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Polipirol Miktarının; Pan/ppy Kompozit İnce Filmlerin Kristal Yapısı, Mekanik Ve Elektriksel Özellikleri Üzerindeki Etkisinin İncelenmesi

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ĐSTABUL TECHICAL UIVERSITY  ISTITUTE OF SCIECE AD TECHOLOGY 

A IVESTIGATIO OF POLYPYRROLE COTET O CRYSTALLIITY, MECHAICAL

AD ELECTRICAL PROPERTIES OF PA/PPY THI COMPOSITE FILM

M.Sc. Thesis by Gamze BAKKALCI

Department : Polymer Science and Technology Programme: Polymer Science and Technology

Supervisor : Prof. Dr. A. Sezai SARAÇ

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ĐSTABUL TECHICAL UIVERSITY  ISTITUTE OF SCIECE AD TECHOLOGY 

M.Sc. Thesis by Gamze BAKKALCI

515061012

Date of submission : 5 May 2008 Date of defence examination: 11 June 2008

Supervisor (Chairman): Prof. Dr. A. Sezai SARAÇ Members of the Examining Committee Assoc. Prof.Dr. Esma SEZER

Assoc. Prof.Dr. Hale KARAKAŞ

JUE 2008

A IVESTIGATIO OF POLYPYRROLE COTET O CRYSTALLIITY, MECHAICAL

AD ELECTRICAL PROPERTIES OF PA/PPY THI COMPOSITE FILM

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Tez Danışmanı : Prof.Dr. A. Sezai SARAÇ Diğer Jüri Üyeleri Doç.Dr. Esma SEZER

Doç.Dr. Hale KARAKAŞ

POLĐPĐROL MĐKTARII; PA/PPY KOMPOZĐT ĐCE FĐLMLERĐ KRĐSTAL YAPISI, MEKAĐK VE

ELEKTRĐKSEL ÖZELLĐKLERĐ ÜZERĐDEKĐ ETKĐSĐĐ ĐCELEMESĐ

ĐSTABUL TEKĐK ÜĐVERSĐTESĐ  FE BĐLĐMLERĐ ESTĐTÜSÜ 

YÜKSEK LĐSAS TEZĐ Gamze BAKKALCI

515061012

Tezin Enstitüye Verildiği Tarih : 5 Mayıs 2008 Tezin Savunulduğu Tarih : 11 Haziran 2008

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ACKOWLEDGEMET

Firstly, I would like to thank my advisor, Prof.Dr. A Sezai SARAÇ, for his encouragement and guidance during my studies.

I would like to thank Assoc.Prof.Dr. Esma SEZER for her guidance and advices. I would like to thank to my colleagues to Koray YILMAZ, C. Metehan TURHAN, Aslı GENCTURK , Sibel SEZGĐN, Şebnem ĐNCEOĞLU, Bilge KILIC and for their invaluable advices, patience and friendship during my MSc study.

I would thank to Đbrahim ĐNANÇ for performing SEM measurements with his friendship also acknowledged.

I also want to thank to Esra OZGUL and Hacer DOLAŞ for their support, encouragement and sincerely friendship.

Especially, I would like to thank Ş. Şebnem TAYYAR and for my homemate Sedef ÇAKIR for their valuable helps and great friendships.

Additionally, I would like to offer the most greatful thanks to Çağdaş KAYA for being beside me whenever I need and endless thanks for his patience and understanding.

And at the end; I am really appreciated to my family for their support about everything at every stage of my life.

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ii TABLE OF COTETS LIST OF ABREVIATOS iv LIST OF TABLES v LIST OF FIGURES vi SUMMARY ix ÖZET xii 1 ITRODUCTIO 1 2 THEORETICAL PART 4 2.1 Conducting Polymers 4

2.1.1 Insolubility and improcessability 4

2.1.2 Mechanical properties 4

2.1.3 Π interaction 5

2.2 Redox polymerization 5

2.3 Synthesis of polypyrrole 6

2.4 Acrylonitrile 6

2.4.1 Acrylonitrile polymerization and copolymerization 7

2.4.2 PAN properties 9

2.4.3 PAN/PPy interactions 9

2.4.4 Applications 9

2.5 Physical categories of polymers 11

2.5.1 Semicrystalline State: 12

2.5.2 Wide-Angle X-Ray Diffraction 13

2.5.3 Bragg's Law 14

2.6 Mechanical Properties 16

2.6.1 Influence of bonding on mechanical properties 16

2.6.2 Dynamic Mechanical Analysis 18

2.6.3 Elasticity and Viscous Flow Behaviors 20

2.6.4 Clamps of Dynamic Mechanical Analyzer 25

2.6.5 Film Tension Clamp Equations 27

2.6.6 DMA Modulus Parameters 27

2.6.7 The Stress-Strain Curve 29

3 EXPERIMETAL WORK 36

3.1 Materials 36

3.2 Preparation of thin films 36

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iii

3.4 Scanning Electron Microscopy (SEM) 37

3.5 X-Ray Diffraction 37

3.6 Electrochemical Impedance Spectroscopy 37

4 RESULTS AD DISCUSSIO 39

4.1 Processing approaches 39

4.1.1 Bulk processing 42

4.1.2 Suspension processing 43

4.1.3 Solution processing 44

4.2 Thin film formation 45

4.3 DMF effect on PAN structure 45

4.4 ATR-FTIR Characterization of PAN/PPy composites. 47

4.5 Pyrrole effect on structural properties of Polyacrylonitrile 56

4.6 Characterization of PAN/PPy composite thin films: 58

4.6.1 Dynamic mechanical behaviour of solution polymerized PAN/PPy composites 59

4.6.2 Stress – Strain Curves 64

4.6.3 Dynamic mechanical behaviours of Bulk composites. 71

4.7 Electrochemical Impedance Spectroscopy 74

4.8 Sem Characterization 76

4.8.1 Deformation characterization 77

5 COCLUSIO 78

REFERECES 81

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iv

ABBREVIATIOS

CA : Cerium (IV) ammonium nitrate PA : Polyacrylonitrile

A : Acrylonitrile

Py : Pyrrole

PPy : Polypyrrole

EIS : Electrochemical Impedance Spectroscopy DMA : Dynamic Mechanical Analyzer

XRD : X-Ray Diffraction

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v LIST OF TABLES

Page No

Table 4.1: The effect of initiator concentration on polymerization. ... 43

Table 4.2: Solvent and monomer concentration effect on polymerization. ... 44

Table 4.3: Monomer, initiator and solution concentration used in solution processing ... 45

Table 4.4: ATR-FTIR absorption bands and peak assignment table for the neat DMF ... 48

Table 4.5: ATR-FTIR absorption bands of bulk copolymerization. ... 50

Table 4.6: ATR-FTIR absorption bands of suspension polymerization. ... 53

Table 4.7: Comparison of Tg values obtained from Loss modulus and tan δ. ... 60

Table 4.8: Storage Modulus, Loss Modulus, Tan Delta and Complex viscosity obtained from DMA of composites, P0, P1, P2, P3 at 25 ˚C ... 63

Table 4.9: Temperatuure effect on toughness, young’s modulus and strain ... 65

Table 4.10: Toughness, young’s modulus and Strain values changing with Py content ... 66

Table 4.11: Storage modulus, complex viscosity loss modulus Tan delta and tg values for P0,P1,P2,P3 ... 70

Table 4.12: Storage modulus, complex viscosity, loss modulus and Tg values for bulk composites. ... 74

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vi LIST OF FIGURES

Page No Figure 2.1: Mechanistic pathways for the thermal degradation of PAN and reactions

involved in the stabilization of PAN fibres ... 11

Figure 2.2: Schematic representation of the semicrystalline state. ... 12

Figure 2.3: Scattering patterns from: a) single crystal and b) unoriented semicrystalline sample. ... 13

