Amorphous Trans Research Articles

Nanostructured TiO2: Analysis and characterization

Nanostructured TiO2 thin films were deposited on quartz glass at room temperature by sol–gel dip coat-ing method. The effects of annealing temperature between 200◦C to 1100◦C were investigated on the structural, morphological, and optical properties of these films. The X-ray diffraction results showed that nanostructured TiO2 thin film annealed at between 200◦C to 600◦C was amorphous trans- formed into the anatase phase at 700◦C, and further into rutile phase at 1000◦C. The crystallite size of TiO2 thin films was increased with increasing annealing temperature. From atomic force microscopy images, it was confirmed that the microstructure of annealed thin films changed from column to nubbly. Besides, surface roughness of the thin films in- creases from 1.82 to 5.20 nm, and at the same time, average grain size as well grows up from about 39 to 313 nm with increase of the annealing temperature. The transmittance of the thin films annealed at 1000 and 1100◦C was reduced significantly in the wavelength range of about 300–700 nm due to the change of crystallite phase. Refractive index and optical high dielectric constant of the n-TiO2 thin films were in- creased with increasing annealing temperature, and the film thickness and the optical band gap of nanostructured TiO2 thin films were decreased.

Effect of clay on melt crystallization, crystallization kinetics and spherulitic morphology of poly(trimethylene terephthalate) nanocomposites

Poly(trimethylene terephthalate) (PTT)/clay nanocomposite films have been prepared via a novel two-step approach. The resulting nanocomposites have been characterized by Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (XRD), differential scanning calorimetry (DSC) and polarized light microscopy (PLM). XRD results showed this two-step process of preparation forms predominantly exfoliated structures. The effect of nanoclay on the isothermal and nonisothermal melt crystallization behaviors of PTT has been investigated and shown that the incorporation of clay accelerates the rate of crystallization and increases the total crystallinity of PTT nanocomposites compared to neat PTT. Nonisothermal melt crystallization kinetics of neat PTT and PTT nanocomposites was analyzed using crystallization isotherm. The Avrami analysis modified by Jeziorny was successfully used to describe the non-isothermal crystallization kinetics of neat PTT and PTT nanocomposites, shown by the decrease in half time (t1/2) of crystallization and increase in rate constant (Zc) as organoclay content increased up to 10%. FTIR spectroscopy has been utilized for the first time to monitor conformational changes during the melt crystallization of PTT nanocomposites. It was demonstrated that the amorphous trans conformation transforms into the crystalline gauche conformation with increasing clay content, which suggests that nanoclay accelerates the polymer chain conformational transition from trans to gauche conformation. No significant change in amorphous gauche conformation was observed either with crystallization temperature or clay content.

Strain-Induced Crystallization and Conformational Transition of Poly(trimethylene terephthalate) Films during Uniaxial Deformation Probed by Polarized Infrared Spectroscopy

The structure development during the drawing of PTT films immediately above the glass transition temperature at two different strain rates (8.33 × 10–3 and 8.33 × 10–4 s–1) has been investigated using differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. It was found that both the melting temperature and the crystallinity development are dependent on the strain rate. The cold crystallization peak decreases with increasing draw ratio and disappears completely beyond a draw ratio of 2.5. DSC results showed that the decreasing strain rate delays crystallization and reduces the rate of crystallization. The bands at 1358 and 976 cm–1 were chosen to determine the gauche and trans conformations of methylene segments in PTT as a function of the draw ratios and strain rates. It was found that the crystalline gauche conformation increases at the expense of the amorphous trans conformation during the strain-induced crystallization of PTT. The conversion of the amorphous trans conformation into the crystalline gauche conformation is also delayed at lower strain rate, which is consistent with our DSC observation. Polarized IR spectroscopy was used to measure the crystalline and the amorphous orientation functions separately with draw ratios and strain rates, and it was demonstrated that the crystalline orientation develops rapidly with strain-induced crystallization and that the amorphous orientation stays constant up to draw ratio of 2.5 and increases slowly above a draw ratio of 2.5, which is typical behavior for flexible chain polymers.

