N. Sanjeeva Murthy

Order parameter measurements in polypeptide liquid crystals

An analysis of x‐ray scattering from magnetically oriented liquid crystalline solutions of poly‐γ‐benzyl‐L‐glutamate (PBLG) in dioxane yields a value for the order parameter S?0.75. At the isotropic–liquid crystal phase transition, the critical order parameter Sc?0.5 appears to agree with predictions of mean‐field theory. Away from the transition, S is independent of PBLG concentration and PBLG axial ratio but does decrease when the helix denaturing agent trifluoroacetic acid is added to the liquid crystal. The magnitude of and changes in S are discussed in terms of the inherent flexibility of helical synthetic polypeptides.

Poly(ethylene terephthalate)–poly(caprolactone) block copolymer. I. Synthesis, reactive extrusion, and fiber morphology

A novel poly(ethylene terephthalate)–poly(caprolactone) block copolymer (PET–PCL) is synthesized in a reactive twin-screw extrusion process. In the presence of stannous octoate, ring-opening polymerization of ϵ-caprolactone is initiated by the hydroxyl end groups of molten PET to form polycaprolactone blocks. A block copolymer with minimal transesterification is obtained in a twin-screw extruder as a consequence of the fast distributive mixing of ϵ-caprolactone into high melt viscosity PET and the short reaction time. The PET–PCL structure is characterized by IV, GPC, 1H-NMR, and DSC. Fully drawn and partially relaxed fibers spun from PET–PCL are characterized by WAXD and SAXS. A substantial decrease in the oriented amorphous fraction appears to be the major structural change in the relaxed fiber that provides the fiber with the desired stress–strain characteristics.

5.5 Å crystallographic structure of penicillin β-lactamase and radius of gyration in solution☆

Abstract An electron density map of crystalline R-TEM Escherichia coli β-lactamase (penicillinase) has been calculated from X-ray diffraction data at 5.5 A resolution with protein phases based on Friedel mates from a high-quality samarium derivative. The mean figure of merit for 854 independent reflections is 0.75. The monomeric molecule is slightly ellipsoidal and contains one and possibly two regions of α-helix which are 25 A long. The Crystallographic search for the substrate binding site has so far been inconclusive. The radius of gyration of the enzyme in solution at pH 7 is 17.1 ± 1.0 A from small-angle X-ray scattering measurements. This compares with 18.6 a calculated from the low-resolution electron density map of the molecule in the crystal.

Effects of Solvent-induced Crystallization on the Amorphous Phase of Polycarbonate of bisphenol A)

Abstract By the combined use of DSC, WAXD and pycnometry techniques, it was demonstrated that the increase in free volume and reduction in Tg observed in semi-crystalline poly(carbonate of bisphenol A) (PC), whose crystallization was solvent-induced, are due to the creation of many microvoids, microcavities and crazes in the amorphous fraction of the polymer during the crystallization. Accordingly, the polymer chains and chain-segments present right at the polymer-air interfaces, forming the walls of these micro-cavities, are more mobile than the chains and segments in the bulk amorphous phase, resulting in a commensurate reduction in Tg . Upon annealing at elevated temperatures the microvoids, etc., heal, the polymer-air interfacial specific surface area decreases, and Tg increases toward the Tg of the bulk amorphous PC containing no microvoids, crazes and internal polymer-air interfaces.

Preliminary crystallographic data for Escherichia coli β-lactamase

Abstract The β-lactamase (penicillinase) from Escherichia coli W3310 has been crystallized from 25% saturated ammonium sulfate solution at pH 6.9. An X-ray examination of the monoclinic crystals shows the space group is C 2, with unit cell dimensions a = 47.2 A , b = 76.9 A , c = 73.3 A and β = 98.6°. With Z = 4 and the molecular weight = 22,000 (Melling & Scott, 1972), the A 3 /dalton ratio is 2.98. The crystals are suitable for structure analysis to at least 2.4 A resolution.

