Article pubs.acs.org/Macromolecules
Analysis of High Molar Mass Branched Polybutadienes by SECMALLS and AF4-MALLS Ashwell C. Makan,† Tino Otte,‡,§ and Harald Pasch*,† †
Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Postnova Analytics GmbH, Max-Planck-Strasse 14, 86899 Landsberg, Germany
‡
ABSTRACT: Industrial polybutadienes synthesized by Ziegler−Natta catalysts may exhibit very high molar masses, significant degrees of branching, and gel species. Such samples were analyzed by size exclusion chromatography (SEC) and asymmetric flow field-flow fractionation (AF4). Both fractionation techniques were coupled to a multiangle laser light scattering (MALLS) detector as well as a refractive index (RI) detector. The initial results obtained from SEC measurements completely contradicted the processing behavior of the materials. SEC seemed not to be suitable for accurate characterization of these materials. SEC separation was strongly influenced by coelution of linear (low molar mass) and branched (high molar mass) macromolecules, shear degradation of the larger macromolecules, and filtration of the gel fraction. For this reason the polybutadienes were subsequently analyzed by AF4 in order to compare the results and identify the limitations of SEC. The results showed that, as a specialized tool, AF4 can be used for the extended and more accurate analysis of synthetic rubber materials, especially in the case of very high molar masses. SEC is not suitable for such samples because it does not properly address the high molar mass fraction and the branched macromolecules.
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INTRODUCTION Polymeric materials with useful properties and well-defined molecular characteristics have become increasingly important for various applications in lightweight construction,1 alternative sources of energy,2−4 drug delivery,5,6 and automotive industry.7 The characterization of complex polymeric materials requires accurate analytical techniques that address the various parameters of molecular heterogeneity. Among them, size exclusion chromatography (SEC) is the most prominent method for the analysis of molar mass distributions (MMD).8−13 SEC coupled to a concentration detector such as a refractive index (RI) or ultraviolet (UV) detectors together with well-characterized calibration standards (e.g., polystyrene) are commonly used for the determination of relative molar mass values. These are then termed apparent molar masses or e.g. polystyrene-equivalent molar masses and have a limited accuracy.14,15 The most frequently used molar mass averages are Mw, Mn, and Mp. When SEC is coupled to molar mass sensitive detectors such as light scattering detectors, absolute molar masses and radii of gyration (Rg) may be determined.16−18 When a viscometer detector is used, molar masses are determined through universal calibration19−22 or directly using the Goldwasser approach.23−25 Additional information regarding molecular conformation and branching can be elucidated26−30 making molar mass © 2012 American Chemical Society
sensitive detectors more useful compared to the classical approach where a calibration curve is needed.31,32 Unfortunately, SEC has limitations when it comes to the analysis of very high molar mass and highly branched macromolecules. When analyzing such materials by SEC poor separation, shear degradation and coelution of linear and branched macromolecules of low and high molar masses, respectively, can be observed.9,10,33 Very strong shear forces in the stationary phase or at the column frits cause degradation of high molar mass macromolecules,10,30,34−39 resulting in wrong molar mass determinations. Another problem is the late elution effect in SEC which is a common occurrence for excessively branched macromolecules. Coelution takes place due to a portion of the highly branched species being retained unusually long on the stationary phase as compared to the corresponding linear macromolecules.28,30,31,38,40−44 In addition, having the same hydrodynamic volume but different coil densities, linear and branched macromolecules will show different molar mass− radius of gyration behavior.3,9,26,27 Based on some of these disadvantages, alternative separation methods are required that minimize shear degradation and coelution effects. Received: April 17, 2012 Revised: May 30, 2012 Published: June 7, 2012 5247
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was not possible in SEC due to shear degradation, filtration, and abnormal elution behavior caused by coelution of branched polymers together with their regular eluting linear counterparts.35 Based on the few applications mentioned above FFF, AF4 and ThFFF appear to be superior tools in areas where SEC fails. In the present work industrial polybutadiene samples synthesized using Ziegler−Natta catalysts78−80 were analyzed via AF4- and SEC-MALLS. Different properties of these materials were measured, in particular the effect of branching and dissolution time upon the elution behavior.
Field-flow fractionation (FFF) has been widely investigated over the years, and several publications have proven that FFF is a useful technique for the characterization of polymers. FFF enables the existing molar mass range for size separation to be extended and offers the possibility to analyze a wide variety of macromolecules and particles ranging from the nanometer to the micrometer size with high resolution.10,45−47 FFF consists of a family of subtechniques, and in each case separation is achieved by applying an external field which is perpendicular to the longitudinal or inlet flow.45−47 In the present case, asymmetric flow field-flow fractionation (AF4) will be used. Some of the most significant advantages of FFF over SEC are (1) no stationary phase is present, so shear degradation is strongly minimized,45,48 (2) the very low surface area of the accumulation wall in FFF avoids unwanted adsorption and secondary separation effects,45,49 (3) filtration is not necessary, since an open channel is used,35,50 (4) the exclusion limit is at least 2 orders higher than in the case of SEC,45 (5) complex mixtures of suspended particles, gels, and soluble polymers can be analyzed in one measurement,45 and (6) the working conditions are conducive for the analysis of sensitive molecules that degrade easily.48 AF4 has been applied in various fields, ranging from synthetic and biological macromolecules to particles of different natures.45 Studies of AF4 coupled to light scattering detection have been used extensively in recent years for in-depth investigations of structural parameters in addition to absolute molar mass detection.51 Intensive studies have been done in aqueous52−56 and nonaqueous35,41,57 mobile phases. Since FFF is based on an empty channel configuration, sample filtration is not required and, therefore, it is possible to fractionate and identify gel containing species.58−61 In the present work AF4- and SEC-MALLS are used to fractionate and analyze polybutadienes regarding molar mass and branching. So far, the molar masses and hydrodynamic sizes of synthetic and natural rubbers have been characterized by SEC,34,62−65 thermal FFF (ThFFF),33,60,66−70,72 and AF4.35,71,73 Organo-soluble polymers have been investigated predominantly by ThFFF since no membranes are used, and no significant interaction or adsorption takes place between sample molecules and the plates of the empty channel. Only a few aqueous applications on ThFFF have been reported.74−76 ThFFF has been used for synthetic organo-soluble polymers especially in the elastomeric field where ultrahigh molar mass (UHMM) polymer fractions might be present as well as insoluble gel fractions. The elastomeric fractions may also be branched which will influence the structure−property relationship of these polymeric materials. Acrylonitrile−butadiene− styrene resins were separated by Shiundu et al. in THF in order to separate the soluble polymer fraction from the insoluble gel fraction. 60 Similarly, van Asten was able to separate polybutadienes of different molar masses from each other in toluene.77 In addition, three polymers with similar hydrodynamic volumes but different chemistries were successfully separated proving that ThFFF can separate according to hydrodynamic volume as well as chemical composition. Several other elastomeric rubbers have been investigated by ThFFF.33,48,58,59,66,68 SBS and PB rubbers among others are some of the few elastomeric polymers already analyzed by AF4. SBR rubbers have been investigated, and it was shown that it is possible to separate free and functionalized SBR from each other.71 The presence of branching and gel species in ultrahigh molar mass polybutadienes were successfully identified which in contrast
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EXPERIMENTAL SECTION
Materials and Sample Preparation. Polystyrene samples used for calibration purposes were obtained from Fluka (Sigma-Aldrich, South Africa), with molar masses Mw of 62, 250, and 1000 kg/mol, respectively, and were dissolved in HPLC grade THF (Sigma-Aldrich, South Africa). The concentrations of the polystyrene samples were 3 mg/mL (62 and 250 kg/mol) and 1 mg/mL (1000 kg/mol), respectively. These concentrations were used for both SEC and AF4 measurements. Each polystyrene sample was dissolved at room temperature for 4 h without adding a stabilizer. The polybutadiene samples used in the study were materials of Bayer, Leverkusen, Germany, having high cis contents with different dispersities and various degrees of branching (Table 1).