Figure 2.4: Diffusion Rayleigh diffraction ... 14

Figure 2.5: Bragg's Law and Diffraction ... 14

Figure 2.6: According to the 2θ deviation, the phase shift causes constructive (left figure) or destructive (right figure) interferences ... 15

Figure 2.7: a) ınteratomic potential energy vs. spacing. b) the effect of thermal vibration on interatomic spacing. c) the force vs. spacing derived by finding F=dE/dr, where r is interatomic spacing. d) the slope of the force vs. spacing relationship approximately linear for small strains, is proportional to Young’s modulus. ... 17

Figure 2.8: Traditional Dynamic Mechanical Analyzer Design ... 20

Figure 2.9: Hooke’ Law and Elastic Deformation ... 21

Figure 2.10: Newton’s Law and graph of Viscous Flow Behaviour ... 22

Figure 2.11: Purely elastic and viscous response functions ... 23

Figure 2.12: Function of viscoelastic response ... 23

Figure 2.13: Tension Clamp of Dynamic Mechanical Analyzer... 23

Figure 2.14: Schematic diagram for DMA measuring ... 23

Figure 2.15: Clamps of Dynamic Mechanical Analyzer ... 26

Figure 2.16: a) Resilience and toughness of modulus of stress strain curves b) A typical stress strain curve ... 29

Figure 2.17: Various regions and points of stress strain curve. ... 30

Figure 2.18: The electrical behavior is simulated by a suitable combination of RC circuits: R = resistivity, C = capacity...32

Figure 2.19: An equivalent circuit representing each component at the interface and in the solution during an electrochemical reaction is shown for comparison with the physical components. Cd, double layer capacitor; Rp, polarization resistor; W, Warburg resistor; Rs, solution resistor...32

Figure 2.20: The dc plotted as a function of overpotential according to the Butler– Volmer equation (solid line)...34

Figure 0.1 Electrochemical Impedance Spectroscopy Set-up……...…...………37

Figure 4.1: Temperature initiated redox initiator ... 39

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vii

Figure 4.3: Most probable tentative copolymerization mechanism of AN & Py with Ce(IV) and in DMF. ... 42 Figure 4.4: DMF structure and molecular interactions and complex formation with PAN ... 46 Figure 4.5: ATR-FTIR spectrum for neat DMF ... 47 Figure 4.6: ATR-FTIR spectrum for Py polymerization with CAN in DMF. ... 48 Figure 4.7: ATR-FTIR spectrum of bulk copolymerization of AN and Py; B0, B1, B2, B3. ... 49 Figure 4.8: Absorbance vs pyrrole content. Py (% 20 (w/w) Py, % 40 (w/w) Py, % 60 (w/w) Py.) in Bulk polymerization. ... 50 Figure 4.9: SEM characterization of a) bulk polymerization of acrylonitrile , b) bulk copolymerization of AN and 0.08M Py . ... 51 Figure 4.10: ATR-FTIR spectrum of bulk copolymerization of AN and N-MethylPy BMP1, BMP2, BMP3, BMP4. ... 51 Figure 4.11: ATR- FTIR spectrum of suspension polymerization of AN and Py in DMF. Py content S1, S2, S3, S4. ... 52 Figure 4.12: Absorbance vs pyrrole content. Py (50µl Py, 100µl Py, 150µl Py.) polymerization in DMF. ... 53 Figure 4.13: SEM characterization of 100µl pyrrole content

poly(pyrrole-co-acrylonitrile) in DMF. ... 54 Figure 4.14: ATR- FTIR spectrum of thin films PAN/DMF ; Py content 40 µl , 50 µl, 60 µl, 75 µl, 200 µl. ... 54 Figure 4.15: SEM image of PAN thin film. ... 55 Figure 4.16: ATR- FTIR spectrum of products prepared by different processes.... 55 Figure 4.17: XRD result of PAN in DMF ... 56 Figure 4.18: XRD results of copolymers obtained process with no solvent. ... 57 Figure 4.19: XRD diagram for pure PPy ... 58 Figure 4.20: Variations of storage modulus of composite thin films, P0, P1, P2, P3

... 59 Figure 4.21: Variations of loss modulus of composite thin films, P0 (%5PAN) , P1 (100 µl Py), P2 (150 µl Py), P3 (200 µl Py) ... 59 Figure 4.22: Tan Delta values of composites, P0 (%5PAN) , P1 (100 µl Py), P2 (150 µl Py), P3 (200 µl Py) ... 60 Figure 4.23: Plots of complex viscosity of composite thin films, P0 (%5PAN) , P1 (100 µl Py), P2 (150 µl Py), P3 (200 µl Py) ... 62 Figure 4.24: Plots of log storage modulus changing with temperature for composite thin films, P0 (%5PAN) , P1 (100 µl Py), P2 (150 µl Py), P3 (200 µl Py) ... 62 Figure 4.25: The Py content effect on storage modulus tan delta, loss modulus and complex viscosity. ... 64 Figure 4.26: Temperature effect on Stress - Strain of %5 PAN 250 µl Py composite

T = 25,50,75,100˚C ... 64 Figure 4.27: Stress- strain curve for PAN, PP1, PP2, PP3 thin films at 25˚C ... 66 Figure 4.28: Young’s modulus values with strain increase ... 67

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Figure 4.29: Temperature dependence of storage modulus of PAN, PP1, PP2, PP3 thin films. ... 67 Figure 4.30: Variations of loss modulus of composite thin films, P0 (%8PAN) , P1 (100 µl Py), P2 (150 µl Py), P3 (200 µl Py) ... 68 Figure 4.31: Temperature dependence of tan delta values of PAN, PP1, PP2, PP3.68 Figure 4.32: Temperature dependence of Complex viscosity for P0, P1, P2, P3 P4.69 Figure 4.33: Temperature dependence of Log Storage Modulus of PAN, PP1, PP2, PP3, PP4 ... 70 Figure 4.34: Variations of storage modulus of composite thin films prepared from bulk polymerization polymer composites . B1, B2, B4, B6. ... 71 Figure 4.35: Variations of loss modulus of composite thin films prepared from bulk polymerization polymer composites . B1, B2, B4, B6. ... 71 Figure 4.36: Temperature dependence of Tan Delta values of bulk composites ... 72 Figure 4.37: Temperature dependence of Complex viscosity of bulk composites. .. 72 Figure 4.38: Temperature dependence of Log Storage Modulus of bulk composites.

... 73 Figure 4.39: Nyquist plot of 150µl and 350 µl Py content composite thin films. Both

measured and calculated plots are shown...74 Figure 4.40: Phase Angle plots for both 150 and 350µl Py included thin film

composites………..75 Figure 4.41: Bode Phase graph for both 150 and 350µl Py included thin film

composites………..75 Figure 4.42: Capacitance plot for both 150 and 350µl Py included thin film

composites………..76 Figure 4.43: SEM image of a) PAN thin film b) %8 PAN/200µl PPy exposed to

300˚C in DMA. ... 76 Figure 4.44: SEM image of %8 PAN/200µl PPy after applied controlled force .. 77 Figure 4.45: SEM image closer view of image %8 PAN/200µl PPy after applied controlled force. ... 77

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ix

A IVESTIGATIO OF POLYPYRROLE COTET O

CRYSTALLIITY, MECHAICAL AD ELECTRICAL PROPERTIES OF PA/PPY THI COMPOSITE FILM

SUMMARY

Conducting polymers exhibit excellent electrical properties; however, the common usage of this material has been restricted due to the poor processability and the lack of stability. Various techniques, such as the modification of the monomer structure, the utilization of a soluble precursors, and the formation of a blend or composite, have been introduced to enhance the processability. The composite formation is one of the simplest methods for providing the processability of a conducting polymer. Electrically conductive fabrics are mainly used for industrial materials like filters, deelectrifying and electro-magnetic interference shield materials, and special purpose clothing, which is dust and germ free. Demand for them has increased strongly in recent years. The reason is that electrostatic and electromagnetic interference have proliferated and become common place due to human lifestyle changes and the increasing sophistication of industrial technology.

