IN TWO
ORGANICCHEMISTRY
NCTIONALIZED
: SURFACE-FU
DIMENSIONS
POLYMERS
AND SELF.ASSEMBLED
MONOLAYERFILMS1
GeorgeM. Whitesidesand GregoryS. Ferguson,zHarvardUniversity
Organic chemistryis largely derived from studiesof the reactivity and propenies of moieculesin homogeneoussolution. and much of the intuidon of
organicchemistsis basedon the behaviorof moleculesin solution.Surfaces
and intertacesr(that is, quasi rwodimensional assembliesof moleculesor
functionalgroups)provideenvironmen$that can be quite differentfrom those
of solutions.and chemicalintuitionderivedfrom solutionis often wrongwhen
applied to processesoccurringat surfaces.The centrai focus of our program
in organic surfacechemistry is on new science:that is. understandingand
controlling the phenomenacharacteristicof surfaces.interfaces.and thin
films. A charm of surfacechemistryis. however. its ability to combine new
sciencewith relevanceto a wide range of technologicalproblems.''sand we
hope to contributeto theseappiiedareasas well.6
Underlying our program in surfacechemistryis a broad interestin the properties of organic surfacesas componentsof materials.In panicular. we hope
to develop the abiiity to rationalize and predict the macroscoprcpropenies
of surtaces-wening, adhesion,friction-by knowing their microscopic,molecular-levelstructures.The issueof stmcture/properryrelationshipsin solids
lies at the base of much of the current research in areas such as matenais
science.condensedmaner and device physics,and polymer physicalchemistry. Surface sciencespans these fields and is curently a researcharea of
particularly great activitv.s'7-6The appeal of surface chemistry as an avenue
into detailed understandingof the relations between microscopic and macroscopicproperties of maner is that interfacesare more accessibleto analysis
and more easily modified by synthesis$att are the interiors of solids.
Organic chemistry has played a surprisinglysmall role in interfacial science.t
Although organic chemistryoffers, in principal, the ability to introduce a wide
range of functional and structural groups into surfaces,in practiceit has been
difrcult to rationalize, much lessdesign and synthesize.ordered two-dimensionalarraysof organicmoieties.aWe havetaken a physical-organicapproach
to the study of organic interfacial chemistlv: We formulare a hypothesisre-
CF{EMTRACTS-ORGANIC
CHEMISTRY 1: 171-187 ( I 988)
0895-444518E/$.m
O 1986Chemtracts
iTAY/JUNEIt'88 / SURFACEfUilCTTONALIZED
PC'LYTERSANO SELFASIIETBI-ED FIL'IS
171
synthuize and charlating molectrlar-scalestnrctlre to macroscopicProPerty'
testing that hypothesis,
orrrirc interfaca having structures aPProPriateto
information concerning
measurethe'propenies it interest, and iruerpret the
The physicalhypothesis.
original
the
of
terms
in
strucrure *a pioperties
and reactiviry
strucnrre
of
pailerns
complex
of
study
the
organic paradigFlior
than on
analogy
on
more
relying
often
one,
is iundam"ntally a qualitadve
prohas,
however,
It
theory.
fundamental
on
numerical calcuiationsbased
compliunderstanding
of
methods
and
useful
vided one of rhe most durable
successes
cated systems.Physical-organicchemistry counts among its many
rationthe
in
solution,
with
reactivities
the correlation of organic structures
of
inference
the
and
catalysis'
and
alization of areastu.l .t photochemistry
will
it
we
believe
intermediates;D
the properties and stnrctures of reactive
also be immenselyvaluable in understandingsurfaces.
SYNTHESISOF SURFACESAND INTERFACES
We have reiied on two seParateryPesof experimentalsystemsin our studies
(Scheme1):
r-j
N=
€
-
Oxi dati on
+
Self-Asscmbly
HeCrOl
pE 4
Au I HS-R-X
s-R-x
-P E -C O2H"
1) P l asma
-PE-.H"
PE +
J-r'-r'
R-X
S,Rr-x'
2) BHe'
Si / SlOz ClrSl-R-X
Left:
Schcme 1. Scfiematicrepresentationof the two mdtods used for productionof functionalizEdsurfaces'
gold
or
on
compounds
of
organosulfur
adsorption
film
by
monolayer
spontaneousself-assemblyof an onented
ftlms.
polydlylene
of
funstionalizEtion
Rigtrc
Oxidative
dioxide.