Table 1. Characteristics of PB 1−6 As Given by the Manufacturera sample PB PB PB PB PB PB
1 2 3 4 5 6
catalyst
branching
dispersity
gel
Nd Nd Co Nd Co Nd
low low low low high low
broad broad broad broad broad broad
no no no no no yes
a Samples were polymerized with different ZN catalysts, varying in Mooney viscosity, branching, gel, and dispersities.
The polybutadiene samples were dissolved in HPLC grade THF with a concentration of ∼3 mg/mL. Butylated hydroxytoluene (BHT) obtained from Sigma-Aldrich (South Africa) was added as a stabilizer (1 mg/mL) to prevent oxidative degradation of the polymers. The basic dissolution procedure was to dissolve the samples for 16 h at room temperature followed by 4 h of heating at 50 °C. This was not the optimal dissolution procedure as will be discussed later. The specific refractive index increment (dn/dc) of polystyrene and polybutadiene in THF were 0.18481 and 0.13282 mL/g, respectively. SEC and AF4 Instrumentation Setup. The experiments were performed on an ambient temperature AF4-Instrument (AF2000, Postnova Analytics, Landsberg/Germany) which was coupled to a MALLS (Dawn DSP, Wyatt Technology, Santa Barbara, CA) (wavelength 632.8 nm, 25 °C) and a RI detector (PN 3140, Postnova Analytics, Landsberg/Germany). The channel was connected to three different pumps (tip, focus, and cross-flow) while the injection port was equipped with a manual injection valve (Rheodyne, Rohnert Park, CA). A regenerated cellulose membrane was installed for the AF4 channel with an average molar mass cutoff of 10 or 5 kg/mol, respectively. The Mylar spacer used for definition of the channel height had a thickness of 350 μm. The SEC setup consisted of a set of two PL Gel columns (mixed B and mixed C) in series. The inlet of the mixed B column was connected to the tip pump while the outlet of the mixed C column was connected to the detectors (MALLS and RI). A schematic setup of the instrument is shown in Figure 1. A mixture of both column types was chosen to ensure proper separation of low (mixed C, 5 μm particles) and medium−high molar mass components (mixed B, 10 μm particles) of the samples. In addition, a mixed A 5248
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Figure 1. AF4 instrumentation setup at Stellenbosch University with a cross-section view of the channel at the bottom and inset of how the SEC columns are connected to the tip pump when the channel is not in use. column (20 μm particles) was used for comparison purposes. For the analysis of the recovery, the RI calibration constant was determined by analyzing the RI peak obtained from injecting a PS standard of 33 kg/ mol. This calibration constant was checked again with a 62 kg/mol PS standard. The calculated RI calibration constant was 1.016 × 10−4. Analysis Conditions for AF4. The samples were introduced into the channel by manual injection with a flow rate of 0.2 mL/min for a time of 4 min (Figure 2). The low inlet flow rate was used to prevent shear degradation. While the samples were injected, the cross-flow was constant at 8.5 mL/min and the focus flow automatically adjusted to 8.8 mL/min to maintain a constant detector flow rate of 0.5 mL/min.
The focus flow causes the injected sample to accumulate in a narrow concentration band minimizing band broadening and longitudinal diffusion. After injection the focus flow rapidly decreased to zero (transition), while the tip flow increased up to 9 mL/min. After 2.5 min, the tip flow and the cross-flow decreased by an exponential decay function to a value of 0.51 and 0.01 mL/min, respectively (elution step). In a further investigation polystyrenes and polybutadienes were analyzed by making use of cross-flow B (Figure 2). The exponential change in the cross-flow profile was the same as in the case of crossflow A, but instead of 8.8 mL/min, a starting cross-flow of 5.5 mL/min was used.
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RESULTS AND DISCUSSION SEC and AF4 Method Validation Using Polystyrene Standards. Both SEC and AF4 separation systems were tested and compared based on the separation of polystyrene standards with known molar masses (62, 250, and 1000 kg/mol). In SEC one PL gel mixed C column was used for the separation of the polystyrene mixture to avoid shear effects. The calibration, normalization, and determination of the correct interdetector delay between MALLS and RI were conducted by making use of narrow disperse polystyrene (33 kg/mol) being an isotropic scatterer. The peak maxima of the RI and MALLS signals were overlaid giving a delay volume of 0.175 mL.
Figure 2. Cross-flow profiles of cross-flows A and B. 5249
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In order to test whether the calibration between the MALLS and RI detectors was correct for both SEC and AF4, a mixture of polystyrene standards was analyzed by AF4 by applying cross-flow A and SEC. The overlay of the SEC and AF4 measurements with RI and MALLS detection is shown in Figure 3. As can be seen, the mixture was separated properly by
Figure 4. Cross-flow profiles of cross flows B, C, and D for AF4.
Figure 3. SEC and AF4 separation for a mixture of polystyrene standards. (A) SEC elugrams and molar mass reading; stationary phase PL gel mixed C. (B) AF4 fractograms and molar mass reading; flow rate (SEC and AF4): 0.5 mL/min.
both SEC and AF4. In AF4 the small molecules elute earlier than the larger molecules, which corresponds to a separation according to the normal mode elution in AF4.83 In SEC the elution order is from high to low molar masses. In the AF4 measurement the lowest molar mass standard does not correspond to the nominal molar mass. This is due to the fact that the higher molar mass standards slightly overlap, resulting in an overestimation of the molar mass. The analysis of these polystyrene standards evidently shows that the system provides sufficient separation at still acceptable analysis time. In addition, the nearly exact retrieval of the molar mass values with both techniques affirms an accurate calibration of the detectors creating an adequate platform for the analysis of the polybutadienes. The effect of changes in the cross-flow on the elution behavior of the polystyrenes is shown for crossflows B−D. Cross-flow gradient B (Figure 4) was applied to the polystyrene mixture, and linear cross-flow gradients C and D were introduced into the existing exponential cross-flow program to improve the separation efficiency. The resultant fractograms (MALLS) obtained from the AF4 measurements are shown in Figure 5. It is obvious that as the linear gradient is elongated, the separation is improved significantly. For cross-flow D, the standards are almost baseline separated. A further linear step could be introduced to baseline separate all three standards completely. This simple model of known standards clearly illustrates that the cross-flow in AF4 is a powerful tool in the separation of macromolecules other than the reference materials. The drawbacks of applying extended cross-flow steps are that the elution time and band broadening are significantly increased while the peak maximum intensity is slightly decreased. The peaks for each cross-flow gradient (A−D) as well as the SEC mixtures were processed
Figure 5. AF4 fractograms for a mixture of polystyrene standards using cross flows B, C, and D. MALLS (90°) detector readings are overlaid.
