ORI GIN AL PA PER

Toughening of poly(butylene terephthalate)by polyacrylic impact modifier

Yunyan Yang • Guohua Li • Xiaoyan Yu •

Huili Ding • Qingxin Zhang • Nongyue Wang •

Xiongwei Qu

Received: 20 February 2014 / Revised: 7 June 2014 / Accepted: 17 June 2014 /

Published online: 1 July 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Core–shell structured polyacrylic (named ACR) impact modifiers con-

sisting of a rubbery poly(n-butyl acrylate) (BA) core and a rigid poly(methyl

methacrylate) shell with a size of about 310 nm were prepared by seed emulsion

polymerization. The ACR modifiers with different core–shell weight ratios (85:15;

80:20; 75:25; 70:30) were used to modify the toughness of poly(butylene tere-

phthalate) (PBT) by melt blending. It was found that the polymerization had a very

high instantaneous conversion ([90 %) and overall conversion (98 %). The ACR

latexes had an obvious core–shell structure confirmed by transmission electron

microscope. The mechanical properties of the PBT/ACR blends were evaluated, and

scanning electron microscope (SEM) was used to observe the fractured morphology.

Dynamic mechanical analysis and differential scanning calorimeter were used to

study the molecular movement and crystallization behaviors of PBT/ACR blends.

The results indicated that with an appropriate value of the core–shell weight ratio,

poly(BA) could disperse well in the matrix and the brittle–ductile transition point

could emerge. As a result, the notch impact strength of PBT/ACR blends with a

core–shell weight ratio of 80:20 was 6.7 times greater than that of pure PBT, and the

mechanical properties agreed well with the SEM observation.

Keywords Emulsion polymerization � Core–shell structured polyacrylic (ACR) �Poly(butylene terephthalate) (PBT) � Toughness � Structure–property relations

Y. Yang � G. Li (&) � X. Yu � H. Ding � Q. Zhang � N. Wang � X. Qu (&)

Institute of Polymer Science and Engineering, School of Chemical Engineering,

Hebei University of Technology, Tianjin 300130, People’s Republic of China

e-mail: nkligh@126.com

X. Qu

e-mail: xwqu@hebut.edu.cn

123

Polym. Bull. (2014) 71:2353–2367

DOI 10.1007/s00289-014-1192-4

Introduction

Poly(butylene terephthalate), PBT, is an important engineering thermoplastic. It has

excellent properties such as high stiffness, high hardness, excellent abrasion

resistance, good chemical resistance, fine electrical insulation and fast crystalliza-

tion rate from the melt [1–6]. However, the remarkable disadvantages of PBT,

namely, the high notch sensitivity and low impact strength, limit its application.

Hence, PBT toughening has raised the interests of researchers greatly in recent

years. To overcome this drawback, elastomeric particles have been utilized to

toughen PBT, resulting in blends with high toughness. However, the domain sizes of

the rubbers are hard to be controlled for their poor process-dependence [4, 7–9]. The

role of the elastomeric particles in the matrix depends on the stress state in the

blend. In a high triaxial stress state around the notch tips during impact, the rubbery

particles are able to withstand the dynamic load together with the matrix, and the

matrix stress concentration becomes low. Many reports have shown that a major

source of toughening in the blends under high triaxial stress state comes from the

ability of these particles in promoting dilatational deformation by rubber cavitations

and interface debonding. As the dilatational deformation takes place, the triaxial

stress plane-strain condition is relieved, initiating extensive plastic deformation in

the matrix and promoting energy absorption mainly when the matrix yielded by

shear yielding [10, 11].

Among rubbery impact modifiers, the core–shell structured polyacrylic (ACR)

fillers are commonly used for polymer toughening because of their predetermined

size and high toughening effects. To minimize the strain, the core has to be made of

a highly elastomeric material. The role of the shell is to ensure compatibility with

the matrix. Core–shell structured polyacrylics, like poly(butyl acrylate)/poly(methyl

methacrylate) (ACR), have been generally used to toughen poly(vinyl chloride)

(PVC) [12, 13]. Clearly, these properties are much dependent on the relative

composition of the core and shell layers and on the structure of the core–shell

particles, such as the size of the particles and the homogeneity of the shell coverage.

