Mukerrem Cakmak

Kinetic studies of polyurethane polymerization with Raman spectroscopy

In this study, the polymerization kinetics of an uncatalyzed polyester based thermoplastic polyurethane formulation was characterized with Raman spectroscopy. Measuring the normalized scattering intensity of a band originating from the TPU diisocyanate, conversion was calculated as a function of time. Kinetic parameters obtained from these experiments correlated well with those obtained from analogous calorimetric experiments and with literature values. It was concluded that Raman spectroscopy is a powerful tool for characterizing the polymerization kinetics of polyurethanes in situ.

Flexible Nanofiber-Reinforced Aerogel (Xerogel) Synthesis, Manufacture, and Characterization

Silica aerogels are sol-gel-derived materials consisting of interconnected nanoparticle building blocks that form an open and highly porous three-dimensional silica network. Flexible aerogel films could have wide applications in various thermal insulation systems. However, aerogel thin films produced with a pure sol-gel process have inherent disadvantages, such as high fragility and moisture sensitivity, that hinder wider applications of these materials. We have developed synthesis and manufacturing methods to incorporate electrospun polyurethane nanofibers into the cast sol film prior to gelation of the silica-based gel in order to reinforce the structure and overcome disadvantages such as high fragility and poor mechanical strength. In this method, a two-stage sol-gel process was employed: (1) acid-catalyzed tetraethyl orthosilicate hydrolysis and (2) base-catalyzed gelation. By precisely controlling the sol gelation kinetics with the amount of base present in the formulation, nanofibers were electrospun into the sol before the onset of the gelation process and uniformly embedded in the silica network. Nanofiber reinforcement did not alter the thermal conductivity and rendered the final composite film bendable and flexible.

Real time development of structure in partially molten state stretching of PP as detected by spectral birefringence technique

Abstract Our main focus in this study is to investigate the deformation behavior of PP in temperature range where the PP is partially molten using a newly developed uniaxial stretching system. This system allows the real time study of the structural reorganization processes as reflected in birefringence coupled with true stress and true strain at temperature and deformation rates used in industrial film process. This instrument revealed that birefringence–stress relationship is linear beyond an initial yielding point until the onset of strain hardening beyond which negative deviation from this linearity is observed. At lower strains the films were found to continue to change significantly during holding stage leading to increase in birefringence. These changes observed in holding stage decreases with decrease of stretching speed and temperature and with increase of total strain. Increased strain rates result in destruction of crystallites that involves block rotations of these regions leading to observation of lower birefringence at faster rates. The latter process generates large amount of stretched amorphous chains that gradually convert to crystalline state during holding.

Growth habits and kinetics of crystallization of poly(ethylene 2,6-naphthalate) under isothermal and nonisothermal conditions

Abstract Poly(ethylene 2,6-naphthalate) (PEN) can be formed into the glassy state by rapid quenching, or it can form semicrystalline structure by either annealing the amorphous precursor at elevated temperature or slow cooling from the melt. During isothermal crystallization, relatively high crystallinity develops, with a time dependence described by the Avrami equation with the exponent n = 2.5. The activation energy for isothermal crystallization was determined to be 60 kcal/mol. The modified Avrami equation proposed by Velisaris was found to describe the primary and secondary crystallization reasonably well. For nonisothermal studies, PEN was crystallized by cooling at rates ranging from 1°C/min to 5°C/min from the melt. The models proposed by Ozawa and Nakamura were used to describe the nonisothermal crystallization kinetics of PEN. The exponent n and rate constant k of the nonisothermal crystallization determined by Ozawa analysis showed good agreement with those of isothermal crystallization. The Na...

Structural gradients developed in injection-molded syndiotactic polystyrene (sPS)

In this experimental research, we investigated the influence of processing history on the development of structural gradients in injection-molded syndiotactic polystyrene (sPS). The structures formed during injection molding of this slow-to-crystallize polymer are explained by the complex interplay between the mechanical history of the sample and the cooling conditions dictated by such variables as mold temperature, cavity geometry, and injection speed. When crystallized under shear, sPS exhibits unusually high preferential chain orientation along the flow direction; this orientation gradually decreases toward the core. The mechanical properties are related to the degree of orientation and crystallinity.

