bài báo cáo vật liệu y sinh( Vũ Văn Kỳ)

Phase morphology, physical properties, and biodegradation behavior ofnovel PLA/PHBHHx blends

Qiang Zhao,1 Shufang Wang,1 Meimei Kong,1 Weitao Geng,2 Robert K. Y. Li,3 Cunjiang Song,2

Deling Kong1

1Key Laboratory of Bioactive Materials, Ministry of Education, College of Life Science, Nankai University,

Tianjin 300071, People’s Republic of China2Department of Microbiology, College of Life Science, Nankai University, Tianjin 300071, People’s Republic of China3Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon,

Hong Kong, People’s Republic of China

Received 21 April 2011; revised 12 June 2011; accepted 16 June 2011

Published online 26 September 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.31915

Abstract: In this study, two biodegradable polyesters [i.e.,

polylactic acid (PLA) and poly(3-hydroxybutyrate-co-3-hydrox-

yhexanoate) (PHBHHx)] with complementarity in terms of

mechanical performance have been combined, and a series

of blends with a broad range of compositions has been

prepared by thermal compounding. The evolution of phase

morphologies with the variation of compositions has been

characterized by using Fourier transform infrared spectro-

scopic imaging together with scanning electron microscope

analyses. Thermal, mechanical, and biodegradation proper-

ties of the PLA/PHBHHx blends were systematically investi-

gated. Mechanical properties were further analyzed by using

theoretical models and correlated with the results of the

morphology/structure and compatibility of the blends.

Results indicate that PLA/PHBHHx blends are immiscible but

can be compatible to some extent at certain compositions

(e.g., PLA/PHBHHx (w/w) ¼ 80/20 and 20/80). The physical

properties of the blend could be fine tuned by adjusting the

blend composition. VC 2011 Wiley Periodicals, Inc. J Biomed Mater

Res Part B: Appl Biomater 100B: 23–31, 2012.

Key Words: biodegradable, blends, phase behavior, poly(3-

hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), polylactic

acid (PLA)

How to cite this article: Zhao Q, Wang S, Kong M, Geng W, Li RKY, Song C, Kong D. 2012. Phase morphology, physical properties,and biodegradation behavior of novel PLA/PHBHHx blends. J Biomed Mater Res Part B 2012:100B:23–31.

INTRODUCTION

Recently, studies of biodegradable polymers have attractedincreasing attention due to the great demand in biomedicalfield, including resorbable sutures, drug delivery systems,and tissue scaffolds.1,2 Currently, commercially available bio-degradable polymers cannot fully satisfy the requirementsof medical devices due to the limitation in performance,including physiochemical and biological properties.

Polylactic acid (PLA) is a biodegradable polyester syn-thesized through chemical polymerization of monomers thatare converted from renewable agricultural feedstock, suchas corn or sugar beets.2 Since the 1980s, PLA has beenstudied extensively for biomedical usages because of itsbiodegradability and biocompatibility. PLA also has otherdesirable properties, including high mechanical strength,good transparent property, and fabricability.3 However, com-modity products made of PLA have not gained widespreadusage because of their relatively high cost and slow biode-gradation rate for the environment.4 In addition, PLA is a

hard material with relatively low fracture toughness, whichoften limits its usage to some extent, especially in thebiomedical field.

Polyhydroxyalkanoates (PHAs) represent another type ofbiodegradable polymers, which are biosynthesized by somemicroorganisms as intracellular carbon and energy storagecompounds.5–8 Because of the bacterial origins, this class ofpolyesters shows good degradability. PHAs have been usedin number of biomedical applications, including tissue engi-neering scaffold, drug delivery system, resorbable surgicalsutures, and so on.6,9 Poly(3-hydroxybutyrate-co-3-hydroxy-hexanoate) (PHBHHx) is a new member of the PHA fam-ily.10–12 Due to the longer alkyl side chain, PHBHHx exhibitslow crystallinity and a broad processing window comparedwith conventional poly(3-hydroxybutyrate) (PHB). Themechanical performance is utterly different from that ofPHB, that is, PHBHHx is a soft and flexible polymer with arelatively higher break elongation but lower tensile strengthand modulus.7

Correspondence to: S. Wang; e-mail: [email protected] or C. Song; e-mail: [email protected]

Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 50830104, 51073081, 81000680

Contract grant sponsor: Tianjin Committee of Science and Technology, PR China (Key Projects); contract grant numbers: 09JCZDJC18400,

09ZCKFSH00800)

