A Novel Application of Hybrid Two-Dimensional Correlation Infrared Spectroscopy: Exploration of the Reversibility of the Pressure- and Temperature-Induced Phase Separation of Poly(N-isopropylacrylamide) and Poly(N-isopropylmethacrylamide) in Aqueous Solution

Wide angle X-ray diffraction patterns and 2D-correlation spectroscopy of crystallization in proton irradiated poly(ether ether ketone)

Proton irradiated poly (ether ether ketone) (PEEK) films were crystallized to different extents, and subsequently characterized by wide angle X-ray diffraction technique. The data were analyzed by two-dimensional correlation mapping (2D-CM), in particular: Generalized, hybrid and multiple perturbations correlation approaches. Two asynchronous correlation peaks at (19.1, 18.7) ^o and at (22.5, 19.1) ^o were utilized as a measure the crystal perfection and the preferred process; orientation/crystal growth respectively. Proton irradiation not only favored the formation of crystal form II, but also changed the type of orientation within the irradiated films. Differential scanning calorimetry and Raman spectroscopic analysis confirmed the contribution of the previous two factors. Raman spectra indicated that the intensity of both bands at 1595 and 1608 cm⁻¹ decreased on samples crystallized from the melt, but increased on cold crystallized samples. 2D-CM combined with other suitable techniques is a promising in evaluating the structure of polymers and revealing the effect of proton irradiation.

Intelligent Poly(N-Isopropylmethacrylamide) Hydrogels: Synthesis, Structure Characterization, Stimuli-Responsive Swelling Properties, and Their Radiation Decomposition

Poly(N-isopropylmethacrylamide) (p(NiPMAm)) is one of the lesser known homopolymers that has significant potential for designing new "intelligent" materials. The aims of this work were the synthesis a series of cross-linked p(NiPMAm) hydrogels by the free radical polymerization method and the application of gamma-ray radiation for additional cross-linking. The synthesized p(NiPMAm) hydrogels were structurally characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The amount of unreacted monomers was analyzed using high pressure liquid chromatography (HPLC) to evaluate conversion of monomers into polymers. The swelling behavior was monitored in dependence of pH and temperature changes. The previous aim of gamma-ray radiation was the further the cross-linkage of the obtained hydrogel sample in the gelatinous, paste-like state, but the gamma-ray radiation caused decomposition. After absorbing irradiation doses, they transformed into the liquid phase. The results obtained by the gel permeation chromatography (GPC) method indicated that only oligomers and monomers were present in the irradiated liquid material, without molecules with a higher average molar mass, i.e., that the decomposition of the hydrogels occurred. Additionally, the irradiated liquid material was analyzed using the static headspace gas chromatography mass spectrometry (HSS-GC/MS) and gas chromatography/flame ionization detection (HSS-GC/FID) methods. The presence of unchanged initiator molecule and a dominant amount of four new molecules that were different from homopolymers and the reactant (monomer and cross-linker) were determined.

Preparation of colloidal molecules with temperature-tunable interactions from oppositely charged microgel spheres

The self-assembly of small colloidal clusters, so-called colloidal molecules, into crystalline materials has proven extremely challenging, the outcome often being glassy, amorphous states where positions and orientations are locked. In this paper, a new type of colloidal molecule is therefore prepared, assembled from poly(N-isopropylacrylamide) (PNIPAM)-based microgels that due to their well documented softness and temperature-response allow for greater defect tolerance compared to hard spheres and for convenient in situ tuning of size, volume fraction and inter-particle interactions with temperature. The microgels (B) are assembled by electrostatic adsorption onto oppositely charged, smaller-sized microgels (A), where the relative size of the two determines the valency (n) of the resulting core-satellite ABn-type colloidal molecules. Following assembly, a microfluidic deterministic lateral displacement (DLD) device is used to effectively isolate AB4-type colloidal molecules of tetrahedral geometry that possess a repulsive-to-attractive transition on crossing the microgels' volume phase transition temperature (VPTT). These soft, temperature-responsive colloidal molecules constitute highly promising building blocks for the preparation of new materials with emergent properties, and their optical wavelength-size makes them especially interesting for optical applications.

