Study on the Crystallization Activation Energy of Poly (L-lactic acid) Nucleated with P-tert-butylcalix[8]arene

2.1 Materials

The PLLA (M_W =100000 g mol⁻¹) used in this paper was manufactured by Jiu Ding Company, China. All the other chemicals were of reagent grade, purchased from commercial sources and used without further purification.

2.2 Preparation of Nucleating Agent tBC8

The p-tert-butylcalix[8]arene was synthesized according to the method of Munch ¹⁷ . The mixture of 50 g (0.33 mol) of p-tert-butylphenol, 17.5 g (0.55 mol) of paraformaldehyde, 300 mL of xylene and 0.415 g (0.01038 mol) sodium hydroxide was put in a 1 L flask with a water separator. Then it was heated to reflux for 4 h under the circumstance of nitrogen. After the mixture was cooled to room temperature, the precipitate was obtained after filtration. The crude product was washed, in succession, with 200 mL portions of toluene, ether, acetone, and water and then dried under reduced pressure. It was added into ethylbenzene. The mixture was kept refluxing for two hours. The boiling solution was concentrated to a quarter of its volume. The solution was cooled to room temperature, and the precipitate was collected by filtration. Then colorless powder nucleating agent tBC8 was obtained after drying under vacuum at 60°C for 12 h.

2.3 Preparation of PLLA Samples

PLLA was mixed with 1 wt% nucleating agent in a self-designed flask, which had an inlet and outlet for nitrogen. After removing air with nitrogen for 10 min, the temperature was raised to 200°C and held for 5 min to reach equilibrium. The mixture was stirred at 200°C for 5 min. The neat PLLA was prepared in the same procedure.

2.4 Measurements

3.1 The Activation Energy of Crystallization for PLLA/tBC8

Crystallization peak temperatures (T _p ) of PLLA and PLLA/tBC8 under different cooling rate are listed in Table 1 . It can be noticed that under the same cooling condition with the addition of 1 wt% tBC8, the T _p of PLLA increased dramatically, which suggested that tBC8 was an efficient nucleating agent for PLLA. Nucleating agent can act as nuclei during PLLA crystallization, thus increasing crystallization temperature.

The activation energy of crystallization for PLLA samples can be obtained by using the Kissinger equation ¹⁰ :

d [ I n ( ϕ / T p 2 ) ] d ( 1 T p ) = − Δ E R

(1)

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Figure 1

Kissinger plots for calculating non-isothermal crystallization activation energies for PLLA samples

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Table 1

Non-isothermal crystallization kinetics parameters for PLLA samples^a,b,c

Sample	Φ (°C/min)	Tp (°C)	-ΔE (kJ mol−1)	Tg	T0 (°C)	T∞ (°C)
PLLA	2.5	122.57	96.06	49.6	130.2	116.4
	5.0	108.29			114.1	101.1
	10.0	99.82			112.2	87.0
	15.0	97.54			111.9	86.9
	20.0	95.03			108.2	81.9
PLLA/PEG	2.5	125.59	79.10	36.0	129.5	121.7
	5.0	117.45			123.6	111.5
	10.0	104.67			116.6	97.0
	15.0	98.64			107.4	88.9
	20.0	91.29			101.1	81.7
PLLA/tBC8	2.5	138.69	165.97	52.6	141.8	136.2
	5.0	134.40			137.7	130.9
	10.0	129.17			133.5	124.9
	15.0	124.09			129.8	118.1
	20.0	121.08			127.7	114.9
PLLA/0.5PEG/tBC8	2.5	139.62	195.10	52.0	142.6	137.0
	5.0	135.68			138.9	132.8
	10.0	130.77			134.8	127.3
	15.0	127.54			132.0	123.1
	20.0	124.52			130.0	119.0
PLLA/1PEG/tBC8	2.5	139.71	204.40	49.9	142.3	137.3
	5.0	135.89			138.6	131.9
	10.0	131.31			133.9	126.7
	15.0	127.98			130.9	121.1
	20.0	125.36			128.8	117.4
PLLA/5PEG/tBC8	2.5	138.22	210.21	38.7	140.9	135.5
	5.0	134.20			137.5	130.1
	10.0	130.95			134.9	127.0
	15.0	128.04			132.2	124.4
	20.0	125.77			130.1	122.2

a

Additional amount of tBC8 was 1 wt%.

b

Additional amount of PEG in PLLA/PEG, PLLA/tBC8/0.5%PEG, PLLA/tBC8/1%PEG, PLLA/tBC8/5%PEG were 5, 0.5, 1 and 5 wt%, respectively.

c

T₀ and T_∞ represented the onset and end temperature of crystallization, respectively.

As listed in Table 1 , ΔE values of PLLA and PLLA nucleated with tBC8 (PLLA/tBC8) during the non-isothermal crystallization process were determined, which were 96.06 and 165.97 kJ/mol, respectively. Obviously the ΔE of PLLA/tBC8 appeared much higher than that of pure PLLA. As mentioned above, tBC8 is a high-performance nucleating agent, which can act as nuclei during PLLA crystallization. Moreover, nucleation is regarded as the controlling step during crystallization. An obvious question arises: Why is ΔE of PLLA/tBC8 much higher than that of PLLA?

ΔE consists of nucleation activation energy ΔE and the transport activation energy ΔE^*12. ΔE is the free energy of formation of the critical-size crystal nuclei, and ΔE^* refers to the activation energy required to transport molecular segments across the phase boundary to the crystallization surface. According to previous presumption^11,13, it can be attributed to the fact that the addition of nucleating agents would baffle the transfer of macromolecular segments from melt to crystal growth surface. It was caused by the interaction between nucleating agents and segments of PLLA, which was related to the increase of ΔE^*.

