Thermally Stable Siloxane Polymers for Gas Chromatography

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 6, 2015 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch012

Chapter 12

Thermally Stable Siloxane Polymers for Gas Chromatography G. M. Day, A. I. Hibberd, J. Habsuda, and G. J. Sharp SGE International Pty. Ltd., 7 Argent Place, Ringwood, Victoria 3134, Australia

Several novel polysiloxane copolymers containing various aryl substituents in the backbone and side chains were prepared and evaluated as possible stationary phases for GC capillary columns. All copolymers prepared were examined using TGA and were found to be thermally stable above 400°C. The new copolymers were evaluated as GC stationary phases and while all were perfectly suitable as such, only two copolymers would be suitable for commercial use due to their superior robustness.

High temperature gas chromatography (GC) is a widely used technique for analyzing mixtures of volatile organic compounds (7,2). The separation system involves coating a thinfilmof polymeric material (stationary phase) onto a fused silica capillary column wall, which is then continuously swept by a stream of carrier gas (mobile phase) containing the mixture of organics to be separated. Since the invention of the open tubular column for gas chromatography several decades ago, much work has been performed on developing stationary phases to improve the application and performance of capillary GC (7,2). The most successful and commonly used stationary phases to date have all been based on polysiloxanes, with poly(dimethylsiloxane) (PDMS) being the most frequently phase used for general analysis (7,2).

© 2003 American Chemical Society

In Synthesis and Properties of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Stationary phase materials must meet certain criteria to be considered satisfactory for use, and each is outlined below. •


• •

•

• •

•

Thermal stability: The phase must be able to withstand high temperatures (200-400°C) for long periods of time without thermal decomposition, the main cause of column bleed, as well as being able to withstand many heating and cooling cycles. Physical stability: The phase must be able to uniformly coat the inner wall of the capillary, and be robust enough to withstand column manipulation. Immobilization: Although not an absolute necessity, it is desirable that the stationary phase be designed such that a cross-linking or immobilization process can be undertaken, to impart greater thermal stability and render them non-extractable by solvents. Partitioning capability: The phase material must allow vaporized molecules to be able to move freely between the mobile (gas) phase and stationary phases at a range of temperatures (0-300°C). Chemical inertness: The phase must be chemically inert to the analytes, the solvent and any impurities in the system. Selectivity: For good retention there must be a strong interaction between the analytes and the stationary phase. Possible modes of interaction are dispersive interactions (Van Der Waals forces), dipole-dipole interactions, dipole-induced interactions, acid-base interactions, and molecular shape interactions (in chiral or liquid crystal phases). Reproducibility: Synthesis of the stationary phase must be reproducible so that the same separations are possiblefromdifferent batches of material.

Polysiloxanes are ideally suited as GC stationary phases owing to their thermal and physical stability, chemical inertness, excellent film forming properties and their ability to facilitate separation of a variety of organic substances. It has been well documented that polysiloxanes incorporating aryl substituents have higher resistance to thermal and oxidative degradation when compared to the non-aryl analogues (2,3). Numerous high temperature polysiloxanes with aryl substituents either as side chains or incorporated into the polymer backbone have previously been successfully prepared (3). The considerable stiffening effect observed upon incorporation of 1,4bis(dimethylhydroxy-silyl)benzene (silphenylene) into polysiloxane backbones has led to this material being used to enhance the thermal stability of a variety of siloxanes for a number of applications (3-5). However, the wide-spread use of arylene siloxanes as GC capillary column stationary phases is a relatively recent occurrence (1,2). The excellent thermal stability (low bleed) and good partitioning properties of existing silphenylene based capillary columns stationary phases are good examples (1,6). With the ever increasing sensitivity


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of GC imtrumentation, there is a constant need to improve the performance of high temperature stationary phases in order to rninimize capillary column bleed levels. New silphenylene polysiloxane analogues are excellent candidates for further investigation in this area. In this study, new polysiloxane materials incorporating aryl substituents in the polymer backbone have been prepared (Figure 1). These random copolymers have been characterized and the inherent thermal stability has been studied by thermogravimetric analysis (TGA). Their suitability as stationary phase materials will be discussed. " CH "

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Structures of PDMS and the copolymers studied in this work.


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Experimental


Copolymer synthesis Copolymers 1, 2, and 6 (7,8) were prepared as previously described. Copolymers 3-5 were prepared according to the procedure outlined by Zhang et al. (5). All materials were obtained as opaque, viscous gums. The copolymers were dried at ca. 25°C under vacuum for more than a week before being studied. Monomer incorporation ratios were confirmed using NMR and FTIR spectroscopy. Copolymer molecular weights were determined via gel permeation chromatography (GPC) relative to polystyrene standards.

