r-

ungy, S

s anHoendbyA ptterPEG

en comd increile, medriendlyHowepoor toion pronding wmprovelends a

PLA/poly (butylenes adipate-co-terephthalate) blends due to their

PEG segment in PEGCA. In the present study, PEGCA was synthesized

round-bottom ask. The ask was tted with inlet and outlet

Materials Letters 71 (2012) 6365

Contents lists available at SciVerse ScienceDirect

Materials

e lspoor miscibility [6]. Plasticization is an economical and easy methodof modifying PLA. Polyethylene glycol (PEG) is an excellent plasticizerfor PLA because of its good miscibility [7]. The elongation at break ofPLA increases in the presence of PEG, but the impact resistance re-mains poor. The miscible blends lack dispersed particles that act asstress concentration points in the matrix which initiate crazing andshear banding, thus absorb energy and hinder crack growth under a

adapters, and was immersed in a silicone bath, preheated and xedat 130 C. After the melting process, the mixture was allowed toreact by stirring under nitrogen for 10 h, and under reduced pressureof 10 kPa for another 50 h. Subsequently, a viscous, transparent, oil-like liquid was obtained. The resulting polymer was poured into aglass ask, which was cooled to room temperature. The sampleswere dried by lyophilization before use.strong external shock [8].Citric acid is a bio-based, inexpensive, no

tional monomer, isolated from citrus frubranched polyester, poly (PEG-co-citric acid

Corresponding author. Tel.: +86 21 64776909; fax:E-mail address: chshj2003@hotmail.com (S. Cheng).

0167-577X/$ see front matter 2011 Elsevier B.V. Aldoi:10.1016/j.matlet.2011.12.045mpatibilization is neces-the impact resistance of

PEGCA was synthesized by a direct esterication reaction. Equi-molar amounts of PEG and citric acid were placed in a four-neck,poor mechanical properties. For example, cosary to enhance the elongation at break andmers such as starch. PLA has attracteindustries, such as the packaging, textdustries. PLA is a very promising, eco-fal petroleum-based polymers [1,2].drawbacks of inherent brittleness andits wide application. Hence, modicathave drawn great attention [35]. Blethe methodmost extensively used to ities of PLA. However, many of these basing interest in variousical, and automotive in-alternative to tradition-ver, PLA also has theughness, which impedecesses that toughen PLAith exible polymers isthe mechanical proper-re immiscible and have

also prepared using an internal mixer. The objective was to investi-gate the mechanical properties of the PLA/PEGCA blends in compari-son with PLA/PEG blends.

2. Experimental

PEG (Mw=1000 g/mol) and citric acid were obtained from theChina Chemical Reagent, Co. PLA (4032D, density=1.25 g/cm3) waspurchased from Natureworks LLC (USA).the advantages of high modulus, htransparency, and biocompatibility, whength, thermoplasticity,pared to other biopoly-

by a condensation reaction between PEG and citric acid, without acatalyst. PLA and PEGCA blends with different compositions werePolylactide (PLA) is a biodegradable and bio-based polymer. It hasigh strNovel polyethylene glycol-based polyeste

Zongyan Gui, Yuanyuan Xu, Yun Gao, Chong Lu, ShujSchool of Materials Science and Engineering, East China University of Science and Technolo

a b s t r a c ta r t i c l e i n f o

Article history:Received 24 October 2011Accepted 9 December 2011Available online 14 December 2011

Keywords:BiomaterialsPolymersPolylactideBlendToughen

Polyethylene glycol (PEG) ielongation at break of PLA.PLA was toughened by blacid) (PEGCA), synthesizedpartially miscible. The PEGCw) blend showed much beWith the addition of 15%reached 103 J/m.

1. Introduction

j ourna l homepage: www.n-toxic, and multifunc-its. The novel highly) (PEGCA) is based on

+86 21 64776549.

l rights reserved.toughened polylactide

Cheng hanghai 200237, China

excellent plasticizer for polylactide (PLA) and can signicantly enhance thewever, the impact resistance of PLA/PEG blend is very limited. In this study,ing with a novel PEG-based polyester, poly (polyethylene glycol-co-citrica condensation reaction between PEG and citric acid. PLA and PEGCA werehase was dispersed in the continuous PLA phase. The PLA/PEGCA (85/15 w/mechanical properties in comparison to the PLA/PEG (85/15 w/w) blend.CA, the elongation at break reached 242%, whereas the impact resistance

2011 Elsevier B.V. All rights reserved.

citric acid and PEG. PEGCA can potentially improve the exibility ofPLA. PLA and PEGCA are supposed to be highly compatible given the

Letters

ev ie r .com/ locate /mat le tBefore blending, PLA was dried in a vacuum at 80 C for 8 h. NeatPLA, PLA/PEGCA blend (85/15 w/w) and PLA/PEG blend (85/15 w/w)were prepared by using a KCCK XSS-300 rheometer equipped witha 60 ml mixing chamber (KCCK, China) at 180 C for 6 min. Afterblending, the samples were hot-press molded into a 1 mm-thickplate, followed by quenching to room temperature. The pressing tem-perature was 180 C, and the pressing time was 2 min, with a pres-sure of 10 MPa.

