Sub-micron dispersed-phase particle size in polymer blends: overcoming the Taylor limit via solid-state shear pulverization

A comparison was made of the fineness of dispersion in immiscible polymer blends achieved by a continuous mechanical alloying technique, solid-state shear pulverization, relative to that achieved by melt mixing. Two polymer blend systems were investigated. A polystyrene (PS)/polyethylene (PE) wax blend was studied because, based on a classic analysis by G.I. Taylor, melt mixing was expected to yield a number-average dispersed-phase domain size, D_n, well above 1 μm. A PS/high density polyethylene (HDPE) blend was also studied because it was known to produce a sub-micron number-average dispersed-phase particle size when mixed by twin-screw extrusion. In the case of the PS/PE wax blend at compositions ranging from 1 to 15 wt% polyethylene wax, pulverization resulted in nearly identical D_n values (typical value of 0.7 μm) independent of minor-phase content; these D_n values were an order of magnitude smaller than the anticipated Taylor limit for melt-mixed blends. In contrast, PS/PE wax blends made by batch, intensive melt mixing yielded D_n values between ∼3 μm at both 1 and 5 wt% minor-phase content and 17.5 μm at 15 wt% minor-phase content. The increase in D_n with increasing dispersed-phase content in the melt-mixed blend is a consequence of coalescence present during melt processing; such effects are disallowed in the pulverization process occurring in the solid state. Scanning electron microscopy of a 95/5 wt% PS/HDPE blend provided D_n values of 500 and 270 nm in the twin-screw extruded and pulverized samples, respectively. Fractionated crystallization studies further corroborated the ability of pulverization to result in a finer, nanoscopic dispersion of the minor phase as compared to extrusion.     

Spinodal clustering induced dewetting and non-monotonic stabilization of polymer blend films at high nanofiller concentrations

We examine the effects of high fullerene nanoparticle (f-NP) concentrations, ?_f-NP ∼ (10–20) mass% on polystyrene (PS)/polybutadiene (PB) blend thin film stability. Dewetting of the polymer blend around spinodally clustered f-NPs in this high concentration limit leads to a spinodal like dewetting morphology. This is in contrast to our previously observed results on the stabilization effects of f-NPs on PS/PB blend thin films in the intermediate f-NP concentration range of 7–10 mass%, wherein, after saturating the polymer–blend interface, the NPs stabilize the film by segregating to the blend–substrate interface. We determine three regimes of polymer blend film stability as a function of filler concentration: a) ?_f-NP < 7 mass% where preferential segregation of the f-NPs to the polymer–polymer interface leads to macroscopic dewetting, b) ?_f-NP ∼ (7–10) mass% where PS/PB blend films exhibit complete film stability, and c) ?_f-NP ∼ (11–20) mass%, where spinodal clustering of the f-NPs gives rise to polymer–NP phase exclusion and subsequent dewetting.     

Solid-state NMR studies on phase behavior and motional mobility in binary blends of polystyrene and poly(cyclohexyl methacrylate)

The phase behavior and motional mobility in binary blends of polystyrene (PS) and poly(cyclohexyl methacrylate) (PCHMA) have been investigated by solid state ¹³C NMR techniques. The blend miscibility has been studied by examining the ¹H spin-relaxation times in the laboratory frame (T₁^H) and in the rotating frame (T_1ρ^H) for the PCHMA/PS blends with various compositions and pure components. The T_1ρ^H results show that PCHMA and PS are intimately mixed at the molecular level within the blends at all compositions. In addition, according to the results of carbon T_1ρ relaxation time measurements, we conclude that mixing is intimate enough to cause a reduction in local chain mobility for PS, but an increase in side chain mobility for PCHMA.     

