J Mater Sci: Mater Electron (2008) 19:1064–1068 DOI 10.1007/s10854-007-9467-2
Refractive index of polymethylmethacrylate oriented by fluid temperature under electrical field Oleksij Lyutakov Æ Vaclav Sˇvorcˇ´ık Æ Ivan Huttel Æ Jakub Siegel Æ Nikola Kasa´lkova´ Æ P. Slepicˇka
Received: 18 July 2007 / Accepted: 29 October 2007 / Published online: 16 November 2007 Springer Science+Business Media, LLC 2007
Abstract We prepared micron and submicron polymethylmethacrylate (PMMA) layers by the spin-coating method. We investigated the possibility to orientate polymer dipoles in electric field in the glass transition area (Tg) and the fluid temperature of PMMA with the aim to increase its refractive index (n) after the layer is cooled below Tg. We have studied the effect of electric field (up to 12 kV cm-1) on change of surface morphology of the layer, dependence of n and contact angle (surface wettability) on the field and dependence of layers orientation on orientation of electric field. The surface morphology was examined using atomic force microscopy (AFM), contact angles were measured by goniometer, film thickness was measured by profilometer, refractive index of films was determined using refractometer. The change of refractive index as dependent on the PMMA layer orientation in electric field depends on temperature and electric field. The highest change in n was found for electric field 11 kV cm-1. The change in contact angle (wettability) on surface of an orientated PMMA layer confirms the dipoles orientation in electric field unambiguously. The orientation of layers causes a ‘‘slight’’ change in their morphology and a ‘‘slight’’ increase of surface roughness only for one direction of field effect. Change in colour for oriented layers does not depend on orientation of electric field.
O. Lyutakov V. Sˇvorcˇ´ık (&) I. Huttel J. Siegel N. Kasa´lkova´ P. Slepicˇka Department of Solid State Engineering, Institute of Chemical Technology, Prague 166 28, Czech Republic e-mail:
[email protected]
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1 Introduction The basic factors for expanding use of polymer materials in electronics, photonics and optoelectronics are their relatively low price, required purity and ‘‘easy’’ treatment. The current development in application of polymers in these areas concentrates not only on study of new polymers, but above all on suitable modifications of properties of already used polymers. A modification of a polymer causes the required change in its properties. Frequently studied methods are ion beam [1–3], chemical modification [4], photoetching [5], electron or proton beam [6, 7], exposition in plasma [8, 9], doping [10, 11], orientation of polymer segments [12] and combination of them [13], and others. The bulk and/or surface features of a polymer can be changed dependent on the selected modification method (physical or chemical). We can mention that electric resistance and optical properties of a polymer layer change because of doping [10, 11], thermal and mechanic stability of polymers increases through chemical modification [4, 14]; electron, proton or ion beam interaction is manifested in change of structure and chemical composition of a polymer, its conductivity and optical properties [1–3, 6, 7]; photoetching results in polymer crosslinking, decrease of its solubility and increased thermal stability [5]. On the other hand, modification of polymer surface does not alter its bulk properties and changes only the surface features, which is required in many applications. Adhesion of metal to polymer and its biocompatibility can be improved through plasma exposition [8, 9], WET chemistry [15, 16] or photoetching [4]. Our former works [17–21] treated modification of a polar polymer by external electric filed during evaporation of solvent in preparation of the layer through spin-coating. The electric field causes turn of dipoles in the polymer layer which is reflected in increase of refractive index [17–21]
J Mater Sci: Mater Electron (2008) 19:1064–1068
and/or electric permittivity [22–24]. Polar groups of dopants (like diphenylsulphoxide) work as dipoles in case of polar polymers (like PMMA) or non-polar polymers [22, 23], respectively. These results are interesting for integrated optics and photonics, as we know, that even a small change in refractive index (Dn = 0.01) suffices for preparation of optical waveguide structures. Other studies have shown that orientated PMMA is not stable and its dipoles orientate back after some time [19]. A similar trend was found by other authors [25]. Significant increase in stability of this way orientated PMMA occurred after doping the layer with non-polar polystyrene while increased growth of refractive index of the layer was maintained [19]. Diluted polymer macromolecules have certain degree of freedom and they are able to change their mutual positions and this way also the position of the polymer segments one against another. Their arrangement is kept under electric field after the solvent evaporates. A similar phenomenon occurs also when heating thermoplastic material above the glass transition temperature (Tg, movement of segments) and particularly above the fluid temperature (Tf, movement of whole macromolecules) [26]. The presented work contained preparation of micron and submicron PMMA layers. We have studied the possibility to orientate polymer dipoles in electric field in the glass transition and fluid temperature areas. We can expect that after a layer is cooled below Tg, its refractive index (n) could increase when compared with the pristine layer. We have further studied impact of electric field on change of surface morphology of the layer, dependence of n on the orientation temperature, effect of electric field on n and contact angle (surface wettability) and effect of pole direction.
