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Neutron reflectivity and soft condensed matter
Current Opinion in Colloid & Interface Science 7 Ž2002. 139᎐147
Neutron reflectivity and soft condensed matter J. PenfoldU ISIS Facility, Rutherford-Appleton Laboratory, CLRC, Chilton, Didcot, Oxon OX11 0QX, UK
Abstract During the last 10᎐15 years neutron reflectivity has emerged as a powerful and important technique for the study of surfaces and interfaces. The selectivity and sensitivity afforded by deuteriumrhydrogen exchange makes the technique particularly attractive for application to the broad field of colloid and interface science. The development of the instrumentation, specialised sample environment equipment and analysis techniques has resulted its application to complex interfaces and environments and in the study of complex multi-component systems. This review provides a summary of those developments in the last two years. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Neutron reflectivity; Adsorption; Thin films
1. Introduction The specular reflectivity of neutrons provides information about the refractive index or scattering length density distribution normal to the surface or interface, and is directly related to the composition or concentration profile in the interfacial region. Grazing incidence geometry and the wavelength range of cold neutrons provides a wave-vector transfer, Q, 4 range Žwhere Qs sin, is the glancing angle of incidence, and , the neutron wavelength. well matched to the length scales of interest Ž; 10 to 4000 ˚ .. The scattering powers of hydrogen and deuterium A are vastly different, and provide the opportunity to manipulate the refractive index distribution by HrD isotopic substitution, without substantially altering the chemistry. The refractive index for neutrons is defined Nb 2 as n s 1 y , where N is the atomic number 2 density, and b the neutron scattering length Ž0.6674
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Tel.: q44-1235-4456-81; fax: q44-1235-4456-42. E-mail address: [email protected] ŽJ. Penfold..
= 10y1 2 cm for deuterium, and y0.374= 10y1 2 cm for hydrogen.. The ability to manipulate the ‘contrast’ is a powerful feature and extensively exploited. It provides the contrast to highlight the interface of a polymer bilayer, and the selectivity to study the adsorption of complex multi-component mixtures. Cold neutrons are also a penetrating probe, and this provides access to ‘buried’ interfaces. Studies are hence not limited to the air᎐solution and air᎐solid interfaces, but can also be made at the solid᎐solid, solid᎐solution and liquid᎐liquid interfaces. The emerging patterns of the application of neutron reflectivity in colloid and interface science are summarised in the main themes of this review. They involve the investigation of more complex interfaces, including bio-membranes, in-situ electrochemistry, and adsorption at the liquid᎐solid and liquid᎐liquid interfaces, and more complex environments, surfaces under shear or confinement. In the study of polymer and surfactant adsorption at interfaces, which have been predominantly the domain of neutron reflectivity, the trend is towards complex structures, mixtures and the development of nano-structures. The greater sophistication of experimental design is also reflected
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in the studies on solid polymer films. The article will briefly review progress over the broad area of applications relevant to colloid and interface science under the following categories; surfactant adsorption, polymer adsorption, polymer films, polymer᎐surfactant mixtures, bio-membranes, electrochemistry, and nano-structured films.
2. Surfactant adsorption The study of the adsorption of surfactants at interfaces has been one of the startling successes of the applications of neutron reflectivity, where the combination with HrD isotopic substitution has enabled not only adsorbed amounts but also the detailed structure of the monolayer to be determined. Lu et al. w1x have produced a comprehensive review of the broad range of investigations of surfactant adsorption at the air᎐water interface. In particular they highlight the detailed structural information that can be obtained, describe the potential for the study of complex mixtures, including polymer-surfactant mixtures, and put the studies in the context of information obtainable from other techniques. Adsorption at the liquid᎐solid interface is an important area, and Penfold et al. w2x have demonstrated the role of the specific interaction with the hy-
drophilic silica surface on the adsorption of cationicrnon-ionic surfactant mixtures, Hexadecyltrimethyl ammonium bromide, C 16 TABrhexaethylene monododecyl ether, C 12 E 6 . In a related study w3x they have investigated the temperature and time dependence of the adsorption of the surfactant mixture of the di-chain cationic surfactant, disterayloydoxylimethyl ammonium chloride and C 12 E 6 at the hydrophilic silica᎐solution interface, and how the adsorption is modified by the L ␣rL  , transition. In more concentrated surfactant solutions more complex surface structures develop, as illustrated by the recent studies by Salamat et al. w4x, and Li et al. w5x. The evolution of the lamellar structure at the liquid᎐solid interface for the surfactant mixture of sodium decyl sulfonate and pentaethylene monododecyl ether, C 2 E 5 with decreasing ionic content was interpreted in terms of undulation forces. Li et al. w5x interpreted both the specular and off-specular scattering arising from concentrated Aerosol-OT, AOT, solutions adsorbed at the air᎐water and liquid᎐solid interfaces. The off-specular scattering Žsee Fig. 1. was attributed to conformal roughness of the fluctuations in the adsorbed multilayer structure. The analysis of both the specular and off-specular scattering was consistent with increasing structural order with increasing temperature, and provided an explanation of the unusual temperature dependence of the lamellar spac-
Fig. 1. Off-specular scattering Ž q x y q z map. for 2% h-AOT in D 2 O at the air᎐solution interface at 25c.
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ing. Lang et al. w6x investigated the effect of the interface on the structure and phase transitions in the liquid crystalline phases of the non-ionic surfactant, C 12 E 4 . The formation of the hexagonal phase was found to be strongly influenced by the air᎐water interface, whereas the lamellar phase showed no dependence on the interface. Neutron reflectivity has provided information about surfactant adsorption on simpler surfactant systems that cannot be obtained by other techniques, and this is illustrated by the recent studies of Green et al. w7x, and Eastoe et al. w8᎐10x. Green et al. w7x have shown how changes in the chemical configuration of the hydrophobic chain affects the adsorption properties of non-ionic surfactants. Comparing the alkyl phenol ethoxylates with the alkyl ethoxylates it was found that the changes in the hydrophobic chain structure affects the structure of the adsorbed monolayer, but does not alter the limiting arearmolecule at the air᎐water interface. Eastoe et al. w9,10x used a combination of neutron reflectivity and surface tension to evaluate the equilibrium adsorption isotherms of the di-chain ionic surfactants, which are in the same structural family as AOT, and some fluorocarbon equivalents w8,9x which are important for the formation of water-in-CO 2 micro-emulsions. Finally the adsorption of surfactant meso-phases at interfaces has been used to template the growth of meso-porous films with well-defined structures. The recent work of White et al. w10,11x is a good example of the complementary use of X-ray and neutron techniques. Holt et al. w11x have studied the initial stages of the growth of silicated films at the solid᎐liquid interface in-situ, and in real time. The initial structure was found to depend upon the nature of silicon surface. However, the final structure and the induction time are independent of this, and driven by the bulk phase behaviour. In a related study Ruggles et al. w12x have investigated, using both X-ray and neutron reflectivity, the role of surfactant chain length and ionic strength on the nature of the silicated films templated at the air᎐water interface, and is related in a predictable way to the surfactant phase diagram.
