Article pubs.acs.org/JPCC
Tailor-Made Fibrous Hydroxyapatite/Polydimethylsiloxane Composites: Insight into the Kinetics of Polymerization in the Presence of Filler and Structure−Property Relationship Nabarun Roy† and Anil K. Bhowmick*,†,‡ †
Indian Institute of Technology, Kharagpur-721302, India Department of Chemistry, Indian Institute of Technology, Patna-800013, India
‡
ABSTRACT: A scrupulously manipulated rate acceleration of anionic ring-opening polymerization of octamethylcyclotetrasiloxane (D4), a precursor for polydimethylsiloxane (PDMS) was designed and executed using hydroxyapatite (HA) fiber surface coated with polyethyleneglycol (PEG) as the rate accelerator. Kinetics of polymerization was investigated through programmed FTIR analysis. In this paper, the beneficial role of HA as a polymerization rate promoter was investigated and hence established. In addition, PEG molecules on the filler surface acted as the activator in the polymerization reaction. Hence, a synergism of these two independent effects resulted in a momentous increase in the rate of polymerization. The rate of reaction was found to increase from 6.35 × 10−2 to 10.32 × 10−2 s−1 in the presence of 4 wt % of HA filler. A morphology−property relationship was established for these tailormade composites to explore the diverse fields of application of these advanced materials. Although the in situ prepared sample showed a 260% increase in tensile modulus, an increase of 210% was observed for the ex situ prepared composite at 2 wt % filler loading. Dispersion of HA filler in the polymer matrix is the crucial parameter in improving the physicomechanical properties of the composites.
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INTRODUCTION The method of polydimethylsiloxane (PDMS) synthesis by anionic ring-opening polymerization of cyclic siloxanes is economical from the industrial point of view owing to the facile polymerization technique and comparatively low cost of the monomers. It is worth mentioning that, in this reaction, the nature of the base has an important role to play in rate enhancement and controlling polydispersity of the polymer synthesized. This genre was investigated independently by Grubb and Osthoff1 and Hurd et al.,2 who found that the rate of polymerization is entirely dependent upon the nature of the countercation. For instance, in the ring-opening polymerization of cyclosiloxanes with alkali metal hydroxide (e.g., KOH), the rate of the reaction is dependent upon the size of the cation. This rate variation takes place owing to the fact that ion pairing is less efficient with bulkier cation thereby facilitating ionization. In the polymerization of octamethyltetrasiloxane (D4) with KOH, the active catalyst is the potassium silanolate ion generated in the reaction medium. Self-association of the catalyst, however, leads to a slower rate of polymerization at low temperature and reduced stability of the products.3−5 Enhancement of the reaction rate is coherent with the extent of solubility of the initiator in the medium as observed by Gilbert and Kantor.6 For instance, the inorganic base KOH is effective as an initiator upon imposition of harsh conditions, whereas alkyl ammonium and phosphonium ions might serve as potential candidates even under milder conditions. Innovations in phosphorus chemistry turned out to be a boon in this regard © 2012 American Chemical Society
since the phosphazene base served as a very effective initiator in the ring-opening polymerization of cyclic siloxanes under milder conditions.7−9 In fact, the past decade also evidenced plenty of works in this field with several intricately designed bases for higher yield of the macromolecule with desired microstructure under milder conditions. While Bessmertnykh et al.10 pursued the polymerization reaction of the siloxanes with phosphorus ylides, hexapyrrolidinediphosphazenium hydroxide was used as initiator by Grzelka et al., who executed a detailed kinetics study of the polymerization reaction.11 The literature shows a lot of work pursued by various research groups from then onward till recent times with variations in the chemical structure of the catalyst.12−16 Our contribution to this field includes polymerization of octamethylcyclotetrasiloxane (D4) in the presence of various inorganic and organic fillers i.e., in situ polymerization technique.17−19 Despite that, the effect of the fillers on the rate of anionic polymerization of D4 into PDMS is yet to be realized. In this paper, we made an attempt to investigate the kinetics of the anionic ring-opening polymerization of D4 in the presence of hydroxyapatite fibers. The drive for this attempt originated from the investigation of Datta et al.,20 who illuminated the accelerating effect of the nanoclay additive in the polymerization of ethyl acrylate through atom transfer Received: June 1, 2012 Revised: November 19, 2012 Published: November 29, 2012 26551
