IN SITU POLYMERIZATION OF TiO2 ...

PHILIPPINE SCIENTIST JOURNAL ISSN: 0079-1466 Issue. 51; 2014; No. 2; Pp. 79-84

IN SITU POLYMERIZATION OF TiO2/PET NANOCOMPOSITES AND THEIR CRYSTALLINITY PROPERTIES

SOMAYEH HOSSEINI RAD Department of Polymer Engineering and Color Technology, Mahshahr Branch, Amirkabir University of Technology, Mahshar, Iran

MEHDI RAFIZADE*

*Corresponding author Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran, P.O. Box 15875-4413, Email: [email protected]

FARAMARZ AFSHAR TAROMI

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran

ABSTRACT In this paper, Nanocomposites of polyethylene terephthalate (PET) including nanoparticles of titanium dioxide, or PET/TiO2 nanocomposites, have been synthesized via in-situ polymerization and crystallization. In addition, in this paper, behavior of these nanocomposites and neat PET was investigated using differential scanning calorimetry (DSC). Fourier Transform Infrared (FTIR) spectra of nanocomposites have been recorded on a Nexus 670 spectrophotometer from Nicolet Co. (Waltham, MA) at room temperature. Intrinsic viscosity of samples was measured using an Ubbelohde type viscometer at 25±0.1 °C. The thermal properties of the neat PET and PET nanocomposites were determined by differential scanning calorimetry (DSC Q1000, TA instruments, New Castle, DE) under N2 atmosphere using 10 °C/min scanning ramp from 30 to 300 °C, following successive cooling and heating cycles. The result has shown that, these nanoparticles have a catalytic effect on the polymerization of PET. In addition, they influence on the crystallinity morphology of PET mostly by acting as a nucleating agent. Molecular weights of nanocomposites are considerably higher in comparison of neat PET. Differential scanning calorimetry (DSC) thermo-grams show that crystallization of nanocomposites are faster than the neat PET and it happens in a higher temperature.

KEYWORDS: POLYETHYLENE TEREPHTHALATE; TITANIUM DIOXIDE; IN-SITU POLYMERIZATION.

Received at 17 December, 2013 Revised at 1 March, 2014 Online from 3 March, 2014

PHILIPPINE SCIENTIST JOURNAL

1- INTRODUCTION Polymer composite materials have attracted much interest due to their potential applications in different fields. These composites show extraordinary advantages in mechanical, thermal, optical, and chemical properties in comparison to neat polymers or conventional composites, including micro-composites [1-7]. These enhancements has attributed to the high accept ration of the nano particles. Solution mixing melt processing, solgel, and in situ polymerization are the various methods to prepare polymer nanocomposites [4, 8-11]. PET (poly ethylene terephthalate) is thermoplastic polyester that used in such diverse fields as the packaging, electrical, automotive, and construction industries because of its low cost and high performance [12]. One of the important properties, which affect the final properties of PET, is the crystallinity and the rate of spherical forming. PET crystallizes relatively slow from the melt state, compared with other semi-crystalline polyesters, such as poly (butylene terephthalate) (PBT), poly (trimethylene terephthalate) (PTT), and thermoplastics. For example, the maximum radial growth rate of PET has been reported to be 10µm/min, whereas this value is about 5000 µm.min-1, for polyethylene. Thus, injection-molding applications of PET are rather limited in engineering applications and require a large cycle time [8]. One way to enhance this property is to prepare PET nanocomposite filled with inorganic materials [13-18]. Several inorganic materials, including silica, clay, zeolites, carbon nanotube and titanium dioxide (TiO2) are used as nanoparticle reinforcing fillers for polymer materials [19-22]. TiO2 is one the additives that usually used in PET as a whitening agent. It has found out that use of this spherical nano particle affected the crystallinity, friction Coefficient, UV blocking of PET/TiO2 nanocomposites [22-23]. Zhu et al synthesized the PET/surface-treated TiO2 nanocomposites by melt-blending process and they investigated the crystallization behavior. Results showed that the presence of TiO2 prolongs the crystallization time and reduces the crystallization rate [22]. Ramesh et al reported that the TiO2 nanoparticles act as a nucleating agent in the PTT matrix thereby reducing t1/2 of the crystallization and leading to easier crystallization of the polymer [24]. As regards few papers dealing with the crystallization behavior of TiO2/PET nanocomposite have been published, so, effects of TiO2 nanoparticles on the crystallization behavior of PET are still not clear. In this study, PET/TiO2 nanocomposites were synthesized via in situ polymerization and crystallization behavior of these nanocomposites and neat PET was investigated using differential scanning calorimetry (DSC).

