Interfacial reciprocal grafting by free radical reactions in polymer blends

Recent developments in the field of reactive compatibilization of polymer blends prepared by melt processing focus on the addition of low molecular weight compounds. This work deals with in situ compatibilization through the formation of graft or crosslinked copolymers at the interface. Mixtures of semicrystalline hydrocarbon polymers have been subjected to free radical reactivity, in a co-rotating twin screw extruder (ZSK 30) in a single step. The particular system, high density polyethylene and polyamide 6, was blended in the presence of a peroxide and a reactive bifunctional monomer, maleic anhydride. Because of a combined effect, the reaction appears to occur mainly at the interface, where the resulting grafted copolymer acts as an anchor for the final stabilization of the biphasic system. Different analytical techniques, such as differential scanning calorimetry, scanning electron microscopy and tensile testing, helped in characterizing the resulting blends and confirmed the high level of interfacial grafting and the expected improvement in mechanical properties.     

Controlled migration of antifog additives from LLDPE compatibilized with LLDPE grafted maleic anhydride

This paper summarizes a study of controlled migration of an antifog (AF) additive; sorbitan monooleate (SMO), from linear low density polyethylene (LLDPE) films containing a compatibilizer, LLDPE grafted maleic anhydride (LLDPE‐g‐MA). LLDPE/LLDPE‐g‐MA/SMO blends were prepared by melt compounding. Bulk and surface properties of compression molded LLDPE films containing SMO and LLDPE‐g‐MA were characterized using Fourier transform infrared spectroscopy and contact angle measurements. Thermal properties were investigated using a thermal gravimetric analyzer. Diffusion coefficient (D) was calculated, and AF properties were characterized using a “hot fog” test. Compression molded films were characterized for their morphology using high‐resolution scanning electron microscopy, and rheological properties were measured using a parallel‐plate rotational rheometer. It was found that the LLDPE/LLDPE‐g‐MA/SMO systems are characterized by a slower SMO migration rate, a lower diffusion coefficient, and lower contact angle values compared with LLDPE/SMO blends. These results are well correlated with results of a hot fog test. Morphological studies revealed a very fine dispersion of SMO in the LLDPE films, when 3 phr LLDPE‐g‐MA was combined with 1 phr SMO. Thermal analysis results show that the incorporation of 3 phr LLDPE‐g‐MA and 1 phr SMO significantly increases the decomposition temperature of the blend at T > 400°C. At high shear rates, the LLDPE blends show that the AF and the compatibilizer have a lubrication effect on LLDPE. Copyright © 2014 John Wiley & Sons, Ltd.     

Glassy polymer solutions: Morphology of in situ crystallized additives

In the present work the phenomenon of in situ crystallization of low molecular weight organic compounds from an amorphous polymer was studied. Phase diagrams of tetrachloroxylene and tetrachlorobenzene with polystyrene showing softening, precipitation, and dissolution have been constructed. Three modes of crystallization, namely, needlelike, dendritic, and tiny crystalline particles have been observed depending on cooling rate. Hot stage optical microscopy and electron microscopy have been used to study crystallization, dissolution, and morphology. Softening temperature was determined by the vicat method. Shearing of samples undergoing crystallization has produced oriented samples.     

Modification of porous suspension‐PVC particles by stabilizer‐free aqueous dispersion polymerization of absorbed acrylate monomers

The present study describes modification of porous PVC particles by polymerization of a monomer/crosslinker/peroxide solution absorbed within the PVC particles. The modifying crosslinked polymers include butyl acrylate (BA) crosslinked with ethylene glycol dimethacrylate (EGDMA) and ethylhexyl acrylate (EHA) crosslinked with EGDMA. The monomer solution is blended with the PVC particles by dry‐blending. The monomer absorbed particles are then polymerized in a stabilizer‐free aqueous dispersion‐polymerization. The modified semi‐IPN PVC particles have better stability than the neat PVC particles in packed columns for absorption of halo‐organics from water, etc. The modified semi‐IPN PVC particles are melt processable and thus have the potential of being interesting and useful modified rigid PVC materials. The modified PVC particles characterization includes polymerization yield, non‐extractables and porosity measurements and also morphology and dynamic mechanical behavior (DMTA). PBA and PEHA polymerization has shown high yield levels. The high conversion of BA and EHA within the particle, is partly due to their low solubility in water. The levels of non‐extractable fractions found are indicative of low chemical interaction between the polyacrylate/PVC phases in the particle. The modified PVC particle's porosity levels indicate that BA and EHA partly polymerize within the PVC particles' bulk and partly in the pores as crusts covering the PVC pore surfaces. This finding is supported by SEM observations of unetched and etched freeze fractured surfaces. Higher crosslinking levels of the polyacrylate modification promote compatibility with the PVC particles' bulk. DMTA measurements show two loss modulus peaks for the 0.5%EGDMA blends in the glass transition temperature region, suggesting imcompatibility. However, at 5%EGDMA a single transition is found exhibiting enhanced compatibility owing to the high degree of crosslinking, which prevents phase separation. Copyright © 2002 John Wiley & Sons, Ltd.     

