Crystallization behavior of poly(lactic acid)/elastomer blends

ORIGINAL PAPER

Crystallization behavior of poly(lactic acid)/elastomer blends

Mujtahid Kaavessina & Ilias Ali & Rabeh H. Elleithy &

S. M. Al-Zahrani

Received: 14 September 2011 /Accepted: 15 December 2011 /Published online: 22 February 2012# Springer Science+Business Media B.V. 2012

Abstract Differential Scanning Calorimetry (DSC) was usedto evaluate the crystallization behavior of poly(lactic acid) andits blends with elastomer. It has been observed that the coldcrystallization temperature of the blends decreased as theweight fraction of elastomer increased as well as the onsettemperature of cold crystallization also shifted to lower tem-perature. In non-isothermal crystallization experiments, thecrystallinity of poly(lactic acid) increased with a decrease inthe heating and cooling rate. The melt crystallization of poly(lactic acid) appeared in the low cooling rate (1, 5 and 7.5 °C/min). The presence of low elastomer tends also to increase thecrystallinity of poly (lactic acid). The DSC thermogram at rampof 10 °C/min showed the maximum crystallinity of poly(lacticacid) is 36.95% with 20 wt% elastomer contents in blends. Inisothermal crystallization, the cold crystallization rate increasedwith increasing crystallization temperature in the blends. TheAvrami analysis showed that the cold crystallization was in twostages process and it was clearly seen at low temperature. TheAvrami exponent (n) at first stage was varying from 1.59 to 2which described a one-dimensional crystallization growth with

homogeneous nucleation, whereas at second stage was varyingfrom 2.09 to 2.71 which described the transitional mechanismto three dimensional crystallization growth with heterogeneousnucleation mechanism. The equilibrium melting point of poly(lactic acid) was also evaluated at 176 °C.

Keyword Poly(lactic acid) . Elastomer . Coldcrystallization . Cold crystallization kinetic

Introduction

Since the last decades, polymers have substituted many con-ventional materials (glass, metal, wood, ceramic, etc.) anddominated in many applications. Due to environmental issuesand rising oil prices, researchers from academia and industryhave been encouraged to develop biodegradable polymers.Biodegradable polymers have the potential to be biomaterialsin medical application [1, 2].

Avérous (2004) has classified of biodegradable polymers intofour families based on the origin different. They are (i) from agroresources (e.g. starches and chitin), (ii) from microbial activity(e.g. poly(hydroxybutyrate co-hydroxyvalerate)/PHBV), (iii)from biotechnology (e.g. poly lactic acid/PLA) and (iv) frompetrochemical product (e.g. polycaprolactone/PCL) [3].

Among these, poly(lactic acid) has been well acknowl-edged as having a better potential of mechanical properties,thermal plasticity, biocompatibility and can be readily fabri-cated [4]. Also, utilization of renewable resources and itsbiodegradability to nontoxic of simple and natural product ina relative short time were able to encourage its use in manyapplications [5, 6]. Much invention has been employed totackle its lack properties and also to tailor the new propertiesin wide range applications, especially in the packaging and thehousehold fields.

M. Kaavessina : I. Ali : S. M. Al-ZahraniChemical Engineering Department, King Saud University,Riyadh 11421, Saudi Arabia

M. Kaavessina : I. Ali (*)SABIC Polymer Research Center (SPRC), King Saud University,Riyadh 11421, Saudi Arabiae-mail: [email protected]

R. H. ElleithyPrintpack Inc,Williamsburg, VA, USA

S. M. Al-ZahraniCenter of Excellence on Research Material (CEREM),King Saud University,Riyadh 11421, Saudi Arabia

J Polym Res (2012) 19:9818DOI 10.1007/s10965-011-9818-9

The main problems faced due to the uses of poly(lacticacid) in those applications are brittleness, stiffness and thermalinstability [7–9]. Thus, several methods have been developedto improve these properties of poly(lactic acid) with highflexibility, ductility and high impact without altering its tensileproperties. Introducing of the other polymer through meltblending or inserting of other polymer chain through copoly-merization have given more significant effects in improvingbulk properties of poly lactic acid [9–12].

