Characterization of PET nanocomposites with different nanofillers Lyudmil V. Todorov a, Carla I. Martins b, Júlio C. Viana c

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1 Solid State Phenomena Vol. 151 (2009) pp Online available since 2009/Apr/16 at (2009) Trans Tech Publications, Switzerland doi: / Characterization of PET nanocomposites with different nanofillers Lyudmil V. Todorov a, Carla I. Martins b, Júlio C. Viana c IPC - Institute for Polymers and Composites, Department of Polymer Engineering, University of Minho, Guimarães, Portugal a lvt@dep.uminho.pt, b cmartins@dep.uminho.pt, c jcv@dep.uminho.pt Keywords: poly(ethylene terephthalate), nanocomposites, mechanical properties Abstract. This study investigates the influence of various nanofillers of different shapes and sizes on the properties of PET nanocomposites. PET was reinforced with 0.3 wt.% of different nanoreinforcements, namely: (i) 1D platelet-like shape of organo-modified layered silicates (montmorillonite) (platelet size approx. 1 x 200 nm) with average agglomerate size of: (a) 30 µm and (b) 8 µm; (ii) 3D spherical shape particles of titanium oxide with an average size of 21 nm and (iii) 3D spherical shape silica with an average particle size of 12 nm. PET nanocomposites were prepared by melt blending in an asymmetric batch minimixer followed by compression moulding process. The effect of nanofillers upon thermal, mechanical and structural properties in comparison to the neat PET are discussed. Introduction Polymer nanocomposite properties are affected by the nature and concentration of the used nanoreinforcements, as well as their characteristics such as shape and size, specific surface area (surface-to-volume ratio) and surface treatment functionality [1]. In terms of shape and size, the nanoparticles used for polymer reinforcement can be classified as follows: (i) 1D, with one dimension in the nanometer range, such as platelet-like shaped montmorillonite (nanoclays), MMT (one to a few nanometers thick and hundreds to thousands nanometers long); (ii) 2D, with two dimensions on the nanometer scale, (i.e. elongated structures such as carbon nanotubes or cellulose whiskers); (iii) 3D, with three dimensions in the nanometer scale, (i.e. spherical silica, SiO 2, and titanium oxide, TiO 2 ) [1]. The macroscopic effects of the nanoparticles incorporated in the polymer matrix generally consist in the enhancement of mechanical performance, barrier properties, thermal stability, fire retardancy, etc. Normally, a very low level of nanoparticles incorporation, typically less than 5%, is needed to observe such properties improvements [1]. The production of polymer nanocomposites, and primarily the incorporation of nanofillers in the polymer matrix can be made either by in situ polymerization, solvent-assisted or melt blending techniques. Melt blending techniques are the most attractive pathways to produce commercial nanocomposites [2]. In this work, melt blending via an asymmetric batch minimixer [3] was used for preparation of PET nanocomposites. Effects of nanoreinforcements upon the polymer bulk degradation, particles dispersion, thermal and mechanical properties of the nanocomposites in comparison to the neat PET were evaluated. Materials and preparation of nanocomposites Materials: In this work the following materials were used: - Poly(ethylene terephthalate), PET, with intrinsic viscosity of 0.74 ± 0.02 dl.g -1 ; density of 1.40 g.cm -3 and average molar mass of M n g.mol nanoreinforcements: (i) two types of platelet-like shape 1D nanoparticles of organo-modified layered silicates, supplied by Süd-Chemie AG, Germany, namely: Nanofil 32 (NF32) with average powder agglomerates (stacks and agglomerates of smaller platelets) of 30 µm, and Nanofil 2 (NF2) All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-04/06/14,15:26:33)

