in the Crystal Growth of Ammonium Perchlorate - ACS Publications

Morphology-controlled ammonium perchlorate (AP) particles were synthesized by using a water-soluble poly(vinyl alcohol) (PVA) polymer as a crystal-hab...
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Role of Poly(vinyl alcohol) in the Crystal Growth of Ammonium Perchlorate Anuj A. Vargeese, Satyawati. S. Joshi,* and V. N. Krishnamurthy Department of Chemistry, UniVersity of Pune, Pune 411007, India

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 1060–1066

ReceiVed May 18, 2007; ReVised Manuscript ReceiVed NoVember 2, 2007

ABSTRACT: One of the key aspects in the development of new inorganic materials is the synthesis of particles with specific sizes and morphologies. Nowadays, water-soluble polymers as crystal-growth modifiers are gaining more importance in the field of morphogenesis of inorganic crystals. Ammonium perchlorate (AP) is an extensively used solid propellant oxidizer. Oxidizer particles with a particular morphology are a challenging requirement in the propellant field. In the present study, AP with different morphologies was synthesized by the polymer-assisted crystal-habit modification route. Poly(vinyl alcohol) and AP salt solutions were prepared and mixed in different proportions. Samples were drawn from the mixture, dried on glass slides, and analyzed by scanning electron microscopy and powder X-ray diffraction (PXRD). AP crystals with a rectangular prism and a rectangular wedge shape were obtained, and the reasons for their shapes are discussed. The PXRD patterns showed that the phase of AP obtained is the same as the one obtained from its saturated solutions. Introduction The synthesis of inorganic materials with a specific size and morphology has recently received much attention in the material science research area. Morphology control or morphogenesis is more important for the chemical industry than size control. Many routes have been reported to control the crystal growth and eventually modify the morphology of the crystals. For crystal-habit modification, crystals are grown in the presence of naturally occurring soluble additives, which usually adsorb or bind to the crystal faces and influence the crystal growth or morphology. A number of recent investigations show that such type of crystal-habit modifiers can be used to obtain inorganic crystals with organized assemblies.1–3 The crystal-habit modifiers may be of a very diverse nature, such as multivalent cations, complexes, surface active agents, soluble polymers, biologically active macromolecules, fine particles of sparingly soluble salts, and so on.4 These crystal modifiers often adsorb selectively on to different crystal faces and retard their growth rates, thereby influencing the final morphology of the crystals.2 The strategy that uses organic additives and/or templates with complex functionalization patterns to control the nucleation, growth, and alignment of inorganic crystals has been universally applied for the biomimetic synthesis of inorganic materials with complex forms.5 The biomimetic process uses an organized supramolecular matrix and produces inorganic crystals with characteristic morphologies.1,6 Understanding the mechanism involved in such a matrix-mediated synthesis has a great potential in the production of engineering materials. Thus, catalyst particles of controlled size and morphology, magnetic materials with appropriate anisotropy, highly porous materials, composites, and well-organized crystallite assemblies can be produced by this synthesis method.7 Using water-soluble polymers as crystal modifiers for controlled crystallization is widely expanding and becoming a benign route for controlling and designing the architectures of inorganic materials.2 Investigators have used different double hydrophilic block copolymers, such as poly(ethylene glycol)block-poly(methacrylic acid), to control the morphology of a * Corresponding Author. Phone: +91-20-25601230 (Ext. 573/569). Fax: +9120-25691728. E-mail: [email protected].

