POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2004; 15: 467–471

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pat.492

Mechanical properties and structural characteristics

of PP/PP-g-SBR nanocomposites prepared by dynamical

photografting

Weizhi Wang, Mouzheng Fu and Baojun Qu*Department of Polymer Science and Engineering, University of Science and Technology of China, 230026, Hefei, Anhui, P. R. China

Received 11 November 2003; Revised 15 March 2004; Accepted 2 April 2004

Poly(propylene) (PP)/PP grafted styrene-butadiene rubber (PP-g-SBR) nanocomposite was pre-

pared by blending PP with PP-g-SBR using dynamical photografting. The crystal morphological

structure, thermal behavior, and mechanical properties of PP/PP-g-SBR nanocomposites have

been studied by photoacoustic Fourier transform infrared spectroscopy (PAS-FT-IR), wide-angle

X-ray diffraction (WAXD), scanning electron microscopy (SEM), differential scanning calorimetry

(DSC), and mechanical measurements. The data obtained from the mechanical measurements show

that the PP-g-SBR as a modifier can considerably improve the mechanical properties of PP/PP-g-

SBR nanocomposites, especially for the notched Izod impact strength (NIIS). The NIIS of the nano-

composite containing 2 wt% PP-g-SBR measured at 208C is about 2.6 times that of the control sam-

ple. The results obtained from PAS-FTIR, WAXD, SEM, and DSC measurements revealed the

enhanced mechanism of impact strength of PP/PP-g-SBR nanocomposites as follows: (i) the b-

type crystal of PP formed and its content increased with increasing the photografting degree of

PP-g-SBR; (ii) the size of PP-g-SBR phase in the PP/PP-g-SBR nanocomposites obviously reduced

and thus the corresponding number of PP-g-SBR phase increased with increasing the photografting

degree of PP-g-SBR. All the earlier changes on the crystal morphological structures are favorable for

increasing the compatibility and enhancing the toughness of PP at low temperature. Copyright #

2004 John Wiley & Sons, Ltd.

KEYWORDS: modification; PP-g-SBR; poly(propylene) (PP); nanocomposites; photografting

INTRODUCTION

Poly(propylene) (PP) as an engineering thermoplastic mate-

rial is widely used in various industries owing to its low den-

sity, good moldability, excellent mechanical properties and

low cost. However, its main disadvantage is poor impact

resistance, especially at low temperature. Therefore, PP is

usually modified with elastomers to improve its impact

strength.1–6 In recent years, many researchers have paid

much attention to the study of dynamic vulcanization or

dynamically photocrosslinking of PP/rubber blends in order

to increase the impact strength at low temperature.7–12 How-

ever, these methods are still very limited because the rubber

phase can not form nanoparticle dispersion even if the shear

strength and system viscosity are high enough. Another rea-

son is that PP very easily degrades during the dynamic vul-

canization or dynamically photocrosslinking processes.

These unfavorable factors lead to the decrease of its stiffness

although the toughness of PP/rubber blend increases.

Recently, Zhang and coworkers reported that the fully

vulcanized styrene-butadiene rubber (SBR) nanoparticles

improved the toughness of PP matrix.13 These vulcanized

nanoparticles provide a good balance of toughness and stiff-

ness for the blends. However, it is very difficult to make nano-

particle dispersion homogeneously in a polymeric matrix

due to the strong tendency of nanoparticles to agglomerate.

In order to solve this problem, many researchers focus on

the approaches of in situ polymerization of monomers in

the presence of nanoparticles, such as sol-gel process14 and

intercalation polymerization technique.15 Although nanos-

cale dispersion of the particles can be obtained from the ear-

lier methods, however, these methods are unsuitable to

produce nanocomposites in mass with low cost and applic-

ability because of their complex polymerization and special

conditions.

In the present work, a combination method of in situ

photografting and mechanical blending is developed to

overcome the defects existing in the earlier-mentioned

methods. The key points are that (i) the SBR nanoparticles

are grafted by PP to form PP-g-SBR copolymer through UV

irradiation, and (ii) the modified nanoparticles PP-g-SBR are

mechanically mixed with pure PP as usual. Owing to the

Copyright # 2004 John Wiley & Sons, Ltd.

