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Transamidation determination and mechanism of long chain-based aliphatic polyamide alloys with excellent interface miscibility
Polymer 59 (2015) 16e25
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Transamidation determination and mechanism of long chain-based aliphatic polyamide alloys with excellent interface miscibility Lili Wang, Xia Dong*, Yunyun Gao, Miaoming Huang, Charles C. Han, Shannong Zhu, Dujin Wang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2014 Received in revised form 13 December 2014 Accepted 24 December 2014 Available online 6 January 2015
Transamidation between PA1012 and PA612 was investigated with combination techniques including differential scanning calorimetry (DSC), rheometry, nuclear magnetic resonance (NMR) and variabletemperature Fourier transform infrared spectroscopy (VT-FTIR). Based on the increase of storage modulus with sweep time, the presence of reactive chain ends were proved, promoting chain growth in either polyamide and interchange reaction in binary blend at high temperature. NMR and VT-FTIR testing signals sufficiently convinced the expected interchange reaction. Quantitative data analysis of DSC and NMR provided direct evidences for evaluating the exchange degree. Various experimental parameters were systematically considered, including reaction temperature and time, blending composition and categories of polyamides. Raising the reaction temperature or prolonging the reaction time would accelerate the transamidation rate and degree. The interfacial miscibility between two LCPA components could be improved by the formation of copolyamides. This work provides a quantitative evaluating method to detect the transamidation extent between two aliphatic LCPAs that overcomes the traditional characterization limitation which is only feasible for whose polyamide component chain is with large volume structure like benzene ring. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Transamidation Interface miscibility Polyamide alloys
1. Introduction Polyamide, presented excellent mechanical and thermal properties, had become a classic important engineering thermoplastic, among which, aliphatic polyamide was generally named as nylon which firstly realized industrialization by DuPont in 1939 last century. In principle, any polyamide having more than 10 repeat units in the main chain was considered as long chain polyamide (LCPA). Due to relatively low density of hydrogen bonds in recurring amide groups, LCPA not only provided a combination of toughness, rigidity and lubrication-free performance but also possessed lower water absorption and better dimensional stability [1e3]. Consequently, based on higher aliphatic portions, it offered more benefits than common polyamide which had high level of absorbed water and hence decreased mechanical properties seriously [4,5]. Thus LCPA provided multiple potential applications in a wide range, such
* Corresponding author. Tel./fax: þ86 10 82618533. E-mail address: [email protected] (X. Dong). http://dx.doi.org/10.1016/j.polymer.2014.12.058 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
as automobile manufacturing, aerospace engineering and electronic appliances [6]. Eveneeven LCPA is the condensation polymerization product of dicarboxylic acids with diamines, where diamines are converted by dicarboxylic acids. Usually dibasic acid currently could be obtained either by chemical synthesis method using non-renewable petrochemical feedstock which produced unwanted hazardous byproducts, or by biological fermentation method using enzymes in microbial cell as catalyst to transform n-alkanes into long chain dibasic acid, which was an environmental friendly conversion process with biobased materials and mild condition [7]. Synthesis of LCPA using biological fermentation based monomers had attracted particular interest in recent years and many commercial polyamides produced in this method were available. In this article, both monomers of PA1012 and PA612 were prepared by biological fermentation method. More recently, the application of pure polyamide was so limited considering its properties and costs, and preparation of polymer alloys had become an economic and efficient alternative to traditional synthesis of novel polymeric material, which made it
L. Wang et al. / Polymer 59 (2015) 16e25
possible to add both advantages [8,9]. As far as polymer alloys are concerned, the key issue, determining success in obtaining better comprehensive properties, is miscibility of two components [10e13]. In general, polymer alloys tend to present poor thermal and mechanical properties caused by thermodynamic immiscibility and thus much emphasis should be put on the studying of the specific interaction between two components as well as corresponding improvement program [14]. In the past years, great effort has been made by researchers to study the compatibility of blends which can be manipulated by some strategies concerning about physical or chemical means. In the case of physical methods, Pang et al. [15,16] extensively studied interaction of isotactic polypropylene/polyethylene-octene copolymer (iPP/PEOc), in which, tailored mechanical properties can be obtained by controlling phase separation and crystallization processes. To compatibilize the ternary immiscible polypropylene/polystyrene/polyamide (PP/PS/PA), a polyolefin-based component PP-g-(MAH-co-St) was produced with maleic anhydride (MAH) and styrene (St) by Xie's group [17]. This multiphase component provided a novel and effective way to achieve good compatibilization. These mentioned modification methods were all without chemical structure change and could effectively improve the components miscibility. Another commonly used method, which was defined as chemical strategies, including estereester [18,19], estereamide [20] and amideeamide exchange reactions, was collectively referred to as effective reactive compatibilization and had an increasing research trend. A lot of polymer alloys were involved in this research like polyamide/ styrene-acrylonitrile (PA/SAN) [21], poly(butylene terephthalate)/ polycarbonate (PBT/PBS) [22], and polyamide/polyamide (PA/PA) [23e27]. Among polyamide alloys, researches based on semicrystalline aliphatic polyamide and amorphous aromatic polyamide blends are mainly performed. G. Groeninckx et al. systematically studied the transamidation occurring in polyamide4,6/polyhexamethylene isophthalamide (PA46/PA6I) blends under molten state. According to the theory developed for polyesters that in all triads the substitution on the left of a central unit does not influence the substitution on the right, number-average block length and degree of transamidation were calculated [23e25]. In order to determine the chain architecture in exchange reactions of polyamide mixtures, different measurements were used. According to that NMR signals have great sensitivity to aromatic polyamide structure with large volume sturture [28], Eersels et al. characterized structure change and degree of randomness by means of high resolution carbon nuclear magnetic resonance (13C NMR) and gradient elution chromatography (GEC) [23e25]. By means of proton NMR (1H NMR), Micaela Vannini et al. distinguished the various sequences in copolyamide efficiently and simply [26]. As reported by Giorgio Montaudo et al., thanks to matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) MS, copolymer formed in situ and its block length in polyamide alloys were accurately analyzed [29,30]. However, since the primary structure of aliphatic polyamide, particularly for LCPA, is mainly composed of methylene units and insoluble in most common solvents, traditional characterization methods like NMR and MALDI-TOF are difficult to distinguish aliphatic copolyamides in exchange reactions. Under this condition, little information is available in previous reports for elucidation of chemical interaction for aliphatic polyamide alloys in molten state. In the present paper, to obtain polyamide alloys with high performance, fundamental understanding of chemical interactions and corresponding effect on miscibility of long chain-based polyamides is required, in which polymer blending was adopted to manufacture mixtures. Moreover, particular attention was given for polyamides of PA1012/612. To overcome the characterization limitation, DSC, NMR, and variable-temperature FTIR were combined to
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comprehensively analyze factors influencing interactions of two polyamides, giving quantitative information about exchange reaction. Simultaneously, mechanisms of improved interface miscibility in aliphatic LCPA alloys were put forward. 2. Experimental 2.1. Materials and sample preparation Polyamide 612 (PA612, e[HN(CH2)6NHCO(CH2)10CO]ne) and polyamide 1012 (PA1012, e[HN(CH2)10NHCO(CH2)10CO]ne) were synthesized from hexamethylene diamine, decamethylene diamine and dodecanedioic acid whose monomers were produced with microorganism fermentation method. The chemical structures of two polyamides were given in Scheme 1 from which it could be obviously seen that PA1012 owns longer diamine chain than PA612. Fundamental physical characterization of four polyamides (PA66, PA610, PA612, PA1012) mentioned in this work was shown in Table 1. All long chain polyamides were commercial grade supplied by Shandong Guangyin New Materials Co., Ltd. Dissolution-precipitation method was used to prepare various polyamide alloys. In this process, trifluoroacetic acid (TFA) and deionized water were used as a common solvent and a poor solvent respectively. Two polyamides were added into TFA vigorously stirring for 10 h, followed by slowly pouring the solvent into deionized water. After precipitation, the blends were intensively washed with deionized water for several times and subsequently dried under vacuum at 60 C for 72 h until a constant weight was reached. The pure PA612 and PA1012 undergone the similar treatments stated above so that all materials kept the same initial state. In this paper, the weight ratio of polyamide alloys was used to express blend composition. (PA1012/612 50/50 denotes weight fraction of PA1012 is 50% in polyamide mixtures.) 2.2. Measurements Thermal behaviors of polyamide alloys were measured by Differential Scanning Calorimeter (TA Q2000) which was calibrated in indium standard. Each specimen of about 7 mg sealed in an aluminum pan was tested under N2 atmosphere at a constant flow rate. All the measurements recorded the first cooling and the second heating runs. The glass transition of polyamide was determined by a Dynamic Mechanical Analyzer (DMA Q800). The specimens with the dimensions of 6 mm 4 mm 0.1 mm (length width thickness) were held by vertical clamps subjected to tensile modes. Measurements were conducted over a range of 100 C to 150 C with a heating rate of 3 C/min under a constant frequency of 1 Hz. The 1H NMR spectra were collected on a Bruker DMX 300 instrument equipped with a BBO double-resonance probe at ambient temperature. The samples were dissolved into trifluoroacetic acidd (20 mg/ml) which served as solvent and locking agent. The spectra were acquired after accumulating 128 scans with a digital
Scheme 1. Chemical structure of repeat units for PA612 and PA1012.
