Polymer Molecular Weight Methods

5 Thin-Layer Methods for Determining Molecular Weight Distribution

Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0125.ch005

E. P. OTOCKA Bell Laboratories, Murray Hill, N. J. 07974

Thin-layer chromatography (TLC) is a common laboratory technique for separating complex mixtures of solutes, usually by an adsorbtion mechanism. Several laboratories have applied the technique to the separation of polymer fractions and characterization of polymer molecular weight distribu tions. This work reviews the experimental results and theoretical approaches to the fractionation mechanisms. ^ p h i n - l a y e r chromatography ( T L C ) is a well-known technique for the separation of mixed solutes by adsorption. Compared with related techniques i n column chromatography, T L C offers several advantages. The use of adsorbents having large surface area ( particle size ~ 1 0 μ vs. 50 μ) results i n excellent resolution. Separation and analysis time are reduced because many samples can be run simultaneously and material need not be eluted for quantitation. Several references provide a com prehensive background on T L C ( 1-3 ). T L C has been a widely accepted technique in biochemistry and or ganic chemistry for a number of years. W i t h modern quantitation T L C is being used today i n such diverse applications as air pollution analysis and clinical medicine. The initial applications of T L C to problems i n polymer chemistry were directed to the separation of polymer blends, stereoisomers, and a variety of copolymers ( 4-8 ). F r o m these investigations occasional molec ular weight effects were noted (8). Recently efforts i n three separate laboratories have been successful i n determining the molecular weight distribution of a polymer sample by T L C techniques (9-12). A n explana tion of the facility and high resolving power observed has been sought through a number of continuing experiments (11-16). The purpose of this report is to review the results of these studies and to comment on the fractionation mechanisms postulated. 55

In Polymer Molecular Weight Methods; Ezrin, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.


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Three types of interactions occur during a T L C run which determine the results. The following is a discussion of these factors, with special emphasis on their relation to the chromatography of polymers. The solvent-substrate interactions depend on the type of adsorption sites on the substrate particles and on the nature of the functional groups and dipole moment of the eluting solvent. The most common expression of the strength of the solvent-substrate interaction is the so-called eluotropic series. Common solvents are rated i n order of polarity and there fore displacing power. A more quantitative rating is found using the solubility parameter δ or solvent strength parameter c° (3, 17). In prac tical terms, the greater the solvent-substrate interaction, the more suc cessful is the solvent in competition with the solute for the adsorption sites on the substrate, and the more completely w i l l the solute be eluted. The interaction of polar solvents with substrates can result in significant changes in the composition of the mobile phase when a mixed solvent elution is being executed. The relative magnitude of this fractionation effect increases as the initial concentration of the more polar component is decreased. Indeed, the chromatographic purification of solvents is the most outstanding example of this phenomenon. These considerations re main largely unaffected by the nature of the solute (i.e., polymeric vs. monomeric). Another consideration unique to T L C is the variation in solvent con centration i n the direction of elution (18). The phase ratio r is simply the weight of solvent per weight of adsorbent measured at various distances from the dip line. The change in r generates the solvent concentration profile. The decrease in phase ratio is an important consideration in sub sequent discussions of the fractionation mechanism. The solute-substrate interactions are responsible for the separation of low molecular weight solutes in conventional T L C . Solutes are par titioned between the mobile phase and substrate. Adsorption takes place at the "head" of the spot and desorption at the "tail." For solutes with varying affinities for the substrate, relatively more or less time is spent in the absorbed state resulting in different migration distances. The adsorption and desorption behaviors of polymers have been the object of extensive study for a number of years (19-21). The low rates of polymer adsorption and desorption as determined by conventional methods would eliminate these processes from consideration as partici pating in T L C separations. A recent study has shown, however, that the removal of polystyrene from silica gel occurs rapidly in benzene, an eluting solvent, and is several orders of magnitude slower in C C U , a noneluting solvent (IS). Both CCI4 and benzene are good solvents for poly styrene, but benzene is a stronger eluent. A n important feature of polymer adsorption is the strong molecular weight dependence of the equilibrium



