Morphology of polymer matrix vs. hybrid 1D/2D type of carbon nanoparticles

PTT-PTMO) and their mutual interactions were studied using imaging techniques, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Nanofiller dispersion is a well-known challenge since CNT and graphene nanoplatelets have an inherent tendency to agglomerate and good dispersion is an important factor for mechanical reinforcement and creation electrical percolation paths. The main reasons behind agglomeration is the van der Waals attraction and the large surface areas of the nanofillers [386], for graphene nanosheets additional π-π bonding also accounts for the stacking of the individual sheets. All nanofillers (1D- nanotubes and 2D- graphene derivatives) were mixed using procedures as written before for single nanofiller and followed by mixing together for another 30 min (15 min with speed stirrer and 15 min with ultrasounds) to avoid agglomeration before synthesis process.

In Fig. 65 a and c a good distribution of the graphene sheets with agglomerates of carbon nanotubes were observed. Both SEM and TEM analysis confirmed that expanded graphite was well dispersed, but nanotubes due to lots of impurities were entangled with one another and create aggregates. The morphology of fractured surfaces depicts the intimate contact and high embedding with polymer matrices, indicating good interfacial bonding between nanoplatelets and PET matrix. Interestingly, some of the nanoplatelets dispersed almost as a monolayer as indicated in figure 65 c (almost completely transparent nanosheets).

In general, nanoplatelets can be easily attracted to each other due to their very high specific surface area and high surface energy. From the cryo-fractured surface (Fig. 65 a) and ultramicrotomed films (Fig. 65 c) it can be seen that the nanoplatelets are well dispersed as thin nanoplatelets. It indicates that the employed mechanical stirring plus the ultrasonic treatment approach was proven to be an effective approach to obtain composites with well-dispersed nanoplatelets in the PET polymer matrix. Importantly, the whole process of preparation the dispersion is carried out just before polymerization reaction, which made the processing simple and effective. In another way, one can say that, during dispersion process, the PET molecules could more sufficiently intercalate galleries of nanoplatelets during in situ polymerization, and as the intercalation process proceeds further, the exfoliated graphene sheets might be further delaminated and exfoliated, which would allow more polymer molecules to enter and enlarge the space between them. However, the agglomeration of SWCNT was observed. The agglomerates of SWCNT disturbed the good connection between graphene nanoplatelets. This might be an explanation of efficiency of using high speed stirrer and ultrasounds. However, in case of PET hybrids agglomerates of SWCNT caused that prepared nanocomposites weren’t conductive (chapter 17.3, pages 134-136) even with the

122 concentration of 0.1 wt % of EG, wherein in the nanocomposites, based on PET with EG only, the percolation threshold was obtained at 0.05 wt %.

a) b)

c) d)

Fig. 65 Scanning and transmission electron micrographs of PET/0.1EG+0.05SWCNT nanocomposite: a) SEM with visible agglomerate of carbon nanotubes; b) SEM of zoomed in agglomerate of SWCNT in polymer matrix; c) TEM at 250 000x and d) TEM at 200 000x.

SEM and TEM were used to visually evaluate the degree of exfoliation and the amount of aggregation of nanofillers in poly(trimetylene terephthalate) matrix. TEM analysis tends to support the findings from SEM but also shows that the SWCNT nanoparticles are well dispersed on the nanoscale in all systems. The efficiency of the hybrid system in modifying the properties of the matrix polymer is primarily determined by the degree of its dispersion in the polymer matrix. The aggregated EG morphology can be characterized with SEM. Because of the difference in scattering density between the nanofiller and PTT, nanoplatelets aggregates can be easily imaged in SEM. However at the same time well-dispersed carbon nanotubes are clearly visible. More direct evidence of the formation of a true nanocomposite is provided by TEM of an ultramicrotomed section. Figure 66 a shows micrograph of PTT hybrid containing different 0.1EG+0.05SWCNT. The dark regions in the photograph are the thicker agglomerates of expanded graphite (less expanded), and the brighter regions show the better dispersed sheets. TEM photography proves that some graphene layers were dispersed homogeneously into the matrix polymer, however mostly clusters or agglomerated particles were detected. This will be cross-checked by ultimate strength and initial modulus in the tensile property section. The SEM and TEM micrographs in Fig. 66 for the hybrid nanocomposite with 0.1EG and 0.05SWCNT KNT 95 indicates that

123 electrical conducting networks were formed by well-dispersed carbon nanotubes.

Incorporation of 0.1 wt % of EG did not make much change in the morphology and electrical conductivity (Fig. 75 b), with some isolated graphene sheets (Fig. 66 a). There was no improvement in electrical conductivity in hybrid system because SWCNT were the main contributor for the formation of network.

.

a) b)

Fig. 66 Transmission electron micrograph at 150 000x (a) and scanning electron micrograph (b) of PTT/0.1EG+0.5SWCNT nanocomposites.

