Main-chain flexibility

There are also some trends when looking at the main chain flexibility. Table II demonstrates that when the main chain flexibility decreases from cyanobiphenyl containing polymethacrylates to polysiloxanes, not only does the Tg drop, but the isotropization temperature increases. However, the trend is the opposite when the mesogen is methoxyphenyl benzoate (18). Therefore, this effect of the main chain flexibility is still ambiguous. 

Table II. Influence of the Main Chain Flexibility on Liquid Crystalline Phase Transitions for Polymers with Cyano-biphenyl as Nesogen...

The chemical features that prohibit crystallinity are main chain flexibility (e.g., rotation), branching, random copolymers or low inter-polymer chain attraction. Normally, polymers are not miscible with each other and on cooling from the melt will separate into different phases. When miscibility is exhibited, e.g., poly(phenylene oxide) (PPO) and PS, crystallisation does not take place. 

Main chain flexibility Main chain rigidity... 

MAIN CHAIN FLEXIBLE SPACER MESOGENIC GROUP LIT... 

Photoresponsive membranes of cellulose-2,4-diacetate incorporating 6-nitro-l, 3, 3 -trimethylspiro-(2H-l-benzopyran-2,2 -indoline) have been prepared and chiroptical and fluorescence properties of optically active co-polymers of acenaphthalene with methyl acrylate/methacrylate have been investigated. Marked optical activity is induced in the aromatic units only for the co-polymer with methyl methacrylate. This difference in behaviour is associated with an overall higher main-chain flexibility and conformational freedom in acrylates compared with methacrylates. Photoisomerization in polyurethanes containing azo-links has been found to be dependent upon the thermal history of the polymer, and photoisomerization of linoleic acid and... 

Polyimides (PI) were introduced in 1962 as thermally non-processable Kapton . To improve processability, the main-chain flexibility was enhanced by incorporating segments with higher mobility, viz. polyamide-imide (PAl), polyether-imide (PEI), polyimide-sulfone (PISO), etc. These polymers are characterized by high T = 150-420°C and thermal resistance. They are blended with PPS to enhance its moldability, thermal stability and mechanical performance. 

Rovlello and Sirigu in 1975 reported the first main chain -(flexible rod pol3nner and soon thereafter the properties of the chemically analogous small molecule family. This allows a comparison of phase types, phase sequences and transition temperatures and is valuable in interpreting the influences of chemical constitution on mesophase behavior. These compounds are shown below. 

Proton NMR has been extensively utilized in the study of orientational ordering in main-chain flexible liquid crystal polymer sys-... 

Flexible groups such as an ether link will enhance main-chain flexibility and reduce the glass transition temperature, with the opposite effect being shown by the introduction of an inflexible group, such as a terephthalate residue. 

Many attempts have been made to optimize polymer structures using this strategy. The results of the structural modifications could be qualitatively understood in most cases on the basis of main chain flexibility, bulkiness of certain structural ele-... 

In the quest for improved performance, better pro-cessibility, and novel applications (1], a wealth of newer compounds with specific chemical architectures have been synthesized. The three most common approaches that have been implemented are greater main-chain flexibility (e.g., polyaniline [2]), side-chain substitution (e.g.. poly(3-alkylthiophenes) [3J), and fabrication from a soluble precursor polymer (e.g., poly-(/ -xylene-a-dimeth-ylsulfonium chloride) to yield poly(/ -phenyleneviny-lene) [4]). All of these modifications can produce materials with a range of structural forms and, in some instances, striking new physical behavior. 

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