PAN is one of the important raw materials for the production of carbon fibers. When PAN is heated in vacuum or air to over 200˚C, a number of reactions take place, resulting in the formation of some by-products which are liberated as volatiles. The carbon ring structure produced during the carbonization of PAN fibers under tension has a crystalline form.

In the case of a conducting composite prepared by the simple coating of conducting polymers onto the surface of the matrix polymer, any interaction between the two components usually does not exist and the conducting polymer can be easily removed from the matrix polymer by repeated friction to result in the ultimate failure of the electrical property. In this regard, it should be stressed that the interaction between the two components should exist in order to provide the desirable properties. In literature, the effect of ionic groups in the matrix on the conducting composite formation was studied in terms of the morphological, electrical, and thermal properties. The environmental stability of the electrical conductivity was also monitored as a function of time and temperature. The application of this type of preparation technique can be utilized in the manufacturing of high performance conducting textile materials.

Synthesis of conducting graft and block copolymers were one of the effective ways to improve poor properties of conducting polymers. The addition of an appropriate functional group to a conventional polymer is another way to impart new properties to conducting polymers. However, a conducting polymer exhibits poor compatibility with common polymers due to the chain rigidity originated from an extended conjugate double bond. In the case of a conducting composite prepared by the simple coating of conducting polymers onto the surface of the matrix polymer, any

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interaction between the two components usually does not exist and the conducting polymer can be easily removed from the matrix polymer by repeated friction to result in the ultimate failure of the electrical property. In this regard, it should be stressed that the interaction between the two components should exist in order to provide the desirable properties. In literature, the effect of ionic groups in the matrix on the conducting composite formation was studied in terms of the morphological, electrical, and thermal properties. The environmental stability of the electrical conductivity was also monitored as a function of time and temperature. The application of this type of preparation technique can be utilized in the manufacturing of high performance conducting textile materials.

The objective of this study is to prepare PAN/PPy thin film composites using different types of polymerization processes. Copolymerization of AN-Py is successfully performed and the interaction between AN-Py is also investigated. One of the polymerization techniques is bulk polymerization of acrylonitrile only presence of initiator and temperature, without any solvent interaction. One of the redox initiators is Ce(IV) ammonium nitrate used as 10-2 M concentration. Copolymerization initiated at 50-60˚C, with addition of Py quantitatively during the process.

The other polymerization technique is solution polymerization. N,N, dimethylformamide is used because it has an ability to solve both monomer and polymer forms of acrylonitrile. And also DMf has a plasticizer effect, because of the interaction with acrylonitrile C≡N group. Thus DMF cannot be removed totally from the composites even if the temperature is up to 110˚C. the addition of Py presence of DMF graft polymer of AN& Py is obtained.

The last polymerization technique is polymerization of Py in PAN/DMF matrix. Homogen distribution is obtained.

According to these three types of polymerization techniques, Py content effect on composite and thin films structure is investigated. Obtained copolymers and composites prepared as thin film, annealed in vacuum for 24 h with 70 ˚C. After solvent evaporated, the remaining film thickness measured as <30µm. And also PPy effect on PAN crystallinity is investigated. Based on the results, semicrystalline structure of PAN shifts as a amourph structure because of PPys’ amourph structure. This interaction is verified with X-Ray diffraction measurements. And also this interaction is investigated with dynamic mechanical analayzer, to find out the influence on the mechanical behaviours. Stress-strain graphics and temperature dependence on modulus is investigated. There are certain differences while Py content changes.

PAN-PPy composites and thin films are prepared and electrical properties are characterized with electrochemical impedance spectroscopy.

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xi

Field Emission Scanning Electron Microscopy (FE-SEM), Attenuated Total Reflectance (ATR-FTIR) and X-Ray Diffraction(XRD) is studied for surface and morphology characterization. And as a result the cauliflower structure is the reason being detrimental to the mechanical properties.

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POLĐPĐROL MĐKTARII; PA/PPY KOMPOZĐT ĐCE FĐLMLERĐ KRĐSTAL YAPISI, MEKAĐK VE ELEKTRĐKSEL ÖZELLĐKLERĐ ÜZERĐDEKĐ ETKĐSĐĐ ĐCELEMESĐ

ÖZET

Đletken polimerler mükemmel elektriksel özelliklere sahip olmalarına rağmen bu materyallerin zayıf işlenebilirliği, çözünmezliği ve stabilite eksikliği nedeniyle özellikle tekstil kullanım alanlarında sınırlılık sözkonusu olmaktadır. Bu özelliklerin, monomer yapısının modifikasyonu, karışım veya komposit oluşturulması ile işlenebilirliği iyileştirilebilir. Đletken polimerlerin çözünebilir klasik polimerlerle kopolimerinin sentezlenmesi ise çözünebilirliği iyileştirebilmek için kullanılan diğer bir yöntemdir. Kompozit oluşturma iletken polimerleri işlenebilir hale getirmenin en basit metodudur.

Đletken elyaflar süzgeç, elektro-manyetik önleyici malzemelerde, ve özel amaçlı toz ve mikrop tutmayan özellikte giyeceklerde çoğunlukla kullanılmaktadır. Elektriksel iletkenliğe sahip polimerler, biosensörler, elektokromik pencereler ve görüntüler, antistatik kaplama ve paketleme, katı faz piller, yarı iletken elektronik cihazlar ve membranlar gibi çok çeşitli yeni teknolojilerde ki potansiyel uygulamaları nedeniyle önemli araştırma alanı oluştururlar.

Karbon fiberler, polimer başlatıcılarla tekstil fiberleri formunda elde edilirler. Bu polimerlerden biri de poliakrilonitrildir. Poliakrilonitril, akrilonitril monomerinin polimerizasyonu ile elde edilir. Poliakrilonitril, spinning teknolojileri ile polimer formdan fiber haline getirilir ve çok yüksek sıcaklıklarda karbonizasyon ve siklizasyon işlemlerine maruz bırakılır. Oluşan karbon fiber dayanıklı mekanik özelliklere sahiptir çoğu polimerlerin güçlendirilmesinde kullanılır.

Đletken polimerlerin basit bir kaplama yöntemiyle matrix polimerin yüzeyine kaplanması sonucu iki bileşen arasında etkileşim çoğunlukla görülmez ve iletken polimer elektriksel özelliklerinin neden olduğu sürtünmenin etkisiyle matrix polimerin yüzeyinden kolayca uzaklaşabilir. Bu göz önüne alındığında, istenen özelliklerde malzeme isteniyorsa iki bileşen arasında mutlaka etkileşim olması gerekir. Bu nedenle bu çalışmada kopolimerizasyon ve kompozit yapıların oluşumuna ağırlık verilmiştir. Yüksek konsantrasyonlardaki kırılgan iletken polimerler, karışımın mekanik özelliklerini zayıflatıcı etkisi vardır ve bu yüzden iletken polimerin kütlece miktarını yeterli oranda tutulması önem taşımaktadır. Bu çalışmada oldukça yüksek iletkenliğe sahip polipirolün(~102 S/cm), akrilonitril gibi bir matrix varlığında farklı polimerizasyon teknikleriyle kompozit ve kopolimer oluşturarak aralarındaki etkileşim incelenmiştir.