number of inorganic salts, namely, CaCO3,8–16 BaCO3, CdCO3, MnCO3, PbCO3,17 BaCrO4,18,19 BaSO4,20–23 tolazamide,24 PbWO4,25 and so forth. In the early stages, gel matrices have been used for the control of nucleation and morphology in aqueoussolution-based crystal growth.26,27 Investigators have used poly(vinyl alcohol) (PVA)-, agar-, gelatin-, and pectin-based gel matrices to control the morphology of inorganic crystals such as PbI2, AgI, Ag2Cr2O7, PbSO4, PbCl2, and so forth.27 The advantage of a gel medium is believed to be the reduction of the nucleation rate and suppression of convection.26 The functional groups, such as amine, amide, carboxylic acid, and so forth, are known to significantly influence the mineralization process.1 Among the reported common gel matrices used as crystal-habit modifiers, PVA is a water soluble synthetic polymer with excellent film-forming and emulsifying properties. PVA is a crystalline polymer with a monoclinic structure and is known for its biological activities.28 Also, PVA is reported to have been used for the morphology control of K2Cr2O7, AgBr, and CaCO3,29 and even for the selective nucleation of CaCO3 polymorphs.30 The morphology-controlling effect of PVA is long known and utilized as a capping agent during the synthesis of nanoparticles. Investigators have successfully synthesized morphology-controlled silver and copper metal nanoparticles by using PVA as a capping agent.31,32 In the presence of crystalhabit-modifying polymers, the crystal growth or nucleation is diverted from the non-uniform to a uniform shape. In most of the earlier studies using PVA as a crystal-habit modifier, a gel matrix made out of PVA has been used for the control of nucleation and morphology in aqueous-solution-based crystal growth.7,28–32 Ammonium perchlorate (AP) is one of the most extensively used solid propellant oxidizers in the propellant industry. The percentage of oxidizer in the propellant formulation varies from 70 to 80% by weight, depending on the energetic requirements and compatibility with the other ingredients. Because of the high percentage in the propellant formulation, the performance of the propellants (specific impulse and burning rate) varies with the oxidizer properties, and in turn, the performance of the oxidizer varies with the particles’ size and morphology.34 Hence,

10.1021/cg070464+ CCC: $40.75  2008 American Chemical Society Published on Web 01/23/2008

Role of PVA in Ammonium Perchlorate Crystal Growth

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Figure 1. SEM images of crystals grown from mixture A (PVA 14000) after (a) 0 h and (b) 24 h.

Figure 2. SEM images of crystals grown from mixture C (PVA 14000) after (a) 0 h and (b) 24 h.

in the present investigation, PVA has been used as a supermolecular matrix to control the morphology of AP. Experimental Section The AP used was synthesized by the double decomposition reaction of ammonium chloride and sodium perchlorate. The sodium perchlorate used for the experiment was synthesized by the reaction between perchloric acid and sodium carbonate. The reagents used for the experiment were of AR grade and used as obtained from the manufacturer. PVA with two different average molecular masses, 14 000 (PVA 14000) and 125 000 (PVA 125000), were used for the current study. An aqueous solution containing 2.5 wt% PVA was prepared by dissolving the polymer in distilled water at 80 °C, and an AP solution of 1 M concentration was prepared by dissolving a weighed amount of AP crystals in distilled water. The salt solution was mixed with the aqueous PVA solution in various proportions. Five different proportions, with the salt-to-PVA solution ratio varying from 20 to 80% (20:80, mixture A; 40:60, mixture B; 50:50, mixture C; 60:40, mixture D; and 80:20, mixture E), were made and thoroughly mixed by vigorous stirring. The temperature during the mixing time was kept near 80 °C by keeping the mixture in a water bath. The mixtures were prepared with both molecular masses of PVA, PVA 14000 and PVA 125000. After thorough mixing of the solutions, two samples were drawn from the mixture. The first sample was drawn immediately after the mixing, put on a glass slide, and dried in a desiccator for one day before doing the analyses. After the first sample was drawn, the mixture was cooled to room temperature and then kept in sealed conditions. The second sample was drawn after 24 h. It was also put on a glass slide and dried in a desiccator. The dried samples on the glass slides were characterized by powder X-ray diffraction (PXRD) with a Bruker AXS D8 Advance XRD instrument to study the composition and structure of the compound. Each sample was scanned over a 2θ range of 10–70° with a sampling interval of 0.01°. The PXRD reflections were obtained at room temperature (25 °C). From the obtained PXRD patterns, the interplanar distances (d) were calculated by Bragg’s equation and compared with the ASTM data. Scanning electron microscopy (SEM) images were obtained at two different magnifications, 30× and 100×,

on a JSM 6360A scanning electron microscope and were used to understand the morphology of the AP particles. In the present study, AP particles with different morphologies were obtained by the polymerassisted crystal-habit modification route.

