Isothermal Crystallization Process Research Articles

Isothermal crystallization of gamma irradiated LDPE in the presence of oxygen

Abstract This work is focused on the study of the effect of oxygen on the isothermal crystallization process of gamma irradiated low density polyethylene (LDPE). The induction time increased with the dose indicating a retarding effect. On other hand, at the same dose, this parameter decreased with the augment in the oxygen content. The classical Avrami equation was used to analyze the crystallization kinetic of these materials. n values suggested that both, the dose and the oxygen content, did not affect the mechanism of crystals growth. An Arrhenius type equation was used for the rate constant (k). Used models correctly reproduced the experimental data. TTT diagrams of studied materials were constructed and also reflected the effects of the doses and the oxygen content.

Isothermal programming of triple shape memory

Here we present a new strategy for enabling triple shape memory (TSM), in which different shapes may be programmed at one constant fixation temperature during isothermal crystallization of a poly(octylene adipate) elastomer. Unlike traditional TSM programming techniques that utilize separate thermal transitions to encode each distinct shape, here we report that multiple shapes may be encoded independently at different time stages of the same isothermal crystallization process, and then recovered sequentially at different temperatures upon heating. As such, the new method extends TSM capabilities to conventional semicrystalline elastomers that exhibit a single and narrow thermal transition. Unexpectedly, shape fixation does not depend on crystallinity acquired prior to sample deformation, which suggests that different shapes are secured by an individual crystalline scaffold with minor contribution of the pre-existing crystals. To that end, the isothermal protocol for TSM programming was utilized to demonstrate a “one-way reversible” shape memory transformation controlled by fixation time.

Polymorphic Behavior and Enzymatic Degradation of Poly(butylene adipate) in the Presence of Hexagonal Boron Nitride Nanosheets

Hexagonal boron nitride nanosheets (BNNSs) were prepared by a facile chemical exfoliation method. Then cosolution film casting method was used to obtain poly(butylene adipate) (PBA)/BNNSs nanocomposites. The incorporated BNNSs changed the formation condition of the polymorphic crystals of PBA. The results of differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) showed that BNNSs significantly facilitated the formation of α-form crystals for isothermal and nonisothermal crystallization. Compared to pure PBA at 28 °C, the α-form crystals would not disappear until the temperature decreased to 13 °C with the addition of 0.5 wt % BNNSs. In addition, the excellent heterogeneous nucleation ability of BNNSs was identified by the nonisothermal and isothermal crystallization process. The enzymatic degradation experiments exhibited that PBA/BNNSs nanocomposites possessed a considerably lower degradation rate than neat PBA. This work demonstrates that BNNSs is an efficient material to regulate the formation of polymorphic crystals and the degradation behavior of PBA.

Isothermal crystallization process of poly(4-methyl-1-pentene)/alkylated graphene oxide nanocomposites: thermal analysis and rheology study

In this work, the nanofiller GO–ODA can accelerate crystallization and improves the appearance temperature of the Type II lamellae. In rheological experiments, crystallization was no longer affected by the frequency.

Effect of biodegradable poly(ethylene adipate) with low molecular weight as an efficient plasticizer on the significantly enhanced crystallization rate and mechanical properties of poly(l-lactide)

The slow crystallization rate and low elongation at break are the main drawbacks of biodegradable poly(L-lactide) (PLLA) from a viewpoint of practical application. Such disadvantages of PLLA have been successfully resolved by blending with low molecular weight biodegradable poly(ethylene adipate) (PEA) in the present work. As a biodegradable plasticizer, PEA not only formed fully miscible and biodegradable polymer blends with PLLA but also apparently reduced the glass transition temperature of the blends. The increased chain mobility favored the nonisothermal cold and melt crystallization behaviors, increased the spherulitic growth rate, and accelerated the overall isothermal melt crystallization process of the blends; however, the blends had the same crystallization mechanism and crystal structures as neat PLLA. One of the exciting results was that the elongation at break value of an 80/20 PLLA/PEA blend was dramatically increased; therefore, for a wider practical application, the preparation of PLLA-based materials with fast crystallization rate, good mechanical properties, and complete biodegradability could be accomplished by the blending with a small amount of PEA with low molecular weight as an efficient plasticizer.

