Dynamic Mechanical Analysis (DMA) can be used to study the effects of blending two polymers together or adding plasticisers to polymeric systems.
The diagram above shows idealised DMA curves of two homopolymers A and B. When they are blended together a completely miscible system results and the glass transition of the resulting system is situated between the Tg's of the parent materials in proportion to the amount of each phase present in the blend. The position of the Tg can be predicted from the Fox equation:
1/Tgblend = WA/TgA + WB/TgB
Where WA & WB are the weight fraction of each polymer who's glass transition temperatures are TgA TgB respectively.
Often the two polymers are not miscible and the resulting blend exhibits two glass-rubber transitions corresponding to those of the parent polymers. Intermediate cases exist where the polymers are partially miscible so that two Tg's are still observed but they are moved towards that of the ideally miscible blend. Of particular interest are interpenetrating polymer networks which have very broad glass transtions and have good damping properties over a broad range of temperatures (and - due to the principle of time-temperature superposition - frequencies). Such polymers make good shock and sound absorbing materials. A less sophisitcated example is presented below for chewing gum, which is a blend of different materials which is "engineered" to have a broad peak in its damping properties around body temperature at a mechanical frequency of 1 Hz or more.
The position of the glass transition of a polymer can be influenced by adding low molecular weight materials which reduce intermolecular forces and essentially "lubricate" the macromolecular chains. This has the effect of reducting the glass rubber transition temperature as shown below for the DMA curves of an acylic fibre in air, water and baths of water plus an additive used to increase the uptake of dye during fabric manufacture.
It can be shown that the increase in dye uptake is directly proportional to the drop in Tg brought about by the additive. The reverse effect can occur during landering of clothes when too hot a water temperature causes colours to run out of fabrics. Note how the plasticisation also causes a broadening of the Tg as well as a reduction in temperature.
In certain circumstances, the low molecular weight compounds can suppress secondary relaxation processes leading to an apparent increase in stiffness over a limited temperature range. This is illustrated above for cellulose acetate exposed to water vapour. Moisture blocks the rotation of the acetate groups which occurs around 120°C and leads to an increase in dynamic modulus between this temperature and the Tg (which is reduced by the presence of water). Unintentional exposure of polymers to potential solvents and plasticisers can seriously compromise the mechanical integrity of fabricated components such as crash helmits and plastic windows.