Introduction

The exceptional mechanical, physical, thermal and magnetic properties of inorganic/organic polymer hybrid nanocomposites have attracted great attention in recent years1,2,3,4,5. Numerous organo-fuctionalized compound based on silica, silicates, transition metal oxides, carbon, or other inorganic cores have been developed6. Among various organic/inorganic nano building block, polyhedral oligomeric silsesquioxane (POSS) has been recognized for its ability to impart thermal stability in polymer composite7. POSS can be regarded as the smallest silica particle (1–3 nm) with hybrid organic-inorganic nature, whose inorganic core imparts molecular reinforcement while its functionalized organic moieties provide schemes for reaction and compatibilization with the host polymer8,9. The flexible chemistry of POSS renders wide variety of substituents to the silica core and enhanced compatibility is expected when POSS substituents and polymer matrix contain similar moieties10. POSS has been widely incorporated with polyurethanes, polyethylene, polystyrene, poly (methyl methacrylate), polycarbonate and thermosets via various technique like copolymerization, grafting, blending and solvent casting11,12,13,14,15,16,17,18,19,20.

In order to develop thermally stable and flame retardant materials, polycarbonate (PC) has allured great interest due to its high tendency of charring during combustion21. However, further improvement in thermal stability has been achieved by incorporating functionalized POSS using various techniques like copolymerization, melt blending and solvent casting10,22. Though, copolymerization has been effectively employed to yield elevation in flammability, thermal or mechanical property, blending is recognized as inexpensive and simple approach for industrial scale application23,24. For instance, Song et al. have demonstrated the enhancement in thermal degradation and combustion behavior of PC/trisilanolphenyl POSS nanocomposite via melt blending while Hao et al. have elucidated the incorporation of phenethyl POSS in polycarbonate matrix by solvent casting technique10,25. The discrepancy in the dispersion level and POSS compatibility has been attributed to the preferential interaction between polymer matrix and POSS moieties during melt blending. Such interaction has been found limited in solvent casting due to the phase separation.

Although, the incorporation of POSS in polycarbonate has been investigated with polycarbonate for applications ranging from heat sink to the flame retardant materials, there is little literature reporting on in-situ quantification of structural orientation distribution of POSS nanoparticles in polymeric composite systems. Molecular interaction and interplay can be examined by temperature assisted in-situ SAXS analysis. In this study, we have attempted to quantitatively evaluate the molecular interaction and interplay between the intercalated POSS layers in the polycarbonate matrices via temperature assisted small angle x-ray scattering, thermal gravimetric analysis, FESEM and hydrophobic analysis at ambient temperature. We have also proposed the possible mechanism involved in structural changes under the various temperatures. In order to contemplate the phase separation due to high loading of POSS nano unit in PC matrix, the dynamic molecular simulation was accomplished for computing the dynamic thermal interaction and mixing energy at various temperatures. Further, we have speculated that the formulation of free volume in PC matrix enacted pronounced effect on molecular distribution and interplay over the hyperrectangle POSS at a wide range of temperature in PC matrix.

Materials and Experimental Details

Polycarbonate (MFI = 10.5 g/10 min, viscosity 22 cp, LEXAN grade 143R, SABIC Innovative Plastics India Pvt. Ltd, India), Octaphenyl (OP) substituted POSS and Chemicals viz. Chloroform (Sigma Aldrich Corp. USA.) were used for the preparation of samples. All reagents and solvents involved were of analytical grade and were used without any further purification. The optimized amorphous unit cell of PC has been constructed by utilizing forcefield module of Materials Studio as shown in Fig. 1(a) while 3-D atomistic model of POSS has been delineated in Fig. 1(b). The grade of the polycarbonate utilized in this study is the amorphous grade with the melt temperature of 293 °C.

Figure 1
figure 1

(a) Force field Optimized Amorphous Unit Cell of PC constructed by Materials Studio. (b) Atomistic representation of single POSS molecule by Materials Studio where R = Phenyl group. (c) Illustration of developed nanocomposite polymer thickness by Cross-Section FESEM of the polymer nanocomposite.

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Solvent casted PC/Ph-POSS nanocomposite films were fabricated by dissolving 20 wt % of polycarbonate in chloroform followed by incorporating 10 to 30 wt % of Ph-POSS in the prepared polymer solution. The solution was then treated by ultrasonication for 5 min and subsequently casted on a glass slide in closed chamber to avoid abrupt evaporation of solvent. The detached polymer nanocomposite was dried under the vacuum at the boiling temperature of chloroform (61 °C) for 24 hr to remove retained traces of solvent. The maximum loading of Ph-POSS in PC matrix was ascertained to be 30 wt %, beyond this concentration the formation “pin hole” was observed under the scanning electron microscopy for the film thickness of approximately 100 μm as elucidated in Fig. 1(c).

