Hümeyra Kılıç, Uğur Yahşi, Ayşe Yumak Yahşi, Cumali Tav  (Marmara University Physics Department)

For the different generations of PAMAM dendrimer macromolecules, the temperature-dependent change of structural free volumes will be investigated by the Positron Annihilation Lifetime Spectroscopy (PALS) technique. In the PALS technique, the observed spectrum will be divided into three components; the longest lasting of these components is the ortho-positronium (o-Ps) formed by the free pick-off of the molecular structure cavities. By examining the relationship of ortho-positronium (o-Ps) with temperature, especially the change in glass transition temperature, the difference in o-Ps annihilation parameters, heating, and cooling processes will be examined. As a result, we will establish a correlation between possible studies related to PAMAM and free volume.

Project

Marmara University BAPKO FEN-A-110718-0395, 11.07.2018 - 11.07.2020 (64.109 ₺).

Thesis

"Free Volume Investigation of PAMAM Dendrimers by Using Positron Annihilation Lifetime Spectroscopy," Hümeyra Kılıç, MSc in Physics, Marmara University, Institute for Graduate Studies in Science and Engineering, İstanbul (2019). Advisors: Doç. Dr. Ayşe Yumak Yahşi and Prof. Dr. Uğur Yahşi. (Thesis in Turkish) 

A. U. Kaya 1, G. Kosem 2, M. Y. Yener 2, C. TAV 2, U. Yahsi 2, K. Esmer 2

Ultrasonically agitated TiO₂ micro powder has been introduced in a PMMA matrix using the mechanical stirrer with a low mixing rate to produce the PMMA/TiO₂ composite films in the free-standing form. The filler contents of composites were selected between 0 and 7% (w/w), since TiO₂ particles cannot be suspended in chloroform at a higher rate than 7% of the filler content by ultrasonic agitation. Optical and dielectric properties of composites were determined by UV-visible and dielectric spectrometry. In the UV region, a 450% improvement of UV absorption was achieved in the TiO₂/PMMA composite compared with neat PMMA. Structural properties and hole-free volume of the composites were determined by X-ray diffractometer and Positron Annihilation Lifetime Spectroscopy (PALS), respectively. The particle size and homogeneous dispersion of the particles in the polymer matrix are related to the hole-free volume of the PMMA.

Keywords: PMMA, TiO₂, Te=2, composite, dielectric properties, UV-Visible, Free Volume, Positron Annihilation Lifetime Spectroscopy

Paper

"Structural and dielectrical properties of PMMA/TiO₂ composites in terms of free volume defects probed by positron annihilation lifetime spectroscopy," A. U. Kaya, G. Kosem, M. Y. Yener, U. Yahsi, C. Tav and K. Esmer, Polymers and Polymer Composites, 2020. (https://doi.org/10.1177/0967391120903533) 

Murat Yavuz Yener, Uğur Yahşi, Cumali Tav, M. Adil Umer, Fatih Dumludağ (Marmara University Physics Department)

Seyfullah Madakbaş, Vezir Kahraman, Emre Baştürk (Marmara University Chemistry Department)

Thermoplastic Polyurethane (TPU), dissolved in dimethylformamide at 80ºC, was doped with different amounts of Boron Nitride (BN). TPU/BN composites, prepared by using a solvent casting technique, were obtained in the form of films. Optical characterization of TPU/BN composites was done by FT-IR in the 400-4000 cm⁻¹ range; thermal by TGA under N₂ atmosphere and in the 30 to 750°C range with a 10°C min⁻¹ heating rate; and DTGA and DSC techniques—heating from 0 to 200°C, all at a heating rate of 5°C min⁻¹, and cooled with the same rate. Finally, structural characterization was done by SEM, and defects on the molecular level were studied using PALS.

Sample synthesis was found to be successful by FT-IR. The thermal stability of the TPU/BN composite was observed to increase with an increase in the amount of BN doping. SEM results indicated that BN particles were dispersed homogeneously throughout the polymer matrix for lower amounts doped, but doping levels higher than 7% result in agglomerations, causing a decrease in the thermal stability of the material. PALS studies revealed that o-Ps lifetime (free volume) increases in all composites with increasing temperature. Free volume intensity (hole fraction) increased till 1% doping weight for all the samples, after which it decreased relatively gently.

Thesis

Murat Yavuz YENER (MSc.), "Characteristic properties of thermoplastic polyurethane/boron nitride composite using thermal and positron annihilation lifetime spectroscopy techniques", Physics MSc. Thesis, Marmara University Institute of Pure and Applied Sciences, İstanbul 2017 (English).

