Two-phase nanoscale morphology of polymer/LC composites
, D.W. Tomlin, M.D. Schulte, T.J. Bunning
Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL /MLPJ and AFRL/MLBP, 3005 P. St., Bldg 651, WPAFB,
OH 45433-7702, USA
Received 10 April 2000; received in revised form 1 June 2000; accepted 12 June 2000
Small-angle X-ray scattering and high-resolution scanning electron microscopy (SAXS/HRSEM) were utilized to examine the nano (1±
100 nm) and meso (100±1000 nm) scale morphology of polymer dispersed liquid crystal (PDLC) ®lms of varying liquid crystal (LC)
concentration. In contrast to the conventional PDLCs derived from photo-initiated step-growth polymerizations, these PDLC ®lms were
formed using photo-irradiation of initially homogeneous syrup comprised of highly functional free-radical monomer and liquid crystal,
resulting in rapid molecular weight increase and network formation prior to or in conjunction with phase separation. Two-phase morphology
observable with HRSEM was absent below 20% LC, although ®ne, small modulation features existed on the fracture surface. In contrast,
SAXS reveals increasing nanoscale heterogeneity with increasing LC content. The scattering behavior is consistent with a structure factor
derived from an Ornstein±Zernicke model indicating that composition ¯uctuations frozen by network formation exist at the lowest LC
concentrations. At higher LC concentrations, a discontinuous LC phase is observed which coalesce into a co-continuous polymer/LC phase
between 35 and 40% LC. Above this regime, aggregated beads of polymer form whose size and uniformity steadily increase with
concentration. These morphological observations are consistent with analysis of the SAXS data via a two component Debye±Bueche
model at low q. The nanoscale features of the PDLCs formed from highly functional free-radical monomers underscore the importance
of the polymerization mechanism in controlling the two-phase morphology in PDLCs. q 2000 Published by Elsevier Science Ltd.
Keywords: Polymer dispersed liquid crystal; Polymerization-induced phase separation; Small angle X-ray scattering
Polymer dispersed liquid crystals (PDLCs) are of techno-
logical importance in the development of switchable
windows, electro-optic shutters, displays, and most recently
switchable gratings [1,2]. The most versatile method to form
PDLC structures is photo-initiated polymerization of an
initially homogenous mixture containing reactive mono-
mers and liquid crystal molecules. These pre-polymer
syrups are typically solvent free and have low viscosity.
Large variations in the ®nal two-phase structure can be
obtained by varying either the LC composition, the intensity
of the curing radiation, or the photo-polymerization chem-
istry (free-radical or step-growth). The breadth of accessible
two-phase structure results in a broad range of electro-
Although the ®nal structure/property relationships for
PDLCs has been examined extensively, most of the empha-
sis has been with respect to the relationship between inher-
ent LC properties (elastic constants, viscosity, dielectric
anisotropy, birefringence) and the electro-optical properties
of the two-phase structure (contrast ratio, switching speeds).
However, the optical characteristics of the PDLC ®lm are
also dependent on the details of the polymer structure and
thus the nature and rate of polymerization mechanism
and the role of small molecule additives. Understanding
and controlling the polymerization mechanism is especially
important for holographic curing of PDLC ®lms (H-PDLC),
in which a spatial variation of light intensity results in a
patterned LC droplet distribution [3±5]. The spatially vary-
ing differences of the two-phase structure are key in deter-
mining the performance of these switchable diffractive
structures (transmissive or re¯ective).
Recent HPDLC developments by our group [6,7] using a
free-radical polymerization of highly functional monomers
have demonstrated the ability to sequester nanoscale LC
domains (,100 nm), thereby eliminating scattering
problems associated with larger two-phase structures result-
ing from step-growth polymerizations. For free-radical
reactions, a high molecular weight polymer is formed
almost instantaneously, while for a step-growth reaction,
Polymer 42 (2001) 1055±1065
0032-3861/01/$ - see front matter q 2000 Published by Elsevier Science Ltd.
* Corresponding author. Tel: 11-937-255-3808; fax: 11-937-255-1128.
E-mail addresses: firstname.lastname@example.org (T.J. Bunning),
email@example.com (R.A. Vaia).
Corresponding author. Tel: 11-937-255-9184; fax: 11-937-255-9157.