M. L. Di Lorenzo

Non-isothermal crystallization of polymers

Abstract Traditional studies of crystallization kinetics are often limited to idealized conditions, in which the parameters of state (temperature, pressure, etc.) are constant. In real situations, however, the external conditions change continuously, which makes the kinetics of crystallization dependent on instantaneous conditions, as well as on rate of change. This article provides an overview of the current state-of-the-art of non-isothermal crystallization of polymers during the cooling from the melt. The majority of the proposed theoretical formulations that predict non-isothermal crystallization kinetics, concern bulk crystallization and are based on modifications of the Avrami equation. The Ziabicki, Nakamura and Ozawa models are examined here in some detail together with treatments from other authors. The basic hypotheses of the various models, as well as their relative drawbacks, are underlined. Alternative empirical approaches to calculate the main parameters of non-isothermal crystallization and to compare the crystallization rate of different polymeric systems are also discussed. This article reviews the data concerning non-isothermal crystallization processes for different classes of polymers. Major attention is directed towards the dynamic crystallization of polyolefins, a class of materials of large industrial interest. Other results for polyoxyolefins, polyesters, polyamides and polyketones are also examined.

Crystallization of isotactic polypropylene/natural terpene resins blends

The influence of two natural terpene resins on the morphology, phase structure and isothermal crystallization process of iPP was investigated by optical (OM) and electron microscopy (SEM) and differential scanning calorimetry (DSC). It was found that in dependence on temperature, composition and chemical nature of the resin, one, two or three phases can be present. The isothermal spherulite growth rate and overall crystallization rate of the blends are depressed compared to plain iPP. This depression was attributed to the increase of the energies relative to the transport of macromolecules in the melt and to the formation of nuclei of critical size, following the addition of resin. The presence of resin seems to lead to the formation of more regular folding surfaces.

Blends of isotactic polypropylene and natural terpene resins. I. Phase structure, thermal, and dynamic‐mechanical properties

This article discusses the influence of two natural terpene resins (NTR), poly(α-pinene) (PαP A115) and poly(d-limonene) (PL C115), on morphology, miscibility, thermal, and dynamic-mechanical properties of their blends with isotactic polypropylene (iPP). The NTR have interesting physical and chemical properties, and they are approved for food contact application. From the results of differential scanning calorimetry and dynamic-mechanical thermal analysis it was deduced that both the resins were completely miscible with the amorphous iPP up to the composition investigated here (70/30 wt %). Scanning electron microscopy (SEM) analysis instead showed that the 70/30 iPP/PαP A115 blend and 80/20 and 70/30 iPP/PL C115 blends contained very small domains homogeneously distributed into the matrix. It is hypothesized that the domains are likely formed by the terpene-rich phase, and the matrix by the iPP-rich phase (besides the crystallized iPP phase). The iPP-rich phase and the NTR-rich phase would have the glass transition temperatures so close that they cannot be resolved by DSC and DMTA. Finally, for the iPP/PαP A115 system an upper critical solution temperature (UCST) is proposed.

Crystallization of poly(1-butene)/hydrogenated oligocyclopentadiene blends

The crystallization process of poly(1-butene) and its blends with hydrogenated oligocyclopentadiene was analyzed in dependence on composition. Both isothermal and nonisothermal crystallization processes were studied, and the isothermal crystallization process was investigated starting from both the glass and the melt states. Results revealed that spherulite growth rate, overall crystallization rate, and morphology are strongly dependent on crystallization conditions and blend composition. Nonisothermal crystallization data were analyzed according to the theories of Ozawa and Ziabicki.

Effect of the thermo-oxidation and natural weather on the structure, morphology, and properties of unstabilized and HALS-stabilized LDPE films

ABSTRACT The aging of unstabilized low density polyethylene (LDPE) films and those stabilized with hindered amine light stabilizers (HALS) was studied as a function of the exposure time to thermo-oxidation at 90°C and to natural weathering. The HALS tested was Tinuvin 783 from Ciba-Geigy and added to the polymer in the concentration of 0.6% (w/w). Modifications in the film characteristics were investigated by using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS) and mechanical testing. The FTIR spectra showed that the thermo-oxidation led to the formation of ketone groups attributed to hydroperoxide decomposition, whereas in natural weathering ketones and vinyl unsaturations are formed due to Norrish II reactions. The mechanism of stabilization involved in thermo-oxidation of the HALS-stabilized samples may be attributed to the classical radical scavenging of alkyl and peroxy radicals formed during propagation by nitroxyl radicals. The stabilizing ...

Thermal and morphological analysis of isotactic poly(1-butene)/hydrogenated oligocyclopentadiene blends

Abstract This contribution discusses the influence of hydrogenated oligocyclopentadiene (HOCP) on the morphology, the phase structure and the thermal properties of its blends with isotactic poly(1-butene) (PB-1) as a function of composition and crystallization conditions. PB-1 and HOCP are partially miscible in the melt state. The blends, in the solid state, form generally a three-phase system: a crystalline phase formed by the polyolefin and two amorphous phases, of which one is rich in PB-1 and the other one in HOCP. The optical micrographs of the solidified blends show a morphology constituted by microspherulites and domains of HOCP-rich phase homogeneously distributed in intraspherulitic regions. Moreover, when the PB-1 and the blends are isothermally crystallized, hedrite-like crystallites are observed. The crystallization process and the melting behavior are strongly influenced by the presence of the oligomer, since the addition of HOCP decreases the overall crystallization rate and the melting point of the blends.

Morphology development of isotactic poly(4-methylpentene-1) during melt crystallization

The morphology of melt-crystallized isotactic poly(4-methylpentene-1) samples, prepared by varying crystallization temperature and time, as well as annealing temperature, has been probed by small and wide angle x-ray scattering, and electron microscopy. This was then correlated with the thermal properties investigated by differential scanning calorimetry. Relationships among melting behavior, morphological evidences and X-ray results have been found. Different populations of lamellae, grown with a time-dependent sequence, can be obtained: their relative amount and thermal stability is determined by crystallization and annealing conditions.

Morphology, phase structure, and properties of polyolefin/hydrogenated oligocyclopentadiene blends

Abstract The paper discusses the influence of an amorphous oligomer (namely hydrogenated oligocyclopentadiene — HOCP) on the morphology and the phase structure of its blends with several polyolefins as a function of composition and crystallization conditions. In particular the following polyolefins were studied: high-density polyethylene (HDPE), isotactic polypropylene (IPP), poly(l-butene) (PB-1), and poly(4-methyl pentene-1) (P4MP1). The blends under investigation are complex polymer systems. In fact, in dependence on temperature, blend composition, and cooling rate, they assume different morphologies and consequently show different thermal and mechanical behaviors. In the solid state the blends form a generally three-phase system: a crystalline phase of polyolefin and two amorphous phases, one rich in the amorphous polyolefin and the other in HOCP. The crystallization process and the properties are determined by the morphology and the phase structure, as well as by the physical state of the HOCP-rich p...