Determination of M-H-S Parameters for Estimation of
PLLA Chemical Recycling（ 九 工 大 院 ・ 生 命 体 ） ○ Wang Yan、 白 井 義 人 、（ 九 工 大 ETC）（ 正 ） 西 田 治 男 *概要
ポリ乳酸のケミカルリサイクルの前処理として行うオリゴマー化反応をより
正 確 に 理 解 す る た め に 、40℃ で の Mark‑Hourwink‑Sakurada 式 の パ ラ メ ー タ を 求 め 、汎用較正曲線法（ＵＣＭ）によるより正確な反応解析を検討した。
INTRODUCTION Poly(L-lactic acid) (PLLA) had been received much interest in medical
and pharmaceutical fields, because of its biodegradability, but nowadays it has been appliedin a wide field, such as electro/electronics devices, parts of automobile, food packages, etc.,
due to its biomass origin. PLLA also has attracted the attention of many researchers becauseof its superior recyclability to raw materials, i.e., L-lactic acid, L,L-lactide, and oligomers.
To achieve the recycling precisely, accurate evaluation of molecular weight andmolecular weight distribution values of resultants in degradation processes is indispensable.
The number and weight-average molecular weights of polymers are conveniently and widely
evaluated with the size-exclusion chromatography (SEC), resulting in calculation as relativevalues based on polystyrene standards. More accurate molecular weights values are given byusing the universal calibration method (UCM) 1 , which utilizes a relationship between themolecular weight and the hydrodynamic volume. The UCM, which needs theMark-Houwink-Sakurada (MHS) parameters: K and a, gives close molecular weight values toexact ones determined by other evaluation method, e.g. osmotic pressure and light scattering
measurements.
Recently, the process temperature for SEC analysis has become set at 40°C to avoid theinfluence of surrounding temperatures. However, so far, no MHS parameters for PLLA andpolystyrene at 40°C have been reported, while there are many parameters measured at25-35 °C. When the molecular weight of PLLA was calculated tentatively using the MHS
parameters measured at different temperatures, it was obvious that the resulting tentativevalues showed a certain extent of errors if compared with the molecular weights evaluatedwith proper MHS parameters.
In order to correct this incongruity in the molecular weight evaluation and use moreexact molecular weight estimated by UCM, in this study, the MHS parameters of PLLA andpolystyrene at 40°C were measured in chloroform and THF solution using viscometry and atriple detector (RI, capillary viscometer, and RALLS) system. The MHS parameters measured
at 40°C will be used for evaluations of various kinetics parameters of PLLA.
EXPERIMENTAL Four PLLA samples with polydispersity indices (PDIs) of around unity
were purchased from Polymer Source, Inc. Two PLLA samples with PDIs of around two wereprepared by high-pressure steam hydrolysis and following fractional precipitation.
Polystyrene (PS) standard samples were obtained from TOSOH Corp.
Viscosity of the samples were measured in chloroform and THF for PLLA and inchloroform for PS using Ubbelohde viscometer in a thermostated water bath at 40±0.1°C.
Intrinsic viscosities of the samples were obtained by the extrapolation of the plots of theinherent and reduced viscosities against concentration of samples.
The relative molecular weights of PLLA samples were analyzed on a TOSOH HLC-8120
GPC system at 40°C using TOSOH TSKgel Super HM-H column and chloroform eluent(0.6 mL min − 1 ). PS standards with M n values from 5.0×10 2 to 1.11×10 6 were used forcalibration.
RESULTS and DISCUSSION
The MHS parameters in THF at
40°C were determined using a PLLA
sample (M n 71 000, M v 120 000, M w130 000) on a triple detector
TDA-302 system (Viscotek. Corp.).
The measured data were analyzed byan Omnisec 4.5 software, resulting in
K=3.57×10 -4 dL·g -1 and a=0.70(Table 1). The viscosity-averagemolecular weights of other PLLA
samples were calculated from intrinsic
Table 1. M-H-S parameters for PLLA and PS
viscosity values measured at 40°C in THF
based on the MHS parameters determined inPolymer
PLLAPS
THF.
solvent
CHCl 3
THF
CHCl 3
To estimate the MHS parameters ofK (×10 4 dL/g)
2.48
3.57
2.07
PLLA at 40°C in chloroform, the intrinsic
a0.72
0.70
0.65
viscosity values of PLLA and PS samples
were measured in a similar manner with the measurement in THF. Figure 1 shows theviscosity-molecular weight relationships of PLLA and PS in chloroform at 40°C. The linear
relationships were represented with high accuracy in both cases. The MHS parameters wereestimated from the plots and summarized in Table 1. The a values were in a range of 0.65 to0.72, indicating that chloroform was good solvents for both polymers. The K value 2.48
dL·g -1 of PLLA decreased by 2.97 from 5.45 dL·g -1 at 30℃ 2 , while in the case of PS, the Kvalue increased from 0.49 to 2.07 dL·g -1 , suggesting different changes in chain variation
tendency between PLLA and PS with temperature.
To clarify influences of the MHS parameters on molecular weight calculation, PLLA
sample (exact M v 120 000) was employed to calculate the molecular weight by UCM,
resulting in M v = 88 000 and 122 000 at 30 and 40°C, respectively. Obviously, the M v value
calculated by using the proper MHS parameters was nearly close to the exact value. Moreover,these values were below the half of a conveniently calculated relative value M n 99 000 andM w 255 000 based on polystyrene standards.
Similar results were obtained in comparisons using hydrolyzed PLLA samples in a M w
range of 3 000~75 000. Molecular weights of the samples calculated using the parameters at40°C were in range of 0.5 to 0.7 when expressed as ratios against the relative molecularweights based on polystyrene standards. These ratios were different from the ratios in a rangeof 0.3 to 0.5 when previously reported MHS parameters measured at 25~35 °C were used.
CONCLUSION It was clearly illustrated that to precisely evaluate the molecular weight ofPLLA, UCM using appropriate MHS parameters gives nearly close to exact molecular weightvalues, which will enable us to control the chemical recycling of PLLA.
12Grubisic, Z.; Rempp, P.; Benoit, H. J. Polym. Sci. Part B 1967, 5, 753-759.
Schindler, A.; Harper, D. J. Polym. Sci.: Polym. Chem. Ed. 1979, 17, 2593-2599.

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