Langmuir 2005, 21, 1539-1546
1539
Nanoporous Thermosetting Polymers Vijay I. Raman and Giuseppe R. Palmese* Department of Chemical Engineering, Drexel University, Philadelphia, Pennsylvania 19104 Received June 29, 2004. In Final Form: October 14, 2004 Potential applications of nanoporous thermosetting polymers include polyelectrolytes in fuel cells, separation membranes, adsorption media, and sensors. Design of nanoporous polymers for such applications entails controlling permeability by tailoring pore size, structure, and interface chemistry. Nanoporous thermosetting polymers are often synthesized via free radical mechanisms using solvents that phase separate during polymerization. In this work, a novel technique for the synthesis of nanoporous thermosets is presented that is based on the reactive encapsulation of an inert solvent using step-growth cross-linking polymerization without micro/macroscopic phase separation. The criteria for selecting such a monomerpolymer-solvent system are discussed based on FTIR analysis, observed micro/macroscopic phase separation, and thermodynamics of swelling. Investigation of resulting network pore structures by scanning electron microscopy (SEM) and small-angle X-ray scattering following extraction and supercritical drying using carbon dioxide showed that nanoporous polymeric materials with pore sizes ranging from 1 to 50 nm can be synthesized by varying the solvent content. The differences in the porous morphology of these materials compared to more common free radically polymerized analogues that exhibit phase separation were evident from SEM imaging. Furthermore, it was demonstrated that the chemical activity of the nanoporous materials obtained by our method could be tailored by grafting appropriate functional groups at the pore interface.
Introduction Nanoporous polymers are used as templates for the synthesis of nanostructured materials,1 polymer electrolytes in electrochemical cells,2,3 separation membranes,4 and substrates in the design of nanocomposites.5 Two fundamental phenomena that form the basis for the application of nanoporous polymers are the diffusion of permeants within the pores and their adsorption onto the pore surfaces. The diffusion characteristics of a given permeant depend on the pore size, shape, and tortuosity of the nanoporous polymer, and the adsorption depends on the pore surface chemistry/activity. Therefore, design of nanoporous polymeric materials entails tailoring the pore size and pore surface chemistry/activity. A novel method of designing nanoporous thermosets in which pore size and pore activity can be tailored is discussed in this paper. Both physical and synthetic methods have been reported in the literature for designing nanoporous polymers. These methods are broadly classified into three categories including (i) methods with no pore-generating agents (porogen), e.g., track etching,6-7 (ii) methods that use templates, e.g., micellar imprinting,8-9 nanotem* To whom correspondence may be addressed: Department of Chemical Engineering, Drexel University, 32nd and Chestnut Streets, Philadelphia, PA 19104; telephone 215 895-5814; fax 215 895-5837; e-mail
[email protected]. (1) Chakaravarti, S. K.; Vetter, J. Radiat. Meas. 1998, 29 (2), 149159. (2) Peled, E.; Livshits, V.; Duvdevani, T. J. Power Sources 2002, 106, 245-248. (3) Liu, Y.; Lee, J. Y.; Kang, E. T.; Wang, P.; Tan, K. L. React. Funct. Polym. 2001, 47, 201-213. (4) Li, T. D.; Gan, L. M.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1996, 12, 5863-5868. (5) Baradie, B.; Dodelet, J. P.; Guay, D. J. Electroanal. Chem. 2000, 489, 101-105. (6) Apel, P. Radiat. Meas. 2001, 34, 559-566. (7) Apel, P.; Blonskaya, I. V.; Orelovitch, O. L.; Root, D.; Vutsadakis, V. A.; Dmitriev, S. N. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 209, 329-334. (8) Zhu, X. X.; Banana, K.; Yen, R. Macromolecules 1997, 30, 30313035. (9) Zhu, X. X.; Banana, K.; Liu, H. Y.; Krause, M.; Yang, M. Macromolecules 1999, 32, 277-281.
