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Chiral Nucleating Agents Affecting the Handedness of Lamellar Twist in the Banded Spherulites in Poly(ε-Caprolactone)/Poly(Vinyl Butyral) Blends
Abstract
Chiral silica, which acts as a nucleating agent of poly(ε-caprolactone) (PCL), was demonstrated to induce excess handedness of lamellar twist in the banded spherulites of PCL blended with poly(vinyl butyral). The d- and l-forms of silica enhanced the right- and left-handed twists, respectively. The influences of chiral silica on the twist handedness were statistically significant. These results indicate that the handedness of twisting can be controlled upon primary nucleation. The organic substances used as chiral templates of silica had no effect on the handedness; silica was shown to govern the handedness. The possible mechanisms of the chirality transfer are discussed.
... Therefore, PEG is rejected from the PLLA bundles, and the blend exhibits interfibrillar segregation (see the schematic representation in Fig. 6). Figure 3b-e shows that after blending with PEG, the spherulites of PLLA exhibit different morphology than that by pure PLLA. Ring-banded spherulites were [35][36][37]. The formation mechanism of banded spherulites has attracted intensive study in the past decades, but controversy still remains [38,39]. ...
The morphology and microstructure of crystalline blends of poly (ethylene glycol) (PEG) and poly (L-lactic acid) (PLLA) were examined using polarized optical microscopy (POM) and scanning electron microscopy (SEM). As PEG was in the melt state during PLLA crystallization, it was rejected from the PLLA bundles. The size of PEG inclusions determined by their extraction is around 1 μm. The PEG/PLLA blends exhibited not only spherulites with Maltese crosses but also distinct extinction rings. The formation of ring-banded spherulites and the periodic distance between the rings were related to the degree of supercooling of the polymer. The ring-banded structure was easily obtained at a high PEG content (70 wt%) and high PLLA crystallization temperature (120 °C). The end group of PEG significantly affected the morphology of PEG/PLLA blends. PLLA blended with PEG containing both end groups as CH3 exhibited the greatest melting temperature depression and lowest degree of supercooling of PLLA, implying the formation of ring-banded spherulites with the smallest PEG content (30 wt%) and lowest PLLA crystallization temperature (85 °C). PEG morphology was also observed using POM after the formation of PLLA crystals. Because PLLA crystals confined the formation of PEG crystals, the chain growth direction of PEG was in association with that of PLLA. Therefore, a brighter POM image was obtained on PEG crystallization.
... It seems that cross-linked polymers can be endowed with chiroptical activity on these SiO 2 nanofibers. To further support this point, other two kinds of polymers are synthesized via the radical polymerization of achiral divinylbenzene (DVB) or N,N′-methylenebisacrylamide (MBA) around SiO 2 [ Figure 7A] [48] . The tart in PEI/tart@SiO 2 was firstly removed by HCl to obtain PEI/SiO 2 . ...
Asymmetric tetrahedral carbon is the basic structural unit of many organic compounds in life and its molecular chirality plays a key role in regulating biological functions. Silica (SiO2) is highly earth abundant and its basic unit is also the tetrahedral form of SiO4. However, much less attention has been paid to the molecular-scale chirality of SiO2 frameworks with repeating SiO4 units because it is challenging to enantioselectively control the molecular structures of SiO2. Research into the chiral molecular structures of SiO2 deserves to be a significant topic for understanding widespread chiral phenomena and for exploring the chiral properties hidden in inorganic matter. This review highlights the asymmetric synthesis strategies that endow SiO2 with chirality transferred from asymmetric carbon at the molecular scale. The chirality transfer ability of SiO2 is also demonstrated for the construction of various inorganic and/or organic chiral materials with a wide range of applications in asymmetric synthesis, circularly polarized luminescence and Raman scattering-based chiral recognition.
Despite excruciating attempts by polymer physicists in last several decades, lamellar assembly in ring-banded spherulites (RBSs) is still an unsolved and highly debated puzzle. In an unprecedented manner, the banded rings, based on a model polymer, are mapped with advanced synchrotron microbeam X-ray diffraction. According to the diffraction map and microscopy-dissection morphology, the periodically banded structure is composed of lamellae self-assembled as structural gratings. The comparison of a real-time micrograph with this mapping shows the sequential arrangement and orientation of lamellae in gratings. Lamellar assembly in RBS is fully explained with experimental evidence. This methodology can be applied to a wide variety of polymers to analyze spherulites displaying periodic self-assembled patterns.