2 114 Nanocomposite Materials with an average of 8 µm. Both nanoclays with a inter-gallery distance of 1.8 nm. The platelets are of a thickness of approx. 1 nm and diameter of approx. 200 nm. These nanoclays were functionalized with long chain hydrocarbon/benzyl groups; (ii) 3D nanosized titanium oxide, TiO 2, hydrophilic particles (AEROXIDE TiO2 P25), average powder agglomerates of 100 nm and an average particle size of 21 nm (specific surface area 50 ± 15 m 2.g -1 ); (iii) 3D nanosized silica, hydrophilic SiO 2, (AEROSIL 200) with spherical shape, average powder agglomerates of 100 nm and an average particle size of 12 nm (specific surface area 200 ± 25 m 2.g -1 ); both from Degussa AG, Germany. Preparation of PET nanocomposites: The PET nanocomposites were prepared via melt blending in an Asymmetric Batch MiniMixer, ABM, similar to those presented in reference [3]. Therefore, PET dried pellets (drying conditions: 170 C for 4 7 h) and nanoparticles were mechanically blended in a tumbler mixer for 15 min. After the blends were melted (for 10 min) and mixed at 280 o C for 5 min into the ABM. Further, the neat and blended PET nanocomposites melts were casted on a preheated stainless steel plate (280 o C) covered with a Teflon foil, and used to prepare plaques with dimension of 200x200x0.3 mm by compression moulding. The plaques were then rapidly quenched in cold water at 5 ± 0.5 o C. Characterization Intrinsic viscosity: The intrinsic viscosity, η, measurements were performed according to the ASTM D standard. A glass capillary viscometer (Ubbelohde type) was used and the solutions were prepared using a solvent mixture composed of 60/40 phenol/1,1,2,2- tetrachloroethane. All PET nanocomposites solutions were prepared with concentration correction of 0.3 wt.%. After complete dissolution, the solutions were centrifuged for 30 min at 3500 rpm and filtered in order to remove the nanoparticles sediments. Transmission electron microscope (TEM): Ultramicrotome cuts made through the thickness of the compression moulded samples, with approx. thickness of 60 nm, were done. TEM observation was carried out under an operation voltage of 100 kv on a JEOL JEM Dynamic mechanical analysis (DMA): The DMA analysis was carried out using a Triton Tritec 2000 from Triton Technology Ltd. All samples were measured in a tension mode over the -140 to 240 o C temperature range, at a heating rate of 4 o C.min -1 and frequency of 1 Hz. Samples were cut into rectangular shapes 3x20x0.3 mm from the compression moulded plaques. An average of three samples was tested (T g was calculated by the peak position of tanδ curve). Differential Scanning Calorimetry (DSC): Thermal properties of neat and PET nanocomposites were measured using a Perkin Elmer DIAMOND PYRIS DSC instrument. The samples were scanned in the temperature range from 30 to 270 o C, at a heating rate of 20 o C.min -1, in a dry nitrogen atmosphere. The reported transition temperatures (T cc and T m ) also are referred to the respective peak maximum positions and the degree of crystallinity (χ c ) was calculated based on a two-phase peak area method, considering the heat of fusion of 100% PET crystalline sample to be 120 J.g -1. The reported results are the average of three samples. Mechanical characterization: Tensile mechanical behaviour of the studied materials was assessed using a universal testing machine - Zwick/Roell Z005. The testes were carried at room temperature, at the rate of 2 mm.min -1. Tensile specimens with a curved axisymmetric shape (30 mm length and cross-section 13.9x0.3 mm) were used. Results and Discussion Intrinsic viscosity: The values of the intrinsic viscosity, η, of the PET nanocomposites are listed in Table 1, in terms of the percentage of η reduction of the processed samples with respect to the pristine PET pellets. A reduction of 28.5% for processed PET in respect to the PET granules is observed, meaning a certain degree of PET degradation occurs during the composite preparation. This result may be attributed to the applied high temperature, shear rate and residence time. The

3 Solid State Phenomena Vol incorporation of TiO 2 leads to a pronounced η reduction as compared to processed PET (around 41.3%). Nevertheless, there is no relation between the shape of the filler and the reduction in viscosity. When comparing the size of used reinforcements, one can observe that the nanoparticles of both 1D and 3D shapes having bigger sizes induce a higher reduction of viscosity, respectively NF32 and TiO 2. Table 1 Percent of reduction of intrinsic viscosity. Sample PET F32 F2 TiO 2 SiO 2 Var. % η * red [%] *(η red percentage of reduction of intrinsic viscosity, η red =((η pristine - η x )/η pristine )x100; where η x is the intrinsic viscosity of each of the processed specimens and η pristine of pristine PET granules; Var. - percentage of variation [(max - min)/min]) Transmission electron microscopy (TEM): TEM micrographs of the different PET nanocomposites are depicted in Fig. 1. TEM revealed the existence of small sized agglomerates for all types of used nanofillers. For the platelet like fillers, shown in Fig. 1 a) and b), an inter-plate distance of 2.7 nm and 1.9 nm, respectively for NF32 and NF2 nanoparticles was measured. For the smaller initial agglomerate size particles (NF2) the inter-plate distance is about the same of the original particles, while for bigger (NF32) the distance is increased, showing some particle intercalation. a) b) c) d) Fig. 1. TEM micrographs of nanocomposites: a) PET- F32, b) PET- F2, c) PET-TiO 2 and d) PET-SiO 2 Dynamic mechanical analysis: In Fig. 2, are shown the storage modulus, E, and the loss factor, tanδ, of the different PET nanocomposites. As observed, the storage modulus varies between 1.7 to 1.9 GPa (at 23 o C) depending on the nanofiller type (Table 2). A slight higher storage modulus in case of PET-SiO 2 nanocomposite relatively to the neat PET is obtained. The incorporation of 0.3%wt. of nanofillers does not influence the results of E. In terms of tan δ, the peak broadening and their shift towards higher temperature, for PET-SiO 2 and PET-NF2 composites, as compared to processed PET, can be observed. These results show an increase of T g for both types of materials. In the first case, the increase is about 1.6ºC, while in the second case is about 1.4ºC. No significant change is observing for the tan δ peak position for the other types of fillers, although the appearance of a broader peak is evidenced. Among the different forms (platelet and spherical types), the small sized particles show the higher influence on T g and on the broadening of this transition. This behaviour was also observed by other authors [4].

