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Polyimides Seminar

• Name

• Enrol. No.

• Section.

• Guide

Nilay shah

200903062003

Polymers. Final year

Dr. VVS. Shertukde

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History of polyimides• Polymer E First polyimide.• Monomers PMDA (pyromellitic dianhydride) and 4,4’-

dimethyl heptamethylene diamine. Prepared by melt or solution polymerisation. For melt processability long chained aliphatic diamine had to be used.

• Properties resembling PET. Polyimide more expensive so were sidelined in early 1950’s.

• Aromatic diamines yielded only intractable polymeric powder.

• Fully aromatic monomeric polyimide great potential • Dupont film research centre in Delaware decided to do

research. A.L.Endrey December 1955. • Approach convertible polymer to deal with intractability.

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• Aim was to prepare an aromatic polyimide film via a soluble intermediate polymeric precursor.

• A suitable solvent had to be found. Dipolar aprotic solvents ( like DMF dimethyl formamide) were found to be very good solvents for poly (amic acid) synthesis.

• A poly (amic acid) film was made and then converted to polyimide film via thermal or chemical conversion.

• The monomers used were PMDA and MPD( m phenylene diamine).

• These films provided excellent thermal stability, chemical resistance, mechanical and electrical insulation properties.

• This set of monomers were also receiving attention in fiber’s department. So if and when commercialized the monomers could be used for films and fibers.

• But for making polyimides moldable a more flexible diamine was desired. That diamine was ODA ( oxydianiline).

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• Because of potential applications in electronics and defence, finance for preparing rolls of ODA – PMDA films and further conducting initial phases of market development was provided.

• Film called H-film.• Testing was done for electrical industry, aircraft companies

and for applications in defence department. Success.• Preparation of roll films began in 1960’s. Capacity - 2000 lbs.• Applications were also found in wire and cable insulation and

flexible circuitry applications.• 1965 three major polyimide products by DuPont:• Kapton (film),Vespel (molding), Pyre – ML (wire insulation).• Today Examples of commercial polyimide films include Apical,

Kapton, UPILEX,VTEC PI, Norton TH and Kaptrex.

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Synthesis of polyimides

• Two methods of preparing polyimides are:1 ) Two step method.2) One step method.

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TWO STEP METHOD• In the first step monomers - diamines and dianhydrides or

tetracarboxylic acid, a diester diacid are reacted at low or moderate temperatures.

• The dipolar aprotic solvent used has high B.P. Examples are Dimethyl formamide (DMF), dimethyl acetate (DMAc), N –

methyl pyrrolidinone (NMP), Tetrahydro furan (THF).• This results in the formation of poly (amic acid) (PAA).• PAA is then converted to polyimide via thermal process or

chemical process. This is the second step.• The first step is an equilibrium reaction. The important

parameters are reactivity of monomers, side reactions, solvents used, mode of addition of monomers and a suitable method to remove condensates.

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• The formation of PAA is a reversible nucleophilic substitution reaction where the amine attacks one of the carbonyl carbons in the anhydride moiety and displaces a carboxylate functionality, followed by proton transfer.

• The reaction is thought to start with the formation of a charge transfer complex between the dianhydride and the diamine.

• Charge transfer complex. ( Monomers – PMDA, PDA).

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• k1 and k3 are desirable while remaining are not. k1 and k4 are second order reactions whereas rest are first order reactions.

• The pathway to k4 depends on the amount of water or alcohol in the reaction medium (as impurities from monomers etc), the electrophilicity of the dianhydride ; and the nucleophilicity of the diamine, Steric hindrance and isomerization; polarity and autocatalytic effects of the solvent.

• Conversion of the anhydride moiety, k4, has the effect of introducing another chemical functionality with a different reactivity that can affect both k1 and k3 via resonance.

• k2 competes with k1 and k3 when the reaction temperature is raised and the thermal dehydration (k5 conversion) to imide is occurring. Once the imide ring is formed, depolymerization is severely limited through removal of the condensate by-product.

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Solvent effects • Polar aprotic solvents do not have a H bonded to an

electronegative atom. • Since they have an electron rich centre, the cations are

solvated very well but the anions are not.• This results in anions being highly reactive and offer faster

reaction. So more basic the solvent faster. In THF reaction rate is slower than dimethyl acetamide as the latter is more basic.

• As the concentration of PAA increases the viscosity of the medium increases because of the association of the acid groups with each other (intermolecular H bonds) & solvent.