*Correspondence to: B. Qu, Department of Polymer Science andEngineering, University of Science and Technology of China,230026, Hefei, Anhui, China.E-mail: qubj@ustc.edu.cn

degradation of PP under UV light and thus the decrease of its

molecular weight, the degradation products may penetrate

into the agglomerated nanoparticles easily and react with the

activated sites of the nanoparticles inside and outside the

agglomerates. Therefore, the nanocomposites with homo-

genous dispersion structures can be obtained from the

decrease of surface tension of nanoparticles and the

improvement of filler/matrix interaction. In the present

work, the preparation of PP/PP-g-SBR nanocomposites

through photografting are described, and their crystal

morphological structures and mechanical properties are

investigated in order to clarify the relationship between

them.

EXPERIMENTAL

MaterialsPP (commercial number F401, melt flow rate 2.7,Mn¼ 55 000)

was supplied by Yangzi Petrochemical Co. Ltd., China. Full-

vulcanized SBR nanoparticles (with 50 wt% butadiene, the

average size is 100 nm) was supplied by SINOPEC Beijing

Research Institute of Chemical Industry, China. Benzil

dimethyl ketal (BDK) as a photoinitiator and hindered amine

(Tinuvin 144) as a photostabilizer were obtained from the

Ciba-Geigy Corporation, Switzerland. Trimethylopropane

triacrylate (TMPTA) as a crosslinking agent was supplied

from UCB, Belgium. All chemicals were used as received

without any purification.

Sample preparation (photografted PP-g-SBR andthe corresponding PP/PP-g-SBR nanocomposites)PP was firstly blended with the same weight fraction of SBR

nanoparticles, 1 wt% BDK, and 4 wt% TMPTA (based on the

total content) for 3 min at 1808C using a double roller mixer.

And then, a 2 kW Philips HPM 15 lamp was applied to irradi-

ate the blend for a given time during mechanical blending.

The obtained blend was named as a PP-g-SBR system con-

taining PP-g-SBR and PP of low molecular weight. The photo-

grafting degrees (PGD) of PP-g-SBR investigated by solvent

extraction are listed in Table 1.

The PP/PP-g-SBR nanocomposite was obtained by PP

blending with PP-g-SBR system, while the corresponding

control sample was obtained by PP blending with unmodi-

fied SBR nanoparticle (PP/SBR). The PP/PP-g-SBR samples

with a given PP and PP-g-SBR system are marked as PPm/PP-

n-SBR where m and n indicate the content of PP blended with

PP-g-SBR system and the irradiation time for PP-n-SBR

system, respectively. It needs to indicate that, such as the

sample PP96/PP-60-SBR, the total content PP in this sample is

98% because of 2 wt% PP in the PP-g-SBR system. The

samples for mechanical measurements were prepared in a

SJSH-30 twin screw extruder (Nanjing Plastic and Rubber

Factory, China) with a speed of 200 rpm.

MeasurementsThe PGD value is defined as the ratio of PP grafted on SBR

particles to total PP in the irradiated blend system. In order

to determine the PGD of PP-g-SBR, the UV irradiated blends

(w0) were packed by ashless filter papers and put into a Sohn-

ley vessel extracting for 120 hr with boiling xylene stabilized

by 0.2 wt% (based on the solvent content) antioxidant Tinu-

vin 144 and with N2 bubbling to prevent oxidation. The sol-

vent was renewed after every 24 hr of extraction. At the end of

extraction, the package was washed with acetone. After being

dried in a vacuum desiccator at about 708C to constant

weight, the insoluble residue (w1) was weighed. The average

PGD (wt%) in the test was calculated as the value of

(w0�w1)/0.5w0. Usually, five samples were analyzed to

determine the average PGD for a given set of experimental

conditions.

The photoacoustic Fourier transform infrared spectro-

scopy (PAS-FT-IR) spectra were recorded with a Nicolet

MAGNA-IR 750 spectrometer. The wide-angle X-ray diffrac-

tion (WAXD) measurements were performed at room

temperature with a D/Max-rA rotating anode X-ray diffract-

ometer (Rigaku Electrical Machine Co., Japan) equipped with

a Cu-Ka tube and a Ni filter. The diffraction patterns were

determined over a range of diffraction angles, 2y¼ 108–408 at

40 kV and 50 mA. The scanning electron microscopy (SEM)

micrographs were recorded using a X650 scanning electron

microscope (Hitachi X650 scanning electron microanalyzer,

Japan). The samples were previously etched by xylene and

coated with a conductive gold layer. The differential

scanning calorimeter (DSC) data were obtained in N2

atmosphere at a heating rate of 108C/min using a Perkin–

Elmer differential scanning calorimeter (model DSC 2). The

degree of crystallization of PP in the nanocomposite was

evaluated from the relative ratio of the values of fusion heat of

the nanocomposite to the fusion heat of PP (DHPP¼ 209 J/

g).16 The mechanical properties were measured using an

Universal Testing Machine (DCS5000, SHIMADZU, Japan)

with the crosshead speed of 50 mm/min at 25� 28C. The

dumb-bell shaped specimens were prepared according to

ASTM D412-87. The notched Izod impact strength was

measured by a Izod impact tester (Chengde, China). The

size of the rectangular specimens was 80� 10� 4 mm3 with a

458 V-shaped notch (tip radius 0.25 mm, depth 2 mm)

according to GB/T 1843–1996. The average value from 10

specimens was used for the data plot.

