Masters Theses

Xiaoyun Ling

Date of Award

8-2003

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

Joseph E. Spruiell

Abstract

Poly(lactic acid) (PLA) is finding many new applications beyond its early use in bioabsorbable sutures. Consequently, there is considerable worldwide interest in understanding the relationship between processing, structure and properties for PLA. There is relatively little discussion in the literature about the best methods to determine such important characteristics as degree of crystallinity and the amount of molecular orientation present in processed PLA materials. And many physical constants for this polymer are either not available, or they exhibit a considerable variability among the various groups that have attempted to measure them. It is with this in mind that the present study was undertaken. The context for the study was the need to characterize the fine structure of PLA fibers. But in view of the current state of understanding of the characteristics and the methods of characterizing PLA, the study became a much broader one which involved finding appropriate means to measure the important structural characteristics (including an understanding of associated errors) and to measure some of the necessary parameters needed in future studies of PLA. Attempts were made to establish the following quantities for PLA: (1) amorphous density, (2) 100% crystalline density, (3) the intrinsic transverse moduli of the amorphous and crystalline phases, and ( 4) amorphous and crystalline intrinsic birefringences. The amorphous density was determined by directly measuring the density of an amorphous fiber that was prepared by melt spinning with no drawdown force other than gravity. This gave Pa= 1.2503 ± 0.002 g/cm3 for the amorphous density. The crystalline density was calculated from the mass and volume in the unit cell of PLA. An X-ray diffraction pattern of a highly oriented PLA rod was indexed as the orthorhombic a-phase with lattice constants: a= 10.57 ± 0.084 A, b = 6.13 ± 0.007 A,' c = 28.91 ± 0.017 A. This cell contains 20 monomer units, which gives the 100% crystalline density Pc =1.2762 ± 0.0013 g/cm3• This value is near the mean of previous values published in the literature. The intrinsic transverse elastic moduli of the crystalline and amorphous phases were obtained from measurements on a series of unoriented (random) samples that contained a range of degrees of crystallinity. The values obtained were E'/.am = 2.72 X 1010 dynes/cm2 and F::.c = 4.12 times 10^10 dynes/cm2 These values were used in characterizing the structure of melt spun fibers in both the as-spun condition and after annealing. Two PLA samples having similar molecular weight but slight difference in the D-isomer content (0.8% versus 1.3%) were melt spun at high speeds. One sample called PLLAl (0.8%D) was spun at 223°C while the second sample called PLLA2 (1.3%D) was spun at 208°C. The crystallinity and orientation development of the as-spun and annealed fibers were then studied by means of differential scanning calorimetry (DSC), density gradient column (DGC), optical birefringence, wide angle x-ray diffraction (W AXD) and sonic modulus. X-ray pattern analysis of both as-spun and annealed fibers suggests that all of them are a. phase PLLA, which is commonly observed in melt spun PLA fibers. The higher extrusion temperature of processing PLLAl compared to PLLA2 leads to lower spinline stress and stress-induced crystallization due to lower viscosity at the upper part of the spinline. Moreover, higher extrusion temperature results in larger extent of thermal degradation of PLLA 1 than PLLA2 and the resulting lower molecular weight was expected to further lower the spinline stress during preparation of PLLA 1 fibers. Due to the higher stress in the spinline of PLLA2 compared to PLLAl, the crystallinity and molecular orientation rise more rapidly with spinning speed. Percentage crystallinity values from both DSC and DGC methods bear the same tendency: as take-up velocity increases, so does the crystallinity, with DGC values closer to reality for as-spun fibers having low values of crystallinity. This conclusion is based on a comparison of x-ray patterns and x-ray crystallinity measurements with the DSC and DGC measurements. Percentage crystallinity improved after annealing and tei:ids to level off at •higher annealing temperature. Even though the basic trends observed are mostly the same, the absolute values exhibit large discrepancies. Although W AXS values were in better agreement with DGC values for as-spun fibers, the DSC and W AXD values are in better agreement for the annealed fibers. From such observation, we believe that at low crystallinity level (especially those fibers that show "cold crystallization" peaks), the DGC values were more accurate than DSC values because the fibers crystallize during the heating procedure and it is difficult to correct for this effect. This problem is not as significant for the annealed samples since there is little or no additional crystallization during heating in the DSC. Therefore, DSC values would be more reliable, while DGC encounters a common problem of trapping air on the fine fibers, which makes accurate measurements difficult. Since an accurate method of separating the scattering from the amorphous and crystalline phases of x-ray patterns is still unavailable, W AXD crystallinity of PLLAs serves more as a qualitative than quantitative method. A method for determining crystalline orientation was developed that uses the (0010) reflection to measure the c-axis orientation factor and the (200) reflection to measure the a-axis orientation factor. The crystalline orientation factor reached fc,z ~ 0.9, fb,z ~ -0.4 and fa,z ~ -0.5 for both PLLAs, but PLLA2 developed the high orientation at much earlier stage of spinning due to the higher stress it experienced. Compared with the development of crystalline orientation factor, the amorphous orientation factor developed much more slowly. Percentage crystallinity improved after annealing and tends to level off at higher annealing temperature. Crystalline orientation factor substantially improved for fibers spun under lower take-up velocities and slightly improved for those spun under higher take-up velocities. The trend of birefringence was in qualitative agreement with the measured crystalline and amorphous orientation factors. · Birefringence values increased gradually with take-up velocity in PLLAl fibers, but reached a maximum in PLLA2 before it slightly decreases. This decrease is believed to result from the radial temperature distribution caused by rapid cooling during high-speed spinning process. The outer skin of fiber cools faster, leading to higher viscosity on the fiber surface, when combined with the spinning stress, it crystallized first, which allows the inner part of the fiber to relax. Thus, the inner part actually experiences less stress-induced crystallization. Using the measured birefringences, crystallinities and orientation factors of the fibers, an attempt was made to determine the intrinsic birefringences of the amorphous and crystalline phases of PLA. The value for the crystalline phase was in reasonable agreement with rough estimates calculated from bond polarizabilities, but this was not the case for the amorphous phase. The latter result may well be due to the accumulated errors associated with its determination, especially the errors in the amorphous orientation factors.

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