The term “physical aging” was introduced by Struik (1978) to distinguish these effects from other aging processes, such as chemical reactions, degradations, and changes in crystallinity. These alterations directly affect enthalpy, volume, and mechanical and diffusion properties. Hence, physical aging may be quantitatively measured as the changes in specific volume or relaxation enthalpy.
The mechanical behavior of low-moisture glassy starch shows a progressive embrittlement upon storage. The mechanical relaxation time increases and the material becomes stiffer. These progressive changes indicate that starch chains become less mobile and the matrix is densified during aging (Lourdin et al., 2002). Data from differential scanning calorimetry and dynamic mechanical analysis suggested that the embrittlement can occur as a result of free volume relaxation during sub-Tg aging as well as a result of plasticizer loss by evaporation (Shogren, 1992). It also seems that amylose retards the relaxation of amorphous regions toward equilibrium state (Chung and Lim, 2004).
After processing, TPS compounds are not usually at equilibrium and exhibit time-dependent changes in structure and macroscopic properties that are associated with aging (Hulleman et al., 1999; Rindlav-Westling, Stading, and Gatenholm, 2002; Smits et al., 2003; Avérous, 2004; Thiré, Andrade, and Simão, 2005). Two different aging behaviors can be detected, depending on water content and glass temperature of films:
- Materials with intermediate to high moisture content (room temperature above Tg) incur plasticizer–starch phase separation and crystallization of starch chains (retrogradation) (van Soest and Knooren, 1997).
- Materials with low moisture content, that is, with water content below 30% of the total wet weight (sub-Tg domain) show physical aging or structural relaxation (Lourdin et al., 2002; Shogren, 1992).
Glassy materials display mechanical and physical properties similar to those of crystalline solids, while keeping a molecular arrangement more characteristic of a liquid. Being far from the thermodynamic equilibrium, glassy materials stored at a temperature below their glass transition temperature are subject to molecular rearrangements, leading to lower states of energy. Thus, physical aging can be considered a molecular rearrangement (structural relaxation) toward an equilibrium as a function of storage time and temperature (Borde et al., 2002; Chung and Lim, 2003).
For rubbery starch materials, at room temperature above Tg, aging occurs mainly due to the formation of an intermolecular double helix and B-type crystallinity structure: the retrogradation process. As amylopectin retrogradation is slow, its crystallization has an important role in TPS aging (van Soest et al., 1996c; Forssell et al., 1999; Hulleman et al., 1999). However, the crystallization of amylose also contributes to time-dependent changes in properties, especially when high content of plasticizers is used (Shi et al., 2007). The V-type crystals, which are formed at the initial storage period, also contribute to changing the mechanical properties during aging and act as physical cross-linking points in the material.
Mechanical properties of rubbery starch materials are strongly influenced by retrogradation. Similar to synthetic polymers, an increase in the amount of B-type crystallinity results in an increase of the elastic modulus and hardness of material (van Soest et al., 1996c; van Soest and Knooren, 1997). Intermolecular crystallinity leads to a reinforcement of the network by the formation of physical cross-links. Intramolecular crystallization of the amylopectin decreases the mobility of the amylopectin and increases the stress in the material at highly crystalline junction zones. At the same time, the elongation between the points is restricted, which results in the decrease of the elongation at breakpoint. At crystalline regions, TPS materials may spontaneously break as a result of internal stress and cracks generated by crystals (van Soest, de Wit, and Vliegenthart, 1996).
Aging effects depend, among other issues, on the botanic origin of the starch. Variations in surface roughness on extruded oat and barley starch films aged 5 weeks, with phase separation in the surface, can be observed by atomic force microscopy on both films (Kuutti et al., 1998), whereas no changes are observed in general morphology and roughness on the surface of cast corn starch films, even after 270 days of storage (Thiré, Andrade, and Simão, 2005). However, an increasing number of ordered domains at the surface of aged films was attributed to amylose recrystallization.
In films from corn starch plasticized with glycerol and stored for 90 days, the water content (3.5 to 13.9%) and the amount of B-type crystallinity (20 to 39%) increased, as the surrounding relative humidity increased from 30 to 90% (Thiré et al., 2003b). It seems that the increase in the surrounding relative humidity led the network of amylose in the matrix and the amylopectin molecules of the granular region to take up water and swell. Hence the plasticization of the starch by increased water content led to higher mobility of the starch molecules and subsequent higher crystallinity of the material.