Investigation on the Brill transition of polyamide 618

The Brill transition of even-even polyamide 618 was investigated using differential scanning calorimetry (DSC), temperature-dependent wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). The X-ray diffraction results indicate that the melt-crystallized sample of polyamide 618 transforms from the triclinic unit cell to the pseudo-hexagonal phase in the range of 120–180°C. In this range, the thermograph of polyamide 618 presents a broad endothermal peak. From the FTIR spectra, it was found that during the transition process of polyamide 618, the intensity of the intra-sheet hydrogen bonds becomes weak. At the same time, the CH₂-amide bonds twist, and the all-trans conformation of methylene sequences is disordered by inserting the gauche conformation. The CH₂ segments are in a mobile state because of the enhanced stretching and twisting vibrations of the C-CO and C-N bonds. Translated from Chemical Journal of Chinese Universities, 2006, 27 (2) (in Chinese)     

Online tracking of the thermal reduction of graphene oxide by two-dimensional correlation infrared spectroscopy

Thermal reduction of graphene oxide (GO) via rapid heating is an environment-friendly and cost-effective method. However, the detailed reduction mechanism remains unclear because of lack of methods for online monitoring of GO thermal reduction. In this study, the thermal reduction of GO (from 20 °C up to 400 °C in argon atmosphere) was successfully monitored online and investigated through temperature-dependent FTIR spectroscopy combined with scaling-MW2D FTIR spectroscopy and generalized 2D correlation analyses. Raman spectroscopy, XPS, and XRD studies were also conducted to characterize the structural changes before and after reduction. SEM and AFM analyses were performed to directly observe the formation of defects in GO after thermal reduction. According to scaling-MW2D results, the thermal reduction of GO was divided into two processes, namely, 35 °C–182 °C (process I) and 182 °C–385 °C (process II). Process I rapidly eliminates oxygen functional groups, and process II gradual removes them. The 2D correlation analysis for each process indicated the sequential order of movements of the oxygen-containing functional groups during thermal reduction. Process I comprised six steps, and process II contained four sequential steps. This work elucidated the complex deoxygenation steps and the mechanism of GO thermal reduction.     

Two-dimensional correlation infrared spectroscopy studies on the thermal-induced mesophase of 4-nitrobenzohydrazide derivative

Two-dimensional (2D) correlation infrared (IR) spectroscopy has been applied to explore the effect of hydrogen bondings (HBs) on the structure of mesophase in the dissymmetrical 4-nitrobenzohydrazide derivative, N-(4-cetyloxybenzoyl)-N′-(4′-nitrobenzoyl) hydrazine (C16-NO₂). The strength and species of HBs as well as the heat-induced structural variations in mesophase have been investigated. It has been found from 2D correlation IR spectroscopy that the sequential order of changes in the different functionalities in the course of liquid crystalline formation is that, firstly, the alkyl chain changes from the significant population of the trans conformation to the significant population of gauche conformation; then, the intermolecular HB between CO and NH groups is weakened, some even being broken, and consequently, the intermolecular distance is enlarged; finally, the skeleton of phenyl ring has enough space to change their conformation to weaken the π–π stacking interaction. In addition, besides a few free and some medium bonded NH and CO groups, strongly bonded NH and CO groups still predominantly exist in the mesophase.     

Spectral insights into microdynamics of thermoresponsive polymers from the perspective of two-dimensional correlation spectroscopy

Generalized two-dimensional correlation spectroscopy (2DCOS) and its derivate technique,perturbation correlation moving window (PCMW),have found great potential in studying a series of physico-chemical phenomena in stimuli-responsive polymeric systems.By spreading peaks along a second dimension,2DCOS can significantly enhance spectral resolution and discern the sequence of group dynamics applicable to various external perturbation-induced spectroscopic changes,especially in infrared (IR),near-infrared (NIR) and Raman spectroscopy.On the basis of 2DCOS synchronous power spectra changing,PCMW proves to be a powerful tool to monitor complicated spectral variations and to find transition points and ranges.This article reviews the recent work of our research group in the application of 2DCOS and PCMW in thermoresponsive polymers,mainly focused on liquid crystalline polymers and lower critical solution temperature (LCST)-type polymers.Details of group motions and chain conformational changes upon temperature perturbation can thus be elucidated at the molecular level,which contribute to the understanding of their phase transition nature.     