Realizing chiral crystals from achiral compounds is an important but challenging issue. Here, a kebab-structured chiral supra-crystallization complex is constructed in accomplishing hierarchical chiral crystallization of achiral fluorescent compound (1,8-naphthalic anhydride, NA) employing synthetic chiral macromolecules (polylactide, PLA) as a template. Chirality transfer efficiently takes place from PLA to NA crystal during the co-crystallization process, endowing the initial achiral NA crystal with intense optical activity. More importantly, the obtained chiral supra-crystals show remarkable circularly polarized luminescence (CPL) properties, with the highest luminescence dissymmetric factor up to 5 × 10-2. Moreover, handedness-tunable and nonreciprocal CPL emissions are further realized in one single sample by simply flipping and rotating the supra-crystallization composite film, following a polarization mechanism. The work presents the first polymer-based hierarchical chiral supra-crystal with switchable CPL emissions and provides a simple and universal strategy for constructing nonreciprocal CPL materials.
To clarify the details of the inner structure of polyethylene spherulites and the mechanism of the lamellar twisting phenomenon, we performed simultaneous step-scan measurements of wide-angle and small-angle X-ray scattering by using a synchrotron X-ray beam of μm size as well as atomistic simulation of the lamellar plate. Assorted new structural information has been extracted: (i) the chain axis is tilted by 15~22° from the normal to the lamellar plane, correcting the previously reported evaluation; (ii) the a- and b-axial lengths of the unit cell change periodically along the radical direction; and (iii) the lamellar twisting pitch is longer in the central part of the spherulite and adopts a constant shorter value in the equilibrated outer part. Second, in association with these experimental results, molecular mechanics calculations were performed to confirm the role of folded chain parts as a trigger of the lamellar twisting phenomenon. The calculated tilting angle of the chain stems was approximately 13°, and the calculated helical pitch for 180o-lamellar twisting was approximately 2 μm, which is in good agreement with the X-ray-observed values. The 2D WAXD/SAXS data collected using a synchrotron X-ray microbeam technique revealed the details of the lamellar twisting phenomenon of high-density polyethylene spherulites. The molecular mechanics calculation, performed for large aggregation models of alkane chains having bulky head groups or folded polyethylene chains, has successfully reproduced the lamellar twisting pitch of a few μm, as extracted from the X–ray data analysis.
Formation of well-organized, periodic spherulites of four-arm star symmetric poly(ε-caprolactone)-b-poly(ethylene oxide) (PEO-b-PCL) via confined slow evaporative crystallization of solution-cast films is reported, and the microstructures were characterized by optical microscopy, atomic force microscopy, and X-ray scattering and diffraction techniques. Having a Maltese cross pattern, the spherulites exhibit a unique concentric-ringed morphology under both polarized and unpolarized lights. The pattern regularity and ring periodicity are both controllable. The tunable periodicity is in the range of 10–45 μm, and the size of the spherulites even exceeds one millimeter. It shows that such birefringent concentric-structure is a reflection of periodic change of thickness caused by rhythmic stacking of lamellar crystals with varied orientation along the radial direction. The lamellae in valley bands are nearly flat-on, while the lamellar orientation becomes gradually inclined with normal thickening, leading to continuously enhanced birefringence in ridges. Finally, a coupling of the poor solubility of the PEO block and its inner geometry in the star topology is then presented to illustrate a possible solvent-assisted topology confined mechanism for inducing PCL-dominated rhythmic crystallization into concentric-ringed spherulites.