VC 2011 WILEY PERIODICALS, INC. 23

Therefore, these two polymers (i.e., PLA and PHBHHx)are both biodegradable and hold complementary potentialin terms of mechanical properties. Blending is a costeffective strategy for combining the advantages of differentpolymers. In addition, some additional desirable propertiesthat neither component possesses alone may also beobtained.13,14,15

In comparison with the studies on commodity polymers,the pertinent literature has paid very little attention to thephase behavior of biodegradable polymer blends, which is akey factor in determining physical performance as well aspotential applications of the final product. Fourier transforminfrared (FTIR) spectroscopic imaging provides an efficienttool for characterizing the phase structure and morphologyof polymer blends.16,17 This method allows spectral andspatial analyses in a single measurement, so the spatialdistribution of different components in a blend can bevisualized by identifying their characteristic IR bands.

In this study, the evolution of the phase morphology ofPLA/PHBHHx blends over a broad range of compositionshas been characterized by using FTIR spectroscopic imaging.The physical properties and degradation behavior also havebeen investigated systematically. With thorough understand-ing of the structure-property relationship, it aims at design-ing and manufacturing PLA/PHBHHx blends with desirableproperties for biomedical applications, such as artificialvascular graft.

EXPERIMENTAL

MaterialsThe PLA (Mn ¼ 47,000; Mw ¼ 192, 000) was obtained fromShimadzu Co. (Japan). PHBHHx (Mn ¼292,000; Mw ¼ 624,000) was provided by Kanaka Co. (Japan), and the moleratio of 3HHx is 20%.

Sample preparationThe PLA/PHBHHx blends were prepared by melt mixingwith a twin-screw extruder (Kurimoto (Japan) (/25 mm,L/D ¼ 10.2) at 190�C and 100 rpm. The extrudates weregranulated into pellets. A series of blend specimens wasprepared with weight ratios (PLA/PHBHHx) of 100/0,80/20, 60/40, 50/50, 40/60, 20/80, and 0/100.

The melt-compounded blends were compression moldedinto sheets of 0.2 mm thickness at 130–190�C.

In vitro biodegradationEach of PLA/PHBHHx blends (100/0, 80/20, 60/40, 40/60,20/80, and 0/100) was submerged in 30 mL of 0.1M phos-phate-buffered saline (PBS) solution in a 50 mL test tube atphysiological temperature (37 6 2 �C). The ionic concentra-tion was adjusted to the physiological range as describedin the ASTM Designation: F 1635-04a.18 Sodium azide(0.1% (w/w)), penicillin (100 U/mL), and streptomycin(100 lg/mL) were added to the solution as the antimicro-bials to prevent bacterial growth. The specimens wereimmersed in the physiological solution, and removed atcertain time intervals, washed with 75% ethanol and dis-tilled water, and dried to constant weight under vacuum.

The weight of the remaining material was measured todetermine the relative biodegradation.19

CharacterizationFTIR spectroscopic imaging analysis was performed byusing a Perkin-Elmer Spotlight 100 system in attenuatedtotal reflectance (ATR) mode (diamond crystal). The size ofone detector pixel was 6.25 � 6.25 lm2. The spectral reso-lution was 4 cm�1, and four scans were co-added for eachspectrum.

Differential scanning calorimetry (DSC) measurementswere conducted using a Netzsch 204 thermal analysis sys-tem in a nitrogen atmosphere. For the DSC measurements,samples of approximately 10 mg in weight were first heatedfrom 25 to 200�C at a heating rate of 10�C/min (first heat-ing scan) and were kept at 200�C for 1 min to eliminatethermal history. The samples were then rapidly cooled to�100�C at a cooling rate of �50�C/min (cooling scan).Finally, the samples were heated from �100 to 200�C at therate of 10�C/min (second heating scan). The glass transitiontemperature (Tg) was taken as the midpoint of the heatcapacity change. The clod crystallization temperature (Tcc),and melting temperature and corresponding enthalpy (Tmand DHm) were determined from exothermic and endother-mic peaks in the second heating scans, respectively.

Thermogravimetric analysis (TGA) measurements wereconducted on a Netzsch TG 209 analyzer. Samples ofapproximately 10 mg in weight were used for each mea-surement. The samples were heated at a heating rate of10�C/min from room temperature to 600�C, under a nitro-gen atmosphere.

Static tensile tests were performed by a Testometric uni-versal tensile tester (United Kingdom) with a crossheadspeed of 10 mm/min at ambient temperature. For thetensile tests, dumbbell-shaped specimens were used. Eachtesting was repeated on five specimens, and the mean val-ues as well as standard deviation (SD) were reported.

