Alan J. Waddon

Structure and properties of polyurethane–silica nanocomposites

Nanocomposites with different concentrations of nanofiller were prepared by adding nanosilica filler to the single-phase polyurethane matrix. A control series was prepared with the same concentrations of micron-size silica. The nanosilica filler was amorphous, giving composites with the polyurethane that were transparent at all concentrations. The nanocomposites displayed higher strength and elongation at break but lower density, modulus, and hardness than the corresponding micron-size silica-filled polyurethanes. Although the nanosilica showed a stronger interaction with the matrix, there were no dramatic differences in the dielectric behavior between the two series of composites.

Structure–property relation in poly(p-phenylene terephthalamide) (PPTA) fibers

Abstract Structure parameters of various poly(p-phenylene terephthalamide fibers have been investigated using WAXD and correlated with mechanical properties. The mechanical properties examined were modulus E and strength σ; the pertinent structural parameters include orientation angle φ200, lattice constants a, b, c, paracrystalline parameter gII,apparent crystal sizes ACS110, ACS200, ACS00l, intensity ratio I110/I200 and transverse crystallinity X. The parameters, c, gII and I110/I200 are found to be interrelated and to provide indications of nonreversible chain conformational changes due to post-treatment. It is concluded that the fiber modulus is determined by the combination of the orientation of the crystallites and the paracrystalline parameter through the following equation: (10) 1/E f =(1/E 0 +D 1 g II 2 )+A〈 sin 2 φ〉 in which Ef is the fiber modulus; gII the paracrystalline parameter; φ the orientation angle; and E0, D1 and A the material constants. This relationship is derived from our proposed morphological model in which crystallites are: (a) formed from chains have nonlinear conformations; and (b) packed with an orientation distribution. The correlation of structure with strength has also been studied. In addition, different types of Kevlar® fibers, Kevlar® 119, Kevlar® 29, Kevlar® 49 and Kevlar® 149 show slight, systematic, variations in structure. In particular, all Kevlar® fibers except Kevlar® 149 show the forbidden 001 diffraction reflection, which has been related to conformational differences.

The evolution of structure and properties in poly(p-phenylene terephthalamide) fibers

Abstract The evolution of the mechanical properties and structure of poly( p -phenylene terephthalamide) (PPTA) fibers with different post-treatment methods involving heat, tension, hydrostatic pressure, and different environments was systematically investigated. Wide-angle X-ray diffraction measurements reveal that the crystal structure of PPTA fiber is not stable and changes upon post-treatment. The cooperative changes in the modulus and two structure parameters — the misorientation angle and the paracrystalline parameter upon treatment — indicate a direct structure–property correlation. After studying free-length annealing and heat-tensioning of fibers, several structure parameters — the c -dimension of lattice constants, the paracrystalline parameter, the intensity ratio between (110) and (200), and the orientation angle — were found to be affected greatly by the tension applied during heat stretching; while other structure parameters such as apparent crystal sizes, equatorial crystallinity and a , b dimensions of the lattice constant are insensitive to the applied tension but determined by the applied temperature and time. A sudden change in the crystal structure at 400°C suggests a α-relaxation in the crystalline region, which is supported by the DMTA and TMA measurements.

Copolymerization of hexafluoropropylene and tetrafluoroethylene: Effect on chain conformation and crystal structure

A series of new copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) containing up to 50 mol % of the hexafluoropropylene comonomer have been investigated with respect to chain conformation and crystal structure using wide-angle X-ray diffraction (WAXD). Increasing HFP content leads to significant departures from the highly ordered crystalline structure of the homopolymer PTFE; the helical conformation of the chain relaxes and untwists to accommodate the larger -CF 3 pendant group in the HFP unit. Simultaneously the lateral hexagonal packing of the helices becomes less ordered and the α-dimension of the hexagonal cell increases. The above effects are progressive with increasing HFP content. At 50 mol % HFP incorporation the structure is a disordered crystalline phase.

Structural transitions during the cold drawing of aliphatic ketone terpolymers

Abstract Cold drawing of aliphatic ketone terpolymers with CH 3 substituents is shown to promote production of the high density α crystal phase. The quantity of the minor α component decreases with increasing CH 3 content and the solid–solid α→β phase transition temperature is shown to be lower compared with the unsubstituted polymer. Draw rate affects the amount of α phase produced: low draw rate favours α phase. This is explained in terms of strain-induced heating effects: high rates increase the temperature above the T α→β transition.

Crystalline and amorphous morphologies of an aromatic polyimide formed on precipitation from solution

Abstract The solution-precipitated morphology of a new sulphonated aromatic polyimide, poly(benzophenone tetracarboxylic dianhydride 3,3′-diaminodiphenylsulphone) (BTDA 3, 3′-DDS), has been studied by transmission electron microscopy (t.e.m.) and wide-angle X-ray diffraction (WAXD). First, it has been shown that if held in solution in dimethylacetamide for an extended period, the polymer can crystallize as sheaf-like, multicrystallite aggregates that take on the appearance of axialites at a sufficiently advanced stage of growth. This morphology is very similar to that displayed by other relatively stiff polymers such as poly(ether ether ketone) (PEEK), poly(ether ketone) (PEK), and poly(phenylene sulphide). The generation of this general type of morphology is attributed to the inflexibility of the backbone, which renders folding difficult and hence leads to the production of a thick amorphous region on the lamellar surface, which then limits lateral crystal growth and creates a high density of cilia that cause nucleation of other crystals, leading to the formation of multilayered aggregates. Secondly, if the solvent is evaporated in moist air before crystallization can occur, a phase separation takes place on absorption of water vapour from the atmosphere. In this case the resulting polymer is amorphous and adopts a bead-like morphology on the scale of 300–2000 A ( 1 A = 10 −10 m ).

Phase Behavior and Properties of Polyurethane-PVC Blends and Fibers

Abstract A polyester urethane elastomer is blended with rigid poly(vinyl chloride) (PVC) and the miscibility of the components studied over the entire range of compositions. The polyurethane elastomer was a block copolymer with a low degree of crystallinity, while PVC was practically amorphous. Differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA) methods showed that polyester urethanes were partially miscible with PVC since two distinct glass transitions, which changed with the change of concentration of components, were observed. Although the PVC content was varied from 0–100%, the aim of the work was to examine if PVC at low concentrations would form fibrils in the urethane matrix and act as a reinforcing agent for the polyurethane elastomer. The morphology of the blends was studied by scanning electron microscopy and x-ray scattering. The blends were then spun into fibers to force the dispersed phase to elongate and form fibrils (draw ratio was ...

The effect of highly specific lattice hydrogen bonding on morphologies of solution-grown crystals

Morphologies of solution-grown crystals in systems containing strong and specific intralattice hydrogen bonding (nylons) are compared with systems displaying only van der Waals intracrystal attractions. The latter are shown to produce crystals that clearly mirror the underlying crystallographic symmetry; in the former systems the morphologies are much more complex and disorganized. This difference is discussed in terms of intralattice forces affecting growth rates and chain rearrangement during crystallization. Arguments deriving from differences in lamellar thickening behavior are also used to demonstrate this point.