4 116 Nanocomposite Materials E [GPa] PETSiO 2 F32 TiO 2 F2 F2 SiO 2 PET Temperature [ o C] TiO 2 F32 Fig. 2. Effect of the nanofillers on E and tanδ peak from DMA. tan δ Table 2 - Thermo-mechanical parameters Sample E [GPa] T g [ o C] T g [ o C] PET 1.80± ± ±0.8 F ± ± ±0.2 F2 1.72± ± ±0.7 TiO ± ± ±0.2 SiO ± ± ±0.3 Var. % (E - storage modulus at 23 o C; T g peak position of tanδ curve; T g - breadth by means half width of full maximum of loss factor, tanδ, peak fitted by Gaussian function; Var. - percentage of variation [(max - min)/min]) Thermal characterization: The thermal parameters assessed by DSC are listed in Table 3. The presence of nanofillers is mainly affecting the cold crystallization temperature, T cc, and the final degree of crystallization of PET nanocomposites. All nanoparticles lead to decreasing of T cc in comparison to neat PET, which means that the used nanofillers are acting as nucleating agents [4]. Among all, the PET-SiO 2 is the composite that presents the biggest reduction. The registered differences might be attributed to the presence of small agglomerates sizes with a better dispersion in the polymer matrix, as suggested elsewhere [4]. Also, due to their behaviour (nucleating agents), all selected nanofillers lead to the increase of PET degree of crystallinity, χ c. The PET-NF32 nanocomposite shows the smallest increment upon χ c as compared with neat PET probably due to the higher level of intercalation observed for this system. According to other authors, the intercalation may reduce the ability of polymer to crystallize [4]. On the other hand, the 3D nanofillers, namely SiO 2 promotes a higher crystallization of the PET matrix. Table 3 Thermal parameters Sample PET F32 F2 TiO 2 SiO 2 Var. % T cc [ o C] 141.7± ± ± ± ± T m [ o C] 249.8± ± ± ± ± χ c [ o C] 7.6± ± ± ± ± (T cc - cold crystallization peak temperature, T m - melting peak temperature, χ c - degree of crystallinity, Var. - percentage of variation [(max - min)/min]) Mechanical behaviour. The stress-strain curves of PET nanocomposites are shown in Fig. 3, and the summary of some mechanical properties is listed in Table 4. The different reinforcing nature of the nanoparticles is evidenced mainly in terms of deformation capabilities (i.e., the strain at break, ε b ). In case of both MMT (NF32 and NF2) nanocomposites, ε b diminishes drastically with respect to the PET value. NF32 fillers contribute for nearly 50% to ε b reduction comparing with neat PET deformability. The smallest NF2 nanofillers contribute to a value of 17.6% for ε b reduction, while in case of TiO 2, the deformation capability is improved by 5.2 %, in agreement with other works [4]. By adding SiO 2 nanoparticles, a significant enhancement of the strain at break, with an increment value of 37.8%, is registered. The strain at break of PET nanocomposites seems to be very sensitive to the shape/size of the incorporated nanofillers.

5 Solid State Phenomena Vol Stress [MPa] F32 15 F2 TiO SiO PET Strain [mm.mm -1 ] Fig. 3. Experimental stress-strain curves Sample Table 4 Mechanical test values E [MPa] σ y [MPa] ε b [%] PET 675.7± ± ±36.9 F ± ± ±22.2 F ± ± ±32.4 TIO ± ± ±37.9 SIO ± ± ±41.3 Var.[%] (E initial modulus; σ y - yield stress; ε b strain at break; Var - percentage of variation [(max - min)/min]) Conclusions Incorporation of nanoparticles within polymer matrix affects polymer degradation during the processing. The interaction at the nanofillers-polymer interface lead to the increase of the glass transition temperature and broadening of this transition. All characterized nanoparticles have nucleating effect regardless their shape/size. Deformability of PET nanocomposites improves with the incorporation of spherical shape and smaller sizes nanofillers. Acknowledgements The authors would like to thank Süd-Chemie AG and Degussa AG, Germany for donation of nanoparticles. This work was supported by FCT Portuguese Foundation for Science and Technology through projects POCTI/CTM/46940/2002 (MICROTEST) and POCI/V.5/A0094/2005 (ADFUN_PACK) References [1] Ray SS and Okamoto M. Progress in Polymer Science (Oxford) 2003;28(11): [2] Koo JH. Polymer Nanocomposites Processing, Characterization, and Applications. New York: McGraw-Hill, [3] Breuer O, Sundararaj U, and Toogood RW. Polymer Engineering and Science 2004;44(5): [4] Jordan J, Jacob KI, Tannenbaum R, Sharaf MA, and Jasiuk I. Materials Science and Engineering A 2005;393(1-2):1-11.

6 Nanocomposite Materials / Characterization of PET Nanocomposites with Different Nanofillers /