Crystallization under shear

The same apparatus was used to measure the kinetics of emulsion crystallization under shear. McClements and co-workers (20) showed that supercooled liquid n-hexadecane droplets crystallize more rapidly when a population of solid n-hexa-decane droplets are present. They hypothesized that a collision between a solid and liquid droplet could be sufficient to act as a nucleation event in the liquid. The frequency of collisions increases with the intensity of applied shear field, and hence shearing should increase the crystallization rate. A 50 50 mixture of solid and liquid n-hexadecane emulsion droplets was stored at 6 -0.01 °C in a water bath (i.e., between the melting points and freezing points of emulsified n-hexadecane). A constant shear rate (0-200 s ) was applied to the emulsion in the shear cell, and ultrasonic velocities were determined as a function of time. The change in speed of sound was used to calculate the percentage solids in the system (Fig. 7). Surprisingly, there was no clear effect of increased shear rate. This could either be because increase in collision rate was relatively modest for the small particles used (in the order of 30% at the fastest rate) or because the time the interacting droplets remain in proximity is not affected by the applied shear. 

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... 

Finally the relation between the anisotropic structure and the shish structure is discussed. In the transmitted electron microscope (TEM) measurements, shish structures with diameter of about several nm have been reported in some polymers [16], It is clear that such a shish structure is different from the anisotropic oriented structure observed here because the spatial scale is very much different. Judging from the size of the anisotropic oriented structure, it must be a bundle of shish-kebab structures at least at the final stage of crystallization. Why is the 4,m scale structure formed in the initial stage of crystallization under shear flow One speculative answer is that tm scale phase separation occurs in the initial stage of crystallization between the oriented and unoriented regions, and then crystallization of stretched polymer chains occurs in the oriented domain after aggregation of the stretched chains. 

Lukkarinen and Kaski [316] have studied the mechanical properties of ER fluids by simulating perfectly ordered structures (single chains, columns of flve chains and bet crystal) under shearing, compression and elongation. They show that ER systems under compressive loading transfer a much larger force from one plane to the other than by shearing. 

XRD provides identification of phases present, proportions of phases and crystalline orientation, whereas NMR provides total crystallinity. Complementing XRD and NMR measurements is necessary to develop reliable quantitative models of crystallization under shear. The possibility of conducting the Rheo-NMR work in a benchtop machine opens an affordable possibility to perform these types of studies in many laboratories. 

Olmsted PD, Goldbart P (1990) Theory of the nonequilibrium phase-tnmsition for nematic liquid-crystals under shear-flow. Phys Rev A 41(8) 4578 581... 

Ramazani SAA, Ait-Kadi A, Grmela M (2001) Rheology of fiber suspensions in viscoelastic media experiments and model predictions. J Rheol 45 945-962 Rienacker G, Hess S (1999) Orientational dynamics of nematic liquid crystals under shear flow. 

Polymers and TAGs crystallize far more quickly when they are exposed to a shear. This phenomenon has been observed experimentally [26-30]. Crystallization under shear is still not fully understood on the theoretical side, yet this is a key parameter for the modeling of industrial processes. A first simple idea is that in the presence of shear the chains of the molecules rotate faster in the liquid and are therefore more easily incorporated into the intracrystaUine network. Morphologies of polymer crystals vary depending on the shear They crystallize preferentially in lamellas under shear whereas they are mostly crystallized as spherulites if they solidify statically, i.e., in quiescent conditions. 

A theoretically advanced model for polymer crystallization under shear was developed by Eder and Janeschitz-Kriegl [31]. The main hypothesis is that dynamic nucleation (i.e., when nuclei appear due to shear) is different from static... 

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