Prepolymer Formation Research Articles

The Formation of RNA Pre-Polymers in the Presence of Different Prebiotic Mineral Surfaces Studied by Molecular Dynamics Simulations.

We used all-atom Molecular Dynamics (MD) computer simulations to study the formation of pre-polymers between the four nucleotides in RNA (AMP, UMP, CMP, GMP) in the presence of different substrates that could have been present in a prebiotic environment. Pre-polymers are C3'-C5' hydrogen-bonded nucleotides that have been suggested to be the precursors of phosphodiester-bonded RNA polymers. We simulated wet-dry cycles by successively removing water molecules from the simulations, from ~60 to 3 water molecules per nucleotide. The nine substrates in this study include three clay minerals, one mica, one phosphate mineral, one silica, and two metal oxides. The substrates differ in their surface charge and ability to form hydrogen bonds with the nucleotides. From the MD simulations, we quantify the interactions between different nucleotides, and between nucleotides and substrates. For comparison, we included graphite as an inert substrate, which is not charged and cannot form hydrogen bonds. We also simulated the dehydration of a nucleotide-only system, which mimics the drying of small droplets. The number of hydrogen bonds between nucleotides and nucleotides and substrates was found to increase significantly when water molecules were removed from the systems. The largest number of C3'-C5' hydrogen bonds between nucleotides occurred in the graphite and nucleotide-only systems. While the surface of the substrates led to an organization and periodic arrangement of the nucleotides, none of the substrates was found to be a catalyst for pre-polymer formation, neither at full hydration, nor when dehydrated. While confinement and dehydration seem to be the main drivers for hydrogen bond formation, substrate interactions reduced the interactions between nucleotides in all cases. Our findings suggest that small supersaturated water droplets that could have been produced by geysers or springs on the primitive Earth may play an important role in non-enzymatic RNA polymerization.

A multifunctional hydrogel fabricated via ultra-fast polymerization by graphene oxide-adsorbed liquid metal nanodroplets

Graphene structures have never been found to play a role in accelerating fabrication of functional hydrogels. In this work, it is initially discovered that multifunctional hydrogels are fabricated via ultra-fast polymerization (∼minutes) by graphene oxide-adsorbed liquid metal nanodroplets (LMNPs@GO) vs. by conventional approaches (∼hours/days). LMNPs@GO are used to rapidly initiate and further cross-link polyacrylic acid (PAA) chains into a three-dimensional (3D) network without any extra molecular initiators, cross-linkers, heat source, and/or protective gas. The polymerization process with LMNPs@GO is extremely faster than that without GO involved (20 s vs. 4 h of prepolymer formation, and then 10 min vs. 3 days of crosslinking) for free radical polymerization of PAA hydrogels. The resulting hydrogel with 2 wt% reduced graphene oxide (rGO) exhibits 600% increase in tensile strength and 950% enhancement in conductivity, as well as excellent self-healing capabilities, in comparison with that of the pure PAA. The sensitivity studies show its great potential for the application of flexible sensors. Furthermore, the hydrogel possesses good dissolving properties, which is greatly beneficial for recyclability of the LM. This creative study not only broadens a novel application of graphene for making advanced multifunctional polymer materials, but also provides a brand-new route to realization of ultra-fast manufacturing technology that is significantly promising for industrial production in wearable devices.

Influence of the Surface Chemistry of Graphene Oxide on the Structure-Property Relationship of Waterborne Poly(urethane urea) Adhesive.

