Self-Assembly of Polystyrene-block-Poly(Ethylene Oxide
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No Job NameAggregate Formation?
Robert B. Cheyne and Matthew G. Moffitt*
Department of Chemistry, UniVersity of Victoria, P.O. Box 3065,
Victoria, BC V8W 3V6 Canada
ReceiVed July 6, 2006
Block copolymer self-assembly at the air-water interface is
commonly regarded as a two-dimensional counterpart of equilibrium
block copolymer self-assembly in solution and in the bulk; however,
the present analysis of atomic force microscopy (AFM) and isotherm
data at different spreading concentrations suggests a
nonequilibrium mechanism for the formation of various
polystyrene-b-poly(ethylene oxide) (PS-b-PEO) aggregates
(spaghetti, dots, rings, and chainlike aggregates) at the air-water
interface starting with an initial dewetting of the copolymer
spreading solution from the water surface. We show that different
spreading concentrations provide kinetic snapshots of various
stages of self-assembly at the air-water interface as a result of
different degrees of PS chain entanglements in the spreading
solution. Two block copolymers are investigated: MW) 141k (11.4 wt
% PEO) and MW) 185k (18.9 wt % PEO). Langmuir compression isotherms
for the 185k sample deposited from a range of spreading
concentrations (0.1-2.0 mg/mL) indicate less dense packing of
copolymer chains within aggregate cores formed at lower spreading
concentrations due to a competition between the interfacial
adsorption of PEO blocks and the kinetic restrictions of PS chain
entanglements. From AFM analysis of the transferred
Langmuir-Blodgett films, it is clear that PS chain entanglements in
the spreading solution also affect the morphological evolution of
surface aggregates for both samples, with earlier structures being
trapped at higher concentrations. At the highest spreading
concentration for the 141k copolymer, the coexistence of long
spaghetti aggregates with cellular arrays of holes, along with
various transition structures, indicates that various surface
aggregates evolve from networks of rims formed as a result of
dewetting of the evaporating spreading solution from the water
surface.
Introduction
Nonlithographic techniques leading to controllable and po-
tentially functional structures occupy a fundamental niche in
modern materials science. For this reason, there has been
considerable interest in systems that spontaneously organize into
predictable surface patterns,1 including significant research
focused on the self-assembly of amphiphilic diblock copolymers at
the air-water interface.2-20 When deposited onto the surface
of water, polymeric amphiphiles form nanoscale surface ag- gregates
with a range of morphologies, including dots, long strands
(spaghetti), planer structures (continents), ring and chainlike
aggregates, and interconnected networks of spaghetti. The
predominant surface features obtained depend on a number of
controllable factors, including the nature and relative lengths of
the two blocks,4,10 the surface pressure,19,21,22and the concentra-
tion of the spreading solution.13,17It is commonly suggested that
these various two-dimensional (2D) aggregates are analogous to the
diverse array of three-dimensional (3D) block copolymer structures
that form under equilibrium conditions in selective solvents and in
the solid state; however, recent results suggest that at least some
of the observed morphological diversity at the air-water interface
can be attributed to kinetic effects related to chain entanglements
between hydrophobic blocks in the spreading solution.13,17 Despite
the immense potential offered by block copolymer self-assembly at
the air-water interface for patterning surfaces with polymeric and
composite features, several key aspects of 2D aggregate formation
continue to be poorly understood, especially compared to the vast
theoretical and empirical literature on 3D block copolymer
morphologies: What is the detailed microstructure of block
copolymer aggregates at the air-water interface, including the
relative positions and conformations of both blocks?11 Are these
structures actually 2D analogues of the various 3D micelle
morphologies that form under equilibrium conditions?13,17 What is
the mechanism of formation of the various surface
morphologies?10,18,19,21
Perhaps the most widely studied system of diblock copolymer
self-assembly at the air-water interface is based on a
hydrophobic
* To whom correspondence should be addressed. (1) Whitesides, G.
M.; Grzybowski, B.Science2002, 295, 2418. (2) Zhu, J.; Eisenberg,
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G.Langmuir2005, 21, 10297. (17) Cheyne, R. B.; Moffitt, M.
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10.1021/la061953z CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/22/2006
polystyrene (PS) block and a hydrophilic and surface-active poly-
(ethylene oxide) (PEO) block.6-13,15,17,18,21,22The formation of
PS-b-PEO surface aggregates has been shown to be the result of
spontaneous copolymer aggregation at the air-water interface,
rather than a transfer of micelles formed in the spreading
solvent.10,17,18The suggested driving force for aggregate formation
is an interplay of attractive interactions between PEO and the
water surface and repulsive interactions between PS and water and
PS and PEO as the spreading solvent evaporates.10Additional insight
into the nature of interfacial aggregate formation comes from
several recent studies that show that the concentration of the
spreading solution can strongly influence the morphologies of the
observed surface features.13,17 For a PS-b-PEO block copolymer with
7 wt % PEO (MW) 51k), Devereaux et al. carried out atomic force
microscopy (AFM) measurements of transferred Langmuir-Blodgett (LB)
films, and determined that self-assembly at the air-water interface
yielded a mixture of dots, spaghetti, and continent features, with
dots favored at lower spreading concentrations and continents
favored at higher concentrations.13Subsequently, we showed that a
relatively high- molecular-weight and hydrophobic PS-b-PEO
copolymer at the air-water interface (MW) 141k, 11.4 wt % PEO
(denoted 141k)) gave rise to dots and spaghetti at high spreading
concentrations, whereas, below a critical spreading concentration,
two new morphologiessrings and chainlike aggregatesswere
observed.17 This spreading concentration dependence points to the
importance of polymer overlap in the spreading solution on the
obtained surface features, and provides evidence that different 2D
morphologies reflect different degrees of PS chain entangle- ment
before vitrification and trapping of the aggregates by solvent
removal. This thinking lead Devereaux et al.,13 and then us,17 to
conclude that the range of observed 2D PS-b-PEO surface features in
our respective systems were not equilibrium structures, but rather
represented kinetic states trapped at different stages of
morphology evolution. However, no mechanism for the formation and
evolution of the various surface morphologies has been
provided.
