Semi-interpenetrating polymer networks synthesis by photocrosslinking of acrylic monomers in a polymer matrix

Semi-interpenetrating polymer networks have been obtained by UV-radiation curing of acrylate monomers dispersed in a polymer matrix, using an arylketone as photoinitiator. The polymerization kinetics was studied quantitatively by infrared spectroscopy for the various polymers examined: polyurethane, poly(vinyl chloride), poly(methyl methacrylate). The fastest reaction occurs in PVC films, where UV-curing develops extensively within a fraction of a second, leading to an insoluble and highly resistant material. The functionality of the acrylic monomer has a strong influence on the formulation reactivity, as well as on the mechanical and chemical properties of the final product. In PMMA, the polymerization was shown to continue to proceed efficiently for a few seconds after the UV exposure, even in the presence of air, due to both the high concentration of initiating radicals generated by the intense irradiation and the slow termination processes in solid media. © 1993 John Wiley & Sons, Inc.     

Effects of added surfactants on thermoreversible gelation of associating polymer solutions

The influence of added surfactants on physical properties of associating polymer solutions was examined by a new statistical‐mechanical theory of associating polymer solutions with multiple junctions and by computer simulation. The sol–gel transition line, the spinodal line, and the number of elastically effective chains in the mixed networks were calculated as functions of the concentration of added surfactants. All of them exhibited nonmonotonic behavior as a result of the following two competing mechanisms. One was the formation of new mixed micelles by binding surfactants onto the polymer associative groups. These micelles serve as crosslink junctions and promote gelation. The other was the replacement of polymer associative groups in the already formed network junctions by added surfactants. Such replacement lowers connectivity of junctions and destroys networks. The critical micelle concentration was also calculated. The results are compared with the reported experimental data on poly(ethylene oxide)‐based associating polymers and hydrophobically modified cellulose derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 733–751, 2004     

Semi-interpenetrating polymer networks of polyurethane and poly(vinyl chloride)

Summary A series of semi-interpenetrating polymer networks (semi-IPN) of polyurethane (PU) and poly(vinyl chloride) (PVC) has been obtained by prepolymer method and characterised by FTIR; morphological features were examined by SEM-EDS. It has been found that PVC spherical aggregates are dispersed in the PU matrix, but Cl atoms location indicates partial miscibility of both polymers at the interphase which is probably due to hydrogen bonding and/or dipole-dipole interactions. The PVC component influences the phase behaviour of PUs hard segments, as evidenced by DSC results. Thermogravimetric analysis (TG) reveals a complex, multi-step decomposition process with the main mass loss at 503-693 K, while the DTG maxima are located between 540 and 602 K.     

Dynamical arrest, percolation, gelation, and glass formation in model nanoparticle dispersions with thermoreversible adhesive interactions

We report an experimental study of the dynamical arrest transition for a model system consisting of octadecyl coated silica suspended in n-tetradecane from dilute to concentrated conditions spanning the state diagram. The dispersion's interparticle potential is tuned by temperature affecting the brush conformation leading to a thermoreversible model system. The critical temperature for dynamical arrest, T*, is determined as a function of dispersion volume fraction by small-amplitude dynamic oscillatory shear rheology. We corroborate this transition temperature by measuring a power-law decay of the autocorrelation function and a loss of ergodicity via fiber-optic quasi-elastic light scattering. The structure at T* is measured using small-angle neutron scattering. The scattering intensity is fit to extract the interparticle pair-potential using the Ornstein-Zernike equation with the Percus-Yevick closure approximation, assuming a square-well interaction potential with a short-range interaction (1% of particle diameter). (1) The strength of attraction is characterized using the Baxter temperature (2) and mapped onto the adhesive hard sphere state diagram. The experiments show a continuous dynamical arrest transition line that follows the predicted dynamical percolation line until ? ≈ 0.41 where it subtends the predictions toward the mode coupling theory attractive-driven glass line. An alternative analysis of the phase transition through the reduced second virial coefficient B(2)* shows a change in the functional dependence of B(2)* on particle concentration around ? ≈ 0.36. We propose this signifies the location of a gel-to-glass transition. The results presented herein differ from those observed for depletion flocculated dispersion of micrometer-sized particles in polymer solutions, where dynamical arrest is a consequence of multicomponent phase separation, suggesting dynamical arrest is sensitive to the physical mechanism of attraction.     

