Local Segmental Dynamics Research Articles

Isobaric and isochoric fragilities and the influence of volume on the temperature dependence of local segmental relaxation in polyvinylethylene networks

The ratio of the isochoric and isobaric activation enthalpies at T(g) is used to quantify the relative contributions of the specific volume and temperature to the local segmental dynamics of polyvinylethylene networks. The dynamics of the networks become more dominated by temperature with increasing cross-link density, consistent with an increasing isobaric fragility. We also find a correlation between the isochoric and isobaric fragilities for the polyvinylethylene networks, in accord with a previously reported relationship between these two quantities for polymers and molecular glass-forming liquids [R. Casalini and C. M. Roland, Phys. Rev. E 72, 031503 (2005)].

The Role of Density and Temperature in the Dynamics of Polymer Blends

The relative effect of volume and thermal energy on the local segmental dynamics, as reflected in the ratio of the isochoric, EV, and isobaric, EP, activation enthalpies, is determined for blends of polystyrene (PS) with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and with poly(vinyl methyl ether) (PVME). We find that neat PPO near Tg has the lowest value of EV/EP = 0.25 ± 0.02 reported for any polymer, indicating volume-dominated dynamics. Addition of the lower Tg PS alleviates constraints on local motion, resulting in a weaker volume effect. The opposite situation prevails with PS/PVME blends. PS has a higher Tg, and in blends segmental relaxation of the PVME becomes more controlled by volume than for neat PVME. We also show herein that the relaxation times for the PVME/PS blends measured at various T and P superpose when plotted vs Tυγ, where υ is the specific volume and γ is a material constant. This scaling, which has previously been demonstrated for various neat glass-formers and is reported herein for the first time for a blend, enables EV/EP to be determined at Tg in the absence of actual measurements near Tg.

Temperature and Density Effects on the Local Segmental and Global Chain Dynamics of Poly(oxybutylene)

Dielectric spectroscopy measurements over a broad range of temperature and pressure were carried out on poly(oxybutylene) (POB), a type-A polymer (dielectrically-active normal mode). There are three dynamic processes appearing at lower frequency, the normal and segmental relaxation modes, and a conductivity arising from ionic impurities. In combination with pressure-volume-temperature measurements, the dielectric data were used to assess the respective roles of thermal energy and density in controlling the relaxation times and their variation with T and P. We find that the local segmental and the global relaxation times are both a single function of the product of the temperature times the specific volume, with the latter raised to the power of 2.65. The fact that this scaling exponent is the same for both modes indicates they are governed by the same local friction coefficient, an idea common to most models of polymer dynamics. Nevertheless, near Tg, their temperature dependences diverge. The magnitude of the scaling exponent reflects the relatively weak effect of density on the relaxation times. This is usual for polymers, as the intramolecular bonding, and thus interactions between directly bonded segments, are only weakly sensitive to pressure. This insensitivity also means that the chain end-to-end distance is invariant to P, conferring a near pressure-independence of the (density-normalized) normal mode dielectric strength.

Segmental Dynamics and Self-Concentration around Chain Ends in Miscible Blend of Poly(cyclohexyl methacrylate) and Poly(cyclohexyl acrylate) As Studied by the Spin-Label Technique

Local segmental dynamics of poly(cyclohexyl methacrylate) (PCHMA) and poly(cyclohexyl acrylate) (PCHA) in their miscible blend was determined by the spin-label technique. The PCHMA and PCHA chains were labeled with nitroxide radicals at chain ends or inside sites, respectively. The temperature-dependent electron spin resonance (ESR) spectra of the spin-labels selectively reflected the segmental dynamics around the labeled sites. Specific chain dynamics of the PCHMA and PCHA components in the blend was separately detected even though a single glass transition was observed in calorimetric measurements.The Lodge−McLeish model (L−M model) on the basis of the self-concentration effect in the blend predicted well the individual chain dynamics detected by the spin-labeling. This agreement implies that the local length scale to influence the dynamics is comparable to the Kuhn length, lk. The chain ends of the PCHA and PCHMA in the homopolymer bulk had higher mobility than those of the inside sites due to flexibility around chain ends. The PCHMA chain end had higher mobility than that of inside segments in the blend with any compositions. On the other hand, the mobility of the PCHA chain end was lower than that of the inside in the blend when the weight fraction of the PCHA was smaller than ca. 0.5. These experimental results were described well by the L−M model with the lower self-concentration around chain ends. The lower self-concentration around chain ends is considered to be caused by the discontinuity of repeat units at chain ends.