Results and Discussion The morphology of crystals depends on the growth rates of different crystallographic faces. The growth of a given face is governed by the crystal structure and defects on one hand and by environmental conditions on the other hand.33 AP is the most commonly used rocket propellant oxidizer and one of the extensively studied ammonium compounds. The morphology of the oxidizer has an important role in the formulation and performance of solid propellants, and the AP crystallized from its saturated solution gives needle-shaped crystals. The nucleation of the crystals was observed immediately after drying began. The crystals grown in the PVA showed entirely different morphologies, such as rectangular prism and rectangular wedge, in comparison to the morphologies of the AP crystals grown in the absence of PVA. The SEM images obtained are shown in Figures 1–6. Three different sets of SEM images were chosen for different concentrations, such as a low salt concentration, equal salt-polymer concentration, and high salt concentration (crystals grown from mixtures A, C, and E). The SEM images of the crystals grown from mixture A were obtained at 30× magnification, and the images of the crystals grown from mixtures C and E were obtained at 100× magnification. The images of the crystals grown immediately after mixing the solution and after 24 h of reaction time are shown in panels a and b of each figure, respectively. Figures 1–3 show the images of the crystals grown from PVA 14000. Figure 1 shows that the crystals have an irregular morphology and do not have any growth orientation toward a particular plane. The images also indicate that the crystals have an irregular

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Figure 3. SEM images of crystals grown from mixture E (PVA 14000) after (a) 0 h and (b) 24 h.

Figure 4. SEM images of crystals grown from mixture A (PVA 125000) after (a) 0 h and (b) 24 h.

Figure 5. SEM images of crystals grown from mixture C (PVA 125000) after (a) 0 h and (b) 24 h.

Figure 6. SEM images of crystals grown from mixture E (PVA 125000) after (a) 0 h and (b) 24 h.

shape, although they tend to grow in an organized manner. This could be due to the polymer-substrate interaction that prevents the crystals from growing in an organized manner. At low salt concentrations, there is too much hydrogen bonding between

the hydroxyl groups of the polymer and the hydrogen of the ammonium ion. Adsorption characteristics of polymers are different from those of other systems because of the polymers’ flexibility. In addition to the usual adsorption factors, such as

