Some Research Projects of the Kramer Group

Controlling Phase Separation in Thin Polymer Films

Our major goal is to understand how to control surface directed spinodal decomposition processes in films of phase separating polymer mixtures to produce two phase structures with patterned morphologies so that the pattern extends in both the lateral and thickness directions. This control would allow us to produce films with three-dimensionally patterned polymer phase morphologies on micrometer to sub-micrometer length scales. The transfer of patterns from the confining interfaces into films of phase separating polymer mixtures can be realized by controlling the interaction between the polymer components and the surfaces of these patterns. The interfacial patterns are being produced principally using self assembled monolayers (SAMs) of alkane thiols on Au using a microcontact printing technique. These experiments exploit the complementary depth profiling capabilities of dynamic secondary ion mass spectrometry (SIMS) and forward recoil spectrometry (FRES) 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. These experiments 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 elements.
Highlights of recent research

Polymer Interfaces for Electronic Packaging

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

Block Copolymer Diffusion

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 , 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

Reactions of Polymer Chains at Polymer Interfaces

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

Lateral Patterning of Domains in Block Copolymer Films

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

Fracture of Semicrystalline Polymer Interfaces

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

Fracture of Polymer Interfaces with Gelatin

Our major goal in this collaboration with Dr. Anthony Dai of Eastman Kodak is to understand and control the fracture of interfaces between gelatin and other polymers such as polyethylene terephthlate (PET). We are developing methods to chemically modify the PET/gelatin interface and to assay quantitatively the areal density of such chemical groups using dynamic secondary ion mass spectrometry (SIMS) and Rutherford backscattering spectrometry (FRES) . We also develop new methods to measure the fracture energy Gc of such interfaces and correlate this fracture energy with the level and type of the chemical modification. Since retaining good fracture resistance under wet conditions is important, we are also collaborating with the group of Prof. Deming who is synthesizing high performance underwater adhesives consisting of random copolypeptides of DOPA and lysine. These synthetic analogs of marine mussel cement proteins may be very useful as thin adhesive layers between gelatin and polymer films. These experiments have the potential to impact the performance of new photographic films currently under development.
Highlights of recent research

Tough Block Copolymers with Brittle Glassy Polymer Matrices

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

Structure of the Low Energy Surfaces of Semifluorinated Polymers

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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