Small-angle neutron scattering of mixed ionic perfluoropolyether micellar solutions

Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios.     

Small-angle neutron scattering on shear-induced micellar structures

Small-angle neutron scattering (SANS) experiments on sheared aqueous surfactant solutions of tetradecyltrimethylammoniumsalicylate (TTMA-Sal) are reported. A5-mM-solution without shear shows a weak correlation peak at a momentum transfer of 0.09 nm^–1 which has its origin in the micellar interaction. For shear rates above a threshold value of =40 s^–1 the scattering pattern shows an irregular increase in anisotropy. The analysis of the anisotropic pattern reveals the existence of two types of micelles: Small rodlike micelles which are weakly aligned and very large rodlike aggregates which are strongly aligned and which are present above the threshold value of. The two micelles are in equilibrium with each other and the equilibrium shifts with increasing shear rate to the side of the large oriented micelles.     

Small-angle neutron scattering from typical synthetic and biopolymer solutions

Small-angle neutron scattering (SANS) has been used to investigate the solution properties of four model polymers, two poly-amino acids [poly(lysine) and poly(proline)], and two water-soluble synthetic polymers [poly(acrylic acid) and poly(ethylene oxide)]. In each case, one of the two polymers is charged, while the other is neutral. SANS measurements were made in the semi-dilute concentration regime in two different solvents [d-water and d-ethylene glycol]. The scattering signals were decomposed into low-Q clustering and high-Q solvation contributions. The temperature dependence of the scattering parameters was determined for poly(lysine) and poly(ethylene oxide) solutions over the temperature range of 13 to 82 °C. Analysis of the SANS spectra revealed that with increasing temperature, the solvation intensity increased in both solvents, while the clustering intensity increased in d-water and decreased in d-ethylene glycol. Significant differences were observed between the SANS spectra of charged and neutral polymer solutions. However, biopolymers and synthetic polymers exhibited qualitatively similar behavior.     

Analysis of small angle neutron scattering data from strongly interacting polydisperse ionic micellar solutions

Small angle neutron scattering (SANS) intensity distributions from ionic micellar solutions without added salt generally show a prominent interaction peak at finite Q (magnitude of the wave vector transfer) related to a certain inter-micellar correlation distance. Analysis of this type of data requires, aside from the structural model of the micelle itself, a statistical mechanical theory for dealing with the inter-micellar correlations. We present a method for a consistent analysis of SANS data taking into account the surfactant chain packing, the aggregation number polydispersity, and the effective micellar charge. The micelle is modelled as a two-region spheroidal particle and the inter-micellar correlations calculated according to a generalized one-component macroion (GOCM) theory. GOCM uses an effective inter-micellar pair potential which is a finite concentration extension of the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) double layer interaction potential. Two micellar solutions are treated as examples, namely that of sodium dodecyl sulfate (SDS) and sodium 1,2-bis(2-ethylhexyloxycarbonyl)ethanesulfonate (AOT). The effects of polydispersity are appreciable immediately above the critical micellar concentrations (CMC). Both the structural parameters of the micelle and the free energies of micellization and micellar growth can be extracted from SANS data.     

Small-angle neutron scattering studies of charged carboxyl-terminated dendrimers in solutions

Small-angle neutron scattering was used to characterize the solution behavior of charged carboxylic acid terminated "cascade" dendrimers (Z-Cascade/methane [4]/3-oxo-6-oxa-2-azaheptylidyne/3-oxo-2-azaheptylidyne/propanoic acids) of third (G3) and fifth (G5) generations as a function of dendrimer concentration, pH, and ionic strength. An increase in dendrimer concentration leads to a single broad peak in the scattering profile arising from interdendrimer interaction. The dissociation of terminal carboxylate groups also gives rise to an interdendrimer interaction peak, which could be suppressed by the addition of excess salt. The results of contrast matching measurements indicate the accumulation of an excess concentration of tetramethylammonium counterions around the surface of these highly charged particles, and the thickness of these counterions lies somewhere between 4 and 6 A. This conclusion is consistent with our previous potentiometric titration (Zhang, H.; et al. J. Phys. Chem. B 1997, 101, 3494) and capillary electrophoresis (Huang, Q. R.; et al. J. Phys. Chem. B 2000, 104, 898) data.     

