Liquid Chromatography under Limiting Conditions: A Tool for

Chapter 14

Liquid Chromatography under Limiting Conditions: A Tool for Copolymer Characterization Downloaded via STONY BROOK UNIV SUNY on June 20, 2018 at 14:48:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

A. Bartkowiak and D. Hunkeler Laboratory of Polymers and Biomaterials, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland

Liquid chromatography under limiting conditions of solubility (LC LCS), a method in which the enthalpic and entropic separation mechanism are perfectly compensated over a large molecular weight range, has been used for the characterization of random copolymers. By eluting macromolecules such as polystyrene-co -methylmethacrylate at the L C LCS for one of the homopolymers (58 vol% THF/42 vol% n-hexane for polymethylmethacrylate) one can distinguish the copolymers according to the molar fraction of polystyrene groups. This can be accomplished through calibrating either according to retention volume shifts or peak area. Such L C LCS conditions can subsequently be coupled with a classical SEC separation in order to deconvolute copolymer composition and molecular size distributions.

Over the past two decades there have been a series of chromatographic methods which combine enthalpic and entropie separations [1-4]. Belenkiiwas the first, in a thin layer chromatography arrangement, to utilize a binary eluent combination to equate the free energy of adsorption with that of exclusion. Given this, "critical conditions" were identified where macromolecules eluted with a retention volume independent of molar mass [1]. Other chapters in this book [5] describe the methods which are now under development including liquid chromatography at the point of the elution-adsorption isotherm (LC-PEAT), as well as liquid chromatography under limiting conditions of adsorption (LC LCA) and solubility (LC LCS). These chapters are complimented by recent reviews [2,6,7]. It is advances in the latter two phenomena which will be described in this chapter, with a particular emphasis on the characterization of random copolymers with high molar masses.

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

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In contrast to LC-PEAT, where the binary eluent involves two solvents for the polymer probe and the composition at the elution-adsorption transition is very sensitive to fractional changes in the solvent ratios, limiting conditions are achieved by combining a non-solvent for the polymer as a component of the mobile phase. Indeed, the thermodynamic quality of the solvent in L C LCS is such that the polymer cannot dissolve in the mobile phase. Given this, polymers are injected in a thermodynamically good solvent. The resulting mechanism involves a continuous process of elution, adsorption and redissolution in which the macromolecules move at a velocity faster then the injection zone, encounter the mobile phase adsorb onto the sorbent surface. As the injection zone reaches the adsorbed polymer, the chains are subsequently redissolved. The net result is the elution of the polymer, independent of its molar mass, just in front of the system peak [8]. It has been shown through cloud point curve measurements [9] that the retention independent composition can occur when the operative forces within the column are a combination of exclusion and adsorption (i.e. L C LCA) or exclusion, adsorption and solubility (LC LCS). These two cases are depicted in Figure 1. Figure 2 shows example of L C L C A identified for polystyrene in THF/n-hexane over a polystyrene/divinylbenzene sorbent. By comparison for the same mobile and stationary phase combination we have observed for the polymethylmethacrylate the L C LCS. Clearly in this case the calibration curve is in the insoluble portion of the cloud point curve (Figure 3). Recently the first hybrid L C L C A - L C S system have been identified [4] for a water soluble polymer (polyacrylamide), as is shown in Figure 4. Although the critical condition, or LC-PEAT, methodologies have been theoretically modeled [10] and experimentally shown to work well for the characterization of functional groups [11], the molecular weight limit for the exclusion-adsorption point is approximately one-hundred thousand daltons. This should not be surprising since the free energies of exclusion and adsorption have quite different molecular weight dependencies. Recently L C - L C A has been shown to be effective in the characterization of polymer blends [6] as well as the tacticity of copolymers [12]. Interestingly, the use of a non-solvent as a component of the mobile phase enables the limiting condition of adsorption or solubility to exceed one million daltons [13]. In the case of L C LCS the use of a second enthalpic separation mechanism, based on solubility, seems to permit the compensation of molar mass dependencies of exclusion and adsorption/solubility over four orders of magnitude [7,8]. This lack of an upper provides hope that these methods will be suitable for the characterization of high copolymers. While experimental data to date has shown the ability to characterize one block of a copolymer, at a limiting condition for the second block, this chapter will report the first finding on the characterization of random copolymers with L C L C S . Clearly this is quite important, since the ability to eliminate a separation according to size permits one to fractionate polymers according to their composition distributions. If an SEC column is added in series the polymer can subsequently be separated according to molecular size. Therefore, the deconvolution of the composition and molecular size distributions is possible with L C LCS, under certain conditions, as will be discussed.