Thermogravimetric analysis (TGA) TGA was carried out using a Setaram TGA instrument. Each copolymer sample weighed ca. 30 mg and was heated from 50 to 1000°C at a rate of 10°C/minute while under a nitrogen atmosphere. The samples were tared at zero time and the weight loss was recorded as a function of temperature measured.

Capillary column preparation Fused silica tubing was prepared in-house. The methods for preparing capillary columns using polysiloxane stationary phases have been well documented (9). Column specifications and run conditions for individual chromatograms are listed in the Figures.

Results and Discussion Characterization !

Characterization of the copolymers by FTIR and H NMR spectroscopy confirmed the repeat unit structures and incorporation ratios. Molecular weights determined by GPC were as follows: copolymer 1, M = 160,000; copolymer 2, M = 126,000; copolymer 3, M = 250,000; copolymer 4, M = 180,000; copolymer 5, M = 200,000; copolymer 6, M = 145,000. w

w

w

w

w

w


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Thermal stability All copolymers were examined using TGA under nitrogen (Figures 2-4). TGA studies of the inherent thermal stability of the polymeric materials in air were not undertaken since nearly all chromatographic analyses are performed under an inert atmosphere. Comparison of the thermal properties of copolymers 1-3 (Figure 2) clearly shows that that the polymers incorporating arylene units in the polymer backbone (copolymer 1 and 3) are more thermally stable than the polymer with aryl substituents solely as pendant groups (copolymer 2). Weight loss for copolymer 2 began at around 480°C, with 98% decomposition having occurred well before the temperature reached 700°C. By contrast, copolymers 1 and 3, both having backbone silphenylene substituents, exhibited weight loss starting at approximately 540°C. The small weight loss exhibited at around 100°C for copolymer 3 was attributed to the presence of moisture and/or solvent still present in the sample. At 650°C, copolymer 1 had lost 68% of its weight, while copolymer 3 has lost only 46%, with no further decomposition evident at 1000°C. All copolymers compare favorably with poly(dialkylsiloxane) based materials currently used as stationary phase materials, which typically decompose at temperatures less than 400°C.

Figure 2

TGA of copolymers 1-3.



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Two analogues of copolymers 3 were prepared, with variations in the type of backbone aryl substituent used; meta-substituted silphenylene analogue (copolymer 4) and a phenyl ether silphenylene analogue (copolymer 5). Their inherent thermal stability was examined and compared to that of copolymer 3 (Figure 3). It is evident that the silphenylene based copolymer 3 is more thermally stable than copolymers 4 and 5, with the onset of degradation at a much higher temperature (540°C). At 1000°C, copolymer 3 had exhibited a 46% weight loss while copolymers 4 and 5 exhibited weight losses of 73% and 69% respectively. While it was not surprising that a polymer having a metasubstituted arylene analogue in the backbone (copolymer 4) would be less thermally stable that a polymer containing a jpara-substituted silphenylene analogue (copolymer 3), previous work had shown that phenyl ether based siloxanes were more thermally stable than the corresponding silphenylene analogues (5). This was shown not to be the case for these particular siloxane analogues and indicates that it may not be true for all siloxane types.

Figure 3

TGA of copolymers 3- 5.


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Figure 4 compares the inherent thermal degradation of copolymer 1 (having a silphenylene moiety in the backbone) and copolymer 6 (having a phenyl ether based moiety in the backbone). It is clear that the silphenylene based copolymer 1 has better thermal stability than the phenyl ether based copolymer 6. The minor weight loss exhibited by copolymer 6 before true degradation onset could indicate the presence of volatile low molecular weight materials remaining in the polymer matrix, even after purification and prolonged drying.

Evaluation of polymers as GC stationary phases The copolymers 1-6 were coated onto the internal surface of fused silica capillary columns, and after undergoing an immobilization process at high temperature, the bleed levels were recorded for each column under the same conditions. As expected, all columns coated with copolymers containing aryl substituents had significantly lower bleed levels at higher temperatures (> 300°C) under an inert atmosphere when compared to columns coated with PDMS. Capillary columns coated with copolymers 1 and 3, both containing the silphenylene moiety in the polymer backbone, exhibited lower bleed levels than


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columns coated with copolymer 2 containing pendant aryl groups only. This is consistent with the TGA results shown earlier. The combination of high temperature and an oxygenated atmosphere will severely damage the stationary phase of most GC capillary columns. Figure 5 shows a typical example of GC partitioning and bleed testing of a capillary column coated with copolymer 1. From the bleed levels shown, it is evident that incorporation of an aryl substituent in the backbone of the polysiloxane stationary phase remarkably aids thermal stability after contact with air at high temperatures. In addition, the copolymer 1 stationary phase also retains its inertness to the test mix constituents after contact with air at high temperature. An example chromatogram for copolymer 3 is shown in Figure 6. Excellent partitioning performance was achieved with this material as stationary phase using the polynuclear aromatic hydrocarbon test mix.