Small-angle neutron scattering from an oil-in-water microemulsion as a function of temperature

A multicomponent oil-in-water microemulsion containing CTAB (C₁₆H₃₃NMe₃Br), n-octane, 1-butanol, NaBr, and D₂O has been studied by small-angle neutron scattering at several temperatures. The size of the microemulsion droplets was found to be independent of temperature. Correlation lengths for the critical concentration fluctuations agree with those determined earlier by light scattering.     

Small-angle neutron scattering study of structural changes in temperature sensitive microgel colloids

The structure of temperature-sensitive poly(N-isopropylacrylamide) microgels in dilute suspension was investigated by means of small-angle neutron scattering. A direct modeling expression for the scattering intensity distribution was derived which describes very well the experimental data at all temperatures over an extensive q range. The overall particle form as well as the internal structure of the microgel network is described by the model. The influence of temperature, cross-linking density, and particle size on the structure was revealed by radial density profiles and clearly showed that the segment density in the swollen state is not homogeneous, but gradually decays at the surface. The density profile reveals a box profile only when the particles are collapsed at elevated temperatures. An increase of the cross-linking density resulted in both an increase of the polymer volume fraction in the inner region of the particle and a reduction of the smearing of the surface. The polymer volume fraction inside the colloid decreased with increasing particle size. The structural changes are in good agreement with the kinetics of the emulsion copolymerization used to prepare the microgel colloids.     

Small angle neutron scattering measurements on micellar solutions of perfluor detergents

Small angle neutron scattering experiments (SANS) were carried out with solutions of the Lithium salt of Perfluoroctanoic acid (LIPFO) in pure D₂O and mixtures of D₂O and H₂O and Diethylammoniumperfluorononanoate (DEAFN). For LIPFO the scattering intensity as a function of the scattering angles could be fitted with calculated scattering functions for spheres. The contrast variation method allowed us to extract accurate values for the radius, the concentrations of the micelles and the aggregation number. For DEAFN the observed scattering function could be fitted on the basis of spherical vesicles with an appreciable variance in diameter. For several solutions these parameters and the concentration of the vesicles could be evaluated.     

Small-angle neutron scattering from two-phase polymeric systems

A small-angle neutron scattering method is presented for the determination of single-chain scattering functions in heterogeneous two-phase polymeric materials such as incompatible polymer blends and microphase-separated diblock copolymers. Development of the method makes use of both the discrete and continuum approaches to scattering theory. In this fashion it is shown that extraction of the single-chain scattering function requires only two experimental observations, regardless of the compressibility of the system. These experiments consist of small-angle neutron scattering profiles from two samples: an unlabeled two-phase material and a similar two-phase material in which a portion of the molecules in one phase have been replaced by molecules which are contrast labeled by isotopic deuterium substitution. Anticipated difficulties associated with actual application of the procedure and requirements of the necessary experimental samples are discussed.     

Small-angle X-Ray and neutron scattering in food colloids

Small-angle and ultra-small-angle X-ray and neutron scattering techniques provide structural information on the nanometre-to-micron length scale. This review describes recent examples and advances of these techniques in the field of food colloids. The article highlights the structural information that can be extracted when applied to all macronutrient classes, namely fats, carbohydrates and proteins, and mixtures thereof, as well as covering emerging applications and future opportunities.     