Formation and Growth of Mesoglobules in Aqueous Poly(N-isopropylacrylamide) Solutions Revealed with Kinetic Small-Angle Neutron Scattering and Fast Pressure Jumps

The phase transition from swollen chains to polymer mesoglobules of an aqueous solution of poly(N-isopropylacrylamide) is investigated with kinetic small-angle neutron scattering with 50 ms time resolution in conjunction with millisecond pressure jumps across the coexistence line. The time-resolved study evidenced three distinct regimes: fractal clusters form during the first second and transform into compact mesoglobules. During the following ∼20 s, these grow by diffusion-limited coalescence. The final step consists of a slow growth characterized by an energy barrier of several k_BT. The method opens opportunities for kinetic structural studies of multicomponent systems over wide length and time scales and gives a structural picture spanning from the chain collapse to mesoscopic phase separation.

Two-dimensional correlation spectroscopic studies on coordination between organic ligands and Ni2+ ions

³A_2g→³T_1g(P) transition band of Ni²⁺ is used to probe the coordination of Ni²⁺. Two-dimensional asynchronous spectra (2DCOS) are generated using the Double Asynchronous Orthogonal Sample Design (DAOSD), Asynchronous Spectrum with Auxiliary Peaks (ASAP) and Two-Trace Two-Dimensional (2T2D) approaches. Cross peaks relevant to the ³A_2g→³T_1g(P) transition band of Ni²⁺ are utilized to probe coordination between Ni²⁺ and various ligands. We studied the spectral behavior of the ³A_2g→³T_1g(P) transition band when Ni²⁺ is coordinated with ethylenediaminetetraacetic acid disodium salt (EDTA). The pattern of cross peaks in 2D asynchronous spectrum demonstrates that coordination brings about significant blue shift of the band. In addition, the absorptivity of the band increases remarkably. The interaction between Ni²⁺ and galactitol is also investigated. Although no clearly observable change is found on the ³A_2g→³T_1g(P) transition band when galactitol is introduced, the appearance of cross peak in 2D asynchronous spectrum demonstrates that coordination indeed occurs between Ni²⁺ and galactitol. Furthermore, the pattern of cross peak indicates that peak position, bandwidth and absorptivity of the ³A_2g→³T_1g(P) transition band of Ni(galactitol)_x²⁺ is considerably different from those of Ni(H₂O)₆²⁺. Thus, 2DCOS is helpful to reveal subtle spectral variation, which might be helpful in shedding light on the physical-chemical nature of coordination.

Moving-window two-dimensional correlation infrared spectroscopic study on the dissolution process of poly(vinyl alcohol)

In this paper, the dissolution process of fully hydrolyzed polyvinyl alcohol (PVA) was investigated by temperature-dependent Fourier transform infrared spectroscopy (FTIR) combined with moving-window two-dimensional (MW2D) correlation infrared spectroscopy (IR). The results show that the FTIR spectra of PVA in OH stretching and bending regions exceed the measuring range of the spectrometer because of the presence of abundant water. The OH stretching and bending peaks reveal that the water mainly diffuses into amorphous region below 45 °C, and water molecules mainly diffuse into crystalline region above 45 °C. The peak at 1141 cm(-1) has ever been thought as the indication of crystallinity of PVA in solid state, but in solution, the peak does not decrease with the dissolution of crystalline region and finally increases when PVA is dissolved completely. The hydrogen bonds between hydroxyl groups in PVA chains are broken by water molecules but abundant new hydrogen bonds between hydroxyl groups in PVA chains and water molecules are formed during the dissolving process. In the 2D correlation analysis of the FTIR spectra, only the correlation movements of hydroxyl groups, including stretching and bending mode, can be observed. The correlation range and intensity are larger than that of PVA in solid state because the dissolution is the interaction process of water molecules diffusing into PVA chains.

Aqueous Solutions of Poly(ethylene oxide)-Poly(N-isopropylacrylamide): Thermosensitive Behavior and Distinct Multiple Assembly Processes

Detailed phase transition and conformational changes taking place as a function of temperature in poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) semidiluted aqueous solutions are elucidated in the present study. By the use of elaborate vibrational spectroscopy techniques in combination with two-dimensional correlation spectroscopy (2Dcos), three transition regions including respective rich domains (<29 °C), loose aggregations (30-36 °C), and dense sphere micelles (>37 °C) are depicted. In particular, subtle variations of hydrogen bonds are detected even under the lower critical solution temperature (LCST), and the respective rich domain regime is marked with strong participation from hydrogen bonding at different concentrations and compositions. Both the formation of intermolecular hydrogen bonds and the less hydration degrees of PNIPAM segments compared with PNIPAM homopolymer at elevated temperatures verify the evolution of PNIPAM from their own domains to loose aggregations with PEO shells. Dense micelles are formed beyond the LCST of PNIPAM, while the outmost PEOs act as buffer layers and postpone the shrinkage of PNIPAM chains. Due to the existence of a buffer layer, higher phase transition temperatures compared with PNIPAM homopolymer are observed.

