Controllable Formation of the α and γ Crystalline Phases of Nylon-6

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Controllable Formation of theαandγCrystalline Phases of Nylon-6

HUANGYing-shan1a,2,JIANGFeng2,LIJie2,PENGShu-min1a,RENXi1a,LUOYu-hang1a,2,YIChun-wang1a,b*

(1a. Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province, b. National and Local Joint Engineering Lab. for New Petro-Chemical Materials and Fine Utilization of Resources, Hunan Normal University, Changsha 410081, China;2. State Key Laboratory of Biobased Fiber Manufacturing Technology, China Textile Academy, Beijing 100025, China)

AbstractIn this paper, controllable formation and characterization ofαandγcrystalline phases of nylon-6 are reported. Differential scanning calorimetry results illustrate that theγ-phase shows a sharp melting peak at about 210 ℃(tm1), with a small shoulder melting peak appearing at about 219 ℃(tm2). Theα-phase is formed above 70 ℃ after 8 h, which appears to be more stable with double melting peaks. FT-IR results further confirm that the obtained sample is indeedγ-phase,α-phase andα&γ-phase structures. From scanning electron microscopy images, we can see that theγ-phase is composed of many holes with different diameters and these holes are stack assembly with very smooth surface, whereas for theα-phase structure we observed irregular lamellae structures with the stack assembly.

Key wordsnylon-6;α-phase;γ-phase; controllable formation

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 andγ-phase structures are widely recognized as major crystalline nylon-6[9-12]. Theα-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 theα-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, theγ-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]. Theγ-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 theγ-phase into theα-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 theα-phase or only for forming theγ-phase. For example, nylon-6 powders precipitated from a formic acid solution trend to form a highly crystallineα-phase, without the trait of theγ-phase. Henceforth, a highly efficient and controllable method for the preparation of bothαandγphases of nylon-6 is desirable.

In this paper, by using the phosphoric acid-ammonia solution method, we preparedα-phase andγ-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.

1Experimental Section

1.1Materials

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.

1.2Preparation of theαandγphases 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.

1.3Characterization of theαandγphases of nylon-6

XRD experiments were carried out on a PANalytical X’Pert Pro MPD X-ray diffraction with nickel-filtered Cu-Kαradiation (λ=1.54 ?) at 45 kV and 40 mA. X-ray scanning was collected in the range 5°<2θ<50° with a scan rate at 2° (2θ)/min. DSC measurements of theαandγphases 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-1and 32 cm-1scans. The surface morphology was examined by a scanning electron microscopy (JSM 6306).

2Results and Discussion

2.1Crystal structures of the samples

For theαphase structure, its characteristic peaks appear at about 20° and 24°, corresponding to (200) and (002)/(202) reflections. The characteristic peaks of theγphase are assigned at about 10° and 21.5° and is attributed to (001) reflections[30].

Figures 1~6 display the XRD patterns of the preparedαandγphases of nylon-6 samples. As are shown in Figs. 1 and 2, it is clear that the crystal form is mainlyγ-phase crystals of nylon-6, with two diffraction peaks at about 10.9° and 21.6°, which can be assigned to the (001) reflections ofγ-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 theα-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 bothα-phase andγ-phase structures at 60 ℃. With an increasing preparation time, the intensity ofα-phase diffraction characteristic peaks gradually increases, while the diffraction peaks characteristic ofγ-phase the gradually decreases. The longer the preparation time was, the more the content ofα-phase formed. As shown in Fig.4, the peaks ofα-phase diffraction peaks become smaller than those ofγ-phase diffraction at 70 ℃ for 6 h. At 70 ℃ and longer than 8 h,γ-phase trends to be transformed toα-phase, and, as a result, the diffraction peaks ofγ-phase are disappeared. From Fig.5, the intensity ofα-phase diffraction peaks are stronger than that ofγ-phase with 80 ℃ and 6h. It can be seen that there was onlyα-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 thatγ-phase is almost all transformed intoα-phase at 90 ℃.

Fig.1　XRD patterns of 40 ℃ at different times　　　　　　　　　　Fig.2　XRD patterns of 50 ℃ at different times

Fig.3　XRD patterns of 60 ℃ at different times　　　　　　　　　　Fig.4　XRD patterns of 70 ℃ at different times

Fig.5　XRD patterns of 80 ℃ at different times　　　　　　　　　　Fig.6　XRD patterns of 90 ℃ at different times

Temperature/℃time/h6810121440γγγγγ50γγγγγ60γ αγ αγ αγ αγ α70γ ααααα80γ ααααα90ααααα

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. Theγ-phase structure could be obtained when the temperature is below 60 ℃ andα-phase is formed when temperature is over 70 ℃. Under the same reaction time,γ-phase is more likely to be transformed intoα-phase as temperature increases, suggesting thatα-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, bothαandγ-phases are formed in 6h at temperatures of 70 ℃ and 80 ℃. However, by elongating the preparation time to 8 h, onlyα-phase is left over.

