Deakin Research Online This is the published version: Wu, Shuying, Guo, Qipeng, Zhang, Taiye and Mai, Yiu-Wing 2013, Phase behavior and nanomechanical mapping of block ionomer complexes, Soft matter, vol. 9, no. 9, pp. 2662-2672. Available from Deakin Research Online: http://hdl.handle.net/10536/DRO/DU:30057695 Reproduced with the kind permission of the copyright owner.
Copyright: 2013, Royal Society of Chemistry.
Soft Matter
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aPolymers Research Group, Institute for Fron
Bag 2000, Geelong, Victoria 3220, AustraliabNewSpec Pty. Ltd., 134 Gilbert Street, AdelcCentre for Advanced Materials Technology
and Mechatronic Engineering J07, The Unive
2006, Australia
Cite this: Soft Matter, 2013, 9, 2662
Received 1st November 2012Accepted 21st December 2012
DOI: 10.1039/c2sm27512k
www.rsc.org/softmatter
2662 | Soft Matter, 2013, 9, 2662–26
Phase behavior and nanomechanical mapping of blockionomer complexes
Shuying Wu,a Qipeng Guo,*a Taiye Zhangb and Yiu-Wing Maic
Block ionomer complexes SSEBS-c-PCL were prepared, as a consequence of proton transfer from the
sulfonic acid of sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SSEBS) to
the tertiary amine of a tertiary amine terminated poly(3-caprolactone) (APCL). The phase behavior of
SSEBS-c-PCL was thoroughly investigated and the results showed that APCL in SSEBS-c-PCL displays
unique crystallization behavior owing to the influence of interactions between the amine and sulfonic
acid groups as well as the effects of confinement. Further, small-angle X-ray scattering study revealed
that SSEBS-c-PCL displays a less ordered micro-phase structure compared to SSEBS. A quantitative
mapping of mechanical properties at the nanoscale was achieved using peak force mode atomic force
microscopy. It is found that the block ionomer complex possesses a higher average elastic modulus after
complexation with crystallizable APCL. Additionally, the moduli for both hard and soft phases increase
and the phase with higher modulus assignable to the hard SPS component shows much more
pronounced changes after complexation, confirming that APCL interacts mainly with the SPS blocks.
This provides an understanding of the composition and nanomechanical properties of these new block
ionomer complexes and an alternative insight into the micro-phase structures of multi-phase materials.
Introduction
Block copolymer ionomers, where one block is fully or partiallyionized, have attracted considerable attention due to thecombination of the individual properties of both ionomers andblock copolymers.1 It is well-known that a block copolymershows a micro-phase separated morphology because of itschemically distinctive polymer segments connected by covalentbonding. The chemically different blocks in a copolymer renderit some interesting properties such as self-assembly, metalcomplexation, micellization, absorption, molecular association,etc.2–4 Block copolymers containing a small amount of ionicgroups exhibit unique properties compared with the corre-sponding non-ionic polymers owing to the strong intermolec-ular association.5 Interaction strength and properties of a blockcopolymer ionomer are governed by several factors such aspolymer backbone, ionic content, degree of neutralization andcounterion types.6–9 And amongst these factors, the effects ofcounterion types on the morphologies and properties have beenextensively studied.6,9 For anion-containing ionomers, thecounterions are usually alkali, alkaline earth, transition, and
tier Materials, Deakin University, Locked
. E-mail: [email protected]
aide, South Australia 5000, Australia
(CAMT), School of Aerospace, Mechanical
rsity of Sydney, Sydney, New South Wales
72
rare earth metal cations. By contrast, the organic cations basedon amine, or pyridine, are also of great interest. Compared toionomers with inorganic cations, relatively fewer studies havebeen reported on ionomers with organic cations which are oenlow molecular weight organic cations.10–12
Sulfonated styrene-based ionomers represent one of themost important and widely studied block copolymer ionomers.The presence of sulfonic acid groups facilitates the preparationof miscible polymer blends or complexes based on somespecic interactions including ion–ion, ion–dipole, hydrogenbonding, or formation of acid–base complexes, etc.13–18 Forexample, Lu and Weiss reported the morphology and phasebehavior of blends of a lightly sulfonated styrenic block copol-ymer ionomer and poly(3-caproactone) (PCL).14 The miscibilityof PCL in the ionomeric micro-phase was greatly enhanced bythe interactions between the sulfonate groups and polyester.Moreover, partially miscible blends of sulfonated polystyreneand polyurethane were prepared due to the proton transferfrom sulfonic acids to tertiary nitrogen of the polyurethaneextender.15 Similarly, this ion–ion interaction was used toprepare complexes of polystyrene ionomers with mono- and bi-functional styrene oligomers (homo-gras) or with butyl acry-late oligomers (hetero-gras).16,17 Lundberg used a tertiaryamine terminated poly(3-caprolactone) to prepare complexesbased on sulfonated styrene-based polymers.18 The resultingcomplex is useful as a pour depressant agent that can promotethe ow of heating fuels, diesel and paraffinic oils effectively atlow temperatures. But, to the best of our knowledge, up to now,
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there have been no published reports on the phase behaviorand nanomechanical properties of these types of complexes.
