*Article* **Synthesis and Study of Morphology and Biocompatibility of Xanthan Gum/Titanium Dioxide-Based Polyurethane Elastomers**

**Shazia Naheed 1,\*, Muhammad Shahid 1, Rashida Zahoor 1, Zumaira Siddique 1, Nasir Rasool 1, Sajjad Haider 2,\* and Shaukat Khan <sup>3</sup>**


**Abstract:** A series of xanthan gum/titanium dioxide-based polyurethane elastomers were synthesized through the prepolymer method by the step growth polymerization. In the present work, xanthan gum was used as a bioactive material, with TiO2 as a nanofiller. The structural characterization of newly prepared polyurethane samples was carried out with the help of Fourier Transform Infrared Spectroscopy. Thermogravimetric Analysis gave us the information about the thermal stability. Differential Scanning Calorimetry directs the thermal changes in the polyurethane samples. The Atomic Force Microscopy technique revealed that the degree of micro-phase separation increases by augmenting the % age of TiO2, which was further confirmed by X-Ray Diffraction results. XRD confirmed the crystallinity of the final sample at about 2θ = 20◦. Antimicrobial activity determined through the Disc Diffusion Method, and the results indicated that the synthesized polyurethane have antimicrobial activity. The water absorption capability of the polyurethane samples showed that these polymer samples are hydrophilic in nature.

**Keywords:** polyurethanes; xanthan gum; titanium dioxide; atomic force microscopy; X-Ray diffraction

### **1. Introduction**

Polyurethanes have become the 6th most used group of polymers in the last few decades and have gained more importance due to multiple use in different fields, such as coatings, adhesives, furniture, and foams [1,2]. Polymeric composites are currently at an important crossroad, where research is shifted toward more sustainable bio-based materials. In the last few years, biomaterials and biocomposites have gained extraordinary attention. Biocomposites have additional benefits, apart from their eco-friendly nature [3]. Polyurethanes are often applied as biomaterials because they have good mechanical properties and low adsorption of biomolecules [4].

In recent years, numerous polymeric materials have utilized for the manufacturing of different medical devices that interact with blood, body liquids, and tissues [5]. The soft, as well as hard, segment of polyurethanes exhibited elastomeric properties [6]. Basically, the polyurethanes are incorporated in medical applications due to their toughness, cost effectiveness, and durability [7–9].

By comparing them with additives, they have gained much more attention due to ease of processing and, specifically, the countless architectural variety for optimizing their stages. The characteristics of the final polyurethane could be modified and controlled by

**Citation:** Naheed, S.; Shahid, M.; Zahoor, R.; Siddique, Z.; Rasool, N.; Haider, S.; Khan, S. Synthesis and Study of Morphology and Biocompatibility of Xanthan Gum/Titanium Dioxide-Based Polyurethane Elastomers. *Polymers* **2021**, *13*, 3416. https://doi.org/ 10.3390/polym13193416

Academic Editor: Edina Rusen

Received: 30 August 2021 Accepted: 27 September 2021 Published: 5 October 2021

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the addition of additives [10]. They have a basic skeleton for lightweight materials, which is necessary for the transport industry. Reducing average weight decreases fuel usage, as well as greenhouse gases. Moreover, most of them are recyclable, bioactive, and biodegradable, depending upon the ingredients [11]. The physical properties of a polyurethane, such as density, tensile strength, high water abrasion, etc., distinguish its functioning [12]. They have admirable properties, such as hardness, tensile, compressive, impact resistance, etc. In general, polyurethanes are used in many different applications. These properties are tightly correlated with the biphasic nature of the segmented polyurethanes in the hard and soft phase. This, in turn, depends upon the chemical nature and composition of both phases. Flexible segment polyether and polyester-based polyurethanes (PUs) are susceptible to degradation under hydrolytic and oxidation environments [13].

The mechanical characterization of microcapsules of polyurethanes exhibits increased mechanical strength by a variable quantity of chain extenders, i.e., 1,4-butanediol (BDO) [14]. In the modern era, the standard of living could be enhanced with the employment of polymeric biomaterials. Generally, such biomaterials are simulated/artificial polymers, which are used in artificial organs, implants, dentistry, abrasion bandages, and drug delivery systems [15]. However, investigation on the degradation, morphology, and thermal and mechanical behavior is crucial to determine the end use of these materials.