Role of PVA in Ammonium Perchlorate Crystal Growth

adsorbate–adsorbent and adsorbate-solvent interactions, a major aspect is the conformation of molecules at the interface and its role in dispersion. PVA is a flexible linear molecule with no charge and which can potentially adsorb on the surface. Bridging is considered to be a consequence of the adsorption of individual intermolecular polymer molecules on the surface. This happens through hydrogen bonding. Because of the high polymer concentration, not all the segments of the polymer are in direct contact with the surface. Also, the diffusion of ions is slow at high polymer concentrations. The possibility that the solution does not contain enough AP to grow in an organized manner cannot be ruled out. The viscous nature of the solutions containing a large quantity of PVA polymer leads to rectangularwedge-shaped crystals because diffusion is predominant and convection is suppressed for the transformation of solutes. As seen from the SEM images, the salt-to-polymer solution ratio change is reflected in the morphology of the crystals. Although the crystals grown from mixture A have an irregular morphology, the particles grown from the other compositions contain individual crystals with a specific morphology. The crystals grown from mixture C have a rectangular prism shape, whereas the crystals grown from the solution containing a high salt concentration (mixture E) have a rectangular wedge shape. The crystals grown from mixtures B and D also show a more or less similar morphology, comparable to the crystals grown from mixtures C and E, respectively. Hence, the crystals grown from mixtures B and D are not discussed here. The crystals obtained from mixture E have grown in an another plane, leading to a rectangular wedge-like morphology in response to the change in polymer-to-salt ratio. This outgrowth to another plane of crystals obtained from mixture E is one of the observed modifications from the crystals grown from mixture C. The morphological evolution of the crystals from rectangular prism to wedge-like shape clearly shows the dependency of the particles’ morphology on the polymer concentration and the minimum salt concentration requirement. Letting the solutions stand for 24 h at ambient condition in a closed vessel before the samples are drawn significantly influences the morphology of the AP crystals. Usually, the change in reaction time is reflected as a slight change in the morphology and size of the crystals. The tendency of AP crystals to grow in a rectangular wedge-like structure seems to be higher after the reaction time is increased. The images of the crystals grown after 24 h are shown in Figures 2b and 3b. Even if the general morphology of the particles is a rectangular wedge, some defects or irregular patterns are seen on the surface of the crystals. But the defects observed in the immediately dried crystals appear to be reduced considerably after the reaction time is increased to 24 h. The SEM images of the crystals with PVA 125000 are shown in Figures 4–6. The images of the crystals grown from mixture A are shown in Figure 4. The crystals immediately grown from the mixture are shown in panel a and the crystals grown after 24 h are shown in panel b. Figures 5 and 6 show the crystals grown from mixtures C and E, respectively. In this case also, the crystals obtained from mixture A have an irregular morphology, confirming the requirement of a minimum quantity of salt for a significant polymer-substrate interaction. Even if most of the crystals grown from mixture A have an irregular shape, the SEM image shows a slightly organized growth of crystals. Also, these crystals seem to grow as twin crystals. The crystals grown from mixture C have a rectangular wedge morphology. This is a variation from the crystals grown from mixture C containing PVA 14000. After the reaction mixture was allowed

Crystal Growth & Design, Vol. 8, No. 3, 2008 1063 Table 1. Habit Modification Effect of PVA 14000 and PVA 125000 on AP

crystal (1 M soln) AP AP AP AP AP AP

habit modifier (2.5 wt% solution) PVA PVA PVA PVA PVA PVA

14000 14000 14000 125000 125000 125000

habit modification salt/PVA observed 20:80 50:50 80:20 20:80 50:50 80:20

no yes yes no yes yes

morphology irregular rectangular rectangular irregular rectangular rectangular

prism wedge wedge wedge

to stand for 24 h, the crystals grown from PVA 125000 show more tendency to grow in a rectangular wedge shape. This phenomenon or growth orientation is not significantly exhibited by the crystals grown from PVA 14000 after 24 h. In the case of the crystals grown from PVA 125000, defects are also observed on the surface of the crystals and are reduced significantly after increasing the reaction time. Other than the surface defects exhibited by the crystals, the different reaction times have no significant influence on the morphology of the crystals and lead to almost the same morphological evolutions. The habit modification of PVA on the AP crystals are summarized in Table 1. The PXRD patterns were measured for the AP crystals grown from the solutions containing both PVA 14000 and PVA 125000. The PXRD reflection of crystals grown from mixtures A, C, and E with PVA 14000 are shown in Figures 7–9, respectively. The PXRD patterns measured for the AP crystals grown from mixtures A, C, and E with PVA 1250000 are shown in Figures 10–12, respectively. The crystals grown from the solutions with the same concentration but with different reaction times are grouped together and shown in the images. The crystals grown immediately from the solutions and after 24 h are shown in panels a and b, respectively. The diffraction lines of the AP crystals coincide with those of the ASTM card (JCPDS 43-648) but differ from one another in relative intensities, confirming that the super structure is composed of well-crystallized AP particles and that there is no change in the basic crystal structure. The d values calculated using Bragg’s equation are in good agreement with the JCPDS 43-648 d values.

Figure 7. PXRD pattern of AP grown from mixture A with PVA 14000.

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Figure 8. PXRD pattern of AP grown from mixture C with PVA 14000. Figure 10. PXRD pattern of AP grown from mixture A with PVA 125000.

Figure 9. PXRD pattern of AP grown from mixture E with PVA 14000.

