Electrospun non-woven mats from stereocomplex between high molar mass poly(l-lactide) and poly(d-lactide)-block-poly(butylene succinate) copoly(ester urethane)s

European Polymer Journal 48 (2012) 1965–1975

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European Polymer Journal

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Macromolecular Nanotechnology

Electrospun non-woven mats from stereocomplex between high molarmass poly(L-lactide) and poly(D-lactide)-block-poly(butylene succinate)copoly(ester urethane)s

Nikoleta Stoyanova a, Rosica Mincheva b, Dilyana Paneva a, Nevena Manolova a,⇑,Philippe Dubois b, Iliya Rashkov a

a Laboratory of Bioactive Polymers, Institute of Polymers, Bulgarian Academy of Sciences, Acad. G. Bonchev St, bl. 103A, BG-1113 Sofia, Bulgariab Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and Polymers (CIRMAP), University of Mons – UMONS,Place du Parc 20, B-7000 Mons, Belgium

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Article history:Received 7 June 2012Received in revised form 24 August 2012Accepted 16 September 2012Available online 25 September 2012

Keywords:ElectrospinningStereocomplexPolylactidePoly(butylene succinate)Poly(ester urethane)

0014-3057/$ - see front matter � 2012 Elsevier Ltdhttp://dx.doi.org/10.1016/j.eurpolymj.2012.09.013

⇑ Corresponding author. Tel.: +359 (2) 979 34 68; faE-mail address: [email protected] (N. M

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By applying electrospinning new fibrous materials from a stereocomplex formed betweena high molar mass poly(L-lactide) (HPLLA) and a copoly(ester urethane) (CPEU) based onpoly(D-lactide) (PDLA) and poly(butylene succinate) (PBS) blocks (PDLA-b-PBS) were pre-pared. The stereocomplex formation was evaluated by means of differential scanning cal-orimetry (DSC) and X-ray diffraction (XRD) analyses. In contrast to electrospun mats ofHPLLA and HPLLA/PDLA-OH in which the polymers were in the amorphous state, it wasfound that the presence of PBS allowed the formation of a crystalline phase in the fibrousmaterials from HPLLA/PDLA-b-PBS. Annealing at 100 �C for 8 h enabled reorganization ofthe polymer segments within the fibers and the appearance for all of the samples of crys-talline diffractions in the XRD patterns. A melting temperature of about 200 �C was regis-tered in the DSC thermograms of pristine and annealed mats from HPLLA/PDLA-OH andHPLLA/PDLA-b-PBS; evidence for stereocomplex formation. It was shown that besides ste-reocomplex crystallites PBS and PLA crystallites existed as well when using CPEUs with ahigher PBS content. The obtained data from the thermogravimetric analysis (TGA) showedthat the thermal stability of the stereocomplex-based fibrous materials depended on thePBS content.

� 2012 Elsevier Ltd. All rights reserved.

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1. Introduction

Nowadays, electrospinning is regarded as a highlypromising method for preparation of non-woven textilesfrom micro- and nanofibers [1,2]. Moreover, due to the pos-sibility for one-pot preparation of fibrous materials fromsolutions under mild and non-destructive conditions (roomtemperature, atmospheric pressure), electrospinning hasjustifiably found its place among the most effectivetechniques for designing diverse micro- and nanofibrousmaterials composed of biocompatible and biodegradable

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x: +359 (2) 870 03 09.anolova).

aliphatic polyesters. Among them, polylactide (PLA),poly(e-caprolactone) (PCL), polyhydroxyalkanoates andpoly(butylene succinate) (PBS) attract increasing interestdue to their beneficial properties such as biocompatibility,biodegradability and non-toxicity, and mostly because ofthe fact that they might (at least partially) be obtained fromannually renewable sources [3]. The polymer materialsthereof can find applications in a great variety of fields,e.g. from medicine and pharmacy to food packaging andpreservation. In contrast to the numerous reports on theelectrospinning of PLA, PCL and their copolymers [4]; thestudies on the preparation of fibrous materials from PBSby electrospinning are still scarce [5–8]. PBS, for instanceknown under the trademark Bionolle�, is obtained through