Glass Transition Temperature and the Nature of the Amorphous Phase in Semicrystalline Polymers: Effects of Drawing, Annealing and Hydration in Polyamide 6

Abstract The influence of the organization of the amorphous chains segments on the glass transition temperature (Tg) in semicrystalline polymers is analysed by studying the effects of drawing, annealing and hydration in polyamide 6 fibers. We consider the role of three of the features of the amorphous phase: orientation (configurational entropy), density (free volume) and confinement (segmental mobility). Three classes of amorphous phases are identified; two of these are constrained in the intercrystaline regions, at the fold surfaces (between the lamellae within the lamellar stack) and at the stem surface (growth surface of the lamellae or between the fibrils). The third species is the bulk amorphous phase outside the lamellar stacks, and constitutes a large fraction of the amorphous phase especially at low crystallinities. Because the small fraction of the amorphous chain segments in the intercrystalline regions, and because they are in confined spaces, we suggest that these interlamellar and the interfibrillar components do not contribute significantly to the observed major glass transition peak. Rather, it is the amorphous region outside the lamellar stack that determines the Tg. Tg increases upon drawing and decreases upon annealing (heat setting). Our data suggest that orientation has a direct influence on Tg and can easily be measured whereas the influence of crystallinity is more complex. The influence of orientation of Tg can be understood in terms of a two Tg model in which the oriented amorphous component has a higher Tg than the unoriented component.

Multilayered crystalline structures and liquid crystalline phases in a mesogen with siloxane tails

Abstract N,N′-Terephthalylidine-bis-4-(2 pentamethyldisiloxyethyl)aniline, TBS2A, which contains a mesogenic group with two di-siloxane tails, and TBS3A with two tri-siloxane tails were synthesized. Liquid crystalline behaviour in these compounds were studied by DSC and polarized light microscopy. Temperature-induced structural transitions in TBS2A were studied using variable temperature X-ray diffraction. At least three structural phase transitions were observed: crystal-crystal transformation (from Cr2 to Cr3) at 125°C, crystalline phase (Cr3) to a smectic liquid crystalline phase (Sb) at 140°C, and the melting of the Sb phase at 152°C. A room temperature crystalline phase (Cr1) with a layer repeat of 29.5 A is also observed. The low temperature Cr2 phase is characterized by a layer repeat of 26.3 A and exhibits a long spacing of 105 A corresponding to a repeat unit of four layers. The high temperature Cr3 phase has a layer repeat of 22.2 A and exhibits a long spacing of 44 A from a repeat unit with two...

Multiscale approach for the construction of equilibrated all-atom models of a poly(ethylene glycol)-based hydrogel.

A multiscale modeling approach is presented for the efficient construction of an equilibrated all-atom model of a cross-linked poly(ethylene glycol) (PEG)-based hydrogel using the all-atom polymer consistent force field (PCFF). The final equilibrated all-atom model was built with a systematic simulation toolset consisting of three consecutive parts: (1) building a global cross-linked PEG-chain network at experimentally determined cross-link density using an on-lattice Monte Carlo method based on the bond fluctuation model, (2) recovering the local molecular structure of the network by transitioning from the lattice model to an off-lattice coarse-grained (CG) model parameterized from PCFF, followed by equilibration using high performance molecular dynamics methods, and (3) recovering the atomistic structure of the network by reverse mapping from the equilibrated CG structure, hydrating the structure with explicitly represented water, followed by final equilibration using PCFF parameterization. The developed three-stage modeling approach has application to a wide range of other complex macromolecular hydrogel systems, including the integration of peptide, protein, and/or drug molecules as side-chains within the hydrogel network for the incorporation of bioactivity for tissue engineering, regenerative medicine, and drug delivery applications.

Optimizing the mechanical performance in semi-crystalline polymers: Roles of melt temperature and skin-core Crystalline morphology of nylon

Crystalline texture and mechanical (tensile and flexural) properties of injection molded nylon 6 were evaluated to understand the influence of one of the key-processing variable, the melt temperature (T mlt). We find that mechanical properties are sensitive to T mlt only below ∼250°C. Rapid quenching of the surface produces a skin with lower crystallinity than the core, which cools more slowly; this difference in the rate of cooling produces γ crystalline form in the skin and α crystalline form in the core. Higher tensile strength at yield, lower elongation at break and higher flexural strength were observed in specimens molded at lower T mlt. These characteristics are associated with thicker and less ordered skin, and a lower crystallinity core. The role of the T mlt on micro-structure and mechanical properties of injection molded nylon 6, the development of skin and core morphologies, and the role of the residual stresses in the core are discussed.