Sasa Andjelic

Time resolved study of shear-induced crystallization of poly(p-dioxanone) polymers under low-shear, nucleation-enhancing shear conditions by small angle light scattering and optical microscopy

Isothermal crystallization of the biodegradable homopolymer polyester poly(p-dioxanone) (PDS), and the p-dioxanone copolymer with glycolide PDS-copolymer were studied in situ and in real-time under quiescent or nucleation-enhancing shear conditions. It was found that the spherulitic growth rates remain unchanged with shear, while the nucleation density increases dramatically. Nucleation-enhancing shear conditions, which do not alter the general spherulitic morphology, consist of a short-duration step-shear. This is in contrast to isothermal crystallization under steady-shear conditions, where at low-shear rates, fibrillar crystalline structures form. At high crystallization temperatures, where under quiescent conditions crystal development requires several days, both PDS, and the PDS-copolymer can be made to crystallize in several hours by the imposition of a step-shear.

Time-resolved isothermal crystallization of absorbable PGA-co-PLA copolymer by synchrotron small-angle X-ray scattering and wide-angle X-ray diffraction

Abstract The isothermal crystallization behavior of absorbable dyed and undyed PGA- co -PLA copolymers was investigated by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) methods with synchrotron radiation. The morphological parameters extracted from time-resolved SAXS profiles show that long period and lamellar thickness decrease slightly after primary crystallization. The unit cell parameters a and b and the apparent lateral crystal sizes L 110 and L 020 were extracted from the corresponding WAXD profiles. A significant decrease in the unit cell parameters and a substantial increase in the apparent crystal sizes are seen during the initial crystallization stage. Both scattering invariant ( Q from SAXS) and crystallinity ( X c from WAXD) results indicate that the crystallization rate is the fastest at 130°C. These copolymers show a bell-shape crystallization rate curve with temperature, where the dyed copolymer has a faster crystallization rate than the undyed one even though the inclusion of the low molecular weight organic dye is very small (ca. 0.2% by weight). We conclude that the dye molecule, which enhances the visibility during surgery, acts as a nucleating agent that increases the overall crystallization rate. The crystallization rate at 90°C is significantly slower than that at 130°C, however, the long period and lamellar thickness formed at 90°C are much lower than those formed at higher temperatures. The dyed and undyed PGA- co -PLA copolymers have almost the same morphological parameters at the same temperature. This indicates that morphological parameters of the lamellar structures in the polymers depend primarily on the crystallization temperature rather than on the crystallization rate. It is evident that the thermodynamic factor driven by temperature principally determines the lamellar morphology. The final unit cell parameters a and b and the final apparent crystal sizes all increase with temperature, indicating that crystal perfection prevails at high temperatures.

Hydrophilic Absorbable Copolyester Exhibiting Zero-Order Drug Release

PurposeA novel absorbable hydrophilic copolyester developed in our laboratory, amorphous 40/60 poly(ethylene diglycolate-co-glycolide), exhibits outstanding physical properties. Films made from this material appear fully transparent, colorless, soft and slightly elastic, but relatively strong and durable materials so that they can be potentially used as stand-alone devices in various in-vivo medical applications. In this study, in-vitro drug release characteristics of this copolyester were examined.MethodsHigh Performance Liquid Chromatography was used to generate release profiles on selected non-steroidal anti-inflammatory agents, NSAIDs. In addition, dielectric relaxation spectroscopy, as well as mid- and near infrared spectroscopy, were used to study specific polymer chain interactions in water and buffer solution as a function of aging time at 37°C.ResultsThis copolyester, compression molded into a film, exhibited nearly constant in-vitro release of various hydrophilic and hydrophobic drugs. The release profile showed minimal or, in most cases, no burst effect. The effect was observed with the three NSAIDs that were tested as model compounds; however, this system may prove generally useful for other drug entities. In-vitro hydrolysis conducted at 37°C on this hydrophilic copolyester revealed an unusually long induction period (no hydrolysis for up to 6 days), followed by the relatively rapid hydrolysis. Data from dipole relaxation spectroscopy indicated that the water molecules do not structurally associate with the polymer chains in phosphate buffer during initial hydrolysis period.ConclusionsThe results suggest unique dynamics of water diffusion through the polymer matrix that may play a critical role in achieving controlled release properties. Furthermore, we suspect that the molecular interactions associated with this new synthetic absorbable material may find a critical utility in important medical applications.

Real-time monitoring of segmental dynamics during crystallization of poly(l(–)-lactide) by simultaneous DRS/SALS technique

Abstract A novel experimental set up consisting of dielectric spectroscopy and small angle light scattering (or potentially any other optical technique used in combination) is introduced. The sample is enclosed in a Dielectric-Optical-Shear (DOS) cell capable of precisely controlling temperature and thickness. This method allows for simultaneous in situ, real-time monitoring of the changes in the molecular dynamics and corresponding development of supramolecular structure of a given polymer system during various time-sensitive processes. The DOS cell is utilized to investigate relaxation phenomena related to the glass transition, T g , of poly( l (−)-lactide) during isothermal crystallization under quiescent, partially constrained conditions. A decrease in T g was observed in situ via a shift in dipolar relaxation to higher frequency (confirmed calorimetrically, ex situ). The shift was most rapid in the spherulitic impingement stage of crystallization, at approximately 30% overall crystallinity.