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Polymer Degradation and Stability 97 (2012) 1822e1828

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Thermal, mechanical and morphological characterization of plasticized PLAePHBblends

Mohamed A. Abdelwahab a,b, Allison Flynn b, Bor-Sen Chiou c, Syed Imamc, William Orts c,Emo Chiellini a,*a Laboratory of Bioactive Polymeric Materials for Biomedical and Environmental Applications (BIOlab), UdR INSTM, Department of Chemistry & Industrial Chemistry,University of Pisa, Via Risorgimento 35, 56126 Pisa, Italyb Lapol, LLC, 55 Castilian Dr., Santa Barbara, CA 93117, USAcUSDA, Agricultural Research Services, WRRC-BCE 800 Buchanan Street Albany, CA 94710, USA

a r t i c l e i n f o

Article history:Received 23 April 2012Received in revised form25 May 2012Accepted 28 May 2012Available online 4 June 2012

Keywords:Poly(lactic acid)Poly(3-hydroxybutyrate)BlendPlasticizerThermal stabilityMechanical properties

* Corresponding author. Tel.: þ39 050 2210301; faxE-mail addresses: m_abedelwahab@ns.dcci.un

emochie@dcci.unipi.it (E. Chiellini).URL: http://syed.imam.ars.usda.gov

0141-3910/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2012.05.0

a b s t r a c t

A blend of poly(lactic acid) (PLA) (75% by weight) and poly(3-hydroxybutyrate) (PHB) (25% by weight)with a polyester plasticizer (Lapol 108) at two different concentrations (5 and 7% by weight per 100 partsof the blends) were investigated by TGA, DSC, XRD, SEM, mechanical testing and biodegradation studies.PLA/PHB blends showed a good distribution of the major components and absence of phase separation.XRD showed that the original crystal structure of PHB in the PLA75/PHB25 blend had been disturbed.DSC curves of PLA or PHB with plasticizer exhibited one Tg value, indicating that both major blendcomponents are miscible. The Tg values also decreased with increased amount of plasticizer and showedgood correlation to the Fox Equation, The melting temperature of PLA and PHB blends mostly did notchange with an increase in plasticizer content, and the thermal stability of PLA and PHB was not affected.Also, the elongation at break of the PLA/PHB blend was greatly improved with the addition of plasticizer.In addition, in preliminary biodegradation studies carried in natural compost neat PHB showed somebiodegradation, whereas the samples containing PLA did not experience a substantial biodegradation.This last aspect is worthy of further investigation in a more comprehensive and detailed approach.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, sustainability, environmental concerns andgreen chemistry have played a big role in guiding the developmentof the next generation of materials, products and processes. Thepersistence of plastics in the environment, dwindling petroleumresources, shortage of landfill space and the concerns over emis-sions of toxic gases during incineration have fuelled efforts todevelop biodegradable polymers from renewable resources.Particularly, renewable agricultural and biomass feedstock haveshown much promise for use in eco-efficient packaging to replacepetroleum feedstock without competing with food crops [1].

Poly(lactic acid) (PLA) is a biodegradable and biocompatiblecrystalline polymer that can be produced from renewable resourceswhich is of great interest to the packaging industry due to its

: þ39 050 2210332.ipi.it (M.A. Abdelwahab),

All rights reserved.36

availability and low cost [2e5]. The properties of PLA are dependenton the ratio between the two enantiomers D and L. PLA can showcrystalline polymorphism,which can lead to differentmelting peaks[6,7]. Poly(3-hydroxybutyrate) (PHB) is a typical biodegradablethermoplastic polymer produced by microorganisms. PHB is highlycrystalline polyester (80%) with a high melting point (173e180 �C)and a glass transition temperature (Tg) of approximately 1e5 �C[8e10].

Both PLA and PHB are biodegradable polyesters and are used inconsumer products by several industrial sectors due to theirbiocompatibility, biodegradability and sustainability. They havecomparable thermal and mechanical properties to those of someconventional polymers and this has generated much interest inexploring their physical and processing properties for potentialapplications [6,11,12].

It is noteworthy that both PLA and PHB have poor processingproperties and are brittle at room temperature [13]. Thus, severalmodifications have been proposed to improve their processing andmechanical properties, which include the preparation of copoly-mers containing monomeric units derived from hydroxyl alkanoicacids and blending these with PLA and PHB.

