Crystallization kinetics of bacterial poly(3-hydroxylbutyrate) copolyesters with cyanuric acid
Crystallization Kinetics of Bacterial Poly(3-hydroxylbutyrate) Copolyesters with Cyanuric Acid as a Nucleating Agent
Pengju Pan,Guorong Shan,Yongzhong Bao,Zhixue Weng
Department of Chemical and Biological Engineering,State Key Laboratory of Chemical Engineering,Zhejiang University,
Hangzhou310027,China
Correspondence to:P.Pan(E-mail:panpengju@https://www.wendangku.net/doc/f014841680.html,)
ABSTRACT:Effects of cyanuric acid(CA)on nonisothermal and isothermal crystallization,melting behavior,and spherulitic morphol-ogy of bacterial copolyesters of poly(3-hydroxybutyrate),i.e.,poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV)and poly (3-hydroxybutyrate-co-3-hydroxyhexanoate)(PHBH),have been investigated.CA has excellent acceleration effectiveness on the melt crystallization of bacterial PHB,PHBV,and PHBH,better than the nucleating agents reported in the literatures,such as boron nitride,uracil,and orotic acid.PHBV and PHBH do not crystallize upon cooling from the melt at10 C/min,while they are able to complete crystallization under the same conditions with an addition of1%CA,with a presence of sharp crystallization exotherm at 75–95 C.Isothermal crystallization kinetics of neat and CA-containing PHBV and PHBH were analyzed by Avrami model.Crystalli-zation half-times(t1/2)of PHBV and PHBH decrease dramatically with an addition of CA.The melting behavior of isothermally melt-crystallized PHBV and PHBH is almost not influenced by CA.Spherulitic numbers of PHBV and PHBH increase and the spher-ulite sizes reduce with an incorporation of CA.Nucleation densities of PHBV and PHBH increase by3–4orders of magnitude with a presence of1%CA.V C2012Wiley Periodicals,Inc.J.Appl.Polym.Sci.129:1374–1382,2013
KEYWORDS:biodegradable;biopolymers and renewable polymers;crystallization;differential scanning calorimetry(DSC);
thermal properties
Received4July2012;accepted13November2012;published online7December2012
DOI:10.1002/app.38825
INTRODUCTION
Polyhydroxyalkanoates(PHAs),a family of bio-based polyesters with diverse structures,are the typical bio-plastics that can be completely synthesized by microorganisms.They have been attracting considerable attentions as potential substitutes for the oil-based plastics.1,2Poly(3-hydroxybutyrate)(PHB)is the most well-studied bacterial polyesters in the family of PHAs.How-ever,it has two major drawbacks.First,it is brittle and rigid due to the high crystallinity,large spherulitic size,and the occurrence of secondary crystallization under ambient condi-tions.Second,PHB has a low thermal decomposition tempera-ture,resulting in a narrow temperature window in melt processing.Copolymerization is a typical approach to modify the physical performances of PHB.3–6Compared with PHB homopolymer,its bacterial copolyesters such as poly(3-hydroxy-butyrate-co-3-hydroxyvalerate)(PHBV)and poly(3-hydroxybu-tyrate-co-3-hydroxyhexanoate)(PHBH)have the better physical properties.For instance,PHBH is tough and flexible,and PHBH containing15mol%hydroxyhexanoate(HH)can elon-gate to ca.760%in tensile deformation.3Besides,the melting temperature and melt viscosity decrease with introducing the commoner units into PHB,conferring PHB copolyesters the broader temperature window in melt processing.
Despite the better physical properties,bacterial copolyesters of PHB have a critical drawback in processing and molding.Crys-tallization of PHB copolyesters is too slow,which further deteri-orates with increasing the commoner unit content.Generally, crystallization does not proceed in the commercial PHBV and PHBH in a cooling condition similar as the practical injection-molding and extrusion,resulting in the long molding-cycle time,low productivity,and high energy consumption in indus-trial processing.Furthermore,the low crystallizability and degree of crystallization(X c)can induce the low strength,mod-ulus,and heat deformation temperature(HDT)of final prod-ucts.The poor crystallizability and processability of PHB bacte-rial copolyesters have been the bottleneck for their large-scale commodity application.
