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Polymer 53 (2012) 1711e1724

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Polymer

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Self-assembled structure formation from interactions between polyhedraloligomeric silsesquioxane and sorbitol in preparation of polymer compounds

Sayantan Roya, Jie Feng a, Vincenzo Scionti b, Sadhan C. Jana a,*, Chrys Wesdemiotis b

aDepartment of Polymer Engineering, The University of Akron, Akron, OH 44325 0301, USAbDepartment of Chemistry, The University of Akron, Akron, OH 44325 3601, USA

a r t i c l e i n f o

Article history:Received 5 January 2012Received in revised form16 February 2012Accepted 19 February 2012Available online 25 February 2012

Keywords:POSSSelf-assemblySorbitol

* Corresponding author. Tel.: þ330 972 8293; fax:E-mail address: janas@uakron.edu (S.C. Jana).

0032-3861/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.polymer.2012.02.034

a b s t r a c t

This study investigated the factors responsible for molecular interactions leading to self-assemblybetween the molecules of sorbitol and polyhedral oligomeric silsesquioxane (POSS) carrying organicand inorganic side groups. The study also assessed the utility of such molecular adducts as processingaids which will aid preparation of polymer compounds, films, and spun fibers. Several POSS moleculescontaining silanol functionalities and alkyl and phenyl substituents were used to separately evaluate theeffects of hydrogen bonding and p�p interactions on formation of molecular adducts. The molecularadducts were studied using differential scanning calorimetry, wide angle X-ray diffraction, scanningelectron microscopy, mass spectrometry, oscillatory shear rheology, and molecular dynamics simulation.The study revealed that POSS-sorbitol self-assembled structures were formed only when hydrogenbonding and p�p interactions worked cooperatively. In addition, self-assembled molecules were ofamorphous nature although POSS and sorbitol were crystalline.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

“Self-assembly” or “self-organization” can be defined as thespontaneous assembly of two ormoremolecular sub-units via non-covalent interactions into higher-order, supra-molecular novelarchitectures [1e5]. In this context, hydrogen bonding [5e11] andp-stacking interactions [12e19] have been found useful indesigning structures in solutions and in solid-states. The p-stackinginteractions often influence hydrogen bonding by lowering theactivation barrier of the systems [18]. This co-operative interplaybetween hydrogen bonding and p-stacking leads to successfulrealization of supramolecular arrays [20e27].

Self-assembled structures have been studied in conjunctionwith many biological [3,25] and organic systems [1,8,9,11,23].However, their occurrencewith hybrid organic-inorganicmaterials,e.g., polyhedral oligomeric silsesquioxanes (POSS), presents aninteresting opportunity for researchers focusing on synthesis ofpolymeric nanomaterials and nanocomposites [28e41]. Consider-able attention has been given to POSS as filler materials for devel-opment of polymer nanocomposites [30,31,42e57]. Theinteractions in POSS-polymer compounds, the degree of dispersion,and the nature of self-assembly all depend on the nature of pendantorganic groups attached to the POSS molecules [58]. In this context,

þ330 258 2339.

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POSS-type organosilanols (silanol-POSS), especially the incom-pletely condensed silsesquioxanes containing the SieOH sidegroups have been known to form useful building blocks forsupramolecular chemistry [37,59,60]. These molecules are capableof producing hydrogen bonding interactions among themselves orwith other host molecules carrying polar functional groups. It wasalso reported that the complexation ability of silanol-POSS greatlyincreases with increasing the number of SieOH groups in themolecular structures [37].

Our previous work [61,62] showed that stronger hydrogenbonding interactions between sorbitol and SieOH groups promoteddispersion of silanol-POSS molecules in isotactic polypropylene(iPP) melt. The finely dispersed silanol-POSS molecules reduced theshear viscosity of the melt compound and formed spherical parti-cles of a few micrometer in diameter as the melt compound wascooled to room temperature. These particles turned into cylindricalnanoparticles of approximately 100 nm in diameter during fiberspinning of the melt compound. A recent publication [62] docu-mented the experimental evidence of interactions of sorbitolmolecules with tri-silanol phenyl-POSS (tri-POSS), tetra-silanolphenyl-POSS (tetra-POSS), or tri-silanol cyclopentyl-POSS (cyclo-POSS). Such interactions were found to be entirely non-covalenttype and were seen to be promoted by hydrogen bonding andpep stacking although it was not established if such self-assemblyoccurred due to only hydrogen bonding, or pep stacking, or due toco-operative action of pep stacking and hydrogen bonding.

S. Roy et al. / Polymer 53 (2012) 1711e17241712

The silanol-POSS/sorbitol interactions also prevented fibrillationof sorbitol during cooling of iPP melt compound. Note that sorbitolsare well-known nucleating agents for iPP and can form both intra-molecular and intermolecular self-assembled superstructures viahydrogen bonding through the free eOH groups in sorbitol[63e65]. This self-aggregation of sorbitol is believed to be the resultof intermolecular hydrogen bonding andpep interactions betweenthe adjacent phenyl rings in the molecular structure of sorbitol.

This paper expands the analysis covered in our earlier publica-tion [62] to a wider range of silanol-POSS and non-silanol POSSmolecules to establish if the concept of molecular assembly can beextended to POSS molecules containing no phenyl side groups orthose containing no silanol functionalities but with phenyl sidegroups. The following POSS molecules were included: tri-silanolisobutyl-POSS (tri-iso-POSS), di-silanol isobutyl-POSS (di-iso-POSS), octa isobutyl-POSS (iso-POSS), and octa phenyl-POSS(phenyl-POSS), the molecular structures of which are presentedin Fig. 1. The paper answers if hydrogen bonding or pep stackingalone or their combinations are responsible for complex formationbetween POSS and DBS.

The rationales for expanding our study to several POSS mole-cules are presented as follows:

(a) The tri-iso-POSS molecules consist of three silanol functional-ities as in tri-POSS and cyclo-POSS, but the side groups aredifferent, e.g., isobutyl in tri-iso-POSS, phenyl in tri-POSS andcyclo-pentyl in cyclo-POSS. It was expected that isobutylgroups would offer compatibility with iPP chains.

(b) The di-iso-POSS molecules have the same isobutyl substituentgroups as in tri-iso-POSS but with one less silanol functionality.Accordingly, the tri-iso-POSS and di-iso-POSS would offer thesame degree of compatibility with iPP, but less hydrogenbonding possibility with itself or with sorbitol moieties.

Fig. 1. Chemical structure of (a) Tetra-POSS, (b) Tri-POSS, (c) Cyclo-POSS, (d)

(c) The iso-POSS consists of isobutyl side groups but no silanolfunctionalities. As a result, together with the tri-iso-POSS anddi-iso-POSS, this molecule would clearly indicate how thepresence of silanol groups hinder or promote compatibilitywith iPP chains.