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Figure 5 GC chromatograms for a capillary column coated with copolymer 1 (12 m, 0.22 mm ID, 0.25 μηι film thickness), shown with bleed levels. Top: Before the use of air as carrier gas. Bottom: After 80 hours with air as carrier gas at 200°C. Peaks: (1) Decane, (2) 4-Chlorophenol, (3) Decylamine, (4) Undecanol, (5) Biphenyl, (6) Pentadecane.


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Figure 6 Separation of polynuclear aromatic hydrocarbons using a fused silica capillary column coated with the copolymer 3 (30 rn, 0.25 mm ID, 0.25 μηι film thickness). Peaks: (1) Naphthalene, (2) Acenaphthylene, (3) Acenaphthene, (4) Fluorene, (5) Phenathrene, (6) Anthracene, (7) Fluoranthene, (8) Pyrene, (9) Benzo(a)anthracene, (10) Chrysene, (11) Benzo(b)fluoranthene, (12) Benzo(k)fluoranthene, (13) Benzo(a)pyrene, (14) Indeno(l,2,3-c,d)pyrene, (15) Dibenzo(a,h)anthracene, (16) Benzo(g,h,i)perylene.

Figure 7 Partitioning capability of fused silica capillary columns coated with copolymer 1 and copolymer 6 (30 m, 0.25 mm ID, 0.25 μηιfilm).Conditions: isothermal, 140°C, 15 min. Peaks: (1) Solvent, (2) Decane, (3) 4Chlorophenol, (4) Decylamine, (5) Undecanol, (6) Biphenyl, (7) Pentadecane.



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A small change to the chemical architecture of a stationary phase polymer can have a pronounced effect on the chromatography of coated capillary columns. This is illustrated in Figure 7 for GC columns coated with copolymers 1 and 6. While the phase materials vary only in the type of aryl substituent present, the chromatography is markedly different due to the dissimilar polarities of the two copolymers. In addition, bleed testing of the columns prepared from the two copolymers showed that the column prepared using copolymer 1 as stationary phase exhibited a much lower bleed level at high temperature than the column prepared using copolymer 6 as stationary phase. This bleed comparison result is consistent with the TGA results for these two copolymers (Figure 4).

Conclusion All copolymers prepared in this work are suitable for use as high temperature GC stationary phases since all materials are thermally stable and chemically inert under standard GC conditions. In all cases the monomer ratios could be modified to suit particular GC applications. However, TGA and bleed testing results indicate that only two of the copolymers tested (1 and 3) could be commercially useful since they are the most robust materials. This work has also shown that for GC applications, it is desirable to include arylene moieties into the backbone of potential stationary phase materials for maximum thermal stability. This is highMghted by the strong resistance to degradation of the silphenylene based copolymer 1 stationary phase after exposure to air at elevated temperature.

References 1.

Jennings, W.; Mittlefehldt, E.; Stremple, P. Analytical Gas Chromatography (2 Edition); Academic Press, NY, 1997. Blomberg, L. LCGC Europe, 2001, 14 (2), 106. Dvornic, P. R.; Lenz, R. High Temperature SiloxaneElastomers;Hulthig and Wepf, Basel, 1990. Zhang, R.; Pinhas, A. R.; Mark, J. E. Macromolecules 1997, 30, 2513. Zhang, R.; Pinhas, A. R.; Mark, J. E. Polym. Prep. 1998, 39(1), 575. Aichholz, R., Lorbeer, E. J. High Resol. Chromatogr. 1998, 21, 363. Zhu, H. D.; Kantor, S. W.; MacKnight, W. J. Macromolecules, 1998, 31, 850. Merker, R. L.; Scott, M. J. J. Polym. Sci.: Part A 1964, 2, 15. Grob, K. Making and Manipulating Capillary Columns for Gas Chromatography; Huethig, Heidelberg, 1986. nd

2. 3. 4. 5. 6. 7. 8. 9.


Thermally Stable Siloxane Polymers for Gas Chromatography

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