*3.6. Morphological Properties*

presence of a compatibilizer [42].

*3.6. Morphological Properties*  The dispersion of HNT within the rubber matrix can be assessed from the SEM images shown in Figure 9. Good dispersion of urea-treated HNT was observed with but shortened lengths of the dispersed HNT particles in the matrix are due to the destruction of HNT structure. This is in agreement with the decreased tensile strength and tear strength observed for comparatively high urea contents. Similar observations were previously reported for micro-fractured surfaces in NR composites having other fillers in the The dispersion of HNT within the rubber matrix can be assessed from the SEM images shown in Figure 9. Good dispersion of urea-treated HNT was observed with but shortened lengths of the dispersed HNT particles in the matrix are due to the destruction of HNT structure. This is in agreement with the decreased tensile strength and tear strength observed for comparatively high urea contents. Similar observations were previously reported for micro-fractured surfaces in NR composites having other fillers in the presence of a compatibilizer [42].

E20U20 0.97 ± 0.03 2.63 ± 0.03 26.87 ± 1.11 615 ± 13 35.60 ± 0.50 43.9 ± 0.7

**Figure 9.** SEM photographs of ENR/HNT composites filled with untreated and urea-treated HNT; E20 (**A**), E20U10 (**B**), E20U14 (**C**), and E20U20 (**D**). **Figure 9.** SEM photographs of ENR/HNT composites filled with untreated and urea-treated HNT; E20 (**A**), E20U10 (**B**), E20U14 (**C**), and E20U20 (**D**).

#### *3.7. Wide-Angle X-ray Scattering 3.7. Wide-Angle X-ray Scattering*

In the section on mechanical properties, the stress–strain behavior of the composites was associated with SIC. Since the nominal strain rate for tensile measurement and SIC is not similar (e.g., 0.42 s−1 and 0.042 s−1 for tensile test and WAXS, respectively). The correlation was made in the view of the stress versus crystallinity only. Previously, it was clear that the treatment of HNT by urea influenced the mechanical properties. The main factor is definitely the improved compatibility between ENR matrix and urea-treated HNT filler. The degree of crystallinity (*XC*) versus strain deformation is shown in Figure 10. Crystallinity was estimated from the areas in diffraction pattern for 200 and 120 plane reflections [43,44]. The *XC* increased with strain due to molecular chain orientation, as expected. The onset strain for SIC was determined from interception of a regression line for *Xc* as a function of strain (see Figure 11). The onset for SIC with urea-treated HNT filler was observed to be lower as a function of urea content. The interaction that takes place in the presence of urea can help to pull the surrounding molecular chains and speeds up the crystallization process. When considering the *XC* and the stress propagation from tensile measurement (see Figure 8), it is clear that the *XC* corresponded well with the stress, and the trend of the curves is also similar as urea content increased. From the stress–strain curves, it is obvious that the stress started to increase towards the urea content. This is responsible to the formation of interfacial contact, as discussed earlier. In the section on mechanical properties, the stress–strain behavior of the composites was associated with SIC. Since the nominal strain rate for tensile measurement and SIC is not similar (e.g., 0.42 s−<sup>1</sup> and 0.042 s−<sup>1</sup> for tensile test and WAXS, respectively). The correlation was made in the view of the stress versus crystallinity only. Previously, it was clear that the treatment of HNT by urea influenced the mechanical properties. The main factor is definitely the improved compatibility between ENR matrix and urea-treated HNT filler. The degree of crystallinity (*XC*) versus strain deformation is shown in Figure 10. Crystallinity was estimated from the areas in diffraction pattern for 200 and 120 plane reflections [43,44]. The *X<sup>C</sup>* increased with strain due to molecular chain orientation, as expected. The onset strain for SIC was determined from interception of a regression line for *X<sup>c</sup>* as a function of strain (see Figure 11). The onset for SIC with urea-treated HNT filler was observed to be lower as a function of urea content. The interaction that takes place in the presence of urea can help to pull the surrounding molecular chains and speeds up the crystallization process. When considering the *X<sup>C</sup>* and the stress propagation from tensile measurement (see Figure 8), it is clear that the *X<sup>C</sup>* corresponded well with the stress, and the trend of the curves is also similar as urea content increased. From the stress–strain curves, it is obvious that the stress started to increase towards the urea content. This is responsible to the formation of interfacial contact, as discussed earlier.

