**7. TPS antiplasticization and its explanation**

Antiplasticization is a well-known phenomenon for synthetic polymers. This phenomenon also exists in TPS and has been receiving increasing attention recently (Chang et al., 2006). Contrary to the orthodox behaviour of plasticization, plasticizer can reduce elongation and gas permeability, and increase tensile strength and modulus of elasticity of TPS when it is introduced in low amount under a critical level (generally <25%) (Godbillot et al., 2006; Lourdin et al., 1997; Zhang & Han, 2010). Fig. 13 shows strain–stress curves of pea starch films plasticized by glycerol and mannose respectively, indicating the starch films plasticized by glycerol and mannose had less stress and strain than the film without any plasticizers.

Fig. 13. Strain–stress curves of pea starch films plasticized by glycerol and mannose respectively, indicating the starch films plasticized by glycerol and mannose had less stress and strain than the film without any plasticizers. (From Zhang & Han, 2010)

Mechanism of anti-plasticization effect of plasticizer on TPS has not been clearly understood, but some efforts have been made to elucidate its mechanism. Zhang & Han (2010) found that starch retrogradation results in antiplasticization phenomena. The crystallinity of the pea starch films increased with plasticizer content increasing from 1% to 20%, leading to decrease in MC, OP, WVP, and E, and increase in EM. Plasticizer performs plasticization or antiplasticization depending on its concentration. Addition of plasticizers at the range of low to intermediate concentration level (1% to 25%) facilitates the formation of crystallites in the starch films, leading to the antiplasticization. Zhang & Han (2010) further proposed an anti-plasticization model. Due to the movement or vibration of the starch polymer chains, water and plasticizer molecules were pushed aside gradually from starch polymers. D-glucosyl residues of the amylose or amylopectin, which used to be separated by water or plasticizers molecules, interacted to form strong hydrogen bonds causing retrogradation or recrystallization. However, when starch polymer was plasticized by high content plasticizer (>25%), the plasticizer molecules could not be pushed aside completely from the starch polymers. Then, the plasticizers performed their plasticization effect, which was to interrupt interaction between the hydrogen bondings of starch polymers, increase the TPS elongation, reduce its *Tg*, and prevent retrogradation of starch chains.

#### **8. Summary**

130 Thermoplastic Elastomers

Antiplasticization is a well-known phenomenon for synthetic polymers. This phenomenon also exists in TPS and has been receiving increasing attention recently (Chang et al., 2006). Contrary to the orthodox behaviour of plasticization, plasticizer can reduce elongation and gas permeability, and increase tensile strength and modulus of elasticity of TPS when it is introduced in low amount under a critical level (generally <25%) (Godbillot et al., 2006; Lourdin et al., 1997; Zhang & Han, 2010). Fig. 13 shows strain–stress curves of pea starch films plasticized by glycerol and mannose respectively, indicating the starch films plasticized by glycerol and mannose had less stress and strain than the film without any

Fig. 13. Strain–stress curves of pea starch films plasticized by glycerol and mannose

and strain than the film without any plasticizers. (From Zhang & Han, 2010)

respectively, indicating the starch films plasticized by glycerol and mannose had less stress

**7. TPS antiplasticization and its explanation** 

plasticizers.

Thermoplastic starch (TPS) has attained more attention for its potential to replace the conventional polymers. Retrogradation occurs in TPS with time and affects its properties and applications. TPS contains normally 10% crystallinity, but this value changes with storage time, temperature, atmosphere relative humidity, and plasticizer content. Starch retrogradation mechanisms are discussed at molecular level. Methods to measure the retrogradation degree, such as differential scanning calorimetry, differential thermal analysis, X-ray, etc. are also reviewed. Changes in TPS property, such as tensile strength, elongation, gas permeability, are due to the retrogradation of starch polymers and these are described.

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**Part 2** 

**Modifications of Thermoplastic Elastomers** 