Elastomers are polymers with low young modulus andhigh yield strain. If an amorphous polymer has a tempera-ture glass transition (Tg) below room temperature, it willhave elastomer properties, i.e. soft and rubbery. By intro-ducing of elastomer to poly (lactic acid), a new class of poly(lactic acid) can be prepared with enhanced elastic andductile properties. In previous study, Zhang et al. [13] andZaman et al. [7] reported the addition of elastomer throughmelt blending. They demonstrated that the tensile modulusof poly(lactic acid) decrease with an enhancement of yieldstrain significantly.

Considering the relationship between molecular struc-ture and property, crystallinity reflected to mechanicaland thermal properties [5, 8, 14]. Suryanegara et al. [8]reported that increasing degree of crystallinity of poly(lactic acid) enhanced the tensile strength, however theelongation at break and elasticity were sacrificed. Theinfluence of molecular weight of poly(lactic acid) oncrystallization characteristic was also reported [15]. Crys-tallization kinetic was an important characteristic thatguide to the optimum conditions of crystallization forthe design of desirable properties of poly (lactic acid).For example, the lower crystallization rate required lon-ger time to obtain the certain crystallinity, the amorphousphase was obtained in short time. Liao et al. [16] havestudied the isothermal cold crystallization of polylactide/nucleating agents. They found that crystallization ratedecreased as increasing temperature. The different resultwas reported by Zhao et al. [6] in their investigation ofpoly (lactic acid)/multiwalled carbon nanotube (MCN) nano-composites. The higher temperature enhanced the crystalliza-tion rate. Due to the most process were operated under nonisothermal. The study of nonisothermal crystallization kineticwas also important. Several researchers reported that increas-ing heating rate improved the crystallization rate [6, 17].

The most of the paper have reported the crystallizationkinetics of poly (lactic acid)/filler blend. There are a fewreports on crystallization kinetic of poly (lactic acid)/elasto-mer blends, whereas some researcher studied to enhance theflexibility, elasticity and ductility by blending poly(lacticacid) with elastomer or other polymer. In this investigation,details of crystallization behavior of poly (lactic acid)/elas-tomer are reported. Poly(lactic acid) and elastomer is blend-ed by injection molding directly. By using DSC, the cold

and melt crystallization phenomenon is elucidated. The coldcrystallization during non-isothermal and isothermal modeis also investigated, including the effect of thermal treatmenton cold crystallization.

Experimental

Materials

Poly (lactic acid), commercial name “PLI003”, was kindlysupplied by Natureplast™ (France). This grade is typicallydesigned for injection molding. Characteristics of PLI003 aretransparent, density of 1.25 g/mL and melt flow index (MFI)of 12–20 g/10 min (190 °C/2.16 kg) as reported by supplier.Elastomer, commercial name “NPEL001”, uses in this studywas also procured fromNatureplast™ (France). NPEL001 hasdensity of 1.23 g/mL and opaque. The poly lactic acid wasmelt blended with various amounts of elastomer (Table 1).

Samples preparation

The samples were prepared by following the procedureprovided by the supplier (Natureplast™), before doing theinjection molding. Poly(lactic acid) and elastomer werepreviously placed in oven during 4 h at 80 °C. ASTMsamples of the blends were prepared by an injection mold-ing machine, Super Master Series SM 120 made by AsianPlastic Machinery Co. The inside conditions are tabulated inTable 2. During the process, the nozzle of injection moldingis opened at 60% with pressure 30 bars. Cooling time andcycle time are 30 and 40 s, respectively. Cold water as aquencher is at 10–11 °C.

Characterization

The crystallization behavior of poly(lactic acid) and itsblends with elastomer were performed by using a ShimadzuDSC60. There were two parts in these investigations, i.e.non isothermal and isothermal crystallization, which haddifferent procedures.

Table 1 List of the samples prepared for this study

Sample Composition, wt%

Poly (lactic acid) Elastomer

PLA 100 0

PLA10 90 10

PLA20 80 20

PLA30 70 30

of 12 M. Kaavessina et al.