Therefore, it is important to prepare the structure-controlled core–shell polyacrylic

modifier with suitable core–shell weight ratio and compatibility with PBT matrix

[14].

In this study, a core–shell structured poly(butyl acrylate)/poly(methyl methac-

rylate) (ACR) modifier, in the absence of other functional monomers, was

synthesized as the latex particle with a predetermined size of about 310 nm, and

various core–shell weight ratios by seed emulsion polymerization. The ACR

modifiers and PBT matrix were blended with a weight ratio of 20:80. The effects of

core–shell weight ratios of ACR particles on the mechanical properties of PBT/ACR

blends were studied. Scanning electron microscope (SEM) was used to observe the

fractured surfaces. Dynamic mechanical analysis (DMA) and differential scanning

calorimetry (DSC) were used to explore the molecular movement and crystallization

of PBT/ACR blends.

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Experiment

Materials

Potassium persulfate (KPS) was purchased from Tianjin Chemical (Tianjin, China),

allyl methacrylate (ALMA) and 1,4-butanediol diacrylate (BDDA) from Tianjiao

Chemical (Tianjin, China), and the anionic surfactant, as Aerosol MA Series, from

Cytec, (Heavens, The Netherlands). ALMA and BDDA were used as cross-linkers

for the core component and ALMA also as a graft-linker between the core and

glassy shell. All above materials were used without further purification.

n-Butyl acrylate (BA) and methyl methacrylate (MMA) were purchased from

Beijing Dongfang Chemical (Beijing, China). The BA monomer was freed of

inhibitor by being washed with a 2 wt% NaOH solution and deionized water until

the filtrated water was neutral, dried with CaCl2 overnight, and distilled under

reduced pressure. The MMA monomer was purified by distillation under reduced

pressure before use. Hydroquinone was used as an inhibitor of the latexes taken

from the emulsion polymerization. Deionized water was used in all experiments.

PBT resin (1100-211M) was provided by Changchun Industry (Taiwan, ROC).

Seed emulsion polymerization process

The core–shell structured polyacrylic latexes were synthesized as 50 % of solid

latexes via a two-stage semicontinuous seed emulsion polymerization. The

surfactant (0.5 g) and water (140 g) were added to a 0.5 L four-neck flask under

nitrogen, and then heated at about 78 �C. After 30 min, the seed-stage BA

monomer (10.0 g, 5 wt% of total monomer) and BDDA (0.054 g) were added to

the surfactant solution and the mixture was stirred for 10 min. Then KPS (0.43 g)

solution in water (20 g) was added, followed by addition of another batch of KPS

(0.11 g) solution in water (10 g) after 55 min. The seed stage was 60 min. The

growth stage involved two layers of pre-emulsified monomers: the first layer of

pre-emulsified monomer of BA (150 g) and ALMA (1.50 g) with surfactant

(1.875 g) and the second layer of pre-emulsified monomer of MMA (40 g) with

surfactant (0.800 g). Based on the weight of the monomer, the pre-emulsified

monomers of the two layers were pumped, using a Watson-Marlow peristaltic

pump (Model 505S), at a rate of *5.34 g/min for 3 h. At the same time, KPS

(0.043 g) dissolved in water (10 g) was added to the reaction flask in three

portions at an interval of 1 h. After 60 min, the latex was cooled down to room

temperature and filtered through a 53-lm sieve to obtain the coagulum content.

The presence of coagulum in the final latex is inevitable, because emulsion

particle collision, interfacial tension, and the demulsification (resulting from

insufficient coating of the emulgator) all lead to its presence. The ACR modifier

sample was obtained by freeze–thaw cycling and washed with deionized water

several times. Based on the different amounts of core and shell added to the above

latex preparation, the weight ratios of core–shell structured particles corresponded

to 85:15, 80:20, 75:25 and 70:30, respectively (Scheme 1).

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123

Blends preparation

The PBT and the ACR modifier were sufficiently dried in a vacuum oven at 120 �C

for 4 h and 40 �C for 24 h, respectively. A TE-34 twin screw extruder (L/D = 28,

Nanjing Institute of Extrusion Machinery, China) was employed to prepare PBT/

ACR blends with the weight ratio of 80/20 at a screw speed of 65 rpm and barrel

temperatures of 227–235–245–240 �C. The palletized materials were dried and

injection molded into standard specimens in an injection-molding machine (JPH-30,

Guangdong Hongli Machine China) at 240 �C.

