Dependence Of Crystallization Rate Research Articles

Effect of melt spinning conditions on the fiber structure development of polyethylene terephthalate

The effects of spinning conditions on fiber properties are not well explained by the fiber structures because the birefringence, crystallinity, and SAXS patterns are often similar. In this study, the effects on the fiber structure development of polyethylene terephthalate after necking was analyzed by simultaneous WAXD/SAXS measurements. An X-shaped SAXS pattern was observed for all fibers drawn at the minimum draw ratio. In contrast, by drawing under a drawing stress of 100 MPa, the strong diffraction of the smectic phase and an obviously larger long period less than 1 ms after necking were observed for fibers spun at 500–1500 m/min, while almost no smectic phase was observed for fibers spun at 2000 m/min. A higher crystallization rate and clear draw ratio dependence of crystallization rate were also observed for the fiber spun at 2000 m/min. The clear differences in structure development can explain their differences in tensile strength and thermal shrinkage.

Synthesis of Grafted Block Copolymers Based on ε‐Caprolactone: Influence of Branches on Their Thermal Behavior

Branched copolymers are a special class of polymeric materials in which are reflected the combined effects of polymer segments and architectural constraints of the branched architecture. This study employed two methodologies to obtain copolymers with different branching density. In the first case, poly(hydroxyethyl methacrylate‐graft‐poly(ε‐caprolactone)‐block‐poly(ε‐caprolactone), P(HEMA‐g‐PCL)‐b‐PCL, copolymers were synthesized by a “grafting through” method in a three‐step reaction pathway involving ring opening polymerization (ROP) and radical addition fragmentation transfer (RAFT) polymerization. In the second case, a combination of simultaneous “grafting through” and “grafting from” methods in a one‐pot RAFT and ROP reaction afforded P(HEMA‐co‐HEMA‐g‐PCL)‐b‐PCL comb‐like copolymers with comparatively less dense branching. Samples with molar masses between 5500 and 46 000 g mol−1 and polydispersity indexes (Mw/Mn) lower than 1.3 were successfully obtained through both approaches. According to thermal analyses, the presence of branches reduces PCL melting temperature by as much as 20 °C, without affecting thermal stability. This fact was particularly evident for the most densely branched copolymers with higher molar masses. Nonisothermal crystallization process was successfully described using Ozawa's method, which showed a clear dependence of crystallization rate and cooling on grafting density. image

Nonisothermal crystallization kinetics and melting behavior of poly(butylene terephthalate) and calcium carbonate nanocomposites

Nonisothermal crystallization kinetics of poly(butylene terephthalate) (PBT)/nano calcium carbonate (CaCO3) composites were investigated by differential scanning calorimetry and polarized optical microscopy. PBT/nano CaCO3 composites containing different wt.% of nano CaCO3 were prepared by melt blending. Analysis of the melt crystallization data by various macrokinetic models like modified Avarami and Liu and Mo models revealed the dependence of crystallization rate on nano CaCO3 content. Nano CaCO3 addition upto 5wt.% greatly accelerated the crystallization rate. However, higher loadings of the filler retarded the crystallization rate. These results were further supported by the effective activation energy calculations by isoconversional method of Friedman. Polarized optical microscopy showed a decrease in spherulites size with increase in filler content. The study showed that small amount of nano CaCO3 acts as additional nucleating agent and is effective in accelerating the crystallization rate whereas higher loadings increase the nucleation density but restrict the mobility of polymer chains and retard the growth process.

Kinetics of nucleation and crystallization of poly(ε-caprolactone) – Multiwalled carbon nanotube composites

The nucleation efficiency of multiwall carbon nanotubes (MWCNT) in poly(ε-caprolactone) (PCL), as an example, was tested for a wide range of temperatures and cooling rates and compared to the efficiency of homogeneously formed nuclei. The temperature range below the maximum of crystallization rate is generally not accessible for non-isothermal cooling experiments because the sample becomes amorphous at the needed cooling rates. Isothermal experiments after fast quenches extend the temperature range down to and below the glass transition. The employed differential fast scanning calorimeter (DFSC) allows cooling at rates up to 100,000K/s and precise adjustment and control of isothermal conditions in the time range from 10−4 to 104s and longer. As shown in previous work, heterogeneous crystal nucleation dominates at low supercooling, revealing a significant dependence of crystallization rate on MWCNT concentration. Nevertheless, no saturation of the nucleation activity at a MWCNT loading of 0.2–0.5wt% as seen in slow DSC experiments was observed at the much higher cooling rates employed here. At high supercooling, where homogeneous nucleation is prevalent, the addition of MWCNT neither enhances nor reduces the crystallization rate. At the temperature of maximum homogeneous nucleation rate, formation of homogeneous nuclei always dominates crystallization.

