Unbalanced Surface Stresses Research Articles

Unique Banded Cylindrites of Polyoxymethylene/Poly(butylene succinate) Blends Induced by Interfacial Shear

Unique banded cylindrites of polyoxymethylene/poly(butylene succinate) blends were induced by the intense interfacial shear, which was applied by pulling a single glass fiber (GF) with non-nucleation ability. The crystalline morphology and structure of banded cylindrites were quantitively correlated with the shear rates to provide revealing insights in shear-induced nucleation and lamellar assembly. Densely packed row-nuclei emerged on the surface of pulled GF and the nucleation density increased dramatically with the shear rate. The subsequent lamellar extension and twisting were restricted perpendicularly to the GF axis, resulting in the formation of banded cylindrites. As the shear rate increased, the band spacing showed a linear decrease and the eccentricity of elliptical bands showed a linear increase. It was attributed to the augment of unbalanced surface stress caused by steric hindrance of serried nuclei. While the growth rate of banded cylindrites remained constant regardless of the shear rate, s...

The sunken morphology of poly(lactic acid) stereocomplex spherulite

AbstractThe mechanical properties and microstructure of poly(lactic acid) (PLA) stereocomplex spherulite at the micrometric scale were investigated using nanoindentation and synchrotron radiation scanning X‐ray microdiffraction, which exhibits a new growth habit with more and more compact lamellae. Using atomic force microscopy and laser scanning confocal microscopy, a unique sunken morphology was found. Unbalanced surface stress and volume shrinkage are proposed as the main factors for the sunken morphology, which is also contributed by a novel growth regime driven by the hydrogen bonding between two PLA enantiomers. © 2019 Society of Chemical Industry

Organization of Twisting Lamellar Crystals in Birefringent Banded Polymer Spherulites: A Mini-Review

In this mini-review, we summarize the evidences of lamellar twisting in the birefringent banded polymer spherulites demonstrated by various characterization techniques, such as polarized optical microscopy, real-time atomic force microscopy, micro-focus wide angle X-ray diffraction, etc. The real-time observation of lamellar growth under atomic force microscopy unveiled the fine details of lamellar twisting and branching in the banded spherulites of poly(R-3-hydroxybutyrate-co-17 mol% R-3-hydroxyhexanoate). Organization of the twisting lamellar crystals in the banded spherulites was revealed as well. The lamellar crystals change the orientation via twisting rather than the macro screw dislocations. In fact, macro screw dislocation provides the mechanism of synchronous twisting of neighboring lamellar crystals. The driving force of lamellar twisting is attributed to the anisotropic and unbalanced surface stresses. Besides molecular chirality, variation of the growth axis and the chemical groups on lamellar surface can change the distribution of the surface stresses, and thus may invert the handedness of lamellar twisting. Thus, based on both experimental results and physical reasoning, the relation between crystal chirality and chemical molecular structures has been suggested, via the bridge of the distribution of surface stresses. The factors affecting band spacing are briefly discussed. Some remaining questions and the perspective of the topic are highlighted.

Ultrathin film crystallization of poly(ε-caprolactone) in blends containing styrene-isoprene block copolymers: The nano-rose morphology

We report the discovery of a new nanostructured morphology in ultrathin blend films of semi-crystalline poly(ε-caprolactone) (PCL) and various styrene-isoprene block copolymers. The nano-rose morphology is visualized by atomic force microscopy as a monolayer of approximately 500 nm diameter crystalline spirals. Our finding that the nano-rose morphology occurs in a narrow film thickness window commensurate with the crystalline lamellar thickness suggests that this distinctive morphology results from redirection of growing PCL crystalline lamellae due to confinement effects during film casting. The nano-rose morphology was located in blends with several block copolymers of various architecture but was absent in blends with the individual homopolymer components. PCL molecular weight did not affect nano-rose size or film thickness window. However, solvent choice was an important factor, with nano-rose formation favored in non-polar and weakly polar solvents. This work supports theories that unbalanced surface stresses and impurity exclusion cause polymer crystals to grow unconventionally.

