Patent Application
20110082274
Formations of polyureas from diamines and diisocyanates have been described. Billmeyer (1984) cited aliphatic polyurea polymers, from aliphatic reactants. Though polymer fibers have long been made from synthetic materials ranging from urethanes, amides, acrylics, esters and many others, no fiber has been fabricated from a polyurea, and particularly not an aromatic polyurea. Polyurea formation chemistry and the physically hard or tough nature of its polymer products led to the widely held conclusion that these materials are intractable with respect to traditional production technology available before the 1980's.
Historically, compared with urethanes, polyureas have long been considered intractable substances from which to manufacture polymeric materials. High chemical reactivity of amines with isocyantes is difficult to control in conventional processing; but more importantly, the high crystallinity of the resultant polyurea products strictly limited further processing into useful products and materials. It was only through a series of developments, aimed initially as solutions to processing other polymer classes, that methods yielding viable polyurea materials became available.
Reporting on the melting points of various homologous polymers, Hill provided some of the earliest such data on urea-linked polymers in 1948, [Billmeyer (1984), reproduced in
Christian Weber of Bayer GmbH patented a diamine chain extender with optimal reactivity and useful for producing reaction injection molded (RIM) elastomers. The chain extender is called diethyltoluenediamine or DETDA (U.S. Pat. No. 4,218,542, issued Aug. 19, 1980), and was discovered as part of a large research effort within Bayer to find a substitute for 4,4′-methylenebis (2-chloroaniline) or MOCA. MOCA was a preferred chain extender for cast urethane polymer materials because of its aromaticity and reduced reactivity, but was classified as a carcinogen in 1973, so a replacement was sought.
Rice and Dominguez filed a patent which built on the Weber patent. This patent, issued Feb. 21, 1984 (U.S. Pat. No. 4,433,067), was the first granted in the United States claiming RIM polyurea materials. However, the principal focus of these early investigators was on development of large, elastomeric molded parts for the automotive industry. The polyether polyol-catalyst package in the Weber patent was substituted with a polyether polyamine, so no catalyst was needed. This polyurea system became the standard in the RIM industry, culminating in the Pontiac Fiero where it was used in all vertical body panels, and the front and rear bumpers. Later developments by Texaco Chemical Company in the 1980's led to spray application of polyurea coatings.
In 2004, Wilkes reported on thermal mechanical measurements from a series of homologous polyurethane and polyurea materials, with only one molecule in the hard block (respectively, meta- or para-phenylene diisocyanate), reproduced in
In contrast to urethanes, polyureas have improved thermal stability, no thermal cycle buckling or warpage, and higher tensile strength and modulus. Recent evidence has emerged that indicates polyureas are preferable for their response to blast and ballistic forces, abrasion resistance, and fuel resistance. The high CED for polyurea materials accounts for much of this behavior.
The present invention represents a progression from a monodentate hydrogen bond to a bi-dentate hydrogen bond (
The properties of para-aramid synthetic fibers (e.g. Kevlar®) are due in large part to a series of intermolecular, mono-dentate hydrogen bonds as shown in
Polyaramids can be made commercially by two practical synthetic protocols. The first is achieved by reacting an aromatic diamine with an aromatic diacid. In practice, this reaction is too slow to be commercially viable. The second method, the one used in commercial practice, is achieved by reacting an aromatic diamine with an aromatic diacid chloride. This reaction is so violent that safeguards need to be in place, and these increase the production cost by significant amounts. Both of these reactions produce by-products, water in the first and HCl in the second. These by-products, particularly HCl which is corrosive to equipment and workers alike, are the most difficult and expensive of the two to address. On the other hand, the reagents used in the investigation of the current invention for the synthesis of aromatic polyureas, aromatic diamines and aromatic diisocyanates need to be handled with care but do not pose the same threat level as an acid chloride. Also, the urea reaction is a polyaddition reaction with no by-products. Thus no expensive systems will be necessary to safeguard against accidental hazards associated with gaseous hydrochloric acid. All these characteristics of the amine-diisocyanate reaction will translate to very significant cost reductions and increased profits in the course of the large scale production of fibers.
