Rheo-optical near-infrared (NIR) spectroscopy study of low-density polyethylene (LDPE) in conjunction with projection two-dimensional (2D) correlation analysis

A rheo-optical characterization technique based on near-infrared (NIR) spectroscopy is developed specifically to probe the submolecular-level deformation caused during a mechanical test. An illustrative example of the mechanical deformation of low-density polyethylene (LDPE) is provided to show how it can be utilized. A set of NIR spectra of the polymer sample were collected by using an acousto-optic tunable filter (AOTF) NIR spectrometer coupled with a tensile testing machine as an excitation device. While the substantial level of variation of spectral intensity was readily captured during the mechanical deformation of the LDPE, main feature of the NIR spectra was overwhelmed by the contribution from the baseline change. Projection 2D correlation analysis was then applied to selectively extract the signal contribution from the baseline fluctuation. The 2D correlation spectra revealed the predominant extension of amorphous tie chains followed by the rotation of crystalline lamellae, which induce elastic and plastic deformation of the LDPE, respectively.     

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.     

Determination of polyphenols in oats by near-infrared spectroscopy (NIRS) and two-dimensional correlation spectroscopy

Two-dimensional correlation spectroscopy (2DCOS) and near-infrared spectroscopy (NIRS) were used to determine the polyphenol content in oat grain. A partial least squares (PLS) algorithm was used to perform the calibration. A total of 116 representative oat samples from four locations in China were prepared and the corresponding near-infrared spectra were measured. Two-dimensional correlation spectroscopy was employed to select wavelength bands for the PLS regression model for the polyphenol determination. The number of PLS components and intervals was optimized according to the coefficients of determination (R²) and root mean square error of cross validation (RMSECV) in the calibration set. The performance of the final model was evaluated using the correlation coefficient (R) and the root mean square error of validation (RMSEV) in the prediction set. The results showed the band corresponding to the optimal calibration model was between 1350 and 1848?nm and the optimal spectral preprocessing combination was second derivative with second smoothing. The optimal regression model was obtained with an R² of 0.8954 and an RMSECV of 0.06651 in the calibration set and R of 0.9614 and RMSEV of 0.04573 in the prediction set. These measurements reveal the calibration model had qualified predictive accuracy. The results demonstrated that the 2DCOS with PLS was a simple and rapid method for the quantitative determination of polyphenols in oats.     

Tensile deformation of isotactic polypropylene (iPP) and iPP-nanocomposite studied by rheo-optical near-infrared (NIR) spectroscopy

Tensile deformations of isotactic polypropylene (iPP) and its nanocomposite were examined by a rheo-optical characterization technique based on near-infrared (NIR) spectroscopy to derive the submolecular-level understanding of the deformation mechanism during a tensile test. Sets of NIR spectra of the iPP samples were collected by using an acousto-optic tunable filter (AOTF) NIR spectrometer coupled with a tensile testing machine as an excitation device. Mechanical deformation of the samples was readily captured as strain-dependent NIR spectra. However, the main feature of the NIR spectra was overwhelmed by the contribution from the baseline change due to the substantial decrease in the sample thickness and subsequent change in the NIR light scattering. The variation of the spectral feature suggests that the deformation of the iPP involves the elongation of the rubbery amorphous chains prior to the displacement of the crystalline lamellae, providing elastic and subsequent plastic deformations during the tensile testing. In addition, it is revealed that the nanoclay layers dispersed within the iPP matrix restrict the elongation of the amorphous chains. Such interaction makes iPP hard and brittle, so that it yields no obvious ductile fracture during the tensile deformation.     