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and this is now transforming our understanding of polymerrsurfactant adsorption. Staples et al. w13x have investigated the nature of the adsorption of the mixture of the cationic polymer, poly-dimethyl diallyl ammonium chloride, poly-dmdaac Žand its co-polymer with poly-acrylamide. and the surfactants sodium dodecyl sulfate, SDS and C 12 E 6 . The strong interaction between the oppositely charged SDS and poly-dmdaac is modified by the non-ionic surfactant C 12 E 6 , and a complex pattern of adsorption and surface tension behaviour is observed. The variations in surfactant and polymer adsorption arise from the competition between surface and bulk complex formation. Cooke et al. w14x have shown that the surface tension plots of gelatinrSDS mixture show features similar to those observed in more weakly interacting systems such as poly-Žethylene oxide., PEOrSDS, but that the underlying behaviour is different due to the stronger interaction. At concentrations above which free SDS micelles form, the surface is similar to that of SDS alone, but at lower SDS concentrations the presence of SDSrgelatin complexes greatly enhances the SDS adsorption. Structural measurements Žsee Fig. 2. reveal a thicker layer consistent with the adsorption of a polymerrsurfactant complex at the interface. In the study of the polymer surfactant mixture of poly Ž nisopropyl acrylamide. Žpoly-NIPAM.rSDS Richardson et al. w15x demonstrated that both SDS and tem-
3. Adsorption of polymer–surfactant mixtures Until recently most of the experimental investigations into polymerrsurfactant complexes have focussed on the solution aggregate behaviour and not their adsorption properties. This is because there have been few techniques which measure directly the surface composition of such layers, and techniques such as surface tension are difficult to interpret comprehensively. Neutron reflection is capable of providing both structural and compositional information,
Fig. 2. Schematic diagram of the structure of the adsorbed SDSrgelatin layer Ža. c ) SDS cmc Ž15 mM. Žb. at 1.15 mM SDS.
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perature modify the adsorption of the poly-NIPAM at the air᎐water interface. Above the lower critical temperature ŽLCST. the thickness of the adsorbed layer and the amount adsorbed increases markedly. Langevin w16x has recently reviewed the complementary use of surface tension, ellipsometry, X-ray and neutron reflectivity, film balance and rheology measurements on a range of polyelectrolytersurfactant mixtures. There are many similarities between the adsorption of polymerrsurfactant mixtures at interfaces and that of proteinrsurfactant mixtures. In a series of measurements Green et al. w17᎐19x have used neutron reflectivity to study the interaction of lysozyme and SDS at the air᎐water and liquid᎐solid interfaces, and of lysozyme and C 12 E 5 at the air᎐water interface. A complex surface tension behaviour is shown to be due to changes in the SDS and lysozyme adsorption at the interface. The associated changes in the surface structure see the progression from the adsorption of protein rich surface complexes to an almost pure surfactant monolayer w17x. The less strong interaction with the C 12 E 5 gives rise to a different pattern of behaviour w18x.
4. Biomembranes In the last few years there has been a greatly increased interest in using neutron reflectivity to study surfaces and thin films of biological or biomimetic interest. These involve protein adsorption Žincluding proteinrsurfactant mixtures, already discussed in the previous section because of their similarity with polymer᎐surfactant mixtures., model bio-membranes, and the nature of protein᎐membrane interactions. Krueger w20x and Fragneto-Cusani w21x have recently reviewed aspects of the applications of neutron reflectivity to systems of biological relevance. Both reviews emphasised primarily the formation and characterisation of model bilayer membranes, and the interaction of peptides or proteins with that layer. Fragneto et al. w21,22x have described in detail how ‘fluid floating’ bilayers of L-␣-di-stearoyl phosphatidylcholine, DSPC and di-palmtoyl phosphatidyl-choline, DPPC, can be established from a combination of Langmuir᎐Blodgett and Langmuir᎐Schaeffer techniques, and have characterised the transition from a rigid Žgel-like. to fluid phase with temperature using neutron reflectivity. Using this approach Fragneto et al. w23x have investigated the interaction of the peptide, the third helix of the antennapedia homeodomain, p-Antp 43 ᎐ 58 . with the membrane. The peptide was found to be associated primarily with the headgroup region of the mixed DPPCrDPPS membrane, and produced an increase in the membrane roughness. Bayerl and co-workers have successfully used
Fig. 3. Models of streptavidin binding to mixed biotinylated fullerenerDPPC monolayers.
lipid monolayers to investigate a range of membrane᎐protein interactions. In their most recent work w24,25x they have investigated the nature of fullerene based monolayers w24x, and used fullerene amphiphiles to control receptor binding. A biotinylated amphiphilic fullerene incorporated into a DPPC monolayer at low surface pressures promoted streptavidin binding, and which then remained bound at high pressures. At high pressures the biotin anchor is retracted into the headgroup region and binding is inhibited Žsee Fig. 3. An important related area of investigation has been the study of protein adsorption at a variety of interfaces, and the similarity between proteinrsurfactant and polymerrsurfactant adsorption has already been highlighted in the previous section. Fragneto et al. w26x have studied the adsorption of -casein and lactaglobulin onto hydrophobic silicon substrates. The pH dependence of the adsorption and the extent to which the tertiary structure is retained is discussed. Nylander et al. w27x have investigated the effect of
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electrolyte on the adsorption of -casein at the hydrophobic solid᎐solution interface.
