- Email: [email protected]
Polyethylene characterization by analytical temperature rising elution fractionation with evaporative light scattering detector
Polymer Testing 72 (2018) 172–177
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Polyethylene characterization by analytical temperature rising elution fractionation with evaporative light scattering detector
T
Adrian Boborodeaa,*, Alan Brookesb, Stephen O'Donohuec a
Certech ASBL, Rue Jules Bordet, Zone Industrielle C, B-7180, Seneffe, Belgium Agilent Technologies LDA UK Ltd, Essex Road, Church Stretton, SY6 6AX, Shropshire, UK c Agilent Technologies LDA UK Ltd, Stokeswood Road, Craven Arms Shropshire, SY7 8NR, UK b
ARTICLE INFO
ABSTRACT
Keywords: TREF Evaporative light scattering Polyethylene LDPE LLDPE HDPE
The temperature rising elution fractionation (TREF) analysis of linear and low-density polyethylene (PE) samples was investigated using an evaporative light scattering detector. Being less sensitive to pressure and temperature variations than the differential refractive index (DRI) detector, the ELSD allows the simplification of the flow path and reduction of the column dimensions. First experiments were done by quenching and capturing the injected polymer solution into a low volume TREF column. The elution profiles during the heating step showed that for each type of PE, clear correlations are found between the peak elution temperature and the PE density. Apart from having a signal to noise about 10 times better than DRI detection, the main advantage of the ELSD detector is the absence of the solvent peak. Because the solvent peak is evaporated by the ELSD, the initial 30 min of analysis can be used for dynamic cooling (meaning that the cooling step is done under solvent flow). Using dynamic cooling, we further develop the method to solve the challenging co-crystallization problem, when a mixture of low density and high-density PE is analyzed by TREF. By selecting specific dynamic cooling programs, it is possible to acquire in 30 min a baseline separation of two compounds and accurate peak elution temperatures, similar with those provided by classical TREF with a low cooling rate, performed over 11 h.
1. Introduction This work develops an advanced method for the characterization of polyolefins in solution, with a new high temperature evaporative light scattering detector (ELSD) with a linearized signal. Temperature rising elution fractionation (TREF) is a complementary technique to gel permeation chromatography (GPC) for polyolefin analysis. As compared with GPC, which allows the measurement of molecular weight distribution (MWD), TREF is usually applied to evaluate the short chain branching distribution (SCBD) of polyethylene (PE). Although analytical TREF is performed by coupling a liquid chromatography (LC) pump with a gas chromatography (GC) oven, the TREF technique is not a chromatographic one. In TREF, the elution is controlled by the crystallization step, when the sample solution is cooled in the column. It is commonly accepted that the separation during crystallization of different polymer species is based on the “onion skin” model, first described by Mirabella in 1987 [1]. However, the separation kinetics of the polymer trapped in the column is essentially driven by phase separation, thus the necessity of using very small cooling rates between 0.1 °C and 0.001 °C/ min [2], leading to a total analysis time of at least 16 h.
*
In a project applying TREF analysis on approximately 50 samples from the reactive extrusion of polypropylene, to understand the influence on mechanical properties of the peroxide type [3], the method avoids the long cooling step by patenting a column filled with metallic wires which allows dynamic cooling (decreasing the column temperature while maintaining the solvent flow through it) [4]. Further, it was showed that by manipulating the density of packed metallic wires and the column dimensions, it is possible to use cooling rates similar to those using instant quenching. With a significant reduction in analysis time, the quenched TREF methods give comparable results with those obtained by the classical overnight cooling method [5–8]. However, those methods were using a differential refractive index (DRI) detector, which is sensitive to pressure and temperature variations. Therefore, the flow path was relatively complex, containing a GPC column to separate the solvent peak, the TREF column, and a second GPC precolumn to smooth the pressure and temperature fluctuations during the elution/heating step. The main drawback was that a large solvent peak was observed for about 30 min at the beginning of elution step [8]. Recent advances in nebulizer technology of the ELSD allow the evaporation of trichlorobenzene (TCB) at much lower temperatures than
Corresponding author. E-mail address: [email protected] (A. Boborodea).
https://doi.org/10.1016/j.polymertesting.2018.10.008 Received 31 August 2018; Received in revised form 7 October 2018; Accepted 8 October 2018 Available online 09 October 2018 0142-9418/ © 2018 Elsevier Ltd. All rights reserved.
