12

21

Research ArticleReceived: 3 August 2009 Revised: 6 November 2009 Accepted: 6 January 2010 Published online in Wiley Interscience: 6 May 2010

(www.interscience.wiley.com) DOI 10.1002/pi.2851

Investigation of the thermal, mechanicaland morphological properties of poly(vinylchloride)/polyhedral oligomeric silsesquioxanenanocompositesRodrigo Silva,a Cleia Salles,b Raquel Maulerc and Ricardo Oliveirac∗

Abstract

Research into organic–inorganic nanocomposites has recently become popular, particularly the development of new polymernanocomposites. Compared to pristine polymers or conventional composites, these nanocomposites exhibit improvedproperties. The storage modulus of a poly(vinyl chloride) (PVC)/polyhedral oligomeric silsesquioxane (POSS) nanocompositeslightly decreased with POSS content, but had a higher modulus from 50 to 100 ◦C. Some of the material appeared to beaggregated with 1 wt% POSS in the polymeric matrix. Conversely, with a POSS content of 5 wt%, a better dispersion of thenanoparticles was observed. The presence of POSS in the plasticised PVC compound had little influence on the final propertiesof the nanocomposites, showing weaker interactions between the POSS and the plasticised PVC compound.c© 2010 Society of Chemical Industry

Keywords: poly(vinyl chloride) (PVC); polyhedral oligomeric silsesquioxane (POSS); nanocomposites; morphology

INTRODUCTIONThe incorporation of nanofillers in polymeric materials is a practicewidely employed to improve the properties of polymers andwiden their applications. This process delivers new materials withsignificantly improved thermal and mechanical properties at lowadditive levels, while still permitting the use of existing commercialprocesses.1 – 4 Hybrid nanocomposites are a class of materialsthat contain monomer molecules, polymer segments, solvents orother species (guests) incorporated into host lattices.5,6 Accordingto Alexandre and Dubois,7 three types of nanocomposites canbe distinguished depending on how many dimensions of thedispersed particles are in the nanometre range. When threedimensions are of the order of nanometres, the nanoparticles areknown as isodimensional, such as spherical silica nanoparticlesobtained by in situ sol–gel methods or by polymerisationpromoted directly from their surface, but this group can alsoinclude semiconductor nanoclusters and others. When twodimensions are of the nanometre scale and the third is larger,forming an elongated structure, the particles are described asnanotubes or whiskers, such as carbon nanotubes or cellulosewhiskers, which are extensively studied as reinforcing nanofillersto produce materials with exceptional properties.

Currently, a new class of materials is being used by researchersto investigate the behaviour of nanoparticles, called polyhedraloligomeric silsesquioxanes (POSS). These are a class of three-dimensional organic–inorganic hybrid silicon–oxygen particleswith the generic formula of (RSiO3/2)n.8 – 10

Recently, investigations of the properties and morphologyof polymer/POSS systems have considerably increased. Thesemolecules contain an inner inorganic framework covered by inertand/or reactive organic substituents. POSS molecules with well-

defined shapes and sizes ranging from 1 to 3 nm have beendescribed as the smallest version of colloidal silica. When thereis covalent bonding between the POSS and polymeric matrix,reinforcement is favoured. POSS polymers may be produced byblending the POSS into a polymer matrix or into a polymerbackbone, or by using POSS as the pendant group of a polymer.Using POSS as a pendant group of a linear polymer gives a linearorganic–inorganic hybrid.11 – 14

Poly(vinyl chloride) (PVC) is an important commercial polymerthat is widely used in industrial fields and has been studiedby academic researchers for many years. However, due tosome inherent disadvantages such as low thermal stability andbrittleness, PVC and its composites are subject to limitationsin certain applications. Thus, there is a need to develop newPVC products with improved qualities and properties in orderto improve and broaden PVC applications.15 In a series ofpapers, Du and co-workers have reported the preparation andcharacterisation of PVC/clay nanocomposites formed by bothmelt and solution blending,16 but little attention has been givento PVC/POSS nanocomposites.

