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Polymer 53 (2012) 1383e1392

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Polymer

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

Gas permeability of melt-processed poly(ether block amide) copolymers and theeffects of orientation

Shannon Armstrong a, Benny Freeman b, Anne Hiltner a, Eric Baer a,*aDepartment of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USAbDepartment of Chemical Engineering, University of Texas at Austin, Austin, TX, USA

a r t i c l e i n f o

Article history:Received 30 November 2011Received in revised form18 January 2012Accepted 20 January 2012Available online 27 January 2012

Keywords:Gas permeationStrain induced crystallizationThermoplastic elastomer

* Corresponding author.E-mail address: exb6@case.edu (E. Baer).

0032-3861/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.polymer.2012.01.037

a b s t r a c t

Poly(ether block amide) (PEBA) thermoplastic elastomers are used in many different applications, withrecent development being on those which show promise as high gas flux membranes. PEBA copolymerswith high polyether soft block content are possible candidates for gas separation applications due totheir high permeability relative to current commercially used polymers and good selectivity for acidgases such as CO2. To be effective and efficient, the high flux must be maintained over the course ofproduction and use. A series of PEBA copolymers containing poly(tetramethylene oxide) and polyamide-12 was studied to explore the influence of mechanical orientation and copolymer composition on gaspermeability and morphology. Upon uniaxial orientation, several compositions of PEBA copolymersexhibited a significant decrease in permeability, both in the oriented and elastically recovered states.Copolymer composition strongly influenced the degree of change seen in the permeability uponorientation. WAXS and DSC were used to identify strain-induced crystallization that occurred duringorientation. Rubbery materials can crystallize under high strains, and PEBA is no exception. Strain-induced crystallization of the polyether blocks produced a tortuous path for gas diffusion, resulting inas much as a 3.5� decrease in permeability for oriented PEBA films. To maintain high flux for membraneapplications, elastic recovery and thermal treatment proved beneficial in reversing the effects of uniaxialorientation on PEBA copolymers.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Recent interest in the permeation properties of polymers hasdeveloped due to the need for more energy efficient and environ-mentally friendly gas separation methods. Poly(ether block amide)(PEBA) copolymers, in particular, have generated interest as a resultof the permeation selectivity of polar to non-polar gases. A high CO2gas selectivity is desirable in applications such as post-combustioncarbon capture, purification of hydrogen, and natural gas sweet-ening, as well as for CO2/O2 control in modified atmosphere pack-aging [1e3]. In this last application, a high flux, in addition to highCO2/O2 selectivity, is necessary to be effective, which is provided bythe amorphous soft block content of many PEBA copolymers.

PEBA copolymers are a useful and diverse set of thermoplasticelastomers used in applications requiring good permeation, abra-sion resistance, elasticity, and toughness. Additional advantageousproperties include chemical resistance, low density, fatigue resis-tance, and good low temperature performance. PEBA copolymers

All rights reserved.

have traditionally been used in toughness applications (e.g.,running shoes, ski boots, and golf ball cores) and recently as highpermeability materials [4,5]. PEBA copolymers with high polyethercontent have generated interest for use in permeation drivenapplications due to their relatively high gas flux, a result of thecopolymer architecture and morphology, and good selectivity forseparations where CO2 (or other acid gases) is one of the compo-nents [6].

The potential use of PEBA copolymers in permeation basedapplications arose from the ability to maintain beneficial poly(-ethylene oxide) (PEO) gas selectivity properties while avoidingcommon PEO limitations of water sensitivity, low permeability(due to high crystallinity levels), and lowmelting temperature. PEOis known for its unique separation properties of polar (or quad-rupolar) gases from mixtures with non-polar gases, and itdemonstrates an affinity for CO2, contributing to high CO2 perme-ability and selectivity [7,8,11]. Many applications that can takeadvantage of the high CO2 selectivity containwater vapor in the gasstreams, rendering PEO less useful as a gas separation membrane.One approach has been to crosslink PEO to reduce water sensitivityand improve mechanical properties; however, moisture continues

1000 1500 2000 2500 3000 3500

A

bs

orb

an

ce

PA-12

3533

4033

5533

6333

7033

7233

Wavenumber (cm-1

)

2533

Fig. 1. FTIR spectra of PEBA copolymers in the PEBAX xx33 series.

