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POSS nanocomposites
Composites Science and Technology 117 (2015) 398e403
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Evaluation of glass transition temperature of PVC/POSS nanocomposites ski a, Jolanta Tomaszewska b, *, Jacek Andrzejewski a, Tomasz Sterzyn rczewska b Katarzyna Sko a b
University of Technology, Institute of Materials Technology, Piotrowo 3, 60-965 Poznan , Poland Poznan Faculty of Technology and Chemical Engineering, UTP University of Science and Technology, Seminaryjna 3, 85-326 Bydgoszcz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2015 Received in revised form 17 July 2015 Accepted 18 July 2015 Available online 21 July 2015
The poly(vinyl chloride) was modified by polyhedral oligomeric silsesquioxane with 3-chloropropyl groups (CPePOSS) by POSS concentration between 0.5 and 5 wt%. Two methods of POSS incorporation were applied, i.e. direct solid powders mixing or addition of POSS to PVC solution in THF. The final samples were produced by melt mixing in the Brabender mixer chamber. By means of the glass transition temperature (Tg) determination the influence of a low contain of CPePOSS on the effect of plasticization of the PVC compounds was investigated. Depending on the measurements method (dynamic mechanical analysis DMA and differential scanning calorimetry DSC) and on the DMA measurement frequency various values of the Tg were observed. It was found that CPePOSS may act as a plasticizer, leading to certain lowering of the glass transition temperature of poly(vinyl chloride). © 2015 Elsevier Ltd. All rights reserved.
Keywords: Nanocomposites Differential scanning calorimetry (DSC) Mechanical properties Dynamic mechanical thermal properties (DMTA) Glass transition Tg
1. Introduction The glass transition temperature (Tg) belongs to the most important material feature of polymers, polymer compounds, copolymers and matrix of polymer composites, regarding its temperature dependent mechanical and application properties [1e5]. The determination of Tg as a function of binary polymer composition may be useful for the description of compounds miscibility [6], and manifests anomalous dependencies on composition [7]. To determine the value of Tg the temperature changes of defined polymer properties, like electrical and heat conductivity, dielectric constant, specific volume, refractive index and others are usually taken into account; however the most often used techniques are differential scanning calorimetry (DSC), thermomechanical analysis (TMA), dielectric analysis (DEA) and dynamic mechanical analysis (DMA) [8e11]. It has to be stressed that depending on the methods used, e.g. various measurement frequency, usually different values of Tg for the same material are observed. These differences are due to various relaxation time of polar groups mobility in the side and
* Corresponding author. E-mail address: [email protected] (J. Tomaszewska). http://dx.doi.org/10.1016/j.compscitech.2015.07.009 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
main chain, as well as thermal stimulation of macromolecular chains movement. There are many factors that affect the value of glass transition temperature, including intermolecular interactions (the presence of polar groups), average molecular weight, size and type of side substituents, degree of crystallinity, degree of crosslinking, and others. Brostow et al. have indicated [6] that the results of Tg determined by DMA method may also be different, whether it is evaluated from the measurements of the loss modulus E00 or from 00 0 the tan v ¼ E /E . By addition of plasticizers the Tg value of polymers, particularly of poly(vinyl chloride) (PVC) might be significantly modified [12]. Plasticizers, also known as softening agents are processing modifiers that influence both, processing and applied properties [13,14]. The addition of plasticizers in general lead to lowering of Tg value, and therefore instead of 80 C for non-plasticized pure PVC, the lowest temperature limit of PVC application may be extended to about 30 C. Such a PVC modification leads to improved properties of the final product like impact strength, elasticity and resistance to low temperature. Other modifiers, especially the ones in a form of nanofillers used in PVC compounds, may also give an impact on the changes of glass transition temperature. For example the addition of zinc and
