Crystallization and orientation development in melt spinning isotactic PP of varying tacticity / D. Choi in INTERNATIONAL POLYMER PROCESSING, Vol. XVII, N° 3 (09/2002)  

[article]  inINTERNATIONAL POLYMER PROCESSING > Vol. XVII, N° 3 (09/2002) . - p. 233-243 Titre : Crystallization and orientation development in melt spinning isotactic PP of varying tacticity Type de document : texte imprimé Auteurs : D. Choi, Auteur ; J. L. White, Auteur Année de publication : 2002 Article en page(s) : p. 233-243 Note générale : Bibliogr. Langues : Anglais (eng) Index. décimale : 668.9 Polymères Résumé : We investigated the effects of tacticity on crystallization and orientation development in melt spinning isotactic polypropylene (iPP) through air. Melt-spun fibers were characterized by wide-angle X-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). The melt-spun fibers usually exhibited the monoclinic α-crystalline form, but the mesomorphic form was found under certain processing conditions. The mesomorphic structure was formed more readily in low tacticity fibers. We found certain amounts of hexagonal β-crystalline form in all of the α-form fiber samples. The amouns were generally negligibly small, but the low tacticity fibers spun at high draw-down ratios exhibited significant amounts of β-form crystals. At low spinline stress levels, some low tacticity fibers exhibited lower birefringence and chain orientation than high tacticity fibers due to a less perfect crystal structure. At certain high spinline stress levels, low tacticity fibers exhibited much higher birefringence and chain-axis orientation levels than expected. This was attributed to a reduction in epitaxial lamellar branching (a*-axis oriented group in bimodal orientation). However, the extent of lamellar branching in low tacticity fibers appeared to increase again at higher stress levels. This led to a significant decrease in birefringence and crystalline orientation. Note de contenu : - Introduction - Experimental : Materials - Material characterization - Melt spinning - Fiber characterization - X-ray diffraction analysis - DSC analysis - Birefringence measurement - Results : Crystallization rates - Spinline stresses - Fiber melting temperatures and crystallinities - WAXD patterns and 2téta scans - Birefringence - Crystalline orientation factors - Amorphous orientation factors - Discussion : Crystal structures - Birefringence and chain orientation - Conclusions DOI : 10.3139/217.1699 En ligne : https://drive.google.com/file/d/1i273l-u443eWki5pSWrhBDEP3xfYL60V/view?usp=drive [...] Format de la ressource électronique : Pdf Permalink : https://e-campus.itech.fr/pmb/opac_css/index.php?lvl=notice_display&id=15999 [article]

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Structure development in melt spinning syndiotactic polypropylene and comparison to isotactic polypropylene / D. Choi in INTERNATIONAL POLYMER PROCESSING, Vol. XV, N° 4 (12/2000)  

[article]  inINTERNATIONAL POLYMER PROCESSING > Vol. XV, N° 4 (12/2000) . - p. 398-405 Titre : Structure development in melt spinning syndiotactic polypropylene and comparison to isotactic polypropylene Type de document : texte imprimé Auteurs : D. Choi, Auteur ; J. L. White, Auteur Année de publication : 2000 Article en page(s) : p. 398-405 Note générale : Bibliogr. Langues : Anglais (eng) Index. décimale : 668.9 Polymères Résumé : Fibers and films - A basic investigation of the development of crystallization and orientation during the melt spinning of syndiotactic and isotactic polypropylene is described. Both polymers were melt spun through ambient air or into ice water at various draw-down ratios and melt temperatures. The melt spun filaments were characterized by wide angle x-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). The polymorphic behavior of isotactic polypropylene during melt spinning is well known. The filaments melt spun through ambient air exhibit the monoclinic α-form while the filaments melt spun into ice water form a smectic structure at low draw-down ratios. Syndiotactic polypropylene also exhibited polymorphic behavior during melt spinning. Disordered Form I with (t2g2)2 helices was found at low spinline stresses and Form III with all trans chain conformation was found at high spinline stresses. The criterion of the spinline stress for this transition was found to be about 6–9 MPa for all samples of various molecular weights, melt temperatures and cooling rates. Syndiotactic polypropylene crystallized much more slowly than isotactic polypropylene and did not exhibit a smectic mesomorphism under ice-water quenched conditions. The melt spun filaments of syndiotactic polypropylene exhibited lower birefringences and amorphous orientations but higher crystalline orientations than those of isotactic polypropylene, compared at the same spinline stresses. Note de contenu : - EXPERIMENTAL : Materials - Material characterization - Melt spinning - Fiber characterization - RESULTS : Spinline stresses - DSC scans - WAXD patterns - Crystalline orientation - Birefringence - DISCUSSION : Polymorphism - Crystallization rate in melt spinning - Orientation and birefringence DOI : 10.3139/217.1608 En ligne : https://drive.google.com/file/d/1a4Re89vyZ13AMP_W-rTU_dFUa_Yvd_HF/view?usp=drive [...] Format de la ressource électronique : Pdf Permalink : https://e-campus.itech.fr/pmb/opac_css/index.php?lvl=notice_display&id=16158 [article]

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