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

Conformational 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 T_a. 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     

Evolution of crystallinity in photodegraded polyethylene films studied by ft-raman spectroscopy

FT-Raman spectroscopic studies of photodegraded polyethylene films have enabled the evolution of the crystallinity process to be measured. Commercial polyethylene films of M_w=90 000 were exposed in a weathering UV-chamber under known conditions of exposure time and radiant energy. The spectral profiles were modelled using Fourier methods. The relative amounts of the orthorrombic crystalline phase, α_c, the amorphous phase, α_a and the interphase, α_b, were calculated using Raman bands at 1416 cm⁻¹ characteristic of the crystalline phase and the bands at 1080, 1305 cm⁻¹, characteristic of the amorphous phase. The interphase content can be calculated from the relationship αb= 1-(α_c+α_a). It was found that the weathering process affects only the relative intensities of the bands attributed to crystalline and amorphous fractions; the crystalline content increases at the expenses of the amorphous fraction. These results are discussed in terms of the changes in the intermolecular forces caused by radiation exposure.     

FTIR spectroscopic characterization of structural changes in polyamide‐6 fibers during annealing and drawing

First, we report the development of Fourier transform infrared (FTIR) spectroscopic methods to determine the α/γ‐crystalline phase ratio of polyamide‐6 fibers and, in combination with density measurements, the total crystallinity. Using density determinations of the crystallinity of pure α and pure γ samples, we found the absorption coefficient ratio for the 930 (α) and 973 cm⁻¹ (γ) bands to be 4.4, from which we could obtain the α/γ ratio for any polyamide‐6 sample. The application of this FTIR method to the quantitative analysis of phase changes during thermal treatment and the drawing of polyamide‐6 was then made. We confirmed that crystallization during thermal treatments involved increases in both phases and did not involve crystal‐to‐crystal transformation, whereas drawing involved both crystallization of the amorphous phase in the α form and γ→α transformation. Finally, we revisited the band assignments for the amorphous phase of polyamide‐6 and found that the band at 1170 cm⁻¹ was not an amorphous band but, because its absorbance was independent of crystallinity, could be used as an internal reference band. The band at 1124 cm⁻¹ was reliably attributed to the amorphous phase. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 536–547, 2001     

Infrared dichroism of polyethylene crystallized by orientation and pressure in a capillary viscometer

Polarized infrared absorption spectra have been obtained by Fourier-transform spectroscopy for several crystalline and noncrystalline absorption bands of polyethylene crystallized by orientation and pressure in capillary viscometer. An analysis of data obtained at room temperature yielded degrees of crystallinity which are in good accord with values obtained from calorimetry and density measurements. The dichroism of the infrared absorption bands for the crystalline region revealed an extreme degree of orientation consistent with previous x-ray studies and also demonstrated that the degree of orientation is a good or better than that obtained from drawn polyethylene films with extension ratios of 20. Dichroism of bands from the amorphous phases revealed that the noncrystalline chain segments are in a comparatively relaxed state compared with results for drawn films having extension ratios of about 2 to 7. This is 1/10 to 1/3 the extension ratio of drawn polyethylene which shows maximum crystalline orientation. The results also indicated that the ratio of the GTG′ to GG segment conformations in the amorphous regions is larger than that of amorphous portions in unoriented polyethylene. The vinyl endgroups were shown to be highly oriented, while the main bulk of the amorphous polymer was fairly relaxed, i.e., of low orientation. It is concluded that the amorphous polyethylene state is strongly dependent on the nature of the crystalline–amorphous interface.     

Determination of crystallinity of swollen poly(vinyl alcohol) by laser Raman spectroscopy

Laser Raman spectra of atactic poly(vinyl alcohol) (PVA) after heat treatment and/or swelling in water have been obtained. An amorphous Raman band is observed at 1124 cm^?1, while a crystalline Raman band is found at 1147 cm^?1. A new method for crystallinity determination is proposed, in which the amorphous band is used instead of the crystalline band. The method is superior to others for water-swollen PVA samples. Laser Raman spectra of swollen PVA revealed that swelling causes destruction of a major fraction of the crystalline regions and the remaining intact crystalline part increases with increasing temperature of heat treatment.     

