neutron reflectivity and soft condensed matter

Ž .Current Opinion in Colloid & Interface Science 7 2002 139�147

Neutron reflectivity and soft condensed matter

J. Penfold�

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 ofsurfaces and interfaces. The selectivity and sensitivity afforded by deuterium�hydrogen exchange makes the techniqueparticularly attractive for application to the broad field of colloid and interface science. The development of the instrumenta-tion, specialised sample environment equipment and analysis techniques has resulted its application to complex interfaces andenvironments and in the study of complex multi-component systems. This review provides a summary of those developments inthe last two years. � 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Neutron reflectivity; Adsorption; Thin films

1. Introduction

The specular reflectivity of neutrons provides infor-mation about the refractive index or scattering lengthdensity distribution normal to the surface or inter-face, and is directly related to the composition orconcentration profile in the interfacial region. Graz-ing incidence geometry and the wavelength range ofcold neutrons provides a wave-vector transfer, Q,

4�Žrange where Q� sin�, � is the glancing angle of�

.incidence, and �, the neutron wavelength wellŽmatched to the length scales of interest �10 to 4000

˚.A . The scattering powers of hydrogen and deuteriumare vastly different, and provide the opportunity tomanipulate the refractive index distribution by H�Disotopic substitution, without substantially altering thechemistry. The refractive index for neutrons is defined

Nb 2as n�1� � , where N is the atomic number2�Ždensity, and b the neutron scattering length 0.6674

� Tel.: �44-1235-4456-81; fax: �44-1235-4456-42.Ž .E-mail address: [email protected] J. Penfold .

�10�12 cm for deuterium, and �0.374�10�12 cm.for hydrogen . The ability to manipulate the ‘contrast’

is a powerful feature and extensively exploited. Itprovides the contrast to highlight the interface of apolymer bilayer, and the selectivity to study the ad-sorption of complex multi-component mixtures. Coldneutrons are also a penetrating probe, and this pro-vides access to ‘buried’ interfaces. Studies are hencenot limited to the air�solution and air�solid inter-faces, but can also be made at the solid�solid,solid�solution and liquid�liquid interfaces.

The emerging patterns of the application of neu-tron reflectivity in colloid and interface science aresummarised in the main themes of this review. Theyinvolve the investigation of more complex interfaces,including bio-membranes, in-situ electrochemistry,and adsorption at the liquid�solid and liquid�liquidinterfaces, and more complex environments, surfacesunder shear or confinement. In the study of polymerand surfactant adsorption at interfaces, which havebeen predominantly the domain of neutron reflectiv-ity, the trend is towards complex structures, mixturesand the development of nano-structures. The greatersophistication of experimental design is also reflected

1359-0294�02�$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.Ž .PII: S 1 3 5 9 - 0 2 9 4 0 2 0 0 0 1 5 - 8

( )J. Penfold � Current Opinion in Colloid & Interface Science 7 2002 139�147140

in the studies on solid polymer films. The article willbriefly review progress over the broad area of applica-tions relevant to colloid and interface science underthe following categories; surfactant adsorption, po-lymer adsorption, polymer films, polymer�surfactantmixtures, bio-membranes, electrochemistry, andnano-structured films.

2. Surfactant adsorption

The study of the adsorption of surfactants at inter-faces has been one of the startling successes of theapplications of neutron reflectivity, where the combi-nation with H�D isotopic substitution has enablednot only adsorbed amounts but also the detailedstructure of the monolayer to be determined. Lu et al.� �1 have produced a comprehensive review of thebroad range of investigations of surfactant adsorptionat the air�water interface. In particular they highlightthe detailed structural information that can be ob-tained, describe the potential for the study of complexmixtures, including polymer-surfactant mixtures, andput the studies in the context of information obtain-able from other techniques.

