the effect of the addition of polypropylene grafted sio2 nanoparticle on the crystallization...

The effect of the addition of polypropylene grafted SiO2

nanoparticle on the crystallization behavior of isotacticpolypropylene

Yoshizo Fukuyama • Takahiko Kawai •

Shin-ichi Kuroda • Masahito Toyonaga •

Toshiaki Taniike • Minoru Terano

Received: 31 October 2012 / Accepted: 13 December 2012 / Published online: 16 January 2013

� Akademiai Kiado, Budapest, Hungary 2013

Abstract The crystallization behavior of isotactic poly-

propylene (iPP)/silica particle (SiO2, 26 nm) nanocomposite

has been investigated. In addition to the non surface-modi-

fied SiO2, iPP grafted SiO2 was synthesized and adopted to

this study with an aim to understand the role of grafted

polymer chain on the crystallization process. The crystalli-

zation rate of non surface-modified iPP/SiO2 composite

stays constant up to 1 vol%. It suggests the very weak

nucleation ability of nano-sized silica particle. While large

acceleration effect was observed for iPP-grafted SiO2/iPP

composite. The spherulite density increased with increasing

SiO2 contents, and more interestingly, the spherulite growth

rate also increased. This finding leads to the conclusion that

the grafted iPP chain has a plasticizing effect that reduces the

chain entanglements at the interface. Further increase in SiO2

contents, the crystallization rate, the spherulite density, and

the spherulite growth rate showed the steep decreases at

higher SiO2 content range regardless of the surface modifi-

cations of SiO2. It was attributed to the confinement of matrix

chain since the inter-particle distance of adjacent SiO2

approaches to the end-to-end distance of matrix chain, which

a large molecular motion is restricted. Moreover, the average

size of SiO2 aggregation in iPP matrix was successfully

evaluated by analyzing the contents dependence of the

growth rate, assuming that the inter-particle distance with

zero growth rate coincided with end-to-end distance of

matrix iPP chain.

Keywords Polypropylene � Nanocomposites � Silica �Crystallization

Introduction

Thermoplastic polymers such as iPP are often reinforced by

inorganic fillers in order to increase the stiffness, tensile

strength, and dimensional stability at elevated temperatures

[1, 2]. Among many composites being developed and widely

used, iPP/SiO2 composite has an advantage of low cost

production as well as good mechanical properties. In recent

years, Inorganic particle filled nanocomposites of semi-

crystalline polymers have been explored extensively [3, 4].

It is well accepted that with decreasing particle size,

the ratio of surface/volume increases, so that the surface

properties become crucial. A tenfold decrease in diameter

of particles leads to a 100-fold increase in their surface

areas. It is, thus, easy to understand that the surface prop-

erties of nano-sized particle have a great influence on the

polymer-nanoparticle composites. It has been reported that

the large specific area of the fillers cause the formation of

an interfacial polymer layer (shell) attached to the particle

core [5]. The polymer chains localized in the shell would

have different mobility compared to that in the bulk. The

attractive force between particle and polymer at the inter-

face leads to the decrease in the mobility of polymer in the

shell, resulting in the increase in the glass transition tem-

perature (Tg). While if there is the repulsive force, the chain

mobility increases yielding to the decrease in Tg.

The properties of semi-crystalline polymers strongly

depend on the crystalline structure including crystallinity,

Y. Fukuyama � T. Kawai (&) � S. Kuroda

Department of Production Science and Technology,

Graduate School of Engineering, Gunma University,

29-1 Honcho, Ota, Gunma 373-0057, Japan

e-mail: [email protected]

M. Toyonaga � T. Taniike � M. Terano

School of Materials Science, Japan Advanced Institute

of Science and Technology, 1-1 Asahidai, Nomi,

Ishikawa 923-1211, Japan

123

J Therm Anal Calorim (2013) 113:1511–1519

DOI 10.1007/s10973-012-2900-7

crystalline form, and morphology. Since the chain mobility

in the interfacial layer is unequal to the bulk as mentioned

above, the higher-order structure of polymer shall be

altered. In semi-crystalline polymer/nano-particle com-

posites, the adhesion between the polymer and particle

strongly depends on the crystalline structure at the inter-

face. The understanding of the effects of the chemical and/

or physical interactions on the crystalline structure, as well

as its crystallization behavior, is of great importance for the

development of polymer nanocomposites.