Although the PXRD lines of the crystals grown from both PVA 14000 and PVA 125000 coincide with the ASTM card values, the intensity variations show a preferential growth of the 002 plane. For the crystals grown from mixture C, the growth of nearly all the other planes is suppressed, in comparison to the crystals grown from mixtures A and E, by the polymer except for the 002 plane. This plane suppression or habit inhibition is visible with both PVA 14000 and PVA 125000. A large hump between 20 and 40° is observed in the PXRD patterns, especially in the samples grown from mixtures A and E. This hump is due to the X-ray reaching the glass substrate through a deposited thick film. By contrast, the hump due to the glass substrate is less noticeable in the PXRD pattern of the crystals grown from mixture C, and this could be due to the large number of the (hkl) reflections. This shows that, as the degree of crystallinity (preferred orientation) of the crystals increases, the relative intensity of the hump decreases. In the literature, the 011 plane (JCPDS-ICDD 43-648) of AP is reported to have a higher growth rate in comparison to that of other planes.35 But in the present study, the 011 growth is

Figure 11. PXRD pattern of AP grown from mixture C with PVA 125000.

factually suppressed by the polymer, and the growth of the 002 plane is favored. This eventually shows as a habit-modification effect of PVA on AP crystals. This could be explained by the fact that the polymer might have suppressed the growth of the 011 plane because of the phase inhibition effect, allowing the growth of the 002 plane. Also, the growth of the 600, 314 plane, which has a reported intensity of one, shows a significant growth rate in crystals grown from mixture E with PVA 14000. Although it has been reported that different polymorphs of CaCO3 could be obtained by using PVA, in the case of AP, inhibition properties are seen without phase changes. During the crystallization from saturated solutions, the PXRD patterns confirm the existence of only orthorhombic crystals. The advantage of using PVA as a habit modifier is a reduced agglomeration, leading to samples containing only individual particles. Studies show that, for a chemical system in which PVA and AP molecules are involved, the PVA induces the crystallization of individual AP crystals irrespectively of the

Role of PVA in Ammonium Perchlorate Crystal Growth

Figure 12. PXRD pattern of AP grown from mixture E with PVA 125000.

polymer-substrate concentration variations. Here, the colloidal action of the PVA or the surfactant activity of the polymer could prevent the particles’ agglomeration. The PVA could possibly get adsorbed on the surface, thereby preventing the agglomeration and leading to the formation of individual particles of AP. That is, immediately after the nucleation has started, the polymer might isolate the salt solution into packets and force them to grow as individual particles. The particles obtained in the current studies are much bigger than the particles obtained by using crystal-habit modifiers. In earlier studies, the reported average particle size or the size range of the particles remains somewhere near 10 µm,1–25 whereas the particles’ size obtained in the current study varies from 80 to 500 µm for individual particles. Although the observations show that PVA significantly influences the crystal morphology of AP particles and forces them to grow as individual particles, the polymer does not seem to be useful in producing AP crystals with different particle sizes. The influence of the different parameters, such as the molecular weight of the polymer used, the change in reaction times, and so forth, on the particle size of the crystals grown is negligible compared to that of the habitmodification effect. Although a slight increase in the particle size is seen after the reaction time is increased to 24 h, this phenomenon cannot be generalized. Also, the examination of Figures 1–6 shows that the crystals are larger in the mixtures with a higher salt-to-polymer concentration. It has also been speculated that the densification of the gel medium increases the random noise for crystal growth because the polymer gel matrices disturb the progress of the growing surface.26 The PVA gel forms an organized matrix under the optimum concentration of components, which ensures the homogeneous distribution of the cations in the polymeric network structures. The chain alignment and interchain separation of PVA, which depend on temperature and concentration, lead to the formation of a polymeric matrix with complex structures. These structures chelate the cations through a process of weak chemical bonding (such as van der Waals, hydrogen bonding) and steric entrapment.29 The polymer matrix not only provides an organized surface of mineralization, but also induces a vector growth on the polymeric surface, the direction of which differs from the characteristically preferred direction of the unit cell.36–39 This results in the formation of extend and nonequi-

