The Column - June 2008 - (Page 26) Clarke The Column www.thecolumn.eu.com June 2008 Figure 3: Structural comparison of three maltodextrin samples showing clear differences in branching. Instrument: Viscotek TDA-GPCmax system with LALS, RI, viscometer. Data collection and calculation with OmniSEC 4.5 software. Chromatographic conditions: 2 30 cm ViscoGEL GMPWXL columns, mobile phase 0.1M NaNO3 at 0.7 mL/min, 35°C. 0.30 Log (Intrinsic viscosity) 0.60 0.90 1.20 1.50 3.50 A B C 4.00 4.50 5.00 5.50 6.00 6.50 7.00 Log (Molecular weight) Figure 4: Comparison of linear and branched polyethylene samples. Instrument: Viscotek 350 High-Temperature GPC system with LALS, RI, viscometer. Data collection and calculation with OmniSEC 4.5 software. Chromatographic conditions: 2 30 cm HT columns, mobile phase TCB at 1.0 mL/min, 140 °C. samples though the instrument. Once determined, the g-factor can then be related mathematically to the number of branches per molecule (branching number — Bn). For star branched molecules, where arms radiate from a central point, Bn will be directly equal to the number of arms. For randomly branched molecules, the value of Bn will generally increase with molecular weight. This is fairly obvious as we would expect the number of branches per molecule to go up as the molecules get longer. A more valuable measure of the branching is the branching frequency ( ). This is obtained by normalizing the Bn to molecular weight and then multiplying by a repeat unit of molecular weight. The value of shows, in effect, the distance between branches along the macromolecular chain. Take the example of polyethylene. The monomer molecular weight is 14 daltons (CH2), so using a repeat unit of 14000 makes the value of read as “branches per 1000 monomer units”. In fact, branching is particularly important in the field of polyolefins where the amount and frequency of branching are often key in determining the end properties. This analysis requires the use of high-temperature triple detection GPC and Figure 4 shows the comparison between standard reference linear and branched polyethylene materials. To illustrate the usefulness of the quantitative branching technique we can use the example of polycarbonates (Figures 5–7). Polycarbonates can exhibit very low levels of branching that can affect the mechanical properties. Figure 5 shows the molecular weight comparison of the two samples and Figure 6 the Mark-Houwink plots (with a linear polystyrene Figure 5: Polycarbonates/molecular weight comparison. 0.45 Log (Intrinsic viscosity) 0.20 0.00 Wf/ (dlog MW) 0.20 0.40 0.60 0.80 3.50 4.00 5.00 5.50 4.50 Log (Molecular weight) Log intrinsic viscosity Log intrinsic viscosity Molecular weight distribution 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 3.00 3.40 3.80 4.20 4.60 5.00 5.40 5.80 PC1 PC2 6.00 2008-05-15_12;51;01_NBS1475_01.vdt: 2008-05-15_13;34;28_NBS1476_01.vdt: Log (Molecular weight) 26 http://www.thecolumn.eu.com
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