Addition Polyimides Research Articles

Methylated Tetrahydrophthalic Anhydrides as End Caps in Addition Polyimides

Thermolysis of methylenedianiline (MDA) 1,2,3,6-tetrahydrophthalic (THP) bismides up to 371 °C, produces aromatic product, with a concomitant lowering in cross-linking. This aromatization is responsible for both the improved thermal oxidative stability of THP end-capped polyimides and their substantial frangibility. In the hope of inhibiting precuring aromatization, 2,3-dimethyl, 3,3-dimethyl, and 1,2,3-trimethyl THP analogues were synthesized and reacted with MDA in a 2:1 ratio, heating gradually from 204 to 371 °C. In the lower temperature range, monoimide transforms to bisimide. At temperatures above 320 °C, cross-linking predominates—generally reaching ∼30% at 371 °C, though it was as much as 69% for 3,3-dimethyl THP. The surprising facility of cross-linking in the latter was correlated to the double bond length of the trans imide formed via thermal enolization. The aromatization (2−25%) observed in these methylated systems at higher temperatures presumably results from oxidative decarboxylation of the pendant methyls, which is the preferred pathway for degradation after cross-linking.

High energy binders: glycidyl azide and allyl azide polymer

AbstractHydroxy‐terminated azido polymers such as poly(glycidyl azide), poly bis(azidomethyl oxetane) and poly(azidomethyl methyloxetane) have been investigated in the past in propellent formulations and as fuels in rocket technology. The high energy released upon the decomposition of the azido group is responsible for their specialized application as high‐energy binders. The present paper describes the synthesis and characterization of new low molecular mass azido polymer i.e. poly(allyl azide). The curing reaction was carried out by using 1,3‐cyclic dipolar addition reaction. The dipolarophiles, such as dimethylene glycol dimethacrylate (EGDMA) and addition polyimides (bismaleimides, bisnadimides and bisitaconimides) were used for curing of azido polymers. The curing reaction was monitored by FT‐IR and differential scanning calorimetry. Curing was carried out at 40°C for 16 h (EGDMA) or 2 days (bismaleimide) and then at 60°C by using different phr of dipolarophiles. The heat of exothermic transition, due to decomposition of azide groups and thermal polymerization of addition polyimides, was very high and an improvement in thermal stability of cured resins was observed.

Reevaluation of Tetrahydrophthalic Anhydride as an End Cap for Improved Oxidation Resistance in Addition Polyimides

Several substituted 1,2,3,6-tetrahydrophthalic anhydride end caps, including the 3-phenyl, 3-methoxy, 3-trimethylsilyloxy, and 3,6-diphenyl analogues, were synthesized via the Diels−Alder condensation of the corresponding butadienes and maleic anhydride. These anhydrides, as well as the commercially available 3-hydro and 4-methyl analogues, were each ground up together with methylenedianiline in a 2:1 ratio and heated gradually from 204 to 371 °C, with the thermolysis followed by NMR. Generally speaking, a transformation via monoimide to bisimide was observed in the lower temperature range, followed by competition between cross-linking and aromatization. We believe that this competition produces a substantial percentage of aromatic product, with the concomitant lowering of the relative amount of cross-linking and is responsible for both improved thermooxidative stability of tetrahydrophthalic end-capped polyimides and their substantial frangibility. The thermolysis of the tetrahydrophthalimides under an inert atmosphere dramatically lowers the amount of aromatization; hence, the mechanism for aromatization is an oxidative one.

Addition Polyimides from Non-Mutagenic Diamines

Studies were conducted to find an acceptable non-mutagenic diamine to replace 4,4′-methylenedianiline (MDA), a suspect carcinogen, which is currently being used in PMR-15 polyimide applications. Several diamines containing fluorine and trifluoromethyl substituent groups were synthesized. The diamines were polymerized with the dimethyl ester of 3,3′,4,4′ -benzophenonetetracarboxylic acid (BTDE), using the monomethyl ester of nadic acid (NE) as an endcap. The effect of diamine structure on rheological properties, glass transition temperature, and thermo-oxidative stability was investigated. Unidirectional laminates were fabricated from selected resins, using carbon fibre as the reinforcement. The results indicate that some of the diamines containing trifluoromethyl groups are non-mutagenic, and have potential to replace MDA in PMR polyimides for long-term applications at temperatures up to 300 °C.

