Jessica M. Torres

Elastic Modulus of Amorphous Polymer Thin Films: Relationship to the Glass Transition Temperature

Understanding the mechanical properties of polymers at the nanoscale is critical in numerous emerging applications. While it has been widely shown that the glass transition temperature (T(g)) in thin polymer films generally decreases due to confinement effects in the absence of strong favorable interactions between the polymer and substrate, there is little known about the modulus of sub-100 nm polymer films and features. Thus, one might use this depressed T(g) as a surrogate to estimate how the modulus of nanoconfined polymeric materials deviates from the bulk, based on constructs such as Williams-Landel-Ferry (WLF) time-temperature superposition principles. However, such relationships have not been thoroughly examined at the nanoscale where surface and interface effects can dramatically impact the physical properties of a material. Here, we measure the elastic modulus of a series of poly(methacrylate) films with widely varying bulk T(g)s as a function of thickness at ambient temperature, exploiting a wrinkling instability of a thin, stiff film on an thick, elastic substrate. A decrease in the modulus is found for all polymers in ultrathin films (<30 nm) with the onset of confinement effects shifting to larger film thicknesses as the quench depth (T(g,bulk) - T) decreases. We show that the decrease in modulus of ultrathin films is not correlated with the observed T(g) decrease in films of the same thickness.

Slip-stick wetting and large contact angle hysteresis on wrinkled surfaces.

Wetting on a corrugated surface that is formed via wrinkling of a hard skin layer formed by UV oxidation (UVO) of a poly(dimethylsiloxane) (PDMS) slab is studied using advancing and receding water contact angle measurements. The amplitude of the wrinkled pattern can be tuned through the pre-strain of the PDMS prior to surface oxidation. These valleys and peaks in the surface topography lead to anisotropic wetting by water droplets. As the droplet advances, the fluid is free to move along the direction parallel to the wrinkles, but the droplet moving orthogonal to the wrinkles encounters energy barriers due to the topography and slip-stick behavior is observed. As the wrinkle amplitude increases, anisotropy in the sessile droplet increases between parallel and perpendicular directions. For the drops receding perpendicular to the wrinkles formed at high strains, the contact angle tends to decrease steadily towards zero as the drop volume decreases, which can result in apparent hysteresis in the contact angle of over 100°. The wrinkled surfaces can exhibit high sessile and advancing contact angles (>115°), but the receding angle in these cases is generally vanishing as the drop is removed. This effect results in micrometer sized drops remaining in the grooves for these highly wrinkled surfaces, while the flat analogous UVO-treated PDMS shows complete removal of all macroscopic water drops under similar conditions. These wetting characteristics should be considered if these wrinkled surfaces are to be utilized in or as microfluidic devices.

Manipulation of the elastic modulus of polymers at the nanoscale: influence of UV-ozone cross-linking and plasticizer.

The mechanical stability of polymeric nanostructures is critical to the processing, assembly, and performance of numerous existing and emerging technologies. A key predictor of mechanical stability is the elastic modulus. However, a significant reduction in modulus has been reported for thin films and nanostructures when the thickness or size of the polymer material decreases below a critical length scale. Routes to mitigate or even eliminate this reduction in modulus, and thus enhancing the mechanically stability of polymeric nanostructures, would be extremely valuable. Here, two routes to modulate the mechanical properties of polymers at the nanoscale are described. Exposure to ultraviolet light and ozone (UVO) cross-links the near surface region of high molecular mass PS films, rendering the elastic modulus independent of thickness. However, UVO cannot eliminate the decrease in modulus of low molecular mass PS or PMMA due to limited reaction depth and photodegradation, respectively. Alternatively, the thickness dependence of the elastic modulus of both PS and PMMA can be eliminated by addition of dioctyl phthalate (DOP) at 5% by mass. Furthermore, an increase in modulus is observed for films with thicknesses less than 30 nm with 5% DOP by mass in comparison to neat PS. Although DOP acts as a plasticizer for both PS and PMMA in the bulk, evidence indicates that DOP acts as an antiplasticizer at the nanoscale. By maintaining or even increasing the elastic modulus of polymers at the nanoscale, these methods could lead to improved stability of polymeric nanostructures and devices.