Raman imaging analysis of pharmaceutical tablets by two-dimensional (2D) correlation spectroscopy

Chemical properties of active substances and insoluble excipient within tablets such as crystalline structures can be seen as an important index for solubility of ingredients. Spectroscopic imaging can potentially be a solid solution to understanding mechanisms at the molecular level and it may bring useful insight in terms of process analytical technique. In the present study, generalized two-dimensional (2D) correlation spectroscopy is utilized for the Raman image analysis of pharmaceutical tablets to reveal molecular interactions between chemical components. By using a spatial distance as a perturbation variable in 2D correlation scheme, synchronous and asynchronous correlation analysis becomes possible. Two kinds of pharmaceutical tablets, pentoxifylline (PTX) as an active substance and palmitic acid (PA) as an insoluble excipient, are prepared with different grinding times, 0.5 and 45 min. The 2D correlation analysis of Raman images of the tablets clearly reveals both physical and chemical effects of grinding process on the properties of the tablets. Asynchronous correlations indicate that a specific molecular structural change of PTX related to the crystallinity is induced by the grinding process. Namely, the crystallinity of PTX based on CH₂ structure is a key factor to control the solubility of the tablets. Some properties of pharmaceutical tablets, i.e. solubility or distribution of components in turn may become possible by the simple grinding process. Detailed analysis of Raman images becomes possible by the 2D correlation spectroscopy.     

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.     

Construction of two-dimensional (2D) H-bonded supramolecular nanostructures studied by STM

In this review,a group of two-dimensional（2D） hydrogen-bonded supramolecular networks developed in our laboratory are discussed.Our attention is mainly focused on:（1） recognition of Fe³⁺ through twocomponent molecular networks;（2） site-selective fabrication of 2D fullerene arrays;and（3） fabrication of the nanoporous structure regulated by photoisomerization reaction process.It is envisioned that special supramolecular nanostructures,through H-bonding interactions,can be constructed or reconstructed to be further investigated toward the research of multi-component systems,molecule recognition,single molecular switches,and host-guest supramolecular chemistry.     

Mechanism in Brill transition of polyamide 66 studied by two-dimensional correlation infrared spectroscopy

The Brill transition of polyamide 66 was investigated by temperature-dependent infrared spectroscopy combined with moving-window two-dimensional (MW2D) correlation spectroscopy. The temperature range of the Brill transition determined by MW2D correlation spectroscopy was 90–170 °C. We employed generalized 2D correlation spectroscopy to study the sequential order of polyamide 66 chains with linear increment of temperature. The movement of the methylene segments near to NH is earlier than those on the CO sides. At the same time, the methylene which is close to NH varies before the inner methylene. Three kinds of NH groups in polyamide 66 were found. The sequential order of their motions is as follows. The free hydrogen-bonded NH groups change first, and then the disordered hydrogen-bonded NH groups. Finally, the ordered hydrogen-bonded NH groups start to change. We also found that the changes of the ordered hydrogen-bonded NH groups follow with the methylene groups.     

Details of thermal behavior of spin-coated film of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer studied by principal component analysis-based two-dimensional (PCA2D) correlation spectroscopy

Principal component analysis-based two-dimensional (PCA2D) correlation spectroscopy was applied to the temperature-dependent infrared-reflection absorption (IRRAS) spectra of a spin-coated film of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HB-co-HHx)) (HHx=7.2 mol%) copolymer. In asynchronous PCA2D correlation spectra, we clearly captured the existence of two components in the crystalline band of the CO stretching mode, well-ordered primary crystals observed at lower wavenumber and less ordered secondary crystals observed at higher wavenumber, which is not readily detectable in the original 1D spectra. Furthermore, the intensity changes of bands at 1298 and 1280 cm(-1) are significantly different in the temperature ranges below and above the transition temperature around 140 degrees C identified by the 2D first derivatives plot. The result further confirms that the sequence of intensity changes with increasing temperature is such that bands for less ordered crystalline components of P(HB-co-HHx) (HHx=7.2 mol%) are changing first at an earlier (i.e., lower temperature) stage.     