The present invention provides a novel alternative polymer material comprising a series of intermolecular, bi-dentate hydrogen bonds.
The present invention provides a novel aromatic polyurea fiber material, and method of synthesis.
In one embodiment, the invention may comprise an aromatic polyurea fiber comprising paraphenylene-diisocyante (PPDI) and paraphenylenediamine (PPDA) linked via urea linkages to form a polymer. The number-averaged molecular weight of aromatic polyurea polymer may be between approximately 10,000 g/mol and 50,000 g/mol.
Another embodiment of the present invention provides a method of synthesizing an aromatic polyurea fiber material. In this embodiment, the method comprises the steps of adding a paraphenylene-diisocyante (PPDI) in anhydrous N-methyl-2-pyrrolidone (NMP) to a paraphenylenediamine (PPDA) and dehydrated calcium chloride to anhydrous NMP. This solution is then mixed vigorously until a change in viscosity occurs, vortexed in a great excess of ethanol, and filtered to collect the aromatic polyurea fiber.
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.
The present invention provides a novel aromatic polyurea fiber material, and method of synthesis.
In one embodiment, the invention may comprise an aromatic polyurea fiber comprising paraphenylene-diisocyante (PPDI) and paraphenylenediamine (PPDA) linked via urea linkages to form a polymer. The number-averaged molecular weight of aromatic polyurea fiber may be greater that 10,000 g/mol, preferably greater than 25,000 g/mol, most preferably greater than 50,000 g/mol.
In another embodiment, the aromatic polyurea fiber may comprise the following structure:
wherein n is approximately 50 or higher, preferrably approximately 100 or higher, most preferably approximately 200 or higher.
In an embodiment of the invention, the aromatic polyurea fiber material comprises a series of intermolecular, hydrogen bonds. In this embodiment, the hydrogen bonds may have an energy greater than 20 kJ/mol, preferably approximately 21.8 kJ/mol. In this embodiment, fibers of the material are capable of being reaction extruded, and produce a fiber with a higher stiffness than para-aramid synthetic fibers.
Another embodiment of the present invention provides a method of synthesizing an aromatic polyurea fiber material. In this embodiment, the method comprises the steps of: a) adding a paraphenylene-diisocyante (PPDI) to anhydrous N-methyl-2-pyrrolidone (NMP) to form Solution A; b) adding a paraphenylenediamine (PPDA) and dehydrated calcium chloride to anhydrous NMP to form Solution B; c) combining Solution A and Solution B to form Solution C and mixing vigorously until a change in viscosity occurs in Solution C; d) adding Solution C to anhydrous ethanol to form Solution D; and e) filtering Solution D to collect the aromatic polyurea fiber.
In one embodiment of the invention, paraphenylene-diisocyante (PPDI) may be present in Solution A at a concentration in the range of 10% to 50% by weight, based on NMP, preferably approximately 20% to 40%, most preferably in the range of 20% to 25%.
In another embodiment of the invention, paraphenylenediamine (PPDA) may be present in Solution B at a concentration of approximately 5% to 15% by weight based on NMP, preferably approximately 5% to 10%, most preferably in the range of 5% to 8%. The concentration of calcium chloride in Solution B may be approximately 10% to 40% by weight, based on NMP, preferably between approximately 20% to 30% by weight, based on NMP, most preferably 20% to 25% by weight, based on NMP.
The method of synthesis may further comprise a step of rinsing the aromatic polyurea fiber with a ketone, preferably acetone, and may also comprise the step of drying the aromatic polyurea fiber in an oven, preferably at above 30° C., most preferably at approximately 110° C.
In an embodiment of the present invention, the synthesis of an aromatic polyurea fiber material may proceed according to the reaction shown in
The reagents used to produce the desired aromatic polyurea polymer include an aromatic diamine and an aromatic diisocyante. Reagents used in the currently disclosed invention are listed in Table 1. These reagents react vigorously, resulting in an exothermic reaction. It is well known in polymer technology that maximization of physical properties is achieved only with a polymer of sufficiently high molecular weight. Three synthetic requirements are necessary to achieve this. First, purities of the reagents must be very high. The diisocyante readily sublimes and this property was used to purify it. The diamine was purchased at purity greater than 99%. Second, a suitable solvent for the reagents and subsequent polymer must be present in which to conduct the synthesis. Polymer solubility is important since the product must remain in solution in order to polymerize to a high molecular weight. Third, it is necessary to control stoichiometery, with the goal of achieving a 1:1 molar ratio.
