Isothermal crystallization and chain mobility of poly(L-lactide)

Isothermal melt crystallization of poly(L-lactide) (PLLA) has been studied in the temperature range of 90 to 135°C. A maximum in crystallization kinetic was observed around 105°C. A transition from regime II to regime III is present around 115°C. The crystal morphology is a function of the degree of undercooling. At crystallization temperatures (T_c) below 105°C, further crystallization occurs upon heating; this behavior is not detected for T_c above 110°C. The analysis of the heat capacity increment at glass transition temperature (T_g) and of dielectric properties of PLLA indicates the presence of a fraction of the amorphous phase which does not relax at the T_g, and the amount of this so-called rigid amorphous phase is a function of T_c. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 911–919, 1997     

Characterization of the thermal properties of PLA fibers by modulated differential scanning calorimetry

Polylactide (PLA) has been melt spun to produce multifilament continuous yarns. The thermal characteristics of PLA filaments have been investigated using modulated differential scanning calorimetry (MDSC). With MDSC, it is possible to separate the different thermal events and to analyze them more precisely. The influence of hot drawing on thermal properties of PLA filaments has been studied. Hot drawing promotes an increase of glass transition temperature (T_g) and a decrease of heat capacity. The cold crystallization spreads on a larger range of temperature and the peak occurs at a lower temperature. The initial degrees of cristallinity of PLA filaments have been calculated thanks to the reversing and non reversing curves of MDSC. Tensile properties of PLA filaments are also investigated.     

Effect of high-performance polymers on crystallization and multiple melting behavior of poly(phenylene sulfide)

The crystallization and multiple melting behavior of poly(phenylene sulfide) (PPS) and its blends with amorphous thermoplastic bisphenol A polysulfone (PSF) and phenolphthalein poly(ether ketone) (PEK-C), crystalline thermoplastic poly(ether ether ketone) (PEEK), and thermosetting bismaleimide (BMI) resin were investigated by a differential scanning calorimeter (DSC). The addition of PSF and PEK-C was found to have no influence on the crystallization temperature (T_c) and heat of crystallization (ΔH_c) of PPS. A significant increase in the value of T_c and the intensity of the T_c peak of PPS was observed and the crystallization of PPS can be accelerated in the presence of the PEEK component. An increase in the T_c of PPS can also be accelerated in the BMI/PPS blend, but was no more significant than that in the PEEK/PPS blend. The T_c of PPS in the PEEK/PPS blends is dependent on the maximum temperature of the heating scans and can be divided into three temperature regions. The addition of a second component has no influence on the formation of a multiple melting peak. The double melting peaks can also be observed when PPS and its blends are crystallized dynamically from the molten state. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 637–644, 1998     

Precipitation of nanosized crystals CuBr and CuCl in potassium aluminoborate glasses

The precipitation of nanosized crystals CuBr and CuCl in potassium aluminoborate glasses containing additives of jointly introduced Cu and Br, as well as Cu and Cl, respectively, has been studied by the methods of small-angle X-ray scattering (SAXS) and X-ray powder diffraction (XRPD) analysis. It has been found that, upon thermal treatments at temperatures close to the glass transition temperature T _g , halide phases precipitate in the form of liquid drops containing CuBr and KBr or CuCl and KCl. The presence of nanocrystals CuBr or CuCl, respectively, has been established in the samples at room temperature by the XRPD method. Liquidus, solidus, and crystallization onset temperatures in the regions of precipitated halide phases in heat treated glass samples have been determined from the temperature dependences of SAXS intensity. The liquid released upon the heat treatment inside a matrix of potassium aluminoborate glass remains in the state of a supercooled liquid at temperatures essentially below the solidus temperature. At the drop sizes of the order of 10 nm, crystallization processes in them start at temperatures of 40?C85°C.     