The sample of PLA/PHBHHx (80/20) was fractured inliquid nitrogen, and the fractured surface was immersed inacetone and heated for 15 min to selectively extract thePHBHHx component. Then, the solvent was replaced, andthe sample remained in the solvent overnight at room tem-perature. The solvent-etched sample was dried in a vacuumto completely evaporate the solvent.

After gold coating, the morphology of the fractured sur-face was analyzed by using a scanning electron microscope(SEM) (JEOL JSM-820) with an accelerating voltage of 20 kV.

RESULTS

Phase morphologyIn this study, the phase morphologies of PLA/PHBHHxblends with a broad range of compositions were investi-gated by using FTIR spectroscopic imaging analysis in ATRmode.

FTIR spectra of neat PLA and PHBHHx were characterizedby their intense absorption bands at 1750 and 1720 cm�1,respectively, which are due to the stretching of the carbonylgroup (C¼¼O) (Figure 1). By mapping the relative intensity of

24 ZHAO ET AL. BIODEGRADABLE PLA/PHBHHX BLENDS

these two bands, the PLA-specific and PHBHHx-specific FTIRspectroscopic images for the blends can be plotted, and theresult is shown in Figure 2. The red and blue colors representthe higher and the lower concentration of the correspondingpolymer, respectively.

PLA/PHBHHx blends with intermediate compositions ofPLA/PHBHHx (with blend ratios that range from 40/60 to60/40) show macroscopic phase separation, and PHBHHx-specific and PLA-specific images were perfectly complemen-tary [Figure 2(A–C)].

The 40/60 PLA/PHBHHx blend exhibits a dual-phasecontinuous morphology. Segregated PHBHHx (e.g., spot 15in Figure 2) and PLA (e.g., spot 17 in Figure 2) phases hadspectra that were typical of the neat component and couldbe clearly distinguished. The dimensions of these domainsapproach about 100 lm. In addition, an inter-phase region(green-colored region, spot 16 in Figure 2) was evident,where two components coexist with comparable peakintensities.

To acquire further insight about phase morphology, wedid additional analyses on the system with equal composi-tion (i.e., PLA/PHBHHx (50/50)). The corresponding FITR-microscopic images indicated that the distribution charac-teristics of two phases apparently were different. In thePHBHHx-rich phase, there is a certain degree of mixingbetween the two components, which resulted in a decreasein the size of the separated PHBHHx domains (by severaltens of microns) compared with those for PLA/PHBHHx(40/60). In contrast, The PLA-rich phase was homogeneousin composition with no signs of mixing. Based on theseresults, it is reasonable to conclude that the PHBHHx-richphase is more compatible than the PLA-rich phase.

Further decreasing the PHBHHx content leads to a finerphase morphology with decreased domain size in PLA/PHBHHx (60/40). The continuous phase is PLA (e.g., spot 7in Figure 2), and the dispersed phases are those irregular-shaped domains that consist of both components with vari-ous compositions, which were reflected by the intensityratios of two characteristic peaks (e.g., spots 5 and 6 in Fig-ure 2). Again, this morphology reflects the high level ofcompatibility in the PHBHHx-rich phase. Some fraction ofPLA mixed with predominant PHBHHx phase with certain

degree of compatibility, while residual PLA has been segre-gated and presented as dispersed domains due to the lim-ited compatibilization capability of PHBHHx matrix.

For the blends in which one of the components has arelatively minor concentration (e.g., PLA/PHBHHx (20/80)and PLA/PHBHHx (80/20)), both PHBHHx-specific and PLA-specific images show a homogenous pattern, and no signsof phase separation can be identified. Only slight, regularstreaks were observed, the appearance of which is due tothe relatively narrow numerical scale over the color scalebar. The spectra of the spots selected from different regionshad two characteristic peaks with the intensity ratiosremaining almost unchanged.

Despite the homogenous pattern reflected by the FTIR-microscopic analysis, it would be arbitrary to draw the con-clusion that these blends (i.e., PLA/PHBHHx (20/80) andPLA/PHBHHx (80/20)) are miscible, one-phase systems. Ashas been mentioned earlier, the resolution of the FITR-microscope is 6.25 lm; therefore, the phase separation withdomain size similar to and/or below this dimension cannotbe effectively differentiated. We further investigated thephase structure by using higher magnification SEM analysesin the case of PLA/PHBHHx (80/20). Before analysis, theminor PHBHHx phase was selectively etched by acetone,because acetone is an effective solvent for PHBHHx,whereas it will not dissolve PLA. The SEM image demon-strated that there were craters (arrow indicated) in the sur-viving matrix, which have been left by the solvent extractionof the PHBHHx phase. The domain size can be approxi-mately estimated within the range of several microns(Figure 3). In contrast with the macroscopic phase-sepa-rated system (such as PLA/PHBHHx (40/60)), the blend ofPLA/PHBHHx (80/20) shows a higher degree of compatibil-ity, that is, the dispersed phase has a finer distribution withremarkably reduced domain size. Furthermore, some desira-ble properties (e.g., improved mechanical performance) canbe anticipated from the compatible blends.15,20

