At

XXa

Sb

c

a

ARRAA

KPPO

1

bratmcatptaTtc

SnT

(

0d

Colloids and Surfaces A: Physicochem. Eng. Aspects 369 (2010) 128–135

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

facile approach to modify polypropylene flakes combining O2-plasmareatment and graft polymerization of l-lactic acid

iaoqing Huaa, Tianzhu Zhanga,c,∗, Jing Rena, Zhigang Zhanga,b, Zhenling Jib,∗,iaoli Jianga,c, Jingjing Linga, Ning Gua,c,∗

Jiangsu Key Laboratory for Biomaterials and Devices, State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering,outheast University, Sipailou 2, Nanjing 210096, ChinaDepartment of General Surgery, Zhong-Da Hospital, Southeast University, Dingjiaqiao 87, Nanjing 210009, ChinaSuzhou Key Lab of Biomedical Materials and Technology, Research Institute of Southeast University in Suzhou, Ren Ai Road 150, Suzhou Industrial Park, Suzhou 215123, China

r t i c l e i n f o

rticle history:eceived 5 May 2010eceived in revised form 18 July 2010ccepted 4 August 2010vailable online 12 August 2010

a b s t r a c t

Modification of polypropylene (PP) surface with poly(l-lactic acid) (PLLA) via a facile approach wasdescribed. The PP flakes were first treated with O2 plasma and then grafted with l-lactic acid (LLA)monomer in the aqueous solution. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were

eywords:olypropyleneoly(l-lactic acid)2 plasma

employed to probe the surface composition and morphology of the flakes. Water contact angle mea-surements were used to monitor the change in hydrophilicity of PP flake surface, and tensile strengthtests were carried out to monitor the mechanical properties of PP flakes along the treatment procedure.It was found that the PLLA grafting degree increases reasonably with the reaction temperature. At thesame time, a grafting polymerization of 6 h of l-lactic acid at 160 ◦C can lead to obvious IR absorption.PLLA-grafted polypropylene (PP-g-PLLA) was eventually obtained. The higher reaction temperature can

uenc

bring the detrimental infl

. Introduction

Polypropylene (PP), as a substantial polymeric material, haseen widely used in industrial field, due to its good chemicalesistance, high impact strength, permeability and flexibility, suchs textiles, packaging materials, laboratory equipment, automo-ive components and so on. Meanwhile, polypropylene exhibits

any desirable properties including very high void volumes, well-ontrolled porosity and chemical inertness, which have beenpplied in medical areas, medical implant materials, microfiltra-ion and ultrafiltration process, to name a few. Unfortunately,olypropylene membranes are poor in biocompatibility and wet-ability as a result of the absence of polar functional groups,

nd they lack of bonding strength and printing strength as well.hese disadvantages limit its applications in many cases. Takenhis into consideration, surface modification is expected to bringhanges in biocompatibility, adhesion, printability, wettability and

∗ Corresponding authors at: Jiangsu Key Laboratory for Biomaterials and Devices,tate Key Laboratory of Bioelectronics, School of Biological Science and Medical Engi-eering, Southeast University, Sipailou 2, Nanjing 210096, China.el.: +86 025 83272476; fax: +86 025 83272460.

E-mail addresses: zhangtianzhuglq@yahoo.com.cn (T. Zhang), zlji@vip.sina.comZ. Ji), guning@seu.edu.cn (N. Gu).

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.08.009

es to the mechanical properties of PP flakes.© 2010 Elsevier B.V. All rights reserved.

hydrophilicity in different instances to PP, without altering the bulkproperties [1–4].

Numerous approaches have been developed to modify the sur-face of polypropylene, including photo-initiated grafting, livingradical grafting, ceric ion-induced grafting, layer-by-layer ionicgrafting, corona discharge, plasma polymerization, and chemicaletching [5].

