Two-Dimensional Liquid Chromatography Analysis of Polystyrene

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Two-dimensional liquid chromatography analysis of polystyrene/polybutadiene block copolymers Sanghoon Lee, Heejae Choi, Taihyun Chang, and Bastiaan B.P. Staal Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00913 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Analytical Chemistry

Two-dimensional liquid chromatography analysis of polystyrene/polybutadiene block copolymers

Sanghoon Lee,+ Heejae Choi,+ Taihyun Chang* Division of Advanced Materials Science and Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang, 37673, Korea

Bastiaan Staal Competence Center Analytics, BASF SE, Ludwigshafen, 67056, Germany

+

Equal contributions

*Corresponding Authors Taihyun Chang TEL) +82-54-279-2787 FAX) +82-54-279-3399 Email) [email protected]

KEYWORDS: Two-dimensional liquid chromatography, branched block copolymer, polystyreneblock-polybutadiene, Styrolux

Submitted to Anal. Chem.

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ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT A detailed characterization of a commercial polystyrene/polybutadiene block copolymer material (StyroluxTM) was carried out using two-dimensional liquid chromatography (2D-LC). The Styrolux is prepared by statistical linking reaction of two different polystyrene-block-polybutadienyl anion precursors with a multivalent linking agent. Therefore, it is a mixture of a number of branched block copolymers different in molecular weight, composition and chain architecture. While individual LC analysis including size exclusion chromatography, interaction chromatography or liquid chromatography at critical condition is not good enough to resolve all the polymer species, 2D-LC separations coupling two chromatography methods were able to resolve all polymer species present in the sample; at least 13 block copolymer species and a homo-polystyrene blended. Four different 2DLC analyses combining a different pair of two LC methods provide their characteristic separation results. The separation characteristics of the 2D-LC separations are compared to elucidate the elution characteristics of the block copolymer species.

INTRODUCTION Polymeric materials are ubiquitous nowadays and the increasing needs for more refined usage demand deeper fundamental understanding of polymers. Synthetic polymers are not homogeneous molecules but complex mixtures with distributions in various molecular characteristics such as molecular weight (MW), chemical composition, chain architecture and so on. These distributions affect the final properties and precise characterization of the distributions is important in the development and quality control of polymeric materials. For the characterization of polymers with multivariate distributions, the separation with respect to a specific distribution is an ideal strategy, if possible. A good example is the size exclusion chromatography (SEC) that separates polymers according to the chain size in a solution and SEC has been used predominantly for molecular weight distribution (MWD) analysis of polymers.1-3 While the size dependent separation is a very powerful method, it is not sufficient to analyze multivariate distributions present in polymeric materials. For example, it is not possible to characterize MWD of copolymers or branched polymers solely by the size since there is no simple correlation between the chain size and MW. To complement the limitation of SEC, other chromatographic techniques such as interaction chromatography (IC) or liquid chromatography at the critical condition (LCCC) have been increasingly used in recent years to meet the growing need in more precise characterization of complex polymers.4-9 When a polymer molecule is subject to pass through a chromatography column packed with porous particles, SEC (when the exclusion effect dominates the interaction effect, 2


