14. Principle of preparation of polymer nanocomposites by in situ method
16.2. Morphology and phase structure
Morphology and phase structure of prepared nanocomposites based on thermoplastic polyesters (PET, PTT) and thermoplastic elastomers (PTT-PTMO) containing graphene derivatives were investigated by using SEM and DSC analysis. The measurements of physical properties (intrinsic viscosity, density etc.) of nanocomposites complimented further analysis of the influence of graphene derivatives on different matrices.
The dispersion of EG in ethanediol produced good exfoliation as shown in the SEM images (Fig. 51 b) where individual EG particles are clearly visualized embedded in the PET polymer matrix. It has been well documented that the effectiveness of nanoadditive dispersion strongly depends on both the method and the process time. As revealed by the SEM images, the use of a sonicator device seems to be rather effective in order to split the “as received”
existing worm-like agglomerates into graphite sheets and to distribute them in the entire volume of ethanediol. SEM images also indicate that the expanded graphite nanosheets were encapsulated by the PET matrix. This suggests a strong interaction between graphene sheets and PET matrix. In order to further investigate the dispersion of expanded graphite in the PET matrix, TEM measurements were also performed. Fig. 51 a shows the microstructure of PET nanocomposites with 0.2 wt% of expanded graphite. The nanosheets appear to be completely embedded in the polyester matrix indicating an exfoliated structure. The high dispersion level of expanded graphite sheets can be attributed to a strong interaction between some residual polar groups on the surface of graphene and the polar groups of PET.
c)
99
a) b)
Fig. 51 TEM (x120.000) and SEM images of PET/0.1 wt % of EG I.
The molecular weight of polyesters and block copolymers during the synthesis were controlled by monitoring the maximum viscosity that can be obtained previous to the extrusion the polymers from the reactor. In our method, the maximum viscosity of the system was set to 14 Pa·s (at a constant temperature of 275 oC and at a stirrer speed of 40 min-1). PET prepared by polymerization without EG had a number-average molecular weight of about 19500 g/mol. Chen et al. [362] reported that, in polymeric nanocomposites, the viscosity increases only after a certain range of volume fraction implying that at low volume fraction (less than 0.4 vol %), nanofluids have lower viscosity than corresponding base fluid, due to lubricating effect of nanoparticles. Accordingly, the molar weight of PET is expected to be dependent on the EG content in the reaction mixture. In Table 20, the values of intrinsic viscosities and molar weights of PET and PET/EG nanocomposites are presented. Both the intrinsic viscosities and molar weights of PET nanocomposites whit an EG content lower than 0.2 wt % were higher than those of neat PET. Thus, the results in Table 20 suggest that even though the viscosity of reaction mixture increased with EG content, for EG loadings below 0.2 wt % the viscosity was lower than the viscosity of reaction mixture without EG nanofiller, in agreement with Chen et al. observations [362]. The molecular weight of PET prepared in the presence of the highest studied EG content, 0.4 wt %, was 15600 g/mol. PET with this molar weight still exhibits suitable rheological properties for extrusion. For the sake of comparison let us remember that textile fibre-grade PET normally has a number-average molecular weight ranging from 15000 to 20000 g/mol, which corresponds to an intrinsic viscosity between 0.55 and 0.67 dl/g [319]. Moreover, it is worth to mention that the polydispersity values (Mw/Mn) remain at the same level as those of neat PET.
100
Table 20 Intrinsic viscosity and molecular weight of PET prepared in the presence of various EG loading.
Last column includes mass crystallinity values, Xc, for the slow cooled (SC) samples as estimated by Wide Angle X-ray scattering
Mv - viscosity average molecular weight ; Mw - weight average molecular weight; Mn - number average molecular weight; Mw/Mn dispersity; PET2- additionally synthesized neat PET with lower molecular weight
Nanofillers usually affect the ability to crystallize semi-crystalline polymers [363]
[364] [365] [366] distributed in the polymer matrix nanoparticles assisted by nucleation and growth of crystallites. In the case of characterized in this work nanocomposites, there was no significant effect of nanoparticles on the physical transitions in PET or they were very small.
