Introduction to Polymer Science and Technology

Thermal properties

The individual graphs of E', E" and tan 5 have not been labelled on the figure for brevity, but should be easily identified from their customary shapes (note that the tan 5-peak always occurs at a higher temperature than the E"-peak). All the polymers show low temperature transitions, and a T at approximately 200 °C (above 200 °C for PES). Clearly the desirable property of the high temperature T has been maintained in the blend, the reduction in its E' value in the glassy region compared with the epoxy resin is small and in composite applications this is insignificant since the strength and stiffness in composites come from the fibre reinforcement. Therefore, blending with PES, improves the toughness of the epoxy resin (see Figure 1.8) without compromising its desirable mechanical and thermal properties.

7.4.1.7 Effect of plasticisers and moisture

Plasticisers are added to polymers to render them soft and flexible and/or to aid their processability, by lowering T and to a lesser extent T_m. This is exemplified for plasticised PVC in Figure 7.27: where the addition of plasticiser diethylhexyl succinate lowers T significantly. The figure also shows that, similar to polymer blending, the plasticisation also broadens the transition region. The broadening is also observed in the storage modulus curves which resemble in shape the ones shown for the PU/PMMA blends, see Figure 7.25. The maxima in logarithmic decrement (a damping term) peaks significantly reduce in height, as well as broaden, with increasing plasticisation; however, around 50 % plasticiser content the damping peaks become narrower and start to increase in peak height. This is probably due to overlapping of the reduced-T ( a-transition) with the P-transition for PVC (around -75 °C) and resulting in a combined effect.

-150 -100 -50 0 50 100 150

temperature, °C

Figure 7.27Logarithmic decrement at 1 Hz vs. temperature for PVC plasticised with diethylhexyl succinate at concentrations (by volume) indicated on the curves (source: Schmieder & Wolf 1952)

In general, the glass-transition broadening is greater with the poor plasticisers than the good ones for the polymer, therefore in PVC, a poor plasticiser such as dioctyl phthalate produces a broader and lower damping-peak height compared with a good plasticiser such as diethyl phalate. Moisture in polymers causes similar effects to that of plasticisers. Some polymers such as polyamides are hygroscopic and can absorb large amounts of water, which alter their physical and mechanical properties, including dynamic mechanical thermal properties. This is described for PA 66 in one of ТА Instruments' application brief. They have conducted DMA in resonance mode with 0.2 mm oscillation amplitude while heating at 5 °C/min over -150 to 150 °C temperature range, using rectangular specimens of 3 mm x 13 mm cross-section with clamps

Introduction to Polymer Science and Technology

Thermal properties

separation of 19 mm. The specimens were conditioned at 0 %, 50 % and 100 % relative humidity (RH). Although the report does not mention the actual moisture uptake for the specimens, however in open literature, moisture contents (mass fraction) of approximately 0.2 % for dry as-moulded, and the saturation/equilibrium moisture level of 8.5 % at 100 % RH and 2.5 % at 50 % RH are quoted for PA 66.

Figure 7.28 shows the plots of E' and tan 5 against temperature for three specimens with different levels of moisture absorption. The tan 5 traces exhibit three peak maxima: a-transition (glass transition) due to long chain segmental motion within the main polymer chain, P-transition is attributed to localised motion within the amide segments, and y-transition is attributed to localised segmental motion of the (-CH₂-) methylene groups between amide groups in the amorphous regions. The main variation, as would be expected, is observed in T : a shift of around 60 °C with increasing moisture content from the "dry as-moulded" to conditioned at 100 % RH. The depression in T is a consequence of the disruption by water of the intermolecular hydrogen bonding. It is also worth noting that the storage modulus in the glassy region below 0 °C increases with moisture content, which has been associated with the crystallization of occluded water (Birkinshaw & Buggy 1987 and Baschek 1999).

Figure 7.28DMTA properties vs. temperature for PA66 with different moisture content:
(--- . --- ) dry; (----) conditioned at 50% RH; ( ) conditioned at 100% RH (source:ТА lnstruments-2)

Dielectric thermal analysis(DETA), or Dielectric Analysis (DEA), is a technique similar to DMTA: dielectric measurements are the electrical analogue of dynamic mechanical measurements, where the mechanical force (stress) is replaced by an alternating voltage across the sample (ax. electrical field) and the alternating strain becomes the stored charge (Q) or the a.c. current in the sample. Q is always measured as its derivative, dQ/dt = I. One can visualise the polymer specimen as being represented by a parallel combination of a resistor (conductor) and a capacitor, akin to the mechanical Kelvin-Voigt model.

Introduction to Polymer Science and Technology Thermal properties

The great advantage of DEA is that it can be employed in situ process monitoring as well as in laboratory testing. In a typical test, the sample is placed in between two metal electrodes (the dielectric sensor) and a sinusoidal voltage (the excitation) is applied to one of the electrodes. The resulting sinusoidal current (the response) is measured at the second electrode. The current and its phase difference with respect to the applied voltage is measured, and from this data the dielectric properties of г (the dielectric constant or the permittivity) and e" (loss factor) are calculated. Another commonly used term for expressing dielectric response is the dissipation factor or the loss tangent, tan5 = e" / s!

Its application in monitoring the curing behaviour of thermosetting resin systems, composite materials, adhesives and paints has been standardised in ASTM E 2038 or E 2039:

ASTM E2038-99 Standard test method for temperature calibration of dielectric analyzers,

ASTM E2039-99 Standard test method for determining and reporting dynamic dielectric properties.

The conventional thermal analysis techniques are suitable for measuring the bulk properties of materials, but for localised characterisation of materials micro and nano-thermal analysis techniques are used. These employ a displacement thermal probe attached to an atomic force microscope (AFM) and measure the probe cantilever deflection (whilst in contact with the sample surface) against probe tip temperature. The instrument is basically an AFM fitted with a tip that acts as heat source and temperature sensor. Material transitions that result in the softening of the material beneath the tip produce a downward deflection of the cantilever, similar in concept to the well established technique of thermo-mechanical analysis (TMA). Localised Thermal Analysis (LTA) can be used, for instance, to explore morphology changes across the surface of a material by measuring the localised transition temperatures. Further information on the technique can be obtained from, for example, www.anasysinstruments.com , www.tainstruments.com or Ehrenstein 2004, рЗОО.

Date: 2015-12-11; view: 927