The Crystallization Kinetics of Polylactic Acid (Pla) Processed Through Solid-State/Melt Extrusion

Polylactic acid (PLA) is a bio-derived, biodegradable polymer with a number of similar mechanical properties to commodity plastics like polyethylene (PE) and polyethylene terephthalate (PETE). There has recently been a great interest in using PLA to replace these typical petroleum-derived polymers because of the developing trend to use more sustainable materials and technologies. However, PLA¿s inherent slow crystallization behavior is not compatible with prototypical polymer processing techniques such as molding and extrusion, and in turn inhibits its widespread use in industrial applications. In order to make PLA into a commercially-viable material, there is a need to process the material in such a way that its tendency to form crystals is enhanced. The industry standard for producing PLA products is via twin screw extrusion (TSE), where polymer pellets are fed into a heated extruder, mixed at a temperature above its melting temperature, and molded into a desired shape. A relatively novel processing technique called solid-state shear pulverization (SSSP) processes the polymer in the solid state so that nucleation sites can develop and fast crystallization can occur. SSSP has also been found to enhance the mechanical properties of a material, but its powder output form is undesirable in industry. A new process called solid-state/melt extrusion (SSME), developed at Bucknell University, combines the TSE and SSSP processes in one instrument. This technique has proven to produce moldable polymer products with increased mechanical strength. This thesis first investigated the effects of the TSE, SSSP, and SSME polymer processing techniques on PLA. The study seeks to determine the process that yields products with the most enhanced thermal and mechanical properties. For characterization, percent crystallinity, crystallization half time, storage modulus, softening temperature, degradation temperature and molecular weight were analyzed for all samples. Through these characterization techniques, it was observed that SSME-processed PLA had enhanced properties relative to TSE- and SSSP-processed PLA. Because of the previous findings, an optimization study for SSME-processed PLA was conducted where throughput and screw design were varied. The optimization study determined PLA processed with a low flow rate and a moderate screw design in an SSME process produced a polymer product with the largest increase in thermal properties and a high retention of polymer structure relative to TSE-, SSSP-, and all other SSME-processed PLA. It was concluded that the SSSP part of processing scissions polymer chains, creating defects within the material, while the TSE part of processing allows these defects to be mixed thoroughly throughout the sample. The study showed that a proper SSME setup allows for both the increase in nucleation sites within the polymer and sufficient mixing, which in turn leads to the development of a large amount of crystals in a short period of time.

A Study of the Diffusion of Electroplated Coatings in the Solid State.

The term diffusion means an equalization or homogenization of diverse materials. Specifically ap­plied to metals, diffusion is the interchange of atoms. It is, in effect, an invasion of one crystal lattice by the atoms of one or more other crystal lattices. Therefore, the study of diffusion must involve the geometry and physics of crystal lattices as well as their energies.

The STEREO/PLASTIC response to solar wind ions (Flight measurements and models)

The Plasma and Supra-Thermal Ion Composition (PLASTIC) instrument is one of four experiment packages on board of the two identical STEREO spacecraft A and B, which were successfully launched from Cape Canaveral on 26 October 2006. During the two years of the nominal STEREO mission, PLASTIC is providing us with the plasma characteristics of protons, alpha particles, and heavy ions. PLASTIC will also provide key diagnostic measurements in the form of the mass and charge state composition of heavy ions. Three measurements (E/qk, time of flight, ESSD) from the pulse height raw data are used to characterize the solar wind ions from the solar wind sector, and part of the suprathermal particles from the wide-angle partition with respect to mass, atomic number and charge state. In this paper, we present a new method for flight data analysis based on simulations of the PLASTIC response to solar wind ions. We present the response of the entrance system / energy analyzer in an analytical form. Based on stopping power theory, we use an analytical expression for the energy loss of the ions when they pass through a thin carbon foil. This allows us to model analytically the response of the time of flight mass spectrometer to solar wind ions. Thus we present a new version of the analytical response of the solid state detectors to solar wind ions. Various important parameters needed for our models were derived, based on calibration data and on the first flight measurements obtained from STEREO-A. We used information from each measured event that is registered in full resolution in the Pulse Height Analysis words and we derived a new algorithm for the analysis of both existing and future data sets of a similar nature which was tested and works well. This algorithm allows us to obtain, for each measured event, the mass, atomic number and charge state in the correct physical units. Finally, an important criterion was developed for filtering our Fe raw flight data set from the pulse height data without discriminating charge states.

