Terence J McMaster

Hydration of Nafion® studied by AFM and X-ray scattering

Nafion® is a commercially available perfluorosulphonate cation exchange membrane commonly used as a perm-selective separator in chlor-alkali electrolysers and as the electrolyte in solid polymer fuel cells. This usage arises because of its high mechanical, thermal and chemical stability coupled with its high conductivity and ionic selectivity, which depend strongly on the water content. The membrane was therefore studied in different states of hydration with two complementary techniques: atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) combined with a maximum entropy (MaxEnt) reconstruction. Tapping mode phase imaging was successfully used to identify the hydrophobic and hydrophilic regions of Nafion. The images support the MaxEnt interpretation of a cluster model of ionic aggregation, with spacings between individual clusters ranging from 3 to 5 nm, aggregating to form cluster agglomerates with sizes from 5 to 30 nm. Both techniques indicate that the number density of ionic clusters changes as a function of water content, and this explains why the bulk volumetric swelling in water is observed to be significantly less than the swelling inferred from scattering measurements.

Heterogeneity and Persistence Length in Human Ocular Mucins

Atomic force microscopy (AFM) has been used to investigate the heterogeneity and flexibility of human ocular mucins and their subunits. We have paid particular attention, in terms of theory and experiment, to the problem of inducing the polymers to assume equilibrium conformations at a surface. Mucins deposited from a buffer containing Ni(2+) ions adopt extended conformations on mica akin to those observed for DNA under similar conditions. The heterogeneity of the intracellular native mucins is evident from a histogram of contour lengths, reflecting, in part, the diversity of mucin gene products expressed. Reduction of the native mucin with dithiothreitol, thereby breaking the S==S bonds between cysteine residues, causes a marked reduction in polymer length. These results reflect the modes of transport and assembly of newly synthesized mucins in vivo. By modifying the worm-like chain model for applicability to two dimensions, we have confirmed that under the conditions employed mucin adsorbs to mica in an equilibrated conformation. The determined persistence length of the native mucin, 36 nm, is consistent with that of an extended, flexible polymer; such characteristics will influence the properties of the gels formed in vivo.

Direct observations of the growth of spherulites of poly (hydroxybutyrate-co-valerate) using atomic force microscopy

Abstract Atomic force microscopy (AFM) has been used to observe, in real time, the growth of two-dimensional poly(hydroxybutyrate-co-valerate) (PHB/V) ‘spherulites’ in thin films. The AFM permits us to image the growth over a wide range of magnifications, from the macroscopic spherulitic growth down to observations of growth of individual lamellae. The lamellar growth images are obtained using a special, high resolution, phase-imaging technique. Low magnification images show, in common with optical microscope techniques, sharp circular growth fronts which move at a constant growth rate. At higher magnifications the rough nature of the growth front on a lamellar scale is clearly revealed with dominant lamellae leading the growth. The most remarkable observation is that these dominant lamellae do not grow at a fixed, constant rate, as predicted by most growth theories, but rather they initially spurt forwards at a rate substantially faster than the macroscopic growth rate, and then slow down or stop. A new theory, in which the spherulite growth rate is controlled not by the growth rate of the individual lamellae, but rather by the rate at which new lamellae nucleate on existing, dormant lamellae, is suggested. It is believed that these observations, although only made on one system, may be more widely applicable.

Atomic force microscopy of the submolecular architecture of hydrated ocular mucins

High-resolution atomic force microscopy has been applied to the imaging of intact human ocular mucins in a near-physiological buffer. The mucins displayed a range of lengths from several hundred nanometers to several microns. By varying the ionic composition of the imaging environment, it was possible to image molecules rigidly fixed to the substrate and the motion of single molecules across the substrate. From static molecular images, high-resolution line profiles show a variation of up to +/-0.75 nm in thickness along the molecule. This variation is localized in regions of several tens of nanometers. It is interpreted in terms of the varying glycosylation along the mucin and is consistent with the known size of oligosaccharides in ocular mucins. The dynamic images indicate the possibility of following mucin interactions in situ.

