α和γ晶型尼龙6的可控合成

黄映珊++姜锋++李杰++彭舒敏++任茜++罗玉航++易春旺����

摘 要 报导了可控晶型尼龙6的制备与表征．DSC结果显示γ晶型在Tm1为210 ℃左右出现一个尖锐的熔融峰，同时在Tm2为219 ℃左右出现一个小肩峰．当温度高于70 ℃，反应时间大于8 h时可以制备稳定的α晶型，α晶型显示双熔融峰．FTIR结果进一步表明所获得的样品为γ晶型、α晶型和混晶．SEM结果显示γ晶型表面是由孔径大小不一样的小孔组成，α晶型表面是由不同厚度的片晶堆积而成．

关键词 PA6；α晶型；γ晶型；可控晶型



Poly(ecaprolactam)s, known as polyamide6 (PA6) or nylon 6, is a kind of typical polycrystalline or semicrystalline polymers with the main chain containing amide groups［12］. There are strong hydrogen bonds among adjacent polymer chains in both crystalline and amorphous regions, which is the determining factor that nylon6 have good chemical stability and mechanical properties. Because of this, the structure and morphology of nylon6 have been of widespread study and use in plastics and synthetic fiber industry ［38］. Currently, αphase and γphase structures are widely recognized as major crystalline nylon6［912］. The αphase, which was first characterized by Holmes［13］, has a monoclinic structure, with hydrogen bonds formed between molecular chains in parallel arrangement［9，14］. It is generally believed that the αphase structure is thermodynamically more stable and can be observed by solution crystallization, annealing, crystallization at high temperatures, or slow cooling of nylon6 melt. In addition, the γphase structure is most commonly observed by melt rapid cooling, high speed spinning, phosphoric acidammonia solution steam precipitation method and treating in aqueous potassium iodideiodine solution［1521］. The γphase also has monoclinic structure that arranges in parallel between the molecular chains in the (200) plane to form hydrogen bonds and twists at an angle between the amide group and carbonitride molecular chain backbone methylene［2224］. Changes of conditions such as heat, pressure, or solvent can transform the γphase into the αphase. Based on the understanding from the current literature［2530］, there are many ways to prepare a single crystal form, but these methods can only be suitable for the preparation of the αphase or only for forming the γphase. For example, nylon6 powders precipitated from a formic acid solution trend to form a highly crystalline αphase, without the trait of the γphase. Henceforth, a highly efficient and controllable method for the preparation of both α and γ phases of nylon6 is desirable.

In this paper, by using the phosphoric acidammonia solution method, we prepared αphase and γphase structures through adjusting the preparation temperature and duration time, and then the crystalline structures and melting behavior of nylon6 are studied carefully by Xray diffraction (XRD) and differential scanning calorimetry (DSC). Our results showed that this method is not only highly efficient, facile and greatly reducing the reaction time, but also suitable for the controllable formation of single crystal phases.

1 Experimental Section

1.1 Materials

Nylon 6 pellets with a relative viscosity of 2.50 were obtained from Guangdong Xinhui Meida Nylon Co.,Ltd (Guangdong, China); Phosphate acid(A.R.) were purchased from Beijing Yili Fine Chemicals Co.,Ltd (Beijing, China); Ammonia solution(A.R.) were purchased from Sinopharm Chemical Reagent Co.,Ltd. All other agents used in this study are commercially available.

1.2 Preparation of the α and γ phases of nylon6 

Firstly, a solution of phosphate acid was added to nylon6 pellets to form the concentration of 6% clear solution, which was placed in a digital electric heated water bath maintained at 80 ℃ for 2 days. Then 15 g Nylon6/phosphate acid solution was shifted in a 50 mL beaker with a sufficient amount of ammonia solution added into a 100 mL beaker. The two beakers were put in a sealed container. Finally the sealed container was placed in a water bath, where the vapor of the ammonia solution diffuses into the nylon6/phosphate acid solution and induces the expected crystallization of nylon6 through controlling the water temperature and diffusing time. The precipitate was filtered, washed with ammonia solution and distilled by water three times. Then the product was ovendried in vacuo at 50 ℃ for 24 h. 

1.3 Characterization of the α and γ phases of nylon6 

XRD experiments were carried out on a PANalytical XPert Pro MPD Xray diffraction with nickelfiltered CuKα radiation (λ=1.54 ) at 45 kV and 40 mA. Xray scanning was collected in the range 5°<2θ<50° with a scan rate at 2° (2θ)/min. DSC measurements of the α and γ phases of nylon6 samples were performed on a DSC 8000 Instrument. The samples were heated from room temperature to 240 ℃ at a constant heating rate of 20 ℃/min under a nitrogen atmosphere. FTIR analysis was conducted on a Nicolet Magna IS10 spectrometer at a resolution of 4 cm-1 and 32 cm-1 scans. The surface morphology was examined by a scanning electron microscopy (JSM 6306).

