Flat-on Lamellae Research Articles

Continuous Spectrum of Morphologies and Phase Behavior across the Contact Zone from Poly(l-lactide) to Poly(d-lactide): Stereocomplex, Homocrystal, and Between.

The enantiomeric ratio is a key factor affecting the crystallization behavior and morphology of poly-l-lactide/poly-d-lactide (PLLA/PDLA) blends. Despite a number of studies on crystallization of nonequimolar PLLA/PDLA blends, a full picture of the effect of the L/D ratio is still lacking. Here, we put the two enantiomers in contact and allow interdiffusion above the melting point of the stereocomplex crystal (SC) to prepare samples with a continuously changing L/D ratio from enantiopure PLLA (ratio 0/100) to enantiopure PDLA (100/0). Using polarized optical microscopy, atomic force microscopy, and microbeam X-ray diffraction, the continuous spectrum of morphologies and phase behaviors across the contact zone is investigated. The blend morphology shows clear evidence of "poisoning by purity" of SC crystallization at all blend compositions. The low birefringence of the 50/50 SC is found to be due to the meandering of broken edge-on lamellae. Its further decrease to near zero as L/D deviates further away from 50/50 is explained by transition from radial edge-on lamellae to fully random meandering lamellae, then to mixed flat-on lamellae, and finally to submicron-sized axialites. In comparison with the smooth and straight homocrystal (HC) lamellae of pure enantiomers, the lamellae in the blends often have serrated edges caused by pinning by rejected excess enantiomer acting as an impurity during lamellar growth. A feature of the binary phase diagram is pure enantiomers acting as an impurity to the SC and counter-enantiomer acting as an impurity to homocrystallization of the enantiomers. Crystallization was found to be most suppressed at 99% enantiomeric purity, where the amount of the counter-enantiomer is insufficient for creation of SC nuclei and HC growth is inhibited by the small amount of the enantio-impurity. These and other intriguing results are less likely to be noticed without the continuous composition gradient of the contact sample.

Dependence on Film Thickness of Guest-Induced c Perpendicular Orientation in PPO Films.

For poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films exhibiting nanoporous-crystalline (NC) phases, c⊥ orientation (i.e., crystalline polymer chain axes being preferentially perpendicular to the film plane) is obtained by crystallization of amorphous films, as induced by sorption of suitable low-molecular-mass guest molecules. The occurrence of c⊥ orientation is relevant for applications of NC PPO films because it markedly increases film transparency as well as guest diffusivity. Surprisingly, we show that the known crystallization procedures lead to c⊥ oriented thick (50–300 μm) films and to unoriented thin (≤20 μm) films. This absence of crystalline phase orientation for thin films is rationalized by fast guest sorption kinetics, which avoid co-crystallization in confined spaces and hence inhibit formation of flat-on lamellae. For thick films exhibiting c⊥ orientation, sigmoid kinetics of guest sorption and of thickening of PPO films are observed, with inflection points associated with guest-induced film plasticization. Corresponding crystallization kinetics are linear with time and show that co-crystal growth is poorly affected by film plasticization. An additional relevant result of this study is the linear relationship between WAXD crystallinity index and DSC melting enthalpy, which allows evaluation of melting enthalpy of the NC α form of PPO (ΔHmο = 42 ± 2 J/g).

Hydrolytic and enzymatic degradation of biobased poly(4-hydroxybutyrate) films. Selective etching of spherulites

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.

Formation of Asymmetric Leaf-Shaped Crystals in Ultrathin Films of Oriented Polyethylene Molecules Resulting from High-Temperature Relaxation and Recrystallization

We melted, annealed, and recrystallized ultrathin films of oriented polyethylene (PE) molecules prepared by melt-drawing. A large number of randomly oriented asymmetric leaf-shaped crystalline structures consisting of preferentially oriented lamellae were formed simultaneously, as observed by optical and atomic force microscopies. The structural arrangement within these leaf-shaped crystalline structures was identified by grazing incidence X-ray diffraction. These structures consisted of two distinct sections, both differing strongly from circular spherulites obtained by crystalizing an isotropic melt. Besides, regions containing groups of stacks of slightly inclined but well-aligned flat-on lamellae and regions of less orderly arranged edge-on lamellae were found. When increasing the annealing temperature and/or annealing time, a change in morphology from the leaf-shaped crystalline structures to spherulites with two symmetric “eyes” was observed. Intriguingly, the annealing times required for such a change in crystalline morphology were about four orders of magnitude longer than the longest bulk relaxation time (reptation time). Because the appearance of spherulites indicates that films became equilibrated and reached a state of an isotropic melt before recrystallization, we may conclude that oriented PE chains in ultrathin films possessed a long-term memory of the preparation-induced chain stretching. Considering that the morphology and relaxation kinetics of PE films depended appreciably on the substrate properties and film thickness, we conclude that the formation of asymmetric leaf-shaped crystalline structures was also affected by the interaction of PE chains with the substrate and spatial confinement. The present results may shed new light on slow relaxation and reorganization processes encountered when long-chain polymers became oriented during sample processing.