In general,polymer crystallization consists of two stages,i.e., nucleation and crystal growth.Crystallization rate of semicrys-talline polymers can be increased by accelerating the nucleation process,which consists of the homogenous and heterogeneous
V C2012Wiley Periodicals,Inc.
nucleation.Nucleating agent(NA)is an efficient additive to accelerate the heterogeneous nucleation in industrial process-ing.7NA can lead to the fast crystallization,high X c,and small spherulitic size of semicrystalline polymers.Thus,NA can improve the processability,productivity in melt processing, HDT,mechanical strength,modulus,toughness,and optical transparency8of the semicrystalline polymers.For example,a typical NA of PHB and its copolyesters,boron nitride(BN),9,10 can improve the mechanical properties of PHBV due to the increased X c and decreased spherulitic size.With the help of BN,PHBV fibers with high strength and X c have been success-fully prepared.11,12Therefore,the utilization of NA would be an
economical and cost-effective approach to modify the physical performances of PHB bacterial copolyesters.
Up to now,some chemicals have been introduced as NAs for the bacterial PHB and its copolyesters,such as talc,13a-cyclo-dextrin,14PHB/a-cyclodextrin inclusion complex,15poly(vinyl alcohol)particle,16terbium oxide,17saccharin,18metal phospho-nate,19thymine,20uracil,21orotic acid(OA).22,23.For example, Qian et al.have reported that thymine increases the overall crystallization rate of PHBV and demonstrates a similar nuclea-tion efficiency as BN.20We have found that uracil and its deriv-ative(i.e.,orotic acid)increase the crystallization rate of PHBH with a HH unit content ranging from0to18mol%and reveal a better nucleation efficiency than BN,especially for PHBH with a high content of HH unit.21–23Although some of NAs reported in literatures are efficient for PHB homopolymer,their nucleation efficiency on melt crystallization of PHB copolyesters such as PHBV and PHBH are relatively limited.Therefore,it is significant to develop the high efficient NAs and investigate their influences on the crystallization kinetics of PHB bacterial copolyesters.
A recent study has reported that cyanuric acid(CA),a nontoxic compound,24can accelerate the crystallization of PH
B and its copolyesters.25However,the effects of CA on crystallization kinetics of PHB and its copolyesters have not been explored.In this work,we introduced CA as a NA for PHB copolyesters,i.e., PHBV and PHBH.The effects of CA on crystallization kinetics of PHBV and PHBH were compared with other NAs of PHAs. CA was found to have the best nucleation efficiency to the melt crystallization of PHBV and PHBH.The effects of CA on noni-sothermal/isothermal crystallization,melting behavior,and spherulitic morphology of PHBV and PHBH were investigated, and the isothermal crystallization kinetics was quantitatively analyzed.
EXPERIMENTAL
Materials
PHB(M w?6.5?105g/mol,M w/M n?2.24)was provided by PHB Industrial S/A(Brazil).PHBV(M w?3.6?105g/mol, M w/M n?2.13)with12mol%HV and PHBH(M w?2.1?105g/mol,M w/M n?2.29)with10mol%HH were kindly sup-plied by Metabolix and Kaneka(Osaka,Japan),respectively.CA, uracil,and OA were purchased from Sinopharm Chemical Reagent(Shanghai,China)and milled before use.BN was pur-chased from Nacalai Tesque(Kyoto,Japan).Chemical structures of PHB,PHBV,PHBH,and CA are illustrated in Scheme1.Sample Preparation
PHB,PHBV,or PHBH was mixed with NA through solution blending to avoid the possible thermal degradation of polymer in melt-mixing.Preweighted NA was added into chloroform and ultrasonically treated for20min.All NAs used here are insoluble in chloroform and dispersed as particles with the sizes less than20l m.PHB or its copolyester was then added into the suspension and stirred rigorously.The mixture was cast onto a polytetrafluoroethylene dish and the solvent was allowed to evaporate under ambient conditions.The film obtained was dried at60 C under vacuum for24h.The concentration of NA in polymer was kept to be1wt%.
Measurements:Differential Scanning Calorimetry(DSC) Nonisothermal and isothermal crystallizations of PHB and its copolyesters were measured by a TA Q200differential scanning calorimeter(TA Instruments,USA)under a nitrogen atmos-phere.The sample(6–8mg)was encapsulated in an aluminum pan and first melted at190 C for2min to erase the thermal history.In the nonisothermal crystallization,the sample was cooled toà50 C from the melt at a cooling rate of10 C/min. In the isothermal crystallization,the sample was cooled to the desired crystallization temperature(T c?40–100 C)with a con-trolled rate of100 C/min.There was no overshoot of either program or sample temperature in the fast cooling process.Af-ter the nonisothermal or isothermal crystallization,the sample was heated to190 C at a rate of10 C/min to detect its melting behavior.
