Hetero-Chain Thermally Stable Polymers For Laser Stereolithography
At the present time the additive technologies are attracting increasing attention of researchers around the world. It is evident from the exponential growth in the number of scientific publications in this field. According to the "Scopus®" database about 8000 articles devoted to this area of research were published over the past 5 years. New technologies and materials for all types of 3D-printing are actively developing (selective laser sintering, electron beam melting, laser stereolithography, extrusion-type printing etc.). The laser stereolithography technologies are also in the active development stage, moreover, the formation of the polymeric three-dimensional (3D) scaffolds for tissue engineering  are of the greatest interest.
Despite the attractiveness of this approach for production of the polymeric parts and units of structural purpose, at the present time application of laser stereolithography is limited only by manufacturing the prototypes and models of different devices and assemblies . Production of the finished structures based on the polymers with a given architectonic through the implementation of the "monomer – finished product" scheme is one of the strategic directions of modern material science. The recent increased interest in the given polymers is also connected with the problems encountered of new technologies, experiencing the need for materials that combine improved heat and thermal resistance with the predetermined optical, electrophysical and other properties.
It is known that most of the high-molecular weight compounds of polymerization type used in the laser stereolithography do not typically have the required thermal and heat resistance that significantly reduces the scope of their practical application at elevated temperatures. One approach to solving these problems is a radical or ionic (co) polymerization of different monomers containing the dissolved polyheteroarylenes [3–6]. In this case, the graft copolymers are produces that have significantly different properties comparing with the corresponding homopolymers and their mechanical mixtures. However, in some cases the glass transition temperature and heat resistance of obtained copolymers do not very significantly, and sometimes it leads to the reduction of these characteristics . Production of three-dimensional cross-linked structures during the radical polymerization with difunctional monomers [8, 9] allows to achieve a considerable improvement of thermal and mechanical properties of the polymeric systems. In connection with the above matter, the development of new approaches to the production of previously unknown heat-resistant high-tension three-dimensional structures with the desired properties that are easy in processing is one of the most important aspects of modern science.
In order to solve the above mentioned problems we have made the design of photoreactive hetero-chain oligomers, polymers, graphene oxide with the terminal or pendant unsaturated groups that are then used to form the photopolymer compositions with no analogues in the world. These approaches favorably differ from the traditional methods and allow the easy and efficient formation of the 3-D products with high heat resistance based on the designed compositions.
RESULTS AND ITS DISCUSSION
Synthesis Of Photopolymerisable Unsaturated Reagents
We have performed the synthesis of aromatic hetero-chain oligomers with terminal (meth) acrylamide groups based on various monomers. The two-step synthesis involves preliminary preparation of oligomers with the terminal amino groups and their subsequent reaction with (meth) acryloyl chloride (Fig. 1).
On the first stage we have obtained oligoamides and oligoimides with the terminal amino groups that were then treated with a twofold molar excess of the acid chloride of acrylic or methacrylic acid in the aprotic dipolar amide solvent medium. The yield of the reaction products, in conversion to the initial monomers was more than 90%.
A significant disadvantage of the above approaches to production of oligomers with (meth) acrylamide groups is the use of the hydrolytically unstable acid chlorides of (meth) acrylic acid. Furthermore, during the synthesis the hydrogen chloride is released that may cause corrosion of equipment. In this regard, we have developed a new approach to the synthesis of unsaturated oligomers of this type consisting in reaction of the terminal amino groups of oligomers with acrylic acid in the presence of N, N’-dicyclohexylcarbodiimide (DCC) as a condensing agent. We have previously performed the research of the model reaction of two equivalents of acrylic acid with 4,4’-diaminodiphenyloxide (DADPO). It has been found that the reaction proceeds readily at room temperature in the aprotic dipolar solvent medium (e. g., NMP) in the presence of the DCC amount equivalent to the acrylic acid. In this case, the model compound, model diacrylamide of DADPO (DAA) is formed almost with the quantitative yield within 30 minutes.
The optimal mode of synthesis of the model compound (DAA) has allowed to implement this process for modification of oligoamides and oligoimides. In both cases, the course of reaction has been rather similar to the model reaction, and the reaction products were formed with the yield of 95%.
