Polyethylene short chain branches

Figure 2 Short chain branching distribution in polyethylenes. Source Ref. 31.

Copolymerization e.g., of 1-butene or 1-hexene with ethylene, gives short-chain branching-, e.g., the branches contain three or five carbon atoms. The random location of the side-chains lowers the crystallinity and density. Long-chain branching refers to branches that are similar in length to the polymer backbone and this type occurs in polyethylene manufactured using the... 

The predominating type of nonlinearity in polyethylene appears to consist of short chain branches three or four chain atoms in length formed by intramolecular chain transfer as follows ... 

There are numerous variations on the basic linear structure of polymers. Returning to our example of polyethylene, we find short chain branches and long chain branches, as shown in Figs. 1.2 and 1.3, respectively. The number and type of these branches strongly influences the way that the molecules pack in the solid state, and hence affect the physical properties. Long... 

At the opposite end of the polyethylene spectrum are the so-called ultra low density products with a high concentration of short chain branching that inhibits crystallization. These materials are soft, flexible, and transparent we encounter these materials in applications such as medical tubing, meat packaging films, and ice bags. 

We can incorporate short chain branches into polymers by copolymerizing two or more comonomers. When we apply this method to addition copolymers, the branch is derived from a monomer that contains a terminal vinyl group that can be incorporated into the growing chain. The most common family of this type is the linear low density polyethylenes, which incorporate 1-butene, 1-hexene, or 1-octene to yield ethyl, butyl, or hexyl branches, respectively. Other common examples include ethylene-vinyl acetate and ethylene-acrylic acid copolymers. Figure 5.10 shows examples of these branches. 

An example of a backbiting reaction that creates the short chain branches is shown in Fig. 18.5. In this example the growing end of a polyethylene chain turns back on itself and abstracts a hydrogen atom from the carbon atom located four bonds away from the chain end, as shown in Fig. 18.5 a). Chain growth proceeds from the newly formed unpaired electron, leaving a pendant butyl group, as shown in Fig. 18.5 b). There are many variants of backbiting, which create a variety of short chain branches. 

We can also incorporate branches by copolymerizing ethylene with vinyl esters and vinyl acids. In addition to their ester or acid side groups, these copolymers also contain the long and short chain branches, which are characteristic of low density polyethylene. 

How do long and short chain branching influence the crystallinity of polyethylene ... 

Favorable rheological properties are an essential requirement for the commercialization of polyolefins like polyethylene. The ease of processability of the polymer melt, obtained through modifications in the microstructural features, is as important as the end use mechanical properties of these polymers. Presence of long-chain as well as short-chain branching, LCB and SCB, respectively, more or less dictates the rheological behavior of most commercial... 

Despite this tremendous versatility, there is still a fundamental limitation for these random copolymers melting point and modulus (stiffness) are inextricably coupled to the density (or percentage short-chain branching from LAO comonomer) as shown in Fig. 1. The same method employed to lower modulus (incorporation of comonomer) results in a thinning of the polyethylene crystals, concomitant with a lowering of the melting point, according to a relationship established by Flory [8],... 

Short-chain branching such as occurs in polyethylene can be considered as equivalent to copolymerization with another olefin of quite low molecular weight its effects are distinct from those of long-chain branching, and it will not be considered here. 

In 1957, Peticolas and Watkins (161) came to the rather surprising conclusion that the low shear-rate melt viscosity of polyethylene was strongly dependent on short-chain branching, but not very dependent on LCB. They gave the equation ... 

Linear low-density polyethylene (LLDPE)440-442 is a copolymer of ethylene and a terminal alkene with improved physical properties as compared to LDPE. The practically most important copolymer is made with propylene, but 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene are also employed.440 LLDPE is characterized by linear chains without long-chain branches. Short-chain branches result from the terminal alkene comonomer. Copolymer content and distribution as well as branch length introduced permit to control the properties of the copolymer formed. Improvement of certain physical properties (toughness, tensile strength, melt index, elongation characteristics) directly connected to the type of terminal alkene used can be achieved with copolymerization.442... 

Whereas in the example just described the sample amount was about 50 mg, a similar procedure developed by another group 129) started with 4 g polyethylene copolymer. The sample was applied as a dilute solution in xylene and precipitated by very slow cooling (1.5 K/h) onto the Chromosorb P packing of a 500 x 127 mm column. The first separation was temperature-rising elution fractionation at a flow-rate of 20 ml/min and a Unear temperature increase by 8 K/h. The MMD of the fractions was measured by SEC at 145 °C in o-dichlorobenzene at 0.7 ml/min flow rate. The column set included a pair of bimodal columns 100 A and 1000 A plus a 4000 A column. The apparatus was equipped with an IR detector. The experimental data is computed to show the distribution of short-chain branching and of molar mass simultaneously. 

Willbourn, A. H. Polymethylene and the structure of polyethylene study of short-chain branching, its nature and effects. J. Polymer Sci. 34, 569—597 (1959). 

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