Miko Cakmak
University of Akron
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Featured researches published by Miko Cakmak.
Polymer | 1995
Miko Cakmak; Song-Wook Lee
Abstract When amorphous poly(ethylene-2,6-naphthalate) (PEN) sheets are stretched between the glass transition temperature and the cold crystallization temperature, they exhibit necking. The occurrence of this unusual neck formation above the glass transition temperature causes large local fluctuations in the thickness of samples. However, uniformity of sample thickness can be re-established when the films are stretched beyond the critical strain at which stress hardening due to crystallization starts to occur. To elucidate the mechanism of neck formation in films stretched up to 20–30°C above the glass transition temperature, a series of samples were prepared under selected conditions and were sliced in the machine direction-normal direction (MD-ND) plane. The structural variations along the necked region were analysed using optical retardation, infra-red dichroism and micro-beam X-ray diffraction techniques. The results obtained from these techniques indicate that, during the course of deformation, the naphthalene groups, which are large and highly planar, rapidly align their broad faces parallel to the surface of the flat film. This behaviour resembles a disorder-nematic order transition and occurs at highly localized regions in the sample, thereby manifesting itself macroscopically as a neck. The free-width uniaxially stretched films were found to exhibit uniplanar axial texture instead of the expected transverse isotropy as a result of this preferential orientation of the naphthalene planes.
Journal of Applied Polymer Science | 1997
Miko Cakmak; J. C. Kim
The structural evolution in fibers produced by high-speed fiber spinning of Polyethylene 2,6 naphthalene dicarboxylate (polyethylene naphthalate) was investigated. The fibers were found to remain amorphous at speeds up to 2500 m/min, and subsequent increases in speed resulted in highly oriented crystalline domains containing primarily α crystalline modification. The fibers processed at and above 3500 m/min were found to contain the β modification together with the α modification. At the highest speed investigated, 4000 m/min, the crystalline regions became disordered, and this was attributed to low deformation temperatures that accompany neck-like deformation. Constrained annealing of the fibers results in relatively unoriented crystalline structure at 500 m/min. Although the WAXS patterns of fibers spun in the 1000–2500 m/min range do not show any crystalline peaks, these fibers develop crystalline regions with significant orientation upon constrained annealing. In addition, these regions were found to have both α and β crystalline phases, indicating that not only the α but also the β phase can be grown from the oriented amorphous precursors upon constrained annealing. From our experimental results we extrapolated a value of 0.791 100% crystalline PEN. The estimated intrinsic birefringence value for the amorphous PEN is 0.75.
Polymer | 1996
C.M. Hsiung; Miko Cakmak; Y. Ulcer
Abstract In this work a Lagrangian approach was used to simulate the crystallinity gradients in injection moulded PPS. This was done through the use of markers whose thermomechanical history was traced as they moved in the cavity. The results of the simulation are compared with our earlier experimental observations. It has been found that there is a good quantitative agreement between the experimental and calculated values of gapwise (NDTD plane) and lengthwise (FDND plane) crystallinity distribution. The calculated results for morphological variables such as the thickness of crystalline layers and their overall span agreed well with the experimentally measured values.
Journal of Applied Polymer Science | 1997
Miko Cakmak; J. C. Kim
The influence of blend composition on the deformation behavior of cast amorphous PEN/PEI blends were investigated above their respected glass transition temperatures. PEN inherently shows a sharp necking phenomenon when stretched at temperatures as high as 20°C above its glass transition temperature. This was attributed to highly localized rapid alignment of naphthalene planes parallel to the surface of the films. The addition of PEI was observed to reduce this necking behavior. The neck formation completely disappears when the PEI fraction exceeds 10% in the blend. X-ray studies indicate that the increase of PEI hinders the rapid alignment of naphthalene planes parallel to the surface of the films. The presence of PEI chains in the blend was found to increase the overall friction between the polymer chains in the system and this was found to prevent the formation of highly localized necks.
Journal of Materials Chemistry | 2013
Jiao Guo; Baochau N. Nguyen; Lichun Li; Mary Ann B. Meador; Daniel A. Scheiman; Miko Cakmak
Silica aerogels are comprised of highly porous three-dimensional networks. They typically are very fragile and brittle due to the inter-particle connections in the pearl-necklace-like fractal network. This behavior prevents their wider utility. The present study aims to reinforce the silica-based gel to improve the poor mechanical strength through crosslinking the silica particles with polyimide and incorporating Lucentite STN clay into the skeletal silica–polyimide network. 3-Aminopropyltriethoxysilane (APTES) end-capped polyamic acid oligomers were first formed followed by gelation with TMOS at a range of clay concentrations to generate a silica network. The incorporation of clay leads to slightly lower BET surface area with little effect on shrinkage, porosity and density. Microscopy revealed that the aerogel preferentially grows from the edges of well dispersed clay particles while minimal growth occurs from clay surfaces. The formation of covalent bonds and hydrogen bonding through the OH functionalized clay edges is thought to enhance the connectivity with silica network and clay, leading to a substantial reinforcement effect as evidenced by an increase in modulus.