Adsorption at the liquid�solid interface is an im-� �portant area, and Penfold et al. 2 have demonstrated

the role of the specific interaction with the hy-

drophilic silica surface on the adsorption ofcationic�non-ionic surfactant mixtures, Hexadecyl-trimethyl ammonium bromide, C TAB�hexaethylene16

� �monododecyl ether, C E . In a related study 3 they12 6have investigated the temperature and time depen-dence of the adsorption of the surfactant mixture ofthe di-chain cationic surfactant, disterayloy-doxylimethyl ammonium chloride and C E at the12 6hydrophilic silica�solution interface, and how the ad-sorption is modified by the L �L , transition. In� �

more concentrated surfactant solutions more complexsurface structures develop, as illustrated by the recent

� � � �studies by Salamat et al. 4 , and Li et al. 5 . Theevolution of the lamellar structure at the liquid�solidinterface for the surfactant mixture of sodium decylsulfonate and pentaethylene monododecyl ether,C E with decreasing ionic content was interpreted in2 5

� �terms of undulation forces. Li et al. 5 interpretedboth the specular and off-specular scattering arisingfrom concentrated Aerosol-OT, AOT, solutions ad-sorbed at the air�water and liquid�solid interfaces.

Ž .The off-specular scattering see Fig. 1 was attributedto conformal roughness of the fluctuations in theadsorbed multilayer structure. The analysis of boththe specular and off-specular scattering was consis-tent with increasing structural order with increasingtemperature, and provided an explanation of the un-usual temperature dependence of the lamellar spac-

Ž .Fig. 1. Off-specular scattering q �q map for 2% h-AOT in D O at the air�solution interface at 25c.x z 2

( )J. Penfold � Current Opinion in Colloid & Interface Science 7 2002 139�147 141

� �ing. Lang et al. 6 investigated the effect of theinterface on the structure and phase transitions in theliquid crystalline phases of the non-ionic surfactant,C E . The formation of the hexagonal phase was12 4found to be strongly influenced by the air�waterinterface, whereas the lamellar phase showed no de-pendence on the interface.

Neutron reflectivity has provided information aboutsurfactant adsorption on simpler surfactant systemsthat cannot be obtained by other techniques, and this

� �is illustrated by the recent studies of Green et al. 7 ,� � � �and Eastoe et al. 8�10 . Green et al. 7 have shown

how changes in the chemical configuration of thehydrophobic chain affects the adsorption propertiesof non-ionic surfactants. Comparing the alkyl phenolethoxylates with the alkyl ethoxylates it was foundthat the changes in the hydrophobic chain structureaffects the structure of the adsorbed monolayer, butdoes not alter the limiting area�molecule at the

� �air�water interface. Eastoe et al. 9,10 used a combi-nation of neutron reflectivity and surface tension toevaluate the equilibrium adsorption isotherms of thedi-chain ionic surfactants, which are in the samestructural family as AOT, and some fluorocarbon

� �equivalents 8,9 which are important for the forma-tion of water-in-CO micro-emulsions.2

Finally the adsorption of surfactant meso-phases atinterfaces has been used to template the growth ofmeso-porous films with well-defined structures. The

� �recent work of White et al. 10,11 is a good exampleof the complementary use of X-ray and neutron tech-

� �niques. Holt et al. 11 have studied the initial stagesof the growth of silicated films at the solid�liquidinterface in-situ, and in real time. The initial structurewas found to depend upon the nature of silicon sur-face. However, the final structure and the inductiontime are independent of this, and driven by the bulk

� �phase behaviour. In a related study Ruggles et al. 12have investigated, using both X-ray and neutron re-flectivity, the role of surfactant chain length and ionicstrength on the nature of the silicated films templatedat the air�water interface, and is related in a pre-dictable way to the surfactant phase diagram.

3. Adsorption of polymer–surfactant mixtures

Until recently most of the experimental investiga-tions into polymer�surfactant complexes have fo-cussed on the solution aggregate behaviour and nottheir adsorption properties. This is because therehave been few techniques which measure directly thesurface composition of such layers, and techniquessuch as surface tension are difficult to interpret com-prehensively. Neutron reflection is capable of provid-ing both structural and compositional information,

and this is now transforming our understanding of� �polymer�surfactant adsorption. Staples et al. 13 have

investigated the nature of the adsorption of the mix-ture of the cationic polymer, poly-dimethyl diallyl

Žammonium chloride, poly-dmdaac and its co-polymer.with poly-acrylamide and the surfactants sodium do-

decyl sulfate, SDS and C E . The strong interaction12 6between the oppositely charged SDS and poly-dmdaacis modified by the non-ionic surfactant C E , and a12 6complex pattern of adsorption and surface tensionbehaviour is observed. The variations in surfactantand polymer adsorption arise from the competitionbetween surface and bulk complex formation. Cooke

