Secondary and Segmental Relaxation in Polybutadienes of Varying Microstructure: Dielectric Relaxation Results

Anomalous toluene transport in model segmented polyurethane-urea/clay nanocomposites

The kinetics of liquid solvent sorption in polymeric systems and their nanocomposites often deviate from normal Fickian behaviour. This needs to be understood and interpreted, in terms of their underlying mechanistic origins. In the present study, the results of time dependent toluene sorption measurements in model segmented polyurethane-urea/clay nanocomposites have been analysed at room temperature. The studies revealed pronounced S-shaped sorption curves and unusually higher swelling of the nanocomposites compared to the neat polyurethane-urea matrix. Dynamic mechanical analysis (DMA) and small angle X-ray scattering (SAXS) measurements on the nanocomposites in the dry and liquid toluene saturated state have been carried out. The DMA studies revealed a significant decrease in the α relaxation temperature and storage modulus of the nanocomposites in the swollen state compared to the dry samples. The SAXS results showed that the nanoclay dispersion morphology transformed from intercalation in the dry state to exfoliation in the swollen state and the interdomain distance between hard segments increased upon swelling. Thermodynamic analysis of the Flory-Huggins interaction parameter (χ) of nanocomposite/toluene systems revealed increasingly negative χ values with increased clay loading. These results imply a significant plasticization effect of toluene on the nanocomposites. An interpretation of these data, which relates the abovementioned results, is presented in the framework of differential swelling stress (DSS) induced deviation from Fickian transport characteristics. We expect that these findings and methods may provide new insight into the analysis of the solvent diffusion process in heterogeneous polymers and their nanocomposites.

Anionic Polymerization of Styrene and 1,3-Butadiene in the Presence of Phosphazene Superbases

The anionic polymerization of styrene and 1,3-butadiene in the presence of phosphazene bases (t-BuP₄, t-BuP₂ and t-BuP₁), in benzene at room temperature, was studied. When t-BuP₁ was used, the polymerization proceeded in a controlled manner, whereas the obtained homopolymers exhibited the desired molecular weights and narrow polydispersity (Ð < 1.05). In the case of t-BuP₂, homopolymers with higher than the theoretical molecular weights and relatively low polydispersity were obtained. On the other hand, in the presence of t-BuP₄, the polymerization of styrene was uncontrolled due to the high reactivity of the formed carbanion. The kinetic studies from the polymerization of both monomers showed that the reaction rate follows the order of [t-BuP₄]/[sec-BuLi] >>> [t-BuP₂]/[sec-BuLi] >> [t-BuP₁]/[sec-BuLi] > sec-BuLi. Furthermore, the addition of t-BuP₂ and t-BuP₁ prior the polymerization of 1,3-butadiene allowed the synthesis of polybutadiene with a high 1,2-microstructure (~45 wt %), due to the delocalization of the negative charge. Finally, the one pot synthesis of well-defined polyester-based copolymers [PS-b-PCL and PS-b-PLLA, PS: Polystyrene, PCL: Poly(ε-caprolactone) and PLLA: Poly(L-lactide)], with predictable molecular weights and a narrow molecular weight distribution (Ð < 1.2), was achieved by sequential copolymerization in the presence of t-BuP₂ and t-BuP₁.

Molecular ring rotation in poly(vinylferrocene)

We investigate the ring rotation dynamics in poly(vinylferrocene) (PVFc) using incoherent neutron spectroscopy. PVFc contains ferrocene units laterally attached to a polymer backbone, allowing for one cyclopentadienyl ring of the organometallic sandwich structure of ferrocene to undergo rotational jump diffusion. The barrier of rotation is found to be broadly distributed, but the dynamics can be well described using a rotation rate distribution model which is well known from the description of methyl group rotation in glassy polymers. As necessary information for the analysis of quasielastic scattering data, we measure the static structure factor of the polymer using polarized neutron diffraction. Neutron time-of-flight and backscattering data are then combined and consistently modeled over the large temperature range from 80 K to 350 K yielding an Arrhenius behavior of the jump rate distribution. The mean value of potential barrier distribution is found to be 〈E_A〉 = 9.61(2) kJ mol^-1 with a root mean square width of σ_E = 3.12(1) kJ mol^-1, being the result of superposition of constant intramolecular and heterogeneous intermolecular rotational barriers.

