In order to determine the reaction parameters of step 2, the thermal properties of the prepolymer are also evaluated by DSC and the crystallization enthalpy [DELTA][H.sub.c], the crystallization temperature [T.sub.c], the heat of fusion [DELTA][H.sub.f] and the melting temperature [T.sub.m] are measured to be 84.6 J/g, 263[degrees]C, 93.3 J/g, and 301[degrees]C, respectively, indicated the prepolymer has high heat-resistance and good crystallizability.
Furthermore, the quite high [DELTA][H.sub.c] (85.6 J/g), high [T.sub.c] (262[degrees]C) and the relative small gap between [T.sub.m] and [T.sub.c] reveal that PA52 possesses outstanding crystallizability.
The high melting temperature (302[degrees]C), crystallization temperature (262[degrees]C), heat of fusion (90.7 J/g) and crystallization enthalpy (85.6 J/g) as determined by DSC reveal that PA52 is a high-temperature resistant polyamide with excellent crystallizability. TGA analysis demonstrates that PA52 has outstanding thermal stability, at least, the same level of PA6.
The fractionation mechanism of Crystaf and TREF relies on differences of chain crystallizabilities in dilute solution: polymer chains with high crystallizabilities are fractionated at higher temperatures, while chains with low crystallizability
are fractionated at lower temperatures .
However, some properties of P (3HB-co-4HB), such as low mechanical strength, poor crystallizability and thermal stability at the processing temperature, fall short of the required properties for its potential applications.
In this study, because of the homogeneous and fine dispersion of silica nanoparticles, it seemed that the 3 wt% loading was an optimal silica loading for the P (3HB-co-4HB) nanocomposites with the best performance such as the good crystallizability and the thermal stability, and the optimal mechanical properties as discussed below.
In this study, we elucidate further the cold crystallization behaviors of FDPLLA comparing with a bulk amorphous PLLA, perform structural characterizations by means of wide-angle X-ray scattering (WAXS), Fourier transform infrared (FTIR) spectroscopy, and density and specific surface area measurements, and try to understand the origin of the high crystallizability characteristic to the FDPLLA.
It is pertinent to assume that the high crystallizability of FDPLLAs is originated from higher mobility of the polymer chain which is specific to the freeze-dried state with high surface-to-volume ratio as will be discussed in the next section.
However, PP exhibits very poor compatibility and low adhesion to other polymers and inorganic fillers due to its nonpolarity and crystallizability. It has been found that in PP/clay nanocomposites crystallization of the PP matrix led to the expulsion of clay platelets from the crystalline phase by thermodynamic forces (41).
The degree of supercooling or the difference between [T.sub.m] and [T.sub.c] is then a good indication of the crystallizability or crystallization rate.
The increases of these two enthalpies, especially the evident increases of [DELTA] [H.sub.m], suggest an increase in the crystallizability
of PPS matrix in the presence of MWCNTs.
To compare the crystallizability
of the PP and PP/[CaCO.sub.3] composites, the undercooling ([T.sub.m.sup.0NLHW] - [T.sub.c]) and crystallization rate constants calculated from Avrami model are shown in Fig.