Peter F. Green

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Name: Green, Peter

Organization: University of Michigan , USA

Department: Department of Materials Science and Engineering

Title: Professor(PhD)

TOPICS

Polymer

POSS Stimuli Responsive Polymers Stimuli Responsive Polymers Block Copolymer Temperature Responsive Polymers

Material Chemistry

Chemicals
References

Confinement Effects on Host Chain Dynamics in Polymer Nanocomposite Thin Films

Co-reporter:Kyle J. Johnson, Emmanouil Glynos, Serafeim-Dionysios Maroulas, Suresh Narayanan, Georgios Sakellariou, and Peter F. Green

Macromolecules September 26, 2017 Volume 50(Issue 18) pp:7241-7241

Publication Date(Web):September 7, 2017

DOI:10.1021/acs.macromol.7b01066

Incorporating nanoparticles (NPs) within a polymer host to create polymer nanocomposites (PNCs) while having the effect of increasing the functionality (e.g., sensing, energy conversion) of these materials influences other properties. One challenge is to understand the effects of nanoparticles on the viscosity of nanoscale thick polymer films. A new mechanism that contributes to an enhancement of the viscosity of nanoscale thick polymer/nanoparticle films is identified. We show that while the viscosities of neat homopolymer poly(2-vinylpyridine) (P2VP) films as thin as 50 nm remained the same as the bulk, polymer/nanoparticle films containing P2VP brush-coated gold NPs, spaced 50 nm apart, exhibited unprecedented increases in viscosities of over an order of magnitude. For thicker films or more widely separated NPs, the chain dynamics and viscosities were comparable to the bulk values. These results—NP proximities and suppression of their dynamics—suggest a new mechanism by which the viscosities of polymeric liquids could be controlled for nanoscale applications.

Component Dynamics in Polymer/Polymer Blends: Role of Spatial Compositional Heterogeneity

Co-reporter:Ravi P. Sharma and Peter F. Green

Macromolecules September 12, 2017 Volume 50(Issue 17) pp:6617-6617

Publication Date(Web):August 18, 2017

DOI:10.1021/acs.macromol.7b00092

The segmental relaxation times τA and τB of the A and B components, respectively, of an A/B polymer/polymer blend typically exhibit dissimilar temperature dependences and can differ by orders of magnitude, thereby manifesting the influence of spatial compositional heterogeneity. We show that for weakly miscible A/B blends the relaxations of the faster A component occur via separate and distinct mechanisms. In the melt state, τA increases in a nonlinear manner as temperature T decreases toward blend glass transition temperature Tg(blend) (or toward the local effective glass transition temperature of its component Tg(A)); this is the typical α relaxation process. For temperatures below the transition, 1/τA exhibits an Arrhenius temperature dependence; this is identified as the α′ process. A third relaxation process, a so-called α0 process, also occurs in the melt state; it is slower than the α process and exhibits a significantly stronger dependence on temperature. Each relaxation process, characterized by a different dependence on temperature, occurs via a different mechanism and associated local composition. This behavior, the existence of the α0 and α′ relaxations that accompany the α relaxations, would occur in miscible and weakly miscible blends whose component Tgs differ significantly.

Glassy Dynamics of Polymers with Star-Shaped Topologies: Roles of Molecular Functionality, Arm Length, and Film Thickness

Co-reporter:Bradley R. Frieberg, Emmanouil Glynos, Malvina Stathouraki, Georgios Sakellariou, and Peter F. Green

Macromolecules May 9, 2017 Volume 50(Issue 9) pp:3719-3719

Publication Date(Web):April 18, 2017

DOI:10.1021/acs.macromol.7b00091

Structural relaxations of a substance quenched to a temperature Tage below its glass transition temperature Tg enable the structure of the substance to approach equilibrium. This phenomenon, also known as physical aging, has been studied for many decades in bulk linear-chain polymer systems, where the aging rates are generally, to first order, independent of chain length. More recently, the phenomenon has been of keen interest in thin films, where the aging rate is shown to be film thickness H dependent, for films in the thickness range of nanometers to a few hundred nanometers. We show here, based on a study of polystyrene star-shaped polymers of a wide range of functionalities 2 ≤ f ≤ 64 and arm molecular weights Marm, that in the limit of sufficiently large values of Marm the aging behavior is similar to that of linear chains—independent of Marm and f. More importantly, in the limit of sufficiently small Marm and large f, the aging rate is independent of film thickness. Otherwise, the rate is a nonmonotonic function of f, for constant Marm and H. We present a general physical picture that rationalizes the H, f, and Marm dependencies of the aging rates of these star-shaped macromolecules—this behavior may be entirely understood in terms of Tage – Tg(x), where Tg(x) is the Tg at distance x from an interface and is a function of H, Marm, and f.

The Elastic Mechanical Response of Supported Thin Polymer Films

Co-reporter:Peter C. Chung, Emmanouil Glynos, and Peter F. Green

Langmuir December 23, 2014 Volume 30(Issue 50) pp:15200-15205

Publication Date(Web):December 23, 2014

DOI:10.1021/la503879v

Nanoindentation studies of the mechanical properties of sufficiently thin polymer films, supported by stiff substrates, indicate that the mechanical moduli are generally higher than those of the bulk. This enhancement of the effective modulus, in the thickness range of few hundred nanometers, is indicated to be associated with the propagation and impingement of the indentation tip induced stress field with the rigid underlying substrate; this is the so-called “substrate effect”. This behavior has been rationalized completely in terms of the moduli and Poisson’s ratios of the individual components, for the systems investigated thus far. Here we show that for thin supported polymer films, in general, information regarding the local chain stiffness and local vibrational constants of the polymers provides an appropriate rationalization of the overall mechanical response of polymers of differing chemical structures and polymer–substrate interactions. Our study should provide impetus for atomistic simulations that carefully account for the role of intermolecular interactions on the mechanical response of supported polymer thin films.

