Overall Rate Of Crystallization Research Articles

Mechanical Properties and Non-Isothermal Crystallization Kinetics of Polylactic Acid Modified by Polyacrylic Elastomers and Cellulose Nanocrystals.

In this paper, a polyacrylic elastomer latex with butyl acrylate (BA) as the core and methyl methacrylate (MMA) copolymerized with glycidyl methacrylate (GMA) as the shell, named poly(BA-MMA-GMA) (PBMG), was synthesized by seeded emulsion polymerization. Cellulose nanocrystal (CNC) was dispersed in the polyacrylic latex to prepare PBMG/CNC dispersions with different CNC contents. The dried product was mixed with polylactic acid (PLA) to fabricate PLA/PBMG/CNC blends. The addition of PBMG and PBMG/CNC improved the mechanical properties of the PLA matrix. Differential scanning calorimetry (DSC) was used to investigate the non-isothermal crystallization kinetics. The Avrami equation modified by the Jeziorny, Ozawa and Mo equations was used to analyze the non-isothermal crystallization kinetics of PLA and its blends. Analysis of the crystallization halftime of non-isothermal conditions indicated that the overall rate of crystallization increased significantly at 1 wt% content of CNC. This seemed to result from the increase of nucleation density and the acceleration of segment movement in the presence of the CNC component. This phenomenon was verified by polarizing microscope observation.

Recovery of lactose from simulated delactosed whey permeate by a low-temperature crystallization process

Delactosed whey permeate is the mother liquor/by-product of lactose manufacture, but it still contains around 20 wt% lactose. The high mineral content, stickiness, and hygroscopic behavior prevent further recovery of lactose in the manufacturing process. Therefore, its use is currently limited to low-value applications such as cattle feed, and more often it is seen as waste. This study investigates a new separation technique operating at sub-zero conditions. At low temperature, precipitation of calcium phosphate is expected to be reduced and the lower solubility at sub-zero temperature makes it possible to recover a large portion of the lactose. We found that lactose could be crystallized at sub-zero conditions. The crystals had a tomahawk morphology and an average size of 23 and 31 µm. In the first 24 h, the amount of calcium phosphate precipitated was limited, whereas the lactose concentration was already close to saturation. The overall rate of crystallization was increased compared with the crystals recovered from a pure lactose solution. Mutarotation was rate limiting in the pure system but it did not limit the crystallization of lactose from delactosed whey permeate. This resulted in faster crystallization; after 24 h the yield was 85%.

Polymorphic selectivity in crystal nucleation.

Crystal nucleation rates have been measured in the supercooled melts of two richly polymorphic glass-forming liquids: ROY and nifedipine (NIF). ROY or 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile is known for its crystals of red, orange, and yellow colors and many polymorphs of solved structures (12). Of the many polymorphs, ON (orange needles) nucleates the fastest with the runner up (Y04) trailing by a factor of 103 when compared under the same mobility-limited condition, while the other unobserved polymorphs are slower yet by at least 5 orders of magnitude. Similarly, of the six polymorphs of NIF, γ' nucleates the fastest, β' is slower by a factor of 10, and the rest are slower yet by at least 5 decades. In both systems, the faster-nucleating polymorphs are not built from the lowest-energy conformers, while they tend to have higher energies and lower densities and thus greater similarity to the liquid phase by these measures. The temperature ranges of this study covered the glass transition temperature Tg of each system, and we find no evidence that the nucleation rate is sensitive to the passage of Tg. At the lowest temperatures investigated, the rates of nucleation and growth are proportional to each other, indicating that a similar kinetic barrier controls both processes. The classical nucleation theory provides an accurate description of the observed nucleation rates if the crystal growth rate is used to describe the kinetic barrier for nucleation. The quantitative rates of both nucleation and growth for the competing polymorphs enable prediction of the overall rate of crystallization and its polymorphic outcome.

