High-Molecular-Weight Samples

III. F.A.B.-M.S. OF High-Molecular-Weight Samples 1. Definition of High Mass... 

Figure 18 Effect of branching on the secondary nucleation and linear growth rates from the work of Lambert and Phillips [59]. (a) Low molecular weight samples. Sample S-5 exhibits only Regime II behavior as explained in the text, (b) High molecular weight samples. Reprinted with permission from Lambert and Phillips [59]. Copyright 1996, Elsevier.

The rate of polymerization may be dramatically accelerated upon addition of a bulky Lewis acid. For example, addition of (184) to a sample of living PMMA generated by irradiation of (181)/MMA causes an increase in polymerization rate by a factor of >45,000.444 The dualcomponent systems (181)/(184), and (181)/(185), have been used to prepare monodisperse, ultra-high-molecular-weight samples of PMMA (Mn > 106, Mw/Mn= 1.2).445... 

Compared with GC and HPLC, the most important advantage of CE is its high peak efficiency. It can give a baseline resolution of peaks even when the separation factor is low. Volatile chiral samples are best analyzed by GC, whereas HPLC and CE are more suitable for nonvolatile samples. CE is the best choice for a charged compound or for a high-molecular-weight sample. 

However, for high molecular weight samples there is a wide disparity among experimental results the reasons have been discussed50. Essentially, the difficulties are not unique to this macromolecule and arise from taking readings at insufficiently low angles. (We have referred to this point earlier in Section III. 1.) For M2 > ca. 

Gravimetric or infrared methods are often preferred for high-molecular-weight samples. These methods can even be used as a check on gas chromatographic data if it is suspected that high-molecular-weight hydrocarbons are present but are not being detected. 

The question arises as to whether or how closely Eq. (D-8) is obeyed by non-randomly coiled macromolecules, especially, by polypeptides in the helix-coil transition region. An answer has been given by a recent work by Norisuye (S3), who measured [fj] and for two high-molecular-weight samples of PBLG... 

Wada et al. (120) reported the observation of a secondary dielectric dispersion with a high-molecular-weight sample of PBLA in wi-cresol and showed that the resulting values of t as a function of / were interpretable in terms of their own theory essentially identical to Schwarz s one. 

Ballman and co-workers have used carbon particles to determine flow patterns for polystyrene melts in plate-cone and capillary viscometers (70). Complex patterns, rather than the simple flow expected, were observed for high molecular weight samples. These may have been caused, however, by differences in viscosity between adjacent layers of pure melt and melt with suspended particles. 

It must be concluded that Bueche s concept does not suffice to explain the melt flow behaviour of very high molecular weight samples. To the knowledge of the present author, Mieras and Van Rijn (735) have first pointed to this fact. The results given in Fig. 4.4 are in fair agreement with those obtained by the mentioned authors on the same samples, using the Weissenberg method (cone-and-plate apparatus, see Section 2.4). 

Fig. 5.4. Ratio of Maxwell-constant to intrinsic viscosity as a function of molecular weight for linear oligomers of poly oxy-propylene glycol in cyclo-hexanol at 20° C (full circles). Ratio of birefringence to shear stress in the limit of zero shear stress for the same oligomers in bulk (temperatures 20—60° C) (open squares). The experimental point at the righthand side is obtained on a high molecular weight sample, Tsvetkov, Garmonova and Stankevich (166)...

This is as a rule not possible as these data are not available for high-molecular-weight samples. In order to be able to exclude the influence of the distribution of degree of polymerization for various polymer structures, polymer analogous reactions may be carried out, where polymers of differing chemical structures and molecular weights, but with the same distribution of degree of polymerization are obtained (e.g., Kulicke, Horl 1985). Of particular interest in this report are polyelectrolytes. 

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