(d) The phenyl-POSS has eight phenyl substituent groups and nosilanol functionalities. This molecule, in comparison with tri-POSS and tetra-POSS, would reveal how the presence of sila-nol groups affect the compatibility of phenyl substituted POSSwith iPP. The phenyl-POSS by itself should not providecompatibility with the iPP chains.

Besides tri-, tetra-, and cyclo-POSS molecules previously re-ported [62], these inclusions covered a wider range of POSSmolecules, from di-silanol to tetra-silanol to non-silanol function-alities along with various side groups e.g., phenyl, cyclopentyl, andisobutyl. Thus, the scope of the non-covalent interactions betweenPOSS and sorbitol molecules could be established much morebroadly to aid future research investigations.

2. Experimental

2.1. Materials

Isotactic polypropylene, P4G2Z-159, was obtained in the form ofpellets from Flint Hill Resources (Wichita, Kansas) with MFI 1.95 g/10 min (ASTM D1238), density 0.90 g/cm3 (ASTM D1505), andcrystalline melting temperature (Tm) 165 �C. This grade of iPP didnot contain any nucleating agent.

Several grades of POSS were obtained from Hybrid Plastics(Hattiesburg, MS). The POSS under considerationwere tetra-silanolphenyl-POSS (tetra-POSS, SO 1460), molecular weight (Mw) of1069.5 g/mol and Tm of 260 �C, tri-silanol phenyl-POSS (tri-POSS, SO

Tri-iso-POSS, (e) Di-iso-POSS, (f) Iso-POSS, (g) Phenyl-POSS and (h) DBS.

S. Roy et al. / Polymer 53 (2012) 1711e1724 1713

1458; Mw 930.07 g/mol and Tm w230 �C), tri-silanol cyclopentyl-POSS (cyclo-POSS; SO 1430; Mw 874.7 g/mol and Tmw250 �C), tri-silanol isobutyl POSS (tri-iso-POSS; SO 1450; Mw 791.42 g/moland Tm w195 �C), di-silanol isobutyl POSS (di-iso-POSS; SO 1440;Mw 891.62 g/mol and Tm w100 �C), octa-isobutyl POSS (iso-POSS;MS 0825; Mw 873.6 g/mol and Tm w270 �C), and octa-phenyl POSS(phenyl POSS; MS 0840; Mw 1033.53 g/mol and Tm w173 �C and426 �C). All POSS materials were available in the form of whitepowder. Fig. 1(aeg) presents the chemical structures of the POSSmolecules. Note that tri-POSS, cyclo-POSS, tri-iso-POSS have threesilanol groups inmolecular structures, but they differ in side groupse phenyl for tri-POSS, cyclopentyl for cyclo-POSS, and isobutyl fortri-iso-POSS. Tetra-POSS has four and di-iso-POSS has two silanolgroups with phenyl and isobutyl side groups respectively. On theother hand, phenyl-POSS and iso-POSS do not have silanol groups,but they contain eight phenyl and isobutyl side groups respectively.

The sorbitol-type dispersion aid chosen for this study wasdi(benzylidene)sorbitol (DBS; Millad 3905; Mw ¼ 358.4 g/mol andTm ¼ 225 �C). DBS has two free eOH groups with phenyl side rings.The structure is shown in Fig. 1(h). This was obtained fromMillikenChemicals (Spartanburg, SC) in the form of white powder.

2.2. Preparation of DBSePOSS mixtures

DBS and POSS were intimately mixed at different weight ratiosin solution in tetrahydrofuran (THF). The solvent was evaporatedand the resultant materials were ground and vacuum dried. Onepart of the mixture was kept in the oven at 200 �C for 5 min toimitate the thermal history experienced in typical polymer mixingexperiments. The resultant material was cooled to room tempera-ture and ground for further analysis.

2.3. Polymer compound preparation

A series of iPP/DBS/POSS compounds were produced in aninternal mixer, Brabender Plasticorder. Before mixing, all theingredients were dried for 24 h at 80 �C in vacuum. First, PP wasallowed tomelt in themixer and then dry-blended powder mixtureof DBS and POSS were added in the mixing chamber. The ingredi-ents were mixed for 5 min at 200 �C at an angular speed of 80 rpm.The resultant compound was cooled down to room temperatureand ground into small pellets for further analysis.

2.4. Characterization

2.4.1. Thermal analysisThe melting behavior of POSS, DBS, and their mixtures were

investigated by differential scanning calorimetry (DSC) usingDSC200 from TA Instruments (New Castle, DE) under continuousnitrogen purge at a flow rate of 50 ml/min. The thermal propertiessuch as Tm, glass transition temperature (Tg), if any, and enthalpychange (DHm) in melting were determined at a heating rate of10 �C/min from 30 �C to 300 �C (in case of phenyl-POSS, until450 �C).

For melt-mixed compounds, recrystallization scans were per-formed by cooling the materials from 200 �C to 30 �C at a coolingrate of 10 �C/min. The compound was kept at 200 �C for 5 min tomelt all crystals. The crystallinity of the melt-mixed material wasdetermined under quiescent condition. The percentage of crystal-linity was determined from Eq. (1) by comparing the enthalpy offusion of polymer to 177 J/g, the enthalpy of fusion of 100% crys-talline iPP (DH0) [66].

Crystallinityð%Þ ¼ DHm=DH0 � 100% (1)

2.4.2. Wide angle X-ray diffractionWide angle X-ray diffraction (WAXD) of DBS, POSS, and their

mixtures produced by mixing in solution and with heat historywere investigated using Bruker wide angle X-ray instrument withsealed tube X-ray generator and CuKa radiation of wavelength (l)of 1.54 Å.

2.4.3. Polarized optical microscopyCrystallization of iPP, iPP/DBS, and iPP/DBS/POSS compounds

were examined by polarized optical microscopy (POM). A LeitzLaborlux 12 Pol S (Oberkochen, Germany) optical microscope wasused in cross-polarized mode. Images were acquired with a Diag-nostic Instruments Inc. 11.2 Color Mosaic digital camera (SterlingHeights, MI). Polymer films of thickness around 100 mm wereprepared by compressing the molten specimens between two glassslides on a hot stage at 200 �C. The sample specimenwas heated to240 �C to melt the residual crystals and afterwards cooled down tothe measurement temperature at a cooling rate of 10 �C/min.

2.4.4. Scanning electron microscopyThe morphology of POSSeDBS mixtures were observed by high

resolution scanning electronmicroscopy (SEM) using JEOL JSM5310scanning electron microscope. For this purpose, POSS, DBS, andtheir mixtures were solution cast to reveal the self-assembledstructures. The melt mixed compounds were fractured at roomtemperature after soaking in liquid nitrogen. The surface of thespecimens was sputter-coated by a layer of silver using SputterCoater, Model ISI 5400 under argon gas atmosphere before takingcorresponding images.

