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PBT blends filled with silica nanoparticles during melt processing
Polymer Degradation and Stability 93 (2008) 1397–1404
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Inhibited transesterification of PET/PBT blends filled with silica nanoparticles during melt processing Feng Wang, Xiangfu Meng, Xinfeng Xu, Bin Wen, Zhongzhong Qian, Xiaowei Gao, Yanfen Ding, Shimin Zhang, Mingshu Yang* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2008 Received in revised form 25 May 2008 Accepted 31 May 2008 Available online 14 June 2008
The blends of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) undergo transesterification reactions between PET and PBT during melt processing. In this research, PET/PBT transesterification has been investigated in the presence of nano-fillers, including pure SiO2 and silanecoupling-agent-modified SiO2. The results show that the incorporation of SiO2 nanoparticles inhibits PET/PBT transesterification, and the influence of pure SiO2 is higher than modified SiO2. The inhibition of SiO2 on transesterification is explained by the fact that the hydroxyl end groups of PET and PBT react with the surface hydroxyl groups of SiO2 before transesterification due to the high activity of surface hydroxyl groups of SiO2, and the reduction of hydroxyl end groups of PET and PBT leads to the inhibition of transesterification between PET and PBT. This has been demonstrated by the experimental data of TGA, FTIR, and XPS. And the reactivity of hydroxyl end groups of PBT is higher than that of PET. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Nanocomposites Poly(butylene terephthalate) Poly(ethylene terephthalate) Silicas Transesterification
1. Introduction Poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) are two members of the most important commercial thermoplastic polyesters. Compared with PET, PBT has the advantages of rapid crystallization rate and good moldability, while heat-deflection temperature and rigidity of PET are superior to those of PBT. Blending offers the potential of property improvements between PET and PBT to obtain a composite material with comprehensive properties. PET/PBT blend is miscible in the amorphous region with the entire blend composition and forms separate crystals rather than cocrystals [1–3]. It is well known that PET/PBT blends undergo ester-interchange (transesterification) reactions above the melting temperature, leading to the formation of block copolymers in the initial stage and random copolymers finally. The practical importance of transesterification consists, first in the possibility to improve the compatibility of polymer blends directly through processing, and second in the chance to prepare novel copolymers with the given structure [4].
* Corresponding author. Tel./fax: þ86 10 82615665. E-mail address: [email protected] (M. Yang). 0141-3910/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2008.05.026
Many works have been focused on the transesterification reaction of PET/PBT blends. Backson et al. [5] and Kim et al. [6] applied 13C NMR to study transesterification extent and sequence structure in PET/PBT blends with the entire blend composition. Jacques et al. [7] carried out melt mixing of PET/PBT in a Brabender Plastograph at 275–300 C. It has been demonstrated by 13C NMR that titanium alkoxide accelerates the reaction, whereas triphenyl phosphite hinders it. And the inhibition mechanism of triphenyl phosphite on transesterification of PET/PBT blends was reported in a series of following papers [8–11]: the observed molecular weight increasing fully confirms the occurrence of high temperature phosphite reactions with hydroxyl chain ends of PET and PBT which lead to the incorporation of significant quantities of phosphorus into the polyester backbone, thus transesterification is inhibited for the hydroxyl end groups of PET/PBT blends were partially capped. Matsuda et al. [12,13] firstly applied 600 MHz 1H NMR to study the sequence distribution of transesterification product between PET and PBT, and the relationship between sequence distribution and thermal properties. The melting temperature of PET/PBT blend decreases as the progress of transesterification due to the decrease of lamellae thickness which was linearly correlated with the inverse of the number–average sequence length. The kinetics of transesterification reaction had also been studied through Monte Carlo simulation and experimental method [14–17]
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F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
as well. The reaction kinetics is consistent with a reversible second order reaction. In conclusion, transesterification of PET/PBT has been sufficiently studied until now. Inorganic nanoparticles are commonly utilized to fill in polymer matrix to prepare PET/PBT nanocomposites which are expected to enhance their stiffness and impact properties simultaneously. Furthermore, inorganic nanoparticles are often organic modified to achieve homogeneous distribution. However, as to PET/PBT nanocomposite, the influence of nanoparticles on transesterification of PET/PBT has been seldom studied. In this work, we study the influence of unmodified and organic modified SiO2 nanoparticles on transesterification of PET/ PBT blends by 13C NMR, and present the inhibition mechanism of nano-SiO2 definitely for the first time. 2. Experimental part 2.1. Materials PET was purchased from Sinopec Yizheng Chemical Fibre Co. Ltd., and PBT was obtained from Nantong Xingchen Synthetic Material Co. Ltd., China. The intrinsic viscosities of PET and PBT in the mixed solvent of phenol/1,1,2,2-tetrachloroethane (TCE) with a weight ratio of 60/40 were 0.875 dL/g and 1.160 dL/g, respectively. The number–average molecular weights of the polymers were 41,500 and 38,900. Aerosil-200 fumed silica (surface area 200 25 m2/g, primary particle size 12 nm, purity 98%) was obtained from Degussa-Hu¨ls, and dried in a vacuum oven overnight at 105 C before use. Two kinds of silane-coupling agents were used: dodecyl-trimethoxysilane (WD10) was a commercial sample from Hubei Wuhan University Silicon New Material Co. Ltd. Trimethylchlorosilane (TMCS) was bought from Beijing Chemical Reagents Company. All other solvents and reagents including 1,1,2,2-tetrachloroethane, phenol, anhydrous toluene, absolute ethanol, trifluoroacetic acid and deuterated trifluoroacetic acid were used as-received.
with the entire blend ratio have been also reported [6]. After comprehensive analysis, the PET/PBT blend with blend composition around 70/30 (wt.%) has excellent properties and its transesterification extent was high enough to be easily detected. Therefore, as in many other papers [7,9–11,27,28], the composition of PET/PBT blend was fixed at 70/30 (wt.%) in this work. The incorporation content of nanosilica (including unmodified SiO2 and modified SiO2) was 5 wt.%. 2.4. Characterizations Scanning electron microscope (SEM) images were taken on a JEOL JSM 6700F field emission scanning electron microscope at 5 kV. The specimens were quenched and fractured in liquid nitrogen, and the fractured surfaces were coated with platinum using GIKO IB-3 ion coater. Infrared spectra were recorded with KBr pellets on a Perkin–Elmer System 2000 infrared spectrum analyzer in the wavenumber range of 4000–370 cm1. X-ray photoelectron spectroscopy (XPS) data was obtained with an ESCALab 220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation. The base pressure is about 3 109 mbar. The binding energies are referenced to the C1s line at 284.6 eV from adventitious carbon. Perkin–Elmer Pyris 1 TGA was used to measure the thermal behavior of the modified silica in nitrogen atmosphere from 50 C to 750 C at a heating rate of 20 C/min. The samples were dissolved in deuterated trifluoroacetic acid at a concentration of ca. 15% w/v. 13C Nuclear Magnetic Resonance (NMR) was performed on a Bruker AVANCE 400 NMR spectrometer operating at 400 MHz. Experimental technique and parameters have been described elsewhere in detail [18]. Intrinsic viscosity of PET/PBT blend was measured with a Ubbelodhe viscometer in 60/40 (wt.%) phenol-1,1,2,2-tetrachloroethane mixed solvent at 110 C. 3. Results and discussion 3.1. Dispersion of silica nanoparticles in PET/PBT/SiO2 nanocomposites
2.2. Preparation of silane-coupling-agent-modified SiO2 Two types of silane-coupling agents were selected to modify the surface of fumed nanosilica: dodecyl-trimethoxysilane (WD10) and trimethylchlorosilane (TMCS). Fumed nanosilica (10.0 g) was dispersed in anhydrous toluene (320 mL). WD10 (26.5 g) or TMCS (16.1 g) was added into the suspension. The mixture was refluxed at 100–110 C under stirring for 24 h. The product was filtered and then washed with anhydrous toluene (300 mL, three times) and absolute ethanol (300 mL, three times). The modified nanosilica was dried in a vacuum oven overnight at 105 C. Nanosilica without modification was labeled as pure SiO2, and nanosilica modified with WD10 and TMCS was labeled as WD10-M-SiO2 and TMCS-MSiO2, respectively. FTIR and TGA results showed that silane-coupling agents were successfully grafted to the surface of SiO2. 2.3. Compounding procedure PET and PBT pellets were dried under vacuum at 110 C for 24 h, and nano-SiO2 was dried under vacuum at 105 C for 24 h before compounding. The PET/PBT/SiO2 nanocomposites were prepared by melt compounding method using a HAAKE Rheomix-600 internal mixer (Mess-Technic GmbH, Germany) at 260 C with a screw speed of 50 rpm for 30 min. The properties of PET/PBT blends with the entire blend composition have been fully researched in many previous studies [1–3], including mechanical property, crystallization property and thermal property. And transesterification extents of PET/PBT blends
The dispersion state of silica nanoparticles in PET/PBT/SiO2 nanocomposites was investigated by SEM observations in Fig. 1. Fig. 1(a), (b) and (c) shows the SEM images of PET/PBT/pure SiO2, PET/PBT/WD10-M-SiO2, and PET/PBT/TMCS-M-SiO2, respectively. It was found that pure SiO2 nanoparticles tended to form large aggregates and agglomerates in PET/PBT matrix. On the contrary, while the surface of SiO2 nanoparticles was modified with silanecoupling agents WD10 and TMCS, most of the surface hydroxyl groups were replaced by organic groups, their hydrophilic surface turned to hydrophobic surface, which was more compatible with polymer matrix, thus WD10-M-SiO2 and TMCS-M-SiO2 dispersed much more uniformly in PET/PBT than pure SiO2. 3.2. Measurement of transesterification extent by
13
C NMR
Plenty of measurements are used to study the ester-interchange reaction and the structure of transesterification product, such as FTIR (Fourier Transform Infrared), SANS (Small Angle Neutron Scattering), NMR (Nuclear Magnetic Resonance), etc. 13C NMR has been proved to be a sensitive and efficient method to analyze the sequence structure of polymer chain qualitatively and quantitatively. In Newmark’s 13C NMR study of random copolyesters, the chemical shifts due to the quaternary aromatic carbon atoms are indications of their environment and are used to characterize and quantify the transesterification product. Typical 13C NMR spectra of PET/PBT blend are shown in Fig. 2. The chemical shifts of the
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
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Fig. 1. SEM images of silica nanoparticles in PET/PBT/SiO2 nanocomposites: (a) PET/PBT/Pure SiO2; (b) PET/PBT/WD10-M-SiO2; (c) PET/PBT/TMCS-M-SiO2.