2 Results and Discussion

2.1 Crystal structures of the samples

For the α phase structure, its characteristic peaks appear at about 20° and 24°, corresponding to (200) and (002)/(202) reflections. The characteristic peaks of the γ phase are assigned at about 10° and 21.5° and is attributed to (001) reflections［30］.

Figures 1~6 display the XRD patterns of the prepared α and γ phases of nylon6 samples. As are shown in Figs. 1 and 2, it is clear that the crystal form is mainly γphase crystals of nylon6, with two diffraction peaks at about 10.9° and 21.6°, which can be assigned to the (001) reflections of γphase［30］. We do not see any notable diffraction peaks at 20.0° and 23.6°, which are allocated to the (200) and (002)/(202) reflections of the αphase. The XRD patterns at 60 ℃ shown in Fig.3 indicate that there are two distinctive peaks at 20.0° and 23.6°, with a diffraction peak at 21.6° with a shoulder at 10.9°, implying that there is the coexistence of both αphase and γphase structures at 60 ℃. With an increasing preparation time, the intensity of αphase diffraction characteristic peaks gradually increases, while the diffraction peaks characteristic of γphase the gradually decreases. The longer the preparation time was, the more the content of αphase formed. As shown in Fig.4, the peaks of αphase diffraction peaks become smaller than those of γphase diffraction at 70 ℃ for 6 h. At 70 ℃ and longer than 8 h, γphase trends to be transformed to αphase, and, as a result, the diffraction peaks of γphase are disappeared. From Fig.5, the intensity of αphase diffraction peaks are stronger than that of γphase with 80 ℃ and 6h. It can be seen that there was only αphase at 80 ℃ above 8h.The XRD patterns from 90 ℃ at different reaction times are shown in Fig.6. From the Figure, we can see that γphase is almost all transformed into αphase at 90 ℃.

The presence of crystals form as a function of the preparation temperature and reaction duration time is summarized in Tab.1. It has been well known that, in general, temperature is one of the most important factors affecting the crystal formation of nylon 6. The γphase structure could be obtained when the temperature is below 60 ℃ and αphase is formed when temperature is over 70 ℃. Under the same reaction time, γphase is more likely to be transformed into αphase as temperature increases, suggesting that αphase is thermodynamically more stable. Tab.1 also reveals that the reaction time plays a vastly important role during the course of crystal formation. For example, both α and γphases are formed in 6h at temperatures of 70 ℃ and 80 ℃. However, by elongating the preparation time to 8 h, only αphase is left over. 

FTIR spectra can be used to distinguish between γ and α phase. Further study using FTIR was conducted for γphase (50 ℃,8 h),α & γphase (60 ℃,10 h) and αphase (80 ℃,12 h).

As are apparent in the FTIR spectra presented in Figs.(7~9), it can be clearly seen that there exist significant differences of characteristic absorption peaks for each crystalline phase of nylon6 between 900 and 1 050 cm-1, which is the most important range for the popular marker bands of α and γphases. The spectra for the single crystalline phase prepared at different conditions are significantly similar, so only one of them was selected to show in above Figures. The corresponding areas where characteristic peaks are located are marked by a red circle and magnified in the lower right corner of the Figure. Taking Fig.7 as an example, we find one forced marker band at 973 cm-1  that belongs to the γphase. Nevertheless, three strong marker bands at 929, 952 and 960 cm-1 are attributed to αphase (see also Fig.9). Some similar marker bands were found as the evidence of the coexistence of both α & γ phases shown in Fig.8, where the bands were found at 929, 952, 960 cm-1 and 973 cm-1 for the αphase and γphase, respectively［19,31］. These FTIR results confirm that the formation of single α and γphases could be controlled by adjusting preparation temperature and time.

Fig.7 FTIR spectra of γphase (50 ℃,8 h) Fig.8 FTIR spectra of α&γphase (60 ℃,10 h)

Fig.9 FTIR spectra of αphase (80 ℃,12 h) Fig.10 DSC melting traces of γphase (50 ℃,8 h)

2.2 Melting behavior of samples

The thermal behavior of γphase (50 ℃,8 h)、α & γphase (60 ℃,10 h) and αphase (80 ℃,12 h) were further investigated using DSC. This is an clearcut to distinguish between γ and α phase structures.

