Studies of macromolecular entanglements. I. Melting behavior of swelling polymers with macromolecular entanglements

Polymer Engineering & Sci

et al. 1992

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Macromolecular entanglements in polyacrylonitrile (PAN) and polyacrylonitrile copolymer (PAC) fibers have been systematically studied by the methods of swelling thermal analysis, i.e., swelling differential scanning calorimetry (SDSC), swelling shrinking stress (SSS), swelling shrinkage (SS), and terminal swelling modulus (TSM). When a PAC fiber sample was immersed in a swelling agent, e.g., 80% aqueous dimethylformamide (DMF) and subjected to a programmed heating, the fiber swells, with the dissociable intermolecular couplings gradually broken until finally the fiber was completely dissolved. If this process was done in a DSC analyzer, a small endothermic peak appears on the latter part of the SDSC thermogram. This represents the disentanglement energy of the entangled bonds in the sample. If done in a thermomechanical analyzer, the SSS, SS, and TSM, measured just before incipient dissolution, i.e., the point that intermolecular couplings were broken and only the entangled loopings junctions were left, should reflect the number of the entanglement loops. Thus, the long argued problem whether an entanglement is topological, or due to loci of chain coupling, or is of dual nature may be experimentally elucidated. By using these methods, problems were detailed concerning entanglements, such as the different textural changes of PAC fiber during ambient storage to below 0°C, the effects of heating at 125°C for various times on the PAC fiber texture, and the textural characteristics of gel‐spun PAN fibers with different molecular weight and different draw ratios.

Section: Applications Of the Methods For The Study Of Some Specific Hsupporting

confidence: 52%

Section: Entanglement Bondsmentioning

confidence: 99%

Methods of studying macromolecular entanglements

Polymer Engineering & Sci

et al. 1992

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“…111 part l. 3 Three percent of polymer concentration in the dope is a limit below which entanglement network seems no longer present. Steam heating seems to be somewhat more efficient to produce entanglements than dry heating.…”

Section: Heating In Dry Air and In Saturated Steammentioning

confidence: 99%

“…The whole swelling and melting process of a polymer fiber during heating can be visually observed under a polarized thermomicroscope. 3 Concerning semicrystalline polymers, the twophase theory is generally accepted. But, for acrylic polymer, the single-phase double-bonded theory is more attractive.l0," The structure of an acrylic polymer may be looked upon as a network structure of amorphous and paracrystalline regions, some long macromolecular chains, interpenetrating both regions without any sharp phase boundary between the two regions.…”

Section: Morphological Structurementioning

confidence: 99%

Macromolecular entanglement. III. Textural development of acrylic fibers

J of Applied Polymer Sci

Qin

et al. 1992

SYNOPSISThe texture of a polymer (or fiber) is effectively developed by heat and stress. In this article, the chief material studied is acrylics, usually polyacrylonitile copolymer (PAC fiber) produced by Jingshan Jinglun Fiber Production Plant. In industry, the Jinglun acrylic fiber is heat treated, using saturated steam, at a temperature of 120-130°C to enhance its loop strength, reduce shrinkage, and stabilize crimp. At 14OoC, the fiber becomes tacky and its good textile properties seem to deteriorate. At 15O-16O0C, the texture of the fiber seems to be deeply modified. Tested by differential scanning calorimetry (DSC) with the fiber immersed in a swelling agent (usually termed swelling DSC, abbreviated as SDSC) , in addition to the entanglement peaks situated on the latter part of the thermogram of the Jinglun fiber, a new endothermic peak before the crystalline peak (actually, paracrystalline in the case of acrylic fiber) was discovered. The elastic recovery properties of the treated fiber under partially swelling conditions were improved. This seems to indicate that this preparacrystalline peak might also be concerned with entanglements.

“…This means that the stress during the beginning part of the curve is due to thermal shrinking stress, as crystallization is a process that requires more time to complete. ( a ) Time-shrinkage stress curve of PAC fibers at constant temperature: ( 1 ) stress is either a positive or a negative value, making the corresponding entanglement always smaller than the central maximum value. The results given in Figure 5 show, however, that entanglements can be completely removed by thermal shrinking and hot recrystallization stress induced isotropically by steam heat setting.…”

Section: Distinction Between Externally Applied Tension and Internal mentioning

confidence: 99%

Studies on macromolecular entanglement. II. Modification of macromolecular entanglements by heat treatment

J of Applied Polymer Sci

et al. 1993

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SYNOPSISThe modifications of macromolecular entanglements in polyacrylonitrile copolymer fibers by heat setting in different mediums such as steam, hot air, and inert liquid are reported. The effects of applied tension, frozen internal stress, temperature, time, medium, crystallization, etc., during heat setting on the modification of entanglements are investigated in detail. These experimental results elucidate clearly the mechanism of entanglement and disentanglement and confirm fully the DSC method of swelling polymers for studying entanglement in solid polymer, as was previously published in Part I of this paper. 0 1993