transparent and ductile poly(lactic acid)/poly(butyl...

European Polymer Journal 48 (2012) 127–135

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European Polymer Journal

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Transparent and ductile poly(lactic acid)/poly(butyl acrylate) (PBA)blends: Structure and properties

Bing Meng, Jingjing Deng, Qing Liu, Zhihua Wu ⇑, Wei YangCollege of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China

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

Article history:Received 10 August 2011Received in revised form 16 October 2011Accepted 18 October 2011Available online 25 October 2011

Keywords:PolylactideRubber tougheningTransparencyPoly(butyl acrylate)

0014-3057/$ - see front matter � 2011 Elsevier Ltddoi:10.1016/j.eurpolymj.2011.10.009

⇑ Corresponding author. Tel.: +86 28 85461716; faE-mail address: [email protected] (Z. Wu).

Poly(butyl acrylate) was prepared by the free radical polymerization of butyl acrylate as aninitiator in the presence of 2,20-Azoisobu-tyronitrile (AIBN) and the average molecularweight, polydispersity and thermal stability were evaluated. PLA and PBA were meltblended using a Haake Rheometer, and the light transmission, thermal properties, dynamicrheological properties, mechanical properties, phase morphology of blends and tougheningmechanism were investigated. Dynamic rheology, SEM and DSC results show that the PLAis partial miscible with PBA. The PBA component improved the crystallization ability of PLAand the crystallinity of PLA increased with content of PBA (

128 B. Meng et al. / European Polymer Journal 48 (2012) 127–135

plasticizer has a tendency to migrate and decrease theglass transition temperature (Tg) significantly. For exam-ple, PLA blend containing 15 wt.% acetyl triethyl citrate(ATC) resulted in a material with a Tg well below roomtemperature [14]. Consequently, the ductile propertiesdeteriorate during storage, and the use temperature is fur-ther limited.

Alternative approaches, more economic and practical,to improve ductile properties is blending PLA with otherflexible polymers that are known to undergo plastic flowin tension rather than exhibit crazing. The most widelypreferred route, however, is the incorporation of a substan-tial volume fraction (typically 10%) of an elastomeric com-ponent in the form of a block or graft copolymer [15]. PLAhas been blended with a variety of biodegradable andbiocompatible rubbery polymers such as poly(butyleneadipate-co-terephthalate) [16], poly(ether-urethane) elas-tomer, [11,17,18] poly(ethylene glycol), polyhydroxyalk-anoate [19], and poly(butylene succinate).[20] Theserubbery polymers are relatively expensive, which increasecosts of the resulting PLA blends. To reduce the costand expand its commercial applications, improving thetoughness of PLA by direct mechanical blending withinexpensive non-degradable polymers such as poly(ethyl-ene oxide), poly(vinyl acetate), polyisoprene, elastomericethylene-butyl acrylate-glycidyl methacrylate terpolymer(EBA-GMA) [12,21], and ethylene/acrylate copolymer(EAC)[22] has also been explored. In many applicationsespecially in packaging, the transparency of the materialis necessary. However, to the best of our knowledge, thereare no published results on transparent and ductile PLAblend, probably due to their poor transparency comparedto the pristine polymer. Blending polymers with differentrefractive indexes could sacrifice the transparency of theresulted material. So, the majority of these PLA blendsstudied by other researchers are opaque due to the differ-ence of the refractive index between components.

Therefore, it is very important to find a cost-effectivetoughening agent that can effectively improves the tough-ness of PLA at low concentrations to maintain its biode-gradability, and maintains the essential transparency ofPLA. Poly(butyl acrylate) (PBA) is a very ductile (�2000%elongation at break), sticky, colorless and transparent rub-bery material. The refractive index of the PBA chosen inthis study is 1.474, very close to that of the PLA matrix(1.45) [23]. In view of their complementary properties,blending PBA with PLA is a natural choice to improve prop-erties including ductility of PLA without compromising itstransparency.

