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Some Current Research Projects of the Kramer Group
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| Fundamentals of Block Copolymer Directed Assembly |
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Our aim is to discover both new directed assembly methods
and their limitations for achieving good translational and orientational order
in 2D arrays of block copolymer domains, methods that could ultimately lead to
nanopatterns that could be transferred to underlying substrates. We investigate
directed self-assembly techniques whereby the edges of regions defined by optical
and electron beam lithography serve to register and template the order.
Emphasis is placed on understanding the ordering and disordering processes in melts
of both single layer and multilayer films of spherical domain block copolymers
and their blends with homopolymers in such regions. State-of-the art scanning
force microscopy (SFM or AFM)
and grazing incidence small angle X-ray scattering
(GISAXS)
at the Advanced Photon Source
Beamline 8_ID-E XOR are used in a
complementary fashion to precisely define the order in these layers.
Directed self-assembly is also of interest for cylindrical domain block copolymer
films with cylinders parallel to the film surface. Here controlling the equilibrium
concentration of dislocations and the thermal unbinding of their component
disclinations using nearby channel edges is the primary challenge.
These experiments
also exploit the depth profiling capabilities of dynamic secondary ion mass spectrometry
(SIMS)
,to provide depth information, transmission electron microscopy, low
voltage scanning electron microscopy and scanning force microscopy to provide
imaging of lateral and cross-sectional film phase structures. Our experiments
will be supplemented with field theoretic numerical simulations of
2D block copolymer ordering in collaboration with Glenn Fredrickson at UCSB. The results
from this project have the potential to impact a range of new
processing technologies, such as new lithographic techniques, new ways
to produce selective membranes and new ways to form thin film optical
or magnetic elements.
Highlights of recent research
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Highlights of recent research
2003
Highlights of recent research
2004
Highlights of recent research
2005
Highlights of recent research
2006
Highlights of recent research
2007
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| Soft Cellular Materials |
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| The UCSB MRL's Interdisciplinary Research Group project
in this area aims to use tailored nanoparticles and nanostructured polymers to stabilize
unique and potentially useful structures in multiphase polymer materials such as bi-continuous
block copolymer films, high internal phase polymer emulsions and polymer foams. Interfacially
active nanoparticles in block copolymers and homopolymer mixtures may be useful as a new class
of surfactants which impart functionality and can lead to new morphologies. We have
discovered that Au nanoparticles coated with the appropriate mixture of short A and B
chains reside at the interfaces between domains in an A-B diblock copolymer. Remarkably mixtures of
A and B ligands on such nanoparticles ranging from 90%A to 10% A all result in
mixed surface nanoparticles being attracted to A-B interfaces. These results suggest
that the A and B end-functional thiol ligands on these Au nanoparticles are mobile
on the nanoparticle surface and can form “Janus” particles in contact with the A-B
interface, particles that will bind much more strongly than particles coated randomly
with A and B ligands. We have developed an even more simple strategy for controlling
the location of gold nanoparticles within block copolymer domains through varying the
surface coverage of gold nanoparticles by end-attached polymer ligands. Gold nanoparticles
coated by short thiol end functional polystyrene homopolymers (PS-SH) (Mn=3.4 kg/mol) are
incorporated into a poly(styrene-b-2-vinylpyridine) diblock copolymer template (PS-b-P2VP)
(Mn=196 kg/mol), the P2VP block of which has a more favorable interaction with a bare gold
particle surface than does the PS block. The areal density of PS chains on the gold
particles is found to be critical to controlling their location in block copolymer templates,
below a critical value the gold particles go to the interface. The adsorption energy of
nanoparticle segregation to the interface can thus be tuned by changing the composition of
the polymer ligands on the nanoparticle and by decreasing the areal chain density of these ligands.
We are developing more controlled inorganic nanoparticle synthesis methods in order to
produce more nearly monodisperse size polymer-coated nanoparticles and with these methods in hand,
we will systematically explore the effects of nanoparticle size on A-B diblock copolymer
morphology where the nanoparticles segregate to the interface.
