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Polymer Degradation and Stability 96 (2011) 2198e2208

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Synthesis and properties of phosphorus polyesters with systematicallyaltered phosphorus environment

Oliver Fischera, Doris Pospiecha,*, Andreas Korwitza, Karin Sahrea, Liane Häußlera, Peter Friedela,Dieter Fischera, Christina Harnischa, Yana Bykovb, Manfred Döringb

a Leibniz Institute of Polymer Research Dresden, Hohe Straße 6, 01069 Dresden, GermanybKarlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

a r t i c l e i n f o

Article history:Received 28 July 2011Received in revised form5 September 2011Accepted 15 September 2011Available online 22 September 2011

Keywords:Thermal decompositionFlame retardantPhosphorus-containing polyestersPolycondensationTransesterification

* Corresponding author. Tel.: þ49 351 4658 497; faE-mail addresses: fischer-oliver@ipfdd.de (O.

(D. Pospiech), korwitz@ipfdd.de (A. Korwitz), sahre@ide (L. Häußler), friedel@ipfdd.de (P. Friedel), fisch@ipfipfdd.de (C. Harnisch), yana.bykov@kit.edu (Y. Byko(M. Döring).

0141-3910/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2011.09.006

a b s t r a c t

Monomers with phosphorus-containing substituents were incorporated into aromaticealiphatic poly-esters to develop polymeric halogen-free flame retardants as additives for poly(butylene terephthalate)(PBT). They were built into the polyester backbone of PBT substituting 1,4-butane diol as monomer byphosphorus-containing aromaticealiphatic diols. Starting from 10-(2,5-bis(2-hydroxyethoxy)phenyl)-9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO-HQ-GE), the chemical structure of thephosphorus monomers was systematically varied resulting in new polymers with diphenyl phosphineoxide substituents and bridged phosphine oxide units. The polymers were prepared by trans-esterification polycondensation in the melt in lab-scale as well as in a 2.4 l-autoclave. The properties ofthe polyesters were determined and compared to the DOPO-based polyester with respect to the achievedmolar mass and polydispersity, solid state structure, glass transition temperature, thermal stability andcombustion behavior.

It was found that the different phosphorus substituents lead to different glass transition temperatures.The polymers containing bridged phosphorus structural units showed higher glass transition tempera-tures Tg and resulted in higher char yields after thermal decomposition. Both phosphine oxide structuresshowed only one-step decomposition with a shoulder at the end of the step. In contrast, two separatesteps were observed in the polyesters with DOPO-substituents. The results indicated that the phosphoruspolyesters under discussion are suitable to adjust the flame retarding mechanism.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Flame retardancy of engineering plastics is essential particularlyin applications in electronics, electrical engineering and in auto-motive parts. Thus, addition of flame retardants (FRs) is necessarybecause themajority of the known organic polymers is not stable infires. Up to now, mostly halogenated FRs have been used whoseemployments in electrical and electronic equipmentwere restrictedin the course of the European Community directives ROHS andWEEE [1]. Alternatives to replace halogenated FRs can be found inphosphorus-containing compounds either as low-molar masscompound or as polymericmaterial. A huge number of phosphorus-

x: þ49 351 4658 565.Fischer), pospiech@ipfdd.depfdd.de (K. Sahre), lili@ipfdd.dd.de (D. Fischer), harnisch@v), manfred.doering@kit.edu

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based additives has been proposed and already examined in therecent years [2e11]. Some of them are already successfullyemployed in commercial applications of polyester materials (forinstance Fyrol� PNX by Supresta, Exolit�-series by Clariant). Theflame retardant action results from inhibition of both ignition aswell as burning by inducing different mechanisms (gas phasemechanism, condensed state mechanism, intumescence) [12].

9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO)is a special structure that has already been investigated in detail. Theaddition of DOPO to epoxy resins was examined by Schartel et al.[13,14], wherein they examined DOPO as FR both as additive and asaminic hardener especially when gas phase mechanism is favored.The employment of DOPO as side group (i.e., substituent) at a poly-meric main chain is another alternative. Yang and Kim [15] usedaDOPOderivative andoligo(ethylene terephthalate) to prepareflameretarded polyesters and discovered that polymers with phosphorus-containing pendant groups are more stable against the harsh condi-tions in melt polymerization than polyesters with phosphorusincorporated in the backbone. Vice versa, copolyesters of poly

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e2208 2199

(ethylene terephthalate) (PET) and a polymerwith phosphorus in thebackbone were less stable but better in terms of flame retardancy,illustrated by higher LOI.

Our group [16,17] studied the synthesis, decomposition andflame retardance efficiency of polyesters with DOPO-substituentsand their nanocomposites with organophilically modifiedlayered silicates [18]. The clay raised the amount of char but didnot change the fundamental decomposition mechanism. Recently,Brehme et al. [19] compared commercially available aluminiumdiethylphosphinate and DOPO-substituted polyester as additive/blend for poly(butylene terephthalate) (PBT), concluding that atthe same phosphorus concentration the fire risk reduction is morepronounced for the phosphorus polymer. Main advantages of theDOPO polyester compared to the aluminium phosphinate aremeant to be the higher char yield, increased time to ignition andintumescence occurring right after ignition. To enhance thephosphorus content in polyesters, Chang et al. [20] synthesizedpolymers with DOPO attached to the polyester backbone as well asphosphorus in the main chain. Blends with PET were preparedwith a phosphorus content of 1.1 wt% and below resulting inunchanged mechanical properties (compared to PET) and goodUL94 ratings (V-0) and LOI values (up to 35.2% O2). Aromaticpolyethersulfones with DOPO as pendant group applied as addi-tive to raise the flame retardancy of epoxy resins were describedby Hoffmann et al. [21,22]. They compared polysulfones with twophosphorus-containing pendant groups and discovered differentmechanisms of action: DOPO with pronounced gas phase andDPPO with pronounced condensed phase mechanism. Thus,different possibilities of application for both phosphorus groupswere concluded.

Progressing these observations, the general aim of this work is toenhance the basic knowledge on the impact of the chemical struc-ture of polymeric flame retardants on the resulting decompositionand combustion behavior by comparing the DOPO-substitutedpolyesters with those having different phosphorus groupsattached. PET-P-DOPO is the starting polymer synthesized fromdimethyl terephthalate and 10-(2,5-bis(2-hydroxyethoxy)phenyl)-9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO-HQ-GE). As shown in Fig. 1, the chemical environment of the phos-phorus was varied by using two phosphine oxide derivatives(O]PR3) instead of a phosphinate (O]P(OR)R2); one having anoxygen bridge between the phenylene rings and one without anybridge. A model polymer without phosphorus substituents but thesame backbone was prepared to compare with.

The resulting polyesters were compared in order to evaluatethe influence of the chemical environment of the phosphorusgroups on the property profile, in particular with respect to thedecomposition behavior. For better understanding molecularmodeling by ab-initio calculation of geometry-optimized mono-mer structures was performed and used for interpretation of theresults.

Fig. 1. Chemical structures of the phosphorus polymers and model polymer withoutphosphorus substituent.

