Strain-Induced Enthalpy Relaxation in Poly(lactic acid)

On Structural and Molecular Order in Cellulose Acetate Butyrate Films

Cellulose acetate butyrate (CAB) is a possible candidate, being a raw material derived from renewable resources, to replace fossil-based materials. This is due to its thermoplastic properties and the relative ease with which it could be implemented within the existing industry. With a significant amount of variation in CAB on the market today, a knowledge gap has been identified regarding the understanding of the polymer structural arrangement in films. This relates to the underlying mechanisms that regulate CAB film material properties, insights that are important in product development. In this study, commercially available CAB was investigated with XRD, SEM, AFM, and TOPEM DSC in order to obtain physicochemical information related to its micro-structural features in solvent-cast films. The film-forming ability relates mostly to the number of hydroxyl groups, and the semi-crystallinity of the films depends on the type and position of the side groups along the cellulose backbone. The appearance of signs of possible cholesteric ordering in the films could be connected to higher amounts of hydroxyl groups along the backbone that disturb the helix arrangement, while the overall order was primarily related to the butyrate substitution and secondarily related to the molecular weight of the particular CAB studied. Cold crystallization was also observed in one CAB sample.

Uniaxial stretching and properties of fully biodegradable poly(lactic acid)/poly(3-hydroxybutyrate-co-4-hydroxybutyrate) blends

In this work, fully biodegradable poly (lactic acid) (PLA)/poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) blends of various compositions were uniaxially stretched at different stretch ratios above the glass transition temperature (T_g) of PLA. These stretched blends exhibited a closed microvoid structure, as evaluated by scanning electron microscopy. Differential scanning calorimetry and wide-angle X-ray diffraction analyses verified that stretching-induced crystallization in the α-form could be achieved in the PLA matrix. This hierarchical structure could improve the multifunctional performance of PLA blends. The density of drawn blends with a P(3HB-co-4HB) content of 30 wt% and stretch ratio of 6 was reduced by 20% as compared to neat PLA. The excellent combination of strength, modulus, and ductility of drawn blends with a P(3HB-co-4HB) content of 10 wt% and stretch ratio of 6 was demonstrated; compared to neat PLA, these parameters increased by 300%, 320%, and 317%, respectively in breaking strength, modulus, and elongation at break (172.2 MPa, 4200 MPa, and 18.4%), respectively. Meanwhile, control over the degradation rate and thermomechanical-property improvement was achieved by adjusting the stretch ratio and/or blend composition. In practical terms, this processing technique provides a new way to manufacture lightweight and high-performance microvoid-containing biopolymers.

Influence of melt-draw ratio on the crystalline behaviour of a polylactic acid cast film with a chi structure

Herein, polylactic acid cast films were prepared with different melt-draw ratios via an extrusion casting process. The oriented structure appeared at first and then a chi structure crystal was formed at higher MDR.

Unusual structural evolution of poly(lactic acid) upon annealing in the presence of an initially oriented mesophase

Uniaxial deformations of amorphous poly(lactic acid) (PLA) films were performed at two different temperatures, 70 and 80 °C, at various draw strains. The samples deformed at 70 °C showed a strain-induced mesophase, and the structural ordering and thermal stability increased as the draw strain increased. Further annealing was performed in situ at constant length at the drawing temperature of 70 °C for the films drawn to strains of 100% and 230%. Unusually, we found that after annealing, the crystal structure of the film at lower strain was more ordered than the one at higher strain. Further investigations revealed that upon annealing the structural evolution followed a distinct molecular mechanism for the samples stretched to the two draw strains. For the sample drawn to 100%, the mesophase melted very quickly upon annealing, resulting in chain randomization and the release of the constraints on the thermodynamic relaxation of the oriented amorphous chains. The chain relaxation motions had a beneficial effect on the occurrence of the conformational rearrangements that are necessary for crystalline ordering. By contrast, for the 230% sample, the melting of the mesophase was slow and most of the chain orientations were preserved upon annealing. As a result, a less ordered crystal structure was formed since the local relaxation motions that are necessary for promoting crystalline order via conformational rearrangements were hindered.