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Poly(L-lactide) and poly(butylene succinate) immiscible blends: from electro-spinning to biologically active materials

Nikoleta Stoyanova, Dilyana Paneva, Rosica Mincheva, Antoniya Toncheva,Nevena Manolova, Philippe Dubois, Iliya Rashkov

PII: S0928-4931(14)00235-5DOI: doi: 10.1016/j.msec.2014.04.043Reference: MSC 4599

To appear in: Materials Science & Engineering C

Received date: 24 October 2013Revised date: 4 April 2014Accepted date: 18 April 2014

Please cite this article as: Nikoleta Stoyanova, Dilyana Paneva, Rosica Mincheva, An-toniya Toncheva, Nevena Manolova, Philippe Dubois, Iliya Rashkov, Poly(L-lactide) andpoly(butylene succinate) immiscible blends: from electrospinning to biologically activematerials, Materials Science & Engineering C (2014), doi: 10.1016/j.msec.2014.04.043

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Poly(L-lactide) and poly(butylene succinate) immiscible blends: from electrospinning to

biologically active materials

Nikoleta Stoyanova,a Dilyana Paneva,

a Rosica Mincheva,

b Antoniya Toncheva,

a

Nevena Manolova,a Philippe Dubois,

b Iliya Rashkov

a,*

1 Laboratory of Bioactive Polymers, Institute of Polymers, Bulgarian Academy of Sciences,

Acad. G. Bonchev St, bl. 103A, BG-1113 Sofia, Bulgaria

2 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

* Corresponding author. Tel./fax: +359 02 9793468/+359 02 8700309

E-mail: rashkov@polymer.bas.bg (I. Rashkov)

Abstract

For the first time the preparation of defect-free fibers from immiscible blends of high molar

mass poly(lactic acid) (PLA) and poly(butylene succinate) (PBS) in the whole range of the

polyesters weight ratios is shown. Electrospinning using the solvent-nonsolvent approach

proved most appropriate. Moreover, electrospinning revealed crucial for the obtaining of

PLA/PBS materials maintaining integrity. DSC and XRD analyses attested for a plasticizing

effect, and for increased PLA crystallinity at PBS addition to PLA. The mechanical properties

of the PLA/PBS mats were controlled by the alignment of the fibers and changed from plastic

to brittle materials upon increasing the PBS content. Drug loading and tests against

pathogenic microorganisms suggested that the obtained mats can find application as

antibacterial fibrous materials.

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Keywords: electrospinning; poly(butylene succinate); poly(L-lactide); immiscible polymer

blends; crystallinity; mechanical properties; drug loading; biological activity

1. Introduction

Materials from biodegradable and non-toxic biocompatible polymers which are

synthesized entirely or partially from annually renewable resources have drawn great interest

[1]. Their use is imposed by the need for replacement of the conventional materials derived

from petroleum sources as well as in the viewpoint of environmental pollution prevention.

Amongst all, the biocompatible and thermoplastic poly(lactide) PLA is one of the most

promising biobased polymers [2], finding applications going from drug-carriers and implants

to packaging and textiles. More recently, PLA-based materials have found more durable

applications in automotive, communication and electronic industries [3, 4]. However,

drawbacks such as mechanical brittleness and low crystallization rate are often obstacles to

PLA practical uses. With this respect, PBS – a partially biobased polyester, with thermal and

mechanical properties comparable with those of low density polyethylene and polypropylene

[5] was studied as a PLA modifier [6-14]. Because of its high flexibility (low glass-transition

temperature), PBS has found applications as films, laminates, molded foams and injection-

molded products in various fields, such as agriculture and packaging. The PBS relatively

rapid crystallization rate and flexibility were believed to improve PLA thermal and

mechanical properties when blended and materials from PLA/PBS blends have been prepared

by extrusion [6-12]. The miscibility, the phase and crystallization behavior, and the

morphology of PLA/PBS blends were reported in the literature [6,15]. Interestingly, despite

the improved crystallization of PLA in the presence of PBS, thermal analyses showed two

distinct melting peaks over the entire composition range, thus classifying the materials as

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semicrystalline/semicrystalline blends. Moreover, small-angle X-ray scattering revealed that,

in a blend system, the PBS component was expelled out of the interlamellar regions of PLA.

Finally, for more than 40 wt% PBS, significant crystallization-induced phase separation was

observed. The thus suggested immiscibility of both polymers [15] is limiting the applications

of PLA/PBS blend materials without compatibilization. A possibility to overcome the

problem might be found in the electrospinning technique.