2.4.5. Mass spectrometryThe samples designated for mass spectrometry (MS) and ion

mobility mass spectrometry (IMMS) analysis were completely dis-solved in THF. An aliquot ofMeOHwas added to the sample solutionin order to get a final concentration of 0.1 mg/mL in 1:1 (v:v) THF/MeOH. All MS measurements were carried out using an SYNAPTHDMS� hybrid quadrupole/time-of-flight (Q/oa-ToF) mass spec-trometer (Waters, Beverly, MA) equipped with a Z-spray electro-spray source [67]. The instrumental settings were optimized tominimize dissociation of the complexes due to the energy providedduring the ionization. The instrument was operated in positivemode using the following settings: capillary voltage 3.5 kV, conevoltage 35V, sampling cone voltage 3.2 V, source temperature 60 �C,and desolvation gas temperature 100 �C. The sample solutions wereelectrosprayed by direct infusion at a flow rate 15 mL/min.

The ion mobility mass spectrometry (IMMS) experiments wereperformed on the same instrument by activating the travelingwave(T-Wave) separation section located between the two analyzers.This section consists of three cells in the order trap cell, ionmobilitycell, and transfer cell. Once the ions generated in the source or byfragmentation in the trap cell (vide infra) enter the ionmobility cell,they move under the influence of a traveling wave electric field, inthe presence of a drift gas (N2) which flows in the opposite direc-tion of the ionsmotion. Separation takes place according to the size,shape, and charge state of the ions [68]. These parameters deter-mine the drift times of the ions through the ion mobility cell, whichcan be measured and converted to experimental collision cross-section that reflects ion shape and size. The electric field used inthe ionmobility experiments was generated by tuning the travelingwave velocity and the traveling wave height at 350 m/s and15 V respectively, while the nitrogen gas flow rate was set at

S. Roy et al. / Polymer 53 (2012) 1711e17241714

22.7mL/min. For tandemmass spectrometry (MS/MS) studies, withor without TWIM separation, a specific non-covalent complex wasfirst mass-selected by the quadrupole and then fragmented in thetrap collision cell using argon as collision gas. The collision energyapplied to disrupt the complexes under study was varied in therange from 6 to 20 eV, depending on the type of complex,POSSePOSS or POSSeDBS, and its stoichiometry. The fragmentswere mass-analyzed by the ToF analyzer either without or withprior TWIM separation.

2.4.6. Rheological propertiesAn Advanced Rheometric Expansion System (ARES) from TA

Instruments (New Castle, DE), operated with 25 mm parallel plateset up was used to measure the temperature dependence ofcomplex viscosity of the polymer compounds in the range of100e220 �C upon cooling from a homogeneous state at 220 �C.Disk-shaped specimens of 25 mm diameter and 2 mm thicknessweremolded at 200 �C in a compressionmold. The specimens weresubjected to 200 �C only for 5 min during molding. The polymercompound in the rheometer was first heated and kept at 220 �C for5 min to reach equilibrium, and was cooled down to 120 �C ata cooling rate of 10 �C/min. The strain was kept fixed at 2% and thefrequency of oscillatory shear flow was kept at 10 rad/s. A linearviscoelastic regime was earlier [61] confirmed under theseconditions.

3. Results and discussion

3.1. Effect of POSS on DBS fibrillation

It was presented earlier [61,62,69e73] that oscillatory shearrheological data provided first evidences of interactions betweenDBS and POSS as the compounds of iPP, DBS, and POSS were cooledfrom a homogeneous state at 220 �C. The same procedure wasfollowed to include several other POSS molecules. The Fig. 2 showsthe values of complex viscosity as function of temperature forsamples containing different types of POSS (DBS/POSS 1/10 wt/wt).The onset of crystallization in unfilled iPP was inferred froma sudden and abrupt increase in complex viscosity at 120 �C, whileDBS in iPP/DBS (1 wt%) compound caused a distinct increase inviscosity atw195 �C prior to the crystallization of iPP. This was dueto formation of nanofibrillar networks (gel formation) by crystal-lized DBS in the polymer melt [61,70,74].

As reported earlier [62,73,75], DBS fibrillation was subdued incompounds of 5 wt% tri-POSS or 2 wt% tetra-POSS. In these cases,

Fig. 2. Temperature dependence of complex viscosity (|h*|) of iPP compounds. Thedata were taken at 2% strain and frequency of 10 rad/s. The vertical lines dropped to x-axis indicate the temperatures at which DBS fibrils began to form.

the viscosity of the compounds did not show an abrupt increaseattributed to DBS fibrillation until iPP began crystallizing at 120 �C.It was also identified in our recent publication [62] that the cyclo-POSS molecules containing silanol groups could not arrest DBSfibrillation even when 10 wt% cyclo-POSS was present in thecompound. This indicated that pep interactions between thephenyl groups of DBS and phenyl groups in tri- and tetra-POSSmolecules might have caused such differences. The pep interac-tion was not possible in compounds of cyclo-POSS, tri-iso-POSS,and di-iso-POSS.

The |h*| vs. T data in Fig. 2 show that both tri-iso- and di-iso-POSS were not able to subdue DBS fibrillation. DBS fibrillationwas inferred from an abrupt increase in viscosity before crystalli-zation of iPP at 170 �C and 190 �C respectively in the presence of10 wt% tri-iso-POSS and di-iso-POSS. The lower viscosity of thecompounds in these cases can be explained in view of the lowmelting points of both tri-iso- and di-iso-POSS which remainedliquid during the experimental conditions (Table 1). Note thatsolubility of tri-iso-POSS and di-iso-POSS in iPP melt would explainthe lowering of compound viscosity in Fig. 2. The tri-iso-POSS wasable to subdue DBS fibrillation in iPP to some extent and more thanthe di-iso-POSS. This is revealed from lower DBS fibrillationtemperature (165 �C vs. 185 �C) in the former. This can be attributedto 30% more silanol groups present in tri-iso-POSS moleculescompared to di-iso-POSS molecules that calls for more abundanthydrogen bonding interactions [37]. Note that non-covalent inter-action between iso-POSS (with no silanol functionalities or phenylsubstituents) and DBS was not expected as reported in earlierstudies [61,70,72]. Non-silanol POSS with substituent phenylgroups (phenyl-POSS) was included in the study to identify if pepstacking was the only contributing factor towards the interactions.As seen in Fig. 2, 10 wt% phenyl-POSS was able to lower thetemperature of abrupt increase in viscosity from w195 �C (iPP/DBS1wt%) tow170 �C. However, it was not sufficient to subdue the DBSfibrillation completely as found in the cases of tri- and tetra-POSSearlier [62,73,75]. This supports the fact that both silanol groupsand phenyl substituents present in the POSS molecules are neededto produce abundant supramolecular complexes with sorbitolsimilar to the phenomenon reported earlier [62]. As will be iden-tified later, molecular dynamics simulation can be used to monitorhow DBS and tri- and tetra-POSSmolecules engage in simultaneouspep interactions and hydrogen bonding.