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

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Figure 1. A schematic plot of the solubility of polymer standards in a mixed eluent (solvent plus non-solvent) system, in interactive liquid chromatography experiments. In domain A , adsorption is the operative enthalpic mechanism which is balanced with exclusion (LC-LCA). In domain C the polymer solvent solubility dominates the enthalpy. Domain Β is a hybrid where the entropie exclusion forces are balanced by the adsorption and solubility. Note that Mi an M represent the range where the retention volume is independent of the polymer molecular weight. 2


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Figure 2. A cloud point curve for polystyrene ( · ) in THF/n-hexane. Measurements were performed at a polymer concentration of 1.0 mg/mL. Line 1 designates the L C L C A point (•), which is clearly in the soluble domain (S).

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Figure 3. A cloud point curve for polymethylmethacrylate ( · ) in THF/n-hexane. Measurements were performed at a polymer concentration of 1.0 mg/mL. Line 1 (•) designates the L C LCS point, which is clearly in the insoluble domain (NS).


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Figure 4. A plot of the solubility of polyacrylamide (PAM) standards ( · ) in a mixed eluent (0.05 M aqueous Na S0 plus methanol) system. P A M of various molecular weights are soluble to the left of cloud point curve (S zone), and insoluble to the right of solid line (NS zone). Line 1 (•) represents the retention independent MW condition (LC-LCS for molecular weights above 2 10 daltons) for P A M ob served on a polyhydroxymethacrylate gel (data from reference [8]). 2

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206 Experimental Liquid Chromatograph. A n L-6000 (Hitachi Instruments, Tokyo, Japan) isocratic pump coupled with an Hitachi L-4000 U V detector operating at a wavelength of 234 nm were utilized in all experiments. A Rheodyne type 7725i valve (Coati, C A , USA) with an injection loop of 20 mL was employed. Chromatograms were collected on a Pentium computer running LaChrom D-7000 Multi HPLC Manager Software (Merck, Germany). The separations involved 1.5 mL/min flowrate, a solute concentration of 1.0 mg/mL with a 2 cm tubing (500 mm ID) connection length between the valve and column. Mobile and Stationary Phases. Spectranalyzed grade THF (Fisher, Norcross, G A , USA and Merck, Switzerland) and HPLC grade n-hexane (Fisher and Merck, Switzerland) were used as received. A Shodex (JM Science, Grand Island, N Y , USA) linear GPC 806L column (0.8 χ 30 cm) packed with 10 pm polystyrene-codivinylbenzene particles was employed for all experiments. Experiments were carried out at 25 ±0.1 °C in a Hitachi L-7300 column oven. Polymers. Polystyrene standards with a molecular weight range of 370-1,400,000 were obtained from American Polymer Standards Corporation (Mentor, Ohio, USA). Atactic polymethylmethacrylates between 6,000 and 1,000,000 daltons and broad polyacrylamides between 7,950 and 725,000 (PDI 1.8-3.0) were also purchased from American Polymer Standards Corporation. Random poly sty rene-comethylmethacrylates with a molecular weight 123,000-325,000 and the polidispersity (1.8-2.5) were prepared at the Slovak Academy of Sciences, Bratislava (Table 1). Water Soluble Polymers. Chromatographic measurements were carried out with 0.02 M N a S 0 and methanol as eluents over a polyhydroxymethacrylate sorbent. The gel was packed in a 300-mm stainless column with 8-mm internal diameter. The Shodex OHpak SB-804 HQ column was obtained from Showa-Denko (Tokyo, Japan). The mobile phase flow rate was 0.5 mL/min and 20 μΐ of a 0.05% wt. aqueous polymer solutions was injected. Polymer samples were injected in a pure solvent (0.02 M Na S0 ). A l l measurements were carried out at ambient temperature. Type I deionized water with a resistivity > 16.7 ιηΩ-cm (Continental Water, San Antonio, T X , USA) was fileter through a 0.2 mm nylon membrane filter. HPLC grade methanol was purchased from Fisher Scientific (Norcross, G A , USA). The HPLC system was identical to that described for organically soluble polymers. 2