Conversion and particle size measurement of the latexes

At 30-min intervals, the samples of the latex (10 mL) were removed into

preweighed vials containing 1 mL of hydroquinone solution (2 wt%) to prevent

further polymerization, which were cooled down with ice to quench the

polymerization and then analyzed gravimetrically to determine the instantaneous

conversion (on the basis of the monomer fed until the sampling time) and overall

conversion (on the basis of the monomer fed in the full emulsion polymerization

process). Particle sizes were measured online with a fixed 90� scattering angle with

dynamic light scattering (DLS) on a Malvern Zetasizer NANO-ZS90 (Worcester-

shire, UK) and the cell temperature was kept at 25 �C. The particle diameters

quoted are the mean values of z-average diameters (dzs) calculated by cumulate

method.

Morphology of the ACR modifier

The ACR latex morphology was examined by TEM (JEM-2100). The latex was

dispersed in water sufficiently with ultrasonic waves before characterization and

then prepared by casting one drop of diluted solution onto a carbon-coated copper

grid.

Impact and tensile tests

All PBT/ACR blends were inject-molded in 80 mm 9 10 mm 9 4 mm specimen

according to GB/T1041-92. The Charpy bars were notched according to GB/T1041-

92 and aged with constant relative humidity (50 % RH) at 23 �C for 24 h before

testing. The notch was formed in the molding process. The tensile testing was

carried out on a universal tensile tester (CMT-6104) according to ASTM D-638. An

Scheme 1 The schematic representation for the seed emulsion polymerization process

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average value of at least five independent measurements was used for each test. The

standard deviations were also calculated.

Thermal analysis

Differential scanning calorimetry (DSC) measurements were conducted with a

differential scanning calorimeter (Perkin-Elmer, Diamond). Samples of PBT/ACR

blends were heated from room temperature to 250 �C at a rate of 10 �C/min and

held for 3 min to destroy any residual nuclei, followed by cooling at 10 �C/min to

room temperature and reheated at 10 �C/min to 250 �C. All DSC measurements

were performed under a nitrogen atmosphere.

Dynamic mechanical analysis

Dynamic mechanical analysis tests of PBT/ACR blends were performed under

nitrogen atmosphere at a frequency of 1 Hz between -80 and 150 �C with a heating

rate of 3 �C/min using Tritec-2000 dynamic mechanical analyzer (Triton, UK)

under a dual-cantilever beam blending mode.

Fractured morphology

The notched Charpy impact-fractured surfaces of PBT/ACR blends were observed

with a JSM-6490LV scanning electron microscope (SEM) operated at 10 kV. The

surface was coated with gold prior to SEM observation.

Results and discussion

Latex preparations with different core–shell weight ratios of ACR latexes

Emulsion polymerizations of ACR latexes were carried out at a temperature of

78 �C. Samples were taken at 30-min intervals during the polymerization to monitor

the conversion and the particle growth. Instantaneous and overall conversions were

calculated from a mass balance of the reagents in the polymerization with the

percentage solid content measured at each sampling time [15]:

Instantaneous conversion ð%Þ ¼ mass of polymers formed

mass of monomers added

� �� 100 ð1Þ

where the mass of monomers added is the sum of the monomer in the seed stage and

all the monomer that has been added during the growth stage.

Overall conversion ð%Þ ¼ mass of polymers formed

mass of total monomers

� �� 100 ð2Þ

where the total mass of monomer is the sum of the monomer in the seed stage and

all of the monomer in the growth stage.

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Figure 1a plots conversion-time data for the ACR latex preparation with a core–

shell weight ratio of 80/20. Table 1 lists the results of final percentage conversion

and percentage coagulum for the latexes with different core–shell weight ratios. For

each preparation listed in Fig. 1a and Table 1, all the ACR polymerizations were

observed to proceed at high instantaneous conversion ([90 %), i.e., the reaction was

carried out in a semicontinuous mode under ‘‘starved conditions’’ where the

addition rate of the monomers was slower than the consumption rate of reaction.