While AFM yields important information on the lateral morphology of
the surface aggregates, Langmuir compression isotherms of PS-b-PEO
monolayers at the air-water interface can offer additional insight
into the aggregate fine structure, including the interfacial
conformation of PS and PEO blocks.10,13,17,18At low surface
density, these aggregates consist of a raised core of clustered PS
chains, in either a collapsed or somewhat brushlike state,13,17
with some of the PEO chains radiating from the periphery of the
aggregate in a 2D corona at the water surface (the so-called
“pancake” conformation).10,11
Cox et al. pointed out that PEO should also reside underneath the
PS core, and proposed a model describing a coherent film of PEO
buffering unfavorable interactions between PS and water.11 When
aggregates on the water surface are laterally compressed, the
surface density (Γ) increases until PEO chains in neighboring
corona eventually overlap, resulting in an increase in the surface
pressure (π), which is the difference between the surface tension
of pure water and the surface containing the film (π ) γ0 -
γ).10,11,13,17,18As the surface area (A) continues to decrease,
overlapping PEO chains submerge into the subphase at constantπ ∼ 10
mN/m to form a PEO brush, as evidenced by a break (pseudoplateau)
in theπ-A isotherm, a conformational change commonly described as
the “pancake-to-brush” phase transition. Further compression
results in the overlap of the relatively incompressible cores and a
sharp increase inπ. From the two regions of increasingπ, two
distinct limiting mean molecular areas can be determined,
describing the average surface
area of chains in the pancake and brush conformations of the
aggregates (A0,p and A0,b, respectively).6,7 The extent of the
pseudoplateau has been found to be dependent on the PEO content:
for copolymers with less than∼15 wt % PEO, the PEO pancake-to-brush
transition is impeded by the large surface area of the PS cores,
such that no break is observed in the isotherm.13
For identical amounts of the 141k PS-b-PEO copolymer deposited at
the water surface, we previously reported that the spreading
concentration significantly affected the resulting Langmuir
compression isotherms, indicating differences in chain
conformations within various trapped aggregates.17 It was found
that the limiting mean molecular area (A0) increased sharply below
a critical spreading concentration (c e 0.25 mg/mL), corresponding
to the appearance of chainlike and ring mor- phologies in the LB
films. This increase inA0 was tentatively ascribed to an increase
in the interfacial adsorption of PEO blocks within the aggregates
due to a decrease in entanglement of PS blocks during spreading.
However, because of the relatively low PEO content of the copolymer
in that study (11.4 wt %), no pseudoplateau was observed under any
of the experimental conditions investigated, limiting the
elucidation of aggregate fine structure from the isotherms.
This paper presents Langmuir isotherm and hysteresis data for
various spreading concentrations of a high molecular weight and
hydrophobic PS-b-PEO copolymer with a somewhat higher PEO content
than that investigated in our previous study: MW ) 185k, 18.9 wt %
PEO (denoted 185k). For this copolymer, Langmuir compression
isotherms show a clear pancake-to-brush phase transition, allowing
the effect of spreading concentration on the limiting areas of the
pancake and brush conformations (A0,p andA0,b, respectively) to be
assessed for the first time. This provides the basis for a model of
the fine structure of surface aggregates under different conditions
of chain entanglements during spreading. As well, we compare the
spreading concentra- tion dependence of the morphologies of
transferred aggregates in LB films of the 141k and 185k copolymers.
For both copolymers, morphology evolution at the air-water
interface is kinetically impeded by chain entanglements in the
spreading solution. As a result, higher spreading concentrations
result in lower chain mobilities before the aggregates are
ultimately trapped by vitrification, providing earlier “snapshots”
in the process of block copolymer aggregate formation.
The kinetic trapping of PS-b-PEO surface aggregates via variations
in the spreading concentration has led us to the first direct
evidence for a mechanism of spontaneous copolymer self- assembly at
the air-water interface. Insight into a mechanism of block
copolymer self-assembly is offered by a relatively high spreading
concentration for the 141k sample, which provides a very early
snapshot of aggregation at the water surface. AFM images of the
resulting LB films show a coexistence of long and interconnected
spaghetti aggregates with cellular networks of holes in relatively
low continent-like regions, with the holes surrounded by raised
rims. These network structures are strongly reminiscent of patterns
formed from the dewetting of ultrathin films of homopolymers from
solid23-32 and liquid33,34surfaces.
(23) Reiter, G.Phys. ReV. Lett. 1992, 68, 75. (24) Reiter,
G.Langmuir1993, 9, 1344. (25) Reiter, G.; Sharma, A.; Casoli, A.;
David, M.-O.; Khanna, R.; Auroy, P.
Langmuir1999, 2551, 1. (26) Ghatak, A.; Khanna, R.; Sharma, A.J.
Colloid Interface Sci.1999, 212,
483. (27) Koplik, J.; Banavar, J. R.Phys. ReV. Lett. 2000, 84,
4401. (28) Tsui, O. K. C.; Wang, Y. J.; Zhao, H.; Du, B.Eur. Phys.
J. E2003, 12,
417. (29) Kaya, H.; Jerome, B.Eur. Phys. J. E2003, 12, 383. (30)
Bollinne, C.; Cuenot, S.; Nysten, B.; Jonas, A. M.Eur. Phys. J.
E2003,
12, 389.
8388 Langmuir, Vol. 22, No. 20, 2006 Cheyne and Moffitt
In addition, various intermediate structures point to the formation
of spaghetti aggregates via the disconnection of junctions within
networks of rims. On the basis of this evidence, we outline a new
nonequilibrium mechanism for the formation and evolution of block
copolymer surface aggregates, beginning with an initial dewetting
of copolymer solutions from the air-water interface. We believe
that this mechanism may be generally applicable to the formation of
various 2D copolymer morphologies at the air- water interface, and
has been elucidated in the present system due to slow polymer
dynamics relative to the time scales of spreading and solvent
evaporation. This is attributed to relatively long PS blocks
leading to strong chain entanglements and low PEO contents leading
to slow copolymer spreading.
Experimental Section
Materials. The two PS-b-PEO copolymers used in this study were
obtained from Polymer Source Ltd. and were characterized by the
manufacturer (Table 1), whereMn is the number-average molecular
weight,Mw/Mn is the polydispersity index, andxn indicates the
degree of polymerization of each block.
Stock solutions of each copolymer (∼5 mg/mL) were prepared by
weighing an appropriate amount of the polymer (dried overnight in a
dark desiccator at room temperature) into new glassware that had
been cleaned three times with acetone and three times with
spectroscopic-grade chloroform (Aldrich, 99.8%); a specific amount
of glass-distilled, HPLC-grade chloroform (Sigma-Aldrich, 99.9+%)
was then added gravimetrically. Spreading solutions of various
concentrations were prepared by diluting appropriately weighed
quantities of this stock solution; all solutions were prepared 24 h
prior to use to allow for equilibration and used within 2 days for
isotherm trials and within 7 days for dipping studies. Prepared
solutions were sealed with Teflon and refrigerated in the dark.
