The Role Of Diisocyanate Structure On The Synthesis And Microphase-separated Morphology Of Polyurethane

Polyurethanes, formed by the step growth polymerization of isocyanates and polyols, proveto be a highly versatile class of materials having applications such as foams, elastomers, coatings,adhesives, fibers and biomaterials. Polyurethanes are composed of soft segments and hardsegments arranged alternately. The soft segments with a low glass transition temperature （T_g）form the continuous matrix, which exhibits low temperature flexibility. The hard segments withhigh T_gor melting points （Tm） tend to self-assemble into domains through physical crosslink.These domains mostly act as reinforcing filler in the low T_gcontinuous matrix and improve theproperties of the material, such as mechanical, thermal performances and solvent resistance. Theexcellent performances of polyurethane are generally attributed to their morphology structure ofthe materials formed by their special chemical structure and processing conditions. Understandingthe structure-property relationship is crucial in the development and application of thesematerials.Polyurethane prepolymer prepared from nonequivalent amounts of toluene diisocyanate（TDI） over trifunctional trimethylolpropane （TMP） was followed by gel permeationchromatography （GPC）. Steric hindrance of TMP is considered as the main factor affecting themolar mass distribution, especially in the higher molecular weight region. An optimum reactioncondition is the initial NCO/OH ratio of3.0and the reaction temperature of50℃. Thenpolyurethane prepolymer could be purified through the thin film evaporator with excellentproperties. The components, such as TDI, TMP-TDI, TMP-2TDI, TMP-3TDI,2TMP-5TDI and3TMP-7TDI, are observed in GPC analyses, and the results are further verified by electrosprayionization mass spectrometry （ESI-MS）. Additionally, other side products such as urea andallophanate are presented in ESI-MS and1H NMR analyses. The formation of allophanates ishighly dependent on reaction temperature. These results indicate that the quantitative analyses forthe TDI-TMP based polyurethane prepolymer do not only favor to avoid or reduce the sidereactions, but also supply fundamental data for designing thin film evaporator and controlling theseparating process.Reaction kinetics of TDI-TMP polyurethane prepolymer at various temperatures is studied. The progress of the polymerization reaction was monitored by measuring the concentration ofisocyanate groups and TDI isomers by means of back-titration and High Performance LiquidChromatography （HPLC）, respectively. The kinetics of dropwise addition method, compared withthe conventional one-shot method, is well described by a second order equation. This procedure isoptimized by comparing the deviations between experimental data and theoretically calculateddata. The effects of temperature, initial stoichiometry and TDI isomers on the amount of excess2,4-TDI and2,6-TDI were investigated. Three commercially available TDI mixtures, that is,65:35,80:20and100:0ratio of2,4-TDI/2,6-TDI respectively were used. A recycling model ofunreacted TDI isomers and solvent is established to reach a stable process and yield polyurethaneprepolymer with good reproducibility. This model has been applied in the chemical plant toprepare polyurethane prepolymer with precisely defined chemical compositions in a continuousprocess.Three diisocyanates with different symmetry and planarity,2,6-TDI,2,4-TDI and4,4’-diphenylmethane diisocyanate （MDI）, were used to synthesize polyureas in solution. Theeffects of diisocyanate symmetry on the phase separated morphology, hydrogen bonding behaviorand molecular dynamics are studied. The symmetrical diisocyanate structure allows excellentself-assemble of hard segments into ribbon like domains by strong bidentate hydrogen bonding.The strongly microphase separated domains in these polymers act as physical crosslinks, and they areexpected to strongly influence molecular dynamics. Small-angle X-ray scattering （SAXS） was utilizedto quantify microphase separation characteristics, and broadband dielectric relaxation spectroscopy（DRS） revealed the sensitivity of polymer dynamics to the presence and changes in microphaseseparated domains in the polymers. SAXS results indicate that the degree of microphase separationis enhanced by the symmetric diisocyanate structure. DRS results show that the soft segmentmobility is significantly reduced due to the hard segment mixing into soft matrix when thesymmetry of the diisocyanate is decreased from2,6-TDI to2,4-TDI, although no pronounceddifference of soft segments T_gis detected in differential scanning calorimeter （DSC）. Thedielectric strength of Maxwell-Wagner-Sillars （MWS） interfacial polarization process in allcopolymers displays a temperature independent plateau extending well over220℃because of the presence of strong bidentate hydrogen bonding between urea containing segments.The temperature dependence of hydrogen bonding for polyureas based on2,6-TDI and2,4-TDI and MDI has been studied by Fourier transform infrared spectroscopy（FTIR）. Theresults show that urea linkages decompose above170℃, and two new peaks at1727cm^-1and1678cm^-1associated with the formation of biuret are evident. At a lower annealing temperature at150℃without side reactions, TDI based polyureas exhibit reversible behavior for hydrogenbonds during heating and cooling processes. The intensity of ordered urea carbonyls decreaseswith temperature while the intensity of the free and disordered urea carbonyls increase gradually.However, in MDI based polyureas with noncoplanar diisocyanate structure, thermal annealingcauses the rise of the ordered urea carbonyls due to the “quenched” morphology formed bysolution casting, consistent with the increased length and width of ribbon like hard domainsobserved in AFM. In combination with the findings from Wide-angle X-ray diffraction （WAXD）and SAXS, the results suggest that the microstructure is also highly dependent on the annealingtemperature. DSC analysis demonstrates that the soft segment T_gdecreases with increasingannealing temperature, indicating a higher degree of microphase separation morphology formed.