Highly Enhanced Mesophase Formation in Glassy Poly(L-lactide) at Low Temperatures by Low-Pressure CO2 That Provides Moderately Increased Molecular Mobility

The mesophase structuring in melt-quenched poly(L-lactide) (PLLA) treated in low-pressure CO2 at 2 MPa and 0-35 degrees C was investigated by using infrared spectroscopy, differential scanning calorimetry (DSC), temperature-modulated DSC, and atomic force microscopy (AFM). It was found that the mesophase formation in glassy PLLA was significantly enhanced, in particular at lower temperature (0 degrees C), which promoted a distinctly faster formation rate. AFM results revealed that the CO2-enhanced mesophase exhibited nodular morphology with dramatically increased nucleation density. A framework of multistage model in combination with the moderately improved molecular mobility exerted by CO2 was proposed to explain the main findings. Because of the moderate molecular mobility, a tremendous number of metastable mesomorphic layers were formed and stabilized by the accompanying development of the rigid amorphous fraction (RAF), leading to the immobilization of the remaining mobile amorphous fraction (MAF). The mesomorphic phases were converted to more stable crystals via cooperative structural reorganization upon the devitrification of the RAF, requiring high chain mobility, showing time and temperature dependence. Consequently, the amorphous PLLA transiently transformed to the mesophase before transforming into the crystal during treating at a relatively high temperature (35 degrees C). Alternatively, upon heating, the mesophase underwent disordering reorganization to form active crystallite, profoundly promoting the cold crystallization of the surrounding restored MAF, resulting in obviously depressed cold crystallization temperatures. The present results have important implications in understanding and regulation of the crystallization of polymers

Mesophase-Mediated Crystallization of Poly(l‑lactide): Deterministic Pathways to Nanostructured Morphology and Superstructure Control

The effect of the CO₂-induced mesophase on the isothermal crystallization of poly­(l-lactide) (PLLA) was investigated by infrared (IR) spectroscopy and microscopy. It was found that the crystallization rate of PLLA was significantly enhanced by the CO₂-induced mesophase, showing that the crystallization was completed even in a short period of 10⁰–10¹ s. Compared with the directly crystallized samples that showed typical spherulites with lamellae, the crystallization via CO₂-induced mesophase led to nonspherulitic (granular or featureless) morphologies consisting of nanorods, whereas the polymorphic behavior remained unaffected by the initial state, resulting in crystallized PLLA containing identical polymorphs of uniquely different nanostrutured morphologies and superstructures. The IR imaging results indicated that the formation of the equilibrium crystal was preceded by the formation of various metastable intermediate phases, including mesomorphic phase, preordering, and metastable crystal, all of which continuously evolved with time. The nucleation process proceeded via a similar pathway. In contrast to the negligible contribution of mesophase to the nucleation in direct crystallization, the CO₂-induced mesophase with extremely high nucleation density underwent disordering–reorganization into the preordering, thereby providing a tremendous number of active nucleation sites for enhancing crystallization and serving as building blocks for nanorods. Importantly, the present results highlight the decisive role of mesophase in directing the nanostructure and superstructure and support a multistep process for the crystallization (including nucleation and crystal growth) of PLLA, validating the Ostwald step rule, providing mechanistic insights into the crystallization of polymers

Nucleation Enhancement in Stereodefective Poly(l-lactide) by Free Volume Expansion Resulting from Low-Temperature Pressure CO2 Preconditioning

Nucleation enhancement in a highly stereodefective poly(l-lactide) (PLLA) with an optical purity of 88% by low-temperature pressure (0 and 35 °C under 2 MPa) CO2 preconditioning was investigated using differential scanning calorimetry (DSC), infrared (IR) spectroscopy, polarized optical microscopy (POM) as well as positron annihilation lifetime spectroscopy (PALS). Despite the preconditioning of the melt-quenched films for 2 h, IR results indicated that no trace of mesophase was generated and the samples remained in the glassy state. However, judging from the results of DSC, IR, and POM, when compared to the untreated sample, both the treated ones showed a significantly enhanced crystal nucleation effect, resulting in the corresponding greatly enhanced crystallization kinetics. Moreover, owing to the existence of the retrograde vitrification, the conditions of the previous low-pressure CO2 conditioning affected the nucleation enhancement effect. When compared to the case of 35 °C, the much lower temperature of 0 °C was more effective for nucleation enhancement. The PALS results indicated that the enlarged free volume, which resulted from the CO2 conditioning, largely accounted for the formation of locally ordered structures, providing many more potential nucleation sites for forming critical nuclei and thus the resulting enhanced crystallization kinetics in glassy PLLA. The present results have implications in understanding the nucleation enhancement effect, in particular in stereodefective PLLA systems, which possess extremely low crystallization ability and are thus probably too problematic to be evaluated by conventional methods

