Hoffman-Lauritzen Theory Research Articles

Crystallization and melting of propylene–ethylene random copolymers. Homogeneous nucleation and β-nucleating agents

The different crystallization behavior of two propylene–ethylene random copolymers, (PP-R and PP-RCT), one of which (PP-RCT) contains a β-nucleating agent is investigated in the present paper. Isothermal and non-isothermal crystallization behavior was thoroughly studied. The subsequent melting behavior after isothermal crystallization was studied by means of standard and Step-Scan DSC. The equilibrium melting point of α crystals determined using the Hoffman–Weeks extrapolation was 11.8°C higher than that for β crystals. The Lauritzen–Hoffman theory was applied and the nucleation constant Kg and the surface free energy σe was calculated. From the isothermal experiments it was concluded that the nucleating agent increased drastically the crystallization rate of the copolymer, also promoting the generation of β crystals. On cooling PP-RCT crystallized at about 8°C higher temperatures than PP-R. The non-isothermal crystallization of the materials was examined and for the kinetics analysis the Avrami, Ozawa and Mo models were elaborated. Nucleation activity was calculated with the use of Dobreva’s method. The effective activation energy was calculated for both polymers according to the iso-conversional method of Friedman and lower values resulted as expected in case of the PP copolymer with nucleating agent.

Nucleation density and growth rate of polypropylene measured by calorimetric experiments

A novel experimental protocol was adopted, based on calorimetric measurements, but varying each time the minimum temperature before the isothermal crystallization step. This method allows to measure nucleation density and growth rate for commercial semicrystalline polymers, for which the nucleation phenomenon is essentially heterogeneous. The protocol was applied to a commercial grade isotactic polypropylene, well characterized in the literature. Crystallization kinetics was evaluated using the Avrami model, through nucleation and growth, describing all experimental results. Morphological characterizations were modeled using the classical Lauritzen–Hoffman theory. Detailed comprehensive description of the crystallization kinetics of i-PP is provided, predicting nucleation and growth with the tailored thermal history.

Microstructure, morphology, crystallization and rheological behavior of impact polypropylene copolymer

Impact polypropylene copolymer (IPC), named polypropylene catalloy, not only possesses excellent impact property, but also presents good rigidity. Its superior performances result from the complicated composition and microstructure. In the present article, recent progress in the studies on microstructure, morphology, crystallization and rheological behavior of IPC is summarized, and findings of the authors and their collaborators are reported. In general, IPC is divided into three components, i.e., ethylene-propylene random copolymer (EPR), a series of different segment lengths ethylene-propylene copolymer (EbP) and propylene homopolymer. The reasonable macromolecular structures of EbP and a multilayered core-shell model of dispersed phase structure in IPC were proposed, in which the dispersed phase consists of an outer EbP shell, an inner EPR layer and an EbP core. It is found that the annealing at melt-state may lead to an abnormal phase inversion, and the phase inversion disappears when temperature cools down to room temperature. The cause of phase inversion is ascribed to the existence of EbP component, which results in the stronger activity of the dispersed phase. The crystalline structure and morphologic results confirm the formation of β-iPP in IPC. Furthermore, it is found that the ethylene content in IPC and cooling rate of the samples have an important influence on the formation of β-iPP. Based on the crystallization kinetics analyzed by Lauritzen-Hoffman theory, crystallization behavior of different IPC samples is discussed and it is proposed that the dilution effect of ethylene-propylene copolymer has a more remarkable influence on surface nucleation than on crystal growth. In addition, annealing at high temperature can result in the changes of chain structure for IPC, and this instability is ascribed to the oxidative degradation and crosslink reaction mainly in iPP component.