� �et al. 14 have shown that the surface tension plots ofgelatin�SDS mixture show features similar to thoseobserved in more weakly interacting systems such as

Ž .polyethylene oxide , PEO�SDS, but that the under-lying behaviour is different due to the stronger inter-action. At concentrations above which free SDS mi-celles form, the surface is similar to that of SDSalone, but at lower SDS concentrations the presenceof SDS�gelatin complexes greatly enhances the SDS

Ž .adsorption. Structural measurements see Fig. 2 re-veal a thicker layer consistent with the adsorption of apolymer�surfactant complex at the interface. In the

Žstudy of the polymer surfactant mixture of poly n-. Ž .isopropyl acrylamide poly-NIPAM �SDS Richard-

� �son et al. 15 demonstrated that both SDS and tem-

Fig. 2. Schematic diagram of the structure of the adsorbedŽ . Ž . Ž .SDS�gelatin layer a c�SDS cmc 15 mM b at 1.15 mM SDS.


perature modify the adsorption of the poly-NIPAM atthe air�water interface. Above the lower critical tem-

Ž .perature LCST the thickness of the adsorbed layerand the amount adsorbed increases markedly.

� �Langevin 16 has recently reviewed the complemen-tary use of surface tension, ellipsometry, X-ray andneutron reflectivity, film balance and rheology mea-surements on a range of polyelectrolyte�surfactantmixtures. There are many similarities between theadsorption of polymer�surfactant mixtures at inter-faces and that of protein�surfactant mixtures. In a

� �series of measurements Green et al. 17�19 haveused neutron reflectivity to study the interaction oflysozyme and SDS at the air�water and liquid�solidinterfaces, and of lysozyme and C E at the air�water12 5interface. A complex surface tension behaviour isshown to be due to changes in the SDS and lysozymeadsorption at the interface. The associated changes inthe surface structure see the progression from theadsorption of protein rich surface complexes to an

� �almost pure surfactant monolayer 17 . The less stronginteraction with the C E gives rise to a different12 5

� �pattern of behaviour 18 .

4. Biomembranes

In the last few years there has been a greatlyincreased interest in using neutron reflectivity to studysurfaces and thin films of biological or biomimetic

Žinterest. These involve protein adsorption includingprotein�surfactant mixtures, already discussed in theprevious section because of their similarity with poly-

.mer�surfactant mixtures , model bio-membranes, andthe nature of protein�membrane interactions.

� � � �Krueger 20 and Fragneto-Cusani 21 have recentlyreviewed aspects of the applications of neutron re-flectivity to systems of biological relevance. Both re-views emphasised primarily the formation and charac-terisation of model bilayer membranes, and the inter-action of peptides or proteins with that layer. Frag-

� �neto et al. 21,22 have described in detail how ‘fluidfloating’ bilayers of L-�-di-stearoyl phosphatidyl-choline, DSPC and di-palmtoyl phosphatidyl-choline,DPPC, can be established from a combination ofLangmuir�Blodgett and Langmuir�Schaeffer tech-niques, and have characterised the transition from a

Ž .rigid gel-like to fluid phase with temperature usingneutron reflectivity. Using this approach Fragneto et

� �al. 23 have investigated the interaction of the pep-tide, the third helix of the antennapedia homeodo-

.main, p-Antp with the membrane. The peptide43 � 58was found to be associated primarily with the head-group region of the mixed DPPC�DPPS membrane,and produced an increase in the membrane rough-ness. Bayerl and co-workers have successfully used

Fig. 3. Models of streptavidin binding to mixed biotinylatedfullerene�DPPC monolayers.

lipid monolayers to investigate a range ofmembrane�protein interactions. In their most recent

� �work 24,25 they have investigated the nature of� �fullerene based monolayers 24 , and used fullerene

amphiphiles to control receptor binding. A biotiny-lated amphiphilic fullerene incorporated into a DPPCmonolayer at low surface pressures promoted strepta-vidin binding, and which then remained bound at highpressures. At high pressures the biotin anchor isretracted into the headgroup region and binding is

Ž .inhibited see Fig. 3An important related area of investigation has been

the study of protein adsorption at a variety of inter-faces, and the similarity between protein�surfactantand polymer�surfactant adsorption has already beenhighlighted in the previous section. Fragneto et al.� �26 have studied the adsorption of �-casein and �-lactaglobulin onto hydrophobic silicon substrates. ThepH dependence of the adsorption and the extent towhich the tertiary structure is retained is discussed.