Thermal propertiesThe thermal properties of PLA/PHBHHx blends wereobtained from the second heating scan of the DSC (Figure 4and Table I). The first heating scan was used to eliminatethe thermal history of the specimens that were prepared bythermal processing. Neat PHBHHx is a partially crystallinepolymer with low crystallinity owing to the bulky sidegroup. The melting of PHBHHx shows a double peak, andthe shoulder peak at the lower temperature corresponds tothe melting of the imperfect crystalline PHBHHx and thesubsequent re-crystallization. After the incorporation of thePLA component, the crystallization of the PHBHHx phasewas further suppressed, and no detectable melting peakwas identified. By comparing the heating scans with thoseof neat PLA and PHBHHx, the exothermal and endothermalpeaks evident in the thermograms of PLA/PHBHHx blendswere ascribed to cold crystallization of the PLA component,followed by subsequent melting. The variation of cold crys-tallization temperatures (Tcc) with blend compositions issomewhat complex, that is, the Tcc values of PLA/PHBHHx

FIGURE 1. FTIR spectra for neat PLA and PHBHHx. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

ORIGINAL RESEARCH REPORT

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | JAN 2012 VOL 100B, ISSUE 1 25

(60/40) and PLA/PHBHHx (40/60) are higher than that ofneat PLA, whereas that of PLA/PHBHHx (80/20) is slightlylowered. PLA/PHBHHx (20/80) shows a small, broad exo-thermal peak, and the peak temperature cannot be precisely

determined. The possible reason explaining the depressedTcc in PLA/PHBHHx (80/20) is that, during the cooling pro-cess, the minor PHBHHx component forms fine spherulites,which can act as nucleating sites to accelerate the

FIGURE 2. FTIR-microscopic images for the PLA/PHBHHx blends and the corresponding spectra for the labeled spots. The size of the image is

250 � 250 lm. Column 1: PHBHHx-specific image; Column 2: PLA-specific image; Column 3: Spectra at certain spots. A: PLA/PHBHHx (40/60); B:

PLA/PHBHHx (50/50); C: PLA/PHBHHx (60/40); D: PLA/PHBHHx (80/20);� and E: PLA/PHBHHx (20/80). [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]


nonisothermal crystallization of the PLA phase in the subse-quent heating process.21

The glass transition can also be identified from the DSCthermograms. Generally, the blends show two distinctiveglass transition temperatures (Tg) that correspond to thetwo components, respectively. Of these two Tg values, thelower value belonged to the PHBHHx component, which isindependent of composition and remains almost unchangedin all systems; the higher Tg value, which is associated withPLA, showed a small, but still evident, shift toward low temper-ature in the cases of PLA/PHBHHx (80/20) and PLA/PHBHHx(20/80). It has been accepted that the glass transition is akinetic phenomenon that is strongly influenced by the localmaterial environment. Previous phase morphology analyseshave indicated that these two blends have more compatiblecharacteristics, which, in turn, results in the shift of Tg.

22

The thermal degradation behavior of the PLA/PHBHHxblends was evaluated by using TGA (Figure 5). The blendsshowed two, well-defined decomposition peaks in the DTGcurves, which were attributed to the thermal degradationsof the PLA and PHBHHx components by comparing themwith the decomposition of neat polymers without com-pounding. Detailed results are summarized in Table I. Thedecomposition temperatures of the PHBHHx component(Tp(I)) in all blends were increased compared with the neat

PHBHHx, and this enhancement is more significant in com-patible systems, such as PLA/PHBHHx (80/20) and PLA/PHBHHx (20/80). Published data have indicated that oneprominent disadvantage of PHA polymers (e.g., PHB) is theirnarrow processing windows. For the PLA/PHBHHx blendsinvestigated in this study, the ranges of the processing tem-perature (i.e., T0(I)�Tm(max)) are relatively broad with val-ues larger than 70�C; therefore, the processability of thiskind of materials is acceptable for large-scale industrialapplications.

