Investigating the crystallization behavior of poly(lactic acid) materials

19 August 2015
Zhong-Yong Fan and Wei Li
The poly(L-lactide)-to-poly(D-lactide) block length ratio of star-shaped asymmetric stereoblock copolymers is a key control on the crystallization of stereocomplex crystallites.

With the continuing decline of oil reserves, as well as the environmental problems caused by petroleum-based plastics, there is likely to be increasing demand and a new market for bioplastics in the near future. Poly(lactic acid)—or PLA—is a biologically based and biodegradable plastic. As such, this material has attracted much interest in various fields (e.g., medical and packaging materials).1–3 The application of PLA, however, is still limited because of several problems. These include its poor thermal resistance and its brittle nature.

In effective and promising methods to improve the mechanical and thermal properties of PLA-based materials, the stereocomplex crystallization behavior of PLA has been exploited. For instance, numerous efforts have been made to synthesize stereoblock PLA with high molecular weight (which is expected to form more stable stereocomplex crystallites). In addition, spherulite size significantly affects the thermo-mechanical properties of materials. A decrease in spherulite size is thus desirable for enhancing the deformability of PLA materials.

We have investigated the effects of different block length ratios (the ratio of different segment lengths in block copolymers) on the crystallization behavior of stereoblock copolymers.4 We have studied how different block length ratios of two PLA entaniomers (i.e., chiral stereoisomers)—poly(L-lactide), or PLLA, and poly(D-lactide), or PDLA—affect the spherulite growth of star-shaped copolymers (i.e., two or more polymerized monomers). The copolymers we examined were poly(propylene oxide) block poly(D-lactide) block poly(L-lactide)—PPO–PDLA–PLLA—stereoblocks.5 We used a polarized optical microscope (POM) to observe the crystallization behavior, which we then analyzed using Lauritzen-Hoffman (LH) theory. The series of PPO–PDLA–PLLA copolymers that we synthesized are given in Table 1. We used three different block length ratios of PPO, PDLA, and PLLA—i.e., 1:7:7, 1:7:14, and 1:7:28—to create this series (we will refer to each sample by the ratio hereafter). Detailed characterizations of these copolymers are also shown in Table 1.


Micrographs of poly(propylene oxide) block poly(D-lactide) block poly(L-lactide)—PPO–PDLA–PLLA—stereoblock copolymers with three different block length ratios (i.e., 1:7:7, 1:7:14, and 1:7:28). These images were obtained at different times during crystal growth at 165°C.

We obtained micrographs of the PPO–PDLA–PLLA stereoblock copolymers at different times during crystal growth, at a temperature of 165°C (see Figure 1). These images clearly reveal that the spherulites become smaller with increasing PLLA/PDLA block length ratio. For the 1:7:7 sample, spherulite diameters reach up to 117μm. For the 1:7:28 sample, however, the diameters are only 60μm, or less. Furthermore, the density of the spherulites increases sharply with increasing PLLA/PDLA block length ratio (as can also be seen in Figure 1). We expect this increase in spherulite nucleation density to enhance the mechanical properties of PLA materials.6, 7

We used the polarized optical photomicrographs that we obtained at different crystallization times to estimate the radial growth rate (G) of the spherulites in our PPO–PDLA–PLLA copolymers. We find that G decreases as the PLLA/PDLA block length ratio and the crystallization temperature (Tc) increase. According to LH theory, the nucleation constant (Kg) and the front constant (G0) can be estimated. G can be expressed as

Characterization of PPO–PDLA–PLLA stereoblock copolymer samples, with three different block length ratios. The number-average molecular weight (Mn) is given, as calculated from both nuclear magnetic spectroscopy (NMR) and gel permeation chromatography (GPC) methods. The melting temperature (Tm) of the samples increases slightly as the PLLA/PDLA block length ratio increases, which indicates that the PLLA/PDLA block length ratio has no significant impact on the Tm. [α]58925: The specific optical rotation measured at 25°C. D-LA: D-lactyl unit content of the PPO–PDLA–PLLA stereoblock copolymer. The melting crystallization temperature (Tmc), however, is about 130°C for all samples and decreases slightly with increasing PLLA/PDLA block length ratio. Tg: Glass transition temperature. Tcc: Cold crystallization temperature.

Mn(NMR)Mn(GPC)[α]58925
Samples(104g mol−1)(104g mol−1)(deg dm−1g−1cm3)D-LA (%)Tg(°C)Tcc(°C)Tm(°C)Tmc(°C)
1:7:76.22.1−6.448.045.082.0192.9134.5
1:7:149.64.1−69.428.349.684.2196.6133.1
1:7:2814.05.8−116.513.653.889.2197.1131.5

where U* is the activation energy for the transportation of polymer chain segments from the melt to the crystallization site, R is the gas constant, T is the hypothetical temperature where all motion associated with viscous flow ceases, ΔT is the degree of supercooling, and f is a correction factor that accounts for the change in heat of fusion as the temperature decreases below the equilibrium temperature. For our stereocomplex PLA samples—we used values for U* (1500cal/mol), T (Tg−30K), and the equilibrium temperature (5521.15K) that were taken from previous work.8 We obtained Tg values from the study of our melt-quenched polymers. We were then able to calculate Kg and G0 from the slopes and intercepts of a G−Tc plot (see Table 2).

Nucleation constant (Kg) and front constant (G0), and fold-surface free energy (σe) values for copolymers, calculated using Lauritzen-Hoffman theory, for the PPO–PDLA–PLLA samples. The results indicate that all the samples should crystallize within regime I.

SamplesKg(105K2)G0(μm/min)Regimeσe(erg/cm2)
1:7:78.933.91×107I77.7
1:7:148.894.72×106I77.3
1:7:288.421.30×106I73.3

In LH theory, crystallization is divided into three regimes, depending on the degree of supercooling. In regime I and regime III, Kg is twice as much as in regime II. According to the temperature range we used in our experiments, and reported Kg values for branched PLA blends (8.22−11.1×105K2),9 all of our PPO–PDLA–PLLA stereoblock copolymers should crystallize in regime I. This indicates that the PLLA/PDLA block length ratio has no effect on the crystal growth mechanism of PPO–PDLA–PLLA stereoblock copolymers. Kg is the thermodynamic nucleation barrier, which can be expressed in terms of the thickness of the polymer stem, the lateral-surface free energy, the fold-surface free energy (σe), the heat of fusion of a perfect crystal, and the Boltzmann constant. We have calculated the fold-surface free energy from a known set of equations,10 and the values for our different samples are given in Table 2. Our results show that σe decreases as the PLLA/PDLA block length ratio increases from 7:7 to 28:7. Nucleation therefore becomes easier with increasing PLLA/PDLA block length ratio. This theoretical result is in good agreement with our POM observations.

We have investigated the effect of PLLA/PDLA block length ratio on the crystallization behavior of PPO–PDLA–PLLA copolymers. Our results indicate that the PLLA/PDLA block length has a significant impact on the size of stereocomplex PLA spherulites. In addition, the size of spherulites decreases as the block length ratio increases. This is a result of an increase in nucleation density. We have also conducted an LH theory analysis, which reveals that the nucleation process is facilitated by the decrease in fold-surface free energy as the block length ratio increases. In our future work we plan to further study the mechanical properties of our stereoblock copolymers.


Authors

Zhong-Yong Fan
Fudan University

Wei Li
Fudan University


References

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DOI:  10.2417/spepro.006076



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