Network fraction and molecular motions in polymer composites: An NMR relaxation and self-diffusion study

We have used NMR T₂ relaxation and pulsed-gradient spin-echo diffusion techniques to study properties of Arco R45M hydroxyl-terminated polybutadiene, either unfilled or containing 65 wt.% filler particles (SiO₂, NaCl, Al) and cured with isophorone diisocyanate (IPDI), as functions of IPDI content. A short T₂ relaxation component arises from the network (gel) whose amount is greatest (up to 92%) near NCO/OH stoichiometry. Gel fraction and stoichiometry concentration both are affected slightly by filler surface reactivity but principally by filler particle size. The diffusion rate of the nonnetwork (sol) molecules has a range of 1–2 orders of magnitude. This range is narrowest near stoichiometry for the smallest filler (SiO₂), i.e., the situation in which the sol molecules are least mobile. Branching theory and the hypothesis of a layer of reduced mobility in a wide vicinity of the filler particles provides semiquantitative explanations of these observations.     

Molecular mobility in rubbery composites: Effect of filler particle size

We have measured proton NMR T₂ relaxation spectra in Arco R45M OH-terminated poly-butadienes (PBs) cured with isophorone diisocyanate and filled with 65 wt % SiO₂ particles of each of six different average sizes. Identifying the short T₂ component with the gel, we find that the gel fraction is displaced from nominal NCO/OH stoichiometry, probably as a result of water adsorbed on filler particle surfaces. Near effective stoichiometry and in the presence of filler, molecular and segmental mobilities decrease, most strongly in specimens with the smallest filler particles. Comparison with parallel Monte Carlo simulations of the PB matrix geometry indicates that segmental mobility and sol migration decrease uniformly in a wide vicinity of the filler particles. Thus the rigidification of the matrix measured via NMR has a range of approximately 1-3 μm from nearby filler particle surfaces, representing the rms diffusion distance of the light components of the sol during the T₂ relaxation. © 1993 John Wiley & Sons, Inc.     

Molecular mobility of ultrasonically devulcanized silica‐filled poly(dimethyl siloxane)

We used NMR relaxation and pulsed‐gradient diffusion measurements at 70 °C in precipitated silica‐filled poly(dimethylsiloxane) (PDMS; silicone rubber) after crosslinking, after subsequent devulcanization by intense ultrasound, and after subsequent revulcanization. As in unfilled PDMS, transverse relaxation displays three distinct rate components attributed to an entangled and crosslinked network (similar in T₂), light sol plus dangling network fragments, and unreactive trace oligomers. Ultrasound produces copious amounts of extractable sol. The T₂ relaxation times decreased modestly with increasing filler content, but they and the components' proportions correlated mainly with the sol fraction, that is, the network degradation. In rupturing the network, devulcanization produces large diffusing fragments and dangling ends; revulcanization largely reverses these effects. The rates and amplitudes of the bimodal diffusivity distribution confirmed this conclusion. The weakness of the effects of filler suggests that ultrasound devulcanization is easily adaptable to the recycling of the preponderantly particulate‐filled industrial rubbers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 454–465, 2003     

Crosslinking Index,Molecular Weight Distribution and Rubber Equilibrium Shear Modulus During Polyfunctional Crosslinking of Existing Polymer, 6. Primary Polymer with a Schulz–Zimm Distribution of Molecular Weights

A network model for the crosslinking of already existing polymer molecules with a so‐called Schulz–Zimm distribution of their molecular weights is presented. It is an extension of previously developed statistical network models applied to the crosslinking of primary polymers with several other molecular weight distributions and with crosslinks of any functionality. The model results in the possibility to obtain more insight into the structure of polymers, especially those with narrow distributions of the molecular weight. In more detail, the model can give a perspective on structural network parameters such as the weight fractions of ideal network, of dangling polymer ends, and of those molecules not connected to the network, i. e., the sol fraction, the number of crosslinks in which a polymer molecule is bound, the functionality of the crosslinks, or the average molar mass of the polymer molecules in between the crosslinks M̄_c. Results of calculations are shown for a hypothetical crosslinking process of polymers with various molecular weight distributions. Moreover, the dependency of the network parameters on the polydispersity index and the type of molecular weight distribution is shown. Finally the increase of the functionality of the crosslinks during the ageing process of a 9.9% poly(vinyl chloride) gel as a function of the polydispersity index of the molecular weight distribution is presented.     

