Study on non-isothermal crystallization kinetics of polymer is relevant for both fundamental as well as application perspective. Hence, the non-isothermal crystallization kinetics of bulk and ...confined poly(ethylene oxide) (PEO) in electrospun nanofibers was investigated using DSC. The nanofibers were prepared from polystyrene (PS)/PEO immiscible blends where PEO weight fraction was varied upto 0.4. It was found that the Avarmi and Jeziorny models successfully predict the nucleation nature of PEO chains and its growth behavior. However, the Ozawa model was not found to be suitable for the confined system. Nevertheless, the Mo's method, which is the combination of Avrami and Ozawa models, effectively described the non-isothermal crystallization behavior of PEO in confined domains. The Kissinger and Takhor methods showed that activation energy for crystallization of PEO chains increased with reduction of PEO weight fraction in the nanofibers. This was plausibly due to the geometrical restriction on the mobility of PEO chains. The present study is the first report on non-isothermal crystallization behavior of confined polymer in electrospun nanofibers systems.
Confined crystallization behavior of poly(ethylene oxide) (PEO) was studied in electrospun core-shell nanofibers and beads, respectively, using DSC. The core-shell structure consisted of PEO core and ...polystyrene (PS) shell. It was observed that the morphology of PEO core changed from continuous to discrete domains when the core spinning solvent was switched from water to chloroform. This significantly influenced the crystallization behavior of PEO such that whereas the crystallization nucleation mechanism was heterogeneous for nanofibers with continuous PEO core, it became fractionated for nanofibers with discontinuous PEO core. The relaxation of the PEO domains, in the nanofibers with discontinuous core, via thermal annealing above the glass transition temperature of PS shell, markedly increased homogenous nucleation contribution in the crystallization. This plausibly was due to relaxation of PEO domains through domain breakup and coalescence process resulting in higher fraction of nanosized PEO domains.
Electrospinning of polymers typically requires high solution concentrations necessitated by the requirement of sufficient chain overlaps to achieve the required viscoelastic properties. Here, we ...report on a novel supramolecular approach, involving polymer/surfactant complexes, which allows for a significant reduction in the solution concentration of polymer for electrospinning. The approach involved supramolecular complexation of poly(4-vinylpyridine) (P4VP) with a surfactant, dodecylbenzenesulfonic acid (DBSA), via ionic interactions. The supramolecular complexation of P4VP with DBSA led to a significant increase in the solution viscosity even at a DBSA/4VP molar ratio as low as 0.05. Furthermore, the solution viscosity of the P4VP/DBSA complex increased significantly with the DBSA/4VP molar ratio. The increase in the viscosity for the P4VP/DBSA complexes was plausibly due to the formation of physical cross-links between P4VP chains driven by hydrophobic interactions between the surfactant tails. The formation of such physical cross-links led to a significant decrease in the solution concentration needed for the onset of semidilute entangled regime. Thus, the P4VP/DBSA complexes could be electrospun at a much lower concentration. The critical solution concentration to obtain bead-free uniform nanofibers of P4VP/DBSA complexes in dimethylformamide was reduced to 12% (w/v), which was not possible for neat P4VP solution even up to approximately 35% (w/v). Furthermore, small-angle X-ray scattering and polarized optical microscopy results revealed that the electrospun nanofibers of P4VP/DBSA complexes self-assembled in lamellar mesomorphic structures similar to that observed in bulk. However, the electrospun nanofibers exhibited significantly improved lamellar order, which was plausibly facilitated by the preferred orientation of P4VP chains along the fiber axis.
