Abstract:

Objective The binary acid method can be used for preparing polyamide 6 (PA6) based elastomer easily and efficiently, but stoichiometric number balance is strictly required when feeding. Once the molecular mass of the soft and hard segments is determined, the proportion of the soft and hard segments cannot be changed, which limits the development of functional products.The paper is to propose a new polymerization method based on the binary acid method to flexibly adjust the relative molecular weight and proportion of the soft and hard segments of PA6-based elastomers and to provide the basis for the subsequent research of PA6-based elastomers.

Method On the basis of the binary acid method, ethylene glycol is introduced to participate in the esterification and ester exchange reaction between polyamide 6 and polyether segments. With the ethylene glycol component, the system can ensure the balance of stoichiometry and adjust the ratio of soft and hard segments more flexibly to obtain the PA6-based elastomer. All reactions for preparing the PA6-based elastomer were performed in a 10 L reactor with a vacuum pump, a vacuum tube, and a nitrogen cyllinder.

Results It can be seen from the infrared spectra of the polymer that there are ester bonds in the product, indicating that ethylene glycol and polyethylene glycol were introduced into the system in the form of copolymerization (Fig.3). The structure of PA6 based elastomer (Fig.4), and combined with the peak (Fig.5), six bonding structures of PA6-based elastomer were made known. The relative integral area of the peak was introduced into equations (5) and (6), and it was proved that the molecular mass and PEG segment content were consistent with the design. The contents of low molecular extractants in PA6 based elastomer (Tab.4). The low content of low molecular extractants was conducive to the subsequent melt spinning of PA6-based elastomer. When the molecular weight of the soft and hard segments was given, the crystallization enthalpy and melting enthalpy of PEG segments would increase with the increase of the content of PEG segments, and the crystallization enthalpy and melting enthalpy of PA6 segments would decrease accordingly (Fig.7). With the same content of soft and hard segments, when the molecular mass ratio of hard segment to soft segment (M_n,PA6/M_n,PEG) increases, the melting and crystallization temperatures of PA6 and PEG segments would increase (Fig.8). It can be seen that the smaller the PEG content, the greater M_n,PA6/M_n,PEG, the higher the thermal stability of the resulting elastomer (Tab.5). It can be seen that the characteristic peaks of PA6 based elastomers were consistent with those of PA6, indicating that the crystal structure of this series of PA6-based elastomers was solely determined by PA6 chain segments(Fig.9). It can be seen that the elasticity of PA6 based elastic fibers increases with the increase of the PEG segment content, while the fracture strength and fracture elongation of fibers decrease sharply (Fig.10). It is evident that with the decrease of PEG segment content and the increase of M_n,PA6/M_n,PEG, the main chain structure of PA6 based elastic fiber is similar to that of pure PA6, and the fracture strength and elongation of the fiber increase (Tab.6).

Conclusion After the introduction of ethylene glycol, a series of PA6-based elastomers were prepared by changing the molecular weight and feeding ratio of polyethylene glycol (PEG) to PA6, making PA6-based elastomers more designable. The molecular structure design of a series of PA6-based elastomers was verified to be effective through the analysis of ¹H-NMR and infrared spectra. The thermodynamic properties, the crystal structure, the fiber mechanical properties and the elastic properties of the series of PA6-based elastomer samples were tested and analyzed. The results show that the crystal structure of PA6-based elastomer is dominated by PA6 segments. With the increase of the PEG segment content, the elastic recovery of fiber increased, and the strength and elongation of fiber decreased. Compared with PA6 fibers, elastic fibers with above 20% PEG content shows higher resilience at high constant elongation (≥10%), the elastic recovery rate are increased by up to 17.5%. PA6-based elastic fiber is found to possess encouraging comprehensive properties, among which the strength is 1.57 cN/dtex, the elongation is 106.89%, and the elastic recovery at 10% constant elongation is 94.3%.

Key words: polyamide 6-based elastomer, polyethylene glycol, diprotic acid method, block copolymerization, elastic fiber