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Platinum catalyst (Pt catalyst in U-10, where U-10 is a vinyl PDMS system with molecular weight 74 400 and viscosity 10 Pa s having a hydride content of 0.05 mmol/g) and the hydride cross-linker polymethylhydrogenosiloxane (V430) with the chemical formula Me3Si(OSiMe2)x(OSiMeH)yOSiMe3, where x and y are 10, having hydride content of 4.3 mmol/g were used for curing purposes. This two pack curing system was provided by Momentive Performance Materials, Bangalore, India. Potassium hydroxide and phosphoric acid were procured from Merck, Mumbai, India. Synthesis of Hydroxyapatite Fibers. Polyethylene glycol and organic ammonium halide such as cetyltrimethylammonium bromide have been widely used in HA filler synthesis as evident from the literature.30−34 In this work, cetrimide has been used as the ionic surfactant for the first time for the HA filler synthesis. In a 250 mL beaker, 3.96 g of (NH4)2HPO4 and 6.73 g of cetrimide were dissolved in 125 mL of deionized water by stirring for half an hour. The temperature of the system was maintained at 45 °C. The solution was labeled as I. Another solution was prepared by dissoluting 11.81 g of Ca(NO3)2.4H2O in 175 mL deionized water which was followed by dropwise addition of 50 mL of PEG 400 with constant stirring. This solution was designated as part II. The next step of the procedure involved dropwise addition of II into I, which was followed by formation of a milky suspension. This latter suspension was subjected to autoclave treatment at 120 °C for 5 h. The product was filtered through Buckner funnel fitted with a nylon membrane and washed thoroughly with ethanol and water. The powdered material was oven-dried for 24 h at 80 °C. In Situ Preparation of HA Fiber/PDMS Composite. In a 250 mL three-necked round bottomed flask provided with B-24 joint, a calculated amount of HA fiber was soaked in 15 g of D4 under dry nitrogen atmosphere. Finely ground KOH (0.08 g) was introduced into the reaction medium. A constant temperature of 140 °C was maintained for 4 h. After 2 h, vinyl terminator was added to the reaction through a dry syringe to initiate chain termination. The samples were extracted from the reaction vessel at regular intervals starting from the initiation of the reaction till the addition of the chain terminator and were analyzed through FTIR spectroscopy. The reaction time was extended for 2 h after injection of the vinyl terminator. The reaction once terminated was left undisturbed overnight. The viscous material obtained was acid neutralized and cured with Pt catalyst (0.05 g) and Si−H cross-linker (0.32 g) mediated curing system. Ex Situ Preparation of Composite through Solution Blending. Ex situ composites were prepared through solution mixing technique. A total of 13 g of the synthesized polymer was dissolved in 20 mL of toluene. To this polymer solution was introduced a homogeneous dispersion of HA in toluene (prepared by ultrasonication), the mixture being subjected to continuous stirring. The resultant mixture was stirred for 2 h, sonicated, and cast in a Teflon Petri dish along with the curing agents. The compositions of the samples along with their designations are compiled in Table 1. In the sample designations, VP and H stand respectively for vinyl end-capped PDMS and hydroxyapatite fiber, whereas in situ and ex situ prepared composites are designated by I and E, respectively. The Arabic numerals stand for the wt % of filler loading. Characterization. 29Si Nuclear Magnetic Resonance (NMR) Studies. 29SiNMR was done with a Bruker AM-360,
radical polymerization (ATRP). Hence, in this work we attempted to investigate whether the filler has any beneficial effect in the polymerization reaction by enhancing the rate of reaction. This is expected to be a simpler method not only to perform the complicated reactions efficiently but also to yield a product of much improved properties because of reinforcement. In this era, when polymer has proved its effectiveness as a potential candidate in several biomimetic applications, hydroxyapatite (HA) based polymer composites have been extensively synthesized owing to their aptness in various biomedical fields.21−25 Moreover, PDMS is highly biocompatible and has been in use in biomedical implants since 1960s. Hence, our aim to develop this hybrid material with hydroxyapatite and PDMS as components has an intention to generate elegant material suitable for biomedical applications. Proper dispersion of the filler in a polymer matrix is the basic key for successful composite preparation and hence property improvement.18,26,27 Literature reveals a handful of evidence where dispersion of the filler in the polymer matrix has been significantly improved by in situ polymerization, covalent and noncovalent functionalization of the filler.17,19,28,29 Hence, in this work we tried to investigate the role of these two factors in improving the extent of dispersion of the filler in the PDMS matrix. The novelty of this work is bifaceted. First of all, the literature hardly presents any evidence on the investigation of kinetics of anionic ring-opening polymerization of D4 to PDMS in the presence of filler. Since ring-opening polymerization in this case proceeds through an ionic pathway, the presence of an inorganic filler might have some electronic influence on the polymerization reaction which, in a way, might affect the rate of the reaction. This aspect is still unexcavated in the literature to the best of our knowledge and here lies the uniqueness of the work. Moreover, the paper also provides a detailed understanding of how the extent of hydroxyapatite fiber dispersion is affected by the method of composite preparation and filler surface modification. This type of investigation has not yet been done for this system to date. In this article, for the first time, we carried out a detailed and comprehensive study on the kinetics of in situ polymerization of PDMS in the presence of HA filler. We made an attempt to investigate the role of filler on the rate enhancement of the ring-opening polymerization of D4 in the presence of a base catalyst. In addition, we pursued a detailed structure−property correlation of the composites. Several factors such as in situ polymerization, noncovalent surface modification of the filler and crystallinity of the polymer were found to contribute to the improvement in properties of the composites. These results are corroborated with morphology analysis of the composites.
