- Email: [email protected]
Characterisation of the optical properties of thin polymer films for their application in detection of volatile organic compounds
MATERIALS ENGINEERING ELSEVIER
Materials Science and Engineering C 5 (1998) 187-191
Characterisation of the optical properties of thin polymer films for their application in detection of volatile organic compounds K. Spaeth *, G. Gauglitz Institute of Physical Chemistl3", University of Tuebingen, A ~ der Morgenstelle 8, D 72076 Tuebingen, Germany Received 24 October 1996
Abstract The interaction of polymers with volatile organic compounds is widely used in sensor applications. Interaction with an analyte changes the optical properties of polymer films. We investigated two commercially available (poly)siloxanes and one (poly)urethane in contact with tetrachloroethene, toluene and cyclohexane vapours. The refractive index of the polymers and the changes in the physical thickness are separately determined by spectroscopic ellipsometry. The information obtained is used to optimise the sensitive layers with respect to sensitivity and selectivity. With the information obtained the most selective polymers can be chosen for every application. © 1998 Elsevier Science S.A. Keywords." Interaction with volatile organic compounds; Polymer films; Swelling of polymers; Sensors; Spectroscopic elIipsometry
1. Introduction Environmental pollution has increased considerably during the last few decades, leading in turn to a growing interest in the detection of pollutants such as pesticides, common organic solvent or volatile organic compounds (VOCs). This accounts for the large number of publications on the development of systems for detection of hazardous molecuIes in the environment, e.g. in air or water. Research in chemical or biochemical sensing has resulted in a great variety of transducers principles, such as surface acoustic wave devices, quartz microbalance devices and optical devices [ I ] . The optimisation of optical transduction principles such as reflectometric interference spectroscopy (RIfS), integrated optics (IO), or surface plasmon resonance (SPR) requires precise information about the optical constants of the film and its stability. With these optical methods it is possible to detect changes in the film thickness d or the refractive index n, or in the product of both values (nd), the so-called optical thickness. For the detection of VOCs, polymer films are often used which show a fast and reversible interaction with these poIlutants. The principle of the optical sensing using polymers is defined as follows. Solvent molecules permeate the polymer films. This permeation leads to an enrichment of the solvent molecules in the polymer film until a state of equilib* Correspondingauthor. 0928-4931/98/$ I9.00 © 1998 Elsevier Science S.A. All rights reserved PIIS0928-493 1 ( 9 7 ) 0004 i-6
rium accompanied by a swelling is reached. The physical thickness, refractive index, mass and other properties of the film will change. These changes depend to a high degree on the concentration and type of the analyte [2]. The interaction is not specific for one single analyte, which would be favourable for the detection of VOCs. Therefore different polymers have to be combined into an array in order to distinguish between different analytes by means of pattern recognition [ 3 ]. An accurate study of the optical parameters is desirable for improvement of the selection of the polymers used, and an evaluation of the data. We used spectroscopic ellipsometry for a detailed study of the optical constants of different polymer films. The refractive index and the physical thickness of the films were determined separately. In addition, the changes in the optical constants and the thickness due to different ambient media were determined.
