## 3 Results and Discussion
### Spherulitic Morphology and Crystal Growth of P3 HBHHx Samples
#### 3.4.3 Crystal Growth Rates Determined from POM  
The crystal growth rates (\(G\)) of neat P3 HBHHx and the nucleated samples at different crystallization temperatures were determined using polarized optical microscopy (POM) by measuring the increase in the spherulite radius over time. The slopes obtained from the linear correlations of the radius versus crystallization time (Figure 17) were used to calculate the growth rates, which are summarized in Table 8.  
For neat P3 HBHHx, the maximum growth rate was observed at 60 \({}^{\circ}\)C, with a \(G\) value of 0.57 \(\upmu\)m/s, suggesting that this temperature offers an optimal balance between the polymer chain mobility and the efficiency of secondary nucleation. In contrast, the nucleated samples exhibited maximum growth rates at higher temperatures, around 80 \({}^{\circ}\)C. This shift indicates that the presence of nucleating agents enhances the nucleation efficiency, allowing effective crystal growth to occur at elevated temperatures compared to the neat polymer.  
Figure 17: Spheruilitic crystal growth at different crystallization temperatures for (**a**) P3 HBHHx, (**b**) PHH_05 BN, (**c**) PHH_05 PHB, and (**d**) PHH_2 Talc.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & **P3 HBHHx** & \(G\) (\(\upmu\)m/s)** & **Induction Time (s)** \\ \hline \multirow{3}{*}{P3 HBHHx} & 50 & 0.23 & 1 \\  & 60 & 0.57 & 45 \\  & 65 & 0.49 & 50 \\ \hline \multirow{3}{*}{PHH\_05 BN} & 80 & 0.71 & 1 \\  & 90 & 0.65 & 71 \\  & 100 & 0.48 & 150 \\ \hline \multirow{3}{*}{PHH\_05 PHB} & 80 & 0.76 & 1 \\  & 90 & 0.73 & 22 \\ \cline{1-1}  & 100 & 0.50 & 80 \\ \hline \multirow{3}{*}{PHH\_2 Talc} & 80 & 0.87 & 1 \\  & 90 & 0.79 & 75 \\ \cline{1-1}  & 100 & 0.43 & 100 \\ \hline \end{tabular}
\end{table}
Table 8: Summarized data of the growth rate of P3 HBHHx and nucleated samples at different temperatures, determined from the slope of the linear correlation of the crystal radium increase with time.  
Among the tested formulations, PHH_2 Talc exhibited the highest crystal growth rate, reaching 0.87 um/s at 80 \({}^{\circ}\)C. Interestingly, this result differs from the trends observed in the DSC-based crystallization data, where PHH_05 BN exhibited the fastest overall crystallization rate. This discrepancy demonstrates the different aspects of crystallization captured by the POM and DSC techniques. In DSC, the measured crystallization rate reflects a combination of primary nucleation (formation of stable nuclei) and crystal growth from existing nuclei. In contrast, POM focuses solely on crystal growth at each temperature, which is directly related to secondary nucleation mechanisms rather than initial nucleation events.  
The superior growth rate of PHH_2 Talc can be attributed to several factors, including the following: Although talc is a less potent nucleator than BN or PHB, its moderate nucleation density at 2 wt% (Figure 16) allows more space for each spherulite to grow, reducing early impingement and enabling faster radial expansion. Furthermore, the well-dispersed lamellar structure of talc may facilitate lateral chain alignment, thereby supporting faster crystal growth once nucleation has occurred. Therefore, although BN excels at initiating nucleation, talc is more effective at promoting rapid crystal expansion under isothermal conditions.  
These findings suggest that while boron nitride excels at initiating crystallization (primary nucleation), talc is more effective at promoting crystal growth (secondary nucleation) once the nuclei are present. As a result, talc exhibited a higher growth rate in POM, but boron nitride remained superior in accelerating the overall crystallization process, as captured by DSC.