Polyphosphazene toughened PMR-type thermosets - CiteSeerX

High Perform. Polym.8 (1996) 455–473. Printed in the UK

Polyphosphazene toughened PMR-type thermosets

O L Abu-Shanab†, C P Chang‡ and M D Soucek†§† Polymers and Coatings Department, North Dakota State University, Fargo, ND 58105, USA‡ Composite Research Laboratories, Auburn University, AL 36849, USA

Received 7 December 1995, accepted for publication 19 June 1996

Abstract. Two new polyphosphazenes, poly(4-maleimidophenoxy/phenoxy)phosphazene andpoly(4-phthalimidophenoxy/phenoxy)phosphazene, were prepared and used to toughen a PMRpolyimide designated LaRCTMRP46. These toughened polyimides were evaluated as thinfilms with a 0–40 wt% range of polyphosphazene to polyimide. The structure–propertyrelationships of these inorganic/organic polymeric matrices were studied and evaluatedin terms of fracture toughness, thermo-oxidative stability, and thermal, mechanical, andtensile properties. The hybrid systems revealed an increase in fracture toughness up to20 wt% polyphosphazene load without any substantial loss in tensile properties. With5 wt% poly(4-phthalimidophenoxy/phenoxy)phosphazene loading, the fracture toughnessof the semi-interpenetrating network was increased by 124%. When 10 wt% poly(4-maleimidophenoxy/phenoxy)phosphazene loading was used, the fracture toughness of the graftedcopolymer was improved by 217%. In addition, substantial enhancement in thermo-oxidativestability was also observed.

1. Introduction

Composite applications of polyimides have been limited in comparison to other polymermatrices because of the processing difficulties due to their poor solubility in commonorganic solvents, the high processing temperatures required, and the evolution ofvolatiles during imidization [1]. Serafini and coworkers at NASA Lewis ResearchCenter developed a much improved process for composite fabrications known asinsitu polymerization of monomer reactants (PMR) [2]. The low-viscosity monomericsolution is subsequently used to impregnate the reinforcing fibres to form a prepreg.In situ polymerization of PMR-15 occurs directly on the fibre surfaces, producing acomposite material with excellent thermal and mechanical properties. The principledrawback of PMR-15 polyimide resins is a lack of toughness and, as a consequencethe development of microcracking. In addition, one of the monomeric reactants (4,4′-methylene dianiline) has been found to be a carcinogen [3]. A modified PMRpolyimide resin designated as LaRCTM RP46 incorporates a less toxic diamine 3,4′-oxydianiline (ODA) instead of the mutagenic 4,4′-methylene dianiline. In addition todecreasing toxicity, the new diamine has been shown to increase the resin flow, enhancetoughness, and improve the overall processability. Even with the improved toughness,the major deficiency in RP46 polyimide resin is the lack of toughness and the ubiquitousmicrocracking.

§ To whom correspondence should be addressed.

0954-0083/96/030455+19$19.50c© 1996 IOP Publishing Ltd 455

at PENNSYLVANIA STATE UNIV on May 12, 2016hip.sagepub.comDownloaded from

http://hip.sagepub.com/

456 O L Abu-Shanab et al

An effective approach to overcome lack of toughness in PMR-type polyimide resinis to form a semi-interpenetrating network (semi-IPN). The combination of a thermosetand thermoplastic polymers results in materials which are tough and relatively easy toprocess [5]. Pater and Morgan [6] have prepared semi-IPN systems based on PMR-15 thermoset and thermoplastic polymers, LARCTM-TPI and NR-150B2. The formationof semi-IPN was shown to enhance the toughness of PMR-15 resin. Sefton andcoworkers [7] formed an epoxy-based semi-IPN in an attempt to study the dependenceof toughness on phase morphology. They found that spinodal morphology exhibitedsuperior toughness in comparison to homogenous or phase-inverted morphologies. Theyalso showed that toughness properties improved with increasing thermoplastic phaseconcentration.

Additives such as plasticizers [8], reactive elastomers, fillers, and rubber toughening [9]have been used in an attempt to enhance the fracture toughness of highly crosslinked polymernetworks. Levita [9] was successful in toughening a series of highly crosslinked diglicidylethers of bisphenol-A by incorporating either amine-terminated butadiene, or butadiene–acrylonitrile copolymer, as a reactive elastomer. Moreover, Cherry and Thompson [8]found that the fracture toughness of the plasticized epoxy systems is higher than that ofthe unplasticized systems. However, the enhancement in fracture toughness is usuallyaccompanied by lowering of the tensile properties, and drastic decrease in thermo-oxidativestability [8].