Small amounts—0.04 wt.%—graphene oxide derivatives with different surface chemistry (graphene oxide—GO-, amine-functionalized GO—A-GO-, reduced GO—r-GO) were added during prepolymer formation in the synthesis of waterborne poly(urethane urea) dispersions (PUDs). Covalent interactions between the surface groups on the graphene oxide derivatives and the end NCO groups of the prepolymer were created, these interactions differently altered the degree of micro-phase separation of the PUDs and their structure–properties relationships. The amine functional groups on the A-GO surface reacted preferentially with the prepolymer, producing new urea hard domains and higher percentage of soft segments than in the PUD without GO derivative. All GO derivatives were well dispersed into the PU matrix. The PUD without GO derivative showed the most noticeable shear thinning and the addition of the GO derivative reduced the extent of shear thinning differently depending on its functional chemistry. The free urethane groups were dominant in all PUs and the addition of the GO derivative increased the percentage of the associated by hydrogen bond urethane groups. As a consequence, the addition of GO derivative caused a lower degree of micro-phase separation. All PUs containing GO derivatives exhibited an additional thermal decomposition at 190–206 °C which was ascribed to the GO derivative-poly(urethane urea) interactions, the lowest temperature corresponded to PU+A-GO. The PUs exhibited two structural relaxations, their temperatures decreased by adding the GO derivative, and the values of the maximum of tan delta in PU+r-GO and PU+A-GO were significantly higher than in the rest. The addition of the GO derivative increased the elongation-at-break, imparted some toughening, and increased the adhesion of the PUD. The highest T-peel strength values corresponded to the joints made with PUD+GO and PUD+r-GO, and a rupture of the substrate was obtained.

Study on the Compressive Strength of Alkali Activated Fly Ash and Slag under the Different Silicate Structure.

Due to its high activation efficiency, waterglass has been widely used for alkali activations in geopolymer. In this study, the n(SiO2)/n(Na2O) (Ms) of waterglass was selected as the variable to investigate the role of the silicate structure on the mechanical properties of harden pastes. Ms was changed by the addition of NaOH to obtain the different silicate group, structure and experiments were performed by employing the liquid-sate 29Si nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS) and gel permeation chromatography (GPC) techniques. Furthermore, selected dissolution, scanning electron microscope (SEM-EDX), X-ray photoelectron spectroscopy (XPS) and FTIR experiments were used to measure the development of the amorphous gel and other materials with different curing condition. Results show that silicate structure of the waterglass was changed via the Si-ONa+ formation and the electric charge effect of Na+. Under the lower Ms waterglass, the Q0, Q1 and QC2 structure reverted to the main structure of the silicate group, which was kind of lower seize, molecule weight, linear or circular chain lower geopolymerization degree silicon structure. It would accelerate the geopolymerization speed of prepolymer formation. In addition, higher activity degree of Q0 and Q1 were useful to increase the formation amount of the gel structure with a low Si/Al ratio and size. Thus, silicate structure of waterglass controls the amorphous gel properties to adjust the compressive strength of alkali-activated materials.

Facile Prepolymer Formation with Ozone-Pretreated Grass Lignin by In Situ Grafting of Endogenous Aromatics

Lignin-based biopolymers are useful precursors for making epoxy resins and other adhesives. Petroleum-derived aromatics such as phenol are often grafted onto natural lignin to increase the number o...

Structure-Properties Relationship in Waterborne Poly(Urethane-Urea)s Synthesized with Dimethylolpropionic Acid (DMPA) Internal Emulsifier Added before, during and after Prepolymer Formation.