From inspection of shear viscosity data in Fig. 2, it appears thatphenyl-POSS showed interaction comparable with tri-iso-POSSunder consideration. However, the systems are more complicatedthan those apparent from the rheological study. It will be presentedwhile discussing mass spectrometry analysis that abundantcomplex formation was associated with DBS and phenyl-POSS, butthe presence of higher melting crystals (Table 1) in phenyl-POSSmolecules would not allow the melting transition [62]. Thusphenyl-POSS and DBS remained as separate phases, althoughmolecular complexes were possible due to pep interactions.

Table 1DSC data on melting of as received specimens and after heating at 200 �C for 5 min(heated).

Component Status Melting transition;Tm (�C)

DHm (J/g)

Tri-iso-POSS As received 195 44Heated 189 36

Di-iso-POSS As received 81, 97 17, 26Heated 70, 92 2, 8

Iso-POSS As received 270 46Heated 270 45

Phenyl-POSS As received 173, 426 49, 32Heated 427 29

Fig. 3. WAXD patterns of (a) tri-iso-POSS, (b) heat treated tri-iso-POSS, (c) di-iso-POSS, (d) heat treated di-iso-POSS, (e) iso-POSS, (f) heat treated iso-POSS, (g) phenyl-POSS (h) heattreated phenyl-POSS; at 200 �C for 5 min.

S. Roy et al. / Polymer 53 (2012) 1711e1724 1715

3.2. Investigation of crystallinity and crystalline order

POSSeDBS mixtures were prepared in solution and the solventwas removed to recover the solid mixtures for further analysis. Thedata in Table 1 present melting temperature (Tm) and heat of fusion(DHm) of POSS molecules. The di-iso-POSS molecules melted earlyas evident from its lower melting point which is almost 100 �Clower than that of tri-iso-POSS. In case of phenyl-POSS, two distinctmelting transitions were identified. These are due to the presenceof a wide array of smaller and bigger sized crystals in the systemwhich will be later identified. Recall that DBS is a crystalline solidwith Tm of 225 �C.

A slight depression in melting point was observed for somePOSS molecules in the presence of sorbitol (Table 2), for example,

Fig. 4. WAXD patterns of (a) DBS/tri-iso-POSS, (b) DBS/di-iso-POSS pre-heated, (c) DBS/di-iso(g) DBS/phenyl-POSS, (h) DBS/phenyl-POSS pre-heated; at 200 �C for 5 min (DBS/POSS 1/2

195 �C to 189 �C for tri-iso POSS and 270 �C to 255 �C for iso-POSS. Asecondmelting peak (218 �Ce224 �C), was identified in all the casesat 1/2 wt/wt ratio of DBS/POSS, clearly signifies lesser DBS/POSSinteractions, as the second melting peak is close to DBS meltingtransition (225 �C). In view of the rheological data presented inFig. 2 and from earlier report [62,70,73,75] we infer that DBSfibrillation occurred even when tri-iso-, di-iso-, iso- and phenyl-POSS were present in the compounds, as will also be presentedthrough visual evidence in later section.

The evidence of DBS melting transition from DSC data supportthat the self-assembly of DBS with itself was more prominent inthese cases instead of forming molecular complexes with POSS.This was clearly evident when POSS molecules had non-phenylsubstituents or when silanol groups were absent from the POSS

-POSS, (d) DBS/di-iso-POSS pre-heated, (e) DBS/iso-POSS, (f) DBS/iso-POSS pre-heated,wt/wt).

Fig. 5. Optical micrographs of iPP compounds taken while cooling the melt under cross-polarizer with adjustable filter; (a) iPP/DBS 1 wt% at 200 �C, (b), (c), (d) iPP/DBS/tri-iso-POSS1/5 wt/wt taken at 200 �C, 170 �C, and 140 �C respectively.

S. Roy et al. / Polymer 53 (2012) 1711e17241716

molecular structure. At 1/5 wt/wt ratio of DBS/tri-iso-POSS, a singlebroad melting transition indicates more abundant interactions incomparison to others, which corroborates the observation ofrheological trend in Fig. 2. These will be discussed again using thedata on phase separated morphology revealed by SEM.

Let us now examine the effect of heating of POSS, and DBS-POSSmixtures at 200 �C for 5 min. This would imitate the changesoccurring during preparation of POSS, DBS, and iPP compounds.Similar data were presented earlier [62], for tri-POSS, tetra-POSSand cyclo-POSS. The crystalline silanol-POSS such as di-iso- and tri-iso-POSS goes through melting transition during heat treatmentdue to lower melting point as seen in Table 1. These heat treatedmaterials showed crystallinity after melting. However, somecondensation products were identified in these cases as revealedfrom mass spectroscopy analysis.

The data in Table 1 show that iso-POSS and phenyl-POSSmolecules with no silanol groups in their structures remained

Fig. 6. The morphology of POSS particles in melt-mixed iPP com

crystalline after heat-treatment at 200 �C for 5 min. A change incrystal structure during heating may have caused the disappear-ance of the lower melting transition of phenyl-POSS after heattreatment. However, the higher melting transition of thesemolecules remains undisturbed. The same phenomenon wasobserved in the case of DBS/phenyl-POSS mixtures e the lowermelting peak of POSS disappeared, as seen in Table 2. Not muchchange in thermal transitions was identified in DBS/POSSmixtures after heat treatment. The DBS and POSS formed twoseparate phases with two thermal transitions each due to POSSand DBS.

It is important to summarize that no liquid complex was iden-tified in any of the above mentioned DBS/POSS mixtures uponheating at 200 �C for 5 min. This is contrary to tri- and tetra-POSSmixtures with DBS where liquid complex was formed, and similarto cyclo-POSS mixture with DBS, where the crystallinity of cyclo-POSS and DBS was retained.

pounds with 5 wt% POSS: (a) tri-iso-POSS, (b) phenyl-POSS.

Fig. 7. SEM images of POSS particles on solvent cast film. (a)-(b) tri-iso-POSS, (c)-(d) di-iso-POSS, (e)-(f) iso-POSS, (g)-(h) phenyl-POSS. As received (a), (c), (e), and (g). Heated at200 �C for 5 min (b), (d), (f), and (h).

S. Roy et al. / Polymer 53 (2012) 1711e1724 1717

The solid mixtures of POSS and DBS before and after heating at200 �C for 5 min were examined by WAXD and their patternswere compared with those of POSS and DBS. The purpose of theWAXD study is to see a possibility of crystalline to amorphoustransformation, if any, which was identified in our earlier publi-cation [62]. The bright arc patterns present the evidence of thecrystalline structure of various POSS. The small bright dots foundin the pattern for tri-iso, di-iso and phenyl-POSS (Fig. 3a, c and g)can be assigned to small crystalline structures present in the asreceived POSS materials. The strongest reflection of all POSSmolecules was found at 2q between 6� and 9� similar to the priorworks [31,76,77]. This can be assigned as a common (101)reflection of the POSS crystals [78]. However, the detailed crystalstructure analysis of the POSS molecules was beyond the scope ofthis work.