Earlier onset for SIC can be found usually for the filled composite. Poompradu et al. [22] reported that the lateral crystallite size was decreased, but the orientational fluctuation increased by the inclusion of filler. The lattice of the SIC changed almost linearly with the nominal stress. In addition, the degree of lattice deformation decreased with the filler content, especially in the CB-filled system. In addition to this, onset for SIC was differently observed depending on the filler's characteristics. Ozbas et al. [45] compared the SIC of graphene and CB-filled composites. They found that the onset of SIC occurs at significantly lower strains for graphene-filled NR samples compared with CB-

change in SIC was promoted by rubber–filler interaction.

change in SIC was promoted by rubber–filler interaction.

*Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 19

**Figure 10.** Strain dependence on degree of crystallinity of ENR/HNT composites filled with untreated and urea-treated HNT. **Figure 10.** Strain dependence on degree of crystallinity of ENR/HNT composites filled with untreated and urea-treated HNT. **Figure 10.** Strain dependence on degree of crystallinity of ENR/HNT composites filled with untreated and urea-treated HNT.

filled NR even at low loadings. Chenal et al. [23] further explained that the onset of SIC is ruled by the strain amplification induced by the filler presence. Moreover, additional interaction in rubber network is also responsible for either accelerating or slowing down the crystallization rate depending on rubber matrix chemical crosslink density. Candau et al. [24] together with the report of Ozbas et al. [45] further emphasized that the rubber–filler interactions may fasten the SIC at low crosslink density. This is because high crosslinking may interfere the chain orientation and results in reduction of the SIC. Therefore, the crosslink density of this composite was also reported and shown in Figure 11. It can be seen that the crosslink density observed was more or less the same over the content of urea. This is a good indication that network chain density is unchangeably involved in the development of SIC regardless of urea content. As a consequence, it can be said that the

filled NR even at low loadings. Chenal et al. [23] further explained that the onset of SIC is ruled by the strain amplification induced by the filler presence. Moreover, additional interaction in rubber network is also responsible for either accelerating or slowing down the crystallization rate depending on rubber matrix chemical crosslink density. Candau et al. [24] together with the report of Ozbas et al. [45] further emphasized that the rubber–filler interactions may fasten the SIC at low crosslink density. This is because high crosslinking may interfere the chain orientation and results in reduction of the SIC. Therefore, the crosslink density of this composite was also reported and shown in Figure 11. It can be seen that the crosslink density observed was more or less the same over the content of urea. This is a good indication that network chain density is unchangeably involved in the development of SIC regardless of urea content. As a consequence, it can be said that the

**Figure 11.** SIC onset strain (primary axis) and crosslink density of ENR/HNT composites filled with untreated and urea-treated HNT. **Figure 11.** SIC onset strain (primary axis) and crosslink density of ENR/HNT composites filled with untreated and urea-treated HNT. **Figure 11.** SIC onset strain (primary axis) and crosslink density of ENR/HNT composites filled with untreated and urea-treated HNT.

Earlier onset for SIC can be found usually for the filled composite. Poompradu et al. [22] reported that the lateral crystallite size was decreased, but the orientational fluctuation increased by the inclusion of filler. The lattice of the SIC changed almost linearly with the nominal stress. In addition, the degree of lattice deformation decreased with the filler content, especially in the CB-filled system. In addition to this, onset for SIC was differently observed depending on the filler's characteristics. Ozbas et al. [45] compared the SIC of graphene and CB-filled composites. They found that the onset of SIC occurs at significantly lower strains for graphene-filled NR samples compared with CB-filled NR even at low loadings. Chenal et al. [23] further explained that the onset of SIC is ruled by the strain amplification induced by the filler presence. Moreover, additional interaction in rubber network is also responsible for either accelerating or slowing down the crystallization rate depending on rubber matrix chemical crosslink density. Candau et al. [24] together with the report of Ozbas et al. [45] further emphasized that the rubber–filler interactions may

fasten the SIC at low crosslink density. This is because high crosslinking may interfere the chain orientation and results in reduction of the SIC. Therefore, the crosslink density of this composite was also reported and shown in Figure 11. It can be seen that the crosslink density observed was more or less the same over the content of urea. This is a good indication that network chain density is unchangeably involved in the development of SIC regardless of urea content. As a consequence, it can be said that the change in SIC was promoted by rubber–filler interactions. *Polymers* **2021**, *13*, x FOR PEER REVIEW 16 of 19