Band bending in conjugated polymer films: role of morphology and implications for bulk charge transport characteristics

Co-reporter:J. K. Wenderott;Ban Xuan Dong

Journal of Materials Chemistry C 2017 vol. 5(Issue 30) pp:7446-7451

Publication Date(Web):2017/08/03

DOI:10.1039/C7TC02302B

The performance of power conversion devices is impacted by the energy level alignment at the interface between the conjugated polymer and conductive substrate. While band bending has been known to vary between conjugated polymers, we show that the degree of band bending within the same polymer can be just as significant with morphology change. Specifically, a significant band bending effect, studied via Kelvin probe force microscopy (KPFM), was exhibited by poly(3-hexylthiophene) (P3HT) films fabricated using matrix assisted pulsed laser evaporation (MAPLE) in contrast to the conventional spin-cast P3HT films. This finding is associated with a broadening of the density of states (DOS) in the MAPLE-deposited P3HT films, originating from the more disordered structure of the film. These findings, to the best of our knowledge, illustrate for the first time a strong connection between morphology, energy level alignment, and bulk transport in conjugated polymer films.

Molecular organization in MAPLE-deposited conjugated polymer thin films and the implications for carrier transport characteristics

Co-reporter:

Journal of Polymer Science Part B: Polymer Physics 2017 Volume 55(Issue 1) pp:39-48

Publication Date(Web):2017/01/01

DOI:10.1002/polb.24237

ABSTRACTThe morphological structure of poly(3-hexylthiophene) (P3HT) thin films deposited by both Matrix Assisted Pulsed Laser Evaporation (MAPLE) and solution spin-casting methods are investigated. The MAPLE samples possessed a higher degree of disorder, with random orientations of polymer crystallites along the side-chain stacking, π–π stacking, and conjugated backbone directions. Moreover, the average molecular orientations and relative degrees of crystallinity of MAPLE-deposited polymer films are insensitive to the chemistries of the substrates onto which they were deposited; this is in stark contrast to the films prepared by the conventional spin-casting technique. Despite the seemingly unfavorable molecular orientations and the highly disordered morphologies, the in-plane charge carrier transport characteristics of the MAPLE samples are comparable to those of spin-cast samples, exhibiting similar transport activation energies (56 vs. 54 meV) to those reported in the literature for high mobility polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 39–48

Enhancing Carrier Mobilities in Organic Thin-Film Transistors Through Morphological Changes at the Semiconductor/Dielectric Interface Using Supercritical Carbon Dioxide Processing

Co-reporter:Ban Xuan Dong, Jojo A. Amonoo, Geoffrey E. Purdum, Yueh-Lin Loo, and Peter F. Green

ACS Applied Materials & Interfaces 2016 Volume 8(Issue 45) pp:31144

Publication Date(Web):October 17, 2016

DOI:10.1021/acsami.6b08248

Charge-carrier mobilities in poly(3-hexylthiophene) (P3HT) organic thin-film transistors (OTFTs) increase 5-fold when OTFTs composed of P3HT films on trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (FTS) monolayers supported on SiO2 dielectric substrates (P3HT/FTS/SiO2/Si) are subjected to supercritical carbon dioxide (scCO2) processing. In contrast, carrier mobilities in P3HT/octadecyltrichlorosilane (OTS)/SiO2 OTFTs processed using scCO2 are comparable to mobilities measured in as-cast P3HT/OTS/SiO2/Si devices. Topographical images of the free and buried interfaces of P3HT films reveal that scCO2 selectively alters the P3HT morphology near the buried P3HT/FTS-SiO2 interface; identical processing has negligible effects at the P3HT/OTS-SiO2 interface. A combination of spectroscopic ellipsometry and grazing-incidence X-ray diffraction experiments indicate insignificant change in the orientation distribution of the intermolecular π–π stacking direction of P3HT/FTS with scCO2 processing. The improved mobilities are instead correlated with enhanced in-plane orientation of the conjugated chain backbone of P3HT after scCO2 annealing. These findings suggest a strong dependence of polymer processing on the nature of polymer/substrate interface and the important role of backbone orientation toward dictating charge transport of OTFTs.Keywords: backbone orientation; conjugated polymer; ellipsometry; organic thin film transistor; scCO2 processing; self-assembled monolayers; structure-transport relationship

Elastic Mechanical Response of Thin Supported Star-Shaped Polymer Films

Co-reporter:Peter C. Chung, Emmanouil Glynos, Georgios Sakellariou, and Peter F. Green

ACS Macro Letters 2016 Volume 5(Issue 4) pp:439

Publication Date(Web):March 15, 2016

DOI:10.1021/acsmacrolett.5b00944

We show evidence of thickness-dependent elastic mechanical moduli that are associated largely with the effects of architecture (topology) and the overall shape of the macromolecule. Atomic force microscopy (AFM) based nanoindentation experiments were performed on linear chain polystyrene (LPS) and star-shaped polystyrene (SPS) macromolecules of varying functionalities (number of arms, f) and molecular weights per arm Mwarm. The out-of-plane elastic moduli E(h) increased with decreasing film thickness, h, for h less than a threshold film thickness, hth. For SPS with f ≤ 64 and Mwarm > 9 kg/mol, the dependencies of E(h) on h were virtually identical for the linear chains. Notably, however, for SPS with f = 64 and Mwarm = 9 kg/mol (SPS-9k-64), the hth was over 50% larger than that of the other polymers. These observations are rationalized in terms of the structure of the polymer for high f and sufficiently small Mwarm and not in terms of the influence of interfacial interactions.