Morphological and Conformational Changes of Poly(trimethylene terephthalate) during Isothermal Melt Crystallization

The isothermal melt crystallization of poly(trimethylene terephthalate) (PTT) has been investigated using differential scanning calorimetry (DSC), optical microscopy, and Fourier transform infrared (FTIR) spectroscopy. Triple-melting endotherms were dominant at crystallization temperatures (T(c)) below 195 °C, while double-melting endotherms were dominant at T(c) above 195 °C. These multiple-melting behaviors were attributed to melting and recrystallization as well as thermal stability of the crystallites. The difference in crystallinity observed between melt-crystallized PTT films before and after cooling to room temperature was attributed to lamellar thickening. The kinetics of isothermal melt crystallization of PTT was analyzed using an Avrami equation, and the rate constant (k) and t(1/2) increased with increasing T(c). An Avrami exponent n = 2-3 was obtained for the PTT melt crystallized from 150 to 205 °C, suggesting two-dimensional to three-dimensional growth with heterogeneous nucleation and was confirmed by polarized optical microscopy. The bands at 1358 and 976 cm(-1) associated with gauche and trans conformations of -OCH(2)CH(2)CH(2)O- were used to determine gauche and trans conformations assuming a two-phase conformational model. The crystalline and the amorphous gauche conformations were separated by combining them with DSC crystallinity. It was shown that the crystalline gauche conformation increased at the expense of the amorphous trans conformation. No significant change in the amorphous gauche conformation was found. On the other hand, the crystalline and amorphous gauche conformations were found to increase with the annealing temperature (T(a)) at the expense of the trans conformations during the annealing of PTT. The difference between melt crystallization and annealing was also discussed.

Structural and conformational changes during thermally‐induced crystallization of poly(trimethylene terephthalate) by infrared spectroscopy

AbstractConformational changes occurring during thermally‐induced crystallization of poly(trimethylene terephthalate) (PTT) by annealing have been studied using density measurement, differential scanning calorimetry (DSC), and mid‐infrared spectroscopy (MIR). Infrared spectra of amorphous and semicrystalline PTT were obtained, and digital subtraction of the amorphous contribution from the semicrystalline PTT spectra provided characteristic MIR spectra of amorphous and crystalline PTT. The normalized absorbance of 1577, 1173, and 976 cm−1 were plotted against the crystallinity showing that these bands can be used unambiguously to represent the trans conformation while the band at 1358 cm−1 can be used to represent gauche conformation of methylene segment. The presence of a weak band at 1358 cm−1 in the amorphous spectrum suggested that a small amount of gauche conformation is present in the amorphous phase. Infrared spectroscopy has been used for the first time as a means to estimate the trans and gauche conformations of methylene segments in PTT as a function of Ta. The amount of gauche conformation was plotted against the crystalline fraction and the extrapolation of this plot to zero crystalline fraction provided a value of 0.07, suggested that the pure amorphous phase consist of ∼ 7% gauche conformation. It was found that the amorphous and crystalline gauche conformation increases at the expense of amorphous trans conformation during thermally induced crystallization of PTT. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1497–1504, 2008

Selective Observation and Quantification of Amorphous Trans Conformers in Doubly 13C-Labeled Poly(ethylene terephthalate), PET, by Zero-Quantum Magic-Angle-Spinning Solid-State NMR

