A nuclear magnetic resonance study of toluene-polyisobutylene solutions. 3. Spin-lattice relaxation: Rotational and segmental motion

The spin-lattice relaxation times are determined for the methylene carbon of polyisobutylene (PIB), as well as for the ortho carbon of toluene in toluene-polyisobutylene solutions. The Hall-Helfand correlation function combined with restricted anisotropic rotational diffusion was used to treat the T₁ data of the methylene carbon of PIB. A simple exponential correlation function was used to describe the local motion of toluene in the solutions which falls in the extreme narrowing limit for the solutions studied. Both models described satisfactorily the temperature and field dependence of the spin-lattice relation times. From the temperature dependence of the correlation times for the polymer segmental motion, the free volume of the solution at each concentration is extracted and compared with the values obtained from previous studies of the translational motion of the toluene penetrant. The free volume values extracted from the T₁ data for the methylene carbon of PIB and the self-diffusion data for the toluene were found to be in substantial agreement. The interrelationship of the timescale of segmental motion of the polymer and the translational diffusion of the toluene was also examined and it was found that the two types of motion seem to be correlated in high polymer concentrated solutions. The toluene reorientational motion was found to be much faster than both the polymer segmental motion and the toluene translational diffusion leading to the conclusion that the toluene reorientational motion is uncoupled from these two motions. ©1995 John Wiley & Sons, Inc.     

Moisture absorption and diffusion in an underfill encapsulant at T > T g and T < T g

Moisture absorption and diffusion behavior of an underfill material used for electronic packaging with a glass transition temperature (T _g) slightly above room temperature have been investigated by the sorption thermogravimetric analysis technique. It has been found that moisture diffusion in this material follows the Fick’s diffusion model, and moisture absorption–desorption is reversible and repeatable. Based on moisture-induced mass gain versus time curve, the diffusion constant can be determined. It was found that below T _g, moisture diffusivity exhibits an Arrhenius temperature dependence, which changes to a different Arrhenius temperature dependence as the temperature increases to T > T _g. The change in diffusivity from T < T _g to T > T _g is accompanied by a significant decrease in the energy barrier for moisture diffusion. Results shed light on the change in moisture diffusion in polymer-based materials in the glassy and the rubbery state.     

Probing polymers with single fluorescent molecules

The use of single molecules to study local, nanoscale polymer dynamics is presented. Fluorescence lifetime fluctuations were used to extract the number of polymer segments (N_s) taking part in the rearranging volume around the probe molecule below the glass transition temperature. N_s was dependent on the temperature and it decreased with increasing temperature. Above the glass transition, rotational motion of single molecules was followed in time and typical time-scales of the rotational diffusion were extracted. These two approaches allowed us to obtain non-averaged information about the heterogeneous dynamics present in polymer systems, on the nanoscale, above and below glass transition temperatures.     

Nuclear magnetic relaxation and diffusion study of the ionic liquids 1-ethyl- and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide confined in porous glass

The molecular dynamics of the room-temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Bmim Tf2N) confined in porous glass is studied by nuclear magnetic resonance (NMR) relaxometry and diffusometry and is compared with the bulk dynamics over a wide temperature range. The molecular reorientation processes for anions and cations alike are found to be significantly affected by the presence of the glass interface at high temperatures. In this respect, the ionic liquid behaves similarly to polar liquids where proton NMR relaxation is governed by reorientations mediated by translational displacements (RMTDs). This process becomes less significant towards lower temperatures when the characteristic translational correlation times of the ions approach a timescale comparable with those of the RMTD process, and the relaxation dispersions in bulk and in confinement become similar below a temperature corresponding to about 1.2T_g, a value where the onset of dynamic heterogeneity has been observed before. The self-diffusion coefficient, on the other hand, is found to be strongly reduced than the bulk within the accessible temperature range of 248 K and above and is significantly slower than expected from the tortuosity effect, suggesting that ion–surface interactions affect the macroscopic properties.     

On the continuity of the diffusion behaviour at amorphous polymer–polymer interfaces on both sides of the bulk glass transition temperature

The diffusion behaviour at amorphous polystyrene (PS)–PS interfaces has been investigated over an interval of temperatures (T) from below to above the bulk glass transition temperature (T _g ^bulk) using the Arrhenius and Vogel-Fulcher approaches. No discontinuity in the variation of the logarithm of the diffusion coefficient versus 1/T has been observed when going through the PS T _g ^bulk over a broad interval of T, from T _g ^bulk???50 °C to T _g ^bulk?+?50 °C. The molecular mechanism of interdiffusion has been discussed.     