2. Experimental details Spectroscopic ellipsometry [4] is a powerful tool for the characterisation of optical properties of thin films or bulk materials. This method is already commonly applied in process control [5] or in the investigation of thin polymer films [ 6 ]. Spectroscopic ellipsometry is based on the measurement of changes in the state of polarisation of linearly polarised
1N8
K. Spaeth, G, Gauglitz/Mazerials Science and Engineering C 5 (1998) 187-I91
light which is reflected by a planar surface or a planar layered system. A schematic representation of the ellipsometer is shown in Fig. 1 (a). For the measurements a commercially available spectroscopic ellipsometer (ES4G, SOPRA, Paris) was used, to determine the so-called etlipsometric ratio p [4] which is the product of the two ellipsometric angles tan k~ and cos A. The ellipsometric ratio is defined as p = t a n gt.eia= rP
(1)
where rp and ts are the coefficients of reflection of light polarised parallel and perpendicularly to the plane of incidence. These coefficients of reflection depend on the physical thickness and the complex refractive index of each layer of the investigated system. For the preparation of gaseous samples with a fixed content of volatile organic compounds we used a gas mixing system which is depicted schematically in Fig. 1 (b). We used two different streams with synthetic air. Of these, one is sent through thermostated bubblers filled with pure liquid analytes such as toluene, cyclohexane and tetrachloroethene. The second flow is synthetic air. By mixing these two flows it is possible to regulate the concentration of the analyte. The gas flows are controlled by mass flow controllers (MKS, Munich, Germany). This allows a reproducible preparation of air with concentrations of VOCs in the range between some ppm and a few thousand ppm. As substrates flat silica on silicon wafers (2 c m × 2 cm, Wacker Chemie, Burghausen, Germany) with a silica thickness in the range of 250-260 nm were used. A four-phase model (silicon/SiO2/polymer film/air) based on the Fresnel equations was used for interpretation of the measurements. The dielectric function of the polymer films was described by means of a Cauchy parametrisation model [7], assuming the films to be transparent materials: (a)
light so
beam line .1
gas flow ,1, t
The measurements were carried out within the range of 300800 nrn. The angle of incidence was set at 70". The substrates were first treated with toluene (analytical grade, Merck, Germany) to remove organic contamination and improve wettability. In a second step, the surface was treated for 30 rain with a so-called piranha solution ( 30 vol.% H202and 70 vol.% concentrated H2SO4),followed by rinsing with deionised water. Finally, the substrates were cleaned with HC1 (half concentrated, analytical grade, Merck Germany), rinsed with deionised water, and dried in vacuum at 80°C for 1 h. The polymer films were prepared by spincoating (Convac I001, Convac, Wiernsheim, Germany) dilute solutions of polymers in toluene. For this study, we used three different commercially available polymers: • Poly(dimethyl)siloxane (SE30) (ABCR, Karlsmhe, Germany) SE30 is well known as an unpolar, stationary phase in gas chromatography. The polymer side chains are not cross-linked. The films are dried in vacuum after preparation in order to remove the solvent. • Poly(dimethyl)siloxane with vinyl-, thiol-, phenyl- and methyl end groups (VP 1529) (Wacker Chemie, Burghausen, Germany) The end groups of VP1529 are crossqinked by polychromatic light. The films were irradiated for 20 h with a mercury lamp to obtain a stable polymer network. • Polyurethane (PUT) (Thermedics Inc., Woburn, USA) The films of PUT were dried in vacuum after preparation. PUT is a polar polymer and its chains are not crosslinked.
monochromator
'~ v I--.-r-A ~ analyzer 4, photomultiplier
(b) analyte, mass flow controller\ thermal stabilised (8 °)
gas mixture
(2)
3. Results and discussion
[--]
rotating polarizer / / ~ J 40 Hz S 1 ,~-", quartz window sample
B C n ( A ) = A + 7 ; + - 7 7 and k(A)=O A- A"
\
synthetic air
Fig. t. Schematicdrawingof the ellipsometer(a) and the gas bubblesystem (b).
The polymer films were prepared as described in Section 2. During the measurements, the samples were mounted in a special measurement cell. The films were investigated in dry synthetic air and synthetic air enriched with analyte molecules. The analyte concentrations were in the range 340-8500 ppm for toluene, 200-5000 ppm for tetrachloroethene, and 1100-28 000 ppm for cyclohexane, respectively. Fig. 2 shows the refractive index of SE30, VP1529 and PUT. The refractive indices of SE30 and VP1529 are relatively low, whereas the refractive index of PUT is much higher. The Cauchy parameters of the investigated films in air are given in Table 1. The measurements show that the films are stable for several months, with no changes in the refractive indices or the film thickness. It is remarkable that the refractive indices of films of one polymer show slight deviations ( < 4 X 10 - s) though the), were prepared in the same manner.
189
K. Spaeth, G. Gauglitz/Materials Science and Engineering C 5 (1998) 187-191
~, 1 . 5 5
. . . . . . . . . .
,-~ 370
y
~=' 360
x 1.5o
350'
°
•~ 1.45.
