Microstructure Regulation And Properties Of Silicon Hybrid Polyurethane Composites

Silicon hybrid polyurethanes have a wide range of applications in coatings,adhesives,biological materials and many other fields due to their good thermal stability,chemical and biological stability,good water resistance and weather fastness.These silicon-containing polyurethanes tend to have unique microphase separation morphology.However,these morphology changes and the dispersion or aggregation morphology of soft and hard segments are still not well understood so far.To solve this problem,in this paper,a two-step method was used to introduce silicon-containing groups into the main chain and side chain of polyurethanes to construct silicon hybrid polyurethanes with different microphase separation forms.The distribution of soft and hard segments in the matrix was observed by energy dispersive spectroscopy analysis with silicon element as the hard segment label.The relationship between structure and properties was further explored through the evolution of micromorphology,thermal stability,mechanical properties and optical properties testing.In addition,polyurethane based conductive composites have been widely concerned for their advantages of flexibility,light weight and adjustable properties.In general,composites with excellent electrical conductivity often require a large amount of fillers,which will undoubtedly reduce the mechanical properties of composites,so reducing the percolation threshold of materials has been a key issue for conductive composites.Surrounding the key issue of regulating the low percolation threshold of conductive polyurethanes,different types or content of hard segments and curing processes were used to modulate the unique microphase separation structure of polyurethanes,which induced the selective distribution of graphite flakes and the formation of conductive networks,and obtained the conductive polyurethane composites with low percolation threshold and good electrical conductivity.The main research content is divided into the following three parts:（1）A series of main chain silicon hybrid polyurethanes（SPUs）elastomers were designed and prepared with diphenylsilanediol（DPSD）as chain extender by two-step method and different curing processes,and then graphite flakes（GFs）as conductive filler were used to construct conductive polyurethane composites through in-situ polymerisation.It was found that the microphase separation of SPUs was gradually enhanced with the increase of DPSD content,and the higher temperature curing process further promoted the microphase separation.Scanning electron microscopy（SEM）and energy dispersive spectroscopy（EDS）analysis further verified that the introduction of DPSD induced the formation of unique micromorphology and flower-like microcrystal structure of the hard segment,which endowed SPUs with reversible thermo-optical response behavior and significantly improved the mechanical properties（tensile strength:14.75MPa,elongation at break:293.31%）.In addition,the high temperature curing process and the introduction of silicon-containing groups could effectively regulate the microphase structure of polyurethanes and improve the distribution of conductive fillers,thereby promoting the formation of conductive networks of the composites,resulting in lower percolation threshold（1.41 wt%）and higher conductivity(3.67×10^-1 S/m at 8 wt%GFs addition).In conclusion,this work provided a meaningful reference for the design and construction of novel functional polyurethane materials by exploring the influence of main chain silicon on the structure and properties of polyurethane elastomers and their conductive composites.（2）A series of side-chain silicon hybrid polyurethane（s-SPUs）elastomers were designed and prepared by using N-[3-（trimethoxysilyl）propyl]ethylenediamine（AEAPTMS）as raw materials to produce isocyanate terminated silicone compounds as hard segments,and then GFs were used as conductive fillers to prepare conductive polyurethane composites through in-situ polymerisation.It was found that with the increase of AEAPTMS content,the proportion of disordered and ordered hydrogen bonds between hard segments increased,indicating that the degree of microphase separation increased.SEM and EDS showed that the changes of s-SPUs morphology were due to the effect of side chain alkoxy group aggregation,and the aggregation ability of side chain decreased with the increase of AEAPTMS content,which may be related to the restriction of chain migration due to the increase of intermolecular hydrogen bonds.For s-SPUs based conductive composites,a low percolation threshold（1.00 wt%）was obtained due to the aggregation of GFs forming linear conductive pathways,while the increase of AEAPTMS content would hinder the formation of conductive pathways and lead to an increase of percolation threshold.However,The higher temperature curing process could promote the redistribution of the filler,formed a more complete conductive network,reduced the percolation threshold,and increased the conductivity by nearly two orders of magnitude(2.96×10^-2 S/m at 5 wt%GFs addition).In summary,this work used the unique properties of the side chain siloxane group to effectively regulate the s-SPUs structure by changing the content of AEAPTMS and curing process,and explored the influence of the side-chain silicon group on the micromorphology and microphase separation,thereby improving the properties of s-SPUs and their conductive composites.（3）Based on the above works,AEAPTMS and DPSD were introduced into the side chain and main chain of polyurethane,and a series of multi-silicon hybrid polyurethanes（m-SPUs）were designed and prepared by different curing processes.Then,conductive polyurethane composites were prepared by using GFs as conductive fillers through in-situ polymerisation.It was found that with the increase of silicon content,the degree of microphase separation increased,and the higher temperature curing process led to further microphase separation.A unique sunflower-like microstructure was found in the SEM image,which was completely different from that before.According to the EDS results,it could be inferred that the special morphology was derived from the alkoxy silane groups in the side chain gathered around the microcrystalline structure formed by the main chain silicon groups,and endowed thermo-optic response performance of m-SPUs.For conductive composites,influenced by the synergic effect of main chain silicon groups and side chain silicon groups,linear conductive pathways were formed to reduce the percolation threshold at low filler content,and the dispersion of GFs was improved to increase the conductivity at high filler content.Furthermore,the percolation threshold（0.68 wt%）was obtained by higher temperature curing process to promote the re-dispersion of the fillers.The conductivity could reach 5.94×10^-2 S/m at 5wt%GFs addition.In conclusion,this work investigated the effects of main-side chain silicon on the microstructure and properties of segmented polyurethane elastomers and their conductive composites,providing a new idea for the design and construction of novel functional polyurethane materials.