Polyphosphazenes are an attractive class of inorganic elastomeric polymers due totheir versatile properties [10]. The most common route used to produce polyphosphazeneconsists of thermally induced polymerization of hexachlorocyclotriphosphazene (trimer).The resultant poly(dichlorophosphazene) can be readily substituted by a variety ofnucleophiles such as alkoxides, aryloxides, or amines to form a range of substitutedpolyphosphazenes [11]. As a consequence, polyphosphazenes have versatile propertiessuch as excellent solvent resistance, inherent flame retardance, good toughness, and awide range of temperature serviceability [12]. In addition, polyphosphazenes are currentlybeing investigated via the formation of IPNs as a tool to toughen thermosetting resins.Visscher and coworkers [13] were the first to report the formation of polyphosphazene-based full IPNs. This consisted of poly(bis(methoxyethoxy)ethoxy)phosphazene (MEEP)and polymethylmethacrylate (PMMA), or MEEP and polystyrene (PS). Recently, Allcockand coworkers [14] reported the formation of semi-IPNs that consist of MEEP and PMMA,PS, polyacrylic acid, and polyacrylonitrile.

In an effort to enhance the fracture toughness of PMR-type polyimide resins,two commercially available thermoplastic polyphosphazenes (ExpelTM A & F) wereused to toughen LaRC RP46 polyimide thermoset via the formation of semi-IPNs [15]. Results showed a substantial increase in fracture toughness withoutany substantial loss in tensile properties. However, phase separation due tothe immiscibility between the inorganic and organic phases was apparent as theweight percentage of polyphosphazenes was increased from 5 to 10 wt%. In thepresent work, modification of the polyphosphazene backbone has been attempted inorder to enhance the miscibility and prevent gross phase separation between theinorganic and organic phases. In an effort to toughen LaRCTM RP46 polyimidethermoset, semi-IPNs and grafted copolymers based on phenylimide and maleimide-substituted polyphosphazenes, respectively, were prepared and evaluated as thinfilms. The synthesis of poly(4-phthalimidophenoxy/phenoxy)phosphazene and poly(4-maleimidophenoxy/phenoxy)phosphazene as well as thin-films mechanical and thermalproperties are discussed in this paper.



Polyphosphazene toughened PMR-type thermosets 457

2. Experimental details

2.1. Materials

Hexachlorocyclotriphosphazene (Ethyl Corporation) was purified by three cycles ofrecrystallization and sublimation. 4-nitrophenol (Aldrich), sodium phenolate (Pfaltz andBayer), sodium hydride (Aldrich), phthalic anhydride (Alfa Product), maleic anhydride (Alfaproduct), platinum oxide (Aldrich), and phosphoric acid (Aldrich) were used as received.5-norbornene-2, 3-dicarboxylic acid (NE, Pharm-Eco Laboratories), 3, 4′-oxidianiline (3, 4′-ODA, Chriskev) and 3, 3′, 4, 4′-benzophenonetetracarboxylic dianhydride (BTDA, Allco)were used as received. 3, 3′, 4, 4-benzophenonetetracarboxylic acid dimethyl ester (BTDE)was prepared from the corresponding anhydride (BTDA) [16]. The solvents, tetrahydrofuran(THF, Aldrich), ethyl acetate (Aldrich), N, N-dimethylacetamide (DMAc, Aldrich), 1-methyl-2-pyrodinone (NMP, Aldrich), methanol (Aldrich), chloroform-d (Aldrich), andhexane (Aldrich), were used as received.

2.2. Analytical and mechanical instruments

IR spectra were recorded on a Mattson 2020 FT-IR spectrometer;1H and NMR spectrawere recorded by a JEOL 270 spectrometer with a resolution of 0.045 ppm or 4.88 Hz.The chemical shift(δ) is given in ppm with tetramethylsilane as the internal standard.31PNMR spectra were recorded by a JEOL 270 spectrometer with a resolution of 0.045 ppm or4.88 Hz. The chemical shift(δ) is given in ppm with phosphoric acid as the externalstandard. Scanning electron micrographs (SEMs) were taken on cross sections of thefilms using a scanning electron microscope (JEOL JSM 6300V), with inclusion of energy-dispersion x-ray analysis (EDAX). Two sets of samples were used, non-etched and DMAc-etched samples. Thin-film etching was conducted by placing the samples in DMAc solventfor 24 h.

Differential scanning calorimetry (DSC) was performed on a DuPont 910 differentialscanning calorimeter with Thermal Analysi 2000 software. Typical resin sample weightswere 3–5 mg. The DSC tests were run at a rate of 10◦C min−1 under nitrogen atmosphere.The DSC procedure varied according to the sample tested. Dynamic thermogravimetricanalysis (TGA) was performed on a DuPont 951 thermogravimetric analyser with ThermalAnalyst 2000 software. Typical sample weights for TGA measurements were 2–4 mg.The TGA procedure consists of heating the samples from 50 to 600◦C at a rate of20◦C min−1 with purged air. Isothermal dynamic TGA was performed on a DuPont 951thermogravimetric analyser with Thermal Analyst 2000 software. Typical sample weightsfor isothermal TGA measurements were 2–4 mg. The isothermal TGA procedure consistedof heating the samples from 50 to 371◦C at a rate of 20◦C min−1 with purged air and stayingat a constant temperature of 371◦C for 60 h. Each of the DSC and TGA measurements wasconducted at least twice.