Çalışılan polimerizasyon tekniklerinden ilki; monomersiz ortamda akrilonitrilin sadece başlatıcı varlığında sıcaklığın da etkisiyle polimerleştirilmesidir. Redox başlatıcılardan biri olan Seryum (IV) amonyum nitrat 10-2M konsantrasyonunda

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kullanılarak, 50-60˚C aralığında polimerizasyon başlatılmış ve kopolimerizasyon aşamasında ise pirol kantitatif olarak reaksiyon süresince polimerizasyon ortamına eklenemiştir.

Kullanılan diğer polimerizasyon yöntemi ise çözelti polimerizasyonudur. N,N-Dimetilformamid, akrilonitrilin hem monomer halini ve hemde polimer halini çözebilme özelliğine sahiptir , ayrıca akrilonitrilin C≡N grubu ile etkileşerek poliakrilonitrile plastifiyan özelliği sağlamakta ve kompleks oluşturma yetisi nedeniyle polimerden kolayca uzaklaştırılamamaktadır. DMF varlığında pirolün kantitatif olarak eklenmesiyle de (AN-Py) graft kopolimeri elde edilmiştir.

Diğer bir polimerizasyon yöntemi (3.Yöntem)ise; pirolün PAN/DMF matriksinde polimerleştirilmesidir. Bu yöntem ile çözünmeyen polimer olan PPy, PAN çözeltisinde düzenli bir dağılım gösterir.

Bu üç polimerizasyonda pirol miktarlarının kompozit ve ince filmlerin yapısındaki etkisi incelenmiştir. Oluşan kopolimerler ve kompozitler ince film haline getirilmiştir. 24 saat vakum etüvünde bekletilmiş ve DMF in uzaklaştırılmasıyla geriye ince film kalması sağlanmıştır. Her bir ince filmin kalınlığı yaklaşık 20-30µm ölçülerindedir. Đletken polimer olan pirolün poliakrilonitrilin kristal yapısına etkisi incelenmiştir. Buna göre yarıkristal yapıya sahip poliakrilonitril polipirolün amorf yapısının etkisiyle kristallikten uzaklaşarak amorf bir yapıya dönüşmüştür. Bu etkileşim X-ışını difraksiyonu ölçümleriyle doğrulanmıştır. Ayrıca bu etkileşimin mekanik özlellikleri üzerindeki etkisi dinamik mekanik ölçüm cihazıyla incelenmiştir. Sonuç olarak uygulanan kuvvete karşı çekme-kopma grafikleri ve modulusun sıcaklığa bağlı değişim grafikleri oluşturulmuştur. Modulus değerleri polipirol etkisiyle değşiklik göstermiştir. Yüzey karakerizasyonu için taramalı elektron mikroskobu (SEM) ile çalışılmıştır. Ve polipirolün karnıbahar yapısının mekanik özelliklerini zayıflatıcı etkisi olduğu gözlenmiştir.

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1 1 ITRODUCTIO

In recent years, research on the preparation of conductive polymer composites in the form of film, fiber, and fabric [1] have increased. Electrically conductive fabrics are mainly used for industrial materials like filters, de-electrifying and electromagnetic interference shield materials, and special purpose clothing, which is dust and germ free. Demand for them has increased strongly in recent years. The reason is that electrostatic and electromagnetic interference have proliferated and become common place due to human lifestyle changes and the increasing sophistication of industrial technology [2].

On the other hand, the commercial use of thin films has been growing at a surprisingly rapid rate in the last two decades and in almost all the industrial fields such as optics, electronics, mechanics and even biotechnology [3]. These films are practically formed by depositing materials onto a supporting substrate to build up thin film through a complicated thin film process rather than by thinning down bulk materials by simple methods. Conducting thin films based on polypyrrole (PPy) have been the subject of interest in polymer materials because of their important electro-optical and chemical stability properties and potential as versatile display members and as coatings to protect against oxidation and corrosion and as chemical sensors and biosensors [4,5].

Conducting polymers exhibit excellent electrical properties; however, the common usage of this material has been restricted due to the poor processability and the lack of stability [ 6 ]. Various techniques, such as the modification of the monomer structure [7,8], the utilization of a soluble precursor [9], and the formation of a blend or composite [ 10 , 11 ], have been introduced to enhance the processability. The composite formation is one of the simplest methods for providing the processability of a conducting polymer [12].

As is well known, the physical and mechanical properties of thin films are closely related to their microstructures. For example, the tensile strength of the material is strongly dependent on the microstructure. If the density of the material is high, the

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2

strength is high because there are fewer defects in the microstructure [3]. Although a doped PPy film in a general synthesis shows good conductivity and stability, their brittleness and other mechanical properties directly cause problems for larger scale industrial applications. Therefore, the mechanical properties of the new polymer films have observably affected these films in industrial applications. But there is still a lack of investigation on the relationship between the mechanical properties and microstructures of conducting PPy films.

The basic aim is to obtain homogeneous composites with good mechanical properties, at least to a certain extent. In these studies low percolation thresholds were achieved with the help of hydrogen bonding between host matrix and polypyrrole (PPy) [ 13 ]. Graft polymers were also obtained via the chemical polymerization method [14]. The graft polymer films showed different behaviors in differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) (ATR), as compared with the mechanical mixture of the two polymers. Insolubility of the insulating polymer in a suitable solvent suggested that the composite may be graft rather than a mixture of two polymers. Copolymers in general exhibit physical and mechanical properties far different from those of blends of the same individual homopolymers [15].

Synthesis of conducting graft and block copolymers were one of the effective ways to improve poor properties of conducting polymers. The addition of an appropriate functional group to a conventional polymer is another way to impart new properties to conducting polymers [16]. A number of techniques for the preparation of polymers with desired end groups have been developed. Living polymerization is widely used polymerization technique to synthesize polymers with desired structure. The final average molecular weight of the polymer can be adjusted by varying the initial monomer/initiator ratio, while maintaining a narrow molecular weight distribution (Mw/Mn/1.5) [ 17 ]. Hence, polymers can be end-functionalized and block copolymerized with other monomers. Thus, it has opened new pathways to create many new materials with vastly differing properties by varying the topology of the polymer (comb, star, dendritic, etc.), the composition/architecture of the polymer (random, periodic, graft, etc.), or the functional groups at various sites of the polymer (end, center, side) [18].

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3

The objective of this study is to prepare PAN/PPy thin film composites using different types of polymerization processes. Copolymerization of AN-Py is successfully performed and the interaction between AN-Py is also investigated. PAN-PPy composites and thin films are prepared and electrical properties are characterized with electrochemical impedance spectroscopy. Field Emission Scanning Electron Microscopy (FE-SEM), Attenuated Total Reflectance (ATR-FTIR) and X-Ray Diffraction(XRD) is studied for surface and morphology characterization.

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4 2 THEORETICAL PART

2.1 Conducting Polymers

Electrically conductive polymers are the focus of considerable current research because of their potential applications in a variety of new technologies, such as biosensors, electrochromic windows and displays, antistatic coatings and packaging, solid state batteries, semiconductor electronic devices and membranes [19]. Although they have good electrical properties, most of them exhibit poor environmental stability, brittleness, low elongation, and poor processibilities [ 20 ]. In order to improve these problems methods of reforming the monomers [21] of the conducting polymers or synthesis of composites [22] and copolymers are reported.

2.1.1 Insolubility and improcessability

The insolubility and infusibility of classical conducting polymers make their processing difficult and limit their applications. One way to improve the solubility is to synthesize copolymers of conducting polymers with soluble classical polymers. Among the manufacturing processes used to produce conductive materials, there have been great expectations of π-electron conjugate polymers, such as polythiophene, polyaniline and polypyrrole [23].