2. Experimental

2.1. Materials

Ethylene carbonate (anhydrous, SigmaeAldrich), dimethylace-tamid (�99.5%, SigmaeAldrich), hydroquinone bis(hydroxyethyl)ether (98%, SigmaeAldrich), p-benzoquinone (�98%, Merck),toluene (absolute, SigmaeAldrich), titanium tetrabutylate (98þ,Alfa Aesar) and antimony(III) oxide (99.9þ, SigmaeAldrich) wereused without any further purification. Potassium iodide (99þ,Acros) was dried in low pressure at 120 �C prior to use. Dimethylterephthalate (DMT, 99þ, Aldrich) was recrystallized twice fromethanol and dried. DOPO-HQ-GE [22], 9,10-dihydro-3,6-dimethyl-9-oxa-10-phospha-anthracene-10-oxide (DPPO) [23] and diphe-nylphosphine oxide (DPhPO) [24] were synthesized according tothe literature procedures. The phosphorus compound DOPO-HQwas kindly provided by Quanta Engineering & Consulting Co.,LTD, as scientific sample.

2.2. Synthesis procedures

2.2.1. Synthesis of the monomer DPPO-HQ-GE10-(2,5-Dihydroxyphenyl)-9,10-dihydro-3,6-dimethyl-9-oxa-

10-phospha-anthracene-10-oxide (DPPO-HQ): DPPO (114.2 g,467 mmol) was dissolved in dry toluene (1200 ml) at 60 �C ina three neck flask equipped with condenser and argon gas inlet.p-Benzoquinone (65.7 g, 608 mmol) was added in portions to thereactionmixture over a period of 15minwhile stirring. The reactionmixture was refluxed over night and than cooled to roomtemperature. The formed precipitate was filtrated off and washedthoroughly with dichloromethane to give an off brown solid. Thecrude product was refluxed over a period of 4 h in ethanol (500 ml)and kept at room temperature over night. After filtration followedby washing with a small amount of ethanol and drying underreduced pressure at 120 �C for 6 h, DPPO-HQ was obtained asa white solid (125.3 g, 299 mmol, 64%): mp 272e274 �C; 31P NMR(121 MHz, DMSO-d6) d �2.5 ppm; 13C NMR (63 MHz, DMSO-d6)d 153.3 (d, J ¼ 3.4 Hz, 2 C), 151.6 (d, J ¼ 4.0 Hz, 1 C), 149.6 (d,J ¼ 14.9 Hz, 1 C), 134.6 (s, 2 C), 132.7 (d, J ¼ 12.6 Hz, 2 C), 129.7 (d,J¼ 5.4 Hz, 2 C),121.4 (d, J¼ 2.1 Hz,1 C),117.8 (d, J¼ 8.0 Hz,1 C),117.7(d, J¼ 113.9 Hz,1 CeP), 117.6 (d, J¼ 6.5 Hz, 2 C), 117.0 (d, J¼ 9.0 Hz,1C), 115.4 (d, J ¼ 124.3 Hz, 2 CeP), 20.0 ppm (s, 2 CH3); 1H NMR(250 MHz, DMSO-d6) d 9.39 (s, 1H, OH), 9.16 (s, 1 H, OH), 7.50e7.25(m, 6 H), 7.15 (dd, J ¼ 14.7 Hz, J ¼ 2.8 Hz, 1 H), 6.79 (dd, J ¼ 8.6 Hz,J¼ 2.8 Hz,1 H), 6.48 (dd, J¼ 8.6 Hz, J¼ 7.0 Hz,1 H), 2.28 ppm (s, 6 H,2 CH3); IR (KBr): n 3177 (s, OeH), 2361, 2337, 1608 and 1585 (w, C]C), 1491, 1471 and 1448 (s-vs, PeCaryl), 1392, 1275 (vs, P]O), 1231,1160, 1138, 1280, 915, 816, 719, 577, 520, 497 cm�1; HRMS (EI)calculated for [12C20H17O4P]þ 352.0864, found 352.0874 Da.

10-(2,5-Bis(2-hydroxyethoxy)phenyl)-9,12-dihydro-3,6-dimethyl-9-oxa-10-phospha-anthracene-10-oxide (DPPO-HQ-GE):A three neck flask with condenser and argon gas inlet was floodedwith argon and charged with DPPO-HQ (90.0 g, 256 mmol),ethylene carbonate (67.5 g, 767 mmol), potassium iodide (ca. 2.5 g)and dry N,N-dimethylacetamide (200 ml). After refluxing overnight followed by cooling to room temperature, the reactionmixture was poured into distilled water (1 l). The formed precipi-tate was filtrated off, washed with distilled water and dried at air togive an off beige solid. The crude product was refluxed over a periodof 30 min in ethanol (150 ml) and kept at room temperature overnight. After filtration followed by washing with a small amount ofethanol and drying in reduced pressure at 190 �C for 48 h, DPPO-HQ-GE was obtained as white solid (92.6 g, 212 mmol, 82%): mp245e247 �C; 31P NMR (121 MHz, DMSO-d6) d �6.1 ppm; 13C NMR(63 MHz, DMSO-d6) d 153.2 (d, J ¼ 3.3 Hz, 2 C), 152.9 (d, J ¼ 3.1 Hz,

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e22082200

1 C), 152.3 (d, J ¼ 14.3 Hz, 1 C), 134.6 (s, 2 C), 132.6 (d, J ¼ 12.6 Hz, 2C), 129.6 (d, J ¼ 4.7 Hz, 2 C), 122.1 (d, J ¼ 112.3 Hz, 1 CeP), 119.9(s, 1 C), 118.4 (d, J ¼ 7.4 Hz, 1 C), 117.6 (d, J ¼ 6.2 Hz, 2 C), 115.2(d, J ¼ 125.5 Hz, 2 CeP), 114.3 (d, J ¼ 8.4 Hz, 1 C), 70.1 (s, 1CH2eOeCaryl), 69.7 (s, 1 CH2eOeCaryl), 59.6 (s, 1 CH2eOH), 58.5 (s, 1CH2eOH), 19.9 ppm (s, 2 CH3); 1H NMR (250 MHz, DMSO-d6)d 7.55e7.24 (m, 7H), 7.12 (dd, J ¼ 3.1 Hz, J ¼ 9.0 Hz, 1 H), 6.86 (dd,J ¼ 6.9 Hz, J ¼ 8.7 Hz, 1 H), 4.92 (t, J ¼ 5.5 Hz, 1 H, OH), 4.56(t, J ¼ 5.5 Hz, 1 H, OH), 4.01 (t, J ¼ 4.8 Hz, 2 H, OeCH2), 3.73(q, J ¼ 5.0 Hz, 2 H, OeCH2), 3.44 (t, J ¼ 6.3 Hz, 2 H, OeCH2), 3.14(q, J ¼ 6.0 Hz, 2 H, OeCH2), 2.28 ppm (s, 6H, 2 CH3); IR (KBr) n 3421and 3282 (s, OeH), 3065 (w, CaryleH), 2944, 2921 and 2869 (m,CeH), 2362, 2339, 1609 and 1586 (m, C]C), 1492, 1472 and 1460(vs, PeCaryl), 1394, 1278 (vs, P]O), 1223 and 1178 (vs, CeO), 1142,1292, 1282, 1260, 1242, 915, 903, 820, 800, 721, 713, 558, 526, 516,495 cm�1; HRMS (EI) calculated for [12C24H25O6P]þ 440.1389, found440.1317 Da.