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EXPERIMENTAL METHODS Materials. HA fibers were synthesized using calcium nitrate tetrahydrate (Ca(NO3)2.4H2O) and diammoniumhydrogenphosphate (NH4)2HPO4. These were supplied by Merck, India. Cetrimide CH3(CH2)13N+(CH3)3 Br− and polyethylene glycol (PEG 400) were used for generating the macromolecular template and were procured from Merck, India. Octamethylcyclotetrasiloxane [(CH3)2SiO] 4 (D4), with boiling point of 175 °C, viscosity of 1.396, density of 0.955 and purity >99% (GC) was obtained from Momentive Performance Materials, Bangalore, India. 1,1,3,3-Tetramethyl-1,3-divinyldisiloxane having a boiling point of 139 °C and density of 0.809 (purity 97%) used as the chain stopper was supplied by Sigma-Aldrich, USA. 26552
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Mechanical Properties. Tensile specimens ASTM D 412-98 were punched out from the solution cast and dried sheets using ASTM Die-C. The tensile tests were executed on a Zwick UTM, model-Z010 (Zwick GmbH and Co., Ulm, Germany) at 25 °C at a cross-head speed of 500 mm/min. The results mentioned are an average of three tests. The error for the measurements is reported in the respective figure. Thermogravimetric Analysis (TGA). TGA analysis of the powdered sample was carried out using a Perkin-Elmer Instrument, Diamond TG-DTA. Around 5 mg of the sample was heated in a programmed manner with a heating rate of 20 °C/min under air atmosphere up to 800 °C. The error for the measurement was found to be ±1 °C.
Table 1. Sample Compositions with Designations sample
amount of HA fiber (wt %)
VPH1I VPH2I VPH4I VPH6I VPH8I VPH1E VPH2E VPH4E VPH6E VPH8E
1 2 4 6 8 1 2 4 6 8
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RESULTS AND DISCUSSION Characterization of the synthesized PDMS. 29Si NMR and GPC Analysis. The synthesized polymer was analyzed through 29Si NMR studies. The results of this analysis have already been reported in our previous publication.17 The signals corresponding to TMS, Si-vinyl, and Si−O−Si (backbone) were found at 0, −4, and −21 ppm, respectively, in the NMR spectrum. Molecular weight was calculated using the end group analysis method and the number average molecular weight was found to be ∼91 000 with a degree of polymerization 1225.The molecular weight of the polymer did not change when polymerization was carried out in presence of the filler. GPC analysis of the synthesized polymer yielded a number average molecular weight Mn of 1 00 776 ± 510 and weight average molecular weight Mw of 1 80 776 ± 720. The polydispersity index PDI was found to be 1.794 ± 0.02. Kinetics of Polymerization of D4 through FTIR Studies. The reaction under consideration in the present study is the anionic ring-opening polymerization of octamethylcyclotetrasiloxane (D4) initiated by KOH. This study focuses on time-bound polymerization kinetics with and without hydroxyapatite in order to exhume the effect of the inorganic filler in the polymerization reaction. The time scale set for the study reveals a first order kinetics of polymerization of D4 without filler.35 The rate of polymerization is influenced by the reaction temperature and the concentration of the initiator. In this study, the effect of filler on the rate of polymerization has been investigated, other conditions being unchanged. Verification of the Polymerization Reaction through FTIR Analysis. The traits in the general absorption patterns in the FTIR spectrum of PDMS and its cyclic precursor D4 are similar since there is no change in the nature of the chemical linkages, except that the strain in the cyclic precursor is minimized by ring-opening during polymerization. However, comparison of the FTIR spectra of PDMS and D4 shows a difference in the absorption pattern in the range of 1100−1000 cm−1. While D4 shows a peak at 1092 cm−1 corresponding to Si−O−Si asymmetric stretching, the polymer generated shows a double humped peak of almost equal intensity in the same region. One hump is found around 1092 cm−1, whereas a new hump appears around 1020 cm−1.36 This change in FTIR pattern warrants a programmed study of the progress of the polymerization reaction through FTIR spectroscopy. The examination of the FTIR plots of the samples in the course of the polymerization reaction shown in Figure 1 reveals an interesting trend in the generation and intensity increase of the peak at 1020 cm−1. The polymerization is initiated after approximately 25 min of attainment of the requisite temper-
