Calorimetry and thermogravimetry as tools for the assessment of thethermal stability of polyoxide-based nonionic surfactants

Claudia R.E. Mansura, Gaspar Gonzalezb, Elizabete F. Lucasa,*aInstituto de Macromoleculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro (IMA/UFRJ), PO Box 68525,

Zip Code: 21945-970, Rio de Janeiro, RJ, BrazilbPetrobras Research Center, Ilha do Fundao, Q.7, Rio de Janeiro, RJ, Brazil

Received 12 November 2002; received in revised form 24 January 2003; accepted 1 February 2003

Abstract

Differential scanning calorimetry (DSC) and thermogravimetry (TGA) have been used to evaluate the thermal stability of non-ionic surfactants. We have studied monofunctional diblock copolymers of poly(ethylene oxide-propylene oxide) (R-PEO–PPO–OH,where R length is linear C4 or C12�14) as nonionic surfactants. It was observed that the thermal stability was dependent on thecopolymer structure. Moreover, the higher the EO/PO ratio in the copolymers the higher the oxidative thermal stability. The

autoxidation exhibits exothermic behaviour and the enthalpy related to the process depends on the EO/PO ratio. The initial tem-peratures of degradation obtained from DSC and TGA were in agreement.# 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Calorimetry; Thermogravimetry; Block copolymers; Polyoxide; Thermal stability

1. Introduction

A main development in the field of nonionic surfac-tants has been the discovery that a stepwise change inhydrophobicity could be obtained by using poly (ethyl-ene oxide)-poly (propylene oxide) (PEO–PPO) blockcopolymers [1]. In spite of its structural similarity withPEO, PPO is not water soluble under normal condi-tions, it constituting the hydrophobic moiety of thesurfactant [2].Following a report that polyoxide-based surfactants

are very sensitive to autoxidation resulting from a step-wise increase in temperature, with two kinetic beha-viours being observed, a study of huge industrialinterest is focused on the thermal stability of polyoxide-based surfactants [3].In this context, such autoxidation as well as its inhi-

bition are particularly relevant to several industrialfields, such as cosmetics, pharmaceuticals, textiles, justto cite a few, in view of the loss of tensile propertyensuing from the breaking of chains. A few publicationshave reported on the topic of the cited autoxidation, as

well as its inhibition in the presence of nonionic surfac-tants based on PEO, PPO and their copolymers, ther-mogravimetry and calorimetry under oxidizingatmospheres (air and oxygen) being used as analyticaltools [4,5].Thermogravimetry under inert atmosphere (N2) has

also been used to study the degradation of the poly-oxides (PEO, PPO and their copolymers). Contrary toother classes of polymers, these compounds do notdepolymerize, with the compounds yielded in majoramounts being ethylene oxide (�4 wt.%), formalde-hyde, ethanol, carbon dioxide and water. As for PPO,besides propylene oxide (�5 wt.%), there is a largerrange of compounds present in major amounts: acetal-dehyde, propene, acetone, dipropyl ether, iso-propylether, methyl-ethyl-ketone, and isopropyl alcoholamong others. It is also observed that PEO is morethermally stable than PPO, leading to the conclusionthat the tertiary carbons of the latter are a source ofweakness in the structure of this polymer [6,7].Other works [4,5] have studied the thermal stability of

PEO-PPO block copolymers. It is observed that thermalstability is strongly influenced by the number of EOand/or PO units that make up the chain. As previouslymentioned the EO units show higher thermal stabilityrelative to the PO units and therefore the higher the EO

0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0141-3910(03)00055-7

Polymer Degradation and Stability 80 (2003) 579–587

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +55-21-270-1035; fax: +55-21-270-

1317.