quaternary aromatic carbon atoms close to the carboxyl group are located between 133 ppm and 134 ppm. Table 1 lists the peak assignments of the quaternary aromatic carbons according to the different environments. The peaks at 133.3 ppm and 133.6 ppm correspond to the quaternary aromatic carbons in the molecular chains of PET and PBT, respectively. Two weak peaks at 133.1 ppm and 133.8 ppm are assigned to the two unequivalent quaternary aromatic carbons in the asymmetric sequence which can be only detected in the transesterification product. Therefore, the transesterification extent and the number–average sequence length can be evaluated through comparison of the peak area of the four different positions.
3.3. Transesterification extent of PET/PBT/SiO2 nanocomposites In order to study the influence of unmodified nanosilica and modified nanosilica on transesterification of PET/PBT blends, three types of nanosilica were used to prepare PET/PBT/SiO2 nanocomposites by melt blending: pure SiO2, WD10-M-SiO2, and TMCSM-SiO2. The category of silane-coupling agent was selected by the following principle: the functional group on the molecular chain of silane-coupling agent must be non-reactive, so that the influence of the functional group on transesterification can be excluded. For this reason, alkylsilane (WD10) and chlorinesilane (TMCS) rather than vinylsilane or aminosilane which has reactive group (vinyl group or amino group) are the appropriate choices.
133.3
Table 1 13 C NMR chemical shifts assigned to the quaternary aromatic carbons of PET/PBT blends 133.6 133.1
133.8
134
133
Position
Chemical shift/ppm
Z1
133.1
X1
133.3
X2
133.6
Z2
133.8
Environment
-O-(CH2)2-O-CO-
-CO-O-(CH2)4-O-
-O-(CH2)2-O-CO-
-CO-O-(CH2)2-O-
-O-(CH2)4-O-CO-
-CO-O-(CH2)4-O-
-O-(CH2)4-O-CO-
-CO-O-(CH2)2-O-
ppm
200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10
ppm Fig. 2. Typical
13
C NMR spectra of PET/PBT blends.
0
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
PET/PBT/TMCS-M-SiO2
PET/PBT/WD10-M-SiO2
PET/PBT/Pure SiO2
PET/PBT
135.5 135.0 134.5 134.0 133.5 133.0 132.5 132.0
Chemical shift (ppm) Fig. 3.
13
C NMR spectra of PET/PBT/SiO2 nanocomposites.
As is shown in Fig. 3, new peaks at 133.1 ppm and 133.8 ppm have been detected in the spectra of all the samples, which indicate that transesterification took place in all of the samples during melt processing. Results of quantitative analysis are summarized in Table 2. Compared with neat PET/PBT blend, the transesterification extent of PET/PBT/SiO2 nanocomposites has been reduced apparently, regardless of the category of nanosilica. Transesterification extent of nanocomposite with pure SiO2 is the lowest, no more than a half of neat PET/PBT blends. Addition of modified SiO2 decreases transesterification extent as well, although the decreasing effect is a little weaker than that of pure SiO2. We stress that this conclusion was not only applicable to the PET/PBT/SiO2 nanocomposites with the 5 wt.% incorporation content but also to the other incorporation contents. In fact, the transesterification extents of PET/PBT/pure SiO2 nanocomposites with other incorporation contents including 1 wt.% and 2.5 wt.% had been also measured by 13C NMR. And we found that transesterification of PET/PBT blends was inhibited by nanosilica within all the incorporation contents. The accuracy of quantitative analysis by 13C NMR is strongly dependent on the precise integration of peak area. However, it is not an easy work to integrate precisely, because the weak peaks due to transesterification product lie adjacent to strong peaks due to PET or PBT chain, and there is no apparent boundary between them. Even though, a great deal of efforts are tried to get the correct
Table 2 Transesterification extent and parameters of chain structure of PET/PBT/SiO2 nanocomposites determined by 13C NMR Sample
Transesterification extent (%)
Sequence length of PET
Sequence length of PBT
Randomness
PET/PBT PET/PBT/pure SiO2 PET/PBT/WD10-M-SiO2 PET/PBT/TMCS-M-SiO2
12.6 5.6 7.0 6.3
11.50 25.42 21.05 23.03
4.33 10.12 7.36 8.93
0.318 0.138 0.183 0.155
results, for example, procedure and condition of measurement should be completely consistent, and selection of initial point and end point of integrating peak should be the same when doing integration, it is still inevitable to prevent inaccuracy. Slight difference was found between the peak area of 133.1 ppm and the peak area of 133.8 ppm which should be theoretically equal. The inaccuracy can be caused not only by integration but also by sample preparation. Nevertheless, some apparent conclusions can be drawn from the results of repeated measurements: nanosilica acts as an inhibitor of transesterification between PET and PBT during melt processing, and the inhibition effect of modified SiO2 is less significant than that of pure SiO2. These conclusions agree with some of the previous research results. Lee et al. [19] studied the influence of BaSO4 filler with average particle diameter less than 0.5 mm on transesterification of PET/PBT blends through qualitative analyzing of crystallization behavior by Differential Scanning Calorimeter, and found that incorporation of BaSO4 can suppress transesterification significantly. Sanchez-Solis et al. [20] researched the transesterification of PET/PEN/montmorrillonite clay nanocomposites, and concluded that sodium montmorrillonite clay reduced the transesterification extent much more than maleic-anhydridemodifed-clay and n-octadecylamine-modified-clay. However, the inhibition mechanism of nanofiller was not illuminated clearly. To clarify the inhibition mechanism of nanoparticle on transesterification, it is necessary to investigate the interaction between nanoparticle and polymer matrix during melt processing. It has been reported [21] that nano-SiO2 tends to react with PET matrix while compounding at 280 C. Therefore, we should make clear above all whether there is chemical reaction between nano-SiO2 and PET/PBT matrices. Such experiment has been done to study the structure change of pure SiO2 during melt processing. First step is to separate the nanoparticle and polymer matrix: PET/PBT/pure SiO2 nanocomposites were dissolved in trifluoroacetic acid solvent with a concentration of 15% w/v, then the solution was centrifuged at 3000 rpm for 10 min, finally the upper solution was removed and white precipitate was in the bottom. There is no doubt that the white precipitate obtained from the above step was the nano-SiO2 after melt processing, and was named extracted SiO2. Second step is to purify the extracted-SiO2: the white precipitate was further washed by trifluoroacetic acid solvent for many repeated cycles of sonication and centrifugation to remove residual polymer matrix in the precipitate completely. The last step is structure characterization of extracted-SiO2. The second step is the critical step, for the reason that the polymer residue in extracted-SiO2 would disturb further characterization of structure change of extracted-SiO2.