EFFECT OF STRUCTURE ON THERMAL BEHAVIOR OF HOMOPOLYMERS AND COPOLYMERS OF ITACONIMIDES

Addition polyimides, such as 5-norbornene-2,3-dicarboximide (nadimide), maleimide, or ethynyl-terminated imide resins, are a leading class of thermosetting polyimides. Since the early 1970s, these ...

7-Hydroxynadic Acid: A New End Cap for Improved Oxidation Resistance in Addition Polyimides

We consider possible replacements for the norbornenyl end cap in addition polyimides that should favor bridge degradation and lead to lower weight loss in the resin system. Our preliminary evaluation demonstrates that molding powder made with 7-hydroxynorbornene-2,3-dicarboxylic acid, 17, as the end cap can be fully imidized at 200 °C. By differential scanning calorimetry (DSC), onset of cross-linking occurs approximately 50 °C lower than that for the unsubstituted end cap. The hydroxy-bearing carbon on 17 is more easily oxidized to carbonyl on aging in the polymer than the parent end cap. Therefore, the new end cap more highly favors bridge degradation. However, processing studies, long-term weight loss, and careful evaluation of degradation mechanisms must be carried out to fully assess 17 as a more thermo-oxidatively stable replacement for the parent norbornenyl end cap.

Effect of structure on thermal behaviour of phosphorus containing addition polyimides and epoxy resins

Addition polyimides, [e.g. endo-5-norbornene-2.3-dicarboximide(nadimides), maleimides etc.] and epoxy resins have been used extensively in the past as matrix resins for advanced fiber-reinforced composites. However, inadequate flame resistance and thermal stability of these resins is one of their serious limitations. Our earlier studies have shown excellent flame resistance properties of addition polyimides based on bis(3-aminophenyl) methyl phosphine oxide (B) or tris(3-aminophenyl) phosphine oxide (T). Carbon fiber reinforced composites based on such matrix resin did not burn even in pure oxygen. In order to investigate the effect of weight percent of phosphorus on thermal behaviour and flame resistance, a series of addition polyimides and epoxy resins were prepared using B, T and amide-amines. The effect of structure on curing behaviour of resins and thermal and flame resistance of cured resins was evaluated. The results demonstrate an improvement in char yield of resins containing higher weight percent of phosphorus.

Structure–Property Relationships in Addition Polyimides. II. Resins from Three-Ring Aromatic Diamines Containing Carbonyl and Methylene Groups

The use of flexibilized three-ring aromatic diamine moieties was investigated in an effort to improve the processing characteristics of addition-type polyimide resins. A series of 10 diamines containing carbonyl and methylene bridging groups was synthesized. The diamines were polymerized with the dimethylester of 3,3′, 4,4′-benzophenonetetracarboxylic acid (BTDE), using the monomethyl ester of nadic acid (NE) as an endcap. The effect of diamine structure on the solubility and rheological properties during cure was determined. The effect of diamine structure and formulated molecular weight on the thermo-oxidative stability and glass transition temperature of the polyimides was also investigated. Unidirectional laminates were fabricated from selected resins, using carbon fibre as the reinforcement. Interlaminar shear strength and flexural properties of the laminates were determined. The results indicate that polyimides from some of the diamines containing methylene bridging groups have potential as matrix resins for long-term applications at temperatures up to 300 °C.

Synthesis and characterization of new siloxane-modified addition polyimides

A new series of polyimides was synthesized by addition polymerization of 4,4′-(bismaleimido)-diphenylmethane, 1,4-piperazine and an amino-terminated polydimethylsiloxane. The synthesis was carried out in m-cresol solution following a two-step procedure. Copolymers containing 15 and 20 wt.-% of the polydimethylsiloxane elastomer were prepared, together with the unmodified base copolymer. Infrared spectroscopy, 13C and 1H NMR, thermal and thermogravimetric analyses were used to characterize the copolymers. The DSC curves of all the samples showed an exotherm in the temperature range 230–280°C, attributable to curing and addition reactions involving the chain-end groups. Different glass transition temperatures (Tg) were observed, depending on the elastomer content. Thermogravimetric analysis indicated that no significant changes occur in the thermal stability of rubber-modified copolymers.

