Mariastella Scandola

Thermal properties and physical ageing of poly (l-lactic acid)

Poly(l-lactic acid) (PLLA) was investigated by differential scanning calorimetry and dynamic mechanical and dielectric spectroscopies. In the dynamic mechanical and dielectric spectra no secondary relaxations were observed below the glass-to-rubber transition (70°C at 3 Hz). The thermal history strongly affects the physical properties of PLLA, inducing changes of crystalline: amorphous ratio as well as large physical ageing effects on the glassy amorphous phase. This latter phenomenon is clearly observed as a consequence of room-temperature storage and develops at a faster rate as the ageing temperature Ta approaches Tg. Ageing experiments carried out at the same undercooling (ΔT = Tg - Ta) on PLLA samples of different molecular weight (Mv = 5300, 20 000, 691 000) show that a decrease in molecular weight increases the magnitude of the enthalpy relaxation at the glass transition.

Binary blends of microbial poly(3-hydroxybutyrate) with polymethacrylates

Abstract Blends of bacterial poly(3-hydroxybutyrate) (PHB) with poly(methyl methacrylate) (PMMA) and poly(cyclohexyl methacrylate) (PCHMA) were prepared by melt compounding followed by quenching to room temperature. PHB/PMMA blends containing up to 20 wt% PHB are single-phase amorphous glasses with a composition dependent glass transition temperature ( T g ). When the concentration of PHB exceeds 20 wt%, partially crystalline pure PHB coexists with a constant-composition PHB/PMMA ( 20 80 ) mixture, which represents the solubility limit of PHB in PMMA. In such blends crystallization of PHB, both isothermally from the melt and with heating from the rubbery state, is retarded by increasing amounts of PMMA in the blend. PHB shows no miscibility at all with PCHMA. In the PHB/PCHMA blends, two phases, consisting of the pure polymer components, are present over the whole composition range and PHB crystallization is unaffected by the presence of the methacrylate polymer.

Physical properties of poly(ester-urethanes) prepared from different molar mass polycaprolactone-diols

Abstract Segmented poly(ester-urethanes) (PEUs) based on poly(ϵ-caprolactone) (PCL) as a soft segment and a non-aromatic diisocyanate in the hard segment were synthesized. The soft segment crystallinity and other physical properties of the PEUs were studied. It was found that the crystallinity and rate of crystallization of the PCL continuous phase in the PEUs decreases and the glass transition temperature of PEUs increases in comparison with the PCL prepolymers. The restriction of the crystallization of the PCL soft segment depends on the hard segment concentration, length of the soft segment, and total molecular weight of the PEUs.

Molecular motions of chitosan in the solid state

Abstract The relaxation behavior of chitosan has been investigated by dynamic mechanical and dielectric spectroscopy over a wide temperature and frequency range. Two dispersion regions are found in the spectrum of the dry polysaccharide: a relaxation (γ) at low temperature (− 102° at 3 Hz; ° H =47 kJ/mol) and another relaxation process above room temperature (130° at 3 Hz; ° H ∼ 100 kJ/mol). Absorbed water strongly modifies the relaxation spectrum of chitosan below room temperature, giving rise to a new relaxation (βd) at temperatures higher than the γ peak. With increasing water content, the γ peak is progressively depressed, while the βd relaxation intensifies and moves to lower temperatures. In room-stored samples (∼10% absorbed water) the βd peak is the only observable relaxation in the viscoelastic spectrum of chitosan below room temperature. The γ and βd loss processes are attributed to local motions of the polysaccharide backbone and of complex polymer-water units, respectively. The characteristic parameters of the high-temperature peak indicate that the relaxation originates from short-range rather than cooperative molecular motions.

Scaffold for tissue engineering fabricated by non-isothermal supercritical carbon dioxide foaming of a highly crystalline polyester

Porous scaffolds of a random co-polymer of omega-pentadecalactone (PDL) and epsilon-caprolactone (CL) (poly(PDL-CL)), synthesized by biocatalysis, were fabricated by supercritical carbon dioxide (scCO(2)) foaming. The co-polymer, containing 31 mol.% CL units, is highly crystalline (T(m) = 82 degrees C, DeltaH(m) = 105 J g(-1)) thanks to the ability of the two monomer units to co-crystallize. The co-polymer can be successfully foamed upon homogeneous absorption of scCO(2) at T > T(m). The effect of soaking time, depressurization rate and cooling rate on scaffold porosity, pore size distribution and pore interconnectivity was investigated by micro X-ray computed tomography. Scaffolds with a porosity in the range 42-76% and an average pore size of 100-375 microm were successfully obtained by adjusting the main foaming parameters. Process conditions in the range investigated did not affect the degree of crystallinity of poly(PDL-CL) scaffolds. A preliminary study of the mechanical properties of the scaffolds revealed that poly(PDL-CL) foams may find application in the regeneration of cartilage tissue.

