Don D. Davis

Kinetic analysis of the crystallization of poly(p-phenylene sulphide)

Growth rates of spherulites were measured in poly(p-phenylene sulphide) crystallized from the melt and the quenched glass over the temperature range 100°C–280°C, possibly the most extensive overall range yet reported for any polymer and, as such, most propitious for study of regime III crystallization. For a medium M.wt. polymer, a regime II → III transition was obtained at 208°C using values of transport parameters common to many polymers (U∗ = 1400 cal mol−1, T∞ − Tg = 30°C) together with experimentally determined values of T0m(315°C) and Tg(92°C). Under these conditions, the regime III/II slope ratio was found to be 2.07 (i.e. only 3.5% higher than predicted by regime theory), and reasonable estimates of surface free energies and of the work of chain folding were obtained. Other choices of the transport terms, including WLF and zero values, did not allow successful kinetic analyses. Although a regime I → II transition is predicted to occur at the high-temperature end of our growth-rate data, we found no experimental evidence for it. For a low M.wt. polymer, our analysis showed that regime III kinetics is obeyed at low temperatures, while at higher ones there is a continuous departure from that behaviour without, however, full attainment of regime II kinetics.

The role of molecular defects on the structure and phase transitions of poly(vinylidene fluoride)

Abstract Having prepared poly(vinylidene fluoride) (PVF 2 ) with 0.2–23.5 mol% head-head/tail-tail (HHTT) defects (by comparison with the commonly obtainable 3.5–6.0 mol%), we examined the influence of such regiodefects on crystal structure, polymorphism and Curie transitions. The structure at ambient temperature changes from α to β as the HHTT content is increased beyond ca. 11.4 mol%. At that composition, either of these two phases may predominate, depending upon temperature of crystallization (higher ones favouring β). For defect contents over 15.5 mol%, the polymer chains show progressive intra molecular disorder, yet are packed with increasing inter molecular order in a pseudohexagonal lattice. When PVF 2 containing 13.5–15.5 mol% regiodefects is heated, clear ferroelectric-paraelectric phase transformations are obtained; these are reversible (with thermal hysteresis) upon subsequent cooling. This confirms our previous findings of Curie transitions in PVF 2 copolymers with tri-or tetra-fluoroethylene, and shows that such Curie behaviour is an inherent property of PVF 2 itself. No solid-state transformations were observed for polymers with over 15.5 mol% defects, their structure remaining akin to that of the paraelectric phase at all temperatures between −10°C and the melting point. Within the same temperature range, the essentially isoregic PVF 2 (0.2 mol% defects) remains predominately in the α-phase. On the other hand, the polymer having 11.4 mol% defects shows a remarkable transformation during heating, from the anti-polar α-phase to the paraelectric phase.

Interchain packing and unit cell of syndiotactic polypropylene

Abstract We have examined the general validity and applicability of our newly proposed alternative interchain packing and unit cell for syndiotactic polypropylene, in relation to the long-standing original structural model. Whereas our recent evidence arose from single crystals in ultra-thin films, we now demonstrate the correctness of our structure for polycrystalline thick films, for bulk specimens obtained directly from the polymerization and for uniaxially oriented specimens. This removes any possible ambiguity that our structure might represent a special case arising from thin-film, substrate or epitaxial effects, or from thermal or mechanical treatments. We also remove any questions about chemical defects as originators of our alternative structure by demonstrating that our specimens have among the very highest syndiotactic contents, ordered-sequence distributions, melting temperatures, heats of fusion and crystallinities in the literature. Finally, by re-examining earlier X-ray evidence from other studies, we find consistent confirmation of our proposed structure. This structure consists of chains with a ( t 2 g 2 ) 2 conformation in a cell similar to the generally adopted one, but with close packing on (010) planes rather than C -centering. The molecular origin of our alternative structure arises from the helical handedness of the molecules. We show that these favour antichiral packing and that this requires close packing on (010) planes. The earlier C -centred unit cell requires exclusively isochiral packing and is not seen in a regular manner in any of our samples but only as a defect structure, with specimens grown at the highest temperatures being essentially defect free.

Electron-microscopic investigation of the morphology of a melt-crystallized polyaryletherketone

We have studied the structure and morphology of the useful high‐temperature/high‐strength polymer polyaryletherketone (PEEK) by transmission electron microscopy and three‐dimensional electron diffraction after finding suitable solvents (α‐chloronaphthalene and benzophenone) that allowed casting of the required ultrathin polymer films. When crystallized from the melt, PEEK grows in the form of spherulites consisting of narrow lamellae and having the b axis of the unit cell radial. Additionally, at high temperatures in these ultrathin films, the spherulites attain an extraordinary cylindrical symmetry as a result of growth of their lamellae on edge, with the c crystallographic direction parallel to the film plane and the a direction corresponding to the cylinder axis. Reasons for this mode of growth are attributed to the highly anisometric molecular cross section normal to the chain direction, which favors crystal nucleation on the substrate with the bc plane. At lower temperatures during crystallization fr...

