Crystallization Rate Constant Research Articles

Polypropylene carbon nanotube composites by in situ polymerization

The preparation of isotactic polypropylene nanocomposites filled with crude, purified and oxidized multi-walled carbon nanotubes (MWCNTs) was accomplished by polymerization of propylene with a metallocene/methylaluminoxane (MAO) catalyst and in situ coating. A good interfacial adhesion between the matrix and the filler is crucial for the successful preparation of nanocomposites; therefore, the polymerizations were performed with a new in situ coating process, which is a promising approach for increasing the matrix adhesion. This coating technique is derived from the Polymerization-Filling Technique (PFT) and based on a covalent bonding of the cocatalyst on the surface of oxidized MWCNT. The resulting nanocomposites contained a filler content of 0.1–8 wt% and were investigated with respect to their properties, such as morphology, crystallization and melting temperature, halftime of crystallization which decreases very strongly in dependence on a rising filler content. Since MWCNTs behave as nucleating agents, a detailed discussion on the Avrami analysis (rate constant of crystallization, dimensionality of the crystalline growth) is made.

Kinetics of Sucrose Crystallization in Whey Protein Films

The kinetics of sucrose crystallization in whey protein isolate (WPI) films was studied at 25 degrees C in four different relative humidity environments: 23, 33, 44, and 53%. The effects of protein matrix, crystallization inhibitors, and storage environment on the rate constants of sucrose crystallization were determined using the Avrami model of crystallization. It was found that a cross-linked, denatured whey protein (WP) matrix more effectively hindered sucrose crystallization than a protein matrix of native WP. The crystallization inhibitors tested were lactose, raffinose, modified starch (Purity 69), and polyvinylpyrrolidone (Plasdone C15). Raffinose and modified starch were determined to be the more effective inhibitors of sucrose crystallization. At lower relative humidities (23, 33, and 44%), the cross-linked protein matrix played a more important role in sucrose crystallization than the inhibitors. As relative humidity increased (53%), the crystallization inhibitors were more central to controlling sucrose crystallization in WPI films.

Crystallization kinetics for simulation of processing of various polyesters

AbstractThe approach to determine crystallization kinetic parameters based on the DSC nonisothermal crystallization experiments is applied to poly(butylene terephthalate) (PBT) and poly(ethylene‐2,6‐naphthalate) (PEN). The differential form of the Nakamura equation and master curve approach are used. The isothermal induction times are obtained from nonisothermal induction times according to the concept of induction time index. The correction of temperature lag between the DSC furnace and the sample is incorporated. The corrected nonisothermal crystallization kinetic data is shifted with respect to an arbitrarily chosen reference temperature to obtain the master curve. By fitting the obtained master curve with the Hoffman‐Lauritzen equation, the model parameters for the crystallization rate constant are obtained. The relative crystallinity measured at different cooling and heating rates is described by these model parameters. © 2006 Wiley Periodicals, Inc. J Appl PolymSci 102: 2847–2855, 2006

High Efficiency Nucleating Agents of Polyoxymethylene

In this research, the effects of different kinds of nucleating agents on the crystallization and mechanical properties of polyoxymethylene (POM) were studied, including inorganic, organic, and polymer nucleating agents and their compounds. These properties showed that nanoCaCO3, sorbitol derivative TMB-5, polyamide PA-4, and compound C-1/polyamide (PA)-4 can make spherulites of POM finer and more perfect, and effectively improve its notched impact toughness, in which the compound C-1/PA-4 displayed more remarkable nucleation effect. The study on the nonisothermal crystallization of POM showed that the crystallization temperature and crystallization growth rate of POM increased when C-1/PA-4 was added. The isothermal crystallization kinetics were also analyzed by the Avrami equation. The addition of C-1/PA-4 can increase the values of the Avrami exponent (n) and crystallization rate constant (k), and reduce the half-time of isothermal crystallization, t1/2, and the time corresponding to the maximum rate of crystallization, tP, indicating its remarkable nucleating effect on POM.

