Pore Morphology of Porous Polymer Particles Probed by NMR Relaxometry and NMR Cryoporometry (original) (raw)
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Probing Pore Size Distribution by Cryogenic- and Relaxation 2 H-NMR
The Journal of Physical Chemistry B, 2002
Cryogenic NMR and NMR spin-lattice relaxation time (NMRT) measurements of pore-confined water (D 2 O) have been performed using deuterium NMR to probe the pore size distribution (PSD) of silica materials and porous membranes. NMRT measurements were performed at a temperature slightly below the normal freezing point (277 K) of bulk water (D 2 O) to ensure that all interparticle water was frozen out. PSD derived from cryogenic NMR was in excellent agreement with PSD obtained from N 2 -adsorption measurements. Also, PSD obtained by NMRT revealed approximately the same average pore dimension as obtained by N 2 adsorption. However, the former experimental technique resulted in somewhat narrower PSD than obtained by cryogenic NMR and N 2 adsorption and is discussed in the text. An attempt to determine the PSD of a TiO 2 membrane on a silica support by NMRT will also be discussed. The main results obtained in this work suggest that a combined use of cryogenic NMR and NMRT may give information on both PSD and pore-connectivity.
NMR Cryoporometry of Polymers: Cross-linking, Porosity and the Importance of Probe Liquid
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019
The morphology of cross-linked polymers plays an important role in their physical and chemical properties. NMR cryoporometry allows for the investigation of these structures over different length scales, through appropriate choice of probe liquid. The different structures of two different polymeric samples, one a cross-linked polymer hydrogel, the other a pore-expanded ion-exchange polymer, are analysed here. The ability for NMR cryoporometry to analyse both polymeric materials in the swollen state is successfully demonstrated, as is the importance of probe-liquid choice for the analysis of different regions of the pore structure. In both cases, water is used to identify populations of pores smaller than ca. 5 nm. The use of t-butanol and menthol reveals the presence of additional mesoporous structures in the ionexchange resin as well as the responsiveness of the pore structure to the liquid used to swell it.
MRS Proceedings, 1994
The molecular dynamics of fluids in porous media has been studied using field-cycling NMR relaxometry and NMR field-gradient diffusometry. The frequency dependences of the 1H and 211 spin-lattice relaxation times T, of various liquids in porous glass reveal weak and strong adsorption behaviour depending on the polarity of the adsorbates. Correlation times eight orders of magnitude longer than in bulk have been observed. The T, dispersion moreover reflects geometrical details of the matrix in a length scale three orders of magnitude longer than the adsorbate molecules. The mean-square displacements of adsorbate molecules on the surface are only one order of magnitude less than in bulk. The global diffusivity is reduced by tortuosity and porosity effects. The observed phenomena may be explained by bulk-mediated surface diffusion, i.e., Ldvy walks. The dynamics of polymer chains much longer than the pore size is characteristicly different from that in bulk melts. There is evidence that the reptation mechanism explains at least a part of the phenomena observed for the porous matrix in contrast to findings with bulk polymer melts.
Pore surface exploration by NMR
Magnetic resonance imaging, 2003
A carefully chosen set of experimental techniques applied to porous media characterization provides results that can be much greater than the sum of the individual parts. The inter-relation and complementarity of a number of techniques will be considered. NMR cryoporometry provides a valuable method of pore size measurement. An NMR method that is more widely used to assess pore dimensions relies on relaxation time analysis of a liquid that fills the pores and the enhanced relaxation that occurs in a liquid at the solid/liquid interface. Thermoporometry, a method based on the application of Differential Scanning Calorimetry (DSC), is closely related to cryoporometry, but employs a different set of assumptions to evaluate pore size distributions. Comparison of the results obtained on the same samples using all these methods together with gas adsorption serves to validate the methods and provide significantly more information about surface-fluid interaction and the behavior of nano-scale material within pores than each method employed in isolation. Technique developments will be discussed and applications of these methods to ideal silicas will be used to illustrate their power, particularly in combination. © 2003 Elsevier Science Inc. All rights reserved. Keywords: Porous media; Nuclear magnetic resonance; Cryoporometry; Relaxometry
The Journal of Chemical Physics, 2007
Reptational dynamics of bulk polymer chains on a time scale between the Rouse mode relaxation time and the so-called disengagement time is not compatible with the basic thermodynamic law of fluctuations of the number of segments in a given volume. On the other hand, experimental field-cycling NMR relaxometry data of perfluoropolyether melts confined in Vycor, a porous silica glass of nominal pore dimension of 4 nm, closely display the predicted signatures for the molecular weight and frequency dependences of the spin-lattice relaxation time in this particular limit, namely T 1 ϰ M −1/2 1/2. It is shown that this contradiction is an apparent one. In this paper a formalism is developed suggesting cooperative chain dynamics under nanoscopic pore confinements. The result is a cooperative reptational displacement phenomenon reducing the root-mean-squared displacement rate correspondingly but showing the same characteristic dependences as the ordinary reptation model. The tube diameter effective for cooperative reptation is estimated on this basis for the sample system under consideration and is found to be of the same order of magnitude as the nominal pore diameter of Vycor.
