Dynamics Of Poly Research Articles

Investigation of Chain Dynamics in Poly(n‐alkyl methacrylate)s by Solid‐State NMR: Comparison with Poly(n‐alkyl acrylate)s

AbstractPnAMAs exhibit a local nanophase separation associated with intriguing chain dynamics (Macromol. Chem. Phys. 2005, 206, 142). PnAMAs of high molar mass, as determined by SEC and MHKS parameters, were investigated in the melt with a recently‐developed solid‐state NMR method (NOE with dipolar filter; Solid State Nucl. Magn. Res. 2005, 28, 160). The correlation times are assigned to the relaxation of the alkyl nanodomains, as coupled motions of the main chain and hindered local modes in the side chain. Comparison with poly(n‐alkyl acrylates) shows a higher anisotropy of the main chain motions and a better organized local nanophase separation in PnAMAs.magnified image

Temperature dependence of molecular dynamics and supramolecular aggregation in MEH-PPV films: A solid-state NMR, X-ray and fluorescence spectroscopy study

This article presents an investigation of the temperature induced modification in the microstructure and dynamics of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) cast films using Wide-Angle X-ray Scattering (WAXS), solid-state Nuclear Magnetic Resonance (NMR), and Fluorescence Spectroscopy (PL). MEH-PPV chain motions were characterized as a function of temperature by NMR. The results indicated that the solvent used to cast the films influences the activation energy of the side-chain motions. This was concluded from the comparison of the activation energy of the toluene cast film, E a = (54 ± 8) kJ/mol, and chloroform cast film, E a = (69 ± 5) kJ/mol, and could be attributed to the higher side-chain packing provided by chloroform, that preferentially solvates the side chain in contrast to toluene that solvates mainly the backbone. Concerning the backbone mobility, it was observed that the torsional motions in the MEH-PPV have average amplitude of ∼10° at 300 K, which was found to be independent of the solvent used to cast the films. In order to correlate the molecular dynamics processes with the changes in the microstructure of the polymer, in situ WAXS experiments as a function of temperature were performed and revealed that the interchain spacing in the MEH-PPV molecular aggregates increases as a function of temperature, particularly at temperatures where molecular relaxations occur. It was also observed that the WAXS peak associated with the bilayer spacing becomes narrower and its intensity increases whereas the peak associated with the interbackbone planes reduces its intensity for higher temperatures. This last result could be interpreted as a decrease in the number of aggregates and the reduction of the interchain species during the MEH-PPV relaxation processes. These WAXS results were correlated with PL spectra modifications observed upon temperature treatments.

How does sensitivity to dynamical heterogeneity in supercooled colloidal liquids depend on tracer size?

Few experimental investigations have examined the dependence of probe length scale on the evolution of dynamical heterogeneity in supercooled liquids. In this study, we use fluorescent tracer probes, which are smaller than that of the constituent unlabeled particles in the matrix suspension, to investigate the tracer size sensitivity to the onset of dynamical heterogeneity in model hard-sphere colloidal suspensions close to the glass transition. The dynamics of poly(methyl methacrylate) (PMMA) tracer particles of radii a(S) = 137 nm embedded into constituent PMMA particles of a(L) = 212 and 637 nm with bulk volume fraction phi varied from 0.50 to 0.57 is examined using fluorescence correlation spectroscopy (FCS). Upon increasing phi for the largest tracer size ratio explored, f (= a(S)/a(L)) = 0.646, the onset of dynamic heterogeneity is observed from the broad distribution of characteristic decay times of the tracers, which is extracted from the measured FCS autocorrelation functions. The strong coupling between tracer size and its sensitivity to dynamical heterogeneity is reflected in the dynamic susceptibility, where dynamic correlation lengths are immeasurable for the smallest size ratio and show a marked increase to five tracer particle diameters for the largest size ratio.

