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polyamide bilayer film and blend
Polymer xxx (2013) 1e8
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Reaction process in polycarbonate/polyamide bilayer film and blend Mingji Wang a, b, Guangcui Yuan a, *, Charles C. Han a, * a State Key Laboratory of Polymer Physics & Chemistry, Joint Laboratory of Polymer Science & Materials, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2013 Received in revised form 22 April 2013 Accepted 6 May 2013 Available online xxx
Bisphenol-A polycarbonate (PC) and amorphous polyamide (aPA) were used as reactive system to study the interfacial interchange reaction between condensation polymers. Aminolysis is the main process during thermal annealing at 160e180 C. The simultaneously scission of PC chains and formation of PCaPA copolymer chains during the reaction process, can act as interfacial compatibilization agents between incompatible homopolymers. Reaction kinetics measured by in-situ Fourier transform infrared spectrum (FTIR) at interfaces of well-separated bilayer films and phase separation blends were compared. The reaction follows a first-order diffusion controlled mechanism and three time regimes were observed. First, the functionalized chains located in the vicinity of the interface participate into the reaction and the annealing time dependence of the integral area of difference IR spectra follows a power law relationship as [Area] w t1/4. Second, a depletion layer of reactants in the interfacial region is formed with the progression of reaction, and center-of-mass diffusion of reactants is required for further reaction which results in a transition of the power law relationship into [Area] w t1/2. Finally, the potential barrier arising from the previously formed copolymers at the interface suppressed the reaction. When the conversion reaches a critical value, thermal fluctuation can induce interface destabilization. An acceleration of reaction rate observed by FTIR for sample annealed at 180 C is synchronous with the interfacial roughness development analyzed by atomic force microscopy. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Interfacial compatibilization Aminolysis Diffusion controlled mechanism
1. Introduction Reactive blending is an important and increasingly accepted way for compatibilization of polymer blends and for improving of their mechanical properties. For example, polyamide 6 (PA6) and bisphenol-A polycarbonate (PC) are important commercial polymers, each of them provides outstanding performances in a variety of applications. PA6 is strongly resistant to most solvents while PC is not; PC is insensitive to moisture and dimensionally very stable while PA6 greatly suffers from its considerable hygroscopicity. Therefore, extensive researches on blends of PA6 and PC have been conducted to obtain an excellent material which is resistant to organic solvents, insensitive to moisture, and durable under various weathering conditions [1e5]. It has been pointed out that PA6 and PC are thermodynamically immiscible over the entire composition range, but if suitable mixing conditions are applied, blends with a relevant extent of interactions and interesting properties can be obtained [1,4].
* Corresponding authors. Tel.: þ86 010 82618089; fax: þ86 010 62521519. E-mail addresses: [email protected] (G. Yuan), [email protected] (C.C. Han).
In the past twenty years, considerable efforts have been focused on understanding the fundamental kinetics and mechanisms of interfacial reaction, investigating the reinforcement of interfacial adhesion and studying the development of morphological structure at polymerepolymer interface induced by interfacial reaction [6e9]. Theoretically, to simplify the treatment, many analyses are base on a simple bilayer model system [10e13], and on which the reaction kinetics can be generally categorized as two different mechanisms: the reaction controlled mechanism and diffusion controlled mechanism. However, the reaction mechanism may be affected by the reaction conditions [14]. The reaction controlled mechanism is suggested to be more appropriate in describing the short time reaction with small reactivity. The diffusion controlled mechanism fits in situation that reaction with high reactivity takes place in longer-time period. The transition from reaction controlled kinetics to diffusion controlled kinetics can be expected for the reactions with high reactivity. Experimentally, the static flat interface of bilayer film is obtained by sequentially spin-coating two polymer solutions onto a substrate or by float-coating technique [14e23]. The mechanism of reaction occurred between bilayer is important for in-situ reactive compatibilization in multilayer packaging film and multilayer
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.05.016
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coextruded tubing applications. However, in many practical applications, reactive compatibilization of immiscible blends proceeds at interface resulted from thermodynamic phase separation or even complex external flow. The reaction kinetics at sharp planar interfaces between bilayer films which are artificially put in touch may be different from that at strongly segregated interface resulted from thermodynamic phase separation between immiscible polymer pairs. One main purposes of this work is to obtain a better understanding of the fundamental factors that influencing the reaction kinetics at interface, by comparing the reaction kinetics in bilayer films and in blends at different temperatures. Moreover, most experimental research works are designed to have an irreversible reaction between end-functionalized polymers at a planar interface [10e23]. For end-coupling reaction leading to block copolymer formation at interface, the influence of the asformed diblock copolymer on the reaction kinetics and on the thermodynamic equilibrium have been investigated [6]. Some major factors influencing the interfacial reaction such as the inherent reactivity of functional polymers [24,25], thermodynamic interaction between polymers [17] and functional groups location along a chain [26] have been studied. However, for interchange reaction that proceeds in condensation polymers containing for example ester or amide bonds, although dozens of polymeric systems have been experimentally studied in homogeneous melt or heterogeneous blend states [1,2,27e35], the studies of interchange reactions at sharp interfaces are still scarce. Interchange reaction between condensation polymers may yield block, segmented, or random copolymers depending on the location of reactive functional groups. The composition and structure of the copolymer formed, is predicted to vary with the extent of reaction, depending both on the reactivity and the concentration of reactive functional groups [36]. This may in turn have significant effects on the morphology of the interface and reaction kinetics. There are many problems to be solved related to the thermodynamics and kinetics of phase separation and compatibilization in systems undergoing interchange reactions [37]. Therefore, another purpose of this work is to obtain a better understanding of the relation between the reaction conversion and interface development by investigate the kinetics of interfacial interchange reaction which was rarely reported. The interfacial reaction kinetics in bilayer systems has been investigated by using different techniques such as dynamic secondary ion mass spectrometry [19], X-ray photoelectron spectroscopy [14], forward recoil spectrometry [20], and neutron reflectometry [38]. Although these methods can provide concentration profile of copolymer product, they can not provide an in-situ investigation. Using FTIR bilayer film model experiments to investigate the in-situ kinetics and extent of reaction at the well-defined interface have been reported before [15]. In general, the interface between strongly immiscible polymers is very sharp, and only a small extent of interpenetration of molecules is allowed due to highly unfavorable enthalpy, so the concentration of functional groups that could meet and induce chemical reaction at the interface is very low. Therefore, monitoring the extent of interfacial reaction in thin film sample is difficult, because the amount of copolymer formed is typically small, especially for end-coupling reaction where increasing average molecular weight leads to a proportional decreasing in the number of reactive end groups. Scott and Macosko [15] have found that bilayer film experiments on reactive polymers with higher molecular weight and low concentrations of reactive functionalities were unsuccessful because the infrared signal from the interface was too weak. In this work, FTIR was used to study the interchange reaction between PC and amorphous polyamide (aPA). The reason for aPA (instead of engineering plastic PA6) is used in this study, is to avoid roughening
induced by crystallization of PA6 and to obtain a smooth surface. The weight-averaged molecular weight of PC and aPA is 35,000 and 30,000 g/mol respectively. Although the infrared signal from the interface is not very strong, the interfacial reactive process was successfully probed. This is because the reaction between PC and aPA is an inner-outer reaction (as discussed later) where each carbonate group along the polycarbonate chain may participate in the reaction. Atomic force microscopy (AFM) was used to visualize the interfacial roughness during annealing of reactive layered samples and to reveal the relationship between interfacial roughness and reaction conversion. 2. Experimental section 2.1. Materials The PC (Scheme la) and aPA (Scheme 1b) were synthesized in our laboratory. Synthesis of PC was adopted from a procedure reported by H. Yoon et al. [29]. The final product PC was purified by dissolving in chloroform and precipitating into ethanol, and then the precipitate was filtered and washed with 75 C hot water. The purification procedure was repeated for three times. The weightaveraged molecular weight of PC was determined to be 35,000 g/ mol using intrinsic viscosity method with chloroform as solvent [39]. The glass transition temperature of PC is 145 C as measured with differential scanning calorimetry (DSC, Mettle Toledo DSC822e). The Mw/Mn was determined to be 1.9 by gel permeation chromatography (GPC, Waters 1515) method with tetrahydrofuran as solvent. The aPA was synthesized by polycondensation reaction of trimethylhexamethylenediamine (2,2,4- and 2,4,4- mixture) with terephthaloyl chloride in N-methylpyrrolidone solution using triethylamine as the acid acceptor. The molecular weight of the desired polymer can be controlled by the ratio of trimethylhexamethylenediamine and terephthaloyl chloride. In order to obtain amino terminated aPA, the quantity of trimethylhexamethylenediamine is slightly larger than that of terephthaloyl chloride. The acquired polymer was purified by dissolving in N,Ndimethylacetamide and precipitating into 75 C hot water. The dissolving and precipitating procedures were repeated for three times. Then the product was washed with ethanol. The weightaveraged molecular weight of the aPA used in this study was determined to be 30,000 g/mol using intrinsic viscosity method in N,N-dimethylformamide [39]. The glass transition temperature of the aPA used was determined to be 147 C by DSC measurement. The Mw/Mn determined to be 2.0 by GPC measurement using N,Ndimethylformamide as solvent.