Polarized Optical Microscopy
Spherulitic morphology was examined on a polarized optical microscopy(Eclipse E600POL,Nikon,Tokyo,Japan).The sam-ple was sandwiched by two glass slides and melted at190 C for 2min in a Linkam THMS600hot stage(Linkam Scientific Instrument,Surrey,UK).It was then quickly cooled to the desired temperature by liquid nitrogen for isothermal crystalli-zation.Spherulitic morphology was recorded after the crystalli-zation completed.To measure the spherulitic growth rate(G), the sizes of growing spherulites were monitored at an appropri-ate time interval before spherulites impinged.G was evaluated from the slope of spherulitic radius(R)vs.growth time(t),that is,G?dR/dt.
RESULTS AND DISCUSSION
Nonisothermal Crystallization Kinetics
Effects of CA on the nonisothermal melt crystallization of PHB, PHBV,and PHBH were studied via DSC.Figure1shows
DSC Scheme1.Chemical structure of PHB,PHBV,PHBH,and CA.
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thermograms of the nonisothermal melt crystallization and subsequent heating scans for neat polymers and those contain-ing 1%CA.On basis of the DSC curves,the thermal parameters including melt-crystallization temperature (T mc ),enthalpy (D H mc )during cooling,cold-crystallization temperature (T cc ),enthalpy (D H cc )during following heating,melting temperature (T m ),melting enthalpy (D H m ),and X c were evaluated.These thermal parameters are tabulated in Table I.Enthalpies were normalized by the weight of polymer matrix.X c was estimated by comparing D H m with the value of an infinitely large crystal (D H m 0),taken as D H m 0?146J/g 26(X c ?D H m /D H m 0?100%).
Neat PHB can complete crystallization in cooling scan,without showing the cold crystallization peak in subsequent heating scan.Because the minor comonomer units of HH and HV retard the crystallization of HB units,crystallizations of neat PHBV and PHBH become very slow and no crystallization peak is shown in a cooling scan at 10 C/min [Figure 1(a)].In the subsequent heating scan,a cold crystallization peak appears in the temperature range 40–80 C for neat PHBV and PHBH
[Figure 1(b)].Due to the introduction of minor comonomer unit,PHBV and PHBH show lower X c values than neat PHB (Table I).Generally,crystallization rate and X c of bacterial copo-lyesters of PHB decrease with increasing the content of minor comonomer unit.3,21As shown in Table I,PHBV with 12mol %HV content has a smaller T cc and higher X c than PHBH with 10mol %HH unit.This indicates the faster crystallization and better crystallizability of PHBV than PHBH,even though the former has a higher content of minor comonomer unit.The better crystallizability of PHBV than PHBH is attributed to the smaller volume/size of BV unit and the inclusion of minor BV unit into PHB crystalline phase,i.e.,isomorphic crystallization of PHBV .27,28
Generally,a high T mc is taken as an indicator of fast crystalliza-tion in nonisothermal melt crystallization.Interestingly,CA accelerates the crystallization of PHB,PHBV ,and PHBH dramatically.T mc value of neat PHB is increased by $6 C with an incorporation of 1%CA.Acceleration effect is more pre-dominant for the bacterial copolyesters of PHB that has slow crystallization rate.After an addition of 1%CA,PHBV
and
Figure 1.DSC curves recorded in (a)nonisothermal melt crystallization and (b)subsequent heating scan for neat PHB,PHBV,PHBH,and those con-taining 1%CA.[Color figure can be viewed in the online issue,which is available at https://www.wendangku.net/doc/f014841680.html,.]
Table I.Thermal Parameters of Neat and NA-Containing PHB,PHBV ,and PHBH Attained in Nonisothermal Melt Crystallization and Subsequent Heating Scan Sample T mc ( C)D H mc (J g à1)T cc ( C)D H cc (J g à1)T m ( C)D H m (J g à1)X c (%)PHB 107.774.6NP 0173.486.559.2PHB/CA 113.676.1NP 0175.487.359.8PHBH NP a
064.834.8111.633.823.2PHBH/CA 77.0
37.8NP 0123.4,140.140.827.9PHBV NP 048.450.1136.6,151.252.335.8PHBV/CA 91.450.4NP 0143.6,154.456.538.7PHBV/BN 84.249.5NP 0141.7,153.454.637.4PHBV/uracil 82.548.5NP 0141.9,153.553.936.9PHBV/OA
86.4
49.4
NP
143.1,154.1
55.0
37.7
a
NP:no peak is shown.
