PI Chain Research Articles

Charge transfer complex induced confinement effect between organic semiconductor and polymer chains for enhancing high-temperature capacitive energy storage

High-temperature polymer dielectrics, renowned for their ultrahigh power densities, robust voltage endurance, and remarkable reliability, present significant potential in optimizing the functionality of microelectronics and electrical power systems. Herein, to elucidate a notable correlation between the confinement effect induced by charge transfer complexes (CTCs) and the discharged energy density (Ue), a series of PI-based composites were synthesized, integrating organic semiconductors with diverse electron affinities. The density functional theory (DFT) simulations revealed that escaping from the CTCs required higher activation energy, besides, a more pronounced electron localization function was observed in the interfacial region between PI chain and F6TCNNQ in comparison to pristine PI, PI/F2TCNQ, and PI/F4TCNQ. Consequently, the CTCs present in PI/F6TCNNQ exhibit the greatest electron localization, significantly impeding electron transport within the composite. Furthermore, at 200 °C, the Ue for the PI/F6TCNNQ composite remains notably high at 5.06 J cm−3 with an efficiency above 90 %, representing a 2.45-fold increase compared to that of pristine PI (2.03 J cm−3). The findings provide evidence for a positive correlation between the confinement effect induced by CTCs and the discharged energy density (Ue) of the composite materials at elevated temperatures, thus offering valuable insights for future investigations focused on all-organic dielectric composite materials.

Simultaneously Improving the Optical, Dielectric, and Solubility Properties of Fluorene-Based Polyimide with Silyl Ether Side Groups

Three fluorene-based polyimides with silyl ether groups (Si–PIs) were successfully synthesized by a simple and efficient silicon etherification reaction of hydroxyl-containing polyimides (OH–PIs) and tert-butylchlorodiphenylsilane (TBDPSCl), and their structures were confirmed by 1H NMR and IR spectra. The bulky nonpolar tert-butyldiphenylsilyl (TBDPS) side groups in the modified PI unit instead of the strong electron donor −OH group is conducive to decreasing electronic conjugation and charge transfer (CT) interaction along the PI chain. Accordingly, the optical, dielectric, and solubility properties of the modified Si–PI films are simultaneously improved compared with the precursor OH–PI films. The modified Si–PI films demonstrate a meaningful enhancement in the transmittances at a wavelength of 400 nm (T400) to 74–81% from 42 to 55% of OH–PI films and the regeneration of fluorescence characteristics. The dielectric constant and loss of Si–PI films are also obviously reduced to 2.63–2.75 and 0.0024–0.0091 at 1 kHz from 4.19 to 4.78 and 0.0173–0.0295 of OH–PI films, respectively, due to substituted with the bulky nonpolar TBDPS groups to increase the free volume and hydrophobicity of Si–PI films. The solubility of Si–PIs in low- or nonpolar solvents (such as CHCl3, CH2Cl2, acetone, and toluene) is significantly improved. Furthermore, Si–PI films still maintain relatively good thermal properties with the 5% weight loss temperature (T5%) in the range 470–491 °C under a nitrogen atmosphere and the glass transition temperature (Tg) in the range 245–308 °C.

White-Light Emission and Tunable Luminescence Colors of Polyimide Copolymers Based on FRET and Room-Temperature Phosphorescence.

Thermally stable copolyimide (CoPI) films exhibiting high optical transparency and room-temperature phosphorescence (RTP) were prepared by copolymerizing fluorescent dianhydride and brominated phosphorescent dianhydride with an alicyclic diamine. The CoPI films underwent a 5 wt % degradation at a temperature higher than 349 °C and exhibited dual fluorescent and phosphorescent emissions owing to their efficient Förster resonance energy transfer from the fluorescent to phosphorescent dianhydride moieties in the main chains, followed by an intersystem crossing from the singlet to triplet state of the latter moiety atoms. The CoPIs displayed bright RTP under a vacuum with various colors produced when adjusting the copolymerization ratio. CoPI with 5 mol % phosphorescent moiety (CoPI-05) emitted white light with high optical transparency owing to the suppression of the PI chain aggregation that causes a yellowish coloration. The copolymerization of fluorescent and phosphorescent PI moieties can control the photoluminescent properties of PI films and is applicable to color-tunable solid-state emitters, ratiometric oxygen sensors, and solar-spectrum converters.

