Linear viscoelastic behavior of a series of 1,4-polyisoprene asymmetric star polymers was investigated experimentally and theoretically in order to determine how branch point motion affects terminal relaxation dynamics. For the systems studied, the branch point is connected to a moderately entangled short arm (M B/M e = 7) and two longer arms with varying numbers of entanglements (M A/M e = 7−43). The measured loss modulus manifests a clear transition from starlike to linear-like dynamics as the molecular asymmetry increases. We show that this behavior can be predicted using a tube-model for branched molecules (Frischknecht et al. Macromolecules 2002, 35, 4801) with a self-consistently determined branch-point diffusivity, D bp = (pa 0)2/2τa,B. Here a 0 is the effective bare tube diameter, τa,B is the arm retraction time, and p 2 = 1/[(2N AR)b 2/a 0 2] is an effective drag produced by unrelaxed backbone segments. We also compare relaxation moduli predicted by the tube model with experimental data and find good agreement over a range of short- and long-arm entanglement densities.
Electrospinning has received a lot of attention in recent years as a simple process to produce sub-micrometer-scale fibers. The process involves continuous stretching of polymer solution or melt in the presence of a strong electric field, which forms ultrathin fibers. A large amount of research is being carried out to achieve control of the diameter, morphology, and spatial alignment of electrospun nanofibers. [1,2] Unlike other 1D nanostructures, such as nanowires and nanotubes, nanofibers exhibit a wide range of unique properties, making them far more attractive for many applications, such as filtration, catalysis, sensing, protective clothing, and tissueengineering scaffolds.[3] Being a continuous process, electrospinning can produce extremely long fibers comparable to those formed via conventional mechanical drawing and spinning techniques. Nanofiber mats exhibit extremely high surface-to-mass ratios, greatly improving their efficiency for catalysis and filtration. [2] Another fascinating feature of this process is that it can be applied to a wide variety of nanostructured materials. Bognitzki et al. [4] fabricated sub-micrometerscale fibers by electrospinning ternary solutions of polylactide and poly(vinylpyrrolidone). They found that the fibers displayed an internal phase morphology, and, by selectively removing one phase, they obtained highly porous fibers. However, these morphologies were largely controlled by rapid phase separation and rapid solidification of the polymer jet, and no method for controlling these morphologies was demonstrated. In the present study, we utilize this simple process to fabricate block copolymer (BCP) nanofibers. BCP solutions and melts are known to self-assemble into a variety of nanoscale morphologies including spheres, rods, micelles, lamellae, vesicles, tubules, and cylinders, [5,6] depending on the volume fraction and interaction parameter between different blocks. We have been able to produce macroscale-length fibers with diameters of a few hundred nanometers that exhibit internal structures of only tens of nanometers in size. Such materials, we believe, can combine the unique properties of continuous nanofibers and BCP self-assembly for use in a variety of applications of nanostructured materials. BCP self-assembly has attracted increasing interest in recent years for applications in nanotechnology.[7] Precise control over the size, shape, and periodicity of these nanoscale microdomains is the key factor needed to realize nanoscale systems. Various methods, including shear and elongational deformation, compressional deformation, electric fields, and temperature gradients, have been utilized to induce orientation of the microdomains. To our knowledge, shear flow has been most extensively studied as a simple means to induce phase transitions and orient self-assembled structures in block copolymers. [8,9] The phenomenon of flow-induced alignment of lamellar BCPs is very well studied in bulk systems. [10][11][12] Three different orientations of lamellar morphology, namely, p...
Coaxial nanofibers with poly(styrene-block-isoprene) (PS-b-PI)/magnetite nanoparticles as core and silica as shell are fabricated using electrospinning.1-4 Thermally stable silica helps to anneal the fibers above the glass transition temperature of PS-b-PI and form ordered nanocomposite morphologies. Monodisperse magnetite nanoparticles (NPs; 4 nm) are synthesized and surface coated with oleic acid to provide marginal selectivity towards an isoprene domain. When 4 wt% nanoparticles are added to symmetric PS-b-PI, transmission electron microscopy (TEM) images of microtomed electrospun fibers reveal that NPs are uniformly dispersed only in the PI domain, and that the confined lamellar assembly in the form of alternate concentric rings of PS and PI is preserved. For 10 wt% NPs, a morphology transition is seen from concentric rings to a co-continuous phase with NPs again uniformly dispersed in the PI domains. No aggregates or loss of PI selectivity is found in spite of interparticle attraction. Magnetic properties are measured using a superconducting quantum interference device (SQUID) magnetometer and all nanocomposite fiber samples exhibit superparamagnetic behavior.
Stress relaxation dynamics of model branched homopolymers with a range of architectures (A2 B (T-shaped) and AB 2 (Y-shaped) asymmetric stars, AB n (combs), B 2-A-B 2 (H-shaped), and B 3-A-B 3 (pom-poms)) are studied using a tube-based theory to evaluate a recent proposal for branch point motion in hierarchically relaxing branched molecules. This model contends that if the random coil size R g, B of relaxed B arms connected via a branch point to an unrelaxed Z A-mer polymer backbone is larger than the equilibrium tube diameter a, the branch point can only move a small distance δ = (ap) of order a/ during each relaxation cycle of the arms. When this prediction is integrated into tube models for branched molecules, it yields a self-consistent theory suitable for describing linear viscoelasticity (LVE) of any branched polymer system. Without artifices, such as arbitrary adjustments of the measured molecular weights, arm functionality, or dilution exponent, we find that this theory yields LVE predictions that are consistent with experimental data from many groups.
Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene-b-isoprene), PS-b-PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post-fabrication annealing of the fibers to obtain equilibrium self-assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self-assembled structures. Various confined assemblies such as co-existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self-assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self-assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block-copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS-b-PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS-b-PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self-assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high-end applications such as drug delivery.
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