ski et al. / Composites Science and Technology 117 (2015) 398e403 T. Sterzyn
aluminum oxide, calcium carbonate, silica as well as MMT nanoparticles and halloysite nanotubes causes an increase of Tg comparing with the unfilled polymer [15e22]. Additionally, higher values of glass transition temperature were found for the CaCO3 nanocomposite with a PVC matrix in a form of a porous structure. However, the modification of PVC with MgAl layered double hydroxide (LDH) causes that the glass transition temperature of the PVC/LDH nanocomposites is slightly lower than of pristine PVC [23]. It should be noted that even a small addition of nanofillers can significantly affect the mechanical, dielectric, thermal and processing properties of PVC [15e22,24,25]. In our previous research we have studied the influence of carbon nanotubes on the PVC glass transition temperature, determined by different methods like DSC, DMTA and electrical loss factor. A considerable influence of both, measurement frequency and CNT content on the glass transition temperature of PVC, was ascertained [26]. The influence of another modifier, like methacryl polyhedral oligomeric silsesquioxanes (POSS) on the Tg of PVC was described by Soong et al. [27]. A decrease of Tg, and related changes of mechanical properties, like lowering of storage modulus has indicated that the addition of POSS may lead to a significant plasticizing effect of the PVC matrix. A similar effect was found in the case of PVC modified with another type of polyhedral oligomeric silsesquioxane containing 3-chloropropyl groups. The addition of this modifier in an amount of above 7 wt% resulted in an improvement of mechanical properties, especially impact strength [28]. Regarding the state of the art, and ecological requirements to lower the contain of phthalate derivatives as specific additives, the main task of our research was to investigate the influence of a low concentration of 3-chloropropyl-POSS on the effect of PVC plasticization, where only a slight amount of processing modifiers was incorporated into the PVC matrix. The mechanical dynamical and calorimetric investigation were chosen in this case. 2. Experimental section 2.1. Materials The compound consisting of 100 parts by weight of PVC S-61 Polanwil (Mn ¼ 47,500, Mw/Mn ¼ 2.25), produced by Anwil-Spolana Wloclawek (Poland), 4 parts by weight of liquid tinorganic stabilizer Patstab 2310, produced by Patcham and 1 part by weight of paraffin wax Naftolube FTP, produced by Chemson was used as polymer matrix. In order to minimize the influence of processing additives, the stabilizer and the paraffin wax were the only additional components used in the PVC composition. As nanofiller the polyhedral oligomeric silsesquioxane (CPePOSS) containing 3-chloropropyl groups on silsesquioxane cage (Fig. 1), in a form of viscous solid synthesized in the Department of Metal-Organic Chemistry of Faculty of Chemistry of Adam (Poland), was used. Mickiewicz University in Poznan
compounds were processed in a kneading chamber of the Brabender mixer. The thermostatic chamber with a capacity of 50 cm3 was filled with 56 g of the PVC dry blend and the compounds were kneaded at temperature of Tch ¼ 185 C, by 30 min1 rotor speed of main rotor, while the kneading friction was 1:1.5. Samples processed by this method were marked as M series. 2.2.2. Method S A solution of 6 wt% of PVC (with the addition of lubricant and stabilizer) in THF was prepared by mixing, and subsequently 0.5, 1, 1.5, 2 and 5 wt% of CPePOSS was added. The thin films were cast on the glass surface. The films were annealed at the temperature of 100 C until the full evaporation of the solvent. At the next stage the films were cut into smaller pieces and filled into the kneading chamber of Brabender along with rest part of PVC compound (S series). The kneading process was performed in the same conditions as mention in the case of M method. After cooling all compounds were grinded, and a part of the samples was used directly for the DSC investigations. The samples for the DMTA and tensile tests, in a form of plates with a thickness of 2 mm, were produced by compression molding at the temperature of 175 C. 2.3. Measurements 2.3.1. Determination of the glass transition The glass transition temperature of the PVC/POSS samples has been determined by DSC