Effect of Bonded Interfacial Structure on Mechanical Properties of Polyimide/SiO2 Composites: Molecular Dynamics Simulations

AbstractThe effect of covalent bonds and the SiO2 concentration on the interface structure and the performance of polyimide/SiO2 (PI/SiO2) composites is investigated by molecular dynamic simulation. To enhance the interface interaction, the silica coupling agent is treated as a bridge to connect PI chains with SiO2 particles. The SiO2 content is controlled by adjusting the number of PI chains. The results show that the bonded interface structure not only increases the interfacial non‐bond energy and the number of the hydrogen bonds, but also strengthens the tensile strength, Young's modulus, shear modulus, and the glass transition temperature of the composites. As the SiO2 concentration increases, the non‐bond interaction energy between each PI chain and SiO2 particle increases and the non‐bond interaction energy in the bonded composites are higher than that of the unbonded. The number of hydrogen bonds in the bonded composites is sensitive to the SiO2 concentration, but there is no obvious relationship in the unbonded composites. Young's modulus and shear modulus of PI/SiO2 composites are higher than that of the unbonded at the same doping concentration. Therefore, establishing the bonded interface structure is an effective way to improve the interface stability and the properties of composite materials.

Synthesis and properties of cardo-type polyimides containing hydroxyl groups for application in specific detection of fluoride ion

Here, four cardo-type polyimides containing hydroxyl groups have been successfully synthesized by one-step solution polymerization of a hydroxyl-containing fluorene diamine monomer and various commercial dianhydride monomers. The obtained PIs with high molecular weights were readily soluble in many polar solvents. Simultaneously, they still maintain the excellent thermal stability (the 5% and 10% weight loss temperatures over 480 °C and 510 °C in nitrogen, respectively) and the high glass transition temperatures (Tgs, 342–442 °C). Furthermore, PI films show high tensile strength (>78 MPa) and relatively high optical transparency (over 82% at 500 nm), except PI-4. Utilizing that hydroxyl group in PI chain is able to form H-bond interaction with F−, these hydroxyl-containing PIs in DMSO solution are successfully applied as colorimetric chemosensors for detection of F− with high selectivity and sensitivity. Finally, the PI film chemosensors is also used for visual and reversible sensing of F−, which enables them to be a fast and portable test kit for visible detection of F−.

Impact of Molecular Architecture on Dynamics of Miktoarm Star Copolymers

Broadband dielectric spectroscopy (BDS) is used to probe the chain and segmental dynamics of A2B2 and AB2 miktoarm star copolymers based on polystyrene (PS, A block) and polyisoprene (PI, B block) that display lamellar morphologies as determined using small-angle X-ray scattering (SAXS). While no changes in the distribution of PI segmental relaxation times are observed with variation of the molecular architecture, an unexpected increase in the normalized PI chain relaxation intensity is realized for AB2 miktoarm star copolymers as well as a change in the distribution of chain relaxation rates as compared to A2B2 and AB diblock copolymer systems. This result is attributed to asymmetry in the molecular architecture near the junction point, which affects the osmotic constraint of the tethered PI chains within the interfacial region of the lamellae. The results highlight the importance of macromolecular design on fundamental chain dynamics in phase-separated thermoplastic elastomers.

Cationic surfactant mediated fibrillogenesis in bovine liver catalase: a biophysical approach

Protein aggregation into oligomers and mature fibrils are associated with more than 20 diseases in humans. The interactions between cationic surfactants dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) with varying alkyl chain lengths and bovine liver catalase (BLC) were examined by various biophysical approaches. The delicate coordination of electrostatic and hydrophobic interactions with protein, play imperative role in aggregation. In this article, we have reconnoitered the relation between charge, hydrophobicity and cationic surfactants DTAB and TTAB on BLC at pH 7.4 and 9.4 which are two and four units above pI, respectively. We have used techniques like turbidity, Rayleigh light scattering, far-UV CD, ThT, ANS, Congo red binding assay, DLS, and transmission electron microscopy. The low concentration ranges of DTAB (0–600 μM) and TTAB (0–250 μM) were observed to increase aggregation at pH 9.4. Nevertheless, at pH 7.4 only TTAB was capable of inducing aggregate. DTAB did not produce any significant change in secondary structure at pH 7.4 suggestive of the role of respective charges on surfactants and protein according to the pI and alkyl chain length. The morphology of aggregates was further determined by TEM, which proved the existence of a fibrillar structure. The surfactants interaction with BLC was primarily electrostatic as examined by ITC. Our work demystifies the critical role of charge as well as hydrophobicity in amyloid formation.

