New nanocomposites based on syndiotactic polystyrene and organo-modified ZnAl layered double hydroxide

New Nanocomposites Based on Syndiotactic Polystyrene and Organo-ModifiedZnAl Layered Double Hydroxide

Fu-An He, Li-Ming Zhang*, Fan Yang, Li-Shan Chen and Qing WuLaboratory for Polymer Composite and Functional Materials, Institute of Optoelectronic and Functional Composite

Materials, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275,

People_s Republic of China

(*Author for correspondence; Tel.: +86-20-84112354; Fax: +86-20-84112354; E-mail: [email protected])

Received 6 April 2006; accepted in revised form 27 June 2006; published online 18 August 2006

Key words: syndiotactic polystyrene, layered double hydroxide, nanocomposites, nonisothermal crystallization

Abstract

New nanocomposites were prepared by mixing syndiotactic polystyrene (sPS) with organo-modified ZnAl layered double

hydroxide (O-ZnAl-LDH) in toluene solution. Both wide-angle X-ray diffraction (WXRD) and transmission electron

microscopy (TEM) examinations confirmed the nanocomposite formation with exfoliated or intercalated O-ZnAl-LDH

distributed in the sPS matrix. Meanwhile, the nonisothermal crystallization behavior of the resulting nanocomposites was studied

using differential scanning calorimetry (DSC) technique at various cooling rates. The results indicated that the O-ZnAl-LDH

particles in nanometer size might act as nucleating agents and accelerate the overall nonisothermal crystallization process.

Introduction

Layered double hydroxides (LDHs), also known as anionic or

hydrotalcite-like clays, are a class of readily synthesizable

layered crystals that can be used as an alternative to the

commonly used silicate crystals for the preparation of

polymeric nanocomposites. They have a layered structure

with aspect ratios similar to or even higher than the ones

observed for aluminosilicate clays [1, 2]. In recent years,

polymer/LDH nanocomposites have attracted a lot of interest

because they exhibit improved physical and performance

properties in comparison to pristine polymers and conventional

composites [3Y11]. For example, the nanoscale dispersion of

LDH into various polymer matrixes such as linear low

density polyethylene (LLDPE) [12], polyamide 6 [13], epoxy

[14], poly(methyl acrylate) [15], poly(ethylene oxide) (PEO)

[16], and poly(vinyl alcohol) (PVA) [17] has been reported.

In the present work, we attempt to develop new nano-

composites based on syndiotactic polystyrene (sPS) and ZnAl

layered double hydroxides (ZnAl-LDH). In order to increase

the system compatibility, the organo-modified ZnAl-LDH

was firstly synthesized. Described here are the preparation

and microstructure characteristics of these nanocomposites

and their nonisothermal crystallization behavior.

Experimental

Materials

The syndiotactic polystyrene (sPS) used in this study has a

weight-average molecular weight (Mw) of 247,500 and a

polydispersity of 1.78. Zn(NO3)2I6H2O, NaOH, ethanol and

oluene were purchased from Guangzhou Chemical Reagent

Company in China. Al(NO3)I9H2O and sodium benzoate

were purchased from Tianjing Maoda Chemical Reagent

Company in China.

Preparation of organo-modified ZnAl-LDH

The organo-modified ZnAl-LDH (O-ZnAl-LDH) was

prepared by the coprecipitation method. Under stirring

vigorously, 0.03 mol of sodium benzoate was dispersed in

100 ml of deionized water. Zn(NO3)2I6H2O (0.03 mol) and

Al(NO3)I9H2O (0.01 mol) were dissolved in another 100

ml of deionized water. The nitrate solution was then added

dropwise to the sodium benzoate solution at room

temperature under stirring and the pH of the solution was

maintained at 10 by the addition of 1 mol/l NaOH solution.

After the addition of the nitrate solution, the mixture was

aged for three days at 70 -C under a nitrogen atmosphere.

The slurry was then filtered and washed by distilled water.