∗ Correspondence to: Ricardo Oliveira, Universidade Federal do Rio Grande do Sul,Av. Bento Goncalves, no. 9500, Instituto de Quımica, Agronomia, 91501-970,Porto Alegre-RS, Brazil. E-mail: ricardo.oliveira@iq.ufrgs.br

a Programa de Pos Graduacao em Ciencia dos Materiais (PGCIMAT), UFRGS,Porto Alegre/RS, Brazil

b Instituto Federal Sul Riograndense (IFSUL), Sapucaia do Sul/RS, Brazil

c Instituto de Quımica, UFRGS, Porto Alegre/RS, Brazil

Polym Int 2010; 59: 1221–1225 www.soci.org c© 2010 Society of Chemical Industry

12

22

www.soci.org R Silva et al.

In the study reported here, the thermal, mechanical andmorphological properties of PVC/POSS nanocomposites wereinvestigated and analysed by means of dynamic mechanicalthermal analysis (DMTA), transmission electronic microscopy(TEM), TGA and the measurement of tensile properties as a functionof POSS content.

EXPERIMENTALMaterialsPVC (NORVIC SP1000, Braskem S/A) and the plasticiser dioctylphthalate (Elekeiroz S/A) were kindly donated by Grandene S/A.Other additives were also used in the preparation of the plasticisedPVC compound, including a stabiliser and lubricants, but theircompositions are under trade secret protection. The POSS usedwas chlorobenzylethylisobutyl POSS, purchased from HybridPlastics, USA.

Preparation of nanocompositesSamples of the plasticised PVC compound and nanocompositeswith 1.0 wt% POSS (PVC/POSS1) and 5.0 wt% POSS (PVC/POSS5)were pre-mixed for 10 min in a rotational mixer at a speed of1710 rpm and then placed in an extruder. These systems wereprepared using a single-screw extruder (Beplast Ltd) at 140 rpmat temperatures from 95 to 105 ◦C (feed to nozzle direction). Theextruded plasticised PVC compound and nanocomposites wereinjection-moulded (Himaco) into dog-bone test bars (ASTM D638)at temperatures from 100 to 115 ◦C (feed to nozzle direction).

CharacterisationDynamic mechanical thermal analysisDMTA (Q800, TA Instruments) was employed for the nanocompos-ite studies. The measurements were taken under tension modefrom −80 to 130 ◦C at a heating rate of 3 ◦C min−1 using a fre-quency of 1 Hz and a clamp dual cantilever. All measurementswere made under an inert atmosphere of nitrogen gas.

Scanning electron microscopyThe PVC/POSS1 and PVC/POSS5 nanocomposites were examinedusing SEM (JSM-6500F, JEOL). The specimens were immersed inliquid nitrogen for 2 h and cryogenically fractured.

Thermogravimetric analysisTGA was performed on the plasticised PVC compound andPVC/POSS nanocomposites using a Q50 thermogravimetric anal-yser (TA Instruments) with a heating rate of 40 ◦C min−1 under anitrogen atmosphere from 20 to 600 ◦C.

Tensile propertiesTensile properties were measured with a universal testing machine(EMIC) with a crosshead speed of 5 mm min−1. The experimentalvalues are the average of at least five independent measurements.

RESULTS AND DISCUSSIONThe viscoelastic behaviour of the PVC/POSS nanocomposites wasinvestigated using DMTA. The results for the storage modulus andtan δ with temperature (−80 to 120 ◦C) for the plasticised PVCcompound and the PVC/POSS1 and PVC/POSS5 nanocompositesare shown in Fig. 1. The storage modulus for the plasticised PVC

Figure 1. Storage modulus and tan δ curves of plasticised PVC compoundand PVC/POSS1 and PVC/POSS5 nanocomposites as a function oftemperature at 1 Hz.

compound is approximately 1.897 MPa at −80 ◦C. With an increasein POSS content, the modulus slightly decreases to 1.752 MPa forPVC/POSS1 and 1.513 MPa for PVC/POSS5 at −80 ◦C.