Biochar/polypropylene composites: A study on the effect of pyrolysis temperature on crystallization kinetics, crystalline structure, and thermal stability

The crystallization behavior of polyolefins continues to gain much attention for the prediction of suitable processing conditions. This study aims to understand the influence of biochar (BC) particles on the crystallization behavior of the selected semi-crystalline polypropylene (PP). For this purpose, BC samples were prepared from waste biomass at various pyrolysis temperature. The prepared BC samples were used to fabricate PP composites via melt processing technique. The crystallization behavior of these composites was studied by differential scanning calorimetry (DSC) calorimetry and validated using Avrami analysis in a non-isothermal conditions. It was observed that BC particles act as nucleating agents and accelerate the overall rate of crystallization. Avrami theory analysis indicated that the addition of BC particles provides polypropylene (PP) composites with a higher crystallization rate, i.e. about one order of magnitude higher than the neat PP. The results of the TGA analysis showed that the BC particles enhanced the thermal stability of the BC/PP composites, and is up to 80 °C higher than the neat PP sample.

Effect of TiO2 Nanofiller Concentration on the Mechanical, Thermal and Biological Properties of HDPE/TiO2 Nanocomposites

The necessity for advanced and effective biomimetic tissue engineering materials has increased massively as bone diseases such as osteoporosis and bone cancer have become a major public health problem. Therefore, the objective of this study is to develop titanium dioxide (TiO2) nanoparticles-enriched high-density polyethylene (HDPE) nanocomposites that could serve as potential biomaterials. HDPE/TiO2 nanocomposites with varying TiO2 nanoparticles content were fabricated by using injection molding technique and were subjected to mechanical, thermal and biological characterization. SEM–EDS analysis confirmed even dispersion of TiO2 nanoparticles into the HDPE matrix. It was observed from the mechanical testing that the addition of TiO2 nanoparticles to HDPE noticeably improved the stiffness (from 345 to 378 MPa) while maintaining almost similar yield strength of pure HDPE. The thermal analyses revealed that TiO2 nanoparticles inclusion to HDPE matrix enhanced the thermal stability of nanocomposites, as the overall rate of crystallization increased by almost 4%. Furthermore, biocompatibility of nanocomposites was also studied by means of various cell culture experiments; human osteoblasts (hFOB) were seeded on the HDPE/TiO2 nanocomposites and were visualized through SEM after 72 h of incubation; surface morphology revealed normal cell growth and spreading with more attachment on PNC-10 that contains 10 wt.% of TiO2. Moreover, cell viability assays (i.e., MTT and cell attachment) revealed consistent increase in cell count and metabolic activity when triplicate cultures were incubated for 1, 3 and 7 days.

Morphology, crystallization and melting behaviour of poly(trimethylene terephthalate)/thermotropic liquid crystalline polymer blends

The miscibility, crystallization and melting behaviour of poly(trimethylene terephthalate)/thermotropic liquid crystalline polymer (LCP) blends were studied using differential scanning calorimetry. The blends were found to form primarily an immiscible system. The addition of LCP accelerated the overall rate of crystallization and caused a depression in equilibrium melting temperature, especially at low LCP content. Lauritzen–Hoffman analysis showed that the addition of LCP caused a reduction in the fold surface energy and increased the regime II to III transition temperature.

The effect of urea and urea-modified halloysite on performance of PCL

The effects of urea and urea-modified halloysite nanotubes (HNT) on structure and properties of poly(ɛ-caprolactone) (PCL) were evaluated using mechanical testing combined with FTIR, DSC, DMA, and various microscopic techniques. The results indicate important changes in mechanical behavior by urea-mediated interchain hydrogen bonding in PCL, whereas no linking between PCL and HNT in the related nanocomposite was found. As a result, the improved mechanical behavior of nanocomposites with urea-modified HNT was caused by combination of the matrix modification and urea-aided enhanced dispersion of HNT. The additives do not have any marked effect on PCL crystallinity. HNT increases and urea reduces the overall rate of crystallization. Both additives show a moderate nucleating effect.

