Advantages of Determining the Molar Mass Distributions of Water

Chapter 9

Advantages of Determining the Molar Mass Distributions of Water-Soluble Polymers and Polyelectrolytes with FFFF-MALLS and SEC-MALLS Downloaded by MICHIGAN STATE UNIV on February 26, 2015 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0731.ch009

W.-M. Kulicke, S. Lange, and D. Heins Institut für Technische und Makromolekulare Chemie, Universität Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany

This paper describes the characterization of the absolute molar mass distribution of water-soluble polymers with the combined fractionation apparatus of size-exclusion chromatography (SEC) and flow field-flow -fractionation (FFFF) coupled with a multi-angle laser light-scattering (MALLS) photometer, which is sensitive to molar mass, and a differential refractometer to measure the differential refractive index (DRI), which is sensitive to concentration. Emphasis is placed on the advantages of these methods of determination with reference to polymers having a variety of structures, such as polysaccharides, polycations, polyanions and synthetic polymers. It is also shown how the separate fractions can be characterized during enzymatic degradation. The same is true for ultrasonic degradation. In the examples illustrated here degradation occurs in the centre of the chain so that homologous series are generated while at the same time the molar mass distribution becomes somewhat narrower. The cause of this is the asymmetric molar mass distribution of the native sample. Examples are used to discuss the merits and limitations.

On account of their properties, water-soluble synthetic and biological polymers and polyelectrolytes have commercial applications in a large number of technological fields, examples include use as flow enhancers, thickening agents and stabilizers (J). Both synthetic polymers and those based on renewable raw materials are mixtures of homologous substances with differing molar masses. For the former this is a consequence of the statistics inherent in every polymer reaction, for the latter the reason is to be found in the lack of reproducibility within nature, and in degradation reactions during pulping or derivatization. The molar mass of a polymer is the product of the molar mass of the monomer, M , (basic component/repeating unit) and the degree of polymerization, P, i.e. the number of repeating units in the respective polymer chain. For relatively low degrees of polymerization (P < 100) the molar mass m o n

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© 1999 American Chemical Society In Chromatography of Polymers; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Downloaded by MICHIGAN STATE UNIV on February 26, 2015 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0731.ch009

distribution can still be described and detected analytically as a distribution of individual species, whereas the molar mass distribution for higher degrees of polymerization turns into a quasi-continuous, generally asymmetric distribution function (see Figure 1). The distribution of molar mass and particle size has a crucial influence on the property profiles of polymers in solution. This paper aims to take an in-depth look at the often problematic determination of the distributions.

Molar mass Figure 1. Schematic representation of an asymmetric distribution function. The determination of molar mass has been a very time-consuming and expensive process, so that mean values are generally given to characterize polymers. These mean values (Figure 1) depend upon the method of determination, hence for example the number-average, M , is determined from osmometric measurements, the weight-average, M , from light-scattering measurements and the viscosity average, Μη, from viscosity measurements. It is known that the mean of Μ (grey shading in Figure 1) varies as a function of the exponent a of the Mark-Houwink relationship (2). The ratio of M to M ( M / M ) is often given as a measure of the polydispersity. In some special applications the declaration of mean values is not sufficient for product optimization and quality assurance. Only knowledge of the entire distribution curve can lead to a distinct characterization of the products and thus allow the structure and technological properties to be correlated. For instance, the length of the soft segments, i.e. diol component, has a crucial influence on the macroscopic properties of segmented polyurethanes (3). The broad distribution of polyethylene is known to be decisive for the quality of processing (Ref. 2, P. 246). Knowledge of the overall distribution assumes almost vital importance for plasma substitutes as highmolar-mass flanks are suspected of triggering anaphylactoid reactions, and mean values do not yield the crucial information (4). One way of determining the molar mass distribution is to fractionate the sample and then determine the molar mass of each separate fraction. Size exclusion chromatography (SEC) (5,6,7) and recently Flow Field-Flow Fractionation (FFFF) (8,9,10) too, can be employed to fractionate polymers by their size. Coupling these fractionating units with a detector system consisting of a lightscattering photometer (MALLS) and a differential refractometer (DRI) makes it possible to separate the polymers and at the same time to carry out an absolute determination of the molar mass and radius of gyration, (RG); hence the entire distributions are determined for molar mass and radius (4,11). n

w

η

w

n

w

n

In Chromatography of Polymers; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

116 This paper aims to demonstrate that these two methods (SEC/MALLS/DRI and FFFF/MALLS/DRI) can be used to determine the distributions of molar mass and radius of gyration for many water-soluble polymers.