silicon/silicon
on
af'fyfricfrbrosilanes
1. Slrfrct-Functirindbed Orgrnic Pollmcrs, Espcddly'Polyethylene Car'
boxylic Acidl (PE-CO2[I). Thesc s),stemsare PrePared by .oxidizing polyethytene (PE) films with chromic acid and using the carborylic acid groups
in6oduccd onto the surface as the staning point for more elaborate chemical
modification (Scheme1).$s The chromic acid oxidation has the advantages
of restraining the funstionality to a very thin (lessthan 10 A in depth) layer
along the surfacc @ntour of the polyurer and of generating a set of functionalities limited to carboxylicacidsand ketonesand/or aldehydes.PE-CO:H
is convenientto prepareand snrdyand is an excellentmaterial for exploratory
snrdies. It also provides an entry into the examination of propenies rePre-
172
SURFACE.FU}GNOil L.trED PC'LY'TERSAT0) SEU!TSSENI-ED FII.G
CHEMISTRY
/ CHEII|TRACTS-ORGANIC
sentadve of a "real" material. It is, however, a complex,
microscopically
heterogeneousand strucrurallyilldefined material.rT
2' self'Assembled Adsorbed Monolayer Films.l&p we
and others have foctsed on rwb classesof monollyersi organosulfurcompounds
(especiait),;;ganicthiols) adsorbedon gold,r5'{)-'rs
and alkyl siloxanemonolayen prepared
by reaction of alkyl trichlorosilaneswith surfacescontaining
hydroryl groups
and/or adsorbedwater.'tsnBoth of thesesystems,and
otheis rilated to rhem,
are excellent models for interprering the characterisrics pE-CO2H
of
and its
derivatives.Immersionof a silicon iuf.r coatedwirh
a thin film (:i000 i;
of evaporatedgold in a solutionof a faqv thiol for t
hour at room remperarure
results in the formation of a highly ordered, quasicrvstailine
monolaverof
fatty thiol anached to tbe gold suriace by ruirui-gold
coordi""',#i;l;r:
1)' The essentialProcesses
occurringduring the adsorptionand organzation
of the thiol on the gold surfaceare stitl iniompleteli
understood,but rhev
are cenainly reiated to the familiar, if complex, coordination
;;;';?
thiols and gold(O)or gold(I).sra
One of the most anracdvefeaturesof organicchemistry
is the wide variarion
in the strucure of organic moleculesthaican be produced
through synthesis.
A challengeto our program in organic surface ihemistry
has been ro bnng
these synthetic techniquesto bear on two separare
classesof problems in
surfacechemistry: first, the introduction of small
fragmentshaving desired
functionalityonto surfacesthrough chemicaireacrion:
second.the preparation/assemblyof thesefragmentsin extendedmacroscopic
arravswith controi
over position and orientadon.The nvo approacheswe
have followed-one
leading to PE-CO2H and its derivatives.and the
other to self-assembled
organic monolayers-are quite diferent. The former
inroduces funcrionai
grouPsonto a preformedheterogeneous
material(Scheme2). This procedure
is convenientand exPenmentati relevantto a broad
rangeof polymer technologies,but it requiresthe srudyand analysisof matenals
rhat are intnnsicallv
structurallv ill-defined. The latter prepareswell-defineo.
approp";;.1:, fu;;:
PE-H
I
I
I
CrO3/H2SO4
I
I
I
ROH
PE-CO2B
<t---
H'
PE-C02H
LiAtH4
or
BHg
->
l*,,
rcoc'
I
I
J
PE-COCI
RoH
//
/\
PE-CO2R
\
PE-CH2OH
PE-CH2OCOR
nnrxz
PE-CONHR
Sghcme 2 Representative
reactionseguencesused to convertthe surface
of polyethylene
fitm(pE-H)to "polyethylene
carborylicacid,,(pE-corH)and
derivatives.
ttiAY/JUNEI98S / SURFACE-FTJilCNOilALJZED
POLYTGRSAr€ SEI.FAS{itr8LED FIL.TIS
17g
on a reactive
tionalizedmolecules,which are then allowed to self-assemble
Self-assembly
stru$ures.