E-mail address: elucas@ima.ufrj.br (E.F. Lucas).

content in the copolymer composition, the higher itsthermal stability. Contrary to what is observed for otherpolymers, EO/PO copolymers of lower molecularweight show higher thermal resistance than corre-sponding products of higher molecular weight. Thisbehaviour has been attributed to the higher hydroxylcontent of the chain ends of these copolymers.The present work reports the thermal characterization of

monofunctional, block PEO–PPO copolymers, one end ofthe chain being blocked by hydrocarbon groups, the studyaiming at evaluating the influence of the blocking and itsstructures on the thermal stability and autoxidation.The present study has two sections:

� Study of the thermal stability of surfactantsobtained from thermogravimetric analysis underinert atmosphere (under nitrogen flow);

� Study of the autoxidation of the surfactants,obtained from both: thermogravimetric andcalorimetric analyses, under oxidizing atmo-sphere (that is, under oxygen flow).

2. Experimental

2.1. Materials

The diblock copolymers monofunctionalized with afour-carbon aliphatic group were obtained from GrupoUltra, Divisao quımica, Sao Paulo, Brazil. The samplescontaining a twelve-carbon terminal group wereobtained from Henkel S. A Industrias quımicas, SaoPaulo, Brazil. The copolymers were polydisperse rela-tive to the number of oxyethylene as well as the numberof oxypropylene groups. For compounds containingtwelve methylene groups this value also represented anaverage. The products were used as received, withoutany further purification.

Copolymer characterization data are summarized inTable 1 [8–10].

2.2. Methods

2.2.1. ThermogravimetryTests were carried out using a Perkin-Elmer model

TGA-7 thermogravimetric instrument. The dynamic modeof analysis was used to characterize PEO, PPO and theirblock copolymers, whereby the sample is submitted to acontrolled heating or cooling rate, the weight change beingmonitored as a function of temperature. The heating andcooling rates may be varied, but usually experiments areconducted at rates between 5 and 10 �C/min [11].The test conditions were as follows:

� For the thermal stability of the surfactants:� Temperature range: 30–600 �C� Heating rate: 10 �C/min� Weight: 4–5 mg� Atmosphere: N2 flow (sample �30 mm3/min;

scale �55 mm3/min)

� For the autoxidation of the surfactants:� Temperature range: 30–400 �C� Heating rate: 10 �C/min� Weight: 4–5 mg� Atmosphere: O2 flow (sample �30mm3/min;

scale �55mm3/min)

The thermogravimetric measurements were done withtwo determinations for each sample.

2.2.2. Calorimetry (DSC) as a tool for the study of theautoxidationThe operation conditions used in the study of the

autoxidation of surfactants with the aid of a Perkin-Elmer model DSC-2 differential scanning calorimeter,are listed below:

Table 1

Characterization of PEO-PPO block copolymers by SEC, VPO and 1H NMR8�10

Copolymera

M� w/M� nb M� n

c (g/mol)

EO/PO ratiod Copolymer structuresd

C12PEO

1.24 415 – C12–(EO)6–OH

C12-PEO

1.20 560 – C12–(EO)9–OH

C4–PEO–PPO–OH

1.24 920 0.36 C4–(EO)4–(PO)11–OH

C4–PPO–PEO–OH

1.23 900 0.60 C4–(PO)10–(EO)6–OH

C12�14–PPO–PEO–OH

1.15 600 1.25 C12�14–(PO)4–(EO)5–OH

C12�14–PEO–PPO–OH

1.15 720 1.20 C12�14–(EO)6–(PO)5–OH

C12�14–PEO–PPO–OH

1.19 910 1.50 C12�14–(EO)9–(PO)6–OH

C12�14–PPO–PEO–OH

1.20 1170 1.50 C12�14–(PO)8–(EO)12–OH

C12�14–PPO–PEO–OH

1.18 1310 2.0 C12�14–(PO)8–(EO)16–OH

a Copolymer structures determined from industries.b Determined by size exclusion chromatography (SEC).c Determined by vapor pression osmometry (VPO).d Determined by nuclear magnetic resonance (1H NMR).

