Solvent rotation polymer molecular weight

If the preceding analysis of hydrodynamic effects of the polymer molecule is valid, K should be a constant independent both of the polymer molecular weight and of the solvent. It may, however, vary somewhat with the temperature inasmuch as the unperturbed molecular extension rl/M may change with temperature, for it will be recalled that rl is modified by hindrances to free rotation the effects of which will, in general, be temperature-dependent. Equations (26), (27), and (10) will be shown to suffice for the general treatment of intrinsic viscosities. 

In view of the preceding analysis, should be a constant independent of both polymer molecular weight and solvent. To some extent, it may, however, vary with temperature, considering that the unperturbed dimension is modi ed by hindrances to free rotation whose effects are,... 

Yoshizaki, et a/. (46) (note also Ref. (47)) measured solvent rotational relaxation times in the presence of very-low-molecular-weight polymer for polyisobutylene benzene and poly(dimethylsiloxane) bromocyclohexane. Both polymers were studied as the dimer an 8.26 kDa PDMS oligomer was also examined. At 500 g/1, the polyisobutylene had no effect on t similar concentrations of PDMS reduced tr of the bromocyclohexane, though by less than 20%. The dimer and c. 75-mer PDMS had the same effect on tr to within experimental error, showing that polymer molecular weight and chain end effects each have minimal effects on the reduction in tr, at least for short polymers. 

Early work on NMR of polymers in dilute solution was reviewed by Heatley(29). It was already clear in that early review that relaxation times of dilute polymers were independent of polymer molecular weight, at least for molecular weights above a few to ten thousand, and were nearly independent of polymer concentration for concentrations up to 100-150 g/1 or so. A revealing exception to this rule was provided by polymers plausibly expected to rotate as nearly rigid bodies, for which Ti continued to depend on M up to much larger M. From these observations, it was plausibly inferred that local chain motions are primarily responsible for the observed relaxation times. Dependences of Ti on solvent temperature and viscosity were concluded to scale linearly with solvent viscosity, at least in most systems, a matter treated in more detail below. Heatley also considers correlations between Ti and chain structure. 

The addition of a polymeric solute to a small-molecule solvent affects translational diffusion, viscosity, and rotational diffusion of solvent and other small molecules in solution. For polymer concentrations 4> < 0.4, the solvent selfdiffusion coefficient follows D exp(—a). The constant a is linear in the probe s molecular volume but is independent of polymer molecular weight. At larger concentrations (/) > 0.4, the simple-exponential dependence of D on concentration is replaced by a stretched-exponential concentration dependence, D and dD/dc both appearing continuous through the transition. The effects of added polymer on solvent self-diffusion and on the diffusion of small-molecule probes are clearly not the same. 

An alternative approach for determining the molecular weight of a polymer in theta solvents includes the determination of the polymer s concentration at the meniscus (c ,) and at the bottom ic, ) (or alternatively at two other positions Xi and X2) in the cell. These two outstanding positions have a distance of x ix ) and Xh(x2), respectively, from the center of rotation. Then, one obtains the weight-average molecular weight of a polydisperse polymer sample via the equation ... 

The polymer of high molecular weight in the solid stage exhibited high crystallinity under a polarized microscope and insoluble in common organic solvents. When the polymer with high optical rotation was used as stationary phase or sorbent for the chromatographic resolution of racemic compounds, it showed the ability of resolution for many kinds of compounds, such as alcohols, amines, esters, and even hydrocarbons (28). 

Fig. 11. Comparison of theoretical curves, which describe the thermal helix-coil transition of a polypeptide system as a function of chain length, with experimental points obtained for poly-->-benzyl-i,-glutamate polymers of varying degrees of polymerization, n. In this solvent mixture, ethylene dichloride and dichloroacetic acid, the helical form, characterized by the more positive rotations, is stable at higher temperatures. The manner in which increased length sharpens the transition, both in theory and in actuality, is here clearly illustrated. For comparative purposes, Tc is defined as the temperature at the mid-point of the transition for the sample of highest molecular weight. (Zimm et al., 1959.)...

An interface between gel permeation chromatography (GPC) and Fourier transform infrared (FTIR) spectrometry has been developed. With this system it is possible to collect solvent free polymer deposition and to measure their infrared spectra as a function of molecular weight. The mobile phase from the GPC effluent is converted into an aerosol and removed using a pneumatic nozzle. The sample is collected on a Ge disc that rotates below the nozzle. After the sample is collected, the disc is transferred to an FTIR spectrometer where the infrared spectrum of the sample is collected. Normal GPC sample concentrations (0.1-0.25 wtJvol%) give sufficient sample for useable FTIR signals. All normal GPC solvents can be effectively removed, and the interface works with both low temperature and high temperature GPC applications. 

This anhydro sugar, polymerized at —60 °C in CHjC12 solvent with 0.5-1.0 mol % PFS initiator, gives high-molecular-weight polymer (Mn up to 7 105) with a >50% yield, which after debenzylation shows a specific rotation between +181° and +197° 29). 

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