Crystallization, shear induced

Relatively speaking the polymer field has not yet reached this stage, to some degree in fibers and films but even here there is much room for improvement. We have many tools to help us produce the optimtim structiire, and optimum properties pressure induced crystallization, shear induced orientation and crystallization, polymer blend composition to control melt rheology as well as the synthesis processes to produce the desired relationship between internal structure and application properties. This is the area of future research that deserves our attention. 

Consequently, the aggregation should take place slowly, but this was not observed during the course of the measurement. Before curve (d) (Fig. 8) was measured, the solution was subjected to shear stress through capillary flow. Here, the particles became oriented and aggregated through crystallization (shear-induced aggregation, i.e. crystallization). The trace of this curve (d) is similar to curve (a). 

Cp is tire number of elasticity active chains per volume unit. The comparison between experimental data and tire prediction by (C2.1.20) shows a reasonable agreement up to large defonnation (figure C2.1.16). For large values of X, strain hardening arises because of tire limited extensibility of tire chains or because of shear-induced crystallization. 

Berrett J F, Molino F, Porte G, Diat O and Lindner P 1996 The shear-induced transition between oriented textures and layer-sliding-mediated flows in a micellar cubic crystal J. Phys. Condens Matters 9513-17... 

Theoretical representation of the behaviour of a hydrocyclone requires adequate analysis of three distinct physical phenomenon taking place in these devices, viz. the understanding of fluid flow, its interactions with the dispersed solid phase and the quantification of shear induced attrition of crystals. Simplified analytical solutions to conservation of mass and momentum equations derived from the Navier-Stokes equation can be used to quantify fluid flow in the hydrocyclone. For dilute slurries, once bulk flow has been quantified in terms of spatial components of velocity, crystal motion can then be traced by balancing forces on the crystals themselves to map out their trajectories. The trajectories for different sizes can then be used to develop a separation efficiency curve, which quantifies performance of the vessel (Bloor and Ingham, 1987). In principle, population balances can be included for crystal attrition in the above description for developing a thorough mathematical model. 

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

Fig. 4.26 Models for the SANS patterns for FCC micellar crystals in Fig. 4.27 (McConnell et al. 1995). (a) Faulted FCC crystal (with equal numbers of FCC and HCP sequences) (b) coexistence between (111) planes (20%) and polycrystalline phase (80%) (c) coexistence between (111) planes (25%) and polycrystalline phase (75%) (d), (e), (f) slipping planes hopping from registry sites with increasing shear-induced flow of layers of micelles. The wavevector range is the same as for Fig. 4.27.

Nogales, A., Hsiao, B.S., Somani, R.H., Srinivas, S., Tsou, A.H., Balta-Callja, F.J. and Eqzuerra, T.A. (2001) Shear-induced crystallization of isotactic polypropylene with different molecular weight distributions in situ small- and wide-angle X-ray scattering studies, Polymer 42(12), 5247-5256... 

Among the three known crystalline structures (a, j3, y) of isotactic polypropylene (PP), the -modification is certainly the most fascinating one. While the stable a-structure develops under standard process conditions, the occurrence of the /J-form has to be forced (i) by directional crystallization in a temperature gradient field [1-3] (ii) by shear-induced crystallization [4-12] or (iii) by the addition of specific nucleating agents [13-31]. This latter technique is preferred at the industrial scale. 

The above features of a sheared colloidal crystal appear to be similar in both BCC and FCC structures. However, there are differences in details, and perhaps even within a given symmetry the flow behavior might vary with particle concentration or charge density. For example, Chen et al. (1994) have shown that between the strained crystal and sliding-layer microstructures there can be a polycrystalline structure, the formation of which produces a discontinuous drop in shear stress (see Fig. 6-33). Ackerson and coworkers gave a detailed description of the fascinating shear-induced microstructures of these systems (Ackerson and Clark 1984 Ackerson et al. 1986 Chen et al. 1992, 1994). 

