- Email: [email protected]
Effect of plasma polymerization conditions on the humidity sorptive properties of thin films obtained from hexamethyldisiloxane in glow discharge
\
Pergamon
Vacuum/volume 51/number 2/pages 217 to 220/1998 © 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042–207X/98 $19.00+.00
PII : S0042–207X(98)00162–6
Effect of plasma polymerization conditions on the humidity sorptive properties of thin films obtained from hexamethyldisiloxane in glow discharge E Radeva* and L Spassov, Institute of Solid State Physics, Bulgarian Academy of Sciences, blv. Tzarigradsko chausse, 72, Sofia, Bulgaria accepted 6 March 1998
The humidity sorptive powers of polymer coatings deposited on quartz resonators has been studied. The polymer was obtained from hexamethyldisiloxane by plasma polymerization under different operating conditions of glow discharge current density, gas phase pressure, polymer film thickness and modification of the polymer surface in an ammonia plasma. The sensitivity of the films to humidity was enhanced by operating at the highest current density and pressure and by raising the film thickness and exposure to the ammonia plasma. © 1998 Elsevier Science Ltd. All rights reserved
0[ Introduction
1[ Experiment
There is a growing interest in plasma grown polymer _lms proper! ties as stable coatings for protection[0Ð3 These properties result from their quite di}erent chemical composition and structure compared to those of the conventional polymers\4 the reason being the mechanism of the plasma polymerization process[ This is a process of a thin _lm formation in which the monomer\ subject of successive fragmentation and recombination\ forms a polymer[5 Because of that\ the chemical composition and struc! ture of the polymer\ obtained in glow discharge plasma\ depend not only on the chemical structure of the monomer\ but on the plasma polymerization conditions as well[ The main parameters that e}ect the polymerization mechanism and\ therefore\ the chemical composition\ structure and properties of the polymers\ are the glow discharge current density and gas phase pressure[ Additional modi_cation of the polymer structure is possible by introducing chemical reagents in the gas phase that copolymerize with the basic polymer[ Our recent studies have shown that polymers\ obtained from hexamethyldisiloxane "HMDSO# in plasma absorb water vapours[6 This paper presents results on the e}ect of glow discharge current density and gas phase pressure on the humidity sorption properties of the HMDSO polymers[ Furthermore\ the in~uence of _lm thickness and ammonia plasma modi_cation on the humidity sorptive are investigated[ Polymer mass changes\ result! ing from water molecules sorption\ are registered by means of a quartz microbalance[
The sensor system\ used in this study\ consists of a humidity sensitive element\ oscillator circuits\ a frequency counter and a computer as shown in Fig[ 0[ The humidity sensitive element consists of AT!cut quartz resonator\ resonating at about 03 MHz[ The entire area of the resonator is coated with a polymer _lm\ obtained from HMDSO in a glow discharge process[ The plasma reactor system\ previously described\6 is used to deposit the _lms[ The glow discharge AC voltage is about 399 V\ 49 Hz[ To produce a large range of selectivities\ the plasma polymers are deposited under the following conditions ] the current density is varied from 9[4 to 2[5 mA:cm1 and the gas phase pressure in the plasma reactor is altered between 15 and 155 Pa[ The thickness of the plasma polymer _lms\ deposited on both sides of the quartz resonators\ is measured by evaluating the frequency shift7 and varies from 9[0 to 9[8 mm[ The modi_cation of the deposited polymer _lms is performed by exposing them to ammonia vapour
To whom all correspondence should be addressed [
Figure 0[ Principal scheme for measuring the humidity!frequency characteristics "HFC# of quartz resonators[ 217
E Radeva et al : Effect of plasma polymerization on thin films
for 6 min[ As described below\ this modi_cation was found to increase the humidity sorption[ The coated quartz resonators with polymer _lms placed in a climatic chamber of Mytron type KPW\ Germany\ are connected to oscillator circuits[ Their fre! quency is monitored by a frequency counter with 9[0 Hz resol! ution[ A personal computer records the counter output\ and evaluates the experimental data[ The humidity frequency charac! teristics "HFC# of the sensing element were measured in terms of the relative humidity "RH# of the atmosphere in the chamber[ 2[ Results and discussion Two typical examples of a coated quartz resonator HFC with plasma polymer _lms are shown in Fig[ 1[ The two curves were obtained by measuring the frequency shifts in increasing and decreasing RH at a constant temperature of 14>C[ As evident from Fig[ 1\ there is a hysteresis e}ect caused by time dependent inertia of the physical sorption[ We observed a substantial decrease in hysteresis with increased sorption time[ The HFCs are non!linear and follow a third order polynomial dependence\ which is consistent also with the results obtained[8 The sensitivity to humidity of a polymer coated quartz res! onator is the frequency change Df as a function of the percentage change in RH[ It depends on the chemical composition and structure of the sorptive _lm[ This is the slope of the HFC in Fig[ 1[ 2[0[ Effects of glow discharge current density on the polymer humidity!sorptive properties The HFCs of the polymer coated quartz resonators\ obtained with di}erent glow discharge current densities I\ were measured[ The gas phase pressure and _lm thickness were held constant at 095 Pa and 9[1 mm\ respectively[ The average sensitivities to humidity CC of the HFCs were cal!