Effect of A2O3 (A = La,Y, Cr,Al) on thermal and crystallization kinetics of borosilicate glass sealants for solid oxide fuel cells

Influence of various intermediate oxides on thermal, structural and crystallization kinetics of 30BaO–40SiO₂–20B₂O₃–10A₂O₃ (A = Y, La, Al, Cr) glasses has been studied. The highest glass transition temperature (T_g) with high thermal stability is observed in Y₂O₃ containing glasses as compared to other glasses. The thermal expansion coefficient (TEC) increases with increasing heat treatment duration in all the glasses. The maximum increase in TEC is observed in Cr₂O₃ containing glass ceramics. FTIR study showed that transmission bands due to silicate and borate chains become sharper with splitting after heat treatment. A selected glass sample (BaCr) has been tested for interaction and adhesion with Crofer 22 APU interconnect material for its application as a sealant in solid oxide fuel cell.     

Thermodynamic insight into the evolution of medieval glassworking properties

We present differential scanning calorimetry (DSC) analyses of seven French stained glasses from the 13th to 16th centuries. These glasses illustrate the dramatic compositional change from the antique soda‐rich glasses to potash‐ and lime‐rich compositions, resulting in drastic temperature and viscosity increases. We investigate the influence of chemical composition on glass thermal properties: glass‐transition (T_g), crystallization, and melting temperatures. We find that T_g varies from 533°C (soda type) to 638°C±17°C (potash type) to 685°C±5°C (lime type). The viscous slowdown of the melt as a function of the temperature, close to T_g, was modeled using the Vogel‐Tammann‐Fulcher equation. This enables temperature‐viscosity profile calculations, and suggests that recipes have been empirically optimized to reach similar thermoelastic properties suitable for glassmaking despite changing the nature of raw materials.     

Thermal properties and non‐isothermal crystallization behavior of poly(trimethylene terephthalate)/poly(lactic acid) blends

Thermal properties and non‐isothermal melt‐crystallization behavior of poly(trimethylene terephthalate) (PTT)/poly(lactic acid) (PLA) blends were investigated using differential scanning calorimetry and thermogravimetric analysis. The blends exhibit single and composition‐dependent glass transition temperature, cold crystallization temperature (T_cc) and melt crystallization peak temperature (T_mc) over the entire composition range, implying miscibility between the PLA and PTT components. The T_cc values of PTT/PLA blends increase, while the T_mc values decrease with increasing PLA content, suggesting that the cold crystallization and melt crystallization of PTT are retarded by the addition of PLA. The modified Avrami model is satisfactory in describing the non‐isothermal melt crystallization of the blends, whereas the Ozawa method is not applicable to the blends. The estimated Avrami exponent of the PTT/PLA blends ranges from 3.25 to 4.11, implying that the non‐isothermal crystallization follows a spherulitic‐like crystal growth combined with a complicated growth form. The PTT/PLA blends generally exhibit inferior crystallization rate and superior activation energy compared to pure PTT at the same cooling rate. The greater the PLA content in the PTT/PLA blends, the lower the crystallization rate and the higher the activation energy. Moreover, the introduction of PTT into PLA leads to an increase in the thermal stability behavior of the resulting PTT/PLA blends. Copyright © 2011 Society of Chemical Industry     