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Ambiguous Surfaces for Biofouling Resistant, High Fouling Release Coatings |
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Our main objective in this project, which is
carried out in collaboration with
Prof. Chris Ober
at Cornell, is to to develop new non-toxic, fouling resistant coatings using the principle
of controlled energy surfaces. The project builds on our current research and involves the
formation of fouling resistant coatings from specifically synthesized surface-active block
copolymers (SABC) that form tough coatings with surfaces of selected polarity and structure
used in combination with commercially available, styrene-ethylene/butylene-styrene (SEBS)
thermoplastic elastomers. As a result of recent successful studies we are
focussing on the preparation and testing of tailored and well characterized “ambiguous surfaces”
with either amphiphilic and/or mixed surface structure in which chemical function is selected
to hinder settlement of marine biofouling organisms and improve ease of removal of such organisms,
a property known as fouling release. The advantage of this approach is that we can create a
low cost coating with molecular level control of the fouling resistant (FR) surface to replace
coatings that depend on leaching toxic metals into the marine environment. Such coatings are likely to
be banned in the near future from partially enclosed waterways, such as harbors, where such metals can
build up to levels that threaten all aquatic life. The goal of this research is thus to produce FR coatings
and to understand the fundamental nature of the surface required for fouling release. The coatings
will be tested for their ability to provide specific surface properties. Surface analytical methods
will include X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS),
and scanning force microscopy (SFM) as well as contact angle studies. In addition, studies of
fouling release in conjunction with investigators such as M. Callow, D. Wendt and M. Hadfield will also
provide direct evaluation of the performance of these surfaces in marine fouling environments.
Such measurements are essential in providing feedback to allow us to discern the connections between the
molecular and higher level structures of our surfaces and their fouling resistance/release.
Highlights of recent research
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Block Copolymers and Networks with Thermally Reversible Hydrogen Bonding |
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| The aim of this project, which is a collaboration with Craig Hawker's and Glenn Fredrickson's groups,
with great help on the synthesis side from Bert Meijer's group at the Technical University of Eindhoven, NL,
is to gain an understanding of how thermally reversible
bonds affect the physical properties and assembly behavior of synthetic polymer materials.
The collaboration has developed the synthetic tools to make well-defined (i.e. low polydispersity) polymers
containing a multiple hydrogen bonding (MHB) functionality at one chain end and are extending these techniques
to telechelic polymers and random copolymers with MHB groups in the side chain.
Our focus is on two MHB dimers in particular—UPy-UPy (self complementary), and UPy-Napy (heterocomplementary)
to study the effects of directionality of the hydrogen bonding group.
Homopolymers functionalized with either UPy or Napy at one end of the chain are blended together in the bulk
to form “supramolecular diblock copolymers,” which are expected to behave like covalent diblocks
in the extreme of high binding strength (low temperature) and homopolymer blends in the extreme of low binding
strength (high temperature).
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Multiblock Polyolefin Copolymers with Semicrystalline and Rubbery Blocks |
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| Conventional thermoplastic elastomers are based on amorphous triblock copolymers
that have glassy end blocks and rubbery mid-blocks, e.g. SBS, SIS and SEBS. The elastomeric properties
of these depend on achieving a microphase-separated structure and the glassy end blocks must be sufficiently
long so that their Tg is well above room temperature and so that these blocks are sufficiently well anchored.
If the blocks are long however the order-disorder temperature will be high and the block copolymers may
be difficult to process at low enough temperatures such that degradation does not occur.
This is a particular issue with SEBS, a block copolymer with a saturated hydrocarbon mid-block and as a
result a rather large Flory-Huggins interaction parameter. Block copolymers with semi-crystalline blocks
can offer potentially attractive alternatives to SEBS and other amorphous-amorphous block copolymers if
the semicrystalline blocks can serve as sufficiently strong “anchors” for the rubbery midblocks allowing
large strains to be achieved at low stresses while allowing the strain to be nearly completely recovered
when the stress is removed. While thermoplastic elastomers from multiblock copolymers with semi-crystalline
blocks are well known (spandex fibers from segmented polyurethanes are important examples and their excellent
properties provide an “existence proof” of what is possible), these require rather expensive monomers.