Electric field enhanced sample preparation for synthetic polymer MALDI-TOF mass spectrometry via Induction Based Fluidics (IBF)

MALDI-TOF mass spectroscopy is used in the characterization of synthetic polymers. MALDI allows for determination of: modal, most probable peak (M_P), molecular number average (M_N), molecular weight average (M_W), polydispersity (PD), and polymer spread (P_SP). We evaluate a new sample preparation method using Induction Based Fluidics (IBF) to kinetically launch and direct nanoliter volumes to a target without contact. IBF offers signal improvement via field enhanced crystallization. This is the first paper to discuss filed enhanced crystallization in MALDI sample preparation. IBF can increase signal/noise (S/N) and signal intensity for polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(ethylene glycol) (PEG) across a mass range of 2500-92,000 Da showing more accurate P_SP. Increases in S/N range up to: 279% for PS, 140% for PMMA, and 660% for PEG. Signal intensities increased up to: 438% for PS, 115% for PMMA, and 166% for PEG. Cross-polarization microscopy indicates dramatic morphology differences between IBF and micropipette. Finally, we speculate as to why IBF nanoliter depositions afford higher S/N values in experiments conducted in different instrumental configurations even without optimization.     

Preparation of submicron-sized porous titanium oxide particles by the swelling process

A method to prepare submicron-sized porous titanium oxide (TiO₂) particles is studied in this work. Polystyrene (PS) template particles were prepared by emulsifier-free emulsion polymerization. The polymer templates dispersed in the aqueous solution have been used for entrapping titanium(IV) isopropoxide (TTIP), by the swelling process in a suitable solvent mixture containing a swelling solvent (good solvent or poor solvent), a TiO₂ precursor (TTIP), and a chelating agent (AcAc), within the polymer templates, followed by hydrolysis/condensation reaction of TTIP confined in PS template particles by the addition of the chelating agent. The influence of various reaction parameters, such as mixtures of different weight ratios between the PS particles and desiccative TTIP, AcAc amounts, and the swelling solvent amounts and type, on the size, bulk, and composition of the particles was investigated. Porous TiO₂ particles have been prepared by thermal decomposition of the PS templates at 500 °C.     

Craze microstructure and molecular entanglements in polystyrene-poly(phenylene oxide) blends

Polystyrene and poly(phenylene oxide) are miscible over the entire range of compositions. Thin films of five blends of high molecular weight polystyrene (PS) with high molecular weight poly(phenylene oxide) (PPO), and four blends of low molecular weight PS (whose molecular weight lies below its entanglement molecular weight M_e) with the same PPO have been prepared. Following bonding of these films to copper grids, crazes were grown by uniaxial straining in air. Suitable crazes were then observed by transmission electron microscopy. From microdensitometry of the image plates it is possible to measure the extension ratio λ_craze within crazes in the nine blends. These measured values are compared with predicted values of λ_max, computed from λ_max = I_ed, where I_e is the chain contour length between entanglements and d is the root mean square end-to-end distance for a chain of molecular weight M_e. For the high molecular weight PS blends λ_max depends on the entanglement properties of both PS and PPO chains. For the low molecular weight PS blends, the PS chains cannot form part of the entanglement network and the correct value of λ_max is obtained from appropriate scaling of the pure PPO value. Comparison of λ_craze and λ_max for both types of blends shows excellent agreement, demonstrating the importance of the entanglement network in determining craze parameters and hence the toughness of a given polymer.     