Polyaniline, polythiophene and polypyrrole also exhibit relatively high conductivity (~102 S/cm), but are more oxidatively stable than polyacetylene and poly(pphenylene) .

Polypyrrole (PPy) , which is intrinsically conductive in its oxidized state, is relatively stable in air at ambient temperatures in its oxidized state [24], and it can be prepared as a film. PPy films are insoluble, hard, and usually brittle. Some improvements have been made by electropolymerization of pyrrole at low temperature and current, which produces conductive and stretchable PPy films [25].

2.1.2 Mechanical properties

Several approaches have been taken to overcome the poor mechanical properties of electrically conductive polymers and still exploit their high electrical conductivity.

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Polypyrrole is an important conducting polymer with high electrical conductivity and appreciable environmental stability [26]. High concentrations of a brittle conducting polymer in a blend may be detrimental to the mechanical properties of the blend, so it is desirable to minimize the amount of the conducting polymer [14]. However, this must be balanced by the requirement of sufficient concentration of the conducting polymer to achieve a percolation threshold for an insulator to conductor transition [27]. For globular aggregates of a conducting polymer in an insulating medium, percolation occurs at a threshold volume fraction of 0.16 [28]. Lower percolation volume fractions have been reported when the conductive polymer phase is anisotropic or if specific interactions occur between the components of the blend [29].

2.1.3 Π interaction

The π-electron structure in these polymers allows the enhanced electrical conductivity at a certain oxidation state. After introducing high electronic conductivity to the conducting polymers, they attracted lots of research interest and became popular basic materials for advanced applications such as plastic batteries, plastic light-emitting diodes, EMI shields, electrochromic display devices, gas separation membranes, smart windows, sensors, and so on [ 30 ]. However, a conducting polymer exhibits poor compatibility with common polymers due to the chain rigidity originated from an extended conjugate double bond. In the case of a conducting composite prepared by the simple coating of conducting polymers onto the surface of the matrix polymer, any interaction between the two components usually does not exist and the conducting polymer can be easily removed from the matrix polymer by repeated friction to result in the ultimate failure of the electrical property. In this regard, it should be stressed that the interaction between the two components should exist in order to provide the desirable properties [9]. In literature, the effect of ionic groups in the matrix on the conducting composite formation was studied in terms of the morphological, electrical, and thermal properties [12]. The environmental stability of the electrical conductivity was also monitored as a function of time and temperature. The application of this type of preparation technique can be utilized in the manufacturing of high performance conducting textile materials [12].

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6 2.2 Redox polymerization

Redox polymerization has the advantages of very short induction time, low activation energy (40–85 kJ/ mol), production of high molecular weight polymers with high yields, easy control of the polymerization reaction at low temperatures due to reduction of the side reactions, and the direct experimental proof of the transient radical intermediates [31]. In redox systems, oxidant forms initially a complex by reacting simply organic molecules which then decomposes unimolecularly to produce free radicals that initiate polymerization.

2.3 Synthesis of polypyrrole

Among the conducting polymers, polypyrrole (PPy) has received a good deal of attention because of its high electrical property, enviromental stability and ease of synthesis.

Highly conductive PPy has been prepared by various methods, such as electrochemical polymerization [ 32 ], chemical polymerization [ 33 ], by mild oxidative transition metal ions.

PPy is commonly synthesized by electrochemical methods, in which pyrrole is dissolved in an appropriate solvent in the presence of an electrolyte [34].

Pyrrole can also be polymerized chemically with an oxidizing agent such as cerium ammonium nitrate, (CAN), ferric chloride (FeCl,), diazonium salts, ozone, nitrous acid, quinone or lead dioxide. Since Ce(IV) is capable of polymerizing vinyl monomers effectively, it was challenging to study polymerization of pyrrole by the use of strong oxidants such as Ce(IV) ions, namely ceric(IV) ammonium nitrate (CAN) and ceric(IV) sulphate (CS) in some detail. The conductivities of PPy complexes obtained showed variations in the range of literature values depending on the conditions [33].

Because of the low solubility of PPy and its intermediates, the reaction mechanism for the chemical polymerization of pyrrole is uncertain, but it is expected to essentially be the same as for the electrochemical polymerization.

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7 2.4 Acrylonitrile

First introduced in 1946, acrylic rubbers have become established as important special purpose rubbers with a useful combination of oil and heat resistance. Acrylic paints have become widely accepted particularly in the car industry whilst very interesting reactive adhesives, including the well-known ‘super-glues’ are also made from acrylic polymers [35].

During the 1970s there was considerable interest for a time in copolymers with a high acrylonitrile content for use as barrier resins, i.e. packaging materials with low permeability to gases. Problems associated with free acrylonitrile have, however, led to the virtual disappearance of these materials from the market. Other developments in recent years have been the appearance of tough and heat-resistant materials closely related to poly(methy1 methacrylate) and to interesting cross-linked polymers. Amongst these are the so-called hydrophilic polymers used in the making of soft contact lenses. Today a very wide range of acrylic materials is available with a broad property spectrum. The word acrylic, often used as a noun as well as an adjective in everyday use, can mean quite different things to different people. In the plastics industry it is commonly taken to mean poly(methy1 methacrylate) plastics, but the word has different meanings, to the fibre chemist and to those working in the paint and adhesives industries. Unless care is taken this may be a source of some confusion.

Acrylonitrile-based copolymers are widely used in the production of acrylic fibers. Polyacrylonitrile fibers suffer from poor hygroscopicity and low dye uptake. Suitable comonomers are, therefore, incorporated into the polymer to overcome these shortcomings [36].

2.4.1 Acrylonitrile polymerization and copolymerization

Although in the pure state it is stable when heated, it is able to polymerize easily under the influence of X-ray, gamma or UV radiation, and ultrasounds or high energy electrons. Organic or inorganic initiators or redox systems are often used. Ce(IV) salts are well known redox initiators for graft copolymerization of vinyl monomers such as acrylonitrile and acrylamide [ 37 ]. The yield of acrylonitrile polymerization increased sharply and reached to about 100% at 20 mmol/l Ce(IV) concentration. Bulk and solution polymerization (organic solvents or concentrated

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mineral salts solution) or aqueous medium polymerization(emulsion, suspension) can be applied. In literature the bulk polymerization of acrylonitrile using AIBN at 40˚C-60˚C is reported in 1979 [ 38 ]. Solution polymerization is generally used for obtaining acrylic polymers (PAN and AN copolymers) suitable for direct wet or dry spinning fiber manufacture. Common solvents are dimethylformamide (DMF), dimethylacetamide, dimethylsulfoxide (DMSO), ethylene carbonate(EC) or propylene carbonate(PC), and concentrated aqueous solutions of NaSCN, HNO3,

H2SO4 and ZnCl2.

Aqueous polymerization permits an easy thermal regulation and leads to a polymer with homogeneous MW and low cost. Because PAN homopolymer has a limited solubility and colorability, it is used commercially with low amounts (5-10%) of other comonomers able to increase PAN solubility and the diffusion of colorants in the fiber, although a third comonomer able to increase colorability much more is preferred in many cases. This is why at present, PAN homopolymer for synthetic fibers has been completely replaced with co- or terpolymers with 5-15% comonomer(s). Comonomers like vinyl acetate, itaconic acid, acrylic acid, methyl acrylate, methacrylonitrile, vinyl-methyl-ketone, morfoline, vinyl-pyridine and α-methyl styrene are often used in this respect [36].

