Christopher L. Soles

A Discussion of the Molecular Mechanisms of Moisture Transport in Epoxy Resins

A typical epoxy formulation can absorb several weight percent of water, seriously degrading the physical properties of the resin. In two preceding publications (Soles, C. L.; Chang, F. T.; Bolan, B. A.; Hristov, H. A.; Gidley, D. W.; Yee, A. F. J Polym Sci Part B: Polym Phys 1998, 36, 3035; Soles, C. L.; Chang, F. T.; Gidley, D. W.; Yee, A. F. J Polym Sci Part B: Polym Phys 2000, 38, 776), the role of electron density heterogeneities, or nanovoids (as measured through positron annihilation lifetime spectroscopy), in the moisture-transport process is elucidated. In this article, the influence of these nanovoids is examined in light of both the specific epoxy-water interactions and the molecular motions of the glassy state to develop a plausible picture of the moisture-transport process in an amine-cured epoxy resin. In this description, the topology (nanopores), polarity, and molecular motions act in concert to control trans- port. Water traverses the epoxy through the network of nanopores, which are also coincident with the polar hydroxyls and amines. In this respect, the nanopores provide access to the polar interaction sites. Furthermore, the sub-Tg (glass-transition temper- ature) molecular motions coincident with the onset of the b-relaxation process incor- porate these polar sites and, hence, regulate the association of water with the epoxy. In effect, the kinetics of the transport mirror the dynamics associated with the local-scale motions of the b-relaxation process, and this appears to be the rate-limiting factor in transport. The volume fraction of the nanopores does not appear to be rate-limiting in the case of an amine-cured epoxy, contrary to popular theories of transport.

Contributions of the nanovoid structure to the moisture absorption properties of epoxy resins

Epoxy resins absorb significant quantities of moisture, typically 1 to 7% by weight for various formulations, which can greatly compromise their physical properties. It is known that polarity of the epoxy is a significant factor in determining the ultimate moisture uptake. However, the contribution from molecular topology still remains vague. In this work, the effects of molecular topology are elucidated by synthesizing novel epoxies where the polarity is maintained constant but the topology is systematically altered. The molecular topology is quantified in part via Positron Annihilation Lifetime Spectroscopy (PALS) in terms of the nanometer-sized voids, or nanovoids, that are also commensurate with typical interchain distances. The nanovoids are separated into their absolute zero and thermally fluctuating fractions by performing PALS measurements over a wide range of temperatures. A strong correlation is observed between the absolute zero hole volume fraction and the ultimate moisture uptake. Although the correlation is clear, the absolute zero hole volume fraction alone is not sufficient to predict the ultimate moisture uptake, and network polarity must also be considered. It is surmised that the role of the nanovoids is to open the epoxy matrix and alleviate steric hindrances that may prevent a water molecule from associating with a polar group.

Contributions of the nanovoid structure to the kinetics of moisture transport in epoxy resins

Absorbed moisture can degrade the physical properties of an epoxy resin, jeopardizing the performance of an epoxy-based component. Although specific water- epoxy interactions are known to be very important in determining transport behavior, the role of network topology is not clear. In this article, a series of epoxies in which the topology is systematically varied (and the polarity held constant) is used to explore how topology influences the kinetics of moisture transport. The topology is quantified via the positron annihilation lifetime spectroscopy technique in terms of the size and volume fraction of electron density heterogeneities 5- 6 A in diameter, a dimension comparable to the 3-A kinetic diameter of a water molecule. Surprisingly, the volume fraction of such nanopores does not affect the diffusion coefficient (D) of water in any of the resins studied. For temperatures at and below 35 °C, there is a mild exponential dependence of D on the average nanopore size observed. Otherwise, the kinetics of moisture transport do not appear to depend on the nanopores. However, the initial flux of moisture into the epoxy does appear to correlate with the intrinsic hole volume fraction. That this correlation persists only in the initial stages of absorption is partially understood in terms of the ability of the water to alter the nanopore structure; only in the initial stages of uptake are the nanopores, as quantified in the dry state, relevant to transport. The role of specific epoxy-water interactions are also discussed in terms of transport kinetics. The lack of a correlation between the topology and transport sug- gests that polar interactions, and not topology, provide the rate-limiting step of trans- port.

Correlations between Mechanical and Electrical Properties of Polythiophenes

The elastic moduli of polythiophenes, regioregular poly(3-hexylthiophene) (P3HT) and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT), are compared to their field effect mobility showing a proportional trend. The elastic moduli of the films are measured using a buckling-based metrology, and the mobility is determined from the electrical characteristics of bottom contact thin film transistors. Moreover, the crack onset strain of pBTTT films is shown to be less than 2.5%, whereas that of P3HT is greater than 150%. These results show that increased long-range order in polythiophene semiconductors, which is generally thought to be essential for improved charge mobility, can also stiffen and enbrittle the film. This work highlights the critical role of quantitative mechanical property measurements in guiding the development of flexible organic semiconductors.

Structural characterization of porous low-k thin films prepared by different techniques using x-ray porosimetry

Three different types of porous low-k dielectric films, with similar dielectric constants, are characterized using x-ray porosimetry (XRP). XRP is used to extract critical structural information, such as the average density, wall density, porosity, and pore size distribution. The materials include a plasma-enhanced-chemical-vapor-deposited carbon-doped oxide film composed of Si, C, O, and H (SiCOH) and two spin cast silsesquioxane type films—methylsilsesquioxane with a polymeric porogen (porous MSQ) and hydrogensilsesquioxane with a high boiling point solvent (porous HSQ). The porous SiCOH film displays the smallest pore sizes, while porous HSQ film has both the highest density wall material and porosity. The porous MSQ film exhibits a broad range of pores with the largest average pore size. We demonstrate that the average pore size obtained by the well-established method of neutron scattering and x-ray reflectivity is in good agreement with the XRP results.

