To establish whether the different PBS samples may be miscible with the organic semiconductor P3HT, the Hansen Solubility Parameters (HSP) of constituents were estimated. HSP is used extensively for polymer processability to understand and optimise polymer blends to suit different applications. To start with, the dispersive (δD), polar (δP) and hydrogen bonding (δH) parameters along with the sphere radius (R0) of each constituent needs to be established to define its solubility space. In-order to do this the solvent gradient method was used, whereby using a ratio of 'good' and 'bad' solvents is used to create a library of solvent mixtures. Then, under controlled dissolving conditions, a 1 or 0 is attributed depending if the material dissolves or does not dissolve, respectively, in each solvent mixture. The resulting HSP parameters and radius are then solved by attempting to optimise the fit to 100 % using the method laid out by Díaz de los Ríos et al. Chlorobenzene was chosen as the 'good' and acetonitrile, methanol, hexane and triethylamine as the 'bad' solvents in ratios of good:bad of 5:0, 4:1, 3:2, 2:3, 1:4 and 0:5. The solubility results are shown in the supporting information (Table -S5), however the R0 outcomes of P3HT and the two different polyborosiloxane elastomers are shown below (Figure ). The sphere created from R0 of P3HT is small (R0 = 1.4) signifying that it is insoluble in most solvents, as any solvent with HSP value inside the sphere should dissolve the polymer. Organic semiconducting polymers, including P3HT, are notoriously insoluble in most non-aromatic and non-chlorinated organic solvents due to the planarity and the resulting strong π-stacking interactions of the polyaromatic backbone. On the other hand, LPBS and HPBS have larger spheres (R0 = 9.4 & 5.4 respectively) reflecting their higher solubility in numerous solvents. Furthermore, LPBS's sphere is much larger than HPBS's, due to a large contribution from δH suggesting LPBS has strong hydrogen bonding interactions however, in the context of this system the interactions are more likely to arise from lone pair donation from an oxygen to an empty p-orbital on the boron. To estimate the compatibility between P3HT and PBS elastomer, the vector distance between two HSP values (Ra) can also be calculated, where a smaller Ra indicates better miscibility between each constituent. As seen in Figure below HPBS:P3HT, has a much smaller Ra than LPBS:P3HT suggesting that they are more miscible. The relative energy density (RED) can also be calculated via the ratio between Ra of the blend and the R0 of the PBS elastomer where RED < 1 is an indication that the two materials in the blend are deemed miscible. For HPBS:P3HT, RED = 0.76 and LPBS:P3HT, RED = 0.86 which shows that P3HT may blend well with each elastomer. However the lower RED value of HPBS:P3HT further indicates a better miscibility than LPBS:P3HT. To initially investigate the miscibility between P3HT and the two elastomers, solution UV-vis absorbance spectroscopy was employed by dissolving the materials in chlorobenzene (Figure ). While keeping the concentration of P3HT in chlorobenzene constant at 100 μg mL -1 , the amount of PBS was increased to observe any aggregation of P3HT, which would suggest immiscibility. The spectra of pure P3HT in chlorobenzene showed a single peak with a maximum absorbance at 458 nm. However, as the concentration of LPBS or HPBS was increased a new absorption band appeared around 600 nm. The HSP experiments revealed that LPBS does not easily dissolve in chlorobenzene at room temperature, which subsequently results in an increase in the background scattering with increasing LPBS concentration. In line with previous studies, the low energy shoulder at 600 nm is related to P3HT aggregates forming in solution. The intensity of the shoulder for LPBS at 99.9 mg mL -1 is much higher than that of HPBS at the same concentration (0.29 vs 0.04) which indicates that LPBS induces higher P3HT aggregation than HPBS. Agreeing with the lower RED value for HPBS:P3HT than LPBS:P3HT from the HSP experiments, the solution UV-vis absorbance results suggest a lower miscibility for P3HT with LPBS manifesting itself in P3HT aggregation. The HSP and solution UV-vis absorbance results suggest HPBS and P3HT are more compatible than LPBS and P3HT and therefore to assess the effects of compatibility the thermal and mechanical characteristics of the blends were measured. To accomplish this, P3HT and PBS were blended in chlorobenzene with increasing P3HT concentration (0.1, 0.5, 1, 5 and 10 wt% in PBS). The complete methodology is detailed in the supporting information, but briefly, the two components were dissolved separately in chlorobenzene, combined, and stirred at 80 °C overnight. The chlorobenzene was removed slowly under reduced pressure by increasing the temperature from 20 °C to 120 °C. Upon examining the resulting solid, samples with more than 5 wt% of P3HT had formed inhomogeneous blends in which the P3HT was visibly phase-separated from the elastomer (see Figure ).