V- 102: Extruder feed vessel; S-103: Extruder/Pelletizer
9. Liquid crystal polyesters
Liquid crystal or mesomorphic polyesters are polymers that, in the quies- cent state and over characteristic temperature or concentration intervals, exhibit a combination of long-range motion and fluidity together with a degree of order, measurable by optical and scattering techniques such as cross-polarized light microscopy, X-ray diffraction and scattering, or small- angle neutron scattering [1O]. This order is a reflection of a cooperative behavior of many polymer chains or their segments. With the exception of very few flexible polymer chains, the cooperative behavior and mesomor- phicity of which are due to inter- and intra-chain hydrogen bonds [11-13], the vast majority of liquid crystal polymers owe their unique behavior to the presence of relatively stiff and elongated units which, because of their stiffness and anisotropy, prefer to order themselves in more or less parallel arrays and impart to the overall system a liquid crystal character. These anisotropic stiff groups are commonly called mesogenic units.
The liquid crystal polymers (LCPs) may be categorized in two broad classes, with several polymers falling in both. One class is that of ther- motropic LCPs and the other is that of lyotropic LCPs. When heated in the quiescent state from a crystalline, semicrystalline, or occasionally a glassy state, the thermotropic LCPs pass through one or more phases, mobile yet ordered to varying degrees, before reaching a fully isotropic molten state. The phase or phases between the low-temperature crystalline or glassy state and the high-temperature isotropic melt are called meso- morphic phases. Based on their structure, thermotropic LCPs are further divisible into two groups. One includes polymers in which mesogenic units of modest lengths are connected along the chain backbone by flexible "spac- ers" , or are connected to a flexible main chain by flexible spacers or tethers, just like pendants to a necklace. The other thermotropic group comprises polymers with rather weak interchain attractive interactions and with back- bones that are uniformly stiff to varying degrees, imparting to these PCPs rodlike or wormlike character. As a group, these polymers are characterized by large persistence lengths and large radii of gyration [1O]. All polyester LCPs, currently manufactured industrially on a large scale, fall into this last category.
These liquid crystal (LC) polyesters combine several interesting fea- tures. They are usually processed at elevated temperatures in their meso-
morphic state, and, because of their high chain stiffness and large persis- tence length, they tend to aggregate in more or less parallel arrays. Gen- erally, this arrangement remains unchanged upon cooling, so that the LC polyesters retain their anisotropic character at the use temperature, at which they are generally crystalline or semicrystalline. Because of this re- tained anisotropy, the LC polyesters are typified at ambient temperature by very high strain modulus and strength, and rather low strains to break.
This is generally the case, regardless of whether the LC polyesters are used alone or as a minor anisotropic reinforcing phase in polymeric compos- ites. In addition to their strong anisotropy and highly desirable mechanical properties, the LC polyesters are characterized by very high use temper- ature, excellent thermal stability, high transition temperatures, and very high isotropization temperature. The latter often leaves no safe melt pro- cessing window, rendering the polymers unprocessable in commonly used equipments under normal processing conditions. Similar to the anisotropic and mechanical properties, these special thermal properties are caused by the high aromatic content of the industrial LC polyesters, by the limitations placed by the restricted torsional mobility on the ability of chain segments to rotate along the chain axis, and by the absence along the chain of ther- mally less stable aliphatic groups. The same structural features render the LC polyesters insoluble or only sparingly soluble in usual organic solvents at room temperature, a feature that may be very desirable for certain ap- plications, but makes the laboratory characterization and study of such polymers very hard.
Unlike all the polyesters previously discussed in this chapter, the highly aromatic LC polyesters contain only aromatic carboxy acids and, more importantly, only phenolic hydroxyl groups. In order to directly esterify carboxy acids, the phenolic hydroxyls must be present in the reaction mix- ture in an activated form. The most common industrial method to achieve this is by means of in situ acetylation of the aromatic hydroxyls, usually with acetic anhydride
O O O
CH3-C-O-C-CH3 + H0^(O) > CH3-C + CH3COOH
Figure 12 shows a flowchart for the industrial production of LC polyesters from two monomers, both being aromatic hydroxy acids.
Industrial-Scale Production of Polyesters 97
HO
COOH
(18)
In Figure 13, three monomers are used together, one being an aromatic diacid, the second an aromatic dihydroxy monomer, and the third an aro- matic hydroxy acid.
COOH COOH
+
-HO- -OHOH OH (19)
— O
There are two common features to both flowcharts. One is the acetic anhydride - acetic acid recycling system, and the second is the extremely high temperature (315-32O0C) at which the polymerization takes place.
This is necessary in order to melt the feed monomers and is also due to the very high transition temperatures of the polymers into melt-processable fluids. Polymerizations at lower temperatures may result in the polymers crystallizing in the reactors into solid masses that may not be removable by pumping. It is important to note here that too high polymerization temperatures may initiate some polymer degradation which may limit the MW of the product.
The more interesting feature of both polymerization facilities (Figures 12 and 13) is that one or more polymerization reactors operate in paral- lel and are flanked by the acetic anhydride - acetic acid recycling system.
As can be seen in both flowcharts, the aromatic monomers bearing free carboxyl and free hydroxyl groups are fed directly into the polymerization reactors. Acetic anhydride is fed into the reactors from a separate tank and the reaction mixture is protected from air by a blanket of nitrogen. For this method of polymerization, no additional catalyst is used. The acetic anhy- dride reacts exclusively with the aromatic hydroxyls to create the reactive aromatic acetoxy groups. From each acetic anhydride molecule, one acetoxy
p-Hydroxyb Acetic inhydride
6-Hydroxy-2-naphthoic acid psia -101 6O0C
34 12O0C 1 *X^•Acetic acid To disposal Acetic anhydride recycle Molten polyme] Figure 12. Preparation of LC polyester from two aromatic hydroxy acid monomers. R-IOl A-D: Polymerization reactors; V-IOl A-D: Reflux drums; T-IOl A&B: Acid/anhydride surge tanks; C-IOl: Acetic acid column; T-102 A&B: Rundown tanks; T-103 A&B: Acetic acid tanks; C-102: Acetic anhydride column; T-104 A&B: Recycle acetic anhydride tanks
Industrial-Scale Production of Polyesters 99
group and one acetic acid molecule are formed, the necessary hydrogen be- ing supplied by the phenolic group. The acetic acid by-product is removed and is passed at atmospheric pressure through two distillation columns where it is dehydrated and converted back to crude acetic anhydride. The latter is purified from residual acetic acid and then pumped back to the head of the polymerization system. It is obvious that the highly corrosive nature of acetic acid and acetic anhydride necessitates the use of corrosion resistant reactors, tanks, columns, pipes and pumps throughout the poly- merization facility. The cost of this and the relatively high price of many of the aromatic monomers are reflected in the high price LC polyesters presently command (see also Chapter 14).