effectively regulated,realizing the control of vertical component distribution along depth direction.Furthermore,we have regulated the polymer-solvent interaction and film formation kinetics through selected solvents,realizing the control of the fibril size in the blends.Finally,we have systematically investigated the microstructure transition of blend films under strains,and summaried the strain energy dissipation mechanism of blend films.1.The N2200 solubility and its miscibility with SEBS were reduced by increasing N2200 molecular weight,which promote the extent of phase separation between N2200 and SEBS along depth direction and thus enhance the enrichment of N2200 at film surfaces.Film-depth-dependent light absorption spectroscopy(FLAS)result shows that the content of N2200 at film surfaces increases from 66%to 85%as the Mn increases from 28 to 143 kDa,leading to the change of morphology from isolated aggregates to entangled nanofibril network within SEBS matrix.As a result,the mobility of the 143 kDa blend film reaches 0.188 cm2 V-1 s-1,much higher than that of 28 kDa blend film(0.043 cm2 V-1 s-1)and does not decrease during stretching up to 150%strain.2.The polymer-solvent interaction was strengthened by using a low Hansen solubility parameter distance of N2200 backbone(Ra(b))and high boiling point solvent 1,2-dichlorobenzene(o-DCB),which greatly suppress the solution aggregation and further assemble of N2200 during film formation process,thus effectively reduce the fibril size.Meanwhile,the long film formation time ensures the polymer molecular order in the final blend film.The blend film presents a sandwiched vertical phase separation structure with N2200 enrich at both the top and bottom surfaces is obtained,where the N2200 layer comprises of nanofibrils with small fibril diameter(<45 nm)to form a continuous network.Compared to the nanofibril bundle with large size(diameter:>230 nm),these nanofibrils with small diameters induce strong nanoconfinement effect,and thus significantly improves the stretchability of the resulting blend film.The strain at fracture of the blend film can reach 153%.The mobility of the resulting blend film gradually increases from 0.113 to 0.255 cm2 V-1 s-1 at 100%strain and exhibit negligible loss at 150%.3.Through the characterizations of film morphology,polymer chain orientation,and molecular packing structure changes during the stretching process,the strain energy dissipation mechanism in the blend system was explored and summarized.Due to the excellent interfacial compatibility between SEBS and N2200 interpentrating polymer network,the good synergistic effect between the two components is conductive for blends to dissipate strain energy through the deformation of SEBS.Furthermore,these nanofibrils with small diameters in SEBS matrix are less constrained under strain.They have greater deformation capacity and can freely rotate in the SEBS matrix when subject to strain.These characteristics ensure superior stretchability of the blend film and the continuity of the polymer layer under strain.In addition,the strain energy can be further dissipated through polymer chains slippage in the crystalline of blend films,which leads to a dislocation structure.The lamellar packing distance in crystalline regions with edge-on orientation decreases and lamellar packing distance in crystalline regions with face-on orientation increases when subject to strain.