Question

1. compare and contrast solubility of polymer vs. solubility parameters.

2. thermal transition, Tg and Tm - what is happening to the polymer?

Answer 1

1. For closer values of the solubility parameters of solute and solvent , the solubility of the solute in the given solvent increases.

a) For polymers, there are several rules of thumb for selecting suitable solvents:

i. Hildebrand solubility parameters are used if the polymer (p) and the solvent (s) have similar polar and hydrogen bonding parameter.

ii.Hansen solubility parameters are used when an approximate spherical "volume" of solubility with radius R can be drawn up for each solute. Only solvents that have Hansen solubility parameters within this volume are likely to dissolve the polymer.

b) The higher the molecular weight of a polymer, the closer the solubility parameter of the solvent and polymer need to be to dissolve the polymer in the solvent.

c) For linear and branched polymers, a plot of solubility versus solubility parameter for a range of solvents will peak when the (Hansen / Hildebrand) solubility parameters of the solute and solvent match.

But for a cross-linked polymer, the swell volume, i.e. the solvent uptake, will peak when the solubility parameters of the solvent match those of the polymer.

d) Generally, the solubility parameters of polymers do not change much with temperature, whereas those of low molecular weight compounds often decrease noticeably with increasing temperature.

2. Tm stands for melt temperature. Amorphous polymers have Tg (temperature range where molecules start to soften from glassy state) and semi-crystalline polymers can have both Tg and Tm (temperature where molecules start to melt). Both of these characteristics are due to the polymer chain structure.

A polymer is flexible at room temperature if its Tg is below room temperature and rigid at room temperature if its Tg is above room temperature.

e.g. Polycarbonate (PC) is used below its Tg of 150°C which is well above room temperature. Due to its unique chemical bond, PC is both a stiff and tough polymer providing great impact resistance below its Tg. If PC is exposed to above 150°C, it will become flexible due to polymer chain mobility.

Whereas, Polyisoprene or natural rubber is used above its Tg of -75°C which is well below room temperature. If natural rubber is exposed to less than -75°C, it will be in the glassy state.