and integrated by applying the random coil data fitting method and the calculated molar masses are presented in Table 2. The SEC measurements presented in Table 2 indicate that the calculated molar masses are incorrect due to the overlapping of peaks in the various elugrams, even after varying the column sets in series (mixed A and B columns). The same applies for the AF4 measurements for cross flows A to C. The molar mass values calculated from cross-flow D is the closest to the values given by the producer and corresponds well to the radius calculation given by R = 0.014M0.585 for linear polystyrene in THF as solvent at 25 °C.32 In SEC the only option for an improvement in separation is by means of adapting the column set and injection flow among others. This could lead to additional problems since high flow velocities and more SEC columns will lead to pronounced shear degradation and filtration effects. In AF4 one of the options is to change the cross-flow gradient for an improvement in separation, making it a superior tool over SEC regarding optimization of the separation. Different from SEC the user is not bound to the separation characteristics of the system that is always determined by the column set. Analysis of Polybutadienes. Effect of Dissolution Time and Temperature on SEC and AF4 Analysis. In this section the effect of dissolution on the elution behavior of the polybutadiene samples was investigated. The samples have quite high molar masses and tend to entangle in solution. Therefore, one must make sure that complete dissolution is 5250
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Table 2. Polystyrene Molar Masses Obtained from Different SEC Column Sets and Different AF4 Cross-Flow Gradients SEC sample PSmix 1a 62 kg/mol 250 kg/mol 103 kg/mol PSmix 2b 62 kg/mol 250 kg/mol 103 kg/mol PSmix 3c 62 kg/mol 250 kg/mol 103 kg/mol
AF4
Mw (kg/mol)
Mn (kg/mol)
Rgd (nm)
DIe
65.7 273 964
60.2 265 878
16.5 22.0 44.5
1.09 1.03 1.10
55.8 221 842
52.2 197 753
15.3 21.7 44.3
1.07 1.12 1.12
98.9 308 910
90.1 289 892
29.2 41.1 56.9
1.10 1.07 1.02
sample cross-flow A 62 kg/mol 250 kg/mol 103 kg/mol cross-flow B 62 kg/mol 250 kg/mol 103 kg/mol cross-flow C 62 kg/mol 250 kg/mol 103 kg/mol cross-flow D 62 kg/mol 250 kg/mol 103 kg/mol
Mw (kg/mol)
Mn (kg/mol)
Rgd (nm)
DIe
81.7 306 1070
70.0 293 1060
11.5 24.8 46
1.17 1.05 1.01
71.5 246 764
70.3 239 749
15.9 26.9 46.3
1.02 1.03 1.02
68.3 244 810
66.1 242 776
17.8 21.4 44.9
1.03 1.01 1.04
61.1 256 1090
59.2 252 1030
9.7 22.4 45.4
1.03 1.02 1.06
a One PL mixed C column used at flow rate of 0.5 mL/min. bOne PL mixed B and one PL mixed C column used at flow rate of 0.5 mL/min. cOne PL mixed A and one PL mixed B column used at flow rate of 0.5 mL/min. dz-average. eDispersity index.
temperature. The sample flasks were rotated regularly to allow proper mixing and homogenization of the solutions. The recovery was checked again by analyzing the samples with SEC, the total dissolution time now being 68 h. The resultant recovery, even after filtration, was in the region of 100% (Table 3), indicating proper dissolution. After prolonged dissolution times the aggregates or large gel-resembling species have been dissolved into individual macromolecules which were able to pass the filter and frits of the columns. The elugrams with the molar masses and Rg of the measurements for both the short and long dissolution times of PB 2 are shown in Figures 6 and 7, respectively. It is clearly
achieved prior to SEC and AF4 analysis. After an initial dissolution step of 16 h at ambient temperature followed by 4 h of heating, polybutadienes PB 1−3 were measured by SEC to see whether the samples have been dissolved completely. For each sample the recovery from the SEC system was measured by comparing the RI signals after injection into the SEC column. The RI calibration constant was determined by analyzing the RI peak obtained from injecting a PS standard of 33 kg/mol. This calibration constant was checked again with a 62 kg/mol PS standard. The calculated RI calibration constant was 1.016 × 10−4. Blockage of the SEC columns was prevented by filtering the samples with a 0.45 μm filter prior to injection. Only 10% of the injected mass was recovered as is shown in Table 3, indicating that after the initial dissolution Table 3. Effect of Different Dissolution Times on Recovery, with the Calculated Molar Masses, Radii of Gyration, and Dispersity Indices for PB 1−3 sample PB 1 (short, 20 h) PB 1 (long, 68 h) PB 2 (short) PB 2 (long) PB 3 (short) PB 3 (long) a
recovery % (eluted mass/injected mass)
Mw (kg/mol)
Mn (kg/mol)
Rga (nm)
DIb
11
343
168
49.5
2.05
93
280
133
55.7
2.11
13
461
172
78.2
2.68
100
386
113
80.8
3.41
13
327
111
53.7
2.94
100
270
99
53.3
2.73
Figure 6. SEC elugrams and molar mass readings for different dissolution times of sample PB2.
z-average. bDispersity index.
visible that the normalized signal intensities of the RI and MALLS signals are much more intense (±5-fold) for the longer dissolution time compared to the shorter dissolution time measurement. The immense differences in intensities and recoveries of the two measurements are a clear indication that complete dissolution took place only after longer dissolution times. The fact that for the short dissolution time not all
step a large percentage of the samples not completely dissolved. As a result of incomplete dissolution, a large part of the macromolecules was probably present in solution as swollen gel particles and was filtered off or trapped in the SEC column. For a further improvement of the sample preparation, the samples were dissolved for another 48 h at ambient 5251
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In another example (PB 4), the effect of dissolution was investigated by comparing SEC and AF4 results. The sample was treated similar to the previous one. In this example different heating steps and filtering were applied to investigate the effect of dissolution and heating upon elution behavior. Figure 9 shows the SEC elugram using the 90° MALLS signal
Figure 7. SEC elugrams and Rg readings for short and long dissolution times of sample PB2.
macromolecules are properly dissolved yet and some entanglements are still present is also visible in the Rg plot given in Figure 7. While for the long dissolution period Rg decreases monotonically with increasing elution time, this is not the case for the short dissolution period. In this case an upswing of the Rg reading is observed. This effect will be discussed later in the article. The molar mass distribution is obtained by extrapolation of the molar mass vs retention time plot and is represented by a differential molar mass plot. The MMD for both short and long dissolution times closely overlap as shown in Figure 8.