FT-IR spectra can be used to distinguish betweenγandαphase. Further study using FT-IR was conducted forγ-phase (50 ℃,8 h),α&γ-phase (60 ℃,10 h) andα-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 ofαandγ-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-1that belongs to theγ-phase. Nevertheless, three strong marker bands at 929, 952 and 960 cm-1are attributed toα-phase (see also Fig.9). Some similar marker bands were found as the evidence of the co-existence of bothα&γphases shown in Fig.8, where the bands were found at 929, 952, 960 cm-1and 973 cm-1for theα-phase andγ-phase, respectively[19,31]. These FTIR results confirm that the formation of singleαandγ-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)

2.2Melting behavior of samples

The thermal behavior ofγ-phase (50 ℃,8 h)、α&γ-phase (60 ℃,10 h) andα-phase (80 ℃,12 h) were further investigated using DSC. This is an clear-cut to distinguish betweenγandαphase 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 ofγ-phase (50 ℃,8 h) in Fig.10 shows that there is only one main melting peak at about 210 ℃ (tm1), with one shoulder melting peak at about 219 ℃ (tm2). According to the literature[30,32-35], the reason for these two melting peaks ofγ-phase can be interpreted as (1) the co-existence of theα-phase andγ-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 ofγ-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 theγ-phase was irreversibly converted to theα-phase at 60 ℃ for 6 h. In addition, according to the literature report, the transformation fromγ-phase toα-phase should be below the melting temperature[28,38]. Therefore, at a scan rate of 20 ℃/min, the instability ofγ-phase might lead to a partial irreversible conversion ofγ-phase intoα-phase as temperature increases. Based on this analysis, we suggest thattm1(210.8 ℃) can be attributed to the melting point ofγ-phase(50 ℃,8 h) andtm2(219 ℃) is the melting peak ofα-phase. Furthermore, the DSC heating curve ofα&γ-phase (60 ℃,10 h) in Fig.11 manifested the existence of double melting peaks at 209 ℃(tm1)and 216 ℃(tm2). 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 thatα-phase andγ-phase were co-existed at 70 ℃ for 6 h. The DSC heating cure ofα-phase (80 ℃,12 h) is shown in Fig.12, with the main melting peak at 217.5 ℃(tm1)coming fromα-phase, which is consistent with the literature report[29-30]. Because no obviousγ-phase characteristic diffraction peaks at about 10.9° and 21.6° were observed in Fig.5, the shoulder melting peak at 202.2 ℃ (tm2) might be contributed toα-phase with different thickness of the crystalline. These results further confirm that the obtainedα-phase is thermodynamically more stable and the transformation fromγtoαoccurs 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)

2.3Morphological Characterization

SEM experiments was also carried out for the same samples,γ-phase (50 ℃,8 h),α&γ-phase (60 ℃,10 h) andα-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 ofαandγphases of nylon 6 samples are examined by SEM, whose images are shown in Fig.13. From these SEM images, it is clear thatγ-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 ofα&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 ofα-phase, which are irregular lamellae with stack assemble. These results suggest that the morphology ofα-phase andγ-phase are markedly different, with the appearance ofγ-phase vesicular andα-phase as the accumulation of irregular lamellae.

3Conclusions

Based on the results obtained from this work, we presented a highly efficient and facile method for the preparation ofαandγphases of nylon-6. By using this controllable formation, singleαandγphases 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. Theγ-phase was obtained below 60 ℃ by the vapor of the ammonia solution diffuses into nylon-6/phosphate acid solution, which can be transformed toα-phase above 60 ℃. The co-existence of theα-phase andγ-phase was found at 60 ℃ and 70 ℃ by extending the preparation duration time. With the time increased, the content of theα-phase gradually increased, while the content of theγ-phase gradually reduced. Finally, allγ-phase could be transformed intoα-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 forα-phase, it is more thermal-stable and could be formed above 70 ℃ for 8 h. From the images of SEM, theγ-phase is composed of holes with many different sizes, but theα-phase is the accumulation of irregular lamellae. These morphology features are important in the sense that the pore structure ofγ-phase could facilitate the penetration of colorings in nylon-6 and improve dying properties. In the study ensued, we will focus on the impact ofα-andγ-phase structures on the dying.

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(編輯WJ)

α和γ晶型尼龍6的可控合成

黃映珊1a,2，姜鋒2，李杰2，彭舒敏1a，任茜1a，羅玉航1a,2，易春旺1a,b*

(1.湖南師范大學(xué) a.資源精細(xì)化與先進(jìn)材料湖南省高校重點(diǎn)實(shí)驗(yàn)室，

b.石化新材料與資源精細(xì)利用國家地方聯(lián)合工程實(shí)驗(yàn)室，中國 長沙410081;

2.中國紡織科學(xué)研究院生物源纖維制造技術(shù)國家重點(diǎn)實(shí)驗(yàn)室，中國 北京100025)

摘要報導(dǎo)了可控晶型尼龍6的制備與表征．DSC結(jié)果顯示γ晶型在Tspan為210 ℃左右出現(xiàn)一個尖銳的熔融峰，同時在Tspan為219 ℃左右出現(xiàn)一個小肩峰．當(dāng)溫度高于70 ℃，反應(yīng)時間大于8 h時可以制備穩(wěn)定的α晶型，α晶型顯示雙熔融峰．FT-IR結(jié)果進(jìn)一步表明所獲得的樣品為γ晶型、α晶型和混晶．SEM結(jié)果顯示γ晶型表面是由孔徑大小不一樣的小孔組成，α晶型表面是由不同厚度的片晶堆積而成．

關(guān)鍵詞PA6；α晶型；γ晶型；可控晶型

中圖分類號TQ342+.11

文獻(xiàn)標(biāo)識碼A

文章編號1000-2537(2016)01-0035-08

*通訊作者,E-mail：cwyi@hunnu.edu.cn

基金項(xiàng)目：國家支撐計劃項(xiàng)目(2013BAE01B03)；湖南省教育廳科研基金項(xiàng)目(12K031)

收稿日期：2015-08-13

DOI:10.7612/j.issn.1000-2537.2016.01.007