Characterization of porous solids by NMR
Physical Review Letters, 1993
We are using a novel NMR method, that has been developed in our laboratory and employs the depression of the freezing point (AT,) of confined liquid within porous media, to investigate the effect of pre-drying silica with average pore diameter ranging from 60 A to 1000 A. Cyclohexane was the confined liquid. Pre-drying was found to affect only the smallest pores. A study to compare partially filled and over-filled samples showed that the average AT, for partially filled samples is greater than for over-filled. We have also investigated the use of water as the absorbed liquid and compared results with those obtained from cyclobexane studies. Reasonable agreement was found but cvclohexane was more sensitive. The method is fast and is suitable for monitoring pore size distributions in the range of 50-1000 A.
Biophysical Journal, 2002
Water is an integral part of the structure in biological porous materials such as wood and starch. A problem often encountered in the preparation of samples for, e.g., electron microscopy is that removal of water leads to a decreasing distance between supermolecular structural elements and a distortion of the structure. It is, therefore, of interest to find methods to investigate these materials in the native water-swollen state. We present a method to study water-swollen biological porous structures using NMR to determine the amount and self-diffusion of water within the porous objects. The contribution of bulk water to the NMR signal is eliminated by performing experiments below the bulk freezing temperature. Further decrease of the temperature leads to a gradual freezing of water within the porous objects. The contribution of the freezing water fraction to the migration of water through the porous network is, thus, estimated. The results are rationalized in terms of the ultrastructure of the samples studied, namely, wood pulp fibers and potato starch granules.
International Journal of Pharmaceutics, 2006
NMR cryoporometry is a unique method permitting the investigation of pores in the microporous and mesoporous regimes for samples in aqueous environments. Here, we apply the technique to porous biodegradable polymer microparticles designed as devices for drug delivery in depot formulations. The results indicate that structural features too small to be captured in surface and fracture images obtained by SEM are able to be accessed using the technique, and that the evolution of pore structure can be studied for several days as the particles swell and degrade in the aqueous environment.
The Journal of Physical …, 1996
The pore size distribution of four commercial silica materials saturated with water have been derived from experimental 1 H NMR intensity vs temperature curves (IT-curves) of the confined water. The observed melting points or transition temperatures of benzene and cyclohexane confined in the same materials are shown to be consistent with corresponding data obtained from the water IT-curves. In general, the freezing point depression (∆T) of these fluids can be related to the pore radius R in accordance with a modified Gibbs-Thompson equation: ∆T ) K/(R + d), where K and d are constants characteristic of the confined fluid. The melting point depressions of benzene and cyclohexane are shown to be more sensitive to pore radius than that of water; i.e., the two former have a larger K-value. Moreover, the average pore radius of these materials can be estimated from 1 H NMR chemical shift measurements of the benzene-saturated samples. Simulation of 1 H NMR spectra of benzene confined in mixtures of silica (mesopores) and zeolite (micropores) will be presented.
The implementation of an easy-to-apply NMR cryoporometric instrument for porous materials
Magnetic Resonance Imaging, 2023
Time-domain NMR has been extensively utilised to study various characteristics of fluid-saturated porous,materials for instance their mobility, dynamics, stiffness, viscosity and rigidity features, particularly for solid hydrocarbons, rubbers and other polymers. As a unique time-domain technique available for over 30 years, NMR cryoporometry (NMRC) may be used to obtain pore-size distributions of the measured samples. To accurately control the sample temperature, a Peltier thermo-electrically cooled variable temperature probe has been developed and integrated with a highly compact precision NMR time-domain relaxation spectrometer, therefore providing the community with a high-performance instrument for NMR Cryoporometry. To extend the application of aforementioned high-performance NMRC instrument into more senarios, we designed a series of lightweight, compact and integral models with optional NMR frequencies from 12 MHz up to 23 MHz. The measured sample temperature can be precisely controlled from about − 60 ◦C to +80 ◦C, with an excellent temperature resolution of 10 mK or better near the probe liquid bulk melting point. Therefore, it offers a fairly wide NMRC pore-size distribution ranging from about 1 nm to 2 μm by using water as the probe liquid in the pores, signifcantly wider than is possible when applying generic NMR Spectrometers for NMRC. A preliminary example of NMR Cryoporometric measurements on two special cement samples is shown in the paper in which the measured pore scales as well as their repeatability are demonstrated. Furthermore, various nano-materials, such as MOF, zeolite and shale kerogen would be potential materials to study by using these new available NMRC instrument models. We aim to offer this technique as a quantitative and easy-to-apply unitary benck-top tool for an even wider range of porous material.