� �Nylander et al. 27 have investigated the effect of


electrolyte on the adsorption of �-casein at the hy-drophobic solid�solution interface.

5. In-situ electrochemistry and other complexenvironments

The favourable transmission of thermal and coldŽneutrons through crystalline materials such as silicon

.and quartz provide the opportunity to investigate‘buried’ interfaces and exploit more complex environ-ments. This has been exploited in some in-situ elec-trochemistry measurements. Although the amount ofmaterial electrochemically deposited can be measuredstraightforwardly using techniques such as the quartzmicro-balance, the structure of the deposited film,and the solvent penetration into that film cannot bemeasured in-situ except by neutron reflectivity. This

� �has been demonstrated by Glidle et al. 28 , and� �Bailey et al. 29 in the electro-polymerised films of

Ž .poly pyrrol-N-propionic acid and in the electro-activeŽ .polymer films of poly o-toluidene . In the former ex-

ample, the degree of solvent penetration was de-termined, and the permeation profile of the Ni2�

ions, which chelate the polymer’s carboxylic acidmoities, the spatial distribution of the polymer and

� �solvent were deduced in-situ. Swann et al. 30 showedthat the same approach can be used to observe the

Ž .swelling of electro-polymerised poly pyrrole film byvapour adsorption; providing important informationfor the development of electronic ‘nose’ sensors.

� �Burgess et al. 31 have used neutron reflectivity tofollow the electro-deposition of the anionic surfactant

Ž .SDS onto an Au 111 electrode surface. The neutronreflectivity data in combination with electrochemical

Ž .data and SPM scanning probe microscopy measure-ments provide a detailed description of the inter-dig-itated condensed bilayer film that is formed. The

� �same group 32 have used electrochemical and neu-tron reflectivity measurements to follow the transfer

Ž .of 4-pentadecyl-pyride an insoluble amphiphile form� Ž .the gas�solution to the metal�solution Au 111 gold

�electrode interface.Measuring adsorption at the liquid�liquid interface

Ž .a buried oil�water interface is experimentally chal-� �lenging using any technique. Bowers et al. 33 have

successfully established a methodology by trapping athin oil layer between a hydrophobic silica surfaceand an aqueous sub-phase. They demonstrated thismethodology and their analysis approach with somerecent measurements on the adsorption of the block

Ž .co-polymer polybutadiene-polyethylene oxide , PB-PEO, at the hexadecane�water interface. The volumefraction distributions of each polymer block at theinterface was determined, and was found to be consis-tent with expectations based on the results at the

� �air�water interface. Strutwolf et al. 34 have used asimilar approach to investigate the interface betweentwo immiscible electrolyte solutions, of 1,2-dichloro-ethane�aqueous potassium hydroxide.

The nature of complex fluids under confinementhas attracted much current interest. To date much ofthe progress has centred around techniques like thesurface force apparatus, SFA, and other scanningprobe microscope techniques. More direct informa-tion has been obtained using X-ray scattering andreflectivity, and other confinement geometries havebeen successfully contrived for thin polymer films.The requirement of a larger illuminated area for

Žneutron reflectivity measurements compared to X-.rays has made such measurements difficult. Kuhl et

� �al. 35 have made remarkable progress in combina-tion with the surface force apparatus approach within-situ neutron reflectivity measurements, and havedemonstrated that it is possible to measure interfer-

˚ence fringes from a separation �1000 A betweensingle crystal substrates of quartz and sapphire.

6. Polymer adsorption

The nature of polymer adsorption is important fora wide range of technologies, and has been exten-sively studied both experimentally and theoretically.Neutron reflectivity has emerged as a powerful tech-nique for determining adsorbed amounts and thestructure of the adsorbed layer at both the air�solu-tion and liquid�solid interfaces.