Nitta et al. [6, 7] have studied the crystallization

behavior of homogeneously dispersed iPP/SiO2 nanocom-

posite. The spherulite growth rate of iPP was found to

decrease with increasing SiO2 content due to the immobi-

lization of polymer chains in the shell. Interestingly, they

also found that the critical distance between adjacent par-

ticles, at which the spherulite growth rate of polymer

becomes zero, coincided with the end-to-end distance of

matrix polymer, hr20i�1=2

. It demonstrates that the crystal

growth of a polymer, which requires large molecular

motion to transport into growing surface of polymer crys-

tal, is strongly affected by the inter-particle distance, i.e.

the size and the contents of particle in the composite.

The surface modification also affects the chain mobility at

the interface and the crystallization of polymer matrix as well

as the dispersibility of the particles. A great number of

modification methods have been reported [8–13]. In addition

to the use of a surfactant and/or silane coupling agents that is

the most common method for industries [10, 11], grafting of

oligomers/polymers has been attracting much attention in

recent years [12]. The surface grafting is, however, very

complex since the end-grafted polymer forms various sur-

face morphologies depending on the surface coverage. It has

been reported that the mushroom, brush, and hyperbrush

structures can be formed according to the distance between

the adjacent grafted polymer chains, Sb [14–16]. The dif-

ferent interfacial structures lead to the various physico-

chemical interactions with matrix polymer even though the

chemical structure is identical. It is thus expected that the

interactions affect the chain mobility at the interface, and

consequently the crystallization of matrix polymer. Our co-

authors have recently developed a method for grafting iPP

onto nano-sized SiO2 to obtain a novel iPP/SiO2 composite

[17, 18]. In addition to the fact that the aggregation of SiO2

became smaller according to its superior interaction at the

surface, the crystallization behavior was also reported to be

different compared to the iPP/SiO2 nanocomposite without

surface modification. The crystallization of the composites

is, however, not fully understood especially for the role of

grafted iPP chains during crystallization.

Our aim is to understand the role of grafted polymer

chain, which has the identical chemical structure to that of

the matrix polymer, on the crystallization process. In this

study, we report the crystallization behavior of iPP/g-SiO2

composite investigated by means of differential scanning

calorimetry and polarized optical microscopy. iPP with the

number average molecular mass of 12 K was directly

grafted onto SiO2 particle with a diameter of 26 nm. The

effects of the surface grafting and the content of SiO2

particles on the crystallization behavior are to be discussed

in terms of overall crystallization rate, nucleating ability,

and spherulite growth rate of iPP.

Experimental

Materials

iPP pellet purchased from Aldrich was used as a matrix

polymer. The mass average molecular mass (Mw) and

molecular mass distribution (MWD) were 250,000 g mol-1

and 3.73 respectively. NanoTek silica with an average par-

ticle size of 26 nm and surface area of 110 m2 g-1 obtained

from Kanto Chemical Corporation was used as filler.

Synthesis of iPP-OH and iPP grafted SiO2

iPP-OH was synthesized with rac-ethylenebis(1-inde-

nyl)zirconium dichloride, MMAO, and triethylaluminium

in toluene solvent at 0 �C for 1 h with propylene flow in

nitrogen ambient atmosphere. Oxygen gas was bubbled in

the reactor at 0 �C for 1 h, followed by the addition of

MeOH and 35 % H2O2 aqueous solution. The number

average molecular mass, Mn, of iPP-OH was controlled

by the concentration of triethylaluminium (TEA), which

was estimated as 12,000 g mol-1. iPP-OH has an isotac-

ticity (mmmm) about 86 mol % and the molecular mass

distribution was about 2.5. iPP-OH was then reacted with

silanol group of silica at 200 �C in tetradecane for 6 h. iPP

grafted SiO2 (g-SiO2) was washed by hot filtration with

o-dichlorobenzene at 140 �C for 1 h.

Preparation of nanocomposites

Nanocomposites were prepared by melt mixing using a two

roll mixer. iPP pellet as matrix polymer and 0.1 wt% of

stabilizer (AO-50 ADEKA) were kneaded at 185 �C for

5 min and then the specific amount of silica was added into

the kneaded iPP. The samples were kneaded at 185 �C for

another 10 min. The composites with various SiO2 con-

tents ranging from 0 to 15 % in volume were prepared. In

addition to the iPP-grafted SiO2 (g-SiO2) described above,

non-surface-treated SiO2 (n-SiO2) was also utilized for a

comparison.