Scheme 1. Chemical structure of (a) PC and (b) aPA {Poly[iminoterephthaloylimino(trimethylhexamethylene)]}, 2,4,4-/2,2,4-trimethyl (1/1, mol/mol).
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2.2. Fourier transform infrared spectrum Bilayer films of PC/aPA for FTIR spectrum experiments were prepared by a two-step casting process. First, a film of aPA was casted on a potassium bromide (KBr) plate from 1,1,1,3,3,3hexafluoro-2-propanol solution (w0.01 g/ml). Then the KBr plate coated with one aPA layer was stored under vacuum at 75 C for 24 h, and then used as the substrate for casting a solution of PC in chloroform (w0.01 g/ml) at room temperature. The blend samples of PC/aPA for FTIR spectrum experiments were prepared by casting the mixture of PC and aPA (w0.02 g/ml, 50/50 w/w) from a mixed solvent of chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol (10/1 v/v). Solvent removal was completed under vacuum prior to all the experiments. In-situ infrared spectra were obtained on a Bruker Tensor 27 FTIR at a resolution of 2 cm1. A heating cell with a maximum heating rate of about 30 C/min was used. Another KBr plate, covered at the top of the second layer, was used to keep the sample being isolated from the outside environment. The sample temperature was calibrated by a thermocouple located between two KBr plates. There is no detectable reaction occurs at 150 C or below even after the bilayer or blend samples had been thermally treated for 10 h. So the amount of reaction that might occur during the heat-up from room temperature to the experimental temperature was considered negligible and was not included in the calculations of all runs reported in this paper. 2.3. Atomic force microscopy
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conditions. The aminolysis was proved occurring much more easily than innereinner exchange reaction between amide and carbonate groups [1,28,41]. Fig. 1(a) shows the FTIR spectra of pure aPA and pure PC, and Fig. 1(b) shows the spectra of PC/aPA bilayer film before annealing and after annealing at 180 C for 600 min over the region from 1850 cm1 to 1000 cm1. The PC and aPA can be identified respectively by the carbonate C]O band at 1780 cm1 and the amide C]O band at 1640 cm1. The slight variation of carbonyl-stretching band from 1780 cm1 to 1765 cm1 indicates the formation of urethane unit structure as product of aminolysis reaction (Scheme 2). The peak located at 1765 cm1 for sample after annealed represents the product of urethane structure, and this was verified by the FTIR spectrum of a PC and trimethylhexamethylenediamine (2,2,4- and 2,4,4- mixture) blends (50:1, w/w) annealed at 180 C in which aminolysis is very easy to take place and produce urethane structure. Accordingly, there is a decrease in the peak of the CeOeC linkages (ranged from 1200 cm1 to 1100 cm1 in Fig. 1b), which indicates that the CeOeC linkages are broken during aminolysis reaction. There is no change at 1640 cm1 during the whole thermal treatment process, indicating that the innereinner exchange reaction between carbonate group of polycarbonate and amide groups of polyamide could be neglected in our analysis. Acidolysis is also excluded since no obvious change in 1740 cm1 and 1070 cm1 peaks were observed which characterizes an aromatic ester structure results from acidolysis reaction [42]. Our results indicate that the aminolysis is the main process, inducing simultaneously scission of PC chains and formation of PCaPA copolymer chains.
The samples prepared for AFM experiment consisted of a PC film on top of an aPA layer with polished silicon wafers as substrates, both layers are approximately 100 nm. The bottom layer was prepared by spinning a 0.02 g/ml solution of aPA in 1,1,1,3,3,3hexafluoro-2-propanol solution directly onto the silicon wafer. The silicon wafer coated with the aPA layer was stored under vacuum at 75 C for 24 h, and then used as the substrate for spinning a 0.01 g/ml solution of PC in chloroform. All the spinning processes were performed at room temperature. Prepared bilayer films were placed under vacuum at 75 C for 24 h to remove residual solvent before using. The interface topology was examined using a Nanoscope Multimode III AFM instrument from Veeco Company in tapping mode. The general silicon nitride cantilevers with a spring constant about 40 N/m (resonance frequency typically around 343 kHz) were used as the scanning probe. The bilayer samples were quenched to room temperature by being quickly placed on a big flat copper block. Before measurement, the unreacted PC homopolymer layer were removed by selective solvent of 1,2-dichloroethane in an ultrasonic bath at room temperature for 10 min and followed by drying the sample in air at room temperature. All the data were acquired in height mode with scan rate of 0.5 HZ and the scan area is 2 2 mm2. The root-mean-square (RMS) roughness was evaluated by analyzing the image with the Nanoscope software. 3. Results and discussion 3.1. Aminolysis between PC and aPA Infrared analysis allows the identification of reaction mechanism and the characterization of reaction kinetics. It is well known that chemical reaction could take place between PC and aPA mixtures when they meet in the molten state. Innereouter exchange reaction (aminolysis and acidolysis) between carbonate group and the terminals of polyamide [1] and innereinner exchange reaction between carbonate group of polycarbonate and amide groups of polyamide [40], should in principle be possible in some appropriate
Fig. 1. (a) FTIR spectra of the pure aPA and PC; (b) PC/aPA bilayer film before annealing (solid line) and after annealing at 180 C for 600 min (dash line).