Phase behavior of diblock copolymer/star-shaped polymer thin film mixtures

Co-reporter:Junnan Zhao, Georgios Sakellariou and Peter F. Green

Soft Matter 2016 vol. 12(Issue 17) pp:3849-3853

Publication Date(Web):06 Apr 2016

DOI:10.1039/C6SM00627B

We investigated the phase behavior of thin film, thickness h ≈ 100 nm, mixtures of a polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer with star-shaped polystyrene (SPS) molecules of varying functionalities f, where 4 ≤ f ≤ 64, and molecular weights per arm Marm. The miscibility of the system and the surface composition varied appreciably with Marm and f. For large values of Marm, regardless of f, the miscibility of the system was qualitatively similar to that of linear chain PS/PS-b-P2VP mixtures – the copolymer chains aggregate to form micelles, each composed of an inner P2VP core and PS corona, which preferentially segregate to the free surface. On the other hand, for large f and small Marm, SPS molecules preferentially resided at the free surface. Moreover, blends containing SPS molecules with the highest values of f and lowest values of Marm were phase separated. These observations are rationalized in terms of competing entropic interactions and the dependence of the surface tension of the star-shaped molecules on Marm and f.

Macroscopic alignment of poly(3-hexylthiophene) for enhanced long-range collection of photogenerated carriers

Co-reporter:Anton Li;David Bilby;Ban Xuan Dong;Jojo Amonoo;Jinsang Kim

Journal of Polymer Science Part B: Polymer Physics 2016 Volume 54( Issue 2) pp:180-188

Publication Date(Web):

DOI:10.1002/polb.23888

ABSTRACT

An epitaxy-directing solvent additive 1,3,5-trichlorobenzene is combined with an off-center spin-casting technique to produce poly(3-hexylthiophene) (P3HT) fibers with uniaxial in-plane alignment on the centimeter scale. Photoconductive atomic force microscopy (pc-AFM) is used to characterize planar heterojunction devices assembled from phenyl-C₆₁ butyric acid methyl ester (PCBM) acceptor and both aligned and unaligned P3HT donor. By varying the relative positions of the laser spot (site of carrier generation) and probe (site of hole extraction), it is found that devices with aligned P3HT exhibit anisotropic and greatly enhanced long-range photocarrier transport, with nearly 10% of original photocurrent measured 400 µm from the laser spot along the direction parallel to the alignment. Complementary thin film transistor (TFT) measurements reveal a factor of ∼3.5 difference in the hole mobilities parallel and perpendicular to the direction of alignment. Together, these findings highlight the importance of macroscopic alignment as a strategy to overcome the low mobilities of disordered polymer semiconductors.1 © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 180–188

Dynamics of Star-Shaped Polystyrene Molecules: From Arm Retraction to Cooperativity

Co-reporter:Kyle J. Johnson, Emmanouil Glynos, Georgios Sakellariou, and Peter Green

Macromolecules 2016 Volume 49(Issue 15) pp:5669-5676

Publication Date(Web):July 26, 2016

DOI:10.1021/acs.macromol.6b00456

The frequency ω dependent storage G′(ω) and loss G″(ω) moduli of star-shaped polystyrene (SPS) molecules of a range of functionalities f and molecular weights per arm Ma were measured under small amplitude oscillatory shear conditions. Star-shaped macromolecules are composed of an inner region, core, where the chain segments are stretched and the “packing” density is higher than that of the outer region, corona. The frequency dependencies of G′(ω) and G″(ω) for low functionality molecules (f < 8) with long arms Ma are well described by the model of Milner and McLeish, indicating that the translational dynamics are facilitated by an arm retraction mechanism. With increasing values of f and decreasing Ma the model fails—the arm retraction process is no longer valid—due largely to the increasing size of the core in relation to the overall size of the molecule. The molecules exhibit evidence of spatial structural order due to entropic, intermolecular interactions, and the translational dynamics of these molecules occur via a cooperative process, akin to that of soft colloids, for sufficiently large values of f and small Ma. The overall dynamics may be summarized in a diagram delineating different mechanisms that facilitate flow as a function of f and Ma.

An all-conjugated gradient copolymer approach for morphological control of polymer solar cells

Co-reporter:Jojo A. Amonoo, Anton Li, Geoffrey E. Purdum, Matthew E. Sykes, Bingyuan Huang, Edmund F. Palermo, Anne J. McNeil, Max Shtein, Yueh-Lin Loo and Peter F. Green

Journal of Materials Chemistry A 2015 vol. 3(Issue 40) pp:20174-20184

Publication Date(Web):07 Sep 2015

DOI:10.1039/C5TA04752H

This work introduces fully π-conjugated gradient copolymers as promising materials to control and stabilize the nanoscale morphology of polymer:fullerene solar cells. Gradient and block sequence copolymers of 3-hexylselenophene (3HS) and 3-hexylthiophene (3HT) are utilized as the donors (D) in bulk-heterojunction (BHJ) solar cells with phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor (A). We show that for the same overall copolymer composition, the ordering of molecular constituents along the copolymer chain (copolymer sequence) significantly influences the nanoscale morphology and phase separation behavior of π-conjugated copolymer:fullerene devices. In addition, energy-filtered transmission electron microscopy (EFTEM) of the blends revealed that relative to the block copolymer:PCBM, the gradient copolymer:PCBM sample formed a more uniform, continuous and interconnected network of polymer fibrils within the acceptor-rich phase, associated with a large D/A interface. Charge extraction of photogenerated carriers by linearly increasing voltage (photo-CELIV) shows that the gradient copolymer:PCBM device possesses the highest initial carrier density, n(0) = (3.92 ± 0.3) × 1018 cm−3, consistent with a larger D/A interfacial area suggested by the observed morphology, albeit at the expense of increased carrier recombination rate. Accelerated degradation studies show that the gradient copolymer:PCBM system maintains the highest efficiency over prolonged heat treatment.

