聚合物等温结晶时间尺度的流变学研究
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摘要
结晶性聚合物的结晶行为是影响其微观结构并决定制品最终性能的关键因素。聚合物在成型加工过程中受到剪切、拉伸等作用时其结晶行为将发生变化。研究聚合物的静态及在剪切场中的结晶行为,对深入了解聚合物的结晶动力学和指导生产工艺过程均具有重要的意义。
本文选择等规聚丙烯(iPP)、高密度聚乙烯(HDPE)为主要研究对象,以流变学方法为主要研究手段,通过对其在静态和剪切流场作用下的结晶行为的研究,探讨其结晶行为与粘弹特性之间的关联,并试图对现有剪切诱导结晶的理论模型进行修正。
利用小应变、低频率(ω)条件下动态流变学方法对材料结构变化的敏感响应,研究了iPP静态等温结晶过程中的液-固转变行为。发现在等温结晶过程中,结晶引起的试样体积收缩会引起沿厚度和半径方向尺寸的减小;如果保持试样的厚度不变,在位于试样上、下方的平行板上将产生一个法向拉应力。在结晶初期,该拉应力快速松弛,当超过一定的时间后,会出现聚集并迅速增大的现象。基于该现象提出了一种表征体系物理凝胶点的新方法,并对这种效应与结晶行为的关联性进行了初步探索。通过动态和静态两种测试方法的对比发现,动态法适用于结晶速率较慢的体系,而静态方法则适用于结晶速率较快的体系;且相对于动态方法而言,静态方法具有不干扰被测试样的优点。
利用流变学的方法研究了HDPE等温结晶行为。结果表明,采用流变学方法测定物理凝胶点具有准确、灵敏的特点。在等温结晶过程中,体系的动态流变行为会发生由类液体行为向类固体行为的转变。我们提出的表征iPP等温结晶过程中物理凝胶点的静态测试方法及表征拉应力变化的数学模型同样适用于HDPE体系。
研究了剪切对iPP体系结晶行为的影响。发现剪切对结晶的影响极为显著。即使剪切速率小幅增大,结晶诱导时间也将成倍减小。在稳态剪切流场作用下,结晶初期的法向应力和粘度基本为一定值;一定时间后,会呈现增大并迅速上升的现象。在高剪切速率下,法向应力突变的时间要早于粘度;剪切速率减小,二
The structure, morphology and ultimate properties of semicrystalline polymers are strongly dependent on the crystallization behavior of them. When a semicrystalline polymer is subjected to flow, as invariably happens during processing, the crystallization behavior change to some extent. In order to understand the crystallization kinetics thoroughly and obtain the better properties, it is important for us to explore the crystallization behavior under both the quiescent and flow conditions of polymers. In this thesis, the relationship of Theological behavior and crystallization behavior for isotactic polypropylene (iPP) and high-density polyethylene (HDPE) were studied. Especially, modeling of shear-induced crystallization was emphasized.Studies on the liquid-solid transition during the isothermal crystallization of three kinds of commercial iPP were carried out. It is found that during isothermal crystallization of the iPPs, volume contraction result in tensile force on the motionless parallel plates. At the early stage, the tensile force quickly relaxes due to shrinkage of the sample's free surface, resulting in that the measured values are noise-like around zero. After a critical time, the tensile force starts to accumulate and grows quickly. Under each crystallization temperature, a threshold can be detected, and beyond which the tensile force grows first exponentially and then linearly. The higher the crystallization temperature, the less steep the linear growth. Accordingly, a new method to determine the liquid-solid transition depending on the static tensile force was proposed. A comparison between it and the classic dynamic methods for detecting liquid-solid transition evidences that the later are preferred to slow crystallization, and former is more appropriate for the crystallization at moderate rates. Moreover, the former has the advantage of almost not disturbing the crystallizing material before the transition.The rheological properties of liquid-solid transition during crystallization for HDPE were investigated through static and dynamic rheological measurements. The results show that with the crystallization proceeding, the dynamic visoelastic
properties of HDPE imply the transit from liquid-like behavior to solid-like behavior. The static method and the mathematical equation for detecting the physical gelation of /PP are also applicable to HDPE.Shear-induced crystallization of /PP was investigated using a rotational rheometer with cone-plate configuration. It is found that shearing has a great impact on the crystallization rate of /PP. The time scale of the crystallization decreases by one hundred times as the shear rate increases from 0.00012s'1 to 1.0s"1. As the specimen is sheared at a constant rate and a constant crystallization temperature, the shear stress and normal force are constant at first, and then start to increase sharply after a certain time. At higher shear rates the transition of the normal force occurred considerably earlier than that of the viscosity, and at medium shear rates the two transition times were coincident. However at extremely low shear rates the data of normal force became noisy and only the data of viscosity was reliable. It is suggested that the