Cold plasmas have drawn much attention since they can providereactive medium at ambient temperature and the modification islimited to the outermost layers of several hundred angstroms whilethe bulk properties can be kept unchanged [6,7]. It has been vali-dated by previous studies that both nitrogen- or oxygen-containingplasma can improve the wettability, printability, topography andbiocompatibility of polymer surfaces although the detailed mech-anism of plasma processes is still unclear. Cold plasma is a rapid,clean and solvent-free process that can introduce specific elementsor functional groups onto the surface of a polymer through choos-ing a suitable gas. Nitrogen-containing functions such as primaryamine (–NH2), imine (CH NH), cyano (–CN) or nitrile can be intro-duced easily via N2 plasmas while O2 plasmas, which can cause

the incorporation of oxygen-containing components, introducesmainly carboxylic groups (–COOH) and hydroxyl groups (–OH). Ofcourse, other functionalities, such as C–O and O C–OO, also canbe found on the surface of polymers after O2-plasma treatment[8–12].

ysicoc

anatvma

ppowt((fg

smlha

2

2

(h(if0

2

tw(owsp

2

fl0

X. Hua et al. / Colloids and Surfaces A: Ph

Poly-l-lactide (PLLA) is broadly used as biodegradable plasticnd biomedical materials in surgery (such as sutures), tissue engi-eering (such as bone fixation and tissue scaffolds), drug deliverynd packaging materials due to its excellent mechanical proper-ies and biodegradable characterization, which can be degraded inivo by hydrolytic deesterification into lactic acid monomers. Theonomers are excreted by the lungs as carbon dioxide and water

fter they enter the carboxylic acid cycle [13–18].High molecular weight PLA is often obtained via ring-opening

olymerization of lactide at high temperature. However, the highrice for preparing PLA this way hindered more general applicationf PLA. Another method is direct condensation of free lactic acid,hich is in need of certain catalyst systems and initiators. Basically,

here are two kinds of initiators, hydroxyl-terminated moleculesincluding polymers) and carboxylic acid-terminated moleculesincluding polymers) [19]. For instance, block copolymers wereormed in solution via LA polymerization from the hydroxyl endroup of poly(ethylene glycol) (PEG) [20,21].

In this paper, we developed an approach to graft PLLA onto theurface of PP flake, which was oxidized first via O2-plasma treat-ent. Different from the ring-opening polymerization of l-lactide,

-lactic acids were polymerized directly on the O2-plasma treatedydroxyl-terminated PP flake. In this polymerization of LLA, PP-OHcts as the co-initiator.

. Experimental section

.1. Materials and general procedures

Methane dichloride (CH2Cl2), tin(II) dichloride dihydrateSnCl2·2H2O) and succinic anhydride were purchased from Shang-ai Chemical Reagent Inc. and used as received. 88% l-lactic acidLLA) aqueous solution was purchased from Jinan Institute of Med-cal Instrument and used as received. Polypropylene was obtainedrom C. R. Bard Inc. The original PP flakes with a thickness of about.25 mm were prepared by pressing molten polypropylene.

.2. Plasma treatment

Polypropylene (PP) flake was first washed with CH2Cl2 for 2 ho remove additives. After dried under vacuum, the flake samplesere treated with oxygen-containing plasma in a plasma chamber

PDC-M, Chengdu Weike spectrum apparatus Inc., China) for 10 sr 30 s, then exposed to air. The plasma treatment was performedith the oxygen flux of 800 mL min−1 and the power of 40 W. The

ample after plasma treatment was named hydroxyl-terminatedolypropylene (which is denoted PP-OH).

.3. Graft polymerization of l-lactic acid on PP flake surface

The four pieces of PP-OH flakes were placed into a two-neckask filled with 10 mL 88% l-lactic acid aqueous solution, with.018 g (0.18 mmol) succinic anhydride and 0.030 g (0.13 mmol)

Scheme 1. Schematic illustration of polymerization graf

hem. Eng. Aspects 369 (2010) 128–135 129

SnCl2·2H2O as catalyst. The polypropylene flake floated on thesolution at the beginning of reaction. The polycondensation reac-tion proceeded at 140 ◦C as well as 160 ◦C for a given time. Afterthis period, the reaction was cooled down to room temperature,the PP flake samples sunk at the bottom of flask and were tookfrom the flask and washed first with distilled water and thenwith methane dichloride thoroughly to remove PLLA physicallyadsorbed the surface of PP flakes. Finally, PP flakes were driedat 60 ◦C for measurements. (Scheme 1). The control experiments,where the original PP was used to replace O2-plasma treated PP,were performed to observe the surface reconstruction of PP flakeson heating with the same grafting polymerization conditions.