two-dimensional
ordered
highly
surfaceand form
will, we beiieve,bec6mea mainstayof orderedmonolayerformadon,"t and
will eventuallyprove invaluablein ratronalsuategiesfor modificationof the
propeniesof interfaces.Prepanngthesesvstemsis, however,expenmentally
roi. complex than generatingfunctionalizedpolymer surfaces'
OF SURFACES
CHARACTERIZATION
We have used rhe usuai array of specroscopictechniquesto characterize
surtaces:ailenuared total reflectance-infrared(ATR-IR) and polarized infrared external reflectivespecrroscopy(PIERS), X-rav photoelectronsPectroscopy (xPS), elecrron spln resonance(ESR), fluorescence.electron
microscopy,and ellipsometryare all usefui (Table). We have, however,aiso
Table. selected Methodsfor Analysis of surtaccs and Intertaces
Applicationand Depth Sensed
Technique
(STM)
Scanningtunnelingmicroscopy
Low-angleX'raYscattenng
(SEM,TEM);
microscopy
Electron
difiractton
eiectron
Contactangle(HrO)
;
spectroscopy
X-rayphotoelectron
(XPS);augerspectroscoPy
Ellipsometry
Attenuatedtotalreflectance'infrareO
I
(ATH-|R)
|
Polarizedinfraredextemalreflecttve)
(PIERS)
spectroscopy
|
(RBS) I
backscattenng
Rutherford
Fluorescence
spectroscopy
Electronspinresonance(ESR)
spectroscopy
Individual
atomicpositionson surfaces
densitymap of the surfaceof
Elec-tron
veryflat solids
anddegreeof
Surfacemorphology
order
crystalline
Polantyof top -10 A'
of top
Atomicand chemicalcomposition
-soA
of filmthicknesswith a
Determination
of 2 A
resolution
analysisof top -1000 A
Vibrational
as a functionof
Atomiccomposition
depthwith resolutionof hunclreds
ofA
after
Assayfor densityof functionality
covalentattachmentof fluorescent
prooes
Locationand mobilityof paramagnetic
centers(e.9.,TMPO)in interfaces
been able to apply to problems in the physical-organicchemistry of surfaces
rwo techniqu"i ior cbaractenzationthat are lessfamiliar to the spectroscopic
community. Ttre first is the measurementand interpretation of liquid-soiid
contact angles. This technique has proven to be the most surface'sensitive
and most convenienr(if not the most easiiyinterpreted) method that we have
It is especially useful in
available to characteiue organic interfaces.!0-35'{r'4r
characterizingthe soli&water interface.The sccondtechniqueinvolves studies of chemicai reactivity at interfaces. This approachis especiallyuseful when
applied using simple, high-yreld reactions that are well undentood in homogeneous,liquid phasechemistry.Ionizationand esterificationof carboxylic
asids and saponificltion of carboxylic acid estersare esPeciallydiagnostic.r
The combination of measurementof contact angle with studiesof ionization
of functional groups has resulted in a techniquewe call "contact angle titration":+hat is, study of the vanation in the contact angle with the pH of the
174
SURFACE-FUI{CflOilAUZEDP(TLYIERS AXO SELFA$iEilBt€D
CHEMISTRY
FILilS / CHEMTRACTS-ORGANIC
aqueoui drop (Scheme3). This techniqueincreasesthe information derived
from the measurementof contactangies.Traditional approachesto studying
contactange generateonly nro.numben (the advancingconractangle0"-anJ
the recedingcontactangle g,)." Recedingconracrangiesare presendyverv
difficult to interpret. EEortsto characterizecomplex,heierogeniousinterfaces
using only advancingcontact anglesare unlikely to be verv broadly userul.
By measunngcontactangleas a function of pH, however, one can often infer
the existence,environment,and narure of ionizable groups present at the
interface.
Contact angle titration is basedon the observationof variationsin contact
angle with pH at surfacesconnining ionizable groups.rsThis variadon plau-
t20
90
60
30
eo
o
t?o
*
OOO$SoO
o
PE-CONHo
PE.H
90
PE-COOCH3
60
PE-CH20H
I
PE-CONHCHzCHzN)
\ PE[>C=OJ[CHzNH2J
30
PE.COOH
ASSORTEDBUFFERS
0
o48t?
pH
Sclrcmc 3. Dependenceof 0. on pH for surface-functlonalized
polyethylenefilm. Iop: Usingunbufieredaqueous
s o f u t t o n s . B u f i e r s : ( DM
0 .p1h o s p h a t e b u f i e r ; (aoi l)o t h e r s ( 0 . 0 5 M ) , p H 1 , 0N. H
1 C I ;g H z , m a l e i c a c i d ; p H 3 ,
tartaricacid;pH 4, succinicacid;pH 5, aceticacid;pH 6, maleicacid:pH 7 andg, HEPES;pH g and 10,CHES,
pH 11, Fiethylamine;
pH 12, phosphate;
pH 13,0.1 N NaOH.The crosshatctred
and labeled"assortedbufiers"
at pH 8 includedatafor phosphates
MoPS,HEPES,TAps, TFlls,andtriethanolamine.
MAY/JUNEI9E8 / SURFACE.FIJi|CNOTAUZED
PC'LYrER9 A}IO SELFASTiETBIID FILTS
175
group: The chargedform of an acid
sibly reflectsionization of the functional
the unclargla form. and the contact angle
or baseis more rr'var"prrnicthan
expranationis fundamentallycorwith water is rower. Although this simpre
one in companngacidities'its simplicity
rect. and the techniqueis a ver.Vuseful
hides a number of iomPlexides'rus
angretiration concernsthe intuitive concept
one interestingaspectof conuct
..quanuty" of a functional group Presentat an interface' our inidal
of the
group chemistry was that' in most
inruition .on."Lirig ,rrrr".. functionai
groups
."p"rimentatiy, th" number of functional
sysremslikely r"i.i*Ji"d
with
compared
would be small
present on a rePresentativearea of surface
expenments
in
volume of soiution used
the quantity of . r""g.nt Pr:s.elt in the
for measurementsof contact
incorrect
largely
on that s,rrface.Tffi Ueiiet is
at high densityon a surface
angles:The number of functional Eoups Present
used for contact angle titration in
is comparable to that preseat in titutibni
the titration curvesobtainedusing
unbufieredsystems.The difierenceberween
(Scheme3) exempiifiesthe phenomenon.3z
bufieredand unbufferedsorutions
to ciarif.vthe conceptof "concentration"
Explanation of this observationhelps
liquid
a iurface *d:
in a heterogeneoussystemconsisungof
:ontaaing
to be
interface
an
at
-ofof-an aqueousdrop
phase. we .onsil; ,i. spreading
at
present
of the functionality
determined in part by the extent ionization
a
in
functionaliqv
of this
that interface.Let us examinethe "concenrration"
polvethvlene
with a denvatized
sysremconsistrngof a 1-p.lgtop in contact
of -1 mm:)' The density of
area
an
covers
surface (a l-p.l drop typically
the order of 6 x 10r4/cm:for a
functionat groups-"'" riti r*i".. can be in
at this density' the concentrationof reagent
surfacewith typical roughness;33
to- react stoichiometricallvwith
in solution in the contactingdrop required
aqueoussolution and a
that funcrionatityis -0.1 m-M.rtFor an unbufiered
of acid or base > 0' 1 mM
monoprodc acidjbasereaction. a concentration
to.achievea stoichiometncreaction'
(i.e., pH < 4 or pH > 10) is thusrequire.d
obtained usingbuffered
duation.crrrves
clearly, the difierencein contactangle
that is itself sufficiently
functionaliry
and unbufferedsolutionsis due ,o ,irf"..
