Structural heterogeneities in starch hydrogels

Hydrogels have a complex, heterogeneous structure and organisation, making them promising candidates for advanced structural and cosmetics applications. Starch is an attractive material for producing hydrogels due to its low cost and biocompatibility, but the structural dynamics of polymer chains within starch hydrogels are not well understood, limiting their development and utilisation. We employed a range of NMR methodologies (CPSP/MAS, HR-MAS, HPDEC and WPT-CP) to probe the molecular mobility and water dynamics within starch hydrogels featuring a wide range of physical properties. The insights from these methods were related to bulk rheological, thermal (DSC) and crystalline (PXRD) properties. We have reported for the first time the presence of highly dynamic starch chains, behaving as solvated moieties existing in the liquid component of hydrogel systems. We have correlated the chains’ degree of structural mobility with macroscopic properties of the bulk systems, providing new insights into the structure-function relationships governing hydrogel assemblies

Block balance in hydrogenated polybutadiene-b-polymethylmethacrylate diblock copolymer for efficient interfacial activity in low-density polyethylene/polymethylmethacrylate blend: Phase morphology development

The objective of the present study was to determine the best molecular balance between the two hydrogenated polybutadiene (HPB) and polymethylmethacrylate (PMMA) blocks that promotes an HPB-b-PMMA diblock copolymer with efficient compatibilization activity in a low-density polyethylene (LDPE)/PMMA immiscible blend. The model blend selected, LDPE/PMMA, is "more immiscible" than the LDPE/polystyrene pair largely reported in open literature. The blends having a composition of 80LDPE/20PMMA exhibit a droplet-in-matrix phase morphology whereas in 20LDPE/ 80PMMA a co-continuous phase morphology was developed. In the droplet-in-matrix phase morphology, the emulsifying efficiency of the copolymer was evaluated based on the maximum reduction of the PMMA droplet size it is able to promote. Whereas, in the co-continuous phase morphology, the copolymer was evaluated based on its ability to stabilize the maximum phase co-continuity. The sequences of the best emulsifying copolymer revealed are not symmetrical. An HPB-b-PMMA where the ratio of molar mass of the blocks, M-n (HPB)/M-n (PMMA), is within 1.8-1.95 exhibits a much better interfacial activity in LDPE/PMMA blends than a copolymer of much lower ratio (longer PMMA block). This is ascribed to the much higher interactions (cohesive energy density) encountered in PMMA (PMMA of the copolymer and PMMA phase of the blend) compared with the LDPE side (HPB of the copolymer and LDPE phase of the blend). (c) 2005 Wiley Periodicals, Inc

Adsorption of polyampholyte copolymers at the solid/liquid interface: the influence of pH and salt on the adsorption behaviour

Polyampholytes are macromolecules that contain oppositely charged groups. We have studied the adsorption of the polyampholyte diblock copolymer poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate), PMAA-b-PDMAEMA, on oxidized silicon surfaces. The amount of polymer adsorbed from aqueous solution of different pH and salt concentration was measured by ellipsometry. The influence of the added salts NaCl, Na2SO4 and CaCl2 was determined. In every case adsorption took place, although the polyampholyte and the substrate exhibit the same sign of net charge. For all types of salt, the adsorbed amount shows two maxima close to the isoelectric point (IEP) of the polymer as a function of pH. Directly at the IEP of the polyampholyte, no adsorption was found. The measured dependences can be explained by the adsorption of one or the other of the two blocks depending on acidity and ionic strength. Furthermore, the lateral structure of the dried films was investigated by scanning force microscopy (SFM)

Effect of block copolymers of various molecular architecture on the phase morphology and tensile properties of LDPE rich (LDPE/PS) blends

The emulsification efficiency of three different block copolymers consisting of hydrogenated polybutadiene (HPB) and polystyrene (PS), i.e. a pure diblock , a tapered diblock and a triblock copolymer has been compared in low density polyethylene/polystyrene (LDPE/PS) blends rich in polyethylene. The comparison relies upon the ability of these potential interfacial agents to stabilize fine phase dispersion and to promote good interfacial adhesion. Based on the phase morphology, the ultimate tensile properties and the dynamic viscosity of the modified blends, the tapered diblock copolymer is clearly the most efficient emulsifier. For instance a plateau is observed in the property-copolymer content dependence when 2 wt% tapered diblock are used compared to ca. 5 wt% in case of the pure diblock. In contrast, no plateau is observed when the triblock copolymer is used. This is assumed to result from a less quantitative localization of these two copolymers i.e. the pue diblock or the triblock at the LDPE/PS interface

Reactively and physically compatibilized immiscible polymer blends: Stability of the copolymer at the interface

This paper reports on the interfacial behaviour' of block and graft copolymers used as compatibilizers in immiscible polymer blends. A limited residence time of the copolymer at the interface has been shown in both reactive blending and blend compatibilization by preformed copolymers. Polystyrene (PS)/polyamide6 (PA6), polyphenylene oxide (PPO)/ PA6 and polymethylmethaciylate (PMMA)/PA6 blends have . been reactively compatibilized by a styrene-maleic anhydride copolymer SMA. The extent of miscibility of SMA with PS, PPO and PMMA is a key criterion for the stability of the graft copolymer' at the interface. For the first 10 to 15 minutes of mixing, the in situ formed copolymer is able to decrease the particle size of the dispersed phase and to prevent it from coalescencing. However, upon increasing mixing time, the copolymer leaves the interface which results in phase coalescence. In PS/LDPE blends compatibilized by preformed PS / hydrogenated polybutadiene (hPB) block copolymers, a tapered diblock stabilizes efficiently a co-continuous two-phase morphology, in contrast to a triblock copolymer that was unable to prevent phase coarsening during annealing at 180°C for 150 minutes