plates,10 molecular imprinting,11 and self-assembled nanostructures,12,13 and (iii) methods that use solvent as a porogen, where the nanoporous morphologies are formed due to phase separation mechanisms induced in a system of polymer-solvent or monomer-polymer-solvent. This last category can be further subdivided into (a) nonreactive and (b) reactive systems. In nonreactive systems, phase separation is induced in a polymer-solvent mixture by changing the temperature14,15 (thermally induced phase separation, TIPS) or by diffusing a nonsolvent/solvent in and out of the system16 (diffusion-induced phase separation, DIPS). In the case of reactive systems, the phase separation is induced by the polymerization reaction (polymerization-induced phase separation, PIPS) as the resulting polymer reduces the solubility of the system. Generally, nanoporous thermosets are synthesized using such systems that exhibit phase separation during reaction.17-20 Kabra et al.21 used TIPS technique along with cross-linking polymerization to lock in the microstructure induced by phase separation. In such systems, the final pore size and structural distribution of the material depend on the thermodynamic path that the system follows. Thus, even small deviations in processing conditions can lead to vastly different porous struc(10) Ma, M.; Li, D. Chem. Mater. 1999, 11, 872-874. (11) Piletsky, S. A.; Panasyuk, T. L.; Piletskaya, E. V.; Nicholls, I. A.; Ulbricht, M. J. Membr. Sci. 1999, 157, 263-278. (12) Zalusky, A. S.; Olayo-Valles, R.; Wolf, J. H.; Hillmyer, M. A. J. Am. Chem. Soc. 2002, 124, 12761-12773. (13) Drzal, P. L.; Halasa, A. F.; Kofinas, P. Polymer 2000, 41, 46714677. (14) Tsai, F.; Torkelson, J. M. Macromolecules 1990, 23, 775-784. (15) Tsai, F.; Torkelson, J. M. Macromolecules 1990, 23, 4983-4989. (16) Stephan, A. M.; Teeters, D. Electrochim. Acta 2003, 48, 21432148. (17) Svec, F.; Frechet, J. M. J. Chem. Mater. 1995, 7, 707-715. (18) Viklund, C.; Svec, F.; Frechet, J. M. J.; Irgum, K. Chem. Mater. 1996, 8, 744-750. (19) Cooper, A. I.; Holmes, A. B. Adv. Mater. 1999, 11 (15), 12701274. (20) Al-Muhtaseb, S. A.; Ritter, J. A. Adv. Mater. 2003, 15 (2), 101114. (21) Kabra, B. G.; Gehrke, S. H.; Spontak, R. J. Macromolecules 1998, 31, 2166-2173.
10.1021/la048393t CCC: $30.25 © 2005 American Chemical Society Published on Web 01/08/2005
1540
Langmuir, Vol. 21, No. 4, 2005
Raman and Palmese
Figure 1. Materials used to synthesize nanoporous gels using step-growth polymerization.
Figure 2. Materials used to synthesize nanoporous gels using free radical polymerization.