> The orientation parameter (OP) indicates indirectly the molecular chain orientation and alignment and can be calculated from the Herman equation [27,28]. The OP for the composites is shown in Figure 12. Completely oriented molecular chains would give OP value 1 [46]. Here, the OP for the composites was lesser at low strains and grew with increasing strain, confirming that stretching oriented the molecular chains. The composites filled with urea-treated HNT showed higher OP values at low strains, indicating that the modification of HNT increases rubber–filler interactions, and accordingly, more molecular chain orientation was found for composites with urea-treated HNT. The orientation parameter (OP) indicates indirectly the molecular chain orientation and alignment and can be calculated from the Herman equation [27,28]. The OP for the composites is shown in Figure 12. Completely oriented molecular chains would give OP value 1 [46]. Here, the OP for the composites was lesser at low strains and grew with increasing strain, confirming that stretching oriented the molecular chains. The composites filled with urea-treated HNT showed higher OP values at low strains, indicating that the modification of HNT increases rubber–filler interactions, and accordingly, more molecular chain orientation was found for composites with urea-treated HNT.

**Figure 12.** Strain dependence on orientation parameter of ENR/HNT composites filled with untreated and urea-treated HNT. **Figure 12.** Strain dependence on orientation parameter of ENR/HNT composites filled with untreated and urea-treated HNT.

From these results, correlation between the SIC and corresponding interaction between ENR and HNT in the presence of urea is represented in a schematic model shown in Figure 13. Referring to the scheme, nothing happened at the un-stretched stage of the sample; the ENR matrix may be in contact with the HNT due to the interfacial interaction created by the unique characteristics of the HNT. When the strain was applied to the sample, crystallization of the ENR was then induced due to the stress concentration point on the HNT surfaces, and the crystallinity increased in association with the orientation of the HNT. HNT is orientated and aligned along the stretching direction. As a consequence, the ENR chains rearrange and crystallize. This always happens regardless of whether untreated or urea-treated HNT is concerned. This kind of phenomenon usually occurs in the filled composites, and it has been reported elsewhere [23,24,45]. However, it is interesting to highlight that the crystallinity of the ENR matrix increases steeply due to the collaborative crystallization of ENR and HNT in association with the contribution of the urea. Higher rubber–filler interactions as indicated by lower Payne effect is responsible for such change. The presence of the urea has played an important role in pulling the surrounding molecular chains. Thus, a significant increase in crystallization is observed at higher strains, and this is in agreement with results observed previously in the stress–strain behaviors and WAXS profiles. From these results, correlation between the SIC and corresponding interaction between ENR and HNT in the presence of urea is represented in a schematic model shown in Figure 13. Referring to the scheme, nothing happened at the un-stretched stage of the sample; the ENR matrix may be in contact with the HNT due to the interfacial interaction created by the unique characteristics of the HNT. When the strain was applied to the sample, crystallization of the ENR was then induced due to the stress concentration point on the HNT surfaces, and the crystallinity increased in association with the orientation of the HNT. HNT is orientated and aligned along the stretching direction. As a consequence,the ENR chains rearrange and crystallize. This always happens regardless of whether untreated or urea-treated HNT is concerned. This kind of phenomenon usually occurs in the filled composites, and it has been reported elsewhere [23,24,45]. However, it is interesting to highlight that the crystallinity of the ENR matrix increases steeply due to the collaborative crystallization of ENR and HNT in association with the contribution of the urea. Higher rubber–filler interactions as indicated by lower Payne effect is responsible for such change. The presence of the urea has played an important role in pulling the surrounding molecular chains. Thus, a significant increase in crystallization is observed at higher strains, and this is in agreement with results observed previously in the stress–strain behaviors and WAXS profiles.

**Figure 13.** Schematic model representing the crystallization development of the ENR/HNT composites filled with ureatreated HNT. **Figure 13.** Schematic model representing the crystallization development of the ENR/HNT composites filled with ureatreated HNT.