Crystallization behaviour of polymeric membrane based on polymer PVdF-HFP and Ionic liquid BMIMBF4

The crystallization behaviour of the polymer poly(vinylidenefluoride)–hexafluoropropylene (PVdF–HFP) in the presence and absence of the ionic liquid (IL) (1-butyl-3-methylimidazolium tetrafluoroborate; [BMIMBF4]) were studied by isothermal and non-isothermal crystallization processes using differential scanning calorimetry. The well-known Avrami equation is used to describe the isothermal crystallization process of pristine PVdF–HFP or PVdF–HFP + x wt% of IL BMIMBF4, where x = 10 and 30, respectively. It was found that the presence of the IL BMIMBF4 in the PVdF–HFP matrix suppresses the crystallization of the polymer PVdF–HFP, resulting in low crystal growth rates. Three kinetic methods (i.e., those of Jeziorny, Ozawa and Mo) were used to analyze the non-isothermal crystallization process. The Avrami equation modified by Jeziorny could only describe the initial stage of crystallization and the Ozawa method failed to describe the non-isothermal crystallization behavior, but Mo's method explains the results better.

Molecular Weight Dependency of Crystallization Line, Recrystallization Line, and Melting Line of Polybutene-1

Isothermal crystallization and subsequent melting processes in semicrystalline polymers can be described by several linear dependencies termed as crystallization line, recrystallization line, and melting line in a phase diagram of inversed crystalline lamellar thickness as a function of temperature. The crystallization line denotes the lamellar thickness dependency on crystallization temperature. Upon subsequent heating, the system either undergoes a process involving a continuous change in lamellar thickness following a recrystallization line or a direct melting at a constant thickness defining a melting line. The limiting temperatures of infinite lamellar thickness for crystallization line and recrystallization line coincide with each other and are higher than that of melting line. The above-mentioned experimental results are interpreted as a consequence of involvement of a mesomorphic phase during crystallization. In this work, the effect of molecular weight on the crystallization line, recrystallizati...

Crystallization kinetics of Zr60Cu25Fe5Al10 bulk metallic glass

Crystallization transformation kinetics of the Zr60Cu25Fe5Al10 amorphous alloy under non-isothermal (continuous heating) and isothermal conditions were investigated by differential scanning calorimetry (DSC). Under non-isothermal condition, the activation energies for the glass transition and crystallization at the onset and the peak crystallization temperatures are determined by means of Kissinger plots to be 383±11, 290±7 and 325±13kJ/mol, respectively. During the isothermal crystallization process, the average activation energy for crystallization deduced by using the Arrhenius equation is about 385±7kJ/mol, which is larger than that obtained by Kissinger equation. The crystallization mechanism under isothermal condition was investigated by using Johnson–Mehl–Avrami (JMA) equation. The Avrami exponent is mainly in the range of 2.64 to 3.07, which indicates that the crystallization is mainly governed by diffusion-controlled three-dimensional growth with increasing nucleation rate.

Carbon nanotubes induced poly(vinylidene fluoride) crystallization from a miscible poly(vinylidene fluoride)/poly(methyl methacrylate) blend

Different contents of carbon nanotubes (CNTs) were introduced into a miscible poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend. The interfacial affinity between CNTs and components of the blend was evaluated by calculating the interfacial tension. The dispersion and microstructure of CNTs in the nanocomposites were investigated through scanning electron microscope and rheological measurement. The effect of CNTs on the crystallization of PVDF was comparatively investigated through nonisothermal and isothermal crystallization processes. The results showed that CNTs exhibited stronger interfacial affinity to PMMA. Homogeneous dispersion of CNTs in the nanocomposites was achieved. Largely enhanced crystallization temperature and increased crystallinity of PVDF were obtained by adding CNTs during the nonisothermal crystallization process. The results obtained from the isothermal crystallization process proved that CNTs induced the concentration fluctuation in the sample, which resulted in the formation of spherulites with different types, i.e., the banded spherulites and compact spherulites. Furthermore, both the crystallization temperature and the content of CNTs exhibited great influence on the crystalline morphology of PVDF.

Shear effects on crystallization behaviors and structure transitions of isotactic poly-1-butene

Different melt pre-shear conditions were applied to isotactic poly-1-butene (iP-1-B) and the effect on the crystallization behaviors and the crystalline structure transitions of iP-1-B were investigated. The polarized optical microscope observations during isothermal crystallization process revealed that the applied melt pre-shear within the experimental range could enhance the nucleation of crystal II and accelerate the diameter growth of the formed spherulites. If the applied melt pre-shear rate was large enough, Shish-Kebabs structure could be formed. After the isothermal crystallization process, the following crystal transition from form II to form I of iP-1-B at room temperature was characterized by X-ray diffraction. It was found that the applied melt pre-shear within the experimental range seemed to have no influence on the crystal transition, implying no strong enough internal stress was formed in the melt pre-sheared iP-1-B samples. Further investigations were applied with synchrotron radiation instruments. Wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS) after the crystal transition showed that the applied melt pre-shear could result in orientated fine crystalline structures. With the melt pre-shear rate increasing, the lattice spaces of the crystallites decreased and the long period, L, and the amorphous layer thickness, La, along the equator direction increased slightly, but L and La along the meridian direction was not affected by melt pre-shear flow. Though the orientated crystalline structures existed in the iP-1-B samples, no accelerating effect on crystal transition from II to I was found. Importantly, the final crystalline structures of iP-1-B in form I was found tunable under different melt pre-shear conditions, even though there was an additional crystal transition process after the isothermal crystallization process of iP-1-B.

