Peter F. Green

Comparison of KWW and power law analyses of an ion-conducting glass

Abstract Measurements of the electrical impedance of LiPO3 glass at frequencies between 1 Hz and 106 Hz were analyzed using two popular procedures: (1) fitting the conductivity and permittivity to power laws in frequency, and (2) fitting the electrical modulus to a stretched exponential. The parameter, β, which expresses the non-exponential nature of the relaxation of electrical perturbations, is substantially different for these two procedures. The value obtained from the frequency dependence of the conductivity via the coupling model, βn, is similar to that seen for mechanical and nuclear magnetic resonance relaxations, suggesting that it may be more physically meaningful, whereas that from the electrical modulus, βm, is significantly larger. This dissimilarity is surprising since others often use these two βs interchangeably when testing theoretical predictions. This result will significantly after previous conclusions drawn from such tests.

Block copolymer thin films: pattern formation and phase behavior

Abstract During the last decade, research on thin, sub-micron thick, block copolymer films was devoted toward understanding and controlling microstructural and topographical features at temperatures T T ODT , where T ODT is the order–disorder transition temperature below which thermodynamic interactions favor the formation of ordered (phase separated) microstructures. Symmetric diblock copolymers, the subject of this review, undergo an isotropic to lamellar transition when T T ODT . Topographical features, ‘islands’ or ‘holes,’ of these films typically reflect the underlying phase separation; and the dimension of these features, normal to the substrate is equal to the interlamellar spacing, L . Two aspects of block copolymer thin films that have not received much attention are discussed in this paper: (1) pattern formation in symmetric block copolymers under conditions of T > T ODT ; and (2) phase behavior of thin film symmetric diblock copolymer/homopolymer mixtures, when T T ODT .

Equilibrium surface composition of diblock copolymers

The surface composition of a series of symmetric diblock copolymers of polystyrene (PS) and polymethylmethacrylate (PMMA) was studied using x‐ray photoelectron spectroscopy (XPS). Each copolymer was composed of N segments where N ranged from 270 to 5200. At large N, where the system is highly segregated, the surface is composed only of PS, the lower surface energy component. For smaller N, where the system is in the weak segregation limit, both the PS and PMMA segments coexist at the surface. The surface excess ψ1 of PS in the weak segregation limit is well described by ψ1=α−βN−1/2, in good agreement with mean field predictions. The constants α and β depend on the interactions between the unlike segments on the copolymer chain and on the surface energy differences between the components. We also show that the strength of the segmental interactions in the vicinity of the surface are weaker than from those in the bulk.

The glass transition of thin film polymer/polymer blends: Interfacial interactions and confinement

We examined the influence of film thickness and composition on the effective Tg of compatible thin film mixtures of polystyrene (PS) and tetramethylbisphenol-A polycarbonate (TMPC) on SiOx/Si substrates using spectroscopic ellipsometry. Our measurements reveal that while the Tg of TMPC films increased with decreasing film thickness, h, the effective Tg of thin film mixtures of PS and TMPC decreased with decreasing film thickness. In these mixtures, Tg was independent of film thickness at large h. We also found that while the Tg of bulk mixtures of TMPC/PS exhibited large negative deviations from additivity with composition, such deviations were negligible in the thin film mixtures. The thickness dependence of Tg is compared with theory.

Scaling Behavior in the Conductivity of Alkali Oxide Glasses, Polymers, and Doped Crystals

Abstract Although the frequency dependent conductivity, σ(ω), of ion-containing glasses displays power law dispersion (σ(ω) ≈ ω″) that can usually be described by a master curve, several findings have suggested that this scaling fails at low temperatures as indicated by a temperature dependence of the scaling exponent, n . This behavior is investigated in the frequency range between 1 Hz and 10 6 Hz for different materials including alkali metaphophate glasses and a polymer. Two distinct regimes of conductive behavior, σ I and σ II , are identified. The first, σ I , is strongly temperature dependent and appears to obey a master curve representation. The second, σ II , exhibits only a weak temperature dependence with a roughly linear frequency dependence. A strong depression of σ I occurs for the mixed alkali case, but σ II is unaffected and occurs at roughly the same location in all the alkali compositions studied. It is proposed that σ II does not arise from cation motion, but rather originates from a second mechanism likely involving small distortions of the underlying glassy matrix. This assignment of σ II is further supported by the approximately universal location of σ II , to within an order of magnitude, of a variety of materials, including a polymer electrolyte and doped crystal. Since σ I ( T ) and σ II ( T ≈ const.) are viewed as separate phenomena, the temperature dependence of the scaling exponent is shown to result merely from a superposition of these two contributions and does not indicate any intrinsic failure of the scaling property of σ I .