400 MHz NMR spectrometer with tetramethylsilane (TMS) as the internal standard. Exact integration was obtained by adding a paramagnetic relaxation agent (chromium acetylacetonate). 50% w/v solutions of the samples in CDCl3 were prepared for analysis. Gel Permeation Chromatography (GPC). GPC analysis of polymer was performed on Shimadzu system equipped with MIXED-D columns (2x PLgel 5 uM, 300 × 7.5 mm)and PLELSD 2100 (Evaporative light scattering detector) as a detector. Polymer solutions were prepared by dissolving in chloroform at a concentration of 2−4 mg/mL. Minute amount of the solution (25 μL) was injected into the column kept at 35 °C. This was eluted with chloroform under isocratic condition at a flow rate of 1 mL/min. The polymer was separated on the basis of molecular size and detected with PL-ELSD. The average Mw of the polymer was obtained by calibrating the column using a narrow set of polystyrene standards. Fourier Transform Infrared (FTIR) Spectroscopy. Samples extracted from the reaction mixture were dissolved in cyclohexane. FT-IR studies were carried out with these aliquots from the reaction mixture using Perkin-Elmer FTIR − spectrophotometer (model spectrum RX I), within the range of 400−4400 cm−1 using a resolution of 4 cm−1. An average of 16 scans was acquired for each sample. In addition, the FTIR spectrum of the powdered sample in the form of KBr pellet was used for analysis. High Resolution Transmission Electron Microscopy (HRTEM). The powdered sample was sonicated in deionized water and a drop of the dispersion was put on a carbon coated copper grid using a microliter syringe. The ultracryomicrotomed composite samples were prepared using a Leica Ultracut UCT (Leica Microsystems GmdH, Vienna, Austria). Microscopy was done with JEOL 2100, Japan. Field Emission Scanning Electron Microscopy (FESEM). Morphology of the powdered sample was studied using a field emission scanning electron microscope (FESEM S4800 Hitachi). The acceleration voltage was set at 10.0 kV with a working distance of 10.1 mm. Dynamic Mechanical Analysis (DMA). The dynamic mechanical analysis of the samples with a typical dimension of 12.59 mm × 6.65 mm × 1.2 mm was pursued using a DMA of TA Instruments (model Q800). A constant frequency of 1 Hz, a strain of 0.05%, and a temperature range from −125 to +50 °C at a heating rate of 3 °C/min was used for sample analysis and the data were analyzed by TA Universal analysis software. Storage modulus (E′) and loss tangent (tan δ) were determined as a function of temperature for the composites under the same conditions. 26553
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dp = k exp( −Ea /RT )f (p) dt
(2)
Ea is the activation energy and R is the universal gas constant. Hence, the widely accepted nth order kinetic equation for polymerization reaction is formulated as38−40 dp = k(1 − p)n dt
(3)
In this work, we have used the nth kinetics model to study the polymerization reaction. To commence with the kinetic study, the absorbance ratio A1020cm−1/A1092cm−1 was calculated. The value of the absorbance ratio was found to exhibit a steady increase followed by attainment of a steady state as shown in Figure 2. The intensity
Figure 2. Comparison of absorbance ratio of the FTIR peaks for PDMS in absence and presence of filler as a function of time. Figure 1. Plot of FTIR spectra of the aliquots during progress of the D4 polymerization reaction as a function of time.