Thermal Stability and Crystallization Kinetics of MgO–Al2O3–B2O3–SiO2 Glasses

To support commercialization of the MgO–Al₂O₃–B₂O–SiO₂-based low-dielectric glass fibers, crystallization characteristics of the relevant glasses was investigated under various heat-treatment conditions. The study focused on the effects of iron on the related thermal properties and crystallization kinetics. Both air-cooled and nucleation-treated samples were characterized by using the differential thermal analysis/differential scanning calorimeter method between room temperature and 1200°C. A collected set of properties covers glass transition temperature (T_g), maximum crystallization temperature (T_p), specific heat (ΔC_p), enthalpy of crystallization (ΔH_cryst), and thermal stability (ΔT=T_p—T_g). Using the Kinssiger method, the activation energy of crystallization was determined. Crystalline phases in the samples having various thermal histories were determined using powder X-ray diffraction (XRD) and/or in situ high-temperature XRD method. Selective scanning electron microscope/energy-dispersive spectroscopy analysis provided evidence that crystal density in the glass is affected by the iron concentration. Glass network structures, for air-cooled and heat-treated samples, were examined using a midinfrared spectroscopic method. Combining all of the results from our study, iron in glass is believed to function as a nucleation agent enhancing crystal population density in the melt without altering a primary phase field. By comparing the XRD data of the glasses in two forms (bulk versus powder), the following conclusions can be reached. The low-dielectric glass melt in commercial operation should be resistant to crystallization above 1100°C. Microscopic amorphous phase separation, possibly a borate-enriched phase separating from the silicate-enriched continuous phase can occur only if the melt is held at temperatures below 1100°C, that is, below the glass immiscibility temperature. The study concludes that neither crystallization nor amorphous phase separation will be expected for drawing fibers between 1200°C and 1300°C in a commercial operation.     

Structure and property studies of bioabsorbable poly(glycolide-co-lactide) fiber during processing and in vitro degradation

Structure and properties of a bioabsorbable poly(glycolide-co-lactide) (PGA-co-PLA) fiber during several processing stages and the final in vitro degradation stage were investigated by means of wide-angle X-ray diffraction, dynamic mechanical analysis and mechanical property tests. In the orientation stage, an increase in the temperature of the first encountered orientation roll resulted in a lower level of crystallinity and larger crystallites. The temperature of the second encountered pre-annealing roll (PR) imposed a smaller effect on the structure. In the hot-stretching stage after fibers were braided, the maximum crystallinity was achieved at around 126 °C. Higher hot-stretching temperatures increased the crystal size, glass transition temperature (T_g) and tensile strength, but decreased the elongation at break and the heat shrinkage near T_g. In the post-annealing stage, it was found that crystallinity, T_g and tensile strength all increased significantly while the heat shrinkage near T_g sharply decreased after annealing. This suggests that the internal stress accumulated in the orientation and hot-stretching stages can be effectively reduced by post-annealing. During in vitro degradation, crystallinity was found to increase with time while the heat shrinkage near T_g and in the supercooling region (T_g<T<T_m) was greatly reduced. These results support the process of cleavage-induced crystallization.     

Glasses in the MnNbOF5–BaF2–FeF3 system: Synthesis,structure and crystallization

Glasses in the MnNbOF₅–BaF₂–FeF₃ system have been fabricated for the first time. The glass structure, thermal characteristics, and crystallization and magnetic properties have been investigated using IR and Raman spectroscopy, Differential scanning calorimetry (DSC), (XRD) X-ray powder diffraction and Superconducting Quantum Interference Device (SQUID) magnetometry. The NbO₂F₄ and FeF_n polyhedra form a glass network. During the devitrification of most of the glasses, the BaFeF₅ phase was found in the crystallization products. The possibility of obtaining crystalline phases in a glass matrix by heat treatment of glasses in the MnNbOF₅–BaF₂–FeF₃ system is demonstrated.     

Aging of poly(lactide)/poly(ethylene glycol) blends. Part 1. Poly(lactide) with low stereoregularity

Poly(lactide) (PLA) is rapidly gaining interest as a biodegradable thermoplastic for general usage in degradable disposables. To improve mechanical properties, a PLA with low stereoregularity was blended with polyethylene glycol (PEG). Blends with up to 30 wt% PEG were miscible at ambient temperature. Blending with PEG significantly decreased the T_g, decreased the modulus and increased the fracture strain of PLA. However, the PLA/PEG 70/30 blend became increasingly rigid over time at ambient conditions. The mechanism of aging primarily under ambient conditions of temperature and humidity was studied. Changes in mechanical properties, thermal transitions and solid state morphology were examined over time. Aging was caused by slow crystallization of PEG. Crystallization of PEG depleted the amorphous phase of PEG and gradually increased the T_g. As T_g approached the aging temperature, reduced molecular diffusivity slowed the crystallization rate dramatically. Aging essentially ceased when T_g of the amorphous phase reached the aging temperature. The increase in matrix T_g and the reinforcing effect of the crystals produced a change in mechanical properties from elastomer-like to thermoplastic-like.     