Role of the polymer microstructure in the thermoreversible gelation of poly(vinyl chloride)

Gels of polyvinylchloride (PVC) and PVC copolymers of different microstructure and molar composition, were prepared in a series of diester-like solvents. The viscoelastic properties of these gels were evaluated as a function of temperature and the molecular structure was studied by small-angle neutron scattering. The degrees of proton association in the polymer solutions were determined by ¹H NMR spectroscopy by measuring the intensity of proton bands of CH₂ monomeric units. The body of experimental data shows clearly that there is a straight relation between molecular structure, viscoelastic properties of the gel, degree of polymer-solvent interaction and polymer microstructure.     

Complex formation through heterogeneous nucleation of thermoreversible gelation

The preparation method and the molecular structure of a composite material consisting of a ternary system polymer/bicoppercomplex/solvent are presented. The properties of each binary system are exposed first. The polymer solutions produce thermoreversible gels while the bicopper organic complex forms a randomly-dispersed, self-assembling structure in organic solvents. It is shown that, in a common solvent, the bicopper complex acts as a nucleation agent for the gelation of the polymer (heterogeneous nucleation). As a result, bicopper complex filaments are encapsulated in a polymer matrix.     

The role of polymer-solvent compounds in thermoreversible gelation

The definition of a thermoreversible gel is first discussed through the use of two criteria dealing with topology and with thermodynamics. According to this definition only fibrillar systems, as opposed to spherulitic packings, ought to be considered gels. A simple mechanical test is presented that helps recognize such gels. The gelation mechanisms are briefly reviewed and the importance of the role of the polymer-solvent compound in preventing chain from folding, which favours fibrillar structures, is stressed. Also, the role of such compounds in gel ageing, particularly in the case of PVC, is highlighted.     

Effect of pharmaceuticals on thermoreversible gelation of PEO-PPO-PEO copolymers

Pluronic F127, a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), has generated considerable interest as a drug delivery vehicle due to its ability to gel at physiological temperatures. This work examines the gelation behavior of Pluronic F127 in the presence of a series of hydrophobic pharmaceuticals, to determine whether there is any correlation between gelation and physicochemical parameters of drug solutes. The study includes the local anesthetics dibucaine, lidocaine, and tetracaine; the pharmaceutical additives methyl paraben, ethyl paraben, and propyl paraben; the anti-cancer agents paclitaxel and baccatin III; and the anti-inflammatory agent sulindac. The results indicate that the presence of local anesthetics and pharmaceutical additives allows F127 solutions to form gels at lower copolymer concentrations; local anesthetics and pharmaceutical additives also shift gelation down to a lower gelation temperature. This behavior is strongly dependent on drug solubility; poorly soluble drugs (paclitaxel, baccatin III, sulindac) do not change the lower gelation temperature or minimum F127 concentration for gelation. An equation relating the decrease in gelation temperature to drug solubility is presented, and the equation fits the data well. The results have significant positive implications on the toxicity and economic issues related to use of Pluronic F127 in drug delivery.     

Critical association and thermoreversible gelation of some selected polymers

The study of associating polymers is complicated by the fact that at a finite concentration only an apparent molar mass is directly measured. This apparent molar mass deviates from the true one because of thermodynamic interaction. In a good solvent the repulsive interaction can be fully described in terms of the second virial coefficient, which can be measured at very low concentrations, and a correction can be made for the true molar mass. The technique has been successively applied to β-galactosidase in a phosphate buffer and to cellulose 2,5-acetate in acetone. The gel point could be determined, and critical behavior was found in very good agreement with percolation prediction. Two other methods were employed and tested. One is based on the condition that the longest relaxation time must diverge at the gel point, and correspondingly the translational diffusion coefficient should go to zero. The second criterion consists of the prediction that power law behavior should be observed for the time correlation function of dynamic light scattering as well as for the frequency dependent storage and loss moduli. These predictions were indeed found to hold for two polysaccharides which form thermoreversible gels in water at a fairly low salt concentration. The two polysaccharides are the Tamarind seeds polysaccharide in 1M Na₂SO₄ and an exopolysaccharide from Rhizobium leguminosarum, strain 8002 in 0.1M NaCl solutions respectively.     