Non-isothermal crystallization kinetics of high density polyethylene/graphene nanocomposites prepared by in-situ polymerization

Abstract High density polyethylene/graphene nanocomposites were synthesized via in-situ polymerization technique using metallocene catalyst and methylaluminoxane (MAO) as co-catalyst. The non-isothermal crystallization of the nanocomposites was studied using differential scanning calorimetry (DSC). Modified Avrami, Ozawa and the model proposed by Mo et al. were applied in the analysis. The activation energy (ΔE) was calculated using the iso-conversional method proposed by Vyazovkin. The effect of degree of transformation and temperature on ΔE was analyzed. It was found that graphene nucleates the crystallization of HDPE by lowering ΔE as well as increasing the crystallization onset temperature (Ton). However, at later stages of crystallization, excess amount of graphene hinder the crystal growth rate thereby retarding overall rate of crystallization, observed from increase of t1/2 and degree of super cooling.

Properties of microinjection molding of polymer multiwalled carbon nanotube conducting composites

AbstractThe effects of processing conditions on the microstructure and properties of polypropylene/multiwalled carbon nanotube (PP/MWCNT) and polycarbonate/multiwalled carbon nanotube (PC/MWCNT) composites were studied. Samples of various MWCNT loadings were prepared by diluting commercial masterbatches. Different processing conditions were then used to systematically change the degree of nanotube alignment, from random to highly aligned. The crystallinity of the PP/MWCNT nanocomposites was found to go through a maximum as a function of nanotube content while the overall rate of crystallization increased. For the highly sheared microinjected PP/MWCNT samples well oriented crystals were formed. Electrical conductivity of the nanocomposites was improved by the presence of the crystalline structure; however, the high degree of nanotube alignment in the microparts resulted in a significant increase in the electrical percolation threshold. The PP nanocomposites exhibited mechanical properties significantly enhanced by nanotube loading; this effect was small in the case of the PC nanocomposites. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Thermal properties and morphology of isotactic polypropylene/acrylonitrile–butadiene rubber blends in the presence and absence of a nanoclay

AbstractA thermoplastic elastomer (TPE) nanocomposite based on polypropylene (PP), acrylonitrile–butadiene rubber (NBR), and a nanoclay (NC) was prepared in a laboratory mixer with a 54/40/6 weight ratio. The effects of NC on the thermal properties, crystalline structure, and phase morphology of the TPE nanocomposite were studied in this work. The results obtained from the nonisothermal crystallization of PP, PP/NBR, and PP/NBR/NC, which was carried out with differential scanning calorimetry, revealed that the overall rate of crystallization of PP decreased with the addition of NBR to PP and increased when NC was incorporated into the nanocomposite. In addition, the crystallite size distribution was more uniform for the PP phase crystallized in the nanocomposite versus the PP itself. Also, although the PP in the reference blend (PP/NBR) crystallized only in the α form, the crystalline structure of the PP incorporated into the nanocomposite was a mixture of α‐ and γ‐crystalline forms. The effects of NC on the phase morphology of PP/NBR blends prepared with three different cooling methods (quenching in liquid nitrogen, cooling between two metal plates at room temperature, and molding at a high temperature in a hot press) were studied. For the samples quenched in liquid nitrogen or cooled between metal plates, a particulate–cocontinuous morphology formed. However, for the samples prepared under a hot press, a laminar‐like morphology was observed. In all three cases, a similar particulate–cocontinuous morphology formed for the reference blend, but the rubber inclusions were always smaller than those of the TPE nanocomposite. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

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.