Crystallization and melting behaviors of poly(trimethylene terephthalate)

The crystallization and melting behaviors of poly(trimethylene terephthalate) (PTT) have been studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and solid-state NMR. At certain crystallization temperatures (T_c) for a given time, the isothermally crystallized PTT exhibits two melting endotherms, which is similar to that of PET and PBT. At higher crystallization temperature (T_c = 210 °C), the low-temperature endotherm is related to the melting of the original crystals, while the high-temperature endotherm is associated with the melting of crystals recrystallized during the heating. The peak temperatures of these double-melting endotherms depend on crystallization temperature, crystallization time, and cooling rate from the melt as well as the subsequent heating rate. At a low cooling rate (0.2 °C/min) or a high heating rate (40 °C/min), these two endotherms tend to coalesce into a single endotherm, which is considered as complete melting without reorganization. WAXD results confirm that only one crystal structure exists in the PTT sample regardless of the crystallization conditions even with the appearance of double melting endotherms. The results of NMR reveal that the annealing treatment increases proton spin lattice relaxation time in the rotation frame, T_1ρ ^H, of the PTT. This phenomenon suggests that the mobility of the PTT molecules decreases after the annealing process. The equilibrium melting temperature (T _m ^o ) determined by the Hoffman-Weeks plot is 248 °C.     

Oxygen solubility and specific volume of rigid amorphous fraction in semicrystalline poly(ethylene terephthalate)

The existence of rigid amorphous fraction (RAF) in semicrystalline poly(ethylene terephthalate) (PET) is associated with the lamellar stack crystalline morphology of this polymer, the regions where several crystalline lamellas are separated by very thin (20-40 Å) amorphous layers. In contrast, regular or mobile amorphous fraction is associated with much thicker interstack regions. The oxygen transport properties of PET isothermally crystallized from the melt (melt-crystallization) or quenched to the glassy state and then isothermally crystallized by heating above T_g (cold-crystallization) were examined at 25 °C. Explanation of unexpectedly high solubility of crystalline PET was attributed to the formation of RAF, which in comparison with mobile amorphous phase is constrained and vitrifies at much higher than T_g temperature thus developing an additional excess-hole free volume upon cooling. Measurements of crystallinity and jump in the heat capacity at T_g were used to determine the amount of mobile and rigid amorphous fractions. Overall oxygen solubility was associated with the solubility of mobile and rigid amorphous fractions. The oxygen solubility of the RAF was determined and related to the specific volume of this fraction. The specific volume of the RAF showed a direct correlation with the crystallization temperature. It was shown that upon crystallization from either melt or glassy state, the constrained between crystalline lamellas PET chains consisting of the RAF, vitrify at the crystallization temperature and resemble the glassy behavior despite high temperature. When cooled to room temperature, the RAF preserves a memory about the melt state of polymer, which is uniquely defined by the crystallization temperature.     

Sulfur‐containing polymers: Effect of composition on melting behavior and crystallization kinetics of poly(butylene terephthalate)

