Polymorphism, Thermal Behavior and Phase Transitions as a Function of Temperature in Semicrystalline Syndiotactic Polystyrene

• Main Authors: Ya-Sen Sun, 孫亞賢
• Other Authors: E. M. Woo
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/q5z2an

博士 === 國立成功大學 === 化學工程學系碩博士班 === 90 === Abstract The first portion of this dissertation focuses on solution-cast and solvent-induced crystallization. To investigate the phase transitions of the sPS-solvent complexes of d or g-crystals subjected to slow heating, three solvents are used in this study: 1,1,2,2-tetrachloroethane, o-dichlorobenzene and 1,2-dichloroethane. According to evidences of X-ray diffraction, transmission electron microscopy, electron diffraction, differential scanning calorimeter combined with Fourier transform infrared analysis, we have confirmed that the g-type crystal is directly yielded without further annealing by solution-cast for sPS/1,1,2,2-tetrachloroethane system, while the similar sample preparations imposed on sPS samples produce d-type crystal for sPS/o-dichlorobenzene and sPS/1,2-dichloroethane systems. Through TEM morphological observation, one has found the small crystallizes of g-form with typical size of <100nm. The TEM morphology and powder-like ED patterns of g-from are shown in Chapter 2. Additionally, upon annealing, two possible helix®trans phase transitions are successfully proposed by using the IR spectral changes in the frequency range of 720~820cm-1 which is associated with the vibration of the C-H out of plane bending of the phenyl ring. On is that an ordered g-type®a¢-type crystal transition occurs at a high temperature of about 200°C. The other is that a disordered g-type®a¢-type crystal transition occurs at approximate 160°C. The detail results are discussed in Chapter 3. Furthermore, on the basis of the IR spectra changes as a function of rising annealing temperature, the degree of crystallinity of mesomorphic form was quantitatively evaluated and determined. A temperature-induced solid-solid transition from mesomorphic form to a¢-crystal has been confirmed. An effect of the miscible blends with various contents of aPS on the polymorphism of the sPS sample subjected to cold-crystallization is also discussed in the study. It has been found that mixed a and b-crystals of various fractions was obtained in the sPS/aPS blends while the a-crystal (or mesomorphic form) becomes discernible species in the glassy sPS samples subjected to cold-crystallization. However, there is no miscible blends effect on a g-type®a¢-type crystal transition by slow heating the solution-cast sPS/aPS blends. Several reasonable models for explanations on structural changes with rising annealing temperatures were also proposed in the text. The second portion of this study focuses on the morphological changes in the sPS samples melt and cold-crystallized isothermally at various temperatures. It has been known that the sPS samples possess multiple melting behaviors. According to the evidences of polarized microscopy, scanning electron microscopy, X-ray diffraction and differential scanning calorimeter, a correlation between lamellar morphology and multiple melting peaks are clarified. There are three melting peaks, which are assigned as P-I, P-II and P-III, in the sPS samples consisting of isolated b¢-type crystal when melt-crystallized at a low temperature of 230°C. A morphology analysis showed that flat-on lamellae develop first, which yield P-I and P-II melting, and during scanning re-crystallize to thickened edge-on lamellae with P-III melting peak. Upon scanning, melting of P-I (crystal of the lowest melting peak) is followed by repacking into thickened P-III crystal, the lamellae of which also reoriented to a perpendicular orientation. The P-II crystal, however, melts at temperatures too close to the melting temperature of P-III; thus, during scanning up, the P-II crystal simply melts without sufficient time to repack into the thickened P-III crystal. In addition, P-III lamellae could also be directly developed in sPS by melt-crystallization at higher temperatures while P-I and P-II crystals were absent. Cold-crystallization of sPS produced only a-type unit cell peaked into crystalline domains of different morphology. The morphology contains a granular-sphere texture when cold-crystallized at low temperatures while high-temperature cold-crystallization produced additional sheaf-like lamellae radiating out from the central spheres.