Influence of ammonia and water sorption on the chemical and electrochemical properties of polyacrylic acid and its derivates [Elektronische Ressource] = Einfluss von Wasser- und Ammoniaksorption auf die chemischen und elektrochemischen Eigenschaften von Polyacrylsäure und deren Derivaten / vorgelegt von Melanie Hörter

Melanie Gervacio

Informations

• Publié par: eberhard_karls_universitat_tubingen
• Publié le: 01 janvier 2008
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Influence of ammonia and water sorption
on the chemical and electrochemical properties
of polyacrylic acid and its derivates

Einfluss von Wasser- und Ammoniaksorption
auf die chemischen und elektrochemischen Eigenschaften
von Polyacrylsäure und deren Derivaten


D I S S E R T A T I O N


der Fakultät für Chemie und Pharmazie
der Eberhard-Karls-Universität Tübingen

zur Erlangung des Grades eines Doktors
der Naturwissenschaften

2008

vorgelegt von

Melanie Hörter

























Tag der mündlichen Prüfung: 18.12.2007

Dekan: Herr Prof. Dr. Lars Wesemann
1. Berichterstatter: Herr PD Dr. Udo Weimar
2. Berichterstatter: Herr Prof. Dr. Günter Gauglitz
Contents
1 Introduction 1
1.1 Motivation and scope of the work ................................................................... 1
1.2 The sorption model system .............. 3
1.2.1 Water vapour as target analyte ............................. 3
1.2.2 Ammonia gas as target analyte ................................ 4
1.2.3 Polyacrylic acid as sensitive material ................... 6
2 Theoretical background and related work 9
2.1 Gravimetric measurements .............................................................................. 9
2.1.1 Acoustic wave devices [55] 10
2.1.2 Gravimetric measurements with QMBs [50, 52] ............................... 12
2.1.3 Further parameters influencing the QMB signals .............................. 14
2.1.4 Interpretation of the QMB measurements .......................................... 16
2.1.4.1 Sorption isotherm ................................. 17
2.1.4.2 Dynamic and static glass transition temperature ................. 19
2.1.5 Literature survey of QMB devices covered with PAA ...................... 20
2.2 Electrochemical measurements ...................................................................... 21
2.2.1 Measurement principle and typical results of AC impedance
spectroscopy ....................................................................................... 22
2.2.2 Interpretation of the AC impedance spectroscopy results .................. 23
2.2.2.1 Impedance of the polymer bulk ........................................... 26
2.2.2.2 Impedance of the polymer/electrode interface .................... 29
2.2.3 Voltage step and cyclic voltammetry measurements ......................... 30
2.2.4 Electrochemical measurements with PAA sensing materials ............ 32
2.3 Measurement of work function changes ........................................................ 32
2.3.1 Basic principle of Kelvin Probe measurements .................................. 33
2.3.2 Work function changes ....................................... 35
2.3.2.1 Work function changes of gold due to sorbed ammonia ..... 37
2.3.2.2 Work fges ofbed water ........... 38 Contents

2.3.2.3 Work function changes of PAA covered gold substrate due to
ammonia ............................................................................... 39
2.4 Spectroscopic studies of PAA ........ 40
2.4.1 Transmittance IR spectra in dry air .................... 40
2.4.1.1 Polyacrylic acid .................................................................... 41
2.4.1.2 PAA derivates ...... 44
2.4.2 IR spectra changes upon water and ammonia sorption [22]............... 45
3 Experimental details 47
3.1 Sensitive materials ......................................................................................... 47
3.1.1 Coating procedures ............. 48
3.1.2 Layer morphology studies .................................................................. 49
3.2 Instrumental equipment.................................................................................. 50
3.2.1 Gravimetric measurements . 50
3.2.2 Electrochemical measurements .......................... 53
3.2.3 Kelvin Probe measurements ............................................................... 56
3.2.3.1 Besocke set-up ..................................... 58
3.2.3.2 McAllister set-up . 59
3.2.4 Infrared measurements ....................................... 61
4 Measurement results and interpretation 63
4.1 Characterisation of the polymer layer morphology ....................................... 63
4.2 Gravimetric measurements ............................................ 65
4.2.1 Water sorption .................................................... 65
4.2.2 Ammonia sorption .............. 67
4.2.3 Water sorption in a background of ammonia ..................................... 69
4.3 Electrochemical measurements ...................................... 72
4.3.1 Impedance measurements ... 72
4.3.1.1 Polymer bulk properties ....................................................... 76
4.3.1.2 Electrode processes .............................. 79
4.3.1.3 Electrochemical properties represented by the R ║C circuitm m
................................................................ 83 Contents