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Table 1 Fundamental physical characterization of four polyamides. Tc ( C)
Polyamide
a
a
PA66 PA610 PA612 PA1012
212 180 191 166
59.28 47.41 37.52 38.59
D Hc
(J/g)
a
Tm ( C)
257 213 216 189
a
D Hm
50.46 36.67 55.27 45.09
(J/g)
b
MFR (g/10 min)
18.1 18.7 25.2 2.3
out at 230 C with the amplitude of 0.5% in linear viscoelastic regime under a continuous purge of nitrogen to study structure change of polyamides. 3. Results and discussion 3.1. Crystallization and melting behaviors
a
Determined by DSC. b Determined by Melt Flow Indexer at 235 C according to ASTM D1238 (2.16 kg) and the tested temperature for PA66 was 265 C.
resolution of 0.138 Hz per point. The spectra width was 9000 Hz, the data points were 65,536 and the acquisition time was 2 s. In all 1 H NMR spectra, chemical shifts were referenced to the residual signals of trifluoroacetic acid-d. The relative peaks areas were measured by automatic integration methods. FTIR spectra were collected using Nicolet 6700 Fourier Transform Infrared Spectrometer with a detector of DTGS. In a transmission mode, the spectra were recorded by accumulating 64 scans with a resolution of 4 cm1. The film samples prepared by solution casting were clipped on a potassium bromide plate which was put in a Linkam FTIR 600 hot stage for viable-temperature process. The hot stage was placed in the compartment of spectrometer so that each sample can be heated in situ to acquire the temperature dependence of infrared spectra. Both pure PA1012 and polyamide alloys (50/50) were heated to 280 C at a rate of 10 C/min, and stayed there for 10 min, and then followed by cooling to room temperature at a rate of 10 C/min. In this process, the spectra were collected at intervals of 30 C. The baselines of the resulting spectra were corrected employing the same standard. WAXD measurements were performed on the beamline BL14B1 located in Shanghai Synchrotron Radiation Facility. Radiation source with wavelength ¼ 1.24 Å was used. The system was equipped with a MAR CCD detector with a resolution of 3072 3072 pixels. The sample to detector distance was 386.8 mm and the exposure time was 60 s. The blend film samples pretreated at 230 C, 240 C, 250 C, 260 C, 270 C and 280 C for 10 min were placed in Linkam stage respectively which served as a holder and scattering patterns were acquired. All the images were corrected for background scattering, air scattering and beam fluctuations. Oscillatory shear tests were performed using a Discovery Hybrid Rheometer (TA Instruments) equipped with parallel plates of 25 mm diameter. Oscillatory time sweep experiments were carried
The influence of thermal condition and chemical structure on the thermal properties of polyamide alloys was investigated, where the thermal condition included annealing temperature and time, and chemical structure included blend composition and polyamide categories. Considering the fact that DSC is not an equilibrium measurement, Cheng et al. [31] has obtained thermodynamic properties including melting temperature and heat of fusion using extrapolation methods. In our research, temperatures without lagcorrected were compared relatively due to the same heating and cooling rate in all runs. Fig. 1 and Fig. 2 illustrate crystallization and melting behaviors under different thermal conditions. In Fig. 1, thermographs with various annealing temperatures are demonstrated which also covers neat PA1012 and PA612 as a contrast. With the increase of annealing temperature, both melting peaks in PA1012/612, obviously assigned to the peaks of PA1012 and PA612 respectively, gradually become weaker and broader indicating the decrease of melting enthalpy. The melting enthalpy of PA1012 component strongly reduces from 27.7 J/g (230 C) to 13.4 J/g (280 C), whereas this is less pronounced in PA612 whose melting enthalpy reduces from 30.1 J/g (230 C) to 24.6 J/g (280 C). To quantitatively evaluate the whole degree of this change in polyamide blends, the melting enthalpy of PA1012 and PA612 is added together, and the total degree of enthalpy decrease j(%) is defined as follow.
jð%Þ ¼ 1
D Hm;T H D m;onset
(1)
where j(%) is the degree of decrease in melting enthalpy, D Hm;T represents the melting enthalpy of polyamide alloys under annealing temperature T, D Hm;onset represents the melting enthalpy of polyamide alloys under lowest annealing temperature. For example, the degree of melting enthalpy decrease for PA1012/612 at 270 C can be calculated by jð%Þ ¼ 1 D Hm;270 =D Hm;230. According to this equation, j as a function of annealing temperature is
Fig. 1. DSC thermal graphs of PA1012/612 (50/50) under various annealing temperatures for (a) melting peaks and (b) crystallization peaks.
L. Wang et al. / Polymer 59 (2015) 16e25
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Fig. 2. DSC thermal graphs of PA1012/612 (50/50) under various annealing time for (a) melting peaks and (b) crystallization peaks.
given in Fig. 3(a). As expected, j exhibits great increase with the increase of annealing temperature in which j can reach to 34.3% at 280 C. The above phenomena indicate that the ability of crystallization is greatly dominated by annealing temperature and higher annealing temperature leads to lower level of crystallization. Corresponding crystallization peaks of polyamide alloys are displayed in Fig. 1(b). Similar to the description above, distinct crystallization peaks of polyamide alloys located between peaks of neat PA1012 and PA612 are assigned to neat polyamide respectively. The blend, exhibiting much broader and weaker crystallization peaks than that of neat materials and demonstrating similar trend for the melting peaks, indicate that the crystallization of polyamide alloys is tremendously impeded. Meanwhile, with the increase of annealing temperature, double peaks at 172.8 C and 188.1 C progressively approach each other and eventually merge into a single peak at 174.6 C. This suggests better miscibility of the two components in another hand which is also convinced in the latter part. Thermal properties of polyamide alloys under various annealing time at a constant temperature of 270 C are shown in Fig. 2 which is remarkably similar to that of annealing temperature discussed in
Fig. 1. PA1012 melting peaks of polyamide alloys gradually disappear and double crystallization peaks at 172.3 C and 184.5 C converge into a single peak at 172.9 C with prolonging annealing time. Corresponding j is calculated in Fig. 3(b) suggesting that longer annealing time can result in larger j which eventually reaches to 26.6% at 270 C for 30 min. It thus confirms that the high j can be acquired by either increasing annealing temperature or prolonging annealing time known as time-temperature equivalence effect mentioned in the previous paper [32]. Thermal behaviors of polyamide alloys with different composition ratios (20/80, 80/20) under various annealing temperatures are represented in Fig. 4. In Fig. 4(a), melting peak areas of PA1012 can hardly be observed due to its low content in PA1012/612 20/80 blends, and after annealed at 250 C, the peak becomes further smaller and almost vanishes. On the contrary, the alloy with the ratio of 80/20 shows a more complex thermal behavior that two melting peaks clearly exist even under high annealing temperature of 270 C. This is also confirmed in Fig. 1 that PA1012 melting peak is more liable to vanish than that of PA612. From a comparison of both figures, it can be strongly concluded that thermal behaviors of polyamide alloys considerably depend on its fractions.
Fig. 3. Variation of j in different conditions for (a) various annealing temperatures for 10 min and (b) various annealing time at 270 C.
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Fig. 4. DSC thermal graphs of melting peaks in polyamide alloys with different composition ratios under various annealing temperatures for (a) PA1012/612 20/80 and (b) PA1012/ 612 80/20.
To explore the general effects of polyamide categories on thermal properties of mixtures, five different polyamide alloys including PA1012/612, PA1012/610, PA66/612, PA66/610 and PA66/ 610 are adopted to reveal the regularities. As is said in the introduction, eveneeven polyamide is a polycondensation product of dicarboxylic acids and diamines. In this section, three kinds of acids and two kinds of amines are involved in the monomers of these polyamides. Fig. 5(a) shows j for different categories of polyamide alloys which were annealed at 30 C, 50 C and 70 C above respective Tm due to different melting temperatures. Apparently, in Fig. 5(a), the j curves of PA1012/612, PA66/610 and PA66/612 under various annealing temperatures lie upper than that of PA1012/610 and PA66/1012. For detailed analysis, a striking character is found that both components of former polyamide alloys have one same monomer at least while the latter includes different monomers. This rule is more obvious when the reasonable comparison methods described in Fig. 5(b) is applied. As is shown by red (in web version) solid curves which include PA1012/612 and PA1012/ 610, considering the effect of acid, two kinds of amine monomers, 10/6, are chosen in both polyamide alloys excluding the effect of
amine categories. For acid monomer, one with 12/12 is the same and the other with 12/10 is different. In Fig. 5(a), it can be seen that PA1012/612 alloys with the same acid have higher j than PA1012/ 610 whose acid monomers are different. Similar method described by blue dot curves in Fig. 5(b) is utilized to investigate categories of amine and the deduction is the same. Therefore, for polyamide alloys having one same monomer at least are always favored with higher j. 3.2. Miscibility of polyamide alloys As is known, the rate of crystallization for polyamide is too fast which makes it difficult to get large portion of amorphous part even under rapid cooling condition. So the glass transition temperature of polyamide is hard to be detected by common DSC without strong cooling equipment [33]. The direct evidence of miscibility in polyamide mixtures was shown in Fig. 6 by DMA analysis concerning about PA1012/612 under annealing temperatures of 230 C, 250 C, 270 C and neat polyamide without any treatment. The glass transition of polyamide represents the motion of long chain
Fig. 5. The effect of polyamide categories on alloys thermal properties for (a) annealing temperature dependence of j and (b) comparison methods of different polyamides.