Gelation Studies, 3

Summary: The sol‐gel transition of one thermoreversible gelling mixture made of xanthan gum and locust bean gum has been studied by using in situ time‐resolved dynamic light scattering (DLS) and measuring the spin‐lattice relaxation time T₁ of several protons. A critical dynamical behavior was observed near the sol‐gel transition, which is characterized by the presence of power‐law spectra over four decades of the delay time in the time‐intensity correlation function g₂(t)−1 ∼ t^−μ at 48 °C. The increase in T₁ with increasing temperature becomes steeper at 50 °C indicating a significant change in the local mobility of one anomeric proton of the xanthan side chain and the anomeric protons of the locust bean gum mannose backbone.

Temperature dependence of the spin‐lattice relaxation time T₁ for the equatorial anomeric proton of the mannopyranosic unit located next to the main chain of the xanthan.     

Alternating copolymerization of carbon dioxide and cyclohexene oxide catalyzed by silicon dioxide/Zn?CoIII double metal cyanide complex hybrid catalysts with a nanolamellar structure

Air‐stable hybrid catalysts of silicon dioxide/double metal cyanide complexes (Si‐DMCCs) based on Zn₃[Co(CN)₆]₂ (ZHCC) were prepared by an in situ sol–gel method. The Si‐DMCCs showed low crystallinity and a nanolamellar structure with a thickness of ～40–60 nm. In particular, a lamellar structure of regular hexagonal shape was observed for Si‐DMCCs with low SiO₂ content. These catalysts had very high catalytic activity for alternating copolymerization of cyclohexene oxide (CHO) and carbon dioxide. A turnover number of 11,444, turnover frequency of 3815 h^?1, and apparent efficiency of 7.5 kg polymer/g ZHCC (～24.0 kg polymer/g Zn) were achieved at 3.8 MPa and 100 °C. The poly(cyclohexenylene carbonate) (PCHC) polymers obtained were completely atactic with a molecular weight (M_n) of ～10 kg/mol and polydispersity of 2.0–3.0. The PCHCs had a structure of nearly alternating CHO and CO₂ units, with a molar fraction of carbonate units of 0.44–0.47. Preliminary investigations of the mechanism suggest that nucleophilic attack by neighboring oxygen atoms is involved in copolymerization initiation with Zn? Co^III DMCCs. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3128–3139, 2008     

Dynamic mechanical and dielectric properties of ORMOCER® incorporating functionalized poly(styrene) latexes

Organically modified aluminosilicate hybrid materials incorporating polystyrene and poly(styrene‐co‐hydroxypropyl acrylate) latexes, (3‐glycidyloxypropyl) trimethoxysilane, and aluminum sec‐butoxide [Al(O^sBu)₃] were synthesized by a sol–gel process. The bulk materials obtained were macroscopically homogeneous dispersions with good mechanical properties. Dynamic mechanical and dielectric analyses of these new hybrid materials as a function of the Al(O^sBu)₃ concentration and copolymer composition revealed a series of transitions that represented relaxation processes of the incorporated polymer (glass transition), ?Al? O? Si?, the ?Si? O? Si? part of the network, and segmental motion of unreacted ?Si? (CH₂)₃OCH₂CHCH₂O chains. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 860–867, 2001     

Gel formation in atom transfer radical polymerization of 2‐(N,N‐dimethylamino)ethyl methacrylate and ethylene glycol dimethacrylate

