Polyethylene aggregation

Water-soluble polymers and polyelectrolytes (e.g., polyethylene glycol, polyethylene imine polyacrylic acid) have been used success-hilly in protein precipitations, and there has been some success in affinity precipitations wherein appropriate ligands attached to polymers can couple with the target proteins to enhance their aggregation. Protein precipitation can also be achieved using pH adjustment, since proteins generally exhibit their lowest solubility at their isoelectric point. Temperature variations at constant salt concentration allow for frac tional precipitation of proteins. 

After 1 minute, add 250pi of 1 percent polyethylene glycol (MW 20,000) per 10ml of gold sol used. The PEG helps to stabilize further the sol against aggregation. 

In a recent study, Jin and Kaplan (2003) demonstrate the formation of silk fibroin aggregates in the presence of polyethylene glycol, and present a step by step model for fiber formation based on the principle of micelle formation, and driven by dehydration as well as flow elongation. During this process, hydrophobic chains are exposed to the solvent, but because of the molecules high free energy, water solvation is unfavorable and phase separation followed by aggregation predominates. 

Stable aggregates have been shown to present a problem in the characterization of polyvinyl chloride (1,2) and it has been suggested that residues of crystalline structures may persist in polyethylene solutions at temperatures below the polymer s crystalline melting point (3-5). 

Table V compares M, M and M values for two polyethylenes analyzed by SEC in TCB solution at l45°C. Sample C is a linear low density material listed in Table 1. NBS 1 476 is low density polyethylene which is stated to be a low conversion tubular reactor product with density 0.931 gem and melt index 1.2 (11). IL and are little af fected by the existence of aggregates in these two samples but values are more severely influenced.

A thermal treatment at 160°C for 1 hour has proved to be adequate for the removal of aggregates that persist at 145°C in TCB or ODCB solutions of polyethylene. Such a treatment enables one to obtain true solutions for use in SEC. Solution of polyethylene in aCN appears to be incomplete even after the 160 C treatment and aCN is therefore not recommended for use in SEC with polyethylene, despite the favorable specific refractive index Increment of its solutions. 

The 1 hour treatment at 160°C will be more severe than necessary for some samples and Inadquate for others. We have observed that storage at 160°C for as much as a day may be required to remove detectable aggregates in solutions of very high molecular weight linear polyethylene samples. In any event, the appropriate duration of such treatments can be assessed by the method discrlbed here. That is to say, the solution history should be adjusted so that direct measurements of by LALLS (without the SEC columns) yields clean recorder traces (as in Figure 1) and second vlrlal coefficients which are in accord with Kok-Rudin predictions of such values for the measured... 

Aggregate elimination in polyethylene Aqueous SEC of narrow molecular weight... 

If crystallization is carried out from concentrated solutions, multilamellar aggregates are formed. In particular, melt crystallization of polyethylene gives bunched-up lamellae with an overall spherical symmetry. The space between the lamellae contains uncrystallized amorphous polymer. These objects are called spherulites, and their radii grow linearly with time, in spite of their intricate morphological features [9]. Another remarkable feature of spheruhtes formed by linear polyethylene is that they are gigantically chiral, although the molecules are achiral. 

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