Short chain branching content

Temperature-risiag elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques usiag variatioas of differeatial scanning calorimetry (dsc) techniques have been developed. 

A blend of 1-octene LLDPE fractions with different short chain branching contents was investigated, i.e., 3 and 33 methyl... 

It should be mentioned that several homopolymers (of which polyethylene is probably the best known sample) also exhibit a complex melting behavior. Branched polyethylene s (LDPE, LLDPE and VLDPE) show multiple melting endotherms, due to the presence of fractions with different branching content [Schouterden et al, 1985 Defoor et al, 1993]. This was clearly illustrated by Defoor et al. who fractionated LLDPE with respect to the short-chain branching content and blended the fractions with the highest and the lowest branching content. It was shown that they both crystallized and melted separately. Both fractions determined the sphemlitic morphology in a co-operative way. 

PVDF/iPEMA shows an LCST phase behavior (LCST =183 °C), the experiments were performed on blends prepared and studied below this temperature A blend of 1-octene LLDPE fractions with different short-chain branching contents was investigated, i.e., 3 and 33 methyl groups per 1000 carbon atoms Rapid crystallization conditions, interfibrillar segregation occurred in blends with a PEI concentration below 50 % at highta- ctmtents of the amorphous component, interspherulitic segregation was observed Slow crystallization conditions... 

Table 3.15 Short chain branch content of lithium aluminium hydride reduced PVC ...

Table 10.13 summarises the short chain branching content obtained by this method for the four low-density polyethylenes along with that found by C-NMR spectroscopy. As a whole, the estimated individual short-branch content and the total values are in fairly good agreement with those obtained by C-NMR spectroscopy. 

More information about the specific type of short-chain branch in PVC can be found from an examination of the Ci-Cu hydrocarbons. Here quantitative differences between the reduced PVC become more apparent. The most obvious differences occur in the amounts of iso-Cy and iso-C products formed, which indicate differences in the total branch content. As the amount of short-chain branching (Cjo) in the reduced PVC increases, there is a decrease in the amount of iso- alkanes formed (Table 10.18). The data in Table 10.18 show small (but distinguishable) differences in the short-chain branch content of the reduced PVC. 

M. J. Roedel in 1953 [14] to account for the short-chain branching content in LDPE. More recent characterization of LDPE by C-NMR has resulted in a more detailed understanding of the SCB in EDPE, which has shown that LDPE is a complex mixture of various types of branching. Methyl, ethyl, propyl, n-butyl, n-amyl and 2-ethyl hexyl are each present along the polymer backbone. Additional references [19] on the characterization of the short-chain branching content in LDPE are recommended. 

They indicate that as the short-chain branching content increases from 2.9 to 28.2 branches per 1000 C atoms, the crystal core thickness decreases from 97 to 18 A and the transition layer thickness increases from 8 to 21 A. These results are in qualitative agreement with Flory s prediction that crystal core thickness... 

Imagine the difficulty this makes in trying to measure the short-chain branch content of polyethylene when the types of branches present are not known. Think about this the next time you see a report of the branches per 1000 carbons measured by IR spectroscopy. 

In addition to the ethylene homopolymers, copolymers of varying composition of -butyl acrylate ( BA), methyl acrylate (MA), vinyl acetate (VAc), n-butyl methacrylate (mBMA) acrylic acid (AA) and methacrylic acid (MAA) were also studied for short-chain branch content. The expected types of backbiting mechanisms that may occur during copolymerization are shown in Fig. 7.5. 

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