effect of antiplasticisation on the volumetric, gas sorption and transport properties of...

Journal of Membrane Science 218 (2003) 69–92

Effect of antiplasticisation on the volumetric, gas sorptionand transport properties of polyetherimide

N.M. Larocca, L.A. Pessan∗Department of Materials Engineering, Universidade Federal de São Carlos, Via Washington Luiz,

Km 235, 13.565-905 São Carlos, SP, Brazil

Received 26 July 2002; received in revised form 7 February 2003; accepted 14 March 2003

Abstract

The incorporation of low molecular mass additives to polyetherimide (Ultem® 1000) led to the polymer’s antiplasticisation,an effect indicated by a reduction of its free volume, an increase of its� sub-Tg (T�) transition temperature and storage modulus,and a reduction of the sorbed carbonic gas level and the gas permeability coefficient. The extent of these changes, however,was found to be strongly dependent on a combination of additive properties, i.e. size, free volume, degree of interaction withthe polymer, and the glass transition temperature.

Based on a theoretical correlation between the diffusion coefficient and the segmental motion frequency, we argue thatthe increase inT�, which indicates intensified segmental restriction, is related to a reduction of the permeability coefficient.However, the real influence of this transition on the permeability could not be analysed because the additives also changeother parameters that influence gas permeability. We assume that several of these parameters are considered in the mixtures’fractional free volume (FFV), since a relatively good correlation was found between this parameter and the permeability. Thescatterings found in this correlation were attributed to alterations in segmental motions and free volume distributions of PEI,which are not considered by the FFV parameter but that affect the gas permeability coefficients.© 2003 Elsevier Science B.V. All rights reserved.

Keywords:Polyetherimide; Antiplasticisation; Free volume; Gas sorption; Gas permeability

1. Introduction

When a polymer is mixed with additives of lowmolar mass the mixture thus obtained is usually moreflexible than the pure polymer. This mixture is char-acterised by a lower vitreous transition temperature,lower tensile strength and higher elongation at break.However, some additives, when incorporated in lowpercentages to vitreous polymers, have some oppositeeffects, i.e. they promote the increase of the material’stensile strength and decrease its rupture elongation.

∗ Corresponding author. Fax:+55-16-261-5404.E-mail address:[email protected] (L.A. Pessan).

Additives with these properties are called antiplasti-cisers.

These additives also increase the elastic modulusand hardness, and diminish the impact resistance andthe thermal distortion temperature. The vitreous tran-sition temperature of such antiplasticised polymerscontinues decreasing, but at a lesser rate than that gen-erated by plasticisation. The magnitude of these effectsdepends on some characteristics of the additives, suchas size, shape, stiffness and concentration in the poly-mer, as well as on the polymer’s characteristics, partic-ularly on the intensity of its secondary transition[1–3].

Robeson[3] evaluated the effects of the addi-tion of antiplasticiser on the intensity of the peak

0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0376-7388(03)00139-X

70 N.M. Larocca, L.A. Pessan / Journal of Membrane Science 218 (2003) 69–92

corresponding to the secondary transition tempera-ture and on the polymer’s permeability. He foundthat antiplasticisation suppresses the secondary tran-sition, an effect that is related to the antiplasticisationmechanism proposed by Jackson and Caldwell[2],whereby the space occupied in the polymer’s freevolume restricts the molecular motions responsiblefor the sub-Tg transitions, which are characterisedby the motions of a few segments in the backbone,involving from 1 to 10 consecutive atoms. Togetherwith the suppression of the sub-Tg relaxation process,Robeson also noted that CO2 permeability decreasedin the polymer, as was expected, because segmen-tal motions for this relaxation also govern the gaspermeability process. Light and Seymour[4] subse-quently reinforced the importance of the polymericmolecules’ segmental motion in the O2 and CO2 per-meability in polyesters. Sefcik et al.[5] demonstratedthe alteration caused by low molecular mass addi-tives in segmental motion in a frequency range of103–105 Hz, with the resulting modification of the gasdiffusion coefficient in PVC. According to Pace andDatyner’s diffusion model[6], segmental motions inthis frequency range are important in both processes:diffusion and permeability of gases in polymers.

Recently, Maeda and Paul[7] and Ruiz-Treviño andPaul [8,9] observed that the substantial decrease inthe gas permeability coefficients, caused by antiplas-ticisation, in poly(phenylene oxide) and polysulfoneis quantitatively well correlated with these polymers’decreased fractional free volume (FFV) following theincorporation of additives, as Lee’s relation predicts[10]. The reduced concentration of sorbed gas, on theother hand, was attributed to the occupancy of excessfree volume (non-equilibrium volume) of the vitreouspolymer by the antiplasticisers’ molecules.

Polyetherimide Ultem® 1000 is an interestingpolymer for antiplasticisation studies because of itsamorphousness, its two sub-Tg transitions and its easysolubility in chloroform, which allow it to be madeinto thin films. Recently, after incorporating imidemonomers into this polymer, Rezac and Schöberl[11]observed reductions in the glass transition temperatureand gas permeabilities and selectivities—behavioursthat are consistent with antiplasticisation.

In the present study, mixtures of polyetherimideUltem® 1000 with additives of low molecular masscontaining no imide moieties were analysed for their

volumetric behaviour, molecular mobility, and gassorption and transport properties. Our purpose in thisstudy was to gain a more in-depth knowledge aboutthis polymer’s molecular structure and the role ofthe antiplasticisation mechanism, as well as to checkmodels of gas sorption behaviour and the correlationbetween gas permeability and free volume.

2. Experimental

2.1. Materials

The polymer used in this work is the Ultem® 1000polyetherimide supplied by General Electric, and itschemical structure is presented inTable 1(in this arti-cle, we use the acronym PEI for this polymer).Table 1also presents the structure of the additives utilised.PNA and BHT are additives normally used as antiox-idants, while TBBPA is a flame retardant[12]. HF-BPA is used to synthesise some polymers. PNA andTBBPA have already been studied as antiplasticisersin polysulfone[3,13,14].

Each additive was chosen based on some impor-tant characteristics that are assumed to strongly affectthe degree of antiplasticisation[1,7,15,16], namely,the size and free volume of the additive molecule,its level of interaction with the polyetherimide chainsand its stiffness. The characteristics of the additivesselected for this study differ significantly from eachother, which enabled us to assess the importance ofeach characteristic on the level of antiplasticisation.Section 3summarises our evaluations of size, level ofinteraction with PEI and stiffness of the additives.

