Strategies in Size Exclusion Chromatography - American Chemical

Chapter 15

Compositional Heterogeneity of Poly(vinyl alcohol): Characterization by Liquid Chromatography Techniques Downloaded by UNIV MASSACHUSETTS AMHERST on October 8, 2012 | http://pubs.acs.org Publication Date: May 30, 1996 | doi: 10.1021/bk-1996-0635.ch015

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Elizabeth Meehan , Simon P. Reid , Frank P. Warner , Michael Patterson , and John V. Dawkins 2

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Polymer Laboratories Limited, Essex Road, Church Stretton, Shropshire SY6 6AX, United Kingdom Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LE11 3TU, United Kingdom

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Samples of partially hydrolysed poly(vinyl alcohol) P V O H may contain several heterogeneities requiring the development of chromatographic techniques for characterization. Size exclusion separations have been carried out using a number of aqueous eluents, incorporating electrolyte, or electrolyte/organic modifier, or surfactant. The most favourable molecular size separation was obtained using 0.25% w/v sodium lauryl sulfate as eluent. Compositional distributions for water-based polymers can be assessed using high-performance liquid chromatography employing a reversed-phase column. For PVOH, gradient elution with water/tetrahydrofuran with a wide pore polystyrene-based packing produced separations dependent on degree of hydrolysis and sequence length distribution. The elution results were verified with a column packed with non-porous beads. Partially hydrolysed P V O H samples appeared to have a broad distribution of composition. Size exclusion chromatography (SEC) is well established as a technique for determining the molar mass distribution (MMD) of homopolymers. Heterogeneous copolymers contain distributions in both molar mass M and copolymer composition. Copolymer characterization based on SEC is often performed with on-line selective concentration detectors (1.2). For heterogeneous copolymers this SEC-based method is only capable of producing average composition data across a chromatogram, because copolymer chains having the same molecular size in solution will have variations in molar mass and composition (3.4). Off-line light scattering has been developed to determine data for M and compositional heterogeneity for copolymers (5-8). The parameters used to quantify the compositional heterogeneity of a copolymer sample, as determined from light scattering data, represent the effect of M on compositional heterogeneity, and overall compositional drift. Therefore, addition of on-line low-angle laser light scattering detection to an SEC system with dual concentration detection can permit for some types of copolymers the calculation of compositional heterogeneity at each elution volume together with overall heterogeneity parameters (9.10). Corresponding author 3

0097-6156/96/0635-0262$15.00/0 © 1996 American Chemical Society

In Strategies in Size Exclusion Chromatography; Potschka, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

Downloaded by UNIV MASSACHUSETTS AMHERST on October 8, 2012 | http://pubs.acs.org Publication Date: May 30, 1996 | doi: 10.1021/bk-1996-0635.ch015

15. MEEHAN ET AL.

Compositional Heterogeneity of Poly (vinyl alcohol)

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When copolymers contain composition heterogeneities, some type of crossfractionation procedure, involving separating by composition fractions previously separated by size (or vice versa), can be attempted, but the experimental work involving transfers between techniques is quite time-consuming. In orthogonal chromatography, on-line transfer can be automated and Balke et al (11-14) demonstrated copolymer separations with two chromatographic systems in which copolymer is separated first by SEC and second by non-exclusion mechanisms. This procedure which may be termed coupled column chromatography (CCC) has been investigated further in order to demonstrate how non-exclusion separations dependent on copolymer composition in the second column can be influenced by choice of polymer-based stationary phase and by the liquid components in the mobile phase (15.16). A major aim of our continuing program on chromatographic techniques is to develop a methodology for separations of water-based copolymers according to composition with a polymer-based stationary phase. Such an objective would suggest studies of high performance liquid chromatography (HPLC) with aqueous eluents for more polar polymers with for example a crosslinked polystyrene-based packing, indicating a reversed-phase separation. A range of partially hydrolysed samples of polyvinyl alcohol) (PVOH) which is normally produced by hydrolysis of polyvinyl acetate) (PVAC) with an acid or a base catalyst was chosen for these investigations. Products with levels of hydrolysis above 70% are water soluble and have many useful applications (17). Characterization of partially hydrolysed PVOH may be regarded as a copolymer problem with heterogeneities in molecular weight M and composition (VOH content). An important extension of this work in the longer term is to perform an orthogonal, or two dimensional, experiment by coupling SEC with H P L C for partially hydrolysed PVOH, but as yet it is unclear whether C C C should operate by SEC-HPLC or HPLC-SEC. Aqueous SEC because of the nature of the analytical columns available and the polymer systems under investigation demands modification of the aqueous eluent in order to minimise secondary interaction effects whilst ensuring full dissolution of the sample (18). Aqueous SEC studies on P V O H have been reported (19), but it is apparent that the variation in properties associated with degree of hydrolysis render the interpretation of data difficult. If partially hydrolysed PVOH is to be characterized by aqueous SEC, the eluent employed needs to be such that there is no secondary effect on the size of the polymer in solution or on the retention mechanism on the column. HPLC methods for P V O H can be developed from observations on adsorption of P V O H onto polystyrene latex for which adsorption has been shown to increase with a decrease in the solvency of the medium, water being a better solvent as the degree of hydrolysis increases (20). This suggests that HPLC by a reversed-phase mechanism on columns packed with polystyrene beads may lead to a separation of P V O H based on V A C content. Gradient elution, where the solvent composition is gradually changed, will facilitate separation of multicomponent samples by HPLC. The aim in this paper is to report independent studies of separations of P V O H by SEC and HPLC. The work on SEC is directed towards the effect of changing the constitution of the mobile phase on the elution characteristics of partially hydrolysed PVOH. For HPLC a separation method suitable for the analysis of P V O H covering a wide range of V A C content is required. Results from these independent studies will permit consideration in the future on how to couple the two chromatographic systems. Experimental A range of P V O H samples was derived from a single source of P V A C by alcoholysis in a methanol/methyl acetate medium using an alkaline catalyst to produce blocky polymer and an acid catalyst to produce random polymer (17). The average degree of hydrolysis was determined by a titration method (17) and the blockiness factor was determined by C - N M R spectroscopy (21). 1 3


