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Liquid Polymer

Polymers can be defined as the molecules which are formed by certain repeating units. These repeating units are called as monomer units. Monomer units can be bonded with chemical bonds which are formed by either addition or condensation of monomer units. The addition of monomer units to form polymers is called as polymerization reaction.

For example the polymerization of ethane results the formation of polyethene and polymerization of vinyl chloride forms PVC or poly vinyl chloride.  The chemical and physical properties of monomer units are quite different from the polymers they form.

The condensation polymerization of monomer units occurs with the loss of some small molecules like water, HCl, CO2 etc. Polymers have several applications in today‚Äôs life.  Polymers can be in crystalline and amorphous forms.

 

Define Liquid Polymer

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Liquid crystal or liquid polymers are the intermediate form of polymers. An Austrian botanist named Friedrich Reinitzer studied a material called as cholesteryl benzoate which showed two distinct melting points. He noticed the change of crystal structure to hazy liquid by increase the temperature of solid sample. 

Later on further heating, the hazy liquid converted to transparent liquid. Reinitzer discovered a new phase of matter that is called as liquid crystal phase. Such materials show unique properties and have several new applications in modern technology.

Liquid Crystal Polymer Properties

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The liquid crystal polymers can be distinguished the arrangement of molecules to point along a common axis. Here molecules are called as mesogens and the common axis is called as director. Remember, no intrinsic order in liquid state molecules while in solid state molecules have ordered arrangement with least freedom to move. The orientation of molecules in liquid crystal state is intermediate liquid and solid phases. This intermediate state I called as mesogenic state or liquid crystal state. 

Liquid Crystal

Liquid crystals have no ordered arrangement as a solid but have some degree of alignment. We can quantify the order in a material with the help of an order parameter (S);
S = $\left ( \frac{1}{2} \right )$$< 3 cos^{2}\theta -1>$
Here,
$\theta$ = Angle between the director and the long axis,
< > = An average of all of the molecules in the sample.

Anisotropy is the tendency of the liquid crystal molecules to point along the director. In such materials the properties of a material depend on the direction in which they are measured. The liquid crystalline structure has positional order, orientational order and bond orientational order.

Liquid Crystal Polymer Applications

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Today liquid crystal polymers have wide applications in various industries.  They are precursor of the high-strength materials which are widely used in optical devices. For example; Kevlar is used in manufacturing of helmets and bullet-proof vests. The liquid crystal polymers with long straight chains are very strong and can be quenched to a highly ordered strong solid. These technologies are mainly used to develop a variety of new materials.

Side chain liquid crystal polymers are used in manufacturing of optically nonlinear devices such as optical wave guides, electro-optic modulators, optically-addressed spatial light modulators, tunable notch filters, optical amplifiers, and laser beam deflectors. They show low warpage , high heat and chemical resistance, low viscosity, high flow, flame retardant, good moldability, good dimensional stability and good adhesion property.

Liquid crystal polymers are also used in manufacturing of engineering parts, food containers, bobbins, switches, aerospace parts, automotive parts, connectors etc.

Liquid Crystal Polymer Substrate

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The semi-crystalline thermoplastic polymers become disordered at temperatures well above the crystalline melting temperature. On the contrary, the liquid crystalline polymers have molecular orders which retained in their viscous liquid state also. Therefore the liquid crystal polymer is used to describe these types of polymer. In liquid crystal polymers, the glass transition temperature indicates the co-operative movements in the amorphous phase.  The crystalline melting temperature is related with the melting of the crystalline phase. 

At temperatures higher than crystalline melting temperature, the transition from order to disorder occurs in liquid crystal polymers.  Liquid crystal polymers are unique combination of various properties which make them ideal for high density electronic substrate applications. They show good electrical properties, excellent barrier properties like glass. These substances are impermeable to moisture, oxygen and other gases.

They have low coefficient of thermal expansion as well as low moisture absorption tendency. In these substances molecules can be mutually aligned and organized like crystal and at the same time they can flow in their molten state like liquids. Due to these properties, they are randomly configured in their molten state and also in solution. When liquid crystal polymer substrate flows, the rigid segments of the molecules create locally oriented domains which in turn, create macroscopic-oriented regions.

Liquid Crystal Polymer Processing

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There are certain steps in the processing of liquid crystal polymers; Image and Etch, drill, desmear, metallization, solder mask and finish metallization. Major disadvantages of liquid crystal polymers are the formation of weak weld lines, high anisotropic properties, and requirement of drying before processing and high cost.
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