2.2.2. Synthesis of the monomer DPhPO-HQ-GE(2,5-Dihydroxyphenyl)diphenylphosphineoxide (DPhPO-HQ):

DPhPO (212.0 g, 1240 mmol) was suspended in toluene (700 ml) ina three neck flask equipped with condenser, mechanical stirrer andargon gas inlet. p-Benzoquinone (146.1 g, 1352 mmol) was added inportions to the reaction mixture over a period of 30 min whilevigorously stirring at room temperature. The reaction mixture washeated using an oil bath to 60 �C, stirred at this temperature overa period of 1 h and cooled to room temperature. The formedprecipitate was filtrated off and washed thoroughly with acetone togive a grey solid. After recrystallisation fromethylene glycol followedby washing with ethanol and drying in vacuum at 130 �C for 12 h,DPhPO-HQ was obtained as a white solid (196.7 g, 634 mmol, 61%):mp 212e212 �C; 31P NMR (121MHz, DMSO-d6) d 28.8 ppm; 13C NMR(63MHz, DMSO-d6) d 152.3 (d, J¼ 3.3 Hz,1 C), 149.8 (d, J¼ 14.2 Hz, 1C), 133.0 (d, J ¼ 125.3 Hz, 2 CeP), 131.7 (d, J ¼ 2.3 Hz, 2 C), 131.3 (d,J ¼ 12.3 Hz, 4 C), 128.4 (d, J ¼ 12.1 Hz, 4 C), 121.4 (d, J ¼ 1.4 Hz, 1 C),118.5 (d, J ¼ 8.1 Hz, 1 C), 117.5 (d, J ¼ 8.9 Hz, 1 C), 116.0 ppm (d,J¼ 122.6 Hz,1 CeP); 1H NMR (250MHz, DMSO-d6) d 9.75 (s,1H, OH),9.12 (s,1 H, OH), 7.71e7.46 (m,12H), 6.94 (dd, J¼ 13.9Hz, J¼ 2.9Hz,1H), 6.85 (dd, J ¼ 8.6 Hz, J ¼ 2.7 Hz, 1 H), 6.70 ppm (dd, J ¼ 8.7 Hz,J ¼ 6.1 Hz, 1 H); IR (KBr) n 3144 (s, OeH), 1606 and 1591 (w, C]C),1431 (vs, PeCaryl), 1252, 1230, 1206, 1130 (vs, P]O), 1288, 1253, 826,752, 734, 716, 691, 560, 569, 534, 520 cm�1; HRMS (EI) calculated for[12C18H15O3P]þ 312.0759, found 312.0778 Da.

(2,5-Bis(2-hydroxyethoxy)phenyl)diphenylphosphine oxide(DPhPO-HQ-GE): A three neck flask with condenser and argon gasinlet was flooded with argon and charged with DPhPO-HQ (80.0 g,258 mmol), ethylene carbonate (68.1 g, 774 mmol), potassiumiodide (ca. 2.0 g) and dry N,N-dimethylacetamide (120 ml). Thereaction mixture was refluxed over a period of 24 h, cooled to roomtemperature, and poured into distilled water (1.5 l). After additionof sodium chloride (ca. 50 g) the crude product appeared as oil. Thewater phase was decanted. The oil was washed by vigorous stirringin freshly distilled water (3 times, 200 ml each), dissolved indichloromethane (ca. 150 ml) and dried over MgSO4. MgSO4 wasfiltrated off and diethyl ether was added carefully to the filtrateuntil a small amount of insoluble fine precipitate formed. Thesolutionwas kept for 4 days at�13 �C. The formed beige precipitatewas separated from the liquid by decantation and washed withacetonitrile. Two additional portions of the crude product wereobtained from the mother liquid. The combined precipitates wererecrystallized from acetonitrile and dried under reduced pressureat 90 �C for 20 h to yield the required product, as a white solid(60.6 g, 152 mmol, 59%): mp: 134e136 �C; 31P NMR (121 MHz,DMSO-d6) d 26.0 ppm; 13C NMR (63 MHz, DMSO-d6) d 154.0(d, J¼ 2.6 Hz, 1 C), 152.5 (d, J¼ 14.1 Hz, 1 C), 132.7 (d, J¼ 126.6 Hz, 2

CeP), 131.7 (d, J ¼ 2.3 Hz, 2 C), 131.2 (d, J ¼ 12.2 Hz, 4 C), 128.3(d, J¼ 12.2 Hz, 4 C), 121.5 (d, J¼ 121.3 Hz, 1 CeP), 119.7 (s, 1 C), 119.4(d, J ¼ 8.5 Hz, 1 C), 115.2 (d, J ¼ 7.4 Hz, 1 C), 71.2 (s, 1 CH2eOeCaryl),70.1 (s, 1 CH2eOeCaryl), 59.5 (s, 1 CH2eOH), 59.1 ppm (s, 1CH2eOH); 1H NMR (250 MHz, DMSO-d6) d 7.73e7.45 (m, 12 H), 7.19(dd, J ¼ 8.9 Hz, J ¼ 2.9 Hz, 1 H), 7.12 (dd, J ¼ 8.9 Hz, J ¼ 5.7 Hz, 1 H),7.00 (dd, J¼ 3.0 Hz, J¼ 14.1 Hz,1 H), 4.96 (t, J¼ 5.6 Hz,1 H, OH), 4.85(t, J ¼ 5.6 Hz, 1 H, OH), 3.91 (t, J ¼ 4.9 Hz, 2 H, OeCH2), 3.82(t, J¼ 5.1 Hz, 2 H, OeCH2), 3.67 (q, J¼ 5.1 Hz, 2 H, OeCH2), 3.22 ppm(q, J ¼ 5.3 Hz, 2 H, OeCH2); IR (KBr) n 3401 und 3319 (s, OeH), 3067and 3057 (w, CaryleH), 2961 and 2875 (m, CeH), 1590 and 1575(w-m, C]C), 1488, 1476, 1450 and 1437 (vs, PeCaryl), 1275 and 1216(vs-s, CeO), 1150 (s, P]O), 1121, 1279, 1257, 1239, 1224, 878, 760,733, 712, 695, 601, 569, 534, 517 cm�1; HRMS (EI) calculated for[12C22H23O5P]þ 398.1283, found 398.1343 Da.

2.2.3. General procedure for melt polycondensationat laboratory scale

The diol monomer (20 mmol), DMT (21 mmol) and the catalystmixture (1 wt.% with respect to DMT) containing Ti(OBu)4/Sb2O3 in1/1 (wt/wt) ratio were placed in a three neck flask with an inert gasinlet, distillation bridge connected to a vacuum line andmechanicalstirrer. After three cycles of reducing pressure and flooding withnitrogen the reaction vessel was placed into a metal bath preheatedto 150 �C. The bath was heated up with 2 K/min to 270 �C havinga slow nitrogen flow applied. After some minutes the solid eductswere completely molten and the stirrer was set to 120 rpm.Reaching 270 �C, the nitrogen flow was slightly increased andremained for further 30 min. After this period the nitrogen inletwas closed and reduced pressure was applied to the system for 5 h.The flask was replaced from the metal bath and the polymer wasremoved while it was still fluid. Yields: 85e95%.

