Synthesis and characterization of structurally well-defined polymer-layered silicate nanocomposites [Elektronische Ressource] / Qinghui Mao

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2007
• Nombre de lectures: 9
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Synthesis and Characterization of
Structurally Well-Defined
Polymer-Layered Silicate
Nanocomposites
Qinghui Mao
Department of Chemistry
University of Mainz
Max-Planck-Institute for Polymer Research
A thesis submitted for the degree of
Ph.D. Thesis
Submit date: January 22, 2007Abstract
Polymer-layered silicate (PLS) nanocomposites are often based on natural clays such as montmoril-
lonite. In EPR and NMR studies of the surfactant layer that compatibilizes the silicate and the
polymer, we found that the iron content in these natural clays is detrimental, as it broadens spec-
tra and shortens relaxation times. The problem was overcome by using as a layered silicate iron-free
and structurally well defined magadiite, which was synthesized by a hydrothermal method. The mor-
phology of the magadiite and the extent of intercalation in melt-prepared polymer-organo-magadiite
nanocomposites were characterized by scanning electron microscopy (SEM) and wide-angle x-ray scat-
tering (WAXS) respectively. Among different types of polymers, those with carbonyl groups appear
to intercalate more easily. With iron-free magadiite, polycaprolactone (PCL) was found to intercalate
into both ammonium surfactant modified and phosphonium surfactant modified organomagadiites. By
using site-specific nitroxide spin probes and applying CW EPR (Continuous Wave Electron Paramag-
netic Resonance) and pulse EPR techniques the dynamics and spatial structure of surfactant layers
2in such nanocomposites, were studied. Static H Nuclear Magnetic Resonance (NMR) on specifically
deuterated cationic surfactants was also applied to access surfactant motion on a complementary time
scale (milliseconds to microseconds) compared to EPR (nanoseconds).
2By CW EPR and H NMR, fast motion of surfactant layers was found to be pronounced by intercalation
of PCL while it diminishes in non-intercalated microcomposites with polystyrene (PS). The rotational
correlation times τ and activation energies E reveal different regimes of reorientation of surfactantc a
molecules with increasing temperature. The transition temperature between the regimes is related
to T of PS in microcomposites and to the melting temperature T of PCL in nanocomposites. Theg m
iron-free magadiite leads to substantially longer electron spin relaxation times and thus allows for pulse
EPR experiments such as four-pulse double electron electron resonance (DEER), electron spin echo
envelope modulation (ESEEM) and electron nuclear double resonance (ENDOR) on spin-labeled and
specifically deuterated surfactants. ENDOR results suggest a model of the surfactant layers with a
well-defined middle layer. This model can explain the dilution effects caused by PCL and PS polymers
in the hybrids. Comprehensive information from these techniques nicely complements information
from conventional characterization techniques such XRD and TEM and provides a much more detailed
picture of structure and dynamics of the surfactant layer in polymer-layered silicate nanocomposites.Contents
Nomenclature v
1 Introduction 1
1.1 Introduction................................................. 1
1.2 Basicsofnanocompositesandorganoclay................................ 2
1.3 Preparationandcharacterizationoforganoclay............................. 4
1.4 Typesofpolymersusedfornanocomposites............................... 6
1.5 Preparationofnanocomposites...................................... 8
1.6 Characterizationofpolymer/claynanocomposites............................ 9
1.7 Thedesignofthisproject......................................... 9
2 Introduction of the synthesis of silicates 13
2.1 The crystal structure of silicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Introduction ............................................ 13
2.1.2 Introduction of silicon compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.3 Basics of silicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.4 Aluminosilicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Introduction of mesoporous silicate materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1 Synthesis of mesoporous silicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.2 Characterizationmethods..................................... 21
2.2.3 States of Si atoms in silicate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3 Alkali silicate minerals: kanemite, magadiite and kenyaite . . . . . . . . . . . . . . . . . . . . . . . 22