Isothermal crystallization kinetics of Kevlar fiber‐reinforced wood flour/high‐density polyethylene composites

AbstractKevlar fiber [KF; poly(p‐phenylene terephthalamide)] can be used as a reinforcing element to improve the mechanical properties of the resulting wood thermoplastic composites. This study was devised to investigate the effects of incorporation of KF on the isothermal crystallization kinetics of high‐density polyethylene (HDPE) in the resulting composites using differential scanning calorimetry. Avrami model was applied to describe the isothermal crystallization process, and the fold surface‐free energy was calculated according to the Hoffman–Lauritzen theory. Comparative study of neat HDPE, wood flour/HDPE composite (WPC), virgin KF‐reinforced WPC (KFWPC), and grafted Kevlar fiber (GKF)‐reinforced WPC (GKFWPC) showed that the overall crystallization rate, the activation energy, the equilibrium melting temperature, and the fold surface‐free energy of the WPC were apparently changed due to the addition of KF; the crystallizability exhibited an order of KFWPC > GKFWPC > HDPE > WPC. The incorporation of virgin KF may cause the heterogeneous nucleation to induce a change in the crystal growth of HDPE from the tridimensional to bidimensional or to the mixed patterns. Avrami exponent values of the composites decreased with time, confirming the change of crystallization behavior. This study demonstrates that both the KF and GKF can act as nucleating agents to improve the crystallization rate of HDPE. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Influence of extrusion temperature on molecular architecture and crystallization behavior of peroxide‐induced slightly crosslinked poly(L‐lactide) by reactive extrusion

AbstractThe influence of temperature during reactive extrusion of poly(L‐lactide) (PLLA) on the molecular architecture and crystallization behavior was investigated for OO‐(t‐butyl) O‐(2‐ethylhexyl) peroxycarbonate‐modified polymer. The long chain–branched PLLA (LCB‐PLLA) content and its structure in the resulting slightly crosslinked PLLA (χ‐PLLA) containing linear and LCB‐PLLA were characterized by both analyses, size exclusion chromatography equipped with multiangle laser light scattering and rheological measurements. A reduction of LCB‐PLLA content in χ‐PLLA and an increase of number of branches in LCB‐PLLA were found with increasing the extrusion temperature. An increase of extrusion temperature induces different process in the polymer: decrease of the lifetime of peroxide, increase of the radical concentration due to rapid peroxide decomposition rate, and increase of the chain diffusion to the amorphous phase. Among these indices, the lifetime of peroxide is a good index for crosslinking behavior of PLLA during extrusion. As for the isothermal crystallization behavior from the melt, the Avrami crystallization rate constant of χ‐PLLA increases as an increase of LCB‐PLLA content in χ‐PLLA. This implies that LCB‐PLLA acts as a nucleating agent for PLLA. Furthermore, regime analysis and the free energy of nucleus of χ‐PLLA were investigated using Hoffman–Lauritzen theory. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Nucleation and crystallization kinetics of poly(lactic acid)

A commercial grade PLA was characterized using differential scanning calorimetry under both isothermal and non-isothermal conditions, and the experimental data were analyzed evaluating the crystallization kinetics constants for melt and cold isothermal crystallization at some temperatures. Furthermore, a peculiar experimental protocol was adopted, varying each time the minimum cooling temperature before the isothermal step, performed monitoring nucleation and growth rate in an adequate range of temperatures. Morphological characterizations were modeled using the classical Lauritzen–Hoffman theory, which assumes that the free energy barrier associated with nucleation has an energetic origin, estimating the dependence on temperature using physical parameters derived from the linear Hoffman–Weeks plot. Crystallization kinetics was evaluated using the Avrami model, through nucleation and growth rate, describing both the melt and cold crystallization experimental results. Detailed comprehensive description of the crystallization kinetics of PLA is provided, predicting the final crystallinity and the morphology with tailored thermal history.

