Adding carbon nanoparticles with different geometries to poly(ethylene terephthalate)

15 June 2015
Sandra Paszkiewicz
The addition of expanded graphite and single-walled carbon nanotubes to poly(ethylene terephthalate) leads to a range of enhanced mechanical and barrier properties in the resultant nanocomposite.

Due to their exceptional mechanical and electrical properties, carbon nanomaterials (e.g., nanotubes and graphite nanoplatelets) inspire unflagging interest for application as fillers in polymer composites.1–7 The way in which the combination of 1D and 2D carbon nanofillers affects the performance of the composites therefore represents a significant area of interest. Although these materials share many similarities with regard to their physical properties, they differ from one another in shape due to the specific arrangements of carbon atoms in the structure of the nanofillers. To discover the effects of adding carbon nanoparticles with different geometries (i.e., shapes and aspect ratios) on the morphology, water absorption, oxygen- and water-vapor permeability, and the mechanical properties of the resultant composites, we investigated their incorporation into poly(ethylene terephthalate) (PET). We prepared these polymer composites by in situ polymerization, with the total content of nanofiller not exceeding 0.1wt%.

We prepared the hybrid nanocomposites by in situ polymerization in a steel polycondensation reactor using two steps: transesterification and polycondensation. The neat polymer and nanocomposites were then extruded from the reactor using compressed pressure. After extrusion, the nanocomposites were granulated and injection-molded into dumbbell-shaped samples.8–13 The carbon nanotubes (CNTs) and expanded graphite (EG) were dispersed for 30min by high-speed stirring and ultrasonication (under a frequency of 29kHz and a power of 200W). To provide better exfoliation, the EG underwent additional dispersion in a low-power sonic bath for 8h.13 To investigate the barrier properties, we pressed the samples into thin amorphous films.9 Using this method, we obtained a series of PET-based nanocomposites with two different nanofillers of varying shape. The technique enables us to control both the polymer architecture and final structure of the composites.

We examined the morphology of the nanofillers with scanning electron microscopy (SEM) and Raman spectroscopy.13 SEM and transmission electron microscopy (TEM) analysis of the nanocomposites, consisting of PET with 0.1wt% EG and 0.05wt% single-walled CNTs (SWCNTs)—see Figure 1(a) and (b)—shows that EG is highly exfoliated into few- and multilayer graphene platelets throughout the PET matrix. Analysis of the SWCNT distribution within the hybrid nanocomposites shows that the CNTs, which are slightly entangled, are located near the graphene nanoplatelets. This agglomeration of SWCNTs together with the good exfoliation of graphene nanoplatelets confirms that the mechanical and barrier properties are more strongly enhanced by the addition of graphene sheets.


Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of poly(ethylene terephthalate) (PET) with 0.1wt% expanded graphite (EG) and 0.05wt% single-walled carbon nanotube (SWCNT) nanocomposites. (a) SEM with visible agglomerate of CNTs and (b) TEM at 250,000×magnification.13

The results obtained for the highest concentration of carbon nanofillers suggest that the incorporation of this hybrid system causes no appreciable change in the molecular weight. Additionally, melting temperature and the degree of crystallinity remain comparable to nanocomposites with only SWCNTs or EG.13 The data that we obtained from differential scanning calorimetry measurements suggests that, when used alone, SWCNTs and EG exert a slight nucleating effect on PET in the nanocomposites, whereas we observed no significant effect resulting from the mixing of both fillers. The presence of the SWCNTs and EG may in fact impede the diffusion and rearrangement of the long polymer chains due to the interaction between both components and the PET matrix, thereby disturbing the overall crystallization process. The greatest oxygen- and water-vapor impermeability is exhibited in nanocomposites containing EG. This is a result of the relatively large surface area of the graphene planes, which inhibit the transport of water/gas molecules into and through the material. Oxygen permittivity and hot water absorption as a function of total content of nanofiller can be seen in Figure 2.


Oxygen (O2) permittivity and hot water absorption as a function of total nanofiller content.

The increase in the barrier effect and mechanical properties in the PET/EG+SWCNT hybrid nanocomposites is moderate compared to the nanocomposites in which these fillers were used individually. This comparably smaller increase may occur due to inhomogeneities in the distribution of nanoparticles in the polymer matrix. Additionally, with the increase in nanofiller concentration, there is also an increase in the number of nanoparticle agglomerates in the material, which leads to the formation of more channels for the molecules to pass through. These conclusions are confirmed by optical microscopy images of the thin-film samples—see Figure 3—in which the nanocomposite consisting of PET with 0.05wt% EG and 0.05wt% SWCNTs shows the highest number of aggregated and agglomerated structures.


Optical microscopy images of the thin films used for permeability measurements of PET with (a) 0.05wt% SWCNTs, (b) 0.05wt% EG, and (c) 0.05wt% SWCNTs and 0.05wt% EG.13

In summary, we obtained a series of PET-based nanocomposites using two different nanofillers with varying morphologies (SWCNTs and EG) by in situ polymerization. The resulting nanocomposites exhibit enhancements in terms of both mechanical properties and oxygen- and water-vapor impermeability. A greater enhancement was observed in composites that incorporate graphene sheets, due to the better dispersion achieved. In the case of the PET/EG+SWCNT hybrid composites, we observed an increase in the barrier effect and the mechanical properties with increasing nanofiller concentration.

In the next stage of our research, we intend to discover optimum concentrations of our hybrid-nanoparticle system for incorporation into other thermoplastic matrices (i.e., thermoplastic polyester and thermoplastic elastomer). Our main goal is to obtain a synergic hybrid effect between CNTs and graphene nanoplatelets for the enhancement of selected properties in polymer nanocomposites.


Author

Sandra Paszkiewicz
West Pomeranian University of Technology

Sandra Paszkiewicz graduated from the Faculty of Chemical Technology in 2009 and from the Faculty of Mechanical Engineering and Mechatronics in 2010. Since 2014, she has been employed as an assistant in the Institute of Materials Science and Engineering.


References

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  11. S. Paszkiewicz, A. Szymczyk, Z. Špitalský, J. Mosnácek, E. Janus and Z. Roslaniec, Effect of addition of expanded graphite (EG) on the synthesis and characteristics of poly(ethylene terephthalate) modified with cyclohexanedimethanol (PETG), Polimery 58 (11-12), pp. 893-899, 2013.

  12. S. Paszkiewicz, Z. Roslaniec, A. Szymczyk, Z. Špitalský and J. Mosnácek, Morphology and thermal properties of expanded graphite (EG)/poly(ethylene terephthalate) (PET) nanocomposites, Chemik 66 (1), pp. 26-30, 2012.

  13. S. Paszkiewicz, M. Kwiatkowska, Z. Roslaniec, A. Szymczyk, M. Jotko and S. Lisiecki, The influence of different shaped nanofillers (1D, 2D) on barrier and mechanical properties of polymer hybrid nanocomposites based on PET prepared by in situ polymerization, Polym. Compos., 2015. First published online: 27 January 2015

DOI:  10.2417/spepro.005867



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