earlier time in transitions for the normal force and viscosity is used to define the onset time (/on) which characterized the crystallization process. It is assumed that the onset time with the smallest shear rate, 0.00012s"1, represents the time scale, denoted by ton>q, of the crystallization in a quiescent state. Plotting normalized onset time (tm ltmA)against the onset strain, yt^, a common critical value for all the undercoolingtemperatures tested, below which the shear has no significant effect on the crystallization rate, can be identified. Furthermore we proposed the dimensionless onset work, scaling with the free energy difference of quiescent undercooling melt. This parameter can make the normalized onset time approximately temperature-invariant within experiments. The results show that there is a critical specific onset work below which the shearing has no significant effect on the rate of crystallization. For the /PP specimen investigated the critical specific onset work is about 0.005.The predictability of the theory of isothermal nucleation and growth rates proposed by Ziabicki were examined. The modeling of quiescent crystallization indicates that crystallization mechanism of the /PP specimen is nucleation-controlled
crystal growth on preexist nuclei. Shear-enhanced crystallization rate is modeled by estimating the excess free energy induced by the flow, using the Theological model of Marrucci. Prediction of the modeling strongly depends on the characteristic relaxation time, which is determined by measuring the relaxation time of the specimen above its melting temperature and shifting to the crystallization temperature. Though the modeling of the shear-enhanced crystallization rate has been only partial successful, it can improves the shortcoming of Coppola's microrheological model to a certainextent. In Coppola's model the raptation relaxation time kd should be determined bynon-linear fitting to the crystallization rates observed. Our modeling suggests that the mechanism of crystallization under small shearing is still two-dimensional, nucleation-controlled crystal growth on preexist nuclei.
本文选择等规聚丙烯(iPP)、高密度聚乙烯(HDPE)为主要研究对象,以流变学方法为主要研究手段,通过对其在静态和剪切流场作用下的结晶行为的研究,探讨其结晶行为与粘弹特性之间的关联,并试图对现有剪切诱导结晶的理论模型进行修正。
利用小应变、低频率(ω)条件下动态流变学方法对材料结构变化的敏感响应,研究了iPP静态等温结晶过程中的液-固转变行为。发现在等温结晶过程中,结晶引起的试样体积收缩会引起沿厚度和半径方向尺寸的减小;如果保持试样的厚度不变,在位于试样上、下方的平行板上将产生一个法向拉应力。在结晶初期,该拉应力快速松弛,当超过一定的时间后,会出现聚集并迅速增大的现象。基于该现象提出了一种表征体系物理凝胶点的新方法,并对这种效应与结晶行为的关联性进行了初步探索。通过动态和静态两种测试方法的对比发现,动态法适用于结晶速率较慢的体系,而静态方法则适用于结晶速率较快的体系;且相对于动态方法而言,静态方法具有不干扰被测试样的优点。
利用流变学的方法研究了HDPE等温结晶行为。结果表明,采用流变学方法测定物理凝胶点具有准确、灵敏的特点。在等温结晶过程中,体系的动态流变行为会发生由类液体行为向类固体行为的转变。我们提出的表征iPP等温结晶过程中物理凝胶点的静态测试方法及表征拉应力变化的数学模型同样适用于HDPE体系。
研究了剪切对iPP体系结晶行为的影响。发现剪切对结晶的影响极为显著。即使剪切速率小幅增大,结晶诱导时间也将成倍减小。在稳态剪切流场作用下,结晶初期的法向应力和粘度基本为一定值;一定时间后,会呈现增大并迅速上升的现象。在高剪切速率下,法向应力突变的时间要早于粘度;剪切速率减小,二
The structure, morphology and ultimate properties of semicrystalline polymers are strongly dependent on the crystallization behavior of them. When a semicrystalline polymer is subjected to flow, as invariably happens during processing, the crystallization behavior change to some extent. In order to understand the crystallization kinetics thoroughly and obtain the better properties, it is important for us to explore the crystallization behavior under both the quiescent and flow conditions of polymers. In this thesis, the relationship of Theological behavior and crystallization behavior for isotactic polypropylene (iPP) and high-density polyethylene (HDPE) were studied. Especially, modeling of shear-induced crystallization was emphasized.Studies on the liquid-solid transition during the isothermal crystallization of three kinds of commercial iPP were carried out. It is found that during isothermal crystallization of the iPPs, volume contraction result in tensile force on the motionless parallel plates. At the early stage, the tensile force quickly relaxes due to shrinkage of the sample's free surface, resulting in that the measured values are noise-like around zero. After a critical time, the tensile force starts to accumulate and grows quickly. Under each crystallization temperature, a