Resumen de Phase segregated copolyetherimides. Optimization of structure and composition for gas separation applications
The work described in this thesis is aimed to the development of polymeric membrane materials for the separation of CO2 from light gases. The augment of carbon dioxide concentration in the atmosphere due to the use of fossil fuels has caused a global warming that, because of its potential dangerous effects, requires energetic actions. 60 % of total CO2 emissions are produced by power generation facilities and industrial factories. Of course, an important amount of research should focus on the optimization of clean energy sources and on the more efficient use of energy. However, it is also necessary to reduce CO2 levels, so carbon capture and storage (CCS) must be considered as an urgent issue. CO2 can be captured by a variety of methods, which can be classified as post-combustion, pre-combustion and oxy-combustion ones. Among these methods, post-combustion appears as one of the most attractive alternatives. This process can be applied to different sources: power, steel, cement or petrochemical plants etc. In this process the flue gas after the combustion step must be concentrated and purified to meet the transport and storage specifications.
The role of polymeric membranes applied to gas separation is gaining importance day by day. Currently, its use is presented as a good alternative for this type of processes when compared with the processes that have been used up to now.
The amount of CO2 in the flue gas ranges from low (4%) to high (30 %) concentrations and therefore the applied technology should consider these differences. Other compounds in the flue gas are O2, H2, CO, NOx or SOx, although the most frequent is N2, which appears in the coal power plants, where CO2 concentrations are typically around 15%.
Polymers to be applied for this type of separation should have an adequate balance of permeability and selectivity. But, it is also necessary to have high gas flow, good mechanical and thermal resistance. Glassy polymers and in particular polyimides are well known for their excellent thermal oxidative stability, good organic solvent resistance and exceptional mechanical properties, along with an extraordinary ability to separate complex mixtures of gases in diverse applications.
Typically these materials have a high selectivity but they sometimes do not exhibit sufficiently high permeability. In order to increase the selectivity to CO2, it is convenient to increase the affinity of the material for this gas. One of the most common approaches to meet these requirements is the use of block-copolymers having moieties able to interact with a certain gas.
Block-copolymers can combine hard and soft blocks. The hard blocks consist in a polymer with well-packed rigid structure; while the soft segments usually contain more flexible chains. The hard segments are glassy while the soft segments behave as rubbery polymers with relatively high free volume fractions. In this way the glassy polymer segments will provide the mechanical resistance. The rubbery segments generally form continuous nanodomains with high gas permeability.
It is widely known that CO2 is highly soluble in polyethylene oxide (PEO) and thus fact has been used to separate carbon dioxide from other light gases. In view of this, the use of block-copolymers combining aromatic diamines with aliphatic ones based on PEO (Jeffamines), appears to be a promising route. These compounds have good permselectivity for the couple CO2/N2, which was attributed mainly to the high solubility-selectivity due to the existence of strong interactions between the hydrophilic and rubbery domains of the oxyethylene groups in PEO and CO2. The role of the interaction between CO2 and ethylene oxide (EO) groups in CO2 selectivity has been discussed and used for the development of new promising membranes.
In addition, it is necessary to reach a good balance between the hard and soft block segments in order to provide good gas separation balance without loss of permeability. For this reason, this research has been focused on the development of membrane materials with a high CO2 permeability. Block copolymers, specifically copoly(ether-imide)s, composed of an aromatic polyimide hard segment, which provide good mechanical properties to the films, and aliphatic ethylene oxide chains as soft segments, which have excellent CO2 separation properties, were prepared. A complete analysis of the influence of the structure and composition of the synthesized copolymers was performed.
The copolymers were synthesized by combination of a dianhydride (BPDA, BKDA or PMDA) with an aromatic amine (ODA, BNZ or PPD), and an aliphatic amine (PEO-900, PEO-2000, PEO-6000, PPO-2000, PPO-4000, RT-1000, pTHF-350 or pTHF-1700). Besides the structure of the monomers, the content of the aliphatic diamines was varied.
The synthesis of the copolymers was optimized. Thus, first, the aliphatic diamine (x mmol), and the aromatic diamine (y mmol), were mixed in different weight ratios from 1:4 to 6:1 (w/w), and dissolved in anhydrous NMP (5 mmol (x+y)/10 mL) in a 100 mL three-necked flask blanketed with nitrogen. Then, the reaction mixture was cooled down to 0 ºC, and under mechanical stirring, a stoichiometric amount of aromatic dianhydride (x+y mmol) was added and the mixture was stirred overnight at room temperature. During this time the dianhydride completely dissolved and the solution reached high viscosity.
The resultant copolyamic acid solution was diluted with NMP to the appropriate viscosity for casting, filtered through a nominal #1 fritted glass funnel, degassed, and cast onto leveled glass plate. The resulting film was covered with a conical funnel to avoid fast evaporation of the solvent, dried at 80 ºC overnight, and finally thermally treated under inert atmosphere at different temperatures. In this way, all the films prepared in this work were obtained, and eventually characterized.
