Study On Time Scales In The Isothermal Polymer Crystallization By Using Rheological Tests

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 visoelasticproperties 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-controlledcrystal 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.