G. R. Brown

Crystallization kinetics in blends of isotactic and atactic poly(β-hydroxybutyrate)

Isothermal crystallization kinetics in the miscible blend system isotactic/atactic poly(β-hydroxybutyrate) (PHB) are examined by optical microscopy. The presence of the atactic component results in a reduction in spherulitic growth rate of the isotactic polymer. Analysis of the growth rates using the Lauritzen-Hoffman model shows that a regime II→III transition is present for all blend compositions investigated (0–50 wt% atactic), and that this transition occurs at a constant temperature of 140°C. The ratio of the nucleation constants for the two regimes, KIIIgKIIg, is found to deviate from the theoretical value of 2.0 in the blend system. It is shown that a modified version of the Lauritzen-Hoffman equation incorporating entropic effects is unable to account for these deviations. The work of chain-folding, q, is calculated to be 5.1 kcal mol−1 for isotactic PHB. The implications of using assumed values of the transport parameters in the kinetic analysis are discussed.

The structure of the mesomorphic phase of quenched isotactic polypropylene

Abstract Scanning transmission electron microscopy (STEM) dark field images reveal that within the essentially amorphous matrix of quench-cooled isotactic polypropylene (iPP) there exist microcrystalline regions, 100 to 200 A in size, that have the α-monoclinic crystal form ( a = 6.67 ± 0.01, b =20.89 ± 0.05, c = 6.48 ± 0.01 A , β = 98.4 ± 0.6° ). Regions of lower crystalline order are also observed which, according to micro-micro diffraction, also possess the a-monoclinic crystal structure but with substantially larger unit cell constants ( a = 7.03 ± 0.01, b = 21.1 ± 0.1, c = 6.47 ± 0.02 A , β = 99 ± 2° ). Differential scanning calorimetry studies indicate that during quench cooling to the glassy state iPP samples attain a low degree of crystalline order to an extent that depends on sample mass. Upon heating from the glassy state, these samples exhibit exothermic transitions, between −15 and 120°C, that correspond to the formation and subsequent perfection of the microcrystalline regions observed by STEM dark field.

The crystallization of poly(ethylene oxide) in blends with neat and plasticized poly(vinyl chloride)

This study concerns the morphology and spherulite growth rates of poly(ethylene oxide) (PEO), a semicrystalline polymer, in binary blends with poly(vinyl chloride) (PVC), an amorphous polymer, and in ternary mixtures containing (i) the amorphous polymer and (ii) tricresyl phosphate (TCP), a plasticizer for PVC. It is shown that the determining factor in the slight melting temperature depression observed in PEO/PVC blends by differential scanning calorimetry is the influence of PVC on the crystalline structure of PEO. PVC does not influence the spherulite radial growth rates of PEO, measured using polarized light videomicroscopy, in the accessible temperature range; however, TCP causes a substantial depression in the growth rates. Both additives cause the spherulite structure to become coarser and less birefringent. Unusual radial extinction bands indicate that TCP is incorporated within amorphous fold surfaces of PEO lamellae. Studies using polarized infrared microspectroscopy demonstrate that PVC causes a disordering of the orientation of the crystalline stems of PEO, and induces a higher incidence of the trans planar zig-zag conformation of the PEO chains within spherulites grown in PEO/ PVC blends. Epitaxial interaction between PEO and PVC also leads to an increased incidence of the planar zig-zag conformation of the PVC. In contrast, TCP has a minimal effect on the orientation and conformation of PEO. In ternary mixtures containing PEO, PVC and TCP, the plasticization of PVC by TCP drastically reduces the effects of PVC on the microstructure of PEO.

Adsorption of Bilirubin with Polypeptide-Coated Resins

Poly-L-lysine, poly-D-lysine and poly-L-ornithine have been coated covalently onto a water-swellable polyamide resin. These sorbents, especially the polylysine coated resins, exhibit high binding affinities for bilirubin in aqueous phosphate buffer solutions at 0 degrees C. The polylysine coated resins have an improved binding affinity for bilirubin over the sorbents with lysine containing oligopeptide pendants and much higher affinity than cholestyramine. The binding capacity is directly proportional to the number of lysine residues on the resin. Replacement of poly-L-lysine with poly-D-lysine does not affect the adsorption capacity.

