Mesomorphism, molecular structure and dynamics of polydiethylsiloxane

The correlation between the phase behaviour of polydiethylsiloxane (PDES) and conformational and motional changes at the various disordering transitions has been investigated by nuclear magnetic resonance (n.m.r.), dielectric relaxation and shear experiments. Diffusive rotation of the chain segments around the long axis of the molecules is indicated by 29Si n.m.r. below the isotropization transition. The remarkably fluid character is explained in part by the coexistence of anisotropic and isotropic motional states of the -OSiEt2- segments, which indicate microscopic domain formation. The molecular motion in the α2- and β2-phases is restricted but still fast with respect to the 29Si n.m.r. timescale. Transmission electron micrographs show, besides chain-folded lamellae, also extended-chain lamellae. These differences in the morphologies can explain why the interconversion of α2-PDES into the thermodynamically more stable β2-polymorph is slow, in spite of the pronounced mobility of the polymer segments. Long-range reorganization processes have to be considered to allow the morphological changes observed by electron microscopy

Thermal loading in flow-through electroporation microfluidic devices

Thermal loading effects in flow-through electroporation microfluidic devices have been systematically investigated by using dye-based ratiometric luminescence thermometry. Fluorescence measurements have revealed the crucial role played by both the applied electric field and flow rate on the induced temperature increments at the electroporation sections of the devices. It has been found that Joule heating could raise the intra-channel temperature up to cytotoxic levels (>45 °C) only when conditions of low flow rates and high applied voltages are applied. Nevertheless, when flow rates and electric fields are set to those used in real electroporation experiments we have found that local heating is not larger than a few degrees, i.e. temperature is kept within the safe range (<32 °C). We also provide thermal images of electroporation devices from which the heat affected area can be elucidated. Experimental data have been found to be in excellent agreement with numerical simulations that have also revealed the presence of a non-homogeneous temperature distribution along the electroporation channel whose magnitude is critically dependent on both applied electric field and flow rate. Results included in this work will allow for full control over the electroporation conditions in flow-through microfluidic devicesThis work has been supported by NSF (CBET 1016547, 1041834, 0967069), the Universidad Autónoma de Madrid and Comunidad Autónoma de Madrid (Project S2009/MAT-1756), and the Spanish Ministerio de Educación y Ciencia (MAT2010-16161). Blanca del Rosal thanks Universidad Autónoma de Madrid for financial support (FPI-UAM grant

Poling influence on the mechanical properties and molecular mobility of highly piezoelectric P(VDF-TrFE) copolymer

The calorimetric, dielectric, and mechanical responses of highly piezoelectric 70/30 P(VDF-TrFE) displaying homogenous d33 of 219 pC N21 are studied. This work aims at better understanding the influence of poling on the mechanical properties of this copolymer. To explain the one decade mechanical modulus drop observed across the Curie transition, a stiffening process of the amorphous phase due to the local electric fields in the ferroelectric crystals is proposed. In poled P(VDF-TrFE), these fields are preferentially aligned resulting in a more stable and higher modulus below the Curie transition. This hypothesis accounts for the lower dielectric signals obtained with the poled sample. Through the Curie transition, the vanishing of these local electric fields, stemming from progressive disorientation and conversion of ferroelectric crystals to paraelectric ones, releases the constraints on the amorphous phase, leading to a storage modulus drop typical of a viscoelastic transition

Characterizing osmotic lysis kinetics under microfluidic hydrodynamic focusing for erythrocyte fragility studies

The biomechanics of erythrocytes, determined by the membrane integrity and cytoskeletal structure, provides critical information on diseases such as diabetes mellitus, myocardial infarction, hypertension, and sickle cell anemia. Here we demonstrate a simple microfluidic tool for examining erythrocyte fragility based on characterizing osmotic lysis kinetics. Hydrodynamic focusing is used for generating rapid dilution of the buffer and producing lysis of erythrocytes during their flow. The lysis kinetics are tracked by monitoring the release of intracellular contents from cells via recording the light intensity of erythrocytes at various locations in the channel. Such release profile reflects sensitively the changes in erythrocyte fragility induced by chemical, heating, and glucose treatment. Our tool provides a simple approach for probing red blood cell fragility in both basic research and clinical settings

D.m.a. and d.s.c. investigations of the β transition of poly(vinylidene fluoride)

The origin of the beta transition in poly(vinylidene fluoride) (PVDF) is still a pending question. This transition has been studied by dynamic mechanical analysis (d.m.a.) and differential scanning calorimetry (d.s.c.) in dependence on sample annealing and dilution with epsilon-caprolactam (CPL). The beta transition temperature is increased upon annealing and thus influenced by the polymer crystallization. Upon addition of CPL, there is no systematic shift in the beta transition temperature, in contrast to the PVDF crystallinity that increases steadily. A shoulder on the low temperature side of the beta transition peak is also observed as a result of annealing. It is shifted to lower temperatures when CPL is added, consistently with a glass transition. It thus appears that the so-called beta-transition is sensitive to the amorphous material, but in a close relationship with the polymer crystallization. Comparison of the observations by d.s.c, and d.m.a. shows that the broad transition observed for the unannealed samples would result from the overlap of two transitions: the glass transition of the unconstrained amorphous phase and the glass transition of chains constrained by the crystalline phase. This situation can account for the complex dependence of the beta transition on the polymer history

Mechanically Strong Graphene/Aramid Nanofiber Composite Electrodes for Structural Energy and Power

Structural energy and power systems offer both mechanical and electrochemical performance in a single multifunctional platform. These are of growing interest because they potentially offer reduction in mass and/or volume for aircraft, satellites, and ground transportation. To this end, flexible graphene-based supercapacitors have attracted much attention due to their extraordinary mechanical and electrical properties, yet they suffer from poor strength. This problem may be exacerbated with the inclusion of functional guest materials, often yielding strengths of <15 MPa. Here, we show that graphene paper supercapacitor electrodes containing aramid nanofibers as guest materials exhibit extraordinarily high tensile strength (100.6 MPa) and excellent electrochemical stability. This is achieved by extensive hydrogen bonding and π–π interactions between the graphene sheets and aramid nanofibers. The trade-off between capacitance and mechanical properties is evaluated as a function of aramid nanofiber loading, where it is shown that these electrodes exhibit multifunctionality superior to that of other graphene-based supercapacitors, nearly rivaling those of graphene-based pseudocapacitors. We anticipate these composite electrodes to be a starting point for structural energy and power systems that harness the mechanical properties of aramid nanofibers