The mode of monomer addition affects the mechanism of particle formation and

their growth as well. Under starved conditions, the surfactant associated to

monomers fed can reside in the water or can diffuse to the growing particles to

stabilize them [16, 17]. So, most of the monomers added have been polymerized, the

copolymer composition would be uniform and approximately equal to the

composition of the BA/MMA comonomer feed mixture. Final conversions were

found to be high ([98 %) for all of the polymerizations, which showed that a

continuation of the polymerization for 1 h after the end of the monomer addition

stage was adequate to allow complete conversion.

The DLS technique was used to obtain quantitative information about the particle

sizes of colloidal systems. In this study, DLS provided rapid means of monitoring

the size of the latex particles during both the seed stage and the growth stage of

polymerization. With this information, it is possible to establish a latex system of

the known particle diameter and to determine whether the latex particles grew

sequentially or the secondary nucleation occurred, during the growth stage of

polymerizations.

Latex particle diameters were determined by DLS and compared with those

theoretically calculated from the following equation [15]:

dt ¼MtIt

MS

� �1=3

�ds ð3Þ

where dt is the diameter of the particle at time t, Mt is the total mass of the monomer

added at time t, It is the instantaneous conversion at time t, Ms is mass of monomer

added in the seed stage, and ds is the seed particle diameter as measured by DLS.

Figure 1b shows the variation of particle size versus the reaction time. The ACR

particle diameter was consistent with the theoretical size. Moreover, the final ACR

particles are of 310 ± 5 nm and the polydispersity index (PDI), referred to as a

Malvern polydispersity index (M-PI), has a very low value (Table 1). M-PI is a

Table 1 Latex parameters for different core–shell weight ratios of ACR particles

Core–shell weight

ratio

Total conversion

(wt%)

Coagulation content

(wt%)

Particle size

(nm)

Polydispersity index

(PDI)

85/15 98.77 2.01 307 ± 9.7 0.012

80/20 98.81 1.16 307 ± 5.9 0.034

75/25 99.05 2.17 314 ± 7.8 0.013

70/30 98.61 2.42 313 ± 6.7 0.011

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dimensionless measure of broadness of the size distribution calculated from the

cumulants analysis, and its value varies from 0 to 1. Values greater than 1 indicate

that the distribution is polydisperse and values lower than 0.08 indicate that the

Fig. 1 Variation with reactiontime of a overall andinstantaneous conversion,b measured and theoreticalparticle diameter and c thedistribution of ACR final latexparticle size with core–shellweight ratio of 80/20

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distribution is nearly monodisperse. Figure 1c shows the distribution of final

particle size PDI with a core–shell weight ratio of 80/20. The PDI of the latex was

0.034. These results showed that the final particle sizes of the latex presented narrow

distribution. The good agreement between the measured and theoretical particle

diameters throughout the polymerization for all the latexes provided strong evidence

that the observed particles grew without significant secondary nucleation. This

showed that the growth stage of the ACR polymerization occurred under optimum

conditions. In conclusion, the instantaneous composition of polymer particles was

close to that of the feed at all times, and the polymerization process could control

the composition and structure of polymer particles with different weight ratios of

core and shell layers. In the next sections, we will discuss the effects of core–shell

weight ratios on the thermal and mechanical properties of PBT/ACR blends.

The morphology of ACR latex with the core–shell weight ratio of 80:20,

observed by TEM, is shown in Fig. 2. It is noted in Fig. 2a that the particles

consisted of a dark core of poly(BA) and a brighter shell of poly(MMA), indicating

the obvious core–shell structure of ACR latex. Based on the Fig. 2b and DLS

results, evidently the final ACR latex was made up of spherical particles with

narrow distribution.

Molecular movement

The melting and crystallization behavior of PBT/ACR blends were studied by DSC.

The results of the second heating scan are shown in Fig. 3 for the ACR modifiers

with different core–shell weight ratios. The second heating showed a minor melting

peak which was a well-known feature of thermoplastic polyesters crystallized in

DSC. This can be ascribed to the occurrence of initial melting, recrystallization, and

remelting processes in the melting region [16, 17]. It has been reported that PBT

could have two kinds of spherulites, depending on the cooling and processing

conditions [18]. No crystallization exotherm was observed despite the rapid cooling

in the injection mold. Figure 3 and Table 2 also show that Tm and degree of

crystallinity of PBT remained constant and showed no visible change, with the

addition of ACR particles. This indicated that the modifier phase did not disturb the

crystallization process of PBT. These results are in agreement with the melting

behavior previously observed in PBT/PEO-g-MA [19] and (PBT–Ph)/PEO-g-MA

[20] blends and with the reports in other semi-crystalline matrix/elastomer blends

[21, 22].