The morphologies and structures of crystalline polymers depend on the crystallization condition, such as the quenching rate, crystallization temperature, pressure, and molecular weight, tacticity, branching, topology and so on. The morphologies and growth of poly (d-lactic acid) (PDLA) spherulites were observed by POM when the samples isothermally crystallized at various temperatures after cooling from 200 °C to 250 °C. The crystallization behavior of PDLA was characterized by DSC measurements at various temperatures and it is not consistent with the kinetics of spherulite growth, which is due to the difference between the characterized partial and total crystallization behavior. It was found that the formation of the banded spherulites of PDLA is not only dependent on the crystallization temperatures, but also on the melting temperatures. Light fluctuation of the POM images was investigated during heating and it was found that the dark bands or black bands disappeared earlier than the bright ones during heating of the banded spherulites. The observed melting phenomenon implies mutiple lamellar stacking model inside the spherulites. The formation condition of the banded spherulites morphology was discussed based on the crystallization rate and diffusion rate. The proposed mechanism would be helpful to understand the structure and formation of PDLA ring-banded spherulites.
Chiral SiO2 nanofibers associated with polyamines on their surfaces could promote polymerization between resorcinol and formaldehyde on silica surface to give phenolic resins (RF). In this process, the chiral information was effectively transferred from SiO2 to the final phenolic resins forming double chiral hybrid.
Constructing novel chiral inorganic nanomaterials is an emerging branch in chirality research. In this work, by employing a solid magnesiothermic reaction at 500-600 °C, we reduced chiral SiO2 nanofibers with average diameter ∼10 nm into chiral Si nanoplates with a size of about several hundred nm. The chirality of the as-prepared Si was judged by the pair of signals with a mirror relationship between 400-500 nm that appeared on the solid-state diffuse reflectance circular dichroism (DRCD) spectra for the l- and d-form Si. Furthermore, the chirality was also confirmed by induced vibrational circular dichroism (VCD) signals corresponding to the absorption bands in the infrared range of achiral organics (polyvinylpyrrolidone K90 and trimethoxyphenylsilane) absorbed onto chiral Si. The as-used SiO2 nanofibers possessed an ultra high-temperature (up to 900 °C) resistant chirality, which would be due to the asymmetric arrangement of Si and O atoms in small chiral domains (<10 nm) on the Si-O-Si network of SiO2. During the removal of oxygen atoms from Si-O-Si by Mg atoms, the arrangement of newly formed Si-Si bonds as well as the growth of Si crystals were still templated without racemization from the chiral information in SiO2. Consequently, the subnano/nano-scale (<10 nm) chiral information was in situ transferred via the so-called self-transfer mechanism, even though there was no retention of the outward shapes of the length-scale nanofiber SiO2 reactants in the Si products. This work offers a feasible chemical method to prepare chiral Si using abundant SiO2 raw materials.
Twisting of polymer lamellae manifested by e.g. appearance of concentric bands in polymer spherulites examined in a polarized optical microscope remains a topic of research and controversy. It has been interpreted variously as resulting from phenomena that take place during growth or from structural features of individual lamellae, or multilamellar aggregates. Phenomena that take place during growth are of general, or even generic character. They include non-linear diffusion processes leading to rhythmic crystallization, or self-induced compositional or mechanical fields generated near the advancing crystal front. Structural features include cumulative reorientation of lamellae at successive isochiral screw dislocations (possibly linked with surface pressure exerted by cilia) or different surface stresses on opposite fold surfaces of individual lamellae, as a result of different levels of congestion of folds.
We present a detailed study of the kinetics of crystallization for thin films of poly(ethylene oxide) (PEO). Measurements of the growth rate have been carried out using optical-microscopy techniques on films of monodisperse PEO. Films with thicknesses from 13 nm to approximately 2 microm were crystallized isothermally at temperatures approximately 20 degrees C below the melting point. A remarkable non-monotonic slowing-down of the crystal growth is observed for films with thickness less than approximately 400 nm. The changes in the growth rate from bulk-like values is significant and corresponds to a factor of 40 decrease for the thinnest films studied. The morphologies of isothermally crystallized samples are studied using atomic-force microscopy. We find that a morphology, similar to diffusion-controlled growth (dendritic growth and densely branched growth), is observed for films with h<150 nm. In addition, changes in the morphology occur for thicknesses consistent with changes in the growth rate as a function of film thickness.