1512 Y. Fukuyama et al.

123

Thermogravimetry

The mass loss of SiO2 particles was measured by means of

Thermogravimetry–Differential Thermal Analysis (TG–

DTA; SII TG/DTA6200). n-SiO2 and g-SiO2 particles with

the mass of ca. 10 mg were subjected to the heating to

500 �C with a rate of 20 �C min-1.

Transmission electron microscopy

The distribution of 26-nm-sized silica particles, embedded

in iPP was observed by transmission electron microscopy

(TEM) (JEOL JEM-1200EX). The films of iPP/n-SiO2

(1.8 vol%) and iPP/g-SiO2 (1.2 vol%) were hardened by

epoxy. The cross-section of the films was then cut to a

thickness of 70 nm using a diamond knife controlled by an

Ultracut S microscope (Reichert). The thin samples were

then laid on a 150 mesh copper grid and dried for 1 day in

a desiccation chamber. The silica nanoparticle image was

examined using an accelerating voltage of 100 kV. TEM

images were analyzed by mean of image analysis software

(Multi Gauge V3.0; FUJIFILM) to quantify the aggrega-

tion size of silica nanoparticles.

Differential scanning calorimetry

Crystallization of iPP nanocomposites was performed in a

Perkin Elmer DSC7 differential scanning calorimeter (DSC)

instrument under nitrogen atmosphere (40 ml min-1).

Samples of 5–6 mg were used. The samples were initially

melted at 200 �C for 5 min in order to erase all previous

thermal history. Isothermal crystallization was performed at

128 �C and crystallization was carried out until it was

completed.

Polarized optical microscopy

Polarized optical microscope (POM; Olympus BX-50) was

employed for the observation of the spherulites formed

during the crystallization at 135 �C. The samples were

sandwiched by cover glasses and subjected to the heat

treatment by means of custom-built hot stage, which

enables the temperature jump with a cooling rate faster

than 1,000 K min-1.

Results and discussion

Characterization of interfacial structure in iPP/SiO2

composites

Figure 1 shows the TG curves of n-SiO2 and g-SiO2 during

heat treatment. Mass of SiO2 stays constant during heating

for n-SiO2, whereas it decreases for g-SiO2. The mass

reduction is attributed to the degradation of iPP grafted

onto SiO2 surface. The change in the mass can be con-

verted to the number density of iPP per area (chains/nm2)

by following equation:

r ¼ W NA qf r 10�21

3 Mn Wf

ð1Þ

where W is the mass loss of the grafted iPP, NA is the

Avogadro number, qf is the density of silica, r is the radius of

silica particle, Mn is the number-average molecular mass and

Wf is the mass of silica. The number of grafted chains per unit

surface area of SiO2 was evaluated to be 0.03 chains/nm2. It

means that the distance, Sb, between adjacent grafted chains

was approximately 6.5 nm. The number of iPP chains on a

single SiO2 particle was thus calculated to be 60 chains. It is

known that the grafted polymer chains form various types of

interfacial morphologies depend on the relationship between

Sb and the radius of gyration, Rg, i.e. the size of the coil of

grated polymer chain [19–24]. If Sb [ 2Rg, grafted polymer

chains are isolated and form mushroom type morphology on

the solid surface. While if Sb � 2Rg, the chain forms high

density brush structure which is extraordinary stretched to

a direction normal to the surface. This structure is obtained

via surface initiated living radical polymerization (LRP)

[21–24], which recently has attracted much attention due to

its superior interfacial properties [23, 24]. In the middle

range where Sb * Rg and Sb \ Rg, grafted chains are

overlapped and form semi-dilute brush structure. The Rg of

iPP is reported to have molecular mass dependence as

follows [25]:

Rg ¼ 0:022 M0:56w ð2Þ

In this study, Rg is evaluated to be 7.1 nm. By comparing

Sb (6.5 nm) and Rg, it is concluded that the adjacent grafted

0 5 10

Time/min15 20 25 30

0

100

200

300

400

500

600101

100

99

98

97

96

95

94

Mas

s/%

Tem

pera

ture

/°Cn-SiO2

g-SiO2

Fig. 1 TG curves of n-SiO2 and g-SiO2 particle during heat

treatment. The temperature is also indicated in the figure

Crystallization behavior of isotactic polypropylene 1513

123

iPP chains are slightly overlapped and form semi-diluted

brush structure.

