Browsing by Author "Chu, PP"

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Conformational changes and molecular motion of poly(ethylene terephthalate) annealed above glass transition temperature
(2000-03-01)
Huang, JM
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Chu, PP
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Chang, FC
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應用化學系
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Department of Applied Chemistry
Conformational conversion and molecular dynamics of the amorphous poly(ethylene terephthalate) (PET) annealed above T-g were investigated by C-13 high-resolution solid-state NMR and differential scanning calorimetry (DSC). Rapid increase in the PET trans content and crystallinity were observed near the onset crystallization temperature (congruent to 120 degrees C). By comparing both changes in conformation and crystallinity during thermal annealing, we found that the trans conformer increases continuously while the degree of crystallinity levels off during the later stage of annealing. The H-1 spin-lattice relaxation time in the rotating frame (T-1 rho(H)) correlates linearly with PET crystallinity but less well with the trans content. A three-domain model containing crystalline (all trans), constrained noncrystalline (trans rich) and amorphous (gauche rich) phases satisfactorily explains the relationship among the PET conformation, crystallinity and annealing conditions. This study complements the prior study of molecular motion by T-1 rho(C), and illustrates the importance of the trans conformer to induce PET crystallization. (C) 1999 Elsevier Science Ltd. All rights reserved.
Conformational conversion and chain ordering in cycle olefin copolymer (COC)
(2000-01-01)
Chu, PP
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Huang, WJ
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Chang, FC
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Fan, SY
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應用化學系
Polynorbornene (PNB) or cycle olefin copolymer (COC) can undergo conformational conversion into the more stable T(T)G conformer from T(T)T and G(T)G. The conversion constitutes the nascent stage for the reordering process of the COG. Interconversion between two major chain conformers leads to two groups of C-13 NMR resonances corresponding to the CH carbon in the norbornene chain at 53 ppm (T(T)T + G(T)G) and 49 ppm (T(T)G). Chain reordering following the conversion leads to the condensation of the COC bulk and raises its T-g, substantially. The presence of random PE segment not only improves its segmental motion but also facilitates the conversion into the most stable T(T)G conformation. (C) 1999 Elsevier Science Ltd. Ail rights reserved.
Crystallization and melting behaviors of poly(trimethylene terephthalate)
(1999-10-01)
Huang, JM
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Ju, MY
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Chu, PP
;
Chang, FC
;
應用化學系
The crystallization and melting behaviors of poly(trimethylene terephthalate) (PTT) have been studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and solid-state NMR. At certain crystallization temperatures (T-c) for a given time, the isothermally crystallized PTT exhibits two melting endotherms, which is similar to that of PBT and PBT. At higher crystallization temperature (T-c = 210 degrees C), the low-temperature endotherm is related to the melting of the original crystals, while the high-temperature endotherm is associated with the melting of crystals recrystallized during the hearing. The peak temperatures of these double-melting endotherms depend on crystallization temperature, crystallization time, and cooling rate from the melt as well as the subsequent heating rate. At a low cooling rate (0.2 degrees C/min) or a high heating rate (40 degrees C/min), these two endotherms tend to coalesce into a single endotherm, which is considered as complete melting without reorganization. WAXD results confirm that only one crystal structure exists in the PTT sample regardless of the crystallization conditions even with the appearance of double melting endotherms. The results of NMR reveal that the annealing treatment increases proton spin lattice relaxation time in the rotation frame, T-1 rho(H), of the PTT. This phenomenon suggests that the mobility of the PTT molecules decreases after the annealing process. The equilibrium melting temperature (T(m)degrees) determined by the Hoffman-Weeks plot is 248 degrees C.
Effects of molecular structure of modifiers on the thermodynamics of phenolic blends: An entropic factor complementing PCAM
(1999-05-04)
Wu, HD
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Chu, PP
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Ma, CCM
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Chang, FC
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應用化學系
Different thermodynamics in phenolic blends with different polymeric modifiers, i.e., phenoxy, poly(decamethylene adipate), poly(ethylene oxide), and poly(vinyl alcohol), calculated by the Painter and Coleman association model (PCAM) are examined. The thermodynamics is calculated based upon the equilibrium constants derived experimentally from infrared spectroscopies of low molecular weight analogues with similar hydrogen-bonding formation. The discrepancies between the PCAM predictions and the experimentally observed T-g and free volume variation with blending composition are attributed to the additional entropic effects introduced by the long repeated units of modifiers. The structural characteristics and hydrogen-bonding heterogeneity as derived from solid-state NMR and IR spectra support the notion that the length and size of the modifier repeated unit are responsible for such discrepancies. These observed nonidealities can be interpreted as competition between inter- and intraassociations (Delta H-m favored), which depend on the entropy rise associated with the amount of increase of the breaking off of the self-association in phenolic and modifiers within blends. While PCAM is based on "true" miscibility, however, minor modification is required to better describe the thermodynamics for "real" blends where microdomain heterogeneity with size greater than that defined by thermodynamic criteria may be present.
Molecular dynamics and mechanical properties correlations of PA6/PPO blends compatibilized with SMA
(1999-06-01)
Chu, PP
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Huang, JM
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Wu, HD
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Chiang, CR
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Chang, FC
The effects of the compatibilizer, styrene maleic anhydride (SMA-8% MA) upon the change of morphology and molecular dynamics of polyamide-6 (PA6) and poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) blends were investigated by means of solid-state NMR techniques. With increasing amounts of SMA, the domains correspond to PA6 and PPO are reduced and the polymer segmental mobility increased. The correlation between NMR relaxation time, T-1p(H), and the bulk mechanical properties provide a molecular level understanding of the modification of molecular dynamics by the compatibilizer (SMA). The correlation shows that the tensile strength is governed mainly by the morphology, but modulated by the PA6 crystallinity, while the tensile elongation and impact strength are closely affected by both the molecular mobility and morphology. The annealing process improved only the tensile strength, but deteriorated tensile elongation and impact strength due to the increase of PA6 crystallinity, which induced phase separation after annealing. This study raised an important point that the polymer mechanical properties are most sensitive to the molecular structure and dynamics take place within the range of 20 Angstrom to few hundred Angstrom. (C) 1999 John Wiley & Sons, Inc.