ISSUE. 51; 2014; NO. 2; PP. 79-84

2- EXPERIMENTAL 2-1- MATERIALS In this work, nano TiO2 Degussa P25 powder (75% anatase and 25% rutile form, surface area 50*15 m2/g) purchased from Nippon Aerosil Co. Ltd (Germany). PET monomers include TerePhthalic Acid (TPA) and Ethylene Glycol (EG) were kindly supplied by Shahid Toundgoyan Petrochemical Complex, Mahshar, Iran. Antimony oxide, as polymerization catalyst and dichloroacetic acid, as solvent of intrinsic viscosity measurement were both obtained from Merck Co., Darmstadt, Germany. All chemical reagents were of analytical grade and were used without further purification.

2-2- SYNTHESIS OF NANOCOMPOSITES PET and its nanocomposites were prepared in a homemade laboratory-scale reactor. This setup consists of a stainless steel reactor, a condenser, a vacuum pump, and an injection system [25]. For all nanocomposites, polymerization and dispersing nanoparticles in matrix happened in a same step. About 245 ppm of antimony trioxide was dissolved in ethylene glycol (EG). Then, EG and terephthalic acid (TPA) (with molar ratio of diol/diacid 1.7) were mixed and poured into the reactor. TiO2 nanoparticles has been dispersed in EG with stirring overnight and added to the reactor with the other monomer and catalyst at the same time. The paste was mixed for 30 min at 200 °C under 4.5 bar. Consequently, temperature was increased to 240 °C. As soon as the temperature reached 240 °C, the esterification step was started and continued until no more water was collected (about 3 hr.). Water vapor was cooled, gathered, and weighed on a regular basis (every 15 min) as an indication of reaction extent. At the end of esterification step, the pressure was reduced to atmospheric pressure. Temperature was then raised to 280 °C, and vacuum was applied to start polycondensation. EG, as a byproduct of the polycondensation reaction was removed by a vacuum pump. This step was continued for 3 h. Finally, the pressure was dropped to atmospheric pressure, and all the materials were evacuated.

2-3- CHARACTERIZATION Fourier transform infrared (FTIR) spectra of nanocomposites have been recorded on a Nexus 670 spectrophotometer from Nicolet Co. (Waltham, MA) at room temperature. Each sample (1 mg) was mixed with 100 mg of KBr to prepare samples. Intrinsic viscosity of samples was measured using an Ubbelohde type viscometer at 25±0.1 °C. The thermal properties of the neat PET and PET nanocomposites were determined by differential scanning calorimetry (DSC Q1000, TA instruments, New Castle, DE) under N2 atmosphere using

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ISSUE. 51; 2014; NO. 2; PP. 79-84

10 °C/min scanning ramp from 30 to 300 °C, following successive cooling and heating cycles.

3- RESULTS AND DISCUSSIONS In-situ polymerized nanocomposites of PET/TiO2 have successfully been synthesized and some of their physical properties have been investigated. FTIR analysis has revealed that the synthesized polymer is PET according to the strong peaks of ester groups.

THE FILLER

Nanoparticles (%wt.) 0 0.5 1 2

IV (dL/g) 0.45±0.01 0.67±0.01 0.58±0.01 0.62±0.1

Mv (g/mol) 11251 20117 16297 17963

3-2- NON-ISOTHERMAL CRYSTALLIZATION KINETICS

3-1- VISCOMETERY MEASUREMENTS One of the concerns in the area of polymerization of PET is about catalysts, which have an important role in the polycondensation step and the final product properties, but these days the highlighted problem is about environmental issues. Most common catalysts like Sb2O3 are not environmental friendly. In the present research, TiO2 nanoparticle as healthy filler is added to PET monomers and the catalytic effect of polymerization of PET has been investigated. It is found that even using a 0.5 %wt. of this additive has increased the polymerization rate of the PET. Table (1) shows the IV of synthesized samples as well as intrinsic molecular weight (Mv). The mentioned Mv values were calculated from the Mark–Houwink equation as follow: [ ]