Local Segmental Dynamics of Poly(2-hydroxyethyl Methacrylate) in Methanolic Solution by Spin Label X-Band ESR

A copolymer of 2-hydroxyethyl methacrylate (HEMA) with spin-labeled methacrylic acid units distributed randomly along the main chain (∼3 mol %) was synthesized. ESR spectra of methanolic solutions of the copolymer at concentrations ranging from 2 to 50 wt % were measured at X-band (9 GHz) over a broad temperature range. Temperature dependence of the parameter RS, characterizing the local segmental dynamics in PHEMA chain, and of the other parameters characterizing local dynamics and conformation of the copolymer were determined by fitting experimental ESR spectra to the theoretical spectra calculated using the MOMD model. Arrhenius plots of the rotational parameters exhibit atypical nonlinear behavior characterized by two breaks at particular temperatures in the RS plot. It is concluded that, similarly to PMMA in a number of solvents, PHEMA in methanol undergoes a conformational transition from a less compact to more compact conformation of the polymer chain when the temperature increases in a particular ...

High-Pressure Study of the Local Dynamics of Melt cis-1,4-Polybutadiene and cis-1,4-Polyisoprene

Time-resolved optical spectroscopy has been employed to study the local melt segmental dynamics of anthracene-labeled cis-1,4-polybutadiene (PB) as a function of pressure (0.1-150 MPa) over a temperature range of 303-323 K PB activation volumes calculated at fixed temperatures were observed to be independent of pressure but dependent on temperature. PB activation energies calculated at fixed pressures were observed to be independent of temperature but dependent on pressure. These results are qualitatively similar to previously reported findings for cis-1,4-polyisoprene (PI). The high-pressure dynamics of PB and PI are shown to be well described by an empirical function of pressure and temperature. An average dT/dP value of 0.15 K MPa -1 for PB is calculated and is observed to be significantly temperature and pressure dependent. The ratio of the activation energy at constant volume to the activation energy at constant pressure, Q V /Q P , increases from 0.76 to 0.91 as the PB density is increased from 0.88 to 0.94 g cm -3 via variations in temperature and pressure.

Effect of solvent on nanoscale polymer heterogeneity and mobility probed by single molecule lifetime fluctuations

In this Letter, we address nanoscale heterogeneity in polymer mobility. Single molecule fluorescence lifetime fluctuations are exploited as a probe for local segmental dynamics in polymer films. Wide distributions in polymer mobility are observed, which depend on film fabrication, treatment and amount of solvent contained. Upon solvent evaporation mobile regions disappear, while more immobile regions settle, effectively increasing the glass transition temperature. Besides insight in polymer nanoscale dynamics and heterogeneity, our results bear specific relevance to photodynamic studies on single chromophores, where routinely polymer matrices are used for immobilization.