The structure of the synthesized oligomers with terminal (meth) acrylate groups is proved by the IR spectroscopy method. The availability of absorption bands at 1370–1390, 1700–1720 and 1760–1780 cm-1 indicates the presence of the imide cycle, and at 1640–1680 cm-1 and 3240–3300 cm-1 (C=O and N-H amides, respectively) confirms the formation of amide groups. The elemental analysis data also indicate the synthesis of oligoimides with the terminal (meth) acrylamide groups.
The resulting oligomers are bright powdered solids that are readily soluble in various organic solvents, including the unsaturated active solvents (N-vinylpyrrolidone, N, N-dimethylacrylamide), forming the high concentrated (up to 50 wt.%) solutions. At the same time the availability of bridge and carder groups in monomers contributes to the better solubility of the oligomers.
All unsaturated oligoamides and oligoimides are characterized by the sufficiently high heat resistance (the weight loss start temperature at a heating rate of 5 degrees•min-1 in the air is 400–450°C).
According to the method described in the article , the reaction of 4 moles of hexamethylene-bis-maleimide with 1 mole of 5,5’-bis-benzotriazoloxide in the melt (Fig. 2) has lead to synthesis of bis-maleimide oligomer (BBTO).
Then, we have carried out a modification of an aromatic polybenzimidazole with the unsaturated allyl groups. The initial poly (2,2’-m-phenylene)–5,5’-dibenzimidazol (MPBI) was obtained with the high-temperature polycondensation of 3,3’-diaminobenzidine and isophthalic acid in the polyphosphoric acid solution at 200–220°C for 16–24 hours according to the well-known procedure . The grafting of allyl groups was carried out in two steps using NaH as the base and allyl bromide (AB) as an alkylating agent (Fig. 3).
On the first stage MPBI dissolved in NMP was treated with 0.5–2 equivalents of NaH per one basic unit of the polymer at 80°C for 4 hours. Then. allyl bromide was added to the solution, and the synthesis was performed for 10–24 hours at 80°C. The degree of modification was determined by 1H NMR spectroscopy. The experimental results are presented in Table. 1.
The 1H NMR spectrum of AB-MPBI-95 contains the signals at 4.89 (N–CH2), 5.1 and 6.08 ppm (–CH=CH2). The NMR spectrum of 13C modified MPBI there is a peak at about 47 ppm corresponding to the carbon atoms of the N–CH2 groups. AB-MPBI were also characterized by IR spectroscopy. The IR spectra have the absorption bands at ~3080 and ~2910 cm-1, confirming the presence of CH2 groups of the allyl fragments. The elemental analysis data also confirm the successful grafting of the allyl groups.
The modified MPBI as opposed to the initial polymer are soluble in chlorinated hydrocarbons (CH2Cl2, CHCl3). Moreover, due to the availability of lateral allyl groups the solubility of polymers in amide solvents is greatly improved, probably due to weakening of the intermolecular hydrogen bonds. In this case, the increase in the modification degree promotes the better solubility of the polymers.
In accordance with the tasks assigned we have implemented two approaches to modification of the materials obtained by introducing the functionalized graphene. The first approach is to obtain the dispersion of oligo-layered graphene in DMAc. The oligo-layered graphene has been previously obtained by breakage of the multilayer graphene (MLG) by ultrasound in an aqueous medium in the presence of the high molecular stabilizer. The second approach is based on the use of the allyl functionalized graphene oxide as a multifunctional crosslinking agent.
The first approach has been implemented in two stages. On the first stage MLG was subjected to deep breakage (up to the monolayer graphene sheets) in water under the action of ultrasound with the use of poly-N-vinylpyrrolidone (PVP) as a dispersion stabilizer. The resulting homogeneous dispersion was vacuum dried to obtain the stable graphene that can be easily redispersed in DMAc. The dispersion was сombined with AB-MPBI-95 and photoinitiator wt.2% (Irgacure 2959) and subjected to UV irradiation (excilamp, λ = 283 nm, power density 20 mW·sm-2). As a result we have obtained a series of composite films with different content of graphene (0.5–5%). It was found that the introduction of the graphene helps to increase the burst strength of the films. Moreover, the composite containing 5% of graphene has high specific electrical conductivity (3 Ч 10–2 Sm·sm-1).