On the dependence of the critical crystallization rate on the initial impurity concentration in melt

The dependence of the critical crystallization rate on the initial impurity concentration in melt is derived by determining the condition at which a nonplanar solution to the stationary diffusion problem arises. It is suggested that the conditions at which defects arise at the interface differ from those obtained when crystallization becomes stationary. The initial transient process of binary melt solidification under crystal pulling at a constant rate has been studied numerically within a 1D model. It is shown that the character of the time dependence of the crystallization rate is determined by the ratio of the crystal pulling rate to solidification rate Vc, when a constitutional supercooling zone is formed in the melt.

Solution-based metal induced crystallization of a-Si

This paper investigates a simplified metal induced crystallization (MIC) of a-Si, named solution-based MIC (S-MIC). The nickel inducing source was formed on a-Si from salt solution dissolved in de-ionized water or ethanol. a-Si thin film was deposited with low pressure chemical vapour deposition or plasma enhanced chemical vapour deposition as precursor material for MIC. It finds that the content of nickel source formed on a-Si can be controlled by solution concentration and dipping time. The dependence of crystallization rate of a-Si on annealing time illustrated that the linear density of nickel source was another critical factor that affects the crystallization of a-Si, besides the diffusion of nickel disilicide. The highest electron Hall mobility of thus prepared S-MIC poly-Si is 45.6 cm2/(V · s). By using this S-MIC poly-Si, thin film transistors and display scan drivers were made, and their characteristics are presented.

Crystallization of Bi Doped Sb8Te2

A rotating disc technique has been applied to measure the crystallization rate dependence of Sb8Te2 on bismuth. The laser amorphised state shows an order of magnitude increase in crystallization rate upon doping with 8 at. % bismuth. This is consistent with static measurements on as-deposited films. Ab-initio methods were used to relax a long period, Sb2Te3, stacking model. The X-ray diffraction pattern was calculated from the relaxed structure and found to be consistent with experimental data; the lattice parameters of the relaxed structures were a=4.22 Å and c=32.80 Å in the case of the undoped film, a=4.22–4.23 Å and c=32.78–33.00 Å for a Bi concentration of 5.5 at. %. The calculated bulk modulus was found to increase upon doping with Bi; suggesting an increase in the crystalline viscosity. The increased crystallization rate is therefore explained in terms of either a decrease in viscosity in the amorphous phase or a reduction in the surface tension of crystalline nuclei upon doping with Bi.

Microscopy and calorimetry as complementary techniques to analyze sugar crystallization from amorphous systems

A comparison of microscopic and macroscopic techniques to evaluate sugar crystallization kinetics is presented using amorphous lactose and lactose–trehalose mixtures. Polarized light video microscopy (PLV) and differential scanning calorimetry (DSC) were applied to measure crystallization kinetics, induction times and time for complete sugar crystallization at different storage temperatures (60–95 °C). DSC was also employed to measure the glass transition temperature ( T ag) of the systems. PLV permitted direct observation, in real time, of growth of individual crystals and morphological aspects at a scale not detected by DSC. Taking the average of several microscopic observations, the results for temperature dependence of crystallization rate and time to complete lactose crystallization were similar to those obtained by DSC. Both PLV and DSC techniques showed that the presence of trehalose delayed lactose crystallization, without affecting the T ag value. For the analysis of sugar crystallization in amorphous systems, PLV and DSC proved to be complementary techniques. Validation of results obtained by PLV with results from DSC opens a new area of microstructural analysis of crystallizing systems.