The objective of the present study was to explore thepossibility of toughening PLA by PBA and to obtain trans-parent blend with improved flexibility for potential usein packaging material. The PBA used was synthesizedthrough a free radical polymerization reaction. The misci-bility, phase morphology, mechanical properties and rheo-logical properties of the PLA/PBA blends were evaluated.The tensile toughness of the blends was probed, and adirect correlation between the morphology and toughnesswas revealed.

2. Materials and methods

2.1. Materials

Butyl acrylate (BA, 128 g/mol) and 2, 20-Azoisobu-tyronitrile (AIBN) were purchased from Aladdin-ReagentChemical Factory. Ethanol, acetone and calcium hydride(CaH2) used in this study were supplied by Yingda Sparse-ness and Noble Reagent chemical factory (Tianjin, China).Polylactide (PLA) resin (2002D) was purchased fromNature-works Company.

2.2. Synthesis of poly(butyl acrylate) (PBA)

2,20-Azoisobu- tyronitrile (AIBN) was recrystallizedtwice from ethanol before use. BA was dried over CaH2and distilled under vacuum prior to use. BA (1 mol,128 g) and AIBN (1.28 g) were dissolved in 100 mL of eth-anol, and then added in a dry three-necked glass flask,equipped with a stirrer and heating bath. Thereafter, themixture was stirred; the reaction temperature was in-creased to 80 �C at 1 �C/min under N2 atmosphere. After3 h reaction under stirring, the mixture was cooled to roomtemperature and poured into 1 L deionized water and stir-red; then the product was precipitated. After filtration, itwas washed by 10% alcohol for 5 times to remove the unre-acted reagents. The product was dried under vacuum at80 �C for 3 days.

2.3. Preparation of PLA blends

PLA was dried under vacuum at 40 �C for 24 h to re-move water before blending. PLA/PBA blends containingdifferent PBA contents (0, 5, 8, 11 and 15 wt.%) were pre-pared in the mixing chamber of a Haake Rheometer XSS-300 at 175 �C, and a mixing rotation of 50 rpm for 7 min.In the following part of this paper, the blends are denotedas BA followed by the mass fraction of the PBA (wt.%). Forexample, BA-5 indicated PLA/PBA blend with 5 wt.% PBA.The blends were preheated at 175 �C for 3 min and thenhot pressed into 1 mm thickness sheets for 2 min using acompression molding machine.

2.4. Characterization

Thermogravimetric analysis (TGA) was performed witha WRT-2P thermogravimetric analyzer from ambient tem-perature to 600 �C at a heating rate of 10 �C/min, under asteady flow of nitrogen of 100 mL/min.

The light transmittance of the neat PLA and PLA/PBAsheets were measured with a UV–vis spectrophotometerinstrument (721, Shanghai Jinhua Group Co., Ltd., China)in the range of 500–900 nm at room temperature. Thesheets used for characterization were prepared by meltingand then quick quenching into a thickness of 0.5 ±0.02 mm.

Thermal analysis was carried out on a differential scan-ning calorimeter (DSC) Instruments (DSC-204F1, Netzsch,Germany) under nitrogen. Both temperature and heat fiow

0 100 200 300 400 500 600

0

20

40

60

80

100

PLA PBA

Temp

Wei

ght (

%)

Fig. 1. Comparison of thermostability between PBA and PLA.

Table 1Characteristic temperatures of thermal degradation of PBA and PLA.

Sample Tone(�C)a T0.1(�C)b T0.5 (�C)c

PLA 358.3 373.8 409.5PBA 348.8 365.6 427.1

a Onset degradation temperature.b Temperature at 10% mass loss.c Temperature at 50% mass loss.

B. Meng et al. / European Polymer Journal 48 (2012) 127–135 129

were calibrated with indium. About 5 mg samples takenfrom the tensile samples were heated to 180 �C at a heat-ing rate of 10 �C /min. The crystallinity of PLA (vC) wasestimated using the following equation:

vc ¼DHm � DHcwf � DH0m

� 100%

Where DHm and DHc are the enthalpies of melting andcold crystallization during the heating, respectively; DH0mis the enthalpy for 100% crystalline PLA homopolymers(93.7 J/g),[24] and wf is the weight fraction of PLA compo-nent in the blend.