As for the second approach, the process of its implementation involves the synthesis of graphene oxide (GO) (by oxidation of natural graphite according to the modified Hummers’ method) and its interaction with allyl isocyanate in the DMF medium. Thus, there is a conversion of the hydroxyl and carboxyl groups of GO into the amide and urethane groups (Fig.4), as evidenced by the IR spectroscopy method due to the presence of amide (1640–1680 cm-1 (C=O amides)) and urethane (1735–1700 and 1710–1690 cm-1 (C=O urethanes)) characteristic absorption bands.
It was shown that the material obtained is readily dispersible in DMAc under the action of ultrasound with synthesis of a highly concentrated (up to 10 mg·ml-1) colloidal dispersions. The resulting dispersions, similar to the above mentioned ones, can be subjected to photo-copolimerization with AB-MPBI-95 with synthesis of the strong and flexible film materials. The potential of production of the 3D products by the laser stereolithography on the basis of the obtained graphene-polymer compositions is currently being studied.
Film Materials And 3D Structures Based On The Photopolymeric Compositions
We have worked to produce the structures based on the photopolymer compositions (FPC) using the laser additive technologies. We have studied the compositions based on poly (N, N’-m-phenylene) isophthalamide (MPA), MPBI and AB-MPBI-95. The crosslinking agents were as follows: bis-maleimide oligomer (BBTO), model diacrylamides DAA (previously mentioned abbreviation "DAA") and oligoamide diacrylamide based on the molar excess of m-phenylenediamine and diacyl chloride of the isophthalic acid (DAAMPA), and the active solvent was N, N-dimethylacrylamide (DMAc). In order to prepare FPC the unsaturated oligomer was dissolved in DMAc, then the heat-resistant polymer was dissolved in the resulting homogeneous mixture then combined with wt.2% photoinitiator (the Michler’s ketone or Irgacure 2959). The FPC compositions (Table. 2) were determined primarily by the solubility of polymers and oligomers in the active solvent. Thus, the content of MPA and MPBI in the FPC did not exceed wt.20% and 10%, respectively. AB-MPBI due to the availability of the lateral allyl substituents can form the more concentrated solutions (up to 40%). In this case, the content of unsaturated oligomers varied from 50 to 150% in relation to the polymer.
In order to identify the most promising systems to obtain the 3D product by the stereolithography methods, we have initially studied the photocured film materials. In order to obtain these materials the FPC was placed between two quartz plates (with the distance of 200 mm) and subjected to UV-curing (excilamp, λ = 283 nm, power density 20 mW·sm-2). The properties of the obtained material are presented in Table 3.
It should be noted that after photocuring the films lose their ability to be dissolved in thte amide solvents. Furthermore, the DSC curves do not show any effects up to the thermal degradation start temperatures. Such data collection leads to the conclusion of the successful formation of three-dimensional polymer grids. In the case of compositions with MPA and MPBI, probably, there is formation of semi-interpenetrating polymer grids (semi-IPGs) composed of the three-dimensional and linear polymers that are not chemically related, but inseparable due to the mechanical intertweaving of chains. However, it should be noted that there is possible chemical interaction of electron-deficient C=C-bonds of the active solvent or unsaturated oligomer with the NH-groups of MPA or MPBI leading to synthesis of the grafted copolymers. The analysis conducted by the phase-contrast microscope at maximum magnification did not reveal any heterogeneous areas in the structure of these films that may be indicative of a good combination of components.
The mechanical and thermal characteristics of the materials vary within the wide limits. Despite the availability of aliphatic fragments the materials have rather high heat resistance (310–447°C) that may serve as further evidence of the formation of the crosslinked 3D polymer. As may be inferred from Table 3, the best mechanical and thermal characteristics are given to the composites based on the modified polybenzimidazole that is probably due to the higher frequency of cross-linking due to the presence of allylic groupd in the MPBI chain.
