The Column - June 2008 - (Page 25) June 2008 www.thecolumn.eu.com The Column Clarke Polymer Branching One very important aspect of macromolecular structure is the presence of branching. In synthetic polymers the branching may be added purposely, introduced by controlling the polymerization process, to tailor the end properties for a specific application. It is, therefore, very important to be able to measure the branching in order to control the process and product. In natural polymers, such as starches, there will be naturally occurring branching that will vary according to the source of the product (eg., potato starch and grain starch). In this instance, measuring the branching is just as critical as in the synthetic case — even more so if the material is undergoing any kind of degradation (eg., de-branching) or modification as part of the production process. To see how we can view branching, both qualitatively and quantitatively by triple detection GPC we need to consider what effect branching has on the molecule. Figure 2 shows a representation of two macromolecules of equal molecular weight. It can be clearly seen that the branched molecule is significantly smaller, occupying less volume in solution, resulting in a lower intrinsic viscosity. This means that if we determine both the molecular weight and viscosity, we will be able to measure the branching. That is exactly what triple detection GPC achieves. Figure 1: Typical output from a modern triple detection system, showing excellent signal to noise on all detectors for a moderate molecular weight synthetic polymer. Sample is polyvinyl chloride with a measured Mw of 137300 daltons. 150 135 125 115 105 95 85 75 65 55 45 35 25 15 Refractive index (mV) Instrument:Viscotek TDA-GPCmax system with LALS, RI, viscometer. Data collection and calculation with OmniSEC 4.5 software Columns: 2 ViscoGEL HHRM 30 cm Run conditions: THF @ 1 mL/min, 35°C For qualitative analysis, the comparisons of the log-log plots of molecular weight versus intrinsic viscosity (Mark-Houwink plot) provide a powerful visual tool. When you compare polymers of the same chemical type, any differences in the branching will result in clear differences in the Mark-Houwink plots. A good example of this is shown in Figure 3. Here, a comparison of the Mark-Houwink plots for three maltodextrin samples show clear differences. Maltodextrin is obtained by the bacterial degradation of starch and is used as an ingredient in foodstuffs. In this instance the three samples have similar bulk viscosity values but the plots show large differences in the branching, clearly explaining the inconsistent properties of the final products. Quantitative Branching Analysis The comparison of Mark Howink plots provides a simple and convenient way to compare samples to ensure consistency in both molecular weight and branching in a single GPC experiment. But, if we wish to go further and attempt to quantify the amount of branching then we must look further into the branching theory developed by Zimm and Stockmayer.1 Here, the g-factor is defined as the difference in intrinsic viscosity between a linear and branched polymer of the same molecular weight. In other words, it is a numerical comparison of the difference in Mark-Houwink plots of the linear and branched samples. Triple detection GPC can determine this quantitative information by simply running both the linear and branched Figure 2: Diagram of the difference in volume occupied by a linear and branched molecule with the same molecular weight. Since the hydrodynamic volume of the branched molecule is smaller, with the same mass enclosed, the density must be higher, producing a lower intrinsic viscosity. 2 4 6 8 10 12 14 16 Retention volume (mL) Low angle light scattering 18 20 22 Refractive Index 2007-02-22_20;09;05_SR1_01.vdt Viscometer DP Linear Branched 25 http://www.thecolumn.eu.com
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