Studies on crystal morphology and crystallization kinetics of polypropylene filled with CaCO3 of different size and size distribution

AbstractChanges in the crystal morphology, crystallinity, and the melting temperature of thermoplastics resulted in significant changes in the mechanical behavior of composites containing them. For this reason, the research of crystal morphology and crystallization kinetics in thermoplastic composites became an important requirement. The thermoplastic filled with the filler of different size gradation was a new method for improving processability of thermoplastic composites. We have previously reported that the melt viscosity of polypropylene (PP) composites, which were filled with 30 wt % CaCO3 of effective size gradation, could be evidently declined. In this study, two sizes of CaCO3, 325 meshes and 1500 meshes, were blended by different proportions and filled into PP matrix with 30 wt %. Crystal morphology and isothermal crystallization kinetics of a series of composites were characterized by differential scanning calorimeter (DSC) and polarizing microscope. The results showed that composites filled with CaCO3 of effective size gradation leaded to a well‐crystalline order and a large crystal size, while their isothermal crystallization kinetics and crystallization rate constant (k) were declined, and their Avrami exponents (n) and crystallization half‐life (t1/2) were increased compared with the composites filled with single size CaCO3. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2437–2444, 2006

Physical properties, crystallization kinetics, and spherulitic growth of well-defined poly(ε-caprolactone)s with different arms

Both well defined star-shaped poly(ε-caprolactone) having four arms (4sPCL) and six arms (6sPCL) and linear PCL having one arm (LPCL) and two arms (2LPCL) were synthesized and then used for the investigation of physical properties, isothermal and nonisothermal crystallization kinetics, and spherulitic growth. The maximal melting point, the cold crystallization temperature, and the degree of crystallinity of these PCL polymers decrease with the increasing number of polymer arms, and they have similar crystalline structure. The isothermal crystallization rate constant ( K) of these PCL polymers is in the order of K 2LPCL> K LPCL> K 4sPCL> K 6sPCL. Notably, the K of linear PCL decreases with the increasing molecular weight of polymer while that of star-shaped PCL inversely increases. The variation trend of K over the number of polymer arms or the molecular weight of polymer is consistent with the analyses of both nonisothermal crystallization kinetics and the spherulitic growth rate. These results indicate that both the number of polymer arms and the molecular weight of polymer mainly controlled the isothermal and nonisothermal crystallization rate constants, the spherulitic growth rate, and the spherulitic morphology of these PCL polymers.

Synthesis, crystallization kinetics, and spherulitic growth of linear and star‐shaped poly(L‐lactide)s with different numbers of arms

AbstractWell‐defined linear poly(L‐lactide)s with one or two arms (LPLLA and 2LPLLA, respectively) and star‐shaped poly(L‐lactide)s with four or six arms (4sPLLA and 6sPLLA, respectively) were synthesized and then used for the investigation of the thermal properties, isothermal crystallization kinetics, and spherulitic growth. The maximal melting temperature, the cold‐crystallization temperature, and the degree of crystallinity of these poly(L‐lactide) polymers decreased with an increasing number of arms in the macromolecule. Moreover, the isothermal crystallization rate constant (K) of these poly(L‐lactide) polymers decreased in the order of KLPLLA > K2LPLLA > K4sPLLA > K6sPLLA2, which was consistent with the variation trend of the spherulitic growth rate (G). Meanwhile, both K and G of 6sPLLA slightly increased with the increasing molecular weight of the polymer. Furthermore, both LPLLA and 2LPLLA presented spherulites with good morphology and apparent Maltese cross patterns, whereas both unclear Maltese cross patterns and imperfect crystallization were observed for the star‐shaped 4sPLLA and 6sPLLA polymers. These results indicated that both the macromolecular architecture and the molecular weight of the polymer controlled K, G, and the spherulitic morphology of these poly(L‐lactide) polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2226–2236, 2006

水熱処理によって焼却灰から合成されるゼオライトの生成速度

In the multi-stages synthesis method proposed by our previous study, the generation rate and the crystallization rate constants of zeolites synthesized from coal fly ash and rice husk was calculated. As a result, the generation rate constant of phillipsite is proportional to the initial concentration ratio of silicate ion to aluminate ion in NaOH aqueous solution. However, that of hydroxysodalite increases to the maximum and then decreases to zero with the initial concentration ratio. Furthermore, the crystallization rate constants of both zeolites in an improved Avrami's equation is independent of the intial concentration ratio.It is also found that the generation behavior and the selectivity of zeolites synthesized with the conventional hydrothermal treatment method can be estimated by the rate constants obtained in the present work.