5. In-situ electrochemistry and other complex environments The favourable transmission of thermal and cold neutrons through crystalline materials Žsuch as silicon and quartz. provide the opportunity to investigate ‘buried’ interfaces and exploit more complex environments. This has been exploited in some in-situ electrochemistry measurements. Although the amount of material electrochemically deposited can be measured straightforwardly using techniques such as the quartz micro-balance, the structure of the deposited film, and the solvent penetration into that film cannot be measured in-situ except by neutron reflectivity. This has been demonstrated by Glidle et al. w28x, and Bailey et al. w29x in the electro-polymerised films of polyŽpyrrol-N-propionic acid. and in the electro-active polymer films of polyŽ o-toluidene.. In the former example, the degree of solvent penetration was determined, and the permeation profile of the Ni 2q ions, which chelate the polymer’s carboxylic acid moities, the spatial distribution of the polymer and solvent were deduced in-situ. Swann et al. w30x showed that the same approach can be used to observe the swelling of electro-polymerised polyŽpyrrole. film by vapour adsorption; providing important information for the development of electronic ‘nose’ sensors. Burgess et al. w31x have used neutron reflectivity to follow the electro-deposition of the anionic surfactant SDS onto an AuŽ111. electrode surface. The neutron reflectivity data in combination with electrochemical data and SPM Žscanning probe microscopy. measurements provide a detailed description of the inter-digitated condensed bilayer film that is formed. The same group w32x have used electrochemical and neutron reflectivity measurements to follow the transfer of 4-pentadecyl-pyride Žan insoluble amphiphile. form the gas᎐solution to the metal᎐solution wAuŽ111. gold electrodex interface. Measuring adsorption at the liquid᎐liquid interface Ža buried oil᎐water interface . is experimentally challenging using any technique. Bowers et al. w33x have successfully established a methodology by trapping a thin oil layer between a hydrophobic silica surface and an aqueous sub-phase. They demonstrated this methodology and their analysis approach with some recent measurements on the adsorption of the block co-polymer polybutadiene-poly-Žethylene oxide., PBPEO, at the hexadecane᎐water interface. The volume fraction distributions of each polymer block at the interface was determined, and was found to be consistent with expectations based on the results at the
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air᎐water interface. Strutwolf et al. w34x have used a similar approach to investigate the interface between two immiscible electrolyte solutions, of 1,2-dichloroethaneraqueous potassium hydroxide. The nature of complex fluids under confinement has attracted much current interest. To date much of the progress has centred around techniques like the surface force apparatus, SFA, and other scanning probe microscope techniques. More direct information has been obtained using X-ray scattering and reflectivity, and other confinement geometries have been successfully contrived for thin polymer films. The requirement of a larger illuminated area for neutron reflectivity measurements Žcompared to Xrays. has made such measurements difficult. Kuhl et al. w35x have made remarkable progress in combination with the surface force apparatus approach with in-situ neutron reflectivity measurements, and have demonstrated that it is possible to measure interfer˚ between ence fringes from a separation ; 1000 A single crystal substrates of quartz and sapphire.
6. Polymer adsorption The nature of polymer adsorption is important for a wide range of technologies, and has been extensively studied both experimentally and theoretically. Neutron reflectivity has emerged as a powerful technique for determining adsorbed amounts and the structure of the adsorbed layer at both the air᎐solution and liquid᎐solid interfaces. Kent w36x has summarised a comprehensive study of the nature of tethered chains under a variety of solution conditions, using Langmuir monolayers of the diblock co-polymer poly-Ždimethylsiloxane.-polystyrene, PDMS-PS. The PDMS acts as a strongly adsorbing block at the air᎐water interface, and neutron reflectivity measurements have determined the segment profile of the PS block as a function of surface density, molecular weight and solution conditions Žfrom good to theta solvent conditions., see Fig. 4. The strong stretching limit assumed from SFC Žself-consistent field., calculations and Scaling theories are found not to be valid over the entire surface density range. Over a wide range of surface densities, the tethered chain profile can be described by theories of weakly interacting or non-interacting chains. In a related study Bowers et al. w37x have investigated the structure of the spread film of polybutadienepolyŽethylene oxide., PB-PEO, diblock co-polymer at the air᎐water interface. The model that was consistent with the data included a thin PB layer, and a brush-like PEO structure. At higher surface coverages there is evidence for surface aggregates in the region adjacent to the monolayer. Shin et al. w38x have
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Fig. 4. Schematic representation of the Langmuir monolayer for PDMS-PS diblock co-polymers for Ža. symmetrical co-polymers, Žb. asymmetrical co-polymers ŽNps 4 NPDMS ..
used both X-ray and neutron reflectivity to determine the nature of the surface ordering of the alkyl side chains of polystyrenerpolyelectrolyte diblock co-polymer Langmuir film at the air᎐water interface. The results, in particular, show that although the polyelectrolyte block is water-soluble it remains adsorbed at the interface throughout compression. Currie et al. w39x have investigated the structure of grafted co-polymers of polystyrene-polyethylene oxide ŽPS-PEO. at high grafting densities Žbrush regime.. At relatively low grafting densities the brush is block-like, and parabolic at higher grafting densities, in good agreement with SCF theories. Bimodal, rather than monodisperse, brushes reveal a more complex structure, in which the longer chains are more strongly stretched than in monodisperse brushes. Yim et al. w40x have determined the segment density profile of the strong polyelectrolyte, polystyrene sulfonate ŽPSS. at the air᎐water interface, as a function of molecular weight and electrolyte. They report higher adsorbed amounts than previously reported for solid surfaces, consistent with a stronger surface attraction. The bilayer profiles obtained are rather different to the theoretically predicted profiles. At low electrolyte concentrations the profile collapses to a simple dense layer, corresponding to the chains lying nearly flat at the interface. The presence of a LCST gives polyŽ N-isopropyl acylamide., poly-NIPAM some interesting properties. Pelton et al. w41x have investigated the effects of temperature and the introduction of the co-solvent methanol, on the adsorption of poly-NIPAM at the air᎐solution interface. The adsorption reflects the solution properties. At high methanol concentrations there is no adsorption, as methanol is a good solvent for poly-NIPAM. At lower methanol concentrations and at temperatures below the LCST an adsorbed monolayer is observed, whereas above the LCST a thick layer Ždue to phase separation. is formed. The study of polymer adsorption at the solid᎐solution interface is equally important. Steitz et al. w42x have studied the structure of polyelectrolyte multilayers built from alternating layers of anions and cations. X-Ray and neutron reflectivity measurements
have provided detailed information about the density gradient of the polyelectrolyte chains across the film, and show the influence of the water content on the film’s internal structure. The equilibrium structure of polymer brushes Žstrongly stretched and terminally attached chains. have been extensively studied Žsee recent studies, w36᎐40x. and many aspects are now well understood. Much less is known about the behaviour of brushes under shear. Specially designed cells to investigate such affects using neutron reflectivity have been developed w43,44x. Baker et al. w43x found that PS-PEO co-polymer brushes, in good solvent, remain remarkably robust under shear, contrary to some theoretical predictions. A similar conclusion was drawn by Ivkov et al. w43x on polystyrene brushes.