100 Pure SiO2 80
Weight (%)
1400
Extracted SiO2 60
40 PET/PBT/Pure SiO2 20 PET/PBT
0 100
200
300
400
500
600
Temperature (oC) Fig. 4. TGA of extracted-SiO2 compared with pure SiO2 and PET/PBT.
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
Compared with pure SiO2, the weight loss of extracted-SiO2 increases evidently from 0.8 wt.% to 22.2 wt.%, indicating that the organic content of pure SiO2 enhanced during melt processing.
Pure SiO2
T/%
2966 730
Extracted SiO2
1411 1728
1268
PET/PBT
4000
3500
1401
3000
2500
2000
1500
1000
500
Wavenumber / cm-1 Fig. 5. FTIR spectra of extracted SiO2 compared with pure SiO2 and PET/PBT.
Therefore, the washing operation should be repeated until no polymer matrix existed in the upper solution which can be characterized by FTIR spectra of the upper solution. 3.4. Characterization of extracted-SiO2 3.4.1. TGA As can be seen in Fig. 4, TGA results show that neat PET/PBT decomposes between 400 C and 500 C, with a weight loss of 91.0 wt.% finally. After the addition of pure SiO2, the weight loss of PET/PBT/pure SiO2 nanocomposite decreases to 87.0 wt.%.
3.4.2. FTIR FTIR is an effective measurement for structure characterization. Fig. 5 shows the FTIR comparison of extracted-SiO2, pure SiO2 and PET/PBT. Compared with pure SiO2 (nanosilica before melt processing), several new peaks appear in the spectra extracted SiO2 (nanosilica after melt processing). The peak at 1728 cm1 is assigned to the stretching vibration of carbonyl group, and the weak peaks at 1411 cm1 and 1268 cm1 are due to in-of-plane bending vibration of C–H bonds on the benzene ring and the stretching vibration of ester group, respectively. The peak at 725 cm1 corresponds to out-of-plane bending vibration of C–H bonds on the benzene ring. The small peak at 2966 cm1 attributes to stretching vibration of CH2 bonds. These peaks are characteristic peaks of polyester, and compared with the corresponding position of PET/PBT blend, all of the peaks shift to the direction of high wavenumber. This could be explained by the reason that after the reaction between hydroxyl end group of polyester and hydroxyl group on the surface of silica, the end group of PET/PBT changes from hydroxyl group to Si–O group, and the electronegativity increase, leading to the blue shift. Furthermore, the peak of hydroxyl group of extracted-SiO2 at 3439 cm1 becomes weaker distinctly. 3.4.3. XPS Fig. 6 shows the XPS spectra of extracted-SiO2 and the peakfitting results of C1s and O1s. A new peak at the binding energy 285.00 eV has been detected in Fig. 6(a), which attributes to C1s. On the basis of the peak-fitting results of C1s, there are three peaks at
a
a
103.80
O1s
Si2p 103.20
Pure SiO2
Extracted SiO2
Si2p C1s
1200
1000
800
600
400
200
0
110
108
Binding Energy (eV)
b C1s(C-C)
106
104
102
100
80
Binding Energy (eV)
C1s
b
O1s
533.55 532.55
C1s(C-O) Pure SiO2
Extracted SiO2
C1s(C=O)
294 292 290 288 286 284 282 280 278
Binding Energy (eV) Fig. 6. XPS of extracted-SiO2 (a) and peak-fitting results of C1s (b).
538
536
534
532
530
528
Binding Energy (eV) Fig. 7. XPS comparison of Si2p (c) and O1s (d) between pure SiO2 and extracted SiO2.
1402
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
polyester (PET PBT)
OH
SiO2
melt processing
polyester (PET PBT)
HO
SiO2
O
Fig. 8. Condensation reaction between PET/PBT and nano-SiO2 during melt processing.
around 285.00 eV: the peak at the binding energy 284.70 eV is assigned to a typical C–C group, and the peak at 286.00 eV corresponds to the C–O group, and the peak at 288.90 eV is due to the C]O group. These results prove that the C–C group, C–O group and C]O group have been grafted to the surface of SiO2 through chemical bonding. XPS of Si2p and O1s comparison between pure SiO2 and extracted SiO2 in Fig. 7 confirms that hydroxyl end group of PET/PBT rather than carboxyl end group reacts with surface hydroxyl group of SiO2. In Fig. 7(c), the Si2p peak of extracted SiO2 at 103.20 eV is lower than that of pure SiO2 (103.80 eV), which results from the formation of Si–O–C bonds. In Fig. 7(d), the O1s peak of extracted SiO2 at 532.55 eV assigned to Si–O bonds is lower than that of pure SiO2 (533.55 eV). As the carbonyl groups (–C]O–) assumed to be bonded with Si–O bonds are electron acceptor groups, the conjoint oxygen atoms on Si–O bonds become electron deficient which results in a shift of the peak of O1s towards high binding energy direction. While the alkyl groups (–C2H4–) are electron donor groups, and the bonding of alkyl groups with Si–O bonds leads to the shift of the peak of O1s towards low binding energy direction. Therefore, the Si–O bonds in extracted SiO2 are linked with alkyl groups (–C2H4–) rather than carbonyl groups (–C]O–), and surface hydroxyl groups of SiO2 react with hydroxyl end groups of PET/PBT rather than carboxyl end groups. All the results about the characterization of extracted-SiO2 could be convincing proofs that the hydroxyl end groups of PET/PBT react with surface hydroxyl groups of nano-SiO2 during melt processing. The reaction is shown in Fig. 8. The increasing intrinsic viscosity of PET/PBT/SiO2 nanocomposite might be an indirect proof of the condensation reaction. Compared with the intrinsic viscosity of neat PET/PBT (0.262 dL/g), the intrinsic viscosity of PET/PBT/pure SiO2 nanocomposite (0.299 dL/g) has been enhanced obviously. To eliminate the influence of pure SiO2 on the intrinsic viscosity, 5 wt.% pure SiO2 was added into the solution of neat PET/PBT, and the intrinsic viscosity of this sample (0.287 dL/g) was still lower than that of PET/PBT/ pure SiO2 nanocomposite. This result might be due to the formation
HO SiO2
HO SiO2 OH
HO SiO2
3.5. Mechanism analysis After the assurance of the condensation reaction between hydroxyl end group of PET/PBT and surface hydroxyl group of nanoSiO2, the transesterification inhibition mechanism of nano-SiO2 could be explained reasonably. During the melt compounding of PET/PBT and pure SiO2 as is shown in Fig. 9, the hydroxyl end groups of PET/PBT react with the surface hydroxyl group of pure SiO2. Considering that large quantity of reactive hydroxyl groups exist on the surface of pure SiO2, the
OH HO SiO2
HO SiO2
of branching or cross-linking structure which accompanies with the condensation reaction. Generally, hydrophobic polymers do not form chemical bonds with hydrophilic inorganic particles directly, but through introducing coupling agent [22,23]. For example, Fadeev and McCarthy [23] obtained the composite film (PET–(SiO2)x–OH) with silica-like reactivity through the following method: PET–CONH(CH2)3Si(OH)3 was first synthesized by attaching 3-aminopropyltriethoxysilane (APTES) to the surface of PET film, and then the composite film (PET–(SiO2)x–OH) was prepared by condensing tetraethyl orthosilicate (TEOS) on the surface of PET–CONH(CH2)3Si(OH)3. However, the occurrence of direct condensation reaction between hydroxyl end group of PET/PBT and surface hydroxyl group of nano-SiO2 is extraordinarily possible for the following reasons: (1) hydroxyl end groups on polyester chains are reactive functional groups which are usually utilized to make chemical modification of PET [22]; (2) surface hydroxyl groups of silica particles are reactive as well which are commonly used to modify nanosilica with silane-coupling agents; (3) melt processing provides favorable condition for condensation reaction with elevated temperature (260 C). Bikiaris et al. [21] firstly proposed the possibility of this condensation reaction and presented some powerful evidences. Nevertheless, the reaction mechanism of this condensation reaction is not elucidated and needs to be further investigated.