Structure of poly(p-phenylene sulphide)

We have examined the unit-cell structure of the important high-temperature and high-strength polymer, poly(p-phenylene sulphide) (PPS); this has recently been in dispute, with two different models proposed from oriented and unoriented polycrystalline specimens, respectively. Having obtained single crystals of PPS from solution and from thin films of the melt (the latter in both flat-on and edge-on orientations), we determined the lattice dimensions of the orthorhombic unit cell using electron diffraction. These are a = 8.68 A, b = 5.66 A and c = 10.26 A, in agreement with the original model based upon alternating ±45° inclinations of the phenyl rings to the plane of the sulphur bonds. We have also determined the rate of expansion of the intermolecular lattice as a function of electron dose, and found PPS to be one of the most radiation-stable polymers, preserving its crystallinity to 3100 C m−2 (i.e. more than 30 times that of polyethylene).

Sequential two-photon-laser-induced fluorescence: a new method for detecting atmospheric trace levels of NO

Sequential two-photon-laser-induced fluorescence (TP-LIF) in NO has been achieved by using 226-nm radiation to excite the X(2)Pi ? A(2)Sigma transition followed by excitation of the A(2)Sigma ? D(2)Sigma transition using the fundamental output of a Nd:YAG laser at 1064 nm. The resulting fluorescence was monitored at wavelengths as low as 187 nm (D(2)Sigma ? X(2)Pi transition), thus permitting major discrimination against conventional noise sources as encountered in single-photon-laser-induced fluorescence (SP-LIF) systems. Also, since the 226-nm radiation used in the TPLIF method is generated in the same manner as in the SP-LIF technique (i.e., the sum-frequency mixing of 288-with 1064-nm radiation), the two-photon sequential-pumping scheme requires no major extra optical hardware. Although the final optimization of the TP-LIF NO method has not yet been completed, extrapolations based on currently measured signal and noise levels suggest a detection sensitivity level of 3 parts in 10(13) (0.3 pptv) under atmospheric conditions. This would represent approximately a 25-fold increase in sensitivity over the SP-LIF NO technique.

Tuning, attenuating, and switching by controlled flexure of long-period fiber gratings

Azimuthal variations in the refractive index that are inherent in CO(2) -laser-induced long-period fiber gratings (LPFGs) coupled to small controlled flexure of the LPFG produce a wide variety of transmission characteristics as a function of LPFG curvature. The particularly useful cases of (1) wavelength tuning at a constant attenuation and (2) variable attenuation (switching) at a constant wavelength are demonstrated by flexing of LPFGs that have been appropriately axially rotationally oriented relative to the plane of curvature.

Morphology and electron-irradiation behaviour of poly(dimethyl silylene)

Abstract We have obtained and studied the first single crystals of the important silicon-backbone polymer, poly(dimethyl silylene) (PDMS), by dissolution in α-chloronaphthalene at 251°C and growth at 238°C. The crystals consist of molecularly folded, rhombic-shaped lamellae, analogous to those of carbon-based polymers. Electron diffraction patterns revealed a weak positional disorder in the intermolecular packing along the b crystallographic axis. The sensitivity of PDMS to electron irradiation was found to be substantially higher than that of typical polymers (e.g. polyethylene), with early structural changes seen at doses as low as 6 C m −2 . By examining changes in composite electron diffraction reflections with radiation dose, we obtained evidence for two competing mechanisms in PDMS and showed that chain scission is much more extensive than crosslinking.

Solid-state thermochromism and piezochromism in the polysilylenes

The polysilylenes exhibit complex solid-state structures and electronic properties. In this report the solid-state conformational structures of symmetrically substituted poly(di-n-alkylsilylenes) are described. Comparisons between the different structures and their respective UV absorption characteristics demonstrate that the relationship is complex and aspects of molecular geometry beyond chain conformation must be considered. The thermochromic and piezochromic behavior of several of these polysilylenes is discussed. The all-trans conformation of the silicon backbone is associated with both phenomena. The properties of several polysilylene copolymers have also been investigated. We find that some of the copolymers form a well-ordered structure and exhibit absorption characteristics similar to the crystalline phases of the corresponding homopolymers.

Comparison of structural and electronic absorption properties of poly(di-n-hexylsilane) and poly(di-n-butylsilane)

Abstract The Si-backbone in crystalline poly(di-n-hexylsilane) (PDHS) is known to be all-trans, whereas the chain conformation of poly(di-n-butylsilane) (PDBS) has been shown to form a 7/3-helix in the solid state. It is demonstrated here that all-trans PDBS can be produced by two methods: precipitation from dilute solution at low temperature, and by application of moderate pressure to the solid polymer. The absorption maximum of all-trans PDBS is red-shifted 40 nm from that of the 7/3-helical form. The effect of pressure on the solid-solid phase transitions in both PDBS and PDHS is presented.