Δ E * = 4120 T c 51.6 + T c − T g

(2)

Based on the equation (2), ΔE^* is regarded to be inversely proportional to the glass transition temperature (T _g )^19–21. To verify that whether ΔE^* of PLLA/tBC8 was higher than that of PLLA, the effect of tBC8 on mobility of chain segment was studied by means of measuring T _g of the samples. The results of T _g also listed in Table 1 . The T _g of PLLA/tBC8 was 52.6°C, 3°C higher than that of PLLA ( Table 1 ), indicating that the chain mobility was retarded. Therefore, lower chain mobility produced by adding tBC8 was one reason for the increase of ΔE^* of PLLA, which appeared in good agreement with the presumption proposed in a previous report^11,13. However, can the increased ΔE of PLLA/tBC8 only be accounted on the rise of ΔE^* caused by tBC8?

To further disclose the mechanism of the increase of ΔE, PEG was added to this system as a nucleation promoter to eliminate the rise of ΔE^* caused by nucleating agent tBC8. As reported^22,23, PEG can improve the mobility of PLLA chain segments and increase the crystallization rate by reducing the inter-chain interaction, which results in a decline of ΔE^*.

First, the crystallization behavior of PLLA/PEG/tBC8 samples was studied to reveal the effect of PEG, so as to help us determine whether inducing PEG was good for the crystallization of PLLA/tBC8 or not ( Figure 2 ). In general, lower the cold peak temperature (T _cp ) is, the faster the overall crystallization rate of PLLA is ²⁴ . As Figure 2 shows, the T _cp of PLLA/1PEG /tBC8 and PLLA/5PEG/tBC8 decreased with the increment of PEG, which indicated PEG had a positive effect on the crystallization of PLLA/tBC8. It could be attributed to the improvement of chain segment mobility. Another evidence was that as Table 1 showed that the addition of PEG remarkably decreased the T _g of PLLA from 49.6°C to 36.0°C. Further, with the increase of PEG content, the mobility of the chain segments improved. POM images ( Figure 3 ) also provide the evidence. The size of PLLA/5PEG/tBC8 crystal appeared smaller than that of PLLA/tBC8.

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Figure 2

Cold crystallization behavior of PLLA samples

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Figure 3

CPOM images of isothermal crystallization of PLLA (a) PLLA/5PEG/tBC8 (b) and PLLA/tBC8 (c) at 140°C

However, T _cp of PLLA/0.5PEG/tBC8 was higher than that of PLLA/tBC8, from Figure 2 . As listed in Table 1 , the T _g of PLLA/0.5PEG /tBC8 was almost the same as that of PLLA/tBC8. Based on previous study, PEG can not only facilitate crystallization growth but also retard nucleation process^18,25. There exists a competition between these two aspects. Moreover, the temperature range of crystallization T _c (between T₀ and T_∞) for these two samples was almost the same. Consequently, according to the equation (2), the ΔE* of PLLA/tBCd8 was approximately equal to that of PLLA/tBC8/0.5PEG. So, the increase of T _cp in PLLA/ tBC8/0.5%PEG was the result of the inhibition by PEG of the PLLA's nucleation process. T _cp of PLLA/1PEG/ tBC8 and PLLA/5PEG/tBC8 appeared lower than that of PLLA/tBC8, indicating greatly accelerated the overall crystallization rate. From Table 1 , with the increment of PEG, T _g of samples decreased, which showed that the mobility of the chain segments for PLLA became better. Specifically, once the additional amount of PEG was higher than 1%, the negative effect on nucleation would be covered by its contribution to the segmental mobility.

Here, we eliminated the rise of ΔE* caused by nucleating agent tBC8 through adding PEG. In addition, we proved that once the additional amount of PEG was higher than 1%, the negative effect on nucleation would be covered by its contribution to the segmental mobility. The additional amount of PEG for PLLA/PEG was the same as that for PLLA/5PEG /tBC8. The T _g for these two samples were both much lower than that of PLLA, furthermore, and the crystallization temperature range of these two sample both were higher than that of PLLA. Therefore, the ΔE* of these two samples should be lower than that of PLLA. Interestingly, the ΔE of PLLA/PEG was lower than that of PLLA, but on the contrary, the ΔE of PLLA/5PEG/tBC8 was 210.2 kJ/ mol, which was much higher than that of PLLA. That is to say, the increased ΔE can not only be accounted for by the rise of ΔE* caused by tBC8.

Based on the previous study, both nucleation and crystal growth are related to T _c , the increment of crystallization temperature would be hindered and enhance the nucleation and crystal growth process, respectively^26,27. In other words, along with the increment of T _c , crystallization of PLLA occurs at higher temperature, then ΔF would be increased. In this case, tBC8 acting as nuclei accelerated the crystallization rate of PLLA, which made the crystallization occur at a high temperature during the cooling process. At the cooling rate of 2.5°C min⁻¹, PLLA/5PEG/tBC8 started crystallizing at 140.9°C, much higher than that of PLLA and PLLA/PEG. Additionally, ΔF increases with the crystallization temperature ²⁷ . Thus, the increment of ΔE could be attributed to the increasing crystallization temperature of PLLA. In other words, if PLLA could crystallize at the same temperature range as PLLA/ tBC8, the ΔF of PLLA must be higher than that of PLLA/tBC8.

Therefore, we could draw a conclusion that the increment of ΔE for PLLA/tBC8 compared to PLLA was the result of the increased temperature range of crystallization and the lowered mobility of the chain segments. Then crystallization temperature range must be taken into consideration when ΔE is calculated by the Kissinger equation.