Nanofibrillar Networks in Poly(ethyl methacrylate) and Its Silica Nanocomposites

to increase the elastic modulus in molten PEMA. Introduction Polymer nanocomposites generally refer to organic/inorganic materials designed so that the matrix consists of a macromolecule to which an inorganic nanoscale particle is physically added or in which an inorganic species is grown under tightly controlled conditions to retain nanoscale dimensions and minimize aggregation. 1,2 Incorporation of such particles provides a versatile and efficient route to multifunctional materials possessing enhanced properties such as electrical conductivity, 3,4 nonlinear optics, 5,6 mechanical toughness, catalytic activity, separation selectivity, 8 and magnetism. 9 In this work, we only consider those nanocomposites prepared by the addition of inorganic particles, such as fumed or colloidal silica, to a polymer matrix. Colloidal silica has been widely used in the production of polymer nanocomposites due to its ability to improve mechanical stability at elevated temperatures

Three-dimensional structural characterization of centrosomes from early drosophila embryos

Abstract. An understanding of the mechanism and structure of microtubule (MT)-nucleating sites within the pericentriolar material (PCM) of the centrosome has been elusive. This is partly due to the difficulty in obtaining large quantities of centrosomes for analysis, as well as to the problem of attaining interpretable structural data with conventional EM techniques. We describe a protocol for isolating a large quantity of functional centrosomes from early Drosophila em-bryos. Using automated electron tomography, we have begun a three-dimensional structural characterization of these intact centrosomes with and without regrown MTs. Reconstructions of the centrosomes to ~6-8 nm resolution revealed no large structures at the minus

Spindle Pole Body Duplication in Fission Yeast Occurs at the G1/S Boundary but Maturation Is Blocked until Exit from S by an Event Downstream of Cdc10(+)

The regulation and timing of spindle pole body (SPB) duplication and maturation in fission yeast was examined by transmission electron microscopy. When cells are arrested at G1 by nitrogen starvation, the SPB is unduplicated. On release from G1, the SPBs were duplicated after 1–2 h. In cells arrested at S by hydroxyurea, SPBs are duplicated but not mature. In G1 arrest/release experiments with cdc2.33 cells at the restrictive temperature, SPBs remained single, whereas in cells at the permissive temperature, SPBs were duplicated. In cdc10 mutant cells, the SPBs seem not only to be duplicated but also to undergo partial maturation, including invagination of the nuclear envelope underneath the SPB. There may be an S-phase–specific inhibitor of SPB maturation whose expression is under control of cdc10(+). This model was examined by induction of overreplication of the genome by overexpression of rum1p or cdc18p. In cdc18p-overexpressing cells, the SPBs are duplicated but not mature, suggesting that cdc18p is one component of this feedback mechanism. In contrast, cells overexpressing rum1p have large, deformed SPBs accompanied by other features of maturation and duplication. We propose a feedback mechanism for maturation of the SPB that is coupled with exit from S to trigger morphological changes

An Improved Strategy for Automated Electron

A prediction-based scheme is proposed and implemented for automated electron microscopic tomography. By assuming that the sample follows a simple geometric rotation and that the optical system can be characterized in terms of an o#set between the optical and mechanical axes, it is found that the image movement in the x, y, and z directions due to stage tilt can be dynamically predicted with desired accuracy (15 nm in x--y position and 100 nm in focus). Thus, the microscope optical system (beam/image shift and focus) can be automatically adjusted to compensate for the predicted image movement prior to taking the projected image at each tilt angle. As a consequence, it is not necessary to either record additional images for tracking and focusing during the course of data collections or to spend valuable setup time in a lengthy pre-calibration of stage motions. Furthermore, this scheme is also found to tolerate a significant degree of non-eucentricity and to be quite robust in the collection of regular and cryo low-dose images on thin or thick samples even at magnifications greater than 62 000# and angular step as large as 10#. For interested users the software can be freely downloaded for non-profit use at http://www.msg.ucsf.edu/tomography

Automated acquisition of electron microscopic random conical tilt sets

Single particle reconstruction using the random conical tilt data collection geometry is a robust method for the initial determination of macromolecular structures by electron microscopy. Unfortunately, the broad adoption of this powerful approach has been limited by the practical challenges inherent in manual data collection of the required pairs of matching high and low tilt images (typically 60 ° and 0°). The microscopist is obliged to keep the imaging area centered during tilting as well as to maintain accurate focus in the tilted image while minimizing the overall electron dose, a challenging and time consuming process. To help solve these problems, we have developed an automated system for the rapid acquisition of accurately aligned and focused tilt pairs. The system has been designed to minimize the dose incurred during alignment and focusing, making it useful in both negative stain and cryo-electron microscopy. The system includes a feature for montaging untilted images to ensure that all of the particles in the tilted image may be used in the reconstruction

Architecture-Induced Phase Immiscibility in a Diblock/

Ordered diblock copolymer blends have recently become the subject of tremendous research interest since they can be used to elucidate the intramicrodomain segregation of blocks differing in length, as well as to identify the molecular and blend parameters yielding phase immiscibility. In this work, we explore the influence of molecular architecture on block copolymer blend miscibility by examining an equimolar mixture of two symmetric styrene (S)/isoprene (I) block copolymers, one an SI diblock and the other an (SI) 4 octablock. Their molecular weights are identical, so that the ratio of block lengths is 4:1 SI:(SI) 4 . While this ratio is expected to yield a single phase in diblock copolymer blends, transmission electron microscopy reveals here that the diblock/multiblock blend is macrophase-separated due to the linear multiblock architecture and midblock conformations of the (SI) 4 copolymer. Electron tomography (3D imaging) permits direct visualization of connected SI and (SI) 4 microdomains at the SI/(SI) 4 interface at relatively high spatial resolution (ca. 3 nm). In addition, the presence of SI molecules in the (SI) 4 phase or (SI) 4 molecules in the SI phase frustrates SI lamellae, resulting in curved microphase boundaries

UCSF tomography: An integrated software suite

A real-time alignment and reconstruction scheme for electron microscopic tomography (EMT) has been developed and integrated within our UCSF tomography data collection software. This newly integrated software suite provides full automation from data collection to real-time reconstruction by which the three-dimensional (3D) reconstructed volume is immediately made available at the end of each data collection. Real-time reconstruction is achieved by calculating a weighted back-projection on a small Linux cluster (five dualprocessor compute nodes) concurrently with the UCSF tomography data collection running on the microscope's computer, and using the fiducial-marker free alignment data generated during the data collection process. The real-time reconstructed 3D volume provides users with immediate feedback to fully asses all aspects of the experiment ranging from sample choice, ice thickness, experimental parameters to the quality of specimen preparation. This information can be used to guide subsequent data collections. Access to the reconstruction is especially useful in low-dose cryo EMT where such information is very di#cult to obtain due to extraordinary low signal to noise ratio in each 2D image. In our environment, we generally collect 2048 2048 pixel images which are subsequently computationally binned fourfold for the on-line reconstruction. Based upon experiments performed with thick and cryo specimens at various CCD magnifications (50000--80000), alignment accuracy is su#cient to support this reduced resolution but should be refined before calculating a full resolution reconstruction. The reduced resolution has proven to be quite adequate to assess sample quality, or to screen for the best data set for full-resolution reconstruction, significantly improving bot..