is reflected in GPC by the detection of a Gaussian distribution of molecular

Is reflected in gpc by the detection of a gaussian

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is reflected in GPC by the detection of a Gaussian distribution of molecular weights foreach sample as a function of time. The curve can be described by a number of termsdepending on the methods used to detect the sample eluted from the column. In this case,we used a refractive index detector that can not tell us anything about a polymer exceptwhen it comes off the column relative to pS of known molecular weight. This allows usto translate elution time (sometimes referred to as elution volume) into molecular weightin pS units.Conversion for sample X = 1 -Ratio for sample XRatio for standard/initial sampleArea under anisole peakArea under monomer peak= Ratio for sample X
8 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Molecular Weight C B A Free-Radical ATRP GPC Standard Type 1.84 1.04 1.05 M w /M n A B C Figure 2 : Samples of refractive index responses for polystyrene standards and samples prepared by ATRP and conventional free radical polymerization along with the corresponding measured polydispersities (from K. Matyjaszewski, et al ., Science , 1996 , 272 , 866). To describe the curve further, the polymer sample can be treated in two different ways and each treatment yields a different value of molecular weight: a number average (M n ) and weight average (M w ) molecular weight. M n tends to be close to the peak of the curve and M w is skewed to the high end of the curve. If these two numbers were equal, there would be only one single spike at one molecular weight and M w /M n would be one. Otherwise M w /M n is used as an indication of how far apart the two values are and hence, how broad the distribution is. Therefore, the closer M w /M n is to one, the narrower the distribution of molecular weights and the more well-defined the sample is by M n . Figure 2 shows a typical GPC trace for polymers prepared by conventional free radical polymerization and ATRP as well as the standards used to calibrate the GPC and their corresponding polydispersities for illustration. 1 H NMR spectra for vinyl polymers are similar to those of small molecules with a few exceptions. Because the linewidth of a given resonance is inversely dependent on segmental flexibility, highly constrained segments of large molecules (such as the main chain of a polymer) tend to have extremely broad lines. Signals corresponding to the side chains tend to be narrower because they are usually less constrained. This means that a
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