An investigation of heat transfer effects in isothermal crystallization studies of low‐density polyethylene

AbstractIsothermal crystallization kinetics of low‐density polyethylene (LDPE) were measured from 96°C‐103°C using a power‐compensating differential scanning calorimetry (DSC). Crystallization kinetics were measured using different sample thicknesses and on samples compounded with nickel, a filler with high thermal conductivity. For the unfilled material, sample thickness and temperature had a significant effect on the rate of crystallization as measured by the Avrami rate constant K, but had no effect on the nucleation mechanism and dimensionality of growth, as measured by the Avrami constant n. The crystallization growth rate as expressed by K1/n scaled approximately with the thickness of the sample. For the filled material, K was much higher and independent of nickel content, suggesting a limiting growth rate for polyethylene at a given temperature in this equipment. The dependence of crystallization rale on sample thickness indicates that barriers to heat transfer can be important. This work shows that for most crystallization rates, thermal conductivity, rather than interfacial resistance between sample and pan, limits heat transfer. Even though thermal conductivity typically dominates heat‐transfer resistance, sample‐pan thermal contact is still important, and some guidelines are given to determine whether good contact is being made.

Isothermal crystallization behavior and melting characteristics of injection sample of nucleated polypropylene

The isothermal crystallization behavior and melting characteristics of pure polypropylene (PP) and PPs nucleated with a phosphate nucleating agent (A) and a sorbitol derivative (D) have been studied by differential scanning calorimetry (DSC). Compared with pure PP, nucleated PPs show a shorter half-times of crystallization. Dependence of crystallization rate of nucleated PP on the crystallization temperature is stronger than that of pure PP at the higher crystallization temperature, whereas the opposite results are obtained at the lower crystallization temperature. Addition of nucleating agent shifts the temperature at the deviation from the baseline of DSC melting curve, T, and the temperature at the completion of melting, T, to higher temperatures, indicating that nucleated PPs exhibit a higher perfection of PP crystals. A shoulder peak in the high temperature range of melting peak of nucleated PP and a wider low temperature region in the melting peak of pure PP are observed. Obviously, PP and nucleated PPs form different distribution of crystal perfection in the isothermal crystallization process. According to the half-time of crystallization, nucleating agent A is more effective than D. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2547–2553, 2000

Isothermal crystallization behavior and melting characteristics of injection sample of nucleated polypropylene

The isothermal crystallization behavior and melting characteristics of pure polypropylene (PP) and PPs nucleated with a phosphate nucleating agent (A) and a sorbitol derivative (D) have been studied by differential scanning calorimetry (DSC). Compared with pure PP, nucleated PPs show a shorter half-times of crystallization. Dependence of crystallization rate of nucleated PP on the crystallization temperature is stronger than that of pure PP at the higher crystallization temperature, whereas the opposite results are obtained at the lower crystallization temperature. Addition of nucleating agent shifts the temperature at the deviation from the baseline of DSC melting curve, T, and the temperature at the completion of melting, T, to higher temperatures, indicating that nucleated PPs exhibit a higher perfection of PP crystals. A shoulder peak in the high temperature range of melting peak of nucleated PP and a wider low temperature region in the melting peak of pure PP are observed. Obviously, PP and nucleated PPs form different distribution of crystal perfection in the isothermal crystallization process. According to the half-time of crystallization, nucleating agent A is more effective than D. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2547–2553, 2000

Crystallization and stress‐strain properties of ultra‐high‐molecular‐weight‐polyethylene gels filled with iron particles

AbstractCrystallization of ultra‐high‐molecular‐weight‐polyethylene (UHMW‐PE) filled solutions and gels was studied by differential scanning calorimetry. It has been found that the structure and properties of the gelating systems depend on solvent type, polymer concentration, and filler. Crystallization rate decreases as the polymer content is increased. The crystallization rate dependence on filler concentration passes through a maximum. The influence of a disperse filler on the structure, the stress‐strain and relaxation properties of UHMW‐PE gels has been studied. The results demonstrate the advantage of using less thermodynamically active solvents (thermodynamic activity means polymer‐solvent interaction). This gel‐technology enables us to obtain UHMW‐PE based materials with a 90‐vol% content of iron particles. Highly loaded fibers based on UHMW‐PE were obtained with a tensile strength of 40–50 MPa and an elastic modulus of 350–400 MPa.