M.A. Abdelwahab et al. / Polymer Degradation and Stability 97 (2012) 1822e1828 1823

Blending is much easier and faster than copolymerizationmethods [14e27]. Previous studies on PLA/PHB blends showed thatthe miscibility between these two polymers depended on themolecular weight of the second component [13,17,19,28]. PHB ismiscible with lowmolecular weight PLA (Mw < 18,000) in the meltover the whole composition range, whereas the PHB blends withhigh molecular weight PLA (Mw > 18,000) show biphasic separa-tion [17,19]. Similarly, the PLA component is miscible with lowmolecular weight PHB (Mw ¼ 9400) in the melt with the PHBcontent up to 50 wt-%, and is immiscible with high molecularweight PHB and commercial-grade bacterial PHB [13,28]. PLA/PHBblends properties also depend on composition, chemical or phys-ical cross-links and processing conditions. Zhang et al. [27] inves-tigated the morphology, structure, crystallization behaviour,thermal properties, mechanical properties, and biodegradation ofPLA/PHB blends, which were prepared by usingmelt compounding.The PLA/PHB 75/25 blend showed improved mechanical propertiesand heat distortion temperature and presence of some interactions.Another way to improve blend compatibility is by using a compa-tibiliser as a third component. Such compatibilisers include glycidylmethacrylate (GMA) [29], poly(ethylene glycol) (PEG) [26] andpoly(vinyl acetate) (PVAc) [25].

PLA can be plasticized using oligomeric lactic acid (OLA), citrateester or low molecular weight additives [7,30,31]. Plasticizationincreases the chain mobility and favours PLA crystallization. Afterplasticization, the crystallinity is in the range between 20 and 30%.The drawback of using these lowmolecular weight additives is thatthey leach out of the PLA matrix, resulting in a brittle plastic overtime.

PHB can be plasticized with dioctyl phthalate, 1,4-cyclohexanedimethanol dibenzoate, tert-butylphenyl phosphate,PEG, triacetyl glycerol, tributyrin, dodecanol, lauric acid, trilaurinand low molecular weight poly(adipate) [32e34]. Grillo Fernandeset al. [35] blended PHB with polyols containing different amountsand types of hydroxyl groups. Glycerol and PEG200 have beenshown to decrease the thermal stability of PHB. In contrast, tri(-ethylene glycol) bis(2-ethylhexanoate) (TEGB) and pentaerythritolhad no effect on thermal stability of PHB. The mechanical testingshowed all samples were brittle, especially with PHB containing20wt-% of PEG. DSC results indicated that PEG200 and TEGB are themost compatible with PHB.

At the USDA, a new polyester plasticizer has been developedfrom mostly renewable resources. This plasticizer is in commer-cial development and goes by the commercial name of Lapol 108[36]. This plasticizer has high molecular weight (>80,000 Da), ishighly branched and is derived from more than 50% renewableresources.

In this paper, we studied the effects of Lapol 108 on PHB, PLAand PLA/PHB blends to create a new type of eco-friendly blendmaterial suitable for single-use applications, such as fast-foodpackaging.

2. Experimental

2.1. Materials

The PLA (4032D, NatureWorks� Co. LLC, USA) had an averagemolecular weight of 52,000 gmol�1, a polydispersity (Mw/Mn) of 1.9and a high optical purity with 98% L-lactide content. The poly(3-hydroxybutyrate) (PHB) was Biocyle� (average molecular weight(Mw) of 425 kDa and polydispersity of 2.51 supplied in kind by PHBIndustrial, SA, Serrana, SP, Brazil. Lapol 108 (Mw) of 80 kDa andpolydispersity of 26.6) was purchased from Lapol, LLC, Santa Bar-bara, CA, USA. It was prepared according to the procedure describedpreviously [36].

2.2. Preparation of the blends

The materials were pre-treated by drying at 85 �C over drieritefor 24 h to eliminate possible absorbed water on the surface of theparticles. Blends were melt processed at 175 �C for 5 min at 45 rpmusing a Haake Rheomix 600with roller type blades. About 5 g of theobtained blend was compression moulded on a laboratory Carverpress to obtain films of w300e500 ɲm. Moulding conditions wereset at 180 �C for 3 min at 200 Bar.

