Yefim Brun

Characterization of synthetic copolymers by interaction polymer chromatography: Separation by microstructure

Gradient elution of synthetic polymers has been studied both theoretically and experimentally using normal and reversed-phase HPLC systems. An accurate equation describing the gradient elution of polymer-homologous series in the context of continuous random-flight model of a flexible polymer chain interacting with attractive surface of the porous material has been derived and experimentally verified against a series of narrow polystyrene standards. Both the theory and the experiment predict the existence of molar mass-independent gradient elution at critical point of adsorption (CPA). The extension of the theory to synthetic copolymers predicts the existence of the CPA for statistical copolymers and describes its dependence on chemical composition and microstructure (blockiness) of the polymer chains. One of the important theoretical conclusions is that blockiness always increases the retention, so that blockier polymer chains elute later than their more random counterparts with the same chemical composition. This prediction has been confirmed experimentally using block and statistical styrene-methylmethacrylate copolymers. Block copolymers do not have CPA and always elute between critical points of the corresponding homopolymers. The retention depends on the polymer molar mass and increases with the length of the blocks from a stronger absorbing monomer. These findings provide theoretical and experimental bases for separation of statistical and block copolymers by chemical composition and microstructure of polymer chains.

Critical conditions of polymer adsorption and chromatography on non-porous substrates

We present a novel thermodynamic theory and Monte Carlo simulation model for adsorption of macromolecules to solid surfaces that is applied for calculating the chain partition during separation on chromatographic columns packed with non-porous particles. We show that similarly to polymer separation on porous substrates, it is possible to attain three chromatographic modes: size exclusion chromatography at very weak or no adsorption, liquid adsorption chromatography when adsorption effects prevail, and liquid chromatography at critical conditions that occurs at the critical point of adsorption. The main attention is paid to the analysis of the critical conditions, at which the retention is chain length independent. The theoretical results are verified with specially designed experiments on isocratic separation of linear polystyrenes on a column packed with non-porous particles at various solvent compositions. Without invoking any adjustable parameters related to the column and particle geometry, we describe quantitatively the observed transition between the size exclusion and adsorption separation regimes upon the variation of solvent composition, with the intermediate mode occurring at a well-defined critical point of adsorption. A relationship is established between the experimental solvent composition and the effective adsorption potential used in model simulations.

High chi polymer development for DSA applications using RAFT technology

Directed self-assembly (DSA) of block copolymers is proving to be an interesting and innovative method to make three-dimensional periodic, uniform patterns useful in a variety of microelectronics applications. Attributes critical to acceptable DSA performance of block copolymers include molecular weight uniformity, final purity, and reproducibility in all the steps involved in producing the polymers. Reversible Addition Fragmentation Chain Transfer (RAFT) polymerization technology enables the production of such materials provided that careful process monitoring and compositional homogeneity measurement systems are employed. It is uniquely suited to construction of multiblocks with components of widely divergent surface energies and functionality. We describe a high chi diblock system comprising partially fluorinated methacrylates and substituted styrenics. While special new polymer separation strategies involving controlled polymer particle assembly in liquid media are required for some monomer systems and molecular weight regimes, we have been able to demonstrate high yield and compositionally homogeneous diblocks of lamellar and cylindrical morphology with polydispersities < 1.1. During purification processes, these diblock materials undergo assembly processes in liquid media, and with appropriate controls, this allows for removal of soluble homopolymer contaminants. SAXS analyses of solid polymer samples provide estimates of lamellar d-spacing, and a good correlation with molecular weight is shown. This system will be described.

Adhesion and Separation of Nanoparticles on Polymer-Grafted Porous Substrates

This work explores interactions of functionalized nanoparticles (NP) with polymer brushes (PB) in a binary mixture of good and poor solvents. NP-PB systems are used in multiple applications, and we are particularly interested in the problem of chromatographic separation of NPs on polymer-grafted porous columns. This process involves NP flow through the pore channels with walls covered by PBs. NP-PB adhesion is governed by adsorption of polymer chains to NP surface and entropic repulsion caused by the polymer chain confinement between NP and the channel wall. Both factors depend on the solvent composition, variation of which causes contraction or expansion of PB. Using dissipative particle dynamics simulations in conjunction with the ghost tweezers free energy calculation technique, we examine the free energy landscapes of functionalized NPs within PB-grafted channels depending on the solvent composition at different PB grafting densities and polymer-solvent affinities. The free energy landscape determines the probability of NP location at a given distance to the surface, positions of equilibrium adhesion states, and the Henry constant that characterizes adsorption equilibrium and NP partitioning between the stationary phase of PB and mobile phase of flowing solvent. We analyze NP transport through a polymer-grafted channel and calculate the mean velocity and retention time of NP depending on the NP size and solvent composition. We find that, with the increase of the bad (poor) solvent fraction and respective PB contraction, NP separation exhibits a transition from the hydrodynamic size exclusion regime with larger NPs having shorter retention time to the adsorption regime with smaller NPs having shorter retention time. The observed reversal of the sequence of elution is reminiscent of the critical condition in polymer chromatography at which the retention time is molecular weight independent. This finding suggests the possibility of the existence of an analogous special regime in nanoparticle chromatography at which NPs with like surface properties elute together regardless of their size. The latter has important practical implications: NPs can be separated by surface chemistry rather than by their size employing the gradient mode of elution with controlled variation of solvent composition.

UV aging performance of Blue Light encapsulant films

To realize the power entitlement of high efficiency selective emitter and lightly doped emitter cells (Blue Light cells), module makers need encapsulants to transmit more usable short wavelength radiation (blue light). To date, most encapsulant manufacturers have done this by changing the stabilizer package added to the encapsulant polymer. Accelerated UV aging of these encapsulant materials was performed and the stability of Blue Light EVA was compared to both standard EVA and Blue Light ionomer film. The results indicate that Blue Light EVA is less stable to UV than standard EVA or Blue Light ionomer. As long term module reliability is clearly as important as initial module efficiency, it is essential that careful consideration and testing be completed before a material change is made.