Structure-property relationships in addition polyimides. I: Resins from four-ring aromatic diamines containing carbonyl and methylene groups

In an effort to improve the processing characteristics of addition-type polyimide resins the use of flexibilized four-ring aromatic diamine moieties was investigated. A series of 12 diamines containing carbonyl and methylene, as well as oxo and thio bridging groups, was synthesized. The diamines were polymerized with the dimethyl ester of 3,3',4,4' benzophenonetetracarboxylic acid (BTDE). using the monomethyl ester of nadic acid (NE) as an end-cap. The effect of diamine structure on the solubility and rheological properties during cure was determined. This paper also describes the effect of diamine structure and formulated molecular weight on the glass transition temperature and thermo-oxidative stability at elevated temperatures after various post-cure regimes. The results indicate that polyimides from some of the diamines containing methylene connecting groups have potential as matrix resins for long-term applications at temperatures up to 300 C.

The synthesis, characterization and polymerization kinetic study of a series of related addition polyimides

A series of eight addition polymides (aspartimides) was prepared in which the imide and amino groups were attached to the ends of aromatic residues containing two or four phenylene rings. The co-monomers (bis-maleimides. BMIs, and diamines) were purified using column chromatography before being fully characterized by spectroscopic techniques. The thermally induced Michael addition and polymerization reactions of the blended co-monomers were monitored using differential scanning calorimetry (Dsc) at several heating rates to enable the kinetics of the processes to be investigated. The thermal stabilities of the thermoset products of these aspartimides were evaluated by thermogravimetric analysis (TGA). Dynamic mechanical thermal analysis (DMTA) was employed to test the physical properties of the neat resins.

Studies on simultaneous curing of nadimides and allyl nadic‐imides and thermal behaviour of cured resins

AbstractThe effect of composition and chemical structure of addition polyimides on thermal characteristics was investigated using commercially available allyl nadic‐imide resins and phosphorus‐containing nadimide resins. Thermogravimetric analysis, in N2 atmosphere, of resins cured at 300°C for 1 h revealed improvement of thermal stability.

Addition polyimides. II. Synthesis and characterization of bisitaconimide prepolymers

AbstractN,N′‐bis(3,3′‐itaconimidophenyl) sulfone has been synthesized by reacting itaconic anhydride with 3,3′‐diamino diphenyl sulfone. The bisitaconimides and the intermediates were characterized by chemical analysis, IR, NMR, TG, and GPC. The degree of polymerization (DP) of the prepolymers of the bisitaconimide was followed by GPC and from the NMR data on vinylidene and aromatic proton ratios. The correlation between DP and intrinsic viscosity of the prepolymers was made use of to determine the Mark–Houwink constants (K and α). The thermal stability and the decomposition kinetics of the prepolymers were evaluated from TG data. The kinetic parameters, viz. energy of activation, E, and preexponential factor A, were computed using four nonisothermal integral equations, and their values are not appreciably affected by the degree of polymerization, within the range studied.

Addition polyimide based on monoitaconamic acid

A monoitaconamic acid was synthesized by reaction of 4,4′-diaminodiphenylmethane and itaconic anhydride in methyl isobutyl ketone. It was characterized by IR, elemental analysis, acid value and 1H-NMR. On heating at 200–210°C, it underwent thermal imidization followed by Michael addition involving the α,β-unsaturated double bond and the NH 2 group present in the molecule leading to formation of a polymer. This polyimide was stable up to 270°C (2.5% weight loss) and underwent rapid degradation above 410°C.