Electrospinning is a cutting-edge technique that allows micro- and nanofibrous

polymer materials to be easily prepared [16-22]. Because of their high surface-to-volume ratio

and porous structure, the fibrous mats are well suited for applications such as filters, scaffolds

for cell and tissue engineering, and carriers of bioactive substances. Electrospinning of

immiscible polymer blends containing PLA is already reported in the literature [23]. The

concept resulted in freezing the phase structure generated during electrospinning, thus

preserving the co-continuous or interpenetrating phase morphologies within the fibers.

Noteworthy, unlike PLA that can be easily electrospun from solutions of diverse in nature

solvent systems based on chloroform or dichloromethane [24], the electrospinning of PBS

solutions encounters certain difficulties and reports on the electrospinning of PBS are still

scarce.

The preparation of fibrous materials from PBS with a number average molar mass of

ca. 75 000 g/mol by electrospinning has been reported [25]. Mixed solvent systems such as

chloroform/2-chloroethanol at a ratio of 7/3 or 6/4 (w/w), dichloromethane/2-chloroethanol at

a ratio of 7/3 or 6/4 (w/w) and chloroform/3-chloro-1-propanol at a ratio of 9/1 (w/w) have

been used. It proved preferable to avoid the use of high boiling solvents such as 2-

chloroethanol and 3-chloro-1-propanol when the polymer materials are intended to find

application in the biomedical domain or simply to come into contact with human body. A

short communication presenting few SEM micrographs of porous fibers prepared by

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electrospinning of PBS with a molar mass of 20 000 g/mol using chloroform as solvent was

published in 2008 [26]. It has been found that the electrospinning of PBS with a molar mass

as high as 200 000 – 300 000 g/mol using chloroform as solvent is very difficult to achieve

when the polymer concentration is lower than 10 wt% or higher than 15 wt% [27]. By using

PBS with a weight average molar mass WM 3 × 105 g/mol and a chloroform/ethanol solvent

system at a ratio of 75/25 (v/v) and polymer concentration of 11 % (w/v) PBS/wollastonite

mats have been electrospun [28]. DCM/trifluoroacetic acid (90/10 v/v) solvent system has

been used for electrospinning of 1,6-diisocyanatohexane-extented PBS [29]. Despite the up-

to-here described interest, and most probably because of the significant difficulties in

performing the electrospinning, there are no literature data on the elaboration of fibrous

materials from these two immiscible commercially available high molar mass PLA and PBS

polymers through this method.

Therefore, the present study aims at preparing defect-free fibers from immiscible

blends of PLA and PBS from 100/0 to 10/90 (w/w). The role of the solvent system and the

total polymer concentration on fiber morphology was studied. DSC and XRD were used to

evaluate crystallinity and crystallization rate of the polyesters in the fibrous mats, and tensile

tests were used to determine their mechanical properties. Drug loading and tests with

pathogenic microorganisms were also performed as possible materials application.

2. Experimental

2.1. Reagents

Poly(butylene succinate) (PBS, Bionolle 1003; Shōwa Denkō Kabushiki-gaisha –

Japan; WM = 97 600 g/mol, WM / nM = 2.56 as determined by size-exclusion

chromatography using polystyrene standards); poly(L-lactide) (PLA, IngeoTM

Biopolymer

4032D, NatureWorks LLC - USA; WM = 259 000 g/mol; WM / nM = = 1.94; as determined

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by size-exclusion chromatography using polystyrene standards), dichloromethane (DCM,

Merck); ethanol (abs. EtOH, Merck); the potassium salt of 5-nitro-8-hydroxyquinoline

(K5N8Q) and 5-chloro-8-quinolinol (5-Chloro-8-quinolinol; 95% 5Cl8HQ; Sigma-Aldrich)

were of analytical grade of purity.

2.2. Preparation of PLA/PBS fibrous materials by electrospinning

Spinning solutions in DCM or DCM/EtOH [DCM/EtOH = 90/10 or 80/20 (w/w)] were

prepared for PLA/PBS blends of PLA/PBS = 100/0; 90/10, 80/20, 70/30, 60/40, 50/50, 40/60,

30/70, 20/80, 10/90 and 0/100 (w/w). For preparation of the solutions, PLA and PBS were

first dissolved separately in DCM; for the dissolution of PBS heating at 40°C for 2 h was

necessary. Then, the solutions were mixed at stirring. The total polyester concentration was

10 or 15 wt%. In the case of the DCM/EtOH solvent systems, ethanol was added after the

mixing of PLA and PBS solutions in DCM.

The dynamic viscosity of the spinning solutions was measured by Brookfield LVT

viscometer equipped with an adaptor for small samples, a spindle, and a camera SC 4-18/13 R

at 20 ±0.1°C; the measurements were performed in triplicate and the mean values with their

standard deviations were used.