OH
OH
HO
O
HO SiO2 OH
OH
SiO2
OH
O HO melt processing
Pure SiO2 HO OH
HO HO
HO
HO HO
O
HO SiO2
SiO2
OH O
HO
HO
OH OH OH OH OH OH HO HO PET/PBT
O OH O
OH OH
HO
SiO2
OH HO
Fig. 9. Melt processing of PET/PBT/pure SiO2 nanocomposite.
SiO2
O
O
O O
SiO2
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
HO SiO2 OH
SiO2
R
R R
SiO2
R
1403
SiO2
HO R
OH
OH
HO OH
HO
SiO2
R
HO SiO2
R
SiO2
R
R
O HO melt processing
Modified SiO2 HO OH
HO HO
HO
OH OH OH OH
OH HO
OH
HO
HO
OH
OH O
OH O
OH
SiO2
R HO
HO
HO
O O
O HO
SiO2
OH
SiO2
HO
SiO2 SiO2
HO R R
HO PET/PBT Fig. 10. Melt processing of PET/PBT/modified SiO2 nanocomposite.
amount of hydroxyl end groups decreases significantly after the condensation reaction. Recently, there are a large number of reports which studied the influence of hydroxyl end group of polyesters on transesterification of polyester blends by both experimental method and simulation method. And the conclusions are similar: the rate of transesterification reaction is highly dependent on the amount of hydroxyl end groups of polyester blends. Kenwright et al. [24,25] found that rate constant for transesterification of PET/PEN was reduced by a factor of almost three when the hydroxyl end groups were completely removed by capping. This conclusion can be explained theoretically as well. Kotliar [26] considered an ester-interchange reaction (transesterification) may be treated as a two-step process, namely, several random chain cleavages followed by the same number of random couplings of chain ends. The chain coupling includes the coupling not only between cleavage positions but also between cleavage position and reactive hydroxyl end group. Therefore, the rate of transesterification strongly depends on the amount of hydroxyl end group of polyester blends. And pure SiO2 inhibits transesterification through decreasing the amount of hydroxyl end group of PET/PBT. As far as PET/PBT/modified SiO2 nanocomposite is concerned, which is shown in Fig. 10, the transesterification extent decreases less than PET/PBT/pure SiO2 nanocomposite. It is not difficult to interpret the result by using the above principle. Apparently,
105 100
Weight (%)
95
Extracted SiO2
modified SiO2 possesses less reactive hydroxyl group on the surface than pure SiO2 for a portion of surface hydroxyl groups were substituted by non-reactive alkyl groups from silane-coupling agent. Consequently, the amount of hydroxyl end groups of PET/ PBT blends reduced relatively less than the pure SiO2 system leading to less inhibition of transesterification. 3.6. Reactivity comparison of the hydroxyl end group To compare the reactivity of hydroxyl end group between PET and PBT, 5 wt.% pure SiO2 was blended with PET and PBT, respectively. Then the pure SiO2 after melt processing was extracted from PET/SiO2 and PBT/SiO2 nanocomposites following the method described previously. TGA result is shown in Fig. 11 and Table 3. It is not difficult to understand that the weight loss of extracted SiO2 represents the organic content which was grafted onto the surface of pure SiO2 during melt processing. For extracted SiO2 from PET/pure SiO2 nanocomposite, the organic content is 21.2 wt.%, a little lower than that of PBT/pure SiO2 nanocomposite (22.5 wt.%). However, the number–average molecular weight of PET (4.15 104) is higher than that of PBT (3.89 104). Therefore, under the same condition, more PET molecular chains were grafted onto the surface of pure SiO2 than PBT molecular chains. Finally, we can conclude that hydroxyl end group of PET is less reactive than that of PBT. The explanation could be that PBT molecular chains are more flexible than PET molecular chains for there are more flexible CH2 groups on the main chains of PBT, so the hydroxyl end groups of PBT is more possible to collide with SiO2 nanoparticles than that of PET. 4. Conclusion
(From PET/SiO2)
90
Extracted SiO2 85
(From PET/PBT/SiO2)
80
Extracted SiO2 (From PBT/SiO2)
75 70 100
200
300
400
500
600
700
Temperature (oC) Fig. 11. Weight loss of extracted SiO2 from (1) PET/SiO2, (2) PBT/SiO2, (3) PET/PBT (70/ 30 wt.%)/SiO2 nanocomposites.
From 13C NMR quantitative analysis of PET/PBT/SiO2 nanocomposites, it is evident that nano-SiO2 acts as an inhibitor of transesterification between PET and PBT during melt processing,
Table 3 Comparison of the amount of polyester which was grafted to the surface of nanoSiO2 Sample
Weight loss (wt.%)
Mn
Extracted SiO2 (from PET/SiO2) Extracted SiO2 (from PBT/SiO2) Extracted SiO2 (from PET/PBT (7:3)/SiO2)
21.2 22.5 22.2
4.15 104 (PET) 3.89 104 (PBT) 4.07 104 (PET/PBT)
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and the inhibition effect of modified SiO2 is less significant than pure SiO2. Condensation reaction between hydroxyl end groups of polyesters and the surface hydroxyl groups of SiO2 nanoparticles was confirmed by TGA, FTIR and XPS data. Therefore, the transesterification inhibition mechanism could be explained that the amount of hydroxyl end group of polyesters decreases due to the condensation reaction, leading to the inhibition of transesterification. The modification of the SiO2 nanoparticles with silane-coupling agent reduces the amount of reactive hydroxyl groups of SiO2 and thus inhibits less the transesterification in the corresponding nanocomposite. And hydroxyl end group of PET is less reactive than that of PBT. As far as we are aware, this work presents a fundamental understanding of the transesterification inhibition mechanism of nano-fillers for the first time, and provides an effective alternative of transesterification inhibitor of PET/PBT blends.
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 50533070).