A thermal oxidative degradation study of triallyl isocyanurate crosslinking moiety in fluorinated rubber by two-dimensional infrared correlation spectroscopy

Thermal degradation of crosslinking moiety in fluorinated rubbers was studied with a new method using spatial-dependent infrared (IR) microscopy and two-dimensional (2D) IR correlation spectroscopy. Upon heating the fluorinated rubber, initially the amount of crosslinker decreased followed by generating another chemical species with carbonyl substituent with IR absorption at around 1730 cm^–1, implying generation of carboxylic acids forming intermolecular hydrogen bonding. Furthermore, projection 2D IR correlation analysis revealed that another chemical species with IR absorption at around 1755 cm^–1 generates, indicating that further degradation progresses upon heating and intermolecular hydrogen bonding were broken. As a result, the multi-step degradation process of the crosslinker in the fluorinated rubber could be detected by combination of spatial-dependent IR microscopy and projection 2D IR correlation spectroscopy.     

Recent developments in two-dimensional (2D) correlation spectroscopy

Recent noteworthy developments in the field of two-dimensional(2D) correlation spectroscopy are reviewed.2D correlation spectroscopy has become a very popular tool due to its versatility and relative ease of use.The technique utilizes a spectroscopic or other analytical probe from a number of selections for a broad range of sample systems by employing different types of external perturbations to induce systematic variations in intensities of spectra.Such spectral intensity variations are then converted into2 D spectra by a form of correlation analysis for subsequent interpretation.Many different types of 2D correlation approaches have been proposed.In particular,2D hetero-correlation and multiple perturbation correlation analyses,including orthogonal sample design scheme,are discussed in this review.Additional references to other important developments in the field of 2D correlation spectroscopy,such as projection correlation and codistribution analysis,were also provided.     

Study of the hydrogen bonding of 1,4-bis[(3,4,5-trihexyloxyphenyl)hydrazide]phenylene in crystalline and liquid crystalline phases using infrared,Raman, and two-dimensional correlation spectroscopy

The vibrational bands of a dihydrazide derivative, 1,4-bis[(3,4,5-trihexyloxyphenyl)hydrazide]phenylene (TC6), observed in the Raman and infrared spectra were assigned. The intermolecular hydrogen bonding vibrational bands due to CO and NH groups in the low-frequency Raman spectra were observed at 111 and 94 cm⁻¹ in the crystalline and liquid crystalline (LC) phases, respectively. The sequential order of changes in the hydrogen bonding and alkyl chains was opposite in the crystalline and LC phases. The modifications in the hydrogen bonding occurred prior to conformational changes in the hydrocarbon chains in the crystalline phase; however, a reverse trend was observed in the LC phase. Simultaneously, the two-dimensional (2D) IR and Raman correlation spectroscopic analysis showed that the amide I band of TC6 in the LC phase comprised at least five distinct bands. In addition, the hetero 2D correlation between the NH and CO groups confirmed that no free NH and CO groups existed in the LC phase.     

Structure and two-dimensional correlation infrared spectroscopy study of two isomeric forms of the octamolybdate cluster

Two isomeric forms of the octamolybdate cluster, (NH₄)₂(Hbpy)₂[Mo₈O₂₆](bpy)₃1 (bpy=bipydine), and [Cu₂(imi)₄]₂[Mo₈O₂₆] 2 (imi=imidazole), were synthesized by hydrothermal method and characterized by X-ray single analysis, one-dimensional (1D) infrared spectroscopy and two-dimensional (2D) correlation infrared spectroscopy under thermal perturbation. In compound 1, the [Mo₈O₂₆] units are β-structure, and octamolybdate anion is ζ-structure in compound 2. The 2D IR correlation spectroscopy study indicates that the intensity changes of MoO band are more sensitive to the temperature variation than that of the Mo-O bond. The μ-Mo-O bond and framework vibrations in compound 1 are more sensitive to the temperature variation than that for compound 2. As the vibrations of μ-Mo-O bond accompanied by the transfer of an oxygen atom, the catalysis ability of compound 1 may be higher than that of compound 2.     

