Crystallization, side chain

Mechanical and Thermal Properties. The first member of the acrylate series, poly(methyl acrylate), has fltde or no tack at room temperature it is a tough, mbbery, and moderately hard polymer. Poly(ethyl acrylate) is more mbberflke, considerably softer, and more extensible. Poly(butyl acrylate) is softer stiU, and much tackier. This information is quantitatively summarized in Table 2 (41). In the alkyl acrylate series, the softness increases through n-octy acrylate. As the chain length is increased beyond n-octy side-chain crystallization occurs and the materials become brittle (42) poly( -hexadecyl acrylate) is hard and waxlike at room temperature but is soft and tacky above its softening point. 

The increase in the length of the side chain results normally in an internal plasticization effect caused by a lower polarity of the main chain and an increase in the configurational entropy. Both effects result in a lower activation energy of segmental motion and consequently a lower glass transition temperature. The modification of PPO with myristoyl chloride offers the best example. No side chain crystallization was detected by DSC for these polymers. 

Thus, important structure/property relationships are emerging that are relevant to electronic and optical materials applications for these materials. In a different vein, side chain crystallization has resulted in the first liquid crystalline inorganic and organometallic macromolecules, viz., unusual poly(dialkoxy-phosphazenes) described by Allcock (p. 250) and Singler (p. 268). 

If the alkyl substituent in a dialkyl substituted polysilane is either too short or is branched, it cannot pack properly to allow side chain crystallization, and hence, one would expect very different, properties from PDHS and its higher homologs. In order to study this possibility, we synthesized and characterized the di-n-butyl (PDBS), di-n-pentyl (PDPS) and di-5-methylhexyl polymers (PDMHS). 

Another interesting feature pertains to the melting points cited for the polymers having m — 6-9. The side chains are now long enough to crystallize themselves, which is apparently the reason that formation of mesophases is suppressed. Such side-chain crystallization has also been involved in polysilane homopolymers... 

Esterification of at least 45% of the hydroxyl groups with long chain fatty acids, e.g., stearic or behenic acid, results in a semi crystalline material (side chain crystallization). The obtained materials are characterized by melting point ranges which are approximately 10 °C lower than the comparable methyl esters. 

Which would be more apt to exhibit side chain crystallization (a) poly(methyl methacrylate) or (b) poly(dodecyl methacrylate) ... 

Many copolymers are said to be internally plastidzed because of the flexibilization brought about by the presence of a second repeating unit in the polymer chain. In contrast. DOP and other liquid plastidzers are said to be external plastidzers. The presence of bulky pendant groups on the polymer increases segmental motion, and the flexibility of the polymer increases as the size of the pendant group increases. However, linear pendant groups with more than 10 carbon atoms reduce flexibility because of side chain crystallization. 

PMA is a tough leathery resin with a low Tg and a solubility parameter of 10.5 H. In polymers of alkyl acrylates the solubility parameter decreases as the size of the alkyl group increases. The flexibility also increases with the size of the pendant groups but because of side chain crystallization this tendency is reversed when the alkyl group has more than ten carbon atoms. Polyalkyi acrylates are readily hydrolyzed by alkalis to produce salts of polyacrylic acid. The copolymer of ethyl acrylate (95%) and chloroethyl vinyl ether (5%) is a commercial oil-resistant elastomer. 

Considerations by V. Shibaev and N. Plate (see this issue) led to a similar conclusion. Investigations on comb like polymers26), where each monomer unit of the macromolecule carried a non-branched alkyl chain of m methylene groups, have shown that for m > 8 side chain crystallization takes place independently to the main chain conformation. Consequently, if mesogenic molecules are linked to the side chains, they should occupy a l.c. order without influence of the backbone conformation. Following these considerations, alkyl chain lengths m > 8 are necessary for the formation of the l.c. order. As shown later, however, nearly only smectic polymers are possible under these conditions (see Chap. 2.3.3.). 

The polymers of 284 are hydrogels and can take up a moderate amount of water. The ROMP of 285 only proceeds to completion if the polystyrene side chains are kept reasonably short (n = 4, 7 or 9). Polymers of monomers such as 286 exhibit a nematic or smectic mesophase resulting from side-chain crystallization. Isotropization temperatures increase with increasing MW, becoming constant at about 30-50 repeat units. Side-chain crystallization tends to be suppressed as the MW increases. 

Since the electronic absorption spectra of polvsilanes and polygermanes also depend strongly on conformation (3), these materials provide a unique opportunity to study the effect of backbone conformation on the NLO properties, particularly since varying alkyl substituents can dramatically influence the backbone structure through intermolecular interactions (e.g., side chain crystallization) while causing... 

MM7525 99SM(101)11), although the emission of substituted PTs is not very predictable due to the interplay of several factors (steric effects, regioregularity, electronic effects, side-chain crystallization, etc.), the full visible range of PL and EL emission, from red to blue, can be covered by variations of the PT structure via substituents in 3,4-positions. 

The long side chains of the acrylate ester group can apparently act as internal plasticizers. Substitution of a carboxyl group on the polymer chain increases brittleness. A more polar substituent, eg, an /V-alkyl amide group, is even less desirable. Copolymers of VDC with N- alkyl a c ryl ami d e s are more brittle than the corresponding acrylates even when the side chains are long (87). Side-chain crystallization may be a contributing factor. 

In the same work, norbornene derivatives with a (4 -methoxy-biphenyl-4-yl)-oxy mesogen linked by an ether group Ill-n (n=4-6, see Fig. 4) were synthesized. Surprisingly, in contrast to ester-hnked polymers (I-n) [38], all of the poly-(III-n) polymers gave highly crystalline polymers (Table 1) caused by side chain crystallization, even in polymers with relatively short spacers ( =4-6). 

This effect was attributed to the higher flexibility of the ether linkage, as there is no rotational restriction of the C-O-C bond in contrast to the ester group, whose asymmetry is believed to disturb the side chain crystallization. 

Low-strain oscillatory measurements show that PDM-PMAS copolymers with longer side-chains (C e and Cig) form a network structure at temperatures below the side-chain Intermolecular side-chain crystallization may be responsible for this behavior. 

The TgS of the comb polysiloxanes increased with the oxyethylene unit content -78 °C for PMMS-8 and -60 for PAGS-12. Melt endotherms resulting from side-chain crystallization were found in the DSC (differential scanning calorimetry) scans at = -1 °C for PAGS-8 and at 24 °C for PAGS-12. A broad melt endotherm centered at -1 °C was found for PMMS-8. The endotherm sharpened considerably when a fractionated sample of methoxypoly(ethylene glycol) was used in the synthesis of PMMS-8. 

PAGS-8 and PAGS-12 are also water insoluble. The tendency toward side-chain crystallization and the hydrophobic backbone of the polymer contributes to their poor water solubility. Gloud points could be measured in 3% aqueous solutions of PM MS-8 in the presence of various salts (Figure 1). Gonsistent with data reported for nonionic surfactants and polymers with oxyethylene moieties, the cloud points are sensitive to the type of salts added, especially that of the anion. Fluorides and sulfates are effective salting-out agents, whereas thiocyanates raise the cloud points. 

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