Stereocomplexation kinetics of enantiomeric poly(L-lactide)/poly(D-lactide) blends seeded by nanocrystalline cellulose

Abstract

Stereocomplexation of poly(lactide) was significantly promoted by seeding with nanocrystal cellulose (NCC). A NCC-accelerated and time-dependent stereocomplex crystal (SC) growth are discovered. Moreover, SC-crystallization regime transitions (II–III) were identified and both the nucleation constant (K_g) and transition temperature (T_tr) were strongly increased in the presence of the NCC.

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Stereocomplex crystallite poly(lactide) (SC-PLA), formed between enantiomeric poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), show a melting point 50 °C higher than that of the PLLA and PDLA homo-crystallite (HC). The SC-PLA have better thermal and mechanical properties and higher hydrolytic stability.^1–4 SC-PLA can be used for applications such as engineering plastics, films, fibers and nucleating agents.⁵ Since Ikada et al. reported the stereocomplexation between enantiomeric PLLA and PDLA in 1987, intensive studies have been carried out on this topic.^6–9

It is known from previous studies that the crystallization of stereocomplex PLA is generally faster than the homo-crystallization of neat PLLA and PDLA and it also possesses higher nucleation density.^10–14 Compared with PLLA and PDLA, rather less study was carried out on the crystallization behaviour such as crystalline morphology, crystalline regimes and kinetics of PLA stereocomplex.^15–17 Till now, researchers are still trying to achieve high(er) efficiency in the formation of SC crystallites, e.g., by plasticizing or copolymerization.^18–21

Tsuji et al. reported that the spherulitic growth, nucleation constant (K_g) and crystallization regime of neat SC-PLA.^11,12 Their results revealed that the spherulitic growth rate of stereocomplex crystallites is higher in comparison with homo-crystallite. Moreover, two crystallization regimes were distinguished, i.e., the regime II at high temperature and the regime III at low temperature for the blends. However, to the best of the authors' knowledge, scarcely attention was paid to the crystallization kinetics and morphology of stereocomplexes in the presence of nucleating agents.

It was found in our study that the stereocomplexation between enantiomeric PLLA and PDLA can be considerably speeded up by using nanocrystalline cellulose (NCC) as a nucleator, which is interesting for the industrial application of SC-PLA. Therefore, the main objective of this investigation was to provide a well understanding on the NCC-enhanced crystallization kinetics of the SC-PLA by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), polarized optical microscopy (POM) and Hoffman–Lauritzen theory. The criteria of regime II and regime III affected by NCC is also studied for wider applications.

Overall stereocomplexation behaviour of SC-PLA in the presence of NCC

Stereocomplexation behaviour of the symmetric PDLA/PLLA in the presence of different amount of NCC (0, 1, 25 wt%) was first investigated by DSC, WAXD and POM, as shown in Fig. 1(A)–(C), respectively. It is found from Fig. 1(A) that the SC in the PDLA/PLLA blend formed at 143 °C, which was increased by 15 °C and 26 °C respectively after addition of 1 wt% and 25 wt% of the NCC, indicating much faster stereocomplexation of the PDLA/PLLA blends in the presence of NCC. This remark was supported by isothermal crystallization of the SC-PLA where a half-crystallization time (t_1/2) of the SC-PLA was reduced by 80% and 95% respectively with 1 wt% and 25 wt% of the NCC at 175–190 °C (Fig. S3 of the ESI†). In addition, two exothermic peaks were observed in the PDLA/PLLA/NCC blends with 25 wt% of the NCC corresponding to the crystallization of SC and HC crystallites, respectively (Fig. 1(A)). The HC crystallized at 135 °C which is far higher than that of neat PLLA or PDLA,²² and also higher than that of SC-seeded PLLA.²³ Therefore, the NCC at high content not only enhanced the stereocomplexation but also promoted the HC crystallization. The DSC results were further evidenced by WAXD patterns (Fig. 1(B)). Characteristic diffraction peaks of the SC crystallites were observed at 2θ values of 12° (110), 21° (300/030) and 24.1° (220),²⁴ however the peak intensities of the NCC-seeded PDLA/PLLA blends are stronger. Being consistent with DSC results, a notable diffraction peak at 2θ = 16.8° (110/200) is detected in the PDLA/PLLA/NCC (25 wt%) blend relating to the α-form PLA crystals.²⁵ The POM micrographs of the non-isothermally crystallized PDLA/PLLA/NCC (0, 1, 25 wt%) blends at 160 °C (Fig. 1(C)) showed that the size of the SC-PLA spherulites was not obviously varied after addition of 1 wt% of the NCC whereas the density of the spherulites was higher. It has to be remarked that the spherulites observed here are only SC crystals because the temperature of 160 °C is too high to form HC crystals, see ESI Fig. S1 and S2.† Interestingly, the spherulites were significantly decreased in size and further increased in numbers with increasing the NCC content from 1 wt% to 25 wt%.