[18]
[19]
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poly(butylene terephthalate) blends: influence on polyester molecular structure and thermal behaviour. Polymer 1996;37(7):1189–200. Jacques B, Devaux J, Legras R, Nield E. Reactions induced by triphenyl phosphite addition during melt mixing of PET/PBT blends: chromatographic evidence of a molecular weight increase due to the creation of bonds of two different natures. Polymer 1997;38(21):5367–77. Matsuda H, Asakura T, Miki T. Triad sequence analysis of poly(ethylene/butylene terephthalate) copolymer using 1H-NMR. Macromolecules 2002; 35(12):4664–8. Matsuda H, Asakura T, Nagasaka B, Sato K. Relationship between sequence distribution and thermal properties of the transesterification product between poly(ethylene terephthalate) and poly(butylene terephthalate). Macromolecules 2004;37(12):4651–7. Backsona SCE, Richards RW, King SM. Small angle neutron scattering investigation of transesterification in poly(ethylene terephthalate) poly (butylene terephthalate) mixtures. Polymer 1999;40(15):4205–11. Collins S, Peace SK, Richards RW, MacDonald WA, Mills P, King SM. Transesterification in poly(ethylene terephthalate). Molecular weight and end group effects. Macromolecules 2000;33(8):2981–8. Collins S, Peace SK, Richards RW, MacDonald WA, Mills P, King SM. Transesterification in polyethylene terephthalate–polyethylene naphthalene–2,6dicarboxylate mixtures: a comparison of small-angle neutron scattering with NMR. Polymer 2001;42(18):7695–700. Shi Y, Jabarin SA. Transesterification reaction kinetics of poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalate) blends. J Appl Polym Sci 2001; 80(13):2422–36. Newmark RA. Sequence distribution in polyethylene/tetramethylene terephthalate copolyesters by 13C-NMR. J Polym Sci Polym Chem Ed 1980;18(2): 559–63. Lee SS, Kim J, Park M, Lim S, Choe CRM. Transesterification reaction of the BaSO4-filled PBT/poly(ethylene terephthalate) blend. J Polym Sci Polym Phys 2001;39(21):2589–97. Sanchez-Solis A, Garcia-Rejon A, Estrada M, Martinez-Richa A, Sanchez G, Manero O. Properties of poly(ethylene terephthalate)–poly(ethylene naphthalene 2,6-dicarboxylate) blends with montmorillonite clay. Polym Int 2005; 54(12):1669–72. Bikiaris D, Karavelidis V, Karayannidis G. A new approach to prepare poly (ethylene terephthalate)/silica nanocomposites with increased molecular weight and fully adjustable branching or crosslinking by SSP. Macromol Rapid Commun 2006;27(15):1199–205. Chen W, McCarthy TJ. Chemical surface modification of poly(ethylene terephthalate). Macromolecules 1998;31(11):3648–55. Fadeev AY, McCarthy TJ. Surface modification of poly(ethylene terephthalate) to prepare surfaces with silica-like reactivity. Langmuir 1998;14(19):5586–93. Kenwright AM, Peace SK, Richards RW, Bunn A, MacDonald WA. Transesterification in poly(ethylene terephthalate) and poly(ethylene naphthalene 2,6-dicarboxylate) blends; the influence of hydroxyl end groups. Polymer 1999;40(21):5851–6. Kenwright AM, Peace SK, Richards RW, Bunn A, MacDonald WA. End group modification in poly(ethylene terephthalate). Polymer 1999;40(8):2035–40. Kotliar A. Effect of interchange reactions on nonequilibrium distributions of condensation polymers and their associated molecular weight averages. J Polym Sci Polym Chem Ed 1973;13(4):973–91. Liu S, Li W, Liang B, Yu Y. Chain structure and thermal behavior of reactive blends of poly(ethylene terephthalate)/poly (butylene terephthalate) by addition of 2,20 -(1,4-phenyl) bisoxazoline. Eur Polym J 2000;36:2159–65. Garcia M, Eguiazabal JI, Nazabal J. Effects of mixing time on phase structure and mechanical properties of poly(ethylene terephthalate)/polycarbonate blends. J Appl Polym Sci 2001;81:121–7.
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Inhibited transesterification of PET/PBT blends filled with silica nanoparticles during melt processing Feng Wang, Xiangfu Meng, Xinfeng Xu, Bin Wen, Zhongzhong Qian, Xiaowei Gao, Yanfen Ding, Shimin Zhang, Mingshu Yang* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2008 Received in revised form 25 May 2008 Accepted 31 May 2008 Available online 14 June 2008
The blends of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) undergo transesterification reactions between PET and PBT during melt processing. In this research, PET/PBT transesterification has been investigated in the presence of nano-fillers, including pure SiO2 and silanecoupling-agent-modified SiO2. The results show that the incorporation of SiO2 nanoparticles inhibits PET/PBT transesterification, and the influence of pure SiO2 is higher than modified SiO2. The inhibition of SiO2 on transesterification is explained by the fact that the hydroxyl end groups of PET and PBT react with the surface hydroxyl groups of SiO2 before transesterification due to the high activity of surface hydroxyl groups of SiO2, and the reduction of hydroxyl end groups of PET and PBT leads to the inhibition of transesterification between PET and PBT. This has been demonstrated by the experimental data of TGA, FTIR, and XPS. And the reactivity of hydroxyl end groups of PBT is higher than that of PET. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Nanocomposites Poly(butylene terephthalate) Poly(ethylene terephthalate) Silicas Transesterification
1. Introduction Poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) are two members of the most important commercial thermoplastic polyesters. Compared with PET, PBT has the advantages of rapid crystallization rate and good moldability, while heat-deflection temperature and rigidity of PET are superior to those of PBT. Blending offers the potential of property improvements between PET and PBT to obtain a composite material with comprehensive properties. PET/PBT blend is miscible in the amorphous region with the entire blend composition and forms separate crystals rather than cocrystals [1–3]. It is well known that PET/PBT blends undergo ester-interchange (transesterification) reactions above the melting temperature, leading to the formation of block copolymers in the initial stage and random copolymers finally. The practical importance of transesterification consists, first in the possibility to improve the compatibility of polymer blends directly through processing, and second in the chance to prepare novel copolymers with the given structure [4].
* Corresponding author. Tel./fax: þ86 10 82615665. E-mail address: [email protected] (M. Yang). 0141-3910/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2008.05.026
Many works have been focused on the transesterification reaction of PET/PBT blends. Backson et al. [5] and Kim et al. [6] applied 13C NMR to study transesterification extent and sequence structure in PET/PBT blends with the entire blend composition. Jacques et al. [7] carried out melt mixing of PET/PBT in a Brabender Plastograph at 275–300 C. It has been demonstrated by 13C NMR that titanium alkoxide accelerates the reaction, whereas triphenyl phosphite hinders it. And the inhibition mechanism of triphenyl phosphite on transesterification of PET/PBT blends was reported in a series of following papers [8–11]: the observed molecular weight increasing fully confirms the occurrence of high temperature phosphite reactions with hydroxyl chain ends of PET and PBT which lead to the incorporation of significant quantities of phosphorus into the polyester backbone, thus transesterification is inhibited for the hydroxyl end groups of PET/PBT blends were partially capped. Matsuda et al. [12,13] firstly applied 600 MHz 1H NMR to study the sequence distribution of transesterification product between PET and PBT, and the relationship between sequence distribution and thermal properties. The melting temperature of PET/PBT blend decreases as the progress of transesterification due to the decrease of lamellae thickness which was linearly correlated with the inverse of the number–average sequence length. The kinetics of transesterification reaction had also been studied through Monte Carlo simulation and experimental method [14–17]
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as well. The reaction kinetics is consistent with a reversible second order reaction. In conclusion, transesterification of PET/PBT has been sufficiently studied until now. Inorganic nanoparticles are commonly utilized to fill in polymer matrix to prepare PET/PBT nanocomposites which are expected to enhance their stiffness and impact properties simultaneously. Furthermore, inorganic nanoparticles are often organic modified to achieve homogeneous distribution. However, as to PET/PBT nanocomposite, the influence of nanoparticles on transesterification of PET/PBT has been seldom studied. In this work, we study the influence of unmodified and organic modified SiO2 nanoparticles on transesterification of PET/ PBT blends by 13C NMR, and present the inhibition mechanism of nano-SiO2 definitely for the first time. 2. Experimental part 2.1. Materials PET was purchased from Sinopec Yizheng Chemical Fibre Co. Ltd., and PBT was obtained from Nantong Xingchen Synthetic Material Co. Ltd., China. The intrinsic viscosities of PET and PBT in the mixed solvent of phenol/1,1,2,2-tetrachloroethane (TCE) with a weight ratio of 60/40 were 0.875 dL/g and 1.160 dL/g, respectively. The number–average molecular weights of the polymers were 41,500 and 38,900. Aerosil-200 fumed silica (surface area 200 25 m2/g, primary particle size 12 nm, purity 98%) was obtained from Degussa-Hu¨ls, and dried in a vacuum oven overnight at 105 C before use. Two kinds of silane-coupling agents were used: dodecyl-trimethoxysilane (WD10) was a commercial sample from Hubei Wuhan University Silicon New Material Co. Ltd. Trimethylchlorosilane (TMCS) was bought from Beijing Chemical Reagents Company. All other solvents and reagents including 1,1,2,2-tetrachloroethane, phenol, anhydrous toluene, absolute ethanol, trifluoroacetic acid and deuterated trifluoroacetic acid were used as-received.