The AB-MPBI-95-DAAMPA-DMAc system (FPC-9) was selected as the model objects for the development of the grounds for synthesis of the three-dimensional structures by laser stereolithography method. In order to carry out the process of laser-induced formation of three-dimensional structures we have used the photoinitiator – the Michler’s ketone (4,4’-bis (dimethylamino) benzophenone) compatible with the developed FPC (in the amount of wt. 1%). For testing the composition during the 3D-structure formation processes using the laser additive technologies we have selected the two-photon polymerization method (2PP). The selection of this approach to the 3D-structure formation for the FPC testing was due to the possibility of controlled selection of parameters (velocity of the laser movement, parameters of the source, etc.) under which the 3D-structure is produced. Also, the 2PP testing method requires a small amount of the FPC (up to 1 g).
Testing was carried out on the 2PP unit in the Institute of Laser and Information Technologies of the Russian Academy of Sciences (Troitsk, Moscow). The laser radiating source was the second harmonic component of the ytterbium femtosecond laser "TEMA-1053/100" (Avesta-Project LLC, Russia) with a wavelength of 525 nm, pulse duration of 200 fs and frequency of 70 MHz, while the operating wavelength of the two-photon polymerization was 262 nm. The unit includes a second harmonic generator ТЕМА ASG-1048–6 with an efficiency of over 50% and a temporary broadening of second-harmonic pulse of less than 120 fs; the input polarization is horizontal; the output polarization is vertical. The pulse frequency is controlled by the acousto-optic modulator serving as an optical shutter capable of operating at frequencies above 1 MHz. The acousto-optic modulator is mounted on a moving turntable, allowing fine-tuning of the optical system. The beam then comes to the mirror that sends it up to the energy regulatory system. In order to set the laser radiation power we have used a half-wave plate mounted on a motorized rotable plate in combination with a polarizing beam splitting cube. The beam polarization is changed by rotation of this plate and the beam splitter cube redirects the part of radiation with different polarization to the head of the power meter mounted on the lateral side of the beam splitting cube and used for continuous monitoring of the laser radiation power on the sample (the power meter displays the power of the laser radiation that is not directed to the sample). The exited cell of the 3-dimensional structure was a hollow cylinder with a height of 150 microns, the external and internal diameter of 250 microns and 150 microns, respectively. On the basis of the selected cell we have provided the duplicated bilayer structures (Figure 5), with the following selected parameters: the laser power of 20 mW, velocity of sample movement of 10,000 nominal units. In these conditions the velocity of formation of the singular structure (cylinder) was 38 sec. With the increase in the sample movement velocity the produced structure was visualized only in the optical microscope. The sample had great inturgescence and foliation while cleaning the structure from the unreacted material (for that purpose we have used the 99% DMF). From the other part, the increase in the laser power was accompanied with the processes of material burning and destruction.
A series of photopolymerizable oligomers with the terminal (meth) acrylamide and maleimide groups has been synthesized. In addition, in order to improve compatibility with the photopolymerizable oligomers and improve the physical and mechanical characteristics of the final products, we have conducted the grafting of unsaturated allyl groups on the hetero-chain polybenzimidazole polymer and nanoparticles of graphene oxide.
On the basis of the compounds obtained containing the chemically reactive multiple bonds, we have developed a series of new photopolymer compositions that can be crosslinked into the three dimensional grid under the action of UV radiation and high-intensity pulsed laser radiation to obtain insoluble, heat-resistant and thermal resistant products.
Using the laser additive technologies we have been produced the 3D-structures based on the synthetic polymers and oligomers. It has been shown that the resulting products have high physical-mechanical parameters both at normal and elevated temperatures. All these results make the materials developed by us very promising for production of the irregular shape products used in critical areas of the industry by the laser additive technological methods.
Thus, we can draw a general conclusion about the prospects of formation of three-dimensional high-heat-resistant structures based on the unsaturated oligomers modified with allyl groups of polybenzimidazoles and graphene oxide by laser stereolithography method and of the undoubted advantage of this approach compared to the traditional ones.
This work was supported by the grants from the Russian Foundation for Basic Research (RFBR) No.14–29–10169 офи_м and No.16–33–00298 мол_а.