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Cloud Point Measurements. A Bausch and Lomb (New York, N Y , USA) Spectronic 20 spectrophotometer operating at 340 nm and ambient temperature was utilized for cloud point measurements. Capped scintillated glass samples vials filled with 2 mL of liquid, were employed. Measurements were performed at a polymer


207

Table 1. Properties of Random Copolymer Poly sty rene-co-methylmethacrylate Samples

Sample

Styrene (Mol %)

M .10

1 2 3 4 5 6 7 8 9 10

0 5.1 10.2 20.4 25.5 50.7 75.5 90.2 95.1 100

325 318 262 247 200 168 137 155 123 152

w

3

M .10 n

181 148 135 106 100 76 65 64 54 61

3

M /M w

1.80 2.15 1.92 2.33 2.00 2.21 2.09 2.40 2.28 2.49


n

208 concentration of 1.0 mg/mL. A l l measurements were carried out at a temperature of 23 ±1 °C. Samples were agitated with magnetic stirring bars.

Results A n d Discussion Application of L C - L C S to the Characterization of Random Copolymers. It has been generally speculated, and mathematically predicted [10], that liquid chromatography under critical conditions or at the exclusion-adsorption isotherm could only be applied to the characterization of one block in a diblock polymer, the central portion of a tri-block copolymer or the backbone of a grafted chain. That is, the L C - C A P condition acts to eliminate the influence of the free end on the separation of the molecule. Hence, the copolymer elutes according to its central block or backbone unit. Clearly, such LC-CAP conditions would not be expected to be applicable to the characterization of random copolymers. While this has not been proven experimentally, sequence lengths can be quite large in copolymers synthesized by radical techniques. Therefore, one might anticipate that methods which combine exclusion with enthalpic separations could be applicable for copolymers produced from monomers having relatively different reactivity ratios. In particular, if one would evaluate a copolymer which was rich in one of the monomers where long blocky have been shown to exist by NMR. Given this, the authors of this chapter sought to investigate a common copolymer based on styrene and methylmethacrylate by L C LCS. Figure 5 shows calibration curves for polymethylmethacrylate in THF/nhexane at various percentages of the non-solvent. It is clear that the L C L C S condition is not as sensitive to eluent composition as LC-CAP compositions are and one may vary the n-hexane non-solvent level by ±2% without any noticeable influence on the retention volume or the vertical nature of the calibration curve. Figure 6 shows a chromatogram for polystyrene-co-methylmethacrylate in pure THF as well as at the L C - L C A for P M M A . It is important to note that this separation was carried out at the L C LCS for P M M A since this involved a lower n-hexane level (42 vol%) then in the L C L C A for polystyrene (73 vol%). Clearly, only at the L C LCS for P M M A can one identify a condition where one component of the copolymer is eluted in the SEC mode. Figure 7 shows chromatograms for a series of polystyrene-comethylmethacrylates of varying compositions at the L C L C A for P M M A (58/42 vol% THF/n-hexane). Shifts in the retention volume and peak area are evident. Figure 8 plots the ratio of peak height obtained under the L C LCS conditions versus the peak height in the pure solvent (THF). Similarly Figure 9 plots the ratio of the peak area at the L C LCS to that in pure THF. Clearly, there is correlation between both peak height (Figure 8) and peak area (Figure 9) with copolymer composition and either can be used as an index of copolymer composition. The authors believe this is the first reported evidence of the use of a coupled (entropic-enthalpic) isocratic separation for the characterization of copolymer composition. Figure 10 illustrates a plot of the


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Figure 5. A plot of the molecular weight (daltons) as a function of the retention volume (mL). The calibration curves for narrow polymethylmethacrylate standards in a mixed eluent (THF/n-hexane) are shown at various compositions, expressed as volume percentages.

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Figure 6. Chromatogram of a random P(S-MMA) copolymer containing 10 mol % styrene in pure T H F as well as at the L C - L C A for PMMA.


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Liquid Chromatography under Limiting Conditions: A Tool for

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