The morphologies of blends were examined by aJSM-5900LV scanning electron microscopy (SEM, JEOLLtd., Akishima, Japan) at an accelerating voltage of 20 kV.All the samples were fractured after immersion in liquidnitrogen for about 10 min. The fracture surface was thencoated with a thin layer of gold prior to observation. Differ-ent zones of the specimens after tensile tests were alsoexamined.

Mechanical properties of samples were measured withan Instron universal tester (model 4302), and the measure-ment was conducted at 25 �C with a cross-head speed of20 mm/min in accordance with ISO527 (type 5). Five repli-cates were tested for each sample and the average valueresults were reported here.

Dynamic rheological properties of the PLA and PLA/PBAblends were assessed using a strain-controlled rheometer(AR 2000EX, TA Instruments, USA). Samples were testedusing a parallel-plate geometry (d = 25 mm) operated at190 �C. A dynamic frequency sweep test was performedto determine the rheological properties of the blends. Thestrain and frequency range used during testing were 5%and 80–0.01 Hz, respectively.

Fig. 2. Complex viscosity curves of PLA and the blends with differentweight fractions of PBA at 190 �C.

3. Results and discussion

3.1. Characterization of PBA

The average molecular weights and polydispersity ofthe synthesized PBA were measured by gel permeationchromatography (GPC) on an Agilent 1100 series liquidchromatography equipped with a refractive index detector(RID A). Tetrahydrofuran was used as the mobile phase at aflow rate of 1.0 ml/min, and the column calibration wasperformed with polystyrene standards. The result showsthat the average weight and polydispersity of poly(butylacrylate) are 13.16 kg/mol and 4.202, respectively.

To evaluate the thermostability of PBA in the blendingprocess with PLA, the thermogravimetric analysis of PLAand PBA was performed (Fig. 1). The TG characteristic tem-peratures of PLA and PBA are included in Table 1. It is evi-dent that the thermal degradation of virgin PLA and PBATstarted at 358.3 and 348.8 �C, respectively, and the charac-teristic temperatures of T0.1 is 373.8 and 409.5 �C as shownin Table 1. As shown in Fig. 1, the TGA curves of PBA sam-ple gives a slower decomposition velocity compared tothat of PLA. Clearly, the thermostability of PBA matchesup to PLA during processing.

3.2. Rheological properties

Dynamic melt rheological properties of neat PLA andPLA/PBA blends are measured to elucidate the interactionbetween both phases and the melt processibility of theblends. The complex viscosities, g⁄, of pure PLA and blendsas a function of frequency, x, are shown in Fig. 2. It can beseen that plain PLA exhibits typical Newtonian behavior inthe low-frequency region and slight shear thinning fromabout 10 rad/s, which was a characteristic for linear poly-mers [25]. Compared with PLA melt, the complex viscosi-ties of PLA/PBA melts show a shear-thinning tendency at

Fig. 4. G0 (a) and G00 (b) versus frequency for PLA and blends withdifferent weight fractions of PBA at 190 �C.


lower frequencies and the tendency became stronger withPBA content. The dynamic complex viscosities of blendsare substantially lower than that of neat PLA and decreasewith PBA content (8%) of the blend.

The variation of the storage modulus, G0, and loss mod-ulus, G00, with small-amplitude oscillatory frequency forneat PLA and the blends is given in Fig. 4. A homopolymerwith narrow molecular weight distribution usually showsa characteristic terminal behavior of G0(x) / x2 [31]. Pure

Fig. 3. Cole–Cole plot of the PLA/PBA blends at 190 �C.