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AA/BB type polycondensation starting from succinic acidand butanediol of petro-chemical or annually renewableorigin [9]. Due to its high degree of crystallinity the prepa-ration of fibrous materials from PBS solutions faces somedifficulties: at high polymer concentrations (required forobtaining defect-free cylindrical fibers) gelation occurs atroom temperature in the spinning solution and, thus, pre-vents electrospinning process from being performed effi-ciently. In contrast, the electrospinning of PLA solutions isa highly effective process [10–13]. An elegant way for thepreparation of composite materials from PLA/PBS can befound in the application of electrospinning by using blendsolutions containing both polyesters. In general, the poly-mer materials consisting of PLA and PBS have gained inter-est because of the possibility to undergo enzymaticdegradation by a greater variety of microorganisms with re-spect to PLA, owing to the PBS presence [14]. So far mostlytwo approaches have been applied for combining the ben-eficial properties of the both aliphatic polyesters: (i) prepa-ration of PBS/PLA blends by melt extrusion [15,16]; and (ii)the controlled synthesis of block copolymers containingPLA and PBS sequences [17–20]. From our literature survey,no data have been found on the preparation of PBS/PLAmaterials by electrospinning.

Very recently, some of us reported on the synthesis ofcopoly(ester urethane)s (CPEUs) built up of PLA and PBSblocks (PLA-b-PBS) by applying a three-step procedure:(i) synthesis of a,x-dihydroxyl PLA (PLA-OH) by ring-opening polymerization; (ii) synthesis of PBS by AA/BBtype polycondensation, and (iii) chain extension reactionusing 2,4-toluenediisocyanate [20]. The PLA block wassynthesized in such a manner that to be built of enantio-merically pure PLLA or PDLA blocks. The presence of enan-tiomerically pure PLA block enables the application of thestereocomplex formation approach (stereoselective associ-ation) between the CPEUs or by using of homo-PLA as oneof the partners in order to prepare new polymer materials.The formation of a stereocomplex comprises only racemiccrystallization to stereocomplex crystallites where PLLAand PDLA chains are packed together due to van der Waalsinteractions [21,22]. However, depending on the PLLA/PDLAratio the formation of PLLA or PDLA homo-crystallites mayexist as well. The stereocomplex-based polymer materialspossess properties that differ significantly from those ofthe neat enantiomers. Depending on the molar mass andrelative content of the PLLA and PDLA chains, they can dis-play a melting temperature that is ca. 50 �C higher thanthat of materials consisting of PLLA or PDLA alone. Interest-ingly, it has been shown that the presence of a block ofcrystalline or amorphous polymer in PLA-based blockcopolymers does not hamper significantly the stereocom-plex formation at stoichiometry ratio in films [21,23–25].Very recently, some of us showed that the PLLA and PDLAblocks in the CPEUs PLA-b-PBS do also preserve theirability to form stereocomplexes even at a narrow rangeout of stoichiometry [20].

The studies in the field of preparation of micro- andnanofibrous materials from a PLA-based stereocomplexby electrospinning are still in an early stage of develop-ment [26–29]. In a previous study we showed the possibil-ity for preparation of fibrous materials by electrospinning

of solutions containing a high molar mass HPL(D)LA andthe amphiphilic diblock copolymer PD(L)LA-b-poly(N,N-dimethylamino-2-ethyl methacrylate) (PD(L)LA-b-PDMA-EMA) in which the PDMAEMA block is amorphous [30]. Itwas found that the stereocomplexation is accompaniedby some hampering of the structural reorganization ofthe PLA segments into crystalline phase due to the pres-ence of the amorphous block of PDMAEMA. PBS is a crys-talline polymer with a melting temperature of ca. 100 �C.The presence of PBS block(s) may have an impact on theproperties of fibrous materials from PLA-based stereocom-plexes (crystallinity; different profile of thermal, hydrolyticand enzymatic degradation, etc.) prepared by electrospin-ning. There are no data on the preparation of fibrous mate-rials from PLA-based stereocomplex by electrospinning byusing of a copolymer containing blocks of two able to crys-tallize polymers.

In the present study fibrous materials were prepared byelectrospinning using a high molar mass PLLA (HPLLA) andtwo CPEUs PDLA-b-PBS differing in PBS content. The ste-reocomplex formation at stoichiometry was evaluated bymeans of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses. The thermal stability ofthe new materials was followed by thermogravimetricanalysis (TGA).