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silicon (crystallographic orientation (100), resistance 0.002 X cm, refractive index n = 3.505). The layers were prepared from 0.5–5.00 wt% solution of PMMA in chloroform. After spin-coating, the Si plate with the PMMA layer was heated to 275 ± 5 C, which is close to Tf of PMMA. The electric field from 0 to 12 kV cm-1 was applied on a part of the sample for 30 min and then the sample cooled spontaneously to room temperature under continuous effect of the electric field. Layer breakdown occurred in electric field over 12 kV cm-1 at higher temperature. The surface morphology was examined using atomic force microscopy (AFM, tapping mode), performed under ambient conditions with Digital Instruments CP II set-up. Veeco oxide-sharpened silicon probes RTESPA-CP with the spring constant 40 N/m were selected. Mean roughness (Ra) represents the arithmetic average of the deviations from the centre plane of the sample. Contact angles, characterizing surface wettability and polarity, were measured by goniometer Surface Energy Evaluation System (Masaryk University, Czech Republic) at 10 positions with distilled water at room temperature. The measured values of the contact angle showed \10% standard deviation. The contact angle was measured 72 h after layer orientation. The samples were stored at room temperature and exposed to ambient atmosphere during ‘‘aging’’. The film thickness was measured (standard error ±10%) by profilometers Hommel 1000 and Talystep. Refractive index of films was determined in the spectral range 250– 750 nm using refractometer Avaspec 2048 as the mean of 6 independent measurements. With the aid of computer code AvaSoft Full 6.1, including code Spectra 3, the dependence of the refractive index (n) on the wavelength (k) for layers deposited on substrate was found. In this way the refractive index n, extrapolated to infinite wavelength, was determined.
2 Experimental 2.1 Materials
3 Results and discussion
Present experiments were performed on polymethylmethacrylate (PMMA) in optical purity supplied by Goodfellow. The heat flow, the glass transition and fluid temperatures (Tg = 112 C, Tf = 275 C) of the polymer were determined by the standard calorimetric method using the DSC 2920 technique. The molecular weights of polymer (Mw = 1,459,000, Mn = 708,000) was measured by the gel permeatic chromatography GPC technique. The typical uncertainties are below ±5% in both cases.
3.1 Refractive index and contact angle after applying electric field
2.2 Methods For measurements, 150–1,500 nm thick polymer films were prepared by spin-coating method (1,500 rpm) [12–14] onto
We have found that the refractive index does not depend on layer thickness in the interval 150–1,500 nm for non-oriented PMMA layers under our experimental conditions. The dependence of the refractive index of the PMMA layer on orientation under electric field is shown in Fig. 1, together with the temperature record of DSC measurement for PMMA. We can distinguish areas with different effect of orientation temperature in the temperature curve for n. The magnitude of refractive index of a layer subjected to field does not depend on temperature below 80 C. Between 80 and 100 C, we can see increasing n, which corresponds to the glass transition area (see DSC results,
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J Mater Sci: Mater Electron (2008) 19:1064–1068 1,482
1,470
PMMA
PMMA 1,479
70
Refractive index 1,476
Heat flow
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Tg
Tf
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Heat flow (W /g)
Refractive index
-0,6 Contact angle
Refractive index
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-1,4 -1,6
C o n t a c t a n g le ( d e g )
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100
150
200
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Temperature (°C)
20 0
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Fig. 1 Dependence of refractive index of PMMA layer on temperature where the layer has been orientated in electric field. The layer thickness was 250 nm and electric field 11.0 kV cm-1. Temperature dependence of heat flow in PMMA was measured using the DSC method to determine the glass transition temperature (Tg) and the fluid temperature (Tf)
Fig. 2 Dependence of refractive index (h) and water surface contact angle () on electric field for 250 nm PMMA layers oriented at 275 C. Circles (; electrical field orientation) and dots (: orientation) represent different directions of field during layer preparation
Fig. 1), where the movement of polymer segments in the layer is enabled. Figure 1 shows further a slight increase of n from 100 to 250 C, where the polymer is in the ‘‘rubber’’ area. A dramatic increase of n occurs above 250 C, because PMMA is then in the fluid area (see DSC results, Fig. 1) where the whole macromolecules can move. PMMA is a polydispersive polymer, this means that macromolecules show distribution of molecular weight. Interpretation of results presented in Fig. 1 shows that the shorter macromolecules orientate first (at lower temperature) in both Tg and Tf areas, and the longer ones only then. Destruction of PMMA layers occurred above 300 C. Temperature 275 ± 5 C was chosen for further experiments based on results shown in Fig. 1. The dependence of the refractive index and surface contact angle on the electrical field strength for a PMMA layer is drawn in Fig. 2. We can see that after the ‘‘critical’’ value of electrical field (5 kV cm-1) is exceeded, a dramatic increase of the refractive index occurs. The highest increase of n was achieved for electric field 11 kV cm-1. Further increase of electric field does not cause increase of the layer refractive index.