Copolymerization in aqueous medium is known as solution-suspension polymerization, due to the fact that it starts from a solution of the monomer in water, and ends as a suspension in the same medium. The process has two phases:

The conversion and MW of the produced PAN depend on the following factors: initiator type and amount, pH (2-2.5), temperature, time and reactants’ throughput. To avoid crosslinkings and branchings which may produce difficulties during polymer dissolution and high viscosity spinning solutions, polymerization temperatures not higher then 60˚C are recommended. Conversions above 80% lead to chain transfer reactions and branched polymers unsuitable for spinning. For dry spinning, the MW has to be 35000-50000 and for wet spinning, 60000-80000.

Continuous polymerization has the advantages of a high productivity, low volume installation, reproducibility and uniformity of the product.

Commercial plants use organic solvents DMF, EC and DMSO. BP (Benzoyl Peroxide) and AIBN are used as initiators, the process taking place at 50-60˚C. In

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this procedure,AN, comonomers, the organic solvent and the initiator as a solution are introduced continuously in a certain ratio into the reactor. Due to the fact that with the increase of conversion the viscosity of the system increases, the stirrer is built to be able to insure the same stirring effects during the copolymerization. The conversion is limited to 60-80% because of the medium viscosity and the desired properties of the polymer.

2.4.2 PA properties

PAN and its copolymers, with AN in a predominant amount, are white powders up to a temperature of 250˚C when they become darker because of the beginning of degradation. Having relatively high values of Tg (90 and 140˚C), they have a low thermal plasticity and cannot be used as a plastic material. Their high crystalline melting point (317˚C), limited solubility in certain solvents and the superior mechanical properties of the fibers are due to the intermolecular forces between the dipoles adjacent − ≡  groups on the same macromolecule. This intramolecular interaction restricts bond rotation and leads to a stiff chain, the stretched PAN fiber presents a high degree of crystallinity.

As mentioned previously, PAN is soluble in a limited number of polar liquids like DMF, DMSO, dimethylacetamide, dimethylsulfonate, tetramethyl-sulfide and aqueous solutions of ethylene carbonate or of some mineral salts. In DMF, concentrations up to 25% and above can be obtained; EC permits formation of solutions with up to 14% PAN.

2.4.3 PA/PPy interactions :

It is reported, that the novelty of the functions provided by electrochemical synthesis technologies for organic thin films is emphasized as an area in which thin film research and development have a significant impact. Remarkable advances have been made in recent years in the science and technology of thin film formation processes. Up to the present time, thin films have been extensively studied from the viewpoint of the relationship between their microstructure/nanostructures and properties[3]. Core Ppy particles were first studied in literature, in the flexible shell solutions and then different core–shell structures can be prepared by electrospinning method. The results showed that: this method is very powerful to form core–shell nanostructures with electroconductive PPy.

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10 2.4.4 Applications

Core–shell structures based on latex particles have gained an interesting importance in many industrial applications; typical examples are paints, coatings diagnostics drug delivery, or support for catalysts. They principally consist of a stiff core and a flexible shell. Typically they are obtained by a two step emulsion polymerization, whereas the core is prepared first and then surrounded with a flexible polymer shell by emulsion polymerization. Electrospinning as a very simple method can form various nanostructures, which include nanofibers, nanospheres etc [53].

Staple acrylic fibers, being soft and resilient, are used as a substitute or diluents for wool, and fabrics made from them show good crease resistance and crease retention(e.g. in permanent pleats); they are also made into resilient bonded batting. The rot and light resistant properties suggest numerous outdoor applications for acrylic fibers in heavy duty and finer fabrics, and in netting and filter fabrics.

PAN is one of the important raw materials for the production of carbon fibers. When PAN is heated in vacuum or air to over 200˚C, a number of reactions take place, resulting in the formation of some by-products which are liberated as volatiles. The carbon ring structure produced during the carbonization of PAN fibers under tension has a crystalline form.

Carbon fiber (graphite fiber) is one of the popular materials that used preparing composites. The synthetic carbon industry starts with the foundation of National Carbon Company in Ohio, and continues Union Carbide Corp. High performance carbon fibers were developed at the Parma Technical Center by Dr. Roger Bacon in 1958. The atomic structure of carbon fiber is similar to graphite. Carbon fibers are classified by the tensile modulus of the fiber; low modulus, standard modulus, intermediate modulus, high modulus, ultrahigh modulus. Polyacrylonitrile is one of the raw materials that used to manufacture carbon fiber.

Fibers are used for plastic industry because of their considerable properties such as hardness, elastic modulus, mechanical strength, impact strength and dimensional control [39]. Fiber reinforced polymer matrix materials in which nano and micro-scale particles have been studied because of their potential and capacity of performance for twenty years [40]. Due to their high specific and stiffness properties, carbon fiber reinforced composites are widely used structural materials for

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aerospace, marine, armor, automobile, railways, civil engineering structures, sport goods industry [41]. Carbon fibers are used for many different industries also carbon fiber composites. For carbon fiber composites, matrix has an important role. Carbon fiber is used for many polymer matrix composites. Nature of interface, bonding properties and surface morphology can change the composite fracture toughness[42].

Figure 2.1: Mechanistic pathways for the thermal degradation of PAN and reactions involved in the stabilization of PAN fibres

2.5 Physical categories of polymers

The chains that make up a polymer can adopt several distinct physical phases; the principal ones are rubbery amorphous, glassy amorphous, and crystalline. Polymers do not crystallize in the classic sense; portions of adjacent chains organize to form small crystalline phases surrounded by an amorphous matrix. Thus, in many polymers the crystalline and amorphous phases co-exist in a “semicrystalline” state.

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12 2.5.1 Semicrystalline State:

Due to the extreme length of polymer molecules, their distribution of chain lengths and the constraints applied to them by entanglements, they cannot crystallize completely. A matrix of amorphous chains – either glassy or rubbery – surrounds the crystallites that form from the molten state. Polymer chains can double back on themselves, contributing several adjacent linear chain segments, linked by folds at the crystallite surface.

In figure 2.2 it is shown that how a single polymer chain can contribute to several crystallites.

Figure 2.2: Schematic representation of the semicrystalline state.

However, too much crystallinity causes brittleness. The crystallinity parts give sharp narrow diffraction peaks and the amorphous component gives a very broad peak (halo). The ratio between these intensities can be used to calculate the amount of crystallinity in the material.

Solid state structure of a polymer refers as its morphology. This includes both the arrangement of the crystalline and amorphous phases, and the orientation of molecules and crystallites. Polymer’s solid state structure can be analyzed using scattering measurements, such as X-ray diffraction and neutron diffraction, to determine electron density and mass fluctuations, which yield information regarding ordering.

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13 2.5.2 Wide-Angle X-Ray Diffraction

Wide-angle X-ray diffraction is used for investigating the interatomic spacings in polymers. It is typically used to analyze regularly arrayed atoms, such as those found in crystallites, but it can also provide information regarding interatomic distances in non-crystalline polymers and in the amorphous regions of semicrystalline polymers. Wide-angle X-ray diffraction measures scattering angles from approximately 2 to 90˚. In practice, semicrystalline polymers contain a multitude of crystallites, each of which has a slightly different orientation relative to the X-ray beam. The unoriented crystallites scatter X-rays as a series of cones, giving rise to concentric rings on the scattering diagram, as shown in fig. b). Non-crystalline regions cretae a diffuse ring, known as an amorphous halo, on the diffraction pattern.

Figure 2.3: Scattering patterns from: a) single crystal and b) unoriented semicrystalline sample.

