Polyimide Networks Research Articles

Melem: A metal-free unit for photocatalytic hydrogen evolution

Melem, the building unit of g-C3N4, is reported for the first time as a metal-free photocatalyst for hydrogen evolution from methanol aqueous solution. Melem is synthesized via a facile thermal condensation of melamine at 425 °C. Its chemical structure is verified by the combination of Fourier transform infrared spectroscopy (FTIR), solid-state 13C nuclear magnetic resonance (NMR) and elemental analysis. Density functional theory (DFT) calculations reveal that melem possesses frontier orbitals quite similar to those of g-C3N4. By conjugating melem with dianhydride monomer, an extended polyimide network with greatly enhanced visible-light absorption is formed. Consequently, the visible-light activity for hydrogen evolution is enhanced by six times and reaches to a rate of 13.1 μmol/h. This work may open up new opportunity to develop efficient metal-free photocatalysts based on melem unit.

Naphthalene-Based Microporous Polyimides: Adsorption Behavior of CO2 and Toxic Organic Vapors and Their Separation from Other Gases

Naphthalene was selected as a building block to prepare three polyimide networks with different topological structures via one-pot polycondensation from naphthalene-1,4,5,8-tetracarboxylic dianhydride with tetrakis(4-aminophenyl)methane, tris(4-aminophenyl)amine, and 1,3,5-tris(4-aminophenyl)benzene. The resultant polymers have moderately large BET surface areas with narrow pore size distribution at around 6 A. Interestingly, it is found that they can uptake 90.5 wt % benzene vapor (298 K, 0.8 bar), and the separation factors of benzene over nitrogen, water, and cyclohexane are as high as 759.3, 40.3, and 13.8, respectively. The high adsorption capacity and selectivity of benzene vapor are attributed to the incorporation of large amount of naphthalene groups in the network since naphthalene is highly hydrophobic in nature and has strong π-electron-delocalization effect. On the other hand, the CO2 uptakes in polymers reach 12.3 wt % (273 K, 1 bar), and the adsorption curves are reversible. Moreover, the se...

Microporous Polyimides with Uniform Pores for Adsorption and Separation of CO2Gas and Organic Vapors

Three microporous polyimides, MPI-1, MPI-2, and MPI-3, with uniform pores were synthesized via one-pot polycondensation from tetrakis(4-aminophenyl)methane, tris(4-aminophenyl)amine and 1,3,5-tris(4-aminophenyl)benzene with pyromellitic dianhydride, respectively. The amorphous networks exhibit excellent thermal stability, large BET surface areas up to 1454 m2 g–1, and narrow pore size distribution in the range from 5 to 6 Å. Their adsorption–desorption isotherms of CO2 are reversible, and the CO2 uptakes at 273 K and 1 bar are up to 16.8 wt %. Moreover, based on the ratios of initial slopes of isotherms, the CO2/N2 and CO2/CH4 separation factors are as high as 102 and 12, respectively. The above CO2 adsorption and separation properties are attributed to the presence of abundant electron-rich heteroatoms in the polyimide networks and the unifrom ultralmicroporous structures. In addition, for MPI-1, the adsorption capacity of benzene vapor is 119.8 wt %, while the separation factors of benzene over nitrogen and water reach 342 and 28, respectively. The outstanding selective adsorptions of CO2 gas and benzene vapor endow the microporous polyimides with promising potential in CO2 capture and separation as well as air- and water-cleaning applications.

Clay reinforced polyimide/silica hybrid aerogel

Silica aerogels are comprised of highly porous three-dimensional networks. They typically are very fragile and brittle due to the inter-particle connections in the pearl-necklace-like fractal network. This behavior prevents their wider utility. The present study aims to reinforce the silica-based gel to improve the poor mechanical strength through crosslinking the silica particles with polyimide and incorporating Lucentite STN clay into the skeletal silica–polyimide network. 3-Aminopropyltriethoxysilane (APTES) end-capped polyamic acid oligomers were first formed followed by gelation with TMOS at a range of clay concentrations to generate a silica network. The incorporation of clay leads to slightly lower BET surface area with little effect on shrinkage, porosity and density. Microscopy revealed that the aerogel preferentially grows from the edges of well dispersed clay particles while minimal growth occurs from clay surfaces. The formation of covalent bonds and hydrogen bonding through the OH functionalized clay edges is thought to enhance the connectivity with silica network and clay, leading to a substantial reinforcement effect as evidenced by an increase in modulus.