Among various methods to estimate the interfacial layer

thickness in composites [4, 26–30], dynamic mechanical

analysis (DMA) was employed in this study. The effective

thickness of the interfacial layer can be evaluated by

rationing the loss modulus of homopolymer E000� �

and the

composite E00c� �

measured at room temperature as follows

[4, 27]:

E00cE000¼ 1

ð1� /fÞBð3aÞ

B ¼ 1þ Dr

r

� �3

ð3bÞ

Here /f and r are the volume fraction and the radius of

the particle, respectively. Dr indicates the effective

interfacial layer thickness. Dr was estimated as 21 nm.

The summary on the interfacial structure of g-SiO2 is listed

in Table 1, and the structure of grafted iPP on the SiO2

particle is schematically illustrated in Fig. 2.

Figure 3 shows the transmission electron microscope

(TEM) images of iPP/n-SiO2 (1.8 vol%) and iPP/g-SiO2

(1.2 vol%) composites quenched from the melt. The dark

circles indicate the nano-sized SiO2 particles in the matrix

iPP. It is clearly seen that the SiO2 particles are aggregated

regardless of the surface treatment. The size of the aggre-

gation was analyzed over the wide range of the sample.

The radiuses being averaged over 300 aggregates were

determined to be 118 nm for iPP/n-SiO2 and 104 nm for

iPP/g-SiO2. Although it is reasonable that the aggregation

becomes smaller for iPP/g-SiO2 due to the enhancement

in the wettability on SiO2 surface, the difference of the

dispersibility of SiO2 is not much obvious in our study.

Crystallization behavior of iPP/SiO2 nanocomposites

Figure 4 shows the DSC curves of iPP/n-SiO2 (a) and iPP/

g-SiO2 (b) composites during isothermal crystallization at

128 �C from the melt. Exothermic peak corresponding to

the half time of crystallization show almost no change in

the iPP/n-SiO2, while shows a shift to shorter time with

increasing SiO2 contents in iPP/g-SiO2. It indicates that the

grafting of iPP onto SiO2 surface accelerates the crystal-

lization of matrix iPP. Figure 5 shows the overall crystal-

lization rate (V) of the composites, estimated by the inverse

half time of crystallization, plotted against SiO2 contents.

In the contents range lower than 1 vol%, V stays almost for

constant both in iPP/n-SiO2 and iPP/g-SiO2, which means

that the surface treatment of SiO2 has no or quite limited

effect on the crystallization of matrix iPP. While in iPP/g-

SiO2, crystallization rates are increased drastically above 1

vol%. Considering that the difference in the dispersity of

SiO2 is not much differed as seen in Fig. 3, the acceleration

effect on iPP crystallization is due to the role of grafted iPP

chains. It is also important to mention that V falls at higher

contents. Since the trend is the same regardless the surface

modifications, it seems to be the characteristics of the

crystallization of polymer with high contents of fillers,

which is to be discussed later.

Table 1 Structural characteristics of grafted iPP chains

Number density r/chains nm-2 Interchain distance Sb/nm Radius of gyration Rg/nm E00c ¼ 1:2 %=E000 Interfacial layer thickness h/nm

0.03 6.5 7.1 1.27 21

h = 21 (nm)

26 (nm)

236

(nm

)

iPP (60 chains/particle)

Sb

= 6

.5 (

nm)

SiO2 SiO2

Fig. 2 Schematic illustration of the hierarchical structure of g-SiO2

in the matrix iPP

Fig. 3 TEM micrographs of

iPP/n-SiO2 (a) and iPP/g-SiO2

(b). The black sphere indicates

the aggregated SiO2 particle


123

The isothermal crystallization kinetics are analyzed by

well known Avrami equation [31, 32]:

Xc tð Þ ¼ 1� exp �k t � sð Þn½ � ð4Þ

where Xc(t) is relative the crystallinity at time (t), n is the

Avrami exponent whose value depends on the mechanism

of nucleation and on the dimension of crystal growth, and

k is a rate constant containing the nucleation and growth

parameters. Xc(t) is equal to the ratio of the heat generated

at time t to the heat generated at infinite time according to:

X tð Þ ¼ Qt=Q1 ¼Z t

0

dH=dtð Þdt

,Z1

0

dH=dtð Þdt ð5Þ

where dH/dt is the rate of heat evolution. Eq. (4) can be

changed to

log � ln 1� X tð Þð Þ½ � ¼ n logðt � sÞ þ log k ð6Þ

The crystallization rate constant k and Avrami exponent

n can be determined from the intercept and slope in the plot of

log [-ln (1 - X(t))] versus log (t - s), respectively. Here, s is

the induction time for the crystallization. swas adjusted so that

the data in the shorter time range shows the straight line in

Avrami plot. The Avrami plots are shown in Fig. 6 for iPP/n-

SiO2 and iPP/g-SiO2. The Avrami exponents are ranging

between 3 and 4, suggesting heterogeneous and homogeneous

nucleation for the three dimensional spherical growth. The

summary of the Avrami analysis is listed in Table 2. The

Avrami exponent decreases slightly with increasing n-SiO2.