Tensile measurements were made on dog-bone-shaped specimens with thicknesses of70–90µm and a width of 12 mm at the centre. The initial gauge length was 4–5 cm. Thetesting was performed on an Instron Universal Tester (model 1000). A crosshead speedof 2 mm min−1 was used for the determination of tensile modulus, tensile strength, andstrain at break. Pneumatically actuated 1 in grips with steel faces were used for clampingthe specimens. A minimum of eight samples were tested for each composition and averagevalues are reported. Error bars are included on all the tensile strength and tensile modulusresults.




A plane-stress fracture toughness test was conducted on rectangular specimens with asingle-edge-notch (SEN) geometry. Rectangular strips 13 mm wide were cut for each samplewith a pair of sharp scissors. Each strip was supported on lightweight cardboard and cutwith a razor blade to create the edge notch. This procedure gave a crack which appearedsharp under a microscope. A microscope stage-top miniature testing system equipped witha 25 lb (100 N) load cell and a variable-speed motor was used to deform the specimen ina tensile mode. The load–displacement curve of each test was recorded on a chart recorderfor a test speed of 1 in min−1. The deformation zone near the crack tip could be observedin situ continuously during loading.

The plane-stress critical stress intensity factors (fracture toughness values) were obtainedfrom the following equation:

KIc = YS√

a (1)

where KIc is the critical stress intensity factor,Y = [3.94(2w/πa)tan(πa/2w)]1/2,S = F/[(w − a)b], w is the specimen width,b the specimen thickness,a the notch depthandF the load at which crack propagation begins.

If a material exhibits linear–elastic stress–strain behaviour prior to rupture (an idealbehaviour approximated by many thermosets), then a simple relationship exists betweena material’s fracture toughness and its surface fracture energy,GIc. This relationship ispresented as

GIc = K2Ic/E (2)

whereGIc is the critical strain release energy,KIc the critical stress intensity factor, andEthe tensile modulus.

Often GIc is called the critical strain energy release rate, but its units are those ofsurface fracture energy. TheKIc andGIc values characterize the resistance of a material tofracture in a neutral environment in the presence of a sharp crack perpendicular to tensileloading. These values are very dependent on sample thickness, crack sharpness, strain rate,and yield strength. Therefore, consistency is very important. Each of the fracture toughnessdata reported represents the average of at least eight specimens. Error bars are included onall KIc andGIc results.

2.3. Preparation of LaRC RP46 prepolymer

The LaRC RP46 moulding powder was prepared from NE, 3, 4′-ODA, and BTDE in a2.000:3.087:2.087 stoichiometric ration which was calculated to yield an imidized, nadic-encapped prepolymer having a formulatedMc of 1500 g mol−1 [17]. A 50 wt% methanolsolution (72.7 g) of BTDE (36.4 g, 0.094 mol) was added to ODA diamine (27.9 g,0.14 mol). The mixture was stirred for 1 h or until the ODA was completely solubilized inBTDE solution. To the mixture, 17.7 g (0.09 mol) of NE was added and stirred for 1/2 hor until complete solubility was achieved. This solution was placed in a vacuum oven andheated to 80◦C for 2 h and at 100◦C for 1.5 h. The resulting product was a yellow powder.

2.4. Synthesis of polydichlorophosphazene (I)

The trimer, hexachlorocyclotriphosphazene, was heated at 240◦C in a constant-temperatureoil bath to produce polydichlorophosphazene(I): IR, 1190–1210 cm−1 (broad P–N–P band)[18]; 31P NMR, δ19.0 ppm [19].




2.5. Synthesis of poly(4-nitrophenoxy/phenoxy)phosphazene (II)

To polydichlorophosphazene (14.1 g), sodium phenoxide (14.22 g, 0.12 mol) was added andreacted in THF (150 ml) solution under nitrogen atmosphere and−70 to −80◦C for 1 h.The poly(chlorophenoxy)phosphazene was further reacted with sodium 4-nitrophenoxide(19.1 g, 0.12 mol) under nitrogen atmosphere and refluxing temperature for 72 h. Theproducts were then filtered and washed three times with each of distilled water (10 ml) andpotassium hydroxide solution (10 ml) to product poly(4-nitrophenoxy/phenoxy)phosphazene(II) (21.3 g): IR, 1180 and 1200 cm−1 (νas P–N–P), 951, 789 cm−1 (νs P–N–P); 1528,1348 cm−1 (νas , νs NO2), and 1591, 1489 cm−1 (aromatic); 31P NMR, δ 8 ppm [–P(OC6H4NO2)(OC6H5)]; 1H NMR, δ 6.8–7.3 ppm (C6H5O) and δ 8–8.3 ppm (NO2–C6H4O).