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polymers elute before the injection solvent peak in the decreasing order of molecular size), LCCC (when the exclusion effect is balanced with the interaction effects, polymers elute near the injection solvent peak independent of molecular size), and IC (when the interaction effect dominates the exclusion effect, polymers elute after the injection solvent peak in the increasing order of molecular weight) elution modes are observed. These separation modes have their own merits and limitations. One separation mode can provide information complementary to the other and a coupling of two separation mode to a two-dimensional LC (2D-LC) can be far more informative than two 1D-LC separations.10-14 In this study, we characterized a commercial copolymer that is a complex mixture of polystyrene/polybutadiene block copolymers. Block copolymers have been an intense research subject in the last few decades due to their formation of ordered nanophases that can be used as templates for various applications in nanotechnology.15-16 Block copolymers are also used in structural materials such as thermoplastic elastomers or high impact polymers.17-18 They are usually prepared by controlled polymerization methods such as anionic polymerization or controlled radical polymerization to produce a well-defined block structure in the polymer chains. For the molecular characterization of block copolymers, SEC has been used routinely, but it often fails to elucidate the details due to their size dependent separation and large band broadening. Other chromatographic methods can do a better job in the characterization of block copolymers. For examples, LCCC successfully characterized individual block in block copolymers.5, 19-21 IC is effective to fractionate homopolymer byproducts from the block copolymers22-24 and able to fractionate individual blocks into narrower fractions.25-26 Nonetheless, most characterization and fractionation efforts have been made on model block copolymers to demonstrate the separation ability of the chromatographic techniques. Unlike the model block copolymers used in nanotechnology research, the commercial materials are far more complex. A good example is the StroluxTM from INEOS Styrolution that consists of many block copolymers of different MW and composition as well as chain architecture. While any 1D-LC was not good enough to fully characterize the polymer species in the sample, 2D-LC was able to resolve and to identify at least 13 different block copolymer species present in the sample. We applied various 2D-LC methods coupling SEC, IC and LCCC and compared the characteristics of 4 different 2D-LC separations combining different pairs of the LC techniques. In addition, we investigated how the MW and composition of the block copolymers affect the IC retention. Experimental Sample

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The polymer sample used in this work is the Styrolux 693D from INEOS Styrolution. According to the information from the provider, the major component of the Styrolux is a mixture of various block copolymers of PS and PB as well as some additives. The block copolymers in the Styrolux sample are synthesized by anionic polymerization using two step initiation at different time in one reactor to obtain a short and a long polystyrene-block-polybutadiene diblock copolymer (PS-bPB) precursors as shown in Scheme 1.17 At the end of the polymerization of the precursors, a coupling agent is added which has up to 7 bonding sites to yield variously branched PS/PB block copolymers. The short PS-b-PB precursor (S) has MW around 30k and 42 wt% butadiene while the long PS-b-PB precursor (L) has MW around 110k and 11.4 wt% butadiene. The commercial sample contains the block copolymer portion consisting of 23.5 wt% butadiene and 66.8 wt% styrene, 3.6 wt% of homoPS blended and 6.1 wt% of stabilizers and processing aids. The polymer sample was reprecipitated in methanol to remove the small molecular species to reduce the excessive solvent peak in the LC analysis.

Scheme 1. Synthetic Scheme of Styrolux

SEC Analyses The specific refractive index increment (dn/dc) of PS and PB was measured in THF as 0.185 and 0.121 mL/g, respectively. Three mixed bed SEC columns (Agilent Polypore 300 x 7.5 mm, Waters Styragel HR4 300 x 7.8 mm, and Jordi mixed bed 300 x 8.0 mm) were used at a column temperature of 40 °C. SEC chromatograms were recorded with a triple detector (Malvern, TDA 305) and a UV detector (Shimazu, SPD-M20A). The solvent was THF (Samchun, HPLC grade) at a flow rate of 0.7 mL/min. Polymer samples for the SEC analyses were dissolved in THF at a concentration of ∼1 mg/mL, and the injection volume was 100 µL. 4


