| Nylon 12(PA12),a long carbon chain polyamide,contains a polar amide group(-CONH-)and eleven methylene chains in its main chain.The special chemical structure gives PA12 excellent performance,so that it maintains the traditional polyamide high strength,wear resistance and chemical corrosion resistance at the same time,but also has lower water absorption and higher flexibility performance than the traditional short-chain polyamide,etc.This effectively improves the common polyamide high water absorption disadvantage,so that it can still maintain good dimensional stability in wet environment.It can be widely used in automotive industry,electronic and electrical appliances,machinery and medical fields,and has strong competitiveness among general-purpose materials.On the other hand,PA12 crystallization is influenced by factors such as hydrogen bonding,chain folding and molecular chain conformation,and there is usually homogeneous polycrystallization,and its crystallization process strongly depends on factors such as temperature,solvent and stress.Therefore,exploring the formation conditions and influencing factors of PA12 polymorphism form can help to further understand the structural properties of PA12.The effects of solvent and crystallization temperature on PA12 polymorphism form were firstly explored,followed by the effects of small molecules(urea and cyclodextrin)that can interact with amide groups on PA12 solution crystallization,and a series of characterizations on the behavior of PA12 crystals in terms of crystal structure,thermal properties and phase transition were carried out to provide fundamental theoretical support for PA12 research.The main research results are as follows:1.The crystal forms of nylon 12 from solution crystallization are related to the crystallizing temperature and the solvent.Nylon 12 solutions were prepared by dissolved it in different solvents,and then casted them into films and isothermally crystallized at varied temperatures.The resulted samples are characterized by wide-angle X-ray diffraction,differential scanning calorimetry(DSC)and Fourier transform infrared spectroscopy,with aim of exploring the influences of solvent and crystallization temperature from the perspective of polycrystalline formation.The results show that with the phenol/formic acid(v/v=1/1)as the solvent the α crystal is formed at room temperature,and the γ crystal is the result at 80 ℃ and above.However,the α crystal coexists with γ crystal when isothermally crystallized at the range of 40-70 ℃,and the content of α crystal gradually decreases with the increasing of crystallization temperature.When the formic acid/dichloromethane(v/v=1/1)is used as the solvent,the white opaque powder with αcrystal form is obtained at room temperature,whereas a translucent film with γ crystal form is obtained at 60 ℃.The DSC results show that the higher the crystallization temperature,the higher the melting point of the α crystal is,and part of α crystals is transferred into γ crystals by Brill transition during the heating process.The peak melting temperatures of the α and γ crystals obtained from the formic acid/dichloromethane solution are higher than those of ones obtained from the phenol/formic acid solution respectively.2.Based on the interaction between urea and amide groups,a series of PA12/urea complexes with different stoichiometric ratios were prepared by adding urea to formic acid/dichloromethane solutions of PA12 and extracting pure PA12 crystals from the corresponding complexes.The formation of PA12/urea complexes was confirmed using WAXD,DSC and FTIR,and the structure,properties and phase transition behaviour of the purified PA12 crystals were characterised.It was found that when the molar content of urea in the complex was low(1-4 mol),the purified PA12 crystals had a double melting peak and a possible Brill transition during DSC warming,showing two melting peaks at 12.3 °C and 12.1 °C above the melting points of pure α and γ crystals of PA12,respectively.When the molar content of urea in the complexes is high(5-15 mol),only one melt peak exists in the purified PA12 crystals,with a melting point 13 °C higher than that of the PA12-γ crystals.The WAXD results show that in addition to the α and γ crystals in all the purified PA12 crystals of the complex,new strong diffraction peaks(2θ of 22.5°)and shoulder peaks(2θof 23.1°)distinct from the α and γ crystals are present,indicating that urea affects the crystalline structure of PA12 and that the increase in its melting point indicates that the molecular chains in the PA12 crystals are better crystallised with the aid of urea,possibly have a relatively extended conformation,resulting in its melting peak appearing in the higher temperature region.However,the fine structure of the PA12 crystals formed remains to be analysed further.3.Both urea and cyclodextrin,as small molecular bodies with a cavity structure,can form inclusion complexes with polymers.The PA12/urea complex system has been prepared in the above study,but the mechanism of the formation of this complex and the fine structure of PA12 crystals obtained from the complex still need to be further explored.Therefore,we prepared PA12/γ-CD complex by adding γ-cyclodextrin(γ-CD)to formic acid/dichloromethane solution of PA12 and extracted pure PA12 crystals from the complex.The formation of PA12/γ-CD complexes was also confirmed by WAXD,DSC and FTIR,and the behaviour of the purified PA12 crystals in terms of structure,properties and phase transition was characterised and compared with the PA12/urea complex system.The differences with the PA12/urea complex system are:(1)the DSC spectra show that both the PA12/γ-CD complex and the PA12 crystals proposed from this complex have a higher melting temperature;(2)the WAXD shows that the PA12/γ-CD complex does not show new weak diffraction peaks,which may be related to the different mechanisms involved in the formation of the complex by the urea/cyclodextrin and polymer chains.The PA12/urea complex and PA12/γ-CD complex are similar in that(1)when the molar content of the main small molecules(urea and cyclodextrin)is relatively low,both of their DSC spectra show double melting peaks,and when the molar content of the main small molecules(urea and cyclodextrin)is high,only one melting peak exists in their DSC spectra;(2)The WAXD spectra showed that almost all the purified PA12 crystals contained,in addition to α and γcrystals,new strong diffraction peaks(2θ of 22.5°)and shoulder peaks(2θ of 23.1°),which were different from α and γ crystals.These results suggest that cyclodextrins have influenced the crystalline structure of PA12,resulting in a more extended conformation and tighter arrangement in the restricted space formed by cyclodextrins,which is reflected in the higher melting point in terms of physical properties.However,the fine structure of the PA12 crystals formed is yet to be analysed further. |
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