Considering that the spherulite morphologies are observed in

all samples, the decrease is attributed to the change in the

nucleation process. The increase in the particle shall enhance

the nucleation probability at the interface although the effect is

quite limited. The same trend is observed for iPP/g-SiO2,

0 0.5 1.51 2 3 42.5 3.5 0 0.5 1.51 2 3 42.5 3.5

Time/min Time/min

Exo

ther

mic

0 %

0.09 %

0.18 %

0.9 %

1.8 %

14.6 %

0 %0.06 %

0.12 %

1.2 %

2.3 %

12.9 %

n-SiO2g-SiO2

Exo

ther

mic

(a) (b)Fig. 4 DSC curves of iPP/n-

SiO2 (a) and iPP/g-SiO2/iPP

(b) during isothermal

crystallization at 128 �C

content/vol.%

V/s

ec–1

SiO2

g-SiO2

n-SiO2iPP/

iPP/

0.02

0.01 0.1 1 10 100

iPP

0.018

0.016

0.014

0.012

0.01

Fig. 5 Plots of the crystallization rate of the composites against the

SiO2 content. The crystallization temperature is 128 �C

1

0 %

1.8 %

14.6 %

0 %

1.2 %

2.3 %

12.9 %

(a) (b)0.5

0

–0.5

–1.5

–1

–2

–2.5

1

0.5

0

–0.5

–1.5

–1

–2

–2.5

log (t–τ ) log (t–τ )3.5 4 4.5 5 3.6 3.8 4 4.2 4.4

log

[–In

(1–

Xt)]

log

[–In

(1–

Xt)]

Fig. 6 Avrami plots of iPP with

various contents of n-SiO2

(a) and g-SiO2 (b). The

crystallization temperature is

128 �C


123

showing the nucleating ability of g-SiO2. The value of s stays

almost constant for entire samples. The small values of s,

compared to the time required for the completion of the

crystallization, indicates that the s has negligible effect for the

Avrami exponent except for very early stage of crystallization.

In order to understand the acceleration effect in the

crystallization process of iPP/SiO2 composite, polarized

optical microscopy was utilized during isothermal crystal-

lization from the melt. Figure 7 shows the optical micro-

graphs of iPP (a), iPP/n-SiO2 (1.8 vol%) (b), and iPP/

g-SiO2 (1.2 vol%) (c) during the isothermal crystallization

at 135 �C after the elapsed time of 155 s. No particular

difference in the number of spherulite, i.e. nucleation

density, is observed between iPP and iPP/n-SiO2, indicat-

ing that the non-treated surface of SiO2 has poor effect on

the nucleation of iPP. While for iPP/g-SiO2, one can see

clearly that the number of spherulite is greatly enhanced. It

should be noted that the nucleation of matrix iPP took place

within a limited range of time, so no further spherulite was

appeared after 155 s. Considering that the dispersibility of

SiO2 in matrix iPP does not show much difference between

n-SiO2 and g-SiO2 system as shown in Fig. 3, grafted iPP

chains on the surface of SiO2 have a particular effect on the

nucleation of iPP. The number density of spherulites

formed during the isothermal crystallization at 135 �C is

plotted in Fig. 8 against the content of SiO2. Although data

shows some variations, the number density for nucleation

is not affected by the addition of n-SiO2. While for iPP/

g-SiO2 composite, the density increases with SiO2 contents

up to 2.3 vol%. It demonstrates the effectiveness of grafted

iPP chains on the SiO2 surface in the nucleation process. It

is interesting that the nucleation density showed maximum

at 2.3 vol%, followed by steep decrease with increasing

SiO2. It suggests that the g-SiO2 has incremental and

decremental effects in the nucleation process of matrix iPP,

which is to be discussed together with the growth rate

analysis.

