Yifu Ding

States of water in different hydrophilic polymers — DSC and FTIR studies

The structure of water molecules sorbed in different hydrophilic polymers was studied by DSC and FTIR. The obtained data shows that, first, the sorbed water molecules are directly bound to the hydrophilic site to form non-freezable water. Then, beyond a certain water content threshold, the sorbed water molecules become freezable, but with a melting point lower than 0°C, due to their location in the second hydration layer. Bulk-like water which can be frozen at 0°C appears at higher water contents, and the two types of freezable water finally merge together at very high water contents. The average number of non-freezable water molecules per site depends on the chemical nature of the polar site: ca. 1 for a hydroxyl, and 4.2 for an amide group. For a polymer with carboxylate sites, it increases with the size of the alkaline counter-ion of the site, due to the increasing ability of the carboxylate counter-ion pair to undergo dissociation.

Mechanisms of multi-shape memory effects and associated energy release in shape memory polymers

Shape memory polymers have attracted increasing research interest due to their capability of fixing a temporary shape and associated deformation energy then releasing them later on demand. Recently, it has been reported that polymers with a broad thermomechanical transition temperature range can demonstrate a multi-shape memory effect (m-SME), where shape recovery and energy release occur in a stepped manner during free recovery. This paper investigated the underlying physical mechanisms for these observed shape memory behaviors and the associated energy storage and release by using a theoretical modeling approach. A multibranch model, which is similar to the generalized standard linear solid model of viscoelasticity, was used for a quantitative analysis. In this model, individual nonequilibrium branches represent different relaxation modes of polymer chains with different relaxation times. As the temperature was increased in a staged manner, for a given temperature, different numbers of branches (or relaxation modes) became shape memory active or inactive, leading to the observed m-SME. For energy release during free recovery, under a tensile deformation of the SMP, stored energy in individual nonequilibrium branches was first transferred into a compressive deformation energy, then gradually declined to zero. Energy release during recovery was a complicated process due to the involvement of multiple relaxation modes.

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

Effect of polymeric structure on the corrosion protection of epoxy coatings

Two epoxy resins, EP and EPA, with similar backbone structure but different water affinity, was obtained by curing o-cresol novolac epoxy resin with phenol novolac resin and phenol novolac acetate resin, respectively. By using these two resins, the effect of the polymeric structure on the corrosion protection of the coatings was studied. The free volume in EPA is larger than that in EP as demonstrated by room-temperature density measurement and positron annihilation, while water sorption of EPA is only half of that of EP. Therefore, water affinity of the resin is more important in determining water sorption of the resin than free volume. The cross-sectional area of water passage at coating/metal interface (Aw) was estimated using the electrochemical impedance spectroscopy and compared with that in the resin matrix (Acs). It was found that, for EPA, Aw is much less than Acs, which suggests a significant narrowing of water passage at the coating/metal interface. This narrowing of water passage at coating/metal interface due to the formation of a hydrophobic layer can greatly improve the corrosion protection of the coating.

Prediction of temperature-dependent free recovery behaviors of amorphous shape memory polymers

Shape memory polymers (SMPs) are active materials that can fix a temporary shape and recover the permanent shape in response to environmental stimuli such as temperature, light, moisture or magnetic field. In order to provide insight into the mechanism for shape memory behavior and to predict the behaviors of targeted design, several constitutive models were developed in the past. Most of these models are complicated and require time-consuming experiments to obtain model parameters. However, for many engineers, an estimation of key features of shape memory behaviors, such as time for free recovery, is sufficient. Such estimation should be based on a simplified model involving only a few key parameters that can be quickly identified experimentally. In this paper, a simple theoretical solution was developed to predict the temperature dependent free recovery behaviors of amorphous SMPs. This solution is based on a modified standard linear solid (SLS) model with a Kohlrausch–Williams–Watts (KWW) stretched exponential function and requires only eight parameters that can be determined by stress relaxation tests. The theoretical predictions of free recovery behaviors show a good agreement with experimental results. Parametric studies using this solution reveal that the free recovery time can be reduced by increasing the equilibrium modulus (E0) or KWW stretching parameter (β), or by decreasing the nonequilibrium modulus (E1) or the relaxation time (τ0) and is most sensitive to the KWW stretching parameter β.

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.

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.

Pervaporation of water–ethanol mixtures with polyacrylate-grafted polyethylene acid (PE-g-AA) membranes: Physico-chemical analysis of the transport mechanism

Polyacrylate-grafted polyethylene (PE-g-AA) membranes were studied in pervaporation. The membrane performances are improved by an increase in the acrylic grafting ratio. The water and ethanol sorption and diffusion properties in the membrane loaded with different counter-ions, as well as the interactions between the solvent and the membrane ion pairs, were measured. The results show that the ion pairs are preferentially solvated by water, and the water content in the liquid mixture at which the ion pair is dissociated is much smaller for the potassium membrane than for the lithium membrane. The easier dissociation of the K carboxylate ion pair in the water–ethanol mixture explains the increase in the permeability according to the sequence Li + < Na + < K + . The apparent contradiction between the sulfonate and the carboxylate membranes with regards to the alkaline cation sequence according to which the permeability changes is explained by the difference in the ionization ability of the sulfonate and carboxylate ion pairs.

Nanoscale Polymer Processing

The established rules for fabricating plastics now require a rethink as feature sizes of the products head toward the nanoscale.