Temperature dependent wide angle X-ray diffraction studies on the crystalline transition in water saturated and dry polyamide 6/66 copolymer

The crystalline transition in water saturated and dry polyamide 6/66 copolymer (ratio 4:1 by mol) was investigated by means of temperature dependent wide angle X-ray diffraction (WAXD). The polyamide 6/66 copolymer (PA6/66) exhibits a poorly developed α-phase at room temperature after being cooled down from the melt. Uptake of water improves the crystalline structure by mobilizing the amorphous phase, thus facilitating chain relaxation in the crystalline phase. Upon heating, the effect of the glass transition on the change of the crystal lattice constants is seen. Further heating leads to a gradual crystalline transition from the α-phase to a pseudohexagonal phase. Different from the behavior in polyamide 6, this pseudohexagonal phase does not further transform to a high temperature α′-phase before melting. The delay of the crystalline transition in the water saturated PA6/66--as compared to the dry material can be understood as a result of the better ordered crystalline structure in it.     

Analysis and discrimination of ten different sponges by multi-step infrared spectroscopy

In this study,a convenient method using multi-step infrared spectroscopy,including Fourier transform infrared spectroscopy(FT-IR),second derivative infrared spectroscopy(SD-IR) and two-dimensional correlation infrared spectroscopy(2DCOS-IR),was employed to analyze and discriminate ten marine sponges from two classes collected from the Xisha Islands in the South China Sea.Each sponge had an exclusive macroscopic fingerprint.From the IR spectra,it was noted that the main ingredient of calcareous sponges was calcium carbonate,but that of demosponges was proteins.For sponges from the same genus or having highly similar chemical profile(IR spectral profile),SD-IR and 2DCOS-IR were applied to successfully reveal the tiny differences.It was demonstrated that the multi-step infrared spectroscopy was a feasible and objective approach for marine sponge identification.     

Tensile deformation of polyamide (PA) 6 probed by rheo-optical near-infrared (NIR) spectroscopy

A rheo-optical near-infrared (NIR) spectroscopy, based on the combination of NIR spectroscopy and mechanical analysis, was applied to polyamide (PA) 6 samples consisting of bundled amorphous chains. Sets of strain-dependent NIR spectra as well as tensile stress of dried and wet treated PA 6 samples were collected during the mechanical elongation of the samples. The spectra were then subjected to two-dimensional (2D) correlation analysis to elucidate fine features of the spectral changes. An asynchronous correlation peak develops between the bands at 2355 and 2300 nm due to the combination modes of CH₂ groups arising from the rubbery amorphous chain and rigid crystalline lamella of the dried PA 6, respectively. It therefore indicates that during the tensile deformation, the orientation of the amorphous chain is induced first to cause the elastic deformation. Further elongation results in the rotation of the crystalline lamella connected with the amorphous chain. This correlation intensity apparently increases by the wet treatment, suggesting that water molecule in the PA 6 disrupts the H-bonding interaction between the adjacent polymer chains and thus makes the polymer more flexible. Accordingly, it is likely the H-bonding between the polymer chains works in a manner somewhat similar to cross-linked polymers, which substantially effects on the mechanical property of the PA 6.     

Application of two-dimensional near-infrared correlation spectroscopy to protein research

The purpose of the present paper is to demonstrate the potential of generalized two-dimensional (2D) near-infrared (NIR) correlation spectroscopy in studies of hydration and structure of proteins. We describe here two examples. The first example is concerned with heat denaturation process of ovalbumin in aqueous solutions and the second one deals with isomerizations with varying pH of serum albumin. New insight has been obtained into the hydration and unfolding process of secondary structures of ovalbumin and serum albumin by studying temperature- or pH-dependent correlation patterns in 2D synchronous and asynchronous spectra. Generalized 2D NIR correlation spectroscopy emphasizes spectral features not readily observable in conventional one-dimensional spectra, enabling to extract subtle but valuable structural changes concerning with the protein denaturation.     