In-line monitoring of alcohol precipitation by near-infrared spectroscopy in conjunction with multivariate batch modeling

Alcohol precipitation is a critical unit operation during the manufacture of Chinese herbal injections. To facilitate enhanced process understanding and develop control strategy, the use of near-infrared spectroscopy (NIRS) combined with multivariate statistical process control (MSPC) methodology was investigated for in-line monitoring of alcohol precipitation. The effectiveness of the proposed approach was evaluated through an experimental campaign. Six batches were run under normal operating conditions to study batch-to-batch variation or batch reproducibility and establish MSPC control limits, while artificial process variations were purposefully introduced into the four test batches to assess the capability of the model for real-time fault detection. Several MSPC tools were compared and assessed. NIRS, in conjunction with MSPC, has proven to be a feasible process analytical technology (PAT) tool for monitoring batch evolution and potentially facilitating model-based advanced process control of the alcohol precipitation during the manufacture of Chinese herbal injections.     

Potential of two-dimensional near-infrared correlation spectroscopy in studies of pre-melting behavior of nylon 12

This review paper discusses the potential of generalized two-dimensional (2D) near-infrared (NIR) correlation spectroscopy in studies of pre-melting behavior and hydrogen bonds of nylon 12. A 2D NIR study on dissociation and hydrogen bonds of N-methylacetamide (NMA) is also reviewed here as a model compound study of nylon 12. Fourier transform (FT)-NIR spectra in the region of 9000–5000 cm⁻¹ of nylon 12 were measured over a temperature range of 30–150°C where gradual weakening of inter- or intramolecular associative interactions and decrease of local order leading to the eventual fusion of nylon 12 crystals are observed. The 2D correlation analysis provided the following conclusions for the pre-melting behavior of nylon 12. (i) There are at least eight bands in the 6800–6100 cm⁻¹ region assignable to free- and hydrogen-bonded NH groups of nylon 12 in various environments. The asynchronous 2D correlation spectrum in the above region indicates that the amide groups with free carbonyl oxygen appear first and then the unassociated free amide and amide groups with free NH follow as the temperature is increased. (ii) The asynchronous spectrum in the 6000–5500 cm⁻¹ region, where the first overtones of the CH₂ stretching modes are expected to appear, indicates that substantial amount of disordered or dissociated components start appearing before the disappearance of more ordered components. It seems that they appear as the pre-melting precursors (or even possibly as the indirect cause) to the precipitous decrease of the ordered components associated with melting of nylon 12 occurring at a much higher temperature.     

Study of the temperature-dependent near-infrared spectra of water by two-dimensional correlation spectroscopy and principal components analysis

Near-infrared spectroscopy (NIR) is an important analytical tool in monitoring properties of systems for that water is a major constituent. For such objects of analysis a quality of information extracted from the NIR spectra depends essentially on used methods of analysis of a massive absorbance of water. Correctly chosen method should be able to identified rational number of overlapped components hidden under the broad band of water. The resolved components have to be justified on grounds of the structure of water and by relation to the properties a hydrogen-bonded network of water molecules. The interest in the correlation is imposed by a fact that hydrogen bonds of water around nonpolar group are significantly strengthened and weakened around polar groups. Intensity variations classified in this context could be valuable source of information on varying properties of the solute molecules embedded in water environment. Therefore, there is a big interest in methods that have a power for detailed analysis of the intensity changes in the broad NIR spectra. Two-dimensional correlation spectroscopy (2DCOS) and principal component analysis (PCA) are our proposition. In the analysis of the temperature-dependent NIR spectra of water by means of the two methods we have focused on the interpretation of the 2DCOS results through the concept of linear and nonlinear relationships. Moreover, a cascaded curve fitting procedure has been employed. Presented approach should be very instructive of how to interpret the features of the 2D results that frequently is not a straightforward task.     