340'
SE30 . . . . . . . . . . . 300 400 500 600 700 800 wavelength [nm] Fig. 2. Refractive index of the investigated polymers in dry synthetic air. 1.40
Table 1 Cauchy parameters for the refractive index of SE30, VP1529 and PUT in dried air Polymer
A
B (nme)
C (nm4)
SE30 VP1529 PUT
1.4151 1.42492 1.4888l
1693.6 3102.2 3740.5
1.4573 × 10~ i.3080 × 10~ 1.1506 × 10s
The swollen film is a mixed phase consisting of the poIymet and the analyte. Therefore the refractive index of the swollen polymers increases or decreases depending on the refractive index of the analyte. For example, the refractive index of PUT decreases due to interaction with cyclohexane (rid = 1.42640), and remains nearly constant with toluene (riD = 1.49985) or tetrachloroethene (rid = 1.5053). In comparison, the refractive index of SE30 or VP1529 remains nearly constant with cyclohexane, and increases with toluene or tetrachloroethene. For example, the changes in the refractive index are 2.6 X i 0 - 4 for SE30, - 3.7 × 10 - 4 for VP 1529, and - 1 . 5 × 10 .2 for PUT at a vapour concentration of 28 000 ppm of cyclohexane. The physical thickness of a SE30 film in dependence on the vapour concentration of tetrachloroethene, toluene and cyclohexane is shown in Fig. 3. The increase in thickness in the case of cyclohexane is less than that for toluene or tetrachloroethene. The increase in thickness for toluene or tetrachloroethene is about the same due to their similar refractive index. This pattern was the same for all polymers. In Fig. 4 the relative changes in the physical thickness of VPI529, SE30 and PUT films are shown for toluene (a), tetrachloroethene (b) and cyclohexane (c). The relative change is determined by dividing the increase of the thickness by the thickness of the film in air as ambient. This allows one to compare the increase in thickness of the different films. For all three polymers a linear dependence between the thickness and the vapour concentration can be observed. With regard to toluene the values for SE30 and VP1529 were similar, but different for PUT. In the case of tetrachloroethene, the increase in thickness is about the same for all investigated polymers, whereas the increase in thickness for cyclohexane is different for each polymer. It is obvious that
the relative changes in the physical thickness of the different
~= 330'
.
,
.
,
.
,
.
,
.
.
.
,
0 5000 15000 25000 vapour concentration [ppm] Fig. 3. Film thickness of SE30 in dependence on the vapour concentration of tetrachloroethene (triangles), tolyene (circles) and cyclohexane (squares). 0.10 0.08 0.06 1
0.044 0.02 i
G) O
0.00! 6
' 4q00
,
.
.
0.06i ty.
.
.
' 8600
.
,
.
,
.
,
, . , . , . , . ,
0.04i ~
0.02i
o.ooi 0 1000 0.12 0.10 £ 0.08 ~ 0.06 ~ 0.04 ~ 0.021 0.00 ~
,
.
,
.
3000 ,
.
,
-
,
5000 .
,
.
,
0 ~606 ' i 5 6 0 0 2 5 0 0 b ' vapourconcentr~ion [ppm] Fig. 4. Relative change in the fiImthickness due to interaction with different analytes for VP1529 (squares), SE30 (circles) and PUT (triangles). polymer films show a similar amount of swelling for each analyte. The optical thickness can be calculated from the refractive index and the physical thickness in dependence on the vapour concentration. For a comparison these values are standardised by dividing the mean value by the optical thickness in air. The relative change in the optical thickness of PUT, VP1529 and SE30 films for toluene (a), tetrachloroethene (b) and cyclohexane (c) is shown in Fig. 5. The relative change in the optical thickness depends linearly on the vapour concentration for the three polymers. SE30 shows the smallest response to each analyte. PUT shows the highest relative changes in the optical thickness in the case of toluene and tetrachloroethene, while in the case of cyclohexane the relative changes in the optical thickness are higher for VP1529.
190
K. Spaerh, G. Gauglitz / Materials Science and Engineering C 5 (1998) 187-191
{D
_5.o
0.24, 0.20i 0.16~ 0.12i 0.08 0.04 O.OOi
1.5E-05 t z 1.0E-05
0 .o
0.16
0 ©
0.12
0
0.08
~
4000
8000
5.0E-06 ~ ~]'~I,~/w- PUT 0 ~~,--~_..~q~VP1529 1 2 3 (a) 3.5E-05 " " / / ' 1 ~ 2.5E-05 t . , ~ . . ~ m / l
0.04! cO
0.00i 0 1000
3000
5000
0.16 0.12 0.08
5.0E-06 ~ } ~ k ~ SE30 0 .... -~--:~-eZ.VP1529 (b) 2 3 Fig. 6. Sensitivityof the polymersfor the differentanalytescalculatedfrom the physical (a) and the optical thickness (b) for tetrachloroethene ( 1), toluene (2) and cyclohexane (3).
