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Rheological Study On The Flow-induced Crystallization Of Semicrystalline Polyolefin

Posted on:2010-11-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:F Y YuFull Text:PDF
GTID:1101360305956699Subject:Polymer Physics and Chemistry
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During the processing process, the polymeric materials experience complex effects of heat and flow, thus the rheological feature and internal structure of the material will be changed, which will greatly affect the final properties of the products. As for the semicrystal polymer, their melts will show an evident flow-induced crystallization behavior under a flow field. Many experimental phenomenons can be related to the flow-induced crystallization of polymeric materials, for example, the necking process of the high-speed fiber spinning process. Different from the quiescent crystallization, the research results show that the effect of flow on the polymer crystallization mainly lies on the acceleration of the nucleation kinetics and change of the crystalline morphology. Thus, study on the flow-induced crystallization and establish the relationship between the macropscopic property of the material and its internal mesoscopic structure are now one of the most important research topics in the polymer processing field.In this thesis, isotactic polypropylene (PP) and high-density polyethylene (HDPE) were selected as the investigated subject with the rheological methods. Through determing the flow-induced crystallization behavior, the relationship of the rheological behavior and crystallization behavior of polyolefin was studied. The theoretical analysis was focused on the effect of flow strength on the crystallization parameters of polyolefin. At the same time, the effect of the molecular structure parameters and the content ratio of the polymer blends on the flow-induced crystallization behavior also be studied in detail. To predict the nucleation number and nucleation induction time, a new coupled solving system, i.e. velocity-pressure-conformation tensor formula, was introduced into the nucleation rate equation. Furthermore, a two-phase model was modified to describe the relationship between the polyethylene's crystallization and viscoelastic properties of the polymer melts. The simulation work was also furthered into the internal structure of material. The main research work and conclusion are introduced as follows:(1) Studies on the liquid-solid transition during the isothermal crystallization of the commercial iPP were carried out with the small amplitude oscillation determination method. The evolutions of the modulus, phase angle and normal force were determined at the same time. According to Winter's physical gelation theory, we employed a new method, namely,"inverse quench"method, to characterize the gel point during the crystallization process of polypropylene. The frequency sweep results showed that the curve of G' deviated the terminal zone behavior of the linear PP as the crystallinity increases, which indicated the formation of a long term relaxation process in the melts. Furthermore, the evolution of the spherical number with time was counted by a polarized light microscopy and simulated by Hoffman nucleation rate equation. Besides the small amplitude oscillation, the different treatments of the finite large amplitude oscillation shear were also performed on the undercooling PP melts. The Fourier transformation method was selected to analysis the waveform during the crystallization of PP melts under the large amplitude oscillation shear.(2) In order to study the rheological properties of a long chain branched isotactic polypropylene (LCB PP) polymers and the effects of its molecular architecture on the crystallization kinetics, self-made LCB PPs were isothermal crystallized under different shear rates. The relationship between the rheological feature and the molecular structure was analyzed through plotting the figures of Han, Cole-Cole and relaxation time spectrum. The results showed that the long branch chain could change the rheological behavior of the samples evidently and result in a longer relaxation process. Two shear modes, including steady shear and preshear treatment, were performed on the melts by rheometer. The crystallization rate was investigated by characterizing the induction time and the half crystallization time. The results showed that the LCB PP samples became more and more sensitive to the flow fields with the content of the branched component increasing; moreover, long branched chains also accelerated the nucleation kinetic of polymer samples. The positive effect of long branched chains on nucleation rate originates from two aspects: firstly, the long branched chains act as a nucleation agent to promote the formation of nuclei; secondly, long branched chain make the FIC process of polymer melts became more sensitive to shear flow and form more nuclei after the finish the preshear treatment. In additional, the crystal form of these LCB PPs after the preshear treatment was investigated by WAXD. The results showed that, even if the different shear time of shear treatment was performed on the polymer melts, there was no evident change being observed at a relative small shear rate 0.1s-1. Namely, the shear flow seems to just promote the crystallization ability of LCB PPs and don't affect the crystal form of LCB PPs.