Isothermal Cold Crystallization Research Articles

A comparison of crystallization behavior for melt and cold crystallized poly ( l-Lactide) using rapid scanning rate calorimetry

A unique rapid scanning rate differential scanning calorimeter is used to examine the differences in melt and cold crystallized poly ( l-Lactide) (PLLA), a biodegradable semi-crystalline polymer. After isothermal melt and cold crystallization at various temperatures, both melt and cold crystallized PLLA are characterized by similar melting temperatures ( T m) and exhibit multiple melting behavior on heating at 500 °C/min. However, cold crystallization results in a higher degree of crystallinity ( w c) compared to melt crystallization. While the overall amorphous fraction is higher for melt crystallization, the mobile amorphous fraction ( w a) is found to be higher for cold crystallization. The rigid amorphous fraction ( w raf) in PLLA is determined to be higher for melt crystallization than for cold crystallization at almost all temperatures. The higher values of w raf also appear to result in higher values of the glass transition temperature ( T g) for melt crystallized samples due to a reduction in mobility of amorphous phase. These dramatic differences depending on whether the material is brought to the crystallization temperature from the melt or the glassy state, could have profound implications for processing and optimizing the properties of PLLA.

Cold Crystallization of Poly(trimethylene terephthalate) As Revealed by Simultaneous WAXS, SAXS, and Dielectric Spectroscopy

This work presents an experimental study on the interplay between structure and dynamics during isothermal and nonisothermal cold crystallization of poly(trimethylene terephthalate). Simultaneous X-ray scattering and dielectric spectroscopy measurements revealed that, during cold crystallization, crystalline lamellae tend to be homogeneously distributed along the sample. A significant amount of rigid amorphous phase is formed during cold crystallization, and its location is assigned to the crystal−amorphous interface. The formation of the rigid amorphous phase during cold crystallization is less effective for poly(trimethylene terephthalate) than for other similar aromatic polyesters like poly(ethylene terephthalate). The mobile amorphous phase, giving rise to the dielectric α relaxation, is mainly located between consecutive crystalline lamellae. The segmental dynamics of the amorphous phase is restricted by the growing crystals during both primary and secondary crystallization.

Molecular Dynamics of Ethylene Glycol Dimethacrylate Glass Former: Influence of Different Crystallization Pathways

The crystallization induced by different thermal treatments of a low molecular weight glass former, ethylene glycol dimethacrylate (EGDMA), was investigated by dielectric relaxation spectroscopy (DRS) and differential scanning calorimetry (DSC). The fully amorphous material, dielectrically characterized for the first time, exhibits three relaxation processes: the alpha-relaxation related to dynamic glass transition whose relaxation rate obeys a Vogel-Fulcher-Tamman-Hesse (VFTH) law and two secondary processes (beta and gamma) with Arrhenius temperature dependence. Therefore, the evaluation of distinct crystallization pathways driven by different thermal histories was accomplished by monitoring the mobility changes in the multiple dielectric relaxation processes. Besides isothermal cold-crystallization, nonisothermal crystallizations coming from both the melt and the glassy states were induced. While an amorphous fraction, characterized by a glass transition, remains subsequent to crystallization from the melt, no alpha-relaxation is detected after the material undergoes nonisothermal cold-crystallization. In the latter, the secondary relaxations persist with a new process that evolves at low frequencies, designated as alpha' that was also detected at advanced crystallization states under isothermal cold-crystallization. Under the depletion of the alpha-relaxation, the beta-process when detected becomes better resolved keeping the same location prior to crystallization leading to a decoupled temperature dependence relative to the alpha-process.