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3.2. Diffusion controlled mechanism
Scheme 2. The aminolysis of PC carbonate groups by the aPA amine end groups.
Spectra of PC/aPA bilayer films and blends, unannealed and annealed at 160, 170 and 180 C for different times (0e600 min) were analyzed, and the spectral regions of interest were chosen, zoomed and evaluated. Fig. 2(a) shows the partial enlarged detail of carbonyl region (1820e1700 cm1) in order to show the change more clearly during the annealing process. The C]O stretching band at 1780 cm1 is found to be slightly shifted to lower wave numbers which indicates that new groups are produced and carbonate groups are consumed in the annealing process. The difference spectra were calculated as follows: Difference spectrum ¼ [Spectrum (4000e600 cm1) of sample annealed for different time] [Spectrum (4000e600 cm1) measured at the zero time which was defined as when temperature reached the set value]. Fig. 2(b) shows the corresponding difference spectra ranged from 1820 to 1700 cm1. In Fig. 2(b), the negative peak located at 1780 cm1 represents the consumption of carbonate units and the positive peak locate at 1765 cm1 represents the production of urethane units. All bilayer samples and blends samples were prepared in uniform thickness respectively for the purpose that the reaction kinetics analysis of them can be reasonably merged in one plot respectively.
Fig. 2. Evolution of IR spectra (a) and the corresponding difference spectra (b) of the carbonyl region (1820e1700 cm1) from PC/aPA bilayer film with increasing annealing time at 180 C.
The integrated area of difference spectra which has the same arbitrary unit as the absorbance spectra is plotted against annealing time as presenting in Fig. 3. For analysis it was assumed that the integrated area was proportional to the areal density of copolymer. This substitution does not affect the determination of kinetic order. The data at 160, 170 and 180 C are compared quantitatively assuming no change in absorptivity of the urethane unit structure over the temperature range indicated. The results show that there are clearly differences in reaction rate with different temperatures, and differences in bilayer and blend samples. Since the aminolysis is the main process, as we mentioned above, the number of carbonate groups of PC which takes part in the reactions can be treated as infinite while the end reactive group of aPA is quite limited compared with the carbonate groups. Based on the fact that the concentrations of reactants were asymmetrical on both sides of the interface, we have Rt z knPA, where Rt is the reaction rate of the system, nPA is the concentration of reaction group in aPA phase and k is the reactivity coefficient. The reaction kinetics should be controlled by the diffusion of aPA to the interface [11,14], thus k z dxt/dt, where xt is the root-mean-square distance of diffusion. According to reptation theory [43], we have xt ¼ a(t/ ta)1/z, where a is reactive group size and ta is the monomer relaxation time. For entangled systems the successive dynamical regimes correspond to z ¼ 4, 8, 4, and when time become even longer, z ¼ 2 can be expected due to the simple center-of-mass diffusion.
Fig. 3. Kinetics of innereouter exchange reaction at PC/aPA interfaces for bilayer films (a) and blends (b) annealed at different temperatures.
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Because the very short time (less than Rouse relaxation time) dynamic is not easy to detect and to clearly define, the areal copolP ymer density t(z xtnPA ¼ a(t/ta)1/znPA) is expected to first follow a P P power law with t wt 1=4 and then transform to t wt 1=2 for longer time. This kind of kinetics is defined as first-order diffusion controlled mechanism [11]. The reaction kinetics obtained at 160 C (the same data in Fig. 3) was analyzed using a double logarithmic plot given in Fig. 4. For bilayer sample (Fig. 4(a)), it can be found that, the relationship between integral area of difference spectrum [Area] and annealing time t, first follows [Area] w t1/4 and then follows [Area] w t1/2. The transition is a direct and strong evidence of diffusion controlled mechanism. The transition indicates that, a depletion hole appears after the reactants located in the vicinity of the interface are consumed, so that the reaction is controlled by the center-of-mass diffusion of reactive homopolymers to the interface. For blend sample, it is found that [Area] w t1/4 in the experiment time scale and no transition is found. The difference between bilayer sample and blend sample is related to the reactants concentration preexisted at the interface. The interface for bilayer sample is more separated, because the two layers are artificially put into contact, but there is an interphase between two phases for blend samples. Once the bilayer is annealed, fast interdiffusion proceeds driven by the artificial concentration gradient should be expected. The quick consumption of the reactants in the vicinity of interface leads to the development of a depletion hole, and centerof-mass diffusion of reactants is required for further reaction. For blend samples, the interface forms as a result of phase separation when the samples are prepared. It should be noted that, the phase domains does not change obviously at our experimental timescales and temperatures as proved by in-situ morphological studies.
Fig. 4. The integral areas of the difference spectra plotted vs. time t in a logelog plot when annealed samples at 160 C. (a) Bilayer; (b) Blend.
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In the diffusion controlled mechanism, three time regimes are expected. First, the reaction will occur using the reactants located in the vicinity of the interface. Second, reactant depletion at the interface will require diffusion of the reactants to the interface in order to have further reaction. At this regime it may take some time to establish a concentration gradient between the interface region and the bulk. Third, the reaction will be impeded by interfacial saturation or by difficulty in finding the counter pair [14]. The diffusion step observed by experiments depends on experimental conditions such as time and temperatures. The first and second regimes are observed in bilayer sample annealed at 160 C, and the third regime is observed in blend sample annealed at 170 C. This is because the relaxation rate at 170 C is much faster than that at 160 C according to the time-temperature correspondence mechanism [44]. As shown in Fig. 3, the conversion of blend sample annealed at 170 C is almost leveled-off after a quick time reaction. It seems that the reaction is suppressed by the brush of copolymers at the interface. The similar phenomena have been reported by Kim et al. [45] before by using a rheological measurement. During the experimental time range, the phenomenon of interfacial saturation was not observed in bilayer sample because it may take times to fill the depletion hole and to establish a concentration gradient between the interface region and the bulk. 3.3. Interfacial destabilization For reaction occurred at 180 C, there is an acceleration in reaction rate in both bilayer and blend samples after reaction for about 150 min (Fig. 3). To our knowledge, this kind of kinetics is very different from any experimental works previously reported on reaction kinetics at polymerepolymer interfaces, whether for reaction between end-functional chains [26,46,47] or for reaction between end-functional chains with mid-functional chains [26]. A conclusion drawn from the end-coupling reaction is that the block copolymer formed at the interface can cause interfacial roughening. If the interfacial areal density of copolymer product reached a certain value, an interfacial instability ensues, which will induce an abrupt interfacial roughening or interfacial emulsification [9]. As we mentioned above, the main reaction process between PC and aPA we concerned here is an inner-outer reaction, where the aPA amide terminals randomly react with the carbonate groups at the main chain of PC. The formation of copolymer in the reactive system that acts as compatibilizer may play a pivotal role on the evolution of interface morphology. Moreover, chemical reactions between the two polymers are supposedly to give rise to PC compounds with low molecular weight [1], these species will improve the compatibility of these two polymers. Based on this point, interchange reaction are easier to induce interfacial instability than end-coupling reaction. Thus, for samples annealed at 180 C, with the extent of reaction increasing, a critical value of copolymers areal density may be achieved. At this point the interfacial tension may be significantly decreased and roughness became greatly increased. Phenomena of interfacial roughening during annealing of reactive layered samples have been observed by some researchers using AFM [17,19,21,45,46] and other techniques [23,46,48,49]. In this study, interfacial roughening in bilayer samples during annealing was visualized by AFM. Selective solvent was used to remove the top PC layer, and AFM investigated the topography of the exposed PA surface. Although the copolymers formed near the interface might be rinsed out together with the polymer (here PC) during the solvent removal sometimes, especially when emulsification occurred, but the AFM measurement will still provide the information of interfacial roughening in a qualitative manner. Fig. 5 showed the 2-dimensional AFM image of typical interfacial morphology for samples annealed at different temperatures for
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Fig. 5. The 2-dimensional AFM image of interfacial morphology development for samples annealed at different temperatures. (a) 160 C; (b) 170 C; (c) 180 C.
