Self-seeding process

Fig. 5.17 SAXS patterns for PEQwPBO.w showing (a) the ordered melt structure (T = 90 °C) (b) a metastable structure at T = 42 °C (c) the equilibrium once-folded structure grown at T - 50°C by a self-seeding process (Ryan et al. 1997). Numbers indicate the order of reflection from a lamellar structure and the arrow indicates the position of the peak in the ordered melt. The calculated repeat lengths for possible molecular conformations are indicated.

While ultrasound-mediated transition of fiber networks is an interesting topic, a clear mechanism has not yet been obtained. Since ultrasound treatment was applied immediately after the formation of spheruHtes, the possible mechanism we postulate is that the micro-segment of the broken fibers serves as seeds for nucleation (nucleation centers). The formation of seeds lowers the concentration of GP-1 dissolved in the solvent, which reduces the thermodynamic driving force (supersaturation of GP-1). This contributes to the one-dimensional growth of fibers into fibrillar networks, as in the thermally controlled self-seeding process described earHer. 

Figure 1 shows the DSC cooling scan of iPP in the bulk after self-nucleation at a self-seeding temperature Ts of 162 °C (in domain II). The self-nucleation process provides a dramatic increase in the number of nuclei, such that bulk iPP now crystallizes at 136.2 °C after the self-nucleation process this means with an increase of 28 °C in its peak crystallization temperature. In order to produce an equivalent self-nucleation of the iPP component in the 80/20 PS/iPP blend a Ts of 161 °C had to be employed. After the treatment at Ts, the cooling from Ts shows clearly in Fig. 1 that almost every iPP droplet can now crystallize at much higher temperatures, i.e., at 134.5 °C. Even though the fractionated crystallization has disappeared after self-nucleation, it should also be noted that the crystallization temperature in the blend case is nearly 2 °C lower than when the iPP is in the bulk this indicates that when the polymer is in droplets the process of self-nucleation is slightly more difficult than when it is in the bulk. In the case of block copolymers when the crystallization is confined in nanoscopic spheres or cylinders it will be shown that self-nucleation is so difficult that domain II disappears. 

Sun, Y. and Xia, Y. (2002). Large-scale synthesis of uniform silver nanowires through a soft, self-seeding, polyol process. Adv. Mater. 14 833-837. 

Five types of nucleation process have been identified for PHB in a recent review.These are homogeneous nucleation, occurring infrequently at random sites when the polymer is cooled from a melt at or above 210°C self-seeding, which has already been described as nucleation by unmelted crystallites when the maximum temperature of the melt does not exceed the melting point by more than a few... 

Figure 4 Kinetic analysis of the self-replication process of Ti. The graphs present the production of Ti over time from Ei and Ni initially seeded with the indicated concentrations of Ti. In the control experiment, the production of Ti in the tonplate-free reaction, shown in gray, was followed for comparison and also presented in the inset to anphasize the amplified growth of Ti over longer time.

Investigation of the crystallization of PEO within the confined lamellar dimension revealed the absence of heterogeneous nucleation, since crystallization took place at temperatures below -5 °C. Crystallographic studies revealed different crystal orientations as a function of the crystallization temperature the crystals formed between -5 and -20 °C were found to be oriented with their c-axis parallel to the glassy PS basal plane, while those crystallized below -20 °C showed random orientation of the c-axis. For temperatures higher than -5 ° C a self-seeding technique was employed to induce the crystallization process. For the self-seeding technique a... 

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

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