Crystallization and melting of biodegradable poly(propylene suberate)

Thermal behavior of poly(propylene suberate) (PPSub) was studied. PPSub is a novel biodegradable polyester which always shows lower crystallinity, and thus faster biodegradation, than poly(ɛ-caprolactone) or poly(butylene succinate). Crystalline structure, multiple melting behavior and re-crystallization of PPSub on heating were studied by using Wide Angle X-ray Diffraction (WAXD), Differential Scanning Calorimetry (DSC) and Step Scan DSC. The equilibrium melting point and the enthalpy of fusion of the pure crystalline polymer were found 79.7 °C and 28.9 kJ/mol, respectively. Polarized Light Microscopy (PLM) tests showed that PPSub forms banded spherulites. The Lauritzen–Hoffman theory was used. Values of spherulite growth rates and also overall crystallization rates after self-nucleation, from DSC data, were used. In the latter case the inverse of crystallization half times or the Avrami constant K values were supposed to be a measure of crystallization rates. Critical breakpoint in the Lauritzen–Hoffman plots appeared at 40 °C showing crystallization regime II to III transition. Non-isothermal crystallization kinetics was also analyzed. Interestingly, the Ozawa model seems to hold for PPSub. The iso-conversional method of Friedman was used to study the activation energy of PPSub crystallization on cooling and the Lauritzen–Hoffman parameters were also calculated by applying the Vyazovkin method.

The Effect of Acid-Treatment of Carbon Nanotubes on the Thermal Kinetics of Isotactic Polypropylene

Isotactic polypropylene (iPP)/maleic-anhydride-grafted polypropylene (PP-g-MA)/raw and acid-treated single-walled carbon nanotube (SWCNT) nanocomposites were prepared in an internal mixer. Thermal and crystallization behaviors of these nanocomposites were evaluated. Thermal properties of the iPP and its nanocomposites were estimated using thermo-gravimetric analysis (TGA) under nonisothermal conditions. The kinetics of nonisothermal degradation of iPP and its nanocomposites were studied using the Friedman model based on the single heating rate method. In terms of onset temperature of degradation (Tonset) and activation energy for decomposition (E), it was found that raw and acid-treated SWCNTs provide improved thermal stability to the iPP matrix. The respective activation energy values were determined to be 84, 94, and 96 kJ/mol for iPP, iPP/SWCNT, and iPP/A-SWCNT, respectively. Differential scanning colorimetry (DSC) was used to study the isothermal crystallization kinetics of iPP and its nanocomposites. During isothermal crystallization, relative crystallinity developed with a time dependence described by the Avrami equation, with the exponent n = 3. Further analysis based on the Hoffman–Lauritzen theory revealed that the surface free energy (σe) for the nanocomposites was lower than that of iPP.

Prediction of non-isothermal crystallization parameters for isotactic polypropylene

Non-isothermal crystallization kinetics of isotactic polypropylene (iPP) was simulated with the assumption that the non-isothermal crystallization process was composed of some finite isothermal crystallization processes, while each isothermal crystallization process consists of three main steps—induction, nucleation and crystal growth. In the simulation, induction time was taken into account, allowing one to make predictions on the start of the non-isothermal crystallization of iPP; nuclei density was treated as a function of temperature; the Hoffman–Lauritzen theory was employed to describe the spherulite growth rate varied with temperature, and the relative crystallinity was determined by the equation of Kolmogorov. Finally, model prediction was verified by quantitative comparison between the theoretical results with the experimental results.

Nonisothermal crystallization kinetics study of LDPE/MWCNT nanocomposites: Effect of aspect ratio and surface modification

AbstractIn this article, the effect of aspect ratio and chemical modification of multiwall carbon nanotubes (MWCNT) on the nonisothermal crystallization kinetics of LDPE/MWCNT nanocomposites was studied. Nine different samples were prepared using different MWCNT to study both effects. The cooling rate (R) was varied in the range 2–10°C/min. In this article, the effect of CNT loading, surface modification, and aspect ratio were studied. For the same MWCNT concentration, aspect ratio and COOH modification had weak influence on both the peak crystallization temperature and the crystallization onset temperature. However, the crystallization onset temperature was significantly affected by the amount of MWCNT. The rate parameters in the modified Avrami method and Mo method [F(T)] of analyses show a very good fit of data. The Vyazovkin and Sbirrazzuoli method of analysis, which is based on Hoffman–Lauritzen theory for secondary crystallization, was also used. Temperature dependency of activation energy was obtained for 30–75% relative crystallinity of the produced nanocomposites. Activation energy based on calculations of Hoffman‐Lauritzen theory showed a decrease with the increase in the concentration of MWCNT and crystallization temperature. A proposed model of the form E = a exp (‐bXT) which relates the activation energy, E, to relative crystallinity, X, and crystallization temperature, T, was able to fit the whole set of data. Incorporation of MWCNT in nanocomposites lowers the activation energy; hence enhances the initial crystallization process as suggested by the different methods of data analyses. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Crystallization behavior of isotactic polypropylene/magnesium salt whisker composites modified by compatibilizer PP-g-MAH