Spinodal demixing,percolation and gelation of biostructural polymers

We present a variety of new experiments which concern the self-assembly of a polymeric network from homogeneous solutions of Agarose, a representative biostructural polysaccharide used for previous studies at our laboratories. They allow deriving a semi-quantitative phase diagram in the T, C plane. The diagram includes both the spinodal and gelation lines. Below a value of about 2% w/v, concentration is not sufficient for direct gelation; however, quenching of the sol from high temperatures to below the spinodal line initiates the spinodal demixing. The latter generates two sets of regions having respectively, higher- and lower-than-average polymer concentrations. In the higher-concentration regions the functional polymer-polymer interaction (that is, self-assembly) is favoured. In fact, as in the course of demixing the point representative of higher-concentration regions reaches the region below the gelation line, gelation is allowed and indeed observed to occur. This evidences the possibly more general role of spinodal demixing (extended to multi-component systems) as a pathway for specific, local and biofunctional enhancements of concentrations. At very low concentrations (e.g. 10⁻⁴ w/v) the kinetics of demixing is still observed to occur, but the set of higher-concentration regions is no longer percolative. In these conditions, gelation occurs only within each individual region, while the specimen remains free-running. This illustrates a novel aspect of the notion of non-gelling concentrations.     

Rapid crystallization and thermoreversible gelation of poly(ethylene terephthalate) in polymer/oligomer binary system

Poly(ethylene terephthalate) (PET) was rapidly crystallized through thermoreversible gelation in a liquid ethylene glycol oligomer or in epoxy resin. The solutions formed gel rapidly on cooling. Polarized light microscopy and small-angle light scattering showed that these gels contain large, regular PET spherulites. The gels may be formed by two consecutive processes: the phase separation and crystallization, and gelation by formation of a three-dimensional PET network in the oligomer solvents, where the nodes of the network are PET spherulites. The crystallinity of PET recovered from polymer/oligomer gels is near 72% measured by wide-angle X-ray diffraction method, which is about 20% higher than PET samples crystallized by solution crystallization in small molecule solvent, high temperature annealing, and stretching techniques. It takes only a few minutes to form the highly crystalline phase PET in the PET/oligomer system, and the crystallinity of the dried gel is independent of the concentration of the original solution. Excimer-fluoresence and Raman spectroscopic studies indicated that PET recovered from the gels are in an ordered state with few chain entanglements. The entanglement density of the recovered PET recovered from a 20 wt % solution in ethylene glycol oligomer is as low as that of freeze-extracted PET from a 0.5 wt % solution in phenol. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1219–1225, 1998     

Organization of nanoscale objects via polymer demixing

We present an extremely versatile method for the lateral organization of nano-scale objects (NOs) based on the phenomenon of polymer demixing. NOs are suspended in a solution of two immiscible polymers, which is used to form a thin polymer film by spin coating. During spin coating the two polymers separate to give a microphase structure, whose length scale depends on the experimental conditions. The NOs spontaneously partition into one or other of the polymer phases resulting in their lateral organization. In this work, the organization of CdSe nanoparticles and fluorescent organic dyes was studied by fluorescence microscopy. The NOs were organized in the polymer film in stochastic patterns or in ordered designs on substrates pre-patterned by soft-lithography techniques. Single-particle measurements, using confocal microscopy, showed that at low concentrations there was little aggregation of the particles.     

Pressure dependence of the demixing of polymer solutions determined by viscometry

Summary From the break-down in the viscosity of a polymer solution, associated with the entrance into the two phase region, the pressure dependence of the demixing temperatures of solutions of polystyrene (M = 600.000) in cyclohexane, cyclopentane, diethylmalonate and 1-phenyldecane was measured up to 1000 bar. The application of pressure increases the solubility of polystyrene in cyclopentane and diethylmalonate, but decreases that in 1-phenyldecane; in the case of cyclohexane, a pressure of optimum miscibility is observed at ca. 120 bar.The discussion of these findings, together with further information from the literature, demonstrates that current theories cannot even predict the pressure influences qualitatively. For an easy forecast of the effects, the distance of the (upper) critical solution temperature,T _c from the melting point of the solvent,T _MP, can, however, serve as a guideline: From the present material it can be concluded thatT _c is increased by pressure if (T —T _MP)/T _MP (K/K) is less than 0,20 but decreased it if it exceeds 0,25.Dedicated to Prof. Dr. K. Ueberreiter in honor of his 70th birthday.     