7. Thin polymer films Neutron reflectivity has been extensively used to study a variety of phenomena in polymer thin films, including the nature of polymer᎐polymer interfaces, micro-phase separated structures, partitioning or segregation at interfaces, and the effects of confinement. The examples discussed in this section summarise some of the recent developments and studies in these areas. Sferrazza et al. w45x have reviewed a number of recent results on the nature of polymer᎐polymer interfaces, and on the interaction between grafted polymer chains and a chemically different polymer matrix Žsee Fig. 5.. The interface between low molecular weight immiscible polymers Žin this case polystyrene- poly-methyl methacrylate, PS-PMMA. was greater than that observed for higher molecular weights, and the interface width grew logarithmically with time or with a weak power law. The conformation of the high molecular weight grafted polymer chain abruptly changes with increasing temperature, from an extended conformation to a sharp interface with the polymer matrix. Sferrazza et al. w46x report in more detail the kinetics of the formation of the interface between the immiscible polymers PSrPMMA
J. Penfold r Current Opinion in Colloid & Interface Science 7 (2002) 139᎐147
Fig. 5. Schematic respresentation of grafted chains as a function of grafting density Ža. low grafting density Žb. high grafting density with extended chains, ‘brush’.
studies on d-PSrPMMA bilayers of high molecular weight polymers, and for d-PS Žwhere d refers to ˚ deuterated. thicknesses in the range 200 to 1000 A show an initial rapid increase, followed by a logarithmic dependence of the interfacial width on time. The results were interpreted as arising from the slow equilibrium time of the long wavelength capillary-wave fluctuations, consistent with the surface tension values and SCFT. Hayashi et al. w47,48x have used both neutron reflectivity and SANS to study the interface between the immiscible polymer pairs of polyamide and polysulfone; and the two methods provide a consistent estimate of the interfacial width. Measurements were made for non-reactive pairs and for pairs with reactive groups on the polysulfone. The interfacial widths for the reactive system are larger, due to the formation of diblock co-polymers at the interface. The scattering measurements showed that co-polymer formation arrests the coarsening of the phase-separated structures in the reactive system. Zhang et al. w49x studied the interfacial structures of the homopolymer poly-butadiene and a terpolymer Žbrominated polyŽisobutylene-cu-p-methylstyrene . by neutron reflectivity, scanning transmission X-ray microscopy and AFM Žatomic force microscopy.. The results showed that the interface behaviour between these elastomer
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blends was a direct function of the terpolymer chemical composition; the interfacial width decreased with increasing bromide functionality. Micro-phase separation, giving rise to ordered structures such as lamellae, have been studied in detail using neutron reflectivity, for a wide range of block co-polymers. Torikai et al. w50x have studied the interfacial structure of block and graft co-polymers of polystyrene-polyŽ2-vinylprridine., PS-PVP forming lamellar micro-phase segregated structures. Huang et al. w51x have studied the structure of lamellar PŽS-bMMA. diblock co-polymer films on a neutral random copolymer PŽS-r-MMA. brush surface. Upon annealing, micro-phase separation occurs quickly forming perpendicular and parallel lamellae emanating from the neutral and air surfaces. With annealing this rich pattern evolves with a slow increase in the amount of parallel lamellae. There is a strong commensurability with film thicknesses, which have a period equal to that of the natural lamellar period, which is due to the formation of defects. Surface or interfacial segregation or partitioning is an important process in adhesion, coatings, and in the many technological applications of thin polymer films. Neutron reflectivity, in combination with other techniques such as FRES Žforward recoil spectroscopy., can provide an important insight into such processes. Oslanec et al. w52x have used FRES and neutron reflectivity to determine the surface excess of the block co-polymer ŽPS-b-PMMA. in a bromostyrene ŽPBr x S. matrix. The degree of bromination is expected to increase the surface excess of the PS, due to the unfavourable PS᎐PBr interaction. However, the opposite was observed, and was attributed to the attractive interaction between the matrix polymer and the silicon surface. Hutchings et al. w53x have synthesised a heterotelechelic polystyrene with a tertiary amine functionality at one end and a fluorocarbon group at the other. Annealing a bilayer, comprising a thin layer of this polymer and thick PS layer, showed from neutron reflectivity and Nuclear Reaction Analysis ŽNRA., that the heterotelechelic polymer formed an excess at both interfaces, with the larger excess remaining at the substrate ᎐film. interface. There was no evidence for bridging by the heterotelechelic polymer, but the detailed distributions depended upon the polymer’s molecular weight and film thickness. Geoghegan et al. w54x have used neutron reflectivity and FRES to measure surface segregation of d-PS in a polystyrene matrix. For a linear polymer of high molecular weight the surface segregation is predicted by mean field theory, where the segregation is a slow function of time due to the larger number of entanglements in the cross-linked mixture. Butler et al. w55x
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showed that the interfacial roughness between a thin d-PS film and a thick high density polyethylene, ˚ depending HDPE, block varied between 11 to 15 A, upon the d-PS thickness. It was expected that the long range Van der Waals forces would destabilise the thinner film. The stability of the films, within the timescale of the measurements, was attributed to a mechanical confinement effect. Pochan et al. w56x showed that thin d-PS films exhibit a strong dependence of the co-efficient of thermal expansion on film thickness and on the nature of the confinement boundaries; and gave a clear indication that the nature of the substrate and superstrate boundaries must be taken into account theoretically.
8. Summary A review of the literature over just the last two years shows an extensive and exciting range applications of neutron reflectivity in colloid and surface science. The emphasis on more complex interfaces, complex environments, and complex multi-component systems is clear. The combination of techniques, neutron and X-ray reflectivity with other surface techniques is also increasingly prevalent. References 䢇 䢇䢇
of special interest of outstanding interest
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J. Penfold r Current Opinion in Colloid & Interface Science 7 (2002) 139᎐147 w51x Huang E, Mansky P, Russell TP et al. Macromolecules 2000;33:80. w52x Oslanec R, Composto RJ, Vlcek P. Macromolecules 2000;33:2200. w53x Hutchings LR, Richards RW, Thompson RL, Bucknall DG, Clough AS. Eur Phys J E 2001;5:451.