Anisotropic, hierarchical surface patterns via surface wrinkling of nanopatterned polymer films.

By combining surface wrinkling and nanopatterned polymer films, we create anisotropic, hierarchical surfaces whose larger length-scale (wrinkling wavelength) depends intimately on the geometry and orientation of the smaller length-scale (nanopattern). We systematically vary the pattern pitch, pattern height, and residual layer thickness to ascertain the dependence of the wrinkling wavelength on the nanopattern geometry. We apply a composite mechanics model to gain a quantitative understanding of the relationship between the geometric parameters and the anisotropy in wrinkling wavelength. Additionally, these results shed light on the effect of surface roughness, as represented by the nanopattern, on the metrology of thin films via surface wrinkling.

Protein dynamics in viscous solvents

The mechanism of protein stabilization by glassy solvents is not entirely clear, and the stabilizer effective for a given protein is often discovered empirically. We use low frequency Raman spectroscopy as an effective tool to directly evaluate the ability of different solvents to suppress the conformational fluctuations that can lead to both protein activity and denaturation. We demonstrate that while trehalose provides superior suppression at high temperatures, glycerol is more effective at suppressing protein dynamics at low temperatures. These results suggest that viscosity of the solvent is not the only parameter important for biopreservation. It is also shown that glycerol and water enhance the high temperature conformational fluctuations relative to dry lysozyme, which explains the lower melting temperatures Tm in the hydrated protein and protein formulated in glycerol.

Effect of initial resist thickness on residual layer thickness of nanoimprinted structures

Quantification and control of the residual layer thickness is a critical challenge facing nanoimprint lithography. This thickness must be known to within a few nanometers, yet there are very few nondestructive measurement techniques capable of extracting such information. Here we describe a specular x-ray reflectivity technique that can be used to not only quantify the thickness of the residual layer with sub-nm resolution, but also to extract the pattern height, the line-to-space ratio, and relative linewidth variations as a function of the pattern height. This is illustrated through a series of imprints where the initial film thickness is varied. For films with sufficient resist material to fill the mold, complete pattern filling is observed and the residual layer thickness is directly proportional to the initial film thickness. When there is insufficient resist material in the film to completely fill the patterns in the mold, a finite residual layer thickness of approximately 50–100A is still observed.

Thermodynamic underpinnings of cell alignment on controlled topographies.

H /H o Surface topography is an important environmental cue for controlling cellular responses such as morphology, adhesion, alignment, migration, and gene expression. [ 1–7 ] Surface topographies with feature sizes covering the range of cell and cell components, i.e., from a few nanometers to tens of micrometers, have been broadly investigated with respect to effects on cell contact guidance (CG). [ 2 , 8 ] Despite the signifi cant work done to date, there has not been a satisfactory general explanation for the phenomenon, although many hypothesize that it is related to a biological response. In this paper, we fabricate a platform with precisely controlled surface topography, and use it to perform systematic cell studies that lead us to a new mechanistic understanding of CG under these conditions, which indicates that the response is rapid and largely physical rather than biological in nature. Below, we describe a two-step approach to fabricate submicrometer polymer gratings with continuous variations in grating height ( H ). First, large-area uniform gratings consisting of equally spaced lines were generated via nanoimprint lithography [ 9 , 10 ] on polystyrene (PS) and polymethylmethacrylate (PMMA). For each polymer, two sets of gratings were created with one-to-one line-to-space ratios, each with a pitch ( Λ ) of approximately 420 and 800 nm. Next, the uniformly patterned area was transformed to a continuous gradient in height by annealing on a thermal gradient stage for a fi xed time (see Supporting Information for details). A sketch of an annealed pattern with a height gradient is shown in the inset of Figure 1 . As indicated, the direction of the gradient is parallel to that of the polymer lines. Figure 1 shows position-dependent grating heights for two PS gratings ( Λ = 420 and 800 nm). The grating heights were characterized by atomic force microscopy (AFM) and are normalized in Figure 1 by the maximum height, H 0 , at x = 0.

Cubic Silsesquioxanes as a Green, High-Performance Mold Material for Nanoimprint Lithography

Optical lithography deep in the UV spectrum is the predominate route for high-resolution, high-volume nanoscale pattering. However, state-of-the-art optical lithography tools are exceedingly expensive and this places serious limitations on the applications, technical sectors, and markets where highresolution patterning can be implemented. To date the only substantial market for high-end optical lithography tools has been semiconductor fabrication. Nanoimprint lithography (NIL) has recently emerged as an alternative to optical lithography and combines the potential of sub-fi ve-nanometer patterning resolution with the low cost and simplicity of a stamping process. [ 1–4 ] This has led to signifi cant efforts to implement NIL methods, not only for semiconductor logic devices, but also in fi elds as diverse as the direct patterning of interlayer dielectrics (ILDs) for back-end-of-line (BEOL) interconnect structures, [ 5–7 ] bitpatterned magnetic media for data storage, [ 8 , 9 ] and high-brightness light-emitting diodes (LEDs). [ 10 ] Some of these are new areas where nanoscale patterning has previously not been considered, and are made possible here by the low cost and simplicity of the NIL stamping processes.