� �Kent 36 has summarised a comprehensive study ofthe nature of tethered chains under a variety ofsolution conditions, using Langmuir monolayers of

Ž .the diblock co-polymer poly- dimethylsiloxane -poly-styrene, PDMS-PS. The PDMS acts as a stronglyadsorbing block at the air�water interface, and neu-tron reflectivity measurements have determined thesegment profile of the PS block as a function ofsurface density, molecular weight and solution condi-

Ž .tions from good to theta solvent conditions , see Fig.4. The strong stretching limit assumed from SFCŽ .self-consistent field , calculations and Scaling theo-ries are found not to be valid over the entire surfacedensity range. Over a wide range of surface densities,the tethered chain profile can be described by theo-ries of weakly interacting or non-interacting chains. In

� �a related study Bowers et al. 37 have investigated thestructure of the spread film of polybutadiene-

Ž .poly ethylene oxide , PB-PEO, diblock co-polymer atthe air�water interface. The model that was consis-tent with the data included a thin PB layer, and abrush-like PEO structure. At higher surface cover-ages there is evidence for surface aggregates in the

� �region adjacent to the monolayer. Shin et al. 38 have


Ž . Ž .Fig. 4. Schematic representation of the Langmuir monolayer for PDMS-PS diblock co-polymers for a symmetrical co-polymers, bŽ .asymmetrical co-polymers N �N .ps PDMS

used both X-ray and neutron reflectivity to determinethe nature of the surface ordering of the alkyl sidechains of polystyrene�polyelectrolyte diblock co-poly-mer Langmuir film at the air�water interface. Theresults, in particular, show that although the polyelec-trolyte block is water-soluble it remains adsorbed atthe interface throughout compression. Currie et al.� �39 have investigated the structure of grafted co-poly-

Ž .mers of polystyrene-polyethylene oxide PS-PEO atŽ .high grafting densities brush regime . At relatively

low grafting densities the brush is block-like, andparabolic at higher grafting densities, in good agree-ment with SCF theories. Bimodal, rather thanmonodisperse, brushes reveal a more complex struc-ture, in which the longer chains are more stronglystretched than in monodisperse brushes. Yim et al.� �40 have determined the segment density profile of

Ž .the strong polyelectrolyte, polystyrene sulfonate PSSat the air�water interface, as a function of molecularweight and electrolyte. They report higher adsorbedamounts than previously reported for solid surfaces,consistent with a stronger surface attraction. The bi-layer profiles obtained are rather different to thetheoretically predicted profiles. At low electrolyteconcentrations the profile collapses to a simple denselayer, corresponding to the chains lying nearly flat atthe interface.

ŽThe presence of a LCST gives poly N-isopropyl.acylamide , poly-NIPAM some interesting properties.

� �Pelton et al. 41 have investigated the effects oftemperature and the introduction of the co-solventmethanol, on the adsorption of poly-NIPAM at theair�solution interface. The adsorption reflects thesolution properties. At high methanol concentrationsthere is no adsorption, as methanol is a good solventfor poly-NIPAM. At lower methanol concentrationsand at temperatures below the LCST an adsorbedmonolayer is observed, whereas above the LCST a

Ž .thick layer due to phase separation is formed.The study of polymer adsorption at the solid�solu-

� �tion interface is equally important. Steitz et al. 42have studied the structure of polyelectrolyte multi-layers built from alternating layers of anions andcations. X-Ray and neutron reflectivity measurements

have provided detailed information about the densitygradient of the polyelectrolyte chains across the film,and show the influence of the water content on thefilm’s internal structure. The equilibrium structure of

Žpolymer brushes strongly stretched and terminally. Žattached chains have been extensively studied see� �.recent studies, 36�40 and many aspects are now

well understood. Much less is known about the be-haviour of brushes under shear. Specially designedcells to investigate such affects using neutron reflec-

� � � �tivity have been developed 43,44 . Baker et al. 43found that PS-PEO co-polymer brushes, in good sol-vent, remain remarkably robust under shear, contraryto some theoretical predictions. A similar conclusion

� �was drawn by Ivkov et al. 43 on polystyrene brushes.

