Recently, we reported the isothermal crystallization behaviors of poly(L-lactic acid) (PLLA) from the melt and glassy states, respectively [J. ]. Surprisingly, the quite different infrared (IR) spectral evolutions occur in the two crystallization processes at different temperatures in which the same crystal modification is expected to be formed. To clarify this unusual phenomenon, the crystal modifications and thermal behavior of PLLA samples prepared under different crystallization temperatures are investigated in detail by TEM, WAXD, and FTIR techniques. On the basis of the WAXD and IR data, a new crystal modification named the Ŕ form is proposed for the crystal structure of PLLA samples annealed at temperature below 120°C. Such crystal modification with loose 103 helical chain packing is less thermally stable than the standard R form of PLLA. This assignment can explain all the experiment observations well. Other possible mechanisms for the IR spectral difference of bulk PLLA samples annealed at different temperatures are also discussed.

Infrared (IR) spectra of new types of bacterial copolyester, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), P(HB-co-HHx) (HHx = 2.5, 3.4, and 12 mol %), and poly(3-hydroxybutyrate) (PHB) were measured over a temperature range of 20 °C to higher temperatures (PHB, 185 °C; HHx = 2.5 mol %, 160 °C; HHx = 3.4 mol %, 160 °C; HHx = 12 mol %, 140 °C) to explore their structure and thermal behavior. The temperature-dependent IR spectral variations were analyzed for the CH stretching, CO stretching, CH3 deformation, and C−O−C stretching vibration regions, and bands characteristic of crystalline and amorphous parts were identified in each region. It has been found from the anomalous frequencies of the CH3 asymmetric stretching bands of the four polymers and the X-ray crystallographic structure of PHB that there is an inter- or intramolecular interaction (C−H···O hydrogen bond) between the CO group in one helical structure and the CH3 group in the other helical structure in PHB and P(HB-co-HHx). The bonding energy of the C−H···O hydrogen bond seems to be smaller than 4 kJ/mol, but considering the heat of fusion (12.5 kJ/mol) of PHB, it is likely that a chain of C−H···O hydrogen bond pairs link two parallel helical structures in the crystalline parts. The temperature-dependent IR spectral variations have shown that the crystallinity of P(HB-co-HHx) (HHx = 12 mol %) decreases gradually from a fairly low temperature (about 60 °C), while the crystallinity of PHB and P(HB-co-HHx) (HHx = 2.5 and 3.4 mol %) remains almost unchanged until just below their melting temperatures. It has also been revealed from the present study that the weakening of the C−H···O interaction starts from just above room temperature and proceeds gradually with increase in temperature, but the collapse of helical structure occurs at a much higher temperature for all the polymers investigated.

The nature of the "peculiarly strong" interaction between the poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains was investigated by real time infrared spectroscopy during the isothermal melt crystallization process of the PLLA/PDLA stereocomplex. A very small low-frequency shift (about 1 cm -1 ) of νas(CH3) and a larger low-frequency shift (about 5 cm -1 ) of ν(CdO) were observed. The typical butterfly pattern in the two-dimensional (2D) asynchronous correlation spectrum and the second-derivative spectra reveal that there is a "peak shift" for ν(CdO). The red shifts of the stretching vibration modes of the methyl and carbonyl groups suggest that the interaction between the PLLA/PDLA stereocomplex is ascribed to CH3‚‚‚OdC hydrogen bonding. Another interesting result is that the peak shift of the ν(CdO) band already occurs in the induction period, which indicates that the CH3‚‚‚OdC interaction is the driving force for forming the racemic nucleation of the PLLA/PDLA stereocomplex. Moreover, the 2D correlation analysis indicates that the structural adjustment of the CH3 group occurs prior to that of the C-O-C backbone during the stereocomplexation process of PLLA/PDLA. The CH3‚‚‚OdC interaction may be the reason for this sequence of structural changes.

A conformational change in the coil−globule transition of poly(N-isopropylacrylamide) (PNiPA) was investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) method and density functional theory (DFT) calculations. ATR/IR spectra of PNiPA in an aqueous solution change dramatically in the vicinity of the coil−globule transition temperature (θ temperature). Below the θ temperature, unimodal peaks are observed at 1624 cm-1 in the amide I region and at 1562 cm-1 in the amide II region, respectively. Above the θ temperature, a new peak appears abruptly near 1653 cm-1 in the amide I region and the amide II band shifts gradually to a lower frequency by 6 cm-1. In the amide III region, the relative intensity of a band at 1173 cm-1 is weaker than that of a band at 1155 cm-1 at lower temperatures, but it becomes larger during the coil−globule transition of PNiPA. DFT calculation for dimer models of PNiPA suggests that the amide I band at 1624 cm-1 is assigned mainly to a stretching vibration of the CO group that forms a strong hydrogen bond with the N−H bond of a neighboring amide group. The band at 1653 cm-1 observed above the θ temperature may be due to a free CO group. It is, therefore, suggested that some of the intramolecular hydrogen bonds between neighboring amide groups are broken during the coil−globule transition. Furthermore, it is deduced from the DFT calculation that the relative intensity of the bands at 1173 and 1155 cm-1 in the amide III region reflects the population change in the gauche and trans conformations in the main chain during the coil−globule transition.