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
previous instruments, thus avoiding the sample loss in the lower molecular weight region [9]. Therefore, the new ELSD opens the possibility to correctly analyze the polymers by TREF, after linearizing the ELSD signal intensity in function of concentration, based on a power law function:
30 cm. The tubing connects the GPC injector with the GC oven, and has a small end zone, of about 1 cm length inside the GC, filled with metallic fibers [4]. This small part of tubing having a volume of about 10 μL is actually the TREF column. The TREF column was held at 30 °C for 30 min, before heating it from 30 °C to 130 °C. The heating rate was 1 °C/min in all experiments except for the tests done to measure the temperature shift, when heating rates of 0.5, 2 and 4 °C/min were used during the elution. The flow rate was kept 0.1 mL/min during the entire test (injection, quenching and elution). When using an isotherm step at 30 °C for 30 min, and a heating rate of 1 °C/min data interpretation is simplified, since the elution time is numerically equal to the elution temperature. For the second set of experiments dealing with the co-crystallization of low density and high density SRM samples, the injector loop was reduced to 100 μL to increase the peak resolution. Also, 6 m of tubing of 1 mm internal diameter was added inside the GC oven to accommodate the dynamic cooling. With this setup, and at a flow rate of 0.1 mL/min, the injected solution reaches the end of the tubing in 30 min, at which time it reaches the low volume TREF column used in the first part of the study. During the first 30 min when the solution is flowing through the tubing inside the GC oven, different linear cooling programs were applied, in which the initial temperature and the cooling rate were selected to reach 30 °C after 30 min. Thus, for a certain cooling rate, the initial temperature can be calculated using the following the equation:
ELSD signal intensity = kELSD *concentrationaELSD In order to find the exponent parameter governing the non-linearity of the ELSD signal, we developed a new method which takes into account not only the peak area, but each point of the ELSD chromatogram [10]. The exponent parameter was found to be dependent on the solvent [11,12], a value of 1.61 being measured for the analysis in TCB of polyolefin and polystyrene samples [9,10]. In this study, we evaluated the possibility to combine a TREF instrument with a high temperature ELSD allowing us to simplify the flow path, and compare the results with the previous elution peak temperatures for different types of PE resins. The absence of the solvent peak and the linearized ELSD signal provided the means for further development of the method to resolve the challenging problem of co-crystallization of a mixture of low density and high-density PE during the cooling step. 2. Experimental 2.1. Solvent 1,2,4-trichlorobenzene (TCB, Netherlands, CAS 120-82-1).
Spectropure
dry,
Initial temperature (°C) = 30 min * cooling rate (°C/min) + 30 °C
Biosolve,
The flow rate was kept at 0.1 mL/min during entire test (injection, dynamic cooling and elution). The heating rate during elution step was 1 °C/min. Once again, by using these parameters, the data interpretation is simplified, after the first 30 min, the elution time being numerically equal to the elution temperature. Agilent GPC software (v1.2) was used to export the ELSD thermograms as Excel files. To calculate the percentage corresponding to each peak, these thermograms were corrected for non-linearity using Excel templates.
2.2. Polymer samples
• Four linear metallocene polyethylene (PE) samples with different • •
densities, obtained by copolymerization with hexene (mPE1 to mPE4), are described in Table 1; Four PE samples containing long chain branching (LCB) obtained by radical polymerization (LDPE1 to LDPE4), are also described in Table 1; A high-density PE standard (SRM 1475a, Mw = 52000 ± 2000 g/ mol, polydispersity = 2.9) and a low-density PE standard (SRM 1476, melt flow index 1.23 g/10 min using ASTM D1238-00 condition 190/2.16), both from National Institute of Standards and Technology, were also included in this study.
2.4. Classical analytical TREF and method To compare the PE separation from the new instrument under dynamic cooling with the classical analytical TREF, the SRM samples and their mixture were also analyzed with a classical analytical TREF system. Details of this method were provided in a previous paper [2]. Important points for comparison with this work are the following:
2.3. Analytical TREF apparatus and methods
• The conventional TREF manufactured by Polymer Char, is situated
The analytical TREF was performed using the pump and the automatic injector of the Agilent PL-GPC 220 High Temperature GPC System connected with an Agilent GC 7890B oven. The concentrations of eluting fractions were measured with an Agilent HT-ELSD 1260 evaporative light scattering detector. The PE samples were dissolved in TCB at 160 °C to obtain solutions with concentrations of 0.1–0.2 mg/mL as described in our previous work [13]. For the first set of experiments, a volume of 200 μL of the hot solution was injected into a tubing of 1 mm internal diameter and a length of about
• •
inside an Agilent GC 6890N oven, and therefore the injector and the column have the same temperature, so it is not possible to run quench cooling programs. The column (100 mm length, 1/4-inch internal diameter) has an internal volume of 3.2 mL for keeping the injected solution volume of 300 μL during the cooling program, which contributes to peak broadening. To counteract the peak broadening, the flow rate at 0.5 mL/min, is five times higher than in the new method, consequently five times
Table 1 Density, melt flow index (MFI, 190 °C/2.16 kg) and principal applications of the linear metallocene PE (mPE) and of the low-density PE samples (LDPE) provided by datasheets. Sample
Density (kg/m3)
MFI (g/10 min)
Applications
mPE1 mPE2 mPE3 mPE4 LDPE1 LDPE2 LDPE3 LDPE4
923 934 947 955 918 921 923 925
0.9 0.9 0.7 1.2 12.0 5.7 0.3 2.0
Blend and co-extrusion with LDPE to obtain films with superior properties Packaging as collation shrink, mailing film, heavy duty sacs, bags Packaging such as bags, heavy duty sacs, automatic packaging, lamination Packaging such as bags, automatic packaging specialty films, lamination Food packaging, extrusion coating Board stock coating industry Blow film extrusion Lamination film, extrusion coating
173
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
Fig. 1. TREF elution profiles for linear metallocene PE samples obtained by quenching of the polymer solution. Each sample was injected twice.
Fig. 3. TREF profiles in function of uncorrected temperature for mPE3 sample, with different heating rates for the elution step. Each sample was injected twice.
more solvent is consumed to perform one analysis.
• The classical TREF has an infrared (IR) detector, limiting the pos• •
sible solvents to di- and tri-chlorobenzene, and not allowing the use of less toxic solvents, such as dibutoxymethane (butylal). Also, the IR is less sensitive than the ELSD, so the concentration of injected solution is typically 2–4 mg/mL, which is about 10 times higher than the concentrations needed for ELSD. The analyses were performed with different cooling rates between 0.01 and 1 °C/min, with temperature decreasing from 100 °C to 30 °C.
The classical TREF thermograms were exported to Excel to correct for temperature shift as explained in a previous paper [14]. 3. Results and discussion Fig. 4. Plot of the uncorrected peak elution temperature vs. heating rate for the mPE3 sample.