Mechanical propertiesThe mechanical performances of PLA/PHBHHx blends wereevaluated by tensile testing (Figure 6 and Table II). Gener-ally, neat PHBHHx is a soft, flexible polyester with relativelylow tensile stress and Young’s modulus (less than 1 GPa),while PLA has higher strength and hardness but lowertoughness and ductility.23,24

Previous studies have gained success in toughening thePLA matrix by the incorporation of second ductile polymers,such as PBAT (poly(butylene adipate-co-terephthalate)25 orPCL.22,26 On the basis of the mechanical characteristic of the

TABLE I. Thermal Transition Data of PLA/PHBHHx Blends Obtained from DSC and TGA Analyses

Sample Code

PLA PHBHHx Blend

Tg (�C) Tcc (�C) Tm (�C) DHm (J/g) Tp (�C) Tg (�C) Tm (�C) Tp (�C)

ProcessingWindowa

PLA 60.7 117.4 168.9 (173.1)b 43.69 372.7 – – – 144.4PLA/PHBHHx (80/20) 55.1 110.4 173.8 37.36 371.2 �5.4 – 289.7 92.4PLA/PHBHHx (60/40) 59.1 118.2 173.0 19.44 366.6 �3.3 – 281.2 72.0PLA/PHBHHx (40/60) 59.6 119.1 173.9 17.40 357.5 �3.9 – 274.2 71.1PLA/PHBHHx (20/80) 55.6 – 172.3 9.19 337.5 �3.0 – 284.0 77.7PHBHHx – – – – – �3.8 108.0 (115.8)b 272.8 124.2

a T0(I) � Tm(max).b Double peaks.

Tg: glass transition temperature; Tcc: cold crystallization temperature; Tm: melting temperature; Tp: peak temperature of the DTG curve;DHm:

melting enthalpy; T0: temperature at which weight loss started to occur; Tm(max): the higher one of the two Tms.

FIGURE 3. Scanning electron micrograph (SEM) of fractured surface

after solvent etching for PLA/PHBHHx blend (80/20).

FIGURE 4. DSC thermograms of PLA/PHBHHx blends during the

second heating scan (10�C/min). [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]



PHBHHx, the incorporation of it can improve the ductilityand flexibility of the PLA matrix. Evident toughening effecthas been observed with the addition of 20 wt % of PHBHHx

(i.e., PLA/PHBHHx (80/20)), the elongation at break (eb) ofwhich is almost 600% higher than that of neat PLA eventaking the relatively large SD into account. The tougheningeffect becomes less effective on further increasing thePHBHHx loading (i.e., PLA/PHBHHx (60/40) and PLA/PHBHHx (50/50)) due to macroscopic phase-separationmorphology illustrated by the FTIR-microscope image,which may induce severe stress concentration, and, there-fore, significantly degrade fracture toughness. On the otherhand, the tensile strength and Young’s modulus wereadversely declined with the PHBHHx addition.

At the other composition extreme, the introduction ofminor amount of PLA effectively reinforced the soft andweak PHBHHx matrix, that is, the tensile stress and Young’smodulus were apparently enhanced in case of PLA/PHBHHx(20/80). In addition, an evident yielding point was identi-fied in the stress-strain curve, and the elongation at break(eb) was increased compared with the neat PHBHHx. Thisreinforcing effect was ascribed to the fine distribution ofstiff PLA particles of small size within the PHBHHx continu-ous phase.

The variation tendency of Young’s modulus with the blendcomposition was further fitted by using theoretical models of‘‘rule of mixture’’ and ‘‘foam model,’’ respectively.26,27

The ‘‘rule of mixture’’ [Eq. (1)] assumes the perfectinter-phase adhesion between the matrix and the dispersedphase and perfect dispersion of the spherical inclusion inthe matrix:

Eb ¼ EdEm

� 1

� �� /d þ 1

� �� Em (1)

where /d is the volume ratio of the dispersed phase whichhas been determined according to the density of the twopolymers (DPLA ¼ 1.24 g/cm3; DPHBHHx ¼ 1.21 g/cm3), Eb,Ed, and Em are the modulus of the blend, dispersed phase,and matrix.

