XRD分析-公式

Crystallization Behavior and Morphological Development of Isotactic Polypropylene with an Aryl Amide Derivative as b-Form Nucleating Agent MU DONG,1 ZHAOXIA GUO,1 JIAN YU,1 ZHIQIANG SU2 1Department of Chemical Engineering, Institute of Polymer Science and Engineering, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China 2Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100084, People’s Republic of China Received 3 March 2008; revised 19 May 2008; accepted 29 May 2008 DOI: 10.1002/polb.21508 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: This article reports crystallization behaviors of isotactic polypropylene (iPP) with an aryl amide derivative (TMB-5) as b-form nucleating agent. The effects of nucleating agent concentration, thermal history and assemble morphology of nucle- ating agent on the crystallization behaviors of iPP were studied by differential scan- ning calorimetry, X-ray diffraction, and polarized optical microscopy. The results indi- cated that the TMB-5 concentration should surpass a threshold value to get products rich in b-iPP. The diverse morphologies of TMB-5 are determined by nucleating agent concentration and crystallization condition. At higher concentrations, the recrystal- lized TMB-5 aggregates into needle-like structure, which induces mixed polymorphic phases on the lateral surface and large amount of b modification around the tip. High b nucleation efficiency was obtained at the lowest studied crystallization tem- perature, which is desirable for real molding process. TMB-5 prefers to recrystallize from the melt at higher concentration and lower crystallization temperature. The dif- ference in solubility, pertinent to concentration and crystallization temperature, determined the distinct crystallization behaviors of iPP. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1725–1733, 2008 Keywords: crystallization; isotactic polypropylene; nucleating agent; nucleation; polarized optical microscopy; spherulite morphology; wide-angle X-ray diffraction INTRODUCTION Isotactic polypropylene (iPP) is a semicrystalline polymer with several crystal modifications such as monoclinic a, trigonal b, and orthorhombic c.1,2 The chain conformation of each modifica- tions is the classical 31 helix. The difference in the crystallography is the manner in which the chains are packed in the unit cell. The differ- ence in the crystal forms of iPP has a dramatic impact on the properties of polymer products. Among all the crystal forms of iPP, the b one has many performance characteristics, such as improved elongation at break and impact strength.3,4 The b form can be formed by various methods such as large temperature gradient, rapid cooling from the melt to 130–135 8C or adding a selective b-nucleating agent.5–7 The most effective and accessible method to obtain iPP with higher level of b crystal form is the addition of some b-nucleating agents. Correspondence to: J. Yu (E-mail: yujian03@mail.tsinghua. edu.cn) or Z. Su (E-mail: suzq@mail.buct.edu.cn) Journal of Polymer Science: Part B: Polymer Physics, Vol. 46, 1725–1733 (2008) VVC 2008 Wiley Periodicals, Inc. 1725 Many different substances possessing the abil- ity to nucleate the b-form have been reported: c- form of linear trans-quinacridone, aromatic com- pounds with coplanar phenyl rings,8–10 N,N-dicy- clohexylnaphtalene-2,6-dicarboxamide, calcium salts of dicarboxilic acids, a mixture of pimelic acid and calcium stearate. Many researchers have reported the satisfactory results using c-form qui- nacridone pigment Permanent Red E3B to pro- mote b-growth in PP, though they added color to the products.11–15 Pigments applied commonly for coloring of polypropylene fibers possess high heat stability and form finely dispersed crystals insolu- ble in the polypropylene melt.16 Binsbergen and de Lange obtained predominantly b-phase PP by doping the resin with aluminum 6-quinizarinsul- fonate.17 Morrow quoted that disodium orthoph- thalate was an effective b-nucleating agent under certain thermal conditions, and that isophthalic and terephthalic acids were also capable but less effective.18 Others19–21 and Huang22 reported the use of pimelic acid as a b-nucleating agent and about 80% b-PP was obtained from the doped resin. The mechanism is still unclear. The efficiency of b-form nucleating agents, depending heavily on the concentration and dis- persion of the additives and the cooling rates, were also described.23–27 Thus the control of the crystallization conditions under which a specific crystalline modification can be formed is essen- tial to the design of materials based on iPP. In our laboratory conditions, we discovered a commercialized nucleating agent TMB-5 proved to be a highly efficient b-type nucleating agent. To our best knowledge, the variable formation parameters for iPP blended with TMB-5 system has not been investigated in details, though