Neutron reflectivity measurements of the translational motion of tris(naphthylbenzene) at the glass transition temperature

The translational dynamics of the low molecular weight glass-former tris(naphthylbenzene) have been studied on the length scale of a few nanometers at the glass transition temperature Tg. Neutron reflectivity was used to measure isotopic interdiffusion of multilayer samples created by physical vapor deposition. Deposition with the substrate held at Tg-6 K allows observation of dynamics characterizing the equilibrium supercooled liquid. The diffusion coefficient measured at q = 0.03 A(-1) was determined to be 1x10(-17) cm2/s at 342 K (Tg). The self-part of the intermediate scattering function I(s)(q,t) decays exponentially. Samples deposited well below Tg show a substantial thermal history effect during subsequent translational motion at Tg.     

Quasielastic neutron scattering of poly(methyl phenyl siloxane) in the bulk and under severe confinement

Quasielastic neutron scattering was utilized to investigate the influence of confinement on polymer dynamics. Poly(methyl phenyl siloxane) chains were studied in the bulk as well as severely confined within the approximately 1-2 nm interlayer spacing of intercalated polymer/layered organosilicate nanohybrids. The temperature dependence of the energy resolved elastic scattering measurements for the homopolymer and the nanocomposites exhibit two distinct relaxation steps: one due to the methyl group rotation and one that corresponds to the phenyl ring flip and the segmental motion. Quasielastic incoherent measurements show that the very local process of methyl rotation is insensitive to the polymer glass transition temperature and exhibits a wave-vector independent relaxation time and a low activation energy, whereas it is not affected at all by the confinement. At temperatures just above the calorimetric glass transition temperature, the observed motion is the phenyl ring motion, whereas the segmental motion is clearly identified for temperatures about 60 K higher than the glass transition temperature. For the nanohybrid, the segmental motion is found to be strongly coupled to the motion of the surfactant chains for temperatures above the calorimetric glass transition temperature of the bulk polymer. However, the mean square displacement data show that the segmental motion in confinement is faster than that of the bulk polymer even after the contribution of the surfactant chains is taken into consideration.     

高分子链的去拥挤转变及低温流动

简述了拥挤理论的基本原理,运用拥挤理论来说明高分子链间弱相互作用对高分子链所处的状态的影响,特别是对高分子玻璃化转变的影响.在实验中,采用固体核磁共振方法探测高分子的链间邻近度,并比较了不同链间邻近度的高分子样品在玻璃化转变温度以下的压力诱导流行行为,发现即使测试温度比高分子玻璃化转变温度低132℃,高分子链在压力下依...     

Tuning polymer melt fragility with antiplasticizer additives

A polymer-diluent model exhibiting antiplasticization has been developed and characterized by molecular dynamics simulations. Antiplasticizer molecules are shown to decrease the glass transition temperature Tg but to increase the elastic moduli of the polymeric material in the low-temperature glass state. Moreover, the addition of antiplasticizing particles renders the polymer melt a stronger glass-forming material as determined by changes in the characteristic temperatures of glass formation, the fragility parameter D from fits to the Vogel-Folcher-Tamman-Hesse equation, and through the observation of the temperature dependence of the size of cooperatively rearranging regions (strings) in each system. The length of the strings exhibits a weaker temperature dependence in the antiplasticized glass-forming system than in the more fragile pure polymer, consistent with the Adam-Gibbs model of glass formation. Unexpectedly, the strings become increasingly concentrated in the antiplasticizer particles upon cooling. Finally, we discuss several structural indicators of cooperative dynamics, and find that the dynamic propensity (local Debye-Waller factor p) does seem to provide a strong correlation with local molecular displacements at long times. The authors also consider maps of the propensity, and find that the antiplasticized system exhibits larger fluctuations over smaller length scales compared to the pure polymer.     