2.6. Synthesis of poly(4-aminophenoxy/phenoxy)phosphazene (III)

An ethyl acetate (100 ml) solution of compound(II) (20 g), catalysed by titaniumoxide (0.07 g), was placed in a pressure reaction apparatus (hydrogenator) and agitatedat 50 psi for 3 h oruntil the nitro groups were reduced to amino groups. Thecatalyst was then filtered off and the solvent (ethyl acetate) was evaporated to formpoly(4-aminophenoxy/phenoxy)phosphazene (12 g) (III): IR, 3400–3460 cm−1 (νs NH2),1628 cm−1 (δs NH2), 1178 and 1195 cm−1 (νas P–N–P), 950, 789 cm−1 (νs P–N–P), and1489 cm−1 (aromatic);31P NMR,δ 10 ppm [–P(OC6H4NH2)(OC6H5)]; 1H NMR, δ 6.8–7.3(C6H5O) andδ 6.5–6.8 (NH2–C6H4O).

2.7. Synthesis of poly(4-phthalimidophenoxy/phenoxy)phosphazene (IV )

To form phenylimide-substituted polyphosphazene (IV), poly(4-aminophenoxy/phenoxy)phosphazene (5.0 g) was reacted in DMAc (10 ml) solution with phthalic anhydride (3.0 g,0.03 mol) under nitrogen atmosphere and ambient conditions for 24 h to form phthalic amicacid intermediate. The intermediate was then placed in a crystalline dish and heated in avacuum oven at 165◦C for 1 h to produce phenylimide-substituted polyphosphazene: IR,1721 and 1778 cm−1 (O=C–N–), 1177 and 1203 cm−1 (νas p–N–P, 945, 781 cm−1 (νas P–N–P), and 1489 cm−1 (aromatic);1H NMR, δ 6.8–7.3 (C6H5O) andδ 8–8.3 (C6H4C2O2N).

2.8. Synthesis of poly(4-maleimidophenoxy/phenoxy)phosphazene (V )

Using the same procedure as for compound (IV), poly(4-aminophenoxy/phenoxy)phosphazene (5.0 g) reacted with maleic anhydride (2.0 g, 0.02 mol) in DMAc (10 ml)to form maleimide-substituted polyphosphazene (V): IR, 1721 and 1777 cm−1 (O=C–N–),1179 and 1204 cm−1 (νas P–N–P), 943, 781 cm−1 (νs P–N–P), and 1489 cm−1 (aromatic).1H NMR, δ 6.8–7.3 (C6H5O) andδ 2.9–3.1 (C4H2N).

2.9. Film preparation

A series of poly(4-phthalimidophenoxy/phenoxy)phosphazenes or poly(4-maleimidophenoxy/phenoxy)phosphazene+RP46 polymides was prepared with 0, 5, 10, 20, 30, and 40 wt%polyphosphazene. A resin solution of each sample was prepared by mixing the appropri-ate quantity ofIV or V and LaRCTM RP46 polyimide resin in DMAc solvent (10 ml).The resin solution was stirred with a magnetic stir bar for 1 h or until all the compo-nents were dissolved. The solutions were then cast with a doctor’s blade on 12 in× 12 in




Figure 1. Synthesis of poly(4-phthalimidophenoxy/phenoxy)phosphazene and poly(4-maleimidophenoxy/phenoxy)phosphazene.

(305 mm×305 mm) Pyrex glass plates. The wet films were held in a dust-free drying cham-ber for 24 h to remove excess solvent. The dried films were placed in a high-temperatureoven where they were cured utilizing the following curing cycle: 1/2 h at 80◦C, 1 h at150◦C, 1/2 h at 200◦C, 1/2 h at 250◦C, and 1 h at 325◦C. The films were cooled toroom temperature and then soaked in deionized water for 24 h. Upon removal, the curedpolymeric films were 70–90µm thick.




3. Results and discussion

The objective of this study is to modify the backbone/substituent groups of polyphosphazenein order to enhance the compatibility between the organic-substituted polyphosphazene andorganic polyimide phases. The modified polyphosphazenes were utilized as tougheningagents in an effort to toughen RP46 polyimide thermoset. The synthesis approach usedin this paper is similar to that used by Kumar and coworkers [20]. In their approach,thechlorine groups of the trimer were replaced by nitrophenoxide and phenoxide by meansof nucleophilic substitution. The nitro groups were then reduced and imidized to formmaleimide [20]. The fully substituted trimer reported by Kumar and coworkers cannot bepolymerized to a polyphosphazene due to the steric and electronic factors. In our approachthe trimer (hexachlorocyclotriphosphazene) was polymerized into polydichlorophosphazene.Then, the backbone was modified via nucleophilic substitution, reduction, and imidization.

