AFM study of lamellar thickness distributions in high temperature melt-crystallization of β-polypropylene

Summary Atomic force microscopy (AFM) was used to study the lamellar thickness and its distribution in the β modification of isotactic polypropylene-(β-iPP) crystallized from the melt at high temperatures. Measurements were performed on lamellae oriented both flat-on and edge-on with respect to the examined surface. Average lamellar thickness was found to be dependent not only on the crystallization temperature, but also on factors such as nucleation density and isothermal lamellar thickening. The limitations and advantages of the AFM technique for evaluation of lamellar thickness are discussed. Received: 2 February 1998/Revised version: 10 July 1998/Accepted: 11 July 1998     

Probing the collapse dynamics of poly(N-isopropylacrylamide) brushes by AFM: effects of co-nonsolvency and grafting densities

Collapse of poly(N-isopropylacrylamide) (PNIPAM) brushes in the mixed solvent system (water/methanol 50% v/v) is studied by in-situ atomic-force microscopy (AFM). PNIPAM brushes with three different grafting densities and similar chain lengths are synthesized via surface-initiated atom-transfer radical polymerization. By changing the solvent from water to a water/methanol (50% v/v) mixture, the polymer brushes switch from a swollen to collapsed state. AFM force measurements using a silica colloidal probe attached to the tip are employed to obtain the Young's moduli of the polymer brushes in different solvation states. The collapse dynamics of the brush is followed by monitoring the pull-off force (adherence) in situ. The modulus of the swollen high-density polymer brush is four times lower than that of the same brush in the collapsed state. It is shown that in the case of the high-density polymer brush with a thickness (t(in water) ) of 900 nm, the collapse takes place in a time scale of ~25 s, whereas the collapse occurs faster for the medium-density brush (t(in water) = 630 nm) and much more rapidly for the low-density brush (t(in water) = 80 nm). This difference in the response kinetics is primarily ascribed to the time needed for solvent exchange in the polymer brushes.     

Quantitative mapping of surface elastic moduli in silica-reinforced rubbers and rubber blends across the length scales by AFM

The surface elastic moduli of silica-reinforced rubbers and rubber blends were investigated by atomic force microscopy (AFM)-based HarmoniX material mapping. Styrene–butadiene rubbers (SBR) and ethylene–propylene–diene rubbers (EPDM) and SBR/EPDM rubber blends with varying concentrations of silica nanoparticles (0, 5, 10, 20, 50 parts per hundred rubber, phr) were prepared to investigate the effect of different composition on the resulting morphology, filler distribution and elastic moduli of a specific rubber or rubber blend sample. For SBR, the elastic modulus values varied from 0.5 MPa for unfilled SBR to 5 MPa for 50 phr reinforced SBR with the increase in the concentration of filler. For EPDM, the corresponding values increased from 1.4 MPa for unfilled EPDM to 4.5 MPa for 50 phr reinforced EPDM. Local stiff and soft domains in silica-reinforced SBR and EPDM rubbers and rubber blends were identified by HarmoniX AFM imaging. While the stiff silica particles show modulus values as high as 2 GPa, the rubber matrix reveals modulus values in the range of ca. 30 MPa for the rubber blends to ca. 300 MPa for the unfilled rubbers. The lower value of elastic modulus of the EPDM phase in the blend, compared to the blank EPDM compound can be attributed to the presence of Sunpar oil in the compound which has a very good affinity with EPDM and decreases the rubber modulus. The elastic moduli maps revealed an increase of the areal fraction of silica particles showing an intrinsic surface modulus value with rising silica content in the compound preparation mixture. HarmoniX AFM measurements revealed the formation of larger silica aggregates in EPDM in contrast to SBR where isolated silica particles were observed. For silica-reinforced rubber blends a phase separation into a soft (ca. 40 MPa) and a significantly harder phase could be observed (ca. 500 MPa–1.5 GPa) indicating the incorporation of silica particles in the SBR phase. Using HarmoniX AFM imaging significantly higher surface elastic moduli were observed compared to those obtained by bulk tensile testing. Possible reasons for the observed differences between bulk modulus values and those measured by AFM are discussed in detail, including the aspect of different averaging procedures like inherent to surface probing by AFM versus bulk tensile testing, different filler distributions in SBR and EPDM and the AFM modulus calibration procedures.     