Real-Time Monitoring of Molecular Dynamics of Ethylene Glycol Dimethacrylate Glass Former

The isothermal cold-crystallization of the glass-former low-molecular-weight compound, ethylene glycol dimethacrylate (EGDMA), was monitored by real-time dielectric relaxation spectroscopy (DRS) and differential scanning calorimetry (DSC). The alpha-relaxation associated with the dynamic glass transition as detected by DRS was followed at different crystallization temperatures, T(cr), nearly above the glass transition temperature, 176 K (1.06 < or = T(cr)/T(g) < or = 1.12). It was found that the alpha-process depletes upon cold-crystallization with no significant changes in either shape or location. At advanced crystallization states, a new relaxation, alpha'-process, evolves that was assigned to the mobility of molecules lying adjacent to crystalline surfaces. From the time evolution of the normalized permittivity, it was possible to get kinetic information that was complemented with the calorimetric data. From DSC measurements that were also carried out under melt-crystallization, an enlarged temperature range was covered (up to T(cr)/T(g) = 1.24), allowing us to draw a diagram of time-temperature crystallization for this system. Dielectric relaxation spectroscopy proved to be a sensitive tool to probe the mobility in the remaining amorphous regions even at high crystallinities.

Isothermal cold crystallization and melting behaviors of poly(L‐lactic acid)s prepared by melt polycondensation

AbstractThe isothermal cold crystallization and melting behaviors of poly(L‐lactic acid)s (PLLAs, weight average molecular weight, Mw, 6000–80,000) prepared via melt polycondensation were studied with differential scanning calorimeter in this work. It is found that the crystallization rate increased with decreasing Mw, reached a maximum at Mw of ca. 21,000 and then decreased again. The crystallinity of PLLA can be controlled in the range 30–50% by crystallization temperature (Tc) and time to fulfill the requirement of subsequent solid state polycondensation. The melting behavior strongly depends on Tc. The samples crystallized at high Tc melted with a single peak but those crystallized at low Tc melted with double peaks. The higher melting point (TmH) kept almost constant and the lower melting point (TmL) increased clearly with Tc. But the TmL changed in jumps and a triple melting peak appeared at the vicinity of a characteristic crystallization temperature Tb, possibly because of a change of crystal structure. The equilibrium melting temperature of PLLA with Mw of 21,300 was extrapolated to be 222°C with nonlinear Hoffman‐Weeks method. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

The observation of rapid surface growth during the crystallization of polyhydroxybutyrate

Abstract A two stage growth during isothermal cold crystallization of PHB has been observed in the temperature range 5–40 °C when a free surface is present. This growth has been investigated with optical and atomic force microscopy both in-situ and ex-situ. Initially, crystal growth is observed to be composed of lamellae oriented approximately flat-on relative to the free surface. At later stages of growth there can be a change to a distinctly different form of crystal growth that is composed of edge-on lamellae which grow at a substantially higher crystallization rate. This change in growth rate at constant temperature gives rise to curved interfaces between the slower growing flat-on growth and the faster growing edge-on growth. Several possible explanations for this change in growth rate are put forward.

Degradation induced by nanostructural evolution of polylactide/clay nanocomposites in the isothermal cold crystallization process

AbstractBACKGROUND: The aim of the work presented was to study the nanostructural evolution of clay tactoids during the isothermal cold crystallization of polylactide/clay nanocomposites (PLACNs). An interesting degradation behavior of the polylactide (PLA) matrix, however, was observed simultaneously with nanostructural evolution. The possible mechanisms of degradation and structural evolution are discussed.RESULTS: The evolution of the nanostructure and the degradation were studied online or offline using parallel plate rheometry, X‐ray diffraction, transmission electron microscopy, differential scanning calorimetry, nuclear magnetic resonance spectroscopy, gel permeation chromatography and thermogravimetric analysis. The results showed that the intercalation level of the clay tactoids further increased during cold crystallization, leading to simultaneous degradation of the PLA matrix, which was further confirmed by the marked change of small‐ and large‐amplitude oscillatory shear flow behavior of the PLACNs before and after cold crystallization.CONCLUSION: The increase of the intercalation level of the tactoids is due to the diffuse in movement of the PLA chain segments during cold crystallization. In this case, chain‐end scission occurs more easily, especially for those PLA chains around the loose edge and defect site of the tactoids during crystallization, which results in a simultaneous degradation of the PLA matrix. Hence, both the linear and nonlinear dynamic viscoelastic properties show a remarkable change after cold crystallization. Copyright © 2009 Society of Chemical Industry