Isocyantes were purified by sublimation, allowing separation of the essential diisocyanate from undesirable dimerization reaction products.
Subsequent preliminary efforts involved determinations of solubility of the primary reactants, p-Phenylene diamine and p-Phenylene diisocyanate, in various organic aprotic solvents, to assess their suitability as carrier media for the reaction and polymer product. These solvents included toluene, parachlorotoluene, dichloromethane, tetrahydrofuran, para-dioxane, dimethylsulfoxide, methylethylketone, n-methylpyrrolidone, and hexamethyl-phosphoramide, see Table 2. The diisocyante was soluble in all of the solvents investigated. The diamine was soluble in all but toluene and parachlorotoluene. Solubility for the diisocyante appeared greater than the diamine in all of the successful solvents, even though all solutions were restricted to 0.1M concentration. Color changes were observed upon dissolution of the diamine in most cases, but not with the diisocyante.
Initial mixture reactions were performed on a small scale to confirm that results could be observed using standard (Fourier transform infrared) FTIR spectroscopy. In this case, mixtures were made in para-dioxane with three different molar ratios of the reactants: excess isocyante, excess amine and equal molar amounts of the two reactants. Infrared spectra from these three combinations of reactant solutions are shown in “stacked” fashion in
Other than evident formation of polyurea products, the most notable finding from comparison of these spectra is that the “equi-molar” mixture actually had an excess of isocyanate. This is visible by comparison of the peak intensities at 2268 cm−1 for the three combinations of reactants. This peak in the spectrum is assigned to stretching of the isocyanate group (—N═C═O). This peak should not be present in the spectrum generated for excess amine, nor should it be present in the spectrum generated by mixtures in which all of both reactants are consumed to form product, that is from equi-molar mixtures of the two reactants. Given that reactant purities were initially no higher than about 98% and the small volumes of material used in these early tests, exact matching of molar quantities for the two reactants was understandably not achieved.
Formation of polyurea was clear in all three cases, as indicated by the strong carbonyl stretches due to Amide I (1634 cm−1) and Amide II (1554 cm−1 and 1510 cm−1) coupling vibrations. In addition to this, the strong, broad peak at 3294 cm−1 was due to hydrogen-bond associated N—H stretching. The lack of a sharp peak at 3450 cm−1, which would be due to freely stretching N—H, is predictable since virtually none of these would be present in polymers so strongly bound together by hydrogen bonds. It is not expected that the bi-dentate hydrogen-bond structure would have formed efficiently in these mixtures, since they were solution-mixed in test tubes, with turbulent stirring and shaking. The tendency of such structures would be increased by proper alignment of the polymer chains drawn in tension, as when a fiber is pulled or spun concomitant with the chemical reaction.
Molecular modeling of potential resonance vibrations through the urea linkage indicated a number of long-range coupling scenarios are feasible within the para-para polyurea material. Many of these are complex vibrations involving different combinations of torsional, or wagging motions of the nitrogen-carbonyl-nitrogen system coupled with various vibrations in the benzene rings. All are low-frequency and assignable to the diminishing cascade of peaks in the spectrum between 1300 cm−1 and 900 cm−1.
Early investigations involved differential scanning calorimetry on mixtures of finely ground powders of the reactants. One scan obtained from these activities merits discussion, shown in
In
Other initial, small-scale experiments showed that the product immediately precipitates in dichloromethane and p-dioxane, which quickly became early solvents of choice for these reactions. Investigations of the literature suggested that the solubility of the polymer product could be increased with a hydrogen bonding blocking agent, such as CaCl2, dissolved in the solvent medium before the reactants were combined. This concept is summarized in
Initial trials in n-methylpyrrolidone indicated this approach allowed the production of dark brown to amber, clear, viscous solutions and gels. Examples are shown in
When water was added to these gels, either fine precipitates or gelatinous masses formed, depending upon the rate of addition. When the reaction product mixtures was quenched in water too quickly, a gelatinous mass formed, shown in
Visual inspection of the gelatinous mass shown in the center image of
Without wishing to be bound by theory, it may be that upon exposure to water during the quench process, calcium ions are solvated and removed from their chelation positions along the polymer chain at carbonyl groups. This may allow amine hydrogens on adjacent chains to bond with the carbonyl oxygens causing the polymer to condense. Thus, quenching removes the blocking effect of calcium ions and the resultant, hydrogen-bonded polymer is not soluble in the resulting solvent mixture.