tures,20,22 and controlling the pore size is challenging in methods involving phase separation. In addition, polymerization in these systems is mostly carried out using free radical mechanisms;17-19 in such cases, the molecular weight between cross-links in the network is affected by a number of factors including the initial monomer ratio, the intrinsic reactivity ratios, and diffusion limitations which tend to affect polymerization behavior as a function of time. Thus, the polymer networks that form via free radical polymerization can be highly heterogeneous with respect to molecular weight between cross-links. In our method, nanoporous thermosets are synthesized by reactive encapsulation of an inert solvent using a step-growth cross-linking polymerization reaction carried out to completion without micro-/macroscopic phase separation.23 This terminology, “reactive encapsulation of a solvent”, or RES, is descriptive of encapsulation at the molecular level. The added condition of miscibility throughout polymerization suggests that for cross-linking systems, the network will encapsulate the solvent during reaction forming nanoporous polymer networks whose typical pore dimensions can be controlled by the amount of solvent. So, potentially, better pore size control can be obtained by using a miscible system. Furthermore, unlike free radical polymerization, step growth polymerization yields homogeneous network structure with respect to molecular weight between crosslinks. Experimental Section Synthesis of Nanoporous Gels by Step-Growth Polymerization. The step-growth polymerization of epoxy-amine resins was carried out in the presence of tetrahydrofuran (THF) as the solvent. The materials used are shown in Figure 1. The difunctional epoxy resin was diglycidyl ether of bisphenol A (DGEBA), EPON 828 (n ) 0.13), purchased from MillerStephenson Chemical Co. The tetrafunctional amine was 4,4′(22) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C.; Pey, K. L. Langmuir 1996, 12, 319-324. (23) Raman, V. I.; Palmese, G. R. Colloids Surf., A 2004, 241, 119125.
methylenebiscyclohexanamine, PACM, purchased from Air Products and Chemicals. THF was purchased from SigmaAldrich. Gels were synthesized by mixing stoichiometric quantities of epoxy and amine (2:1 mole ratio, as each amine reacts with two epoxy) with varying amounts of solvent as discussed in the subsequent discussion and reacting at 60 °C. The reactions were carried out in sealed 20 mL vials. Other solvents such as dimethylformamide (DMF) and acetone were also evaluated for their suitability. DMF and acetone were purchased from SigmaAldrich. Synthesis of Nanoporous Gels by Free Radical Polymerization. The free radical polymerization of vinyl ester resin was carried out in the presence of THF as the solvent. The materials used in this method are shown in Figure 2. The vinyl ester resin was synthesized by reacting methacrylic acid with the epoxy groups of EPON 828 and the resin thus obtained was designated as VE 828. Gels were synthesized by mixing VE 828 with varying amounts of solvent, and to this mixture 1 wt % of thermal initiator Trigonox 239A (45% cumyl hydroperoxide) was added along with the accelerator cobalt naphthanate, CoNap (6% metal content). A 4:1 ratio of Trigonox to CoNap was used. The chemical structures of Trigonox and CoNap are reported elsewhere.24 Trigonox was purchased from Akzo Nobel Chemicals, Inc., Chicago, IL, and CoNap was purchased from OMG Americas, Inc., Cleveland, OH. The reaction was carried out at 60 °C and the reaction mixture turned turbid during polymerization indicating micro/macroscopic phase separation. Fourier Transform Infrared (FTIR). FTIR spectroscopy in the near-infrared (NIR) region was used to monitor the reactions in the presence of the solvent. A Nexus 670/870 FTIR spectrometer (Thermo Nicolet Corp.) was used. The step-growth polymerization of the epoxy-amine was monitored by following the epoxy peak at 4528 cm-1, the primary amine peak at 4925 cm-1, and the primary and secondary amine peak at 6510-6470 cm-1. The free radical polymerization of the VE 828 system was monitored by following the vinyl group peak at 6160 cm-1. FTIR analysis was used to monitor the cross-linking reactions in order to ensure adequate conversion and to compare the reaction time of both these systems at 60 °C. In addition, FTIR was also used to monitor the increase in hydrophilicity after tailoring the pore environment using hydrophilic grafts. The grafted and ungrafted (24) Ziaee, S.; Palmese, G. R. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 725-744.
