# Lactic Acid Modified Natural Rubber-Bacterial Cellulose Composites
## 4 Discussion
Hydrogen bonding led to a decrease in polymer mobility, resulting in increased T\({}_{\text{m}}\). Significant improvement in mechanical strength of the composite films was observed by acid modification. In general, pure NR has a low Young's modulus and tensile strength, but a high elongation at break. Conversely, BC has a high Young's modulus and tensile strength, but a low elongation at break. The Young's modulus and tensile strength of both NRBC and ANRBC films were enhanced with an increase in the BC fiber content [4]. Maximal Young's modulus and tensile strength were obtained with BC loading at 80 wt.%. Moreover ANRBC80 showed a higher modulus and tensile strength than those of NRBC80, which was probably due to the OH- inter- and intra-molecular interaction obtained by lactic acid modification. Acid modification increased the rigidity of the composites because the cellulose whiskers with hydrogen bond forming reduced the brittleness of the composite backbone. The enhanced rigidity was possibly due to the increased crystallinity of the ANRBC80, as previously observed from the XRD result (in Section 3.5). ANRBC80 had the highest tensile strength, which was approximately 209 times and 1.6 times that of the NR and NRBC80 films, respectively. In addition, with lactic acid modification, the elongation at break of ANRBC20 was improved to \(\approx\)1.3 of NRBC20. The acid modified films of ANRBC showed significant improvement in solvent resistances. The interaction between the BC fibers and the NR chains in the NR matrix limited the absorption of the non-polar solvent of the composite films. The strong interface between the BC fibers and the NR chains constrained the swelling of the polymeric chains located in the interfacial zone [25; 26]. In addition, the BC fibers provided a tortuous path for toluene because of its polarity difference. The decrease in solvent uptake in the composites was probably due to the increase in the tortuous path and the reduction in the transport area [27]. Furthermore, the ability of the composites to swell in toluene was also due to the behavior of the filler itself. BC and most of its derivatives were insoluble in non-polar solvents [28]; therefore, the addition of BC improved the non-polar resistance of the composites. Because of acid modification, ANRBC composite films had less swelling and were not dissolved in either the non-polar or polar solvents. Under 20 wt.% BC loading, ANRBC composites swelled in toluene less than did the NRBC composites. Acid modification partially restricted the swelling of the composites in non-polar solvent. The hydrogen bond probably induced intra- and inter-species interactions within the composite films. Structure and molecular packing behavior of the composites with high molecular packing density swelled less than those with loose molecular packing. Furthermore, hydrogen bond formation improved the interfacial interactions and subsequently restricted the diffusion of toluene molecules in the vicinity of the cellulosic surface. However, there was no significant improvement in toluene resistance between ANRBC50 and NRBC50 or ANRBC80 and NRBC80. Therefore, at high BC loading, the effect of acid modification might be inferior to the effect of interfacial polar and non-polar interactions between cellulose and toluene. On the other hand, the increase in BC loading in the composite films enhanced the degree of swelling in water, which was due to the hydrophilicity of the BC fibers. This can be attributed to the increasing presence of OH groups in the composite films. However, the hydrophilization of the NR chains increased their sensitivity to water [22]. In addition, the presence of natural fibers in the compound disturbs the structural homogeneity in the material, which produces voids at the interface and may increase the ability of water molecules to penetrate the composite through capillary transport. The compatibility between the phases should lead to a decrease of voids in the composites [29].