Cooperative Effects of Electron Donors and Acceptors for the Stabilization of Elusive Metal Cluster Frameworks: Synthesis and Solid-State Structures of Pt-19(CO)(24)(mu(4)-AuPPh3)(3)](-) and Pt-19(CO)(24){mu(4)-Au-2(PPh3)(2)}(2)]

The anionic cluster Pt-19(CO)(22)](4-) (1), of pentagonal symmetry, reacts with CO and AuPPh3+ fragments. Upon increasing the Au:Pt-19, molar ratio, different species are sequentially formed, but only the last two members of the series could be characterized by X-ray diffraction, namely, Pt-19(CO)(24)(mu(4)-AuPPh3)(3)](-) (2) and Pt-19(CO)(24){mu(4)-Au-2(PPh3)(2)}(2)] (3).The metallic framework of the starting cluster is completely modified after the addition of CO and AuL+, and both products display the same platinum core of trigonal symmetry, with closely packed metal atoms. The three AuL+ units cap three different square faces in 2, whereas four AuL+ fragments are grouped in two independent bimetallic units in the neutral cluster 3. Electrochemical and spectroelectrochemical studies on 2 showed that its redox ability is comparable with that of the homometallic 1.

A Cruciform Electron Donor-Acceptor Semiconductor with Solid-State Red Emission: 1D/2D Optical Waveguides and Highly Sensitive/Selective Detection of H2S Gas

In this paper, a new cruciform donor–acceptor molecule 2,2'-((5,5'-(3,7-dicyano-2,6-bis(dihexylamino)benzo[1,2-b:4,5-b']difuran-4,8-diyl)bis(thiophene-5,2-diyl))bis (methanylylidene))dimalononitrile (BDFTM) is reported. The compound exhibits both remarkable solid-state red emission and p-type semiconducting behavior. The dual functions of BDFTM are ascribed to its unique crystal structure, in which there are no intermolecular face-to-face π–π interactions, but the molecules are associated by intermolecular CN…π and H-bonding interactions. Firstly, BDFTM exhibits aggregation-induced emission; that is, in solution, it is almost non-emissive but becomes significantly fluorescent after aggregation. The emission quantum yield and average lifetime are measured to be 0.16 and 2.02 ns, respectively. Crystalline microrods and microplates of BDFTM show typical optical waveguiding behaviors with a rather low optical loss coefficient. Moreover, microplates of BDFTM can function as planar optical microcavities which can confine the emitted photons by the reflection at the crystal edges. Thin films show an air-stable p-type semiconducting property with a hole mobility up to 0.0015 cm2V−1s−1. Notably, an OFET with a thin film of BDFTM is successfully utilized for highly sensitive and selective detection of H2S gas (down to ppb levels).

Electronic tuning effects via π-linkers in tetrathiafulvalene-based dyes

Four new tetrathiafulvalene (TTF)-based dyes featured with a donor–bridge–acceptor (D–π–A) structure were synthesized and characterized. All of them undergo two reversible oxidations to form stable radical cation and dication species. The electronic interactions between the TTF donor and the cyanoacrylic acid acceptor through the different π-linkers have been demonstrated by the presence of a photo-induced intramolecular charge-transfer (ICT) absorption band in the visible region. A red shift of the ICT state can be finely tuned by the degree of aromaticity and extended conjugation of π-bridges. To some extent, the oxidation potentials of these dyes are affected by the nature of π-bridges. They have been applied in organic dye-sensitized solar cells, showing relatively low power conversion efficiencies of up to 0.87% due to substantial charge recombination losses.

On the Methyl-Transfer Reaction in Crystalline Methyl 2-(Methylthio)benzenesulfonate: a Thermally Induced Non-Topochemical Solid-State Reaction

The rearrangement of methyl 2-(methylthio)benzenesulfonate (1) to the zwitterionic 2-(dimethyl-sulfonium)benzenesulfonate (2) is known to proceed in solution by intermolecular Me transfers. The same rearrangement has been observed to occur in crystalline 1, but the crystal structure shows that the molecular packing is not conducive to intermolecular Me transfer. The reaction has been carried out with mixed crystals composed of 1 and deuteriomethylated (D6)-l. By fast-atom-bombardment mass spectroscopy, it has been shown that the product consists of a 1:2:1 mixture of the non-, tri-, and hexadeuterated species, the mixture expected, if the solid-state reaction proceeds by intermolecular Me transfers. From this result, together with the slower rates of conversion in the single crystal compared with the melt, it can be concluded that the reaction must occur not topochemically but rather at defects such as microcavities, surfaces, and other irregularities in the ordered crystal arrangement.