Chain flexibility, crystallinity

Characteristic Property D.P Branching Cross- Linking Polar Structures Chain Flexibility Crystallinity Size of Spherulites Orientation Molecular Weight Dist. 

This condition means that for f < 0.63 the disordered arrangement of molecules is thermodynamically unstable and the system is spontaneously reorganized into an ordered liquid crystalline phase of a nematic type (Flory called this state crystalline ). This result has been obtained only as a consequence of limited chain flexibility without taking into account intermolecular interactions. 

Optical and electro-optical behavior of side-chain liquid crystalline polymers are described 350-351>. The effect of flexible siloxane spacers on the phase properties and electric field effects were determined. Rheological properties of siloxane containing liquid crystalline side-chain polymers were studied as a function of shear rate and temperature 352). The effect of cooling rate on the alignment of a siloxane based side-chain liquid crystalline copolymer was investigated 353). It was shown that the dielectric relaxation behavior of the polymers varied in a systematic manner with the rate at which the material was cooled from its isotropic phase. 

Incorporation of flexible siloxane spacers into side chain or main chain liquid crystalline polymers have been shown to drastically reduce the transition temperatures 255,267,271,272,277) anc[ aiso increase the response time of the resultant systems to the applied thermal, optical or electrical fields 350-353>. In addition, siloxanes also provided elastomeric properties and improved the processibility (solution or melt) of the resulting liquid crystalline copolymers. 

The Tg of P-plastomers changes as a function of ethylene content. The Tg decreases with increasing ethylene content, primarily due to an increase in chain flexibility and loss of pendant methyl residues due to incorporation of ethylene units in the backbone. It is well known that PP has a Tg of 0°C, and polyethylene a Tg< —65°C. The addition of ethylene to a propylene polymer would therefore be expected to decrease the Tg, as is observed here. A secondary effect would be the reduction in the level of crystallinity associated with increasing ethylene content, which is expected to reduce the constraints placed upon the amorphous regions in proximity to the crystallites. Thus, an increase in ethylene content will result in a lower T as well as an increase in magnitude and a decrease in breadth of the glass transition. 

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

Lastly, it was demonstrated with PPO substituted with a series of alkyl side-chains as we have here, that the glass transition temperature decreases with an increase in the side-chain length (28). At the same time, the Tg s of the more flexible side-chain liquid crystalline polymers investigated to date are always much higher than those of the corresponding polymers without the mesogenic side-chains (3). Therefore, it is quite likely that we may obtain side-chain liquid crystalline polymers of approximately the same Tg from PPO and PECH. 

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. 

The variation in the initial Tg s show the effect on processability of the structures of the diol and the dibromobenzene. Two of the systems, those based on thiodiphenol and dihydroxybiphenyl, gave crystalline products which could not be made amorphous upon heat treatment. The effect on chain flexibility of dibromobenzene structure could be seen with the two hydroquinone systems. In this case, the meta isomer gave an initial Tg of 12°C while the more rigid para system had a Tg of 59°C. Variation of diol structure shows a parallel effect as shown with the resorcinol/j>-dibromobenzene system. This material has a softening temperature of 20°C. 

By building - in combinations of aromatic rings into the polymer chains, chemists are able to produce polymer chains with very low chain flexibility. In the limit they reach rigid-rod-type op polymers. Such polymers show substantial temperature - pressure -concentration regions in which the stiff polymer chains arrange in some form of orientation. This phase behaviour gave them the name Liquid Crystalline Polymers (LCP) and LCP have unique properties. 

The molecular weight distribution of the polymer can have a dramatic effect on rates of depolymerisation. This effect has been demonstrated for a number of polymers, where a critical lower limit must be present before the process will start. The molecular origin for this effect is still subject to speculation, and has been attributed to a range of causes such as changes in enzyme accessibility, chain flexibility, fits with active sites, crystallinity or other aspects of morphology. 

In random copolymers chain flexibility and crystallinity can be quite different from those in either of the component /zomopolymers. In consequence, the copolymer may well present an entirely new set of physical properties. On the other hand, the different chain segments in block or graft copolymers often segregate into effectively separate phases, when the properties of the composite resemble those of a mixture of the individual homopolymers. 

Side-chain liquid-crystalline polymers were also prepared using double hydrogen bonds (Scheme 11) [243]. All these examples have in common a flexible spacer connecting the mesogenic unit to the polymer backbone. That this is not always required is demonstrated by yet another class of supramolecular hquid-crystalhne polymers obtained by hydro-... 

Anhydride (-C-C(0)-0-C(0)-C-) Amide (-NH-C(O)-) SUoxane (-R3Si-0-) Chain flexibility, water sensitive (especially for short aliphatic chains) Chain rigidity, crystalline, water sensitive Chain flexibility and low Tg (especially for small substituents), stable toward oxidation, acid/base reactive... 

This study is the first step towards a quantitative prediction of chain flexibility based on conformational analysis. Torsional relaxation of the adjacent bonds is very important. The present approach differentiates between the polymer relaxations in the glassy state and in the melt state. It provides insight into the crystallinity of a system and succeeds in explaining the isomorphic transformations of PDES. Chain flexibility is influenced by at least two types of factors the number of isomeric states available and the torsional freedom in a given state, which is determined by the shape of the potential well. 

The effect of crystallinity and backbone chain flexibility has been examined on the photodegradation rates of aromatic polyure-... 

The concept of a side chain liquid crystal polymer has been demonstrated in a number of laboratories and is well documented in the literature (J ). Most of the side chain liquid crystalline polymers reported to date contain polysiloxane, polyacrylate or polymethacrylate main chains. More recent studies on the effect of backbone flexibility now include the use of flexible poly(ethylene oxide) or more rigid poly( a-chloroacrylate) chains. 

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