Polystyrene thin films with incorporated borane clusters for electronic applications investigated by infrared reflection-absorption spectroscopy

The paper is focused on the investigation of thin polystyrene (PS) films and PS films with incorporated anti-octadecaborane(22) (anti-B18H22) clusters (PS+B18H22) with a thickness of 450 nm and 290 nm, respectively. The main aim is to describe the vibrational behavior of thin films in dependence on temperature and determine the changes connected with the glass transition of PS. Infrared reflection-absorption spectra were measured and from the plot of spectral region area (Arange) vs. temperature, glass transition temperature (Tg) of PS was extracted. The abrupt change in the course of A of Car-H stretching vs. temperature was assigned to the glass transition. Tg of PS and PS+B18H22 thin film was determined to be 74 °C and 77 °C, respectively. On the other hand, only a continuous decrease of A of C-H stretching vs. temperature was found for both samples indicating that each vibrational motion reacts uniquely on temperature increase. It was further found that anti-B18H22 is not fixed firmly in the thin layer at higher temperatures, because no B-H spectral response was detected above 91 °C. This result was also supported with TGA and DSC analysis showing a particular mass decrease at temperature 75 °C.


INTRODUCTION
Polymer thin films have attracted attention due to their specific properties related to the thin layer arrangement of macromolecules. Semi-conductive polymer layers (e.g. MEH-PPV, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene]) have found their use in light-emitting diodes preparation [1] and insulating polymer layers (PS, polystyrene or PMMA, poly[(methyl) methacrylate] etc.) have been used as matrices for the incorporation of low molecular species. For example, blue-emitting borane clusters, especially anti-octadecaborane(22) (anti-B18H22) and its derivatives [2], are one of the highly investigated low molecular species nowadays, even though octadecaborane was synthesized already in the 1960s by Hawthorne and his co-workers [3]. Anti-B18H22 absorption and the emission maximum is located at 330 nm and 430 nm, respectively, and it also provides fluorescence that could be used in LED applications. However, some of its properties are not viable and several derivatives were investigated to improve solubility and to tune fluorescence, e.g. iodine- [4], pyridine- [5], sulfur- [6] derivative.
Properties of substrate-supported thin polymer films (the glass transition temperature, Tg) are influenced by chain confinement near the substrate surface, interactions between substrate and film (such as hydrogen bonding) [7], or surface roughness [8]. The Tg of a thin film can thus be lower or reach to higher values than the Tg of the bulk polymer. However, it cannot be investigated by routine measurement (DSC). Glass transition is related with the unlocking of polymer segments motion, which can cause conformational changes of chains observable by spectroscopic methods [9,10]. Infrared Reflection-Absorption Spectroscopy (IRRAS) has been used for the investigation of thin films on reflective substrates. Besides routine measurements related with confirmation of film structure or composition [11], it can be used for monitoring of the phase separation of thin films made from polymer blends, e.g. PMMA/poly(4-vinyl phenol) [12], or other important structure-and thickness-dependent properties, such as Tg [13,14] of thin films. For instance, the strong dependence of the Tg of poly(ethylene terephthalate) was observed below 100 nm thin layers, and Tg reduction up to 25 °C in comparison with bulk was found for film thickness below 65 nm [15].
In the contribution, we focus on the behaviour of anti-B18H22 incorporated in the PS thin film. Our preliminary study deals mainly with the effect of temperature on the IR spectra of PS films and PS+B18H22 films on the gold substrate. The experiments were performed with the advanced technique IRRAS that provide high spectral quality. We investigated vibrational motions of PS and PS+B18H22 films and the effect of temperature on the change in IR spectra and the glass transition process. The purpose of all our studies in this field is to unlock the potential of borane clusters in electronics as they represent a new class of materials based neither on πnor on σelectron delocalization.