Viscoelastic and thermal properties of bacterial poly(d-(−)-β-hydroxybutyrate)

Abstract Three poly( d -(-)-β-hydroxybutyrate) (PHB) samples from Rhbizobium spp. have been characterized in order to evaluate the effects of different extraction procedures of the polymer. Only the molecular weight is found to change, being 6 × 10 4 for the sample extracted with HCl (1 m and of the order of 10 6 for the samples extracted with acetone. Thermogravimetric results on PHB with different molecular weights, indicate that the temperature at which weight loss becomes significant is lower than 230°C only for the lowest molecular weight examined (6 × 10 4 ). The calorimetric properties strongly depend on thermal history. The d.s.c. curves of ‘room stored’ samples show only a melting endotherm at 177°C, whose area increases with annealing. Quenching from the melt shows evidence an intense glass transition (ΔCp = 0.43 J/g deg) in the vicinity of 0°C, followed by a ‘cold crystallization’ peak preceding melting. The viscoelastic spectrum shows three relaxations: a water-related low temperature relaxation (−80°C), the glass transition (apparent activation energy ΔH a = 356 KJ/mol) and a broad relaxation in the temperature range between T g and T m due to motions in the crystalline phase.

Viscoelastic relaxations and thermal properties of bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate)☆

3-Hydroxybutyrate-3-hydroxyvalerate (3HB-3HV) as well as 3-hydroxybutyrate-4-hydroxybutyrate (3HB-4HB) copolyesters have been investigated by differential scanning calorimetry, thermogravimetric analysis and dynamic mechanical spectroscopy, over a wide range of compositions (0-95 mol% 3HV; 0-82 mol% 4HB). Both series of isolated copolyesters are partially crystalline at all compositions. Quenched samples show a glass transition that decreases linearly with increasing co-monomer molar fraction, more markedly when the co-monomer is 4HB. Above Tg, all copolyesters, rich in 3HB units, show a cold crystallization phenomenon followed by melting, while at the other end crystallization on heating is observed only in 3HB-3HV copolymers. The viscoelastic spectrum, strongly affected by thermal history, shows two relaxation regions: the glass transition, whose location depends on copolymer type and composition, and a secondary dispersion region at low temperatures (-130/-80 degrees C). The latter results from a water-related relaxation analogous to that of P(3HB) and, in 3HB-4HB copolymers, from another overlapping absorption peak centered at -130 degrees C, attributed to local motion of the methylene groups in the linear 4HB units.

Viscoelastic properties of cellulose derivatives: 1. Cellulose acetate

Abstract The dynamic mechanical spectrum of cellulose acetate (CA) from −130°C to 240°C has been determined at different frequencies (from 0.1 to 30 Hz). Three relaxations, designated α, β and γ in order of decreasing temperature, and one shoulder (β∗) above room temperature were found. Comparison with calorimetric and thermogravimetric measurements yields the conclusion that the α relaxation (197°C at 3 Hz) is related to the glass-to-rubber transition and the β∗ shoulder (50°C–100°C) is due to loss of moisture. The β relaxation (−38°C at 3 Hz, ΔH = 100 kJ mol−1) is tentatively assigned to local motions of the main chain (glucopyranose rings). The low-temperature γ relaxation (−88°C at 3 Hz, ΔH = 46 kJ mol−1), is humidity-dependent: its intensity decreases when the samples are dried to moisture contents lower than that obtained by normal room conditioning (about 3%). Higher water contents shift the relaxation to lower temperatures without increasing the intensity of the mechanical loss. It is suggested that water associated with the unesterified methylol groups of cellulose acetate is responsible of the dynamic mechanical γ dispersion.

Miscibility of bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with ester substituted celluloses

SummaryBlends of bacterial Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP) were prepared by melt blending. When CAB or CAP content in the blends is ≥ 50%, the blends are stable, homogeneous mixed glasses, characterized by a glass transition that follows the Fox equation. When the cellulose ester content is lower than 50%, PHBV crystallizes upon room storage and the blends are partially crystalline. Besides the strongly composition dependent glass transition, another slightly composition dependent relaxation is observed by both DMTA and DSC measurements in the vicinity of the glass transition of PHBV. It is suggested that both blend components partake in both mobilization processes.

Structural investigation of poly(3-hydroxybutyrate) spherulites by microfocus X-ray diffraction

A banded spherulite of bacterial poly(3-hydroxybutyrate) (band spacing 120 μm) was linearly scanned along the radial direction in 3 μm steps by means of microfocus X-rat diffraction using a synchroton source with a beam diameter of 3 μm. A large number of X-ray patterns was collected inside each band; Those taken at the center fo the bands represented the two limiting diffraction patterns from which the exact orientation of the unit cell was inferred. The intensity of the reflections (020) and (002) , taken as indicators of parellelism to the X-ray beam of the unit cells c- or b-axis, respectively, changed periodically along the spherulite radius, alternating maximal with zero-intensity zones. When the reflection (020) showed a maximum, the reflection (002) was practically nil and vice versa. This behavior clearly shows that the unit cells mean position smoothly rotates around the a-axis with increasing distance from the spherulite center. The identity of the cell rotation period with the band spacing derived from optical microscopy observations provides structural evidence that the extinction bands originate from a regular twisting of the lamellar crystals during their growth.