Sol Molecules Research Articles

Effect of Aluminum Doping on the Performance and Microstructure of Methyl-Modified SiO2 Sol

With methyltriethoxysilane as hydrophobic precursors and aluminum isopropoxide as the aluminum source, the Al2O3/SiO2 sols and their gel materials were prepared. The effects of aluminum content (nAl) on the viscosity (η), density (ρ), reaction rate constant (k), Gibbs energies of activation for viscous flow (ΔG*), particle size of Al2O3/SiO2 sols were studied. And the high temperature calcined materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). The results show that, with the increase of nAl, the η, n and ΔG* values of the Al2O3/SiO2 sols decrease while the k and ρ values and average particle sizes increase. There is strong intermolecular interaction between the Al2O3 and SiO2 sol molecules. The Al-O-Si bond exists in the Al2O3/SiO2 materials before and after calcination at 350 °C. Calcination at 350 °C in N2 atmosphere can change the phase structure of Al2O3/SiO2 sample greatly, which can make the γ-AlOOH in the Al2O3/SiO2 gel material convert to γ-Al2O3 through dehydration.

Free‐Radical Polymerization with Long‐Chain Branching and Scission: Markovian Solution of the Weight‐Average Molecular Weight

Analytic solution for the weight‐average chain length in a matrix formula, , is derived for free‐radical polymerization with simultaneous long‐chain branching and scission. Illustrative calculations are conducted for a batch polymerization. With bimolecular termination by combination, gelation could be observed. Assuming the same polymerization kinetics for branching and scission in the post‐gel period, the formed gel molecule could be degenerated into sol molecules again, i.e., degelation might occur. Both the gelation and degelation points are defined as the point when the largest eigenvalue of M is unity. The matrix formula is suitable to determine accurate values, while the Monte‐Carlo simulation can give much more detailed information. These two approaches are nicely complementary.

Hierarchical wrinkling on elastomeric Janus spheres

Hierarchical wrinkling on elastomeric Janus spheres is permanently imprinted by swelling, for different lengths of time, followed by drying the particles in an appropriate solvent. First-order buckling with a spatial periodicity (λ11) of the order of a few microns and hierarchical structures comprising of 2nd order buckling with a spatial periodicity (λ12) of the order of hundreds of nanometers have been obtained. The 2nd order buckling features result from a Grinfeld surface instability due to the diffusion of the solvent and the presence of sol molecules.

Photostability and retention of UV absorber molecules in sol–gel hybrid UV-protective coatings

Highly efficient UV-protecting coatings were prepared, consisting on UV absorber molecules embedded in transparent sol–gel phenyl-functionalized silica thin-films. The photostability and retention of the molecules in the films depend strongly on parameters such as the composition of the hybrid host matrix, the UV-absorber loading in the matrix and the sol–gel preparation conditions. The amount of the modifying phenyl group was found to affect strongly the retention of the UV absorber molecules in the matrix. The retention of the molecules incorporated in the thin-films showed an increase of 21 times as the amount of the phenyl groups is reduced by a factor of 4. On the other hand, the incorporation of increasing amounts of Ph groups lead to a slight decrease of the photostability of the UV absorber molecules. The ability to control the parameters affecting the durability of the UV absorber molecules in the sol–gel matrices allows us increase the effectiveness of the UV-protective films and hence their potential usage in both, indoors and outdoors applications.

Reversible association and network formation in 3 : 1 ligand–metal polymer solutions

Formation of reversible metallo-supramolecular networks based on 3 : 1 ligand–metal complexes between end-functionalized oligomers and metal ions was studied using Monte Carlo simulations. The fraction of 1 : 1, 2 : 1 and 3 : 1 ligand–metal complexes was obtained and analyzed using an analytical approach as a function of oligomer concentration, c and metal-to-oligomer ratio, r. At low concentration the maximum in the number-average molecular weight is achieved near the stoichiometric composition (r = 2/3), while the weight-average molecular weight maximum is shifted to higher r (≃ 0.75). With an increase in oligomer concentration the molecular weight maxima shift to higher metal-to-oligomer ratios (similar to the experimentally observed viscosity) due to diminished ring formation. Network formation as determined from the analysis of the molecular weight distribution and reduced average cluster size occurs in a limited range of metal-to-oligomer ratios at sufficiently large oligomer concentrations. At oligomer concentrations slightly exceeding the onset of network formation its growth occurs by means of incorporation of sol molecules into the dangling parts of the network. In the range of r close to the stoichometric composition the overall number of oligomers in the sol and dangling parts is found to remain nearly constant as a function of oligomer concentration and equal to that at the onset of network formation. The average molecular weight between effective crosslinks decreases with oligomer concentration and reaches its minimum at the stoichiometric composition, where the high-frequency elastic plateau modulus approaches its maximal value. At high oligomer concentrations the plateau modulus follows a c1.8 concentration dependence, similar to recent experimental results for metallo-supramolecular networks.