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Temperature Gradient Interaction Chromatography (TGIC) Analyses For normal phase (NP)-TGIC analyses, a bare silica column (Nucleosil, Macherey-Nagel, 250 x 4.6 mm, 5 µm, 100 Å pore) was used. A mixed eluent of THF and n-hexane (38/62, v/v, Samchun, HPLC grade) was used at a flow rate of 0.4 mL/min with a HPLC pump (Shimadzu, LC20AD). For reversed phase (RP)-TGIC analysis, a C18 bonded silica column (Nucleosil C18, Macherey-Nagel, 150 x 4.6 mm, 5 µm, 300 Å pore) was used. The mobile phase was a mixture of CH2Cl2/CH3CN (64/36, v/v, Samchun, HPLC grade) at a flow rate of 0.4 mL/min. Temperature of the column was controlled by circulating fluid from a programmable bath/circulator (Julabo, F25-HL) through a homemade column jacket. In NP-TGIC, the column temperature was varied linearly from 5 to 78°C for 45 min. In RP-TGIC, the column temperature was varied linearly from 21 to 65 °C for 40 min. Sample solutions (∼1 mg/mL) were prepared by dissolving the polymers in a small volume of the corresponding eluent and the injection volume was 100 µL. The chromatograms were recorded with a light scattering (LS) detector (Wyatt MiniDawn) and a UV absorption detector (Younglin, UV730) operating at a wavelength of 260 nm. LCCC analyses For NP-LCCC analyses, a bare silica column (Nucleosil, Macherey-Nagel, 250 x 4.6 mm, 5 µm, 300 Å pore) and a mixed eluent (THF/n-hexane, 42/58, v/v) were used at the critical adsorption point (CAP) of PS. The column temperature was 58.5 °C and the flow rate was 1.5 mL/min. For RPLCCC analyses, a C18 bonded silica column (Nucleosil C18, Macherey-Nagel, 250 x 4.6 mm, 5 µm, 300 Å pore) was used. The mobile phase was a mixture of CH2Cl2/CH3CN (78/22, v/v) at a flow rate of 1.15 mL/min. The column temperature was 28.5 °C at the CAP of PB. The PS and PB standard samples (homemade) were used to find each CAP condition of PS and PB, respectively. The microstructures of PB standard samples and of the PB block in the Styrolux were confirmed identical by 1H-NMR measurement. Preparation of the injection samples and the detection were done in the same manner as in TGIC analyses.

2D-LC Analyses TGIC x SEC: For the first dimension (1st-D) TGIC analysis, NP-TGIC or RP-TGIC was carried out at the same separation conditions as described in the TGIC experimental section above except for the column temperature program and the flow rate. In the NP-TGIC, the column temperature was varied from 5 to 78°C for 450 min while in the RP-TGIC, column temperature was varied from 21 to 65°C for 400 min. The flow rate was set at 0.04 mL/min for both NP- and RP-TGIC in order to synchronize with the 2nd-D SEC separation. For the 2nd-D SEC analysis, two columns (Shodex, KW-804, 300 x 8.0 mm and Agilent, Polypore, 300 x 7.5 mm) were used. The eluent was THF (Samchun, HPLC 5



grade) and the flow rate was 2.5 mL/min at 90°C. The 2nd-D SEC run was repeated every 5 min while the 1st-D TGIC eluted the effluent to fill up a 200 µL storage loop. The SEC analysis takes about 9 min from the sample injection to the injection solvent elution but it could be repeated every 5 min without losing any polymers since no polymer eluted out in the first 4 min (See Figure S1 in the supporting information). The overlap-injection method helped shorten the total analysis time.27-28 The 2D-LC system is similar to the one reported previously.29 Two HPLC pumps (Shimadzu, LC-20AD) and two UV detectors (Younglin, UV730) were used. The two LC systems were connected via an electronically controlled 10-port 2-position switching valve (Alltech, SelectPro) equipped with two 200 µL storage loops. Control of the switching valves and data acquisition were carried out by a homemade software.

TGIC x LCCC: The separation conditions of the 2nd-D LCCC were the same as in the LCCC condition described above. In the NP-TGIC x NP-LCCC, two LC systems were connected via an electronically controlled 10-port 2-position switching valves (Alltech, SelectPro) equipped with two 100 µL storage loops. In the RP-TGIC x RP-LCCC, two short bare silica columns (Nucleosil, Macherey-Nagel, 30 x 4.6 mm, 5 µm, 300 Å pore) was used instead of the storage loops. The 2nd-D LCCC run was repeated every 2.5 min while the 1st-D TGIC eluted the effluent to fill up a 100 µL storage loop or a short column. Otherwise the 2D-LC system is the same as the TGIC x SEC 2D-LC system.