2. Experimental

2.1. Materials

D-lactide (D-LA, Purasorb� D, with optical purity >99.5%,MW = 144 g/mol, free acid <1 meq/kg, water content<0.02%) was supplied by Purac Biochem BV (Netherlands)and stored in a glove box prior to use. Dimethyl succinate(DMSu, MW = 146 g/mol, >98%, Kosher), 1,4-butanediol(BDO, MW = 90 g/mol, >98%, Kosher), tin(II) 2-ethylhexano-ate (Sn(Oct)2, MW = 405 g/mol,�95%, Aldrich), tetrabutoxytitane (TBT, MW = 340.32 g/mol, 99%, Acros) and 2,4-toluenediisocyanate (TDI, MW = 174, P98%, Aldrich) wereused as-received. Triphenylphosphine (Ph3P, MW = 262 g/mol, P99%, Merck) was recrystallized from diethyl ether,dried at 25 �C under reduced pressure overnight and thenby three consecutive azeotropic distillations with dry tolu-ene prior to use. Toluene (Labscan, 99%) was dried using anMbraun solvent purification system under N2 flow. Highmolar mass poly(L-lactide) (HPLLA, Unitika� 6201, �MW =95000 g/mol, �MW / �Mn = 1.63, D-enantiomer content = 1.7%;according to the supplier, Terramac Unitika Ltd., Japan)was dried at 60 �C under reduced pressure overnight priorto use. Chloroform (Chem-Laboratory, 99.8%), heptane(Labscan, 99%), methanol (Chem-Laboratory, 99.8%),dichloromethane (DCM, Merck); dimethyl sulfoxide(DMSO, Fluka) and all other reagents were of analyticalgrade of purity and used as-received.

2.2. Synthesis of the copoly(ester urethane)s (CPEUs) PDLA-b-PBS

The route of the three-step synthesis of the block CPEUs(PDLA-b-PBS) includes ring-opening polymerization,

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polycondensation and chain coupling via reaction withTDI, all performed in bulk. The synthesis of CPEUs has beendescribed in details elsewhere [20]. The first stage con-sisted in the synthesis of a,x-dihydroxy poly(D-lactide)(PDLA-OH) with �Mn 4000 g/mol by ring-opening polymer-ization of D-LA in bulk. The synthesis of a,x-dihydroxypoly(butylene succinate) (PBS-OH) was performed in twosteps: (1) polycondensation of DMSu and BDO to a-meth-oxy-x-hydroxy poly(butylene succinate) (H3CO-PBS-OH),and (2) end-capping reaction of the obtained oligoesterwith BDO to PBS-OH. For the synthesis of CPEUs (PDLA-b-PBS) PDLA-OH and PBS-OH with �Mn 4000 and 2000 g/mol, respectively, were charged into a 50 ml stainless steelreactor (Autoclave, France) at a PDLA/PBS molar ratio(x/y) = 9/1 or 1/1 and heated at 160 �C until reaching com-plete melting. Then, 1.2 eq. TDI/eq. OH and 2 � 10�4 eq.Sn(Oct)2/eq. OH were added via a syringe as chain extenderand catalyst, respectively, and the reaction was allowed toproceed for 4 h.

The relative composition of the CPEU samples wasdetermined by 1H NMR spectroscopy (Bruker AMX-300)using CDCl3 as solvent at room temperature while themacromolecular parameters were determined by size-exclusion chromatography (SEC) in CHCl3 at 30 �C bymeans of an Agilent liquid chromatograph equipped withan Agilent degasser, an isocratic HPLC pump (flow rate =1 ml/min), an Agilent autosampler (loop volume = 200 lL,solution concentration = 2.5 mg/ml), an Agilent-DRI refrac-tive index detector and three columns: a PL gel 10 lmcolumn and two PL gel Mixed-D 10 lm columns (linearcolumns for separation of MWPS ranging from 500 to106 g/mol). Polystyrene standards were used for calibra-tion. The specific optical rotation [a]20 of the polymerswas determined on their solutions in chloroform at apolymer concentration of 1 g/dL at 20 �C using a digitalPolarimeter OA AA5 (Optical Activity Ltd., UK) at a wave-length of 589 nm.

