D. J. Hourston

Polymer/layered clay nanocomposites: 2 polyurethane nanocomposites

A kind of novel polyurethane/Na+-montmorillonite nanocomposites has been synthesised using modified 4,4′-di-phenymethylate diisocyanate (M-MDI), modified polyether polyol (MPP) and Na+-montmorillonite (layered clay). Here, MPP was used as a swelling agent to treat the layered clay. Experimental results indicated that with increasing the amount of layered clay, the strength and strain-at-break increased. The storage modulus below the glass transition temperature of the soft segments in the polyurethane was increased by more than 350%. With increased loading of layered clay, the thermal conductivity decreased slightly rather than increased. This finding will provide valuable information for polyurethane industry.

The toughening of epoxy resins with thermoplastics: 1. Trifunctional epoxy resin-polyetherimide blends

A series of blends has been prepared by adding a polyetherimide, in varying proportions, to a trifunctional epoxy resin, triglycidylparaaminophenol, cured with 4,4′-diaminodiphenylsulphone. All the materials showed two-phase morphology when characterized by dynamic mechanical thermal analysis and scanning electron microscopy. Addition of the thermoplastic resulted in improved fracture properties (K1C and G1C), as measured by three-point bending experiments, although no obvious correlation with blend morphology was observed.

Localized thermal analysis using a miniaturized resistive probe

We describe a novel thermal characterization technique based on a differential arrangement, which achieves spatially localized calorimetric analysis. It involves the use of an active probe which acts both as a highly localized heat source and a thermometer. This ability opens the way for the implementation of scanning calorimetric microscopy where image contrast will be created from thermal analysis data. For a number of polymers we have recorded events such as glass transitions, meltings, recrystallizations and thermal decomposition within volumes of material estimated at a few μm3. The data obtained are compared with those obtained from conventional calorimetry and the events registered in both cases are found to match. For a full quantitative analysis of the results obtained, mathematical modelling of the operation of the technique, taking account of physical and other changes in materials, is required.

Scanning thermal microscopy: Subsurface imaging, thermal mapping of polymer blends, and localized calorimetry

We have used a platinum/10% rhodium resistance thermal probe to image variations in thermal conductivity or diffusivity at micron resolution and to perform localized calorimetry. The probe is used as an active device that acts both as a highly localized heat source and detector; by generating and detecting evanescent temperature waves, we may control the maximum depth of sample that is imaged. Earlier work has shown that subsurface images of metal particles buried in a polymer matrix are consistent with computer simulations of heat flows and temperature profiles, predicting that a 1 μm radius probe in air will give a lateral resolution of ∼200 nm near the surface, with a depth detection of a few μm. We have a special interest in polymer blends, and we present zero‐frequency mode and temperature‐modulation mode thermal images of some immiscible blends in which the image contrast arises from differences in thermal conductivity/diffusivity between single polymer domains. The behavior of domains is observed i...

Modulated differential scanning calorimetry: 6. Thermal characterization of multicomponent polymers and interfaces

A quantitative thermal method of determining the weight fraction of interface and the extent of phase separation in polymer materials is described. It is based on the differential of heat capacity signal from modulated-temperature differential scanning calorimetry. By measurement of the increment of heat capacity at the glass transition temperature, the total interface content can be determined. The method assumes that the interface and the rest of the system can be modelled as a series of discrete fractions each with its own glass transition temperature. Several examples, including block copolymers, block copolymers blended with homopolymer, and two-phase and four-phase systems are given to illustrate the range of the method. The calculated results were close to the experimental data for two-phase and four-phase systems.

Modulated differential scanning calorimetry: 1. A study of the glass transition behaviour of blends of poly(methyl methacrylate) and poly(styrene-co-acrylonitrile)

Abstract The glass transition and the effect of specific interactions on this transition process in a binary polymer blend of poly(methyl methacrylate) (PMMA) and poly(styrene- co -acrylonitrile) (SAN), which have very similar glass transition temperatures, have been investigated by means of modulated differential scanning calorimetry. The blends investigated were miscible blends of PMMA and SAN and a physical mixture of the two constituent polymers. Using the differential of heat capacity versus temperature signal, the technique has been shown to be able to resolve the two glass transitions so long as their difference is not less than about 5°C. The value of heat capacity of the miscible blend does not satisfy simple linear addition of the heat capacities of the two components polymers over the glass transition region. It is believed that this enhancement of heat capacity in the glass transition region results from specific interactions between segments of the two polymers.