to bufier the pH of
drop
and
concentratedin the systemcomprising surface
the qualitative idea that a .
the aqueoussolurion in the ranie pH :-? Tlur.
small in quantitv commonolayer of organic functiona-liryis insignificantly
if one is concerned
incorrect,
is
paredwith the fuictionaliry preseniin solution
with small volumesof soludon'
interpretadonof the contact
A secondinterestingissueconcernsthe detailed
should the solid-iiquid inangle titration .o*Ir. h particular, we ask how
groups present on the
terfacial free enerSy "Ysrbe related to the functional
the conracrangle to igterfacial
surface?The fundamental relation connecdng
free energy tem$ is Young's equation (Eq' 1)'5t
^l
Tsu
c o s 0=
176
FII'IS
ITRFACE.FUrCNOilALtrED POLYTERSAXO ITELFASSEMLED
rsL
Jsv
- 'Ysr
'lw
/ CHEMTRACTS{RGANIC CHEMISTFIY
(1)
For aqueoussolutionsconstituted with appropriate buffers, the
liquid-vapor
interfacial free enerily ^lw is the same as that for pure water. vanationJin
interfacial free energiesare thus related to the obsCrvedvalue g
of pr,,',*iiv
by the terrns15vand "y51.
Thesetenns, in turn, dependon a number of factors:
&g tyP!, densiry, and distribution of funaional groups present
at the solidvaPor (liquid) interface; their extent of ionization; the
roughnessof the surface; tbe relative humidiry of the vapor.
As a fint approximation, we have proposed that the
interfacial free energy
can be expressedas a linear combinatibnof funcdonal
group contnbutions.
multiplied by the normalized fraction
9i of these groups on the surfacei2r4
(Eq. 2). The parameters"yisr and
1,.5yrefled intrinsic
'Isr
"Isv
=?
=?
9i 1,se
(2a)
9i lisv
(2b)
hydrophiiiciryand group sizeor:rea. Comparisonsof infrared
specroscopic
data with contact anglesindicate that this gypgof analysis
is approximatelv
o
V
o
oe
I
'a-A-
- r,lL911-tf
l-v
?rs
ry
/sv
! -
t
I
rr!
I
:
I---
lE
A - v
I
I - + 7
- - T
-I
I I I
4
_ _ _ I
S
Figurc' scfiematicrepresentationof an ideal
1np leftl and real (top right, bottomldropof tiquid(L) in contact
with a solict(s) and vapor (v) with contactangle
0. The symbolsinirre upperright picturerepresent(c) water
molecules'(A) dissolvedsolutes(phosphate,
bunersalts),io, e) polarsurfacegroups(co2H,cor, c=o,. . .),
(t) nonpolarsurfacegrcups (CH;,
.). t f,p of liquid(,bo6m; not drawnto scate),the ,.precu6orfitrn,,,
extendsmicronsbeyondthe edge of9H.,
thi drop in ."rt",n orcumstances.
MAY/JUNEl!)gE / SURFACERJ[crloilAU"Fn
POLYI|ERSatrto sELFAssEtBLED FtLfls
1TT
but that interactions
derivatives,rs
correctfor PE-CO2Hand someof its acidic
heterogeneity,make the problem
berweengroups, ana perrrapsinterfacial
Jr*g tiris simpleapproach'35
more complexthan can be discribed completely
and complexities remains to be esRn unaerltanding of these interactions
tablished.
