Peter A. Mirau

Chain conformations and dynamics of crystalline polymers as observed in their inclusion compounds by solid-state NMR

Abstract Certain small molecules, such as urea (U), perhydrotriphenylene (PHTP) and cyclodextrins (CDs), can be co-crystallized with polymers to form inclusion compounds (ICs). The guest polymer chains are confined to narrow, cylindrical channels created by the host, small-molecule lattice. The number and conformation of included polymer chains depend on the relative cross-sectional dimensions of polymer chains and the host channel diameter. For the hosts U, PHTP and α-CD ( D ≈5xa0A), only highly extended single chains can be squeezed inside the channel and are separated from neighboring polymer chains by the IC channel walls composed exclusively of the small-molecule lattice. However, for the host γ-CD ( D ≈8xa0A), two side-by-side, parallel extended polymer chains can be incorporated inside the channel, and thus, are also decoupled from all other neighboring chains by the channel walls. Therefore, the unique solid-state environment for polymers residing in IC channels can be utilized as model systems for ordered, bulk polymer phases. Comparison of the behavior of isolated, extended polymer chains in different host environments with the behavior observed for ordered, bulk phases of polymers permits an assessment of contributions made by the inherent, single chain, interactions between adjacent side-by-side pairs of chains and the overall co-operative, interchain interactions to the properties of ordered, bulk polymers. Solid-state NMR spectroscopy is an efficient technique to study the conformations and molecular motions of polymer ICs. This review paper mainly discusses the solid-state NMR study of the conformations and dynamics of a series of crystalline polymers observed in their ICs. In order to facilitate interpretation of the NMR observations, at the beginning of this review, we also discuss the related modeling results obtained by rotational isomeric state modeling and molecular dynamics simulations.

Characterization of nanoporous ultra low-k thin films templated by copolymers with different architectures

Abstract Triblock, diblock and random copolymers of poly(ethylene oxide) and poly(propylene oxide) are used as molecular templates in poly(methyl silsesquioxane) (MSQ) matrices to fabricate ultra low-k dielectric materials (k⩽2.0). Solid-state NMR shows that polymer architecture plays an important role in the polymer domain size and the polymer–matrix interface in the nanocomposites. Positronium annihilation lifetime spectroscopy reveals that porous MSQ film templated by triblock copolymers ( M n in the range of ∼5000–12,000xa0g/mol) have smallest pores and highest percolation threshold compared to those templated by diblock and random copolymers.

Molecular motions in the supramolecular complexes between poly(ε-caprolactone)-poly(ethylene oxide)-poly(ε-caprolactone) and α- and γ-cyclodextrins

The structure and molecular motions of the triblock copolymer PCL-PEO-PCL and its inclusion complexes with α- and γ-cyclodextrins (α- and γ-CDs) have been studied by solid-state NMR. Different cross-polarization dynamics have been observed for the guest polymer and host CDs. Guest-host magnetization exchange has been observed by proton spin lattice relaxation T 1 , proton spin lattice frame relaxation T 1ρ and 2D heteronuclear correlation experiments. A homogeneous phase has been observed for these complexes. Conventional relaxation experiments and 2D wide-line separation NMR with windowless isotropic mixing have been used to measure the chain dynamics. The results show that for localized molecular motion in the megahertz regime, the included PCL block chains are much more mobile than the crystalline PCL blocks in the bulk triblock copolymer. However, the mobility of the included PEO block chains is not very different from the amorphous PEO blocks of the bulk sample. The cooperative, long chain motions in the midkilohertz regime for pairs of PCL-PEO-PCL chains in their γ-CD channels seem more restricted than for the single PCL-PEO-PCL chains in the α-CD channels, however, they are not influencing the more localized, higher frequency megahertz motions.

Controlling the chain dynamics of polymers at interfaces

Abstract Solid state proton NMR with fast magic angle spinning has been used to study a low Tg acrylate polymer in bulk and polymerized in a vycor glass with 40 A pores. The combination of chain motion and fast magic angle sample spinning averages the proton–proton dipolar interactions such that high resolution spectra can be observed at ambient temperature. Multiple-quantum NMR shows that some of the chains are severely restricted relative to the bulk material by incorporation into the porous glass. These restricted chains can be returned to their bulk-like state by surface treatment of the porous glass.

Fast magic-angle spinning proton NMR studies of polymers at surfaces and interfaces

Solid-state proton NMR with fast magic-angle sample spinning has been used to study the structure and dynamics of polymers and the water interface in porous glass composites. The composites were prepared by photopolymerization of poly(ethyl acrylate) and other acrylate formulations in a high surface-area rigid glass matrix with 40-A interconnected pores. High resolution solid-state proton spectra were obtained for polymer films and composites with 15 kHz magic-angle sample spinning at temperatures above the polymer glass transition temperature. The solid-state proton spectra can be detected with high sensitivity and used to determine the composition of polymer and water filling the pores. These results and spin diffusion studies using 1H-29Si 2D heteronuclear correlation and wideline separation NMR show that the polymer fills the central 30 A of the pore, and that the remaining volume is filled with surface hydroxyl groups and water.

Solid-state multipulse proton Nuclear Magnetic Resonance (NMR) characterization of self-assembling polymer films.

Multipulse solid-state proton Nuclear Magnetic Resonance (NMR) has been used to study the domain structure in poly(styrene-b-isoprene-b-styrene) triblock copolymers in clear and self-assembled polymer films. Films containing ordered arrays of microcavities (3-5 microm) were obtained by casting the polymer from carbon disulfide solution in a moist environment, while clear films were obtained by solvent evaporation under nitrogen. The domain sizes for the polystyrene and polyisoprene blocks were measured by proton spin diffusion using the dipolar filter pulse sequence. The domain sizes for the dispersed phase and the long period were measured to be in the range of 3-6 nm, depending on the polymer molecular weight, and no differences domain size were observed for the clear and the self-assembled polymer films.

NMR CHARACTERIZATION OF POLYMERS

NMR spectroscopy is an important analytical method that is extensively used to study the structure and properties of macromolecules. NMR methods developed rapidly after these initial observations, both for polymers in solution and in the solid state. Solution NMR has emerged as one of the premier methods for polymer characterization because of its high resolution and sensitivity. It is observed in the early studies that the chemical shifts are sensitive to polymer microstructure, including polymer stereochemistry, regioisomerism, and the presence of branches and defects. This chapter gives an overview of the applications of NMR for the determination of polymer structure and dynamics. NMR is a complex method that is being used for a wide variety of applications ranging from microstructural characterization to structure determination in solid polymers. On one level, these NMR methods are relatively simple and can be performed on routine instruments. On another level, there is a very active research effort to develop new methods to probe the structure and dynamics of polymers at an ever more detailed level. The new research efforts are aided by developments in magnets, probes, spectrometers, and pulse sequences. This advance is driven in part by the desire to understand the molecular level structure and dynamics of polymers, and how these properties relate to the observed macroscopic properties.

Solid-state NMR characterization of resist formulations for 193-nm lithography: chain dynamics and length scale of mixing

Solid-state carbon NMR with cross polarization and magic-angle spinning has been used to study the chain dynamics and length scale of mixing in resist formulations of norbornene-maleic anhydride copolymers for 193 nm lithography. Two-dimensional wide line separation NMR has been used to measure the chain dynamics via the indirectly detected proton line shapes. The results show that the polymers do not experience large amplitude atomic fluctuations at the high temperatures (155 degrees Celsius) currently used for resist processing. Additional NMR experiments using proton spin diffusion demonstrate that the polymers and dissolution inhibitors are mixed on a molecular length scale.