Shear-induced crystallization had a much greater effect in bulk systems than emulsified systems (Fig. 6) and resulted in an accelerated rate of crystallization. Prior to, and during, the initial stages of crystallization, intradroplet fat is protected from interdroplet crystallization by the spherical shape and pressure of the droplet and is not directly available to the shear field, i.e., no protruding crystals. This observation is consistent with microstructure work where limited destabilization was observed in droplets with no visible crystals. Initially, droplet interfaces in the PSCO system showed that the crystallized fat was not available at the surface, limiting the occurrence of crystal-induced flocculation and coalescence. Droplets remained stable until their interfaces were disturbed by the shear fleld or crystal interaction. 

Exposure to a shear fleld causes more rapid crystallization as the presence of protruding crystals leads to more aggregation of crystalline droplets, similar to bulk-fat behavior. The rapid increase in the rate of droplet destabilization following crystal growth and protrusion is due to the shear induced by the less spherical... 

However, for polymers with low degree of crystallization it was assessed that the melting point was depressed at high shear stresses (4). Shear Induced melting has been theoretically predicted from non-equilibrium molecular dynamic simulations (5,... 

If so, then PE chains had to exist in the PP phase to start with, i.e., under high shear rate in injection molding machine PP and PE could be locally miscible. Upon removal of the shear field the PE chains crystallized in PP-rich matrix. Also note that in the PE phase, there are the highly disordered regions in which normal PE lamellar growth may be prevented by the presence of PP chains as impurities. This implies that shear-induced mixing took place in the PE-rich phase as well. 

Abuzaina, F., Fitz, B. D., Andjelic, S. and Jamiolkowski, D. D., Time resolved study of shear-induced crystallization of poly(p-dioxanone) polymers imder low-shear, nucleation-enhancing shear conditions by small angle Ught scattering and optical microscopy. Polymer, 43, 4699, 2002. 

MAD Madbouly, S.A. and Wolf, B.A., Shear-induced crystallization and shear-induced dissolution of poly(ethylene oxide) in mixtures with tetrahydronaphthalene and ohgo(dimethylsiloxane-i-ethylene oxide), A/ocra/wo/. Chem. Phys., 204,417, 2003. 

P. Deng, K. Liu, L. Zhang, H. Liu, T. Wang, J. Zhang, Micro and macro injection molded parts of isotactic polypropylene/polyethylene blends Shear-induced crystallization behaviors and morphological characteristics. J. Macromol. Sci. Part B Phys. 53, 24-39 (2014)... 

In this section, we review three representative cases of flow-induced mesophases. PET serves as an example of a semi-rigid polymer with intrinsic rigid building blocks. The shear-induced smectic ordering in the flexible-chain polymer iPP provides a special case of induced rigidity, which is still far from fully understood. PDES has been chosen as an example of a stretch-induced condis crystal. Various experimental techniques have been employed to characterize flow-induced mesophases. While scattering methods probe the ordering, a direct technique to measure the molecular mobility is nuclear... 

Figure 6a gives a typical SAXS picture of the shear-induced smectic peak at 200 °C an accompanying smectic WAXS peak may be hidden in the amorphous halo of the corresponding pattern in Fig. 6b [152]. At this temperature no crystallization is expected at all, as confirmed by the absence of any crystalline reflections in both SAXS and WAXS. The smectic SAXS intensity is...

Fig, 19 Relative SAXS intensities vs crystallization time of iPP (a) in presence of shear-induced smectic bundles (b) without smectic bundles (after Li and de Jeu [6])... 

To understand the mechanism leading to enhancement of crystallization under shear, the structure of the so-call shish or row nuclei, the primary nucleus induced by flow, is essential. The common textbook picture is that the shish consist of extended-chain crystals that maybe defective [184-189]. It is generally accepted that structures with some degree of order are induced by the flow before the onset of crystallization, which can be long living at shallow supercooling [127-150]. These shear-induced precursors have been described... 

Obviously, shear induces in the PPS a thread-like crystal structure. This consists of stable nuclei that align tightly in the direction of shear. 

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