A white O-ZnAl-LDH solid was obtained by drying in a

vacuum oven at 60 -C.

For a comparison, pure ZnAl-LDH was also prepared.

The conditions were similar to the synthesis of the O-

ZnAl-LDH in the absence of sodium benzoate.

Preparation of sPS/O-ZnAl-LDH nanocomposites

The sPS/O-ZnAl-LDH nanocomposites were prepared by

refluxing the mixture of desired amounts of O-ZnAl-LDH

and sPS in 100 ml of toluene at 140 -C under a nitrogen

Journal of Polymer Research (2006) 13: 483Y493 # Springer 2006

DOI:10.1007/s10965-006-9071-9

atmosphere for 24 h. The product was then poured into

200 ml of ethanol for rapid precipitation and kept in a

vacuum oven at 140 -C for another 24 h.

Characterization

A Rigaku (Japan) D/max-RB wide-angle X-ray diffractom-

eter (WAXD) was used to characterize the layer structure of

the O-ZnAl-LDH in the nanocomposites. The operation

parameters were CuYKa radiation at a generator voltage of

40 kV and a current of 100 mA. The scanning rate was

2-/min at an interval of 0.02-. Nonisothermal crystalliza-

tion kinetic measurements were carried out with a Perkin

Elmer DSC-7 differential scanning calorimeter calibrated

with indium under a nitrogen atmosphere. Sample was

initially heated to 300 -C at a rate of 100 -C/min. It was

held for 5 min at this temperature to eliminate previous

thermal histories before cooling at a specified cooling rate.

The cooling rates employed were 10, 20, 30, and 40 -C/min,

respectively.

Results and discussion

The XRD patterns in the range of 2 theta = 1.5-)13- for

ZnAl-LDH and O-ZnAl-LDH samples are shown in

Figure 1. By the Bragg equation, the d003 spacing value

of ZnAl-LDH was calculated to be 0.876 nm. After the

anion exchange with benzoic group, the d003 spacing was

expanded to 1.556 nm for O-ZnAl-LDH. This result

indicates that the benzoic group is indeed intercalated into

the interlayer of ZnAl-LDH by anion exchange reaction.

The intercalation of benzoic group not only enlarges the

gallery between the nanolayers, but also improves the

interfacial properties between the O-ZnAl-LDH and syn-

diotactic polystyrene matrix because the benzoic group is

compatible with syndiotactic polystyrene chains and allows

their intercalation into O-ZnAl-LDH layers.

The XRD patterns in the range of 2 theta = 1.5-)10-for pure sPS, O-ZnAl-LDH, and fabricated sPS nano-

composites with various O-ZnAl-LDH loading amounts

(see Table 1) are shown in Figure 2. The peaks around 2

theta = 6.7- was a-form crystal peak of sPS itself [18]. The

characteristic X-ray peaks of O-ZnAl-LDH disappeared

completely in the XRD patterns of the sample PSL-1

containing the O-ZnAl-LDH loading of 2.0 wt.%, suggest-

ing that the stacking layers of the O-ZnAl-LDH in this

sample were fully separated and an exfoliated nano-

structure was formed. In the case of samples PSL-2 and

PSL-3, the intercalation peaks appeared around 2 theta =

3.65- and 2 theta = 5.06-, respectively. This result indicates

that sPS intercalated nanocomposites are formed for

sample PSL-2 and PSL-3. By the Bragg equation, we

could know that the d-space of O-ZnAl-LDH in sample

PSL-2 and PSL-3 increased to 2.419 nm and 1.746 nm,

respectively. Table 1 lists the compositions of the samples

and their XRD data.

Figure 3 shows typical DSC curves of heat flow as a

function of temperature at different cooling rates for sPS

2 4 6 8 10 12

(006)

(003)

(003)

a

b

Inte

nsity [a.u

.]

2 theta [degree]

Figure 1. The XRD patterns for (a) ZnAl-LDH and (b) O-ZnAl-LDH samples.

