Multidetector Thermal Field-Flow Fractionation for the Characterization

Jan 9, 2017 - Different from single component solvents, binary solvent systems of cyclohexane and methyl ethyl ketone can provide an additional drivin...
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Multidetector Thermal Field-Flow Fractionation for the Characterization of Vinyl Polymers in Binary Solvent Systems Guilaume Greyling* and Harald Pasch Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, 7602 Matieland, South Africa S Supporting Information *

ABSTRACT: Thermal field-flow fractionation (ThFFF) is applied to the characterization of vinyl polymers such as poly(vinylcyclohexane) (PVCH) in terms of size, molar mass and chemical composition. It is shown by 1H NMR that, although limited, ThFFF using single component solvents can separate polystyrene (PS) and its hydrogenated product, PVCH, according to differences in molecular microstructure. Different from single component solvents, binary solvent systems of cyclohexane and methyl ethyl ketone can provide an additional driving force (in the form of solvent partitioning) to dramatically improve the separation of PS and PVCH by ThFFF. It is found that the separation of PS and PVCH improves with increasing methyl ethyl ketone content in the mobile phase up to 30 vol %. Additionally, it is shown that the compositional distribution of PVCH in the binary solvent systems can be obtained by the online coupling of infrared spectroscopy to ThFFF.

1. INTRODUCTION

DT and D shows that ThFFF is able to separate analytes according to size (D) and chemical composition (DT).1,6,10 The capabilities of ThFFF to separate polymers according to size and/or chemical composition has well been established and an excellent overview on ThFFF applications for polymers has been presented by Malik and Pasch.11 ThFFF is a powerful alternative to traditional column-based techniques (such as size exclusion chromatography (SEC)) as the relatively gentle separation conditions and the absence of a stationary phase make it a suitable technique for the characterization of high molar mass and fragile compounds without shear degradation taking place.1,7 Despite its remarkable separation capabilities and advantages over column-based techniques, ThFFF is not often used for the characterization polyolefins. In order for ThFFF to be used for polyolefin characterization the cold wall must have a minimum temperature of 130−150 °C in order to keep the sample in solution which would result in a hot wall temperature of 180−200 °C when using a typical temperature drop of 50 °C.12 This situation is referred to as high temperature ThFFF (HT-ThFFF). To date only a single publication has reported the analysis of polyolefins by ThFFF.13 In this case, a single polypropylene sample was analyzed by HT-ThFFF (solvent: decalin) and it was found that the sample showed no retention as it coeluted with the void peak. Also, molar mass distributions were determined using SEC and not HT-ThFFF. However, a more pressing issue is that HT-ThFFF is generally not a suitable technique for the analysis of most polyolefins as they tend

Thermal field-flow fractionation (ThFFF) is a subtechnique of the field-flow fractionation family that employs a temperature gradient perpendicular to a carrier liquid flowing through an open, ribbon-like channel in order to fractionate analytes according to size, chemical composition and microstructure.1−5 The applied temperature gradient drives analytes from the hot wall toward the cold wall (accumulation wall) of the channel. This temperature-induced migration is termed thermal diffusion and is described by the thermal diffusion coefficient, DT, which (among other factors) is dependent on the chemical nature of the analytes and the solvent.1,2,6,7 Thermal diffusion is balanced by normal diffusion which is the migration of analytes away from the accumulation wall toward the center of the channel due to increasing analyte concentration.6,8 Normal diffusion is described by the normal diffusion coefficient, D, and is dependent on the size of the analytes in solution.1,6,7 Because of the high aspect ratio of the channel, a laminar parabolic flow velocity profile develops with faster flow streams toward the center of the channel and slower flow streams near the channel walls.1,2 Retention is determined by the analyte cloud’s average position in the flow velocity profile and therefore by its distance away from the accumulation wall.1,9 Thus, the further the analyte cloud is from the accumulation wall (faster flow streams), the shorter the retention time will be. The analyte cloud’s average distance from the accumulation wall is governed by its Soret coefficient, ST, which is the ratio of DT/D and describes the interplay between the two forces (Figure S1 in Supporting Information).1,2,7 As a result, analytes with similar ST values will reside in similar flow streams and coelute from the channel. Moreover, this interplay between © XXXX American Chemical Society