L. Wang et al. / Polymer 59 (2015) 16e25
Fig. 6. Tan delta-temperature behaviors of polyamide alloys and neat polyamide (heating rate: 3 C/min; frequency: 1 Hz).
segments in amorphous regions [34,35]. To enhance this transition, after annealed, all samples quenched directly by liquid nitrogen to get the largest degree of amorphous phase. The glass transition temperature (Tg) of neat PA1012 and PA612 is approximately 40 C and 50 C respectively which is in accordance with previous literatures [36,37]. Likewise, for physical blends without annealing treatment, there are two glass transition peaks appeared separately in DMA curves. Although the respective Tg value of the alloys are close, two peaks still can be distinguished which indicates the immiscibility of two components. However, after annealed, the blends show a single peak and the shape of the peak becomes sharp with increasing annealing temperature. Theses shifts intuitively suggest that the miscibility of two components is remarkably improved by the annealing process. Accordingly, the convergency of two crystallization peaks shown in Section 1 indeed indicates the improved miscibility too. In another hand, for polymers with ester or amide end groups, there are estereester and estereamide exchange reactions happened in wider high temperatures. Many papers have studied reactive compatibilization method like estereester and estereamide exchange reactions [14,25,38]. Therefore, for PA1012/ PA612 system, the essence of different thermal properties and improved miscibility above is impossible to be clearly explained by means of DSC alone. Considering the previous research, transamidaiton occurring in aromatic polyamides [23e25] should be taken into account in our system. However, for traditional characterization method like NMR, the signal shift is only sensitive to reactions in which large volume structure such as benzene ring unit incorporates into reaction products. For MALDI-TOF, giving detailed information about chain sequence, some mild solvents are used to dissolve polymer. For long chain aliphatic polyamides, composed of large amount of methylene groups and insoluble in most common solvent, the reactions of which are hard to be distinguished by traditional characterization means. In the following work, in order to overcome above limitation, NMR, variable-temperature FTIR and rheological methods are adopted to comprehensively study the essence of thermal behaviors in molecular level.
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which is adopted by Zou et al. and Zhang et al. to successfully [39e41] probe the structure evolution under shear filed. In this part, oscillatory time sweep is carried out above polyamide melting temperature in a time range of 4000 s detecting time dependence of storage modulus (G0 ) for PA1012, PA612 and PA610 as shown in Fig. 7. The observed rheological response in polyamide, significant increase of G0 with prolonging time, suggests occurrence of the chain-growing reactions which have been discussed previouly [42,43]. Upon annealing, the increasing of molecular weight for polyamide originating from chain-growing reactions contributes to the increase of G0 . The analysis of Fig. 7 reveals that the end-groups including carboxyl groups and amido groups indeed exist in polyamides that we used, which promotes the occurrence of transamidation [44]. The variable temperatures-FTIR spectra of polyamide alloys undergone a heating-cooling process are demonstrated in Fig. 8. The spectra demonstrate a notable stretching vibration bands at 3300 cm1 assigned to hydrogen bonded NeH groups and whose intensity can be an evaluation of average strength of the hydrogen bond [45,46]. Upon heating, a noticeable decrease in the intensity and another characteristic peak at about 3450 cm1 are observed. As reported by Yayoi Yoshioka et al. [47,48] Upon heating, the broadness and decrease in the intensity at 3300 cm1 result from a decrease of absorptivity coefficient reflecting transformation of hydrogen-bonded NeH groups to free NeH groups appeared at 3450 cm1 [49]. The spectra collected at high temperature were resolved by the Peak Solve Fitting program to calculate the peak area at 3300 cm1 and 3450 cm1. The peak areas at 3450 cm1 accounts for 10% of the total areas representing the large number of free NeH groups at high temperature. As a consequence, with increasing temperature, the strength of hydrogen bond decreases allowing polymer chain moving freely to participate in transamidation. Both FTIR results and rheological results corroborate that the system is capable of having this exchange reaction. Proton NMR spectra and relevant assignments are listed in Table 2. In chemical structure of PA1012 and PA612, methyl proton of H1, H2, H4 and H5 have similar chemical surroundings, and the striking difference lies in the concentration of methyl proton associated with H3 and H6. However, these units probably are too far away from amide groups so that chemical shifts are almost the same for PA1012 and PA612, which is clearly revealed in Table 2. Therefore, conventional methods of detecting chemical shift
3.3. Transamidation of polyamide alloys Oscillatory sweep in linear viscoelastic regime is an effective method to investigate structure changes under certain temperature
Fig. 7. Storage modulus (G0 ) as a function of time at 230 C for polyamide (frequency: 1 Hz; strain: 0.5%).
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L. Wang et al. / Polymer 59 (2015) 16e25
Fig. 8. Temperature dependence of infrared spectra of polyamide for PA1012/612 (50/ 50) in the region of 3000 cm1 to 3600 cm1.
Fig. 9. Integration in the region of 1e2 ppm for polyamides under various temperature treatments.
corresponding to 16 CH2 units and 12 CH2 units in per repeating structure. For polyamide alloys like physical blends the integration values are between that of neat PA1012 and PA612. However, for etched polyamide alloys, with increasing annealing temperature, the integration becomes smaller evidently showing that transamidation really takes place in PA1012/612 mixtures. Because if exchange reaction occurs, chain segments of PA612 can be incorporated into PA1012, that is to say, chain segment of eNH(CH2)10NHCO(CH2)10CONH(CH2)6NHCO(CH2)10COe will form in the chain of PA1012. So PA612 chains incorporated in PA1012 can't be etched and the integration of the etched PA1012/612 alloys after annealed is smaller than physical blends. Without etching treatment, integration values under high temperature treatment have no change as opposed to physical blend because the total amount of methyl proton is constant whether transamidation occurs or not. The decrease of integration value for PA1012/612 due to PA1012-co-PA612 generated in this reaction verifies that transamidation occurs in this system. Therefore, based on different
changes after transamidation mentioned before [38,50] are unfeasible in our system. Nonetheless, it is noticed that the integration value of 1e1.5 ppm for PA1012 are bigger than that of PA612 representing different concentration of methyl proton assigned to H3 and H6 and other peak areas are the same due to the same amount of methyl proton for PA1012 and PA612. In this research, it is found that polyamide blends immerging into formic acid at 50 C for 2 min can almost remove PA612 phase completely, yet there is no influence on PA1012. Based on this feature, we can reliably estimate transamidation in aliphatic LCPA alloys. Assuming that this reaction takes place in PA1012/612 alloys, the integration value of 1e1.5 ppm in polyamide alloys after etching should be different from neat PA1012 and blends without reaction. In order to enhance the accuracy of integration, 1e2 ppm assignments including H2, H5, H3, H6 was integrated in the following as indicated in Fig. 9 including samples with various treating methods. As is observed in Fig. 9, the integration values in the region of 1e2 ppm of neat PA1012 and PA612 are 8.45 and 6.43 respectively
Table 2 Assignments of the characteristic peaks for PA1012 and PA612 in proton NMR spectra. Polyamide
Proton
Chemical shifts (ppm)
PA1012
H1 H4 H2 H3 H1 H4 H2 H3
3.49 2.65 1.65 1.20 3.49 2.65 1.66 1.25
PA612
þ H5 þ H6
þ H5 þ H6
1
H NMR spectra of PA1012 and PA612
L. Wang et al. / Polymer 59 (2015) 16e25
dissolving ability in solvent, combination of etching and NMR enables an excellent possibility to distinguish exchange reaction for LCPA. Due to the broad distribution of MW for long chain polyamide synthesized by microorganism method, transamidation evidence by MW is not adopted in this research. Variable-temperature FTIR is carried out to provide the concrete information concerning about structure change of polyamide under temperature treatment. Temperature dependence of infrared spectra of neat PA1012 in the fingerprint region are represented in Fig. 10(a), which show that all the bands in the fingerprint region decrease gradually in intensity and some even vanish ultimately in the heating process. According to the research before [47,51,52], this phenomenon arise from the so called Brill transition and enhanced motion of some units. Subsequently, the system undergoes a cooling process and the intensity of the bands recovers to some extent. Infrared spectra of PA1012/612 50/50 are shown in Fig. 10(b) which has a similar trend as neat PA1012. However, interesting information is given by the bands at 720 cm1 and 690 cm1, which are assigned to methylene rocking mode and NeH out of plane mode (amide Ⅴ) respectively [47,53]. It is clearly seen that the intensity at 720 cm1 is stronger than that of 690 cm1 in the beginning, whereas after temperature treatment, the intensity at 720 cm1 is weaker than that of 690 cm1. But this significant change isn't observed in neat PA1012. As reported earlier, with the ratio of CH2/CONH increasing in polyamide, the intensity ratio of the peak at 720 cm1 and 690 cm1 should decrease which also can be confirmed by FTIR spectra of neat PA1012 and PA612 in Fig. 10(c).
