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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 CH 2 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.

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.

Non-equilibrium behavior of liquid mixture of polyethylene glycol (PEG) and polypropylene glycol (PPG) is studied. During the phase variation of PEG/PPG mixture, time-resolved Fourier transform infrared (FTIR) spectra were collected by attenuated total reflection (ATR) technique to selectively capture the development and subsequent variation of PEG-rich phase, appearing within a layer of the sample adjacent to the measuring surface of the ATR diamond prism. The FTIR spectra were then subjected to a two-dimensional (2D) correlation method to elucidate subtle but important changes in the absorbance of spectral bands. 2D correlation spectra revealed fine details of the non-equilibrium system. During the phase variation, dissociation between PEG and PPG generates free PEG molecules first. The emergence of the free PEG molecules triggers the subsequent self-assembly via the development of intermolecular H-bonding between PEG molecules, which eventually generates bigger PEG aggregates.

Degradations of polyester (-COO-) and polyether (-O-) polyurethane (PU) samples in KOH aqueous solution were examined by microscopic infrared (IR) spectroscopy coupled with two-dimensional (2D) correlation analysis. The PU sample were subjected to degradation treatment in the KOH aqueous solution. Sets of IR spectra were collected as a function of distance under varying the distance to the sample surface where the degradation predominantly occurs. The spatial changes in the IR spectra were sorted out by 2D correlation spectroscopy. Namely, the hydrolysis of the polyester and polyether PU initially occurs in hard segment by dissociating weak intermolecular hydrogen bonding between the urethane groups of adjacent PU chains. Further degradation eventually induces the hydrolysis of C=O groups present in the soft segment domain.

Photo-Fries rearrangement of phenyl salicylate was examined by real-time infrared (IR) monitoring in conjunction with density functional theory (DFT) calculation. Changes in the spectral features were readily captured during the photo-induced chemical reaction of the phenyl salicylate by means of two-dimensional (2D) correlation spectroscopy. The obvious variations of spectral intensities due to the production of 2,2′-dihydroxybenzophenone and 2,4′-dihydroxybenzophenone are clearly identified to provide in-depth understanding to the photo-Fries rearrangement. Namely, when exposed to UV-irradiation, the majority of the phenyl salicylate undergoes photothermal degradation. Further irradiation to the rest of the phenyl salicylate then results in the production of 2,2′-dihydroxybenzophenone followed by the development 2,4′-dihydroxybenzo-phenome products. Such delay in the production of 2,4′-dihydroxybenzo-phenome implies the presence of some more pathways, which makes its reaction efficiency lower than that of 2,2′-dihydroxybenzophenone.

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.

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.

Projection two-dimensional (2D) correlation analysis, achieved by selectively eliminating specific portion of dynamic spectra synchronized with the projecting vector, was used to study compression-induced variation of cellulose tablet monitored by near-infrared (NIR) spectroscopy and water sorption test. The increased density of the tablet by compression provided apparent variation of spectral intensity much larger than those caused by the change in the cellulosic structure. The direct calculation of 2D NIR correlation spectra from the raw spectra generated a relatively uninformative synchronous correlation spectrum and a very noisy asynchronous correlation spectrum due to the predominant intensity variation arising from the light scattering. In contrast, significant correlation features were elucidated without being hampered by the baseline fluctuation when the projection-corrected NIR spectra were constructed by the projection onto the space spanned orthogonal to the baseline change. Fine features of the compression-induced variation of the system were also elucidated by 2D hetero-correlation analysis based on the NIR spectra and water sorption profiles. The 2D correlation analysis revealed that the compression produces a disordered amorphous component of cellulose. The development of mobile amorphous phase results in a more tightly packed matrix with less porosity, which in turn prevents the penetration of water into the tablet and delays the water sorption.

The thermal behavior of polyethylene (PE) below its melting temperature ( T m ) in relation to the density variation was studied by Raman spectroscopy combined with two-dimensional (2D) correlation analysis. The Raman spectra of homo and 1-octene copolymerized PEs were collected by varying the sample temperature from 30 to 100 °C, where pre-melting would occur. The spectral variation upon temperature perturbation was dependent on the density. For a deeper understanding of the thermal behaviors, 2D hetero correlation analysis was performed for the temperature-dependent Raman spectra of homo and 1-octene copolymerized PEs. The hetero-sample correlation analysis revealed that even below T m , co-monomer side chains excluded from the crystalline lattice of the PE underwent partial fusion, indicating that such a conformational defect was closely related to the pre-melting of PE.

Isothermal melt crystallization behavior of neat poly(l-lactide) (PLLA) and PLLA/organically modified lay- ered silicate (OMLS) nanocomposite was studied by using time-dependent infrared (IR) spectroscopy and transmission electron microscopy (TEM). Sets of time-dependent IR spectra reflecting the crystallization from the melt of the neat PLLA and PLLA/OMLS nanocomposite were collected at 105 ◦ C. Fine details of the crystallization behaviors were analyzed by two-dimensional (2D) correlation spectroscopy, and sig- nificant differences in the crystallization behaviors depending on the presence of OMLS were revealed. Namely, it was found that the crystalline lamellae of the PLLA nanocomposite grow along the layered silicate, while the orientation of the neat PLLA resulted in more disordered orientation because of the absence of the OMLS. The result derived from the 2D correlation analysis of the time-dependent IR spectra of PLLA samples showed good agreement with corresponding TEM images.

The crystalline structure of a poly(lactic acid) (PLA) nanocomposite is probed by near-infrared (NIR) hyperspectral imaging to derive penetrating insight into its improved mechanical properties by inclusion of organoclay particles. A set of PLA nanocomposite samples are prepared under varying levels of clay content. The variation of the crystalline structure of PLA and corresponding mechanical strength are examined by NIR hyperspectral imaging and thermomechanical analysis (TMA), respectively. By carrying out the detailed analysis of the NIR hyperspectral image, it effectively provided a more detailed picture of the nanocomposite system. Namely, the addition of the clay substantially increases the frequency of the spontaneous nucleation of the PLA crystals. Corresponding decrease of the amorphous portion, in turn, restricts mechanical deformation of the samples under a certain level of load. Consequently, by carrying out detailed band position shift analysis of the NIR image spectra, the chemically meaningful information concerning the nanocomposite system can be effectively extracted.

Highly selective two-dimensional (2D) correlation analysis, achieved by attenuating a major principal component in spectral data with eigenvector manipulating transformation (EMT) technique, was demonstrated. A spontaneous evaporation process of a binary mixture solution of oleic acid (OA) and carbon tetrachloride (CCl 4 ) was monitored by attenuated total reflection infrared (ATR-IR) spectroscopy. Fine details of the dynamic behavior of OA molecules undergoing self-assembly into smectic liquid crystals were analyzed by 2D correlation spectroscopy scheme in conjunction with EMT technique. Correlation feature derived from the original spectral data is dominated with the contribution from the major dimer components of OA. On the other hand, 2D correlation spectra generated form the reconstructed data by attenuating the first principal component (PC1) showed selective enhancement of the correlational feature associated with the minor monomer component, which makes it possible to identify distinct populations, each having different dynamic behavior during the self-assembly.

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 CH 2 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.