Molecular dynamics simulation of the glass transition temperature of fullerene filled cis-1,4-polybutadiene nanocomposites

t In this work,the effect of the fullerene (C60) weight fraction and PB-C60 interaction on the glass transition temperature (Tg) of polymer chains has been systemically investigated by adopting the united atom model of cis-1,4-poly(butadiene) (cis-PB).Various chain dynamics properties,such as atom translational mobility,bond/segment reorientation dynamics,torsional dynamics,conformational transition rate and dynamic heterogeneity of the cis-PB chains,are analyzed in detail.It is found that Tg could be affected by the C60 weight fraction due to its inhibition effect on the mobility of the cis-PB chains.However,Tg is different,which depends on different dynamics scales.Among the chain dynamics properties,Tg is the lowest from atom translational mobility,while it is the highest from the dynamic heterogeneity.In addition,Tg can be more clearly distinguished from the dynamic heterogeneity;however,the conformational transition rate seems to be not very sensitive to the C60 weight fraction compared with others.For pure cis-PB chains,Tg and the activation energy in this work can be compared with those of other polymers.In addition,the temperature dependence of the dynamic properties has different Arrhenius behaviors above and below Tg.The activation energy below Tg is lower than that above Tg.This work can help to understand the effect of the C60 on the dynamic properties and glass transition temperature of the cis-PB chains from different scales.     

Probing the two-stage transition upon crossing the glass transition of polystyrene by solid-state NMR

A two-stage transition upon crossing the glass transition of polystyrene with increasing temperature was precisely determined and interpreted by using solid-state nuclear magnetic resonance(SSNMR), ~1H-~1H dipolar couplings based double quantum-filtered(DQF) and dipolar filter(DF) experiments and ~(13)C chemical shift anisotropy(CSA) based centerband-only detection of exchange(CODEX) experiment are used to fully characterize the time scale of molecular motions during the glass transition. While differential scanning calorimetry(DSC) and CODEX experiment predicted the first stage of glass transiton, DQF and DF experiments provided the evidence for the second stage transition during which the time scale of molecular motions changed from very slow(t ms) to very fast(t μs). The first stage of glass transition begins with the occurrence of remarkable slow re-orientation motions of the polymer backbone segments and ends when the degree of slow motion reaches maximum. The onset and endpoint of the conventional calorimetric glass transition of polystyrene can be quantitatively determined at the molecular level by SSNMR. In the second stage, a subsequent dramatic transition associated with the melting of the glassy components was observed. In this stage liquid-like NMR signals appeared and rapidly increased in intensity after a characteristic temperature T_f(~1.1T_g). The signals associated with the glassy components completely disappeared at another characteristic temperature T_c(~1.2T_g).     

Effect of diluents on poly(vinyl acetate) dynamics

The effect of diluents and temperature on segmental motion in poly(vinyl acetate) was investigated by both NMR and ESR spectroscopy. Three classes of diluents were studied: chloroform, a thermodynamically good solvent; water, a poor solvent which slightly swells the polymer and lowers its glass transition temperature; and decane, a nonsolvent so poor it does not appear to swell the polymer nor lower the calorimetric glass transition temperature. At all temperatures investigated each type of diluent increased the segmental motion of the polymer over that of the bulk sample. Under the conditions studied, ¹³C and ²H NMR and nitroxide spin-label ESR data gave similar views of segmental motion of the polymer, indicating that in this amorphous polymer the segmental motion of the polymer may be safely inferred from spin-label data.     

Size effects on miscibility and glass transition temperature of binary polymer blend films

After determining the size dependent miscibility of binary polymer blend films using molecular dynamics simulation and thermodynamics, the size dependent glass transition temperatures Tg(w,D) of several polymer blend films in miscible ranges are determined by computer simulation and the Fox equation where w is the weight fraction of the second component and D denotes thickness of films. The Tg(w,D) function of a thin film can decrease or increase as D decreases depending on their surface or interface states. The computer simulation results are consistent with available experimental results and theoretical results for polymer blend films of PPO/PS [poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene] and stereoregular PMMA/PEO [poly(methyl methacrylate)/poly(ethylene oxide)]. The physical background of the above results is related to the root of mean square displacement of thin films in their different regions.     