Chloroform was used as the solvent for both PEIand additives, while heptane and carbon tetrachloridewere used to measure the samples’ density, and methylethyl ketone (MEK) was used to determine the specificvolumes of each additive.

2.2. Evaluation of the PEI–additive interactionlevel

As discussed in the preceding paragraphs, we as-sume that the level of interaction between PEI andthe additives has a crucial effect on the level ofthe antiplasticisation. To evaluate the interaction be-tween polymer and additives it is common to use

N.M. Larocca, L.A. Pessan / Journal of Membrane Science 218 (2003) 69–92 71

Table 1Structures of the polymer and additives used in this study

a solubility parameter-based approach which pre-dicts that the compatibility between polymer andadditive is inversely proportional to the quantity(δpolymer − δadditive)

2, where δ is the global solu-bility parameter [17]. Based onTg measurementsof polymer-low molecular weight additive mix-tures, however, Slark[18,19] found that a solubil-ity parameter that combines polar and hydrogenbonding forces (δph) is a better indicator of inter-action than the global solubility parameter (whichalso considers dispersion forces). This parameter isgiven by

δ2ph = δ2

p + δ2h (1)

whereδp and δh the solubility parameters resulting,respectively, from polar and hydrogen bonding forces.

Thus, we chose to useδph, rather than solelyδ, to cal-culate the(δph PEI− δph additive)

2 parameter, using it asan indication of PEI–additive interaction. Theδp andδh parameters were calculated by the Hoy method ofgroup contribution for the PEIs and additives’ averagerepeating unit[17].

2.3. Preparation of PEI+ additive mixtures

PEI + additives films were produced by dissolvingthese components in the desired ratio in chloroform,with a total solid concentration of 10 wt.%. The solu-tion was then poured on a glass plate, using a bladeinside a cold chamber at 0◦C to reduce the chloroformevaporation rate, thereby preventing surface irregular-ities on the films. After a 1-day stay in the chamber,


the film was stripped from the glass plate by immer-sion in water.

2.4. Drying and elimination of the samples’differential thermal history

The solvent was removed from the films by dryingthem in a vacuum oven at 70◦C for 4 days. The glasstransition in each mixture was then determined bydifferential scanning calorimetry (DSC), after whichthe films were placed in the vacuum oven again.This time, however, the temperature was increasedgradually in each case (about 0.5◦C/min) to 10◦Cabove the corresponding glass transition temperature(it should be noted that this temperature,Tg + 10◦C,is not the same for all the mixtures, since each mix-ture has a differentTg). The films were held at thistemperature for 60 min, followed by a quick dropto room temperature. To prevent the films from de-forming in response to the increase in temperatureabove Tg, each film was sandwiched between twoKapton® sheets and this sandwich, in turn, was placedbetween two heavy glass plates. This procedure ef-fectively prevented the formation of blisters, indicat-ing that the solvent was fully removed by drying at70◦C for 4 days and possibly also by the gradualheating from this temperature to 10◦C above eachmixture’sTg.

This procedure, analogous to that used by Pfrommand Koros[20] and by Rezac and Schöberl[11], erasesthe previous thermal history of each mixture and isnecessary to minimise dissimilar structural modifica-tions caused by drying at 70◦C, because the mixtureswhoseTg is close to that temperature would undergomore intense thermal ageing than those with a highTg[21,22].

2.5. Thermal characterization

The glass transition temperatures were measuredusing a Perkin-Elmer DSC-7 calorimeter. Each runwas made from room temperature to 250◦C at aheating rate of 20◦C/min in a nitrogen atmosphere,and the glass transition was measured from thefirst run. A procedure similar to that described bySlark [18] was used for the additives. To determinethe temperature at which the additives decomposed(thermal decomposition temperature,Td), thermal

gravimetric analyses (TGA) of the additives weremade using a TGA DuPont 2000 device from 50to 400◦C, a 10◦C/min heating rate and nitrogenatmosphere.

The mechanical relaxation spectra of PEI and its ad-ditive mixtures were determined using a RheometricsScientific DMTA MK-IV dynamic-mechanical ther-mal analyser in the tension mode, at a 10 Hz frequency,with temperatures ranging from−120 to 220◦C anda heating rate of 3◦C/min.

2.6. Volumetric characterization

For a detailed volumetric characterization of PEI+ additive mixtures, each component’s specific vol-ume must be determined in the amorphous state. Forthe additives, which are crystalline solids at room tem-perature, specific volumes were obtained based on amethod devised by Maeda and Paul[13], which deter-mines the density of the additive+ solvent mixture atvarious additive concentrations, using MEK solvent.Because these mixtures follow a volumetric additiverule, the partial specific volume of each additive waseasily identified.

The specific volumes for PEI+ additive mixturesat 25◦C were measured by a flotation method, usinga mixture of two miscible liquids with very differentdensities (heptane and carbon tetrachloride). Eachliquid was mixed at a ratio designed to promote theflotation of PEI+ additive film when the liquid so-lution density equalled film density. After 24 h andconfirmation of flotation, liquid solution densitieswere determined using a pycnometer. Four distinctsections of the same film were analysed for eachPEI + additive mixture, the maximum variation ob-served being±0.005 g/cm3. The temperature wascontrolled at 25.0 ± 0.5◦C in a thermostatic waterbath.

2.7. Determination of equilibrium sorptionisotherms of CO2

To measure the amount of CO2 sorbed at equilib-rium by the PEI+ additive mixtures, a technique andapparatus similar to those described by Koros et al.[23] were used. The isotherms were measured at 35◦Cand the maximum pressure applied was 2026.50 Pa(20 atm).


Fig. 1. Illustration of diffusion cell used in this work.

2.8. Determination of the gas permeabilitycoefficient

A diffusion cell such as the one illustrated inFig. 1,which was first developed by Gilbert and Pegaz[24],was employed to measure the permeability coefficientsof O2 and CO2 in the mixtures. A constant stream ofpermeant gas was produced in the external chambers,diffusing onto the polymeric sample situated betweenthe external and the middle chambers. The gas accu-mulated there in an airtight environment. In this ap-paratus, the partial pressure gradient of gas permeant(�p) was kept at a nearly constant level by the con-tinuous flow of gas in the external chambers. Sincethe atmospheric pressure in the lab was 700 mmHg,this was also the�p value. The pressure gradient wasconsidered constant until the concentration of perme-ant gas in the intermediate chamber reached 3% of thepartial pressure of the gas in the external chambers.

