*4.5. Atomic Force Microscopy (AFM)*

The change in surface morphology of XTPU samples was mainly investigated by AFM. It provides important information relating surface characterization. In the current study, the final morphology of XTPUs films is visualized in Figure 9. In these phase images, the darker regions correspond to soft segments, while lighter domain or crystalline regions correspond to hard segments [2]. The XTPUs can be visualized for the microphase structures as a roughness gradient of 2.5 μm, 158 nm, and 300 nm for XTPU-1, XTPU-3, and XTPU-6 respectively. The complete and better dispersion of nanofiller, also with biomaterial, was also observed within samples. These results showed (the circled region) homogeneous structure of polymer and pattern of distribution of xanthan gum with TiO2 in the polyurethane polymer. AFM images of XTPUs have shown that hard segments are completely dispersed in the soft segment matrix. Moreover, it was inferred that, by increasing the content of TiO2 from 0% to 5% in 6 polymer samples from XTPU-1 to XTPU-6, the order in pattern crystalline character and stability increases and cracking decreases in the polymer, according to reported literature [25]. Morover, in regard to

polyurethane composition, it was also reported that hydrogen bonding was present in urethane groups, which may be the cause of phase segregation. The higher molecular weight of macrodiol and xanthan gum provided ordered and compact arrangments of soft and hard phases. However, various factors can be reponsible for phase segregation in PU, such as the diminsions of soft and hard segments, the polarity of groups, the chemical nature, and molecular weight [37,39].

**Figure 8.** X-ray diffractograms of XTPU-2, XTPU-3, XTPU-5, and XTPU-6 with varying titanium dioxide % age.

**Figure 9.** Three-dimensional AFM images of XTPU-1, XTPU-3, and XTPU-6 with 0%, 2%, and 5% titanium dioxide, respectively.

#### *4.6. Evaluation of Water Absorption*

The water absorption test and hydrophobicity/hydrophilicity of polyurethane samples are used for evaluation of biodegradability of synthesized polymer. The degree of swelling of polymer components and their affinity to water is one of the indices of decomposition rate of polymeric materials under the influence of environmental factors. Due to presence of soft segment HTPB and XG into the back bone, the prepared polymers are hydrolytic resistant and can be utilized to perform indoor and outdoor environments. So, water absorption capability can be used to determine the hydrolytic degradation of them. The investigation of the hydrolytic stability of XTPUs has shown an increase of the water absorption. Such an increase may be due to the presence of free polar fragments (OH groups), which determine the hydrophilic properties of the polymer. Probably, a part of XG hydrophilic hydroxyls involved in the formation of intermolecular bonds with the polar groups of XTPUs remains unengaged [40]. Water absorption as a function of time

and type of samples is summarized in Table 4. Water absorption was calculated by the following formula:

$$\text{Water absorption} \left(\% \right) = \frac{\text{m}\_{\text{W}} - \text{m}\_{\text{d}}}{\text{m}\_{\text{d}}} \times 100 \tag{2}$$

where md and mw are the masses of dry and wet XTPU samples, accordingly [38]. There was a remarkable water absorption indicated with passage of time in these prepared samples of polyurethanes. The results noticeably indicated that the polymer samples are hydrophilic in nature [41].



#### **5. Conclusions**

Xanthan gum/Titanium dioxide-based polyurethane elastomers with concordant crystallinity and hydrophilicity were synthezied. The reactants were HTPB, IPDI, and Titanium dioxide that produced prepolymer, extended with BDO. Xanthan gum was used as a bioactive material. Molecular characterization of the prepared elastomers was carried out by FT-IR spectroscopy, and the appearance of NH peaks with disappearance of NCO and OH peaks indicates that the proposed structure of polyurethane has been accomplished. The AFM technique revealed that the degree of micro-phase separation increases with augmenting % age of TiO2, which was further confirmed by XRD results. It was revealed by XRD that the crystalline behavior of the synthesized samples due to amorphous region of macrodiol. The % age of TiO2 influence on TGA and DSC values gave the information about the thermal stability and thermal changes in the polyurethane samples. The higher value of DSC is due to segments' mobility in the hard blocks' microregions (hard domain) and their destruction. Such changes may be caused by three-dimensional XG molecules, which form steric hindrence during polymer formation. Antimicrobial activity determined through the Disc Diffusion Method and the results indicated that the synthesized polyurethane had antimicrobial activity against the *Macrococcus*, *E. coli*, and *Aspergillus flavus*.

**Author Contributions:** Conceptualization, S.N. and S.H.; methodology, S.N. and M.S.; formal analysis, S.N. and N.R.; investigation, M.S. and S.N.; data curation, M.S., S.N. and N.R.; writing original draft preparation, M.S.; writing—review and editing, M.S., S.N., Z.S. and R.Z.; supervision, S.N.; project administration, S.N., N.R. and S.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors sincerely appreciate funding from Researchers Supporting Project number (RSP-2021/399), King Saud University, Riyadh, Saudi Arabia.

**Institutional Review Board 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.

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