Expansion and crystallization of a sediment of charged colloidal spheres

An amorphous sediment of charged colloidal particles in a nonaqueous solvent was prepared by centrifugation. The evolution of sediment height and volume fraction profile to equilibrium were studied with light scattering. A simple sedimentation–diffusion equation allows the experimental data to be modeled quantitatively. The top of the sediment crystallizes within a week, whereas the lower part of the sediment does not show any sign of crystallization after several months, indicating a strong concentration dependence of crystallization rate.

Relationship between plagioclase crystallization and cooling rate in basaltic melts

Rock textures commonly preserve a record of the near-surface crystallization history of volcanic rocks. Under conditions of simple cooling without convection or mixing, textures will reflect sample cooling rate, the temperature at which crystallization was initiated, and the distribution of mineral phase precipitation across the crystallization interval. Compilation of plagioclase size and number density data on natural (dike, sill and lava lake) and experimental samples suggests that (1) growth and nucleation rates of plagioclase in natural basaltic samples are a predictable function of cooling rate, and (2) the observed crystallization rate dependence on cooling rate is similar to that observed in experiments initiated at subliquidus temperatures. Comparison of natural and experimental samples thus suggests that most basalts crystallize under conditions of heterogeneous nucleation, with the number density of preexisting nucleii partially controlling textural responses to cooling rate changes. Time scales of crystallization and cooling in magmatic systems are intimately linked through a balance between heat removal from the system and heat evolved through crystallization. Evaluation of textural data in the context of recent numerical models of crystallization in simple (one- and two-component systems) provides new insight into regularities in the crystallization behavior of basaltic magmas. For example, the rate of change in crystal size (and number density, as dictated by mass balance) has been used as a measure of the relative importance of time scales of crystallization and cooling in numerical models of crystallizing systems. In natural samples, plagioclase size scales with the length scale of cooling such that a logarithmic plot of grain size as a function of normalized distance across the dike has a slope that appears approximately independent of dike width (solidification time). Comparison with available textural data for other phenocryst phases suggests that the same may be true for pyroxene and magnetite crystallization, with each phase having a characteristic slope probably controlled by the thermodynamic properties of the crystallizing phase. Measured crystal size distributions are unimodal and show maximum frequencies in the smaller size classes; distributions broaden and the grain size at peak frequency increases with increasing crystallization times (decreasing cooling rates). In contrast, partially crystallized Makaopuhi lava lake samples have crystal size distributions that decrease exponentially with increasing crystal size. Measured size distributions in dikes can be explained by late stage modification of Makaopuhi-type distributions through loss of small crystals, possibly the consequence of growth without nucleation. Finally, this compilation of the textural response of basaltic magmas to changes in cooling rate suggests that empirical calibrations of crystallization rate dependence on cooling rate from natural samples provide a reasonable model for plagioclase crystallization in near-surface basaltic systems. Predicted growth rates will be slow and relatively constant (10-10–10-11 cm/s) for crystallization times expected in most shallow volcanic systems (<1000 years).

Crystallization of filled Nylon 6 II. Isothermal crystallization

Isothermal crystallization kinetics from the melt of nylon 6 containing various amounts of untreated glass beads and Aerosil (series GB and A) and aminosilane-treated glass beads (series GBA) was studied by DSC method. Treatment of experimental data in terms of Kolmogorov-Avrami equation has shown that time exponentn is relatively insensitive to presence of GBA filler, whereas it was found to decrease gradually with increasing filler content in samples of GB and A series to value as low as 0.5 for sample A–67. Nucleation free energy was the same in pure nylon 6 and filled samples of GBA series, but dropped suddenly in samples of GB series when mean interparticle separation x decreased below “critical” value ξcrit =(5–10) x 10−6 m. Taking into account the concomitant drop of equilibrium melting temperature, it was concluded that these effects are a consequence of specific structural change in polymer phase accompanying transition into a “boundary state”. Temperature dependence of polyer crystallization rate in highly filled samples of A series at small supercoolings was found to obey them=4 law, which is characteristic of transformations in strained media. Experimental data seem to be consistent with the idea that the postulated strain energy might originate from negative capillary pressure between high-surface energy solid particles wette d by polymer melt. Quantitative analysis of overall crystallization rate dependence on interparticle distance x has shown that ξcrit may be identified with change of growth morphology from a three-dimensional to a two-dimensional one, while slowing down of growth rate at lower values of ξ was attributed to increase of nucleation free energy due to increasing contribution from capillary forces.