The DMA results are shown in Fig. 4 and Table 3, illustrating the temperature

dependence of the loss tangent (tan d) and storage modulus of the PBT/ACR blends

with different core–shell weight ratios of ACR and pure PBT. Figure 4a and Table 3

show that there is only one peak for the pure PBT and the Tg of it is 66.4 �C, but for

the PBT/ACR blends, there are two peaks, the lower peaks correspond with the PBA

phase and the higher peaks that are very broader than the pure PBT correspond with

the matrix PBT and the PMMA shell. The Tg2 for the higher peaks are located about

70 �C, but Tg of pure PMMA is about 105 �C; they are miscible for PBT and

PMMA shell.

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On the other hand, for these blends, the PBT Tg2 peaks with the addition of core–

shell structured modifier was found to shift to lower temperature when the core–

shell weight ratio was 80:20. Meanwhile, the two Tg peaks got closer for the blends.

For other blends Tg2 peaks moved to higher temperature relative to the pure PBT,

Fig. 2 TEM images a and b of the ACR particles with core–shell weight ratio of 80/20

Polym. Bull. (2014) 71:2353–2367 2361

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but the Tg2 peaks were broader than the pure PBT. The Tg1 peaks did not change

visibly. The detail value can be seen in Table 3. It demonstrated that the chain

movement of PBT was more easily affected by the rubbery modifier, than the

rubbery modifier was affected by PBT. This was consistent with that PS was

modified by PBA/PMMA [23]. As we know, if the blend displayed two Tgs, at or

near the two components, then it was immiscible. On the other hand, if it showed a

single transition or two transitions at temperature intermediate between those of the

pure components, then the blend was miscible or partially miscible [23]. So Fig. 4

and Table 3 indicate good compatibility between PBT and the ACR particles, which

is the necessary qualification for an effective toughening modifier.

The storage modulus which represents the rigidity of the material is shown in

Fig. 4b. With addition of ACR, the storage modulus of PBT/ACR blends decrease

notably, compared to pure PBT. When the core–shell was 80:20, the storage

modulus showed the biggest drop, corresponding to maximum toughness.

Table 2 Effect of the core–shell weight ratios of ACR on the melting and crystallization parameters of

the PBT/ACR blends

Blends of PBT/ACR First heat Cooling Second heat

Tm (�C) Xc(PBT)(%) Tc (�C) 4Hc (J/g) Tm1 (�C) Tm2 (�C) Xc

(PBT)(%)

PBT 224.5 26.4 192.6 47.6 214.8 224.4 25.2

PBT/ACR (85:15) 225.5 35.1 189.8 35.1 215.3 224.5 26.1

PBT/ACR (80:20) 228.2 37.0 190.3 39.3 217.4 227.3 29.4

PBT/ACR (75:25) 227.2 36.1 190.9 39.9 215.3 224.9 27.4

PBT/ACR (70:30) 224.6 33.4 191.4 38.9 214.3 223.3 27.7

Fig. 3 DSC heating scans of the pure PBT and PBT/ACR blends with different core–shell weight ratios

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Table 3 Some glass transition temperature parameters for the PBT/ACR blends with different core–shell

weight ratios of ACR particles

PBT 85:15 80:20 75:25 70:30

Tg1 (�C) – -46.6 -48.8 -47.7 -47.7

Tg2 (�C) 66.4 70.4 58.5 75.5 75.5

Tg2–Tg1 (�C) – 117.1 107.3 123.2 123.2

Fig. 4 DMA spectra of the PBT/ACR blends with different core–shell weight ratios in ACR: a tand andb storage modulus

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Mechanical properties

The impact strength of the PBT/ACR blends as a function of the core–shell weight

ratio of ACR modifier is shown in Fig. 5a. The impact strength of the blends

changed noticeably with the addition of ACR with different core–shell weight

ratios. It increased rapidly up to 59.68 kJ/m2 for the blends with the ratio of 85:15.