Polyolefin-based semi-crystalline thermoplastic elastomers, especially those based on polypropylene,
offer attractive alternatives but until very recently were not able to be synthesized. With the development of
new single site catalysts, by the groups of our collaborators Geoff Coates, Cornell and Gui Bazan, UCSB,
that can carry out “living” polymerizations of ethylene and propylene and
produce stereo-regular blocks of the latter, this situation have changed. In this project we investigate the
relationships between the chain architecture, the microstucture and the mechanical properties of this interesting
new class of block copolymer materials.
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| Selected Recently Completed Projects |
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| Polymer Interfaces for Electronic
Packaging |
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Direct chip attach microelectronic assemblies
undergo hydro-thermal fatigue during field operation and
reliability testing. Delamination of the underfill/passivation
interface due to hydro-thermal fatigue is the cause for
majority of the failures during reliability testing of these assemblies and is
currently a major concern in such assemblies. Hence, it is essential to
characterize the hydro-thermal fatigue resistance of the interface and
to understand the mechanisms operating to cause the failure of this
interface under hydrothermal fatigue. Our goals in this project are
to develop methods to measure the fracture toughness, hydro-thermal
fatigue crack growth resistance, hydrolysis and moisture concentration
profiles at the interface between the underfill and the polyimide
passivation of DCA microelectronic assemblies. We plan to use
these methods to guide materials and process modification aimed at
strengthening these interfaces and minimizing water absorption and attack. A
major effort will be made to develop methods to characterize the
hydrolysis
depth profiles of the epoxy since our initial results suggest
that it is
very prone to network damage near the interface by water attack. Both
dynamic SIMS and RBS
will be
used as new analytical tools for this effort as well as
FTIR for
analyses of
bulk samples.
Highlights of recent research
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| Block Copolymer Diffusion
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Diffusion of block copolymers is interesting to us
for
several reasons: Potentially, diffusion of block copolymers is
exquisitively sensitive to the thermodynamic factors driving
the self-assembly of the microdomain structure. The slow kinetics of
segregation of block copolymers to interfaces between molten
homopolymers may be one of the important limitations for using such
preformed block copolymers as so-called compatibilizers for two phase
polymer mixtures. In turn, these kinetics almost surely depend on
either the diffusion on individual block copolymer chains, or block
copolymer micelles, in one or both of the homopolymers. The chain
topology of the block copolymer can be very important - we expect that
A-B-A triblock copolymer will diffuse very differently from A-B
diblock copolymers. Finally the time required to achieve the
equilibrium ordered structure of the block copolymer will depend on
the
individual block copolymer chains. Our experiments use
polystyrene-b-poly(2vinylpyridine) (PS-PVP) diblock copolymers and
poly(2vinylpyridine)-b-polystyrene-b-poly(2vinylpyridine) (PVP-PS-PVP)
triblock
copolymers that we synthesize using anionic polymerization
methods.
Even though this system is strongly ordered, PS and PVP have almost
the same
glass transition temperature and monomer friction coefficient. In
addition we
can synthesize the matching block copolymers (dPS-PVP and PVP-dPS-PVP)
with
deuterium replacing hydrogen on the PS block. By measuring, using
either
SIMS
or FRES
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the depth profile of deuterium , after diffusion of a thin
initial
layer placed on the surface of an underlying unlabeled polymer
(homopolymer,
block copolymer or a mixture of the two), we can determine the
diffusion
coefficient.
Prof. Fredrickson
has been a leader in developing both
theory and
simulation of block copolymer diffusion and we are actively
collaborating with
him in devising suitable tests for the theory.
Highlights of recent research
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Reactions of Polymer Chains at Polymer Interfaces |
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Reactions of polymer chains at interfaces between
two molten polymer phases are important as a way to form graft or
block copolymer interfacial compatibilizers in two phase polymer
blends. The compatibilizers act as interfacial strengthening agents,
as inhibiters of droplet coalescence and as polymeric surfactants.