3.1. Synthesis of poly(phthalimidophenoxy/phenoxy)phosphazene andpoly(maleimidophenoxy/phenoxy)phosphazene

The synthesis procedure of poly(phthalimidophenoxy/phenoxy)phosphazene and poly(maleimidophenoxy/phenoxy)phosphazene is illustrated in figure 1. Polydichlorophos-phazene(I) was prepared by thermal ring-opening polymerization of the cyclic trimer. Thechlorine groups on the backbone of the polymer were replaced in two stages by nucleo-philic substitution to form poly(4-nitrophenoxy/phenoxy)phosphazene. In the first stage,sodium phenoxide, prepared from sodium hydride and phenol, was added toII at −78◦C.In the second stage, poly(chloro/phenoxy)phosphazene was further reacted with sodium4-nitrophenoxide by refluxing in THF at 66◦C to produceII.

CompoundII was characterized by FTIR, DSC, and31P and1H NMR. The FTIR ofII exhibited the IR bands 1528 and 1348 cm−1 (νas and νs NO2) and 1591, 1489 cm−1

(aromatic). The31P NMR spectrum exhibited a single sharp resonance atδ 8 ppm due to thepresence of−P(OC6H4NO2)(OC6H5). This suggests that full chlorine atom replacementhad been achieved. The1H NMR spectra revealed a broad multiplet in the range ofδ 6.8–7.3 ppm representing the presence of phenoxy groups and fromδ 8 to 8.3 ppm representingthe presence of 4-nitrophenoxy groups. In addition, the integration results showed 1:1substitution ratio of phenoxy and 4-nitrophenoxy. The DSC graph showed a glass transitiontemperature (Tg) of −1.1 ◦C (onset temperature).

The nitro groups of compoundII were then reduced into amino groups to form poly(4-aminophenoxy/phenoxy)phosphazene (III). The reduction of nitro groups was monitoredby FTIR. The FTIR revealed a shrinkage in the nitro group bands (1528, 1438 cm−1)concomitant with an increase of the amino group bands at 3400–3460 cm−1 (νs NH2) and1628 cm−1 (δs NH2). CompoundIII was further characterized by DSC and31P and1HNMR. The31P NMR spectrum exhibited a single sharp resonance atδ 10 ppm confirming thepresence of –P(OC6H4NH2)(OC6H5). The down-field shift exhibited byII is a result of thenitro group electrons de-shielding the phosphorus nucleus more than the amino group. InIII,the 1H NMR spectra revealed a broad multiplet in the range ofδ 6.8–7.3 ppm representingthe presence of phenoxy groups and fromδ 6.5 to 6.8 ppm representing the presence ofthe 4-aminophenoxy group. The DSC displayed aTg of −15.6◦C (onset temperature). Thedecrease inTg of III is attributed to the fact that the amino group will cause less sterichindrance than the nitro group.

CompoundIII was reacted with phthalic anhydride to form the corresponding maleamicacid intermediate which was imidized to form poly(4-phthalimidophenoxy/phenoxy)




Figure 2. A schematic representation of the formation of poly(phthalimidophenoxy/phenoxy)phosphazene/LaRCTMRP46 semi-IPNs.

phosphazene(IV). The emergence of the imide carbonyl bands at 1718 and 1778 cm−1

and the disappearance of amic acid carbonyl bands were revealed by the FTIR spectrum,proving the complete imidization of amino groups. The1H NMR spectra ofIV revealeda broad multiplet in the range ofδ 8–8.3 ppm representing the presence of phenylimideprotons in addition to the phenoxy group protons. The DSC graph ofIV showed aTg of19.6◦C and an exotherm at 275◦C which is polymer decomposition.

Similarly, III was reacted with maleic anhydride to form poly(4-maleimidophenoxy/phenoxy)phosphazene (V), and was characterized by FTIR and DSC. The spectral datawere consistent with the structure ofV. The FTIR spectrum ofV displayed imide carbonyl




Figure 3. Tensile properties of semi-IPNs: (a) tensile strength; (b) tensile modulus.

bands at 1718 and 1778 cm−1. The presence of maleimide vinyl proton NMR bands atδ 2.9–3.1 ppm in addition to the phenoxy group bands further confirmed the structure ofV. The DSC of V exhibited aTg of 14.5◦C, and exothermic peaks at 220 and 350◦Crepresenting the crosslinking reaction of maleimide through its double bond and polymerdecomposition respectively.

3.2. The semi-interpenetrating network

A sequential method [21] was used to prepare the semi-IPNs as shown infigure 2. The LaRCTMRP46 prepolymer was crosslinked in the presence of poly(4-




Figure 4. Toughness properties of semi-IPNs: (a) fracture toughness(KIc); (b) critical releaseenergy(GIc).

phthalimidophenoxy/phenoxy)phosphazene. Thin-film samples with 0, 5, 10, 20, 30 and40 wt% poly(4-phthalimidophenoxy/phenoxy)phosphazene in LaRC RP46 were prepared.These thin films were evaluated in terms of fracture toughness, tensile properties,and thermo-oxidative stability. With lower polyphosphazene loads (0–20 wt%), thepolyphosphazene was compatible with RP46 and no solubility problems were observed. Incontrast, the miscibility between the two phases decreased above 20 wt% polyphosphazene.At 40 wt% polyphosphazene load, longer time interval and heat had to be applied in order




to achieve partial phase mixing.Figure 3(a) and (b) shows the tensile strength and tensile modulus data of semi-