2.3. Preparation of HPLLA, HPLLA/PDLA-OH and HPLLA/PDLA-b-PBS fibrous materials by electrospinning

For the preparation of fibrous materials by electrospin-ning, solutions were prepared using a mixed DCM/DMSOsolvent system at a weight ratio of 9/1 and with a totalpolymer concentration of 15 wt.%. For preparation of fi-brous materials from stereocomplexes based on eitherHPLLA/PDLA-OH or HPLLA/PDLA-b-PBS the relative contentin PDLA-OH or copolymer was selected in such a mannerthat the PLLA/PDLA molar ratio to be equal to 1/1. Thespinning solutions were prepared by mixing of the DCMsolutions of HPLLA and either PDLA-OH or PDLA-b-PBS fol-lowed by the addition of the required amount of DMSO. Forthe PDLA71-b-PBS29 copolymer its dissolution in DCMnecessitated heating of the solution at 40 �C for 2 h.

Regarding the electrospinning a home-made set-up wasused consisted in a high-voltage power supply (up to30 kV); a syringe pump for delivering the spinning solutionat a controlled feed rate, a syringe (5 mL) equipped withpositively charged metal nozzle with inner diameter of600 lm for delivering the spinning solution, and agrounded rotating drum aluminum collector (rotating rate:

500 rpm). The electrospinning was performed under thefollowing conditions: spinning solution feed rate of0.0167 mL/min, voltage of 25 kV, tip-to-collector distance:16 cm, humidity of 70%, and at 23 �C. The mats were addi-tionally dried under reduced pressure (320 Pa) at roomtemperature for 8 h.

2.4. Characterization of the electrospun materials

The morphology of the fibrous materials was evaluatedby scanning electron microscopy (SEM). The samples wereimpulsively sputtered (45 s) with gold under vacuum andthen observed by SEM (Jeol JSM-5510). The morphologyof the fibers was evaluated by applying the criteria for acomplex evaluation of electrospun materials described indetails in [31] using the software program Image J [32]by measuring the diameter of at least 20 subjects per eachSEM micrograph. Attenuated total reflection Fourier trans-form infrared (ATR-FTIR) spectroscopic analyses wereperformed using an IRAffinity-1 spectrophotometer (Shi-madzu Co., Japan) equipped with a MIRacle ATR (diamondcrystal, depth of penetration of the IR beam into the sam-ple about 2 lm) accessory (PIKE Technologies, USA). Thespectra were recorded from 4000 to 600 cm�1 with a spec-tral resolution of 4 cm�1 using a DLATGS detector equippedwith a temperature controller. The spectra were correctedfor H2O and CO2 using IR solution internal software.

The differential scanning calorimetry (DSC) analyseswere performed by means of DSC Q2000 TA Instrumentunder nitrogen (heating rate of 10 �C/min). In order to en-hance the crystallinity of the fibrous materials prepared byelectrospinning some annealing was performed at 100 �Cfor 8 h under vacuum. X-ray diffraction (XRD) analyses ofpristine and annealed fibrous materials were performedon a Siemens D5000 diffractometer using Cu-Ka radiation(wavelength: 1.5406 Å) at room temperature. The sampleswere step-scanned between 2h = 10�–25� at a step of 0.02�with fixed counting time of 4 s (40 kV, 30 mA). The ther-mogravimetric analyses (TGA) were performed by usingInstrument TGA Q5000 V3.11 Build 259 under nitrogenwith a heating rate of 10 �C/min.

3. Results and discussion

The copoly(ester urethane)s (CPEUs) PDLA-b-PBS(Scheme 1) were synthesized applying a previously dis-cussed procedure [20].

The macromolecular characteristics and [a]20 values ofthe two studied CPEUs and PDLA-OH are listed in Table 1.

As seen from the [a]20 values, PDLA-OH and CPEUs con-sist of optically pure enantiomeric poly(D-lactide). In a pre-vious study some of us have shown that incorporated inCPEUs the PDLA block preserves its ability to form a stereo-complex with the PLLA block [20]. It is well-known that thepolymer molar mass is a crucial parameter for preparationof polymer solutions able to be electrospun into fibrousmaterials [30,33,34]. Since the molar masses of PDLA-OHand CPEUs are not high enough to perform effective elec-trospinning (Table 1), a PLLA counterpart of high molarmass (HPLLA; �MW = 96 000 g/mol) is to be used as a

OO

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lx

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Scheme 1. Structure of the copoly(ester urethane)s PDLA-b-PBS, where x/y = 9/1 or 1/1 (mol/mol); l = 41; m = 12. At x/y = 9/1 the experimentallydetermined PBS content was 4 wt.% (PDLA96-b-PBS4), and at x/y = 9/1 it was 29 wt.% (PDLA71-b-PBS29).