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This is most likely attributed to the fact that the methylene rocking mode is restricted harder with increasing density of hydrogen bond. In comparison with Fig. 10(a) and (b), it is evident that the ratio of CH2/CONH in PA1012/612 blends changed after temperature treatment which is associated with the presence of PA612 chain or PA1012 chain incorporated in the other polyamide. These results are in good consistent with the results obtained by NMR. 3.4. Degradation and crystal transition analysis Thermal induced polyamide degradation is an irreversible change which has been widely studied by Davis et al. [54,55] Because of the high temperature thermal history, polyamide degradation should also be taken into account for the analysis of thermal properties described in Section 1. In order to investigate the effect of degradation, neat PA1012 and PA612 are subjected to the same thermal history of polyamide blends and both the melting peaks and crystallization peaks of PA1012 and PA612 keep invariant with increasing annealing temperature (see Supporting Information). So polyamide degradation is not involved in our system with the protection of N2 environment in all experiments. It's well known that polyamide can form a, b and g crystalline phases under various conditions and the crystal transition of transforming from g-form into a-form can be realized through an appropriate way. Generally, a structure is more thermodynamically stable than g structure below Brill temperature [49,56]. It is necessary to discuss crystal transition of polyamide which may lead
Fig. 10. Temperature dependence of infrared spectra of polyamide for (a) PA1012 in the region of 1400e700 cm1, (b) PA1012/612 (50/50) in the region of 1400e700 cm1, (c) infrared spectra of PA1012 and PA612 at room temperature.
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L. Wang et al. / Polymer 59 (2015) 16e25
Scheme 2. Schematic of mechanism of improved miscibility of PA1012/PA612 blends.
to the change of thermal properties in Section 1. A series of WAXD patterns of samples (see Supporting Information) under various temperatures treatment clearly show that all samples display two strong diffraction peaks at 2q ¼ 16.2 and 19.1 which is characteristic of a form. The peak at 2q ¼ 16.2 is assigned to 100 plane reflection related to interchains distance while the peak at 2q ¼ 19.1 is assigned to 010/110 plane reflection related to intersheet distance. Simultaneously, there's some difference in peak intensity of various samples caused by different degrees of crystallinity indicated in Fig. 1 and different thickness of samples. So crystal transition doesn't take place in polyamide alloys under various temperature treatments which is also commented and verified in other research [57].
3.5. Mechanism of improved interface miscibility Most of the researches evidence finds that binary blends of aliphatic polyamides form an immiscible system due to a weak attractive interaction [58]. However, we find that the miscibility of polyamide blends can be improved to some extent by increasing annealing temperature, prolonging annealing time, changing blend composition and altering categories of polyamide alloys indicated in Section 1. The actual reason leading this thermal behavior is verified in Section 2 that transamidation takes place in our system. The correlation of polyamide transamidation and miscibility is illustrated in Scheme 2. As is proposed in the Scheme, for physical PA1012/612 alloys, they show poor miscibility proved by two separate crystallization peaks and two distinctive glass transition temperatures from DSC and DMA results. Triggered by appropriate high temperature, average strength of hydrogen bonds weakens (Fig. 8) which leads to a relative free chain motion and amido linkage is prone to be broken. The resultant chain segments with active group attack each other and ultimately link together, through which, PA1012 chain segments incorporated into PA612 backbone and form block copolymer of PA1012-co-PA612 and vice versa. In this way, the in-situ formation of copolyamide, consisting of two components, detaches from neat polyamide to act as a compatibilizer at the alloys interfaces. This in turn reduces interfacial tension between separate phases and enhances the interfacial adhesion, which improves the miscibility of PA1012/612 blends. It is actually possible that the transamidation reaction did not limit in the interfaces but converted the composite to a mixture of random copolymers of PA612 and PA1012 under higher temperature for longer time. According to the above discussion, an appropriate mechanism to explain the phenomenon revealed in Section 1 is proposed. With increasing annealing temperature, the degree of transamidation develops and copolyamide formed locally becomes a driving force to improve miscibility of blends, leading to the progressively merging of two crystallization peaks and two glass transition peaks in macro-scale properties. Accordingly, the decrease of heat
enthalpy j can reflect the degree of transamidation to some extent. Remind that the higher degree of transamidation, the greater decrease of heat enthalpy. This explanation is also suited for annealing time. In Section 1, it is also found that thermal behaviors depend considerably on blend fractions showing that PA1012 melting peaks are more liable to vanish in blends with the ratio of 20/80 than PA612 melting peaks in alloys with the ratio of 80/20. This strongly implies that the regular structure of PA1012 is easy to be disrupted. Greater changes of crystallization and melting peaks of PA1012 than those of PA612 may be resulted by the influence of first PA612 crystallization on the succeeding crystallization behavior of PA1012. For different categories, chain segments are easier to link together in polyamide alloys with a kind of same monomer due to the similar reactivity. 4. Conclusions The determination difficulty of the transamidation between two kinds of aliphatic LCPAs was solved by using the combination measurements of DSC, NMR and variable-temperature FTIR and the reaction degree was clearly calculated in this work. The annealing temperature, annealing time, blend compositions and categories of aliphatic polyamides had significant impacts on the transamidation and its degree. Reference to heat enthalpy measured by DSC, total degree of enthalpy decrease ratio j was calculated which was firstly used to evaluate relative extent of exchange reaction. It was analyzed that the total degree of enthalpy decrease ratio j, indicating the extent of reaction, was mainly dominated by reaction temperature, time, blend composition and categories of polyamides, according to which, the interchange degree could be controlled. The high j could be acquired either by increasing annealing temperature or by prolonging annealing time. Altering blend composition and polyamide categories also contributed to j, for those couple polyamides that with at least one same diamine or dicarboxylic acids monomer always favored bigger j, which means the high extent exchange reaction occurring between two LCPAs blends. As a result, the interface of polyamide mixtures, suggested by new observed peaks in DSC and DMA curves, was refined efficiently by means of copolyamide locally generated. From this work, a relative precise method to evaluate reaction degree in aliphatic LCPAs was put forward and tailored LCPAs could be established by regulating the transamidation extent. In other hand, it is an effective guide to prepare or produce polyamides alloy materials with good mechanical properties and relative lower cost to extend the applications of high performance LCPAs based materials. Acknowledgments We would like to thank Prof. Jinguang Wu of Peking University for many good discussions about the FTIR analysis in this work and
L. Wang et al. / Polymer 59 (2015) 16e25
the generous financial support by following grants: R&D Program of the Ministry of Science and Technology (2013BAE02B02) and National Natural Sciences Foundation of China (51173195). We also thank the beam time on BL14B1 and BL16B1 in SSRF of China. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2014.12.058. References [1] Kohan MI. Nylon plastics. John Wiley; 1973. [2] Li WL, Liu JL, Zhang XQ, Zhao Y, Wang DJ. J Appl Polym Sci 2012;125(2): 1077e83. [3] Li HZ, Wu YJ, Sato H, Kong L, Zhang CF, Huang K, et al. Macromolecules 2009;42(4):1175e9. [4] Yang HJ, Zhao JP, Yan MQ, Pispas S, Zhang GZ. Polym Chem 2011;2(12): 2888e92. [5] Ellis TS. J Appl Polym Sci 1988;36(3):451e66. [6] Wang XC, Zheng Q, Yang GS. J Polym Sci Part B Polymer Phys 2007;45(10): 1176e86. [7] Mao JH, Durchholz M, Biermann M. U. S. Patent 2011, 0,028,679, A1. [8] Pereira PSC, Mendes LC, Ramos VD. Macromol Symp 2010;290:121e31. [9] Aravind I, Eichhorn KJ, Komber H, Jehnichen D, Zafeiropoulos NE, Ahn KH, et al. J Phys Chem B 2009;113(6):1569e78. [10] Safapour S, Seyed-Esfahani M, Auriemma F, de Ballesteros OR, Vollaro P, Di Girolamo R, et al. Polymer 2010;51(19):4340e50. [11] Lu XB, Zhang HM, Zhang Y. J Appl Polym Sci 2014;131(7). [12] Zhang GZ, Yoshida H, Kawai T. Thermochim Acta 2004;416(1e2):79e85. [13] Wang MJ, Yuan GC, Han CC. Polymer 2013;54(14):3612e9. [14] Soccio M, Lotti N, Finelli L, Gazzano M, Munari A. Eur Polym J 2008;44(6): 1722e32. [15] Pang YY, Dong X, Zhang XQ, Liu KP, Chen EQ, Han CC, et al. Polymer 2008;49(10):2568e77. [16] Pang YY, Dong X, Zhao Y, Han CC, Wang DJ. Polymer 2007;48(21):6395e403. [17] Wang D, Li Y, Xie XM, Guo BH. Polymer 2011;52(1):191e200. [18] Gurusamy-Thangavelu SA, Emond SJ, Kulshrestha A, Hillmyer MA, Macosko CW, Tolman WB, et al. Polym Chem 2012;3(10):2941e8. [19] Connell LS, Jones JR, Weaver JVM. Polym Chem 2012;3(10):2735e8. [20] Desroches M, Caillol S, Auvergne R, Boutevin B, David G. Polym Chem 2012;3(2):450e7. [21] Sailer C, Handge UA. Macromolecules 2007;40(6):2019e28. [22] Li WD, Zeng JB, Lou XJ, Zhang JJ, Wang YZ. Polym Chem 2012;3(5):1344e53. [23] Aerdts AM, Eersels KLL, Groeninckx G. Macromolecules 1996;29(3):1041e5. [24] Eersels KLL, Aerdts AM, Groeninckx G. Macromolecules 1996;29(3):1046e50. [25] Eersels KLL, Groeninckx G, Mengerink Y, VanderWal S. Macromolecules 1996;29(21):6744e9.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Transamidation determination and mechanism of long chain-based aliphatic polyamide alloys with excellent interface miscibility Lili Wang, Xia Dong*, Yunyun Gao, Miaoming Huang, Charles C. Han, Shannong Zhu, Dujin Wang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2014 Received in revised form 13 December 2014 Accepted 24 December 2014 Available online 6 January 2015