Work [9] has shown that wettability (polarity) of materials can be characterised through the contact angle, where higher polarity of surface corresponds to a lower water contact angle. Dependence of contact angle on electric field is also presented in Fig. 2. As we can see, PMMA is a polar polymer and therefore its contact angle has value approximately 67 and this angle is constant up to 5 kV cm-1. When electric field increases further, dramatic drop of contact angle (to 25) appears, and it is constant for electric field above 10 kV cm-1. Decrease of contact angle of the layer proves that the applied field has caused orientation of PMMA dipoles along the field. Figure 2 shows that the change in the layer refractive index has been caused by orientation of PMMA dipoles in electric field. Dependence of n and contact angle on electric field for samples that have been orientated in the opposite field direction than the layers discussed above is also drawn in Fig. 2. We can see that the n value does not depend on direction of electric field. The refractive index increases with electric field also in this case. On the other hand we can see a dramatic change on the curve contact angle vs. electric field caused by changed direction of the field. It is
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evident that the contact angle does not depend on electric field under this field orientation and it is the same as for pristine PMMA, within measurement error. The prepared PMMA layers are coloured because of interference. We have used this effect to show the results discussed above: we have photographed them after heating and subsequent orientation of a part of samples in electric field (see Fig. 3). PMMA does not absorb in the visible spectrum [20] and surface of prepared layers has ‘‘relatively’’ low surface roughness (to be discussed, see Fig. 4). Colouration of layers is generally a function of their thickness, of refractive index and sensitivity of human eye. Figure 3 shows that the samples have the same colour after heating to 275 C (see the left part of the samples). A dramatic change in colour is visible in samples that have been subjected to electric field after heating (see the right part of the samples). The different colour of orientated layers (after correlation with results from Fig. 2) confirms a change in their refractive index which depends on electric field. The same changes in the layer colour documented in Fig. 3 for orientated layers as dependent on electric field were evident even after changing the field direction. This is an expected result, because the change in field direction does not change the refractive index (see Fig. 2).
PMMA ± electric field 0
7.6
1 cm ______
0
8.7
9.7
0
Fig. 3 Digital photo (under 45 angle) of PMMA layers 250 nm thick prepared on Si substrate. The layers were heated to 275 C and a part of them (the right one in the figure) was modified in electric field. The numbers in the figure show electric field in kV cm-1
from solution. The contact angle generally depends also on surface roughness of the sample [9]. Thus we studied (see Fig. 4) morphology and surface roughness (Ra) of nonorientated and orientated PMMA layers for both directions of electric field. We can see in Fig. 4 that the surface morphology of a PMMA layer after orientated in field depends on its direction. While only a ‘‘slight’’ change in morphology and roughness occurs in one direction, no changes in morphology or Ra beyond measurement error are evident in the opposite direction. Changed surface morphology is manifested in less clusters for an orientated layer, and the clusters are ‘‘smoother’’. A ‘‘slight’’ increase of surface roughness occurred in this direction. We cannot expect that these changes in Ra would influence the
3.2 Surface morphology after electric field It has been shown [20] that neither surface morphology nor roughness change in preparation of orientated PMMA films Fig. 4 AFM images of 150 nm pristine PMMA layer, oriented by electric field 9.7 kV cm-1 prepared by spin-coating on Si substrate. Ra is surface roughness in nm and symbols : or ; mark direction of electric field during layer preparation
0
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non-oriented layer
Ra=0.28
Ra=0.12
↑
Ra=0.16
Ra=0.14
↓
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magnitudes of monitored water contact angles significantly, because water drops with volume 8 lL were inserted on the samples using micropipette.
4 Conclusion The results can be summarised as follows: •
•
•
•
•
the change in refractive index with orientation of a PMMA layer in electric field depends on temperature or rather temperature area where the layer is orientated. The increase in n is more significant in the PMMA fluid area, than in the glass transition area, refractive index of orientated layers depends on electric field strength, increase in n is evident from 5 kV/cm on and achieves the maximum magnitude at 11 kV/cm, for layers of the same thickness, their orientation is manifested by a changed colour due to interference because of changed refractive index. Changed colour for orientated layers does not depend on direction of electric field, orientation of dipoles caused by electric field can be documented through changed contact angle (wettability). The change in contact angle depends on direction of electric field, orientation of layers causes a ‘‘slight’’ change in morphology and a ‘‘slight’’ increase of surface roughness for one field direction only.
Acknowledgements The work was supported by the Czech Grant Agency within the project No. 102-06-0424, by GA ASCR within the project KAN400480701 and by the Czech Ministry of Education within Research Programs No. MSM 6046137302 and LC 06041.
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