Structural changes above the glass transition and crystallization in aluminophosphate glasses: an in situ high-temperature MAS NMR study

We present an in situ high-temperature nuclear magnetic resonance study on the structural changes in aluminophosphate glasses occurring in the temperature range between the glass transition temperature Tg and the crystallization temperature Tc, Tg < T < Tc. Decisive changes in the network organization between Tg and Tc in potassium aluminophosphate glasses in the compositional range 50K2O-xAl2O3-(50 - x)P2O5 with 2.5 < x < 20 could be monitored for the first time employing 1D 31P- and 27Al-MAS NMR. Accompanying ex situ NMR experiments (31P-RFDR NMR and 31P-{27Al} CP-HETCOR NMR) on devitrified samples were performed at room temperature to further characterize the phases formed during the crystallization process. The structural role of boron-which is known to inhibit the crystallization process in these aluminophosphate glasses-on short and intermediate length scales was analyzed employing 11B-MQMAS, 11B-{27Al} TRAPDOR and 11B-{31P} REDOR NMR spectroscopy.     

Analysis of one-dimensional pure-exchange NMR experiments for studying dynamics with broad distributions of correlation times

One-dimensional (1D) exchange NMR experiments can elucidate the geometry, time scale, memory, and heterogeneity of slow molecular motions (1 ms-1 s) in solids. The one-dimensional version of pure-exchange (PUREX) solid-state exchange NMR, which is applied to static samples and uses the chemical shift anisotropy as a probe for molecular motion, is particularly promising and convenient in applications where site resolution is not a problem, i.e., in systems with few chemical sites. In this work, some important aspects of the 1D PUREX experiment applied to systems with complex molecular motions are analyzed. The influence of intermediate-regime (10 micros-1 ms) motions and of the distribution of reorientation angles on the pure-exchange intensity are discussed, together with a simple method for estimating the activation energy of motions occurring with a single correlation time. In addition, it is demonstrated that detailed information on the motional geometry can be obtained from 1D PUREX spectral line shapes. Experiments on a molecular crystal, dimethyl sulfone, confirm the analysis quantitatively. In two amorphous polymers, atactic polypropylene (aPP) and polyisobutylene (PIB), which differ only by one methyl group in the repeat unit, the height of the normalized exchange intensity clearly reveals a striking difference in the width of the distribution of correlation times slightly above the glass transition. The aPP shows the broad distribution and Williams-Landel-Ferry temperature dependence of correlation times typical of polymers and other "fragile" glass formers. In contrast, the dynamics in PIB occur essentially with a single correlation time and exhibits Arrhenius behavior, which is more typical of "strong" glass formers; this is somewhat surprising given the weak intermolecular forces in PIB.     

Orientation dynamics in isotropic phases of model oligofluorenes: glass or liquid crystal

Orientation molecular dynamics were investigated in a series of "defect-free" oligofluorenes by depolarized dynamic light scattering and dynamic NMR spectroscopy. Typical liquid crystalline pretransitional dynamics were observed upon cooling the isotropic phase to the liquid crystalline phase with strong increase of the scattered intensity and slowing down of the characteristic time of the probed collective relaxation. This is well accounted for by the Landau-de Gennes theory, however, with a strong temperature dependence of the viscosity coefficient, reflecting the proximity of the glass transition. For the trimer the two transitions almost overlap and the molecular orientation coincide with the alpha-relaxation associated with the glass transition. The NMR measurements confirm that the time scale of the dynamics is completely governed by the glass process, yet the geometry of the motion is anisotropic, yielding order parameters ranging from 0.15 to 0.25 for the long axis in the liquid crystalline phase. The glass transition is therefore geometrically restricted with poorly ordered mesophase which is consistent with the weak transverse phonons in the light scattering experiment down to Tg+20 K.     