Preparation and characterization of inherently heat‐sealable polyimides with high glass transition temperatures

ABSTRACTA series of inherently heat‐sealable copolyimides (CPIs) with high glass transition temperatures were synthesized from 2,3,3′,4′‐oxydiphthalic anhydride (aODPA) and bicomponent diamines, 4,4′‐oxydianiline (ODA) and para‐phenylenediamine (PDA). The PI chain rigidity was manipulated by the regulation of the diamine ratio, and its effects on the heat sealability and thermal resistance of the derived CPI films were investigated in detail. The results show that these films are in possession of both good heat sealability and thermal resistance due to the synergetic effect of the asymmetry of aODPA and the rigidity of PDA. It is also found that there exists one critical PDA content that distinguishes the heat‐sealing behaviors of the CPI films, and the relevant mechanism was established. Especially for CPI‐5, the heat‐sealing strength is up to 350 N m−1 simultaneously with a relatively high Tg of 310°C. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43058.

Molecular-dynamics simulations of thin polyisoprene films confined between amorphous silica substrates

Constant temperature-constant pressure (NpT) molecular-dynamics computer simulations have been carried out for the united-atom model of a non-crosslinked (1,4) cis-polyisoprene (PI) melt confined between two amorphous, fully coordinated silica surfaces. The Lennard-Jones 12-6 potential was implemented to describe the polymer-silica interactions. The thickness H of the produced PI-silica film has been varied in a wide range, 1 < H/R(g) < 8, where R(g) is the individual PI chain radius of gyration measured under the imposed confinement. After a thorough equilibration, the PI film stratified structure and polymer segmental dynamics have been studied. The chain structure in the middle of the films resembles that in a corresponding bulk, but the polymer-density profile shows a pronounced ordering of the polymer segments in the vicinity of silica surfaces; this ordering disappears toward the film middles. Tremendous slowing down of the polymer segmental dynamics has been observed in the film surface layers, with the segmental relaxation more than 150 times slower as compared to that in a PI bulk. This effect increases with decreasing the polymer-film thickness. The segmental relaxation in the PI film middles shows additional relaxation process which is absent in a PI bulk. Even though there are fast relaxation processes in the film middle, its overall relaxation is slower as compared to that in a bulk sample. The interpretation of the results in terms of polymer glassy bridges has been discussed.

Dendritic Block and Dendritic Brush Copolymers through Anionic Macroinimer Approach

Anionic macroinimer is prepared through selective monoaddition of polyisoprenyllithium (PILi) toward 1,3-bis(1-phenylvinyl)benzene (MDDPE) in THF/cyclohexane mixture. The resulting macroinimer, possessing a living anion and a diphenylethylene (DPE) group at the same end of the PI chain, undergoes self-condensing vinyl copolymerization (SCVCP) with styrene after the solvent has been switched from mixture of THF/cyclohexane into pure cyclohexane. Dendritic block copolymers of styrene and isoprene are obtained, in which PI chains are linked to the branch points. The molecular weights of the dendritic products mainly depend on the ratio of monomer to macroinimer, whereas PI chain length has only slight effects. The PI segments are subsequently epoxidized and grafted with PILi or polystyrenyllithium (PSLi) to give dendritic polymer brushes. Both dendritic block copolymers and dendritic polymer brushes show decreased intrinsic viscosities. Dendritic block copolymer of styrene and isoprene undergoes a specially slow process of self-assembly, from unimolecular micelles and intermediate compound micelles to large compound micelles, in mixed solvent (n-heptane/THF = 19/1, v/v). In addition, conversion of large compound micelles into lamellar structure is observed for the first time in self-assembly of dendritically branched polymers.

Analysis of Molecular Aggregation Structures of Fully Aromatic and Semialiphatic Polyimide Films with Synchrotron Grazing Incidence Wide-Angle X-ray Scattering