Visual appearance of the precipitates in
Sample 55 was synthesized following R. J. Gayman's protocol (No. 18, see below) with the following exceptions. The reaction was stirred with a vibrating agitator, the second component was dissolved in NMP and then added instead of being added in molten liquid form, the reaction started at room temperature and the temperature was allowed to rise naturally and the polymer was precipitated with EtOH instead of H2O. Gayman produced a polyaramid that he described as “a crumbled mass.” On the other hand, the product produced by the currently disclosed process was a viscous fluid. Since the aromatic polyurea product of the currently disclosed product should theoretically be crystalline and have a higher degree of hydrogen bonding, the physical difference between the viscous solution taught here and the teaching of Gayman is related to the difference in molecular weight of the products. The reaction in Gayman's protocol No. 18 may be kinetically more vigorous than the currently disclosed reaction. Sample 69 is made in the same manner as Sample 55, except that the mixing was done using a rotating carousel.
Sample 79 was made differently from sample 55, based on dilution of the reactants prior to mixing. The molar ratio of CaCl2 to polyurea is lower in sample 79 as compared to sample 55. Also, the diamine in sample 79 is dissolved in a larger portion of the total NMP due to its lower solubility compared to the diisocyante. This sample was also mixed on a carousel.
Comparison of the images in
Quenching with a vortexing medium having a lower dipole moment than that of water resulted in the most fibrous precipitate observed at this point in the project. Without wishing to be bound by theory, it appears that shear force imparted by the vortex on the polymer to align its chain was nearly balanced with calcium removal from carbonyl groups along the chain. Considering these results, an attempt was made to draw the fiber. In this trial, a small portion of the polymer solutions was covered by a thick layer of ethanol. Using a hooked probe, a small portion of the interface between the solution and ethanol was drawn slowly from the vessel. This resulted in a fiber mass being drawn from the vessel as shown in
After allowing this fiber to dry overnight in room conditions, photomicrographs were obtained from several segments to view its structure. In
Aromatic polyurea fiber was prepared as follows:
Table 3 provides a summary of the key polymer compositions, experimental conditions, and general results obtained after the decision to use n-methylpyrrolidone as the carrier medium and calcium chloride as the stabilizer for synthesis reactions. Table 3 is organized according to experimental sequence number in the left hand column. The second and third columns give the concentrations of diisocyanate and diamine in total n-methylpyrrolidone. Similarly, the fourth column gives the expected concentration of polymer product in the final mixture, and the fifth gives the percent excess calcium chloride. The sixth column shows the reaction temperature used when the two component solutions were mixed to form product. Visual observations on the product solution are given in the seventh column; the quench conditions are given in the eighth.
Without wishing to be bound by theory, it has been observed in the experiments reported herein that increases in molecular weight of the product may be achieved by retaining the product in solution as long as possible, slowing the addition rate of diisocyante to the solution of product and unreacted diamine, and modest, but continuous vortex mixing of the reaction medium. It appears important to ensure greater than 99% purity of the reactants and n-methylpyrrolidone, and to ensure anyhydrous conditions are maintained with respect to this solvent and the calcium chloride.
To test the effect of constant and thorough mixing on the resultant product solutions, an alternative method to the method provided in Example 4 was carried out.