Lamellar orientation in isotactic polypropylene thin films: a complement study via grazing incidence X-ray diffraction and surface/cross-sectional imaging

The crystalline structures of isotactic polypropylene (iPP) thin films were investigated using grazing incident wide-angle X-ray diffraction (GIWAXD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The out-of-plane (110) reflection split with 99° inclination in the GIWAXD pattern. The film surface was covered with flat-on mother lamellae with orthogonal daughter lamellae. The cross-sectional TEM image and the fast Fourier transform-processed image showed vertically aligned daughter lamellae and cross-hatched lateral mother lamellae. Flat-on lamellae may be preferentially produced at the substrate interface, after which the mother lamellae may yield daughter lamellae from the ac plane to give vertically aligned lamellae. The daughter lamellae yield in-plane lamellae with flat-on alignment from the ac plane, resulting in the global growth of a cross-hatched lamellar structure.

Nonbirefringent bands in thin films of a copolymer melt: rapid rhythmic crystal growth with an unusual crystal–melt interface

Rhythmic-growth-induced nonbirefringent bands that are comprised of repetitive stacks of discrete flat-on lamellae along the growth directions are reported in a thin film of an asymmetric PCL-b-PEO during isothermal melt crystallization.

Nanoscale Lamellar Assembly and Segregation Mechanism of Poly(3-hydroxybutyrate)/Poly(ethylene glycol) Blends

Atomic force microscopy coupled with infrared spectroscopy (AFM-IR) was combined with scanning electron microscopy (SEM) to investigate the tridimensional lamella assembly and polymer segregation mechanism at the nanoscale of equimolar crystalline/crystalline poly(3-hydroxybutyrate) (PHB) and poly(ethylene glycol) (PEG) blends, isothermally crystallized at different temperatures. The combination of solvent etching and thermal quenching-induced fractures of spherulites enabled the direct visualization of the lamella assembly and interior 3D spherulitic structure by SEM. Two different spherulitic patterns (banded and cone-like) were investigated after removing the PEG by ethanol etching. The banded spherulites are composed of perpendicularly oriented lamellae while flat-on lamellae are observed in the valleys. The transition ridge–valley region contains right-handed, helically twisted lamellae. Cone-like spherulites are composed of highly ordered lamellae about 500 nm deep from the sample–air interface, and...

Temperature-Dependent Recrystallization Morphologies of Carbon-Coated Isotactic Polypropylene Highly Oriented Thin Films

Temperature-dependent recrystallization morphologies of carbon-coated iPP oriented thin films were studied by electron microscopy combined with electron diffraction. It was found that the vacuum evaporated carbon layer always results in oriented melt-recrystallization of the preoriented iPP thin films. The morphology of the recrystallized carbon-coated iPP thin films is, however, dependent on both melting and crystallization temperatures. At low melting temperatures, e.g., lower than 200 °C, parallel-aligned edge-on lamellae with the same molecular chain orientation as the prepared samples were always generated regardless of crystallization temperature. When melting at 300 °C and crystallization at low temperature, e.g., 120 °C, both edge-on and flat-on lamellae were observed. At elevated crystallization temperature, e.g., 150 °C, the flat-on iPP crystals show more or less a regular lath shape. At the same time, quadrites caused by wide-angle lamellar branching originating from homoepitaxy of α-iPP were s...

Lamellar and spherulitic crystallization of poly(s-2-hydroxybutanoic acid) and its stereocomplexes

Crystallization kinetics of poly (S-2-hydroxybutanoic acid) [P(S2HB)] and P(S2HB)/P (R2HB) stereocomplexes were investigated by in situ atomic force microscopy, polarized optical microscopy, and differential scanning calorimetry. For the homopolymer, an equilibrium melting temperature Tm° of 130.3 °C is obtained according to the Hoffman-Weeks method. In the case of thin films of 50 nm in thickness, a large number of edge-on lamellae form rapidly on the edge of the scratch line induced by the AFM tip. The edge-on lamellae change their orientation to flat-on lamellae at crystallization temperatures equal or higher than 75 °C. The lamellar and radial growth rates exhibit a bell-shaped temperature dependence with a maximum around 400 ± 50 nm/min at a crystallization temperature of 77 ± 2 °C, in contrast to the stereocomplexes whose growth rates peak at 19.6 μm/min. Analysis of the growth rates using the Lauritzen-Hoffman model shows that crystallization of homopolymers and stereocomplexes occurs in Regime II, with an end surface free energy σe of 6.2 × 10−2 J/m2 in the first case and 7.6–8.5 × 10−2 J/m2 in the second, and work of chain folding q between 18 and 24 kJ/mol for the homopolymer and 26–30 kJ/mol for the stereocomplexes, which is in the range of moderately stiff chains without bulky side-groups.

