Giuseppina Ceccorulli

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.

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.

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.

Composition dependence of the glass transition temperature of polymer-diluent systems: 1. Experimental evidence of a dual behaviour in plasticized PVC

The glass transitions of six PVC-diluent systems have been studied using differential scanning calorimetry (d.s.c.). The diluents were: di-(methyl, ethyl, n-propyl, n-butyl)-phthalate, n-butylacetate and tritolylphosphate. A large number of compositions have been examined over the whole concentration range, from pure polymer to pure diluent. For all systems but PVC-butylacetate, the Tgconcentration dependence shows a ‘cusp’ in the vicinity of 40% diluent content. The width of the glass transition increases in the concentration range of the ‘cusp’ reaching values as high as 50°C. PVC-butylacetate mixtures show two glass transitions, both decreasing with increasing diluent content, in a narrow composition range. For all systems two Tgconcentration dependencies can be identified: the fast decrease of the polymer Tg caused by plasticizer addition and the much slower increase of the diluent Tg due to the presence of the dissolved polymer molecules. Also the specific heat increment at Tg(ΔCp) shows a dual concentration dependence, suggesting that it is the component present ‘in excess’ that dictates the ΔCpcomposition behaviour.

Composition dependence of the glass transition of polymer-diluent mixtures: 2. Two concomitant glass transition processes as a general feature of plasticized polymers

The available theoretical and empirical treatments concerning the glass transition temperature, Tg, of polymer-diluent mixtures do not account for the occurrence of the ‘cusp’ in the Tgcomposition dependence or of the two concomitant glass transitions recently reported by these authors. The only treatment predicting a change of curvature in Tg vs. concentration, due to Braun and Kovacs, does not describe satisfactorily the behaviour of the seven polymer-diluent systems examined. In order to compare the results with the available literature data, a normalization procedure is developed, whose main effect is to evidence that the Tgconcentration dependence of all polymer-diluent systems is composed of two branches. It is concluded that in polymer-diluent mixtures still considered homogeneous at a macroscopic level two different mobilization phenomena occur, related to mobilization of ‘hindered diluent’ and ‘plasticized polymer’, respectively. The possibility of revealing the two phenomena individually depends strongly on the technique being used as well as on the peculiar physical properties of the binary system being examined.

Dynamic mechanical relaxations of poly(vinylidene fluoride)-poly(vinylpyrrolidone) blends

Abstract Dynamic mechanical measurements on blends of poly(vinylidene fluoride) (PVDF) and poly(vinylpyrrolidone) (PVP) over the whole composition range are reported from −150 to 220°C. The results provide evidence of miscibility of PVDF with PVP. Two ranges of compositions are identified. When the PVP content is higher than 40%, crystallization of PVDF is inhibited and single-phase, homogeneous mixtures are obtained. At lower PVP concentrations, three phases coexist: a homogeneous blend of quasi-constant composition, crystalline PVDF and a pure amorphous PVDF phase, the so-called ‘interphase’ evidenced by a relaxation at about −40°C. For the partially crystalline blends, the viscoelastic spectrum is shown to depend strongly on thermal history.

Effect of thermal history on Tg and corresponding Cp changes in PVC of different stereoregularities

Abstract Samples of poly(vinyl chloride) polymerized at temperatures from −60°C to +90°C, and therefore of different degrees of syndiotacticity and crystallinity, have been thermally treated in an attempt to reduce their content of ordered structures. Changes in order have been monitored by measuring Tg and the corresponding specific heat increment Δcp. Partially syndiotactic samples have higher Tg and lower Δcp than samples that are nearly atactic. After thermal treatment the differences decrease but do not vanish, even for samples heat-treated to about 250°C. This seems to indicate that some ordered structures are very stable even at relatively high temperatures. A quantitative estimate of the ordered structure content, based on Δcp data, is proposed.

Viscoelastic properties of cellulose derivatives: 2. Effect of diethylphthalate on the dynamic mechanical relaxations of cellulose acetate

Abstract The dynamic mechanical spectra of cellulose acetate (CA) with various amounts of diethylphthalate (DEP), from 3 to 50 wt%, have been obtained over a range of temperatures (−130°C to 240°C) and frequencies (0.1 to 30 Hz). The effect of DEP addition on the main (α) relaxation is to shift the dispersion and corresponding modulus drop to lower temperatures. The depression of the transition temperature is smaller than commonly found for polymer-diluent mixtures. The influence of DEP on the secondary β and γ relaxations is quite different: while the low-temperature γ dispersion is progressively depressed by DEP addition, the intensity of the β relaxation increases strongly at DEP contents higher than 15%. The dynamic modulus increases with γ peak depression and shows a more intense drop with the strengthening of the β relaxation. The activation energy of the β dispersion remains fairly constant up to 15% DEP content, then increases to approach, at 50% DEP, the ΔH value of the α relaxation of the pure diluent. At high DEP contents, active participation of the diluent molecules in the motion responsible for the β relaxation is suggested.