Nonisothermal cold crystallization behavior and kinetics of polylactide/clay nanocomposites

Abstract

The nonisothermal cold crystallization behavior of intercalated polylactide (PLA)/clay nanocomposites (PLACNs) was studied using differential scanning calorimetry, polarized optical microscope, X-ray diffractometer, dynamic mechanical thermal analysis, and Fourier transform infrared spectrometer. The results show that both the cold crystallization temperature (T cc ) and melting point (T m ) of PLA matrix decreases monotonously with increasing of clay loadings, accompanied by the decreasing degree of crystallinity (X c %) at the low heating rates ( 5 8C/min). However, the X c % of PLACNs presents a remarkable increase at the high heating rate of 10 8C/min in contrast to that of neat PLA. The crystallization kinetics was then analyzed by the Avrami, Jezioney, Ozawa, Mo, Kissinger and Lauritzen-Hoffman kinetic models. It can be concluded that at the low heating rate, the cold crystallization of both the neat PLA and nanocomposites proceeds by regime III kinetics. The nucleation effect of clay promote the crystallization to some extent, while the impeding effect of clay results in the decrease of crystallization rate with increasing of clay loadings. At the high heating rate of 10 8C/min, crystallization proceeds mainly by regime II kinetics. Thus, the formation of much more incomplete crystals in the PLACNs with high clay loadings due to the dominant multiple nucleations mechanism in regime II, may have primary contribution to the lower crystallization kinetics, also as a result to the higher degree of crystallinity and lower melting point in contrast to that of neat PLA.

Isothermal and Non-isothermal Crystallization Kinetics of Poly(L-Lactide)/Carbonated Hydroxyapatite Nanocomposite Microspheres

“…The values of fold surface energy σ e were obtained by solving Equation (11) and are listed in Table 3. For neat PLLA, the value of σ e varies in the range of 91-117 erg/cm 2 (1 erg/cm 2 = 1×10 -3 J/m 2 ), which agrees well with the result (107 erg/cm 2 ) by Miyata and Masuko (1998) but higher than 40.5 erg/cm 2 by Di Lorenzo (2001) and 43.5 erg/cm2 by Wu et al (2007). It has been shown that the value of σ e is dependent on the molecular weight of PLLA (Miyata and Masuko 1998) and the cooling or heating rate to the isothermal crystallization temperature (Wu, Wu et al 2007).…”

“…For neat PLLA, the value of σ e varies in the range of 91-117 erg/cm 2 (1 erg/cm 2 = 1×10 -3 J/m 2 ), which agrees well with the result (107 erg/cm 2 ) by Miyata and Masuko (1998) but higher than 40.5 erg/cm 2 by Di Lorenzo (2001) and 43.5 erg/cm2 by Wu et al (2007). It has been shown that the value of σ e is dependent on the molecular weight of PLLA (Miyata and Masuko 1998) and the cooling or heating rate to the isothermal crystallization temperature (Wu, Wu et al 2007). Miyata and Masuko (1998) implied that the value of σ e increases with increase in the loose-loop chains on the fold surface of high molecular weight PLLA.…”

“…These results suggest that the CHAp nanoparticles promoted initial glass transition and cold crystallization of the PLLA matrix, indicating an enhanced crystallization ability of PLLA in the presence of CHAp, which might behave as nucleating agents. Similar observations were made recently in the PLLA/nano-clay system (Wu, Wu et al 2007) with a decrease of T g value of 1-2 °C. The addition of 10 wt% CHAp nanospheres reduces the T g of PLLA by about 2.89 °C.…”

“…The increased crystallization rate due to the addition of CHAp into PLLA matrix was also confirmed by the higher values of isothermal crystallization parameter k listed in Table 2. Similar observations of inorganic particle reinforced PLLA composites exhibited increased bulk crystallization rates relative to neat polymer (Krikorian and Pochan 2004;Wu, Wu et al 2007). This phenomenon is usually attributed to the higher heterogeneous nucleation rates, as demonstrated by increases in the isothermal crystallization parameter k. However, the effects of inorganic fillers on radial spherulite growth rates vary, depending on the properties and content of the fillers.…”

Twin-screw extrusion impact on natural fibre morphology and material properties in poly(lactic acid) based biocomposites

“…There was a competition between fibre nucleating effect and hindrance of chain mobility, limiting the crystallite growth. This competition between the filler nucleating effect and its impeding effect on crystallization at high loading was also observed in PLA filled with clay by Wu et al (2007).…”