Self-order in flexible non-mesogenic macromolecules

Godovsky, Yu. K.; Макарова, Н. Н.

doi:10.1098/rsta.1994.0080

Cited by 12 publications

(5 citation statements)

References 15 publications

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“…The reason that our compressed PDES networks did not transform into the mesophase can be understood from the mesophase microstructure. The mesophase has a columnar structure in which the Si−O chain backbones align in a common direction (the axes of the columns) . In uniaxial extension, the mesophase can form because the chains are stressed in a common direction.…”

Section: Resultsmentioning

confidence: 99%

“…The difference in properties between PDES and PDMS is due to the difference in segment−segment interactions in the two polymers. It has been suggested that segment alignment is energetically favorable in PDES because overlap of ethyl groups on neighboring chain segments is maximized when the siloxane backbones are oriented in a common direction. ,, The orientational enthalpic coupling between segments in PDES arises as nonpolar ethyl groups associate with each other in preference to the polar Si−O backbone. Alignment of the chain backbones decreases the internal energy of the network, a result that has been verified by calorimetric measurements. ,,− For the mesophase transition to be spontaneous, this favorable decrease of internal energy must offset the entropic penalty for alignment.…”

Section: Resultsmentioning

confidence: 99%

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Effects of Molecular Structure on Segment Orientation in Siloxane Elastomers. 1. NMR Measurements from Compressed Samples

et al. 2001

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2 H NMR spectral data are reported for two types of compressed, deuterium-labeled siloxane elastomers: 2 H-labeled poly(diethylsiloxane) (PDES) networks and 2 H-labeled poly(dimethylsiloxane) (PDMS) free chains dissolved at low concentration in PDES host networks. We find that compressed PDES networks have higher segment orientation under stress than PDMS networks, a result that follows partially from the larger segment size (persistence length) of PDES. The internal motions of PDES chain segments are found to be substantially less isotropic than in PDMS, which leads to larger 2 H NMR line widths. Chain segments in compressed PDES elastomers have enhanced orientation compared to predictions from excluded-volume calculations. The 2 H NMR spectral splitting for compressed PDES networks is not directly proportional to (λ 2λ -1 ), unlike the case of conventional elastomers. The behavior of the compressed PDES networks is tentatively attributed to an enthalpic orientational coupling between PDES segments.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Effects of Molecular Structure on Segment Orientation in Siloxane Elastomers. 1. NMR Measurements from Compressed Samples

et al. 2001

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“…The discovery of the mesophase in PDES in 1975 − stimulated characterization of this polymer by DSC, optical and electron microscopy, X-ray diffraction, NMR, and other techniques in order to understand the mesophase structure and its relation to the crystalline polymorphs. − The crystal−mesophase transition observed on heating at sub-RT temperatures gives samples with almost 100% mesophase content. − ,,, The comparison of the PDES density in the mesophase with the theoretical crystallographic density showed 7 that the chain packing in this phase can be described by a monoclinic unit cell, which slightly differs from the hexagonal packing. Polarized optical microscopy revealed a formation of extended lamellar structures in the mesophase obtained by slow cooling of the melt …”

Section: Introductionmentioning

confidence: 99%

Atomic Force Microscopy Visualization of Morphology Changes Resulting from the Phase Transitions in Poly(di-n-alkylsiloxane)s: Poly(diethylsiloxane)

2001

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The results of visualization of morphology and nanostructure of polydiethylsiloxane (PDES) are presented. The study was performed in a temperature range, which covers melting, crystallization, mesophase formation, and isotropization of PDES, on polymer samples of different molecular weights and different thicknesses. The morphological identification of α2- and β2-polymorphs of PDES was further extended by demonstrating, for the first time, that the α2- and β2-crystalline polymorphs transform to two morphologically incompatible α-mesophase and β-mesophase. Domains, which are several microns in size, are typical morphologic patterns for the α-phase material, whereas large lamellae (from a few to tens of microns in length and several hundreds of nanometers in width) are typical structures of the β-phase PDES. The above polymorphs and amorphous polymers have appeared in different ratios in the samples depending on their preparation (deposition way, thermal history) and thickness. Shearing of PDES into thin film on Si substrate induced the formation of the mesomorphic β-phase structures, which are embedded in amorphous material. Crystallization of the amorphous PDES occurred at much lower temperatures due to its constrained geometry. The width of the β-phase lamellae in the crystalline and mesomorphic state as measured from AFM images correlates with the length of the extended PDES chains in the crystalline state. Sublamellae structures were revealed by AFM imaging the mesomorphic lamellae with different tip−sample forces. Each lamella has a skeleton formed of 10−15 nm thick linear structures, which are separated by 40−50 nm and tapered at the ends to form a single entity. This skeleton is wrapped with numerous 0.8 nm thick layers, which are most likely formed of partially ordered domains of extended chains. Crystallization of the β-mesomorphic lamellae leads to more ordered and stiffer top layers of lamellae. In some cases individual lamellae are broken into small crystalline blocks, and in other cases crystalline blocks incorporate two neighboring lamellae. The domains of the mesomorphic α-phase are less organized, and upon crystallization large domains are broken into smaller blocks due to shrinkage. This is accompanied by surface roughening. Some of the blocks exhibit periodic surface structures with a repeat distance of 50−60 nm. At least partial chain extension in the α-phase PDES is suggested.

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“…The cyclolinear polysiloxanes have bulk columnar liquid crystal phases, and it has been proposed that the multilayers form by a sliding mechanism . In order to understand the mechanism of collapse better and to examine the validity of this model, we have studied the collapse of monolayers of a cyclolinear polysiloxane by several methods.…”

Section: Introductionmentioning

confidence: 99%

Structures of Collapsed Polysiloxane Monolayers Investigated by Scanning Force Microscopy

et al. 1997

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Surface pressure−area isotherms of monolayers of cyclolinear polysiloxanes on water exhibit several plateaus that have been associated with the formation of multilayers. This stepwise collapse has been studied in one of these polymers by scanning force microscopy under topographic, frictional, and electric force modes. Topographic images show that the growth of the second layer begins with the nucleation of three-dimensional islands, which then appear to spread in the form of ribbons 0.5 μm wide and 7.5 Å high. The number of ribbons increases as the extent of the second layer increases, but the width of the ribbons remains constant. Subsequent layers do not exhibit this morphology.

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Self-order in flexible non-mesogenic macromolecules

Cited by 12 publications

References 15 publications

Effects of Molecular Structure on Segment Orientation in Siloxane Elastomers. 1. NMR Measurements from Compressed Samples

Effects of Molecular Structure on Segment Orientation in Siloxane Elastomers. 1. NMR Measurements from Compressed Samples

Atomic Force Microscopy Visualization of Morphology Changes Resulting from the Phase Transitions in Poly(di-n-alkylsiloxane)s: Poly(diethylsiloxane)

Structures of Collapsed Polysiloxane Monolayers Investigated by Scanning Force Microscopy

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