Received: October 25, 2016 Revised: December 5, 2016

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Macromolecules to exhibit low thermo-oxidative stability. Thus, a “low temperature” ThFFF approach (cold wall temperature of 25 °C and temperature drop of 60 °C) for the characterization of polyolefins would be of fundamental importance. Indeed, if a low temperature ThFFF approach is not capable of fractionating amorphous polyolefins according to chemical composition or microstructure then efforts to possibly study semicrystalline polyolefins at elevated temperatures would be futile. With regards to other FFF techniques, high temperature asymmetric flow field-flow fractionation (HT-AF4) was shown to successfully characterize high molar mass polyolefins such as high density and low density polyethylene.12 However, although both AF4 and ThFFF can determine size and molar mass distributions, only ThFFF is capable of providing additional information regarding chemical composition distributions. For ThFFF it was shown that, in addition to the composition of the analyte, the composition of the carrier liquid (mobile phase) is a key parameter influencing retention behavior.6,14,15 Moreover, it was demonstrated that the solvent components in a binary solvent system can segregate under the influence of the temperature gradient and that this partitioning of solvent components inside the channel could either enhance or reduce the retention of an analyte.6,15 Despite having only been demonstrated for well characterized, single component analytes such as polystyrene standards, the partitioning effect can greatly broaden the application range of ThFFF to potentially include the characterization of polyolefins, such as poly(vinylcyclohexane), and thereby circumventing the problems associated with HTThFFF. Hydrogenated polystyrene, poly(vinylcyclohexane) (PVCH), is a useful polyolefin that exhibits a higher glass transition temperature as well as improved thermal, oxidative and UV stability than polystyrene (PS) (Figure 1).16,17 Thus, the incorporation

is not suitable for separating polymers according to microstructure. SEC separates according to hydrodynamic size in solution which is mainly determined by the polymer chain length. Thus, SEC is not suitable for determining changes in microstructure as this has little influence on the hydrodynamic size. With regards to NMR, FTIR, and UV−vis, even though these techniques can provide information on the average composition of the polymer, it is not possible to obtain information regarding molar mass or compositional distributions. Therefore, there is currently no single technique that can provide comprehensive information on molar mass, chemical composition or their respective distributions for the hydrogenation of polystyrene. In the present study we shall address the fundamental question whether ThFFF is capable of fractionating polyolefins according to chemical composition or microstructure. This study will demonstrate that ThFFF is capable of characterizing polyolefins such as PVCH in terms of size, molar mass and chemical composition. Moreover, it will also be demonstrated that ThFFF can fractionate PS and PVCH according to microstructure while simultaneously determining size, molar mass and chemical composition (as well as their respective distributions) from a single analysis. This is shown for PS and PVCH standards having identical molar masses. It will further be demonstrated that solvent composition can have a significant influence on retention behavior of the polymers and that binary solvent systems can greatly extend the application range of ThFFF to polyolefins. This work, for the first time, describes the fractionation of PS and PVCH according to microstructure by means of ThFFF coupled online to ultraviolet (UV), multiangle laser light scattering (MALLS), differential refractive index (dRI), dynamic light scattering (DLS) and FTIR detection as well as by 1H NMR. The multidetector ThFFF setup allows for the simultaneous online determination of molar mass (MALLS) and chemical composition (FTIR) distributions while DLS is used for the determination of D (and subsequently DT). The dRI detector was required as universal concentration detector while the UV detector was used to monitor the polystyrene content.

