Iskender Yilgor

Electrospinning of polyurethane fibers

A segmented polyurethaneurea based on poly(tetramethylene oxide)glycol, a cycloaliphatic diisocyanate and an unsymmetrical diamine were prepared. Urea content of the copolymer was 35 wt%. Electrospinning behavior of this elastomeric polyurethaneurea copolymer in solution was studied. The effects of electrical field, temperature, conductivity and viscosity of the solution on the electrospinning process and morphology and property of the fibers obtained were investigated. Results of observations made by optical microscope, atomic force microscope and scanning electron microscope were interpreted and compared with literature data available on the electrospinning behavior of other polymeric systems.

Chemical modification of matrix resin networks with engineering thermoplastics: 1. Synthesis, morphology, physical behaviour and toughening mechanisms of poly(arylene ether sulphone) modified epoxy networks

Abstract Bisphenol A-based epoxy resins were modified with either phenolic hydroxyl or aromatic amine functionally-terminated poly(arylene ether sulphone) oligomers and thermally cured with 4,4′ diaminodiphenyl sulphone. The resulting networks displayed significantly improved fracture toughness, with little sacrifice in modulus. The bisphenol A-based polysulphones were molecularly miscible with the epoxy precursors over the entire range of compositions and molecular weights investigated, but developed a two phase structure upon network formation. The molecular weights and composition of polysulphone chemically incorporated into the network were varied and their effect on several important physical properties was investigated. The dynamic mechanical analysis and scanning electron microscopy (SEM) studies showed that it is possible to generate a two-phase morphology in the cured networks wherein polysulphone composite particles are dispersed in the epoxy matrix. Despite the two-phase structure, the modified crosslinked systems are nearly transparent, due to a similarity in component refractive index values. The fracture toughness of these modified networks under plane strain conditions improved significantly with minimal sacrifice of the flexural modulus.

Chemical modification of matrix Resin networks with engineering thermoplastics

SummaryFunctionally terminated bisphenol-A polysulfone oligomers were used in the modification of Epon Resin 828/4,4′-diamino-diphenylsulfone (DDS) network system. Phenolic hydroxyl terminated PSF oligomers were first capped with a large excess of bisphenol-A diglycidyl ether or Epon Resin 828 at both ends and then the resulting system was cured with DDS, in a two-step process. During these studies molecular weight and the amount of PSF oligomers incorporated into the network were varied and their effect on the overall properties of the resulting systems were investigated. The capping and curing reactions were followed by using FT-IR and NMR spectroscopy, GPC, HPLC and DSC techniques. As a function of the oligomer molecular weight, SEM studies showed the formation of two-phase structures with ductile PSF particles dispersed in the continuous epoxy matrix. Mechanical characterization and fracture toughness measurements showed a remarkable increase in KIC or gIC values of the modified networks over that of control, without significant loss in the modulus. This work would appear to be one of the first studies where well bonded ductile glassy modifiers have significantly improved the fracture toughness of highly crosslinked networks.

Hydrogen bonding and polyurethane morphology. I. Quantum mechanical calculations of hydrogen bond energies and vibrational spectroscopy of model compounds

Abstract Advanced quantum mechanical calculations within ab initio molecular orbital theory and density functional theory were performed using gaussian 98 programs in quantitative determination of hydrogen bond (H-bond) energies between various model compound pairs. Model compounds studied contained functional groups or segments that were similar to those in segmented polyurethanes and polyureas. These model compounds included urea, 1,3-dimethylurea, 1,3-dimethylcarbamate, diethyl ether, methyl acetate and ethyl alcohol. Optimized conformations, H-bond energies and H-bond lengths of the complexes were determined. Quantum mechanical calculations indicated that based on relative magnitudes of H-bond energies, appreciable amount of phase mixing between hard and soft segments in polyether or polyester based polyurethanes and polyureas should be expected. Vibrational spectra of individual compounds and their hydrogen-bonded complexes (with themselves and other compounds) were determined. Correlation between theoretical and experimental spectra was found to be very good.

Segmented organosiloxane copolymers: 2 Thermal and mechanical properties of siloxane—urea copolymers

Abstract The structure-property behaviour of new siloxane-urea containing segmented copolymers has been investigated. Amino-propyl terminated poly(dimethylsiloxane) oligomers of from 900–3660 〈 M n 〉 were reacted with various diisocynates to form segmented copolymers with urea linkages. The length of the hard segments in these copolymers corresponds approximately to the length of the diisocynate unit employed. A number of mechanical and thermal properties were investigated for these phase separated materials. It was found that the performance of these copolymers was effected by varying the hard segment type and/or content and that high strength necessitates a microphase texture. The two phase nature of these copolymers was verified by dynamic mechanical, thermal and SAXS studies. The phase separation was found to occur in these copolymers even with 6% hard segment by weight. In conclusion, these materials displayed a behaviour similar to the segmented polyurethanes and were found to be superior to the unfilled silicone elastomers.

