*3.4. Mechanical Properties of Latex and Composite Films*

The mechanical properties of NRL and composite films containing NRL and varying concentrations of NOCNF is shown in Table 2. The stress–strain curve was plotted for all the composite films and is presented in Figure 6. The stress on the y-axis was calculated by dividing the load by the cross-sectional area of the film. The thickness and length of the film is measured using a caliper and all nanocomposites showed the same thickness of 0.08 cm2. There could be a slight variation of thickness due to the increased amount of NOCNF added, but the difference would be negligible since the NRL solid amount contributes to most of the sample's volume. The stress–strain curves for the NRL and composite films also indicates their ultimate tensile strength (UTS).



**Figure 6.** Stress–strain curves on (**i**) NRL film (control), and composite films made of NRL and NOCNF with varying concentration of NOCNF (**ii**) 0.1 wt.%, (**iii**) 0.2 wt.%, (**iv**) 0.4 wt.%.

It was found that the NRL film exhibited the UTS value of 0.77 Mpa. However, the addition of NOCNF increased the UTS value of the film quite notably (e.g., the films with 0.1, 0.2 and 0.4 wt. % of NOCNF showed the UTS value of 2.5, 5.2 and 6.2 MPa, respectively). We note that the ultimate tensile strength reported for the pure NOCNF extracted from jute fibers using the nitro-oxidation method was 108 MPa [31] and the chemically crosslinked NRL film typically exhibited the UTS value of 27 Mpa [39]. In the above study [39], vulcanized (chemically crosslinked) NRL was reinforced by addition of cellulose nanocrystals (CNC) where the 3 wt. % of CNC was found to be most effective to increase the ultimate tensile strength of NRL (by about 29%). In this study, the authors have used the term cellulose nanofibers to describe nanocellulose isolated from coconut spathe using the acid hydrolysis method. We believe that the length of such nanocellulose particles extracted by the acid hydrolysis approach should be shorter, the cross-sectional dimensions larger and the crystallinity higher than those in NOCNF, and they should be termed CNC. It is interesting to note that the overlap concentration of CNC usually varies between 1.5 wt. % to 3 wt. %, depending on the source of the biomass [40]. We hypothesize the ultimate tensile strength of vulcanized CNC-NRL also takes place near the overlap concentration of CNC.

On the other side, the maximum elongation (λ*max*, %) observed in the tested films showed an opposite trend. For example, the pure NRL film exhibited λ*max* at about 234 %, where 0.1 wt.% of NOCNF in the composite film decreased the λ*max* value to 31.4 %. The increase in the NOCNF content further decreased the λ*max* value (e.g., 0.2 and 0.4 wt. % of NOCNF films showed the λ*max* value of around 2.5% and 3.5%), rendering the films to be quite brittle. These results were consistent with the SEM images (Figure 5), which showed that the addition of NOCNF decreased the roughness of the NRL film and increased the content of cracks owing to phase separation between NOCNF and NRL.

The ductile–brittle transition was also noticeable from the Young's modulus (Ym) evaluation shown in Figure 7. The pure noncrosslinked NRL was very ductile, showing an Ym value of merely 3.5 KPa, where 0.1 wt. % of NOCNF addition increased the Ym value to 79.6 KPa and 0.2 wt. % of NOCNF addition increased the Ym value maximum to 2080 kPa. The further increase of NOCNF content decreased the Ym value to 1770 kPa. As a result, the higher NOCNF content would not lead to any property enhancement, where the best content of NOCNF addition appeared to occur near its overlap concentration (0.2 wt. %).

**Figure 7.** Graph represents the relationship between the Young's modulus (kPa) and the NOCNF concentration in the composite films.

## **4. Conclusions**

This study showed that the nitro-oxidized carboxycellulose nanofibers (NOCNF) extracted from raw jute fibers could be incorporated into the NRL matrix to increase the mechanical properties even in a nonvulcanized state. The optimal amount of the NOCNF for the overall property improvement seems to take place around the overlap concentration of NOCNF (around 0.2 wt. %). This is not surprising as the overlap concentration of nanocellulose represents the transition point from a viscous state to a gel state. Addition of NOCNF into non-vulcanized rubber has changed the NRL film from elastic to brittle. The more schematic study on latex composite preparation can explore the use of these inexpensive and sustainable nanofibers material into the preparation of other rubber-based (e.g., Guayule) composite materials.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/4/706/s1.

**Author Contributions:** Methodology, H.C. and K.J.; software, S.L.; validation, P.R.S., S.K.S. and B.S.H.; formal analysis, R.W. and W.B.; data curation, C.Z.; writing—original draft preparation, S.K.S. and S.L.; writing—review and editing, P.R.S. and B.S.H.; supervision, P.R.S., S.K.S. and B.S.H.; funding acquisition, B.S.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** The financial support for this work was provided by a grant from the Polymer Program of the Division of Materials Science of the National Science Foundation, United States (DMR-1808690).

**Acknowledgments:** Authors would like to acknowledge ThINC facility at AERTC, Stony Brook University for the AFM, TEM, DMA, SEM characterizations of the samples.

**Conflicts of Interest:** The authors declare no conflict of interest.