Study of Phase Transition by FTIR/SAXS/WAXD Simultaneous Measurement System

FTIR/SAXS/WAXD simultaneous measurement system has been developed for the study of the structural phase transition behaviors of soft materials including crystalline polymers. The system consists of a transmission-type miniature FTIR spectrometer set around the sample stage and the 2-dimensional detectors for WAXD and SAXS measurements in a synchronized mode, as shown in Figure. The infrared and X-ray beams can pass through the same position of a sample, which is set on the various types of equipments such as a stretcher, a heater, a temperature-jump cell for isothermal crystallization, etc. The system has been successfully applied to the studies of the structural evolution processes of crystalline polymers. For example, an intimate relationship has been clarified between the structural change in the crystal lattice and the change in lamellar stacking mode during the stress-induced phase transition of poly(tetramethylene terephthalate). Another example is the study of phase transition of aliphatic nylons in the heating and cooling processes. So far the aliphatic nylons were considered to show the phase transition at a so-called Brill transition point, but the simultaneous measurement revealed another phase transition occurring in a temperature region immediately below the melting point. The structural evolution has been also studied for the isothermal crystallization process of crystalline polymer from the melt, as seen in the case studies of polyethylene, nylon, polyester, and so on. The concrete description was made for the regularization process viewed from the various hierarchical levels of molecular chain conformation, chain packing mode, and lamellar stacking mode or higher-order structure.

Isothermal and non-isothermal crystallization of isotactic polypropylene nucleated with 1,3,5-benzenetricarboxylic acid tris(cyclohexylamide)

Abstract Isothermal and non-isothermal crystallization kinetics of isotactic polypropylene (iPP) nucleated with 1,3,5-benzenetricarboxylic acid tris(cyclohexylamide) (BTCA-TCHA) were investigated by differential scanning calorimetry (DSC). The classical Avarmi equation and the Mo method were used to analyze the experimental data. The results showed the Avrami equation and the Mo method described the isothermal and non-isothermal crystallization processes of iPP accurately, respectively. During isothermal crystallization, addition of BTCA-TCHA shortened half crystallization time (t1/2) and increased crystallization rate of iPP greatly. In addition, addition of BTCA-TCHA decreased the crystallization activation energy ΔE from 370 kJ/mol of virgin iPP to 312 kJ/mol. During non-isothermal crystallization, half time of the crystallization (t1/2) of virgin iPP was larger than that of nucleated iPP under the same cooling rate and the required cooling rate of virgin iPP to reach the same relative crystallinity was higher than that of nucleated iPP, indicating that addition of BTCA-TCHA increased the crystallization rate obviously.

Formation of a Rigid Amorphous Fraction in Poly(3-(2'-ethyl)hexylthiophene).

Herein, we detail the formation of a rigid amorphous fraction in poly(3-(2'-ethyl)hexylthiophene) (P3EHT) at high relative crystallinity, yielding a more complete picture of the solid-state structure. In the differential scanning calorimetry (DSC) heating scans of isothermally crystallized P3EHT a distinct endothermic peak appears slightly above the crystallization temperature. This previously undescribed endothermic feature of P3EHT's thermal behavior is observed consistently ∼20 °C above the crystallization temperature-shifting to higher temperatures with increasing crystallization temperature-and increases in magnitude with both time and crystallization temperature. Here, we determine the origins of this endothermic peak with DSC and temperature-modulated DSC (TMDSC). TMDSC reveals that the annealing peak observed in the total heat flow (THF)-heat flow equivalent to that of conventional DSC-is a consequence of an enthalpic relaxation observable as an endothermic peak in the nonreversible heat flow (NHF) and a glass transition evident as a step increase in the reversible heat flow (RHF). In conjunction with conventional DSC observations, these results indicate that the observed annealing peak is a consequence of the formation of distinct amorphous regions-a mobile amorphous fraction (MAF) and a rigid amorphous fraction (RAF)-during the isothermal crystallization process and not the melting of a distinct crystallite population or melt recrystallization.

