Monica Bertoldo

Modification of surface and mechanical properties of polyethylene by photo-initiated reactions

Low density polyethylene thin films were crosslinked in the bulk and on the surface by means of photo-initiated reactions. The purpose of these experiments was to study if there is a direct relationship between the distribution of crosslinks in the depth of the films and the mechanical properties of the films themselves. By bulk treatment with a PI1 photoinitiator and a dimethacrylate, samples with enhanced Youngs modulus values were obtained. The addition of an inorganic filler had no significant influence on the rate of the crosslinking reaction, but resulted in an enhancement of the oxygen barrier of the treated films. Samples with enhanced modulus and hydrophilicity were obtained by hot-surface treatment with a PI2 photoinititator and multifunctional allylic and acrylic monomers.

Oxidation of glycogen “molecular nanoparticles” by periodate

Glycogen, a natural hyperbranched polymer of glucose characterized by a substantially spherical macromolecular structure, was oxidized by periodate to obtain reactive intermediates for the preparation of nanostructured materials. The reaction was carried out at room temperature in water, methanol, isopropanol or N,N-dimethylformamide as solvents. The oxidized glycogen was analyzed by 1H NMR and the oxidation degree was determined by titration. The repartition of the aldehydes between the surface and the volume of the macromolecular nanoparticles was studied by selective cleavage of the oxidized monomeric units under Smith degradation conditions and SEC analysis. Tuning of the oxidation degree in the range of 1–40% (mole fraction of aldehyde per glucoside unit) was achieved by controlling the reagent feed ratio (periodate/glycogen), while the topology of the aldehyde group distribution was controlled by exploiting the kinetics of mass transport. When the reaction was carried out in water, at low oxidation degree values (0.5–3 mol% of oxidized monomeric units), the aldehyde groups were found to be mostly near the nanoparticle surface. A gradient of distribution of the aldehydes from the surface to the particle core was obtained at higher oxidation degrees (>5%). When the reaction was carried out in organic solvents the functionalization occurred mostly at the nanoparticle surface and also at a high oxidation degree. In particular, in N,N-dimethylformamide at 5% oxidation degree, the aldehyde groups were found to be near the macromolecular chain-ends, mostly located on the particle external shell.

Local structure of temperature and pH-sensitive colloidal microgels

The temperature dependence of the local intra-particle structure of colloidal microgel particles, composed of interpenetrated polymer networks, has been investigated by small-angle neutron scattering at different pH and concentrations, in the range (299÷315) K, where a volume phase transition from a swollen to a shrunken state takes place. Data are well described by a theoretical model that takes into account the presence of both interpenetrated polymer networks and cross-linkers. Two different behaviors are found across the volume phase transition. At neutral pH and T ≈ 307 K, a sharp change of the local structure from a water rich open inhomogeneous interpenetrated polymer network to a homogeneous porous solid-like structure after expelling water is observed. Differently, at acidic pH, the local structure changes almost continuously. These findings demonstrate that a fine control of the pH of the system allows to tune the sharpness of the volume-phase transition.

Oxygen and water vapor barrier properties of MMT nanocomposites from low density polyethylene or EPM with grafted succinic groups.

LDPE, EPM and their derivatives containing a moderate amount (0.08-1.8 by mol) of diethylsuccinate or succinic anhydryde groups were used as matrices in blending with different amount of organophilic montmorillonites and the resulting composite morphology and structure (by XRD, SEM, TEM microscopy, DSC analysis and selective solvent extraction) were studied with reference to the polar groups/MMT ratio. Exfoliated, intercalated and mixed morphologies were achieved. High concentrations of polar groups grafted to the polyolefin and montmorillonite loading not larger than 5% wt were favourable for obtaining high exfoliation degree. Particularly in the exfoliated MMT composite LDPE had lower crystallinity degree, while EPM showed increased glass transition temperature and reduced solubility in hot toluene. Moreover, oxygen and water vapor barrier property improvement was observed in films where MMT exhibits either exfoliated or intercalated morphologies. Strong interactions with the montmorillonite particle surface through the polar groups grafted to the polyolefin seems to be the basic effect responsible for the morphology and peculiar properties. A model based on the reduced mobility of the polymer located near the particle surface or inside the MMT gallery (confined phase) was proposed to explain the observed oxygen permeability reduction, the T(g) increase and solubility of poly(ethylene-ran-propylene)/MMT nanocomposites.

Swelling of responsive-microgels: experiments versus models

Abstract Interpenetrated Polymer Network (IPN) microgels of PNIPAM and PAAc have been investigated and the experimental data have been compared with theoretical models from the Flory–Rehner theory. We confirm that the swelling behavior of PNIPAM microgels is well described by this theory by considering the second order approximation for the volume fraction ϕ dependence of the Flory parameter χ(ϕ). Indeed the Volume-Phase Transition (VPT) of the PNIPAM-PAAc IPN microgel at neutral conditions and in D2O solvents can be well described only considering a third-order approximation. Interestingly we empirically find that sharper is the transition higher is the order of the χ(ϕ) relation which has to be considered. Moreover the VPT can be experimentally controlled by tuning the polymer/solvent interactions through pH and solvent allowing to directly modify the delicate balance between energetic and entropic contributions and to explore the swelling behavior in a wide range of environmental conditions. In particular we find that the most advantageous condition for swelling is in water at acidic pH.