Figure 9. SEC elugrams of PB 4 with MALLS signal and molar mass readings.
and the molar mass reading. Measurements were taken after filtration with a 0.45 μm filter 1 and 3 days after the initial sample treatment and after a second heating step of 5 h. An additional measurement after the second heating step was taken with a 0.20 μm filter. Both 1 and 3 days after initial heat treatment (thick black and gray lines) a strong noise signal is observed at the high molar mass side in the MALLS signal which is possibly due to very small amounts of large gelresembling species present in solution. Their concentration is too small to be detected by the RI detector (Figure 10). The assumption is made that these gel-resembling species might be not cross-linked macromolecules but entangled macromolecules that are only partially dissolved. The decrease of the noise with increasing heating or dissolution time is a clear indication that this assumption is correct. After 3 days (thick gray line) of dissolution the noise decreased compared to the measurement after 1 day (thick black line). The measurement taken after the second heating step (thin black line) also suggests an increasing amount of properly dissolved macromolecules. The measurement after filtration with a 0.20 μm filter (thin gray line) shows almost no noise present in the light scattering signal. This is a possible indication that most of the gel-resembling species were filtered off or got stuck on the inlet frit of the column.35 The corresponding RI signals and Rg readings of the SEC measurement shown in Figure 10. The smooth RI curves do not show any noise or peaks at an elution time of about 21 min, emphasizing the low sensitivity of the RI detector for elution slices containing large macromolecules that are very low in concentration.58,68 At high retention times both the molar mass and Rg vs retention time plots (Figures 9 and 10) show abnormal SEC behavior.35,84 In Figure 9, the molar mass shows a minor increase at high elution time for the
Figure 8. Cumulative weight and molar mass distribution of PB2 at short and long dissolution times. Symbols □ and △ are nonextrapolated data while all the other plots are fitted with fit orders given in parentheses.
The cumulative weight fraction plots of both measurements are overlaid before and after extrapolation. It is visible that the different dissolution procedures have only a minor impact on the very low and high molar mass fractions which is one of the reasons why the improper dissolution is hard to identify just by comparison of the average molar mass values, radii, and dispersities as shown in Table 3. Consequently, longer dissolution times of minimum 68 h were used as a basis for the analysis of all other polybutadiene samples since they provided more reliable results with regard to sample recovery and reproducibility. 5252
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Figure 11. AF4 fractograms of PB 4 with RI, MALLS, and molar mass readings. Figure 10. SEC elugrams of PB 4 with RI signal and radius of gyration readings showing abnormal radius behavior at high elution times.
species. Even for the measurement after filtration with a 0.2 μm filter, a small amount of gel-resembling species can be observed. This possibly indicates that the small amount of this material which was able to pass the 0.2 μm filter in SEC got stuck on the column frit or in narrow interparticle cavities of the packing material, since a smooth curvature was observed in the SEC measurement. In AF4 no frit is used and the MALLS signal is a true reflection of the total material in solution which was able to pass the 0.2 μm filter. The molar mass readings of the three measurements are in excellent agreement until the gelresembling spikes are observed at higher elution times, showing that the measurements in AF4 are reproducible. The RI and MALLS traces support this assumption. Beyond this point data fitting is not accurate since the scattering is very intense although concentration is very low making data fitting very difficult or even impossible. As has been pointed out, these peaks are a possible indication of incomplete dissolution, entanglements between polymer chains, or the presence of insoluble gel in solution. A possible explanation for the noise could be that if some very large species pass the light scattering cell, the movement of the structure can cause spikes since the low concentrations of the diluted sample are caused by only a few very large molecules. Consequently, a movement of such highly scattering species through the laser beam will have a huge impact on the scattered light intensity as the scatter volume is typically low and the number of molecules is not sufficient to average the effect. These findings underline the importance of a light scattering detector as a highly sensitive tool for the detection of very small quantities of high molar mass or gel-containing macromolecules in solution. Effect of Branching on Molar Mass Analysis in SEC and AF4. For a given molar mass, branched species have a smaller hydrodynamic volume than the linear species of similar molar mass.32 Since separation in SEC takes place according to hydrodynamic volume, two species of equal molar mass but different hydrodynamic volume can elute at different elution times.28,87 Since branched macromolecules may coelute with linear macromolecules, this results in a region of medium to high dispersity regarding molar mass. Having the same hydrodynamic volume but different coil densities linear and branched macromolecules will show different molar mass−
shorter dissolution procedures (filled squares). The corresponding radius values (filled squares in Figure 10) show this behavior much stronger as a clear curvature is visible toward the later eluting species. Such behavior is well-known for highly branched molecules and was already discussed in various publications.40,42−44,85 Explanations for the observed phenomenon range from anchoring of branched species in the column packing to molecular topology fractionation in small cavities.84 In PB 4, however, the reason for the detected results is the unusually late elution of high molar mass species which are supposed to be highly branched together with regular eluting linear species of low molar mass.28 The Rg reading is more sensitive to this phenomenon than the molar mass reading. The particular slice of the chromatogram which is affected by coelution is an averaged value for the different radii and molar masses.26,31 The molar mass represents a weighted average value while the radius represents a z-averaged value in the case of these polydisperse portions.32 This causes the radius plots to be more responsive to very small amounts of larger molecules compared to the molar mass plots, which results in a more pronounced upswing of the radii.32,35,41 The effect of branching on the elution behavior will be discussed later in the article. However, until now it can be stated that for both samples PB 2 and PB 4 the abnormal elution behavior in SEC is increased at short dissolution times or improper dissolution or sample preparation conditions. The same sample (PB 4) was measured using AF4 after the second heating step by making use of cross-flow B. Three different measurements were done with a 0.2 μm filter (thick black line, filled black squares) and a 0.45 μm filter (dark gray line, filled dark gray squares) and without filtration (light gray line, open squares). Figure 11 shows the AF4 fractograms and molar mass readings of PB 4. The molar mass increases with increasing elution time which is indicative of normal mode AF4.86 The MALLS signal shows that for the unfiltered sample larger gel species are present in the sample in comparison with the two filtered samples. An unfiltered sample could not be measured in SEC due to column sensitivity toward microgel 5253
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radius of gyration behavior. For linear macromolecules Rg will increase proportionally with molar mass. This effect is lowered or might not be the case for branched macromolecules. If the branching density increases, then for the same molar mass Rg decreases. This will result in a decreased slope of the Rg vs M plot for samples that are branched over the whole mass range. Samples that show significant branching only for the high molar mass molecules often show a decrease of the Rg vs M slope for the high molar mass fractions.87 A second effect which is often found for branched material is the abnormal upswing of the radius and (to a lesser extent) the molar mass at low elution times in SEC-MALLS. This effect is also visible for sample PB 5 (Figures 12 and 13). The reason for
between the polybutadiene molecules and the cellulose membrane. However, care must be taken to avoid membrane fouling which would affect the separation. Until now, the reason for the upswing in SEC is not completely clear. Since the effect is preferably found for branched material,28,38,40,42−44,85 the branching seems to be one of the reasons for the abnormal late elution of large macromolecules. In addition, the presence of small linear macromolecules and large excessively branched material might further increase the dispersity effect, therefore resulting in the differences between the detected molar mass and Rg values which are weight (w)- and z-averages for disperse fractions, respectively. Consequently, the bigger the dispersity effects of the late-elution fractions, the higher the impact of the different averaging of Rg and M. The degree of branching from SEC-MALLS is typically calculated by the interpretation of the slope in the conformation plot which is the relationship between Rg and molar mass (Figure 14).28 Because of the upswing effect in the
Figure 12. MALLS signals of SEC and AF4 with molar mass readings overlaid, sample PB 5.