TABLE 1: FOUR NANOCOMPOSITES WITH DIFFERENT %WT. OF

(1)

All the samples are synthesized in a same conditions and just the amount of filler content is different. As it can be seen, the molecular weight of nanocomposites that has been polymerized in a same period of time is more than neat PET. Thus, it can be concluded that TiO2 nanoparticles are increasing the speed of the polymerization and can act as catalyst. However, increasing the percentage of the filler doesn’t lead to higher IV, which it can be due to aggregation of the particles or saturation of the catalytic sites.

Non-isothermal crystallization conditions are closer to real processing conditions. This is why the study of nonisothermal crystallization has a great practical importance [26]. With the aid of DSC measurement technic non-isothermal crystallinity properties of the nanocomposites have been investigated. The crystallinity of the PET and PET nanocomposites used in this paper was calculated using the following formula: (2) where Φ is the weight fraction of TiO2 nanoparticles, ΔHm represents the enthalpy of melting, ΔHcc is the enthalpy of cold crystallization and ΔH0 refers to the heat of fusion of 100% crystalline PET, which is 140 J/g [27]. In our samples there is no cold crystallization peak. Figure (1) represents the variation of crystallinity percentage regarding to weight percent of loaded nano particles. The percentage of crystallinity decreases in comparison to neat polymer by adding nano particles content. This can be a result of lower molecular weight of the neat polymer. The DSC thermo-grams of non-isothermal crystallization for PET, PET/TiO2 nanocomposites at different filler content (0.5, 1, 2 %wt.) are presented in Figure (2). The details of crystallization data are listed in Table (2).

TABLE 2: NON-ISOTHERMAL CRYSTALLIZATION KINETICS VALUES FOR PET AND ITS NANOCOMPOSITES Tg Melting cycle Onset Inflection End Tm Tm* Area PET 66.9 72.6 73.9 251.0 56 PET+ 0.5% TiO2 66.1 74.6 85.6 248.7 240.5 47.6 PET+1% TiO2 71.8 76.1 85.1 247.1 235.4 38.7 PET+2% TiO2 70.3 77.6 84.4 247.1 238 49.7 Time [min], Temperature [oC], Area [J/g], X [percentage] * Tm is the secondary peak in the melting double peak. ** Temperature at 1% relative crystallinity. *** Temperature at 99% relative crystallinity. Sample

It can be seen from Table (2) that, the crystallization peak temperature (Tc) and the initial crystallization temperature (Ti) for the PET/ TiO2 nanocomposites are

T0.01** 168.4 194.3 178.5 186.5

Tc 183.7 206.4 191.7 199.4

Cooling cycle T0.99*** Area Xc 196.1 46.8 40 210.7 49.1 33.8 201.2 40.1 27.4 210.7 43.2 34.8

183.5 205,5 190.7 199.9

t1/2 1.85 1.5 1.61 1.77

higher than the PET sample. This indicates that the crystallization process has been facilitated in the presence of the nano particles. This observation can be

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ISSUE. 51; 2014; NO. 2; PP. 79-84 FIGURE 2: DSC THERMO-GRAMS OF NON-ISOTHERMAL CRYSTALLIZATION (COOLING CYCLE) FOR NEAT PET AND ITS NANOCOMPOSITES 100 Relative crystallinity %

originated by different factors of molecular weight of the chains and the role of nucleating agent of the nanoparticles. As it is mentioned above, incorporating nano particles increases the molecular weight of polymeric matrix. Crystallization of the low molecular weight polymer chain is easier due to easier movements. So increasing molecular weight will retard crystallization process. In the other hand, nanoparticles can act as nucleating agent and accelerate the nucleation step of the crystallization. This observation also can suggest that nanoparticles have a good dispersion state.

80 60 40 20 0

50 40

180

190

200

210

PET+TiO2 0.5% PET+TiO2 2%

220

Temp (°C)

FIGURE 3: RELATIVE CRYSTALLINITY FUNCTION OF TEMPERATURE FOR PET AND ITS NANOCOMPOSITES WITH DIFFERENT TiO2 %WT.