MATERIALS AND METHODS
Thin films were prepared from PS (Mw 350000, Mn 170000) purchased from Sigma-Aldrich and anti-B18H22 obtained from the Institute of Inorganic Chemistry of the Academy of Sciences of the Czech Republic by spincoating from solution. Chloroform (HPLC) from J. T. Baker was used as a solvent. The concentration of the PS solution was 30 mg/mL, and the concentration of B18H22 solution was 0.4 mg/mL. Solutions were mixed and stirred until full dissolution to achieve the ratio of PS:B18H22 equal 75:1 in the final thin film.
Thin films were deposited on gold substrates with spin coater Laurell WS-650Mz-23NPPd. The spin rate was set to 1000 rpm and the substrate was spun for 30 s. After, substrates with deposited solutions were removed and placed to vacuum oven at 40 °C to enable solvent evaporation.
The thickness of the prepared PS and PS+B18H22 films was measured with profilometer Dektak XT-E (Bruker).
IRRAS spectra were collected with spectrometer Nicolet iN10/iZ10 (Thermo Scientific) with a heated chamber Refractor-Reactor (Harrick) that enables reflection-absorption measurement of thin layers. The chamber is equipped with ZnSe windows and provides the angle of incidence of 75°. The spectra were recorded with a polarizer set to maximize p-polarized radiation. Thin films were heated from room temperature to 140 °C with a rate of 2.6 °C/min and spectra were recorded with resolution 4 cm -1 and 16 scans. This setting enabled measurement of one spectrum per 1 °C of film heating, which eliminated adverse effects connected with spectrum measurement and simultaneous sample heating.
TGA and DSC analysis of PS pellet and anti-B18H22 powder was performed with Labsys Evo DTA/DSC instrument from Setaram. Measurement was set in the air with heating from 30 °C to 500 °C with a rate of 10 °C/min.

RESULTS AND DISCUSSION
Thin films of PS and PS+B18H22 were prepared in two series for (i) analyses of film thickness and (ii) IRRAS measurement. Thickness determination, as well as a surface visual check of thin films, was done with a mechanical profilometer and its digital camera with 100x zoom. Three repeated measurements were performed on different positions of the film surface. It was found that PS+B18H22 has a lower thickness of 290 nm if compared with the 450 nm PS thin film. This is probably caused by the presence of B18H22, which may act as a surfactant and change the properties of the spin-coated liquid layer. The surface of both films was smooth without significant differences between PS and PS+B18H22 samples. Both films surface evinced radial inhomogeneities which origin from the spin-coating process.
Further, IRRAS analysis was performed with both thin films. IR spectra were collected during film heating and after that spectral regions 3120 -2985 cm -1 , 2975 -2800 cm -1 and 2660 -2500 cm -1 of Car-H stretching vibrations of phenyl rings, C-H stretching vibrations of aliphatic PS part and B-H stretching vibrations of anti-B18H22, respectively, were examined. Spectra were baseline corrected, and area (Arange) was calculated. Figure 1 depicts the dependence of A3120-2985 and A2975-2800 on temperature. As can be seen from the graph on the left side of Figure 1, the area of Car-H stretching decreases with increasing temperature until the minimum is reached, then the trend is reversed and A3120-2985 begins to grow. The temperature, which can be extracted at the minimum of this dependence, is assigned to the Tg of the thin film. The drop of region area can be explained by the fact that the density of thin films can decrease at a higher temperature and thus IR intensity of the region declines. On the other hand, the intensity of IR vibration increases as the free volume increases and segmental motion is enabled. The latter effect becomes to prevail at a temperature close to the Tg, which corresponds with the area minimum in the given plot. The Tg of PS thin film was therefore extracted to be 74 °C, while the Tg of PS+B18H22 was determined to be 77 °C. Anti-B18H22 does not influence the PS glass transition at a significant extent. However, both of these values are 31 -34 °C lower than the Tg of bulk PS 108 °C that was determined with a standard DSC method. This result indicates that no strong interaction between the gold substrate surface and PS are present and that high Mw of PS does not increase the thin layer Tg above the bulk Tg [16,17].
The right part of Figure 1 is dedicated to the temperature dependence of the aliphatic C-H stretching area. The clear difference between aromatic and aliphatic C-H stretching vibrations behaviour was found. The A2975-2800 does not evince any abrupt changes with increasing temperature, and it decreases linearly in the whole temperature range. This spectral region is thus not affected by the glass transition of PS, and no transition temperature can be determined. The reason for this different course of the plot probably originates in the structure of PS. Car-H vibrations are definitely influenced with mutual positions of phenyl rings and related non-bonding interactions, while aliphatic CH2 on PS backbone are not. Moreover, CH2 cannot rotate. Glass transition process influences Car-H vibrations to a great extent, which gives the opportunity of the Tg detection.