of the peak at 1020 cm−1 showed an abrupt increase in the presence of 1 wt % uncalcined HA filler. The increase in the peak intensity was persistent with increase in filler concentration as was observed for the in situ polymerization in presence of 4 wt % HA. In this study, we tried to investigate the effect of HA fiber on rate enhancement of polymerization. In other words, we tried to scrutinize the effect of the filler in increasing the rate of polymerization reaction. The FTIR analysis of the aliquots extracted from the reaction medium with and without filler at different stages of the polymerization revealed an interesting observation. Figure 3a-b shows respective comparison of the FTIR spectra of the aliquots extracted at two different times (after 14 and 90 min) from initiation of the reaction for the samples with and without filler. It was observed that the peaks at 1092 cm−1 and 814 cm−1 experienced prominent shift in position when polymerization was carried out in presence of HA nanoparticles. The shift in position of the peaks in FTIR spectra of PDMS is probably because of the fact that the Ca2+ ions on the filler surface act as the co-ordinating sites for the monomer molecules. The O atoms in D4 molecules interact with the Ca2+ ions through nonbonding forces of attraction. This kind of interaction between the monomer molecules and the filler provides us with an explanation for the shift in the position of the peaks corresponding to asymmetric Si−O−Si stretching (1092 cm−1) and Si−O−Si skeletal stretching (814 cm−1) respectively toward lower frequency. Hence, it might be postulated that HA fibers act as the template to nucleate the polymerization reaction. The monomer anchorage of the Ca2+
ature for reaction. The reaction initiation is evidenced by the appearance of a small hump around 1020 cm−1. This change in the FTIR profile suggests ring-opening and hence formation of small oligomeric chains. Progress of the polymerization reaction is substantiated by a steady increase in the intensity of this hump. The intensity of the hump at 1092 cm−1, however, does not necessarily change owing to the presence of unchanged total concentration of polymer and its precursor. Here, we initiated a time-bound investigation (a time period of 2 h) since in the composite synthesis the possibility of a propagation step was eliminated after 2 h by addition of the chain terminator. On addition of the filler, there was a change in both the peak position and the intensity which are discussed below. The progressive change in the peak intensities with time during the polymerization reaction of D4 necessitates a detailed kinetic study of polymerization through a controlled FTIR program. In this context, the basics of kinetics of polymerization reaction need a quick review. The rate of the reaction is the function of degree of polymerization and temperature. Hence, the kinetic equation can be formulated as follows:37 dp = k(T )f (p) dt
(1)
where k is the rate constant of the polymerization reaction, T is the temperature of the reaction, and p is the extent of conversion. Under isothermal conditions, the equation assumes the form 26554
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fibers as accelerator in the polymerization reaction was further activated by the PEG molecules. The presence of PEG molecules on the HA fiber surface was verified from FTIR and TGA analysis of the powdered samples as shown in Figure 4. In addition to the presence of absorption
Figure 3. Changes in the nature and position of the peaks for PDMS with time in the presence of HA filler.
Figure 4. (a) FTIR spectrum of HA fibers and (b) TGA and DTG plots of PEG coated HA fibers.
ions of the filler probably aids an easy attack of the D4 molecules by the anions and hence proliferation of the PDMS chains along the filler surface. This probable mechanism of attack enunciates a reason for the increased rate of the reaction in presence of the filler and hence provides a ground for the strong polymer−filler interface formation. The conversion of the monomer to polymer p has been calculated using eq 4 as follows: A − A∞ p=1− t A 0 − A∞ (4)
peaks for PO43‑ moiety of HA in the FTIR spectrum (Figure 4a), additional peaks confirmed the presence of organic PEG coating on the surface of the filler. A quantitative estimation of the amount of PEG wrapped on the surface of the HA was done from the maximum weight loss in the TGA trace of the powdered filler at 480 °C (Figure 4b). Calculations revealed that for 0.016 mol of HA, 0.0014 mol of PEG was wrapped onto the filler surface. Figure 5 shows the proposed mechanism for rate acceleration of PEG molecules in the anionic ringopening polymerization of D4. In order to confirm the accelerating effect of the HA fiber on the ring-opening polymerization of D4, the reaction was carried out in the presence of 4 wt % of filler. The rate constant was found to enhance significantly compared with 1 wt % filler
Here, A0, At, and A∞ are the normalized peak intensities at 0, t, and infinite time. The plot of p at various times follows exactly the same trend as Figure 2 (hence not shown here). The conversion, p, increased drastically at a much earlier stage of the reaction in the presence of the filler. Here it is worth mentioning that the HA filler possessed a layer of PEG molecules on the surface by virtue of its method of preparation. The role of the HA nanoparticles in polymerization rate enhancement is explained earlier. There is an additional factor involved in the rate increase. PEG is also known as “the poor chemist’s crown” due to its crowning effect of various alkali metal ions.41 In this case, the PEG molecules on the filler surface were involved in crowning of the K+ ions of KOH and hence ionizing it. Thus, reaction was initiated at an earlier stage. This kind of fastened reaction initiation is coherent with the observation of Neumann and Sasson.42 Hence, the role of HA