For the incorporation of K5N8Q and 5Cl8HQ spinning solutions of PLA/PBS = 80/20

(w/w) in DCM/EtOH = 90/10 (w/w) at total polymer concentration of 10 wt% were used; the

concentration of K5N8Q and 5Cl8HQ was 10 or 20 wt% in respect to the polymer weight,

respectively. In the case of K5N8Q due to its poor solubility, the electrospinning was

performed from a stable dispersion.

The electrospinning was carried out using a electrospinning set-up consisting of a high

voltage power supply (up to 30 kV); a pump [NE-300 Just Infusion™ Syringe Pump (New

Era Pump Systems Inc., USA)] for delivering the spinning solution at a constant rate; a

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syringe provided with a positively charged metal needle with inner diameter of 0.6 mm and a

custom-made grounded rotating aluminum collector. The electrospinning was performed

under the following conditions: feeding rate: 3 mL/h, applied voltage: 25 kV, tip-to-collector

distance: 17 cm, collector rotating speed: 600 or 1900 rpm, humidity: 70%, temperature:

20°C. The obtained mats were dried additionally under reduced pressure (320 Pa) at r.t. for 8

h.

2.3. Characterization of the electrospun materials

The morphology of the fibers was examined by a scanning electron microscope

(SEM). The samples were vacuum-coated with gold and observed by a Jeol JSM-5510 SEM.

The fiber morphology was evaluated by applying the criteria for complex evaluation of

electrospun materials as described elsewhere [30] using the ImageJ software by measuring the

diameters of at least 20 fibers from each SEM micrograph.

The thermal behavior of the prepared new fibrous materials was followed by

differential scanning calorimetry (DSC). The analyses were performed using a Perkin Elmer

DSC 8500 differential scanning calorimeter in nitrogen atmosphere at heating rate of

10°С/min. PLA (cPLA

, %) and PBS (cPBS

, %) crystallinity degree was calculated using the

following equations:

cPLA

, % = [(HmPLA

- HccPLA

)/(HmPLA,0

×WPLA

)] × 100 (1)

cPBS

, % = [HmPBS

/(HmPBS,0

×WPBS

)] × 100 (2),

where WPLA

is PLA weight fraction, and WPBS

– PBS weight fraction in the spinning solution;

Hm0 is the heat of fusion of 100 % polymer crystal: Hm

PLA,0 = 93.0 J/g [31]; Hm

PBS,0 =

210.0 J/g [32]. The measurements were performed in triplicate and the mean values with their

standard deviations were used for interpretation the obtained results.

The absence/presence of a crystalline phase in the fibrous materials was assessed by

X-ray diffraction analysis (XRD). XRD spectra were recorded at r.t. using a computer

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controlled D8 Bruker Advance powder diffractometer with a filtered CuKα radiation source

and a luminescent detector. The analyses were performed in the 2θ range from 5° to 50° with

a step of 0.02° and counting time of 1 s/step.

Mechanical properties were evaluated by tensile measurements performed on the

fibrous scaffolds using a Zwick/Roell Z 2.5 apparatus, load cell 2 mV/V, type Xforce P,

Nominal Force 2.5 kN, test Xpert II. The specimens’ thickness was around 50 μm. The

stretching rate was 10 mm/min, and the initial length between the clamps was 60 mm. The

fibrous materials were tested along the circumferential direction of the collector on which the

mat was deposited, and the mean values were calculated.

The water contact angle of the mats was determined using Easy Drop DSA20E

KRÜSS GmbH device at r.t. A drop of deionized water (10 μL) was deposited on the mats

surface. From the images of the droplets on the surface of the mats acquired by a digital

camera and processed by a software program the average values of the water contact angles

were determined based on at least 10 measurements. The samples for the water contact angle

measurements were cut in the collector rotation direction and collector axis direction.

Microbiological tests. The antibacterial activity of PLA/PBS = 80/20 (w/w) mats

containing 10 wt% K5N8Q or 20 wt% 5Cl8HQ was monitored against the pathogenic

microorganism S. aureus (ATCC 6538 P). In order to measure the zones of inhibition, in vitro

studies were performed using Tryptone glucose extract agar (DIFCO Laboratories, Detroit,

USA) solid medium. The surface of the solid agar was inoculated with a suspension of S.

aureus bacterial culture with a cell concentration of 1×105 cells/ml and on the surface of the

agar in each Petri dish two mats (disks with diameters of 10 mm and weight 1.0 mg) from

drug-(non)containing PLA/PBS = 80/20 (w/w) were placed. The Petri dishes were incubated

for 24 h at 37°C and subsequently the zones of inhibition around the disks were measured.