2.2.4. General procedure for melt polycondensationin 2.4 l-autoclave

The diol monomer (330 mmol), DMT (347 mmol) and thecatalyst mixture (1 wt.% with respect to DMT) were added into thevessel of a pre-dried 2.4 l-stainless-steel-autoclave. The attach-ments to the reactor were similar to the lab-scale synthesis, buthaving additional sensors for pressure, core temperature and tor-que. After closing the reactor and three cycles of evacuation andpurging with nitrogen, the vessel was heated quickly to 200 �C.After setting the stirrer to 120 rpm, a heating rate of 2 K/min wasapplied until 275 �C were reached. 30 minutes after reaching thistemperature reduced pressure was applied and remained for 7 h.The reaction was stopped by cooling down and the polycondensatewas removed mechanically from the autoclave. Yields: 85e93%.

For PET-P-DOPO synthesis a further scale-up process wasexecuted: DOPO-HQ-GE (453 g, 1.1 mol), DMT (224 g, 1.15 mol) andcatalyst mixture (2.2 g) were placed into the pre-dried 2.4 l-auto-clave. The heating and reduced pressure phases were appliedaccording to the general procedure described above. Yield: 93%.

2.3. Methods

Solution viscosity was measured in an Ubbelohde viscometer at25 �C in pentafluorophenol/chloroform mixture (1:1 vol:vol) ata concentrationof 5 g polymer/l solvent, giving the inherent viscosityhinh ¼ loghrel/c [dl/g]. Size exclusion chromatography (SEC) wasconducted using a SEC system (Knauer, Germany) with RI detectorand Plgel 5 mm MiniMIX-D column with a 1:2 vol:vol mixture ofpentafluorophenol andchloroformas eluent. Themolarmasseswerecalculated relative to PS-standards. NMR spectroscopy was carriedout on a DRX 500 spectrometer (Bruker, Germany) operating at500.13 MHz for 1H, 202.40 MHz for 31P (proton-decoupled) and on

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e2208 2201

a BZH 250/52 (Bruker, Germany) at 250 MHz for 1H,121 MHz for 31P(proton-decoupled) and 63 MHz for 13C (proton decoupled, phos-phorus coupled). The solvents used were DMSO-d6 (monomers) andTFA-d/CDCl3 (1:1 vol:vol, polymers). Melting ranges (uncorrected)were measured with a B-545 instrument (Büchi).

High resolution mass spectrometry (HR MS) analyses of thephosphorus monomers were performed on a MicroMass GCT (timeof flight (TOF); electron ionization (EI), 70 eV; Waters, USA). IRspectra were recorded with a Varian 660-IR (Agilent Technologies,USA) and Golden Gate Diamant ATR (Agilent Technologies, USA).

Mass spectra of the polymers were acquired on a MALDI-TOFsystem (Bruker Biflex, Daltonics GmbH) in reflector mode. Formeasuring, the samples were dissolved in chloroform and thenmixed with 2-(4-hydroxyphenylazo)benzoic acid (HABA) as matrixdissolved in tetrahydrofuran. Subsequently, 1 ml-portions weresuperimposed on the target. After removing of solvent the sampleswere exposed to desorption/ionization processes induced bypulsed N2 laser (337 nm). The ionized molecules were acceleratedand reflected by electric fields and projected on a mass sensitivedetector. Eachmass spectrum represents the accumulation of about200 single-laser-spot spectra.

Differential scanning calorimetry (DSC) was performed usinga DSC Q1000 (TA Instruments, USA) under nitrogen in modulatedmode in the temperature range from �30 �Ce250 �C at a scan rateof 2 K/minwith an amplitude of� 0.31 K and a period of 40 s. Glasstransition temperatures were calculated from reversing heat flowsignal using the half-step method.

Thermogravimetric analysis (TGA) was done with a Q5000(TA Instruments, USA) coupled with Fourier-transformed infraredspectrometer Nicolet 380 (Thermo Electron, USA) (TGA-FTIR) in thetemperature range from 30 to 800 �C at a heating rate of 10 K/minand 30e900 �C at a heating rate of 60 K/min under nitrogenatmosphere (25 ml/min). The FTIR spectra of the gas evolved wereacquired from 8 scans with a resolution of 4 cm�1. Pyrolysis-GC/MSwas done using a Pyroprobe 5000 (CDS Analytical, USA) as pyro-lyzer, a GC 7890A (Agilent Technologies, USA) for gas chromatog-raphy and 5975C MSD (EI 70 eV, scan range between 15 and550 m/z; Agilent Technologies, USA). Pyrolysis Combustion FlowCalorimetry (PCFC) was executed with a FAA Micro Calorimeter(Fire Testing Technology Ltd., UK) with a heating rate of 1 K/s in thetemperature range from 75 �C to 650 �C and 0.16667 K/s in a rangeof 85e750 �C.

The phosphorus content was measured by elemental analysis(Mikroanalytisches Labor KOLBE, Höhenweg 17, D‑45470 Mülheima. d. Ruhr, Germany).

Quantum mechanical ab-initio calculations of the phosphorusmonomers were performed by an energy minimization procedurewith the program package GAMESS [25] by applying the 6-31Gbasis set (without diffuse orbitals) of the restricted open-shellHartree-Fock (ROHF) self-consistent field (SFC) approach. If theSCF procedure had convergence problems due to less distinct

Fig. 2. Geometry-optimized structures of the monomers a) DOPO-HQ-GE; b) DPPO-HQ-GE alight grey.

minima of the potential energy surface, the unrestricted Hartree-Fock (UHF) approach was used for these optimization steps. Then,the last optimization step was repeated as long as ROHF could beapplied for holding the energetic results consistent. The gradienttolerancy for the deepest descent method was chosen to 0.0001Hartree/Bohr as convergence criterion in each case.

3. Results and discussion

3.1. Chemical structure of the phosphorus monomers

The phosphorus monomers included in this study consist ofbulky phosphorus-containing substituents attached to glycolizedhydroquinone rings. Fig. 2 shows the geometry-optimized molec-ular models of the phosphorus-containing monomers obtained byquantum mechanical ab-initio calculation. The chemical environ-ment of the phosphorus substituent was altered systematicallystarting with DOPO as phosphinate derivative with one phenylgroup directly linked to the phosphorus atom and another one asphosphoryl ester group. The phenyl groups are linked by a -CeC-bridge to form a six-membered ring, as illustrated in Fig. 2a. DPhPOand DPPO are both phosphine oxide derivatives with two phenylgroups attached to the phosphorus which are either non-linked(DPhPO, Fig. 2c) or linked by an ether bridge forming again a six-membered ring (DPPO, Fig. 2b). p-Bis(hydroxyethoxy) benzenewas used as model monomer having comparable basic structurebut without phosphorus substituent giving the model (reference)polyester abbreviated here PET-P-0.

Due to the bulkiness of the substituents as well as the aroma-ticealiphatic character of the backbone, glass transition tempera-tures Tg of the resulting polyesters significantly higher than for PETor PBT were expected. Indeed, PET-P-DOPO polyester exhibited a Tgof 143 �C as reported in the previous study [17]. The two additionalmethyl groups in DPPO compared to DOPO increase the bulkinessof PET-P-DPPO even more. For this reason, a further raise of Tg. forPET-P-DPPO was supposed. Vice versa, a Tg drop in PET-P-DPhPOcompared to PET-P-DOPO was assumed due to the two non-linked phenyl groups resulting in a higher degree of freedom(see 3.3 for results).

