Effect of pressure on the crystallization of poly(ethylene terephthalate)

The effect of pressure on the crystallization of poly(ethylene terephthalate) (PET) was studied. The Instron capillary rheometer was adapted as a high-pressure/high-temperature dilatometer to carry on experiments up to 40,000 psi. Isothermal measurements of PET melt density were made with a precision of ±0.5%. Analysis of the kinetics of crystallization of PET melt at high pressures reaffirmed the existence of low Avrami exponents and their noninteger values. To rationalize the crystallization mechanism with the observed low exponents it is proposed that an appropriate increase in pressure would effectively reduce the free volume of a crystallizable substance to a point at which an alteration of the crystallization mechanism or nucleation mode could occur. It is further shown that PET clearly exhibits two different and sharply defined stages of crystallization behavior at pressures above 10,000 psi. Based on the Avrami equation, the fraction of uncrystallized polymer for the initial stage is defined as an empirically determined function of time and pressure. There is good agreement between the predicted and experimental values over the pressure range investigated.     

Effects of pressure on the compressibility and crystallization of fiber-forming polymers

The effects of pressure on the compressibility and crystallization of three fiber-forming polymers, poly(tetramethylene terephthalate), nylon 66, and Qiana® nylon, have been studied. The Instron capillary rheometer was adapted as a high-pressure dilatometer for all the high-pressure experiments. The compressibility results reaffirmed that polymers are highly compressible, and their compressibilities are nonlinear at temperatures above the glass transition temperatures. Polymer melts show higher compressibility than do polymers in the solid state. The kinetics of crystallization of these polymer melts under high pressures were studied. Analysis of the data revealed low Avrami exponents at high pressures. It seems that the kinetics of crystallization of these polymers from the melt under high pressure are different from those at normal pressure. Crystallization temperatures of these polymers were also measured. The crystallization temperatures are considerably higher at higher pressures.     

Crystallization of polypropylene,nylon-66 and poly(ethylene terephthalate) at pressures to 200 MPa: Kinetics and characterization of products

The crystallization kinetics of polypropylene (PP), polyamide (PA66), and poly(ethylene terephthalate) (PET) were studied, using a pressure dilatometer (to 200 MPa) to follow the volume changes associated with the crystallization process. The commonly used Avrami equation fitted the isothermal/isobaric crystallization data of PP and PA66 well. The Avrami exponent n was between 1.3 and 1.7, independent of crystallization pressure and temperature. Lines of constant Avrami rate parameter Z in the P-T plane were essentially parallel to the pressure dependence of the melting points and crystallization temperatures. However, the Avrami equation was not suitable for PET. The Malkin, Dietz, and Kim equations provided better fits. The crystallization half-time of PET increased with pressure at constant supercooling, in contrast to PP and PA66, for which it remained essentially unchanged. X-ray diffraction, differential scanning calorimetry, and pressure dilatometry were used to study the effect of formation pressure on the crystal structure, the melting point, and the density of products which were crystallized for short times (minutes) at various temperatures and pressures. No new crystal structures were found for PA66 and PET, but a mixture of monoclinic and triclinic crystals existed in PP above a formation pressure of 50 MPa. The melting points increased with formation pressure for PET, but remained unchanged for PP and PA66. Density at ambient conditions decreased with formation pressure for PP, but increased for PET and PA66. © 1994 John Wiley & Sons, Inc.     