To plot a graph of the concentration of permeablegas versus time, an aliquot of 300�l of gas was peri-odically collected from the intermediate chamber byinserting a hypodermic syringe through the sept of thischamber and measuring the concentration of perme-ant gas in this aliquot by gas chromatography. Slopeα of the graph allows for the permeability coefficientP to be calculated, usingEq. (2):

P = α

A �p(2)

where is the film thickness andA the permeatedarea.

Two samples with the same composition were anal-ysed for most of the mixtures, although in some cases,when a significant dissimilarity was observed betweentwo results or when the graphic correlation coefficientwas small, a third sample was analysed.

Because permeability measurements are very sen-sitive to surface defects on polymeric films, sampleswith a permeation area equal to the aperture area of thediffusion cell (Fig. 1) could not be used, since therewould always be some defects in such a large area inthe films. To circumvent this problem, each film’s per-meation area was limited to a clean part of the film us-ing an adhesive aluminium mask with a circular 5 cm2

opening.

3. Results

3.1. Properties of polyetherimide and additives

Table 2 lists several parameters of the PEI andadditives used. The specific volume at 25◦C (V25◦C)indicated the relative size of each additive molecule,which then decreased in the following sequence:TBBPA > BHT > HFBPA > PNA. The occupiedvolume (Vo Bondi) calculated by the Bondi method[17]allowed us to calculate the fractional free volume at


Table 2Properties of the polymer and additives used in this study

Mixture component V (25◦C) (cm3 mol−1) Vo Bondi (cm3 mol−1) FFV Tg (◦C) (δph PEI− δpb additive)2 (J cm−3)

PEI 462.98 396.46 0.144 215 –PNA 188.59 162.93 0.136 −11 25TBBPA 255.09 236.05 0.075 30 12BHT 253.63 193.25 0.238 −29 63HFBPA 232.34 193.36 0.168 −2 30

25◦C (FFV = (V25◦C−Vo)/V25◦C), whose order was:BHT > HFBPA > PNA > TBBPA. The glass tran-sition temperature (Tg) yielded the relative stiffness,i.e. the higher theTg, the greater the molecule’s stiff-ness. Thus, the stiffness ranked as follows: TBBPA�HFBPA > PNA � BHT. TheTg of all the additiveswas found to be slightly lower than that of the PEI.

Table 2also shows the(δph PEI − δph additive)2 pa-

rameter for each pair of PEI–additive, which was usedas an indicator of the PEI–additive interaction. Basedon this criterion, the lower this parameter the higherthe interaction. Thus, the additives were classifiedin the following order in terms of their interactionwith PEI: TBBPA > PNA > HFPBA � BHT. Allthese additive molecule characteristics (size, free vol-ume, stiffness and interaction level) are assumed tostrongly affect the level of antiplasticisation of PEI,as discussed earlier inSection 2.2.

3.2. Thermal analysis

3.2.1. Glass transition temperatures of the mixturesTable 3shows the glass transition temperature val-

ues of each PEI–additive mixture (Tg mixture) and theadditives’ thermal degradation temperatures. Eachmixture was heated to a maximum temperature ofTg mixture + 10◦C during the procedure to eliminatethe samples’ differential thermal history (as describedin Section 2.4). The mixture’s glass transition tem-perature is lower the higher the concentration ofadditive and as can be observed, in mixtures contain-ing high concentrations of additive,Tg mixture+ 10◦Cis much lower than theTg of pure PEI. The PEI+ 32% TBBPA mixture, for instance, was subjectedto a maximum treatment temperature of 130◦C, sincethe Tg of this mixture is 120◦C. Because this max-imum treatment temperature is lower than theTd ofTBBPA, it is unlikely that this additive underwent

thermal degradation in this mixture. A comparison ofTg mixture andTd in Table 3also led to the assumptionthat none of the additives became thermally degradedin any mixture containing TBBPA, PNA and HFBPA.

However, the maximum treatment temperature ofthe PEI+ 23% BHT mixture exceeded theTd of BHT.Although some degradation of BHT possibly occurred,we assume that this degradation was sufficiently slightand did not promote a meaningful alteration of BHTproperties in terms of the antiplasticisation of PEI.This assumption is based on the results of the PEI+ BHT mixture shown below, which are congruentwith the qualitative behaviour expected for a mixtureof PEI containing a large-sized additive having a highfree volume, lowTg and low affinity with PEI.

We did not attempt to analyse the PEI+ BHTmixtures with a BHT content of<23 mol% becausethe Tg of these mixtures would be much higher thantheTd of BHT (for instance, a PEI+ 10% BHT mix-ture would have aTg of around 180◦C). The processof increasing the temperature toTg mixture + 10◦Cin these mixtures would likely promote severe

Table 3Glass transition temperatures of each mixture PEI–additive withthe respective thermal degradation temperature (Td) of additive

Additive Additive in PEI(mol%)

Tg of mixture(◦C)

Td of additive(◦C)

PNA 12.5 160 17523 137

TBBPA 5.5 195 21511 16621.5 14032 120

BHT 23 135 120

HFBPA 16.5 168 17531 14843 134


Fig. 2. Glass transition temperatures for PEI and mixtures with additives measured by DSC at 20◦C/min.

thermal degradation of the BHT additive. Moreover,we could not make films with larger amounts of BHTbecause BHT concentrations of over 23 mol% led tothe onset of macroscopic phase separation in suchmixtures.

Fig. 2 shows the glass transition temperature ofeach mixture as a function of the additive concentra-tion. The trend observed and the decrease inTg inten-sity are comparable to those found for other vitreouspolymer+ additives systems[7,8,25,26].

3.2.2. Dynamic-mechanical analysisFig. 3 depicts the dynamic-mechanical relaxation

spectrum (tanδ versus temperature) for PEI and itsmixtures. This group of figures show that PEI hastwo secondary sub-Tg transitions, which are associ-ated with segmental motions: a� transition at about−90◦C and a� transition in the vicinity of 110◦C.The additives modify the temperature and intensity ofthese transitions, indicating that the PEI’s segmentalmobility is also modified. The� transition shifts tohigher temperatures with increasing amounts of ad-ditive, as shown inFig. 4, indicating that the addi-tive restricts the segmental motions associated withthis transition, whose peak magnitude also changes,

although the intensity of these changes depends onthe additive. The effect of additives on the� transi-tion is more difficult to analyse because, when addi-tive concentrations exceed 10 mol%, the substantialTgreduction causes the� transition (Tg) to overlap the� transition.

