rheological and mechanical characterization of biodegradable aliphatic polyester and...

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Rheological and Mechanical Characterization ofBiodegradable Aliphatic Polyester andPoly(epichlorohydrin) Blends

Jinho Kim,1 Sung T. Lim,1 Hyoung J. Choi,* 1 Myung S. Jhon2

1 Department of Polymer Science and Engineering, Inha University, Inchon, 402-751, KoreaE-mail: [email protected]

2 Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

IntroductionThe production of synthetic plastics and fibers has drasti-cally increased during the last several decades, and thesuccess of these products lies in their strength, low cost,and resistance to chemical and biological attack. Conse-quently, these properties created a disposal problem,since the plastic wastes accumulated in the environmentlead to long-term environmental and waste managementproblems. Thereby, extensive research has focused onvarious biodegradable polymers as potential candidates toreduce pollution problems caused by plastic wastes,including bacterial aliphatic polymers produced by manytypes of micro-organisms such as poly(3-hydroxybuty-rate) (PHB) and its copolymer.[1–3] However, the biode-gradable polymers developed previously are expensive.Furthermore, their physical properties and processabilityare often inferior to non-biodegradable, synthetic poly-mers. Thereby, to obtain both biodegradability and

desired physical properties, a blending or copolymeriza-tion technique has been widely employed.[4–5]

PHB and its copolymers, which are thermoplastic, bio-degradable polymers obtained by bacterial fermenta-tion,[6] are biosynthetic aliphatic polyesters and are misci-ble with various synthetic polymers such as poly(ethyleneoxide) (PEO),[7–9] poly(vinyl acetate) (PVAc),[10] andpoly(methyl methacrylate) (PMMA).[11] PHB/poly(epi-chlorohydrin) (PECH) blends were observed to have sin-gle glass transition temperatures (Tg) and demonstrategood agreement with both the Fox equation and thedepression of equilibrium melting temperature T0

m.[5, 12, 13]

The miscibility of PHB and its copolymers with PHB andits copolymers with poly(vinyl chloride) (PVC)[14] andpoly(e-caprolactone)[15] has been also examined.

On the other hand, biodegradable aliphatic polyesters(BDP)[9, 16] are known to be completely biodegradable inthe environment and possess high melting points and

Full Paper: Rheological, thermal, and mechanical proper-ties for blends of synthetic biodegradable aliphatic poly-ester (BDP) and poly(epichlorohydrin) (PECH) wereexamined. The synthetic BDP was a copolymer synthe-sized from the polycondensation reaction of aliphatic gly-cols and aliphatic dicarboxylic acids. From differentialscanning calorimetry (DSC) and dynamic mechanicalthermal analysis, we confirmed the BDP and PECHblends were miscible. The DSC results suggest that thedegree of crystallization of BDP in blends decreased shar-ply as PECH content increased. We also show a greatincrease of both elongation at the break point and tensileimpact strength increase with PECH content. From thestatic and dynamic rheological experiments, we found thatshear viscosity increases with PECH content, which isconsistent with the storage moduli increasing with PECHcontent. This suggests that PECH acts as a diluent forBDP, and these two polymers are compatible in the meltstate. The storage moduli of the BDP homopolymer arelower than that of blends.

Macromol. Chem. Phys. 2001, 202, No. 12 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/1208–2634$17.50+.50/0

Macromol. Chem. Phys. 2001, 202, 2634–2640

Tc exotherm peaks of BDP and its blends from DSC experi-ments.

Rheological and Mechanical Characterization of Biodegradable Aliphatic Polyester ... 2635

excellent mechanical strength. It can be degraded biologi-cally in soil, fresh water, and seawater, but is stable in theatmosphere. In addition, BDPs have similar physicalproperties to polyethylene and polypropylene and arethereby considered to be one of the most effective, envi-ronmentally friendly packing materials. However, someapplication difficulties exist due to its low melting tem-perature, weak thermal stability, and low molecularweight. Therefore, by blending this biodegradable mate-rial with conventional thermoplastics, low-cost materialswith improved properties can be obtained. Even thoughBDP is comparable with PHB in cost, applications of thispolymer are still limited due to its higher cost comparedto non-biodegradable polymers. Despite its necessity, lit-tle work has performed on the miscible blend systemswith synthetic BDP.