Two-dimensional correlation spectroscopy in polymer study

This review outlines the recent works of two-dimensional correlation spectroscopy (2DCOS) in polymer study. 2DCOS is a powerful technique applicable to the in-depth analysis of various spectral data of polymers obtained under some type of perturbation. The powerful utility of 2DCOS combined with various analytical techniques in polymer studies and noteworthy developments of 2DCOS used in this field are also highlighted.

Pressure-induced structural and hydration changes of proteins in aqueous solutions

The effects of elevated hydrostatic pressure on four representative proteins, lysozyme, human serum albumin, ubiquitin and RNase A, were investigated by using Fourier transform infrared (FTIR) spectroscopy, by principal component analysis (PCA) and by moving-window two-dimensional (MW2D) correlation analysis. In addition, we revealed the pressure-induced changes of secondary structure elements using curve fitting. With pressure increase, the amide I band shifted to lower wavenumbers, with a transition at 200 MPa, which was indicative of hydration enhancement. Moreover, the pressure-induced behavior of pure water was studied, similar transition pressure was observed with protein in aqueous solution, suggesting that structure change of water around 200 MPa caused a hydration enhancement of protein. Under pressure higher than 200 MPa, the structural changes of the four proteins were obviously different except for the common features shifting to lower wavenumbers with pressure, basically due to the distinct structural differences among them.

Moving-window two-dimensional correlation infrared spectroscopy study on structural variations of partially hydrolyzed poly(vinyl alcohol)

In the present study, the molecular chain changes and structural transitions of partially hydrolyzed poly(vinyl alcohol) (PVA) having a 12 mol% acetate unit were analyzed by moving-window two-dimensional (MW2D) correlation infrared spectroscopy combined with differential scanning calorimetry and thermogravimetric analysis. The results show the glass-transition temperature (T(g)) of PVA is clearly distinguished by MW2D correlation infrared spectroscopy, and the acetate groups start to be eliminated around the melting temperature, whereas the free water molecules in PVA are eliminated above T(g). The correlation movements of the O-H stretching modes, including the free hydroxyl groups and the hydrogen bonds, are clearly determined using MW2D correlation infrared spectroscopy. The spectral variations in the C=O stretching region caused by the elimination of the acetate unit from polymer chains are also discussed on the basis of the results of the MW2D correlation analysis. Such results cannot be obtained by traditional infrared spectroscopy owing to the complex overlapping peaks.

Concatenated two-dimensional correlation analysis: a new possibility for generalized two-dimensional correlation spectroscopy and its application to the examination of process reversibility

We propose a new application of generalized two-dimensional (2D) correlation spectroscopy called "concatenated" 2D correlation analysis, which is useful in identifying the presence of strict similarity or very subtle difference between two spectral data sets having a similar origin. This approach is very efficient and can offer many potential applications. In this study, the detailed examination of process reversibility is explored. Two forms of concatenation, horizontal and vertical concatenation of data matrices, are introduced and the latter is discussed in detail. Concatenated 2D correlation analysis allows one to investigate directly the correlation between two independent but related spectral data sets. It can extract more detailed information, such as the comparison of effects of two different perturbations or different systems. We describe the principle of the "mirror-image concatenation" in 2D correlation analysis, which is applied to demonstrate its reliability and efficiency on three spectral models: a synthetic simulation data set; experimental Fourier transform infrared (FT-IR) spectra of the thermally induced unfolding-refolding transition of bovine pancreatic ribonuclease A (RNase A) in aqueous solution; and a set of FT-IR spectra of traditional Chinese medicines (TCM) of similar origin. The concatenated 2D correlation analysis shows its power in revealing the irreversibility of the thermally induced conformation transition of RNase A as well as the comparison of different species of TCM.