Fig. 5 shows the dynamic storage modulus (E′) asa function of temperature for PEI and its mixtures,while Fig. 6 summarises theE′ curves versus temper-ature, showingE′ at 25◦C (the temperature used forvolumetric and gas transport analysis) as a functionof additive type and level. As this figure indicates,all the additives actually promote stiffening of PEIglassy phase. The strength of this stiffening dependson the additive and apparently follows the sequenceTBBPA ∼= PNA > BHT ∼= HFBPA.

3.3. Volumetric analysis

Dilatometry data determined by Simon et al.[21]was used to accurately estimate the PEI-occupiedvolume. Using mercury dilatometry, these authors de-termined PEI densities above itsTg, i.e. PEI density inthe liquid state. Based on an exhaustive empirical anal-ysis, Sanchez and Cho[27] determined that polymer


Fig. 3. tanδ at 10 Hz for: (a) PEI and mixtures with TBBPA; (b) PEI and mixtures with PNA; (c) PEI and mixture PEI+ 23% BHT; (d)PEI and mixtures with HFBPA.

Fig. 4. Effect of additive type and level on peak temperature ofPEI � transition.

densities in the liquid state are a temperature linearfunction. Thus, an extrapolation of the data of Simonet al.[21] to 0 K yields the characteristic PEI density,ρ∗, which is the equilibrium density at that temper-ature. The reciprocal ofρ∗ is the specific volume atabsolute zero,v∗, which is the PEI-occupied volume.A straight line was plotted based on the data of Simonet al. [21] and by extrapolation to 0 K, it was foundthat v∗ = 0.666 cm3/g (or 396.83 cm3/mol). Thisvalue is very similar to that one found using Bondi’sempirical method [17], which was estimated at0.670 cm3/g.

The specific free volume of each mixture was cal-culated by subtracting the occupied volume from thespecific volume, while the fractional free volume wascalculated by dividing the specific free volume by thespecific volume. The occupied volume of the mixtures


Fig. 5. Storage modulus for: (a) PEI and mixtures with TBBPA; (b) PEI and mixtures with PNA; (c) PEI and mixture PEI+ 23% BHT;(d) PEI and mixtures with HFBPA.

(Vmo) was determined by assuming the following ad-ditive function:

Vmo = mVdo + (1 − m)Vpo (3)

wherem is the molar fraction of additive andVdo andVpo are the additive and polymer occupied volume,respectively. Bondi’s empirical method[17] was usedto calculate the additives’ occupied volumes, whereasthe aforementioned occupied volume was used for thePEI. Figs. 7–9show, respectively, the mixtures’ spe-cific volume, specific free volume and fractional freevolume as a function of the additive molar concen-tration. The dashed lines inFig. 8 correspond to an

ideal additive behaviour of the specific free volume ofthe mixtures, which is calculated considering that thespecific volume of mixtures followsEq. (4):

Vmi = mVd + (1 − m)Vp (4)

whereVmi is the ideal specific volume of mixture inthe glassy state, andVd and Vp are the specific vol-umes of additive and PEI, respectively. As illustratedin Fig. 8, the specific free volumes of mixtures all de-viate from the ideal behaviour. To quantify this devi-ation, we used the excess specific volume (or excessspecific free volume),�V, defined below:

�V = Vm − Vim (5)


Fig. 6. Effect of additive type and level on PEI storage modulusat 25◦C.

where Vm is the experimental specific volume ofmixture. Fig. 10 shows this parameter for all thePEI–additive mixtures. One can see from this figurethat, although there is no clear trend of�V with lowconcentrations of additive, at concentrations exceed-ing 20 mol%, the extent of deviation from volumeadditivity with PEI occurs in the order of BHT>PNA > HFBPA > TBBPA.

Fig. 7. Specific volumes for PEI–additives mixtures at 25◦C.

3.4. Gas sorption and permeation

Fig. 11shows sorption isotherms for CO2 at 35◦Cfor the neat PEI and for its mixtures. As has beenobserved in previous studies[28,29], gas sorptionisotherm for PEI exhibits a strong downward curvepattern. This behaviour is well described by thedual-mode sorption model[6,23], which assumes thatgas molecules can be located on a vitreous matrix intwo molecular environments. The first environmentis where there is an ordinary solution, described byHenry’s law:

CD = Kdp (6)

whereKd is Henry’s law solubility constant andp thepartial pressure of the gas.

In the second environment, the gas molecules arearranged on non-equilibrium frozen microvoids cre-ated by cooling the polymeric matrix to below its glasstransition temperature. This ambient is described byLangmuir’s isotherm:

CH = C′Hbp

1 + bp(7)


Fig. 8. Specific free volume for PEI–additives mixtures at 25◦C.

whereCH is the concentration of adsorbed moleculeson microvoids,C′

H the Langmuir capacity constantandb the Langmuir affinity constant. The dual-modesorption model can thus be described as

Fig. 9. Fractional free volume for PEI–additives mixtures at 25◦C.

C = CD + CH = kdp + C′Hbp

1 + bp(8)

Values for the dual sorption parameters listed inTable 4 were determined by nonlinear regression


Table 4Dual sorption model parameters for CO2 in PEI and mixtures (at 35◦C)

System Kd (cm3(STP)/cm3 atm) C′H (cm3 (STP)/cm3 atm) b ((Pa−1) atm−1))

PEI 0.51 17.7 39.52 (0.39)PEI + 23% PNA 0.93 4.3 85.11 (0.84)PEI + 11% TBBPA 0.66 7.8 58.77 (0.58)PEI + 21.5% TBBPA 0.72 5.0 42.56 (0.42)PEI + 32% TBBPA 0.71 3.7 29.39 (0.29)PEI + 23% BHT 0.78 6.3 40.53 (0.4)PEI + 16.5% HFBPA 1.06 4.6 –

analysis usingEq. (8). The average solubility coeffi-cients (S) for the pure PEI and for its mixtures withadditives, at 202.650 Pa (2 atm) pressure (the first pres-sure measured) and 35◦C, were also calculated con-sidering thatS = C(p)/p, whereC(p) is the concen-tration of gas sorbed on equilibrium at a pressurep.The correlation betweenS and additive concentrationsin PEI is shown inFig. 12.