Miscibility behavior and rheological properties ofblends of BDP with both linear low density polyethylene(LDPE)[9] and PVAc[17] have been investigated andrevealed that both blend systems were immiscible, byshowing two distinct Tg values in all compositions.Recently, Kim et al.[18] found the miscibility of a blend ofBDP with PECH by both thermal characterization fromdifferential scanning calorimetry (DSC; Perkin-ElmerDSC 7) and dynamic mechanical thermal analyzer(DMTA; Rheometric Scientific), and morphological stu-dies from scanning electron microscopy (SEM) experi-ments.

In this paper, we investigated thermal, rheological, andmechanical properties of BDP/PECH blends, which areknown to be miscible. We also examined how the mole-cular structure and characteristics of a second componentinfluence the miscibility in the melt and solid state forBDP and PECH, where the second component is rubbery.In addition, the rheological properties of synthetic BDPand its blends provide valuable information for determin-ing processing conditions. Han and Jhon[19] have shownthat the logarithmic plots of storage modulus (G9) vs. lossmodulus (G99) for miscible blends give correlations vir-tually independent of temperature and blend composition.G9–G99 plots are very useful for interpreting the rheologi-cal behavior of both compatible and incompatible poly-mer blends, analogous to the logarithmic plots of the firstnormal stress difference (N1) vs. shear stress (s12).

Experimental Part

Materials

The synthetic BDP (Skygreen 2109), acquired from Sun-kyong Industries (SKI, Korea), was the copolymer synthe-sized from the polycondensation reaction of diols (ethyleneglycol and 1,4-butanediol) and aliphatic dicarboxylic acids(succinic acid and adipic acid). The weight-average molecu-lar weight (M

—w) of the BDP measured from gel permeation

chromatography was 6.06104 g/mol. The PECH sample

(M—

w = 7.06105 g/mol) was purchased from Scientific Poly-mer Products (U.S.A.). The noncrystallizable, rubberyPECH, which is a homopolymer of epichlorohydrin, is a use-ful rubber that has excellent resistance to ozone, oils, heatand weathering and also has very low gas permeability.Structures of both BDP and PECH are as follows:

A BDP/PECH solution in dichloromethane was initiallyblended by stirring for 24 h at room temperature, and thendried to constant weight under vacuum at 50 8C. BDP/PECHblend ratios were 100/0, 90/10, 80/20, 70/30, and 60/40 byweight; their code names were BDP100, BDP90, BDP80,BDP70, and BDP60. Each sample was used to make a filmvia a hot press at 1508C.

Analysis

Influence of the second component, composition, and ther-mal history on the crystallinity, melting, and crystallizationtemperature of BDP and its blend were measured via DSC.At first, the blended films were heated from 30 to 1008C at arate of 108C/min and then held at 1008C for 2 min (run I).The samples were then quenched in liquid nitrogen to–1008C. Finally, after approximately 2 min at –1008C, thetemperature was raised to 1508C at a rate of 108C/min (runII). Tg was obtained, and Tm and the crystallinity were meas-ured from the DSC endotherms. For non-isothermal crystal-lization behavior the samples were cooled down to 08C after1 min at 1508C at a rate of 108C/min.

DMTA was also employed to examine the miscibility ofthe blends. For the DMTA test, polymeric film samples (0.7cm62 cm60.05 cm) were clamped in a frame and sub-jected to three point bending. We applied a frequency of1 Hz and a heating rate of 38C/min from –100 to 508C,while the film was under tension. The variation of the G9,G99, and loss tangent (tan d) was then recorded.

Rheological properties of the blend systems were exam-ined using a rotational rheometer (Physica MC-120, PhysicaInc., Germany). Cylindrical samples having 1 mm thicknessand 6 mm radius were analyzed in the parallel plate geome-try at a fixed temperature of 1508C. Shear viscosity andshear stress were measured as a function of shear rate.Dynamic experiments were carried out with the cone andplate geometry (radius: 12.5 mm, angle: 18, truncation: 0.05mm). G9, G99 were measured versus frequency for six differ-ent temperatures with a 6% deformation.

