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 disordered structures are found deeper. The AFM-IR images and nanoscale infrared spectra reveal that the PEG components are mainly trapped into various interlamellae regions of PHB depending on the structure of the PHB crystal template. We report herein, for the first time, several tridimensional chemical images with full information on the diffusion of PEG in the PHB structure at the nanoscale.
Ultrathin films (12–125 nm) of poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) blends of different compositions have been crystallized between 180 and 210 °C, i.e., above the melting point of each ...polymer crystallized separately. The overall crystal shape depends on the temperature, film thickness and ratio of the two polyenantiomers in the blends. In nonequimolar blends, lamellae show curvatures, and the sense of the curvature is determined by the chirality of the polyenantiomer in excess, blend ratio, film thickness, crystallization temperature and lamellar orientation (flat-on or edge-on). The curvature of the stereocomplex lamellae is ascribed to the unequal amount of PLLA and PDLA segments at the crystal growth front, creating an unbalanced mechanical stress at the chain folding surfaces which can be released by a curvature of the growth tip.
Geopolymers are amorphous three-dimensional aluminosilicate binder materials that may be synthesized at room or slightly higher temperature by alkaline activation of aluminosilicates obtained from ...industrial wastes, calcined clays, natural minerals or mixtures of two or more of these materials. Among the different families of geopolymers, those based on potassium show modified thermal and mechanical properties due to the larger size of the potassium ion compared to sodium. This work deals with the preparation of geopolymer foams based on potassium silicate, industrial waste and various types of clays (kaolin, metakaolin, illite or montmorillonite). The influence of the clays used is assessed in terms of clay reactivity using structural data determined by FTIR spectroscopy, thermal analysis, XRD, and SEM characterizations.
In situ geopolymer foam was obtained from all of the clays but its characteristics depended on the nature of the clays, including their structural alteration and chemistry. The extent of destruction of the clay structure was partial for kaolinite but was greater for illite, followed by montmorillonite. These inorganic foams have a potential use in housing construction, since they display thermal insulating properties.
►Geopolymer foam obtained from clays (kaolin, metakaolin, illite, and montmorillonite). ►Geopolymer foam characteristics depended on the nature of clays ►Importance of cristallo-chemistry of clay ►Inorganic foams have a potential use in housing construction.
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.
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•Poly (S-2-hydroxybutanoic acid) (P(S2HB)) has an equilibrium melting point of 130.3 °C.•In ultrathin films, lamellae begin to grow edge-on but, later, they orient flat-on (the transition depends on temperature).•Stereocomplexes of P(S2HB) with P (R2HB) exhibit a melting temperature of more than 100 °C above those of the homopolymers.•Stereocomplexes have a growth rate maximum of 19.6 μm/min as compared to 400 μm/min for the homopolymers.
The preparation of superhydrophobic textiles with high mechanical and chemical durability is challenging. Here, facile and fluorine-free methods, using alkali and plasma-etching treatments, followed ...by the addition of silica nanoparticles and tetraethyl orthosilicate (TEOS), were used to prepare superhydrophobic cotton surfaces. With different input variables and etching techniques, superhydrophobic cotton fabrics with high chemical and mechanical durability were successfully prepared, with contact angles up to 173°. A control of the surface architecture at the nanoscale in combination with a homogeneous repellent layer of TEOS in the cotton surface was achieved. The repellent properties of the as-prepared cotton remain stable under accelerated laundering and abrasion test conditions. The etching pretreatment by alkali or plasma plays a key role in obtaining superhydrophobic cotton surfaces.
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.
Morphologies, growth rates, and melting of isothermally crystallized ultrathin (200−1 nm) poly(ε-caprolactone) (PCL) films have been investigated in real time by atomic force microscopy. The flat-on ...orientation of the lamellar crystals relative to the substrate was determined by electron diffraction. The truncated lozenge-shaped PCL crystals observed at low undercooling become distorted for films of thicknesses equal to or thinner than the lamellar thickness, which depends on the crystallization temperature but not on the initial film thickness. The melting behavior of distorted crystals differs from that of undistorted ones, and their growth is slower and nonlinear. The crystal growth rate decreases greatly with the film thickness. All these observations are discussed in terms of the diffusion of the polymer chains from the melt to the crystal growth front.
Block copolymer films obtained by dip-coating have been much less studied than those obtained by spin-coating, although it is an important industrial process. In our investigations of supramolecular ...diblock copolymer films of PS–P4VP (∼30 wt % P4VP) dip-coated from THF solutions, we initially found that films containing a monohydroxy-functionalized small molecule (SM), 1-naphthol (NOH), have a dot morphology, whereas those containing a monocarboxylic acid-functionalized small molecule, 1-naphthoic acid (NCOOH), have a stripe morphology. The COOH functionality is known to hydrogen bond more strongly to pyridine than OH. Yet the total amount of SM in the two types of films under given conditions is the same and PS–P4VP/NOH and PS–P4VP/NCOOH (equimolar SM:VP) in THF solution are both micellar, with the same spherical shape and size as PS–P4VP alone. On the other hand, the stripe morphology can be transformed to dot morphology by decreasing the dip-coating rate, the solution concentration or the SM:VP molar ratio, and vice versa. Vertical TEM indicates that the stripes correspond to horizontal cylinders, whereas the dots appear to be essentially spherical micelles. Although the films tend to be subject to terracing or dewetting, the average thickness of the films was observed to decrease toward a minimum with increase in dip-coating rate. This was related to the slow rates used, shown recently in sol–gel films to correspond to the so-called capillarity regime, and never before, to our knowledge, investigated in connection with its influence on block copolymer morphology. The minimum film thickness corresponds to the brush copolymer regime, which can be obtained almost uniformly for NOH-containing films, but is accompanied by stripe regions in the NCOOH-containing films investigated. Possible reasons for the differences in morphology evolution in films containing NOH vs NCOOH are discussed in the light of the experimental findings.
Aluminous cement, when mixed with calcium sulfate and water, produces a binder called an ettringite binder. A model of the hydration of an ettringite binder that accounts for the reactive surface of ...the solid phases in a solute was proposed. Mechanisms of precipitation and dissolution were considered. Calibration was performed based on the kinetics of the conductivity of the solute. Experimental pH kinetics are in accordance with the model prediction. Based on this good agreement, the final part of this article discusses the model predictions of the hydration of an ettringite-based binder.
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