Broadband Dielectric Investigation of Amorphous and Semicrystalline l-Lactide/meso-Lactide Copolymers

Development of force field parameters for molecular simulation of polylactide

Polylactide is a biodegradable polymer that is widely used for biomedical applications, and it is a replacement for some petroleum based polymers in applications that range from packaging to carpeting. Efforts to characterize and further enhance polylactide based systems using molecular simulations have to this point been hindered by the lack of accurate atomistic models for the polymer. Thus, we present force field parameters specifically suited for molecular modeling of PLA. The model, which we refer to as PLAFF3, is based on a combination of the OPLS and CHARMM force fields, with modifications to bonded and nonbonded parameters. Dihedral angle parameters were adjusted to reproduce DFT data using newly developed CMAP dihedral cross terms, and the model was further adjusted to reproduce experimentally resolved crystal structure conformations, melt density, volume expansivity, and the glass transition temperature of PLA. We recommend the use of PLAFF3 in modeling PLA in its crystalline or amorphous states and have provided the necessary input files required for the publicly available molecular dynamics code GROMACS.

Segmental relaxation in semicrystalline polymers: a mean-field model for the distribution of relaxation times in confined regimes

The effect of confinement in the segmental relaxation of polymers is considered. On the basis of a thermodynamic model we discuss the emerging relevance of the fast degrees of freedom in stimulating the much slower segmental relaxation, as an effect of the constraints at the walls of the amorphous regions. In the case that confinement is due to the presence of crystalline domains, a quasi-Poissonian distribution of local constraining conditions is derived as a result of thermodynamic equilibrium. This implies that the average free-energy barrier DeltaF for conformational rearrangement is of the same order of the dispersion of the barrier heights, delta(DeltaF) , around DeltaF. As an example, we apply the results to the analysis of the alpha-relaxation as observed by dielectric broad-band spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized from either an isotropic or an oriented glass. It is found that in the latter case the regions of cooperative rearrangement are significantly larger than in the former.

Continuum electrostatics for electronic structure calculations in bulk amorphous polymers: application to polylactide

The bond rotational energy landscapes of polylactide (PLA) oligomers were estimated using electron density functional theory (DFT) at the B3LYP/6-31G** level, both in vacuo and with a self-consistent reaction field (SCRF) method to simulate the electronic environment within the condensed phase. The SCRF method was evaluated for application to polymeric systems, and we demonstrate the difficulties involved in applying the method to bulk amorphous polymers with specific attention to the selection of the solvent probe radius. In addition, rotational isomeric states (RIS) calculations were performed, showing the effect of accounting for the bulk phase reaction field on the bond rotational energetics and characteristic ratio. We conclude that present methods of accounting for bulk environments in electronic structure calculations are not well suited for use with polymeric systems, and the development of improved methods is needed in this area.

Effect Of Structural Relaxation On The Preparation And Drug Release Behavior Of Poly(lactic-co-glycolic)acid Microparticle Drug Delivery Systems

Control of burst release is a major challenge in the development of poly(lactide-co-glycolide) (PLGA) microparticle drug delivery systems. It has been well-documented in previous literature that formulation and processing variables determine particle morphology, which in turn, governs drug diffusivity and burst release. However, it is not generally appreciated that PLGA polymers used for microparticle systems are typically amorphous, and as such, undergo structural relaxation during processing and storage, characterized by enthalpy and volume reduction. Volume reduction due to structural relaxation can decrease drug diffusivity within microparticles and affect burst release. The magnitude of the driving force leading to structural relaxation is linked to the rate of particle hardening, and is affected by process parameters. Studies that directly address structural relaxation in PLGA microparticles indicate that the manufacturing process and residual solvent levels, as well as the nature of the interaction between drug and polymer affect the rate of structural relaxation. Therefore, the conditions chosen for particle fabrication may be a major source of variability in the burst release and may affect the stability of the drug release profile during storage. The potential effects of structural relaxation on drug release are likely to be formulation specific. Additional work is required to understand and control the relationship between microparticle processing, structural relaxation, and performance of PLGA microparticle drug delivery systems.

Crystallization of polylactide and its stereocomplex investigated by two-dimensional fourier transform infrared correlation spectroscopy employing carbonyl overtones

The band origins and transitions of weak vibrational modes developed in the 3500 cm(-1) region of polylactide (PLA) spectra during crystallization are investigated. The band assignment to the OH stretching mode of terminal hydroxyls is unlikely because the trace amount of chain-ends is negligible considering the long chain of high molecular weight polymer. The band intensity can be enhanced for quantitative study by increasing the sample film thickness. The results show that the transition patterns of these bands mimic those of C=O stretching modes. Therefore, these are assigned to C=O overtones. Two bands associated with crystalline and amorphous characteristics are revealed during cold crystallization. The crystalline C=O bands of PDLA and its stereocomplex counterpart are located at 3510 cm(-1) and 3482 cm(-1), respectively, indicating a weaker C=O bond in the latter crystal structure. Two-dimensional Fourier transform infrared (2D-FT-IR) correlation spectroscopy is then applied to study the correlation between C=O overtones and the crystalline characteristic band located near 900 cm(-1). The transitions of the two vibrational modes observed in crystallization of the stereocomplex are in-phase with each other. This reflects an involvement of short-range hydrogen bonding in the stereocomplex crystal structure. In contrast, crystallization of PDLA shows that the C=O overtone varies prior to that of the C-H character, indicating that dipole-dipole force is a crystal-induced interaction.

Study of the Segmental Dynamics in Semi-Crystalline Poly(lactic acid) using Mechanical Spectroscopies

The glass transition of poly(L-lactic acid) (PLLA) occurs not far above typical service temperatures (room or body temperatures) which has consequences on the material properties during its use, such as damping or the occurrence of structural relaxation. This work aims at characterising the glass transition dynamics of a semi-crystalline PLLA using both dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). The main viscoelastic parameters have been characterised at 1 Hz using DMA and the master curve obtained after isothermal experiments at different temperatures provided a full picture of the solid-state rheological behaviour throughout a wide frequency range. The activation energies calculated from the shift factors agree with the TSR ones, exhibiting a maximum near the T(g). Above the T(g), the results can be described with the WLF model. In the glassy state, the activation energy decreases with decreasing temperatures being always higher than the prediction of the Adam and Gibbs theory, at least down to temperatures 30 degrees C below the T(g). This suggests the existence of non-arrested degrees of freedom in the glassy state, being consistent with the existence of a significant degree of cooperativity in the TSR results.