CO2‐delayed crystallization of isotactic polypropylene: A kinetic study

The effect of CO₂ on the nonisothermal crystallization of isotactic polypropylene (iPP) was studied with high‐pressure differential scanning calorimetry at cooling rates of 0.2–5 °C/min. CO₂ significantly delayed the melt crystallization of iPP, and both the crystallization temperature and the heat of crystallization decreased with increasing CO₂ pressure. The crystallization rate of iPP, as characterized by the half‐time, was also prolonged by the presence of CO₂. With a modified Ozawa model developed by Seo, the Avrami crystallization exponent n of iPP was calculated. This value was depressed by the addition of CO₂ and was strongly dependent on the CO₂ pressure at low cooling rates. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1518–1525, 2003     

Isothermal crystallization kinetics and melting behavior of poly(ethylene terephthalate)/barite nanocomposites

Poly(ethylene terephthalate) (PET)/Barite nanocomposites were prepared by direct melt compounding. The effects of PET‐Barite interfacial interaction on the dynamic mechanical properties and crystallization were investigated by DMA and DSC. The results showed that Barite can act as a nucleating agent and the nucleation activity can be increased when the Barite was surface‐modified (SABarite). SABarite nanoparticles induced preferential lamellae orientation because of the strong interfacial interaction between PET chains and SABarite nanoparticles, which was not the case in Barite filled PET as determined by WAXD. For PET/Barite nanocomposites, the Avrami exponent n increased with increasing crystallization temperature. Although at the same crystallization temperature, the n value will decrease with increasing SABarite content, indicating of the enhancement of the nucleation activity. Avrami analyses suggest that the nucleation mechanism is different. The activation energy determined from Arrhenius equation reduced dramatically for PET/SABarite nanocomposite, confirming the strong interfacial interaction between PET chains and SABarite nanoparticles can reduce the crystallization free energy barrier for nucleus formation. In the DSC scan after isothermal crystallization process, double melting behavior was found. And the double endotherms could be attributed to the melting of recrystallized less perfect crystallites or the secondary lamellae produced during different crystallization processes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 655–668, 2009     

Non-isothermal crystallization kinetics of a Si–Ca–P–Mg bioactive glass

In this work, the crystallization process of a SiO₂–3CaO·P₂O₅–MgO glass was studied by non-isothermal measurements using differential thermal analysis carried out at various heating rates. X-ray diffraction at room and high temperature was used to identify and follow the evolution of crystalline phases with temperature. The activation energy associated with glass transition, (E _g), the activation energy for the crystallization of the primary crystalline phase (E _c), and the Avrami exponent (n) were determined under non-isothermal conditions using different equations, namely from Kissinger, Matusita & Sakka, and Osawa. A complex crystallization process was observed with associated activation energies reflecting the change of behavior during in situ crystal precipitation. It was found that the crystallization process was affected by the fraction of crystallization, (x), giving rise to decreasing activation energy values, E _c(x), with the increase of x. Values ranging from about 580 kJ mol^?1 for the lower crystallized volume fraction to about 480 kJ mol^?1 for volume fractions higher than 80 % were found. The Avrami exponents, calculated for the crystallization process at a constant heating rate of 10 °C min^?1, increased with the crystallized fraction, from 1.6 to 2, indicating that the number of nucleant sites is temperature dependent and that crystals grow as near needle-like structures.     

SAXS/WAXS/DSC studies on crystallization of a polystyrene-b-poly(ethylene oxide)-b-polystyrene triblock copolymer with lamellar morphology and low glass transition temperature

Crystallization of a polystyrene-b-poly(ethylene oxide)-b-polystyrene (S-EO-S) triblock copolymer, S₄₀EO₁₃₆S₄₀, with lamellar morphology in the melt and low glass transition temperature (T_g=47 °C) of the S block was studied. The triblock copolymer was cooled from ordered melt and isothermal crystallization was conducted at crystallization temperatures (T_c) near the T_g of the S block. It is found that crystallization behavior of S₄₀EO₁₃₆S₄₀ strongly depends on T_c. When T_c is far below T_g, an Avrami exponent n=0.5 is observed, which is attributed to diffusion-controlled confined crystallization. As T_c slightly increases, the Avrami exponent is 1.0, indicating that crystallization is confined and crystallization rate is determined by the rate of homogeneous nucleation. When T_c is just below the T_g of the S block, crystallization tends to become breakout and accordingly Avrami exponent changes from 1.0 to 3.2. Crystallinity and melting temperature of the EO block in breakout crystallization are slightly higher than those in confined crystallization. Time-resolved small and wide angle X-ray scattering (SAXS/WAXS) were used to monitor isothermal crystallization of S₄₀EO₁₃₆S₄₀. It shows that the long period is constant in confined crystallization, but it gradually increases during breakout crystallization. WAXS result reveals that confined or breakout crystallization has no effect on the crystal structure of the EO block.     