A new method for identifying and quantifying conformations of 13 C-labeled polymer segments in magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra is introduced. The observed magnetization is dephased by the difference of the recoupled anisotropic chemical shifts, reflecting relative orientation, of two sites with a significant 13 C- 13 C dipolar interaction. Signals of trans conformations decay more slowly than those near gauche or cis if the chemical-shift tensors in the trans conformation are related by inversion symmetry. This permits an estimate of the trans:gauche ratio. The experiments, which are based on zero-quantum (ZQ) or double-quantum/single-quantum (DQ-2SQ) dephasing under the recoupled chemical-shift anisotropy (CSA), yield selective MAS spectra of methylene groups in trans or gauche conformations. For the O-CH 2 -CH 2 -O moieties in amorphous and semicrystalline poly(ethylene terephthalate) (PET), the ZQ-CSA-dephasing and (DQ-2SQ)-CSA-dephasing methods enable selective observation of the amorphous trans components, which cannot be detected by traditional CP/ MAS experiments due to complete overlap with dominant gauche and crystalline trans signals. The observed amorphous trans chemical shift is close to that in PET crystallites; it cannot be rationalized based on the empirical γ-gauche effect. These experiments also provide signal decay curves from which torsion angles can be determined, with high angular resolution around the trans state. The data yield 12 ± 3% trans in amorphous PET, with a approximate 16° standard deviation of the torsion angles from the ideal trans state. In semicrystalline PET, the total trans fraction is 43 ± 3% and the best estimate of the crystalline-trans fraction, which corresponds to the degree of crystallinity, is 23 ± 5%; the amorphous trans fraction is 20 ± 5%. The trans fraction in the amorphous regions of semicrystalline PET, 26 ± 5%, is significantly higher than the 12 ± 3% in amorphous PET. The experiment is easily combined with double-quantum spectroscopy of isotropic chemical shifts, which provides further information on the isotropic chemical shifts of specific conformers. In amorphous PET, the two CH 2 groups in trans conformers are found to experience similar local fields.

Injection molding of poly(ethylene terephthalate): Differential scanning calorimetry and confocal micro-raman spectroscopy investigations of the skin-core morphology

The skin-core structure of injection-molded plates of poly(ethylene terephthalate) (PET) has been investigated using Confocal Micro-Raman Spectroscopy (CMRS) and Differential Scanning Calorimetry (DSC). CMRS has proved to be a useful tool for finding out about quantitative conformational changes in polymeric materials. In order to obtain a complete picture of isomer distributions in the injection molded industrial PET samples, spectroscopic measurements were correlated with DSC measurements. The spectroscopic results reveal two different structural zones related to the fabrication process: a skin layer and the core. The correlation between the apparent degree of crystallinity obtained by DSC and the percentage of the trans isomer obtained by CMRS has allowed the latter parameter to be separated into crystalline and amorphous trans isomer and to follow its change along the injection molded plate.

Infrared spectroscopic studies of melting behavior of poly(ethylene 2,6-naphthalenedicarboxylate)

The FTIR method was used to study the melting behavior of poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) samples of different thermal history of crystallization. With subtraction of the spectra of PEN samples of different crystallinities and amorphous PEN, the spectra of crystalline trans conformers, amorphous trans conformers, and amorphous gauche conformers were obtained. By following the changes in intensity of various gauche and crystalline bands during heating, the phenomenon of multiple melting endothermic peaks was shown to be due to the melting of imperfect crystalline conformation formed during crystallization. For two-step annealed samples, the crystalline conformational defects change into a more perfect one during annealing at higher temperatures.

Fourier transform Raman study of the conformers in poly(ethylene terephthalate)

AbstractFourier transform (FT) Raman spectroscopy has been proved to be an adequate technique for the study of the quantitative structural changes which take place in poly(ethylene terephthalate) on annealing. The two‐phase conformational model proposed previously in the infrared domain using photoacoustic FTIR has been extended to the Raman domain. Moreover, typical Raman spectral changes with the thermal process have been directly related to conformational rather than crystallinity changes. The correlation between the percentage of the trans isomer obtained by FT‐Raman and skin layer crystallinity values obtained by differential scanning calorimetry allows the crystalline and amorphous trans isomer contents to be obtained and their evolution with the annealing process to be followed.

