Isothermal Crystallization Kinetics of Poly (3-hydroxybutyrate/ Poly(ethylene-co-vinyl acetate) Blends Enhanced by NH₄Cl as a Nucleating Agent

Abstract

The isothermal crystallization kinetics of poly (3-hydroxybutyrate) (PHB)/NH₄Cl composites, PHB/poly(ethylene-co-vinyl acetate), with vinyl acetate contents of 80% (EVA80) blends and PHB/EVA80/1 wt% NH₄Cl composites were investigated by using differential scanning calorimetry (DSC). The Avrami approach was applied to estimate the kinetic parameters of the PHB samples under isothermal crystallization conditions. It was found that the values of The Avrami rate constant, $k,$ of PHB were increased with the incorporation of NH₄Cl, declining EVA80 content in the PHB blends, and decreasing crystallization temperature. Moreover, the Avrami exponent, n, remained unchanged in spite of the change in the crystallization temperature, EVA80 content, and presence of NH₄Cl. The activation energy, $E_A,$ calculated using the Arrhenius equation, for PHB/NH₄Cl was much lower than that of pure PHB and PHB/EVA80, suggesting that the NH₄Cl induced heterogeneous nucleation. The $E_A$ values for PHB/EVA80/1 wt% NH₄Cl increased as the EVA80 content was increased, indicating a retardation effect of EVA80. Furthermore, a modified Lauritzen–Hoffmann treatment (LH) was used to evaluate the nucleation constant, $K_g,$ of the PHB samples based on the assumption that U* was 13.8 kJ/mol and the equilibrium melting temperatures were 470 K, intercepts of HL equation,$\mathrm{ }\mathbf{ln}\left(1/{\mathit{t}}_{0.5}\right),$ were 25.9 for all PHB samples. The $K_g$ values indicated that the crystallization of PHB in its composites and blends, described here, occurred in regime III. The maximum degradation rate temperatures of the PHB/NH₄Cl composites were higher than that of PHB, by 9 ^oC. No significant change in thermal stability was detected by the addition of EVA80.

Keywords:

• Poly(3-hydroxybutyrate) (PHB)
• poly (3-hydroxybutyrate)/NH₄Cl composite
• poly(3-hydroxybutyrate)/poly(ethylene-co-vinyl acetate) NH₄Cl
• isothermal crystallization kinetics
• differential scanning calorimetry (DSC)
• activation energy
• thermal gravimetric analysis

Acknowledgment

Authors would like to express their gratitude and appreciation to Dr. Günter Höhne, University of Ulm, Germany for his fruitful discussions about the DSC results.