Study On Crystal Structure Of Nylon 12 During Heating,Cooling And Thermal Drawing

Nylon 12（PA12）is a long carbon chain nylon,with many good properties,such as very low water absorption,low density,good flexibility,good dimensional stability,insulation,low temperature resistance,oil resistance,chemical stability,good abrasion resistance,low melting point,and easy molding and processing.Accordingly it is widely used in various fields.However,there are relatively few articles using wide-angle X-ray diffraction（WAXD）to study the change of crystal structure of PA12during heating,cooling and thermal drawing.In this work,the crystal transition,d-spacing,crystallinity,crystallite size and crystal orientation of PA12 under different conditions were studied,for a more comprehensive understanding of the structural information of PA12.This study not only provides theoretical support for the basic research of PA12,but also provides theoretical basis for its application and processing.PA12 samples were prepared in three ways:solution casting at different temperatures,slow melt cooling and melt quenching.First,the samples were characterized by wide-angle X-ray diffraction（WAXD）,differential scanning calorimetry（DSC）,Fourier transform infrared spectroscopy（FTIR）,and mechanical properties test.Then,the PA12 samples was heated through WAXD online heating,and the effect of heating on the crystal transition,d-spacing,crystallinity,and crystallite size of the PA12 samples were analyzed.Afterwards,the PA12 samples was heated to the temperature above the melting point through WAXD online heating for 5 minutes to remove the thermal history and then cooled.The effect of cooling on the crystal transition,d-spacing,crystallinity,and crystallite size of the PA12 samples were analyzed.Finally,by equipped with tensile platform and thermal platform of Wide-angle X-ray diffractometer,?and?’-PA12 were drawn different drawing ratios（?=1.00、1.25、1.50、1.75、2.00、2.25、2.50、2.75、3.00）at different temperatures（30?C、50?C、110?C）.The effect of the temperature and drawing ratios on the crystallinity,orientation,crystallite size and tensile force of the samples were analyzed.The following main conclusions are drawn as follows:（1）WAXD tests were performed on the PA12 samples prepared by three methods of solution casting at different temperatures,slow melt cooling and melt quenching.It was found that the sample casted at 30?C was the?-form.The samples casted at 40?C to 80?C were the（?+?）-form.The samples casted at 90?C and above were the?-form.The sample obtained from slow melt cooling was the?-form.The sample obtained from melt quenching was the??-form witch belongs a metastable structure of the?-form.（2）DSC tests of the samples showed that there were two melting endothermic peaks in the?-form and the（?+?）-form.This was related to the crystal transition of the?-form to the?-form during heating.And the?-form and the??-form has only one melting endothermic peak.（3）FTIR tests of the samples showed that the amide V and VI bands of PA12were sensitive to the crystal form.The amide VI band（C=O out-of-plane bending）characteristic peak appeared at 626 cm^-1 in the?-form and the??-form.While in the?-form,it shifted to the position of 577 cm^-1.The amide V band（N-H out-of-plane bending）characteristic peak appeared at the position of 680 cm^-1 in the?-form.But the it was not found in the?-form and the??-form.（4）By testing the mechanical properties of the samples,it was found that?and?’-PA12 belong to strong and tough polymer materials.The tensile strength of?-PA12is 63.9 MPa and the elongation rate of fracture is 618%.The tensile strength of??-PA12 is 44.1 MPa and the elongation rate of fracture is 446%.But?-PA12 belongs to brittle and weak polymer material.Its tensile strength is only 25.2 MPa and its elongation rate of fracture is only 18%.（5）By using WAXD to heat up the PA12 samples,it was found that the?-form gradually changed to the?-form as the temperature increased,namely,the Brill transition occurred.The??-form will also change into the?-form above 110?C.The?-form is stable below the melting point.And due to the change of the thickness of the lamellae,the melting of crystal defects,and the thermal expansion of the molecular chains during heating,the d-spacing,crystallinity,and crystallite size are also changed.（6）By using WAXD to cool the PA12 samples,it was found that with the increase of the supercooling degree,the heat released increased,the crystallization rate accelerated,and the crystallinity increased.（7）By using WAXD to conduct online thermal drawing tests of?and?’-PA12,it was found that the?-form will change to the??-form under the action of tensile force.The larger the drawing ratio,the greater the crystallinity,orientation and tensile force.When?≤2.00,they increased with a greater degree.And they tended to be stable when?≥2.00.In addition,the temperature has a certain role in promoting the crystallization and orientation of the sample.And drawing the same rate,the higher the temperature,the lower the tensile force used.