Yunlong Guo

Core–Shell Fe3O4 Polydopamine Nanoparticles Serve Multipurpose as Drug Carrier, Catalyst Support and Carbon Adsorbent

We present the synthesis and multifunctional utilization of core-shell Fe3O4 polydopamine nanoparticles (Fe3O4@PDA NPs) to serve as the enabling platform for a range of applications including responsive drug delivery, recyclable catalyst support, and adsorbent. Magnetite Fe3O4 NPs formed in a one-pot process by the hydrothermal approach were coated with a polydopamine shell layer of ~20 nm in thickness. The as prepared Fe3O4@PDA NPs were used for the controlled drug release in a pH-sensitive manner via reversible bonding between catechol and boronic acid groups of PDA and the anticancer drug bortezomib (BTZ), respectively. The facile deposition of Au NPs atop Fe3O4@PDA NPs was achieved by utilizing PDA as both the reducing agent and the coupling agent. The nanocatalysts exhibited high catalytic performance for the reduction of o-nitrophenol. Furthermore, the recovery and reuse of the catalyst was demonstrated 10 times without any detectible loss in activity. Finally, the PDA layers were converted into carbon to obtain Fe3O4@C and used as an adsorbent for the removal of Rhodamine B from an aqueous solution. The synergistic combination of unique features of PDA and magnetic nanoparticles establishes these core-shell NPs as a versatile platform for multiple applications.

Ultrastable nanostructured polymer glasses

Owing to the kinetic nature of the glass transition, the ability to significantly alter the properties of amorphous solids by the typical routes to the vitreous state is restricted. For instance, an order of magnitude change in the cooling rate merely modifies the value of the glass transition temperature (T(g)) by a few degrees. Here we show that matrix-assisted pulsed laser evaporation (MAPLE) can be used to form ultrastable and nanostructured glassy polymer films which, relative to the standard poly(methyl methacrylate) glass formed on cooling at standard rates, are 40% less dense, have a 40 K higher T(g), and exhibit a two orders of magnitude enhancement in kinetic stability at high temperatures. The unique set of properties of MAPLE-deposited glasses may make them attractive in technologies where weight and stability are central design issues.

Structural relaxation of polymer nanospheres under soft and hard confinement: isobaric versus isochoric conditions.

We have measured the glassy-state structural relaxation of aqueous suspended polystyrene (PS) nanoparticles (the case of soft confinement) and the corresponding silica-capped PS nanoparticles (the case of hard confinement) via differential scanning calorimetry. Suspended and capped PS nanoparticles undergo physical aging under isobaric and isochoric conditions, respectively. With decreasing diameter, suspended and capped PS nanoparticles exhibited reduced and bulk glass transition temperatures (T(g)), respectively. To account for T(g) changes with confinement, all physical aging measurements were performed at a constant value of T(g) - T(a), where T(a) is the aging temperature. With decreasing diameter, aqueous suspended PS nanoparticles exhibited enhanced physical aging rates in comparison to bulk PS. Due to differences in thermodynamic conditions during aging and interfacial effects from nanoconfinement, at all values of T(g) - T(a) investigated, capped PS nanoparticles aged at reduced rates compared to the corresponding aqueous suspended PS nanoparticles. We captured the physical aging behavior of all nanoparticles via the Tool, Narayanaswamy, and Moynihan model of structural relaxation.

Fragility of an Isochorically Confined Polymer Glass.

We report the effect of isochoric confinement on the dynamic fragility of a polymeric glass-former, that is, polystyrene (PS). Utilizing silica-capped PS nanospheres as a model system, the fictive temperature (Tf) and the isochoric heat capacity (Cv) are measured as a function of diameter via differential scanning calorimetry. By examining Tf as a function of cooling rate for each sample, the isochoric fragility (mv) is obtained, which decreases significantly as the diameter of the nanospheres is reduced from 260 to 129 nm. Hence, the temperature dependence of structural relaxation near the glass transition is weakened with isochoric confinement.