0.04 0.00 0 5000 15000 25000 vapourconcentr~ion [ppm] Fig. 5. Relative change in the optical thickness due to interaction with different analytemoleculesfor VP1529 (squares), SE30 (circles) and PUT (triangles). These values show a higher variance than the values for the physical thickness. The slope of the linear regression of the relative changes of the physical and optical thickness is called sensitivity. This value describes the strength of the interaction between a polymer and an analyte. It is obvious that a big slope demonstrates that the polymer shows a strong effect with analyte molecules. In Fig. 6, the sensitivity of the polymer films to the different analytes are shown. In Fig. 6(a), the sensitivity values calculated from the physical thickness are shown. For all polymers the highest sensitivity is detected with tetrachloroethene, while the smallest values are obtained for cyclohexane. Each polymer shows significant differences in its sensitivity to the different anatytes. The differences between the sensitivity values of the polymers are even more pronounced if the sensitivity calculated from the optical thickness is considered. The pattern of the sensitivity values changes. For example, the sensitivity of both PUT and VP1529 to cyclohexane shows a much higher value when considering the optical thickness, than when considering the physical thickness. The sensitivity of SE30 to the different analytes is much lower than that of VP 1529 and PUT in regard to the optical thickness, while it is comparable to VP1529 with regard to the physical thickness. The measurements show that a separate determination of the refractive indices and the physical thickness of polymers films is possible. Changes of these values due to interaction
are also determined. With respect to their application as sensitive materials, the polymers show no significant differences in the increase of their physical thickness. The sensitivity pattern calculated from the optical thickness shows more differences, because the different refractive indices of the polymer films are taken into account. The investigated polymers are not the ideal combination for the determination of the investigated analytes. The sensitivity value of each polymer should be high for one analyte and low for the others.
4. Conclusion
We used spectroscopic ellipsometry for the determination of the swelling of polymers caused by interaction with volatile organic compounds. The measurements proved a linear increase in the physical thickness with increasing vapour concentration of tetrachloroethene, toluene and cyclohexane for the investigated polymers. Also, small changes in the refractive index where determined. The effects of swelling and changes in the refractive index can be clearly separated. Because the interaction of polymers with VOCs is not specific, an array of different polymers has to be used to distinguish between the different analytes. The results show that the refractive index of the polymers and the anatyte have to be taken into account when selecting polymers as sensing films. Polymers with pronounced variances in the refractive index, such as PUT and VP1529, are preferable to polymers with small variances, such as SE30 and VP1529. Even polymers showing strong interaction with only one selected analyte are desirable. For example, a polymer which interacts more strongly with cyclohexane than with toluene or tetrachloroethene would be a good complement for the investigated polymers. If these points are considered when selecting pol-
K. 5paerh, G. Gauglitz/ Materials Science and Engineering C 5 (199~3) 187-19l
ymers for an array, the selectivity pattern for the different analyte should be more characteristic.
Acknowledgements This work was supported by the " F o n d der C h e m i s c h e n I n d u s t r i e " , the Deutsche F o r s c h u n g s g e m e i n s c h a f t ( D F G Forschergruppe "Molekulare Mustererkennung mit Supramolekularen Strukturen und P o l y m e r e n ' D F G - G 0 3 0 I / 2 3 - 1 ) and the Graduiertenkolleg 'Analytische C h e m i e ' at the University of Tuebingen.
191
References
[ 1] G, Gauglitz et al., Sensors and Actuators B 11 ( 1993 ) 2 t. [2] K. Spaeth et al., Fresenius J. Anal. Chem., 357 (1997) 292. [3] G. Kraus, G. Gauglitz, Chemometrics and Intelligent Laboratory Systems 30 (1995) 2tl. [4] R.M.A. Azzam. N.M. Bashara, Ellipsometry and Polarized Light, North Holland. Amsterdam, 1988. [5] N. Blayo et al., J. Opt. Soc. A 12 ( 19951 59i. [6] E. Toussaere, J. Zyss, Thin fihns for optical systems, SPIE 1782 (1992) 271. [ 7] H.-J. Eichler, in Bergmann Schaefer ( Ed./, Lehrbuch der Experimentalphysik, Band 3, Optik, Waiter de Gruyter, Berlin, 1993, p. 249.