In our recent studies, we reported a class of block ionomercomplexes as a template to prepare tough nanostructured epoxythermosets.19,20 These novel block ionomer complexes, namelySSEBS-c-PCL, were prepared based on sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SSEBS) andtertiary amine terminated poly(3-caprolactone) (i.e., 3-dimethy-laminopropylamine-terminated poly(3-caprolactone) (APCL)).However, to date, little work has been done on the phasestructure and properties of these novel block ionomercomplexes formed due to the protonation from the sulfonic acidgroups to the tertiary amine end group. The complex SSEBS-c-PCL consists of SSEBS as the hydrocarbon backbone and APCLas the side chains, leading to a kind of amphiphilic block ion-omer complex, which can also be viewed as ionomers contain-ing counterions with long organic tails or gra copolymer-likematerials.
Herein, we report a detailed study of the phase behavior andnanomechanical properties of these new block ionomercomplexes. A unique micro-phase morphology is expected dueto the presence of a rubbery phase (poly(ethylene-ran-butylene),EB blocks), a hard phase (sulfonated polystyrene, SPS blocks),and semicrystalline component (PCL side chains) in the SSEBS-c-PCL. This study will also provide new insights into how thecrystallization behavior is affected by a restricted geometry(block copolymer ionomer micro-domains) and interactionswith the block copolymer ionomer. Further, nanomechanicalmapping techniques based on peak force mode atomic forcemicroscopy (AFM) were used to obtain elastic modulus maps,which have enabled the investigation of the physico-chemicalproperties of these chemically and mechanically heterogeneousmaterials at the nanoscale.
Table 1 Characteristics of block ionomer complexes
SulfonatedSEBS(SSEBS)
Sulfonationdegree (mol%)
Block ionomercomplex
Content ofAPCL (wt%)
Amine/acidmolar ratio
14.2SSEBS 14.2% 1SSEBS-c-PCL 11.8% 0.1118.8SSEBS 18.8% 2SSEBS-c-PCL 21.1% 0.1629.8SSEBS 29.8% 3SSEBS-c-PCL 28.6% 0.1741.5SSEBS 41.5% 4SSEBS-c-PCL 44.4% 0.23
Experimental sectionMaterials and preparation of samples
Polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene(SEBS) containing 29 mol% of styrene blocks was purchasedfrom Sigma-Aldrich Co. The average molecular weightMw of theSEBS block copolymer was 95 600 withMw/Mn ¼ 1.05 (andMn isnumber-average molecular weight) measured by GPC in tetra-hydro-furan (THF) relative to the polystyrene standard. All thechemicals including acetic anhydride, concentrated sulfuricacid (96%), 1,2-dichloroethane (DCE), isopropyl alcohol (IPA),3-caprolactone, stannous octanoate (Sn(Oct)2), 3-dimethylami-nopropylamine and THF were reagent grade.