0 20 40 60 80 100

0

20

40

60

80

100

120

140

160

180

200

P(CO2)

Perm

eab

ility (B

arrer)

PEBA Copolymer Composition (mol % PTMO)

P(O2)

0 20 40 60 80 100

0

5

10

15

20

P(CO )

Pe

rm

ea

bility

(B

arre

r)

PEBA Copolymer Composition (mol % PTMO)

P(O )

Fig. 2. O2 and CO2 permeabilities of as-extruded, 30 mm thick PEBAX films over therange of PEBA copolymer compositions.

-25 0 25 50 75 100 125 150 175 200

PTMO

PA-12

Heat F

lo

w (J/g

)

Temperature ( C)o

7033

2533

3533

6333

5533

4033

7233

PA-12

Fig. 3. DSC thermograms showing the effect of copolymer composition on the meltingbehavior of as-extruded unoriented PEBAX films.

Table 2PEBAX crystallinity and thermal characterization.

PEBAX grade Mol% PTMO Tm PTMO (�C) Tm PA�12 (�C) PEBAX %Xca

2533 86 6 138 1<3533 75 5 142 1.1

S. Armstrong et al. / Polymer 53 (2012) 1383e13921384

to have an effect [9]. Current gas separationmembrane applicationsrequire that the membrane materials possess high selectivity aswell as moisture resistance and relatively high flux [10]. Towarda solution to water sensitivity, PEBA and other polyether basedcopolymers can provide reasonable gas flux, CO2 gas selectivities,and water resistance [11e14].

Copolymers have unique and beneficial properties which arisefrom the nature of the individual polymer components and theinteractions between them. The copolymer architecture, mor-phology, and composition directly relate to observed macro-scaleproperties. PEBA copolymers are microphase separated thermo-plastic elastomers, in which the polyether blocks often exist abovetheir melting point and act as soft blocks within the copolymer,while the polyamide hard blocks act as physical crosslinks[13,15,16]. PEBA acquires mechanical strength from the hard poly-amide blocks while elasticity and relatively high permeation area result of the polyether soft blocks [17].

The combination of permeation and elastomeric propertiesprovides the opportunity to explore how orientation and perme-ability are interrelated for PEBA copolymers. Previous studiesfocused on structural changes occurring as a result of orientation,usually explored with the aid of x-ray scattering and atomic forcemicroscopy [18,19]. Until now, the effect of uniaxial orientation onthe permeation properties of PEBA has not been studied, mostlikely owing to the fact that most PEBA membranes are cast fromsolution directly onto porous substrates rather than being extruded[4,5]. This study will further explore the effects of mechanicalorientation on PEBA copolymer structure and morphology bycomparing oxygen and carbon dioxide permeation, as well as x-rayscattering and differential scanning calorimetry, of as-extruded anduniaxially oriented PEBA extruded thin films.

2. Materials and methods

Poly(ether block amide)s that are part of the PEBAX xx33 serieswere purchased from Arkema in pellet form. The PEBAX xx33

Table 1PEBAX copolymer composition and CO2/O2 selectivity.

PEBAX grade 2533 3533 4033 5533 6333 7033 7233 PA�12Mol% PTMO 86 75 71 47 37 25 20 0P(CO2)/P(O2) 9.9 9.3 9.3 9.2 7.9 6.2 4.8 4.2

grades are a series of copolymers consisting of poly(tetramethyleneoxide) (PTMO) and polyamide-12 (PA-12) with the xx denoting theshore D hardness [20]. The xx33 series varies greatly in copolymercomposition, providing a wide range in material properties.

The copolymer composition for each grade in the series wasdetermined by Fourier transform infrared spectroscopy (FTIR)

4033 71 1 161 3.45533 47 e 160 7.86333 37 e 169 10.77033 25 e 171 14.27233 20 e 170 17.5PA�12 0 e 175 23.4

a PEBAX %Xc calculated at room temperature (21 �C).