Materials Science and Engineering C 5 (1998) 187-191
Characterisation of the optical properties of thin polymer films for their application in detection of volatile organic compounds K. Spaeth *, G. Gauglitz Institute of Physical Chemistl3", University of Tuebingen, A ~ der Morgenstelle 8, D 72076 Tuebingen, Germany Received 24 October 1996
Abstract The interaction of polymers with volatile organic compounds is widely used in sensor applications. Interaction with an analyte changes the optical properties of polymer films. We investigated two commercially available (poly)siloxanes and one (poly)urethane in contact with tetrachloroethene, toluene and cyclohexane vapours. The refractive index of the polymers and the changes in the physical thickness are separately determined by spectroscopic ellipsometry. The information obtained is used to optimise the sensitive layers with respect to sensitivity and selectivity. With the information obtained the most selective polymers can be chosen for every application. © 1998 Elsevier Science S.A. Keywords." Interaction with volatile organic compounds; Polymer films; Swelling of polymers; Sensors; Spectroscopic elIipsometry
1. Introduction Environmental pollution has increased considerably during the last few decades, leading in turn to a growing interest in the detection of pollutants such as pesticides, common organic solvent or volatile organic compounds (VOCs). This accounts for the large number of publications on the development of systems for detection of hazardous molecuIes in the environment, e.g. in air or water. Research in chemical or biochemical sensing has resulted in a great variety of transducers principles, such as surface acoustic wave devices, quartz microbalance devices and optical devices [ I ] . The optimisation of optical transduction principles such as reflectometric interference spectroscopy (RIfS), integrated optics (IO), or surface plasmon resonance (SPR) requires precise information about the optical constants of the film and its stability. With these optical methods it is possible to detect changes in the film thickness d or the refractive index n, or in the product of both values (nd), the so-called optical thickness. For the detection of VOCs, polymer films are often used which show a fast and reversible interaction with these poIlutants. The principle of the optical sensing using polymers is defined as follows. Solvent molecules permeate the polymer films. This permeation leads to an enrichment of the solvent molecules in the polymer film until a state of equilib* Correspondingauthor. 0928-4931/98/$ I9.00 © 1998 Elsevier Science S.A. All rights reserved PIIS0928-493 1 ( 9 7 ) 0004 i-6
rium accompanied by a swelling is reached. The physical thickness, refractive index, mass and other properties of the film will change. These changes depend to a high degree on the concentration and type of the analyte [2]. The interaction is not specific for one single analyte, which would be favourable for the detection of VOCs. Therefore different polymers have to be combined into an array in order to distinguish between different analytes by means of pattern recognition [ 3 ]. An accurate study of the optical parameters is desirable for improvement of the selection of the polymers used, and an evaluation of the data. We used spectroscopic ellipsometry for a detailed study of the optical constants of different polymer films. The refractive index and the physical thickness of the films were determined separately. In addition, the changes in the optical constants and the thickness due to different ambient media were determined.
2. Experimental details Spectroscopic ellipsometry [4] is a powerful tool for the characterisation of optical properties of thin films or bulk materials. This method is already commonly applied in process control [5] or in the investigation of thin polymer films [ 6 ]. Spectroscopic ellipsometry is based on the measurement of changes in the state of polarisation of linearly polarised
1N8
K. Spaeth, G, Gauglitz/Mazerials Science and Engineering C 5 (1998) 187-I91
light which is reflected by a planar surface or a planar layered system. A schematic representation of the ellipsometer is shown in Fig. 1 (a). For the measurements a commercially available spectroscopic ellipsometer (ES4G, SOPRA, Paris) was used, to determine the so-called etlipsometric ratio p [4] which is the product of the two ellipsometric angles tan k~ and cos A. The ellipsometric ratio is defined as p = t a n gt.eia= rP