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The average diameters of the zones of inhibition were determined using the ImageJ software

based on 15 measurements in 15 different directions for each zone.

3. Results and Discussion

3.1. Electrospinning and fiber morphology

Attempts to obtain uniform PLA/PBS fibers from both immiscible polymers were

performed by electrospinning their blend solutions at 30 kV and at collector rotating speed of

600 rpm. DCM – a common solvent for both polyesters was first studied at total polymer

concentration of 10 wt% (Fig. S1). Thus, cylindrical defect-free fibers were obtained up to

PLA/PBS = 70/30 (w/w). Further increase in PBS content resulted in spindle-like defects

along the fibers (to 50 wt% PBS) or beaded fibers (PBS content from 80 wt% to 100 wt%).

Further experiments were performed at a total polymer concentration of 15 wt% (DCM, 30

kV, 600 rpm) allowing the obtaining of defect-free fibers at 50 wt% PBS (mean diameter of

7.3±2.1 µm) (Fig. S2). However, the electrospinning process was ineffective (and even

impossible at higher PBS content) due to rapid phase separation and gelation of the spinning

solution. For overcoming these problems, the solvent-nonsolvent approach was attempted. It

is known that adding a certain amount of a poor solvent to the spinning solution assists in the

formation of fibers during electrospinning [33,34]. Thus, in the present study DCM was

partially replaced by EtOH - a poor solvent for both PBS and PLA, while the total polymer

concentration, the voltage and the collector rotating speed were kept constant (10 wt%, 30 kV

and 600 rpm, respectively). It was found that the addition of only 10 wt% EtOH to DCM

significantly improves the electrospinning of PLA/PBS = 50/50 (w/w) solutions. In contrast to

DCM, the fibers obtained from DCM/EtOH = 90/10 (w/w) were cylindrical and defect-free.

Their mean diameter was 1.1±0.4 µm. Further increase of EtOH content in the spinning

solution [DCM/EtOH = 80/20 (w/w)] was counterproductive as spindle-like defects appeared

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along the fiber axis (Fig. S2). Therefore the preparation of defect-free fibrous materials from

PLA/PBS at the whole range of weight ratios was studied in the system DCM/EtOH = 90/10

(w/w) at total polymer concentration of 10 wt% (further denoted as DCM/EtOH).

Figure 1

SEM micrographs of the obtained fibers are presented in Fig. 1. As seen, the use of

DCM/EtOH (30 kV, 600 rpm) allowed the obtaining of randomly deposited defect-free fibers

from the immiscible PLA and PBS up to 80 wt% PBS. Even at PLA/PBS = 10/90 (w/w) only

few spindle-like defects appeared and the mats were predominantly composed of defect-free

fibers. For neat PBS the obtained fibrous material consisted of fibers (mean diameter: 170±48

nm) and bead-like defects (mean diameter: 7.8±2 µm). This pattern is quite different as

compared to DCM solutions, thus confirming the pertinence of the solvent-nonsolvent

approach. The mean diameters of PLA/PBS fibers ( d ) fabricated using DCM/EtOH were in

the range 1800-600 nm depending on the PBS content (Fig. 2A).

Figure 2

As seen, d continuously decreased upon increasing PBS content and the thinnest defect-free

fibers obtained at 90 wt% PBS (590±140 nm). This effect might be related to the changes in

the dynamic viscosity of the spinning solutions (Fig. 2B). Indeed, the dynamic viscosity

decreased upon increasing PBS content – the polyester of lower molar mass ( WM = 97 600).

3.2. Crystallinity and mechanical properties of the fibrous mats

The as prepared fibrous mats were further characterized in terms of crystallinity and

mechanical properties.

Representative DSC thermograms of PLA/PBS mats depending on the PBS content

are shown in Fig. 3A. Concerning the PLA component, Tg, Tcc and Tm were detected at low

PBS content. This occurrence of cold crystallization is in agreement with literature data on

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electrospun PLA materials and might be explained by the rapid solvent evaporation

hampering the polyester crystallization [35-40].