Table 1Macromolecular characteristic and [a]20 values of PDLA-OH and CPEUsPDLA-b-PBS.

Content Macromolecular characteristicsa [a]20 (�)b

�Mn�MW

�MW / �Mn

PDLA-OH 4 700 6 000 1.28 143PDLA96-b-PBS4 22 900 37 100 1.62 137PDLA71-b-PBS29 12 000 22 800 1.90 102

a Macromolecular characteristics as obtained by SEC in chloroformusing PS standards;

b Specific optical rotation as measured in chloroform at 1 g/dL.

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partner for the preparation of fibrous materials built onHPLLA/PDLA-OH and HPLLA/CPEU. In our previous studyit has been shown that the difference in the molar massesat a [L-lactide]/[D-lactide] molar ratio of 1/1 does not ham-per the stereocomplex formation [24]. SEM micrographs offibers issued from HPLLA/PDLA-b-PBS are presented inFig. 1.

For sake of comparison, SEM micrographs of fibers so-prepared by electrospinning of HPLLA and HPLLA/PDLA-OH are shown as well. Under the applied conditions HPLLAfibers are generated with a mean diameter of 2070 ±460 nm. The substitution of low molar mass PDLA-OH for50 wt.% HPLLA, i.e., allowing the preparation of fibers from

HPLLA

HPLLA/PDLA96-b-PBS4

Fig. 1. SEM micrographs of the fibers prepared by electros

a stereocomplex between the two enantiomeric partnersat a [L-lactide]/[D-lactide] molar ratio = 1/1, led to fiberswith smaller mean diameter value: 1290 ± 250 nm. At thesame time, some defects appeared along the fibers. The de-crease in the mean diameter value as well as the presence ofdefects is attributed to the presence of the PDLA-OH havinga lower molar mass. When CPEU blends with HPLLA wereelectrospun, it was found that the presence of PBS block doesnot affect substantially the mean diameter value of HPLLA/PDLA96-b-PBS4 and HPLLA/PDLA71-b-PBS29 fibers, with val-ues of 1350 ± 280 and 1520 ± 290 nm, respectively. How-ever, in contrast to HPLLA/PDLA-OH fibers, no defects areobserved along the HPLLA/PDLA-b-PBS fibers. This is morelikely due to the higher molar mass of CPEUs as comparedto PDLA-OH (Table 1). It is worth pointing out that for thedissolution of the copolymer containing higher PBS amount(PDLA71-b-PBS29) heating of the solution was required. Thisis clearly attributed to the PBS block. Indeed, for electrospin-ning of high molar mass homo-PBS Liu et al. [6] and Wu et al.[7] have also applied heating for the preparation of theirspinning solutions in chloroform.

The characteristic FTIR absorption bands of HPLLA/PDLA-OH and HPLLA/PDLA71-b-PBS29 mats appearing inthe range from 1950 to 1550 cm�1 are juxtaposed in Fig. 2.

As seen in the FTIR spectrum of HPLLA/PDLA-OH mat aband at 1749 cm�1 is detected. It is attributed to the

HPLLA/PDLA-OH

HPLLA/PDLA71-b-PBS29

pinning at a total polymer concentration of 15 wt.%.

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stretching vibrations of the ester C@O group from PLA (forneat HPLLA mat the band for the ester C@O group appearsat 1751 cm�1). In the spectrum of HPLLA/PDLA71-b-PBS29

mat a new band at 1715 cm�1 is registered as well. The lastis attributed to the PBS ester C@O group and the urethaneC@O group spread along the PDLA-b-PBS copolymer [35].

The thermal behavior of pristine and annealed fibrousmaterials from HPLLA, HPLLA/PDLA-OH and HPLLA/CPEUswas evaluated in the temperature range from �80 to230 �C by differential scanning calorimetry (DSC). As seenfrom Fig. 3A(1), HPLLA mat possesses a glass transitiontemperature (Tg) of 60 �C, cold crystallization temperature(Tcc) of ca. 80 �C and melting temperature (Tm) of 164 �C.