IPNs as a function ofIV weight percentage respectively. As a general trend, tensilestrength decreases as the weight percentage ofIV incorporated into the polymer matrixincreases. At composition of greater than 20 wt%IV, the drop of tensile properties becomesmore apparent. Above 30 wt%IV, the thin films sustained numerous cracks, nicks, andimperfections upon removal from the glass substrate and were especially brittle. The tensilemodulus values also generally decreased as the weight percentage ofIV incorporated into thepolymeric matrix increased. Above 10 wt%IV, the difference in tensile modulus becomesnegligible.

Table 1. Dynamic thermo-oxidative stability of semi-IPNs.

Weight loss percentage

Wt% polyphosphazene 5% 10% 25% 50%

LaRC RP46 452◦C 517◦C 561◦C 589◦C5% PIPP 494◦C 546◦C 588◦C 620◦C10% PIPP 459◦C 525◦C 574◦C 600◦C20% PIPP 427◦C 508◦C 572◦C 594◦C30% PIPP 399◦C 499◦C 561◦C 583◦C40% PIPP 378◦C 475◦C 570◦C 623◦C

Table 2. Isothermal thermo-oxidative stability of semi-IPNs at 371◦C.


Wt% polyphosphazene 10 h 20 h 40 h 60 h

LaRC RP46 22.5% 33% 57% 78.9%5% PIPP 31.5% 41.7% 59.5% 73.2%10% PIPP 19.1% 27.1% 43.1% 57.5%20% PIPP 25.7% 30.6% 37.7% 45.5%30% PIPP 25.9% 32.6% 43.2% 59.1%40% PIPP 51.5% 69.9% 86% 95%

Fracture toughness(KIc) data of semi-IPNs as a function ofIV weight percentage areshown in figure 4(a). TheKIc initially increased for composition containing up to 5 wt%polyphosphazene load followed by a decrease thereafter. At 5 wt%IV loading, theKIc was1.5 MPa m1/2, which represents an increase of 124% in comparison to non-toughened RP46film. At compositions higher than 20 wt% polyphosphazene, a large decrease inKIc valueswas observed. Declines ofKIc values of 74.5 and 78.8% were reported at 30 and 40 wt%polyphosphazene, respectively. Figure 4(b) show the critical release energy(GIc) of semi-IPN films as a function ofIV weight percentage.GIc initially increased for compositionscontaining up to 5 wt% polyphosphazene load followed by a decrease thereafter. At 5 wt%polyphosphazene, the value forGIc was 141.2 J m−2, which represents a 228% increaserelative to non-toughened RP46 film.

Dynamic and isothermal thermo-oxidative data of semi-IPNs are shown in tables 1 and2, respectively. In general, the inclusion ofIV in RP46 thermoset surprisingly enhancedthe thermo-oxidative stability of the polymer matrix. In comparison to RP46, the thermo-




oxidative stabilities of the semi-IPNs were enhanced up to a 30 wt% polyphosphazeneloading. At 10 wt% polyphosphazene loading, isothermal thermo-oxidative stabilityenhancements of 15, 18, 24 and 27% over RP46 were observed at 10, 20, 30 and 40 h,respectively. At 40 wt% polyphosphazene loading, the thermo-oxidative properties of thesemi-IPN significantly deteriorated (see tables 1 and 2). This decrease can be attributed tothe suspected phase separation between the two phases.

Figure 5. A representative micrograph of semi-IPN at 1200× magnification; (a) non-etchedsample; (b) DMAc solvent-etched sample.

Figure 5(a) shows a representative micrograph ofIV/RP46 polyimide film at 1200×magnification. The micrograph displayed a two-phase morphology, a continuous phase




Figure 6. A schematic representation of the formation of poly(maleimidophenoxy/phenoxy)phosphazene/LaRCTMRP46 grafted copolymers.

representing RP46 polyimide and a discontinuous phase representing the polyphosphazene.The semi-IPN film displayed considerable roughness in comparison with the topologicalmorphology exhibited by the RP46 film. Figure 5(b) shows a representative SEMmicrograph of DMAc solvent-etchedIV/RP46 polyimide film at 1200× magnification. Theetched films displayed cracks and void formation representing the position occupied bythe polyphosphazene chains. This further indicates that the two polymers interact throughphysical entanglements rather than chemical bonds. The SEM micrograph of semi-IPN filmwith 40 wt% polyphosphazene load showed void formation due to phase immiscibility andpartial phase separation.