3.2. Polyester synthesis

The phosphorus diols were used in transesterification poly-condensation of DMT to yield phosphorus polyesters PET-P-x. Thereaction was performed in melt and was catalyzed by trans-esterification catalysts. The syntheses were carried out undereffective stirring at lab-scale in glassware equipment and in up-scaled procedures in a 2.4 l-stainless steel stirring autoclave withvacuum line. The resulting products were brittle at room temper-ature, whereas the reference polymer PET-P-0 was very tough and

nd c) DPhPO-HQ-GE, carbon: black; hydrogen: white; oxygen: dark grey; phosphorus:

Table 1Chemical characterization of the phosphorus polyesters by means of size exclusion chromatography (SEC), solution viscosity and differential scanning calorimetry (DSC).

Samplea Synthesissize (g)

SEC Viscosity P-content DSC

Mn (kg/mol) Mw (kg/mol) Mw/Mn hinh (dl/g) calc. (wt%) meas. (wt%) Tg (�C)

PET-P-0 10 71.5 145.9 2.04 1.28 0 0 73PET-P-DOPO-1 10 12.7 29.5 2.32 0.31 5.71 5.72 145PET-P-DOPO-R6 560 20.2 309.4 15.32 0.60 5.71 5.95 147PET-P-DOPO-R11 160 18.8 141.0 7.52 0.45 5.71 5.97 146PET-P-DPPO-1 10 14.2 43.1 3.04 0.32 5.43 5.48 160PET-P-DPPO-R1 170 11.7 36.5 3.12 0.27 5.43 5.46 160PET-P-DPhPO-1 10 14.7 78.1 5.33 0.33 5.86 5.88 119PET-P-DPhPO-R1 150 11.1 72.9 6.57 0.27 5.86 5.62 115

a R indicates those products synthesized in the 2.4 l-autoclave.

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e22082202

could only be crushed after cooling with liquid nitrogen. Thesynthesis results obtained are summarized in Table 1.

The polymerizations at lab-scale performed under comparableconditions yielded phosphorus-containing polymers with number-averaged molar masses relative to PS in the range of 12 000 g/mol.PET-P-DOPO and PET-P-DPPO had weight-averaged molar massesof about 30 000 to 40 000 g/mol, while PET-P-DPhPO gave higherMw and thus, higher polydispersity.

The synthesis of PET-P-DOPO was carried out repeatedly underlaboratory (PET-P-DOPO-1) and up-scaled conditions (PET-P-DOPO-R11, PET-P-DOPO-R6 as examples of a series of experiments)to monitor the influence of the up-scaling process. The poly-condensation times at low pressure were increased in the 2.4 l-autoclave from 5 h (the time used at lab-scale) to 8 h. Enlargingentry and polycondensation time increased the molar mass of PET-P-DOPO significantly, also indicated by higher solution viscosities.However, the molar mass distribution (polydispersity) broadenedsignificantly. As possible cause, the occurrence of a reversible sidereaction is discussed, as outlined in Fig. 3. Opening of the DOPO-ring at the PeOeC linkage under alkaline conditions was analyti-cally already proven by Hoffmann et al. [21]. In case of trans-esterification polycondensation the phosphinate may undergotransesterification similar to carboxylic esters [26] resulting in theformation of branches distributed along the polyester chain. Thus,branching would enhance the molar mass distribution. Owing tosterical hindrance and the phenolate leaving group connected viaCeC-bond, the chemical equilibrium of this reactionwill remain onside of the educts. However, after higher molar masses werereached, i.e., at higher conversion, the amount of branches perpolyester chain as well as the molar mass of the branches shouldincrease resulting in broader molar mass distribution. This wasindeed observed. The concentration of the branches is supposed tobe rather low from statistical point of view. Therefore, an analyticalproof, e.g., by NMR, was not possible.

As indication, the comparison of 31P-NMR spectra of PET-P-DOPO prepared at lab-scale (Fig. 4a) and in the stirring autoclavewith longer reaction time (Fig. 4b) shows that the purity of the lab-scale polymer is higher than that of the autoclave product, asindicated by the ratio between the phosphorus main signal at43.3 ppm to the additional phosphorus signals caused by side

Fig. 3. Proposed mechanism leading to ring opening in DOPO-HQ derivatives.

reactions. Similar NMR results were obtained for PET-P-DPhPO,where the phosphorus content of the up-scaled products waslower then that of the lab product. We assume separation of thediphenylphosphine oxide group as a side reaction due to thesomewhat higher reaction temperature and duration in the up-scaled process.

Neither PET-P-DPPO nor PET-P-DPhPO syntheses resulted insuch high molar masses and polydispersities. The lab-scale and up-scaled products were comparable.

The growth of the polymer chains in the polycondensation of allthree phosphorus monomers appears to be inhibited by the bulkysubstituents. This is indicated by the significantly raised molarmass and solution viscosity of the reference polyester PET-P-0. Atleast one side of the phosphorus-containing diols is stericallyhindered and has reduced reactivity, thus resulting in a decrease ofthe achieved molar mass. NMR, MALDI-TOF-MS and elementalanalytical determination of phosphorus content were taken intoaccount to examine this hypothesis.

The experimental phosphorus contents are also given in Table 1.Comparing lab-scale and up-scaled PET-P-DOPO polymers indi-cates a rather large difference between calculated and found values.Two possible reasons for that can be discussed: first, end-capping ofthe polyester by DOPO-HQ-GE owing to steric hindrance as dis-cussed above, and second, ring closure.

MALDI-TOF was used to further clarify the chemical structure ofPET-P-DOPO products. No MALDI data of DOPO-based phosphoruspolyesters were found in the literature, so, to the best of ourknowledge, this is the first description of MALDI results for this

Fig. 4. 31P NMR (202.40 MHz, TFA-d/CDCl3 1:1 (vol/vol)) of PET-P-DOPO prepared bylab scale (a) and up-scaled procedures (b), shifts related to slightly different solventmixtures.

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e2208 2203

complicated polymer species. The samples were purified beforeanalysis by re-precipitation from chloroform to methanol toremove catalyst residues. Fig. 5 illustrates results obtained withPET-P-DOPO-1 (lab-scale) and PET-P-DOPO-R6 (autoclave).

It should be noted that the spectra contain only informationabout the polymeric structures of the sample with lower molarmass between 1200 and 5000 Da. In this region, two main distri-butions were identified, marked in Fig. 5 by different symbols. Theperiodicity in these distributions corresponding to the mass of therepeating units is the same (542 Da). This fact proves unambigu-ously the formation of the desired phosphorus polymer PET-P-DOPO. One of the main distributions could be assigned to linearPET-P-DOPO macromolecules end-capped at both ends by DOPO-phenylene glycolether units ionized by Naþ resulting fromsynthesis (n� 542 g/molþ 412 g/molþ 23 g/mol). This distributionis indicated by (L) in Fig. 5. The second distribution represents cyclicpolymer molecules (n � 542 g/mol þ 23 g/mol) (indicated by (o) inFig. 5). The formation of cycles in polycondensation reactions iswell-known [27] and favored by extension of reaction time. Thus,the autoclave product PET-P-DOPO-R6 with higher reaction time(Fig. 5b) contains mainly cyclic polymer molecules in the molarmass region up to 4000 Da and only traces of linear polymermolecules.