Figure 1 Area of Car-H (left) and C-H (right) stretching region plotted against temperature for PS and PS+B18H22 films
A surprising result was obtained from the detailed analysis of B-H stretching region that belongs to anti-B18H22 incorporated in PS+B18H22 thin film. It has been observed during the measurement that the intensity of the absorption band at 2585 cm -1 decreases at higher temperatures until it is undetectable. The area of this region vs. temperature is given in Figure 2. The region of temperature, in which the area decrease occurs is 77 -91 °C. It is probably related to the Tg of PS thin film. As was stated above, the Tg was determined to 77 °C. Above this temperature, the anti-B18H22 molecules are not trapped in PS matrix anymore due to the release of segmental motions, and they start to migrate. This process might be joined with the sublimation of anti-B18H22 above 77 °C until it is fully released from the thin film.

Figure 2 Area of B-H stretching region plotted against temperature for film PS+B18H22
To reveal the reason of the described release of anti-B18H22 from the thin film, TGA and DSC analysis of pure anti-B18H22 powder was performed in the air atmosphere to simulate similar conditions as in the case of IRRAS analysis. The curves are shown in Figure 3. A small weight loss of 2 % was detected at 75 °C on TGA curve and also heat flow decrease at 105 °C. This decrease in the TGA curve can be connected with sample moisture and its evaporation. It is a general fact that boranes are very sensitive to humidity, and thus thin films measured in the air atmosphere can suffer the same problem. However, there is observable a gradual decrease of the sample mass starting at about 120 °C while the main peak of anti-B18H22 melting is found at 180 °C in the DSC curve. It indicates the sublimation of the material. The mass loss step in the TGA curve smoothly continues with no disrupt change correlated with the anti-B18H22 melting, so the sublimation verges into evaporation and possible thermal degradation unnoticeably. Nevertheless, the effect of anti-B18H22 release from thin PS film needs to be explained yet, and the work on this issue continues.

CONCLUSION
Thin films of PS and PS+B18H22 deposited on gold substrates were investigated with IRRAS analysis. IR spectra were used for the description of temperature influence on particular vibrational motions. We have focused on Car-H, C-H and B-H characteristic spectral regions and development of their area in dependence on temperature. It was found that vibrational motions respond to temperature increase differently. Whereas a clear turning point in the plot of area vs. temperature was detected for Car-H, only a slight continuous decrease of the C-H area was observed. This is probably caused by the effect of non-bonding interactions between phenyl rings that also influence vibrational motions and are more sensitive to temperature change. As a result, the determination of the Tg of PS thin film is enabled. An influence of anti-B18H22 presence on Tg of PS thin film was only marginal because the Tg of PS film was found to be 74 °C and Tg of PS+B18H22 77 °C, which is 31 -34 °C lower than Tg of bulk PS. As a result, different thin film Tg should always be considered if temperature demanding applications of polymer thin films are required.
During experiments, we also found that the intensity of the anti-B18H22 characteristic vibration decreases with increasing temperature (77 -91 °C). Anti-B18H22 is not fixed firmly in the thin film structure and at higher temperatures, is not stable in the case of PS+B18H22 films. TGA and DSC analysis of pure anti-B18H22 powder revealed partial weight loss at 75 °C and heat flow decrease at 105 °C. This can be connected with the evaporation of humidity trapped in powder and a similar effect could act on the thin film. Another finding is that the Tg of PS+B18H22 thin film is identical with temperature, at which the B-H intensity decrease starts. It is possible that the glass transition triggers the migration and evolution of anti-B18H22 molecules from the film. However, the mechanism of anti-B18H22 release from thin PS film needs further investigation.