Synthesis and Properties of Syndiotactic Poly(propylene)/Carbon Nanofiber and Nanotube Composites Prepared by in situ Polymerization with Metallocene/MAO Catalysts

AbstractSummary: The preparation of syndiotactic poly(propylene) (sPP) nanocomposites with multi‐walled carbon nanotubes (MWNTs), carbon nanofibers (CNFs), and carbon black (CB) as fillers was accomplished by the in situ polymerization of propylene with a metallocene/methylalumoxane (MAO) catalyst. Different pre‐treatments were applied to achieve a homogeneous dispersion of the fillers in the matrix. The resulting nanocomposites were investigated with respect to their properties, which were then compared to those of the pure polymer and among each other. The thermal stability of the nanocomposites was slightly enhanced compared to the pure polymer. In addition, the yield strength of the nanocomposites could be slightly raised in comparison to the neat sPP. The most significant influence of the nanofillers was detected on the crystallization behavior. The crystallization temperature was increased with rising filler content in all cases. Moreover, the half‐time of crystallization was significantly reduced in the nanocomposites. The rate constant of crystallization was also greatly enhanced for all nanocomposites as compared to the neat sPP. The nanofillers acted, therefore, as nucleating agents. This effect was most pronounced in the case of MWNTs as fillers.Influence of the type of filler on the degradation temperature (temperature of maximum weight loss).magnified imageInfluence of the type of filler on the degradation temperature (temperature of maximum weight loss).

Crystallization behavior of poly(butylene succinate)/corn starch biodegradable composite

AbstractThe effects of corn starch (CS) filler and lysine diisocyanate (LDI) as a coupling agent on the crystallization behavior of a poly(butylene succinate) (PBS)/CS ecocomposite were investigated using differential scanning calorimetry. In isothermal crystallization, n values for pure PBS were from 2.33 to 2.82. On the other hand, both composites showed values of 3 < n < 4. In nonisothermal crystallization, the Avrami exponent varied from 2.12 to 2.55 for pure PBS, from 1.58 to 1.96 for the composite without LDI, and from 1.79 to 1.91 for the composite with LDI, depending on the cooling rate. There was not a large difference of the crystallization rate constant (k) as adjusted by the Jeziornay suggestion. The activation energy for nonisothermal crystallization was also calculated on the basis of three different equations (Augis–Bennett, Kissinger, and Takhor equations). However, the values of the activation energy were in contradiction with the results of the kinetics. The addition of the filler (CS) and coupling agent (LDI) affected the morphological structure of PBS spherulites. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1107–1114, 2005

SiC/Si3N4 Nano-Composites: Crystallization Kinetics of Precursor-derived Amorphous Ceramic Solids of Type Si-C-N

The crystallization kinetics of precursor-derived amorphous Si-C-N ceramics is investigated by X-ray diffractometry. Annealing leads to crystallization where composites of nano-crystalline SiC and micro-crystalline Si3N4 are formed. Both crystalline phases are developed during a simultaneous diffusion controlled nucleation and growth process. The rate constants of crystallization obey an Arrhenius behaviour with a very high activation enthalpy, which is the reason for a relatively uniform temperature stability of the amorphous state.

Crystallization kinetics of poly(trimethylene terephthalate)/poly(ether imide) blends

The crystallization kinetics of a melt-miscible blend, consisting of poly(trimethylene terephthalate) (PTT) and poly(ether imide) (PEI) prepared by solution precipitation, has been investigated by means of optical polarized microscopy and differential scanning calorimeter. It was found that both the PTT spherulitic growth rate (G) and overall crystallization rate constant (kn) were depressed, with increasing PEI composition or crystallization temperature (Tc). The kinetic retardation was attributed to the decrease in PTT molecular mobility, and the dilution of PTT concentration due to the addition of PEI, which has a higher glass transition temperature (Tg). According to the Lauritzen–Hoffman theory of secondary nucleation, the crystallization of PTT in blends was similar to that of neat PTT as regime III, n = 4 and regime II, n = 2 growth processes, while the transition point of regime III to II has been shifted from 194°C for neat PTT to 190°C for blends. POLYM. ENG. SCI. 46:89–96, 2006. © 2005 Society of Plastics Engineers

Effects of modified kaolin on the crystallization property of PP/Kaolin nanocomposites

Modified kaolin-filled polypropylene was prepared by blending different kinds of modified kaolin intercalated master batches with polypropylene. The microstructure of PP/ Kaolin was studied with X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) photos. The effects of the organic kaolin master batches on the crystallization property of polypropylene were investigated by means of non-isothermal differential scanning calorimetry analysis (DSC) and polarized light microscopy (PLM). Non-isothermal crystallization kinetics of polypropylene was studied by the use of modified Avrami Equation improved by Jeziorny. The results indicate that some organic kaolin can be exfoliated by polypropylene, proved by an invisible 001 peak on XRD pattern and individual layer on transmission electron microscopy (TEM) photos, and the heterogeneous nucleus increased with the addition of modified kaolin, proved by the PLM photos. The filled polypropylene crystallizes fastest at 395 K. The results also show that organic kaolin master batches not only can effectively accelerate the heterogeneous nucleation of polypropylene but also can increase the rate of crystallizing point and the temperature of crystallization. However, the rate constant of crystallization remains unchanged.