Solid copolymer samples were stored in a sealed vial covered in
aluminum foil at -20 °C. These precautions were necessary to
minimize the degradation of the PEO block via oxidation.35
Isotherms of Langmuir Films. Surface pressure-area (π-A) isotherms
were obtained using a KSV 3000 Langmuir trough (KSV Instruments
Ltd.) secured inside a dust shield. The total trough surface area
was 150× 515 mm2 and the total trough volume was∼1 L. The trough
area was robotically controlled by two hydrophobic paddles that
compressed the spread film symmetrically and bilaterally at a rate
of 10 mm/min. For hysteresis measurements, a 10 min delay was used
following each compression or expansion step, prior to the next
compression or expansion. House-distilled, deionized water
(Barnstead NANOpure Diamond, 18.2 m‚cm) was used as the subphase in
all trials. For all experiments, the subphase temperature was 21(
0.5°C. Prior to each trial, the water surface was cleaned by
aspirating off any debris, such that the measured surface pressure
remained<0.20 mN/m over a full compression. The LB components
were cleaned daily with absolute ethanol, and the deionized water
subphase was replaced regularly. Surface pressure measurements were
made from a roughened platinum Wilhemly place (perimeter of 39.240
mm), which was flamed prior to each trial to ensure cleanliness.
PS-b-PEO solutions in chloroform were spread on the subphase from a
gastight Hamilton syringe. Spreading solution volumes depended on
the concentration of the spreading solution
and ranged from 10 to 1000µL, so that a constant mass of copolymer
was deposited for each trial. The spreading solution was deposited
dropwise (∼2 µL drop volume) at regularly spaced locations on the
trough. In all trials, a 15 min evaporation period between the last
deposited drop of solution and the beginning of compression was
employed to ensured complete solvent evaporation.
One aspect of the of the present isotherm investigations is that
the volume of solution deposited must proportionately increase with
decreasing concentration to maintain a constant mass of deposited
copolymer. This introduces an increased risk of contaminating the
surface with impurities that may contribute to the isotherms. To
examine this possibility, three trials using 1000µL of pure 99.9+%
chloroform (the same volume as the 0.10 mg/mL trial) were carried
out. For the resulting compression isotherms, the observed surface
pressure did not exceed 0.4 mN/m over a full compression,
indicating that impurities from the largest solvent volumes
employed do not contribute significantly to the isotherms.
Langmuir -Blodgett Films. Langmuir films were prepared as described
above for isotherm measurements, except that the volume of the
spreading solution was exactly half. This was necessary to optimize
the transfer ratio (1.1( 0.2), and independent AFM analysis
confirmed that the volume of solution deposited had no discernible
influence on the surface morphologies. Following the solvent
evaporation period, the films were compressed and maintained at the
desired transfer pressure for 10 min, followed by vertically
lifting the submerged substrate though the monolayer at a rate of 1
mm/ min. Glass substrates (VWR Scientific, 18× 18 mm2) were first
cleaned with methanol, sonicated (Branson 3510) in spectroscopic-
grade chloroform (99.8%, Aldrich) for 20 min, dried, and used
immediately. All transferred thin films were dried vertically for a
minimum of 12 h and imaged within a week.
Atomic Force Microscopy.All AFM imaging was conducted on a Veeco
(ThermoMicroscopes Explorer) instrument equipped with a Veeco tip
(NanoProbe-MLCT-EXMT-A) run in contact mode. Tip velocity was
typically maintained at 10µm/s, and the resolution of each image
was held at 500 scans/image. The AFM tips have a radius of<50
nm, resulting in a maximum systematic error in lateral dimensions
of<20 nm (calculated on the basis of geometrical considerations
of the tip and surface features); this error is within the reported
standard deviations of feature widths. To ensure that the friction
of the tip did not perturb the surface features, a 10µm × 10 µm
area was imaged several times followed by a 20µm × 20 µm scan of
the same region; there was no obvious agitation of the observed
aggregate structures from tip contact during the repeated scans.
The AFM probe was housed within a vibration-resistant case on a
vibration isolation platform maintained at 80 psi (Newport). Each
sample was imaged several times at different locations on the
substrate to ensure reproducibility. In all cases, the imaging of
LB films was performed far from the edge of the glass substrate to
minimize any local effects caused by turbulent water flow at the
boundary and meniscus effects during transfer.
Results and Discussion
The Results and Discussion section of this paper is divided into
the following parts: In the first part, we describe the effect of
spreading concentration on the compression isotherms of the 185k
copolymer; these results, together with previous isotherm data from
the 141k copolymer,17 are used to develop a model describing the
packing and conformation of hydrophobic and hydrophilic blocks
within PS-b-PEO surface aggregates for different spreading
concentrations. The second part describes the spreading
concentration dependence of isotherm hysteresis. This is followed
by a description of the spreading concentration dependence of
aggregate morphologies in LB films, including a comparison of
morphology evolution in the 141k and 185k copolymers. We finally
present AFM data of surface aggregates for the 141k polymer
obtained at a high spreading concentration (c ) 2.0 mg/mL), showing
evidence that various surface morphologies of PS-b-PEO evolve from
patterns formed by
(31) Muller-Buschbaum, P.J. Phys.: Condens. Matter2003, 15, R1549.
(32) Sharma, A.; Verma, R.Langmuir2004, 20, 10337. (33) Segalman,
R. A.; Green, P. F.Macromolecules1999, 32, 801. (34) Lu, G.; Li,
W.; Yao, J.; Zhang, G.; Yang, B.; Shen, J.AdV. Mater.2002,
14, 1049. (35) Bailey, F. E.; Koleske, J. V.Poly(ethylene oxide);
Academic Press:
NewYork, 1976.
Table 1. Characteristics of the Copolymers Used in This Study
PS-b-PEO Mn
(g/mol) xn,PEO xn,PS
141k 141 100 11.4 1.04 16 100 125 000 370 1200 185k 185 000 18.9
1.09 35 000 150 000 800 1440
Dewetting in the Formation of PS-b-PEO Copolymers Langmuir, Vol.
22, No. 20, 20068389
dewetting of the evaporating solution from the water subphase. On
the basis of this evidence, we propose a nonequilibrium mechanism
for PS-b-PEO self-assembly at the air-water interface, involving an
initial dewetting step, following by subsequent morphology
evolution driven by interfacial PEO adsorption.
