Summary: The liquid‐liquid phase separation (LLPS) is often coupled with other ordering processes such as crystallization. In a polyolefin blend system, overwhelming changes in crystallization kinetics due to concentration fluctuation caused by spontaneous spinodal LLPS have been observed. Consequently, we are proposing a new mechanism of “fluctuation‐assisted crystallization”. In this process, the usual nucleation barrier could be overcome (or at least partially) by the spontaneous fluctuation growth of LLPS in the spinodal region.
Time‐resolved polarized optical micrographs for poly(ethylene‐co‐hexene) (PEH)/poly(ethylene‐co‐butene) (PEB) = 40:60 isothermally crystallized at 117 °C for 2 min after LLPS at 135 °C for the times shown and the nucleation rates at 117 °C as a function of LLPS time at 135 °C. 相似文献
The nucleation and crystallization of poly(ethylene oxide) (PEO) and poly(ε-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.
Figure
a Illustration of PEO and PCL segments orientation caused by interdiffusion at the interface with concentration fluctuation and PCL spherulite induced by oriented PEO chains. b Illustration of PCL spherulites induced by the surface of PEO spherulite. PEO-rich and PCL-rich domains form and grow toward the liquid–liquid coexistent compositions during LLPS. The moving PEO and PCL chains could induce some segmental alignment or orientation (relative to adjacent chains) during the reptative interdiffusion. The oriented PEO segments have the heterogeneous nucleation ability on PCL, leading to the PCL nuclei occurs at the interface of the phase domains, illustrated in a. The PEO crystal has more regular chain alignment, so PCL nucleates easier on PEO crystal surface than on oriented PEO melt surface, such as illustrated in b. 相似文献
Influence of inter-diffusion on the crystallization dynamics in polyethylene/poly(ethylene-alt-propylene) (PE/PEP) blends was studied by a combination of optical microscopy (OM), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). OM measurements showed that the crystal nuclei may be first generated at phase separated interface where concentration fluctuation is greatly enhanced in the temperature quench process. After the formation of crystal nuclei, the only crystallizable components, PE chains, are necessary to reach the nucleation site via inter-diffusion to continue the secondary nucleation and growth process. DSC showed that there is only one 96 °C crystallization peak when PE (M(W) = 52 kg/mol) is blended with low molecular weight PEP (M(W) = 32 kg/mol); while there are two crystallization peaks, which are 96 °C and 72 °C, respectively, when the same PE is blended with high molecular weight PEP (M(W) = 110 kg/mol). The origin of the 72 °C crystallization peak was studied by DSC isothermal crystallization and time resolved FTIR. It was proved that the 72 °C crystallization peak is resulted from the smaller inter-diffusion coefficient in the PEP-rich region. Both slow mode theory and fast mode/constraint release models of inter-diffusion can be used to explain the smaller inter-diffusion coefficient in the PEP-rich region, which dynamically results in the disappearance of the 72 °C crystallization peak after isothermal crystallization at 90 °C for 60 min. Therefore, inter-diffusion plays an important role on crystallization dynamics in multi-component and multi-phase polymeric blends. 相似文献
The morphology, crystallization and self nucleation behavior of double crystalline diblock copolymers of poly(p-dioxanone) (PPDX) and poly(ϵ-caprolactone) (PCL) with different compositions have been studied by different techniques, including optical microscopy (OM), atomic force microscopy (AFM) and differential scanning calorimetry (DSC). The two blocks crystallize in a single coincident exotherm when cooled from the melt. The self-nucleation technique is able to separate into two exotherms the crystallization of each block. We have gathered evidences indicating that the PPDX block can nucleate the PCL block within the copolymers regardless of the composition. This effect is responsible for the lack of homogeneous nucleation or fractionated crystallization of the PCL block even when it constitutes a minor phase within the copolymer (25% or less). Nevertheless, we were able to show that decreasing amounts of PCL within the diblock copolymer still produces confinement effects that retard the crystallization kinetics of the PCL component and decrease the Avrami index. On the other hand evidence for confinement was also obtained for the PPDX block, since as its content is reduced within the copolymer, a depression in its self-nucleation and annealing temperatures were observed. 相似文献