Characterization and crystallization behavior of poly(ethylene‐co‐trimethylene terephthalate) copolymers

AbstractA series of poly(ethylene‐co‐trimethylene terephthalate) (PETT) copolymers were prepared by polycondensation. The synthesized PETT are block copolymers and the content of poly(trimethylene terephthalate) (PTT) units incorporated into the copolymers are always larger than that fed in the polymerization. The nonisothermal crystallization at the different cooling rates was studied by means of differential scanning calorimetry. The copolymers develop the crystallization later and show the lower melting temperature than the corresponding enriched homopolymers. The modified Avrami analysis fit well the nonisothermal crystallization of these polymers. The overall rate of crystallization of PTT is fastest and that of PET is slowest, whereas the copolymers are between them at the same cooling rate. The minor PET units incorporated into PTT polymer chains reduce the crystallization of PTT segments, but the present minor PTT units in the PET chains seem to accelerate the crystallization of PET segments. The Avrami exponent nvaries in the range of 3–4, indicating that the nonisothermal crystallization follows the homogeneous nucleation and two‐ to three‐dimensional growth mechanism. Wide angle X‐ray diffraction analysis explains that the PET and PTT units do not cocrystallize and it is considered as the enriched polymer segments to crystallize during crystallization. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Alteration of crystal structure of bismuth confined in cylindrical nanopores

The effect of nanoscopic cylindrical confinement on the crystal structures and crystallization behaviors of Bi was investigated. While the crystallites of Bi nanorods were found to have a higher orientation proportional to the degree of confinement, an alteration of the crystal orientation between electrodeposited and recrystallized Bi nanorods was observed. Thermal analysis confirmed a structural difference between the as-deposited and recrystallized nanorods and that a crystallization mechanism where nucleation determines the overall rate of crystallization occurred during recrystallization. The crystal structure, including the preferred crystal orientation, is discussed in terms of the dominant crystallization mechanism under the cylindrical nanoconfinement.

Isothermal crystallization behavior of poly(trimethylene terephthalate) at high undercoolings

After successfully being commercialized in the 1990s, poly(trimethylene terephthalate) (PTT) has drawn more and more attention due to its superior characteristics, such as elastic recovery, chemical resistance and resilience in comparison with poly (ethylene terephthalate) (PET) and poly (butylene terephthalate) (PBT) [1, 2]. PTT is a typical semicrystalline polymer material and many reports were focused on its crystallization behaviors recently. Huang and Chang [3] investigated the crystallization behavior for PTT from the melt and analyzed the kinetics in the temperature range from 451 K to 483 K, some important parameters for crystallization such as equilibrium melting temperature(Tm 0 = 521 K), lateral surface-free energy (r),work of chain folding (q) were also reported. Crystallization kinetics parameters of PET, PTT and PBT were compared by Chuah [4], it could be concluded that PTT crystallized faster than PET but slower than PBT at the same degree of undercooling. Hong and his coworkers [5] took note of the inaccuracy of Chang and Huang in determining the work of chain folding (q) and gave a more reasonable value of 27.2 kJ mol, they also investigated the regime behaviors of PTT crystallization using the secondary crystallization theory [6] and found the boundaries between the classical regime I fi II and regime II fi III are located at 488 and 468 K, respectively. We also carried out the kinetics analysis of PTT crystallization including the secondary crystallization process using a new mathematical model constructed by us [7]. Much of the above work has concentrated on the measurement of overall rate of crystallization or the measurement of spherulitic growth rate of PTT crystallization from melt at low and moderate degrees of undercooling (DT = Tm –Tc, Tc is crystallization temperature), yet a detailed overall kinetics analysis at high undercoolings (DT > 70 K), especially in the temperature range where the fastest crystallization rate would occur is still lacking. However, study of the behavior of a semicrystalline polymer crystallizing in this temperature range is of great importance from both theoretical and practical view points. Of theoretical importance is the mechanism of nucleation, the growth geometry of crystals and the formation of polymer fine structure during crystallization. The practical importance is that such crystallization data may be very useful to polymer processing such as fiber spinning and injection molding, because most polymer shaping processes need a very fast crystallization rate to shorten the solidifying time. The importance also arises from the effect of final crystallinity on the physical and chemical properties of polymers. In this study, we present the isothermal crystallization behavior of PTT from 356 K to 465 K, the overall crystallization kinetics data in the temperature range 356–450 K was reported for the first time. Poly(trimethylene terephthalate) samples with different molecular weights were obtained by the polycondensation of terephthalic acid in the melt phase with 1,3-propandediol using different reaction time Y. Xu (&) H.-b. Jia Department of Polymer Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China e-mail: njust_xuy@hotmail.com