Synthesis, Characterization and Dielectric Properties of Rodlike Zinc Oxide-Polyimide Nanocomposites

Novel polyimide-rodlike zinc oxide hybrid nanocomposite (PI-RL-ZnO) has been developed from the poly (amic acid) with different weight percentages (1, 3, 5 wt%) of using N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc) as aprotic solvents. The prepared zinc oxide-polyimide nanocomposites were characterized for their structure, morphology, and thermal behavior employing Fourier transform infrared spectroscopy (FTIR), scanning electron micrograph (SEM), transmission electron micrograph (TEM), X-ray diffraction (XRD) and thermal analysis (DTA/TGA/DSC) techniques. These studies showed the homogenous dispersion of zinc oxide in the polyimide matrix with an increase in the thermal stability of the composites on zinc oxide loadings. More importantly, we intend to explore the possibility of incorporating ZnO particles through dispersion into the polyimide network to achieve the polyimide hybrid with lower dielectric constant (low-k). The lowest dielectric constant achieved of the PI-RL-ZnO material is 2.82 by incorporating 5 wt% ZnO (pure PI, k = 3.22). The reduction in the dielectric constants of the PI-RL-ZnO hybrids can be explained in terms of creating ZnO particles and the free volume increase by the presence of the ZnO particles resulting in a loose PI structure.

Synthetic control of network topology and pore structure in microporous polyimides based on triangular triphenylbenzene and triphenylamine units

Four new polyimide networks were prepared via a one-step polycondensation method by using tris(4-aminophenyl)amine and 1,3,5-tris(4-aminophenyl)benzene to react with pyromellitic dianhydride (PMDA) and naphthalene-1,4,5,8- tetracarboxylic dianhydride (NTDA), respectively. It was found that the topological structures and pore morphologies of the networks were tightly related to the geometrical shape of the net nodes, the molecular volume of the connecting struts, the reactivity of the monomers as well as the polymerization conditions. The results revealed that the reactions between triangular triamines and rod-like PMDA tended to form six-membered ring units, which were then extended to yield polyimide crosslinking networks, reflected by the complete disappearance of the endgroup signals in the FTIR and 13C CP MAS spectra, the significantly high thermal stability, large accessible surface area (818 cm2 g−1) and narrow pore size distribution centered at 4–8 Å. On the contrary, the low reactivity and steric bulky naphthyl group of the NTDA monomer led to the generation of branched or hyperbranched oligomers, which were further mutually crosslinked at a high temperature to produce the polyimde networks. In this way, a small amount of unreacted groups unavoidably remained, and the polymers exhibited a heterogeneous topological architecture, a broad pore size distribution and a low surface area of only 232 cm2 g−1. The influences of synthetic control and monomer structures on the pore morphologies and properties are investigated.

Synthesis of cost-effective porous polyimides and their gas storage properties

Porous polyimide (PI) networks with surface area up to 660 m(2) g(-1) were synthesized by planar structure monomers without detrimental catalysts.

Microporous polyimide networks with large surface areas and their hydrogen storage properties

Microporous polyimide networks with BET surface areas up to 1407 m(2) g(-1) and pore size distribution of 4-8 Å were synthesized. The respective effect of surface area and affinity between hydrogen molecule and polyimides on hydrogen storage properties were investigated.

Predicting crystalline polyamic acids as precursors to porous polyimides

We present here structural models for three distinct crystalline polyamic acids which are potential precursors to polyimide networks. In each case, the two different isomeric forms are considered.