0 100 200 300 400 500

0

5

10

15

20

25

30

35

7033

2533

3533

6333

5533

4033

7233

Stress (M

Pa)

Strain (%)

PA-12

Fig. 4. Stressestrain behavior of extruded, 30 mm thick, PEBAX xx33 series films.

Fig. 5. WAXS reflection patterns of as-extruded unoriented PEBAX films. (a) 2

S. Armstrong et al. / Polymer 53 (2012) 1383e1392 1385

using a Nicolet Nexus 870 with a Continuum microscope attach-ment and a Germanium crystal in the ATR mode. Each block in thePEBA copolymer has a distinct peak in the FTIR spectrum that doesnot overlap with the other component, allowing for direct corre-lation to composition, Fig. 1. The PTMO blocks exhibit an ether peakat 1100 cm�1 not seen in PA-12, while the PA-12 exhibits a carbonylpeak at 1640 cm�1 [21,22]. Composition was calculated based onthese two peak heights by the following:

% PTMO ¼�

h1100h1100 þ h1640

�� 100

where h1100 is the PTMO ether peak height, while h1640 correspondsto the PA-12 carbonyl peak height. Peak height was determinedafter establishing a baseline andmeasuring from the baseline to thepeak maximum [23]. With increasing PTMO content, a clearincrease was observed in the 1100 cm�1 peak with a simultaneousdecrease in the 1640 cm�1 peak. The PEBA copolymer compositionwas observed to range from 20 mol% to 86 mol% PTMO, Table 1.

Seven PEBA grades in the xx33 series were extruded at 190 �Cusing a 1400 film die inlinewith a film casting roll at 55 �C to producemonolith films 25e50 mm in thickness. PEBAX 4033 films were45e50 mmwhile all otherfilmswere 25e30 mm.Thicker PEBAX4033films were used due to thinner films breaking at very low strains,

533, (b) 3533, (c) 4033, (d) 5533, (e) 6333, (f) 7033, (g) 7233, (h) PA-12.

0 100 200 300 400 500

0

20

40

60

80

100

120

140

160

180

200

2533 3533 4033 5533 6333 7033 7233

CO

2 P

erm

eab

ility (B

arrer)

Final Strain (%)

7033

6333

5533

40333533

2533

7233

0 100 200 300 400 500

0

2

4

6

8

10

12

14

16

18

20

b

a 2533 3533 4033 5533 6333 7033 7233

7033

63335533

4033

3533

2533

Oxyg

en

P

erm

eab

ility (B

arrer)

Final Strain (%)

7233

Fig. 6. PEBAX film O2 and CO2 permeability dependence on uniaxial strain acrossa range of strains from 100 to 400%.

S. Armstrong et al. / Polymer 53 (2012) 1383e13921386

possiblya result of extrusion inducedorientationof thePA-12,whichis partially observed in the 50 mm film WAXS patterns in Fig. 5.

Uniaxial orientation of film samples with a gage length of 2.5 cmwas conducted with an MTS Alliance RT/30 testing machine.Samples with dimensions of 7 cm � 4 cm were cut from theextruded films with the smaller dimension parallel to the extrusiondirection. Films were oriented along the extrusion direction atroom temperature (21 �C) with a strain rate of 50% min�1 tomaximum strains of 100, 150, 200, 300, and 400%. While in tensionat each strain, the oriented samples were constrained in thestretched state by applying a self-adhesive aluminummask to bothsides of the film. The aluminummask possessed a 5 cm2 hole in thecenter for permeation and wide angle x-ray scattering measure-ments. Additionally, a series of the oriented samples were allowedto elastically recover by removing the applied stress before beingadhered to the aluminum mask.

Permeabilitymeasurements were conducted on PEBA filmswithdifferent thermo-mechanical histories: (1) as-extruded, (2) intension, (3) elastically recovered, and (4) after annealing while intension. The O2 and CO2 permeabilities were measured usingMOCON OX-TRAN 2/20 and PERMATRAN-C 4/40 permeationinstruments. Testing was done at 23 �C and 0% humidity witha sample test area of 5 cm2 for all samples. The O2 and CO2 testgases were used at 2% concentration in nitrogen to prevent sensorsaturation due to high flux values.