(1)
where rp and ts are the coefficients of reflection of light polarised parallel and perpendicularly to the plane of incidence. These coefficients of reflection depend on the physical thickness and the complex refractive index of each layer of the investigated system. For the preparation of gaseous samples with a fixed content of volatile organic compounds we used a gas mixing system which is depicted schematically in Fig. 1 (b). We used two different streams with synthetic air. Of these, one is sent through thermostated bubblers filled with pure liquid analytes such as toluene, cyclohexane and tetrachloroethene. The second flow is synthetic air. By mixing these two flows it is possible to regulate the concentration of the analyte. The gas flows are controlled by mass flow controllers (MKS, Munich, Germany). This allows a reproducible preparation of air with concentrations of VOCs in the range between some ppm and a few thousand ppm. As substrates flat silica on silicon wafers (2 c m × 2 cm, Wacker Chemie, Burghausen, Germany) with a silica thickness in the range of 250-260 nm were used. A four-phase model (silicon/SiO2/polymer film/air) based on the Fresnel equations was used for interpretation of the measurements. The dielectric function of the polymer films was described by means of a Cauchy parametrisation model [7], assuming the films to be transparent materials: (a)
light so
beam line .1
gas flow ,1, t
The measurements were carried out within the range of 300800 nrn. The angle of incidence was set at 70". The substrates were first treated with toluene (analytical grade, Merck, Germany) to remove organic contamination and improve wettability. In a second step, the surface was treated for 30 rain with a so-called piranha solution ( 30 vol.% H202and 70 vol.% concentrated H2SO4),followed by rinsing with deionised water. Finally, the substrates were cleaned with HC1 (half concentrated, analytical grade, Merck Germany), rinsed with deionised water, and dried in vacuum at 80°C for 1 h. The polymer films were prepared by spincoating (Convac I001, Convac, Wiernsheim, Germany) dilute solutions of polymers in toluene. For this study, we used three different commercially available polymers: • Poly(dimethyl)siloxane (SE30) (ABCR, Karlsmhe, Germany) SE30 is well known as an unpolar, stationary phase in gas chromatography. The polymer side chains are not cross-linked. The films are dried in vacuum after preparation in order to remove the solvent. • Poly(dimethyl)siloxane with vinyl-, thiol-, phenyl- and methyl end groups (VP 1529) (Wacker Chemie, Burghausen, Germany) The end groups of VP1529 are crossqinked by polychromatic light. The films were irradiated for 20 h with a mercury lamp to obtain a stable polymer network. • Polyurethane (PUT) (Thermedics Inc., Woburn, USA) The films of PUT were dried in vacuum after preparation. PUT is a polar polymer and its chains are not crosslinked.
monochromator
'~ v I--.-r-A ~ analyzer 4, photomultiplier
(b) analyte, mass flow controller\ thermal stabilised (8 °)
gas mixture
(2)
3. Results and discussion
[--]
rotating polarizer / / ~ J 40 Hz S 1 ,~-", quartz window sample
B C n ( A ) = A + 7 ; + - 7 7 and k(A)=O A- A"
\
synthetic air
Fig. t. Schematicdrawingof the ellipsometer(a) and the gas bubblesystem (b).
The polymer films were prepared as described in Section 2. During the measurements, the samples were mounted in a special measurement cell. The films were investigated in dry synthetic air and synthetic air enriched with analyte molecules. The analyte concentrations were in the range 340-8500 ppm for toluene, 200-5000 ppm for tetrachloroethene, and 1100-28 000 ppm for cyclohexane, respectively. Fig. 2 shows the refractive index of SE30, VP1529 and PUT. The refractive indices of SE30 and VP1529 are relatively low, whereas the refractive index of PUT is much higher. The Cauchy parameters of the investigated films in air are given in Table 1. The measurements show that the films are stable for several months, with no changes in the refractive indices or the film thickness. It is remarkable that the refractive indices of films of one polymer show slight deviations ( < 4 X 10 - s) though the), were prepared in the same manner.
189
K. Spaeth, G. Gauglitz/Materials Science and Engineering C 5 (1998) 187-191
~, 1 . 5 5
. . . . . . . . . .
,-~ 370
y
~=' 360
x 1.5o
350'
°
•~ 1.45.