2 min and 540 min, respectively. To quantitatively analyze the interfacial roughening, the RMS roughness was extracted from the images and was plotted vs. time in Fig. 6. The in-plane characteristic length scales which are also interested in the topology analysis were not found in power spectral density and fast Fourier transform analyses of the AFM data in our experiments. Thus, only RMS values were taken into consideration in interface development analysis. For a reasonable comparison, we first prepared a bilayer film with a large substrate (3 inch diameter) and then cut it into several pieces, these pieces were assumed to have similar initial state before they
were annealed at the given temperature for different times. It should be noticed that, the RMS roughness for the samples without annealing is not zero (around 0.5 nm). This is probably caused by the surface thermal fluctuations and/or perturbation by the solvent treatment, but is less than that caused by reaction. As shown in Fig. 6 for samples annealed at 160 C and 170 C, the interface became rougher in 10 min, but from 10 to 600 min, the RMS roughness increased very slowly. For samples annealed at 180 C, there is an abrupt increase in RMS roughness at about 200 min and this may be due to interfacial corrugation or the emulsification
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Fig. 6. Root-mean-square (RMS) roughness vs. annealing time at different temperatures. (Dashed lines drawn to guide the eye, and the error bars were obtained from at least three independent measurements.)
transition induced by the interfacial instability. The important observation we should point out is that the development of interfacial roughness and reaction conversion are seemingly in synchronous. The comparison of the onset of this acceleration in interfacial roughening observed by AFM with the speed-up point of the reaction rate observed by FTIR indicates that the roughness
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increased with the extent of reaction will give fresh area that allows further exchange reaction of polymers. When the maximum value of interfacial copolymer density (saturation density) achieved, the interfacial reactions should be judged by the total interphase which denote as the intrinsic interfacial thickness multiply interfacial area. As the equilibrium intrinsic interfacial thickness should be constant at long time annealing process, the conversion growth at late stage of 180 C annealing process was mainly controlled by the increase of interfacial area which is directly related to the growth of “grain” number and “grain” sizes. The word “grain” was used to describe the wave like interface here. The reaction kinetics of late stage for samples annealed at 180 C was analyzed using a double logarithmic plot given in Fig. 7. It was found that the conversion increase with t1/3 in bilayer sample and t2/3 in blend sample for this short time range. By using the average grains size analysis, the interfacial roughness r was found to follow r w t1/3 relation in many works [50e54] and the diffusion controlled mechanism was accepted widely to be responsible for the growth law. The same relationship between interfacial roughness and time was also found in the simulation work based on the Kardar-Parisi-Zhang (KPZ) theory [55]. For bilayer model, with the confinement effects of the substrate, the grain is confined at a two dimensional interface, the number of grains decrease when grain size increase, and the total interfacial area A increase can be estimate as A w r [56], where r is the average height of the roughness, i.e. the average grains size at the interface. For blend model, without the confinement of the substrate, the interface can be deformed freely, the number of grains do not need to decrease when grain size increase, and the total interfacial area A increase can be estimate as A w r2. The results is consistent with the notion that interface destabilization due to surface tension reduction by reaction product was the major factor to perpetuate reactions by generating new interface area. 4. Conclusions
Fig. 7. The integral areas of the difference spectra plotted vs. time t in a logelog plot when annealed samples at 180 C. (a) Bilayer; (b) Blend.
In this work, the interfacial interchange reaction at bilayer samples and at blend samples of two immiscible condensation polymers, PC and aPA, were investigated experimentally. Aminolysis was the main process at 160e180 C with the random attack of amino terminal groups of aPA on the inner carbonate groups of PC, and it followed a first-order diffusion controlled mechanism in most period of the reaction process. Although the diffusion step observed by experiments depends on experimental conditions such as time and temperatures, three stages of interfacial interchange reaction were observed which is similar to that of interfacial endcoupling reaction. First, the functionalized chains located in the vicinity of the interface would first participate into the reaction. The integral area of difference spectra (which is proportional to the areal density of copolymer) follows a power law relationship with annealing time as [Area] w t1/4. Second, a depletion hole of reactants in the interfacial region is formed with the progression of reaction, and center-of-mass diffusion of reactants is required for further reaction. The integral area of difference spectra follows a power law relationship with annealing time as [Area] w t1/2. Finally, the potential barrier arising from the previously formed copolymers at the interface suppressed the reaction. The phenomenon of interfacial saturation was observed in blend samples annealed at 170 C where the integral area of difference spectra remains almost constant with time in last regime. As products of interchange reaction, copolymer was formed and polymer with low molecular weight was released. Both copolymer and low molecular weight polymer can play the role of compatibilizer. Therefore, unlike the end-coupling reaction reported before, there was an acceleration of the reaction rate when the conversion reached to a
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certain value for sample annealed at 180 C. The reaction kinetics was explained from the viewpoint that the product formed in the reactive system tends to decrease the interfacial tension of the system. The interfacial roughness development analyzed by AFM and reaction conversion development obtained from FTIR were synchronous, which indicating that interface destabilization due to surface tension reduction by reaction product perpetuates reactions by generating new surface area. Interfacial roughening enhances the creation of fresh interfacial area, thereby greatly accelerates the reaction process. Acknowledgment This work was supported by the Chinese National Science Foundation (Project 21004069). References [1] Gattiglia E, Turturro A, Lamantia FP, Valenza A. J Appl Polym Sci 1992;46(11): 1887e97. [2] Gattiglia E, Turturro A, Pedemonte E. J Appl Polym Sci 1989;38(10):1807e18. [3] Gattiglia E, Turturro A, Pedemonte E, Dondero G. J Appl Polym Sci 1990;41(7e 8):1411e23. [4] Gattiglia E, Mantia F, Turturro A, Valenza A. Polym Bull 1989;21(1):47e52. [5] Wang Q, Jiang Y, Li L, Wang P, Yang Q, Li G. J Macromol Sci Part B 2012;51(1): 96e108. [6] Litmanovich AD, Plate NA, Kudryavtsev YV. Prog Polym Sci 2002;27(5): 915e70. [7] Plate NA, Litmanovich AD, Kudryavtsev YV. Theoretical considerations on the reactions in polymer blends. Dordrecht: Springer; 2004. [8] Macosko CW, Jeon HK, Hoye TR. Prog Polym Sci 2005;30(8e9):939e47. [9] Jiang GJ, Wu H, Guo SY. Polym Eng Sci 2010;50(12):2273e86. [10] O’Shaughnessy B, Sawhney U. Phys Rev Lett 1996;76(18):3444e7. [11] O’Shaughnessy B, Vavylonis D. Macromolecules 1999;32(6):1785e96. [12] Fredrickson GH, Milner ST. Macromolecules 1996;29(23):7386e90. [13] Fredrickson GH. Phys Rev Lett 1996;76(18):3440e3. [14] Oyama HT, Inoue T. Macromolecules 2001;34(10):3331e8. [15] Scott C, Macosko C. J Polym Sci Part B Polym Phys 1994;32(2):205e13. [16] Jiao J, Kramer EJ, de Vos S, Möller M, Koning C. Polymer 1999;40(12):3585e8. [17] Jones TD, Schulze JS, Macosko CW, Lodge TP. Macromolecules 2003;36(19): 7212e9. [18] Kim BJ, Fredrickson GH, Kramer EJ. Macromolecules 2007;40(10):3686e94. [19] Kim BJ, Kang H, Char K, Katsov K, Fredrickson GH, Kramer EJ. Macromolecules 2005;38(14):6106e14. [20] Schulze JS, Cernohous JJ, Hirao A, Lodge TP, Macosko CW. Macromolecules 2000;33(4):1191e8. [21] Zhang JB, Lodge TP, Macosko CW. Macromolecules 2005;38(15):6586e91.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Reaction process in polycarbonate/polyamide bilayer film and blend Mingji Wang a, b, Guangcui Yuan a, *, Charles C. Han a, * a State Key Laboratory of Polymer Physics & Chemistry, Joint Laboratory of Polymer Science & Materials, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2013 Received in revised form 22 April 2013 Accepted 6 May 2013 Available online xxx