Figure 9 shows the change in the spherulite size during

the isothermal crystallization at 135 �C. It is well known

that the spherulite grows linearly with time when the

polymer is subjected to the isothermal crystallization. No

non-linear growth, as is often seen in the crystallization of

polymer/plasticizer, is observed [33]. Although the induc-

tion time for crystallization is not much differed in each

sample, the gradient of linear radial growth, which corre-

sponds to the growth rate of iPP crystal, shows the sample

dependence. The spherulite growth rates of the composites

are plotted in Fig. 10 against the logarithmic SiO2 content.

The dotted line indicates the growth rate of iPP without

SiO2 particle. It is clearly seen in iPP/n-SiO2 composites

that the growth rate stays almost constant up to 0.9 vol%,

followed by the steep reduction. This decelerating effect is

Table 2 Summary of Avrami analysis for iPP/n-SiO2 and iPP/g-SiO2

SiO2/vol% s/s n k/s–n

iPP 0 3.5 3.38 6.81 9 10-7

iPP/n-SiO2 1.8 3.0 3.21 1.53 9 10-6

iPP/n-SiO2 14.6 3.5 3.08 5.25 9 10-7

iPP/g-SiO2 1.2 2.5 3.31 1.49 9 10-6

iPP/g-SiO2 2.3 3.0 3.25 2.28 9 10-6

iPP/g-SiO2 12.9 3.5 3.54 2.74 9 10-7

Fig. 7 Polarized optical microscope of iPP (a), iPP/n-SiO2 (b), and iPP/g-SiO2 (c) at the elapsed time of 155 s during the isothermal

crystallization at 135 �C

250

content/vol%

Sph

erul

ite n

umbe

r de

nsity

/mm

–3

SiO2

g-SiO2

n-SiO2iPP/

iPP/200

150

100

50

00.01 0.1 1 10 100

iPP

Fig. 8 Plots of the spherulite number density as a function of SiO2

content. The crystallization temperature is 135 �C


123

well explained by the model of the confinement of the

matrix chains due to the reduction in the inter-particle

distance of SiO2. The distance between the surfaces of

adjacent particles d can be estimated by the following

[6, 34]:

d ¼ 4pffiffiffi2p

3uf

� �1=3

�2

2

4

3

5r ð7Þ

where /f and r are the volume fraction and the radius of the

particle or aggregate, respectively. Nitta et al. [6] have

reported that the growth rate of iPP approached to zero

with increasing SiO2 content as following:

Gc ¼ G� k1

dð8Þ

where Gc and G are the spherulite growth rate of the

composite and homopolymer, respectively. k is the

constant that has no molecular mass dependence, and

d the inter-particle distance. They have also concluded that

the critical distance which Gc become zero coincides with

the end-to-end distance, hr20i�1=2

, of matrix iPP chains. Our

results in Fig. 10 qualitatively agreed with their results.

With an assumption that the radius r equals to 26 nm,

fitting of Eq. (7) to our data leads to obtain the critical

distance as 2 nm, which is much smaller than the end-to-

end distance of polymer. The end-to-end distance of iPP

is reported to have the molecular mass dependence as

follows [35]:

r20

� �Mw

�1=2¼ 0:83 ð9Þ

hr20i�1=2

of matrix iPP used in this study is estimated to

be 41.5 nm. The difference between the evaluated critical

distance and the end-to-end distance of iPP strongly

suggests the aggregation of the SiO2 particles. Here, we

tried to estimate the effective radius of aggregation by

assuming dc ¼ hr20i�1=2

. The fitted curve is shown in

Fig. 10 with bold line, implying that it fits well with the

data of n-SiO2 composites. The radius of the aggregates of

n-SiO2 was then estimated as 143 nm, which shows good

agreement with the radius directly observed by TEM

(118 nm). It is important to note that the method described

above enable us to evaluate the average aggregation size of

the particle in semi-crystalline polymer matrix.