Non isothermal crystallization mode

The procedure of this part was: first heating with aramp of 10 °C/min from room temperature up to200 °C and kept at this temperature for 5 min todiscard any anterior thermal history, and cooled downat a ramp of 10 °C/min down to 25 °C, second heatingscan from 25 °C to 300 °C with a ramp of 10 °C/min.The various ramps were employed at 1, 5, 7.5, 15 and20 °C/min for all blend samples, except for theannealed samples on cold crystallization experiment,which was carried out at 10 °C/min only.

Isothermal crystallization mode

The procedure of this study was: first heating with a ramp of10 °C/min from room temperature up to 200 °C and kept atthis temperature for 5 min to discard any anterior thermalhistory and subsequently cooling at 10 °C/min to 25 °C. Thesecond heating scan from 25 °C to various Tiso at 50 °C/minand held for 5 min, then heated at 10 °C/min to 250 again toget the melt temperature.

Results and discussion

Non isothermal crystallization mode

Thermal properties analysis

Crystalline structures of poly(lactic acid) occurred betweenglass transition temperature (Tg) and melting temperature(Tm). There are two types of crystallization which are dis-tinguished by the difference of their initial state. Melt

Table 2 Details of injection molding conditions

Condition Injection Zonea

I II III IV V VI

Temperature, °C 180 190 190 170 160 140

Speed of screw, rpm 60 40 30 25 20 20

a Zone I 0 die zone and zone VI 0 feed zone

Fig. 1 Dynamic scanning thermogram of PLA20 with different heating/cooling rate: (a) 10 °C/min, (b) 1 °C/min and (c) 7.5 °C/min

Crystallization behavior of poly(lactic acid)/elastomer blends of 12

crystallization means that the crystallization occurred duringcooling scan. The initial stage of this type is the melt state,thus the poly(lactic acid) samples should in temperature thathigher than its Tm. The second type is cold crystallizationwhere the crystallization occurred during heating scan andits initial state was started from temperatures lower than itsTg. This phenomenon is clearly visible in Fig. 1.

Figure 1a shows the dynamic scanning thermogram ofPLA20 with heating/cooling rate at 10 °C/min. In the firstheating scan, the thermogram of PLA20 revealed the glasstransition temperature (Tg051.1 °C), cold crystallizationtemperature (Tcc085.5 °C) and melting point (Tm0167.3 °C)clearly. During the cooling scan from 200 °C, there were noappeared exothermic peak, signifying that sample cooled fromthe melt state with rate greater than 10 °C/min remain amor-phous. Evidently, in the second heating scan, it still exhibitedthe cold crystallization (Tcc090.0 °C) and also the exother-mic enthalpy shift was closer to the melting enthalpy (Tm0

166.8 °C), indicating that there was no crystallizationduring the cooling scan. The similar results were reportedby other researchers [15, 16].

The different phenomena occurred in the heating/coolingrate at 1 °C/min (Fig. 1b). Although, the thermogram ofPLA20 exhibited also the glass transition temperature (Tg),cold crystallization (Tcc) and melting temperature (Tm) inthe first heating scan. The values of Tg and Tcc have shifteddown to 46.5 °C and 68.7 °C, respectively. The melt crys-tallization (Tmc) appeared at 110.0 °C when cooled scanfrom 200 °C. In the second heating scan, the thermogramof PLA20 exhibited only Tm (166.5 °C) while Tg and Tccwere absent, indicating that there was complete crystalliza-tion during the cooling scan. This phenomenon is clearlyobserved from Fig. 1c. The second heating scan shows thatthe thermogram of PLA20 still exhibited Tcc (81.1 °C) andTm (166.2 °C), although the exothermic peak also appearedduring cooling. It can be concluded that crystallization wasnot perfect at cooling rate of 7.5 °C/min. As a whole, therewas not enough time for poly(lactic acid) chains to rearrangeinto a crystalline structure in the fast cooling. It is anagreement with other reported results [5, 8]. The quantifica-tion of Fig. 1 and the others samples was tabulated in theTable 3.

