Current issues in research on structure–property relationships in polymer nanocomposites

The irruption of polymers from renewable resources on the scene of macromolecular science and technology

Polymorphism and isomorphism in biodegradable polyesters

Crystallization of drugs formulated in the amorphous form may lead to reduced apparent solubility, decreased rate of dissolution and bioavailability and compromise the physical integrity of the solid dosage form. The purpose of this work was to develop thermodynamic approaches, both practical and theoretical, that will yield a better understanding of which factors are most important for determining the ability of polymers to stabilize amorphous active pharmaceutical ingredients (API). Lattice based solution models were used to examine miscibility criteria in API-polymer blends. Different methods were used to estimate the Flory‐Huggins interaction parameter for model API-polymer systems consisting of felodipine or nifedipine with poly(vinylpyrrolidone) (PVP). These were melting point depression and determination of solubility parameters using group contribution theory. The temperature and enthalpy of fusion of crystalline API alone and the fusion temperature of the API in the presence of the polymer were measured by differential scanning calorimetry. The resultant thermal data were used to estimate the reduced driving force for crystallization and the solubility of the API in the polymer. Flory‐Huggins theory predicts that, for typical API-polymer systems, the entropy of mixing is always favorable and should be relatively constant. Due to the favorable entropy of mixing, miscibility can still be achieved in systems with a certain extent of unfavorable enthalpic interactions. For the model systems, interaction parameters derived from melting point depression were negative indicating that mixing was exothermic. Using these interaction parameters and Flory‐Huggins theory, miscibility was predicted for all compositions, in agreement with experimental data. A model was developed to estimate the solubility of the API in the polymer. The estimated solubility of the model APIs in PVP is low suggesting that kinetic rather than thermodynamic stabilization plays a significant role in inhibiting crystallization. The thermodynamics of API-polymer systems can be modeled using solution based theories. Such models can contribute towards providing an understanding of the compatibility between API and polymer and the mechanisms of physical stabilization in such systems.

Theoretical and Practical Approaches for Prediction of Drug–Polymer Miscibility and Solubility

From monomers to polymers from renewable resources: Recent advances

This paper reports a DSC and FTIR study of blends of poly(l-lactide) (PLLA) with poly(vinylphenol) (PVPh). According to the single Tg criterion, miscibility has been found in all the compositions range for the blends obtained by solution/precipitation in a dioxane/hexane pair. However, phase separation has been observed for PVPh-rich blends obtained by solvent casting from dioxane solutions. The Tg of the blends shows negative deviation from linearity. Hydrogen bonding has been found, and the band attributed to hydrogen-bonded carbonyl groups is shifted about 18 cm-1, suggesting relatively weak hydroxyl−ester hydrogen bonds. The equilibrium melting points of pure PLLA and different blends have been recorded, and the values of the interaction parameter χ12 = −0.42 and the interaction energy density B = −8.8 cal/cm3 have been calculated. The negative value of the interaction parameter confirms a thermodynamically miscible blend. The value of B is similar to the value found in poly(e-caprolactone) (PCL)/PVPh...

Miscibility and Specific Interactions in Blends of Poly(l-Lactide) with Poly(Vinylphenol)

This paper reports the analysis of the C=O stretching region of poly(L-lactide). This spectral band splits into up to four components, a phenomenon that a priori can be explained in terms of carbonyl-carbonyl coupling or specific interactions (such as C-H...O hydrogen bonding or dipole-dipole). Hydrogen bonding can be discarded from the analysis of the C-H stretching spectral region. In addition, low molecular weight dicarbonyl compounds of chemical structure similar to that of PLLA, such as diacyl peroxides, show a remarkable splitting of the carbonyl band attributed to intramolecular carbonyl-carbonyl coupling. Several mechanisms can be responsible for this behavior, such as mechanical coupling, electronic effects, or through-space intramolecular TDC (transition dipole coupling) interactions. Intermolecular dipole-dipole interactions (possible in the form of interchain TDC interactions) are proven to be of minor relevance taking into account the spatial structure of the PLLA conformers. The Simply Coupled Oscilator (SCO) model, which only accounts for mechanical coupling, has been found to predict adequately the relative intensity of the symmetric and asymmetric bands of dicarbonyl compounds. The dispersion curves predicted for PLLA by the SCO model also match those given by more general treatments, such as Miyazawa's first-order perturbation theory. Hence, the SCO model is adopted here as an adequate yet simple tool for the interpretation of band splitting caused by intramolecular coupling of polylactide. The four components observed in the C=O stretching band of semicrystalline PLLA are attributed to the four possible conformers: gt, gg, tt, and tg. The narrow bands observed for the interlamellar material are attributed to highly ordered chains, indicating the absence of a truly amorphous phase in the crystalline polymer. The interphase seems to extend over the whole interlamellar region, showing the features of a semiordered metastable phase. In amorphous PLLA, bands corresponding to gt, gg, and tt conformers also can be resolved by second derivative techniques, and curve-fitting results provide information about the conformational population at different temperatures.

Conformational behavior of poly(L-lactide) studied by infrared spectroscopy.

Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions

Binder Jetting Metal Additive Manufacturing (BJ-MAM), known also as metal 3D-printing, is a powder bed-based additive manufacturing technology. It consists of the deposition of liquid binder drople...

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A review on recent developments in binder jetting metal additive manufacturing: materials and process characteristics

Bioresorbable polylactides are one of the most important materials for tissue engineering applications. In this work we have prepared scaffolds based on the two optically pure stereoisomers: poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA). The crystalline structure and morphology were evaluated by DSC, AFM and X-ray diffraction. PLLA and PDLA crystallized in the α form and the equimolar PLLA/PDLA blend, crystallized in the stereocomplex form, were analyzed by a proliferation assay in contact with mouse L-929 and human fibroblasts and neonatal keratinocytes for in vitro cytotoxicity evaluation. SEM analysis was conducted to determine the cell morphology, spreading and adhesion when in contact with the different polymer surfaces. The preserved proliferation rate showed in MTT tests and the high colonization on the surface of polylactides observed by SEM denote that PLLA, PDLA and the equimolar PLLA/PDLA are useful biodegradable materials in which the crystalline characteristics can be tuned for specific biomedical applications.

Ester Zuza

Papers

Miscibility and Specific Interactions in Blends of Poly(l-Lactide) with Poly(Vinylphenol)

Conformational behavior of poly(L-lactide) studied by infrared spectroscopy.

Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions

A review on recent developments in binder jetting metal additive manufacturing: materials and process characteristics

Crystallinity assessment and in vitro cytotoxicity of polylactide scaffolds for biomedical applications.