Dynamics of Poly(l-lysine) in Hyaluronic Acid/Poly(l-lysine) Multilayer Films Studied by Fluorescence Recovery after Pattern Photobleaching

Poly( L-lysine) (PLL)/hyaluronic acid (HA) multilayers are films whose thickness increases exponentially with the number of deposition steps. Such a growth process was attributed to the diffusion, in and out of the whole film, of at least one of the polyelectrolytes constituting the film. In the case of PLL/HA, PLL is known to be the diffusing species. In order to better understand the growth mechanism of such films, it is of primary importance to well characterize the diffusion process of the polyelectrolytes in the multilayer. This process is studied here by fluorescence recovery after pattern photobleaching. We show that the diffusion behavior is different when we consider either PLL chains that are deposited on top of the film or PLL chains embedded in the film, even below only one HA layer. For chains that are embedded, we find two populations: a mobile one with a diffusion coefficient, D, of the order of 0.1 microm(2) x s(-1) and a population that appears immobile ( D < 0.001 microm(2) x s(-1)). For chains deposited on top of the multilayer, a third population appears which is rapidly diffusing ( D congruent with 1 microm(2) x s(-1)). These results confirm the validity of the model generally accepted for the exponential growth process and in particular the existence of up to three subgroups of PLL chains from the point of view of their diffusion coefficient.

Unique Structure and Dynamics of Poly(ethylene oxide) in Layered Silicate Nanocomposites: Accelerated Segmental Mobility Revealed by Simulating ESR Spectra of Spin-Labels, XRD, FTIR, and DSC

The structure and dynamics of poly(ethylene oxide) (PEO) intercalated in the galleries of a fluoromica inorganic clay were studied by spin-label electron spin resonance (ESR), XRD, FTIR, and DSC. The polymer was end-labeled by attachment of a nitroxide radical. Basic structural information was determined by XRD, while DSC and FTIR revealed that the crystallization of the PEO was inhibited in the clay galleries. The temperature variation of the ESR spectra from the spin-label was simulated based on the “macroscopic order with microscopic disorder” (MOMD) model. For PEO intercalated in the narrow (0.33 nm) clay galleries, the ESR spectra indicated a very low segmental mobility even at high temperature, 410 K, which was attributed to the strong polymer interaction with the charged mica platelets. In wider (0.83 nm) galleries, however, the parameters used to simulate the ESR spectra of the nitroxide labels reflected a lowering of the PEO segmental density: In this sample, the ESR spectrum consisted of two distinct contributions from slow- and fast-motional components, and the relative intensity of the fast component increased with an increase in temperature. The two spectral components were attributed to segments located close to, and away from, the polar solid walls in the gallery, respectively. Interestingly, the fast-motional component had higher mobility compared to that of PEO chains adsorbed on the fluoromica surface. In addition, the activation energy of the segmental motion in the fast-motional component was lower compared to that of bulk PEO. The low segmental density and reduced cooperative motion with neighboring segments are considered the main factors leading to the fast PEO chain motion with low activation energy.

Molecular Dynamics of Poly(Ethylene Terephthalate)/Muscovite Mica Composite by Low-Field NMR

Poly(ethylene terephthalate) (PET) composites containing muscovite mica as reinforcing filler were prepared by melt mixing, and their molecular dynamics were investigated by low-field NMR. Spin-lattice and proton spin–spin relaxation times of composites containing different amounts of the filler from 2 to 15 wt% were evaluated and the results compared with pure PET. DSC showed that for systems containing muscovite mica in percentages above 2 wt% a high level of crystallinity is achieved. By proton spin-lattice relaxation studies, a slight decrease in the T1H is observed for the composites with filler content up to 8 wt% and a drastic decrease at the higher filler content (15 wt%). Changes in T1H were associated with the content of amorphous phase in the composite.

Dipolar Stabilization of Emissive Singlet Charge Transfer Excited States in Polyfluorene Copolymers

