I.
Kelnar
*a,
J.
Kratochvíl
a,
I.
Fortelný
a,
L.
Kaprálková
a,
A.
Zhigunov
a,
M.
Nevoralová
a,
M.
Kotrisová
b and
V.
Khunová
b
aInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského nám. 2, 162 06 Praha, Czech Republic. E-mail: kelnar@imc.cas.cz
bThe Slovak University of Technology, Faculty of Chemical and Food Technology, Radlinského 9, 812 37 Bratislava, Slovakia
First published on 10th March 2016
The effect of two clays with different geometries, viz. organophilized montmorillonite nanoplatelets (oMMT) and halloysite nanotubes (HNT), on the structure and properties of poly(lactic acid) (PLA)/thermoplastic polyurethane (TPU) 80/20 and 70/30 has been studied. The reinforcement of both the components and the blend is higher for oMMT due to its higher specific surface area and aspect ratio. The effect of both nanofillers (NFs) on the structure, reflected in a decrease of the dispersed TPU size, is comparable. A larger effect on the viscosity by oMMT is eliminated by its more marked less favourable localization in the dispersed phase in comparison with HNT. The fact that the toughness of the HNT-containing nanocomposite (NC) is markedly higher (with maximum at 1% HNT) than that with oMMT is in contradiction with PLA NC. The results indicate that the application of nanofillers can lead to a polymer system with a balanced mechanical behaviour. However, a different impact of NFs on the polymer components and interface parameters may also lead to antagonistic effects. The influence of NFs, including their geometry, on the polymer blend is more complex in comparison with single-matrix nanocomposites.
Another natural NF with a fair reinforcing ability is HNT19,20 which also has a pronounced effect on biodegradable polymers, including PLA,21,22 even without any modification. However, the application of HNT in multicomponent systems is still in its infancy.23–28
This work deals with the effect of oMMT and HNT on the structure and properties of a PLA/commercial TPU system. The goal was a more detailed study of the effect of nanofiller content and a comparison of the reinforcing/structure-directing abilities of nanosilicates with various geometries.
Tensile impact strength, at, was measured on specimens notched on one-side using a CEAST Resil impact junior hammer (CEAST S.p.A., Torino, Italy) with an energy of 4 J (variation coefficient 10–15%). The reported values are averages of twelve individual measurements. Dynamic mechanical analysis (DMA) was performed in single-cantilever mode using a DMA DX04T (RMI, Pardubice, Czech Republic) apparatus at 1 Hz and a heating rate of 1 °C min−1 from −120 to 250 °C.
DSC analysis was carried out using Perkin-Elmer 8500 DSC apparatus. Cyclohexane and indium were used for calibration. The instrument was cooled with liquid nitrogen using an LN2 accessory at a set-point of −30 °C and flushed with dry nitrogen as a purge gas. The samples were scanned from 0 to 200 °C at 10 °C min−1. The melting point and cold crystallization temperature were identified as a maximum and minimum of the melting endotherm and crystallization exotherm, respectively. A value of 93.1 J g−1 was used as the heat of melting of 100% crystalline PLA29 in calculating its crystallinity.
Wide-angle X-ray scattering (WAXS) experiments were performed using a pinhole camera (Molecular Metrology System, Rigaku, Japan) attached to a microfocused X-ray beam generator (Osmic MicroMax 002) operating at 45 kV and 0.66 mA (30 W). The camera was equipped with an interchangeable imaging plate of 23 × 25 cm (Fujifilm). The experimental setup covered a momentum transfer (q) range of 0.25–3.5 Å−1. q = (4π/λ)sinθ, where λ = 1.54 Å is the wavelength and 2θ is the scattering angle. Calibrations of the centre and sample-to-detector distance were made using Si powder. Samples were measured in the transmission mode. Due to their isotropic character, 2D diffractograms were transformed into 1D using the whole 0–360° azimuthal range.
Fig. 1 SEM images of (a) PLA/TPU 80/20 blend, (b) with 5 phr oMMT, and (c) with 5 phr HNT, and (d) PLA/TPU 70/30 blend, (e) with 5 phr oMMT, and (f) 5 phr HNT. |
Fig. 2 Effect of nanofiller content on size of dispersed TPU in the nanocomposite with PLA/TPU 80/20 matrix. |
Fig. 3 shows beneficial14 interfacial localization in both cases, but it is more significant for oMMT. This undoubtedly supports the “compatibilizing” effect of NF. This figure also shows more significant localization of oMMT inside the dispersed phase in comparison with HNT. This difference is quite important with respect to possible effects on the dynamic phase behaviour. A high content of well dispersed oMMT inside TPU apparently increases its viscosity (see below), which hinders particle break-up in the course of mixing. This obviously eliminates the above mentioned effect of more significant interfacial localization of oMMT and seems to be the reason for the relatively lower effectivity of oMMT in comparison with HNT. Probably, this effect is only partially compensated by the higher viscosity of the oMMT containing system. At the same time, the lower specific surface area of HNT is apparently compensated by a relatively low increase of viscosity, which is beneficial in the case of HNT localized in the dispersed phase. Moreover, the mentioned more significant content of HNT in the matrix may lead to a favourable increase of matrix viscosity and decrease of viscosity ratio, which supports the break-up of the dispersed phase. Taking into account the more marked effect of HNT on PLA degradation (see below), this effect can be considered to take place in the early stage of mixing.