Vitrification of Thin Polymer Films: From Linear Chain to Soft Colloid-like Behavior

Co-reporter:Emmanouil Glynos, Bradley Frieberg, Alexandros Chremos, Georgios Sakellariou, David W. Gidley, and Peter F. Green

Macromolecules 2015 Volume 48(Issue 7) pp:2305-2312

Publication Date(Web):March 26, 2015

DOI:10.1021/ma502556n

We show that the vitrification of star-shaped polystyrene (PS), of functionality f and molecular weight per arm Mwarm, thin films supported by silicon oxide, SiOx, is strongly dependent on Mwarm and f. When f is small, the vitrification behavior is similar to that of linear-chain PS where the average glass transition, Tg, decreases with decreasing film thickness (ΔTg < 0). However, for sufficiently large f and small Mwarm, Tg becomes independent of film thickness (ΔTg ≈ 0). In this region, where ΔTg ≈ 0, the star-shaped macromolecules self-assemble into ordered, periodic structures, similar to that of soft spheres or colloids, as revealed by simulations and experiments. This is identified as the soft-colloidal region. The transition from the linear-chain-like to the soft-colloidal-like region occurs over an intermediate range of functionalities and arm lengths; throughout this transition range ΔTg > 0. We show that the overall vitrification behavior of these thin film star-shaped polymers is due to competing entropic interactions associated with changes in f and Mwarm. The vitrification behavior of thin star-shaped PS films on SiOx is summarized in terms of a “diagram of states”.

The Elastic Mechanical Response of Nanoscale Thin Films of Miscible Polymer/Polymer Blends

Co-reporter:Peter C. Chung and Peter F. Green

Macromolecules 2015 Volume 48(Issue 12) pp:3991-3996

Publication Date(Web):June 12, 2015

DOI:10.1021/acs.macromol.5b00392

Nanoindentation measurements of the elastic moduli Er of thin polymer films supported by stiff substrates with moduli Es ≫ Er show an increase of Er with decreasing h, for h less than a threshold thickness ht. In the thickness range h < ht , the value of the modulus manifests the influence of various interactions associated with the “stiff” substrate. We show that ht is a function of composition for the miscible blend of polystyrene (PS) and tetramethyl bisphenol-A polycarbonate (TMPC). The modulus Er,TMPC of TMPC films supported by SiOx increases for h < ht(TMPC) ∼ 300 nm while ht(PS) ∼ 450 nm for the modulus Er,PS of PS films, supported by the same substrate. The threshold thicknesses of the blends and the moduli of the blends appear to be reasonably described by an effective medium approximation. This behavior is rationalized in terms of the vibrational force constants f, determined using incoherent neutron scattering experiments, of the materials.

Enhancing Photovoltaic Performance Using an All-Conjugated Random Copolymer to Tailor Bulk and Interfacial Morphology of the P3HT:ICBA Active Layer

Co-reporter:Anton Li;Jojo Amonoo;Bingyuan Huang;Peter K. Goldberg;Anne J. McNeil

Advanced Functional Materials 2014 Volume 24( Issue 35) pp:5594-5602

Publication Date(Web):

DOI:10.1002/adfm.201401058

Bulk heterojunction (BHJ) solar cells are fabricated using active material blends of poly(3-hexylthiophene) (P3HT) donor, indene-C₆₀ bisadduct (ICBA) acceptor, and an all-conjugated random copolymer (RCP) additive. By optimizing RCP loading, power conversion efficiencies (PCEs) up to 20% higher than those of a binary P3HT:ICBA mixture are achieved. The improved device characteristics are rationalized in terms of the differences between the photoactive thin film morphologies. Energy-filtered transmission electron microscopy reveals that incorporation of the RCP improves the degree of structural order of the BHJ fibrillar network and increases the extent of microphase separation between P3HT and ICBA. Additionally, a combination of atomic force microscopy and X-ray photoelectron spectroscopy analysis indicates segregation of the RCP at the free interface, leading to a shift in the surface potentials measured by Kelvin probe force microscopy. These changes, both in the bulk morphology and in the interfacial composition/energetics, are correlated to improved carrier collection efficiency due to a reduction of non-geminate recombination, which is measured by charge extraction of photogenerated carriers by linearly increasing voltage.

Wetting of Macromolecules: From Linear Chain to Soft Colloid-Like Behavior

Co-reporter:Emmanouil Glynos, Alexandros Chremos, Bradley Frieberg, Georgios Sakellariou, and Peter F. Green

Macromolecules 2014 Volume 47(Issue 3) pp:1137-1143

Publication Date(Web):January 31, 2014

DOI:10.1021/ma4024119

The ability to control the wetting properties of a polymeric liquid on a given surface is important for several emerging technological applications including protective coatings, lubricants and sensors. Here we show that star-shaped polystyrene (PS) molecules exhibit notably different wetting properties than their linear analogues of the same chemical structure and their wetting properties can be controlled through changes of their functionality f (number of arms per molecule). Unlike linear chains, the wetting of star-shaped macromolecules is determined by the competition between entropic forces. Wetting is enhanced due to reductions in the loss of entropy upon adsorption of the stars with increasing f; soft colloidal-like entropic repulsion effects suppress the ability of the stars to exhibit a high degree of surface adsorption and to efficiently pack, for sufficiently large values of f. This phenomenon is manifested in the existence of a minimum in the macroscopic contact angle and other related microscopic parameters embodied in the effective interface potential as a function of f.

Spatial Organization of Nanoparticles in Thin Film Block Copolymer/Homopolymer Hosts

Co-reporter:Junnan Zhao and Peter F. Green

Macromolecules 2014 Volume 47(Issue 13) pp:4337-4345

Publication Date(Web):June 19, 2014

DOI:10.1021/ma5007045

Polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymers (BCP) form spherical micelles, wherein each micelle is composed of an inner core of the P2VP block and an outer corona of the PS block of BCP, within thin film PS homopolymer hosts. The spatial distribution of PS-grafted gold nanoparticles (PS-Au NPs) within this thin film BCP/homopolymer system is characterized by a morphological diagram of the curvature of the Au cores 1/RC vs the degree of polymerization N of the grafted PS chains. Five basic morphological regimes, largely dictated by competing entropic and enthalpic intermolecular interactions, characterize the distribution of nanoparticles. The NP distributions include one case wherein the NPs reside primarily at the external interfaces (free surface and substrate) and another where the nanoparticles preferentially “decorate” the surfaces of the micelles. The existence of these different regimes is rationalized in terms of various competing short- and long-range intermolecular interactions.