threshold can be detected, and beyond which the tensile force grows first exponentially and then linearly. The higher the crystallization temperature, the less steep the linear growth. Accordingly, a new method to determine the liquid-solid transition depending on the static tensile force was proposed. A comparison between it and the classic dynamic methods for detecting liquid-solid transition evidences that the later are preferred to slow crystallization, and former is more appropriate for the crystallization at moderate rates. Moreover, the former has the advantage of almost not disturbing the crystallizing material before the transition.The rheological properties of liquid-solid transition during crystallization for HDPE were investigated through static and dynamic rheological measurements. The results show that with the crystallization proceeding, the dynamic visoelastic
properties of HDPE imply the transit from liquid-like behavior to solid-like behavior. The static method and the mathematical equation for detecting the physical gelation of /PP are also applicable to HDPE.Shear-induced crystallization of /PP was investigated using a rotational rheometer with cone-plate configuration. It is found that shearing has a great impact on the crystallization rate of /PP. The time scale of the crystallization decreases by one hundred times as the shear rate increases from 0.00012s'1 to 1.0s"1. As the specimen is sheared at a constant rate and a constant crystallization temperature, the shear stress and normal force are constant at first, and then start to increase sharply after a certain time. At higher shear rates the transition of the normal force occurred considerably earlier than that of the viscosity, and at medium shear rates the two transition times were coincident. However at extremely low shear rates the data of normal force became noisy and only the data of viscosity was reliable. It is suggested that the earlier time in transitions for the normal force and viscosity is used to define the onset time (/on) which characterized the crystallization process. It is assumed that the onset time with the smallest shear rate, 0.00012s"1, represents the time scale, denoted by ton>q, of the crystallization in a quiescent state. Plotting normalized onset time (tm ltmA)against the onset strain, yt^, a common critical value for all the undercoolingtemperatures tested, below which the shear has no significant effect on the crystallization rate, can be identified. Furthermore we proposed the dimensionless onset work, scaling with the free energy difference of quiescent undercooling melt. This parameter can make the normalized onset time approximately temperature-invariant within experiments. The results show that there is a critical specific onset work below which the shearing has no significant effect on the rate of crystallization. For the /PP specimen investigated the critical specific onset work is about 0.005.The predictability of the theory of isothermal nucleation and growth rates proposed by Ziabicki were examined. The modeling of quiescent crystallization indicates that crystallization mechanism of the /PP specimen is nucleation-controlled
crystal growth on preexist nuclei. Shear-enhanced crystallization rate is modeled by estimating the excess free energy induced by the flow, using the Theological model of Marrucci. Prediction of the modeling strongly depends on the characteristic relaxation time, which is determined by measuring the relaxation time of the specimen above its melting temperature and shifting to the crystallization temperature. Though the modeling of the shear-enhanced crystallization rate has been only partial successful, it can improves the shortcoming of Coppola's microrheological model to a certainextent. In Coppola's model the raptation relaxation time kd should be determined bynon-linear fitting to the crystallization rates observed. Our modeling suggests that the mechanism of crystallization under small shearing is still two-dimensional, nucleation-controlled crystal growth on preexist nuclei.
引文
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