Fig. 4 Temperature dependence of the loss modulus of PLA/TPU 70/30 blend containing (a) HNT, and (b) oMMT. |
Fig. 6 shows a rather different effect of both of the NFs’ content on tensile strength, especially in the dependence on the matrix composition. Although higher values for oMMT have been found in all systems, the effect of the NF content is different. In neat PLA, a slight increase with oMMT content and decrease with HNT content has been found. This is in accordance with the fact that NF-reinforcement predominantly consists of an increase of stiffness.33 In the case of the PLA/TPU 80/20 matrix, Fig. 6 shows a practically unaffected tensile strength in the whole range of NF content (up to 10%) for HNT and a slight maximum at 5% content for oMMT. This may correspond to reduced PLA crystallinity (CRp in Table 1) at a high oMMT content in comparison with the analogous HNT-containing system. Compared with neat PLA, the increase of the strength of the blends with both NFs is more marked. We consider the positive effect of the finer structure and reinforcement of TPU (see the predominant localization of oMMT in dispersed TPU, Fig. 3) because the addition of a low-modulus elastomer decreases strength significantly. Surprisingly, in the 70/30 system only a slight decrease, more significant for HNT, was found. Although the reinforcing effect seems to be partially eliminated by the higher particle size in the 70/30 system (Fig. 2), this difference undoubtedly confirms the complex effect of NFs in a two-component matrix nanocomposite.
Composition | Filler (phr) | T g, °C | Exo | Endo | CRp, % | ||
---|---|---|---|---|---|---|---|
T c, °C | CRc, % | T m, °C | CRm, % | ||||
PLA/TPU 80/20 | 0 | 60.0 | 106.6 | 24.6 | 149.0 | 29.3 | 4.7 |
PLA/TPU 80/20 + HNT | 1 | 59.1 | 106.2 | 24.2 | 150.3 | 29.3 | 5.2 |
3 | 59.9 | 105.9 | 26.4 | 149.2 | 31.0 | 4.6 | |
5 | 60.4 | 105.6 | 25.9 | 149.8 | 30.9 | 5.0 | |
10 | 59.2 | 107.7 | 29.1 | 148.6 | 33.3 | 4.2 | |
PLA/TPU 80/20 + oMMT | 1 | 59.7 | 105.1 | 22.8 | 148.0 | 28.2 | 5.4 |
3 | 58.0 | 105.7 | 25.4 | 149.6 | 29.6 | 4.2 | |
5 | 58.1 | 109.7 | 28.3 | 149.9 | 30.9 | 2.6 | |
10 | 57.4 | 109.8 | 30.8 | 152.1 | 31.5 | 0.7 |
On the contrary, higher toughness values were found in the HNT-containing systems (Fig. 7). In the PLA nanocomposite, the increase corresponding to the results of other authors13 indicates the fair dispersion of NF. The slightly lower toughness in the case of oMMT may correspond to a higher reinforcement and thus a higher overall degree of unfavourable immobilization of the polymer chains. A relatively low gain in toughness due to 20% and especially 30% TPU addition indicates the limited toughening ability of the TPU applied. In this respect, a marked increase of PLA/TPU 80/20 toughness with 1% HNT is important. This undoubtedly indicates certain synergistic effects of the nanofiller on the multi-component matrix NC found also by others8, mostly with platelets and particles, especially in the case of their more significant interfacial localization. In PLA/TPU 80/20, this can be explained by the lower content of HNT inside the TPU inclusions and its presence at the interface (Fig. 3).
From Fig. 7, it follows that the toughness of all other PLA/TPU/nanofiller systems (80/20 with oMMT and 70/30 with both NF) decreases monotonously, less markedly with HNT. The main reason is NF localization inside TPU which increases its rigidity and reduces the toughening effect. This negative effect is more marked in the oMMT system with its increased content inside TPU and higher reinforcing ability. As a result, a more significant decrease of toughness with oMMT content has been found. This is in accordance with other authors who found a similar decrease even in the case of the absence of clay in a dispersed phase.34,35 These results indicate that the influence of clay on multicomponent systems may lead to rather antagonistic effects and further confirms the complex effect NF has on these systems. Certain negative effects in the oMMT-system can also originate from reduced PLA crystallinity, which may lead to less favourable energy-absorbing microdeformations.36 Further differences, especially in the crystalline structure, between the oMMT- and HNT-systems are obvious from WAXS. Different degrees of the immobilization of polymer chains is also shown by DMA (see above). Finally, strain at break (Fig. 8) decreases predominantly with a higher NF content in accordance with increased rigidity, but it partially follows the synergistic increase of toughness with 1% HNT, surprisingly also in the case of oMMT. All the mentioned facts, especially the differences between the HNT- and oMMT-systems, also confirm the complex and different effects on multiphase NC of both NFs with dissimilar geometry.
