Low molecular weight polyesters were end-functionalized with ammonium and carboxylate salts and used in ionic exchange reactions with respectively cationic (MMT) and anionic (LDH) clays. The hybrid ...organic-inorganic substrates were structurally analysed to determine the ester oligomers' modification degree and their thermal behaviour owing to confinement effects. The dispersion of such hybrids in polylactic acid (PLA) matrix was performed and the ultimate structural, morphological and thermal properties of the collected nanocomposites were investigated and correlated to the tailored interfacial properties with the different inorganic substrates. While the composites with MMT proved to be stable under thermo-oxidative conditions, the samples obtained by dispersing the LDH hybrid suffered from poor final thermostability owing to molecular weights decrease. Deeper insights about the effect of the interactions at interface (polymer chain-surfactant and polymer chain- inorganic surface) evidenced that by promoting an intimate contact between PLA chains and LDH surface (through oligoester used as inorganic substrate modifier) a certain extent of PLA hydrolysis triggered by both surfactant and inorganic surface (LDH) occurred and cannot completely avoided.
Low density polyethylene/layered double hydroxide (LDH) composites were prepared via melt compounding using different kinds of organo-LDHs and polyethylene-grafted maleic anhydride as the ...compatibilizer. The organo-LDHs were successfully prepared by converting a commercial MgAl-carbonate LDH into a MgAl-nitrate LDH, which was later modified by anion exchange with linear and branched sodium alkyl sulfates having different alkyl chain lengths (nc = 6, 12 and 20). It was observed that, depending on the size of the surfactant alkyl chain, different degrees of polymer chain intercalation were achieved, which is a function of the interlayer distance of the organo-LDHs, of the packing level of the alkyl chains, and of the different interaction levels between the surfactant and the polymer chains. In particular, when the number of carbon atoms of the surfactant alkyl chain is larger than 12, the intercalation of polymer chains in the interlayer space and depression of the formation of large aggregates of organo-LDH platelets are favored. A remarkable improvement of the thermal-oxidative degradation was evidenced for all of the composites; whereas only a slight increase of the crystallization temperature and no significant changes of both melting temperature and degree of crystallinity were achieved. By thermodynamic mechanical analysis, it was evidenced that a softening of the matrix is may be due to the plasticizing effect of the surfactant.
The functionalization of polyolefins is a useful methodology to chemically convert these extremely important commodities into value-added polymers by generating materials with additional properties, ...such as adhesion, dyeability, paintability, wettability, and compatibility that are not characteristic of the starting macromolecules with a paraffin structure. Owing to the intrinsic chemical resistance and low reactivity of polyolefins, the post-modification processes involve the use of free-radicals and are generally carried out at high temperature, rendering the reactions non-selective. Cross-linking and degradation set boundaries of the desired grafting reaction and their extent of grafting depends on the type of polyolefin and nature/type of chemicals used. The occurrence of these side reactions can severely damage the structure of polyolefins with poor control of ultimate properties. The main purpose here is to describe the conventional process in terms of chemical reactions involved, down to the molecular mechanism, and the dependence on chemicals/reagents nature and amounts. Particular attention is focused on methodologies aimed to hinder side reactions. The corresponding new functionalization approaches are based on well-known process chemistry and are all directed to control the grafting levels and structure/architecture of the initial polymer. Finally, a section is devoted to problems related to the radical post-modification processes of polypropylenes and obtaining new interesting materials by controlling their structure/architecture with the use of specific mono- or poly-functional co-agents as macroradical stabilizers.
