The crystallization behavior of b-nucleated isotactic polypropylene in multi-component systems

Abstract

In this project, the crystallization behavior of isotactic polypropylene (iPP) in multicomponents systems, including compatibilized iPP/polyamide-6 (PA6) and iPP/zincoxide (ZnO), was investigated in the presence of a β-nucleating agent. In the investigated systems, iPP played the role of matrix and PA6 or ZnO served as the second components. It was observed that for the compatibilized iPP/PA6 blend containing 0.3 wt % of β- nucleating agent (designated as Blend-0.3) subjected to slow cooling, the PP crystallized predominantly in β-crystal form. On the other hand, when Blend-0.3 was cooled at fast cooling rate (comparable to that found in injection molding process) mainly α-crystal was formed. As PA6 was an effective α-nucleating agent for PP crystallization, Blend-0.3 could be considered as a system containing both α and β-nucleating agent simultaneously. Results from isothermal crystallization of Blend-0.3 suggested that mainly β-phase crystal was formed when it isothermally crystallized at relatively high temperature (crystallization temperature > 125oC). The folds free energy (σe) of Blend-0.3 was lower than that of the compatibilized iPP/PA6 blend without β-nucleating agent (Blend-0) at high temperature. This indicated that the β-nucleating agent used in this study (i.e. TMB- 5) was much more active than the α-nucleating agent (i.e. PA6) in Blend-0.3. Meanwhile, the crystallization rate of Blend-0.3 was also faster than that of Blend-0 in the high crystallization temperature range employed in the study. However, the β-nucleating agent was less active than the α-nucleating agent at lower temperature. If Blend-0.3 was isothermally crystallized at lower temperature (120oC), mainly α form crystal was formed. The slower the cooling rate, the longer the residence time at high temperature, and hence the higher β-PP content was formed. Although it was impossible to prepare a Blend-0.3 sample that contains β-PP only, Blend-0.3 with high β-PP content could be obtained when it was crystallizing between 125 and 141oC. High β-crystal content in Blend-0.3 could be achieved under compression molding. Tensile tests were performed at temperatures of 20, 30, 40 and 50 oC, and the occurrence of β to α transformation was monitored by DSC and WAXD measurements. It was observed that β to α transformation in Blend-0.3 could only be activated at elevated tensile testing temperatures. This was related to the increase in tensile elongation at break with the increase in tensile testing temperature. DSC measurements showed that for ZnO nano-particles filled neat iPP (ZnO-aPP), only α crystal form of PP crystals could be formed. On the other hand, for ZnO-nanoparticles and TMB-5 filled PP (ZnO-bPP), β crystal was predominant and accompanied by a small amount of α crystal. Avrami equation was used to describe the crystallization kinetics under isothermal crystallization. The resulting kinetics parameters such as the Avrami exponent n, t1/2 were compared. From Hoffman theory, the nucleation chain folded free energy (σe) of ZnO-aPP and ZnO-bPP was slightly lower than the corresponding neat aPP and bPP respectively. This indicated that ZnO was a weak nucleating agent for the crystallization of PP in both ZnO-aPP and ZnO-bPP. On the other hand, the relatively larger t1/2 in the ZnO-aPP and ZnO-bPP system implied that the addition of ZnO in PP could prohibit the mobility of PP molecule chains and defer the overall crystallization rate. The dispersion and the crystallization of hyperbranched poly(urea-urethane)s-grated carbon nanotubes/polyamide-6 nanocomposites were studied. Considering that hyperbranched polymers showed low melt-viscosity and multifunctional groups, CNTs modified by these hyperbranched polymers should provide good processability and functionality for the resultant composites. In this study, multiwalled carbon nanotubes (MWNTs) were first functionalized by hyperbranched poly(urea-urethane)s (HPUs) through a polycondensation technique, and forming multi-hydroxyl groups on the MWNTs surface (MWNTs-HPUs). PA6/MWNTs-HPUs nanocomposites with different MWNT contents were then prepared by a simple melt-compounding approach. MWNTs- HPUs were dispersed uniformly in PA6 matrix, because the grafted HPUs made MWNTs-HPUs more compatible with the PA6 matrix. Dynamic mechanical properties of PA6 were effectively improved by the addition of MWNTs-HPUs when compared with the neat PA6. Due to the enhanced dispersion of MWNTs-HPUs in PA6, it resulted in an effective transfer of load from polymer to MWNTs. In addition, because of the hydrogen-bonding interaction (OH···O=C) between hydroxyl groups (OH) on the surface of MWNTs-HPUs and carbonyl groups (C=O) within PA6 chains, the interfacial adhesion of PA6 and MWNTs has been improved. In addition, the effect of MWNTs- HPUs and MWNTs on the crystallization behavior of PA6 was also compared.