| Waterborne polyurethanes(WPU)have shown increasingly applications in coatings,adhesives and jointing resins for polymer composites due to their environmental friendly benefits,diverse copolymerization structures and superior performance.However,on the one hand,how to overcome the performance degradation caused by the introduction of hydrophilic components in WPU through the relatively simple copolymerization design has been a long-standing research topic in related fields;on the other hand,the application of tailoring mofits with spatial recognition function to induce higher ordered aggregation structures of small molecules,macromolecules and colloidal particles is a current research hotspot nowadays although few studies have been involved concerning the applying of aggregation induced motifs in WPU systems with a strong intrinsic tendency to amphiphilic association.In this dissertation,a multi-step chain extension(MCE)method based on the stepwise copolymerization reaction of WPU was designed to improve the strength,toughness,hydrophobicity and interfacial adhesion of WPU with para-aramid fabrics by using a small molecule urea with di-blocked hydroxyl endgroups as one of the chain extenders accompanied with the mixed polyol.As a further exploration of the MCE method,amino end-capped oligomers prepared by the copolymerization of terephthaloyl chloride and pentamethylene diamine were used also as one chain extender for the copolymerization of WPU,and the assembly of WPU macromolecules in selective solvents as well as the particle association behavior in aqueous dispersion environment are both investigated under the induction of semiaromatic amide motifs.The main contents of this dissertation are as follows:(1)One novel chain extender of 1,3-bis(hydroxymethyl)urea(DMU)was used to introduce urea groups into the main chain of WPU through the chain extension reaction of hydroxyl and isocyanate.By using isophorone diisocyanate(IPDI)as the isocyanate monomer,polytetrahydrofuran ether diol(PTMG)and polycaprolactone diol(PCL)as the mixed polyol,2,2-dihydroxymethylbutyric acid(DMBA),(1,4-butanediol)BDO,DMU and aminopropyl terminated polydimethylsiloxane(PDMS)as the chain extenders in separated chain extension steps,the MCE method was established for the preparation of WPU.Dynamic light scattering(DLS),Fourier transformed infrared spectroscopy(FTIR),dynamic mechanical analysis(DMA)and tensile mechanical tests were applied to systematically study the effects of DMU dosages,chain extender types,chain extension sequences and mixed diol ratios on the particle size and distribution of WPU dispersion,the distribution of different carbonyl hydrogen bonding/non-bonding states,the microphase separation structures and the physical properties of WPU films.It has been found that when the ratio of PTMG/PCL was controlled at 6:4,and2% of DMU and 1% of PDMS was incorporated as chain extenders in the third and fourth chain extension steps,respectively,the WPU films exhibited not only excellent tensile strength of 30 MPa,elongation of break of about1300 %,improved hydrophobicity indicated by the water contact angle of 98°,but also effectively modified interfacial adhesion to para-aramid fabrics.The peeling strength of the joint based on the polysiloxane incorporated WPU after four steps of chain extension was 430% higher than that prepared through only two steps of chain extension.Moreover,about 44% of the peeling strength was sustained after the joint had been boiling for 40 min in water,suggesting the potential resistance to heat and humidity.(2)Further based on the MCE method,WPU modified with semiaromatic amide motifs were prepared by using the amino-terminated oligomers(PASA)prepared by the low-temperature condensation of plantderived biogenic pentamethylene diamine with terephthaloyl chloride as the chain extender in the third step of the chain extension reactions.The assembly of polyurethane macromolecules in selective solvents and the association behavior of polyurethane colloidal particles in aqueous dispersion were both investigated by transmission electron microscopy(TEM)and rheological analysis.It has been found that morphologies of the molecular self-assemblies in the selective solvent were sensitive to the kinetic and thermodynamic equilibrium conditionss,and evolved from nanoparticles,nanoribbons,nanosheets to nano-octopuses when the polymer concentration,non-solvent content and addition rate had been changed.In the appropriately diluted aqueous dispersion of the same WPU,dendritic particle aggregations were observed under the amide hydrogen bonding association of PASA segments.For the WPU dispersion with the solid content of 15%,the interparticle hydrogen bonding network contributed to the enhanced dispersion viscosity and modulus,which increased as the content of PASA increased,showing the self-thickening effect.The effect was more obvious when the hard segmental content of WPU was also properly increased.But when the hard segmental content was increased to 7/1,the viscosity and the modulus would show the descending trend.However,further evolution of the interparticle association could also be achieved by the introduction of 1D para-aromatic amide nanofibers,whose incorporation strengthened the associated network formed by dispersed nano-objects. |
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