Copolymers of 2‐(N,N‐dimethylamino)ethyl methacrylate (DMAEMA) and ethylene glycol dimethacrylate (EGDMA) were synthesized via atom transfer radical polymerization using ethyl 2‐bromoisobutyrate as the initiator, Cu(I)Br as the catalyst, and 1,1,4,7,10,10‐hexamethyltriethylene tetramine as the ligand. At low crosslinker levels, the polymerizations followed the first‐order kinetics. However, when the crosslinker level was above 10 mol %, the ln([M]₀/[M]) versus time curves showed deceleration at medium conversions because of the higher reactivity of EGDMA than that of DMAEMA. An acceleration at high conversions was also observed and probably caused by the diffusion limitations of catalyst/ligand complex in the polymer network. The hydrogels were characterized by swelling experiments, and the sol polymers were characterized by the size exclusion chromatographic technique to determine the number‐average molecular weight and polydispersity. The gel data were analyzed and, via a comparison to Flory's gelation theory, found to be more homogeneous than similar hydrogels prepared by conventional free‐radical polymerization methods. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3780–3788, 2001     

Quantitative nuclear magnetic resonance imaging of liquids in swelling polymers

The variation of nuclear magnetic resonance (NMR) relaxation parameters (T₁, T₂) within a polymer during swelling, limits the absolute accuracy with which liquid concentration profiles can be obtained using NMR imaging. In this article a study of the diffusion of decalin into ultra-high molecular weight polyethylene (UHMWPE) is reported. The study illustrates the use of a method of analysis whereby quantitative solvent profiles can be obtained from data influenced by both T₁ and T₂ contrast effects. A T₁ and T₂ map are obtained at a point in the uptake of liquid where the greatest range in liquid concentration is obtained at a point in the uptake of liquid where the greatest range in liquid concentration is observed. The intensity of signal corresponding to liquid in the polymer is compared to that of pure liquid in a reference sample, and correlations for T₁ and T₂ values versus signal intensity are used to deconvolve relaxation contrast, to yield the true liquid concentration. The technique was used to study the effect of degree of crosslinking of UHMWPE on the swelling kinetics and decalin transport within the polymer. A spin-echo imaging technique was used with a recycle delay approximately equal to the average spin-lattice relaxation time of the liquid, and an echo time approximately half the average spin-spin relaxation time. Under these conditions the relaxation contrast was significant, yet the mass uptake data derived from the concentration profiles obtained, using the method of analysis described, agreed well with gravimetric data. © 1995 John Wiley & Sons, Inc.     

Comparison of number–average molecular weights calculated by GPC and membrane osmometry

Data are presented which show that when a polymer contains an appreciable amount of low molecular weight species below the diffusion limit of the osmometer membrane, the osmotic molecular weight, M?_n, is generally higher than the M?_n calculated from gel-permeation chromatography (GPC). Experiments were performed on samples of poly(vinyl chloride) (PVC) and high-cis polybutadiene polymers. Osmotic data were obtained in the usual manner, while GPC data were obtained using the universal calibration approach. It was found that when all polymer species below approximately 10,000 molecular weight were excluded from the calculation of M?_n by GPC, agreement in M?_n was obtained between membrane osmometry and GPC. The data obtained suggest that the choice of M?_n as measured by membrane osmometry in the calibration of the GPC should not be done casually, as the measured M?_n may not reflect the “true” value of that sample, especially when the polymer sample contains an appreciable amount of low molecular weight material.     

Real-Time NMR. How Fast Can We Do It?