A t h i r d i m p o r t a n t i s s u e i s t h e m e a n i n g o f h y s t e r e s i s i n t h e m e a sa usimple'
rementof
to provide
For derivatinesof PE-CO'H, 0" aPPear-s
conuct
group character and
"ngl"rsemiquantitadvelyinterpretable measureof interfacial
obsimple
a
Parameterderived from
density.connct anglesare, however,
contacl
receding
and
Advancing
servationsof a complex realiry (Figure)'
derivativesof PE-CO2H (particanglesdifier on many surfaces,anOltt the
in their contact ansies:
hystereses
utarly polar derivatives)display very large
vaiues of 0"' Large
large
fairly
0, is frequently 0 even for'syste*i tt"t'ing
systemfar from
heterogeneous
a
hysteresisis usuallyinterpr.,id -,o indicats
equation'
Young's
on
based
Yet analysesof 0"
thermodynu.i. equilibrium.se
give
interpretable
to
seem
- .qu"rion *ru,n]ng th;odynamic equilibrium,
rreat a systemthat is
shourd
one
how
crear
not
is
It
results.
and reasonable
cormeasurements
physical
nor at thermodynamicequilibnum. but for which
to
analogies
Processes
relate with those expeded basedon physical-organic
solution'
in
equilibrium
occurring at
RESULTS
mon'
and self-assembled
Both functionatizedpolvethvleneand its derivatives,
at
occumng
reactions
olayer fiims. provide systemswith which to examine
strucand
interfacesand to test hypothesesconcerningstnrcture/reactivit;many of the resultswe
rure/properryrelationships.In so doing' we find that
in solution (often with
obtain can be rationaiizedby anaiogyto phenomena
and contrast the
cbaracteristicdifierencesthai can uJinterpreted to comPare
interfaces)'we also
environmentsprovided by homoseneoussolutionsand
that any models
frequentlyencoun,., un.ipectedlh.no*.na, which suggest
with solution'
analogies
on
exclusively
based
of organicreactivityat interfaces.
examples'
provide
follow
are not complete. The studiesthat
acids and many' but
Surface Acidities. Scheme3 indicates that carboxylic
of the drop used in
not all, aminesshow inflectionsin plots of 0, vs the pH
Asiuming that the midpoint of the inflectton
measuringthe conractangie.s?r5
(an assumption sup.orr.rporids to half-ionizltion of the functional group
surfaces),31
ported by independentATR-IR measurem.ntsoJ.arboxylic acid
are
solution
in
we infer that acidiriesof functional groups at an interface and
with
a
connct
very difierent. For example, the value of pH lol " solution in
at
grouPs
aod
surface required to achiive half-ionizarion of the carboxylic
very large aPParent
that surfari ott be as high as 12. What is the oriFn of this
increase in the
decrease in acidity of &rUorylic acids (and con"sponding
of theseshifts
apparentacidiryof ammoniurn-ions)?We believethai the origin
at the Polycan ultimately be anributed to the locally low dielectric constant
values
anomalous
these
of
rationeli-a3isn
but
ethylene-water interface,x{-CI
of p& is not Yet comPlete.
Wettebility of In'
Relstions between Func{ional Group Hydrophilicity and
more hydrophilic
that
terfaces. We assumedat the outset of our studies
as the
interfacial groups (as measuredby some convenient Parameter such
exfact,
In
would lead to more wettable surfaces.
Hansch ,r, i"o*eiet',)
hydrophiiiciry
group
perimentalobservationsreiadng wenability to functional
178
POLYTERSAI{D SEI-FASSETELEDRtrIS
SUFFACE.FUI{CTIOIIAIJZED
/ CHEMTRACTS{RGANIC CHEMISTRY
difier significantlyfrom those expected(Schemea). As the value of n for the
functional group on the surfacedecreases.0oalso decreases,but only up to
a point. Beyondtharpoint, further increasesin functionaigroup hydrophilicity
result in no further increasein wettability: that is. the hydrophilicirv of the
surface "sarurates."35We postulate that the origin of this efiect lies in condensationof watervaporat polarsolid-vaporinterfaces(Scheme5). Nonpolar
interfacescondenserelatively little water. All of our experimentsinvoiving
contad angJeswith water nre carried out at LAUVorelarive humidiry in order
to Elssure
that the systemis as closeto therrnodynamicequilibrium as possible.
Polar functional groups at interfacesare undoubtedly associatedwith hydrating warer adsorbedfrom the vapor phase.We postulatethat. beyond a
certain vaiue of the Hansch tr pzuameter.the polar surfacefunctional groups
becomecompletelysurroundedby condensed.hydrating water, producing a
solid-vapor interfacewhosepolarity is essentiallyindependentof the underlying functionai group. Under these circumstances.the wettabilitv of the
surfaceis determined primarily by the area fraction of the surface converted
to polar funcdonaliry,and then hydrated by condensedwater.
\(
160
MceHrz
tt
I
-cxro8c,r,?.
/
{xro
//
120
?rr/t
o
CONHCaH,
.^\
i
/
05, /PE-H
lo /o
c-/
,.\ o \/
oo
.4
(:Ar/r**
/o^\ 6
/ro
|tt' ?-oo
o o 6o
9O
-3
'2
/3
o
n
'1
Schcme4. Contactanglesof water for derwatvesof PE-CO.H,PE-R,with
a me€lsure
of thetrc,upR. lr is the Hanschparameter,
a rangeof fryctrophilicities
of functionalgroup hydrophilioty,derivedtrom the equilibriumconstantfor
partioningbetweenaqueousand hyclrocarbonphases(rnset).