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PHBH can completely crystallize in cooling scan,with an appearance of sharp exotherm.This indicates that CA is an efficient NA for the bacterial copolyesters of PHB.
Effects of CA on the crystallization kinetics of PHB copolyest-ers were compared with those of BN,20,21uracil,21and OA,22,23all of which were good NAs of PHB and its copolyesters.PHBV was taken as an example to compare the nucleation effi-ciency of different NAs.DSC curves recorded in nonisothermal melt crystallization and melting process of PHBV containing different NAs are compared in Figure 2.The corresponding thermal parameters derived from these DSC results are sum-marized in Table I.As shown in Figure 2and Table I,T mc value of PHBV containing 1%CA is 5–10 C higher than that containing the equal amount of BN,uracil,or OA.Similar results are recognized for PHBH through comparing with our previous results of BN,uracil,or OA-nucleated PHBHs.21–23As shown in panel b of Figures 1and 2,NA does not influence the melting behavior of PHB,PHBV,and PHBH.In contrast to the single melting peak of PHB,PHBV and PHBH show two melting peaks upon heating,ascribing to the melt-recrystalliza-tion mechanism.29
Isothermal Crystallization
Isothermal crystallization kinetics of neat and CA-containing PHBV and PHBH were investigated by means of DSC.Parts a and b of Figure 3show the DSC curves of isothermal melt crys-tallization at T c ?40–100 C for the neat and CA-containing PHBVs,respectively.Parts c and d of Figure 3depict the corre-sponding DSC curves of PHBH.As shown in this figure,the crystallization peaks of neat PHBV and PHBH are broad.They become sharp and shift to the short time side with an incorpo-ration of CA,indicating the increase of crystallization rate.As seen in part b and d of Figure 3,CA-containing PHBV and PHBH show the incomplete DSC traces at T c <80 C,indicat-ing the occurrence of crystallization in cooling process.This is because of the fast crystallization of CA-containing polymers and the too low T c .30
To evaluate the isothermal crystallization kinetics,the heat flow in isothermal DSC curve was integrated to attain the relative degree of crystallinity (X t )at different crystallization time (t ).X t was calculated from the integrated area of DSC curve from t ?0to t ?t divided by the integrated area of whole heat flow curve.A horizontal line from a point after the crystallization exotherm was used as the baseline for integration.Half time of crystallization (t 1/2)was evaluated from the time with X t ?50%.Figure 4shows the typical X t $t plots of neat polymers crystallized at 80 C and CA-containing polymers crystallized at 100 C.Isothermal crystallization kinetics were analyzed by Avrami equation.31,32Considering the presence of crystallization induction time t 0,Avrami equation can be written as 30
1àX t àt 0?exp ?àk et àt 0Tn
(1)
where n is the Avrami index and k is the overall rate constant.Linear form of eq.(1)can be written as
log ?àln e1àX t àt 0T ?log k tn log et àt 0T
(2)
k and n can be estimated from the linear fitting of log ?àln e1àX t àt 0T vs.log(t–t 0).Because the incomplete DSC traces usually induce severe errors in Avrami analysis,30the ki-netic modeling of CA-containing polymers crystallized at high temperature (100 C)and neat polymers was only performed.To ensure the accuracy of Avrami analysis,the data in a limited conversion range of 3–30%was employed for linear fitting.30Figure 5shows the typical Avrami plots of neat and CA-con-taining polymers.Excellent fits were attained with the correla-tion coefficients above 0.999.
Kinetic parameters t 1/2,n ,and k of neat and CA-containing PHBV and PHBH are tabulated in Table II.Predicted data by Avrami equation are compared with the experimental values in Figure 4.As seen in this figure,predicted values of neat polymers agree well with the experimental data within whole
conversion
Figure https://www.wendangku.net/doc/f014841680.html,parison of different NAs on nonisothermal melt crystallization and melting behavior of PHBV.[Color figure can be viewed in the online issue,which is available at https://www.wendangku.net/doc/f014841680.html,.]