Dimethylolpropionic acid (DMPA) internal emulsifier has been added before, during and after prepolymer formation in the synthesis of waterborne poly(urethane-urea)s (PUDs) and their structure–properties relationships have been assessed. PUDs were characterized by pH, viscosity and particle size measurements, and the structure of the poly(urethane-urea) (PU) films was assessed by infra-red spectroscopy, differential scanning calorimetry, X-ray diffraction, thermal gravimetric analysis, plate–plate rheology and dynamic mechanical thermal analysis. The adhesion properties of the PUDs were measured by cross-hatch adhesion and T-peel test. The lowest pH value and the highest mean particle size were found in the PUD made by adding DMPA after prepolymer formation, all PUDs showed relatively ample mono-modal particle size distributions. The highest viscosity and noticeable shear thinning were obtained in the PUD made by adding DMPA during prepolymer formation. Depending on the stage of addition of DMPA, the length of the prepolymer varied and the PU films showed different degree of micro-phase separation. Because the shortest prepolymer was formed in the PU made with DMPA added before prepolymer, this PU film showed the lowest storage moduli and early melting indicating higher degree of micro-phase separation. The highest storage modulus, later melting, higher temperature and lower modulus at the cross between the storage and loss moduli corresponded to the PU made by adding DMPA after prepolymer formation, because the longer prepolymer produced during synthesis. The lowest thermal stability corresponded to the PU made by adding DMPA during prepolymer formation and the structures of all PU films were dominated by the soft domains, the main structural differences derived from the hard domains. Whereas DMPA-isophorone diisocyanate (IPDI) urethane and urea hard domains were created in the PU film made by adding DMPA during prepolymer formation, the other PU films showed DMPA-IPDI, polyester-IPDI and two different DMPA-IPDI-polyester hard domains. Finally, the adhesion properties of the PUDs and PU coatings were excellent and they were not influenced by the structural differences caused by adding DMPA in different stages of the synthesis.

Addition of small amounts of graphene oxide in the polyol during the synthesis of waterborne polyurethane urea adhesives for improving their adhesion properties

Mixtures of polyadipate of 1,4-butanediol polyol and different small amounts (0.01–0.10 wt%) of graphene oxide (GO) were added during the synthesis of waterborne polyurethane urea dispersions (PUDs) carried out with the acetone method. The addition of 0.02–0.04 wt% GO increased the T-peel strength of plasticized poly (vinyl chloride)/PUD/plasticized poly (vinyl chloride) joints whereas the addition of 0.05–0.10 wt% GO increased the single lap-shear strength of stainless steel/PUD/stainless steel joints. The improved adhesion of the PUDs containing GO was ascribed to the creation of covalent interactions between the surface groups on the GO surface and the NCO groups of the isocyanate during prepolymer formation, creating new urethane groups; during the phase inversion, the covalently bonded GO sheets were embedded between the polyurethane urea chains inside the PUD particles, this changed the degree of phase separation between the hard and the soft segments. When more than 0.04 wt% GO was added, some GO particles cannot be covalently bonded to the prepolymer during the PUD synthesis but they agglomerated and physically “trapped” between the polyurethane urea chains. As a consequence, the adhesion properties of the PU + GO films were different in the PU films containing 0.01–0.04 wt% GO and the ones containing 0.05–0.10 wt% GO.

Addition of Graphene Oxide in Different Stages of the Synthesis of Waterborne Polyurethane-Urea Adhesives and Its Influence On Their Structure, Thermal, Viscoelastic and Adhesion Properties.

In this study, 0.04 wt % graphene oxide (GO) was added in different stages (before and after prepolymer formation, and during water addition) of the synthesis of waterborne polyurethane-urea dispersions (PUDs) prepared by using the acetone method. The structural, thermal, mechanical, viscoelastic, surface and adhesion properties of the polyurethane-ureas (PUUs) containing 0.04 wt % GO were studied. The addition of GO before and after prepolymer formation produced covalent bonds between the GO sheets and the NCO groups of the isocyanate, whereas the GO sheets were trapped between the polyurethane chains when added during water addition step. As a consequence, depending on the stage of the PUD synthesis in which GO was added, the degree of micro-phase separation between the hard and soft segments changed differently. The addition of GO before prepolymer formation changed more efficiently the polyurethane-urea structure, i.e., the covalently bonded GO sheets disturbed the interactions between the hard segments causing lower percentage of free urethane groups, higher crystallinity, lower storage modulus, higher yield stress and T-peel strength. The interactions between the GO sheets and the polymeric chains have been evidenced by plate-plate rheology, thermal gravimetric analysis and spectroscopy. On the other hand, physical interactions between GO and the polyurethane-urea chains were produced when GO was added in water during the synthesis, i.e., GO was acting as a nanofiller, which justified the improved mechanical properties and high lap-shear strength, but poor T-peel strength.

