Mao Xu

Polyurethanes having shape memory effects

Segmented polyurethanes (PUs) were prepared from polycaprolactone diols (PCLs), 4,4′-diphenylmethane diisocyanate, and 1,4-butanediol, and tested for shape memory effects. Effects of soft segment molecular weight (Mn = 2000, 4000 and 8000), soft segment content (50–90%), and maximum strain (em = 100, 200, and 600%) on the cyclic tensile properties as well as the dynamic mechanical, and mechanical properties below (25°C) and above (65°C) the shape recovery temperatures were studied. With increasing soft segment contents: i) glassy state modulus increased and rubbery state modulus decreased; ii) hardness increased at room temperature, and decreased at 65°C; iii) recovery strain decreased with PCL 2000, and increased with PCL 8000 based PUs. On the other hand, the increase in soft segment length resulted in: i) increased rubbery state modulus as well as glass state modulus; ii) increased hardness at room and high temperatures; iii) increased recovery strain at high soft segment content. Tensile yielding became clear with increasing soft segment length and content. Strain upon cooling and unloading (eu) and residual strain (ep) increased, and recovery strain (er) decreased with cycling. Among these, residual strain was most sensitive to the cycling. Most of the cycling effects were confined during the first one or two cycles. These results were interpreted in terms of soft segment-hard segment phase separation and soft segment crystallization.

Studies on thermally stimulated shape memory effect of segmented polyurethanes

The shape memory behavior of a series of polycaprolactone/diphenylmethane diisocyanate/butanediol (PCL/MDI/BDO) segmented polyurethanes of different composition was studied. The molecular weight of PCL diols was in the range of 1600-7000, and the hard segment content varied from 7.8 to 27% by weight. Film specimens for shape memory measurements were prepared by drawing at temperatures above the melting temperature of the soft segment crystals and subsequent quick cooling to room temperature under constrained conditions. The shape memory process was observed and recorded in a heating process. Parameters describing the recovery temperature, ability, and speed were used to study the influence of structure and processing conditions on the shape memory behavior of the sample. It was found that the high crystallinity of the soft segment regions at room temperature and the formation of stable hard segment domains acting as physical crosslinks in the temperature range above the melting temperature of the soft segment crystals are the two necessary conditions for a segmented copolymer with shape memory behavior. The response temperature of shape memory is dependent on the melting temperature of the soft segment crystals. The final recovery rate and the recovery speed are mainly related to the stability of the hard segment domains under stretching and are dependent on the hard segment content of the copolymers.

Substituted polyaniline-polypropylene film composites: preparation and properties

Abstract Electrically conductive polymer composites containing polyaniline and substituted polyanilines were prepared by one-step chemical oxidation polymerization. Substituted anilines used were o-toluidine and o-chloroaniline. The matrix polymer was a polypropylene (PP) microporous film which was prepared by a biaxial stretching technique. The structure of the conducting polymers in composites was characterized by IR, UV-Vis and X-ray photoelectron spectra. The conductivity of these three kinds of composites was decided mainly by the conductivity of the corresponding conducting polymers, and decreased in the following order: polyaniline/polypropylene, poly(o-toluidine)/polypropylene and poly(o-chloroaniline)/polypropylene. Their conductivities were in the same order of magnitude as those of the corresponding pure polyaniline family of polymers, which were synthesized by the conventional polymerization method of chemical oxidation. The conductivity of the composite was influenced by the reaction conditions, including the reaction temperature, concentrations of monomer and oxidant, reaction time, etc. Different surface morphology was observed on the two sides of the composites. The mechanical properties of the composites were quite similar to those of the PP microporous film. Their elongation at break was much larger than that of the pure polyaniline.

A novel polypropylene microporous film

The structure and properties of a polypropylene microporous film prepared by biaxial stretching of non-porous polypropylene film of high β-crystal content were investigated. The porosity of these films can be as high as 30–40%, and the average pore size was around 0.05 μm. The films were found to have the structure of a two-phase interpenetrating network; both the polypropylene and the pore regions were three-dimensionally continuous. The advantages of the biaxially stretched microporous films are the combination of high permeability to fluids with good mechanical properties and almost circular pore shape with narrow pore size distribution. The application of this microporous film for battery separators, filtration membranes and substrates of functional polymer composites is discussed.

Morphological study of oriented films obtained from side-chain liquid crystalline polymers

Abstract The morphological features of side-on fixed liquid crystalline polymers in an oriented state are investigated using polarizing microscopy, small angle light scattering (SALS) and infra-red dichroism techniques. A conventional side-chain polymer with mesogenic units attached longitudinally to the backbone is also studied for comparison. Banded texture usually only reported for main-chain liquid crystalline polymers is observed for these side-on fixed polymers under the crossed polarizing microscope, and SALS measurements also show typical grating diffraction patterns. This implies that side-on fixed rigid mesogens may significantly affect the conformation of the main chain of molecules, and the molecule as a whole may become rather rigid, exhibiting behaviour similar to that of the main chain. Infra-red dichroism measurements indicate that the main-chain axes of the molecules tend to orient in directions close to the shear direction. The side-on fixed mesogens are packed around the backbone, but their axes are not parallel to the latter.