Transamidation between PA1012 and PA612 was investigated with combination techniques including differential scanning calorimetry (DSC), rheometry, nuclear magnetic resonance (NMR) and variabletemperature Fourier transform infrared spectroscopy (VT-FTIR). Based on the increase of storage modulus with sweep time, the presence of reactive chain ends were proved, promoting chain growth in either polyamide and interchange reaction in binary blend at high temperature. NMR and VT-FTIR testing signals sufficiently convinced the expected interchange reaction. Quantitative data analysis of DSC and NMR provided direct evidences for evaluating the exchange degree. Various experimental parameters were systematically considered, including reaction temperature and time, blending composition and categories of polyamides. Raising the reaction temperature or prolonging the reaction time would accelerate the transamidation rate and degree. The interfacial miscibility between two LCPA components could be improved by the formation of copolyamides. This work provides a quantitative evaluating method to detect the transamidation extent between two aliphatic LCPAs that overcomes the traditional characterization limitation which is only feasible for whose polyamide component chain is with large volume structure like benzene ring. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Transamidation Interface miscibility Polyamide alloys
1. Introduction Polyamide, presented excellent mechanical and thermal properties, had become a classic important engineering thermoplastic, among which, aliphatic polyamide was generally named as nylon which firstly realized industrialization by DuPont in 1939 last century. In principle, any polyamide having more than 10 repeat units in the main chain was considered as long chain polyamide (LCPA). Due to relatively low density of hydrogen bonds in recurring amide groups, LCPA not only provided a combination of toughness, rigidity and lubrication-free performance but also possessed lower water absorption and better dimensional stability [1e3]. Consequently, based on higher aliphatic portions, it offered more benefits than common polyamide which had high level of absorbed water and hence decreased mechanical properties seriously [4,5]. Thus LCPA provided multiple potential applications in a wide range, such
* Corresponding author. Tel./fax: þ86 10 82618533. E-mail address: [email protected] (X. Dong). http://dx.doi.org/10.1016/j.polymer.2014.12.058 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
as automobile manufacturing, aerospace engineering and electronic appliances [6]. Eveneeven LCPA is the condensation polymerization product of dicarboxylic acids with diamines, where diamines are converted by dicarboxylic acids. Usually dibasic acid currently could be obtained either by chemical synthesis method using non-renewable petrochemical feedstock which produced unwanted hazardous byproducts, or by biological fermentation method using enzymes in microbial cell as catalyst to transform n-alkanes into long chain dibasic acid, which was an environmental friendly conversion process with biobased materials and mild condition [7]. Synthesis of LCPA using biological fermentation based monomers had attracted particular interest in recent years and many commercial polyamides produced in this method were available. In this article, both monomers of PA1012 and PA612 were prepared by biological fermentation method. More recently, the application of pure polyamide was so limited considering its properties and costs, and preparation of polymer alloys had become an economic and efficient alternative to traditional synthesis of novel polymeric material, which made it
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possible to add both advantages [8,9]. As far as polymer alloys are concerned, the key issue, determining success in obtaining better comprehensive properties, is miscibility of two components [10e13]. In general, polymer alloys tend to present poor thermal and mechanical properties caused by thermodynamic immiscibility and thus much emphasis should be put on the studying of the specific interaction between two components as well as corresponding improvement program [14]. In the past years, great effort has been made by researchers to study the compatibility of blends which can be manipulated by some strategies concerning about physical or chemical means. In the case of physical methods, Pang et al. [15,16] extensively studied interaction of isotactic polypropylene/polyethylene-octene copolymer (iPP/PEOc), in which, tailored mechanical properties can be obtained by controlling phase separation and crystallization processes. To compatibilize the ternary immiscible polypropylene/polystyrene/polyamide (PP/PS/PA), a polyolefin-based component PP-g-(MAH-co-St) was produced with maleic anhydride (MAH) and styrene (St) by Xie's group [17]. This multiphase component provided a novel and effective way to achieve good compatibilization. These mentioned modification methods were all without chemical structure change and could effectively improve the components miscibility. Another commonly used method, which was defined as chemical strategies, including estereester [18,19], estereamide [20] and amideeamide exchange reactions, was collectively referred to as effective reactive compatibilization and had an increasing research trend. A lot of polymer alloys were involved in this research like polyamide/ styrene-acrylonitrile (PA/SAN) [21], poly(butylene terephthalate)/ polycarbonate (PBT/PBS) [22], and polyamide/polyamide (PA/PA) [23e27]. Among polyamide alloys, researches based on semicrystalline aliphatic polyamide and amorphous aromatic polyamide blends are mainly performed. G. Groeninckx et al. systematically studied the transamidation occurring in polyamide4,6/polyhexamethylene isophthalamide (PA46/PA6I) blends under molten state. According to the theory developed for polyesters that in all triads the substitution on the left of a central unit does not influence the substitution on the right, number-average block length and degree of transamidation were calculated [23e25]. In order to determine the chain architecture in exchange reactions of polyamide mixtures, different measurements were used. According to that NMR signals have great sensitivity to aromatic polyamide structure with large volume sturture [28], Eersels et al. characterized structure change and degree of randomness by means of high resolution carbon nuclear magnetic resonance (13C NMR) and gradient elution chromatography (GEC) [23e25]. By means of proton NMR (1H NMR), Micaela Vannini et al. distinguished the various sequences in copolyamide efficiently and simply [26]. As reported by Giorgio Montaudo et al., thanks to matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) MS, copolymer formed in situ and its block length in polyamide alloys were accurately analyzed [29,30]. However, since the primary structure of aliphatic polyamide, particularly for LCPA, is mainly composed of methylene units and insoluble in most common solvents, traditional characterization methods like NMR and MALDI-TOF are difficult to distinguish aliphatic copolyamides in exchange reactions. Under this condition, little information is available in previous reports for elucidation of chemical interaction for aliphatic polyamide alloys in molten state. In the present paper, to obtain polyamide alloys with high performance, fundamental understanding of chemical interactions and corresponding effect on miscibility of long chain-based polyamides is required, in which polymer blending was adopted to manufacture mixtures. Moreover, particular attention was given for polyamides of PA1012/612. To overcome the characterization limitation, DSC, NMR, and variable-temperature FTIR were combined to
17
comprehensively analyze factors influencing interactions of two polyamides, giving quantitative information about exchange reaction. Simultaneously, mechanisms of improved interface miscibility in aliphatic LCPA alloys were put forward. 2. Experimental 2.1. Materials and sample preparation Polyamide 612 (PA612, e[HN(CH2)6NHCO(CH2)10CO]ne) and polyamide 1012 (PA1012, e[HN(CH2)10NHCO(CH2)10CO]ne) were synthesized from hexamethylene diamine, decamethylene diamine and dodecanedioic acid whose monomers were produced with microorganism fermentation method. The chemical structures of two polyamides were given in Scheme 1 from which it could be obviously seen that PA1012 owns longer diamine chain than PA612. Fundamental physical characterization of four polyamides (PA66, PA610, PA612, PA1012) mentioned in this work was shown in Table 1. All long chain polyamides were commercial grade supplied by Shandong Guangyin New Materials Co., Ltd. Dissolution-precipitation method was used to prepare various polyamide alloys. In this process, trifluoroacetic acid (TFA) and deionized water were used as a common solvent and a poor solvent respectively. Two polyamides were added into TFA vigorously stirring for 10 h, followed by slowly pouring the solvent into deionized water. After precipitation, the blends were intensively washed with deionized water for several times and subsequently dried under vacuum at 60 C for 72 h until a constant weight was reached. The pure PA612 and PA1012 undergone the similar treatments stated above so that all materials kept the same initial state. In this paper, the weight ratio of polyamide alloys was used to express blend composition. (PA1012/612 50/50 denotes weight fraction of PA1012 is 50% in polyamide mixtures.) 2.2. Measurements Thermal behaviors of polyamide alloys were measured by Differential Scanning Calorimeter (TA Q2000) which was calibrated in indium standard. Each specimen of about 7 mg sealed in an aluminum pan was tested under N2 atmosphere at a constant flow rate. All the measurements recorded the first cooling and the second heating runs. The glass transition of polyamide was determined by a Dynamic Mechanical Analyzer (DMA Q800). The specimens with the dimensions of 6 mm 4 mm 0.1 mm (length width thickness) were held by vertical clamps subjected to tensile modes. Measurements were conducted over a range of 100 C to 150 C with a heating rate of 3 C/min under a constant frequency of 1 Hz. The 1H NMR spectra were collected on a Bruker DMX 300 instrument equipped with a BBO double-resonance probe at ambient temperature. The samples were dissolved into trifluoroacetic acidd (20 mg/ml) which served as solvent and locking agent. The spectra were acquired after accumulating 128 scans with a digital
Scheme 1. Chemical structure of repeat units for PA612 and PA1012.
