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No / Simply melt blended polypropylene/carbon nanotubes composites (PP/CNTs) usually present mechanical deterioration. In this work, multilayered sheet of PP/CNTs with improved tensile property was obtained by solid-phase orientation and hot compression. The initially blended PP/CNTs were highly orientated by employing a constrained slit die and hot compressed under a certain temperature and pressure by stacking eight layers together. The effects of compression temperature and pressure on the tensile property and AC conductivity of the multilayered sheets were examined to explore the evolution of hierarchical crystallization morphology and CNT networks. The multilayered sheet which was hot compressed at 184°C and 5 MPa demonstrated an optimum tensile strength of ∼132.5 MPa and an elongation at break ∼52.7%, respectively, raised by almost 3-fold compared with those initially blended PP/CNTs. By increasing compression temperature and decreasing pressure, the AC conductivity showed an increase of 2 to 4 orders of magnitude. / China Scholarship Council. Grant Number: 201806465028.

A small scale, highly accurate elongational viscometer was developed explicitly for the rheological investigation of well characterized polyethylene samples. Elongational stress growth measurements as well as dynamic shear experiments demonstrated that the rheological response of molten polyethylene was sensitive to both molecular weight distribution and shear modification. The effects of molecular weight distribution and shear modification were rationalized from a molecular point of view. A model is proposed which is based on the concept of a molecular network and incorporates polymer chain entanglement disruption and regeneration. Direct observations of polymeric semicrystalline morphologies in a copolyester by scanning electron microscopy were made possible by the development of a novel chemical etch. Spherulitic textures were consistent with classic spherulite growth mechanisms and structure theories. Uniaxial deformation of a single spherulite was successfully studied in a model system consisting of isolated spherulites embedded in an amorphous polymer matrix. By isolating the spherulite, the mechanical influence of surrounding and often impinging spherulites found in most semicrystalline polymers on the mechanical response of an individual spherulite was avoided. The mechanical response of the amorphous matrix was characterized and found to correlate with the effectiveness of a cold draw neck in elongating an embedded spherulite. The observed mechanism and morphology of isolated spherulite deformation were rationalized within the context of existing theories of spherulite-to-microfibrillar transitions. Optically active poly(L-lactic acid) and racemic poly(lactic acid) were synthesized in a ring opening polymerization scheme with stannous octoate as a catalyst and lactic acid as a molecular weight controlling initiator. Binary polymer blends composed of these isomeric polymer pairs were found to be miscible at 40,000 molecular weight and immiscible at 120,000 molecular weight. Strain hardening and the level of strain induced crystallization which occurred in the biaxial deformation of poly(lactic acid) blend films were found to be contingent on the concentration of optically active poly(L-lactic acid). Temperature, molecular weight, and biaxial strain rate were also found to have an influence on strain hardening and strain induced crystallization of these thermodynamically ideal polymer blends. / Ph. D.

Research is presented on the crystallization and melting behavior of an aromatic semi crystalline polyimide known as LaRC CPI - 2 (Langley Research Center Crystalline rolyimide - second generation). This aromatic polyimide, a structural variant of the well known LaRC CPI, is synthesized froml 1,4-bis( 4-aminophenoxy-4' -benzoyl)benzene (1,4-BABB) and 4,4' -oxydiphthalic dianhydride (ODPA). The multiple melting behavior at ca. 334°C and 364°C, characteristic of this polymer, is analyzed and explained. Transmission electron microscopy (TEM), wide angle X-ray diffraction (W AXD), and small angle X-ray scattering (SAXS) studies, in conjunction with differential scanning calorimetry (DSC) analysis, show that the high melting transition results from the melting of lamellae which were melt recrystallized during heating in a DSC. The effects of synthesis and process variables upon the crystallization and melting properties of this polyimide were also studied. The DSC response of LaRC CPI - 2 films and powders of varying molecular weight are compared and contrasted. Powders generally melt at a higher temperature than films of the same molecular weight. The glass transition temperature, T g' of both the powders and films are found to be dependent upon molecular weight. An interpolated value of T g for the infinite molecular weight LaRC CPI - 2 is ca. 234°e. In a study of the development of crystallinity in films during standard thermal imidization, crystallinity was found to initiate off the glass surface very early in the process leaving an amorphous layer at the air surface after the full thermal treatment. LaRC CPI - 2 powder 'types' synthesized in different solvents, dimethylacetamide (DMAc) and m-cresol present with dramatically different melting, melt stability and crystallization properties. Though the chemical architecture and crystal lattice structure are the same, the powders synthesized in DMAc display dual melting transitions at ca. 334°C and 364°C, while the powders from m-cresol melt singularly at ca. 409°C. Rheological analysis, along with annealing data, revealed a higher temperature melt stability in the powders from m-cresol. It is postulated that the differences in the properties of the two powders are the result of differing initial molecular weights and a suppression of a cross-linking ketimine reaction in the polymer produced in m-cresol. / Ph. D.

