Short chain branching level

The kinetic coefficients are estimated by measuring the level of quaternary carbons (equated to short-chain branch level) and terminal unsaturations (D=) by NMR. 

NMR remains a very useful technique to estimate branch content in hydrocarbon polymers and constitutes a direct quantitative approach. However, using NMR in quantifying branch content has a drawback in that the results for the branch content obtained will always overestimate long-chain branching, i.e., branches longer than about six carbons. Hence, the sensitivity of NMR to determine branch content is limited to high levels of short-chain branching. But... 

In terms of characterizing the microstrac-ture of polymer chains, the two most useful techniques are infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) spectroscopy. Commercial infrared spectrometers were introduced after the end of the second world war and quickly became the workhorse of all polymer synthesis laboratories, providing a routine tool for identification and, to a certain degree, the characterization of microstructure (e.g., the detection of short chain branches in polyethylene). In this regard it can no longer compete with the level of detail provided by modem NMR methods. Nevertheless, IR remains useful or more convenient for certain analytical tasks (and a powerful tool for studying other types of problems). So here we will first describe both techniques and then move on to consider how they can be applied to specific problems in the determination of microstructure. 

The advent of 13C NMR allowed an analysis of branching at a far greater level of detail because the 13C shifts of paraffinic hydrocarbons depend strongly on their proximity to tertiary carbons (i.e., the branch points). As a result, the 13C NMR spectrum of a (short chain) branched polyethylene sample is rich in detail, as can be seen in the example shown in Figure 7-24. It was originally thought that... 

In addition, short-chain branches of C5 and C7 are seen at levels of 1 per 1000 carbon atoms, compared with a maximum of 15 n-butyl branches. This also arises from a backbiting reaction of the propagating radical and the resultant intramolecular chain transfer, and the relative amounts of the branches of various lengths may vary depending on the conditions of synthesis. 

With the development of FTIR, infrared detection attached to GPC with a flow-through cell became interesting in the 1990s for obtaining the comonomer incorporation (short chain branches) in polyolefin copolymers by measuring, besides concentration, the number of methyl groups per 1,000 carbon atoms (CH3/IOOOC) along the molar mass [34-36]. The measurement of very low levels... 

Blend analysis by TREF was first described by Knobeloch and Wild [14] and was specifically directed toward the evaluation of LLDPE/LDPE blends. This type of blend is sold commercially and is very difficult to analyse because the two components overlap both in terms of molecular weight and more importantly with respect to the level of short-chain branching. However, there is considerable difference in overall SCB distribution of the two types of PE (Fig. 34). There is a part of the low branched region of a LLDPE which has no counterpart in a similar density LDPE and is thus free from any overlap from the LDPE component. As a result, the high separation temperature peak in such a blend can be used as a measure of the LLDPE component. 

The long chain branching (LCB) already known from LDPE is not typical for LLDPE. While LDPE has high levels of LCB, there is little LCB in LLDPE however, there are high levels of short-chain branching (SCB) contributed by the incorporated comonomer. Molecular weight distribution is narrow (LDPE and HDPE tend to be broader). 

The use of NMR makes it possible to probe details of molecular features not accessible using other techniques. TREF and CRYSTAF are techniques based on crystaUizability in solution and are essential in the determination of the level and distribution of the short-chain branches resulting from the introduction of a comonomer. Much of the material in this chapter is presented in more detail by Sperling [101] and in the book edited by Pethrick and Dawkins [102],... 

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