Figure 3

Moreover, Tg and Tcc decreased with ca. 20 °C upon increasing PBS content to 20 wt% (Table

1). In parallel, the enthalpy of cold crystallization decreased with ca. 15 %, thus suggesting

some miscibility between the polyesters in the amorphous state at low PBS content. These

data are in agreement with the literature on PLA/PBS materials prepared by extrusion

[6,8,17], evidencing the role of PBS as plasticizer for PLA. Further increase in PBS content

did not influence the PLA Tg, but led to the disappearance of Tcc, suggesting that blending

with PBS most probably influenced the PLA crystallization and crystallization rate. In

agreement, the melting endotherm of PLA turned bimodal and the corresponding crystallinity

increased. The second Tm newly occurring at higher temperatures (max at 176 °C for

PLA/PBS = 20/80, w/w), might be ascribed to the formation of crystals which are organized

to a greater extent, as observed for PLA/PBS extruded blends [8]. It is to be noted that the

PBS Tg was difficult to observe due to the high crystallinity and crystallization rate of the

polyester. Therefore, avoiding any speculation on electrospinning induced miscibility of the

PLA/PBS blends based on Tg determination is preferable.

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Table 1. Thermal characteristics of PLA/PBS mats as obtained from DCM/EtOH at 600 rpm.

PLA/PBS

PLA

gT

°C

PLA

ccT

°C

PLA

ccΔH

J/g*

PLA

mT

°C

PLA

mΔH

J/g*

χPLA

%**

PBS

ccT

°C

PBS

mT

°C

PBS

mΔH

J/g*

χPBS

%**

100/0 67 90 21.2 167 31.3 10.9 - - - -

90/10 58 78 15.3 168 33.6 21.9 - - - -

80/20 51 70 18.1 166 34.4 21.9 - 114 6.0 14.3

70/30 53 - - 168 26.3 22.7 91 114 16.8 26.7

60/40 54 - - 168 24.2 25.1 89 114 27.2 32.4

50/50 52 - - 166 19.2 29.5 88 110 35.6 29.8

40/60 52 - - 168 20.4 35.0 89 114 38.6 30.6

30/70 52 - - 168 9.2 30.4 88 114 52.5 35.7

20/80 48 - - 167 16.2 50.0 88 112 52.0 31.0

10/90 48 - - 168 6.5 70.0 - 114 67.9 36.0

0/100 - - - - - - - 116 77.0 36.0

* The enthalpies are not recalculated taking into account the respective polyester content. **

Crystallinity degrees are calculated as described in the Exp. Part. HccPBS

are not presented

since the cold crystallization overlapped PBS melting.

Regarding PBS, no significant difference in thermal behavior was detected between the neat

and the blended fibrous mats. PBS is rapidly crystallizing polyester of high crystallinity [32],

making its Tg difficult to assess. Therefore, no values were collected from the DSC analyses.

The polyester melting appeared at ca. 116 °C and was directly preceded by a cold

crystallization exotherm in agreement with the literature data [32]. Interestingly, Tm was not

influenced by the mat composition and the corresponding enthalpy of melting increased upon

increasing PBS content. The obtained results were confirmed by the performed XRD

analyses.

XRD data on films of PLA and PBS homopolyesters are available in the literature

[41,42]. According to the literature, the PLA pattern presents diffraction peaks at 2θ = 14.8°,

16.6°, 19.0°, 22.4°, 29.2° distinctive for the α-form of optically pure PLLA (or PDLA)

crystallizing in a pseudo-orthorhombic unit cell containing l03 helices [42], while PBS

presents diffraction peaks at 2θ = 19.6°, 21.9°, 22.8° and 29.1° ascribed to its α-form

(monoclinic system of 21 helices) [41]. As seen from Fig. 3B, the XRD patterns of PLA/PBS

= 100/0 and 80/20 (w/w) mats revealed only the presence of an amorphous halo. Diffractions

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for crystal phase were not observed, thus confirming the hindered crystallization of the

polyesters during the electrospinning process as evidenced by DSC. In the XRD pattern of

PLA/PBS = 50/50 (w/w) mat a diffraction at 2θ = 22.6° was registered, and in the XRD

pattern of PLA/PBS = 20/80 (w/w) mat: two diffractions were observed at 2θ = 19.6° and

22.6°, respectively. These values of 2θ are characteristic for both PLA and PBS crystal

structure, and their attribution cannot be done only on the basis of the XRD data. However

and taking into consideration their crystallinity degree calculated by DSC (Table 1), it can be

assumed that the registered diffractions were due to the crystal structure of both PLA and

PBS.

It is worth to be mentioned that solvent-cast films from PBS and PLA/PBS lost their

integrity and became very brittle on drying. In sharp contrast, PLA/PBS fibrous mats

maintained integrity and could be easily handled without fragmentation. In order to determine

their mechanical properties tensile tests on mats of defect-free fibers were performed.

The stress-strain curves of PLA/PBS fibrous materials at different PBS contents are

presented in Fig. F4A. Digital images of the samples after testing are shown as well.