Inverse Vulcanization Polymers with Enhanced Thermal Properties via Divinylbenzene Homopolymerization-Assisted Cross-Linking.

High-refractive-index sulfur-rich polymers with significantly improved thermal properties are prepared using divinylbenzene (DVB) as a comonomer in a modified, low-temperature inverse vulcanization with elemental sulfur. Differential scanning calorimetry and Fourier transform infrared studies reveal that under the modified inverse vulcanization conditions, homopolymerized DVB segments form, leading to high glass-transition temperatures (Tg > 100 °C) and thermal stability previously unattainable from the inverse vulcanization of bifunctional olefin comonomers. On the basis of the modified procedures, a three-step molding process of the inverse vulcanization product of DVB, poly(S-r-DVB), involving (1) prepolymer formation, (2) hot-press compression molding of the soft prepolymer, and (3) thermal annealing of the molded product is demonstrated. The molded high-sulfur-content poly(S-r-DVB) exhibits a high refractive index (n > 1.85), along with high midwave infrared transmittance. Combined with a high Tg, these properties render poly(S-r-DVB) with properties highly desirable in applications involving infrared optics.

On the Effect of Microwave Energy on Lipase-Catalyzed Polycondensation Reactions.

Microwave energy (MWe) is, nowadays, widely used as a clean synthesis tool to improve several chemical reactions, such as drug molecule synthesis, carbohydrate conversion and biomass pyrolysis. On the other hand, its exploitation in enzymatic reactions has only been fleetingly investigated and, hence, further study of MWe is required to reach a precise understanding of its potential in this field. Starting from the authors’ experience in clean synthesis and biocatalyzed reactions, this study sheds light on the possibility of using MWe for enhancing enzyme-catalyzed polycondensation reactions and pre-polymer formation. Several systems and set ups were investigated involving bulk and organic media (solution phase) reactions, different enzymatic preparations and various starting bio-based monomers. Results show that MWe enables the biocatalyzed synthesis of polyesters and pre-polymers in a similar way to that reported using conventional heating with an oil bath, but in a few cases, notably bulk phase polycondensations under intense microwave irradiation, MWe leads to a rapid enzyme deactivation.

Optimization of Self-Healing Automotive PU Coating Using Desmodure N3800 as Isocyanate and the Different Desmophens as Polyalcohols

An isocyanate agent (Desmodure N3800) and three polyalcohol agents (Desmophen 670BA, 1800 and A665, respectively) were used for formulation of the polyurethane coating with self-healing property. The effect of the chain extender agent (ethylene glycol) and the optimum ratio of the isocyanate to polyalcohol agent were studied. The obtained results show that a hard polymer coating is obtained using Desmophen A665 polyalcohol in the absence of any chain extender; however, a corrosion resistant and transparent coating is obtained using Desmophen 1800 and a ratio of NCO/OH = 2 for pre-polymer formation stage and NCO/OH = 0.85 in polymerization stage by adding proper amount of ethylene glycol. This obtained PU shows the best self-healing properties.

Synthesis, Characterization and Applications of Polyurethane Aryl Cyanate Ester Resin

Polyurethane-based aryl cyanate ester resin (PU-CER) with a biphenyl backbone obtained from polymeric 4,4‘-diphenylmethanediisocyanate, bisphenol A and cyanogen bromide was synthesized to get a polymer with better functional properties such as better adhesion and impact resistance. Synthesis of polyurethane based 4,4‘-(oxy biphenyl propane) cyanate ester involved two steps of polyurethane hydroxyl-terminated prepolymer (PU-PP) formation and esterification (PU-CER). PU-CER was cyclotrimerized to yield a triazine ring-containing polymer that possesses better adhesion at high temperature and has greater impact strength. PU-PP and PU-CER were characterized by wet chemical analysis, spectral methods and thermal methods. PU-CER showed better thermal stability and adhesion with single part polyurethane lamination adhesive and also functioned as a better toughening agent in two-part epoxy laminate system..