Self-assembled structure and relaxation dynamics of diblock copolymers made of polybutadiene and styrene/butadiene rubber

Interrelations between self-assembled structure and cooperative α dynamics are systematically studied based on two series of poly(butadiene-block-(styrene-stat-butadiene)) diblock copolymers.

Atomic motions in the alphabeta-merging region of 1,4-polybutadiene: a molecular dynamics simulation study

We present fully atomistic molecular dynamics simulations on 1,4-polybutadiene in a wide temperature range from 200 to 280 K, i.e., in the region where the alpha- and beta-relaxations merge and above. A big computational effort has been performed-especially for the lowest temperatures investigated-to extend the simulation runs to very long times (up to 1 mus for 200 K). The simulated sample has been carefully validated by using previous neutron scattering data on the real sample with similar microstructure. Inspecting the trajectories of the different hydrogens in real space, we have observed a heterogeneous dynamical behavior (each kind of hydrogen moves in a different way) with signatures of combined hopping and diffusive motions in the whole range investigated. The application of a previously proposed model [Colmenero et al., Europhys. Lett. 71, 262 (2005)] is successful and a characterization of the local motions and diffusion is possible. The comparison of our results to those reported in the literature provides a consistent scenario for polybutadiene dynamics and puts into a context the different experimental observations. We also discuss the impact of the hopping processes on the observation and interpretation of experimentally accessible magnitudes and the origin of the deviations from Gaussian behavior in this system.

Glass transition in 1,4-polybutadiene: Mode-coupling theory analysis of molecular dynamics simulations using a chemically realistic model

We present molecular dynamics simulations of the glass transition in a chemically realistic model of 1,4-polybutadiene (PBD). Around 40 K above the calorimetric glass transition of this polymer the simulations reveal a well-developed two-stage relaxation of all correlation functions. We have analyzed the time-scale separation between vibrational degrees of freedom (subpicosecond dynamics) and the alpha relaxation behavior (nanosecond to microsecond dynamics) using the predictions of mode-coupling theory (MCT). Our value for the mode-coupling critical temperature Tc agrees perfectly with prior experimental estimates for PBD. The predictions of MCT for the scaling behavior of the so-called beta relaxation, the plateau regime separating vibrational dynamics and the alpha relaxation, are well fulfilled. Furthermore, we are able to derive a consistent set of MCT exponents, completely characterizing the scaling behavior of relaxation processes in the vicinity of Tc. For the temperature dependence of the alpha relaxation we find deviations from MCT predictions which we trace to the influence of torsional barriers on the atomic motions.

Spatial regimes in the dynamics of polyolefins: Self-motion

Molecular dynamics simulations are used to investigate the spatial dependence of dynamics in a series of polyolefins. The dynamic indicator used is the self-intermediate scattering function, which parallels the observable in an incoherent quasielastic neutron scattering experiment such as time of flight or backscattering. As with neutron time of flight experiments, two processes are evident. The fast process is a single exponential, and has relaxation times that scale as q(-2), where q is the momentum transfer. The slow process is the stretched exponential decay usually associated with the motion underlying the glass transition. The stretching exponent is a function of spatial scale, with the minimum values occurring near the spatial scale of interchain packing. Relaxation times for the slow process scale as q(-2/beta) for all materials investigated. The relative contribution of the two processes is a function of spatial scale, with the crossover from fast to slow dynamics at the location of closest possible interchain contacts, which is approximately three times the cage size. These observations apply equally well to the four materials considered. We consider the relative ordering of relaxation times of the series in light of their local chain architecture. This ordering varies depending on the observable calculated..

Intramolecular caging in polybutadiene due to rotational barriers

We present molecular dynamics simulations of a chemically realistic model of 1,4-polybutadiene and a freely rotating chain model derived from the first model by neglecting all dihedral potentials. We show that the presence of energy barriers hindering dihedral rotation leads to an intermediate plateau regime in the tagged particle mean-squared displacement reminiscent of the cage effect underlying the mode-coupling description of the liquid-glass transition. This intramolecular caging, however, occurs already at temperatures well above the glass transition regime. Because of its different physical origin, it also does not comply with the theoretical predictions of the mode-coupling theory. Consequences for the applicability of the mode-coupling theory to the glass transition in polymer melts are discussed.