Crystallization and morphology of starch-g-poly(1,4-dioxan-2-one) copolymers

The thermal transition, crystallization and spherulitic morphology of starch-g-poly(1,4-dioxan-2-one) copolymers were studied by means of differential scanning calorimetry (DSC) and polarized optical micrographs (PM). It is found that the graft structures of copolymers have obvious effects on the thermal and crystallization behaviors. Because there were more defect sites in the crystalline phase originating from the short grafted chains of poly(1,4-dioxan-2-one) (PPDO), the crystal structure of the copolymers was much less perfect than that of PPDO. PM revealed that the spherulitic morphology of the graft copolymers depended on graft structures and crystallization temperatures. From the single polarized micrograph of the graft copolymers it was observed clearly that the starch segments acted as nucleation sites. The Avrami equation was used to analyze the overall isothermal crystallization of the graft copolymers. Avrami exponents were almost constant at crystallization temperatures Tc ranging from 45 to 60°C. Both the PM observation and the DSC investigation (crystallization rate constant, K values) indicated that the graft copolymers crystallize faster than pure PPDO, especially at higher crystallization temperatures.

Crystallization and morphology of a novel biodegradable polymer system: poly(1,4-dioxan-2-one)/starch blends

Poly(1,4-dioxan-2-one) (PPDO)/starch blend is a novel biodegradable polymer system, and it has potential applications in preparing various low-cost environmentally friendly materials. In order to understand the relationship between their structure and properties, the thermal transition, crystallization and spherulitic morphology of PPDO/starch blends have been studied by means of differential scanning calorimetry (DSC) and polarized optical microscopy. It was found that when 5 wt% starch was added to PPDO, a clear nucleating effect was observed. When the blends were crystallized from the melt, they formed banded spherulites that exhibited a well-defined Maltese cross depending on the crystallization temperature. Avrami exponents and crystallization rate constants of the blends with 5 wt% starch were higher than those of PPDO and those of the blends with 10 wt% starch. Both the dynamic DSC results and isothermal crystallization data indicated that the starch, with a content of 5 wt% in the blends, acted as the nucleating agent for PPDO.

Formation kinetics of crystalline Si 3N 4/SiC composites from amorphous Si–C–N ceramics

The crystallization kinetics of amorphous precursor-derived bulk ceramics of composition Si 26C 41N 33 are investigated in the temperature range between 1500 and 1645 °C using X-ray diffractometry. Isothermal annealing in nitrogen leads to crystallization of microcrystalline α-Si 3N 4 and nanocrystalline SiC. At 1500 °C only Si 3N 4 is crystallized, while with increasing temperature a growing amount of additional SiC and a decreasing amount of Si 3N 4 is formed in a simultaneous crystallization process, until at 1645 °C only SiC is observed. Crystallization can be described according to the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism, assuming a three-dimensional, diffusion controlled grain growth with a time dependent nucleation rate. The determined rate constants of crystallization are temperature dependent, but equal for both crystallized phases. An Arrhenius behavior with a very large activation enthalpy of about 12.5 eV is observed, where the enthalpies of diffusion and nucleation both contribute substantially to that value. Two kinetic processes are identified which may influence phase composition and microstructure of the crystallized material in order to produce tailor-made composites: (a) the direct crystallization process of amorphous Si 26C 41N 33 and (b) a transformation process where already crystallized α-Si 3N 4 reacts with carbon, embedded in the residual amorphous Si–C–N matrix, to SiC at high temperatures.

Influence of molecular characteristics on overall isothermal melt-crystallization behavior and equilibrium melting temperature of syndiotactic polypropylene

Overall isothermal melt-crystallization and subsequent melting behavior of metallocene-catalyzed syndiotactic polypropylene resins of various molecular weights were investigated using differential scanning calorimetry (DSC) technique. Two sets of molecular weight range were synthesized with two different metallocene catalyst systems. The kinetics of the overall isothermal melt-crystallization process was analyzed based on various macrokinetic models, i.e. the Avrami, Malkin and Urbanovici–Segal models. The effective activation energy describing the overall isothermal crystallization process over the crystallization temperature range studied was estimated based on an Arrhenius approximation of the obtained Avrami crystallization rate constants. The equilibrium melting temperature for each of these resins was estimated based on the linear and non-linear Hoffman–Weeks extrapolative methods.

