Non-isothermal Crystallization Experiments Research Articles

Nucleation of polyvinyl alcohol crystallization by single-walled carbon nanotubes

Non-isothermal crystallization experiments were performed on poly(vinyl alcohol) mixed with single-walled carbon nanotubes. Non-isothermal crystallization experiments showed that nanotubes nucleate crystallinity at weight fractions as low as 0.1%. An Ozawa model was applied to the non-isothermal crystallization data, and this approach confirmed the results deduced from the qualitative examination of the data: there was a clear difference in crystallization kinetics for the 0.1 and 1% sample vs. the sample without nanotubes. Nanotubes were also found to promote the thermal degradation of poly(vinyl alcohol).

EVALUATION OF THE MASTER CURVE APPROACH FOR THE NON-ISOTHERMAL CRYSTALLIZATION OF PP/EPR

The purpose of the present work was to test the validity of the Master Curve Approach in order to determine the nonisothermal crystallization constant for a heterophasic polypropylene (PP/EPR). Nonisothermal crystallization experiments were carried out in a DSC at several cooling rates, and the original curves were corrected because of the temperature lag between the samples and the furnace. Through the use of the nonisothermal kynetic crystallization constant, determined by the Master Curve Approach, and the use of the Nakamura equation, the curves of relative cristallinity in relation to temperature were simulated and compared with the corresponding curves obtained from experimental data. A relatively good agreement with the experimental curves was obtained. Key words: ethylene-propylene rubber, crystallization, master curve approach, heterophasic polypropylene

Crystallization kinetics and solidified structure in iPP under high cooling rates

A wide set of isothermal and non-isothermal crystallization experiments were carried out in this work on an iPP resin. Several experimental techniques were adopted in order to characterize crystallization kinetics and final morphology of the material, also under cooling rates comparable to those encountered during material processing (up to several hundred K/s). The whole set of data was taken as a reference to identify a kinetic model which describes the evolution of the structural organization of iPP (α crystalline phase and mesomorphic phase) as a parallel of two non-interacting kinetic processes competing for the available amorphous volume. Kolmogoroff equation was adopted to describe the crystallization of the α form. Avrami–Evans–Nakamura isokinetic approach was adopted to describe the evolution of the mesomorphic phase. Resulting kinetic model satisfactorily describes the whole set of experimental data including those obtained on samples solidified under high cooling rates, and reveals that a correct description of the evolution of the α phase during solidification can be attained only if the evolution of the competing mesomorphic phase is kept into account. The effect of cooling rate during solidification from the melt on diameters of spherulites, observed on solidified samples, is also satisfactorily described by model predictions.

Nucleation of Polypropylene Crystallization by Single-Walled Carbon Nanotubes

Nonisothermal and isothermal crystallization experiments were performed on polypropylene mixed with carbon nanotubes produced by disproportionation of CO on Co−Mo catalysts. Functionalization of the nanotubes with octadecylamine made the tubes hydrophobic and allowed the tubes to be solubilized in an organic solvent. Mixing of the nanotubes with the polymer was accomplished by adding the nanotubes to a Decalin solution that contained dissolved polypropylene, followed by evaporation of the solvent. Dynamic mechanical analysis indicated very little difference in the small-strain mechanical properties between filled and unfilled polymers at the very low solid levels that were tested. By contrast, the crystallization behavior of the filled and unfilled polymer was quite different. Nanotubes promoted growth of the less-preferred beta form of crystalline polypropylene at the expense of the alpha form. In nonisothermal crystallization, the total amount of crystalline material in the sample was the same for the filled and unfilled materials. However, for isothermal crystallization experiments, the percent crystallinity in the filled materials was slightly higher. Most importantly, the rate of crystallization was substantially higher in the filled system. The results presented in this paper clearly show that carbon nanotubes nucleate crystallinity in polypropylene.

Semi-rigid thermotropic polyester containing a rigid, bent spirobicromane moieties—primary characterizations and the thermal behavior

Thermotropic copolyester TBH with inherent spirobicromane (SPI) moieties was prepared by polycondensation reaction between mixed diols of 4,4-bis(6-hydroxyhexanoyloxy)spirobicromane (BHS, 10mol%), 1,7-heptanediol (HD, 90mol%) and terephthaloyl bis(4-oxybenzoyl chloride) (TOBC). In addition, homopolyester TH (or TB) synthesized from TOBC and HD (or BHS) diol was prepared for comparison. With the incorporated SPI moieties, polyester TB is a nematic material, in contrast to the smectic C2 mesophase observed for polyester TH and copolyester TBH. The rigid, spiro SPI moieties in polyester TB are supposed to interrupt the ordered packing of the mesogenic triads required for a smectic C2 mesophase. Under diffraction scanning calorimetry (DSC) scans, copolyester TBH exhibited multiple endothermic and exothermic peaks, which are different from the much simpler thermal behavior observed for homopolyesters TH and TB. The multiple melting and crystallization transitions for copolyester TBH was due to the facile crystallization of the inherent flexible spacers and the potential registry of neighboring chains in the mesomorphic state according to the results from isothermal, non-isothermal crystallization experiments, and the X-ray diffraction study. Suggestively, the rigid, bent SPI moieties would impose a nonlinear geometry on the connected spacers and mesogenic units and consequently, influence its chain arrangement in the mesomorphic state.