The singlet excited-state dynamics in poly[(9,9-dioctylfluorene)-(dibenzothiophene-S,S-dioxide)] (PFSx ) random copolymers with different contents of dibenzothiophene-S,S-dioxide (S) units have been studied by steady-state and time resolved fluorescence spectroscopies. Emission from PFSx copolymers shows a pronounced solvatochromism in polar chloroform, relative to the less polar toluene. An excited intramolecular charge transfer state (ICT) is stabilized by dipole-dipole interactions with the polar solvent cage, and possibly accompanied by conformational rearrangement of the molecular structure, in complete analogy with their small oligomer counterparts. The spectral dynamics clearly show that the ICT stabilization is strongly affected by the surrounding medium. In the solid state, emission from PFSx copolymers depends on the content of S units, showing an increase of inhomogeneous broadening and a red shift of the optical transitions. This observation is consistent with stabilization of the emissive ICT state, by the local reorientation of the surrounding molecules at the location of the excited chromophore, which results in favorable dipole-dipole interactions driven by the increase in the dielectric constant of the bulk polymer matrix with increasing S content, in analogy to what happens in polar solvent studies. Furthermore, in clear agreement with the interpretation described above, a strong increase in the emission quantum efficiency is observed in the solid state by decreasing the temperature and freezing out the molecular torsions and dipole-dipole interactions necessary to stabilize the ICT state.

The Structure and Dynamics of Poly(l-lysine) in Templated Silica Nanocomposites

Biologically templated nanoparticles are of interest for a variety of biological applications and for controlling the porosity and other properties in nanocomposites. Solid-state NMR has been used to characterize polypeptide/silica composites templated from the poly(L-lysine) solutions in the α-helix, β-sheet, and random coil conformations. The results show that the poly(L-lysine) retains its solution conformation in the nanocomposites when templating from the α-helix or /3-sheet conformation, but the random coil conformation is partially converted to the α-helical form during nanocomposite formation. The dynamics of poly(L-lysine) are restricted by incorporation into the composite compared to bulk poly(L-lysine) at the equivalent relative humidity. Bulk and film poly(L-lysine) samples undergo transitions from the random coil to the /?-sheet to the α-helix conformation with increasing relative humidity, but these transitions are suppressed in the silica composite. { 1 H- 31 P} NMR studies show that inorganic phosphate is incorporated into the random coil-templated composite and that part of the phosphate is closely associated with the poly(L-lysine). The implications for biomimetic nanocomposites formation are considered.

Effect of Silica Nanoparticles on the Local Segmental Dynamics in Poly(vinyl acetate)

The effect of nanosized silica particles on the properties of poly(vinyl acetate) (PVAc) was investigated for a range of silica concentrations encompassing the percolation threshold. The quantity of polymer adsorbed to the particles (“bound rubber”) increased systematically with silica content and was roughly equal to the quantity shielded from shear stresses (“occluded rubber”). This bound and occluded polymer attained a level of ∼12% at a silica volume content of 28%; nevertheless, the glass transition properties of the PVAc, including the glass transition temperature, local segmental relaxation function and relaxation times, and the changes in thermal expansion coefficient and heat capacity at Tg, were unaffected by the interfacial material. That is, there is no indication that the local segmental dynamics of the chains adjacent to silica particles differ from the motions of the bulk chains. Interestingly, the volume sensitivity of the segmental dynamics, as determined from the scaling exponent γ in th...

Anomalous relaxation of self-assembled alkyl nanodomains in high-order poly(n-alkyl methacrylates)

We have exploited the selectivity of neutron scattering combined with isotopic substitution to study the structure and dynamics of poly(n-alkyl methacrylates). Our diffraction data strongly support the suggested nanosegregation of main-chains and side groups. Moreover, we have been able to separately follow the dynamics of both subsystems at a molecular scale. While the structural relaxation observed at the main-chain level is standard, for high-order members the correlations involving side groups within the alkyl nanodomains relax through an exotic logarithmic decay. In these polymers, a strong dynamic asymmetry also develops. We discuss possible theoretical frameworks for the anomalous relaxation observed.