Addition polyimides. I. Kinetics of cure reaction and thermal decomposition of bismaleimides

AbstractThe thermal polymerization of four structurally different bismaleimide resins, prepared by reacting maleic anhydride with four aromatic diamines, viz., 4,4′‐diaminodiphenyl methane, 4,4′‐diamino diphenyl ether, 4,4′‐diamino diphenyl sulfone, and 3,3′‐diamino diphenyl sulfone, was followed by differential scanning calorimetry (DSC). The enthalpy change and the kinetic constants for the polymerization reactions were evaluated from the DSC curves. Thermal stability of the cured polymers was studied by thermogravimetry (TG). The kinetic parameters, viz., activation energy E and preexponential factor A, for the thermal decomposition of the cured bismaleimides were calculated from the TG curves using three nonmechanistic integral equations. The kinetic constants (E and A) follow a trend similar to the thermal stability of the polymers.

Isomeric bismaleimides and polyaspartimides

AbstractA study was conducted of the effects of meta‐para isomerism on the synthesis and properties of aromatic bismaleimides and polyaspartimides. Three isomers, 3,3′‐, 3,4′‐, and 4,4′‐diaminodiphenylmethanes (methylenedianilines), were used to prepare three isomeric bismaleimides. The bismaleimides then were reacted with their respective isomeric diamines in m‐cresol solution to give a series of isomeric polyaspartimides. The properties of each of the isomeric series were measured and compared. Strong flexible films were solvent cast from the two polyaspartimides derived from the 3,4′‐ and 4,4′‐diamines and their respective bismaleimides. Tensile properties of the films from the 3,4′‐diamine/3,4′‐bismaleimide combination polyaspartimide were equivalent to those from the 4,4′‐diamine‐derived polymer. That finding, together with that polymer's lower softening temperature and the nonmutagenic nature of the 3,4′‐diamine monomer, suggested a potential usefulness for 3,4′‐diaminodiphenylmethane as a replacement for 4,4′‐diaminodiphenylmethane in addition polyimides.

Addition polyimides. V. Effect of vinyl ester resin on thermal and mechanical properties of chain‐extended bismaleimides

Addition polyimides. V. Effect of vinyl ester resin on thermal and mechanical properties of chain‐extended bismaleimides

Acetylene-terminated polyimide adhesives

In a research programme to develop an addition polyimide adhesive that is easily processed and exhibits good retention of strength after long term ageing, three different acetylene-terminated polyimide adhesives systems were prepared and characterized. The most promising system, a polyimidesulphone, was selected for a long term ageing study. The results indicate that this polyimidesulphone has improved strength retention over LARC-13 after ageing for extended time at high temperatures. In addition, it exhibited surprising adhesive strength for a short time at 593°C. The synthesis and characterization of these adhesives are discussed.

Addition polyimides. I

AbstractCrosslinking of 4,4′(bismaleimido)diphenyl ether (BM) was investigated in presence of bis(m‐aminophenyl)methylphosphine oxide (BAP), tris(m‐aminophenyl)phosphine oxide (TAP), diaminodiphenyl ether (E) and 3,3‐bis(p‐aminophenyl)phthalide (AP). These crosslinked resins were examined for thermal stability by thermogravimetric analysis. A slight decrease in initial decomposition temperature, the temperature of maximum rate of weight loss, was observed, though the char yield in nitrogen atmosphere at 800°C did not change appreciably. Glass‐cloth‐reinforced laminates were fabricated from BM and amine mixtures. An improvement in mechanical properties was observed in the presence of phosphorus‐containing di‐ and triamines.

Addition polyimide adhesives containing various end groups

AbstractAddition polyimide oligomers have been synthesized from 3,3′,4,4′‐benzophenone tetracarboxylic acid dianhydride and 3,3′‐methylenedianiline using a variety of latent crosslinking groups as end‐caps. The nominal 1300 molecular weight imide prepolymers were isolated and characterized for solubility in amide, chlorinated and ether solvents, melt‐flow and cure properties, glass transition temperature, and thermal stability on heating in an air atmosphere. Adhesive strengths of the polyimides were obtained both at ambient and elevated temperatures before and after aging at 232°C. Properties of the novel addition polyimides were compared to a known nadic end‐capped adhesive, LARC‐13.