The structural changes, if any, in pre-heated POSS and theirmixtures with DBS were inferred from WAXD patterns as seen inFig. 4. It is noted that the non-phenyl silanol-POSS (such as, tri-iso-POSS and di-iso-POSS) did not undergo crystalline to amorphoustransformation as was observed [62] in the case of tri-POSS afterheat treatment at 200 �C for 5 min. Although, tri-iso-POSS and di-iso-POSS melted at lower temperature than tri-POSS, they did notundergo significant condensation reactions. This can be attributed

Fig. 8. SEM images of 1/2 wt/wt DBS and POSS mixtures and their pre-heated mixtures castPOSS, (g)-(h) DBS/phenyl-POSS. As received (a), (c), (e), and (g). Heated at 200 �C for 5 min

to the phenyl substituents in tri-POSS, which influenced intra-molecular condensation reactions. This trend was also supportedby the data from thermal analysis presented in Table 1 for the heattreated materials.

The disappearance of small bright dots in the pre-heatedsamples signifies that crystal structures changed for tri-iso-, di-iso-, and phenyl-POSS as seen in Fig. 3(b), (d) and(h), respectively.The structural changes possibly resulted from the partial conden-sation reactions of the silanol-POSS molecules (tri-iso- and di-iso-POSS) and aggregation of the smaller crystals in case of thephenyl-POSS. The disappearance of the lower melting transition ofphenyl-POSS from the DSC data as presented in Table 1 also sup-ported the fact. No such change was expected for iso-POSS as itdoes not contain free silanol groups.

The crystalline structures were not significantly influenced bythe heat treatment of the DBS/POSS mixtures under consideration.This is evident from a comparison of theWAXD patterns of DBS andPOSS before and after heat treatment (Fig. 4). Only difference isevident in the case of tri-iso-POSS (Fig. 4a, b) where small crys-talline aggregates can still be seen as identified by the small dots inthe WAXD pattern which eventually disappeared with heat treat-ment. The patterns for phenyl-POSS (Fig. 4g, h) remainedunchanged.

as film from solution, (a)-(b) DBS/tri-iso-POSS, (c)-(d) DBS/di-iso-POSS, (e)-(f) DBS/iso-(b), (d), (f), and (h).

Fig. 9. ESI mass spectra of: di-iso-POSS (top); tri-iso-POSS (center); phenyl-POSS(bottom).

Table 2DSC data of POSSeDBS mixtures at different weight ratios prepared by solutionmixing (solution) and pre-heating at 200 �C for 5 min (heated).

Components wt. ratio status Melting transition (Tm�C) DHm (J/g)

First Tm Second Tm

DBS/tri-iso-POSS

1:2 Solution broad; 189 broad; 217 29 & 651:2 Heated broad; 164 broad; 214 8 & 411:5 Solution broad; 190 e 601:5 Heated broad; 181 e 37

DBS/di-iso-POSS

1:2 Solution broad; 71, 194 sharp; 224 4,9, & 511:2 Heated e sharp; 224 501:5 Solution broad; 93, 162 sharp; 223 15, 7 & 181:5 Heated broad; 141 broad; 221 6 & 13

DBS/iso-POSS

1:2 Solution sharp; 222 broad; 255 41 & 341:2 Heated broad; 181 broad; 242 9 & 321:5 Solution sharp; 225 sharp; 263 20 & 441:5 Heated sharp; 212 sharp; 258 26 & 40

DBS/phenyl-POSS

1:2 Solution broad; 141 broad; 220 27 & 451:2 Heated e broad; 220 431:5 Solution broad; 130, 195 broad; 218 9, 13, & 91:5 Heated e broad; 219 9

S. Roy et al. / Polymer 53 (2012) 1711e17241718

3.3. Effect of POSS on DBS nucleation and iPP crystallization

It is known [61,70,74] that endless nanofibrillar networks(Fig. 5a) are formed due to sorbitol crystallization in the polymermelt. The fibrils in turn act as heterogeneous nucleation sites for iPPcrystallization that leads to spherulites. The nucleation efficiency of

Fig. 10. ESI mass spectra of iso-POSS.

iPP can be increased by using higher concentration of DBS, and dueto the formation of dense network of fibrils, more nucleation sitesare generated. Thus the size of spherulites is reduced significantlyat higher concentration of DBS [70]. The introduction of tri-POSSand tetra-POSS in iPP/DBS system was found to reduce the DBSnucleating efficiency, as already explained in our earlier publication[62]. This also can be attributed to the formation of the liquidcomplex between tri- or tetra-POSS and DBS which preventedorganization of DBS into fibrillar networks, which was absent in thepresent cases with the POSS molecules under consideration.

The DBS fibrillation and spherulite size of iPP/DBS or iPP/DBS/POSS system can be visually examined by optical microscope withimages taken between two cross-polarizers equipped with a hot-stage. The optical image revealed no DBS nanofibrils (data notshown) in case of iPP/DBS/tetra-POSS system upon cooling of themelt at the rate of 10 �C/min contrary towhat was observed for iPP/DBS system [70e72]. On the contrary, and in corroborationwith thedata obtained from rheological and thermal study, DBS can stillfibrillate in presence of tri-iso-POSS in iPP compound as evidentfrom Fig. 5(b) and (c). Notable difference can be seen in the lengthof the DBS fibrils with reduced dimensions in presence of tri-iso-POSS in the iPP/DBS compound. This can be attributed toa certain extent of interaction that is possible between DBS and tri-iso-POSS as explained earlier through rheological study of thecompound. This interaction provides some hindrance in DBSnanofibril formation yielding shorter length of fibrils.

Crystallization of the iPP/DBS/tri-iso-POSS compound was alsomonitored by POM as appeared in the window of observation inFig. 5(d), and similar crystallization temperature as of iPP/DBScompound (around 140 �C) [70] was observed with the formationof small sized spherulites. It should be noted here that tri-iso-POSSoffers compatibility with iPP chains due to the presence of itshydrocarbon type isobutyl substituent groups, thus POSS particlesare not visible at a lower magnification in POM images.

To support the hypotheses of compatibility for various POSSwith non-interacting iPP, the morphology of POSS particles in iPPcompounds prepared in batch mixer was examined by high reso-lution SEM. As apparent in Fig. 6(a), tri-iso-POSS dispersed well iniPP and formed close to spherical particles of 75e200 nm indiameter range shown in circles. Similar observation was found incase of di-iso-POSS (data not shown). The long chain isobutylsubstituent group particularly was found responsible for thiscompatibility. On the other hand, phenyl-POSS with phenylsubstituents did not provide enough compatibility with iPP chain

S. Roy et al. / Polymer 53 (2012) 1711e1724 1719

and remained as several micrometer size random aggregates asshown in circles in Fig. 6(b).