In various extra and intracorporeal devices, biomedical polyurethanes are extensively applied. Typical illustrations include pacemaker leads insulation, indwelling tubes, heart pump tubes, and balloons of angioplasty [16]. Aliphatic diisocyanates, such as hexamethylene diisocyante (HDI), or cycloaliphatic diisocyanates, such as 4,4 methylenebis(cyclohexyl isocyanate) (H12MDI) and isophorone diisocyanate (IPDI), have bid superior stability over the aromatic isocyanate. The aliphatic diisocyanates show improved phase separation compared to corresponding aromatic diisocyanates. They also show improved phase separation behavior over the corresponding aromatic diisocyanates [3]. Today, extensive studies have been accomplished on polyurethane due to their inimitable possessions, as well as to illustrate their different behaviors [17].

By supplementing the diols of lower molecular weight as chain extender, Barikani and Hepburn reported that thermal stability of polyurethanes was enhanced [18]. Polyurethane biocompatible organic polymers, with an alginate nucleus and chitosan shell, used as nanoparticles, were prepared for increasing encapsulation efficiency, more regular insulin liberation, and better insulin accessibility [19]. Today, polysaccharides extracted from plants, such as guar gum or pectin, and extracted from algae, such as alginate, are replaced by some bacterial exopolysaccharides, such as xanthan gum and gellan gum [20]. *Xanthomonas campestris* produces the xanthan gum, which is basically heteropolysaccharide. Xanthan gum has extensive applications in industries, such as food, oil, pharmaceuticals, etc., due to its rheological properties, i.e., pseudo plasticity, high viscosity, etc. [1].

Biopolymer collection has been turned into a noteworthy natural issue today because of expanding use of biomaterials for pharmaceutical applications. Reprocessing of polymeric wastes by recycling them has been extensively applied. Now, there is a dire need to synthesize environmentally friendly biodegradable polymer [15].

Biocompatibility is an index which is a basic requirement for a polymeric material to be appropriate as an ideal biomaterial, which, in turn, depends upon degradable nature, cytotoxicity, and the other mechanical properties [21].

A few biomaterials out of bulk commodity polymers have been found promising. The novel polymeric materials of nanocomposites of nylon 6/clay has led to emanation having unanimity of incomparable characteristics [22]. For structural development, the amalgamation of layered nanofillers can radically influence the blends' and polymers' microphase morphology by acting as templates [23]. In order to promote biodegradability and biocompatibility, current studies have marked the porosity worthy of acritical stuff, more distinctively, porosities distribution and structure [24].

In this study, the xanthan gum/TiO2-based polyurethanes (XTPUs) were prepared by augmentation of weight (% age) of TiO2, by the prepolymer method. The microstructure biocompatibility of the nanocomposites was investigated. It was hoped that the introduction of nano TiO2 could not only improve the physical properties and biocompatibility of PU but also inhibit the growth of bacteria even when nanofiller TiO2 were used in low concentrations and were embedded in the polyurethane (PU) matrix.

#### **2. Experimental**

#### *2.1. Materials and Synthesis*

Analytical grade chemicals were used in this research work, provided by Sigma Chemical Co. (St. Louis, MO, USA), including Hydroxyl-terminated polybutadiene (HTPB, Mn = 3000 g/mol); Isophorone Diisocyanate (IPDI), 1,4-butanediol (BDO) was used as chain extender, titanium dioxide (TiO2) was used as a nanofiller, and the commercial xanthan gum (XG) was used as bioactive material. These were used to synthesize polyurethanes' elastomers by the step growth synthesis method. Before the use of these chemicals, for removal of moisture, they were dried at 80 ◦C in an electrical oven. During the drying process, they were placed in a vacuum for 24 h.

The prepolymer synthesis was done, according to a reported method [25]. Firstly, TiO2 (% by weight) and XG (% by weight) were dispersed and mechanically stirred in one mole of HTPB for 2 h at 100 ◦C until it completely dispersed in four-necked speciallydesigned apparatus equipped with round bottom flask, heating oil bath, magnetic stirrer, reflux condenser, nitrogen inlet, and dropping funnel at 100 ◦C. Then, it was reacted with two moles of IPDI for 2 h at 80–100 ◦C, in order to obtain isocyanate terminated (NCO) polyurethane prepolymer. The stirring continued until NCO-terminated prepolymer was synthesized. Titration with n-butylamine (ASTM D 2572-80) was conducted to obtain the NCO contents of the polymer. The synthesis of prepolymer was confirmed using FT-IR spectroscopy [26].