4.3.2 Voltage step and cyclic voltammetry measurements ......................... 84
4.4 Kelvin Probe measurements .......................................................................... 87
4.4.1 Work function changes of the uncovered gold substrate .................... 88
4.4.2 Work fges of the polymer covered gold substrates ........ 89
4.5 Spectroscopic studies ..................................................................................... 91
4.5.1 Hydrogen bonded water and CH stretching vibrations (3750 to 2
-12500 cm ) ........................... 91
-14.5.2 Range of C=O stretching vibrations (1800 to 1600 cm ) .................. 94
-14.5.3 Stretching modes of the carboxylate anion (1600 to 1000 cm ) ........ 96
4.5.4 Irreversible changes of PAA due to interaction with gaseous ammonia
............................................................................................................ 97
5 Discussion and modelling 99
5.1 Processes in the polymer bulk ...................................................................... 101
5.1.1 Water sorption .................. 101
5.1.1.1 Mass changes due to water sorption .. 101
5.1.1.2 Electrochemical property changes due to water sorption .. 102
5.1.2 Ammonia sorption ............................................................................ 106
5.1.3 Water sorption in a background of ammonia ... 108
5.2 Processes at the electrode ............. 111
5.2.1 Electrochemical processes at the electrode ...................................... 111
5.2.2 Kelvin Probe signal of polymer coated gold electrodes ................... 112
5.2.2.1 Kelvin Probe signals in dry air .......... 113
5.2.2.2 be signals in humid air ..................................... 114
6 Summary and outlook 117
Bibliography 121
List of abbreviations 135
List of publications 141
Acknowledgements 143
Curriculum Vitae 147
1 Introduction
In everyday life the human nose is well adapted for perception of odours in the
atmosphere. However, the detection of gases and vapours with the nose is not suitable
for industrial applications because it is extremely subjective as human smell
assessment is affected by many parameters [1] and inapplicable if odourless or harmful
substances have to be detected. For this reason, assistive techniques as for example gas
chromatography and mass spectrometry have been employed to control the
atmosphere, to raise an alarm if a maximum or minimum value is exceeded or to
assess the quality of products through odour evaluation. However, there are several
drawbacks of customary analytical techniques: They are not portable and tend to be
expensive and furthermore are relatively slow [2]. Compared with these techniques,
chemical microsensors have several advantages as small size, low power consumption
and the potential to be produced in a low priced batch fabrication manner. Nowadays,
chemical sensors are used in and optimised for many applications such as for example
the identification of purity, process and quality control, environmental analysis,
medical diagnosis [3], food evaluation and flavour and fragrance testing [4], but there
are still many applications remaining for which optimal sensors have not yet been
developed. Therefore, further research into the area of chemical sensors is required.

1.1 Motivation and scope of the work
The entire field of chemical sensors suffers from a common malady: The development
of chemically sensitive and selective interfaces is far behind the technology of the
physical transduction platforms, which translate energy from a chemical system to a
useful analytical signal [5]. Additionally, to meet the demands on chemical sensors the
sensing material must not only have interesting and useful interactions with the key
analytes, but they must be cheap and commercially viable in terms of
manufacturability, reproducibility, and longevity.
Inorganic sensing materials, e.g. metal oxides show good sensing properties [6] but
have the disadvantage to work at elevated temperatures only. The requi