Meta-diisopropylbenzene is reacted with propylene over the acid form of the molecular sieves SAPO-5, mordenite, offretite, beta, hexagonal and cubic faujasite (hex and FAU), L, SAPO-37, and an amorphous silica-alumina at temperatures around 463 K in a flow-type fixed-bed reactor. A small amount of cracking is observed. The main reactions of meta-diisopropylbenzene are isomerization and alkylation. This alkylation is proposed as a new test reaction to characterize the effective size of the voids in larger pore (12 T-atom rings or above) molecular sieves by measuring the amount ratio of formed 1,3,5- to 1,2,4-triisopropylbenzene. In most cases, this ratio increases with the increasing effective void size of the molecular sieves in the order: SAPO-5 < mordenite < offretite < beta < hex ≈ FAU < L < SAPO-37 < amorphous silica-alumina. Since samples with the FAU topology show lower selectivities to 1,3,5-triisopropylbenzene than the mesoporous, amorphous silica-alumina, pore curvature has an influence on alkylation selectivity even for voids of 13 A size. / Ph. D.

The time dependence of the physical properties of ethylene/1-octene (EO)-copolymers after primary crystallization is investigated by calorimetry, density, and creep measurements. The temporal evolution of the multiple melting of EO-copolymers is monitored by differential scanning calorimetry. The low temperature endotherm displays an evolution similar to that observed for the enthalpy recovery in glasses after physical aging. Using this analogy, a calorimetry-aging rate is defined, which quantifies the change in the low endotherm temperature with time. Similarly a density-aging rate is defined from the evolution of density with time. A non-classical creep behavior is observed for short aging times, consistent with crystallization-induced shrinkage. The change in crystallinity during aging leads to a change in the shape of the relaxation spectrum. Hence, analysis of creep data cannot be carried out using Struik's superposition method. For both short and long aging times, the creep rate exhibits a dependence on copolymer composition similar to those associated with the calorimetry- and the density-aging rates, suggesting a common origin for the evolution of the low endotherm, the creep behavior and the bulk density. The calorimetry, density, and creep data are reexamined based on the following assumptions: First, a single population of small crystals is formed during crystallization at low temperature; Second, these small crystals increase in stability under isothermal conditions, easily melt and recrystallize during heating and serve as efficient thermo-reversible cross-links to increase the conformational constraints in the residual amorphous fraction. These assumptions appear to be consistent with all observations made to date. / Master of Science

No / A precipitation process is considered in which completely miscible solution and supercritical antisolvent are passed through premixing and diluting zones of a turbulent flow. The influence of flow velocity on particle size and nuclei concentration is discussed in terms of mixing and precipitation time constants and their supersaturation dependencies. The proposed model allowed the major process parameters such as supersaturation profile, mixed fluid fraction and mean particle size to be calculated and compared with experimental data. For the crystallization system paracetamol/ethanol/CO2 studied, the supersaturation profile becomes established at Re104. The particle size and shape are defined, firstly, by increase of supersaturation and relative volume of mixed (on molecular scale) fluid with increase of flow velocity and, secondly, by decrease of residence time available for nucleation with increase of flow velocity. These competitive processes can result in minimum particle size at a defined flow rate.