#### *2.2. Synthesis of Final Polymer*

The final xanthan gum/TiO2-based polyurethane (XTPU) polymer was obtained by stirring, for 30 min, the polyurethanes prepolymer with the chain extender 1,4-butanediol (1.2 mol). On the appearance of homogeneity and completion of dispersion of the chain extender in the reaction mixture, the polymer, in liquid form, was poured on the Teflon plate in order to develop a sheet having thickness of almost 2–3 mm. Then, at 100 ◦C for 24–48 h, the circulating hot air oven was used to cure the synthesized polymer. Before testing, i.e., characterization by various techniques, the cured sheets was stored at 25 ◦C for one week in order to attain almost 40% humidity. A series of six polymer samples were prepared by keeping constant the weight % age of XG, i.e., 1% and by varying weight % age of TiO2 from 0% to 5%, as shown in Table 1. Figure 1 presents the schematic demonstration of the chemical route for synthesis of the polymer.


**Table 1.** General formulation of polyurethanes.

**Figure 1.** Synthesis of polyurethane.

#### **3. Characterization**

#### *3.1. Fourier Transform Infrared Spectroscopy (FT-IR)*

Structural characterization of synthesized polymer was done by BRUKER TENOSR II FT-IR spectrometer (Bruker, Billerica, MA, USA). The infrared spectra recorded after 15-s interludes, by using 8 scans over a resolutions of 2 cm−1. The KBr beam splitter and DTGS detector were provided to the spectrometer. Data was collected, processed, and presented by thermo scientific spectroscopy software against a time-dependent series. The mechanism of reaction, as well as crosslinking behavior, was interpreted by FT-IR [27].

#### *3.2. Atomic Force Microscopy (AFM)*

AFM (CP-II, Veeco, Newport Beach, CA, USA) was used to assess surface morphology of prepared samples. A phosphorus-doped silicon-integrated pyramidal tip was used as support, in order to acquire images in tapping mode with a triangular cantilever (force constant of 20–80 N/m). Simultaneously, the images of topography and phase separation were recorded. The root-mean square average of the surface roughness was calculated within the given area as the standard deviation of all heights. Image Pro Plus 4.5 software (Media Cybernetics, Rockville, MD, USA) was used to measure the average hard and soft domain size from the phase images [28].

### *3.3. Thermogravimetric (TGA) and Differential Scanning Calorimeter (DSC)*

Thermogravimetric and Differential Scanning Calorimeter analysis were accomplished using an SDT Q600 V20.9 Build 20 (TA Instruments, Newcastle, DE, USA) under a dry nitrogen flow. The temperature range was set at 0–600 ◦C, and tamp (heating) rate was retained at 10 ◦C/min. The samples for TGA and DSC were primed on a glass substrate by spin coating of the mixed solution. Then, it was cured at various temperature steps. Aluminium sample pans were utilized to seal the almost 4 mg of each sample. Then, the prepared materials were analyzed under dry nitrogen by DSC over a heating rate of 10 ◦C/min from 0 to 500 ◦C. The glass transition temperature (Tg) crystallization temperature (Tc) and melting temperature (Tm) of the prepared polymers were investigated with the help of differential scanning calorimeter SDT Q600 V20.9 Build 20 [29].

#### *3.4. Antimicrobial Activity*

The capacity of a substance to have contact with the human body tissues without affecting the human body is called biocompatibility. The samples were subjected to assess the antimicrobial activity. The inhibition studies were done by actively growing bacterial cells. First of all, a nutrient agar media of 1000 mL was prepared, and 150 mL was poured in separate 150-mL flasks. The autoclave was used for 15 min at 120 ◦C by putting in agar medium nutrients flasks. Later on, it was cooled and bacteria, i.e., *Escherichia coli* (gram-negative) and *Macrococcus* (Gram-positive), were added to about 15 μL in the above two flasks. The sterile petri plates were used, and almost 20 mL of each agar nutrient medium was transferred in these plates, while room temperature was maintained. For 24 h, the samples were incubated at 37 ◦C, and they were then placed in petri dishes. The zone of inhibition, in which growth of bacteria was inhibited, was calculated by the diffusion active compound in the surrounding of the sample [30,31].