Evolution of Intermediate and Highly Ordered Crystalline States under Spatial Confinement in Poly(3-hydroxybutyrate) Ultrathin Films

The crystallization behavior and crystalline structures of poly(3-hydroxybutyrate) (PHB) ultrathin films (∼52 nm) under spatial confinement were investigated using infrared reflection–absorption spectroscopy (IR-RAS) and two-dimensional grazing incidence X-ray diffraction (2D-GIXD). Intermediate and highly ordered crystalline states were observed during heating and melt-cooling of these films. In the ultrathin films, the intermediate state was noticeably stable at lower temperatures, whereas the highly ordered state was more stable at higher temperatures. A transformation from the intermediate state into the highly ordered state occurred as the temperature increased, as the crystals in the intermediate state acquire sufficient thermal energy to overcome the energy barrier. 2D-GIXD results show that the intermediate state was dominant in edge-on lamellae configuration where the crystallographic b-axis is normal to the film surface. Meanwhile, the highly ordered state was predominant in flat-on lamellae con...

Crystal and multiple melting behaviors of PCL lamellae in ultrathin films

In this paper, isothermal crystallization at different temperatures and multiple melting of four molecular weights (MW) PCL ultrathin films were investigated by using Atomic Force Microscopy (AFM). The results showed that: two different crystal structures, Flat-on and Edge-on lamellae, were simultaneously formed in PCL ultrathin films when the Tc lower than 30°C. During the heating process, Edge-on lamellae was melted firstly. This demonstrated that the multiple melting behavior of PCL in ultrathin films was caused by the two different lamellar structures. With the increase of MW, or with the decrease of Tc, PCL chains in ultrathin films changed from Flat-on to Edge-on, and the controlled factors of growth process in ultrathin films transformed from surface nucleation-limited (NL) to melt diffusion-limited (DL).

Branched Crystalline Patterns of Poly(ε-caprolactone) and Poly(4-hydroxystyrene) Blends Thin Films.

The chain organization of poly(ε-caprolactone) (PCL) in its blend with poly(4-hydroxystyrene) (PVPh) in thin films (130 ± 10 nm) has been revealed by grazing incident infrared (GIIR) spectroscopy. It can be found that PCL chains orient preferentially in the surface-normal direction and crystallization occurs simultaneously. The morphology of the PCL/PVPh blends films can be identified by optical microscopy (OM). When crystallized at 35 °C, the blends film shows a seaweed-like structure and becomes more open with increasing PVPh content. In contrast, when crystallized at higher temperatures, i.e., 40 and 45 °C, dendrites with apparent crystallographically favored branches can be observed. This characteristic morphology indicates that the diffusion-limited aggregation (DLA) process controls the crystal growth in the blends films. The detailed lamellar structure can be revealed by the height images of atomic force microscopy (AFM), i.e., the crystalline branches are composed of overlayered flat-on lamellae. The branch width has been found to be dependent on the supercooling and PVPh content. This result differs greatly from pure PCL, in which case the crystal patterns controlled by DLA process developed in ultrathin film or monolayers of several nanometers. In the PCL/PVPh blends case, the strong intermolecular interactions and the dilution effect of PVPh should contribute to these results. That is to say, the mobility of PCL chains can be retarded and diffusion of them to the crystal growth front slows down greatly, even though the film thickness is far more than the lamellar thickness of PCL.