Crystallization of poly(ethylene terephthalate–co–isophthalate)

Melt crystallization behaviors of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate‐co‐isophthalate) (PETI) containing 2 and 12 mol % of noncrystallizable isophthalate components were investigated. Differential scanning calorimetry (DSC) isothermal results revealed that the introduction of 2 mol % isophthalate into PET caused a change of the crystal growth process from a two‐dimensional to a three‐dimensional spherulitic growth. The addition of more isophthalate up to 12 mol % into the PET structure induced a change in the crystal growth from a three‐dimensional to a two‐dimensional crystal growth. DSC heating scans after completion of isothermal crystallization at various T_c's showed three melting endotherms for PET and four melting endotherms for PETI‐2 and PETI‐12. The presence of an additional melting endotherm is attributed to the melting of copolyester crystallite composed of ethylene glycol, tere‐phthalate, and isophthalate (IPA) or the melting of molecular chains near IPA formed by melting the secondary crystallite T_m (I) and then recrystallizing during heating. Analyses of both Avrami and Lauritzen‐Hoffman equations revealed that PETI containing 2 mol % of isophthalate had the highest Avrami exponent n, growth rate constant G_o, and product of lateral and end surface free energies σσ_e. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2515–2524, 2000     

Cold crystallization and postmelting crystallization of PLA plasticized by compressed carbon dioxide

The cold crystallization at temperature T_cc (melting > T_cc > glass transition) and the postmelting crystallization of polylactic acid plasticized by compressed carbon dioxide (CO₂) were studied using a high-pressure differential scanning calorimeter. The kinetics of the two kinds of crystallization were evaluated by the Avrami equation as a function of pressure at certain temperatures. The effects of using talc as a nucleation agent on the two types of crystallization under pressure were also investigated. The results show that compressed CO₂ increased the mobility of the polymer chains in solid state, resulting in an increased rate of cold crystallization. The decreased rate of postmelting crystallization was mainly in the nucleation-controlled region, which indicates that the number of nuclei was decreased by the compressed CO₂. The growth rate of the two crystallization types followed the Avrami equation, but the kinetics of each depended upon temperature and pressure. The inclusion of talc accelerated postmelting crystallization but had little effect on cold crystallization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2630–2636, 2008     

Crystallization kinetics of polyethylene under high pressure

The kinetics of isothermal crystallization of polyethylene under high pressures ranging from 840 to 5300 kg/cm² has been studied dilatometrically. The crystallization rate estimated from the half-time of the overall transformation increases markedly with pressure. The Avrami exponent n becomes smaller with increasing pressure. Values of n ≈ 2 for the crystallization at 840 and 1950 kg/cm², and n ≈ 1 at 5100 and 5300 kg/cm² were obtained. Differential scanning calorimetry and electron microscopy data are presented. It is concluded that extended-chain crystals grow rapidly, predominantly in one dimension, at high pressure. Relations between log k and T_m/T(ΔT) and T_m²/T(ΔT)² are nearly linear. Here, k is the crystallization rate constant from an Avrami equation, ΔT = T_m – T, T_m is the melting point, and T is the temperature of crystallization. From the dependence of the slope of the straight line on the crystallization pressure it is concluded that the surface energy of crystal nuclei decreases with increasing pressure.     