The structure of latex polystyrenes blends: dynamic properties of PS/XPS systems

The dynamic behavior of latex polystyrene/crosslinked polystyrene (PS/XPS) systems has been studied. PS and XPS lattices were either physically blended, or used as seeds in semi-continuous sequential two-stage polymerizations. The Cox–Merz equation is not met by any PS/XPS system. Power-law relaxation has been observed for the physically blended PS–b–XPS system in the terminal region. A two-parameter equation suggested for polymers at the critical gel point fits the experimental data for XPS contents beyond 60%. This has been attributed to the formation of a “continuous” XPS network. This network is able to disrupt upon flow, due to the unique morphology of the XPS phase. It is possible that, for the sequential system with XPS seed particles, such a network may appear at a lower XPS content, due to a PS occlusion effect. A qualitative model is presented for these PS/XPS systems.     

Morphology of ternary immiscible polymer blends

Ternary blends consisting of thermoplastic and thermotropic immiscible polymers were studied. Both thermodynamic and kinetic considerations were found to affect their multiphase structure. Thermodynamics is expressed by means of spreading coefficients, whereas the kinetic effect is driven by the dispersed phase viscosity ratio. Some morphologies could be predicted, when both effects acted cooperatively. However, in cases where the effects were opposing, kinetics hindered the development of the expected structure; interpenetration between the two minor phases, rather than engulfing or separately dispersed morphology, took place. In cases where two relatively polar phases were dispersed in a nonpolar matrix (e.g., nylon and polycarbonate in polypropylene), the interaction between the two dispersed minor phases always existed due to their low interfacial tension. Spreading of one minor phase over another, rather than penetration, is the dominating mechanism of encapsulation in polymer blends, contrary to low molecular weight liquids where both spreading and penetration play an important role in the structurization.     

Effect of Molecular Modification on PCL Foam Formation and Morphology of PCL

Poly(ϵ-caprolactone) was chemically modified by using dicumyl peroxide from 0.25 to 2 % (w/w) and the effects of molecular architecture on the density and morphology of PCL foams were examined. The polymer was first blended with dicumyl peroxide at a low temperature (80°C), to prevent premature peroxide decomposition. The peroxide modification was then performed at different temperatures, from 110°C to 150°C. The reaction kinetic was followed by measuring the dynamical rheological properties of the melt in isothermal experiments by using a parallel plate rheometer. The evolution of the macromolecular structure during the chemical reaction was followed by analyzing the time evolution of the complex viscosity. Foams were prepared from the peroxide modified PCL with a batch foaming process using nitrogen as the foaming agent under different process conditions. As expected, the increase of the molecular modification led to a shift towards higher temperatures of the foaming window and, moreover, influenced the viscoelastic behavior of the expanding polymeric matrix so that the final foam properties are affected.     

Conductivity and structure of melt‐processed polyaniline binary and ternary blends

In the present study, conductive binary and ternary blends containing polyaniline (PANI) were developed through melt blending. The binary blends' investigation focused on the morphology, in light of the components' interaction, and the resulting electrical conductivity. Similar solubility parameters of a given doped PANI and a matrix polymer lead to dispersion of fine PANI particles within the matrix, and to formation of conducting paths at low PANI contents. A plasticizer acting also as a compatibilizer improves the matrix polymer/PANI interactions. In ternary blends consisting of PANI and two immiscible polymers, the PANI preferrentially locates in one of the components, affecting the blend's morphology. This “concentrating” effect leads to relatively high electrical conductivity at a low PANI content. The electrical conductivity of the studied ternary blends is almost independent of the components' sequence of addition into the hot melt mixing device, exhibiting the selectivity of PANI towards one of the components. Copyright © 2000 John Wiley & Sons, Ltd.     

Computer-aided selection of a polymer matrix for polyaniline conductive blends

The selection of a polymer matrix for a conductive blend with polyaniline and para-toluene sulfonic acid (PANI-pTSA) was performed using molecular simulation techniques, both a fast quantitative structure–properties relationship method as a first screening phase followed by atomistic simulation. Using the atomistic simulation method, the solubility parameters and the heat of mixing of each blend were calculated to enable the determination of compatible matrices in blends with PANI-pTSA, which was validated by experimental scanning electron microscopy fractographs. Based on such calculations, polycaprolactone (PCL)/PANI-pTSA phase diagrams were estimated, showing slight miscibility of polydispersed PANI in PCL, particularly the short chains fraction, at the elevated melt processing temperature. It was suggested that this partial miscibility at the elevated temperature might lead to a conductive network morphology of PANI in PCL at room temperature, because of phase separation and precipitation of soluble PANI molecules, upon cooling and solidification of the melt. © 1998 John Wiley & Sons, Ltd.