2.3. Scanning electron microscopy (SEM)

Blend materials were fractured in liquid nitrogen to observe theinterior of the unstressed composite. A samplewas dropped directlyinto liquidnitrogen and fracturedwith a pre-chilled razor bladeheldin a vice-grip. The fractured pieces were picked out of the liquidnitrogen using pre-chilled forceps andplaced in a desiccator to thawand reduce the condensation ofwater on the surface of thematerial.All specimens were coatedwith gold-palladium for 45 s in a DentonDesk II sputter coating unit (Denton VacuumUSA, Moorestown, NJ).Specimens were viewed in a Hitachi S4700 field emission scanningelectron microscope (Hitachi HTA, Japan) at 2 kV.

2.4. Tensile properties

Young’s modulus, tensile strength, and elongation at break weredetermined according to ASTM method D638M under ambientconditions, using an Instron 5500R Universal Testing Machine(Instron Corp., Canton, MA). Prior to testing, samples were equili-brated at 50% relative humidity in a chamber containing saturatedsolutions of calcium nitrate. The testing conditions usedwere: crosshead speed of 5 mm/min and load cell of 0.1 kN. Dumbbell samples(0.2e0.5 mm thick) were tested with a gauge length of 20 mm. Thereported values were the average of at least 6 measurements. Datapoints were the mean value of each measurement, with error barsin each graph representing 95% confidence intervals.

2.5. Thermal properties

Thermogravimetric analysis (TGA)wasperformedundernitrogenatmosphere in a TA Instruments TGA 2950. The sampleswere heatedfrom room temperature to 510 �C at a heating rate of 10 �C/min anda nitrogen gas flow rate of 60 mL/min. The derivative of TGA curves(DTG)was obtained using TA analysis software. Differential scanningcalorimetry (DSC) was performed with a Perking Elmer Jade DSC8000 instrument calibrated with indium (T, DH) and zinc (T). DSCsamples of 10e15 g were weighed in a 40 ɲl stainless steel pan andmeasured against an empty pan as reference. Measurements werecarried out under 80 ml min�1 nitrogen flow rate according to thefollowing protocol: first and second heating from �20 to200 �C.min�1; first cooling (quenching after first heating) from 200to �20 �C at 100 �C.min�1 and 2 min of isotherm at the end. Inparticular, the first heating scanwas used to erase any prior thermalhistory of the sample and the second heating scan was used toevaluate melting temperature (Tm) and crystallinity (Xc). The degreeof crystallinityXc was calculated fromDSC curves as follows (Eq. (1)):

Xc ¼ DHm

ð1� fÞDH�m� 100 (1)

where f is the weight fraction of the dispersed phase in the blends,DHm is the melting enthalpy (J/g) that was calculated from thefusion peak in the DSC curve and DHm

* is the heat of fusion forcompletely crystallized PLA (93.1 J/g) and for completely crystal-lized PHB (146 J/g) [37,38].

M.A. Abdelwahab et al. / Polymer Degradation and Stability 97 (2012) 1822e18281824

2.6. X-ray diffraction

X-ray diffraction (XRD) analyses were performed with a Philips1820 diffractometer operated at 45 kV and, 40 mA using Cu-Ka

radiation with a graphite diffracted beam monochromator. Datawere acquired in a 2q scale from 2� to 40�.

2.7. Biodegradation test

A Micro-Oxymax Respirometer System (Columbus InstrumentsInc., Columbus, OH) was used tomonitor biodegradation of the neatPHB, neat PLA, PLAePHB blend and PLAePHB-7%Lapol samples.Pregelatinized wheat starch was used as a positive control. Eachsample (0.2 g) was mixed with 20 g of compost and kept in a 250-mL sample chamber connected to a fully computerized, closed-circuit system. Carbon dioxide evolution from each sample wasmeasured every 6 h for a total of 56 days at room temperature.

3. Results and discussion

3.1. Morphological properties

The morphologies of the fracture surfaces were investigated bySEM, as shown in Fig. 1. Neat PLA presents a smooth and uniformsurface of an amorphous polymer containing some holes [Fig. 1(a)],

Fig. 1. SEM images of PLA (a), PHB (b), PLA-7%Lapol (c), PHB-7%Lapol (d), PL

whereas neat PHB shows an irregular fracture surface due to itscrystalline structure [Fig.1(b)]. PLAandPHBwith7wt-%Lapoldidnotshowphase separation [Fig.1(c&d)].Coarsenesswasobservedonthefracture surface of PLA75/PHB25, and the fractures in the blendmainly developed along the samedirection, indicating the resistanceto thedevelopmentof fractureswasquite low [Fig.1(e)]. On theotherhand, the surfaces of specimens with Lapol were rougher than thosewithout Lapol, and fractures developed in different directions, asshown in Fig.1(f & g). Shear-yield and plastic deformation formed onthe fracture surfaces of blends with Lapol, especially for the blendwith high plasticizer content [(Fig. 1(g)]. Plastic deformation and thedifferent fracture directions required more energy and thus thematerials with Lapol should have better toughness.