Banded spherulite resulting from lamellar twisting due to the imbalanced stresses at opposite fold surfaces can be formed by isothermal crystallization of chiral polylactide and its blends with poly(ethylene glycol) (PEG). Using a polarized light microscope, the handedness of the twisted lamella in banded spherulite is determined. With the same growth axis along the radial direction as evidenced by wide-angle X-ray diffraction (WAXD) for isothermally crystallized samples at different temperatures, the twisted lamellae of chiral polylactides (poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA)) display opposite handedness. The split-type Cotton effect on the C═O stretching motion of vibrational circular dichroism (VCD) spectra helps determine the helix handedness (i.e., conformational chirality). The results indicate that the conformational chirality can be defined by the molecular chirality through intramolecular chiral interactions. Moreover, the preferred sense of the lamellar twist in the banded spherulite corresponds to the twisting direction identified by the C–O–C vibration motion of VCD spectra, reflecting the role of intermolecular chiral interactions in the packing of polylactide helices. Similar results are obtained in the blends of chiral polylactides and poly(ethylene glycol) (PEG, a polymer compatible with polylactide), indicating that the impact of chirality is intrinsic irrespective of the specific crystallization conditions. In contrast to the chiral polylactides, the spectrum of the crystalline stereocomplex that associates PLLA and PDLA shows VCD silence. The spectroscopic results are in line with the morphological observations. No banded spherulites are observed in the stereocomplex crystallites due to the symmetric packing of mirror L- and D-chain conformations in the fold surfaces and the crystallites core.
Herein, we suggest a unique approach to control the handedness of twisted lamellae in banded spherulites of a stereoregular polymer, isotactic poly(2-vinylpyridine) (iP2VP). When (R)- or (S)-hexahydromandelic acid (HMA), which can associate with iP2VP, was introduced as a chiral dopant, mirror-image CD spectra in the complex systems showed induced circular dichroism (ICD) of the iP2VP by chiral HMA. Banded spherulites resulting from lamellar twisting due to the imbalanced stresses at the opposite folding surfaces could be formed by crystallization of the iP2VP/HMA complexes, which had a crystalline structure similar to that of neat iP2VP. A preferential sense of the twisted crystalline lamellae was found in the iP2VP/HMA complex, thus suggesting homochiral evolution from conformational to hierarchical chirality.
In the course of structural studies of aliphatic polyesters, the crystal structure of poly-ε-caprolactone, [–(CH2)5–CO–]n, was determined by interpretation of X-ray diffraction patterns. The crystallographic data are: a=7.47Å, b=4.98Å, c(fiber axis) =17.05Å, the orthorhombic space group P212121-D24, two molecular chains in the unit cell. The chain conformation of poly-ε-caprolactone is almost planar zigzag, but evidently deviates from the fully extended form, i.e., the CH2 sequences are planar but the plane of atoms of the ester group tilts slightly to the fiber axis. The molecular chains are arranged side by side in the same way as in polyethylene, but the carbonyl groups of the two chains in the unit cell are separated by an amount of 3/14c along the fiber axis.
It is widely accepted that achiral polymers crystallize in lamella helicoids of both handedness, whereas chiral polymers are known to form helicoids of only one handedness. By employing nanofocus X-ray diffraction, we show that the situation is more complex and that the lamella helicoids of achiral polymers can be in fact hybrid, i.e., containing a left- and a right-handed part. Moreover, our data demonstrate that the poly(trimethylene terephthalate) lamellae invert chirality at a certain crystallization temperature. The chirality correlates with the growth axis polarity (i.e., a versus −a) and handedness of the lamellar helicoid (L versus R) and can be conveniently expressed in terms of the chirality parameter pairs. In addition, the overall chain tilt does not affect the rate and sense of twisting, calling into question the premise of the Keith and Padden model. Instead, the inclination of the terminal segment of the crystalline stem protruding the lamellar surface is proposed to be the key factor controlling the surface stresses.
Banded spherulites are formed by crystallization of a chiral polymer that is end-capped with chromophore. Induced circular dichroism (ICD) of the chromophore can be found in the crystallized chiral polymers, giving exclusive optical response of the ICD. The ICD signals are presumed to be driven by the lamellar twisting in the crystalline spherulites, and the exclusive optical activity is attributed to the chirality transfer from molecular level to macroscopic level. To verify the suggested mechanism, the sense of the lamellar twisting in the crystalline spherulite is determined using PLM for the comparison with the ICD signals of the chromophore in the electron circular dichroism spectrum. The conformational chirality of the chiral polymer is determined by the vibrational circular dichroism spectrum. On the basis of the chiroptical results, evolution of homochirality from helical polymer chains (conformational chirality) to lamellar twisting in the banded spherulite (hierachical chirality) is suggested.
