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Research Papers

Effect of Laser Surface Modification on the Crystallinity of Poly(L -Lactic Acid)

[+] Author and Article Information
Anubha Bhatla, Y. Lawrence Yao

Department of Mechanical Engineering, Columbia University, New York, NY 10027

J. Manuf. Sci. Eng 131(5), 051004 (Sep 04, 2009) (11 pages) doi:10.1115/1.3039519 History: Received October 11, 2007; Revised October 03, 2008; Published September 04, 2009

Crystallinity of semicrystalline polymers such as aliphatic homopolymer poly(L -lactic acid) (PLLA) affects their degradation and physical properties. In this paper, the effects of laser irradiation using the third harmonic of a Nd:YAG laser on the crystallinity, long-range order, and short-range conformations at the surface of PLLA films are investigated. The factors affecting the transformation are also studied. Detailed characterization of the effect of laser treatment is accomplished using microscopy, X-ray diffraction, and infrared spectroscopy. The cooling rates in the process and the spatial and temporal temperature profiles are numerically examined. The simulation results in conjunction with melting and crystallization kinetics of PLLA are used to understand the effect on sample crystallinity. The effects of laser fluence and annealing conditions on the crystallinity of the processed films are examined. Since degradation profiles depend on crystallinity, laser processing can potentially be used to achieve a modified spatially controlled polymer surface with promising applications such as controlled drug delivery.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Experimental setup

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Figure 2

Optical transmission micrograph of sample PLLA film annealed at 110°C

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Figure 3

α-form structure of PLLA indicating the a, b, and c directions of the orthorhombic 103 unit cell (20)

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Figure 4

WAXD profiles of untreated sample annealed at 110°C, laser treated (40 J/cm2), amorphous sample, and background scatter. Note reduction in 110/200 and 203 peaks.

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Figure 5

WAXD profiles indicating effects of laser fluence on the long-range order of PLLA sample. Profiles represent the crystalline fraction in the material. The variation in crystallinity with fluence can be seen in Fig. 1.

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Figure 6

SEM images of 300 μm (a) untreated sample and samples treated at (b) 30 J/cm2 and (c) 50 J/cm2, indicating microtome induced crazing in treated samples, which is indicative of increase in the amorphous fraction in the polymer due to laser irradiation. Also, the affected depth increases with increase in fluence.

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Figure 7

FTIR spectra of untreated specimen annealed at 110°C, laser treated (30 J/cm2), and amorphous specimens from 2000 cm−1 to 700 cm−1, indicating the important bending, stretching, and rocking modes of PLLA

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Figure 8

Carbonyl CO stretching region of PLLA samples: untreated, laser treated (30 J/cm2) and amorphous indicating factor group splitting; the primary conformations in the original sample being gt and tt, respectively. Amorphous sample does not show any splitting while the laser treated sample shows reduction in gt conformers.

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Figure 9

Derivative spectrum of the carbonyl stretching region indicating the distribution of conformers. The primary conformation consists of gt conformers at 1757 cm−1 followed by tt conformers at 1747 cm−1.

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Figure 10

Lower crystalline perfection is seen in the samples represented by broader bands. The mean factor group splitting is seen at 1222 cm−1.

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Figure 11

970–890 cm−1 region of FTIR spectrum indicating reduction in the band at 920 cm−1 (corresponding to coupling of the νC–C backbone stretch with CH3 rocking mode in 103 helical conformation of PLLA) with laser treatment

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Figure 12

Curve fitting for various conformers in the carbonyl region of PLLA films for (a) untreated sample and (b) sample treated at 50 J/cm2

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Figure 13

Effect of laser fluence on the crystallinity and crystallite thickness of PLLA film samples obtained from WAXD and gt peak absorbance values from curve fitting of carbonyl region of FTIR spectra

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Figure 14

Temperature distribution in the film cross section (30×30 μm2) at (a) 50 ns during which heat flux is supplied and (b) 1 ms after single pulse laser irradiation at 30 J/cm2 as obtained from the numerical analysis

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Figure 15

Numerically obtained time history of temperature at the center of the laser beam (as seen in Fig. 1) at various distances from the surface at laser repetition rates of 1 kHz and 1.5 kHz, respectively

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Figure 16

Comparison of peak cooling rates in the material on laser irradiation at different laser fluences as obtained from the numerical analysis

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Figure 17

(a) Comparison of sample crystallinity versus laser fluence at annealing conditions of 110°C and 135°C. (b) Comparison of absorbance of the gt to tt conformer peaks in the carbonyl (C=O) stretching region versus laser fluence at annealing conditions of 110°C and 135°C.

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