Tough, biodegradable poly(L-lactic acid) polymer blends

11 September 2013
Ahmed El-Hadi
Blending poly(L-lactic acid) and polyhydroxybutyrate, with tributyl citrate as the plasticizer, increases molecular mobility and the rate of crystallization.

Poly(L-lactic acid) PLLA is an aliphatic polyester that is used in textile fibers, paper coatings, and biomedicine. It has many advantageous properties: it is insoluble in water, completely biodegradable by composting, and is an optically active and transparent thermoplastic. However, slow crystallization rates under supercooling conditions permit large spherulites (spherical semi-crystalline regions) with cracks to form.1,2 In addition, the glass transition temperature (Tg) of PLLA is 60°C, such that the polymer chain is immobile at room temperature. As a result, it is brittle and unsuitable for many applications, such as in food packaging.

Four methods are well known to improve the mechanical properties and thermal stability of PLLA, namely, copolymerization with poly(glycolic acid)—also known as PGA—and polycaprolactone (PCL), chemical crosslinking with dicumyl peroxide (DCP), radiation, and physical crosslinking.3–9However, these methods have disadvantages. Copolymerization with PGA and PCL is not suitable for mass production, and the product of chemical crosslinking with DCP is not biodegradable. Radiation causes a scission of the side chain, which reduces the molecular weight and changes the physical properties of the product, rendering it non-biodegradable.

I considered how to increase PLLA's rate of crystallization and reduce its Tg from an industrial point of view. Physical crosslinking techniques, such as adding plasticizers, fillers, nucleation agents, or blending with another polymer, are the most promising for producing a tougher, biodegradable polymer on a large scale at reasonable cost. To this end, I developed novel biopolymer blends. Using the plasticizer tributyl citrate (TBC), I could increase polymer chain mobility and reduce the Tg of PLLA from 60°C to 10°C in the blends. The crystallization rate of PLLA was also improved by adding 5–20% by weight of the accelerating agent 3-polyhydroxybutyrate (PHB), which increases the number of smaller crystals and minimizes their size.

I first mixed PLLA with TBC and PHB by dissolving them in chloroform at room temperature in different ratios (see Figure 1 and Table 1). When studying the the thermal behavior of the blends using differential scanning calorimetry (DSC), the Tg decreased from 60°C1 for neat PLLA to 10°C for blend 4, which has the highest TBC and PHB content of the blends examined (see Figure 1). The cold temperature (Tcc) and the melting point (Tm) of the blends also changed as the additive content increased. For instance, Tcc of pure PLLA is 105°C and decreases to 80°C for blend 4. Tm of pure PLLA is 174°C and decreases to 153°C for blend 4. The reduction in Tm is a typical indicator of miscible blends containing a crystalline polymer and the size of crystallite is smaller accordingly. Figure 2 shows the thermal degradation behavior of PLLA and its blends. The mass loss of PLLA blends occurs in three stages, the first of which can be attributed to moisture content in the range 100–170°C with the maximum loss occurring at 150°C (weight loss 5–11%). The second stage is the thermal degradation of PHB in the range 240–280°C, and the third stage, from 280°C to 370°C, with a maximum at 343°C, is attributable to thermal degradation of the main chains of PLLA.


The differential scanning calorimetry second heating run at a rate of 10°C/min for poly(l-lactic acid)—PLLA—blends.

Percentage weight compositions of the different blends of PLLA with tributyl citrate (TBC) and polyhydroxybutyrate (PHB).

BlendPLLATBCPHB
19055
2801010
3701515
4602020

Thermal behavior of PLLA and its blends, obtained by thermogravimetric analysis.

I used polarized optical microscopy to investigate the morphology of the PLLA blends. This showed that adding a small amount of PHB improves the PLLA crystallization process. Figure 3 shows the spherulitic growth of blend 4, crystallized isothermally at 120°C and then cooled at 80°C for different lengths of time. Isothermal crystallization at 120°C showed that approximately 13.0 big PLLA spherulites per unit area are formed at first. These spherulites grew very slowly, reaching a diameter of up to 50μm after 5min. Reducing the isothermal crystallization to Tc=80°C resulted in remarkably rapid crystal growth and enhanced PLLA nucleation density. That is, thousands to millions of small spherulites had been formed, which means that crystallization can occur faster.


Polarized optical microscopy of spherulite texture of blend 4, crystallized isothermal at 120°C and then at 80°C after (a) 2min, (b) 3min, (c) 5min, (d) 6min, (e) 7min, and (f) 8min.


Fourier-transform-IR spectra of PLLA and its blends from 700 to 4000cm−1.

Figure 4 shows the Fourier-transform IR (FTIR) spectra of PHB, PLLA, and its blends. PLLA has a strong carbonyl stretching absorption (C=O of ester groups) at about 1749cm−1. The spectrum of pure PHB consists of two peaks, one of which is attributed to the stretching vibrations of crystalline carbonyl groups at 1720cm−1. The other, a small ‘shoulder’ at 1744cm−1, is characteristic of the amorphous carbonyl vibration. With additives, the C=O band position is shifted to a lower wave number at 1734cm−1, which indicates physical interactions between PLLA and the additives. The ~15cm−1 shift in the C=O peak can be attributed to hydrogen bonding (–C=O… H3C–), showing that the blend systems are miscible.

Wide-angle x-ray diffraction (WAXD) patterns of PLLA and its blends show that typical diffraction peaks of the α-form crystal of PLLA blends appear at 2θ=14.6°, 16.6°, and 22.3°, corresponding to (010), (200)/(110), and (205) reflections, respectively (see Figure 5). These diffraction peaks are very similar to the diffraction patterns of the α-form PLLA crystals reported in the literature.10 The β-form of the crystallite appears at a scattering angle 2θ=19° (203). The crystalline structure of the PLLA matrix is significantly affected by the presence of PHB.


Wide-angle x-ray diffraction (WAXD) diagrams of pure PLLA and its blends.

In summary, Tg, Tc, and Tm of PLLA blends with PHB and TBC shift to lower temperatures as the proportion of additives increases, thereby enhancing the molecular mobility of PLLA. FTIR spectrometry reveals molecular interaction between PLLA and additives, as indicated by the change in band positions of the ester group C=O from 1749 to 1734cm−1 in comparison with pure PLLA. WAXD shows that with increasing PHB content, the intensities of both peaks at (200/110) and (203) increase, meaning that the PHB component crystallizes in the PLLA matrix, and PHB acts as heterogeneous bionuclei. The novel blends are suitable for use in household films for food packaging, deep drawing articles (i.e., formed by being punched into a deep die), and as medical sutures. As well as continuing to work to improve the properties of PLLA biopolymer blends, I am now studying how to prepare medical sutures and other medical applications from this and other biopolymers by electrospinning.


Author

Ahmed El-Hadi
Department of Physics, Faculty of Science Umm Al-Qura University

Ahmed Mohamed El-Hadi is an associate professor of polymer physics. His research focuses on biodegradable polymers such as PHB, PLA, starch, and polysaccharides that include cellulose and chitin. He studies the relationships between the macromolecular structures and physical properties of polymers with a view to developing new applications for biopolymer materials for medical and packaging applications. He is a reviewer for several scientific journals.


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DOI:  10.2417/spepro.005028