In the ‘‘Foam model’’ [Eq. (2)], dispersed phase is consid-ered as noninteracting phase equivalent to a void or pores:

Eb ¼ ð1� /2=3d Þ � Em (2)

The plot of Young’s modulus against blend ratio followsthe ‘‘rule of mixture’’ more closely and is apparently higherthan the prediction based on ‘‘foam model,’’ revealing a

FIGURE 5. TGA curves of PLA/PHBHHx blends at a heating rate of

10�C/min: (a) TGA curves; (b) DTG curves. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 6. Stress-strain curves of PLA/PHBHHx blends obtained from

static tensile testing. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]

TABLE II. Mechanical Properties of PLA/PHBHHx Blends

Determined from Stress-Strain Curves

Sample CodeYoung’s

Modulus (GPa)Stress at

Max (MPa)Elongationat Break (%)

PLA 1.39 6 0.09 36.4 6 3.9 13.8 6 5.7PLA/PHBHHx (80/20) 1.32 6 0.04 29.5 6 0.9 99.6 6 69.4PLA/PHBHHx (60/40) 1.24 6 0.16 33.5 6 4.0 7.68 6 1.60PLA/PHBHHx (50/50) 0.91 6 0.07 22.1 6 0.9 7.26 6 2.72PLA/PHBHHx (40/60) 1.25 6 0.15 27.7 6 1.8 11.5 6 4.3PLA/PHBHHx (20/80) 0.59 6 0.18 23.6 6 0.7 83.5 6 64.2PHBHHx 0.37 6 0.12 17.6 6 1.4 19.3 6 1.4


certain degree of interfacial adhesion (Figure 7). Direct evi-dence for the interfacial adhesion has already been demon-strated by FTIR microscope image [Figure 2(A,B)].

In vitro biodegradation behaviorThe in vitro biodegradation behavior of PLA/PHBHHxblends with various compositions was evaluated in thesimulated body fluid (PBS buffer, pH ¼ 7.4). Figure 8 showsthe relationship between weight loss of PLA/PHBHHx-blendfilms and the degradation time. In general, all blendsdegraded slowly as well as the neat polymers up to 5months, which agreed with the reported data under similarconditions.28 It has been reported PLA began to experiencesignificant weight loss after a time period of 16 months;therefore, the degradation remained in the introduction pe-riod during the time period investigated in this study. It isreasonable to predicate that the in vivo degradation of thesematerials should be faster than in vitro process becausesome enzymes present in human body can catalyze the hy-drolysis of the polymer chains.

Despite the relative low absolute value for weight loss,they still show a regular variation trend with blend compo-sition, that is, the weight retention decreased with thedecrease of PHBHHx loading in general, which is the inverseof that reflected by the simulated soil medium (data notshown). It is worth noting that no significant change wasobserved for the neat PLA and blend with PLA as predomi-nant phase (i.e., PLA/PHBHHx (80/20)) during the firstmonth. This phenomenon could be attributed to the rela-tively high crystallinity of PLA phase compared withPHBHHx (as shown in Table I), which can inhibit the diffu-sion of degraded product from the bulky material.

DISCUSSION

PLA and PHBHHx are two biodegradable polyesters withevident complementary potential in terms of mechanical

properties. The two polymers were combined via blendingwith the aim to fabricate novel material with comprehen-sively optimal performance. As far as we know, the finalphysical properties of the polymer blend are determined bythe compatibility and phase structure/ morphology, andmacroscopic phase separation often results in the poor me-chanical performance.

In essence, the blend developed in this study is thermo-dynamically immiscible, because the two components (i.e.,PLA and PHBHHx) are both semicrystalline polyesters andcannot undergo cocrystallization. The FTIR-microscopic andSEM analyses indicate that the phase morphology evolvescontinuously with the variation of compositions, which leadsto phase domains that gradually vary in size.

Furthermore, it is well established that, even in the caseof immiscible blend systems, molecular chains of differentcomponents can interpenetrate at the inter-phase locationsto some extent.29 In addition, the constituents of polyesterblends readily undergo interchange reactions near or abovethe melting points of the blends. For the processing condi-tions used in this study, some transesterifications may takeplace at the interface of two phases, and block copolymersmay thus be formed. These copolymers can improve thelocal miscibility, which in turn promotes further contactbetween the two components.14

As a result, certain degree of compatibility has beenobserved in the blends with relatively minor second compo-nent (i.e., PLA/PHBHHx (w/w) ¼ 80/20 and 20/80), due tothe fine distribution of separated phase with reduced size.Enhanced compatibility was also supported by the shift inglass transition temperature demonstrated in DSCthermogram.

The mechanical properties are generally in agreementwith the described phase structure and morphology andconfirm the complementarity of these two polymers. Indetail, the incorporation of a minor amount of the secondcomponent can effectively overcome the drawbacks ofthe predominant matrix due to the better compatibility

FIGURE 7. Plot of Young’s modulus against blend ratio based on ex-

perimental data and theoretical prediction. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 8. Changes of weight remaining for PLA/PHBHHx blends in

PBS buffer at 37�C. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]



and fine phase distribution. Evident toughening and rein-forcing effect have been observed in the case of PLA/PHBHHx (w/w) ¼ 80/20 and 20/80, respectively. Themechanical performance of as-prepared blends could satisfya wide range of biomedical usages through fine tuning theblend composition.