Molecular imaging of polyimide formation

The reactions of 3,4,9,10-perylenetetracarboxylic dianhydride with 4,4'-diamino-p-terphenyl and with 2,4,6-tris(4-aminophenyl)-1,3,5-triazine on an Au(111) surface have been followed by low-temperature UHV-STM in the range of 300 to 700 K at coverages of up to one monolayer. Well-ordered, H-bonded structures are observed even after annealing at temperatures up to 470 K, while above 550 K reaction is initiated with evidence of amic acid intermediates. At higher temperatures, full imidisation leads to a covalently linked, surface polyimide networks. There is evidence of gold reconstruction playing a role in the early stages of imidisation and signs of limited order in the final polyimide. With the diamine as precursor, 1D-interconnectivity results, while with the triamine as partner, full 2D-connectivity is possible. In contrast to the situation in the bulk, the intermediate amic acid seems to be less stable and iso-imides are common in the final networks, as witnessed by the conformation of the product and the prevalence of triangular features in the case of the polyimide formed from the triamine precursor.

Fabrication of Surface-Supported Low-Dimensional Polyimide Networks

Interest in thermal and chemical stability of surface-supported organic networks has stimulated recent attempts to covalently interlink adsorbed molecular species into extended nanostructures. We show, using low-temperature scanning tunneling microscopy, that imidization of anhydrides and amines adsorbed on Au(111) can be thermally initiated under controlled ultrahigh vacuum conditions. Using two types of amine-functionalized polyphenyl molecules together with the organic semiconductor PTCDA, monolayer thick linear polymeric strands and a porous polymeric network with nanoscale dimensions are obtained.

Crosslinked sulfonated polyimide networks as polymer electrolyte membranes in fuel cells

AbstractSulfonated polyimides with tertiary nitrogen in the polymer backbone were synthesized with 1,4,5,8‐naphthalenetetracarboxylic dianhydride, 4,4′‐diaminobiphenyl 2,2′‐disulfonic acid, 2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane, and diaminoacrydine hemisulfate. They were crosslinked with a series of dibromo alkanes to improve the hydrolytic stability. The crosslinked sulfonated polyimide films were characterized for their thermal stability, ion‐exchange capacity (IEC), water uptake, hydrolytic stability, and proton conductivity. All the sulfonated polyimides had good thermal stability and exhibited a three‐step degradation pattern. With an increase in the alkyl chain length of the crosslinker, IEC decreased as 1.23 > 1.16 > 1.06 > 1.01, and the water uptake decreased as 7.29 > 6.70 > 6.55 > 5.63. The order of the proton conductivity of the crosslinked sulfonated polyimides at 90 °C was as follows: polyimide crosslinked with dibromo butane (0.070) > polyimide crosslinked with dibromo hexane (0.055) > polyimide crosslinked with dibromo decane (0.054). The crosslinked polyimides showed higher hydrolytic stability than the uncrosslinked polyimides. Between the crosslinked polyimides, the hydrolytic stability decreased with an increase in the alkyl chain length of the crosslinker. The crosslinked and uncrosslinked sulfonated polyimides exhibited almost the same proton conductivities. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2370–2379, 2005

Polyimide and polyhedral oligomeric silsesquioxane nanocomposites for low-dielectric applications

A novel polyimide (PI) hybrid nanocomposite containing polyhedral oligomeric silsesquioxane (POSS) with well defined architecture has been prepared by copolymerization of octakis(glycidyldimethylsiloxy)octasilsesquioxane (Epoxy–POSS), 4,4′-oxydianiline diamine (ODA), and 4,4′-carbonyldiphthalic anhydride (BTDA). In these nanocomposite materials, the equivalent ratio of the Epoxy–POSS and ODA are adjustable, and the resultant PI–POSS nanocomposites give variable thermal and mechanical properties. More importantly, we intend to explore the possibility of incorporating POSS moiety through the Epoxy–POSS into the polyimide network to achieve the polyimide hybrid with lower dielectric constant (low- k) and thermal expansion. The lowest dielectric constant achieved of the POSS/PI material (PI-10P) is 2.65 by incorporating 10 wt% Epoxy–POSS (pure PI, k=3.22). In addition, when contents of the POSS in the hybrids are 0, 3, 10 wt% (PI-0P, PI-3P, PI-10P), and the resultant thermal expansion coefficients (TEC) are 66.23, 63.28, and 58.25 ppm/°C, respectively. The reduction in the dielectric constants and the resultant thermal expansion coefficients of the PI–POSS hybrids can be explained in terms of creating silsesquioxane cores of the POSS and the free volume increase by the presence of the POSS–tethers network resulting in a loose PI structure.