PLA deviates from the terminal behavior markedly (G0(x)/ x1.58). The values are in consistency with the results re-ported in literatures [16,32]. The G0 of PLA/PBA blendsslightly deviates from the terminal behavior, from G0(x))/ x1.51 for the blend with 5 wt.% PBA to G0(x)) / x1.2 forthe blend with 15% PBA. Molecular weight distributionstrongly affects the shape of the terminal region. Chainsof different sizes relax to equilibrium at different times,which ‘‘smears out’’ the typical relaxation behavior of amonosized polymer melt. The relatively broad molecularweight distribution of PBA (PI = 4.202) likely accounts forthe different relaxation behavior.[33] On the other hand,the partially miscible characteris of blends might be theother reason for the deviation.[34] The incorporation ofPBA affected the storage moduli and loss moduli of themelts at all frequencies; they decreased with the increaseof PBA content as compared with pure PLA, except thatthe G0 of BA-15 increased at low frequencies. For the blendwith 15 wt.% PBA, the lower slope values and higher abso-lute values of dynamic modulus indicate the formation ofentanglement structures in PLA/PBA melts [16]. It is knownthat molecular chain PBA is more flexible than that of PLAand is easier to entangle. The entanglement density washigher than that of PLA, leading to high reversible elasticdeformation (G0) of the BA-15 melts [16].

Table 2DSC results of PLA and PLA/PBA blends.

sample Tg (�C) a Tcc (�C) b Tm (�C) c Tcc - Tg (�C) vc (%)d

PLA 62.4 122.5 151.1 60.1 3.9BA-5 61.5 117.9 152.1 56.4 5.6BA-8 58.3 113.4 150.5 55.1 9.2BA-11 58.1 111.8 149.4 53.7 6.2BA-15 58 111.7 151.3 53.7 5.7

a Glass transition temperature.b Cold crystallization peak temperature.c Melt peak temperature.d Degree of crystallinity of PLA (vc) was determined from the first

heating curves.


3.3. Thermal properties

Neat PLA and the blends exhibit very similar thermo-grams, all showing cold crystallization of PLA and subse-quent melting of PLA crystals. DSC results of PLA andblends are shown in Table 2, including the degree of crys-tallinity. Because the crystalline state of the PLA moldedsamples finally influences the mechanical properties ofthe blends, only the DSC data from the first heating scanare presented.

From Table 2, the difference in melting temperature (Tm)between neat PLA and PLA in the blends is not significantand seems to be independent of loading of PBA. The glasstransition temperature (Tg) slightly decreases with theincreasing of PBA. It also indicates that PBA is partially mis-cible with PLA [35], which well accords with the SEM obser-vation. However, the cold crystallization peak temperature(Tcc) and characteristic temperature DT ¼ Tcc � Tg of PLA inblends is slightly decreased with the increase of PBA con-tent. Such decrease in Tg and enhanced cold crystallization

a b

c d

Fig. 5. Cryo-fractured SEM images of blends: (a

process are commonly observed for plasticized PLA systemsand are due to the increased segmental mobility of the PLAchains by plasticization [36,37]. The decrease of characteris-tic temperature (DT) suggests that the blend shows higherkinetic crystallizability and crystallization rate than purePLA [38]. As shown in Table 2, the crystallinity of the PLAmatrix increases with increasing content of PBA except forBA-15 sample. It is well known that for polymer blends con-taining a semi-crystalline component, the variations in thevalues of vc are usually due to the interactions betweencomponents. In fact, the interface of the phase-separateddomains may provide favorable nucleation sites for crystal-lization and, therefore, the crystallization rate of PLA pro-motes by a lower nucleation barrier than that of pure PLAmelt.[30] Therefore, the enhanced crystallization rate ofPLA is resulted from the increase in the nucleation rate.