Network structure development during cross-linking photopolymerization of polyethylene glycol di-acrylate and its mixture with a mono-functional 2-ethylhexyl acrylate was studied using real-time proton NMR T₂ relaxation analysis. The time resolution of the method is typically in the order of seconds. The results reveal largely heterogeneous origin of network build up at the intermediate stages of photocuring. Domains of nano-gel are already formed on initial stages of UV-curing where hardly any change in viscosity is observed. Upon increasing curing time the fraction of gel increases at the expence of sol, the molar mass of network chains decreases and the molar mass of sol increases. The presence of mono-acrylate slows down the curing rate. The curing continues after UV-illumination causing a significant increase in the amount of gel and cross-link density in the gel. Thus, the NMR method is a valuable tool for characterization of the kinetics of photopolymerization, the development of molecular structure and the resultant molecular scale heterogeneity during photocuring.     

A relaxation time study of the electron-beam induced polymerization of 1,6-hexanediol diacrylate

NMR measurements have been used to characterize the electron-beam (EB) induced polymerization of 1,6-hexanediol diacrylate. Hydrogen T₁, T_1θ, and T₂ values have been measured as a function of radiation dose. The T₂ signal consists of a Gaussian component and a longer component consisting of two exponentials. The sum of the Gaussian and the short exponential intensities is strongly correlated with the polymer fraction determined by gel extraction. T₂ values associated with the exponential component decrease rapidly with increasing dose. The T₁ relaxation consists of two exponentials for low dose and a single exponential for high dose. The T_1θ relaxation consists of from two to three exponential components, and the departure from a single exponential decreases with increasing dose. T₁ and T_1θ intensities are poorly correlated with gel extraction results. T₁ and T_1θ minima occur at intermediate radiation dose, and T_1θ depends on H₁ at high dose. Limited spin diffusion plays an important role in the T₁ and T_1θ relaxation. Hydroquinone has little effect on the EB-induced polymerization.     

Prediction of microstructures for polydisperse block copolymers,using continuous thermodynamics

A generalization of an earlier theory (Leary–Henderson–Williams) developed for microphase separation in monodisperse block copolymers is made for copolymers having moderate degrees of polydispersity and illustrated for the Schultz molecular weight distribution (MWD). First, an explicit study is made of molecular weight (M) effects for monodisperse poly (styrene–butadiene) diblock (SB) and triblock (SBS) copolymers. For a fixed temperature, it is shown how the critical molecular weight (M_c)—above which the copolymer is phase-separated at equilibrium —varies with molecular composition (?_S, volume fraction of S component) for both molecular architectures. Also predicted are the microstructural parameters ΔT(M) and f(M)—interphase thickness and volume fraction, respectively—and the high-M limiting functions ΔT ∝? M^α2, f ∝? M^α3, D ∝? M^α4 (D is domain repeat distance) and T_s ∝? M^α5 (T_s is separation temperature). Then, for polydisperse systems in the range 1 ? p ? 3 ( where \[ P = \bar M_w /\bar M_n \] ) corresponding predictions at constant \[ \bar M_n \] are made after identifying the mixture free-energy-minimum state with a weight average of the free energy minima of each fraction of the MWD. Calculations are made specifically for ?_S = 0.50 and T_s = 298 K. It is shown that, even when \[ \bar M_n < M_c \] , polydispersity can induce microphase separation if p is sufficiently large. Good success is obtained in comparisons of D predictions with data on blends of two polydisperse diblock samples.     

Proton spin relaxation in liquid polydimethylsiloxane: Molecular motion and network formation

Proton spin relaxation time measurements are reported for a series of liquid polydimethylsiloxanes covering the temperature range from ?50 to 200°C. These include fractionated and whole polymer samples (M _w from 10₄ to 10₆), binary mixtures, and crosslinked material. This paper is mainly devoted to the behavior of the spin-spin relaxation (T₂) which is sensitive to the low-frequency long-range configurational motions occurring in the liquid phase. These motions determine the time scale over which final motional averaging of the nuclear dipolar interactions occur, prior to the onset of normal liquid-like behavior of T₂. When these motions are too slow (e.g., at low temperatures of high molecular weight) to lead to appreciable averaging of the small residual static dipolar interactions present in the liquid phase, the spin-spin relaxation exhibits pseudo-solid-like behavior. A further aspect of this work concerns the observed sensitivity of T₂ to network formation which considerably restricts the long-range motions that determine T₂. It is possible to determine the fraction of material present in the form of a network from the relative proportions of network and non-network T₂ components. This is not only true for a permanent crosslinked network but also for a network arising from transient entanglement couplings. The dynamic nature of the latter type of a network is clearly revealed in the proton nuclear magnetic resonance measurements reported here.     