These observationsand interpretations imply the existenceof a thin. condensedwater film on polar surfaces.The nature of this film, and especially
the relAtion of its stnrcnrre to that of bulk water, remains an important and
complex problem.
The Rangeof InteractionsDetermining Wetting. Scheme4 displaysan astonishingobservation:Although a surfaceincorporatingamides(PE'CONH:) is
relatively hydrophilic,the analogousprimary amide PE-CONHC3H' is more
hydrophobicthan unfunctionalizedpolyethylene.Some of the apparent hy-
II/TAY/JUNE1988 I
SURFACE.FI,NCTIOTAU:ZED FOLYTERS ANO SELFASSEilBLED
FIL.TIS
179
oHz
Hro
P
Scheme5. Schematicillustrationof the degreeof hydrationof a functional
groupp at the solid+raporinterface.WhenP is nonpolar,the equilibnumlies
io tne bft; when P is polar,it lies to the righl
drophobiciry of PE-CONHCTH7and its analogsundoubtedly reflectsthe mtcroscopicroughnessof the surfaceof these.materials(generatedduring the
oxidanve surfice functionalization).Nonetheless.we find that it takes oniy
a small hydrophiiic or hydrophobic group to determine the wenability of a
surface.Furthermore, a smail hydrophobicgroup is capableof completelv
maskingan underlying,intnnsicallyhydrophiliccore functionaliry'Thus. for
example,replacementof a terminal CHi grouP in one of the well-defined.
monolayersystemsby a CH2OHgroupchangesthe monolaver
seif-assemblid
from being very hydrophobic to very hydrophilic,'r and reacvlationof the
terminal n-ydroiyt(CHTOCOR) once again makes it very hydrophobic-The
interactionsthat determinemacroscopicwettability are, aPParently,verv short
of contactangieis the most
in range.r0Webelieve,in fact, that measurement
rechniquepresentlyavailablefor examiningthe sclid-liquid
surface-sensitive
great
advantagesof wening as a probe of surface stnrcture
The
interface.
(reladve, for eiample, to XPS) are that its measurementis very simple,
convenient,and ineipensive,and that it is inrinsically appiicableto the soli#
iiquid interfaceand to heterogeneous,noncrystallinesurfaces.Its disadvanmges are that contact angle measurementsare information Poor, that they
requirea liqui&solid interface,and that their physicalbasisis complex and
still incompletelyunderstood.
DesignedInterfaces.The materialsPE-CO-X are convenientbut heterogeneous.The best characterizedand strucnrrally best defined organic interfaces
now availableare rhoseformed by adsorbinglong-chainalkvl thiols on gold,
or by atlowing long-chainalkyl trichlorosilanesto react with surfacehydroxyl
groups and adsorbedwater prescnt on the surface of glassor silica. Both of
these systemshave the alkyl groups in completely rraru'extended conformationi, provided that the terminal functional group is relatively srnall. For
organicttriotson gold, the chainsare tilted -30o from the normal to the metal
surfacc;€'rs for attcyl siloxanes on silicon/silicon dioxide, they are aPProxrmately perpendicoi"t to the subsrate surface (Scheme 6).e Transmission
electron microscopyindicates that the thiol/gold system has at least microsrystralline order in the plane of the monolayer.az
Theseordered monolayer systemspermit an exquisite degreeof control over
sttucture and dimensionality at the interface. As one example, consider a
monolayer formed by adsorptionof HS(CH?),9OHon gold. Formation of
sucha monolayeris experimintallyvery straightforward:one simply dips the
in a solventsuch
gold-coatedsubstrateinto a solution of the c,,rrr-thioalcohol
as acetonitrile for t hour at room temperature, withdraws it, and washesit
briefly. At the conclusionof this procedure,the entire accessiblesurfaceof
180
suRFAcE-FuilcnoilALeED FoLynERs Ar{o SELFASSETELEDFrLrs
cHEMlsrFY
r CHEMTRACTS-ORGAN|C
variable
tunctionality
it-
I
polymethylene I
I
chains
I
I
I
(o'''ro'
))
lr
a(
(a
))
((
))
I
a{ aa
-l'-o'l'
Au Au Au
L
tsl-o,sl
L
suPPortorganic
interface
OH'"OH
r
I
I
))
\HSHS
tl
tl
Scheme6. Schematic
illustration
of conformation
and packingorderin monolayersof organicthiolson goldand alkylsiloxanes
on silicon/silicon
dioxide.
The monolayeris composedof three importantregions:the head groups
(portionbindingto solidsubstrate),the polymethytene
chains(forformationof
van der Waalssurface),and the tail groups(termrnalfunctronality
that determinesthe characterof the soli#liquidand solid-vaporinterfaces).
OH
looH
tf,,r,,
I
SH
ll
U
,J-.