Applicability of photochemically generated pendant benzoyl peroxides in both “grafting from” and “grafting to” techniques

Methyl methacrylate and styrene copolymers containing pendant benzil groups, such as 1-[4-(2-methacroyloxyethoxy)phenyl]-2-phenyl-1,2-ethanedione-co-methyl metacrylate (BzMA/MMA), 1-[4-(2-methacroyloxyethoxy)phenyl]-2-phenyl-1,2-ethanedione-co-styrene (BzMA/S), and 1-phenyl-2-(4-propenoylphenyl)-1,2-ethanedione-co-styrene (PCOCO/S), were prepared and used as precursors for photochemically generated pendant benzoyl peroxides. Decomposition of the pendant benzoyl peroxides was subsequently used in grafting processes. Either irradiation or a combination of irradiation with subsequent thermal treatment was adopted for grafting a thin layer of BzMA/MMA copolymer onto the surface of LDPE films. The grafting resulted in a significant decrease in contact angle of the film surface. The same activation strategy was successfully adopted to initiate the polymerisation of acrylic or methacrylic acids from the surface of styrene copolymer films containing the initiator precursor in the polymer side chains (BzMA/S and PCOCO/S). The successful surface grafting was proved by contact angles measurement as well as by infrared spectroscopic analysis.

Evidence of a low-temperature dynamical transition in concentrated microgels

The dynamical transition of hydrated proteins also occurs in nonbiological macromolecules, such as colloidal microgels. A low-temperature dynamical transition has been reported in several proteins. We provide the first observation of a “protein-like” dynamical transition in nonbiological aqueous environments. To this aim, we exploit the popular colloidal system of poly-N-isopropylacrylamide (PNIPAM) microgels, extending their investigation to unprecedentedly high concentrations. Owing to the heterogeneous architecture of the microgels, water crystallization is avoided in concentrated samples, allowing us to monitor atomic dynamics at low temperatures. By elastic incoherent neutron scattering and molecular dynamics simulations, we find that a dynamical transition occurs at a temperature Td ~ 250 K, independently from PNIPAM mass fraction. However, the transition is smeared out on approaching dry conditions. The quantitative agreement between experiments and simulations provides evidence that the transition occurs simultaneously for PNIPAM and water dynamics. The similarity of these results with hydrated protein powders suggests that the dynamical transition is a generic feature in complex macromolecular systems, independently from their biological function.

On the thermal behavior of protein isolated from different legumes investigated by DSC and TGA: Legume proteins investigated by DSC and TGA

BACKGROUND Pea, lentil, faba bean, chickpea and bean proteins are potentially renewable raw materials for bioplastic production that can be obtained from agricultural waste. Plastics are usually processed under heating, and thus thermal stability is a mandatory requirement for the application. In this study, the thermal behavior of several legume protein isolates at different purity degrees was investigated. RESULTS The thermal stability of proteins extracted from legumes was maximum for chickpeas and minimum for beans and decreased with decreasing protein purity in the range 30-88%. A similar dependence on purity was observed for the glass transition temperature. On the contrary, the denaturation temperature was found not to depend on sample purity and origin and was lower than the degradation temperature only in the case of protein samples with purity higher than 60%. CONCLUSION Proteins from legumes are suitable to produce thermoplastic biopolymeric materials if isolated at purity higher than 60%. In fact, under this circumstance, they can be denaturized without degrading and thus are suitable for extrusion processing.

Interpenetrating Polymer Network Microgels in Water: Effect of Composition on the Structural Properties and Electrosteric Interactions

Microgels of cross-linked interpenetrating polymer networks (IPNs) are very versatile systems combining the properties of colloids and polymers. Herein we study IPN microgels composed of poly(N-isopropylacrylamide) and poly(acrylic acid) to understand how weight composition and reactant concentrations affect their structural, conformational and electrosteric properties in water. The results show that it is possible to drive the formation of microgels with the desired properties by adjusting IPN composition and preparation method during the synthesis. During synthesis, the polymerization of acrylic acid triggers the merging among IPNs via covalent linking, giving rise to microgels with larger mass and size, the effect being larger for higher concentration of the reactants. In addition, a close relation between the microgel internal conformation and the colloidal stability is observed, due to the presence of screened groups inside the microgel.

Study of the compounding process parameters for morphology control of LDPE/layered silicate nanocomposites

Abstract A careful insight into melt compounding procedure is proposed in order to achieve a better understanding and control of the dispersion and orientation mechanisms of organo-clay platelets into LDPE nanocomposites. The method involved is the preparation of a maleic anhydride grafted polyethylene masterbatch containing 10 wt% organo-clay via twin-screw extrusion. A substantial nanodispersion and orientation of clay platelets was obtained as observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. Moreover, the nanocomposites prepared by diluting the master-batch through the blend mixing with additional LDPE preserved or improved the exfoliation and lamellae orientation. Finally, the thermo-gravimetric analysis (TGA) showed a significant improvement of the thermal stability while both differential scanning calorimetry (DSC) and XRD evidenced a slight increase of the LDPE crystallinity degree with respect to neat polymer matrices thus suggesting the occurrence of orientation also for the polymer.