Crystallization behavior of isotactic polypropylene (iPP)/magnesium salt whisker (MSW) composites modified by compatibilizer PP-g-MAH was studied under both isothermal and nonisothermal conditions. Analysis of the isothermal crystallization showed that the Avrami model successfully described the crystallization process. On the basis of Lauritzen–Hoffman theory, a regime transition was observed at about 139 °C for the iPP/MSW composite, and a decrease in the fold surface free energy was calculated with the addition of MSW and PP-g-MAH. The addition of MSW filler and PP-g-MAH compatibilizer distinctly improved the crystallization temperature and accelerated the total crystallization rate of iPP. It was observed that MSW induced the formation of β-iPP but PP-g-MAH suppressed the formation of β-iPP.

Crystallization behavior and crystallization kinetic studies of isotactic polypropylene modified by long‐chain branching polypropylene

AbstractIn this article, we discuss the crystallization behavior and crystallization kinetics of isotactic polypropylene (iPP) modified by long‐chain‐branching (LCB) high‐melt‐strength iPP over a wide composition range, that is, LCB‐iPP from 10 to 50 wt %. Over the entire range we investigated, the presence of LCB‐iPP accelerated crystallization in both the isothermal crystallization process and nonisothermal crystallization process, even when the LCB‐iPP content was as low as 10%, and both crystallization processes were enhanced more significantly as the LCB‐iPP content increased. Hoffman–Lauritzen theory analysis revealed that the fold‐free energy decreased effectively with the occurrence of the LCB structure, although the growth rate of spherulites was depressed, as shown by polarized optical microscopy. Meanwhile, the regime III–regime II transition temperature was about 15° higher for all of the LCB‐iPP compositions than that of iPP because the LCB structure reduced the mobility of the polypropylene chains. Furthermore, the γ‐form crystal structure was favored by LCB compared to the β form, which was supported by wide‐angle X‐ray diffraction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Crystallization Behavior of the Blends of Isotactic Polypropylene and Ethylene-Propylene Blocky Copolymers

Two ethylene-propylene copolymer fractions (EP90 and EP120) were separated from a polypropylene in-reactor alloy by extraction with n-octane at different temperatures. 13C-NMR shows that these two fractions have a blocky structure and WAXD reveals that both ethylene and propylene sequences in these two fractions are crystallizable. However, EP90 has higher propylene content and the average length of propylene sequences is longer. These two fractions were blended with isotactic polypropylene (PP) at various proportions, respectively, and crystallization behavior and morphology of the blends were investigated. It is found that both EP90 and EP120 are partially compatible with PP. The phase-separated domains have a nucleation effect on crystallization of PP, leading to increase in crystallization temperature and crystallinity of PP in the blends. EP90 and EP120 also affect the relative content of β crystals in an irregular way. The number of EP90-rich domains in PP/EP90 blends is larger than that of EP120-rich domains in PP/EP120 blends, but the size of EP90-rich domains is smaller, indicating that EP90 has better compatibility with PP than EP120. Spherulites are formed in all the blends. The data were analyzed with Hoffman-Lauritzen theory of crystallization regime and the free energy of the folding surface (σe) was derived. Addition of EP90 and EP120 has little effect on the transition temperature from regime II to regime III. The value of σe for the PP/EP90 blends is similar to that of neat PP, but σe of the PP/EP120 blends is a little higher than that of neat PP.