3.1. First experiments with samples containing a single type of PE Reproducibility and discrimination of samples having different physical properties are important requirements for newly developed polymer testing analyses. For the new analytical TREF method using fast cooling by quenching the polymer solution together with an ELSD, these requirements are demonstrated for linear PE samples in Fig. 1, and for PE samples containing long chain branching in Fig. 2. In order to verify the accuracy of the method, it is necessary to take into account the temperature shift due to the time lag between when
the sample is dissolved in the column and when the sample reaches the detector. To evaluate this time lag, the sample mPE3 was analyzed using different heating rates, and the obtained profiles as a function of temperature are given in Fig. 3. The peak elution temperatures for different experiments are over a 4 °C range (between 94.8 °C and 98.7 °C). However, the extrapolation to zero heating rate, plotted in Fig. 4, gives the temperature of 94.6 °C, which is in excellent agreement with the previously obtained values, between 94.6 °C and 94.8 °C, achieved with classical TREF [14]. For a heating rate of 1 °C/min, the time delay between when the sample is dissolved in the column and when the sample reaches the detector is 0.6 min. The corrected peak TREF temperature is simplified to the following relationship: peak TREF temperature (°C) = elution time (min) – 0.6 (min) As expected based on the difference in column dimensions, the time delay is much lower than the value of 5.1 min for classical TREF [14]. The corrected peak TREF elution temperatures do not depend on the instrument and heating rates, thus “universal” calibration curves can be generated to predict the density of PE samples as a function of the PE type, as presented in Fig. 5. Requiring less than 0.1 mg to evaluate both properties for a certain PE sample, we expect that these calibration curves are useful in forensic science or for evaluation of contaminants in pharmaceutical industry, where small polymer particles have to be identified by comparing with different possible sources.
Fig. 2. TREF elution profiles for low-density PE samples obtained by quenching the polymer solution. Each sample was injected twice. 174
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
Fig. 5. Corrected elution peak temperature vs. density for PE samples.
Fig. 7. Classical TREF elution profiles for a SRM 1475a/SRM 1476 mixture (50/ 50 = w/w) analyzed using different cooling rates (0.01, 0.1 and 1.0 °C/min).
Fig. 6. Fast and classical TREF results for SRM 1475a and SRM 1476.
Fig. 8. Plot of the proportion of low density PE peak area to total peak area in function of the cooling step duration.
3.2. Second experiments with mixtures of low-density and high-density PE samples Fig. 6 shows a comparison between the thermograms obtained for SRM 1475a and SRM 1476 samples with fast/quench cooling and those given by classical analytical TREF. As for the first analyzed samples, the peak elution times are similar with both methods. Both methods clearly show that the peak of SRM 1476 elutes at a different time as compared with SRM 1475a, but the peaks obtained with the classical method are significantly broader due to the increased column dimensions. Chemists accustomed to chromatographic techniques will expect that the difference in elution profiles of PE having different densities will be translated in a good peak resolution when mixtures of PE are analyzed by TREF. However, TREF not being a chromatographic method, the separation of compounds takes place during the cooling step and is governed by several physical phenomena such as: co-crystallization kinetics of different species, diffusion of low melting chains from the already crystallized linear PE, and dilution by diffusion in the TREF column. The elution step only reflects the image in the column after the cooling step was finished, and usually does not help compound separation. To show an example, the classical TREF elution profiles of the mixture of PE SRM 1475a and PE SRM 1476 for different cooling rates, are presented in Fig. 7. Obviously, for 1 °C/min the peaks separation is completely different from 50:50, and a correct ratio is obtained only for cooling rates lower than 0.1 °C, corresponding to a cooling program of about 11 h. For a complete image of the low speed of the separation kinetics in the presence of co-crystallization, Fig. 8 presents the variation of the fraction of the peak area corresponding to low density in function of the cooling time.
Fig. 9. TREF elution profiles of 50/50 mixture of SRM 1475a and SRM 1476 for dynamic cooling rates of 1, 2 and 3 °C/min.
Our challenge was to obtain the same separation in a reduced time of 30 min, by using a dynamic cooling, in which the flow turbulence during crystallization will allow the low-density species to escape from the linear high-density chain trap. The results of tests done using cooling rates of 1 °C/min, 2 °C/min and 3 °C/min presented in Fig. 9, show that the crystallization of the mixture behaves completely differently as a function of the dynamic 175
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
Fig. 10. TREF elution profiles of 50/50 mixture of SRM 1475a and SRM 1476 for dynamic cooling rates of 1.85, 1.95 and 2.05 °C/min.
Fig. 12. TREF elution profile of SRM 1475a with dynamic cooling conditions corresponding to a cooling rate of 1.95 °C/min (initial temperature 88.5 °C).
For all dynamic cooling rates between 1 °C/min and 3 °C/min, two peaks were visible, one corresponding to low density PE and the other one to high density PE, the threshold temperature being 90 °C. By plotting the ratio of the peak area corresponding to low density polyethylene and the total area (of both peaks of low and high density polythylenes) as a function of the initial temperature of the cooling program, as shown in Fig. 11, it is possible to get a clear image of the behavior of crystallization of the LDPE and HDPE mixture. Finally, starting from the observation of the small front shoulder presented at low temperatures in the SRM 1475a peak obtained with classical TREF (Fig. 6), we tried to obtain a better separation of the two peaks using dynamic cooling. Therefore, we decided to run the same HDPE sample in the critical conditions defined by the dynamic cooling rate of 1.95 °C (starting temperature of 88.5 °C), where only the lowdensity PE is present in the thermogram of mixture. The result, presented in Fig. 12, undoubtedly confirms the presence of two high density PE species in SRM 1475a sample.
