Crystallization Behavior And Electrochemical Properties Of Comb-like PEG-based Copolymer Electrolyte

Polyethylene glycol?PEG?has been used as the matrix of polymer electrolytes because of plentiful number of ether oxygen bond,high dielectric constant and low glass transition temperature.But at room temperature,the low molecular weight PEG is in the liquid state,which has no mechanical strength,and the high molecular weight PEG has a strong tendency to crystallize while the process of ionic conduction mainly occurs in the amorphous region.In order to make the polymer electrolyte obtain a certain mechanical strength and high ionic conductivity,the inhibition of crystallization of PEG through copolymerization and cross-linking and the support of self-standing film are required.This paper adopted one pot polycondensation method to synthesize PEG-based copolymers,which polyethylene glycol monomethyl ether?MPEG?acted as side chains,PEG as segments in main chains,and hexamethylene-1,6-diisocyanate homopolymer?HDI trimer?as cross-linking agent.Then corresponding polymer electrolytes were prepared after addition of certain amount of perchloric acid lithium?LiClO₄?.First,the results of infrared spectroscopy?FT-IR?and nuclear magnetic resonance?NMR?verified the synthesis and the high decomposition temperature in thermo-gravimetric analysis?TGA?revealed good stability of the copolymers.Then,through characterizations of the swelling ratio and differential scanning calorimetry?DSC?,it was discovered that cross-linking inhibited the crystallizaiton of chains of the copolymer.Hence we have to discuss the influence of the molecular weight of the side chain and the molecular weight of the main chain segment on the crystallization of the copolymer at a certain degree of cross-linking.When the molecular weight of PEG/MPEG was 1000g/mol or less than 1000g/mol,the state of cross-linked comb-like copolymer was amorphous.When the molecular weight of PEG2000/MPEG2000 reached 2000g/mol,PEG/MPEG started to crystallize in cross-linked comb-like copolymer.When the molecular weight of PEG was 2000g/mol,more molecular weight of MPEG led to lower crystallinity of PEG2000.The low molecular weight of PEG also affected the crystallization behavior of MPEG2000.Further,after adding LiClO₄ into the solution to form membrane,a series of polymer electrolytes were obtained.For amorphous polymer electrolytes,the existence of side chains improved the ionic conductivity.The ionic conductivity raised with the rise of side chains' molecular weight.However,as the molecular weight of main chain segments became larger,the ionic conductivity declined due to the reduced density of side chains.At this time,the ionic conductivity at room temperature was up to a maximum of 7.7? 10-4 S/cm.For the crystallized polymer electrolytes,the ionic conductivity of the polymer electrolyte was greatly dropped because of the presence of the crystallization which suppressed the motion of chains.In addition,in order to further enhance the ionic conductivity of polymer electrolytes,inorganic particles,boron oxide?B₂O₃?were added.The images of scanning electron microscope?SEM?and transmission electron microscope?TEM?indicated that B₂O₃ had reacted with PEG as Lewis acid.The results showed that the ionic conductivity of the polymer electrolyte has been improved to some extent.At last,mechanical strength of polymer electrolytes were tested.It was worked out that the fracture strength of amorphous polymer electrolytes can reach 4⁴.5 MPa,and the fracture strength of crystallized polymer electrolytes were higher than that of non-crystallized polymer electrolytes.Compared with the fluid state of linear copolymer,the solid polymer electrolyte provided a certain guarantee for the safety of the battery.