| Waterborne polyurethane(WPU)and polyurethane-polyacrylate composite latex(WPUA)has been widely used in varies of fields of like waterborne adhesives,coatings and inks due to the advantages on environmental protection and comprehensive performance.In the material design of WPU and WPUA,crosslinking is one of the most used methods to improve the performance of products.On the other hand,the construction of dynamic covalent crosslinking networks by introducing dynamic covalent bonds is also much highlighted in current researches.The formation of crosslinking network with dynamic reversible character can endow the polymer not only the excellent thermoset-like properties but the signal responsive self-healing,reprocessing and recycling capabilities.It would be promising to construct dynamic crosslinking networks based on waterborne polymer matrix,which takes into account both environmental protection and resource conservation.In this dissertation,WPU and WPUA based on dynamic crosslinking systems were prepared in the form of one-pot two-component post-crosslinking system utilizing the flexible WPU synthesis and the heterophase characters of aqueous dispersion with acylhydrazone and disulfide bonds used as the dynamic covalent bonds,respectively.The relationship between the structure and properties of the two kinds of dynamic covalent crosslinking networks were systematically investigated.The main research contents of this dissertation are as follows:1.WPU dispersion was prepared by using ketone carbonyl containing1,3-dihydroxyacetone(DHA)as the chain extender.Then,the water soluble adipic acid hydrazide(ADH)was introduced post-added into the system,and the obtained one-pot two-component system based on WPU-DHA showed excellent storage stability.After film-casting,with the evaporation of water and amine,the chemical crosslinking bonds of acylhydrazone were formed through the reaction of ketone carbonyl groups on the polyurethane chains and the hydrazide groups on ADH.The particle sizes and stabilities of the dispersion were characterized by dynamic laser light scattering(DLS)and optical stability analyzer.The results showed that the particle size of the prepared WPU dispersion was distributed in a range of 45～60 nm.The mixed system of WPU-DHA and ADH was of good stability.The enhanced solvent resistance and gel contents of the films proved the post curing reaction.By the characterization of Fourier Transform Infrared spectroscopy(FTIR),it was found that the chemical crosslinking network based on reversible acylhydrazone bonds was formed.In addition,the acylhydrazone structure also contributed significantly to the strengthen H-bonding physical crosslinking network.Dynamic mechanical analysis(DMA)results showed that the degree of the film microphase separation increased with the increase of chemical crosslinking degree.The results of stress relaxation experiments suggested that the system behaved as the associative type of covalent adaptive networks(CANs)in the range of medium temperature(30～80℃).When the temperature rose to above 120℃,the thermo-dissociative chain exchange would occur.The structure evolution of the film during the heating process was revealed by using in-situ infrared spectroscopy and two-dimensional correlation(2D-COS)analysis.It was found that the two kinds of dynamic dissociation sequentially occurred and mutually related in response to the temperature elevation signals.The formation of the double crosslinking networks in the films could significantly improve the mechanical properties.When the content of DHA was 3 wt%,the tensile strength was increased from11 MPa(no-crosslinking)to 28 MPa,while the elongation at break was maintained above 760%.Importantly,the dynamic reversible characteristics of the networks enabled the films to be repaired from the completely cut-off state.The self-healing efficiencies based on tensile strength(H_σ)and elongation at break(H_ε)were more than 95%after 12 hours of repairing at 120℃.The films were cut into pieces and reprocessed by simple hot-pressing.It was seen that the macroscopic state and the mechanical properties of the film could be fully recovered after 2 min at the condition of 120℃and 5 MPa.The WPU-DHA was used as an adhesive for the bonding of metal aluminum plates,and showed high bonding strength.After four times tests of total destruction and re-bonding,the bonding strength was still steadily maintained.2.The miniemulsion polymerization was carried out by the use of the mixture of double bond end-capping WPU as the macromonomer,commercial acrylate monomers and functional monomer acetylacetoxyethyl methacrylate(AAEM)as the oil phase.After the optimize the WPU structure,the polyacrylate(PA)content and the soft to hard monomer ratios of acrylate monomers,a series of polyurethane-polyacrylate(WPUA)composite latex with different AAEM contents were prepared in the presence of water-soluble free radical initiator.And the one-pot two-component crosslinking system based on WPUA-AAEM was formed through the post addition of4,4’-diaminodiphenyl disulfide(DTDA).In the process of film formation,with the evaporation of water and amine,the-C=C-NH-crosslinking structure was formed by the reaction between acetyl acetoxyl and amino groups while the disulfide bond was integrated into polymer networks.The formation of the crosslinking structure and the incorporation of S-S bonds were confirmed by FTIR characterization.The morphology of WPU latex particles was studied by transmission electronic microscopy(TEM),which showed that the WPUA latex particles were of core-shell structure with WPU locating in the core and PA in the shell.The particle sizes were characterized by DLS,and the results showed that the particle size of the prepared latex were in the range of 90～150nm depending on the components.The compatibility between WPUA-AAEM and DTDA was confirmed by the optical stability analyzer by which the system presented good stability.The formation of chemical crosslinking network was verified by the solvent resistance and gel content tests.DMA characterization revealed obvious phase separation of PU and PA in the WPUA-AAEM films,but the degree of phase separation decreased with the increase of AAEM content.The reversible exchange of disulfide bond made the film display the characteristics of associative CANs.Moreover,the tensile strength of the films were improved from 8 MPa to 11 MPa by crosslinking.At the same time,the H_σand H_εof the cut-off films could reached 95%and more after self-healing at 70℃for 2h. |
