Pressurized interfacial failure of soft adhesives.

Interfacial separation of soft, often viscoelastic, materials typically cause the onset of instabilities, such as cavitation and fingering. These instabilities complicate the pathways for interfacial separation, and hence hinder the quantitative characterization of bulk and interfacial contributions to soft material adhesion. To overcome these challenges, we developed a method termed pressurized interfacial failure (PIF), in which the interfacial separation is controlled by applying a positive pressure at the contact interface between a rigid, annular probe and a thin adhesive. We conducted experiments on model and commercially-available acrylic adhesives. Surprisingly, all the materials studied here fail by an inside-out growth of an interfacial cavity and show similar trends in the interrelationship between the cavity radius, applied pressure and change of contact force. In contrast, the force-displacement relationships of the same materials measured by conventional tack tests vary significantly. Accordingly, we conclude that the PIF method allows for controlling the interfacial failure mechanism. Furthermore, we have applied a linear elastic fracture mechanics framework and conducted finite element analysis to develop analytical models to calculate the critical energy release rate for interfacial separation, Gc. For model acrylic adhesives and commercially available adhesives, the values of Gc are similar to values determined by sphere-probe tack tests. Collectively, the herein introduced PIF method and analysis work provide a new foundation for quantitatively decoupling the interfacial and bulk contributions to soft polymer adhesion.

Mechanics of adhesives under annular confinement: internal pressure, force, and interfacial area.

Closed annular adhesive interfaces are commonly found in nature as well as in many existing and developing technologies. Such contacts provide enhanced control of interfacial history by prescribing whether interfacial separation occurs at the outer or inner edge, and whether internal pressure affects the required force for separation. To facilitate the development of technologies involving annular contacts, we have experimentally measured the relationship between applied displacement, resulting force and internal pressure, and annular interface dimensions for the contact between a rigid annular probe and an adhesive layer with finite thickness. Experiments were validated by finite element analysis models, which were used to develop semi-empirical analytical relationships for the changes in contact compliance as a function of material properties and geometric constraints. Additionally, the change in internal pressure was modeled as a function of annular contact dimensions and adhesive layer material properties. This model predicts the critical volume where internal pressure changes alters critical force for separating an annular contact interface. The results discussed here provide a foundation for new experimental protocols for characterizing soft materials, including pressure-sensitive adhesives, as well as guidelines for designing annular interfacial materials with controlled separation histories.

Multidentate block-copolymer-stabilized ultrasmall superparamagnetic iron oxide nanoparticles with enhanced colloidal stability for magnetic resonance imaging.

Ultrasmall superparamagnetic iron oxide nanoparticles (USPIOs) with diameters <5 nm hold great promise as T1-positive contrast agents for in vivo magnetic resonance imaging. However, control of the surface chemistry of USPIOs to ensure individual colloidal USPIOs with a ligand monolayer and to impart biocompatibility and enhanced colloidal stability is essential for successful clinical applications. Herein, an effective and versatile strategy enabling the development of aqueous colloidal USPIOs stabilized with well-defined multidentate block copolymers (MDBCs) is reported. The multifunctional MDBCs are designed to consist of an anchoring block possessing pendant carboxylates as multidentate anchoring groups strongly bound to USPIO surfaces and a hydrophilic block having pendant hydrophilic oligo(ethylene oxide) chains to confer water dispersibility and biocompatibility. The surface of USPIOs is saturated with multiple anchoring groups of MDBCs, thus exhibiting excellent long-term colloidal stability as well as enhanced colloidal stability at biologically relevant electrolyte, pH, and temperature conditions. Furthermore, relaxometric properties as well as in vitro and in vivo MR imaging results demonstrate that the MDBC-stabilized USPIO colloids hold great potential as an effective T1 contrast agent.

Dual redox and thermoresponsive double hydrophilic block copolymers with tunable thermoresponsive properties and self-assembly behavior.

The synthesis, tunable thermoresponsive properties, and self-assembly of dual redox and thermoresponsive double hydrophilic block copolymers having pendant disulfide linkages (DHBCss) are reported. Well-defined DHBCss composed of a hydrophilic poly(ethylene oxide) block and a dual thermo- and reduction-responsive random copolymer block containing pendant disulfide linkages are synthesized by atom transfer radical polymerization. Their lower critical solution temperature (LCST) transitions are adjusted through modulating pendant hydrophobic-hydrophilic balance with disulfide-thiol-sulfide chemistry. Further, these DHBCss derivatives are converted to disulfide-crosslinked nanogels at temperatures above LCST through temperature-driven self-assembly and in situ disulfide crosslinking. They exhibit enhanced colloidal stability and further reduction-responsive degradability, thus demonstrating versatility of dual thermo- and reduction-responsive smart materials.

Continuous controlled radical polymerization of methyl acrylate in a copper tubular reactor.

The use of copper tubing as both the reactor and as a catalyst source is demonstrated for continuous controlled radical polymerization of methyl acrylate at ambient temperature and at low solvent content of 30%. The high surface area provided by the copper walls mediates the reaction via the single electron transfer-living radical polymerization (SET-LRP) mechanism. The polymerizations proceeded quickly, reaching 67% conversion at a residence time of 16 min. Ligand concentration could also be reduced without a sharp drop in polymerization rate, demonstrating the potential for decreased raw material and post-process purification costs. Chain extension experiments conducted using synthesized polymer showed high livingness. The combination of living polymer produced at high polymerization rates at ambient temperature and low volatile organic solvent content demonstrate the potential of a copper reactor for scale up of SET-LRP.

A dual location stimuli-responsive degradation strategy of block copolymer nanocarriers for accelerated release.

A new strategy that centers on the incorporation of stimuli-responsive cleavable linkages in dual or multiple locations as in both micellar cores and at polymer/solution interfaces leading to synergistically enhanced release of encapsulated anticancer drugs in cancer cells.

Tuning LCST with thiol-responsiveness of thermoresponsive copolymers containing pendant disulfides.

A new approach that centers on modulating the hydrophobic-hydrophilic balance by conversion of pendant disulfides to thiols and further to sulfides enables facile tuning of the thermoresponsive properties of thiol-responsive copolymers.