Investigation of the structure of poly(vinyl alcohol)–iodine complex hydrogels prepared from the concentrated polymer solutions

The structure and dynamics of poly(vinyl alcohol) (PVA)–iodine complex hydrogels that were prepared from concentrated PVA solutions have been characterized by high-resolution solid-state 13C NMR spectroscopy. The fully relaxed dipolar decoupling (DD)/MAS 13C NMR spectrum indicates that the hydrogels contain at least two components, a highly mobile and broader components. The former is assigned to the soluble or well water-swollen PVA chains that are not closely associated with the PVA–iodine complexes, whereas the latter may be mainly ascribed to the aggregated PVA chains that are produced by the formation of the PVA–iodine complexes because no diffraction peaks due to the conventional PVA crystallites are observed by wide-angle X-ray diffractometry. Furthermore, 13C spin-lattice relaxation time (T1C) analyses reveal that the broader component is composed of the highly restricted component probably assignable to PVA molecular chain aggregates containing the PVA–iodine complexes and the less mobile component. As for the former component, their CH resonance line measured by the T1C-filtering method is successfully resolved into 7 constituent lines by the least-squares curve fitting. The statistical analysis of the integrated intensities of the constituent lines thus obtained also reveals that the probability fa for the formation of intramolecular hydrogen bonding in the successive two OH groups along each chain and another probability ft of the trans conformation are, respectively, as high as 0.86 and 0.88. This fact indicates that the PVA molecular chain aggregates containing the PVA–iodine complexes should be composed of PVA segments with the trans-rich conformation and the PVA–iodine complexes therein may also be formed with these several trans-rich segments surrounding the rod-like polyiodine cores in agreement with the so-called aggregation model. Moreover, several new diffraction peaks that should be interpreted in terms of the hexagonal structure are observed for the PVA–iodine complex hydrogels in the low 2θ region in the wide-angle X-ray diffraction (WAXD) profile measured by a strong X-ray source at SPring-8. This suggests the necessity of more detailed WAXD characterization to propose a new structure model, which should be referred to as the hexagonal aggregation model, for the PVA–iodine complexes.

Structure and Dynamics of Poly(T) Single-Strand DNA: Implications toward CPD Formation

The formation of cyclobutane pyrimidine dimers between adjacent thymines by UV radiation is thought to be the first event in a cascade leading to skin cancer. Recent studies showed that thymine dimers are fully formed within 1 ps of UV irradiation, suggesting that the conformation at the moment of excitation is the determining factor in whether a given base pair dimerizes. MD simulations on the 50 ns time scale are used to study the populations of reactive conformers that exist at any given time in T18 single-strand DNA. Trajectory analysis shows that only a small percentage of the conformations fulfill distance and dihedral requirements for thymine dimerization, in line with the experimentally observed quantum yield of 3%. Plots of the pairwise interactions in the structures predict hot spots of DNA damage where dimerization in the ssT18 is predicted to be most favored. The importance of hairpin formation by intra-strand base pairing for distinguishing reactive and unreactive base pairs is discussed in detail. The data presented thus explain the structural origin of the results from the ultrafast studies of thymine dimer formation.

Dynamics of PEO–PPO–PEO triblock copolymer in aqueous solutions investigated byinternal friction

The dynamics of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethyleneoxide) (PEO–PPO–PEO) in aqueous solutions in the range of the polymer concentrationϕ = 10.0–45.0 wt% has been investigated by the tube inversion method and the internal friction technique.At ϕ≤31.0 wt%, two internal friction peaks are observed with the characteristics of the first-order phasetransition, corresponding to the opaque liquid-to-liquid transition and the phase separation.The opaque liquid-to-liquid transition is due to the sphere-to-rod transition in shape of themicelles, and the phase separation is caused by the destruction of core–shell structure. Atϕ≥32.0 wt%, besides the internal friction peak associated with phase separation, two other peaksare observed, which correspond to the solution-to-gel (liquid-to-crystal) transition andgel-to-solution (crystal-to-liquid) transition, respectively. The former is due to the micellecrystallization, whereas the latter is the result of spherical micelles fusing into wormlike orrodlike micelles at elevated temperatures.

