A. Etxeberria

Synthesis, structure and properties of poly(L-lactide-co-ε-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.

Inverse gas chromatography in the characterization of polymeric materials

Different experimental results are presented for the use of inverse gas chromatography in the determination of the physico-chemical properties of pure polymers, polymer solutions and polymer blends. Using poly(hydroxy ether of bisphenol-A) (Phenoxy), its solutions and its blends with other polymers, thermal transitions, polymer solubility parameters, polymer-solvent and polymer-polymer interaction parameters and diffusion coefficients of small molecules in pure Phenoxy have been measured by inverse gas chromatography.

Effects of chain microstructures on mechanical behavior and aging of a poly(L-lactide-co-ε-caprolactone) biomedical thermoplastic-elastomer

Three statistical poly(L-lactide-co-ε-caprolactone) (PLCL) copolymers of 70% L-lactide content having different chain microstructures ranging from moderate blocky to random (R=0.47,0.69 and 0.92, respectively) were characterized by DSC, GPC and (1)H and (13)C NMR. The results demonstrate that higher randomness character (R→1) limits the capability of crystallization of LA-unit sequences shifting the melting temperature of the copolymers to lower values and reducing the crystallinity fraction substantially. The effect of different distributions of sequences of PLCL on crystallization and phase behavior was also studied for different storage times at room temperature (21±2°C) by DSC. The mechanical properties were evaluated by tensile tests during aging. The PLCL showing a random character closest to the Bernoullian distribution of sequences (l(LA)=1/CL) was found to exhibit higher strain capability and strain recovery values and is less prone to supramolecular arrangements. However, as a result of aging, L-lactide sequence blocks in the other PLCLs of smaller randomness character tend to crystallize prompting to a double T(g) behavior indicative of the existence of phase separation into two compositionally different amorphous phases. Physical aging leads also to dramatic changes in tensile behavior of the moderate blocky PLCLs that evolved from being an elastomeric to be partly a glassy semicrystalline thermoplastic, and, thus, can eventually condition its potential uses for medical devices.

Crystallization and melting behaviour of poly(bisphenol A hydroxy ether)/poly(ethylene oxide) blends

Abstract The crystallization and melting behaviour of solution-cast films of poly(bisphenol A hydroxy ether)/poly(ethylene oxide) (PEO) blends were investigated as a function of composition by means of optical microscopy, differential scanning calorimetry and dilatometry. All blends show only one single value of glass transition temperature, intermediate between those of the pure polymers. The growth rate of PEO spherulites (G) as well as the observed equilibrium melting temperature, for a given ΔT, decrease as the phenoxy content increases. These results suggest that this blend is miscible for every composition investigated. Temperature and composition dependences of the radial growth rate G and the overall kinetic constant K have been employed to calculate the surface energy of folding σe. The interaction energy density B has been determined from the dependence of the equilibrium melting-point depression upon composition.

Polymer–solvent interaction parameters in polymer solutions at high polymer concentrations

Molecular mass and temperature dependences of the polymer-solvent interaction parameter have been investigated in the extreme interval of high polymer concentration using inverse gas chromatography (IGC). The observed molecular mass dependence has been compared with the predictions of a theoretical model, which emphasises the role of intramolecular contacts in the polymer chain. The model reproduces reasonably well the molecular mass dependence. However, enthalpic and entropic contributions of the interaction parameter, obtained from IGC measurements at different temperatures, exhibit behaviours difficult to explain in the framework of the current models.

Miscibility of poly(vinyl chloride)/poly(ethylene oxide) blends—I. Thermal properties and solid state 13C-NMR study

Abstract Blends of poly(vinyl chloride), PVC, and poly(ethylene oxide), PEO, have been studied for a complete range of composition, by differential scanning calorimetry and high-resolution solid state 13C-NMR spectroscopy. Proton spin-lattice relaxation times in the rotating frame, T1ρ(H), as well as calorimetric data indicate miscibility of the components of these blends in the amorphous phase for mixtures containing ≥40% PVC. In this range of composition and from the melting point depression for PEO in the mixtures, the binary interaction parameter is found to be −0.094.

