Nanoparticle-Driven Intermolecular Cooperativity and Miscibility in Polystyrene/Poly(vinyl methyl ether) Blends

Phase separation behavior of poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) in the presence of hollow silica nanotubes

The phase separation behavior of poly(methyl methacrylate) (PMMA)/poly(styrene-co-maleic anhydride) (SMA) blends with and without one-dimensional hollow silica nanotubes (HSNTs) was investigated using time-resolved small-angle laser light scattering. During isothermal annealing over a range of 100 °C above the glass transition temperature, the Arrhenius equation is applicable to describe the temperature dependence of phase separation behavior at the early and late stages of spinodal decomposition (SD) for unfilled and filled PMMA/SMA systems. The mechanical barrier effect of HSNTs on the macromolecular chain diffusion of the blend matrix may retard the concentration fluctuation at the early stage and slow down the domain coarsening at the late stage of SD phase separation for the blend matrix to result in the decrease of apparent diffusion coefficient D app, the postponement of the relaxation time and the decline of temperature sensitivity for the phase separation rate.

Phase miscibility and dynamic heterogeneity in PMMA/SAN blends through solvent free reactive grafting of SAN on graphene oxide

The spatial distribution of nanoparticles in a particular host polymer matrix can be improved by using brush coated nanoparticles. In this work we have grafted styrene-acrylonitrile (SAN) onto the surface of graphene oxide (GO) and investigated as to how the demixing temperature, morphology and volume cooperativity of PMMA/SAN blends are influenced. Grafting of polymer chains on the surface of nanoparticles usually involves the use of large amounts of solvents, many which are detrimental to the environment besides involving cumbersome processes. SAN-g-GO was prepared by a robust solvent-free strategy wherein the cyano group in SAN was replaced by oxazoline groups during melt mixing in the presence of zinc acetate and ethanol amine. These newly created oxazoline groups reacted with the COOH group of GO under melt extrusion resulting in grafting of SAN on the surface of GO sheets. The effect of SAN-g-GO nanoparticles on the demixing, local segmental motions and morphology evolution for different annealing times was carefully investigated in a classical LCST system, PMMA/SAN blend, using melt rheology, modulated DSC and AFM, respectively. The changes in viscoelastic behavior in the vicinity of demixing are investigated systematically for the control, and blends with GO and SAN-g-GO. Various models were used to gain insight into the spinodal decomposition temperatures of the blends. Interestingly, the demixing temperature determined rheologically and the spinodal decomposition temperature increased significantly in the presence of polymer grafted nanoparticles in comparison to the control and blends with GO. The evolution of the morphology, interfacial driven coarsening as a function of temperature and the localization of nanoparticles were assessed using atomic force microscopy. The cooperatively re-arranging regions estimated from calorimetric measurements begin to suggest enhanced dynamic heterogeneity in the presence of GO and SAN-g-GO in the blends. Taken together, our study reveals that the solvent-free approach of grafting SAN onto GO delays demixing, suppresses coalescence and alters cooperative relaxation in PMMA/SAN blends.

Phase separation and physico-chemical processes at microscopic and macroscopic levels in MWCNT laden polymer blends using a unique droplet based architecture

We propose a unique contact-free droplet based architecture in which thermally induced instabilities can be used to precisely alter the phase separation behavior in a dynamically asymmetric polymer blend (solution of PS/PVME in toluene) by controlling the external heating rates and concentration of added nanoparticles (multi-walled carbon nanotube particles, MWCNTs). In addition, by tuning the heating rates, distinctly different macroscopic morphologies (hollow shell or globular mass) can be obtained as a final structure in such droplets. Furthermore, the process of separation is temporally aggravated by several orders (about 3-5 orders) as compared to the traditional bulk processing techniques (thin film of blends). Faster production rate and high throughput promise a new spray-based architecture for producing phase separated structures. Addition of MWCNTs in the polymer blend delays the separation phenomenon as it interacts with the polymers and alters the stability criteria. Furthermore, addition of nanoparticles also introduces a different mode of instability at higher external heating rates. Heat accumulation due to particles causes boiling of the solvent (toluene) trapped inside the droplet which leads to subsequent explosion of the entire droplet, in addition to the phase separation phenomena (at the microscopic level). Volumetric expansion due to bubble growth leads to the formation of a unique hollow structure which is distinctly different from the globular mass obtained at lower heating rates.

Thermally induced phase separation in levitated polymer droplets

We report thermally induced rapid phase separation in PS/PVME polymer blends using a unique contact free droplet based architecture. De-mixing of homogeneous blends due to inter component dynamic asymmetry is aggravated by the externally supplied heat. Separation of polymer blends is usually investigated in the bulk which is a tedious process and requires several hours for completion. Alternatively, separation in droplet configuration reduces the process timescale by about 3-5 orders due to a constrained micron-sized domain [fast processing and high throughput] while maintaining similar separation morphologies as in the bulk. We observed the effect of heating rates on the phase separation length and timescales. Furthermore, the separation length scale can be precisely controlled across one order by simply tuning the heating rate. The methodology can be scaled up for applications ranging from surface patterning to pharmaceutics.

Is kinetic polymer arrest very specific to multiwalled carbon nanotubes?