Dewetting kinetics of thin polymer bilayers: Role of under layer

A combined experimental and computational study is presented to uncover the dewetting kinetics of the PS/PMMA system by changing the film thickness of the PMMA under layer. On the low M_w PMMA (M_w = 15 kg/mol) layer, the dewetting velocity of PS film firstly rapidly decreases (regime I), and then becomes almost invariant (regime II) with the increase of the film thickness of the liquid lower layer. Experiments suggest that the transition from regime I to regime II is correlative with the property of the solid substrates. The linear stability analysis of thin bilayers uncovers a bimodal behaviour of the instability under the experimental conditions and changeover of dominant mode of instability from one interface to the other is the major reason behind the switching of regimes. The nonlinear simulations closely mimic the experimental interfacial morphologies and suggest two different pathways of hole growth for regimes I and II under experimental condition. The simulations also indicate that the rapid reduction in the dewetting velocity is because of the increased excursion or penetration of the upper layer into the lower layer near the three phase contact line as the viscous resistance at the more viscous PMMA layer reduces with its increasing thickness. A qualitative match is thus found between the experimental and theoretical trends of the dewetting velocities. In addition, the experiments show that on a high M_w PMMA (M_w = 365 kg/mol) layer, the kinetics of hole growth of the PS layer is not affected by the PMMA layer thickness.     

Transition of the bounded polymer layer to a rigid amorphous phase: A computational and DSC study

The study focuses on the transition of the bounded to solid surfaces polymer layer to a rigid amorphous phase (RAP). Based on previous Monte Carlo (MC) simulation studies on bulk polyethylene (PE), we refine our numerical variable density Self Consistent Field (nSCF) method in order the calculated density in bulk to follow the predictions of the MC simulation. The proposed modification of the Sanchez–Lacombe equation of state allows us to examine thermodynamic aspects of the glass transition. By imposing a glass transition (T_g = 220 K) in the bulk we predict an earlier (during cooling), stronger transition of the bounded layer to a RAP, at ∼350 K. Also at short separation distances we record the appearance of undisturbed polymer bridges. Differential scanning calorimetry (DSC) experiments on polydimethylsiloxane (PDMS) and polyamide 6 (PA6) nanocomposites suggest that although the crystallization can be significantly suppressed by the addition of nanoparticles, the RAP layer may locally equilibrate, above T_g, for long experimental times with the melt phase. Our results support the idea that a significant free energy barrier of entropic origin appears due to the RAP formation, below the melting temperature (T_m) and above the T_g.     

Theoretical representation of shear and pressure influence on the phase separation behavior of PVME/PS mixture

In the framework of lattice fluid model, the Gibbs energy and equation of state are derived by introducing the energy (E_s) stored during flow for polymer blends under shear. From the calculation of the spinodal of poly(vinyl methyl ether) (PVME) and polystyrene (PS) mixtures, we have found the influence of E_s on equation of state in pure component is inappreciable, but it is appreciable in the mixture. However, the effect of E_s on phase separation behavior is extremely striking. In the calculation of spinodal for the PVME/PS system, a thin, long and banana miscibility gap generated by shear is seen beside the miscibility gap with lower critical solution temperature. Meanwhile, a binodal coalescence of upper and lower miscibility gaps is occurred. The three points of the three-phase equilibrium are forecasted. The shear rate dependence of cloud point temperature at a certain composition is discussed. The calculated results are acceptable compared with the experiment values obtained by Higgins et al. However, the maximum positive shift and the minimum negative shift of cloud point temperature guessed by Higgins are not obtained. Furthermore, the combining effects of pressure and shear on spinodal shift are predicted.     

Surface fluctuations of polymer brushes probed by diffuse X-ray scattering

The diffuse scattering from the surfaces of melt and glassy polymer brushes has been studied systematically for the first time using polystyrene (PS) and poly(n-butylacrylate) (PnBA) brushes synthesized by free radical polymerization. The data show unambiguously that the diffuse scattering behavior varies systematically with brush thickness for both types of brushes. We attribute a cross-over in scattering with q_x, the in-plane scattering vector, to the presence of surface thermal fluctuations and their suppression for longer wavelengths, a phenomenon already reported for films of untethered chains. Long wavelength fluctuations are suppressed more strongly on the surface of a PS brush than on the surface of a film of untethered (‘free’) PS chains of comparable thickness, so that even in films of thickness, d, such that d/R_g>5 clear evidence of the suppression of fluctuations can still be seen in the experimentally available range of q_x. Fluctuations are suppressed for q_x less than a lower wavevector cut-off, q_l,c, which changes with film thickness, though much more weakly than for films of free chains. For values of d/R_g<4, where R_g is the unperturbed radius of gyration of a comparable free chain, q_l,c drops as d increases. For d/R_g>4 q_l,c begins to increase with brush thickness, in qualitative agreement with theory, indicative of a transition to a true ‘brush’ state in which stretching of the chains makes longer wavelength fluctuations at the surface unfavorable. Measurements with PnBA brushes having T_g substantially below room temperature confirm the trends mentioned above. Further, they give evidence that the value of q_l,c is temperature insensitive above T_g.     

