**4. Polymeric nanoparticles: structures, gelation and preparation**

These are also known as polymer nanoparticles (PNP). Natural and synthetic (anthropologic) polymeric materials can be used to prepare nanoparticles. Whereas synthetic polymers can be derived from petroleum raw materials and are understandable due to controllable chemical composition, natural polymers are derived from animal and plant based sources [16]. Biocompatibility, biodegradability, nontoxicity at extensive scope of concentrations and economic viability are the incentives to use natural polymers. They are extracted or synthesized from different natural sources and have varying compositions as well as ubiquitous. These processes can make control difficult during application. Proteins, polysaccharides and other biodegradable polymers are examples. It is these polymeric materials that are applicable in drilling fluids for improved performance. Most natural polymers are cellulosic and gellable. As such, they can act as viscosifiers. In other applications in drilling fluid, polymer nanoparticles can be used to prevent and control corrosion [17]. Structures


#### **Table 2.**

*Reported nanoparticles used in drilling fluids, size and concentration [11].*

of some polymers are shown in **Figure 4**. It is seen that they contain chains of carbon held by strong covalent bonds resulting in very long molecules. However, two types of polymers (i) cross-linked and (ii) linear are common in the oil and gas industry for various purposes [18]. In linear polymers the carbon-carbon bonds form continuous chain. The outstanding valence bonds link with hydrogen. Physical attractions keep the polymeric chains together. Conversely, cross-linked polymers are formed by short covalent or ionic bonds of polymer chains linked with one another. There could be natural or synthetic types of this kind of polymers. Cross-linked polymers are stronger and more stable polymer materials. It is seen that the natural polymeric structures (**Figure 4**) could be either linear or cross-linking formed. Cellulose is a linear polymer of glucose units connected by β-1,4-glycosidic links. Starch, a mixture of amylose and amylopectin which are linear and branched chain polysaccharides is abundant. It is the chain of carbon atoms that make up organic materials that form polymers. They are categorized as organic compounds because of the presence of carbon. Another common element found is hydrogen. Whereas chitosan is a polycationic linear polymer, alginate is linear polyuronic and lignin is a 3D cross-linked irregular polyphenolic polymer. Pullulan is a linear polysaccharide formed by α-1,6 glycosidic assembly linkages. Therefore, these structures can form gels that affect the

**Figure 4.**

*Structures of (a) cellulose (b) chitosan (c) alginate (d) gliadin (e) lignin and (f) pullulan polymeric materials [16].*

rheological behavior of drilling fluid formulations by influencing viscosity property. Structural units known as monomers combine to form these polymers. Moreso the long molecules from chains of carbon results in greater viscosity since the attraction due to intermolecular forces increases with longer chains. The higher intermolecular forces in the long chains give the polymers high melting point.

Moreso, based on their chemical structures, natural polymers could be proteins, polysaccharides or polyesters. Chitosan, alginates and cellulose are polysaccharides. Cellulose is more crystalline than starch, and it is the most abundant organic compound on earth. Its chemical formula is [C6H10O5]n. Chitosan derived from chitin is the second most abundant. Recently, some nano-fillers which include graphene, graphene oxide, fullerene, nanodiamond, carbon black, carbon nanotube, nanoclay and inorganic nanoparticles have been developed for application in polymeric matrices to utilize corrosion prevention characteristics. By functionality, polymers can form hydrogels and encapsulate solid particles for effective interaction and control. Ultimately, they have the capacity to influence drilling fluid properties by their (i) film forming and (ii) gelatinization characteristics, adduced to their thermal, mechanical and barrier properties**.**

#### *Polymeric Nanoparticles in Drilling Fluid Technology DOI: http://dx.doi.org/10.5772/intechopen.106452*

An example of natural polymer with nanoparticles is starch. Most plants store food in the form of starch. They are polysaccharides with animal or plant origins. Studies on mechanism of starch gelatinization are not new. According to a study, the summary of its application and gelation mechanism of starch was presented [19]. The researchers concluded that their applications include oil-drilling, coating, water-holding, viscous enhancing, gelling, emulsifying, protective and encapsulating agents. Also, they hinted that starch morphology is difficult to prove. Moreso, starch macroscopic properties and internal structure (supermolecular and molecular) were recognized to have an interconnection. They highlighted temperature and timedependence of events in the gel forming process. It was pointed that shear conditions during preparation also affect viscoelastic behavior of gelatinized starch dispersions. "It has been observed that the gelation process due to intra and inter molecular associations that result in hydrogen bonding or van der Waals attractive forces is due to hydroxyl or methyl groups and hemiacetal oxygen of sugar residues". Similarly, they projected that during the process of dissociation of amylopectin double helices coupled with increased shear; the swollen granules of a starch sample were disrupted and gave rise to amorphous gel with subsequent viscosity increase. Similarly, they highlighted the purpose of starch in food industry which was to control structure and rheology by addition of starch hydrocolloids, irrespective of whether plant, animal or microbial origin. They showed that starches swell in aqueous environments, and increase the viscosity of the system and that gels can be produced by altering the solvent's pH. Gelling (rheological) characteristics were shown to be different.

