Highly Thermal Stable Main‐Chain Nonlinear Optical Polyimide Based on Two‐Dimensional Carbazole Chromophores

Summary: A new NLO‐active lambda‐shaped main‐chain polyimide that comprises a two‐dimensional carbazole chromophore was synthesized. This polyimide exhibits high thermal and temporal stability. It can endure temperatures of up to 240 °C for a transient time and maintain a large SH signal at 100 °C for a long time because embedding the two‐dimensional chromophores into the polymer backbone effectively suppresses the randomization of the oriented dipole at high temperatures.

     

Spontaneous Three‐Layer Formation in the Curing of Polyimide/Epoxy Blends

Morphological structures of epoxy resin toughened by a fluorinated polyimide were observed by SEM. Three layers formed spontaneously parallel to the film surface in the range of polyimide weight fraction w = 0.14–0.20, when the samples were cured at 130 °C. The two outer layers consisted mostly of the epoxy‐rich phase. The middle layer consisted of bicontinuous domains of polyimide‐ and epoxy‐rich phases with a characteristic domain size much smaller than the outer layer thickness. At w = 0.10–0.13, aggregates of polyimide‐rich particles formed. Observation of the morphological development at w = 0.15 suggested that polyimide‐rich domains were distributed over the whole sample in the early period.

     

Thermal degradation of polyacrylonitrile in the temperature range 280–450°c.

This paper presents a study of the thermal degradation of polyacrylonitrile of 13,000 to 43,000 number-average molecular weight in vacuum over the temperature range 280–450°C. Sixteen products of the decomposition were identified by chromatographic and infrared analytical techniques. The five major products, i.e., cyanogen, hydrogen cyanide, acrylonitrile, acetonitrile, and vinylacetonitrile, were monitored at intervals during the decomposition using gas chromatography. Activation energies of 15 and 23 kcal./mole were calculated from initial rates of formation of HCN and cyanogen, respectively. The overall activation energy of the polymer degradation was found to be 3.6 kcal./mole. The residue of the decomposition in the temperature region 280–450°C. was suggested by infrared absorption measurements and elemental analysis to be a polymer of the structure The rate of production of vinylacetonitrile was found to be proportional to the production of the residual black poly-1,4,4-trihydronaphthyridine. A new photothermal degradation cell is also presented.     

Thermostable organic polymers devoid of hydrogen and other pendent groups

Aromatic hydrogen atoms in thermostable polymers can be surprisingly labile at relatively low temperatures and may participate extensively in degradative reactions. If hydrogen were eliminated from the polymer chain, a number of degradative pathways would be excluded. Replacement of hydrogen with other pendent groups such as fluoride or trifluoromethyl still leaves a potential “handle” to enter into unwanted reactions, therefore, all bonds not required to create the chain were eliminated. The polyimide devoid of hydrogen from pyrazinetetracarboxylic dianhydride and 2,5-diamino-1,3,4-thiadiazole was synthesized. This polymer exhibits extraordinary high temperature oxidation stability. Films of this composition support weight in air up to 592°C, and rupture at this temperature is accompanied by no charring whatever. By comparison, the corresponding polymer from pyromellitic dianhydride, with two hydrogen atoms per repeating unit begins to char at 320°C.     

Effect of an Organically Modified Nanoclay on Low‐Surface‐Energy Materials of Polybenzoxazine

Novel low surface free energy materials of polybenzoxazine/organically modified silicate nanocomposites have been prepared and characterized. The CPC (cetylpyridinium chloride)/clay10%/poly(3‐phenyl‐3,4‐dihydro‐2H‐1,3‐benzoxazine) (PP‐a) material possesses an extremely low surface free energy (12.7 mJ · m⁻²) after 4 h curing at 200 °C, which is even lower than that of poly(tetrafluoroethylene) (22.0 mJ · m⁻²) calculated on the basis of the three‐liquid geometric method. X‐Ray photoelectron spectroscopy (XPS) shows a higher silicon content on the surface of the nanocomposites than for an average composition, which implies that the clay is more preferentially enriched on the outermost layer. In addition, the glass transition temperature (T_g) of the polybenzoxazine (PP‐a) in the nanocomposite is 22.6 °C higher and its thermal decomposition temperature is also 31.5 °C higher than the pure PP‐a. This finding provides a simple way to prepare low surface energy and high thermal stability materials.

     

In situ Collapse of Phase‐Separated Structure by Covalent Bond Cleavage at a Branching Point upon Heating

A branched copolymer containing a degradable polyperoxide linkage at a branching point was synthesized by the radical copolymerization of dienyl‐functionalized polystyrene and polyisoprene macromonomers with molecular oxygen. The ternary mixture of the branched copolymer and the macromonomers showed phase‐separated structure after annealing at 45 and 90 °C. The adjacent spacing of the phase‐separated structure was in the order of submicron to micrometer, which is larger than that of general microphase separated structures, due to the presence of homopolymers (macromonomers). Annealing at 110 °C induced thermal decomposition of the polyperoxide followed by in‐situ collapse and a drastic morphology change in the phase‐separated structure.