The Elastic Mechanical Response of Supported Thin Polymer Films

Co-reporter:Peter C. Chung, Emmanouil Glynos, and Peter F. Green

Langmuir 2014 Volume 30(Issue 50) pp:15200-15205

Publication Date(Web):2017-2-22

DOI:10.1021/la503879v

Role of Domain Size and Phase Purity on Charge Carrier Density, Mobility, and Recombination in Poly(3-hexylthiophene):Phenyl-C61-butyric Acid Methyl Ester Devices

Co-reporter:Bingyuan Huang ; Jojo A. Amonoo ; Anton Li ; X. Chelsea Chen

The Journal of Physical Chemistry C 2014 Volume 118(Issue 8) pp:3968-3975

Publication Date(Web):January 31, 2014

DOI:10.1021/jp411525t

The power conversion efficiency (PCE) of a bulk heterojunction (BHJ) organic solar cell is influenced by the morphology (domain size and connectivity, phase purity, and interfacial structure) of the donor:acceptor blend active layer. The design of experiments to understand interrelationships between structure, transport properties, and device performance remains an important challenge. To this end, we created different types of morphologies in the poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC61BM) active layer by exploiting different processing strategies: conventional solvent casting, supercritical carbon dioxide (scCO2) processing, and thermal annealing. We investigated the device characteristics and transport behavior (carrier densities, mobilities and recombination) of samples possessing comparable domain sizes, which exhibited comparable initial carrier densities upon illumination. Notably, however, one morphology exhibited PCE, short circuit current (JSC), and carrier mobility that were each approximately a factor of 3 larger than the other morphologies. We also investigated another case where, in spite of significant differences between the domain dimensions, the PCE and JSC values were quite similar. These observations are rationalized on the basis of interrelations between aspects of the active material morphology, the transport properties, and the device efficiencies. This work provides insight into morphological design of active layers for optimum device performance.

Surface Dynamics of Miscible Polymer Blend Nanocomposites

Co-reporter:Bradley Frieberg, Jenny Kim, Suresh Narayanan, and Peter F. Green

ACS Nano 2014 Volume 8(Issue 1) pp:607

Publication Date(Web):December 23, 2013

DOI:10.1021/nn405233a

Diverse processes that include energy conversion, wettability, lubrication, adhesion, and surface-directed phase separation in mixtures fundamentally depend on the structure and dynamics of materials’ surfaces and interfaces. We report an unusual phenomenon wherein the surface viscosity of polymer nanocomposites of polystyrene (PS), polyvinyl methyl ether (PVME), and PS-coated gold nanoparticles (PS/PVME/PS-Au) is over an order of magnitude smaller than that of the neat miscible PS/PVME blend. Our X-ray photon correlation spectroscopy studies of the surface dynamics also reveal that the polymer chains manifest dynamics associated with two separate average compositional environments: a PVME-rich region, significantly in excess of its bulk concentration, and a separate PS-rich environment, where the dynamics are approximately 2 orders of magnitude slower. The unusually rapid surface dynamics in the PS/PVME/PS-Au nanocomposite are due largely to the excess PVME chains and the polymer/brush-coated nanoparticle interactions at the free surface.Keywords: blends; nanocomposites; surface dynamics; viscosity; XPCS

Electrorheology of Suspensions Containing Interfacially Active Constituents

Co-reporter:Carl McIntyre, Hengxi Yang, and Peter F. Green

ACS Applied Materials & Interfaces 2013 Volume 5(Issue 18) pp:8925

Publication Date(Web):August 26, 2013

DOI:10.1021/am4017544

We recently showed that a suspension of micrometer-sized polystyrene (PS) particles in a PDMS liquid, mixed with small (1 wt %) amounts of a nanocage, sulfonated polyhedral oligomeric silsesquioxane (s-POSS), exhibited significant electrorheological (ER) behavior. This behavior was associated with the formation of a thin adsorbed layer of s-POSS onto the surfaces of PS and the subsequent formation of polarization-induced aggregates, or structures, responsible for the ER effect in an applied electric, E, field. Current theory suggests that the ER effect would largely be determined by the dielectric and conductive properties of the conductive layer of core/shell particles in ER suspensions. We show here that sulfonated-PS (s-PS)/PDMS suspensions exhibit further increases in the yield stress of over 200%, with the addition of s-POSS. The yield stress of this system, moreover, scales as τy ∝ E2. The dielectric relaxation studies reveal the existence of a new relaxation peak in the s-POSS/s-PS/PDMS system that is absent in the s-POSS/PS/PDMS suspension. The relative sizes of these peaks are sensitive to the concentration of s-POSS and are associated with changes in the ER behavior. The properties of this class of ER fluids are not appropriately rationalized in terms of current theories.Keywords: colloids; core/shell particles; electrorheological fluid; self-assembly; stimuli-responsive materials;

Surface Layer Dynamics in Miscible Polymer Blends

Co-reporter:Bradley Frieberg, Jenny Kim, Suresh Narayanan, and Peter F. Green

ACS Macro Letters 2013 Volume 2(Issue 5) pp:388

Publication Date(Web):April 22, 2013

DOI:10.1021/mz400104p

In thin film A/B polymer/polymer mixtures, the formation of a layer at the free surface, with average composition that differs from the bulk, due to the preferential segregation of the lower cohesive energy density component, is well understood. While much is also understood about this surface layer formation and growth to date, virtually nothing is known about the surface dynamics of the chains in such mixtures. Questions about the surface chain dynamics in relation to the bulk have remained unanswered. With the use of X-ray photon correlation spectroscopy (XPCS) we show that the dynamics of poly(vinyl methyl ether) (PVME) chains at the free surface of polystyrene (PS)/PVME thin film mixtures can be orders of magnitude larger than the PVME chains in the bulk. These dynamics manifest from differences between the local compositions of the blend at the free surface and the bulk, as well as film thickness constraints.