Synthesis of thermally-induced phase separating polymer with well-defined polymer structure by living cationic polymerization. I. Synthesis of poly(vinyl ether)s with oxyethylene units in the pendant and its phase separation behavior in aqueous solution

Living cationic polymerization of alkoxyethyl vinyl ether [CH₂?CHOCH₂CH₂OR; R: CH₃ (MOVE), C₂H₅ (EOVE)] and related vinyl ethers with oxyethylene units in the pendant was achieved by 1-(isobutoxy)ethyl acetate ( 1 )/Et_1.5AlCl_1.5 initiating system in the presence of an added base (ethyl acetate or THF) in toluene at 0°C. The polymers had a very narrow molecular weight distribution (M?_w/M?_n = 1.1–1.2) and the M?_n proportionally increased with the progress of the polymerization reaction. On the other hand, the polymerization by 1 /EtAlCl₂ initiating system in the presence of ethyl acetate, which produces living polymer of isobutyl vinyl ether, yielded the nonliving polymer. When an aqueous solution of the polymers thus obtained was heated, the phase separation phenomenon was clearly observed in each polymer at a definite critical temperature (T_ps). For example, T_ps was 70°C for poly(MOVE), and 20°C for poly(EOVE) (1 wt % aqueous solution, M?_n ～ 2 × 10⁴). The phase separation for each case was quite sensitive (ΔT_ps = 0.3–0.5°C) and reversible on heating and cooling. The T_ps or ΔT_ps was clearly dependent not only on the structure of polymer side chains (oxyethylene chain length and ω-alkyl group), but also on the molecular weight (M?_n = 5 × 10³-7 × 10⁴) and its distribution. © 1992 John Wiley & Sons, Inc.     

Water penetration in nylon 6,6: Absorption,desorption, and exchange studied by NMR microscopy

The uptake of water by nylon 6,6 [42DB Adipure (trade name of Dupont Canada Inc.)] at 100°C has been monitored by a combination of one-dimensional proton NMR spectroscopy, relaxation time (T₁ and T₂) measurements and proton microscopic NMR imaging techniques. The relaxation times of the water absorbed into the nylon matrix are very short at room temperature, (T₂ < 1 ms and T₁ ≈ 1 s) indicating that the water is located in a highly restricted environment and suggesting that strong interactions exist between the absorbed water and the polymer. The diffusion profiles measured at room temperature indicate that the diffusion of water into nylon 6,6 at 100°C is Case I Fickian diffusion. The spatial dependence of the T₂ relaxation time constant and its variation with the water content was also examined. The results reveal that both T₂ and T₂^* decrease toward the center of the sample in samples that have a concentration gradient of sorbed water. In fully saturated samples, no spatial dependence was observed. The overall values of T₂ and T₂^* are also observed to increase as a function of exposure time. An evaluation of the desorption process at room temperature and at 100°C was performed. A continuous, exponentially decreasing solvent profile was observed for the desorption process which again indicates Case I Fickian kinetics. The exchange process of external bulk and atmospheric water with deuterium oxide (D₂O) saturated nylon rods has also been studied using the microscopic imaging technique. © 1993 John Wiley & Sons, Inc.     