As it was mentioned before, the strong intertube attraction between carbon nanotubes is one of the main problems to overcome making the dispersion of CNTs in polymer matrices challenging and hence limit its effective use as nanofiller in polymer matrix. Many approaches to overcome the affinity of the tubes result in excessive modification or even damage to the unique morphology of the CNTs. An efficient alternative to tailor the polymer/CNT interface while preserving the integrity of the tubes is mixing carbon nanotubes together with the nanofiller with different shape (plate-like shape). In Fig. 67 we observe a good distribution of the CNT on the surface of the graphene platelets which should together act as a stronger reinforcing agent for the polymer causing synergistic effects in electrical conductivity and mechanical properties. The uniformly dispersed bright dots and lines are attributed to the ends of the broken PTT-PTMO-wrapped SWCNTs/Graphene clearly observed due to their high electrical conductivity. For use in the CNT, graphene nanoplatelets mixture samples were hitched to the CNTwalls to facilitate better attachment to the defect sites and residual oxide groups on the Graphene Angstron surface and also to the elastomer matrix. The dual-filler strategy appears to yield a more efficient dispersion of SWCNT+Graphene-PTT-PTMO. The formation of conducting networks (chapter 17.3, page 134-139) and dispersion state in the composites with hybrid CNT - graphene particles was further confirmed by transmission microscopy.

124

a) b)

Fig. 67 SEM micrographs of PTT-PTMO/0.1SWCNT+0.3Graphene hybrid nanocomposites.

The TEM images in Figure 67 (with two different magnifications) present the real distribution of conducting fillers in the respective nanocomposites. They may shed some insight into the mechanisms behind the synergy in enhancing the conductivity of nanocomposites due to the hybrid fillers of CNT and graphene platelets. In Fig 68 a there were both randomly dispersed individual CNTs and a round CNT agglomerate but also well-dispersed graphene sheets. In some places carbon nanotubes and graphene plates seemed to connected to one another. It is better seen in Fig. 68 b, were the dispersion of both nanofiller was even better and only nanotubes “hitched” to graphene planes were observed. This should be the reason why the hybrid nanocomposites filled non-conductive Graphene Angstron and SWCNT achieved enhanced electrical conductivity (chapter 17.3, page 134-139) in comparison to PTT-PTMO composite with 0.1 wt % of SWCNT.

a) b)

Fig. 68 Transmission electron microscopy (TEM) micrographs of PTT-PTMO/0.1SWCNT+0.3Graphene Ang nanocomposites at a) 100 000x and b) 150 000x.

The SEM images of the PTT-PTMO/0.3SWCNT+0.1 Graphene Angstron are shown in Fig. 69. Both the SWCNTs and Graphene Angstron show fairly good distribution in Fig. 69 c and d. The graphene nanosheets in Figure 69 appear to show better dispersion than those in the PTT-PTMO/Graphene in Figure 54. This indicates that co-addition of SWCNT improves the dispersion of graphene nanoplatelets, which agrees with the electrical conductivity results that exhibit conductivity improvement by the coaddition of SWCNT.However, because of the van der Waals interactions, the as-reduced graphene sheets tend to form irreversible agglomerates and even restack to form graphite (Fig. 69 b). Additionally, the SEM image of PTT-PTMO/SWCNT+Graphene in Figure 69 (c and d) show that the SWCNTs and graphene

125 nanoplatelets form some kind of network structure. The SWCNTs appear to a have good affinity for Graphene Angstron. This kind of conductive network formation due to graphene nanosheets may explain the good electrical conductivity results from PTT-PTMO/0.3SWCNT+0.1Graphene Angstron (chapter 17.3, pages 134-139).

a) b)

c) d)

Fig. 69 SEM micrographs of PTT-PTMO/0.3SWCNT+0.1Graphene hybrid nanocomposites.

To examine exactly the dispersion of the SWCNT/graphene in the polymer hybrids, TEM studies were carried out. A TEM allows a qualitative understanding of the internal structure through direct observation. Typical TEM photographs for hybrids with 0.3 wt % of SWCNT and 0.1 wt % of Graphene Angstron are shown in Fig. 70 a (lower magnification) and 70 b. Fig. 70 a shows that both nanofillers were well dispersed in polymer matrix, although some parts of agglomerated layers still exist. To observe the agglomerated structures of 1D+2D hybrid system the TEM micrograph were done at higher magnification.

Here agglomerates of CNTs strongly connected to agglomerates of graphene nanoplatelets were seen. However, also some single nanotubes were observed. Interestingly, with the concentration of 0.1SWCNT+0.3 graphene sheets more homogenously dispersion was observed. On the other hand, the “pull-out” mechanism of SWCNTs has been observed in Fig 69 c and d. Carbon nanotubes counteracted the break of polymer matrix, which is clearly seen at higher magnification. In this case, higher concentration of SWCNT:Graphene (3:1) seemed to create more agglomerates than in case of SWCNT:Graphene (1:3). However, a few studies suggest that CNT agglomeration could favor the formation of a percolating network [86] [92]

[387]. As observed from these figures, agglomeration promotes to-CNT and

CNT-126 Graphene interactions through surface contact (or tunneling) and hence it should increase the electrical conductivity of the composite, as it will be further discussed.

a) b)

c) d)

Fig. 70 Transmission electron microscopy (TEM) micrographs of PTT-PTMO/0.3SWCNT+0.1Graphene Ang nanocomposites at a) 25 000x , b) and c) 150 000x and d) 175 000x.