In the first alternative method, experiment number 87, a “Drink Master” electric blender was used to induce a higher energy vortex than any earlier experimental procedure. All reactants were added drop-wise in the quantities described in Example 4, and after fifteen minutes a highly coagulated product resulted. At this point 50% more n-methylpyrrolidone was added to dilute the product solution so that the material could be poured or transferred. Even this solution was considered quite viscous after that dilution. In the final moments of mixing the mixer motor failed due to the highly viscous solution. A higher-power, handheld mixing drill was used to repeat the procedure with sample 93 as shown in
A second alternative method to the method provided in Example 4 was carried out.
The second alternative method, experiment number 89, involved initial dilution of the para-phenylenediamine in an effort to make subsequent dilution at the end of polymerization unnecessary. Because of the additional solvent, the reaction was easily mixable at higher energy for a longer time. However, the solution never became as viscous as in experiment number 87. This experimental procedure was repeated to ensure validity (sample 91).
Upon repeating the two alternative methods described in Example 5 and Example 6 (experiment numbers 91 and 93, respectively) two solutions that only differed in the dilution protocol were obtained. These experiments resulted in a viscosity difference between the two product solutions of approximately 8000 centi-Poise, suggesting a higher molecular weight for the first reaction was obtained (where the reactants were present at a greater mass concentration, compared to n-methyl-pyrrolidone). Thus, the amount of solvent present during initial stages of reaction has a direct effect on the viscosity and hence apparent molecular weight of the final product.
The polymeric product obtained from experimental trial number 55 (see Table 3) was subjected to thermal gravimetric analysis (TGA). TGA measures and tracks weight loss as the temperature of the sample is raised. The weight loss scale in plots of these analyses starts at 100% since the material does not decompose and begin to lose weight until higher temperatures are reached. Thus, as temperature increases, the percent remaining material decreases. This can be seen from the trace labeled “sample”, indicated by the decreasing curves in the plots of
Sudden changes in slope of these curves represent the onset of new thermal regimes that are more thermally stable than material evaporated at lower temperatures. Sample number 55, run in air (
The analysis was repeated in a nitrogen atmosphere with a second sample of experiment number 55 to determine the extent air oxidation played in this thermal decomposition. The general patterns of the weight loss and derivative traces was the same as that obtained in air, except about 25% char residue was obtained above 600° C. (see middle plot,
The same analysis was repeated with Kevlar 49® (poly paraphenylene terephthalamide). As expected, there was little evidence of thermal decompositional weight loss until temperatures above 400° C. were reached, and then weight loss was sudden and immediate. At temperatures above 600° C., approximately 20% char residue remained, when the analysis was done in nitrogen. Evidently, much of this high thermal stability in Kevlar 49® was due to the high crystallinity of the drawn fiber used to make the sample. With this in mind, the analysis was repeated with less crystalline Kevlar®, so that the results would be more reflective of the process history experienced by the aromatic polyurea fiber disclosed herein (sample 55). That is, the fiber disclosed herein had not been spun-drawn and thermally tensioned to optimize degree of crystallinity and thermal-physical properties, as had the Kevlar®. A sample of Kevlar 49® was dissolved in hot high-purity 99% sulfuric acid, followed by slow quenching in vortexing, room temperature water. The resultant fibrous mass was air dried over night and then oven dried at 100° C. for 24 hours. A sample of this post-processed, para-crystalline Kevlar 49 was then analyzed using the same thermal gravimetric procedure as above. The plotted result of the analysis in nitrogen is shown at the bottom of
Continued investigations, after synthesis of number 55 described above, were focused on increasing molecular weight of the polyurea product. Thermal analysis of the two of these experiments, number 69 and number 73, are plotted in
Following the thermal analytical assessments on fibrous precipitates described above, we next considered drawn films of the polymer product. In these cases sample films were prepared by drawing a metered edge over the product solution (in NMP) after it was poured onto a clean glass plate. The metered edge ensured a uniform thickness of solution was obtained on the glass. Afterward, the glass and polymer solution film were gently submerged in an alcohol (e.g., ethanol, n-propanol) to dissolve and remove calcium ions and the NMP. This resulted in gelation of the polymer. Gentle swirling of this combination was continued until the gelled film detached from the glass plate. Following this, the film was consecutively air dried at room temperature for 12 to 24 hours, and then at 100° C. overnight. The resultant film was brittle and variously warped due to shrinkage.