Miscibility of poly(vinyl chloride)/poly(ethylene oxide) blends—II. An inverse gas chromatography study

The application of inverse gas chromatography (IGC) to measure polymer-polymer interaction parameters (X or B) requires attention to the dependence of these parameters for the selected probes. A phenomenological alternative to solve this problem, recently proposed, has been applied to poly(vinyl chloride)/poly(ethylene oxide) blends. A composition dependent B parameter was obtained, in reasonable agreement with the results reported in the previous paper of this series.

Key role of entropy in nanoparticle dispersion: polystyrene-nanoparticle/linear-polystyrene nanocomposites as a model system

Dilution of contact, hard sphere-like, nanoparticle-nanoparticle interactions upon mixing plays a key role in explaining nanoparticle dispersion in athermal all-polymer nanocomposites, as illustrated for polystyrene-nanoparticle/linear-polystyrene blends as a model system.

Tensile behavior and dynamic mechanical analysis of novel poly(lactide/δ-valerolactone) statistical copolymers.

Lactide-co-δ-valerolactone copolymers (PLVL) have not attracted as much research interest as the more popular poly(lactide-co-ε-caprolactone) (PLCL) elastomeric materials. In this work the study of the mechanical performance is focused on the former with the aim of identifying the potential advantages of these thermoplastic elastomers for their application in the biomedical field. Mechanical testing (at 21°C and at 37°C) of at least 5 specimens and dynamic mechanical analysis (DMA) in duplicate were carried out on various PLVL, which include a moderately blocky l-lactide/δ-valerolactone copolymer (~70% of l-LA and R=0.68) and several that showed a random distribution of sequences (R~1): some terpolymers based on l-lactide, d-lactide and δ-valerolactone (with a lactone content of ~25 and ~14%) and a series of copolymers of l-LA and δ-VL having l-LA molar contents ranging from 69 to 74%. In view of the results, it can be concluded that noteworthy improvements in stiffness and strength were achieved by adding δ-VL to the reaction mix instead of ε-CL, although both monomers have analogous chemical properties. For example, a PLVL with a 75:25M composition of l-LA/δ-VL at 21°C presented a secant modulus of 213.7±36.5MPa and σu=14.7±1.4MPa whereas a previously studied PLCL of equal composition had a secant modulus and an ultimate stress value of 19.4±1.3MPa and 3.2±0.6MPa, respectively. At 37°C, the differences in the mechanical properties between the different PLVLs of this work were far less relevant, with most of them showing a fully elastomeric behavior. Referring to the DMA measurements, the reduction in the peak of tan δ (from ~2.5 to 0.5) through the glass transition was a clear indicator that crystalline domains formed during hydrolytic degradation in some of the polymers. However, the more amorphous PLVLs with short l-LA average sequence lengths (ll-LA<2.91) did not undergo changes in the storage modulus and tan δ curves after two weeks submerged in PBS at 37°C.

A Nanotechnology Pathway to Arresting Phase Separation in Soft Nanocomposites

Direct observation of the miscibility improving effect of ultra-small polymeric nanoparticles (radius ≈4 nm) in model systems of soft nanocomposites is reported. We have found thermodynamically arrested phase separation in classical poly(styrene) (PS)/poly(vinyl methyl ether) blends when PS linear chains were totally replaced by ultra-small, single chain PS nanoparticles, as determined by thermo-optical microscopy measurements. Partial arrested phase splitting on heating was observed when only some of the PS chains were replaced by unimolecular PS nanoparticles, leading to a significant increase of the lower critical solution temperature (LCST) of the system (up to 40 °C at 15 vol.-% nanoparticle content). Atomic force microscopy and rheological experiments supported these findings. Thermodynamic arrest of the phase separation process induced by replacement of linear polymer chains by unimolecular polymer nanoparticles could have significant implications for industrial applications requiring soft nanocomposite materials with excellent nanoparticle dispersion in a broad temperature range.