SPRINGER BRIEFS IN MOLECULAR SCIENCE Guangming Liu Guangzhao Zhang QCM-D Studies on Polymer Behavior at Interfaces SpringerBriefs in Molecular Science Series Editor Sanjay K. Sharma For furthervolumes: http://www.springer.com/series/8898 Guangming Liu Guangzhao Zhang • QCM-D Studies on Polymer Behavior at Interfaces 123 Guangming Liu Guangzhao Zhang Department of Chemical Physics Faculty ofMaterials Science and Universityof Science andTechnology Engineering of China South ChinaUniversity ofTechnology Hefei Guangzhou People’s Republic ofChina People’s Republic ofChina ISSN 2191-5407 ISSN 2191-5415 (electronic) ISBN 978-3-642-39789-9 ISBN 978-3-642-39790-5 (eBook) DOI 10.1007/978-3-642-39790-5 SpringerHeidelbergNewYorkDordrechtLondon LibraryofCongressControlNumber:2013945155 (cid:2)TheAuthor(s)2013 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation,broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionor informationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodology now known or hereafter developed. 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While the advice and information in this book are believed to be true and accurate at the date of publication,neithertheauthorsnortheeditorsnorthepublishercanacceptanylegalresponsibilityfor anyerrorsoromissionsthatmaybemade.Thepublishermakesnowarranty,expressorimplied,with respecttothematerialcontainedherein. Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) Preface For the solid/liquid interfaces modified with polymers, the interfacial properties would be significantly influenced by the polymer behavior. Understanding the interfacial polymer behavior is a prerequisite to control the interfacial properties and to prepare well-designed polymeric interfacial materials. Actually, the char- acterization and analysis of polymer behavior at interfaces still remain a great challenge, particularly for the dynamic behavior. In the past 10 years, quartz crystal microbalance with dissipation (QCM-D) has been successfully applied to study the polymer behavior at various solid/liquid interfaces. In comparison with the conventional QCM which can merely provide the change in frequency, QCM-Dgivestheinformationaboutnotonlythevariationsinmass,thicknessand rigidity of the polymer layer but also the viscoelastic properties of the polymer layer. Therefore, it is anticipated that QCM-D would provide a clearer picture on the molecular interactions of the macromolecular systems at the solid/liquid interfaces. This book is intended to give an overview of our recent studies on polymer behavior at the solid/liquid interfaces by use of QCM-D. It starts with a brief introduction of the basic principles of QCM-D to give readers a straightforward impression on what is QCM-D and what can be obtained from QCM-D mea- surements.Inthefollowingchapters,wewillshowthathowQCM-Dcanbeused to investigate the conformational change of grafted polymer chains, the grafting kinetics of polymer chains, the growth mechanism of polyelectrolyte multilayers, andtheinteractionsbetweenpolymersandphospholipidmembranes.Thephysical meaning of the shifts in frequency and dissipation and the advantages of QCM-D technique will be presented. We hope this book will be helpful to the readers to understand how to use QCM-D to study the polymer behavior at the solid/liquid interfaces. We also hope it can serve as a reference material for academic and industrial researchers working in the fields of polymers and interfaces. Guangming Liu Guangzhao Zhang v Contents 1 Basic Principles of QCM-D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2 Conformational Change of Grafted Polymer Chains . . . . . . . . . . . 9 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2 Temperature-induced Conformational Change of Grafted PNIPAM Chains with a Low Grafting Density . . . . . . . . . . . . . 10 2.3 Temperature-induced Conformational Change of Grafted PNIPAM Chains with a High Grafting Density. . . . . . . . . . . . . 13 2.4 Solvency-induced Conformational Change of PNIPAM Brushes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.5 pH-induced Conformational Change of Grafted Polyelectrolytes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.6 Salt Concentration and Type-induced Conformational Change of Grafted Polyelectrolytes . . . . . . . . . . . . . . . . . . . . . 