culated and shown in Fig[ 2 for current densities varying from 9[14 to 2[0 mA:cm1[ The results show that the increase of glow discharge current density leads to an increase of water vapour sorption\ which is probably due to stronger interactions between the polymer and the water molecules because of the higher oxygen content in the polymer[ This e}ect had been registered by Fou! riere transform infra!red "FTIR# spectroscopy and X!ray photo! electron spectroscopy "XPS# at higher current densities[09\00 2[1[ Effects of gas phase pressure on the polymer humidity!sorptive properties The HFCs of samples obtained at di}erent gas phase pressure P\ ranging from 15 to 155 Pa\ were measured[ The current density and _lm thickness were held constant at 0[4 mA:cm1 and 9[1 mm\ respectively[ The _lm sensitivity to humidity increases with an increase of the gas phase pressure as evident from Fig[ 3[ The highest pressure results in the highest degree of cross!linking and formation of structure close to those of the "SiO1#n compounds[09 These changes in the chemical structure of the polymer _lms explain the results in Fig[ 3[ 2[2[ Effects of _lm thickness on the polymer humidity!sorptive properties The HFCs of the samples with di}erent _lm thickness h in the range of 9[0Ð9[8 mm were measured[ In these experiments the glow discharge current density was kept constant at 2[5 mA:cm1 and the gas phase pressure is 155 Pa[ As evident from Table 0 the sensitivity to humidity CC increases with the _lm thickness[ This is explained by water sorption in the bulk of the polymer _lm[ 2[3[ Effects of ammonia plasma treatment on the humidity!sorptive properties The HFCs of the samples obtained at di}erent plasma parameters*glow discharge current density\ gas phase pressure\
Figure 1[ Humidity!frequency characteristics "HFCs# of a polymer covered quartz resonator[ 218
E Radeva et al : Effect of plasma polymerization on thin films
Figure 2[ E}ect of glow discharge current density I on the sensitivity to humidity CC[
Figure 3[ E}ect of gas phase pressure P on the sensitivity to humidity CC[
polymer _lms with di}erent thicknesses\ with and without modi! _cation in ammonia plasma were measured[ The results were summarized in Table 1[ It is evident that the sensitivity to humidity of _lms\ treated with ammonia\ is higher with _lms without treatment\ if the same deposition conditions are used[
The results in Table 1 show that structural changes of plasma polymer _lms obtained from HMDSO and observed by FTIR and XPS09\00 lead to an increase in water sorption of the modi_ed polymer _lms[ The increased amount of the oxygen and nitrogen content in the polymers and the formation of structures with 219
E Radeva et al : Effect of plasma polymerization on thin films
3[ Conclusion
Table 0[ Polymer thickness "mm# 9[0 9[1 9[1 9[3 9[6 9[7 9[8
Current density "mA:cm#
Gas phase pressure "Pa#
Sensitivity to humidity "Hz:)OB#
2[0 2[0 9[4 2[0 9[5 9[5 9[5
155 155 095 155 022 022 022
6[9 01[4 5[8 07[9 7[4 09[2 00[7
References
Table 1[ Sensitivity to humidity "Hz:)OB# Current density "mA:cm#
Gas phase pressure "Pa#
with NH2
without NH2
0[4 0[4 2[0 9[14 9[53 0[04
095 155 155 095 022 095
7[8 00[4 01[4 4[6 00[7 7[9
3[4 7[9 09[8 1[7 5[5 4[0
higher degree of cross!linking\ compared to the polymers without modi_cation\ is probably the reason for increased sensitivity[
220
The humidity sorptive properties of the polymer _lms\ obtained from HMDSO in a glow discharge plasma were investigated[ It was shown that the higher glow discharge current density\ gas phase pressure during plasma polymerization\ and _lm thickness result in higher polymer sensitivity to humidity[ Modi_cation with ammonia vapors in the plasma after poly! merization additionally increases the humidity sorption of the polymer _lms[ The sensitivity to humidity depend on changes in the chemical structure of the polymers\ mainly due to oxygen content and cross linking of _lms[ We feel that polymers\ obtained from HMDSO and plasma modi_cation with ammonia may be used as hygroscopic elements for sensing the humidity of ambient air[
