**3. Results**

## *3.1. Thermal and Chemical Analysis*

DSC curves illustrating the thermal characteristics of the base polymers and blends are shown in Figure 2 and quantified in Table 2. For the base materials investigated preand post-extrusion (Figure 2a,b), there is a notable difference for PLLA and a subtle change for PCL post-extrusion in comparison to pre-extrusion. After extrusion, PLLA presents a crystallisation peak, along with lower Tg and Tm peaks. In contrast, the melting peak of PCL does not appear to be affected by the extrusion process. PHBV pre-extrusion shows two melt peaks (Figure 2c) which relate to the two constituents of the polymer, PHB and HV, with PHB representative of the higher melting peak. Tg and Tm remain close to that of PLLA in all blends, but the location of the peak is modified by parts of the blends becoming more mobile at lower temperatures.

The two-polymer blend profiles are illustrated in Figure 2d,e. Incorporation of 30% PCL in a blend with PLLA eliminates the crystallisation process for PLLA, whereas it remains present with PHBV incorporation. The step change glass transition for PLLA is accentuated with the incorporation of PCL (Figure 2d). Here, we see the fusion of the Tg of PLLA together with the Tm of PCL at 68 ◦C. For both two-polymer blends, the Tg and Tm are similar to those for PLLA post-extrusion. The three-polymer blend profiles are shown in Figure 2f,g, and there is a clear change in the glass transition and crystallisation peaks when more PCL/PHBV is incorporated into PLLA. The melting temperatures of the three-component blends remain in a 4 ◦C range of the PLLA post-extrusion Tm.

**Figure 2.** DSC curves (**a**) PLLA, (**b**) PCL, (**c**) PHBV, (**d**) PLLA/PCL (70:30), (**e**) PLLA/PHBV (85:15), (**f**) PLLA/PCL/PHBV (80:10:10) and (**g**) PLLA/PCL/PHBV (90:5:5).

**Table 2.** Thermal characteristics and crystallinity. *omr* indicates effect outside the measurement range; *nm* indicates effect not measurable. Results for blended materials are all post-extrusion. Δ*Hcc* for samples which did not show a cold crystallinity peak assumed to be 0. PLLA and PHBV cold crystallisation peaks or melt peaks closely align, so a single value is reported. PCL melting peaks were distinct, and so the blends containing PCL values are reported separately for PCL and the other components of the blend.


Table 2 also details the melt enthalpy ( Δ *Hm*) values for the polymers and the cold crystallisation enthalpy ( Δ *Hcc*), which allows the degree of crystallisation to be estimated. The effect of thermal processing on PLLA and PCL can be seen from the decline in material crystallinity from pre to post-extrusion, with decreases of 49.4% and 8.9% for PLLA and PCL, respectively. PHBV has the lowest melt enthalpy of all the polymers. For PLLA/PCL (70:30), the melting peak at ~178 ◦C is reduced by PCL incorporation into PLLA, lowering the melt enthalpy by 26.39 J/g. The melt enthalpy for PLLA/PHBV (85:15) is not as significantly affected by PHBV incorporation (4.12 J/g enthalpy reduction). The material crystallinity of PLLA/PCL is impacted when the two polymers are combined; however, the crystallinity of the PLLA/PHBV (85:15) blend is improved when combining these two polymers.

Figure 3 shows the ATR-FTIR spectra, which have been normalised and baseline corrected. All the spectra display similar profiles. Peaks at ~2920 and ~1720 cm<sup>−</sup><sup>1</sup> are characteristic peaks of C–H bonds and C=O carbonyl bonds, respectively. The peak located at ~1270 cm<sup>−</sup><sup>1</sup> represents the C–O saturated ester bonds [36]. PLLA shows an increase in the intensity of the peaks at ~1200 (C–C), ~1094 (C–O), ~1750 (C=O), ~1079–1189 (CH3) and ~1184/1088 (C–O–C) after extrusion [16,37–39]. With PCL, there is an increase in intensity of the fingerprint region (500–1500 cm<sup>−</sup>1) post-extrusion. PLLA, PCL and PHBV all have ATR-FTIR profiles that are indicative of them being aliphatic polyesters.

**Figure 3.** ATR-FTIR spectra for (**a**) PLLA, (**b**) PCL and (**c**) PHBV; the black lines are indicative of pre-extrusion and red lines of post-extrusion. Vertical lines indicate key wavelengths for ease of interpretation.

## *3.2. Mechanical Properties*

The results of the tensile tests on extruded filaments are summarised in Figure 4 and Table 3. PCL, when tested as a single polymer, displays the anticipated low modulus and high elongation behaviour. In the binary blend with PLLA, the inclusion of PCL reduces the average modulus and yield stress compared to PLLA alone but increases elongation at break. The addition of PHBV to PLLA slightly increases the average modulus and yield stress compared to PLLA alone, but also increases the elongation at break.

**Figure 4.** Representative load–displacement responses from filament tensile testing.



For the ternary blends, small amounts of PHBV increase the average modulus and yield stress, but as the amount of PCL increases, the effect of PCL on lowering the average modulus and yield stress also increases. For the ternary blends, the effect on the elongation at break is variable, but the average elongation at break achieved is always greater than that shown by PLLA alone.

Figure 5 illustrates curves for material strain under constant load with respect to time for each of the polymers and blends at room temperature (22 ◦C), and Table 4 shows the steady state creep rate for the materials. For PCL, the strain rate is very high (the maximum test displacement of 500 mm was reached in 3000 s), so no creep rate was calculated for this material. For all other materials, the absolute strain remains below 3% over the course of the test. For the PLLA/PCL/PHBV 80:10:10 and PLLA/PHBV 85:15 blends, there is a sharper increase in strain over time in comparison to the other materials. PLLA is the polymer with the lowest deformation rate. The deformation rate increases when PCL and PHBV are added to the PLLA matrix, and the PLLA/PCL/PHBV (90/5/5) blend has the most comparable strain relationship with time to PLLA.

**Figure 5.** Representative time–strain curves for polymers and blend filaments under a 40 N constant load.

**Table 4.** Average creep rate at room temperature under a 40 N constant load, and measured viscosity at 210 ◦C.


For the binary blends, the viscosity of the materials (Table 4) broadly follows the trends indicated by the modulus and strength: blending PHBV with PLLA increases the viscosity, while blending with PCL reduces the viscosity. In the ternary blends, the effect of the PCL dominates, giving a reduced viscosity compared to PLLA alone. All of the materials show a reduction in viscosity with increasing frequency.