The intermolecular aggregation structures of fully aromatic polyimides (Ar-PIs) prepared from pyromillitic dianhydride (PMDA) and those of semialiphatic polyimides (Al-PIs) from 4,4′-diaminocyclohexylmethane (DCHM) were characterized by a grazing incidence wide-angle X-ray scattering (GIWAXS) technique. The aggregation structures of both Ar- and Al-PI thin films formed on Si substrates were identified as a mixture of a liquid-crystalline-like ordered domain and an amorphous matrix. For Ar-PIs whose glass transition temperatures (Tg) are higher than the imidization temperature (Ti), the aggregation structures are significantly influenced by the three-dimensional structures of the PI chain. Rodlike molecular structures with high planarity are prerequisites for the growth of ordered domains of Ar-PIs, whereas an Ar-PI having a bent and nonplanar structure displays the highest intensities of an amorphous halo. In addition, the bulky −CF3 groups in the diamine moiety increase the interchain distance in the ordered domains. On the other hand, for Al-PIs whose Tgs are lower than Ti, the degrees of interchain ordering in the ordered domains were higher than those of Ar-PIs, but the orientation of the ordered domains was decreased significantly by decreasing their Tgs. This is due to the vigorous motion of PI chains during thermal imidization.

Viscoelastic and Dielectric Behavior of a Polyisoprene/Poly(4-tert-butyl styrene) Miscible Blend

Linear viscoelastic and dielectric measurements were conducted for a blend of polyisoprene (PI, M = 19.9 × 103) and poly(4-tert-butyl styrene) (PtBS, M = 69.5 × 103) with a PI/PtBS composition of 8/2 (w/w). In general, PI and PtBS exhibit the lower-critical-solution-temperature (LCST) type phase behavior. At temperatures examined, T ≤ 70 °C, our PI/PtBS blend was in a statically homogeneous state. The PI chain has the so-called type-A dipoles parallel along the backbone, and its large-scale (global) motion activates prominent dielectric relaxation, while the PtBS chain has no type-A dipoles and its global motion is dielectrically inert. In fact, at angular frequencies ω between 101 s-1 and 105 s-1 and at T ≤ 70 °C, the dielectric signal of the blend was exclusively attributed to the PI chains therein. The time−temperature superposition failed for the dielectric loss e‘ ‘ of the PI chains, despite the fact that the blend was statically homogeneous. This result suggested that the frictional environment for ...

Effect of chemical interaction on morphology and mechanical properties of CPI‐OH/SiO2 hybrid films with coupling agent

AbstractA novel hybrid film composed of copolyimide with hydroxyl group, silica and γ‐glycidyloxypropyltrimethoxysilane (CPI‐OH/SiO2/GOTMS) was prepared by the sol–gel process based on hydrolyzed tetraethoxysilane (TEOS) under acidic condition. GOTMS, as the coupling agent, and hydroxyl group in PI chain were used to improve the compatibility between the PI and SiO2. The components, morphologies, and mechanical properties of the hybrids were investigated by FTIR, UV–vis, SEM, stress–strain tests, and DMA. The results showed that SiO2 particle size significantly decreased, fractured cross sections of hybrid were rougher, and the surfaces of spherical SiO2 particles were more widely covered by PI component. The tensile mechanical properties of hybrids increased when adding GOTMS. The critical points of maximum tensile strength and elongation at break move from 11 to 16 wt % SiO2 content. DMA results showed that the storage moduli of hybrids with GOTMS, when above 260°C, were obviously higher than those without GOTMS; the tan δ transition temperature of hybrid films went up from 317 to 337°C. It suggests that chemical interaction between CPI‐OH and SiO2 is formed and the PI molecular mobility is restricted by the chemical interaction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3530–3538, 2007

Phase Behavior of a Mixture of Poly(isoprene)−Poly(oxyethylene) Diblock Copolymer and Poly(oxyethylene) Surfactant in Water

The phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer (PI-PEO or C250EO70) and poly(oxyethylene) surfactant (C12EO3, C12EO5, C12EO6, C12EO7, and C12EO9) in water was investigated by phase study, small-angle X-ray scattering, and dynamic light scattering (DLS). The copolymer is not soluble in surfactant micellar cubic (I1), hexagonal (H1), and lamellar (Lalpha) liquid crystals, whereas an isotropic copolymer fluid phase coexists with these liquid crystals. Although the PI-PEO is relatively lipophilic, it increases the cloud temperatures of C12EO3-9 aqueous solutions at a relatively high PI-PEO content in the mixture. Most probably, in the copolymer-rich region, PI-PEO and C12EOn form a spherical composite micelle in which surfactant molecules are located at the interface and the PI chains form an oil pool inside. In the C12EO5/ and C12EO6/PI-PEO systems, one kind of micelles is produced in the wide range of mixing fraction, although macroscopic phase separation was observed within a few days after the sample preparation. On the other hand, small surfactant micelles coexist with copolymer giant micelles in C12EO7/ and C12EO9/PI-PEO aqueous solutions in the surfactant-rich region. The micellar shape and size are calculated using simple geometrical relations and compared with DLS data. Consequently, a large PI-PEO molecule is not soluble in surfactant bilayers (Lalpha phase), infinitely long rod micelles (H1 phase), and spherical micelles (I1 phase or hydrophilic spherical micelles) as a result of the packing constraint of the large PI chain. However, the copolymer is soluble in surfactant rod micelles (C12EO5 and C12EO6) because a rod-sphere transition of the surfactant micelles takes place and the long PI chains are incorporated inside the large spherical micelles.