Studies of the positron lifetime and Doppler-broadened annihilation radiation of polypropylene-polystyrene alloys

Positron annihilation lifetime spectroscopy and Doppler-broadened annihilation radiation (DBAR) experiments were performed on polypropylene-polystyrene (PP-PS) alloys (0-100% styrene) prepared by in situ polymerisation of styrene in a PP host matrix. The mean size of free-volume holes estimated from the ortho-positronium lifetime τ₃ shows a continuous decrease from 0.119 nm³ in PP to 0.095 nm³ in PS. The intensity of the o-Ps component, I₃, the average positron lifetime τ_av, the curve-shape parameter S and the peak height H of the DBAR spectra increase linearly with the styrene concentration. This is attributed to a linear superposition of the Ps yields of PP and PS in the PP-PS alloys. The DBAR spectra were fitted by a sum of three Gaussians, the narrowest of them is attributed to self-annihilation of para-positronium confined within holes. After its subtraction, a ‘broad component’ is obtained which represents the momentum distribution of electrons bound to molecules. Its normalised peak height does not show any change with the composition which reflects the fact that both constituents of the PP-PS alloys contain only hydrogen and carbon atoms in their chemical units. Immersing of PP into styrene liquid leads to a very pronounced increases in the lifetime parameters which is attributed to the plasticisation of PP.     

Thermal degradation in bulk and thin films of 2-, 4-, and 6-arm polystyrene stars with a C60 core

The thermal stability (TS) of hexa-, tetra-, and di-arm polystyrene (PS) stars with a C₆₀ core was studied by thermal gravimetric analysis and mass-spectrometry. The quantitative production of volatile products, their composition and their formation kinetics during heating of (PS_xC₆₀) are reported. A bimodal release of styrene is observed. The first release takes place about 100 °C before the depolymerization temperature of styrene and all the C₆₀ comes out at this lower temperature. That results from a complete breaking of the weak PS-C₆₀ bonds followed by a partial depolymerization of the PS arms initiated by the so formed radicals. The amount of PS ‘surviving’ this first depolymerization step increases with the length of the arms and its TS is close to that of pure PS. The thermal stability of the PS_xC₆₀ stars decreases if the number of arms increases and, from the activation energy of the release of styrene and C₆₀, it was possible to estimate the PS-C₆₀ bond strength for these three adducts.     

Nanoscale artifacts in RuO4-stained poly(styrene)

This research studies the effects of RuO₄ exposure on various samples of poly(styrene) (PS). Transmission electron energy-loss spectroscopy (EELS) shows a decrease in the 7 eV π-π∗ transition characteristic of aromatic rings in PS indicating that RuO₄ covalently alters aromatic character. Imaging and selected-area electron diffraction show that a layer of RuO₂ solid forms on the surface of bulk PS specimens exposed to RuO₄. High-resolution TEM imaging (HREM) shows that RuO₂ nanocrystals consistently condense on specimen surfaces, independent of the chemical nature of the specimen below. These nanocrystals modulate contrast at length scales on the order of 2-5 nm in TEM images and limit resolution at nanometer length scales.     