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w54x Geoghegan M, Boue F, Menelle A et al. J Phys: Codens Matt 2000;12:5129. w55x Butler SA, Fliggins JS, Bucknall DG, Sferrazza M. Macro Chem Phys 2001;202:2275. w56x Pochan DJ, Lin EK, Satija SK, Wu W. Macromolecules 2001;34:3041.
Neutron reflectivity and soft condensed matter J. PenfoldU ISIS Facility, Rutherford-Appleton Laboratory, CLRC, Chilton, Didcot, Oxon OX11 0QX, UK
Abstract During the last 10᎐15 years neutron reflectivity has emerged as a powerful and important technique for the study of surfaces and interfaces. The selectivity and sensitivity afforded by deuteriumrhydrogen exchange makes the technique particularly attractive for application to the broad field of colloid and interface science. The development of the instrumentation, specialised sample environment equipment and analysis techniques has resulted its application to complex interfaces and environments and in the study of complex multi-component systems. This review provides a summary of those developments in the last two years. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Neutron reflectivity; Adsorption; Thin films
1. Introduction The specular reflectivity of neutrons provides information about the refractive index or scattering length density distribution normal to the surface or interface, and is directly related to the composition or concentration profile in the interfacial region. Grazing incidence geometry and the wavelength range of cold neutrons provides a wave-vector transfer, Q, 4 range Žwhere Qs sin, is the glancing angle of incidence, and , the neutron wavelength. well matched to the length scales of interest Ž; 10 to 4000 ˚ .. The scattering powers of hydrogen and deuterium A are vastly different, and provide the opportunity to manipulate the refractive index distribution by HrD isotopic substitution, without substantially altering the chemistry. The refractive index for neutrons is defined Nb 2 as n s 1 y , where N is the atomic number 2 density, and b the neutron scattering length Ž0.6674
U
Tel.: q44-1235-4456-81; fax: q44-1235-4456-42. E-mail address: [email protected] ŽJ. Penfold..
= 10y1 2 cm for deuterium, and y0.374= 10y1 2 cm for hydrogen.. The ability to manipulate the ‘contrast’ is a powerful feature and extensively exploited. It provides the contrast to highlight the interface of a polymer bilayer, and the selectivity to study the adsorption of complex multi-component mixtures. Cold neutrons are also a penetrating probe, and this provides access to ‘buried’ interfaces. Studies are hence not limited to the air᎐solution and air᎐solid interfaces, but can also be made at the solid᎐solid, solid᎐solution and liquid᎐liquid interfaces. The emerging patterns of the application of neutron reflectivity in colloid and interface science are summarised in the main themes of this review. They involve the investigation of more complex interfaces, including bio-membranes, in-situ electrochemistry, and adsorption at the liquid᎐solid and liquid᎐liquid interfaces, and more complex environments, surfaces under shear or confinement. In the study of polymer and surfactant adsorption at interfaces, which have been predominantly the domain of neutron reflectivity, the trend is towards complex structures, mixtures and the development of nano-structures. The greater sophistication of experimental design is also reflected
1359-0294r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 0 2 9 4 Ž 0 2 . 0 0 0 1 5 - 8
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in the studies on solid polymer films. The article will briefly review progress over the broad area of applications relevant to colloid and interface science under the following categories; surfactant adsorption, polymer adsorption, polymer films, polymer᎐surfactant mixtures, bio-membranes, electrochemistry, and nano-structured films.
2. Surfactant adsorption The study of the adsorption of surfactants at interfaces has been one of the startling successes of the applications of neutron reflectivity, where the combination with HrD isotopic substitution has enabled not only adsorbed amounts but also the detailed structure of the monolayer to be determined. Lu et al. w1x have produced a comprehensive review of the broad range of investigations of surfactant adsorption at the air᎐water interface. In particular they highlight the detailed structural information that can be obtained, describe the potential for the study of complex mixtures, including polymer-surfactant mixtures, and put the studies in the context of information obtainable from other techniques. Adsorption at the liquid᎐solid interface is an important area, and Penfold et al. w2x have demonstrated the role of the specific interaction with the hy-
drophilic silica surface on the adsorption of cationicrnon-ionic surfactant mixtures, Hexadecyltrimethyl ammonium bromide, C 16 TABrhexaethylene monododecyl ether, C 12 E 6 . In a related study w3x they have investigated the temperature and time dependence of the adsorption of the surfactant mixture of the di-chain cationic surfactant, disterayloydoxylimethyl ammonium chloride and C 12 E 6 at the hydrophilic silica᎐solution interface, and how the adsorption is modified by the L ␣rL  , transition. In more concentrated surfactant solutions more complex surface structures develop, as illustrated by the recent studies by Salamat et al. w4x, and Li et al. w5x. The evolution of the lamellar structure at the liquid᎐solid interface for the surfactant mixture of sodium decyl sulfonate and pentaethylene monododecyl ether, C 2 E 5 with decreasing ionic content was interpreted in terms of undulation forces. Li et al. w5x interpreted both the specular and off-specular scattering arising from concentrated Aerosol-OT, AOT, solutions adsorbed at the air᎐water and liquid᎐solid interfaces. The off-specular scattering Žsee Fig. 1. was attributed to conformal roughness of the fluctuations in the adsorbed multilayer structure. The analysis of both the specular and off-specular scattering was consistent with increasing structural order with increasing temperature, and provided an explanation of the unusual temperature dependence of the lamellar spac-
Fig. 1. Off-specular scattering Ž q x y q z map. for 2% h-AOT in D 2 O at the air᎐solution interface at 25c.
