Structure–property relationship in high urethane density polyurethanes

The structure–property relationship of polyurethane (PU) homopolymers synthesized by the stoichiometric combination of 1,6‐hexamethylene diisocyanate (HDI) and low‐molecular‐weight glycols with different structure is investigated by the analysis of PU elastic modulus obtained by means of high‐resolution peak force atomic force microscopy. Different experimental techniques are employed to understand the role of block structure influence on hydrogen bonding distribution, crystallinity, thermal transitions, and crystal packaging within the PU. The results show that glycols with bulky side groups have more impediments to self‐assemble into hydrogen‐bonded and packed macromolecular structures. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 739–746     

A Solvent‐Based Strategy for Tuning the Internal Structure of Metallo‐Folded Single‐Chain Nanoparticles

Controlling the spatial distribution of catalytic sites in metallo‐folded single‐chain nanoparticles (SCNPs) is a first step toward the rational design of improved catalytic soft nano‐objects. Here an unexplored pathway is reported for tuning the internal structure of metallo‐folded SCNPs. Unlike the conventional SCNP synthesis in good solvent (protocol I), the proposed new route (protocol II) is based on the use of amphiphilic random copolymers and transfer, after SCNP formation, from selective to good (nonselective) solvent conditions. The size and morphology of the SCNPs obtained by the two protocols, and the corresponding spatial distribution of the catalytic sites, have been determined by combining results from size exclusion chromatography with triple detection, small‐angle X‐ray scattering and molecular dynamics (MD) simulations. Remarkably, the use of these protocols allows the tuning of the internal structure of the metallo‐folded SCNPs, as supported by MD simulations results. While the conventional protocol I yields a homogeneous distribution of the catalytic sites in the SCNP, these are arranged into clusters in the case of protocol II.

     

Synthesis and Characterization of Double Crystalline Cyclic Diblock Copolymers of Poly(ε‐caprolactone) and Poly(l(d)‐lactide) (c(PCL‐b‐ PL(D)LA))

The synthesis of symmetric cyclo poly(ε‐caprolactone)–block–poly(l (d )‐lactide) (c(PCL–b–PL(D)LA)) by combining ring‐opening polymerization of ε‐caprolactone and lactides and subsequent click chemistry reaction of the linear precursors containing antagonist functionalities is presented. The two blocks can sequentially crystallize and self‐assemble into double crystalline spherulitic superstructures. The cyclic chain topology significantly affects both the nucleation and the crystallization of each constituent, as gathered from a comparison of the behavior of linear precursors and cyclic block copolymers. The stereochemistry of the PLA block does not have a significant effect on the nonisothermal crystallization of both linear and cyclo PCL‐b‐PDLA and PCL‐b‐PLLA copolymers.

     

Toward microphase separation in epoxy systems containing PEO–PPO–PEO block copolymers by controlling cure conditions and molar ratios between blocks

Nanostructuring of thermosetting systems using the concept of templating and taking advantage of the self-assembling capability of block copolymers is an exciting way for designing new materials for nanotechnological applications. In this first part of the work, reactive blends based on stoichiometric amounts of a diglycidylether of bisphenol-A epoxy resin and 4,4′-diaminodiphenylmethane cure agent modified with three poly(ethylene oxide)-co-poly(propylene oxide)-co-poly(ethylene oxide) block copolymers were studied. Cure advancement of these systems was analyzed by differential scanning calorimetry. The experimental results show a delay of cure rate, which increases as copolymer content and PEO molar ratio in the block copolymer rise. Infrared spectroscopy shows that PEO block is mainly responsible of physical interactions between the hydroxyl groups of growing epoxy thermoset and ether bonds of block copolymer. These interactions are mainly responsible for the delaying of cure kinetics. The molar ratio between blocks also has a critical influence on the delaying of the cure rate. A mechanistic approach of cure kinetics allows us to relate the delay of cure as a consequence of block copolymer adding to physical interactions between components.     

Component dynamics in polyvinylpyrrolidone concentrated aqueous solutions

(2)H-nuclear magnetic resonance (NMR) and neutron scattering (NS) on isotopically labelled samples have been combined to investigate the structure and dynamics of polyvinylpyrrolidone (PVP) aqueous solutions (4 water molecules/monomeric unit). Neutron diffraction evidences the nanosegregation of polymer main-chains and water molecules leading to the presence of water clusters. NMR reveals the same characteristic times and spectral shape as those of the slower process observed by broadband dielectric spectroscopy in this system [S. Cerveny et al., J. Chem. Phys. 128, 044901 (2008)]. The temperature dependence of such relaxation time crosses over from a cooperative-like behavior at high temperatures to an Arrhenius behavior at lower temperatures. Below the crossover, NMR features the spectral shape as due to a symmetric distribution of relaxation times and the underlying motions as isotropic. NS results on the structural relaxation of both components-isolated via H/D labeling-show (i) anomalously stretched and non-Gaussian functional forms of the intermediate scattering functions and (ii) a strong dynamic asymmetry between the components that increases with decreasing temperature. Strong heterogeneities associated to the nanosegregated structure and the dynamic asymmetry are invoked to explain the observed anomalies. On the other hand, at short times the atomic displacements are strongly coupled for PVP and water, presumably due to H-bond formation and densification of the sample upon hydration.     

On the origin of the stereoselectivity in the alkylation of oxazolopiperidone enolates

The origin of the diastereoselective alkylation of enolates of oxazolopiperidones is studied by means of theoretical calculations and experimental assays. For the unsubstituted oxazolopiperidone, the alkylation with methyl chloride is predicted to afford mainly the exo product, a finding further corroborated from the analysis of the experimental outcome obtained in the reaction of the racemic oxazolopiperidone. However, such a preference can be drastically altered by the presence of substituents attached to the fused ring. In particular, when the angular carbon adopts an R configuration in a phenylglycinol-derived oxazolopiperidone, the presence of a phenyl ring at position 3 forces the pseudo-planarity of the bicyclic lactam, and the diastereoselectivity is dictated by the internal torsional strain induced in the enolate. However, when the angular carbon adopts an S configuration, the preference for the exo alkylation stems from the intermolecular steric hindrance between the enolate and the alkylating reagent. Interestingly, the intramolecular hydrogen bond formed between the phenyl ring and the carbonyl oxygen in the enolate largely reduces the difference in stability of the two TSs compared to the unsubstituted oxazolopiperidone.     

Metamorphosis of a Commodity Plastic like PVC to Efficient Catalytic Single-Chain Nanoparticles

We perform the conversion of a commodity plastic of common use in pipes, window frames, medical devices, flexible hoses, etc. like polyvinyl chloride (PVC) to single-chain nanoparticles (SCNPs). SCNPs are versatile, protein-mimetic soft nano-objects of growing interest for catalysis, sensing, and nanomedicine, among other uses. We demonstrate that the metamorphosis process -as induced through metal-free click chemistry- leads to well-defined, uniform SCNPs that are stable during storage in the solid state for months. All the conversion process (from PVC isolation to PVC-SCNPs synthesis) can be run in a green, dipolar aprotic solvent and involving, when required, a simple mixture of ethanol and water (1/1 vol.) as non-solvent. The resulting PVC-SCNPs are investigated as recyclable, metalloenzyme-mimetic catalysts for several representative Cu(II)-catalyzed organic reactions. The method could be valid for the metamorphosis and valorization of other commodity plastics in which it is feasible to install azide functional groups in their linear polymer chains.