Nonsolvent-induced morphological changes and nanoporosity in poly(l-lactide) films

Temperature and Time Dependence of the Solvent-Induced Crystallization of Poly(l-lactide)

The role of organic solvents in governing the crystallization and morphology of semi-crystalline poly-l-lactide (PLLA) sheets was systematically investigated. Three different organic solvents; ethyl acetate (EA), o-dichlorobenzene (ODCB), and nitrobenzene (NB), with a solubility parameter analogous to PLLA and with a high capability of swelling, were chosen. It has been witnessed that the degree of crystallization and crystal morphology depends highly on the degree of swelling and evaporation rate of the solvent. Besides, the temperature and time of treatment played a significant role in the crystallization of polymers. The effect of different solvents and curing times are reflected by the measured X-ray diffraction (XRD) peaks and the differences are best shown by the unit cell size. The largest variation is observed along the c-axis, indicating shorter bonds, thus, showing better conformation after NB and ODCB treatment. The percentage of crystallinity calculated using the classical relative crystallinity index of XRD shows closer values to those calculated with differential scanning calorimetry (DSC) data, but a huge variation is observed while using the LeBail deconvolution method. The strong birefringence of polarised optical micrograph (POM) and the crystal morphology of scanning electron micrograph (SEM) also evidenced the orientation of polymer crystallites and increased crystallinity after solvent-supported heat treatment.

Investigation of the influence of pressurized CO2 on the crystal growth of poly(l-lactic acid) by using an in situ high-pressure optical system

Since CO2 is a kind of nontoxic, non-flammable and biocompatible fluid, introducing CO2 in the PLLA formation process has been regarded as a green way to the manufacture of biological products or medical supplies. However, it is still a challenge to understand the influence of CO2 on the crystal growth behavior of PLLA. Here, we developed an in situ high-pressure observation system, composed of optics, polarization optics and a small angle laser scattering system, to record the growth process of PLLA crystals in a pressurized CO2 environment. It is found that, at a low temperature (near Tg), low pressure CO2 (0.5 MPa in this work) can still induce the formation of numerous micron-sized spherulites of PLLA. Therefore, the introduction of CO2 can significantly enhance the crystallization ability of PLLA and decrease the crystallization temperature, which is helpful in improving the mechanical properties of PLLA products. We also found that a snowflake-shaped crystal was assembled by rhombic lamellae under pressurized CO2. There is a melt accumulation zone surrounding the growth front of the snowflake-shaped crystal, indicating that the growth front nucleation is limited by the pressurized CO2. This melt accumulation zone is quite different from the melt depletion zone existing ahead of the reported dendritic crystal front. Interestingly, in a high-pressure CO2 environment, a kind of bamboo-like branch is formed in a rhythmic growth mode. The repeating unit of the bamboo-like branch is constructed by an asymmetric terrace crystal originated from screw dislocation in the melt accumulation zone. These results demonstrated that CO2 has a remarkable tunability on the polymer crystal morphology.