with the entire blend ratio have been also reported [6]. After comprehensive analysis, the PET/PBT blend with blend composition around 70/30 (wt.%) has excellent properties and its transesterification extent was high enough to be easily detected. Therefore, as in many other papers [7,9–11,27,28], the composition of PET/PBT blend was fixed at 70/30 (wt.%) in this work. The incorporation content of nanosilica (including unmodified SiO2 and modified SiO2) was 5 wt.%. 2.4. Characterizations Scanning electron microscope (SEM) images were taken on a JEOL JSM 6700F field emission scanning electron microscope at 5 kV. The specimens were quenched and fractured in liquid nitrogen, and the fractured surfaces were coated with platinum using GIKO IB-3 ion coater. Infrared spectra were recorded with KBr pellets on a Perkin–Elmer System 2000 infrared spectrum analyzer in the wavenumber range of 4000–370 cm1. X-ray photoelectron spectroscopy (XPS) data was obtained with an ESCALab 220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation. The base pressure is about 3 109 mbar. The binding energies are referenced to the C1s line at 284.6 eV from adventitious carbon. Perkin–Elmer Pyris 1 TGA was used to measure the thermal behavior of the modified silica in nitrogen atmosphere from 50 C to 750 C at a heating rate of 20 C/min. The samples were dissolved in deuterated trifluoroacetic acid at a concentration of ca. 15% w/v. 13C Nuclear Magnetic Resonance (NMR) was performed on a Bruker AVANCE 400 NMR spectrometer operating at 400 MHz. Experimental technique and parameters have been described elsewhere in detail [18]. Intrinsic viscosity of PET/PBT blend was measured with a Ubbelodhe viscometer in 60/40 (wt.%) phenol-1,1,2,2-tetrachloroethane mixed solvent at 110 C. 3. Results and discussion 3.1. Dispersion of silica nanoparticles in PET/PBT/SiO2 nanocomposites
2.2. Preparation of silane-coupling-agent-modified SiO2 Two types of silane-coupling agents were selected to modify the surface of fumed nanosilica: dodecyl-trimethoxysilane (WD10) and trimethylchlorosilane (TMCS). Fumed nanosilica (10.0 g) was dispersed in anhydrous toluene (320 mL). WD10 (26.5 g) or TMCS (16.1 g) was added into the suspension. The mixture was refluxed at 100–110 C under stirring for 24 h. The product was filtered and then washed with anhydrous toluene (300 mL, three times) and absolute ethanol (300 mL, three times). The modified nanosilica was dried in a vacuum oven overnight at 105 C. Nanosilica without modification was labeled as pure SiO2, and nanosilica modified with WD10 and TMCS was labeled as WD10-M-SiO2 and TMCS-MSiO2, respectively. FTIR and TGA results showed that silane-coupling agents were successfully grafted to the surface of SiO2. 2.3. Compounding procedure PET and PBT pellets were dried under vacuum at 110 C for 24 h, and nano-SiO2 was dried under vacuum at 105 C for 24 h before compounding. The PET/PBT/SiO2 nanocomposites were prepared by melt compounding method using a HAAKE Rheomix-600 internal mixer (Mess-Technic GmbH, Germany) at 260 C with a screw speed of 50 rpm for 30 min. The properties of PET/PBT blends with the entire blend composition have been fully researched in many previous studies [1–3], including mechanical property, crystallization property and thermal property. And transesterification extents of PET/PBT blends
The dispersion state of silica nanoparticles in PET/PBT/SiO2 nanocomposites was investigated by SEM observations in Fig. 1. Fig. 1(a), (b) and (c) shows the SEM images of PET/PBT/pure SiO2, PET/PBT/WD10-M-SiO2, and PET/PBT/TMCS-M-SiO2, respectively. It was found that pure SiO2 nanoparticles tended to form large aggregates and agglomerates in PET/PBT matrix. On the contrary, while the surface of SiO2 nanoparticles was modified with silanecoupling agents WD10 and TMCS, most of the surface hydroxyl groups were replaced by organic groups, their hydrophilic surface turned to hydrophobic surface, which was more compatible with polymer matrix, thus WD10-M-SiO2 and TMCS-M-SiO2 dispersed much more uniformly in PET/PBT than pure SiO2. 3.2. Measurement of transesterification extent by
13
C NMR
Plenty of measurements are used to study the ester-interchange reaction and the structure of transesterification product, such as FTIR (Fourier Transform Infrared), SANS (Small Angle Neutron Scattering), NMR (Nuclear Magnetic Resonance), etc. 13C NMR has been proved to be a sensitive and efficient method to analyze the sequence structure of polymer chain qualitatively and quantitatively. In Newmark’s 13C NMR study of random copolyesters, the chemical shifts due to the quaternary aromatic carbon atoms are indications of their environment and are used to characterize and quantify the transesterification product. Typical 13C NMR spectra of PET/PBT blend are shown in Fig. 2. The chemical shifts of the
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
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Fig. 1. SEM images of silica nanoparticles in PET/PBT/SiO2 nanocomposites: (a) PET/PBT/Pure SiO2; (b) PET/PBT/WD10-M-SiO2; (c) PET/PBT/TMCS-M-SiO2.
quaternary aromatic carbon atoms close to the carboxyl group are located between 133 ppm and 134 ppm. Table 1 lists the peak assignments of the quaternary aromatic carbons according to the different environments. The peaks at 133.3 ppm and 133.6 ppm correspond to the quaternary aromatic carbons in the molecular chains of PET and PBT, respectively. Two weak peaks at 133.1 ppm and 133.8 ppm are assigned to the two unequivalent quaternary aromatic carbons in the asymmetric sequence which can be only detected in the transesterification product. Therefore, the transesterification extent and the number–average sequence length can be evaluated through comparison of the peak area of the four different positions.
3.3. Transesterification extent of PET/PBT/SiO2 nanocomposites In order to study the influence of unmodified nanosilica and modified nanosilica on transesterification of PET/PBT blends, three types of nanosilica were used to prepare PET/PBT/SiO2 nanocomposites by melt blending: pure SiO2, WD10-M-SiO2, and TMCSM-SiO2. The category of silane-coupling agent was selected by the following principle: the functional group on the molecular chain of silane-coupling agent must be non-reactive, so that the influence of the functional group on transesterification can be excluded. For this reason, alkylsilane (WD10) and chlorinesilane (TMCS) rather than vinylsilane or aminosilane which has reactive group (vinyl group or amino group) are the appropriate choices.
133.3
Table 1 13 C NMR chemical shifts assigned to the quaternary aromatic carbons of PET/PBT blends 133.6 133.1
133.8
134
133
Position
Chemical shift/ppm
Z1
133.1
X1
133.3
X2
133.6
Z2
133.8
Environment
-O-(CH2)2-O-CO-
-CO-O-(CH2)4-O-
-O-(CH2)2-O-CO-
-CO-O-(CH2)2-O-
-O-(CH2)4-O-CO-
-CO-O-(CH2)4-O-
-O-(CH2)4-O-CO-
-CO-O-(CH2)2-O-
ppm
200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10
ppm Fig. 2. Typical
13
C NMR spectra of PET/PBT blends.
0
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
PET/PBT/TMCS-M-SiO2
PET/PBT/WD10-M-SiO2
PET/PBT/Pure SiO2
PET/PBT
135.5 135.0 134.5 134.0 133.5 133.0 132.5 132.0
Chemical shift (ppm) Fig. 3.
13
C NMR spectra of PET/PBT/SiO2 nanocomposites.