Results and Discussions

It has been ascertained that, the polymeric chain in a solvent cast film can preferentially align in a plane of casting substrate due to the freezing of molecular conformation into the film during the drying process26,27. Prest et al. demonstrated that the evaporation of volatile solvent caused development of entangled polymer chain by the viscosity upturn of the polymer solution followed by film disruption which rendered, unorganized and aggregated structural domain28. These localized aggregation of structural domains presumably lead to the development of partial orderliness as shown in SAXS spectra of PC at ambient temperature in Fig. 2(a). The temperature assisted in-situ small angle X-ray pattern of pristine PC at various temperatures has been exemplified in Fig. 2(a,b). In instance, for all the temperature, the first peak can be associated with the lamellar periodicity produced by those entangled domains and imperfect crystals of polymeric matrix29. As shown in Fig. 2(b), the intensity of the SAXS spectra was contemplated to be exceptionally reduced till 90 °C which can be successfully explained by the hypothesis of the Siegmann et al. Siegmann et al. have annotated that all solution casted PC thin film retains a small amount of solvent and this residual solvent (less than 1%) is sufficient to affect molecular structure of the film30. Therefore, the increase in the temperature results in the disruption of redundant excess free volume formed during the solvent evaporation due to the high degree of molecular mobility when the film was unveiled to the critical boiling temperature of chloroform (61 °C). The molecular mobility of film continues to the 90 °C till all the retained solvent was not completely exhausted from the film. Beyond this temperature we have observed partial change in peak intensity with the constant scattering vector at 0.041 nm. Further, pair distance distribution function was obtained from generalized indirect Fourier transformation technique. The interatomic distances present in the sample and each pair of the molecules accord rise to a peak as shown in Fig. 2(c) 31,32. Figure 2(c) shows three correlation peaks corresponding to the existence of three neighboring molecules at a distance of 1.25 nm, 2.3 nm and 3.35 nm within the maximum distance of 3.75 nm. Bandhopadhyay et al. demonstrated that, the presence of maxima and minima in pair correlation function was likely due to overlapping of the neighboring domains33. The unaltered correlation maxima for all the temperatures may be the unchanged average intermolecular distance, independent of the disruption of the aggregated domain in the presence of retained solvent.

Figure 2
figure 2

SAXS spectra of pristine PC at various temperature.

(a) Full scattering curve (b) Scattering curve for lower q region (c) Pair distance correlation function for pristine PC at various (d) Electron Density correlation function for Pristine PC.

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Polycarbonate has been regarded as crystallizable amorphous polymer with granular structure that can be affected by thermal and mechanical treatment below glass transition temperature34,35,36,37. However, in contrast to the crystallizable polymers, the process of crystallization in polycarbonate is very slow. In our study, we are expecting no change in the amorphous character of PC during its exposure to the temperature assisted SAXS spectra. In order to enumerate the temperature dependence crystal/amorphous ratio during SAXS spectra, electron density correlation function has been employed in conjunction with Bragg’s law as illustrated in eqs 1 and 2 respectively38. This methodology was utilized by Men et al. to calculate linear crystallinity of cold-drawn polyethylene38,39. It must be noted that it is impossible to decide whether the deduced thickness is amorphous or crystalline obtained from correlation function without prior knowledge of the crystallinity.

where K(z) = one dimensional correlation function, Ce(z) is electron density distribution along the stack normal, dac is long spacing obtained from Brag’s law (dac = da + dc, where da and dc are the average thickness of amorphous and crystalline region as shown in Fig. 2d).

The inter-lamellar distance or long spacing values for all the temperature (from 30 °C to 150 °C) obtained from Brag’s law and electron density correlation function are indexed in Table 1. As evidenced form Table 1, the amorphous regions were not found to be altered significantly and it has been postulated that the crystalline region marked in electron density correlation function is very small to demonstrate the effect of crystallite in surrounding amorphous region38. Therefore, we have postulated that there is no significant change in the amorphousity of PC during the in-situ temperature assisted SAXS spectra.

