Dae Up Ahn

Programmable, pattern-memorizing polymer surface.

However, all current SMP applications focus on harvesting the macroscopic scale deformation, i.e. employing the SMP as structural materials. An intriguing capability of all SMPs, which remains largely unexplored, is their ability to memorize and recover nanoscale patterns or structures. Here we demonstrate that SMPs can memorize and faithfully recover their lithographically fabricated, permanent or even temporary surface patterns. More signifi cantly, tunable multi-pattern memory capability can be achieved in Nafi on fi lms. Considering the prevalence of nanostructured surfaces in emerging nanotechnologies, such pattern-memorizing surfaces could potentially transform these technologies. During a typical shape memory cycle, an SMPs permanent shape is fi rst “programmed” into a temporary shape under mechanical loading at a temperature higher than the transition temperature (either glass transition temperature, T g , or melting temperature, T m ) of the SMP. At the permanent shape, the polymer chains between crosslinking points can be considered at the equilibrium state, or the lowest energy state. The mechanical loading during the programming deforms the chains into a higher energy state (with lower entropic freedom), forming the temporary shape. Without the mechanical constraints, the SMP sample will return to their permanent shape to minimize the system energy. However, this temporary shape can be “fi xed” as the temperature decreases below the T g (or T m ) of the SMP before releasing the mechanical loading and remains stable indefi nitely. The SMP softens and recovers its permanent shape when exposed to an environmental stimuli such as heat, [ 4 , 5 ] light, [ 1 ] or even solvent vapors. [ 6 ] During the recovery, strain or stress can be harvested under free or constrained conditions, respectively. [ 7 , 8 ] However, beyond such structural applications, the potential applications of SMP surfaces have yet to be explored. King et al. reported that AFM-indented holes on a SMP surface can be recovered via heating which enables the AFM-based data storage. [ 9 , 10 ] Recently, Burke et al. showed that micron scale patterns embossed on liquid crystalline elastomer can be erased

Time and Temperature Dependent Recovery of Epoxy-Based Shape Memory Polymers

Shape memory polymers (SMPs) are a group of adaptive polymers that can recover the permanent shape from a temporary shape by external stimuli on demand. Among a variety of external stimuli for polymer actuation, temperature is the most extensively used. In SMP applications, one of the major design considerations is the time necessary to recover the shape without external deformation constraints, or free recovery, and the amount of the recoverable strain. This paper investigates the amount of the recoverable strain and the recovery rate of an epoxy-based SMP (Veriflex ® E, VFEI-62 (CRG, Dayton, OH)) under different thermal conditions. In particular, the free recovery behaviors of the SMPs under two experimental protocols, isothermal and shape memory (SM) cycle, are studied. It is found that free recovery in isothermal experiments is much faster than that in a SM cycle at the same recovering temperature and the material is fully recoverable at the temperature above differential scanning calorimetry Tg. Furthermore, for the recovery in SM cycle experiments, reshaping the sample at a low temperature and recovering from the deformation at a high temperature yield the fastest recovery rate, while reshaping at a high temperature and recovering at a low temperature cannot recover the original shape within this works experimental time frame. The possible mechanism for these observations is discussed.

Hierarchical polymer patterns driven by capillary instabilities at mobile and corrugated polymer–polymer interfaces

We investigate the spontaneous formation of hierarchical structures during the annealing of polystyrene (PS) films cast on topographically patterned poly(methylmethacrylate) (PMMA) substrates at temperatures higher than the glass transition temperatures for both polymers. The mobile and corrugated PS/PMMA interfaces give rise to two sequential events of capillary instability of PS stripes segregated on the PMMA mesas and PS threads confined in the PMMA trenches. The latter occurred through a simultaneous “out-of-phase” fashion between the neighboring confined PS threads. Both the size and the spatial distribution of the PS droplets formed are determined by the geometric parameters of the patterns and films, and thus can be controlled for different applications. In addition to both capillary instabilities, a larger scale “wrinkling” fluctuation was also observed, with a wave vector along the original patterned line direction. The morphological evolution and structure formations are unique to the current system, and are drastically different from the polymer film dewettings on a planar surface, a chemically patterned surface, or a topographically patterned rigid surface.

Vertical pattern decay and lateral in-phase capillary breakup of nanoimprinted bilayer polymer films

We report the morphological evolutions of nanoimprinted polystyrene (PS)/polymethyl methacrylate (PMMA) bilayers during thermal annealing. At an annealing temperature above the glass transition temperatures of both polymers, the PS/PMMA bilayer patterns undergo a sequential event of morphological evolutions. At the early stage, the bilayer patterns decay vertically, which leads to a continuous PS layer over the PMMA patterns. Further annealing leads to an intriguing “in-phase” capillary breakup of the PS stripes. As a result, strings of PS droplets develop at the direction orthogonal to the original pattern line directions. At the final stage, these PS droplets anisotropically coalesce into thicker stripes, which eventually break up into larger discrete droplets due to the Rayleigh instability. Our study demonstrates that the mobile and corrugated PS/PMMA interfaces dictate both the kinetics of the vertical pattern decay and the correlated lateral instability. Both polymers directly participate in the pattern evolutions, different from the well-documented dewetting of polymers confined on elastic topographic substrates.

Topographically uniform but chemically heterogeneous nanostructures by nanoimprinting demixed polymer blends.

Nanoimprint lithography is applied to fabricate topographically uniform patterns onto demixed polymer blend films. The high fidelity of pattern replications is achieved for 150 nm polystyrene (PS)/polymethylmethacrylate (PMMA) blend films with varying compositions. When imprinted at 150 °C, the morphology of the blend across the patterned structures is similar to that in the as-casted films. Significant morphological evolutions occur for patterns imprinted at 180 and 210 °C. For all the patterns, PMMA is found to segregate into the residual layer, driven by the preferential wetting of PMMA onto the SiO(x) surfaces. The combined domain coarsening and preferential wetting of PMMA leads to the formations of unique encapsulated structures within the topographically uniform features, ranging from blocks to threads.

Time Dependent Recovery of Shape Memory Polymers

Shape memory polymers (SMPs) are polymers that can recover the permanent shape from a temporary shape by external stimuli. In SMP applications, one of the major design considerations is the time necessary to recover the shape without external constraints, or free recovery rate. This paper investigates the free recovery behaviors of an epoxy-based SMP under shape memory (SM) cycles. We found that reshaping the sample at a low temperature and recovering from the deformation at a high temperature yields the fastest recovery rate whilst reshaping at a high temperature and recovering at a low temperature cannot recover the original shape within this work’s experimental time frame. The possible mechanism for these observations is discussed.