Enhancing Poly(lactic acid) crystallization with the aid of polypeptides

Petroleum-based plastics possess a high durability and usually are low-cost materials. However, reducing our carbon footprint is becoming more important every day, so these plastics probably have to be replaced by a more sustainable alternative such as polymers based on renewable bio-resources. At present, different biobased plastics (or biobased polymers) are slowly finding their way towards the commodity plastic market. The biodegradability of some biobased plastics is an advantage when they are disposed in the environment and it also facilitates efficient recycling. Polylactic acid (PLA), being a thermoplastic biodegradable biobased polymer, can serve in many classic polymer applications. Although PLA is an eco-friendly biobased plastic with excellent biocompatibility, overall processability, and low energy consumption during production, it has drawbacks as well, which currently limits its use in certain applications. The main limitation of PLA that was addressed in this work is the low crystallization rate. Conventional polymer processing methods operate at very high cooling rates. Therefore, processing a plastic with a low crystallization rate would lead to poorly crystalline or fully amorphous materials. The low level of crystallinity in these materials leads to a low quality and inconsistent polymer products. PLA’s slow crystallizing nature therefore restricts fast and/or mass manufacturing of PLA based products. Enhancing the PLA crystallinity and crystallization rate could boost the possibilities of use significantly and concomitantly would do good to the material properties such as stiffness, strength, heat deflection temperature and chemical resistance. Efficient routes to enhance the crystallization kinetics of PLA include the increase of nucleation density by adding a heterogeneous nucleating agent and the increase of chain mobility by addition of plasticizers. When using a biodegradable nucleating agent in combination with a biodegradable biopolymer, the resulting product could be completely biodegradable. In the presented research the attention was directed to increasing the nucleation density by the addition of crystallizable, bio-based, biodegradable molecules such as polypeptides. From the perspective of mutual compatibility, as determined by molecular simulations, the nucleating ability of some polypeptides appeared to be promising. Ultimately, polyalanine was selected as the best candidate to experimentally evaluate its nucleating efficiency in a PLA matrix. Polyalanine was synthesized and characterized. It was found that polyalanine has a very high thermal stability. After synthesis, polyalanine mainly adopts an α-helical conformation which, in absence of any solvent, converts into β-sheet structures at temperatures higher than 210°C. An α-helical to β-sheet crystal transition at these temperatures was not reported before. The resulting β‑sheet crystals degraded at much higher temperatures without showing a prior melting transition. Polyalanine fibers were produced and characterized to evaluate whether polyalanine exhibited any nucleating abilities for a PLA matrix. The fibers were added to a PLLA matrix and from POM measurements it was clear that they acted as nucleating agents. Further, polyalanine was also dispersed more finely in the PLA matrix. via two different techniques: solvent blending and melt blending. In the case of solvent blended samples, it was suggested that polyalanine is an efficient nucleating agent but that at high concentrations it may also impede the PLA crystal growth. For the melt blended samples, polyalanine effectively acted as a nucleating agent as well and, in contrast to the solvent blended samples, higher concentrations of polyalanine seemed to be more effective. By adding polyalanine to the PLA matrix, the PLA crystallization was sped up significantly and the PLA crystallinity almost doubled. Increasing the crystallinity and mainly increasing the crystallization rate is an important step in the future development of PLA based products. In industry the increased crystallization rate means an important time gain in industrial plastic article manufacturing processes which employ high cooling rates.status: publishe