Fig. 11. Plot of the proportion of low density PE peak area to total peak area in function of the initial temperature of the cooling program.
4. Conclusions
cooling parameters. We provide for these results the following explanation:
This is the first study in which the crystallization step in TREF experiments is done in an empty tubing, and not on a column packing support. A small amount of metallic wires plugging the end of the empty tubing, only impedes the advancement of the precipitated/ crystallized species to the detector. This setup allows for focusing the relatively large band of crystallized polymer solution into a narrow region, which elutes as a narrow peak, even for low flow rates of 0.1 mL/min. A good reproducibility and a clear discrimination were obtained for linear and long chain branching PE resins with different densities. When the temperature shift is taken into account, the peak elution temperatures are within 0.1 °C accuracy as compared with classical TREF results. These first results opened the way to further investigate the cocrystallization of low and high-density PE mixtures under dynamic cooling conditions, by using a longer tubing in which the solution can flow during the cooling step. By carefully selecting the cooling step parameters, it is possible to crystallize high density linear PE species under turbulent conditions, thus shifting the co-crystallization process control from high to low density PE species. From the academic point of view, it is our hope that this new technique will attract more research to better understand the complex phenomena related to co-crystallization under turbulent flow conditions. Still, from a practical point of view, the method can be immediately applied even when the available PE sample is scarce (less than 0.1 mg) such as in forensic or pharmaceutical studies related to identification of the source of small polymer particles. More research is
• For 1 °C/min, the mixture enters in the column at 60 °C, and its fast • •
crystallization does not allow the low density species to separate from the matrix of linear PE chains; For 3 °C/min, the mixture enters in the column at 120 °C, and it crystallizes much later in the tubing inside the GC oven, where the laminar flow does not help the diffusion of low density species, which once again remain trapped inside the linear chains; For 2 °C/min, the mixture enters in the column at 90 °C, so the crystallization of linear chains takes place in the turbulent regime, which switches the control of the co-crystallization process to the low-density PE chains.
In order to fully investigate these phenomena, we performed experiments using different cooling rates around 2 °C/min. Fig. 10 shows the elution profile at the two cooling rates where peak areas are close to the expected value for a 50:50 mixture, as well as the thermogram at the critical cooling rate where the entire mixture crystallizes as lowdensity PE. The thermogram obtained for the cooling rate of 1.85 °C/ min (starting cooling program at 85.5 °C) is similar to the expected result for the mixture given in Fig. 7 at a cooling rate of 0.1 °C/min. The important difference is the cooling time between the two experiments: 11 h in classical TREF compared with only 30 min under dynamic cooling. 176
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
under way, but in our opinion the method opens the possibility to confirm the heterogeneity of high density PE chemical composition.
[8] A. Boborodea, A. Brookes, Characterization of polyethylene using fast analytical temperature rising elution fractionation with triple detection (ATREF 3D), Int. J. Polym. Anal. Char. 20 (2015) 277–283. [9] A. Boborodea, S. O'Donohue, New evaporative light scattering detector for high temperature gel permeation chromatography, Int. J. Polym. Anal. Char. 22 (2017) 631–638. [10] A. Boborodea, S. O'Donohue, Linearization of evaporative light scattering detector signal, Int. J. Polym. Anal. Char. 22 (2017) 685-681. [11] A. Boborodea, A. Brookes, Polyolefin characterization in dibutoxymethane by high temperature gel permeation chromatography with a new evaporative light scattering detector, Polym. Test. 64 (2017) 217–220. [12] A. Boborodea, F.M. Mirabella, S. O'Donohue, Polyolefin characterization in Xylene by high-temperature gel permeation chromatography with a new evaporative light scattering detector, Chromatographia 81 (3) (2018) 419–424. [13] A. Boborodea, A. Brookes, Investigation of sample preparation for high temperature gel permeation chromatography using a low solvent consumption method, Polym. Test. 63 (2017) 210–213. [14] A. Boborodea, A. Luciani, Temperature Shift Evaluation Method in Analytical Temperature Rising Elution Fraction (ATREF) – LC GC North America, 31 (2013), pp. 414–418.
References [1] F.M. Mirabella, Characterization of multicomponent polymer systems using temperature rising elution fractionation, Proc. Int. Symp. GPC ’87 (1987) 180. [2] F.M. Mirabella, A. Boborodea, Updated “onion skin” model for temperature rising elution fractionation, Int. J. Polym. Anal. Char. 20 (2015) 435–441. [3] Albe, L. K.; Vandeurzen, P. Application Priority Data 8 September 2000. Controlled Rheology Polypropylene Heterophasic Copolymers, EP 1186618A1. [4] Boborodea, A. G.; Bailly, C.; Daoust, D.; Jonas, A.; Nysten, B. Application Priority Data 9 October 2002. Column for Analytical Temperature Rising Elution Fractionation (ATREF), US 7389678B2. [5] A. Boborodea, A. Luciani, J. Michel, Fast analytical temperature rising elution fraction (ATREF) method, Int. J. Polym. Anal. Char. 19 (2014) 124–129. [6] A. Boborodea, A. Luciani, Progress on fast analytical temperature rising elution fractionation (ATREF) technique for practical applications, Int. J. Polym. Anal. Char. 19 (2014) 475–481. [7] A. Boborodea, F.M. Mirabella, Fast analytical temperature rising elution fractionation (TREF) of polypropylenes, Int. J. Polym. Anal. Char. 20 (2015) 261–267.