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Transparent wood (TW) biocomposites combine high optical transmittance and good mechanical properties and can contribute to sustainable development. The safety against fire is ...important for building applications. Here, a “green” bleached wood reinforcement is impregnated by water soluble and flame-retardant melamine formaldehyde (MF) in a scalable process, for a wood content of 25 vol%. FE-SEM is used for characterization of optical defects and EDX to examine MF distribution at nanoscale cell wall pore space. Curing (FTIR-ATR), mechanical properties, optical transmittance (74% at 1.2 mm thickness) and flame-retardant properties are also characterized (self-extinguishing behavior and cone calorimetry), and scattering mechanisms are discussed. The fire growth rate of transparent wood was less than half the values for neat wood. Transparent wood/MF biocomposites show interesting wood-MF synergies and are of practical interest in building applications. Critical aspects of processing are analyzed for minimization of optical defects.
The confined crystallization behaviour of poly(ethylene oxide) (PEO) has been studied in electrospun nanofibers of the phase-separated blends of polystyrene (PS) and PEO compatibilized with ...polystyrene-
-poly(ethylene oxide) (PS-
-PEO) block copolymer. The PS was present as the majority component such that the electrospun nanofibers consisted of PEO domains dispersed in the PS matrix. The phase separation in the blend occurred under the radial constraint of the nanofibers which led to the formation of small-sized fibrillar PEO domains. The use of block copolymer compatibilizer resulted in a noticeable decrease in the PEO domain size in the as-spun nanofibers. Moreover, the decrease in the domain size and domain connectivity was more substantial in the thermally annealed blend nanofibers due to the suppression of the domain coalescence mechanism resulting from the localization of the PS-
-PEO block copolymer at the interface. Consequently, the fraction of PEO domains crystallizing
homogeneous nucleation increased in the compatibilized blend nanofibers due to the presence of higher number of heterogeneity free PEO domains and disruption in their spatial connectivity. Interestingly, in the compatibilized blend nanofibers consisting of low molecular weight PEO, additional crystallization event attributed to surface nucleation was observed. The surface nucleation, plausibly, resulted from the formation of wet-brush structures where the PEO homopolymers homogeneously wet the PEO blocks present at the interface. In such a scenario, the PEO crystallization occurred
surface nucleation at the domain interface. The surface nucleated crystallization was absent in the compatibilized blend nanofibers composed of high molecular weight PEO presumably due to the formation of morphology with dry-brush structures.
Study on non-isothermal crystallization kinetics of polymer is relevant for both fundamental as well as application perspective. Hence, the non-isothermal crystallization kinetics of bulk and ...confined poly(ethylene oxide) (PEO) in electrospun nanofibers was investigated using DSC. The nanofibers were prepared from polystyrene (PS)/PEO immiscible blends where PEO weight fraction was varied upto 0.4. It was found that the Avarmi and Jeziorny models successfully predict the nucleation nature of PEO chains and its growth behavior. However, the Ozawa model was not found to be suitable for the confined system. Nevertheless, the Mo's method, which is the combination of Avrami and Ozawa models, effectively described the non-isothermal crystallization behavior of PEO in confined domains. The Kissinger and Takhor methods showed that activation energy for crystallization of PEO chains increased with reduction of PEO weight fraction in the nanofibers. This was plausibly due to the geometrical restriction on the mobility of PEO chains. The present study is the first report on non-isothermal crystallization behavior of confined polymer in electrospun nanofibers systems.
In this study, a bundle of poly(l‐lactic acid) (PLLA) hollow nanofibers was prepared through anodic aluminum oxide (AAO) membrane wetting method. The infiltrated PLLA in the nanopores of AAO membrane ...were isolated via etching of the membrane using phosphoric acid, preserving their original nanostructure. The outer diameter of isolated PLLA nanofibers and wall thickness were found to be 235 ± 17 nm and 38.5 ± 9 nm, respectively. The differential scanning calorimetry analysis showed that the hollow PLLA nanofibers exhibited lower melting and crystallization temperatures than the bulk PLLA sample. Furthermore, the wide‐angle X‐ray diffraction measurement suggested that PLLA crystallized preferentially along the 110/200 plane direction due to confinement effect of the AAO walls. Such hollow PLLA nanofibers may have potential application as a drug delivery system.