3.2. XRD analysis

X-ray diffraction analysis was used to determine crystallinestructure and degree of crystallinity in the blends. PHB is a highlyordered polymer and is known to crystallize in an orthorhombiccell [39,40]. The lattice parameters of PHB are: a ¼ 0.572 nm andb ¼ 1.312 nm with its chain conformation in the left-handed 21helix. In the orthorhombic cell, the three axis are mutuallyperpendicular (¼b ¼ g ¼ 90�) and of unequal length (a s b s c)[37]. PLA can be crystallized from the melt as an orthorhombic unitcell with lattice parameters of a ¼ 1.037 nm and b ¼ 0.598 nm2.

A75-PHB25 (e), PLA75-PHB25-Lapol5 (f) and PLA75-PHB25-Lapol7 (g).

Fig. 3. DSC traces of the 2nd heating scan of PLA, PHB, Lapol and PLAePHB-Lapolblends.

M.A. Abdelwahab et al. / Polymer Degradation and Stability 97 (2012) 1822e1828 1825

Neat PLA is usually formed from the -type helix 103. PLA can formtwo types of crystals, depending on the crystallization conditions.These are the form with a 103 helical shape and the b form witha spiral shape 31 [2,22,41e43]. PHB has reflection peaks at 13.5� and16.8�, corresponding to the (020) and (110) planes, and three weakpeaks at 19.1�, 22.2� and 25.5�, with the first two peaks typical ofthe form orthorhombic structure [2]. The PLA had only one broaddiffraction peak at 2 of 15� corresponding to the (110/200) reflec-tions (d ¼ 0.536) and smaller peaks at 19.1� and 32� correspondingto (010) and (203), respectively.

In general, the patterns of PLA and PHB with Lapol blends arevery similar to that of neat PLA and neat PHB (Fig. 2). It can beobserved that diffraction at 2 ¼ 13.5� and 16.9� for PHB-7%Lapolhad increased in intensity. For the PLA75/PHB25 blend, the peaklocated at 16.9� is too weak to be observed in the pattern.Furthermore, the presence of amorphous PLA significantly reducesthe crystallinity of the PLA/PHB samples. This is due to the reduc-tion in the PHB content and the PLA interfering with crystallizationof PHB. For the PLA75/PHB25 blend, as shown in Fig. 2, the intensityratio I(020)/I(110) is different from that of neat PHB. This indicatesthat the crystal structure of PHB in the PLA75/PHB25 blend hasbeen altered from interactions between PLA and PHB. Since thecrystallinity and the crystal growth rate of PHB are higher and fasterthan those of PLA, the reflection peaks of PHB are observed evenwith higher PLA content. PLA75/PHB25/7Lapol did not have a peak(110) at 16.9� and had a weak peak (020) at 13.5�. This implies thatthe addition of Lapol significantly improves the crystallinity andcrystallization rate of the PLA75/PHB25 blend.

3.3. Thermal and crystallization behaviour

The thermal properties of the blends could be significantlyaffected by the crystallization characteristics of PHB and PLA. DSCtraces recorded during the second heating cycle of the polymersamples are presented in Fig. 3. Themeasurements were performedimmediately after the melt-quenching scans, thus the samples hadthe same thermal history and could be compared directly. Thecurves revealed similar thermal events for all samples, as follows:the glasserubber transition (characterized by Tg), cold crystalliza-tion process (characterized by Tcc and the cold crystallizationenthalpy (DHcc)), and themelting process (characterized by Tm andthe melting enthalpy (DHm)). The neat PLA sample showed a Tg of63 �C, an exothermic crystallization peak at 113 �C and an endo-thermic melting peak at 173 �C. Similarly, the neat PHB sampleshowed a Tg of 5.2 �C, an exothermic crystallization peak at 48 �C

Fig. 2. XRD of PLA, PHB and PLAePHB-Lapol blends.