Solvent Dependence of Polystyrene Local Segmental Dynamics in Dilute Solution by Spin-Label X-Band ESR

A styrene copolymer with methacrylic acid containing less than 5 mol % of spin-labeled methacrylic acid chain units distributed randomly along the polymer main chain (SL−ST−MA) was synthesized. ESR spectra of the copolymer were measured at X-band (9 GHz) in dilute solutions (ca. 1 wt %) over a broad temperature range in four solvents differing in thermodynamic quality, viscosity, and activation energy of viscous flow. Temperature dependences of the parameter RS characterizing local dynamics in polystyrene chains and of other parameters characterizing dynamics in the copolymer were determined by fitting experimental ESR spectra to the MOMD model. Unlike so far published conclusions, the data show that the copolymer in the solvents usedϑ solvent dioctyl phthalate (DOP), marginal solvent dibutyl phthalate (DBP), and good solvents toluene (TOL) and dimethylformamide (DMF)exhibits non-Kramers behavior characterized by the parameter α = 0.73 ± 0.02 and by the height of the potential barrier for local conformational transitions Ea = 10.5 ± 0.6 kJ/mol.

Pressure and temperature dependence of the melt segmental dynamics of cis-1,4-polyisoprene via time resolved optical spectroscopy

The local segmental dynamics of melt anthracene-labeled cis-1,4-polyisoprene have been studied as a function of temperature (318–333 K) and pressure (0.1–140 MPa). Activation volumes for the motions range from 24 to 39 cm3 mol−1 and are observed to be temperature dependent, decreasing with increasing temperature. Activation energies at constant pressure range from 32 to 75 kJ mol−1 and are significantly pressure dependent, increasing with increasing pressure. These activation energies are decomposed at a series of pressures and temperatures into a term arising from the energy required to execute the motions at constant volume (i.e., a constant volume activation energy) and a term describing the energy required to create the needed free volume for the observed motions. Values of the constant volume activation energy for the local motions are compared where nearly the same density is reached either via the combination of a high pressure and high temperature or the combination of a lower pressure and lower temperature. These comparisons reveal that the constant volume activation energy reached via the higher pressure and temperature is larger.

Large-scale computer simulation of local segmental dynamics in amorphous atactic polystyrene

Abstract Molecular dynamics computer simulations of amorphous atactic polystyrene have been performed for chains of up to 320 monomer units in a temperature range from 100 to 600 K and in a pressure range from 0.1 to 1000 MPa. The MD-determined values of the glass transition temperature T g are in a good agreement with experimental PVT data at different values of applied pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers as a function of time. The long-time asymptotic slope of these dependencies is close to 0.6 at T > T g , showing diffusive behaviour. The cage effect when local translational motions are essentially frozen in the glassy state has been studied. Local orientational mobility is studied with the help of Legendre polynomials of the first, P 1 ( t ), and second, P 2 ( t ), order for the bonds in the main chain and along the phenyl side groups. Temperature dependencies of the orientational relaxation times at different values of pressure are well described by the Vogel–Fulcher law.

Segmental dynamics of disordered styrene–isoprene tetrablock copolymers

The local segmental dynamics of four styrene-b-isoprene-b-styrene-b-isoprene (SISI) tetrablock copolymers with different styrene composition fs and constant total degree of polymerization N≈120 has been studied in the disordered state in the nano-picosecond time scale, by incoherent quasielastic neutron (QENS), and Brillouin (BS) and depolarized Rayleigh (DRS) light scattering. Far above the glass transition temperature, all three techniques demonstrate the presence of two distinct time scales from which the fast segmental relaxation was quantitatively resolved. This process is associated with the mobility of the polyisoprene (PI) component, and is moderately slower and possesses a broader distribution of relaxation times than in bulk PI. The comparison between the correlation times of DRS and the characteristic times of QENS suggest that segment (hydrogen nucleus) diffusion over a distance of ≈0.8 nm suffices for the loss of local orientation correlations. The faster times of the BS experiment correspond to shorter displacements, ≈0.3 nm. These results demonstrate that the segmental dynamics of the PI are much faster than would be inferred from the monomeric friction factor of PI previously extracted by diffusion and viscosity measurements in the same tetrablock matrices. This, in turn, indicates a substantial local spatial heterogeneity in the segmental dynamics. The slow process is due to the PS segments, which do not relax, appreciably on the time scales accessible here.