The βα growth transition of isotactic polypropylene during stepwise crystallization at elevated temperature

Growth transition from β-isotactic polypropylene (iPP) to its α phase during the stepwise crystallization process is investigated. β-iPP hexagonites and spherulites are prepared first at 135 °C by using pimelic calcium as a nucleation agent, and transformation of them to their α-counterparts is realized by continuous isothermal crystallization at elevated tempera- tures, e.g., 151 °C. The β to α growth transition regions are investigated by optical and scanning electron microscopies. The results show that β-iPP hexagonites and spherulites grow in thin and thick films, respectively, while α-iPP spherulites are always observed in both thin and thick films. After abrupt temperature jump from 135 to 151 °C, α-iPP spherulites grow continuously with edge-on lamellae extending smoothly from the region crystallized at 135 °C to the area grown at 151 °C. On the contrary, the growth of the β-iPP crystals stops. At the growth front of β-iPP crystals, α-iPP crystallization takes place via nucleation and subsequent crystal growth, leading to spo- radically dispersed α-iPP crystals around the growth front of β- iPP crystals. The similar structure of the transformed α-iPP crystals with that produced directly by isothermal crystalliza- tion at 151 °C also supports this conclusion. Moreover, the same cross-hatched lamellar structure of α-iPP in contact with both the flat-on and edge-on β-iPP lamellae demonstrates that there is no fixed structure relationship between the newly formed α- and the original β-iPP crystals.

Influence of film thickness on the crystalline morphology of a copolyesterurethane comprising crystallizable poly(ɛ-caprolactone) soft segments.

In this work, a model approach to investigate changes in crystalline morphology during heating/cooling procedures in the context of programming and induction of the shape-memory effect is presented. Atomic-force microscopy (AFM) was performed to investigate the variations in poly(ɛ-caprolactone) (PCL) crystalline morphology in nm thin films on a silicon substrate and a film with 20 μm thickness, prepared from a copolyesterurethane (named PDLCL) consisting of crystallizable poly(ω-pentadecalactone) (PPDL) hard segments and crystallizable PCL segments forming switching domains. PCL crystals in switching domains melted/recrystallized repeatedly during heating/cooling cycles between 20 and 70 °C, while no evident variation in PPDL crystals forming hard domains was observed. When film thickness was 20 nm, PCL edge-on lamellae were observed, confined in the phase-separated morphology. Flat-on PCL lamellae, which broke out from the previous phase-separated morphology, were obtained at a film thickness of 170 nm. In contrast, large PCL spherulites were observed in the 20 μm thick film. PCL crystalline morphology in PDLCL as well as the competition between crystallization and phase separation can be tailored by the film thickness and the substrate. Such AFM investigations on thin films can be a helpful approach for predicting the crystal morphology in micro-/nanoscaled objects.

Morphology of poly(propylene azelate) gratings prepared by nanoimprint lithography as revealed by atomic force microscopy and grazing incidence X-ray scattering

Nanostructured gratings of semicrystalline poly(propylene azelate) (PPAz) have been prepared over spin-coated thin films by Nanoimprint Lithography (NIL). The structure and morphology of the gratings have been investigated by combining Atomic Force Microscopy (AFM) and Grazing Incidence X-ray Scattering at small angle (GISAXS) and wide angle (GIWAXS). The results reveal that NIL affects significantly the orientation of the crystalline lamellae. PPAz gratings are more abundant in edge-on lamellae than the reference non-printed films. We attribute this effect to the PPAz preferential crystallization as flat-on lamellae on silicon surfaces either the stamp trench walls or the substrate surface. Thus, the flat-on lamellae on the trench walls appear to be edge-on lamellae in the printed sample. These results further support NIL as an appropriate procedure in order to control polymer crystal orientation.

Thickness-dependent orientation structure in poly(ethylene oxide) multi-layer crystals

Poly(ethylene oxide) multi-layer crystals were obtained and the re-crystallization behavior was studied to give insight into how melt thickness and temperature affect the lamellar orientation. For a special re-crystallization temperature, there exists a critical transition thickness range for the occurrence of edge-on lamellar orientation. Below the critical thickness, only flat-on lamellae were observed. While above the critical thickness, both flat-on and edge-on lamellae were found and the proportion of the edge-on lamellae increases with thickness. At low re-crystallization temperatures (below 30 °C), the critical transition thickness gradually increases from about 15 nm to 35 nm when the re-crystallization temperature was increased from 20 °C to 30 °C. However, when the re-crystallization temperature is above 30 °C, the critical transition thickness becomes constant. Our results demonstrated that the lamellar orientation could be specially modified by changing the melt thickness and re-crystallization temperature.