2. EXPERIMENTAL SECTION 2.1. Materials. The 100 kg mol−1 polystyrene and PVCH (sample code: P18054-VCH) samples were purchased from Sigma-Aldrich (South Africa) and Polymer Source Inc. (Montreal, Canada), respectively. The PVCH sample was prepared and characterized by the manufacturer as follows: PS (100 kg mol−1) was hydrogenated in the presence of a Pd-based catalyst with hydrogen pressure of 1200 psi at 140 °C. The hydrogenation was carried out for 3 days. The polymer was recovered after removing the catalyst by passing through a silica column and precipitation in ethanol. The polymer was finally dried at 100 °C for 24 h under vacuum. The molar mass was obtained by SEC in THF. SEC analysis was performed on a Varian liquid chromatograph equipped with refractive and UV light scattering detectors. Three SEC columns from Supelco (G6000−4000−2000 HXL) were used with triple detectors from Viscotek Co. Furthermore, the polystyrene and PVCH sample solutions were prepared in 3 mg.mL−1 concentrations using HPLC grade tetrahydrofuran, toluene, chloroform, cyclohexane, and methyl ethyl ketone purchased from SigmaAldrich (South Africa). All binary solvent systems are prepared in v/v ratios. The refractive index and solvent viscosity values for each of the various binary solvent systems were determined from the molar ratios of cyclohexane and methyl ethyl ketone, as reported in literature, while dn/dc values were also calculated as reported in literature.14,18 The refractive index, solvent viscosity and dn/dc values are reported in Table S1 in the Supporting Information.

Figure 1. Structure of (A) polystyrene and (B) poly(vinylcyclohexane).

of PVCH into block copolymers can offer materials of superior stability which can be used for high temperature applications. Indeed, PVCH−PMMA block copolymers were shown to exhibit attractive thermal and photoresist properties which place these copolymers at the forefront of advanced nanopatterning applications.16 However, proper characterization of PVCH is still limited and commonly used techniques include SEC, NMR, FTIR, or UV−vis. SEC is used to determine possible chain scission during hydrogenation by monitoring changes in molar mass distributions while NMR, FTIR, and UV−vis are used to determine the conversion of benzene units to cyclohexane units. SEC is the standard technique to determine molar mass distributions, but it has its limitations for very high molar mass polymers due to shear degradation as a result of interactions with the stationary phase and the column frits. Moreover, SEC B

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and subsequently analyzed. Data were processed by use of MestReNova software version 7.1.1. Quantitative information obtained from the spectra is accurate within 5%. 2.4. FTIR Spectroscopy. The online coupling of FTIR to ThFFF was adapted from the online coupling of FTIR to SEC as performed by Beskers et al.19 FTIR analyses were performed on a Tensor II Spectrometer (Bruker, Ettlingen, Germany), equipped with a zinc selenite flow cell. Spectra were recorded at a resolution of 4 cm−1 with 64 scans being recorded for each spectrum. Composition as a function of retention time was determined by monitoring the specific wavenumbers associated for polystyrene at 2920 cm−1. Bruker Optik Opus (version 7.5), Origin 8 and time resolved infrared spectroscopy for molecular online SEC detection (TIMO) software were used for data collection and processing.

Table 1. Solubility of Poly(vinylcyclohexane) (PVCH) in Cyclohexane (CH), Toluene (TOL), Dichloromethane (DCM), Tetrahydrofuran (THF), Chloroform (CF), Methyl Ethyl Ketone (MEK), Dioxane (DOX), and Dimethylformamide (DMF) PVCH solubility solvent

polarity index [P′]

soluble

CH TOL DCM THF CF MEK DOX DMF

0.2 2.4 3.1 4.0 4.1 4.7 4.8 6.4

X X

insoluble

X X X X X X

3. RESULTS AND DISCUSSION As polyolefins, such as PVCH, exhibit low solubility in most organic solvents at ambient temperatures, the solubility of PVCH in various organic solvents was first investigated. Of the various solvents tested, it was found that only tetrahydrofuran (THF), toluene (TOL), chloroform (CF) and cyclohexane (CH) dissolved both PS and PVCH. Table 1 lists the solubility of PVCH in the various organic solvents as well as their Snyder polarity indexes (P′). Once suitable solvents for PVCH were determined, PS and PVCH standards of similar molar masses were analyzed in CH, TOL, THF, and CF. The three solvents TOL, THF and CF are all low viscosity, thermodynamically good solvents for PS and are expected to yield short analyses times. Thus, these three solvents serve as a good starting point for the study. 3.1. ThFFF Analysis in Tetrahydrofuran, Toluene, and Chloroform. Figure 2 shows the MALLS fractograms of the PS and PVCH samples in TOL, THF and CF and it can be seen that no separation could be achieved. Moreover, these results show that solvent polarity did not significantly influence the retention behaviors of PS and PVCH samples even though the two samples exhibit different Hildebrand solubility parameters (δ) of 18.3 and 16.9 MPa1/2, respectively.20 The Hildebrand solubility parameter, which is a numerical estimate of the degree of intermolecular interactions, can be used to describe a polymer’s solubility and relative polarity.21 Therefore, the different δ values for PS and PVCH indicate that the samples exhibit different polarities. The molar masses for the PS and PVCH samples were determined to be 103.5 ± 8.2 and 98.1 ± 4.7 kg mol−1, respectively, which is in good agreement with the molar masses reported by the manufacturers. The retention times (tr), diffusion (D), thermal diffusion (DT) and Soret (ST) coefficients for the PS