It has been suggested that parameterC′H is related

to the excess or non-equilibrium volume of the glassypolymer[30], which means that the additives caused alarge reduction in volume, sinceTable 4shows a strongreduction of the PEI’sC′

H parameter. The proposed re-lation betweenC′

H and the non-equilibrium volume is

C′H = 22414

VCO2(l)

�αm(Tg − 35◦C) (9)

Fig. 10. Calculated excess specific volumes for PEI–additives mix-tures at 25◦C.

where�αm is the difference in thermal expansion co-efficients between the liquid and glassy states of thepolymer andVCO2(l)

is the molar volume of CO2 in a

liquid-like state. InFig. 13, the mixtures’C′H values

were plotted againstTg−35◦C. As can be seen, themixtures do not follow the dashed line, which wascomputed fromEq. (9)using�αm = 2.4×10−4 ◦C−1

for pure PEI[21] andVCO2(l)= 55 cm3/mol [13,30].

It is worth noting that besides the alteration inTg, theadditives also caused a substantial reduction of thePEI’s �αm. The observed reduction in�αm is cor-roborated by the work of Kinjo and Nakagawa[31],who, based on dilatometry, found that low molecularweight additives promote a remarkable change in thethermal expansion coefficient of glassy PVC.

Permeability coefficients of O2 and CO2 in PEI andits mixtures are listed inTable 5and plotted againstadditive concentrations inFig. 14. A strong reductionoccurred in the level of gas permeability as the addi-tives were added to PEI. The PEI/32% TBBPA com-position, for instance, underwent a CO2 permeability

Table 5Permeability coefficients for O2 and CO2 in PEI and mixtureswith additives at pressure gradient 700 mmHg and 25◦C

System PO2 (barrers) PCO2 (barrers)

PEI 0.60 1.27PEI + 23% PNA 0.15 0.25PEI + 5.5% TBBPA 0.35 –PEI + 11% TBBPA 0.27 0.45PEI + 21.5% TBBPA – 0.27PEI + 32% TBBPA 0.16 0.20PEI + 23% BHT 0.32 0.49PEI + 16.5% HFBPA – 0.67PEI + 31% HFBPA – 0.33PEI + 43% HFBPA – 0.55

1 barrer= 10−10 cm3 (STP) cm/cm2 s cmHg.


Fig. 11. CO2 sorption isotherms at 35◦C for: (a) PEI and its mixtures with TBBPA; (b) PEI and its mixtures with PNA, BHT and HFBPA.

reduction of about 80%. Another interesting featurewas the behaviour of the CO2 permeability in thePEI–HFBPA mixtures. As shown inFig. 14b, thepermeability decreased down to a load of 31 mol%of HFBPA, but further incorporation of this additivecaused the CO2 permeability to increase.

4. Discussion

4.1. Mechanism of Tg decrease

TheTg decrease phenomena due to the addition oflow molecular weight additives in glassy polymers


Fig. 12. Effect of additives contents on the PEI average solubilitycoefficient at 202.650 Pa (2 atm) and 35◦C.

can be understood if one considers the process ofpreparation of glassy mixtures from solution, e.g. PEI+ chloroform solution. As the solvent is removedby drying, the relaxation of PEI macromolecularsegments slows down to the point at which largescale segments can no longer relax. This promotesthe formation of high-energy frozen voids and theglassy phase thus formed is not at thermodynamicequilibrium. If a small amount of lowTg additive ispresent while the glass is being formed, it can relax

Fig. 13. Correlation of Langmuir mode sorption capacity of themixtures with the difference between glass transition and 35◦C(temperature of sorption measurement).

those high-energy regions because, as the additive’sTg is lower than the polymer’s, its intrinsic mobilityis higher than the polymer’s[32]. In other words, theadditive promotes higher densification of the glassyphase, causing the polymer in the glassy state to ap-proach its equilibrium state, which in turn implies adecrease ofTg.

Based on this discussion, we assume that themixtures’ decrease inTg is enhanced as the additives’Tg decreases. However, in this study we found that,although the additives’ glass transitions increased inthe order BHT< PNA < HFBPA < TBBPA, Fig. 2shows that theTg of the PEI+ PNA, PEI+ TBBPAand PEI+ BHT mixtures showed only minor differ-ences, while the PEI+ HFBPA mixtures showed highTg values. This may be attributed to the fact that theTgof polymer+ low molecular weight additives mixturesalso depends on the degree of interaction between thepolymer and the additives[18,19,33], which can beevaluated by the(δph polymer− δph additive)

2 parameter.Thus, although PNA has a higherTg than BHT, theformer additive is able to decrease theTg of PEI asmuch as BHT does owing to its higher interactionwith PEI chains (low value of(δph PEI− δph additive)

2)than the interaction of BHT. The case of TBBPAis more remarkable, because its highTg (30◦C) isoffset by its relatively high level of interaction withPEI ((δph PEI − δph additive)

2 = 12). On the otherhand, because HFBPA interacts only weakly withPEI chains (high value of(δph PEI − δph additive)

2)and its Tg is not very low, this additive cannot de-crease the PEI’sTg to the same extent as the otheradditives can.

The intensity of the volumetric contraction shownin Fig. 10 is also expected to have some correlationwith the additives’Tg, assuming that the additive’sability to increase the polymer’s density depends onthis parameter. This may explain the large volumetriccontraction of PEI+ BHT mixtures, since BHT hasa very lowTg and can therefore promote large reduc-tions of PEI density. The increasing intensity of |�V|in mixtures with additive concentrations of 20 mol%upward seems to be correlated with the decreasingorder of theTg of additives (TBBPA> HFBPA >

PNA > BHT). The assumption of this correlationis supported by the work of Ruiz-Treviño and Paul[14], who concluded that the negative departure fromvolume additivity is the result of the relaxation of


Fig. 14. (a) O2 permeability coefficients at 700 mmHg and 25◦C for PEI+ additives mixtures. (b) CO2 permeability coefficients at700 mmHg and 25◦C for PEI + additives mixtures.

the excess volume of the glassy mixture in relationto the equilibrium state promoted by mixing twocomponents with different glass transition temper-atures. Although we have reached this qualitativeinterpretation of |�V|, the non-equilibrium state ofthe glassy phase precludes us from making a thermo-dynamic analysis of the mixtures[7,34].

