Article pubs.acs.org/ac
Determination of Percent Crystallinity of Side-Chain Crystallized Alkylated-Dextran Derivatives with Raman Spectroscopy and Multivariate Curve Resolution Ashok Zachariah Samuel,† Mengbo Zhou,‡ Masahiro Ando,§ Robert Mueller,∥ Tim Liebert,‡ Thomas Heinze,‡ and Hiro-o Hamaguchi*,† †
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan ‡ Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University of Jena, Humboldtstrasse 10, D-07743 Jena, Germany § Waseda University, Consolidated Research Institute for Advanced Science and Medical Care, 513 Wasedatsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan ∥ Leibniz Institute of Photonic Technology e.V. (IPHT), Postfach 100239, D-07702 Jena, Germany ABSTRACT: We demonstrate a methodology to estimate the percent crystallinity of polymers directly with Raman spectroscopy and multivariate curve resolution (MCR) by alternating least-squares (ALS). In this methodology, the Raman spectrum of semicrystalline polymer is separated into two constituent components (crystalline and molten) and their corresponding concentrations. The percent crystallinity can be estimated as the change in area intensity of the molten spectral-component when polymer cools from a temperature above melting point to room temperature. The number of carbons in the crystalline lattice has also been estimated from the position of longitudinal acoustic (LA) Raman bands with the correlation established by Mizushima and Simanouti [Mizushima, S.; Simanouti, T. J. Am. Chem. Soc. 1949, 71, 1320]. The new method allows direct Raman estimation of absolute percent crystallinity of polymers. Until now, Raman spectroscopic estimation of percent crystallinity was possible only in conjunction with other techniques or by using internal standards.
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The spectroscopic determination of absolute crystallinity of the semicrystalline material requires the measurements of the concentration of the crystalline and amorphous content selectively. For instance, the absolute percent crystallinity of regioregular poly(3-hexylthiophene) has been determined by solid-state 13C single pulse excitation (SPE) NMR.26 The terminal CH3 groups of hexyl chain give specific signals for chains existing in crystalline and amorphous fractions. The integrated intensities of these two signals give direct measures of the concentrations of chains existing in crystalline as well as amorphous domains. The absolute percent crystallinity is then calculable from these values. X-ray diffraction techniques also provide an absolute value of polymer crystallinity. This technique relies on the different shapes of the scattering patterns from amorphous and crystalline domains of the polymer.27 However, rigorous theoretical considerations are required to quantitatively separate these two contributions.27
rystallinity crucially regulates the performance of polymeric materials from utility plastics to photovoltaics.1−5 For example, a strong correlation is generally observed between hole mobility and crystallinity.6−8 However, the relationship between crystallinity and electronic properties is hard to predict owing to the difficulty in characterizing all the parameters that regulate them.3,9−15 Attaching side chains to the polymer backbone has long been adapted as a means to modulating crystallinity.9−15 However, control over molecular organization can not be guaranteed by molecule design alone. Several methods such as electron microscopy,16−18 wide-angle X-ray scattering (WAXS),19,20 dilatometry,21−23 differential scanning calorimetry (DSC),24,25 etc. are widely used for studying the crystallinity of polymer materials. Dilatometry requires prior knowledge of the specific volumes of the amorphous and crystalline domains to obtain the degree of crystallinity and hence of limited practical usefulness. The estimate of crystallinity of polymers by DSC is relative and requires completely crystalline analogues, which are often not easy to obtain. Thus, determination of absolute percent crystallinity of polymers still remains a challenging task.26−34 © XXXX American Chemical Society
Received: October 27, 2015 Accepted: April 7, 2016
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DOI: 10.1021/acs.analchem.5b04075 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
iminium chloride is prepared by conversion of N,Ndimethylformamide (DMF) with oxalyl chloride and subsequently with the fatty acid. The activation of the fatty acid as well as the esterification occurs under mild and efficient conditions53 resulting in pure products of defined properties. Products with degree of substitution (DS) in the range from 0.92 to 2.69 were accessible. The samples for the recent investigations were prepared with dextran with Mw 6000 g/ mol. The molar ratio between dextran (anhydroglucose unit) and fatty acid is 1:5 (palmitate) and 1:4 (myristate), respectively, and leads to DS values of about 2.9 and 2.5, respectively (Figure 1). The DS values have been calculated
Several fundamental investigations have demonstrated the specific utility of Raman spectroscopy for polymer characterization.35−42 Since the chemical signatures of different segments give different vibrational bands, there is a natural chemical contrast in the Raman spectrum that allows one to investigate domains in a semicrystalline polymer selectively. Moreover, the crystalline and amorphous regions give characteristic spectra, which can be effectively utilized to estimate the percent crystallinity. There had been studies in the past intended to estimate percent crystallinity of polymers (e.g., polyethylene,35,36 polystyrene,37,38 peptides,39 polyethylene terephthalate,40 polypropylene,41 polylactide,42 etc.) with Raman spectroscopy. Most of these studies assume a two-state phasetransition model (crystalline and amorphous/molten), and intensity ratios were converted to percent crystallinity values by applying different calibration methods. Considerable differences can be observed between percent crystallinity values estimated using different experimental techniques.37 The Raman spectrum of a semicrystalline polymer is a composite spectrum of amorphous and crystalline regions. The intensities of these spectral components are proportional to the concentration of individual domains. The relative contributions of individual components toward the overall Raman spectrum of the polymer sample change as a function of temperature during melting. In such circumstances, we can apply multivariate statistics42 to separate the spectral components belonging to crystalline and amorphous domains and their respective concentrations directly.43−49 Hence, the method offers a direct way to estimate the percent crystallinity of the sample from variable temperature Raman measurements. We demonstrate here a Raman spectroscopic method to estimate the percent crystallinity of polymers directly in conjunction with multivariate curve resolution (MCR) by alternating least-squares (ALS).43−49 Biologically benign alkylated (myristic acid and palmitic acid) dextran derivatives are promising as future biocompatible coatings for medical equipment and implants.50−52 The adhesive’s tensile strength depends on the polymer−surface interface and the crystallization tendency of the alkyl chains. Magnetic nanoparticle dispersed alkylated dextran derivatives have remote melting capabilities under the influence of alternating magnetic field. The uniform distribution and optimum performance of the composite film depends on the interface chemistry and crystallinity of alkyl side chains.53 We address the specific questions regarding the side-chain crystallization of these dextran derivatives with Raman spectroscopy. We have employed Raman spectroscopy and MCR-ALS for estimating absolute percent crystallinity of alkyl side-chain domains in the dextran derivatives. We have also studied the length of the side chain (number of carbon atoms) that exists in the crystalline domain. Raman spectroscopy has a unique sensitivity toward the carbon chain organization in lamellar lattices.54,55 For instance, accordion type vibrations (longitudinal acoustic mode) of C−C chain in the crystalline state reveal a multitude of information about polymer lamella formation.56−59 Here, we report a detailed characterization of crystalline alkyl side-chain domains in alkylated dextran derivatives with low frequency Raman microspectroscopy.
Figure 1. Molecular structure of the dextran polymers.
from 1H NMR spectra after peracetylation.60 Dextran esters are free of chlorine containing byproduct as elucidated by elemental analysis. FT-IR spectra confirm the structure and the purity of the dextran esters (absence of stretch vibration of carbonyl group of the fatty acid at 1700 cm−1). Raman Spectroscopy. A confocal Raman microspectrometer with the following components was used for the measurements. The second harmonic (532 nm) of a cw Nd:YVO4 laser (Verdi-V5, Coherent) was used as the excitation light. A small portion (
