Growth rejection process

In previous sections we have shown that the redistribution of additives at the spherulite boundaries during polymer crystallization leads to the additives uneven distribution, whose form is determined by the kinetics of the growth rejection process. In time, this initial dynamic distribution should relax to an equilibrium form in which the noncrystalline polymer is uniformly permeated by the additive, whose distribution reflects that of the noncrystalline polymer. The relevanoe of these observations to oxidative degradation processes in semi-crystalline polyolefins is discussed in this section. 

Ryan studied the rejection process during crystallisation (9). A wave of rejected additive builds up ahead of the growing spherulite as shovm in Figures 2 and 3. The shape of this wave depends on the diffusion coefficient of the additive and the growth rate of the spherulite and has been compared with computer simulations of the rejection. 

It is well-established that fractionation occurs during crystallization for broad band molecular weight homopolymers [86, 148] that is, segregation occurs such that the lower molecular weight components remain in the melt (or solution) and only the higher molecular weight components become crystalline. The rejected components depend upon the growth conditions, which indicates that fractionation is directly linked to the crystallization process. Hence a study of the conditions... 

HO-1 is controlled at the level of transcription by oxidative stress and several inducers such as porphyrins, metals, and progesterone (8). The absence of HO-1 is associated with severe growth retardation, anemia, and enhanced endothelial cell injury 156, 157). In addition, HO-1 has been implicated in protection against transplant rejection 158, 159). HO-2 is constitutively expressed, and its presence in brain and the noted similar effects of CO and NO has led to the proposal that CO-generated by HO-2 is involved in signaling processes 8, 160-162). 

Among the controversies summarized in Table 9.1, the argument that the rounded forms are due to growth and represent the as-grown morphology has been completely rejected, and it is now believed that all these forms represent the forms appearing due to the dissolution process. 

The sequence of measurements described in this section are designed to increase the efficiency of the screening process by sending a crystal on to a more involved stage of characterization only if it is likely to have the desired properties on the basis of the simpler measurements. The accuracy of each of the techniques is limited, so that in practice there is considerable chance that useful crystals will be rejected - and non-useful crystals will be selected - for further characterization. Nontheless, these methods greatly enhance our ability to identify useful crystals before any serious crystal growth efforts are necessary. 

Doubts about the impact on crystallization of such processes have already been raised by the present author in a paper that, very gracefully, Gert Strobl allowed to be published in parallel with his own contribution that presented a different viewpoint [8]. In that paper, the preeminence of a more classical nucleation and growth scheme (Fig. 1) was advocated crystallization is viewed as a more sequential process in which incoming stems probe the crystal growth face and are accepted if they fulfill the correct criteria. If not, the incoming stems are rejected or must undertake conformational adjustments. In other words, the classical nucleation and growth process can be seen as dominated or controlled by the crystal (substrate structure, or... 

Scrap, a rejected and nonprime material from the semiconductor was the main supply route in the early days of PV. Due to the fast growth of the market, scrap is not sufficient and the main source today is nonprime polysilicon, deliberately produced by operating the conventional Siemens process with more economical parameters (e.g., faster production rates, lower energy... 

Until 1998, the silicon photovoltaics industry was able to rely on off-specification EG material, including rejects from polysilicon manufacture (material from aborted processes, faulty seeds, ends of seed rods close to the carbon contacts, fines and chips, dendritic growths, small granules, sawn sections of seed rods) and from crystal growth (heads and tails of... 

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