Results and discussion SEC analysis separates polymers according to the chain size in a good solvent. Polymer chain size depends on not only MW but also chemical composition as well as chain architecture. Therefore, SEC separation often fails to resolve different polymer species present in this type of complex polymer mixture. We recently reported a good example of polystyrene-g-polyisoprene to demonstrate the limitation of SEC separation.30 For an SEC analysis of polymers with chemical composition and/or architectural distribution, multiple detection method is helpful to examine the homogeneity of the polymers. Since different detectors give signals related with different molecular characteristics of the polymers, non-overlapping chromatograms obtained by different detectors provide with good clues for the heterogeneity in the samples.31-32

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Figure 1. SEC chromatogram of the Styrolux recorded by a RI detector (∆n, black dotted line), a LS detector (red dashed line), and a UV detector at 260 nm (green solid line). Separation condition: 3 mixed bed columns. Eluent: THF at a flow rate of 0.7 mL/min; column temperature: 40 °C. MW calibration curve made with PS standards (Mp: 1090k, 360k, 136k, 30.9k, and 11.9k) is also shown. SEC chromatograms of the Styrolux sample are shown in Figure 1. In the SEC analysis, a refractive index (RI) detector (∆n, black dotted line), a LS detector at 90o (R90, red dashed line), a UV detector at 260 nm (A260, green solid line) were used. While the SEC chromatogram shows a broad polymer size distribution (30k ~ 1000k with respect to the PS calibration), it is somewhat different from typical commercial polymers with a broad MW distribution. The Styrolux contains a number of polymer species prepared by anionic polymerization and the SEC chromatogram consists of many narrow peaks overlapped one another. Each peak shows a different A260/∆n ratio indicating that they are of different PS/PB composition. (PB does not absorb UV at 260 nm). For an example, the small peak eluting at tE ~ 36 min has the lowest A260/∆n ratio that indicates the highest PB content among the block copolymer species in the Styrolux sample. It turns out that the peak corresponds to the small PS-b-PB precursor (S1) as described later. Among the possible polymer species present in the Styrolux sample, the short PS-b-PB precursor (S1) and its multimers (S2, S3 ..) have the lowest PS content while the long PS-b-PB precursor (L1) and its multimers (L1, L2 ..) have the highest PS content other than the homo-PS blended. All coupled block copolymers having both short and long PS-b-PB precursor chains have a PS content in between. The variation of A260/∆n with the elution time shows that it does not correlate with the polymer size monotonously. Further analysis of the sample is not possible with the SEC/multiple detection result and different separation methods need to be adopted. For such a heterogeneous polymer sample containing polymer species of different composition and chain architecture, IC separation is often more effective than SEC separation. IC primarily separates block copolymers according to the MW of the interactive block in the block 7



copolymers.25, 33-34 We used temperature gradient elution method in which the column temperature is varied during the elution to control the interaction strength.35-36 80

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Figure 2. (a) NP-TGIC and (b) RP-TGIC chromatograms of the Styrolux recorded by a LS detector (red dashed line) and a UV detector at 260 nm (green solid line). The column temperature is raised linearly as shown in the plot. Separation conditions: (a) Nucleosil bare silica, 250 ⅹ 4.6 mm, 5 µm, 100 Å pore. Eluent: THF/n-hexane (38/62, v/v) at a flow rate of 0.4 mL/min. (b) Nucleosil C18, 150 ⅹ 4.6 mm, 5 µm, 300 Å pore. Eluent: CH2Cl2/CH3CN (64/36, v/v) at a flow rate of 0.4 mL/min. NP-

TGIC and RP-TGIC separate the polymer species according to the MW of PS and the MW of PB mainly, respectively. Figure 2a shows an NP-TGIC chromatogram of the Styrolux. The NP-TGIC separation condition is the IC mode for PS and the SEC mode for PB. Therefore, the polymer species in the Styrolux sample are supposed to be separated according to the MW of PS mainly.25 Two detectors, 8