177
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Polyethylene characterization by analytical temperature rising elution fractionation with evaporative light scattering detector
T
Adrian Boborodeaa,*, Alan Brookesb, Stephen O'Donohuec a
Certech ASBL, Rue Jules Bordet, Zone Industrielle C, B-7180, Seneffe, Belgium Agilent Technologies LDA UK Ltd, Essex Road, Church Stretton, SY6 6AX, Shropshire, UK c Agilent Technologies LDA UK Ltd, Stokeswood Road, Craven Arms Shropshire, SY7 8NR, UK b
ARTICLE INFO
ABSTRACT
Keywords: TREF Evaporative light scattering Polyethylene LDPE LLDPE HDPE
The temperature rising elution fractionation (TREF) analysis of linear and low-density polyethylene (PE) samples was investigated using an evaporative light scattering detector. Being less sensitive to pressure and temperature variations than the differential refractive index (DRI) detector, the ELSD allows the simplification of the flow path and reduction of the column dimensions. First experiments were done by quenching and capturing the injected polymer solution into a low volume TREF column. The elution profiles during the heating step showed that for each type of PE, clear correlations are found between the peak elution temperature and the PE density. Apart from having a signal to noise about 10 times better than DRI detection, the main advantage of the ELSD detector is the absence of the solvent peak. Because the solvent peak is evaporated by the ELSD, the initial 30 min of analysis can be used for dynamic cooling (meaning that the cooling step is done under solvent flow). Using dynamic cooling, we further develop the method to solve the challenging co-crystallization problem, when a mixture of low density and high-density PE is analyzed by TREF. By selecting specific dynamic cooling programs, it is possible to acquire in 30 min a baseline separation of two compounds and accurate peak elution temperatures, similar with those provided by classical TREF with a low cooling rate, performed over 11 h.
1. Introduction This work develops an advanced method for the characterization of polyolefins in solution, with a new high temperature evaporative light scattering detector (ELSD) with a linearized signal. Temperature rising elution fractionation (TREF) is a complementary technique to gel permeation chromatography (GPC) for polyolefin analysis. As compared with GPC, which allows the measurement of molecular weight distribution (MWD), TREF is usually applied to evaluate the short chain branching distribution (SCBD) of polyethylene (PE). Although analytical TREF is performed by coupling a liquid chromatography (LC) pump with a gas chromatography (GC) oven, the TREF technique is not a chromatographic one. In TREF, the elution is controlled by the crystallization step, when the sample solution is cooled in the column. It is commonly accepted that the separation during crystallization of different polymer species is based on the “onion skin” model, first described by Mirabella in 1987 [1]. However, the separation kinetics of the polymer trapped in the column is essentially driven by phase separation, thus the necessity of using very small cooling rates between 0.1 °C and 0.001 °C/ min [2], leading to a total analysis time of at least 16 h.
*
In a project applying TREF analysis on approximately 50 samples from the reactive extrusion of polypropylene, to understand the influence on mechanical properties of the peroxide type [3], the method avoids the long cooling step by patenting a column filled with metallic wires which allows dynamic cooling (decreasing the column temperature while maintaining the solvent flow through it) [4]. Further, it was showed that by manipulating the density of packed metallic wires and the column dimensions, it is possible to use cooling rates similar to those using instant quenching. With a significant reduction in analysis time, the quenched TREF methods give comparable results with those obtained by the classical overnight cooling method [5–8]. However, those methods were using a differential refractive index (DRI) detector, which is sensitive to pressure and temperature variations. Therefore, the flow path was relatively complex, containing a GPC column to separate the solvent peak, the TREF column, and a second GPC precolumn to smooth the pressure and temperature fluctuations during the elution/heating step. The main drawback was that a large solvent peak was observed for about 30 min at the beginning of elution step [8]. Recent advances in nebulizer technology of the ELSD allow the evaporation of trichlorobenzene (TCB) at much lower temperatures than
Corresponding author. E-mail address: [email protected] (A. Boborodea).
https://doi.org/10.1016/j.polymertesting.2018.10.008 Received 31 August 2018; Received in revised form 7 October 2018; Accepted 8 October 2018 Available online 09 October 2018 0142-9418/ © 2018 Elsevier Ltd. All rights reserved.
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
previous instruments, thus avoiding the sample loss in the lower molecular weight region [9]. Therefore, the new ELSD opens the possibility to correctly analyze the polymers by TREF, after linearizing the ELSD signal intensity in function of concentration, based on a power law function:
30 cm. The tubing connects the GPC injector with the GC oven, and has a small end zone, of about 1 cm length inside the GC, filled with metallic fibers [4]. This small part of tubing having a volume of about 10 μL is actually the TREF column. The TREF column was held at 30 °C for 30 min, before heating it from 30 °C to 130 °C. The heating rate was 1 °C/min in all experiments except for the tests done to measure the temperature shift, when heating rates of 0.5, 2 and 4 °C/min were used during the elution. The flow rate was kept 0.1 mL/min during the entire test (injection, quenching and elution). When using an isotherm step at 30 °C for 30 min, and a heating rate of 1 °C/min data interpretation is simplified, since the elution time is numerically equal to the elution temperature. For the second set of experiments dealing with the co-crystallization of low density and high density SRM samples, the injector loop was reduced to 100 μL to increase the peak resolution. Also, 6 m of tubing of 1 mm internal diameter was added inside the GC oven to accommodate the dynamic cooling. With this setup, and at a flow rate of 0.1 mL/min, the injected solution reaches the end of the tubing in 30 min, at which time it reaches the low volume TREF column used in the first part of the study. During the first 30 min when the solution is flowing through the tubing inside the GC oven, different linear cooling programs were applied, in which the initial temperature and the cooling rate were selected to reach 30 °C after 30 min. Thus, for a certain cooling rate, the initial temperature can be calculated using the following the equation:
ELSD signal intensity = kELSD *concentrationaELSD In order to find the exponent parameter governing the non-linearity of the ELSD signal, we developed a new method which takes into account not only the peak area, but each point of the ELSD chromatogram [10]. The exponent parameter was found to be dependent on the solvent [11,12], a value of 1.61 being measured for the analysis in TCB of polyolefin and polystyrene samples [9,10]. In this study, we evaluated the possibility to combine a TREF instrument with a high temperature ELSD allowing us to simplify the flow path, and compare the results with the previous elution peak temperatures for different types of PE resins. The absence of the solvent peak and the linearized ELSD signal provided the means for further development of the method to resolve the challenging problem of co-crystallization of a mixture of low density and high-density PE during the cooling step. 2. Experimental 2.1. Solvent 1,2,4-trichlorobenzene (TCB, Netherlands, CAS 120-82-1).