Morphology, crystallization and mechanical properties of biodegradable poly(butylene succinate-co-butylene carbonate)/multi-walled carbon nanotubes nanocomposites

Biodegradable poly(butylene succinate-co-butylene carbonate) (PBSC)/carboxyl-functionalized multi-walled carbon nanotubes (f-MWCNTs) nanocomposites were prepared through a solution and casting method at low f-MWCNTs loadings. Scanning electron microscopy observations revealed a fine dispersion of f-MWCNTs in the PBSC matrix. Both the nonisothermal and isothermal melt crystallization processes of PBSC were enhanced, apparently by the presence of f-MWCNTs in the nanocomposites, because f-MWCNTs acted as an efficient heterogeneous nucleation agent for the crystallization of PBSC. The crystallization mechanism remained unchanged for both neat PBSC and the PBSC/f-MWCNTs nanocomposites, regardless of crystallization temperature; moreover, the crystal structures of PBSC were not modified by f-MWCNTs in the nanocomposites. The dynamical mechanical properties were also improved in the nanocomposites. Upon incorporating only 1 wt% of f-MWCNTs, the storage modulus of PBSC improved by about 30% at −60 °C. In addition, the glass transition temperature values of PBSC were increased slightly in the nanocomposites, relative to that of neat PBSC. Open image in new window

Experimental study of crystallization of PolyEtherEtherKetone (PEEK) over a large temperature range using a nano-calorimeter

The recently developed fast scanning differential calorimetry is used for the first time to determine the crystallization kinetics of Poly(EtherEtherKetone) (PEEK). In our experiments, crystallization is studied in isothermal conditions over a large temperature range from 170 °C to 310 °C. Two different measurement protocols were employed. Between 200 °C and 300 °C the heat flow was directly measured during isothermal crystallization. Outside this temperature range we measured the heat of fusion on heating after interrupted isothermal crystallization. We show that data can be analyzed with the Avrami approach incorporating a term describing secondary crystallization. The crystallization half-times are measured. The Avrami kinetic coefficient KAv associated with primary crystallization is evaluated from isothermal crystallization between 170 °C and 310 °C where data were not previously available. The kinetics of crystallization of PEEK has only one maximum located around 230 °C and its Avrami exponent is close to 3, suggesting instantaneous nucleation with subsequent spherical growth. The whole isothermal crystallization process is modeled in terms of Hillier's model since it takes secondary crystallization kinetics into account. Finally, it is shown that the double melting peak behavior observed after isothermal crystallization (below 260 °C) is a consequence of the reorganization process during heating.

Isothermal crystallization behavior and crystal structure of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters

Poly(ethylene terephthalate) (PET) and a series of poly(ethylene terephthalate‐co‐1,4‐cyclohexylene dimethylene terephthalate) (P(ET/CT)) copolyesters with different molar ratios of ethylene glycol (EG) to 1,4‐cyclohexanedimethanol (1,4‐CHDM) were investigated by their isothermal crystallization behavior, transparency, crystal structure and morphology changes in different isothermal crystallization process using differential scanning calorimetry (DSC), digital photos, wide‐angle X‐ray diffraction (WAXD) and polarized optical microscopy (POM). The results revealed that P(ET/CT) copolyesters with ≤ 15 mol% 1,4‐CHDM and ≥ 50 mol% 1,4‐CHDM, such as P(ET/CT)(85/15), P(ET/CT)(50/50) and P(ET/CT)(30/70), were crystallizable, while that with 30 mol% 1,4‐CHDM, namely P(ET/CT)(70/30), was amorphous. The crystallization rate, crystallinity and transparency of these copolyesters underwent the same isothermal crystallization process, i.e. it first decreased and then increased remarkably with the increase of 1,4‐CHDM content. Accordingly, the crystal structure of these copolyesters changed from PET‐type lattice to PCT‐type lattice when the copolyester with around 30 mol% 1,4‐cyclohexylene dimethylene terephthalate (CT) unit. Interestingly, the small incorporation of 1,4‐CHDM into PET could lead to the formation of larger spherulite crystals than that of PET. But small and grainy crystallites appeared with further increase in the 1,4‐CHDM content.

