Novel Hyperbranched Polymers as Host Materials for Green Thermally Activated Delayed Fluorescence OLEDs

A series of novel hyperbranched polymers （HBPs） consisting ofa 2,7-subsituted 9-（heptadecan-9-yl）-9H-carbazole unit （A2＋A2＇） and a tetra-substituted green thermally activated delayed fluorescence （TADF） dye of 2,3,5,6-tetra（9H- carbazol-9-yl）-4-pyridinecarbonitrile （4CzCNPy, B4） have been synthesized via Suzuki cross-coupling reaction following an ＂Az＋A2＇＋B4＂ method. The polymers are named according to the polymerization ratio of 4CzCNPy monomer （5 mol%, 10 mol% and 15 mol% for HBPs of P2-P4 respectively, and 0 mol% for the control linear polymer P1）. Their thermal, optoelectronic and electrochemical properties have been characterized by a combination of techniques. All the polymers exhibit high thermal stability with the decomposition temperatures （Ta） above 400 ℃ and glass transition temperatures （Tg） up to 98℃. Unfortunately, the incorporation of TADF moiety into these HBP materials induced non-TADF characteristics. However, when the HBPs functionalized as the host for our previously developed 4CzCNPy TADF dopant in solution processed devices, maximum external quantum efficiency of 5.7% and current efficiency of 17.9 cd/A have been achieved in P3-based device, which is significantly higher than those of 1.5% and 4.2 cd/A for the linear polymer P1.

Boron nitride/agarose hydrogel composites with high thermal conductivities

Hydrogels are cross-linked polymers suitable for various applications,but the thermal conductivities of hydrogel-based composites have not been thoroughly investigated.In this study,agarose hydrogel-based composites with various boron nitride(BN)fillers were synthesized and their thermal conductivities were systematically investigated.With the increase in the agarose content from 1.5 wt%to 3.0 wt%,the thermal conductivity of the composite decreased.The composites with BN micropowder had larger thermal conductivities than those of the composites with BN nanopowder at the same filler loading,as the BN micropowder provided better thermal conduction pathways in the hydrogel matrix than those provided by the nanopowder.The maximum thermal conductivity of 2.69 W m-1·K-1 was achieved when 15 wt%microscale BN fillers were added into 1.5 wt%agarose hydrogel,which was 3.5 times larger than that of the pure agarose hydrogel.Additionally,a theoretical model was used to calculate the thermal conductivities of the BN/agarose hydrogel composites;a good agreement was achieved between the experimental and fitting ones.This study demonstrated that the thermal conductivities of hydrogel-based materials can be efficiently and significantly enhanced using BN fillers.