Study On Physics Of Hydrogen-Bonded Semi-Crystalline Polymer Stretch Processing

Due to the existence of hydrogen bonds,the hydrogen-bonded semi-crystalline polymers have excellent heat-resistance and mechanical properties,which are widely used in high-tech fields.However,the hydrogen bond hinders the arrangement and orientation of polymer chains,which brings great difficulties to the processing of hydrogen-bonded semi-crystalline polymer materials.The processing of hydrogen-bonded semi-crystalline polymer materials involves the tensile deformation process under complex conditions.The tensile deformation process is an important process for polymer materials to obtain high orientation and high performance.Therefore,studying the structural evolution of hydrogen-bonded semi-crystalline polymers during tensile deformation under different external field conditions can help us grasp the influence law of hydrogen bonds on the structural evolution of semi-crystalline polymers and understand the effects of tensile on hydrogen bonding network during tensile deformation,which can provide the theoretical basis and technical support for the improvement of the processing and performance of the polymer materials.However,no systematic study focused on the structural evolution of hydrogen-bonded semi-crystalline polymer materials during tensile processing,and the effects of hydrogen bond on the structural evolution of the polymer materials during processing is still unclear.In this study,polyamide 46?PA46?and polyamide 66?PA66?are taken as representatives of flexible chain hydrogen-bonded semi-crystalline polymers,and cellulose triacetate?TAC?is taken as representative of rigid chain hydrogen-bonded semi-crystalline polymers.The structural evolutions of the above hydrogen-bonded semi-crystalline polymer and the effects of water vapor with different conditions on the structural evolution of TAC film during the tensile deformation process were studied by in-situ synchrotron radiation wide-angle X-ray scattering?WAXS?,Fourier transformation infrared?FTIR?and other technologies,which promote the further understanding of the physical problems such as stretched-induced phase transition,the effects of stretching on the hydrogen bonding network,and the tensile deformation mechanism of the hydrogen-bonded semi-crystalline polymer.Besides,this study provides a theoretical basis for the processing of hydrogen-bonded semi-crystalline polymer materials to obtain high-performance.The main research contents and results of this study are as follows:?1?Using in-situ synchrotron radiation WAXS,in-situ FTIR and dynamic mechanical analysis?DMA?,the structural evolutions of PA46 films during the tensile deformation in the temperature range of 37-249?were studied.The experimental temperature range can be divided into three regions.In region I?37??T<180??,stretch enlarges the d-spacing gap between?100?and?010/110?planes of?phase?triclinic?,which is opposite with that during heating.In the interval II?180??T<230??,stretch induces a reverse Brill transition,which is characterized with the single diffraction of?phase?hexagonal?splitting into the two diffractions of?phase.The hydrogen bonds arrange from disorderly to orderly.Whilst further increasing strain drives the remerging of the splitting two diffractions into a single diffraction.As in-situ FTIR measurements reveal the continuously increasing content of trans conformation,we speculate that stretch may induce a new form,which is an intermediate structure close to both?and?phases?named as??phase?at large strain.In region III?230??T?249??,the?phase may skip?phase and directly transform into??phase.The above research results indicate that the effect of stretching on the crystal structure of PA46 is opposite to that of heating and stretching can induce the reverse Brill transition of PA46.In this work,the non-equilibrium crystalline phase diagram of PA46 in strain-temperature and true stress-temperature spaces and the schematic diagram of the structural evolution during stretching at different temperatures were provided,which are expected to provide theoretical guidance for the processing of PA46.?2?The structural evolutions of the PA66 film during stretching at different temperatures were studied by using in-situ synchrotron radiation WAXS and FTIR.The experimental temperature ranging from 34 to 213?can be divided into four regions.In region I?34??T<78??,stretching increases the gaps between the diffractions of?100?and?010/110?planes of the?phase?room-temperature triclinic structure?,which is opposite to the trend of that during heating.In regions I and II?78??T<185??,stretching induces the transition of??phase?high-temperature triclinic structure?to?phase?room-temperature triclinic structure?,and the d-spacing of?002?plane shows a rapidly increase before the transition.Furthermore,FTIR result indicates that the content of trans conformation increases with strain.In region IV?185??T?213??,stretching induced the reverse Brill transition with the single diffractions of?phase splitting into the double diffractions of??phase,and meanwhile the corresponding absorption band of trans conformation in FTIR curves appears.As the strain further increases,the content of trans conformation continues to increase.With the strain reaching a value,the stretch-induced??