3.3. Grafted copolymers

The formation of poly(4-maleimidophenoxy/phenoxy)phosphazene/LaRCTM RP46 graftedcopolymer matrix is shown in figure 6. At elevated temperatures, the double bond ofmaleimide was cleaved to produce a free radical. The free radical then facilitated thegrafting of polyphosphazene on the backbone of the thermoset to form grafted copolymers.Thin-film samples with 0, 5, 10, 20, 30 and 40 wt%V with RP46 were prepared. Thesethin films were evaluated in terms of fracture toughness, tensile properties, and thermo-oxidative stability. As in the semi-IPN case, no miscibility problem was observed at lowerpolyphosphazene loads and phase separation started to appear at higher polyphosphazeneloads, especially at 40 wt%.

Figure 7(a) and (b) shows the tensile strength and tensile modulus data of the graftedcopolymers respectively. As in the semi-IPN case, the tensile properties decrease withincreasing polyphosphazene. At 5 wt% polyphosphazene load, the tensile properties ofgrafted copolymer film were improved by 119% in comparison with the semi-IPN film.At composition higher than 10 wt% polyphosphazene loads, the semi-IPN films showedrelatively higher tensile properties than the analogous grafted copolymer films. The tensilemodulus values generally decreased as the weight percentage ofV incorporated into theRP46 matrix increased.

Fracture toughness(KIc) data of grafted copolymers as a function ofV weightpercentage are shown in figure 8(a). TheKIc initially increased and the maximum value ofKIc occurred at 10 wt% polyphosphazene. At 10 wt% polyphosphazene loading, theKIc

was 2.6 MPa m1/2, which represents an increase of 217% in comparison to non-toughenedRP46 film. At composition higher than 20 wt% polyphosphazenes, a large drop inKIc

values was observed. Declines ofKIc values of 47.5 and 61% were reported at 30 and40 wt% polyphosphazene loads, respectively. Figure 8(b) shows the critical release energy(GIc) of the grafted copolymer films as a function of polyphosphazene weight percentage.GIc initially increased up to 10 wt% polyphosphazene, followed by a decrease thereafter. At10 wt% polyphosphazene loading, the value forGIc was 636 J m2 which represents 1028%increase in comparison to non-toughened film. In general,GIc values in both the semi-IPNsand grafted copolymer systems significantly improved for up to 20 wt% polyphosphazeneloading. Thus, the decrease in tensile properties was compensated by the increase in fracturetoughness. In addition, grafted copolymer films were relatively tougher than the analogoussemi-IPN films especially at 10, 20 and 30 wt% polyphosphazene loads.

Table 3. Dynamic thermo-oxidative stability of grafted copolymers.


Wt% polyphosphazene 5% 10% 25% 50%

LaRC RP46 452◦C 517◦C 561◦C 589◦C5% MIPP 490◦C 541◦C 582◦C 601◦C10% MIPP 478◦C 536◦C 581◦C 605◦C20% MIPP 448◦C 519◦C 567◦C 589◦C30% MIPP 448◦C 511◦C 566◦C 604◦C40% MIPP 455◦C 517◦C 575◦C 602◦C

Dynamic and isothermal thermo-oxidative data of grafted copolymers are shown intables 3 and 4, respectively. The grafting ofV into the organic thermoset showedan unexpected improvement in the thermo-oxidative stability. At 5 and 10 wt%




Figure 7. Tensile properties of grafted copolymers: (a) tensile strength; (b) tensile modulus.

polyphosphazene, the films showed superior thermo-oxidative stability in comparison tothe non-toughened RP46 film. At 5 wt% polyphosphazene loading, isothermal thermo-oxidative stability enhancements of 15, 20, 31 and 35% were observed at 10, 20, 30and 40 h, respectively. In comparison to the non-toughened RP46 film, the 5 wt%polyphosphazene film required an additional 38, 24, 21 and 12◦C to lose 5, 10, 25, and50 wt%. At 40 wt% polyphosphazenes, the thermo-oxidative properties of the graftedcopolymer relatively decreased due to the phase separation between the inorganic/organicphases. In comparison, grafted copolymer films displayed relatively better thermo-oxidativestability than the analogous semi-IPN films. This can be attributed to the higher crosslink




Figure 8. Toughness properties of grafted copolymers: (a) fracture toughness(KIc); (b) criticalrelease energy(GIc).

density of the copolymers due to the capability of the maleimide double bond to crosslink.Figure 9(a) shows a representative micrograph of aV/RP46 polyimide film at 1200×

magnification. The micrograph displayed a two-phase morphology, a continuous phaserepresenting RP46 polyimide, and a discontinuous phase representing the polyphosphazene.The semi-IPN film displayed considerable roughness in comparison with the topologicalmorphology exhibited by the RP46 film. Figure 9(b) shows a representative micrographof a DMAc solvent-etchedV/RP46 polyimide film at 1200× magnification. The etchedsample showed no cracks or voids formed at its surface. This can be attributed to the




Figure 9. Representative micrographs of 5 wt% grafted copolymers at 1200× magnification:(a) non-etched sample; (b) DMAc solvent-etched sample.

chemical bonds formed betweenV and RP46 (copolymerization). The micrograph at 40 wt%polyphosphazene load showed void formation due to phases immiscibility and partial phaseseparation.