2.4. Characterizations of PP flake surface

The surfaces of the PP flake were probed by atomicforce microscopy (AFM, Agilent, PicoPlus). The attenuated totalreflectance Fourier transform infrared (ATR-FTIR) spectra wereobtained using a Nicolet 5700 spectrometer (Thermo, USA) withan internal reflection accessory ZnSe crystal at an angle of 45◦ anda DTGS KBr detector. Spectra were recorded at 4 cm−1 resolutionbetween 4000 cm−1 and 400 cm−1 and were the sum of 256 indi-vidual scans. The sessile drop method was used for contact anglemeasurements at 20 ◦C using a commercial contact angle meter(CAM 200, KSV Instruments Ltd., Finland). Ultra pure water droplets(10 �L) were placed at six different positions for one sample. Thenthe average value was obtained. The experimental error of the mea-surements was about ±2◦. X-ray photoelectron spectroscopy (XPS)(PHI 5300 spectrometer Perkin-Elmer Corporation, USA) was usedto determine the surface composition of polypropylene flakes atthe different process stage. The monochromatic Al K� X-rays sourcewas operated at 1486.7 eV and the anode X-ray source was oper-ated at 15 kV and 8.9 mA. Survey spectra were acquired from 0 eVto 1200 eV binding energy (BE) with a pass energy of 160 eV, a stepsize of 1.0 eV and a dwell time of 50 ms. For high-resolution spec-tra, a pass energy of 17.9 eV, an energy step of 0.1 eV and a dwelltime of 1.2 s were employed with a typical average of 12 scans.The operating pressure of the spectrometer was ∼10−9 mbar. Alldata were collected and analyzed using software provided by themanufacturer. At last, tensile strength tests of untreated PP, PP flakesamples treated with oxygen-containing plasma and PP-OH graftedwith PLLA was done using Instron Corporation Series IX AutomatedMaterials Testing System 7.43.00 (XYZ Corporation, Main Street,Anytowm, USA). Testing included the displacement at peak (mm),load at peak (kN), stress at max. load (MPa) and modulus (AutY-oung) (MPa). In the experiments, Sample rate was 10.0000 pts/s,crosshead speed was 20 mm min−1, full scale load range was 0.5 kNunder the humidity of 40% and temperature of 17 ◦C.

3. Results and discussion

The surface of original PP flake is relatively smooth and flat.After treated with O2 plasma, the surface became rougher. With

ting of l-lactic acid on polypropylene flake surface.

130 X. Hua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 369 (2010) 128–135

ith t

aot

aiat1tap

Fc

Fig. 1. AFM images of original PP flake (a) compared w

general observation, images of AFM indicate that treatment ofxygen-containing plasma has destroyed the surface spherulites ofhe original PP flake, as can be seen in Fig. 1.

The ATR-FTIR spectra of the original PP flakes (below curve)nd PP flakes treated with O2 plasma (above curve) are shownn Fig. 2, where no significant difference was found. There was nobsorption peak around 1757 cm−1 of carbonyl (C O) in the spec-rum for both two PP flakes [5]. After O -plasma treatment, only at

2632 cm−1 there appeared an IR absorption, which can be assignedo fractionary carbonyl groups caused by O2 plasma. The wide bandround 3350 cm−1 possibly indicates typical hydroxyl groups afterlasma treatment. AFM observation and ATR-FTIR measurement

ig. 2. ATR-FTIR spectra obtained from the original PP flake (below) and oxygen-ontaining plasma treated PP flake (above).

hat of oxygen-containing plasma treated PP flake (b).

demonstrate that O2-plasma treatment alters the flakes in bothphysical and chemical way.