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).     

Multiple glass transitions in block copolymer films cast from cyclohexane solutions at two temperatures

Films of block copolymers of polystyrene + isoprene, cast from cyclohexane at temperatures above and below a conformational transition temperature (T_p) derived from the plot of [η] vs. T, have been examined for glass transition temperatures. In every case, two glass transitions were found, T_g1 (polyisoprene) and T_g2 (polystyrene) or T_g1 and T_g (an intermediate species). T_g is assumed to be characteristic of a mixed phase thus providing further evidence that T_p marks a conformational transition from a phase-separated to a phase-mixed form.     

Structure and optical characteristics of the polymer-dye composites prepared via solvent crazing

Optical characteristics of the composites based on an amorphous polymer and a luminescent dye [poly(vinyl chloride)-Rhodamine 6G] prepared via solvent crazing are studied. After annealing at temperatures above glass transition temperature T _g, the samples show a dramatic increase in the intensity of absorption and luminescence bands of Rhodamine Y and a marked decrease in light scattering. The observed changes are due to the diffusion of dye molecules in the polymer matrix and healing of the porous structure of crazes. The above processes are independent and proceed simultaneously. As a result, transparent composites with a low level of light scattering and with a uniform distribution of dye molecules in the polymer matrix are formed. When the PVC-Rhodamine 6G samples are annealed at temperatures above T _g, dramatic changes in their spectral luminescent characteristics and color allow the above composites to be considered specific photochromic materials.     

Dynamics of junction point motions in model network polymers

³¹P solid-state exchange 2D NMR and spin-lattice relaxation times (T₁^P) have been used to investigate the motion of a crosslink unit in model networks. The networks were formed from tris(4-isocyanatophenyl) thiophosphate with telechelic poly(propylene glycol) or poly(tetrahydrofuran). From the variation of the 2D NMR pattern with temperature and mix time, the motion of the crosslink is identified as Brownian reorientational diffusion. Good simulations of the spectra were obtained using the Williams-Watts distribution of correlation times. The temperature dependence of the crosslink motion follows the WLF equation. The parameters derived from the NMR data are sufficient to describe the temperature dependence and breadth of both the dielectric and mechanical loss associated with the glass transition. The T₁^P relaxation data fitted equally well to the Cole-Cole or the Williams-Watts relaxation functions. The motion of the crosslinks can be described quantitatively by the activation energies and the coupling parameters.     

The proper glass transition temperature of amorphous polymers on dynamic mechanical spectra

Glass transition is crucial to the thermal and dynamical properties of polymers. Thus, it is important to detect glass transition temperature (T _g) with a sensitive and proper method. Dynamic mechanical analysis (DMA) is one of the most frequently used methods to determine T _g due to its advantage of high sensibility. However, there is controversy in the past literatures to determine the proper glass transition temperature among three transition temperatures, i.e., T _g1, T _g2 and T _g3 in the dynamic mechanical spectra, which correspond to the temperature abscissa of intersect value of two tangent lines on storage modulus (E′), the peak of the loss modulus (E″) and the peak of the loss tangent (tan δ). In this work, these three transition temperatures were compared with the glass transition temperature determined by DSC (T _gDSC). Based on the discussion of different modes of molecular motion around the glass transition region, it is demonstrated that T _g1 and T _g2 have the same molecular mechanism as T _gDSC, i.e., local segmental motion which is enthalpic in nature and determines the proper glass transition temperature, while T _g3 is assigned to the transition temperature of entropic Rouse modes, thus cannot be used as the proper glass transition temperature.     

Is adhesion between amorphous polymers sensitive to the bulk glass transition?

Two bulk samples of one and the same or of different amorphous polymers were brought into contact and held for a chosen period of time at a constant healing temperature (T) over the interval of T from below the bulk glass transition temperature (T _g ^bulk) by ~50 °C to above T _g ^bulk by ~10 °C. As formed adhesive joints were shear-fractured in tension at room temperature, and lap-shear strength (σ) was measured as a function of T. It has been found that σ develops with T as logσ?~?1/T both at symmetric and asymmetric interfaces of polystyrene, poly (methyl methacrylate) and poly (2,6-dimethyl-1,4-phenylene oxide). This behaviour implies that there is no discontinuity in the evolution of σ when going through T _g ^bulk, and that this process is controlled by one and the same diffusion mechanism both below and above T _g ^bulk. The results obtained indicate that the contact layer of the polymers investigated is in the viscoelastic state at T well below T _g ^bulk and support the concept of a decrease in the T _g of a near-surface layer with respect to T _g ^bulk.     