Highly active thermally stable ?-nucleating agents for isotactic polypropylene

Calcium salts of suberic (Ca-Sub) and pimelic (Ca-Pim) acids were synthesized and implemented as in different grades of isotactic polypropylene (iPP). Propylene homopolymer, as well as random and block copolymers containing these additives, crystallized iPP into pure or nearly pure β modification in the isothermal and nonisothermal crystallization experiments. Recently, Ca-Sub proved to be the most effective β-nucleating agent of iPP. The Ca-Sub nucleating agent widens the upper crystallization temperature range of pure β-iPP formation up to 140°C. In this study the effect of the these additives on the crystallization, melting characteristics, and structure of the PP were studied. The degree of crystallinity of β-iPP was markedly higher than that of α-iPP. A widening in the melting peak of the samples crystallized in a high temperature range was first observed and discussed in regard to literature results of the same phenomenon for α-iPP. The morphology of the β-iPP samples was revealed by scanning electron microscopy. Independent of the type of polymer or nucleating agent, hedritic structures were found in the early stages of growth of the β-spherulites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2357–2368, 1999

Isotactic polypropylene/ethylene-co-propylene blends: Influence of the copolymer microstructure on rheology, morphology, and properties of injection-molded samples

Meltrheological behavior, phase morphology, and impact properties of isotactic-polypropylene (iPP)-based blends containing ethylene–propylene copolymer (EPR) synthesized by means of a titanium-based catalyst with very high stereospecific activity (EPRTi) were compared to those of iPP/EPR blends containing EPR copolymers synthesized by using a traditional vanadium-based catalyst (EPRV). The samples of EPR copolymers were synthesized ad hoc. They were characterized by comparable propylene content, average molecular masses, and molecular mass distribution in order to assess the effects of distribution of composition and sequence lengths of the structural units on the structure–properties correlations established in the melt and in the solid state while studying different iPP/EPR pairs.1–5 Differential scanning calorimetry, (DSC), wide-angle X-ray spectroscopy (WAXS), small-angle X-ray (SAXS), and scanning electron microscopy (SEM) investigations showed that the EPRTi chain is characterized by the presence of long ethylenic sequences with constitutional and configurational regularity required for crystallization of the polyethylene (PE) phase occurring, whereas a microstructure typical of a random ethylene–propylene copolymer was exhibited by the EPRV copolymer. The different intra- and intermolecular homogeneity shown by such EPR phases was found to affect their melt rheological behavior at the temperatures of 200 and 250°C; all the EPRTi dynamic–viscoelastic properties resulting were lower than that shown by the EPRV copolymer. As far as the melt rheological behavior of the iPP/EPRV and iPP/EPRTi blends was concerned, both the iPP/EPR pairs are to be classified as “negative deviation blends” with G′ and G" values higher than that shown by the plain components. The extent of the observed deviation in the viscosity values and of the increase in the amounts of stored and dissipated energy shown by such iPP/EPR pairs was found to be dependent on copolymer microstructure, being larger for the melts containing the EPRTi copolymer. The application of the Cross–Bueche equation also confirmed that, in absence of shear, the melt phase viscosity ratio is the main factor in determining the viscosity of iPP/EPR blends and their viscoelastic parameters. The general correlation established between EPR dispersion degree (range of particle size and number-average particle size), as determined in injection-molded samples, and melt phase viscosity ratio (μ) was ratified; the type of dependence of EPR size upon μ value was in qualitative agreement with the prediction of the Taylor–Tomotika theory. Contrary to expectation,1–5 for test temperature close to iPP Tg, EPRV particles ranging in size between 0.75 and 1.25 μm resulted and were more effective than EPRTi particles, ranging in size between 0.25 and 0.75 μm, in promoting multiple craze formation. Also taking into account the SAXS results, revealed that the molecular superstructure (i.e., crystalline lamellar thickness and amorphous interlayer) of the iPP matrix is unaffected by both the presence of EPRTi and EPRV phase. The above finding was related to the ethylenic crystallinity degree shown by the EPRTi copolymer. In particular, such a degree of crystallinity was supposed to deteriorate toughening by decreasing the tie molecules density in the EPRTi domains, notwithstanding the beneficial effect of the ethylenic lamellar buildup. For test temperature close to room temperature, the ductile behavior exhibited by the iPP/EPRTi blends was accounted for by a predominant shear yielding fracture mechanism probably promoted by a high concentration of interlamellar tie molecules among iPP crystallites in agreement with DSC results. Nonisothermal crystallization experiments showed, in fact, that the crystallization peak of the iPP phase from iPP/EPRTi melt is shifted to higher temperatures noticeably, thus indicating a material characterized by a comparatively higher number of spherulites per unit value grown at lower apparent undercooling values. Accordingly, WAXS analysis revealed comparatively higher iPP crystal growth in the directions perpendicular to the crystallographic planes (110) and (040) of the iPP. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 701–719, 1999

