The Formation And Evolution Of Microstructures In Thermoplastic Polyurethane

Thermoplastic polyurethane(TPU)is a kind of linear segmented copolymers that contains alternating soft and hard segments.Soft segments are formed by macrodiol,while hard segments are constituted by diisocyanate and chain extender.Generally,thermodynamic incompatibility between soft segments and hard segments results in microphase separation,leading to the formation of soft domains and hard domains.Such microstructures enable TPUs to exhibit both the elasticity of rubbers and the processibility of thermoplastics,as the hard domains serve as physical cross-linking points.Notably,TPUs are endowed with remarkable mechanical properties,weatherability,hydrolysis resistance,biocompatibility,and so forth.Due to diverse chemical formula,TPUs could be processed into different products with various applications and thus widely applied to national economy fields,such as automobiles,textile,constructions,medical industry,and so on.Therefore,optimizing processing methods and enhancing product performance are crucial.This requires a deep understanding of the microstructure formation and evolution,as well as the establishment of the structure-property relationship in TPUs.In this thesis,prototype polyether TPUs with different hard segment contents are employed.Fast scanning chip calorimetry(FSC)is applied to investigate the dynamics of enthalpy relaxation and crystallization.By means of X-ray diffraction/scattering(WAXD/SAXS/USAXS)techniques,DSC and time-domain NMR(TD-NMR),we explored the effects of temperature,strain and blending on microstructures in TPUs.The main conclusions are summarized as follows:1.The dynamics of enthalpy relaxation and crystallization,as well as the effect of crystallization on enthalpy relaxation in TPU wereelucidated by employing FSC experiments.In single-step annealing,different thermodynamic behaviors between enthalpy relaxation and crystallization wereobserved due to molecular motions with different scales.Enthalpy relaxation originates from local motion of chain segments,while crystallization result from nucleation and growth of polymer chains.Depending on annealing temperature,a transition from α-relaxation to β-relaxation can be observed.Moreover,enthalpy relaxation leads to an increase in Tg due to the denser amorphous phase.Conversely,the crystallization resulted in the reduction of Tg in that phase separation causes change in the amorphous phase composition.Double-step annealing revealed that crystallization reduces enthalpy relaxation in two ways:(1)the change in amorphous composition results in the change in Tg,(2)crystallization restricts segmental motion.FSC results,combined with microbeam SAXS results,show that the system crystallizing at a lower temperature possesses a stronger suppression effect on enthalpy relaxation despite having the same crystallinity.This was because system with lower crystallization temperature has a larger specific surface area that restrains chain motion.2.Temperature-induced evolutions of microstructure and chain dynamics in TPU were studied by DSC,in-situ synchrotron SAXS and time-domain NMR(TD-NMR)experiments.For TPU with low hard segments content,the sequence length of hard segments was approximately four MDI-BD units,as calculated by reaction kinetics,SSA experiments and SAXS results.It is indicated by in-situ SAXS and TD-NMR results that the glassy fraction of hard blocks softens first,followed by the melting of nanocrystals,accompanied by the mixing of hard segments and soft segments.In the melt,hard and soft blocks are mixed on a scale above nanometers.Nonetheless,in the TPU with high hard segments content,hard domains undergo melting and recrystallization into more stable crystals and hard domains with better thermal stability.3.The multi-scale structure evolutions of TPUs with different hard segment contents were investigated under different temperatures and strain rates utilizing realtime WAXD,SAXS and USAXS.During stretching,soft domains deform first,followed by destruction and reorganization of hard domains,resulting in the formation of strain-induced crystals and fibril-like structures.Fibril-like structures then slip past each other and be destructed until breaking.At large strains,the occurrence of stresswhitening phenomenon was caused by craze,as shown by USAXS results.Crazes were induced by the defects in fibril-like structures.Furthermore,the effects of strain rates on domain structures were demonstrated by SAXS.The microstructure evolutions at high speeds are similar to those at low speeds,but there is a delay in the change of microstructures at high-speed stretching due to the viscoelasticity of the polymer.Additionally,high strain rates induce microstructure defects,which aggravate the strain-whitening phenomenon.4.The microstructures of TPU,micro-crosslinking thermoset polyurethane(PU)and a TPU/PU blend(weight ratio 3:2)were studied by TD-NMR,DSC,SAXS and Raman spectra.PU sample presents a very weak degree of phase separation.The hard domains content in blend sample is lower than that in pristine TPU sample.Interestingly,the results show that the physical structure is not simply fragmented during blending process,as new hard domains form during this process.SAXS results also demonstrate that multiple melting peaks observed during DSC heating scans of the TPU sample originated from melting and recrystallization.And the degree of melting and recrystallization in TPU was higher than that of blend.Heating experiments in combination with Raman spectra indicate that the hydrogen bond content in blend sample is similar to that in TPU sample.This suggests that the hard segments in TPU and PU can link through hydrogen bonding,leading to the formation of new hard domains and ultimately resulting in a homogenous mixture.Therefore,the blend sample can be processed by injection molding.