Table 3 Thermal characteristics of poly(lactic acid) and its blends obtained from cooling and second heating scans

Sample Θ, Melt crystallization Cold crystallization Melting Xc,%

°C/min Tmc±0.2% ΔHmc±0.15% Tcc±0.2% ΔHcc±0.15% Tm±0.2% ΔHm±0.15%

PLA 1 104.4 23.19 – – 167.6 −31.34 33.45

5 91.2 20.12 95.4 24.72 169.0 −53.75 30.98

7.5 62.3 10.60 98.9 29.05 169.2 −51.42 23.87

10 – – 102.9 29.79 170.1 −49.27 20.79

15 – – 106.2 27.64 170.9 −38.29 11.37

20 – – 113.9 29.25 172.5 −34.33 5.42

PLA10 1 112.3 33.77 – – 166.7 −30.41 36.06

5 95.6 32.22 – – 167.1 −29.53 35.02

7.5 91.0 10.87 87.1 8.12 168.3 −36.02 33.08

10 – – 93.5 20.03 168.9 −47.17 32.18

15 – – 101.2 19.03 169.3 −39.90 24.75

20 – – 104.8 27.82 170.9 −34.42 7.83

PLA20 1 110.0 35.15 – – 166.5 −29.74 39.67

5 93.2 33.65 – – 165.8 −28.85 38.48

7.5 88.2 15.49 81.1 9.21 166.2 −37.53 37.78

10 – – 90.0 15.58 166.8 −43.28 36.95

15 – – 94.8 13.24 167.2 −29.21 21.30

20 – – 99.4 18.19 168.7 −23.04 6.47

PLA30 1 102.3 20.60 – – 160.4 –22.44 34.21

5 88.8 19.53 – – 162.1 −21.80 33.23

7.5 62.7 12.14 71.8 6.89 163.4 −27.58 31.54

10 – – 81.3 13.02 164.5 −32.09 29.07

15 – – 85.8 9.04 164.9 −22.47 20.48

20 – – 90.2 16.04 165.1 −20.63 6.99

Θ 0 heating/cooling rate, °C/min; Xc 0 degree of crystallinity,%; ΔH 0 melting enthalpy, J/mol


Table 3 listed the thermal characteristics of poly(lactic acid)and its blends with elastomer at various heating/cooling rates(Θ). The melt crystallization temperature (Tmc) shifted downto lower temperature with increasing cooling rate. In parallel,the exothermic enthalpy (ΔHmc) was also decreased. More-over, the melt crystallization peaks disappeared above coolingrate at 7.5 °C/min. As mentioned earlier, the presence of peaksin the cooling scan reflected the crystallization during cooling.It proved that higher crystallinity of poly(lactic acid) wasobtained by decreasing cooling rate.

The quantification of crystallinity fraction (Xc) was calcu-lated by using of the following equation [18]:

Xc ¼ ΔHm �ΔHcc

x ΔHom

� 100% ð1Þ

Where,ΔHm andΔHcc are the experimental melting and coldcrystallization enthalpy, respectively. x is the weight fraction

of poly(lactic acid) in blends. The value of ΔHom ¼ 93:7 J=g

[18] was used according to a reported enthalpy of melting of100 wt% crystalline poly (lactic acid). The crystallinity (Xc)values for all samples and different heating rate (°C/min) werealso documented in Table 3.

At heating/cooling rate 1 °C/min, the addition of elasto-mer (10, 20 and 30 wt%) increased crystallinity of poly

Fig. 2 Neat PLA and PLA/elastomer blends plot by using:(a) Kissinger and (b) Takhor

Table 4 Calculation of ΔEcc by equation of Kissinger’s and Takhor

No Sample Activation energy of cold crystallization, J/mol

Kissinger Takhor

1 PLA 81643.5 (R200.981) 87920.6 (R200.984)

2 PLA10 55254.8 (R200.983) 61390.6 (R200.986)

3 PLA20 53176.3 (R200.941) 59203.9 (R200.952)

4 PLA30 49468.3 (R200.922) 55346.3 (R200.936)


(lactic acid) from 33.45% to 36.06, 39.67 and 34.21%,respectively. In the PLA30 blend, crystallinity of poly(lacticacid) found to decrease. The addition of 10 and 20 wt%elastomer may act as diluent agent on the crystallization ofpoly (lactic acid) by giving the extra mobility of its chains toform crystal [19]. However, at PLA30, the elastomer in-duced higher levels of free volume and tended to inhibitthe crystal growth of poly(lactic acid) [5, 7]. In addition, thedispersion of formed PLA crystals in blends could be alsoresponsible for the decrease of crystallinity [13].

