Construction Of Dynamic Polymers Based On Knoevenagel Reaction:Synthesis And Application

Generally,chemically crosslinked thermosetting polymer materials are difficult to recycle and reprocess.By introducing dynamic covalent bonds(DCBs)into the main chain or side chain of the polymer,it is possible to endow traditional covalently crosslinked polymers with properties such as reprocessability,self-healing,controllable degradation,and recyclability.The Knoevenagel condensation(KC)reaction is a classic reaction between aldehydes(or ketones)and compounds containing active methylene groups,resulting in the formation of C=C double bond,which has been extensively utilized in both academia and industry.The C=C double bond formed by KC reaction is a typical dynamic covalent bond.At present,the research on the dynamics of the C=C double bond is mainly concentrated in the field of small molecules,and there are few reports on the use of this reaction to build dynamic polymers.Following that,two new dynamic polymer materials were fabricated based on the KC reaction and their mechanical and thermal properties,controllable degradation,reprocessing,self-healing and recovery abilities were systemically studied.The details are as follows:(1)Methoxy poly(ethylene glycol)s(m PEG)modified with benzaldehyde(BA)and cyanoacetate(CA)were used to construct the model compound(m PEG-CCA-m PEG)by KC reaction which verified that the formed C=C double bond could be cleaved and formed reversibly in water.Furthermore,the crosslinked dynamic polymer(PPEG-CCA-PCL)was prepared by KC reaction using poly(ethylene glycol)modified with bis(benzaldehyde)(BA-PEG-BA)as the main chain,and polycaprolactone ternary alcohol modified with tris(cyanoacetate)(PCL-TCA)as the crosslinking agent.Thermogravimetric analysis showed that the decomposition temperature of PPEG-CCA-PCL was 321 ~oC,and it had good thermal stability;The tensile test showed that the tensile strength and Young’s modulus of PPEG-CCA-PCL were 9.72 MPa and 156 MPa respectively,and the elongation at break was305%,which had good mechanical strength and ductility.Due to the reversibility of dynamic C=C double bond in water,the degradation behavior of PPEG-CCA-PCL in water was controllable,and it could be re-crosslinked after removing water.The dynamic polymer could be reprocessed by hot pressing.(2)In order to reduce the water absorption of dynamic polymer materials,improve the stability,and endow the materials with self-healing properties at room temperature,on the basis of the previous system,we selected polydimethylsiloxane(PDMS)with hydrophobicity,heat resistance,and cold resistance as the main chain of the polymer.A new self-healing polydimethylsiloxane elastomer(PDMS-CCA-TMP)was prepared by KC reaction with polydimethylsiloxane modified bis(benzaldehyde)(BA-PDMS-BA)as the main chain and tris((2-cyanoacetoxy)methyl)propane(TMP-TCA)as the crosslinking agent.The thermogravimetric analysis showed that the decomposition temperature of PDMS-CCA-TMP was 357 ~oC,and it had good thermal stability.The tensile test showed that the tensile strength and elongation at break of PDMS-CCA-TMP were 0.57 MPa and 266%,respectively,which had good tensile properties.Due to the existence of dynamic covalent bonds and the extremely low glass transition temperature(T_g=-117 ~oC),the elastomer had the ability of self-healing at room temperature and could also be reprocessed by hot pressing.The elastomer has good application prospects in the fields of adhesives and renewable flexible electronic devices.