Infrared spectroscopic characterization of oriented polyamide 66: Band assignment and crystallinity measurement

Having found much ambiguity in the infrared band assignments for polyamide 66 (PA66), we revisited some of these assignments before using infrared spectroscopy to assess microstructure changes resulting from multiple thermal treatments. We discovered that earlier assignments of the 1144 and 1180 cm⁻¹ bands to the amorphous (noncrystalline) phase were incorrect, whereas the bands at 924 and 1136 cm⁻¹ can be attributed unambiguously to the noncrystalline phase. We also confirmed that PA66 bands at 936 and 1200 cm⁻¹ are crystalline bands. The normalized absorbance of the 1224‐cm⁻¹ fold band increases in proportion to crystallinity, indicating that chain folding is the predominant mechanism of thermal crystallization in PA66. We demonstrated that infrared spectroscopy can be used to estimate the degree of crystallinity of PA66, and two methods were explored. One is a calibration method in which the band ratio of 1200 and 1630 cm⁻¹ is plotted against crystallinity measured by density. The other is an independent infrared method based on the assumption that PA66 satisfies a two‐phase structure model. The crystallinity determined by the independent infrared method showed good agreement with the crystallinity obtained from density measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 516–524, 2000     

A Raman spectroscopic study of the morphological structure of the polyethylenes

The method described by Strobl and Hagedorn to analyze the Raman spectrum internal modes of semi-crystalline polyethylene has been applied to a set of selected polyethylene samples crystallized under controlled conditions. The crystallite structure can be described in terms of the relative amounts of the crystalline orthorhombic phase, the liquid-like amorphous phase and the interfacial region. The dependence of the level of crystallinity on molecular weight and crystallization conditions is very similar to that found by other methods. However, this method allows for the quantitative determination of the interfacial content which becomes significant for molecular weights greater than about 1×10⁵ for linear polyethylene fractions, and for all the branched samples and copolymers. The degree of crystallinity determined from density measurements is equal to the sum of the crystallinity and interfacial content obtained from the Raman analysis while enthalpy of fusion measurements yield values which are equal to just the crystallinity content. The difference between the level of crystallinity obtained from density and enthalpy of fusion is thus found to be primarily due to interfacial contributions.Dedicated to Prof. Dr. F. H. Müller     

Oxygen‐barrier properties of cold‐crystallized and melt‐crystallized poly(ethylene terephthalate‐co‐4,4′‐bibenzoate)

This study examined the oxygen‐transport properties of poly(ethylene terephthalate‐co‐bibenzoate) (PETBB55) crystallized from the melt (melt crystallization) or quenched to glass and subsequently isothermally crystallized by heating above the glass‐transition temperature (cold crystallization). The gauche–trans conformation of the glycol linkage was determined by infrared analysis, and the crystalline morphology was examined by atomic force microscopy. Oxygen solubility decreased linearly with volume fraction crystallinity. For melt‐crystallized PETBB55, extrapolation to zero solubility corresponded to an impermeable crystal with 100% trans glycol conformations, a density of 1.396 g cm^?3, and a heat of melting of 83 J g^?1. From the melt, PETBB55 crystallized as space‐filling spherulites with loosely organized lamellae and pronounced secondary crystallization. The morphological observations provided a structural model for permeability consisting of impermeable platelets randomly dispersed in a permeable matrix. In contrast, cold‐crystallized PETBB55 retained the granular texture of the quenched polymer despite the high level of crystallinity, as measured by the density and heat of melting. Oxygen solubility decreased linearly with volume fraction crystallinity, but zero solubility corresponded to an impermeable defective crystal with a trans fraction of 0.83 and a density of 1.381 g cm^?3. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2489–2503, 2002     

Infrared studies of drawn polyethylene. II. Orientation behavior of highly drawn linear and ethyl-branched polyethylene