• The step kinetics increases, because of proton attack on anhydride. This is the auto catalytic effect.

• Because of the polar aprotic solvents stabilizing the carboxylic acid proton auto catalytic effects are not observed.

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• Dipolar aprotic solvents are difficult to remove in imidization step.

• The interaction between carboxylic acid groups and amide solvents and the corresponding electron withdrawing amide group is a source of huge energy dissipation.

• Note when the protonof amic acid associates itself with oxygen atom of anhydride, there are chances of Carbon on anhydride being attacked by oxygen leading to ring opening.

• Sometimes acidic catalysts are added to increase the rate of the reaction because acyclation of amine can be activated by the presence of acidic catalyst.

• The rate of reactions is max in DMAc than acetonitrile and finally THF.

• There is a huge influence of the reactivity of monomers and the solvent used.

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Monomer reactivity

• The more deficient carbonyl carbon of dianhydride is more is the reactivity. Measured by Ea – electron affinity.

• Charge transfer interaction and electrostatic interaction also contributes to the high reactivity.

• As the reactivity of the dianhydrides decreases their tendancy to accept water decreases.

• The more available the electron pair of nitrogen, the more reactive diamines are. More pKa stronger the diamine.

• If OPDA (oxy phenylene dianhydride) is reacted with 4,4’ diaminobenzophenone only low molecular weight poly(amic acid) will be obtained. The solvent used here was a dipolar aprotic amide solvent. Use of acidic solvent seems to increase the molecular weight. The reason sems to be reduced rate of backward reaction.

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Dinahydride Name Ea

PMDA 1.90

BTDA 1.55

ODPA 1.30

BPADA 1.12

Diamine Name pKa

pDA 6.08

ODA

5.20

mDA 4.80

DABP 3.10

Structure Table

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• PMDA = pyromellitic dianhydride• BTDA = 3,3’,4,4’ benzophenone tetracarboxylic dianhydride• ODPA = 4,4’ – Oxy DiPhthalic Anhydride• BPADA = BisPhenolA DiAnhydride• pDA = p - diamine• ODA = 4,4’ - oxy dianiline• mDA = m - diamine• DABP = 4,4’ DiAmino BenzoPhenone• Ea = electron affinity• PAA = poly(amic acid)

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Side reactions

• The presence of small quantities of water can decrease the amount of dianhydrides. It is found that poly (amic acid) and small amount of dianhydrides are in equilibrium.

• This unexpected loss of dianhydrides results in equilibrium going towards left. The polyamic acid groups can have on one side the amic acid group and on the other two carboxylic groups instead of reverting to dianhydride.

• Moreover the nucleophilicity of water is enhanced in a polar aprotic solvent.

• Imidization also results in water being liberated. Hence it is found that there might be a small decrease in molecular weight of PAA over time though rate of imidization is very low.

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• It is observed that after initial few hours of polymerization

there is a significant decrease in viscosity.• Insoluble PMDA is added to the solution of diamine to reduce

effects of water and maintain a huge concentration of PAA.• Interfacial polymerization takes place, initial viscosity is very

high as the PMDA react with diamine ends which are at the surface. Hence there is a huge difference in Mn & Mw.

• After some time these differences seem to level off by re-equilibriation, Mn remaining constant.

• The presence of water results in unreactive ortho dicarboxylic groups. Because of this there will be unreacted diamines and low M.W. So by adding dianhydride, there will be an increase in viscosity resembling formation of PAA.

• Also ,when cyclo dehydrating, ortho carboxylic groups will dehydrate resulting in dianhydride formation, complicating situations.

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Imidization• Imidization is PAA cyclodehydrated to form polyimides.• The stiffness of the polymeric chain and the solvents used has

a huge influence on the degree of imidization.• Incomplete imidization will affect the desired properties.• Rate of imidization is faster in the initial stages but the rate

goes on decreasing. • Types of imidiztion are:• Thermal imidization.• Chemical imidization.

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Thermal imidization• Mainly used for preparation of fibers, coatings, films and

powders so that diffusion of byproduct from solvent and evaporation of solvent is not a problem.

• Cast films are dried and heated heated from 250°C – 350°C depending on Tg & stability.

• Imidization takes place in a concentrated viscous solution initially and is faster in presence of dipolar amide solvents.

• This is because of specific solvation to allow favorable conformation of amic acid group to cyclize.