-?phase transition occurs.The above results indicate that the effect of stretching on the crystal structure of PA66 is also opposite to that of heating and stretching can induce the Brill reverse transformation of PA66,which are the same as the conclusion of PA46.It further validates the conclusion of PA46 and shows that these conclusions may be the commonality of polyamide.In addition,stretching can also induce the transition of the high-temperature triclinic structure???phase?to the room-temperature triclinic structure??phase?for PA66.In this work,the non-equilibrium crystalline phase diagram of PA66 in strain-temperature space and true stress-temperature space and the model diagram of the structural evolution of PA66 during stretching at different temperatures are constructed,which are expected to provide assistance for the processing of PA66.?3?Through in-situ synchrotron radiation WAXS,DMA and DSC,the structural evolutions of the TAC film during stretching at different temperatures were studied.The research results show that the TAC film has different structural evolution mechanisms during stretching in different temperature ranges.In the temperature range of 60 to 125?,the structure evolutions of the TAC film during the stretching process are severely affected by the moisture absorption,which can weaken the influence of hydrogen bonds,resulting in a large change in crystallinity.At temperatures around T_g?195??,stretch-induced orientations for crystal can be achieved more easily although₍?8? and crystal size have no large change compared with the flexible-chain semi-crystalline polymers like polyethylene.Interestingly,the TAC film presents various deformation mechanisms including the elongation of amorphous and slipping,formation,destruction and recrystallization of crystals during stretching at different temperatures,which are often observed for the flexible-chain semi-crystalline polymers.This work promotes the understanding of the deformation mechanism of rigid chain hydrogen-bonded semi-crystalline polymers and the effect of hydrogen bonding on the structural evolution of the polymers during tensile deformation,which can provide valuable guidance for the post hot-stretching process of TAC retardation films.?4?Using the home-made water vapor stretching rheological device,the effects of water vapor with different temperatures on the structure and properties of TAC films were investigatedwere carried out by in-situ synchrotron radiation WAXS,DSC,DMA and FTIR.The results show that the presence of water vapor can weaken the hydrogen bonds between molecular chains and effectively increase the fracture strain of the TAC film.Furthermore,the higher temperature of the water vapor results in the larger fracture strain of the TAC film.Compared with the stretching of TAC film in a dry environment,the existence of water vapor can improve the movement ability of the TAC molecular chain,and can also improve the stability and uniformity of the evolution of the crystallinity and crystal size during the stretching process,which is beneficial for TAC film to obtain a uniform structure.As the temperature of water vapor increases,the hydrogen bonds between the molecular chains is further weakened,and the movement ability of TAC molecular chains is further improved.The orientation of TAC film obtained in water vapor at 140?is comparable to that obtained in dry air at high temperature around T_g?195??.The birefringence results show that the TAC film stretched in dry air at temperature between 80 and 140?can obtain a certain reverse wavelength dispersion,while the stretched in water vapor at lower temperatures?60 and 80??can obtained a better reverse wavelength dispersion.This work indicates that the mechanical properties,optical properties,and structural evolution during stretching of TAC films can be effectively controlled by the water vapor at temperatures much lower than T_g,which is expected to provide new method for the post-processing of TAC retardation films.In summary,the influence of hydrogen bonding on the structure of the flexible chain hydrogen-bonded semi-crystalline polymer is mainly reflected in restricting the movement ability and changing the conformation of the molecular chain,while that of rigid chain hydrogen-bonded semi-crystalline polymer is mainly reflected in limiting the movement ability of the molecular chain due to the limitation of the rigid main chain.In aliphatic polyamides,the effect of stretching on the hydrogen-bonded network structure is opposite to that of heating and stretching can induce Brill reverse transition.For TAC,the presence of rigid chains weakens the influence of hydrogen bonds on its structure.Water vapor can effectively weaken the hydrogen bonds between molecular chains and improve the processing performance of this kind of polymer materials.