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Table 1 Fundamental physical characterization of four polyamides. Tc ( C)
Polyamide
a
a
PA66 PA610 PA612 PA1012
212 180 191 166
59.28 47.41 37.52 38.59
D Hc
(J/g)
a
Tm ( C)
257 213 216 189
a
D Hm
50.46 36.67 55.27 45.09
(J/g)
b
MFR (g/10 min)
18.1 18.7 25.2 2.3
out at 230 C with the amplitude of 0.5% in linear viscoelastic regime under a continuous purge of nitrogen to study structure change of polyamides. 3. Results and discussion 3.1. Crystallization and melting behaviors
a
Determined by DSC. b Determined by Melt Flow Indexer at 235 C according to ASTM D1238 (2.16 kg) and the tested temperature for PA66 was 265 C.
resolution of 0.138 Hz per point. The spectra width was 9000 Hz, the data points were 65,536 and the acquisition time was 2 s. In all 1 H NMR spectra, chemical shifts were referenced to the residual signals of trifluoroacetic acid-d. The relative peaks areas were measured by automatic integration methods. FTIR spectra were collected using Nicolet 6700 Fourier Transform Infrared Spectrometer with a detector of DTGS. In a transmission mode, the spectra were recorded by accumulating 64 scans with a resolution of 4 cm1. The film samples prepared by solution casting were clipped on a potassium bromide plate which was put in a Linkam FTIR 600 hot stage for viable-temperature process. The hot stage was placed in the compartment of spectrometer so that each sample can be heated in situ to acquire the temperature dependence of infrared spectra. Both pure PA1012 and polyamide alloys (50/50) were heated to 280 C at a rate of 10 C/min, and stayed there for 10 min, and then followed by cooling to room temperature at a rate of 10 C/min. In this process, the spectra were collected at intervals of 30 C. The baselines of the resulting spectra were corrected employing the same standard. WAXD measurements were performed on the beamline BL14B1 located in Shanghai Synchrotron Radiation Facility. Radiation source with wavelength ¼ 1.24 Å was used. The system was equipped with a MAR CCD detector with a resolution of 3072 3072 pixels. The sample to detector distance was 386.8 mm and the exposure time was 60 s. The blend film samples pretreated at 230 C, 240 C, 250 C, 260 C, 270 C and 280 C for 10 min were placed in Linkam stage respectively which served as a holder and scattering patterns were acquired. All the images were corrected for background scattering, air scattering and beam fluctuations. Oscillatory shear tests were performed using a Discovery Hybrid Rheometer (TA Instruments) equipped with parallel plates of 25 mm diameter. Oscillatory time sweep experiments were carried
The influence of thermal condition and chemical structure on the thermal properties of polyamide alloys was investigated, where the thermal condition included annealing temperature and time, and chemical structure included blend composition and polyamide categories. Considering the fact that DSC is not an equilibrium measurement, Cheng et al. [31] has obtained thermodynamic properties including melting temperature and heat of fusion using extrapolation methods. In our research, temperatures without lagcorrected were compared relatively due to the same heating and cooling rate in all runs. Fig. 1 and Fig. 2 illustrate crystallization and melting behaviors under different thermal conditions. In Fig. 1, thermographs with various annealing temperatures are demonstrated which also covers neat PA1012 and PA612 as a contrast. With the increase of annealing temperature, both melting peaks in PA1012/612, obviously assigned to the peaks of PA1012 and PA612 respectively, gradually become weaker and broader indicating the decrease of melting enthalpy. The melting enthalpy of PA1012 component strongly reduces from 27.7 J/g (230 C) to 13.4 J/g (280 C), whereas this is less pronounced in PA612 whose melting enthalpy reduces from 30.1 J/g (230 C) to 24.6 J/g (280 C). To quantitatively evaluate the whole degree of this change in polyamide blends, the melting enthalpy of PA1012 and PA612 is added together, and the total degree of enthalpy decrease j(%) is defined as follow.
jð%Þ ¼ 1
D Hm;T H D m;onset
(1)
where j(%) is the degree of decrease in melting enthalpy, D Hm;T represents the melting enthalpy of polyamide alloys under annealing temperature T, D Hm;onset represents the melting enthalpy of polyamide alloys under lowest annealing temperature. For example, the degree of melting enthalpy decrease for PA1012/612 at 270 C can be calculated by jð%Þ ¼ 1 D Hm;270 =D Hm;230. According to this equation, j as a function of annealing temperature is
Fig. 1. DSC thermal graphs of PA1012/612 (50/50) under various annealing temperatures for (a) melting peaks and (b) crystallization peaks.
L. Wang et al. / Polymer 59 (2015) 16e25
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Fig. 2. DSC thermal graphs of PA1012/612 (50/50) under various annealing time for (a) melting peaks and (b) crystallization peaks.
given in Fig. 3(a). As expected, j exhibits great increase with the increase of annealing temperature in which j can reach to 34.3% at 280 C. The above phenomena indicate that the ability of crystallization is greatly dominated by annealing temperature and higher annealing temperature leads to lower level of crystallization. Corresponding crystallization peaks of polyamide alloys are displayed in Fig. 1(b). Similar to the description above, distinct crystallization peaks of polyamide alloys located between peaks of neat PA1012 and PA612 are assigned to neat polyamide respectively. The blend, exhibiting much broader and weaker crystallization peaks than that of neat materials and demonstrating similar trend for the melting peaks, indicate that the crystallization of polyamide alloys is tremendously impeded. Meanwhile, with the increase of annealing temperature, double peaks at 172.8 C and 188.1 C progressively approach each other and eventually merge into a single peak at 174.6 C. This suggests better miscibility of the two components in another hand which is also convinced in the latter part. Thermal properties of polyamide alloys under various annealing time at a constant temperature of 270 C are shown in Fig. 2 which is remarkably similar to that of annealing temperature discussed in
Fig. 1. PA1012 melting peaks of polyamide alloys gradually disappear and double crystallization peaks at 172.3 C and 184.5 C converge into a single peak at 172.9 C with prolonging annealing time. Corresponding j is calculated in Fig. 3(b) suggesting that longer annealing time can result in larger j which eventually reaches to 26.6% at 270 C for 30 min. It thus confirms that the high j can be acquired by either increasing annealing temperature or prolonging annealing time known as time-temperature equivalence effect mentioned in the previous paper [32]. Thermal behaviors of polyamide alloys with different composition ratios (20/80, 80/20) under various annealing temperatures are represented in Fig. 4. In Fig. 4(a), melting peak areas of PA1012 can hardly be observed due to its low content in PA1012/612 20/80 blends, and after annealed at 250 C, the peak becomes further smaller and almost vanishes. On the contrary, the alloy with the ratio of 80/20 shows a more complex thermal behavior that two melting peaks clearly exist even under high annealing temperature of 270 C. This is also confirmed in Fig. 1 that PA1012 melting peak is more liable to vanish than that of PA612. From a comparison of both figures, it can be strongly concluded that thermal behaviors of polyamide alloys considerably depend on its fractions.