Nanoporous Thermosetting Polymers
Langmuir, Vol. 21, No. 4, 2005 1541
Figure 3. Pictures of the reaction mixtures of epoxy-amine-acetone, epoxy-amine-DMF, and epoxy-amine-THF (3:1 solvent/ monomers weight ratio) systems, showing that the system with acetone phase separates whereas systems with THF and DMF remain miscible through out the polymerization reaction. samples were immersed in water and allowed to equilibrate. NIR spectra of these water-saturated samples were taken. The water peak (5240 cm-1) to reference peak (4620 cm-1) height ratio was used to measure the hydrophilicity of the sample. Scanning Electron Microscopy and Small-Angle X-ray Scattering. The pore structures of the gels synthesized were investigated using scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) after drying the samples under supercritical conditions to maintain the morphology of the pores. The critical point dryer used was purchased from SPI supplies, West Chester, PA. An environmental field emission scanning electron microscope (ESEM), model XL30 ESE FEG, was used. The fractured surfaces of the supercritically dried samples were evaluated using SEM. Samples were gold sputtered prior to SEM analysis. The SAXS data were collected using a Bruker Hi-Star 2D multiwire area detector and a modified AntonPaar HR-PRK small-angle camera. X-rays were generated using a Rigaku Ultrax18 rotating anode generator operated at 2.4 kW with a Cu anode. A pyrolytic graphite monochromator selects only Cu KR radiation, λ ) 1.5418 Å. The sample to detector distance was approximately 65 cm. Two-dimensional data were azimuthally averaged and corrected for background noise, detector noise, and sample transmission.
Results and Discussion Selection of Materials for the System. The selection of a step-growth monomer-polymer system and a solvent is important for synthesizing nanoporous thermosets by the RES technique. In this work, the step-growth polymerization of the epoxy-amine system shown in Figure 1 was used. There are two important criteria for the selection of a suitable solvent; they are (i) the miscibility criteria, which requires that the solvent be miscible with the monomers and the resulting polymer network, and (ii) the homogeneous network structure criteria, which requires that the solvent remain chemically inert during polymerization, otherwise, the solvent will influence the molecular weight between cross-links by chemical modification and will affect the homogeneous nature of the networks that could be formed by the step-growth polymerization. Polar solvents such as acetone, dimethylformamide (DMF), and tetrahydrofuran (THF) were evaluated with respect to these criteria. Miscibility. The miscibility of acetone, DMF, and THF was tested by mixing stoichiometric quantities of EPON 828-PACM monomers with the solvent (3:1 solvent/
monomers weight ratio) in sealed vials and observing the reaction mixture for micro/macroscopic phase separation at a reaction temperature of 60 °C. Figure 3 shows the reaction mixtures of these three systems before and after polymerization. The system with acetone phase separates upon polymerization and therefore cannot be used, whereas the systems with DMF and THF do not show micro/macroscopic phase separation during this reaction. Both of these solvents were further tested for their chemical inertness. Homogeneous Network Structure. The chemical inertness of the proposed solvents was evaluated by monitoring the epoxy-amine reaction in the presence of the solvent using FTIR spectroscopy. Stoichiometric quantities of epoxy and amine were mixed with the solvent (0.5:1 solvent/monomers weight ratio) and reacted at 60 °C. Figure 4 shows the spectra of the reaction mixture with THF at time t ) 0 and 4000 min. The fact that all epoxy, primary amine, and secondary amine moieties react to completion, as shown by the disappearance of the epoxy peak at 4530 cm-1, the primary amine peak at 4930 cm-1, and the primary and secondary amines peak at 6510 cm-1, indicates that the reaction taking place is the desired reaction and that THF does not react with epoxy or amine functional groups since such reaction of THF would result in incomplete reaction of epoxy or amine. Thus, THF is chemically inert and satisfies the homogeneous network structure criteria. Similar experiments with DMF showed complete reaction of amine and incomplete epoxy reaction indicating that DMF reacts with amine curing agent and hence cannot be used. Thus, the epoxy-amine-THF system satisfies both the criteria required by the RES technique and represents a novel system used in this work to synthesize nanoporous materials. Epoxy-Amine-THF Miscibility. The swelling behavior of the final cross-linked polymer in THF can be useful in understanding the miscibility/solubility effects of the epoxy-amine-THF system. For example, the cross-linked polymer synthesized without any solvent by the stepgrowth polymerization of the DGEBA-PACM system swelled 40% of its weight in THF. In contrast, the crosslinked polymer obtained by the homopolymerization of DGEBA exhibited minimal swelling (∼1%) in THF. The thermodynamics of swelling can be used to explain the
1542
Langmuir, Vol. 21, No. 4, 2005
Raman and Palmese
Figure 5. Schematic of reaction steps involved in the stepgrowth polymerization of epoxy-amine and homopolymerization of epoxy, showing that unlike hompolymerization stepgrowth chemistry results in a polar OH group that could interact with THF via hydrogen bonding.
