## 3 Results and Discussion
between fibers and polymers when the hydrophilic group (hydroxyl group) of fibers was substituted by the hydrophobic (acetyl) group. The second reason was that the composites contain acetylated kenaf that were more thermally stable, as discussed in the preceding paragraph. These results are directly related to changes in the thermal decomposition properties of the preceding kenaf (Figure 6). Since the initial thermal decomposition temperature of the acetylated kenaf greatly increases, the initial thermal decomposition temperature of the composite material also increases accordingly. Figure 8 shows the storage moduli of kenaf-PLA composite as a function of temperature based on dynamic mechanical analysis (DMA) The storage moduli of the untreated kenaf-PLA composite was higher than PLA composites containing briefly (0.5 h and 1 h) acetylated kenaf. The reason was due to poor interfacial bonding, as suggested by an examination of the fractured surfaces of the composites (Figure 5) and as reported in another study [31]. On the other hand, the storage moduli of PLA composites containing kenaf acetylated for 2 h and 3 h were higher than for the untreated kenaf-PLA composite. This behavior was due to the improvement of interfacial adhesion, as concluded from fractured surface morphology studies of the composites. However, in the high-temperature region (over 60 \({}^{\circ}\)C), the lowest modulus is obtained when using kenaf without acetylation treatment. The modulus of PLA rapidly decreases after the glass transition temperature (T\({}_{\text{g}}\)) region, and the modulus of elasticity changes abruptly, according to the degree of interaction with kenaf. As acetylation treatment strengthens the interaction with PLA, the decrease in elastic modulus becomes smaller. Figure 9 indicates damping ratio of the loss modulus/storage modulus. The damping ratio was lower for PLA composites containing kenaf acetylated for 2 h or longer. This observation can be interpreted as lower mobility of polymer chains attributable to the improved interfacial adhesion. However, glass transitions of all the composites were not markedly altered. Figure 8: Storage modulus (E’) of untreated and acetylated kenaf-filled PLA composites. (**a**) Log scale; (**b**) Linear scale. Figure 7: Thermogravimetric curves for PLA composites with untreated and acetylated kenaf. The hygroscopicity of composites was evaluated using a water immersion test conducted at room temperature. Figure 10 shows changes in water uptake for untreated and acetylated kenaf-PLA composites. All of the composites showed rapid water absorption on the first day, but the water uptake eventually reached a plateau state, similar to that observed by [PERSON] et al. [32,33]. Since the water absorption rate of PLA is less than 1%, the overall result can be interpreted as a result of water absorption of kenaf or water absorption at the interface of PLA/kenaf [34]. PLA composites of acetylated fibers showed a lower rate and extent of water uptake when compared to composites of untreated fibers. The reduced affinity of the composites to water was due to the replacement of sorption sites (hydroxyl groups) of the constituting kenaf with acetyl groups. On the other hand, untreated kenaf-PLA composites showed faster and more absorption of water, and this was due to relatively greater hydroxyl availability of the untreated kenaf fibers, and poor interfaces (affinity) between fibers and PLA.