Thermophysical properties of azomethine ether main-chain liquid crystalline polymers at pressures to 200 MPa

This article reports on an experimental investigation of the equation of state and the transition behavior of main-chain thermotropic liquid crystalline polymers over a wide temperature range, and at pressures to 200 MPa. The materials studied were a series of azomethine ether polymers. A varying number n (= 4, 7, 8, 9, 10 and 11) of methylene spacer units in the backbone provided systematic variation of the structure. Experimental techniques used included high-pressure dilatometry (PVT measurements) to 200 MPa, high-pressure differential thermal analysis, also to 200 MPa, and conventional (atmospheric-pressure) differential scanning calorimetry (DSC). The equation of state of the materials can be well represented by the Tait equation in distinct regions, separated by a glass transition, T_g(P), a first-order transition to a nematic state, T_k-n(P), and a first-order transition to an isotropic melt state T_c(P). The atmospheric pressure values of T_k-n and T_c decreased with increasing number of spacer units and showed a clear odd-even effect. T_g and T_k-n both increased with pressure. The pressure dependence of T_c could not be observed due to the onset of degradation in the same temperature region. On isobaric cooling at 3°C/min, the crystallization from the nematic state occurred a few tens of degrees below T_k-n. This supercooling was independent of pressure for some materials, while for others it increased with increasing pressure. The values of the enthalpy and entropy associated with the first-order transition into the nematic state were lower than those of typical isotropic polymers at their melting transitions. The transition enthalpy did not have any systematic variation with increasing number of spacer units. Values of the transition enthalpy calculated from the Ciapeyron equation did not always agree with the values measured by DSC. This may be due to the two-phase nature of the low-temperature state. At the transition to the isotropic state, the transition enthalpy at P = 0 decreased with n and showed an odd-even effect. © 1994 John Wiley & Sons, Inc.     

Strain-induced crystallization of swollen,crosslinked polyethylene

The kinetics of strain-induced crystallization of swollen polyethylene networks have been measured using dynamometry coupled with optical birefringence. Fibers were prepared by gel-spinning ultrahigh-molecular-weight polyethylene followed by crosslinking in dicumyl peroxide and swelling in xylene. Retractive force on crystallization was monitored at various temperatures and draw ratios. Fiber visualization was achieved using optical illumination coupled with video recording and digital analysis of the in situ transformation. Avrami plots of the transformation data showed good linear fits for low draw ratios in the range α < 1.2 and moderate undercoolings (60 < T_c < 70°C). Time exponents of unity were found, indicating a one-dimensional, heterogeneous growth mechanism whose temperature dependence could be described by a formalism similar to that recently used for flow-induced growth.     

Isothermal crystallization kinetics of Fe<Subscript>75</Subscript>Cr<Subscript>5</Subscript>P<Subscript>9</Subscript>B<Subscript>4</Subscript>C<Subscript>7</Subscript> metallic glass with cost-effectiveness and desirable merits

In this work, the isothermal crystallization kinetics of cost-effective Fe₇₅Cr₅P₉B₄C₇ metallic glass with a combination of desired merits synthesized by industrial ferro-alloys without high-purity materials was evaluated by Johnson–Mehl–Avrami approach using differential scanning calorimeter. The Avrami exponents at all isothermal annealing temperatures range from about 2.93 to 4.61, indicating a three-dimensional diffusion-controlled growth with an increasing nucleation rate during the isothermal crystallization. Meanwhile, the Avrami exponent firstly increases from 2.93 at the initial time to a maximum value of 4.61 and then decreases to 4.09 with the increment of the isothermal annealing temperature, which can be attributed to the atomic diffusion in the alloy. Additionally, the trend of the local Avrami exponent variations at different isothermal annealing temperatures reflects a variable crystallization mechanism during the crystallization process. Moreover, the local activation energy determined by Arrhenius equation gradually decreases from about 412 to 383 kJ mol^?1 during the present isothermal crystallization, further revealing that the process is dominated by a three-dimensional diffusion-controlled growth with an increasing nucleation rate, which provides useful insights into the formation of the present alloy.     