Differential scanning calorimetry (DSC) measurements were per-formed with a modulated DSC2910 (TA Instruments, USA) in a nitro-gen atmosphere. The sample weights were approximately 8 mg.Samples were rst heated from room temperature to 200 C, andthen held for 3 min to eliminate the thermal history. They werethen cooled to 50 C at 10 C/min. After holding for another3 min, the samples were reheated to 200 C at 10 C/min. Only thesecond heating curves were measured.

The fracture surfaces were studied under a JSM-6360LV (JEOL,Japan) scanning electron microscopy (SEM) instrument. The sampleswere submerged in liquid nitrogen and broken down. The fracturesurfaces were studied by SEM after all surfaces were sputtered withgold.

The tensile testing was carried out using a universal testing ma-chine (CMT6104, Sans, China) at a crosshead speed of 10 mm/minwith a gauge length of 25 mm. Notched Izod impact tests were car-ried out according to ASTM D256A procedures using a standard im-pact tester (JJ-20, China). All samples were stored under ambient

between the PLA molecular chain and the PEG segment of PEGCA.Some PEGCA molecules were soluble in the PLA matrix. The shiftedglass transition temperature represented the improved segmentalmobility of the PLA segment.

The tensile and impact properties of neat PLA, PLA/PEGCA blend aswell as PLA/PEG blend are shown in Fig. 4. Neat PLA was typicallyrigid and brittle. It had high tensile modulus and strength, as well asvery limited elongation at break and impact resistance. After the ad-dition of PEGCA or PEG, the tensile strength and modulus of theblend dropped, whereas the elongation at break signicantly in-creased. PEG had no effect on the toughness of PLA. However, thePLA/PEGCA blend shows signicantly higher impact resistance than

Fig. 2. SEM images of (a) neat PLA and (b) PLA/PEGCA (85/15 w/w) blend.

64 Z. Gui et al. / Materials Letters 71 (2012) 6365conditions (23 C and 50% relative humidity) for 48 h before testing.Each sample included ve to seven tested replicates, to obtain a reli-able mean and standard deviation.

3. Results and discussion

Fig. 1 shows the 1H NMR spectrum of citric acid, PEG and PEGCA.In PEGCA spectra, the peaks around 2.93.1 ppm were assigned tothe underlined protons in\CH2\ from citric acid, which were shiftedtowards higher values in comparison to the NMR spectrum of purecitric acid (peaks around 2.72.9 ppm). The peaks around 4.3 ppmwere attributed to the protons signal of esteried\COOCH2\ groupsin PEG segments. All these peaks indicated the formation of esterbonds [9,10].

Fig. 2 shows the SEM images of neat PLA and PLA/PEGCA blend.The blend sample exhibited a phase-separated morphology. ThePEGCA phase was spherical or elliptical, and was dispersed in the con-tinuous PLA phase. This nding meant that PLA and PEGCA were notmiscible.

Fig. 3 shows the DSC heating curves of neat PLA and PLA/PEGCAblend. The neat PLA curve showed three transitions upon heating: aPLA glass transition, a cold-crystallization exothermic peak, and amelting endothermic peak. The blend curve showed an additionalmelting endothermic peak around 30 C, regarded as the meltingpeak of PEGCA. The PLA/PEGCA blend exhibited a lower glass transi-tion temperature of 51.5 C than neat PLA of 61.0 C. The DSC andSEM data indicated that PLA and PEGCA were partially miscible, andthat some molecular interactions existed between the two compo-nents. The interaction may have originated from the miscibility

1Fig. 1. H NMR spectra of citric acid, PEG and PEGCA. Fig. 3. DSC curves of neat PLA (P0) and PLA/PEGCA (85/15 w/w) blend (P15).

EGC

65Z. Gui et al. / Materials Letters 71 (2012) 6365the other samples due to the dispersed phases in the matrix of PLA[8]. Therefore, the PLA was successfully toughened after the additionof PEGCA.

4. Conclusions

Fig. 4. Tensile and impact properties of neat PLA, PLA/PPEGCA can toughen PLA and make it exible. PEGCA was partiallymiscible in PLA, as shown by SEM and DSC analyses. Glass transitiontemperature of PLA decreased with the addition of PEGCA. The PLA/PEGCA (85/15 w/w) blend showed much improved mechanical prop-erties when compared to the PLA/PEG (85/15 w/w) blend. With theaddition of 15% PEGCA, the elongation at break reached 242%, where-as the impact resistance reached 103 J/m.

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Novel polyethylene glycol-based polyester-toughened polylactide1. Introduction2. Experimental3. Results and discussion4. ConclusionsReferences