Small-angle neutron scattering study of temperature-induced emulsion gelation: the role of sticky microgel particles

In this work, small-angle neutron scattering (SANS) is used to probe the structural transformations that accompany temperature-induced gelation of emulsions stabilized by a temperature-responsive polymer. The latter is poly(NIPAM-co-PEGMa) (N-isopropylacrylamide and poly(ethyleneglycol) methacrylate) and contains 86 mol% NIPAM. Turbidity measurements revealed that poly(NIPAM-co-PEGMa) has a lower critical solution temperature (T(LCST)) of 36.5 degrees C in D(2)O. Aqueous polymer solutions were used to prepare perfluorodecalin-in-water emulsions (average droplet size of 6.9 mum). These emulsions formed gels at 50 degrees C. SANS measurements were performed on the poly(NIPAM-co-PEGMa) solutions and emulsions as a function of temperature. The emulsion was also prepared using a D2O/H2O mixture containing 72 vol% D2O in order to make scattering from the droplets negligible (on-contrast). The SANS data were analyzed using a combination of Porod and Ornstein-Zernike form factors. The results showed that the correlation length (xi) of the polymer scaled as xi approximately phi(p)(-0.68) at 32 degrees C, where phi(p) is the polymer volume fraction. The xi value increased for all systems as the temperature increased, which was attributed to a spinodal transition. At temperatures greater than T(LCST), the polymer solution changed to a polymer dispersion of poly(NIPAM-co-PEGMa) aggregates. The aggregates have features that are similar to microgel particles. The average size of these particles was estimated as 160-170 nm. The particles are "sticky" and are gel-forming. The on-contrast experiments performed using the emulsion indicated that the interfacial polymer chains condensed to give a relatively thick polymer layer at the perfluorodecalin-water interface at 50 degrees C. The gelled emulsions appear to consist of perfluorodecalin droplets with an encapsulating layer of collapsed polymer to which sticky microgel particles are adsorbed. The latter act as a "glue" between coated droplets in the emulsion gel.     

The influence of hydrophobic counterions on micellar growth of ionic surfactants

This review covers the effects of hydrophobic counterions on the phase behavior of ionic surfactants and the properties of the phases. Mixing hydrophobic counterions with ionic surfactant micellar solutions may initiate the micellar growth and transform the micellar microstructure into different morphologies. This behavior may also be achieved by mixing ionic surfactants with hydrophilic counterions, although higher counterionic concentrations are then required. First, the role of hydrophilic and hydrophobic counterions in regards to micelle growth is discussed. Second, the effect of the hydrophobic counterion on the self-assembly of cationic and anionic surfactants and their viscoelastic behavior are presented. Third, the relationships between geometry, hydrophobicity and their consequences on micellar growth for different hydrophobic counterions are reviewed. Forth, the influence of hydrophobic counterion substituents (substitution pattern) on the phase behavior is discussed. Some results we previously obtained for different isomers of hydroxy naphthaoic acids and the cationic surfactant cetyltrimethylammonium hydroxide are included. With these systems the effect that the hydrophobic counterion microenvironment has on the phase behavior, rheological behavior and the micellar microstructure is discussed. The results from other research groups are also discussed.     

Small-angle neutron and x-ray scattering study of swollen nylon-6

Results of swelling and small-angle scattering experiments on samples of nylon-6 swollen with heavy water are discussed on the basis of the lamellar and switchboard models. The small-angle neutron scattering (SANS) intensity is very sensitive to the distribution of water in swollen samples, while the small-angle x-ray scattering (SAXS) data characterize the dry samples. The observed values of the mean-square fluctuation of scattering-density can be explained by a model with assumed inhomogeneous swelling of the amorphous phase.     

Small-angle x-ray and neutron scattering studies of amorphous polymer blends

Dilute mixtures of poly(ε-caprolactone) (PCL) in poly(vinyl chloride) (PVC), poly(p-iodostyrene) (PpIS) in polystyrene, and deuterated polystyrene (d-PS) in protonated poly(o-chlorostyrene) (PoCIS) were investigated by means of small-angle x-ray scattering and small-angle neutron scattering. The radii of gyration of PCL in PVC and of d-PS in PoCIS were found to be expanded over the θ values by 50 and 30%, respectively. Values of the second virial coefficient were positive although errors prevented definitive evaluation of the interaction parameter. PpIS was found to aggregate, with both the radius of gyration and molecular weight increasing with concentration. Concentrated mixtures of PCL and PVC were also investigated by using the Debye–Bueche correlation function analysis. Correlation distances indicated good mixing throughout the entire composition range.     