Spectropure
dry,
Initial temperature (°C) = 30 min * cooling rate (°C/min) + 30 °C
Biosolve,
The flow rate was kept at 0.1 mL/min during entire test (injection, dynamic cooling and elution). The heating rate during elution step was 1 °C/min. Once again, by using these parameters, the data interpretation is simplified, after the first 30 min, the elution time being numerically equal to the elution temperature. Agilent GPC software (v1.2) was used to export the ELSD thermograms as Excel files. To calculate the percentage corresponding to each peak, these thermograms were corrected for non-linearity using Excel templates.
2.2. Polymer samples
• Four linear metallocene polyethylene (PE) samples with different • •
densities, obtained by copolymerization with hexene (mPE1 to mPE4), are described in Table 1; Four PE samples containing long chain branching (LCB) obtained by radical polymerization (LDPE1 to LDPE4), are also described in Table 1; A high-density PE standard (SRM 1475a, Mw = 52000 ± 2000 g/ mol, polydispersity = 2.9) and a low-density PE standard (SRM 1476, melt flow index 1.23 g/10 min using ASTM D1238-00 condition 190/2.16), both from National Institute of Standards and Technology, were also included in this study.
2.4. Classical analytical TREF and method To compare the PE separation from the new instrument under dynamic cooling with the classical analytical TREF, the SRM samples and their mixture were also analyzed with a classical analytical TREF system. Details of this method were provided in a previous paper [2]. Important points for comparison with this work are the following:
2.3. Analytical TREF apparatus and methods
• The conventional TREF manufactured by Polymer Char, is situated
The analytical TREF was performed using the pump and the automatic injector of the Agilent PL-GPC 220 High Temperature GPC System connected with an Agilent GC 7890B oven. The concentrations of eluting fractions were measured with an Agilent HT-ELSD 1260 evaporative light scattering detector. The PE samples were dissolved in TCB at 160 °C to obtain solutions with concentrations of 0.1–0.2 mg/mL as described in our previous work [13]. For the first set of experiments, a volume of 200 μL of the hot solution was injected into a tubing of 1 mm internal diameter and a length of about
• •
inside an Agilent GC 6890N oven, and therefore the injector and the column have the same temperature, so it is not possible to run quench cooling programs. The column (100 mm length, 1/4-inch internal diameter) has an internal volume of 3.2 mL for keeping the injected solution volume of 300 μL during the cooling program, which contributes to peak broadening. To counteract the peak broadening, the flow rate at 0.5 mL/min, is five times higher than in the new method, consequently five times
Table 1 Density, melt flow index (MFI, 190 °C/2.16 kg) and principal applications of the linear metallocene PE (mPE) and of the low-density PE samples (LDPE) provided by datasheets. Sample
Density (kg/m3)
MFI (g/10 min)
Applications
mPE1 mPE2 mPE3 mPE4 LDPE1 LDPE2 LDPE3 LDPE4
923 934 947 955 918 921 923 925
0.9 0.9 0.7 1.2 12.0 5.7 0.3 2.0
Blend and co-extrusion with LDPE to obtain films with superior properties Packaging as collation shrink, mailing film, heavy duty sacs, bags Packaging such as bags, heavy duty sacs, automatic packaging, lamination Packaging such as bags, automatic packaging specialty films, lamination Food packaging, extrusion coating Board stock coating industry Blow film extrusion Lamination film, extrusion coating
173
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
Fig. 1. TREF elution profiles for linear metallocene PE samples obtained by quenching of the polymer solution. Each sample was injected twice.
Fig. 3. TREF profiles in function of uncorrected temperature for mPE3 sample, with different heating rates for the elution step. Each sample was injected twice.
more solvent is consumed to perform one analysis.
• The classical TREF has an infrared (IR) detector, limiting the pos• •
sible solvents to di- and tri-chlorobenzene, and not allowing the use of less toxic solvents, such as dibutoxymethane (butylal). Also, the IR is less sensitive than the ELSD, so the concentration of injected solution is typically 2–4 mg/mL, which is about 10 times higher than the concentrations needed for ELSD. The analyses were performed with different cooling rates between 0.01 and 1 °C/min, with temperature decreasing from 100 °C to 30 °C.
The classical TREF thermograms were exported to Excel to correct for temperature shift as explained in a previous paper [14]. 3. Results and discussion Fig. 4. Plot of the uncorrected peak elution temperature vs. heating rate for the mPE3 sample.