Mechanism of the temperature-dependent degradation of polyamide 66 films exposed to water

Experiments were performed to elucidate the degradation mechanism of hot-pressed polyamide 66 upon exposure to water. For films exposed to water over the temperature range 25 °C-90 °C, degradation was monitored using FTIR and solid-state ¹³C NMR spectroscopies. The data are consistent with a mechanism in which (1) a radical is formed on the methylene carbon adjacent to the amide nitrogen, (2) this radical reacts with oxygen to form a hydroperoxide, and (3) the hydroperoxide decomposes to form an imide or a hydroxylated amide, both of which may cleave leading to chain scission. Water appears to facilitate degradation by increasing the flexibility of the polymer matrix through swelling rather than acting as a reactive species, at least at the early stages of the process. An apparent activation energy of 15 ± 2 kJ/mol is observed for the early stages of degradation, suggesting that segmental motions in the polymer associated with water and oxygen sorption or inter-chain radical reactions are indeed key components of the degradation process.     

Studies on crystal transition of polyamide 11 nanocomposites by variable-temperature X-ray diffraction

Polyamide I1 （PAll） and its nanocomposites with different organoclay loadings were prepared by melt-compounding and subsequent pelletizing. The crystal phase transitions of PAl 1 and its clay nanocomposites were investigated by variable-temperature X-ray diffraction. It was found that the Brill transition of the nanocomposite was 20 K higher than that of the neat PAl 1 for both heating and cooling processes. The PAl 1 d-spacings of the nanocomposites were observed to be smaller than those of the neat PAl 1 for melt crystallization. The constraints imposed by the addition of layered clay, restricting the thermal expansion of the polymer chains, are probably responsible for such a reduction of the d-spacing.     

Thermal and crystallization studies of nano-hydroxyapatite reinforced polyamide 66 biocomposites

The thermal and crystalline behaviour of nano-hydroxyapatite (n-HA) reinforced polyamide 66 (PA66) biocomposites was studied by thermogravimetry (TG) and differential scanning calorimetry (DSC). The thermal properties of PA66 and n-HA/PA66 composites were analysed by TG. The effect of hydroxyapatite on the melting and crystallization of PA66 was evaluated by DSC. DSC measurements exhibited an increase in the crystallization temperature, however, decrease in crystallinity with the addition of n-HA to the PA66 matrix, which was attributed to the hydrogen bonds between the n-HA surface and polyamide 66 molecules. With increase of n-HA content, the melting peak of the PA66 component shifted to higher temperature, suggesting constrained melting. The addition of n-HA to PA66 played the role of nucleating agent and enhanced the crystallization rate. Non-isothermal parameter a measured by Liu method varies from 1.13 to 1.18, from 1.02 to 1.07, and from 1.18 to 1.21 for PA66, 30 wt% n-HA/PA66 and 40 wt% n-HA/PA66, respectively, and the values of K_(T) systematically increase with rise in relative degree of crystallinity.     

Glass transition of atactic polystyrene probed at the submolecular level by dynamic infrared linear dichroism (DIRLD) spectroscopy

Dynamic infrared linear dichroism (DIRLD) spectroscopy is a rheo-optical characterization technique developed specifically to probe the submolecular dynamics of polymer segments. The technique combines the measurement of submolecular orientation based on the directionally selective absorption of polarized IR light with a small-amplitude oscillatory tensile deformation used in dynamic mechanical analysis. A DIRLD spectroscopic study of atactic polystyrene reveals that a dramatic change in the reorientation behavior of aromatic side groups is observed around the glass transition temperature of 100 °C. The transition point for the main chain backbone, on the other hand, is observed at a much higher temperature around 125 °C. Thus, the macroscopically observable glass transition of polystyrene seems to be dominated by the dynamics of side groups rather than that of the coordinated motions of polymer segments along the backbone. This result suggests a fundamental similarity between the glass transition phenomena of polymers and those of small-molecule inorganic glasses.     