ACS Applied Materials & Interfaces | 2015
Zhe Qiang; Yuanhao Guo; Hao Liu; Stephen Z. D. Cheng; Miko Cakmak; Kevin A. Cavicchi; Bryan D. Vogt
Roll-to-roll (R2R) processing enables the rapid fabrication of large-area sheets of cooperatively assembled materials for production of mesoporous materials. Evaporation induced self-assembly of a nonionic surfactant (Pluronic F127) with sol-gel precursors and phenolic resin oligomers (resol) produce highly ordered mesostructures for a variety of chemistries including silica, titania, and tin oxide. The cast thick (>200 μm) film can be easily delaminated from the carrier substrate (polyethylene terephthalate, PET) after cross-linking the resol to produce meter-long self-assembled sheets. The surface areas of these mesoporous materials range from 240 m(2)/g to >1650 m(2)/g with these areas for each material comparing favorably with prior reports in the literature. These R2R methods provide a facile route to the scalable production of kilograms of a wide variety of ordered mesoporous materials that have shown potential for a wide variety of applications with small-batch syntheses.
Asaio Journal | 2009
Sharon Grundfest-Broniatowski; Gurkan Tellioglu; Ken S. Rosenthal; Jungmee Kang; Gabor Erdodi; Baris Yalcin; Miko Cakmak; Judith Drazba; Ana E. Bennett; Lina Lu; Joseph P. Kennedy
We have developed a replaceable bioartificial pancreas to treat diabetes utilizing a unique cocontinous amphiphilic conetwork membrane created for macroencapsulation and immunoisolation of porcine islet cells (PICs). The membrane is assembled from hydrophilic poly(N,N-dimethyl acrylamide) and hydrophobic/oxyphilic polydimethylsiloxane chains cross-linked with hydrophobic/oxyphilic polymethylhydrosiloxane chains. Our hypothesis is that this membrane allows the survival of xenotransplanted PICs in the absence of prevascularization or immunosuppression because of its extraordinarily high-oxygen permeability and small hydrophilic channel dimensions (3–4 nm). The key components are a 5–10 μm thick semipermeable amphiphilic conetwork membrane reinforced with an electrospun nanomat of polydimethylsiloxane-containing polyurethane, and a laser-perforated nitinol scaffold to provide geometric stability. Devices were loaded with PICs and tested for their ability to maintain islet viability without prevascularization, prevent rejection, and reverse hyperglycemia in three pancreatectomized dogs without immunosuppression. Tissue tolerance was good and there was no systemic toxicity. The bioartificial pancreas protected PICs from toxic environments in vitro and in vivo. Islets remained viable for up to 3 weeks without signs of rejection. Neovascularization was observed. Hyperglycemia was not reversed, most likely because of insufficient islet mass. Further studies to determine long-term islet viability and correction of hyperglycemia are warranted.
Review of Scientific Instruments | 2012
Miko Cakmak; M. Hassan; E. Unsal; C. Martins
An instrumented and highly integrated biaxial stretching system was designed and constructed to obtain true stress, true strain, and optical behavior of polymeric films during biaxial stretching. With programmable drive motors, any form of temporally varying biaxial deformation profiles, including linear, exponential, logarithmic as well as cyclic, can be applied to a square-shaped films. This machine allows the investigation of mechano-optical behavior of films under profiles captured in industrial processes. To overcome the edge effects, the samples are painted with a dot pattern that is imaged using a high speed video capture system. This system accurately determines the locations of the each dot matrix in subsequent images acquired and calculates the true strains in both directions. The in-plane optical retardation is determined using spectral birefringence method that uses polarized white light and optical spectrometer in the optical train. This is carried out automatically at less than 10 nm in retardation resolution with the light beam passing through the symmetry center of the sample. Out of plane retardation is measured with an identical optical train tilted 45° to the plane of the film with its light beam going through the same spot on the sample as 0° beam. The true stress and birefringences are calculated with the determined instantaneous thickness of the film. With this system, the stress optical behavior of PETs is determined up to very large deformation levels at moderate to high deformation rates. Beyond the initial linear stress optical behavior, these films exhibit sudden positive deviation from linearity and this start of nonlinearity was directly associated with the stress induced crystallization.
RSC Advances | 2015
Sara Liparoti; Andrea Sorrentino; Gustavo Guzman; Miko Cakmak; Giuseppe Titomanlio
It is widely accepted that mold temperature has a strong effect on the amount of molecular orientation and morphology developed in a non-isothermal flowing melt. In this work, this effect was investigated in fast and asymmetric thermal conditions. Therefore, a well-characterized isotactic polypropylene was injected in a rectangular mold cavity conditioned by a purpose developed thin electric heater. Temperature evolution on the mold surface influences the cooling rates near the surface that, in turn, reduces flow stresses and facilitates molecular relaxation. Moreover, asymmetrical thermal conditions have a strong influence on the melt flow field by changing its distribution along the cavity thickness. As a consequence, the morphology distribution of the molded samples was asymmetric and showed complex and peculiar features. It was accurately characterized by optical microscopy and FESEM analysis and compared with the orientation distribution obtained by birefringence measurements.
Journal of Applied Polymer Science | 1996
Y. Ulcer; Miko Cakmak
The influence of annealing at temperatures where thermally activated crystallization rates become significant on the structural variations in the injection-molded polyethylene naphthalate (PEN) was investigated. In these studies, two distinct shear crystallized layers are observed. The shear crystallized layer near the skin forms during the filling stage, and the second shear crystallized layer forms during the packing stage but remains optically unobservable until the annealing takes place. Upon annealing, additional layers with larger crystallite sizes form around these preformed highly oriented layers. The α and β phases were observed in both of the shear crystallized layers. The β phase is generally grown either under high deformation fields or under quiescent conditions at high temperatures.