Thermal analysis was done under nitrogen using a Perkin ElmerDSC-7. The heating scans ranged from �60 to 200 �C at a rate of10 �C min�1 with sample sizes of 4e6 mg. For all figures containingDSC thermograms, only the first heating scan is shown.

Wide angle x-ray scattering (WAXS)was conductedwith an x-raygenerator (MicroMax-002, Rigaku, Woodlands, TX) equipped withtwo laterally gradedmultilayer optics in a side-by-side arrangement,giving a highly focused parallel beam of monochromatic CuKaradiation (l ¼ 0.154 nm). The monochromatic X-ray beam, operatedat 45 kV and 0.88 mA, was collimated using three pinholes; thediameter of the X-ray beam at the sample position was w700 mm.The images were collected using Fujifilm magnetic imaging plates,which were processed by a Fujifilm FLA-7000 image plate reader.Measurements were conducted in the normal direction (ND) undervacuum and at room temperature (21 �C) on PEBA films with thefollowing thermo-mechanical histories: (1) as-extruded, (2) intension, (3) elastically recovered, and (4) after annealing while intension. As-extruded, unoriented samples were exposed for 7 hwhile the oriented samples were exposed for 15 h. PEBAX hasa relatively lowcrystallinity, necessitating the long exposure times toachieve strong reflection patterns. Additionally, the oriented filmsrequired extended exposure times relative to the controls due toa reduction in thickness during orientation, which results in lessscattering.

3. Results and discussion

3.1. Unoriented PEBAX copolymer films

To determine and understand the changes that occur uponorientation of PEBA copolymers, as-extruded unoriented filmswereinitially studied. As-extruded films exhibited an interesting trend inthe O2 and CO2 permeabilities over the range of PEBA copolymercompositions. As the amount of PTMO increased, permeabilityincreased exponentially, as shown in Fig. 2. The dramatic increasein permeability was a result of low copolymer crystallinity andincreased PTMO soft block content, which exists above its meltingtemperature, facilitating a less tortuous, high permeability pathwayfor gas to diffuse through the film. Additionally, while both O2 andCO2 permeabilities increase exponentially, CO2 increases at a faster

rate, which was evident in the CO2/O2 gas selectivity, Table 1. Manypolymers exhibit a CO2/O2 gas selectivity of approximately four[24], while the PEBAX copolymers exhibit selectivity values rangingfrom 4 to almost 10. PEBAX grades with greater than approximately47% PTMO showed a consistently high selectivity of about 9,probably due to most of the gas transport occurring through thepolyether soft blocks, which have an affinity for CO2. From litera-ture, the increased selectivity has been attributed to the polyetherblocks having an affinity, or increased solubility, toward CO2molecules [12e14].

The thermal properties of PEBAX copolymers provide insightinto the observed permeation trend. At room temperature, thePTMO exists above its melting temperature (Tm ¼ 15 �C) [13] whilethe polyamide-12 (PA-12) exists below its glass transitiontemperature (Tg ¼ 50 �C). PEBAX is a microphase separated ther-moplastic elastomer as seen by DSC. In DSC thermograms of 25 mmextruded films for each grade in the xx33 series, two distinct peakswere seen in the first heating scans of grades with at least 70%PTMO, Fig. 3. As the amount of PTMO was increased, the meltingpeak of the PTMO blocks shifted to higher temperature, at the sametime, the PA-12 melting peak diminished and showed a shift to

Table 3Permeability change upon orientation of PEBAX copolymers.