Interfacial behavior of polymer blends (e.g., perfect ad-hesion and no adhesion) is very important in determiningthe final properties. Various approaches have been usedto characterize the interfacial behavior, including SEM,dynamic viscoelasticity, and so forth. In addition, predic-tion models offer an expedient alternative for recognizingand quantifying to some extent the interfacial behavior ofpolymer blends.22,26,27 In this study, the variation tend-ency of mechanical performance with the blend composi-tion has also been fitted by using theoretical models of‘‘rule of mixture’’ and ‘‘foam model.’’ Results indicate thereis a certain degree of interfacial adhesion in PLA/PHBHHxblends, which has been supported by FTIR microscopyanalysis.

Biodegradation behavior of PLA/PHBHHx blends eval-uated in simulated body fluid environment show adecreasing trend with the increase of PHBHHx loading.The dominant mechanism for in vitro degradation of thepolyester is the scission of ester bonds, and macromolecu-lar chains were hydrolyzed into water-soluble oligomersand then released from the bulky film into the surround-ing medium, which results in the weight loss. The acidicdegradation product can catalyze and therefore acceleratethe hydrolysis process.30 It has been accepted that thehydrolysis rate is proportional to the concentration of car-boxyl groups of the degraded products. Hence, PLA withshorter repeating unit (C3) is more liable to be hydrolyzedthan PHBHHx which consists of six carbon atoms in eachrepeating unit.

CONCLUSIONS

In this study, we have prepared a type of biodegradablePLA/PHBHHx blends by melt mixing. The phase structuresand morphologies of these blends were characterizedby using FTIR-spectroscopic imaging together with SEManalyses. Thermal, mechanical, and in vitro biodegradationproperties of the PLA/PHBHHx blends were systematicallycharacterized. The variation tendency of mechanical per-formance was further fitted by using theoretical models toexplore the inherent structure-property relationship. Theresults indicate that PLA/PHBHHx blends are immisciblebut can be compatible to some extent at certain composi-tions. The phase structure and morphology evolve continu-ously with the variation of compositions, leading to gradu-ally varied sizes of the phase domains. The two comprisingpolymers hold complementary capability in terms of me-chanical properties. Incorporation of a minor amount of thesecond component can improve the mechanical performanceremarkably by providing a toughening or a reinforcing effectin compatible systems, such as PLA/PHBHHx (80/20) andPLA/PHBHHx (20/80).

REFERENCES1. Griffith LG. Polymeric biomaterials. Acta Mater 2000;48:263–277.

2. Gross RA, Kalra B. Biodegradable polymers for the environment.

Science 2002;297:803–807.

3. Engelberg I, Kohn J. Physico-mechanical properties of degradable

polymers used in medical applications: A comparative study. Bio-

materials 1991;12:292–304.

4. Ramsay BA, Langlade V, Carreau PJ, Ramsay JA. Biodegradability

and mechanical properties of poly-(b-hydroxybutyrate-co-b-hydroxyvalerate)-starch blends. Appl Environ Microbiol 1993;59:

1242–1246.

5. Anderson AJ, Dawes EA. Occurrence, metabolism, metabolic role,

and industrial uses of bacterial polyhydroxyalkanoates. Microbiol

Rev 1990;54:450–472.

6. Doi Y. Microbial Polyesters. New York: VCH Publishers; 1990.

7. Gao Y, Kong LJ, Zhang L, Gong YD, Cheng GG, Zhao NM, Zhang

XF. Improvement of mechanical properties of poly(DL-lactide)

films by blending of poly(3-hydroxybutyrate-co-3-hydroxyhexa-

noate). Eur Polym J 2006;42:764–775.

8. Inoue Y, Yoshie N. Structure and physical properties of bacterially

synthesized polyesters. Prog Polym Sci 1992;17:571–610.

9. Misra SK, Valappil SP, Roy I, Boccaccini AR. Polyhydroxyalka-

noate (PHA)/inorganic phase composites for tissue engineering

applications. Biomacromolecules 2006;7:2249–2258.

10. Chen GQ, Wu Q. Polyhydroxyalkanoates as tissue engineering

materials. Biomaterials 2005;26:6565–6578.

11. Qu XH, Wu Q, Liang J, Qu X, Wang SG, Chen GQ. Enhanced vas-

cular-related cellular affinity on surface modified copolyesters of

3-hydroxybutyrate and 3-hydroxyhexanoate (PHBHHx). Biomateri-

als 2005;26:6991–7001.