X-ray patterns can be quantified by plotting the scattering intensity as a function of diffraction angle. The measured spectrum can be deconvoluted into its component crystalline peaks and an amorphous halo. The angular position of the peaks reveals the interatomic spacings in the unit cell according to well established crystallographic principles. We can calculate the sample’s degree of crystallinity from the relative areas of the crystalline peaks and the amorphous halo. The widths of the crystalline peaks can be analyzed to obtain a measure of the perfection of the

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atomic arrays within crystallites. The broader the peak, the less well ordered and smaller are the crystallites within a sample. Th

indication of the range of interatomic distances found in the non of a sample.

2.5.3 Bragg's Law Since a beam of X-interact with one another.

in the bundle are in phase, that is their crests and troughs occur at exactly the same position (the same as being an integer number of wavelengths out of phase, nλ= 1, 2, 3, 4, etc.), the waves will interfere with one another and their

together to produce a resultant wave that is has a higher amplitude (the sum of all the waves that are in phase.

Figure 2.4: Diffusion

Two such X-rays are shown occurs over the distance, d. equal to its angle of incidence. at the same angle θ. While Ra farther than Ray 1. If this distance

(nλ), then Rays 1 and 2 will be in phase on their exit from the crystal and constructive interference will

Figure 2.5: Bragg's Law

14

atomic arrays within crystallites. The broader the peak, the less well ordered and smaller are the crystallites within a sample. The breadth of the amorphous halo is an indication of the range of interatomic distances found in the

non--rays consists of a bundle of separate waves, the waves can interact with one another. Such interaction is termed interference

in the bundle are in phase, that is their crests and troughs occur at exactly the same position (the same as being an integer number of wavelengths out of phase, nλ= 1, 2, 3, 4, etc.), the waves will interfere with one another and their amplitudes will add together to produce a resultant wave that is has a higher amplitude (the sum of all the waves that are in phase.

Diffusion Rayleigh diffraction

rays are shown here, where the spacing between the atomic planes occurs over the distance, d. Ray 1 reflects off of the upper atomic plane at an angle θ equal to its angle of incidence. Similarly, Ray 2 reflects off the lower atomic plane

While Ray 2 is in the crystal, however, it travels a distance of 2a If this distance 2a is equal to an integral number of wavelengths 1 and 2 will be in phase on their exit from the crystal and constructive interference will occur.

Bragg's Law and Diffraction

atomic arrays within crystallites. The broader the peak, the less well ordered and e breadth of the amorphous halo is an -crystalline regions

rays consists of a bundle of separate waves, the waves can interference. If all the waves in the bundle are in phase, that is their crests and troughs occur at exactly the same position (the same as being an integer number of wavelengths out of phase, nλ= 1, 2, amplitudes will add together to produce a resultant wave that is has a higher amplitude (the sum of all the

here, where the spacing between the atomic planes Ray 1 reflects off of the upper atomic plane at an angle θ Similarly, Ray 2 reflects off the lower atomic plane y 2 is in the crystal, however, it travels a distance of 2a 2a is equal to an integral number of wavelengths 1 and 2 will be in phase on their exit from the crystal and

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In figure 2.5, Ray 1 comes to the surface with θ angle and diffracts again with θ angle. Using this angle and Bragg’s equation we can find the distance between atomic planes. This gives us information about the solid state structure of the material.

Figure 2.6: According to the 2θ deviation, the phase shift causes constructive (left figure) or destructive (right figure) interferences

If the distance 2a is not an integral number of wavelengths, then destructive interference will occur and the waves will not be as strong as when they entered the crystal. Thus, the condition for constructive interference to occur is

nλ = 2a

but, from trigonometry, it can be figured out what the distance 2a is in terms of the spacing, d, between the atomic planes.

a = d sin θ or 2a = 2 d sin θ thus, nλ = 2d sin θ

This is known as Bragg's Law for X-ray diffraction.

The wavelength, λ, of the X-rays going in to the crystal, and the angle θ of the diffracted X-rays coming out of the crystal can be measured, then the spacing (referred to as d-spacing) between the atomic planes can be known.

d = nλ /2 sin θ

In theory, then the crystal can be re-oriented so that another atomic plane is exposed and measure the d-spacing between all atomic planes in the crystal, eventually leading us to determine the crystal structure and the size of the unit cell.

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16 2.6 MECHAICAL PROPERTIES

Mechanical properties of most materials are dependent on the microstructure inherited from processing , the test temperature, and often the loading rate. Elastic properties of most metals and ceramics are not as greatly influenced by processing and microstructural changes as are the elastic properties of polymers and composites. 2.6.1 Influence of bonding on mechanical properties,

The nature of the bond in materials determines the responses of the materials to applied stress as much as the melting temperature and the crystalline or molecular arrangement. The three categories of strong bonds-ionic, covalent, and metallic-comprising the bonds between atom and ions within crystals and molecules and the secondary bonds between crystals and molecules determine the mechanical response of materials.

2.6.1.1 Linear elasticity

The net result of attractive and repulsive energies,  =   +  , for a particular bond is an asymmetric energy relationship for the spacing r between atom or ion centers shown in fig. as the influence of atomic vibration increases at higher temperatures, the average spacing  increases, representing the thermal expansion shown in fig. the relationship of the force F between atoms on ions with spacing is found by taking the slope  of the energy relationship. 

Elastic properties are only defined over small strains so that the slope of the F versus r curve defining the effective spring constant for small stretches of the bonds can be treated as linear. Then, if F=kr, young’s modulus, E, must scale with K.

The shapes of the energy and force curves in Figure 2.7 determine the stiffness of the bonds. They also give the corresponding strengths of the bonds. The effects of thermal vibration have an impact on the elastic stiffness and also the thermal expansion coefficient Young’s modulus decreases with increasing temperature. This interactions shows the elastic stiffness of the bonds indicated by the room-temperature Young’s modulus scales with the melting room-temperature.

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Figure 2.7: a) Interatomic potential energy vs. spacing. b) the effect of thermal vibration on interatomic spacing. c) the force vs. spacing derived by finding F=dE/dr, where r is interatomic spacing. d) the slope of the force vs. spacing relationship approximately linear for small strains, is proportional to Young’s modulus.

In figure 2.7 force and potential energy interaction because of interatomic spacing is shown. And also the effect of thermal vibration on interatomic spacing interaction can be seen.

2.6.1.2 Shear deformation of noncrystalline materials

As is clear from the preceding discussion, if al volume change does not occur, the deformation can always be resolved into shear components. For the deformation mechanism of grain boundary sliding and diffusion creep is discussed in literature[43]. For noncrystalline materials, which are normally noncrystalline owing to cooling rates from a liquid state more rapid than would allow the formation of crystalline materials or glasses, a difference in the bond strength between bonds within the molecular unit and between molecular units can allow flow of the individual molecules past one another whenever the strength of the weaker bonds is exceeded.

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18 2.6.1.3 Fluid flow Analogy of the liquid

A solid can be defined in terms of physical of mechanical properties easier than almost any other characteristic. The property that clearly distinguishes a liquid from a solid is that you cannot stir a solid. The property that determines the difficulty of stirring a liquid is called viscosity, designated by η , the proportionality constant between a shear stress τ and the rate velocity change with distance from a surface , where fluid flows in the x-direction over the y-plane with

 = −



For a solid, in the equivalent expression we consider shear strain rate (or rate of deformation) and

 = !"#$ = %&

Where %& is defined as the shear strain rate in the x-direction, ' ( .

The size of the crystals in a polycrystalline material does indeed influence the effective viscosity of the solid. Although the diffusive deformation mechanisms that occur near the melting temperature in many crystalline solids have a linear relationship, the relationship between stress and strain rate is often nonlinear. In noncrystalline materials consisting of large molecules, a nonlinear relationship for stress and strain rate is also quite common. Nonlinear expressions can be defined as

)** = +&**,

Where C is a constant, with units that depend on the magnitude of m, which is called the strain rate sensitivity.