Understanding and controlling gold nanoparticle formation from a robust self-assembled cyclodextrin solid template

Recently, we discovered that cyclodextrin (CD)-stabilized gold nanoparticles could be synthesized in an aqueous medium from a self-assembled supramolecular structure of CD and gold salt. We showed that the self-assembled structure of the CD complex induced by the gold salt acted as a solid template for the formation of nanoconfined gold seeds and that gold seeds grew into CD-stabilized gold nanoparticles in water without the necessity of other reducing or stabilizing agents. Here, we extensively investigate the supramolecular self-assembled structure of the CD/gold salt complex under various synthetic conditions, the mechanism of the α-CD-stabilized gold nanoparticles formation, and the processing parameters for controlling the size of gold nanoparticles. We demonstrate that gold salts were confined between two different crystalline phases of the supramolecular CD solid template via a gold salt-induced molecular self-assembly process and that thermal treatment of the CD/gold salt complex led to the formation of nanosized gold seeds geometrically confined within the crystalline interface region. Placement of the thermally treated complex in water without the addition of any supplementary additives proliferated the growth of CD-stabilized gold nanoparticles via stabilization of the growing gold seed intermediates by CD molecules. In addition, various processing parameters such as Au salt concentration are shown to affect the size of AuNPs.

Spatially Distributed Rheological Properties in Confined Polymers by Noncontact Shear

When geometrically confined to the nanometer length scale, a condition in which a large portion of the material is in the nanoscale vicinity of interfaces, polymers can show astonishing changes in physical properties. In this investigation, we employ a unique noncontact capillary nanoshearing method to directly probe nanoresolved gradients in the rheological response of ultrathin polymer films as a function of temperature and stress. Results show that ultrathin polymer films, in response to an applied shear stress, exhibit a gradient in molecular mobility and viscosity that originates at the interfaces. We demonstrate, via molecular dynamics simulations, that these gradients in molecular mobility reflect gradients in the average segmental relaxation time and the glass-transition temperature.

Structural Relaxation of Confined Glassy Polymers

Glasses are non-equilibrium, amorphous materials. They undergo glassy-state structural relaxation towards thermodynamic equilibrium. Consequently, glasses exhibit time-dependent engineering properties in a process termed physical aging. Understanding structural relaxation is important for predicting long-term material properties and useful lifetimes. Here, we highlight the influence of nanoscale confinement and interfaces on the structural relaxation of polymers. Whenever possible, we also discuss correlations or lack thereof between the well-documented influence of confinement on the glass transition temperature (Tg). How confinement might be used to engineer different aging responses in polymer glasses is also mentioned. Finally, we conclude with perspectives and future work.

Slow Down Dewetting in Polymer Films by Isocyanate-treated Graphite Oxide

Isocyanate-treated graphite oxides (iGOs) were well-dispersed into the polystyrene (PS) thin films and formed a novel network structure. With control in fabrication, an iGOs-web layer was horizontally embedded near the surface of the films and thus formed a composite slightly doped by iGOs. This work demonstrated that the iGOs network can remarkably depress the dewetting process in the polymer matrix of the composite, while dewetting often leads to rupture of polymer films and is considered as a major practical limit in using polymeric materials above their glass transition temperatures (Tg). Via annealing the 50–120 nm thick composite and associated neat PS films at temperatures ranging from 35 °C to 70 °C above Tg, surface morphology evolution of the films was monitored by atomic force microscopy (AFM). The iGOs-doped PS exhibited excellent thermal stability, i.e., the number of dewetting holes was greatly reduced and the long-term hole growth was fairly restricted. In contrast, the neat PS film showed serious surface fluctuation and a final rupture induced by ordinary dewetting. The method developed in this work may pave a road to reinforce thin polymer films and enhance their thermal stability, in order to meet requirements by technological advances.

Structural Relaxation near the Glass Transition: Observing Kovacs Kinetic Phenomenology by Mechanical Measurements

paper presents the structural relaxation behavior near the glass transition temperature (T g ) of poly (phenylene sulfide) (PPS). Since the test temperature is close to T g , the specimen is able to reach equilibrium after isothermal or nonisothermal temperature jumps in the experimental time scale; as such, the classic phenomenology of structural relaxation into equilibrium can be pursued by mechanical testing (physical aging) in a temperature range near T g . In our study, physical aging effects were observed by sequential creep test method using a dynamic mechanical analyzer (DMA). The physical aging of PPS near T g from mechanical testing clearly showed the similar behavior from the classic volumetric responses found by Kovacs in 1960s. The mechanical aging shift factors manifested all of the three “essential ingredients” in the kinetics of structural relaxation which constitute the most physical phenomenology of glassy-forming materials: intrinsic isotherms; asymmetry of approaching equilibrium; and memory effects. The findings in this study offer a valuable approach to investigate the relationship between the mechanical response and thermodynamic properties (volume, enthalpy, etc.) during structural relaxation of glassy materials.