Suryanegara, et al. [8] concluded that quenching of poly(lactic acid) from melt state will produce the amorphousphase. Their statement also occurred in this investigation.The quenched poly(lactic acid) from melt state (200 °C) tosolid state (25 °C) with cooling rate 20 °C/min producedlow crystallinity (5.42%). At the same cooling rate, thecrystallinity of PLA10, PLA20 and PLA30 were 6.76%,6.47%and 6.99%, respectively.

For poly (lactic acid)/elastomer blends, it can be seenthat cold crystallization was not appeared at heating/cooling rate 1 and 5 °C/min. The crystallinity of poly(lactic acid) should be same because the crystallizationduring cooling was completed. In fact, there was a littlechange about 1% of crystallinity. It may caused byinsensitivity of the DSC at low heating/cooling rate.Thus, at heating rate 5 °C/min, the cold crystallizationcannot be detected easily.

Table 3 also presents the dependence of cold crystalliza-tion behavior on the various heating rate of neat poly(lacticacid) and its blends with elastomer. By increasing the heat-ing rate from 7.5 to 20 °C/min, the exothermic cold crystal-lization peaks became wider and shifted to a highertemperature. It indicated that there was not enough timeduring crystallization at high heating rate [6]. The sametrend was reported by other researchers [6, 17, 20, 21]. Inthe same heating rate, the cold crystallization temperature

Fig. 3 Fractional crystallinityof poly(lactic acid) as a functionof (a) temperature and (b) timeat various heating rates


decreased significantly as increasing elastomer contents inthe blends. This result suggests that poly(lactic acid) andelastomer are thermodynamically compatible as reported byothers [7, 21]

Cold crystallization activation energy

The activation energy of the cold crystallization was also animportant parameter that indicates the crystallization ability[20]. There are some mathematic models to estimate crys-tallization activation energy (ΔE). In this investigation, Kis-singer’s [20, 23] and Takhor’s [20] equation as shown inEqs. 2 and 3, respectively, were employed to calculate thisenergy by considering the effect of the various heating rate

in the non isothermal cold crystallization. They were pre-sented as following equation:

d ln Θ=T2cc

� �� d 1=Tccð Þ ¼ �ΔE

Rð2Þ

d ln Θð Þ½ �d 1=Tccð Þ ¼ �ΔE

Rð3Þ

where, R is the universal gas constant (8.314 J/mol K), Θ isheating/cooling rate, °C/min and Tcc is cold crystallizationtemperature, °C

By plotting Kissinger’s and Takhor’s equation, linearrelationship can be obtained as shown in Fig. 2 and its slopegave the value of ΔE/R. The activation energy of coldcrystallization was summarized in Table 4. Considering theregression coefficients (R2), both of them fitted experimen-tal data very well.

Table 4 shows the activation energy of cold crystallization(ΔEcc) for neat poly(lactic acid) and its blends. As explainedby Huang et al. [20], activation energy (ΔE) is the minimumenergy required to transport molecular part to the crystalliza-tion surface. Thus, the low ΔE value exhibited the highcrystallization ability [22]. Both Kissinger and Takhor equa-tion showed the same tendency, the presence of elastomer inblends tends to decrease the activation energy. It can beconcluded that elastomer may give an extra movement abilityof chain segments, thereby making it easier for crystallization.The activation energy of poly(lactic acid) as presented inTable 4 was closed to the other reports [6, 17].

Kinetics of non-isothermal cold crystallization

Fractional crystallinity (Xt) of poly(lactic acid) and itsblends as a function of cold crystallization temperature canbe proposed as follows [20, 21]:

Xt ¼R TT0

dHcc=dTð ÞdTR T1T0

dHcc=dTð ÞdTð4Þ

where, To and T∞ reflect the initial and the end of coldcrystallization temperature, respectively. The (dHcc/dT) rep-resent the enthalpy difference of cold crystallization for thecertain temperature range. The kinetic of cold crystallizationcan be explained by making relationship between fractionalcrystallinity versus crystallization time. Thus, cold crystal-lization temperature in the Eq. 4 should be converted tocrystallization time. Some researchers have given a strictlyvalid conversion as follows [6, 20]:

t ¼ To � T

Θð5Þ

where T is the temperature at crystallization time t andΘ is the heating rate, °C/min. Figure 3 represents the

