Peter J. Ludovice

Influence of molecular weight and film thickness on the glass transition temperature and coefficient of thermal expansion of supported ultrathin polymer films

Abstract The influence of film thickness and molecular weight on the glass transition temperature and coefficient of thermal expansion for polystyrene thin films supported on silicon native oxide surfaces has been studied in detail using variable angle spectroscopic ellipsometry. It was observed that Tg exhibits a dependence on both film thickness and polymer molecular weight. The Tg of polystyrene was found to decrease with decreasing film thickness below a critical film thickness of approximately 10 times the polymer radius of gyration. The coefficient of thermal expansion was also found to increase sharply as film thickness is decreased below approximately 10 times the polymer radius of gyration. It was found that the Tg of polystyrene thin films supported on silicon native oxide surfaces can be modeled using a ‘master’ curve based on reduced thickness (i.e. thickness relative to the polymer radius of gyration) and reduced glass transition temperature (i.e. Tg relative to the polymer bulk Tg). Using this concept, a single equation has been generated that describes the dependence of Tg on molecular weight and film thickness for the polystyrene on silicon native oxide system.

The bigger, the better: Ring-size effects of macrocyclic oligomeric Co(III)-salen catalysts

Macrocyclic oligomeric Co(III)-salen complexes derived from cyclooctene salen monomers are among the most active catalysts for asymmetric epoxide ring-opening reactions. Due to the uncontrollable feature of the ring-expanding olefin metathesis step during catalyst synthesis, the macrocyclic oligomeric Co(III)-salen complexes are produced as mixtures of oligomers with different ring sizes. We rationalize that the ring size of the Co(III)-salen oligomers might have a significant effect on the catalytic efficiency and selectivity and report here a purification protocol to isolate macrocyclic dimers, trimers, tetramers and oligomeric mixtures with larger size rings. Hydrolytic kinetic resolution (HKR) tests using allyl glycidyl ether as substrate show that the dimer is inactive at a catalyst loading of 0.01 mol%. Increasing ring size shows a remarkable effect on reaction rates with the largest ring-size species exhibiting superior selectivities and activities. NMR studies reveal that the dimeric catalyst is strained which is not observed for the larger ring-size catalysts. Computational modeling studies indicate that the dimer is lacking the flexibility to allow adjacent Co(III)-salen groups to form a bimetallic complex. Further catalytic tests of larger ring-size Co(III)-salen complexes (tetramer to hexamer mixture) by investigating the HKR of various racemic terminal epoxides and the asymmetric epoxide ring-opening with different nucleophiles demonstrate the superior catalytic activity of large ring-size macrocyclic catalysts. Furthermore, this study demonstrates again the structural (or configurational) sensitivity of Co(III)-salen catalyst towards the selectivity and efficiency of cooperative bimetallic reactions.

Biochemical enhancement of transdermal delivery with magainin peptide: Modification of electrostatic interactions by changing pH

Magainin is a naturally occurring, pore-forming peptide that has recently been shown to increase skin permeability. This study tested the hypothesis that electrostatic forces between magainin peptides and drugs mediate drug transport across the skin. Electrostatic interaction between positively charged magainin and a negatively charged model drug, fluorescein, was attractive at pH 7.4 and resulted in a 35-fold increase in delivery across human epidermis in vitro when formulated with 2% N-lauroylsarcosine in 50% ethanol. Increasing to pH 10 or 11 largely neutralized magainins charge, which eliminated enhancement due to magainin. Shielding electrostatic interactions with 1-2M NaCl solution similarly eliminated enhancement. Showing the opposite dependence on pH, electrostatic interaction between magainin and a positively charged anti-nausea drug, granisetron, was largely neutralized at pH 10 and resulted in a 92-fold increase in transdermal delivery. Decreasing to pH 5 increased magainins positive charge, which repelled granisetron and progressively decreased transdermal flux. Circular dichroism analysis, multi-photon microscopy, and FTIR spectroscopy showed no significant pH effect on magainin secondary structure, magainin deposition in stratum corneum, or stratum corneum lipid order, respectively. We conclude that magainin increases transdermal delivery by a mechanism involving electrostatic interaction between magainin peptides and drugs.

