Poly(lactic acid) stereocomplex formation in the melt : limitations and prospectives

Conventional synthetic polymers (plastics) are almost exclusively based on fossil feedstock, notably oil. At present, approximately 5 % of the world production of oil is used to produce plastics but in view of the strong projected growth in this century, more than 25 % of the oil production will be needed, normalized on current production volumes, which is not sustainable in view of oil depletion. Currently, a lot of attention is paid, both in industry and academia, to develop so-called bio-based plastics, viz. plastics derived from biomass, notably to derive the monomers from biomass. A well-known example in this respect is poly(lactic acid) (PLA) in which case the monomer lactic acid is obtained from corn (maize) by fermentation and polymerized in industrial reactors. PLA as a plastic suffers from severe drawbacks notably the low softening temperature (Tg), which is approximately 55 °C, and a low speed of crystallization. Consequently, PLA products such as cups and bottles cannot be used at elevated temperatures, e.g. as coffee cups. Due to the optical activity of the lactic acid monomer (l- and d- lactic acid), two types of PLA polymers could be synthesized, namely PLLA and PDLA. Upon blending of those enantiomers in solution or in the melt, a so-called stereocomplex PLA (sc-PLA) is obtained. The very high melting point of approximately 220 °C of the sc-PLA (40–50 °C above the melting point of the homopolymers) gives possibility to extend the applications of PLA to the field of engineering plastics. However, the problem is to form highly crystalline sc-PLA without any formation of homocrystallites via melt processing (e.g. via extrusion). The reason is that during extrusion/melt-blending of PLLA and PDLA many problems are encountered like thermal degradation of the homopolymers at high temperatures, blockage of the extruder at lower processing temperatures and difficulties to avoid homopolymers crystallization. Therefore the aim of the dissertation is to study the fundamentals of the process of stereocomplexation of PLLA and PDLA in the melt via a step-by-step approach targeting to obtain at the end as much as possible sc-PLA with as high as possible crystallinity. In chapter 2 the very early stage of melt-blending without flow is studied by probing sc-PLA formation from solid-state mixed powders/flakes in 1:1 weight ratio homopolymers. The blends were then subjected to thermal treatment between 190 °C and 220 °C. Due to the sc-PLA formation, a non-linear increase of the viscosity with time - between 40 to 100 % was measured at 190 °C to 210 °C. The lower crystallinity of the formed sc-PLA (5 to 10 %) in those blends is explained by the limited chain diffusion and strongly reduced mobility of the melt caused by the "locking" effect of the sc-PLA crystals formed at the interface between PLLA and PDLA domains. At higher temperature (220 °C) the thermal degradation which was competing with sc-PLA formation prevailed and dominated the melt behavior of the blend. In chapter 3 one step further is taken to understand sc-PLA formation from the melt via considering the effect of homopolymers mixing on stereocomplexation. Comparison between the previously discussed solid-state mixed (SSM) blend and blends of PLLA and PDLA prepared via melt mixing in extruder is done (ME). Both blends had no initial sc-PLA. Melt crystallization of sc-PLA at temperatures between 190 °C and 220 °C was followed in the rheometer and in DSC, after initial heating of the blends to 250 °C. A much more pronounced increase of viscosity and a much higher crystallinity of the sc-PLA were observed for the ME blend, both attributed to more favorable melt mixing of PLLA and PDLA in this blend. Based on the results from this study, an optimum temperature of 210 °C was selected for further direct melt extrusion of sc-PLA (without subsequent treatment). The obtained extrudate contained 30 % crystallinity of the sc-PLA, without any homopolymers crystallized. In chapter 4 we discuss in more details the effect of the initial mixed state, melt memory, cooling rate and flow on the stereocomplexation of PLLA and PDLA from melt, in their equimolar blend. Using the combined approach of DSC and synchrotron SAXS/WAXD measurements, we show that highly crystalline (with crystallinity up to 60 %) sc-PLA could be formed from well mixed blends when making use of sc-PLA self-nucleation, isothermal crystallization or flow-induced crystallization. The target of chapter 5 is to show possible routes to increase even more the crystallinity of the sc-PLA by using organoclay (Nanomer 1.44P MMT) as a nucleating agent for sc-PLA. Melt extrusion of PLLA and PDLA in a weight ratio of 1:1 in presence of 0.5 to 5 wt. % organoclay resulted in sc-PLA with very high crystallinity- up to 90 % vs. 50 % for sc-PLA crystallinity without clay. The nucleation effect of organoclay is found to be increased with higher organoclay loadings. WAXD results and TEM images showed intercalation and partial exfoliation of the organoclay in the nanocomposite extrudates. The highly crystalline sc-PLA was found to be very brittle. The dissertation discusses the major problems encountered during melt-processing of sc-PLA. The fundamental aspects of the initial state of mixing, chain mobility, applied deformation (flow) on the melt, melt temperature and nucleating agents addressed here provide explanations and also feasible routes for melt-processing of sc-PLA.