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ing. Lang et al. w6x investigated the effect of the interface on the structure and phase transitions in the liquid crystalline phases of the non-ionic surfactant, C 12 E 4 . The formation of the hexagonal phase was found to be strongly influenced by the air᎐water interface, whereas the lamellar phase showed no dependence on the interface. Neutron reflectivity has provided information about surfactant adsorption on simpler surfactant systems that cannot be obtained by other techniques, and this is illustrated by the recent studies of Green et al. w7x, and Eastoe et al. w8᎐10x. Green et al. w7x have shown how changes in the chemical configuration of the hydrophobic chain affects the adsorption properties of non-ionic surfactants. Comparing the alkyl phenol ethoxylates with the alkyl ethoxylates it was found that the changes in the hydrophobic chain structure affects the structure of the adsorbed monolayer, but does not alter the limiting arearmolecule at the air᎐water interface. Eastoe et al. w9,10x used a combination of neutron reflectivity and surface tension to evaluate the equilibrium adsorption isotherms of the di-chain ionic surfactants, which are in the same structural family as AOT, and some fluorocarbon equivalents w8,9x which are important for the formation of water-in-CO 2 micro-emulsions. Finally the adsorption of surfactant meso-phases at interfaces has been used to template the growth of meso-porous films with well-defined structures. The recent work of White et al. w10,11x is a good example of the complementary use of X-ray and neutron techniques. Holt et al. w11x have studied the initial stages of the growth of silicated films at the solid᎐liquid interface in-situ, and in real time. The initial structure was found to depend upon the nature of silicon surface. However, the final structure and the induction time are independent of this, and driven by the bulk phase behaviour. In a related study Ruggles et al. w12x have investigated, using both X-ray and neutron reflectivity, the role of surfactant chain length and ionic strength on the nature of the silicated films templated at the air᎐water interface, and is related in a predictable way to the surfactant phase diagram.
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and this is now transforming our understanding of polymerrsurfactant adsorption. Staples et al. w13x have investigated the nature of the adsorption of the mixture of the cationic polymer, poly-dimethyl diallyl ammonium chloride, poly-dmdaac Žand its co-polymer with poly-acrylamide. and the surfactants sodium dodecyl sulfate, SDS and C 12 E 6 . The strong interaction between the oppositely charged SDS and poly-dmdaac is modified by the non-ionic surfactant C 12 E 6 , and a complex pattern of adsorption and surface tension behaviour is observed. The variations in surfactant and polymer adsorption arise from the competition between surface and bulk complex formation. Cooke et al. w14x have shown that the surface tension plots of gelatinrSDS mixture show features similar to those observed in more weakly interacting systems such as poly-Žethylene oxide., PEOrSDS, but that the underlying behaviour is different due to the stronger interaction. At concentrations above which free SDS micelles form, the surface is similar to that of SDS alone, but at lower SDS concentrations the presence of SDSrgelatin complexes greatly enhances the SDS adsorption. Structural measurements Žsee Fig. 2. reveal a thicker layer consistent with the adsorption of a polymerrsurfactant complex at the interface. In the study of the polymer surfactant mixture of poly Ž nisopropyl acrylamide. Žpoly-NIPAM.rSDS Richardson et al. w15x demonstrated that both SDS and tem-
3. Adsorption of polymer–surfactant mixtures Until recently most of the experimental investigations into polymerrsurfactant complexes have focussed on the solution aggregate behaviour and not their adsorption properties. This is because there have been few techniques which measure directly the surface composition of such layers, and techniques such as surface tension are difficult to interpret comprehensively. Neutron reflection is capable of providing both structural and compositional information,
Fig. 2. Schematic diagram of the structure of the adsorbed SDSrgelatin layer Ža. c ) SDS cmc Ž15 mM. Žb. at 1.15 mM SDS.
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perature modify the adsorption of the poly-NIPAM at the air᎐water interface. Above the lower critical temperature ŽLCST. the thickness of the adsorbed layer and the amount adsorbed increases markedly. Langevin w16x has recently reviewed the complementary use of surface tension, ellipsometry, X-ray and neutron reflectivity, film balance and rheology measurements on a range of polyelectrolytersurfactant mixtures. There are many similarities between the adsorption of polymerrsurfactant mixtures at interfaces and that of proteinrsurfactant mixtures. In a series of measurements Green et al. w17᎐19x have used neutron reflectivity to study the interaction of lysozyme and SDS at the air᎐water and liquid᎐solid interfaces, and of lysozyme and C 12 E 5 at the air᎐water interface. A complex surface tension behaviour is shown to be due to changes in the SDS and lysozyme adsorption at the interface. The associated changes in the surface structure see the progression from the adsorption of protein rich surface complexes to an almost pure surfactant monolayer w17x. The less strong interaction with the C 12 E 5 gives rise to a different pattern of behaviour w18x.
4. Biomembranes In the last few years there has been a greatly increased interest in using neutron reflectivity to study surfaces and thin films of biological or biomimetic interest. These involve protein adsorption Žincluding proteinrsurfactant mixtures, already discussed in the previous section because of their similarity with polymer᎐surfactant mixtures., model bio-membranes, and the nature of protein᎐membrane interactions. Krueger w20x and Fragneto-Cusani w21x have recently reviewed aspects of the applications of neutron reflectivity to systems of biological relevance. Both reviews emphasised primarily the formation and characterisation of model bilayer membranes, and the interaction of peptides or proteins with that layer. Fragneto et al. w21,22x have described in detail how ‘fluid floating’ bilayers of L-␣-di-stearoyl phosphatidylcholine, DSPC and di-palmtoyl phosphatidyl-choline, DPPC, can be established from a combination of Langmuir᎐Blodgett and Langmuir᎐Schaeffer techniques, and have characterised the transition from a rigid Žgel-like. to fluid phase with temperature using neutron reflectivity. Using this approach Fragneto et al. w23x have investigated the interaction of the peptide, the third helix of the antennapedia homeodomain, p-Antp 43 ᎐ 58 . with the membrane. The peptide was found to be associated primarily with the headgroup region of the mixed DPPCrDPPS membrane, and produced an increase in the membrane roughness. Bayerl and co-workers have successfully used
Fig. 3. Models of streptavidin binding to mixed biotinylated fullerenerDPPC monolayers.
lipid monolayers to investigate a range of membrane᎐protein interactions. In their most recent work w24,25x they have investigated the nature of fullerene based monolayers w24x, and used fullerene amphiphiles to control receptor binding. A biotinylated amphiphilic fullerene incorporated into a DPPC monolayer at low surface pressures promoted streptavidin binding, and which then remained bound at high pressures. At high pressures the biotin anchor is retracted into the headgroup region and binding is inhibited Žsee Fig. 3. An important related area of investigation has been the study of protein adsorption at a variety of interfaces, and the similarity between proteinrsurfactant and polymerrsurfactant adsorption has already been highlighted in the previous section. Fragneto et al. w26x have studied the adsorption of -casein and lactaglobulin onto hydrophobic silicon substrates. The pH dependence of the adsorption and the extent to which the tertiary structure is retained is discussed. Nylander et al. w27x have investigated the effect of
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electrolyte on the adsorption of -casein at the hydrophobic solid᎐solution interface.