Crystallinity obtained from DSC for the semi-crystalline poly-mers like iPP shows direct evidence of its nucleation phenomenon.Thus the percent crystallinity of the iPP/DBS/POSS compoundscrystallized under quiescent conditions was examined by DSC, andwas compared with that of iPP/DBS compound. The iPP/DBScompound showed higher crystallinity than iPP due to increasednucleation efficiency in the presence of DBS fibrils [70]. In corrob-oration with the rheology data as reported earlier [62], theoptimum combinations (2 wt% tetra-POSS or 5wt% tri-POSS with1wt% DBS) in iPP produced the lowest crystallinity. In these cases,the nucleating function of DBS was ‘arrested’ due to participation ofDBS in DBS-POSS complex formation. As explained through rheo-logical study, and will also be summarized through mass spectraanalysis, marginal complex formation is evident in the cases ofsilanol-POSS with non-phenyl substituents or with non-silanolPOSS with DBS. This observation was also reflected in marginal orno such change in crystallinity of the iPP/DBS/POSS compounds(until DBS/POSS at 1/10 wt/wt in iPP) in comparison to iPP/DBScompound (data not shown).

3.4. Organization of POSS molecules

The organization of various silanol and non-silanol POSS inmixtures with DBS was investigated by SEM. For this purpose,solutions of DBS, POSS, and POSS/DBS mixtures were cast as filmson SEM stubs and their morphology compared as presented inFigs. 7 and 8. Small crystalline aggregates with size of a fewmicrometers was identified in the case of tri-iso POSS (Fig. 7a)which corresponds to the small bright dots identified in WAXDpatterns in Fig. 3(a). The lamellar structure was identified for di-isoPOSS (Fig. 7c). However, lamellar growth identified from singlenuclei signifies the existence of small crystal structures in thesystem as corroborated by theWAXD pattern presented in Fig. 3(c).The small bright dots in WAXD pattern (Fig. 3e) were not seen inthe case of iso-POSS. This is reflected in SEM morphology (Fig. 7e)as apparent from its endless lamellar crystalline structures. Perfectbuilding blocks in the range of less than 1 mm to several micrometerdimensions were identified in the case of phenyl POSS as seen inFig. 7(g). This was reflected earlier in WAXD analysis as in Fig. 3(g)with bright dots signifying small crystal patterns in the self-assembled structure of phenyl-POSS.

The heat treatment somewhat changed the crystallinemorphology of the as-received POSS as previously identified fromWAXD patterns. Fig. 7 also presents SEM images of the materialsheated at 200 �C for 5 min. In all the cases, they retained theircrystalline structures, but smaller crystal aggregates are now visiblein the case of tri-iso-, di-iso-, and phenyl-POSS molecules inFig. 7(b), (d), and (h) respectively. Some clearly identifiable defec-tive blocks are seen in the crystalline morphology of heat-treatedphenyl-POSS molecules, which can be attributed to preheating.No such changes appeared in the case of iso-POSS (Fig. 7f) asidentified in previous sections dealing with DSC and WAXDanalysis.

Distinct aggregates of a few micrometers of POSS phase asevident in Fig. 8 for POSSeDBS mixture indicate greaterPOSSePOSS affinity over POSSeDBS supramolecular complexes. Anexpected exception was identified for phenyl-POSS/DBS mixtureFig. 8(g), where an intermingledmorphology of DBS nanofibrils andPOSS was present, possibly a consequence of the intermolecularpep interactions present in this combination. This phenomenonwill be further supported by mass spectrometry analysis in latersection. The crystalline morphology of POSS molecules somewhatchanged in the presence of DBS, which is due to marginal

interactions between them as previously identified from therheological study. The fibrillar domains of DBS are apparent in allthe cases in Fig. 8.

Crystalline DBS form nanofibrillar networks, and in the case ofsilanol-POSS with phenyl substituents and DBS mixtures muchsmaller crystalline aggregates of POSS were identified as presentedin our earlier publication [62]. This was attributed to abundantintermolecular interactions between DBS and silanol-POSS mole-cules with phenyl substituent groups. In the case of non-phenylsilanol-POSS and non-silanol POSS molecules under consider-ation, it is evident that none of these can fulfill the criteria of rep-resenting cooperative hydrogen bonding and pep interactionsbetween the molecules [20e27]. This in turn results a phaseseparated crystal structures of POSS in the mixtures with DBS asevident from Fig. 8.

The morphological features of DBS/POSS mixtures were exam-ined after heating these materials at 200 �C for 5 min and furtherthe resultant materials were solution cast on SEM stubs. Inconjunction with the DSC data as presented in Table 2, and afterheat treatment, the POSS crystal structures are identified in thepresence of DBS. The present result is somewhat similar to theearlier presented data on cyclo-POSS/DBS combinations [62], whichupon heating still retained its crystalline domains and exhibitedDBS fibrils.

Clearly separated and aggregated POSS crystals were identifiedin Fig. 8(b), (d), and (f) for the pre-heated tri-iso-, di-iso-, and iso-POSS molecules in the presence of DBS. Heat treatment couldhave adversely affected the marginal intermolecular non-covalentbonding between DBS and POSS and a stronger POSSePOSS inter-action may have resulted during cooling. DBS can still retain itsentity in the presence of tri-iso POSS, di-iso-POSS, and iso-POSS,which is apparent from the appearance of its fibrillar network inthe morphological structure after the heat treatment. The phenyl-POSS and DBS remain intermingled with each other (Fig. 8h) asa consequence of the existence of intermolecular pep interactionsin the system. This will be further explained in the next sectionthrough mass spectrometry analysis.

3.5. POSSeDBS molecular complex from mass spectroscopy

The characterization of the different systems by mass spec-trometry was carried out on an ESI-Q/ToF mass spectrometer. ESIwas the ionization technique of choice as it is gentler, compared toother methods, and it keeps intact weaker interactions during thetransfer of the different complexes from the liquid phase to the gasphase [79]. Each system was investigated before and after beingheated at 200 �C, which is the temperature used to mix thedifferent POSS/DBS combinations with the polymer matrix. Eachcomponentwas also investigated by itself to determine its chemicalbehaviors under mass spectrometry analysis conditions.

The tri-POSS, tetra-POSS and cyclo-POSS, with and without DBS,were already analyzed in our earlier publication [62], in which theoccurrence of non-covalent interactions, not only among eachsingle species (homocomplex) but also between the POSS andsorbitol molecules (heterocomplexes), was confirmed. A higherorder complex formation was observed in the POSSeDBS systemsinvolving POSS molecules consisting both of the silanol function-alities and the phenyl substituents. This finding indicated that boththe hydrogen bonding and the pep interactions took place. Boththese contributions lower the energy of the complex and in turnincrease the stability of the complex [37].

