The rich −SO3H groups enable sulfonated poly (ether ether ketone) (SPEEK) to possess excellent proton conductivities in proton exchange membrane (PEM), but cause excessive water absorption, resulting in the decline of dimensional stability. It is a challenge to resolve the conflict between conductivity and stability. Owing to its unique structural designability, covalent organic frameworks (COFs) have been used to regulate the performances of PEMs. The authors propose the use of COFs with acidic and basic groups for meeting the requirements of proton conductivity and dimensional stability. Herein, COFs containing different groups (sulfoacid, pyridine, and both) were uniformly dispersed into the SPEEK matrix by in situ synthesis, and the effects on the properties of SPEEK matrix PEMs were revealed. The sulfoacid group significantly improves proton conductivities. At 60 °C, under 95% RH, the conductivity of the SPEEK/TpPa−SO3H-20 composite membrane was 443.6 mS·cm−1, which was 3.3 times that of the pristine SPEEK membrane. The pyridine group reduced the swelling ratio at 50 °C from 220.7% to 2.4%, indicating an enhancement in dimensional stability. Combining the benefits of sulfoacid and pyridine groups, SPEEK/TpPa−(SO3H-Py) composite membrane has a conductivity of 360.3 mS·cm−1 at 60 °C and 95% RH, which is 1.86 times that of SPEEK, and its swelling ratio is 11.8%, about 1/20 of that of SPEEK membrane. The method of in situ combination and regulation of groups open up a way for the development of SPEEK/COFs composite PEMs.
Research on the rheological performance and mechanism of polymer nanocomposites (PNCs), mainly focuses on non-polar polymer matrices, but rarely on strongly polar ones. To fill this gap, this paper explores the influence of nanofillers on the rheological properties of poly (vinylidene difluoride) (PVDF). The effects of particle diameter and content on the microstructure, rheology, crystallization, and mechanical properties of PVDF/SiO2 were analyzed, by TEM, DLS, DMA, and DSC. The results show that nanoparticles can greatly reduce the entanglement degree and viscosity of PVDF (up to 76%), without affecting the hydrogen bonds of the matrix, which can be explained by selective adsorption theory. Moreover, uniformly dispersed nanoparticles can promote the crystallization and mechanical properties of PVDF. In summary, the viscosity regulation mechanism of nanoparticles for non-polar polymers, is also applicable to PVDF, with strong polarity, which is of great value for exploring the rheological behavior of PNCs and guiding the process of polymers.
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