340'
SE30 . . . . . . . . . . . 300 400 500 600 700 800 wavelength [nm] Fig. 2. Refractive index of the investigated polymers in dry synthetic air. 1.40
Table 1 Cauchy parameters for the refractive index of SE30, VP1529 and PUT in dried air Polymer
A
B (nme)
C (nm4)
SE30 VP1529 PUT
1.4151 1.42492 1.4888l
1693.6 3102.2 3740.5
1.4573 × 10~ i.3080 × 10~ 1.1506 × 10s
The swollen film is a mixed phase consisting of the poIymet and the analyte. Therefore the refractive index of the swollen polymers increases or decreases depending on the refractive index of the analyte. For example, the refractive index of PUT decreases due to interaction with cyclohexane (rid = 1.42640), and remains nearly constant with toluene (riD = 1.49985) or tetrachloroethene (rid = 1.5053). In comparison, the refractive index of SE30 or VP1529 remains nearly constant with cyclohexane, and increases with toluene or tetrachloroethene. For example, the changes in the refractive index are 2.6 X i 0 - 4 for SE30, - 3.7 × 10 - 4 for VP 1529, and - 1 . 5 × 10 .2 for PUT at a vapour concentration of 28 000 ppm of cyclohexane. The physical thickness of a SE30 film in dependence on the vapour concentration of tetrachloroethene, toluene and cyclohexane is shown in Fig. 3. The increase in thickness in the case of cyclohexane is less than that for toluene or tetrachloroethene. The increase in thickness for toluene or tetrachloroethene is about the same due to their similar refractive index. This pattern was the same for all polymers. In Fig. 4 the relative changes in the physical thickness of VPI529, SE30 and PUT films are shown for toluene (a), tetrachloroethene (b) and cyclohexane (c). The relative change is determined by dividing the increase of the thickness by the thickness of the film in air as ambient. This allows one to compare the increase in thickness of the different films. For all three polymers a linear dependence between the thickness and the vapour concentration can be observed. With regard to toluene the values for SE30 and VP1529 were similar, but different for PUT. In the case of tetrachloroethene, the increase in thickness is about the same for all investigated polymers, whereas the increase in thickness for cyclohexane is different for each polymer. It is obvious that
the relative changes in the physical thickness of the different
~= 330'
.
,
.
,
.
,
.
,
.
.
.
,
0 5000 15000 25000 vapour concentration [ppm] Fig. 3. Film thickness of SE30 in dependence on the vapour concentration of tetrachloroethene (triangles), tolyene (circles) and cyclohexane (squares). 0.10 0.08 0.06 1
0.044 0.02 i
G) O
0.00! 6
' 4q00
,
.
.
0.06i ty.
.
.
' 8600
.
,
.
,
.
,
, . , . , . , . ,
0.04i ~
0.02i
o.ooi 0 1000 0.12 0.10 £ 0.08 ~ 0.06 ~ 0.04 ~ 0.021 0.00 ~
,
.
,
.
3000 ,
.
,
-
,
5000 .
,
.
,
0 ~606 ' i 5 6 0 0 2 5 0 0 b ' vapourconcentr~ion [ppm] Fig. 4. Relative change in the fiImthickness due to interaction with different analytes for VP1529 (squares), SE30 (circles) and PUT (triangles). polymer films show a similar amount of swelling for each analyte. The optical thickness can be calculated from the refractive index and the physical thickness in dependence on the vapour concentration. For a comparison these values are standardised by dividing the mean value by the optical thickness in air. The relative change in the optical thickness of PUT, VP1529 and SE30 films for toluene (a), tetrachloroethene (b) and cyclohexane (c) is shown in Fig. 5. The relative change in the optical thickness depends linearly on the vapour concentration for the three polymers. SE30 shows the smallest response to each analyte. PUT shows the highest relative changes in the optical thickness in the case of toluene and tetrachloroethene, while in the case of cyclohexane the relative changes in the optical thickness are higher for VP1529.
190
K. Spaerh, G. Gauglitz / Materials Science and Engineering C 5 (1998) 187-191
{D
_5.o
0.24, 0.20i 0.16~ 0.12i 0.08 0.04 O.OOi
1.5E-05 t z 1.0E-05
0 .o
0.16
0 ©
0.12
0
0.08
~
4000
8000
5.0E-06 ~ ~]'~I,~/w- PUT 0 ~~,--~_..~q~VP1529 1 2 3 (a) 3.5E-05 " " / / ' 1 ~ 2.5E-05 t . , ~ . . ~ m / l
0.04! cO
0.00i 0 1000
3000
5000
0.16 0.12 0.08
5.0E-06 ~ } ~ k ~ SE30 0 .... -~--:~-eZ.VP1529 (b) 2 3 Fig. 6. Sensitivityof the polymersfor the differentanalytescalculatedfrom the physical (a) and the optical thickness (b) for tetrachloroethene ( 1), toluene (2) and cyclohexane (3).
