Jose-Ramon Sarasua

Synthesis, structure and properties of poly(L-lactide-co--caprolactone) statistical copolymers

Four poly(L-lactide-co-ε-caprolactone) (PLCL) copolymers were synthesized at 120, 130, 140 and 150 °C by ring opening polymerization using stannous octoate catalyst at a 2000:1 comonomer:catalyst ratio. Gel permeation chromatography (GPC) and (1)H NMR measurements were performed to determine the molecular weight, composition and chain microstructure of copolymers of L-lactide(LA):ε-caprolactone(CL) synthesized using 90:10, 80:20, 75:25 and 70:30 feed ratios. The overall conversion of these PLCL copolymers was in the range of 80%-90% leading to weight average molecular weights (M(w)) between 98,500 and 226,000 g mol(-1) depending on feed composition and polymerization temperature. At temperatures lower than 140 °C, the incorporation of CL units into polymer chains was incomplete because of the low reactivity of CL, thus at 120 °C the copolymer composition was difficult to control obtaining more LA in the copolymer than the desired, hence the blocky character of PLCL copolymers also increased. At 150 °C the catalyst was less effective and the molecular weights of the copolymers took lower values. A temperature of 140 °C was established as optimal to obtain highest yields and molecular weight. The number average crystallizable lactide sequence lengths (l(LA)) shifted from 6.5 to 16.7 LA repeat units for PLCL polymerized at 140 °C while the randomness character (R) value shifted from 0.4 for polymerization at 130 °C to 0.6, at 150 °C. Increasing the LA content in the copolymers the glass transition temperature and the crystallizability and melting temperature of PLCLs approached to that of PLLA homopolymer. The aging sensitivity of PLCLs increased with CL content. A double T(g) behavior due to phase separation associated to crystallizing LA blocks was observed during aging. The mechanical properties, however, evolved toward the PLLA character when the molar content of LA in PLCL was increased from 66% to 90%, observing a shift from an elastomeric thermoplastic behavior to that of a glassy plastic, reflected by an increase in tensile modulus (from 12.0 to 1343.1 MPa) and a decrease in strain recovery after break (from 93.5% to 25.0%). Small amounts of CL content in the copolymers produced large improvements in their deformability with regard to PLLA. In addition, thermogravimetric analysis demonstrated that PLCLs are more stable to thermal degradation than PLLA and they undergo a more complex degradation mechanism than those of the corresponding homopolymers.

Rheological Behavior and Modeling of Thermal Degradation of Poly(∊-Caprolactone) and Poly(L-Lactide)

Abstract Growing environmental concerns have led to the development of alternative biodegradable polymers with properties comparable to the conventional poly(ethylene), poly(propylene) or poly(ethylene terephthalate). In this paper the thermal degradation of poly(∊-caprolactone) (PCL) and poly(L-lactide) (PLLA) melts was investigated by rheometry and thermogravimetry under different temperatures and inert atmosphere. The chain scission process was modeled to explain the phenomenon. The analysis suggests that PLLA is degraded by an unzipping depolymerization process from the hydroxyl end of the polymer chains. In contrast, the thermal degradation behavior of PCL was very complex because various reactions occurred concurrently: post-polymerization, loss of structural regularities and random chain scissions.

Competing specific interactions investigated by molecular dynamics: analysis of poly(p-dioxanone)/poly(vinylphenol) blends.

Molecular dynamics simulations (MD) were carried out to model the miscibility behavior of blends of poly(p-dioxanone) (PPDO) with poly(vinylphenol) (PVPh). The Hildebrand solubility parameters of the pure polymers and the Flory-Huggins interaction parameters of the blends at different compositions were computed. Negative interaction parameters were found across the whole range of compositions, suggesting the miscibility of the system, in agreement with the experimental results. The interaction parameter obtained from melting point depression studies was also found to be in good agreement with the value computed from the simulations. The repeat unit of PPDO contains one ether and one ester group, and both can act as hydrogen bond acceptors. The radial distribution functions (RDFs) between those groups and the hydroxyl groups of PVPh were computed to investigate the competence between the acceptor groups for the specific interactions. The RDFs indicate that interassociation occurs mainly with the ester groups, which is detrimental to the ether groups. This result was also corroborated by the analysis of the hydroxyl stretching region of the blends using Fourier transform infrared spectroscopy (FTIR). The good overall agreement found between the simulated and the experimental data reveals the importance of the molecular modeling techniques in the analysis of the miscibility behavior of polymer blends.

Analysis of the Miscibility of Polymer Blends Through Molecular Dynamics Simulations

Molecular Dynamics Simulations are important tools in the prediction of the properties of polymer materials and have therefore become invaluable aids in the design of new materials suited to particular applications. The combination of Molecular Simulations and the FloryHuggins theory allows the study of the compatibility of polymer blends. MD Simulations constitute the simplest type of simulation for complex systems containing different types of interactions between bonded and non bonded atoms. These interactions are summarized in the forcefield files. The systems may be very sensitive to some details of the model; particularly the choice of the forcefield. Among different properties, the interaction parameters can be calculated from the numerical trajectories of polymer blends, which constitute statistical samples.

Biodegradable Polylactide‐Based Composites

The aim of this chapter is to introduce to the use and possible applications of polylactide‐based composites. Polylactides are biodegradable aliphatic polyesters, which are widely used in medical and ecological‐friendly fields. First of all, a deep description of main characteristics of polylactides is shown. This chapter summarizes many concepts, which comprehend a general view of polylactide biopolymers such as synthesis and structures, physical‐chemical and mechanical characterization and possible applications of final products. Then, an overview of composites based on polylactides and their benefits compared with bare polylactides are described.