     

Alternating copolymerization of propylene oxide and carbon dioxide with an alumina supported diethylzinc catalyst

Diethylzinc was allowed to react with γ-alumina in n-heptane at 50°C, and the copolymerization of propylene oxide and carbon dioxide was investigated in some detail at 30–90°C by using the reaction product as a catalyst. From an analysis of the catalyst it was found that diethylzinc reacted with the surface hydroxyl groups of γ-alumina mainly to give the following A-type species by evolving ethane: The catalyst showed considerably high activity for the copolymerization. The polymer obtained was a white solid with a high molecular weight soluble in benzene, acetone, dioxane, and methylene chloride and insoluble in diethyl ether and water. It was confirmed as an alternate copolymer of propylene oxide and carbon dioxide. The copolymerization was also conducted with a reaction mixture of the catalyst and catechol in which the molar ratio of catechol to the A-type species was varied. The copolymerization activity decreased linearly with an increase in the molar ratio and disappeared completely at the molar ratio of unity. On the basis of these results it has been concluded that the A-type is the true active species for the copolymerization.     

Thermal decomposition of azoisopropane. I. Primary decomposition processes

The thermal decomposition of azoisopropane (AIP) was studied by detailed product analysis in the temperature and pressure intervals 498–563 K and 0.67–5.33 kPa. Besides the predominant termination and hydrogen-abstraction reaction of the 2-propyl radical, the decomposition is characterized by a very short chain process. The following rate constants were determined from the measurements for the following reactions:      

Improvement of the Thermal Stability of TEMPO‐Oxidized Cellulose Nanofibrils by Heat‐Induced Conversion of Ionic Bonds to Amide Bonds

Improving thermal stability of TEMPO‐oxidized cellulose nanofibrils (TOCNs) is a major challenge for the development and preparation of new nanocomposites. However, thermal degradation of TOCNs occurs at 220 °C. The present study reports a simple way to improve thermal stability of TOCNs by the heat‐induced conversion of ionic bonds to amide bonds. Coupling amine‐terminated polyethylene glycol to the TOCNs is performed through ionic bond formation. Films are produced from the dispersions by the casting method. Infrared spectroscopy and thermogravimetric analysis confirm conversion of ionic bonds to amide bonds for the modified TOCN samples after heating. As a result, improvement of TOCNs' thermal stability by up to 90 °C is successfully achieved.

     

Application of Alkyne‐TCNQ Addition Reaction to Polymerization

The polymerization using a high‐yielding addition reaction between electron‐rich alkynes and 7,7,8,8‐tetracyanoquinodimethane (TCNQ) derivatives is described. The bifunctional monomer containing two TCNQ moieties and the counter comonomer bearing two dialkylaniline (DAA)‐substituted alkynes are reacted in 1,2‐dichloroethane under mild heating conditions. At the high monomer concentrations, high molecular weight linear polymers are obtained, while the reaction at the low monomer concentrations produces a significant amount of the cyclic compounds. A clear relationship between the monomer concentration and the cyclic compound amount is demonstrated. The obtained polymers feature a sufficient thermal stability with the decomposition temperature exceeding 300 °C as well as strong charge‐transfer (CT) bands and redox activities ascribed to the produced donor–acceptor moieties. These features are also used to optimize the polymerization conditions and to estimate the chemical structures.

     

Novel Cobalt (II) Phthalocyanine‐Containing Polyimides: Synthesis,Characterization, Thermal and Optical Properties

Summary: A novel class of polymer has been prepared by reaction of a copolyimide containing carboxylic acid groups, and a cobalt phthalocyanine peripherally substituted with a reactive hydroxyl group. The incorporation of the phthalocyanine ring was achieved by esterification of the carboxylic acid groups of the polyimide with the free hydroxyl group of the phthalocyanine. In this work, a limited number of phthalocyanine rings have been introduced in order to avoid extensive modification of the copolyimide properties. The extent of modification (percentage of esterified carboxylic groups) was estimated to be about 18%. The resulting material showed good thermal stability and a high glass transition temperature (above 300 °C). The incorporation of the bulky phthalocyanine ring brought about a light decrease of thermal properties, relative to those of the initial polyimide. The presence of phthalocyanine was confirmed by the absorption band at 670 nm showing in the visible region for the modified polyimide. The novel copolyimide also exhibited a remarkable emission peak at 430 nm, when excited at 350 nm, corresponding to the emission of the cobalt phthalocyanine moiety.

Schematic representation of the PI‐CoPc molecule and the chemical structure of the repeat unit.     

Fluorinated Hyperbranched Polyimide for Optical Waveguides

A novel fluorinated hyperbranched polyimide (HBPI) is synthesized by using a new triamine monomer, 1,3,5‐tris(2‐trifluoromethyl‐4‐aminophenoxy)benzene (TFAPOB) (A₃), as a ‘core’ molecule, 4,4‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) as a B₂ monomer, and 3,5‐ditrifuoromethylphenyl as an endcapping reagent. The polymer shows a glass transition temperature of 232 °C and a temperature of 10% weight loss at 535 °C. The HBPI presents a birefringence as low as 0.002 at 650 nm. Near‐IR results indicate that HBPI exhibits desirable properties for low loss optical waveguide applications.