Pressure and Temperature Dependence of the Dilute Solution Segmental Dynamics of Anthracene-Labeled Polyisoprene

The local segmental dynamics of anthracene-labeled cis-1,4-polyisoprene in dilute solution have been studied as a function of temperature (298−323 K) and pressure (0.1−150 MPa) in several solvents (dioctyl phthalate, n-dodecane, squalane, and toluene). This range of temperature and pressure afforded a viscosity range of 0.55−494 mPa·s. Previous studies of cis-1,4-polyisoprene as a function of temperature at ambient pressure have revealed that these motions scale as a fractional power of the solvent viscosity, η, thereby violating Kramers' theory which predicts τc ∼ η. Findings reveal quantitatively similar behavior is observed when the solvent viscosity is varied by pressure at fixed temperatures.

Time-resolved optical spectroscopy study of the local dynamics of cis-1,4 and vinyl-1,2-polybutadiene in dilute solution at high pressure

The local segmental dynamics of anthracene labelled cis-1,4-polybutadiene in dilute solution and vinyl-1,2-polybutadiene in dilute solution have been studied by means of time-resolved optical spectroscopy as a function of temperature (298–323 K) and pressure (0.1–150 MPa) in several solvents (dioctyl phthalate, squalane, cis-decalin, n-dodecane and toluene) of good thermodynamic quality. This range of temperatures and pressures afforded a viscosity range of three decades. Kramers' theory predicts local motions that scale as the first power of the steady shear solvent viscosity. However, findings from this study reveal motions that scale as the 0.61±0.03 power of the solvent viscosity for cis-1,4-polybutadiene and 0.75±0.04 for vinyl-1,2-polybutadiene over the entire pressure and temperature range.

Development of cooperativity in the local segmental dynamics of poly(vinylacetate): synergy of thermodynamics and intermolecular coupling

The properties of the α- and Johari–Goldstein β-relaxations in conjunction with configurational entropy data are used to show how cooperativity of the dynamics of poly(vinylacetate) develops as temperature is decreased towards Tg from above. Such behavior is also characteristic of small molecule glass-forming liquids, demonstrating that the combined effects of configurational entropy changes and intermolecular coupling underlie structural relaxation phenomena. Any theoretical model must include both thermodynamics and many-body dynamics in order to accurately describe the general properties of the glass transition.

An investigation into the local segmental dynamics of polyethylene: An isothermal/isobaric molecular dynamics study

Molecular dynamics simulations of unentangled linear polyethylene melts have been performed for systems composed of 10 chains of 100 united atoms over a pressure range of 1 to 5000 bar and a temperature range of 375 to 475 K. Transition rates, activation volumes, and activation energies are in good agreement with values from similar simulations quoted in literature for systems well above Tg. Second-neighbor torsional angle coupling is observed to increase with increasing pressure and decreasing temperature. The lifetime of this coupling between conformational events is presented for the first time. Geometric autocorrelation functions are analyzed in terms of their distribution of relaxation times and reveal a process on the time scale of a few picoseconds and another on the time scale of a few nanoseconds. An intermediate process develops between these two time scales at high pressure and low temperature.

High-resolution solid-state 13C n.m.r. study on phase structure of the compositionally fractionated bacterial copolyester poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid)s

Solid structures of the compositionally fractionated bacterial poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid)s [P(3HB-co-3HP)], bearing much narrower comonomer compositional distributions, were studied by high-resolution solid-state 13C n.m.r. 1H– 13C cross-polarization relaxation behaviour ( T CH and T 1 ρ H) of chain-carbon nuclei was found to strongly depend on the micro-environments they were involved in, and the resonance contributions pertaining to different phases were therefore distinguished. Employing CP/MAS and PST/MAS methods, 13C resonance signals from the crystalline and non-crystalline regions were selectively enhanced. The observed chemical shifts demonstrated that co-crystallization did not occur in the P(3HB-co-3HP) system. Spin-lattice relaxation times ( T 1 C) of carbon nuclei attributed to 3HB and 3HP units were found to be considerably different, reflecting their distinguishable difference in local segmental dynamics. From the lineshape analyses of 3HB methyl carbon resonance measured by a DD/MAS method, it was revealed that the presence of minor 3HP comonomer units significantly suppressed the crystallization of a 3HB-rich semicrystalline copolyester. In addition, a correlation was found between average block length of 3HB sequences estimated by solution-state 13C n.m.r. spectra and crystallizability of semi-crystalline copolyesters.