• As the degree of imidization increases, the Tg of polyimide goes on increasing the chance of precipitation and hence complete imidization can be a problem.

• Also because of higher Tg the conformational change of the group to be imidized is less.

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• In almost all cases, on increasing the temperature there is first a decrease in viscosity and then a subsequent rise till final imidized product is obtained.

• Using IR two peaks of dianhydride formation was observed. One at 153°C and other at 227°C.

• At 153 anhydride is formed by reversion from PAA . This temperature is true for dipolar aprotic solvents, for ether solvents 130 °C.

• Then at 227°C, the water released reacts with anhydrides to form dicarboxylic acid.

• Because of presence of such high temperatures, dehydration takes place and anhydride is formed.

• These anhydrides react with diamines around 300°C as the chain mobility is regained.

• If Tg is very high the dianhydrides will decompose. CO2 is formed.

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Mechanism of thermal imidization

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Chemical imidization

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Chemical imidization

• The advantage is the use of ambient temperature for carrying out cyclodehydration. A fine PI powder is obtained. The imide formed should be soluble for completing imidization. Heat treatment is required to remove isoimide and solvents.

• Acid anhydrides like acetic anhydride, n – butryic anhydride etc are used as dehydrating agent, an amine catalyst is used like pyridine, trialkyl amines.

• Higher the pKa of amines, no isoimides are formed.• The isoimides formed get rearranged to imides in the

presence of acetate ions which has a higher concentration with trialkyl amines(pKa = 10.65) than pyridine (pKa = 5.5). And also high M.W. are obtained.

• With pyridine though high molecular weight is obtained isoimides are present which give a intense bright yellow color.

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Problems with PAA

• PAA poly(amic acid) as mentioned earlier when kept for a prolonged period of time under ambient conditions, forms a very low concentration of polyimide.

• As water is lberated in the process, the above mentioned side reactions will occur due to which the molecular weight is reduced.

• Also the presence of moisture can result in undesirable reactions. This will also depend on the monomers used.

• Hence, it is very important to store PAA under refrigeration.• A problem with thermal imidization can be that the substrate

should be able to withstand temperatures requiered for imidization.

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One step method• At polymerization temperatures if a polyimide is soluble in a

high boiling point solvent one step method can be used.• For making polyimides soluble various approaches are

designed. • The methods which will be discussed here are:• Reaction of diisocyanates and dianhydrides.• Synthesis of thermoset PMR –15 (diesters of dicarboxylic acids• Synthesis of Thermid IP – 600 ( polyisoimides as precursor).

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Di isocyantes and dianhydrides• Phthalic anhydride does not readily react with aromatic

isocyanate. Only at 180°C N - phenyl phthalimide are formed.• In dipolar aprotic solvents medium temperatures were

required whereas high temperature in polar protic solvents. In the former base catalysts are used.

• When a controlled amount of water was added, the isocyanate converted to carbamic acid which decomposed to form primary amines which reacted with dianhydrides easily.

• The catalytic action of water is enhanced in dipolar aprotic solvents.

• Also ,alcohols or phenol can react with isocyanates to produce urethanes which react with dianhydrides to form imides.

• Isocyanates undergo dimerization or polymerization on C-N bond. Alkali metal alkoxides (catalyst) hinder this reaction.

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PMR - 15• PMR is a thermosetting polyimide developed at NASA.• The monomers used are BTDA, MDA, the endcapping agent is

norborene dicarboxylic anhydride.• BTDA and endcapping agent are first converted into their

ester derivatives in methanol. BTDA into dimethyl ester and norborene dicarboxylic anhydride into monomethyl ester.

• Once converted MDA is added. There is no reaction at ambient conditions. Hence the viscosity is low. The resin can easily be impregnated into graphite fiber cloth.

• Methanol is readily removable. At 100°C – 150°C dianhydrides start forming and start reacting with diamines to form PAA.

• Around 200°C, the amines would attack the methyl group of diesters getting converted into N and N,N’ dimethyl group.

• Also over esterification of BTDA into triesters resulted in inferior mechanical properties. Scheme next page.

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Thermid IP - 600

• Polyisoimides are even in their final form more soluble, possess lower viscosities and have a lower Tg because of their syn, anti conformations of imino group.

• Acetylene terminated oligoisoimides were developed as adhesives and matrix resin for graphite fiber composites.

• After shaping the resin, they are crosslinked and subsequently heated around 250°C to form polyimides.