Sulfonation of SEBS was conducted in 1,2-dichloroethane at50–55 �C under nitrogen atmosphere described in detail inour previous paper.19 The sulfonation degree of SSEBS, i.e.,the percentage of polystyrene blocks graed with sulfonicacid groups, was obtained by titration with standardsodium hydroxide solution (0.1 N) using phenolphthaleinas indicator. SSEBS with four different sulfonation degreeswere prepared by adjusting the feed amount of acetylsulfonate. In terms of appearance, the color of SSEBSbecomes darker with increasing degree of sulfonation. 3-
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Dimethylaminopropylamine-terminated poly(3-caprolactone)(APCL) was synthesized by ring-opening polymerization of3-caprolactone with 3-dimethylaminopropyl-amine as initiatorin the presence of the catalyst, Sn(Oct)2.19 The resultant SSEBSand APCL were characterized by FTIR and 1H NMR and Mn ofAPCL was estimated to be �2000 g mol�1.19
The block ionomer complex SSEBS-c-PCL was synthesized byneutralization of SSEBS with APCL as previously described.19 Inthe present work, four block ionomer complexes SSEBS-c-PCLwere prepared from SSEBS with four different degrees ofsulfonation. The characteristics of these four block ionomercomplexes are given in Table 1. The sulfonation degree, i.e., themolar percentage of polystyrene graed with sulfonic acidgroups, varies from 14.2 to 41.5 mol%. The corresponding blockionomer complexes are denoted by 1SSEBS-c-PCL, 2SSEBS-c-PCL, 3SSEBS-c-PCL and 4SSEBS-c-PCL, respectively. Theformation of the block ionomer complex is due to the protontransfer from the sulfonic acid group, which is naturally acidic12
to the tertiary amine end group of APCL, leading to an ioniclinkage. The block ionomer complex SSEBS-c-PCL was charac-terized by FTIR and 1H NMR.19
Fourier transform infrared (FTIR) spectroscopy
FTIR spectra of all samples were measured with a Bruker Vertex70 FTIR spectrometer. The THF solutions of samples weredropped onto the KBr disks. The solvent was evaporated atroom temperature and the disks were further dried undervacuum at 100 �C before measurement. The spectra wererecorded by the average of 32 scans in the standard wave-number range 600–4000 cm�1 at a resolution of 4 cm�1.
Differential scanning calorimetry (DSC)
Calorimetric measurements were made on a TA Q200 differen-tial scanning calorimeter in dry nitrogen. Indium and tinstandards were used for calibration of the low- and high-temperature regions, respectively. Samples of about 8 mg wereplaced in the DSC pan. All samples, except when indicatedotherwise, were rst heated to 100 �C from 0 �C at a rate of 20 �Cmin�1 (rst heating scan) and kept at that temperature for 5min; they were subsequently cooled at a rate of�10 �Cmin�1 todetect crystallization (cooling scan). Following the cooling scan,a second scan was conducted at the same heating rate as therst. The Tg values were taken as the mid-point of the transitionin the second scan of the DSC curves. The crystallizationtemperature (Tc) was determined from the minimum of the
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exothermic peak, whereas the melting temperature (Tm) wasobtained from the maximum of the endothermic peak.
Polarizing Optical Microscopy (POM)
The semi-crystalline morphology of SSEBS-c-PCL was examinedusing a Nikon eclipse-80i optical microscope under polarizedlight. Solutions of SSEBS-c-PCL were spread as thin lms onglass slides and dried in a vacuum oven.
Small-angle X-ray scattering (SAXS)
The SAXS experiments were performed at the AustralianSynchrotron on the small/wide angle X-ray scattering beam-lineutilizing an undulator source that allowed measurement at avery high ux to moderate scattering angles and a good ux atthe minimum q limit (0.012 nm�1). The intensity proles wereinterpreted from the plot of scattering intensity (I) versus scat-tering vector, q ¼ (4p/l) sin(q/2) (where q is scattering angle andthe wavelength l is 0.062 nm).
Measurements of nanomechanical properties
Nanomechanical measurements were performed using thePeakForce QNM (Quantitative NanoMechanics) mode which isbased on the Derjaguin–Muller–Toropov (DMT) model on anAFM system under ambient conditions. The AFM system is aBruker MultiMode� 8 SPM equipped with a Nanoscope Vcontroller and Nanoscope analysis soware (Bruker NanoSurface Business, Santa Barbara, CA 93117, USA). Following aproper calibration procedure, samples were scanned using theSNL-A probe with a nominal radius of 2 nm and a nominalspring constant of 0.35 N m�1.
Peak-force tapping AFM is an operating mode that cancontrol the maximum normal force (“peak force”) applied onthe samples at each point of the map. Nanoscale property and
Fig. 1 DSC curves of SEBS and SSEBS: (a) second heating scan at 20 �C min�1; and
2664 | Soft Matter, 2013, 9, 2662–2672
peak force are obtained by collecting a force curve at each pixel.The force–separation curves are subsequently analysed toobtain information on sample adhesion, surface deformationand topography. Adhesion force is the minimum forcedepending on the interaction between the tip and sample whiledeformation is the difference of the separation from the forceequal to zero to the peak force. The reduced elastic modulus E*is obtained by tting the experimental data using the Derja-guin–Muller–Toropov (DMT) model given by:21
Ftip ¼ (4/3)E*(Rd3)1/2 + Fadh (1)
where Ftip is the force on the tip, Fadh is the constant adhesionforce during contact, R is the tip end radius, and d is the tip tosample separation. The reduced modulus E* is related to thesample elastic modulus Es by:
E* ¼ [(1 � nt2)/Et + (1 � ns
2)/Es]�1 (2)
where n and E are the Poisson's ratio and Young's modulus andthe subscripts “t” and ”s” stand for the tip and sample,respectively. In our materials system, the tip modulus, Et, ismuch larger than Es so that the rst term of eqn (2) can beneglected. Hence, Es is calculated easily given the Poisson'sratio ns.