PEBAX grade 2533 3533 4033 5533 6333 7033 7233 PA�12Mol% PTMO 86 75 71 47 37 25 20 0P(O2) decrease 3.5� 3.0� 2.1� 1.8� 0 0 0 0

S. Armstrong et al. / Polymer 53 (2012) 1383e1392 1387

lower temperature, Table 2. With lower PA-12 content, the overallcrystallinity significantly decreased and the copolymer becamemostly amorphous, particularly in the materials with the highestpolyether content, which also exhibited the highest permeabilityand selectivity values. Crystallinity for all PEBA copolymers wascalculated at room temperature, which only accounts for the PA-12crystallinity due to the PTMO having a melting point below roomtemperature. PA-12 crystallinity was determined assuming that theheat of fusion for PA-12 is 246 J/g and the total PEBA crystallinitywas calculated by accounting for the composition of PA-12 in thespecific copolymer [13].

The influence of PEBAX copolymer composition was alsoapparent in the mechanical behavior. Stressestrain curves forthe high PTMO grades showed a very broad curve with nodistinct yield point, indicative of low crystallinity and an elas-tomeric behavior, Fig. 4. The polyether soft blocks allow thefilms to stretch to high strains, up to 400% for PEBAX 2533,

Fig. 7. WAXS reflection patterns of uniaxially oriented PEBAX films in tension. (a) 2533, (b)strain, while (f) 7033 is at 100% strain.

providing the possibility of high alignment and orientation ofthe polymer chains. The low PTMO containing grades showedstronger yield points and higher moduli, shown in the Fig. 4inset, and additionally tended to fracture at lower strains,closely representative of PA-12, which prevented high orienta-tion levels.

WAXS patterns of the as-extruded unoriented films showed themostly amorphous structure of PEBAX. With increasing PA-12content, a PA-12 crystal reflection appeared overlapping theamorphous PTMO halo as well as at the meridian in the center ofthe WAXS pattern, Fig. 5. Similar patterns have also been observedbyWilkes [32]. The PA-12 blocks show broad reflection arcs, due toslight orientation resulting from the film extrusion conditions;however, they were assumed to have no effect on the permeability.

3.2. Uniaxially oriented PEBAX copolymer films

From the mechanical characterization of the xx33 PEBAX series,it was observed that several of the high polyether content copoly-mers exhibited an elastomeric behavior and fairly high fracturestrains. Until now, the effect of strain induced orientation on thepermeability properties of PEBAX has not been studied, likelyowing to the fact that most PEBAX membranes are cast fromsolution [18]. In addition, previous studies focused on structural

3533, (c) 4033, and (d) 5533 at 200% strain. (e) 6333, (g) 7233, and (h) PA-12 at 150%

0 20 40 60 80 100 120

0

5

10

15

20

5533 6333

Stress (M

Pa)

Strain (%)

6333

5533

0 20 40 60 80 100 120

0

5

10a

b

2533 3533 4033

Stre

ss

(M

Pa

)

Strain (%)

4033

3533

2533

Fig. 8. Hysteresis of PEBAX films oriented to 100% strain. (a) 2533, 3533, and 4033, (b)5533 and 6333.

S. Armstrong et al. / Polymer 53 (2012) 1383e13921388

changes occurring as a result of orientation, usually characterizedbyWAXS and SAXS, without relating the changes in the structure toa measurable property [25].

Gas permeability is an important property that directlyresponds to changes in polymeric structures, specifically uponorientation and changes in crystallinity of the polymer chains[26e29]. To determine the influence of mechanical orientation onPEBA copolymers, gas permeability was measured for the xx33series PEBAX films while in tension at varying strains, presumed tobe an oriented state.

Upon uniaxial orientation, 86% PTMO (PEBAX 2533) down to47% PTMO (PEBAX 5533) copolymers showed a significant andlinear decrease in permeability at increasing strains. For PEBAX2533, both the O2 and CO2 permeabilities decreased by 3.5�between the unoriented control and the 400% strain film, Fig. 6.Because a similar decrease in permeability occurred for O2 and CO2,there was no change in the CO2/O2 selectivity. The significance ofthe permeability decrease in each grade was highly dependent onthe copolymer composition. A smaller decrease in permeabilityoccurred with lower PTMO soft block content, Table 3. Copolymerscontaining less than approximately 37% polyether showed nochange in permeability upon orientation. The greatest change inpermeability was observed with higher polyether content copoly-mers; therefore, the change was likely a result of a structuralchange in the PTMO soft blocks.