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UV and LS, are used for rough estimation of relative MW of the eluted peaks. The peaks eluting earlier than the injection solvent peak (tE < 8 min) can be either homo-PB or block copolymers with high PB content. Since the presence of homo-PB is not expected from the synthetic scheme and the peaks show a strong UV signal, they are likely block copolymers with high PB content. The peaks show a decreasing trend of R90/A260 as tE increases that indicates the SEC mode elution, i.e., high MW first. In the IC mode separation region (tE > 8 min), many peaks show up, but they are not fully resolved. While it is expected that the separation has been done mainly by the length of the PS block, the ratio of R90/A260 does not show a monotonous trend with tE indicating that they are separated in a complex manner by both MW and composition. Figure 2b shows an RP-TGIC chromatogram of the Styrolux. The RP-TGIC separation condition is the IC separation condition for PB while PS elutes in the SEC mode. Therefore, the polymer species in the Styrolux sample are supposed to be separated in IC mode according to the MW of PB mainly. The peaks eluted in the SEC mode before the injection solvent peak (tE < 5 min, high MW first) are likely due to the homo-PS blended with the block copolymers and the block copolymers seem to elute out in the IC mode after the solvent peak (tE > 5min). Again, the RP-TGIC resolution is not good enough to fully resolve the polymer species in the sample. Although the IC separations resolve the Styrolux into a larger number of peaks than SEC, it is still not possible to fully resolve the polymer species present in the sample. It is in part due to the limited peak capacity of the 1D-LC separation for such a mixture of multivariate distributions both in MW and composition. For the analysis of this kind of complex polymers, 2D-LC is a more powerful method with a larger peak capacity. Not only the high resolution due to the large peak capacity, but it also provides a visual overview for easy appreciation of the landscape of complex samples and sometimes it can show features that is completely hidden in one dimensional LC separations. The Styrolux sample is characterized by a few different 2D-LC combinations, TGIC x SEC and TGIC x LCCC. In addition, taking advantage of the full resolution of the block copolymer species in the Styrolux sample, the effects of MW and composition on the IC retention of block copolymers are investigated. First, we carried out 2D-LC separations combining NP-TGIC with SEC or NP-LCCC. The NP-TGIC separates the Styrolux according to mainly MW of the PS block and the SEC separates the NP-TGIC fractions according to the hydrodynamic volume of the present polymer species. The 2DLC analysis with 2nd-D NP-LCCC at the CAP of PS is expected to give somewhat different information from the 2D-LC separation with SEC. LCCC is an attractive analysis method for analysis of block copolymer. For example, at the CAP of A block for a AB diblock copolymer, the A block is chromatographically ‘invisible’ and the block copolymer elute according to the B block length only. 5, 20-21

There are two kinds of blocks in the Styrolux, PS and PB block. In the NPLC separation, PS 9



block is more interactive with the stationary phase than PB block while in the RPLC separation, PB block is more interactive with the stationary phase than PS block. Therefore, in the NP-LCCC separation at the CAP of PS, the block copolymer species are expected to be separated by PB block size only in the SEC mode and vice versa. The 1D-LC chromatograms of SEC and LCCC analyses of Styrolux for the 2D-LC separations are shown in the supporting information (Figures S1 and S2). Figure 3 shows the two contour plots of (a) NP-TGIC x SEC and (b) NP-TGIC x NP-LCCC 2D-LC chromatograms of the Styrolux. Both contour plots look similar at a glance and show wellseparated, at least 13 peaks, but there are distinct differences between them. First, the blended homoPS (and some PS prematurely terminated during the synthesis of PS-b-PB precursors as well) eluting out last (tE ~ 380 min) in the 1st-D NP-TGIC separation elutes quite differently in the 2nd-D LC separation. In the 2nd-D SEC separation, it elutes as a broad peak covering the whole elution time range of the block copolymer species while it elutes as a rather narrow peak near the solvent peak position (tE ~ 2.1 min) in the 2nd-D SEC LCCC separation at the CAP of PS. Since the high MW (also broad MWD) homo-PS does not elute well with the temperature control of the column, a small amount of THF was injected at 300 min after the sample injection to expedite the elution. Therefore, the homo-PS peak in the LCCC separation appears to have a long tail due to the THF peak. Next notable difference is the elution time of the block copolymers in the 2nd-D LC axis. While SEC separates the polymer species according to the size of the whole polymer chains, NP-LCCC is supposed to separate them according to the PB blocks only. Therefore, the LCCC elution times of the 13 peaks form 4 groups (5 groups if the homo-PS is included) as marked with 4 vertical dashed lines for visual aid while the SEC elution times are shifted reflecting the size of the whole polymer chains.