3.4. Morphology

Since the mechanical properties of multiphase polymerblends depend largely on the phase morphologies, SEMwas employed to identify the phase structure of the binaryblends. Fig. 5 shows cryo-fracture surface SEM micro-graphs of various PLA-based blends. As observed fromFig. 5, the round particles indicated the PBA phases distrib-uted within the PLA matrix. As shown in Fig. 5 (a), the ovalparticles were well dispersed in PLA matrix for the blendcontaining 5 wt.% PBA. With increasing content of PBA,the amount of the big rubber particles increases. It canbe seen that the interface between PBA and PLA phases isindistinct (Fig. 5). This indicates that in the present blendsa relatively strong interaction between PBA and PLA matrixexists. The interaction is thought to be a hydrogen bondamong polar groups like –C'O of PBA and PLA [2,4].

) BA-5, (b) BA-8, (c) BA-11 and (d) BA-15.

Fig. 6. Stress–strain curves of composites; curves were shifted along thestrain axis (increments of 10%) for clarity.


3.5. Mechanical properties and toughening

The stress–strain curves of neat PLA and the blends areshown in Fig. 6, and the details of the tensile properties aregiven in Table 3. From Table 3, it can be found that the frac-ture behavior of the specimens in the tensile tests changesfrom brittleness of neat PLA to ductile fracture of binaryblends. It is also demonstrated in the stress–strain curvesas shown in Fig. 6. Neat PLA deforms in brittle fashionand is very rigid and brittle. On the other hand, the binaryblends show an initial strain softening after yielding andthen undergo considerable cold drawing. The stress–straincurve after the yield point shows a combination of strainsoftening and cold drawing. In this region, there is compe-tition between the orientation of PLA chains and crack for-mation. Hence, there is a drop in stress with increasingstrain. After about 10–15% of strain, cold drawing domi-nates at a constant stress. This suggests that large energydissipation occurs owing to the presence of PBA in the bin-ary systems.

The elongation at break of PLA blends is improved dra-matically with the increase of PBA content. The elongationat break of the blends increases from 4% for neat PLA to31% for the blend containing 5 wt.% PBA, and reaches themaximum of 174.52% for the blend containing 15 wt.%PBA. The tensile toughness also increases with PBA contentdramatically from 2.13 MJ/m3 for neat PLA to 47.02 MJ/m3

for the PLA/PBA blend with 15 wt.% PBA. This is quite anunusual result, as PLA/PBA blend maintains a quite hightensile modulus value of 1.33 GPa even with 15 wt.% PBA.

Table 3Mechanical properties of blends and PLA.

Sample PBA content Tensile strength Elonga(wt.%) (MPa) (%)

PLA – 68 4.52BA-5 5 51.77 31.52BA-8 8 44.79 74.62BA-11 11. 40.82 173.98BA-15 15 41.01 174.52

a Calculated as the area under stress–strain curve.

The tensile strength of the binary blends decrease slowlywith the continuous increase of PBA content (Table 3).The tensile strength is still 41.01 MPa for the blend con-taining 15 wt.% rubber, which is still greater than that ofgeneral petroleum based package polymer, such as PEand PP. The result is very significant in obtaining a bio-based transparent material with remarkable stiffness-toughness balance.

To elucidate the reason why PBA toughens PLA, themorphologies of the fracture surface and the skin of thestretched samples were observed by SEM, and the micro-graphs of the samples are shown in Fig. 7. The presenceof the rubber domains alters the deformation mechanism.As shown in Fig. 7(a–d), there are many voids in the PLAmatrix that occurred in uniaxial extension. The micro-graphs of fractured surface of the blend show a ductilefracture which is evident from the presence of more andlonger fibrils pulled out during the test. For the blend with5 wt.% PBA, it can be seen that a few fibrils are formed dur-ing crazing (Fig. 7e). For the blend with PBA more than5 wt.%, the fracture surfaces show more features becauseof larger plastic deformation caused by more crazes withincreasing rubber contents. On the fracture surface ofblends numerous fibrils are drawn out of the polymer, withcavities residing among them, as shown in Fig. 7(e–h). TheSEM observation is well consistent with the results ofmechanical properties.