• Even the crosslinked isoimide is found to be soluble.• In another experiment, polyisoimides prepared from PMDA

and substituted p-phenylenediamines were found to be soluble.

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THERMID IP - 600

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Modification of polyimides

• The difficulty to make polyimides soluble or melt processable in their final form has limited their potential applications.

• The approaches to overcome this disadvantages are:• To make polyimide soluble.• To increase thermoplasticity of melt making PI processable.• The current example of a thermoplastic polyimide being

ULTEM which is a polyetherimide and NASA’s LARC – 1500.• While enhancing solubility and/or melt processability, it is also

very important to maintain the thermal, mechanical & electrical properties which define polyimides.

• LARC - 1500

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Concepts to overcome solubility restrictions• Molecular stiffness, high polarity and high intermolecular

association forces make these polymers virtually insoluble in any organic medium, and raise the Tg and subsequently Tm to well above the decomposition temperature.

• New monomers that provide less molecular order, torsional mobility and lower intermolecular bonding. The cost of synthesizing monomers should be justified.

• The introduction of free volume in a polymer decreases the number of polarizable groups per unit volume.

• The ways are by incorporating a flexible methylene chain(1), tert butyl side groups(2), rigid zigzag structures, (3) flexible bridging functionalities ( ether or siloxane) (4), adamantane groups(5), pendant flouro and trimethyl flouro groups(6) & polyimide siloxane group(7).

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(1) ……..m<12.

(2)

(3)

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(4)

(5)

(6)

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(7)

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Brief description(1) Flexible links do help in making the final polyimide structure

flexible. Tg decreases. Though, it should be noted that the more flexible and regular and long the chain is (m>12) , more will be the tendancy of interchain crystallinity which will again make the polyimide insoluble. Though solubility is obtained with 6 FDA, solubility is not obtained with ODPA and BTDA. This shows that it is not only solubility but also irregularity affecting solubility.

(2) The disruption of planarity in the dianhydrides containing bulky tert-butyl groups prevented efficient molecular packing and formation of 3D order hence increasing the fractional free volume and contributing to enhanced . The resulting polyimides showed two-stage decomposition with thermal stability as high as 450°C showing that though t butyl groups decreases intermolecular order, it also hinders the rotation stiffening the chain .

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(3) In (1) there is a reduction in thermal stability because of the flexible groups. In (3) introduction of bulky, propeller-shaped packing disruptive triphenylamine (TPA) groups into the polymer backbone increases the solubility of polyimides without sacrificing thermal stability. The polyimides showed a good film forming ability. Introducing 2,7-triptycenediyl zigzag structure will reduce the interchain packing, improve solubility and maintain desired thermal properties. Same in (5).

(4) The poly(ether imide), marketed under the trade name Ultem is an amorphous, soluble polymer that shows Tg values around 217°C . It can be processed from the melt by conventional means. The ‘‘kink’’ linkages between aromatic rings cause a breakdown of the planarity and an increase of the torsional mobility. Furthermore, the additional bonds lead to an enlargement of the repeating separating the imide rings. SO2 further helps being a little bulky though stiff.

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(7) Incorporation of siloxane oligomeric segments increases solubility and flexibility compared to the polyimide homopolymer, leading to increased overall processability. The introduction of the siloxane component allows for increased impact resistance, excellent adhesion, reduced water absorption, decreased dielectric constants, and increased gas permeability, while maintaining the thermal and mechanical stability that is adequate for most microelectronic applications.

• Although some thermal and thermooxidative stability is compromised through introduction of the siloxane segment, a number of improvements are also observed in processability, toughness, flexibility and in electrical properties. Block copolymers arranged alternately will give rise to thermoplastic elastomers.

• Exceptionally high solubility of the poly(imide siloxane)s could be ascribed to the presence of flexible aminopropyl linkages

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Flourinated polyimides• Flourinated polyimides induce transparency. Transparency is a

must in optoelectronics and to cover solar cells.• The bond between F and C is very strong giving very good thermal

stability. It has very low polarity resulting in a small dielectric constant making flourinated polyimides a choice in electrical applications. The low polarity ensures minimal water uptake, good chemical resistance and low surface energy.

• The coefficient of thermal expansion and poor adhesion can be the drawbacks.

• Solution polymerization is useful for synthesis.• Flourinating diamine reduces it’s reactivity to a huge extent. This

can result in low molecular weight polymers. • 6 FDA is the most popular flourinated dianhydride used.