and a melting temperature of 178.5 �C. These values were similar tothose reported in literature [35]. As summarized in Table 1, Lapolhad only one Tg at �0.7 �C. In general, the DSC curves of PLA or PHBwith plasticizer (Lapol) blends exhibit one Tg value, which indicatesthat both blend systems are miscible systems [44e47]. Thedependence of Tg, blend (glass transition temperature of the blend)on the composition of the blends follows the Fox relationship [44](Eq. (2)):

1Tg;blend

¼ W1

Tg1þW2

Tg2(2)

where W1 is the mass fraction of component 1, W2 is the massfraction of component 2, Tg1 is the glass transition temperature ofcomponent 1 and Tg2 is the glass transition temperature ofcomponent 2. Using Tg values of 62.9 �C, 5.2 �C and �0.7 �C for neatPLA, PHB and Lapol, respectively, we calculated theoretical Tg, blendvalues of 59 �C for PLAwith 5 wt-% Lapol, 57.5 �C for PLAwith 7 wt-% Lapol, 4.9 �C for PHB with 5 wt-% Lapol and 4.78 �C for PHB with7 wt-% Lapol. These values are in good agreement with the exper-imental values.

On the other hand, the DSC curves of PHB75/PLA25 blendsexhibit two Tg values, which indicates that these blends areimmiscible systems. However, addition of Lapol to the PLA75-PHB25 sample resulted in disappearance of the Tg of Lapoland a decrease in Tg values of PHB and PLA. These behaviourssuggest that Lapol has a significant effect on the PLAePHBmacromolecules.

Table 1DSC parameters (2nd heating) of PLAePHB/Lapol Blends.a

Code Tg1(�C)

Tg2(�C)

Tcc(�C)

Tm(�C)

DHcc

J/gDHm

J/gXc

(%)

PLA e 63 115 173 30 33 35Lapol �0.7 e e e e e e

PLA-5%Lapol e 60 110 173 29 31 35PLA-7%Lapol e 58 110 172 29 31 37PHB 5.2 e 48 179 22 72 49PHB-5%Lapol 5.8 e 49 177 25 69 50PHB-7%Lapol 4.6 e 50 178 25 63 48PLA75-PHB25 1.7 62 115 173 16 13 36PLA75-PHB25-Lapol 5 2.7 59 125 172 24 26 40PLA75-PHB25-Lapol 7 0.6 58 120 173 24 24 38

a Tg, Tm are the glass crystallization and melting peak temperatures, respectively,DHm is the enthalpy of melting, DHcc is the cold crystallization enthalpy and Xc is thedegree of crystallinity.

Fig. 4. a) TGA of PLA, PHB, PLAePHB based blends and b) DTG of PLA, PHB andPLAePHB based blends.

Table 2TGA data of PLA/PHB/Lapol blends.a

Code Tonset (�C) Tp PHB (�C) Tp PLA (�C) R500 (%)

PLA 324 e 357 0.12Lapol 278 e 340 8.94PLA-5%Lapol 313 e 356 0.16PLA-7%Lapol 312 e 357 0.86PHB 260 279 e 0.25PHB-5%Lapol 264 281 e 0.54PHB-7%Lapol 264 282 e 0.53PLA75-PHB25 270 283 356 0.10PLA75-PHB25-Lapol 5 273 284 357 0.86PLA75-PHB25-Lapol 7 271 283 341 0.93

a Td is the onset temperature; Tp is the first derivative peak; and R500 is theresidual weight at 500 �C.

M.A. Abdelwahab et al. / Polymer Degradation and Stability 97 (2012) 1822e18281826

The cold crystallization temperature (Tcc) of PLA decreased andthat of PHB increased with an increase in plasticizer content. Thiscorresponded to a decrease of DHcc in PLA blends and an increase ofDHcc in PHB blends. However, PLA75/PHB25 had higher cold crys-tallization temperature and its peak became broader with anincrease in Lapol concentration. This is possibly due to the smallfinely dispersed PHB crystals acting as nucleating agents in PLA. Thepresence of one crystallization temperature suggested that the PLA/PHB blend has a significant single homogeneous phase through theheating process. These results agree with previous studies by Huet al. [20], Zhang et al. [48] and El-Hadi [25]. From Table 1, the shiftsin the cold crystallization peaks (by approximately 11 �C and 7 �Cfor 5 wt-% and 7 wt-% of Lapol, respectively) is related to thedifference in flexibility of the chains and their ability to forma crystalline structure.

