356
S. Saito
development of materials with high self-interacting property led to the realization of
sufficient adhesive strength for temporary fixation, because the adhesion force on the
glass substrates was stronger than the cohesive force of the molecular aggregation.
The reason of the high adhesion force between the light-melt adhesive and the glass
substrate is not yet elucidated.
The high cohesive force is derived from the V-shaped molecular structure that is
easy to stack. This V-shaped molecular skeleton interacts strongly with each other to
form an ordered structure with columnar π stacking (Fig. 20.8c). By taking advantage
of the stacking nature of this molecular framework, we have developed a columnar
liquid crystal material with high cohesive force. Generally speaking, liquid crystal is
thought of as a material with high fluidity that is used in displays, but the developed
columnar liquid crystal material has low fluidity due to strong intermolecular interactions, and the two glass plates are firmly fixed. In addition, heat-resistant adhesion
has been achieved by adjusting the temperature indicating the columnar liquid crystal
phase to a high temperature range by molecular design.
Some polymer materials have been reported to lose adhesion by various mechanisms when exposed to light. This includes dicing tape that, when exposed to ultraviolet light, molecules are polymerized like a network and the material is cured
to induce peeling [20]. On the other hand, liquid crystal materials composed of
small molecules are known to show a phenomenon in which the ordered structure
of molecules is spontaneously collapsed and liquefied by mixing a few impurities
with different molecular shapes. In particular, by using liquid crystal materials based
on molecules that change shape when exposed to light such as azobenzene derivatives, molecules that have different shapes (impurities) can be made inside the liquid
crystal by light irradiation, and materials that become liquid by light can be created
[21, 22]. The functional properties of these functional liquid crystals that change the
phase of substances with light have attracted attention for use in memory materials
that record information with light. On the other hand, due to the soft nature of liquid
crystals, development as an adhesive material has not received much attention until
recently [23, 24].
The light-melt adhesive has a columnar liquid crystal structure in which V-shaped
molecules are stacked, maintaining high cohesive force (Fig. 20.9a). When ultraviolet
light is applied in the temperature range (70–135 °C) with the liquid crystal phase,
the V-shaped molecule changes its conformation to a planar form in the lowest
singlet excited state, and then those that become close to the next molecule dimerize
(Fig. 20.9b). Molecular shape of the dimers generated in this way are unsuitable
for ordered stacking, so they act as impurities and destroy the V-shaped molecular
packing structure (Fig. 20.9c). As a result, the columnar liquid crystal with strong
adhesive strength collapses, and the adhesive strength of the fluid mixture is greatly
reduced.
To use a light-melting material as a temporary fixing adhesive in manufacturing
processes, it must be removed immediately using a general light irradiation device.
For that purpose, it is desirable that photomelting occurs with small energy of light.
When light-melt adhesive is sandwiched between two glass plates and irradiated with
S. Saito
development of materials with high self-interacting property led to the realization of
sufficient adhesive strength for temporary fixation, because the adhesion force on the
glass substrates was stronger than the cohesive force of the molecular aggregation.
The reason of the high adhesion force between the light-melt adhesive and the glass
substrate is not yet elucidated.
The high cohesive force is derived from the V-shaped molecular structure that is
easy to stack. This V-shaped molecular skeleton interacts strongly with each other to
form an ordered structure with columnar π stacking (Fig. 20.8c). By taking advantage
of the stacking nature of this molecular framework, we have developed a columnar
liquid crystal material with high cohesive force. Generally speaking, liquid crystal is
thought of as a material with high fluidity that is used in displays, but the developed
columnar liquid crystal material has low fluidity due to strong intermolecular interactions, and the two glass plates are firmly fixed. In addition, heat-resistant adhesion
has been achieved by adjusting the temperature indicating the columnar liquid crystal
phase to a high temperature range by molecular design.
Some polymer materials have been reported to lose adhesion by various mechanisms when exposed to light. This includes dicing tape that, when exposed to ultraviolet light, molecules are polymerized like a network and the material is cured
to induce peeling [20]. On the other hand, liquid crystal materials composed of
small molecules are known to show a phenomenon in which the ordered structure
of molecules is spontaneously collapsed and liquefied by mixing a few impurities
with different molecular shapes. In particular, by using liquid crystal materials based
on molecules that change shape when exposed to light such as azobenzene derivatives, molecules that have different shapes (impurities) can be made inside the liquid
crystal by light irradiation, and materials that become liquid by light can be created
[21, 22]. The functional properties of these functional liquid crystals that change the
phase of substances with light have attracted attention for use in memory materials
that record information with light. On the other hand, due to the soft nature of liquid
crystals, development as an adhesive material has not received much attention until
recently [23, 24].
The light-melt adhesive has a columnar liquid crystal structure in which V-shaped
molecules are stacked, maintaining high cohesive force (Fig. 20.9a). When ultraviolet
light is applied in the temperature range (70–135 °C) with the liquid crystal phase,
the V-shaped molecule changes its conformation to a planar form in the lowest
singlet excited state, and then those that become close to the next molecule dimerize
(Fig. 20.9b). Molecular shape of the dimers generated in this way are unsuitable
for ordered stacking, so they act as impurities and destroy the V-shaped molecular
packing structure (Fig. 20.9c). As a result, the columnar liquid crystal with strong
adhesive strength collapses, and the adhesive strength of the fluid mixture is greatly
reduced.
To use a light-melting material as a temporary fixing adhesive in manufacturing
processes, it must be removed immediately using a general light irradiation device.
For that purpose, it is desirable that photomelting occurs with small energy of light.
When light-melt adhesive is sandwiched between two glass plates and irradiated with