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surface coverage and total number of adsorbed segments per molecule (19—21). It is very difficult for significant amounts of polymer to adsorb onto any type of substrate from a good solvent. Some mention should be made concerning the state of the polymer at the beginning of T L C experiment. Normally the sample is applied to the plate from solution in a relatively nonpolar solvent. This solvent is then evaporated, and the chromatographic plate is eluted with the chosen eluent. The drying step results in a polymer deposit which would be difficult to characterize; it is not simply a precipitate, and it probably is not a simple adsorbed (multi) layer. Redissolution and entry into the mobile phase under displacement conditions occur in a minute or less (13). The effects of solute-solvent interactions play a greater role in the chromatography of polymers than in conventional T L C . Normally, solu bility plays a very small role i n the T L C behavior of monomeric com pounds. For polymers, however, the effects of solute-solvent interaction are critical. Solvents with 3s which match that of the polymer are "good" thermodynamic solvents, and should displace the polymer during devel opment.

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Figure 1. Determination of molecular weight distribu tion for narrow polystyrene standard. Curve I, calibra tion of Rf vs. M ; curve 11, densitogram of two dimensionally developed sample; curve III, molecular weight distribution: M /M — 1.02 (12). w

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Polymer Behavior in TLC Figures 1, 2, and 3 show examples of molecular weight distributions ( M W D ) determined by T L C . F o r the analyses of Otocka and Inagaki, similar gradient elution methods were employed. Quantitation was achieved by densitometry in situ in one case and by densitometry of a photographic record of the plate in the other cases. Belenkii and Gankina employ a two-dimensional development method to overcome "chroma tographic spreading" followed by photographic densitometry to deter mine the M W D of polystyrene fractions. These results are indicative of the broad range of polydispersities for which the technique is applicable. In order to advance i n more generalized utility where other polymers can be studied, the mechanism of this high-resolution technique must be understood. Several experiments by Inagaki lead to the postulation of a precipita tion mechanism as the prime source of fraction in T L C . Samples of isotactic poly (methyl methacrylate ) show migration with 0 < R < 1 ( R is defined as the solute migration from the starting line divided by the solvent front travel from the starting line ) in mixed C H C I 3 - C H 3 O H i n two separate eluent composition regions. In the first region (—80% C H C 1 ) , samples of different molecular weight show no difference i n R . The secf

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Figure 2. Comparison of GFC and TLC molecular weight distributions_of polystyrene= 76,600, M = 45,400 from TLC; M = 71,800, M = 39,300 from GFC (11). n

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ond region ( ~ 7 0 % C H O H ) , where precipitation is imminent, shows a strong dependence of R on molecular weight (14). Another indication that precipitation and not adsorption is operative depends on the observation that samples of the same molecular weight migrate identical distances from the dip line, independent of their start ing positions on the T L C plate (15). This of course would not be the case if adsorption governed chromatographic mobility. To explain these phe nomena, Inagaki has proposed that the solvent concentration profile results 3


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in precipitation. A typical solvent concentration profile is shown sche matically in Figure 4. The exact contours and values of the solvent con centration profile depend on the method of development (ascending, descending, horizontal) and the nature of substrate (void volume) and solvent (polarity, vapor pressure). It was found that a measured solvent concentration profile and a series of R * (distances from dip line) meas urements indicated a polymer concentration of 0.01 volume fraction at precipitation. A separate measurement in the eluent gave ^ 0 . 0 3 as the concentration of polymer at precipitation for the same temperature (15). More recently, Otocka measured the R * values for several polysty renes using dioxane-methanol and dioxane-2-propanol θ solvents (16). The results shown i n Table I indicate that despite a great variety i n the f

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type of adsorbent nature, R * values are quite similar for similar develop ment techniques. However, the agreement in calculated concentrations at precipitation is not very good between ascending, descending, and horizontal development. Based on Inagaki's solvent concentration profile and our dip line phase ratio, Otocka calculated concentrations at precipi tation for ascending development between 0.003 and 0.006 gram of polymer per gram of solvent depending upon molecular weight. Lightscattering studies showed no turbidity at concentrations of polymer to 0.01 gram of polymer per gram of solvent (molecular weight = 160,000). Thus, while the precipitation theory of fractionation offers good quali tative explanation for the behavior of polymers in T L C , assessment of the quantitative aspects is not yet fully developed.


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Polymer Molecular Weight Methods

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