Engineering Dynamic Bonds Into Elastomeric Materials-Reinforcement And Reprocessability

Elastomers have become an indispensable part of day-to-day life.To acquire sufficient elasticity and mechanical strength for practical applications,elastomers need to be chemically crosslinked and are often reinforced by nanofilling.The latter one causes significant constraints in terms of nanofiller dispersion/aggregation,complicated interfacial regulation,and increased processing viscosity.The permanent crosslinks prevent elastomers from reprocessing and reuse,which makes end-of-life rubber products one of the major challenges nowadays faced by solid waste management and the circular economy.Therefore,it is highly desirable to explore alternative approaches to realize the reinforcement and closed-loop recycling of elastomers.In recent years,engineering dynamic sacrificial bonds to strengthen and toughen elastomers or incorporating dynamic covalent bonds for reprocessable elastomers have received widespread attention,and are considered to be very promising.This thesis presents several new approaches for strengthened and reprocessable elastomeric materials by programming dynamic bonds,in combination with interfacial engineering,heterogeneous or layered structural design.The main content of the present joint Ph.D.thesis is outlined as follow:(1)Sacrificial hydrogen bonds have been incorporated into a sulfur-crosslinked solution-polymerized styrene-butadiene rubber(SSBR)via triazolinedione(TAD)click chemistry.It was found that the“cage effect”of the pre-crosslinked SSBR facilitates the formation of hydrogen-bonded clusters,which are microphase-separated from the SSBR matrix.The clusters can not only act as hard reinforcements,but also play a role as sacrificial bonds,promoting energy dissipation and preventing stress concentration,and thereby significantly improving the mechanical performance of SSBR.With only 2.75 mol%of urazole groups(relative to double bonds in SSBR),the tensile strength and fracture energy of the modified SSBR are 2 and 3 times those of the unmodified SSBR,respectively.Additionally,SSBR with a high modification degree show promising thermally-triggered triple-shape memory behavior due to an additional broad transition temperature region associated with the clusters.Because of the high reactivity of TAD towards dienes,this strategy can be expanded to other diene rubbers.(2)By introducing pyridine-Zn²⁺-catechol coordination bonds at the interface of butadiene-styrene-vinylpyridine rubber(VPR)and graphene,rubber materials with greatly improved mechanical properties were obtained.When 5 wt%graphene(relative to the weight of VPR)was added,the tensile strength,300%modulus,and fracture energy of the resulting rubber are 4 times,3 times,and 6 times higher than those of pure VPR,respectively.It was found that the strong pyridine-Zn²⁺-catechol coordination bonds facilitate the uniform dispersion of graphene and efficient interfacial stress transfer.Moreover,they serve as sacrificial bonds under large deformation,which can dissipate a large amount of energy and facilitate chain orientation.This work presents a promising methodology for designing high-performance elastomers by engineering strong yet sacrificial bonds at the interface of an elastomer composite.(3)A C-N transalkylation of pyridinium has been demonstrated and used for the preparation of reprocessable covalently crosslinked elastomeric materials.After a model study for evidencing the exchangeable reaction,this chemistry was implemented at the interface of VPR and silica by using bromo-functionalized silica as crosslinking agent.Due to the strong covalent interfacial bridges,silica is well dispersed in VPR,and the obtained composites have robust mechanical properties,which improve with the increasing content of silica.Moreover,the materials are malleable and reprocessable at high temperatures attributed to network topology rearrangement induced by the interfacial C-N transalkylation reaction.After three times reprocessing,the materials can recover most of their initial mechanical properties.This work provides a promising dynamic covalent chemistry platform for the reprocessable materials containing pyridine groups.(4)A facile strategy to construct mussel byssus cuticle-like heterogeneous structure in elastomers towards mechanical reinforcement,functionalization and reprocessability,is demonstrated.Specifically,ground epoxidized natural rubber(ENR),highly crosslinked by dynamic?-hydroxyl ester bonds,was mechanically mixed with ENR and sebacic acid(SA),followed by hot-press molding.The resultant biomimetic heterogeneous vitrimeric elastomers(hetero-VEs)comprise a dispersed densely crosslinked hard phase and a continuous loosely crosslinked soft matrix.The two phases are well connected by covalent bonds attributed to the interfacial co-vulcanization and transesterification reactions.The hard phases serve as reinforcements,meanwhile they can deform upon loading,which can dissipate a large amount of energy.As a result,the mechanical properties of hetero-VEs are significantly improved,and they can be tuned by varying the weight ratio and the cross-linking degree of the two phases.With 9.5 wt%SA(relative to the weight of ENR)incorporated,the tensile strength and fracture energy of the resultant hetero-VEs are both 3 times those of its homogeneous counterpart.Moreover,when introducing CNTs into the soft matrix,the prepared hetero-VEs exhibit improvements of 8-10 orders of magnitude in the conductivity.In addition,the hetero-VEs can be reshaped and reprocessed at high temperatures due to the dynamic covalent crosslinks.Furthermore,the relatively high T_g of the hard phases combined with their malleability allows hetero-VEs with a high hard phase content to display thermadapt shape memory effects.*(5)Naturally occurring lipoic acid(LA),has been used to prepare recyclable ionic elastomers and nacre-mimetic nanocomposites based on these ionic elastomers by a straightforward water evaporation process.The physical properties of both,the obtained ionic elastomers and nanocomposites are tailored by varying the counter cations.Interestingly,the composites containing triethanolammonium cations show enhanced ductility with an elongation at break up to about 16%when compared to other reported nacre-mimetic nanocomposites with similar montmorillonite(MTM)content.Furthermore,the ionic elastomers can be reversibly depolymerized/repolymerized by diluting and evaporating.This dynamic property of the ionic elastomers and the supramolecular architecture within the nanocomposites enable water-assisted self-healing and closed-loop recycling for materials with up to 70 wt%MTM.The mechanical performance of the recycled nanocomposites is consistent with that of the original one.In addition,the nanocomposites were found to exhibit flame-retardant properties attributed to the high MTM content and their layered arrangement.