In this study we have assessed, using dielectric relaxation spectroscopy (DRS), the confinement effects of the more mobile chain in partially miscible polymeric blends of PS/PVME (polystyrene/poly(vinyl methyl ether)) in the presence of anisotropically shaped MWCNTs (multiwalled carbon nanotubes). To understand if this confinement effect is very specific to MWCNTs, the characteristic dimensions of which are often close to the radius of gyration of the polymeric chains, a few other particles like spherical silver, stacked clay tactoids and platy graphene sheets at similar weight fractions were also incorporated and systematically studied. The DRS studies reveal that the more mobile chain (here PVME) experiences possibly two different environments in the presence of frozen PS and more importantly in the presence of MWCNTs at temperatures close to and not so far from the blend T_g. The presence of bimodal relaxations with a weak temperature independent faster relaxation in the blends is composition dependent (PS rich blends). Assuming that there are no chemical interactions of PVME with the particles, these confinement effects seem to be very specific to MWCNTs as the bimodal relaxations were completely absent in the case of other nanoparticles. In the case of polymer blends, when two different chains are brought together, a loss in the deformational entropy is expected due to the excluded volume interaction and chain connectivity effects. In the presence of nanoparticles, especially MWCNTs, the polymer coils are subjected to perturbation leading to entropic loss in the system, which determine the miscibility in the blends. The configurational entropy near glass transition was assessed to understand the improved miscibility due to MWCNTs in this particular blend. The length of cooperativity suggests a cooperative motion of PS and PVME over shorter length scales in the case of MWCNTs as compared to other particles. This also hints at perturbed PVME motion in the network of MWCNTs. Taken together, our study reveals that the kinetic PVME arrest results in two different environments and is dependent on the effective concentration of MWCNTs in the blends.

Segmental dynamics, morphology and thermomechanical properties of electrospun poly(ε-caprolactone) nanofibers in the presence of an interacting filler

The effect of addition of an interacting nanofiller (Cloisite 10A) on the segmental dynamics, morphology and thermal stability of poly(ε-caprolactone) (PCL) is subjected to detailed investigation.

Nanoparticle induced miscibility in LCST polymer blends: critically assessing the enthalpic and entropic effects

The use of polymer blends widened the possibility of creating materials with multilayered architectures.

Mapping the intriguing transient morphologies and the demixing behavior in PS/PVME blends in the presence of rod-like nanoparticles

The demixing behavior, transient morphologies and mechanism of phase separation in PS/PVME blends were greatly altered in the presence of a very low concentration of rod-like particles (multiwall carbon nanotubes, MWNTs). This phenomenon is due to the specific interaction of one of the phases (PVME) with the anisotropic MWNTs, which creates a heterogeneous environment in the blend. This specific interaction alters the chain dynamics in the interfacial region as against the bulk. A comprehensive analysis using isochronal temperature sweep was performed to understand the demixing temperature in the blends. The evolution of phase morphology as a function of time and temperature was assessed by polarizing optical microscopy (POM), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The addition of MWNTs increased the rheological demixing temperature and the spinodal temperature in almost all the compositions. The intriguing transient morphologies were mapped, which varied from nucleation and growth to coalescence-induced viscoelastic phase separation (C-VPS) in PVME-rich blends, to spinodal decomposition in the near-critical compositions, to transient gel-induced VPS (T-VPS) in the PS-rich compositions. Mapping of the morphology development displayed two types of fracture mechanisms: ductile fracture for near-critical compositions and brittle fracture for off-critical composition. The change in the phase separation mechanism in the presence of MWNTs was due to the variation in dynamic asymmetry brought about by these anisotropic particles. All these observations were correlated by POM, SEM and AFM studies. The length of the cooperatively rearranging region (CRR), as evaluated using modulated differential scanning calorimetry (MDSC) measurements, was found to be composition-independent. The observed variation of effective glass transition of PVME (low Tg component) on blending with PS (high Tg component) and by the addition of MWNTs accounts for the dynamic heterogeneity introduced by MWNTs in the system.

The key role of polymer grafted nanoparticles in the phase miscibility of an LCST mixture

A lower graft density leads to entropic penalty, further facilitating PS-g-nAg particles to localize in the PVME phase of the blends. Further, the PS-g-nAg particles delayed the demixing temperature by 18 °C in PS-Br–PVME blends.

A unique strategy towards high dielectric constant and low loss with multiwall carbon nanotubes anchored onto graphene oxide sheets

Multiwall carbon nanotubes (MWNTs) were anchored onto graphene oxide sheets (GOs) via diazonium and C–C coupling reactions and characterized by spectroscopic and electron microscopic techniques.

Simultaneous enhancement in mechanical strength, electrical conductivity, and electromagnetic shielding properties in PVDF–ABS blends containing PMMA wrapped multiwall carbon nanotubes

A unique approach was adopted to drive the multiwall carbon nanotubes (MWNTs) to the interface of immiscible PVDF–ABS blends by wrapping the nanotubes with a mutually miscible homopolymer (PMMA).

Manipulating the kinetics and mechanism of phase separation in dynamically asymmetric LCST blends by nanoparticles

The addition of nanoparticles in dynamically asymmetric LCST blends is used to induce the preferred phase-separating morphology by tuning the dynamic asymmetry, and to control the kinetics of phase separation by slowing down (or even arresting) the domain growth.