Figure 4

As seen, before reaching the ultimate tensile strength, the PLA/PBS = 100/0 (w/w) and

PLA/PBS = 80/20 (w/w) mats displayed typical strain behavior of thermoplastic materials. At

low stress values (in the elasto-plasticity range) they rapidly reached the yield point after

which necking was observed. This necking after the yield point gave evidence for the

occurrence of inelastic deformation. The subsequent increase of the stress led to hardening of

the samples most probably due to additional orientation of the fibers during specimens’

elongation. Partial tearing of the samples was observed after reaching the maximum tensile

stress. Further increase in PBS content (PLA/PBS = 50/50 (w/w) and PLA/PBS = 20/80

(w/w)) resulted in distinct break point, and absence of necking, indicating moderately ductile

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fracture behavior of these materials. One possible explanation of this finding might be

alignment of the fibers during stretching. However, based on the DSC and XRD data

alteration in the blend morphology: a transition from dispersed structure to bicontinuous one

and again to dispersed structure; alteration in the crystallinity of the blends and by interfacial

adhesion [43] should not be neglected. Effects of additional orientation of the macromolecules

during specimen elongation [44] are not to be excluded. To verify the hypothesis, fibrous

materials were obtained at higher collector rotation speed (1900 rpm), known to increase the

degree of fibers’ alignment. According to theoretical simulations confirmed by experimental

measurements [44], mats with higher degree of fibers alignment are characterized by

improved mechanical strength, but reduced failure strain (shorter linear part of the stress-

strain curves) in comparison to less aligned and randomly deposited fibers. Similar trends

were observed for the PLA/PBS fibrous materials. As seen from Fig. 4B, the higher collector

rotating speed affected significantly the yield point value. Furthermore, the linear part of the

stress-strain curves for the samples prepared at 1900 rpm narrowed significantly upon

increasing the collector rotation speed (Fig. 4B), attesting for reduced flexibility of the fibrous

materials. Thus, higher degree of alignment of the fibers in the materials obtained at the

higher collector rotating speed might be stated (Fig. S3A and B).

At low collector rotation speed (600 rpm), the Young’s modulus, the tensile strength

and the elongation did not significantly depend on the PBS content and were determined to be

ca. 100 MPa, 2.5 MPa and 90 %, respectively (Fig. F5A, B and C). At higher collector

rotation speed (1900 rpm), the detected Young’s modulus and tensile strength increased by a

factor of ca. 4 (350 MPa and 9.0 MPa, respectively), remaining almost independent on the

PBS content. The obtained results are in correlation with the calculated higher crystallinity of

PLA in PLA/PBS = 100/0 (w/w) mats prepared by applying higher collector rotating speed

(Fig. 5D). Concerning the elongation (Fig. 5C), the highest value (130±5 %) was obtained for

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PLA/PBS = 80/20 (w/w). In order to explain this result, several hypotheses including

polyester miscibility, blend morphology and crystallinity need more exhaustive investigations.

Figure 5

3.3. Drug loading and biological activity

One of the potential applications of the new fibrous PLA/PBS materials is as carriers

of low molecular weight bioactive substances with targeted behavior, e.g. having antibacterial

and antimycotic activity [45]. In addition, the PLA/PBS mats prepared in this study were

rather hydrophobic (the determined water contact angles were in the range 127-132°).

Regarding biomedical applications, the incorporation of an antibacterial agent in PLA/PBS

mats would often be advisable since such materials may be a good substrate for biofilm

formation. In the present study two 8-hydroxyquinoline (8HQ) derivatives were used as

model drugs: the potassium salt of 5-nitro-8-hydroxyquinoline (K5N8Q) and 5-chloro-8-

quinolinol (5Cl8HQ). As a polymer carrier PLA/PBS = 80/20 (w/w) mats prepared by using

of DCM/EtOH as solvent and at a collector rotating speed of 1900 rpm was selected due to

the lower diameter values and adequate mechanical behavior (especially regarding their

elasticity behavior). SEM micrographs of the prepared fibers as well as their mean diameters

values are shown in Fig. 6.

Figure 6

The incorporation of these 8HQ derivatives led to decrease of the mean diameter value of the

fibers, which might be attributed to the ionogenic nature of the drugs used. This decrease is in

accordance with previous findings about fibrous materials prepared by electrospinning of

polymer solutions containing an ionogenic low molecular weight additive [40,46,47]. The

performed measurements of the water contact angle showed that the incorporation of the

bioactive compounds did not alter the water contact angle value, i.e. the mats remained

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hydrophobic. K5N8Q and 5Cl8HQ are broad-spectrum antibacterial agents with minimum

inhibitory concentration against S. aureus of 0.25 µg/mL for 5Cl8HQ [48] and 16 µg/mL for

K5N8Q [49]. The antibacterial activity of the electrospun mats was assessed by performance

of microbiological tests against this microorganism. The results obtained by determination of

the zones of inhibition after a 24-h contact of the fibrous materials with the bacterial cells are

shown in Fig. 7.