Table 1 Illustration of long spacing value obtained from Bragg’s law and one dimensional correlation function.
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The work of Zheng et al. have demonstrated that octa substituted POSS nanoparticles with various end groups (hydrogen, ethyl, methyl, isopropyl, cyclohexyl or phenyl) form rhombohedral (equivalent hexagonal crystal structure with similar X-ray fingerprints40,41. These cubical silsesquioxane can be regarded as nearly spherical unit which are arranged in a plane on a hexagonal array. The HRTEM image of the stacked POSS units has been shown in Fig. 3(a,b). POSS containing polymer nanocomposite acquire high thermal and mechanical stability either from aggregates of POSS nano units into the larger clusters or the degree of compatibility with the host polymer components41. In agreement, mechanically reinforced Ph-POSS nano units in PC polymer matrix has been elucidated in Fig. 3(c) which demonstrates flocculated, overlapping POSS nano units in various domains. We have also perceived analogous results in SAXS pattern of the developed polymer nanocomposite. In contrast to the SAXS spectra of PC, the stacking of POSS units form crystalline aggregates in the polymer matrix, can be clearly observed in the SAXS spectra as shown in Fig. 3(d,e). Figure 3(d) exhibits the enhancement in the SAXS intensity at lower q region resulting from crystalize POSS aggregates which has been also illustrated in Table 2.

Table 2 The intensity comparison between pristine PC and PC/POSS polymer nanocomposite at lower q region.
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Figure 3
figure 3

(a) TEM image of the single POSS unit demonstrating hyper-rectangle structure (b) HRTEM image of the single POSS moiety. (c) FESEM images of polymer Ph-POSS/PC nanocomposite (d) Full scattering pattern for polymer nanocomposite (PC/POSS) for various temperature (e) Scattering pattern for lower q region, exemplifying lamellar periodicity of PC at lower q region (f) SAXS pattern delineating primary and secondary aggregates at various temperature (higher q region).

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According to the Gibbs phase rule, there are only two possible phases at constant temperature and pressure. Therefore, we have annotated that the incorporation of utmost possible POSS loading in 100 μm thin film accorded non uniform distribution of POSS units and formed flocculated clusters with two different phases (POSS rich domain and PC rich phase). In addition, the duplex peak in SAXS spectra of nanocomposite at higher q region (Fig. 3f) has been interpreted as peak obtained from primary domain and secondary domain. As stated earlier, the evaporation of the volatile solvent can cause the localized aggregation of the structural domain which presumably lead to the development of primary and secondary domain observed in PC/POSS composite as shown in Fig. 3(d).

As annotated by Prest et al. as solvent evaporates, the molecules of the film reduce their ability to respond the increased relaxation time26. Further evaporation of the solvent locks out the molecules at their respective positons and they are no longer able to relax to their new equilibrium state. Based on this postulation, we have concluded that primary domains are associated with those aggregates which are locked in the matrix with the higher concentration. While secondary domains correspond to the lower concentration of POSS unit with the reduced inter lamellar stacking as delineated in Table 2. SAXS spectra of the primary domains renders intensified and stable peak with constant inter lamellar distance of 11.14 Å till 150 °C while the alteration of secondary peak followed by its abundance with temperature augmentation was noticed for secondary domain. As stated earlier, polymer nanocomposite contains various domains, coined as primary and secondary domain whose temperature assisted conducts has been substantially controlled by available free volume in both the domains for given volume at any instant. In this context, we have contemplated that primary domains are dominated by POSS rich phase and secondary POSS domains are dominated by PC rich phase. In the consequence of increasing temperature from ambient to 60 °C, we have postulated intercalation of PC moieties in stacked hyper-rectangle POSS units resulting in enhanced inter lamellar distance in contrast to consistent primary aggregates. Further increased temperature i.e. 90 °C can be assigned as a critical temperature for developed polymer nanocomposite because at this temperature we have discerned intensified SAXS spectra for primary aggregates, speculated to the partial reduction in interfacial layer that continues to120 °C and endowed to be constant till 150 °C. While disruption of secondary domain extended till 150 °C by the virtue of continuous intercalation of PC unit in stacked POSS moieties. It is important to note that, diminished SAXS spectra at lower scattering vector region was ascertained beyond 120 °C as elucidate in Fig. 3(f). This phenomenon can be associated to the thermally assisted randomization of PC rich domains due to the co-operative movement of the phenyl group with the molecular motion of the carbonyl group42. Therefore, secondary domain undergoes complete disruption and fragmentation with the reduced extractability under the SAXS spectra at 180 °C due to the segmental movement of the polymer matrix without altering primary domain. At the 180 °C temperature we have noticed single spectra at higher scattering vector region with reduced inter lamellar distance due to the complete disruption of secondary domains.