Results and discussionPhase behavior and crystallization
The thermal behaviors of SEBS, SSEBS and SSEBS-c-PCL wereinvestigated by DSC. The second scan DSC thermograms ofSSEBS and block ionomer complex SSEBS-c-PCL are shown inFigs. 1 and 2.
Fig. 1a presents the second heating scan of SEBS and SSEBSwith different degrees of sulfonation. The corresponding
(b) cooling scan at �10 �C min�1.
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Fig. 2 DSC curves of APCL and block ionomer complexes SSEBS-c-PCL: (a) second heating scan at 20 �C min�1; and (b) cooling scan at �10 �C min�1.
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cooling scan is shown in Fig. 1b. The parent SEBS with twoimmiscible distinct blocks is known to have a micro-phaseseparated structure consisting of a rubbery phase of EB blocksand hard micro-domains of PS blocks. Thus, two different Tg,i.e., one at��40 �C for the EB block and another at ca. 80–90 �Cfor the PS block should be observed.14,22,23 However, from theDSC curve for SEBS (Fig. 1a), it is noted that there is nodiscernible Tg, which might be due to the insensitivity of DSCfor detecting Tg. Meanwhile, a broad endotherm between �20and 30 �C can be ascribed to the melting of small crystallitesformed by long sequences of ethylene.14,23 Aer sulfonation, anobvious Tg was found, which can be ascribed to the SPS block ofSSEBS. For 14.2SSEBS, the Tg is located at 107 �C and increasesgradually with increasing degree of sulfonation, reaching�122 �C for 41.5SSEBS. It is known that strong hydrogenbonding interactions usually occur between SO3H groups in SPSblocks, restricting the chain mobility and thereby generallyresulting in a higher Tg for PS block.22
From the cooling scan (Fig. 1b), an exothermic peak isreadily seen for all the SSEBSs which can be attributed to thecrystallization of ethylene segments.14 It is also noticed that thecrystallization temperatures (Tc) of SSEBSs were slightly lowerthan that of the parent SEBS. The hydrogen bonding interactionbetween the SO3H groups in the SPS block can act as thephysical crosslink, which simultaneously connes the mobilityof the EB chains owing to the covalent bonding between SPSand EB blocks. Therefore, the restriction of chain mobility canbe responsible for the depression of crystallization of theethylene segments in SSEBS.24 Furthermore, it is shown that Tcof the ethylene segments decreases slightly with increasingsulfonation degree.
Fig. 2a shows clearly that neat APCL displays a melting point(Tm) at �48 �C. Aer the neutralization with SSEBS, noticeablechanges in the crystallization behavior of APCL can be seen.There are sharp endothermic peaks assignable to the melting of
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APCL in the DSC curves of all block ionomer complexes except1SSEBS-c-PCL which has the least APCL content (11.8 wt%).APCL in block ionomer complexes shows almost the same Tm asneat APCL. 1SSEBS-c-PCL, however, does not display themelting peak of APCL, indicating that APCL is either dissolvedor at least partially dissolved in SSEBS, since the melting pointdepression is a typical characteristic of a miscible polymerblend.
Fig. 2b shows the crystallization behavior of APCL in blockionomer complexes. Neat APCL displays a sharp exothermicpeak at �29 �C assignable to the crystallization of APCL. Bycontrast, SSEBS-c-PCLs with relatively higher APCL contents(3SSEBS-c-PCL and 4SSEBS-c-PCL) exhibit two exothermicpeaks assignable to the crystallization of APCL. Compared toTc of neat APCL (�29 �C), one of these two Tc values is slightlyhigher (31 and 32 �C) and the other is lower (21 and 28 �C). Bycontrast, 2SSEBS-c-PCL shows a weak exothermic peak at 20 �Cdue to the crystallization of APCL, which is much lower thanthat of neat APCL whereas 1SSEBS-c-PCL does not show anoticeable exothermic peak assignable to the crystallizationof APCL.