Infrared dichroism is employed to study the orientation of chain molecules in linear and ethyl-branched polyethylene in the crystalline and noncrystalline regions during drawing and subsequent annealing. A crystalline (1894 cm^?1) and a noncrystalline (1368 cm^?1) band, as well as the bands at 909 cm^?1 and 1375 cm^?1 resulting from vinyl endgroups and methyl endgroups and sidegroups, are studied. For these bands relative orientation functions are derived and compared as a function of draw ratio and annealing temperature. It is shown that the relative orientation functions as derived from the dichroism of the noncrystalline, vinyl and methyl bands follow the same curve while the orientation function for the crystalline bands does not. These results support a two-phase model for partially crystalline polyethylene and additionally favor segregation of the endgroups and sidegroups in the noncrystalline component during crystallization. It is further shown that shrinkage occurs at the temperature at which the noncrystalline chain molecules start to disorient. From the dichroism of the methyl groups in ethyl-branched polyethylene, a value for the mean orientation of the noncrystalline chain molecules is calculated. We obtain for the orientation function of the noncrystalline regions at highest draw ratios (λ = 15–20), f = 0.35–0.57, while the chain molecules in the crystallites are nearly perfectly oriented (f ≈ 1.0). On the assumption that the noncrystalline component consists of folds, tie molecules, and chain ends, the different contributions of these components to the overall orientation are estimated. From these the relative number of CH₂ groups incorporated into folds, tie molecules, and cilia can be derived. Further, on the basis of a simple structural model, the relative number of chains on the crystal surface contributing to the different noncrystalline components and their average length are estimated.     

The conformation and vibrational spectra of isocyanato and isothiocyanatocyclohexane

The IR spectra of isocyanato and isothiocyanatocyclohexane (C₆H₁₁NCX) as liquids and as amorphous and crystalline solids at low temperatures have been recorded in the region 4000-50 cm^?1. High pressure (0–30 kbar) IR spectra of the neat samples were obtained in a diamond anvil cell and various high-pressure solid phases were studied. Raman spectra of the compounds as liquids and as low-temperature solids were obtained.Isocyanatocyclohexane crystallized directly as anisotropic solids containing equatorial molecules at low temperature and axial molecules at high pressure. Isothiocyanatocyclohexane formed a possibly plastic phase between 225 and 260 K where both equatorial and axial conformers are present. A solid high-pressure phase (1–3 kbar) at ambient temperature appeared anisotropic and contained both the e and a conformers. Below 225 K (atmospheric pressure) and above 10 kbar (ambient temperature) anisotropic crystals were formed which both contained equatorial conformers only.Normal coordinate analyses were carried out for the equatorial and axial conformers of the two molecules with different orientations (C_s and C_l symmetries) of the side chain. Force constants were transferred from various halo and pseudohalocyclohexanes. Tentative assignments of the fundamentals belonging to both the e and a-conformers are presented.     

Structure development of polyamide‐66 fibers during drawing and their microstructure characterization

Structure development during drawing was studied for three sets of polyamide‐66 (PA66) fibers with density, optical microscopy, wide‐angle X‐ray diffraction, and Fourier transform infrared spectroscopy. The crystallinity, estimated by density measurements, remained virtually constant with increasing draw ratios, indicating that stress‐induced crystallization did not occur for the PA66 fibers drawn at room temperature, but there was a rapid transformation from a hedrite morphology to a fibrillar one. The absence of stress‐induced crystallization differed from the behavior of polyamide‐6, and this was attributed to the stronger hydrogen bonding between polyamide chains and the higher glass‐transition temperature of PA66. Polarized infrared spectroscopy was used to measure the transition‐moment angles of the vibrations at 936 and 906 cm^?1, which were found to be 48 and 60°, respectively. The crystalline orientation was estimated from the band at 936 cm^?1, and the increase with an increasing draw ratio was in close quantitative agreement with X‐ray diffraction data; this showed that infrared spectroscopy could be used reliably to measure the crystalline orientation of PA66 fibers. Because we were unable to obtain the transition‐moment angle of the amorphous bands, the amorphous orientation was obtained with Stein's equation. The amorphous orientation developed more slowly than the crystalline orientation, which is typical behavior for flexible‐chain polymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1940–1948, 2002     

Temperature‐dependent polymorphic crystalline structure and melting behavior of poly(butylene adipate) investigated by time‐resolved FTIR spectroscopy