5. In-situ electrochemistry and other complex environments The favourable transmission of thermal and cold neutrons through crystalline materials Žsuch as silicon and quartz. provide the opportunity to investigate ‘buried’ interfaces and exploit more complex environments. This has been exploited in some in-situ electrochemistry measurements. Although the amount of material electrochemically deposited can be measured straightforwardly using techniques such as the quartz micro-balance, the structure of the deposited film, and the solvent penetration into that film cannot be measured in-situ except by neutron reflectivity. This has been demonstrated by Glidle et al. w28x, and Bailey et al. w29x in the electro-polymerised films of polyŽpyrrol-N-propionic acid. and in the electro-active polymer films of polyŽ o-toluidene.. In the former example, the degree of solvent penetration was determined, and the permeation profile of the Ni 2q ions, which chelate the polymer’s carboxylic acid moities, the spatial distribution of the polymer and solvent were deduced in-situ. Swann et al. w30x showed that the same approach can be used to observe the swelling of electro-polymerised polyŽpyrrole. film by vapour adsorption; providing important information for the development of electronic ‘nose’ sensors. Burgess et al. w31x have used neutron reflectivity to follow the electro-deposition of the anionic surfactant SDS onto an AuŽ111. electrode surface. The neutron reflectivity data in combination with electrochemical data and SPM Žscanning probe microscopy. measurements provide a detailed description of the inter-digitated condensed bilayer film that is formed. The same group w32x have used electrochemical and neutron reflectivity measurements to follow the transfer of 4-pentadecyl-pyride Žan insoluble amphiphile. form the gas᎐solution to the metal᎐solution wAuŽ111. gold electrodex interface. Measuring adsorption at the liquid᎐liquid interface Ža buried oil᎐water interface . is experimentally challenging using any technique. Bowers et al. w33x have successfully established a methodology by trapping a thin oil layer between a hydrophobic silica surface and an aqueous sub-phase. They demonstrated this methodology and their analysis approach with some recent measurements on the adsorption of the block co-polymer polybutadiene-poly-Žethylene oxide., PBPEO, at the hexadecane᎐water interface. The volume fraction distributions of each polymer block at the interface was determined, and was found to be consistent with expectations based on the results at the
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air᎐water interface. Strutwolf et al. w34x have used a similar approach to investigate the interface between two immiscible electrolyte solutions, of 1,2-dichloroethaneraqueous potassium hydroxide. The nature of complex fluids under confinement has attracted much current interest. To date much of the progress has centred around techniques like the surface force apparatus, SFA, and other scanning probe microscope techniques. More direct information has been obtained using X-ray scattering and reflectivity, and other confinement geometries have been successfully contrived for thin polymer films. The requirement of a larger illuminated area for neutron reflectivity measurements Žcompared to Xrays. has made such measurements difficult. Kuhl et al. w35x have made remarkable progress in combination with the surface force apparatus approach with in-situ neutron reflectivity measurements, and have demonstrated that it is possible to measure interfer˚ between ence fringes from a separation ; 1000 A single crystal substrates of quartz and sapphire.
6. Polymer adsorption The nature of polymer adsorption is important for a wide range of technologies, and has been extensively studied both experimentally and theoretically. Neutron reflectivity has emerged as a powerful technique for determining adsorbed amounts and the structure of the adsorbed layer at both the air᎐solution and liquid᎐solid interfaces. Kent w36x has summarised a comprehensive study of the nature of tethered chains under a variety of solution conditions, using Langmuir monolayers of the diblock co-polymer poly-Ždimethylsiloxane.-polystyrene, PDMS-PS. The PDMS acts as a strongly adsorbing block at the air᎐water interface, and neutron reflectivity measurements have determined the segment profile of the PS block as a function of surface density, molecular weight and solution conditions Žfrom good to theta solvent conditions., see Fig. 4. The strong stretching limit assumed from SFC Žself-consistent field., calculations and Scaling theories are found not to be valid over the entire surface density range. Over a wide range of surface densities, the tethered chain profile can be described by theories of weakly interacting or non-interacting chains. In a related study Bowers et al. w37x have investigated the structure of the spread film of polybutadienepolyŽethylene oxide., PB-PEO, diblock co-polymer at the air᎐water interface. The model that was consistent with the data included a thin PB layer, and a brush-like PEO structure. At higher surface coverages there is evidence for surface aggregates in the region adjacent to the monolayer. Shin et al. w38x have
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Fig. 4. Schematic representation of the Langmuir monolayer for PDMS-PS diblock co-polymers for Ža. symmetrical co-polymers, Žb. asymmetrical co-polymers ŽNps 4 NPDMS ..
used both X-ray and neutron reflectivity to determine the nature of the surface ordering of the alkyl side chains of polystyrenerpolyelectrolyte diblock co-polymer Langmuir film at the air᎐water interface. The results, in particular, show that although the polyelectrolyte block is water-soluble it remains adsorbed at the interface throughout compression. Currie et al. w39x have investigated the structure of grafted co-polymers of polystyrene-polyethylene oxide ŽPS-PEO. at high grafting densities Žbrush regime.. At relatively low grafting densities the brush is block-like, and parabolic at higher grafting densities, in good agreement with SCF theories. Bimodal, rather than monodisperse, brushes reveal a more complex structure, in which the longer chains are more strongly stretched than in monodisperse brushes. Yim et al. w40x have determined the segment density profile of the strong polyelectrolyte, polystyrene sulfonate ŽPSS. at the air᎐water interface, as a function of molecular weight and electrolyte. They report higher adsorbed amounts than previously reported for solid surfaces, consistent with a stronger surface attraction. The bilayer profiles obtained are rather different to the theoretically predicted profiles. At low electrolyte concentrations the profile collapses to a simple dense layer, corresponding to the chains lying nearly flat at the interface. The presence of a LCST gives polyŽ N-isopropyl acylamide., poly-NIPAM some interesting properties. Pelton et al. w41x have investigated the effects of temperature and the introduction of the co-solvent methanol, on the adsorption of poly-NIPAM at the air᎐solution interface. The adsorption reflects the solution properties. At high methanol concentrations there is no adsorption, as methanol is a good solvent for poly-NIPAM. At lower methanol concentrations and at temperatures below the LCST an adsorbed monolayer is observed, whereas above the LCST a thick layer Ždue to phase separation. is formed. The study of polymer adsorption at the solid᎐solution interface is equally important. Steitz et al. w42x have studied the structure of polyelectrolyte multilayers built from alternating layers of anions and cations. X-Ray and neutron reflectivity measurements
have provided detailed information about the density gradient of the polyelectrolyte chains across the film, and show the influence of the water content on the film’s internal structure. The equilibrium structure of polymer brushes Žstrongly stretched and terminally attached chains. have been extensively studied Žsee recent studies, w36᎐40x. and many aspects are now well understood. Much less is known about the behaviour of brushes under shear. Specially designed cells to investigate such affects using neutron reflectivity have been developed w43,44x. Baker et al. w43x found that PS-PEO co-polymer brushes, in good solvent, remain remarkably robust under shear, contrary to some theoretical predictions. A similar conclusion was drawn by Ivkov et al. w43x on polystyrene brushes.
