SURFACE MODIFICATION OF SILICON THROUGH THERMAL ANNEALING AND RINSING OF SOLVENT CAST POLYSTYRENE FILMS A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Polymer Engineering Steven V. Kalan August, 2011 SURFACE MODIFICATION OF SILICON THROUGH THERMAL ANNEALING AND RINSING OF SOLVENT CAST POLYSTYRENE FILMS Steven V. Kalan Thesis Approved: Accepted: Co-Advisor Department Chair Dr. Kevin Cavicchi Dr. Sadhan Jana, Ph.D Co-Advisor Dean of the College Dr. Alamgir Karim Dr. Stephen Z.D. Cheng, Ph.D. Faculty Reader Dean of the Graduate School Dr. Mark Soucek Dr. George R. Newkome, Ph.D. Date i ABSTRACT Surface modification is an important process for many applications. It has been found that an ultrathin polymer film can be generated on silicon substrates with a native oxide layer by spin-coating, thermally annealing, and washing films of thiol terminated polystyrene (PS-SH) or anionically polymerized polystyrene (PS). This is a useful modification technique as it requires little pretreatment of the substrate or specialized polymerization chemistry for the case of PS films. These ultrathin films were characterized using contact angle (CA), x-ray reflection (XR), and x-ray photoelectron spectroscopy (XPS). Experimental results indicated that a thin polymer film formed characteristic of the grafted polymer for both PS-SH and PS due to the physical adsorption of the PS chains. It is suggested that the benzene rings of polystyrene all share a bond with the silicon surface creating a relatively overall strong bond from the polymer to the silicon wafer. Differences in the PS-SH and PS films are consistent with additional end- tethering of the PS-SH chains. Spin coated films underwent different levels of rinsing with multiple solvents. After all levels of rinsing, the polymer was still present on the surface, indicating a relatively strong physical bond between the polymer and silicon oxide. Control experiments using non-annealed versus annealed polystyrene polymers showed that grafting depends on the mobility of the chains during thermal annealing of ii the polystyrene films. Two other samples, polytert-butyl styrene (PtBS) and block copolymer poly (styrene-b-dimethylsiloxane) (PS-b-PDMS), were also used and compared with PS to show the effects of thermal annealing of PS to a silicon surface. XR results show an increase in residual layer thickness with molecular weight (Mw). Analysis of water contact angle of PS residual layers as a function of annealing time and annealing temperature illustrates directly proportional relationships. Further surface modification analysis by ultra violet light and ozone (UVO) treatment demonstrates that the surface energy can be roughly tuned for a small range of UVO treatment times. iii ACKNOWLEDGEMENTS I would like to thank first and foremost my advisors Dr. Cavicchi and Dr. Karim for their continued support and guidance. I also would like to thank Dr. Christopher Stafford for his role at NIST in collecting valuable data for XR and XPS analysis. Mr. Jon Page for allowing me the time to use the Polymer Science contact angle equipment. My fellow group members for their admirable kindness and time they gave to demonstrate or help with aspects of my research, especially Yuqing Liu whom would drop what he was doing at any time to help another. To these people I express my deepest gratitude, for I would not have accomplished this without them. iv TABLE OF CONTENTS Page LIST OF TABLES ........................................................................................................................... vii LIST OF FIGURES ......................................................................................................................... viii CHAPTER I-INTRODUCTION ........................................................................................................................... 1 1.1 Covalently Bonded Polymer Brushes and Mats .................................................................... 7 1.1.1 Grafting-To Approach .................................................................................................. 11 1.1.2 Grafting-From Approach .............................................................................................. 15 1.2 Non-Covalent Bonding of Polymer Brushes and Mats ........................................................ 21 1.2.1 Formation of Polymer Mats by Non-Covalent Bonding from Solution ........................ 21 1.2.2 Guiselin Brush .............................................................................................................. 22 1.2.3 Polymer Brush Through Non-Covalent Bonding .......................................................... 24 1.2.4 Polymer Adsorption Through Thermal Annealing ....................................................... 26 II-EXPERIMENTAL SECTION ....................................................................................................... 28 2.1 Materials .............................................................................................................................. 28 2.1.1 Anionically Polymerized Polystyrene ............................................................................ 28 2.1.2 RAFT Polymerized Polystyrene ..................................................................................... 29 2.1.2.1 RAFT Agent ............................................................................................................. 29 2.1.2.2 Styrene Monomer/n-Butylamine ........................................................................... 30 2.1.3 PS-b-PDMS .................................................................................................................... 30 2.1.4 PtBS .............................................................................................................................. 30 2.1.5 Silicon Substrate ............................................................................................................ 30 2.2 Experimental Process ........................................................................................................... 31 2.2.1 RAFT Polymerization of Polystyrene ............................................................................. 31 2.2.2 Aminolysis of RAFT Polymerized Polystyrene ............................................................... 32 2.2.3 Preparation of samples ................................................................................................. 33 v 2.2.3.1 Sample Preparation ............................................................................................... 33 2.2.3.2 Levels of Rinsing ..................................................................................................... 34 2.3 Equipment/Measurement Methods .................................................................................... 34 2.3.1 Gel Permeation Chromatography (GPC) ....................................................................... 34 2.3.2 Contact Angle (CA) ........................................................................................................ 35 2.3.3 Optical Microscopy (OM) .............................................................................................. 36 2.3.4 X-ray Photon Spectroscopy (XPS) & X-ray Reflection (XR) ............................................ 36 III-SURFACE MODIFICATION OF SILICON OXIDE VIA THIOL TERMINATED PS (COVALENT) ................................................................................................................................. 37 3.1 Introduction ........................................................................................................................ 37 3.2 Materials and Water Contact Angle .................................................................................... 38 3.3 Effect of Different Levels of Rinsing .................................................................................... 42 3.4 Effect of Different Solvents ................................................................................................. 44 3.5 Topological Changes (OM) .................................................................................................. 46 3.6 Dewetting (OM) .................................................................................................................. 50 3.7 Control Experiments (PS-SH vs PS_RAFT vs PS) .................................................................. 54 3.8 Summary ............................................................................................................................. 56 IV-SURFACE MODIFICATION OF SILICONO OXIDE VIA THERMALLY ANNEALED PS (NON-COVALENT) ........................................................................................................................ 58 4.1 Introduction ........................................................................................................................ 58 4.2 Effect of Different Solvents and Different Levels of Rinsing ............................................... 59 4.3 Adsorption of PS vs PtBS vs PS-b-PDMS .............................................................................. 64 4.4 X-Ray Reflectivity (XR) and X-ray photon spectroscopy (XPS) ........................................ 65 4.5 Effect of Annealing Time and Annealing Temperature ....................................................... 69 4.6 UVO Surface Energy Modification......................................................................................