23 2.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3 Grafting Kinetics of Polymer Chains . . . . . . . . . . . . . . . . . . . . . . 33 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2 Pancake-to-Brush Transition. . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.3 Mushroom-to-Brush Transition . . . . . . . . . . . . . . . . . . . . . . . . 38 3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4 Growth Mechanism of Polyelectrolyte Multilayers . . . . . . . . . . . . 45 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4.2 Roles of Chain Interpenetration and Conformation in the Growth of PEMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4.3 Specific Ion Effect on the Growth of PEMs . . . . . . . . . . . . . . . 54 4.4 Effects of Chain Rigidity and Architecture on the Growth of PEMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 vii viii Contents 5 Interactions between Polymers and Phospholipid Membranes . . . . 71 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.2 Role of Hydrophobic Interaction in the Adsorption of PEG on Lipid Membrane Surface . . . . . . . . . . . . . . . . . . . . 72 5.3 Effect of Length of Hydrocarbon End Group on the Adsorption of PEG on Lipid Membrane Surface . . . . . . . . . . . . . . . . . . . . 77 5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Chapter 1 Basic Principles of QCM-D Abstract The solution of wave equation relates the eigenfrequency to the thicknessofcrystalwhentheacousticwavepropagatesinacircularAT-cutquartz crystal, which makes the resonator as a quantitatively ultrasensitive mass sensor possible. When a RF voltage is applied across the electrodes near the resonant frequency, the quartz crystal will be excited to oscillate in the thickness shear modeatitsfundamentalresonantfrequency.Forarigidlyadsorbedlayerwhichis evenly distributed and much thinner than the crystal, the added mass on the res- onator surface is proportional to the frequency shift (Df), i.e., they are related by the Sauerbrey equation. On the other hand, the energy dissipation (D) during the oscillation of resonator is an indication of the rigidity of the adsorbed layer. The frequency and dissipation are measured by fitting the oscillation decay of the freelyoscillatingresonator.InaNewtonianliquid,DfandDDarerelatednotonly to the inherent properties of the resonator but also to the solvent viscosity and density.Thetheoreticalrepresentationsbased ontheVoigt model can beused for the viscoelastic film in the liquid medium where the Sauerbrey equation may not bevalid.Therefore,QCM-Dnotonlygivesthechangesinmassandrigidityofthe adsorbedlayer,butalsocanprovidetheinformationontheviscoelasticproperties of the adsorbed layer such as hydrodynamic thickness and shear modulus. Keywords AT-cut quartz crystal(cid:2) Sauerbrey equation(cid:2) Thickness shear mode (cid:2) Oscillation (cid:2) Eigenfrequency (cid:2) Dissipation factor (cid:2) Viscoelasticity (cid:2) Voigt model In 1880, Pierre Curie and his elder brother Jacques Curie found that crystals of Rochellesaltcouldgenerateelectricpotentialbetweenopposingsurfaceswhenthe crystals were compressed in certain directions (i.e., piezoelectricity) [1]. Two years later, they confirmed that the reverse effect could also occur when the crystals were subjected to an electric field. However, the phenomenon of piezo- electricityanditsconversepiezoelectriceffectdidnotreceivemuchattentionuntil the World War I when it was demonstrated that quartz crystals could be used as transducers and receivers ofultrasoundinwater todetect the submarine. In 1921, Cady made the first quartz crystal resonator based on the X-cut crystals [2]. But, the X-cut crystals exhibited very high temperature sensitivity, so that they could G.LiuandG.Zhang,QCM-DStudiesonPolymerBehavioratInterfaces, 1 SpringerBriefsinMolecularScience,DOI:10.1007/978-3-642-39790-5_1, (cid:2)TheAuthor(s)2013 2 1 BasicPrinciplesofQCM-D merely be applied in the fields where the variation of temperature is little importance. The first AT-cut