Strong enhancement of the twisting frequency of achiral orthorhombic lamellae in poly(ε-caprolactone) banded spherulites via evaporative crystallization

Besides the rather poor and irregular bands with large periods formed at high temperatures, banded spherulites were seldom spotted in pure poly(e-caprolactone) (PCL). Herein, well-ordered extinction banded spherulites with the smallest spacing less than 10 μm are encountered in solution-cast PCL films via evaporative crystallization, and structural analyses unveil that such narrow bands suggest strong twisting of achiral orthorhombic PCL lamellae. It is proposed that solvent evaporation can enhance dramatically the disorder on opposite lamellar surfaces that stems from a slightly asymmetrical structural feature from the disposition of ester groups within a PCL unit cell, which gives rise to a striking enhancement of the unbalanced surface stresses resulting in the intensification of twisting frequency. The dependence of band spacing on crystallization conditions is also investigated to verify this intensifying effect. The present findings are envisaged to improve our understanding of the mechanical origin of unequal surface stresses for periodic twisting of polymer lamellar crystals.

Influence of Flexible Poly(Trimethylene Sebacate) Segments on the Lamellar Twisting Behavior of Poly(Trimethylene Terephthalate) Ring-Banded Spherulites

Ring-banded spherulite is an important morphology of polymer crystals, and its formation mechanism is still under debate. In this study, we synthesized poly(trimethylene terephthalate)-co-poly(trimethylene sebacate) copolyesters (PTT-co-PTS) by melting copolycondensation and studied the influence of PTS segments on the crystallization and ring-banded morphology of poly(trimethylene terephthalate) spherulites by using a moderate etching technology. It is found that he band spacing of PTT-co-PTS spherulites decreased sharply with PTS segments content. It is confirmed by experiments that there are twofold reasons for the sharp decrease of band spacing in PTT-co-PTS ring-banded spherulites. First, PTS segments acted as diluents for PTT molecules, which intensified the unbalanced surface stress at the fold surfaces for the PTT lamellar twisting. Second, incorporation of PTS segments decreased the thickness of PTT lamellae, and the greater the PTS content, the thinner the lamellae. Both these two changes signif...

Macroscopically oriented hierarchical structure of the amphiphilic tetrathiafulvalene molecule

Since the physical properties of electronic and biological molecules strongly depend on the nature of molecular self-assembly and organization, it is essential to control their molecular packing structure and morphology on the different length scales. In this aspect, a programmed amphiphilic tetrathiafulvalene (TTF) molecule (abbreviated as amph-7TTF14) was newly designed and synthesized. Differential scanning calorimetry (DSC) combined with one-dimensional (1D) wide angle X-ray diffraction (WAXD) techniques revealed that a highly ordered crystalline phase emerged below the isotropic phase. From the 2D WAXD pattern of the macroscopically oriented amph-7TTF14 film, the crystal structure was identified to be a monoclinic unit cell. The face-to-face π–π interaction between TTF groups and the nanophase separation between rigid TTF groups and flexible hydrophobic alkyl and hydrophilic tri(ethylene oxide) tails were the main driving forces for the self-assembly of amph-7TTF14. The morphological observations using transmission electron microscopy (TEM), atomic force microscopy (AFM), and polarized optical microscopy (POM) indicated that the amph-7TTF14 formed not only flat ribbons but also scrolls and helixes, in which the ribbons further aggregated to create the fibrous hierarchical structures. Based on the experimental results and careful analyses, it was realized that the scrolls and helices were induced by the unbalanced surface stresses generated during the crystallization process. When the macroscopically oriented fibrous hierarchical structure is properly applied to the electrooptical and bio-mimetic devices, the targeted physical properties may be significantly improved and tuned for the specific practical applications.

Rhythmic Growth Combined with Lamellar Twisting Induces Poly(ethylene adipate) Nested Ring-Banded Structures.

Previously, poly(ε-caprolactone) (PCL) classical ring-banded spherulites with periodic twisted lamellae and nonclassical concentric ringed structures induced by rhythmic growth were obtained by modulating the competition between diffusion flux and spherulitic growth. In this study, the modulation of diffusion flux and spherulitic growth on ring-banded structures is further studied, and hierarchical nested ring-banded patterns with a banded structure nested in the ridge rings of the other concentric ringed structure are prepared during slow solvent evaporation in poly(ethylene adipate) (PEA) solution films. The structural characterizations reveal that the big concentric ringed structure derives from periodic variation of thicknesses and that the small inner banded structure consists of periodic twisted lamellae. A diffusion-induced rhythmic growth mechanism and an unbalanced surface stress induced lamellar twisting model are proposed to illustrate the formation of the big concentric and the inner banded structures, respectively.