4.2. Antiplasticisation and free volume

Although additives cause the glass transition tem-perature of PEI to decrease, they also promote anincrease in the dynamic storage modulusE′at 25◦C(storage modulus of the glassy phase).Fig. 15summarises these conflicting trends. This enhanced


Fig. 15. Variation of storage modulus at 25◦C with the glass transition temperatures of mixtures PEI+ additives.

long-range mobility combined with stiffening of theglassy phase is a characteristic feature of an antiplas-ticised system[13,25,34,35].

This stiffening is explained by a free volume ap-proach to antiplasticisation[15], which can be sum-marised by schematic diagrams of specific volumeversus temperature devised by Duda et al.[16] andillustrated inFig. 16. As this figure shows, what dif-ferentiates the antiplasticisation from the plasticisa-tion process is the specific volume of the glassy phaseof the mixture, which can be either below or abovethe specific volume of the pure polymer at room tem-perature. From these diagrams, one can conclude thatthese features are determined mainly by the alterationof the equilibrium specific volume of the polymer attemperatures exceedingTg polymerwhen the additive ispresent. If the additive has a much higher free vol-ume than the polymer, plasticisation takes place. Onthe other hand, if the additive free volume is not muchhigher or even if it is lower than that of the polymer,antiplasticisation takes place.

However, as the diagrams indicate, the glass tran-sition temperatures of the mixtures in both cases arelower than that of pure polymer, regardless of whetherit is plasticisation or antiplasticisation that occurs. Asdiscussed inSection 4.1, the decrease mechanism ofpolymer Tg appears to depend on the additives’Tg

and on the polymer-additive interaction. Thus, this de-crease is expected to occur in both these situations,because theTg of low molecular weight additives isusually lower than that of the polymer (as in mixtureswith PEI). This can also be attributed to the fact that,in both these situations, the free volume above themixture’s Tg is higher than that of the pure polymer,facilitating the mobility of larger segments.

From this discussion, one can conclude that an-tiplasticisation produces a reduction in theTg and freevolume of glassy polymers. This reduction in free vol-ume increases the interchain cohesion and representsa restricted freedom for the macromolecules to absorbmechanical energy, which is reflected in the increasedmodulus. This theory is supported by the good corre-lation between the modulusE′ at 25◦C and the frac-tional free volumes of mixtures, which are shown inFig. 17. This correlation has also been found in otherantiplasticised polymers[7,34].

Based on the free volume approach, we argue thatthe highest level of antiplasticisation (i.e. the highestdensification in the glassy state) is promoted by theadditive having the lowest free volume.Fig. 6 showsthat the additives having the lowest free volume, PNAand TBBPA, in fact promote greater stiffening of PEIthan the BHT and HFBPA additives, which have ahigher fractional free volume. However, although the


Fig. 16. Schematic diagram of the volumetric behaviour of a polymer and a mixture polymer+ additive exhibiting (a) plasticization belowthe glass transition; (b) antiplasticization below the glass transition.

free volume of PNA is very dissimilar from that ofTBBPA, theE′ of the PEI+ PNA and PEI+ TBBPAmixtures are very similar with the same concentra-tion of additive. A comparison of BHT and HFBPAreveals a similar behaviour, since the densification inthe glassy state also depends on the volumetric con-traction, as discussed earlier, and this contraction, inturn, appears to increase as the glass transition temper-ature of the additive decreases. Thus, it is possible thatsome compensating effect involving FFV andTg oc-

curs between the PEI+ PNA and PEI+ TBBPA mix-tures, and between the PEI+ BHT and PEI+ HFBPAmixtures. For example, since the FFV of TBBPA ismuch lower than that of PNA, it is to be expectedthat PEI+ TBBPA mixtures would have a higherE′than PEI+ PNA mixtures. However, because PNAhas a much lowerTg than TBBPA, PEI+ PNA mix-tures undergo a higher volumetric contraction, with theend result that the two mixtures have almost identicalFFVs.


Fig. 17. Variation of the storage modulus at 25◦C with the frac-tional free volume for PEI–additives mixtures.

The free volume approach of antiplasticisation isalso useful to interpret the behaviour ofE′ in PEI+ HFBPA mixtures. AsFig. 6 illustrates, this additiveincreases theE′of PEI up to 16.5% of concentration,but further incorporations of additive do not producea continuous increase ofE′, indicating that concen-trations of HFBPA in excess of 16.5% do not leadto further decreases of the FFV of mixtures. In fact,Fig. 9 shows that the FFV curve of PEI+ HFBPAmixtures apparently reached a minimum at around30% HFBPA, with a trend to increase with higherconcentrations. Thus, according to the free volumeapproach, the onset of plasticisation is like to oc-cur at concentrations exceeding the aforementionedone.

These findings strongly suggest that an additive isnot always intrinsically an antiplasticiser or plasticiserand that the same additive may promote either an-tiplasticisation or plasticisation, according to its con-centration in the polymer, as suggested byFig. 9. Thisfinding has been reported in the literature by other au-thors[5,34,35].

The molecular event that promotes this inversion isbelieved to be an aggregation of highly concentratedadditive molecules, since pair-wise contacts of addi-tives are more probable in this condition. This ag-gregation forms microscopic clusters whose size maynot suffice to promote phase separation but suffices tophysically separate the polymer chains, resulting in anincrease of the polymer’s free volume[5,32,34–36].

We assume that large clusters are more easilyformed when the additive is large size and its in-teraction with the polymer chains is minor.Table 2shows that BHT and HFBPA possess these two char-acteristics, considering these additives’ highV and(δph PEI − δph additive)

2 values. Although TBBPA alsohas a highV, this additive is not prone to form clus-ters because its relatively strong interaction with PEI(small (δph PEI − δph additive)

2) far outweighs its largemolecular size.

The HFBPA molecules’ strong tendency to formclusters was corroborated by the PEI+ 43% HFBPAmixture’s high free volume and low storage mod-ulus. This tendency of BHT molecules was clearlyevidenced by the macroscopic phase separation thatoccurred in the PEI+ BHT mixture with BHT con-centrations of more than 23 mol%. This macroscopicphase separation did not occur in the PEI+ 43% HF-BPA mixture, probably because the PEI+ HFBPAinteraction was stronger than the BHT+ PEI pair, asindicated by the(δph PEI− δph additive)

2 values.