Quercetin (Q), a natural antioxidant molecule, is physically immobilized onto multi-walled carbon nanotubes (CNTs) bearing covalently-linked long-chain alkyl functional groups, and the so obtained ...hybrid-nanoparticles are used to prepare Ultra High Molecular Weight PolyEthylene-based nanocomposite films with enhanced thermo-oxidation resistance. The effective immobilization of the Q molecules is confirmed by spectroscopic (micro-Raman, ATR-FTIR, and FTIR) and thermo-gravimetric analyses, and the influence of the nanoparticles on the rheological behaviour and thermo-oxidative stability of the nanocomposites are investigated. Rheological analyses (linear viscoelasticity and stress relaxation tests) and morphological observations reveal that the Q-functionalized CNTs disperse better than bare CNTs in the host matrix. Quercetin confirms to be an excellent anti-oxidant for polyethylene, but the study of the thermo-oxidation behaviour shows that a remarkable stabilizing action only emerges when Q is physically immobilized on the CNTs. In particular, a ten-fold increase of the onset of degradation phenomena in thermo-oxidative environment was found. Such an excellent result is due to a synergic effect stemming from the physical interaction between Q and CNTs, which cannot provide a similar stabilizing action if used separately. In particular, we argue that the process of physical immobilization of the Q molecules causes the formation of structural defects onto outer CNTs surfaces, thus remarkably improving the CNTs radical scavenging activity and probably promoting Q regeneration. In addition, CNTs seem acting as efficient nano-carriers for the quercetin molecules, improving the dispersion of the latter in the host matrix in spite of their poor solubility.
Isotactic polypropylene (iPP) gives very extensive degradation when treated with peroxides above its melting temperature in mechanical mixers or extruders. This undesired reaction is very modestly ...affected by maleate molecules which on the contrary actively compete with side reactions of macroradicals in the case of ethylene polymers. In this work iPP was treated in a Brabender mixer at 180 °C with peroxide and different selected molecules capable of promptly reacting with the macroradical formed on iPP chain and converting it into a more stable free radical. Furan derivatives, successfully used for iPP cross-linking without any remarkable increase of MFR, were used as free radical removers and maleic anhydride as functionalizing monomer. The results indicate a detectable improvement with respect to the use of maleic monomers and peroxide only, allowing to us get a significant grafting of functional groups and only partial degradation. Moreover, furan derivatives bearing various reactive substituents were used as functionalizing molecules. The results are discussed in the frame of the general mechanism proposed for the free radical functionalization of polyolefin in the melt.
The synthesis of polystyrene chains covalently bound to the surface of cross‐linked rubber particles from recycled tires (ground tire rubber, GTR) was investigated via free radical polymerization in ...situ by using azobisisobutyronitrile (AIBN) and dibenzoyl peroxide (BPO) as initiators. Indeed, the graft polymerization provides a significant route to modify the physical and chemical properties of these particles allowing to improve their compatibility with other polymers. Polymerization reactions were carried out in bulk by changing the styrene/GTR ratio as well as the amount of free radical initiator. Appreciable amounts of polystyrene (PS) were grafted on GTR when BPO was used as confirmed by particle characterizations.
Two different bulk modifications have been carried out aimed to the introduction of carboxyl groups into atactic polystyrene (PS) based on the reaction at 180°C with diethylmaleate (DEM) or ...4‐carboxybenzensulfonyl azide (CBSA) respectively. The former system proceeds with a free radical mechanism giving functionalization around 0.5% by mol but with reduction of molecular weight and Tg increase. On the other side CBSA which grafts by addition to the C‐H bond gives appreciable functionalization with rather limited degradation and moderate Tg increase. Detailed analysis of the molecular structure of the functionalized PS was performed to propose possible reaction mechanisms.
The possibility to submit a semi‐crystalline polyolefin (or a mixture of different polyolefins) to a phase selective crosslinking process, in order to produce a material with unchanged crystallinity ...but a partially crosslinked amorphous fraction, has been investigated. The crosslinking process has been performed in the melt in the presence of radical initiators. Three different polymer systems have been considered: a LLDPE sample (an ethylene‐1‐butene 91/9 w/w copolymer), two EVA samples (ethylene‐vinyl acetate copolymers containing a different amount (10 and 15 wt.‐%) of acetate monomer units), and mixtures of LLDPE with polybutadiene (BR) (90/10 w/w). All the reaction products have been characterized by selective solvent extractions to determine the gel content, by analysis with different spectroscopies (IR and NMR) and by viscosity measurements to evaluate changes of the structure and the molecular weight (MW) of the partially crosslinked samples. Finally DSC and DMTA analyses have been used to correlate the variations of the macromolecule structure to the thermo‐mechanical properties and, in the case of the mixtures, TEM analysis has been carried out to gather information about the morphology.
TEM micrograph of the partially crosslinked polyethylene/polybutadiene blend 90/10 (w/w).
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