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Polyamide imides• Polyamide imides ( PAI) contain both amide and heterocyclic imide

strucuture along main chain. The commercial polyamide imides are polytrimellitamideimides derived from trimellitic acid and its derivatives. Amines define Tg (Glass transition) of the polymer.

• As trimellitic acid is less reactive with aromatic diamines, low molecular weight PAI are obtained around 180.

• So trimellitic anhydride chloride and aromatic diamines or trimellitic anhydride and aromatic diisocyanates are used.

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Polyamideimides from trimellitic anhydride cholride

Ar can have Oxygen, methylene, SO2 connected to two benzene rings.

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Properties of polyamide imides (PAI)• PAI has an exceptional thermal stability. It is totally amorphous. • It can be functional from temperature of liquid nitrogen to as high

as 260°C.• It is highly resistant to radiation and is the one of the most stable

thermoplastic against radiation. • PAI has very good flame retardancy without the addition of any

flame retardant additives. It generates very low level of smoke and noxious fumes.

• PAI has excellent stress rupture resistance, high fatique resistance and exceptional mechanical properties. Similar to polyimides. PAI are used with graphite to result in a composite matrix.

• PAI is subjected to chemical attack by highly alkaline pH systems and some oxidizing acids. NaOH, benzene sulfonic acid and steam will attack PAI above 160°C.

• PAI is hygroscopic, so precautions must be taken before processing.• For maximum friction and wear resistance it is important to cure PAI

at around 265°C.

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• The major application include moldings, wire coating enamels and high temperature adhesives.

• PAI are also used in industrial compression rod packings, valve plates. Ball bearings in marine apps. Have replaced many zinc and aluminium die castings, steel gears, bronze and brass bearings.

• Because of high melt viscosity if wall thickness is high take care of internal stresses in extrusion. 2.5% water max at 21 °C. Compression ratio 1.5:1. In injection molding, the pressures used have to be very high. The pressure drop should be smooth. NO. Location Injection molding Temperature 3 control zone

1. Feed zone 304°C

2. Middle zone 327°C

3. Front zone 343°C

4. Nozzle 371°C

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Applications of polyimides• Electronics : For opto electronics where transpareny and high

thermal stability is required fluorinated polyimides are ideal. Flourinated polyimides also have a huge scope as interlevel dielectrics in IC farication, wire insulation, corrosion protection mainly because of their resistance to water vapor absorption and a very low dielectric constant. It is important to note that electronics industry frequently uses plasma for fabrication, hence the polyimide should be able to withstand it. There is considerable crosslinking and reorganization of the backbone in polyimidedue to thermo oxidative degradation. 6FDA has gained prominence for higher temperature usage.

• On polyimides thin film of copper is adhered and the entire integrated circuit is etched on copper. This is flexible and is used in laptops.

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• Polyimide composites have been used in continuous sliding (lubricating) applications where PTFE cannot be used due to them not being thermally stable above 250°C. Same is the case with UHMWPE which has low thermal stability. Example includes PI with graphite. These composites can be used for bearings, bearing separators, ball bearing retainers, gears, bushings, sliding seals and for space and high temperature applications. (Maximum continuous temperature being upto 316°C).Polyimides compounded with graphite or glass fiber reinforcements have flexural strengths of up to 50,000 p.s.i. (345 MPa) and flexural moduli of 3 million p.s.i. (20,684 MPa). Thermoset polyimides exhibit very low creep and high tensile strength.

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• The adhesion substrate can be a metal, ceramic or another PI film. The product interfaces need to tolerate severe stresses, exposures to elevated temperatures and humidity etc. Polyimide films can be deposited via vapor phase deposition or by casting from solution.

• Polyimides can also be used as a membrane for gas separation. Here m,m’ derivatives decrease permeability and increase selectivity as compared to p,p amine derivatives. Increasing monomer rigidity decreases permeability and increases selectivity. Presence of fluorine groups increases permeability. All the above modifications are made to diamines. In case of dianhydrides, BPDA and BTDA offer maximum selectivity while 6FDA exhibits maximum permeability. Because of the polar groups present Polyimide membranes would have an affinity towards water vapor. Hence it could be used for dehumidification of gases, air and organic vapors.

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• Photosensitive PI’s :The main applications of polyimides are photosensitive polyimides where the properties of the material can directly be altered by the exposure of light.