Figure 7

A blank sample (without any fibrous material) is presented as well. As expected PLA/PBS

mats did not exhibit any antibacterial activity. Well-distinguished zones of inhibition of the

bacterial cells growth were detected for the drug-containing PLA/PBS = 80/20 (w/w) mats.

The diameter of the zones of inhibition of PLA/PBS/K5N8Q and PLA/PBS/5Cl8HQ mats did

not differ sufficiently, being 37.5 mm and 40.0 mm, respectively. The observation of zones of

inhibition is evidence that the incorporated drugs in the mats retain their antibacterial activity.

4. Conclusions

For the first time, immiscible blends of PLA and PBS were electrospun to obtain

defect-free fibrous mats in the entire range of PLA/PBS weight ratios. Different conditions as

solvent system and total polymer concentration were tested in order to determine the optimal

electrospinning conditions. The fiber morphology patterns showed the pertinence of the

solvent-nonsolvent approach. DSC and XRD analyses attested for the role of PBS on the

crystallinity of the electrospun mats. Blending PLA with PBS decreased its Tg and suppressed

its cold crystallization, suggesting for a plasticizing effect of PBS. Moreover, a second Tm for

PLA appeared at higher temperatures, confirming the influence of PBS on PLA crystallization

and crystallization rate. These observations were clearly confirmed by XRD.

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The electrospinning process revealed crucial for the obtaining of PLA/PBS blend

materials maintaining integrity and allowed the preparation of fibrous materials which

mechanical properties changed from plastic to brittle depending on the PBS content. The

alignment of the blend fibers was controlled by the collector rotation speed and was found to

increase the modulus and the tensile strength of the fibrous materials. Drug loading and tests

involving pathogenic microorganisms suggested that the obtained mats can find application as

antibacterial fibrous materials.

Acknowledgements

Financial support from the Bulgarian Science Fund (Grant DCVP 02/2/2009) is

gratefully acknowledged. The authors thank the bilateral agreement between WBI/FRS-FNRS

and BAS and the financial support from Région Wallonne and European Commission in the

frame of SINOPLISS-POLYEST. N.S. acknowledges the OP-HRD Grant BG051PO001-

3.3.06-006 of the European Social Fund.

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Figure Captions

Fig. 1. Representative SEM images of PLA/PBS fibers prepared at PLA/PBS from 100/0 to

0/100 (w/w) at total polymer concentration of 10 wt% and by using a mixed solvent system

DCM/EtOH; collector rotating speed: 600 rpm; magnification: ×1000.

Fig. 2. Mean fiber diameter (A), and dynamic viscosity (B) for different PLA/PBS weight

ratios; total polymer concentration: 10 wt%; collector rotating speed: 600 rpm.

Fig. 3. DSC thermograms (first heating run, A) and XRD patterns (B) at selected PLA/PBS

weight ratios. DCM/EtOH; collector rotating speed: 600 rpm.

Fig. 4. Stress-strain curves recorded during mechanical tests of PLA/PBS mats prepared at a

collector rotating speed of 600 (A) or 1900 (B) rpm; solvent system: DCM/EtOH; the samples

were cut in such a manner so that their length axis coincided with the direction of the collector

rotation.

Fig. 5. Dependence of Young’s modulus (A), tensile strength (B) and elongation at break (C)

of PLA/PBS mats, and the crystallinity degree of PLA and PBS (D) in the fibrous materials

on PBS content in the mats, prepared at collector rotation speed of 600 or 1900 rpm; solvent

system: DCM/EtOH. The samples for which a break during the tests was detected are marked

with an asterisk; thus, the obtained values may be considered as elongation at break.

Fig. 6. SEM micrographs of PLA/PBS fibers prepared at total polymer concentration of 10

wt% using DCM/EtOH in absence (A) and presence of the drug: 10 wt% K5N8Q (B) or 20

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wt% 5Cl8HQ (C) (in respect to the total polymer weight); collector rotating speed: 1900 rpm;

the collector rotation direction is indicated by a white arrow.

Figure 7. Digital images of the zones of inhibition against S. aureus, detected after a 24-h

contact of the drug-(non)containing PLA/PBS fibrous materials with the bacterial cells. Left

image - blank control.