Figure 5. Proposed mechanism for rate acceleration of PEG molecules in anionic ring-opening polymerization of D4 (graphic created using ArgusLab freeware version 4.0.1). 26555
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loaded sample (Table 2). In Table 2, k is the rate constant, Er, standard error of rate constant, and SDF, standard deviation of
morphology and properties of the in situ and ex situ prepared composites. Morphology of the HA Particles and HA Filled PDMS Composites. In order to determine the dimension of the HA fibers, HRTEM and FESEM micrographs were carefully studied and size of the particles was resolved through image analysis. It is well observable from the HRTEM micrograph in Figure 7a
Table 2. Comparison of the Rate Constants of the Polymerization Reaction with and without HA Fiber sample
k (s−1)
VPH0 VPH1I VPH2I VPH4I
6.35 × 10−2 7.84 × 10−2 8.38 × 10−2 10.32 × 10−2
Er 3.19 2.96 4.94 9.11
× × × ×
SDF 10−3 10−3 10−3 10−3
0.322 0.329 0.519 0.785
the linear fit. In order to explain this enhancement, two factors need to be considered, viz., the templating effect of HA fibers and the K+ ion crowning effect of PEG. According to Gokel et al.,43 each PEG molecule, in spite of having more than one binding site, will cause one reaction at one time. Hence, PEG will complex with equivalent moles of KOH required for polymerization. Calculations revealed that the exact amount of PEG required for complete ionization of KOH is 0.0014 mol. This is exactly the amount of PEG present in the case of 1 wt % HA. An excess of PEG will be ineffective in increasing the reaction rate. Thus, when reaction was pursued in presence of 4 wt % of HA, the excess PEG molecules were nonfunctional in rate enhancement thereby ruling out the second factor. Hence, the rate increase in this case was entirely due to the excess amount of HA fibers present in the reaction medium. In the presence of higher concentration of HA particulates, the templating effect was much more significant to fasten polymerization and hence increase in the rate was observed. This rate enhancement is observed from the ln(1 − p) versus time plot for VPH4I plot in Figure 6 and the rate constant values provided in Table 2. The
Figure 7. (a) HRTEM micrograph (scale bar = 100 nm) and (b) FESEM micrograph of HA fibers (scale bar = 3 μm).
that the thickness of the individual particles varies and is around 30−80 nm. This is also confirmed from the FESEM image in Figure 7b. The HA particles, however, are found to aggregate forming larger particles. Hence, the particles formed here are nanoparticles which self-associate to form microparticles. The structure exhibited by the HA fibers is a quasi-nano structure. The mean aspect ratio of the HA fibers was determined through HRTEM analysis using Image J software as mentioned in our previous publication29 and was 10.3. Figure 8a,b are the representative HRTEM images of the ex situ and in situ prepared composites at 2 wt % filler concentration. In the micrographs, the fiber-like features dispersed in the continuous PDMS matrix are HA fibers. For the ex situ prepared composite at 2 wt % fiber loading shown in
Figure 6. Plot of ln(1 − p) as a function of time for the polymerization of D4 in absence and in presence of HA filler.
rate constant for the anionic ring-opening polymerization in absence of HA was found to be 6.35 × 10−2 s−1.The rate constant value increased to 7.84 × 10−2 s−1 when polymerization was carried out in the presence of 1 wt % of PEG coated filler. The composites prepared by initiating polymerization in the presence of HA filler are termed as in situ prepared composites, whereas those prepared by solution casting technique are ex situ prepared composites. The details of the methods of synthesis are provided in the Experimental Methods section. The subsequent sections deal with a comparative study of the
Figure 8. Representative HRTEM images of ex situ and in situ prepared HA fiber/PDMS composites at 2 wt % (a) and (b) and at 8 wt % (c) and (d) HA loadings respectively (scale bar = 100 nm). 26556
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Figure 8a, good filler dispersion was observed. Individual particles with dimension in the range of 30−80 nm were dispersed in abundance in the PDMS matrix along with some bigger fibers (shown by arrows). On the other hand, the in situ prepared composite at 2 wt % filler loading showed better dispersion of HA fibers compared with the ex situ prepared composite. The HRTEM micrograph of the in situ prepared composite is shown in Figure 8b. With increase in fiber concentration, agglomeration was predominant for both composites. For the ex situ prepared composite at 8 wt % filler loading (Figure 8c), individual fibers were observable along with prominent agglomeration. In the case of ex situ prepared composites, however, the fibers are coated with PEG molecules. Hence, dispersion was better compared with the in situ prepared composite at 8 wt % filler loading. Owing to the virtue of the preparative method, the PEG molecules were removed from the surface of HA in case of in situ prepared composite. Hence, HA fibers agglomerated pronouncedly at 8 wt % filler loading as shown in Figure 8d. A thick fiber like structure of more than 200 nm thickness is observed in Figure 8d. It is probably formed by aggregation of numerous HA fibers as some are discernible from the micrograph (shown by arrows). From here also, the phenomenon of quasi-nano structure formation of HA fibers can plausibly be justified. Properties of the in Situ Prepared Composites. Mechanical Properties. Figure 9 shows the plot of tensile