Synthesis and characterization of polyimides containing a chiral dimine link that mimics the salen ligand

We report the synthesis of a covalently linked complex that mimics the salen ligand within the main chain of the polyimide network. The thermally-stable solid polyimides offer suitable sites for metal coordination, which is a crucially important step in establishing steroselectivity. The ligand synthesized by the sequential treatment of a chiral trans-1,2-diaminocyclohexane and p-dimethylaminobenzaldehyde was used to prepare polyimides by a one-step polymerization in the dipolar media. A series of stable polyimides was synthesized from (1S,2S)- and (1R,2R)-bis(p-dimethylaminobenzylideneamino)-1,2-cyclohexyl and various aromatic dianhydrides had inherent viscosities ranging from 1.45 to 2.21 dl/g and were soluble in polar solvents. The glass transition temperatures were 264–319°C, and the 10% weight loss temperatures were above 406–487°C.

Preparation of novel, high‐modulus, swollen‐ or jungle‐gym‐type polyimide gels end‐crosslinked with 1,3,5‐tris(4‐aminophenyl)benzene

AbstractA novel preparation approach for high‐performance polyimide gels that are swollen or have a jungle‐gym‐type structure is proposed. A new rigid and symmetric trifunctional amine, 1,3,5‐tris(4‐aminophenyl)benzene (TAPB), was synthesized as a crosslinker. Three different kinds of amic acid oligomers derived from pyromellitic dianhydride (PMDA), 4,4′‐oxydiphthalic anhydride (ODPA), p‐phenylenediamine (PDA), and 4,4′‐oxydianiline (ODA) were end‐crosslinked with TAPB at a high temperature to make polyimide networks with different structures. Transparent polyimide gels were obtained from the ODPA–ODA/TAPB series with high compression moduli of about 1 MPa at their equilibrium swollen states in N‐methylpyrrolidone. Microscopic phase separation occurred during the gelation–imidization process when polyimide networks were generated from PMDA–PDA/TAPB and PMDA–ODA/TAPB. After these opaque polyimide networks were dried, a jungle‐gym‐like structure was obtained for the PMDA–PDA/TAPB and PMDA–ODA/TAPB series; that is, there was a high void content inside the networks (up to 70%) and little volume shrinkage. These polyimide networks did not expand but absorbed the solvent and showed moduli as high as those of solids. Therefore, using the highly rigid crosslinker TAPB combined with the flexible monomers ODPA and ODA and the rigid monomers PMDA and PDA, we prepared swollen, high‐performance polyimide gels and jungle‐gym‐type polyimide networks, respectively. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2501–2512, 2002

Synthesis and microwave characterizations of crosslinked oligoimide

In this paper, we present the microwave characterization of polymers for electro-optic devices, carried out as a collaboration between chemical and physical research groups. We begin by reporting the synthesis and characterization of telechelic oligoimides functionalized with trialkoxysilane end-groups. Addition of a crosslinking agent, tris(4-hydroxyphenyl)ethane, and thermal curing leads to network formation, with the trialkoxysilyl end-groups as crosslinking sites. The network formation is confirmed by FTIR spectroscopy. The resulting material exhibits a glass transition temperature above 300°C and a thermal decomposition temperature above 450°. These oligomers offer an attractive processing alternative as low viscosity precursors to polyimide networks. The permittivity from 250 MHz to 40 GHz is determined by spin-coating the polymer on a structure as close as possible to a travelling wave electro-optic modulator. The permittivity is obtained from the measured scattering parameters and a spectral domain approach (S.D.A.)-based simulation The scattering parameters (measured), constant propagation (simulated) and characteristic impedance (simulated) are linked together using two alternative physical representations, the multiple reflection method and the attenuation method. This approach was first validated with a commercial polymer and then applied to the oligoimides.

