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Temporal change of radius of spiral spherulite with time showing a stepwise growth for the 7030 PVDF/PVAc blend at the isothermal crystallization temperature of 167 ± C.
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Numerical calculations reveal that the target and spiral growth patterns in spherulites can be generated from the time-dependent Ginzburg-Landau equations (model C) by coupling a conserved compositional order parameter and a nonconserved crystal ordering parameter. Of particular interest is that the periodic concentric rings (target) or the spirals...
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... we investigated the growth of PVDF spherulites in the blend with PVAc at various compositions by capturing optical microscope images as a function of time. As depicted in Fig. 3, a stepwise growth was uniquely observed in the plot of the radius of spiral spherulites versus time. At the length scale of lamellar growth, it is well accepted that amorphous chains are rejected from the crystallizing fronts into the interlamellar region. At the larger length scale of spherulitic growth, the noncrystallizing PVDF and ...
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Secondary nucleation, wherein crystal seeds are used to induce crystallization, is widely employed in industrial crystallizations. Despite its significance, our understanding of the process, particularly at the molecular level, remains rudimentary. An outstanding question is why do a few seeds give rise to a many-fold increase in new crystals? Usin...
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... For example, ring-shaped spherulites are formed during thermally induced crystallization from a melt in an air atmosphere, in which acicular crystals (whiskers) of L-menthol selforganize into radially oriented periodic regions with high and low densities of the solid phase of the substance, or non-banded continuous spherulites, consisting of densely packed lamellar crystals [15]. Such rhythmic crystallization could be due to the chirality of L-menthol [16], as well as nonlinear diffusion of the substance during mass transfer [17,18] and the "unbalanced" surface energy of the system [19]. During thermally induced crystallization in an aqueous medium (antisolvent), the phase transition from the liquid-liquid region to the solid-liquid one is accompanied by the formation of the stable α-polymorphic form of L-menthol [20,21]. ...
... It is a powerful way to predict microstructure evolution in the process of crystalline phase transformation, considering the microscopic, high-sharpness interface between two phases as the diffusion interface region in the computational domain, which conform to the non-equilibrium thermodynamic principle [28]. After Kyu's pioneering work on the polymer crystallization phase-field model [29], the method was also developed and applicated in polymer research, covering the investigations of the crystal pattern formation, internal field generation during crystallization, and the nucleus's effects [30][31][32][33][34][35][36]. However, as reported so far, there is no report on this method's application in PTFE crystallization phase transformation. ...
We investigated, via a phase-field model simulation, the effects of a matrix’s properties and a filler’s characters on the polytetrafluoroethylene (PTFE) crystal growth process in composites under various supercooling degrees. The results show that the supercooling degree has a deciding influence on the crystal growth process. The intrinsic properties of PTFE polymer, such as anisotropic strength and phase transition latent heat, affect the growth rate, orientation, and interfacial integrity of the crystal trunk and the branching of the PTFE crystal growth process. The factors of the PTFE crystallization process, such as anisotropic strength and phase translation interface thickness, affect the uniformity and crystallization degree of the PTFE crystal. In the composites, the biphasic interface induces the crystal growth direction via the polymer chain segment migration rate, of which the degree depends on the shapes of the filler and the PTFE crystal nucleus. According to the results, choosing the low molecular weight PTFE and mixture filler with various particle sizes and surface curvatures as the raw materials of PTFE-based composites improves the crystallization of the PTFE matrix.
... A ringed texture is usually associated with the twisting of the constitutive edge-on lamellae [36], but other interpretations have also been postulated (e.g., development of interlamellar screw dislocation [37] or the presence of different polymorphic structures [38]). However, the crystallization derived as a result of slow evaporation from dissolution and the low sample molecular weight could also be related to a rhythmic crystal growth derived from the presence of depletion zones in the growth front [39,40]. It has been postulated that the final spherulitic morphology is the consequence of a balance between the diffusion rate (vd) of polymer chains and the crystallization rate (vc) [40][41][42]. ...