The crystallization behavior of APCL in block ionomercomplexes can also be inferred from the FTIR results as shownin Fig. 3. The carbonyl stretching vibration band of neat APCL islocated at 1724 cm�1, which can be ascribed to the crystallineconformation of APCL. By contrast, we can clearly see a slightlybroader main peak at 1724 cm�1 with a shoulder at 1737 cm�1
in the spectra of block ionomer complexes, which are ascribedto the crystalline and amorphous conformations of APCL,respectively.25 The crystalline peak decreases in intensity andshis to a higher frequency (1730 cm�1), whereas the peak at1737 cm�1 corresponding to the amorphous state becomesmore discernible with decreasing APCL content and is espe-cially noticeable in the spectra of 1SSEBS-c-PCL containing theleast amount of APCL.
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Fig. 3 Carbonyl stretching region in FTIR spectra of the block ionomercomplexes.
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These unique crystallization characteristics may result fromthe complex multi-phase morphology and the miscibilitybetween the components. For the 1SSEBS-c-PCL, the APCL isprobably completely dissolved in SPS micro-domains due to theinteraction between the sulfonic acid groups and tertiary amineend group of APCL. Hence, no APCL crystallites are present inthis block ionomer complex, which is consistent with the SAXSresults to be discussed later. With increasing molar ratio ofamine/acid from 0.11 for 1SSEBS-c-PCL to 0.16 for 2SSEBS-c-PCL(see Table 1), there may be some APCL not completely dissolvedbut crystallized with connement in the SPS micro-domains. Itis known that in the case of a block copolymer when the crys-tallization is conned in micro-domains, the crystallization isoen depressed compared with the neat crystallizable homo-polymer.14,26 Therefore, 2SSEBS-c-PCL exhibits a lower Tc thanneat APCL. For block ionomer complexes with a higher APCLcontent, the presence of two crystallization peaks most possiblyoriginates from two different population of APCL crystallites,i.e., inside the SPS micro-domains and APCL matrix outside theSPS micro-domains.14 For APCL conned within SPS micro-domains, the crystallization is depressed showing lower crys-tallization temperature, but the APCL matrix shows highercrystallization temperature arising from possible heteroge-neous nucleation effects of the neighboring micro-domains inthis multi-phase system.
Apart from the crystallization of APCL, another obviouscrystallization peak can also be found for all block ionomercomplexes which could be ascribed to the crystallization of theethylene block (see Fig. 2b). Compared to neat SSEBS, thecrystallization temperature of the ethylene block in block ion-omer complexes is slightly increased. For example, Tc of theethylene block in 4SSEBS-c-PCL increases to 7 �C while the
2666 | Soft Matter, 2013, 9, 2662–2672
ethylene block in the corresponding SSEBS (41.5SSEBS) showsa crystallization peak at 3 �C. Moreover, Tc increases withincreasing APCL content in the block ionomer complexes,conrming that the incorporation of APCL has some effect onthe crystallization of the ethylene block. The crystallization of apolymer involves two crucial steps, i.e., nucleation and growthof the crystallites.26 The crystallization temperature of theethylene block is lower than that of APCL which, hence, may actas the nucleus for crystallization of ethylene. Also, the presenceof APCL might interfere with the hydrogen bonding interactionbetween SO3H groups in the SPS block, enabling better mobilityof ethylene segments. Therefore, it becomes easier to rearrangedue to the plasticization effects of APCL resulting in a highercrystallization temperature.
The semi-crystalline morphology of the block ionomercomplexes was examined using a polarizing optical microscope(POM). Fig. 4 shows the polarized images revealing that blockionomer complexes SSEBS-c-PCL contain spherulites. For neatAPCL, a very well-developed spherulitic structure can be found.The spherulitic morphology becomes coarser in SSEBS-c-PCLs.Indeed, the spherulites become smaller and less regular withdecreasing APCL content. For 1SSEBS-c-PCL, small but clearspherulitic structures are visible, possibly caused by the exis-tence of ethylene crystallites. This speculation is based on theabsence of crystallization and melting peaks of APCL but thepresence of the corresponding peaks for the ethylene block inthe DSC curves of 1SSEBS-c-PCL.
Self-assembly and nanostructures
SAXS measurements for parent SEBS, SSEBS and block ionomercomplexes SSEBS-c-PCLs were performed at room temperatureto study the micro-phase structures. Fig. 5a shows the SAXSproles of SSEBS with different degrees of sulfonation andFig. 5b shows SAXS results of their corresponding block ion-omer complexes.
