Nonlinear optical polymers with novel benzoxazole chromophores II. Synthesis of polyurethanes with good thermal stability

Novel diol monomer having a π-conjugated benzoxazole ring, 2-(4-nitrophenyl)-6-[N,N-bis(2-hydroxyethyl)amino]benzoxazole was successfully synthesized to improve the thermally stability of nonlinear optical chromophores. Several polyurethanes bearing pendant benzoxazole chromophores were obtained by polyaddition of the diol to various aromatic diisocyanates. The thermal stability of these polyurethanes were monitored by thetrnogravimetry and IR spectroscopy, and compared with other polyurethanes having general pendant azo chromophores. The poled polyurethane films exhibited large and stable second-order nonlinear optical properties.     

Blocked polyisocyanates containing monofunctional polyhedral oligomeric silsesquioxane (POSS) as crosslinking agents for polyurethane powder coatings

Blocked polyisocyanate crosslinkers for powder coatings were synthesized using alicyclic diisocyanates (TMDI and IPDI), formic acid, (methylaminopropyl)hepta(isobutyl)Si₈O₁₂ (POSS), ?-caprolactam, dibutyltin dilaurate as well as triethylamine as catalysts. The chemical structures of these compounds were characterized by means of IR, ¹H NMR and ¹³C NMR spectroscopy. The three-dimensional surface topography and surface chemical structure of the resulting powder coatings were investigated by using confocal microscope and ATR FT-IR. The values of surface roughness parameters were calculated. The surface topography was correlated with the chemical structure of the coatings and macroscopic surface behaviour: surface free energy, abrasion resistance, hardness, adhesion to the steel surface and impact resistance. Thermogravimetric analysis was employed to assess the hardening property of powder coatings and the thermal decomposition processes.     

Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation

Photodegradation of polymer coatings generally involves photooxidation, resulting in the formation of oxidized products, chain scission, and crosslinking. On severe exposure to ultraviolet (UV) light in the presence of air, chemical degradation transforms into substantial changes in the physical and mechanical properties, leading to failures of the coatings. Systematic research by NIST on service life prediction of polymeric coatings indicates that the degradation of polymer coatings starts from the sub-micrometer degradation-susceptible regions at the surface and then grows in width and depth. Additionally, due to the oxygen diffusion effect and the attenuation of the UV light passing through the polymer, the degradation can be spatially heterogeneous. In this study, the changes with depth of the mechanical and chemical properties of a UV-exposed epoxy/polyurethane system were measured by nanoindentation and Fourier transform infrared spectroscopy (FTIR) microscopy using cross-sectioned specimens. Multilayers of epoxy/polyurethane samples were prepared by a draw-down technique. After curing, samples were exposed to the outdoors in Gaithersburg, MD, for four months. Cross-sectioned slices of the exposed and unexposed samples, approximately 500 nm thick as-prepared by microtoming, were used for micro-FTIR imaging. Samples for nanoindentation were prepared by embedding the epoxy/polyurethane multilayers (both exposed and unexposed) in a molding compound, followed by microtoming and polishing the embedded films in the thickness direction. Micro-FTIR images clearly show that, for the outdoor exposed samples, substantial amounts of oxidation products are distributed in the 60 μm deep region from the surface to the epoxy bulk, decreasing in the center of epoxy region and increasing again toward the epoxy/urethane interface. Nanoindentation results also show that the modulus significantly increases in the first 60 μm region after UV degradation, and then decreases gradually with depth until a value slightly higher than the modulus of the undegraded epoxy is reached. The modulus rises again in the region near the epoxy/urethane interface. These similarities in the depth profiles of the properties indicate the linkage between the chemical degradation and the mechanical degradation. The study clearly shows that the spatial distribution of chemical species and mechanical properties is heterogeneous in the thickness direction for polymer coatings after UV degradation. It also demonstrates that cross-sectional analysis using nanoindentation and micro-FTIR imaging techniques is a useful method to characterize the mechanical and chemical depth profiles of polymer coating degradation.

A comparative study of the structure-property behavior of highly branched segmented poly(urethane urea) copolymers and their linear analogs

The solid-state structure-property behavior of highly branched segmented poly(urethane urea) (PUU) copolymers and their linear analog was investigated. A limited study of their solution rheological behavior was also undertaken. The linear PUUs were synthesized by the two-step prepolymer method, whereas the oligomeric A₂+B₃ methodology was utilized to synthesize the highly branched materials. The soft segments (SS) were either poly(tetramethylene oxide) (PTMO) or poly(propylene oxide) (PPO). All copolymers utilized in this study, with one exception, contained 28 wt% hard segment (HS) content. DMA, SAXS, and AFM studies indicated that the linear as well as the highly branched PUUs were microphase separated. The SS T_g of the highly branched PUUs was nearly identical to that of their respective linear analogs. However, the linear copolymers exhibited broader and less temperature sensitive rubbery plateaus, both attributed to one or both of two reasons. The first is better hydrogen bonding organization of the HS phase as well as greater HS lengths than in the highly branched analogs. The second parameter is that of a potentially higher chain entanglement for the linear systems relative to the branched analogs. Tapping-mode AFM phase images confirmed the microphase morphology indicated by SAXS and DMA. Ambient temperature strain-induced crystallization was observed in the PUU based on PTMO 2040 g/mol at a uniaxial strain of ca. 400%, irrespective of the chain architecture. Stress-strain, stress relaxation, and mechanical hysteresis of the highly branched copolymers were in general slightly poorer than that of their linear analogs. Ambient temperature solution viscosity of the highly branched materials in dimethyl formamide was substantially lower that that of the linear samples of nearly equal molecular weight.     