Transition and relaxation processes of polyethylene,polypropylene, and polystyrene studied by positron annihilation

Transition and relaxation processes of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were studied by the positron annihilation technique. From measurements of lifetime spectra of positrons as a function of temperature, the lifetime of ortho-positronium, τ₃, and its intensity, I₃, were found to increase above 260 K for PP. This fact was attributed to a cooperative motion of large segments of molecules above the glass transition temperature, T_g. For PE, above T_g (140 K), the value of τ₃ increased, but the temperature coefficient of I₃ was negative below 230 K. From this fact, for PE, the molecular motions that cause the glass transition were associated with a rearrangement of molecules by local motions such as kink motions. The discrepancy between the results for PE and PP was attributed to the presence of methyl groups in PP and the resultant suppression of the local motions. For PS (T_g = 340 K), the molecular motions were found to start above 260 K, but those were suppressed by an interphenyl correlation. Detailed annihilation characteristics of positrons in polymers were also discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1601–1609, 1997     

Molecular dynamics study of orientational order and rotational melting in clusters of TeF6

Molecular dynamics simulations of the behavior of molecules in crystalline clusters of TeF₆ were carried out on systems of 100, 150, 250, and 350 molecules. Several diagnostic functions were applied to investigate whether rotational melting occurred before translational melting. These functions included the coefficient of rotational diffusionD _θ(T), the “orientational Lindemann index” δ_θ(T), the “orientational angular distribution function”Q(θ,T), and the “orientational pair-correlation function”g _θ(r, T). All indicators implied that rotational melting occurred before translational melting, that it began with the outermost molecules, and that its onset for smaller clusters was at lower temperatures than for larger clusters. Results also showed that the rotational transition coincided with the transition from a lower symmetry phase (monoclinic) to cubic, a phenomenon that had been noted by others to occur with some regularity for systems of globular molecules.     

Glass Transitions of Ordinary and Heavy Water within Silica‐Gel Nanopores

The dynamic properties of water confined within nanospaces are of interest given that such water plays important roles in geological and biological systems. The enthalpy‐relaxation properties of ordinary and heavy water confined within silica‐gel voids of 1.1, 6, 12, and 52 nm in average diameter were examined by adiabatic calorimetry. Most of the water was found to crystallize within the pores above about 2 nm in diameter but to remain in the liquid state down to 80 K within the pores less than about 1.6 nm in diameter. Only one glass transition was observed, at T_g=119, 124, and 132 K for ordinary water and T_g=125, 130, and 139 K for heavy water, in the 6‐, 12‐, and 52‐nm diameter pores, respectively. On the other hand, two glass transitions were observed at T_g=115 and 160 K for ordinary water and T_g=118 and 165 K for heavy water in the 1.1‐nm pores. Interfacial water molecules on the pore wall, which remain in the noncrystalline state in each case, were interpreted to be responsible for the glass transitions in the region 115–139 K, and internal water molecules, surrounded only by water molecules in the liquid state, are responsible for those at 160 or 165 K in the case of the 1.1‐nm pores. It is suggested that the glass transition of bulk supercooled water takes place potentially at 160 K or above due to the development of an energetically more stable hydrogen‐bonding network of water molecules at low temperatures.     

Unique evolution of spatial and dynamic heterogeneities on the glass transition behavior of PVPh/PEO blends

