Giovanni C. Alfonso

Mechanism of densification and crystal perfection of poly(ethylene terephthalate)

Abstract Amorphous poly(ethylene terephthalate) has been isothermally annealed in a wide range of temperatures from Tg to Tm. The effects of temperature and time of thermal treatment on density, melting curve and WAXS have been systematically explored. The results showed that densification and crystal perfection take place through a step mechanism involving partial melting and diffusion. The evaluation of the activation energies suggests that diffusion is the rate determining step in the low temperature range.

Crystallization kinetics of oriented poly (ethylene terephthalate) from the glassy state

Abstract Crystallization kinetics of oriented poly(ethylene terephthalate) have been studied in a temperature range close to Tg. It has been shown that orientation of the amorphous phase promotes a substantial increase in crystallization rate. This effect, in turn, depends on the crystallization temperature: the higher the temperature, the stronger is the effect of orientation. From experimental results it was possible to make an estimation of parameters describing quantitatively the crystallization kinetics in the oriented state.

Polymer crystallization studies under processing-relevant conditions at the SAXS/WAXS DUBBLE beamline at the ESRF

Recent developments on the experimental infrastructure and the acquisition of new detectors on the Dutch–Belgian beamline BM26B at the ESRF offer novel and promising possibilities for synchrotron X-ray experiments in the field of polymer crystallization under processing-relevant conditions. In this contribution, some of the most recent experiments mimicking conditions similar to those relevant for the plastics processing industry are discussed. Simultaneous thermal analysis and wide-angle X-ray scattering (WAXS) experiments, at the millisecond time-frame level, on β-nucleated isotactic polypropylene (i-PP) samples subjected to ballistic cooling up to 230 K s−1, show that the efficiency of the nucleating agent can be suppressed when quenched cooling rates higher than 130 K s−1 are used. In situ WAXS experiments using small-scale industrial equipment during a real film blowing process reveal the dependence of the onset of crystallinity (the so-called freeze line) and the crystal orientation as a function of different take-up and blow-up ratios. In situ small-angle X-ray scattering (SAXS) experiments during high-flow fields reveal the formation of shish and kebab structures in i-PP as a function of the imposed stress. Quantitative analysis of i-PP flow-induced structures is presented. The beamline specifications required to obtain high quality and industrially relevant results are also briefly reported

Memory effects in isothermal crystallization. I: Theory

A theoretical analysis of transient isothermal crystallization, including relaxational and athermal effects is given. The analysis is based on Kolmogoroff-Avrami-Evans transformation equation and nucleation theory of Turnbull and Fisher. The memory of previous structures manifests itself in the distribution of atomic (molecular) clusters which determines the initial number of crystal nuclei and the initial rate of thermal nucleation.

Three-block copolymers: morphologies and stress - strain properties of samples prepared under various experimental conditions

Abstract The structural and morphological characterization of a styrene-butadiene-styrene thermoelastic three-block copolymer (Kraton 1101) is reported together with the stress-strain properties of specimens prepared with different techniques (compression moulding, extrusion, solution casting). The influences of the extrusion and of the annealing treatment at high temperature on the morphology (and therefore on the stress-strain properties) are considered in detail because the structure of the specimen becomes particularly simple; it consists of polystyrene cylinders arranged along the extrusion direction and embedded into the rubbery matrix. The rods along the axis lead to a continuous polystyrene phase, so that the plugs exhibit the Mullins and hardening effects already observed for other copolymers. Films cast from toluene are also investigated; the rate of evaporation of the solvent shows a remarkable influence on the morphology of the material and, of course, on its physical properties. An explanation for the hardening effects at high values of deformation is also suggested, supported by optical birefringence measurements on compression moulded specimens after strain.

Time‐Resolved SAXS/WAXS Studies of the Polymorphic Transformation of 1‐Butene/Ethylene Copolymers

Abstract The melting behavior of isothermally crystallized it‐1‐butene/ethylene copolymers with different ethylene content has been investigated by differential scanning calorimetry (DSC) and simultaneous small‐ and wide‐angle x‐ray diffraction (SAXS/WAXS) using synchrotron radiation. A very important influence of the aging time on the structural characteristics of these copolymers has been observed. The thermal behavior determined by DSC is very complex, especially for the copolymer with the highest ethylene content. In the DSC curves, as well as the two endotherms that can be clearly assigned to Form II and Form I, several endotherms can be observed at low temperatures, which increase in intensity with increasing aging time. The WAXS measurements have ruled out the existence of a different crystalline modification. Simultaneous real‐time SAXS/WAXS experiments performed during the heating of aged samples have demonstrated that the multiple endotherms correspond to the melting of crystals with different thermodynamic stability.

Crystallization of poly(aryl-ether-ether-ketone)s blends

Abstract The crystallization and melting behaviour of poly(aryl-ether-ether-ketone) (PEEK) in blends with another polymer of the same family containing a bulky pendant phenolphthalein group (PEK-C) have been investigated by thermal methods. The small interaction energy density of the polymer pair (B = −8.99 J/cm3), evaluated from equilibrium melting point depression, is consistent with the Tg data that indicate partial miscibility in the melt. Two conjugated phases are in equilibrium at 430°C: one is crystallizable and contains about 35 wt% of PEK-C; the other, containing only 15 wt% of PEEK, does not form crystals upon cooling and it interferes with the development of spherulites in the sample. The analysis of kinetic data according to nucleation theories shows that crystallization of PEEK in the explored temperature range takes place in Regime III and that a transition to Regime II might be a consequence of an increase in the amount of non-crystallizable molecules in the PEEK-rich phase. A composition independent value of the end surface free energy of PEEK lamellae has been derived from kinetic data ( ρ e = 40 ± 4 erg cm 2 ) in excellent agreement with previous thermodynamic estimates. A new value for the equilibrium melting temperature of PEEK (T°m = 639 K) has been obtained; it is about 30°C lower than the commonly accepted value and it explains better the “memory effect” in the crystallization from the melt of this high performance polymer.

Morphology and structure of poly(ethylene-cocarbon monoxide) single crystals

Abstract Random copolymers from ethylene and carbon monoxide, containing different numbers of carbonyl groups in the main chain, have been crystallized from dilute solutions. The self-seeding technique has been applied. Monolayer single crystals are obtained and some results concerning their morphology and the unit cell dimensions as functions of the carbonyl content are reported. The annealing treatment supports a structural transition previously noted for other polymers.

A simple model of flow-induced crystallization memory

Flow induced crystallization of polymer systems exhibits strong memory effects. Crystalline structures gradually change when the flow is switched off and the polymer is relaxed prior to crystallization. A simple model based on the multidimensional theory of crystal nucleation [1] is proposed. Steady, potential flow applied to a polymer fluid above melting temperature (T p > T m ) results in molecular orientation of crystallizing units. The flow controls formation of molecular clusters which convert into athermal nuclei when the system is cooled down to crystallization temperature, T c < T m . Orientation effects gradually disappear when the melt is relaxed above T m in the absence of flow or stress.

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.