Robert T. Kean

Crystallinity Determination of Polylactide by FT-Raman Spectrometry:

The complete Raman spectra of isotropically crystalline and amorphous polylactide (PLA) have been successfully obtained with the use of a Fourier transform instrument equipped with a near-IR laser source. The Raman spectra of PLA were recorded at a resolution of 4 cm−1 from a backscattering sampling geometry. The changes in band intensity and shape of Raman spectra in the crystalline PLA samples facilitated the quantification of the crystallinity, which was primarily determined by the change in enthalpy via differential scanning calorimetry (DSC). A comparison of quantitative analysis of crystallinity by a multivariate technique, partial least-squares (PLS), and a univariate method at the carbonyl stretching band is described. Quantitative analysis of PLA crystallinity was performed after the data preprocessing with the standard normal variate (SNV) method in the combined spectral regions 3100–2800 cm−1 and 2000–200 cm−1 that were determined by the analysis of the correlation spectrum. The use of the calibration model containing mostly 1% d-PLA samples to separately analyze the samples with 1% and 5% d-PLA is discussed. Superior results obtained from the multivariate over the univariate method make PLS the preferred method of choice in quantification of crystallinity. The standard error of prediction (SEP) for the analysis of 1% d-PLA samples with the use of the calibration model described is 0.85 (—J/g), and the SEP for 5% d-PLA samples is 1.35 (—J/g) with the outliers excluded. The discrepancy in analyzing these two types of PLA samples with the use of the same calibration model is also discussed.

An alternative interpretation of the HETCOR NMR spectra of poly(lactide)

An alternative to the assignments proposed recently by Chisholm et al. for the 1H and 13C NMR resonances of poly(lactide) based on their HETCOR spectra is presented; we find that the HETCOR spectra are consistent with older assignments of the tetrad and hexad stereosequence resonances in the 1H and 13C NMR spectra and we believe that the influence of adjacent chiral units on the NMR chemical shift extends asymmetrically and is opposite in the case of 1H and 13C nuclei; this is in contrast to the assumption that the observed chiral repeat unit for any particular stereosequence resonance is the same for 1H and 13C NMR spectra as used by Chisholm et al. in their analysis.

1H NMR Spectroscopy in the Analysis and Characterization of Poly(lactide)

Abstract High resolution 500 MHz 1H NMR spectroscopy is a powerful tool for the analysis and characterization of poly(lactide) (PLA). It accurately provides information about the distribution of a few stereosequences in the polymer whose resonances are well-resolved in the NMR spectrum. Here, the splitting of the methine resonance due to coupling to the methyl protons in the polymer is removed by homonuclear decoupling. 13C NMR provides complementary stereosequence information, but due to the poor signal-to-noise ratio its accuracy is not comparable to that of 1H NMR. Through the analysis of the stereosequence distribution in a number of PLA spectra, it was determined that there is a preference for syndiotactic addition during lactide stereo copolymerization. It was shown that the normalized intensity of a few well-resolved resonances in the 1H spectrum of PLA can be used to quantitatively determine the lactide stereoisomer composition incorporated in the polymer. The change in stereosequence distribution...

The Effect of Crystallinity on the Rheological Behavior of Poly(lactide)

Abstract The rheological behavior of poly(lactide) samples, with different levels of crystallinity, was investigated. The dynamic shear storage and loss modulus were obtained as a function of temperature at 1 rad/s. From the glassy to the rubbery region, the dynamic shear modulus decreased more than two decades for the initially amorphous poly(lac-tide) (containing 1% of D-lactide) and about one decade for the annealed (semicrystaJ-line) sample. The dynamic shear modulus data showed that the crystallization rate increased with a decrease of D-lactide in poly(lactide) and an increase of talc (a nucleating agent) in the sample. The steady-state and dynamic shear viscosity were measured at 210°C. The results indicated that the Cox-Merz rule can be used to relate the dynamic shear viscosity to the steady-state shear viscosity. A master curve for the amorphous poly(lactide) was obtained, using the time-temperature superposition principle. The result showed that the temperature dependence of the shift factor ca...