the b-nucleating agent of aromatic amide deriva- tives have been reported a lot.28–31 The main objective of this study is to investigate the b nucleating activity of TMB-5 under nonisother- mal and isothermal crystallization condition and further discuss the relationship of additive con- centration, thermal history and assemble mor- phology of nucleating agent with b form crystal content and the spherulitic morphology of iPP. EXPERIMENTAL Materials and Samples Preparation The matrix polymer used in this wok was com- mercial grade iPP, 2401, with melt flow index of 2.5 g/10 min, Mw ¼ 4.4 3 105 g/mol, and melt- ing temperature of 165 8C, produced by Yanshan Petroleum and Chemical, China. The commer- cial b-nucleating agent (TMB-5) is an aryl am- ide-based system (the chemical structure is undisclosed) and was supplied by Shanxi Pro- vincial Institute of Chemical Industry, with melting point of 197 8C.32 IPP/TMB-5 blends were produced at 190 8C for 10 min, with a rotor speed of 60 rpm, using a HAAKE (Thermo Haake Rheomix) batch melt mixer. TMB-5 proportion in the blends was 0.01, 0.05, 0.1, 0.3, and 0.6% by weight. For conven- ience, samples with TMB-5 are coded with its concentration. PP0.01 stands for iPP nucleated with 0.01% TMB-5. The crystallization procedure and thermal history are prime factors in determining the morphological features and physical-mechanical properties of a given polymer. In this work, to ensure the consistency of thermal histories of different samples, all the samples with different nucleating agent content were prepared in the film form. The procedure used was as follows: the samples were placed between two pieces of aluminum foil. This sandwich was then trans- ferred to a pair of steel platens, at a tempera- ture of 230 8C. After the pellets have melted, pressure was then given to the desired film thickness (40 lm). After melting for 5 min, the pressure was then released, the foil/polymer sandwich was then transferred quickly to another pair of hot press temperature controlled at Tc, where it was between the two steel platens for a holding time tc. The final samples were then rapidly cooled to room temperature for test. In the various isothermal crystalliza- tions, Tc ¼ 100, 110, 120, 130, 140, and 150 8C, respectively. Measurements For optical microscopy observation, a Nikon type 104 optical microscope was used in this study. The optical microscope was equipped with cross- polarizer, with a camera system (Panasonic wv- CP240/G) incorporated, and a programmable heating stage. WAXD experiments were conducted with a Panalytical (Holland) X’ pert Pro MRD diffrac- tometer (Cu Ka, k ¼ 0.154 nm, 40 kV, 40 mA, reflection mode). The experiments were per- formed with a 2h range of 108–308, at a scanning rate of 48/min and a scanning step of 0.028. 1726 DONG ET AL. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb The relative amount of different crystal forms present in these samples was measured from the X-ray diffraction profiles. In iPP WAXD pro- files, (110) at 2h ¼ 14.18, (040) at 16.98, (130) at 18.58 are the principal reflections of the a-crystal form of iPP while (300) at about 15.98 is the principal reflection of the b-crystal form, and they are considered as the characteristic peaks for a-crystals and b-crystals, respectively. The relative content of b-form crystal, Kb, can be evaluated according to eq 1, following the forma- tion of Turner–Jones criterion:33 Kb ¼ Abð300Þ Aað110Þ þ Aað040Þ þ Aað130Þ þ Abð300Þ ð1Þ where Ab(300) represents the area of the (300) reflection peak; Aa(110), Aa(040), and Aa(130) repre- sent the areas of the (110), (040), (130) reflection peaks, respectively. The overall crystallinity is calculated by Xc ¼ P AcrystallineP Acrystalline þ P Aamorphous 3 100% ð2Þ X Acrystalline ¼ Abð300Þ þ Aað110Þ þ Aað040Þ þ Aað130Þ ð3Þ where Acrystalline and Aamorphous are the fitted areas of crystalline and amorphous, respectively. In this work, the peak intensities of WAXD pro- files were calculated by a curve-fitting soft by using the mixed function of Gauss and Lorenz, background scattering being subtracted. The noncrystallization behaviors of iPP/TMB- 5 were studied using differential scanning calori- metry. Samples of approximately 2–3 mg were placed into aluminum pans and tested in nitro- gen atmosphere in a Q2900 calorimeter (TA Instruments). The first step in the thermal treatment was always annealing at 230 8C for 5 min to erase earlier thermomechanical histor- ies. Next, samples were cooled to 80 8C at pro- grammed cooling rate of 10 8C/min, respectively. For all nucleated and pure iPP samples, Tc,m were determined from the DSC curves as the maxima of the exothermic peak. RESULTS AND DISCUSSIONS Nonisothermal Crystallization Behaviors of iPP/TMB-5 A crystallizable polymer, nucleation may caused by homogeneous or by heterogeneous (impur- ities, nucleating agent, or others), which