The appearance of a rather intensive peak for cold crys-tallization is an indication that in the electrospun materialHPLLA is mostly in its amorphous state. This is further con-firmed by the registered amorphous halo in the XRD pat-tern of the HPLLA mat (Fig. 3B(1)). The obtained resultsare in accordance with other data concerning the thermalbehavior of PLA-based fibrous materials prepared by elec-trospinning [30,36–38]. The preparation of fibrous materi-als in which PLA is in its amorphous state is attributed tothe fast evaporation of the solvent during electrospinninghampering PLA crystallization. In the DSC thermogram ofHPLLA/PDLA-OH mat again a peak for cold crystallizationis registered (Fig. 3A(2)), i.e., the enantiomeric partners inthe fibers again are in the amorphous state as evidencedby the recorded amorphous halo in the XRD pattern ofthe fibrous material (Fig. 3B(2)). Interestingly enough, theDSC thermogram recorded on these fibers does not displayany melting peak at ca. 164 �C but a peak corresponding tothe melting of the stereocomplex crystallites is clearly de-tected at 204 �C. This is an indication that during therecording of the DSC thermograms at a heating rate of10 �C/min chain mobility allows for reorganization of theenantiomeric partners leading to stereocomplex formation.The presence of a CPEU with low PBS content, i.e. (PDLA96-

Fig. 2. FTIR spectra of HPLLA/PDLA-OH

b-PBS4), in the fibrous materials, leads to decrease of the Tg

value down to 55 �C (Fig. 3A(3)). The increase of the PBScontent up to 29 wt.% led to further decrease of the Tg va-lue up to 47 �C. Thus, increasing the PBS content within theCPEUs reduces the Tg values. As far as the melting enthalpy(DHm) of the stereocomplex crystallites is concerned, therecorded value is somehow decreased in presence of thecopolymer (Table 2).

One can observe diffraction peaks in the XRD pattern at2h of ca. 21� and 23� of relatively low intensity (Fig. 3B(3)).Thus, the presence of PBS block able to crystallize faster ascompared to homo-PLA can support the obtaining of acrystalline phase in the new fibrous materials during theelectrospinning process. This assumption is confirmed bythe enhancement of the peak intensity at 2h of 21� and23� upon using a copolymer enriched in PBS: PDLA71-b-PBS29 (Fig. 3B(4)). Noteworthy, the electrospinning of PBSalone leads to obtaining of mats in which a crystal phasefrom the polyester is present [5,6]. At this stage of thestudies the crystalline phase composition cannot be de-fined precisely since more detailed studies are needed.The DSC thermogram of HPLLA/PDLA71-b-PBS29 mat(Fig. 3A(4)) shows that in addition to the melting endo-therm appearing above 200 �C and attributed to the stereo-complex crystallites, Tm of PBS blocks at ca. 100 �C, and athird Tm at ca. 160 �C (with low intensity) that could be as-signed to PLA sequences are detected as well. This is anindication that at higher PBS content the interactionbetween HPLLA and PDLA block leading to stereocom-plex formation is somehow hampered, and thus, besidesthe stereocomplex crystallites, some PL(or D)LA homo-crystallites are formed as well. The obtained result is inaccordance with data on the thermal behavior of solvent-cast films from a stereocomplex between PLLA-b-PBS andPDLA-b-PBS [20]. Interestingly enough, the presence ofTm of a block from a crystallizable copolymer in films froma stereocomplex between PLLA and PDLA has been

and HPLLA/PDLA71-b-PBS29 mats.

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Fig. 3. DSC thermograms (A; first heating run) and XRD patterns (B) of as-obtained by electrospinning (pristine) fibrous materials from: HPLLA (1); HPLLA/PDLA-OH (2), HPLLA/PDLA96-b-PBS4 (3) and HPLLA/PDLA71-b-PBS29 (4).

Table 2Thermal characteristics of pristine and annealed (100 �C; 8 h) mats from HPLLA, HPLLA/PDLA-OH, HPLLA/PDLA96-b-PBS4 and HPLLA/PDLA71-b-PBS29.