7. Thin polymer films

Neutron reflectivity has been extensively used tostudy a variety of phenomena in polymer thin films,including the nature of polymer�polymer interfaces,micro-phase separated structures, partitioning or seg-regation at interfaces, and the effects of confinement.The examples discussed in this section summarisesome of the recent developments and studies in theseareas.

� �Sferrazza et al. 45 have reviewed a number ofrecent results on the nature of polymer�polymer in-terfaces, and on the interaction between graftedpolymer chains and a chemically different polymer

Ž .matrix see Fig. 5 . The interface between low molec-Žular weight immiscible polymers in this case

.polystyrene- poly-methyl methacrylate, PS-PMMAwas greater than that observed for higher molecularweights, and the interface width grew logarithmicallywith time or with a weak power law. The conforma-tion of the high molecular weight grafted polymerchain abruptly changes with increasing temperature,from an extended conformation to a sharp interface

� �with the polymer matrix. Sferrazza et al. 46 report inmore detail the kinetics of the formation of the inter-face between the immiscible polymers PS�PMMA


Fig. 5. Schematic respresentation of grafted chains as a function ofŽ . Ž .grafting density a low grafting density b high grafting density

with extended chains, ‘brush’.

studies on d-PS�PMMA bilayers of high molecularŽweight polymers, and for d-PS where d refers to

˚.deuterated thicknesses in the range 200 to 1000 Ashow an initial rapid increase, followed by a lo-garithmic dependence of the interfacial width on time.The results were interpreted as arising from the slowequilibrium time of the long wavelength capillary-wavefluctuations, consistent with the surface tension val-

� �ues and SCFT. Hayashi et al. 47,48 have used bothneutron reflectivity and SANS to study the interfacebetween the immiscible polymer pairs of polyamideand polysulfone; and the two methods provide a con-sistent estimate of the interfacial width. Measure-ments were made for non-reactive pairs and for pairswith reactive groups on the polysulfone. The interfa-cial widths for the reactive system are larger, due tothe formation of diblock co-polymers at the interface.The scattering measurements showed that co-polymerformation arrests the coarsening of the phase-sep-arated structures in the reactive system. Zhang et al.� �49 studied the interfacial structures of the homopo-

Žlymer poly-butadiene and a terpolymer brominatedŽ .poly isobutylene-cu-p-methylstyrene by neutron re-

flectivity, scanning transmission X-ray microscopy andŽ .AFM atomic force microscopy . The results showed

that the interface behaviour between these elastomer

blends was a direct function of the terpolymer chemi-cal composition; the interfacial width decreased withincreasing bromide functionality.

Micro-phase separation, giving rise to orderedstructures such as lamellae, have been studied indetail using neutron reflectivity, for a wide range of

� �block co-polymers. Torikai et al. 50 have studied theinterfacial structure of block and graft co-polymers of

Ž .polystyrene-poly 2-vinylprridine , PS-PVP forminglamellar micro-phase segregated structures. Huang et

� � Žal. 51 have studied the structure of lamellar P S-b-.MMA diblock co-polymer films on a neutral random

Ž .copolymer P S-r-MMA brush surface. Upon anneal-ing, micro-phase separation occurs quickly formingperpendicular and parallel lamellae emanating fromthe neutral and air surfaces. With annealing this richpattern evolves with a slow increase in the amount ofparallel lamellae. There is a strong commensurabilitywith film thicknesses, which have a period equal tothat of the natural lamellar period, which is due tothe formation of defects.

Surface or interfacial segregation or partitioning isan important process in adhesion, coatings, and in themany technological applications of thin polymer films.Neutron reflectivity, in combination with other tech-

Ž .niques such as FRES forward recoil spectroscopy ,can provide an important insight into such processes.