For parent SEBS, multiple scattering peaks can be observedin the SAXS prole, which indicates that ordered nanoscalestructures may exist in SEBS. The rst-order scattering peak iscentered at a value of the scattering vector q* corresponding to along spacing of 34 nm. There are higher order reections clearlyvisible at the positions of q/q* ¼ 1, 2, 3, 4 and 5, which arecharacteristic of a lamellae arrangement.22,27 For all SSEBSs, theSAXS proles show well-dened peaks, indicating they aremicro-phase separated at the nanoscale. But, compared to theSAXS prole of SEBS, the rst-order scattering peaks becomebroader and a smaller number of secondary peaks are observedin the SAXS proles of SSEBSs. This suggests that a less orderedmicro-phase structure exists in SSEBSs where the sulfonategroups are attached to PS blocks. According to previous studies,light sulfonation of PS blocks may introduce two competingeffects on morphology and thereby the properties.7,28,29 Firstly,the introduction of ionic groups onto the PS blocks leads to agreater driving force for phase separation owing to theincreased difference in solubility parameters between constit-uent blocks. Secondly, phase separation may be simultaneouslyhindered by the reduced mobility of ionic blocks due to some
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Fig. 5 SAXS profiles of (a) SEBS and SSEBS with different sulfonation degrees; and (b) block ionomer complexes from the corresponding SSEBS at room temperature.Each profile is shifted vertically for clarity.
Fig. 4 Polarized optical microscopy images (Mag. �100) of (a) neat APCL, (b) 4SSEBS-c-PCL, (c) 3SSEBS-c-PCL, (d) 2SSEBS-c-PCL, and (e) 1SSEBS-c-PCL.
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Fig. 6 SAXS profiles of block ionomer complexes SSEBS-c-PCLs at 70 �C. Eachprofile is shifted vertically for clarity.
Fig. 7 Schematic illustration of morphological transition from SEBS to SSEBS-c-PCL, i.e., the SPS phase is remarkably swollen and the micro-phase structurebecomes poorly ordered after the incorporation of APCL.
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specic interactions, e.g., hydrogen bonding or ion–ion inter-actions. Even though the results of these competing effects arenot yet fully understood, based on the obtained SAXS results, itseems that the second effect dominates.
Also, it is shown from the SAXS prole of 14.2SSEBS that theprimary scattering peak is located at a position correspondingto a long spacing of 27 nm. With increasing sulfonation degree,the primary scattering peak shis slightly to a lower scatteringvector, reecting an increase in the average distance betweenneighboring micro-domains. This probably originates from thefact that more sulfonate groups are graed onto the PS block forSSEBS with a higher sulfonation degree. Further, it is clear thatthe characteristic scattering peaks become broader, whichindicates reduced ordering for SSEBS with higher sulfonationdegree. Thus, there is only one broad secondary scattering peakfor 41.5SSEBS and the long spacing increases to 34 nm.
Aer neutralization with APCL, the micro-phase structure ofthe block ionomer becomes less ordered, which is demon-strated by the presence of broader scattering peaks and asmaller number of secondary peaks (Fig. 5b). Block ionomercomplexes with a higher APCL content (4SSEBS-c-PCL and3SSEBS-c-PCL) exhibit no higher order reections in SAXSproles, indicating the deterioration of the micro-phase struc-ture. Themicro-phase structure of the block copolymer ionomergoes through a transition from a relatively ordered arrangementto a disordered structure by introduction of APCL. It is alsonoticed that the long spacing between micro-domains increasesremarkably for all complexes compared with the correspondingSSEBS. For example, aer the incorporation of APCL into18.8SSEBS, the long spacing increases from 30 to 39 nm for thecorresponding complex 2SSEBS-c-PCL. This is consistent withthe morphology studies using AFM in our previous paper,19 i.e.,the SPS phase is remarkably swollen aer the incorporation ofAPCL. The interaction between APCL and SPS enhances themiscibility leading to a lower Tg of the SPS block due to theplasticization effect and also disrupts the hydrogen bondingbetween SO3H groups within the SPS phase.14 From this view-point, it should be easier for the block ionomer complex to self-assemble into well-ordered structures caused by the increasedmobility of the SPS sub-chains. However, it is well-establishedthat the micro-phase structure of a block copolymer iscontrolled by the phase parameter cN, where c is interactionparameter and N is the degree of polymerization.30 As cNincreases, the equilibrium block copolymer micro-phase struc-ture transforms from the disordered phase, to bcc spheres,hexagonally packed cylinders, and lastly to lamellae.31 In theblock ionomer complex SSEBS-c-PCL, the introduction of APCLhas disrupted the strong interaction between SO3H groupswhich acts as a physical crosslink in SSEBS. Thus, the apparentmolecular weight (degree of polymerization) N decreases and sodoes cN.29 This possibly explains why the micro-phase structureexperiences a transition to a poorly ordered structure aer theintroduction of APCL into SSEBS.