Nanoparticle encapsulation in thin film micellar structures: a physical method for functional materials design

Co-reporter:Junnan Zhao, Xi Chelsea Chen and Peter F. Green

Soft Matter 2013 vol. 9(Issue 26) pp:6128-6134

Publication Date(Web):29 May 2013

DOI:10.1039/C3SM50175B

We investigate the confinement of gold nanoparticles, onto which poly(2-vinylpyridine) (P2VP) are end-tethered, within diblock copolymer polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) micelles in a polystyrene (PS) host. The micellar nanocomposites were prepared by spin casting mixtures of the nanoparticles, the copolymer and the homopolymer chains onto a substrate, to form films approximately 110 nm thick. The samples were then annealed above the glass transitions of the polymers, resulting in the formation of spherical micelles, composed of inner cores of the P2VP segments and outer coronas comprising the PS blocks, throughout the PS homopolymer thin film host. All nanoparticles were encapsulated within micelle cores; on average each micelle contains one, or no, nanoparticle. The micelles exhibited a strong tendency to self-organize at interfaces when the PS homopolymer chain length is large compared to the PS corona chain length; they otherwise remain distributed throughout the film. In comparison to pure PS-b-P2VP/PS blends, the nanoparticle/PS-b-P2VP/PS blends contain a higher density of, on average, smaller micelles. This sample fabrication procedure is straightforward and compliments the current “toolbox” used to create functional materials from block copolymer–nanoparticle systems.

Role of Spatial Compositional Heterogeneity on Component Dynamics in Miscible Bulk and Thin Film Polymer/Polymer Blends

Co-reporter:Hengxi Yang and Peter F. Green

Macromolecules 2013 Volume 46(Issue 23) pp:9390-9395

Publication Date(Web):November 22, 2013

DOI:10.1021/ma401813p

A study of the poly(vinyl methyl ether) (PVME) segmental dynamics of bulk miscible blends of polystyrene (PS) and PVME reveals that while at high temperatures, T, there is evidence of a single α-relaxation process, at lower T, two separate dominant relaxation processes, associated with the change in structure of the blend with decreasing T, emerge. One relaxation process decreases with a much stronger dependence on T and “freezes” at a temperature comparable to the glass transition temperature, Tg, of the blend measured using differential scanning calorimetry. The other exhibits a weaker T dependence and persists at much lower T, becoming Arrhenius (the so-called α′-process) at sufficiently low T. In thin PVME/PS films confined between aluminum substrates, a new relaxation process, αint, associated with PVME chains that preferentially segregate to the substrates, emerges. These observations are considered in light of the influence of spatial compositional heterogeneities on blend dynamics.

Segmental Dynamics of Chains Tethered at Interfaces of Varying Curvatures

Co-reporter:Hengxi Yang, X. Chelsea Chen, Ga Ram Jun, and Peter F. Green

Macromolecules 2013 Volume 46(Issue 12) pp:5036-5043

Publication Date(Web):June 11, 2013

DOI:10.1021/ma400098u

The segmental dynamics of a low glass transition temperature Tg polymer under different conditions of morphological confinement are shown to manifest the influence of the frustration of the local “packing” of chain segments at interfaces of varying curvatures, unique to the morphology. Of particular interest are the polyisoprene (PI) segmental dynamics in polystyrene-b-polyisoprene (PS-b-PI) micelles and in layered onion-like structures in high Tg, “frozen,” polystyrene (PS) hosts. At temperatures close to the Tg of PI domains, the segmental relaxation times of PI chains in the micellar structures, τmicelle, are more than 1 order of magnitude shorter than those of PI chains in the onion-like structures, τonion. These rates are appreciably faster than those of homopolymer PI and of PI in the neat copolymers possessing lamellar and gyroid phases, τhomo ∼ 0.5τlam ∼ 0.5τgyroid, respectively. At high temperatures τhomo ∼ τlam ∼ τgyroid ∼ τonion ∼ τmicelle. These observations are readily reconciled with trends in the local Tg of the PI phase in each sample.

Out-of-Plane Carrier Transport in Conjugated Polymer Thin Films: Role of Morphology

Co-reporter:Hengxi Yang ; Emmanouil Glynos ; Bingyuan Huang

The Journal of Physical Chemistry C 2013 Volume 117(Issue 19) pp:9590-9597

Publication Date(Web):April 16, 2013

DOI:10.1021/jp402254r

The influence of morphology on the dc conductivity σdc, the charge carrier density n and the out-of-plane mobilities μ were investigated in conjugated polymer films of poly(3-hexylthiophene) (P3HT) using impedance spectroscopy (IS). IS was used for the first time to discern this information from P3HT films. Values of μ, which were found to be film-thickness dependent, increasing with increasing film thickness, h, for films of thickness h > 700 nm, are in excellent agreement with those measured using time-of-flight (ToF) and the method of charge extraction by linearly increasing voltage (CELIV). Both σdc and n are shown to decrease appreciably with increasing h. The thickness dependent trends in μ, σdc and n are consistent with changes in the morphology of these films. Conductive atomic force microscopy (CAFM) provided corroborating information, showing an appreciable dependence of carrier transport on the morphology of P3HT.