3.1. First experiments with samples containing a single type of PE Reproducibility and discrimination of samples having different physical properties are important requirements for newly developed polymer testing analyses. For the new analytical TREF method using fast cooling by quenching the polymer solution together with an ELSD, these requirements are demonstrated for linear PE samples in Fig. 1, and for PE samples containing long chain branching in Fig. 2. In order to verify the accuracy of the method, it is necessary to take into account the temperature shift due to the time lag between when
the sample is dissolved in the column and when the sample reaches the detector. To evaluate this time lag, the sample mPE3 was analyzed using different heating rates, and the obtained profiles as a function of temperature are given in Fig. 3. The peak elution temperatures for different experiments are over a 4 °C range (between 94.8 °C and 98.7 °C). However, the extrapolation to zero heating rate, plotted in Fig. 4, gives the temperature of 94.6 °C, which is in excellent agreement with the previously obtained values, between 94.6 °C and 94.8 °C, achieved with classical TREF [14]. For a heating rate of 1 °C/min, the time delay between when the sample is dissolved in the column and when the sample reaches the detector is 0.6 min. The corrected peak TREF temperature is simplified to the following relationship: peak TREF temperature (°C) = elution time (min) – 0.6 (min) As expected based on the difference in column dimensions, the time delay is much lower than the value of 5.1 min for classical TREF [14]. The corrected peak TREF elution temperatures do not depend on the instrument and heating rates, thus “universal” calibration curves can be generated to predict the density of PE samples as a function of the PE type, as presented in Fig. 5. Requiring less than 0.1 mg to evaluate both properties for a certain PE sample, we expect that these calibration curves are useful in forensic science or for evaluation of contaminants in pharmaceutical industry, where small polymer particles have to be identified by comparing with different possible sources.
Fig. 2. TREF elution profiles for low-density PE samples obtained by quenching the polymer solution. Each sample was injected twice. 174
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
Fig. 5. Corrected elution peak temperature vs. density for PE samples.
Fig. 7. Classical TREF elution profiles for a SRM 1475a/SRM 1476 mixture (50/ 50 = w/w) analyzed using different cooling rates (0.01, 0.1 and 1.0 °C/min).
Fig. 6. Fast and classical TREF results for SRM 1475a and SRM 1476.
Fig. 8. Plot of the proportion of low density PE peak area to total peak area in function of the cooling step duration.
3.2. Second experiments with mixtures of low-density and high-density PE samples Fig. 6 shows a comparison between the thermograms obtained for SRM 1475a and SRM 1476 samples with fast/quench cooling and those given by classical analytical TREF. As for the first analyzed samples, the peak elution times are similar with both methods. Both methods clearly show that the peak of SRM 1476 elutes at a different time as compared with SRM 1475a, but the peaks obtained with the classical method are significantly broader due to the increased column dimensions. Chemists accustomed to chromatographic techniques will expect that the difference in elution profiles of PE having different densities will be translated in a good peak resolution when mixtures of PE are analyzed by TREF. However, TREF not being a chromatographic method, the separation of compounds takes place during the cooling step and is governed by several physical phenomena such as: co-crystallization kinetics of different species, diffusion of low melting chains from the already crystallized linear PE, and dilution by diffusion in the TREF column. The elution step only reflects the image in the column after the cooling step was finished, and usually does not help compound separation. To show an example, the classical TREF elution profiles of the mixture of PE SRM 1475a and PE SRM 1476 for different cooling rates, are presented in Fig. 7. Obviously, for 1 °C/min the peaks separation is completely different from 50:50, and a correct ratio is obtained only for cooling rates lower than 0.1 °C, corresponding to a cooling program of about 11 h. For a complete image of the low speed of the separation kinetics in the presence of co-crystallization, Fig. 8 presents the variation of the fraction of the peak area corresponding to low density in function of the cooling time.
Fig. 9. TREF elution profiles of 50/50 mixture of SRM 1475a and SRM 1476 for dynamic cooling rates of 1, 2 and 3 °C/min.
Our challenge was to obtain the same separation in a reduced time of 30 min, by using a dynamic cooling, in which the flow turbulence during crystallization will allow the low-density species to escape from the linear high-density chain trap. The results of tests done using cooling rates of 1 °C/min, 2 °C/min and 3 °C/min presented in Fig. 9, show that the crystallization of the mixture behaves completely differently as a function of the dynamic 175
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
Fig. 10. TREF elution profiles of 50/50 mixture of SRM 1475a and SRM 1476 for dynamic cooling rates of 1.85, 1.95 and 2.05 °C/min.
Fig. 12. TREF elution profile of SRM 1475a with dynamic cooling conditions corresponding to a cooling rate of 1.95 °C/min (initial temperature 88.5 °C).
For all dynamic cooling rates between 1 °C/min and 3 °C/min, two peaks were visible, one corresponding to low density PE and the other one to high density PE, the threshold temperature being 90 °C. By plotting the ratio of the peak area corresponding to low density polyethylene and the total area (of both peaks of low and high density polythylenes) as a function of the initial temperature of the cooling program, as shown in Fig. 11, it is possible to get a clear image of the behavior of crystallization of the LDPE and HDPE mixture. Finally, starting from the observation of the small front shoulder presented at low temperatures in the SRM 1475a peak obtained with classical TREF (Fig. 6), we tried to obtain a better separation of the two peaks using dynamic cooling. Therefore, we decided to run the same HDPE sample in the critical conditions defined by the dynamic cooling rate of 1.95 °C (starting temperature of 88.5 °C), where only the lowdensity PE is present in the thermogram of mixture. The result, presented in Fig. 12, undoubtedly confirms the presence of two high density PE species in SRM 1475a sample.