Only for the highest content of EG in investigated nanocomposites based on PET it can be stated repeatedly the increase of crystallization temperature (Tc) and melting temperature (Tm) (Fig. 52). EG content does not significantly affect the degree of crystallinity of PET, which is between 34.4-36.1 (Table 21). This is due perhaps to the fact that the sizes of individual expanded graphite nanoplatelets were below the critical nucleation agents, as a result, may not constitute active centers of growth of crystallites. It also describes the lack of changes in the glass transition temperature (Tg) of semicrystalline PET regardless of the EG content
50 100 150 200 250 300
Fig. 52 DSC thermograms for PET and PET/EG nanocomposites during the cooling and 2nd heating.
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Table 21 Thermal properties of neat PET and PET/EG nanocomposites determined by DSC
Symbol
Tg
oC
Tm
oC
ΔHm
J/g
Tc
oC
ΔHc
J/g
Xc %
PET 85 257 47.0 214 46.9 33.5
PET/0.025 EG 83 251 49.4 207 48.8 35.3
PET/0.05 EG 83 253 49.4 210 49.1 36.1
PET/0.1 EG 83 252 50.5 214 49.8 36.0
PET/0.2 EG 83 252 48.2 217 47.5 34.4
PET/0.4 EG 83 253 50.1 217 49.5 35.8
Tg - glass transition temperature; Tm - melting temperature; Tc – crystallization temperature; Hm, Hc - enthalpy of melting and crystallization; xc - mass fraction of crystallinity determined from DSC
To confirm the morphological features of EG in the PTT matrix, SEM images of the fractured surfaces for the nanocomposite films were examined, as can be seen in 53 b. For further investigation, the TEM analysis was used to assess the degree of exfoliation of the expanded graphite platelets and the morphology of the nanocomposites (Fig. 53 a). From this micrograph, the more or less transparent graphene platelets providing a high degree of exfoliation of EG in a matrix has been observed. The fractured surface of PTT/EG composite film with 0.3 wt % EG also exhibits the smooth fractured surface without exhibiting any aggregates of graphene sheets. Nonetheless, it should be mentioned that the partial aggregates of graphene sheets in the nanocomposites (Fig. 53 b) with higher EG contents are not crystalline but disordered, as confirmed from density and DSC measurements of the nanocomposites in Tables 22 and 23. In addition, the accordion-like structures detected in the PTT/EG nanocomposites means that the graphene sheets of EG exist in a completely exfoliated and disordered state in the matrix of nanocomposites due to the high shear force generated during dispersion preparation followed by in situ polymerization. Direct evidence of the exfoliation of expanded graphite was provided by TEM analysis of PTT/0.3EG nanocomposites. The exfoliated morphology with clearly visible flakes (partially transparent) were observed. The flakes size supported from TEM was around 0.5x1μm2, which was probably due to the in situ procedure and dispersion process before polymerization, that causes wrapping and breaking of graphene sheets. It was observed that the expanded graphite sheets were dispersed homogeneously in PTT matrix. The occurrence of this exfoliation morphology may be attributed to the strong shear field in the mechanical stirrer and treatment of ultrasounds, and the strong interactions between EGII and PTT.
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a) b)
Fig. 53TEM (x100.000) and SEM images of PTT/0.3 wt % of EG II.
Nanocomposites with concentrations of 0.1-0.5 wt % of EGII were prepared.