After grafted with PLLA under different conditions, there aresome new absorption peaks in the ATR-FTIR spectra. The IR absorp-tion peak at 1757 cm−1 or 1737 cm−1 was assigned for the abundantcarbonyl C O stretching of ester groups in bulk PLLA graft, and peakat 1186 cm−1 and 1166 cm−1 are for the symmetric stretching ofC–O–C. According to Scheme 1, PLLA grafts grew on the PP substrateand finally were theoretically terminated with hydroxyl groups.The IR spectra exhibited characteristic absorption peaks of ester (at1737 cm−1 or 1757 cm−1 for C O and 1103 cm−1 or 1091 cm−1 for–O–, respectively) and –CH2–, –CH3 groups (at 2850–3050 cm−1)[17]. One broad absorption band can be seen at 3350 cm−1, andanother peak was found around 1041 cm−1 for the PLLA-graftedPP flakes, which can be assigned to the hydroxyl groups of thecarboxylic acid of PLLA grafts. Figs. 3 and 4 reveal the ATR-FTIRspectra of the PLLA-grafted PP flakes reacted for 2 h (below), 6 h(middle), 10 h (above) at 140 ◦C and 160 ◦C. Although very small,there exists a slight variation of typical peak of C O at differentreaction temperatures of 140 ◦C and 160 ◦C [22,23]. According tothe IR absorption intensity, for 140 ◦C, only the reaction of 10 h canlead to an obvious PLLA graft. For 160 ◦C, a shorter reaction time,say 2 h, can lead to PLLA graft on PP surface. However, comparedwith the IR spectra in Fig. 2, the difference is not obvious. The pos-sible reason is that PLLA graft layer is too thin to cause obvious IRabsorption.

The change in morphologies of PLLA-grafted PP flake surface is

also very obvious compared with that of both original PP flakes andO2-plasma treated PP flakes. The granules or apexes ranged on thesurface of flakes in irregular ways, which apparently are differentfrom the pristine PP flakes or that treated with oxygen-containingplasma. Figs. 5 and 6 show the AFM images of the PLLA-grafted PP

X. Hua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 369 (2010) 128–135 131

Fig. 3. ATR-FTIR spectra of the PLLA-grafted PP flakes reacted for 2 h (below), 6 h(middle), and 10 h (above) under the temperature of 140 ◦C. Fig. 4. ATR-FTIR spectra of the PLLA-grafted PP flakes reacted for 2 h (below), 6 h

(middle), and 10 h (above) under the temperature of 160 ◦C.

Fig. 5. AFM images of the PLLA-grafted PP flakes reacted for 2 h (a), 6 h (b), and 10 h (c) under the temperature of 140 ◦C.

132 X. Hua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 369 (2010) 128–135

for 2 h

flospflfmeno

fl(scfi

Fig. 6. AFM images of the PLLA-grafted PP flakes reacted

akes reacted for 2 h (a), 6 h (b), 10 h (c) under the temperaturef 140 ◦C and 160 ◦C. In consideration of possible surface recon-truction of PP flakes on heating, three control experiments wereerformed and results show that the surface reconstruction of PPake after heating at 140 ◦C or 160 ◦C occurred. (Fig. 7) After a care-

ul comparison of these AFM morphologies, it was found that AFMorphologies of PLLA-grafted PP and surface reconstructed PP are

xtremely similar. Therefore, PLLA graft layer on PP flakes can-ot be effectively distinguished through only AFM morphologiesbservations.

Fig. 8a shows the XPS C1s core-level spectrum of the original PP

akes. A predominant C1s peak component at the binding energyBE) of about 284.6 eV, characteristic of C–C/C–H species, is con-istent with the chemical composition of the PP flake. The C1sore-level spectrum of O2-plasma treated PP (Fig. 8b) can be curve-tted with two peak components, having BEs at about 284.6 eV and

(a), 6 h (b), and 10 h (c) under the temperature of 160 ◦C.

286.3 eV, attributed to the C–H and C–O, respectively. Fig. 8c showsthat the three peak components, 283.31 eV, 284.7 eV, and 286.3 eV,attributed to the C–H, C–O and O–C O of PP-g-PLLA obtained at140 ◦C.

Following the treatment procedure, the water contact angle ofPP flake changed accordingly. For the original PP flake, the watercontact angle measurements are 105◦ and indicate that the PPsurface is hydrophobic. After the treatment of oxygen-containingplasma, the formed hydroxyl groups on the PP surface leads to aslight decrease in hydrophobicity with a contact angle of 94◦, andthe further grafting polymerization of LLA on PP surface affect the

water contact angle differently according to the reaction time andtemperature (Table 1).