Meanwhile, as the weight ratio of core to shell of the ACR approached 80:20, the

notch impact strength increased up to the maximum with 70.49 kJ/m2, which is 6.7

times as high as that of pure PBT (10.52 kJ/m2). Figure 5b presents the tensile

strength of the PBT/ACR blends with different core–shell weight ratios. It shows

that the blends with different core–shell weight ratios have a lower tensile strength

Fig. 5 Notch impact strength (a) of pure PBT and PBT/ACR blends as a function of core–shell weightratio, tensile strength (b) of pure PBT and PBT/ACR blends as a function of core–shell weight ratios

2364 Polym. Bull. (2014) 71:2353–2367

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compared with that of the pure PBT, but it increased slightly as the core–shell

weight ratio of ACR modifier decreased.

The impact property of PBT blends is very important for the end use. An

important factor which influenced the impact strength in rubber toughened plastic

blends is the core–shell weight ratio of the modifier. Because the rubber core, rather

than the shell, plays an important role in toughening PBT, the toughness of PBT/

ACR blends depends on the rubber content. If the rubbery phase (PBA) in the

modifier is too low to absorb energy effectively in the failure process, the toughening

effect will not be enough. Nevertheless, the PMMA with a hard shell plays two

important roles: it acts as compatibility agent in PBA and PBT system, as thickening

PMMA shell may increase the interfacial adhesion, and it provides a means of stress

transfer from matrix to particles in the modified PBT. For these reasons, just when

the shell is thick enough, poly(BA) could disperse well in the matrix and the brittle–

ductile transition point could emerge [24]. Considering the above factors, the core–

shell weight ratio has an optimum value of 80:20, as shown in Fig. 5a.

Fractured morphology

The fractured morphologies of pure PBT and PBT/ACR blends are shown in Fig. 6.

As expected, only a few smooth lines could be seen. A large area of the fracture

Fig. 6 SEM micrographs of the fracture surface of notched impact samples of PBT and its blends:a PBT; b PBT/ACR-85:15; c PBT/ACR-80:20; d PBT/ACR-75:25

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surface was smooth while almost no stress whitening could be found on the

fractured surface, as shown in Fig. 6a for pure PBT, indicating the absence of a

significant energy absorption-related deformation mechanism. Fractured morphol-

ogies of PBT/ACR blends are shown in Fig. 6b–d. Compared with pure PBT, as the

core–shell weight ratios decreased, the fractured surface showed morphological

variation. When the core–shell weight ratio changed from 85:15 to 80:20, the

fractured surface became rougher. Especially, Fig. 6c displays the fractured

morphology of PBT/ACR-80/20 blend. The fractured surface showed the charac-

teristics of ductile fracture and extensive plastic deformation, indicating that the

shear yielding of the PBT matrix had taken place. The notched impact surface

morphology was consistent with the mechanical testing results. When the core–shell

weight ratio of the ACR particles further decreased to 70:30, the fractured surface

became smooth again. These evidences proved that the core–shell weight ratio of

80:20 for ACR modifier was more suitable for the toughening of PBT.

Conclusions

ACR modifiers were synthesized by seed emulsion polymerization and PBT/ACR

blends were prepared by melt blending. The morphology, mechanical properties and

crystallization behavior were investigated. The ACR latex particles had a visible

core–shell structure and the mechanical properties showed that the notched impact

strength of the PBT/ACR blends was greatly improved. When the core–shell weight

ratio was 80:20, the notched impact strength was highest (70.49 kJ/m2) i.e., 6.7

times of pure PBT (10.52 kJ/m2), corresponding to the SEM morphology for

fracture surfaces. DMA results indicated good compatibility between PBT and the

ACR particles which was required for an effective toughening modifier. SEM

observation showed that the PBT matrix had yielded by the stress concentrations

associated with the rubber particles.

Acknowledgments This work was partially supported by Natural Science Foundation of Hebei

Province; contract grant number: E2010000107, and National Natural Science Foundation of China;

contract grant number: 51073049 and 21001039. The authors would like to thank Professor Maryam

Grami, Winona State University, MN, USA, for her helpful discussions during revision.

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