A significant enhancement in isothermal crystallization kinetics of biodegradable polylactide (PLA) in its immiscible blends can be accomplished through blending it with a comb-like copolymer. PLA was blended with poly(ethylene glycol) methyl ether acrylate (PEGA) and poly[poly(ethylene glycol) methyl ether acrylate] (PPEGA, a comb-like copolymer), respectively. The results measured from phase contrast optical microscopy (PCOM) and differential scanning calorimetry (DSC) indicate that PLA and PEGA components are miscible, whereas PLA and PPEGA components are immiscible. The study of crystallization kinetics for PLA/PEGA and PLA/PPEGA blends by means of polarized optical microscopy (POM) and DSC indicates that both PEGA and PPEGA significantly increase the PLA spherulitic growth rates, G, although PLA/PPEGA blends are immiscible and the glass transition temperatures of PLA only have slight decreases. PPEGA component enhances nucleation for PLA crystallization as compared with PEGA component owing to the heterogeneous nucleation effect of PPEGA at the low composition of 20 wt%, while PLA crystallization-induced phase separation for PLA/PEGA blend might cause further nucleation at the high composition of 50 wt%. DSC measurement further demonstrates that isothermal crystallization kinetics can be relatively more enhanced for PLA/PPEGA blends than for PLA/PEGA blends. The “abnormal” enhancement in G for PLA in its immiscible blends can be explained by local interfacial interactions through the densely grafted PEGA side chains in the comb-like PPEGA, even though the whole blend system (PLA/PPEGA blends) represents an immiscible one. 相似文献
Differential scanning calorimetry (dual furnace, null-balance, DSC) and optical microscopy (OM) have been used to study the isothermal crystallization kinetics of poly(oxymethylene)-POM. The non-isothermal crystallization of the same material has also been studied by optical microscopy. A very controversial problem is whether the isothermal kinetic parameters may be applied to describe the non-isothermal crystallization. The results show that the kinetic spherulite growth parameters obtained by non-isothermal optical microscopy are, within the experimental errors involved, the same as those obtained by isothermal optical microscopy or isothermal DSC. The importance of this finding is highlighted. 相似文献
In a previous paper (Vivarès, D.; Bonneté, F. Acta Crystallogr., Sect. D 2002, 58, 472), protein-protein interactions of Aspergillus flavus urate oxidase (Uox) in solution were determined by small-angle X-ray scattering in the presence of different poly(ethylene glycol)s (PEG) in order to correlate second virial coefficient measurements with crystallization conditions. In this paper, we have characterized the experimental phase diagram of urate oxidase in the case of PEG 8000 by determining the solubility curve and the dilute part of the liquid-liquid phase separation (LLPS). Within this phase diagram, different mechanisms of urate oxidase crystal growth and LLPS can be observed by optical video microscopy. The influence of the LLPS on both the mechanisms and kinetics of urate oxidase crystal growth was observed by optical microscopy and small-angle X-ray scattering (SAXS). Interactions between the macromolecules were studied by SAXS in the dilute and dense phases of the demixed solution. It was observed that the LLPS precedes and slows down the crystallization. This study shows that urate oxidase is a good model to study protein/PEG mixtures in the general context of protein crystallization. 相似文献
In this work, the film thickness (l0) effect on the phase and dewetting behaviors of the blend film of poly(methyl methacrylate)/poly(styrene-ran-acrylonitrile) (PMMA/SAN) has been studied by in situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The thinner film shows the more compatibility of the blend, and the phase separation of the film occurs at l0>5Rg (radius of gyration). An initially time-independent q*, the characteristic wavenumber of the phase image, which is in good agreement of Cahn's linearized theory for the early stage of spinodal decomposition, has been obtained in real space and discussed in detail. For 5Rg>l0>3Rg, a "pseudo-dewetting/(phase separation+wetting)" behavior occurs, where the pseudo-wetting is driven by the concentration fluctuation mechanism. For l0<3Rg, a "real dewetting/(phase separation+wetting)" behavior occurs. 相似文献
The melt crystallization of poly(vinylidene fluoride) in a static electric field was studied for different fields strengths and undercooling conditions. The γ-phase nucleation process was examined directly by polarized optical microscopy and indirectly by small-angle light scattering. The crystal phase content was assessed by differential scanning calorimetry. It is shown that the γ-phase nucleation density and γ-phase content increase with field strength and that the higher the crystallization temperature, the larger the effect of the field. These experimental results confirm the predictions of the model of nucleation in an electric field that we published previously. It is also noted that the degree of crystallinity and the perfection of crystal orientation along the γ-phase spherulite radical direction decrease with field strength. The homogeneity of morphology resulting from the crystallization in the field is also examined by polarized optical microscopy on specimens microtomed across their thickness. When the crystallization is carried out under high field (E ≈ 0.1 MV/cm) and high temperature (T > 166°C) a nonuniform morphology results, characterized by a higher nucleation density at the positive electrode than at the negative electrode. Near the negative electrode very large disklike spherulites are seen to grow parallel to the substrate. 相似文献