0[ Freitag\ W[ O[\ Yasuda\ H[ and Sharma\ A[ K[\ J[ Appl[ Polym[ Sci[\ 0873\ 27\ 074[ 1[ Yasuda\ H[\ J[ of Membr[ Sci[\ 0873\ 07\ 162[ 2[ Sacher\ E[ J[\ Klemberg!Sapieha\ E[\ Schreiber\ H[ P[ and Werthei! mer\ M[ R[\ J[ Appl[ Polym[ Sci[\ 0873\ 27\ 052[ 3[ Schreiber\ H[ P[\ Wertheimer\ M[ R[ and Wrobel\ A[ M[\ Thin Solid Films\ 0879\ 61\ 376[ 4[ Hamman\ C[ and Kampfrath\ G[\ Vacuum\ 0873\ 23\ 0942[ 5[ Yasuda\ H[\ J[ Appl[ Polym[ Sci[\ Macromol[ Rev[\ 0870\ 05\ 088[ 6[ Radeva\ E[\ Bobev\ K[ and Spassov\ L[\ Sensors and Actuators\ 0881\ B"7#\ 10[ 7[ Malov\ V[ V[\ Piezoresonancnie datchiki\ Ener`oatomizdat\ M[\ 0878\ p[ 57 "in Russian#[ 8[ Smirnow\ Y[ T[\ Malugin\ W[ P[\ Karaulnik\ A[ E[ and Chernih\ G[ G[\ Electronnaia tehnica\ Seria 4\ Radiodetali i radiokomponenti\ W[\ 0866\ 2\ 2[ 09[ Radeva\ E[\ Tsankov\ D[\ Bobev\ K[ and Spassov\ L[\ J[ Appl[ Polym[ Sci[\ 0882\ 49\ 054[ 00[ Radeva\ E[\ Stefanov\ P[ and Spassov\ L[\ Comp[ Rend[ Acad[ Bul`[ Sci[\ "in press#[
Pergamon
Vacuum/volume 51/number 2/pages 217 to 220/1998 © 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0042–207X/98 $19.00+.00
PII : S0042–207X(98)00162–6
Effect of plasma polymerization conditions on the humidity sorptive properties of thin films obtained from hexamethyldisiloxane in glow discharge E Radeva* and L Spassov, Institute of Solid State Physics, Bulgarian Academy of Sciences, blv. Tzarigradsko chausse, 72, Sofia, Bulgaria accepted 6 March 1998
The humidity sorptive powers of polymer coatings deposited on quartz resonators has been studied. The polymer was obtained from hexamethyldisiloxane by plasma polymerization under different operating conditions of glow discharge current density, gas phase pressure, polymer film thickness and modification of the polymer surface in an ammonia plasma. The sensitivity of the films to humidity was enhanced by operating at the highest current density and pressure and by raising the film thickness and exposure to the ammonia plasma. © 1998 Elsevier Science Ltd. All rights reserved
0[ Introduction
1[ Experiment
There is a growing interest in plasma grown polymer _lms proper! ties as stable coatings for protection[0Ð3 These properties result from their quite di}erent chemical composition and structure compared to those of the conventional polymers\4 the reason being the mechanism of the plasma polymerization process[ This is a process of a thin _lm formation in which the monomer\ subject of successive fragmentation and recombination\ forms a polymer[5 Because of that\ the chemical composition and struc! ture of the polymer\ obtained in glow discharge plasma\ depend not only on the chemical structure of the monomer\ but on the plasma polymerization conditions as well[ The main parameters that e}ect the polymerization mechanism and\ therefore\ the chemical composition\ structure and properties of the polymers\ are the glow discharge current density and gas phase pressure[ Additional modi_cation of the polymer structure is possible by introducing chemical reagents in the gas phase that copolymerize with the basic polymer[ Our recent studies have shown that polymers\ obtained from hexamethyldisiloxane "HMDSO# in plasma absorb water vapours[6 This paper presents results on the e}ect of glow discharge current density and gas phase pressure on the humidity sorption properties of the HMDSO polymers[ Furthermore\ the in~uence of _lm thickness and ammonia plasma modi_cation on the humidity sorptive are investigated[ Polymer mass changes\ result! ing from water molecules sorption\ are registered by means of a quartz microbalance[