For tensile tests, the premixed material was compression-molded at 1708C and of 10000 psi in a heated press to pro-duce sheets (0.2 mm thick). The sheets were cut into dumb-bell-shaped specimens on which each tensile mechanical testand tensile impact test were performed. A universal testingmachine (Zwick) was used to determine the ultimate tensilestrength, elongation at break with a speed of 50 mm/min anda sample length of 10 mm at room temperature. The tensile

2636 J. Kim, S. T. Lim, H. J. Choi, M. S. Jhon

impact tester (Custom Scientific Instrument Inc.) was alsoused to determine the tensile impact strength.

Results and DiscussionThe DSC thermogram of BDP and its blends showed thatonly one endotherm peak is observed, and it also showedthat when heated from –100 to 1508C (run II), the peakshifted to a lower temperature as PECH contentincreased. A single Tg was observed for all BDP/PECHblend ratios showing an excellent agreement with theore-tical values obtained from the Fox equation and indicat-ing that BDP and PECH are miscible.[18] Figure1 showsthat the addition of PECH causes a depression in theexperimental Tm of BDP in the blend. The Tm depressionis a common phenomenon for the miscible blends con-taining one crystallizable component.[5, 12] Similar resultsare found in all the miscible PHB blends when theamount of the second polymer is increased.[5, 7–11] Thedecrease in the heat of fusion (DHf) also suggests that theaddition of PECH results in a decrease in the crystallineperfection of BDP. The decrease of the Tm and DHf cantherefore be attributed to morphological and dilutioneffects. A decrease in the T0

m and the negative value of theinteraction parameter (v12) of the PHB was observed inblends when the PECH was added.[12] Because the spher-ulite growth rate of BDP is faster than that of PHB, andthe isothermal crystallization peak cannot be detected inDSC, we could not get the T0

m and v12 , which means themiscible blend theoretically exists for negative values ofthe v12 of BDP/PECH in our system.

Furthermore, from the DSC thermograms of BDP andBDP/PECH blends obtained during the non-isothermalcrystallization runs, we were able to observe the Tc andthe area of the exotherm peak, given in Figure 2, are dras-tically influenced by the composition. The degree of crys-tallization of BDP in blends decreased as PECH content

increases. In addition, the BDP/PECH blends with morethan 40% PECH by weight did not show a crystallizationexotherm. This result implies that crystallization of BDPin the blends becomes more difficult with increasedPECH content, suggesting that BDP is miscible withPECH in the melt. It is also related to the depression ofthe spherulite growth rate due to the mutual dilutioneffect of the two polymers, implying that the noncrystal-lizable component is incorporated in the interlamellar orinterfibrillar regions of the BDP spherulites. Avella etal.[7] reported similar results for PHB/PEO blends.

These interactions are likely to occur for polymers thatcontain chlorine or oxygen atoms.[20] Riedel and Prud’-homme[21] found that PVC/poly(e-caprolactone) blendshave a specific interaction between the hydrogen of theClCH and the ester group. Many researchers demon-strated that PVC is miscible with other, but not all, ali-phatic polyesters.[21, 22] These findings can be conveni-ently organized in terms of the ratio of the number of ali-phatic carbons, CH2 or CHx units depending on whetherthe structure is linear or branched, to the number of estergroups, COO, in the polyester repeating unit as found forblends of other polymers with this homologous series.[22]

Miscibility of P(HB-HV)/PVC blends has been reportedbased on the existence of single Tg value.[23] Holmes etal.[24] also reported the partial miscibility of PHAs withpolymers containing chlorine or nitrile groups. Blends ofPHAs and several chlorine and nitrile containing poly-mers show improvement in mechanical properties. It wassuggested that the chlorine and nitrile groups promotehydrogen bonding with the carbonyl groups of the PHApolymers. Since PECH has similar structure to PVC, con-taining chlorine groups, the interactions between BDPand PECH are probably due to hydrogen bonding. There-fore, it can be considered that PHB/PECH blends havesimilar interaction as BDP/PECH blend.

