Materials Science and Engineering MS Thesis Defense by Enes Buz

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KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MATERIALS SCIENCE AND ENGINEERING

MS THESIS DEFENSE BY ENES BUZ

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Title: Evaluation of CdX (X = S, Se) Quantum Dots As Photoinitiator In Free Radical Polymerizations

Speaker: Enes Buz

Time: August 01, 2017, 11:00

Place: ENG-B21

Koc University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof. Havva F. Yağcı Acar (Advisor, Koc University)

Assoc. Prof. Uğur Ünal (Koc University)

Prof. Duygu Avcı (Bogazici University)

Abstract:

Free radical photopolymerization of vinyl monomers is highly popular since initiation can be performed at room temperature which is beneficial in especially medicine, paint and coating applications. Photoinitiators (PI) usually do have strong absorbance in the UV and therefore decompose to produce free radicals for initiation upon excitation in UV which creates a safety issue and limited penetration depth.  Lack of 100% decomposition, toxicity of the PI and/or degradation products, bleaching out of these species and need for a co-catalyst are additional problems which are the subjects of many current studies.

Quantum dots (QD) are luminescent semiconductor nanocrystals with size tunable optical and electrical properties due to quantum confinement and used widely in energy, medicine, fluorescent labeling, optical diagnosis and therapy. Their combination with polymers usually involve polymeric coatings on QDs to provide stability, functionalization and processability to QDs or blending QDs with polymers to introduce the optical and electrical properties of QDs to the polymers.

QDs generate an electron-hole pair which is the basis of their potential as a PI. Size tunable and continues absorbance of QDs allow excitation at different wavelength and hence render them as a potential long-wavelength initiators. There has been a very limited amount of reports on utilization of QDs as PIs within the last 20.   Results of these studies are usually conflicting with each other. General belief involves the need for a hole-scavenging alcoholic solvent and luminescence quenching for successful photoinitiation with QDs.

Motivation of this thesis work is four fold: 1. Understanding of the influence of QD type, size, surface quality and coating composition on PI efficiency in hydrophobic solvents; 2. Elucidating the initiation mechanism; 3.  Investigating QDs as long-wavelength PIs; 4. Producing QD/polymer nanocomposites in a simple way.

In the first part, colloidal and hydrophobic CdS QDs with different crystal size and particle quality, but with the same coating (oleic acid) were used as a PI in batch photopolymerization of MMA under UV-excitation (360 nm) wherein the QD concentration and reaction time on monomer conversion were studied.   Kinetics of the photopolymerization and initiation in the visible range were studied with photo-DSC using PEGDA and HDDA as monomers.  Studies indicate that more than the size of the nanocrystals, surface quality and defect states differentiate the initiation efficiency of QDs. Small, colloidally stable and luminescent CdS/PMMA were produced. Decarboxylation of Oleic acid upon photoexcitation was suggested as the source of initiating radical. In the second part, octadecylamine coated CdSe and CdSe-ZnS core-shell QDs were studied to shed light on the initiation mechanism further. However, successful photopolymerization were not achieved. Possible reasons and suggestions for future studies are provided in the last part.