Melting temperature (Tm) of the PLA and PHB blends mostly didnot change with increasing amount of plasticizer. It is interesting tonote that the PLA/PHB blend exhibits two melting peaks at 169 �Cand 173 �C (Table 1 shows only one melting peak for the blends),which are attributed to recrystallization or two different lamellathickening. These results are in agreement with the results ofGunaratne et al. [49]. However, the presence of double meltingtemperatures in the PLAePHB-Lapol blends corresponds to themelting of “as formed” and recrystallized PHB crystallites. Ikeharaet al. [50] stated that when miscible crystalline/crystalline blendsare crystallized from a homogeneous melt, the crystallizationbehaviour largely depends on the difference in the melting point ofthe two components. When the Tm difference is large, thecomponent with higher Tm crystallizes first, and its spherulitesusually fill the whole volume. The lower Tm component crystallizesat a lower temperature in spatially limited regions inside thespherulites of the other component. When the difference in Tm issmall enough, both components have a high possibility to crystal-lize simultaneously. The Tm difference in most of the misciblecrystalline/crystalline blends reported so far was approximately100 �C. In these cases, the two components crystallize at differenttemperatures. In this study, the Tm difference between PHB and PLAis about 5.8 �C, indicating that simultaneous crystallization hasoccurred.

The degree of crystallinity (Xc) of PLA and PHB based blend wasbasically uniform. A higher value of Xc was observed for PLA with7 wt-%Lapol and for PHB with 5 wt-%Lapol. According to Azumaet al. [51] and Janigova et al. [52], an increase in crystallinity is dueto higher chain mobility and a better packing of segments. Theincorporation of Lapol in PLA75-PHB25 slightly increased Xc by 4%for the 5 wt-% Lapol sample and by 2% for the 7 wt-% Lapol sample,as shown in Table 1. This behaviour could be dependent on thechain mobility of the polymer. Also, the interface interactionsbetween PHB and PLA with Lapol could influence nucleation assupported by the XRD results presented in Fig. 2. In addition, thedecrease in DHm values of all blends might imply formation ofa more stable structure, perhaps due to increased movement of thepolymer segments that facilitate polymer chain mobility.

3.4. Thermogravimetric analysis (TGA)

Thermal degradation of plasticized PLA and PHB samples isshown in Fig. 4(a & b). In general, the thermal weight-losses of PHBand PLA were quite sensitive to temperature, with narrowdecomposition temperature ranges. The temperature correspond-ing to the onset of decomposition (Tonset) for a polymer is essentialfor evaluating its thermal stability and is summarized in Table 2.PLA is more thermally stable than PHB. Lapol decreased the Tonset ofPLA by 11 �C for the 5 wt-% Lapol sample and by 12 �C for the 7wt-%Lapol sample. In contrast, Lapol increased the Tonset of PHB by 4 �C

for the 5 and 7 wt-% Lapol samples. PHB and PLA thermally degradeto produce polymeric chains terminated with carboxyl and vinylgroups [53]. Carboxyl end groups of polyester catalyse hydrolysisreaction. In the present study, it is feasible that a synergistic effectoccurred to accelerate thermal degradation of PLA by moisture andpolyol [54,55]. The Tonset of PLA75/PHB25 blend was between thoseof PLA and PHB. The addition of Lapol led to a slight shift in Tonset ofPLA/PHB blend to a higher temperature, as shown in Table 2. Thisphenomenon was attributed to the higher thermal stability ofLapol.

Another important thermal property is the temperature corre-sponding to the maximum rate of weight loss (Tp), which is defined

Fig. 5. Carbon dioxide evolution of the pregelatinized wheat starch, neat PHB, neatPLA, PLAePHB blend and PLAePHB-7%Lapol samples as a function of time.

M.A. Abdelwahab et al. / Polymer Degradation and Stability 97 (2012) 1822e1828 1827

as the peak value of the first derivative of the TGA curve. The firstand second derivatives curves for neat PLA, neat PHB and PLAePHBbased blends are shown in Fig. 4(b) and their Tp values are listed inTable 2. All neat polymers exhibited a single peak, indicating thatthe PLA and PHB degraded in only one step. In addition, the Tp ofPLA and PHB with Lapol shifted slightly to higher temperature.However, the Tp value of PLA75/PHB25 showed a decrease withincreasing amounts of Lapol. All samples had less than 1 wt-%residue at 500 �C and this value increased with increasing plasti-cizer content.

