Chapter 14. Molar Mass Distribution.

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Colin Owen
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1 Chapter 14. Molar Mass Distribution.

$oin early polymer science, it was necessary to fractionate polymers, into fractions which have a narrow(er) MWD. These were then represented by a histogram of fractions.$

2 Difficulty with M n and M w, etc. osome polymers are hard to describe from just M n, M w, etc. o Examples: Bimodal, multimodal, nonuniform, broad, etc. MWDs. oin early polymer science, it was necessary to fractionate polymers, into fractions which have a narrow(er) MWD. These were then represented by a histogram of fractions. (Particle size distributions for each cut-off size used)

3 Fractionation of polymers The simplest procedure for polymer fractionation is to dissolve the polymer at low concentration in a good solvent and then to bring about stepwise phase separation (i.e. precipitation ) of polymer fractions. Change the temperature Add the poor solvent of a solvent pair to the dissolved polymer, but SLOWLY. The highest molar mass species phase separate first. Fractions are obtained in order of decreasing molar mass. (Just like gpc )

4 Fractionation of polymers Phase separation of a very dilute polymer solution is brought about by causing the polymer to border on non-solubility. Again, either by add a non-solvent to the solution or by changing the temperature. Non-solvent is preferred and involves addition of non-solvent to the polymer solution until phase separation is clearly evident. The addition of non-solvent is stopped at this point and the solution heated to re-dissolve the concentrated phase. UCST This solution then is cooled slowly to achieve equilibrium phase separation. After allowing the concentrated phase to settle at the bottom of the fractionation flask, it is removed and the remaining dilute solution phase subjected to further similar fractionations. Remember recrystallization in organic lab? There is considerable overlap of the distribution curves of the individual fractions and this has to be taken into account when constructing the distribution curve for the original polymer. UCST = Upper critical solution temperature

5 Fractionation of polymers The procedure just described is rather time-consuming and so attempts have been made to automate fractionation by phase separation using methods of chromatography. One was to do this experimentally involves depositing the polymer at the top of a column of glass beads and then passing a mixture of solvent and non-solvent through the column which usually has a gradient of temperature along its length (decreasing temperature for UCST behavior). The concentration of solvent in the eluant is gradually increased so that the low molar mass species dissolve first, the eluant stream being collected in successive portions and the different polymer fractions recovered by removal of the solvent from each fraction. Another method: continuous liquid liquid extraction procedures, where the polymer initially is dissolved in one solvent and is extracted continuously from this solution by another solvent which is only partially miscible with the first solvent. UCST = Upper critical solution temperature

7 How does one determine the molecular weight and the mwd? This is usually done by gel permeation chromatography (also, hint-hint, called size exclusion chromatography). How does this work? Uses size exclusion for separation. The stationary phase contains solvent filled pores of certain sizes that allow smaller molecules to enter while excluding larger molecules. Technique is mostly used to characterize the MW distribution of polymer materials or separation of high MW weight proteins.

8 Gel Permeation Chromatography

9 GPC In order to convert a GPC chromatogram into a MMD (Molar Mass Distribution) curve it is necessary to know the relationship between molar mass M and V e. This relationship results from the dependence of molecular size upon M The molecular size of a polymer molecule in solution can be taken as its hydrodynamic volume which from is proportional to [η]m [η] is the intrinsic viscosity. So log([η]m) will decrease approximately linearly with V e. A calibration plot of log M against V e should be approximately linear. The calibration plot is specific to the polymer under study (through the values of K and a, Mark Houwink Sakurada Constants. See Table 13.2 in Young) Obtained by measuring V e for a series of narrow MMD samples of the polymer with different M (see Figure 14.6). The value of V e usually is taken as that at the peak of the chromatogram for this purpose.

10 GPC Calibration FIGURE 14.6 Some typical GPC calibration curves for polystyrene in tetrahydrofuran obtained using a GPC system comprising a series of six different GPC columns packed with gels of porosity: (a) 10 5 nm, (b) 104 nm, (c) 103 nm, (d) 102 nm, (e) 50 nm, (f) 5 nm. In each case, the calibration curve is approximately linear over the range of resolvable molar masses. From Young, chapter 14, p 320.

11 GPC to MWD A GPC chromatogram and (b) the weight-fraction MMD into which it transposes

12 Universal Calibration via hydrodynamic volume ***For given solvent/temperature/gpc column system

13 GPC Schematic illustration of a GPC apparatus: A, solvent reservoir; B, solvent pump; C, injection valve; D, syringe for injecting the polymer solution; E, injection loop; F, column oven; G, guard column (which protects the more expensive GPC columns from adventitious blockages); H, GPC columns; I, detector; J, analogue-to-digital converter; K, computer; and L, waste solvent/solution reservoir.

14 flow-fff (field flow fractionation) Description Utility?

15 Mass Spectrometry Needless to say, this is different than for small molecules. Creation of molecular ions from polymers requires methods of soft ionization in order to prevent (or at least minimize) fragmentation of the polymer molecules. 1. Electrospray Ionization 2. Matrix-Assisted Laser Desorption / Ionization

16 Electrospray Ionization 1980 s ESI involves spraying a polymer solution into a chamber through a needle, the tip of which is subjected to a high voltage. The resulting tiny, highly charged droplets are heated by a stream of nitrogen gas and divide rapidly into smaller and smaller droplets due to Coulombic repulsions as the solvent evaporates, ultimately producing individual charged molecular species that pass into the mass separator/detector system. A feature of ESI is that species with multiple charges are generated easily, but not necessarily reproducibly; i.e. not all macromolecules of the same molecular mass are vaporized/ionized into molecular ions with the same charge (i.e. giving molecular ions with different m/z values from species of identical molecular mass). Less of a complication for biopolymers which have a unique molar mass since it gives m/z peaks with separations equal to well-defined fractions of the molar mass. In analysis of polymers with molar mass dispersity, even for synthetic polymers with narrow molar distributions, the generation of molecular ions species with different numbers of charges creates considerable uncertainty in the interpretation of ESI mass spectra.

17 Matrix-Assisted Laser Desorption / Ionization (MALDI) MALDI involves vaporization of a solid solution of polymer Polymer dispersed at low concentration ( wt%) within a low molar mass crystalline matrix material in which metal ions also are present. The sample is vaporized by a short pulse (a few nanoseconds) of high-energy laser radiation (usually UV light of 337 nm wavelength) Each polymer molecule associates with a metal ion and is carried into the vapor phase. After a short delay (typically ns), the molecular ions are moved into the mass separator/detector system. Procudes singly charged species

18 10.2 K 19.8 K

Molecular weight of polymers. Molecular weight of polymers. Molecular weight of polymers. Molecular weight of polymers. H i

Molecular weight of polymers. Molecular weight of polymers. Molecular weight of polymers. Molecular weight of polymers. H i Gel Permeation Chromatography (GPC) : Size Exclusion Chromatography GPC : 1. Chromatogram (V R vs H) H i Detector response Baseline N i M i 130 135 140 145 150 155 160 165 Elution volume (V R ) (counts)