Bisphenol-A polycarbonate (PC) and amorphous polyamide (aPA) were used as reactive system to study the interfacial interchange reaction between condensation polymers. Aminolysis is the main process during thermal annealing at 160e180 C. The simultaneously scission of PC chains and formation of PCaPA copolymer chains during the reaction process, can act as interfacial compatibilization agents between incompatible homopolymers. Reaction kinetics measured by in-situ Fourier transform infrared spectrum (FTIR) at interfaces of well-separated bilayer films and phase separation blends were compared. The reaction follows a first-order diffusion controlled mechanism and three time regimes were observed. First, the functionalized chains located in the vicinity of the interface participate into the reaction and the annealing time dependence of the integral area of difference IR spectra follows a power law relationship as [Area] w t1/4. Second, a depletion layer of reactants in the interfacial region is formed with the progression of reaction, and center-of-mass diffusion of reactants is required for further reaction which results in a transition of the power law relationship into [Area] w t1/2. Finally, the potential barrier arising from the previously formed copolymers at the interface suppressed the reaction. When the conversion reaches a critical value, thermal fluctuation can induce interface destabilization. An acceleration of reaction rate observed by FTIR for sample annealed at 180 C is synchronous with the interfacial roughness development analyzed by atomic force microscopy. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Interfacial compatibilization Aminolysis Diffusion controlled mechanism
1. Introduction Reactive blending is an important and increasingly accepted way for compatibilization of polymer blends and for improving of their mechanical properties. For example, polyamide 6 (PA6) and bisphenol-A polycarbonate (PC) are important commercial polymers, each of them provides outstanding performances in a variety of applications. PA6 is strongly resistant to most solvents while PC is not; PC is insensitive to moisture and dimensionally very stable while PA6 greatly suffers from its considerable hygroscopicity. Therefore, extensive researches on blends of PA6 and PC have been conducted to obtain an excellent material which is resistant to organic solvents, insensitive to moisture, and durable under various weathering conditions [1e5]. It has been pointed out that PA6 and PC are thermodynamically immiscible over the entire composition range, but if suitable mixing conditions are applied, blends with a relevant extent of interactions and interesting properties can be obtained [1,4].
* Corresponding authors. Tel.: þ86 010 82618089; fax: þ86 010 62521519. E-mail addresses: [email protected] (G. Yuan), [email protected] (C.C. Han).
In the past twenty years, considerable efforts have been focused on understanding the fundamental kinetics and mechanisms of interfacial reaction, investigating the reinforcement of interfacial adhesion and studying the development of morphological structure at polymerepolymer interface induced by interfacial reaction [6e9]. Theoretically, to simplify the treatment, many analyses are base on a simple bilayer model system [10e13], and on which the reaction kinetics can be generally categorized as two different mechanisms: the reaction controlled mechanism and diffusion controlled mechanism. However, the reaction mechanism may be affected by the reaction conditions [14]. The reaction controlled mechanism is suggested to be more appropriate in describing the short time reaction with small reactivity. The diffusion controlled mechanism fits in situation that reaction with high reactivity takes place in longer-time period. The transition from reaction controlled kinetics to diffusion controlled kinetics can be expected for the reactions with high reactivity. Experimentally, the static flat interface of bilayer film is obtained by sequentially spin-coating two polymer solutions onto a substrate or by float-coating technique [14e23]. The mechanism of reaction occurred between bilayer is important for in-situ reactive compatibilization in multilayer packaging film and multilayer
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.05.016
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coextruded tubing applications. However, in many practical applications, reactive compatibilization of immiscible blends proceeds at interface resulted from thermodynamic phase separation or even complex external flow. The reaction kinetics at sharp planar interfaces between bilayer films which are artificially put in touch may be different from that at strongly segregated interface resulted from thermodynamic phase separation between immiscible polymer pairs. One main purposes of this work is to obtain a better understanding of the fundamental factors that influencing the reaction kinetics at interface, by comparing the reaction kinetics in bilayer films and in blends at different temperatures. Moreover, most experimental research works are designed to have an irreversible reaction between end-functionalized polymers at a planar interface [10e23]. For end-coupling reaction leading to block copolymer formation at interface, the influence of the asformed diblock copolymer on the reaction kinetics and on the thermodynamic equilibrium have been investigated [6]. Some major factors influencing the interfacial reaction such as the inherent reactivity of functional polymers [24,25], thermodynamic interaction between polymers [17] and functional groups location along a chain [26] have been studied. However, for interchange reaction that proceeds in condensation polymers containing for example ester or amide bonds, although dozens of polymeric systems have been experimentally studied in homogeneous melt or heterogeneous blend states [1,2,27e35], the studies of interchange reactions at sharp interfaces are still scarce. Interchange reaction between condensation polymers may yield block, segmented, or random copolymers depending on the location of reactive functional groups. The composition and structure of the copolymer formed, is predicted to vary with the extent of reaction, depending both on the reactivity and the concentration of reactive functional groups [36]. This may in turn have significant effects on the morphology of the interface and reaction kinetics. There are many problems to be solved related to the thermodynamics and kinetics of phase separation and compatibilization in systems undergoing interchange reactions [37]. Therefore, another purpose of this work is to