Additionally, for comparison purposes, unmodified PTT was synthesized and characterized in the same manner as nanocomposites. In order to prepare high molecular weight polyesters, the polycondensation of these oligomers ensued at higher temperature, with the simultaneous application of high vacuum. The intrinsic viscosity of neat PTT was 0.781 dl/g. The presence of the expanded graphite in the polymerization mixture affected the reaction, leading to the decrease of intrinsic viscosity. As shown in Table 22, the intrinsic viscosity wasn’t affected by the addition of EG. All obtained values of [η] are comparable to one another. Only the PTT/0.3EG exhibited slightly higher value of intrinsic viscosity to around 0.783 dl/g. As can be also seen in Table 22, the synthesized nanocomposite have also comparable molecular weights to the neat PTT, and only slight increase with the addition of 0.3 EG to 3.85·104 g/mol for PTT/0.5EG was observed. Even if the dumbbell shape samples were injection moulded to obtain amorphous samples, some crystallites have been formed, as it can be seen in increase of density. The prepared nanocomposites in comparison to the neat polymer showed slightly higher density due to the increase number of crystallites with an increasing content of nanosheets. Distinctive increase in weight degree of crystallinity was due to the nucleating influence of expanded graphite, also confirmed by DSC analysis, Table 23.
Table 22Physical properties of PTT/EG nanocomposites
Sample
[η]
dl/g
Mv x 104 g/mol
d g/cm3
Xcw
%
PTT 0.781 3.84 1.323 19.8
PTT/0.1 EG 0.777 3.81 1.328 23.4
PTT/0.3 EG 0.783 3.85f 1.329 24.4
PTT/0.5 EG 0.778 3.82f 1.332 26.5
Mv - viscosity average molar weigh; f- measured after filtration of EG; d- density; Xc degree of crystallinity estimated from density measurement
Figure 54 shows DSC heating and cooling thermograms of neat PTT and PTT/EG nanocomposites with various EG contents. In the cooling thermograms of Figure 54 a the
103 melt-crystallization temperatures (Tm) of PTT/EG nanocomposites are shifted to higher temperatures, as the EG content in the nanocomposites increased up to 0.5 wt %. The melt-crystallization exothermic area ( ΔHm) also increased with increasing the EG content up to 0.5 wt %. Contrarily, in the heating thermograms of Figure 54 b, two thermal transitions are observed apparently with increasing temperature, i.e., the glass transition and melting. The glass transition temperatures (Tg) and melting temperatures (Tm) of PTT/EG nanocomposites remained practically unchanged, regardless of the EG content, as summarized in Table 23.
These results indicate that the overall crystallization rates of PTT/EG nanocomposites become faster with the increasing content of EG content up to 0.5 wt %, which is due to the effective nucleating agent effect of the graphene nanosheets of EG for PTT matrix component.
50 100 150 200
Fig. 54 DSC thermograms for PTT and PTT/EG nanocomposites during the cooling and 2nd heating.
Table 23Thermal properties of neat PTT and PTT/EG nanocomposites determined by DSC
Sample
Tg - glass transition temperature; Tm - melting temperature; Tc - crystallization temperature Hm, Hc - enthalpy of melting and crystallization; xc - mass fraction of crystallinity determined from DSC.
The dispersion and orientation of graphene platelets within the thermoplastic elastomer was examined using SEM microscopy. In general, the surface of the cryuofracture injection molded specimens exhibited a high degree of graphene sheets alignment along the polymer. Figure 54a shows the SEM images for PTT-PTMO/0.3 wt % of Graphene Angstron, whose particle distribution morphology includes overall projection and microdomain. Due to the residual groups on the surface of graphene, after preparation process, it can be observed that the graphene platelets are covered by polymer matrix (Fig. 55 b).
104 The morphology of the ultramicrotomed nanocomposites was analyzed in detail using transmission electron microscope (TEM). Exfoliated graphene-based materials are often compliant, and when dispersed in a polymer matrix are typically not observed as rigid disks (flate platelets), but rather as bent or crumpled/wrinkled platelets. Moreover, graphene has been shown to ‘scroll up’ irreversibly when its polymer host is heated above its glass transition temperature (Tg) [345]. Compatibility between the polymer matrix and graphene platelets also can reportedly affect the platelets’ conformation. In Fig. 55 c and d randomly oriented, exfoliated platelets was observed possibly due to restacking of the platelets. The processing technique by means of in situ polymerization could induce orientation of the dispersed platelets, which can be beneficial for reinforcement but may also raise the percolation threshold.
a) b)
c) d)
Fig. 55 SEM (a and b) and TEM (c-d, x75.000) images of PTT-PTMO/0.3wt % of Graphene Angstron.