Table 5 Non-isothermal cold crystallization kinetic parameter

Sample Θ, (°C/min) t½ (min) 1/t½ (min−1) CRP

PLA 7.5 1.56 0.64 0.0528

10 1.34 0.75 (R200.9934)

15 0.94 1.06

20 0.78 1.28

PLA10 7.5 1.48 0.68 0.0563

10 1.26 0.79 (R200.9816)

15 0.86 1.16

20 0.74 1.35

PLA20 7.5 1.42 0.70 0.0587

10 1.24 0.81 (R200.9535)

15 0.80 1.25

20 0.72 1.39

PLA30 7.5 1.22 0.82 0.0618

10 1.1 0.91 (R200.9920)

15 0.78 1.28

20 0.64 1.56

Fig. 4 Crystallization rate parameter (CRP) of poly(lactic acid) and itsblends


plots of fractional crystallinity versus crystallizationtemperature (Fig. 3a) and time (Fig. 3b) at variousheating rate.

Figure 3a describes that the crystallization temperature ofpoly(lactic acid) shifts down to lower temperature range asdecreasing the heating rate and Fig. 3b describes that thecrystallization time becomes longer as decreasing the heat-ing rate. They indicated that the lower heating rate promotedcrystallization by given an appropriate crystallization timeand were proved that higher crystallinity can be obtained inlower heating rate (Table 3). However, crystallization ratewas also lower than that of higher heating rate. The sametendency was also obtained for poly (lactic acid)/elastomerblends.

Quantification of non-isothermal crystallization ratecould be proposed by half time of crystallization [20].The half time crystallization (t1/2) was obtained directlyby tracing the fractional crystallinity of 0.5 to re-quired time (Fig. 3). The t1/2 and its inverse value(1/t1/2) were documented in Table 5. Clearly, it can beknown that the higher 1/t1/2 reflected faster crystalli-zation rate.

The other quantification technique of crystallizationrate during non isothermal was also proposed by Zhanget al. [24]. They introduced crystallization rate parame-ter (CRP) which can be determined from the linearslope of 1/t1/2 vs. heating rate (Θ) as presented inFig. 4. Considering the regression coefficients (R2), thisplot fitted experimental data very well and CRP valuewas tabulated in Table 5. The lower CRP value reflectsthe slower crystallization rate [6, 20]. Based on CRPvalue, there is a similar conclusion with result of pre-vious quantification. The kinetic of cold crystallization

results confirmed previously statement that elastomermay act predominantly as diluent agent.

Effect of annealing on cold crystallization

As explained by some researchers [5, 8, 25], differentprocessing techniques and its conditions influenced thebulk properties of poly (lactic acid), especially degreeof crystallinity. Injection molding is one of commonmeans of industrial processing technique for manufac-turing molded articles. Faster cooling rate of poly(lac-tic acid) melt during injection molding obtained lowercrystallinity due to fact that there was not enough timefor PLA chains to rearrange themselves into a crystal-line structure [5]. Annealing is a thermal treatmentprocess at higher temperature. It was intended to re-covery of crystals that were lost during injection mold-ing process. Figure 5 shows the different annealingtime of poly(lactic acid) at 80 °C. Clearly, the longer annealingtime decreased gradually the enthalpy of cold crystallization(ΔHcc). Considering the Eq. 1, the lower ΔHcc value meantthe higher crystallinity would be obtained. This result was alsofound by others [5, 8]. The cold crystallization peak did notappear at annealing time of 1 h. It can be concluded thatrearranging of crystalline structure would be completed afterone hour at 80 °C.