As the temperature increases, the modulus decreases in allcases, and a sharp decrease is observed in the temperature range−30 to 50 ◦C. PVC/POSS1 has a higher modulus from −40 to 0 ◦Cas compared to the plasticised PVC compound, probably due tobetter synergism between PVC and POSS at this temperature. Also,the PVC/POSS1 nanocomposite has a higher modulus from 50 to100 ◦C. In contrast, the plasticised PVC compound shows a lowermodulus between 0 and 100 ◦C. PVC/POSS5 has a lower modulusbetween −80 and −30 ◦C. However, from 0 to 100 ◦C, the modulusof PVC/POSS1 is slightly higher than that of the plasticised PVCcompound, and the values are similar for the PVC/POSS1 andPVC/POSS5 nanocomposites from 20 to 50 ◦C.

The α-transition temperature decreases for PVC/POSS1 andPVC/POSS5, which may be due to a free volume increase in thePVC/POSS nanocomposites with an increase in the POSS content,suggesting that the nanoparticles act as plasticisers.17

The tan δ peak value for the plasticised PVC compoundand the nanocomposites, seen in Fig. 1, are lower for higherPOSS contents. A low value for the loss modulus indicates anelastic polymeric behaviour, and a high value indicates a viscouspolymeric behaviour. As expected, upon increasing the POSScontent, there is a decrease in the nanocomposite tan δ peak. Inagreement with the results of Soong et al.,18 the plasticised PVCcompound chains, being polar, can resist viscous flow, whereaswith the POSS phase, they show lower resistance to viscous flowat higher temperatures. This behaviour can have implicationsin terms of the morphology of the nanocomposite material,which may change at higher temperatures, thus affecting thedegradation process. The temperature at which there is energyloss during cyclic stress on the sample, due to additional degreesof freedom, is a measure of the glass transition temperatureassociated with the α-relaxation of the polymeric material.

It has been previously reported that for PVC/POSS nanocompos-ites with POSS contents below 15 wt%, samples are transparent,while with 20 wt% POSS they are opaque.17 The miscibility ofthe PVC/POSS blends was further examined using SEM. Figure 2shows SEM micrographs for PVC/POSS5, showing the relation-ship between the diameter and the dispersed phase particles ofthe POSS. These SEM images illustrate the heterogeneous and

www.interscience.wiley.com/journal/pi c© 2010 Society of Chemical Industry Polym Int 2010; 59: 1221–1225

12

23

Properties of PVC/POSS nanocomposites www.soci.org

Figure 2. SEM micrographs of the PVC/POSS5 nanocomposite.

Figure 3. TEM micrographs of the nanocomposites: (a) PVC/POSS1; (b) PVC/POSS5.

homogeneous distribution of the microstructure that forms in thematrix of the plasticised PVC compound during domain develop-ment. As can be seen in Fig. 2 for the PVC/POSS5 nanocomposite,the POSS is not evenly dispersed in the polymeric matrix, but formsboth small and large aggregates. These results verify that the sili-con/oxygen hybrid nanocomposites are not well interspersed anduniformly distributed in the plasticised PVC compound matrix.

Figure 3 shows TEM images of composites with 1 wt% and 5wt% POSS contents. As can be seen, there are some aggregatedparticles in the 1 wt% POSS polymeric matrix, revealing a lowerlevel of interaction between the POSS nanocomposite and theplasticised PVC compound matrix. In contrast, with a POSS contentof 5 wt%, a better dispersion of the POSS nanoparticles in thepolymeric matrix is evident. For the 5 wt% POSS content material,the presence of tug regions (white areas) is clear, showing lessadhesion and some aggregates (dark areas). According to Soonget al.,17 the POSS aggregates become squeezed and deformedduring microtoming, resulting in the long elliptic morphology.

The thermal degradation of the plasticised PVC compoundand the nanocomposites with the two POSS contents under aninert atmosphere was characterised using TGA. The results areshown in Fig. 4. As in the case of the plasticised PVC compound,the PVC/POSS nanocomposites reveal two-stage degradation: thefirst stage is attributed to the volatilisation of hydrogen chloridemolecules followed by the formation of conjugated polyenesequences; the second stage corresponds to thermal cracking ofthe carbonaceous conjugated polyene sequences. The two peaksin the first-derivative thermograms, indicative of the maximumweight loss rate of degradation, correspond to the temperaturesof the fastest degradation of each stage.