The polymorphic crystalline structure and melting behavior of biodegradable poly(butylene adipate) (PBA) samples melt‐crystallized at different crystallization temperatures were studied by differential scanning calorimetry (DSC) and fourier transform infrared (FTIR) spectroscopy. The crystalline structure and melting behavior of PBA were found to be greatly dependent on the crystallization temperature. By comparison of the FTIR spectra and the corresponding second derivatives between the α‐ and β‐crystal of PBA, the spectral differences were identified for the IR bands appeared at 1485, 1271, 1183, and 930 cm^?1 and the possible reasons were presented. Especially, the 930 cm^?1 band was found to be a characteristic band for the β‐crystal. Combining the DSC data with the analysis of normalized intensity changes of several main IR bands during the melting process, the melting behaviors of the α‐ and β‐crystal were clarified in detail. It is demonstrated by the in situ IR measurement that the β‐crystalline phase would transform into the α‐crystalline phase during the melting process, and the solid–solid phase transition from the β‐ to α‐crystal was well elucidated by comparing the intensity changes of the 1170 and 930 cm^?1 bands. The dependence of the β‐ to α‐crystal phase transition on the heating rate was revealed by monitoring the intensity ratio of the 909 and 930 cm^?1 band. It was suggested that at the heating rate of 0.5 or 1 °C/min, the percent amount of the transformed α‐crystal from the β‐crystal was much higher than that at the higher heating rate. The β‐crystal transforms into the α‐crystal incompletely at the higher heating rate because of the less time available for the phase transition. In addition, the β‐ to α‐crystal phase transition was further confirmed by the IR band shifts during the melting process. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1997–2007, 2009     

The conformation and vibrational spectra of trans-1,4-dihalocyclohexanes: Part II. trans-1,4-Chlorobromocyclohexane and trans-1,4-chloroiodocyclohexane

The IR spectra of trans-1,4-chlorobromo- and trans-1,4-chloroiodocyclohexane were recorded in the region 4000–30 cm^?1 as solutes in various solvents, as KI and polyethylene pellets and as solids under high pressure (1–50 kbar at ambient temperature). Additional spectra of the melts, amorphous and annealed crystalline solids at 90 K and dichroism of oriented polycrystalline films were obtained above 200 cm^?1. Raman spectra of the compounds were recorded in the amorphous and crystalline states at 90 K, and polarization measurements were made in CCl₄ CS₂ and C₆H₆ solution.The title compounds existed as an equilibrium mixture of ee and aa conformers in solution, in the melts and in the amorphous solids at 90 K. When heated to temperatures in the region 165–195 K the amorphous solids apparently crystallized into a metastable form containing the aa conformer, while above 200 K the solids were converted to a stable crystal containing the ee conformer. Under high pressure the concentration of the aa conformer increased and this form was almost exclusively present at ca. 50 kbar nominal pressure.The fundamental frequencies for both conformers were assigned in terms of C_smolecular symmetry. An extensive normal coordinate analysis on six trans-1,4-dihalocyclohexanes was carried out using the overlay technique.     

Morphology and modulus–temperature behavior of semicrystalline poly(ethylene terephthalate) (PET)

The influence of the thermal history on the morphology and mechanical behavior of PET was studied. The degree of crystallinity (density measurements) and the morphological structure (electron microscopy and small-angle x-ray diffraction) depend on the crystallization temperature. The viscoelastic parameters obtained from the modulus–temperature curves are mainly determined by the morphology of the samples. The glass-transition temperature, T_i, is a function of the crystallinity and the crystallization temperature. It is maximum for a crystallinity between 0.34 and 0.39 for a sample crystallized isothermally between 120 and 150°C. This dependence on crystallization conditions is ascribed to the conformation of the amorphous chain segments between the crystalline lamellae as well as the concentration and the molecular weight of the polymer material rejected during isothermal crystallization. Both factors are supposed to be temperature-dependent. The value of the rubbery modulus is a function of both the volume concentration of the crystalline lamellae and the structure of the interlamellar amorphous regions (chain folds, tie molecules, chain ends, and segregated low molecular weight material). Annealing above the crystallization temperature of isothermally crystallized samples has a marked influence on their morphology and mechanical behavior. The morphological structure and the viscoelastic properties of annealed PET samples are completely different from those obtained with samples isothermally crystallized at the same temperature.     

Thermodynamics of crystalline linear high polymers. V. Extended-chain copolymers of polyethylene

Ethylene—propylene and ethylene—butene-1 copolymers with up to 1.7 side groups per 100 carbons have been crystallized at 227°C. and under 4100–4900 atm. pressure. The resulting crystalline polymers are at least partially of extended-chain crystal morphology. Comparison with the same polymers crystallized at atmospheric pressure, which gives folded-chain crystal morphology, revealed: (1) a density higher by 0.008–0.019 g./cm.³ depending on copolymer content; (2) a similar decrease of crystallinity with side group concentration; (3) a similar decrease of the beginning of melting from 125°C. for homopolymer to 65°C. for 1.7 side groups per 100 carbons; (4) a higher (138 ± 0.8°C.) experimental maximum melting point which, in contrast, is independent of copolymer content and seems to vary only with the fraction of low molecular weight material; (5) a decreasing amount of high-melting crystals with increasing copolymer content (72–8%) and an increasing amount of low-melting crystals (27–53%) with increasing copolymer content. In addition, superheating, which reached 5.5°C. for 50°C./min. heating rates, was detected. It was concluded that high-pressure crystallization leads, at least for part of the crystals, to solid solution formation, while atmospheric pressure crystallization does not. Which mode of crystallization is achieved seems kinetically determined. Experimental techniques were dilatometry, DTA, and calorimetry.     