In order to examine the miscibility of each blend sys-tem, we measured Tg of the blends by evaluating Tg at the

Figure 1. Tm endotherm peaks of BDP and its blends fromDSC experiments.

Figure 2. Tc exotherm peaks of BDP and its blends from DSCexperiments.


point where tan d from the DMTA exhibits a maximum.As shown in Figure 3, the blends have single Tg peaks,which are dependent on composition. These resultsstrongly suggest that polymers of each blend are miscible.Similar miscibility behavior has also been observed forbiodegradable polybutylene succinate adipate copolymerand PECH blends.[24]

Rheological properties (e.g., viscosity and elasticity)are important to polymer processing fundamentals,including pressure drops, and heating & stress relaxa-tions. As shown in Figure 4, the shear viscosity (g)increases with PECH content. In the case of homopoly-mers, slight non-Newtonian fluid behavior is observed ata relatively high shear rate (_cc). However, by increasingPECH content, we observed a drastic increase in shearthinning behavior. Remarkably, the shear viscosityincreases with the rubber content (even at 10 wt.-%). Inaddition, BDP60 has an enormous shear viscosity, indi-cating that the BDP60, similar to PECH, is rubber-like. Itis difficult to detect shear viscosity above 60 wt.-%

PECH content because of the high torque requirement.To examine the dependence of g on _cc, we fitted g to theCarreau model:

g ¼ g0½1þ ð _cckÞ2�ðnÿ1Þ=2 ð1Þ

Here, g0 is the zero shear rate viscosity, k is the relaxa-tion time, and n is a dimensionless power law parameter.The slope in the power-law region is (n–1). Note that thespecial case of n = 1 or _ccke 0, Equation (1) reduces tothe Newtonian fluid model with constant viscosity, and ifn a 1, the model predicts shear-thinning behavior. Thecalculated values for g0 , n, and k are shown in Table 1.From this, we can predict that g0 , (1 – n), and k increasewith PECH content. The shear thinning viscosities ofBDP/PVAc blends[17] and PEO-clay nanocomposites[26]

were also observed to fit the Carreau model quite well.It should be noted that G9 represents the energy stored

in the fluid (elastic property) and G99 represents theenergy loss (viscous property) during oscillatory shearflow. Figure 5 shows G9 as a function of frequency.Because PECH acts as a diluent for BDP, and the twopolymers are compatible in the melt phase, G9 of the BDPhomopolymer are lower than that of the blends. This isrelated to the fast relaxation processes via an increase inmolecular weight with PECH content. Figure 6 gives G99

as a function of frequency, showing that G99 increases

Figure 3. Tan d vs. temperature for BDP/PECH blends(DMTA).

Figure 4. Shear viscosity vs. shear rate for BDP and its blendsat 150 8C.

Table 1. Carreau model (Equation (1)) for BDP/PECH blendsystems.

g0610ÿ4

Pa N sk

sn

BDP 100 0.319 1.70 0.78BDP 90 3.19 3.36 0.24BDP 80 9.98 3.80 0.15BDP 70 15.5 6.89 0.23BDP 60 24.7 6.71 0.19

Figure 5. Storage moduli vs. frequency for BDP and its blendsat 150 8C.


slightly with PECH content. For the BDP homopolymer,G99 is observed to be higher than G9. This means thatenergy dissipation is greater than energy storage. How-ever, by increasing PECH content, an opposite behavioris observed, due to elastic rubber particles. In general,concentrated suspensions exhibit similar qualitativeresponses, at only small strains. However, concentrated,high molecular weight polymeric liquids show behaviorsimilar to rubber at shorter times with a nearly constantmodulus plateau followed by flow relaxation.[27] ThePECH homopolymer shows a fast relaxation followed bya constant modulus, and the BDP homopolymer exhibitstypical polymeric liquid behavior. However, as PECHcontent increases, it behaves similarly to the concentratedsuspension. Above 60 wt.-% PECH content, it shows arubbery behavior. In addition, the dynamic viscosities ofthe high and low PECH blends cross over shear rate,showing that the shear rate dependent viscosity variesstrongly with the blend composition.