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Figure 3. 2D-LC chromatograms of the Styrolux. (a) NP-TGIC x SEC and (b) NP-TGIC x NP-LCCC.

NP-LCCC is at the condition of CAP of PS blocks. NP-TGIC separates the polymer species according to MW of PS block mainly. Each peak is labeled with LX (X arms of long PS-b-PB) and SY (Y arms of short PS-b-PB) for the polymer species eluting in the peak. The vertical dashed lines in (b) are visual aid to show the identical LCCC elution time of the block copolymer species with the same PB blocks.

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The NP-TGIC x NP-LCCC 2D-LC chromatogram (Figure 3b) provides a valuable clue to identify the polymer species in the Styrolux sample. Since there is only one size PB block in the two PS-b-PB precursors (L & S), the PB blocks in the coupled final products form star-shaped PB with different number of uniform MW arms at the core of the branched block copolymers. Therefore, the peaks in the 2D-LC chromatogram are located on the parallel vertical lines (a common LCCC elution time for the same PB block) reflecting their quantized size of the star-shaped PB regardless of the corona PS block size attached onto them. Such an LCCC analysis of the ‘visible’ blocks of a block copolymer was found to be not very accurate quantitatively since the block at its CAP is not completely transparent but influences the retention of block copolymers to some extent.21,

37-38

Nonetheless, it is still good enough to provide a useful guide to identify the polymer species. The lines connecting the polymers with the same PB blocks (i.e., the same branch numbers) in SEC (Figure 3a) are no longer vertical and the elution times get shorter for larger polymers.

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NP-LCCC Figure 4. Schematic NP-TGIC x NP-LCCC 2D-LC chromatogram of the 13 major block copolymer species in the Styrolux.

Combining the elution times of the three LC separations (PS block size by NP-TGIC, PB block size by NP-LCCC, and the whole polymer size by SEC), the polymer species in each peak can be assigned as illustrated in Figure 4. The assignment was confirmed by separate offline 2D-LC 12


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experiments of the NP-TGIC fractionation (9 fractions shown in Figure 2a) and the subsequent analysis of the fractions by SEC-multiple detection as described in the supporting information (Figure S3 and Table S1). Based on these analyses, the molecular structure of each peak is labeled in the contour plots of Figure 3. There are up to 4 arm star-shaped block copolymers present clearly in the Styrolux sample and the presence of small amounts of 5 arm species (S5, L4S1, and L3S2) is detectable as tails at the left of the 4 arm line. Considering the presence of excess PS-b-PB precursor and the use of a linker with 7 bonding sites, the coupling reaction was far from complete. It was probably due to the slow linking reaction rate of the polymer anions with a small coupling agent as the number of arms increases.39

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Figure 5. (a) RP-TGIC x SEC and (b) RP-TGIC x RP-LCCC 2D-LC contour plots. The polymer species in each peak are labeled with LX (X arms of long PS-b-PB) and SY (Y arms of short PS-b-PB) for the polymer species eluting in the peak. RP-LCCC is at the condition of CAP of PB blocks.

RP-TGIC separates the polymer species according to MW of PB block mainly. Figure 5 shows two contour plots of (a) RP-TGIC x SEC and (b) RP-TGIC x RP-LCCC 2DLC separations of the Styrolux. The separation conditions of RP-TGIC, SEC and RP-LCCC are 14


