Enhancing Thermomechanical Properties and Heat Distortion Resistance of Poly( l -lactide) with High Crystallinity under High Cooling Rate

In this work, a novel, effective and simple approach to largely improve the thermomechanical properties and heat distortion resistance of biodegradable poly( l -lactide) (PLLA) by using a new nucleating agent (NA), i.e., itself high-melting-point homocrystallites ( h PLLA crystallites) is reported. Specially, h PLLA crystallites with a melting temperature ( T m ) of 187 °C were introduced into the PLLA matrix with a lower T m, i.e., 168 °C via simply melt blending at 170 °C which is between the T m s of the two PLLAs. Nonisothermal and isothermal crystallization results reveal that h PLLA crystallite is an efficient nucleating agent for PLLA. Also, h PLLA crystallites show much more prominently promoting effect on the crystallization rate of PLLA in comparison with two widely reported NAs for PLLA, talc and stereocomplex crystallites. Most importantly, this promoting effect is still efficient at very high cooling rate, leading to a crystallinity of 39.1% at a cooling rate of 100 °C/min, which can help to obtain high-crystallinity PLLA products in conventional manufacturing processes. The optical microscopic observation reveals that the remarkable crystallization promotion can be attributed to the outstanding heterogeneous nucleation effect, as a result of both identical chemical constitution and lattice constitution between h PLLA crystallites and PLLA matrix. Further characterizations indicate that the enhancement of PLLA crystallinity by using such a new efficient NA can enhance the thermomechanical properties and heat distortion resistance of PLLA remarkably. For instance, at 80 °C (above T g of PLLA), the elastic modulus increases by 60 times from 8 to 477 MPa with the incorporation of 5 wt % h PLLA.