Synthesis And Structure Of Copolyamide PA5T/54

AABB-type polyamide is prepared from diamine and dibasic acid,and is widely used due to its excellent mechanical properties and chemical resistance.Succinic acid is a dibasic acid with a wide range of sources,low price and simple preparation process.High melting point polyamide prepared from succinic acid has great application potential.However,previous studies have found that the number-average molecular weight of succinic acid-based polyamides obtained by conventional methods is usually within 1000-2000,and the highest is not more than 6000.Furthermore,the product also possessed false-color problem,which severely restricted the preparation and application of the polyamide using succinic acid as the raw material,thus industrialized product has not yet appeared.As a result,it is a very valuable research topic to study the reasons for the low molecular weight and false-color phenomenon of succinic acid-based polyamides,and to adopt appropriate methods to solve the problems existing in the synthesis of succinic acid-based polyamides.This topic is to synthesize PA5T/54 with succinic acid as the reactive monomer,and explore the reasons for the low molecular weight and the false-color problems of the product prepared by the melt polymerization method using infrared spectroscopy,nuclear magnetic resonance and time-of-flight mass spectrometry as research methods,it is found that imide end-capping is the main reason for the low molecular weight and false-color problems of the product.At the same time,it was found that the higher the reaction temperature,the more serious the end-capping phenomenon.In order to avoid the end-capping phenomenon,this article uses direct solid-state polymerization method to prepare PA5T/54 under the low-temperature reaction condition,which is 30?below the melting point of nylon 5T/54 salt.A white polymer product with a number-average molecular weight close to 9000 g/mol was obtained.The research results show that there is no imide end-capping phenomenon in the product.At the same time,the effects of polymerization temperature and vapor pressure on the polymerization process in the direct solid-state polymerization process are studied.The results show that when the temperature is lower than 190?,the increase in temperature is beneficial to the growth of the molecular weight,and when the temperature continues to increase,the molecular weight of the product decreases,and the product is slightly yellow.When the vapor pressure is lower than 0.7 MPa,the increase of the vapor pressure is conducive to the increase of the molecular weight,and when the vapor pressure continues to increase,the molecular weight of the polymerized product decreases.This article further studied the chain structure,aggregate structure and crystallization behavior of the products of melt polymerization and direct solid-phase polymerization.The main chain sequence structure of the products prepared by the two different polymerization methods was studied by ¹H-NMR.The results showed that the length of the PA54 segment of the melt polymerization product was shorter than that of the direct solid-state polymerization product,and the length of the PA5T segment was greater than that of the direct solid-state polymerization.The reason for this phenomenon is that the cyclization reaction reduces the relative concentration of succinic acid,which leads to an excess of pentane diamine in the system and an increase in the probability of reaction with terephthalic acid,thereby increasing the length of the PA5T segment.At the same time,¹³C-NMR also proves excessive 1,5-diaminopentane leads to the existence of amine end-capping.In the direct solid-state polymerization process,imide end-capping phenomenon will not occur,so that the length of the PA54segment of the direct solid-state polymerization product is greater than that of the molt polymerization product.X-ray diffraction showed that the direct solid-state polymerization product appeared the crystalline diffraction peaks of PA54 and PA5T,indicating that PA5T and PA54 appeared two-phase separation and crystallized independently.In the melt polymerization,only the crystalline diffraction peak of PA5T appears but the crystalline diffraction peak of PA54 is not shown.This is due to the fact that PA5T has a longer molecular segment that can be crystallized,while PA54 has a smaller molecular segment that is difficult to crystallize.The SEM study found that the surface of the molten polymerized product was smooth and flat,while the surface of the direct solid-state polymerization had needle-like substances.The results of the study showed that the needle-like substances were the aggregate phase formed by the molecular chain segments of PA54.