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described in the experimental section. Since the 1st-D RP-TGIC eluent is different from the 2nd-D RP-LCCC eluent and causes a problem of solvent compatibility, two short bare silica columns (Nucleosil, Macherey-Nagel, bare silica, 30 x 4.6 mm, 300 Å pore, 5µm) were used in place of the storage loops to prevent the breakthrough. The molecular structure of the polymer species in each peak is labeled in the chromatograms with reference to the elution sequence of RP-TGIC relative to the NP-TGIC in the supporting information (Figure S4). The first distinct feature of the RP-TGIC separation in contrast to the NP-TGIC separation is the elution sequence of the polymer species. Homo-PS elutes first in the SEC mode, then block copolymers elute in the IC mode from the high PS content to low PS content in general. Next, the resolution of RP-LCCC appears poor relative to NP-LCCC and SEC. As explained before, the long and short PS-b-PB precursors (L and S) have an identical MW PB block but different MW PS block. Therefore, coupling of these PS-b-PB precursors yields well quantized PB block size from 1 to 4 arm stars that the NP-LCCC separates well. On the other hand, there are two different MW PS blocks in the PS-b-PB precursors and the statistical coupling of the precursors yielded the final PS block size to be poorly quantized that cannot be resolved by RP-LCCC as well as NP-LCCC. Furthermore, a single column was used in the LCCC separation and the column temperature had to be fixed at the CAP temperature that results in a poorer resolution than the SEC separation in which two columns were used at 90 oC. Since the Styrolux contains PS/PB block copolymers of various MW and composition, it is a good model sample to consider the IC elution behavior of block copolymers. While SEC and LCCC separate the polymers according to the whole chain size and the ‘visible’ block size, respectively, the IC retention is more complicated since it is affected by both MW and composition of the polymers. For the purpose, the TGIC elution time vs. MW of the interactive block and the TGIC elution time vs. chemical composition are plotted in Figure 6 and 7, respectively. S1-S4 are not shown in the NP-TGIC plot since they elute in the SEC mode before the injection solvent peak (tE < 80 min). As in Figure 6, the IC elution time of the block copolymers certainly does not scale with MW of the interactive block since the data points are scattered widely. Therefore, the noninteractive block must play a significant role in the IC retention of the block copolymers. In general, the IC retention of random copolymers is known to depend on chemical composition for high MW polymers.40 As in Figure 7, however, the retention of the block copolymers does not scale with the composition either although the data points appear less scattered than Figure 6. Neither S1-S4 nor L1L3 set (groups of the same composition) shows the same retention in the IC region but the retention increases with increasing MW of the interactive block.

15



NP-TGIC

4.8

log MWPS

5.4

L 3 S1 L2S2

5.2

5.0

L 1 S3

L 1 S2

L2S1

RP-TGIC

L2

L 1 S1 L1

4.4

L2

L 1 S2

S3

L 1 S1

S2

S1

L1 200

L 2 S1

L3

4.2

4.6 150

S4

4.6

4.8

100

L 1 S3

L 2 S2

L 3 S1

L3

log MWPB

5.6

250

300

4.0 100

350

tE (min)

150

200

250

300

350

tE (min)

Figure 6. Plots of MW of PS block vs. NP-TGIC elution time of block copolymers and MW of PB block vs. RP-TGIC elution time of block copolymers in the Styrolux. The experimental peak elution times are from Figures 3 and 5.

50

NP-TGIC

90

L2

L1 L 1S 1

80

L 1S 2

L 2S 1

RP-TGIC

L 3S 1

L 2S 2

L1 S3 70

30

L1 S3 L 1S 1 L S 2 2 L 2S 1 L1

60 150

200

250

300

S3 S4

L1 S2 20

100

S2

S1

40

L3

wt% of PB

100

wt% of PS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

350

10 100

L2 L3 150

L3 S1 200

250

300

350

tE (min)

tE (min)

Figure 7. Plots of wt% of PS block vs. NP-TGIC elution time of block copolymers and wt% of PB block vs. RP-TGIC elution time of block copolymers in Styrolux. The experimental peak elution times are from Figures 3 and 5.

Therefore, it is clear that the IC retention of the block copolymers is a complex function of both MW and composition. Nonetheless, we can find a few general trends. First, at the same (or similar) MW of the interacting block, retention increases as the composition of the interacting block increases: L1S3 < L1S2 < L1S1 < L1, L2S2 < L2S1 < L2, and L3S1 < L3 in NP-TGIC, and L1 < S1, L2 < L1S1 < S2, L3 < L1S2 < L2S1 < S3, and L3S1 < L2S2 < L1S3 < S4 for RP-TGIC. In other words, as the size of noninteractive block increases, the block copolymers are retained less. This type of behavior was 16


Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observed in other diblock copolymers38 and graft copolymers.41 Secondly, the addition of a short PSb-PB precursor (L1 → L1S1 → L1S2 → L1S3, L2 → L2S1 → L2S2, L3 → L3S1) decreases the retention in NP-TGIC while it increases the retention in RP-TGIC. The addition of a short precursor increases the MW of both PS and PB while it increases the PB content (except for S1-S4 for which the composition does not change.). On the other hand, the addition of a long PS-b-PB precursor shows not a simple trend. The addition of a long precursor also increases the MW of both PS and PB while it decreases the PB content (except for L1-L3 for which the composition does not change.). In NP-TGIC, the addition of a long precursor always increases the retention (L1 → L3, S1 → L3S1, L1S2 → L2S2). In RPTGIC, however, it increases the retention for L1 → L3, decreases the retention for S2 → L2S2 and S3 → L1S3 while it does not change the retention much for S1 → L3S1. The observed trends could be explained qualitatively either as the effect of the noninteracting block weakening the interaction of the interacting block38 or as the exclusion effect playing a role in the IC separation mode.42 When a short precursor is added, the composition of the interacting block decreases in NPLC (PS block) but increases in RPLC (PB block). The effect of the composition change (interaction effect) appears to dominate the size change of the polymer chain (exclusion or interference effect) and the retention increases as the composition of interactive block increases in both NPLC and RPLC. It is likely due to the fact that the chain size increase upon addition of a short precursor is not large. On the other hand, the addition of a long precursor increases the PS content much more than PB. Therefore, in NP-TGIC, retention increases upon addition of a long precursor while, in RP-TGIC, the effect of noninteracting PS block is large to lead to the complex retention variation. In addition, the branching chain architecture would also have contributed to the retention variation. Therefore, it is not simple to elucidate the quantitative effect of interacting and noninteracting blocks of block copolymers on the IC retention. Further theoretical and/or simulation study would be helpful to understand the complex elution behavior. In summary, we have demonstrated that 2D-LC can be used successfully to characterize a complex commercial block copolymer sample. The combination of IC x LCCC 2D-LC can be more informative than more frequently used IC x SEC 2D-LC. Since LCCC is an isocratic elution technique and can be repeated as rapidly as SEC. Furthermore, most of IC separation condition (TGIC in particular) is close to the CAP condition, solvent compatibility problem is not serious. Even though it was not used in this study, overlap-injection method can be also used in LCCC as in SEC. The IC retention of block copolymers is a complex function of both MW and composition and some general features of the retention variation of block copolymers upon MW and composition are discussed.

ASSOCIATED CONTENT

17



Page 18 of 21

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

Figures S1−S4 (PDF)

Figure S1: Overlap-injection of the 2nd-D SEC, Figure S2: LCCC analyses of the Styrolux for 2nd-D LCCC, Figure S3: SEC chromatogram of the Styrolux fractions, Figure S4: RP-TGIC chromatogram of the Styrolux fractions, Table S1: SEC-light scattering analysis results of the Styrolux fractions

AUTHOR INFORMATION Corresponding Author Taihyun Chang: [email protected] Author Contributions Sanghoon Lee and Heejae Choi contributed equally. Notes The authors declare no competing financial interest.

Acknowledgments This

study

is

supported

by

grants

from

NRF-Korea

(2015R1A2A2A01004974

2016K2A9A1A06919960).

for Table of Contents use only Two-dimensional liquid chromatography analysis of polystyrene/polybutadiene block copolymers. Sanghoon Lee,† Heejae Choi, † Taihyun Chang*, Bastiaan Staal

18


and

Page 19 of 21

2D-LC 350

L3

L3

NP-TGIC tE (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


L3S1

L2

L2S1

L 2S 1

250

L1

L1S1

L1S1

0

L1S2

L1S2 L1S3

50

S4 S3 1.4

L1

L2S2

L2S2 150

L2

L3S1

L1S3

S2

S4

S1

4 arm 1.6

S3 3 arm

S2

2 arm

S1

1 arm

1.8

NP-LCCC tE (min)

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Two-Dimensional Liquid Chromatography Analysis of Polystyrene

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