STRATEGIES IN SIZE EXCLUSION CHROMATOGRAPHY


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Sample solutions for chromatography were prepared by stirring an accurately weighed sample of polymer in eluent (SEC) or water (HPLC) and heating to 90°C for dissolution. SEC analysis was carried out using a system comprising a model 64 pump, a model 98 refractive index detector (both Knauer, Germany) and a model 7125 injection valve (Rheodyne, USA). The first columns used (two in series) were polymeric based, with a particle size of 8μτη and an exclusion limit based on poly(ethylene oxide) of 200,000 (PL aquagel-OH 40, 300 χ 7.5 mm, Polymer Laboratories, UK). Further analysis used the same type of packing but with a higher exclusion limit of 1,000,000 (PL aquagel-OH 50, 300 χ 7.5 mm). An eluent flow rate of lOmL/rnin was used and samples were analyzed in various eluent systems with an injection volume of 200|jL and sample concentration of 0.5% w/v. The eluent modifiers NaN03, methanol, sodium lauryl sulfate (Fisons, UK) required no further treatment and were used as supplied. H P L C analysis was carried out using a gradient system comprising two model 64 pumps controlled by a model 50 HPLC programmer, a dynamic mixing chamber (Knauer, Germany), a model 7125 injection valve (Rheodyne, USA), and a model PLE M D 950 evaporative mass detector (Polymer Laboratories, UK). The column used was a polymeric-based reversed-phase packing of polystyrene/divinylbenzene (PS/DVB) with a particle size of 8μηι and a pore size designation of 4000Â (PLRP-S 4000Â 8μηι 50 χ 4.6mm, Polymer Laboratories, UK). Further studies employed an experimental column of the same dimensions packed with non-porous PS/DVB particles of the same diameter. An eluent flow rate of l.OmL/min was used throughout and samples were analyzed at room temperature using a linear gradient of water and tetrahydrofuran (THF) with an injection volume of 50-1000pL and sample concentrations of 0.2-0.5% w/v. HPLC grade water (Rathburn, UK) and H P L C grade THF (Fisons, UK) were used throughout. The mass detector was operated at an evaporation temperature of 90-100°C using compressed air as nebulizer gas at a flow rate of 12-14I7min. Results and Discussion Polyvinyl alcohol). The two series of model P V O H polymers were derived by hydrolysis from a single source of P V A C , with blocky or random architectures dependent on the method of hydrolysis (17). In C - N M R studies of P V O H when the methylene carbon is considered, the spectrum contains three peaks corresponding to the three possible chain sequences which can be used to calculate relative sequence lengths (21). Typically, alkaline hydrolysed samples with a degree of hydrolysis of 70% had a blockiness factor, n, of 0.4, which increased to 0.43 for a degree of hydrolysis of 80%. These results may be compared with acid hydrolysed samples with degrees of hydrolysis between 72 and 76%, having a value of η of 0.83. As values for η increase, the blockiness of the polymer decreases, (a perfectly random polymer would exhibit a value of n=l). These results confirmed that the alkaline hydrolysed samples were blocky and also the blockiness decreased with increasing degree of hydrolysis. 13

