What Determines the Lamellar Orientation on Substrates?

Anisotropic material depletion in epitaxial polymer crystallization

The physical properties of a semicrystalline polymer thin film are intimately related to the morphology of its crystalline domains. While the mechanisms underlying crystallization of flat-on oriented polymer crystals are well known, similar mechanisms remain elusive for edge-on oriented thin films due to the propensity of substantially thin films to adopt flat-on orientations. Here, we employ an epitaxial polymer-substrate relationship to enforce edge-on crystallization in thin films. Using matrix-assisted pulsed laser evaporation (MAPLE), we deposit films in which crystal nucleation is spatially separated from subsequent epitaxial crystallization. These experiments, together with phase-field simulations, demonstrate a highly anisotropic and localized material depletion during edge-on crystallization. These results provide deeper insight into the physics of polymer crystallization under confinement and introduce a processing motif in the crystallization of ultrathin structured films.

A combined experimental and molecular simulation study on stress generation phenomena during the Ziegler-Natta polyethylene catalyst fragmentation process

The morphology of particles obtained under different pre-polymerization conditions has been connected to the stress generation mechanism at the polymer/catalyst interface. A combination of experimental characterization techniques and atomistic molecular dynamics simulations allowed a systematic investigation of experimental conditions leading to a certain particle morphology, and hence to a final polymer with specific features. Atomistic models of nascent polymer phases in contact with magnesium dichloride surfaces have been developed and validated. Using these detailed models, in the framework of McKenna's hypothesis, the pressure increase due to the polymerization reaction has been calculated under different conditions and is in good agreement with experimental scenarios. This molecular scale knowledge and the proposed investigation strategy would allow the pre-polymerization conditions to be better defined and the properties of the nascent polymer to be tuned, ensuring proper operability along the whole polymer production process.

Temperature and Thickness Dependence of the Time Scale of Crystallization of Polymers under 1D Confinement

Confined in nanodomains, polymers crystallize much slower than in bulk due to both finite size and interfacial effects. These two factors are successfully disentangled in our phenomenological framework, which provides a measurement of the time scale of crystallization via a product of probabilities involving nucleation and of chain diffusion. In this Letter, we demonstrate that our model allows determining the Gibbs free energy of the formation of a critical size nucleus indicated by the classical nucleation theory for bulk polymer melts. In addition to that, by means of segmental mobility data and one single set of isothermal crystallization measurements at different confinement degrees, our model predicts the right temperature and thickness dependence of the crystallization time.

Effect of a Novel Flame Retardant on the Mechanical, Thermal and Combustion Properties of Poly(Lactic Acid)

Poly(lactic) acid (PLA) is one of the most promising biobased materials, but its inherent flammability limits its applications. A novel flame retardant hexa-(DOPO-hydroxymethylphenoxy-dihydroxybiphenyl)-cyclotriphosphazene (HABP-DOPO) for PLA was prepared by bonding 9,10-dihydro-9-oxy-10-phosphaphenanthrene-10-oxide (DOPO) to cyclotriphosphazene. The morphologies, mechanical properties, thermal stability and burning behaviors of PLA/HABP-DOPO blends were investigated using a scanning electron microscope (SEM), a universal mechanical testing machine, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), limiting oxygen index (LOI), vertical burning (UL-94) and a cone calorimeter test (CCT). The LOI value reached 28.5% and UL-94 could pass V-0 for the PLA blend containing 25 wt% HABP-DOPO. A significant improvement in fire retardant performance was observed for PLA/HABP-DOPO blends. PLA/HABP-DOPO blends exhibited balanced mechanical properties. The flame retardant mechanism of PLA/HABP-DOPO blends was evaluated.

Tunable electroresistance and electro-optic effects of transparent molecular ferroelectrics

Recent progress in molecular ferroelectrics (MOFEs) has been overshadowed by the lack of high-quality thin films for device integration. We report a water-based air-processable technique to prepare large-area MOFE thin films, controlled by supersaturation growth at the liquid-air interface under a temperature gradient and external water partial pressure. We used this technique to fabricate ImClO₄ thin films and found a large, tunable room temperature electroresistance: a 20-fold resistance variation upon polarization switching. The as-grown films are transparent and consist of a bamboo-like structure of (2,[Formula: see text],0) and (1,0,[Formula: see text]) structural variants of R3m symmetry with a reversible polarization of 6.7 μC/cm². The resulting ferroelectric domain structure leads to a reversible electromechanical response of d₃₃ = 38.8 pm/V. Polarization switching results in a change of the refractive index, n, of single domains, [Formula: see text]. The remarkable combination of these characteristics renders MOFEs a prime candidate material for new nanoelectronic devices. The information that we present in this work will open a new area of MOFE thin-film technologies.

Fractionated crystallization, polymorphism and crystal transformation of poly(butylene adipate) confined in electrospun immiscible blend fibers with polystyrene

Utilizing electrospun immiscible blend fibers of poly(butylene adipate) (PBA) and polystyrene (PS) and following coating by the high glass transition temperature poly(4-tert-butylstyrene) (P4tBS), confined PBA specimens in nanometer space were effectively prepared.

Modeling of polystyrenic nanoparticles driven β-trans-crystalline efficiency in isotactic polypropylene

Addition of four styrenic soft nano-particles induced relative β-polymorphism as much as 20, 27, 34 and 10% during 10 minutes annealing of iPP at 116 °C. The kinetic and thermodynamic aspects of the phenomenon rationalized by a bi-exponential function

A grazing incident XRD study on the structure of poly(3-hydroxybutyrate) ultrathin films sandwiched between Si wafers and amorphous polymers

The crystallization behavior and morphology of poly(3-hydroxybutyrate) (PHB) ultrathin films sandwiched between Si wafers and amorphous thin polymer layers were studied by using grazing incident X-ray diffraction (GIXD) technology.

Are polymers glassier upon confinement?

Glass forming systems are characterized by a stability against crystallization upon heating and by the easiness with which their liquid phase can be transformed into a solid lacking of long-range order upon cooling (glass forming ability). Here, we report the thickness dependence of the thermal phase transition temperatures of poly(l-lactide acid) thin films supported onto solid substrates. The determination of the glass transition, cold crystallization and melting temperatures down to a thickness of 6 nm, permitted us to build up parameters describing glass stability and glass forming ability. We observed a strong influence of the film thickness on the latter, while the former is not affected by 1D confinement. Further experiments permitted us to highlight key structural morphology features giving insights to our ellipsometric results via a physical picture based on the changes in the free volume content in proximity of the supporting interfaces.

Critical formation conditions for β-form hybrid shish-kebab and its structural analysis

Critical formation conditions for β-form hybrid shish kebab and its unique condensed state structure.

Band-to-Nonband Transition into Unique Poly(ε-caprolactone) Crystals by Modulating the Interplay of Diffusion and Growth

Previously, by modulating the interplay of chain diffusion and crystal growth rate in evaporating solution-cast poly(ε-caprolactone) (PCL) thin films, discrete banded crystals were obtained when diffusion and growth are competitive. In this study, we further investigated the effect of diffusion and growth, here mainly the growth rate on the crystallization of PCL. We found that there is a threshold (or range of) growth rate above which the banding cannot develop, and thus a band-to-nonband transition leads to the emergence of a novel crystal pattern. Accompanied by the band-to-nonband transition, the lamellar orientation also changes from flat-on to mainly tilted or even edge-on. The result reveals the significant impact of the diffusion and growth, especially the growth rate on the crystal construction and then the crystal morphology.