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librium morphologies, as well as metastable phases with lattice parameters on the order of the spacing available in the polymeric matrix. However, the nucleation of a particular space group on a charged polymeric surface not only depends on the lattice geometry, but also includes spatial charge distribution, hydration, defect sites, and surface relaxation.40 These factors affect the collision frequency and in turn the activation energy for nucleation; hence, the transition state theory might be considered to explain the nucleation of biominerals.29 The shape of inorganic crystals is normally related to the intrinsic unit cell structure, and the crystal shape is usually the outside embodiment of the unit cell replication and amplification.41–43 The diverse crystal morphologies that a mineral, identical to that for calcium carbonate, can have are due to the different surface energies and external growth environments of the crystal faces.44 The morphological evolutions (from irregular to organized crystal assemblies) of the AP crystals are seen in the SEM images. The polymer-substrate interaction is clearly seen in the polymer pattern observed near the crystals. The polymer seems to have grown in the form of dendrites surrounding the crystals with primary and secondary branches. Usually, the rate of nucleation is governed by the temperature, the degree of supersaturation, and the interfacial tension. Crystals often grow from the center of the face and spread outward toward the edges in layers, and these layers may have a thickness of several 1000 Å. During this growth, dissolved impurities may affect the thickness and shape of the layers, which in turn change the morphology of the crystals. Usually, the effect of these impurities is highly specific and depends upon a number of parameters. The growth rate of a crystal face is usually related to its surface energy, if the same growth mechanism acts on each face. The fast growing faces have high surface energies, and they will vanish in the final morphology, and vice versa. This treatment assumes that the equilibrium morphology of a crystal is defined by the minimum energy resulting from the sum of the products of the surface energy and the surface area of all exposed faces (Wulff rule).2 The driving force for this spontaneous oriented attachment is that the elimination of the pairs with a high surface energy will lead to the substantial reduction of the surface free energy from a thermodynamic viewpoint.45,46 The surface roughness on the molecular level is governed by energetic factors arising from fluid-solid interactions at the interface between the crystal and its growth environment. A change in the solvent often changes the crystal habit, and this may sometimes be explained in terms of interface structure changes. The structure of the growing crystal surface at its interface with the growth medium has an important effect on the particular mode of crystal growth adopted. A functional group with a high affinity ensures the anchoring of the molecule on a particular phase, and the polymeric chain protects the surface from coalescing with the next one through electrostatic repulsion or steric hindrance.31 This result suggests a significant interaction between the polymeric hydroxyl groups and the crystallizing AP, resulting in the considerable influence on both the primary crystallization and the superstructure.20 It is possible that the hydroxyl groups of PVA can adsorb onto the growing crystal face and suppress further growth, a mechanism that was observed in the presence of magnesium ions.47 Conclusions Morphology-controlled AP particles were synthesized by using a water-soluble PVA polymer as a crystal-habit modifier.

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Two different average molecular-mass PVA were used, and both significantly influenced the morphology of the crystals obtained. Rectangular wedge/prism-shaped particles were obtained via this method. Although the exact mechanism of the nucleation and growth control of the crystals is not clear, the observed formation of rectangular wedge/prism-shaped particles of AP in PVA matrices with different concentrations may be attributed to the steric control exerted by the PVA matrix and the adsorption of the hydroxyl group onto the growing faces of the AP crystals. Also, the molecular mass does not seem to affect the particle size of the crystals, although it had a considerable role in the morphology control of the AP particles. Acknowledgment. The authors thank the Indian Space Research Organization for its financial support of this study. Supporting Information Available: The d values calculated using Bragg’s equation are tabulated in Tables 3-6. Tables 3 and 4 show the d values of AP crystallized with different proportions of PVA14000. Table 3 is for immediately crystallized AP and Table 4 is for AP crystallized after 24 h. Tables 5 and 6 are for AP crystallized with PVA125000 at two different time intervals. This information is available free of charge via the Internet at http://pubs.acs.org.

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