**2. Poly(methyl methacrylate) (PMMA): synthesis, morphology, and physical properties**

Acrylic acid (C3 H4 O2 ) gives rise to the so-called acrylic, where the poly(methyl methacrylate) (PMMA) is the most important thermoplastic in this group, which is commercially known as Plexiglas, Lucite, and Perspex [16].

PMMA is an amorphous polymer formed by the polymerization of MMA monomer carried out using different mechanisms [free radical vinyl polymerization, anionic polymerization, group transfer polymerization (GTP), or atom transfer radical polymerization (ATRP)] [16–20]. The bulk or solution (homogeneous polymerization) and emulsion or suspension (heterogeneous polymerization) techniques are used to obtain PMMA [18, 20– 22]. Among them, suspension polymerization is a good route to produce PMMA with high molecular weight (36,100), high yield (83%), and a polydispersity of 2.4 (polydispersity index: Mw/Mn) [18].

## **2.1. Suspension polymerization**

**1. Introduction**

44 Acrylic Polymers in Healthcare

portion of everyday clinical practice [4].

**physical properties**

H4 O2

Plexiglas, Lucite, and Perspex [16].

Acrylic acid (C3

of the deficiencies of the materials used at that time [6].

important to choose a suitable material for dental prosthesis.

The dynamic development of new multidisciplinary areas has a direct impact over the possible treatments and the rehabilitation of the dental function. Teeth rehabilitation with removable denture prosthesis is an established form of treating both partial and complete dentition in edentulous patients [1]. The developments in recent decades with dental implants dominate the current dental research, not only medical contraindications but also a negative attitude toward implants [2] and economic limitation [3] are the major disadvantages for their universal applicability, so the rehabilitation with dental prostheses still makes up a significant

The PMMA material revolutionized the preparation techniques used so far since Walter Wright introduced the acrylic resin as the denture base material in 1937 [5]. The acrylic resin became the preferred material for making denture bases, due to its ability to overcome many

Conversely, removable dentures are used in critical conditions of the oral cavity. There are about 500 microorganisms in the mouth, which produce a biofilm in an acidic environment causing several diseases [7], such as denture stomatitis [8], deterioration of the periodontal status of the remaining teeth [9], or carious lesions in abutment teeth [10]. Therefore, it is very

Poly(methyl methacrylate) (PMMA) is an acrylic resin usually used with a long tradition for prosthetic purposes [11]. It can be classified as chemically or thermally polymerized material depending on the factors that initiate the reaction. For dental prosthesis, thermally polymerized materials are used and the heat can be generated by hot water bath or microwave energy [12]. It was suggested that residual monomer concentration is the most important parameter in the determination of the final properties of the PMMA for dental prosthesis [12, 13]. It was found that in the chemical structure of PMMA, the alpha methyl groups tend to remain in the outer layer surface, whereas the methylene groups are in the inner layer of the PMMA surface, which gives an idea of the arrangement of the polymer [13]. In other words, PMMA has

The aim of this chapter is to present the trends for the processing of PMMA, including the chemical synthesis, conventional processing (thermal polymerization), the new technique of thermal polymerization assisted with ultrasound, the antibacterial effect on PMMA with

(PMMA) is the most important thermoplastic in this group, which is commercially known as

) gives rise to the so-called acrylic, where the poly(methyl methacrylate)

exhibited moderate cytotoxicity in bulk material and polymerized form [14, 15].

nanoparticles, and biocompatibility (cytotoxicity, genotoxicity, and mutagenesis).

**2. Poly(methyl methacrylate) (PMMA): synthesis, morphology, and** 

Hoffman and Delbruch developed suspension polymerization in 1909 for the first time [23]. In this technique, the initiator and the monomer are miscible with each other (**Figure 1**) and it involves droplet formation by the initiator/monomer (polymerizing phase) dispersed into water (oil/water system), where the volume ratio of monomer about 0.5 or less is suggested [22]. Water works as a heat-transfer agent and a dispersion medium, which improves the reaction rate and the yield in the polymerizing phase. To prevent settling or creaming, the suspension polymerization was kept under stirring during polymerization. In this polymerization, the addition of a soluble stabilizer in water [gelatin, clay or clay derivative, cellulose derivatives, water-soluble polymers such as poly(vinyl alcohol) (PVA) or starch] helps to prevent the breakup of droplets or avoids the droplet from adhering to each other [20–22, 24]. This process could be assisted with low temperature or ultrasonic waves [20, 24–27].

#### *2.1.1. Spherical microparticles: effect of stabilizer agent on size*

Suspension polymerization is adequate technically to obtain PMMA spherical microparticles with controlled sizes ranging from 5 to 1000 µm [22]. Alginate stabilizer produces microparticles from 5 to 80 µm (**Figure 2a**), whereas microparticles below 30 µm are obtained with gelatin stabilizer (**Figure 2b**) as previously reported [24–26]. These sizes are within the range of commercial PMMA (10 to 100 µm) used for prosthodontics (**Figure 2c** and **d**). Therefore, polydisperse particles could influence the surface roughness of PMMA without affecting their mechanical properties [28].

**Figure 1.** Suspension polymerization of MMA monomer. In the first step, the initiator, benzoyl peroxide interacts with the monomer in water in order to form emulsion oil/water, where the volume of water is twice as that of the monomer. Soluble water-stabilizer helps to obtain smooth and controlled size spherical microparticles.

**Figure 2.** Experimental PMMA microparticles obtained by suspension polymerization with (a) alginate or (b) gelatin stabilizer agents. Commercial PMMA microparticles: (c) Opticryl®, (d) Lucitone® used for prosthodontics.

#### **2.2. Physical properties**

PMMA has different characteristics and properties, such as chemical stability, hardness, stiffness and high transparency, resistance in atmospheric conditions and greater impact resistance than glass, and thermal and acoustic insulation. **Table 1** enlists the physical properties of PMMA [17, 29].



**Table 1.** Properties of poly(methyl methacrylate) [17, 29].

**2.2. Physical properties**

46 Acrylic Polymers in Healthcare

of PMMA [17, 29].

**Properties Values** Relative molecular mass 100.12 Elastic modulus 2.4–3.1 GPa Tensile strength 80 MPa Flexural strength 140 MPa Elongation at break 2–5%

Volatility 3.87 kPa at 20°C

PMMA has different characteristics and properties, such as chemical stability, hardness, stiffness and high transparency, resistance in atmospheric conditions and greater impact resistance than glass, and thermal and acoustic insulation. **Table 1** enlists the physical properties

**Figure 2.** Experimental PMMA microparticles obtained by suspension polymerization with (a) alginate or (b) gelatin

stabilizer agents. Commercial PMMA microparticles: (c) Opticryl®, (d) Lucitone® used for prosthodontics.

Stability Highly inflammable vapor, lower explosive limit 2.1 vol%

These properties are important for the final application, such as optical device, airplane windows, lenses, covers, automotive taillight, dental articles, and bioengineering [29]. Also, PMMA is a material widely used daily in dental practice, such as dental prosthesis for edentulous patients [24, 26]. For this particular application, PMMA (experimental or commercial acrylic resin) must be processed by heat, which can be generated by hot water bath or microwave energy [12].