Perkalite as an Innovative Filler for Isotactic Polypropylene-Based Nanocomposites

Perkalite, an organically modified synthetic layered double hydroxide, was used as a filler in the preparation of nanocomposites based on isotactic polypropylene. The polymorphism of the polymer matrix was investigated, finding that Perkalite favors beta phase in certain conditions and is not a nucleant for the gamma polymorph. Perkalite promoted the formation of a dual population of lamellae, as evidenced by SAXS data. The dispersion of the filler was studied by WAXD, SAXS, and TEM. Perkalite was intercalated by the polymer and formed small, rather disordered tactoids, that determined an increase in physical mechanical properties, such as modulus, impact strength, thermal stability and heat deflection temperature. The Tm(0) of the materials was calculated by the method proposed by Marand, the kinetics of crystallization was evaluated by the Avrami analysis and also the Hoffman-Lauritzen theory of crystallization regimes was applied. Perkalite was found to depress Tm(0), to greatly enhance the rate of crystallization and to ease the chain folding of macromolecules.

Effect of plasticizer on the crystallization behavior of poly(lactic acid)

AbstractIn this article, the spherulitic morphology and growth rate of the neat and plasticized poly(lactic acid) (PLA) with triphenyl phosphate (TPP) were compared and analyzed by polarizing optical microscopy with hot stage at a temperature range of 100−142°C. The spherulitic morphology of the neat PLA underwent a series of changes such as the typical Maltese Cross at less than 132°C, the disappearance of the Maltese Cross at 133°C, the irregular and distorted spherulites at higher than 134 and 142°C, respectively. For plasticized PLA, the spherulitic morphology exhibited the same changes as neat PLA, but these changes were shifted to lower temperature when compared with neat PLA. In the case of the spherulitic growth, neat PLA had the maximum value of 0.28 μm/s at 132°C, and plasticized PLA had higher values than that of neat PLA. Further analysis based on the Lauritzen–Hoffman theory was presented and results showed that the values of nucleation parameter Kg increased with TPP content. The crystallization behavior of PLA was analyzed by differential scanning calorimetry and wide‐angle X‐ray diffraction. The results showed that the degree of crystallinity of plasticized PLA markedly increased when compared with neat PLA sharply with the incorporation of plasticizer. The crystallization kinetics for the neat and plasticized PLA under isothermal crystallization at 114°C was described by the Avrami equation and the Avrami exponent is close to 2, implying that the crystallization mechanism did not change. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Effect of carbon nanotubes content on crystallization kinetics and morphology of polypropylene

Thermoplastic nanocomposites were prepared in a laboratory mixer using polypropylene (PP) and different amounts of single-walled carbon nanotubes (SWNT) in the range 0.25–2 wt%. The effect of SWNT content on the thermal and mechanical properties and also morphology of the PP/SWNT nanocomposites were studied. The results obtained from nonisothermal crystallization of PP and the nanocomposites, which were carried out using the differential scanning calorimetry technique, showed that not only the overall rate of crystallization of PP increased when SWNT was added to the polymer but also the rate of nucleation was higher and the crystallite size distribution was more uniform for the nanocomposites than for PP. From the optical microscopy studies, it was found that the PP spherulites decreased in size when SWNT was introduced into the polymer and also the mature spherical shaped crystals of PP changed in part to the immature kidney- or bean-shaped crystal forms in the nanocomposites. In addition, the crystallization kinetics was also studied by using isothermal spherulitic growth rate, and the values of nucleation constant, K g, and end surface free energy, σ e, were calculated for PP and the nanocomposites according to Lauritzen–Hoffman theory. The reductions of these two parameters were in agreement with the fact that the rate of crystallization of PP in nanocomposites was higher than that of the pristine polymer.

Non-isothermal crystallization kinetics of iPP/sPP blends

Non-isothermal crystallization and subsequent melting behaviour of iPP/sPP blends of various compositions were investigated by differential scanning calorimetry (DSC). Results revealed that sPP crystallizes at a slower rate than iPP. This effect is attenuated when the cooling rate is increased up to 20 °C/min and 25 °C/min. DSC scans showed that stability of primary crystallites of sPP and perfection of iPP crystallites were both increased by decreasing cooling rate. Effective activation energy ( Δ E X t ) of sPP, calculated using Friedman iso-conversional method, was found to be lower in iPP/sPP blend than in neat sPP whereas Δ E X t of iPP is not modified. Results were analysed through Avrami, Ozawa and Mo models. They both showed a decrease of crystal growth dimensionality for both iPP and sPP in iPP/sPP blends compared with neat iPP and sPP. The kinetic parameters U and K g were determined by the Hoffman–Lauritzen theory.