Sample PBS melting PLA melting stereocomplex melting

Tm (�C) DHm (J/g) Tm (�C) DHm (J/g) Tm (�C) DHm (J/g)

HPLLA (pristine) – – 165 37 – –HPLLA (annealed) – – 165 50 – –HPLLA/PDLA-OH (pristine) – – – – 206 51HPLLA/PDLA-OH (annealed) – – – – 205 53HPLLA/PDLA96-b-PBS4 (pristine) – – – – 203 42HPLLA/PDLA96-b-PBS4 (annealed) – – – – 205 40HPLLA/PDLA71-b-PBS29 (pristine) 102 22 158 4 205 18HPLLA/PDLA71-b-PBS29 (annealed) 103 23 159 5 202 17

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registered for stereocomplexes formed by using diblockand triblock copolymers containing enantiomeric PLA andPCL as well [23]. Additional evidence for the reduction ofthe stereocomplexation ability between HPLLA and thePDLA block is found in the detected decrease of Hm valuefor the stereocomplex crystallites with increasing of PBScontent (Table 2). In addition, evidence for the presence

Fig. 4. DSC thermograms (A; first heating run) and XRD patterns (B) as recordeHPLLA/PDLA-OH (2), HPLLA/PDLA96-b-PBS4 (3) and HPLLA/PDLA71-b-PBS29 (4).

of PLA homo-crystallites and PBS crystallites in theHPLLA/PDLA71-b-PBS29 mats also comes from the recordedcharacteristic diffractions in the XRD patterns of the fi-brous materials after their annealing at 100 �C for 8 h(Fig. 4B(4)).

It is known that the annealing of PLA materials enablesthe occurrence of crystallization of the polymer and leads

d after annealing at 100 �C for 8 h of fibrous materials from: HPLLA (1);

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to the appearance of characteristic peaks for crystallinephase in the XRD pattern. Thus, the samples from HPLLA,HPLLA/PDLA and HPLLA/CPEUs were annealed at 100 �Cfor 8 h. In the DSC thermograms a peak for PLA Tcc is ab-sent; an indication that the annealing of the samples hasled to occurrence of crystallization of the polymers(Fig. 4A). Annealing of HPLLA mats led to the appearanceof clearly detected peaks at 2h = 16.58 and 19.06�(Fig. 4B(1)) attributed to the a form of PLLA crystallizedinto pseudo-orthorhombic unit cell with dimensions ofa = 1.07 nm, b = 0.595 nm and c = 2.78 nm that containstwo 103 helices [22,26]. As seen from Fig. 4B(2), in theXRD pattern of annealed HPLLA/PDLA-OH mats intensivepeaks at 11.98�, 21.48�, 23.12� and 23.82� are observed.These peaks are attributed to crystallites of PLA-based

Fig. 5. TGA (A) and DTG (B) thermograms of mats annealed at 100 �C for 8 hthermograms of PDLA-OH are presented as well (3).

stereocomplexes crystallized in triclinic unit cell of dimen-sions: a = 0.916 nm, b = 0.916 nm and c = 0.870 nm,a = 109.2�, b = 109.2� and c = 109.8�, with lateral packingof one left-handed 31 helices (b-conformation) of the oneenantiomeric polymer and right-handed helices of theother [21,22]. This is a clear indication for the presenceof a crystalline phase in the stereocomplexed PLA-based fi-brous material. In addition, peaks for homo-crystallitesfrom PDLA or PLLA are absent; evidence that the mat onlyconsists of stereocomplexed PLA phases. Similarly to theDSC thermogram in which no PLA Tm is registered(Fig. 4A(3)), no peak for PLA homo-crystallites is detectedin the XRD pattern as well (Fig. 4B(3)). This is an indicationthat the presence of 4 wt.% PBS in the copolymers does nothamper substantially the stereocomplex formation

and made of HPLLA (1) and HPLLA/PDLA-OH (2). For comparison the

N. Stoyanova et al. / European Polymer Journal 48 (2012) 1965–1975 1973

between HPLLA and PDLA block. However, the increase inPBS content in the case of HPLLA/PDLA71-b-PBS29 mat leadsto observation in the XRD pattern, besides the stereocom-plex diffraction peaks, of an intensive peak at 2h =16.58� due to the homo-crystallites of PLA blocks non-participating in the stereocomplex formation process(Fig. 4B(4)). This result is consistent with the presence ofTm for PLA in the DSC thermogram of the sample(Fig. 4A(4)) and is attributed to impeding the stereocom-plex formation between HPLLA and PDLA block from thecopolymer. Concerning PBS it is known that its crystal unitcell is monoclinic and diffraction peaks from [020], [021]and [110] are observed at 2h of ca. 19.4�, 21.5� and 22.5�,respectively [39,40]. Thus, its characteristic peaks mostprobably are a part from the diffraction patterns registeredin the 2h range from 20� to 25� (Fig. 4B(4)). The obtainingof information about the intimate crystalline structure of

Fig. 6. TGA (A) and DTG (B) thermograms of mats annealed at 100 �C for 8 h mathe thermograms of PDLA71-b-PBS29 copolymer is presented as well (1).

the new fibers and crystallinity degree of the differentcrystalline species will be subject of further research usingspecific methodological approaches for study of fibrousmaterials [41,42].