The reported value of water contact angle of a smooth and flatPLLA film is about 77◦ [24,25]. Practically, the water contact anglehas something to do with the surface structure and roughness.

X. Hua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 369 (2010) 128–135 133

F nditiof

Auccfhh6

TW

ig. 7. The surface construction of the original PP flakes under different heating coor 10 h.

s seen in Figs. 5 and 6, especially in the three-dimensional fig-res, most of the surfaces of PLLA-grafted PP contain concentratedonvexity structures, which lead to the hydrophobic surface, the

◦ ◦

ontact angle ranges from 100 to 108 (Table 1). With the dif-erent reaction time or temperature, the different topography andydrophobicity appeared. An exception also was found: the onlyydrophilic sample obtained under the temperature of 160 ◦C forh has a water contact angle of 62◦. For the surface-reconstructed

able 1ater contact angle of PP flakes reacted under different times and temperatures.

Time Temperature

2 h 6 h 10 h

140 ◦C 105◦ 106◦ 106◦

160 ◦C 108◦ 62◦ 100◦

ns. (a) Heated at 140 ◦C for 6 h, (b) heated at 140 ◦C for 10 h and c) heated at 160 ◦C

pure PP flakes obtained at 160 ◦C, the contact angle is 94.5◦ for6 h heating and 97.7◦ for 10 h heating, respectively. When thereconstruction temperature is 140 ◦C and heating time is 10 h, theobtained contact angle is 105.3◦. This further indicates the relationof the hydrophobicity and the topography of PP flake. Therefore,for the contact angle of 62◦, a possible reason can be figured outfrom its AFM image that there are nano-structured little holes onits surface, in other words, the rougher surfaces of high hydropho-bicity together with the smooth surfaces of low hydrophobicityspace each other, which leads to its hydrophilicity. Nevertheless,the further experiment needs to be carried out in order to probe the

relation of the hydrophobicity and PLLA-grafted PP surface struc-ture [26].

What’s more, there exist changes in tensile strength tests corre-sponding to the experimental procedure. Table 2 shows the detailedtesting results.

134 X. Hua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 369 (2010) 128–135

Table 2Average results of tensile strength tests.

Pure PP O2-plasma treated PPa O2-plasma treated PPb PLLA-grafted PPc PLLA-grafted PPd

Displacement at peak (mm) 2.953 1.860 1.557 1.847 1.837Load at peak (kN) 0.045 0.093 0.063 0.074 0.069Stress at max. load (MPa) 33.189 35.849 34.082 36.489 31.759Strain at max. load (%) 17.094 9.300 7.783 9.233 9.183Young modulus (MPa) 1157.245 937.490 1100.646 830.592 1058.893

tOcfometLrdbi

Fa

a PP that has been treated with O2-plasma for 10 s.b PP that has been treated with O2-plasma for 30 s.c Graft polymerization of LLA at 140 ◦C onto PP treated with O2-plasma for 10 s.d Graft polymerization of LLA at 140 ◦C onto PP treated with O2-plasma for 30 s.

At max. load, there is no apparent difference in the stress forhese five samples. How, the strain (%) values indicate that either2-plasma treatment or subsequent LLA grafting polymerizationan leads to the obvious decrease of elasticity of PP. For example,or the original PP, the strain is 17.094%, after O2-plasma treatmentf 10 s, the value decreased to 9.300%, the further LLA grafting poly-erization still did not change the value remarkably within the

rror range. The measurement of Young modulus also confirmedhe loss of elasticity of PP after O2-plasma treatment or subsequentLA grafting polymerization. It was worthy noting that the higher

eaction temperature is major factor which leads to the severeecrease of the elasticity of PP. In fact, at 160 ◦C, some PP flakesecame smashed and mixed with PLLA during reaction, therefore

t is impossible to separate PP flakes from the reaction. So, the lower

ig. 8. XPS C1s core-level spectra of (a) original PP, (b) O2-plasma-treated for 10 s,nd (c) O2-plasma-treated for 10 s and reacted with LA for 10 h at 140 ◦C.

reaction temperature is critical factor to maintain the excellentmechanical properties of PP.