Progress of Polymerization of D,L-Lactide Through Differential Scanning Calorimetry and Gel Permeation Chromatography

Melt polymerization conditions for D,L-lactide initiated with tetraphenyltin were studied with regard to polymer molecular weight. The present study was undertaken to investigate the progress of polymerization of D,L-lactide through differential scanning calorimetry (DSC), and also to explore the correlation between melt polymerization conditions and molecular weight. The physical characteristics, such as glass transition temperature (Tg) of the polymer and melting transition (Tm) of D,L-lactide are correlated with GPC data. DSC data shows that the Tm of D,L-lactide is 122.8 at 150°C polymerization time. ΔHf is 83.2 J g-1, and Tg of polymer is untraceable. At 180°C the Tm is 101.4°C, ΔHf is 34 J g-1, and Tg is around 29.5°C. The drop in Tm and ΔHf clearly shows the conversion of D,L-lactide to polymer. The maximum increment to molecular weight of polymer is achieved at 160°C and 8 h. After a short induction period, the slow polymerization of D,L-lactide resulted in maximal molecular weight followed by an almost constant value of molecular weight. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phase biaxiality in nematic liquid crystalline side-chain polymers of various chemical constitutions

In a previous deuterium NMR study conducted on a liquid crystalline (LC) polymer with laterally attached book-shaped molecules as the mesogenic moiety, we have revealed a biaxial nematic phase below the conventional uniaxial nematic phase (Phys. Rev. Lett. 2004, 92, 125501). To elucidate details of its formation, we here report on deuterium NMR experiments that have been conducted on different types of LC side-chain polymers as well as on mixtures with low-molar-mass mesogens. Different parameters that affect the formation of a biaxial nematic phase, such as the geometry of the attachment, the spacer length between the polymer backbone and the mesogenic unit, as well as the polymer dynamics, were investigated. Surprisingly, also polymers with terminally attached mesogens (end-on polymers) are capable of forming biaxial nematic phases if the flexible spacer is short and thus retains a coupling between the polymer backbone and the LC phase. Furthermore, the most important parameter for the formation of a biaxial nematic phase is the dynamics of the polymer backbone, as the addition of a small percentage of low molar mass LC to the biaxial nematic polymer from the original study served to shift both the glass transition and the appearance of detectable biaxiality in a very similar fashion. Plotting different parameters for the investigated systems as a function of T/Tg also reveals the crucial role of the dynamics of the polymer backbone and hence the glass transition.     

Ion and polymer dynamics in polymer electrolytes PPO-LiClO4. II. 2H and 7Li NMR stimulated-echo experiments

We use 2H NMR stimulated-echo spectroscopy to measure two-time correlation functions characterizing the polymer segmental motion in polymer electrolytes PPO-LiClO4 near the glass transition temperature Tg. To investigate effects of the salt on the polymer dynamics, we compare results for different ether oxygen to lithium ratios, namely, 6:1, 15:1, 30:1, and infinity. For all compositions, we find nonexponential correlation functions, which can be described by a Kohlrausch function. The mean correlation times show quantitatively that an increase of the salt concentration results in a strong slowing down of the segmental motion. Consistently, for the high 6:1 salt concentration, a high apparent activation energy Ea=4.1 eV characterizes the temperature dependence of the mean correlation times at Tg相似文献   

Slow dynamics in glassy methyl alpha-l-rhamnopyranoside studied by 1D NMR exchange experiments

It is widely known that the ability of sugar glasses to preserve anhydrobiotic systems in nature is important but the process is not yet fully understood. Molecular motions in the glassy state are likely to be important in the process but until now have remained largely uncharacterized. Here we describe the use of 1D 13C NMR exchange experiments using CODEX (centreband only detection of exchange) methods to study the dynamics of the well characterised model glassy monosaccharide, methyl alpha-l-rhamnopyranoside. The glass was prepared by fast cooling of a melt inside an NMR rotor. Molecular motions in the range of seconds to milliseconds were observed in the glass, whereas identical experiments using the crystalline material displayed no observable motions in the time-scales covered by the experiment. At 13 to 14 K above Tg the nature of the motion in the glass changed probably due to the onset of larger scale reorientation. A bimodal distribution of jump angles combined with a broad distribution of correlation times was found to best represent the observed motions.     