Figure 14. Conformation plots (Rg vs molar mass) for SEC (gray squares) and AF4 (black squares) of sample PB 5. MMD of SEC (gray line, extrapolated) and AF4 (black line) are overlaid.
SEC plot, calculations related to branching will be falsified (slope of 0.36 in Figure 14) since the coelution effect leads to wrong values of the slope. Two overlapping effects both related to branching are observed in this case. First a decrease in the Rg−M slope is visible for (1) partially branched molecules at high molar masses and (2) fully branched molecules throughout the molar mass range. Second, the upcurling of Rg at high elution times/low molar masses which is related to branching but cannot be correlated since the reincreasing radii are due to the high polydispersity of the size fractions in this part of the conformation plot. Consequently, the effect cannot be assigned to a change of shape or density of the polymer coil since this would only be possible if proper separation with size fractions that are monodisperse in hydrodynamic volume are obtained. The upswing influences the regular molecular contraction as described in the first effect. Thus, SEC may be very sensitive toward branching since upswing and molecular contraction get visible, but a data evaluation is not possible due to the coexistence of a huge variety of effects caused by branching. Table 4 shows the calculated molar masses and radii for the upswing in Figures 12 and 13. The extrapolated values are also
Figure 13. MALLS signals of SEC and AF4 with Rg readings overlaid, sample PB 5.
the reincrease is the abnormal late coelution of large and high molar mass molecules together with regular eluting small molecules of low molar mass. Species which differ highly in molar mass and size are coeluting which was proven for polyethylene samples by fractionation of the late eluting part from SEC and reinjection of the fraction in FFF.84 Low and high molar mass molecules can be isolated by proper FFF separation provided that their hydrodynamic volumes are different. In addition, no interactions are observed 5254
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0.43, which is below the reference value of 0.588 for linear molecules.87 The reduced slope indicates that long chain branching was present in the sample. The conformation plot as well as the MMD in Figure 14 shows that in AF4 much higher molar masses are detected compared to the measurement in SEC. Higher molar masses are also confirmed in Table 4 for both the filtered and unfiltered samples in AF4. This decrease in molar mass in SEC is possibly an indication that shear degradation and/or removal of ultrahigh molar mass components of the sample caused by the stationary phase occurred. The MMD in Figure 14 also shows that lower molar masses are obtained for the SEC measurement if extrapolation is applied. This was due to the polymer chains above the molar mass cutoff of the membrane being able to pass through the pores of the membrane. The cutoff value (10 kg/mol) is an averaged value; therefore, it is possible that higher molar mass fractions can be lost through the membrane as well. The lower recoveries observed for the AF4 measurements compared to SEC in Table 4 supports the analogy made. Additionally, it can be speculated that improper dissolution, and as a consequence, the presence of compact and only partially dissolved agglomerates of multiple polymer chains may lead to increased late elution behavior in SEC. As a result, they will elute late, and thus they will be masked due to the local mass and radius average effects. Finally, the species are invisible in the MMD and overall molar mass/radius average values. It is known that dissolution irregularities could cause polymer structures to behave like branched or even highly branched species since the contact points of the different polymer chains will act like physical branching points, and thus they could also
Table 4. Calculated Molar Masses, Radii, and Dispersity Indices of PB5 from SEC and AF4a sample PB 5 (SEC, filter) PB 5 (SEC, extrapolated) PB 5 (AF4, filter) PB 5 (AF4, no filter) PB 5 (AF4, reinjection1) PB 5 (AF4, reinjection2
recovery % (eluted/injected mass)
Mw (kg/mol)
Mn (kg/mol)
Rgb (nm)
DIc
97
450
154
69.1
2.9
420
103
60.9
4.1
87
920
187
141.4
4.9
89
982
204
161.1
4.8
465
160
122.5
2.9
490
168
121.7
2.9
a Filtering was done with a 0.45 μm filter. The collected fractions of the late eluting fraction in SEC were re-injected in AF4 without filtering. b z-average. cDispersity index.
calculated by applying a linear fit to the molar mass and Rg plots in SEC, giving the values that are estimated if no upswing was present. As can be seen, the extrapolated molar mass and Rg values are smaller than in the case of the calculation without extrapolation. In AF4, no stationary phase is present, which avoids these unwanted irregularities (black lines, black filled squares in Figures 12 and 13). The effects of shear degradation and coelution are minimized, and the resultant branching calculations can be achieved accurately (Figure 14, gray lines and squares). Cross-flow A was applied for this measurement, and the slope observed in the conformation plot for AF4 was
Figure 15. Upswing elugram from SEC vs AF4 of the collected fractions for PB 5: (A) Molar mass readings showing that the lower molar masses are masked in SEC due to coelution compared to AF4. (B) Cumulative molar mass plot confirming the missing low molar mass fraction in SEC, but present in the collected fractions measured in AF4. The Rg−M relationships of the two distributions observed in (A) are overlaid in (B). 5255
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might also suffer partially from unsufficient resolution. Therefore, the determination of Rg is challenging in this regime. To overcome this problem, the use of a membrane having a lower cutoff might be helpful. Investigation of Gel Species in Polybutadienes. The size heterogeneity of polymers is often correlated to the modality of the elugrams obtained by SEC.32 In the present example (Figure 16), the MALLS signal (thin gray line, open squares) illustrates that the sample PB 6 is monomodal over the entire elution profile.
undergo the second separation mechanism which is often claimed to be a reason for late elution.35 This observation is supported by the increased upswing phenomenon of molar mass and radius for sample PB 2 and PB 4 if poor dissolution conditions were applied. The late eluting fraction in PB 4 (retention time of 26−31 min) was fractionated multiple times in order to investigate whether or not artifacts were present in the interpretation of the SEC data. The late eluting fractions were combined and evaporated at room temperature to obtain a concentration sufficient for reinjection. The combined late eluting fraction was reinjected into AF4 in duplicate and compared to the initial SEC measurement. Figure 15 shows the molar mass, cumulative weight fraction, and conformation (Rg vs M) plots. In this case a membrane with a cutoff of 5 kg/mol was used. Figure 15A shows that lower and higher molar masses were observed in AF4 compared to the SEC measurements. The lower molar mass molecules are well separated from the higher molar masses in AF4, resulting in smaller values being detected compared to SEC. In SEC the low molar masses were masked by the upswing observed in the molar mass and radii plots caused by the coelution of normal linear eluting species with branched species, respectively. The most remarkable observation is that molar masses as high as 50 000 kg/mol were detected in the collected fractions reinjected in AF4, confirming the presence of large structures of high molar masses present in the late eluting fraction. In SEC molar masses only up to 10 000 kg/mol were detected since this is the exclusion limit of the mixed B column. The molar mass plots in AF4 also show two distinct molar mass regions which is an indication that different species are present in the late eluting SEC fraction, possibly a combination of linear, branched, and gel species. The cumulative molar mass plot in Figure 15B confirms the detection of much lower molar masses in AF4 compared to SEC. The weight fraction of polymer chains above 100 kg/mol is ∼70% of the collected fraction. This indicates that a significant amount of high molar mass species eluted later than expected, either due to branching, retardation of high molar mass species in the pores of the stationary phase (slalom chromatography), or a combination of the two.49,88 Similar work has been done for the investigation of the late eluting fraction of branched polyolefins using high temperature SEC/AF4-MALLS analysis of lowdensity polyethylene.84 The conformation plots of the reinjected fractions are presented in Figure 15B. Two distinct slopes are clearly visible for the two observed molar mass regions in Figure 15A. The first slope resembles molar mass species up to 400 kg/mol that are highly branched (slope = 0.34) while the second slope of 0.14 is indicative of very high molar mass species which are spherical and compact in nature. These species are most likely highly cross-linked gel material present in very small amounts. The fact that the molar mass increases without a significant change in size supports this statement. The molar mass values observed in the measurements before (SEC, AF4) and after (AF4) fractionation are shown in Table 4. The average molar mass (Mw) is in the region of 500 kg/mol, which is well above 80 kg/mol, the value where the upswing in SEC initiated (Figure 15A). The presence of late eluting branched species together with high molar mass material is validated by the observed values in Table 4. It has to be mentioned, however, that the direct comparison of the small fraction in Figure 14 with the results of Figure 15
Figure 16. MALLS signals of SEC and AF4 (filtered and unfiltered) for sample PB 6; molar mass readings are overlaid.