Electrorheology of Polystyrene Filler/Polyhedral Silsesquioxane Suspensions

Co-reporter:Ernest C. McIntyre, Hengxi Yang, and Peter F. Green

ACS Applied Materials & Interfaces 2012 Volume 4(Issue 4) pp:2148

Publication Date(Web):March 19, 2012

DOI:10.1021/am300149u

An important challenge in the field of electrorheology is identifying low-viscosity fluids that would exhibit significant changes in viscosity, or a yield stress, upon the application of an external electric field. Our recent research showed that optimal compositions of mixtures, 10 wt % sulfonated polyhedral oligomeric silsesquioxanes (s-POSS) mixed with polydimethyl siloxane (PDMS), exhibited significant electrorheological activity. Here we show that s-POSS/PDMS mixtures containing polystyrene (PS) fillers, of micrometer-sized dimensions, containing as little as ∼1 wt % s-POSS, exhibited an increase in ER activity by an order of magnitude, beyond that of s-POSS/PDMS mixtures. The dynamic yield stress was found to scale with the particle diameter, a, as τy ∝ a0.5 and with the electric field as τy ∝ E1.5–2.5; this behavior is reasonably well understood within the context of dielectric electrorheological theory.Keywords: ; additives; electrorheological fluid; nanocage; polar molecule (PM) ER fluid; polystyrene; silsesquioxane;

Effect of Thickness-Dependent Microstructure on the Out-of-Plane Hole Mobility in Poly(3-Hexylthiophene) Films

Co-reporter:Bingyuan Huang, Emmanouil Glynos, Bradley Frieberg, Hengxi Yang, and Peter F. Green

ACS Applied Materials & Interfaces 2012 Volume 4(Issue 10) pp:5204

Publication Date(Web):September 6, 2012

DOI:10.1021/am3011252

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a widely used donor material for bulk heterojunction polymer solar cells. While much is known about the structure and properties of RR-P3HT films, important questions regarding hole mobilities in this material remain unresolved. Measurements of the out-of-plane hole mobilities, μ, of RR-P3HT films have been restricted to films in the thickness regime on the order of micrometers, beyond that generally used in solar cells, where the film thicknesses are typically 100 to 200 nm. Studies of in-plane carrier mobilities have been conducted in thinner films, in the thickness range 100–200 nm. However, the in-plane and out-of-plane hole mobilities in RR-P3HT can be significantly different. We show here that the out-of-plane hole mobilities in neat RR-P3HT films increase by an order of magnitude, from 10–4 cm2/V·s, for a 80 nm thick film, to a value of 10–3 cm2/V·s for films thicker than 700 nm. Through a combination of morphological characterization and simulations, we show that the thickness dependent mobilities are not only associated with the differences between the average morphologies of thick films and thin films, but specifically associated with changes in the local morphology of films as a function of distance from the interfaces.Keywords: conjugated polymers; disordered semiconductors; mobility; poly(3-hexylthiophene); polymer photovoltaic devices; thickness dependence;

Physical Aging of Star-Shaped Macromolecules

Co-reporter:Bradley Frieberg, Emmanouil Glynos, Georgios Sakellariou, and Peter F. Green

ACS Macro Letters 2012 Volume 1(Issue 5) pp:636

Publication Date(Web):April 24, 2012

DOI:10.1021/mz300129e

Time-dependent structural relaxations, physical aging, of films with thicknesses in the range 0.4 μm < H < 2 μm of star-shaped polystyrene (SPS) macromolecules were investigated. Our studies reveal that the aging rates of star-shaped PS macromolecules are appreciably slower than their linear chain analogs. The magnitude of the difference between the aging rates of the linear and star-shaped macromolecules increases with increasing functionality, f, and decreasing molecular weight per arm, Mnarm, of the stars. Our results are consistent with the notion that constraints imposed due to the architecture of the macromolecule suppress relaxations associated with and accommodate the reduction of the free volume of the system.

Tailoring the Refractive Indices of Thin Film Polymer Metallic Nanoparticle Nanocomposites

Co-reporter:Jenny Kim, Hengxi Yang, and Peter F. Green

Langmuir 2012 Volume 28(Issue 25) pp:9735-9741

Publication Date(Web):May 29, 2012

DOI:10.1021/la300374w

We demonstrate how to tailor the spatial distribution of gold nanoparticles (Au-NPs) of different sizes within polystyrene (PS) thin, supported, film hosts, thereby enabling the connection between the spatial distribution of Au-NPs within the polymer film and the optical properties to be determined. The real, n, and imaginary parts, k, of the complex refractive indices N = n(λ)+ik(λ) of the nanocomposite films were measured as a function of wavelength, λ, using multivariable angle spectroscopic ellipsometry. The surface plasmon response of films containing nearly homogeneous Au-NP distributions were well described by predictions based on classical Mie theory and the Drude model. The optical spectra of samples containing inhomogeneous nanoparticle distributions manifest features associated with differences in the size and interparticle spacings as well as the proximity and organization of nanoparticles at the substrate and free surface.

An Alternative Processing Strategy for Organic Photovoltaic Devices Using a Supercritical Fluid

Co-reporter:Jojo A. Amonoo ; Emmanouil Glynos ; X. Chelsea Chen

The Journal of Physical Chemistry C 2012 Volume 116(Issue 39) pp:20708-20716

Publication Date(Web):September 10, 2012

DOI:10.1021/jp304976x

We propose a sustainable low temperature alternative, using supercritical carbon dioxide (scCO2), to the conventional high temperature thermal annealing protocol for processing poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC61BM) organic photovoltaic devices. This new strategy enabled the fabrication of devices that exhibited comparable, and often better, short circuit currents, JSC’s, and efficiencies than those prepared using the conventional heat treatment protocol. While the fill factors (FF) of devices processed using either of the prorocols were comparable, the best performing scCO2 processed devices provided the largest JSC’s. With the use of energy-filtered transmission electron microscopy (EFTEM), and electron energy loss spectroscopy (EELS), we show that the supercritical solvent protocol enabled the formation of similar macro- and nanoscale morphology as the heat treatment protocol. The active materials in the devices with the best efficiencies possessed purer P3HT and PC61BM phases, as shown by EFTEM/EELS. UV–vis measurements, moreover, corroborated this observation, revealing a higher average degree of packing and order of P3HT chains throughout the films. Conductive- and photoconductive-atomic force microscopy (cAFM and pcAFM) revealed that the higher efficiency devices possessed larger fractions of photoactive regions throughout the active material. The variations in photoconductivity are associated with changes in the local composition throughout the active material.