Also, a study on a starch-hydrocolloids system with leguminous *Mucuna sloanei* reported the gelatinization temperature range of 29.52–98.0°C. It was concluded that starch was converted from a semi-crystalline to an amorphous form that involved initial hydration of the amorphous regions that facilitated mobility of the molecules in the amorphous regions that was followed by reversible swelling. The reversible swelling led to dissociation of the double helices within the regions of the crystal and subsequent granule expansion as the biopolymer hydrated.

Nanoparticles generally have a core surrounded by shell as an additional layer, and surface molecules covalently linked. Whereas the core controls some properties such as the electrical and magnetic properties, the surface layers of molecules control the binding affinity. It is the right combination of the core and surface molecules that afford design flexibility and preparation. Several methods exist for production of polymeric nanoparticles. They include ball milling and polymer nanoprecipitation. Whereas ball milling involves grinding by the balls under high energy depending on the size, number of balls, slipping velocity and residence time, polymer precipitation involves dissolution in an organic solvent, and introduction into a poor solvent where the polymer chains collapse, agglomerate and precipitate out of solution. Other methods include solvent evaporation, emulsification/reverse salting-out, emulsification/ solvent diffusion, sulfuric acid hydrolysis [20], ultrasound, cold plasma, thermosonication and use of enzymes.

#### **4.1 Polymeric nanoparticles: properties, shapes**

Nanoparticles exhibit a wide variety of structures as a result of the significant physical properties which include; (i) much higher surface area to volume ratio when compared with micro and macro sized materials. That provides higher surface area for contact with surrounding substances for increased reactivity. This results in the use of smaller quantity and corresponding economic advantage (ii) high degree of mobility

in free-state (iii) display of quantum effects (iv) electronic (super conductivity) and optical (light adsorption and emission) activity (v) very high mechanical strength (vi) magnetic (superparamagnetism) properties (vii) thermal (fast cooling) properties (viii) chemical (catalytic) reactivity and (ix) barrier properties [10].

Particle shape plays major roles in formulation and prediction of behavior of materials. Both end properties and processes are affected by particle shape. Though most polymeric nanoparticle shapes exist as nano-spheres, challenges of creation of other shapes has been reported in nanomedicine where nanotechnology has had widespread applications [21]. Other particle shapes can be dispersed with the nanospheres to yield desirable properties as reported when chitosan was dispersed in bentonite formulation to produce improvements in thixotropy, shear thinning tendencies and better yield stress [22]. The mechanical property of a polymer has been adjusted by this approach [23]. Similarly, it has reported that particle shape has significant effects on drilling fluid properties such as fluid loss and rheological behaviors [24]. Whereas rods and plate shaped sharp-edged particles by jamming and interlocking can cause quick thickening [25] in fluids, flow would be negatively affected. Nanospherical polymeric particles would build viscosity less aggressively, in addition to enhancement of rheology (**Figure 5**).

Both organic and inorganic nanoparticles exist in diverse shapes. Whereas cellulose nanocrystals (CNC) are needle shaped, starch nanoparticles (SNP) are spherical in shape and starch granules are angular shaped for maize, pentagonal and angular shaped for rice, disk-like or lenticular for wheat; which is also roughly spherical or polygonal in shape. Similarly, chitosan has an agglomeration and nano-aggregates of particles similar to what was observed by a research [19]. The nano-aggregates of *Mucuna solannie* biodegradable polymer were presented as responsible for its utilization and influence in mud formulations. Its formulations were stable and exhibited predictable characteristics in accordance with American Petroleum Institute (API) specifications [2]. Polycaprolactone nanoparticles have spherical shape and no aggregates occur, while alginate nanoparticles can be spherical. Nanocapsules and nanospheres differ in morphology and architecture and both are nanostructures. Synthetic, biodegradable and biocompatible polymers are used to prepare polymeric nanospheres. Most polymeric nanoparticles have nanosphere structure (**Figure 6**) and a particular biopolymer was observed to have nano-aggregate structure (**Figure 7**). Whereas nanocapsule is seen as a reservoir system, the nanospheres are seen as matrix systems. Various nanoparticle shapes can be combined to influence a desired property such as barrier property. These shapes are subunits of the shapes presented in **Figure 2**.

**Figure 5.** *Effect of particle shape on viscosity.*

*Polymeric Nanoparticles in Drilling Fluid Technology DOI: http://dx.doi.org/10.5772/intechopen.106452*

#### **Figure 6.**

*Nanocapsule and nanosphere schematics.*

**Figure 7.** *Mucuna solannie scanning electron micrograph showing nano-aggregate particle shapes [19].*