Several samples of these films were sent for structural analysis, and one sample (experiment number 79) was assessed for thermal stability by thermal gravimetric analysis. Again, residual solvent loss was observed below 230° C. However, above this temperature four sizable polymer fractions were evident from significant drops in sample mass at about 330° C., 390° C., 530° C., and 600° C. Little or no residual char remained at temperatures above 625° C. Drawing the metered edge to obtain uniform solution film thickness on the glass plate will tend to align polymer molecules in the solution. Once the metered edge passes over a particular polymer molecule it may retract to various extents, depending upon its internal tendency to coil up on itself; but this will tend to decrease with higher and higher molecular weight polymers, as a result of dispersive attractive forces between adjacent chains. Nevertheless, the pattern observed for sample number 79 in
Dynamic mechanical analysis was next performed on a sample film obtained from experiment number 79. A straight break occurred when the sample failed at about 285° C. This analysis held the sample in tension, and 1 Hz frequency was used. The plotted results of measurements of storage modulus and tan delta up to the failure temperature of the sample are shown in
Molecular weights of selected experimental polyureas in NMP solution were sent to Polymer Solutions, Inc. There, the molecular weights were measured using gel permeation chromatography against a polystyrene standard. The numerical results are summarized in Table 4. Plots of the data exhibited near normal distributions, with slight skewing toward lower weights (see
In Table 4 Mn is the number-averaged molecular weight, Mw is the weight averaged molecular weight and Mw/Mn is a measure of the spread in the distribution, known as its polydispersity. According to Billmeyer (1984), number averaged molecular weights of commercial polymers lie in the range 10,000 to 100,000, and in most cases, the physical properties associated with typical high polymers are not well developed if Mn is below about 10,000.
Interestingly, the values of polydispersity shown in Table 4 lie in the range of polymers synthesized by an autoacceleration route, such as a free radical mechanism. These are usually characterized by an increase in reaction rate with molecular weight, known as the gel effect, and this occurs when the rate limitation results from diffusion of the polymer in a viscous medium. While we do not believe the mechanism of the current polyurea forming reaction proceeds by free radical polymerization, the product solutions do become increasingly viscous over time. Without wishing to be bound by theory, it is very possible that a high degree of hydrogen bonding between the tertiary amine of the solvent (NMP) and nitrogen protons on the polymer backbone is responsible for our observed increases in viscosity, and this leads to characteristics of autoacceleration, which may be misleading in terms of the chemical mechanism.
When samples were sent to Polymer Solutions, Inc. for molecular weight measurements, they were kept suspended in solution and stabilized with calcium chloride. Samples were only diluted with additional NMP to approximately 4% by weight of product. The molecular weights reported above in Table 4 and
It is also of interest to compare our measured values of polydispersity, listed above for the aromatic polyurea fiber of the present invention, to those reported for Kevlar® (poly paraphenylene terephthalamide). Again, according to Yang (1991), Mw/Mn ranges between 2 and 3. This is roughly half of the values measured for our polyurea, indicating that its distributions in molecular weight are much wider than that achieved for Kevlar® (poly paraphenylene terephthalamide).
Molecular modeling of reactants, potential intermediates and oligomer products was conducted using HyperChem® version 5.0 from Hypercube. The purpose of these efforts was to gain insight into reaction chemistry and product properties to support conjectures that originated from experimental observations and analytical results and to build a coherent picture of the anticipated polyurea polymer derived from para-phenylene diisocyanate and para-phenylene diamine. Thus, early models were constructed to understand oligomer topology as the polymerization reaction proceeded in n-methylpyrrolidone. Later models involved potential constructions of end product structure; and these were supported, or validated, by simultaneous constructions of Kevlar® (poly paraphenylene terephthalamide) molecular structure
To achieve optimum physical properties in the currently-disclosed aromatic polyurea polymer, it is believed that molecular weight must be maximized. In such a reactive system as this, achieving high molecular weight is not necessarily a readily achievable goal. The growing molecule can quickly become entangled and knotted, and this limits access to reactive end groups by additional reactants. Thus, n-methylpyrrolidone became a logical choice as a solvent, and addition of calcium chloride as a stabilizing chelating agent.