Since the MALLS detector is very sensitive toward the presence of small amounts impurities or different size regimes, it can be assumed that the SEC measurement is rather monomodal with respect to size, and no additional size distribution is present in solution. However, the AF4 measurements, where cross-flow B was used, clearly show a bimodal distribution which is observed in the MALLS signal. The measurements with and without filtration are seen in Figure 16 (thick black and gray lines and filled squares). Even in the measurement after filtration a bimodal peak is observed. This phenomenon is possibly a result of very small amounts of cross-linked or gel-resembling macromolecules which were able to pass through the 0.45 μm filter.33,68 It can be assumed that cross-linked structures have an increased mechanical stability retaining their high flexibility.89 It is likely that such structures are able to pass the filter pores in a stretched conformation with very low effective hydrodynamic radius. The filtration will not degrade the chains due to the network-like structure which is more stable than an entirely linear chain. After filtration the structure may go back to its original conformation with an increased diameter, and thus it will elute later as a second peak due to the lower diffusion ability. In the SEC measurement these cross-linked networklike molecules possibly got stuck or were degraded by the frits and pores of the columns. The concentration profiles of the measurements together with the Rg readings are shown in Figure 17. It is evident that the two AF4 measurements overlap each other very closely throughout the fractogram. At higher elution times, where the concentration signal intensity is close to zero, the later eluting species are observed in the fractogram of the MALLS 90° signal. The unfiltered sample shows a very noisy peak (MALLS) which could be an indication of improper 5256
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species since a rodlike structure would have a lower hydrodynamic radius/diffusion coefficient than a coil of the same molar mass.90−92 Further adjusting the cross-flow or making use of mixed mobile phases could be a solution for optimal separation in AF4. Another explanation for the second observed peak could be the onset of steric FFF, whereby coelution in AF4 takes place due to overlapping of normal with steric mode of separation. In steric mode very narrow peaks are observed due to the lower sensitivity of the steric separation mechanism toward differences in size. A very small amount of very large material above the steric point of inversion is present in solution (usually 800− 1000 nm depending on diffusion properties of the material and coil density). This material coelutes with the well-separated smaller molecules as discrete peaks which cannot be changed in shape even not at very high cross-flows. At the steric point of inversion one gets a plateau in the Rg reading due to the presence of both size separations according normal and steric FFF mode. Poor separation will always be observed as long as steric FFF mode is overlaid since the strengthening of the cross-flow force will lead to better normal mode FFF separation but also a stronger steric mode separation (which is opposite). As a consequence, no improvement in separation will be visible due a complete shift in retention time of both peaks to the same extent (Figure 18A,B). If a spacer of larger width would be used, the steric inversion point would be shifted to larger radii values and an increase toward higher molar masses and radii would be observed. The second peak would appear to be much smaller or even disappear. Since the second peak is still visible and relatively constant in size after filtration, these species can also be due to very small amounts of aggregates that elute together with properly separated polymer chains. A solution to this problem is, therefore, to either heat the mobile phase up to higher temperatures (increase in diffusion coefficient) or to increase the channel width by replacing the spacer with a larger size, which will allow for a normal mode of separation only and proper calculations of molar mass and radii. Thermal FFF (ThFFF) separates according to chemical composition and hydrodynamic volume and could also be a possible solution for this problem since the diffusion can be adjusted by temperature programming, and thus the fractionation efficiency may be increased.
Figure 17. RI signals of SEC and AF4 (filtered and unfiltered) for PB 6; radius of gyration readings are overlaid.
dissolution and/or extremely large species as already explained above. As a consequence of the noise, the software is not able to calculate average molar mass or Rg values in this region for the random coil fitting method. The measurement after filtration gave molar mass and Rg readings in this region, since the intensity of the late eluting species is less pronounced and the signal is smoother than the measurement without filtration which made data fitting possible. The interesting question was to see whether it is possible to make use of the uniqueness of the AF4 cross-flow system and investigate the possibility of separating the bimodal peaks by adjusting the cross-flow gradients. Numerous attempts were made by making use of the cross-flows observed in Figure 18A, and the resultant fractograms with MALLS 90° signals for the unfiltered sample are given in Figure 18B. It was found that there was no major improvement in separation upon cross-flow gradient but merely a shift in elution time. The reason for this could be that the second observed peak might be due to cross-linking within the sample. This could result in similar diffusion coefficients for both species despite a difference in size in THF as solvent, and as a consequence AF4 is not suitable for a complete separation. Similar diffusion coefficients may also be caused by differences in shape of both
Figure 18. (A) Cross-flow profiles used in an attempt to separate the bimodal peaks of PB 6 observed in Figure 16 and (B) corresponding MALLS signals of the fractograms. 5257