Compression Isotherms.Compression isotherms obtained for the more
hydrophilic 185k copolymer all showed a pseudo- plateau atπ ∼ 10
mN/m, consistent with a pancake-to-brush transition for overlapping
PEO chains during compression. Representative isotherms of Langmuir
films cast fromc) 0.10- 1.0 mg/mL spreading solutions of the 185k
copolymer are shown in Figure 1a, highlighting a marked spreading
concentration dependence of the isothermal film behavior. For each
spreading concentration, the limiting mean molecular areas of the
pancake and brush conformations (A0,p, and A0,b, respectively) were
determined by extrapolating tangents to the inflection points in
the two regions of increasingπ to π ) 0, as shown in Figure 1b. For
the range of spreading concentrations, the isotherms in Figure 1a
show a constant initial onset in the surface pressure increase and
a constant pancake limiting areaA0,p; the latter value is plotted
versus the spreading concentration in Figure 2a, with errors
determined from repeat runs carried out on separately prepared
Langmuir films. When we normalize the determined A0,p values with
respect to the number of EO repeat units in the copolymer, we
obtain an average value of∼31 Å2 per EO monomer over the range of
spreading concentrations. This value is consistent with the pancake
limiting areas reported for PS- b-PEO copolymers of various
compositions,6,7,12,13 and is significantly lower than the limiting
areas reported for PEO homopolymer at the air-water interface
(40-48 Å2 per EO).36
In contrast to the constant limiting area of the pancake
conformation, the isotherms in Figure 1a clearly show a progressive
increase in the brush limiting areaA0,bwith decreasing spreading
concentration. The earlier onset of the brush region with constant
onset of the pancake region results in a clear decrease in the
extent of the pseudoplateau as the spreading concentration
decreases, which appears as only a slight break in the isotherm for
c ) 0.10 mg/mL. The dependence of the spreading concentration on
the brush limiting area (Figure 2b) shows that A0,b is relatively
constant for higher spreading concentrations (c g 1.0 mg/mL),
although it starts to increase considerably forc e 0.5 mg/mL.
Interestingly, this trend inA0,b with respect to spreading solution
concentration is similar to that observed previously forA0 values
determined from compression isotherms of the 141k copolymer, which
were constant for a range of spreading concentrations but sharply
increased forc e 0.25 mg/ mL.17
The observed spreading concentration dependence of com- pression
isotherms points to significant differences in chain conformations
and packing within surface aggregates of the 185k copolymer
following solvent evaporation. Figure 3 shows a qualitative model,
based on the observed trends in the isotherm data, describing the
effect of spreading concentration on the conformations of PS and
PEO blocks within surface aggregates
(36) Sauer, B. B.; Yu, H.; Yazdanian, M.; Zografi, G.; Kim, M. W.
Macromolecules1989, 22, 2332.
Figure 1. (a) Representativeπ-A compression isotherms of the 185k
copolymer at the air-water interface, obtained following spreading
from different concentrations of copolymer in chloroform (0.10-1.0
mg/mL). From each isotherm, the brush limiting areaA0,b and the
pancake limiting areaA0,p were determined as shown in panel
b.
Figure 2. Plots of pancake limiting areaA0,p (a) and brush limiting
areaA0,b (b) versus spreading concentration of the 185k copolymer
determined from compression isotherms.
Figure 3. Qualitative model of copolymer packing and its effect on
the pancake-to-brush transition for PS-b-PEO surface aggregates
formed from (a) concentrated and (b) dilute spreading solutions,
based on concentration-dependent compression isotherm data for the
185k copolymer.
8390 Langmuir, Vol. 22, No. 20, 2006 Cheyne and Moffitt
after solvent removal, before and after compression past the
pancake-to-brush transition. This picture assumes that the PS cores
are vitrified after solvent evaporation, so that, upon compression,
peripheral PEO chains extending from the cores can undergo a
pancake-to-brush transition, but no fundamental rearrangement of
chains within the cores can occur. Consistent with the model of
PS-b-PEO surface aggregates described previously by Cox et al.,11
Figure 3 shows PEO blocks (dark blue) located both in a corona at
the periphery of the PS core (red) and also in a sublayer
underneath the core. The key feature of the present model is the
difference in copolymer packing densities within the aggregates
under different spreading condi- tions and the consequential
differences in the PEO chain conformation within the sublayer. For
concentrated spreading (Figure 3a), the packing of copolymers
within the core is relatively dense, resulting in a high local
density of PS blocks inside the cores. The surface concentration of
PS blocks directly affects the nature of the interaction between EO
monomers and the interface,9
such that strong repulsive interactions are expected underneath the
densely packed cores, forcing PEO chains in the sublayer to
submerge into the water, even before the aggregates are compressed.
Upon compression, the peripheral PEO chains overlap as aggregates
are brought into contact, and will eventually undergo a
pancake-to-brush transition until the neighboring cores start to
overlap. In contrast, for dilute spreading (Figure 3b), the packing
density of copolymers within the cores is significantly lower, with
weaker repulsion between the EO monomers and the interface
underneath the aggregates reflected in the partial interfacial
adsorption of PEO chains in the sublayer. When these aggregates are
compressed, peripheral PEO begins to overlap at an areal density
similar to that in the concentrated spreading case; however, the
less dense cores occupy a larger area per molecule, and will begin
to overlap at an earlier stage of compression (Figure 3b) compared
to the cores from concentrated spreading (Figure 3a). The earlier
overlap of aggregate cores will impede compression-induced
desorption of PEO chains in the corona, explaining the decrease in
the extent of the pseudoplateau as the spreading concentration
decreases (Figure 1a). Although this model is based on trends
inA0,p and A0,b
measured from compression isotherms for the 185k copolymer, the
previously reported concentration dependence of the limiting area
of the 141k copolymer17 suggests that a similar picture of
copolymer packing from different spreading conditions is generally
applicable to copolymers in which chain entanglements influence
surface self-assembly on the time scale of solvent evaporation, as
discussed below.
The observed differences in chain conformations within aggregates
formed from different spreading concentrations can be explained by
increased entanglements between PS blocks in more concentrated
spreading solutions. On the basis of the model in Figure 3, the
interfacial adsorption of EO monomers should provide a driving
force for the system to decrease the density of packing within the
aggregates, although this tendency can be kinetically impeded by
the overlap of PS blocks in the spreading solvent if chain
entanglement effects are significant: when the spreading
concentration is relatively high, the polymer chain dynamics will
be slower, such that denser, less energetically favorable aggregate
conformations are ultimately trapped by solvent removal and
vitrification. We note that the structures in Figure 3 describe
only differences in the average density of copolymers within
aggregates from various spreading solutions, as inferred from
isotherm data, and do not address the spreading concentration
dependence of the lateral morphologies of the
aggregates. The latter related issue will be discussed in
connection with the AFM data of transferred LB films of
self-assembled PS-b-PEO.
