Polylactide stereocomplex crystallites as nucleating agents for isotactic polylactide

Abstract

A nucleation efficiency scale for isotactic poly(L-lactide) (PLLA) was obtained with self-nucleation and nonisothermal differential scanning calorimetry experiments. The maximum nucleation efficiency occurred at the highest concentration of self-nucleating sites, and the minimum efficiency occurred in the absence of these sites (pure PLLA polymer melt). Blends of PLLA and isotactic poly(D-lactide) (PDLA) led to the formation of a 1/1 stereocomplex. In comparison with the homopolymer (PLLA), the stereocomplex had a higher melting temperature and crystallized at higher temperatures from the melt. Small stereocomplex crystallites were formed in PLLA/PDLA blends containing low concentrations of PDLA. These crystallites acted as heterogeneous nucleation sites for subsequent PLLA crystallization. Using the PLLA nucleation efficiency scale, we evaluated a series of PLLA/PDLA blends (0.25-15 wt % PDLA). A maximum nucleation efficiency of 66% was observed at 15 wt % PDLA. The nucleation efficiency was largely dependent on the thermal treatment of the sample. The nucleating ability of the stereocomplex was most efficient when it was formed well before PLLA crystallization. According to the efficiency scale, the stereocomplex was far superior to talc, a common nucleating agent for PLLA, in its ability to enhance the rate of PLLA crystallization. In comparison with the PLLA homopolymer, the addition of PDLA led to reduced spherulite sizes and a reduction in the overall extent of PLLA crystallization. The decreased extent of crystallization was attributed to the hindered mobility of the PLLA chains due to tethering by the stereocomplex.

Differences Between Stereocomplex Spherulites Obtained in Equimolar and Non‐Equimolar Poly(L‐lactide)/Poly(D‐lactide) Blends

“…It is believed that the as-formed stereocomplex crystals can act as nucleation agents for the polyenantiomer in excess, as already reported in the literature. [8,20,21] Enthalpies of fusion of the homopolymer and stereocomplex crystals calculated from the T c ¼ 160 8C curve of Figure 5 are 30.8 and 14.8 J Á g À1 , respectively. Whereas the enthalpy of fusion of the stereocomplex crystallized at the same T c from an equimolar blend is 46.7 J Á g À1 .…”

Selective Localization of Nanofillers: Effect on Morphology and Crystallization of PLA/PCL Blends

“…It is reported that the transition temperature of PLA from regime II to regime I is about 125 8C. [80][81][82] Thus, the multiple nucleations occur in the ternary systems because the crystallization proceeds in regime II; while the crystallization of the neat PLA and the binary blend is still in regime I, in which the surface nucleation is dominant. In this case, the ternary systems show higher crystallization rate (lower t 1/2 ) than that of the neat PLA.…”

“…The changing slopes indicate that the Ozawa exponent is not constant with temperature during the primary crystallization process. [83] This is due to the cold crystallization temperature dependence of lamellar thickness [71,82] [70,71] ); c) X c ¼ DH m DH 0 m , DH 0 m ¼ 137 J Á g À1 , such a low value is attributed to the incomplete cold crystallization at high heating rates. physical variables relating to nonisothermal crystallization process is relative degree of crystallinity but not temperature, [83] can provide a satisfactory description to the cold crystallization for all samples, as shown in Figure 12 Table 5.…”

Mechanical and Thermal Properties of Poly(L‐lactide) Incorporating Various Inorganic Fillers with Particle and Whisker Shapes

“…T c of PLLAwas observed at 94-107 8C in the composites, about 30 8C lower than that of the unfilled PLLA. This fact suggests that the inorganic fillers can work as the crystal nucleator for PLLA, [16,17] although the size growth of the crystals is restricted by the increased nucleation to make DH f of the composites significantly lower than that of the unfilled PLLA. Table 3 summarises the mechanical properties of the ifR-PLA composites containing 20 phr of fillers.…”