The DSC analysis was treated like FTIR with only one representative curve for each type of crystalline phase shown in the Figures. The DSC heating curve of γphase (50 ℃,8 h) in Fig.10 shows that there is only one main melting peak at about 210 ℃ (tm1), with one shoulder melting peak at about 219 ℃ (tm2). According to the literature［30,3235］, the reason for these two melting peaks of γphase can be interpreted as (1) the coexistence of the αphase and γphase and (2) a crystal with different degrees of perfection. However, as can be seen from the results of XRD and FTIR, the sample of nylon6 prepared at 50 ℃ for 8 h only exhibits characteristic diffraction peaks of γphase in Figs.2 and 7. Figures 1~6 and Tab.1 clearly demonstrate the impact of temperature on the transformation of nylon6 crystal phases and a good agreement has been obtained with the experimental findings. That is, part of the γphase was irreversibly converted to the αphase at 60 ℃ for 6 h. In addition, according to the literature report, the transformation from γphase to αphase should be below the melting temperature［28,38］. Therefore, at a scan rate of 20 ℃/min, the instability of γphase might lead to a partial irreversible conversion of γphase into αphase as temperature increases. Based on this analysis, we suggest that tm1(210.8 ℃) can be attributed to the melting point of γphase(50 ℃,8 h) and tm2(219 ℃) is the melting peak of αphase. Furthermore, the DSC heating curve of α & γphase (60 ℃,10 h) in Fig.11 manifested the existence of double melting peaks at 209 ℃（tm1）and 216 ℃(tm2). The difference between the two melting peaks was only 10 ℃ so the two melting peaks overlap with each other. As shown by the XRD pattern of this sample obtained at 60 ℃,10 h in Fig.4, it has been known that there are two distinctive peaks at 20.0° and 23.6°, and a diffraction peak at 21.6° with a shoulder at 10.9°, indicating that αphase and γphase were coexisted at 70 ℃ for 6 h. The DSC heating cure of αphase (80 ℃,12 h) is shown in Fig.12, with the main melting peak at 217.5 ℃（tm1）coming from αphase, which is consistent with the literature report［2930］. Because no obvious γphase characteristic diffraction peaks at about 10.9° and 21.6° were observed in Fig.5, the shoulder melting peak at 202.2 ℃ (tm2) might be contributed to αphase with different thickness of the crystalline. These results further confirm that the obtained αphase is thermodynamically more stable and the transformation from γ to α occurs below the melting point.

Fig.11 DSC melting traces of α&γphase (60 ℃,10 h) Fig.12 DSC melting traces of αphase (80 ℃,12 h)

2.3 Morphological Characterization

SEM experiments was also carried out for the same samples, γphase (50 ℃,8 h), α&γphase (60 ℃,10 h) and αphase (80 ℃,12 h).

Fig.13 SEM images of γphase(a and b), α&γphase(c and d) and αphase(e and f) with various morphologies

The morphology of α and γ phases of nylon 6 samples are examined by SEM, whose images are shown in Fig13. From these SEM images, it is clear that γphase is composed of many holes with different diameters. These holes are stacked assembly with very smooth surface (shown in Fig.13 (a and b)). Fig.13 (c and d) present the coexistence morphology of α & γphase. It is interesting that not all holes with different diameters can clearly be seen in those images and a portion of the irregular lamellae with stack assembly also can be observed in them. Fig.13(e and f) are SEM images of αphase, which are irregular lamellae with stack assemble. These results suggest that the morphology of αphase and γphase are markedly different, with the appearance of γphase vesicular and αphase as the accumulation of irregular lamellae.

3 Conclusions

Based on the results obtained from this work, we presented a highly efficient and facile method for the preparation of α and γ phases of nylon6. By using this controllable formation, single α and γ phases of nylon6 could be prepared. Based on the information from XRD, FTIR and DSC analysis, the temperature is one of the most important factors affecting the crystal formation of nylon 6. The γphase was obtained below 60 ℃ by the vapor of the ammonia solution diffuses into nylon6/phosphate acid solution, which can be transformed to αphase above 60 ℃. The coexistence of the αphase and γphase was found at 60 ℃ and 70 ℃ by extending the preparation duration time. With the time increased, the content of the αphase gradually increased, while the content of the γphase gradually reduced. Finally, all γphase could be transformed into αphase as long as the preparation time is long enough. Besides, at a certain temperature, the higher the preparation temperature, the shorter the transition time. As for αphase, it is more thermalstable and could be formed above 70 ℃ for 8 h. From the images of SEM, the γphase is composed of holes with many different sizes, but the αphase is the accumulation of irregular lamellae. These morphology features are important in the sense that the pore structure of γphase could facilitate the penetration of colorings in nylon6 and improve dying properties. In the study ensued, we will focus on the impact of αand γphase structures on the dying.

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