Nanostructuration by self-assembly in N-alkyl thiazolium and triazolium side-chain polymethacrylates

Amphiphilic polymers are tunable systems to construct supramolecular hierarchically self-assembled structures. Six families of heterocyclic polymethacrylates (PMTAs) bearing 1,3-thiazole and 1,2,3-triazole pendant groups with alkyl and succinate spacers were chemically modified by quaternization reaction of the azole heterocycles with five alkylating agents (methyl, butyl, octyl, dodecyl, and hexadecyl iodide) leading to a library of 30 different amphiphilic poly(ionic liquid)s (PMTAs-RI). These polymers have been characterized in bulk by small- and wide-angle X-ray scattering (SAXS, WAXS) and differential scanning calorimetry (DSC). Quaternization induces a dramatic effect (increase) on the glass-transition temperature Tg, being strongest for methyl iodide members. Increasing the length of the quaternizing agent, plasticization is first observed, followed by a further increase of Tg. This effect, together with evidence of a second Tg and crystallization for the members with the longest quaternizing agents, could be attributed to the presence of well-developed alkyl nanodomains evidenced by the structural investigation. WAXS and SAXS results have been consistently interpreted by assuming nanostructuration driven by the amphiphilicity balance of poly(ionic liquid)s. The different morphologies revealed by SAXS have been characterized, assigning a plausible chemical nature to the phases involved in each case. The nonpolar fraction has been considered as the control parameter defining the main features of the achieved morphology. By increasing this parameter, structures ranging from hexagonally packed nonpolar cylinders in a polar matrix to the inverse situation have been found, passing through lamellar phases. Under some conditions, within the polar lamellae a third phase formed by cylinders of heterocycles has even been determined. We have checked the validity of the scenario proposed by comparing the sizes deduced from the SAXS analysis with the expected characteristic lengths of the associated moieties, inferring thereby how alkyl side groups arrange within the nanodomains. On the basis of the complete picture achieved, the type of nanostructures formed by this class of polymers can be predicted, if the chemical composition including the quaternization degree is known.This work has been supported by MAT2013-47902 and MAT2011-24797 (Spanish-MINECO and EU). R.T. acknowledges CSIC for his JAE-Pre grant. A.A. acknowledges financial support from the Projects MAT2012-31088 (Spanish-MINECO and EU) and IT-654-13 (Basque Government).Peer Reviewe

A synergetic combination of neutron scattering, atomistic simulations and theoretical modelling

This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0.Trabajo presentado a: QENS/WINS 2014 - 11th International Conference on Quasielastic Neutron Scattering and 6th International Workshop on Inelastic Neutron SpectrometersMotivated by the proposition of a new theoretical ansatz [V.N. Novikov, K.S. Schweizer, A.P. Sokolov, J. Chem. Phys. 138, 164508 (2013)], we have revisited the question of the characterization of the collective response of polyisobutylene at intermediate length scales observed by neutron spin echo (NSE) experiments. The model, generalized for sublinear diffusion –as it is the case of glass-forming polymers– has been successfully applied by using the information on the total self-motions available from MD-simulations properly validated by direct comparison with experimental results. From the fits of the coherent NSE data, the collective time at Q → 0 has been extracted that agrees very well with compiled results from different experimental techniques directly accessing such relaxation time. We show that a unique temperature dependence governs both, the Q → 0 and Q →∞ asymptotic characteristic times. The generalized model also gives account for the modulation of the apparent activation energy of the collective times with the static structure factor. It mainly results from changes of the short-range order at inter-molecular length scales.We thank support from the projects IT-654-13 (GV) and MAT2012-31088.Peer Reviewe