compared with the ex situ prepared ones. The probable reason for the minimal increase in strength and modulus with high filler content for this system is the filler agglomeration at higher concentration. Comparison with ex Situ Composites. The plot of comparison of tensile strength and tensile modulus of the in situ prepared composites and ex situ composites is shown in Figure 9. In this study, the mechanical property data for the composite containing 2 wt % HA are taken from our previous publication.29 The composites prepared by solution casting (ex situ method) exhibited decent but steady increase in tensile strength and modulus with increasing filler loading. At low filler concentration, the composites showed lower magnitude of mechanical properties compared with the in situ prepared composite. However, at higher filler loading property improvement was persistent. As for instance, an improvement of 103% in tensile strength was observed for 8 wt % filler loading. This increase was probably because dispersion is facilitated in this case by the noncovalently attached PEG molecules on the HA filler surface. This phenomenon was absent in the case of in situ prepared composites by the virtue of composite preparation technique. Following the concept of percolation theory for tensile modulus,44−47 the filler particles including their interface regions are considered as percolation clusters.48 At low filler concentration, these percolation clusters are well separated and disconnected from each other. Once filler concentration increases, the effective number and volume fraction of percolation clusters significantly increase. This increase provides well-defined percolation pathways which lead to an increase in tensile modulus. For ex situ prepared composites, good dispersion was observed even at higher filler concentration. Hence proper distribution of the percolation clusters along with increase in effective volume fraction of the latter favored a steady increase in tensile modulus. The in situ prepared composite, on the other hand, showed significant agglomeration at high filler concentration. At 2 wt % filler loading, dispersion was good which facilitated formation of well connected percolation pathways, and hence, a prominent increase in tensile modulus was observed. With increase in filler amount in the polymer matrix, filler agglomeration takes place. The filler aggregates of large size were dispersed quite unevenly in the polymer matrix. This factor accounted for only marginal improvement in tensile modulus for the in situ prepared composites at higher concentration. Tensile strength was found to increase up to a certain concentration of filler which was followed by a steady decrease with increasing concentration for both in situ and ex situ prepared composites. The ex situ prepared composites showed increase in tensile strength up to 4 wt %, whereas for the in situ prepared composites, the increase was restrained at 2 wt %. This was due to better filler dispersion in the case of the ex situ prepared composites even at high filler loading facilitated by the presence of PEG molecules on the filler surface. Dynamic Mechanical Properties. Dynamic mechanical analysis yielded a handful of complicated results as shown in Figure 10a,b. The storage modulus as well as the tan δ plots of the composites show unique trends. As observed from the plot, the in situ prepared composite at 2 wt % filler loading exhibited an improved storage modulus value throughout the entire temperature range. At 25 °C, there was a 57% improvement in modulus. Moreover, modulus improvement was 92% for the same composite at −120 °C. In addition, although the virgin
Figure 9. Comparison of the tensile strength and tensile modulus of the in situ prepared composites with the ex situ composites.
strength and tensile modulus of the in situ prepared composites with increasing filler concentration. Significant improvement in properties was observed for the composites prepared by this in situ polymerization technique. For instance, tensile modulus exhibited enhancement of 260%, 235%, 161%, and 71% for 2, 4, 6, and 8 wt % of HA filler loading. In a similar way, improvement in tensile strength was estimated to be 130%, 89%, 40%, and 21% in the same sequence of filler content. From the results it was observed that, with lower filler content, the improvement in mechanical properties was well noticeable. However, with higher filler volume fraction, tensile strength improvement became less discernible. In situ composite preparation has an added advantage in property improvement since polymerization takes place in presence of the filler.17−19 Polymer chain growth in the presence of the filler results in a stronger polymer−filler interface formation which is the necessary criterion for property improvement in the composites. Hence, in general, in situ prepared composites show more pronounced improvement in the properties 26557
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lower storage modulus values in the low temperature region, even lower than the unfilled cross-linked polymer. The result for 2 wt % is used from our previous publication.29 The unexpectedly reduced low-temperature storage modulus for ex situ composites was probably due to the plasticizing effect of the PEG molecules. This phenomenon of plasticization lowers the low temperature storage modulus of the ex situ composites at low temperature. In addition, improvement in high temperature storage modulus was more distinct for the ex situ composites compared with the in situ prepared ones. This is shown in Figure 10a and Table 3 which is a compilation of the storage modulus values at different temperatures. Table 3. Storage Modulus E′ Values at Different Temperatures storage modulus E′ sample
at −120 °C (MPa)
VPH0 VPH1I VPH2I VPH4I VPH2E VPH4E
1210 ± 11 1603 ± 17 2318 ± 09 1980 ± 22 720 ± 13 651 ± 19
at −75 °C (MPa) 376 583 568 448 170 160
± ± ± ± ± ±
07 15 11 20 15 16
at +25 °C (MPa) 0.164 0.232 0.257 0.243 0.335 0.368
± ± ± ± ± ±
0.004 0.070 0.006 0.009 0.004 0.006
