Quiescent crystallization

Warner, S. B, and Jaffe, M. Quiescent crystallization in thermotropic polyesters. J. Crystal Growth 48, 184 (1980)... 

Three different isothermal crystallization experiments were performed in this work classical static (i.e., quiescent) crystallization in the DSC apparatus, dynamic crystallization with the apparatus described above, and dynamic-static crystallization. Dynamic isothermal crystallization consisted in completely solidifying cocoa butter under a shear in the Couette apparatus. Comparison of shear effect with results from literature was done using the average shear rate y. This experiment did not allow direct measurement of the solid content in the sample. However, characteristic times of crystallization were estimated. The corresponded visually to the cloud point and to an increase of the cocoa butter temperature 1 t) due to latent heat release. The finish time, was evaluated from the temperature evolution in cocoa butter. At tp the temperature Tit) suddenly increases sharply because of the apparition of a coherent crystalline structure in cocoa butter. This induces a loss of contact with the outer wall and a sharp decrease in the heat extraction. 

Similar to extended chain crystals, the shish-kebob morphology is not observed in samples, which have been subjected to quiescent crystallization conditions. We introduce several examples already in this chapter in order to present a complete cross-section of all levels of structural hierarchy in polymers. Further examples related to oriented crystallization of polymers in practical applications will be the focus of Sect. 3.5. 

XCjmax Maximum crystalline fraction attainable under isothermal quiescent crystallization. Xj The i th one among structural parameters xl5 x2,..., X13 defined for correlating Tg. 

The predicted maximum attainable crystalline fraction (xc max) under isothermal quiescent crystallization is a rough indicator of intrinsic crystallizability for polymers [64], Here, "isothermal" means that crystallization is being performed at a constant temperature, while... 

Figure 6.25. Relationship between maximum crystalline fraction under isothermal quiescent crystallization and the ratio Tg/Tm where the temperatures are in degrees Kelvin.

Figure 6.27. Predicted maximum possible crystalline fraction (xC)lllax) under isothermal quiescent crystallization, for aliphatic polyesters -[(CH2)k-COO-]n and polyamides -[(CH2)k-CON(H)-]n.

Some useful relationships for quiescent crystallization, defined as crystallization in the absence of any applied deformation, will be summarized in this section. These relationships cannot generally be used in a truly predictive manner at this time. Their practical utility is limited mainly to providing a framework within which to analyze and correlate data. The reader interested in very extensive further details, including many literature references and examples using polypropylene and polyethylene terephthalate), is referred to our review article [64],... 

Yu, Y. White, J.L. (2001). Comparison of structure development in quiescent crystallization, die extrusion and melt spanning of isotactic polypropylene and its compounds containing fillers and nucleating agents. Polym.Eng.Sci, Vol. 41, Issue 7, pp.1292-1298. 

A special case of interest is reinforced polypropylene with various fibers. Often transcrystallinity in polypropylene occurs which is due to dense heterogeneous nucleation by a substrate. The occurrence of transcrystallinity depends on the type of fiber and the temperature. In contrast to transcrystallinity in quiescent crystallization, the application of stress at the interface between a fiber and a PP melt results in the crystallization of polypropylene on a row-nuclei around a fiber. This effect is caused by strain-induced nucleation via some self-nucleation mechanism and is independent of the type of fiber and less dependent on the temperature of crystallization [5,6]. Axial stress arises also during cooling of two materials with a large difference in thermal expansion coefficients. As such, the stress-induced nucleation in reinforced PP depends also on the cooling rate, fiber length, position along the fiber and viscoelastic properties of the PP melt [5]. 

For quiescent crystallization under a constant temperature, in the case of instantaneous nucleation with a constant number density No, one obtains Nq t) = NoH t) for the activated quiescent nuclei number density, where H(t) is the Heaviside unit step function, zero for f < 0 and unity for t > 0. Then the rate of the nuclei number density is Ng = NoS t), with (5(f) being the Dirac delta function concentrated at f = 0. Equations 4.1 and 4.3 lead to the familiar Avrami equation ... 

Syndiotactic polystyrene will strain-induce crystalhze, as well as quiescently crystallize as discussed previously. Furthermore, imder certain conditions, it may also be quenched to the amorphous state. In injection molded parts, these processes may lead to skin/core differences in morphology that can be observed (111,112). With higher temperature molds (>150°C) the parts are generally fully... 

The properties of polymer materials are determined by morphology and structure. For single polymer composites, the only way to enhance the mechanical properties is to manipulate the morphology and structure in them. There are three types of crystals for polymer materials spherulites, shish kebabs and extended-chain crystals. It is difficult to get extended-chain crystals due to the harsh processing conditions of high pressure. Comparatively, the former two crystals are more easily formed. Spherulites are isotropic and form upon quiescent crystallization conditions. However, shish kebabs, generated from... 

Polymorphs - 1 (quiescent crystallization), II (quiescent crystallization), III (orientation) ... 

For linear polyethylene fibers made in this fashion, the tensile modulus (a measure of the strength of a material, which is defined more rigorously in Chapter 16) of 44GPa (6.4 X lO psi) compares well with steel (which is about 206 GPa or 30 x lO psi) but has a density 7.6 times as great PE fibers with a tensile strength of 1.8 GPa (2.6 x 10 psi) have also been claimed [9]. Compare these figures with those for ordinary, quiescently crystallized HOPE in Table 4.1. 

Primary nuclei, forming when thermal fluctuations overcome a free energy barrier, appear randomly in space and in time, at a rate fluctuating around the momentary mean value, dictated for a given polymer by its thermomechanical history and crystallization conditions. Quiescent crystallization of homopolymers from the melt occurs usually in the form of spherulites growing radially from primary nuclei, as illustrated by... 

The work presented in this chapter is focused on the effect of flow on crystallization of polymer melts quiescent crystallization is discussed in Chapter 7, and for crystallization of polymer solutions we refer to Chapter 5 and to the literature [2], The modeUng presented is on a continuum level (coarse-grained) molecular modeling is presented in Chapter 6. The backgrounds of the experimental methods applied are discussed in more detail in Chapter 1. This chapter can, of course, not be exhaustive and more background information on specific topics and in-depth discussions can be found in the literature [9-12] and references therein. 

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