Fluorescence Spectroscopy to Investigate the Effect of End-tethered Chains and Loops on the Local Glass Transition Temperature of Polystyrene Thin Films Restricted; Files Only

Abstract

I investigated the effect of grafted monocarboxy-terminated polystyrene (PS-COOH) chains and dicarboxy-terminated polystyrene (HOOC-PS-COOH) loops grafted to silica substrates on the local glass transition temperature ($T_g$) of 12 nm thick PS layers intermixed with the grafted polymers, and capped by a 590 nm bulk PS layer. $T_g$ was measured via fluorescence spectroscopy, using pyrene dye copolymerized to PS as the probe. Temperature of the sample was ramped down steadily at 1 $^o$C/min. $T_g$ was located by identifying the change in slope of the linear dependence of fluorescence intensity vs. temperature. Overall, I found that the local $T_g$ next to 50 and 100 kg/mol end-grafted chains increases by approximately $41 \pm 2^o$C independent of the grafting density compared to the $T_g$ next to bare silica substrates, which is equal to $T_{g,bulk}$. These results contributed to a publication with James Merrill, a graduate student of our lab [J.H. Merrill, R. Li and C.B. Roth, \textit{ACS Marco Lett.}, \textbf{2023}, 12, 1-7]. As for the loops, I found that the local $T_g$ next to 50 and 100 kg/mol grafted loops increase by $36 \pm 2 ^o$C and $27 \pm 2 ^o$C compared to $T_{g,bulk}$, respectively, independent of the intermixing time between the 12 nm thick pyrene-labeled PS layer and the grafted loops. In addition, together with a current graduate student in our lab, James Merrill, we found a broad transition when analyzing the intensity vs. temperature data collected over a wide temperature range, which was characterized by identifying a lower $T_g$, denoted as $T_g^{mid}$. $T_g^{mid}$ is still elevated compared to $T_{g,bulk}$, and is around $115 ^o$C next to 50 and 100 kg/mol grafted loops, and varies between $113 ^o$C and $130 ^o$C for 50 and 100 kg/mol grafted chains. The difference $\Delta T_g = T_g - T_g^{mid}$ may inform about the breadth of glass transition, but its physical significance is yet to be determined.

Table of Contents

Chapter 1: Introduction and Background.......................... 1

1.1 Introduction of Polymers and the Polymer Glass Transition . . . . . . . . . . . . . 1

1.2 The Effect of a Modified Interface on the Polystyrene Glass Transition Temperature .....4

1.2.1 Literature of How Grafted Chains Impact Polystyrene Glass Transition . . . 5

1.2.2 Impact of How Adsorbed Layers Impact the Polystyrene Glass Transition . 7

1.2.3 Summary of Previous Literature ....................... 10

1.3 Goals of the Project.................................. 10 

Chapter 2: Experimental Methods.............................. 12

2.1 Preparation of the Multilayer Sample......................... 12

2.1.1 Preparation of Grafted Layers ........................ 12

2.1.2 Preparation of Pyrene-labeled PS Layer, the Bulk PS Layer and Their Assembly..................................... 14

2.1.3 Measuring the Thicknesses of the Layers via Ellipsometry . . . . . . . . . 15

2.2 Fluorescence Spectroscopy to Probe the Glass Transition Temperature . . . . . . . 17

2.3 Our Fluorescence Spectroscopy Method of Sensing the Glass Transition . . . . . . 21 

Chapter3: Results and Discussion.............................. 25

3.1 Control Experiments ................................. 25

3.2 Effect of End-Grafted Chains and Loops on the Local Glass Transition Temperature ..........27

3.2.1 Effect of Grafted Chains on the Local Glass Transition Temperature . . . . 27

3.2.2 Effect of Grafted Loops on the Local Glass Transition Temperature . . . . 30

3.3 Discovery of Broad Glass Transition Over Extended Temperature Range . . . . . . 34

3.3.1 The ”Midpoint” Glass Transition Temperature of Grafted Chains . . . . . . 37

3.3.2 The Midpoint Glass Transition Temperature of Grafted Loops . . . . . . . 37 

Chapter 4: Conclusion and Further Works ......................... 40

4.1 Summary of the Results................................ 40

4.2 The Next Steps of the Project............................. 41

References ........................................... 43 

About this Honors Thesis

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School	Emory College
Department	Physics
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Physics, Condensed Matter Chemistry, Polymer
Keyword	Grafted layer Polystyrene glass transition
Committee Chair / Thesis Advisor	Connie B. Roth, Emory University
Committee Members	Jed Brody, Emory University Jennifer Rieser, Emory University

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