Highly Crystalline Poly(l-lactic acid) Porous Films Prepared with CO₂-philic, Hybrid, Liquid Cell Nucleators

Abstract

Supercritical CO2 (scCO2) foaming of poly­(l-lactic acid) composite films with liquid polyhedral oligomeric silsesquioxanes (PlLA-POSS) was carried out to obtain polymer matrices for drug delivery applications. Highly crystalline (>45%) PlLA generally requires high supercritical processing (saturation) temperatures close to its melting temperature (∼450 K) and pressures about or over 20 MPa for foaming with scCO2. To decrease the saturation temperature and obtain ductile PlLA films with uniform and homogeneous porosity, we used two different, novel CO2-philic organic–inorganic hybrid liquid POSS as cell nucleators. The solvent-casting of the polymer with these additives resulted in porous thin films. ScCO2 processing at 313 K and 21 MPa increased the pore density and enhanced the mechanical and thermal properties such as elastic modulus and T g of the highly crystalline PlLA porous films. ScCO2 processing led to the extraction of the cell nucleators, allowing the recovery of these additives from the porous films. Drug release studies with a model antibiotic (Ceftriaxone Sodium) showed that the porous films exhibit burst release characteristics, which is suitable for local antibiotic applications.

Controlling the foam morphology of supercritical CO2‐processed poly(methyl methacrylate) with CO2‐philic hybrid nanoparticles

“…A syringe pump (model 260D, Teledyne ISCO) was used to deliver CO 2 to the vessel. The schematic representation of the set‐up is given elsewhere 45,47 . Teflon sheets with appropriate dimensions were placed in the vessel to form individual sections, which prevented specimens to stick to each other.…”

“…The increased cell nucleation sites and improved interactions of the polymer with CO 2 provided by these CO 2 -philic nanoparticles overall enhanced the foaming performances of these composites, decreasing the pore size and increasing the pore density. 45,47,53,54 Among all the studied conditions the highest pore density was obtained with the PMMA-5% POSS composite processed with scCO 2 at the experimental conditions of 35 C, 32 MPa, 24 h of processing time and fast-venting rate. This was the condition that also provided the smallest average pore diameter in the PMMA-based foams.…”

“…The recent studies on P L LA foaming with supercritical CO 2 using other CO 2 -philic POSS types such as fluoroalkyl POSS, isooctyl POSS or methacrylisooctyl POSS as cell nucleators also showed enhancement in the foaming behavior of the polymer, and allowed foaming at low temperatures such as 40 C, at which the polymer cannot be foamed without the POSS additives due to its highly crystalline nature. 45,47 Among all the specimens, PMMA-5% POSS composite processed at the experimental conditions of 35 C, 32 MPa, 24 h of processing time and fast-venting rate resulted in the smallest average pore diameter of 0.3 ± 0.6 μm.…”

“…The schematic representation of the set-up is given elsewhere. 45,47 Teflon sheets with appropriate dimensions were placed in the vessel to form individual sections, which prevented specimens to stick to each other. After the specimens were placed in these sections, the vessel was sealed and heated to the set temperature.…”

“…ScCO 2 foaming process slightly increased the glass transition temperature of PMMA and PMMA-POSS composites when they were processed at 35 C. During the foaming process, as CO 2 dissolved in the polymer behaves as a plasticizer decreasing the T g of the polymer, the chains of PMMA possibly realign to a more ordered, lower energy state, which leads to stronger intermolecular interactions between the chains and increases the T g of the polymer. 47 Another reason to this increase might be the formation of the nanoscale pores leading to the confinement of the polymer chains within the cell walls, increasing the T g of the polymer foam. 57 This effect manifested itself in the foams of neat PMMA and PMMA-0.5% POSS composites processed at 35 C, which had at least an order of magnitude smaller average pore diameter than those processed at 100 C. While T g was influenced mainly by temperature, other process parameters and the POSS addition did not have a distinct effect on the T g of PMMA composite foams.…”