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

Abstract

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 (hPLLA crystallites) is reported. Specially, hPLLA 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 ms of the two PLLAs. Nonisothermal and isothermal crystallization results reveal that hPLLA crystallite is an efficient nucleating agent for PLLA. Also, hPLLA 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 hPLLA 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 % hPLLA.

Fully bio-based, highly toughened and heat-resistant poly(L-lactide) ternary blends via dynamic vulcanization with poly(D-lactide) and unsaturated bioelastomer

“…The poor heat resistance, namely low HDT, arises from the low degree of crystallinity during thermal processing, ascribed to the extremely slow crystallization rate of PLLA. Thus, enhancing the degree of crystallinity (X c ) has been considered one of the most convenient approaches to improve the heat resistance of PLLA [35]. Annealing is useful in increasing the X c of PLLA but requires a very long time to finish crystallization and is thus not favorable for practical processing [36].…”

“…Many nucleating agents including carbon fillers, cellulose nanocrystals, talc, crystallites of high-melting-point PLLA (hPLLA), and stereocomplex (SC) crystallites of PLLA and poly(D-lactide) (PDLA) have been employed to enhance the crystallization rate of PLLA [36][37][38][39][40][41][42]. Investigations by Yang et al [35,37] indicated that hPLLA showed the most effective nucleation effect for PLLA when compared to talc and SC crystallites. The melting temperature of hPLLA is very close to that of PLLA; thus, the narrow processing temperature window is unfavorable for practical thermal processing.…”

“…The melting temperature of hPLLA is very close to that of PLLA; thus, the narrow processing temperature window is unfavorable for practical thermal processing. SC crystallites could be used as the optimal nucleating agent for PLLA, as it has a much higher melting temperature and shows excellent miscibility with PLLA as well as a more effective nucleation effect than that of talc [35,43]. In addition, incorporation of PDLA would change the rheological behavior of PLLA and facilitate morphology regulation of PLLA-based blends [44,45].…”

Improvements in mechanical strength and thermal stability of injection and compression molded components based on Poly Lactic Acids

“…[31][32][33][34] Alternatively, the introduction of advanced polymers, featuring higher heat resistance, [35][36][37][38][39] inside the blend, can represent another viable strategy. The use of nucleating agents in the form of mineral fillers can promote the formation of the crystalline phase and at the same time confer additional thermal resistance to the biodegradable polymer.…”

Engineered poly(lactic acid)‐talc biocomposites for melt processing: Effects of co‐blending with poly(butylene succinate) and poly(butylene terephthalate) on thermal and mechanical behavior

“…A high crystalline content can also affect by also promoting the amorphous phase that becomes more rigid in the surrounding of the crystalline phase that results in less sensitive to temperature increase. For this purpose, natural nucleating agents or mineral fillers, which can tune the crystallization behavior of PLA and, in turn, confer additional thermal resistance to the polymer and lower the production costs, are widely investigated in the literature .…”