Isothermal melt and cold crystallization kinetics of poly(aryl ether ketone ether ketone ketone)

The isothermal melt and cold crystallization kinetics of poly(aryl ether ketone ether ketone ketone) are investigated by differential scanning calorimetry over two temperature regions. The Avrami equation describes the primary stage of isothermal crystallization kinetics with the exponent n ≈ 2 for both melt and cold crystallization. With the Hoffman–Weeks method, the equilibrium melting point is estimated to be 406 °C. From the spherulitic growth equation proposed by Hoffman and Lauritzen, the nucleation parameter (K_g) of the isothermal melt and cold crystallization is estimated. In addition, the K_g value of the isothermal melt crystallization is compared to those of the other poly(aryl ether ketone)s. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1992–1997, 2000     

Melting behavior of isotactic polystyrene

The melting behavior of isotactic polystyrene, crystallized from the melt and from dilute solutions in trans-decalin, has been studied by differential scanning calorimetry and solubility measurements. The melting curves show 1, 2, or 3 melting endotherms. At large supercooling, crystallization from the melt produces a small melting endotherm just above the crystallization temperature T_c. This peak originates from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increases linearly with T_c, yielding an extrapolated value for the equilibrium melting temperature T_c° of 242 ± 1°C as found before. By self-seeding, crystallization from the melt could be performed at much higher temperature to obtain melting temperatures as high as 243°C, giving rise to doubt about the value of T_c° found by extrapolation. For normal values of T_c and heating rate, an extra endotherm appears on the melting curve. Its peak temperature is the same for both melt-crystallized and solution-crystallized samples, and independent of T_c, but rises with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas and of previous heat treatment on dissolution temperature, it is concluded that this peak arises from the second one by continuous melting and recrystallization during the scan.     

New insight into melting and crystallization behavior in semicrystalline poly(ethylene terephthalate)

After isothermal crystallization, poly(ethylene terephthalate) (PET) showed double endothermic behavior in the differential scanning calorimetry (DSC) heating scan. During the heating scans of semicrystalline PET, a metastable melt which comes from melting thinner lamellar crystal populations formed between the low and the upper endothermic temperatures. The metastable melt can recrystallize immediately just above the low melting temperature and form thicker lamellae than the original ones. The thickness and perfection depends on the crystallization time and crystallization temperature. The crystallization kinetics of this metastable melt can be determined by means of DSC. The kinetics analysis showed that the isothermal crystallization of the metastable PET melt proceeds with an Avrami exponent of n = 1.0 ∼ 1.2, probably reflecting one‐dimensional or irregular line growth of the crystal occurring between the existing main lamellae with heterogeneous nucleation. This is in agreement with the hypothesis that the melting peaks are associated with two distinct crystal populations with different thicknesses. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 53–60, 2000     

The effect of prior crystallinity on the flow-induced crystallization of poly(ethylene oxide)

After flowing in a dilatometer bulb for a small fraction of the duration of the transformation, a relaxed melt of poly(ethylene oxide) (M?_n = (5.9 ± 0.1) × 10³) showed marked increases in isothermal crystallization rate. The extent of increase was greater when flow was imposed at modestly later stages rather than at the earliest stage of a crystallization. Kinetic parameters for the flow-induced crystallizations were obtained via modification of the conventional mathematical treatment of the kinetics of phase change, thereby allowing the analytical resolution of the overall process into flowinduced and quiescent components. Determination of the flow-induced crystallization parameters required independent determination of the kinetic parameters for quiescent crystallizations at that temperature. The Avrami exponents n_f which characterized the flow-induced portions of the crystallizations were larger for those instances in which flow was imposed at the more advanced stages of the crystallizations, thus indicating a transition in crystallization mechanism. It is suggested that prior crystallinity present at the time of flow contributed to the crystallization by serving as a source of nucleation sites. However, in light of the experimental procedure employed, values of n_f approximating 4 that were obtained are not susceptible to mechanistic interpretations now extant.     