Crystallization and Melting of β-Nucleated Isotactic Polypropylene

Ca salts of suberic (Ca-Sub) and pimelic acid (Ca-Pim) were synthesized and used as β-nucleating agents in different grades of isotactic polypropylene (IPP). Propylene homo-, random- and block-copolymers containing these additives crystallize principally in pure β-modification as demonstrated in isothermal and non-isothermal crystallization experiments. Ca-Sub proved the most effective β-nucleating agent known, so far. It broadens the upper crystallization temperature range of pure β-IPP formation up to 140°C. The effect of the additives on the crystallization and melting characteristics of the polymers was studied. The degree of crystallinity of the β-modification was found to be markedly higher than that of α-IPP. High temperature melting peak broadening was first observed and discussed in literary results regarding the same phenomenon for α-IPP.

Crystallization and microstructure in quenched slabs of various molecular weight polypropylenes

AbstractThe purpose of this study was to accumulate material data for use in a simulation of morphology and crystallization in quenched slabs of isotactic polypropylenes (i‐PP's). The molecular weight effect on the crystallization kinetics of various i‐PP's were studied using isothermal and nonisothermal experiments. An experimental setup was constructed to measure the spherulite growth rate of i‐PP's using isothermal and nonisothermal melt crystallization experiments. The molecular weight effect on spherulite growth rate of i‐PP's was determined. Molecular weight effect on nuclei concentrations as a function of temperature was determined from growth and crystallization rate constants. The morphology and crystallization in quenched slabs of i‐PP's were determined from experiments and compared to computer simulations. The molecular weight effect on the degree of crystallinity and morphology was determined for i‐PP during quenching.

Poly(p-phenylene sulfide)/liquid crystalline polymer blends. II. Crystallization kinetics

The crystallization kinetics of blends made of poly(p-phenylene sulfide) (PPS), with a liquid crystalline polymer (LCP) was studied. The blends were found to be immiscible by dynamic mechanical thermal analysis (DMTA). Results of non-isothermal and isothermal crystallization experiments made by differential scanning calorimetry (DSC) showed that both components had their crystallization temperatures increased ; also the LCP melting temperature was found to increase in the blends. It was concluded that the addition of LCP to the PPS increased the PPS overall crystallization rate due to heterogeneous nucleation. The fold interfacial free energy, σ e , of the PPS in the blends was observed not to vary with composition.

The effect of electric field on the crystallization of polyamide-6,6 by rheological measurement

The effect of the electric field on crystallization behaviour of polyamide-6,6(PA-6,6) was investigated by a rheological measurement. The crystallization kinetics were estimated by monitoring the changes in the absolute value of complex viscosity during crystallization. In both isothermal and nonisothermal crystallization experiments, it is observed that the crystallization rate of PA-6,6 decreases with increasing the applied electrical field. This phenomenon is explained by the orientation of dipole moments in amide groups under the electric field. In the PA-6,6, dipoles of amide groups are aligned antiparallel to each other to yield a stable crystal structure. The external electric field disturbs the antiparallel orientation of dipoles and thus prevents the chain packing during crystallization.

側鎖にバルキーな置換基を有する芳香族コポリエステルの結晶化における活性化エネルギー

The activation energy for the crystallization of an aromatic copolyester with bulky side group was obtained by using a differential scanning calorimeter (DSC). The results of non-isothermal crystallization experiments indicate that there are two crystallization processes; the fast process and the slow process. The crystallization during cooling is controlled by diffusion. The activation energy for the slow process is quite large because of difficulty in diffusion of a molecule due to the rigid rod structure. On the other hand, the isothermal crystallization takes place by nucleation and growth. The activation energy based on kinetic data was 645kJ·mol-1. The value is somewhat larger than those of flexible polymers because of the large energy barrier in molecular movement due to the rigid backbone and bulky side groups. The equilibrium melting temperature and heat of fusion were determined, and the equilibrium entropy change of 0.63J·mol-1·K-1 was calculated from the thermodynamical relationship, ΔSf°=ΔHf°/Tm°. Compared to the literature values for flexible polymers such as polyethylene (9.88J·mol-1·K-1) and polypropylene (15.1J·mol-1·K-1), the obtained value is rather small. This is because the transition from nematic melt to crystals induces less entropy change than that from isotropic melt to crystals.