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Relationship between the Size of the Latex Beads and the Solid-Solid Phase Transitions in Emulsion Polymerized Poly(tetrafluoroethylene)
No full textTen virgin powder samples of poly(telrafluoroethylene) (PTFE) were examined by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). The samples, prepared by emulsion or nanoemulsion polymerization, differed by average molecular weight and particle size. DSC was used to study the trends of the temperature and enthalpy of solid-solid transitions around room temperature. The samples with smaller particle size and lower molecular weight departed heavily from the behavior reported in the literature. The major role of particle size in determining this behavior was established by experiments of crystallization from the melt. WAXS was employed to study the crystalline phases of PTFE as a function of temperature and to confirm the hypotheses put forward in the DSC analysis. WAXS data showed that the degree of order of the crystallites in the virgin powders is controlled by particle size. Therefore, the confinement of the as-polymerized PTFE crystals in small entities (tens of nanometers) has a similar effect on the crystal correlation length and on the solid-state transitions, while it appears that the molecular weight per se is not able to induce such a behavior in the melt-crystallized polymers
Preparation and Properties of PTFE-PMMA Core-Shell Nanoparticles and Nanocomposites
Full text linkhe preparation of polytetrafluoroethylene-poly(methyl methacrylate) (PTFE-PMMA) core-shell particles was described, featuring controlled size and narrow size distribution over a wide compositional range, through a seeded emulsion polymerization starting from a PTFE seed of 26 nanometers. Over the entire MMA/PTFE range, the particle size increases as the MMA/PTFE ratio increases. A very precise control over the particle size can be exerted by properly adjusting the ratio between the monomer and the PTFE seed. Particles in the 80240 nm range can be prepared with uniformity indexes suited to build 2D and 3D colloidal crystals. These core-shell particles were employed to prepare nanocomposites with different compositions, through an annealing procedure at a temperature higher than the glass transition temperature of the shell forming polymer. A perfect dispersion of the PTFE particles within the PMMA matrix was obtained and optically transparent nanocomposites were prepared containing a very high PTFE amount
Two-dimensional non-close-packed arrays of nanoparticles via core-shell nanospheres and reactive ion etching
No full textNanosized PTFE/polystyrene core-shell particles were prepared by seed emulsion polymerization technique starting from PTFE seeds of 20nm. At the end of the reaction, no residual PTFE nor secondary nucleation was observed and by appropriately choosing the ratio between the monomer and the PTFE seed it was possible to obtain particles, with predetermined size in the range 60-100nm, featuring an extremely narrow size distribution. These particles were successfully employed as building blocks for the preparation of large scale nanosized monolayers through the floating technique. Reactive ion etching was further applied to modulate the size characteristics of the resulting 2D ordered nanostructure. Although for relatively short RIE times a peculiar continuous morphology was observed in which the particles are interconnected through thin arms, on further increasing the RIE time a well-organized 2D arrangement of particles with size of about 30 nm was obtained. Considering the shell as an expendable ordering and spacing tool, the use of core-shell nanospheres allows a wide variety of controlled morphologies to be designed and prepared thus opening new perspectives for nanostructure fabrication processes through nanosphere lithography (NSL)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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