Second, it was observed that the peak height at Tg reduced more prominently for the ex situ composite compared with the in situ prepared one. This was due to better dispersion of the filler in the case of the ex situ prepared composite. Effect of Filler Dispersion and Noncovalent Surface Modification in Composite Property Upliftment. A critical examination of the results revealed that the storage modulus values in the high temperature range were in accordance with the results of the mechanical properties for the in situ prepared composites. This was due to better dispersion of the filler in the composite. However, on slight increase in filler concentration, the mechanical properties drastically deteriorated for the in situ prepared composites. In contrast, the ex situ prepared composites showed a decent and steady increase in tensile strength and modulus values with increasing filler loading. This increase is also explainable in terms of dispersion. In the case of the in situ prepared composites, by virtue of polymerization technique, the improvement in the extent of dispersion was overwhelming particularly at low filler concentration. With the increase in filler concentration, agglomeration was significant. This was reflected in the properties of the composites with varying filler content. The reason for this is that during in situ polymerization the PEG molecules were removed from the surface of the filler. Since HA is very difficult to disperse in the polymer matrix, the in situ composite preparation criterion, though successful in dispersing the filler at lower concentration within the polymer matrix, failed to meet the essential issue of good dispersion at higher filler concentration. Hence, for the in situ prepared composites, property enhancement was significant at lower filler concentration, whereas it was just nominal at higher concentration of filler. However, in the case of ex situ composites good dispersion was assured by the PEG coating on the inorganic filler. This was reflected in the steady property improvement with filler concentration for the ex situ prepared composites. This phenomenon was not reported earlier for such systems.
Figure 10. (a) Comparison of storage modulus versus temperature plots of the in situ and ex situ prepared composites at 2 wt % HA filler loading. (b) Comparison of tan δ versus temperature plots of the in situ and ex situ prepared composites at 2 wt % HA filler loading.
PDMS elastomer shows three well-defined drops, the first two drops (one around −117 °C corresponding to the Tg and the other around −57 °C due to the formation of the crystalline domains) are not quite prominent for the composites. The drop around −50 °C showed complete disappearance for both in situ and ex situ composites. The tan δ plots comparison shown in Figure 10b shows two prominent observations. First, the tan δ plot for the unfilled elastomer exhibits three well-defined peaks corresponding to the drops in the storage modulus plot. The first one at −117 °C corresponds to the glass transition temperature (Tg) of PDMS. The other two peaks at −57 and −30 °C account for the crystalline domain formation and melting respectively.49,50 It is significant that in the tan δ plot of the composites, the second peak is replaced by a “dwarf” hump covering a temperature range of 50 °C. Moreover, the melting peak is also ill-defined compared with the virgin elastomer which is due to restraint in microcrystalline domain formation and growth in the presence of filler.17 The height of the peak at Tg, however, undergoes pronounced reduction accounting for the polymer−filler interaction. Comparison with ex Situ Composites. In comparison with the in situ prepared composites, the ex situ prepared ones at the same filler loading yielded some unpredicted results. While the in situ prepared composites showed improvement in storage modulus in the low temperature zone (below −20 °C), the trend was just the opposite for the ex situ prepared composites. Interestingly, the ex situ composites displayed 26558
dx.doi.org/10.1021/jp305373w | J. Phys. Chem. C 2012, 116, 26551−26560
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CONCLUSIONS In this study, we investigated the kinetics of anionic ringopening polymerization of D4 in presence of HA fibers. The reaction rate was found to be enhanced in the presence of uncalcined HA fibers. The rate enhancement was hypothetically due to the templating effect of the HA fibers which was further supplemented by the crowning of K+ ions by the PEG molecules present on the surface of HA nanoparticles. Although in the absence of the filler the rate constant for the anionic ring-opening polymerization was 6.35 × 10−2 s−1, the rate constant enhanced to 7.84 × 10−2 s−1 in presence of 1 wt % of PEG coated filler. A kinetics study was also executed by varying the concentration of the filler. The rate of reaction increased with increasing filler concentration thereby confirming the role of HA as polymerization rate accelerator. The in situ composite preparation facilitated finer dispersion of the filler at lower concentration, as was evident from the morphology analysis through HRTEM studies. With the increasing filler concentration, however, agglomeration was prominent. The consequence of agglomeration was reflected in the mechanical properties of the in situ prepared composites with prominent improvement in tensile strength and modulus for lower filler loading but having marginal improvement at high filler loading. The ex situ composites, on the other hand, exhibited a decent but steady increase in magnitude of several properties owing to the filler surface modification with low molecular weight polymer molecules.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Phone: 91-3222-283180; 91-612-2277380. Fax: 91-3222-220312; 91612-2277384. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the financial assistance for this work from Council of Scientific and Industrial Research (CSIR), New Delhi, India.
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