580 C.R.E. Mansur et al. / Polymer Degradation and Stability 80 (2003) 579–587

� Temperature range: 50–250 �C� Heating rate: 10 �C/min� Weight: 1–2 mg� Atmosphere: O2 flow (�30 mm3/min)

First order exothermic transitions relative to therelease of heat during the autoxidation process of thesurfactants were observed. Areas under the first orderexothermic transition peaks were calculated with the aidof a planimeter. The conversion of the obtained area intoenthalpy change (�H) was carried out by measuring thearea under the melting peak of a standard substance, inthis case indium (In). Calculations of the enthalpy werecarried out according to Eq. (1) [12].

�Hsam ¼ �Hin � mln=msam

� �

� asam=alnð Þ � Ssam=Slnð Þ � Pln=Psamð Þð1Þ

where:

�Hsam

= enthalpy change of the sample (cal/g); �Hin = In melting enthalpy change

(�Hin=6.80 cal/g);

mIn = weight of indium (3.3 mg); msam = weight of sample (mg); asam = area under the sample peak (cm2); aIn = area under the indium melting peak

(1.6 cm2);

Fig. 1. Thermogravimetric analyses of pure PEO and PPO polymers.

C.R.E. Mansur et al. / Polymer Degradation and Stability 80 (2003) 579–587 581

Ssam

= apparatus sensitivity in the sample test(20 mcal/seg);

SIn

= apparatus sensitivity in the indium test(20 mcal/seg);

PIn

= paper speed recording in the indium test(40 mm/min);

Psam

= paper speed recording in the sample test(10 mm/min).

The calorimetric measurements were done with twodeterminations for each sample.

3. Results and discussion

3.1. Study of the thermal stability of the diblockmonofunctional copolymers

Fig. 1 illustrates the thermogravimetric tests of purePEO and PPO polymers, respectively. These tests werecarried out in order to aid in the understanding of theresults obtained for the studied copolymers. The num-ber average molecular weights of the selected PEO andPPO polymers were comparable to the molecularweights of the other materials under test.Besides these tests, nonionic surfactants of the kind

C12–(EO)6–OH and C12–(EO)9–OH, which were con-sidered also useful for the discussion of results, weretested. Fig. 2 shows the analysis obtained for the C12–(EO)9–OH polymer.Fig. 3 illustrates the thermogravimetric analysis of a

monofunctional PEO–PPO block copolymer blocked bya four-carbon hydrocarbon chain as well as a copolymerblocked by a 12-carbon atom chain.Table 2 lists all the results obtained from these testes,

as well as the number average molecular weights andEO/PO ratio of all the tested materials.Results obtained for pure polymers show that, as

expected, thermal stability of PEO is higher than that ofPPO and that such stability was further increased whenmolecular weight was increased. This behaviour is inagreement with the literature [6,7], that cites PEO asbeing more stable than PPO, this latter having hydrogenatoms linked to tertiary carbon, this leading in generalto lower thermal stability.By comparing the results obtained for the C12–(EO)6–

OH and C12–(EO)9–OH polymers with the pure poly-mers it is observed that the presence of the C12 blockingin the EO chains reduces its thermal stability.By separately analysing the copolymer families

according to the kind of blocking present in theirchains, it may be observed that the figures for initialdegradation temperature (Ti=237 and 240 �C) of thecopolymers blocked by C4 chains, having a low EO/PO ratio and similar molecular weights (Table 2), areclose to those observed for pure PPO (M� n=900,

Ti=250 �C), that is, propylene oxide is responsible forthe onset of the degradation. However, temperatureswhere the degradation rate is maximum (Tmax=380 and381 �C) were intermediate to those observed for purePPO (Tmax=365 �C) and pure PEO (Tmax=396 �C),suggesting that the degradation of the PO moietymay be leading to the degradation of the EO moietyat lower temperatures. Besides, it is observed that thevariation of the EO/PO ratio for these copolymersdid not alter their initial temperatures nor the tem-peratures where the degradation rate is maximum(Tmax).Results obtained for the copolymers blocked with C12