2.2. Thermal Field Flow Fractionation. The thermal FFF system TF2000 (Postnova Analytics, Landsberg, Germany) was coupled online to UV (PN 3212 at 254 nm, Postnova Analytics), MALLS (PN 3070, Postnova Analytics), dRI (PN 3150, Postnova Analytics), and DLS detectors (Zen 1600, Malvern Instruments, Worcestershire, U.K.). The TF2000 channel had a tip-to-tip length of 45.6 cm, breadth of 2 cm, thickness of 127 μm, and void volume of 1.14 mL. The temperature of the cold wall was 24 °C and a constant ΔT of 60 degrees was used to achieve fractionation. The analytes were introduced into the channel via a Rheodyne manual injection valve and carrier flow was generated by an isocratic pump (PN 1130, Postnova Analytics). THF, toluene and chloroform were used as carrier liquids with a flow rate of 0.3 mL·min−1, while analyses in cyclohexane and cyclohexane/ methyl ethyl ketone mixtures were all performed at a flow rate of 0.2 mL·min−1 in order to reduce the influence of flow rate on the solvent partitioning. Diffusion data were gathered online via a quartz flow cell and values for DT were calculated according to1 DT =

6Dtr ΔTt 0

where t is the void time, tr is the retention time of the analyte, and ΔT is the temperature drop between the hot and cold wall. The analytes were injected through a 100 μL capillary sample loop, and triplicate analysis of each sample was performed under nonoverloading sample concentrations. 2.3. 1H NMR Spectroscopy. The NMR experiments were conducted on a 400 MHz Varian Unity Inova spectrometer (Agilent/ Varian, Palo Alto, California, USA). The measurements were performed with a 5 mm dual broadband pulsed field gradient probe. A total of 256 scans using 45° pulses were acquired with an acquisition time of 2.0 s and a relaxation delay of 1 s. The polymer samples were fractionated by ThFFF, collected in 25 runs, and evaporated at room temperature. The collected fractions were dissolved in 0.7 mL CDCl3 0

Figure 2. MALLS 90° fractograms of PS (blue) and PVCH (red) in (A) toluene, (B) THF, and (C) chloroform. C

