Poly(butylene terephthalate) (PBT)/multiwalled carbon nanotube (MWCNT) composites were prepared using a simple and reliable melt mixing technique to understand how the addition of nanotubes affects the crystallization behavior and thermal properties of PBT. The non-isothermal crystallization process was studied using differential scanning calorimetry (DSC), while hot-stage polarized optical microscopy (HSPOM) was used to observe changes in microstructure at different cooling rates.

From a practical point of view, adding MWCNTs changes how PBT crystallizes. As the nanotube content increases, the viscosity of the system also increases, which restricts the movement of PBT chains and makes crystallization more difficult. Kinetic analysis shows that the basic crystallization mechanism remains similar, but the rate is influenced by the presence of MWCNTs. For pure PBT, the modified Avrami exponent (n) is 3.0 at a cooling rate of 15 °C min⁻¹ and increases to 3.7 at 30 °C min⁻¹. When 1 wt% MWCNT is added, the values slightly decrease to 2.9 and 3.1 at 15 °C min⁻¹ and 30 °C min⁻¹, respectively, indicating heterogeneous nucleation with minor changes in growth behavior. Kissinger analysis also suggests that higher energy is required for crystallization after adding MWCNTs.

Another important observation is the reduction in the crystal growth parameter after adding MWCNTs. It decreases from 6.1× 10⁴ for pure PBT to 3. × 10⁴ for the 1 % MWCNT into PBT. This shows that although MWCNTs help in nucleation, they also act as barriers that slow down the growth of crystals.

X-ray diffraction (XRD) results confirm that the crystal structure of PBT does not change with the addition of MWCNTs, as no new peaks are observed. However, a slight increase in crystallinity is seen due to the nucleating effect of the nanotubes.

Overall, MWCNTs play a dual role in the PBT matrix: they promote nucleation while also restricting chain movement and crystal growth. This balance leads to a more controlled crystallization process, which can be useful for improving material performance in practical applications.