Poly(l‐lactic acid) holo‐nanofibers were developed inside the anodic aluminum oxide membrane via template wetting methods (melt‐infiltration, spin coating, and vacuum suction). By preserving original nanostructure, these holo‐nanofibers were isolated using selective etching method by phosphoric acid. The crystallization behavior of these nanotubes was studied.
The preparation of wood substrates modified by charged inorganic nanoparticles (NPs) diffusing into the internal cell wall structure is investigated for generating functional properties. The ...flammability problem of wood biocomposites is addressed. NPs applied from colloidal sols carry charge to stabilize them against aggregation. The influence of charge on particle diffusion and adsorption should play a role for their spatial distribution and localization in the wood substrate biocomposite. It is hypothesized that improved dispersion, infiltration, and stability of NPs into the wood structure can be achieved by charge control diffusion, also restricting NP agglomeration and limiting distribution to the wood cell wall. Cationic and anionic silica NPs of ≈30 nm are therefore allowed to diffuse into bleached wood. The influence of charge on distribution in wood is investigated as a function of initial sol concentration. Transparent wood is fabricated by in situ polymerization of a thiol‐ene in the wood pore space. These biocomposites demonstrate excellent flame retardancy with self‐extinguishing characteristics. The approach has potential for commercial fabrication of flame retardant transparent composites for glazing and other building applications.
Controlled electrostatic interactions help to achieve homogeneous silica distribution throughout the bulk of a wood based structure. This uniform silica‐rich surface layer protects the wood composite against thermal degradation. This research elucidates a straightforward technique for impregnating commercially available functional colloids into hierarchical wood structures without chemical modifications, and directs a path for commercial fabrication of functional wood biocomposites.
The effect of molecular weight on the crystallization behavior of poly(ethylene oxide) (PEO) confined in the glassy polystyrene (PS) matrix in electrospun nanofibers of PS/PEO blends were ...investigated. It was found that, irrespective of the PEO molecular weight, the crystallization behavior was fractionated when the weight fraction of PEO (wPEO) in the blend was 0.2, whereas purely homogeneous nucleated crystallization was observed when the wPEO ~ 0.1. Nevertheless, the nature of homogeneous nucleated crystallization was found to be significantly affected for very low molecular weight PEO in the blend nanofibers. The peak temperature of homogeneous nucleated crystallization was observed to shift to high supercoolings when the PEO molecular weight was decreased, and the effect was most pronounced for PEO with lowest molecular weight, that is, MPEO ~ 4000 g/mol. Furthermore, relaxation of PEO domains by thermally annealing the nanofibers above the glass transition temperature of PS resulted in an increased polydispersity in the PEO domain size when the PEO molecular weight was 4000 g/mol. This evidently led to the observation of dual homogeneous nucleated crystallization peaks which was not observed in blend nanofibers composed from PEO with MPEO ≥ 35 000 g/mol. The confinement was also found to impose a higher degree of mobility frustration on longer polymer chains such that the depression in the degree of crystallinity was significantly higher for high molecular weight PEO in the blend nanofibers. Furthermore, in the case of blend nanofibers containing mixture of two PEO with different molecular weights, the crystallization behavior was closely related to the dominant nucleation mechanism. Hence, in the case of blend nanofibers where the heterogeneous nucleation was the dominating crystallization mechanism, two melting peaks where observed revealing phase segregated crystallization behavior. However, the blend nanofibers which crystallized solely through homogeneous nucleation mechanism, the mixture of PEO fractions were found to cocrystallize as revealed from the observation of single melting peak. The cocrystallization was observed irrespective of the crystallization conditions and plausibly was a result of mobility restrictions imposed by the confinement at high degree of supercoolings.
Mixture of PEO, of two different molecular weights, were found to cocrystallize under confinement in electrospun nanofibers composed of PS/PEO blends. The cocrystallization was observed irrespective of the crystallization conditions and plausibly was a result of mobility restrictions imposed by the confinement at high degree of supercoolings. Under similar conditions, the bulk blend samples of the PEO mixtures and PS exhibited phase segregated crystallization.
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