Crystallization of APCL may also have some impact on themicro-phase structure of SSEBS-c-PCL.14 From the DSC studiesdiscussed above we know that there are APCL crystallites in theblock ionomer complexes except 1SSEBS-c-PCL. To study the
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effect of crystallization of APCL on the micro-phase structure,SAXS tests were conducted at 70 �C, which is above the meltingpoint of APCL. At this temperature, the crystalline APCLbecomes a melt, resulting in a micro-phase separated structureof amorphous APCL and SSEBS. The obtained SAXS proles areshown in Fig. 6. We can see that there is almost no change in theprole of 1SSEBS-c-PCL, indicating that no changes occur in themicro-phase structure at this temperature. By contrast, multiplescattering peaks are discernible for the other three complexesunlike the proles obtained at room temperature, indicatingthe presence of some ordered nanostructures. This remarkablechange is particularly obvious for 4SSEBS-c-PCL. Hence, it isevident that crystallization of APCL has a profound impact onthe micro-phase structure of block ionomer complexes. Basedon the SAXS results and AFM investigation in our previouspaper,19 a schematic illustration of the self-assembly is givenin Fig. 7.
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Quantitative mechanical property mapping at the nanoscale
Recent development in peak force mode AFM techniques offersthe potential for imaging themechanical properties of amaterialat the nanoscale.32–35 It has now become possible to obtain trulyquantitative material property mapping with high resolutionand precision, including elastic modulus, adhesion, deforma-tion, and dissipation maps of nanostructured materials. Suchmaps can be used to identify and characterize successfully the
Fig. 8 Topographic images and elastic modulus maps of (a) SEBS, (b) SSEBS and (c
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composition and properties of multi-phase materials. However,little workhas been reportedusing this technique to characterizeblock copolymers. Herein, we use it to evaluate the nano-mechanical properties of SSEBS-c-PCL. The results discussedbelow, except when indicated otherwise, are for 18.8SSEBS andits corresponding block ionomer complex 2SSEBS-c-PCL.
The triblock copolymer SEBS consists of hard and socomponents and usually displays micro-phase separatedmicrostructures as revealed by AFM investigations.36–38 Fig. 8
) SSEBS-c-PCL.
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shows simultaneously generated topography and elasticmodulus maps of SEBS, SSEBS and SSEBS-c-PCL by peak forceQNM. A distinct contrast in these maps suggests that there areat least two different phases with different properties. Surfacetopography images in Fig. 8a1, b1 and c1 demonstrate worm-likemicro-phase structures in SEBS which remain almostunchanged in SSEBS and SSEBS-c-PCL except for slight changesin the domain sizes. From the corresponding elastic modulusmaps, it can be found that there are two distinct phasesshowing different elastic moduli. Consider SEBS, the brightregion in Fig. 8a2 shows a higher elastic modulus. By contrast,there is another phase showing a lower modulus. Owing to thedifference in viscoelasticity between PS and PEB components,the phase with the higher elastic modulus can be attributed tothe hard PS-rich phase while that with the lower elastic modulusis probably due to the so PEB-rich phase.36–38
Fig. 9 Elastic modulus maps of (a1) SEBS, (b1) SSEBS and (c1) SSEBS-c-PCL. Numericaand (c1) are shown in (a2), (b2) and (c2), respectively. The dark and bright dots markeby the dashed lines, exhibiting higher and lower moduli, respectively.
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To further study the properties of each phase and theirvariation from SEBS, SSEBS to SSEBS-c-PCL, DMT modulus (Es)maps and the corresponding histograms are shown in Figs. 9and 10, respectively. The DMT elastic modulus proles alongthe dotted lines in the modulus maps are also displayed inFig. 9, which clearly indicate that the bright region has highermodulus values and the dark region, lower values. The dark andbright dots marked on the modulus maps are two typical pointsshowing higher and lower moduli, respectively, for the corre-sponding hard and so phases. The elastic modulus (Es) valuesvary from 5.87 to 35.84 MPa for SEBS, 10.24 to 36.91 MPa forSSEBS, and 11.27 to 51.46 MPa for SSEBS-c-PCL along the cross-sections.