Local Segmental Dynamics of Low Molecular Weight Polystyrene: New Results and Interpretation

The molecular dynamics of an 1100 low molecular weight polystyrene are reexamined by photon correlation spectroscopy measurements. The new experimental data combined with recoverable creep compliance data taken previously on the same sample provides information on the local segmental motion in a wide time window of more than 11 decades. Significant reduction of the “fragility” (i.e., a scaled temperature dependence of the retardation time) of the low molecular weight polystyrene compared with high molecular weight polystyrene, seen first by Santangelo and Roland by shear modulus measurement, is confirmed. The broader time window available in the present work shows for the first time that, unlike high molecular weight polystyrene, the dispersion of the low molecular weight sample is strongly temperature dependent and narrows with increasing temperature. By considering the low molecular weight polymer as a blend of chain ends and inner repeat units, the results are interpreted by the coupling model.

Viscosity Dependence of the Local Segmental Dynamics of Anthracene-Labeled 1,2-Polybutadiene in Dilute Solution

Time-correlated single-photon-counting measurements have been performed on anthracene-labeled 1,2-polybutadiene. The local segmental dynamics were observed in seven solvents covering a viscosity range of more than three decades and over the temperature range of 280−350 K. Correlation times were extracted from the fluorescence anisotropy decays and were determined as a function of temperature in all solvents. The observed temperature and viscosity dependence has been examined in terms of Kramers' theory in the high friction limit and two empirical equations, those of Fleming and co-workers and Robinson, both of which incorporate the idea of frequency-dependent friction developed by Grote−Hynes. It was found that the dynamics scaled as the viscosity to the power 0.82 ± 0.04.

Detailed Analysis of Differential Dynamics and Morphology in Polyisobutylene and Poly(isobutylene-co-p-methylstyrene) by Solid State NMR

Polyisobutylene is unique among elastomeric polymers in that while the glass transition temperature is low (−70 °C), segmental dynamics at temperatures above Tg are relatively slow due to steric constraints resulting from the molecular packing of methyl groups. We describe results from NMR experiments on solid polyisobutylene (PIB) and copolymers with p-methylstyrene (PIB−PMS) in which the dynamics of chain motion in PIB homopolymers and PIB−PMS copolymers are compared as a function of temperature. Our one- and two-dimensional solid-state 1H, 13C, and 2H results clarify previously published reports on local vs correlated segmental dynamics for the PIB homopolymer, which differ by 4−5 orders of magnitude in their correlation times. Distinct differences in aromatic ring dynamics as a function of PMS concentration are also observed for PIB−PMS copolymers. While PMS dynamics are found to be sensitive to PMS concentration in the copolymer, PIB motions appear to be independent of the comonomer incorporation. Th...

Confinement Effects on The Local Motion in Nanocomposites

AbstractThe local segmental dynamics of polymers confined within the 15–20 Å interlayer spacing of nanocomposites consisting of poly(methyl phenyl siloxane) intercalated within organically modified silicates, has been investigated utilizing Dielectric Relaxation Spectroscopy. The effect of confinement on the local reorientational dynamics is evident by the observation of a relaxation mode, which is much faster than the segmental α-relaxation of the bulk polymer and exhibits much weaker temperature dependence. This is attributed to the restrictions placed by the interlayer spacing on the cooperative volume required for the ax-relaxation.