Comparison Between Isothermal Cold and Melt Crystallization of Polylactide/Clay Nanocomposites

The isothermal cold and melt crystallization behavior of intercalated polylactide (PLA)/clay nanocomposites (PLACNs) were studied using differential scanning calorimetry (DSC), polarized optical microscope (POM), X-ray diffractometer (XRD) and Fourier Transform Infra-Red Spectrometer (FT-IR). The results show that the degree of crystallinity of PLA matrix decreases monotonously with increasing clay loadings for both the cold and melt crystallization. The cold crystallized sample shows a double melting behavior and lower melting temperature compared to that of melt-crystallized sample, especially in the presence of clay. The crystallization kinetics was then analyzed by the Avrami and Lauritzen-Hoffman methods for further comparison between these two crystallization behaviors. The results reveal that PLA and its nanocomposites present higher activation energy in melt crystallization than that in cold crystallization due to the reptation of entire polymer chains. The addition of clay facilitates the overall kinetics of melt crystallization, which is attributed to both the nucleation effect of clay and enhanced diffusion of PLA chains. However, for cold crystallization, only very small amounts of clay can slightly increase the kinetics, while larger amounts impede the process. The presence of clay leads to a diffusion-controlled growth of nucleation of PLA matrix in the cold crystallization process and, the hindrance effect of clay hence becomes the dominant factor gradually with increasing clay loadings in the case of high-rate nucleation.

Comparison Between Isothermal Cold and Melt Crystallization of Polylactide/Clay Nanocomposites

The thermoluminescence (TL) of undoped and Dy3+ doped ZrO2 nanocrystals under beta-ray irradiation is reported. The TL glow curves are the result of the overlapping of four TL peaks produced partly by the intrinsic defect of highly asymmetrical monoclinic structure and partly due to defects produced during the synthesis process. The introduction of dopant ions induces changes in the glow curve due to the enhancement of high temperature peaks intensity. The results show that both undoped and doped ZrO2 nanocrystalline phosphor present good TL efficiency as well as good dose response which qualify them as a potential beta-ray dosimeter.

Effect of the Cooling Rate on the Nucleation Kinetics of Poly(l-Lactic Acid) and Its Influence on Morphology

Although the crystallization kinetics of PLLA is slow enough to allow one to obtain amorphous samples by cooling from the melt at moderate rates, surprisingly, at the same time, the formation of crystallization nuclei is difficult to avoid. Their amount depends on the cooling rate from the melt: the higher the cooling rate, the lower the amount of available nuclei after reaching the glass state. Cooling at controlled rates of 5−300 °C/min was performed by making use of a relatively new high-speed calorimetry technology, high-performance DSC (HPer DSC). Subsequent isothermal cold crystallization of amorphous PLLA, at different temperatures distributed across the bell-shaped crystal growth curve, was shown to depend on the rate at which the glass state was attained, reflecting the number of nuclei formed. However, (only) after complete cold crystallization, the DSC heating curves and the resulting morphology measured by optical microscopy and AFM were shown to be independent of the previous cooling rate into the glass, i.e., of the number of nuclei formed during the cooling process. In cases where the DSC heating curves subsequent to isothermal cold crystallization show two melting peaks, their origin, recrystallization, was clarified by heating at rates varying from 10 to 300 °C/min. The morphologies of the cold crystallized systems have been assessed by AFM and were correlated with the calorimetric results. Using HPer DSC, isothermal cold and hot crystallization at the same temperature has been studied successfully.