Han and co-workers[19, 28] have shown that G9 – G99 plotsgive rise to correlations that are virtually independent oftemperature and are very useful for interpreting the com-patibility of polymer blends. When a blend system is trulycompatible on the molecular level, N1 – s12 and G9 – G99

plots become composition-independent. It should beremembered that s12 and G99 are interpreted as energy dis-sipation, and N1 and G9 are interpreted as energy storagein the molecules during the shearing deformation. There-fore, the ratio of the energy stored and the energy dissi-pated during the shear deformation is expected to be inde-pendent of the blend composition. Yang et al.[29] havestudied G9 – G99 plots of PMMA/PVDF, blends of twohomopolymers LDPE blends having different molecularweights, and heterogeneous blends of PMMA/polystyrene(PS) and PMMA/poly(styrene acrylonitrile) (PSAN). TheG9 – G99 plot for BDP60 at various temperatures is givenin Figure 7. It was found that BDP60 collapsed into a sin-

gle line, which is independent of temperature, and alsoindicated that the two polymers are compatible in themelt. It also shows that the slope is less than 2 due topolydispersity. G9 – G99 plots for different blend composi-tions of BDP/PECH are shown in Figure 8, which exhibita composition-dependent correlation. This might becaused by the large difference in molecular weightsbetween BDP (M

—w = 6.46104 g/mol) and PECH (M

—w =

7.06105 g/mol).The mechanical properties of BDP/PECH blends (up to

40 wt.-% of PECH) were also studied. From the results oftensile tests shown in Figure 9, it is observed that elonga-tion at break increases with PECH content, while the ten-sile strength for each blend is almost constant. Note thatabove 1500% elongation, BDP60 shows rubbery proper-ties. Furthermore, as shown in Figure 10, tensile impactstrength also increases with PECH content. The increaseof elongation at break and tensile impact strength resultsin better compatibility and better adhesion of the rubberto the matrix due to the formation of interactions betweenBDP and PECH.[30] From the relation between morpho-

Figure 6. Loss moduli vs. frequency for BDP and its blends at150 8C.

Figure 7. G9 vs. G99 for BDP60 (40 wt.-% of PECH) at six dif-ferent temperatures.

Figure 8. G9 vs. G99 for BDP and its blends at 150 8C.


logy and mechanical properties, it is known that thisblend system is miscible.

Even though the biodegradability of the BDP/PECHblends was not investigated in this paper, previous studyby Sadocco et al.[13] on biodegradation of PHB/PECHblends suggests that the addition of the PECH affects thebiodegradability of our blend systems. They[13] reportedthat the biodegradation rate decreased with PECH con-tent, for films containing up to 60 wt.-% PECH, afterwhich biodegradation was completely inhibited. The bio-degradation of pure BDP used in this study can be refer-enced from the fact that the biodegradation rate of PHB/poly(3-hydroxybutylate) (PHB) A Skygreen (BDP) A

Mate-Bi.[31]

ConclusionsWe examined the miscibility of BDP/PECH blends andalso studied thermal, rheological, and mechanical proper-ties. From the DSC experiments, we observed a single Tg

for the blends, which suggests that BDP/PECH form mis-cible blends for the entire composition range. In addition,Tc of BDP/PECH blends was found to be sharplydepressed by the presence of the second component, and

Tm of BDP/PECH decreased as the second componentincreased. Furthermore, Tg with a single peak is depen-dent on the composition obtained from DMTA. The rheo-meter data show that shear viscosity (as a function ofshear rate) increases with PECH content. The storagemoduli (as a function of frequency) also increase withPECH content, which PECH plays a role of a diluent forBDP, and the two polymers are compatible in the meltphase. This implies that the storage moduli of the BDPhomopolymer are lower than that of blends. For mechan-ical properties, both elongation at break and tensileimpact strength also increase with PECH content.

Acknowledgement: This study was supported by grant fromthe KOSEF through Applied Rheology Center at Korea Univer-sity, Korea.

Received: September 29, 2000Revised: December 19, 2000

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