Effect of supercritical carbon dioxide on the crystallization and melting behavior of linear bisphenol A polycarbonate

The crystallization and melting behavior of bisphenol A polycarbonate treated with supercritical carbon dioxide (CO₂) has been investigated with differential scanning calorimetry. Supercritical CO₂ depresses the crystallization temperature (T_c) of polycarbonate (PC). The lower melting point of PC crystals increase nonlinearly with increasing treatment temperature. This indicates that the depression of T_c is not a constant at the same pressure. T_c decreases faster at a higher treatment temperature than at a lower temperature. The leveling off of the depression in T_c at higher pressures is due to the antiplasticization effect of the hydrostatic pressure of CO₂. The melting curves of PC show two melting endotherms. The lower melting peak moves to a higher temperature with increasing treatment temperature, pressure, and time. The higher temperature peak moves toward a higher temperature as the treatment temperature is increased, whereas this peak is independent of the treatment pressure, time, and heating rate. The double melting peaks observed for PC can be attributed to the melting of crystals with different stability mechanisms. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 280–285, 2004     

High-pressure studies on the coacervation of copoly(<Emphasis Type="Italic">N</Emphasis>-vinylformamide–vinylacetate) and copoly(<Emphasis Type="Italic">N</Emphasis>-vinylacetylamide–vinylacetate)

The effects of pressure on the temperature-induced coacervation of copoly( N-vinylformamide–vinylacetate) and copoly( N-vinylacetylamide–vinylacetate) in aqueous solution were investigated by measuring the apparent light scattering, and the effects of the concentrations of polymer and inert salt were also studied. At lower pressures, the apparent temperature of transition, T _c, increased with an increase in the pressure, but further increases in pressure decreased T _c. In contrast, the apparent pressure of transition, P _c, monotonously increased with decreasing fixed temperature. The T– P diagram was elliptical, but the curvature was not large and extrema were not observed in the low-temperature and high-pressure range. Anions with low lyotropic numbers induced an almost linear decrease in the T _c and P _c, whereas anions with a high number increased the T _c and P _c. In both polymers, the T _c and P _c depended on the concentrations of the polymers, reflecting the association/aggregation mechanism of coacervation. This is in contrast to typical thermoresponsive polymers such as poly( N-isopropylacrylamide) and poly( N-vinylisobutyramide), which show the coil-collapse transition as a single molecular event.     

Miscibility and crystallization behavior of PBT/epoxy blends

The miscibility and the isothermal crystallization kinetics for PBT/Epoxy blends have been studied by using differential scanning calorimetry, and several kinetic analyses have been used to describe the crystallization process. The Avrami exponents n were obtained for PBT/Epoxy blends. An addition of small amount of epoxy resin (3%) leads to an increase in the number of effective nuclei, thus resulting in an increase in crystallization rate and a stronger trend of instantaneous three‐dimensional growth. For isothermal crystallization, crystallization parameter analysis showed that epoxy particles could act as effective nucleating agents, accelerating the crystallization of PBT component in the PBT/Epoxy blends. The Lauritzen–Hoffman equation for DSC isothermal crystallization data revealed that PBT/Epoxy 97/3 had lower nucleation constant K_g than 100/0, 93/7, and 90/10 PBT/Epoxy blends. Analysis of the crystallization data of PBT/Epoxy blends showed that crystallization occurs in regime II. The fold surface free energy, σ_e = 101.7–58.0 × 10^?3 J/m², and work of chain folding, q = 5.79–3.30 kcal/mol, were determined. The equilibrium melting point depressions of PBT/Epoxy blends were observed and the Flory–Huggins interaction parameters were obtained. It indicated that these blends were thermodynamically miscible in the melt. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1320–1330, 2006