From the modulus maps in Fig. 9, the average, maximumand minimum modulus values determined by QNM can beobtained. The mean modulus increases from 18.20 MPa for
l values in each image across the sections indicated by the dotted lines in (a1) (b1)d on the modulus maps correspond to the points in the modulus profiles indicated
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Fig. 10 Histograms of elastic modulus of (1) SEBS, (2) SSEBS and (3) SSEBS-c-PCL. The histograms are based on the same scan areas as those shown in Fig. 9.
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SEBS and 19.51 MPa for SSEBS to 36.1 MPa for SSEBS-c-PCL.Further, the maximum modulus increases only slightly from40.20 MPa for SEBS to 42.49 MPa for SSEBS, but dramatically to63.11 MPa for SSEBS-c-PCL. The minimum modulus values forSSEBS and SSEBS-c-PCL (9.30 and 6.5 MPa, respectively) alsoincrease compared to SEBS (4.58 MPa). The histograms inFig. 10 demonstrate the distribution of the modulus values. It isclearly seen that the elastic modulus of the majority of thescanned surfaces (peak in the distribution curve) increases,especially for SSEBS-c-PCL.
All results obtained above indicate that sulfonation andsubsequent complexation with APCL increase the modulus ofboth phases consisting of stiff PS blocks and so PEB blocks.The increase in modulus with sulfonation is most likely causedby the presence of strong interactions between sulfonic acidgroups in SPS blocks which serve as physical crosslinks.39 Forthe block ionomer complex, the presence of APCL side chainsionically linked to the SPS block may bring about twocompeting factors contributing to the modulus. First, APCLforms ionic linkage with SO3H groups in SPS blocks leading tothe interference of hydrogen bonds between SO3H groups.Second, APCL tends to crystallize in the block ionomer complexeven though this process is depressed to some extent comparedto neat APCL. From the obtained results, it may be inferred thatthe latter factor is more dominant, yielding an increasedmodulus. Also, the phase with the higher modulus assignable tothe hard PS components shows much more pronouncedchanges aer sulfonation and complexation. Especially,complexation of SSEBS with APCL increases the modulus of thehard phase (SPS phase) dramatically, conrming that APCLinteracts mainly with SPS blocks. Although no chemicalchanges take place for PEB blocks during sulfonation andcomplexation, there exist some changes in the properties of theso phase (PEB component) from SEBS to SSEBS-c-PCL, which
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is very likely due to the covalent bonding with the hard phaseconsisting of PS or SPS.
There exist some inconsistencies between the properties atnanoscale and those of bulk samples. The minimum modulusobtained from peak force QNM is slightly lower than that ofbulk PEB samples. The maximum modulus, however, is muchlower than that of bulk PS samples. These results suggest thatthe so and hard segments may mutually affect each other.Similar results have been reported by other groups, namely, theproperties at nanoscale may not agree with those of the bulksamples due to the microstructure effects exerted on eachother.33,37,38,40,41 For example, there are rubbery layers in theprobed volume such that the so PEB blocks may surround orlie underneath the hard PS blocks leading to dramatic decreasesof the modulus values. Furthermore, other possible factors likecontact area, tip geometry and local value of Poisson's ratio mayalso contribute to the elastic modulus reductions.
Conclusions
Block ionomer complexes, SSEBS-c-PCL, were prepared basedon sulfonated SEBS and a tertiary amine-terminated PCL. DSCresults revealed that APCL exhibited a unique crystallizationbehavior due to the effects of miscibility and restrictionimposed by the SPS micro-domains. Crystallization was gener-ally depressed compared with neat APCL and two crystallizationpeaks were observed for the complexes with a relatively higherAPCL content (3SSEBS-c-PCL and 4SSEBS-c-PCL). Spheruliticsemi-crystalline structures were observed for neat APCL, whichbecame smaller and irregular in block ionomer complexes.SAXS results showed that the micro-phase structure of the blockionomer underwent a transition from a relatively orderedstructure to a poorly ordered morphology by the introduction ofAPCL. Quantitative mapping of mechanical properties at the
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nanoscale using AFM indicated that the block ionomer complexshowed a higher average elastic modulus than SEBS and SSEBS.In addition, the elastic moduli for both hard and so phasesincreased, and the phase with higher modulus assignable to thehard SPS component shows much more noticeable change aersulfonation and complexation.
Acknowledgements
This work was nancially supported by the Australian ResearchCouncil under the Discovery Project Scheme (DP0877080). SAXSmeasurements were conducted on the SAXS beam-line at theAustralian Synchrotron, Victoria and we would like to thankDr Nigel Kirby for his assistance. We also gratefully thankInderpreet Gill and Neil McMahon, NewSpec Pty. Ltd., Adelaide,Australia for their support with the AFM investigation.
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