RESULTS AND DISCUSSION

PAS-FT-IR characterization of different PP-g-SBRparticlesFigure 1 shows the PAS-FT-IR spectra of unmodified SBR

particle and various photografting degrees of PP-g-SBR par-

ticles obtained by extracted PP-g-SBR systems by boiling

xylene. The intensities of the two wide absorption bands at

about 2926 and 2855 cm�1 increase with increasing UV irra-

Table 1. Photografting degree of PP-g-SBR system (wt%,

PP:SBR¼ 1:1)

System code Irradiation time (sec) PGD (%)

1 60 14.92 90 21.03 120 32.44 180 43.25 240 40.5

Copyright # 2004 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2004; 15: 467–471

468 W. Wang, M. Fu and B. Qu

diation time, which have been assigned to the antisymmetry

and the symmetry stretching vibration of –CH2– group,

respectively. Similarly, the intensities of two absorption

bands at 1450 and 1380 cm�1 increase with increasing the irra-

diation time due to the antisymmetry and the symmetry

bending vibration of –CH3 group, respectively. These results

indicate that the –CH2– and –CH3 groups in the UV irra-

diated samples increase with increasing irradiation time

due to PP molecular chains grafted to SBR nanoparticles.

As listed in Table 1, the contents of PP molecular chain

grafted on SBR nanoparticles are 14.9, 21.0, 32.4, 43.2, and

40.5% for the irradiation times of 60, 90, 120, 180, and

240 sec, respectively. Therefore, the photografting degree

data also give the stronger evidence to support the positive

effect of UV irradiation on the formation of PP-g-SBR.

Crystal structures of PP in the PP/PP-g-SBRnanocompositesThe WAXD spectra of PP98/SBR, PP96/PP-60-SBR, PP96/PP-

90-SBR, PP96/PP-120-SBR, and PP96/PP-180-SBR are shown

in Fig. 2. The 2y peaks of all samples at 14.18, 16.88, 18.58and 21.28 due to the 110, 040, 130 and the overlapping 131,

041 and 111 planes are characteristics of a-type monoclinic

crystal structures of PP. This indicates that all samples con-

tain a-type crystal structure. However, when the samples

are compared with the sample PP98/SBR, it is obvious that

there is a new peak appeared at 16.28 owing to UV irradiated.

This new peak has been assigned to the (300) reflection of

b-type hexagonal crystal structure of PP. Moreover, the

b phase fraction in the crystalline part of irradiated samples

can be assessed from the ratio of the height of (300) b reflec-

tion to the sum of the heights of four main crystalline reflec-

tions, i.e. (110), (040) and (130) from the a phase plus (300)

from the b phase, as proposed by Turner–Jones.17 The calcu-

lated results of b phase fraction are 18.2, 18.8, 21.4 and 23.3%,

respectively. It means that the b phase fraction increases with

increasing the irradiation time, owing to the b crystal nucle-

ating effect of PP-g-SBR particles. The b-type hexagonal crys-

tal can obviously improve the impact strength of PP because

of its ductility and better strength than a-type monoclinic

crystal with little loss of stiffness.18 The results are also

proved by mechanical measurements stated later.

Morphological structures of PP/PP-g-SBRnanocompositesThe SEM micrographs of PP95/SBR, PP90/PP-60-SBR, PP90/

PP-90-SBR, PP90/PP-120-SBR, and PP90/PP-180-SBR sam-

ples are shown in Fig. 3. The small balls observed in these

micrographs are resulted from the crosslinked SBR particles,

which can not be etched by xylene due to the gelled network

nature. Figure 3(a) was obtained from the sample of unirra-

diated SBR particles blending with PP. It clearly shows that

the rubber balls have large and non-spherical size. Moreover,

these rubber particles are easily revealed by etching because

there is no interaction with the PP matrix. After the PP/SBR

samples were irradiated for different times, the morphologi-

cal structures of PP/PP-g-SBR nanocomposites are signifi-

cantly different. Figure 3(b) was obtained from PP90/PP-60-

SBR sample, the rubber particles obviously decrease and

the surface of PP and PP-g-SBR becomes fuzzy compared

with PP95/SBR sample. With further increasing irradiation

time, the trend is more obvious as shown in Fig. 3(c) and

3(d). The numbers of rubber particle in the photografted sam-

ples become more and their average size becomes smaller

than that in the corresponding PP95/SBR control sample.