Table 1. The compositions of the samples and their XRD data.

Abbreviation sPS (wt.%) O-ZnAl-LDH (wt.%) d-Space (nm)

sPS 100 0 YPSL-1 98 2 YPSL-2 95 5 2.419

PSL-3 90 10 1.746

O-ZnAl-LDH 0 100 1.556

484 Fu-An He et al.

2 4 6 8 10

e

c

b

d

a

Inte

nsity [a.u

.]

2 theta [degree]

Figure 2. The X-ray diffraction patterns of (a) pure sPS, (b) PSL-1, (c) PSL-2, (d) PSL-3, and (e) O-ZnAl-LDH.

195 200 205 210 215 220 225 230 235 240 245 250 255

1:10oC/min

2:20oC/min

3:30oC/min

4:40oC/min

a

4

33

2

1

En

do

Temperature(oC)

195 200 205 210 215 220 225 230 235 240 245 250 255

1:10oC/min

2:20oC/min

3:30oC/min

4:40oC/min

b

4

2

1

En

do

Temperature(oC)

195 200 205 210 215 220 225 230 235 240 245 250 255

1:10oC/min

2:20oC/min

3:30oC/min

4:40oC/min

c

43

32

2

1

En

do

Temperature(oC)

195 200 205 210 215 220 225 230 235 240 245 250 255

1:10oC/min

2:20oC/min

3:30oC/min

4:40oC/min

d

4

1

En

do

Temperature(oC)

Figure 3. DSC curves of (a) sPS, (b) PSL-1, (c) PSL-2, and (d) PSL-3 at various cooling rates.

Nanocomposites Based on Syndiotactic Polystyrene and O-ZnAl-LDH 485

and its nanocomposites. As seen, each DSC curve at a

lower cooling rate has a higher trigger temperature of

crystallization and a smaller apparent peak area. The peak

crystallization temperature (Tp) and the heat of crystalliza-

tion (DHc) of sPS and its nanocomposites, which are listed

in Table 2, can be obtained from these curves. It is clear

that the Tp values of all nanocomposites are higher than

those of pure sPS at a given cooling rate. This phenomenon

can be explained by the heterogeneous nucleation effect of

the O-ZnAl-LDH nanoparticles on sPS macromolecule

segments, which leads to the crystallization of sPS at a

higher crystallization temperature. On the other hand, DHc

values of the nanocomposites were lower than that of pure

sPS, which indicates that the crystalline portion was

Table 2. The values of Tp, DHc, t1/2, CRP, CRC, n, Zt and Zc for sPS and its nanocomposites at various cooling rates.

Sample Cooling rate (-C/min) Tp (-C) DHc (J/g) t1/2 (min) CRC CRP n Zt Zc

sPS 10 235.09 23.46 0.93 1.98 0.0563 4.18 0.98 0.99

20 229.19 24.57 0.63 4.23 6.17 1.09

30 224.13 23.18 0.42 4.05 25.02 1.11

40 220.13 23.48 0.37 4.37 47.46 1.10

PSL-1 10 235.16 22.48 0.91 2.25 0.0599 4.74 1.15 1.01

20 229.67 22.98 0.59 5.03 10.07 1.12

30 225.35 21.92 0.40 4.81 61.55 1.15

40 221.99 22.41 0.35 5.22 131.63 1.13

PSL-2 10 235.66 20.75 0.83 2.27 0.0697 4.45 1.08 1.01

20 229.84 20.46 0.51 4.27 13.61 1.14

30 225.82 21.03 0.37 4.27 53.52 1.14

40 222.53 19.92 0.31 4.49 145.48 1.13

PSL-3 10 237.11 19.19 0.72 2.35 0.0761 4.43 3.26 1.13

20 231.05 19.82 0.42 4.42 31.51 1.19

30 227.65 19.88 0.32 4.41 104.59 1.17

40 224.42 19.92 0.27 4.61 228.15 1.15

205 210 215 220 225 230 235 240 245

0

20

40

60

80

100

a

10oC/min

20oC/min

30oC/min

40oC/min

XT (

%)