Preparation and Characterization of Polyurethane Microspheres Containing Theophylline

Microencapsulation of theophylline in polyurethane was developed with 4, 4'-methylenediphenyl isocyanate (MDI), castor oil (CO) and ethylene diamine as a chain extender. Polyurethane microspheres were prepared in two steps pre-polymer preparation and microspheres formation. CO, MDI and theophylline were mixed first before suspending the mixture in an aqueous medium (stirred at 4, 000rpm) of the second reaction. Fourier transform infrared (FTIR) was employed to confirm polyurethane formation during the course of reactions. SEM illustrated microspheres with uniform spherical morphology. Particle size investigation with optical microscopy revealed size distribution of 27–128 m. Controlled release experiment of theophylline was performed in phosphate buffered saline (PBS) at pH 7.4 with ultraviolet (UV) spectrometer at 274nm. Drug release profiles showed initial release of 2–40% and further release for more than 10 days.

Production and Characterization of Carbon−Silica Nanocomposites from the Pyrolysis of Polyacrylonitrile in a Porous Silica Matrix

Carbon−silica nanocomposites are fabricated from the pyrolysis of polyacrylonitrile (PAN), which has been photopolymerized in the ∼7 nm voids of porous Vycor glass. The effect of the pores on the pyrolysis process was investigated by thermal analysis. Temperatures at which key processes in the pyrolysis of bulk PAN occur are not significantly changed in the porous constraints of the Vycor, although prepolymer (nitrile cyclization) formation (∼270 °C) was split into two exotherms. The carbon that forms in the pores was found to exist as graphitic nanoparticles. In contrast to bulk pyrolyzed PAN, little amorphous carbonaceous material is formed when carbonization is carried out in the pores. Moreover, the graphitic nanoparticles were oriented in the pores with the 002 graphitic planes aligned from crystallite to crystallite. A mechanism for this orientation is proposed on the basis of how the polymer aligns in the pores when the monomer is photopolymerized. Heating to 1100 °C closed the pores around the car...

Ternary phase separation in polyurethane elastomers with immiscible soft segments

AbstractPolyurethane elastomers with two immiscible hard‐segment polyols have been synthesized from homogeneous prepolymer mixtures, to obtain incompatibility on a molecular scale from reaction driven phase separation. Miscibility of a series of different soft‐segment polyols and of their prepolymers with 4,4′‐diisocyanatodiphenyl methane (MDI) has been investigated by cloud point analysis. UCST‐behavior was observed for all binary combinations of polyols and of prepolymers; prepolymer formation lowered UCST with respect to polyols with the option to adjust it by further addition of MDI. Polyurethane elastomers have been synthesized and characterized with respect to their thermal, mechanical and viscoelastic (DMTA) behavior. Broad temperature ranges of loss factor above 0.1 were observed from polyurethanes with two immiscible soft‐segment polyols. Rebound resilience, however, was still significant at room temperature, because of the low transition temperatures of the polyols used. Introduction of a third partially miscible component, a hard‐segment from tetraethyleneglycol and MDI gave materials with a loss factor above 0.1 over a temperature range from −50 to 120 °C and higher. The studies proved that the concept of intimate mixing of immiscible soft‐segments in polyurethanes works for the design of viscoelastic materials with good damping properties.