4. Conclusions

The O2-plasma treated polypropylene (PP-OH) flakes can beused effectively as a co-initiator to undergo a polycondensationreaction with l-lactic acid (LLA) monomer in aqueous solution atheating condition. Consequently, poly(l-lactic acid) (PLLA) grew onthe surface of PP-OH flakes, which means that LLA was grafted even-tually onto the surface of PP flakes. ATR-FTIR spectra measurementsand XPS spectroscopy confirmed the changes in chemical aspectof the PLLA-grafted PP flakes compared with that of the originalPP or PP-OH flake surface. Due to the surface reconstruction of PPflakes on heating, AFM morphologies observations cannot effec-tively distinguish PLLA graft layer on PP flakes. The hydrophilicitychanges along the O2-plasma treatment and the subsequent graft-ing process were monitored by the static water contact anglemeasurements. Mechanical properties were examined via tensilestrength tests, which indicate that PP flakes treated with O2-plasmaas well as the grafted PP flakes were fragile and lost it applicationvalue due to the higher reaction temperature. The further improve-ment to decrease the reaction temperature and retain excellentmechanical properties of polypropylene has to be considered.

Acknowledgments

The work is supported by the Ministry of Education of China KeyScience and Technology Research Project (109072), Suzhou High-Tech Enterprise Innovation Fund (SG0921), National ImportantScience Research Program of China (2006CB933206), National Nat-ural Science Foundation of China (Nos. 50872021 and 90406023),International cooperation program awarded by Ministry of Scienceand Technology of China (2008 DFA51180) and Qing Lan Project.The authors also thank Ms. Yan Huang for IR measurements andDr. Hui Jiang for water contact angle measurements.

References

[1] J.M. Goddard, J.H. Hotchkiss, Polymer surface modification for the attachmentof bioactive compounds, Prog. Polym. Sci. 32 (2007) 698–725.

[2] N.V. Bhat, D.J. Upadhyay, R.R. Deshmukh, S.K. Gupta, Investigation of plasma-induced photochemical reaction on a polypropylene surface, J. Phys. Chem. B107 (2003) 4550–4559.

[3] D.E. Bergbreiter, Polyethylene surface chemistry, Prog. Polym. Sci. 19 (1994)529–560.

[4] Q. Yang, Z.-K. Xu, Z.-W. Dai, J.-L. Wang, M. Ulbricht, Surface modification ofpolypropylene microporous membranes with a novel glycopolymer, Chem.Mater. 17 (2005) 3050–3058.

[5] G. Tao, A. Gong, J. Lu, H.-J. Sue, D.E. Bergbreiter, Surface functionalizedpolypropylene: synthesis, characterization, and adhesion properties, Macro-

molecules 34 (2001) 7672–7679.

[6] J Hopkins, R.D. Boyd, J.P.S. Badyal, Plasma fluorination versus oxygenation ofpolypropylene, J. Phys. Chem. 100 (1996) 6755–6759.

[7] M.-J. Wang, Y.-I. Chang, F. Poncin-Epaillard, Acid and basic functionalities ofnitrogen and carbon dioxide plasma-treated polystyrene, Surf. Interface Anal.37 (2005) 348–355.

ysicoc

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

X. Hua et al. / Colloids and Surfaces A: Ph

[8] T. Desmet, R. Morent, N. De Geyter, C. Leys, E. Schacht, P. Dubruel, Nonthermalplasma technology as a versatile strategy for polymeric biomaterials surfacemodification: A review, Biomacromolecules 10 (9) (2009) 2351–2378.

[9] S.C. Choi, W.K. Choi, H.J. Jung, B.C. Chung, Y.S. Yoo, S.K. Koh, Relation betweenhydrophilicity and cell culturing on polystyrene petri dish modified by ion-assisted reaction, J. Appl. Polym. Sci. 73 (1999) 41–46.

10] J. Meichsner, In situ characterisation of polymer surfaces and thin organic filmsin plasma processing, Contrib. Plasma Phys. 39 (1999) 427–439.