Viscoelastic and Dielectric Behavior of Entangled Blends of Linear Polyisoprenes Having Widely Separated Molecular Weights: Test of Tube Dilation Picture

Viscoelastic and dielectric experiments were conducted for entangled binary blends of linear cis-polyisoprenes (PI) having widely separated molecular weights, M1 = 21.4 × 103 ≅ 4Me and M2 = 308 × 103 ≅ 62Me with Me (≅5 × 103) being the entanglement molecular weight. The PI chain had type-A dipoles and its global motion (end-to-end vector fluctuation) was dielectrically active. The volume fraction of the high-M chain, υ2, was varied from 0.005 to 1. The Struglinski-Graessley parameter for the blends, M2Me2/M13 = 0.79, was larger than a threshold value ≅0.5 necessary for the thermal constraint release (CR) mechanism to dominate the relaxation of the dilute high-M chains. Indeed, the Rouse-like CR relaxation was experimentally detected for the blends with small υ2 ≤ 0.01. For large υ2 (≥0.05), the high-M chains were entangled with each other (as well as with the low-M chains) and exhibited solution-like relaxation behavior at long times in their terminal regime. Comparison of viscoelastic and dielectric data...

Polyimides reinforced with the sol‐gel derived organosilicon nanophase as low dielectric permittivity materials

AbstractPolyimide (PI) nanocomposites prepared by the in situ generation of crosslinked organosilicon nanophase (ON) through the sol‐gel process were characterized by densities, thermally stimulated depolarization currents and dielectric relaxation spectroscopy.Both a looser molecular packing of PI chain fragments adjacent to the ON and a loose inner structure of the spatial aggregates of ON were assumed to be responsible for a non‐additive decrease of the experimental values of dielectric permittivity for the nanocomposites. The pattern of composition dependence of the apparent dielectric permittivity of the ON suggested a probability of a morphological change around the composition PAAS/MTS = 100/16 (presumably, a sort of percolation transition from small‐size, individual clusters into large‐size, infinite clusters). Thus, PI reinforced with the sol‐gel derived nanophase may have a reasonably good potential as low dielectric permittivity materials. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Molecular dynamics in nanostructured polyimide–silica hybrid materials and their thermal stability

AbstractMolecular motion and thermal stability in two series of nanophase‐separated polyimide–silica (PI–SiO2) hybrid materials with chemically bound components were studied. The hybrids were synthesized from p‐aminophenyltrimethoxysilane‐terminated poly(amic acid)s as PI precursors and tetramethoxysilane as a silica precursor via a sol–gel process. The hybrids differed in their PI chemical structure and chain length (number‐average molecular weight = 5.000, 7.500, or 10.000) and in their SiO2 content, which ranged from 0 to 50 wt %. Differential scanning calorimetry, laser‐interferometric creep rate spectroscopy, and thermally stimulated depolarization current techniques were used for studying the dynamics from 100 to 650 K and from 10−3 to 10−2 Hz. Comparative thermogravimetric measurements were also carried out from 300 to 900 K. Silica nano‐ or submicrodomains that formed affected PI dynamics in two opposite directions. Because of the loosening of the molecular packing of PI chains confined to nanometer‐scale spaces between silica constraints, an enhancement of small‐scale motion, mostly at temperatures below the β‐relaxation region, occurred. However, a partial or total suppression of segmental motion could be observed above the β‐relaxation temperature, drastically so for the shortest PI chains at elevated silica contents and within or close to the glass‐transition range, because of the covalent anchoring of chain ends to silica domains. Large changes in thermal stability, including a 2.5‐fold increase in the apparent activation energy of degradation, were observed in the hybrids studied. A greater than 100 °C rise in long‐term thermal stability could be predicted for some hybrids with respect to pure PI. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1056–1069, 2002