The ESI spectrum of each single species is reported first, fol-lowed by the investigation of the spectrum of various POSS/DBScombinations. The mass spectrum arising from the analysis of DBSshowed the quasi-molecular ion [M þ Na]þ as base peak as well as

S. Roy et al. / Polymer 53 (2012) 1711e17241720

the respective dimeric species [2MþNa]þ, before and after thermaltreatment. Among the different POSS molecules chosen for thisstudy, only di-iso-POSS, tri-iso-POSS, and phenyl POSS, providedclear evidence of non-covalent complex occurrence with the samespecies (homocomplexes), Fig. 9.

In all the spectra, the quasi-molecular ions detected were eithersodium, [M þ Na]þ,or ammonium, [M þ NH4]þ, adducts. Thesecations are believed to be present in both the glassware and thesolvents used during the sampling.

On the other hand, the analysis of iso-POSS did not showcomplex formation, as shown in Fig. 10. Such difference in behaviorcan be rationalized by taking into consideration the nature of thesubstituents on different POSS molecules. Both di-iso-POSS and tri-iso-POSS have silanol functionalities which can be involved inhydrogen bonding. The phenyl-POSS consist of phenyl substituentgroups that can promote pep interactions with the phenyl groupsin between several molecules present in the system. In contrast,iso-POSS has only the isobutyl groups as substituents which cannot

Fig. 11. ESI mass spectra of: di-iso-POSS (top); tri-iso-POSS (center); phenyl-POSS(bottom) after thermal treatment at 200 �C.

Fig. 12. ESI mass spectra of: di-iso-POSS/DBS (top), tri-iso-POSS/DBS (bottom). M and Lindicate the POSS and DBS units, respectively.

be engaged in any type of measurable interaction with in the samespecies.

No significant change was observed in the ESI mass spectraarising in the same set of POSS molecules after their thermaltreatment. Non-covalent interactions between the same species,homocomplexes, were detected in case of di-iso-POSS, tri-iso-POSS,and phenyl-POSS molecules, as seen in Fig. 11. The tri-iso-POSSmolecules appeared to undergo condensation reactions, as indi-cated by the water loss from its dimeric species ([2M]Naþ). Thiswater loss, which was already mentioned in our earlier publication[62], occurs by intramolecular condensation reactions in the POSSmolecules having silanol groups, before these undergo complexformation.

The ESI spectrum of iso-POSS molecules did not show any peakrelated to non-covalent complex formation as was anticipated.

Fig. 13. ESI mass spectra of phenyl-POSS/DBS. M and L indicate the POSS and DBSunits, respectively. The difference in values of m/z between two consecutive DBShomocomplexes is 179 units, which is half value of the mass of one DBS molecule.

Fig. 14. ESI mass spectra of iso-POSS/DBS. No peak related to a POSS-containingcomplex is observed. L indicates DBS unit.

Fig. 16. ESI mass spectra of di-iso-POSS/DBS (top) and iso-POSS/DBS (bottom) afterthermal treatment at 200 �C. M and L indicate the POSS and DBS units, respectively.

S. Roy et al. / Polymer 53 (2012) 1711e1724 1721

When mixed with DBS, each POSS/DBS combination behaveddifferently towards complex formation, depending on the POSScomponent involved. The di-iso-POSS and tri-iso-POSS (M), whichare similar in composition, gave rise to analogous spectra whenmixed with DBS (L), as seen in Fig. 12. Along with the dimerformation between two POSS molecules, [2M]Naþ, the complexPOSS-sorbitol, [M þ L]Naþ, was also observed in both systems. Notethat the highest order complex in both systems exist in 1:1 stoi-chiometry, in comparison to higher order stoichiometries presentin the case of tri- and tetra-POSS molecules as reported in ourearlier publication [62].

The phenyl-POSS/DBS mixture gave rise to completely differentresults. Non-covalent interactions between the two componentswere observed along with the homocomplexes, Fig. 13. Note the

Fig. 15. ESI mass spectra of tri-iso-POSS/DBS (top) and octa phenyl POSS/DBS (bottom)after thermal treatment at 200 �C. M and L indicate the POSS and DBS units,respectively.

intensity and complex order of the heterocomplexes present in thiscombination compared to the other POSS/DBS mixtures. Only pepinteractions among the phenyl rings of POSS and DBS are possiblein these complexes, as no hydroxyl group is present in the phenyl-POSS. Moreover, the presence of phenyl-POSS in the mixtureseemed to promote the formation of higher order DBS homo-complexes [Ln], as is evident in the spectrum.

No interaction between iso-POSS and DBS was detected. Theonly non-covalent interaction observed in the resulting spectrumwas among the DBS molecules themselves, which formed highorder doubly charged sodiated homocomplexes (Ln), Fig. 14.

The same analysis was performed after the POSS/DBS mixtureswere subjected to thermal treatment at 200 �C. The tri-iso-POSS/DBS and phenyl-POSS/DBS combinations gave rise to both typesof complexes, homocomplexes (POSSePOSS, DBSeDBS) and het-erocomplexes (POSSeDBS), as presented in Fig. 15.

a

b c

Fig. 17. Schematic representation of transitions of different states in POSS/DBSsystems. (a) individual POSS and DBS; (b) POSSePOSS and DBSeDBS complexes; (c)POSSeDBS complex.

Table 3Transition energies (kcal/mol) of various POSS/DBS systems between different states.

Energy Tri-POSS Tetra-POSS Phenyl-POSS Cyclo-POSS Tri-iso-POSS Di-iso-POSS Iso-POSS

DE1 �34.8 �44.8 �27.4 �33.5 �28.7 �31.0 �24.2DE2 �45.0 �48.1 �27.4 �31.7 �27.9 �27.9 �22.9DE2 � DE1 �10.2 �3.3 0.0 1.8 0.8 3.1 1.3

DE1: Transition energy from individual 2 POSS and 2 DBS to POSSePOSS and DBSeDBS complexes.DE2: Transition energy from individual 2 POSS and 2 DBS to 2 POSSeDBS complexes.

S. Roy et al. / Polymer 53 (2012) 1711e17241722

The peaks indicating intramolecular condensation reactionswere observed in the spectrum of the tri-iso-POSS/DBS system asseen in Fig. 15, in which the POSS units carry three silanol func-tionalities. The nature of the condensation was investigated bytandem mass spectrometry, in our earlier publication [62]. It wasconfirmed that intramolecular condensation reactions in the POSSpart occurred before the same molecule underwent complexformation.