2.6.2 Dynamic Mechanical Analysis

Dynamic Mechanical Analysis has become more popular because of their significant properties and to provide information about materials in particular polymers. The first experiments to measure elasticity of materials could be done by Poynting in 1909 [44]. In the 1950’s, the Weissenberg Rheogoniometer and the Rheovibron instruments were invented for usage of commercially [45]. Viscoelastic properties of polymers publicated by Ferry in 1961 [ 46 ]. This book explains dynamic measurements and the best theory on viscoelastic measurements. “Torsional Braid

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Analyzer” was developed by J. Gilham and modern era of DMA began [47]. J. Starita, C. Macosko and Bohlin developed a commercial dynamic mechanical analyzer in the 1970’s. The first instruments have some disadvantages about using instrument and limited resulting properties. At the same time, characterization of material by dynamic mechanical analysis was reported by Murayama, Read and Brown [48,49]. After these developments on dynamic mechanical analysis, Polymer Labs, Du Pont and Perkin Elmer developed new instruments about dynamic mechanical analysis. With computer technology, dynamic mechanical analysis has become more effective and useful in the world of science.

There are several components which are critical to the design and resultant performance of a dynamic mechanical analyzer. These components are the drive motor (which supplies the sinusoidal deformation force to the sample material), the drive shaft support and guidance system (which transfers the force from the drive motor to the clamps which hold the sample), the displacement sensor (which measures the sample deformation [oscillation amplitude] that occurs under the applied force), the temperature control system (furnace), and the sample clamps. DMA gives the information about rheological and thermal properties of polymers. Rheology is very sensitive to small changes of the material’s polymer structure – thus ideal for characterization of polymers. The rheology structure relationship is the key to the development of new materials.

Dynamic Mechanical Analysis (DMA) measures the mechanical properties of materials as function of temperature, frequency and time and also it is a thermal analytical method by which the mechanical response of a sample subjected to a specific temperature program is investigated under periodic stress. Dynamic mechanical analyzer is a thermal analytical instrument used to test the mechanical properties of many different materials.

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Figure 2.8: Traditional Dynamic Mechanical Analyzer Design

The Dynamic Mechanical Analysis is a high precision technique for measuring the viscoelastic properties of materials. Viscoelasticity is about elastic behaviors of material. Most real-world materials exhibit mechanical responses that are a mixture of viscous and elastic behavior.

2.6.3 Elasticity and Viscous Flow Behaviors

Materials are often referred to as solids or liquids, depending on whether or not they retain their shape under the force of gravity. An ideal solid is a material that is purely elastic. When adequate stress is applied to a purely elastic material, it undergoes a deformation, i.e., a change in shape instantaneously. When that stress is removed, the sample instantaneously regains its initial shape. The energy involved in the application of stress is stored within the material due to its elasticity, and in turn, it can use that energy to do work when the stress is released. It is important that there is no time dependence in the behavior of the material, in that the deformations occur the very instant the stress is changed (applied or removed in the example). This ideal mechanical behavior is described by Hooke’s law in which stress and strain are related through a proportional constant called the modulus of rigidity or simply, modulus (E or G).

There is a constant ratio between stress and strain. This equation is also known as “Hooke’s Law”.

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21 Hooke’s Law:

σ = Eε (Tension, Compression or Bending) τ = Gγ (Shear)

Where:

σ and τ are stress terms ε and γ are strain terms

Figure 2.9: Hooke’ Law and Elastic Deformation

An ideal liquid is a material that is absolutely devoid of any elasticity or rigidity. Thus, when placed in a container, an ideal liquid will conform to the shape of the container and find its own level under gravity. An ideal liquid is unable to store any energy imparted to it upon the application of a stress. Instead, the liquid undergoes continuous deformation until the stress is removed. Water is a good approximation to an ideal liquid. When placed in a beaker, it assumes the shape of the beaker and finds its own level in it. Next, consider an inclined plane, upon whose top end, one pours the water out of the beaker. Since gravity is acting as a stress on the water, there will be a continuous flow down the incline. If one could imagine that we are able to eliminate gravity, then the water would stop flowing farther down the incline and stay in that location. The absence of elasticity prevents it from flowing back up the incline to its initial location at the top. This kind of viscous behavior is addressed by Newton’s Law of Viscosity.

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A simple graphical representation of the behavior of an ideal fluid is shown in the Figure 1.9. An ideal fluid will deform continuously under the application of a stress but will not recover when the stress is removed. The strain developed under the application of the stress is a function of time until the stress is removed. The stress is independent of the strain but proportional to the rate of strain. The proportionality factor η is called the coefficient of viscosity. A Newtonian Fluid is one whose viscosity is independent of the applied shear rate.

Dynamic Mechanical Analyzer (DMA) deforms a sample mechanically and after that it measures the sample response. When a force is applied on a material it suffers a change in shape, that is, it deforms. The deformation can be applied sinusoidally, in a constant (or step) fashion, or under a fixed rate. The response to the deformation can be monitored as a function of temperature or time. A force to resist the deformation is also set up simultaneously within the material and it increases as the deformation continues. If the material is unable to put up full resistance to external action, the process of deformation continues until failure takes place. The deformation of a body under external action and resistance to deform are referred to by strain and stress respectively.

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23 Figure 2.11: Purely elastic and viscous

response functions Figure 2.12: Function of viscoelastic response

DMA is a useful instrument to measure mechanical properties for materials. DMA results for solid polymers can be used to set the polymer morphology and structure for industrial end-use products. Polymers are viscoelastic fluids, which behave viscous or elastic, depending on how fast they flow or are deformed in the process. For instance, glass transition temperature and damping behavior can be used to determinate material’s using conditions such as temperature, stiffness. In addition, DMA measurements explain how a material behaves at the moment and future. For some industrial products, it is more important to know how a material will behave weeks, months, and years. DMA is a non destructive technique. Small specimens can be used, so it is a decided advantage for evaluating experimental materials.

Figure 2.14: Schematic diagram for DMA measuring

Motor Applies Force (Stress)

Displacement Sensor - Measures Strain

Sample

Figure 2.13: Tension Clamp of Dynamic Mechanical Analyzer

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Dynamic mechanical analysis can be applied to following materials: thermoplastic polymers, thermosetting polymers, elastomer, composites (including fillers, reinforcing fibers...), wood, paper, glass ceramics, metals, alloys, food, cosmetics … Dynamic or oscillation experiments can be performed to characterize many properties of materials. The following is a partial list.

 Glass-transition temperature, Tg, and other secondary transitions -  Viscoelastic Response and Spectrum

 Linear Viscoelastic Region (Oscillatory Stress or Strain Sweep)  Damping characteristics (tan d)

 Stress fatigue

 Structure-related properties, especially with regard to:  Crosslinking, cure state

 Crystallinity  Molecular Weight  Additives  Plasticizers  Blending  Aging  Orientation

Dynamic Mechanical Analyzer is useful for these tests: mechanical properties, morphology of polymers, loss factor (Tan delta), loss angle (delta), impact resistance, dynamic viscosity, curing kinetics, correlation with materials formulation, ageing, damping, glass transition temperature (Tg), industrial products stiffness, polymer compatibility, relationships mechanical properties/molecular structure, relaxation time, rheological properties, secondary transitions, specimen stiffness, stress relaxation test, thermal properties, viscoelastic properties, young Modulus, thermal stability, prediction of long term mechanical behavior , optimization of curing process, dynamic viscosity, complex viscosity, modulus values, dynamic test, Creep behavior, gel time, melting point, dimensional stability, impact resistance, secondary transitions, tension test, stress-strain.

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