A related aspect of the work involved close examinations of molecular geometry in the urea linkage. Simultaneous examinations of analogous Kevlar® (poly paraphenylene terephthalamide) moieties were enlightening to understand potential differences in thermal and mechanical properties of these two materials based on differences in their structure. Thus,
The polyurea is capable of bi-dentate hydrogen bonding to the carbonyl oxygens of adjacent polymer chains. The aramid is only capable of mono-dentate hydrogen bonding. What is remarkable about observations from the modeling captured in
In the next two figures potential long-range polymeric structure of the polyurea in accordance with the instant invention (
Without wishing to be bound by theory, it appears that other forms of hydrogen bonds are possible in both polymer structures; namely, hydrogen bonding between nitrogens on adjacent chains. Until now, only the possibility of intermolecular hydrogen bonding was considered, which is the modality whereby chains could be linked together in a fiber or strand of the polymer. These models also suggested the chains could become knotted and entangled when intra-molecular hydrogen bonds formed. These are more likely between nitrogen centers, due to their greater number, but also between nitrogen center and carbonyl, though formation of the latter is also further limited in possibility by geometric constraints. However, the hydrogen bond to carbonyl appears to be thermodynamically more favorable than between nitrogen centers based on these models.
In
The models shown in
A C2 symmetry element is evident in the urea linkage, while no symmetry element is present in the amide linkage. This distinction has structural implications which support the beneficial physical properties of the currently-disclosed aromatic polyurea fiber compared with those of polyaramids.
It is commonly thought that the presence of even minor symmetry within an aggregate structure increases the probability of long-range order within the aggregate. This is even more the case when the element of symmetry is repetitive. Recurrence of a symmetry element in the repeat units of a polymer has an ordering effect on the long-range spatial structure of the molecular chain. This in turn yields higher order within aggregates of the polymer, by improving dispersive contact and hydrogen bonding between the molecules. The C2 symmetry element in the urea linkage, and the absence of symmetry in a homologous amide, could therefore provide a beneficial differential in long-range order to aromatic polyureas, compared to aromatic polyamides.
This concept of a symmetry element translating into long-range structured order within a polymer might be exemplified by an analogy to liquid versus solid water. In this case, water also has a C2 symmetry element. In its liquid form, any structured order is short-ranged and transient, because the molecules have thermal energy and are free to move. In the solid phase, the symmetry becomes “locked in” and long-range order is pervasive and often evident. Considering the results of some of the computer models of polyurea, this trend also seems feasible, as shown in
Fourier Transform Infrared Spectroscopy (FTIR), proton Nuclear Magnetic Resonance Spectroscopy (NMR), Gel Permeation Chromatography (GPC) and elemental analysis were performed to characterize polymer samples from experiments 77c, 79c, and 57a, according to the present invention.
The proton NMR spectrum of Sample 77c is consistent with a small amount of p-phenylene diisocyante (PPDI) and p-phenylene diamine (PPDA) based aromatic polyurea in a large amount of N-methylpyrrolidone (NMP) solvent. The profile of the chemical shifts of polymer portion shows two broad single chemical shifts near 10 ppm [urea group, —NHC(O)NH—] and 7.5 ppm (aromatic positions). The very weak chemical shifts from end groups shown in the proton NMR spectrum are consistent with a p-phenylene amine. The approximate end Ar-amine groups in the polymer is about 12.3±1.2%.
The elemental (C, H, N, O) analysis results for Sample 57a (Table 3) and comparison with calculated element results (no end groups) are shown in Table 4.
The relative molecular weight (to polystyrene) for two liquid samples was measured with GPC, N-methylpyrrolidone (NMP) was used as the eluent. The summary of GPC analysis results are provided in Table 5.
Fourier Transform Infrared (FT-IR) Spectroscopy is a tool of choice for material identifications. In FT-IR, the infrared absorption bands are assigned to characteristic functional groups. Based on the presence of a number of such bands, a material under consideration can be identified. Availability of spectra of known compounds increases the probability of making a positive identification. Horizontal Attenuated Total Reflectance (HATR)-FT-IR probes for molecular structure at depth in polymer films.
