Swelling polyethylene

Keywords Crosslinked, foam formation, low density polyethylene, swell ratio parameter... 

In some cases, steel filters are undesirable. An analyte may adsorb to the steel or may decompose in the presence of steel. In these cases, plastic frits made from PEEK (polyetheretherketone) or polyethylene can be substituted for the stMl filters. They are not as strong as steel frits and are not completely compatible with all mobile phases. Polyethylene swells in solvents such as hexane, methylene chloride, or tetrahydrofuran (THE). PEEK also is incompatible with methylene chloride and THF. In combination with PEEK or glass tubing, these frits can be used for the construction of columns with a metal-free flow path. 

Crystalline polymers dissolve usually less readily than amorphous polymers. Dissolution of crystalline polymers requires large expenditures of energy for chain separation. Polyethylene swells in hexane at the room temperature and dissolves at elevated temperature. Isotactic polystyrene does not dissolve at the room temperature in solvents capable to dissolve ataetic polystyrene. To be dissolved, isotactie polystyrene must be brought to elevated temperature. 

Chlorosulfonated polyethylene swells in organic acids. Glacial acetic acid, concentrated butyric acid, and others initiate strong swelling even at room temperature. In dilute organic acids, the service life of CSM is also limited to weeks or months. Most organic acids diffuse easily into CSM glacial acetic acid penetrates a 1 mm thick CSM layer at 20 °C within ten hours [32],... 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

There are thus no solvents at room temperature for polyethylene, polypropylene, poly-4 methylpent-l-ene, polyacetals and polytetrafluoroethylene. However, as the temperature is raised and approaches F , the FAS term becomes greater than AH and appropriate solvents become effective. Swelling will, however, occur in the amorphous zones of the polymer in the presence of solvents of similar solubility parameter, even at temperatures well below T. ... 

In the case of crystalline polymers better results are obtained using an amorphous density which can be extrapolated from data above the melting point, or from other sources. In the case of polyethylene the apparent amorphous density is in the range 0.84-0.86 at 25°C. This gives a calculated value of about 8.1 for the solubility parameter which is still slightly higher than observed values obtained by swelling experiments. 

Figure 8.9. Swelling rates against shear rate for a low-density polyethylene at six temperatures. (After...

Since polyethylene is a crystalline hydrocarbon polymer incapable of specific interaction and with a melting point of about 100°C, there are no solvents at room temperature. Low-density polymers will dissolve in benzene at about 60°C but the more crystalline high-density polymers only dissolve at temperatures some 20-30°C higher. Materials of similar solubility parameter and low molecular weight will, however, cause swelling, the more so in low-density polymers Table 10.5). 

High-density polyethylene (p = 0.94-0.96 g/cm ) has up to five times the stiffness of low-density polyethylene at ambient temperatures and can be used at much higher temperatures. Its chemical resistance is similar to that of the low-density grades, but the resistance to swelling by solvents is higher. 

In the case of crystalline polymers it may be that solvents can cause cracking by activity in the amorphous zone. Examples of this are benzene and toluene with polyethylene. In polyethylene, however, the greater problem is that known as environmental stress cracking , which occurs with materials such as soap, alcohols, surfactants and silicone oils. Many of these are highly polar materials which cause no swelling but are simply absorbed either into or on to the polymer. This appears to weaken the surface and allows cracks to propagate from minute flaws. 

A similar example is the formation of nonstoichiometric interpolymeric complexes between mutually complementary polyelectrolytes — polycation and polyanion [69,70], They behave like true polymer networks and are capable of swelling the interpolymeric complexes between PAAc and polyethylene piperazine swells, for instance, 16-18 times [70], Also advantageous in this case is the possibility to carry out this type of crosslinking in open systems, such as soil. 

Time of Swelling (h) Styrene Absorption (mg styrene / g polyethylene) ... 

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