Poly(butylene succinate) (PBS) scaffolds with 3D microporous interconnected architecture and crystallized PBS substructures have been prepared by thermally-induced phase separation (TIPS). Curcumin (CUR) and piperine (PIP) natural drugs were incorporated into the matrices during a one-step fabrication protocol. Alterations in TIPS parameters such as solvent system (1,4 dioxane (DXN) or THF) and cooling condition (intensity and direction of thermal gradient) affected the phase separation process and the scaffold properties. Controlling the crystallization of DXN in a uniaxial direction, resulted in the formation of microtubular scaffolds with oriented porosities. Well-developed PIP crystals and matrix-integrated CUR agglomerates were uniformly distributed throughout the scaffolds. The integration of CUR to the polymer matrix was confirmed by physicochemical evaluations, attributed to a possible interaction with PBS, resulting in a slower release than PIP. The scaffolds with uniaxially-oriented porosities displayed a greater sustained release (only 50–60% after 170 h) due to the restricted drug diffusion through their dense spherulitic pore walls. Due to such structure and smoother well-arranged surfaces, the oriented scaffolds also exhibited greater biocompatibility. Our results reveal the positive influence of TIPS-derived structural orientation on properties of PBS matrices for cell/drug delivery.
... A ringed texture is usually associated with the twisting of the constitutive edge-on lamellae [36], but other interpretations have also been postulated (e.g., development of interlamellar screw dislocation [37] or the presence of different polymorphic structures [38]). However, the crystallization derived as a result of slow evaporation from dissolution and the low sample molecular weight could also be related to a rhythmic crystal growth derived from the presence of depletion zones in the growth front [39,40]. It has been postulated that the final spherulitic morphology is the consequence of a balance between the diffusion rate (v d ) of polymer chains and the crystallization rate (v c ) [40][41][42]. ...
Different copolymers incorporating terpene oxide units (e.g., limonene oxide) have been evaluated considering thermal properties, degradability, and biocompatibility. Thus, polycarbonates and polyesters derived from aromatic, monocyclic and bicyclic anhydrides have been considered. Furthermore, ring substitution with myrcene terpene has been evaluated. All polymers were amorphous when evaluated directly from synthesis. However, spherulites could be observed after the slow evaporation of diluted chloroform solutions of polylimonene carbonate, with all isopropene units possessing an R configuration. This feature was surprising considering the reported information that suggested only the racemic polymer was able to crystallize. All polymers were thermally stable and showed a dependence of the maximum degradation rate temperature (from 242 °C to 342 °C) with the type of terpene oxide. The graduation of glass transition temperatures (from 44 °C to 172 °C) was also observed, being higher than those corresponding to the unsubstituted polymers. The chain stiffness of the studied polymers hindered both hydrolytic and enzymatic degradation while a higher rate was detected when an oxidative medium was assayed (e.g., weight losses around 12% after 21 days of exposure). All samples were biocompatible according to the adhesion and proliferation tests performed with fibroblast cells. Hydrophobic and mechanically consistent films (i.e., contact angles between 90° and 110°) were obtained after the evaporation of chloroform from the solutions, having different ratios of the studied biobased polyterpenes and poly(butylene succinate) (PBS). The blend films were comparable in tensile modulus and tensile strength with the pure PBS (e.g., values of 330 MPa and 7 MPa were determined for samples incorporating 30 wt.% of poly(PA-LO), the copolyester derived from limonene oxide and phthalic anhydride. Blends were degradable, biocompatible and appropriate to produce oriented-pore and random-pore scaffolds via a thermally-induced phase separation (TIPS) method and using 1,4-dioxane as solvent. The best results were attained with the blend composed of 70 wt.% PBS and 30 wt.% poly(PA-LO). In summary, the studied biobased terpene derivatives showed promising properties to be used in a blended form for biomedical applications such as scaffolds for tissue engineering.
... A spherulite structure then forms with alternating dark and light rings around the nucleation center [4]. In subsequent works, Keith and Padden [5] and Kyu et al. [6] showed that banded spherulites may also result from structural discontinuity caused by rhythmic crystal growth. Lamellae twisting model and rhythmic deposition model are among the most acceptable models that explain the growth mechanism of banded spherulites. ...
