Deformation morphology, polymer processing

Fig. 10.13 Melting of low density polyethylene (LDPE) (Equistar NA 204-000) in a starve-fed, fully intermeshing, counterrotating Leistritz LMS 30.34 at 200 rpm and 10 kg/h. (a) The screw element sequence used (h) schematic representation of the melting mechanism involving pellet compressive deformation in the calender gap (c) the carcass from screw-pulling experiments. [Reprinted by permission from S. Lim and J. L. White, Flow Mechanisms, Material Distribution and Phase Morphology Development in Modular Intermeshing counterrotating TSE, Int. Polym. Process., 9, 33 (1994).]...

In this example of model reactive polymer processing of two immiscible blend components, as with Example 11.1, we have three characteristic process times tD,, and the time to increase the interfacial area, all affecting the RME results. This example of stacked miscible layers is appealing because of the simple and direct connection between the interfacial layer and the stress required to stretch the multilayer sample. In Example 11.1 the initially segregated samples do create with time at 270°C an interfacial layer around each PET particulate, but the torsional dynamic steady deformation torques can not be simply related to the thickness of the interfacial layer, <5/. However, the initially segregated morphology of the powder samples of Example 11.1 are more representative of real particulate blend reaction systems. 

Various process steps were used to determine their Influence on the morphological nature of liquid crystalline copolyester films. Compression molding was used to form quiescent films, while extenslonal deformation above and below the onset of fluidity, as well as shear deformation above the onset of fluidity was used to make non-quies-cent films. It Is a basic result that molecular orientation can only be achieved when the deformation is done while the polymer is In a liquid crystalline melt state. Experimental details are given In the subsection Materials and Processing, while an interpretation is offered in the discussion in the subsection Morphological and Process Consideration. ... 

During polymer processing non-isothermal crystallization conditions, mechanical deformation, and shear forces may alter the morphology and orientation of polymers both at the surface and in the bulk. In addition, orientation effects of semicrystalline polymers that crystallize in contact with solids are considered. 

At low concentration of a second polymer, blends have dispersed-phase morphology of a matrix and discrete second phase. As the concentration increases, at the percolation threshold volume fraction of the dispersed phase, (f>c 0.16, the blends structure changes into co-continuous. Maximum co-continuity is achieved at the phase inversion concentration, (py. The morphology as well as the level of stress leads to different viscosity-composition dependencies. The deformation and dispersion processes are best described by microrheology, using the three dimensionless parameters the viscosity ratio (2), the capillarity number (k), and the reduced time (f ), respectively (Taylor 1932) ... 

This portion of the chapter can be summarized by noting that there is a substantial body of evidence demonstrating that formal phase-equilibrium thermodynamics can be successfully applied to the fusion of homopolymers, copolymers, and polymer-diluent mixtures. This conclusion has many far-reaching consequences. It has also been found that the same principles of phase equilibrium can be applied to the analysis of the influence of hydrostratic pressure and various types of deformation on the process of fusion [11], However, equilibrium conditions are rarely obtained in crystalline polymer systems. Usually, one is dealing with a metastable state, in which the crystallization is not complete and the crystallite sizes are restricted. Consequently, the actual molecular stmcture and related morphology that is involved determines properties. Information that leads to an understanding of the structure in the crystalline state comes from studying the kinetics and mechanism of crystallization. This is the subject matter of the next section. 

Drop deformation and breakup plays a decisive role in the evolution of polymer blend morphology. The breakup mechanism during polymer blending is very complex and is influenced by many variables, such as shear stress, viscosity ratio, stress ratio, Deborah numbo-and first normal force difference [1-3]. Visualization was used to get realtime information during the drop deformation and breakup process [1-5]. It is shown that drops can break up in simple shear flow via different modes such as breakup in the flow axis, erosion, parallel breakup, tip streaming and breakup along the vorticity axis [1-7]. 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

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