Molecular dynamics of poly(glycolide‐co‐L‐lactide) copolymer during isothermal cold crystallization

AbstractPoly(glycolide‐co‐l‐lactide) (PGA/PLLA) is a random copolymer with 92 wt % PGA, being the basic resin for Vicryl® suture. Molecular dynamics of PGA/PLLA in its wholly amorphous state and during isothermal cold crystallization at 70 and 80°C have been analyzed. Experimental results were generated over a wide range of frequency and temperature by broad‐band dielectric spectroscopy (DRS). The variation of the average relaxation time (defined as τ = [1/2]πfmax where fmax is the frequency at maximum loss for the main α relaxation) has been studied during cold crystallization and the temperature dependence of this average relaxation time for wholly amorphous and crystallized samples has been analyzed. This behavior has been modeled by Havriliak–Negami, Vogel–Fulcher–Tammann, and Kohlrausch–Williams–Watts equations. The evolution profile of the dynamics (frequency at which the maximum loss appears, fmax) depends on the crystallization temperature, being different at 80°C relatively to 70°C, which could reflect different progress of the spherulitic morphology, as it is shown by the evolution of the morphologies obtained during the crystallization processes, followed by optical microscopy. While the loss maximum (and consequently relaxation times) remains almost unmodified during the crystallization process at 70°C, for the process at 80°C the maximum first moves slightly to higher frequencies (shortening of relaxation times) and at the final stages of crystallization it moves to lower frequencies (increasing the relaxation times). Supporting evidence about the thermal behavior of the polymers has been obtained with DSC. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Isothermal cold crystallization kinetics of polylactide/nucleating agents

AbstractThe isothermal cold crystallization kinetics of polylactide (PLA)/nucleating agents (CaCO3, TiO2, and BaSO4, content from 0.5–2.0 wt %) was investigated by differential scanning calorimetry in the temperature range of 120–124°C. With blending nucleating agents, the crystallinity of PLA had a maximum crystallinity of 14.9%. Crystallization rate decreased with increasing crystallization temperature in the researched content range. The crystallization rate followed the Avrami equation with the exponent n around 4.5. From Lauritzen–Hoffman equation, the nucleation parameter Kg was estimated. And from the value of Kg, regime II crystallization behavior can be concluded. Then the lateral and fold surface free energy were calculated from Kg. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 310–317, 2007

Molecular dynamics of poly(glycolide) and poly(glycolide-co-L-lactide) during isothermal cold crystallization

An investigation was carried out on the molecular dynamics of poly(glycolide) and poly(glycolide-co-L-lactide) copolymer in their wholly amorphous state and during cold crystallization at 70 and 80°C. Experimental results were generated over a wide range of temperature and frequency by broad-band dielectric relaxation spectroscopy. The variation of the average relaxation time (related to the frequency at maximum loss for the main α-relaxation) has been studied during cold crystallization. This variation has been found to be dependant on the crystallization temperature and type of polymer, suggesting different progress of the spherulitic morphology in each case. The temperature dependence of the average relaxation time for wholly amorphous samples has been analyzed. This behavior has been modelled by Havriliak–Negami and Vogel–Fulcher equations obtaining the most important parameters. Supporting evidence about the thermal behavior of the polymers has been obtained with differential scanning calorimetry. Complementarily, the evolution of the morphologies obtained during crystallization processes has been followed by optical microscopy.

Molecular Dynamics of PGA Bioabsorbable Polymer During Isothermal Cold Crystallization

AbstractAn investigation was carried out on the molecular dynamics of poly(glycolide) (PGA) in its completely amorphous state and during isothermal cold crystallization. Experimental results were generated over a wide range of frequency and temperature by broad‐band dielectric spectroscopy (DRS). The variation of the average relaxation time (defined as τ= ½πfmax where fmax is the frequency at maximum loss for the main α relaxation) has been studied during cold crystallization and the temperature dependence of this average relaxation time for completely amorphous and crystallized samples has been analyzed. This behaviour has been modelled by Havriliak‐Negami and Vogel‐Fulcher equations. The sensitiveness of the segmental dynamics to the degree of crystallinity has been analyzed, taking into account the relaxing segments and the amorphous layers between lamellae. Supporting evidence about the thermal behaviour of the polymers has been obtained with DSC. Complementarily, the evolution of the morphologies obtained during crystallization processes has been followed by optical microscopy.