Isothermal crystallization mode

Kinetics of isothermal cold crystallization

Evaluation of isothermal cold crystallization kinetic can bedone as in non-isothermal part. Fractional crystallinity (Xt)

Fig. 5 The first heating DSCcurves of neat poly(lactic acid)at 10 °C/min with differentannealing time: (a) 0, (b) 15, (c)30, (d) 45 and (e) 60 minutes at80 °C


of poly(lactic acid) and its blends as a function of coldcrystallization time can be proposed as follows [16, 26]:

Xt ¼R tt0dHcc=dtð ÞdtR t1

t0dHcc=dtð Þdt ð6Þ

where, t0 and t∞ reflect the initial and the end of coldcrystallization process, respectively. The (dHcc/dt) representthe enthalpy difference of cold crystallization for the certaintime range. Plots of fractional crystallinity of poly(lacticacid) versus time were presented in Fig. 6a. The higher coldcrystallization temperature (Tcc) required less crystallizationtime, which meant that crystallization rate was increasing. Itcan be highlighted that at fractional crystallinity about 0.9,the crystallization rate decreased significantly due to thecrystallite collision may detain the crystal growth. This

condition caused the existence two different stages duringcold crystallization. The first stage occurred without crys-tallite collision effects and the second stage greatly influ-enced by crystallite collision. Further, It will be proved inAvrami’s plot. The different Tcc did not change the crystal-lization mechanism that illustrated by similar shapes of thecurves in Fig. 6a. This result was also found by others [6,16, 27].

From the half time of crystallization (t1/2) as tabulat-ed in Table 6, the presence of elastomer in poly(lacticacid) blends decreased the value of t1/2 at the same Tcc.It meant that elastomer may act as diluent agent whichgiving extra mobility of chains segment which enhancedthe crystallization rate of poly (lactic acid). Besides that,elastomer shifted down the crystallization peak to thelower temperature.

Fig. 6 Isothermal coldcrystallization of poly(lacticacid) at different temperature:(a) fractional crystallinity vs.time and (b) Avrami plots


There were some kinetic model to determined the iso-thermal crystallization [27, 28]. Avrami model was a com-mon and powerful model as claimed by some researchers[20, 21, 23] which develops by assuming a constant growthand nucleation rate in random nucleation. This model can bewritten as follows:

1� X ðtÞ ¼ exp �Ktnð Þ ð7Þwhere, x(t) is fractional crystallinity at a certain time, n is theAvrami exponent of time which represents the nucleationmechanism and growth dimension, and K is overall crystal-lization rate constant. By linearization of Eq. 7, the Avramimodel was easier to be solved as proposed below [26]:

log �In 1� X ðtÞð Þ½ � ¼ logK þ n log t ð8ÞFigure 6b represented the Avrami plots of Fig. 6a and its

parameter was tabulated in Table 6. Evidently, the Avramimodel shows the powerful model (R200.99) to describe anisothermal cold crystallization kinetic. There were two stagesof cold crystallization of poly(lactic acid) and its blends. It wasclearly visible when lower temperature was employed duringisothermal cold crystallization. The first and second stagereferred to primary and secondary crystallization stage.

Table 6 shows that the value of Avrami constant (n) in thefirst stage of poly(lactic acid) varies from 1.59 to 2. It indicat-ed a one-dimensional crystallization growth with homoge-neous nucleation mechanism [23]. In the second stage, thevalue of n is higher than that the first stage. However, thenucleation mechanism tended to occur in the transitionalmechanism between homogeneous and heterogeneous nucle-ation due to the value of n varied between 2.09 and 2.71. The nvalue of poly(lactic acid) and its blends at the same tempera-ture are almost same. It can be noted that the presence ofelastomer did almost not influence the nucleation mechanism.

Table 6 Kinetics parameters obtained from isothermal cold crystalli-zation experiments and Avrami analysis

Sample Tcc (°C) n K (s−1).10−3 t½ (s)

PLA 90 1.80 0.104 132.961

2.39a 0.010 104.615

94 1.83 0.202 86.264

2.30 0.035 73.768

98 1.88 0.256 66.685

2.24 0.075 58.400

102 1.99 0.376 43.847

PLA10 82 1.77 0.118 135.222

2.71 0.002 122.262

86 1.84 0.164 92.758

2.33 0.072 73.949

90 1.88 0.431 50.682

2.14 0.037 47.989

94 1.99 0.998 26.884

PLA20 78 1.59 0.398 108.886

2.35 0.018 90.285

82 1.61 0.984 59.175

2.09 0.237 45.999

86 1.89 1.230 28.795

90 2.00 2.636 16.170

PLA30 74 1.77 0.387 68.498

2.69 0.015 53.721

78 1.83 0.855 38.940

2.21 0.305 33.102

82 1.85 2.393 21.522

86 1.87 4.266 15.192

a Bold number 0 second stage of crystallization

Fig. 7 Determination ofequilibrium melting point ofneat poly(lactic acid) and itsblends by Hoffman-Weeksequation