Characterization of L-isoleucine crystal morphology from molecular modeling

An accurate prediction of the L-isoleucine crystal morphology is demonstrated through the application of a molecular mechanics simulation using a suitable force field. The model prediction is validated against crystals grown by several experimental methods and in multiple environments. Semiempirical quantum chemistry techniques were required for determination of the electronic structure of the isoleucine molecules. Stable simulated crystal morphologies were obtained upon application of the point atomic charges from these techniques. Additionally, no explicit hydrogen bonding energy term was needed for energy minimization or morphology prediction of the amino acid crystal when using these atomic charges. The significant nonbonded energy within the crystal demanded morphology calculation procedures that considered these contributions to the crystal lattice energy. Attachment energy morphology calculations performed on the potential energy minimized model using the generic DREIDING2.21 force field and developed minimization protocol with the derived partial charges ultimately proved successful in simulating the macroscopic L-isoleucine crystal shape.

Microstructure of 2,3 erythro di-isotactic polynorbornene from atomistic simulation

A new RIS model was developed for erythro di-isotactic polynorbornene that included long-range steric interactions that are needed to properly model its conformation. These interactions were included by extracting RIS states from an approximate energy state map for the heptamer of this polymer. The RIS model predicted a novel helix-kink conformation for this polymer in which helices were occasionally disrupted by a backbone kink and a value of approximately 1.9 for the exponent “a” from the Mark‐Houwink‐Sakurada equation at the u condition. These RIS results were consistent with independent single chain Monte Carlo simulations and were also used to generate bulk periodic structures of this polymer. The results of these simulations compare well to both experimental viscometry and wide angle X-ray diffraction results for two polymer samples synthesized using Pd and zirconocene homogeneous polymerization catalysts. These results indicated that the likely stereochemical configuration for polymers produced using these catalysts is the erythro di-isotactic form. They also suggested that polymers of similar structure such as cis poly(t-butyl acetylene) may also adopt this kink-helix conformation. q 2000 Elsevier Science Ltd. All rights reserved.

Characterizing High-Energy-Density Propellants for Space Propulsion Applications

A technique for computationally determining the thermophysical properties of high-energy-density matter propellants is presented. High-energy-density matter compounds are of interest in the liquid rocket engine industry due to their high-density and high-energy content relative to existing industry-standard propellants. To accurately model rocket engine performance, cost, and weight in a conceptual design environment, several thermodynamic and physical properties are required over a range of temperatures and pressures. The approach presented here combines quantum mechanical and molecular dynamic calculations and group additivity methods. A method for improving the force field model coefficients used in the molecular dynamics simulations is included. This approach is used to determine thermophysical properties for two high-energy-density matter compounds of interest: quadricyclane and 2-azido-N, N-dimethylethanamine. The modified force field approach provides results that more accurately match experimental data than the unmodified approach. Launch vehicle and lunar lander case studies are presented to quantify the system-level impact of employing quadricyclane and 2-azido-N, N-dimethylethanamine rather than industry-standard propellants.In both cases, the use of high-energy-density matter propellants provides reductions in vehicle mass compared with industry-standard propellants. The results demonstrate that high-energy-density matter propellants can be an attractive technology for future launch vehicle and lunar lander applications.

Effects of block copolymer polydispersity and χN on pattern line edge roughness and line width roughness from directed self-assembly of diblock copolymers

This paper addresses two fundamental issues: (1) the connection between block copolymer polydispersity (as measured by a polydisperisty index (PDI)) and pattern LER/ LWR limits and (2) the connection between block copolymer χN value and pattern LER/LWR limits. In this work, we have used coarse grained molecular dynamics (MD) simulations of BCP DSA to study the effect of block copolymer PDI on DSA properties including LER/LWR and patterning capability. It is observed that as PDI increases from 1 to values of ~1.3, there is little effect on pattern LER/LWR, and as PDI increases above ~1.3 the LER/LWR increases slowly with increasing PDI. This suggests that LER/LWR concerns are not a major determinant in terms of specifying block copolymer PDI requirements for DSA processes. Concerning χN and LER/LWR, there is a sharp increase in roughness for χN<30. Because of the sharp increase at such low χN values, it is unlikely that BCP DSA processes for semiconductor manufacturing will be able to operate at low χN values even though microphase separation still occurs at these low χN values.