Understanding the structure development in hyperbranched polymers prepared by oligomeric A2+B3 approach: comparison of experimental results and simulations

Structure development in highly branched segmented polyurethaneureas based on oligomeric A₂+B₃ approach was investigated by experimental studies and kinetic Monte-Carlo simulations. In both simulations and experiments, hyperbranched polymers were produced by the slow addition of A₂ onto B₃. Experimental studies showed strong influence of solution concentration on the gel point and the extent of cyclization in the polymers formed. In polymerizations conducted at a solution concentration of 25% by weight gelation took place at the stoichiometric ratio [A₂]/[B₃]=0.886. This is somewhat higher than the theoretical ratio of 0.75. In very dilute solutions, such as 5% solids by weight, no gelation was observed although the stoichiometric amount of A₂ added well exceeded the theoretical amount for gelation. Both experimental studies by size exclusion chromatography (SEC) and kinetic Monte-Carlo simulations demonstrated a gradual increase in polymer molecular weights as more A₂ is added onto B₃. This was followed by a sharp increase in the polymer molecular weight as the gel point is approached. A very similar behavior was observed for the polydispersity values of the polymers formed. Kinetic Monte-Carlo simulations performed at different cyclization ratios showed very good agreement with experimental results.     

A systematic series of ‘model’ PTMO based segmented polyurethanes reinvestigated using atomic force microscopy

Approximately 30 years after their preparation, the nanoscale morphology of a series of ‘model’ segmented polyurethane elastomers has been further elucidated using the technique of tapping mode AFM. The materials investigated are based on 1,4-butanediol extended piperazine based hard segments and employ poly(tetramethylene oxide) soft segments. The chemistry of these polyurethanes was specifically controlled in a manner which yielded monodisperse hard segments precisely containing either one, two, three, or four repeating units. Phase images obtained via AFM, for the first time, enable visual representation of the microphase separated morphology of these materials. AFM images also confirmed the presence of a spherulitic morphology, as shown several years ago using SALS and SEM. In addition, applying AFM to films of freshly prepared solution cast samples, the observed lath-like hard domains are suggested to preferentially orient with their long axis along the radial direction of the spherulites, while the respective crystalline hard segments comprising the hard domains are, in turn, preferentially oriented perpendicular to the spherulitic radius. The hard domain connectivity was found to increase with increasing percentage hard segment content of the polymers.     

Cyclodextrin polyurethanes polymerized with multi-walled carbon nanotubes: Synthesis and characterization

Insoluble cyclodextrin polymers co-polymerized with multi-walled carbon nanotubes were synthesized by polymerizing β-cyclodextrin with acid-functionalized multi-walled carbon nanotubes and diisocyanate linkers; hexamethylene- and toluene-2,4-diisocyanate. The polymers are useful in removing some organic pollutants from water, and we now report the full characterization of these polymers using infrared spectroscopy (IR), Raman spectroscopy, scanning and transmission electron microscopy (SEM and TEM) and thermal techniques such as thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC).The polymers could be synthesized as either powders or amorphous solids. Results of the IR analysis showed the presence of functional groups such as CO, CC, CH and CO, indicating that polymerization indeed took place. Characterization of the polymers by scanning electron microscopy and BET analysis showed that these polymers had a spongy appearance indicating a hierarchical pore structure. Incorporation of small amounts (<5%) of multi-walled nanotubes (MWNTs) improved the thermal stability of the polymers. This observation was further confirmed by differential scanning calorimetry (DSC) measurements.     

Isophorone diisocyanate in blocking agent free polyurethane powder coating hardeners: analysis, selectivity, quantumchemical calculations

Blocking agent free polyurethane powder coating formulations are emission free and thus comply with the highest ecological requirements. In addition the resulting coatings exhibit an excellent performance in non-yellowing light-stable applications for exterior use. Essential component of the formulations is isophorone diisocyanate (IPDI)-uretdione, an internal blocked derivative of IPDI. For the first time it is now possible to identify each of the 10 isomers by ¹³C-NMR-spectroscopy. This opportunity leads to fundamental conclusions concerning the selectivity of IPDI in the uretdione generating step and the reactivity of the different uretdione isomers in the crosslinking reaction. Quantumchemical calculations confirm the experimental results.