The Synthesis And Crystallization Kinetics Of The High-flow Nylon 6

Nylon with high flowability was often used in molded products of small parts with thin walls or precision parts, which has attracted a great deal of interest over several decades. Lots of work showed that high flow polyamides were almost prepared by melting blend and the flow behavior was improved by feeding low content of organic, inorganic or oligomer components and so on, unfortunately, however it might be hard to maintain their mechanical properties.Since 1980s, polyamidoamine (PAMAM) dendrimers have been paid more and more attentions due to their unique features such as three-dimensional architecture, low intrinsic viscosity, good solubility and high reactivity because of the presence of large amount of terminal functional groups. PAMAM dendrimers have attracted much more attention for their many potential applications, including nanoscale catalysts, drug or gene carriers, chelating agent, building blocks for assembly nanoconjugates and so forth. But few applications have been found in plastic engineering area.A new synthetic strategy for high flow nylon 6 was developed in this article. Generation 1, 2, 3 (G1, G2, G3) PAMAM dendrimers reacted with p-phthalic acid by equimolar terminal groups in water solution respectively,mother salt solution was then prepared. The high flow nylon 6 was prepared with suitable quantity of mother salt solution, end capping agent andε-caprolactam by in situ polymerization. Blue shifts are found for the peaks of NH (γN-H and 2δN-H) of the high flow nylon 6 compared with pure nylon 6 in the IR spectra. Comparing with the pure nylon 6, the high flow nylon 6 containing low content of PAMAM units, has high flow property and almost the same mechanical property. The high flow nylon 6 with low content of PAMAM units has greater melt-flow index (MFI) (the value of MFI increased by 70-90%). Hardly any decrease in the tensile strength is observed with the elongation at break decreasing by 20-35%. But the izod impact strength of the high flow nylon 6 increases. The SEM images show that the high flow nylon 6 presents brittle fracture with conglomeration-like structure while pure nylon 6 exhibits plastic fracture with island-like structure. DSC thermograms of non-isothermal crystallization exhibit that the peak of high flow nylon 6 broadens comparing with the pure nylon 6 and the broader peak means the wider processing temperature. The isothermal crystallization kinetics was investigated by differential scanning calorimetry for high-flow nylon 6. The Avrami equation has been adopted to study the crystallization kinetics. Comparing with pure nylon 6, the high-flow nylon 6 has lower crystallization rate, which varies with the generation and content of PAMAM units in nylon 6 matrix. The traditional analysis indicates that the values of Avrami parameters calculated from the half time of crystallization might be more in agreement with the actual crystallization mechanism than the parameters determined from the Avrami plots. Meanwhile, the plots of parameters n or K versus time were also obtained by local linear fitting. The results imply the parameters n or K from local linear fitting approach the actual crystallization mechanism more precisely than the parameters calculated from the half time of crystallization. The n values of the high-flow nylon 6 range between 2.1 and 2.4, meaning the crystallization of the high-flow nylon 6 is a two-dimensional growth process. The activation energies were determined by Arrhenius'method. The activation energies decrease with the increase in the generation of PAMAM units, but decrease quite slowly with the increase in the content of PAMAM units in nylon 6 matrix.The crystallization kinetics of the high-flow nylon 6 containing PAMAM dendrimers units in nylon 6 matrix was investigated by differential scanning calorimetry. Ozawa and Mo equation were used to describe the crystallization kinetics under non-isothermal condition. The values of Avrami exponent m and the cooling crystallization function F(T) were determined from the Ozawa plots, which showed bad linearity and were divided into three sections depending on different cooling rates. The plots of the m and lgF(T) values versus crystallization temperatures were obtained, which indicated that the actual crystallization mechanisms might change with the crystallization temperatures. The high-flow nylon 6 has higher values of m and lgF(T) than pure nylon 6, which implied that the high-flow nylon 6 had more complicated crystallization mechanisms and slower crystallization rate than pure nylon 6. The good linearity of the Mo plots verified the success of this combined approach. The activation energies of the high-flow nylon 6 were determined by the Kissinger method. TheΔE values were lower than that of pure nylon 6, and meanwhile theΔE was affected by both the generation and the content of PAMAM units in the nylon 6 matrix.The crude MWNTs were oxidized using sulfuric and nitric acid solution, then reacted with thionyl chloride, resulting in MWNTs-COCl, which reacted respectively with an excess of G1 PAMAM and G2 PAMAM respectively. FTIR and TGA indicated the successful functionalization of MWNTs. The MWNTs-G1 PAMAM and MWNTs-G2 PAMAM were respectively put into an autoclave along withε-caprolactam, distilled water and acetic acid, and then MWNTs-G1/PA6 and MWNTs-G2/PA6 composites were obtained by in situ polymerization. For comparison, PA6 and crude MWNTs/PA6 were also prepared under the same condition. Compared with pure nylon 6, the bending strength and izod impact strength of MWNTs-G1/PA6 and MWNTs-G2/PA6 composites were significantly improved. The SEM images indicated that MWNTs-G1 PAMAM was more uniformly dispersed than MWNTs-G2 PAMAM and crude MWNTs in the PA6 matrix. DSC analysis showed the crude and functionalized MWNTs might act as heterogeneous crystal nuclei of the matrix.