Cracking in spherulites of poly(hydroxybutyrate)

Poly(hydroxybutyrate) (PHB) is a biodegradable thermoplastic. Melt-cast PHB sheets normally become brittle on storage. The embrittlement has been associated with cracks formed during cooling due to differences in radial and circumferential thermal expansion coefficients. We show that the cracks are in fact due to differences in thermal expansion between the PHB film and the constraining glass slides. The circumferential features, previously identified as cracks, are shown to be surface features formed due to volume reduction during crystallization constrained by the substrate.

Atomic force microscopy studies of rubbed polyimide surfaces used for liquid crystal alignment

Liquid crystal(LC)surface alignment, which controls off‐state contrast in LC displays, is commonly achieved by directional rubbing of polyimide‐coated substrates. In order to better understand the alignment mechanism we have used tapping‐mode atomic force microscopy to study the dependence of polymer surface morphology on rubbing conditions. Based on results from three polyimides rubbed at different strengths with two cloths, we have identified five surface feature types induced by the rubbing: (1) isolated point defects, (2) occasional deep scratches (≥5 nm), (3) numerous very shallow scratches (mostly ≤1 nm deep), (4) nanoscale surface islands (3–10 nm high, number density ≳109 cm−2), and (5) modified background texture. The last feature confirms that the entire surface has been affected by the rubbing process. The finest resolution achieved, about 10 nm full width, was insufficient to resolve individual polymer chains. Of the features observed, only occasional very deep off‐axis scratches clearly affected azimuthal LC alignment. Quasiperiodic features observed under weak rubbing conditions suggest that initial rubbing occurs by a stick‐slip mechanism, and the widths of such features gives an estimate of fiber‐surface contact size.

Atomic Force Microscopy (AFM) Study of Interactions of HMW Subunits of Wheat Glutenin

ABSTRACT Atomic force microscopy (AFM) has been used to study the noncovalent interactions of alkylated HMW subunit 1Dx5 and a M r 58,000 peptide derived from the central repetitive domain. Both protein and peptide align side-by-side to form fibrils, the HMW subunit forming a branched network, and the peptide forming linear rods. The N- and C-terminal domains of the subunit would, therefore, appear to contain regions that interact through noncovalent interactions in the absence of disulfide bond formation. These regions may be of importance in facilitating disulfide bond formation during protein body development.

Scanning tunneling microscopy of biomolecules

We have used STM to study a range of biological molecules. The necessity of immobilizing the molecules on the surface was recognized and achieved by the formation of ordered layers or by the addition of chemical groups which strongly adsorbed to the graphite substrate. Clear images of the helical structure of the synthetic polypeptide poly γ‐benzyl‐L‐glutamate were obtained. Images of protein molecules isolated from wheat were found to be consistent with the rare β‐spiral conformation. The binding of the mutagen ethidium bromide to DNA was also studied and images of the cyclic glucan β‐cyclodextrin, tosylated to improve adsorption of the molecule to graphite, were also obtained. Although the contrast mechanism is unclear in some of these examples, these results suggest that STM can be applied successfully to a wide range of biomolecules.

Chromosome classification by atomic force microscopy volume measurement

Historically, metaphase chromosomes have been differentiated (or karyotyped), within a species using chemical banding techniques and optical microscopy. This chemical approach alters the structure of the chromosomes. Following successful work on high‐resolution imaging of plant chromosomes, the volumes of complete metaphase sets of maize and barley chromosomes have been calculated from three dimensional atomic force microscopy data. These results correlated extremely well with classification by arm lengths and ratios. This study demonstrates the novel application of atomic force microscopy as a high‐resolution measuring instrument for volumes of biological systems, a hitherto underused facility of the microscope. This approach will permit chromosomes to be identified and experiments performed on them without recourse to destructive chemical treatment.

Adsorption of liquid crystals imaged using scanning tunneling microscopy

Scanning tunneling microscopy has been used to obtain images of the surface conformations of 4‐n‐octyl‐4’‐cyano‐biphenyl, a room‐temperature smectic liquid crystal. Working in air and using the constant‐current mode, it has been possible to resolve both the aliphatic and aromatic parts of the molecule, and to observe individual benzene rings. Two previously unreported structures have been observed: an overlapping bilayer of layer spacing 37 A, and a densely packed monolayer of spacing 24 A. Periodic dislocations in the molecular packing have also been observed. Surface forces and intermolecular interactions are discussed as factors which could lead to these observed structures.