Fig. 3. Variation of j in different conditions for (a) various annealing temperatures for 10 min and (b) various annealing time at 270 C.
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Fig. 4. DSC thermal graphs of melting peaks in polyamide alloys with different composition ratios under various annealing temperatures for (a) PA1012/612 20/80 and (b) PA1012/ 612 80/20.
To explore the general effects of polyamide categories on thermal properties of mixtures, five different polyamide alloys including PA1012/612, PA1012/610, PA66/612, PA66/610 and PA66/ 610 are adopted to reveal the regularities. As is said in the introduction, eveneeven polyamide is a polycondensation product of dicarboxylic acids and diamines. In this section, three kinds of acids and two kinds of amines are involved in the monomers of these polyamides. Fig. 5(a) shows j for different categories of polyamide alloys which were annealed at 30 C, 50 C and 70 C above respective Tm due to different melting temperatures. Apparently, in Fig. 5(a), the j curves of PA1012/612, PA66/610 and PA66/612 under various annealing temperatures lie upper than that of PA1012/610 and PA66/1012. For detailed analysis, a striking character is found that both components of former polyamide alloys have one same monomer at least while the latter includes different monomers. This rule is more obvious when the reasonable comparison methods described in Fig. 5(b) is applied. As is shown by red (in web version) solid curves which include PA1012/612 and PA1012/ 610, considering the effect of acid, two kinds of amine monomers, 10/6, are chosen in both polyamide alloys excluding the effect of
amine categories. For acid monomer, one with 12/12 is the same and the other with 12/10 is different. In Fig. 5(a), it can be seen that PA1012/612 alloys with the same acid have higher j than PA1012/ 610 whose acid monomers are different. Similar method described by blue dot curves in Fig. 5(b) is utilized to investigate categories of amine and the deduction is the same. Therefore, for polyamide alloys having one same monomer at least are always favored with higher j. 3.2. Miscibility of polyamide alloys As is known, the rate of crystallization for polyamide is too fast which makes it difficult to get large portion of amorphous part even under rapid cooling condition. So the glass transition temperature of polyamide is hard to be detected by common DSC without strong cooling equipment [33]. The direct evidence of miscibility in polyamide mixtures was shown in Fig. 6 by DMA analysis concerning about PA1012/612 under annealing temperatures of 230 C, 250 C, 270 C and neat polyamide without any treatment. The glass transition of polyamide represents the motion of long chain
Fig. 5. The effect of polyamide categories on alloys thermal properties for (a) annealing temperature dependence of j and (b) comparison methods of different polyamides.
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Fig. 6. Tan delta-temperature behaviors of polyamide alloys and neat polyamide (heating rate: 3 C/min; frequency: 1 Hz).
segments in amorphous regions [34,35]. To enhance this transition, after annealed, all samples quenched directly by liquid nitrogen to get the largest degree of amorphous phase. The glass transition temperature (Tg) of neat PA1012 and PA612 is approximately 40 C and 50 C respectively which is in accordance with previous literatures [36,37]. Likewise, for physical blends without annealing treatment, there are two glass transition peaks appeared separately in DMA curves. Although the respective Tg value of the alloys are close, two peaks still can be distinguished which indicates the immiscibility of two components. However, after annealed, the blends show a single peak and the shape of the peak becomes sharp with increasing annealing temperature. Theses shifts intuitively suggest that the miscibility of two components is remarkably improved by the annealing process. Accordingly, the convergency of two crystallization peaks shown in Section 1 indeed indicates the improved miscibility too. In another hand, for polymers with ester or amide end groups, there are estereester and estereamide exchange reactions happened in wider high temperatures. Many papers have studied reactive compatibilization method like estereester and estereamide exchange reactions [14,25,38]. Therefore, for PA1012/ PA612 system, the essence of different thermal properties and improved miscibility above is impossible to be clearly explained by means of DSC alone. Considering the previous research, transamidaiton occurring in aromatic polyamides [23e25] should be taken into account in our system. However, for traditional characterization method like NMR, the signal shift is only sensitive to reactions in which large volume structure such as benzene ring unit incorporates into reaction products. For MALDI-TOF, giving detailed information about chain sequence, some mild solvents are used to dissolve polymer. For long chain aliphatic polyamides, composed of large amount of methylene groups and insoluble in most common solvent, the reactions of which are hard to be distinguished by traditional characterization means. In the following work, in order to overcome above limitation, NMR, variable-temperature FTIR and rheological methods are adopted to comprehensively study the essence of thermal behaviors in molecular level.
21
which is adopted by Zou et al. and Zhang et al. to successfully [39e41] probe the structure evolution under shear filed. In this part, oscillatory time sweep is carried out above polyamide melting temperature in a time range of 4000 s detecting time dependence of storage modulus (G0 ) for PA1012, PA612 and PA610 as shown in Fig. 7. The observed rheological response in polyamide, significant increase of G0 with prolonging time, suggests occurrence of the chain-growing reactions which have been discussed previouly [42,43]. Upon annealing, the increasing of molecular weight for polyamide originating from chain-growing reactions contributes to the increase of G0 . The analysis of Fig. 7 reveals that the end-groups including carboxyl groups and amido groups indeed exist in polyamides that we used, which promotes the occurrence of transamidation [44]. The variable temperatures-FTIR spectra of polyamide alloys undergone a heating-cooling process are demonstrated in Fig. 8. The spectra demonstrate a notable stretching vibration bands at 3300 cm1 assigned to hydrogen bonded NeH groups and whose intensity can be an evaluation of average strength of the hydrogen bond [45,46]. Upon heating, a noticeable decrease in the intensity and another characteristic peak at about 3450 cm1 are observed. As reported by Yayoi Yoshioka et al. [47,48] Upon heating, the broadness and decrease in the intensity at 3300 cm1 result from a decrease of absorptivity coefficient reflecting transformation of hydrogen-bonded NeH groups to free NeH groups appeared at 3450 cm1 [49]. The spectra collected at high temperature were resolved by the Peak Solve Fitting program to calculate the peak area at 3300 cm1 and 3450 cm1. The peak areas at 3450 cm1 accounts for 10% of the total areas representing the large number of free NeH groups at high temperature. As a consequence, with increasing temperature, the strength of hydrogen bond decreases allowing polymer chain moving freely to participate in transamidation. Both FTIR results and rheological results corroborate that the system is capable of having this exchange reaction. Proton NMR spectra and relevant assignments are listed in Table 2. In chemical structure of PA1012 and PA612, methyl proton of H1, H2, H4 and H5 have similar chemical surroundings, and the striking difference lies in the concentration of methyl proton associated with H3 and H6. However, these units probably are too far away from amide groups so that chemical shifts are almost the same for PA1012 and PA612, which is clearly revealed in Table 2. Therefore, conventional methods of detecting chemical shift
3.3. Transamidation of polyamide alloys Oscillatory sweep in linear viscoelastic regime is an effective method to investigate structure changes under certain temperature
Fig. 7. Storage modulus (G0 ) as a function of time at 230 C for polyamide (frequency: 1 Hz; strain: 0.5%).
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Fig. 8. Temperature dependence of infrared spectra of polyamide for PA1012/612 (50/ 50) in the region of 3000 cm1 to 3600 cm1.
Fig. 9. Integration in the region of 1e2 ppm for polyamides under various temperature treatments.
