Interfacial Confinement and Coalescence

In all the systems examined in this study a third phase is situated between two others. Clearly this spatial limitation of the intermediate phase imposes a form of confinement. These systems thus present the very interesting possibility of studying the influence of interfacial confinement on coalescence for both the partially wet droplets and the completely wet layers observed in this work.

Partially wet confined droplets. Figure 5.7 compares the dependence of the PE number average

droplet size on composition for the confined droplets showing strong partial wetting in PLA/PE/PBAT and that of the corresponding two binary systems PLA/PE and PBAT/PE. These data are obtained immediately after melt mixing and without any annealing whatsoever. At 5% PE, the ternary system already demonstrates a much higher phase size of 3.5 µm as compared to 0.6 and 0.8 µm for the two binary systems respectively. The slope of the ternary curve at 0.22 is also significantly greater than that of the binary systems, 0.02 (PLA/PE) and 0.03 (PBAT/PE). Thus, as

Figure 5.7. PE number-average droplet size as a function of composition comparing the confined partially wet droplets in the ternary blends with that for dispersed PE droplets in binary blends.

composition increases, the effects of coalescence for the confined PE droplets in the ternary system become even more marked. This is a clear and quantitative indication of the important effects of interfacial confinement on coalescence for a strong partially wet system.

The enhanced coalescence for partially wet confined droplets is even more evident when examining the systems after static annealing. The diagrams in Figure 5.8 demonstrate the third power of number average droplet size as a function of annealing time in which all of them follow the relationship of 𝑅‡_{~𝑘𝑡 in which k is the coalescence rate i.e. particle growth rate. The results of the}

interfacial presence (%) and coalescence rate of the partially wet systems are presented in Table 5.5. It was found that more than 90% of the PE phase with the strong partial wetting tendency is located at the interface of PLA/PBAT while about 73% of PBAT and 75% of PHBV with the weak partial wetting tendency were present at the interfaces of PLA/PE and PLA/PBS, respectively, right after the melt mixing process.

The growth rates of these strong and weak partially wet droplets with annealing time were compared to the growth rates reported for binary matrix/dispersed systems reported in the literature (see Table 5.5). It can be seen that the interfacial growth rates of the partially wet systems, either with weak or strong partial wetting tendencies, are much higher than those of binary systems. It has been reported in the literature that geometrical confinement can significantly increase the

Figure 5.8. Diagrams of the third power of the number average diameter (d3) as a function of annealing time: a) 5% PHBV in PLA/PHBV/PBS, 5% PBAT in PLA/PBAT/PE and 5% PE in PLA/PE/PBAT; b) 5, 10 and 20% PE in PLA/PE/PBAT.

coalescence of droplets [43,44]. The growth rate of the strong partially wet PE droplets at the interface of PLA/PBAT during annealing dramatically increases with composition which further confirms the strong influence of interfacial confinement on the morphology development at the interface.

Comparing the 5% data in Table 5.5, it can be seen that significant differences in the coalescence rate, k, with annealing time can be observed. Firstly, the strong partially wet PE droplets show a much lower rate of coalescence (0.5 µm3_{/min) than the weak partially wet systems PHBV (1.8}

µm3_{/min) and PBAT (13.2 µm}3_{/min). This significant difference is likely due to a high dewetting}

rate for the strong partially wet PE droplets and this will be discussed in more detail later in the paper. In the case of the two weak partially wet droplets, it should also be noted that the coalescence rate for 5% PBAT droplets is significantly higher than that of 5% PHBV.

The significant differences in the coarsening rate during annealing between 5% PBAT and 5% PHBV can be attributed, in part, to the differences in the coarsening rate of the supporting co- continuous phases. In the absence of any external forces, Brownian motion is usually held

Table 5.5. Interfacial concentration and particle growth rates of partially wet droplets at the interface during annealing. Comparison of growth rates with binary systems.

Ternary systems with

PHBV 5% in PLA/PHBV/PBS (weak partial wetting) PBAT 5% in PLA/PBAT/PE (weak partial wetting) PE 5% in PLA/PE/PBAT (strong partial wetting) PE 10% in PLA/PE/PBAT (strong partial wetting) PE 20% in PLA/PE/PBAT (strong partial wetting) PW phase (%) at the interface 75 73 95 93 92

Particle growth rate k

(µm3/min) 1.8 ± 0.1 13.2 ± 2.1 0.5 ± 0.06 6.2 ± 0.6 56.0 ± 5.8

Supporting phases (50/50) growth rate k’ (µm/min)

PLA/PBS PLA/PE PLA/PBAT

1.5±0.5 17.2±1.4 2.1±0.5

Binary systems with 10-20 wt% typically show particle growth rates k (µm3_/min)

in the range of 10—ƒ_{− 1.0*}

responsible to cause contact between droplets in a binary dispersed/matrix system, but its effect is negligible due to high melt viscosity in polymer blends [28]. It is believed that the internal flow caused by the coarsening of the two co-continuous phases induces the force required for contact of the partially wet droplets confined at the interface [15,20]. The capillary flow inside continuous channels creates a mobile interface, and consequently the confined partially wet droplets move towards each other. It has been reported that the higher the interfacial tension in co-continuous systems, the faster the coarsening and consequently the higher the contact probability [45–47]. The growth rate 𝑘= _{of co-continuous PLA/PBS and PLA/PE systems were measured to be 1.5±0.5}

µm/min and 17.2±1.4 µm/min, respectively. These results correlate well with the coalescence rates of the PHBV and PBAT droplets in Table 5.5 and imply that although they both have weak partial wetting behavior, the higher growth rate of the supporting PLA/PE phases contributes to the significantly higher coalescence rate of PBAT, i.e. 13.2±2.1 µm3_{/min, while the lower growth rate}

of the supporting PLA/PBS phases only results in a PHBV particle coalescence rate of 1.8±0.1 µm3_/min.

Completely wet confined layer. In this part of the paper, the growth rate of the completely wet

intermediate layer as a function of annealing time will be examined. In this case the coarsening rate, 𝑘=_{, is estimated by following the growth of the thickness of the layer in the ternary systems}

using an approach to study the coarsening of co-continuous systems studied previously [46]. In addition, the coarsening rate 𝑘= _{of the middle phase in the ternary systems is compared to those of}

the constituent binary blends of the two ternary (PLA/PHBV/PBS and PLA/PBAT/PE) systems (see Table 5.6).

The growth rate of the completely wet intermediate layer comprised of 20% PHBV and 33% PHBV in PLA/PHBV/PBS is considerably higher than the coarsening rates estimated for the constituent co-continuous binary systems, PLA/PHBV (0.4 µm/min) and PHBV/PBS (0.6 µm/min) (see Figure 5.3b) which appears to indicate that a confined completely wet layer in ternary systems also demonstrates enhanced coalescence as was observed for the partially wet case. However, in contrast, the coarsening rate of the PBAT intermediate layer in PLA/PBAT/PE has a significantly lower coarsening rate than that for the constituent binary co-continuous systems of PLA/PBAT and PBAT/PE. Understanding this latter behavior will require further work.