For the banded spherulites of poly(∈-caprolactone), PCL, grown from the blends with miscible polymers of polyvinyl butyral and poly(styrene-co-acrylonitrile), the effects of blended amorphous polymers on the band spacing have been examined experimentally. The results reconfirmed the strong influence of the second components even with small amount (c.a. 0.09 wt%). For the crystallization under the strong influence of the second components probably on the lamellar surface, we have examined the applicability of our modeling of spherulitic growth and its limit. Important findings in this paper are the followings: 1) On the confirmation of the applicability of the modeling for the amount of the second component small enough and the band spacing long enough. 2) On the violation of the predicted relationship of the modeling with increasing amount of the second component, which caused sharp decrease in the band spacing. 3) On the observation of the lower bound of the band spacing, to which the band spacing approached with the increase in the second component. With approaching the lower bound, the band spacing eventually became independent of other growth conditions such as crystallization temperature.
The crystallization behavior and morphology of poly(ϵ-caprolactone) (PCL)/poly(vinyl butyral) (PVB) blends containing carbon black (CB) were studied as functions of PVB and CB content. The presence of CB had no influence on the primary nucleation of PCL crystals or the spherulitic growth rate. They were only influenced by the blend ratio of PVB. The growth rates of spherulites were unchanged throughout the crystallization process, regardless of the CB content. The results indicate that the concentration of PCL at the front of growing spherulite remains constant during crystallization. The distribution of CB in the spherulites was observed using atomic force microscopy to explain these results. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 797–802, 1997
In poly(ε caprolacton) (PCL)/poly(vinyl butyral) (PVB) blend which forms band spherulite, some straight lines along the radial direction of the spherulites are observed under polarized optical microscope (POM). By using microbeam scanning WAXS, we have investigated the handedness of lamella twisting on the left and right sides of the straight line. As a result, we have observed that the manner of periodic change of 110 reflection in WAXS is reversed between left and right sides of the straight line with respect to the direction of the X-ray beam scanning. This result clearly shows that the straight line observed under POM is the boundary line of handedness in band spherulite, and further, that both handednesses co-exist within one spherulite. It also implies that the lamellae growing from one nucleus are twisted cooperatively only with neighboring lamellae but not with all lamellae in the spherulite of PCL/PVB.
Electrical properties of poly(ε-caprolactone) (PCL)/poly(vinyl butyral) (PVB) blends containing carbon black (CB) were studied as a function of a small amount of PVB content and a wide range of molecular weight of PVB. For samples with the same CB content, the intensity of positive temperature coefficient (IPTC, defined as the ratio of peak resistivity to resistivity at room temperature) of the blends was increased, with PVB content greatly and molecular weight of PVB weakly. As the band spacings of PCL spherulites in PCL/PVB blends decrease with PVB content and molecular weight of PVB, the changes of the positive temperature coefficient property are ascribed to the morphological difference (i.e., period of twisted lamellae) in the blends. We confirmed our previous conclusion that the origin of the positive temperature coefficient phenomenon is the changes of the distribution of the CB on the melting of the crystalline phase. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 193–199, 1998
We investigated lamellar twisting manner in a banded spherulite, the blend of poly-(ɛ-caprolactone) and poly-(vinyl butyral), with scanning microbeam X-ray diffraction. We obtained the diffraction contour intensity map with a scanning pitch of 1 μm by employing a rotation of a spherulite around its radial direction along which the microbeam scans. The results confirm that the twisting manner depends on the crystallization temperature and that it changes from continuous twisting to step-wise twisting with the increase of crystallization temperature. Moreover, we observed that the phase of long-period lamellar twisting advanced by about 15° compared to that of short-period lamella. In addition, it was confirmed that c-axis of packing structure was normal to lamella, which was represented by dominant short-period lamella.Graphical abstract
The study of twisting crystalline orientation in banded polymer spherulites has recently been revitalized by availability of optically active macromolecules that crystallize exclusively in isochiral (uncompensated) helical conformations. In such cases, handedness of twisting appears to be rigidly controlled by chirality of molecular conformation, and the twisting itself appears to derive from causes other than those believed to apply in optically inactive polymers. We discuss and evaluate methods for determining handedness of twisting, particularly in tightly banded spherulites, and illustrate them by application to poly(3-hydroxybutyrate), poly(3-hydroxyvalerate), and to enantiomeric polymer pairs in the cases of poly(epichlorohydrin) and poly(propylene oxide). Although rigidly controlled twisting in such polymers commonly shares the handedness of the underlying molecular conformation we have clear evidence in poly(3-hydroxyvalerate) that such a correlation is not universal.