30 20 10

100

0 0

0.5

1

1.5

2

2.5 TiO2 %wt.

FIGURE 1: RELATIVE CRYSTALLINITY VS. TiO2 %WT., FOR PET/TiO2 NANOCOMPOSITES Figure (2) shows the shift of the crystallization temperature and heat flow changes, which is an effect of addition of TiO2 nanoparticles in PET matrix. Relative crystallinity function of temperature and time of PET for all samples presented in Figure (3) and (4), respectively. Clearly, Figure (3) shows that all crystallization process of nanocomposites happened in higher temperatures and the higher rate of this process can be seen in Figure (4). The t1/2 (half time of crystallization) is also listed in Table (2). It is evident that t1/2 decreases with adding TiO2 content, which indicates faster crystallization rate in the presence of the filler. This is probably due to fact that the TiO2 nanoparticles act as nucleating agents. The relatively poor interaction between the TiO2 nanoparticles and the polymer chains in the rubbery temperature could be also responsible to suppress the crystallization process. [22]. 20 16 Heat Flow(mW)

170

PET PET+TiO2 1%

12 8 4 0 50

100 PET PET+TiO2 1%

150

200 250 300 PET+TiO2 0.5% Temp (°C) PET+TiO2 2%

Relative crystallinity %

Relative crystallinity (%)

160

80 60 40 20 0 0 PET

1

2

3 PET+TiO2 0.5%

PET+TiO2 1%

PET+TiO2 2%

4 Time (min)

FIGURE 4: RELATIVE CRYSTALLINITY FUNCTION OF TIME FOR PET AND ITS NANOCOMPOSITES WITH DIFFERENT TiO2 %WT. Second heating cycles of DSC tests for different nanocomposites presented in Figure (5). Difference melting peak is observed for nanocomposites and neat PET. As it is explained above, the mechanism of crystallinity has changed in the presence of the nanoparticles, which is leading to different crystalline morphology. It can be seen from a double peak on the exothermic curve in the nanocomposite samples, which may be due to the second reorientation of the chain in the crystalline and it is originated from different crystallize size or type in the cooling cycle. Tg values of all samples also listed in Table (2). Increment of Tg by increasing nano fillers is obvious as it is expected from adding mineral filler to a polymeric matrix. Obviously, double melting endotherms are observed in thermograms. Theses peaks can be related to the melting of recrystallized crystallites of different stabilities that were formed during a heating scan Based on the strength of these secondary peaks, it can be concluded that some portion of the formed crystalline during previous cooling cycle are not stable, so during subsequent heating, they

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Heat Flow (mW)

found this opportunity to recrystallize again and melt upon further heating [28]. 0 -2

ISSUE. 51; 2014; NO. 2; PP. 79-84 [4] Chen Z, Luo P and Fu Q. Preparation and properties of organo-modifer free PET/MMT nanocomposites via monomer intercalation and in situ polymerization. Polymers for Advanced Technologie. 2009, 20:916–925. [5] Changa JH, Ana YU, Kima SJ, et al.; Poly(butylene terephthalate)/organoclay nanocomposites prepared by in situ interlayer polymerization and its fiber (II). Polymer, 2003, 44: 5655–5661.

-4 -6 -8

-10 -12 30 PET

80

130

180 230 Temp.(°C) PET+ TiO2 0.5%

PET+TiO2 1%

PET+TiO2 2%

FIGURE 5: DSC THERMO-GRAMS OF NON-ISOTHERMAL CRYSTALLIZATION (SECOND HEATING CYCLE) FOR NEAT PET AND ITS NANOCOMPOSITES

4- CONCLUSION Successfully, PET/TiO2 nanocomposites have been synthesized via in situ polymerization. Presence of nanoparticles in the reactor from the beginning of the polymerization leads to a higher molecular weight in a same period of time, which proves that these nanoparticles can act as a catalyst. By adding just 0.5 %wt. of this filler, the IV of final polymer is increased about 0.22 dL/g. The non-isothermal melt-crystallization kinetics and the subsequent melting behavior of PET nanocomposites were investigated by DSC measurement. Results show that nanocomposites have a higher crystallization temperature and a smaller crystallization period that means nanoparticles facilitates the crystallization process. This can be due to nucleating effect of nanoparticles.

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