Methylcellulose, a Cellulose Derivative with Original Physical
Properties and Extended Applications
Chemically modified polymers have been extensively investigated in order to develop new biomaterials with innovating physic–chemical properties. Important classes of modified polymers are cellulose ethers, such as methylcellulose (MC), hydroxypropylmethylcellulose (HPMC), hydroxyethylcellulose (HEC) and carboxymethylcellulose (CMC). Cellulose is the most abundant polysaccharide found in nature; it is a regular and linear polymer composed of (1→4) linked β- D -glucopyranosyl units. This particular β-(1→4) configuration together with intramolecular hydrogen bonds gives a rigid structure. Aggregates or crystalline forms are a result of inter-molecular hydrogen bonds occurring between hydroxyl groups. The water insolubility of cellulose is assigned to this association between the single molecules, leading to the formation of highly ordered crystalline regions . This morphology, with the consequent low accessibility to reactants, is related to the origin of cellulose and controls its reactivity. Then, derivatives prepared under heterogeneous conditions have often an irregular distribution of substituents along the cellulosic backbone. Methylcellulose (MC) is one of the most important commercial cellulose ethers and it has been used in many industrial applications [2,3]. MC is the simplest cellulose derivative, where methyl groups (–CH 3 ) substitute the hydroxyls at C-2, C-3 and/or C-6 positions of anhydro- D -glucose units (Figure 1).
This cellulose derivative has amphiphilic properties and original physico–chemical properties. MC becomes water soluble or organo-soluble when the degree of substitution (DS) varies from 0 to 3. It shows a singular thermal behavior in which aqueous solution viscosity is constant or slightly decreasing when temperature increases below a critical temperature point (29 ± 2 °C). If temperature continues to increase, viscosity strongly increases resulting in the formation of a thermoreversible gel . These characteristics classify MC as a lower critical solution temperature polymer (LCST). The formation of the thermoreversible MC gels is a two-stage process as studied earlier and it is accompanied by an increase in turbidity of the solution and macroscopic phase separation at high temperatures (>60 °C) [5,6]. Although the gelation of MC has been studied extensively by various techniques, a variety of different gelation mechanisms has been proposed [4,7–15]. During the gelation process, the first step denominated “clear loose gel” (or pre-gel) is mainly driven by hydrophobic interaction between highly methylated glucose zones, and the second step, is a phase separation occurring at temperatures >60 °C with formation of a “turbid strong gel”. Additionally, MC gelation is influenced by the substitution pattern  and co-ingredients like salts, sugars, and alcohols [16,17]. The influence of the molecular weight (MW) is still under discussion [6,7,18–20] as well as the structure of the strong turbid gel [15,21–23]. This review covers particularly the influence of the MW of methylcelluloses on their most original physical properties using MC with the same degree of substitution as described in our recent works [20,24].