By FTIR-ATR the imidization process was followed. A complete imidization was achieved at relatively low temperatures (120-160 ºC depending on the PEO content). This is an evident processing advantage respect to fully aromatic polyimides, for which very high temperatures, generally above 300 ºC, are necessary to achieve almost complete imidization.
The TGA technique allowed the analysis of the thermal stability of the samples. It was observed that it was possible to eliminate in a selective way the aliphatic part of the copolymers by controlled thermal treatment. In this way, it was possible to make partial pyrolyzed membranes (PPM) with properties comparable to that of the carbon membranes but with the stability and mechanical properties of pure aromatic polyimides.
From SAXS, the phase segregation was analyzed, and the improvement on the phase separation with the increase in the treatment temperature demonstrated. Two parameters can be calculated from these scattering curves: the relative invariant, Q¿, as the integral below the curve Iq2 vs q, which is related to the extent of the phase separation; and the maximum on the scattering curve, qmax, related to the size scale of the separated phases, calculated also from the curve Iq2 vs q. Following the changes of Q¿ and qmax at real time, it was possible to analyze the evolution of the segregation as a function of the structure and composition, and the thermal treatment.
DSC was employed in order to determine the Tg and Tm of the aliphatic part of the sample and to determine the percentage of crystalline PEO in the sample. Besides, it was possible to analyze the evolution of these parameters with the thermal treatment. No transition for the aromatic polyimide segments could be detected by DSC. However, it was possible to obtain the Tg of the aromatic block by TMA. The detection of two Tg clearly demonstrated the existence of a phase-segregated morphology in our systems. The values obtained for these parameters gave an idea of the purity of the phases. Thus, a closer value for the glass transition temperature of the aliphatic part to the value for the pure aliphatic monomer, was related to a higher purity of this phase, and consequently to a better phase separation. These results confirmed the results observed by SAXS. These copolymers were able to experiment phase segregation, and it was observed that when the rigidity of the structure was higher the ability of the copolymers to segregate increased.
Tensile tests were performed in order to determinate the mechanical properties of the samples. In general, the mechanical properties were good for all copolymers; as expected, the mechanical properties of copolymers improved when the amount of soft segments decreased.
Regarding the gas permeation properties, interesting relations with the previous properties were found. The thermal treatment increased the purity of the phases, or in other words, increased the phase segregation, and when these results were related with the permeability, it was found that permeability improved with the thermal treatment for all the gases and in all the copolymers synthesized without a significant change on the selectivity.
It was found a direct relationship between permeability and phase segregation, thus, copolymers with higher ability for segregation, that is, with more rigid structures; showed better CO2 permeability values, whilst the selectivity remained constant. This behavior was confirmed using diverse aromatic diamines and dianhydrides (with different rigidity).
When different polyether diamines were used, in terms of permselectivity, the gas performances when the gases had different polar natures followed the sequence of polarities of the aliphatic part of the copolymers. It was observed that the ability of PEO crystallization was a crucial factor when the influence of the length of PEO in permselective properties was evaluated. Thus, when the temperature of measurement for the permeability was sufficiently high (at 50 ºC the main part of the polyethylene glycol is in the amorphous state), the permeability increased with the length of the polyether.
In general, the permeability increased when the PEO percentage on the copolymer increased, without observing any effect on the selectivity. For the samples with the longer PEO (PEO-6000) it was observed a decrease due to its higher crystallization ability. However, this effect was not detected at higher measurement temperatures.
For PEO contents below 50%, the more rigid aromatic monomers produced better segregation, and in turn higher permeability values. For PEO proportions over 50%, the permeability obtained was similar regardless of the rigidity of the aromatic monomer. In this case, the permeability only depended on the PEO percentage in the polymer.
The model based on the Effective Medium Approximation (EMA) succeeded to predict the main features of the experimental results with much more accuracy than other existing models. Specifically, it was able to calculate the volume fraction for the maximum increase of permeability, a common feature for all the studied segregated copolymer membranes. In particular, the model was even able to predict the permeabilities of this kind of copolymers. The model has the advantage of being able to be adapted for three phase (or multiphase) compositions where the structure of each phase in the mixture is effectively random in nature. This fact has the added advantage of making unnecessary any consideration on the detailed phase-to-phase morphology.
When it¿s represented some of the existing data in the literature for different families of polymers and the results obtained for the copolymers prepared in this work, it was possible to observe with greater clarity the goodness of these copolymers on gas separation applications.
As main conclusion, it seems clear that these copolymers are suitable for separations where CO2 is involved and other non-condensable gases are involved, especially for the separation CO2/N2. These separations have a great importance in the remediation of greenhouse effect and the results presented in this work make these membranes good candidates for their application at industrial level.