Effect of processing consitions on the development of morphology in clay nanoparticle filled nylon 6 fibers

The effect of melt temperature on the phase behavior and preferential orientation development in Nylon 6/montmorillonite nanocomposites were investigated at melt spinning temperatures ranging from 230° to 250°C. The fibers were found to exhibit mostly y crystalline form that is typical of Nylon 6 filled with montmorillonite nanoparticles. At higher take-up speeds α-crystals begin to appear in the crystalline phase. The presence of nanoparticles was found to impart substantial chain orientation levels even at low to moderate take up speeds reaching a plateau at moderate take up speeds. This was attributed to the increased spin line stress in the presence of nanoparticles that increase the overall viscosity due to their large contact areas with the polymer chains. This increased spinline tension was found to cause fiber breakup at moderate speeds. Increasing melt temperature from 230 to 250°C alleviated this problem.

Effect of composition on orientation, optical and mechanical properties of bi-axially drawn PEN and PEN/PEI blend films

Abstract The influence of biaxial deformation on the development of thermal, optical and mechanical properties of PEN and PEN/PEI blends were investigated. The refractive indices in the normal direction of biaxially stretched films decreased with the areal expansion ratio, λ MD × λ TD . The Wide angle X-ray results showed that this is caused by the orientation of naphthalene planes that become parallel to the film surface as the expansion ratio increases. Blending PEI with PEN at concentration up to 20% was also found to reduce the naphthalene plane orientation, thereby improving the deformation behavior of PEN by eliminating highly localized neck formation attributed to this orientation behavior. The WAXS pole figure studies further indicate that biaxially oriented PEN films show bimodal (‘cross-hatched’) orientation of the chain axes one population in the MD1 direction and another distinct population in MD2 direction. This is also reflected in the mechanical properties where moduli exhibit equal values in MD1 and MD2 directions but lower values in other directions in the film plane. When blended with PEI at 10%–20% concentrations, the bimodal orientation is eliminated and the equal biaxially stretched films was found to exhibit in-plane isotropy.

Crystal structures and morphology of thin-film, melt-crystallized, and polymerized poly(ethylene naphthalate)

Abstract Application of electron diffraction (ED) to poly(ethylene naphthalate) (PEN) crystallized isothermally from the melt at 260°C has permitted determination of the α-and β-form unit cells. While the α-form lattice parameters and cell agree well with those proposed in the literature, a different cell is proposed for the β-form (monoclinic, P21/n11 space group; 4 chains, with a = 1.304 nm, b = 0.926 nm, c = 1.300 nm, α = 131.47°, β = γ = 90°, ρ = 1.368 g/cm3). The β-form crystals were large, resembling hedrites (axialites). A number of different, single-zone ED patterns from at least one additional two-chain unit cell (γ) were found, indicating a large degree of PEN polymorphism. The γ unit cell has not yet been determined. Nascent melt-polymerized samples were also examined, giving rise to both β-and γ-type patterns. The γ-patterns arise from extended-chain single crystals of variable thickness.

Hierarchical structural gradients in injection moulded poly(ethylene naphthalene-2,6-dicarboxylate) parts

Structural development in injection moulded poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) was studied as a function of processing parameters including mould temperature, injection speed and holding time. This polymer exhibits a relatively low thermal crystallization rate and as a result quenches into wholly amorphous form when moulded into thick cavities at low mould temperatures. It, however, exhibits a three-layer structural gradient (an amorphous skin, a shear crystallized intermediate layer and an amorphous core) when moulded into thin cavities at mould temperatures up to about its cold crystallization temperature. Above this temperature, thermally activated crystallization starts playing a role particularly in the structural formation at the interior of the samples. At these temperatures the holding time in the mould becomes an important factor and as it gets longer the overall crystallinity in the sample increases. Thermal analysis results suggested that at mould temperatures up to its glass transition temperature the crystal structure of PEN consists of a mixture of ordinary α form and β that exhibits high melting temperature. Mechanical properties of injection moulded PEN were found to mostly depend on mould temperature and at high moulding temperatures on holding time.

Hierarchical structure gradients developed in injection-molded PVDF and PVDF--PMMA blends. I. Optical and thermal analysis

The structural gradients developed along and across the flow direction of injection-molded PVDF and PVDF/PMMA parts were investigated by optical microscopy and thermal analysis techniques. The spatial variation of crystallinity across the thickness direction was found to be insensitive to the process variables: injection speed and mold temperature. This relatively flat crystallinity profile across the thickness of the parts was found to decrease with the increase of PMMA concentration. The blends become noncrystallizable beyond about 40–45% PMMA concentration. The influence of flow history on the structural evolution across the thickness was observed in the peak position of the cold crystallization region. This peak temperature showed a minimum at depths where shear effects are at their maximum. This was attributed to the increased levels of chain orientation frozen in the amorphous portions of these regions which crystallize at lower temperatures upon heating.