Detailed molecular dynamics studies of block copolymer directed self-assembly: Effect of guiding layer properties

Detailed molecular dynamics simulations have been performed to explore the effect of guiding layer properties and errors on resulting directed self-assembly pattern properties produced in block copolymer (BCP) thin films. Guiding patterns that are noncommensurate to the natural BCP pitch are considered, as are guiding lines that have correlated or anticorrelated line edge deviations. The process window is detailed for noncommensurate line widths. Guiding lines with various correlated and anticorrelated roughnesses show that under the high χ conditions used here, very significant guiding roughness is required to have any effect on the BCP film, and most of the guiding roughness is damped out within 5 nm of the bottom surface of the BCP film. Also, pitch subdivision patterns (where the BCP natural periodicity is some integer multiple smaller than the guiding pattern periodicity) damp out guiding line roughness more easily than pitch replicating patterns where a guiding pattern exists for each line formed in...

Coarse grained molecular dynamics model of block copolymer directed self-assembly

A model has been developed for the simulation of block copolymer (BCP) directed self-assembly (DSA) based on a coarse grained polymer model that anneals using molecular dynamics. The model uses graphics processing units (GPUs) to perform the calculations; this combined with the coarse graining means simulations times approach the speed of other more commonly used simulation techniques for BCPs. The model is unique in how it treats the pure phase blocks interactions with themselves (i.e. A-A and B-B interactions) and their interactions with each other. This allows for simulations that can potentially more accurately capture the differences between the properties of each block such as density and cohesive energy. The model is fully described and used to examine some of the issues that are unique to DSA lithographic applications of BCPs. We describe a method to calculate χ for the off-lattice MD system based on observation of the order-disorder transitions (ODT) for different degrees of polymerization N. The model is used to examine the transient, complex, non-classical morphologies that can occur through film thickness during a DSA process. During the phase separation process from a mixed initial state, the BCPs first locally phase separate to form small aggregate type structures. These aggregates then coalesce into larger features that approach the size of the equilibrium domain. These features then shift to match the guiding pattern on the underlayer followed by the slow elimination of defects. We also studied how the guiding patterns work in chemo-epitaxy DSA. The guiding patterns have a strong immediate effect on the BCP film nearest the interface and induce locally aligned self-assembly. Over time, this induced pattern tends to propagate up through the thickness of the film until the film is uniformly aligned to the guiding pattern. We also clearly see that the observed morphology at the top of the film gives no indication of the morphology through the depth, especially during the transient portions of the self-assembly process.

Magainin-Mediated Disruption of Stratum Corneum Lipid Vesicles

Drug delivery across the skin has had great success for drugs such as nicotine, estradiol, and a few others (1,2). However, the vast majority of drugs cannot cross skin at therapeutic rates, due primarily to the formidable barrier presented by skin’s outer layer, the stratum corneum. This barrier to transdermal transport is formed primarily by a series of multilamellar lipid bilayers found in stratum corneum’s extracellular spaces. Strategies to enhance drug delivery across the skin have focused to a large extent on chemical and physical methods to disrupt lipid structure (1,2). However, most enhancers have side effects, including irritation or safety concerns. The ideal enhancer for transdermal drug delivery would be one that is specifically targeted to disrupt stratum corneum lipids without damaging cells found deeper within the skin. The use of magainin peptides may present an opportunity to do this. Magainins are a family of peptides originally isolated from the skin of the African clawed frog, Xenopus laevis (3), which show a broad spectrum of antimicrobial activity. They exhibit potent antibacterial behavior at low concentrations (4,5) and belong to a class of antimicrobial peptides that interact directly with the lipid bilayer as opposed to specific membrane proteins (6,7). This antimicrobial activity appears to be derived from the peptides’ ability to increase the porosity of the membrane. Magainins specifically target bacteria because of the favorable interaction between the positivelycharged magainins and the typically negatively-charged bacterial membranes. Bacterial membranes generally contain large amounts of lipid with negatively-charged head groups such as phosphatidylserine and phosphatidylglycerol. In contrast, magainins are generally not as effective against most eukaryotic cells because of unfavorable interaction with the positive charges residing on their numerous zwitterionic lipid head groups such as phosphatidylethanolamine and phosphatidylcholine. Given this charge-dependent mechanism of magainins’ effect, we sought to determine if magainins could disrupt bilayers made of lipids found in human stratum corneum. Although analysis of stratum corneum lipids is complicated by spatial and inter-individual variation, they are reported to contain fewer zwitterionic phospholipids (∼5 wt. %) than typical eukaryotic cells, while containing ∼16 wt. % negativelycharged fatty acids (8). Given the significant negative charge and limited zwitterion content of stratum corneum, we propose the hypothesis that magainins can disrupt stratum corneum lipid bilayers.