Plastic deformation in rubber-toughened glassy poly-mers can occur through several mechanisms includingcrazing, cavitation of the rubber phase, and shear yielding.Inelastic deformation occurs via the phenomenon of shearyielding, multiple crazing, and cavitation in the softer dis-persed phase. Dispersed rubber phase can provide controlover nucleation and growth of crazes and increase crazeplasticity in glassy polymers leading to the decrease inbrittleness. The true stress acting on the craze fibrils is in-versely proportional to the volume fraction of the craze[39]. So increasing the volume fraction of the craze can re-duce the true stress on an individual craze. Thus, if thesmall size of the rubber domains in the PLA matrix causesthe critical stress for cavitation to be greater than that of analternative deformation mechanism (e.g., matrix crazing),the matrix phase may yield prior to cavitation. [40] Conse-quently, the phenomenon of multiple crazing was likelythe possible energy dissipation mechanism.

3.6. Transparency of PLA blends

An effective way to judge the miscibility is the opticalclarity of the blend; immiscible blends usually are cloudy

tion at break Tensile modulus Tensile toughnessa

(GPa) (MJ/m3)

3.51 2.131.54 3.71.49 17.01.44 41.741.33 47.02

Skin of stretched sample Fracture surface

a e

b

c

d h

g

f

Fig. 7. SEM images of tensile blends: a) and e) BA-5, b) and f) BA-8, c) and g) BA-11, d) and h) BA -15.


or milk white in appearance because of the differentrefractive indices of the components. In Fig. 8(a–d) showthe photos of PLA and PLA blends. PLA sheet is completelytransparent and the blends maintain high transparency.However, the blends containing PBA show a reduction ofoptical clarity compared to pure PLA with increasing rub-ber content. In spite of this, the blends still show very hightransparency even at high loading of PBA, which meets therequirements of transparent package material.

To further study the effect of the PBA rubber on thetransparency of the binary, the visible light transmittanceof PLA and PLA/PBA were measured by a UV–vis spectro-photometer instrument (Fig. 9). It is obvious that the trans-mittance decreases with increasing content of PBA, whichis in consistent with the optical observation. But the trans-mittance of the blends is still above 75% in the range of500–900 nm, also indicating that the blends are miscibleor semi-miscible [41].

a b

c d

Fig. 8. Photographs of the PLA and PLA/PBA blends sheets (1.5 mmthickness) on a graphic pattern: (a) PLA; (b) BA-5; (c) BA-8; (d) BA-11.

Fig. 9. Visible transmission spectra of PBA/PLA blends.


4. Conclusions

Poly(butyl acrylate) rubber was successfully preparedby the free radical polymerization of butyl acrylate in thepresence of 2,20-Azoisobu- tyronitrile (AIBN); and the aver-age molecular weight, polydispersity and thermal stabilitywere evaluated. Then, PLA/PBA binary blends wereprepared by melt blending to investigate the effects ofrubber loadings on the optical transparency, thermal prop-erties, mechanical properties, phase morphology anddynamical rheological properties of the blends. Dynamicrheology, SEM and DSC results show that the PLA wassemi-miscible with PBA. Rheological results reveal thecomplex viscosity and melt elasticity of the blends de-crease with the concentration of PBA. In addition to tough-ening PLA, PBA increases the processability of PLA byacting as a plasticizer. Phase segregation occurred at PBA

loading above 11 wt.%. DSC results showed the additionof PBA accelerates the crystallization rate of PLA and thecrystallinity increased slowly with the increase of PBAcontent (


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Transparent and ductile poly(lactic acid)/poly(butyl acrylate) (PBA) blends: Structure and properties1 Introduction2 Materials and methods2.1 Materials2.2 Synthesis of poly(butyl acrylate) (PBA)2.3 Preparation of PLA blends2.4 Characterization

3 Results and discussion3.1 Characterization of PBA3.2 Rheological properties3.3 Thermal properties3.4 Morphology3.5 Mechanical properties and toughening3.6 Transparency of PLA blends

4 ConclusionsAcknowledgmentReferences

transparent and ductile poly(lactic acid)/poly(butyl...

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