As is shown in Fig. 3, new peaks at 133.1 ppm and 133.8 ppm have been detected in the spectra of all the samples, which indicate that transesterification took place in all of the samples during melt processing. Results of quantitative analysis are summarized in Table 2. Compared with neat PET/PBT blend, the transesterification extent of PET/PBT/SiO2 nanocomposites has been reduced apparently, regardless of the category of nanosilica. Transesterification extent of nanocomposite with pure SiO2 is the lowest, no more than a half of neat PET/PBT blends. Addition of modified SiO2 decreases transesterification extent as well, although the decreasing effect is a little weaker than that of pure SiO2. We stress that this conclusion was not only applicable to the PET/PBT/SiO2 nanocomposites with the 5 wt.% incorporation content but also to the other incorporation contents. In fact, the transesterification extents of PET/PBT/pure SiO2 nanocomposites with other incorporation contents including 1 wt.% and 2.5 wt.% had been also measured by 13C NMR. And we found that transesterification of PET/PBT blends was inhibited by nanosilica within all the incorporation contents. The accuracy of quantitative analysis by 13C NMR is strongly dependent on the precise integration of peak area. However, it is not an easy work to integrate precisely, because the weak peaks due to transesterification product lie adjacent to strong peaks due to PET or PBT chain, and there is no apparent boundary between them. Even though, a great deal of efforts are tried to get the correct
Table 2 Transesterification extent and parameters of chain structure of PET/PBT/SiO2 nanocomposites determined by 13C NMR Sample
Transesterification extent (%)
Sequence length of PET
Sequence length of PBT
Randomness
PET/PBT PET/PBT/pure SiO2 PET/PBT/WD10-M-SiO2 PET/PBT/TMCS-M-SiO2
12.6 5.6 7.0 6.3
11.50 25.42 21.05 23.03
4.33 10.12 7.36 8.93
0.318 0.138 0.183 0.155
results, for example, procedure and condition of measurement should be completely consistent, and selection of initial point and end point of integrating peak should be the same when doing integration, it is still inevitable to prevent inaccuracy. Slight difference was found between the peak area of 133.1 ppm and the peak area of 133.8 ppm which should be theoretically equal. The inaccuracy can be caused not only by integration but also by sample preparation. Nevertheless, some apparent conclusions can be drawn from the results of repeated measurements: nanosilica acts as an inhibitor of transesterification between PET and PBT during melt processing, and the inhibition effect of modified SiO2 is less significant than that of pure SiO2. These conclusions agree with some of the previous research results. Lee et al. [19] studied the influence of BaSO4 filler with average particle diameter less than 0.5 mm on transesterification of PET/PBT blends through qualitative analyzing of crystallization behavior by Differential Scanning Calorimeter, and found that incorporation of BaSO4 can suppress transesterification significantly. Sanchez-Solis et al. [20] researched the transesterification of PET/PEN/montmorrillonite clay nanocomposites, and concluded that sodium montmorrillonite clay reduced the transesterification extent much more than maleic-anhydridemodifed-clay and n-octadecylamine-modified-clay. However, the inhibition mechanism of nanofiller was not illuminated clearly. To clarify the inhibition mechanism of nanoparticle on transesterification, it is necessary to investigate the interaction between nanoparticle and polymer matrix during melt processing. It has been reported [21] that nano-SiO2 tends to react with PET matrix while compounding at 280 C. Therefore, we should make clear above all whether there is chemical reaction between nano-SiO2 and PET/PBT matrices. Such experiment has been done to study the structure change of pure SiO2 during melt processing. First step is to separate the nanoparticle and polymer matrix: PET/PBT/pure SiO2 nanocomposites were dissolved in trifluoroacetic acid solvent with a concentration of 15% w/v, then the solution was centrifuged at 3000 rpm for 10 min, finally the upper solution was removed and white precipitate was in the bottom. There is no doubt that the white precipitate obtained from the above step was the nano-SiO2 after melt processing, and was named extracted SiO2. Second step is to purify the extracted-SiO2: the white precipitate was further washed by trifluoroacetic acid solvent for many repeated cycles of sonication and centrifugation to remove residual polymer matrix in the precipitate completely. The last step is structure characterization of extracted-SiO2. The second step is the critical step, for the reason that the polymer residue in extracted-SiO2 would disturb further characterization of structure change of extracted-SiO2.
100 Pure SiO2 80
Weight (%)
1400
Extracted SiO2 60
40 PET/PBT/Pure SiO2 20 PET/PBT
0 100
200
300
400
500
600
Temperature (oC) Fig. 4. TGA of extracted-SiO2 compared with pure SiO2 and PET/PBT.
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
Compared with pure SiO2, the weight loss of extracted-SiO2 increases evidently from 0.8 wt.% to 22.2 wt.%, indicating that the organic content of pure SiO2 enhanced during melt processing.
Pure SiO2
T/%
2966 730
Extracted SiO2
1411 1728
1268
PET/PBT
4000
3500
1401
3000
2500
2000
1500
1000
500
Wavenumber / cm-1 Fig. 5. FTIR spectra of extracted SiO2 compared with pure SiO2 and PET/PBT.
Therefore, the washing operation should be repeated until no polymer matrix existed in the upper solution which can be characterized by FTIR spectra of the upper solution. 3.4. Characterization of extracted-SiO2 3.4.1. TGA As can be seen in Fig. 4, TGA results show that neat PET/PBT decomposes between 400 C and 500 C, with a weight loss of 91.0 wt.% finally. After the addition of pure SiO2, the weight loss of PET/PBT/pure SiO2 nanocomposite decreases to 87.0 wt.%.
3.4.2. FTIR FTIR is an effective measurement for structure characterization. Fig. 5 shows the FTIR comparison of extracted-SiO2, pure SiO2 and PET/PBT. Compared with pure SiO2 (nanosilica before melt processing), several new peaks appear in the spectra extracted SiO2 (nanosilica after melt processing). The peak at 1728 cm1 is assigned to the stretching vibration of carbonyl group, and the weak peaks at 1411 cm1 and 1268 cm1 are due to in-of-plane bending vibration of C–H bonds on the benzene ring and the stretching vibration of ester group, respectively. The peak at 725 cm1 corresponds to out-of-plane bending vibration of C–H bonds on the benzene ring. The small peak at 2966 cm1 attributes to stretching vibration of CH2 bonds. These peaks are characteristic peaks of polyester, and compared with the corresponding position of PET/PBT blend, all of the peaks shift to the direction of high wavenumber. This could be explained by the reason that after the reaction between hydroxyl end group of polyester and hydroxyl group on the surface of silica, the end group of PET/PBT changes from hydroxyl group to Si–O group, and the electronegativity increase, leading to the blue shift. Furthermore, the peak of hydroxyl group of extracted-SiO2 at 3439 cm1 becomes weaker distinctly. 3.4.3. XPS Fig. 6 shows the XPS spectra of extracted-SiO2 and the peakfitting results of C1s and O1s. A new peak at the binding energy 285.00 eV has been detected in Fig. 6(a), which attributes to C1s. On the basis of the peak-fitting results of C1s, there are three peaks at
a
a
103.80
O1s
Si2p 103.20
Pure SiO2
Extracted SiO2
Si2p C1s
1200
1000
800
600
400
200
0
110
108
Binding Energy (eV)
b C1s(C-C)
106
104
102
100
80
Binding Energy (eV)
C1s
b
O1s
533.55 532.55
C1s(C-O) Pure SiO2
Extracted SiO2
C1s(C=O)
294 292 290 288 286 284 282 280 278
Binding Energy (eV) Fig. 6. XPS of extracted-SiO2 (a) and peak-fitting results of C1s (b).
538
536
534
532
530
528
Binding Energy (eV) Fig. 7. XPS comparison of Si2p (c) and O1s (d) between pure SiO2 and extracted SiO2.
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polyester (PET PBT)
OH
SiO2
melt processing
polyester (PET PBT)
HO
SiO2
O
Fig. 8. Condensation reaction between PET/PBT and nano-SiO2 during melt processing.