Figure 4. NIR spectra of the reaction mixture with 0.5:1 THF/ monomers weight ratio at 60 °C at time ) 0 and 4000 min, showing that the stoichiometric reaction of epoxy-amine goes to completion in the presence of THF as seen by the disappearance of the epoxy peak at 4530 cm-1, the primary amine peak at 4930 cm-1, and the primary and secondary amines peak at 6510 cm-1.
notable difference in the THF uptake of these systems. Equation 1 gives the change in Gibbs free energy (∆G) that takes place during the swelling of these polymer networks
∆G ) ∆Hmix - T∆Smix + T∆Selastic
(1)
The equilibrium solvent uptake of the final cross-linked polymer depends on the chemical attributes and the flexibility of the polymer network. The entropy of mixing (∆Smix) between the polymer network and the solvent has to overcome the elastic resistance (∆Selastic) offered by the network and the enthalpy of mixing (∆Hmix) due to the interactions between solvent and the network. The enthalpy of mixing could be favorable (∆Hmix < 0) or unfavorable (∆Hmix > 0) depending on the types of interactions. The elastic resistance depends on the rigidity of the network structure. The elastic rubbery modulus of the polymer networks obtained via step-growth and hompolymerization of epoxy are not significantly different. Hence, the significant difference in the solvent uptake due to swelling in these systems cannot be explained in terms of the elastic resistance offered by these networks. The difference in the swelling behavior of these systems could be due to the difference in the interaction effects of the polymer network with the solvent. During the epoxyamine reaction, there is a loss of amine hydrogen and a concomitant generation of hydroxyl (OH) groups as shown in Figure 5, and thus the final cross-linked polymer obtained through this polymerization method has polar
OH groups equal in concentration to the initial concentration of epoxy groups. This is not the case with homopolymerization of epoxies wherein the glycidyl oxygen becomes an ether linkage as shown in Figure 5. The OH group in the polymer obtained via step-growth polymerization interacts with the oxygen in THF by forming hydrogen bonds.25,26 The contribution to the ∆Hmix due to the interaction between OH groups in the polymer and the cyclic ether group in THF gives insight into the swelling behavior. The hydrogen bond interaction energy between OH group and ether group has been reported to be -19.95 ( 0.25 kJ/mol by Letcher and Bricknell.26 The negative contribution of this interaction to the ∆Hmix, i.e., the favorable OH-THF interactions between the solvent and the polymer network obtained via step growth polymerization, could explain the higher solvent uptake in this case as compared to hompolymerization. In addition, as the epoxy-amine reaction takes place in the presence of THF, the enthalpy of mixing decreases, i.e., becomes more favorable for mixing as the weaker hydrogen bonds between NH-THF (∆HH-bond ) -2.84 ( 0.05 kJ/ mol, Letcher and Bricknell26) disappear and stronger hydrogen bonds between OH-THF (∆HH-bond ) -19.95 ( 0.25 kJ/mol) are formed. Thus, the formation of more favorable interactions such as OH-THF interactions, as the reaction proceeds is required in order for the polymer formed to be miscible in the solvent as is the case with the RES technique. Thus, the selection of polymerization method like the step-growth system of epoxy-amine that would result in polymers having favorable interactions with solvent is a requirement for synthesizing nanoporous polymer networks via reactive encapsulation. Gels were synthesized by mixing stoichiometric quantities of epoxy-amine with THF at 60 °C as reported in the Experimental Section. No micro/macroscopic phase separation was observed during this polymerization reaction. This was the case for all the reaction mixtures of this (25) Letcher, T. M.; Govender, U. P. J. Chem. Eng. Data 1995, 40, 1097-1100. (26) Letcher, T. M.; Bricknell, B. C. J. Chem. Eng. Data 1996, 41, 166-169.