The present study is aimed at investigating structure, dispersibility, and crystallinity of poly(3-hydroxybutyrate) (PHB) and poly(l-lactic acid) (PLLA) blends by using FT-IR microspectroscopy and differential scanning calorimetry (DSC). Four kinds of PHB/PLLA blends with a PLLA content of 20, 40, 60, and 80 wt % were prepared from chloroform solutions. Micro-IR spectra obtained at different positions of a PHB film are all very similar to each other, suggesting that there is no discernible segregated amorphous and crystalline parts on the PHB film at the resolution scale of micro-IR spectroscopy. On the other hand, the micro-IR spectra of two different positions of a PLLA film, where spherulite structures are observed and they are not observed, are significantly different from each other. PHB and PLLA have characteristic IR marker bands for their crystalline and amorphous components. Therefore, it is possible to explore the structure of each component in the PHB/PLLA blends by using micro-IR spectroscopy. The IR spectra of a position of blends except for the 20/80 blend are similar to that of pure PHB. On the other hand, the IR spectra of another position of the blend consist of the overlap of those of pure PHB and PLLA. For the 20/80 blend, it is difficult to find a position whose spectrum is similar to that of pure PHB. However, a crystalline peak due to the CO stretching band is observed at 1718 cm-1. This means that PHB crystallizes as very small spherulites or immature spherulites under such blend ratio. DSC curves of the blend show that the heat of crystallization of PHB varies with the blending ratio of PHB and PLLA. The recrystallization peak is detected for PLLA and the 20/80 blend respectively at 106.5 and 88.2 °C. The lowering of recrystallization temperature for the 20/80 blend compared with that of pure PLLA suggests that PHB forms small finely dispersed crystals that may act as nucleation sites of PLLA. The results for the PHB/PLLA blends obtained from IR microspectroscopy indicate that PHB crystallizes in any blends. However, crystalline structures of PHB in the 80/20, 60/40, and 40/60 blends are different from those of the 20/80 blend.

Time-dependent infrared (IR) spectral variations during isothermal melt-crystallization process of poly(3-hydroxybutyrate) (PHB) have been analyzed for different wavenumber regions (C-H, CdO, C-O-C, and C-C stretching vibration regions) by difference spectra, second derivatives, and twodimensional (2D) correlation analysis, and the bands characteristic of crystalline and amorphous parts are identified in each region. By the 2D correlation analysis, it has been found that the intensity changes in the 1731 cm -1 band, which may be due to the intermediate state, and in the 1722 cm -1 band due to the crystal packing occur out of phase with each other, whereas those in the two amorphous CdO stretching bands (1747 and 1739 cm -1 ) ascribed to the different conformations of the main chain are synchronous, which represents a cooperative conformational rearrangement for the CdO groups in the amorphous state during the crystallization process. Meanwhile, a distinct delay between the intensity change rates of bands at 1184 and 825 cm -1 indicates that the adjustment of C-O-C backbone occurs faster than that of C-C backbone in PHB. This result has also been confirmed by investigating the 2D correlation spectra of PHB in various spectral regions. On the basis of these observations, a physical picture on the molecular evolution of PHB during the melt-crystallization process has been derived.

Temperature-dependent X-ray diffraction and infrared (IR) spectra were measured for poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) P(HB-co-HHx) (HHx = 2.5, 3.4, 10.5, and 12 mol %) to explore their crystal and lamella structure and the C−H···OC hydrogen bonding in them. The X-ray diffraction and IR measurements of PHB and P(HB-co-HHx) revealed that the smaller the a lattice parameter, the higher the frequency (∼3008 cm-1) of the C−H stretching band of the C−H···OC hydrogen bonding along the a axis between the CH3 group of one helix and the CO group of another helix. Therefore, it seems that the C−H···OC hydrogen bonding becomes strong with the decrease in the a lattice parameter. To investigate the relation between the C−H···OC hydrogen bonding and the lamella structure, we estimated the number of C−H···OC hydrogen bonding along the c axis (the direction of the lamella thickness) based on the reported lamella thickness. It is about 8 or 9 for PHB and about 3 for P(HB-co-HHx) (HHx = 10.5 and 12 mol %). It is very likely that the C−H···OC hydrogen bondings break much more easily in P(HB-co-HHx) than in PHB because of the bulkiness of large amounts of amorphous parts. However, the polymer chains still keep the lamella structure even in the copolymers with the HHx content of more than several percent. This is the reason why the P(HB-co-HHx) copolymers show high crystallinity and essentially have the same lattice spacing as the PHB homopolymer even if the HHx content is more than 10%. We have concluded that the C−H···OC hydrogen bonding stabilizes the chain folding in the lamella structure of PHB and P(HB-co-HHx) and the high crystallinity of PHB and P(HB-co-HHx) partly comes from the C−H···OC hydrogen bonding.

No abstract