0.04 0.00 0 5000 15000 25000 vapourconcentr~ion [ppm] Fig. 5. Relative change in the optical thickness due to interaction with different analytemoleculesfor VP1529 (squares), SE30 (circles) and PUT (triangles). These values show a higher variance than the values for the physical thickness. The slope of the linear regression of the relative changes of the physical and optical thickness is called sensitivity. This value describes the strength of the interaction between a polymer and an analyte. It is obvious that a big slope demonstrates that the polymer shows a strong effect with analyte molecules. In Fig. 6, the sensitivity of the polymer films to the different analytes are shown. In Fig. 6(a), the sensitivity values calculated from the physical thickness are shown. For all polymers the highest sensitivity is detected with tetrachloroethene, while the smallest values are obtained for cyclohexane. Each polymer shows significant differences in its sensitivity to the different anatytes. The differences between the sensitivity values of the polymers are even more pronounced if the sensitivity calculated from the optical thickness is considered. The pattern of the sensitivity values changes. For example, the sensitivity of both PUT and VP1529 to cyclohexane shows a much higher value when considering the optical thickness, than when considering the physical thickness. The sensitivity of SE30 to the different analytes is much lower than that of VP 1529 and PUT in regard to the optical thickness, while it is comparable to VP1529 with regard to the physical thickness. The measurements show that a separate determination of the refractive indices and the physical thickness of polymers films is possible. Changes of these values due to interaction
are also determined. With respect to their application as sensitive materials, the polymers show no significant differences in the increase of their physical thickness. The sensitivity pattern calculated from the optical thickness shows more differences, because the different refractive indices of the polymer films are taken into account. The investigated polymers are not the ideal combination for the determination of the investigated analytes. The sensitivity value of each polymer should be high for one analyte and low for the others.
4. Conclusion
We used spectroscopic ellipsometry for the determination of the swelling of polymers caused by interaction with volatile organic compounds. The measurements proved a linear increase in the physical thickness with increasing vapour concentration of tetrachloroethene, toluene and cyclohexane for the investigated polymers. Also, small changes in the refractive index where determined. The effects of swelling and changes in the refractive index can be clearly separated. Because the interaction of polymers with VOCs is not specific, an array of different polymers has to be used to distinguish between the different analytes. The results show that the refractive index of the polymers and the anatyte have to be taken into account when selecting polymers as sensing films. Polymers with pronounced variances in the refractive index, such as PUT and VP1529, are preferable to polymers with small variances, such as SE30 and VP1529. Even polymers showing strong interaction with only one selected analyte are desirable. For example, a polymer which interacts more strongly with cyclohexane than with toluene or tetrachloroethene would be a good complement for the investigated polymers. If these points are considered when selecting pol-
K. 5paerh, G. Gauglitz/ Materials Science and Engineering C 5 (199~3) 187-19l
ymers for an array, the selectivity pattern for the different analyte should be more characteristic.
Acknowledgements This work was supported by the " F o n d der C h e m i s c h e n I n d u s t r i e " , the Deutsche F o r s c h u n g s g e m e i n s c h a f t ( D F G Forschergruppe "Molekulare Mustererkennung mit Supramolekularen Strukturen und P o l y m e r e n ' D F G - G 0 3 0 I / 2 3 - 1 ) and the Graduiertenkolleg 'Analytische C h e m i e ' at the University of Tuebingen.
191
References
[ 1] G, Gauglitz et al., Sensors and Actuators B 11 ( 1993 ) 2 t. [2] K. Spaeth et al., Fresenius J. Anal. Chem., 357 (1997) 292. [3] G. Kraus, G. Gauglitz, Chemometrics and Intelligent Laboratory Systems 30 (1995) 2tl. [4] R.M.A. Azzam. N.M. Bashara, Ellipsometry and Polarized Light, North Holland. Amsterdam, 1988. [5] N. Blayo et al., J. Opt. Soc. A 12 ( 19951 59i. [6] E. Toussaere, J. Zyss, Thin fihns for optical systems, SPIE 1782 (1992) 271. [ 7] H.-J. Eichler, in Bergmann Schaefer ( Ed./, Lehrbuch der Experimentalphysik, Band 3, Optik, Waiter de Gruyter, Berlin, 1993, p. 249.