Swelling Behavior of Partially Cross-Linked Polymers: A Ternary System

Cross-linked polymer networks, irrespective of the method of cross-linking, contain a fraction of unattached polymer (sol macromolecules). During swelling in a solvent of low molecular weight, the sol macromolecules participate in the swelling process as an athermal solvent. Also, sol macromolecules begin to diffuse out of the swollen polymer into the surrounding solvent bath until equilibrium is reached between the sol macromolecules within the polymer specimen and the molecules in the surrounding bath. Thus, a partially cross-linked polymer during swelling should be treated as a ternary system consisting of polymer network, a high molecular weight solvent, and a low molecular weight solvent. On the basis of the Flory-Rehner theory, a first model has been developed to predict the swelling behavior of such ternary system. Transient states are considered as quasi-steady state, and the final condition of equilibrium is evaluated. The effect of polymer molecules in the surrounding solvent bath on the swelling behavior was also studied. Swelling experiments of partially cross-linked high-density polyethylene in p-xylene were performed to evaluate the model predictions. The presence of sol macromolecules within the swollen gel increased the swelling while sol molecules in the surrounding bath decreased the swelling of the partially cross-linked polymer.

Effect of Coulombic Interactions on Rotational Mobility of Guests in Sol−Gel Silicate Thin Films

We investigate the effect of Coulombic interactions on the mobility of rhodamine 6G (R6G) and Oregon Green 514 (ORG) in sol−gel silicates by measuring their mobility distributions using single molecule polarization measurements. While R6G is a cationic dye, ORG (pKa = 3.69 ± 0.08) is an anionic dye at neutral pH. The presence of more tumbling ORG molecules in sol−gel silicates indicates that R6G and ORG experience opposite Coulombic interactions with a predominately anionic silica sol−gel surface. On the other hand, the fact that tumbling ORG only represents a minor portion of ORG investigated, even at pH 7, clearly illustrates that Coulombic interaction alone does not control the mobility of an encapsulated guest molecule in sol−gel silicates.

C60Based Hybrid Nanocomposites Obtained in the Presence of Ultrasounds

Since their discovery, fullerenes in general and buckminsterfullerene C60 in particular, became a subject of great interest for studies. Being compatible with the sol–gel process, one of the promising approaches is to incorporate the fullerene molecules in sol–gel oxide matrices. Great part of studies deals with SiO2 sol–gel oxide as the optimal matrix for entrapment of organic molecules. C60-doped silica matrices used through our present study have been prepared by sol–gel processing, using different alkoxide precursors, as a silicon oxide source: tetraethoxysilane (a), methyltriethoxysilane (b), phenyltriethoxysilane (c) and a mixture of phenyltriethoxysilane and tetraethoxysilane (d). C60-to-Si molar ratio was chosen to be 1.0 × 10−3:1 for all materials synthesized, final oxide composition remaining unchanged in all cases. The effect of ultrasounds on the gelation process was established by preparing two series of samples, either via sonication or in the absence of ultrasound processing. The properties of the resulted materials were also established. The prepared samples were characterized by XRD, IR, RPE and UV-VIS spectrometry. All methods have put in evidence the embedment of the fullerene into the silica matrix.

Gelation in Multiple Link System

AbstractThe critical point theory is generalized to include gelation in multilink system with f functional units and J junction points. The equations derived include, as special cases, the cyclotrimerization model of J = 3, and the R–Af model of J = 2. The theory is applied to the recent observation of the cyclotrimerization of bisphenol‐A dicyanate. The theoretical prediction agrees exactly with the Stutz‐Simak observation, Dc = 0.504, and accords with the Georjon‐Galy‐Pascault observation, giving a confirmation of the physical soundness of the theory. Under the smoothness assumption, we derive post‐gelation relationships with loop formation, the result suggesting the formation of permanent sol molecules that resist being absorbed into gel phase throughout an entire reaction process.Representation of binary branching.imageRepresentation of binary branching.

Formation of Poly(dimethylsiloxane) Gels

One of the most important reactions used for room temperature vulcanization of elastomers, hydrosilylation, was investigated by end-linking samples of vinyl-terminated poly(dimethylsiloxane) (PDMS) with silanes having number-average functionalities of 3, 4, and 40−50. The reaction was monitored through SiH consumption up to and beyond the gel point via Fourier transform infrared spectroscopy (FTIR). Size exclusion chromatography (SEC) was used to determine the molecular weights of the sol fractions throughout the reaction. Gel samples were swollen in toluene so that the cycle rank (the proportion of useful intramolecular bonds) could be estimated. The experimental results were compared to results from simulations generated by Accelrys' polymer modeling software. The simulations were also used to predict fractions of loop and dangling chain defects in the gel and distributions of cycle ranks in the sol molecules. The success of this study should encourage extension to other systems, particularly the commer...

