Effects Of Hydrogen-Bonding Density On Polyamide Crystallization Kinetics

Polyamide(PA)commonly known as nylon is a linear polymer which has repetitive sequences of C=O×××N-H hydrogen bonds between the chains.Polyamide is useful for industrial applications because of its enhanced mechanical properties,high chemical resistance and other superior properties.Hydrogen bond orientation enhances the chains arrangement in order to facilitate the crystallization of polyamide.However,the relationship between crystallization kinetics and hydrogen bonding density has not been discussed systematically.We employed Flash DSC1 apparatus for fast-scan chip-calorimeter measurement in a broad temperature range to compare the isothermal crystallization rates among six polyamide(PA)samples with different hydrogen-bonding density,i.e.polyamide 46(PA 46),polyamide 66(PA 66),polyamide 610(PA 610),polyamide 612(PA 612),polyamide 1012(PA 1012)and polyamide 12(PA 12).In Chapter 1,we fully introduced our sample polyamides about crystallization structure and crystal transition.The molecular chains in polyamide are mainly zigzag arrangement,which can maximize the hydrogen-bonding.Polyamides are composed of chain-folded and hydrogen-bonded sheets.There are two possible hydrogen-bonded sheet schemes in polyamides: terraced or alternating staggered.Differences in arrangement of molecules and hydrogen bonding sheets lead to diversity of polyamide crystal structure.Different crystal structure can be obtained in different experiment conditions.Brill transition can be observed in polyamides in heating or cooling process.It is pretty meaningful to study the crystallization kinetics of polyamides because of its superior applications.In Chapter 2,we introduced the knowledge background that used in this project: polymer crystallization nucleation kinetics,Avrami equation and the principles and applications of Flash DSC.The harvest of crystallinity in nylon industrial processing is mainly dominated by crystallization kinetics,so it is helpful to understand the polymer crystallization nucleation kinetics.The polymer crystal nucleation kinetics was presented here,and the homogeneous and heterogeneous crystal nucleation was mainly discussed.The Avrami analysis as a helpful tool for a time-scaling kinetic analysis can help us understand the nucleation method in polyamide crystallization nucleation,so we introduced Avrami equation in this chapter.In this project,we employed the fast-scan chip-calorimeter measurement Flash DSC1 to study the isothermal crystallization kinetics.In the end of this Chapter,we introduced the principle of Flash DSC and some work about crystallization kinetics of polymer materials which were studied by this kind of commercially available chip-calorimeter(Flash DSC1).In Chapter 3,the suppliers of samples,the glass transition temperature,melting point and other basic data were presented.Furthermore,Flash DSC is not very common and it is not very easy to process samples,so we also detailed the use of the instrument and sample processing.In addition,we introduced the four experimental steps: critical erasing thermal history experiment,critical cooling experiment,critical heating experiment and isothermal crystallization experiment,and we explained the results can be obtained,then the function of each step was presented in the end of this Chapter.In Chapter 4,the experiment methods and results of erasing thermal history,critical cooling and heating experiment was discussed in more details.The first three steps helped us to make sure the experimental conditions for isothermal crystallization experiment in the last step,that is the critical temperature that can erase all of the thermal history,and the critical cooling rate that can suppress the melting crystallization in the cooling process and the critical heating rate that can suppress the cooling crystallization in the heating process.The crystallization half-time curves were obtained by analyzing the melting curves,which reveals the significant effects of hydrogen-bonding density in polymer crystallization kinetics.The hydrogen-bonding densities in the six polyamides are different with each other,and the hydrogen-bonding density has a great influence on crystallization kinetics of the samples.Furthermore,Avrami indexes were obtained as a function of crystallization temperature for six polyamide samples,and their Avrami indexes approve the idea of mode switching between homogeneous and heterogeneous crystal nucleation separately favored in the high-and low-temperature regions.The Chapter 5 is summary and outlook.