The thermoplastic poly(ether-ester) segmented block copolymer, containing 50 wt % of PTT, as the rigid segment, and 50 wt % of PTMO as the flexible one, was used as polymer matrix. The nanocomposites containing from 0.1 to 1.0 wt % of graphene nanoplatelets (Graphene Angstron) were synthesised by a two-stage melt transesterification and polycondensation. The physical properties of the obtained nanocomposites are presented in Table 24. Values of [] for copolymers in nanocomposites varied between 1.305 and 1.289 dl/g (from the lowest to the smallest concentration (Table 24), and are lower than the value obtained for the neat PTT-PTMO block copolymer (1.325 dl/g). As can be seen in Table 16, the obtained high values of [] for matrix in nanocomposites show that the synthesized copolymers have high molecular weights and comparable to neat copolymer, which were confirmed by SEC analysis (Table 24). Only the slight increase in dispersity was observed for
105 the highest concentration of nanofiller (1.0 wt %). Nanocomposites in comparison to the neat copolymer have also slightly higher density due to the presence of nanofiller with higher density.
Table 24 Physical properties of PTT-PTMO/Graphene Ang nanocomposites
Sample
[η]
dl/g
Mw g/mol
Mn g/mol
Mw/Mn d
g/cm3
PTT-PTMO 1.325 102 066 56 773 1.797 1.174
PTT-PTMO/0.1 G 1.305 - - - 1.174
PTT-PTMO/0.3 G 1.327 - - - 1.175
PTT-PTMO/0.5 G 1.322 101 002 57 543 1.755 1.176
PTT-PTMO/1.0 G 1.289 100 849 54 249 1.859 1.177
Mw - weight average molecular weight; Mn - number average molecular weight; Mw/Mn dispersity; d - density
As mentioned earlier, the neat PTT–PTMO block copolymer has two phase morphology: a PTT crystalline phase and PTMO-rich amorphous phase. It was found in many semicrystalline systems that the addition of nano-additives have affected the crystalline structure and crystallization rate. The effect of Graphene (<1nm) on the thermal behaviour of the nanocomposites and neat PTT-PTMO block copolymer during heating and cooling is examined by DSC (Figure 56, Table 25). As can be seen from the Table 25, the glass transition temperature (Tg), corresponding to the amorphous soft PTMO-rich phase, and the meting point (Tm) derived from the crystallized rigid PTT segments (Tm) are not affected by the presence of graphene nanoplatalets in polymer matrix. Degrees of crystallinity of the nanocomposites are comparable to the neat PTT-PTMO block copolymer, with only slight decrease with the highest concentration of nanofiller. However, the crystallization traces in Fig. 54a indicate that graphene worked as a nucleation agent for the crystallization of PTT segments. At the same cooling rate, the nanocomposites have higher crystallization temperatures (Fig. 56 a, Table 25) with the increase of nanoplatelets content than neat PTT-PTMO copolymer. Crystallization rate of a polymer is determined by the nucleation rate and mobility of polymer chains. The introduction of nanoscale platelets produces a hindrance on the molecular chains movement, which will reduce the tendency for molecular chains to be crystallized, though the confined molecular chains may be well ordered in the lamellar space.
Hence, in our system only increase of crystallization rate was observed, but degree of crystallinity is not affected by the presence of the graphene sheets. Similar effect was previously observed for PTT-PTMO and PTT nanocomposites after introduction of carbon nanotubes
106
Table 25Thermal properties of neat PTT-PTMO and PTT-PTMO/Graphene Angstron nanocomposites determined by DSC crystals, respectively; xc - mass fraction of crystallinity.