3 Electron Paramagnetic Resonance 26
3.1 BasicsofElectronParamagneticResonance............................... 26
3.1.1 Energy levels of the free electron in amagneticfield...................... 26
13.1.2 Classical description of motion of an ensemble of electron spins (system with spin S = ). 28
2
3.1.3 The effects of an additional small magnetic field B (B B )................ 30
1 1 0
3.1.4 SpinrelaxationandBlochequations............................... 33
3.2 Spin Hamiltonian of radicals in a static external magnetic field . . . . . . . . . . . . . . . . . . . . 37
3.2.1 GeneralspinHamiltonian..................................... 37
iiCONTENTS
3.2.2 Theg-factorforradicalsinastaticmagneticfield ....................... 38
3.2.3 Thehyperfinetensor........................................ 39
3.3 BasicsofPulseEPRtechniques...................................... 41
3.3.1 Energy levels of coupled spins with one electron S=1/2 and one nucleusI=1/2....... 41
3.3.2 Primaryecho(Hahnecho)andphasecycling.......................... 43
3.3.3 Thestimulatedecho........................................ 46
3.4 Nuclearmodulationexperiments:ESEEM................................ 48
3.4.1 Introduction of the density operator and coherence . . . . . . . . . . . . . . . . . . . . . . . 48
3.4.2 PrinciplesoftwopulseESEEM.................................. 50
3.4.3 Principleofthre-pulseESEEM ................................. 53
3.4.4 AnalysisandinterpretationofESEEM.............................. 54
3.5 Doubleresonancetechniques:ENDOR.................................. 56
3.5.1 PrincipleofENDOR........................................ 56
3.5.2 DataanalysisofMimsENDOR.................................. 59
3.6 Doubleresonancetechniques:four-pulseDEER............................. 60
3.6.1 PrincipleofDEER......................................... 60
3.6.2 Dataanalysisoffour-pulseDEER................................ 63
3.7 CWEPRspectrumofnitroxidespinprobes............................... 64
3.7.1 SpinHamiltonianofnitroxides.................................. 64
3.7.2 Powderspectrumofnitroxides.................................. 66
3.7.3 SlowtumblingEPRspectraofnitroxides ............................ 67
3.7.4 Line shape analysis for tumbling nitroxide radicals . . . . . . . . . . . . . . . . . . . . . . . 69
3.8 EPRandNMRspectrometers....................................... 70
3.9 EPRmethodsappliedinthisthesis.................................... 71
24 H Solid State NMR 73
4.1 BasicsofNuclearMagneticResonance.................................. 73
4.1.1 Energylevelsofnucleiinamagneticfield............................ 73
4.1.2 Thechemicalshift......................................... 75
4.1.3 ThespinHamiltonianinsolidstateNMR............................ 78
24.2 HsolidstateNMR ............................................ 79
4.2.1 Thenuclearquadrupoleinteraction................................ 79
24.2.2 The HNMRexperiment..................................... 81
24.2.3 HNMRspectraintherigidlimit................................ 82
4.2.4 Studiesofmolecularmotion.................................... 83
24.2.5 Analysis of the HsolidechoNMRspectrum.......................... 84
iiiCONTENTS
5 Experimental results 86
5.1 Synthesis and characterization of magadiite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.2 Preparation of organosilicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.2.1 Preparation of organosilicates without spin probes . . . . . . . . . . . . . . . . . . . . . . . 87
5.2.2 Characterization of the organosilicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5.2.3 Preparation of organosilicates with spin probes for DEER and ENDOR . . . . . . . . . . . 88
5.2.4 Preparation of organosilicates with spin probes for ESEEM . . . . . . . . . . . . . . . . . . 90
25.2.5 Preparation of organosilicates for HNMR........................... 90
5.3 Intercalation of organosilicates with polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
5.4 Instrumentalanalysis............................................ 91
5.4.1 WAXS/SAXSmeasurements................................... 91
5.4.2 SEMmeasurements ........................................ 91
5.4.3 TGAandDSCmeasurements................................... 91
315.4.4 PNMRmeasurements...................................... 91
25.4.5 HNMRmeasurements...................................... 92
5.4.6 EPRmeasurements ........................................ 92
5.5 Experimentresultsonsamplepreparation................................ 94
5.5.1 Optimum parameters for synthesis of magadiite according to WAXS and SEM . . . . . . . 94
5.5.2 CEC scale of magadiite found by titration .