Microhardness studies of chain-extended PE. I. Correlations to microstructure

The hardness–microstructure correlation of various polyethylene (PE) samples crystallized at high pressure from the melt (chain-extended), with different molecular weights, has been investigated and compared to melt crystallized samples at atmospheric pressure (chain-folded). The hardness, H, of melt crystallized PE is confirmed to increase linearly with the logarithm of the annealing time, t_a, at a constant annealing temperature. The H increase with t_a is discussed in terms of the crystallinity and crystalline lamellar thickness variation. Unusually high hardness values are obtained for samples crystallized or annealed at high pressure as a consequence of the resulting high degree of crystallinity and large crystalline lamellar thickness values. However, it is shown that the high surface free energy value of the chain-extended crystals considerably lowers the hardness values from that of an ideal infinitely thick PE crystal. Analysis of the crystal hardness and the melting temperature data of different polymeric materials emphasizes the close existing relationship between both quantities. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3151–3158, 1999     

Two‐dimensional Fourier transform infrared spectroscopy study of biosynthesized poly(hydroxybutyrate‐co‐hydroxyhexanoate) and poly(hydroxybutyrate‐ co‐hydroxyvalerate)

Generalized two‐dimensional (2D) Fourier transform infrared correlation spectroscopy was used to investigate the effect of the comonomer compositions on the crystallization behavior of two types of biosynthesized random copolymers, poly(hydroxybutyrate‐co‐hydroxyhexanoate) and poly(hydroxybutyrate‐co‐hydroxyvalerate). The carbonyl absorption band around 1730 cm^?1 was sensitive to the degree of crystallinity. 2D correlation analysis demonstrated that the 3‐hydroxyhexanoate units preferred to remain in the amorphous phase of the semicrystalline poly(hydroxybutyrate‐co‐hydroxyhexanoate) copolymer, resulting in decreases in the degree of crystallinity and the rate of the crystallization process. The poly(hydroxybutyrate‐co‐hydroxyvalerate) copolymer maintained a high degree of crystallinity when the 3‐hydroxyvalerate fraction was increased from 0 to 25 mol % because of isodimorphism. The crystalline and amorphous absorption bands for the carbonyl bond for this copolymer, therefore, changed simultaneously. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 649–656, 2002; DOI 10.1002/polb.10126     

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

Recently 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.     

Orientation of isotactic polypropylene in crystalline and amorphous phases: IR methods

Two methods based on polarization infrared spectroscopy are described for measurement of polypropylene orientation. One method, for the crystalline phase, allows precise measurement of polypropylene orientation in three orthogonal directions. It is based on the polarization characteristics of two crystallinity bands, those at 841 and 809 cm^?1, which exhibit opposite polarization characteristics. The other method, for the amorphous phase, provides a semiquantitative estimate and is suitable for uniaxially oriented films. It is based on polarization characteristics of the 1155-cm^?1 band. Both methods are applied to the measurement of orientation in polypropylene films uniaxially stretched at various speeds and temperatures.     

Transport properties of poly(ethylene terephthalate) crystallized from the glassy state

Poly(ethylene terephthalate) was crystallized from the glassy state at 120 and 200°C. The structural organization of samples, after the primary and secondary crystallizations, was analyzed by density, X-rays, infrared and transport properties of dichloromethane vapor. The values of crystallinity derived with different methods do not agree, indicating that the crystallized samples cannot be considered simply biphasic. Since the fraction of the impermeable phase is much higher than the fraction of the crystalline phase, it suggested that the presence of mesomorphic form, impermeable to the vapors at low activity. With this hypothesis, the complete composition of the crystalline samples, in terms of fraction of crystalline, amorphous and mesomorphic form was derived. A value of density of the mesomorphic form of 1.39 g/cm³ was also derived.