The melting behavior and crystallization kinetics of poly(butylene terephthalate/thiodipropionate) (PBT) copolymers were investigated using the differential scanning calorimetry technique. Multiple endotherms typical of PBT were observed in the copolymers under investigation and were found to be influenced both by crystallization temperature (T_c) and composition. Wide‐angle X‐ray diffraction measurements permitted the identification of the crystalline structure of PBT in all the copolymers investigated. By applying the Hoffman–Weeks method, the equilibrium melting temperature of the copolymers was derived. Isothermal crystallization kinetics were analyzed according to Avrami's treatment. Values of the exponent n close to 3 were obtained, independent of T_c and composition, results in agreement with it being a crystallization process originating from predetermined nuclei and characterized by three‐dimensional spherulitic growth. The introduction of butylene thiodipropionate units was found to decrease the PBT crystallization rate. The heat of fusion (ΔH_m) was correlated to the specific heat increment (Δc_p) of samples of different degrees of crystallinity, and the results were interpreted based on there being an interphase, whose amount was found to increase as the sulfur‐containing unit content was increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2003–2009, 2003     

Crystallization behaviour and properties of BaO-CaO-B2O3-SiO2 glasses and glass-ceramics for LTCC applications

The influence of BaO content (up to 15?mol%) on the crystallization behaviour, structure, thermal properties and microwave dielectric properties of the BaO-CaO-B₂O₃-SiO₂ glasses and glass-ceramics system was investigated. The glasses were produced by melting at 1400?°C and quenching into water, and the glass-ceramics were produced via heat treatment at temperatures between 750 and 800?°C. The results of X-ray diffraction analysis showed that increasing the BaO content raised the resistance of the glass against crystallization and favoured the transformation of β-CaSiO₃ and α-CaSiO₃ phases, which crystallized in the Ba-free and in low BaO content compositions, into SiO₂ and Ba₄Si₆O₁₆, which crystallized in compositions with higher concentrations of BaO. The BaO content had little influence on the glass transition temperature (T_g) and the linear coefficient of thermal expansion (CTE), but strongly reduced the softening point (T_s). Even the addition of BaO as minor additives resulted in a dramatic reduction of the T_s; for example, the T_s decreased from 902?°C for the Ba-free composition to 682?°C for the BaO-containing one (5%). Low values of the dielectric constant (5.9?≤?ε_r ≤?6.63) and dielectric loss (1.12?×?10^?3 ≤?tanδ?≤?3.15?×?10^?3) were measured.     

Morphology–property relationship in oriented PET films: Microstructural reorganization during heat treatment

Morphology–property relationships for simultaneously biaxially stretched films and heatset with fixed dimensions in the temperature range of 100–240°C have been studied. The observed transition in various properties at 180°C can be explained on the basis of microstructural changes caused by competition among several processes, such as crystallization, solid-state thickening, melting, and molecular relaxation as well as by melting and recrystallization. The resulting structures and, thereby, the properties are different in temperature Regime-II (T_g to T_max) and Regime-III (T_max to T_m). In Regime-II, the high rate of crystallization compared to the rate of molecular relaxation develops a constrained amorphous phase, whereas the predominant melting and recrystallization process in Regime-III generates the relaxed amorphous phase. The structural reorganization during heat treatment is almost the same for uniaxially oriented film, fibers, and biaxially oriented films prepared under similar processing conditions. © 1994 John Wiley & Sons, Inc.     

Effect of crystallization temperature on crystal modifications and crystallization kinetics of poly(L‐lactide)

The crystalline structure of poly(L ‐lactide) (PLLA) have been found to quite depend on the crystallization temperatures (T_cs), especially in the range of 100?120°C, which is usually used as the crystallization temperature for the industrial process of PLLA. The analysis of wide‐angle X‐ray diffraction and Fourier transformed infrared spectroscopy revealed that 110°C is a critical temperature for PLLA crystallization. At T_c < 110°C and T_c ≥ 110°C, the α′ and α crystals were mainly produced, respectively. Besides, the structural feature of the α′‐form was illustrated, and it was found that the α′‐form has the larger unit cell dimension than that of the α‐form. Moreover, the crystallization kinetics of the α′ and α crystals are different, resulting in the discontinuousness of the curves of spherulite radius growth rate (G) versus T_c and the half time in the melt‐crystallization (t_1/2) versus T_c investigated by Polarized optical microscope and Differential scanning calorimetry, respectively. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation of transparent Bi2O3-ZnO-B2O3 glass by conventional sintering at low temperatures