Miscibility in crystalline polymer blends: Isotactic polypropylene and linear low‐density polyethylene

AbstractThe phase behavior and the crystallization kinetics of blends composed of isotactic polypropylene (iPP) and linear low‐density polyethylene (LLDPE) were investigated by differential scanning calorimetry. The phase behavior indicates the formation of separate crystals of iPP and LLDPE at each investigated blend composition. The crystallization trace reveals that iPP crystallizes in its normal range of temperatures (i.e., at temperatures higher than that of LLDPE), when its content in the blend is higher than 25% by weight. In the blend whose iPP content is as high as 25%, at least a portion of iPP crystallizes at temperatures lower than that of LLDPE. This behavior has been proposed by Bassett to be attributed to a change in the kind of nucleation from heterogeneous to homogeneous. From the Avrami analysis of the isothermal crystallization of iPP in the presence of molten LLDPE, n values close to 2 are always obtained. According to our previously proposed interpretation of the Avrami coefficient, it can be related to the crystallite fractal dimension, through d = n + 1, which gives values close to 3, according to the spherulitic observed morphology. The kinetics parameter, i.e., the half‐time of crystallization, and the kinetic constant k show that a decrease in the overall rate of crystallization of iPP occurs on blending. Optical microscopy photographs, taken during the cooling of the samples from the melt, confirm the above results and show increasingly less resolved spherulite texture on increasing LLDPE content in the blend. The diffusion parameters evaluated for the neat polymers and for the blends in dichloromethane, which give information on the miscibility in the amorphous state, show that the diffusional behavior of the blends is governed by iPP, suggesting a two‐phase amorphous state. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3338–3346, 2003

Effects of La3+‐containing additive on crystalline characteristics of isotactic polypropylene

AbstractThe influence of a mixed additive of lanthanum stearate and stearic acid on the crystalline characteristics of isotactic polypropylene (iPP) has been investigated. The results of the wide‐angle X‐ray diffraction (WAXD) measurements and the melting behaviour examination by differential scanning calorimetry (DSC) show that the additive might induce a high proportion of β‐form and act as a β‐form nucleating agent. The relative content of β‐form estimated by WAXD is 33.1% in a PP containing 2.5% (by weight) of the additive. Isothermal crystallization at 130 °C, examined by DSC, reveals that the additive considerably accelerates the overall rate of crystallization: the half crystallization period t1/2, decreases from 11.7 min for pure PP to 7.3 min for PP containing 2.5% of the additive. However, the additive has no obvious influence on the nucleation mechanism and crystal growth mode. Polarized light microscopy (POM) examinations indicate that the addition of the additive to PP causes spherulites to become much finer.© 2003 Society of Chemical Industry

Crystallization and melting behavior of poly(ethylene oxide)/poly( n-butyl methacrylate) blends

The phase diagram, crystallization and melting behavior of poly(ethylene oxide) (PEO)/poly( n-butyl methacrylate) (PnBMA) blends have been investigated using differential scanning calorimetry and optical microscopy. The results show that the blends are miscible up to 85 °C and show an lower critical solution temperature-type demixing at a higher temperature. The isothermal crystallization studies of the blends indicate a reduction in the overall rate of crystallization. Analysis of isothermal crystallization data by means of Avrami equation leads to average values of the Avrami index of 2.5 for pure PEO and 3.0 for the different blend compositions. The melting behavior of the blends reveals double endotherms, which is ascribed to both secondary crystallization and recrystallization. The melting point depression study yielded χ 12=0, indicating a relatively low interaction strength.