Strategies for encapsulating biomolecules in sol–gel matrices

The encapsulation of biological molecules in sol–gel matrices has received considerable attention because of the prospect of creating novel materials which exhibit the characteristic chemical and biochemical functionalities of enzymes and other proteins. This paper explores the nature of the interactions between a sol–gel derived silica matrix and the electron transfer protein, cytochrome c. These interactions were found to be of central importance in determining the synthesis conditions for these materials, in creating the pores which confine the protein and in stabilizing the protein so that it retains its structure and chemical function.

MC‐determination in elastomers by 1H‐NMR relaxation and 2H‐NMR spectroscopy

AbstractWe show how 1H–NMR transversal relaxation and 2H–NMR spectroscopy can be used for the determination of the number average molecular mass MC in typical elastomers at temperatures well above the glass transition temperature. MC‐results of the different NMR methods are compared among one another and with MC‐results of other common independent methods. Moreover, the NMR measurements provide a number of additional useful parameters: correlation times, portions of dangling chain ends and of sol molecules, molecular order.

Study of sol extraction from model poly(dimethylsiloxane) (PDMS) networks using pulsed gradient nuclear magnetic resonance (NMR) diffusion measurements

Model PDMS networks have been prepared from a blend of hydroxyl-terminated molecules of two sizes using tetraethoxysilane as cross-linking agent in the presence of a catalyst. Sol was extracted for various lengths of time by swelling in toluene; up to 4.2 % of sample weight was extractable in 5 days. There was evidence that the largest sol molecules are entangled with the network. Results suggest that prolonged extraction is required for PDMS networks destined as host materials for the study of the kinetics of polymeric trace additives

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

AbstractWe 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.

Analysis of crosslinked and entangled polymer networks as studied by nuclear magnetic resonance

Recent proton n.m.r. measurements on a range of irradiated polydimethyl- siloxane fluids have shown that the spin-spin relaxation (which can comprise two components T 2L and T 2S ) provides information on the density of crosslinking, which is proportional to radiation dose r . For material of high molecular mass it is necessary to add a virtual dose r e , even in the absence of radiation-induced crosslinks. This addition is ascribed to the presence of entanglements - not induced by radiation - which on the time scale of the n.m.r. measurements can behave as crosslinks. With the slower methods of network analysis such as solubility, swelling or slow elastic deformation these entanglements, which are in a state of dynamic equilibrium, play little part. These observations have been applied to the statistical theory of network formation, in which the crosslink density q o r is replaced by the crosslink + entanglement density q o ( r + r e ). The sol fraction s is replaced by the quantity f , the non-network fraction, which is directly measurable by n.m.r. since it represents the fraction of protons relaxing with the longer spin-spin relaxation time T 2L . The gel fraction g (= 1 - s ) is replaced by 1 - f , the network component, which possesses the shorter relaxation time T 2S . Corresponding changes are also made in the other parameters used in network theory. In the separated gel fraction T 2S is found to be directly proportional to the average molecular mass M ce between crosslinked and/or entangled units. At 10 °C T 2S ~ 0.6 x 10 -6 M ce , when the crosslinked or entangled unit density per molecule is changed by radiation dose or initial molecular weight respectively. For the sol fraction T 2L is related to the number average molecular mass; T 2L M n 0.5 ≈ 15 at 10°C. This relation is derived from ( a ) unirradiated commercial material, ( b ) mixtures, ( c ) the residual sol fraction of a fluid irradiated to varying extents. For high molecular weight sol fractions a T 2S component is also observed due to entanglements. Its magnitude, and the dose at which it disappears agree with the theory of networks, as modified to take entanglements into account. When both sol and gel fractions are present simultaneously, both T 2S and T 2L change, due to swelling of the network, and reduced mobility of the sol molecules. Pulsed n.m.r. promises a simple and quantitative method of studying mobile polymer networks, including the effect of short-lived entanglements whose presence affects the visco-elastic behaviour of these fluids.

Flow behavior of partially crosslinked low‐density polyethylene

AbstractThe viscosity of low‐density crosslinked polyethylene was studied as a function of gel content and shear rate (3.75–33 sec−1). A simple model relating viscosity with gel content is suggested. It is shown that sol viscosity decreases with cross‐linking propagation. The rheological parameters of the sol fraction are changed as a result, and it is natural that this effect should be utilized for flow calculations. Experimental data indicate a high degree of interaction between the sol molecules and the gel, and an experimental technique is presented for measuring it.The rheological parameters of crosslinked polyethylene are closely dependent on the gel content, viscosity and pseudoplasticity increasing with the latter.