obtain a better understanding of the relation between the reaction conversion and interface development by investigate the kinetics of interfacial interchange reaction which was rarely reported. The interfacial reaction kinetics in bilayer systems has been investigated by using different techniques such as dynamic secondary ion mass spectrometry [19], X-ray photoelectron spectroscopy [14], forward recoil spectrometry [20], and neutron reflectometry [38]. Although these methods can provide concentration profile of copolymer product, they can not provide an in-situ investigation. Using FTIR bilayer film model experiments to investigate the in-situ kinetics and extent of reaction at the well-defined interface have been reported before [15]. In general, the interface between strongly immiscible polymers is very sharp, and only a small extent of interpenetration of molecules is allowed due to highly unfavorable enthalpy, so the concentration of functional groups that could meet and induce chemical reaction at the interface is very low. Therefore, monitoring the extent of interfacial reaction in thin film sample is difficult, because the amount of copolymer formed is typically small, especially for end-coupling reaction where increasing average molecular weight leads to a proportional decreasing in the number of reactive end groups. Scott and Macosko [15] have found that bilayer film experiments on reactive polymers with higher molecular weight and low concentrations of reactive functionalities were unsuccessful because the infrared signal from the interface was too weak. In this work, FTIR was used to study the interchange reaction between PC and amorphous polyamide (aPA). The reason for aPA (instead of engineering plastic PA6) is used in this study, is to avoid roughening
induced by crystallization of PA6 and to obtain a smooth surface. The weight-averaged molecular weight of PC and aPA is 35,000 and 30,000 g/mol respectively. Although the infrared signal from the interface is not very strong, the interfacial reactive process was successfully probed. This is because the reaction between PC and aPA is an inner-outer reaction (as discussed later) where each carbonate group along the polycarbonate chain may participate in the reaction. Atomic force microscopy (AFM) was used to visualize the interfacial roughness during annealing of reactive layered samples and to reveal the relationship between interfacial roughness and reaction conversion. 2. Experimental section 2.1. Materials The PC (Scheme la) and aPA (Scheme 1b) were synthesized in our laboratory. Synthesis of PC was adopted from a procedure reported by H. Yoon et al. [29]. The final product PC was purified by dissolving in chloroform and precipitating into ethanol, and then the precipitate was filtered and washed with 75 C hot water. The purification procedure was repeated for three times. The weightaveraged molecular weight of PC was determined to be 35,000 g/ mol using intrinsic viscosity method with chloroform as solvent [39]. The glass transition temperature of PC is 145 C as measured with differential scanning calorimetry (DSC, Mettle Toledo DSC822e). The Mw/Mn was determined to be 1.9 by gel permeation chromatography (GPC, Waters 1515) method with tetrahydrofuran as solvent. The aPA was synthesized by polycondensation reaction of trimethylhexamethylenediamine (2,2,4- and 2,4,4- mixture) with terephthaloyl chloride in N-methylpyrrolidone solution using triethylamine as the acid acceptor. The molecular weight of the desired polymer can be controlled by the ratio of trimethylhexamethylenediamine and terephthaloyl chloride. In order to obtain amino terminated aPA, the quantity of trimethylhexamethylenediamine is slightly larger than that of terephthaloyl chloride. The acquired polymer was purified by dissolving in N,Ndimethylacetamide and precipitating into 75 C hot water. The dissolving and precipitating procedures were repeated for three times. Then the product was washed with ethanol. The weightaveraged molecular weight of the aPA used in this study was determined to be 30,000 g/mol using intrinsic viscosity method in N,N-dimethylformamide [39]. The glass transition temperature of the aPA used was determined to be 147 C by DSC measurement. The Mw/Mn determined to be 2.0 by GPC measurement using N,Ndimethylformamide as solvent.
Scheme 1. Chemical structure of (a) PC and (b) aPA {Poly[iminoterephthaloylimino(trimethylhexamethylene)]}, 2,4,4-/2,2,4-trimethyl (1/1, mol/mol).
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2.2. Fourier transform infrared spectrum Bilayer films of PC/aPA for FTIR spectrum experiments were prepared by a two-step casting process. First, a film of aPA was casted on a potassium bromide (KBr) plate from 1,1,1,3,3,3hexafluoro-2-propanol solution (w0.01 g/ml). Then the KBr plate coated with one aPA layer was stored under vacuum at 75 C for 24 h, and then used as the substrate for casting a solution of PC in chloroform (w0.01 g/ml) at room temperature. The blend samples of PC/aPA for FTIR spectrum experiments were prepared by casting the mixture of PC and aPA (w0.02 g/ml, 50/50 w/w) from a mixed solvent of chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol (10/1 v/v). Solvent removal was completed under vacuum prior to all the experiments. In-situ infrared spectra were obtained on a Bruker Tensor 27 FTIR at a resolution of 2 cm1. A heating cell with a maximum heating rate of about 30 C/min was used. Another KBr plate, covered at the top of the second layer, was used to keep the sample being isolated from the outside environment. The sample temperature was calibrated by a thermocouple located between two KBr plates. There is no detectable reaction occurs at 150 C or below even after the bilayer or blend samples had been thermally treated for 10 h. So the amount of reaction that might occur during the heat-up from room temperature to the experimental temperature was considered negligible and was not included in the calculations of all runs reported in this paper. 2.3. Atomic force microscopy
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conditions. The aminolysis was proved occurring much more easily than innereinner exchange reaction between amide and carbonate groups [1,28,41]. Fig. 1(a) shows the FTIR spectra of pure aPA and pure PC, and Fig. 1(b) shows the spectra of PC/aPA bilayer film before annealing and after annealing at 180 C for 600 min over the region from 1850 cm1 to 1000 cm1. The PC and aPA can be identified respectively by the carbonate C]O band at 1780 cm1 and the amide C]O band at 1640 cm1. The slight variation of carbonyl-stretching band from 1780 cm1 to 1765 cm1 indicates the formation of urethane unit structure as product of aminolysis reaction (Scheme 2). The peak located at 1765 cm1 for sample after annealed represents the product of urethane structure, and this was verified by the FTIR spectrum of a PC and trimethylhexamethylenediamine (2,2,4- and 2,4,4- mixture) blends (50:1, w/w) annealed at 180 C in which aminolysis is very easy to take place and produce urethane structure. Accordingly, there is a decrease in the peak of the CeOeC linkages (ranged from 1200 cm1 to 1100 cm1 in Fig. 1b), which indicates that the CeOeC linkages are broken during aminolysis reaction. There is no change at 1640 cm1 during the whole thermal treatment process, indicating that the innereinner exchange reaction between carbonate group of polycarbonate and amide groups of polyamide could be neglected in our analysis. Acidolysis is also excluded since no obvious change in 1740 cm1 and 1070 cm1 peaks were observed which characterizes an aromatic ester structure results from acidolysis reaction [42]. Our results indicate that the aminolysis is the main process, inducing simultaneously scission of PC chains and formation of PCaPA copolymer chains.