Effect of silica nucleants on the rates of crystallization of poly(ethylene terephthalate)

Abstract Studies of the crystallization of poly(ethylene terephthalate) (PET) by d.s.c., both isothermally and non-isothermally, show that the addition of silica causes marked increases in rate at loadings

Adsorption of poly(dimethyl siloxanes) from solution on silica: 1

The adsorption of a broad range of poly(dimethyl siloxanes) on silica, treated and untreated, has been studied. Three solvents, n-hexane, CCl4, and benzene were employed. Adsorption was molecular weight dependent and more polymer adsorbed from n-C6H14 than from CCl4. Only negative adsorption from C6H6 solution was observed. The results are rationalized on the basis of the polymer—solvent interaction values (χ), the solvent—adsorbent and the solute—adsorbent adsorption energies. It is concluded that the loss in configurational entropy on adsorption of the macromolecules is a major factor in determining the adsorption isotherms.

Computer Simulations of Microstructure Changes Resulting from the Thermal Degradation of PVC

Abstract Recent studies of the dehydrochlorination of PVC in the solid state offer convincing evidence in support of a mechanism that involves initiation at gttg − conformations in isotactic stereosequences generally considered to be normal units. Attempts have been made to verify such a mechanism by direct measurements, using solution 13C NMR, of the changes in tacticity that result from the degradation. In this paper, computer simulations are reported that use Monte Carlo methods to simulate preferential degradation at isotactic sequences of various minimum sequence lengths. The results indicate that at the levels of degradation accessible to experimental verification by solution NMR (<2%), the predicted changes in triad probabilities are close to the limits of experimental error and are not a sensitive indicator of the mechanism. The simulations also indicate that for the degradation to result in polyene sequence lengths that correspond to experimentally determined values requires that the dehydrochlor...

Binding of Bile Acids by Polymerized Cyclodextrin Resins

Abstract Copolymers of β-cyclodextrin and epichlorohydrin were prepared by a crosslinking reaction of β-cyclodextrin with different amounts of epichlorohydrin. These copolymers were tested for their ability to bind several bile salts (including the sodium salts of cholic acid, glycocholic acid, taurocholic acid, and chenodeoxycholic acid), individually and competitively, from phosphate buffer solutions at room temperature. The polymer resins with a lower degree of crosslinking had a higher binding capacity for cholic acid. In all cases the binding of chenodeoxy-cholate by the polycyclodextrin resins was much more effective than that of cholate and its conjugates, which indicates the marked importance of the host cavity size relative to that of the guest molecules. In addition, there were indications that hydrophobicity plays a role in the binding of the bile salts by these resins.

13C CP/MAS NMR studies of morphological changes in polypropylene: 2. The double melting endotherm of spunbonded fabrics

The double melting endotherm of spunbonded isotactic polypropylene (iPP) fabrics was investigated by monitoring changes in the solid-state NMR spectrum that result from thermal annealing. The DSC melting thermogram was found to change from a double to a single endotherm at anneal temperatures ≥156°C, with a concomitant increase in percent crystallinity. All of the carbon resonances in the CP/MAS NMR spectrum of the purely crystalline phase of iPP were found to be composed of multiple peaks with relative intensities that depend on anneal temperature. By monitoring the changes in the distribution of intensity among the various peaks of a given resonance, a transition temperature of 156°C was identified. Arguments are presented that this redistribution of intensity within a given carbon resonance characterizes the transformation from the α1 to the α2 monoclinic crystal form. The exothermicity associated with this transformation is responsible for the observation of a double melting endotherm by DSC. The splitting patterns observed in the NMR spectrum are discussed in terms of interlayer distances between layers of isochiral helices and the density of exposed methyls at the contact faces of these interlayers.

Enhancement of tensile properties of isotactic polypropylene spunbonded fabrics by a surface-modified silica/silicone copolymer additive

Previous work at Kimberly–Clark Corp. (R. S. Nohr and J.G. MacDonald, Kimberly–Clark Corp., unpublished results, 1990) demonstrated that the addition of an ultrafine-particle (submicron-size), surface-modified silica predispersed in an alkyl silicone to isotactic polypropylene (iPP) results in dramatic improvements in the tensile properties of fibers and spunbonded fabrics. For both fibers and spunbonded fabrics prepared with a high concentration of the low melt flow index (MFI) resin the incorporation of the additive resulted in markedly improved tensile properties. In this article it is shown that under quiescent conditions the surface-modified silica/silicone copolymer additive acts as an effective nucleating agent for iPP. This results in a reduction in the size of the crystalline entities, hence a more homogeneously distributed crystalline phase and load-bearing “tie” molecules. Indeed, scanning transmittance electron microscopy (STEM) studies show that the crystalline entities are substantially smaller at the surface and mid-radius of fibers prepared with additive containing iPP. Furthermore, the bond points of the resulting spunbonded fabrics have a dramatically increased nucleation density and smaller crystallite dimensions.