'::"'
7' Stylized illustrauonsof monolayerstructures.'rProposedstructures (A) pure
$!1c
of
monotayerof
Hs(cHJ'eoH;(B) monolayercomposedof 50o/oHS(CHJ,eOH
and 50o/oHS(CHJ,,OH;(C) pure monolayerof
HS(CHJ'OH' Struaureswe believedo not occurin the systemsstudiedhere:(D)
disorderedmonolayerand (E)
monolayercontaininga mixtureof componentsand strowingphaseseparationinto
islands.
ITIAY/JUNE1988 / SURFACEfiJrcnor
r r?FF FoLyrcns
Al|O SETFASSEilBLED FtLrS
1g1
the gold is coveredwith a uniform monolayer?3 A thick, and the exposed
thicknessof
surfaceis a denselypackedmonolayerof hydroryl groups.The
varying the
by
angstroms
at
the
of
scale
controlled
the monolayer is
"aiitygroups in the thiol chain; the surface ProPertiesate
number of methylene
functionai
independently controllable through variations in the terminal
are used'
thiols
funcdonalized
terninally
group. If mixtures of rwo difierent
7)'
(Scheme
surface
the
on
rwo
mixed
the
monolayerscan be made having
CURRENTPROBLEMS
provide new materiais
The physicakrganic chemistryof surfacespromisesto
interfaces'new anand
basedon rationil syntheticmodificadonof surfaces
deeper levels of
and
alytical methods with which to characterizesurfaces,
wetting,
adsorption.
understandingof familiar processessuch as dissolution.
phenomena.being
and adhesiorioccurringarlnrerfacesand in solutions.The
in hoobsewed are, however, usually more complex than those occurring
at
understood
mogeneoussolution, and are, consequently,still incompietely
fundaeven the simplest levels. Tbe fieid presentsa number of fascinating
mentai problemsin interfacialchemistry,amongwhichwe placethe following:
'
de1. Molecular-LevelOrder. How shouldthe order in thesesvstemsbe
that
fined and measured?One advantageof a two-dimensionalsystemis
it is, in principle, less complex structurally than a three-dimensional
svstem:The componentsof a two-dimensionalsystemare by definition
restrictedto a plane rather than free to nanslate and rotate in three
dimensions.In practice.however, the problem of defining order in
surface-functionalized poIymers and seif-assembled monoiayers remains
very complex. All of ih.s. svstemsare, in realiry, only quasi rwo-dimensionai.Matenalssuchas functionalizedpolyethyleneare obviously
mictoscopically rough and heterogeneousand have functionaliry distributed nonuniformiy in a thin interfacial layer. Contact of these
systemswith a liquid phase may result in interfacial swelling and
monolaversare bener definedstrucrurreconsrruction.Seli-assembled
ally, but even with thesesysrems,subtle issuesof order in the plane of
thi monolayer.ar the gold-monolayerand monolayer-liquidinterfaces
and berweenadja..niorganic moleculesrequire the developmentof
new analyticaltechniquesand new criteria for order.
2. Kinetics vs Thermodynamics. The extent to which any of the systems
currently studied :re ar thermodynamicequilibrium. and the influence
of deparnrresfrom equilibrium on their behavior, is almost completely
uncertain at Present.
3. Wetting. Despite interesting and provocative theoretical contributions
there is no
to the ih"ory'of wening inlenain idealized systems,e'6a-7t
hetmisroscopically
usefully detailed theory of wening relevant to real,
the
in
hysteresis
.rog"n.ous surfaces.The current rationalizadon of
Detaiied
measgrement of conracr angles is especially unsatisfaaory.
examination of hysteresis,bJth theoretically and experimentally would
be particularly uieful, becausehysteresisapPearsto be very sensidve
to order; an understandingof the relation betweeninterfacial stnrcture
and hysteresismight provide a new avenueof approachto this important
subject.
4. IVlolecularDesign of Monolayers. Essentially all work so far carried
out with self-asslmbledmonolayershas focusedon derivadvesof fary
1&l
ltUFFACE-Ffrt{61OtrA!.trEDFOLYUeRSAt{O SELFASSETBLEDRIJS
CHEMISTRY
t CHEMTRACTS-ORGANIC
acids. These systemshave the two virnres that they are easyto manipulate syntheticallyand that they do, for whatever reason, form *.itordered monolayers.They are, however. not stable at even modesdv
elevated temperaturesand have no strong intermolecular interactions
contributing to order in the plane of the monolayeror to thermal or
oxidarivestability. It is important to develop molecrrlarstnrcturesother
than fattv acids that form ordered, stable rwodimensional sheet strucnrres.
CONCLUSIONS
Organic chemistryat interfacesis a field offering major oppornrnitiesfor borh
the conductof basicscienceand the developmenrof new technology.It also
provides,through the synthesisof extendedfuncrionalizedinterfacej, a bridge
benveenthe scienceof isolated moleculesand the ssienceand technology6f
materiais.Sincechemicalreactivity and we$abiliw provide whar we beiieve
wiil prove to be invaluableprobesof interfacialstructurefor organicsvsrems.
these s)'stemsare particularly attractive for srudiesaimed at understandine
the characteristicsof soli&liquid interfaces.