Hysteresis. Hysteresis effects have proved to be very informative
in elucidating the dynamic behavior of amphiphilic systems under
variable stress at air-liquid6-8,37-43 and liquid- liquid
interfaces.44 It has been observed that PS-b-PEO copoly- mers at
the air-water interface have compression isotherms that can be
significantly different from the subsequent expansion isotherms.6-8
Goncalves da Silva et al.6 studied hysteresis in PS-b-PEO
monolayers at both high and low surface pressures (brush and
pancake regimes, respectively) through a series of
compression/expansion isotherms on PS-b-PEO copolymers of variable
PEO block length. However, the effect of spreading concentration on
the hysteresis of PS-b-PEO monolayers has not been previously
investigated.
Hysteresis of the 185k copolymer in the pancake regime (compression
toπ ) 10 mN/m, just above the plateau region, followed by
expansion) was found to show a clear spreading concentration
dependence, as shown by representative compres- sion/expansion
cycles for spreading concentrations of 0.25 and 0.50 mg/mL (Figure
4a). This hysteresis was quantified by
(37) Bijsterbosch, H. D.; de Haan, V. O.; de Graaf, A. W.; Mellema,
M.; Leermakers, F. A. M.; Cohen Stuart, M. A.; van Well, A.
A.Langmuir1995, 11, 4467.
(38) Xu, Z.; Holland, N. B.; Marchant, R. E.Langmuir2001, 17, 377.
(39) Seo, Y.; Im, J.-H.; Lee, J.-S.; Kim, J.-H.Macromolecules2001,
34, 4842. (40) Baskar, G.; Gaspar, L. J. M.; Mandal, A.
B.Langmuir2003, 19, 9051. (41) Francis, R.; Taton, D.; Logan, J.
L.; Masse, P.; Gnanou, Y.; Duran, R.
S. Macromolecules2003, 36, 8253. (42) Beija, M.; Relogio, P.;
Charreyre, M.-T.; Silva, A. M. G. d.; Brogueira,
P.; Farinha, J. P. S.; Martinho, J. M. G.Langmuir2005, 21, 3940.
(43) Baekmark, T. R.; Elender, G.; Lasic, D. D.; Sackmann,
E.Langmuir
1995, 11, 3975. (44) Milling, A. J.; Hutchings, L. R.; Richards, R.
W.Langmuir2001, 17,
5305.
Figure 4. (a) Representative compression-expansion cycles of the
185k copolymer in the pancake regime (compression toπ ) 10 mN/m)
following spreading from 0.25 mg/mL (solid line) and 0.50 mg/mL
(dashed line) copolymer solutions in chloroform. (b) Hysteresis in
the pancake regimeA0,p versus spreading concentra- tion.
Dewetting in the Formation of PS-b-PEO Copolymers Langmuir, Vol.
22, No. 20, 20068391
calculating the differenceA0,p ) A0,p - A′0,p, whereA′0,p is the
pancake limiting area determined from the expansion isotherm.
Figure 4b showsA0,pto be constant in the range of high spreading
concentrations, but decrease sharply for monolayers cast from
spreading solutions below 0.50 mg/mL (Figure 4b), corresponding to
the increase inA0,b in Figure 2b. Hysteresis in the pancake regime
can be explained by PEO entanglement during the pancake-to-brush
transition.6 We note that the extent of PEO entanglements during
compression will depend on the amount of PEO that undergoes the
pancake-to-brush transition. On the basis of the model in Figure 3,
less extensive PEO entanglements should therefore form upon
compression for lower spreading concentrations, since earlier
overlap of less densely packed cores will impede the conformational
transition of peripheral PEO chains. The observed trend inA0,p
therefore supports the proposed differences in chain packing and
conformation within aggregates under different spreading
conditions. Another possible cause of hysteresis at low pressures
is the irreversible desorption of PEO homopolymer impurity from the
interface.6,37We carried out multiple compression-expansion cycles
in the pancake regime of the isotherm (Supporting Information) and
found that A0,p decreases steadily with each cycle for all
spreading concentrations, although significant hysteresis persists
after the first cycle. This indicates that PEO homopolymer may
contribute to A0,p, although it is not the main cause of hysteresis
in this system.
Hysteresis was also observed in the high surface pressure region of
the 185k copolymer isotherm (brush regime) by compression toπ ) 40
mN/m followed by expansion. For both spreading concentrations shown
in Figure 5a (0.10 and 0.50 mg/mL), a sharp drop in the surface
pressure is observed upon expansion, followed by a shallow minimum
(inset). A similar
minimum was previously observed in expansion isotherms obtained by
Goncalves da Silva et al. for PS-b-PEO monolayers expanded from 35
mN/m, and was attributed to the local ordering and cohesion of
helical PEO chains in the brush conformation.6
To compare hysteresis for different spreading concentrations, we
calculatedA0,b) A0,b- A′0,b, analogous to the determination of
A0,p. As shown in Figure 5b, the observed trend inA0,bwith
decreasing spreading concentration is the opposite of that found
for A0,p, increasing for spreading concentrations below 0.50 mg/mL.
An increase in hysteresis in the brush regime for lower spreading
concentrations can also be explained in terms of the conformational
differences described in Figure 3. In the dilute spreading case,
the increased compressibility of less densely packed cores should
lead to improved organization and cohesion of submerged PEO chains
upon compression to high surface pressures, explaining the observed
increase in hysteresis compared to aggregates from more
concentrated spreading.
Aggregate Morphologies in Langmuir-Blodgett Films. Figure 6
summarizes the spreading concentration dependence of LB film
topologies for both 141k and 185k copolymers, showing AFM images of
aggregates obtained from spreading concentra- tions from 1.0
(Figure 6a,b) to 0.25 mg/mL (Figure 6e,f). In all AFM micrographs,
the light regions represent the raised PS cores of the aggregates,
which are formed at the air-water interface and then transferred to
glass at constant surface pressure. Since the aggregates become
kinetically frozen upon solvent evapora- tion,10,13,17the observed
features can be assumed to represent the
Figure 5. (a) Representative compression-expansion cycles of the
185k copolymer in the brush regime (compression toπ ) 40 mN/m)
following spreading from 0.10 mg/mL (solid line) and 0.50 mg/mL
(dashed line) copolymer solutions in chloroform. The inset details
the pressure minimum during expansion for the 0.50 mg/mL case. (b)
Hysteresis in the brush regimeA0,b versus spreading concentra-
tion.
Figure 6. AFM images of LB films of 141k (a,c,e) and 185k (b,d,f)
copolymers deposited at the air-water interface from various
spreading concentrations: 1.0 mg/mL (a,b), 0.50 mg/mL (c,d), 0.25
mg/mL (e,f). Transfer pressures,π ) 2.0 mN/m (a,c,e) andπ ) 5.0
mN/m (b,d,f). Images are 10× 10 µm.