In order to substantiate the phase separation in polymer nanocomposite, thermodynamic interaction parameter or solubility parameter (χ) and mixing energy (Emix) has been delineated via Materials Studio that utilizes off lattice method which combined extended Flory Huggins model and molecular simulation technique. Utracki has evinced that the conversion of room temperature to high temperature impart the effect of decreasing solubility parameter43,44. We have obtained analogous result of solubility parameter with the increase in the temperature as exemplified in Fig. 4. It has been ascertained that the positive χ and Emix value obtained at ambient temperature allied to the phase separation as conferred in SAXS spectra of polymer nanocomposite previously while reduction in the value with the amelioration in temperature relinquish some extent of intercalation or mixing45,46.

Figure 4
figure 4

Computation of Thermodynamic Interaction Parameter (χ ) and Mixing Energy (Emix) via Materials Studio.

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Thermal stability and thermal oxidative degradation of the nacreous composite were delineated by thermal gravimetric analysis as shown in Fig. 5. The obtained result reveals that the initiation of thermal decomposition and temperature at maximum decomposition was eloquently improved in nano Ph-PC/POSS polymer nanocomposite by the formation of silicon containing heat shield layer a high temperature. In this framework, the suppressed mass loss was imputed by uncovering of the entrapped nano asperities of POSS by the complete mass loss of PC matrix followed by conversion of POSS in silicates after degradation47,48,49,50. The suppressed mass loss can also be ascertained by the ordered distribution of POSS in PC lamellae space which yielded shielding effect until the external temperature is high enough to the melting point of the polymer matrix which further concluded by the calculation of latent heat of fusion.

Figure 5
figure 5

Thermal gravimetric analysis of pristine PC and Ph-POSS/PC nanocomposite.

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The hydrophobic characteristics of nanocomposite were outlined by measuring the contact angle of the sessile drop based on the apprehension developed by Balasubramanian et al. They have extensively explored the contact angle of various surfaces which predominantly depends on the surface roughness, surface energy and surface tension based on the proposed model of Cassie and Baxter51,52,53. In this context, we have contemplated hydrophobic attribute of developed polymer nano composite based on alteration in surface energy by the amalgamation of POSS units in PC matrix. Mishra et al. in their abstraction manifested that increasing POSS content displayed the ability to reduce the surface energy of the polymer nanocomposite54. We have noticed analogous effect when Ph-POSS was incorporated in PC matrix, which yield reduction (surface energy of pristine PC- 32 ± 2 mN/m and for nanocomposite 23 ± 3 mN/m) in surface energy, contributing to the enhancement in contact angle by 11% of enhanced contact angle as demonstrated in Fig. 6(a,b). The peculiarity of hydrophobic surface is commonly interpreted via the hypothesis of Cassie-Baxter state and Wenzel state intended by Wenzels and Cassie–Baxter. Cassie-Baxter state unravel the state in which the liquid does not penetrate hallows of corrugated surface while Wenzel state exist when liquid is in contact with entire exposed surface of the solids55,56,57. In disparity to aforementioned hypothesis, we assumed that polymer nanocomposite in our abstraction pursue the perception of Gao et al., which report that the contact angle of sessile drop on solid surface depends on the three phase contact line formed due to the perpetuation of the drop on surface58,59,60,61. As previously stated, polymer nanocomposite encompasses primary and secondary aggregated domain in PC matrix, therefore, we envision that instead of conceding the formation of Cassie-Baxter and Wenzel state, the sessile drop will form the measurable contact by the virtue of contact line pinning in the presence of stacked POSS units.

Figure 6
figure 6

Contact angle measurement for (a) pristine PC and (b) Ph-POSS/PC composite.

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Conclusions

In this contemplation, we have delineated the temperature assisted in-situ SAXS spectra of solvent casted phenyl substituted POSS/Polycarbonate nanocomposite for first time to deduce the possible structural changes and interplay of POSS moieties in PC matrix. We have observed the formation of duplex peak at higher scattering vector region which was associated to the strongly bonded primary domain and loosely bonded secondary domain containing various amount of free volume via stacking of hyper-rectangle POSS unit. The increase in the local free volume in polymer nanocomposite portrayed indispensable conduct under the advancement of temperature. The secondary peak was found to be abundant and un-extractable with temperature augmentation during temperature assisted in-situ SAXS analysis that is presumably assumed for those clusters which contains sizable free volume by stacking of large number of hyper-rectangular POSS units. As expected, we have contemplated improved thermal stability of the polycarbonate film by incorporating POSS nano units in the matrix. We envision that the hydrophobic attribute of the polymer nanocomposite presumably follows the Gao’s hypothesis instead of conceding the formation of Cassie-Baxter and Wenzel state.

Additional Information

How to cite this article: Yadav, R. et al. Temperature Assisted in-Situ Small Angle X-ray Scattering Analysis of Ph-POSS/PC Polymer Nanocomposite. Sci. Rep. 6, 29917; doi: 10.1038/srep29917 (2016).