Crystallization of tethered polyethylene in confined geometries

Ordered poly(ethylene)-poly(vinylcyclohexane) (PE-PVCH) block copolymers are employed to study the crystallization of tethered PE in confined geometries. The high Tg of the PVCH component of these materials forces PE chains to crystallize in well-defined geometries dictated by the mesophase structure of the block copolymer. Effects of chain tethering on crystallization are examined through comparison of singly-tethered PE chains in PE-PVCH (EV) diblocks and doubly-tethered PE in PVCH-PE-PVCH (VEV) triblocks. Crystallinity is independent of the block copolymer mesophase structure in both the EV and VEV systems, although crystallinity in VEV depends on the molecular weight of the PE block of the copolymer. Melting temperature data indicate that spatial confinement reduces crystallite size in EV and VEV, and that the double tethering of PE chains in VEV reduces crystallite size further through topological constraints. Crystal nucleation and growth depend strongly on the type of microstructure in both EV and VEV block copolymers. Differences in the overall rate of crystallization are correlated with the dimensional continuity of the PE microdomains. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37:2053–2068, 1999

Crystallization of tethered polyethylene in confined geometries

Ordered poly(ethylene)-poly(vinylcyclohexane) (PE-PVCH) block copolymers are employed to study the crystallization of tethered PE in confined geometries. The high Tg of the PVCH component of these materials forces PE chains to crystallize in well-defined geometries dictated by the mesophase structure of the block copolymer. Effects of chain tethering on crystallization are examined through comparison of singly-tethered PE chains in PE-PVCH (EV) diblocks and doubly-tethered PE in PVCH-PE-PVCH (VEV) triblocks. Crystallinity is independent of the block copolymer mesophase structure in both the EV and VEV systems, although crystallinity in VEV depends on the molecular weight of the PE block of the copolymer. Melting temperature data indicate that spatial confinement reduces crystallite size in EV and VEV, and that the double tethering of PE chains in VEV reduces crystallite size further through topological constraints. Crystal nucleation and growth depend strongly on the type of microstructure in both EV and VEV block copolymers. Differences in the overall rate of crystallization are correlated with the dimensional continuity of the PE microdomains. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37:2053–2068, 1999

Crystallization in Polyamide 6/Polysulfone Blends: Effect of Polysulfone Particle Size.

By reactive blending of polyamide 6 (PA) with polysulfone (PSU) using a gram-scale mixer (Mini-Max Molder), we prepared a series of PA/PSU (80/20 wt.ratio) with various diameters of PSU particles: 90 nm by using phthalic anhydride-terminated PSU (PSU-PhAH), 0.2 µm by carboxylic acid-grafted PSU (PSU-COOH), and 1.0 µm by nonreactive PSU (nf-PSU). Isothermal crystallization was carried out at 200 degreesC. Light scattering studies showed that spherulites in the blends can grow to more than 10 µm in diameter, as in the case of neat PA. It suggests that crystal growth in the radial direction can be achieved by circumventing the PSU obstacles; i.e., it does not matter how big or small they are in the range of 90 nm to 1 µm. The smaller the PSU particles, the slower was the overall rate of crystallization. The slower rate may be caused by the higher population of brush PA chains attached to PSU particles as block or graft chains, since the brushes may be hard to organize into crystal lamellae, compared with free chains. The blends showed rather broad wide-angle X-ray diffraction profiles. The smaller the PSU particles, the broader was the profile. The scattering from the particles themselves should appear only at extremely low angles (<0.1 degrees) and hardly affects the wide-angle X-ray profiles so that the results may imply that the presence of smaller PSU particles renders the less perfection of the PA crystal. This may be caused also by the higher population of brush chains. Note that the brush chains are formed only in the reactive systems and the population may be higher in the smaller particle systems. Even for the blend with smallest particle size, a small-angle X-ray scattering peak appeared at a magnitude of scattering vector of approximately 0.8 nm-1, suggesting the evolution of interlamellar spacing of 8.7 nm. However, the scattering profile was much broader than that of neat PA. It implies a less ordered lamellar stacking in the blend.