8392 Langmuir, Vol. 22, No. 20, 2006 Cheyne and Moffitt
morphology of aggregates as they existed at the air-water
interface. Evidence for the kinetic stability of aggregates at the
air-water interface, attributed to the glassy nature of high-Tg PS
chains following solvent evaporation, was provided by AFM film
morphologies obtained under identical spreading conditions but for
different surface pressures during transfer (π ) 2.0-20.0 nN/m).
Compression to higher surface pressures before transfer resulted in
a significant decrease in the distance between features, although
the observed aggregate morphologies were found to be identical
(Supporting Information).
For both the 141k and 185k copolymer samples, clear morphology
changes are observed as a function of the spreading concentration.
Both samples show a mixture of dots and spaghetti from 1.0 mg/mL
spreading (Figure 6a,b), with longer spaghetti and fewer dots
observed for the 141k sample than for the 185k sample. The mean
widths of dots and spaghetti for the two copolymers are the same
within error (Table 2), although the 185k sample forms dots and
spaghetti that are somewhat taller (20 and 15 nm, respectively)
than those formed from the 141k sample (16 and 12 nm,
respectively), consistent with a somewhat taller brush from the
longer PS blocks in the 185k copolymer. As described in our earlier
paper,17 a decrease in the spreading concentration of the 141k
sample results in a progression from spaghetti (Figure 6a), to
“budding spaghetti” (Figure 6b) with dimples at the ends and
depressions along the length of the aggregates, to a predominance
of interesting rings and chainlike aggregates (Figure 6e). In
contrast, for the 185k sample, decreasing the spreading
concentration results in an increase in the number of dots relative
to spaghetti, with transition structures (Figure 6f, inset)
indicating that dots are formed by pinching from spaghetti
aggregates. In addition, as the spreading con- centration
decreases, the 185k sample shows a dramatic increase in the number
of structures classified as small irregular aggregates (Figure
6d,f), with lateral dimensions significantly smaller than dots
(<125 nm) and irregular, noncircular topologies. Unlike the 141k
sample, no rings or chainlike aggregates were observed from the
185k copolymer for any of the spreading concentrations
studied.
These AFM data from different spreading concentrations indicate
that varying degrees of PS chain entanglement, resulting in
different copolymer mobilities in the spreading solvent, can
significantly influence the aggregate morphologies. The depen-
dence of chain entanglements on the observed surface aggregates
strongly suggests that they are nonequilibrium structures,
kinetically trapped at different stages of morphology evolution
upon solvent removal; this sharply contrasts the situation of block
copolymer micelles in solution where a thermodynamic balance of
interfacial energy and chain stretching determines the aggregate
size and morphology.45-49 AFM images obtained for higher spreading
concentrations, where chain mobilities are more restricted before
freezing, can therefore be regarded as earlier
snapshots in the evolution of aggregate morphology. From the
predominance of spaghetti aggregates at higher concentrations for
both the 141k and 185k copolymers, along with the observed
transition morphologies at lower concentrations, it can be
concluded that spaghetti forms at a relatively early stage of self-
assembly for both copolymers, and then evolves by different
pathways in a manner that depends on the composition. Since
isotherm data and the resulting chain-packing model (Figure 3)
suggest that changes in copolymer packing are driven by an increase
in interfacial PEO adsorption underneath the aggregates, this same
driving force should play an important role in directing the
evolution of spaghetti aggregates into the various observed
morphologies. In the 141k system, the development of dimples
(budding spaghetti, Figure 6c) and then holes along the length of
spaghetti (chainlike aggregates, Figure 6e) should allow PEO blocks
trapped underneath the aggregates to become adsorbed. In the 185k
system, the pinching of dots from spaghetti will increase the
contact line of the aggregates at the air-water interface, also
allowing submerged PEO to become adsorbed. Another driving force
for the pinching of dots from spaghetti will be provided by a more
favorable PS surface-to-volume ratio, similar to the development of
a Rayleigh instability in a liquid ribbon.31The different pathways
for the two copolymers may be explained by the more hydrophobic
character of the 141k sample, which results in slower chain
dynamics upon spreading compared to the more hydrophilic 185k
copolymer; on the basis of this argument, it is possible that the
formation of budding spaghetti and then chainlike aggregates (141k
copolymer) requires less lateral movement of chains than the
pinching of dots from spaghetti (185k copolymer).
An important feature of the copolymer packing model in Figure 3
that cannot be confirmed by isotherm measurements is the relative
heights of the aggregates from high and low spreading
concentrations. For concentrated spreading, where the average
packing of copolymers in the cores is found to be relatively dense,
the aggregates should be, on average, higher than when the
spreading concentration is more dilute, where the average packing
is less dense and therefore less brushlike (Figure 3). The relative
occurrence and heights (Table 2) of the features observed by AFM
from high and low spreading concentrations of both copolymers
appear to support this conclusion. For the 141k copolymer, the
rings and chainlike features formed at low spreading concentrations
are significantly lower (10 nm) than the dots and spaghetti formed
at higher spreading concentrations (16 and 12 nm) and are closer to
the expected height of an individual collapsed PS coil from the
Kumaki area (8 nm).50 For the 185k copolymer, dots, spaghetti, and
small irregular aggregates are observed at all spreading
concentrations, although the occurrence of small irregular
aggregates increases significantly as the spreading concentration
decreases (Figure 6d,f); these irregular aggregates are also much
lower (10 nm) than the corresponding dots and spaghetti (20 and 15
nm), further
(45) Zhang, L.; Eisenberg, A.Science1995, 268, 1728. (46) Zhang,
L.; Eisenberg, A.J. Am. Chem. Soc.1996, 118, 3168. (47) Zhang, L.;
Eisenberg, A.Macromolecules1999, 32, 2239. (48) Yu, K.; Eisenberg,
A.Macromolecules1996, 29, 6359. (49) Yu, K.; Eisenberg,
A.Macromolecules1998, 31, 3509. (50) Kumaki, J.Macromolecules1988,
21, 749.
Table 2. Mean Dimensions of PS-b-PEO Surface Aggregates Observed by
AFMa
141k 185k
(walls) dots spaghetti irreg. aggs.
height (nm) 16( 2 12( 2 10( 2 20( 2 15( 2 10( 3 width (nm) 170( 30
160( 20 170( 20 180( 40 170( 20 110( 20
a Errors represent the standard deviation of at least 75 measured
surface features.
Dewetting in the Formation of PS-b-PEO Copolymers Langmuir, Vol.
22, No. 20, 20068393
supporting the conclusion from isotherm data that the average
density of chain packing decreases with the spreading concentra-
tion.