Photoinitator surface segregation induced instabilities from polymerization of a liquid coating on a rigid substrate

Periodic wrinkled surfaces have generated significant interest over the past decade as these structures can be easily fabricated over large areas with minimal fabrication cost, but these structures have generally been limited to thin films on soft elastomeric substrates, which limits their general applicability and utility. Here we present a simple methodology to generate wrinkled surfaces on rigid substrates by surface segregation of a photocatalyst. Upon ultraviolet light (UV) induced photopolymerization, increased catalyst concentration yields a cross-linked layer at the free surface that is supported on top of a more liquid-like bulk film due to differences in polymerization rate. Further polymerization of the underlayer provides the requisite mechanical stress (contraction due to polymerization) to create a wrinkled pattern. A system based upon the renewable monomer, furfuryl alcohol, that is cross-linked with a photoacid generator, triphenyl sulfonium triflate, is utilized to illustrate this concept. Moreover, the polymerized furfuryl alcohol can be transformed into amorphous carbon by heating at elevated temperatures in an inert environment. The role of photoacid generator concentration and substrate temperature on the wrinkle formation and morphology is presented. Finally, exposure through a simple mask can generate hierarchical structures with the wrinkled structure conforming to the geometric constraints of the photopatterned area, including curvilinear structures. This photocatalyzed surface segregation-based methodology provides a promising route to the facile fabrication of microstructured surfaces based upon the wrinkling instability.

Thickness dependence of the elastic modulus of tris(8-hydroxyquinolinato)aluminium

The intrinsic flexibility of organic molecular films has been suggested to enable bendable electronics in comparison to their stiffer, inorganic counterparts. However, very little is known regarding the mechanical properties of these molecular glasses that are commonly utilized in organic electronics. To begin to address these issues, the elastic modulus of vapor deposited tris(8-hydroxyquinolinato)aluminium (Alq3) films, commonly used in organic light emitting devices (OLEDs), is determined as a function of thickness from 10 nm to 100 nm using a wrinkling-based metrology. These thicknesses correspond well with the range actually utilized in OLEDs. The direct deposition of Alq3 onto a polydimethylsiloxane (PDMS) elastomer results in anomalous results due to diffusion of Alq3 into the PDMS substrate. Conversely, a thin polystyrene (PS) film can be used as a diffusion barrier, enabling the elastic moduli to be accurately measured. Similar to most organic glasses, the Youngs modulus of Alq3 is on the order of 1 GPa and is statistically invariant for thicknesses >20 nm. Interestingly, there is a significant increase in the Youngs modulus for thinner films. The modulus of a 10 nm Alq3 film is found to be nearly twice that of a thicker film. Corresponding to this change in modulus is a loss of the optical adsorption peak associated with aggregation of Alq3, which suggests that the modulus change is related to the local packing of the molecule. This result illustrates that the thickness of active layers in OLEDs impacts not only the device performance but also their elastic properties, both of which are important for use in flexible devices.

Elastic constants and dimensions of imprinted polymeric nanolines determined from Brillouin light scattering

Elastic constants and cross-sectional dimensions of imprinted nanolines of poly(methyl methacrylate) (PMMA) on silicon substrates are determined nondestructively from finite-element inversion analysis of dispersion curves of hypersonic acoustic modes of these nanolines measured with Brillouin light scattering. The results for the cross-sectional dimensions, under the simplifying assumption of vertical sides and a semicircular top, are found to be consistent with dimensions determined from critical-dimension small-angle x-ray scattering measurements. The elastic constants C(11) and C(44) are found to be, respectively, 11.6% and 3.1% lower than their corresponding values for bulk PMMA. This result is consistent with the dimensional dependence of the quasi-static Youngs modulus determined from buckling measurements on PMMA films with lower molecular weights. This study provides the first evidence of size-dependent effects on hypersonic elastic properties of polymers.