Effects of branches on the crystallization kinetics of polypropylene-g-polystyrene and polypropylene-g-Poly(n-butyl acrylate) graft copolymers with well-defined molecular structures

Effects of branches on the crystallization kinetics of polypropylene-g-polystyrene (PP-g-PS) and polypropylene-gpoly( n-butyl acrylate) (PP-g-PnBA) graft copolymers with well-defined molecular structures were systematically investigated by DSC. The Avrami equation was used to analyze the isothermal crystallization process, while the analysis of nonisothermal crystallization process was based on the Jeziorny-modified Avrami model and Mo model. The kinetics results of isothermal and nonisothermal crystallization verified the peculiar effects of branches on the crystallization process of PP backbones in PP-g-PS and PP-g-PnBA graft copolymers: on one hand, the interaction between branches (π-π interaction between PS branches, or dipole-dipole interaction between PnBA branches) restrained the mobility and reptation ability of the PP backbones, which hindered the crystallization process; on the other hand, the heterogeneous nucleation effect resulting from the branched structure and fluctuation-assisted nucleation mechanism (caused by microphase separation between the PS or PnBA rich phase and the PP rich phase) became more pronounced with increasing branch length, which facilitated the crystallization process.

Preparation of magnetite core-titania shell and hollow titania nanoparticles via layer-by-layer (LbL) assembly method

Time-resolved small-angle X-ray scattering (SAXS) analysis was performed on a series of poly(ethyleneco-1,4-cyclohexyldimethylene terephthalate)s (PECT copolymers) containing 1.6, 5.3, and 9.8 mol% 1,4-cyclohexyldimethylene (CHDM) units during isothermal crystallization and subsequent melting processes. The measured SAXS data were quantitatively analyzed to yield detailed information (scattering invariant quantity, Q; long period, Lp; lamellar crystal layer thickness, dc; and amorphous layer thickness, da) about the crystal structure evolution and melting devolution behaviors. The Q value was found to be a very sensitive powerful probe for monitoring the crystallization and crystal melting processes. The structural evolution of the copolymers was dominated by the primary crystallization transition. The secondary crystallization effects contributed little to the structural evolution. The few secondary crystals present most likely formed fringed micelle structures that were very small and included a high degree of imperfections. The poor secondary crystal formation was attributed to the presence of bulky, kinked CHDM units, which introduced a high degree of steric hindrance. The high steric hindrance of the CHDM units resulted in their exclusion from the lamellar crystal layers and secondary crystals, and in their insertion into amorphous regions and layers. Overall, the CHDM comonomer units strongly perturbed the crystallization process and the morphological structure of the PECT copolymer. The effects of CHDM as a chemical modifier of poly(ethylene terephthalate)-based polymers may potentially be optimized in an effort to enhance the properties and processability of the polymer. Open image in new window

Effects of Preexisting Poly(butylene succinate-co-24 mol % hexamethylene succinate) Crystals on the Crystallization Behavior and Crystalline Morphology of Poly(butylene adipate) in Their Melt-Miscible Polymer Blend

The effects of preexisting poly(butylene succinate-co-24 mol % hexamethylene succinate) (PBHS) crystals on the crystallization behavior and crystalline morphology of poly(butylene adipate) (PBA) were studied in their fully biodegradable melt-miscible crystalline/crystalline blend. Regardless of crystallization conditions, PBHS crystallized first, and PBA must crystallize in the presence of preexisting PBHS crystals. The preexisting PBHS crystals may not only enhance the nonisothermal melt crystallization behavior but also favor the nucleation and growth of only α-form crystals of PBA, providing an easy method of controlling the polymorphic crystals of PBA through polymer blending; moreover, the preexisting PBHS crystals could accelerate the overall isothermal melt crystallization process of PBA in the blend. PBA must crystallize as tiny crystals at the same orientation as the crystallites of the host PBHS spherulites in the blend, unlike the spherulitic morphology in its neat state. Both components crysta...

The combination of fluctuation-assisted crystallization and interface-assisted crystallization in a crystalline/crystalline blend of poly(ethylene oxide) and poly(ε-caprolactone)

The nucleation and crystallization of poly(ethylene oxide) (PEO) and poly(e-caprolactone) (PCL) in the PEO/PCL blends have been investigated by means of optical microscopy (OM) and differential scanning calorimetry (DSC). During the isothermal or nonisothermal crystallization process, when the adjacent PEO is in the molten state, PCL nucleation preferentially occurs at the PEO and PCL interface; after the crystallization of the adjacent PEO, much more PCL nuclei form on the surface of the PEO crystal. However, PEO crystallizes normally and no interfacial nucleation occurs in the blend. The concentration fluctuation caused by liquid–liquid phase separation (LLPS) induces the motion of PEO and PCL chains through interdiffusion and possible orientation of chain segments. The oriented PEO chain segments can assist PCL nucleation, and the heterogeneous nucleation ability of PEO increases with the orientation of PEO chains. Oriented PCL chain segments have no heterogeneous nucleation ability on PEO. It is postulated that the interfacial nucleation of PCL in the PEO/PCL blend follows the combination of “fluctuation-assisted crystallization” and “interface-assisted crystallization” mechanisms.