The sensor system\ used in this study\ consists of a humidity sensitive element\ oscillator circuits\ a frequency counter and a computer as shown in Fig[ 0[ The humidity sensitive element consists of AT!cut quartz resonator\ resonating at about 03 MHz[ The entire area of the resonator is coated with a polymer _lm\ obtained from HMDSO in a glow discharge process[ The plasma reactor system\ previously described\6 is used to deposit the _lms[ The glow discharge AC voltage is about 399 V\ 49 Hz[ To produce a large range of selectivities\ the plasma polymers are deposited under the following conditions ] the current density is varied from 9[4 to 2[5 mA:cm1 and the gas phase pressure in the plasma reactor is altered between 15 and 155 Pa[ The thickness of the plasma polymer _lms\ deposited on both sides of the quartz resonators\ is measured by evaluating the frequency shift7 and varies from 9[0 to 9[8 mm[ The modi_cation of the deposited polymer _lms is performed by exposing them to ammonia vapour
To whom all correspondence should be addressed [
Figure 0[ Principal scheme for measuring the humidity!frequency characteristics "HFC# of quartz resonators[ 217
E Radeva et al : Effect of plasma polymerization on thin films
for 6 min[ As described below\ this modi_cation was found to increase the humidity sorption[ The coated quartz resonators with polymer _lms placed in a climatic chamber of Mytron type KPW\ Germany\ are connected to oscillator circuits[ Their fre! quency is monitored by a frequency counter with 9[0 Hz resol! ution[ A personal computer records the counter output\ and evaluates the experimental data[ The humidity frequency charac! teristics "HFC# of the sensing element were measured in terms of the relative humidity "RH# of the atmosphere in the chamber[ 2[ Results and discussion Two typical examples of a coated quartz resonator HFC with plasma polymer _lms are shown in Fig[ 1[ The two curves were obtained by measuring the frequency shifts in increasing and decreasing RH at a constant temperature of 14>C[ As evident from Fig[ 1\ there is a hysteresis e}ect caused by time dependent inertia of the physical sorption[ We observed a substantial decrease in hysteresis with increased sorption time[ The HFCs are non!linear and follow a third order polynomial dependence\ which is consistent also with the results obtained[8 The sensitivity to humidity of a polymer coated quartz res! onator is the frequency change Df as a function of the percentage change in RH[ It depends on the chemical composition and structure of the sorptive _lm[ This is the slope of the HFC in Fig[ 1[ 2[0[ Effects of glow discharge current density on the polymer humidity!sorptive properties The HFCs of the polymer coated quartz resonators\ obtained with di}erent glow discharge current densities I\ were measured[ The gas phase pressure and _lm thickness were held constant at 095 Pa and 9[1 mm\ respectively[ The average sensitivities to humidity CC of the HFCs were cal!
culated and shown in Fig[ 2 for current densities varying from 9[14 to 2[0 mA:cm1[ The results show that the increase of glow discharge current density leads to an increase of water vapour sorption\ which is probably due to stronger interactions between the polymer and the water molecules because of the higher oxygen content in the polymer[ This e}ect had been registered by Fou! riere transform infra!red "FTIR# spectroscopy and X!ray photo! electron spectroscopy "XPS# at higher current densities[09\00 2[1[ Effects of gas phase pressure on the polymer humidity!sorptive properties The HFCs of samples obtained at di}erent gas phase pressure P\ ranging from 15 to 155 Pa\ were measured[ The current density and _lm thickness were held constant at 0[4 mA:cm1 and 9[1 mm\ respectively[ The _lm sensitivity to humidity increases with an increase of the gas phase pressure as evident from Fig[ 3[ The highest pressure results in the highest degree of cross!linking and formation of structure close to those of the "SiO1#n compounds[09 These changes in the chemical structure of the polymer _lms explain the results in Fig[ 3[ 2[2[ Effects