Mechanism of the Formation of PS-b-PEO Surface Ag- gregates:
Dewetting-Induced Self-Assembly at the Air- Water Interface. The
results discussed in previous sections demonstrate that, for both
141k and 185k block copolymers, spaghetti aggregates appear to be a
common starting point from which a number of other surface
aggregates evolve (dots, chainlike aggregates, rings, and small
irregular aggregates). However, this conclusion gives rise to the
question, How do spaghetti aggregates initially form? Long surface
aggregates with high aspect ratios (termed spaghetti,13,17rods,4,10
worms,18 or strands19 in various references) are often regarded as
2D counterparts to 3D rodlike block copolymer micelles, which form
under conditions of a dynamic equilibrium between single chains and
a thermody- namically optimum morphology.45-48 However, the
observed spreading concentration dependence of surface behavior and
aggregate morphology reported here makes a strong case that
spaghetti aggregates, unlike 3D block copolymer cylindrical
micelles, are frozen rather than equilibrium structures. In this
section, we consider compelling evidence that dewetting of
evaporating copolymer solutions from the air-water interface
initiates self-assembly and the formation of the various aggregate
morphologies.
The conditions and mechanism for the dewetting of liquid polymer
films from surfaces have been widely studied in recent years.23-34
The stability of a thin (,1 µm) liquid film on a surface is
governed by a combination of long-range and short- range
intermolecular forces. Under conditions where long-range van der
Waals interactions predominate, the tendency of a thin film to
undergo dewetting is determined by the sign of the effective
Hamaker constant,A ) ALL - ASL, whereALL is the Hamaker constant
for liquid-liquid interactions within the film andASL
is the Hamaker constant for substrate-liquid interactions.30 If A
is positive, then cohesive interactions within the film are
stronger than interactions between the film and the substrate, and
the film will dewet below a critical thickness to minimize contact
with the substrate. Film rupture results in the formation of holes
throughout the unstable or metastable film, via either spinodal
decomposition or nucleation by local defects or impurities; the
holes then expand as material flows from wet regions into raised
rims surrounding the growing holes. Advancing rims eventually come
into contact, forming a polygon pattern of interconnected liquid
ribbons that finally breaks down into droplets via a Rayleigh
instability.23,24,31
Figure 7 shows AFM images of LB films of the 141k copolymer
obtained from a spreading concentration of 2.0 mg/mL, which is
twice as concentrated as the spreading solution from which the
spaghetti aggregates in Figure 6a were prepared. The large number
of chain entanglements present in this solution gives rise to an
even earlier snapshot of the self-assembly process, which shows a
large number of long and interconnected spaghetti aggregates, in
addition to cellular networks of holes in relatively low
continent-like regions. These networks of polygonal holes are
strongly reminiscent of patterns formed in the intermediate stages
of dewetting of thin polystyrene films from solid surfaces,23,24and
provide evidence that the evaporating film of copolymer solution
underwent dewetting from the air-water interface before
vitrification. Moreover, the coexistence of spaghetti aggregates
with the apparent dewetting patterns, along with various
intermediate structures, suggests that spaghetti evolves from the
networks of merging rims formed by hole formation and growth.
Figure 8 shows close-up images of transition structures in
different regions of the 141k LB film from a 2.0 mg/mL spreading
concentration, representing various stages of the formation of
spaghetti aggregates. Investigation of the various structures
within this film suggest a series of kinetic snapshots in which the
earliest structures are networks of holes in a continuous film
(Figure 8a, i) with holes surrounded by raised rims (Figure 8a,
ii). The pattern in Figure 8a is very consistent with the
intermediate stages of liquid film dewetting described by Reiter,24
showing regions where advancing rims are about to come into contact
(Figure 8a, ii), along with strands formed by the merging of two
rims (Figure 8a, iii). As described for other dewetting films, the
developing network consists of polygonal holes with vertexes
connecting three edges.24 Another transition structure (Figure 8b)
shows a region where a portion of the resulting network of rims has
broken into large chains (on a larger length scale than the chains
observed for the same sample at low spreading concentrations), with
buds of accumulated material on the chains arising from the breaks
in connectivity (Figure 8b, iv). Other regions indicate that the
large chains then break at junction points to form spaghetti, with
sharp bends in the spaghetti (Figure 8c, v), suggesting their
former connectivity within the large chains. This evolution of
spaghetti aggregates from a network of rims arising from a
dewetting process is supported by a topographic line profile
(Figure 8d,e) revealing that the heights (∼12 nm) and widths (∼150
nm) of the walls of a “broken” network and the neighboring
spaghetti are nearly identical. As well, the height and width of
spaghetti aggregates from this higher spreading concentration (2.0
mg/mL) of the 141k copolymer are completely consistent
Figure 7. AFM images of the LB film of the 141k copolymer deposited
at the air-water surface from a spreading concentration of 2.0
mg/mL, showing coexistence of dewetting patterns and spaghetti
aggregates. Transfer pressure,π ) 2.0 mN/m. Images are 10 × 10
µm.
8394 Langmuir, Vol. 22, No. 20, 2006 Cheyne and Moffitt
with spaghetti aggregates observed for the same copolymer under
more dilute spreading (e.g., Figure 6a). Contrasting the 141k
copolymer, the 185k copolymer did not reveal dewetting patterns
even at the highest investigated spreading concentration of 2.0
mg/mL, which resulted in a mixture of mainly spaghetti with some
dots (not shown). This is explained by faster chain dynamics during
spreading of the more hydrophilic polymer, with a larger PEO
content resulting in a stronger driving force for aggregate
evolution against the restrictions of PS entanglements. However,
considering the similarities of spaghetti aggregates observed for
the two copolymers, it is reasonable to conclude that spaghetti
within the 185k films also formed via a dewetting process, although
the early dewetting patterns could not be trapped on the time scale
of solvent evaporation under the spreading conditions
investigated.