#### **4. Conclusions 4. Conclusions**

Urea-treated HNT filler was successfully prepared via a mechano-chemical process. Attachment of urea on HNT was verified by FTIR, where new peaks appeared around 3505 cm−1 and 3396 cm−1, corresponding to urea's functionalities. The intercalation of urea in the gallery of HNT was revealed by XRD results. The development of two peaks at 21.54° and 22.53° in the range of saw tooth diffraction peak and the shifting of the peak at 2θ of 12.05° to 8.23° correlated to the decrease of the lattice strain and/or to the increment of the crystallite size. The ts2 and tc90 decreased with urea content due to alkaline nature of urea. The MH was optimal at 14% urea content. Increased tensile strength and tear strength were also observed due to the improved filler–matrix interfacial adhesions of HNT with the rubber matrix. This is confirmed by the dynamic properties of the composites, as the value of Payne effect was clearly reduced to 62.38% after using urea for treatment. The SIC in the composites exhibited a clear change, as the strain upturn was an earlier strain during stretching, indicating faster crystallization caused by better interfacial interactions within the composites. The *Xc* was directly observed with XRD during stretching, and it corresponded well with the tensile moduli of the composites. It is noted that correlation between SIC and mechanical strength may not be enough; an alternative testing, such as a fatigue test, is recommended in future work. Urea-treated HNT filler was successfully prepared via a mechano-chemical process. Attachment of urea on HNT was verified by FTIR, where new peaks appeared around 3505 cm−<sup>1</sup> and 3396 cm−<sup>1</sup> , corresponding to urea's functionalities. The intercalation of urea in the gallery of HNT was revealed by XRD results. The development of two peaks at 21.54◦ and 22.53◦ in the range of saw tooth diffraction peak and the shifting of the peak at 2θ of 12.05◦ to 8.23◦ correlated to the decrease of the lattice strain and/or to the increment of the crystallite size. The ts2 and tc90 decreased with urea content due to alkaline nature of urea. The M<sup>H</sup> was optimal at 14% urea content. Increased tensile strength and tear strength were also observed due to the improved filler–matrix interfacial adhesions of HNT with the rubber matrix. This is confirmed by the dynamic properties of the composites, as the value of Payne effect was clearly reduced to 62.38% after using urea for treatment. The SIC in the composites exhibited a clear change, as the strain upturn was an earlier strain during stretching, indicating faster crystallization caused by better interfacial interactions within the composites. The *X<sup>c</sup>* was directly observed with XRD during stretching, and it corresponded well with the tensile moduli of the composites. It is noted that correlation between SIC and mechanical strength may not be enough; an alternative testing, such as a fatigue test, is recommended in future work.

Based on the observations overall, it can be concluded that a urea content of about 14% in HNT filler is highly recommended for preparing composites with ENR matrix to ensure the compatibility and strength of the composite. On top of that, the use of urea can be the solution of choice for improving the interaction between ENR and HNT. It can offer improvements in processing behavior without the need to use complicated and costly silane coupling-agent systems. Based on the observations overall, it can be concluded that a urea content of about 14% in HNT filler is highly recommended for preparing composites with ENR matrix to ensure the compatibility and strength of the composite. On top of that, the use of urea can be the solution of choice for improving the interaction between ENR and HNT. It can offer improvements in processing behavior without the need to use complicated and costly silane coupling-agent systems.

**Author Contributions:** Investigation and writing—original draft preparation, I.S. and K.W.; methodology and validation, S.S., H.I., and N.O.; methodology, validation, writing—review and editing, supervision, N.H. All authors have read and agreed to the published version of the manuscript. **Author Contributions:** Investigation and writing—original draft preparation, I.S. and K.W.; methodology and validation, S.S., H.I. and N.O.; methodology, validation, writing—review and editing, supervision, N.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, through a SAT-ASEAN research grant (Grant No. SAT590594S). **Funding:** This research was funded by Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, through a SAT-ASEAN research grant (Grant No. SAT590594S).

**Institutional Review Board Statement:** Not applicable. **Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable. **Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. **Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We would like to thank the Research and Development Office (RDO), Prince of Songkla University, and Seppo Karrila for assistance with manuscript preparation.

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