Glass components fabricated by the sintering route have wide-ranging applications. However, one issue is that the crystallization tendency of glass powders often leads to residual pore-glass interfaces and crystal-glass interfaces, thereby causing strong light scattering and rendering the sintered glass opaque. This issue is particularly pronounced in glasses with a low glass transition temperature (T_g) due to their weak bonding and thus high crystallization tendency. In the present study, a Bi₂O₃-ZnO-B₂O₃ glass with a low T_g of 364°C was fabricated using the conventional sintering method to explore whether transparent glass materials can be obtained. The temperature range of crystallization of the glass powders was analyzed using differential scanning calorimetry. X-ray diffraction was employed to analyze the crystalline phases formed in the sintered glasses. The microstructure of the sintered glasses was examined using scanning electron microscopy. The optical transmittance of the sintered glasses was measured using ultraviolet-visible spectroscopy. The results show that transparent sintered glasses with the highest transmittance of 54% at the wavelength of 650 nm can be obtained by using a coarser initial particle size, lower forming pressure, and an appropriate sintering temperature/time (430°C/30 min). It is suggested that this combination of processing parameters can suppress glass crystallization while maintaining a low glass viscosity during sintering.     

Influence of heat treatment temperature on the properties of the lithium disilicate-fluorcanasite glass-ceramics

This study aims to investigate the influence of heat treatment temperatures on the mechanical properties and chemical solubility (CS) of lithium disilicate-fluorcanasite glass-ceramics and to develop new dental materials. The glasses and glass-ceramics were prepared using CaF₂-SiO₂-CaO-K₂O-Na₂O-Li₂O-Al₂O₃-P₂O₅-based glass system using a conventional melt quenching method followed by a two-stage crystallization process. This two-stage method involves two heating temperature steps: first at a constant temperature (T_S1) of 600°C and second step at varying temperatures (T_S2) of 650, 700, 750, and 800°C. The crystallization behavior, phase formation, microstructure, translucency characteristic, density, hardness, fracture strength, and CS were investigated. It was found that the lithium disilicate crystal acted as the main crystalline phase, and the crystalline phase of fluorcanasite occurred at the heat treatment temperatures of 750 and 800°C. In addition, it was found that density, hardness, fracture strength, and CS increased while the translucency values decreased with increasing heat treatment temperatures. Furthermore, the CS increased dramatically when the fluorcanasite phases occurred in the glass-ceramic samples. The maximum density values, Vickers hardness, fracture toughness, and flexural strength are 2.56 g/cm³, 6.73 GPa, 3.38 MPa.m^1/2, and 259 MPa, respectively. These results may offer a possibility to design a new material for dental applications based on lithium disilicate-fluorcanasite glass-ceramics.     

Melting behavior of poly(l-lactic acid): Effects of crystallization temperature and time

Effects of crystallization temperature and time on the melting behavior of poly(l-lactic acid) were studied with differential scanning calorimetry (DSC). The isothermal crystallization was performed at various temperatures (T_cs), and DSC melting curves for the isothermally crystallized samples were obtained at 10 K min⁻¹. When T_c was lower than T_d (∼135 °C), the double melting peaks appeared. The melting behavior, especially T_c dependence of the melting temperature (T_m), discretely changed at T_b (=113 °C), in accordance with the discrete change of the crystallization behavior at T_b, which was previously reported. When T_c was higher than T_d, a single melting peak appeared. In addition, T_c dependence of dT_m/dT_c discretely changed at T_d. That is, the melting behavior, especially T_c dependence of T_m and dT_m/dT_c, are different in three temperature regions of T_c divided by T_b and T_d: Regions I (T_c ≤ T_b), II (T_b ≤ T_c ≤ T_d), and III (T_d ≤ T_c). The effect of crystallization time on the melting behavior, melting temperature and heat of fusion in each temperature region of T_c is also discussed.