A modified Surface Forces Apparatus was used for making rheological and tribological measurements of thin fluid films, and polybutadiene (PBD) of Mw ≈ 7000 was studied in detail. At low shear rates, PBD exhibits bulklike properties in films thicker than about 200 nm. In thinner films (200−20 nm), the shear viscosity ηeff and moduli G‘ and G‘‘ become quantitatively modified from those of the bulk. However, a sinusoidal input still produces a sinusoidal output, and a modified WLF representation remains applicable to such films. On entering the tribological regime (film thickness <30 nm) polybutadiene exhibits highly nonlinear behavior and yield points, indicative of phase transitions to “glassy” or “solidlike” states. Other features are (i) a shift of the slip plane, (ii) an additional normal force component, and (iii) the emergence of new scaling relationships for ηeff, G‘, G‘‘, and the friction force F as a function of frequency, sliding speed, and other system parameters.
The periodical cooperative twisting of lamellar crystals in poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) ring-banded spherulites was studied using mild methylamine vapor etching method. The samples were first dissolved in chloroform and then precipitated with methyl alcohol. The precipitates were collected by centrifugation and dried in vacuum at room temperature for 24 h. The configurational chiralities of all samples (including PHB and PHBV) were measured to be right using a polarimeter. The number-average molecular weight (Mn) and polydispersity index (Mw/M n) of PHBV with 12 mol % HV were measured to be 182 000 g/mol and 1.52, respectively. The isothermal crystallization of the films was performed by heating the sample at 190°C for 3 min and then cooling it to 90°C at a cooling rate of 100°C min-1. Both extinction bands and Maltese cross are clearly observed under crossed-polarized light which are similar to the classical ring banded spherulites in other different polymeric materials.
The crystallization of poly(d-lactide) (PDLA) and poly(l-lactide) (PLLA) in ultrathin films (15 nm) has been followed between 125 and 160 °C using in situ atomic force microscopy. Using a forced nucleation technique, edge-on lamellae were observed, showing a curvature which is related to the polymer chirality. In the case of PLLA, the lamellae are S-shaped, contrary to the PDLA lamellae which are Z-shaped. This behavior was also observed on TEM pictures of PLLA and PDLA films crystallized in the same conditions, without any external nucleation. As shown by electron diffraction patterns, the crystalline unit cells of the two enantiomers are identical. For the first time, a relationship has been established between the molecular chirality of poly(lactide)s and their macroscopic behavior. Moreover, the direction of curvature of the lamellae can be linked with the sense of twisting of the poly(lactide) lamellae in banded spherulites, and the temperature dependence of the radius of curvature can be correlated with the distance between the extinction rings. Those observations are consistent with Keith and Padden's model since the curved crystals in ultrathin films can be considered as “half-lamellae”, which give, when associated together, twisted complete lamellae.
We report on the isothermal crystallization behavior of thin (film thickness d < 500 nm) and ultrathin (d < 100 nm) films of poly(ethylene oxide) (PEO), as well as pyrene end-labeled PEO, on native silicon studied by in situ hot stage atomic force microscopy (AFM). Individual lamellae were imaged during crystallization and melting. Using AFM, we have directly measured lamellar growth rates, lamellar thicknesses, and melting ranges as a function of film thickness (ca. 15−>500 nm), crystallization temperature (40−62 °C), and molar mass (11−100 kg/mol). On the basis of the Hoffman−Weeks extrapolation, the Gibbs−Thomson equation, and the Hoffman−Lauritzen theory, we show that the crystallization of PEO in thin and ultrathin films can be described with the same laws as the bulk crystallization. In addition, we find that the equilibrium melting points and surface free energies of the fold surfaces agree quantitatively with literature data for bulk crystallization and hence are not altered due to confinement in ultrathin films. However, there is a monotonic decrease of lamellar growth rates with decreasing film thickness for films thinner than ca. 250 nm. The growth rates decrease to below 1% of their bulk value in the thinnest films; this is attributed to an increase in glass transition temperature of up to 30 °C for the confined PEO and the concomitant reduction of molecular mobility.