� �Oslanec et al. 52 have used FRES and neutronreflectivity to determine the surface excess of the

Ž .block co-polymer PS-b-PMMA in a bromostyreneŽ .PBr S matrix. The degree of bromination is ex-x

pected to increase the surface excess of the PS, due tothe unfavourable PS�PBr interaction. However, theopposite was observed, and was attributed to theattractive interaction between the matrix polymer and

� �the silicon surface. Hutchings et al. 53 have synthe-sised a heterotelechelic polystyrene with a tertiaryamine functionality at one end and a fluorocarbongroup at the other. Annealing a bilayer, comprising athin layer of this polymer and thick PS layer, showedfrom neutron reflectivity and Nuclear Reaction Anal-

Ž .ysis NRA , that the heterotelechelic polymer formedan excess at both interfaces, with the larger excessremaining at the substrate�film. interface. There wasno evidence for bridging by the heterotelechelic poly-mer, but the detailed distributions depended upon thepolymer’s molecular weight and film thickness. Ge-

� �oghegan et al. 54 have used neutron reflectivity andFRES to measure surface segregation of d-PS in apolystyrene matrix. For a linear polymer of highmolecular weight the surface segregation is predictedby mean field theory, where the segregation is a slowfunction of time due to the larger number of entan-

� �glements in the cross-linked mixture. Butler et al. 55


showed that the interfacial roughness between a thind-PS film and a thick high density polyethylene,

˚HDPE, block varied between 11 to 15 A, dependingupon the d-PS thickness. It was expected that the longrange Van der Waals forces would destabilise thethinner film. The stability of the films, within thetimescale of the measurements, was attributed to a

� �mechanical confinement effect. Pochan et al. 56showed that thin d-PS films exhibit a strong depen-dence of the co-efficient of thermal expansion on filmthickness and on the nature of the confinement boun-daries; and gave a clear indication that the nature ofthe substrate and superstrate boundaries must betaken into account theoretically.

8. Summary

A review of the literature over just the last twoyears shows an extensive and exciting range applica-tions of neutron reflectivity in colloid and surfacescience. The emphasis on more complex interfaces,complex environments, and complex multi-componentsystems is clear. The combination of techniques, neu-tron and X-ray reflectivity with other surface tech-niques is also increasingly prevalent.

References

� of special interest�� of outstanding interest

� �1 Lu JR, Thomas RK, Penfold J. Adv Con Int Sci 2000;84:143.� �2 Penfold J, Staples EJ, Tucker I, Thomas RK. Langmuir

2000;12:8879.� �3 Penfold J, Staples E, Tucker I, Soubiran L, Creeth A, Hubbard

J. PCCP 2000;2:5230.� �4 Salamat G, deVries R, Kaler EW, Satija S, Sung L. Langmuir

2000;16:102.� �5 Li ZX, Lu JR, Thomas RK, Weller A et al. Langmuir

2001;17:5858.� �6 Lang P, Steitz R, Braun Chr. Coll Surf 2000;163:91.� �7 Green SR, Su TJ, Lu JR, Penfold J. J Phys Chem B

2000;104:1507.� �8 Eastoe J, Downer A, Paul A et al. PCCP 2000;2:5230.� �9 Eastoe J, Nave S, Downer A et al. Langmuir 2000;16:4571.

� �10 Nave S, Eastoe J, Penfold J. Langmuir 2000;16:8732.� �11 Holt SA, Reynolds PA, White JW. PCCP 2000;2:5667.� �12 Ruggles JL, Holt SA, Reynolds PA, White JW. Langmuir

2000;16:4613.� �13 Staples E, Tucker I, Penfold J, Warren N, Thomas RK. J

Phys: Condens Matt 2000;12:6023.� �14 Cooke DJ, Dong CC, Thomas RK, Howe AM, Simister EA,

Penfold J. Langmuir 2000;16:6541.� �15 Richardson RM, Pelton R, Cosgrove T, Zhang T. Macro-

molecules 2000;33:6269.� �16 Langevin D. Adv Coll Int Sci 2001;89:467.

� �17 Green RJ, Su TJ, Joy H, Lu JR. Langmuir 2000;16:5795.� �18 Green RJ, Su TJ, Lu JR, Webster J, Penfold J. PCCP

2000;2:5222.� �19 Green RJ, Su TJ, Lu JR, Penfold J. J Phys Chem B

2001;105:1594.� �20 Krueger S. Cur Opin Coll Int Sci 2001;6:111.� �21 Fragneto-Cusani G. J Phys: Codens Matt 2001;13:4973.� �22 Fragneto G, Charitat T, Graner F, Mecke K, Perino-Gallice