Intrachain Photoluminescence Dynamics of MEH−PPV in the Solid State

The photoluminescence (PL) dynamics of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) blended in host polymer (polypropylene, PP) matrix as well as that in the neat film has been studied. The concentration of MEH-PPV in the PP blend is designed to be fairly low (0.01 wt %) in order to observe the intrinsic intrachain PL property of MEH-PPV in the solid state. The steady-state 0-0 PL band of the blend sample shows a blue-shift of 0.12 eV with respect to that of the neat film of MEH-PPV. The PL-excitation (PLE) spectra of the blend sample exhibit definite vibronic structure, and hence we can determine the magnitude of the Stokes shift as 0.06 eV. The blend sample shows a single-exponential PL decay at 4 K with a time constant of 850 ps. We emphasize that this single-exponential-type PL decay is an intrinsic property of the intrachain PL species. Time-resolved PL measurements confirm dynamical red-shift of the PL band in the neat film, whereas this trend is not found in the case of the PP blend. These observations indicate that the energy transfer between finite segments, which can cause exciton migration, is much less efficient within the isolated MEH-PPV polymer chain compared to the case of the interchain transfer. The time-resolved measurements further demonstrate that the Stokes shift identified in the blend sample takes place at the early stage within 50 ps following photoexcitation. We attribute this Stokes shift to the rapid increase of the planarity of the MEH-PPV chain caused by the torsion of some constituent phenyl rings following photoexcitation. Finally, based on an argument on the different magnitudes of Stokes shift between the blend sample and the neat film, we conclude that the PL of MEH-PPV in the neat film predominantly occurs at the site of interchain excitations via the interchain migration of excitons.

Glass transition and segmental dynamics in poly(dimethylsiloxane)/silica nanocomposites studied by various techniques

We report new results on segmental dynamics and glass transition in a series of poly(dimethylsiloxane) networks filled with silica nanoparticles prepared by sol-gel techniques, obtained by differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), broadband dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). The nanocomposites are characterized by a fine dispersion of 10nm silica particles and hydrogen bonding polymer/filler interactions. The first three techniques indicate, in agreement with each other, that a fraction of polymer in an interfacial layer around the silica particles with a thickness of 2–3nm shows modified dynamics. The DSC data, in particular measurements of heat capacity jump at Tg, are analyzed in terms of immobilized polymer in the interfacial layer. The dielectric TSDC and DRS data are analyzed in terms of slower dynamics in the interfacial layer as compared to bulk dynamics. We employ a special version of TSDC, the so-called thermal sampling (TS) technique, and provide experimental evidence for a continuous distribution of glass transition temperatures (Tg) and molecular mobility of the polymer in the interfacial layer, which is consistent with the DRS data. Finally, DMA results show a moderate slowing down of segmental dynamics of the whole polymer matrix (increase of glass transition temperature by about 10K as compared to the pure matrix).

Heterogeneous dynamics and dynamic heterogeneities at the glass transition probed with single molecule spectroscopy

We studied the temperature dependence of the structural relaxation in poly(vinyl acetate) near the glass transition temperature with single molecule spectroscopy from Tg-1 K to Tg+12 K. The temperature dependence of the observed relaxation times matches results from bulk experiments; the observed relaxation times are, however, 80-fold slower than those from bulk experiments at the same temperature. We attribute this factor to the size of the probe molecule. The individual relaxation times of the single molecule environments are distributed normally on a logarithmic time scale, confirming that the dynamics in poly(vinyl acetate) is heterogeneous. The width of the distribution of individual relaxation times is essentially independent of temperature. The observed full width at half maximum (FWHM) on a logarithmic time axis is approximately 0.7, corresponding to a factor of about 5-fold, significantly narrower than the dielectric spectrum of the same material with a FWHM of about 2.0 on a logarithmic time axis, corresponding to a factor of about 100-fold. We explain this narrow width as the effect of temporal averaging of single molecule fluorescence signals over numerous environments due to a limited lifetime of the probed heterogeneities, indicating that heterogeneities are dynamic. We determine a loose upper limit for the ratio of the structural relaxation time to the lifetime of the heterogeneities (the rate memory parameter) of Q<80 for the range of investigated temperatures.

