C. Serbutoviez et al., POLYMERIZATION-INDUCED PHASE-SEPARATION .3. MORPHOLOGIES AND CONTRASTRATIOS OF POLYMER-DISPERSED LIQUID-CRYSTALS, Liquid crystals, 22(2), 1997, pp. 145-156
Polymerization induced phase separation in mixtures of liquid crystals
(LCs) and acrylates (Merck TL205/PN393) proceeds by liquid-gel demixi
ng, in most cases of practical interest. At high LC content or low tem
perature of polymerization liquid-liquid separation cannot be excluded
. Depending on the elasticity and homogeneity of the polymer network a
t the onset of phase separation, spherical or non-spherical LC domains
are observed; non-spherical domains reflect an inhomogeneous gel stru
cture. The change from spherical to non-spherical occurs in a very nar
row range of LC concentrations and curing temperatures. The transition
between these two morphologies can be explained using conversion phas
e diagrams obtained from the Flory-Huggins-Dusek theory. The contrast
ratio of PDLCs made from the Merck mixture passes through a maximum wh
en the droplet shape at the onset of phase separation changes from sph
erical to non-spherical. Lowering the LC content or increasing the tem
perature leads to smaller LC domains which scatter less efficiently. T
he reverse changes lead to early phase separation and large LC domains
which also scatter inefficiently. It is speculated that the maximum o
f the contrast ratio is related to secondary phase separation, leading
to subdomains of an appropriate size.