Dielectric behaviour of Gelatin solutions and Gels

 Sol and Gel state properties of aqueous gelatin solutions of concentrations 4%, 6%, 8% and 10% (w/v) have been investigated through dielectric relaxation studies done at various temperatures in the range T=20–60 °C carried out over a frequency range f=20 Hz–10 MHz and no relaxation of any nature was observed. The sharp transition observed at the gelation temperature T _gel provided an excellent matching with the same measured through differential scanning calorimetry (DSC). The capacitance (C _p) values above f=100 kHz became increasingly negative as the gel was melted to the sol state. However, in the gel state C _p was found to be almost independent of temperature for frequencies above 100 kHz. At frequencies lower than 10 kHz, C _p measured was ∼10⁵ F, implying pronounced interfacial polarization either due to electro-chemical reaction or because of ions getting trapped at some interface within the bulk. Received: 10 February 1997 Accepted: 2 September 1997     

基于NMR技术的溶菌酶在琼脂糖凝胶中的动力学研究

运用脉冲梯度场测量自扩散系数及自旋自旋弛豫时间测量核磁共振技术对鸡蛋清溶菌酶(HEWL)在琼脂糖凝胶中的动力学进行了研究.试验结果表明,HEWL在琼脂糖凝胶中的自扩散系数及自旋自旋弛豫时间较其在纯乙酸钠溶液中变小,说明琼脂糖凝胶的三维网状结构使HEWL分子整体运动及局部运动都受到阻碍.并且随着琼脂糖浓度的增大,凝胶网孔尺寸不断减小,HEWL分子运动受限程度加剧,从而蛋白质分子可以较长时间内停留在高浓度区,分子间更容易互相碰撞,发生反应,晶核生长得以促进.同时琼脂糖凝胶较小流体力学网孔尺寸抑制聚晶或沉淀的出现,晶体质量获得提高.     

Quantitative NMR measurements in copolymers of vinylidene chloride and acrylonitrile: The effect of saturation and varying nuclear Overhauser enhancements

Spin-lattice relaxation times and nuclear Overhauser enhancements (NOE) for ¹³C nuclei in copolymers of acrylonitrile and vinylidene chloride were measured at 20.1 and 67.9 MHz. In the ? CCl₂? region of the spectrum T₁ and NOE values of the various resonances are equal within experimental error and are invariant to changes in composition. The T₁ and NOE values of the ? CCl₂? region, however, are not equal to those of the ? CH? or ? CN region. As a result compositions cannot be calculated by direct comparison of the areas in the ? CCl₂? region and either the ? CH? or the ? CN region. Discrepancies can be corrected for the ? CH? resonances by multiplication of the area by an empirical constant. A similar constant for the ? CN region is composition-dependent at 20.1 and 67.9 MHz. A chemical shift anisotropy mechanism is postulated as important for relaxation of the ? CN resonances. The overall influence of variable T₁ and NOE values on quantitative determination of polymer composition is considered.     

Development of superconducting phases in BSCCO and Ba-BSCCO by sol spray process

The effects of barium substitution for Bi or Sr sites on the growth of superconducting phases have been studied. The sol spray process has been used to synthesis the Bi-Sr-Ca-Cu-O (BSCCO) and Ba-BSCCO homogeneous ceramic powders. Thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques were employed to characterize the synthesized materials. The electrical resistance was measured by classical d.c. fourprobe technique. It has been observed that sol spray process has affected the physico-chemical properties of the materials and also avoid the use of chelating agent as in the case of sol gel process. In addition to the Bi₂Sr₂CaCu₂O₈ (2212) phase Ba doped specimens also contained Bi₂Sr₂Ca₂Cu₃O₁₀ (2223), BaBiO₃, BaCuO₂ and CuO phases. The results revealed that the specific effect of barium-doping on either sites (Bi or Sr) seems to avoid the formation of higher volume fraction of the low T _c phase and promoting the formation of BiBaO₃, BaCuO₂ and CuO along with formation of a high T _c 2223 phase. The substitution of Ba on either sites (Bi or Sr) lower the sintering temperature for the formation of high T _c (small volume fraction) however, the Ba doped specimens also contained non-superconducting phases.