Nanoporous Thermosetting Polymers
Figure 6. Schematic showing polymer networks/structures with different connectivity that could be formed during the stoichiometric step-growth polymerization of a difunctional monomer with a tetrafunctional monomer in the presence of a solvent.
system used in this work. The porous structure of the gels synthesized using these materials was investigated by SEM and SAXS after extraction and supercritical drying using carbon dioxide as discussed in the next section. Investigation of Polymer Network Pore Structure. Design of nanoporous polymeric materials using the reactive encapsulation technique entails a fundamental understanding of the development of the porous structure by step growth polymerization as a function of solvent content. The presence of a chemically inert and completely miscible solvent during polymerization affects the way the monomer units come together, i.e., network connectivity. Figure 6 shows schematic diagrams of infinite polymer networks having different connectivity (threedimensional (3D), two-dimensional (2D), and one-dimensional (1D)) that could be obtained by step growth stoichiometric reaction of a difunctional monomer with a tetrafunctional monomer. Ideally, the monomers would react to form a 3D structure as shown schematically in Figure 6. The presence of solvent is expected to affect the connectivity in such a way as to enhance the creation of networks of lower dimensions (2D and 1D) that would be geometrically required to enclose the solvent. In the theoretical limit of infinite solvent content, one might expect 1D or linear structures to form as shown in Figure 6. Intermediate 3D network structures and finite cyclic structures could also be formed between the limits of no solvent and infinite solvent concentration. At complete conversion of a stoichiometric mixture of reactants, the intermediate 3D structure and the finite cyclic structures would form the insoluble and soluble part of the gel, respectively. Thus, the gels synthesized by the reactive encapsulation technique were immersed in THF to extract the soluble part of the gel. These extraction mixtures were analyzed by high-performance liquid chromatography (HPLC) for the presence of finite cyclic structures. HPLC experiments showed a soluble fraction of less than 0.5% for gels synthesized with solvent content up to 3:1 solvent/ monomers weight ratio. Thus, the presence of solvent during polymerization primarily results in intermediate 3D network structures, as shown in Figure 6, that have different network connectivity than the 3D network obtained without the solvent. The intermediate 3D network structures would be 3D units enclosing 2D interfaces. The 2D interfaces are created due to the presence of solvent, which influences network connectivity. Thus, it is proposed that the presence of a chemically inert
Langmuir, Vol. 21, No. 4, 2005 1543
and completely miscible solvent during network formation affects the network connectivity and thereby influences the porous morphology at the nanoscopic level. A direct method of obtaining information on network pore structure is by SEM microscopy. As part of the synthesis scheme used herein, the volatile organic solvent occupies the pores and it was not possible to obtain SEM micrographs of the wetted porous structure directly. Therefore, the materials were dried before the SEM analysis. However, drying these materials by conventional methods caused the existing pore structure to collapse because of strong surface tension forces at the vapor liquid interface within the capillary pores27 and via polymer relaxation. Liquid extraction and supercritical drying using carbon dioxide (CO2) was used to circumvent this problem. It should be noted that the epoxy-amine-THF gels phase separate/shrink during the extraction of THF with liquid CO2 prior to supercritical drying. This was observed through the sapphire window of the extraction/supercritical drying apparatus. The phase separation/shrinkage is due to the insolubility of the liquid CO2 with the polymer network. Thus, final porous morphology imaged using SEM is due to the reactive encapsulation of the solvent and phase separation/ shrinkage during extraction with liquid CO2. Even though this method of drying is not completely free from shrinkage, it is far superior to thermal drying where most of the porosity is lost due to