of _lm thickness on the polymer humidity!sorptive properties The HFCs of the samples with di}erent _lm thickness h in the range of 9[0Ð9[8 mm were measured[ In these experiments the glow discharge current density was kept constant at 2[5 mA:cm1 and the gas phase pressure is 155 Pa[ As evident from Table 0 the sensitivity to humidity CC increases with the _lm thickness[ This is explained by water sorption in the bulk of the polymer _lm[ 2[3[ Effects of ammonia plasma treatment on the humidity!sorptive properties The HFCs of the samples obtained at di}erent plasma parameters*glow discharge current density\ gas phase pressure\
Figure 1[ Humidity!frequency characteristics "HFCs# of a polymer covered quartz resonator[ 218
E Radeva et al : Effect of plasma polymerization on thin films
Figure 2[ E}ect of glow discharge current density I on the sensitivity to humidity CC[
Figure 3[ E}ect of gas phase pressure P on the sensitivity to humidity CC[
polymer _lms with di}erent thicknesses\ with and without modi! _cation in ammonia plasma were measured[ The results were summarized in Table 1[ It is evident that the sensitivity to humidity of _lms\ treated with ammonia\ is higher with _lms without treatment\ if the same deposition conditions are used[
The results in Table 1 show that structural changes of plasma polymer _lms obtained from HMDSO and observed by FTIR and XPS09\00 lead to an increase in water sorption of the modi_ed polymer _lms[ The increased amount of the oxygen and nitrogen content in the polymers and the formation of structures with 219
E Radeva et al : Effect of plasma polymerization on thin films
3[ Conclusion
Table 0[ Polymer thickness "mm# 9[0 9[1 9[1 9[3 9[6 9[7 9[8
Current density "mA:cm#
Gas phase pressure "Pa#
Sensitivity to humidity "Hz:)OB#
2[0 2[0 9[4 2[0 9[5 9[5 9[5
155 155 095 155 022 022 022
6[9 01[4 5[8 07[9 7[4 09[2 00[7
References
Table 1[ Sensitivity to humidity "Hz:)OB# Current density "mA:cm#
Gas phase pressure "Pa#
with NH2
without NH2
0[4 0[4 2[0 9[14 9[53 0[04
095 155 155 095 022 095
7[8 00[4 01[4 4[6 00[7 7[9
3[4 7[9 09[8 1[7 5[5 4[0
higher degree of cross!linking\ compared to the polymers without modi_cation\ is probably the reason for increased sensitivity[
220
The humidity sorptive properties of the polymer _lms\ obtained from HMDSO in a glow discharge plasma were investigated[ It was shown that the higher glow discharge current density\ gas phase pressure during plasma polymerization\ and _lm thickness result in higher polymer sensitivity to humidity[ Modi_cation with ammonia vapors in the plasma after poly! merization additionally increases the humidity sorption of the polymer _lms[ The sensitivity to humidity depend on changes in the chemical structure of the polymers\ mainly due to oxygen content and cross linking of _lms[ We feel that polymers\ obtained from HMDSO and plasma modi_cation with ammonia may be used as hygroscopic elements for sensing the humidity of ambient air[
0[ Freitag\ W[ O[\ Yasuda\ H[ and Sharma\ A[ K[\ J[ Appl[ Polym[ Sci[\ 0873\ 27\ 074[ 1[ Yasuda\ H[\ J[ of Membr[ Sci[\ 0873\ 07\ 162[ 2[ Sacher\ E[ J[\ Klemberg!Sapieha\ E[\ Schreiber\ H[ P[ and Werthei! mer\ M[ R[\ J[ Appl[ Polym[ Sci[\ 0873\ 27\ 052[ 3[ Schreiber\ H[ P[\ Wertheimer\ M[ R[ and Wrobel\ A[ M[\ Thin Solid Films\ 0879\ 61\ 376[ 4[ Hamman\ C[ and Kampfrath\ G[\ Vacuum\ 0873\ 23\ 0942[ 5[ Yasuda\ H[\ J[ Appl[ Polym[ Sci[\ Macromol[ Rev[\ 0870\ 05\ 088[ 6[ Radeva\ E[\ Bobev\ K[ and Spassov\ L[\ Sensors and Actuators\ 0881\ B"7#\ 10[ 7[ Malov\ V[ V[\ Piezoresonancnie datchiki\ Ener`oatomizdat\ M[\ 0878\ p[ 57 "in Russian#[ 8[ Smirnow\ Y[ T[\ Malugin\ W[ P[\ Karaulnik\ A[ E[ and Chernih\ G[ G[\ Electronnaia tehnica\ Seria 4\ Radiodetali i radiokomponenti\ W[\ 0866\ 2\ 2[ 09[ Radeva\ E[\ Tsankov\ D[\ Bobev\ K[ and Spassov\ L[\ J[ Appl[ Polym[ Sci[\ 0882\ 49\ 054[ 00[ Radeva\ E[\ Stefanov\ P[ and Spassov\ L[\ Comp[ Rend[ Acad[ Bul`[ Sci[\ "in press#[