Surface'funcdonaiized
polymers (of which the best developedis PE-CO'H)
are provingto be convenientsvstemswith which to conductexploratorvwork.
They are easilypreparedand manipulated.and becausethev presenrsolidvaPor interfacesthat have low surfacefree energies,thev are relativelv resistant to contaminationby atmosphenccontaminanc. Funher. since they
are physicallyrobust, surface-modifiedpolvmers can be used to examine
complexmaterialsproblemssuchas biocompadbilirv,D.s
adhesion.srgas
permeation.s friction,s3and the influenceof bending, stretching,sand su*o..
reconstructionson interfacialpropenies.
Self-assembled
monolayerswill, we believe, prove to be the ultimare cornerstoneof the basicsciencein organicsurfacechemisrry.They will cenainly
also find technologicalapplication in areassuch as promotion of adhesion,
inhibition of corrosion,and control of friction, and they may prove important
in the production of sensorsand microelectronic devices. The remarkable
easewith which very complex monolayer stnrcrurescan be assembledfrom
moleculesof very modestcomplexirywill be invaluablein studyingthe propernes of organizedmolecularassemblies.The best defined of thesesuitems
is presently obtained by adsorption of orfunctionalized fatty thiols on gold.
althoughorganosiliconcompoundson silicon dioxide and glassmay ultimitely
prove equally ordered. Alkyl thiols on gold have as their major advantage
the comparibility of the thiol moiety with a wide range of organic functional
grouPs, and the fact that these systerm lead to highly ordered monolayers.
Silaneson silica are more economical, bener adapted to the formation of
multilayer stnrctures, and more robust srnrctrrrally.
Given the astonishingsensitiviry of wenabiliry to local surfacestructure. its
srudy should provide a range of imponant new q?es of informadon about
interfaces,especiallysolid-liquid interfaces.Designingand interpreting these
experirnentswill require a physical-organic approach-the sysremsbeing
studiedare too complexto be defined usingconventional,spectroscopy-based
physicalchemistry.Becausewerdng is directly relevant to a broad range of
technologicalproblems, these studies should be exceptionallyvaluable in
applicarions.The experimentaltechniquesrequiredto srudywening are verv
simple. Surfacesciencebasedon studiesof wettabilir,vshould rhus be accessible even to those without routine accessto the instrumenmtionof highir Y/JUNE l98E / suRFAcE-FuilctoNALuED
poLyrEns Ailo sELFAssEilBLED Frlrs
1&l
basis for wening is desperately
vacuum physics. A more realistic theoretical
with su.bjectssuch as
adequately
deal
needed. Current fieatments do not
deviations from thermodynamic
molecular-scaieheterogeneity, hysteress,.
and liquids.
na-rure6i tnl interactions berween sorids
equitibrium, *otl.
ACKNOWLEDGMENTS
been the.result of-skilled experiThe researchcarried out in our Eoup has
a numand lreatiue designand interpretarion by
menrarion and ;;;G;rfrl
Tom
H-olmes-Fariey'
Randy
ber of individuals, including Jim Rasmussen,
Lou
Biebuyck'
Hans
Laibinis'
McCarthy, Cotin il"in, n"rry Troughton'-Paul
grateful
also
We
are
M. Scarmoutzos'
Suong, StephenWasserman,and l-luis
t o o u r c o l l e a g u e s P e t e r p e n t r a n ( D i v i s i o n : { n p p l l : d S c i e(Mrr)
n c e , Hfor
a r vimard),
and Maiii wrighton
Rarph Nuzzo (AT&T Bell Labor"rri.r),
of surfaces.
pori"n, contributions to our understanding
AND NOTES
REFERENCES
l . T h e w o r k d e s c r i b e d i n t h i s p a P e r w i r s s u p p o l l d l n p Agency'
artbythe
o 6by
c ethe
o f NNSF
aval
and
Projects
Researchand the Defensc ndvlced Rescarch
Research
Materials
oruw (cT{E g54S702)and to the Harvard
through g-nl.
(DMR
86-14003)'
I-aboratory
2. NIH PostdoctoralFellow, 1988-1989'
..sur{ace,.and ,.interface"have cleariy defined meaningsoniy in the
3. The words
vapor in contact wtth a smooth' hocontexl of an ideal system.a liquid and
here (espeoally functionalized
mogeneoussolid. ln the real sysiemsdiscussed
as indicating the more
understood
polyethylenefilms). thesewords should be
the comincorporates
which
reglon"'
"interphase
complicatednorion of an
etc')
swelling'
reconstructionanC
piexines (roughness.inhomo;;;;,y. surfice
inevitabtY encountered'
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6 ' ( ' T1986:
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MAY/JUNE 1988 ,' SURFACE.FUilCNOXALEED
PCILYTERS AflD SELFAIISEilBLED
FIL.iISi
187