The composite with g-SiO2, on the other hand, shows

peculiar content dependence in the spherulite growth rate

as seen in Fig. 10. The growth rate starts to increase at

2.3 vol%, reaches to the maximum at 5.6 vol%, and then

starts to decelerate. It indicates that iPP/g-SiO2 composites

have reciprocal effects on the crystallization of matrix iPP

chains. The deceleration effect at higher content range is

attributed to the confinement effect of matrix polymer

chains, which was discussed above. The acceleration

effect, on the other hand, in the middle content range seems

Time/sec0 50 100 150 200 250 300

Time/sec

0 50 100 150 200 250 300S

pher

ulite

rad

ius/

μm

Sph

erul

ite r

adiu

s/μm

15

10

5

0

15

10

5

0

0 %

1.8 %

7.1 %

14.6 %

0 %

2.3 %

5.6 %

12.9 %

(a) (b)Fig. 9 Changes in the radius of

spherulite of iPP/n-SiO2 (a)

and iPP/g-SiO2 composites

(b) during the crystallization

at 135 �C

0.01

content/vol%

Sph

erul

ite g

row

th r

ate/µm

/sec

SiO2

g-SiO2

0.1 1 10 100

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

n-SiO2iPP/

iPP/

Fig. 10 Plots of spherulite growth rate of iPP/SiO2 composites as a

function of SiO2 content. The crystallization temperature is 135 �C.

The broken and bold line indicates the growth rate of iPP and the

fitted curve of Eqs. 7 and 8 assuming the particle radius of 143 nm,

respectively


123

to originate in iPP chains grafted onto nano-SiO2 surface

since n-SiO2 composite shows no acceleration effect.

It is well accepted that the growth rate of polymer is

depending on its molecular mass. The spherulite growth

rate (G) of polymer spherulite is written as follows [36]:

G ¼ G0 exp � U�

R Tc � T1ð Þ

� exp

�Kg

TcDTf

� ð10Þ

where G0 is the coefficient and dependent on the molecular

mass, U* is the effective activation energy related to chain

motion, R is the gas constant, Tc is the crystallization tem-

perature, T? is the temperature at which chain motion cea-

ses, DT is the supercooling defined as DT ¼ T�m � Tc, Kg is

the nucleation constant and f is corrective factor,

f ¼ 2Tc

Tc þ T

�

m

� �. G0 has been recognized to have a

molecular mass dependence as G0 * M-0.5 [37]. It

demonstrates that shorter grafted iPP chains have higher

growth rate during the crystallization except at high Tc. Thus,

the increase in the growth rate with increasing g-SiO2 content

in Fig. 10 is attributed to the increase of shorter iPP chains at

the interface of iPP/g-SiO2 composite. As already mentioned

in this manuscript, grafted iPP chains form semi-dilute brush

structure at the interfacial region with the thickness of

21 nm. iPP chains of matrix might be able to enter the

interfacial region since the density at the interfacial layer is

extremely small. The grafted chain reduced the chain

entanglements and allows iPP crystal to grow faster. The

higher mobility of grafted chain also helps to nucleate at the

interface, resulting in the increase in the nucleation rate as

well as the nucleation density at the early stage of crystalli-

zation process. Moreover, the co-crystallization of matrix

and grafted chains could be occurring due the same primary

structure, as proposed in the previous paper [18]. It is rea-

sonable that the grafted iPP accelerates the crystal growth by

acting as plasticizer, and then it is packed in same crystalline

lamellae. This might be the characteristic of the crystalli-

zation of composites that the chemical structure of grafted

chains and matrix are the same. If they have different

chemical structures, the grafted chains have to be extracted

from the surface of growing crystal yielding to the very weak

interaction with the matrix polymer, i.e. forming very soft

interface. The increase in crystal which includes both matrix

and graft chains should increase the mechanical, thermal,

and electrical properties of the interfacial layer.

Conclusions

The effect of the addition of iPP/SiO2 nanocomposite on the

crystallization behavior of iPP was investigated by means of

DSC and polarized optical microscopy. In this study, iPP/SiO2

composite having the same primary structure for the matrix and

grafted chain was synthesized. No acceleration effect was

observed for the crystallization of non-surface treated SiO2/iPP

composite, suggesting the weak nucleation ability of SiO2

surface. While large acceleration effect was observed for iPP-

grafted SiO2/iPP composite. The acceleration of spherulite

growth rate of iPP confirmed that the grafted iPP acted as a

plasticizer. At SiO2 contents higher than 5.6 vol%, steep

decrease in the crystallization rate were seen regardless of the

surface modifications. It was concluded that the confinement of

matrix chain took place according to the decrease in the inter-

particle distance with increasing SiO2 content. Moreover, the

average size of SiO2 aggregation in iPP matrix was successfully

evaluated by analyzing the contents dependence of the growth

rate, assuming that the inter-particle distance with zero growth

rate coincided with end-to-end distance of matrix iPP chain.

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