corresponding to 16 CH2 units and 12 CH2 units in per repeating structure. For polyamide alloys like physical blends the integration values are between that of neat PA1012 and PA612. However, for etched polyamide alloys, with increasing annealing temperature, the integration becomes smaller evidently showing that transamidation really takes place in PA1012/612 mixtures. Because if exchange reaction occurs, chain segments of PA612 can be incorporated into PA1012, that is to say, chain segment of eNH(CH2)10NHCO(CH2)10CONH(CH2)6NHCO(CH2)10COe will form in the chain of PA1012. So PA612 chains incorporated in PA1012 can't be etched and the integration of the etched PA1012/612 alloys after annealed is smaller than physical blends. Without etching treatment, integration values under high temperature treatment have no change as opposed to physical blend because the total amount of methyl proton is constant whether transamidation occurs or not. The decrease of integration value for PA1012/612 due to PA1012-co-PA612 generated in this reaction verifies that transamidation occurs in this system. Therefore, based on different
changes after transamidation mentioned before [38,50] are unfeasible in our system. Nonetheless, it is noticed that the integration value of 1e1.5 ppm for PA1012 are bigger than that of PA612 representing different concentration of methyl proton assigned to H3 and H6 and other peak areas are the same due to the same amount of methyl proton for PA1012 and PA612. In this research, it is found that polyamide blends immerging into formic acid at 50 C for 2 min can almost remove PA612 phase completely, yet there is no influence on PA1012. Based on this feature, we can reliably estimate transamidation in aliphatic LCPA alloys. Assuming that this reaction takes place in PA1012/612 alloys, the integration value of 1e1.5 ppm in polyamide alloys after etching should be different from neat PA1012 and blends without reaction. In order to enhance the accuracy of integration, 1e2 ppm assignments including H2, H5, H3, H6 was integrated in the following as indicated in Fig. 9 including samples with various treating methods. As is observed in Fig. 9, the integration values in the region of 1e2 ppm of neat PA1012 and PA612 are 8.45 and 6.43 respectively
Table 2 Assignments of the characteristic peaks for PA1012 and PA612 in proton NMR spectra. Polyamide
Proton
Chemical shifts (ppm)
PA1012
H1 H4 H2 H3 H1 H4 H2 H3
3.49 2.65 1.65 1.20 3.49 2.65 1.66 1.25
PA612
þ H5 þ H6
þ H5 þ H6
1
H NMR spectra of PA1012 and PA612
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dissolving ability in solvent, combination of etching and NMR enables an excellent possibility to distinguish exchange reaction for LCPA. Due to the broad distribution of MW for long chain polyamide synthesized by microorganism method, transamidation evidence by MW is not adopted in this research. Variable-temperature FTIR is carried out to provide the concrete information concerning about structure change of polyamide under temperature treatment. Temperature dependence of infrared spectra of neat PA1012 in the fingerprint region are represented in Fig. 10(a), which show that all the bands in the fingerprint region decrease gradually in intensity and some even vanish ultimately in the heating process. According to the research before [47,51,52], this phenomenon arise from the so called Brill transition and enhanced motion of some units. Subsequently, the system undergoes a cooling process and the intensity of the bands recovers to some extent. Infrared spectra of PA1012/612 50/50 are shown in Fig. 10(b) which has a similar trend as neat PA1012. However, interesting information is given by the bands at 720 cm1 and 690 cm1, which are assigned to methylene rocking mode and NeH out of plane mode (amide Ⅴ) respectively [47,53]. It is clearly seen that the intensity at 720 cm1 is stronger than that of 690 cm1 in the beginning, whereas after temperature treatment, the intensity at 720 cm1 is weaker than that of 690 cm1. But this significant change isn't observed in neat PA1012. As reported earlier, with the ratio of CH2/CONH increasing in polyamide, the intensity ratio of the peak at 720 cm1 and 690 cm1 should decrease which also can be confirmed by FTIR spectra of neat PA1012 and PA612 in Fig. 10(c).
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This is most likely attributed to the fact that the methylene rocking mode is restricted harder with increasing density of hydrogen bond. In comparison with Fig. 10(a) and (b), it is evident that the ratio of CH2/CONH in PA1012/612 blends changed after temperature treatment which is associated with the presence of PA612 chain or PA1012 chain incorporated in the other polyamide. These results are in good consistent with the results obtained by NMR. 3.4. Degradation and crystal transition analysis Thermal induced polyamide degradation is an irreversible change which has been widely studied by Davis et al. [54,55] Because of the high temperature thermal history, polyamide degradation should also be taken into account for the analysis of thermal properties described in Section 1. In order to investigate the effect of degradation, neat PA1012 and PA612 are subjected to the same thermal history of polyamide blends and both the melting peaks and crystallization peaks of PA1012 and PA612 keep invariant with increasing annealing temperature (see Supporting Information). So polyamide degradation is not involved in our system with the protection of N2 environment in all experiments. It's well known that polyamide can form a, b and g crystalline phases under various conditions and the crystal transition of transforming from g-form into a-form can be realized through an appropriate way. Generally, a structure is more thermodynamically stable than g structure below Brill temperature [49,56]. It is necessary to discuss crystal transition of polyamide which may lead
Fig. 10. Temperature dependence of infrared spectra of polyamide for (a) PA1012 in the region of 1400e700 cm1, (b) PA1012/612 (50/50) in the region of 1400e700 cm1, (c) infrared spectra of PA1012 and PA612 at room temperature.
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Scheme 2. Schematic of mechanism of improved miscibility of PA1012/PA612 blends.
to the change of thermal properties in Section 1. A series of WAXD patterns of samples (see Supporting Information) under various temperatures treatment clearly show that all samples display two strong diffraction peaks at 2q ¼ 16.2 and 19.1 which is characteristic of a form. The peak at 2q ¼ 16.2 is assigned to 100 plane reflection related to interchains distance while the peak at 2q ¼ 19.1 is assigned to 010/110 plane reflection related to intersheet distance. Simultaneously, there's some difference in peak intensity of various samples caused by different degrees of crystallinity indicated in Fig. 1 and different thickness of samples. So crystal transition doesn't take place in polyamide alloys under various temperature treatments which is also commented and verified in other research [57].
3.5. Mechanism of improved interface miscibility Most of the researches evidence finds that binary blends of aliphatic polyamides form an immiscible system due to a weak attractive interaction [58]. However, we find that the miscibility of polyamide blends can be improved to some extent by increasing annealing temperature, prolonging annealing time, changing blend composition and altering categories of polyamide alloys indicated in Section 1. The actual reason leading this thermal behavior is verified in Section 2 that transamidation takes place in our system. The correlation of polyamide transamidation and miscibility is illustrated in Scheme 2. As is proposed in the Scheme, for physical PA1012/612 alloys, they show poor miscibility proved by two separate crystallization peaks and two distinctive glass transition temperatures from DSC and DMA results. Triggered by appropriate high temperature, average strength of hydrogen bonds weakens (Fig. 8) which leads to a relative free chain motion and amido linkage is prone to be broken. The resultant chain segments with active group attack each other and ultimately link together, through which, PA1012 chain segments incorporated into PA612 backbone and form block copolymer of PA1012-co-PA612 and vice versa. In this way, the in-situ formation of copolyamide, consisting of two components, detaches from neat polyamide to act as a compatibilizer at the alloys interfaces. This in turn reduces interfacial tension between separate phases and enhances the interfacial adhesion, which improves the miscibility of PA1012/612 blends. It is actually possible that the transamidation reaction did not limit in the interfaces but converted the composite to a mixture of random copolymers of PA612 and PA1012 under higher temperature for longer time. According to the above discussion, an appropriate mechanism to explain the phenomenon revealed in Section 1 is proposed. With increasing annealing temperature, the degree of transamidation develops and copolyamide formed locally becomes a driving force to improve miscibility of blends, leading to the progressively merging of two crystallization peaks and two glass transition peaks in macro-scale properties. Accordingly, the decrease of heat
enthalpy j can reflect the degree of transamidation to some extent. Remind that the higher degree of transamidation, the greater decrease of heat enthalpy. This explanation is also suited for annealing time. In Section 1, it is also found that thermal behaviors depend considerably on blend fractions showing that PA1012 melting peaks are more liable to vanish in blends with the ratio of 20/80 than PA612 melting peaks in alloys with the ratio of 80/20. This strongly implies that the regular structure of PA1012 is easy to be disrupted. Greater changes of crystallization and melting peaks of PA1012 than those of PA612 may be resulted by the influence of first PA612 crystallization on the succeeding crystallization behavior of PA1012. For different categories, chain segments are easier to link together in polyamide alloys with a kind of same monomer due to the similar reactivity. 4. Conclusions The determination difficulty of the transamidation between two kinds of aliphatic LCPAs was solved by using the combination measurements of DSC, NMR and variable-temperature FTIR and the reaction degree was clearly calculated in this work. The annealing temperature, annealing time, blend compositions and categories of aliphatic polyamides had significant impacts on the transamidation and its degree. Reference to heat enthalpy measured by DSC, total degree of enthalpy decrease ratio j was calculated which was firstly used to evaluate relative extent of exchange reaction. It was analyzed that the total degree of enthalpy decrease ratio j, indicating the extent of reaction, was mainly dominated by reaction temperature, time, blend composition and categories of polyamides, according to which, the interchange degree could be controlled. The high j could be acquired either by increasing annealing temperature or by prolonging annealing time. Altering blend composition and polyamide categories also contributed to j, for those couple polyamides that with at least one same diamine or dicarboxylic acids monomer always favored bigger j, which means the high extent exchange reaction occurring between two LCPAs blends. As a result, the interface of polyamide mixtures, suggested by new observed peaks in DSC and DMA curves, was refined efficiently by means of copolyamide locally generated. From this work, a relative precise method to evaluate reaction degree in aliphatic LCPAs was put forward and tailored LCPAs could be established by regulating the transamidation extent. In other hand, it is an effective guide to prepare or produce polyamides alloy materials with good mechanical properties and relative lower cost to extend the applications of high performance LCPAs based materials. Acknowledgments We would like to thank Prof. Jinguang Wu of Peking University for many good discussions about the FTIR analysis in this work and
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