Electron diffraction intensities of two zones, (Okl) and (hhl), obtained from poly(ε-caprolactone) epitaxially crystallized on benzoic acid, were used for a quantitative crystal structure determination. Combining the data from epitaxially oriented samples with those obtained from solution-grown crystals permitted a nearly complete three-dimensional structure analysis to be carried out. The structural analysis demonstrates that the chains pack in the nonplanar structure in space group P212121 proposed by Chatani et al. rather than an alternative planar chain conformation. The R-factor is 0.199.
Unexpected morphological changes have been found in a number of aliphatic polyesters (poly(ε-caprolactone) and the suberate, azelate, and sebacate polyesters of ethylene glycol) upon blending with small concentrations (approximately 1%) of compatible polar polymeric diluents, notably poly(vinyl butyral) and poly(vinyl chloride). It is believed that adsorption of diluent on crystal boundaries (growth faces and fold surfaces of lamellae) underlies most of these effects. Preliminary experiments indicate that similar morphological changes may occur in nylon 66 and nylon 610 when blended with poly(vinylpyrrolidone).
The lamellar morphology in banded spherulites of poly(ε-caprolactone) blended with an amorphous polymer, poly(vinyl butyral), was investigated by three-dimensional transmission electron tomography. It showed a local lamellar twist on a smaller scale than the band spacing by 2 orders of magnitude. It also indicated wavy lamellae and frequent variation in the direction of the lamellar plane. All these results indicated an S-profiled lamellar structure; that is, the cross section perpendicular to the lamellar growth direction was S-shaped. S-profiled lamellae show these structures when they are sliced at a certain angle to the lamellar surface direction. Lamellar branching was also observed, but no screw dislocations that led to the formation of extinction rings were observed in this work. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1122–1125, 2007
Crystalline and chiral supramolecular complexes can be self-assembled from linear polyethyleneimine and D- or L-tartaric acid. These complexes act as templates to direct nanofiber-based chiral silica, which showed remarkable chiraoptical properties and remained chiral even after sintering at ultrahigh temperatures. Achiral molecules and gold nanoparticles bound to the chiral silica became active in CD spectra owing to induced chirality.
A possible connection is suggested and explored between non-planer crystal habits in banded polymer spherulites and disordered chain folding in polymers crystallized relatively rapidly from the melt. It is proposed that, when lateral growth faces and fold surfaces are not orthogonal (because chain stems are tilted with respect to the lamellar normal), different degrees of disorder develop at opposite fold surfaces. Resulting differences in surface stress give rise to bending moments, but these are likely to be of transient existence. It is shown that, on this basis, an appealingly simple rationale can be developed to account for the complex and hitherto puzzling observations of Bassett and Hodge on polyethylene spherulites, including S-bending and non-uniform axial twisting in lamellae, and also an empirical correlation between these deformations. Much depends, however, upon interactions between interleaved crystals and upon relaxation of bending moments. Existing evidence in support of the rationale is outlined. Implications with respect to polymers other than polyethylene, and to kinetics of crystallization in general, are discussed briefly. Calculations concerning axial twisting under the influence of surface stresses suggest that the twisted crystals incorporate twist boundaries, possibly formed by aggregation of dislocations generated during the growth of what must initially be relatively disordered crystals. The ‘chiral’ factor determining handedness of twisting in a given crystal is the direction in which chain stems tilt with respect to the lamellar normal.
With the recent availability of chiral main chain polymers in the form of both of its enantiomers, it is demonstrated that the molecular chirality determines the sense of the crystal rotation underlying the banded structure of spherulites — an issue of a century standing. In addition, an attempt is made to provide an historical perspective, and the role of asymmetries on all dimensional levels of the structure hierarchy is discussed.