Polypyrrole—polypropylene composite films: preparation and properties

Abstract Polypyrrole—polypropylene conducting polymer composite films with good mechanical properties and high electrical conductivity were prepared using a two-direction permeation/diffusion polymerization method. The oxidant used was iron( III ) chloride. Polypropylene microporous films prepared by the biaxial stretching technique were used as the matrix material. The maximum conductivity of these composite films, 6.0 S cm−1, is of the same order of magnitude as that of pure polypyrrole, 4.5 S cm−1, which was synthesized by chemical oxidation of pyrrole monomer in aqueous solution. The composite films exhibit quite uniform mechanical properties in the film plane. Youngs modulus is in the range 0.4–0.8 GPa, tensile strength 40–85 MPa and elongation at break 90–200%, maintaining the mechanical properties of polypropylene in the composite. Polymerization conditions, including reaction time, reaction temperature and concentrations of the pyrrole and FeCl3 solutions, have a great influence on the structure and properties of the composite films. The structure of the conducting polymer in the composite films was characterized by X-ray photoelectron spectroscopy and the surface morphology of the films was investigated using scanning electron microscopy. Two kinds of surface structure were observed, one is a loose particle structure and the other is a compacted surface structure.

Kinetics of phase transitions for fractions of a main-chain liquid-crystalline polyester

Abstract The kinetics of crystallization from the mesophase and liquid crystallization from an isotropic melt of fractions of an aromatic thermotropic polyester were investigated by differential scanning calorimetry and depolarizing transmittance measurements. The isothermal process of these transitions can be described by the Avrami equation. The values of the exponent n are found to be ∼4 and ∼1 for crystallization and liquid-crystallization transitions, respectively. The temperature dependence of the crystallization rate from the mesophase is similar to that for small-molecule liquid crystals and also to that for conventional polymers obtained from an isotropic melt. Compared to crystallization, the liquid-crystallization rate shows a very weak temperature dependence. This implies that the surface free energy of mesophase domains is much less than that of crystallites, which may explain the experimental fact that liquid crystallization from an isotropic melt can proceed with high transformation rate at very small undercoolings, whereas crystallization only proceeds at very large undercoolings.

Novel interpenetrating polymer networks of polypropylene/poly(n-butyl acrylate)

Abstract Novel interpenetrating polymer networks of polypropylene/poly( n -butyl acrylate) have been prepared via in-situ polymerization within a biaxially stretched microporous polypropylene film. The volume fraction of poly( n -butyl acrylate) in composite films was higher than 40%. The fine-dispersed anisotropic dual-phase continuous interpenetrating network structure was confirmed by surface analysis and transmission electron microscope observation. The interfacial action between polypropylene and poly( n -butyl acrylate) was relatively clear. The elastic moduli of the composite films were lower than that of the matrix film. The swelling of poly( n -butyl acrylate) in organic solvent was strikingly limited by the polypropylene network. All these properties were in agreement with the anisotropic interpenetrating network structure.

The peculiarity of side-chain liquid crystalline polymers with mesogenic units attached directly in a side-on mode

The peculiarity of a series of side-chain liquid crystalline polymers with mesogenic units attached directly in a side-on mode was studied by using nuclear magnetic resonance, wide-angle X-ray diffraction, polarizing microscopy and molecular simulation techniques. It was found that in special cases molecules of side-chain liquid crystalline polymers may become rather rigid as a whole and take the more extended conformation, like a rod with the main chain approximately parallel to the rod axis. The side-chain mesogens are packed around the main chain with their axes almost perpendicular to the main chain. These side-chain polymers may exhibit behaviour similar to that of main-chain polymers. The effect of interaction between main-chain and side-chain mesogens on the mesophase formation of side-chain polymers and the influence of mesogen location in molecules on the behaviour of liquid crystalline polymers are discussed.

Morphological changes and SALS of thermotropic polyesters during isotropic and liquid crystallization transitions

Abstract The morphology of two thermotropic polyesters with mesogenic units and flexible spacers along their backbones during isotropic and liquid crystallization transitions was studied by means of polarizing microscopy and small angle light scattering (SALS). These transition processes were found to be composed of the initiation of the new phase at local places of the old phase matrix and growth of the new phase domains. A change of Hv scattering patterns was observed in the transition temperature ranges and is explained by models proposed in previous papers as the result of heterogeneity of the mesophase systems.