Scrolled Polymer Single Crystals Driven by Unbalanced Surface Stresses: Rational Design and Experimental Evidence

To understand the formation mechanism of nonflat polymer single crystals, two types of triblock copolymers with a middle crystalline block and two amorphous, immiscible end-blocks were designed and synthesized. Specifically, polystyrene-block-poly(ethylene oxide)-block-poly(1-butene oxide) and polystyrene-block-poly(ethylene oxide)-block-polydimethylsiloxane were examined. When the end-blocks possess different volumes and are microphase separated onto the opposite sides of the single crystal lamella formed by the middle crystalline block, unbalanced surface stress can be generated. As a result, large scrolled single crystals (∼80 μm) were grown from dilute solution using the self-seeding procedure at low supercoolings. The scrolling direction was identified to be along the (120) planes based on transmission electron microscopy (TEM) observations of the sedimented scrolled single crystals, which is in line with the fact that the scrolling occurs along the planes with the highest coefficient of thermal expansion. Using high-resolution TEM at high tilting angles, three layers of distinct chemical compositions can be clearly identified from the edges of the single crystals after RuO4 staining. It suggests the formation of microphase separated domains of the amorphous end-blocks on the opposite sides of PEO single crystals. Although the tethering densities of these amorphous end-blocks are identical, their reduced tethering densities are different, resulting in dissimilar volumes and surface crowdedness on the opposite sides of PEO single crystal. The unbalanced surface stress is thus generated to scroll the lamellar single crystal. Macroscopically, based on the observed curvature and the assumption of a solid plate cylinder, the strain energy for each individual single crystal with lateral size of 80 μm was estimated to be ∼3 × 10–9 erg, which, though small, is sufficient to maintain the scrolling of single crystal in solution at room temperature (the thermal energy is approximately kT ∼ 4 × 10–14 erg). Microscopically, the difference of the reduced surface free energy of the tethered blocks at the opposite sides of the PEO lamellar single crystal is analyzed and understood to be the driving force of the scrolling.

Increasing Lamellar Twisting Frequency with Poly(lactic acid) Segments Incorporation in Poly(trimethylene terephthalate) Ring-Banded Spherulites

In this study, a series of random poly(trimethylene terephthalate)-co-polylactide copolyesters (PTT-co-PLA) were synthesized by melting copolycondensation, and the influence of PLA segments on the lamellar twisting frequency of PTT was studied. Etching technology was used to study lamellar morphologies of ring-banded spherulites and distinct images of twisting lamellae in crystalline PTT and PTT-co-PLA were observed by scanning electron microscopy. Comparing with PTT homopolymer, ring-banded spherulites in PTT-co-PLA have much smaller band spacing and the band spacing decreases sharply with the increase of PLA segments content. It was confirmed by experiments that there are 2-fold reasons for the sharp decrease of band spacing in PTT-co-PLA ring-banded spherulites. First, the unbalanced surface stress at the fold surfaces for the PTT lamellar twisting can be intensified by amorphous PLA segments. Second, the thickness of PTT lamellae decreases with the increase of PLA segments content. Both changes significantly enhance the helical twisting frequency of twisted lamellae.

Microstructure of Banded Polymer Spherulites: Studies with Micro-Focus X-ray Diffraction

Micro-beam X-ray diffraction has been used to investigate the texture of banded spherulites of melt-crystallized poly(trimethylene terephthalate), PTT, formed in films of approximately 30 to 50 μm thickness. The WAXS micro-diffraction patterns show that at the local scale, the PTT texture is close to that of a single crystal. In agreement with previous studies using selected-area electron diffraction, it is shown that the crystal growth direction is parallel to the a-axis of the unit cell. When plotted as a function of the distance to the spherulite center, the intensity of different diffraction peaks reveals the same periodicity. This means that the lamellar twist is strictly uniform. The latter observation is more compatible with the model explaining the twist as a result of unbalanced surface stresses than that of isochiral giant screw dislocations. The main features of the experimental diffractograms can be understood using the numerical approach, which is developed in the approximation of a purely geometric broadening of X-ray reflections. In particular, the simulation can predict the sequence of appearance of different diffraction peaks and their shape on the 2D micro-diffraction patterns.

