These constants are then used to determine the viscous average molecular weight of an unknown polymer. Pour into a working cup about 25 mL of the selected polymer solution.
From typical industrial synthesis a fairly broad distribution in molecular weight results. Renormalization is one of the major success stories of polymer physics in the past 20 years. There may be some minor differences but the instruments are basically all the same thing.
The molecular weight is then given in terms of the equivalent polystyrene molecular weight and must be so noted.
Intrinsic viscosity measurement is typical of an absolute technique and the purpose of this lab is to introduce the problems which are common to such techniques. Rarely, it is necessary to develop a new set of standards for an unknown sample, most commonly using light scattering.
The intrinsic viscosity, [h] is the limit of the reduced viscosity as the polymer solute concentration approaches zero.
The units of intrinsic viscosity are inverse concentration. An absolute determination of molecular weight for a single sample using light scattering, for example, might take the better part of a week including sample preparation, filtration, running the experiment and preparation of Zimm plots.
Since the viscosity method is not based on rigorous physical laws, it must be calibrated by standards of known molecular weight with narrow molecular weight distributions. The Mark-Houwink "a" constant is close to 0.
Allow time about 15 minutes for the sample fluid to attain bath temperature and become free of air bubbles. In usual practice, at non-theta conditions, intrinsic viscosity relies on knowledge of the Mark-Houwink coefficients for the thermal and solvent conditions of measurement.
The main consequence is that measurements of molecular weight must involve dilute conditions so as to avoid the overlap concentration and complications which basically can not be resolved. See Figure 4 and Table I Select a Cannon-Fenske viscometer that will give a efflux flow time greater than sec or the minimum time specified in Table I, shown below.
Many primary or absolute techniques are based on colligative properties of the polymer such as melting point depression, vapor pressure and osmotic pressure. In a "poor" solvent, one that shows a positive heat of mixing, segments of a polymer moleculeattract each other in solution more strongly than they attract surrounding solvent molecules.
GPC results in measurement of the entire molecular weight distribution see figure above from which any of the moments of molecular weight or common polydispersity indices can be calculated. In the characterization lab two techniques for determination of molecular weight are introduced, intrinsic viscosity and use of the GPC.
For skewed distributions the third moment and perhaps higher order moments are needed such as in the Maxwellian distribution.
These new parameters are assumed to have the same molecular weight dependence as the end-to-end distance and radius of gyration for the coils.
If the polymer solution concentrations are low and the polymer molecular weights are also low, then the solution viscosities measured for the flow curve are constant or independent of the shear rate.
A long-chain polymer molecule in solution takes on a somewhat kinked or curled shape, intermediate between a tightly curled mass coil and a rigid linear configuration. Such a bimodal distribution could not be fully describe by just Mw and Mn for instance.
Values greater than 0. The number average, weight average, z-average and viscosity average. In polymer science it is the convention to define several special parameters to describe molecular weight distributions. Under these conditions the coil can be shown to display Gaussian statistics.
The time it takes the polymer to elute from the column is called the retention time, tR, and the elution volume for this time is called the retention volume, VR. Repeat steps 1 through 4 for the unknown molecular weight polymer solutions. Under high pressure flow some of the polymer chains are forced into the pores of the gel, while others pass by the gel beads.
The output of a GPC is considered to reflect the number of chains at a given retention volume or molecular weight. To minimize the unwanted shear thinning effect, it is desirable to operate a viscometer at low shear rates.Measuring Molecular Weight of Polymer from Kinematic Viscosity?
how do you decide which method to use (assuming GPC isn't allowed)? Can we calculate average molecular weight of a polymer. established through molecular weight measurements. Vol.
10 MOLECULAR WEIGHT DETERMINATION Molecular Weight Distribution Functions perse solutes, the number-average molecular weight is the average determined. At higher concentrations and molecular weights, the solutions become nonideal.
Determination of Molecular Weight. The number average molecular weight, M n is the same as the first moment about the origin or the mean.
Gel permeation chromatography involves passing a dilute polymer solution through a tubular column packed with polymeric gel (crosslinked) beads. Under high pressure flow some of the polymer chains are. Molecular Weights, Polymers, & Polymer Solutions (Part I - Chapter 2 in Stevens)1 I Number and Weight Average Molecular Weight - An Introduction A) Importance of MW and MW Distribution 1) a) 2) a) b) 3) a) b) ¾ ¾ Optimum MW, MW Distribution, etc.
an absolute method that does not need. Determination of the molecular weight of a polymer from intrinsic viscosity measurements Fig. 1. Experimental set-up. more rapidly through the capillary arm of the viscometer. To Determination of the molecular weight of a polymer from. Determination of number-average and weight-average molecular weights of polymer sample from diffusion and sedimentation velocity measurements in theta solvent.Download