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(5) Grayson, A. C. R.; Choi, I. S.; Tyler, B. M.; Wang, P. P.; Brem, H.; Cima, M. J.; Langer, R. Nat. Mater. 2003, 2 (11), 767−772. (6) Yoon, H.-J.; Jang, W.-D. J. Mater. Chem. 2010, 20 (2), 211−222. (7) Narula, C. K.; Allison, J. E.; Bauer, D. R.; Gandhi, H. S. Chem. Mater. 1996, 8 (5), 984−1003. (8) Striegel, A. M. Anal. Bioanal. Chem. 2008, 390, 303. (9) Kostanski, L. K.; Keller, D. M.; Hamielec, A. E. J. Biochem. Bioph. Methods 2004, 58 (2), 159−186. (10) Messaud, F. A.; Sanderson, R. D.; Runyon, J. R.; Otte, T.; Pasch, H.; Williams, S. K. R. Prog. Polym. Sci. 2009, 34 (4), 351−368. (11) Striegel, A. M.; Kirkland, J. J.; Yau, W. W.; Bly, D. D. Modern Size Exclusion Chromatography, 2nd ed.; Wiley: Hoboken, NJ, 2009; p 494. (12) Gaborieau, M.; Castignolles, P. Anal. Bioanal. Chem. 2011, 399, 1413. (13) Trathnigg, B. Size-Exclusion Chromatography of Polymers; John Wiley & Sons, Ltd.: New York, 2006. (14) Netopilik, M.; Kratochvil, P. Polymer 2003, 44, 3431. (15) Guillaneuf, Y.; Castignolles, P. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 897. (16) Podzimek, S. J. Chromatogr., A 1994, 677 (1), 21−24. (17) Gray-Weale, A. A.; Cave, R. A.; Gilbert, R. G. Biomacromolecules 2009, 10 (9), 2708−2713. (18) Mendrek, B.; Trzebicka, B. Eur. Polym. J. 2009, 45 (7), 1979− 1993. (19) Pang, S.; Rudin, A. Polymer 1992, 33, 1949.. (20) Striegel, A. M.; Krejsa, M. R. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 3120. (21) Garamszegi, L.; Nguyen, T. Q.; Plummer, C. J. G.; Manson, J. A. E. J. Liq. Chromatogr. Relat. Technol. 2003, 26, 207. (22) Gaborieau, M.; Nicolas, J.; Save, M.; Charleux, B.; Vairon, J.-P.; Gilbert, R. G.; Castignolles, P. J. Chromatogr., A 2008, 1190, 215. (23) Yau, W. W.; Rementer, S. W. J. Liq. Chromatogr. 1990, 13, 627. (24) Goldwasser, J. M. ACS Symp. Ser. 1993, 521, 243. (25) Harrison, C. A.; Mourey, T. H. J. Appl. Polym. Sci. 1995, 56, 211. (26) Podzimek, S. J. Appl. Polym. Sci. 1994, 54 (1), 91−103. (27) Kastánek, A.; Podzimek, S.; Dostál, J.; Simek, L.; Bohdanecký, M. Polymer 2000, 41 (8), 2865−2870. (28) Podzimek, S.; Vlcek, T.; Johann, C. J. Appl. Polym. Sci. 2001, 81 (7), 1588−1594. (29) Zagar, E.; Zigon, M.; Podzimek, S. Polymer 2006, 47 (1), 166− 175. (30) Angoy, M.; Bartolomé, M. I.; Vispe, E.; Lebeda, P.; Jiménez, M. V.; Pérez-Torrente, J. J.; Collins, S.; Podzimek, S. Macromolecules 2010, 43 (15), 6278−6283. (31) Wyatt, P. J. Anal. Chim. Acta 1993, 272 (1), 1−40. (32) Podzimek, S. Light Scattering, Size Exclusion Chromatography and Asymmetric Flow Field Flow Fractionation: Powerful Tools for the Characterization of Polymers, Proteins and Nanoparticles; John Wiley & Sons: New York, 2011. (33) Lee, S.; Molnar, A. Macromolecules 1995, 28 (18), 6354−6356. (34) Aust, N. J. Biochem. Biophys. Methods 2003, 56, 323−334. (35) Otte, T.; Pasch, H.; Macko, T.; Brüll, R.; Stadler, F. J.; Kaschta, J.; Becker, F.; Buback, M. J. Chromatogr., A 2011, 1218 (27), 4257− 4267. (36) Slagowski, E. L.; Fetters, L. J.; McIntyre, D. Macromolecules 1974, 7 (3), 394−396. (37) Zammit, M. D.; Davis, T. P.; Suddaby, K. G. Polymer 1998, 39 (23), 5789−5798. (38) Stadler, F. J.; Kaschta, J.; Münstedt, H.; Becker, F.; Buback, M. Rheol. Acta 2009, 48, 479−490. (39) Cave, R. A.; Seabrook, S. A.; Gidley, M. J.; Gilbert, R. G. Biomacromolecules 2009, 10 (8), 2245−2253. (40) Gerle, M.; Fischer, K.; Roos, S.; Muller, A. H. E.; Schmidt, M. Macromolecules 1999, 32, 2629−2637. (41) Mes, E. P. C.; de Jonge, H.; Klein, T.; Welz, R. R.; Gillespie, D. T. J. Chromatogr., A 2007, 1154 (1−2), 319−330. (42) Percec, V.; Ahn, C. H.; Cho, W. D.; Jamieson, A. M.; Kim, J.; Leman, T.; Schmidt, M.; Gerle, M.; Möller, M.; Prokhorova, S. A.;
CONCLUSIONS The AF4-MALLS-RI separation technique was successfully implemented for very high molar mass polybutadienes. Comparisons of SEC with AF4 using polystyrene standards as reference materials were in good agreement with the expected values. The versatility of the cross-flow field in AF4 is an invaluable tool for the optimization of separation as was shown for the mixture of polystyrene standards. As a result, the correct molar mass and radius information can be obtained in the case of perfect separation. SEC analysis of polybutadienes showed overlapping effects of chain fragmentation and entanglements of large molecules, while the coelution of large macromolecules together with regularly eluting species of smaller molar mass causes problems with regard to calculating the correct molar mass and radius distributions. In contrast, AF4 analysis of the polybutadienes showed a number of advantages over SEC, especially in the case of very high molar mass, branched and gel-containing samples. Because of the higher exclusion limit and absence of a stationary phase in AF4, accurate molar mass and radius distributions can be calculated for the branched or ultrahigh molar mass species since coelution due to branching or shear degradation is minimized. The collection of the coeluting fraction and reinjection in AF4 confirmed the presence of high molar mass species, which is an important step for SEC-MALLS interpretations of data in future. AF4 is also a valuable tool for investigating the dissolution process compared to SEC where filtration is necessary. The MALLS detector is very sensitive to high molar mass species; therefore, as a result of the empty channel in AF4 it was possible to analyze very small amounts of gel species without filtering the samples. The experiments indicate that it is possible to use AF4 as a special tool for the extended analysis of synthetic rubber materials especially in the case of very high molar mass or excessive branching. In future the new information from AF4 could offer the possibility to explain some differences in the processing behavior of such samples.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address §
Hamilton Bonaduz AG, Via Crusch 8, 7402 Bonaduz, Switzerland.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Freeworld Coatings, PPS Insurance, and the National Research Foundation (NRF) for financial support. We greatly thank Robert Bruell for providing the polybutadiene samples.