(where the EO/PO ratio is higher) show as a generaltrend that as the molecular weight increases, Ti figuresalso increase. By comparing the C12�14(PO)4–(EO)5OHand C12�14(EO)6–(PO)5OH copolymers it is observedthat in spite of the latter having a higher molecularweight, the Ti of this copolymer is a little lower, thisbeing probably due to its higher PO content. A similarbehaviour may be observed relative to Tmax, which isapparently influenced by molecular weight as well as theEO/PO ratio.To conclude on the influence of the EO/PO ratio it

can be stated that low PO content copolymers showthermal stability figures that suggest that they are moresusceptible to the PO content in the structure of themolecule. It can be confirmed by literature, whichshowed that the EO units show higher thermal stabilityrelative to the PO units and therefore the higher the EOcontent in the copolymer composition, the higher itsthermal stability [4,5].From a comparison of the copolymers blocked with

C4 and the C12�14–(EO)9–(PO)6–OH, all having similarmolecular weights, it is observed that the Ti of this latter

Table 2

Results obtained from thermogravimetric analyses, in N2 atmosphere,

as well as the number average molecular weights and EO/PO ratio of

all the tested materials

Polymer

Tia (�C) Tmax

b (�C)

EO/PO ratio M� n (g/mol)

PPO

150 284 – 400

PPO

250 365 – 900

PEO

180 348 – 400

PEO

296 396 – 1000

C12–EO6–OH

137 352 – 415

C12–EO9–OH

163 372 – 560

C4–PO10–EO6–OH

237 380 0.60 900

C4–EO4–PO11–OH

240 381 0.36 920

C12�14–PO4–EO5–OH

180 370 1.25 600

C12�14–EO6–PO5–OH

176 370 1.20 720

C12�14–EO9–PO6–OH

250 392 1.50 910

C12�14–PO8–EO12–OH

260 393 1.50 1170

C12�14–PO8–EO16–OH

265 389 2.0 1310

a Initial temperature of degradation.b Temperature where the degradation velocity is maximum.

582 C.R.E. Mansur et al. / Polymer Degradation and Stability 80 (2003) 579–587

is slightly higher than the Ti of the two copolymers. Inthis case it seems that a balance in contributions mayoccur: the higher EO/PO ratio could contribute to anincrease in Ti; however, an increase in the chain lengthof the blocking group would contribute to a reductionin Ti.

3.2. Study of the autoxidation of the PEO–PPOmonofunctional block copolymers

3.2.1. Thermogravimetric analysisTable 3 lists the results obtained from thermogravi-

metric analyses for the autoxidation for all analysedmaterials. Figs. 4 and 5 show analyses obtained for

the homopolymers (PEO and PPO) and for onemonofunctional copolymer of each analysed family,respectively. The first observation is that as expected thefigures for the degradation temperatures in this case arelower than those observed for the tests under N2. Theseresults suggest that the copolymers that show higherdegradation temperature under N2 are most affected asregards the reduction in thermal stability when theatmosphere is switched to O2.Values of Ti and Tmax obtained for the PEO pure

polymers show that under O2, the influence of themolecular weight on the thermal stability is reduced.The molecular weights of monofunctional block

copolymers studied are similar to those of the pure

Fig. 2. Thermogravimetric analyses of the C12–(EO)9–OH polymer.

Table 3

Results obtained from thermogravimetric analyses for the autoxidation process for PEO-PPO block copolymers

Polymer

Tia (�C) Tmax

b (�C)

EO/PO ratio M� n (g/mol)

PPO

148 203 – 900

PEO

138 207 – 400

PEO

150 211 – 1000

C12–EO6–OH

110 212 – 415

C12–EO9–OH

143 210 – 560

C4–PO10–EO6–OH

140 200 0.60 900

C4–EO4–PO11–OH

140 204 0.36 920

C12�14–PO4–EO5–OH

150 211 1.25 600

C12�14–EO6–PO5–OH

152 206 1.20 720

C12�14–EO9–PO6–OH

148 215 1.50 910

C12�14–PO8–EO12–OH

148 208 1.50 1170

C12�14–PO8–EO16–OH

150 208 2.0 1310

a Initial temperature of degradation.b Temperature where the degradation velocity is maximum.