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range.However,predicted data of CA-containing polymers devi-ate from the experimental values at high conversion.This may be ascribed to the impingement of spherulites and secondary crystal-lization in the latter stage,which produces nonlinearity in Avrami plot.30n is between 2and 3and insensitive to CA.It should be noted that the n values evaluated from Avrami analysis are
lower
Figure 3.DSC curves recorded in isothermal melt crystallization of neat,CA-containing PHBV and PHBH.[Color figure can be viewed in the online issue,which is available at
https://www.wendangku.net/doc/f014841680.html,.]
Figure 4.Variation of relative degree of crystallinity with crystallization time for neat,CA-containing PHBV and PHBH.Isothermal crystallization tempera-tures are 80and 100 C for neat and CA-containing polymers,respectively.Solid lines represent the predicted data from Avrami equation.[Color figure can be viewed in the online issue,which is available at
https://www.wendangku.net/doc/f014841680.html,.]Figure 5.Avrami plots of neat and CA-containing PHBV and PHBH within conversion range of 3–30%.Isothermal crystallization temperatures are 80and 100 C for neat and CA-containing polymers,respectively.Solid lines represent the linear fitting.[Color figure can be viewed in the online issue,which is available at https://www.wendangku.net/doc/f014841680.html,.]
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than the predicted values of polymers crystallized in the3D spherulitic morphology,that is,4for neat polymer nucleated sporadically and3for NA-modified polymers nucleated instanta-neously.30,33As reported in the literatures,some neat and NA-containing polymers crystallized in spherulitic morphology also show the n values between2and3,for example the neat hydro-genated polybutadiene,30neat,NA-containing poly(lactic acid),34,35and poly(butylene succinate).36,37Neat polymer usually contains some impurities that can induce the heterogeneous nucleation to some extent,which can surpass the ideal sporadic and homogenous nucleation in crystallization.This may induce the decrease of n values of neat polymer.As shown in Table II,t1/2values of PHBV and PHBH decrease dramatically with an incorporation of CA.At T c?80 C,t1/2 values decrease to0.72and1.50min from19.6and38.0min of neat PHBV and PHBH,respectively,with an addition of1% CA.This implies the high nucleation efficiency of CA to PHBV and PHBH,consistent with the DSC results of nonisothermal melt crystallization.
Melting behavior of PHBV and PHBH was studied after the isothermal melt crystallization.Panels a and b of Figure6show the DSC heating curves of neat and CA-containing PHBVs after isothermal melt crystallization at T c?40–100 C,respectively. Parts c and d of Figure6show the corresponding DSC curves
Table II.Kinetic Parameters of Neat,CA-Containing PHBV,and PHBH Crystallized at80and1008C a
sample
T c?80 C T c?100 C
t1/2(min)n k(minàn)t1/2(min)n k(minàn)N(/mm3)
PHBV19.6 2.369.88?10à483.2 2.23 4.05?10à563
PHBV/CA0.72–– 2.22 2.420.10 1.6?105 PHBH38.0 2.49 1.43?10à495.7 2.41 1.17?10à537
PHBH/CA 1.50–– 5.42 2.520.018 5.7?104 a Avrami analysis was not performed for CA-containing polymers crystallized at80 C due to their incomplete DSC
traces.
Figure6.DSC heating curves of(a)neat PHBV,(b)CA-containing PHBV,(c)neat PHBH,and(d)CA-containing PHBH crystallized at different tem-peratures.[Color figure can be viewed in the online issue,which is available at https://www.wendangku.net/doc/f014841680.html,.]
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of PHBH.As shown in this figure,CA has no discernible effect on melting behavior of PHBV and PHBH.Neat and CA-containing polymers melt-crystallized at the same T c show the similar DSC melting curves.Melting behavior of PHBH is more complex than PHBV ,indicating the larger effects of HH units on the crystallization of HB units than HV.Different from the DSC curves of PHBV ,an annealing peak,usually ascribed to the secondary crystallization during long time annealing,38,39is present at 10–20 C above T c in the DSC heating curves of PHBH.This suggests that secondary crystallization of PHBH is more distinct than that of PHBV,due to the larger retardant effects of minor HH unit on the crystallization of HB unit.On basis of the DSC melting curves,T m and X c of neat and CA-containing PHBV and PHBH crystallized at different T c were eval-uated.T m was taken as the temperature of peak located at lower temperature in melting region,which corresponded to the melt of crystals formed in isothermal crystallization.T m and X c are plotted
as a function of T c in Figure 7.As seen in this figure,CA does not affect T m and the CA-containing PHBV and PHBH have similar T m as neat polymers.X c values of PHBV and PHBH increase slightly with an addition of CA.Because more prefect crystals with larger lamellar thickness are formed at high T c ,T m and X c of both neat and CA-containing PHBV and PHBH increase with T c .