can controls the microstructure and physical proper- ties of polymer. Therefore, overall crystallization behavior of polymers can be analyzed by the ob- servation of the rates of nucleation and growth or the overall rate of crystallization of polymers, which can forecast relationship between the microstructures and preparation conditions of the polymers. Thus, Figure 1 shows the noniso- thermal crystallization behavior of iPP/TMB-5 as a function of TMB-5 content at cooling rate of DSC. For pure iPP and iPP/TMB-5 blends, sin- gle exothermic peaks were observed. The crys- tallization temperature increased sharply with TMB-5 percentage initially and grows gradually with further addition, meaning that TMB-5 mol- ecules essentially act as effective nucleating agent for iPP matrix and promote the crystalli- zation rate of iPP during nonisothermal crystal- lization. In our experiments, 0.01%, 0.05%, 0.3% can be considered as subcritical, critical, super- critical concentrations, respectively. Effects of Crystallization Temperatures on the Isothermal Crystallization Behaviors According to the earlier results, the effect of TMB-5 content on the efficiency of heterogene- ous nucleation is very important. XRD measure- ments were conducted for iPP/TMB-5 with dif- ferent concentrations of TMB-5 isothermally crystallized at different temperatures, to deter- mine the temperature and the concentration at Figure 1. Crystallization temperature as a function of concentration of TMB-5. The inset is DSC scans in a cooling for samples containing different concentra- tions of TMB-5. BEHAVIOR AND DEVELOPMENT OF iPP 1727 Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb which the changes of the crystallization behav- iors of a and b may be observed. Figure 2 provides the X-ray diffraction pro- files of iPP nucleated with 0.01, 0.05, and 0.3% TMB-5, crystallized under different Tc as exam- ples. A small amount (0.01%) of TMB-5 does not enhance much b-iPP. However, further increase (0.05% and 0.3%) in the concentration enhances the intensities of b reflections at 2h ¼ 168 [Fig. 2(b and c)]. The similar phenomenon occurs in PP0.6, though their WAXD profiles are omitted here. These results indicate that a critical con- centration of nucleating agent (here is 0.05%) is responsible for the considerable creation of the trigonal crystalline phase in iPP. And the higher crystallization temperature leads to reduction of the b-form content, as has been shown that the intensity of b reflection at 2h ¼ 15.98 decreases at Tc ¼ 140, 150 8C. With the aid of eqs 1 and 2, the total crystal- linity (Xc) and b relative content (Kb) can be cal- culated from WAXD profiles of all samples. The total crystallinity has grown from 55% (pure iPP) to 70% on the addition of 0.01% nucleating agent. With higher concentration, the crystallin- ity decreases slightly due to agglomeration of nucleating agent. Figure 3 illustrates that b relative content (Kb) changes as a function of crystallization tem- perature. The result corresponds to the exother- mic morphologies in Figure 1. After a critical concentration (0.05%), the Kb value increases greatly. Generally speaking, the b phase has a faster growth rate than a phase in the conven- tional range, that is, for T < 140 8C. With the aid of enough number of b nuclei provided by nucleating agent, large amount of nucleation active sites are created. When concentration is higher than 0.05%, higher content of b-phase (Kb � 0.8) is obtained when Tc ¼ 100, 120, 130 8C. At higher Tc, Kb decreases remarkably due to lower melting temperature of b-iPP. As mentioned earlier, the growth rate of b phase is higher as compared with a phase in special temperature domain. The high critical temperature T* of growth transition, which takes place above 140 8C is observed by Varga.34 Lotz observes that the b-growth transition occurs if the samples are cooled below the low critical temperature T** at �100 8C during the crystal growth. A number of different interpre- tations for the mechanism of the b-a transforma- tion were suggested. Samuels and Yee proposed the formation of a liquid phase because of the Figure 2. WAXD profiles of the iPP samples with different TMB-5 percentage crystallized from the melt at various crystallization temperatures: 100, 110, 120, 130, 140, 150 8C. The TMB-5 concentrations are listed upright: (a) 0.01%; (b) 0.05%; (c) 0.3%. 1728 DONG ET AL. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb considerable differences in the unit cell of the two structures;35 Asano and Fujiwara suggested that this transformation occurs by unfolding, melting, and recrystallization;36 and Garbarczyk et al. suggested that b-a recrystallization is pre- ceded by the transition of the phase into a disor- dered state.10 The b-a transformation does not take place automatically with the disappearance of the form; it occurs only when the necessary energy to overcome the energy barrier of the transformation is reached. Extensive DSC stud- ies by Varga et al. demonstrated partial melting of the b form during the heating cycle after cool- ing below 100 8C, results that were confirmed by Fillon et al. by DSC and optical microscopy measurements.37–39 However, in our experiments, we discovered that the decline of b-phase content at Tc ¼ 140 8C is depressed when TMB-5 concentration is increased, which we did not expect. According to Garbarczyk, some additives exhibited retarda- tion influence on the b ? a transition dependent on their chemical structures.9 And Varga and Marco discovered that on adding some additives the b ? a transition temperature could be increased.40,41 In our case, we do not provide evidence that TMB-5 does retard b ? a transi- tion, however, the relatively high Kb values of PP0.3 and PP0.6 may support that the final polymorphic structure depends heavily on TMB- 5 concentration. We prolonged the crystalliza- tion to 24 h and discovered that the Kb value dropped greatly compared with samples crystal- lized for 1 h, which testified that crystallization in 2 h at 140 8C is not complete and the relative high Kb value of PP0.3 and PP0.6 may be ascribed to secondary crystallization during quenching from the given crystallization temper- ature 140 8C. At 140 8C, samples comprise mixed polymorphic dominant a crystals and minor b-nuclei induced on the surface of TMB-5. The number of effective b-nuclei is determined by nucleating agent concentration. Upon cooling, fast growing b nuclei develop quickly and rela- tive b-phase content increases. At Tc ¼ 150 8C, none of b nuclei can be obtained regardless of nucleating agent concentration. During quench- ing, a-phase is still predominant. Another noticeable point is that iPP/TMB-5 possessed high content b-phase even at 100 8C. In other experiments, we also confirmed that quite amount of b-phase was present in PP/ TMB-5 even at much lower temperatures. In this matter, a broader temperature range could be provided for b-iPP, which is favorable for obtaining high content b-phase under real pro- cessing conditions. It is even expected that high content b-phase can be obtained when quenched in room temperature. Development of Spherulitic Morphologies of iPP/TMB-5 Blends The spherulitic morphological development of isothermal crystallization procession for iPP/ TMB-5 at various loading of TMB-5 were also investigated with POM at 120 8C as shown in Figures 4–6. In Figure 4 the morphology of large spherulites for the pure iPP, with a diameter about 100 lm, is clearly visible. The addition of 0.05 wt % of TMB-5 results in a significant decrease of the spherulite dimension and increase of crystallization rate. In Figure 5(a), the particles of the dispersed nucleating agent cannot be detected by POM in melt films pre- pared from the polymer containing TMB-5. After several seconds TMB-5 recrystallized during cooling and quickly induced quantity of the b- nuclei. In Figure 5(c), these bright b nuclei developed in a much faster rate than pure iPP with a diameter 20–30 lm. For well-developed spherulites a very fine morphology does not allow us to determine precisely the size of the spherulites [Fig. 5(d)]. It is interesting to note that PP0.3 shows het- erogeneous nucleation process with needle-like structure in the initial step of crystallization as shown in Figure 6. The dissolved nucleating agent recrystallizes from the melt in the form of Figure 3. b relative content (Kb) of samples crystal- lized at different temperature. The TMB-5 content is listed upright. BEHAVIOR AND DEVELOPMENT OF iPP 1729 Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb Figure 4. Spherulite morphology of pure iPP with various crystallization times at 120 8C, magnification 3100. Figure 5. Spherulite morphology of PP0.05 with various crystallization times at 120 8C, magnification 3100. 1730 DONG ET AL. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb needle-like crystals. The birefringent transcrys- talline structure forms on the lateral surface of the needle crystals, while a ‘‘flower-like’’ struc- ture appears around the tips of the needles. These structures can be unambiguously regarded as b-iPP by the XRD results in Figure 2. Varga observed that ab-transcrystalline layer developed on the lateral surface of the needle crystals of nucleating agent and attributed this to dual nucleating nucleation ability of nucleat- ing agent.42 In our case, when crystallized at 120 8C, the b-modification, which grows faster than the a phase, occludes completely the pri- mary a-crystallites as crystallization proceeds. Later, the growing crystalline front contains only the b-phase of iPP. a-crystallites cannot be detected in the b-phase formed after occlusion. In the case of PP0.6, the similar phenomenon was observed, accompanied with denser needle structures. It is