Paper

“Effects of Boron Nitrite in Thermoplastic Polyurethane on Thermal, Electrical and Free Volume Properties,” F. Dumludag, M. Y. Yener, E. Basturk, S. Madakbas, V. Kahraman, M. A. Umer, U. Yahsi and C. Tav, Polymer Bulletin https://doi.org/10.1007/s00289-018-2560-2 Link to download 

Gonca ERDEMCİ, Bilgehan ÇOŞKUN, Cumali TAV, Uğur YAHŞİ (Marmara University Physics Department)

Hüseyin DELİGÖZ, S. Yılmaztürk, M. Yılmazoğlu (Chemical Preparation and Characteristics) (İstanbul University Chemistry Engineering Dept.)

Kemal ULUTAŞ, Deniz ULUTAŞ, Şahin Yakut (Electrical Conductivity Measurements)(İstanbul University Physics Dept.)

        Polymer-based ionic conducting materials are of important interest because of their potential applications in polymer electrolytes and membranes used as fuel cells. The ionic conductivity mechanism is explained with the free volume model. The free volume notion in polymers is significant to explain viscosity, ionic conductivity, glass transition temperature, and physical quantities such as mechanical and thermodynamic properties. In the study of Cohen and Turnbull and later Bamford et al., the transport phenomena in polymers have been stated in terms of free volume arising from the structural disorders in amorphous polymers. To interpret the ionic conductivity mechanism, Yahsi et al. have proposed a model in terms of temperature- and pressure-dependent hole fraction as a measure of free volume.On the other hand, the size and the amount of free volume can also be measured by ortho-positronium (o-Ps) lifetime using the Positron Annihilation Lifetime Spectroscopy (PALS), a unique tool to probe the holes.

            For this purpose, PVdF-co-HFP (polyvinylidene fluoride-co-hexafluoropropylene) was used as a polymer matrix in this work because of its high ionic conducting features and superior mechanical properties. Polymer electrolytes containing different types of lithium salt (LiClO₄, LiPF₆, and Li triflate) and various amounts of salt (1, 3, 5, 10, 15, and 20% (weight)) were prepared by the solution casting method. The changes of the nanoscopic free volume and hole fraction were investigated with respect to temperatures using the PALS technique. Eventually, the hole fraction has an inflection point at about a 0.03 molar ratio. The free volume increases when it is less than 0.03. It is the fact that the formation of polymer and salt complications, which occur between the fluorine groups in the polymer backbone and the Li⁺ ion in salt, retards the ordering of polymer crystalline and causes the formation of amorphous phases.

Key words: Positron Annihilation Lifetime Spectroscopy, Doppler Broadened Annihilation Radiation, Free Volume, PVdF-co-HFP.

Thesis

Bilgehan ÇOŞKUN (MSc.), "An Investigation of the Relationship Between Free Volume and Ionic Conductivity of Polymer-Salt Electrolytes By Using Positron Annihilation Lifetime Spectroscopy," Physics MSc. Thesis, Marmara University Institute of Pure and Applied Sciences, İstanbul 2009 (Turkish).

Gonca ERDEMCİ (MSc.), "The Correlation Between Conductivity and Free Volume in Polymer Electrolytes Using Positron Techniques," Physics MSc. Thesis, Marmara University Institute of Pure and Applied Sciences, İstanbul 2009 (Turkish).

Projects

• "Polymer-Ionic Salt Electrolytes: (I) Investigation of Conductivity-Free Volume Relations by Positron Annihilation Techniques (PALS) and (II) Usage in Fuel Cells of PEM Type," U. Yahşi, C. Tav, H. Deligöz, and K. Ulutaş, TÜBİTAK (107T133)—1 July 2007–1 July 2010.
• "Investigation of Nanometric Defects in Materials by Using Doppler Broadened Positron Annihilation Radiation with Double Detectors," C. Tav and U. Yahşi, Marmara University Scientific Research Projects Units (BAPKO), June 2006-June 2008.
• "Investigation and Characterization of Nanometric Defects of Thin Films by using Slow Positron Beams," C.Tav and U. Yahşi, the State Planning Organisation, Research & Development Projects, January 2006-December 2009.
• "Investigation of Nanometric Defects in Materials by using Doppler Broadened Positron Annihilation Radiation (DBAR)," C. Tav and U. Yahşi, TÜBİTAK (106T041), 3 May 2006-2 May 2009.