Determination of thermal expansion coefficient of a monofilament polyamide fiber using digital image correlation

Coiled polymer actuators are a type of artificial muscles that are a promising development in the field of smart materials. The coefficient of thermal expansion of monofilament polyamide fibers is a crucial parameter for understanding the actuation of coiled fibers. The main purpose of this work is to develop a new methodology for estimating the coefficient of thermal expansion and the transition temperature of monofilament polymer fibers. In the experimental procedure, axial deformations of monofilament polyamide fiber samples were induced by temperature variations using a controlled thermal system. These deformations were determined from images of polyamide samples using the digital image correlation method. Two different approaches based on distinct temperature conditions were conducted. An alternative model with three parameters, including the coefficient of thermal expansion, was introduced to describe the thermal-mechanical behavior of monofilament polyamide fibers. Moreover, polyamide samples were also characterized using four conventional methodologies. Results indicated that the coefficient of thermal expansion changed of a modest negative value to a large negative value and this transition occurred around the glass transition temperature of the polyamide. The thermal expansion curves demonstrate good repeatability and all estimated parameters were in accordance with literature, indicating that the proposed approach can be suitable for the proposed study. This investigation may help in understanding of the intrinsic thermal-mechanical behavior of polymeric monofilaments employed as actuators.     

Synthesis, crystal structure and two-dimensional infrared correlation spectroscopy of a layer-like transition metal (TM)-oxalate templated polyoxovanadium borate

A novel polyoxometalate (POM) Na₂(H₂en)₂{(VO)₁₀[B₁₄O₃₀(OH)₂]₂}{Mn₄(C₂O₄) [B₂O₄(OH)₂]₂}Mn(H₂O)₂·(H₃O)₁₂(H₂O)₁₉1 (en=ethylenediamine), which is a layer-like transition metal (TM) oxalate templated polyoxovanadium borate, has been synthesized under hydrothermal conditions and characterized by EPR, elemental analysis, thermal analysis, single crystal X-ray diffraction and 2D IR correlation spectroscopy studies, respectively. This compound crystallized in the triclinic space group with , , , α=64.48(1)°, β=64.68(1)°, γ=63.93(1)°, and Z=2. The cluster anion with an open central cavity has a central band of ten edge-sharing VO₅ square pyramids, which is capped top and bottom by two crown-like polyborate ligands of formula [B₁₄O₃₀(OH)₂]²⁰⁻. There is a fragment of {Mn₄(C₂O₄)[B₂O₄(OH)₂]₂}²⁻ fixed in the central cavity as a guest part. The cluster units are connected to form a two-dimensional (2D) framework by octahedral Mn(II) and Na⁺ sites. In addition, we first introduce the generalized 2D correlation spectroscopy to explore the POMs and obtain the dynamic information about structural changes of POMs.     

Structure and phase transition in self-assembled films of an anti-ferroelectric liquid crystal studied by two-dimensional correlation FTIR spectroscopy

The FTIR spectra were measured for a self-assembled film of the anti-ferroelectric liquid crystal (AFLC) molecules over a temperature range of 40-150 degrees C. The frequency and intensity variations of the infrared spectra were discussed in terms of changes in molecular orientation, conformation and intra- and/or inter-molecular interaction during phase transitions. The two-dimensional (2d) correlation analysis revealed that two hydrocarbon chains have different orientation behaviors, the shorter hydrocarbon chain is parallel to molecular long axis and the longer one is perpendicular to it. The splitting of the C=O stretching band indicated that the s-cis and the s-trans conformers of the ester group coexist in the self-assembly film. During the phase transitions, in mesogen part the C=O group adjacent to the chiral part is first response with temperature changing, following the C=O group between two phenyl rings, and eventually the phenyl rings; and the respondence with temperature of methyl is slower than that of methylene in alkyl chains.