Polymer-solvent co-crystallization during thermoreversible gelation in solutions of the helical polypeptide PBLG

Macromolecular aggregation during thermoreversible gelation in solutions of the helical polypeptide poly(γ-benzyl-L-glutamate) [PBLG] in benzyl alcohol [BA] were studied by small angle neutron and small angle X-ray scattering. Gelation is apparent as a large increase in the intensity scattered at low angles, signifying formation of a microfibrillar PBLG network. The aggregated phase in isotropic gels from semidilute solutions contains about 28% solvent. A periodic structure is observed when gelation is induced by rapid cooling to a low temperature, but not by slow cooling or gelation at a higher temperature. In gels from concentrated liquid crystal solutions, two crystalline structures are observed, depending on whether the solution is rapidly quenched and then annealed or slowly gelled at an elevated temperature. A phase diagram for the PBLG/BA system is presented and the observed microstructural transitions are rationalized in terms of a gelation mechanism involving a combination of liquid-liquid phase separation and crystallization in the form of polymer-solvent co-crystals.     

Characterization of thermoreversible polymer gels by the degree of crystallinity by surface area

A characterization for crystalline junction polymer gels has been extended by introducing a new parameter which implies a measure of the number of the polymer molecules passing through the crystallizing junctions. The parameter has been considered in a theoretical gel melting relation derived by Takahashi, Nakamura, and Kagawa and a crystallite size distribution has also been considered. The equation obtained was examined on ethylene–propylene copolymer gels by using the previous data of melting temperatures determined from DSC, crystallite sizes from x-ray and DSC measurements, and reaction probabilities from ¹³C-NMR measurements. Consequently, the parameter was found to be helpful as one additional measure. Also, it was confirmed that the average crystallite sizes estimated in several different ways were all in good agreement with the conventional data. © 1996 John Wiley & Sons, Inc.     

Nanoscale memory provided by thermoreversible stochastically structured polymer aggregates on mica

Stimuli-responsive polymers are used in a large variety of applications due to the controlled manner in which their physical properties can be reversibly altered. In this study, we demonstrate the thermoreversible structuring of poly(N-isopropylacrylamide)-based polymer. By temperature-controlled atomic force microscopy, we demonstrate that polymer aggregates form on mica above the polymer lower critical solution temperature and disperse below it, and in so doing, display positional "memory" in that the nanodomains are retained in the same positions and with the same shapes during repeated cooling/heating cycles. Such positional "memory" may be useful for multiple applications in nano-microscale devices.     

Effect of sodium poly(styrene sulfonate) on thermoreversible gelation of gelatin

The effect of an added polyanion, sodium poly(styrene sulfonate) (NaPSS), on the thermoreversible gelation and remelting of gelatin gels has been investigated by polarimetry and rheology. The presence of NaPSS can either enhance or reduce collagenlike helix formation, depending on the polymer concentration relative to that of gelatin and the gelation temperature. At temperatures < 20°C, the helical content is reduced by increasing the amount of added NaPSS, demonstrating the disruption of helical structure of gelatin by the polyanion. Synchronous measurements of optical rotation and modulus at 25°C, in both gelation and remelting, indicate that the optical rotation at the gel point for the pure gelatin is lowered on addition of NaPSS. At low frequency, the storage modulus of gelatin is increased by the addition of a small amount of NaPSS relative to that of gelatin, but decreased with excess NaPSS. The mechanical properties of gelatin with and without NaPSS will be discussed in light of the competition between network junction formation by strands of triple helices among gelatin chains and temporary ionic crosslinking between gelatin and the polyanion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2287–2295, 1999     

Synthesis of template-free zeolite nanocrystals by using in situ thermoreversible polymer hydrogels

A simple effective strategy was developed by using thermoreversible polymer hydrogels as space-confining nanoreactors to control zeolite nucleation and growth. In particular, the synthesis of zeolite 4A nanocrystals 20-180 nm in size and zeolite X nanocrystals 10-100 nm in size from template-free precursor solution was demonstrated. The synthesized zeolite nanocrystals exhibit good dispersibility, as evidenced by dynamic light-scattering measurements.     

Facile fabrication and adsorption property of a nano/microporous coordination polymer with controllable size and morphology

A porous coordination polymer [Cu(3)(btc)(2)] with controllable size and morphology from nanocube to microoctahedron was readily synthesized in an ethanol-water mixture at room temperature by adjusting the concentration of the surfactant and the polymer shows size- and morphology-dependent sorption properties.