Temperature(oC)

210 215 220 225 230 235 240 245

0

20

40

60

80

100

b

10oC/min

20oC/min

30oC/min

40oC/min

XT (

%)

Temperature(oC)

210 215 220 225 230 235 240 245

0

20

40

60

80

100

c

10oC/min

20oC/min

30oC/min

40oC/min

XT (

%)

Temperature(oC)

215 220 225 230 235 240 245

0

20

40

60

80

100

d

10oC/min

20oC/min

30oC/min

40oC/min

XT (

%)

Temperature(oC)

Figure 4. The relative degree of crystallinity with temperature for the crystallization of (a) sPS, (b) PSL-1, (c) PSL-2, and (d) PSL-3 at various cooling

rates.

486 Fu-An He et al.

0.0 0.5 1.0 1.5

0

20

40

60

80

100

a

10oC/min

20oC/min

30oC/min

40oC/min

Xt (

%)

Temperature(0C)

0.0 0.5 1.0 1.5

0

20

40

60

80

100

b

10oC/min

20oC/min

30oC/min

40oC/min

Xt (

%)

Temperature(0C)

0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5

0

20

40

60

80

100

c

10oC/min

20oC/min

30oC/min

40oC/min

Xt (

%)

Temperature(0C)

0

20

40

60

80

100

d

10oC/min

20oC/min

30oC/min

40oC/min

Xt (

%)

Temperature(0C)

Figure 5. The relative degree of crystallinity with time for crystallization of (a) sPS, (b) PSL-1, (c) PSL-2, and (d) PSL-3 at various cooling rates.

492 494 496 498 500 502 504 506 508 510 512

10

15

20

25

30

35

40

Co

olin

g r

ate

(oC

/min

)

Tp(K)

sPS

PSL1

PSL2

PSL3

Figure 6. The plot of cooling rate versus crystallization peak temperature (Tp) for pure sPS and its nanocomposites.

Nanocomposites Based on Syndiotactic Polystyrene and O-ZnAl-LDH 487

5 10 15 20 25 30 35 40 45

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1/t

1/2(1

/min

)

Cooling rate(oC/min)

sPS

PSL1

PSL2

PSL3

Figure 7. The Plot of 1/t1/2 versus cooling rate for pure sPS and its nanocomposites.

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

-7

-6

-5

-4

-3

-2

-1

0

1

2

b

219 C

223 C

227 C

231 C

235 C

239 C

ln(-

ln(1

-XT))

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

-6

-5

-4

-3

-2

-1

0

1

2

219 C

223 C

227 C

231 C

235 C

239 C

ln(-

ln(1

-XT))

lnB

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

-6

-5

-4

-3

-2

-1

0

1

2lnB

c

a

219 C

223 C

227 C

231 C

235 C

239 C

ln(-

ln(1

-XT))

lnB

2.5 3.0 3.5

-7

-6

-5

-4

-3

-2

-1

0

1

2

d

223 C

227 C

231 C

235 C

239 C

ln(-

ln(1

-XT))

lnB

Figure 8. Ozawa plots of ln[jln(1 j XT)] against lnb for crystallization of (a) sPS, (b) PSL-1, (c) PSL-2, and (d) PSL-3.

488 Fu-An He et al.

reduced. This may be attributed to higher interfacial area

and adhesion between the sPS matrix and O-ZnAl-LDH

nanoparticles, which would act to reduce the transportation

ability of sPS chains during the crystallization process.

From the DSC curves of melting crystallization, the

values of relative crystallinity at different temperature, XT,

can be calculated according to the following equation:

X T ¼R T

T0dHc=dTð ÞdT

R T1T0

dHc=dTð ÞdTð1Þ

where T0 and T1 are the temperatures at which crystalli-

zation starts and ends, and dHc/dT is the heat flow rate.