Melting behavior and isothermal crystallization kinetics of polypropylene/metallocene‐catalyzed polyethylene elastomer blends

AbstractPolypropylene (PP)/metallocene‐catalyzed polyethylene elastomer (mPE) blends were prepared in a twin‐screw extruder. The melting behavior, crystallization behavior, and isothermal crystallization kinetics of the blends were studied with differential scanning calorimetry. The results showed that PP and mPE were partially miscible and that the addition of mPE shifted the melting peak of PP to a lower temperature but the crystallization temperature to a higher temperature, demonstrating a dilution effect of mPE on PP. The isothermal crystallization kinetics of the blends were described with the Avrami equation. The values of the Avrami exponent indicated that the nucleation mechanism of the blends was heterogeneous, the growth of spherulites was almost three‐dimensional, and the crystallization mechanism of PP was not affected much by mPE. At the same time, the Avrami exponents of the blends were higher than that of pure PP, and this showed that the addition of mPE helped PP to form more perfect spherulites. The crystallization rate of PP was increased by mPE because the dilution effect of mPE on PP increased the mobility of PP chains. The crystallization activation energy was estimated with the Arrhenius equation, and the nucleation constant was determined by the Hoffman–Lauritzen theory. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Melting behavior and isothermal crystallization kinetics of PP/mLLDPE blends

The melting/crystallization behavior and isothermal crystallization kinetics of polypropylene (PP)/metallocene-catalyzed linear low density polyethylene (mLLDPE) blends were studied with differential scanning calorimetry (DSC). The results showed that PP and mLLDPE are partially miscible and interactions mainly exist between the mLLDPE chains and the PE segments in PP molecules. The isothermal crystallization kinetics of the blends was described with the Avrami equation. Values of the Avrami exponent indicated that crystallization nucleation of the blends is heterogeneous, the growth of spherulites is almost three-dimensional, and the crystallization mechanism of PP is not affected much by mLLDPE. The Avrami exponents of the blends are higher than that of pure PP, showing that the mLLDPE helps PP to form perfect spherulites. The crystallization rates of PP are decreased by mLLDPE because the crystallization temperature of PP was decreased by addition of mLLDPE and consequently the supercooling of the PP was correspondingly lower. The crystallization activation energy was estimated by the Friedman equation, and the result showed that the activation energy increased by a small degree by addition of mLLDPE, but changed little with increasing content of mLLDPE in the blends. The nucleation constant (Kg) was determined by the Hoffman–Lauritzen theory.

Effect of β‐nucleating agents on crystallization and melting behavior of isotactic polypropylene

AbstractTwo kinds of β‐nucleating agents, named a rare earth complex (WBG) and a N,N′‐dicyclohexylterephthalamide (TMB5), were introduced into isotactic polypropylene (iPP), and their effect on crystallization and melting behavior of iPP was comparatively investigated. Wide angle X‐ray diffraction measurements revealed that both the two additives were highly effective in inducing β modification. At their respective optimum concentrations of 0.08 wt % for WBG and 0.06 wt % for TMB5, the relative amount of β‐form calculated by Turner‐Jones equation both exceeds 92%. However, the isothermal crystallization kinetics investigated by differential scanning calorimetry demonstrated that WBG had more pronounced effect than TMB5 in accelerating the overall crystallization rate. The Lauritzen–Hoffman theory analysis also revealed that WBG was more effective not only in increasing the nucleus number but also in accelerating the growth rate of crystallization. After completing isothermal crystallization process, the subsequent melting behavior examination suggested that the addition of WBG expanded the upper limit temperature of forming β modification, and therefore was more effective in delaying the β‐α transformation than TMB5. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008