around 285.00 eV: the peak at the binding energy 284.70 eV is assigned to a typical C–C group, and the peak at 286.00 eV corresponds to the C–O group, and the peak at 288.90 eV is due to the C]O group. These results prove that the C–C group, C–O group and C]O group have been grafted to the surface of SiO2 through chemical bonding. XPS of Si2p and O1s comparison between pure SiO2 and extracted SiO2 in Fig. 7 confirms that hydroxyl end group of PET/PBT rather than carboxyl end group reacts with surface hydroxyl group of SiO2. In Fig. 7(c), the Si2p peak of extracted SiO2 at 103.20 eV is lower than that of pure SiO2 (103.80 eV), which results from the formation of Si–O–C bonds. In Fig. 7(d), the O1s peak of extracted SiO2 at 532.55 eV assigned to Si–O bonds is lower than that of pure SiO2 (533.55 eV). As the carbonyl groups (–C]O–) assumed to be bonded with Si–O bonds are electron acceptor groups, the conjoint oxygen atoms on Si–O bonds become electron deficient which results in a shift of the peak of O1s towards high binding energy direction. While the alkyl groups (–C2H4–) are electron donor groups, and the bonding of alkyl groups with Si–O bonds leads to the shift of the peak of O1s towards low binding energy direction. Therefore, the Si–O bonds in extracted SiO2 are linked with alkyl groups (–C2H4–) rather than carbonyl groups (–C]O–), and surface hydroxyl groups of SiO2 react with hydroxyl end groups of PET/PBT rather than carboxyl end groups. All the results about the characterization of extracted-SiO2 could be convincing proofs that the hydroxyl end groups of PET/PBT react with surface hydroxyl groups of nano-SiO2 during melt processing. The reaction is shown in Fig. 8. The increasing intrinsic viscosity of PET/PBT/SiO2 nanocomposite might be an indirect proof of the condensation reaction. Compared with the intrinsic viscosity of neat PET/PBT (0.262 dL/g), the intrinsic viscosity of PET/PBT/pure SiO2 nanocomposite (0.299 dL/g) has been enhanced obviously. To eliminate the influence of pure SiO2 on the intrinsic viscosity, 5 wt.% pure SiO2 was added into the solution of neat PET/PBT, and the intrinsic viscosity of this sample (0.287 dL/g) was still lower than that of PET/PBT/ pure SiO2 nanocomposite. This result might be due to the formation
HO SiO2
HO SiO2 OH
HO SiO2
3.5. Mechanism analysis After the assurance of the condensation reaction between hydroxyl end group of PET/PBT and surface hydroxyl group of nanoSiO2, the transesterification inhibition mechanism of nano-SiO2 could be explained reasonably. During the melt compounding of PET/PBT and pure SiO2 as is shown in Fig. 9, the hydroxyl end groups of PET/PBT react with the surface hydroxyl group of pure SiO2. Considering that large quantity of reactive hydroxyl groups exist on the surface of pure SiO2, the
OH HO SiO2
HO SiO2
of branching or cross-linking structure which accompanies with the condensation reaction. Generally, hydrophobic polymers do not form chemical bonds with hydrophilic inorganic particles directly, but through introducing coupling agent [22,23]. For example, Fadeev and McCarthy [23] obtained the composite film (PET–(SiO2)x–OH) with silica-like reactivity through the following method: PET–CONH(CH2)3Si(OH)3 was first synthesized by attaching 3-aminopropyltriethoxysilane (APTES) to the surface of PET film, and then the composite film (PET–(SiO2)x–OH) was prepared by condensing tetraethyl orthosilicate (TEOS) on the surface of PET–CONH(CH2)3Si(OH)3. However, the occurrence of direct condensation reaction between hydroxyl end group of PET/PBT and surface hydroxyl group of nano-SiO2 is extraordinarily possible for the following reasons: (1) hydroxyl end groups on polyester chains are reactive functional groups which are usually utilized to make chemical modification of PET [22]; (2) surface hydroxyl groups of silica particles are reactive as well which are commonly used to modify nanosilica with silane-coupling agents; (3) melt processing provides favorable condition for condensation reaction with elevated temperature (260 C). Bikiaris et al. [21] firstly proposed the possibility of this condensation reaction and presented some powerful evidences. Nevertheless, the reaction mechanism of this condensation reaction is not elucidated and needs to be further investigated.
OH
OH
HO
O
HO SiO2 OH
OH
SiO2
OH
O HO melt processing
Pure SiO2 HO OH
HO HO
HO
HO HO
O
HO SiO2
SiO2
OH O
HO
HO
OH OH OH OH OH OH HO HO PET/PBT
O OH O
OH OH
HO
SiO2
OH HO
Fig. 9. Melt processing of PET/PBT/pure SiO2 nanocomposite.
SiO2
O
O
O O
SiO2
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
HO SiO2 OH
SiO2
R
R R
SiO2
R
1403
SiO2
HO R
OH
OH
HO OH
HO
SiO2
R
HO SiO2
R
SiO2
R
R
O HO melt processing
Modified SiO2 HO OH
HO HO
HO
OH OH OH OH
OH HO
OH
HO
HO
OH
OH O
OH O
OH
SiO2
R HO
HO
HO
O O
O HO
SiO2
OH
SiO2
HO
SiO2 SiO2
HO R R
HO PET/PBT Fig. 10. Melt processing of PET/PBT/modified SiO2 nanocomposite.
amount of hydroxyl end groups decreases significantly after the condensation reaction. Recently, there are a large number of reports which studied the influence of hydroxyl end group of polyesters on transesterification of polyester blends by both experimental method and simulation method. And the conclusions are similar: the rate of transesterification reaction is highly dependent on the amount of hydroxyl end groups of polyester blends. Kenwright et al. [24,25] found that rate constant for transesterification of PET/PEN was reduced by a factor of almost three when the hydroxyl end groups were completely removed by capping. This conclusion can be explained theoretically as well. Kotliar [26] considered an ester-interchange reaction (transesterification) may be treated as a two-step process, namely, several random chain cleavages followed by the same number of random couplings of chain ends. The chain coupling includes the coupling not only between cleavage positions but also between cleavage position and reactive hydroxyl end group. Therefore, the rate of transesterification strongly depends on the amount of hydroxyl end group of polyester blends. And pure SiO2 inhibits transesterification through decreasing the amount of hydroxyl end group of PET/PBT. As far as PET/PBT/modified SiO2 nanocomposite is concerned, which is shown in Fig. 10, the transesterification extent decreases less than PET/PBT/pure SiO2 nanocomposite. It is not difficult to interpret the result by using the above principle. Apparently,
105 100
Weight (%)
95
Extracted SiO2
modified SiO2 possesses less reactive hydroxyl group on the surface than pure SiO2 for a portion of surface hydroxyl groups were substituted by non-reactive alkyl groups from silane-coupling agent. Consequently, the amount of hydroxyl end groups of PET/ PBT blends reduced relatively less than the pure SiO2 system leading to less inhibition of transesterification. 3.6. Reactivity comparison of the hydroxyl end group To compare the reactivity of hydroxyl end group between PET and PBT, 5 wt.% pure SiO2 was blended with PET and PBT, respectively. Then the pure SiO2 after melt processing was extracted from PET/SiO2 and PBT/SiO2 nanocomposites following the method described previously. TGA result is shown in Fig. 11 and Table 3. It is not difficult to understand that the weight loss of extracted SiO2 represents the organic content which was grafted onto the surface of pure SiO2 during melt processing. For extracted SiO2 from PET/pure SiO2 nanocomposite, the organic content is 21.2 wt.%, a little lower than that of PBT/pure SiO2 nanocomposite (22.5 wt.%). However, the number–average molecular weight of PET (4.15 104) is higher than that of PBT (3.89 104). Therefore, under the same condition, more PET molecular chains were grafted onto the surface of pure SiO2 than PBT molecular chains. Finally, we can conclude that hydroxyl end group of PET is less reactive than that of PBT. The explanation could be that PBT molecular chains are more flexible than PET molecular chains for there are more flexible CH2 groups on the main chains of PBT, so the hydroxyl end groups of PBT is more possible to collide with SiO2 nanoparticles than that of PET. 4. Conclusion
(From PET/SiO2)
90
Extracted SiO2 85
(From PET/PBT/SiO2)
80
Extracted SiO2 (From PBT/SiO2)
75 70 100
200
300
400
500
600
700
Temperature (oC) Fig. 11. Weight loss of extracted SiO2 from (1) PET/SiO2, (2) PBT/SiO2, (3) PET/PBT (70/ 30 wt.%)/SiO2 nanocomposites.
From 13C NMR quantitative analysis of PET/PBT/SiO2 nanocomposites, it is evident that nano-SiO2 acts as an inhibitor of transesterification between PET and PBT during melt processing,
Table 3 Comparison of the amount of polyester which was grafted to the surface of nanoSiO2 Sample
Weight loss (wt.%)
Mn
Extracted SiO2 (from PET/SiO2) Extracted SiO2 (from PBT/SiO2) Extracted SiO2 (from PET/PBT (7:3)/SiO2)
21.2 22.5 22.2
4.15 104 (PET) 3.89 104 (PBT) 4.07 104 (PET/PBT)
1404
F. Wang et al. / Polymer Degradation and Stability 93 (2008) 1397–1404
and the inhibition effect of modified SiO2 is less significant than pure SiO2. Condensation reaction between hydroxyl end groups of polyesters and the surface hydroxyl groups of SiO2 nanoparticles was confirmed by TGA, FTIR and XPS data. Therefore, the transesterification inhibition mechanism could be explained that the amount of hydroxyl end group of polyesters decreases due to the condensation reaction, leading to the inhibition of transesterification. The modification of the SiO2 nanoparticles with silane-coupling agent reduces the amount of reactive hydroxyl groups of SiO2 and thus inhibits less the transesterification in the corresponding nanocomposite. And hydroxyl end group of PET is less reactive than that of PBT. As far as we are aware, this work presents a fundamental understanding of the transesterification inhibition mechanism of nano-fillers for the first time, and provides an effective alternative of transesterification inhibitor of PET/PBT blends.
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 50533070).
[18]
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