Branching level detection, in polymers

Scorah MJ, Dhib R, Penlidis A. Branching level detection in polymers. In Lee S, editor. Encyclopedia of Chemical Processing (ECHP). London Taylor Francis 2005. p 25Iff... 

The ability of the complementary Py-GC-MS technique to differentiate between linear and branched polysiloxanes has been investigated. Erzin and co-workers [56] investigated the ability of Py-GC-MS to detect trace levels of silicone polymer in recycled paper, and to differentiate between linear and branched polysiloxanes. The mass spectrometer had a mass range of 100-700 m/z and was operated by single ion monitoring and by selected ion data collection to enhance resolution and detection. A pyrolysis temperature of 750 °C was used. Silicone polymer scraped from the backing sheet was used for calibration. Thermal desorption GC-MS at 225 °C was used for the analysis of volatile components. It was concluded that concentrations of parts per million, and possibly as low as parts per billion could be measured. 

Contaminants in recycled plastic packaging waste (HDPE, PP) were identified by MAE followed by GC-MS analysis [290]. Fragrance and flavour constituents from first usage were detected. Recycled material also contained aliphatic hydrocarbons, branched alkanes and alkenes, which are also found in virgin resins at similar concentration levels. Moreover, aromatic hydrocarbons, probably derived from additives, were found. Postconsumer PET was also analysed by Soxhlet extraction and GC-MS most of the extracted compounds (30) were thermally degraded products of additives and polymers, whereas only a few derived from the original contents... 

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 presence of a small number of defects in a polymer chain can have a large influence on a polymer s properties. Two important aspects of polymer chemistry involve detection and structure identification of chain branching and junctions between segments in block copolymers, as low levels of these structures can have a large effect on a polymer s crystallinity. 

Thus, perhaps one of the most important potential practical uses of rheology in the entire field of polymer science is as a method to detect and quantify the presence of long-chain branching. This potential use of rheology is more important than its use in measuring molecular weight distributions, because there are analytical methods for the latter, but for the measurement of LCB there is simply no alternative method that can detect minute levels of LCB (i.e, less than about 0.1 branch per 1,000 carbons) see Section 5.12.2. 

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