Note the high intensity of the heterocomplex in the phenyl-POSS/DBS mixture compared to those arising from the tri-iso-POSS/DBS combination, which implies a superior stability afterthe thermal perturbation. No interaction between POSS and DBSwas detected in the di-iso-POSS/DBS and iso-POSS/DBS systemsafter heating them up at 200 �C, Fig. 16. Peaks arising fromhomocomplex formation and some complex degradation are

Fig. 18. Self-assembly kinetics of DBS and tetra-POSS by dynamic simulation; snapshots takshown in Ǻ. Grey: Carbon; Red: Oxygen; Yellow: Silicon; and White: Hydrogen atoms. Binterpretation of the references to colour in this figure legend, the reader is referred to the

observed in the spectrum of di-iso-POSS/DBS, while just the singlycharged sodiated DBS species appeared in the iso-POSS/DBS spec-trum. The presence of fewer silanol groups in case of di-iso POSSdid not necessarily contribute towards intermolecular self-assembled structure [37].

3.6. Simulation of POSSeDBS complex formation

Employing a simple model and PCFF [80] force field basedmolecular dynamics, the affinity between POSSePOSS andPOSSeDBS was demonstrated. As shown in Fig. 17, three scenarioswere designed: a) individual POSS and DBS; b) POSSePOSScomplex and DBSeDBS complex; c) POSSeDBS complexes. Foreach system, molecular dynamics simulation was executed for 5 nsand the total energy was averaged in the last 1 ns.

en at different time-frame as labeled. Distance between the C atoms of phenyl ring islue dash: hydrogen bond; Green dash: distance measurement between atoms. (Forweb version of this article.)

S. Roy et al. / Polymer 53 (2012) 1711e1724 1723

As schematically presented in Fig. 17, the energy differences ofDE1 (transition energy from individual status to POSSePOSS andDBSeDBS complexes), and DE2 (transition energy from individualstatus to two POSSeDBS complexes) represent the binding strengthbetween the individual molecules. In each combination the bindingstrength was calculated, and listed in Table 3. The theoreticalcalculation showed that the generation of any POSS dimer from theindividual POSS molecules is an energy favorable process (DE1,DE2 < 0). Several silanol and non-silanol POSS were tested in oursimulation asmentioned in the experimental section, with varietiesof substituent groups ranging from phenyl to cyclopentyl to iso-butyl. These different kinds of POSS could be classified into“phenyl” group and “non-phenyl” group. The former includes tri-POSS, tetra-POSS and phenyl-POSS and cyclo-POSS, iso-POSS, di-iso-POSS, and tri-iso-POSS with non-phenyl substituents can beincluded in the latter group. The “phenyl” group POSS with anexception of phenyl-POSS prefer forming complexes with DBSinstead of forming complexes with themselves. In contrast, in the“non-phenyl” systems, it is more favorable to formPOSSePOSSþDBSeDBS complexes in comparison to 2 (POSSeDBS)complexes.

In our dynamic simulation, pep stacking conformations wereformed between phenyl groups in between DBS and “phenyl” POSSvia molecular assembly and the interaction between phenyl groupsis supposed to be the driving force to cause POSSeDBS complexes.However, the different results between the systems of tri- or tetra-POSS and phenyl-POSS indicated that only interaction betweenphenyl groups may not be strong enough to drive the formation ofthe POSSeDBS complex. The absence of the hydrogen bondingbetween phenyl-POSS and DBS resulted into an unsuccessfulPOSSeDBS assembly.

Experimentally it was found [62] that more silanol groupspresent in the tetra-POSS molecules showed more affinity towardsDBS than in tri-POSS/DBS systems. However, the theoreticalcalculation showed that the energy difference (|DE2 � DE1|) intetra-POSS is lower than in tri-POSS. This anomaly can be explainedthrough the different spatial distribution of silanol groups in tri-and tetra-POSS molecules. As seen in the chemical structures pre-sented in Fig. 1, the four silanol groups present in tetra-POSS aredistributed at the two sides of the molecule and only two groups atthe same side can interact (form hydrogen bond) with a single DBSmolecule. On the contrary, three SieOH groups located at the sameside of tri-POSS molecules possess higher probability to formhydrogen bonds with a single DBS molecule.

As seen in the dynamic simulation in Fig. 18, it is observed thathydrogen bond is not necessary to be the first driving force to formthe DBSePOSS complex. Before the formation of the hydrogen bondbetween DBS and POSS, the phenyl groups in DBS and POSSapproach each other due to the interaction between phenyl groupsand other van-der Waals interactions. Hydrogen bond between thetwo molecules form when the donoreacceptor distance is closeenough and the donor-acceptor angle is suitable. Nevertheless,hydrogen bond is beneficial to stabilize the DBSePOSS complexes.

4. Conclusions

The following conclusions can be made from the results of thisstudy

(a) It is inferred from oscillatory shear data that POSS moleculeswith no phenyl side groups and POSS molecules with phenylside groups but no silanol groups cannot subdue DBS fibrilla-tion even at 10 wt% POSS loading in iPP compounds containing1wt% DBS. Thus the presence of both silanol groups and phenyl

groups is conducive to POSS-sorbitol interactions leading toself-assembly.

(b) Non-silanol POSS and silanol-POSS with non-phenyl substitu-ents retained their crystalline organization even after beingexposed to iPP compound processing temperature of 200 �C for5 min.

(c) The mixture of phenyl-POSS and DBS showed an intermingledmorphology before and after heating indicating partial inter-actions. However, other non-phenyl and non-silanol POSSshowed phase separated and aggregated crystals.

(d) Marginal complex formation was observed in silanol-POSSwith isobutyl side groups and no complex formation wasfound in iso-POSS molecules with DBS.

(e) The kinetics of POSS-sorbitol self-assembly studied by molec-ular dynamics simulation revealed that p�p stacking broughtthe phenyl groups in POSS and sorbitol molecules togetherbefore hydrogen bonding interactions could give the final‘shape’ to the molecular adducts. This, aided by mass spec-trometry data, establishes that POSS and sorbitol moleculescontaining both phenyl side groups and hydrogen bondingagents are amenable to form self-assembledmolecular adducts.

(f) The self-assembled molecular adducts were amorphous withglass transition temperature much lower than the processingtemperature of iPP.

Acknowledgement

The authors thank National Science Foundation for providingfinancial support through grant number CMMI-0727231, CHE-1012636, and DMR-0821313, and Hybrid Plastics for partnership,and FHR Plastics, and Milliken Chemicals for assistance with thematerials. SCJ acknowledges Dr. David A. Schiraldi of Case WesternReserve University for introducing him to POSS and for numerousdiscussions on the subject. SR acknowledges Dr. Bojie Wang fromThe Department of Polymer Science, for his guidance regardingmorphology characterization, and Dr. Rong Bai from The Depart-ment of Polymer Engineering, The University of Akron for herguidance in several analytical characterizations.

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