Fig. 1 Non-isothermal crystallization behaviour of the PDLA/PLLA/NCC (0, 1, 25 wt%) blends cooled from their melt state (250 °C for 2 min): (A) DSC curves, (B) WAXD patterns taken at room temperatures and (C) POM images taken at 160 °C. The cooling rates were fixed at 10 °C min⁻¹ for all the treatment. ((a), NCC 0%; (b), NCC 1%; (c), NCC 25%.)

These results convincingly demonstrate that the NCC is efficient nucleator to promote the crystallization of SC-PLA.

Spherulitic growth kinetics of the SC-PLA

It is well-known that crystallization consists of two steps, i.e., nucleation and crystal growth. It has been shown in the non-isothermal crystallization by POM (Fig. 1(C)) that the NCC could remarkably accelerate the nucleation of the SC-PLA, notably at high NCC content. In the second part of this communication we show that the initial crystal growth rate of SC-PLA (G-SC) was also increased by incorporation of the NCC (Fig. 2(a)). The crystal growth rates show typical bell-shaped temperature dependence, being consistent with general polymer crystallization characteristics.^26,27 For the PDLA/PLLA blends, the G-SC increased gradually with reducing the temperature from 200 °C to 165 °C and then decreased, exhibiting a maximum (17 μm min⁻¹) at 165 °C. The same trend and maximum position (165 °C) were observed for the PDLA/PLLA/NCC blends as well. However, the G-SC was obviously increased after addition of the NCC at all examined temperatures, e.g., the G-SC at 180 °C was increased by 38% and 110%, respectively, after addition of 1 wt% and 25 wt% of NCC. According to crystallization theory of polymers, the crystal growth rate is mainly controlled by chain mobility that is associated with temperatures.^20,28 The NCC, notably at high content, have steric hindrance and hydrogen bonds with PLA molecules,²⁹ which are not beneficial to the crystal growth. Therefore, it is interesting to observe the increased G-SC after addition of the NCC. A multi-nucleation mechanism within spherulites was proposed here for the enhanced G-SC, i.e., nucleation still occurs during spherulitic growth since the NCC is finely dispersed in the spherulites.³⁰ The differences in packing manners between stereocomplexation and normal folded-chain crystallization may be another reason for the unusual phenomenon. However, direct experimental evidence is lacking and needs to be studied further.

Fig. 2 (a) The temperature-dependence of crystal growth rates (G-SC) of the PDLA/PLLA/NCC (0, 1, 25 wt%) blends measured in the initial stage of the growth; (b) the time-dependence of crystal growth rates of the PDLA/PLLA/NCC (0, 1, 25 wt%) blends monitored by POM at 190 °C.

The higher nuclei density of the SC-PLA with NCC was also observed during isothermal crystallization (Fig. S4 of the ESI†). Meanwhile, an interesting time-dependence of the G-SC was discovered in the isothermal crystallization, i.e., the radii of SC-PLA spherulites increase linearly with time in the initial stage, however, the growth slowed down gradually with time and, at a certain point, it was levelled off, as shown in Fig. 2(b) and S4 of the ESI.† It has to be remarked that this phenomenon occurred prior to the impingement between neighbouring spherulites and was observed at different temperatures as well. The loading of NCC did not change the nature of the time-dependence of the G-SC but varied the appearance time of stopping points which is associated with the inter-spherulite distance. Moreover, the NCC accelerated the completion of PLA stereocomplexation at high loadings (e.g., 20 min for the blend with 25 wt% of NCC in comparison with more than 2 h for the blend without NCC).

In general, crystal growth rate is initially constant with time,³¹ whereas a sizeable slowdown of G was observed when two poly(ethylene oxide) spherulites approached each other to distances of less than 150 μm ascribing to (1) an increase in temperature expected in the vicinity of the crystal growth front and/or (2) the accumulation of non-crystallizable materials before the growing crystals.^32,33 However, rather limited study in literature was reported on such phenomenon, especially on such phenomenon of SC-PLA. In this study, the G varied already far prior to the spherulitic impingement at high temperatures (175–190 °C). As a consequence, the accumulation of non-crystallizable materials and possible changes in molecular weight (degradation) are regarded as possible explanations for the time-dependence of the G-SC which, however, have to be evidenced further in the future.

Crystallization regime analysis

In order to further investigate the crystallization kinetics of the SC-PLA in the presence of the NCC that isothermally crystallized from the melt, the well-known Hoffman–Lauritzen analysis is applied. According to this analysis,^34,35 three regimes are distinguished for crystallization of polymeric materials, i.e., regimes I, II and III.³⁴ The crystal growth rate is a function of the surface nucleation rate (i) and the velocity with which the surface nucleus spreads on the crystal surface (g). When the i value is much smaller than the g value, once a nucleus is formed, it will spread rapidly across a substrate length on the growth front resulting in regime I type of crystallization at high temperatures. When the i value is comparable to the g value, it will be the regime II type of crystallization. When the i value is far higher than the g value, it will generate multi-nucleation before completion of one lamellae,³⁶ i.e., the so-called regime III type of crystallization. Thus, it is important to study the regime transition behaviour for both of understanding the crystallization kinetics and the application of the SC-PLA.

