Crystallization in poly(l-Lactic Acid) Foams

Poly(l-lactic acid) (PLLA) is a biodegradable and renewable polyester that has gained popularity because of its sustainability and eco-friendly properties, as well as for the variety of possible applications, which range from packaging and agriculture to biomedicine. In biomedicine, PLLA is often used as an expanded (foamed) polymer for tissue engineering and medical implants. PLLA foams have also several additional applications, such as thermal and sound insulation in the construction and food packaging industry. A specific feature of PLLA is its copolymer nature, with several grades on the market, where both uncrystallizable and crystallizable grades are available. Crystallization that occurs during processing poses an additional challenge in the production of foams, as it plays an important role in determining the structure and properties of the foam. Understanding the crystallization behavior of PLLA foams is crucial for controlling their final properties, such as mechanical strength, thermal stability, and gas permeability. The rate and extent of crystallization, as well as the crystal structure and size, are influenced by various factors, such as the cooling rate, the nucleation density, and the presence of additives. Recent studies have shown that the addition of nucleating agents, such as talc, carbon nanotubes, and cellulose nanocrystals, can significantly enhance the crystallization rate and expansion of PLLA foams. The blowing agents can also influence the crystallization and cellular structure of PLLA foams. Furthermore, the thermal and pressure profile during foaming can have a significant impact on the crystal structure and size of PLLA foams. Despite notable progress in understanding the crystallization behavior of PLLA foams, there are still challenges in controlling morphology and structure of the material. Recent research in this area, focused on developing new techniques to tailor the crystal size and morphology of PLLA foams, in order to enhance material properties, are reviewed in this chapter.