Hydrogen bonding and polyurethane morphology. II. Spectroscopic, thermal and crystallization behavior of polyether blends with 1,3-dimethylurea and a model urethane compound

Abstract Thermal, structural and spectroscopic behavior of the blends of poly(ethylene oxide)glycol (PEO) with a model urethane compound bis(4-butylcarbamatocyclohexyl)methane and 1,3-dimethylurea (DMU) were investigated by differential scanning calorimetry (DSC) and hot-stage optical microscopy (HOM). Blends with a wide range of compositions were prepared in tetrahydrofuran (THF) solutions and dried. DSC results indicated the formation of two-phase structures consisting of a pure polyether phase and a highly mixed DMU–polyether phase. As the amount of polyether in the blends was increased, the melting endotherm of DMU became much broader and shifted to lower temperatures, indicating extensive mixing with PEO. The mixed phase was also crystalline. This was strongly supported by HOM results. While pure PEO and DMU crystals showed spherulitic structures, mixed DMU–PEO phase showed fibrillar crystals. Consecutive heating–cooling cycles of the blends did not result in any changes in the blend morphologies. Formation of strong hydrogen bonding between DMU and PEO was also demonstrated by FTIR spectroscopy from the shifts in (N–H and CO) absorption peaks.

Structure‐Morphology‐Property Behavior of Segmented Thermoplastic Polyurethanes and Polyureas Prepared without Chain Extenders

A comprehensive review of the structure‐morphology‐property relations in segmented thermoplastic elastomers (STPE) prepared by the stoichiometric reactions of soft segment oligomers and hard segment precursors is provided. Although the main focus of this study is on linear, segmented, thermoplastic polyurethanes and polyureas, other systems such as linear segmented polyamides and polyesters are also discussed for comparison. Special emphasis is made on the influence of soft segment structure and molecular weight, hard segment symmetry and crystallinity, and the strength of the hydrogen bonding on the morphology and properties of segmented, non‐chain extended thermoplastic elastomers.

Effect of Symmetry and H‐bond Strength of Hard Segments on the Structure‐Property Relationships of Segmented, Nonchain Extended Polyurethanes and Polyureas

Segmented, nonchain extended polyurethanes and polyureas based on PTMO soft segments (SS) and hard segments (HSs) based on only single molecules of a diisocyanate were synthesized. Type and nature of the diisocyanate was systematically varied in order to analyze the effect of HS symmetry and type of linkage between the HS and SS on the structure‐property relationship of these segmented copolymers. Results showed that the increased symmetry of the diisocyanates allows a more efficient packing of the HSs which leads to a microphase‐separated structure with the crystalline hard ribbon or thread‐like domains percolated throughout the SS matrix, even with a low HS content (ca. 13 wt.%). The service window of these segmented copolymers was significantly influenced by the symmetry and type of linkage between the HS and SS. Most copolymers also showed evidence of strain hardening accented by the strain induced crystallization of the PTMO SS.

Siloxane-urea segmented copolymers

SummarySiloxane-urea copolymers were synthesized from MDI and α, ω-bis(aminopropyl)polydimethylsiloxane of different molecular weights by solution polymerization, using 2-ethoxyethyl ether as the solvent. Chain extenders were also employed in some reactions. Formation of urea linkages were followed by FTIR spectroscopy. The products were characterized by GPC chromatography and intrinsic viscosity measurements. Thermal (DSC) and thermomechanical (TMA) characterization of the products were also carried out. The results indicate the formation of novel, strong multiphase elastomeric siloxane-urea copolymers.

Structure-property behavior of new segmented polyurethanes and polyureas without use of chain extenders

Abstract New novel segmented polyurethane and polyurea copolymers have been synthesized without chain extenders and the structure-property behavior of these systems has been investigated. It is shown that by the proper choice of diisocyanate and its symmetry, one can obtain highly microphase separated systems without chain extenders and that the materials also display useful mechanical behavior. In particular, it is shown that due to the bidentate hydrogen bonding achieved in the segmented ureas, a significant modulus “service temperature window” is also obtained. It is also verified that not only can strong microphase separation be obtained with low weight fraction hard segment content (14%) but that the hard phase, which is comprised of monodisperse “single molecule” units, also displays a percolated thread-like structure throughout the dominant soft segment material — the latter being based on ca. 1000g/mol polytetramethylene oxide.