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THF

0.143 ± 0.001 0.149 ± 0.001 0.114 ± 0.002 0.113 ± 0.001

CF TOL

0.159 ± 0.001 0.165 ± 0.002 0.632 ± 0.061 0.695 ± 0.076

THF CF

0.502 ± 0.053 0.510 ± 0.018 0.634 ± 0.069 0.647 ± 0.071

TOL THF

4.42 ± 0.43 4.67 ± 0.51 4.41 ± 0.47 4.51 ± 0.15

CF TOL

3.99 ± 0.41 3.93 ± 0.41

CF

9.8 ± 0.1 10.1 ± 0.1

TOL

14.5 ± 0.1 15.0 ± 0.1

sample

PVCH PS

THF

and PVCH samples in the various solvents are reported in Table 2. In addition to the fractograms, the similar values for ST also indicate that PS and PVCH are not separated in either THF, TOL or CF. Table 2 also shows that PS and PVCH exhibit similar D and DT values. The similar values for D indicate that two samples exhibit similar sizes in solution, which is as expected, while the similar values for DT indicate that the two polymers interact with the solvents in a similar manner. Additionally, DT can also yield information on polymer backbone flexibility with flexible polymers exhibiting smaller DT values than more rigid polymers.22 The DT values for PS and PVCH show that PVCH is slightly more flexible than PS as it has smaller DT values in the three solvents. This is confirmed by reported Kuhn lengths of 18.0 and 13.9 nm for PS and PVCH, respectively.20,23 In Figure 3 the UV fractogram of the PVCH sample is presented and, unexpectedly, it shows the presence of some PS content. The PS content in the PVCH standard was calculated to be 26.5 ± 2.9% by using the relative areas of the UV and dRI fractograms in THF. 1 H NMR was also used to determine the PS content by taking the ratio of the aromatic protons (7.2−6.4 ppm) and the methylene protons of the cyclohexane ring (1.5−1.2 ppm) (Figure S2 in the Supporting Information). The NMR results showed a PS content of 20.3 ± 3.7%, which is in good agreement with the ThFFF data. Furthermore, the NMR spectra showed no evidence for the presence of other hydrogenation products such as cyclohexene or cyclohexadiene. In addition to ThFFF, the PS and PVCH samples were also subjected to SEC analysis in THF. It was found that PS and PVCH exhibited similar retention times of 13.7 and 13.9 min, respectively, and that a blend of the two samples could not be separated (Figure S3, S4 and S5 in Supporting Information). The PS and PVCH blend exhibited a retention time of 13.7 min. 3.2. ThFFF Analysis in Cyclohexane. Previous studies on microstructure-based separations by ThFFF showed that a theta solvent can be a promising solvent to significantly influence retention behavior because polymer−solvent interactions are significantly different for a theta solvent at various temperatures (especially at the theta temperature) than for a thermodynamically good solvent.3−5,24,25 Thus, CH was selected for this study as it is a theta solvent for PS. Figure 4 shows the UV, dRI, and MALLS fractograms for the PS and PVCH samples in CH and in contrast to the other solvents, the MALLS and dRI fractograms of PVCH indicate a bimodality in the elution profile. In order to exclude the influence of possible aggregation, the PVCH sample was analyzed at several dilutions and filtered but the bimodality persisted. Moreover, the MALLS and DLS data showed no evidence of aggregation but instead show uniform molar mass and size distributions (Figure 5). These results suggest that a separation according to microstructure could potentially be achieved. Table 3 shows the diffusion (D), thermal diffusion (DT), and Soret (ST) coefficients for the samples in CH and it can be seen that PVCH and PS exhibit different ST values. Different ST values are indicative of a separation. Thus, if the samples exhibit different ST values (where ST = DT/D) but similar D values, then the separation is due to difference in DT which in turn is influenced by polymer composition or microstructure. From Table 3 it can be seen that the samples exhibit similar D values but different DT values. The similar D values are as expected

12.9 ± 0.1 13.4 ± 0.1

ST [K‑1] DT [10‑7 cm2 s‑1 K‑1] D [10‑7 cm2 s‑1] tr [min]

Table 2. Retention Times (tr), Diffusion (D), Thermal Diffusion (DT), and Soret (ST) Coefficients for Poly(vinylcyclohexane) (PVCH) and Polystyrene (PS) Determined in Toluene (TOL), Chloroform (CF), and Tetrahydrofuran (THF)

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Figure 3. (A) UV and (B) dRI fractograms of PS (blue) and PVCH (red). Solvent: THF. Void time (t0) for fractogram A is 6.6 min while t0 = 9.9 min for fractogram B.

Figure 4. (A) UV, (B) dRI, and (C) MALLS fractograms of PS (blue) and PVCH (red) in cyclohexane.

Figure 5. (A) Molar mass distributions of PVCH determined from online MALLS data and (B) size distributions of PVCH determined from online DLS data. The green dots represent the Z-average size (Dh), and the red line shows the corresponding elution profile.