3.5. Mechanical properties

Young’s modulus, strain at break and stress at break values ofmelt-mixed PLA, PHB and PLAePHB based blends are presented inTable 3. The mechanical behaviour of the neat PLA showed char-acteristics of glassy polymers with low deformation at break. Incontrast, the neat PHB has a low melt viscosity and is a brittlematerial with a higher modulus. The blends of PLA and PHB withLapol showed different behaviour. PLA with 5 wt-% Lapol hada higher Young’s modulus (YM) than that for neat PLA. However,PHB with 5 wt-%Lapol had a lower YM than that of neat PHB. TheYM of the PLA75/PHB25 sample was lower than that of neat PLAand addition of plasticizer resulted in even lower values. The tensilestrength was significantly reduced by addition of Lapol, but theelongation at break increased in value. Tensile strength of PLA75/PHB25 was lower than those of PLA and PHB, with no change in theelongation at break (w7%). The addition of Lapol markedly changedthe stress-strain behaviour of the PLA75/PHB25 blend. The PLA75/PHB25 blend with 5 wt-%Lapol had a higher elongation at breakvalue than neat PLA and neat PHB. However, its tensile strengthwaslower than neat PLA and neat PHB. The PLA/PHB/Lapol samplesshowed some characteristics of flexible materials. These improve-ments might be due to the finely dispersed Lapol acting as fillers,which enhanced the properties of neat polymer. They can also bedue to possible interactions between PHB and PLA, with Lapolcausing strong interfacial bonding.

3.6. Biodegradation results

The data herewith reported are referred to a preliminary studyrelevant to the biodegradation of PHB, PLA and two blends of PLA/PHB.

The neat PHB sample showed some biodegradation during themeasurement time period, whereas the other polymer samples didnot degrade much. This is shown in Fig. 5, which displays carbondioxide evolution of the samples as a function of time. The starchsample seemed to degrade the fastest since it generated the largestamount of carbon dioxide. Meanwhile, the neat PHB sampleexhibited an increase in carbon dioxide content up to approxi-mately 40 days, but this content levelled off thereafter. In

Table 3Tensile properties of PLA/PHB/Lapol blend films.a

Code Young’smodulus (MPa)

Tensilestrength (MPa)

Elongationat break (%)

PLA 1400 � 130 42 � 18 7.2 � 2PLA-5%Lapol 1450 � 180 14 � 2 14.4 � 1PLA-7%Lapol 1200 � 60 16 � 1 13.7 � 3PHB 1950 � 140 31 � 4 7.3 � 1PHB-5%Lapol 1750 � 110 29 � 2 7.2 � 1PHB-7%Lapol 1830 � 140 26 � 2 5.6 � 1PLA75-PHB25 1270 � 110 16 � 3 7.1 � 1PLA75-PHB25-5Lapol 1150 � 40 13 � 2 15.5 � 2PLA75-PHB25-7Lapol 1120 � 60 15 � 1 15.1 � 3

a Errors were calculated at 95% of confidence of student’s t-test.

comparison, the samples containing PLA did not generate muchcarbon dioxide, indicating these samples did not degrade muchunder the experimental conditions. This last aspect is worthy offurther investigation in a more comprehensive and detailedapproach.

4. Conclusions

Biodegradable PLAePHB blends have been successfullyprepared by melt mixing PLA, PHB and a new type of plasticizer(Lapol 108). This compounding technique is easy and potentiallycommercially viable. SEM showed that PLA and PHB with plasti-cizer exhibited a homogeneous distribution of Lapol 108 in thepolymer matrix. Samples of ternary blends also showed homoge-nous distribution. XRD data showed that Lapol 108 was affectingthe interlayer space of the blends. Moreover, Lapol 108 essentiallydecreased the Tg for all of the binary and ternary blends. Also,crystallinity in the PLA75-PHB25 sample slightly increased withincreased loading of Lapol 108. On the other hand, thermaldecomposition properties of the blends were not affected by Lapol108. The addition of Lapol caused the elongation at break toincrease and the Young’s modulus to somewhat decrease comparedwith neat PLA and neat PHB. Also, the neat PHB sample showedsome biodegradation, whereas the samples containing PLA did notdegrade much during the experimental time period.

Acknowledgement

The work reported here is part of the activities performed byM.A. Abdelwahab for the achievement of the PhD title in Bioma-terials as granted by the University of Pisa (Italy).

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