Confined crystallization in phase-separated poly(ethylene terephthalate)/poly(ethylene naphthalene 2,6-dicarboxilate) blends

The isothermal cold crystallization of poly(ethylene terephthalate)(PET) in cryogenic mechanical alloyed blends of PET and Poly(ethylene naphthalene 2,6-dicarboxilate)(PEN) 1:1 by weight has been investigated by simultaneous small and wide angle X-ray scattering (SAXS and WAXS) and dielectric spectroscopy (DS). For transesterification levels higher than 23% the blends tend to transform into a one-phase system and the crystallization of PET is strongly inhibited due to the significant reduction of the PET segment length. For lower levels of transesterification the blends are phase separated and the overall crystallization behaviour can be explained considering the confined nature of the PET domains in these blends. The formation of a rigid amorphous phase in the intra-lamellar stack amorphous regions is reduced in the blends due to a lower probability of stack formation in the confined PET-rich domains. The more effective filling of the space by the lamellar crystals in the blends provokes a stronger restriction to the amorphous phase mobility of PET in the blends than in pure PET.

The kinetics of isothermal cold crystallization and tensile properties of poly(ethylene terephthalate)

The exothermic peak that is frequently observed during the heating scan of a differential scanning calorimetry (DSC) experiment of poly(ethylene terephthalate) (PET) is due to a cold crystallization process, originating from the rearrangement of amorphous regions into a crystalline phase. In this work the isothermal cold crystallization kinetics of PET was investigated by using DSC, X-ray diffraction and tensile experiments. The isothermal crystallization rate was determined as a function of temperature, and the Avrami analysis was conducted. The results showed that at low temperatures the cold crystallization is a two-regime process, whereas at high temperatures just one stage is observed. The rate constant for isothermal crystallization K increased and the half time of crystallization (t½) decreased with increasing crystallization temperature. The Avrami exponent n was close to 2, and this corresponds to a disc-like morphology formed by heterogeneous nucleation. Cold crystallization increased the crystallinity and therefore the tensile properties of the samples were enhanced.

Structural Changes and Crystallization Dynamics of Poly(l-lactide) during the Cold-Crystallization Process Investigated by Infrared and Two-Dimensional Infrared Correlation Spectroscopy

The structural evolution and crystallization dynamics of poly(l-lactide) (PLLA) during isothermal cold-crystallization process are studied by infrared (IR) spectroscopy and two-dimensional correlation analysis. In the CO stretching region, the band shift to a higher wavenumber taking place during the crystallization of PLLA is attributed to the dipole−dipole interaction between the CO groups. A detailed analysis is performed for the range of 1500−1000 cm-1 where bands are highly overlapped. It was found that the 1458 cm-1 band reflects the structural order of the CH3 group, and the band at 1109 cm-1 is related to the C−O−C trans conformation in the crystalline phase of PLLA. The band at 1193 cm-1 is sensitive not only to the structural adjustment of the C−O−C backbone but also to the structural order of CH3 group in crystalline phase. From the analysis of the difference spectra and 2D correlation spectra in the 1500−1000 cm-1 region, it is shown that the structural adjustment of the CH3 group unambiguousl...

Structure-dynamics relationship in crystallizing poly(ethylene terephthalate) as revealed by time-resolved X-ray and dielectric methods

The isothermal cold crystallization of poly(ethylene terephthalate) was investigated by simultaneous small and wide angle X-ray scattering (SAXS and WAXS) and dielectric spectroscopy (DS). By this experimental approach (SWD), simultaneously collected information was obtained about the specific changes occurring in both crystalline and amorphous phases during crystallization. The main features which are directly derived from our experiments can be explained assuming the formation of a heterogeneous multiple lamellar population arrangement. The rigid amorphous phase can be associated with the intra-lamellar stack amorphous phase. The restriction of the amorphous phase mobility mainly occurs in the inter-lamellar stacks regions probably due to the formation of secondary lamellae.