Our experimental research seeks to uncover the links between polymer interface
thermodynamics, polymer reaction rates at these interfaces and the
interface instabilities that develop when the interfacial tension of
the flat interface is driven negative as more polymer compatibilizer
molecules are created. Issues of special interest are the dependence
of the rate of the grafting reaction on the length of the grafting
polymer chains, how the build-up of grafted chains affects the rate of
reaction leading to further formation of grafted chains and, for chains
with reactive end groups, how the chemical architecture of the end
affects its reactivity. To investigate these questions quantitatively
we carry out these reactions at initially flat interfaces
between two molten polymer thin films in which one of the components
is labelled with deuterium. The areal chain density of the
grafted chains at the interface is determined by depth profiling the
the deuterium in the films after the reaction has proceeded for a certain
time using either secondary ion mass spectrometry
(SIMS)
or forward recoil spectrometry
(FRES)
. Where the bottom film is a glassy polymer, we can wash away the
top film with a non-solvent for the bottom polymer and investigate the
morphology of the now-frozen interface using scanning force
microscopy. In this way we are able to observe the response of the
interface as the interfacial tension is driven to zero and even
(transiently) negative. We also are using our methods as part of a
collaborative project with
Prof. Leal,
Prof. Pine and
Prof. Fredrickson
that is aimed at understanding the fundamentals and rheology of drop
break-up and coalescence under conditions where compatibilizers have
been formed at the drop interface by reaction of chains with reactive
end groups.
Highlights of recent research
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Lateral Patterning of Domains in Block Copolymer Films |
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Here we are interested in producing patterns of
ordered block copolymer, not only in the thickness direction of polymer
films but also in the plane of the film. Such films would have uses in
nanometer scale lithography, for optical devices and as substrates for
biomedical manipulation of biomacromolecules and cells. To accomplish
these aims we are interested in developing substrates that can be
patterned so that a given area of a substrate attracts the
A-block of an A-B block copolymer, attracts the B-block of the
copolymer or is neutral, i.e. attracts without preference either
block. Microcontact printing of self-assembled monolayers
is one useful such method, using
either alkane thiols or alkylsilanes to form the monolayers, but
the thermal stability of such monolayers often leaves something to be
desired. Surface relief structures, such as mesas and wells, are also
useful in templating block copolymer order even if all surfaces
attract the same block.
Another goal is to develop a fundamental understanding of
the kinetic processes by which such block copolymer films order. To
achieve this we need to develop methods to determine the structure of
these films starting from when they are deposited by spin casting from
solution. A third objective is to understand the effect of confining
the block copolymer during the ordering step. Thin block copolymer
films often form "islands" and "holes" on the surface
layer if they are annealed
without confinement (i.e. with a free surface). These allow the block
copolymer to adopt its optimum chain conformation locally and still
present the "right" block to the surface or substrate. Confinement of
the film by a stiff layer can require the chains to stretch for
certain alignments of block copolymer domains and hence can favor
other alignments. To achieve these goals we use
forward recoil spectrometry
(FRES or
TOF-FRES)
or dynamic secondary ion mass spectrometry
(SIMS)
to determine the concentration of the various blocks through the
thickness of the film. The topology of film surfaces (e.g. the
island or hole structure) can be easily determined by
Nomarski interference optical
microscopy or by scanning force microscopy (SFM). Transmission
electron
microscopy (TEM) to give both plan and crossectional views of the film
can also provide very useful information, especially when the interface is
close to being neutral for the two blocks.
Over the past few years we have developed techniques
for removing block copolymer films from Au layers on silicon and
cross-sectioning them by ultramicrotomy. The combination of reactive
ion etching and low voltage scanning electron microscopy (LV-SEM) has also
proved useful in revealing the exact 3-D microstructure of block copolymer
films and provides an excellent complement to TEM imaging of these films.