Considering the K value, crystallization rate of poly(lac-tic acid) could be enhanced by increasing cold crystalliza-tion temperature (Tcc) and introducing elastomer in poly(lactic acid) blends. For example: increasing Tcc of poly(lactic acid) from 90 to 94, 98 and 102 °C enhanced Kvalues from 0.104×10−3 to 0.202×10−3, 0.256×10−3 and0.376×10−3 1/s, respectively. This result was also found inits blends with elastomer. While introducing elastomer10 wt% in PLA blends at 90 °C increased K value from0.104×10−3 to 0.431×10−3 1/s. Clearly, the addition elasto-mer is more significant to increase the crystallization rate ofpoly(lactic acid) than temperature during isothermal crystal-lization. Comparison the first and second stage crystalliza-tion in lower temperature for each sample proved theprevious statement. In the second stage, crystallization ratewas detained by the present of crystallite collision. It wasdescribed by lower K value than the first stage. Someresearchers were also reporting this phenomenon [17, 23].

Equilibrium melting temperature

The equilibrium melting temperature (Tom) of polymer may

describe the melting point of a crystallite aggregate [29].The calculation of To

m has been solved by extrapolationlinear trend line of Tm-Tc data to the line Tm0Tc. Thismethod was known as Linear Hoffman and Weeks (LHW)equation because it was based on their theory and can bewritten mathematically as follows:

Tm ¼ Tc2b

þ Tom 1� 1

2b

� �ð9Þ

where, Tm, Tc and β are the melting temperature, the coldcrystallization temperature and the ratio thickness of lamel-lar, respectively

Figure 7 illustrates the extrapolation of LHW equation.The equilibrium melting point of PLA, PLA10, PLA20 andPLA30 were obtained at 176, 170.5, 166, 164.8 °C, respec-tively. Considering the To

m of neat poly (lactic acid), it wasclose to the other report [27]. The addition of elastomertended to decrease the To

m of PLA. It may be understoodthat the presence of elastomer decreased the quantities ofcrystallite aggregate. Also, it proved that PLA and elastomerwere thermodynamically compatible as claimed previously.

Conclusion

The crystallization behavior of poly(lactic acid) and itsblends with elastomer were evaluated by DSC. There weretwo kind of crystallizations appeared in poly(lactic acid)during isothermal scanning, i.e. melt crystallization and coldcrystallization. The appeared crystallization was only melt

crystallization at low heating/cooling rate, indicating therewas enough time to rearrange its crystalline structure. Inother hand, the cold crystallization tended to appear athigher heating/cooling rate, indicating the amorphous struc-ture was obtained. The presence of elastomer tends to in-crease the crystallinity of poly(lactic acid) at lowcomposition as well as its cold crystallization kinetics dur-ing non-isothermal mode. The cold crystallization peaksshifted down to lower temperature which addressed bydecreasing their activation energy. The enhancement ofcrystallization rate can be done by increasing heating/cool-ing rate. Thermal treatment through annealing disappearedcold crystallinity completely after 1 h. Nucleation phenom-enon of poly(lactic acid) was almost not influenced bypresence of elastomer during isothermal mode. The highercrystallization rate can be obtained by increasing crystalli-zation temperature and introducing elastomer in poly(lacticacid) blends. The equilibrium melting temperature de-creased as increasing the elastomer content, indicating thatpoly(lactic acid) and elastomer were thermodynamicallycompatible.

Acknowledgement Authors would like to thank SABIC PolymerResearch Chair at King Saud University for providing their equipmentto conduct these tests. One of the authors M Kaavessina also thanksCEREM at KSU for the financial grant for this research work.

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Crystallization behavior of poly(lactic acid)/elastomer blends

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