Polyethylene long branching

The chemical iadustry manufactures a large variety of semicrystalline ethylene copolymers containing small amounts of a-olefins. These copolymers are produced ia catalytic polymerisation reactions and have densities lower than those of ethylene homopolymers known as high density polyethylene (HDPE). Ethylene copolymers produced ia catalytic polymerisation reactions are usually described as linear ethylene polymers, to distiaguish them from ethylene polymers containing long branches which are produced ia radical polymerisation reactions at high pressures (see Olefin POLYMERS, LOWDENSITY polyethylene). 

Branching can to some extent reduce the ability to crystallise. The frequent, but irregular, presence of side groups will interfere with the ability to pack. Branched polyethylenes, such as are made by high-pressure processes, are less crystalline and of lower density than less branched structures prepared using metal oxide catalysts. In extreme cases crystallisation could be almost completely inhibited. (Crystallisation in high-pressure polyethylenes is restricted more by the frequent short branches rather than by the occasional long branch.)... 

The reaction temperature is above the critical temperature of ethylene so that the ethylene is in gas phase. High pressures are needed for propagation reaction. Only about 6-25 per cent of ethylene is polymerised. Rest of monomer is recycled. Extensive chain transfer reactions takes place during polymerisation to yield a branched chain polyethylene. In addition to long branches, it also contains a large number of short branches of upto 5 carbon atoms produced by intra-molecular chain transfer reactions. A typical molecule of Low density polyethylene contains a short branch for about every 50 carbon atoms and one or two long branches per molecule. 

The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts (Sec. 8-1 lb). The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene. Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40-60% vs. 70-90%) and density (0.91-0.93 g cm 3 vs. 0.94-0.96 g cm-3). 

This covers methods that depend upon the fact that branching introduces groupings with different chemical structure from that of the repeat units of linear chain, namely branch-points and end-groups. These can sometimes be detected and estimated by physical or chemical methods. However, short branches as well as long ones introduce these groups, and it may not be justifiable to attribute them, or all of them, to long branches. Methyl groups in polyethylene are a case in point. 

Similar effects are observed in y-irradiated n-C H (530 kGy) in the molten state. Three new structures are identified as a) one-bond crosslinks (H-structure), b) trans-vinylene groups and c) long branches (T- or Y-structure)144). However, highly crystalline polyethylene y-irradiated in the solid state at low doses (up to 40 kGy) yields predominantly the branched Y-structure. A failure to detect the cross-linked H-structure could arise from a) insufficient abundance of crosslinks to give a detectable signal and b) insufficient mobility of crosslinked chains in the polyethylene gel which results in very broad resonance lines, not observable during normal data acquisition in the solution 13C NMR experiment145). 

These two branching processes decrease the regularity of the polyethylene macromolecules. Individual polymer chains may have long branches or butyl branches that occur at random positions. As we will see shortly, this irregularity in the structure dramatically affects the physical properties of the polymer. 

What makes the method using Ziegler-Natta catalysts so important The resulting polymers are much more regular than those produced by other methods. For example, polyethylene produced by using a coordination catalyst is linear. It does not have the short or long branches that characterize polyethylene that is produced by a radical... 

A branched macromolecule forms a more compact coil than a linear polymer with the same molecular weight, and the flow properties of the two types can differ significantly in the melt as well as in solution. Controlled introduction of relatively long branches into diene rubbers increases the resistance of such materials to flow under low loads without impairing processability at commercial rates in calenders or extruders. The high-speed extrusion of linear polyethylene is similarly improved by the presence of a few long branches per average molecule. 

Molecular weight distributions in commercial polymers are characterized by ratios of about 3 for substances like polystyrene in which transfer to polymer does not appear to be important. Where long branches can be formed by chain transfer to polymer, the molecular weight distribution will be even broader and M /M ratios of 50 and more are observed in some polyethylenes made by free-radical syntheses. 

Three different polyethylenes referred to as HD, LD, LLD have been investigated. They mainly differ in their molecular weight distribution and in the structure of the chains which can be either very hnear as in the case of HD (high density polyethylene) or short-branched in the case of LLD (linear low density polyethylene) or long-branched in the case of LD (low density polyethylene). The weight average molecular weight and the polydispersity index of the samples are siunmarized in Table la. 

Fig. 18. Branched Polyethylene. Long spacing (L) and crystal thickness (d ) for various temperatures. Histograms obtained from electron micrographs...

In this chapter the solid state extrusion of different grades of polyethylene is discussed. The term coprdyetfaylene stands as weU for short and long branched PE as for the nearly alternating 1 1 oipolymer poly(ethjdene-aj-chlorotrifluoroethylene) (PECTFE). It is well known that even HDPE cxsitains a certain amount of ort branches. Therefore, it isi of interest to note that already one butyl side group per thousand main diain carboni atoms effects the solid state extrusion properties of PE remarkably. 

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