Figure 4 shows the relative degree of crystallinity (XT)

versus the temperature (T ) for sPS and its nanocomposites

at various cooling rates.

In nonisothermal crystallization, the time t is related

to the temperature T according to the following equation:

t ¼ T0 � Tð Þ=� ð2Þ

where T is the temperature at crystallization time t, T0 is

the temperature at which crystallization begins (t = 0) and

b is the cooling rate. According to Equation (2), the value

of T on the x-axis in Figure 4 can be transformed into the

crystallization time t, as shown in Figure 5. The half

crystallization time (t1/2) of sPS and its nanocomposites

can be determined from Figure 5, and the results are listed

in Table 2. It is apparent that the t1/2 values of the

nanocomposites at various cooling rates are lower than

those of neat sPS. Moreover, the t1/2 values appear to

decrease with the increase of O-ZnAl-LDH loading

amount. These results indicate that the O-ZnAl-LDH

particles could act as heterogeneous nucleating agents to

facilitate the crystallization.

The nonisothermal crystallization rates of sPS and its

nanocomposites were estimated by the crystallization rate

coefficient (CRC) proposed by Khanna [19] and crystalli-

zation rate parameter (CRP) proposed by Zhang et al. [20].

The CRC can be obtained from the plot of cooling rate

against crystallization peak temperature (Figure 6) and the

CRP can be obtained from the plot of the reciprocal of t1/2

against the cooling rate (Figure 7). Table 2 lists the

obtained the CRP and CRC values for all samples

investigated. It is found that the CRC values and CRP

values for the nanocomposites are both higher than those

for neat sPS. This suggests that the addition of nanoscale

O-ZnAl-LDH particles could accelerate obviously the

crystallization rate of sPS.

In order to understand better the evolution of crystal-

linity during the nonisothermal crystallization, the Ozawa,

Avrami, and modified AvramiYOzawa methods were

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

ln(t/min)

10 C/min

20 C/min

30 C/min

40 C/min

a

(ln

[-ln

(1-X

)])

ln(t/min)-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

10 C/min

20 C/min

30 C/min

40 C/min

b

(ln

[-ln

(1-X

)])

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

10 C/min

20 C/min

30 C/min

40 C/min

c

(ln

[-ln

(1-X

)])

ln(t/min)

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

10 C/min

20 C/min

30 C/min

40 C/min

d

(ln

[-ln

(1-X

)])

ln(t/min)

Figure 9. Avrami plots of ln[jln(1 j XT)] against lnt for crystallization of (a) sPS, (b) PSL-1, (c) PSL-2, and (d) PSL-3.

Nanocomposites Based on Syndiotactic Polystyrene and O-ZnAl-LDH 489

employed to analyze the nonisothermal crystallization

kinetics of sPS and its nanocomposites.

Assuming that the polymer melt was cooled at a

constant rate and the mathematical derivation of Evans

was valid, Ozawa [21] extended the Avrami equation to the

nonisothermal condition:

X T ¼ 1� exp �K Tð Þ=�mð Þ ð3Þ

where XT is the relative degree of crystallinity at tem-

perature T; K is the Ozawa crystallization rate constant; m

is the Ozawa exponent and b is the cooling rate. The

double natural logarithm of the Ozawa equation gives the

following relationship:

ln � ln 1� X Tð Þ½ � ¼ ln K Tð Þ � m ln� ð4Þ

A plot of ln[jln(1 j XT)] against lnb at a given

temperature should result in a straight line if the Ozawa

method is valid. Thus, K(T ) and m can be obtained from the

intercept and the slope of the lines, respectively. For the sPS

and its nanocomposites, the Ozawa plots of ln[jln(1 j XT)]

against lnb are shown in Figure 8. However, the obvious

curvature in the plot indicated that the Ozawa method could

not fit well the crystalline behavior of the sPS and its

nanocomposites. This is probably due to the factors such as

secondary crystallization of sPS, the dependence of lamellar

thickness on crystallization temperature and the constant value

of cooling function over the entire crystallization process [22].