Dynamic mechanical properties of polyimide/poly(silsesquioxane)‐like hybrid films

AbstractPolyimide/poly(silsesquioxane)‐like (PI/PSSQ‐like) films have three‐dimensional structure with linear PI blocks and a crosslinked PSSQ‐like structure. They have higher thermal stability and char yields than pure PI from 4,4′‐diaminodiphenyl ether and 3, 3′‐oxydiphthalic anhydride (ODPA). In a series of X‐PIS [PI modified with p‐aminophenyltrimethoxysilane (APTS), where X is the molecular weight of each PI block] hybrid films, decreasing the PI block chain length enhances the storage modulus, tensile modulus, and glass‐transition temperature (Tg) but reduces the α‐relaxation damping peak intensity, density, and elongation. The change in the former three properties may be caused by an increase in the crosslinking density and rigidity of the network structure. The changes in the next two properties are caused by an increase in the free volume or the PI interblock separation and decreases in the interblock PI chain interaction. The change in the last property (i.e., a decrease in elongation) is related to the increase in the rigidity of the network structure. The activation energy of the α transition depends on the chain length of the PI block. A maximum value is reached at the chain length of 10,000 because of two different factors: crosslinking density and free volume. In a series of X‐PIS‐y‐PTS [X‐PIS modified with phenyltrimethoxysilane (PTS), where y is the weight‐ratio percentage of PTS to APTS–polyamic acid] films with a constant PI block length, the storage modulus, tensile modulus, Tg, density, and α‐relaxation damping peak intensity decrease with the PTS content. This could be because of the increase of the PSSQ‐like domain size with the PTS content, which leads to the introduction of more free volume or interblock separation and to a decrease in the interblock chain interaction force. © 2001 John Wiley &amp; Sons, Inc. J Appl Polym Sci 81: 2500–2516, 2001

Micellization of Model Macromolecular Surfactants as Studied by Static Light Scattering

Micellization of ionically end-capped polystyrene-block-polyisoprene copolymers (PS-b-PI) in N,N-dimethylacetamide (DMAc) is investigated by static light scattering. DMAc is a polar selective solvent for PS which ensures for sufficient dissociation of ion pairs in solution. The use of polyisoprene as the nonsoluble block provides micelles in thermodynamic equilibrium since PI is a liquid at ambient temperature. Two different types of ionic end functionality are considered: (i) a monofunctional diblock with a lithium sulfonate group at the PI chain end and (ii) an α,ω-macrozwitterionic species with an additional ammonium group at the PS chain end. They are both compared to a neutral diblock of same mass and block sizes (12 kg/mol per block). It is shown that micelles formed by block copolymers containing a sulfonate group at the PI end (core of the micelle) exhibit an aggregation number (Nagg) which is only about 30% of Nagg of the neutral block copolymer. The critical micelle concentration (cmc) in this case is increased by 1 order of magnitude. The macrozwitterionic samples behave qualitatively as the monofunctionalized polymer, however, presenting a larger aggregation number and a lower cmc with respect to the monofunctionalized sample. This effect is attributed to a self-screening mechanism in micelles of the α,ω-macrozwitterions, where counterions are bound through chains of the soluble block.

Molecular dynamics in nanostructured polyimide‐silica hybrid materials and their thermal stability

AbstractMolecular motion and thermal stability in two series of nanophase‐separated polyimide‐silica (PI‐SiO2) hybrid networks with chemically bound components were studied. The hybrids were prepared via a sol‐gel process and differed in PI structure and chain length, and in SiO2 content ranging from 10 to 50 wt.%. Differential scanning calorimetry, laser‐interferometric creep rate spectroscopy, dielectric relaxation spectroscopy, thermally stimulated depolarization current techniques, and thermogravimetry were used covering, on the whole, the ranges of 100–900 K and 10−3‐109 Hz. Silica domains influenced PI dynamics in two opposite directions. Loosened segmental packing in chains confined to nanovolumes resulted mainly in rise of small‐scale motion below β‐relaxation region, while anchoring of chain ends to ‘rigid walls’ caused, contrarily, a partial or total suppression of segmental motion above Tβ, especially drastically at the temperatures close to and within glass transition. The latter resulted in a large change in thermal stability, e.g., 2.5‐fold increasing of the apparent activation energy of thermooxidative degradation, and more than 100° rise of predicted long‐term thermal stability for the hybrids as compared to that for PI.