Transport and Stability of Laser-Deposited Amorphous Polymer Nanoglobules

We characterized the transport, i.e., time-of-flight, and nanoscale thermal properties of amorphous polymer nanoglobules fabricated via a laser-deposition technique, Matrix-Assisted Pulsed Laser Deposition (MAPLE). Here, we report the first experimental measurement of the velocity of polymer during MAPLE processing and its connection to nanostructured film formation. A nanoscale dilatometry technique using atomic force microscopy was employed to directly measure the thermal properties of MAPLE-deposited polymer nanoglobules. Similarly to bulk stable polymer glasses deposited by MAPLE, polymer nanoglobules were found to exhibit enhanced thermal stability and low density despite containing only thousands of molecules. By directly connecting the exceptional properties of the nanostructured building blocks to those of bulk stable glasses, we gain insight into the physics of glassy polymeric materials formed via vapor-assisted techniques. C glasses are liquids that have lost their ability to flow. For millennia, the primary route to form vitreous materials has been rapid cooling from the liquid state. If the liquid is cooled at high rates, crystal nucleation can be avoided, and the liquid becomes supercooled. Upon further cooling, molecular motions become progressively slower, and eventually, equilibrium is unattainable on the experimental time scale. The temperature at which the liquid falls out of equilibrium into the nonequilibrium glassy state on cooling is termed the glass transition temperature (Tg). A result of the kinetic nature of glass formation is that Tg, density, and other key material properties can be tuned by varying the rate of cooling from the liquid. However, the use of supercooling to access glasses with dramatically different properties is limited. Varying the rate of cooling by one order-of-magnitude during quenching from the liquid state will result in only a ∼ 3−5 K change in the value of Tg and a minute change in density. 3 In an effort to overcome the kinetic limitations of glass formation and to facilitate control over the properties of disordered solid materials, the past few years have seen intense effort in the use of unconventional processing methodologies to form glasses. In a seminal study, Ediger and co-workers demonstrated that organic molecular glasses formed via slow physical vapor deposition (PVD) exhibited exceptional kinetic and thermal stability. Termed “stable” glasses, these materials were characterized by a ∼15 K increase in the onset temperature of the glass transition (Tonset) during heating, 2% greater density, and ∼8 J/g lower enthalpy compared to an analogous glass formed via supercooling. By all accounts, stable glass formation is a general phenomenon for organic small molecules suitable for PVD. The diversity of materials that have been transformed into stable glasses via PVD includes indomethacin, trisnaphthylbenzene and isomers, decalin, and toluene. Recent work has focused on the challenge of expanding the range of materials that can be transformed into stable glasses, with particular focus on those that are not amenable to PVD processing. These include both metallic and polymeric glasses. Here, we focus on stable polymer glasses formed via Matrix-Assisted Pulsed Laser Deposition (MAPLE), a versatile vapor-assisted deposition technique by which films of polymers, proteins, small molecules, and nanoparticles can be fabricated. In the MAPLE process, thin films of macromolecules can be grown at slow rates, i.e., less than 1 nm/s. Stable polymer glasses formed via MAPLE exhibit significantly enhanced Tonset and improved high-temperature kinetic stability relative to glasses formed via supercooling. Both these characteristics are in accord with organic small molecule stable glasses. In contrast with small molecule stable glasses, stable polymer glasses uniquely have ∼30 J/g higher enthalpy and dramatically reduced density (40%). This unusual combination of material properties is only realized when MAPLEdeposited polymer glasses exhibit a novel nanostructured morphology, as illustrated in the Figure 1b inset. In our prior work, we explored the origins of nanostructured film formation via MAPLE. As confirmed by Atomic Force Microscope (AFM) topology images and Scanning Electron Microscope (SEM) cross-sectional images, stable polymer films prepared by MAPLE are formed by the assembly of intact Received: September 2, 2014 Accepted: September 30, 2014 Published: October 2, 2014 Letter pubs.acs.org/macroletters © 2014 American Chemical Society 1046 dx.doi.org/10.1021/mz500546u | ACS Macro Lett. 2014, 3, 1046−1050 nanoglobules consisting of 10 to 10 polymer molecules. However, the conditions under which polymer nanoglobules may impact the substrate without damage or disintegration remain an open question. The investigation of surface nanoglobule coalescence in bulk films as a function of deposition temperature demonstrated that nanoscale features persist well above the material’s standard Tg, indicating