The isothermal crystallization process of polymers can

be analyzed in terms of the Avrami equation [23]:

Xt ¼ 1� exp �Zttnð Þ ð5Þ

While the Avrami equation was developed to describe

isothermal crystallization kinetics, it has also been used to

describe the nonisothermal process. The double natural

logarithm of the Avrami equation gives the following

relationship:

ln � ln 1� Xtð Þ½ � ¼ ln Zt þ n ln t ð6Þ

where t is the crystallization time, Xt is the relative degree

of crystallinity at time t, the exponent n is a mechanism

constant with a value depending on the type of nucleation

-1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8 a

Xt=0.2

Xt=0.4

Xt=0.6

Xt=0.8

lnB

ln(t/min)

-1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

b

Xt=0.2

Xt=0.4

Xt=0.6

Xt=0.8

lnB

ln(t/min)

-1.8 -1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

c

Xt=0.2

Xt=0.4

Xt=0.6

Xt=0.8

lnB

ln(t/min)

-1.8 -1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

d

X =0.2

X =0.4

X =0.6

X =0.8

lnB

ln(t/min)

Figure 10. Modified AvramiYOzawa plots of lnb against lnt for crystallization of (a) sPS, (b) PSL-1, (c) PSL-2, and (d) PSL-3.

490 Fu-An He et al.

and the growth process, and the parameter Zt is a growth

rate constant involving both nucleation and growth rate

parameters. Since the rate of nonisothermal crystallization

depends on the cooling rate, Jeziory [24] suggested that the

rate parameter Zt should be corrected for the influence b of

cooling rate of the polymer. The parameter characterizing

the kinetics of nonisothermal crystallization was given as

follows:

log Zc ¼log Zt

�ð7Þ

The Avrami plots of ln[jln(1 j Xt)] vs ln t for sPS and

its nanocomposites are shown in Figure 9. The values of

Avrami parameters n, Zt and Zc are given in Table 2. The

exponent n values for the sPS/O-ZnAl-LDH nanocompo-

sites are higher than that for pure sPS at every cooling rate,

indicating that the nanoscale O-ZnAl-LDH layers act as the

nucleating agents for the sPS matrix. The Zc values of the

sPS/O-ZnAl-LDH nanocomposites are, as expected, higher

than that of the pure sPS at the same cooling rate, showing

that sPS/LDHs nanocomposites could crystallize at a

quicker rate than pure sPS. These results are similar to

those reported for the sPS/clay systems [25Y27].

Many researchers have shown that the kinetics equation

combining Avrami and Ozawa equation, which was

proposed by Liu et al. [28], is applicable in many

nanocomposites systems such as polyamide 6/clay nano-

composites [29], PE/copper nanocomposites [30] and

POM/clay nanocomposites [31]. Therefore, the combined

Avrami and Ozawa equation was used to study the

nonisothermal crystallization kinetics of the sPS/O-ZnAl-

LDH nanocomposites. As the degree of crystallinity was

related to the cooling rate b and the crystallization time t

(or temperature T ), the relation between b and t could be

defined for a given degree of crystallinity. Consequently,

using Equation (2) and combining Equations (4) and (6),

the following equation can be obtained under a certain

crystallinity degree:

ln k þ n ln t ¼ ln K Tð Þ � m ln� ð8Þ

and by rearrangement

log � ¼ ln F Tð Þ � � ln t ð9Þ

where F(T ) = [K(T )/k]1/m, which refers to the cooling rate

that must be selected within a unit of crystallization time

when the measured system reaches a certain degree of

crystallinity, and a is the ratio of the Avrami exponent (n)

to the Ozawa exponent (m), that is n/m. It can be seen that

F(T ) has a definite physical and practical meaning.

According to Equation (9), at a given degree of crystallin-

ity, plotting lnb vs lnt should yield a linear relationship.

