Structure-property behavior of poly(dimethylsiloxane) based segmented polyurea copolymers modified with poly(propylene oxide)

Poly(propylene oxide) (PPO) was incorporated in a controlled manner between poly(dimethylsiloxane) (PDMS) and urea segments in segmented polyurea copolymers and their solid state structure-property behavior was investigated. The copolymers contained PDMS segments of MW 3200 or 7000 g/mol and an overall hard segment content of 10-35 wt%. PPO segments of MW 450 or 2000 g/mol were utilized. Equivalent polyurea copolymers based on only PDMS as the soft segment (SS) component were used as controls. The materials (with or without PPO) utilized in this study were able to develop microphase morphology as determined from dynamic mechanical analysis (DMA) and small angle X-ray scattering (SAXS). DMA and SAXS results suggested that the ability of the PPO segments to hydrogen bond with the urea segments results in a limited inter-segmental mixing which leads to the formation of a gradient interphase, especially in the PPO-2000 co-SS containing copolymers. DMA also demonstrated that the polyureas based on only PDMS as the SS possessed remarkably broad and nearly temperature insensitive rubbery plateaus that extended up to ca. 175 °C, the upper temperature limit depending upon the PDMS MW. However, the incorporation of PPO resulted in more temperature sensitive rubbery plateaus. A distinct improvement in the Young's modulus, tensile strength, and elongation at break in the PPO-2000 and PDMS-7000 containing copolymers was observed due to inter-segmental hydrogen bonding and the formation of a gradient interphase. However, when PPO was incorporated as the co-SS, the extent of stress relaxation and mechanical hysteresis of the copolymers increased relative to the segmented polyureas based on the utilization of only PDMS as the soft segment component.     

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.     

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.     

Improved Adhesion of Waterborne Polyurethanes by Hybridizations

Aqueous polyurethane dispersions based on isophorone diisocyanate (IPDI), poly (tetramethylene adipate) glycol (PTAd), and dimethylolproprionic acid (DMPA) were synthesized by a prepolymer mixing process. Effects of the molecular weight of PTAd and types of hybridizations, viz. blending, semi-interpenetrating polymer network (IPN), and full IPNs with polybutylacrylate have been determined. It was found that thermal, mechanical, and adhesion properties of the polyurethane dispersions increased with increasing molecular weight of polyols.

Regarding the effects of hybridization, full IPNs gave the greatest tensile strength and elongation at break with a fast drying rate, whereas semi-IPNs gave the greatest initial as well as final adhesion, implying that a certain degree of chain mobility would augment the penetrations of adhesive molecules into the soft polyurethane foam substrates.     

Molecular modeling of the H-bonds in polyurethane with multiple donors and acceptors

The molecular mechanics (MM) method with COMPASS force field was used to study the H-bonds in polyurethane with carboxyl (PUc), which has multiple donors and acceptors. 2-Methyl-3-{[(methylamino)carbonyl]oxy}propanoic acid was used as the model molecule. It was found that the model PUc possesses four conformers with lowest energy. Considering six possible H-bond types such as OHOC(OH) (Type I), OHOH(CO) (Type II), OHOC(NH) (Type III), NHOC(OH) (Type IV), NHOH(CO) (Type V), NHOC(NH) (Type VI), in such system 192 H-bond complexes are simply expected. All the complexes were simulated in this modeling. Obtained total energies of the complexes were used to analyze the existence probability of each H-bonding configuration. The results show that for the six types of H-bonds, Types I (61%) and VI (37%) are the main H-bonding configurations in PUc, Types III and IV have the low probability (2%) and mostly coexist with other H-bond types, and Type II and V hardly exist.     

Three-dimensional compositional gradients in water-borne urethanes and latexes: spectroscopic approaches

Stratification of individual components in thermosetting coatings plays an important role during film formation and affects a number of macroscopic properties. This behavior is particularly pronounced for water-borne coatings, and therefore molecular level understanding of interactions in latexes and responsiveness of individual components during coalescence of water-borne polyurethanes are of particular interest. The presence of macromolecular arrangements and interactions among various components near the film–air (F–A) and the film–substrate (F–S) interfaces can be effectively monitored using attenuated total reflectance (ATR) and step-scan photoacoustic (SSPA) Fourier transform infrared (FT-IR) spectroscopy. Both approaches are capable of obtaining information from various surface depths and complement each other if one seeks molecular level information from 0 to 150 μm into the film. In this presentation combined ATR and PA information along with IR and/or Raman surface imaging will be presented to establish three-dimensional representation of polyurethane film formation and the effect of copolymer/polymer blend latexes on coalescence processes in Sty/n-BA.     

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.     

Polyurethanes from Alcell® lignin fractions obtained by sequential solvent extraction

Lignin-derived polyurethane films were synthesized by solution casting using three fractions of distinct molecular weight and chemical functionality from Alcell® lignin. A three-component system of lignin fraction, polyethylene glycol (PEG) of MW=400 g mol⁻¹ and polymeric methyl-diisocyanate (MDI) was used. For each lignin fraction, the effects of varying the NCO/OH ratio and lignin content on the crosslink density, ultimate tensile properties and dielectric strength of the polyurethanes were studied. The crosslink density was found to increase with molecular weight of lignin. Strong polyurethanes were produced with the high molecular weight fraction of Alcell® lignin, but the polyurethanes from the medium molecular weight fraction appeared to be tougher and more flexible. For both the medium and high molecular weight fractions of Alcell® lignin used, polyurethanes produced with lignin contents above 35 wt% were found to be too brittle and glassy to be tested. A maximum of 18 wt% of the low molecular weight lignin fraction could be used to prepare polyurethanes. Any higher compositions attempted produced polyurethanes which were too brittle to be tested. For all three fractions, the dielectric constant of the polyurethanes, decreased with increasing lignin content or NCO/OH ratio.     

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.     

Re-Mendable Polyurethanes

Novel re-mendable polyurethanes were prepared by the Diels-Alder cycloaddition reaction of urethane bismaleimides to bisfuryl monomers. The urethane bismaleimides were synthesized by an addition reaction of 4-maleimidophenylisocyanate to macrodiols, such as polycaprolactone diol (M_n ? 1250 and 2000), PEA-2000 and PBA-2000. The bisfuryl monomers were obtained by the reaction of 2-furfuryl alcohol with hexamethylene diisocyanate or by open-ring reaction of the oxirane ring from diglycidyl bisphenol A with furfuryl amine.