Structural analysis of injection-moulded semicrystalline polymers by Fourier transform infra-red spectroscopy with photoacoustic detection and differential scanning calorimetry: 1. Poly(ethylene terephthalate)

Fourier transform infra-red spectroscopy coupled with photoacoustic detection (PA FTi.r.) has proved to be a useful tool for finding out about quantitative structural changes in solid materials. Poly(ethylene terephthalate) (PET) is a well known semicrystalline polymer that shows important changes on annealing. In order to obtain a complete picture of isomer distributions in industrial PET samples, spectroscopic measurements were correlated with differential scanning calorimetry (d.s.c.). The results obtained indicate that the structural characteristics of the thermally treated samples are related to the fabrication process. Two different strata in the plates can be distinguished: a skin layer and the core. The correlation between the apparent degree of crystallinity of the surface obtained by d.s.c. and the percentage of trans isomer obtained by PA FTi.r. allows the latter parameter to be separated into crystalline and amorphous trans isomer and to follow its evolution with the annealing process. Amorphous trans isomer vanishes at the primary isomerization (∼ 100°C) while crystalline trans and gauche conformations show sigmoidal evolution. At higher annealing temperatures (> 140°C) the ordered trans conformation shows an approximately linear increase at the expense of the gauche conformation. Finally, a direct correlation between the ordered trans isomer and the apparent degree of surface crystallinity can be made.

Optical properties of conjugated segments embedded in polyvinylidene-chloride

Oriented conjugated segments embedded in polyvinylidene-chloride (PVDC) were prepared by thermal degradation of PVDC-films under applied mechanical stress. These samples exhibit stable optical properties while stored in air in contrast to the oxidative instability of pure polyacetylene. Transmission spectra in the visible range show highly anisotropic properties even at low concentration of conjugated bonds. Distinct oscillatory behavior of the absorption of light polarized perpendicular to the stretching direction is observed and the positions of the extrema are extrapolated to infinite conjugation length assuming a simple model. A comparison with optical data for dense trans-(CH) x films yields an average conjugation length of n = 36 for oriented crystalline trans-(CH) x-films and n = 11–14 for amorphous trans-(CH) x films.

Spectroscopic characterization of the rotational conformations in the disordered phase of poly(ethylene terephthalate)

AbstractRecently developed data processing techniques have been applied to determine the complete spectra of the trans and gauche isomers of PET and to use these spectra to determine the conformational composition of PET samples isothermally crystallized from the glassy state between 75 and 240°C. A linear correlation was found between normalized absorbance of conformational bands and density measurements, with the trans bands showing a positive slope and the gauche bands a negative slope. Extrapolation of the results from trans bands to zero percent of trans yields the density of the pure gauche isomer, 1.326 ± 0.002 g/cm3. Similarly, from the gauche bands the density of 100% crystalline trans is found to be 1.510 ± 0.014 g/cm3. Meanwhile, the density of the amorphous trans is calculated to be 1.430 ± 0.003 g/cm3. From these values and the combined density and infrared measurements, the content of trans isomers in the amorphous phase is calculated. It is found that the content of amorphous trans is a function of both annealing temperature and time. It is observed that the amorphous trans content decreases as the crystalline trans content increases. At high crystallinity, the amorphous trans content approaches zero. These amorphous trans isomers are associated with the extended units making up the interlamellar links. The loss of these extended trans interlamellar links is reflected in the mechanical properties of PET.

Application of fourier transform infrared spectroscopy to the study of semicrystalline polymers: Poly(ethylene terephthalate)

AbstractAn infrared absorbance subtraction technique has been used to “isolate” bands in the composite spectrum of semicrystalline polymers according to their crystalline or amorphous character. Amorphous and crystalline spectra for annealed, melt‐quenched, and solution‐cast poly(ethylene terephthalate) have been separated. The spectra of the amorphous component show an increased intensity of bands associated with the trans configuration of oxygen about the CC bond when the polymer is annealed. This increased “trans” band intensity reflects the increased proportion of trans structures as a result of annealing. The amorphous trans bands are shifted approximately 1–3 cm−1 from their positions in the crystalline “trans” spectrum. The frequency shift of these bands can be attributed to the differences in chain interactions that exist in the amorphous phase and the crystalline lattice. We have also found that under identical anealing conditions the amorphous phase of the melt‐quenched polymer contains an increased intensity of conformational trans bands compared to the sample cast from solution.