Molecular dynamics investigation of a density-driven glass transition in a liquid crystal system

Molecular dynamics simulations are carried out to address the density-driven glass transition in a system of rodlike particles that interact with the Gay-Berne potential. Since crystallization occurs in this system on the time scale of the simulations, direct simulation of the glass transition is not possible. Instead, glasses with isotropic orientational order are heated to a temperature T, and the relaxation times by which nematic orientational order develops are determined. These relaxation times appear to diverge at a critical density rho(c); i.e., the system can equilibrate at rhorho(c) (at the temperature T). The relaxation times follow a power-law scaling as the critical density is approached, suggesting that this density-driven glass transition concurs with mode coupling theory.     

Molecular dynamics of oligofluorenes: a dielectric spectroscopy investigation

The molecular dynamics were investigated in a series of "defect-free" oligofluorenes up to the polymer by dielectric spectroscopy (DS). The method is very sensitive to the presence of keto "defects" that when incorporated on the backbone give rise to poor optical and electronic properties. Two dielectrically active processes were found (beta and alpha process). The latter process (alpha) displays strongly temperature dependent relaxation times and temperature- and molecular weight-dependent spectral broadening associated with intramolecular correlations. The glass temperature (Tg) obeys the Fox-Flory equation and the polymer Tg is obtained by DS at 332 K. The effective dipole moment associated with the alpha process is 0.27 +/- 0.03 D.     

Local chain dynamics of a model polycarbonate near glass transition temperature: A molecular dynamics simulation

Constant pressure constant temperature molecular dynamics method is employed to investigate the atomistic scale dynamics of a model Bisphenol A polycarbonate in the vicinity of its glass transition temperature. First, the glass transition temperature and the thermal expansion coefficients of the polymer are predicted by performing simulations at different temperatures. To explore the significance of different modes of motion, various types of time correlation functions are utilized in analyzing the trajectories. In these nanosecond scale simulations, the motion of the chain segments is found to be highly localized with little reorientation of the vectors representing these segments. Detailed analysis of trajectories and the correlation functions of the backbone dihedrals and side methyl groups indicates that they exhibit numerous conformational transitions. The activation energies of the conformational transitions obtained from the simulation are generally larger than the potential barriers for the rotations of these dihedrals, however, both show the same trend. We also have estimated the phenylene ring flip activation energy as 12.6 kcal/mol and the flip frequency as 0.77 MHz at 300 K. These values fall either fall within the range determined by various NMR spectroscopy experiments or slightly out of the range. The study shows that the conformational transitions between the adjacent dihedrals are strongly correlated. Three basic cooperative modes are identified from the simulation. They are: a positive synchronous rotation of two phenylene rings, a negative synchronous rotation of two phenylene rings, and a carbonate group rotation. Above the glass transition temperature, the large scale cooperative motions become much more significant.     

Dual nature of a tin-containing liquid crystalline side group homopolymer

Two tin-containing homopolymers, one liquid crystalline and the other non-mesomorphic, were synthesized and characterized by different relaxation methods (dielectric, calorimetric, NMR). The results prove the existence of two glass transition temperatures related to the dynamics of the main chain and of the liquid crystalline side group, respectively. The reason for this effect is based on a phase separation on nanometer scale.     

Effect of physical aging on the Johari-Goldstein and alpha relaxations of D-sorbitol: a study by thermally stimulated depolarization currents

The relaxations in amorphous D-sorbitol have been studied by thermally stimulated depolarization currents during annealing at 255 K, which is 17 K below its calorimetric glass transition temperature Tg=272 K. As the glass structurally relaxes on aging, the features of the alpha relaxation and of the Johari-Goldstein (JG) relaxation change with time. For the alpha relaxation (i) the dielectric strength decreases; (ii) the activation energy decreases; and (iii) the relaxation time increases. For the JG relaxation the dielectric strength also decreases but with a different time dependence, and there is no evidence for any modification of the kinetic features of the mobility. The amplitude of response to aging is higher for the higher temperature motional components of the Johari-Goldstein relaxation compared with the lower temperature ones.