In this paper the influence of different aspects of crystallization process on the formation of ring-banded spherulites and the transition between banded and non-banded structures in polylactide was analyzed. Based on the measurements using the differential scanning calorimetry (DSC) and polarized optical microscopy (POM), it was found that the banding phenomenon of spherulites in polylactide and its blends with triethyl citrate (TEC) exclusively depends on the degree of supercooling (ΔT). In the work, for the first time, the equilibrium melting point (Tm⁰) of polylactide was correlated with the isothermal crystallization temperature (Tic) in the context of banded spherulites formation. To determine the Tm⁰ of polylactide blends, due to the possibility of modifier migration towards the sample surface a correction of the methodology was proposed. It has been shown that ΔT necessary to initiate the growth of spherulites with a ring pattern in polylactide ranges from 46 to 56 °C. The inclusion of TEC into polylactide matrix shifts the Tic range in which banded spherulites appear towards lower values, but does not change the required value of supercooling degree. A direct correlation between the band space of the ring-banded spherulites and the degree of supercooling was also demonstrated.
... Therefore, reported experimental data were not useful to discern between the two typical explanations formulated, to elucidate the ringed morphology of spherulites. Namely, a continuous twisting of constitutive lamellae [20] or alternatively a rhythmic growth derived from the presence of depletion zones in the growth front [21]. ...
Hydrolytic degradation of poly(4-hydroxybutyrate) (P4HB) films has been studied considering media of different pH values (i.e., 3, 7 and 10) and temperatures (i.e., 37 and 55 °C). Enzymatic degradation has also been evaluated at physiological conditions using two different lipases: Pseudomonas cepacia and Rhizopus oryzae.
Different bulk and surface erosion mechanisms with random chain scissions and successive removal of monomer units have been supported through weight loss measurements, molecular weight determinations by GPC and NMR spectroscopy and changes on thermal properties by DSC. Thermal annealing during exposure to different media and even degradation influenced on the melting temperature and crystallinity of samples, as well as on the lamellar geometrical parameters as evaluated by SAXS. Enzymatic degradation was ideal to selectively eliminate the amorphous regions and highlight the spherulitic morphology. Presence of ringed textures were therefore evident in bright field optical micrographs in addition to SEM images, namely observations under polarized light was not necessary to distinguish the presence of banded spherulites. Rhizopus oryzae was revealed to be the most suitable enzyme to crop out the P4HB spherulites that form part of the initial smooth surfaces of solvent casting films. After determining the appropriate activity and exposure time, the presence of rings constituted by cooperative C-shaped edge-on lamellae and flat-on lamellae was highlighted.
... Crystallization of P4HB from the melt state rendered spherulitic morphologies with a banded texture (Figure 10a) at all assayed temperatures (i.e., between 36 • C and 49 • C). This kind of morphology is usually observed in the crystallization of different polymers and has merited different explanations based on: the development of interlamellar screw dislocations [25,26], the continuous twisting of lamellae [27], the rhythmic growth derived from the presence of depletion zones in the growth front [28] and the presence of different polymorphic structures [29]. It has also been postulated that final morphology is a consequence of a balance between the diffusion rate (v d ) of the melted polymer and the crystallization rate (v c ) [30,31]. ...
... Crystallization of P4HB from the melt state rendered spherulitic morphologies with a banded texture (Figure 10a) at all assayed temperatures (i.e., between 36 °C and 49 °C). This kind of morphology is usually observed in the crystallization of different polymers and has merited different explanations based on: the development of interlamellar screw dislocations [25,26], the continuous twisting of lamellae [27], the rhythmic growth derived from the presence of depletion zones in the growth front [28] and the presence of different polymorphic structures [29]. It has also been postulated that final morphology is a consequence of a balance between the diffusion rate (vd) of the melted polymer and the crystallization rate (vc) [30,31]. ...