Spherulitic Morphology
Spherulitic morphology of neat and CA-containing PHBV and PHBH was investigated by POM.Figure 8shows the photomi-crographs of spherulites of neat and CA-containing PHBV and PHBH after isothermal melt crystallization at 60and 80 C.Neat PHBV and PHBH show huge spherulites with a diameter of several micrometers,which are much larger than those of other aliphatic polyesters such as poly(lactic acid)and poly (butylene succinate).The difficulty of nucleation may account for the slow crystallization and large spherulites of PHB bacte-rial copolyesters.PHBV and PHBH containing 1%CA
exhibit
Figure 7.Dependences of (a)melting temperature and (b)degree of crystallinity on crystallization temperature for neat,CA-containing PHBV ,and PHBH.[Color figure can be viewed in the online issue,which is available at
https://www.wendangku.net/doc/f014841680.html,.]
Figure 8.POM photographs of neat and CA-containing PHBV and PHBH crystallized at 60and 80 C.The scale bar represents 50l m.[Color figure can be viewed in the online issue,which is available at https://www.wendangku.net/doc/f014841680.html,.]
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much smaller size and larger density of spherulites than their neat counterparts.Because of the difficulty of nucleation at high temperature,spherulitic sizes of neat PHBV and PHBH increase and their spherulitic densities decrease with increasing T c.On the other hand,the crystalline structure of CA-containing PHBV and PHBH were examined via wide-angle X-ray diffrac-tion(data not shown).It is found that CA does not affect crys-talline structures of PHBV and PHBH.CA-containing PHBV and PHBH form the typical a-form PHB-type crystals in melt crystallization,analogous to neat polymer.
Effects of CA on nucleation densities of PHBV and PHBH were quantitatively analyzed.Nucleation density(N)in isothermal crystallization was estimated according to a model of three-dimensional spherulitic growth simultaneously initiated from the active nuclei(n%3)40
N?
3k
4p G3
(3)
where k is the Avrami rate constant;G is the linear radius growth rate of spherulite,which can be determined from POM measurements.G values of PHBV and PHBH melt-crystallizing at100 C were measured to be5.4and4.2l m/min,respectively. NA does not change the spherulite growth rate of polymers because it only affects the primary nucleation but not secondary nucleation.14N values of neat and CA-modified PHBV and PHBH crystallized at100 C were estimated from eq.(3),as shown in Table II.It can be seen that N values of PBHV and PHBH increase by3–4orders of magnitude with a presence of CA,indicating the good nucleation effectiveness of CA.The unit of k is minàn and n values of neat and CA-containing polymers are not the same.This may result in errors in the esti-mation of nucleation density from eq.(3).
CONCLUSIONS
Effects of CA on nonisothermal and isothermal melt crystalliza-tion of PHB,PHBV,and PHBH have been investigated.CA accel-erates the melt crystallization of PHB bacterial copolyesters remarkably and its acceleration effect is more significant than the typical NAs of PHAs reported previously.With an incorporation of1%CA,PHBV and PHBH finish crystallization upon cooling at10 C/min.Isothermal crystallization kinetics was analyzed by Avrami model.Crystallization half-time of PHBV and PHBH decreases remarkably with an addition of CA.Spherulitic num-bers of PHBV and PHBH increase and their spherulitic sizes decrease significantly with a presence of CA.Nucleation densities of PHBV and PHBH were evaluated.They increase by3–4orders of magnitude with an addition of1%CA.NA reported in this work may widen the applications of bacterial PHAs in the com-modity applications,especially in the cases where the physical properties such as high strength,modulus,and HDT are desired. ACKNOWLEDGMENTS
This work was financially supported by the State Key Laboratory of Chemical Engineering(No.SKL-ChE-11D05and SKL-ChE-12D06),Fundamental Research Funds for the Central Universities, and National Natural Science Foundation of China(51103127).REFERENCES
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