Effect of chain length on fragility and thermodynamic scaling of the local segmental dynamics in poly(methylmethacrylate)

Local segmental relaxation properties of poly(methylmethacrylate) (PMMA) of varying molecular weight are measured by dielectric spectroscopy and analyzed in combination with the equation of state obtained from PVT measurements. Significant variations of glass transition temperature and fragility with molecular weight are observed. In accord with the general properties of glass-forming materials, single molecular weight dependent scaling exponent gamma is sufficient to define the mean segmental relaxation time taualpha and its distribution. This exponent can be connected to the Gruneisen parameter and related thermodynamic quantities, thus demonstrating the interrelationship between dynamics and thermodynamics in PMMA. Changes in the relaxation properties ("dynamic crossover") are observed as a function of both temperature and pressure, with taualpha serving as the control parameter for the crossover. At longer taualpha another change in the dynamics is apparent, associated with a decoupling of the local segmental process from ionic conductivity.

Dynamics of Poly(cyclohexyl methacrylate): Neat and in Blends with Poly(α-methylstyrene)

Pressure−volume−temperature measurements, calorimetry, and dielectric spectroscopy at ambient and elevated pressures were carried out on poly(cyclohexyl methacrylate) (PCHMA) and its blend with poly(α-methylstyrene) (PaMS). Both the glass transition temperature and local segmental relaxation times, τα, go through a maximum as a function of blend composition, plausibly due to a negative excess volume. Using a nonlinear function for the composition dependence of Tg, a value of the self-concentration parameter of the Lodge−McLeish model is determined, which is close to that calculated from the PCHMA chain statistics. For both neat PCHMA and the blend, the τα superpose as a function of TVγ. Since the breadth of the local segmental relaxation dispersion is a unique function of τα, this means that the parameter γ defines both the magnitude and distribution of relaxation times. The thermodynamic scaling for the blend was corroborated by calculating the exponent γ from the isochronal thermal expansion coefficient, determined from the pressure coefficient of Tg. γ increases upon blending, consistent with a larger activation volume of PCHMA when mixed with the PaMS. PCHMA exhibits a weak β-relaxation, which lacks correspondence to the segmental dynamics and thus is presumably due to local motion of the carbonyl group. A more intense γ-relaxation and a frequency-insensitive loss at higher frequencies are also observed. Unlike the segmental process, these secondary relaxations are insensitive to blending.

Structure and Dynamics of N-Isopropylacrylamide/Acrylic Acid Copolymer Gels Prepared by Cross-Linker-Free UV−Induced Polymerization

The structure and dynamics of poly(N-isopropylacrylamide-co-acrylic acid) (NIPAm/AAc) gels prepared either by cross-linker-free UV-induced polymerization or by redox polymerization were compared using swelling measurement and dynamic/static light scattering. Either AAc monomer or AAc polymer units are included in the polymer network. As a result, four types of NIPAm/AAc gels were obtained, namely (A) redox-mono, (B) UV-mono, (C) redox-poly, and (D) UV-poly. A is the reference polymer mostly investigated in the literature. The following facts emerged: Compared to A, B contains a large portion of defects, e.g., dangling chains, loops and branched chains, which leads to characteristic behavior, i.e., (i) larger swelling ratio and (ii) stronger temperature dependence of the spatial inhomogeneities. Charged groups in both C and D are topologically localized on AAc polymer chains, which result in different temperature dependencies of the structure and dynamics.

Distribution of Phenols in Thermoresponsive Hydrogels

The effect of three hydrotropic molecules, phenol, resorcinol and phloroglucinol, on the thermodynamic equilibrium and the dynamics of poly(N-isopropylacrylamide) (PNIPA) hydrogels is reported. With increasing hydroxyl number of this molecular sequence both the depression of the temperature and the width of the volume phase transition (VPT) increase, reflecting the nonuniform distribution of these molecules inside the gel. Small-angle X-ray scattering detects no notable change in the local structure of the polymer chains in the presence of phenol. Dynamic light scattering observations, however, show that phloroglucinol, unlike phenol and resorcinol, produces a small but significant decrease in the mobility of the polymer in the solvent. Isothermal microcalorimetry reveals an anomalous increase in the exothermic enthalpy of mixing just below the transition, both with phenol and phloroglucinol, which indicates a pretransition state in the gel in which the aromatic molecules partly replace the bound water. This finding is corroborated by small-angle neutron scattering measurements under contrast matched conditions showing that at low phenol concentration the aromatic molecules are uniformly dispersed in the solvent, but in pretransition conditions a sparsely populated layer of phenol forms close to the polymer chains.