The nucleation effect of modified carbon black on crystallization of poly(lactic acid)

AbstractThe nonisothermal crystallization of poly(lactic acid) (PLA), PLA/carbon black (CB), and PLA/modified carbon black (MCB) composites were investigated using differential scanning calorimetry (DSC) analysis. The rate of crystallization and the spherulitic morphologies of the PLA and PLA/MCB composites during isothermal crystallization were investigated using DSC and observed by means of polarizing optical microscopy (POM), respectively. The results show that either CB or MCB acts as an efficient nucleating agent for PLA. The nucleation activities of CB and MCB were quantitatively determined. It is shown that MCB has higher nucleating activity than CB in PLA. An isoconversional method correlates the temperature dependence of the effective activation energy and was used to evaluate the effective activation energy of PLA and PLA/MCB composites. It is confirmed that MCB advances the nuclei density and promotes the crystallization rate of the PLA matrix significantly. In addition, an interesting phenomenon of the periodic cracks of PLA spherulites observed by using POM is reported. The unbalanced surface stresses arising from growth features and thermal shrinkage may be the two main factors accounting for the formation of target pattern cracks. POLYM. ENG. SCI., 50:1658–1666, 2010. © 2010 Society of Plastics Engineers

Left- or Right-Handed Lamellar Twists in Poly[(R)-3-hydroxyvalerate] Banded Spherulite: Dependence on Growth Axis

Novel morphology of banded spherulite was observed in poly[(R)-3-hydroxyvalerate) (PHV) during its isothermal crystallization at temperatures between 50 and 80 °C. A novel optical technique, Polscope was applied to obtain both the birefringence retardation and azimuth angle of the slow optical axis at each pixel in the micrograph of the polycrystalline specimen. It is revealed that two different morphological regions, normal and eye-like regions, are derived from different growth axes of the lamellae along the radial direction. In the normal region, lamellae grow and twist along b axis, leading to the alternative strong and weak positive birefringence. In the eye-like region, they grow and twist along a axis, resulting in the alternative positive and negative birefringence. It is of particular interest to note that PHV lamellae in the two regions exhibit the opposite twist senses. To the best of our knowledge, it is the first report that an optically pure chiral polymer contains two lamellar twist senses in one spherulite. We attribute the driving force of lamellar twist in PHV spherulite to the unbalanced anisotropic surface stresses. A mechanical model is proposed to interpret the opposite twisting senses along the two orthogonal growth axes. The results demonstrate that the growth axis is one of the factors determining the twist handedness of the lamellae of chiral polymers.

Tailor‐Made Onionlike Stereocomplex Crystals in Incompatible, Enantiomeric, Polylactide‐Containing Block Copolymer Blends

Stereocomplexes formed by blending enantiomeric PLA block copolymers have demonstrated great potential for applications in biomedical devices. Here, we successfully synthesized well-defined enantiomeric PLA containing block copolymers by living ring-opening polymerization of L- and D-lactides from hydroxyl-terminated hydrophilic [poly(ethylene oxide) or PEO] and hydrophobic [poly(ethylene-co-1,2-butylene) or PEB] oligomers. Quantitative stereocomplex formation was achieved by equimolar mixing of the incompatible PEO-b-PLLA and PEB-b-PDLA. Intriguingly, in the blend of PEB-b-PDLA and PEO-b-PLLA with different PEB and PEO molecular weights, onion-like stereocomplex crystals were observed because of unbalanced surface stresses caused by different PEO and PEB molecular weights.