Submolecular dynamics of amorphous polymers probed by two-dimensional infrared (2D IR) spectroscopy

Two-dimensional infrared (2D IR) spectroscopy was used to probe the submolecular dynamics of atactic polystyrene. 2D IR is a powerful analytical technique especially suited for the elucidation of localized motions of polymer segments. In 2D IR, a polymer sample is excited by a small-amplitude oscillatory strain at a frequency in the acoustic range. The fluctuation of IR dichroism signals resulting from the strain-induced reorientation of electric dipole-transition moments is monitored with a time-resolved spectrometer. Spectra defined by two independent wavenumber axes are constructed by applying a correlation analysis to such signals. The 2D spectra provide detailed information about the local dynamics of submolecular constituents of the system. From the sign of cross peaks in the synchronous 2D IR spectrum of glassy polystyrene, it is shown that the main-chain backbone of polystyrene reorients in the direction of applied strain. Cross peaks in an asynchronous 2D IR spectrum reveal highly localized reorientational motions of phenyl side groups occurring more or less independently of the main-chain realignment. In the glassy state, the phenyl ring tends to fold back along the main-chain, indicating that there exists a highly constrained local distortion of side groups during deformation.     

二维相关红外光谱研究再生蚕丝蛋白膜的构象与温度之间的关系

通过二维相关红外光谱,研究了再生蚕丝蛋白膜的构象及其转变与温度之间的关系.实验结果表明,将样品从130℃升温到220℃、或在180℃的恒温过程中,丝素蛋白分子链的构象会发生变化,且不同构象对温度升高过程或180℃恒温过程响应的顺序是无规线团变化先于β-折叠、α-螺旋的形成.     

Characterization by fourier transform infrared spectroscopy (FT-IR) and 2D IR correlation spectroscopy of PAMAM dendrimer

FT-IR and 2D correlation spectroscopy were employed to study the microstructural changes occurring during phase transitions of a liquid crystal poly(amidoamine) codendrimer (PAMAM (L1)16(L2)16) generation 3, functionalized on the terminal groups by one-chain promesogenic calamitic units (4-(4'-decyloxybenzoyloxy)salicylaldehyde (L1)) and two-chain promesogenic calamitic units (4-(3',4'-didecyloxybenzoyloxy)salicylaldehyde (L2)). Spectral modifications associated with molecular conformation rearrangements allowing for molecular shape change on going from a liquid-crystalline organization to another were found. The transition temperatures were calculated, and they are in good agreement with the DSC data. Spectral analysis gives evidence of the LC phase transitions and to an additional transition associated with the existence of conformers. Various types of hydrogen bonding have been established.     

pH-induced structural changes of surface immobilized poly(L-lysine) by two-dimensional (2D) infrared correlation study

This paper reports the pH-induced structural changes in the surface immobilized poly(L-lysine)(PLL)film.Two-dimensional(2D) correlation analysis was applied to the Fourier transform infrared(FTIR)spectra of the surface-immobilized PLL film to examine the spectral changes induced by the alternations of the protonation state of the amino group in the side chain.Significant spectral changes in the FTIR spectra of the PLL film were observed between pH 7 and 8.The decrease in the protonation state of the amino group in the side chain induced spectral changes in the amino group as well as conformational changes in the alky]group in the side chain.From pH 1-8,the spectral changes in the amino and alkyl groups in the side chain occurred before those of the amide group in the main chain of the surface immobilized PLL film.     

Aggregation behavior of amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution studied by fluorescence correlation spectroscopy

The diffusional behavior of amphiphilic poly(2-oxazoline) diblock copolymers in aqueous solution is studied using photon correlation spectroscopy (PCS) and fluorescence correlation spectroscopy (FCS). The polymers were synthesized by living cationic polymerization and were fluorescence-labeled with tetramethyl rhodamine isothiocyanate either at the end of the hydrophilic or the hydrophobic block. Temperature-resolved PCS showed that, at room temperature, large metastable aggregates are present along with unimers and micelles. An annealing above ~40 °C resulted in stable equilibrium micellar solutions. By means of FCS, the hydrodynamic radii of the unimers and the micelles were measured simultaneously in a broad concentration range, and the critical micelle concentration could be determined. Comparison of the results from conventional PCS measurements with this first FCS study showed excellent agreement and the high potential of the FCS technique.Dedicated to Prof. Dr. Erhard W. Fischer on the occasion of his 75th birthdayAn erratum to this article can be found at