4.3. Antiplasticisation and secondary relaxation

Polymer-low molecular weight additive mixturesthat undergo antiplasticisation display another char-acteristic behaviour: while the additive shifts thepolymer’s α relaxation to shorter times, i.e.Tg isreduced, the secondary relaxation occurring at lowtemperatures (below 0◦C) shifts to longer times,representing an increase in the temperature of thistransition[37]. This behaviour also prevails in the PEI+ additive mixtures because, while additives causethe PEI’sTg to drop, they also promote an increaseof this polymer’sT�.

This increase ofT� is also explained by the freevolume approach, considering that the densificationpromoted by the additives also suppresses free vol-ume fluctuations in the glassy state[32,37]. This sup-pression restricts the available space for the polymersegmental motions responsible for low temperaturetransitions, causing a shift of the relaxation to longertimes or, equivalently, a rise in transition temperature.

The hypothesis of the tendency for cluster forma-tion is consistent with some deviations in the DMTArelaxation spectra of the PEI+ HFBPA and PEI+ BHT mixtures. As shown inFig. 3d, the � peakintensity for HFBPA mixtures appears to increase


as more additive is added to PEI. This behaviourmakes sense considering that the formation of HF-BPA molecule clusters begins even at low loadingin PEI, although the initial clusters are small. Thesesmall clusters are not expected to modify the averagefractional free volume of PEI. However, we believethey cause increased fluctuations of local free volume,which are associated with local motions of small seg-ments of the polymer chain[37]. We argue that thisincrease may enable the motions of longer segmentswhose relaxation times are longer, that is, highertransition temperature and greater amplitudes, whichmeans a higher intensity of relaxation. At this point,however, we cannot affirm unequivocally that the lowtemperature transitions of PEI–HFBPA mixtures arethe result of an alteration of the original� transitioncaused by clusters or that these clusters create a newrelaxation. New relaxations have been seen in otherpolymer-low molecular weight additive systems,chiefly in high additive concentrations[25,31,32].

Fig. 3c more clearly depicts a new relaxation forthe PEI + 23% BHT relaxation spectrum at about−20◦C. We speculate that this relaxation is also asso-ciated with cluster formation in this mixture due to thehigh tendency of BHT to form molecule aggregates,as in the case of HFBPA. However, these clusters havedissimilar effects on each mixture, probably becauseof the differences in each additive’s individual char-acteristics.

4.4. Antiplasticisation and transport properties

According to the diffusion theory of Pace andDatyner[5,6], gas transport through the glassy poly-mer depends on the segmental mobility of macro-molecular chains in this state. The correlation betweenthe diffusion coefficientD and the frequencyυ of thechain openings allowing for the passage of a penetrantmolecule is given by:

D =(

d2υ

6

)exp

(−E

RT

)(10)

where d is the mean-square jump displacement,Ethe activation energy for the macromolecule openingsandR the gas constant.

The average frequency of cooperative polymer mo-tions important for the diffusion of gas molecules isin the range of 105–108 Hz [5]. Considering the PEI

relaxation map relating to the� transition (frequencyversus� transition temperature) determined by Belanaet al.[38], the frequency of segment motion associatedwith the� transition is about 1.3×105 Hz at 25◦C (thetemperature at which gas permeability was measured).Thus, it can be inferred that the� transition can influ-ence gas diffusion through the PEI, since it is likelythat the macromolecular motions associated with thistransition create openings in the polymeric matrix thatsuffice to allow for the diffusion of gas molecules.

To confirm the veracity of this speculation, we at-tempted to find a qualitative correlation between thegas permeability andT� and compare it with the cor-relation betweenD andυ given byEq. (10). To findthis correlation,T� must first be transformed into aparameter proportional to�, considering that the cor-relation between the frequency and temperature of aviscous elastic relaxation can be expressed as an Ar-rheniusEq. (11) [39]:

υ� = A exp

(−E�

RT�

)(11)

whereυ�, T� and E� are the frequency, temperatureand activation energy of� relaxation, respectively, andA is a constant. Our use ofEq. (11) was based onthe assumption that the additives cause no substantialchanges inE�. Ngai et al.[37] and Sauer et al.[40]found that this holds true when the intensity of thetransition is slight, like the� transition of PEI. Thus,from Eq. (11), we expect that the relative changes inlog(υ�) promoted by the additives are proportional to1/T�. This means that plotting 1/T� versus log(P) is thesame as plotting log(υ�) versus log(P), which allowsone to check whether the linear relationship predictedby Eq. (10)holds true.Fig. 18a and bshow these plotsfor the O2 and CO2 permeability values, respectively.

The above figures suggest that some connection ac-tually exists between the modification of� transitionand the gases’ permeability. However, the correlationdoes not appear to follow a linear behaviour when allthe mixtures are evaluated together, although a cer-tain linear dependence can be identified among themixtures containing TBBPA. Nevertheless, these fig-ures show that it is rather simplistic to expect the gaspermeability to be governed only by the temperatureof � transition of PEI and/or gas permeability coeffi-cients to be proportional to gas diffusivity coefficients.Several factors confirm this speculation. For example,


Fig. 18. Correlation between of peak temperature of PEI� transition and (a) O2 gas permeability for PEI–additives mixtures; (b) CO2 gaspermeability for PEI–additives mixtures.

we analysed a transition at about−90◦C and perme-ability measured at room temperature. This temper-ature exceeded that of the additives’ glass transition(seeTable 2), modifying their molecular mobility inthe polymeric matrix, as well the degree of interac-tion with the segments associated with the� transition.Thus, the additive’s effect at−90◦C probably differsfrom that at room temperature.

Another inaccuracy source of usingEq. (10) asa basis to correlate the results of this work involv-ing chain dynamics with gas transport parameters is

based on the fact that the permeability of mixturesmay not be proportional to the diffusion coefficientsif the additives also alter the gas solubility of PEI,sinceP = D × S. This appears to be the case heresince, as shown inFig. 12, the presence of additivesmodifies the solubility coefficient significantly. Thus,it is likely that the qualitative behaviour ofP does notreflect the behaviour ofD.

It is also worth noting that, at room temperature, thePEI’s � transition influences the polymer’s propertiesto a considerable extent. In polyimides, this transition


is associated with low frequency and cooperative mo-tions of large molecular segments[38,41]. Such mo-tions can also influence the permeability coefficientthrough their influence on the d parameter ofEq. (10).Coleman and Koros[41] observed that alterations of� transition in polyimides by modification of theirmolecular structure affect the gas permeability coeffi-cients. AsFig. 3suggests, the additives also alter the�transition. Hence, this alteration can also modify gaspermeability, a fact that is neglected when only thealteration of the� transition by the additives is takeninto account.