C.R.E. Mansur et al. / Polymer Degradation and Stability 80 (2003) 579–587 583

polymers therefore, by comparing the results obtainedfor these copolymers with those obtained for the purepolymers it is observed that when the EO/PO ratio islow, the values are close to those observed for purePPO and for higher ratios the values are close to thoseobserved for pure PEO.

3.2.2. Calorimetric analysisFig. 6 shows the calorimetric analyses obtained for

pure PEO and PPO polymers and for PEO–PPO

monofunctional block copolymers; in all cases stronglyexothermic behaviour was observed. DSC curves relatedto the degradation of the C12–EO6–OH polymer and ofthe copolymers C12(EO)6–(PO)5OH and C12(PO)4–(EO)5OH are not shown since the exothermic peaksobtained in these analyses did not show a definite baseline. This in turn did not allow us to calculate the peakarea and consequently, from Eq. (1), to calculate theenthalpy change of the exothermic process. The resultsobtained from the calorimetric study are best presented

Fig. 3. Thermogravimetric analyses of monofunctional PEO–PPO block copolymer blocked by a four-carbon hydrocarbon chain as well as a

copolymer blocked by a 12-carbon atom chain, in N2 atmosphere.

584 C.R.E. Mansur et al. / Polymer Degradation and Stability 80 (2003) 579–587

in Table 4 after extrapolation of the curves for obtain-ing the onset of the degradation and the calculation ofthe enthalpy change considering the sample weight usedin each measurement.Slight changes in the atmospheric composition exert

a relatively significant influence on the degradationtemperature of materials [3–5]. The values for thedegradation temperature obtained by DSC are onlycomparable to those obtained by TGA. Since the

change in the degradation temperatures has alreadybeen discussed, the DSC results will be used in thediscussion of the enthalpy change in the degradationprocess, such information not being provided for byTGA.The enthalpy figure related to the polymer degrada-

tion seems to be linked mainly to the EO/PO ratio;PEO-400 and PEO-1000 did not show a very importantchange in �H.

Fig. 4. Thermogravimetric analyses obtained for the homopolymers (PEO and PPO), in O2 atmosphere.

C.R.E. Mansur et al. / Polymer Degradation and Stability 80 (2003) 579–587 585

The �H for PEO polymers is higher than that ofPPO. Thus, within a same family, the copolymers thatshow higher EO group contents in their chains have alsohigher enthalpy values.The family of C4 blocking copolymers cannot be

compared to that of C12�14 since the blocking exerts aconsiderable influence on the �H value. This may beobserved by comparing the values obtained for the PEOhomopolymers (�H=7900 J/g) and C12–PEO–OH(�H=3300 J/g).

Upon analysis of the results obtained for copolymersblocked with C12�14 chains, it is observed, as expected,that the location of the PO and EO groups in the copo-lymer chains did not exert any influence on theresults of enthalpy change. Besides, in spite of thefact that these copolymers present higher EO/POratios, they have shown values of enthalpy changessimilar to those shown by the copolymer C4–(PO)10–(EO)6–OH, leading to the conclusion that this kindof blocking may reduce these values. This may be sug-

Fig. 5. Thermogravimetric analyses of monofunctional PEO–PPO block copolymer blocked by a four-carbon hydrocarbon chain as well as a

copolymer blocked by a 12-carbon atom chain, in O2 atmosphere.

586 C.R.E. Mansur et al. / Polymer Degradation and Stability 80 (2003) 579–587

gested by analysing the values obtained for pure poly-mers and for the C12–EO9–OH polymer, for which theenthalpy value was much lower than those presented bypure polymers.

References

[1] Schmolka IR. Polyalkylene oxide block copolymers. In: Schick

MJ, Fowkes FM, editors. Nonionic surfactants. New York:

Marcel Dekker, Inc; 1966. p. 300–34 [Chapter 10].