L, Bellet-Amalric E. Europhys Lett 2001;53:100.� �23 Fragneto G, Graner F, Charitat T, Dubos P, Befiet-Amaltic

E. Langmuir 2000;16:4581.� �24 Maierhofer AP, Brettreich M, Burghardt S et al. Langmuir

2000;16:8884.� �25 Maierhofer AP, Braun M, Vestrosky O, Hirsch A, Langridge

S, Bayerl TM. J Phys Chem B 2001;105:3639.� �26 Fragneto G, Su TJ, Lu JR, Thomas RK, Rennie AR. PCCP

2000;2:5214.� �27 Nylander T, Tiberg F, Su TJ, Lu JR, Thomas RK. Bio-

molecules 2001;2:278.� �28 Glidle A, Swann MJ, Gadegaard N, Cooper JM. Physica B

2000;276�78:359.� �29 Bailey L, Henderson MJ, Hillman AR, Gadegaard N, Glidle

A. Physica B 2000;276-278:273.� �30 Swann MJ, Glidle A, Gadegaard N, Cui L, Barker JR, Cooper

JM. Physica B 2000;276�278:357.� �31 Burgess I, Zamlynny V, Szymanski G et al. Langmuir

2001;17:3355.� �32 Zamlynny V, Burgess I, Szymanski G et al. Langmuir

2000;16:7861.� �33 Bowers J, Zarbakhsh A, Webster JRP, Hutchings LR,

� Richards RW. Langmuir 2001;17:140.� �34 Strutwolf J, Barker AL, Gonsolves M et al. J Electroanal

Chem 2000;483:163.� �35 Kuhl TL, Smith GS, Isrealachivili JN, Mejewski J,

�� Hamilton W, Rev Sci Inst 2001;72:1715.� �36 Kent MS. Macromol Rapid Comm 2000;21:243.� �37 Bowers J, Zarbakhsh A, Webster JRP, Hutchings LR,

Richards RW. Langmuir 2001;17:131.� �38 Shin K, Rafailovih MH, Sokolov J et al. Langmuir

2001;17:4955.� �39 Currie EPK, Wagemaker M, Cohen Stuart MA, Van Well

AA. Physica B 2000;283:17.� �40 Yim H, Kent MS, Matheson A et al. Macromolecules

2000;33:6126.� �41 Pelton R, Richardson R, Cosgrove T, Ivkov R. Langmuir

2001;17:5118.� �42 Steitz R, Leiner V, Siebrecht R, Klitzing RV. Coll Surf A

2000;163:63.� �43 Baker SM, Smith GS, Anasiaopoules DL, Toprakicioglu C,

Vradis AA, Bucknall DG. Macromolecules 2000;33:1120.� �44 Ivkov R, Butler PBD, Satija SK, Fetters LJ. Langmuir

2001;17:2999.� �45 Sferrazza M, Jones RAL, Penfold J, Bucknall DG, Webster

JRP. J Mat Chem 2000;10:127.� �46 Sferrazza M, Mao C, Jones RAL, Penfold J. Phil Mag Lett

2000;80:561.� �47 Hayashi M, Hashimoto T, Hasegawa H et al. Macromolecules

2000;33:8375.� �48 Hayashi M, Grull H, Ester AR et al. Macromolecules

2000;33:6485.� �49 Zang Y, Li W, Tang B, Hu X et al. Polymer 2001;42:9137.� �50 Torikai N, Matsushita Y, Langridge D, Bucknall D, Penfold

J, Taleda M. Physica B 2000;283:17.


� �51 Huang E, Mansky P, Russell TP et al. Macromolecules2000;33:80.

� �52 Oslanec R, Composto RJ, Vlcek P. Macromolecules2000;33:2200.

� �53 Hutchings LR, Richards RW, Thompson RL, Bucknall DG,Clough AS. Eur Phys J E 2001;5:451.

� �54 Geoghegan M, Boue F, Menelle A et al. J Phys: Codens Matt2000;12:5129.

� �55 Butler SA, Fliggins JS, Bucknall DG, Sferrazza M. MacroChem Phys 2001;202:2275.

� �56 Pochan DJ, Lin EK, Satija SK, Wu W. Macromolecules2001;34:3041.

neutron reflectivity and soft condensed matter

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