Structure of thin film polymer/nanoparticle systems: polystyrene (PS) coated-Au nanoparticle/tetramethyl bisphenol-A polycarbonate mixtures (TMPC)

Co-reporter:X. Chelsea Chen and Peter F. Green

Soft Matter 2011 vol. 7(Issue 3) pp:1192-1198

Publication Date(Web):08 Dec 2010

DOI:10.1039/C0SM00684J

The morphologies of thin film blends of polystyrene (PS)-brush coated Au nanoparticles with tetramethyl bisphenol-A polycarbonate (TMPC) were investigated. Our results reveal that entropic effects, associated with the brush/host chain interactions, nanoparticle diameter, D, and asymmetries in monomer sizes of the host and grafted chains, can play a more important role than the favorable PS/TMPC enthalpic interactions toward determining the phase miscibility of the system. A diagram of states is constructed to show the phase separated and dispersed regimes as a function of D, N, the degree of polymerization of the grafted chains, and P, the degree of polymerization of the host chains, at a constant grafting density. These results have important implications on the design of brush coated nanoparticle/homopolymer mixtures for various applications.

Control of Morphology and Its Effects on the Optical Properties of Polymer Nanocomposites

Co-reporter:X. Chelsea Chen and Peter F. Green

Langmuir 2010 Volume 26(Issue 5) pp:3659-3665

Publication Date(Web):December 3, 2009

DOI:10.1021/la903108u

Chain-grafted Au nanoparticles were synthesized and incorporated into a fluorescent polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), host. We show that control of the Au nanoparticle distribution within MEH-PPV is achieved by manipulating the enthalpic and entropic interactions between the grafted brush layers and the host chains. Further, we show that the fluorescence of these Au/MEH-PPV nanocomposite thin films may be “tailored” by as much as an order of magnitude through changes in the nanoparticle distribution, brush length, and nanoparticle size. The ideas presented herein represent reliable strategies for materials design for devices.

Aggregation and Coarsening of Ligand-Stabilized Gold Nanoparticles in Poly(methyl methacrylate) Thin Films

Co-reporter:Luciana Meli and Peter F. Green

ACS Nano 2008 Volume 2(Issue 6) pp:1305

Publication Date(Web):June 5, 2008

DOI:10.1021/nn800045s

Dodecanethiol-stabilized gold nanoparticles (5 nm diameter) are shown to self-organize to form a two-dimensional hexagonal structure in poly(methyl methacrylate) films upon spin-casting from solution onto a substrate, using high-angle annular dark-field scanning transmission electron microscopy. Through use of the distribution functions describing particle distributions, we show that the particle coarsening dynamics is self-similar, characterized by two distinct growth stages. During the initial stage, coarsening occurs via simultaneous Ostwald ripening and coalescence mechanisms, whereas during the second stage, the dominant coarsening mechanism is coalescence.Keywords: coalescence; coarsening mechanisms; dodecanethiol desorption; nanoparticle; Ostwald ripening; polymer nanocomposite; thin film

Role of interfacial interactions on the anomalous swelling of polymer thin films in supercritical carbon dioxide

Co-reporter:Yuan Li;Eun J. Park;Kwon T. Lim;Keith P. Johnston

Journal of Polymer Science Part B: Polymer Physics 2007 Volume 45(Issue 11) pp:1313-1324

Publication Date(Web):18 APR 2007

DOI:10.1002/polb.21159

It has recently been shown that thin polymer films in the nanometer thickness range exhibit anomalous swelling maxima in supercritical CO₂ (Sc-Co₂) in the vicinity of the critical point of CO₂. The adsorption isotherm of CO₂ on carbon black, silica surfaces, porous zeolites, and other surfaces, is known to exhibit anomalous maxima under similar CO₂ conditions. It is believed that because CO₂ possesses a low cohesive energy density, there would be an excess amount of CO₂ at the surfaces of these materials and hence the CO₂/polymer interface. This might cause excess CO₂ in the polymer films near the free surface, and hence the swelling anomaly. In addition, an excess of CO₂ would reside at the polymer/substrate and polymer/CO₂ interfaces for entropic reasons. These interfacial effects, as have been suggested, should account for an overall excess of CO₂ in a thin polymer film compared to the bulk, and would be responsible for the anomalous swelling. In this study, we use in situ spectroscopic ellipsometry to investigate the role of interfaces on the anomalous swelling of polymer thin films of varying initial thicknesses, h₀, exposed to Sc-CO₂. We examined three homopolymers, poly(1,1′-dihydroperflurooctyl methacrylate) (PFOMA), polystyrene (PS), poly(ethylene oxide) (PEO), that exhibit very different interactions with Sc-CO₂, and the diblock copolymer of PS-b-PFOMA. We show that the anomalous swelling cannot be solely explained by the excess adsorption of CO₂ at interfaces. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1313–1324, 2007

An all-conjugated gradient copolymer approach for morphological control of polymer solar cells

Co-reporter:Jojo A. Amonoo, Anton Li, Geoffrey E. Purdum, Matthew E. Sykes, Bingyuan Huang, Edmund F. Palermo, Anne J. McNeil, Max Shtein, Yueh-Lin Loo and Peter F. Green

Journal of Materials Chemistry A 2015 - vol. 3(Issue 40) pp:NaN20184-20184

Publication Date(Web):2015/09/07

DOI:10.1039/C5TA04752H