     

Optical Non‐Linearity from Montmorillonite Intercalated with a Chromophore‐Containing Dendritic Structure: A Self‐Assembly Approach

Chromophore‐containing dendritic structures (G1, G2) are utilized to intercalate layered silicates, which results in a large d‐spacing up to 126 Å. An exfoliated morphology is obtained by mixing the dendritic structure intercalated layered silicates with polyimide in N,N‐dimethylacetamide solution. The dendritic structures attached on the clay template would arrange in a non‐centrosymmetric manner. This self‐assembled arrangement brought about the electro‐optical coefficients of 5–6 pm · V⁻¹ for these relatively low chromophore‐containing organic/inorganic nanocomposites without resorting to poling. Excellent temporal stability (100 °C) is also achieved.

     

Highly Fluorescent Conjugated Copolymers Containing Dithieno[3,2‐b:2′,3′‐d]pyrrole

Three copolymers that incorporate dithieno[3,2‐b:2′,3′‐d]pyrrole with fluorene, carbazole, or pyridine have been prepared by Suzuki reaction and characterized by NMR spectroscopy and GPC. A new homopolymer of dithieno[3,2‐b:2′,3′‐d]pyrrole was also synthesized for the comparison of their structure–property relationships. Their thermal, optical, and electrochemical properties have been investigated. All the polymers exhibit good thermal stability with decomposition temperatures around 400 °C. The fluorescence quantum efficiencies of all these polymers in solution are in the range of 33.5–55.5%. The copolymers also show high film fluorescence quantum efficiencies of about 20% while the fluorescence of the homopolymer film is almost quenched.

     

The Nucleating Effect of Carbon Nanotubes on Crystallinity in R‐BAPB‐Type Thermoplastic Polyimide

Summary: The relatively high degree of crystallinity of a new thermoplastic polyimide (PI) makes it a favorable matrix candidate for fiber reinforced composites. This advantage is the result of the ability of the matrix to recrystallize during the composite manufacturing process. In this study we examine the potential nucleating effect of carbon nanotubes on a thermoplastic PI. In addition to their inherent mechanical contribution, the carbon nanotubes help recover a significant proportion of the original crystallinity.

Scanning electron micrograph of the spherulitic structure of a recrystallized carbon nanotube/polyimide film.     

Thermal decomposition of azoisopropane. II. Secondary decomposition processes

The thermal decomposition of azoisopropane (AIP) was studied in the presence of various quantities of propylene in the temperature and pressure intervals 498–553 K and 3.33–5.33 kPa. The inhibition functions relating to formation of the products were determined; these proved a good basis for interpretation of the formation of the secondary decompositon products of AIP. The experimental data support the conception that the βμ radical—radical reaction occurs. The product of this is not stable; its decomposition is one of the sources of the secondary products. The ratio of the rate constants was determined for the following reactions:      

The Resting State Makes the Difference: The Influence of the Anchor Group in the ROMP of Norbornene Derivatives

Summary: A model study with four norbornene derivatives with different groups connecting a functional unit to the norbornene reveals that the “anchor groups” influence the polymerisation rate constants of ring opening metathesis (ROM) polymerisations. Consequently, polydispersities and molecular weights are affected. The observed effects are explained by different tendencies of the anchor groups to coordinate to the ruthenium centre during the resting state of the propagation.

A functionalised monomer where F is the functional unit, S is a spacer, A is an anchor group and P is the polymerisable group.     

A Low Curing Temperature Silver Ink for Use in Ink‐Jet Printing and Subsequent Production of Conductive Tracks

Summary: A drop‐on‐demand ink‐jet printer has been used to print a silver‐organic solution onto glass substrates. Conductive silver tracks were obtained by heat treatment of the ink‐jet printed deposits at temperatures ranging from 125 °C–200 °C in air. Resistivity values were found to have dropped to two to three times the theoretical resisitivity of bulk silver after temperatures of 150 °C and above were used.

Resistivity values of a silver‐based ink.     

Thermal properties of thermotropic polyesters with rigid chains

The relationship between chemical structure and thermal properties of three polyesters composed of rigid backbone units, and here X = Br (A), Y = Cl (B), and Y = Methyl (C), was studied by differential scanning calorimetry, x-ray diffraction, and polarized-light microscopy. The effect of annealing on the polyesters was also investigated. Polymer A forms an anisotropic melt phase at temperatures above 280°C, but decomposes at temperatures in the range of the anisotropic melt phase. The thermal properties of polymer B are very similar to those of polymer C. Anisotropic-isotropic melt transitions were observed at temperatures below 400°C in both heating and cooling runs for polymers B and C. These two polymers do not decompose thermally up to 400°C. A polymer with chlorine as a substituent on the hydroquinone ring in the main chain shows a wider range of temperatures at which the anisotropic melt phase exists. Quenched films of the polymers are very poorly crystalline or almost amorphous. Textures observed in anisotropic melt phases are largely preserved in the quenched films. The temperatures of the crystal-anisotropic melt and anisotropic-isotropic melt phase transitions become higher with annealing.