Polyethylene, branching molar mass

Samples of branched polyethylene were investigated by the combination of crystal-lizability fractionation and fractionation by molar mass. The first step was the precipitation of the polymer sample onto the glass-beads in a column (150 x 8 mm) used for the fractionation by crystallinity. The precipitation was performed by cooling a solution of 10 g/1 polyethylene in o-dichlorobenzene from 140 to 40 °C within 60 min. Subsequently, the polymer was extracted from the column by the same solvent at stepwise increased temperature. In an example given, the first fraction was eluted at 40 °C and additional 17 fractions at temperatures each raised by 2-10 K. The finer steps were employed in the middle of the fractionation procedure the last one reached even from 110 to 140 °C. About 10 min equilibration time proved to be adequate. The fractions were analyzed subsequently by SEC on a polystyrene-gel column. The whole process was automated 128). 

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. 

Abstract The morphology of polyethylene has been an important theme in polymer science for more than 50 years. This review provides an historical background and presents the important findings on five specialised topics the crystal thickness, the nature of the fold surface, the lateral habit of the crystals, how the spherulite develops from the crystal lamellae, and multi-component crystallisation and segregation of low molar mass and branched species. 

The morphology of a polyethylene blend (a homopolymer prepared from ethylene is a blend of species with different molar mass) after crystallisation is dependent on the blend morphology of the molten system before crystallisation and on the relative tendencies for the different molecular species to crystallise at different temperatures. The latter may lead to phase separation (segregation) of low molar mass species at a relatively fine scale within spherulites this is typical of linear polyethylene. Highly branched polyethylene may show segregation on a larger scale, so-called cellulation. Phase separation in the melt results in spherical domain structures on a large scale. 

The morphology and crystallisation behaviour of a series of binary blends based on a low molar mass linear polyethylene (Mw=2500 g mol-1 Mw/Mn= 1.1) and two higher molar mass branched polyethylenes... 

Branching of polyethylenes provides the second dimension, after molar mass, with which to control properties. Tables 3.2 and 3.3. Branching of polyethylenes is a complex topic in this review it will be treated starting with the ideal copolymer structures formed with the new metallocene catalysts. Branched polyethylenes (BPE) provide increased toughness though decreased modulus and strength compared with LPE. Branches are obtained by copolymerization with 1-alkenes, such as... 

Polyethylene imines, PEI, are strongly cationic and strongly branched polymers with a molar mass between 100,000 and 1,000,00 (g/mole)... 

Transfer to polymer. The transfer reaction with a polymer chain leads to branching rather than initiation of a new chain so that the average molar mass is relatively unaffected. The long- and short-chain branching detected in polyethylene is believed to arise from this mode of transfer. 

In the previous section, we have seen that the combination of GPC with IR detection provides a measurement of the comonomer incorporation (composition) versus molar mass however, this does not teU us about the intermolecular distribution of branches (or any other polar comonomer incorporated) into the linear chains, which we refer as the CCD. This also needs to be measured independently in most polyethylene copolymers. 

Blends of linear and branched polyethylene normally crystallize in two stages. The components crystallize separately provided that they are of similar molar mass. Linear polyethylene will crystallizes at the highest temperatures, forming regular shaped crystal lamellae. Branched polymers crystallize at lower temperatures in finer, S-shaped lamellae located between the stacks of the dominant lamellae. Although linear and branched polyethylenes are chemically very similar they can phase separate in the molten state. A characteristic of phase separated behaviour is the observation of a dominant lamella structure (Figure 6.14). ... 

If R in the monomer is hydrogen, the monomer is ethylene, C2H4, and the polymer is polyethylene. Ethylene is at the heart of the petrochemical industry. Its annual production in the United States is measured in the billions of pounds. Erom that mass, billions of pounds of polyethylene are produced, equating to dozens of pounds per person each year (Eig 21.33). Polyethylene with a molar mass of about 15,000 g/mol is called low-density polyethylene (Fig. 21.34[a]). It is a supple material that folds and bends easily because the intermolecular (induced dipole) forces between the branched carbon chains are weak. These properties make it ideal for sandwich bags and trash bags. 

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