This is because the surface modification particles are easily

broken into smaller ones and dispersed homogeneously

into PP under the condition of high mixing torque. In

Fig. 3(e), the PP-g-SBR particles are almost not observed

because of the disappearance of coalescence of these small

particles during mechanical shearing. Apparently, PP graft-

ing on SBR particles severely increase its compatibility with

PP matrix. These results give more evidence of the effect of

SBR surface modification.

Thermal behavior of PP/PP-g-SBRnanocompositesFigure 4 shows the DSC melting curves for PP98/SBR, PP96/

PP-60-SBR, PP96/PP-90-SBR, PP96/PP-120-SBR, and PP96/

Figure 1. PAS-FT-IR spectra of unmodified SBR nanopar-

ticles and photografted PP-g-SBR nanoparticles for different

UV-irradiation times.

Figure 2. WAXD patterns of PP/SBR and PP/PP-g-SBR

nanocomposites.

PP/PP-g-SBR nanocomposites prepared by dynamical photografting 469

Copyright # 2004 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2004; 15: 467–471

PP-180-SBR samples over the temperature range of 100–

2008C. It can be seen that the melting points for all irradiated

samples decrease compared with unirradiated PP98/SBR

sample. This can be interpreted that the non-crystallizable

component SBR inhibits the crystal growth of PP and thus

leads to the formation of small and unperfected PP crystal.

Photografting modification improves the compatibility of

PP-g-SBR with PP matrix, which aggregates the attenuation

and unperfected PP crystal. Another obvious view is that

the melting endotherm has a slight broadening of the curves

of all irradiated samples compared with unirradiated PP98/

SBR sample. This result can be explained by the fact that

photografted PP-g-SBR particle acts as a b-type nucleation

agent for PP, which results in the coexistence of a-type and

b-type crystal in PP/PP-g-SBR sample, thus broadening the

melting endotherm. The melting behavior and crystallinity

of these samples determined by DSC method are listed in

Table 2. It shows the crystallinity of PP96/PP-60-SBR, PP96/

PP-90-SBR, PP96/PP-120-SBR, and PP96/PP-180-SBR sam-

ples are higher than that of the corresponding PP98/SBR con-

trol sample, which might be due to the formation of b-type

crystal and interface crystalline of PP-g-SBR copolymer.

These thermal analysis data are in accord with the results of

WAXD patterns on forming PP-g-SBR graft copolymer.

Mechanical properties of PP/PP-g-SBRnanocompositesThe impact and tensile strength of PP/SBR and irradiation

treated PP/PP-g-SBR samples are listed in Table 3. The

notched Izod impact strength (NIIS) data show that the value

Figure 3. SEM micrographs of PP/SBR and PP/PP-g-SBR

nanocomposites: (a) PP95/SBR; (b) PP90/PP-60-SBR;

(c) PP90/PP-90-SBR; (d) PP90/PP-120-SBR; (e) PP90/PP-

180-SBR.

Figure 4. DSC traces of PP/SBR and PP/PP-g-SBR

nanocomposites.

Table 2. Melting behavior and crystallinity of PP, PP/SBR,

and PP/PP-g-SBR nanocomposites

Blend systemsHeat of fusion,

DH(J/g)

Meltingtemperature,

Tm (8C)Crystallinity

(%)

PP 84.1 169.1 40.2PP98/SBR 79.3 167.2 37.9PP96/PP-60-SBR 114.6 167.1 54.8PP96/PP-90-SBR 115.8 166.7 55.4PP96/PP-120-SBR 109.6 166.4 52.4PP96/PP-180-SBR 109.4 166.6 52.3

Table 3. Impact strength and tensile strength of PP/SBR

and PP/PP-g-SBR nanocomposites

Blend systemsImpact strength at

208C (kJ/m2)Tensile strength

(MPa)