The various kinetic snapshots provided by AFM and isotherm data at
different spreading concentrations therefore suggest a mechanism
for interfacial PS-b-PEO self-assembly (Figure 9). Immediately
following deposition of the spreading solution on water, a
continuous film of the copolymer solution forms at the air-water
interface, stabilized by the positive spreading coef- ficient of
chloroform on water. Within the film, PEO blocks (dark blue) will
orient toward the water surface as chloroform evaporation occurs,
resulting in the formation of a continuous monolayer of diblock
copolymer, with the PS blocks (red) highly swollen with chloroform
(Figure 9a). With further evaporation of chloroform, increasing
contributions from unfavorable PS- water interactions eventually
result in film dewetting and hole formation in various regions of
the film (Figure 9b,c). As the
holes expand, the monolayer material is pushed from the continuous
wet regions into rims surrounding the holes, where the copolymer
chains become more densely packed and brushlike as a result of the
local lateral flow fields (Figure 9b);27 PEO underneath the rims is
forced into the subphase by the local dense packing of PS blocks
arising from the dewetting flow. The growth of holes will continue
until the advancing rims merge to form a 2D network of merged rims
(Figure 9d), with an internal structure best described by a densely
packed brush of copolymer chains, once most of the chloroform has
evaporated (Figure 9e). In most examples of dewetting of liquid
films from solid substrates, the liquid ribbons that form from
contact between advancing rims undergo immediate breakup into
droplets via a Rayleigh instability, in a process driven by the
surface tension of the liquid.23,24,31In contrast, the evolution of
the 2D network formed by dewetting of copolymer solutions from the
water surface appears to follow a different path, with the network
breaking at various junctions to form long spaghetti aggregates
(Figure 9f), which then further evolve into chainlike aggregates
(141k copolymer) or dots (185k copolymer) (Figure 9g). We point out
that, although PS surface tension will be an important factor in
the evolution of aggregates following the dewetting stage, our
isotherm results suggest that increased adsorption of PEO chains
trapped underneath the aggregates provides an additional driving
force for aggregate evolution. Material flow toward the rims during
dewetting can explain the initial dense packing of copolymer chains
within the spaghetti aggregates and the resulting desorption of PEO
chains under the aggregate cores. Thus, subsequent PEO adsorption,
along with kinetic restrictions of PS entanglements, may account
for the unique
Figure 8. (a-d) AFM images of various transition structures found
in different regions of the LB film of the 141k copolymer deposited
at the air-water surface from a spreading concentration of 2.0 mg/
mL. The indicated features are described in the text. Transfer
pressure, π ) 2.0 mN/m. Images are 4× 4 µm. (e) Surface feature
topology profile from the line in panel d.
Figure 9. Dewetting mechanism for the formation of PS-b-PEO
aggregates at the air-water interface.
Dewetting in the Formation of PS-b-PEO Copolymers Langmuir, Vol.
22, No. 20, 20068395
structures formed via PS-b-PEO self-assembly at the air-water
interface compared to the patterns of droplets formed in more
conventional dewetting processes of single-component liq-
uids.23,24,31
This dewetting mechanism for surface aggregate formation within
PS-b-PEO monolayers also explains the observation of large
continent structures previously observed in PS-b-PEO
monolayers.10,13,19From the discussion above, we propose that
continents are simply extended regions of the original continuous
film, which become frozen by solvent evaporation before undergoing
dewetting or before being subsumed by an advancing rim. In fact,
although we did not observe large continents from either of our
samples at any of the studied spreading concentra- tions, the flat
regions between rims in Figure 8a suggest smaller versions of the
continents shown by Devereaux et al.,13consisting of relatively
low, flat aggregates surrounded by raised rims.
To our knowledge, only one previous paper has suggested dewetting
as a mechanism for block copolymer aggregate formation at the
air-water interface to explain the formation of 2D nanofoams from
polystyrene-b-poly(sodium acrylate) block copolymers.5 Here we have
shown evidence that dewetting appears to be the critical first step
in the formation of a wide range of more common surface aggregates
from self-assembly PS-b-PEO copolymers, including continents,
spaghetti, and dots. Of course, dewetting as a mechanism for
aggregate formation can only explain structures that form during
solution spreading/ evaporation, and cannot therefore account for
features which have been recently observed to form at elevated
surface pressures upon compression of PS-b-PEO starlike copolymers,
that is, “micelle chaining”.21
A key question arising from this investigation is, How general is
the proposed dewetting mechanism for block copolymer self- assembly
at the air-water interface? The main hurdle to answering this
question by the current methodology is that, to trap the initial
dewetting step by solvent evaporation, extremely slow chain
dynamics are required. For the two copolymers investigated, both
possess long PS blocks and therefore significant chain
entanglements, and yet only the 141k copolymer showed trapped
dewetting patterns and transitional structures at high concentra-
tions, presumably due to its lower PEO content and slower spreading
dynamics compared to the 185k copolymer. However, the similarity of
spaghetti aggregates formed from both copoly- mers suggests a
similar mechanism of formation. These two copolymers therefore
highlight the possibility that the mechanism may be quite general,
although its initial stages can only be observed ex situ via
kinetic trapping, which, for normal rates of chloroform
evaporation, requires a combination of long PS blocks
and sufficiently low PEO content. Increasing the rate of solvent
evaporation (e.g., by elevating the subphase temperature) may
provide one avenue by which dewetting structures could be trapped
for a wider range of copolymers, thus providing some insight into
the generality of the mechanism.
Concluding Remarks The apparent role of dewetting in the formation
of PS-b-PEO
surface aggregates emphasizes fundamental differences between
diblock copolymer self-assembly in aqueous solutions (3D) and at
the air-water interface (2D). While 3D block copolymer micelle
morphologies represent, for the most part, equilibrium structures
with minimum free energy, the 2D features described here and
elsewhere appear to be frozen at various stages of a nonequilibrium
process starting with a dewetting stage which seeks to minimize
contact between an evaporating copolymer solution and the water
surface. In fact, it appears that, for block copolymers exhibiting
a variety of aggregate morphologies at the air-water interface, the
resemblance with 3D structures formed from self-assembly in
solution or in the bulk may be coincidental and not reflective of
analogous formation mech- anisms operating in two and three
dimensions. It appears to be problematic, for example, to
meaningfully compare the 2D spaghetti aggregates observed here with
3D cylindrical micelles formed from PS-b-PEO block copolymers in
aqueous medias the former structures arising from the
nonequilibrium convergence of rims in a dewetting film, and the
latter structures representing an optimization of curvature and
interfacial tension under equilibrium conditions. This insight into
the role of dewetting in block copolymer self-assembly at the
air-water interface should provide new understanding of surface
aggregate formation in various systems, along with potentially new
strategies of controlled patterning for specific
applications.
Acknowledgment. The authors gratefully acknowledge the National
Science and Engineering Research Council (NSERC), the Canadian
Foundation for Innovation (CFI), and the British Columbia Knowledge
Development Fund (BCKDF) for their generous support of the
research.
Supporting Information Available: Plots of hysteresis in the brush
(A0,b) and pancake (A0,p) regimes for multiple compression/
expansion cycles of the 185k copolymer at different spreading
concentrations. AFM images of the 185k copolymer at constant
spreading concentration (c ) 0.50 mg/mL) and various transfer
pressures (π ) 5.0, 10.0, and 20.0 mN/m). This material is
available free of charge via the Internet at
http://pubs.acs.org.
LA061953Z