All these techniques are available to us at
UCSB for structure determination and are used as appropriate. These
methods
of determining the ordered structure of block copolymer films also
provide needed information for our block copolymer diffusion studies
while the block copolymer diffusion information helps us in our
investigation of block copolymer ordering kinetics.
Highlights of recent research
More highlights of recent research
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Fracture of Semicrystalline Polymer Interfaces |
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Our major goal in this collaboration with the group
of
Prof. Frank Bates at the University of Minnesota
is to understand how crystals in semicrystalline polymers with a
rubbery amorphous phase act to anchor chains during plastic
deformation and fracture. The effects of block copolymers, one block
of which is semicrystalline (SC), on the
fracture of interfaces between a glassy homopolymer and the
semicrystalline one, can give important information about such
anchoring. For example, we expect there to be a critical SC block length
below which the SC block pulls out of its side of the interface. To
determine
this block length we will synthesize block copolymers in which the SC
block is labelled with deuterium and detect the pull-out after fracture
using dynamic secondary ion mass spectrometry
(SIMS)
and forward recoil spectrometry
(FRES)
to determine the deuterium content of both sides of the fracture
surface. These
experiments have the potential to impact our understanding
not only of the fracture of interfaces between glassy and
semicrystalline polymers,
but also the crack growth mechanisms in important semicrystalline
homopolymers such as polyethylene. Since polyethylene is being used more
and more for applications (e.g., pipe for natural gas distribution)
where resistance to slow crack growth is a major concern, such
mechanisms are important.
Highlights of recent research
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Tough Block Copolymers with Brittle Glassy Polymer Matrices |
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This is a collaboration with the theoretical group
of
Prof. Fredrickson
with the polymer synthesis effort of Steve Hahn at Dow Chemical Co.
aimed at understanding the fracture behavior of block copolymers
consisting of blocks of brittle, glassy polymer (majority component)
and blocks of ductile, semicrystalline polymer (minority component).
While the physical mechanisms that control fracture
and toughness of amorphous
homopolymer glasses are well established, little is known about the
relationships between practical toughness and chain architecture,
composition,
or mesoscale morphology in block copolymer materials. Working with a
set of model hydrogenated block copolymer materials, we are investigating these
relationships, using transmission electron microscopy, low
voltage scanning electron microscopy and scanning force microscopy to
provide
lateral and cross-sectional views of the plastically deformed
microphase separated structures in thin block copolymer films. Small
angle
X-ray scattering, from both samples under static strain and
under rapid loading are also planned as are theoretical
studies to support the experiments. These
experiments have the potential to throw light on the surprising
toughness of these materials.
Highlights of recent research
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Structure of the Low Energy Surfaces of Semifluorinated Polymers |
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Our main objective in this project, which is
carried out in collaboration with
Prof. Chris Ober
at Cornell, is to understand
the role of semi-fluorinated (SF) molecules attached to the
backbones of diblock copolymers in forming highly
organized, stable, low-energy surfaces. To accomplish this goal,
Chris Ober has developed novel
synthetic routes to attach either single SF group or multiple
SF groups ("monodendrons" to pendent double bonds of the
polymer blocks. A palette of
analytical techniques is used to characterize the structure and
properties of the surfaces of thin films made of such materials.
In particular, the surface
morphology is investigated using scanning force microscopy
(SFM). In addition, near-edge absorption fine structure (NEXAFS)
experiments designed to measure the orientation of the SF groups on the surface
are carried out on the Dow/NIST beamline U7A at the National
Synchrotron Light Source at Brookhaven National Lab.
To investigate the role of the SF groups on the stability of the thin
films, both NEXAFS and SFM experiments are under way on samples annealed
in-situ at elevated temperatures and samples exposed to water.
The orientational changes of the SF groups and the morphology
of the surfaces of thin films will be correlated with contact angle
measurement and the bulk behavior of these materials as measured
differential scanning calorimetry, transmission electron microscopy and X-ray diffraction.
Highlights of recent research
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Link to the Web Page of the UCSB SIMS Facility |
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