that enhanced stability is present at the nanoscale. However, because these experiments examine structure at the surface of a bulk film, this measurement of nanoscale stability is indirect. To conclusively attribute the measured macroscopically enhanced stability to the material’s nanoscale morphology, a direct measurement of the thermal properties of individual nanoglobules is warranted. Such study is necessary to obtain a better understanding of the relationship between material properties at the nanoand macroscales of MAPLE-deposited glassy films. In this letter, we address two major questions related to polymer stable glass formation. First, to understand how nanoglobules remain intact after transport and deposition onto the substrate (or growing film), we report for the first time, the measurement of the time-of-flight and average velocity of material during MAPLE processing. Second, we directly investigate the nanoscale thermal stability of isolated nanoglobules via AFM. Combined, these two novel characterizations will allow us to relate the bulk and nanoscale material properties to the physics of the laser-deposition process. Because the combination of bulk properties observed in our material is unique (enhanced stability, low density, and high enthalpy), a better understanding of the nanoscale properties of these glasses provides insight into the physics of amorphous materials prepared via vapor-assisted deposition. As opposed to direct polymer ablation, MAPLE provides a gentler mechanism for the deposition of polymer films with thicknesses ranging from a few nanometers to microns. In the technique, a pulsed laser ablates a target consisting of a frozen dilute solution of the desired polymer producing thin films of the material. The mechanism of film growth has recently been refined by insight obtained via simulations performed by Zhigilei and co-workers. They provided evidence that polymer molecules are ejected from the target within polymer− solvent clusters. These clusters form due to explosive decomposition of the solvent molecules when a short laser pulse superheats the target beyond the ablation threshold. During transport from target to substrate, solvent is rapidly removed from the clusters, forming polymer nanoglobules. Simulations revealed that depending on a nanoglobule’s velocity upon impact with the substrate it may remain intact or disintegrate into polymer fragments. It was illustrated that polymer nanoglobules with velocities less than 100 m/s do not lose structural integrity upon impact with the substrate. Yet, the velocity of ejected nanoglobules during the MAPLE process has not been experimentally measured to confirm how nanoglobules can deposit intact without coalescence. Figure 1a shows a schematic of the MAPLE setup equipped with a custom-built time-of-flight (TOF) system based on a quartz crystal microbalance (QCM). Isolated nanoglobules of poly(methyl methacrylate) (PMMA) were deposited from a chloroform target solution, and TOF was measured by tracking the deviation of the quartz crystal frequency in real time as the nanoglobules arrived at the crystal surface. A detailed experimental protocol is provided in the Supporting Information (SI). Figure 1b shows the normalized QCM oscillation frequency as a function of time after laser ablation of the target. Deviation from the initial steady-state frequency began at ∼10 ms, thus indicating the arrival of the first measurable portions of the plume to the sensor. The distance between the sensor and target was set to ∼5 cm, yielding an average velocity of ∼5 m/s for depositing material. This value is significantly lower than the threshold velocity (100 m/s) required for structural integrity of the depositing nanoglobules upon impact with the substrate. Because this velocity measurement is conducted at the process conditions under which bulk stable glasses are formed by MAPLE, we demonstrate that the formation of bulk stable glasses is made possible by the low impact velocity of depositing nanoglobules. In comparison with velocity measurements made on other laser-material transfer techniques, such as MALDI, Pulsed Laser Deposition, and Laser-Induced Forward Transfer (LIFT), our measured velocity is low. Direct laser ablation of PMMA results in a velocity of 200 m/s, with a fluence (energy per area) 4× higher than that used in this study. Studies of LIFT of live cells and hydrogels reported velocities of 122 and 66 m/s, respectively. The low velocity reported for MAPLE-deposited material may be attributed to a combination of many factors. Neutral species travel more slowly than ionized species, and larger material clusters travel more slowly than small clusters. MAPLE-deposited polymer is neutral and deposited in clusters Figure 1. (a) Schematic of MAPLE, equipped with a time-of-flight (TOF) system. (b) Normalized deviation of the quartz crystal sensor frequency vs time after the laser ablates the target. Inset: SEM image of a MAPLE-deposited PMMA bulk film. ACS Macro Letters Letter dx.doi.org/10.1021/mz500546u | ACS Macro Lett. 2014, 3, 1046−105