Understanding the impact of loyal user behaviour on Internet access pricing: a game-theoretic framework

In this paper, we investigate the impacts of user behaviour—user loyalty in particular—on pricing strategies of Internet Service Providers (ISPs) for a profitable yet sustainable Internet access marketplace. We carry out an extensive empirical analysis of customer loyalty issues of ISP markets including our own survey in the Hungarian ISP market. Based on the empirical results, we propose a realistic user loyalty model, the price difference dependent loyalty model. Next, we apply the loyalty model in game-theoretical framework where optimal Internet access pricing strategies are expressed. Our game-theoretic framework includes both short-term and long-term interaction cases (single-shot and repeated games, respectively) and is capable of dealing with uncertain as well as dynamic scenarios (Bayesian and Stackelberg games, respectively). Finally, we present the impacts of user loyalty on the prices and profits of ISPs in different scenarios based on simulation results.     

Nanostructured Polymer Brushes by UV‐Assisted Imprint Lithography and Surface‐Initiated Polymerization for Biological Functions

Functional polymer brush nanostructures are obtained by combining step‐and‐flash imprint lithography (SFIL) with controlled, surface‐initiated polymerization (CSIP). Patterning is achieved at length scales such that the smallest elements have dimensions in the sub‐100 nm range. The patterns exhibit different shapes, including lines and pillars, over large surface areas. The platforms obtained are used to selectively immobilize functional biomacromolecules. Acrylate‐based polymer resist films patterned by SFIL are first used for the selective immobilization of ATRP silane‐based initiators, which are coupled to unprotected domains of silicon substrates. These selectively deposited initiators are then utilized in the controlled radical SIP of poly(ethylene glycol)methacrylates (PEGMA). Nanostructured brush surfaces are then obtained by removal of the resist material. The areas previously protected by the SFIL resist are passivated by inert, PEG‐based silane monolayers following resist removal. PEGMA brush nanostructures are finally functionalized with biotin units in order to provide selective attachment points for streptavidin proteins. Atomic force microscopy and fluorescence spectroscopy confirm the successful immobilization of streptavidin molecules on the polymer grafts. Finally, it is demontrated that this fabrication method allows the immobilization of a few tens of protein chains attached selectively to brush nanostructures, which are surrounded by nonfouling PEG‐functionalized areas.     

Aligned carbon nanotube-reinforced silicon carbide composites produced by chemical vapor infiltration

A chemical vapor infiltration (CVI) technique was used to overcome most of the challenges involved in fabricating exceptionally-tough CNT/SiC composites. Nanotube pullout and sequential breaking and slippage of the walls of the CNTs during failure were consistently observed for all fractured CNT/SiC samples. These energy absorbing mechanisms result in the fracture strength of the CNT/SiC composites about an order of magnitude higher than the bulk SiC. The CVI-fabricated CNT/SiC composites have an strongly-bonded tube/matrix interface and an amorphous, crack-free SiC matrix, enabling the composites to withstand oxidization at 700–1600 °C in air.     

Assessment of Tractable Cysteines for Covalent Targeting by Screening Covalent Fragments

Targeted covalent inhibition and the use of irreversible chemical probes are important strategies in chemical biology and drug discovery. To date, the availability and reactivity of cysteine residues amenable for covalent targeting have been evaluated by proteomic and computational tools. Herein, we present a toolbox of fragments containing a 3,5-bis(trifluoromethyl)phenyl core that was equipped with chemically diverse electrophilic warheads showing a range of reactivities. We characterized the library members for their reactivity, aqueous stability and specificity for nucleophilic amino acids. By screening this library against a set of enzymes amenable for covalent inhibition, we showed that this approach experimentally characterized the accessibility and reactivity of targeted cysteines. Interesting covalent fragment hits were obtained for all investigated cysteine-containing enzymes.     

采用NE555定时器的低成本电压／频率转换器

1971年,飞利浦公司的前身Signetics推出了NE555定时器．现在,制造商每年仍要提供超过10亿片这种产品。通过为NE555增加一些元件,可以建立一个不到50美分的简单电压／频率转换器。该转换器使用了由一只TL071组成的Miller积分器和～只NE555定时器（图1）。此应用的输入电压范围为0V～10V,