Crystallization and Melting Behavior of Poly(3‐hydroxybutyrate)‐Based Blends

AbstractSummary: Blends of high molecular weight poly(R‐3‐hydroxybutyrate) (PHB) ($\overline M _{\rm w}$ = 352 000 g · mol−1), comprising of either low molecular weight poly(R‐3‐hydroxybutyrate) (D‐PHB) ($\overline M _{\rm w}$ = 3 900 g · mol−1) or poly[(R‐3‐hydroxybutyrate)‐co‐(R‐3‐hydroxyvalerate)] (PHBV) ($\overline M _{\rm w}$ = 238 000 g · mol−1) with 12 mol‐% hydroxyvalerate (HV) content as a second constituent, were investigated along with the thermal properties and morphologies. After isothermal crystallization, a lowering of the melting temperature of PHB can be observed with increasing content of the second component in the blends. This behavior points towards miscibility of the constituents both in the liquid and the solid state. Crystallization kinetics was studied under isothermal and non‐isothermal conditions. The overall kinetics of isothermal crystallization was analyzed in terms of the Avrami equation. Only one crystallization peak is observed in all cases for the PHB/D‐PHB and PHB/PHBV blends under the conditions studied. This demonstrates co‐crystallization of the constituents. The addition of D‐PHB or PHBV to PHB reduces the rate of crystallization of the blends compared to that of neat PHB. The corresponding activation energies of crystallization also decrease with an increasing concentration of the second constituent. Non‐isothermal crystallization, carried out with different cooling rates held constant, is discussed in terms of a quasi‐isothermal approach. The corresponding rate constants as functions of reciprocal undercooling show Arrhenius‐like behavior in a certain range of temperatures. At sufficiently high undercooling, the rate constants of crystallization for the isothermal process exceed those reflecting non‐isothermal conditions, whereas in the limit of low undercoolings, the rate constants become similar. Ring‐banded morphologies are observed when PHB is in excess. When the respective second component is the major component, fibrous textures of the spherulites develop.Polarized micrograph of PHB/PHBV 90/10.magnified imagePolarized micrograph of PHB/PHBV 90/10.

Modeling the nucleation and crystallization kinetics of a palm stearin/canola oil blend and lard in bulk and emulsified form

Using high-pressure homogenization to generate different droplet size distributions, the nucleation and crystallization of two fat systems [lard or a plam stearin/canola oil blend (PSCO)] were compared in bulk and emulsified form. Droplet size reduction decreased the final volume fraction of solid fat primarily in the lard system vs. the PSCO system, with a greater reduction in volume fraction using the homogenization regime that led to smaller droplets. Homogeneous and heterogeneous models showed that the nucleation rate generally decreased with a reduction in droplet size. However, the Gibbs surface energy (γ) was significantly underestimated using the homogeneous model, whereas the heterogeneous model fit the data adequately (P<0.05). The temperature sensitivity of the calculated impurity concentration in all emulsified systems was droplet size dependent. The Avrami model showed the emulsified fats to have lower Avrami indices relative to the bulk fat as well as lower crystallization rate constants. Differences in the Avrami indices and the rate constants were more pronounced in the bulk and emulsified PSCO compared with lard.

Algal meal supplementation of the cows' diet alters the physical properties of milk fat.

A three-week algal meal supplementation of the cows' basal diet resulted in an increase in the firmness of milk fat crystallized isothermally at 5 degrees C for 24 h--the apparent elastic constant increased from 100 to 224 N/mm. This was accompanied by a decrease in solid fat content, from 47.7% to 44.4%. The crystallization behaviour of milk fat was also modified significantly. The rate constant of crystallization (Avrami constant) of the enriched milk fat at 19 degrees C was approximatly 20 times higher than that of control milk fat. A shorter induction time of nucleation was also observed in the temperature range [20, 27 degrees C]. These effects were attributed to a higher degree of supersaturation of the enriched milk fat. Enriched milk fat nucleated in a more stable beta' polymorphic form at 5 degrees C, while control milk fat nucleated in the metastable alpha form, as determined by powder X-ray diffraction and differential scanning calorimetry. Changes in the microstructure of the material were observed by polarized light microscopy at 5 degrees C. The enriched milk fat displayed a greater amount of crystal clustering than the control. This effect was reflected in a decrease in the box-counting mass fractal dimension (Db) of the fat crystal network from 1.853 to 1.809. The decrease in Db closely predicted the observed 2.2-fold increase in the elastic constant of the fat. These changes in mechanical properties, crystallization behaviour and microstructure were driven by an increase in the 18:1 trans and a decrease in the 18:1 cis fatty acid content of the enriched milk fat.