Influence of the temperature on the formation of a phenolic resol resin catalyzed with amine

The influence of the condensation temperature on the phenolic resol resin prepolymer formation was studied by liquid chromatography and carbon nuclear magnetic resonance (13C NMR) spectroscopy. Four resols catalyzed with triethylamine and with initial pH=8.0 and F/P=1.8 were synthesized at 60, 80, 95°C, and reflux (98–102°C). On increasing the condensation temperature, reactant consumption rates and first formed addition products increased, condensation times decreased as well. Prepolymer synthesized at refluxing temperatures showed higher formaldehyde addition onto ortho positions with respect to the other resols. Different types of phenolic rings connecting bonds were observed: para, para and ortho, para bridges as well as ether bonds were detected. No ortho, ortho bonds were observed for the condensation with the catalyst used. 13C NMR measurements did not show qualitative differences between prepolymers. Anyway, residual phenol and formaldehyde concentrations and the amount of free unreacted ortho and para positions decreased with temperature.

Influence of the amount of catalyst and initial pH on the phenolic resol resin formation

The influence of the added amount of triethylamine as phenolic resol resin catalyst and that of the initial pH were studied for resols synthesized at 80°C with F/P=1.8. The evolution of phenol and first formed addition products were quantitatively followed by high-performance liquid chromatography (HPLC) and final prepolymers were analyzed by 13C NMR spectroscopy. The catalyst amount kinetically and mechanistically affected the prepolymer formation. The higher the amount of added catalyst the higher reactant consumption rates and product formation rates. The ortho directing characteristics of triethylamine for the addition of formaldehyde onto phenolic sites were observed. It could be stated that two mechanisms for the addition of formaldehyde occurred simultaneously. On the one hand, hydroxyl groups favoured the formation of phenolate ions, which would favour addition onto para phenolic positions. On the other hand, formaldehyde, phenol, and triethylamine could be involved in the formation of an intermediate transition state, favouring addition onto ortho sites, due to the ortho directing properties of triethylamine. The influence of the studied pH range did not give rise to significant differences on the resol resin prepolymer formation.

New Segmented Polyurethanes Containing Perfluoropolyether Blocks 1. Morphological Characterization

The morphology of a new class of segmented polyurethanes (PU) containing perfluoropoly-ethers soft blocks was investigated by SEM and TEM. Effective, selective staining was obtained using ruthenium tetroxide that easily reacts with PU hard phase, leaving the fluori-nated soft phase unaltered. The presence of both a micro- and a macro-phase separation was revealed. Various morphological features including shell-core structures and crystalline lamellae were detected, depending on composition and synthesis procedures. The macrophase separation was related to the immiscibility of the fluorinated macromers with the other components and to its segregation during the prepolymer formation step. The consequent formation of fluorine-rich molecules that self-segregate and the related compositional heterogeneity of the polymer chains lead to the attainment of a multiphase materials more similar to a blend than to a real statistical segmented copolymer.

Monomer sequence length distributions of hard segments of linear segmented poly(urethane–urea)s 1: chromatographic study

AbstractAn attempt was made to determine, by the chromatographic method, monomer sequence length distributions of the hard segments in segmented poly(urethane–urea)s (PURs) synthesised in a two‐step process: (1) a prepolymer formation reaction of an excess of methylenebis(4‐phenylisocyanate) (MDI) with poly(tetramethyleneoxide) (PTMO) and (2) a polymerisation reaction of the prepolymer containing unreacted MDI with ethylenediamine, giving PUR. The monomer sequence length of the hard segment was found to become longer and its distribution to broaden as the initial concentration ratio of MDI to PTMO increased. This result is attributable to an increase of the concentration ratio of the unreacted MDI to the prepolymer, leading to gelation of the polymer solution. By comparing the chromatographic results with the theoretical data calculated in the simple case of the equal reactivity of the reactants in the prepolymerisation and polymerisation reactions, respectively, the existence of unequal reactivities of the reactants in both reactions, is predicted.

The effect of reaction conditions on the urethane prepolymer formation

In the two step synthesis of urethane copolymers, most statements from the literature assume that all of the diisocyanate will be used up in the formation of the isocyanate-endcapped polyol. However, a computerized simulation indicated the formation of three kinds of product.