Study On The Structure And Rheological Properties Of Polyoxometalates/Polymer Composites

Elastomer materials are usually classified as thermosets and thermoplastics.The thermosets "set" in shape after the thermal curing.In comparison,the thermoplastics are more environmental friendly:they are reprocessible and thus can be reused many times.Commercially available thermoplastics are usually crosslinked via glassy or crystalline blocks,and thus their processing windows are usually relatively fixed:they are set by the glass transition or crystalline melt temperatures of the corresponding blocks.Supramolecular networks are transient networks based on noncovalent bonds,such as ionic interactions and hydrogen bonds.These networks attracted a lot of attention in the past few decades thanks to the excellent properties,including dynamic reversibility,self-healing ability,and stimuli-response property.This thesis is focused on materials that combine advantages of both traditional thermoplastic elastomers and reversible supramolecular networks based on polyoxometalates(POM,Keggin structure)/polymer interactions.The main research contents are as follows:Firstly,an ABA-type copolymer,poly(vinyl imidazole-b-n-butyl acrylate-b-vinyl imidazole),was synthesized through the reversible addition-fragmentation chain transfer(RAFT)polymerization.The two short vinyl imidazole(?m)blocks were short to ensure low ?N and accordingly no phase segregation.The introduced POM protonated A monomer,and the strong electrostatic attraction between protonated A monomers and macroions(from POM)induced the phase segregation.The average size of the microdomains after the phase segregation,seen in the X-ray scattering,and the plateau modulus and solid-liquid transition temperature,seen in the linear viscoelasticity,strongly rely on the content of POM and thus they could be adjusted over wide ranges.In particular,the solid-liquid transition temperature enabled the quantitative estimation of the electrostatic energy,based on which the relationship between structure and rheological properties was established.The second part of work switched the focus on structure and property of the intrinsically phase-segregated ABA block copolymer crosslinked by POM.To this end,poly(4-vinyl pyridine)(P4VP)and poly(n-butyl acrylate)(PnBA)were chosen as the A and B blocks,respectively,and sufficient ?N was chosen to ensure their segregation.In addition,small amount of POM was introduced so to selectively crosslink only a fraction of the P4VP blocks.In other words,only a fraction of P4VP blocks are protonated and attracted electrostatically by POM.These blocks exhibit relaxation time much longer than those unprotonated P4VP blocks.This dynamic contrast enabled the materials to exhibit the double relaxation behavior,where the fast and slow relaxation processes were controlled by ?N and the electrostatic interaction energy,respectively.The plateau modulus of these two processes were predicted quantitatively using the relative ratio of the two groups of P4VP blocks(crosslinked and uncrosslinked by POM).Finally,we examined the structural and rheological properties of random copolymer crosslinked by POM.we synthesized the poly(2-vinyl pyridine-co-styrene)(P2VP-PS)via RAFT polymerization,and varied the content of 2VP that can be crosslinked by POM over a wide range from 0 to 1.The introduced POM protonated the 2VP monomers,and the electrostatic attraction between the protonated 2VP monomers and POM induced the phase separation.Three types of phase-segregated structures were obtained with increasing the 2VP content:(1)the isolated 2VP-rich domains in the continuous S-rich phase,(2)bicontinuous structure composed of 2VPrich and S-rich phases,and(3)isolated S-rich domains in the continuous 2VP-rich phase.Analysis of the linear viscoelasticity indicated that the glassy part of modulus was governed by the continuous phase,and the terminal relaxation seen in the rubber part of modulus was controlled by local Tg as well as the average number of 2VP/POM interactions per chain.