246 Introduction to Renewable Biomaterials
Table 8.6Main properties of thermoplastic starch plasticized by glycerol
and 1-butyl-3-methylimidazolium chloride [BMIM]Cl.
Properties
TPS/glycerol
100/30
TPS/[BMIM]Cl
100/30
Water absorption (wt%) 20 13
Glass transition temperature (Tg)(∘C) − 21 − 13
Electrical conductivity (S cm−^1 )10−5.4 10 −4.6
Young’s modulus (MPa) 8.3 0.5
Stress at break (%) 88 392
Sankriet al. 2010 [17]. Reproduced with permission of Elsevier.
8.2.2 Thermoplastics Blends
Polymer blending is an easy method to obtain polymers with modified properties
and versatile applications. In fact, polymeric blend production was half of all plastic
production in 2010 [18].
Different kinds of polymer blends exist, but, in this section, we discuss thermoplastic
blend, focusing on oil-based and bio-derived thermoplastic polymers.
In general, a polymer blend is a mixture of two or more different polymers, which,
in the molten state, are usually immiscible. As a consequence, the polymer in the lower
amount assumes the geometric shape that is thermodynamically more stable, a sphere.
In fact, considering the second law of thermodynamics, in order to mix polymers, the
variation of free energy has to be negative.
ΔG=ΔH−TΔS
whereΔGis the variation of free energy,ΔHis the variation of enthalpy,Tis the
temperature, andΔSis the entropy.
ΔHis negative just in case the affinity between polymers is higher than the affinity
of the polymer itself: thusΔH is generally positive.ΔSis inversely proportional to
polymerization degree, which for polymers is generally very high. As a consequence,
the variation of free energy is generally positive, causing immiscible blends between
polymers [19].
In order to predict the solubility between two polymers, the Flory and Huggins theory
is often used. They studied this question in the early 1940, and proposed a model based
on the following equation:
휒AB=
Vref(훿A−훿B)^2
RT
where휒ABis the Flory–Huggins interaction parameter,Vrefis a reference volume,
generally 100 cm^3 mol−^1 ,훿is the solubility parameter of the polymer,Ris the gas
constant andTthe temperature. The higher the휒AB, the lower the miscibility between
both polymers [20]. The solubility parameter was also deeply studied by Bicerano in his
book ‘Prediction of polymer properties’ [21] because of its importance for compatibility
and permeation prediction, bulk and solution properties of polymers [22]. Hildebrand