To verify that the permeability decrease was caused by a changein the PTMO blocks, WAXS patterns were obtained for all filmswhile in tension. A definitive difference was observed in theoriented films as compared to the as-extruded films. The highPTMO copolymers, which showed the largest permeabilitydecrease, exhibited two strong equatorial reflections where previ-ously only an amorphous polyether halo existed, Fig. 7. Thesereflections lessen in intensity as the PTMO content decreased, whilethe broad PA-12 reflections at the equator and the meridianintensified. It is evident from the new reflections that PTMO couldbe crystallizing during orientation.

In the past, authors have shown that soft, elastomeric polymerscould strain crystallize when a large strain was applied. Straininduced crystallization occurred when the polymer chains werehighly oriented and aligned, allowing for easier chain packing[25,30]. Natural rubber and some copolymers, such as PEBA, havepreviously exhibited this property [31,32]. Also, polymer crystalsare generally considered to be impermeable and have the ability toslow down or block the gas diffusion pathway through the bulk film[27e29]. Considering possible strain induced crystallization andthe generally impermeable nature of polymer crystals, in combi-nationwith the permeation andWAXS data presented, the PTMO inPEBAX copolymers undergo strain induced crystallization, whichincreases the tortuosity of the gas diffusion pathway and reducesthe permeability. Strain induced crystallization of the PTMO wouldproduce smaller changes in permeability as the concentration ofPTMO was decreased, due to fewer PTMO crystals to block the gasflow, as was observed over the range of copolymer compositionspresented.

3.3. Elastically recovered PEBAX copolymer films

The previous uniaxially oriented samples were measuredwhile in tension, showing strong orientation and strain inducedcrystallization of the PTMO soft blocks. Elastic recovery ofstrained natural rubber is known to cause strain induced crys-tallinity to melt upon release of the stress [31]. This phenom-enon may lead one to consider whether the strain inducedcrystallinity in PEBA copolymers diminishes upon recovery forany or all of the copolymers, particularly because the degree of

elastic recovery varied with copolymer composition, with thehigh polyether containing copolymers exhibiting the mostrecovery, Fig. 8.

Upon measurement of the elastically recovered PEBAX filmpermeability, a trend similar to the permeability of films in tensionwas observed. The grades with greater than approximately 47%PTMO content showed a decrease in permeability with increasingresidual strain, Fig. 9. The permeability of the recovered films,particularly for the 2533 and 3533 grades, was lower than the filmsin tension at similar strains. Even when orienting within the rangeof total elastic recovery, a permeability lower than that of the as-extruded films and oriented films in tension was observed. Thegas flow through the high PTMO containing copolymers wascontinuing to be inhibited after recovery, suggesting that residualPTMO crystallization may exist in the film after elastic recovery.

Two methods were used to determine whether residual PTMOcrystals were in fact present in the recovered PEBAX films andsubsequently causing decreased permeability. The first methodused was WAXS patterns of the recovered films, which showeddramatically different scattering than the films in tension andmoreclosely resembled unoriented films at high PTMO content, Fig. 10.Upon recovery, the two strong equatorial PTMO reflections were

0 100 200 300 400 500

0

2

4

6

8

10

12

14

16

18

20

2533 3533 4033 5533

Oxyg

en

P

erm

eab

ility (B

arrer)

Final Strain (%)

Oriented

Recovered

6333

5533

40333533

2533

Fig. 9. Oxygen permeability of PEBAX xx33 series films after elastically recoveringfrom various strains.

S. Armstrong et al. / Polymer 53 (2012) 1383e1392 1389

diminished to two very fine arcs and lessen as the PTMO contentwas decreased, evidence that residual crystals exist in films ofPEBAX with greater than 47% PTMO; however, to a lesser extentthan while in tension.