Fig. 5. MALDI-TOF spectra of a) PET-P-DOPO-1 (lab product) and b) PET-P-DOPO-R6(autoclave, longer reaction time). (L): signals representing DOPO-phenylene glycolend-capped, linear PET-P-DOPO; (O): signals representing cyclic PET-P-DOPOmolecules.

3.3. Thermal behavior and solid state structureof the phosphorus polyesters

Aromaticealiphatic poly(alkylene terephthalates), like PETor PBT, possess a semicrystalline structure with glass transitiontemperatures between 40 and 80 �C and melting ranges between200 and 300 �C, depending on the length of the aliphatic spacer.Poly(oxyethylene-1,4-oxyphenylene-oxyethylene-oxyterephthaloyl)(PET-P-0) has a reduced flexibility compared to PET and PBT due tothe higher aromatic content. However, both, melting range as wellas crystallinity are decreased compared to PET and PBT (Fig. 6).

In contrast, the phosphorus polyesters under discussion arecharacterized by complete loss of crystallinity. The bulky substitu-ents disturb ordering of main chains and lead to formation ofamorphous polymers only with glass transition, as illustrated in theDSC curves (Fig. 6).

The glass transition temperatures Tg of the phosphorus poly-esters are all above 120 �C, i.e., much higher than those ofpoly(alkylene terephthalates). Tg raises with bulkiness andconformation in the order of PET-P-DPhPO < PET-P-DOPO < PET-P-DPPO.

With its two non-bridged, rotable phenyl groups, PET-P-DPhPOshows the lowest Tg because of its higher degree of freedom.The highest Tg is observed in PET-P-DPPO with the bridged,phenanthrene-like ring on the phosphine oxide unit. It is expectedthat this non-crystalline structure of the phosphorus polyesterswill give rise to deterioration of mechanical properties which are inpolyesters mainly determined by the crystallinity [28].

3.4. Investigation of thermal decompositionof the phosphorus polyesters

The thermal decomposition and combustion of the polymerswas investigated by combination of three methods, TGA/FTIR,Pyrolysis-GC/MS and PCFC. To compare the different phosphoruspolymer structures, samples with matching relative molar mass,solution viscosities and low deviation of the phosphorus contentfrom the calculated value were selected. TGA measurements areusually carried out with a heating rate of 10 K/min, while theheating rate often applied for PCFC is 60 K/min. For a better

Fig. 6. DSC scans (2nd heating) of phosphorus-containing polymers and model poly-mer PET-P-0.

Table 2Thermal decomposition and combustion behavior of the phosphorus polyesters.

Sample Heating rate(K/min)

TGA PCFC

Tmaxa (�C) Residue at

750 �C (wt%)Tmax (�C) MaxHRC (J/g K) HR (kJ/g) HOC (kJ/g) Residue at

750 �C (wt%)

PET-P-0 10 422 (sh)b/446 22.7 409 � 2 260 � 4 14.3 � 0.2 18.1 � 0.3 21.0 � 0.4523 493 � 6 23 � 2

60 460(sh)/480 20.5 445 � 1 253 � 3 14.5 � 0.1 18.5 � 0.1 21.3 � 0.1562 (sh) 529 � 5 22 � 3 (650 �C)

PET-P-DOPO-1 10 427 36.9 399 � 4 175 � 8 13.0 � 0.7 19.3 � 1.5 32.8 � 2.7476 (700 �C)c 436 � 5 116 � 10

60 470 33.1 445 � 3 163 � 7 12.0 � 0.5 20.6 � 1.2 41.6 � 0.7515 490 � 3 103 � 7 (650 �C)

PET-P-DPPO-1 10 415 (sh)/429 34.6 394 � 2 329 � 15 13.7 � 0.7 20.1 � 0.8 32.3 � 1.3465 (sh)

60 467 30.7 444 � 1 284 � 5 13.1 � 0.3 20.8 � 0.5 37.2 � 0.2535 (sh) (650 �C)

PET-P-DPhPO-1 10 424 23.4 394 � 2 369 � 19 18.6 � 0.3 23.2 � 0.3 19.6 � 0.9490 (sh) (700 �C)

60 466 20.7 444 � 1 317 � 13 17.5 � 0.1 23.7 � 0.1 26.3 � 0.1530 (sh) (650 �C)

a Maximum temperature of the derivative weight curve.b Shoulder, no separate peak observed.c Temperature maximum used in measurement.

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e22082204

comparison of TGA and PCFC, additional measurements wereperformed: TGA measurements with 60 K/min and PCFC with10 K/min (0.16667 K/s). All data obtained are summarized inTables 2 and 3.

Table 3Assigned fractions resulting from pyrolysis-GC/MS at 520 �C (PET-P-0), 465 �C (PET-P-pyrograms.

time (min) Mþ (m/z) Assigned structure

1.75 44 CO2, acetaldehyde2.55 78 benzene3.63 92 toluene4.91 106 ethyl benzene5.33 104 styrene5.75 108 benzoquinone6.37 106 benzaldehyde6.35e6.50 ? not identified7.48 134 ethyl benzaldehyde7.55 120 phenyl acetaldehyde7.88 120 acetophenone7.97 108 cresol8.27 136 methyl benzoate8.81 148 vinyl benzoate9.17 146 5-vinyl-2,3-dihydrobenzofuran9.25 122 þ 150 benzoic acid þ ethyl benzoate9.52 162 p-divinylether benzene9.85 120 2,3-dihydrobenzofuran10.25 136 p-vinylether-phenol10.30 148 p-vinylether benzaldehyde10.45 160 5-benzofuranyl-acetaldehyde10.55 110 p-hydroquinone10.72 152 p-benzoquinone mono glycol ace11.05 134 1,4-benzodioxine11.65 122 p-hydroxy benzaldehyde11.95 136 p-hydroxyphenyl acetaldehyde13.08 194 dimethylterephthalate13.25 162 p-vinyloxyacetophenone13.41 206 methylvinyltherephthalate13.85 208 ethylmethylterephthalate14.15 222 diethylterephthalate14.38 198 p-ditolylether15.28 184 ethylene phenylphosphonate15.42 194 diphenylcyclopropane17.11 ? not identified17.15 232 methyl diphenylphosphinate17.21 244 vinyl diphenylphosphinate13.30e15.80 overlapping, including: p-

(2-hydroxyethoxy)phenol (13.71and diethylterephthalate (14.15)

Fig. 7 shows the thermal decomposition curves and thederivative weight curves of the samples obtained with heatingrate of 10 K/min having a main decomposition step occurring in allphosphorus polymers at about 430 �C. The main step of the

DPPO) and 490 �C (PET-P-DPhPO), values are given as area% of the corresponding

PET-P-0 PET-P-DPPO PET-P-DPhPO

53 60 415 4 5<1 <1 1<1 e <1<1 <1 102 e e

1 34e <1 e

e e <1<1 <1 2e 5 e

<1 2 <1<1 <1 <1<1 e e

<1 2 44 2 22 e <1<1 5 61 e e

<1 e e

5 e e

tal 3 e e

1 e e

1 e <12 1 <1e 4 e

<1 e e

e 4 <1e <1 e

<1 <1e 1 e

e e 15e e 1e 2 e

e e 1e e 2

)16

Fig. 7. TGA curves of PET-P-DOPO, PET-P-DPPO, PET-P-DPhPO and PET-P-0.