Small-angle neutron scattering by two-component particles: Application to block copolymers

General equations for small-angle neutron scattering are derived for block copolymers in dilute solution. The effects of excluded volume and polydispersity are examined. The applicability of the method of variable contrast to such systems is discussed.     

Small-angle neutron scattering from a hexagonal phase under shear

Summary The shear orientation of a micellar hexagonal liquid crystalline phase was investigated by small-angle neutron scattering. The hexagonal phase in the quiescent state showed a symmetrical scattering pattern typical of a polydomain structure. Enhanced scattering along the flow direction was observed during shear and the anisotropy of scattering intensity became stronger with increasing shear rate. The anisotropic scattering pattern corresponds to an orientation perpendicular to the flow direction and can be interpreted as a log-rolling state. The oriented sample did not relax after cessation of shear. The results from small-angle neutron scattering confirm data obtained previously from rheo-small angle light scattering measurements and are discussed in comparison to shear alignment of lyotropic liquid crystalline polymer solutions.     

Small-angle neutron scattering of a model crystallizable polymer: Crystallization and size-mismatch effects

Small-angle neutron scattering (SANS) experiments have been conducted on mixtures of nearly monodisperse hydrogenated polybutadiene (HPB) and its deuterated counterpart (DPB) in molten and semicrystalline states. A direct comparison of scattering patterns from molten and quench-crystallized samples shows unambiguously that chain conformation is unchanged by crystallization over large and intermediate size scales. The HPB's are shown to have slightly larger coil dimensions than linear polyethylenes of the same molecular weight. The scattering law for mixtures of different chain lengths was also investigated. Mismatched mixtures of monodisperse components give rise to scattering with an angle dependence well described by the Debye function for a Gaussian chain, even in the 50/50 composition range. This permits evaluation of SANS parameters [apparent radius gyration (R_g)_e and apparent degree of polymerization N_e] by analysis of the full scattering curve, removing the restrictions that the Guinier condition (qR_g < 1) be satisfied. The values of (R_g)_e and N_e obtained from experiment are in good agreement with predictions of the scattering law for ideal mixtures.     

Small-angle X-ray and neutron scattering studies of the morphology of ionomers

Preliminary small-angle neutron scattering (SANS) studies have been made of different ionomers in the dry state and after saturation with water. Scattering from the dry samples arises from differences in the neutron scattering cross sections of the ionic and nonionic units in the polymer. The SANS technique is complementary to previous small-angle x-ray scattering (SAXS) studies since the SANS contrast differences are generally quite different than those for SAXS. A quantitative comparison is made of SANS and SAXS intensities for a dry cesium salt of an ethylene-methacrylic acid (E-MAA) copolymer. For water-saturated samples the technique of isotopic replacement can be used in conjunction with SANS since saturation can be effected with either H₂O or D₂O. In this case information about the chemical composition of the phases is obtained from an analysis of the intensity ratio I/I. Results are consistent with the presence of a separate phase containing water molecules and ions in a matrix of the nonionic units. A Guinier analysis gives a radius of gyration of 17 Å for a water-saturated cesium salt of an E-MAA copolymer.     

Small-angle neutron scattering study on the transamidition of polyamide-4.6

The interchange or so-called transamidation process in polyamide-4.6 was investigated by means of small-angle neutron scattering (SANS). For this purpose, a 50/50 blend of partially deuterated and fully hydrogenous polyamide-4.6 was processed at 300°C for variable times (1–20 min). The obtained results are in line with the theory as described by Benoit to quantify the transesterfication process in homopolyesters. An exponential relaxation time τ of approximately 1500 s was obtained. On the basis of this relaxation time τ, it can be calculated that approximately four interchange reactions per chain occur in polyamide-4.6 during a typical processing time of 3 min at 300°C. The data interpretation is not complicated by the presence of some crystallinity in polyamide-4.6. © 1996 John Wiley & Sons, Inc.