The kinetic parameter F(T ) and a are determined from the

0.00196 0.00197 0.00198 0.00199 0.00200 0.00201 0.00202

2.2

2.4

2.6

2.8

3.0

3.2

3.4

iPS

PSL1

PSL2

PSL3

lnB

1/Tp(K

-1)

-10.2

-9.9

-9.6

-9.3

-9.0

-8.7

iPS

PSL1

PSL2

PSL3

ln(B

/Tp

2)

a

b

Figure 11. Determination of activation energy DE for crystallization of sPS and its nanocomposites based on (a) Kissinger approach or (b) Takhor

approach.

Nanocomposites Based on Syndiotactic Polystyrene and O-ZnAl-LDH 491

intercept and the slope of the lines, respectively. Plots of

lnb vs lnt at various degree of crystallinity for sPS and its

nanocomposite are presented in Figure 10. From Figure 10,

it can be seen that these plots show a good linearity, which

verifies the advantage of the combined approach applied in

this case. The values of lnF(T ) and a are listed in Table 3.

The value of a varies from 1.34 to 1.40 for pure sPS and

from 1.34 to 1.58 for sPS/O-ZnAl-LDH nanocomposites.

Almost all a values of neat sPS are lower than those of its

nanocomposites at the same relative degree of crystallinity.

This phenomenon is similar to that reported by Qian and

He [32]. The F(T ) values of sPS and its nanocomposites

seem to increase with the increase of the relative degree of

crystallinity. Thus, at a unit crystallization time, a higher

cooling rate is needed to achieve a higher degree of

crystallinity. At the same relative degree of crystallinity,

the values of F(T ) for sPS/O-ZnAl-LDH nanocomposites

are smaller than that for pure sPS, implying that the

addition of O-ZnAl-LDH nanoparticles can accelerate the

overall crystallization process.

The activation energy DE for the transport of the

polymeric segments to the growing crystal surface can be

determined using Kissinger approach [33] and Takhor

approach [34] by calculating the variation of Tp with the

cooling rate b. The activation energy DE can be evaluated

based on the plots of the following forms: (1) Kissinger

approach,

d ln �.

T 2p

� �h i

d 1�

Tp

� � ¼ ��E

Rð10Þ

where R is the gas constant and other parameters have the

same meaning as earlier indicated, and (2) Takhor

approach,

d ln �ð Þ½ �d 1�

Tp

� � ¼ ��E

Rð11Þ

Figure 11 shows the plots based on the Kissinger

approach and the Takhor approach, respectively. The

obtained DE values are listed in Table 4. As seen, all DE

values for the sPS/O-ZnAl-LDH nanocomposites are

higher than that for neat sPS, implying that neat sPS

could crystallize more quickly than sPS/O-ZnAl-LDH

nanocomposites.

Conclusions

Syndiotactic polystyrene/O-ZnAl-LDH nanocomposites

were prepared by mixing the sPS with the organo-modified

ZnAl-LDH in toluene solution. The microstructures of

nanocomposites were confirmed by WXRD and TEM.

DSC has been used to investigate the influence of O-ZnAl-

LDH nanoparticles on the non-isothermal crystallization

behavior of syndiotactic polystyrene/O-ZnAl-LDH nano-

composites. Several methods based on Avrami, Ozawa,

and modified AvramiYOzawa models were employed to

analyze the nonisothermal crystallization kinetic data of

the nanocomposites. It was found that Ozawa method

failed to provide an adequate description for the non-

isothermal crystallization of sPS and its nanocomposites. In

contrast, Avrami and the modified AvramiYOzawa models

have been proven to be successful. The O-ZnAl-LDH

particles in nanometer size might act as the nucleating

agents and accelerate the overall nonisothermal crystalli-

zation process of sPS.

Acknowledgments

This work was supported by NSFC (20273086; 30470476),

NSFG (021769; 039184), Department of Science and

Technology of Guangdong Province (2004B33101003),

and NCET Program in Universities as well as SRF for

ROCS, SEM, China.

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