This paper presents an investigation of the phase diagram of BPTI (bovine pancreatic trypsin inhibitor)/350 mM KSCN at pH 4.9 by direct observation and numerical simulations. We report optical microscopy and light and X-ray scattering experiments coupled with theoretical data analysis using numerical tools. The phase diagram is thoroughly determined, as a function of temperature. Two polymorphs are observed by video microscopy and their solubility measured. In this phase diagram, the liquid-liquid phase separation (LLPS) is metastable with respect to the solid-liquid phase separation. Above the T(L-L) boundary curve, solutions are composed of a mixture of BPTI monomers and decamers. Attractive interactions are stronger between decamers than between monomers. Below the T(L-L) boundary curve, the dense phase is highly concentrated in protein and composed of BPTI decamers alone. Thus, the driving force for liquid-liquid or liquid-solid phase separation is the attraction between decamers at low pH. The structure factors of the dense phases are characteristic of repulsive dense phases because of a hard sphere repulsion core, meaning that in the dense phase proteins are actually in contact (interparticle distance of 53 A). In agreement with the Oswald rule of stages, LLPS occurs prior to and impedes the solid nucleation. 相似文献
The effects of self-seeding nucleation on the crystallization behavior and properties of poly( trimethylene terephthalale) were studied. Differential scanning calorimetry (DSC) results indicated that the crystallization temperature of paly(trimethylene terephthalate) increased obviously(increased about 20℃) after the process of selfseeding nucleation. The results of polarized light microscopy (PLM) showed that the spherulite size decreased markedly from 40μm to 8μm. 相似文献
Thermally induced phase separation (TIPS) has prompted a great deal of interest, especially as an effective approach to fabricate ultra-high molecular weight polyethylene (UHMWPE) microporous membranes. However, the existing utilized diluents for the TIPS process of UHMWPE suffer from environmental and health issues. Herein, we utilized low molecular weight polybutene (PB) bearing similar structure with liquid paraffin (LP) but inferior miscibility with UHMWPE relative to UHMWPE/LP blend, as a diluent for the TIPS process of UHMWPE. The phase separation behavior of UHMWPE/PB blends were investigated by the combination of rheological measurements, optical microscopy as well as differential scanning calorimeter (DSC). The results suggest that PB is fully miscible with UHMWPE at elevated temperature, but yielding a more sensitive phase separation behavior in respect to LP in TIPS process, because PB has weaker interaction with UHMWPE. The Jeziorny method analysis indicates that the crystallization mechanism of UHMWPE/LP blends is in line with that of UHMWPE/PB blends, which includes nucleation and growth as well as their dynamic competition. Moreover, compared to those of UHMWPE/LP blends, UHMWPE/PB blends display higher TIPS temperature and faster TIPS rate along with faster overall crystallization rate, further demonstrating that PB can accelerate phase separation rates and enhance the efficiency of TIPS process. 相似文献
Miscibility and morphology of poly(ethylene 2,6-naphthalate)/poly(trimethylene terephthalate)/poly(ether imide) (PEN/PTT/PEI)
blends were investigated by using a differential scanning calorimeter (DSC), optical microscopy (OM), wide-angle X-ray diffraction
(WAXD), and proton nuclear magnetic resonance (1H-NMR). In the ternary blends, OM and DSC results indicated immiscible properties for polyester-rich compositions of PEN/PTT/PEI
blends, but all compositions of the ternary blends were phase homogeneous after heat treatment at 300 °C for more than 30 min.
An amorphous blend with a single Tg was obtained in the final state, when samples were annealed at 300 °C. Experimental results from 1H-NMR identified the production of PEN/PTT copolymers by so-called “transesterification”. The influence of transesterification
on the behaviors of glass transition and crystallization was discussed in detail. Study results identified that a random copolymer
promoted the miscibility of the ternary blends. The critical block lengths for both PEN and PTT hindered the formation of
crystals in the ternary blends. Finally, the transesterification product of PEN/PTT blends, ENTT, was blended with PEI. The
results for DSC and OM demonstrated the miscibility of the ENTT/PEI blends. 相似文献