Effect of Chain Stiffness on the Structure of Single-Chain Polymer Nanoparticles

Polymeric single-chain nanoparticles (SCNPs) are soft nano-objects synthesized by purely intramolecular cross-linking of single polymer chains. By means of computer simulations, we investigate the conformational properties of SCNPs as a function of the bending stiffness of their linear polymer precursors. We investigate a broad range of characteristic ratios from the fully flexible case to those typical of bulky synthetic polymers. Increasing stiffness hinders bonding of groups separated by short contour distances and increases looping over longer distances, leading to more compact nanoparticles with a structure of highly interconnected loops. This feature is reflected in a crossover in the scaling behaviour of several structural observables. The scaling exponents change from those characteristic for Gaussian chains or rings in $\theta$-solvents in the fully flexible limit, to values resembling fractal or `crumpled' globular behaviour for very stiff SCNPs. We characterize domains in the SCNPs. These are weakly deformable regions that can be seen as disordered analogues of domains in disordered proteins. Increasing stiffness leads to bigger and less deformable domains. Surprisingly, the scaling behaviour of the domains is in all cases similar to that of Gaussian chains or rings, irrespective of the stiffness and degree of cross-linking. It is the spatial arrangement of the domains which determines the global structure of the SCNP (sparse Gaussian-like object or crumpled globule). Since intramolecular stiffness can be varied through the specific chemistry of the precursor or by introducing bulky side groups in its backbone, our results propose a new strategy to tune the global structure of SCNPs.Comment: 20 pages, 17 figure

Structure and dynamics of self-assembled comb copolymers: Comparison between simulations of a generic model and neutron scattering experiments

12 páginas, 15 figuras, 1 tabla.-- El pdf del artículo es la versión post-print.We have performed extensive molecular dynamic simulations on a simple bead−spring model for copolymers with comblike architecture. Monomers located at the main chain and at the arms are respectively denoted as S (“slow”) and F (“fast”). The model parameters are selected in order to induce segregation in domains rich in one component and poor in the other. In particular, we investigate the case in which the linear homopolymer of F-monomers exhibits much faster intrinsic dynamics than the S-counterpart. As a consequence, a strong dynamic asymmetry between both components is still present in the self-assembled copolymer system. We investigate static and dynamic properties as a function of arm length and temperature. The fast component exhibits decoupling of self- and collective dynamics as well as strongly stretched relaxation. Stretching is an intrinsic feature and is not necessarily related to gradients of mobility. The observed qualitative trends are fully consistent with recent neutron scattering experiments on poly(n-alkyl methacrylates), suggesting that they are generic in comb copolymers with strong dynamic asymmetry.We acknowledge financial support from the projects FP7-PEOPLE-2007-1-1-ITN (DYNACOP, EU), MAT2007-63681 (Spain), and IT-436-07 (GV, Spain).Peer reviewe

Sequential crystallization and morphology of triple crystalline biodegradable PEO-b-PCL-b-PLLA triblock terpolymers

et al.The sequential crystallization of poly(ethylene oxide)-b-poly(ε-caprolactone)-b-poly(l-lactide) (PEO-b-PCL-b-PLLA) triblock terpolymers, in which the three blocks are able to crystallize separately and sequentially from the melt, is presented. Two terpolymers with identical PEO and PCL block lengths and two different PLLA block lengths were prepared, thus the effect of increasing PLLA content on the crystallization behavior and morphology was evaluated. Wide angle X-ray scattering (WAXS) experiments performed on cooling from the melt confirmed the triple crystalline nature of these terpolymers and revealed that they crystallize in sequence: the PLLA block crystallizes first, then the PCL block, and finally the PEO block. Differential scanning calorimetry (DSC) analysis further demonstrated that the three blocks can crystallize from the melt when a low cooling rate is employed. The crystallization process takes place from a homogenous melt as indicated by small angle X-ray scattering (SAXS) experiments. The crystallization and melting enthalpies and temperatures of both PEO and PCL blocks decrease as PLLA content in the terpolymer increases. Polarized light optical microscopy (PLOM) demonstrated that the PLLA block templates the morphology of the terpolymer, as it forms spherulites upon cooling from the melt. The subsequent crystallization of PCL and PEO blocks occurs inside the interlamellar regions of the previously formed PLLA block spherulites. In this way, unique triple crystalline mixed spherulitic superstructures have been observed for the first time. As the PLLA content in the terpolymer is reduced the superstructural morphology changes from spherulites to a more axialitic-like structure.We gratefully acknowledge funds received through the following projects: “MAT2014-53437-C2-P, MAT2012-31088 (Spanish-MINECO and EU)”, UPV/EHU (UFI 11/56) and GIC IT-586-13, IT-654-13 (Basque Government).Peer Reviewe