Structure–property correlation for thin films of semi‐interpenetrating polyimide networks. II. Anisotropy in optical, electrical, thermal, and mechanical properties

Structure–property correlation for thin films of semi‐interpenetrating polyimide networks. II. Anisotropy in optical, electrical, thermal, and mechanical properties

Structure–property correlation for thin films of semi‐interpenetrating polyimide networks. I. Miscibility, curing, and morphology studies

Three different semi-interpenetrating polymer network (semi-IPN) polyimide systems were prepared through blending in solution by using 2 different polyimides, BPDA–PDA and PMDA–ODA (E), and 2 different oligomers, bismaleimide (MDAB) and phenylthynyl-terminated BPDA–PDA (BPDA–PDA–PEPA) oligomers. The oligomers are used as crosslinkers to modify the morphology of polyimides. The results show that both MDAB and PEPA are miscible with BPDA–PDA, but MDAB is immiscible with PMDA–ODA (E). Fourier transform infrared spectrum, dynamic mechanical thermal analysis data, and calculated crosslinking density indicate that there are crosslinking networks in these semi-IPN polyimide systems. In addition, the density and wide-angle X-ray diffraction results confirm that the molecular ordering and packing order are reduced by the addition of oligomers for these semi-IPN polyimide systems. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 261–272, 1998

Structure-property correlation for thin films of semi-interpenetrating polyimide networks. I. Miscibility, curing, and morphology studies

Three different semi-interpenetrating polymer network (semi-IPN) polyimide systems were prepared through blending in solution by using 2 different polyimides, BPDA–PDA and PMDA–ODA (E), and 2 different oligomers, bismaleimide (MDAB) and phenylthynyl-terminated BPDA–PDA (BPDA–PDA–PEPA) oligomers. The oligomers are used as crosslinkers to modify the morphology of polyimides. The results show that both MDAB and PEPA are miscible with BPDA–PDA, but MDAB is immiscible with PMDA–ODA (E). Fourier transform infrared spectrum, dynamic mechanical thermal analysis data, and calculated crosslinking density indicate that there are crosslinking networks in these semi-IPN polyimide systems. In addition, the density and wide-angle X-ray diffraction results confirm that the molecular ordering and packing order are reduced by the addition of oligomers for these semi-IPN polyimide systems. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 261–272, 1998

Structure-property correlation for thin films of semi-interpenetrating polyimide networks. II. Anisotropy in optical, electrical, thermal, and mechanical properties

Three different semi-IPN polyimide systems were prepared through blending in solution by using two different polyimides, such as poly (p-phenylenebiphenyl tetracarbonyimide) (BPDA–PDA) and poly (4,4′-oxydiphylene pyromellitimide) [PMDA–ODA(E)], and two different oligomers, such as bismaleimide (MDAB) and phenylthynyl-terminated BPDA–PDA (BPDA–PDA–PEPA) oligomers. These crosslinkable oligomers are used to modify the morphology of polyimides and, therefore, change the anisotropic properties of the resulting semi-IPN polyimide systems. The results show that the crosslinkable oligomers can disrupt the in-plane molecular arrangement which gives rise to an increase in lateral CTE and vertical CTE and a decrease in birefringence and the Young's modulus. The dielectric constant and water absorption increase with increasing MDAB content but decrease with increasing PEPA content. An empirical correlation between density and volume expansion is observed, indicating a correlation between the volume expansion and free volume expansion and free volume in the semi-IPN polyimide systems. The in-plane modulus decreases but the vertical moduli increases with increasing content of crosslinkable oligomers. In addition, the vertical modulus is affected by the free volume at low stress levels and can be enhanced by crosslinks or by a crystallinelike phase at higher stress levels. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 273–285, 1998