Lamellar orientation of polyamide 6 thin film crystallization on solid substrates

We studied the lamellar orientation of polyamide 6 (PA6) thin films with the thickness ranged from ∼5 μm to ∼20 nm on different solid substrates. The lamellar morphology was observed using atomic force microscopy and transmission electron microscopy, and the lamellar orientation was characterized using two-dimensional (2D) grazing incidence wide-angle X-ray diffraction (GI-WAXD) and selected area electron diffraction (SAED). The 2D GI-WAXD results indicate that in the films thicker than ∼50 nm PA6 lamellae of the α-form are preferentially edge-on with the ab-planes (b is the chain axis) parallel to the substrate. Decrease of film thickness leads to higher order of the orientation. This edge-on orientation is insensitive to the surface properties of the substrates changed from hydrophilic to hydrophobic, suggesting that the low energy hydrogen bonded sheets of PA6 are prone to lying down in the film. For the films on the hydrophilic substrate with thickness less than ∼50 nm, the edge-on orientation remains. On the other hand, on a more hydrophobic substrate, flat-on lamellae can be detected by SAED when the film thickness is reduced to be less than 50 nm. An unexpected result is that in the ∼30 nm-thick thin films the strong confinement effect can result in flat-on lamellae of PA6's γ-form, although the crystallization temperature applied did not favor the γ-form formation. We consider that in comparison with that of the α-form lamellae with a lath-like shape, the growth of more symmetrical lamellae of the γ-form with large lateral dimensions can reduce more the free energy of the very thin film.

Formation of banded and non-banded poly(l-lactic acid) spherulites during crystallization of films of poly(l-lactic acid)/poly(ethylene oxide) blends

In this study, a series of poly(l-lactic acid) (PLLA)/poly(ethylene oxide) (PEO) blends with different PLLA concentrations was prepared. Films of these blends crystallized with and without a coverslip were characterized by the presence and absence of banded structures, respectively. This difference in morphology was observed because the PEO component of the blends was oxidized at a high temperature (125 °C) in air without the protection of a coverslip. X-ray photoelectron spectroscopy (XPS) results showed that the surface of the blends crystallized in nitrogen without a coverslip contained mostly PLLA while the surfaces of the same blends crystallized under a coverslip contained only a moderately higher concentration of PLLA than their bulks. The migration of PLLA to the surface of the blends during crystallization in nitrogen when no coverslip was used was due to its low surface tension. Phase images obtained using atomic force microscopy (AFM) indicated that the banded structures consisted of valleys and ridges, which were in fact flat-on and edge-on lamellae, respectively. A detailed time-of-flight secondary ion mass spectrometry (ToF-SIMS) examination suggested that PLLA and PEO were located mainly on the surfaces of the ridges and valleys, respectively.

Crystallization under One-Dimensional Confinement in Alumina Nanopores of Poly(trimethylene terephthalate) and Its Composites with Single Wall Carbon Nanotubes

We report the preparation of semicrystalline polymer nanorods of PTT and of its nanocomposites with SWCNTs by infiltration of the molten polymer into disordered anodic alumina membranes. An accurate study of the crystalline orientation of these systems has been accomplished by means of X-ray microdiffraction. While polymer residual film exhibits isotropic character, edge-on lamellae are formed upon approaching the polymer/membrane interface. This effect might be due to the elongational flow that takes place in the molten state as polymer chains infiltrate the AAO membrane. At the interface, edge-on and flat-on crystalline lamellae coexist as a consequence of the strong interaction between the polymer and the AAO surface. Inside the nanopores, the confined environment induces a kinetic selection of polymer crystals which only allows the growth of crystalline lamellae with its a-axis parallel to that of the pore. In the case of PTT/SWCNT nanocomposites, this effect, in conjunction with the strong interaction between polymer and AAO surface, seems to prevail over the templating effect of the carbon nanotubes and a similar orientation to that of the neat PTT case is observed.

Structure and Physical Properties of Zein/Pluronic F127 Composite Films

Triblock copolymer Pluronic F127 has been used to form composites with zein, a corn protein and coproduct of the bioethanol industry, to alleviate its natural brittleness. At low F127 loadings (0-35%), the plasticizing effect was dominant, and the elongation at break of zein composite film containing 35% F127 was about 8-fold higher than that of the zein film with 10% F127. At high F127 loadings (50-100%), a large number of lamellae crystals were formed in the film matrix as verified by a combination of differential scanning calorimetry (DSC), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS). The F127 crystallization surpassed its plasticizing effect, resulting in an increased brittleness of zein film with the further increase of F127 loading. Compared with the flat-on lamellae of pure F127, F127 chains folded into branch-like lamellar structures in the zein composite film containing 50% F127 due to the confinement of amorphous zein. Besides, the crystals in zein films were composed of extended chain integral folding (IF = 0) and once-folded chain (IF = 1) polyethylene oxide (PEO) crystals, and the portion of once-folded chain (IF = 1) PEO crystals increased with F127 loading. Thus, through investigation of the competition of plasticizing effect and crystal formation under different F127 loadings, the optimized F127 loading in zein/F127 composite film with a good overall performance was determined to be at around 35%.