The samples prepared for AFM experiment consisted of a PC film on top of an aPA layer with polished silicon wafers as substrates, both layers are approximately 100 nm. The bottom layer was prepared by spinning a 0.02 g/ml solution of aPA in 1,1,1,3,3,3hexafluoro-2-propanol solution directly onto the silicon wafer. The silicon wafer coated with the aPA layer was stored under vacuum at 75 C for 24 h, and then used as the substrate for spinning a 0.01 g/ml solution of PC in chloroform. All the spinning processes were performed at room temperature. Prepared bilayer films were placed under vacuum at 75 C for 24 h to remove residual solvent before using. The interface topology was examined using a Nanoscope Multimode III AFM instrument from Veeco Company in tapping mode. The general silicon nitride cantilevers with a spring constant about 40 N/m (resonance frequency typically around 343 kHz) were used as the scanning probe. The bilayer samples were quenched to room temperature by being quickly placed on a big flat copper block. Before measurement, the unreacted PC homopolymer layer were removed by selective solvent of 1,2-dichloroethane in an ultrasonic bath at room temperature for 10 min and followed by drying the sample in air at room temperature. All the data were acquired in height mode with scan rate of 0.5 HZ and the scan area is 2 2 mm2. The root-mean-square (RMS) roughness was evaluated by analyzing the image with the Nanoscope software. 3. Results and discussion 3.1. Aminolysis between PC and aPA Infrared analysis allows the identification of reaction mechanism and the characterization of reaction kinetics. It is well known that chemical reaction could take place between PC and aPA mixtures when they meet in the molten state. Innereouter exchange reaction (aminolysis and acidolysis) between carbonate group and the terminals of polyamide [1] and innereinner exchange reaction between carbonate group of polycarbonate and amide groups of polyamide [40], should in principle be possible in some appropriate
Fig. 1. (a) FTIR spectra of the pure aPA and PC; (b) PC/aPA bilayer film before annealing (solid line) and after annealing at 180 C for 600 min (dash line).
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3.2. Diffusion controlled mechanism
Scheme 2. The aminolysis of PC carbonate groups by the aPA amine end groups.
Spectra of PC/aPA bilayer films and blends, unannealed and annealed at 160, 170 and 180 C for different times (0e600 min) were analyzed, and the spectral regions of interest were chosen, zoomed and evaluated. Fig. 2(a) shows the partial enlarged detail of carbonyl region (1820e1700 cm1) in order to show the change more clearly during the annealing process. The C]O stretching band at 1780 cm1 is found to be slightly shifted to lower wave numbers which indicates that new groups are produced and carbonate groups are consumed in the annealing process. The difference spectra were calculated as follows: Difference spectrum ¼ [Spectrum (4000e600 cm1) of sample annealed for different time] [Spectrum (4000e600 cm1) measured at the zero time which was defined as when temperature reached the set value]. Fig. 2(b) shows the corresponding difference spectra ranged from 1820 to 1700 cm1. In Fig. 2(b), the negative peak located at 1780 cm1 represents the consumption of carbonate units and the positive peak locate at 1765 cm1 represents the production of urethane units. All bilayer samples and blends samples were prepared in uniform thickness respectively for the purpose that the reaction kinetics analysis of them can be reasonably merged in one plot respectively.
Fig. 2. Evolution of IR spectra (a) and the corresponding difference spectra (b) of the carbonyl region (1820e1700 cm1) from PC/aPA bilayer film with increasing annealing time at 180 C.
The integrated area of difference spectra which has the same arbitrary unit as the absorbance spectra is plotted against annealing time as presenting in Fig. 3. For analysis it was assumed that the integrated area was proportional to the areal density of copolymer. This substitution does not affect the determination of kinetic order. The data at 160, 170 and 180 C are compared quantitatively assuming no change in absorptivity of the urethane unit structure over the temperature range indicated. The results show that there are clearly differences in reaction rate with different temperatures, and differences in bilayer and blend samples. Since the aminolysis is the main process, as we mentioned above, the number of carbonate groups of PC which takes part in the reactions can be treated as infinite while the end reactive group of aPA is quite limited compared with the carbonate groups. Based on the fact that the concentrations of reactants were asymmetrical on both sides of the interface, we have Rt z knPA, where Rt is the reaction rate of the system, nPA is the concentration of reaction group in aPA phase and k is the reactivity coefficient. The reaction kinetics should be controlled by the diffusion of aPA to the interface [11,14], thus k z dxt/dt, where xt is the root-mean-square distance of diffusion. According to reptation theory [43], we have xt ¼ a(t/ ta)1/z, where a is reactive group size and ta is the monomer relaxation time. For entangled systems the successive dynamical regimes correspond to z ¼ 4, 8, 4, and when time become even longer, z ¼ 2 can be expected due to the simple center-of-mass diffusion.
Fig. 3. Kinetics of innereouter exchange reaction at PC/aPA interfaces for bilayer films (a) and blends (b) annealed at different temperatures.
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Because the very short time (less than Rouse relaxation time) dynamic is not easy to detect and to clearly define, the areal copolP ymer density t(z xtnPA ¼ a(t/ta)1/znPA) is expected to first follow a P P power law with t wt 1=4 and then transform to t wt 1=2 for longer time. This kind of kinetics is defined as first-order diffusion controlled mechanism [11]. The reaction kinetics obtained at 160 C (the same data in Fig. 3) was analyzed using a double logarithmic plot given in Fig. 4. For bilayer sample (Fig. 4(a)), it can be found that, the relationship between integral area of difference spectrum [Area] and annealing time t, first follows [Area] w t1/4 and then follows [Area] w t1/2. The transition is a direct and strong evidence of diffusion controlled mechanism. The transition indicates that, a depletion hole appears after the reactants located in the vicinity of the interface are consumed, so that the reaction is controlled by the center-of-mass diffusion of reactive homopolymers to the interface. For blend sample, it is found that [Area] w t1/4 in the experiment time scale and no transition is found. The difference between bilayer sample and blend sample is related to the reactants concentration preexisted at the interface. The interface for bilayer sample is more separated, because the two layers are artificially put into contact, but there is an interphase between two phases for blend samples. Once the bilayer is annealed, fast interdiffusion proceeds driven by the artificial concentration gradient should be expected. The quick consumption of the reactants in the vicinity of interface leads to the development of a depletion hole, and centerof-mass diffusion of reactants is required for further reaction. For blend samples, the interface forms as a result of phase separation when the samples are prepared. It should be noted that, the phase domains does not change obviously at our experimental timescales and temperatures as proved by in-situ morphological studies.
Fig. 4. The integral areas of the difference spectra plotted vs. time t in a logelog plot when annealed samples at 160 C. (a) Bilayer; (b) Blend.