[2] Bailey FE, Koleske JV. Configuration and hydrodynamic prop-

erties of polyoxyethylene chain in solutions. In: Schick MJ,

Fowkes FM, editors. Nonionic surfactants. New York: Marcel

Dekker, Inc; 1966. p. 794–5.

[3] Gann RG, Dipert RA, Drews MJ. Flammability. In: Mark HF,

Bikales NM, Overberger CG, Menges G, editors. Encyclopedia

of polymer science and engineering, vol. 7. New York: John

Wiley & Sons; 1987.

[4] Santacesaria D, Gelosa M, DiSerio, Tesser R. Thermal stability

of nonionic polyoxyalkylene surfactants. J Appl Polym Sci 1991;

42:2053–61.

[5] Haken JK, Tan L. Mechanism of thermal degradation of poly

(alkyl acrylate)s using pyrolysis-gas chromatography/mass spec-

trometry. J Polym Sci 1998;26:1315–22.

[6] Jones GK, Ghie AR, Farrington GC. Studies of the stability of

poly(ethylene oxide) and PEO-based solid electrolytes using

thermogravimetry-mass spectrometry. Macromolecules 1991;24:

3285–90.

[7] Diab MA, El-Sonbati AZ. Thermal stability of poly(ethylene

glycol) homopolymer and polymer complexes of poly(ethylene

glycol) with some transition metal chlorides. Polym Degrad Stab

1990;29:271–7.

[8] Mansur CRE, Oliveira CMF, Gonzalez G, Lucas EF. The influ-

ence of hydrotropic agent in the properties of aqueous solutions

containing poly(ethylene oxide)-b-poly(propylene oxide) surfac-

tants. Colloids and Surfaces, A: Physicochem Eng Aspects 1999;

149:291–300.

[9] Mansur CRE, Oliveira CMF, Gonzalez G, Lucas EF. Phase

behavior of aqueous systems containing block copolymers of

poly(ethylene oxide) and poly(propylene oxide). J Appl Polym Sci

1997;66:1767–72.

[10] Mansur CRE, Spinelli LS, Oliveira CMF, Gonzalez G, Lucas

EF. Behavior of aqueous solutions of poly(ethylene oxide-b-pro-

pylene oxide) copolymers containing a hydrotropic agent. J Appl

Polym Sci 1998;69(2):2459–68.

[11] Felisberti MI. Caracterizacao de blendas polimericas atraves de

analise termica e termonecanica. Brasil: ABPOL, IQ-UNICAMP;

1992.

[12] PerkinElmer. Instructions-model DSC-2, differential scanning

calorimetry. Norwalk (USA): Perkin Elmer Corp; 1977.

Fig. 6. Calorimetric analyses obtained for pure PEO and PPO poly-

mers and for PEO–PPO monofunctional block copolymers.

Table 4

Results obtained from the calorimetric study after extrapolation of the curves for obtaining the onset of the degradation and the calculation of the

enthalpy change

Polymer

Tia (�C) Tmax

b (�C)

Tfc (�C) �Hd (J/g) EO/PO ratio M� n (g/mol)

PPO

160 217 216 5550 – 900

PEO

140 204 230 7600 – 400

PEO

140 201 227 7900 – 1000

C12–EO9–OH

148 – 340 3300 – 560

C4–PO10–EO6–OH

135 194 224 6500 0.60 900

C4–EO4–PO11–OH

141 194 223 5800 0.36 920

C12�14–EO9–PO6–OH

119 195 232 6200 1.50 910

C12�14–PO8–EO12–OH

133 196 226 6200 1.50 1170

C12�14–PO8–EO16–OH

132 195 224 6500 2.00 1310

a Initial temperature.b Maximum temperature of the endothermic peak.c Final temperature.d Enthalpy variation.

C.R.E. Mansur et al. / Polymer Degradation and Stability 80 (2003) 579–587 587