Size Exclusion Chromatography. As the P V O H samples were all produced from a single source of P V A C , it was expected that the molecular size in solution of all the hydrolysed polymers would be similar, that is the SEC chromatograms would exhibit similar profiles. The first SEC eluent used was 0.05M NaN03 which had been reported in the literature as being suitable for a relatively high degree of hydrolysis P V O H (88% or greater) (22). Initial experiments were performed with alkaline hydrolysed P V O H covering the hydrolysis range 77.7% to 100. A general trend was observed, that as the degree of hydrolysis decreased the detector response decreased and the peak retention time increased. This type of behaviour could be caused by either adsorption of the polymer onto the column packing material or a decrease in molecular size or a change in polymer refractive index as the V A C content of a sample increased. In order to



15. MEEHAN ET AL.

Compositional Heterogeneity of Polyvinyl alcohol)

265

investigate this further the eluent was modified to include a higher salt concentration (0.2M) and 20% by volume methanol. Previous studies (23) have indicated that the addition of this organic modifier is sufficient to suppress hydrophobic interactions for synthetic polymers. With this eluent modification chromatograms with improved consistency were obtained, presumably as a result of suppressed interaction or improved solubility, but there was still a gradual decrease in detector response as the degree of hydrolysis decreased. It has been suggested (24.25) that in aqueous solution sodium lauryl sulfate (SLS) binds readily with P V O H thus irihibiting intermolecular interactions resulting in dissociation of multimers which will reduce molecular size. If this were the case, then an eluent based on SLS may be useful since it will present the polymer molecules in an expanded form irrespective of degree of hydrolysis, and thus yield a separation based on true size in solution. However, SLS must be used at a level below its critical micelle concentration in order to avoid induced micellization. Initial SEC chromatograms for alkaline hydrolysed PVOH samples indicated similar elution patterns for all degrees of hydrolysis above 70%, but chromatograms for the P L aquagel-OH 40 columns exhibited peaks showing excluded chains. When the pore size was increased for the P L aquagel-OH 50 columns and the same eluent employed, all samples eluted with very similar profiles, see Figures 1 and 2, which suggested that a true size exclusion separation mechanism was taking place in the absence of secondary interaction effects. Determination of M M D from the chromatograms in Figures 1 and 2 must be determined carefully. Procedures based on molar mass calibrations should be attempted cautiously in view of the results on composition heterogeneity (see next section). Furthermore, application of a universal calibration approach may have to assess whether a blocky versus random chain architecture of V A C units may influence the dependence of chain size on M . Also it is possible that partially hydrolysed PVOH may contain long-chain branching dependent on the conditions of polymerization of V A C . A full characterization of some partially hydrolysed PVOH samples would therefore require an extensive programme of experiments to determine distributions in M , composition, sequence length and branching. High Performance Liquid Chromatography. Samples of P V O H were analysed using a PLRP-S 4000À 8μπι 50 χ 4.6mm column with gradient elution from 99% water/1% THF to 30% water/70% THF in 8 min. Typical HPLC chromatograms obtained for alkaline hydrolysed samples of different degrees of hydrolysis are shown in Figure 3. Fully hydrolysed P V O H always exhibited a relatively sharp early eluting peak, indicating limited interaction with the packing material. For samples of partially hydrolysed PVOH, broader and later eluting peaks were observed indicating stronger interactions at higher levels of V A C . Typical chromatograms for acid hydrolysed samples, shown in Figure 4, exhibited a similar trend to those for the alkaline samples. At lower degrees of hydrolysis, the increased V A C content enhanced interactions with the PS/DVB packing material at the start of the gradient (99% water/1% THF). Thus, a higher concentration of THF is required to release the samples resulting in increased elution time. A strong correlation was observed between elution time and degree of hydrolysis for both acid and alkaline hydrolysed samples. In general, the samples of alkaline hydrolysed (blocky) PVOH eluted later than the acid hydrolysed (random) samples for the same degree of hydrolysis. A more blocky distribution of acetate groups, that is a longer sequence length, presents a larger segment for column adsorption and requires correspondingly higher THF content to elute the sample. This difference is not observed at high degrees of hydrolysis, greater than 90%, where the sequence lengths in random and blocky samples are similar. As with the SEC experiments, it was desirable to ensure that the HPLC separations operated by a single mechanism. A n HPLC column with very wide pores (designation 4000Â) was selected following consideration of the molecular size




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5. MEEHAN ET AL.

Compositional Heterogeneity of Polyvinyl alcohol)

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Figure 2. Effect of degree of hydrolysis on SEC chromatograms for acid hydrolysed (random) PVOH. Columns: 2 χ P L aquagel-OH 50, 8μ, 300x7.5mm, eluent: 0.25% sodium lauryl sulfate. 1. 75.6%, 2. 84.6%, 3. 93.9%, 4. 97.2%.


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Strategies in Size Exclusion Chromatography - American Chemical

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