7. Thin polymer films Neutron reflectivity has been extensively used to study a variety of phenomena in polymer thin films, including the nature of polymer᎐polymer interfaces, micro-phase separated structures, partitioning or segregation at interfaces, and the effects of confinement. The examples discussed in this section summarise some of the recent developments and studies in these areas. Sferrazza et al. w45x have reviewed a number of recent results on the nature of polymer᎐polymer interfaces, and on the interaction between grafted polymer chains and a chemically different polymer matrix Žsee Fig. 5.. The interface between low molecular weight immiscible polymers Žin this case polystyrene- poly-methyl methacrylate, PS-PMMA. was greater than that observed for higher molecular weights, and the interface width grew logarithmically with time or with a weak power law. The conformation of the high molecular weight grafted polymer chain abruptly changes with increasing temperature, from an extended conformation to a sharp interface with the polymer matrix. Sferrazza et al. w46x report in more detail the kinetics of the formation of the interface between the immiscible polymers PSrPMMA
J. Penfold r Current Opinion in Colloid & Interface Science 7 (2002) 139᎐147
Fig. 5. Schematic respresentation of grafted chains as a function of grafting density Ža. low grafting density Žb. high grafting density with extended chains, ‘brush’.
studies on d-PSrPMMA bilayers of high molecular weight polymers, and for d-PS Žwhere d refers to ˚ deuterated. thicknesses in the range 200 to 1000 A show an initial rapid increase, followed by a logarithmic dependence of the interfacial width on time. The results were interpreted as arising from the slow equilibrium time of the long wavelength capillary-wave fluctuations, consistent with the surface tension values and SCFT. Hayashi et al. w47,48x have used both neutron reflectivity and SANS to study the interface between the immiscible polymer pairs of polyamide and polysulfone; and the two methods provide a consistent estimate of the interfacial width. Measurements were made for non-reactive pairs and for pairs with reactive groups on the polysulfone. The interfacial widths for the reactive system are larger, due to the formation of diblock co-polymers at the interface. The scattering measurements showed that co-polymer formation arrests the coarsening of the phase-separated structures in the reactive system. Zhang et al. w49x studied the interfacial structures of the homopolymer poly-butadiene and a terpolymer Žbrominated polyŽisobutylene-cu-p-methylstyrene . by neutron reflectivity, scanning transmission X-ray microscopy and AFM Žatomic force microscopy.. The results showed that the interface behaviour between these elastomer
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blends was a direct function of the terpolymer chemical composition; the interfacial width decreased with increasing bromide functionality. Micro-phase separation, giving rise to ordered structures such as lamellae, have been studied in detail using neutron reflectivity, for a wide range of block co-polymers. Torikai et al. w50x have studied the interfacial structure of block and graft co-polymers of polystyrene-polyŽ2-vinylprridine., PS-PVP forming lamellar micro-phase segregated structures. Huang et al. w51x have studied the structure of lamellar PŽS-bMMA. diblock co-polymer films on a neutral random copolymer PŽS-r-MMA. brush surface. Upon annealing, micro-phase separation occurs quickly forming perpendicular and parallel lamellae emanating from the neutral and air surfaces. With annealing this rich pattern evolves with a slow increase in the amount of parallel lamellae. There is a strong commensurability with film thicknesses, which have a period equal to that of the natural lamellar period, which is due to the formation of defects. Surface or interfacial segregation or partitioning is an important process in adhesion, coatings, and in the many technological applications of thin polymer films. Neutron reflectivity, in combination with other techniques such as FRES Žforward recoil spectroscopy., can provide an important insight into such processes. Oslanec et al. w52x have used FRES and neutron reflectivity to determine the surface excess of the block co-polymer ŽPS-b-PMMA. in a bromostyrene ŽPBr x S. matrix. The degree of bromination is expected to increase the surface excess of the PS, due to the unfavourable PS᎐PBr interaction. However, the opposite was observed, and was attributed to the attractive interaction between the matrix polymer and the silicon surface. Hutchings et al. w53x have synthesised a heterotelechelic polystyrene with a tertiary amine functionality at one end and a fluorocarbon group at the other. Annealing a bilayer, comprising a thin layer of this polymer and thick PS layer, showed from neutron reflectivity and Nuclear Reaction Analysis ŽNRA., that the heterotelechelic polymer formed an excess at both interfaces, with the larger excess remaining at the substrate ᎐film. interface. There was no evidence for bridging by the heterotelechelic polymer, but the detailed distributions depended upon the polymer’s molecular weight and film thickness. Geoghegan et al. w54x have used neutron reflectivity and FRES to measure surface segregation of d-PS in a polystyrene matrix. For a linear polymer of high molecular weight the surface segregation is predicted by mean field theory, where the segregation is a slow function of time due to the larger number of entanglements in the cross-linked mixture. Butler et al. w55x
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showed that the interfacial roughness between a thin d-PS film and a thick high density polyethylene, ˚ depending HDPE, block varied between 11 to 15 A, upon the d-PS thickness. It was expected that the long range Van der Waals forces would destabilise the thinner film. The stability of the films, within the timescale of the measurements, was attributed to a mechanical confinement effect. Pochan et al. w56x showed that thin d-PS films exhibit a strong dependence of the co-efficient of thermal expansion on film thickness and on the nature of the confinement boundaries; and gave a clear indication that the nature of the substrate and superstrate boundaries must be taken into account theoretically.
8. Summary A review of the literature over just the last two years shows an extensive and exciting range applications of neutron reflectivity in colloid and surface science. The emphasis on more complex interfaces, complex environments, and complex multi-component systems is clear. The combination of techniques, neutron and X-ray reflectivity with other surface techniques is also increasingly prevalent. References 䢇 䢇䢇
of special interest of outstanding interest
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