The second method was to use thermal analysis to extract thepresence of residual PTMO crystals. In unoriented samples, the softPTMO blocks melted below room temperature at around 7 �C andwere observed only in the three highest PTMO containing copoly-mers. Thermograms of PEBAX films recovered from various strains,with approximately 47% PTMO and higher, were obtained andcompared with the unoriented as-extruded films, Fig. 11. As theapplied orientation strain was increased, a second endothermappeared at 42 �C, along with a decrease in the PTMOmelting peak

Fig. 10. WAXS reflection patterns of recovered PEBAX films. (a) 2533, (b) 3533, (c) 403

at 7 �C [33,34]. It appears that the PTMO crystal morphologychanged due to orientation and results in a higher melting point,present above room temperature, which allowed the crystals toremain as gas barriers in the film. Interestingly, the 42 �C endo-therm is close to the reported 43e46 �C equilibrium meltingtemperature, T

�m, for low molecular weight pure PTMO [35e37].

The T�m is achieved by extended-chain crystals that can be created

through high orientation of the polymer chains, similar to theconditions described in this work.

PEBAX 2533 (86% PTMO) showed the strongest effect of orien-tation on the formation of the 42 �C melting peak, which corre-sponds to the strong PTMO crystal reflections in theWAXS patternsand the most significant drop in permeability. The presence andintensity of the new peak was less dominant as PTMO contentdecreased due to less PTMO available to strain crystallize duringorientation. Further evidence that the additional 42 �C peak wasa result of PTMO crystals came from the second heating thermo-grams, which showed only a PTMOmelting peak at around 7 �C andno melting peak at 42 �C.

From the heats of melting, a dramatic change in the crystallinemelting behavior was observed as a result of uniaxial orientation.PEBAX 2533 had a strong strain crystallized peak, particularly at400% strain. At 400% strain, the heat of melting decreased fromapproximately 20.5 to 2.9 J/g for the 7 �C endotherm, while the42 �C peak heat of melting was as high as 17.8 J/g. The effect wasless evident in lower PTMO containing copolymers, however, wasstill present at 75% and 71% PTMO, particularly at the higheststrains, Fig. 12. The decreased effect was due to a lower PTMOcontent, which was prevented from strain crystallizing duringuniaxial orientation by the increased number of PA-12 hard blocks.During uniaxial orientation and elastic recovery, the PA-12 crys-tallinity remained constant with no effects of orientation on theamount of crystallinity present. However, in all cases, the increasedPTMO crystallinity present at room temperature produced a moretortuous path for gas flow as compared to the unoriented films,which resulted in a substantial decrease in permeability of orientedand elastically recovered PEBAX films.

3, and (d) 5533 recovered from 200% strain. (e) 6333 recovered from 100% strain.

-25 0 25 50 75 100 125 150 175 200

Control

100%

200%

300%

400%

Heat F

lo

w

Temperature ( C) o

Temperature ( C) o

Temperature ( C) o

Temperature ( C) o

Strain Crystallized PTMO

T

a

c

b

d

m = 42

o

C

-25 0 25 50 75 100 125 150 175 200

400%

Control

300%

200%

He

at F

lo

w

Strain Crystallized PTMO

Tm = 42

o

C

-25 0 25 50 75 100 125 150 175 200

Control

100%

200%

H

ea

t F

low

150%

-25 0 25 50 75 100 125 150 175 200

Control

100%

300%

He

at F

lo

w

Strain Crystallized PTMO

Tm = 42

o

C

200%

Fig. 11. Thermograms of uniaxially oriented PEBAX films after elastic recovery from various stages of strain: (a) 2533, (b) 3533, (c) 4033, (d) 5533.

0 100 200 300 400 500

0

5

10

15

20

25

2533 3533 4033

3533

4033

He

at o

f M

eltin

g (J

/g

)

Maximum Strain before Recovery (%)

2533

Fig. 12. Melting enthalpy of residual strain-induced crystallinity of PEBAX filmsrecovered from various strains.

40 50 60 70 80 90 100

0

2

4

6

8

10

12

14

16

18

20

Annealed

Oriented

Unoriented

3533

2533

Unoriented Oriented Annealed

Oxyg

en

P

erm

eab

ility (B

arrer)

PEBA Copolymer Composition (mol% PTMO)

4033

Orientation Direction

Fig. 13. Oxygen permeability of PEBAX 2533, 3533, and 4033 films as extruded, intension (200%), and after annealing at 60 �C for 30 min.