 Papers

1. “Dielectric properties and conductivity of PVdF-co-HFP/LiClO₄ polymer electrolytes”, K. Ulutaş, U. Yahsi, H. Deligöz, C. Tav, S. Yılmaztürk, M. Yılmazoğlu, G. Erdemci, B. Coşkun, Ş. Yakut, and D. Değer, Can. J. Phys. 96, 786–791 (2018). (https://doi.org/10.1139/cjp-2017-0678) Link
2. “Ionic Conductivity of PVdF-co-HFP with LiClO₄: Free Volume Effects Probing by Positron Annihilation Lifetime Spectroscopy”, U. Yahsi, H. Deligöz, C. Tav, K. Ulutaş, D. Değer, S.Yılmaztürk, G. Erdemci, B. Coşkun, M.Yılmazoğlu, Ş. Yakut, Radiation Effects and Defects in Solids (2019). https://doi.org/10.1080/10420150.2018.1552959

Uğur YAHŞİ and Cumali TAV  (Marmara University Physics Department)

Renwu John ZHANG (Department of Chemistry and Biochemistry, California State University, San Bernardino)

           The dependence of free volume on the nanometric structure of a polyurethane sample has been investigated for the broad range of temperature 18-373 K using positron annihilation lifetime spectrometer (PALS). Free volume and hole fraction deviations have been studied in five separate regions. Particularly the transitions of the glass state of polyurethane’s soft polyester-polyol and hard haxametylenediisocynate structures have been observed at Tg = 214.1 K and T'g = 339.4 K temperatures, respectively. The phase transition of the robust part is in satisfactory agreement with the DSC measurement given in the literature. Yet the existence of the trapped oxygen and water molecules on synthesizing the sample has been observed, and the effect of their phase transitions on the polyurethane structure has also been studied.

            A plateau has occurred under Tg at the glassy-state transition. It’s also been observed that the existence of oxygen in the plateau regime has caused the structural collapse due to the melting-state transition of oxygen and kept the plateau regime invariable while constructing new free volume. For T > Tg, the soft part of polyurethane has come off the glassy state; therefore, the size and number of the free volume have increased. It’s been expected that the glassy portion of the hard part of polyurethane has reached a plateau regime before. However, a structural collapse of the hard part of polyurethane has occurred, and restructuring of free volume has arisen. The transition from glassy state to plastic state has happened for T > T'g. Thermal energy at this state has caused the excitation and flexibility in the structure of hydrogen bonds of the hard part, and as a result, there is a slow growth in free volume size and no difference in the hole numbers.

Key Words: Positronium Annihilation Lifetime Spectroscopy (PALS), Free Volume in Polymers, Phase Transformation, Polyurethane

Projects

• "Investigation of Nanometric Defects in Macromolecular Structures by using Positron Annihilation Spectroscopy (PAS)," C. Tav and U. Yahşi, TÜBİTAK, 1 September 2000-August 2004.
• "Nanometric Defects in Macromolecular Structures by using Positron Annihilation Lifetime Spectroscopy," C. Tav and U. Yahşi, Marmara University Scientific Research Projects Units (BAPKO), January 2003-July 2004.
• "Investigation of Viscose and Viscoelastic Properties in Macromolecular Structures by using Positron Annihilation Spectroscopy," Uğur Yahşi, the State Planning Organization, Research & Development Projects, January 1998-December 2001.

Cumali TAV  (Marmara University Physics Department)

Serpil YILMAZTURK (Istanbul University, Engineering Faculty, Chemical Engineering)

Polyvinylidene fluoride-co-hexafluoropropylene (PVdF-co-HFP) with conducting polyaniline (PAni) was studied to measure free-volume intensity (), ortho-positronium (o-Ps) lifetime (), and Doppler momentum distribution parameters (S and W) as a function of temperature varying from 273 to 373 K and PAni weight percentage varying from 1% to 20% using the techniques of Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler Broadening Annihilation Radiation (DBAR). It has been observed that the free-volume intensity and the o-Ps lifetime increased sharply with PAni weight percentage up to 4%, after which they decreased slowly. DBAR momentum distribution parameters behaved similarly as well, and it’s assumed that in the studied samples one type of defect existed. The thermal expansion coefficient as a function of PAni weight was also extracted from PALS data.

Keywords: Positron Annihilation Lifetime Spectroscopy, Free Volume, Conducting Polymers, PAni, PVdF-co-HFP, Thermal Expansion Coefficient.

Paper

“Temperature and Weight Dependence of Free Volumes in Conducting Composite Polymer Using Positron Annihilation Techniques,” C. Tav and S. Yılmazturk, High Temperature Materials and Processes, 0334-6455, 30, 1-2, 193-198 (2011). (https://doi.org/10.1515/htmp.2011.028) Link