TGA reveals that the thermal stability in this case is slightlydependent on the PVC/POSS interactions. As can be seen in Fig. 4,the TGA traces show similar behaviour for the plasticised PVC

Figure 4. TGA thermograms and first derivatives for plasticised PVCcompound and PVC/POSS1 and PVC/POSS5 nanocomposites in inertatmosphere.

compound and the nanocomposites. The temperature of the firststage of degradation is shifted towards a higher value when POSS1and POSS5 are added compared to the plasticised PVC compoundalone.

A higher content of POSS slightly improves the thermal stabilityof PVC/POSS due to the low level of interaction between theplasticised PVC compound matrix and the POSS. However, thisbehaviour is not observed in the second stage of degradation,where PVC/POSS1 undergoes a minor weight loss in comparisonto the behaviour observed for PVC/POSS5.

The percentages of weight loss and the residue at 600 ◦Care listed in Table 1. The amount of weight loss is divided intotwo groups. The first group (wt1) includes the first loss in thetemperature range 220–260 ◦C. In the second group (wt2), the

Polym Int 2010; 59: 1221–1225 c© 2010 Society of Chemical Industry www.interscience.wiley.com/journal/pi

12

24

www.soci.org R Silva et al.

Table 1. Percentages of relative weight according to temperatureregions

Sample wt1 (mg) wt2 (mg) Residue at 600 ◦C

Plasticised PVC compound 15.80 3.60 1.40

PVC/POSS1 14.50 3.30 1.15

PVC/POss5 14.10 3.00 1.40

Table 2. TGA results for the plasticised PVC compound andnanocomposites

POSS content (wt%) Tonset (◦C) Tendset1 (◦C) Tendset2 (◦C)

0 232 376 517

1 232 374 518

5 233 376 514

Table 3. Mechanical properties of the plasticised PVC compound andnanocomposites

Sample

Tensilestrength

(MPa)

Elongationat

break (%)

Elasticmodulus

(MPa)

Plasticised PVC compound 6.95 305 4.8

PVC/POSS1 6.55 310 4.6

PVC/POSS5 6.20 278 4.6

loss corresponds to the degradation region between 430 and500 ◦C. As can be seen, all of the changes are almost the same.

For the onset and endset decomposition temperatures of thenanocomposites, there are only slight differences between theplasticised PVC compound and the PVC/POSS nanocomposites.In Fig. 4 and Table 1, it can be observed that the maximum de-composition rate of the nanocomposites is higher for PVC/POSS1.The residues of the plasticised PVC compound and PVC/POSS5nanocomposites are similar, revealing that the presence of POSS5in the plasticised PVC compound matrix enhances the formationof char, i.e. carbonisation of the polymer.19 The temperatures ofdecomposition onset (Tonset) and endset (Tendset) are summarisedin Table 2.

This anomalous behaviour of temperature is perhaps related toerrors during preparation of the nanocomposites. In fact, intimatemixing favouring covalent bonding between the organic and in-organic components can prevent the formation of microscopicallyseparate phases.20

The tensile properties of the plasticised PVC compound and thePVC/POSS1 and PVC/POSS5 nanocomposites were measured andthe results are listed in Table 3. The mechanical properties for allof the samples are practically the same.

The results for the strain/stress properties show the tensilestrength, elongation at break and elastic modulus properties. Theexperimental tensile test curves are shown in Fig. 5. As can beobserved, the presence of 1 or 5 wt% POSS particles results inmodifications in the form of a reduction in the tensile strength ofthe PVC/POSS1 and PVC/POSS5 nanocomposites when comparedwith the plasticised PVC compound.

In comparison with the plasticised PVC compound, PVC/POSS1shows lower tensile strength (up to 6.55 MPa) and higher

Figure 5. Stress/strain curves of plasticised PVC compound and PVC/POSS1and PVC/POSS5 nanocomposites.

elongation at break (up to 310%). With an increase in POSScontent (PVC/POSS5), there is a decrease in the tensile strength(to 6.20 MPa) and a reduction in the elongation at break (to 278%).The plasticised PVC compound shows brittle behaviour and thePVC/POSS1 and PVC/POSS5 nanocomposites have lower yieldpoints than the plasticised PVC compound.