The (HATR)-FT-IR spectrum of the ‘as received’ sample “polymer solid 57a” is provided in
NMR analysis is an important method of organic material characterization. The chemical shifts (NMR signals) of the nuclei of atoms in the molecule depend on the magnetic environment of NMR active nuclei and the local fields they experience. Since the chemical shifts of the active nuclei are determined by the local magnetic field, NMR methods provide valuable information at the atomic scale.
The proton NMR spectrum of the “as received” sample “polymer solution 77c” is provided in
Ratio of repeated polyurea over end Ar-amine group: 800/8:56/4˜100:4
End Ar-amine %: ˜14/114×100%=12.3%
1.2% deviation was reported in conclusion in consideration of deviation of integration, especially for such weak chemical shifts for the end groups.
Elemental analysis is a measurement that determines the amount (typically as weight percent) of an element in a compound. Just as there are many different elements, there are many different methods for determining elemental composition. The most common type of elemental analysis is for carbon, hydrogen, and nitrogen (CHN analysis). This type of analysis is especially useful for organic compounds (compounds containing carbon-carbon bonds).
The elemental analysis was performed on the sample “polymer solid 57a.” A combustion method was used to determine total carbon, hydrogen, and nitrogen. Pyrolysis was the method used to determine content of oxygen in this sample. The analysis was duplicated and the results are summarized in Table 6.
Gel Permeation Chromatography is used to determine the molecular weight of distribution of polymers. In GPC analysis, a solution of the polymer is passed through a column packed with a porous gel. The sample is separated based on molecular size with larger molecules eluting quicker than smaller molecules. The retention time of each component is detected and compared to a calibration curve, and the resulting data is then used to calculate the molecular weight distribution for the sample.
A distribution of molecular weights rather than a unique molecular weight is characteristic of all types of synthetic polymers. To characterize this distribution, statistical averages are used. The most common of these averages are the “number average molecular weight” (Mn) and the “weight average molecular weight” (Mw). The ratio of these two values (Mw/Mn) is referred to as the polydispersity index (PI). The larger the PI, the more disperse the molecular weight distribution is. The lowest value that a PI can have is 1, which represents a monodispersed sample—a polymer with all of the molecules in the distribution being the same molecular weight. Also sometimes included is the peak molecular weight, Mp. The peak molecular weight value is defined as the mode of the molecular weight distribution. It signifies the molecular weight that is most abundant in the distribution. This value also gives insight into the molecular weight distribution.
Most GPC measurements are made relative to a known polymer standard (usually polystyrene). The accuracy of the results depends on how closely the characteristics of the polymer being analyzed match those of the standard used. The expected error in reproducibility between different series of determinations, calibrated separately, is ca. 5-10% and is characteristic of the limited precision of GPC determinations. Therefore, GPC results are most useful when a comparison between the molecular weight distribution of different samples is made during the same series of determinations.
The summary of GPC analysis parameters and conditions are provided below:
The results are provided in Table 7. The calibration curve and MWD curves of two samples are provided in
The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/220,354, filed on Jun. 25, 2009, and to U.S. Provisional Patent Application Ser. No. 61/222,292, filed on Jul. 1, 2009, entitled NOVEL POLYUREA FIBER, the entire content of each of which is hereby incorporated by reference.
This invention was made in part during work supported by a grant from the Defense Advanced Research Projects Agency (DARPA) of the Department of Defense, in the form of an SBIR Phase I project funded by DARPA and managed under oversight from the U.S. Army Aviation and Missile Command (Contract No. W31P4Q-09-C-0120). The government may have certain rights in the invention. This document contains information which falls under the purview of the U.S. Munitions List (USML), as defined in the International Traffic in Arms Regulations (ITAR), 22 CFR 120-130, and is export controlled. It shall not be transferred to foreign nationals in the U.S. or abroad, without specific approval of a knowledgeable TR1 export control official, and/or unless an export license/license exemption is obtained/available from the United States Department of State. Release or distribution of information is restricted under the Export Control Act.
| Number | Date | Country | |
|---|---|---|---|
| 61220354 | Jun 2009 | US | |
| 61222292 | Jul 2009 | US |