Thermal properties and crystallization kinetics of poly(4-hydroxybutyrate) (P4HB) have been studied. The polymer shows the typical complex melting behavior associated to different lamellar populations. Annealing processes had great repercussions on properties and the morphology of constitutive lamellae as verified by X-ray scattering data. Kinetics of isothermal crystallization was evaluated by both polarizing optical microscopy (POM) and calorimetric (DSC) measurements, which indicated a single crystallization regime. P4HB rendered banded spherulites with a negative birefringence when crystallized from the melt. Infrared microspectroscopy was applied to determine differences on the molecular orientation inside a specific ring according to the spherulite sectorization or between different rings along a determined spherulitic radius. Primary nucleation was increased during crystallization and when temperature decreased. Similar crystallization parameters were deduced from DSC and POM analyses (e.g., secondary nucleation parameters of 1.69 × 105 K2 and 1.58 × 105 K2, respectively). The effect of a sporadic nucleation was therefore minimized in the experimental crystallization temperature range and a good proportionality between overall crystallization rate (k) and crystal growth rate (G) was inferred. Similar bell-shaped curves were postulated to express the temperature dependence of both k and G rates, corresponding to the maximum of these curves close to a crystallization temperature of 14–15 °C.
... Kyu et al. suggested that as a consequence of the nonlinear diffusion during the crystallization the crystal growth becomes rhythmic, which causes banded spherulites. 31 Takayanagi and Yamashita observed ringed spherulites in the case of poly(ethylene adipate). 32 Padden and Keith found a similar structure in polypropylene (PP). ...
In this work, we investigate the growth of spherulites in the wake of a solidification front in a thin film of liquid menthol. We observed that the two enantiomeric forms of menthol ((–)-menthol and (+)-menthol)) form ring-banded spherulites, in which needle-like crystals self-assemble into high- and low-crystal density regions. Interestingly, the racemic mixture produces non-banded spherulites consisting of closely packed plate-like crystals. In the ring-banded spherulitic growth, we could clearly identify the curvature effect of the solidification front on the pattern formation at a millimeter spatial scale. We developed a numerical model based on the Cahn-Hilliard equation, which qualitatively describes the main features observed in experiments, namely the formation of periodic ring-banded structures and the curvature effect of the propagating solidification front.
... The driving force for twisting is attributed to anisotropic or unbalanced surface stresses. Growth asymmetry introduced by chain tilt, 51 reordering of fold surfaces, 52 repetition of isochiral screw dislocations, 53 rhythmic crystallization process due to nonlinear diffusion, 54 and self-induced compositional or mechanical fields generated near advancing growth front 55 are proposed to be the origin of lamellar twist. Yet, the ultimate driving force is still an open question. ...
... Generally, the unbalanced stress at a lamellar surface drives the lamellar twisting to yield ring-banded spherulite. 60 Compared with pure trans configuration entry 15, the introduction of the amorphous cis isomer intersperses among the crystalline lamellae, thus restricting lamellar in twisting and scrolling of trans unit moieties. As a result, the distorted lamellae can only form spherulites with loose coarse and blurry bands. ...
Maleate and fumarate-based unsaturated copolyesters are of interest as biomedical materials with tailorable properties. However, the preparation of unsaturated copolyester with controllable maleate (cis) and fumarate (trans) isomer compositions is still a great challenge in polymer chemistry, because cis units always unbridledly isomerize to trans units in the polymerization process. Herein, we presented a universal method for the facile synthesis of stereochemistry-controllable poly(butylene maleate-co-butylene fumarate) (P(BM-co-BF)) copolyesters. Then, we prepared a series of comparable high-molecular weight (Mw >30kDa) biobased copolyesters with a wide range of well-controlled cis-trans composition. The solid-state microstructure, morphology, and crystallization behavior of P(BM-co-BF) copolyesters were systematically investigated to explore the respective role of cis and trans isomers on the physical properties of copolymers. With the increment of the cis content, the copolyesters transform the phase state from semicrystalline to completely amorphous with tunable thermal and mechanical properties, which can be accounted for the conformational geometry effect. Consequently, the variations of stereochemistry composition allow tailoring the physical properties over a broad range. Furthermore, a merit of these copolyesters is to provide a chemoselective polymer platform for introduction of various functional groups onto backbone through a green aza-Michael addition reaction under extremely mild conditions, such as PEG, hydroxyl, alkynyl, etc. These incorporated functional groups can be used for selective tailoring of the polyesters, which dramatically increase their potential on the wide versatility applications.
