Polymers 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). 

Polymers with long branches do not fit these equations and different relations exist with polymers of different degrees of long branching. In many cases the equation... 

A polymer molecule may have just a linear chain or one or more hranches protruding from the polymer hackhone. Branching results mainly from chain transfer reactions (see Chain Transfer Reactions later in this chapter) and affects the polymer s physical and mechanical properties. Branched polyethylene usually has a few long hranches and many more short hranches... 

Star-shaped polymer molecules with long branches not only increase the viscosity in the molten state and the steady-state compliance, but the star polymers also decrease the rate of stress relaxation (and creep) compared to a linear polymer (169). The decrease in creep and relaxation rate of star-shaped molecules can be due to extra entanglements because of the many long branches, or the effect can be due to the suppression of reptation of the branches. Linear polymers can reptate, but the bulky center of the star and the different directions of the branch chains from the center make reptation difficult. 

In some processes, a diluent, like benzene or chlorobenzene are used as the solvent. At high pressure and temperature, both the polyethylene and the monomers dissolve in these solvents so that the reaction occur in a solution phase. In a typical process, 10-30 per cent of the monomer is converted to polymer per cycle. Rest of monomer is recycled. Extensive chain transfer reactions take place during polymerisation to yield a branched polyethylene. Apart from long branches it is also having a large number of short branches of unto 5 carbon atoms formed by intramolecular chain transfer reactions. A typical molecule of Low density polyethylene is having a short branch for about every 50 carbon atoms and one or two long branches per molecule. 

Other polymers, called branched polymers, are a long chain with monomers that stick out to the sides. 

Make sketches or diagrams showing (a) a linear polymer, (b) a polymer with pendant groups, (c) a polymer with short branches, (d) a polymer with long branches, and cross-linked polymers with (e) low and (f) high cross-linked density. 

Long branches in PVC, up to about one branch per 2000 monomer units, arise from hydrogen abstraction at the CHC1 group in the polymer chain. 

The third possibility to prepare graft copolymers is termed grafting onto . This means that a growing chain B attacks the polymer backbone A with formation of a long branch. This attack can be a chain-transfer reaction or a copolymerization with unsaturated groups, for example, in polydienes. These reactions play an important role in the preparation of high impact polystyrene (see... 

The viscoelastic properties of long-branched polymers in the melt are understood even less well than their solution properties the former are profoundly affected by entanglements, unless the polymer is of low DP, and it is intuitively obvious that entanglements involving branched molecules may be more difficult to unravel than those of linear molecules, especially those involving segments between two branch points but to treat this quantitatively would be difficult. 

In 1959, Zimm and Kilb (34) made some calculations of the intrinsic viscosities of certain branched polymer molecules, taking into account the hydrodynamic interaction between portions of the polymer chain, using a modification of the Rouse procedure. They carried out these difficult calculations for a quite restricted range of models, obtaining numerical results for equalarmed stars with 3, 4, and 8 branches, and for one modified star with 2 long branches and 8 shorter branches. They found that their numerical results for this set of structures could be approximately represented by ... 

Since the measurements most commonly used as an index of long branching are those of intrinsic viscosities, whereas the most readily calculable theoretical measures of branching are the purely geometrical quantities or g0, and since the theoretical problem of relating these is not completely solved (Section 4), one of the applications of model branched polymers is to test proposed relationships. 

Pannell (38) has studied a range of polystyrenes with comb-like branching, but with relatively long branches. He has correlated the low-shear melt viscosities with calculated values of , finding i/o°c(so)4 8, whereas the exponent for linear polymers is about 3.4. Fujimoto s results can be correlated in a similar way, but with a rather higher exponent, 5.1, though rather better correlations would be obtained if separate lines were used for each branching frequency. 

It is now generally accepted that the GPC retention volume is a function of the product M tf, independent of the nature or structure of the polymer 46, 47) though Pannell 45) found that it failed to correlate the elution behaviour of his highly branched polystyrenes, it may be accepted that M rf will be determinable from GPC retention volumes for moderately branched polymers. To estimate branching, it is necessary to separate this product so that M and [rf are both known and the relation between them can then be used, subject to the uncertainties mentioned in Subsection 9.2.2, for this purpose. It is usual to measure rf rather than M in order to make the separation, as it is easier. The combination of GPC and intrinsic viscosity measurements is now the most usual method for studying long branching. 

This equation subsequently proved to be inadequate, for it was found that some long-branched LDPE samples have higher low-shear melt viscosities than linear ones of the same MW (56, 60,163) similar results have been found for other polymers as discussed in Section 5. 

As a result, the formation of long branches in the polymerization of vinyl acetate by the free-radical mechanism is better understood than in the polymerization of any other monomer, though there is even so still some disagreement about the transfer coefficient with the polymer, and also in estimates of the proportion of the total branching that takes place through acetate groups in either monomer or polymer. However, the state of knowledge is still less satisfactory for other monomers such as ethylene or vinyl chloride. 

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