Servlets

In this part, the manipulation steps for performing the drop retraction method are drafted in a work flow chart. An example of the use of this method to measure the interfacial tension between PLA and PBAT will also be presented. We also investigated the influence of polymer molecular weight on the interfacial tension. The different deformation direction

a

b

- 83 -

versus shear direction of PA drops in PBAT, and the influence of drop tilt angle correction on the result of interfacial tension will also be discussed.

IV.2.1 Working flow chart of drop retraction

The second method employed to characterize the interfacial tension is based on the kinetics of the retraction of a slightly deformed drop. The principle of the method has also been introduced in Chapter II. The experimental steps to apply this method are shown in Fig. IV.6.

Fig. IV.6 Working flow chart for drop retraction

Since knowledge of the rheological properties of the polymers is also mandatory to implement this method to characterize the interfacial tension, it was also necessary to assess the potential decrease of polymer viscosities during the test. Unlike the severe thermomechanical conditions in the mixer, the observation conditions in rheo-optics are quite gentle. Therefore, the polymers were used as received, considering that they do not suffer degradation during the rheo-optical test.

As for the samples for rheo-optical observations, the matrix was shaped as a flat disk with thickness 0.6 mm and diameter 25 mm. The size of fragments serving as dispersed phase was 10 to 100 µm. Then, the observation was conducted following the protocol introduced in Chapter III.

IV.2.2 Retraction of PBAT drop in PLA as example

The retraction of a PBAT drop in the matrix of PLA after stopping shear is taken as an example in Fig. IV.7:

- 84 -

Fig. IV.7 Retraction of PBAT drop in PLA after shear cessation

The evolution of the drop deformation versus time during relaxation is plotted in a semi-logarithmic scale in Fig. IV.8. Using the equation given by Taylor [Taylor, 1934] (Eq. 28) to fit the data, a value of interfacial tension of 3 mN/m is obtained in the case of PLA/PBAT. Measurements were systematically performed on five droplets in order to have an average value. The measurement results by this method are summarized in Table IV.2.

IV.2.3 Influence of molecular weight and inversed matrix on interfacial tensions on PLA/PBAT

We took advantage of the degradation of PBAT with time (see paragraph III.1) to study the effect of the PBAT molecular weight on interfacial tension. To examine this effect, four series of drop retraction tests were conducted on the pairs PLA/PBAT: PBAT 2011 and PBAT 2007 respectively dispersed in PLA, and PLA dispersed in two matrices PBAT 2011 and PBAT 2007. The results of interfacial tensions are shown in Table IV.1:

Fig. IV.8 Deformation versus time during the retraction of PBAT drop in PLA 100 m

- 85 -

PBAT 2007 in PLA 3 ± 0.4 mN/m PBAT 2011 in PLA 3.3 ± 0.7 mN/m PLA in PBAT 2007 3.6 ± 0.8 mN/m PLA in PBAT 2011 3.4 ± 0.8 mN/m

Table IV.1 Interfacial tensions between PLA and the batches of PBAT

Although some literature [Anastasiadis et al., 1988] claimed that lower molecular weight leads to a lower interfacial tension, the results in Table IV.1 indicate that there is almost no effect from this parameter on the value of interfacial tensions.

To avoid any possible confusion in this chapter, “PBAT” mentioned without an indication of batch refers to PBAT 2007.

IV.2.4 Drop deformation along the vorticity axis of PA in PBAT

During the characterization of interfacial tension between PA and PBAT when the drop retraction test was conducted with a PA drop in the PBAT matrix, an unexpected phenomenon was observed while imposing the deformation of PA drops: In Fig. IV.9, under a shear in horizontal direction as indicated in the figure, the direction of drop deformation was observed to be perpendicular to the direction of shear.

Fig. IV.9 Drop retraction of a PA drop in PBAT, drop deformed along the vorticity axis. The

arrow indicates the shear direction

Mighri et al. [Mighri et al., 2002, 2005] conducted an extensive study on the phenomena and concluded that this vorticity deformation takes place when the elasticity of the dispersed phase is two times larger than that of the matrix at high shear rates with a quasi- Newtonian rheological behaviour of the matrix. From the rheological curves of PBAT(2007) and PA (Fig. III.1), it is found that both conditions are satisfied since the elastic modulus of PA is higher than that of PBAT over a large frequency range, and the viscosity of PBAT is constant and around 200 Pa.s over the whole frequency range.

t=0 s t=38 s t=60 s t=122 s

- 86 -

IV.2.5 Influence of angle correction on the result of interfacial tension

As mentioned in the experimental part, during the characterization of interfacial tension using the drop retraction after a shear, the direction of the major axis of the drop has a 45° deviation from the shear direction. Under this circumstance, the observed length of the major axis is not the true long axis. Then it is interesting to know how large the difference between the observed length and the true length is, since it can affect the calculated interfacial tension (see Fig. III.6). The measurement of retraction of PBAT drops in the PLA matrix was taken as an example here to show the difference with and without correction in the following table:

Interfacial tension without angle correction (mN/m)

Interfacial tension with angle correction (mN/m) Drop 1 2.7 2.6 Drop 2 2.8 2.6 Drop 3 3.3 3 Drop 4 3.2 3.1 Drop 5 3.7 3.4 Average value 3.1 3

Table IV.2 Interfacial tensions between PLA and PBAT with and without the angle

correction

From Table IV.2, the interfacial tension obtained with correction is always slightly smaller than the case without correction, although the difference between them is not very significant. It is understandable that both measurements give close results since the drop deformation is very small.