Studies On Preparation And Properties Of Dithioacetal And Aminal Dynamic Covalent Bond Crosslinked Rubbers

Rubbers are strategically important materials,which have been widely used in tires,seals,wearable electronics and damping materials due to their excellent entropy elasticity.Crosslinking of rubbers is essential to acquire resilience and practical applicability.Thus far,traditional covalent crosslinking technologies are mainly adopted for rubber crosslinking,resulting in a permanently crosslinked network.The irreversible crosslinking networks cannot be recycled or reprocessed,causing the problems of resource waste and environmental pollution.To solve these issues,increasingly growing efforts have been focused on incorporating elegant dynamic covalent bond into chemically crosslinked network.The dynamic covalent bond can be reversibly broken and reconstructed under external stimuli(heat,light,p H,chemical stimulation,etc.)to alter the network topologies,imparting the crosslinked networks with the ability to be healed and recycled.In this thesis,two kinds of dynamic covalent bonds,dithioacetal and aminal,are introduced into epoxy natural rubber and butadiene rubber,respectively.Based on the dynamic features of these bonds,the covalently cross-linked networks can rearrange themselves at elevated temperatures,endowing the elastomers with healing and recyclable ability.The effects of crosslinking density and molecular weight of rubber precursors on the mechanical and dynamic properties of the samples are studied.The main work is as follows:Exchange reaction of dithioacetals in the absence of catalyst is demonstrated in small molecule model studies.Then,the as-synthesized 2,2'-(phenylmethylene)bis(sulfanediyl)diacetic acid(BTA)containing dithioacetal structure is utilized as a crosslinker for epoxidized natural rubber(ENR)based on the chemical reaction between carboxy and epoxy groups,by which the covalent crosslinking of ENR is achieved and the dynamic dithioacetals are introduced into the network.Due to the thermoactivated dithioacetal exchange reaction,the crosslinked ENR can release stress at elevated temperatures and acquire reprocessability.With the increase of BTA crosslinker fraction,the tensile strength and modulus of the crosslinked networks are consistently improved,and the stress relaxation rate is gradually decreased.A series of aldehyde group-terminated polybutadiene rubber(APB)with different molecular weight are prepared by epoxidation and chain cleavage reactions.Based on the reaction between the secondary amine of piperazine and the aldehyde group in APB,dynamic aminal bond crosslinked PB(PAPB)is obtained.Due to the exchange reaction of aminals,the crosslinked networks can be reprocessed and recycled.The molecular weights of APB can be adjusted by changing the degree of epoxidation and chain cleavage.With the decrease in molecular weight of APB,the modulus of PAPB samples is enhanced while the elongation at breaking decreases,and the healing efficiency decreased.The Ea value decreases initially and then increases due to the influence of two competitive factors,restricted network and high aminal bond concentration.