the above-mentioned reasons. Gels used for SEM analysis were synthesized using 0:1, 4:1, 6:1, and 8:1 solvent/monomers weight ratio. Liquid extraction and supercritical drying were carried out with the wet gels; SEM micrographs of the fractured surfaces of these samples are shown in Figure 7. In this figure, the effect of solvent content on the porous morphology of the samples synthesized by the reactive encapsulation process is evident. Comparing the SEM micrograph of the sample synthesized with no solvent (0:1) with other SEM micrographs of the samples synthesized with solvent, it is clear that solvent content influences the structural attributes such as porosity and pore size. Micrographs with higher magnification (35000× and 50000×) for 4:1, 6:1, and 8:1 compositions were used to estimate the average pore sizes. They are 28, 35, and 50 nm, respectively. Thus, the average pore size of the gels, synthesized by the reactive encapsulation technique, increases with solvent content. SEM analysis also shows that a bicontinuous structure is obtained. These morphologies result in relatively high values of specific surface area (200 m2/g, BET analysis). SEM imaging of samples synthesized with lower solvent content ( 150 °C. Hydrophilic pore interfaces are required to obtain a hybrid of hydrophobic substrate and hydrophilic/electroactive sensor materials. Epoxy-amine network hydrophilicity can be enhanced by grafting acrylamide onto the pore interface via Michael addition reaction. A series of epoxy-amine networks were synthesized with nonstoichiometric quantities of epoxy and amine in the presence of THF (0.75:1 solvent/monomers weight ratio) by the reactive encapsulation technique. The use of excess amine during synthesis results in pendant secondary amine moieties in the pore environment. The concentration of secondary amines in the pore environment is proportional to the amount of excess amine used during the synthesis. Subsequently, Michael’s addition was used to add acrylamide to the secondary amines present in the polymer network. The unmodified and modified gels were immersed in water and allowed to equilibrate. FTIR was used to measure equilibrium water uptake of the system before and after grafting as discussed in the Experimental Section. The results are given in Figure 12 and they show a striking increase in hydrophilicity after acrylamide grafting, as indicated by the increase in the water peak height to reference peak height ratio. Thus, nanoporous polymeric materials synthesized by the reactive encapsulation technique are amenable to chemical modification
Conclusions The step-growth polymerization of EPON 828-PACM in the presence of THF as a miscible and chemically inert solvent represents a novel system used in this work to synthesize nanoporous polymeric materials. The selection of a polymerization method that would result in polymers having favorable interactions with solvent like the OHTHF interactions of the step-growth system is a requirement for synthesizing nanoporous polymer networks using the reactive encapsulation of a solvent (RES) technique. Investigations of pore structure using SEM and SAXS show that porous materials with average pore sizes less than 50 nm are obtained. The pore size and porosity of the nanoporous materials synthesized by this technique can be tailored by changing the solvent content. Furthermore, the miscible step-growth system results in a homogeneous co-continuous morphology when compared to a free radically polymerizing and phase separating system. The pore environment of these materials can be chemically modified for specific control of pore activity, for example, improving the hydrophilicity of epoxy-amine networks using the grafting technique based on Michael addition reaction or sulfonation. In summary, the RES technique and associated chemical grafting methods is a novel technique that can be used to design nanoporous thermosets by tailoring pore structure, size, and pore interface chemistry for specific applications. Acknowledgment. The authors acknowledge financial support for this work from the Army Research Laboratories (ARL) under Cooperative Agreement DAAD 19-02-2-0010 and Dr. Federick Beyer from ARL for the help in obtaining SAXS data. In addition, National Science Foundation support under Grant 0216343 for the Drexel University ESEM facility is greatly appreciated. LA048393T