Mechanical and thermal characterization of native Brazilian Coir fiber

Coir fiber native to the Brazilian northeast coast has been characterized by mechanical, thermal, and microscopy techniques. The tensile strength, initial modulus, and elongation at break were evaluated for untreated and alkaline-treated fibers. The results showed an enhancement of mechanical properties after 48-h soaking in 5 wt % NaOH. The thermal stability slightly decreased after this alkaline treatment. A thermal event was observed between 28 and 38°C. The heat capacity, Cp, as a function of temperature curves between −70 and 150°C, were obtained for the untreated and alkaline-treated coir fibers. The morphologies of the coir-fiber surfaces and cross sections were observed by scanning electron microscopy. The properties and the morphologies were discussed, comparing the native Brazilian coir fiber with the more extensively studied native Indian coir fiber.

Sub-surface imaging by scanning thermal microscopy

Scanning probe thermal microscopy has been used to achieve sub-surface imaging of metallic particles embedded in a polymer matrix, using a probe which can act as both ohmic heater and thermometer. A lateral resolution of the order of a micron and a depth detection of a few microns were achieved. Together with the description of the technique and the experimental results obtained, a basic theoretical framework is presented which describes heat flow and temperature distributions within a sample consisting of inclusions buried within a bulk material. Computer models have been developed to give theoretical heat flows and temperature profiles: these are compared here with the experimental data. The theoretical lateral resolution was found to be in good agreement with the experimental observation. We show that theoretical modelling can be used to calibrate the instrument for specific investigations. For example, the technique could be used quantitatively to determine and map thermal conductivity variations across heterogeneous samples, or to determine the depth at which inclusions are located in the case where the thermal conductivities of both the inclusions and the enclosing material are known as well as the geometry of the inclusions.

Poly(ether urethane)/poly(ethyl methacrylate) interpenetrating polymer networks: Morphology, phase continuity and mechanical properties as a function of composition

The composition range of polyurethane (PUR)/poly(ethyl methacrylate) (PEMA) interpenetrating polymer networks was investigated with respect to morphology and phase continuity using mechanical and dynamic mechanical methods and transmission electron microscopy (TEM). Dynamic mechanical data revealed one main tanδ transition with a shoulder for the intermediate compositions from 70:30 to 40:60 indicating a semi-miscible system. For the remaining compositions only one peak, indicating a higher degree of miscibility was observed. The storage and elastic moduli were related to the Davies, Kerner and Budiansky modulus-composition models. The Budiansky modulus-composition model, which indicates phase inversion at the mid-range composition, resulted in the best fit. However, it was found that the shape of the modulus versus composition curves was strongly temperature-dependent. In previous studies, not much attention had been given to the temperature at which the modulus-composition studies were conducted. Tensile testing revealed a strong synergistic effect at the 70:30 PUR/PEMA composition with maxima occurring at this composition for both the elongation at break and the toughness index. The tensile strength increased in a three-step regime corroborating the dynamic mechanical thermal analysis results. TEM micrographs confirmed a co-continuous system at the 70:30 to 40:60 PUR/PEMA mid-range compositions.

Highly localized thermal, mechanical, and spectroscopic characterization of polymers using miniaturized thermal probes

In this article, we demonstrate the versatility of use of cantilever-type resistive thermal probes. The probes used are of two kinds, Wollaston wire probes and batch-microfabricated probes. Both types of probe can be operated in two modes: a passive mode of operation whereby the probe acts as a temperature sensor, and an active mode whereby the probe acts also as a highly localized heat source. We present data that demonstrate the characterization of some composite polymeric samples. In particular, the combination of scanning thermal microscopy with localized thermomechanometry (or localized thermomechanical analysis, L-TMA) shows promise. Comparison with data from conventional bulk differential scanning calorimetry shows that inhomogeneities within materials that cannot be detected using conventional bulk thermal methods are revealed by L-TMA. We also describe a new mode of thermal imaging, scanning thermal expansion microscopy. Finally, we outline progress towards the development of localized Fourier tr...