Structural correlations in the ac conductivity of ion-containing glasses

Abstract The power law dispersion of the conductivity in most ion-containing oxide glasses appears to be universal. This universality indicates that no correspondence exists between the degree of non-Debye behavior and the microscopic structure of a glass host. Despite its universal frequency dependence, we demonstrate how the ac conductivity defines a special length scale, L , that represents average diffusional displacement of an ion per activated jump and which can be correlated to properties of the glass host. We compare this length scale to the inter-ionic spacing and demonstrate how both increase with decreasing ion concentration. Although there is much evidence for universality, we suggest that the conductivity exponent may in fact be influenced by the ‘dimensionality’ of the conduction process.

Water-in-carbon dioxide microemulsions for removing post-etch residues from patterned porous low-k dielectrics

A hierarchy of CO2-based solvents is used to remove post-etch residues from vias and trenches in low-k patterned porous methylsilsesquioxane (pMSQ) interlayer dielectrics. Spectroscopic ellipsometry measurements indicate that, upon pressurization and depressurization with CO2, the thickness and refractive index return to values near the original values indicating that collapse or voiding of the pores did not occur. Post-etch residue could not be removed by pure water, CO2, with or without cosolvent, or dry CO2 with a surfactant. These results suggest that the particles could not be removed by a dissolution mechanism alone. The via and the bottom trench were cleaned with a solvent containing water, CO2, and a hydrocarbon surfactant. In this mild detergent system, particles may be removed both as a suspension and in the dissolved state in microemulsions and macroemulsions. The low interfacial tension prevents collapse of the pMSQ during water removal. This mild detergent system does not produce voids in the...

Pattern formation and evolution in diblock copolymer thin films above the order–disorder transition

Research on diblock copolymer thin films has been devoted primarily toward understanding and controlling microstructural and topographical features at temperatures below the order–disorder transition (ODT), where ordered phase separated structures exist. Recently, we showed that the topography of thin liquid diblock films above the ODT form a hierarchy of patterns, depending on the film thickness. One of these topographies is a “spinodal-like” pattern. Through the use of a pair correlation analysis we show that the structural evolution of this pattern is characterized by four stages. The first involves the amplification of surface perturbations, followed by the formation of an interconnected, “spinodal-like,” pattern. The onset of the third stage is associated with the breakup of the interconnected pattern and the eventual formation of droplets of irregular shapes. The final stage involves evolution toward the formation of circular droplets.

Brillouin scattering in alkali metaphosphate glasses and melts

Abstract We report the results of Brillouin scattering measurements performed on a series of alkali metaphosphate glasses and melts including the mixed alkali composition. Using independent viscosity data, we have applied a time–temperature superposition approach to obtain from the relevant features of the Brillouin spectra the relaxation characteristics of the complex longitudinal elastic modulus. A mixed alkali effect is evidenced in the Brillouin scattering which occurs in the gigahertz range. We find a relaxation process that can be described by a Kohlrausch function with β ≈0.30 for single alkali compositions, but is narrower ( β ≈0.40) at the mixed alkali composition.

Relaxations in mixed alkali metaphosphate glass

Abstract A study of the materials response to time-dependent mechanical and electrical perturbations was performed on a mixed alkali, sodium and lithium, metaphosphate glass over a range of overlapping frequencies. The characteristic timescale that characterizes the mechanical relaxations, τ μ , was observed to be two orders of magnitude slower than the characteristic response, τ σ , of the system to electrical perturbations. This is unlike the situation in single alkali glasses where τ μ and τ σ are equivalent. Interestingly, it was also noted that the breadth of the distribution of relaxation times that characterize the response of the mixed mobile ion system is slightly narrower than that of the single alkali (Li) metaphosphate. A comparison of the characteristic timescales, τ 1 , that describes the nuclear magnetic resonance spin-lattice relaxation time response revealed that τ 1 is comparable to τ σ . This is also by contrast with the single alkali case where τ σ is approximately one order of magnitude faster than τ 1 . However, as observed in single alkali glasses, the breadth of the distribution of relaxation times that characterize the loss processes of the mechanical response is considerably larger than that of the electrical response.