Fig. 11. Plot of the proportion of low density PE peak area to total peak area in function of the initial temperature of the cooling program.
4. Conclusions
cooling parameters. We provide for these results the following explanation:
This is the first study in which the crystallization step in TREF experiments is done in an empty tubing, and not on a column packing support. A small amount of metallic wires plugging the end of the empty tubing, only impedes the advancement of the precipitated/ crystallized species to the detector. This setup allows for focusing the relatively large band of crystallized polymer solution into a narrow region, which elutes as a narrow peak, even for low flow rates of 0.1 mL/min. A good reproducibility and a clear discrimination were obtained for linear and long chain branching PE resins with different densities. When the temperature shift is taken into account, the peak elution temperatures are within 0.1 °C accuracy as compared with classical TREF results. These first results opened the way to further investigate the cocrystallization of low and high-density PE mixtures under dynamic cooling conditions, by using a longer tubing in which the solution can flow during the cooling step. By carefully selecting the cooling step parameters, it is possible to crystallize high density linear PE species under turbulent conditions, thus shifting the co-crystallization process control from high to low density PE species. From the academic point of view, it is our hope that this new technique will attract more research to better understand the complex phenomena related to co-crystallization under turbulent flow conditions. Still, from a practical point of view, the method can be immediately applied even when the available PE sample is scarce (less than 0.1 mg) such as in forensic or pharmaceutical studies related to identification of the source of small polymer particles. More research is
• For 1 °C/min, the mixture enters in the column at 60 °C, and its fast • •
crystallization does not allow the low density species to separate from the matrix of linear PE chains; For 3 °C/min, the mixture enters in the column at 120 °C, and it crystallizes much later in the tubing inside the GC oven, where the laminar flow does not help the diffusion of low density species, which once again remain trapped inside the linear chains; For 2 °C/min, the mixture enters in the column at 90 °C, so the crystallization of linear chains takes place in the turbulent regime, which switches the control of the co-crystallization process to the low-density PE chains.
In order to fully investigate these phenomena, we performed experiments using different cooling rates around 2 °C/min. Fig. 10 shows the elution profile at the two cooling rates where peak areas are close to the expected value for a 50:50 mixture, as well as the thermogram at the critical cooling rate where the entire mixture crystallizes as lowdensity PE. The thermogram obtained for the cooling rate of 1.85 °C/ min (starting cooling program at 85.5 °C) is similar to the expected result for the mixture given in Fig. 7 at a cooling rate of 0.1 °C/min. The important difference is the cooling time between the two experiments: 11 h in classical TREF compared with only 30 min under dynamic cooling. 176
Polymer Testing 72 (2018) 172–177
A. Boborodea et al.
under way, but in our opinion the method opens the possibility to confirm the heterogeneity of high density PE chemical composition.
[8] A. Boborodea, A. Brookes, Characterization of polyethylene using fast analytical temperature rising elution fractionation with triple detection (ATREF 3D), Int. J. Polym. Anal. Char. 20 (2015) 277–283. [9] A. Boborodea, S. O'Donohue, New evaporative light scattering detector for high temperature gel permeation chromatography, Int. J. Polym. Anal. Char. 22 (2017) 631–638. [10] A. Boborodea, S. O'Donohue, Linearization of evaporative light scattering detector signal, Int. J. Polym. Anal. Char. 22 (2017) 685-681. [11] A. Boborodea, A. Brookes, Polyolefin characterization in dibutoxymethane by high temperature gel permeation chromatography with a new evaporative light scattering detector, Polym. Test. 64 (2017) 217–220. [12] A. Boborodea, F.M. Mirabella, S. O'Donohue, Polyolefin characterization in Xylene by high-temperature gel permeation chromatography with a new evaporative light scattering detector, Chromatographia 81 (3) (2018) 419–424. [13] A. Boborodea, A. Brookes, Investigation of sample preparation for high temperature gel permeation chromatography using a low solvent consumption method, Polym. Test. 63 (2017) 210–213. [14] A. Boborodea, A. Luciani, Temperature Shift Evaluation Method in Analytical Temperature Rising Elution Fraction (ATREF) – LC GC North America, 31 (2013), pp. 414–418.
References [1] F.M. Mirabella, Characterization of multicomponent polymer systems using temperature rising elution fractionation, Proc. Int. Symp. GPC ’87 (1987) 180. [2] F.M. Mirabella, A. Boborodea, Updated “onion skin” model for temperature rising elution fractionation, Int. J. Polym. Anal. Char. 20 (2015) 435–441. [3] Albe, L. K.; Vandeurzen, P. Application Priority Data 8 September 2000. Controlled Rheology Polypropylene Heterophasic Copolymers, EP 1186618A1. [4] Boborodea, A. G.; Bailly, C.; Daoust, D.; Jonas, A.; Nysten, B. Application Priority Data 9 October 2002. Column for Analytical Temperature Rising Elution Fractionation (ATREF), US 7389678B2. [5] A. Boborodea, A. Luciani, J. Michel, Fast analytical temperature rising elution fraction (ATREF) method, Int. J. Polym. Anal. Char. 19 (2014) 124–129. [6] A. Boborodea, A. Luciani, Progress on fast analytical temperature rising elution fractionation (ATREF) technique for practical applications, Int. J. Polym. Anal. Char. 19 (2014) 475–481. [7] A. Boborodea, F.M. Mirabella, Fast analytical temperature rising elution fractionation (TREF) of polypropylenes, Int. J. Polym. Anal. Char. 20 (2015) 261–267.
177