Table 3. Retention Times (tr), Diffusion (D), Thermal Diffusion (DT), and Soret (ST) Coefficients for Poly(vinylcyclohexane) (PVCH) and Polystyrene (PS) Determined in Cyclohexane sample

tr [min]

D [10‑7 cm2 s‑1]

DT [10‑7 cm2 s‑1 K‑1]

ST [K‑1]

PVCH PS

22.3 ± 0.1 18.4 ± 0.2

2.67 ± 0.43 2.76 ± 0.21

0.765 ± 0.098 0.644 ± 0.036

0.286 ± 0.001 0.233 ± 0.002

Table 4. Polystyrene Content Determined from 1H NMR Spectra for the Four Collected Fractions as Well as Their Respective Collection Times fraction

polystyrene content [mol %]

fraction elution times [min]

1 2 3 4

36.5 31.2 15.0 6.1

15.0−17.7 17.7−20.3 20.3−23.8 23.8−26.0

while the differences in DT for PS and PVCH indicate that a separation according to microstructure can be achieved. In order to demonstrate that a separation is indeed achieved, the PVCH sample was fractionated into four fractions by ThFFF. The four fractions were subsequently subjected to 1H NMR analysis in order to determine the PS content of each fraction. Table 4 shows the PS content for each fraction (as well as the elution times at which the fractions were collected) while Figure 6 illustrates the general trend in PS content across the elution profile. E

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system it is expected that the solvent partitioning will drive PS and PVCH into different flow streams and, as a consequence, improve the separation. For this study several binary solvents systems of increasing MEK content were prepared from CH and MEK. The MEK content of the binary solvent systems was 10, 20, 30 and 40%. Attempts to increase the MEK content above 40% resulted in precipitation of PVCH. In a first series of experiments, the principle of solvent partitioning was applied to the separation of a 1:1 blend of PS and PVCH. Figures 7 and 8 show the MALLS 90° and dRI fractograms of the blend analyzed in each of the solvent systems and it can be seen that the separation improves with increasing MEK content. Furthermore, the molar mass data shows uniform distributions for each of the solvent systems, which is as expected. Following the analysis of the PS−PVCH blend at various MEK contents, the PVCH sample was subsequently also analyzed in the various binary solvent systems. Figures 9 and 10 show the MALLS 90° and dRI fractograms of PVCH analyzed in each of the binary solvent systems and it can be seen that, similar to the blend, the separation of PVCH improves with increasing MEK content. Figure 9 also shows that uniform molar mass distributions in each of the solvent systems is obtained, which is as expected. Additionally, the differences in retention times between PS and PVCH in each binary solvent system also show that the separation improves with increasing MEK content. The differences in retention times between PS and PVCH increase from 4.6 to 6.6 and 7.3 min for MEK content of 10, 20 and 30%, respectively. Moreover, these results show a significant improvement in separation compared to pure CH which exhibits a difference in retention times of 3.3 min. From MEK contents of 10 to 20%, the retention data show that both PVCH and PS exhibit an increase in retention time from 26.9 to 35.6 min and from 22.3 to 29.0 min, respectively. The increase in PS retention time can be attributed to its

Figure 6. Fractionation of PVCH by ThFFF in cyclohexane for 1H NMR analysis. General trends in polystyrene content are superimposed on the elution profile.

The decreasing trend in PS content as a function of elution time clearly illustrates that ThFFF is capable of separating PVCH and PS according to microstructure. 3.3. ThFFF Analysis in Binary Solvent Systems. As the separation limit of single component solvents for PS and PVCH has been reached, the use of binary solvent systems was applied in an attempt to further improve the separation. In order to improve the separation with binary solvent systems, one of the analytes should preferentially dissolve in the solvent that partitions to the cold wall while the other analyte should preferentially dissolve in the solvent that partitions to the hot wall. As a consequence, the solvent partitioning should force the different analytes into different flow streams and thus change their relative retention in the channel. The two solvents selected for the binary solvent system are CH and MEK. These two solvents were selected because, first, MEK is a good solvent for PS but a nonsolvent for PVCH while CH is a Θ solvent for PS but a good solvent for PVCH. Thus, it is expected that in this binary solvent system PVCH will concentrate in CH while PS will preferably dissolve in MEK. Second, it was reported that a binary solvent system consisting of CH and MEK is miscible and will segregate with CH enriching the cold wall.6 Therefore, in this binary solvent

Figure 7. Molar mass distributions superimposed on the MALLS 90° fractograms of the PS−PVCH blend in various binary solvent systems of cyclohexane and methyl ethyl ketone. The methyl ethyl ketone content is (A) 0%, (B) 10%, (C) 20%, (D) 30%, and (E) 40%. F

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Figure 8. dRI fractograms of the PS−PVCH blend in various binary solvent systems of cyclohexane and methyl ethyl ketone. The methyl ethyl ketone content is (A) 0%, (B) 10%, (C) 20%, (D) 30%, and (E) 40%. Void time (t0) = 9.9 min.