Component dynamics in polyvinylpyrrolidone concentrated aqueous solutions

2H-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, J. Chem. Phys. 128, 044901 (2008)]10.1063/1.2822332. 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. © 2012 American Institute of Physics.Financial support from the projectsMAT2007-63681 and IT-436-07 (G.V.) is acknowledged.Peer Reviewe

Dynamics of tetrahydrofuran as minority component in a mixture with poly(2-(dimethylamino)ethyl methacrylate): A neutron scattering and dielectric spectroscopy investigation

We have investigated a mixture of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and tetrahydrofuran (THF) (70 wt. % PDMAEMA/30 wt. % THF) by combining dielectric spectroscopy and quasielastic neutron scattering (QENS) on a labelled sample, focusing on the dynamics of the THF molecules. Two independent processes have been identified. The >fast> one has been qualified as due to an internal motion of the THF ring leading to hydrogen displacements of about 3 Å with rather broadly distributed activation energies. The >slow> process is characterized by an Arrhenius-like temperature dependence of the characteristic time which persists over more than 9 orders of magnitude in time. The QENS results evidence the confined nature of this process, determining a size of about 8 Å for the volume within which THF hydrogens' motions are restricted. In a complementary way, we have also investigated the structural features of the sample. This study suggests that THF molecules are well dispersed among side-groups nano-domains in the polymer matrix, ruling out a significant presence of clusters of solvent. Such a good dispersion, together with a rich mobility of the local environment, would prevent cooperativity effects to develop for the structural relaxation of solvent molecules, frustrating thereby the emergence of Vogel-Fulcher-like behavior, at least in the whole temperature interval investigated.Financial support from the Project Nos. MAT2012-31088 (Spanish-MINECO and EU) and IT-654-13 (Basque Government) is acknowledged. This work is based on experiments performed at TOFTOF and DNS (Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany) and has been supported by the European Commission under the 7th Framework Programme through the “Research Infrastructures” action of the “Capacities” Programme, NMI3-II Grant No. 283883.Peer Reviewe

Contribution Rules (in Japanese)

International audienceQuasielastic neutron scattering, x-ray diffraction measurements, and fully atomistic molecular dynamics simulations have been performed on poly(vinylpyrrolidone) homopolymer above its glass transition temperature. A “prepeak” appears in the x-ray diffraction pattern that shows the typical features of a first amorphous halo. From an effective description of the experimentally accessed incoherent scattering function of hydrogens in terms of a stretched exponential function, we observe enhanced stretching and a momentum-transfer dependence of the characteristic time different from that usually reported for more simple polymers (main-chain polymers or polymers with small side groups). The comparison with both kinds of experimental results has validated the simulations. The analysis of the simulated structure factor points to a nanosegregation of side groups (SG) and main-chains (MC). The detailed insight provided by the simulations on the atomic trajectories reveals a partial and spatially localized decoupling of MC and SG dynamics at length scales between the average SG–SG distance and the characteristic length of the backbone interchain correlations. Anomalous behavior in correlators calculated for the SG subsystem are found, like e.g., logarithmiclike decays of the density–density correlation function. They might be a consequence of the existing large dynamic asymmetry between SG and MC subsystems. Our results suggest that, as the SGs are spatially extended and chemically different from the backbone, they form transient nanosegregated domains. The dynamics of these domains show similar behavior to that found in other systems displaying large dynamic asymmetry

Atomic motions in the αβ\alpha\beta-region of glass-forming polymers: Molecular versus Mode Coupling Theory approach

We present fully atomistic Molecular Dynamics simulation results on a main-chain polymer, 1,4-Polybutadiene, in the merging region of the $\alpha$- and $beta$-relaxations. A real space analysis reveals the occurrence of localized motions (``$\beta$-like'') in addition to the diffusive structural relaxation. A molecular approach provides a direct connection between the local conformational changes reflected in the atomic motions and the secondary relaxations in this polymer. Such local processes occur just in the time window where the $\beta$-process of the Mode Coupling Theory is expected. We show that the application of this theory is still possible, and yields an unusually large value of the exponent parameter. This result might originate from the competition between two mechanisms for dynamic arrest: intermolecular packing and intramolecular barriers for local conformational changes (``$\beta$-like'').Comment: 10 pages, 6 figure