(217el) Mesoscale Ordering and Mechanical Response in Polydomain Smectic Networks | AIChE

(217el) Mesoscale Ordering and Mechanical Response in Polydomain Smectic Networks


Yu, Z. - Presenter, Texas Tech University
Hedden, R. C., Texas Tech University

Smectic Main-Chain Liquid Crystalline Networks
(S-MCLCN) are semi-flexible polymer networks with main-chain mesogenic units.   These
materials behave like normal isotropic rubbers at temperature above their
clearing temperatures (Ti), while showing strong deviations from ordinary
rubber-like mechanical behavior below Ti due to nanometer-scale
segmental layering.  Properties
like shape-memory, soft actuation, and energy-dissipative characteristics have
attracted much attention in recent years. The anomalous mechanical behavior of
S-MCLCN is partially attributed to the large internal energy cost for deformation
of smectic microdomains, which promotes mechanical instability during

factors affecting mechnical response of S-MCLCN have been considered in this
study. First, the effects of thermal history on the domain thickness were characterized
to elucidate the reasons for stiffening of the material during aging or annealing.
Second, the effects of crosslink density on domain size were examined to
explain an unexpected trend in the Young's modulus at low strains.

X-ray lineshape analysis
was applied to quantify domain size after different thermal treatments and identify
morphological factors relating to mechanical behavior.  A significant growth in domain thickness with
increased annealing time was observed, similar to the thickening of crystal
lamellae in semicrystalline polymers during annealing.  Because
there is a greater internal energy penalty for deforming larger, more stable
domains, the material stiffens and mechanical instability becomes more
pronounced as annealing continues. 

The effects of crosslink
concentration on the modulus can be understood in terms of a competition
between domain size effects and elastic chain concentration effects.  Surprisingly,
under certain conditions, the Young's modulus can actually decrease as
crosslinker concentration increases because the crosslinkers decrease the
average thickness of smectic microdomains.   As
crosslink density increases from zero to some small value, smaller domains are
at first produced because crosslinkers cannot fit into the smectic lattice,
generating defects in the layering.  Thus,
the internal energy penalty for deformation of the domains decreases, and the
material softens.  However, as the concentration of crosslinker
molecules increases further, the increase in the number of elastically
effective chains outweighs the effects of domains size, and the material
stiffens again.

In addition, a recent study
of flexible or semiflexible
guest molecules in S-MCLCN will be described in this work. Two kinds of guest
molecules have been synthesized.  Low
molar mass liquid crystals having nearly the same structure as the repeat unit
of the S-MCLCN were used to decrease the glass transition temperature of a
MCLCN without destroying smectic ordering. 
These guest molecules act as plasticizers that can both lower the Tg
of the material and hinder the formation of higher order mesophases, both of
which are desirable changes.  A second class
of guest molecules are modified chromophores with flexible oligosiloxane tails.
The orientational response of the modified dichroic dyes during elongation of
the host network reveals how host-guest coupling affects the mechanical
response of plasticized S-MCLCN.