Compatibilising action of random and triblock copolymers of poly(styrene-butadiene) in polystyrene/polybutadiene blends: A study by electron microscopy, solid state NMR spectroscopy and mechanical measurements

Scanning electron microscopy, solid-state proton NMR spectroscopy and static mechanical analysis have been performed in order to evaluate the compatibilising action of random copolymers of polystyrene and polybutadiene and triblock copolymers of poly(styrene-butadiene-styrene) in incompatible polystyrene/polybutadiene (PS/PB) blends. Scanning electron microscopic examination of the cryofractured and etched surfaces showed high degree of compatibilising action of the triblock copolymers as evidenced by the very sharp decrease of the domain size of the dispersed phase followed by an increase at higher concentrations. This is a clear indication of interfacial saturation. These results were in agreement with the theoretical predictions of Noolandi and Hong. The random copolymer was not effective in compatibilising the system. Solid-state proton NMR experiments were performed on the uncompatibilised and compatibilised blends. The proton spin-lattice relaxation times in the laboratory frame, T₁(H), and in the rotating frame, T_1ρ(H), and spin-spin relaxation times, T₂(H), were carefully measured for the systems. Significant changes were observed for the systems compatibilised with triblock copolymers due to the preferential localisation of the copolymers at the PS/PB interface. However, the random copolymer did not have any compositional drift and is not an effective interface modifier in agreement with microscopy study. The static mechanical properties of the blends have also been analysed. The addition of triblock copolymers increased the mechanical properties of the blends. Finally, attempts have been made to correlate the NMR results with the microstructure and mechanical properties of the blends.     

Phase separation and dewetting in polystyrene/poly(vinyl methyl ether) blend thin films in a wide thickness range

Phase separation and dewetting processes of blend thin films of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in two phase region have been studied in a wide film thickness range from 65 μm to 42 nm (∼2.5R_g, R_g being radius of gyration of a polymer) using optical microscope (OM), atomic force microscope (AFM) and small-angle light scattering (LS). It was found that both phase separation and dewetting processes depend on the film thickness and were classified into four thickness regions. In the first region above ∼15 μm the spinodal decomposition (SD) type phase separation occurs in a similar manner to bulk and no dewetting is observed. This region can be regarded as bulk. In the second region between ∼15 and ∼1 μm, the SD type phase separation proceeds in the early stage while the characteristic wavelength of the SD decreases with the film thickness. In the late stage dewetting is induced by the phase separation. In the third region between ∼1 μm and ∼200 nm the dewetting is observed even in the early stage. The dewetting morphology is very irregular and no definite characteristic wavelength is observed. It is expected that the irregular morphology is induced by mixing up the characteristic wavelengths of the phase separation and the dewetting. In the fourth region below ∼200 nm the dewetting occurs after a long incubation time with a characteristic wavelength, which decreases with the film thickness. It is considered that the layered structure is formed in the thin film during the incubation period and triggers the dewetting through the capillary fluctuation mechanism or the composition fluctuation one.     

Electrical, mechanical, and glass transition behavior of polycarbonate-based nanocomposites with different multi-walled carbon nanotubes

Five commercially available multi-walled carbon nanotubes (MWNTs), with different characteristics, were melt mixed with polycarbonate (PC) in a twin-screw micro compounder to obtain nanocomposites containing 0.25-3.0 wt.% MWNT. The electrical properties of the composites were assessed using bulk electrical conductivity measurements, the mechanical properties of the composites were evaluated using tensile tests and dynamic mechanical analysis (DMA), and the thermal properties of the composites were investigated using differential scanning calorimetry (DSC). Electrical percolation thresholds (p_cs) were observed between 0.28 wt.% and 0.60 wt.%, which are comparable with other well-dispersed melt mixed materials. Based on measurements of diameter and length distributions of unprocessed tubes it was found that nanotubes with high aspect ratios exhibited lower p_cs, although one sample did show higher p_c than expected (based on aspect ratio) which was attributed to poorer dispersion achieved during mixing. The stress-strain behavior of the composites is only slightly altered with CNT addition; however, the strain at break is decreased even at low loadings. DMA tests suggest the formation of a combined polymer-CNT continuous network evidenced by measurable storage moduli at temperatures above the glass transition temperature (T_g), consistent with a mild reinforcement effect. The composites showed lower glass transition temperatures than that of pure PC. Lowering of the height of the tanδ peak from DMA and reductions in the heat capacity change at the glass transition from DSC indicate that MWNTs reduced the amount of polymer material that participates in the glass transition of the composites, consistent with immobilization of polymer at the nanotube interface.     