Glass transition behavior of hydrogen bonded polymer blends of poly(vinyl phenol)(PVPh) and poly(ethylene oxide)(PEO) is systematically investigated using normal differential scanning calorimetry(DSC) and recently developed multifrequency temperature-modulated DSC(TOPEM),in combination with Fourier transform infrared spectroscopy(FTIR) and nuclear magnetic resonance(NMR) techniques,focusing on the effect of the PEO molecular weight on the spatial and dynamic heterogeneity.It is found,for the first time,that both the glass transition temperature(T_g) and activity energy(E_a) of the blends strongly depend on PEO molecular weight,and a common turning point,which separates the rapid and slow increasing regions,can be found.The interchain hydrogen bonding interactions,both determined by FTIR measurements and obtained from the Kwei equation,decrease with increasing PEO molecular weight,indicating a decrease of the componential miscibility.A series of parameters related to the microscopic spatial and dynamic heterogeneity,such as the activity energy, fragility,nonexponential factor and the size of cooperatively rearranging regions,are calculated from frequency dependency complex heat capacity measured using TOPEM.It is found that each of these parameters monotonically changes with increasing the PEO molecular weight during the glass transition process,demonstrating that hydrogen bonding interaction is the key factor in controlling the spatial and dynamic heterogeneity,thus the glass transition.NMR relaxation results reveal the existence of obvious phase separation large than 5 nm,implying that the cooperatively rearranging regions should be closely related to the interphase region between the two components.The above obtained origin and evolution of spatial and dynamic heterogeneity provide a new insight into the glass transition behavior of polymer blends.     

Study of relaxation processes in polyethylene and polystyrene by positron annihilation

Relaxation processes in polyethylene (PE) and polystyrene (PS) were studied by positron annihilation technique. For PE, above the glass transition temperature, T_g, the size of free volumes and its concentration were increased by the micro-Brownian motion of molecules. For PS, local motions of molecules in backbone chains were found to start above 260 K. However, these local motions were suppressed by an interphenyl correlation. For both PE and PS, below 250–260 K, the formation probability of positronium atoms increased with decreasing temperature. This fact was assigned to the freezing in of the local motions of molecules. For PS, an onset of the local motions of molecules was observed above 100 K. These motions were expected to be associated with liberation of phenyl groups. © 1996 John Wiley & Sons, Inc.     

Glass transition and crystallization kinetics analysis of Sb–Se–Ge chalcogenide glasses

Differential thermal analysis (DTA) has been employed to investigate the effect of Ge addition on the glass transition behavior and crystallization kinetics of Sb₁₀Se_90?xGe_x (x = 0, 19, 21, 23, 25, 27) alloys. The three characteristic temperatures viz. glass transition (T _g), crystallization (T _c), and melting (T _m) have been determined and found to vary with the heating rates and Ge content. Thermal stability and glass forming tendency have been evaluated in terms of ΔT (= T _c ? T _g) and reduced glass transition temperature. The activation energies for glass transition and crystallization have been used to analyze the nucleation and growth process. The activation energy analysis also determines the suitability of alloys to be used in switching applications. Results have been interpreted in terms of bond energies and structural transformations in the investigated alloys.     

Carbon nanotube–polyurethane shape memory nanocomposites with low trigger temperature

Ester-based polyurethane (PU) with low glass transition temperature was used to develop shape memory nanocomposites with low trigger temperature. Pristine carbon nanotubes (CNTs) and oxidized CNTs (ox-CNTs) were introduced by melt mixing to improve the mechanical and shape memory properties of the PU matrix. The dispersion of CNTs on the mechanical properties and shape memory behaviors of the nanocomposites were also investigated. The results show that better dispersion of ox-CNTs contributes to more stiffness effect below glass transition temperature (T_g) while lower storage modulus (E′) above T_g. The nanocomposites exhibit high shape fixity and recovery ratio above 98%. The ox-CNT/PU nanocomposite shows higher shape recovery ratio for the first cycle, faster recovery due to better dispersion of CNTs and have potential applications for controlling tags or proof marks in the area of frozen food. The trigger temperature can be tailored by controlling the T_g of the PU matrix or the content of the nanofillers.     

Temperature dependence of the dynamic mechanical properties of filled polymers

A theory has been developed to explain the jump in the relative modulus of filled polymers near the glass transition temperature T_g and the subsequent decrease in relative modulus at temperatures above the glass transition temperature. The theory is based upon the concept that there are some particle–particle contacts in doublets and in agglomerates containing a larger number of particles. Below T_g motion of particles at the contact points is possible because of the high modulus of the polymer. At T_g particle–particle motion mostly ceases because of the low modulus of the polymer. At higher temperatures, the mismatch in the coefficients of expansion allows some motion to occur at points of contact and slippage may occur at the polymer–particle interfaces, so the modulus decreases. It is shown theoretically and experimentally that both the elastic modulus and the mechanical damping depend upon the nature of the surface of the particles.