S. Armstrong et al. / Polymer 53 (2012) 1383e13921390

Fig. 14. WAXS reflection patterns of 200% strain, uniaxially oriented PEBAX films before and after annealing at 60 �C for 30 min: (a,d) 2533, (b,e) 3533, (c,f) 4033.

S. Armstrong et al. / Polymer 53 (2012) 1383e1392 1391

3.4. Annealed PEBAX copolymer films

From the thermal analysis of recovered PEBAX films, specificallythose with higher PTMO contents, an additional melting peak wasobserved at 42 �C due to residual strain induced crystallization. Inmany gas separation membrane applications, the decrease inpermeation caused by residual crystallization would be an unde-sired consequence of orienting PEBAX. A solution for increasing thegas flux would be to incorporate a post process annealing step toeliminate the residual strain induced crystallinity without nega-tively affecting the membrane structure or properties.

Toward a solution to resolve the decreased permeability uponorientation, PEBAX films oriented to 200% strain were annealed at60 �C while in tension. A temperature of 60 �C was chosen becauseit was well enough above the 42 �Cmelting endotherm of the straininduced crystallinity to ensure complete melting. Annealing above42 �C should remove the PTMO crystallinity and thereby reduce theresistance to gas flow through the film.

The permeability of PEBAX copolymers which show effects ofstrain induced crystallinity was measured in tension and afterannealing while in tension. As seen previously, the permeabilitydramatically decreased after uniaxial orientation. However, afterannealing, the permeability increased to levels comparable to thatof the as-extruded unoriented films, Fig. 13. The increase inpermeability was a result of melting the PTMO crystals and recre-ating a high gas diffusion pathway through the polyether blocks.

In addition, WAXS patterns were collected at the same points atwhich permeability measurements were conducted to determinestructural changes that occurred during orientation and afterannealing. Upon annealing, the two equatorial PTMO crystalreflections, present in the oriented films, were eliminated, corre-sponding to the observed increase in permeability, Fig. 14. Only thehighly oriented PA-12 reflections were seen after annealing of theoriented films, which have no effect on the permeability of PEBAXcopolymers.

Upon examination of the permeability after annealing, theannealed films showed values slightly higher than the unoriented

control films. The increase in permeability was larger than thedecrease caused by uniaxial orientation, whichmeant that a changein the copolymer, other than melting of the PTMO, had occurred. Itwas possible that upon orientation, the polyamide-12 blocks hadchanged morphology. Uniaxial orientation of all samples was doneat room temperature, well below that of the glass transition of thePA-12. During orientation, the PA-12 crystals could have beenbroken into small blocks as a result of the high strain used to orientthe films [25,30]. Smaller PA-12 crystals would provide a lesstortuous diffusion pathway, increasing the overall permeability.

4. Conclusions

Uniaxial orientation has a significant effect on the gas perme-ability of poly(ether block amide) (PEBA) copolymers, with up toa 3.5� decrease from the unoriented control to the 400% strainsample. The decrease in permeability was attributed to crystallinityresulting from strain-induced crystallization of the poly(tetra-methylene oxide) (PTMO) soft blocks. The PTMO, usually melted atroom temperature, strain crystallizes in a form that melts aboveroom temperature (at 42 �C), which allowed for the crystals toblock the flow of gas through the oriented and elastically recoveredfilms. The diffusion path becomes more tortuous resulting ina much lower permeability. To reverse the effect of orientation onPEBA copolymers, particularly for high fluxmembrane applications,an annealing step was employed. Annealing melted the PTMOcrystals and increased the permeation back to levels comparable tothat of the unoriented as-extruded films. The ability to remove theeffects of orientation in PEBA copolymers allows for a broad rangeof applications, particularly in extruded gas separationmembranes.

Acknowledgments

The authors are grateful to the National Science Foundation forfinancial support from the Science and Technology Center forLayered Polymeric Systems (DMR 0423914).

S. Armstrong et al. / Polymer 53 (2012) 1383e13921392

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