quartz crystal was introduced in 1934, which had nearly zero frequency drift with temperature around room temperature [3]. The advantageoftheAT-cutquartzcrystalrendersthisparticularcutthemostsuitable for the mass-detection sensor [4]. In 1959, the linear relationship between the deposited mass and the frequency response was established by Sauerbrey, which formed the fundamental basis of the quartz crystal microbalance (QCM) meth- odology[5].Nonetheless,QCMwasjustusedasamassdetectorinvacuumorair until the beginning of 1980s when scientists realized that a quartz crystal can be excitedwithastableoscillationinaviscousliquidmedium[6,7].Afterwards,the applications of QCM were extended to many research areas including biology, chemistry,physics,medicine,polymerscience,andenvironmentalscience[8–10]. Generally, the cut angle of quartz crystal determines the mode of induced mechanical vibration of resonator. Resonators based on the AT-cut quartz crystal with an angle of 35.258 to the optical z-axis would operate in a thickness shear mode(TSM)(Fig. 1.1)[4].Clearly,theshearwaveisatransversewave,thatis,it oscillates in the horizontal direction (x-axis) but propagates in the vertical direc- tion(y-axis).Whenacousticwavespropagatethroughaone-dimensionalmedium, the wave function (w) can be described by [11]: o2w 1o2w (cid:3) ¼0 ð1:1Þ ox2 v ot2 where v is the wave speed which depends on the elastic and inertial properties of themedium,xisthepositionatwhichthewavefunctionisbeingdescribed,tisthe time. The eigenfrequency (f ) can be obtained by solving the above equation: n nv f ¼ ð1:2Þ n 2L where n is the overtone number and L is the length of the one-dimensional medium. Likewise, the wave equation for the propagation of acoustic waves in a three- dimensional medium (e.g., quartz crystal) can be described by [11]: (cid:2)o2w o2w o2w(cid:3) 1o2w þ þ (cid:3) ¼0 ð1:3Þ ox2 oy2 oz2 v ot2 The corresponding eigenfrequency (f ) for the quartz crystal can be calcu- nmk lated by solving the wave equation with appropriate boundary conditions [4]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi v n2 m2 k2 f ¼ þ þ ð1:4Þ nmk 2 h2 l2 w2 q q q where h , l , and w are the thickness, length, and width of the quartz crystal, q q q respectively,andn,m,k = 1,3,5,….Obviously,theeigenfrequencyofthequartz crystal is determined by its size. Similarly, the eigenfrequency for the acoustic 1 BasicPrinciplesofQCM-D 3 Fig.1.1 Schematic illustration of the thickness-shear-mode resonator, where the shear wave oscillatesinthehorizontaldirectionbutpropagatesintheverticaldirection waves traveling in a circular AT-cut quartz crystal with a radius of r can be q obtained by solving the wave equation using the cylindrical coordinates [4]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi v n2p2 v2 f ¼ þ mk ð1:5Þ nmk 2p h2 r2 q q wheren = 1,3,5,…,m = 0,1,2,3,…,k = 1,2,3,…,andv isthekthrootof mk Bessel function of order m. If r (cid:4) h , the propagation of the shear wave can be q q treated one-dimensionally [4]: sffiffiffiffiffiffiffiffiffi sffiffiffiffiffi v n2p2 nv n l f (cid:5) ¼ ¼ q ¼nf ð1:6Þ n 2p h2 2h 2h q 0 q q q q wheref isthefundamentalresonantfrequency,l istheshearmodulusofquartz, 0 q q isthedensityofquartz,andv = (l /q )1/2.ItcanbeseenfromEq. (1.6)thatthe q q q fundamentalresonantfrequencyismainlyinfluencedbythethicknessofthequartz crystal since otherparameters are constant inthis equation, which makesthe AT- cutquartzcrystalasaquantitativelyultrasensitivemasssensorpossible.TheQCM sensor generally has a sandwich structure, namely, the quartz crystal is placed betweenapairofgoldelectrodes(Fig. 1.1).Whenaradiofrequency(RF)voltage isappliedacrosstheelectrodesneartheresonantfrequency,thequartzcrystalwill be excited to oscillate in the thickness shear mode at its fundamental resonant frequency.Assumingthattheadditionofathinlayertotheelectrodeswillinduce a change in the crystal thickness from h to h 0, then the frequency will change q q from f to f 0: n n nv f0 ¼ ð1:7Þ n 2h0 q Therefore, the frequency shift (Df) caused by the deposition of the additional layer can be expressed as: Df ¼f0(cid:3)f ¼(cid:3)nvðh0q(cid:3)hqÞ¼(cid:3)nvDhq ð1:8Þ n n 2h h0 2h h0 q q q q