PP98/SBR 2.8 36.8PP96/PP-60-SBR 4.2 39.3PP96/PP-90-SBR 5.4 39.7PP96/PP-120-SBR 6.0 40.7PP96/PP-180-SBR 7.2 40.0PP96/PP-240-SBR 6.5 39.2PP95/SBR 3.3 36.5PP90/PP-60-SBR 4.9 37.3PP90/PP-90-SBR 6.0 37.8PP90/PP-120-SBR 5.5 38.7PP90/PP-180-SBR 5.2 38.0PP90/PP-240-SBR 5.0 37.4

470 W. Wang, M. Fu and B. Qu

Copyright # 2004 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2004; 15: 467–471

of PP98/SBR is 2.8 kJ/m2. After photografting modification,

the NIISs of PP96/PP-60-SBR, PP96/PP-90-SBR, PP96/PP-

120-SBR, and PP96/PP-180-SBR are 4.2, 5.4, 6.0, and 7.2 kJ/

m2, respectively. When further increasing the irradiation

time, the NIIS of PP96/PP-240-SBR decreases significantly

to 6.5 kJ/m2. The sample of PP96/PP-180-SBR appears to

give the maximum value of 7.2 kJ/m2, which is about 2.6

times that of the untreated PP98/SBR sample. When increas-

ing the content of nanoparticles in the PP matrix, the NIIS of

untreated PP95/SBR increased to 3.3 kJ/m2 from 2.8 kJ/m2

for PP96/SBR. This can be explained by the fact that the excess

amount of nanoparticles in the PP matrix might be aggre-

gated to cluster, which may inhibit plastic deformation of

PP matrix by constraining effects or simply by splitting. After

photografting modification, the NIIS of all samples increased.

Moreover, PP90/PP-90-SBR shows the maximum value of

6.0 kJ/m2 after irradiated for 90 sec, which is almost 2 times

that of the control sample PP95/SBR. However, when further

increases irradiation time, their NIIS values decrease. The

appearance of NIIS maximum values can be explained by

that the increasing irradiation grafting time may accelerate

the degradation of PP-g-SBR copolymer, which decrease

the effect of photografting degree with inferior dispersion.

The mechanism of enhanced toughness by rubber nano-

particles can be also attributed to two main reasons. First, the

rubber nanoparticles act as stress concentration sites for

dissipation of shock or impact energy by controlling and

promoting matrix deformation. Therefore, the addition of

rubber into PP leads to relaxation of the stress concentration

and suppresses the formation of matrix crazes or deforma-

tion. Second, the discrete crosslinked PP-g-SBR particles

greatly decrease the possibility of rubber cohesion into bulky

particles during mechanical mixing. As a result, the sites for

dissipation of shock or impact energy are greatly increased in

the PP/PP-g-SBR nanocomposites.

Table 3 also shows the effects of photografting modified

PP-g-SBR particles on the tensile strength of PP matrix

compared with unmodified SBR particles. It shows that, after

photografting the values of tensile strength increase com-

pared with 36.8 MPa for PP98/SBR. When increasing the

content of SBR particles in PP matrix, the tensile strength

value of sample PP95/SBR is 36.5 MPa, which is almost the

same as that for PP98/SBR. After photografting, the tensile

strength values increase. Similarly, the tensile strength

appears to reach a maximum value after an irradiation time

of 120 sec for both series of samples.

The NIIS and tensile strength results show that the PP-g-

SBR nanoparticles filled PP can improve the toughness and

stiffness of composites at the same time. The earlier results

also prove that the interface adhesion of PP and PP-g-SBR

positively influences the mechanical behavior of composites.

It attributes to the homogeneous dispersion of PP-g-SBR in

PP matrix and the miscibility between the photografting

polymer and the matrix.

CONCLUSIONS

The PAS-FT-IR and DSC results gave positive evidence for

the formation of graft copolymer PP-g-SBR, which greatly

enhances the compatibility with PP, and thus increases the

impact strength of PP/PP-g-SBR nanocomposites. The

WAXD data show that the formation of b-type crystal of PP

partly enhances the impact strength of PP/PP-g-SBR nano-

composites. The SEM micrographs demonstrate that the

decrease of particle size and the increase of particle number

of the rubber phase are caused by the UV irradiated photo-

grafting modification and mechanical shearing. The PP-g-

SBR particles not only enhance interfacial adhesion but also

increase the sites for dissipation of shock and impact energy

in the PP/PP-g-SBR nanocomposites. Therefore, the UV irra-

diated photografting modified PP-g-SBR nanoparticles can

considerably improve the mechanical properties of PP/PP-

g-SBR nanocomposites, especially for the NIIS.

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PP/PP-g-SBR nanocomposites prepared by dynamical photografting 471

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