Confined crystallization phenomena in immiscible polymer blends with dispersed micro- and nanometer sized PA6 droplets, part 3: crystallization kinetics and crystallinity of micro- and nanometer sized PA6 droplets crystallizing at high supercoolings

Crystallization kinetics and crystallinity development of PA6 droplets having sizes from 0.1 to 20 μm dispersed in immiscible uncompatibilized PS/PA6 and reactively compatibilized (PS/Styrene-maleic anhydride copolymer=SMA2)/PA6 blends are reported. These blend systems show fractionated crystallization, leading to several separate crystallization events at different lowered temperatures. Isothermal DSC experiments show that micrometer-sized PA6 droplets crystallizing in an intermediate temperature range (T_c∼175 °C) below the bulk crystallization show a different dependency on cooling rate compared to bulk crystallization, and an athermal crystallization mechanism is suggested for PA6 in this crystallization temperature region. The crystallinity in these blends decreases with PA6 droplet size. Random nucleation, characteristic for a homogeneous nucleation process, is found for sub-micrometer sized PA6 droplets crystallizing between T_c 85 and 110 °C using isothermal DSC experiments. However, crystallization in the PA6 droplets is most likely initiated at the PA6-PS interface due to vitrification of the PS matrix during crystallization. Very imperfect PA6 crystals are formed in this low temperature crystallization region, leading to a strongly reduced crystallinity. These crystals show strong reorganization effects upon heating.     

Micromechanical properties of glassy and rubbery polymer brush layers as probed by atomic force microscopy

Carboxylic acid terminated polystyrene and polybutylacrylate were grafted from melt onto a silicon substrate modified with the epoxysilane monolayer. The tethered layers fabricated from polymers of different molecular weights are smooth, uniform, mechanically stable, and cover homogeneously the modified silicon surface. Micromechanical properties of the dry glassy and rubbery brush layers were measured with atomic force microscope. We observed that for the PS layers with the thickness higher than 7 nm, the average value of the elastic moduli reached 1.1 GPa, which is close, but still lower than the expected for bulk polymer. The elastic modulus of PS polymer brush layers dramatically depends upon molecular weight and follows the inverse law with segment molecular weight, M_c of 18,000 known for bulk PS. This result indicates that the process of the formation of the physical network within polymer melt of chains tethered to a solid substrate is similar to that occurring in unconstrained polymer melt. Under these conditions, three PS brush layers studied in this work represent different cases of chains without stable entanglements for M<M_c as well as chains with stable entanglements for brushes with M∼M_c. This transition shows itself in significant reduction of the compliance reflected in twofold increase in elastic modulus. Our estimation predicts that modest lowering of ‘limiting’ elastic modulus of 1.4 GPa can be expected for thicker polymer brushes.     

Use of functionalized WS2 nanotubes to produce new polystyrene/polymethylmethacrylate nanocomposites

Multiwall WS₂ nanotubes of 40-50 nm diameter were functionalized with n-octadecyl phosphonic acid by sonication in toluene and blended with mixtures of polystyrene (PS) and polymethylmethacrylate (PMMA) to form new nanocomposite (NC) materials. The surface and domain structures were studied by atomic force microscopy (AFM), scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM) for various levels of loading of nanotubes up to 20 wt%. Phase-separated domain size and surface roughness of the nanocomposite films were found to be dramatically reduced relative to the pure homopolymer blend and good dispersal of the nanotubes in the blend matrix was attained.