11] J. Lub, F.C.B.M. van Vroonhoven, E. Bruninx, A. Benninghoven, Interaction ofnitrogen and ammonia plasmas with polystyrene and polycarbonate studiedby X-ray photoelectron spectroscopy, neutron activation analysis and staticsecondary ion mass spectrometry, Polymer 30 (1989) 40–44.

12] N Shahidazadeh-Ahmadi, M.M. Chehimi, F. Arefi-Khonsari, N. Foulon-Belkacemi, J. Amouroux, M. Delamar, A physicochemical study of oxygenplasma-modified polypropylene, Colloids Surf. A: Physicochem. Eng. Asp. 105(1995) 277–289.

13] R.D. Tayrac, S. Chentouf, H. Garreau, C. Braud, I. Guiraud, P. Boudeville, M. Vert,In vitro degradation and in vivo biocompatibility of poly(lactic acid) mesh forsoft tissue reinforcement in vaginal surgery, J. Biomed. Mater. Res. B 85 (2)(2008) 528–536.

14] P. Makela, T. Pohjonen, P. Tormala, T. Waris, N. Ashammakhi, Strength retentionproperties of self-reinforced poly l-lactide (SR-PLLA) sutures compared withpolyglyconate (Maxon®) and polydioxanone (PDS) sutures. An in vitro study,

Biomaterials 23 (2002) 2587–2592.

15] E. Chiellini, R. Solaro, Biodegradable polymeric materials, Adv. Mater. 8 (1996)305–313.

16] S.M. Li, M. Vert, Morphological changes resulting from the hydrolytic degrada-tion of stereocopolymers derived from l- and dl-lactides, Macromolecules 27(1994) 3107–3110.

[

[

hem. Eng. Aspects 369 (2010) 128–135 135

17] Z. Lei, Y. Bai, S. Wang, Synthesis of high molecular weight polylactic acid fromaqueous lactic acid co-catalyzed by tin(II)chloride dihydrate and succinic anhy-dride, Chin. Sci. Bull. 50 (20) (2005) 2390–2392.

18] S.I. Woo, B.O. Kim, H.S. Jun, H.N. Chang, Polymerization of aqueous lactic acidto prepare high molecular weight poly(lactic acid) by chain-extending withhexamethylene diisocyanate, Polym. Bull. 35 (1995) 415–421.

19] K. Hiltunen, M. Härkönen, J.V. Seppälä, T. Väänänen, Synthesis and character-ization of lactic acid based telechelic prepolymers, Macromolecules 29 (1996)8677–8682.

20] I. Rashkov, N. Manolova, S.M. Li, J.L. Espartero, M. Vert, Synthesis, characteri-zation, and hydrolytic degradation of PLA/PEO/PLA triblock copolymers withshort poly(l-lactic acid) chains, Macromolecules 29 (1996) 50–56.

21] S.M Li, I. Rashkov, J.L. Espartero, N. Manolova, M. Vert, Synthesis, characteri-zation, and hydrolytic degradation of PLA/PEO/PLA triblock copolymers withlong poly(l-lactic acid) blocks, Macromolecules 29 (1996) 57–62.

22] A. Borissova, S. Khan, T. Mahmud, K.J. Roberts, J. Andrews, P. Dallin, Z.-P. Chen, J.Morris, In situ measurement of solution concentration during the batch coolingcrystallization of l-glutamic acid using ATR-FTIR spectroscopy coupled withchemometrics, Cryst. Growth Des. 9 (2009) 692–706.

23] I.S. Choi, R. Langer, Surface-initiated polymerization of l-lactide: coating ofsolid substrates with a biodegradable polymer, Macromolecules 34 (2001)5361–5363.

24] T.N. Paragkumar, D. Edith, S. Jean-Luc, Surface characteristics of PLA and PLGA

films, Appl. Surf. Sci. 253 (2006) 2758–2764.

25] S.L. Ishaug-Riley, L.E. Okun, G. Prado, M.A. Applegate, A. Ratclie, Human articularchondrocyte adhesion and proliferation on synthetic biodegradable polymerfilms, Biomaterials 20 (1999) 2245–2256.

26] J. Xi, L. Jiang, Biomimic superhydrophobic surface with high adhesive forces,Ind. Eng. Chem. Res. 47 (2008) 6354–6357.