Figure 9. MALLS 90° fractograms and molar mass distributions of PVCH in various binary solvent systems of cyclohexane and methyl ethyl ketone. The methyl ethyl ketone content is (A) 0%, (B) 10%, (C) 20%, (D) 30%, and (E) 40%.

shows that at 20% MEK content and above, PS equilibrates into similar flow velocity streams. Therefore, above 20% MEK content PS appears to be interacting exclusively with the MEK partition. With regards to the increase in PVCH retention from 10 to 20% MEK content, this increase in retention can be explained by the partitioning theory. As the CH gradient decreases across the channel with increasing MEK content, PVCH is progressively concentrated closer to the cold wall and into slower flow velocity streams, which results in longer retention. The retention data further show that from 20 to 30% MEK content PVCH continues to show an increase in retention.

preferential solubility in MEK as well as its stronger retention in MEK than in CH. Therefore, it is expected that PS would show an increase in retention with the addition of MEK and also that the PS retention behavior would stabilize or “flatten off’” above a certain MEK content. Figure 11A shows the superimposed dRI fractograms of the 100 kg mol−1 polystyrene standard in the various binary and single component solvent systems while Figure 11B shows a plot of PS retention time as a function of MEK content. Parts A and B of Figure 11 clearly show that PS exhibits an increase in retention time with increasing MEK content. This increase in retention continues up to 20% MEK content which G

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Figure 10. dRI fractograms of PVCH in various binary solvent systems of cyclohexane and methyl ethyl ketone. The methyl ethyl ketone content is (A) 0%, (B) 10%, (C) 20%, (D) 30%, and (E) 40%. Void time (t0) for fractogram A is 9.9 min while t0 = 15.2 min for fractograms B, C, D, and E.

Figure 11. (A) Superimposed dRI fractograms of the 100 kg mol−1 polystyrene standard in each of the binary and single component solvent systems. (B) Plot of polystyrene retention time versus methyl ethyl ketone content. Indicated on the graph are the polystyrene retention times at corresponding methyl ethyl ketone contents. Void time (t0) = 15.2 min.

was dependent on the DCE content with maximum partitioning occurring at 55−60% volume DCE. Therefore, similar to previous reports, it is believed that at 40% MEK content the solvent partitioning is less efficient which in turn influences the retention behavior of PVCH. Unfortunately, due to the limited solubility of PVCH above 40% MEK content, the partitioning efficiency of the solvents (and its influence on retention behavior) could not experimentally be tested for the CH:MEK solvent system. Figure 12 shows the size distributions (radius of gyration, Rg) of PVCH in each of the binary solvent systems and it can be seen that uniform size distributions are obtained. These results show that the separation of PS and PVCH is not due to differences in size and that no aggregation occurs in the binary solvent systems, which is as expected. ThFFF’s capability to separate analytes according to composition can yield valuable information on the composition distributions of eluting species, if a suitable detector is used.

The retention time of PVCH increases from 35.6 to 37.8 min while PS shows only a minor increase in retention time from 29.0 to 30.4 min. The increase in PVCH retention is due to solvent partitioning while the nearly constant PS retention is as expected. Unexpectedly, the retention data also show that at a MEK content of 40% the separation is weaker with a difference in PS and PVCH retention times of 5.1 min. This weaker separation is not due to a change in the PS retention behavior as the PS retention times remain fairly constant at 30.4 and 30.5 min for a MEK content of 30 and 40%, respectively. However, the retention times of PVCH show a decrease from 37.8 to 35.4 min for a MEK content of 30 and 40%, respectively. Thus, the addition of MEK above 30% seems to negatively influence the partitioning as only the retention behavior of PVCH is influenced and not that of PS. Similar trends were reported for various other binary solvent systems such as dichloroethane (DCE) and CH.6 It was found that solvent partitioning (and retention) H

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Figure 12. Size (Rg) distributions of PVCH in various binary solvent systems of cyclohexane and methyl ethyl ketone superimposed on their respective MALLS 90° fractograms. The methyl ethyl ketone content is (A) 10%, (B) 20%, (C) 30%, and (D) 40%.