Highly Enhanced Mesophase Formation in Glassy Poly(l‑lactide) at Low Temperatures by Low-Pressure CO₂ That Provides Moderately Increased Molecular Mobility

The mesophase structuring in melt-quenched poly­(l-lactide) (PLLA) treated in low-pressure CO₂ at 2 MPa and 0–35 °C was investigated by using infrared spectroscopy, differential scanning calorimetry (DSC), temperature-modulated DSC, and atomic force microscopy (AFM). It was found that the mesophase formation in glassy PLLA was significantly enhanced, in particular at lower temperature (0 °C), which promoted a distinctly faster formation rate. AFM results revealed that the CO₂-enhanced mesophase exhibited nodular morphology with dramatically increased nucleation density. A framework of multistage model in combination with the moderately improved molecular mobility exerted by CO₂ was proposed to explain the main findings. Because of the moderate molecular mobility, a tremendous number of metastable mesomorphic layers were formed and stabilized by the accompanying development of the rigid amorphous fraction (RAF), leading to the immobilization of the remaining mobile amorphous fraction (MAF). The mesomorphic phases were converted to more stable crystals via cooperative structural reorganization upon the devitrification of the RAF, requiring high chain mobility, showing time and temperature dependence. Consequently, the amorphous PLLA transiently transformed to the mesophase before transforming into the crystal during treating at a relatively high temperature (35 °C). Alternatively, upon heating, the mesophase underwent disordering–reorganization to form active crystallite, profoundly promoting the cold crystallization of the surrounding restored MAF, resulting in obviously depressed cold-crystallization temperatures. The present results have important implications in understanding and regulation of the crystallization of polymers

Thermal Behavior of Poly(L-lactide) Having Low L-Isomer Content of 94% after Compressed CO2 Treatment

The effect of compressed CO2 treatment on the thermal behavior of poly(L-lactide) (PLLA) with low L-isomer content of 94% was studied by using differential scanning calorimetry (DSC) and temperature modulated DSC. It was shown that the treated samples displayed rich thermal transition signals during DSC heating, which is differ from the PLLA having high L-isomer content, including enthalpy relaxation, endothermic annealing, melting-recrystallization process, and cold crystallization. The results suggested that the crystalline phase obtained has less perfection and low crystallinity regardless of the treatment conditions because of the low crystallizability of the PLLA. This PLLA was induced to crystallize after treatment under CO2 at 2 MPa and temperature >= 65 degrees C and at pressures of 4-16 MPa and temperatures as low as 0 degrees C. At 2 MPa, the alpha crystal form is formed predominantly in the crystallized samples. The results also indicated that the ELLA-CO2 system exhibited the property of retrograde vitrification

Multipole Radiations from Large Gold Nanospheres Excited by Evanescent Wave

We proposed the use of the evanescent wave generated in a total internal reflection configuration to excite large gold nanospheres and investigated the radiations of the high-order plasmon modes supported in gold nanospheres. It was revealed that the evanescent wave excitation is equivalent to the excitation by using both the incident and reflected light, offering us the opportunity to control the orientation of the electric field used to excite nanoparticles. In addition, it was found that the scattering light intensity is greatly enhanced and the background noise is considerably suppressed, making it possible to detect the radiations from high-order plasmon modes. Moreover, the influence of the mirror images on the scattering induced by a metal substrate is eliminated as compared with the surface plasmon polariton excitation. By exciting a gold nanosphere with s-polarized light and detecting the scattering light with a p-polarized analyzer, we were able to reveal the radiation from the electric quadrupole mode of the gold nanosphere in both the spatial and the frequency domains. Our findings are important for characterizing the radiations from the high-order modes of large nanoparticles and useful for designing nanoscale photonic devices

Multipole Resonance in Arrays of Diamond Dielectric: A Metamaterial Perfect Absorber in the Visible Regime

Excellent characteristics and promising application prospects promote the rapid development of metamaterials. We have numerically proposed and demonstrated a novel subwavelength broadband metamaterial perfect absorber (BMPA) based on diamond dielectric arrays. The proposed absorber is composed of an ultra-thin two-layer structure covering the dielectric periodic array on a metal substrate. The materials of dielectric silicon (Si) and gold (Au) substrate are discussed in detail. In addition, different dielectric and refractory materials are also applied to achieve broadband absorption, which will make the proposed absorber greatly broaden the application field. A perfect absorption window (i.e., absorption rate exceeding 90%) can be obtained from near-ultraviolet to the visible range. The average absorption rate of 93.3% is achieved in the visible range. The results of multipole decomposition show that broadband absorption is mainly caused by electromagnetic dipole resonance and lattice resonance in a periodic array of Si. The proposed absorber can be extended freely by adjusting the structural parameters. The polarization-independent and incident angle insensitivity are proved. The proposed absorber may well be used in light energy acquisition, as well as for the scalability of optoelectronic and sensing devices