Nowadays nanoscale thin polymer films are widely used in many fields like coatings, membranes, sensors, electronic devices and so on. Meanwhile, a lot of research work has evidenced the fact that many physical properties (glass transition, crystallization, dewetting, physical aging, etc.) of ultrathin polymer films show strong deviations from their bulk behavior. Since the aforementioned properties of polymer are closely related to their application and functionality, the discrepancies motivated us to obtain a more complete understanding of how nanoscale confinement affects the physical properties of polymer.
The research work presented in this thesis is focused on understanding how the free surface (air-polymer interface), the polymer-substrate interface and the film thickness influence the glass transition temperature (Tg) and the related segmental dynamics (α-relaxation process) in both homopolymers and miscible polymer blends of thin films. Complementary experimental techniques including Differential Scanning Calorimetry (DSC), Capacitive Scanning Dilatometry (CSD), Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS) have been used to investigate the glass transition of thin polymer films from both the thermodynamic and the kinetic point of view.
In the thesis the film thickness dependence of Tg and segmental dynamics of different thin polymer films have been investigated. For ultrathin polycarbonate (PC) films capped between two aluminum (Al) layers an increase of both the glass transition temperature (Tg) and Vogel temperature (T0) with decreasing film thickness (d) was observed when the thickness became lower than 20 nm. The segmental relaxation time at a fixed temperature was found to increase for the ultrathin PC film of 19 nm measured by BDS, whereas no thickness dependency of the segmental dynamics was detected within the experimental error limit for the PC films supported on silicon dioxide (SiO2) (10-192 nm) in the SHS measurements. These properties are discussed in terms of the thin film geometry and the relevant interfacial interaction between the polymer and the substrate. In the case of thin polystyrene (PS) films with high molecular weight (Mw), Tg is decreasing with reducing film thickness while the segmental dynamics is independent of film thickness. Moreover, the effects of the Mw and the annealing protocol performed on thin PS films on their Tg and segmental dynamics is studied. In the part of thin poly(vinyl methyl ether) (PVME) films, no thickness dependence of the segmental dynamics was observed in the SHS measurements. The last part of the thesis was concentrated on the thin films of a miscible polymer blend, PS/PVME with the weight fraction of 50/50. It was observed that the segmental dynamics became faster with reducing the film thickness. This phenomenon is explained in terms of surface enrichment of PVME in the polymer blend system where PVME has a lower surface energy than PS. The segmental dynamics of the PVME-enriched free surface layer are faster than the bulk dynamics. Such free surface effect becomes so predominant with reducing the film thickness that it affects the segmental dynamics of the whole films detected by SHS using differential AC chip-based calorimetry. X-ray photoelectron spectroscopy (XPS) was used to probe the surface composition in order to confirm such surface enrichment phenomena.

ACKNOWLEDGMENTS IX

ABSTRACT XI

ZUSAMMENFASSUNG XIII

1 INTRODUCTION 1

2 BACKGROUNDS 5

2.1 Glass Transition and Segmental Dynamics in Bulk Homopolymers 5

      2.1.1 Glass Formation and the Glass Transition Temperature 5

      2.1.2 Segmental Dynamics 6

      2.1.3 Models of the Glass Transition 10

      2.1.4 Dynamic Heterogeneity 12

2.2 Glass Transition and Segmental Dynamics in Bulk Miscible Polymer Blends 13

      2.2.1 Miscibility of Binary Polymer-Polymer Blends 13

      2.2.2 Dynamic Heterogeneity in Miscible Polymer Blends 14

      2.2.3 Surface Enrichment 19

      2.2.4 Theories of Segmental Dynamics in Miscible Polymer Blends 20

2.3 Glass Transition and Segmental Dynamics in Thin Polymer Films 22

      2.3.1 The Glass Transition Temperature of Thin Polymer Films 22

      2.3.2 The Segmental Dynamics in Thin Polymer Films 27

      2.3.3 The Glass Transition Temperature and Segmental Dynamics in Miscible Polymer Blend Films 28

3 PRINCIPLES OF EXPERIMENTAL TECHNIQUES 29

3.1 Broadband Dielectric Spectroscopy 29

      3.1.1 Electrostatics 29

      3.1.2 Dielectric Relaxation 31

      3.1.3 Analysis of Dielectric Relaxation Spectra 34

      3.1.4 Fitting HN Function to the Experimental Results 37

3.2 Specific Heat Spectroscopy 38

      3.2.1 Complex Heat Capacity 38

      3.2.2 Differential AC Chip-based Calorimetry 39

3.3 Capacitive Scanning Dilatometry 41

3.4 Differential Scanning Calorimetry 42

4 EXPERIMENTAL SECTION 43

4.1 Methods 43

      4.1.1 Broadband Dielectric Spectroscopy 43

      4.1.2 Specific Heat Spectroscopy 43

      4.1.3 Capacitive Scanning Dilatometry 43

      4.1.4 Differential Scanning Calorimetry 43

      4.1.5 Spin-coating 44

      4.1.6 Annealing 44

      4.1.7 Metal Deposition 44

      4.1.8 Atomic Force Microscopy 44

      4.1.9 Contact Angle Measurement 45

      4.1.10 X-ray Photoelectron Spectroscopy 45

4.2 Materials 45

      4.2.1 Poly(bisphenol A carbonate) 45

      4.2.2 Polystyrene 46

      4.2.3 Poly(vinyl methyl ether) 47

      4.2.4 PS/PVME (50/50 wt%) 48

4.3 Sample Preparation 49

      4.3.1 Sample Preparation for Dielectric Measurement 49

      4.3.2 Sample Preparation for Calorimetric Measurement 51

5 RESULTS & DISCUSSION 53

5.1 Glass Transition of Ultrathin Poly(bisphenol A carbonate) Films 53

      5.1.1 Broadband Dielectric Spectroscopy on Thin Poly(bisphenol A carbonate) Films 53

      5.1.2 Specific Heat Spectroscopy on Thin Poly(bisphenol A carbonate) Films 68

      5.1.3 Discussion on the Dielectric and Calorimetric Results in Terms of Interfacial Interaction 75

5.2 Glass Transition of Ultrathin Polystyrene Films 80

      5.2.1 Glass Transition Temperature Depression and Invariant Segmental Dynamics 80

      5.2.2 Role of Molecular Weight and Annealing Protocol 85

5.3 Glass Transition of Ultrathin Poly(vinyl methyl ether) Films 93

5.4 Glass Transition of Ultrathin Films of A Miscible Polymer Blend 96

6 CONCLUSIONS 103

REFERENCES 106

LIST OF ABBREVIATIONS, SYMBOLS AND CONSTANTS 115

LIST OF PUBLICATIONS 117

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