Cell nucleation

A material, such as (a) a terpolymer of propylene, ethylene and butene-1, (b) a polyolefin composition, which includes about 31 to 39% of a copolymer of propylene and ethylene and about 58 to 72% of a terpolymer of propylene, ethylene and butene-1 or (c) a polyolefin composition, which includes about 30 to 65% of a copolymer of propylene and butene-1 and about 35 to 70% of a copolymer of propylene and ethylene, is irradiated and extruded through a die in the presence of a physical expanding agent and a cell nucleating agent to produce a structure having a density, which is at least 10 times less than the initial density of the material. The foam articles exhibit improved flexibility and low temperature toughness compared to conventional propylene polymer materials. 

EFFECT OF TALC ON CELL NUCLEATION IN EXTRUSION FOAM PROCESSING OF POLYPROPYLENE WITH CARBON DIOXIDE AND ISOPENTANE... 

Antec 96. Volume II. Conference proceedings. Indianapolis, 5th-10th May 1996, p.1941-7. 012 EFFECT OF BRANCHED STRUCTURE ON THE CELL MORPHOLOGY OF EXTRUDED POLYPROPYLENE FOAMS. I. CELL NUCLEATION... 

Figure 17.9 Rat in vivo hematotoxicity evaluated using flow cytometry (based on Saad et al. [59]). Gating strategy of flow cytometry evaluated rat bone marrow samples. N = at least 10000 cells. Results include absolute number nucleated cells, myeloid cells, nucleated erythroid cells and lymphoid cells.

The 1980 view assumed that the prokaryote-to-eukaryote transition occurred via gradualist mechanisms such as point mutation and hence did not involve symbiosis at all (van Valen and Maiorana 1980 Doolittle 1980) and culminated with a cell that possessed a nucleus, but lacked mitochondria. This is what Doolittle (1998) has called the standard model . In this view, mitochondria are interpreted as a small tack-on to, and mechanistically unrelated to, the process that made eukaryotic cells nucleated and complex (Cavalier-Smith 2002). In the standard model, mitochondria (and chloro-plasts) are descended from endosymbionts, but the nuts-and-bolts of the prokaryote-to-eukaryote transition (the origin of eukaryote-specific traits) was seen as having occurred independently from, and prior to, the origin of mitochondria. The paper by van Valen and Maiorana (1980) expresses this view in clear physiological terms the host was assumed to be an amoeboid, anaerobic, fermenting cell related to archaebacteria, the advantage of the mitochondrial endosymbiont was to supply ATP. 

For evaluating the efficiency of the nanostructured interface for cell nucleation, the particle density of PPE, as a measure for the number of nucleating sites available for nucleation, is plotted versus the nucleation density observed for the foam (Fig. 21). For comparison, the previously found values of the uncompatibilized PPE/SAN blend are added. For PPE/SAN, even the relatively high number of PPE particles of around 5 x 10ncm-3 only leads to nucleation of approximately 2.5 x 1010 cells cm-3, i.e., only 1/20 of the potentially available PPE particles act as cell nucleating agents. Via compatibilization, however, not only the particle density of PPE and the nucleation density can be increased, but also the efficiency is strongly enhanced. While the number of cells directly scales with particle density, more than two foam cells are nucleated by one PPE particle. 

A strong similarity is found for the present blends with a PPE/PS ratio of 50/50, as reflected by a similar bimodal cell size distribution for all SAN contents. Small differences can be related to the distinct foaming kinetics of the PPE/PS blend phase. Compared to the PPE/PS 75/25 blend phase, the higher content of PS in the PPE/PS 50/50 phase leads to a cell nucleation and growth kinetics close to the SAN phase. Nevertheless, the PPE/PS phase still appears to restrict the cell growth and expansion in the SAN phase to some extent, and smaller cells are found within the cell walls. Independent of the SAN content, cell growth within the dispersed SAN phase proceeds under the constraints of the continuous, higher Tg PPE/PS phase. 

At high SAN contents of 40 wt%, cell nucleation initially starts in the SAN phase and rapid cell growth appears. The elongated phase structure and the generally elevated phase size of SAN promote rapid cell coalescence, leading to an in-... 

At SAN contents of 20 wt%, a finer dispersed SAN phase and therefore an elevated cell density is observed. Carbon dioxide is subsequently transferred from the more viscous PPE/PS phase to the less viscous SAN phase showing a higher tendency to nucleate. The PPE/PS phase rapidly vitrifies and cell nucleation is suppressed. 

By compatibilizing the immiscible PPE/SAN blend with SBM triblock terpoly-mers, the overall size of the dispersed PPE particles was reduced, increasing the number of potential nucleating sites and easing the incorporation of PPE in the cell walls. Moreover, nanostructured interface between PPE and SAN was formed, which turned out to be highly beneficial for cell nucleation. As a result, a dramatic increase of the cell density and reduction of the cell size was observed, keeping the density reduction at a similar level. 

While the bimodal cell structure is still present for low SBM contents of 5 wt% (Fig. 33b), it vanishes at elevated contents. In parallel, the overall cell size is strongly reduced from 1 [im to several 100 nm (Fig. 33c,d). Similar to the discussed PPE/SAN/SBM blend systems, the interfacial structure developed by the SBM appears highly effective for cell nucleation. Therefore, a dramatic increase in nu-cleation density can be detected with increasing SBM content (Fig. 34). 

In order to overcome this drawback, the concept of selective blending was exploited. Selective blending of PPE with low-viscous PS allowed one to control the microstructure, to refine the phase size, and to adjust the foaming characteristics of the individual phases of PPE/SAN blends. Appropriate blend compositions allowed simultaneous nucleation and cooperative expansion of both phases, generally leading to bimodal cell size distributions in the micron range. Due to cell nucleation and growth in both blend phases, the density could be further reduced when compared to PPE/SAN blends. Moreover, the presence of coalesced foam structure and particularly macroscopic defects could be avoided, and the matrix of the foamed structure was formed by the heat resistant PPE/PS phase. 

Park CB, Baldwin DF, Suh NP (1995) Effect of the pressure drip rate on cell nucleation in continuous processing of microcellular polymers. Polym Eng Sci 35 432—440... 

Most expandable polystyrene processes involve aqueous suspension systems in which pentane fractions of petroleum are introduced before, during, or after polymerization of styrene. Water-free systems may also be used. Particle size is controlled by suspension polymerization or by chopping fine filaments. The quenched pellet process for expandable polystyrene can consume off-size particles and is a convenient way to add colorants and cell-nucleating additives. 

Microtubule assembly in cells differs in some ways from assembly in vitro. In cells, nucleation of microtubules requires a third type of tubulin, which is called y-tubulin, that functions in concert with other proteins in the form of a y-tubulin ring complex. In most animal cells, the y-tubuIin ring complex is located at the pericentriolar region of the microtubule organizing center (or centrosome) where it nucleates microtubule assembly at the minus ends (7). The y-tubulin does not become incorporated into the microtubule, but rather it only localizes to the minus ends. Assembly of tubulin to form microtubules during the early stages of polymerization in vitro can be considered a pseudo first-order reaction. A steady state is eventually attained in which both the soluble tubulin concentration and the microtubule polymer mass attain stable plateaus (8). The critical concentration at apparent equilibrium (actually a steady state, see below) is the concentration of soluble tubulin in apparent equilibrium with the microtubule polymers. 

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