Molecular relaxation of isotactic polystyrene: Real‐time dielectric spectroscopy and X‐ray scattering studies*

AbstractThe molecular relaxation processes and structure of isotactic polystyrene (iPS) films were investigated with real‐time dielectric spectroscopy and simultaneous wide‐ and small‐angle X‐ray scattering. The purpose of this work was to explore the restrictions imposed on molecular mobility in the vicinity of the α relaxation (glass transition) for crystallized iPS. Isothermal cold crystallization at temperatures of Tc = 140 or 170 °C resulted in a sigmoidal increase of crystallinity with crystallization time. The glass‐transition temperature (Tg), determined calorimetrically, exhibited almost no increase during the first stage of crystal growth before impingement of spherulites. After impingement, the calorimetric Tg increased, suggesting that confinement effects occur in the latter stages of crystallization. For well‐crystallized samples, the radius of the cooperativity region decreased substantially as compared with the purely amorphous sample but was always smaller than the layer thickness of the mobile amorphous fraction. Dielectric experiments directly probed changes in the amorphous dipole mobility. The real‐time dielectric data were fitted to a Havriliak–Negami model, and the time dependence of the parameters describing the distribution of relaxation times and dielectric strength was obtained. The central dipolar relaxation time showed little variation before spherulite impingement but increased sharply during the second stage of crystal growth as confinement occurred. Vogel–Fulcher–Tammann analysis demonstrated that the dielectric reference temperature, corresponding to the onset of calorimetric Tg, did not vary for well‐crystallized samples. This observation agreed with a model in which constraints affect primarily the modes having longer relaxation times and thus broaden the glass‐transition relaxation process on the higher temperature side. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 777–789, 2004

Crystallization and melting behavior of poly(ether ether ketone)/poly(aryl ether sulfone) blends

AbstractThe crystallization and melting behavior of poly(ether ether ketone) (PEEK) in blends with poly(aryl ether sulfone) (PES) prepared by melt mixing are investigated by differential scanning calorimetry (DSC) and wide‐angle X‐ray scattering (WAXS). The presence of PES is found to have a notable influence on the crystallization behavior of PEEK, especially when present in low concentrations in the PEEK/PES blends. The PEEK crystallization kinetics is retarded in the presence of PES from the melt and in the rubbery state. An analysis of the melt crystallization exotherm shows a slower rate of nucleation and a wider crystallite size distribution of PEEK in the presence of PES, except at low concentrations of PES, where, because of higher miscibility and the tendency of PES to form ordered structures under suitable conditions, a significantly opposite result is observed. The cold crystallization temperature of the blends at low PES concentration is higher then that of pure PEEK, whereas at a higher PES concentration little change is observed. In addition, the decrease in heat of cold crystallization and melting, which is more prevalent in PEEK‐rich compositions than in pure PEEK, shows the reduction in the degree of crystallinity because of the dilution effect of PES. Isothermal cold crystallization studies show that the cold crystallization from the amorphous glass occurs in two stages, corresponding to the mobilization of the PEEK‐rich and PES‐rich phases. The slower rate of crystallization of the PEEK‐rich phase, even in compositions where a pure PEEK phase is observed, indicates that the presence of the immobile PES‐rich phase has a constraining influence on the crystallization of the PEEK‐rich phase, possibly because of the distribution of individual PEEK chains across the two phases. The various crystallization parameters obtained from WAXS analysis show that the basic crystal structure of PEEK remains unaffected in the blend. Further, the slight melting point depression of PEEK at low concentrations of PES, apart from several other morphological reasons, may be due to some specific interactions between the component homopolymers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2906–2918, 2003