Comments on: ‘A critical assessment of unbalanced surface stresses: Some complementary considerations’, by DC Bassett

The complementary considerations developed by Bassett are welcome additions to our paper that surveyed the origin of twist in polymer lamellae. They point to alternate mechanisms than those suggested by Keith and Padden. Polyethylene is not, however, an archetypical system, and the mechanisms uncovered may not have general value. Development of non-planar lamellar geometries associated with unbalanced surface stresses can have different origins. They all rest on structural asymmetries (with chains normal or not to the fold surface), either produced directly on crystallization, or after reorganization. This is best illustrated by the analysis of the structural origin of scrolled lamellae, for which different origins of unbalanced surface stresses have been uncovered for the three systems analyzed so far. In ultimate analysis, the unbalanced stresses are linked with the unbalanced fold structure and its orientation relative to the growth direction of the lamella.

A critical assessment of unbalanced surface stresses: Some complementary considerations

The wide-ranging survey of twisted growth in polymers by Lotz and Cheng cites extensive evidence consistent with the relief of surface stress being the underlying cause. This complementary note contributes to the discussion by making three main points. First, it is necessary to go further and explain the key issue of how a consistent twist is maintained when, as commonly, this habit has a lower symmetry than the crystallographic lattice. Detailed study has shown that, in polyethylene, this occurs by reorganization of the initial fold surfaces. Second, the suggested explanation by Keith and Padden that, in polyethylene, the asymmetric habit derives from molecules adding to lamellae with inclined fold surfaces is invalid being doubly inconsistent with observation. Third, twisting has now been linked to faster growth by study of row structures in polyethylene. This produces inherently rough fold surfaces in Régime II whose internal stresses drive reorganization and twisting. For slower (Régime I) growth, fold surfaces form with and maintain ordered packing so providing no basis for twisting. These new insights radically alter the context of twisted growth and provide a firm factual basis for further work.

A critical assessment of unbalanced surface stresses as the mechanical origin of twisting and scrolling of polymer crystals

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.This contribution reviews evidence that has accumulated in favor of lamellar twist induced by surface stresses that result from differential congestion of fold surfaces, as suggested initially (in 1984) and advocated for many years by Keith and Padden. Such differences in fold surface structure are occasionally amenable to experimental (even if only qualitative) verification, as illustrated by polymer decoration of polyethylene single crystals. Twist is expected when a two-fold symmetry parallel to the growth direction exists in the lamellar structure (crystalline core and fold surface). This symmetry often stems from chirality: most frequently atomic (configurational) or stem (conformational) chirality but chirality (or at least asymmetry) may also be introduced by chain tilt.Possible origins of twisting in chiral polymers are also reviewed. In β sheets of fibrous proteins, the origin of twist stems from the atomic chiral centers in the crystalline core of the lamellae and its transfer to higher structural levels via the strong structural identity of the hydrogen-bonded β sheets. However, in a series of synthetic liquid-crystalline main-chain nonracemic chiral polyesters, the lamellar twist sense depends on the odd or even numbers of atoms in the aliphatic segment. For these and other more flexible chiral polymers, often with helical chain conformation, twisting appears to result from surface stresses associated with different fold structure or conformations at opposite fold surfaces, as suggested by a preliminary analysis of the Form III of isotactic poly(1-butene). Such differences in fold conformations result from, but are not directly related to, the specific helical hand of the polymer since they rest on the details of the chain conformation as it reaches the fold surface. This analysis accounts for the lack of one-to-one correspondence between configurational or conformational chirality of the polymer and lamellar twist sense (the one-to-one correspondence applies however for stereoenantiomers of a given polymer).Twist is not the only known non-planar geometry of polymer lamellae. In a few cases, the lamellae are scrolled. Scrolling of polymer lamellae is also easily accounted for by the existence of surface stresses when the two-fold symmetry parallel to the growth direction is absent. Such surface stresses are again linked to disparities in fold volume, as first suggested for poly(vinylidenefluoride) in its γ Form and later for two long paraffins substituted near their middle carbon atom and that crystallize in hairpin fashion, and for scrolled crystals of polyamide 66.The different nature and structure of polymer crystal fold surfaces, therefore, offer an unusual opportunity to decouple surface and bulk contributions and to analyze the origin of non-planar lamellar geometries at a sub-molecular level. Fold structure disparities and resulting unbalanced surface stresses provide a unified explanation for the formation of non-planar (both twisted and scrolled) lamellar crystals. They account for both the diversity of lamellar morphologies produced under the same crystallization conditions and for the similarity of lamellar morphologies produced under very different crystallization conditions.