Study On Crystallization Behavior Of Linear Aliphatic Polyesters

Fourie transfer infrared spectra?FTIR?is a characterization method based on the interference between molecular vibrations and IR radiation to uncover the inner structure of materials.Compared to other common methods like DSC,POM and AFM,FTIR is a more accurate means to study the relationship between crystal structure of the polymer and its performance.In this paper,the relationships between structures and crystallization behaviors of poly?butylene succinate??PBS?and poly?propylene fumarate??PPF?which are typical biodegradable linear aliphatic polyesters have been investigated in detail mainly by FTIR.The research contents and main achievements are as follows:Firstly,we determined that the C=O stretching modes of PBS can respond to three distinguish absorption bands in the FTIR spectrum for the first time,at around 1736,1720 and 1714 cm^-1 respectively,and further confirmed that they correspond to the stretching modes of the C=O groups in free amorphous fraction?MAF?,C=O groups in rigid amorphous fraction?RAF??or intermediate phase which locates between MAF and crystalline phase?and hydrogen-bonded C=O groups in the crystalline phase,respectively.Further investigation by differential scanning calorimetry?DSC?and wide-angle X-ray diffraction?WAXD?supported that the three particular C=O absorption bands can indeed reveal the typical three-phase structure for PBS.More important,the FTIR spectrum of PBS is very sensitive to the crystallization temperature?T_c?,measured thickness and measurement mode.The higher T_c or thicker of the sample is,the more RAF content appears when measured at room temperature,then stronger intensity of the peak at 1720 cm^-1 appears;the thicker penetration thickness of IR in sample,the more RAF content can be detected,then more distinguish the peak at 1720cm^-1 too.Thus,the particular three absorption bands of C=O groups in PBS force us to carefully reconsider the previous reports on structure and the interaction state obtained by FTIR spectroscopy in PBS and its composites.Secondly,FTIR is used to characterize the intriguing morphology of PPF with two-domain structures in one spherulite.When PPF is crystallized at temperature within64⁸8°C,novel morphology with two different banded regions named as regular bands and irregular bands can be found in one spherulite whose centers display as either round or diamond shape.The relative content of regular banded area increases with T_c,so does to its band spacing.By employing the in-situ POM,it is to found that the two kinds of morphology regions exhibit similar growth rates and totally the same melting process.By employing DSC,FTIR,small angle X-ray scattering?SAXS?and other techniques,the crystallinity of irregular banded region is confirmed to be higher than the regular banded region.Importantly,it is revealed by micro-polarized FTIR that the different morphology regions are derived from different growth axes of the lamellae along the radial direction,i.e.lamellar grow and twist along a axis in regular banded region but along b axis in irregular banded region,plausibly.We attribute the dual twist axes to its peculiar molecular structure and speculate that the segments existing at the surface between the amorphous region and crystalline region should be different when lamellae twist along different axes.When the lamellae grow and twist along a axis,the folded segments may be the rigid-C=C-units;while them grow and twist along b axis,the folded segments may be the flexible-CH₂-CH₂-CH₂-units.The lower flexibility of the segment is,the more crowded molecular chains at the folded surface,leading to ring-bands with more regular structure.Moreover,longer chain segments should be required in the amorphous fraction if the rigid units construct the fold surface,which lowers the degree of crystallinity.