In order to evaluate the effect of the stereocomplex for-mation as well as the presence of PBS in the new fibrousmaterials onto their thermal stability annealed mats fromHPLLA, HPLLA/PDLA-OH and HPLLA/CPEUs were subjectedto thermogravimetric analyses (TGA). The TGA andDTG (derivative thermal gravimetry) thermograms ofHPLLA, HPLLA/PDLA-OH and PDLA-OH are compared inFig. 5.Since the used in the present study HPLLA and PDLAdiffer significantly in molar mass ( �MW of HPLLA is 95000 g/mol, and �MW of PDLA-OH is 6000 g/mol) they have differ-ent degradation temperature values. The high molar massHPLLA mats have a degradation temperature of ca.324 �C, while PDLA-OH is characterized by a lower thermal

de of HPLLA/PDLA71-b-PBS29 (2) and HPLLA/PDLA-OH (3). For comparison

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stability: its degradation temperature is 284 �C. Thestereocomplex formation between HPLLA and PDLA-OHmat at equimolar ratio (weight ratio HPLLA/PDLA-OH = 1/1) leads to fibrous materials with a degradation tempera-ture (328 �C) 4 �C higher than that of HPLLA and 44 �Chigher than that of PDLA-OH. This increase in the degrada-tion temperature of HPLLA/PDLA fibrous materials can beattributed to stereocomplex formation between the twoenantiomeric partners. The obtained result is consistentwith data reported by Tsuji et al. [43] for films producedfrom equimolar stereocomplex between PLLA and PDLAhaving close molar masses. The presence of a PBS blockin the CPEUs leads to significant alteration of the thermaldegradation profile of the fibrous materials from HPLLA/CPEUs and this is more pronounced for the mats consistingof the copolymer with the higher PBS content (PDLA71-b-PBS29). The thermogram recorded for HPLLA/PDLA71-b-PBS29 mat is presented in Fig. 6.

They are juxtaposed with those obtained for HPLLA/PDLA-OH mat and PDLA71-b-PBS29 copolymer. In accor-dance with data obtained from other studies on the ther-mal stability of CPEUs PLA-b-PBS with similar PBS blockcontent [13–15], PDLA71-b-PBS29 degrades in severalstages and the highest weight loss (ca. 95%) is detected atca. 400 �C. This temperature is attributed to the PBS com-ponent in the copolymer. As seen from Fig. 6, HPLLA/PDLA71-b-PBS29 mat degrades in two stages and possessesthermal behavior which is in between that of HPLLA/PDLAmat and PBS block from the copolymer. The measuredweight losses during the two stages are 60% and 40% atdegradation temperatures ca. 290 and 378 �C, respectively.The higher weight loss at the first stage at 290 �C is attrib-uted to the HPLLA chains incorporated into the stereocom-plexed phase.

4. Conclusions

The preparation of fibrous materials based on stereo-complexes formed between HPLLA and PDLA-b-PBS CPEUsby electrospinning proved a readily efficient route.Interestingly enough this is the first reported contributiondealing with the preparation of fibrous materials by elec-trospinning and combining the beneficial properties ofPLA and PBS aliphatic polyesters. The results obtained fromDSC, TGA and XRD analyses clearly show that the thermalproperties and the crystal structure of the new materialsdepends on PBS content in the copolymers. Having in mindthe alteration on the thermal degradation profile of theCPEUs-containing mats it is to be expected that the new fi-brous materials possess hydrolytic and enzymatic degra-dation behavior different from that of the neat PLA andPBS. The obtained results are of importance for the poten-tial application of these new biodegradable fibrous materi-als, e.g., in the biomedical domain.

Acknowledgment

The authors thank the agreement between the WBI, theFRS-FNRS and the Bulgarian Academy of Sciences. N.S.,D.P., N.M. and I.R. are thankful to the National Fund of Sci-entific Research (Grant DO 02-237/2008). R.M. and P.D.

gratefully acknowledge financial support from Région Wall-onne and European Commission in the frame of SINOPLISS-POLYEST project and OPTI2MAT program of excellence andfrom FNRS-FRFC.

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