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REFERENCES
(1) Halliwell, S. Polymers in Building and Construction; Rapra Technology Ltd.: Shawbury, UK, 2002. (2) Gurunathan, K.; Murugan, A. V.; Marimuthu, R.; Mulik, U. P.; Amalnerkar, D. P. Mater. Chem. Phys. 1999, 61 (3), 173−191. (3) Heeger, A. J. Synth. Met. 2001, 125 (1), 23−42. (4) Mark, J. E. Polymer Data Handbook; Oxford University Press: New York, 1999. 5258
dx.doi.org/10.1021/ma3007812 | Macromolecules 2012, 45, 5247−5259
Macromolecules
Article
Sheiko, S. S.; Cheng, S. Z. D.; Zhang, A.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120, 8619−8631. (43) Johann, C.; Kilz, P. J. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1991, 48, 111−122. (44) Wintermantel, M.; Antonietti, M.; Schmidt, M. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1993, 52, 91−103. (45) Schimpf, M. E.; Caldwell, K.; Giddings, J. C. Field Flow Fractionation Handbook; John Wiley & Sons: New York, 2000. (46) Cölfen, H.; Antonietti, M. Adv. Polym. Sci. 2000, 150, 67−187. (47) Janca, J. Field-flow fractionation. In Heftmann, E., Ed.; Journal of Chromatography Library; Elsevier: Amsterdam, 1992; pp A449−A479. (48) White, R. J. Polym. Int. 1997, 43 (4), 373−379. (49) Liu, Y.; Radke, W.; Pasch, H. Macromolecules 2006, 39 (5), 2004−2006. (50) Moon, M. H. J. Sep. Sci. 2010, 33 (22), 3519−3529. (51) Scherer, C.; Noskov, S.; Utech, S.; Bantz, C.; Mueller, W.; Krohne, K.; Maskos, M. J. Nanosci. Nanotechnol. 2010, 10, 6834. (52) Adolphi, U.; Kulicke, W.-M. Polymer 1997, 38 (7), 1513−1519. (53) Arfvidsson, C.; Wahlund, K.-G. J. Chromatogr., A 2003, 1011 (1−2), 99−109. (54) Dubascoux, S.; Von Der Kammer, F.; Le Hécho, I.; Gautier, M. P.; Lespes, G. J. Chromatogr., A 2008, 1206 (2), 160−165. (55) Lee, H.; Cho, I.-H.; Moon, M. H. J. Chromatogr., A 2006, 1131 (1−2), 185−191. (56) Ratanathanawongs Williams, S. K.; Lee, D. J. Sep. Sci. 2006, 29 (12), 1720−1732. (57) Otte, T.; Brüll, R.; Macko, T.; Pasch, H.; Klein, T. J. Chromatogr., A 2010, 1217 (5), 722−730. (58) Lee, D.; Williams, S. K. R. J. Chromatogr., A 2010, 1217 (10), 1667−1673. (59) Lohmann, C. A.; Haseltine, W. G.; Engle, J. R.; Williams, S. K. R. Anal. Chim. Acta 2009, 654 (1), 92−96. (60) Shiundu, P. M.; Remsen, E. E.; Giddings, J. C. J. Appl. Polym. Sci. 1996, 60 (10), 1695−1707. (61) Williams, S. K. R.; Runyon, J. R.; Ashames, A. A. Anal. Chem. 2010, 83 (3), 634−642. (62) Stojanov, C.; Shirazi, Z.; Audu, T. Chromatographia 1978, 11 (5), 274−281. (63) Parth, M.; Aust, N.; Lederer, K. Macromol. Symp. 2002, 181 (1), 447−456. (64) Kim, C.; Morel, M.-H.; Beuve, J. S.; Guilbert, S.; Collet, A.; Bonfils, F. J. Chromatogr., A 2008, 1213 (2), 181−188. (65) Kim, C.; Beuve, J. S.; Guilbert, S.; Bonfils, F. Eur. Polym. J. 2009, 45, 2249−2259. (66) Lee, S.; Eum, C. H.; Plepys, A. R. Bull. Korean Chem. Soc. 2000, 21 (1), 69−74. (67) Lewandowski, L.; Sibbald, M. S.; Johnson, E.; Mallamaci, M. P. Rubber Chem. Technol. 2000, 73 (4), 731−742. (68) Sibbald, M.; Lewandowski, L.; Mallamaci, M.; Johnson, E. Macromol. Symp. 2000, 155 (1), 213−228. (69) Kim, W.-S.; Eum, C. H.; Molnar, A.; Yu, J.-S.; Lee, S. Analyst 2006, 131 (3), 429−433. (70) Ratanathanawongs Williams, S. K.; Benincasa, M.-A.; Ashames, A. A. Encyclopedia of Analytical Chemistry; John Wiley & Sons, Ltd.: New York, 2009; pp 1−32. (71) Bang, D. Y.; Shin, D. Y.; Lee, S.; Moon, M. H. J. Chromatogr., A 2007, 1147 (2), 200−205. (72) Eslahian, K. A.; Maskos, M. Colloids Surf., A 2012, http://dx.doi. org/10.1016/j.colsurfa.2012.02.003. (73) Awan, M. A.; Dimonie, V. L.; Ou-Yang, D.; El-Aasser, M. S. Langmuir 1997, 13 (2), 140−146. (74) Mes, E. P. C.; Tijssen, R.; Kok, W. T. J. Chromatogr., A 2001, 907 (1−2), 201−209. (75) Shiundu, P. M.; Munguti, S. M.; Williams, S. K. R. J. Chromatogr., A 2003, 983 (1−2), 163−176. (76) Shiundu, P. M.; Williams, S. K. R. Thermal Field-Flow Fractionation for Particle Analysis: Opportunities and Challenges. Particle Sizing and Characterization; American Chemical Society: Washington, DC, 2004; pp 185−198.
(77) van Asten, A. C.; Venema, E.; Kok, W. T.; Poppe, H. J. Chromatogr., A 1993, 644 (1), 83−94. (78) Kwag, G.; Kim, P.; Han, S.; Choi, H. Polymer 2005, 46, 3782− 3788. (79) Nasirov, F. A. Iran. Polym. J. 2003, 12 (4), 281−289. (80) Pires, N. M. T.; Ferreira, A. A.; de Lira, C. H.; Coutinho, P. L. A.; Nicolini, L. F.; Soares, B. G.; Coutinho, F. M. B. J. Appl. Polym. Sci. 2006, 99, 88−99. (81) Mori, S.; Ishikawa, M. J. Liq. Chromatogr. Relat. Technol. 1998, 21 (8), 1107−1117. (82) Angelo, R. J.; Ikeda, R. M.; Wallach, M. L. Polymer 1965, 6 (3), 141−156. (83) Giddings, J. C. Science 1993, 260 (5113), 1456−1465. (84) Otte, T.; Klein, T.; Brüll, R.; Macko, T.; Pasch, H. J. Chromatogr., A 2011, 1218 (27), 4240−4248. (85) Frater, D. J.; Mays, J. W.; Jackson, C. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 141−151. (86) van Bruijnsvoort, M.; Wahlund, K. G.; Nilsson, G.; Kok, W. T. J. Chromatogr., A 2001, 925 (1−2), 171−182. (87) Podzimek, S.; Vlcek, T. J. Appl. Polym. Sci. 2001, 82 (2), 454− 460. (88) Hirabayashi, J.; Ito, N.; Noguchi, K.; Kasai, K. Biochemistry 1990, 29 (41), 9515−9521. (89) Bell, W.; Pethrick, R. A. Polymer 1982, 23 (3), 369−373. (90) Chung, S. E.; Chung, I. J. Polym. Bull. 1989, 21 (1), 105−112. (91) Farzaneh, S.; Hossein, F. J. Chem. Phys. 2011, 133 (23), 234− 904. (92) Zero, K. M.; Pecora, R. Macromolecules 1982, 15 (1), 87−93.
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