Principal component analysis based interconversion between infrared and near-infrared spectra for the study of thermal-induced weak interaction changes of poly(N-isopropylacrylamide)

The use of a novel spectral interconversion scheme, principal component analysis (PCA) based spectral prediction, to probe weak molecular interactions of a polymer film is reported. A PCA model is built based on a joint data matrix by concatenating two related spectral data matrices (such as infrared (IR) and near-infrared (NIR) spectra) along the variable direction, then the obtained loading matrix of the model is split into two parts to predict the desired spectra. For a better PCA-based prediction, it is suggested that the samples whose spectra are to be predicted should be as similar as possible to those used in the model. Based on the PCA model, the thermal-induced changes in the weak interaction of poly(N-isopropylacrylamide) (PNiPA) film is revealed by the interconversion between selected spectral ranges measured between 40 and 220 degrees C. The thermal-induced weak interaction changes of PNiPA, expressed as either the band shift or intensity changes at a specific region, have been probed properly. Meanwhile, the robustness of the spectral prediction is also compared with that achieved by a partial least squares (PLS2) model in detail, illustrating its advantages in predicting more subtle structural changes such as C-H groups.

Role of methyl in the phase transition of poly(N-isopropylmethacrylamide)

Poly(N-isopropylmethacrylamide) (PiPMA) has one more methyl group at each monomeric unit than poly(N-isopropylacrylamide) (PiPA). By use of laser light scattering (LLS) and ultrasensitive differential scanning calorimetry (US-DSC) we have investigated the association and dissociation of PiPMA chains in water. LLS studies reveal that PiPMA chains form larger aggregates at a temperature above its lower critical solution temperature (LCST) as the chain molar mass (Mw) decreases. In comparison with PiPA aggregates, PiPMA aggregates show a larger ratio of average radius of gyration to average hydrodynamic radius (<Rg>/<Rh>), indicating that PiPMA aggregates are looser. US-DSC studies show PiPMA chains have smaller enthalpy change (DeltaH) and entropy change (DeltaS) than PiPA chains during the phase transition, indicating that PiPMA chains have smaller conformational change. Our experiments demonstrate that the additional methyl groups in PiPMA chains restrain the intrachain collapse and interchain association, leading the phase transition to occur at a higher temperature.

Structure analysis of poly(N-isopropylacrylamide) using near-infrared spectroscopy and generalized two-dimensional correlation infrared spectroscopy

Based on a detailed study of the fundamental vibrations in the mid-infrared (MIR) region and supported by NH-proton deuteration results, the assignment of the overtone and combination bands in the near-infrared (NIR) spectrum of poly(N-isopropylacrylamide) (PNIPAM) is presented. Variable-temperature experiments and two-dimensional correlation infrared spectroscopy are used to determine the chemical mechanism and changing sequence of groups in PNIPAM; we conclude that bonded NH groups turn into free NH groups during the heating process, while the CH groups on the side-chains change prior to those on the main chains. A heterospectral dynamical correlation between the NIR and MIR regions or H-included groups in both regions was also performed. The temperature-induced dissociation of the hydrogen-bonded NH groups is found to proceed earlier than the conformational changes in the hydrocarbon chains.

FT-IR and 2D-IR Spectroscopic Studies on the Effect of Ions on the Phase Separation Behavior of PVME Aqueous Solution

Thermosensitive phase transition behavior of poly(vinyl methyl ether) (PVME) in an aqueous solution and the effect of inorganic ions on the coil-globule transition have been investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) accessory. ATR-IR spectra of PVME aqueous solution indicate that in water-PVME-inorganic salts system, the phase separation temperature of PVME aqueous solution decreased with the increase of ion concentration and the increase of anion electronegativity. Meanwhile, two-dimensional infrared (2D-IR) measurements have been made to clarify the microcosmic conformational changes of PVME during the coil-globule transition. Results show that the conformation changes of main chains occur earlier than those of ether groups during heating. Furthermore, the 2D correlation spectroscopy of PVME aqueous solution during heating and the increase of concentration of potassium chloride have been studied. The features of 2D-IR spectra during heating did not change compared to the features of PVME aqueous solution during the increase of concentration of potassium chloride. This result implies that, although the addition of inorganic ions shifts the phase separation temperature, it does not alter the internal mechanism of the coil-globule transition of PVME.