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In the diffusion controlled mechanism, three time regimes are expected. First, the reaction will occur using the reactants located in the vicinity of the interface. Second, reactant depletion at the interface will require diffusion of the reactants to the interface in order to have further reaction. At this regime it may take some time to establish a concentration gradient between the interface region and the bulk. Third, the reaction will be impeded by interfacial saturation or by difficulty in finding the counter pair [14]. The diffusion step observed by experiments depends on experimental conditions such as time and temperatures. The first and second regimes are observed in bilayer sample annealed at 160 C, and the third regime is observed in blend sample annealed at 170 C. This is because the relaxation rate at 170 C is much faster than that at 160 C according to the time-temperature correspondence mechanism [44]. As shown in Fig. 3, the conversion of blend sample annealed at 170 C is almost leveled-off after a quick time reaction. It seems that the reaction is suppressed by the brush of copolymers at the interface. The similar phenomena have been reported by Kim et al. [45] before by using a rheological measurement. During the experimental time range, the phenomenon of interfacial saturation was not observed in bilayer sample because it may take times to fill the depletion hole and to establish a concentration gradient between the interface region and the bulk. 3.3. Interfacial destabilization For reaction occurred at 180 C, there is an acceleration in reaction rate in both bilayer and blend samples after reaction for about 150 min (Fig. 3). To our knowledge, this kind of kinetics is very different from any experimental works previously reported on reaction kinetics at polymerepolymer interfaces, whether for reaction between end-functional chains [26,46,47] or for reaction between end-functional chains with mid-functional chains [26]. A conclusion drawn from the end-coupling reaction is that the block copolymer formed at the interface can cause interfacial roughening. If the interfacial areal density of copolymer product reached a certain value, an interfacial instability ensues, which will induce an abrupt interfacial roughening or interfacial emulsification [9]. As we mentioned above, the main reaction process between PC and aPA we concerned here is an inner-outer reaction, where the aPA amide terminals randomly react with the carbonate groups at the main chain of PC. The formation of copolymer in the reactive system that acts as compatibilizer may play a pivotal role on the evolution of interface morphology. Moreover, chemical reactions between the two polymers are supposedly to give rise to PC compounds with low molecular weight [1], these species will improve the compatibility of these two polymers. Based on this point, interchange reaction are easier to induce interfacial instability than end-coupling reaction. Thus, for samples annealed at 180 C, with the extent of reaction increasing, a critical value of copolymers areal density may be achieved. At this point the interfacial tension may be significantly decreased and roughness became greatly increased. Phenomena of interfacial roughening during annealing of reactive layered samples have been observed by some researchers using AFM [17,19,21,45,46] and other techniques [23,46,48,49]. In this study, interfacial roughening in bilayer samples during annealing was visualized by AFM. Selective solvent was used to remove the top PC layer, and AFM investigated the topography of the exposed PA surface. Although the copolymers formed near the interface might be rinsed out together with the polymer (here PC) during the solvent removal sometimes, especially when emulsification occurred, but the AFM measurement will still provide the information of interfacial roughening in a qualitative manner. Fig. 5 showed the 2-dimensional AFM image of typical interfacial morphology for samples annealed at different temperatures for
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Fig. 5. The 2-dimensional AFM image of interfacial morphology development for samples annealed at different temperatures. (a) 160 C; (b) 170 C; (c) 180 C.
2 min and 540 min, respectively. To quantitatively analyze the interfacial roughening, the RMS roughness was extracted from the images and was plotted vs. time in Fig. 6. The in-plane characteristic length scales which are also interested in the topology analysis were not found in power spectral density and fast Fourier transform analyses of the AFM data in our experiments. Thus, only RMS values were taken into consideration in interface development analysis. For a reasonable comparison, we first prepared a bilayer film with a large substrate (3 inch diameter) and then cut it into several pieces, these pieces were assumed to have similar initial state before they
were annealed at the given temperature for different times. It should be noticed that, the RMS roughness for the samples without annealing is not zero (around 0.5 nm). This is probably caused by the surface thermal fluctuations and/or perturbation by the solvent treatment, but is less than that caused by reaction. As shown in Fig. 6 for samples annealed at 160 C and 170 C, the interface became rougher in 10 min, but from 10 to 600 min, the RMS roughness increased very slowly. For samples annealed at 180 C, there is an abrupt increase in RMS roughness at about 200 min and this may be due to interfacial corrugation or the emulsification
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Fig. 6. Root-mean-square (RMS) roughness vs. annealing time at different temperatures. (Dashed lines drawn to guide the eye, and the error bars were obtained from at least three independent measurements.)
transition induced by the interfacial instability. The important observation we should point out is that the development of interfacial roughness and reaction conversion are seemingly in synchronous. The comparison of the onset of this acceleration in interfacial roughening observed by AFM with the speed-up point of the reaction rate observed by FTIR indicates that the roughness
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increased with the extent of reaction will give fresh area that allows further exchange reaction of polymers. When the maximum value of interfacial copolymer density (saturation density) achieved, the interfacial reactions should be judged by the total interphase which denote as the intrinsic interfacial thickness multiply interfacial area. As the equilibrium intrinsic interfacial thickness should be constant at long time annealing process, the conversion growth at late stage of 180 C annealing process was mainly controlled by the increase of interfacial area which is directly related to the growth of “grain” number and “grain” sizes. The word “grain” was used to describe the wave like interface here. The reaction kinetics of late stage for samples annealed at 180 C was analyzed using a double logarithmic plot given in Fig. 7. It was found that the conversion increase with t1/3 in bilayer sample and t2/3 in blend sample for this short time range. By using the average grains size analysis, the interfacial roughness r was found to follow r w t1/3 relation in many works [50e54] and the diffusion controlled mechanism was accepted widely to be responsible for the growth law. The same relationship between interfacial roughness and time was also found in the simulation work based on the Kardar-Parisi-Zhang (KPZ) theory [55]. For bilayer model, with the confinement effects of the substrate, the grain is confined at a two dimensional interface, the number of grains decrease when grain size increase, and the total interfacial area A increase can be estimate as A w r [56], where r is the average height of the roughness, i.e. the average grains size at the interface. For blend model, without the confinement of the substrate, the interface can be deformed freely, the number of grains do not need to decrease when grain size increase, and the total interfacial area A increase can be estimate as A w r2. The results is consistent with the notion that interface destabilization due to surface tension reduction by reaction product was the major factor to perpetuate reactions by generating new interface area. 4. Conclusions
Fig. 7. The integral areas of the difference spectra plotted vs. time t in a logelog plot when annealed samples at 180 C. (a) Bilayer; (b) Blend.
In this work, the interfacial interchange reaction at bilayer samples and at blend samples of two immiscible condensation polymers, PC and aPA, were investigated experimentally. Aminolysis was the main process at 160e180 C with the random attack of amino terminal groups of aPA on the inner carbonate groups of PC, and it followed a first-order diffusion controlled mechanism in most period of the reaction process. Although the diffusion step observed by experiments depends on experimental conditions such as time and temperatures, three stages of interfacial interchange reaction were observed which is similar to that of interfacial endcoupling reaction. First, the functionalized chains located in the vicinity of the interface would first participate into the reaction. The integral area of difference spectra (which is proportional to the areal density of copolymer) follows a power law relationship with annealing time as [Area] w t1/4. Second, a depletion hole of reactants in the interfacial region is formed with the progression of reaction, and center-of-mass diffusion of reactants is required for further reaction. The integral area of difference spectra follows a power law relationship with annealing time as [Area] w t1/2. Finally, the potential barrier arising from the previously formed copolymers at the interface suppressed the reaction. The phenomenon of interfacial saturation was observed in blend samples annealed at 170 C where the integral area of difference spectra remains almost constant with time in last regime. As products of interchange reaction, copolymer was formed and polymer with low molecular weight was released. Both copolymer and low molecular weight polymer can play the role of compatibilizer. Therefore, unlike the end-coupling reaction reported before, there was an acceleration of the reaction rate when the conversion reached to a
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Please cite this article in press as: Wang M, et al., Reaction process in polycarbonate/polyamide bilayer film and blend, Polymer (2013), http:// dx.doi.org/10.1016/j.polymer.2013.05.016