Polymers investigated Bovine serum albumin (BSA, Fluka, Neu-Ulm, Germany) is a member of the large albumins group. Together with the globulins and prolamines, these form the most important group of proteins. B S A is obtained from the blood of cattle and has a molar mass of about 66,000 g/mol (72). The refractive index has been determined as 0.170 mL/g (Wood RF-600 (Wood Co., PA, USA), 0.1 M N a N 0 solution with 0.02% azide added, 633 nm, 298 K). The tobacco mosaic virus (TMV) consists of a ribonucleic acid helix which is stabilized by 2,130 protein sub-units that are suspended in an outward direction. The result is a hollow cylinder with a length of 300 nm and a width of 15-18 nm, the inside diameter is approx. 4 nm (13). The T M V investigated here was kindly provided by M . Mackay, University of Queensland, Australia. A transmission electron micrograph of the tobacco mosaic virus is shown in Ref. 14. The starch derivatives hydroxyethyl starch and acetyl starch were prepared from so-called waxy starches with an amylopectin content of > 95 % (15,16). Amylopectin is the highly branched component of starch and its aqueous solutions are stable. The main chain consists of a-(l-^4)-linked D-glucose, with a-(l-»6)-linked branching site every 18 to 27 glucose residues (see Figure 2) (77). The desired mean molar mass is adjusted by partial hydrolysis. The hydroxyethyl starch samples investigated were commercial products used as plasma substitutes. Hydroxyethylation was carried out by means of ethylene oxide in alkaline medium (18). Esterification to acetyl starch was accomplished by means of acetic anhydride in alkaline medium (16). 3

Figure 2. Structure of the starch derivatives acetyl and hydroxyethyl starch: oc-(1^4) - linked chain of glucose units with a-(l->6) - branches.


117 The refractive index increments were determined in 0.1 M N a N 0 solution with 0.02% azide added (T = 398 K) at 0.133 mL/g for hydroxyethyl starch and at 0.138 mL/g for acetyl starch. The cellulose derivatives hydroxyethyl cellulose (HEC) and carboxymethyl cellulose (CMC) (Figure 3) were synthesized from alkaline cellulose as the starting material. This involved activating the cellulose with caustic soda in an inert solvent (e.g. isopropanol), i.e. the hydrogen bonds in cellulose are stretched or broken (19). Hydroxyethylation to produce H E C was performed by epoxide reaction with ethylene oxide at a temperature of 30-80°C. The maximum average degree of substitution for cellulose is DS = 3.0 because the substituent can also react with the reagent the molar degree of substitution (MS) may be higher. For commercial samples the DS lies within a range from 0.8 to 1.2, whereas the MS takes values from 1.7 to 3.0. An M S of 2.5 was given for the samples from Polysciences investigated here. The refractive index increment was determined as 0.145 mL/g. Carboxymethylation to C M C was carried out by a Williamson ether synthesis in a heterogeneous reaction using chloroacetic acid and involving the formation of NaCl. The samples investigated here came from the company Wolff Walsrode A G . Sample C M C 1 has a DS of 1, sample C M C 2 a DS of 2.4. A refractive index increment of 0.136 mL/g was used.


3

HEC: R = - ( C H C H 0 ) - C H C H O H 2

2

n

2

2

3

CMC: R = -CH COO" N a

+

2

Figure 3. Structure of the cellulose derivatives. β-(1—»4) - linked chain of glucose units.

The sodium polystyrene sulphonate standards (NaPSS) (left-hand formula in Figure 4) used come from the company Polymer Standard Service (Mainz). According to the distributor, the samples were manufactured by anionic polymerization of styrene followed by sulphonation. The degree of sulphonation is given as greater than 90% and the polydispersity as M / M less than 1.1 ( M / M (NaPSS-7) < 1.3). According to the information supplied, the samples underwent dialysis and freeze-drying prior to delivery. The refractive index increment was determined after Equilibrium dialysis in 0 . 1 M N a N O solution with 0.02% azide added (633 nm, 298 K) as 0.195 mL/g (20). The non-ionic polyacrylamide (PAAm) (right-hand formula in Figure 4) was synthesized in the laboratory by the radical polymerization of acrylamide, for details refer to Kulicke in Houben-Weyl amongst others (21,22). A value of 0.177 mL/g was determined for the refractive index increment after Equilibrium dialysis in 0.1 M N a N 0 solution with 0.02% azide added (633 nm, 298 K). w

n

3

3


w

n

118

η


polystyrene sulphonate

polyacrylamide

Figure 4. Monomer units of polystyrene sulphonate (left) and polyacrylamide (right). Poly(diallyldimethylammonium chloride) (poly-DADMAC) is a cationic polymer that is used in water treatment (flocculation and dewatering). Synthesis is by radical cyclopolymerization in aqueous solution (23). The product contains the three possible structures shown in Figure 5. The refractive index increment was determined after Equilibrium dialysis in 0.1 M N a N 0 solution with 0.02% azide added (633 nm, 298 K) as 0.12 mL/g (16). 3

84%

14%

Advantages of Determining the Molar Mass Distributions of Water

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