According to the kinetic theories by Hoffman–Lauritzen,^34,35 the G at the crystallization temperature, T_c, can be expressed by eqn (1) as follows:

where G₀ is a pre-exponential factor, R is the gas constant, T_c is the crystallization temperature, T_∞ (T_∞ = T_g − 30 K) is the hypothetical temperature where all the motion associated with viscous flow ceases, U* (1500 cal mol⁻¹) is the activation energy for the transport of chain segments to the site of crystallization, ΔT is the degree of supercooling given by T⁰_m − T_c, T⁰_m is the equilibrium melting point of stereocomplex crystals. And f is a factor given by 2T_c/(T⁰_m + T_c) which accounts for the variation in the enthalpy of fusion as the temperature decreased below T⁰_m, K_g is the nucleation constant as shown in eqn (2), i.e.,where σ and σ_e are the lateral and end-surface free energy, respectively, b₀ is the layer thickness, k is the Boltzmann constant, Δh_f is the heat of fusion per unit volume, and the value of m dependents on the crystallization regime. The m values are 4, 2, and 4 for crystallization regimes I, II and III, respectively. In literature, the crystallization regime transition of I–II was only discussed in a couple of polymers such as homo-crystallization of poly(L-lactide)³⁷ and poly(chlorotrifluoro-ethylene).³⁸ Whereas, more polymers such as isotactic and syndiotactic poly(propylene), poly(3,3-dimethylthietane), poly(hydroxybutyrate), poly(butylenes succinate) and poly(ethylene succinate) showed the crystallization regime transitions of II–III.^39–44 However, the crystallization regime transitions of SC-PLA at high temperatures are seldom reported.

For practically convenient use, eqn (1) is usually rewritten as follows:

The Lauritzen–Hoffman plots of ln G + U*/R(T_c − T_∞) versus 10⁵/T_c(ΔT)f for the three SC-PLA blends with different NCC contents are shown in Fig. 3. Obviously, the optimum fitting of the plots of each sample is by two straight lines among different ranges of T_c. Hence, two crystallization regimes were clearly identified in Fig. 3, including the regime II at higher temperatures and the regime III at lower temperatures for all the examined SC-PLA blends. It has to be remarked that the regime I usually appears at too high temperatures to be detected directly by experiment. The K_g is obtained from the slopes of the fitted lines. In the regime III, the values of K_g are determined as 0.95 × 10⁵ K², 1.23 × 10⁵ K² and 1.75 × 10⁵ K² for the PDLA/PLLA, PDLA/PLLA/NCC (1%) and PDLA/PLLA/NCC (25%) blends, respectively. In the regime III, the values of K_g are determined as 0.40 × 10⁵ K², 0.48 × 10⁵ K² and 0.53 × 10⁵ K² for the PDLA/PLLA, PDLA/PLLA/NCC (1%) and PDLA/PLLA/NCC (25%) blends, respectively. Hence, the K_g values of the PDLA/PLLA/NCC blends are much larger than those of the PDLA/PLLA blend regardless of temperature ranges and NCC loadings. The higher K_g means a higher crystallization rate and nucleation activity.⁴⁵ From the K_g values of the three samples, it can be concluded that the NCC as a nucleating agent of SC-PLA is effective in both crystalline regimes II, and III. It is known from Fig. 3 that the transition temperatures (T_tr) from regime II to regime III are 185 °C, 190 °C, and 195 °C for the PDLA/PLLA, PDLA/PLLA/NCC (1%) and PDLA/PLLA/NCC (25%) blends, respectively. The significant shift of T_tr to high temperatures with increasing the NCC content is resulted from the strong nucleation effect of the NCC, notably at high loadings.⁴⁶

Fig. 3 Lauritzen–Hoffman analysis on crystallization regime transitions of the PDLA/PLLA/NCC (0, 1, 25 wt%) blends.

To summarize, the melt crystallization behaviour of stereocomplex poly(lactide) (SC-PLA) with different amount of nanocrystal cellulose (NCC) was investigated by means of DSC, POM, WAXD and Hoffman–Lauritzen theory. The experimental results conclusively revealed that the NCC, notably at high loadings (25 wt%), can be effect heterogeneous nucleating agents for SC-PLA that increased both of the nuclei density and crystallization temperature (by 26 °C) while reduced the half-life crystallization time by 95% (175–190 °C). The linear stereocomplex crystal growth rate (G-SC) in the examined samples showed both temperature and time dependences. The maximum G-SC occurs at 165 °C and the G-SC slows down with time in isothermal crystallization. The addition of the NCC increased the G-SC values and shortened the appearance time of growth stopping points. According to Hoffman–Lauritzen analysis, all the SC-PLA samples exhibit crystallization regime I to II and regime II to III transitions. The crystallization regime transition temperatures from regime II to regime III shift to high temperatures with increasing the NCC loadings. Therefore, the investigation on the melt crystallization kinetics of SC-PLA in the presence of NCC has both academic and industrial interest which may potentially broaden the application range of SC-PLA.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51303067), the Natural Science Foundation of Jiangsu Province (BK20130147) and the Fundamental Research Funds for the Central Universities (JUSRP51408B).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12586c

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