These observations demonstrate that it is difficultto establish a correlation between gas permeabilityand segmental mobility because of the complexity ofpolymer chain motions. However, when one consid-ers that many of these motions directly influence thepolymer’s free volume, as evidenced by the FFV pa-rameter at room temperature, there should be a goodcorrelation between FFV and gas permeability. Lee[10] developed the following empirical model of thesetwo parameters:

P = Ad exp

(−Bd

FFV

)(12)

whereAd andBd are constants for each type of gas.Fig. 19a and bshow the permeability coefficients forO2 and CO2, respectively, as a function of FFV, re-vealing a clear trend of decreasing permeability asFFV decreases, despite some dispersion of the lineardependence predicted byEq. (12).

A remarkable deviation occurred between thePEI/23% PNA and PEI/23% BHT compositions,whose FFV values were almost identical but whose gaspermeability coefficients differed. A possible explana-tion for the dissimilarities is that these compositions’molecular motions are different, possibly sufficing tochange the gas permeability but not the FFV parame-ter. This is consistent with our previous discussion inSection 4.3, where we speculated that BHT moleculeclusters are formed in the PEI/23% BHT mixture.Because these are large size clusters, the polymerchains are likely to become physically separated,presumably increasing the free volume fluctuationand possibly causing a new relaxation. Although thisproduces higher BHT mixture permeability, the FFVparameter is not sensitive to that difference. However,this difference seems to be accounted for by theC′

H

Fig. 19. (a) O2 permeability-fractional free volume correlationat 700 mmHg and 25◦C for PEI–additives mixtures; (b) CO2permeability-fractional free volume correlation at 700 mmHg and25◦C for PEI–additives mixtures.

parameter because, as shown inTable 4, the C′H of

the PEI/23% BHT mixture is much larger than thatof the PEI/23% PNA composition, indicating thatthe former mixture contains a larger concentration ofnon-equilibrium voids than the latter.

Section 4.3discussed the formation of clustersnot only in the PEI–BHT mixture but also in thePEI–HFBPA mixtures. This phenomenon is the prob-able reason for the PEI+ 31% HFBPA composition’sgreater CO2 permeability (Fig. 14b). However, unlikethe PEI+ 23% BHT mixture, the higher permeabilityof the HFBPA-containing composition was followedby an increase of the FFV.Fig. 19bshows that this


Fig. 20. CO2 average solubility coefficient-fractional free volumecorrelation at 202.650 Pa (2 atm) and 25◦C for PEI–additives mix-tures.

HFBPA mixture also apparently followed the linearcorrelation based onEq. (12).

At this point, one must consider the possibility thatthe differences in permeability could also be causedby discrepancies in free volume size distribution pro-duced by different additives. Such discrepancies mayoccur when using the FFV calculation mode (based ongroup contribution constants), a parameter that cannotdetect the small variations in free volume distribution,which may suffice to alter gas permeability.

Lee’s empirical correlation between gas perme-ability and FFV is based on the hypothesis that gassolubility changes in a polymer family are slight orfollow a linear correlation with FFV similar to thatshown inEq. (12). Fig. 20shows that this correlationappears to be followed, which means that differencesin gas–mixture interactions are reflected linearly inthe FFV parameter (the linear correlation observed at35◦C is assumed to be kept at permeability measure-ments temperature). From these observations it cantherefore be inferred that, in the case of the mixturesstudied here, deviations from Lee’s relation probablydid not result from differences in solubility amongthe films.

5. Conclusions

The incorporation of low molecular weight addi-tives in PEI produces substantial alterations in sev-

eral properties of this polymer, such as glass transitiontemperature, fractional free volume, secondary relax-ations temperatures, storage modulus and gas solubil-ity and permeability. The extent of these alterations islargely dependent on the additive’s properties and onits concentration in the polymer. The results of thisstudy suggest that a high degree of antiplasticisation inPEI is attained when the additive has small moleculesand little free volume, is highly interactive with thePEI and has a lowTg. The low Tg of the additive isnecessary because, with the additives used here, wefound that the lower the additive’sTg the higher thevolumetric contraction of the glassy phase. However,this parameter should not be so low as to reduce thepolymer’sTg to below room temperature.

Although these requirements are usually associatedwith antiplasticisers, additives cannot always be de-signed as antiplasticisers because, when present inhigh concentrations in the polymer, they may triggerplasticisation, possibly through the formation of clus-ters of additive molecules.

The mechanical behaviour of the PEI+ additivemixtures is apparently well correlated with the frac-tional free volumes, indicating that this parameter ac-counts for the free volume involved in relaxations oflarge segments of the polymer that are important forthe absorption of mechanical energy. As the additivesreduce the polymer’s free volume, interchain cohesionincreases, restricting the available space for the macro-molecules to relax. However, with high concentrationsof additive, the presence of bulky clusters leads to thephysical separation of the polymer chains, therebyincreasing the FFV and consequently decreasingtheE′.

According to Pace and Datyner’s theory of molec-ular diffusion, reductions in permeability coefficientscan be considered to result from modifications of thePEI’s segmental mobility promoted by the additives.However, gas permeability does not depend only onthe� transition temperature and other parameters mustbe accounted for in order to quantify the alterationscaused by additives on the transport coefficients. Theseparameters include the� transition temperature, alter-ations of the mixtures’ solubility coefficients and thedifference between the� transition temperature andthe temperature at which the permeability coefficientswere determined. However, these and other parame-ters seem to be accounted for by the fractional free


volume, since a good correlation was found betweenFFV and gas permeability, according to Lee’s rela-tionship. The deviations observed can be attributed toslight differences in macromolecular segmental mo-tions or differences in free volume size distributions.Neither characteristic is considered in calculating theFFV, but both are probably important in the mixtures’gas permeation.

Acknowledgements

The authors would like to thank the Brazilianresearch funding agencies Conselho Nacional deDesenvolvimento Cientıfico e Tecnológico (CNPq)and Programa de Núcleos de Excelencia (PRONEX/FINEP/CNPq) for their financial support of this workand ITAL/CETEA—Centro de Tecnologia de Embal-agem for the gas permeability measurements.

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