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e2208 2205

non-phosphorus polymer PET-P-0 is at 450 �C, but a shoulder wasdetected in the region around 430 �C that was used for compar-ison of the respective FTIR spectra. Spectra at 450 �C were alsoacquired but showed a mixture of superimposing bands neithercomparable with the first nor the second decomposition steps.

3.4.1. Infrared spectra of evolved gasesIndicating acetaldehyde (3495, 2733, 1761, 1386, 1121,

933 cm�1), carbon monoxide (2170, 2112 cm�1) and carbon dioxide(2300e2400, 669 cm�1), the FTIR spectra of the gases evolvedduring the first step of decomposition (in case of PET-P-0: theshoulder at 430 �C) are similar (Fig. 8). Bands at 950 and 1270 cm�1

interpreted as ethylene (950 cm�1) and ester (1270 cm�1) byBalabanovich et al. [18] investigating the decomposition of PET-P-DOPO, were found in all the polymers. Additionally, p-substitutedaryl-alkyl ethers (Fig. 8a; 1506, 1228, 837, 719 cm�1) were observedin the spectrum of PET-P-0. They are also present in the spectra ofthe phosphorus polymers. Thus, the degradation of the backbone ofthe phosphorus-containing polymers appears to be similar to thedecomposition of the unmodified polymer.

Fig. 8. FTIR spectra of the gases evolved at the 1st degradation step at 430 �C (a: PET-P-0: b: PET-P-DOPO-1; c: PET-P-DPPO-1; d: PET-P-DPhPO-1).

The decomposition of the substituent of PET-P-DPPO (Fig. 8c)leads to diphenyl ether derivatives (1471, 1275, 1227 cm�1) withbands superimposing the ester band at 1270 cm�1. Furtherdecomposition of these biphenyl structures leads to p-cresol (3589,3020, 1506, 1246 (shoulder), 1180, 820 cm�1) and toluene (no clearevidence due to superimposing bands). Bands of phosphorus-carbon or phosphorus-oxygen bonds are not observable for thispolymer.

The bands at 3050, 1463, 1242, 931, 731, 693 cm�1 observed inthe PET-P-DPhPO spectrum (Fig. 8d) refer to a monosubstitutedphenyl ring bonded to a phosphoryl group indicating that thediphenyl phosphine oxide group is separated completely from thepolymer backbone. That is the only indication for the formation ofphosphorus-related pyrolysis products during the first decompo-sition step.

In all spectra of the second decomposition step the amount ofacetaldehyde is strongly decreased and benzoic acid (3580, 3074,1767, 1358, 1276, 1180, 721 cm�1) can be observed. The bands thatwere assigned to aryl-alkyl ether are still present but superimposedby other pyrolysis products and accompanied by aryl-aryl ether(Fig. 9bed, w1480, w1240 cm�1), except for PET-P-0, for whichonly shoulders can be guessed at these wavenumbers (Fig. 9a). Thebands of both ether types are very strong for PET-P-DPPO.

The second decomposition step of PET-P-DOPO at 476 �C evolvesDOPO derivatives in the vapor phase indicated by infrared bands at1592, 1478, 1432, 929, 755 cm�1 as already reported by Balabano-vich et al. [18]. These bandsmay also be present during the first stepbut cannot be observed due to the predominant bands of acetal-dehyde. They also prove the existence of volatile phosphorus-containing products that can act in the gas phase as flame retardant.

Bands indicating monosubstituted benzene (731 and 693 cm�1)are present in the spectrum of gases evolved from PET-P-DPhPO at430 �C, but not at 488 �C because they are superimposed by intensebands of non-substituted benzene at 3050, 1491, 690, 673 and655 cm�1. Phosphorus-related pyrolysis products are reflected bybands of aryl phosphoryl compounds with low intensity.

3.4.2. Pyrolysis-gas chromatography/mass spectrometryIn contrast to TGA measurements no heating rate is applied in

Py-GC/MS. The samples were pyrolyzed at a given constant

Fig. 9. FTIR spectra of the gases evolved at the 2nd degradation step (a: PET-P-0 at520 �C; b: PET-P-DOPO-1 at 475 �C; c: PET-P-DPPO-1 at 465 �C; and d: PET-P-DPhPO-1at 490 �C).

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e22082206

temperature. The temperatures chosen for each polymer to pyro-lyse were those of the second decomposition steps or the shoul-ders, respectively. The substances evolved in the vapor phase wereanalyzed by GC/MS. Table 3 summarizes the results of the analyzedmass spectra. Some mass spectra could not be clarified yet eitherdue to the absence of a molecular ion or to the occurrence of peakoverlapping.

As assumed, products relating to backbone decomposition canbe observed in all of the three pyrolysis experiments, leading toCO2, acetaldehyde and various esters of benzoic acid as mainproducts. Additionally, a wide bunch of aryl aldehydes was found,as well as acetophenone and its derivatives that are related to intra-or intermolecular rearrangements during pyrolysis.

Differences between the polymers are not only caused byderivatives of the degraded pendant groups. For example, benzo-quinone and hydroquinone were only found for the model polymerPET-P-0. 2,3-Dihydrobenzofuran derivatives that were formed byring closure of a phenyl-vinyl-ether can be found only in traces inthe pyrolysis products of the phosphorus polymers, but also toa lower extent in the gases of the non-phosphorus polymer.Surprisingly, tri-substituted arenes, found among the pyrolysisgases of PET-P-0, which were expected after decomposition ofsubstituted hydroquinone derivatives were not observed for thephosphorus containing polymers. So either the CaryleO bond isweaker when phosphorus is attached to the aromatic ring andcleavage will occur easier at this bond or re-protonation due torearrangement is easier after the PeC bond is cloven. Supportingthis hypothesis, the amount of hydroquinone derivatives found justas vinyl-ether-phenol or divinylether-benzene is relatively high.However, there is a lack of benzoquinone and hydroquinone thatcannot be explained with this assumption.

The decomposition of PET-P-DPPO should lead to phosphorus-containing volatiles. At least derivatives of the cleaved DPPOgroup were expected, but as observed in TGA-FTIR measurementsno phosphorus-containing structures were found. Instead, p-ditolylether and cresol could be identified as decomposition product ofthe DPPO group. In fact, there are still unidentified pyrolysis frac-tions, but without indications of phosphorus-related decomposi-tion products in these pyrograms.

Changes in the decomposition of the polymer backbone can beobserved for PET-P-DPPO indicated by the esters, in particular theterephthalates that were identified. Thus, dimethyl terephthalatewas only observed for PET-P-DPPO. The total content of tere-phthalates is about 4 area%, in contrast to PET-P-0 revealing noterephthalate at all. Usually the decomposition of the terephthaloylgroup leads to the generation of carbon dioxide and benzoates, asfound for PET-P-0 and PET-P-DPhPO wherein the cleavage of thealkylic CeC bond occurs but plays only a minor role.