An increase in tensile modulus is expected due to the highYoung’s modulus of POSS (ca 10 GPa). The plasticised PVCcompound exhibits a much lower modulus, so the incorporationshould increase the modulus of the nanocomposite. This decreasecould be a function of the weak incorporation of the POSS into thePVC matrix.

CONCLUSIONSThe PVC/POSS1 and PVC/POSS5 nanocomposites show a reducedstorage modulus, loss modulus and tan δ peak values in relationto the plasticised PVC compound with increasing temperature,indicating behaviour as a plasticiser. The TEM micrographs showless interaction between the plasticised PVC compound matrixand POSS nanoparticles, which have poor interfacial adhesion.The thermal stability of these nanocomposites exhibited, in thiscase, a slight dependence on the PVC/POSS interactions. Theformation of residues is lower for PVC/POSS1, thereby reducingthe thermal stability of this nanocomposite. The tensile propertiesof the two nanocomposites, PVC/POSS1 and PVC/POSS5, showslight differences compared with the plasticised PVC compound.

ACKNOWLEDGEMENTSThe authors thank CAPES and CNPQ for financial support, andGrendene, Beplast and Federal Institute Sul-rio-grandense (IFSul)for supplying materials, equipment and other technical support.

REFERENCES1 Varley RJ, Groth AM and Leong KH, Polym Int 57:618–625 (2008).2 Santiago F, Mucientes AE, Osorio M and Poblete FJ, Polym Int

55:843–848 (2006).3 Calcagno CIW, Mariani CM, Teixeira SR and Mauler RS, Compos Sci

Technol 68:2193–2200 (2008).

www.interscience.wiley.com/journal/pi c© 2010 Society of Chemical Industry Polym Int 2010; 59: 1221–1225

12

25

Properties of PVC/POSS nanocomposites www.soci.org

4 Castel CD, Bianchi O, Oviedo MAS, Liberman SA, Mauler RS andOliveira RVB, Mater Sci Eng C 29:602–606 (2009).

5 Pomogailo A, Inorg Mater 41:47–74 (2005).6 Yang DY, Liu QX, Xie XL and Zeng FD, JThermAnalCalorim 84:355–359

(2006).7 Alexandre M and Dubois P, Mater Sci Eng R 28:1–63 (2000).8 Iwamura T, Adachi K, Sakaguchi M and Chujo Y, Prog Org Coat

64:124–127 (2009).9 Zhang Y, Lee S, Yoonessi M, Liang K and Pittman CU, Polymer

47:2984–2996 (2006).10 Suto N, Fujimori A and Masuko T, Polymer 46:167–172 (2005).11 Phillips SH, Haddad TS and Tomczak SJ, Curr Opin Solid State Mater Sci

8:21–29 (2004).12 Pielichowski K, Njuguna J, Janowski B and Pielichowski J, Polyhedral

oligomeric silsesquioxanes (POSS)-containing nanohybridpolymers, in Supramolecular Polymers Polymeric Betaines Oligomers.

Advances in Polymer Science, vol. 201. Springer, Berlin, pp. 225–296(2006).

13 Xu H, Kuo SW, Lee JS and Chang FC, Macromolecules 35:8788–8793(2002).

14 Zhao Y and Schiraldi DA, Polymer 46:11640–11647 (2005).15 Wan C, Qiao X, Zhang Y and Zhang Y, Polym Test 22:453–461 (2003).16 Du J, Wang D, Wilkie CA and Wang J, Polym Degrad Stab 79:319–324

(2003).17 Soong SY, Cohen RE, Boyce MC and Mulliken AD, Macromolecules

39:2900–2908 (2006).18 Soong SY, Cohen RE and Boyce MC, Polymer 48:1410–1418 (2007).19 Gong FL, Feng M, Zhao CG, Zhang SM and Yang MS, Polym Degrad

Stab 84:289–294 (2004).20 Messori M, Toselli M, Pilati F, Fabbri E, Fabbri P, Pasquali L, et al,

Polymer 45:805–813 (2004).

Polym Int 2010; 59: 1221–1225 c© 2010 Society of Chemical Industry www.interscience.wiley.com/journal/pi