The results of the DSC analysis are summarized in Table 1 as averages of four measurements of the glass transition temperature Tg, the temperature of minimum of the cold crystallization exotherm Tc, i.e. of maximum crystallization rate, the crystallinity connected with cold crystallization CRc, the melting temperature Tm, the total crystallinity CRm, and the crystallinity of the as-prepared sample CRp.
Fig. 9 shows that the cold-crystallization exotherm passes smoothly to the melting endotherm as the processes of cold crystallization and melting partially overlap. It is thus impossible to unambiguously determine the areas of these two peaks. Therefore, an integration was performed by leading the baseline between 70 and 160 °C. The CRc value in Table 1 was obtained from the area of the exotherm between 70 °C and the cross-section of the baseline with the DSC trace. Analogously, the CRm value was obtained from the area of the endotherm between the cross-section and 160 °C. The as-prepared crystallinity CRp, as the difference CRm − CRc, results in the neat thermal balance of the integration between 70 and 160 °C.
The results in Table 1 indicate that, with the exception of slightly rising CRc and CRm, the determined parameters are virtually independent of the added HNT concentration. On the other hand, some dependence on the concentration of the added oMMT has been found. The Tg smoothly decreases with increasing oMMT content. The values of Tc, CRc, Tm, and CRm have a generally increasing trend, however, with a minimum at about 1% oMMT. The crystallinity of the as-prepared samples, CRp, passes through a maximum at about 1% and then decreases sharply with the oMMT content. Anyway, all of the as-prepared samples show very low PLA crystallinity of about 1–5%. On heating, this crystallinity increases to about 30% due to the intensive cold crystallization.
It is hard to speculate about the decomposed peak’s position when it originates from such broad spectra. Nevertheless, analysis shows that, in the case of the nanofiller-free PLA/TPU 80/20 sample, the position of the first peak is shifted to lower angles. We can exclude possible TPU crystallization, as this should produce a shift to higher angles.37,38 A possible explanation could be in partly transformed PLA into the α-crystalline form, the most common and stable polymorph of PLA. During such transformation, a diffraction peak at 2θ = 14.8 appears.39 This effect is more pronounced when the oMMT filler is added. It is also seen that HNT produces rather the opposite effect. Better chain organization (α-form) leads to improved mechanical properties. This difference, especially if possibly localized in the vicinity of the TPU inclusions, may be a further reason for the differences in mechanical behaviour between PLA/TPU containing both NFs (see above).
The comparison of the dependence of η* on the frequency for neat PLA and the PLA/oMMT nanocomposite (Fig. 11b) indicates that, in the nanocomposite with 1% of oMMT an increase of η* due to the presence of hard fillers prevails over the decrease of η* caused by degradation. The degradation of PLA by the presence of a higher oMMT content also causes a lower viscosity of the nanocomposite at moderate and high frequencies.
At low frequencies, a higher η* of the nanocomposites containing 3% oMMT apparently indicates the tendency of oMMT to form a weak physical network in the PLA matrix. Its contribution to η* competes with the effect of the degradation which is lower in comparison with the HNT-systems.
In the case of the PLA/TPU (80/20)/HNT system (Fig. 12a), rheological behaviour, especially the lower reduction of η* with HNT content, indicates a certain competition between the effect of the amount of HNT on the degradation of the component, on the one hand, and the tendency to increase η* by energy dissipation in the presence of the fillers and formation of a physical network on the other. This is shown by the curves for PLA/TPU and the nanocomposites with 0.5 and 5% of HNT.
Fig. 12 Effect of NF addition on complex viscosity of the PLA/TPU/HNT system at 190°: (a) 80/20 and (b) 70/30. |
The dependence of η* on the frequency for PLA/TPU (70/30) filled with HNT (Fig. 12b) also demonstrates competition between the effect of the blend degradation and viscosity increase by the presence of hard fillers. The direct effect on the viscosity increase and the formation of a physical network is more pronounced in the PLA/TPU (70/30) blends in comparison with the 80/20 systems. Similarly to PLA/oMMT, the degradation of PLA/TPU/oMMT (Fig. 13a and b) is also less marked in comparison with the analogous HNT-system.
Fig. 13 Effect of NF addition on complex viscosity of the PLA/TPU/oMMT system at 190°: (a) 80/20 and (b) 70/30. |
In the case of the oMMT-systems, Fig. 13a and b show an increase of viscosity with the concentration of the added clays in the region of low frequencies. The increase of viscosity with the amount of clay is more pronounced for the PLA/TPU (70/30) blend. The effect of the clay on the blend degradation is indicated by the values of η* in the region of high frequencies. Comparison of the effect of the addition of HNT and oMMT on the rheological properties of the PLA/TPU blends shows that the increase of η* at low frequencies is more pronounced in the PLA/TPU/oMMT than in the PLA/TPU/HNT nanocomposites. The decrease of η* with frequency is more pronounced for the PLA/TPU/oMMT than for the PLA/TPU/HNT nanocomposites.
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