Stabilization of a cocontinuous phase morphology by a tapered diblock or triblock copolymer in polystyrene-rich low-density polvethylene/polstyrene blends

The stability against the thermal annealing of a cocontinuous two-phase morphology developed in polystyrene (PS)/low-density polyethylene (LDPE) blends containing 80 wt % PS was investigated. Blends containing 1, 5, and 10 wt % of a tapered diblock poly(styrene-block-hydrogenated butadiene) (P(S-b-hB)) or triblock poly(styrene-block-hydrogenated butadiene-block-styrene) (P(S-hB-S)) copolymer were melt-blended with roll-mill mixing equipment. The efficiency of each of the two copolymers in stabilizing against coalescence the cocontinuous morphology was examined. The tensile properties of the resulting blends, annealed and nonannealed, were also examined in relation to the morphology induced by thermal annealing. The phase morphology was studied by optical and scanning electron microscopy. With computer-aided image analysis, it was possible to obtain a measurable characteristic parameter to quantify the cocontinuous phase morphology. When it was necessary, the extraction of one phase with a selective solvent was performed. Although the observed differences were subtle, the tapered diblock exhibited a more efficient compatibilizing activity than the triblock copolymer, particularly at a low concentration of about 2 wt %. The superiority of the tapered diblock over the triblock might be due to its ability to quantitatively locate at the LDPE/PS interface and consequently form a more efficient barrier against the subsequent breakup of the elongated structures of the cocontinuous phase morphology. The tensile properties of the triblock-modified blends were more sensitive to thermal annealing than the tapered-modified ones. This deficiency was ascribed to the phase morphology coarsening of the dispersed polyethylene phase

Miscibility, crystallization κinetics and real-time small-Angle x-ray scattering investigation of the semicrystalline morphology in thermosetting polymer blends of epoxy resin and poly(ethylene oxide)

Thermosetting polymer blends of poly(ethylene oxide) (PEO) and bisphenol-A-type epoxy resin (ER) were prepared using 4,4&prime;-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA) as curing agent. The miscibility and crystallization behavior of MCDEA-cured ER/PEO blends were investigated by differential scanning calorimetry (DSC). The existence of a single composition-dependent glass transition temperature (Tg) indicates that PEO is completely miscible with MCDEA-cured ER in the melt and in the amorphous state over the entire composition range. Fourier-transform infrared (FTIR) investigations indicated hydrogen-bonding interaction between the hydroxyl groups of MCDEA-cured ER and the ether oxygens of PEO in the blends, which is an important driving force for the miscibility of the blends. The average strength of the hydrogen bond in the cured ER/PEO blends is higher than in the pure MCDEA-cured ER. Crystallization kinetics of PEO from the melt is strongly influenced by the blend composition and the crystallization temperature. At high conversion, the time dependence of the relative degree of crystallinity deviated from the Avrami equation. The addition of a non-crystallizable ER component into PEO causes a depression of both the overall crystallization rate and the melting temperature. The surface free energy of folding &sigma;e displays a minimum with variation of composition. The spherulitic morphology of PEO in the ER/PEO blends exhibits typical characteristics of miscible crystalline/amorphous blends, and the PEO spherulites in the blends are always completely volume-filling. Real-time small-angle X-ray scattering (SAXS) experiments reveal that the long period L increases drastically with increasing ER content at the same temperatures. The amorphous cured ER component segregates interlamellarly during the crystallization process of PEO because of the low chain mobility of the cured ER. A model describing the semicrystalline morphology of MCDEA-cured ER/PEO blends is proposed based on the SAXS results. The semicrystalline morphology is a stack of crystalline lamellae; the amorphous fraction of PEO, the branched ER chains and imperfect ER network are located between PEO lamellae.<br /

Phase morphology development in poly(ethylene terephthalate) (PET)/ low density poly(ethylene) (LDPE) blends: compatibilizer precursors effect

In attempt to enhance the compatibility of PET/LDPE blends by using a proper functionalized polymer as third component, diethyl maleate (DEM)-functionalized ultralow density poly(ethylene) (ULDPE-g-DEM) and styrene-b-(ethylene-co-1-butene)-b-styrene triblock copolymer (SEBS-g-DEM) were prepared by radical functionalization in the melt. Immiscible PET/LDPE blends having compositions of 70/30 and 80/20 by weight were then extruded in the presence of 1–10% by weight of ULDPE-g-DEM and SEBS-g-DEM as compatibilizer precursors and ZnO (0.3% by weight) as transesterification catalyst. In both cases, evidences about the occurring of compatibilization between the two immiscible phases, thanks to the studied reactive processes, were obtained. Moreover, the phase distribution and particle size of blends were deeply investigated. Completely different kinds of phase morphology were achieved, as ULDPE-g-DEM stabilized a dispersed phase morphology, whereas SEBS-g-DEM favored the development of a cocontinuous phase morphology. The observed differences are tentatively explained on the basis of reactivity and physical features of polymers