Figure 13. Changes in the FTIR signal intensities of polystyrene in various binary solvent systems of cyclohexane and methyl ethyl ketone superimposed across the elution profile of a PS−PVCH blend. The methyl ethyl ketone content is (A) 10%, (B) 20%, (C) 30%, and (D) 40%.

In order to further demonstrate that PS and PVCH are indeed separated according to chemical composition (and to determine the composition distributions in the PVCH sample), a FTIR detector was coupled online to the ThFFF. The online ThFFF-FTIR approach was first applied to determine the PS content distributions of the PS−PVCH blend in each of the binary solvent systems. Figure 13 shows the changes in the PS content in the blend as a function of retention time for each of the binary solvent systems and it can be seen that the PS content decreases across the elution profile.

This shows that PS and PVCH are indeed separated according to chemical composition in the various binary solvent systems. After showing that the online ThFFF-FTIR approach is suitable to determine compositional distributions in a PS−PVCH blend, the PVCH sample was subsequently analyzed in order to determine if ThFFF is capable to determine the PS content distributions resulting from an incomplete hydrogenation reaction. Figure 14 shows the changes in the PS content in the PVCH sample as a function of retention time for each of the binary solvent systems. From Figure 14 it can be seen that, I

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Figure 14. Changes in the FTIR signal intensities of polystyrene in various binary solvent systems of cyclohexane and methyl ethyl ketone superimposed across the elution profile of PVCH. The methyl ethyl ketone content is (A) 10%, (B) 20%, (C) 30%, and (D) 40%.

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similar to the blend, the PS content shows a decreasing trend across the elution profile which demonstrates that ThFFF can indeed determine the PS content distribution resulting from incomplete hydrogenation reactions. Furthermore, these online FTIR results are also in good agreement with the decreasing trend in PS content as determined from the 1H NMR results reported in Table 4 and illustrated in Figure 6.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02314. Representation of ThFFF channel and separation mechanism, refractive index, dn/dc and solvent viscosity values, 1H NMR spectra and size exclusion data (PDF)

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4. CONCLUSIONS This study successfully addressed the fundamental question whether ThFFF is capable of fractionating vinyl polymers according to chemical composition or microstructure. It was demonstrated that not only can ThFFF successfully characterize vinyl polymers such as PVCH in terms of size, molar mass, and chemical composition but also it can also separate PS and PVCH based on differences in microstructure. It was shown by means of fraction collection and 1H NMR that a separation of PS and PVCH based on microstructure can be achieved in a single component solvent system such as cyclohexane but that this approach is limited. Moreover, it was demonstrated that the use of binary solvent systems can overcome the limitations of single component solvents by providing an additional driving force (in the form of solvent partitioning) to improve the separation. The influence of solvent partitioning on the separation was shown by the increase in resolution with increasing MEK content. Furthermore, the microstructure-based separation of PS and PVCH in various binary solvent systems was confirmed by the first reported use of FTIR coupled online to ThFFF. This study highlights the sensitivity of ThFFF toward polymer microstructure (as well as its flexibility toward the carrier composition) and that the use of binary solvent systems can greatly broaden the application range of ThFFF to polyolefins such as PVCH. More importantly, this study opens the door for future investigations into the characterization of a broader range of amorphous and possibly semicrystalline polyolefins by ThFFF.

AUTHOR INFORMATION

Corresponding Author

*(G.G.) E-mail: [email protected]. ORCID

Guilaume Greyling: 0000-0001-6513-9669 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Ms. N. W. Radebe and Dr. T. Beskers for their assistance with the online FTIR data collection.

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DOI: 10.1021/acs.macromol.6b02314 Macromolecules XXXX, XXX, XXX−XXX