Overall, the inclusion of either polyphosphazene (IV and V) to RP46 polyimide resinresulted in a polymeric matrix with outstanding thermo-oxidative stability. This showsthe enhanced temperature serviceability of the toughened systems. The fracture toughnessproperties of RP46 matrix were improved significantly by the inclusion of polyphosphazenes.




Table 4. Isothermal thermo-oxidative stability of grafted copolymers at 371◦C.


Wt% polyphosphazene 10 h 20 h 40 h 60 h

LaRC RP46 22.5% 33% 57% 78.9%5% MIPP 19.1% 26.4% 39.5% 51.6%10% MIPP 20.5% 26.4% 42.6% 58%20% MIPP 26.3% 39.6% 57% 72.8%30% MIPP 28% 36% 47.7% 56.4%40% MIPP 40.8% 52.2% 67.8% 77%

The maximum values ofKIc andGIc were obtained at 5 wt% semi-IPN and 10 wt% graftedcopolymer film, respectively. In comparison, the grafted copolymer films were relativelytougher than the semi-IPN films. The initial increase in fracture toughness outweighedthe decline in tensile properties for both the semi-IPNs and grafted copolymers. At higherpolyphosphazene loads, fracture toughness values decreased significantly due to the initiationof gross phase separation between the two phases. The phase separation can be observedby void formation monitored by SEM micrographs at higher polyphosphazene loading.

4. Conclusion

The LaRCTMRP46 polyimide resins were toughened by the inclusion of poly(4-phthalimidophenoxy/phenoxy)phosphazene or poly(4-maleimidophenoxy/phenoxy)phospha-zene. The highest fracture toughness values were required when 5 wt%IV and 10 wt%V were incorporated into RP46 resin. TheGIc values showed that the enhancement offracture toughness properties outweighed the decrease in tensile properties up to 20 wt%polyphosphazene for both the semi-IPNs and grafted copolymers. The inclusion ofIV andV in RP46 polymide resin resulted in polymeric matrices with outstanding thermo-oxidativestability. In comparison to non-toughened RP46 film, the thermo-oxidative stability of semi-IPNs and grafted copolymers was improved by a maximum of 27 and 35% respectively.

References

[1] Southcott M, Amore M, Singer J, Wong A, Polio A and Sheppard C H 1994High Perform. Polym.6 1–12[2] Serafini T T, Delvigs P and Lightsey G R 1972J. Appl. Polym. Sci.16 905[3] Pater R H 1991Proc. Int. SAMPE Symp.36 78[4] Delaney E, Riel F, Vuong T, Beale J, Hirshbnehler K and Leane A 1992SAMPE J.28 31[5] Stuart M L 1990International Encyclopedia of Compositesed S M Lee (New York: VCH)[6] Pater R H and Morgan C D 1988SAMPE J.24 25[7] Sefton M S, McGrail P T, Peacock J A, Wilkinson S P, Crick R A, Davis M and Almen G 1987Proc. 19th

Natl SAMPE Tech. Conf.pp 700–10[8] Cherry B W and Thompson K W 1981J. Mater. Sci.16 1913–24[9] Levita G 1989Rubber-Toughened Plastics (Advances in Chemistry Series 222)(Washington, DC: American

Chemical Society) pp 99–118[10] Mark J E, Allcock H R and West R 1992Inorganic Polymers (Polymer Science and Engineering Series)

(Englewood Cliffs, NJ: Prentice-Hall) pp 61–131[11] Allcock H R and Kugel R L 1965J. Am. Chem. Soc.87 4216[12] Allcock H R 1992J. Org. Organomet. Polym.2 197[13] Visscher K B, Manners I and Allcock H R 1990Macromolecules23 4885[14] Allcock H R, Visscher K B and Manners I 1992Chem. Mater.40 1188




[15] Soucek M D, Tuman S J and Pater R H 199553rd Ann. Tech. Conf. Soc. Plastic Eng.2676[16] Pater R HProc. 36th Int. SAMPE Symp. (San Diego, CA, 1991)p 78[17] Soucek M D and Pater R H 1993J. Plastic Film Sheeting19 67[18] Allcock H L, Kugel R L and Valen K J 1966J. Inorg. Chem.5, 10 1711–12[19] Crutchfield M, Dungan C, Letcher J, Mark V and Van Wazer JTopics in Phosphorus Chemistry.31P Nuclear

Magnetic Resonance vol 5(New York: Interscience) ch 3[20] Kumar D and St Clair T L 1992Polyhedron11 13[21] Sperling L H 1977 Current Status of IPN Research, Emphasizing Miscibility and Phase Separation

(Contemporary Topics In Polymer Science)(New York: Plenum)



Polyphosphazene toughened PMR-type thermosets - CiteSeerX

Documents

Transcript of Polyphosphazene toughened PMR-type thermosets - CiteSeerX