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PLA PLA/PBS = 90/10 (w/w) PLA/PBS = 80/20 (w/w)

PLA/PBS = 20/80 (w/w) PLA/PBS = 10/90 (w/w) PBS

Fig. 1. Representative SEM images of PLA/PBS fibers prepared at PLA/PBS from 100/0 to

0/100 (w/w) at total polymer concentration of 10 wt% and by using a mixed solvent system

DCM/EtOH; collector rotating speed: 600 rpm; magnification: ×1000.

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0 10 20 30 40 50 60 70 80 90 1000

500

1000

1500

2000

2500

3000

3500

100/0 80/20 60/40 40/60 20/80 0/100

PLA/PBS, w/w

Mean

fib

er

dia

mete

r valu

e,

nm

0 10 20 30 40 50 60 70 80 90 1000

100

200

300

400

500

100/0 80/20 60/40 40/60 20/80 0/100

PLA/PBS, w/w

Dyn

am

ic v

isco

sit

y,

cP

A B

Fig. 2. Mean fiber diameter (A), and dynamic viscosity (B) for different PLA/PBS weight

ratios; total polymer concentration: 10 wt%; collector rotating speed: 600 rpm.

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A

B

Fig. 3. DSC thermograms (first heating run, A) and XRD patterns (B) at selected PLA/PBS

weight ratios. DCM/EtOH; collector rotating speed: 600 rpm.

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A

B

Fig. 4. Stress-strain curves recorded during mechanical tests of PLA/PBS mats prepared at a

collector rotating speed of 600 (A) or 1900 (B) rpm; solvent system: DCM/EtOH; the samples

were cut in such a manner so that their length axis coincided with the direction of the collector

rotation.

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0 20 50 800

100

200

300

400

500 600 rpm

1900 rpm

100/0 80/20 50/50 20/80

PLA/PBS, w/w

Yo

un

g's

Mo

du

lus

, M

Pa

0 20 50 800.0

2.5

5.0

7.5

10.0

12.5 600 rpm 1900 rpm

100/0 80/20 50/50 20/80

PLA/PBS, w/wT

en

sile

str

en

gth

, M

Pa

* *

*

*

A B

0 20 50 800

25

50

75

100

125

150 600 rpm 1900 rpm

* *

*

*

100/0 80/20 50/50 20/80

PBS content, wt.%

Elo

ng

ati

on

, %

0 20 50 800

10

20

30

40

50

60PLA/600 rpm

PLA/1900 rpm

PBS/600 rpm

PBS/1900 rpm

100/0 80/20 50/50 20/80

PLA/PBS, w/w

Cry

sta

llin

ity

de

gre

e, %

C D

Fig. 5. Dependence of Young’s modulus (A), tensile strength (B) and elongation at break (C)

of PLA/PBS mats, and the crystallinity degree of PLA and PBS (D) in the fibrous materials

on PBS content in the mats, prepared at collector rotation speed of 600 or 1900 rpm; solvent

system: DCM/EtOH. The samples for which a break during the tests was detected are marked

with an asterisk; thus, the obtained values may be considered as elongation at break.

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A B C

1985±400 nm 953±475 nm 760±310 nm

Fig. 6. SEM micrographs of PLA/PBS fibers prepared at total polymer concentration of 10

wt% using DCM/EtOH in absence (A) and presence of the drug: 10 wt% K5N8Q (B) or 20

wt% 5Cl8HQ (C) (in respect to the total polymer weight); collector rotating speed: 1900 rpm;

the collector rotation direction is indicated by a white arrow.

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S. aureus PLA/PBS PLA/PBS/K5N8Q PLA/PBS/5Cl8HQ

Figure 7. Digital images of the zones of inhibition against S. aureus, detected after a 24-h

contact of the drug-(non)containing PLA/PBS fibrous materials with the bacterial cells. Left

image - blank control.

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Graphic for Graphical Abstract only

Poly(L-lactide) and poly(butylene succinate) immiscible blends: from electrospinning to

biologically active materials

Nikoleta Stoyanova, Dilyana Paneva, Rosica Mincheva, Antoniya Toncheva, Nevena

Manolova, Philippe Dubois, Iliya Rashkov*

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Highlights

Poly(L-lactide) and poly(butylene succinate) immiscible blends: from electrospinning to

biologically active materials

Nikoleta Stoyanova, Dilyana Paneva, Rosica Mincheva, Antoniya Toncheva, Nevena

Manolova, Philippe Dubois, Iliya Rashkov*

The solvent-nonsolvent approach enabled electrospinning of immiscible PLA/PBS

blends

Fibrous materials maintained integrity in contrast to cast films

The mechanical properties depended on PBS content and collector rotation speed

PBS acts as plasticizer for PLA and increases its crystallinity and crystallization rate

Drug-loaded PLA/PBS mats exhibit antibacterial properties against S. aureus