Phosphorus compounds found in TGA-FTIR were also observedin pyrolysis-GC/MS. The mass spectra of the pyrolysis products ofPET-P-DPhPO contain a fragment ion of 47 m/z which can beassigned to P]O radicals. Three different phosphorus-containingstructures were identified, two diphenylphosphinates and one

Fig. 10. Assigned structures of phosphorus-containing pyrolysis products of PET-P-DPhPO.

phenylphosphonate, with an estimated total amount of 18 area%(Fig. 10). The phenylphosphonate is the second largest fraction ofthis pyrolysis (15 area%) described as rearrangement with theglycol ether of the backbone by simultaneous cleavage of two of thethree CaryleP bonds. The occurrence of this pyrolysis product isquite interesting, but until yet there is no obvious explanation whythis rearrangement is favored. Moreover, styrene as a product ofthese rearrangments has a fraction of 12 area%, while for PET-P-0 and PET-P-DPPO less than 1 area% was observed. The genera-tion of these two structures, ethylene phenylphosphonate andstyrene, seems to be quite complex taking the original polymerstructure into account. Nevertheless, the amount of these rear-ranged structures is higher than the amount of structures gener-ated by bond cleavage with hydrogen shifting.

These changes in the decomposition of the parental polymer arecalled solid phase mechanism, but due to the small amount of charthat is formed by the DPhPO substituent in addition to the charformed by the model polymer it may not increase flame retardancybased on this mechanism. In case of the high amount ofphosphorus-containing volatiles we assume gas phase mechanismis favored.

3.4.3. Pyrolysis combustion flow calorimeterPCFC is a screening tool to estimate the fire risk of materials. The

microcalorimetric measurements were analyzed by calculating thearea below the heat release rate versus time curve to achieve thetotal heat release (HR). The effective heat of combustion of thevolatiles is calculated using equation h0c ¼ HR=1� m [29], whereinh0c is the heat of combustion (HOC) and m the char yield. The thirdessential parameter is the heat release capacity which is calculatedby dividing the heat release rate maximum by the heating rate.

Table 2 summarizes PCFC as well as TGA data obtained with twoheating rates (10 and 60 K/min). Increasing the heating rate to60 K/min shifts the decomposition maxima in both methods, PCFCand TGA, by about 40 K to higher temperatures. There are differ-ences between PCFC and TGA measurements done with compa-rable heating rate. The TGA maxima were found at highertemperature compared to the maxima in PCFC (difference about30 K). This behavior is illustrated in the Suppl. Fig. 1 showing theresults obtained for PET-P-DOPO-1. The differences found traceback to the fact that it is impossible to hold all setup parameters inthe two methods constant: pan material, pan geometry, gas flow,direction of gas inlet to the cell, and other influencing parameters.

Fig. 11. Heat release rate versus temperature curves measured by PCFC for thedifferent P-containing polyesters and the model polymer.

O. Fischer et al. / Polymer Degradation and Stability 96 (2011) 2198e2208 2207

Therefore, for the discussion of the results concerning theinfluence of chemical structure of the PET-P polyesters the standardmeasuring parameters with heating rates of 10 K/min (TGA) and1 K/s (PCFC) were used.

The heat release rate curves of the polymers (Fig. 11) are similarto the TGA derivative weight loss curves. All global maxima of theheat release rates are positioned in a small temperature rangebetween 443 and 446 �C. Polymers like PET or PBT exhibit only oneglobal maximum. It is known for DOPO-derivatives [17,21] that twomaxima occur. This is in accordance with the decomposition stepsobserved by TGA measurements and was also found for PET-P-0.But this behavior is in contrast to the decomposition observed inTGA for PET-P-DPPO and PET-P-DPhPO, wherein both exhibiteda shoulder at the high-temperature side of the global maximuminstead of a second step (Fig. 7). In PCFC, only tailing of the high-temperature side of the main decomposition step was found.

The substitution of the polymer backbone by DPhPO increasedthe amount of residue slightly, but the heat generation significantly.Both PET-P-DOPO and PET-P-DPPO generated a higher char yieldthan PET-P-0, but in both cases the heat that is produced by thevolatiles (HOC, Table 2) is also higher. So, all three phosphoruspolymers degrade to pyrolysis products that generate more heatthan those of the non-substituted polymer PET-P-0.

The total heat release per initial mass (HR) of PET-P-DPPO isslightly higher than that of PET-P-DOPO (Table 2). Despite thesecond decomposition step of PET-P-DOPO both polymers producea similar quantity of heat. The differences concerning HR can beoriginated from the lower char yield of PET-P-DPPO. Although HOCis similar, PET-P-DOPO dispense the evolving heat in two separatelocal peak heat release rates, thus maxHRC is quite lower than PET-P-DPPO. According to Lyon et al. [30] the lower HRC of PET-P-DOPOpredicts a lower flammability.

Based on the TGA-FTIR results (see Figs. 8 and 9) the DOPO-group remains in the residue during the first decomposition stepand is released within the second step, leaving a high char yield.PET-P-DPPO seems to vary the decomposition mechanism byreleasing the diphenyl ether but keeping the phosphorus in thechar to further redirect backbone decomposition and enhance charforming. Even at higher temperatures the phosphorus seems to bebonded to the residue.

In contrast PET-P-DPhPO exhibits a different behavior, withsignificantly higher HR and HRC. The char yield is lower and a largeramount of volatiles produces a higher effective heat of combustion.

4. Conclusions

The decomposition of three different polyesters withphosphorus-containing substituents was investigated using variousmethods. The results for the PET-P-DOPO polyester were alreadypublished by Balabanovich et al. and Pospiech et al. [16e18]. Thesedata could be verified and provided a serious basis for this study.

TGA-FTIR and pyrolysis-GC/MS measurements assessed thechemical structure of volatile pyrolysis products. This way wewereable to detect the influence of the polyester backbone and founddifferences for each polymer with different phosphorus substit-uent. Large amounts of phosphorus-containing volatiles were ob-tained in PET-P-DPhPO pyrolysis, while phosphoros products werenot found in PET-P-DPPO pyrolysis. Additionally, pyrolysiscombustion flow calorimetric measurements were performedwhich revealed an increased heat release of PET-P-DPhPOcompared to the other polymers. Both, PET-P-DOPO and PET-P-DPPO exhibited similar values of HR and HOC. The decompositionbehavior reflected by TGA curves differed significantly, showingone (PET-P-DPPO) and two thermal decomposition steps (PET-P-DOPO), which finally lead to different heat release capacities.

The ratio between condensed phase mechanism and gas phasemechanism could not be quantified with the data of the methodsused here. Therefore, bench scale fire tests like UL94, LOI and conecalorimeter will be applied in the next examination step. However,the results obtained in this work allow some predictions. First, it isexpected PET-P-DPhPO will exhibit a less pronounced condensedphase mechanism than PET-P-DOPO. Second, it is derived that PET-P-DPPO will show a weaker gas phase action compared to PET-P-DOPO. Thus, we assume that the different polymer structuressynthesized enable the adjustment of flame retardancy for differenttypes of application.

Acknowledgements

Financial support by German Research Foundation (DFG PO 575/11-1, DO 453/7-1) is gratefully acknowledged. The authors thank DrBernhard Schartel andMr. Sven Brehme from BAM Federal InstituteforMaterial Research and Testing, Berlin, Germany for collaborationand helpful discussions as well as Dr. Hartmut Komber (IPF Dres-den) for NMR measurements and structural assignment. Further-more, O. F. would like to thank Mrs. Maria auf der Landwehr(IPF Dresden) for introduction in PCFC measurements.

Appendix. Supplementary data

Supplementary data associated with this article can be found inthe online version, at doi:10.1016/j.polymdegradstab.2011.09.006.

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