12. Cheng S, Chen GQ, Leski M, Zou B, Wang Y, Wu Q. The effect of

D,L-b-hydroxybutyric acid on cell death and proliferation in L929

cells. Biomaterials 2006;27:3758–3765.

13. Wang XL, Du FG, Jiao J, Meng YZ, Li RKY. Preparation and prop-

erties of biodegradable polymeric blends from poly(propylene

carbonate) and poly(ethylene-co-vinyl alcohol). J Biomed Mater

Res 2007;83B:373–379.

14. Groeninckx G, Sarkissova M, Thomas S. Chemical reactions

in blends based on condensation polymers: Transreactions

and molecular and morphological characterization, Chapter

14. In: Paul DR, Bucknall CB, editors. Polymer Blends, Vol. 1,

Formulation, USA: Wiley; 1999.

15. Hobbs SY, Watkins VH. Morphology characterization by

microscopy techniques, Chapter 9. In: Paul DR, Bucknall CB, edi-

tors. Polymer Blends, vol.1, Formulation. USA: Wiley; 1999.

16. Vogel C, Wessel E, Siesler HW. FT-IR imaging spectroscopy of

phase separation in blends of poly(3-hydroxybutyrate) with

poly(L-lactic acid) and poly(e-caprolactone). Biomacromolecules

2008;9:523–527.

17. Malheiro VN, Caridade SG, Alves NM, Mano JF. New poly(e-cap-rolactone)/chitosan blend fibers for tissue engineering applica-

tions. Acta Biomater 2010;6:418–428.

18. ASTM Designation: F 1635-04a, Standard test method for in vitro

degradation testing of hydrolytically degradable polymer resins

and fabricated forms for surgical implants.

19. Tao J, Song CJ, Cao MF, Hu D, Liu L, Liu N, Wang SF. Thermal

properties and degradability of poly(propylene carbonate)/poly(b-hydroxybutyrate-co-b-hydroxyvalerate) (PPC/PHBV) blends. PolymDegrad Stab 2009;94:575–583.

20. Zhang L, Xiong C, Deng X. Miscibility, crystallization and mor-

phology of poly(b-hydroxybutyrate)/poly(d,l-lactide) blends. Poly-

mer 1996;37:235–241.

21. Furukawa T, Sato H, Murakami R, Zhang J, Duan YX, Noda I,

Ochiai S, Ozaki Y. Structure, dispersibility, and crystallinity of pol-

y(hydroxybutyrate)/poly(L-lactic acid) blends studied by FT-IR

microspectroscopy and differential calorimetry. Macromolecules

2005;38:6445–6454.

22. Broz ME, VanderHart DL, Washburn NR. Structure and mechanical

properties of poly(D,L-lactic acid)/poly(e-caprolactone) blends. Bio-materials 2003;24:4181–4190.

23. Takayama T, Todo M. Improvement of impact fracture properties

of PLA/PCL polymer blend due to LTI addition. J Mater Sci 2006;

41:4989–4992.


24. Todo M, Park SD, Takayama T, Arakawa K. Fracture microme-

chanisms of bioabsorbable PLLA/PCL polymer blends. Eng Fract

Mech 2007;74:1872–1883.

25. Jiang L, Wolcott MP, Zhang J. Study of biodegradable polylac-

tide/poly(butylene adipate-co-terephthalate) blends. Biomacromo-

lecules 2006;7:199–207.

26. Simoes CL, Viana JC, CunhaAM.Mechanical properties of poly(e-capro-lactone) andpoly(lactic acid) blends. JAppl PolymSci 2009;112:345–352.

27. Tomar N, Maiti SN. Mechanical properties of PBT/ABAS blends.

J Appl Polym Sci 2007;104:1807–1817.

28. Tsuji H. In vitro hydrolysis of blends from enantiomeric poly(lacti-

de)s Part 1. Well-stereo-complexed blend and non-blended films.

Polymer 2000;41:3621–3630.

29. Sakai F, Nishikawa K, Inoue Y, Yazawa K. Nucleation

enhancement effect in poly(L-lactide) (PLLA)/poly(e-caprolac-tone) (PCL) blend induced by locally activated chain mobility

resulting from limited miscibility. Macromolecules 2009;42:

8335–8342.

30. Cha Y, Pitt CG. The biodegradability of polyester blends. Biomate-

rials 1990;11:108–112.



Documents

bài báo cáo vật liệu y sinh( Vũ Văn Kỳ)