Polyethylenes of Low and High Molecular Weight

The most common commercial grades of polyethylene have numb average molecular weights of the order of 10000-40000 (with corresponding values in the range 50000-300000). There are, however, a number of special purpose grades on either side of this range. 

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Cao J, Gao X, Shen K (2012) Morphologies and mechanical properties of high-density polyethylene induced by the addition of small amounts of both low- and high-molecular-weight polyolefin under shear stress applied by dynamic packing injection molding. J Macromol Sci Part B 51 2519-2526... 

Figure 4.25 (page 122) shows results obtained on TSK-GEL SW and TSK-GEL PW columns for low molecular weight polyethylene glycol (PEG) oligomers and high molecular weight dextrans. The TSK-GEL G2000PW column successfully resolved components of PEG 200, whereas the TSK-GEL G2000SW column did not (Fig. 4.25A). Therefore, the TSK-GEL G2000PW column would be preferable for this analysis.

In the absence of substantial unsaturation and of active groups on the chain for either polymer, only linear block copolymers are formed, according to the initiation Reactions 1 and 4. Low density polyethylene and high molecular weight polyisobutylene are typical of polymers which form block copolymer fractions on intensive mechanical working. The composition of block copolymers is related also to the relative rates of reaction, (Reactions 2 and 3) which is determined by the relative radical reactivity. 

A modified liquid phase, based on OV-225 and high molecular weight polyethylene glycol, and exhibiting a low level of bleeding from columns, has been used in the g.l.c. separation of methylated alditol acetates. 

In the past, industrial and academic researchers traditionally focused on developing stable and durable polymeric materials that resisted exposure to natural forces such as heat, sunlight, oxygen (O2), water and microbial attack. The most widespread modern plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) are inexpensive, easily processable, resistant and durable. In fact, most man-made polymers on the market cannot be biologically degraded because their carbon components are not broken down by microbial enzymes. The hydrophobic character of plastics, their low surface area and high molecular weights (MW) are all features which inhibit or decrease enzyme activity and enhance resistance to microbial attack [1]. 

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