(3) The researcher often encounters the processing of the binary polymer mixtures to obtain satisfactory mechanical properties in practical applications. Therefore, it is also important to verify the relationship between the kinetic and rheological parameters in the flow-induced crystallization of thermoplastic polymers. The shear-induced crystallization of pure PP and PP/PPEOc blends has been studied in this thesis. In the dynamic measurements, the PP/PEOc blends showed a long characteristic time, which was a second relaxation mechanism in the blends, corresponding to droplet-matrix structure. Through observation of normalized viscosity, it has been found that, similar to the pure PP, the blends also showed the sensitivity to the change of shear rate and the crystallization induction time of blends also decreased as the increasing of shear rate. The presence of high viscosity of elastomer PEOc trend to promote the chain of iPP oriented in the shear flow field, thus acting as nucleation promoter in the blends. In addition, a new microstructure rheological model based on the conformation tensor will be proposed to describe the flow-induced crystallization behavior of PP/POE blending system. With conformation tensor equation, we estimated the value of free energy change, which was induced by flow field. Then we predicted the induced time of PP/POE system with Coppla's FIC model. The results showed a good agreement with experiments, even though our blend was not a homogeneous system. This result might help us to comprehend the FIC process in a more complex system. (4) The acceleration of nucleation kinetics manifests that there is a significant difference between flow-induced crystallization (FIC) and quiescent crystallization in polymer melting. In this thesis, the effect of pre-shear flow on the subsequent crystallization process was investigated and a FIC model based on the conformation tensor incorporating the pre-shear effect was proposed. The model is capable of predicting the overshoot of the stress and the flow-induced free energy change of the polymeric system at high pre-shear rates. Under the condition of flow, the increase in the activated nuclei number was contributed by the flow-induced free energy change, which showed an overwhelming effect on the nuclei formation during the pre-shear process at high shear rates. The half crystallization time ( t1 /2) of PP as functions of pre-shear rate and pre-shear time at different crystallization temperature was predicted and compared with the experiment data. A good agreement between the model predictions and experimental data was found. Both numerical and experimental results showed that t1 /2 of PP decreased dramatically when the flow started but leveled off at long times. It was found that two transformation stages in t1 /2 existed within a wide range of shear rates. For the first stage where the melting polymer experienced a relatively weak shear flow, the acceleration of crystallization kinetics was mainly contributed by the steady value of free energy change while in the second stage for high shear rates, strong overshoot in flow-induced free energy change occurred and the crystallization kinetics was thus significantly enhanced. Therefore, the overshoots in stress and flow-induced free energy change reflected an important role of flow on the primary nucleation especially when the flow was strong enough.(5)In the thesis, the isothermal flow-induced crystallization (FIC) of high density polyethylene (HDPE) under a simple shear flow was investigated. To determine the viscosity and modulus of the crystallization process, two experimental modes, including steady shear and preshear treatment, were performed on the polymer melt. Based on the non-equilibrium thermodynamic theory, the FIC process of HDPE was predicted through the modification of a continuum FIC model. The theoretical predictions of the evolution of both the viscosity in steady shear flow and the complex modulus under preshear treatment were essentially related to the crystallinity of HDPE, in agreement with the experimental findings. Both experimental and predicted results showed that the applied flow field could accelerate the crystallization kinetics of HDPE significantly. However, the effect of the intensity of shear flow on the crystallization of HDPE was finite, showing a saturation phenomenon, namely, the accelerated degree of crystallization tending to level off when the shear rate was large enough. In additional, it was found that the predicted crystallinity of HDPE was very low in induction period either in steady shear flow or by preshear treatment. The theoretical calculated results indicted that the melt viscosity was sensitive to the crystallinity. Moreover, the results also showed that the FIC process of HDPE melt illustrated a more evident acceleration in the extensional flow field than in the shear flow field.Therefore, the main innovations of the present research work are listed as follows:1. Considering the free energy change of polymer melt under the flow, a new flow-induced crystallization model was developed incorporating the effect of orientation and stretch of polymer chains. The predicted induction time of crystallization process of the PP/PEOc blend under the shear flow agreed with the experimental data well. Through introducing the conformation tensor equation into the nucleation rate equation, a system of simultaneous equations was developed to predict the PP's crystallization process. The predicted results showed that the evolution curves of half crystallization time with shear rate have two inflexions indicating there was the different crystallization mechanism as the change of shear rate.2. The McHugh's work was extended. Based on their theory, a modified two-phase microstructure rheological model was developed to predict the flow-induced crystallization of the undercooling polymer melts. The model could predict the evolutions of the viscosity and modulus with time under the flow. The predicted results showed a good agreement with the experimental data. It was first time to theoretical discuss the relationship between inflexion point of viscosity curve and crystallinity.3. Study on the nonlinear rheology during the polymer phase transition by using the rotation rheometer. It was the first time that, through the Fourier transition, the relationships between the stress and strain, and the strain and phase transition were analyzed. The rheological properties of a long chain branched isotactic polypropylene (LCB PP) polymers and the effects of its molecular architecture on the crystallization kinetics were also determined by the rheometer. Subsequently, the complete crystal samples were determined by WAXD. There was no apparent crystalline form change in the observation.
Keywords/Search Tags:Rheology, Polyolefin, Flow, Crystallization kinetics, Physical model
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