and DMTA measurements. The dynamic mechanical properties were analyzed by the means of Anton Paar MCR 301 apparatus in oscillatory mode, operating at frequencies (G) of 1 and 10 Hz, by heating rate of 2 C/min within the range of temperature between 20 C and 150 C, with the deformation level of 0.01%. Two different techniques of the Tg evaluation were used. First one based on the loss modulus curves, second one on the tangent d. The temperature by which the loss modulus or tangent d reveals the maximum value, was taken as the glass transition temperature. The calorimetric investigations were performed by DSC, using the Phoenix DSC 204 F1 Netzsch apparatus in standard conditions, with samples of 10 mg operating at heating rate of 5 C/min at the range from þ30 to þ120 C. The glass transition value was
2.2. Processing The unmodified PVC compound (PVCR) and PVC mixtures with 0.5, 1.0, 1.5, 2 and 5 wt% of CPePOSS were processed by kneading using a Brabender mixer. Two diverse procedures of compound preparation, differed by way of additive incorporation were used, thus two series of samples were obtained. 2.2.1. Method M The PVC in a form of a dry blend with following CPePOSS amount 0.5, 1.0, 1.5, 2 and 5 wt% was mixed in the Brabender chamber (type Plasti-Corder Pl 2200-3), operating at temperature 120 C, by rotor speed 30 min1, during 10 min. Subsequently,
399
Fig. 1. The structure of 3-chloropropyl-POSS T8 [28].
400
ski et al. / Composites Science and Technology 117 (2015) 398e403 T. Sterzyn
estimated by DSC base line inflection point, as a middle point on the tangent line to the curve (comp. Fig. 2). The samples detached from the lumps, taken out from the Brabender mixing chamber, were crumbled into the form of powder. In the DSC experiment the samples of powder has been applied, originated from a mixture taken from various positions of the crumbled lumps. 2.3.2. Mechanical properties Static tensile tests were carried out by means of the Instron 5985 machine operated by BlueHill 2 program. Measurements were performed according to PN-EN ISO 527-1, always for 5 samples of each of the PVC/POSS composites, using a tensile rate of 10 mm/ min. The Young's modulus was evaluated for the elastic part of the strain e stress curve. The results of the Young's modulus, as a function of POSS content are presented on Fig. 11. 3. Results and discussion An exemplary DSC heating trace for PVC/POSS nanocomposites is presented in Fig. 2. For all samples the run of the DSC curve at the domain of the glass transition was similar, and the Tg values (Table 1) varied in the range between 78 C and 75 C (samples of M series), and between 79 C and 76 C for the samples of S series. The values of Tg, determined by DSC, for both series of PVC/ CPePOSS samples, alone with the additive concentration are presented in Table 1. As it may be seen, the value of Tg depends on the procedure of the compounds' preparation (M and S series). In general, for the samples prepared in solution a somehow higher values of Tg were noted; indicating that any plasticizing effect, due to the possible residual THF existed. Moreover, for the CPePOSS concentration below 1 wt% in the case of samples prepared by S method any changes of Tg may be observed. Only after addition of CPePOSS above 1.5 wt% causes decrease of glass transition temperature of from 1 to 3 C; an indication of a slight plasticizing effect of PVC. An identical value of Tg decrease in the case of PVC/POSS samples prepared by M method was noted; however it takes place already by the POSS concentration of 0.5 wt%. Similar result has been described in the literature for PVC modified with methacryl polyhedral oligomeric silsesquioxane [27], thought but this effect was found for higher concentration of modifier, namely between 5 and 15 wt%. The effect of POSS addition and preparation method may clearly be seen on Fig. 3 where the relative decrease of DSC evaluated Tg vs.
Fig. 2. DSC trace of sample prepared by M method (PVC with 0.5 wt% CPePOSS).
CPePOSS content of PVC/POSS samples, is presented. The relative changes of the Tg were estimated by following formula:
DTg 100%; TgPVC
(1)
where: DTg ¼ TgPVC TgPVC=POSS The data on Fig. 3 indicate that the difference in Tg values, related with the methods of samples preparation, are not significant if the modifier's concentration is above 2 wt%. In this case the relative decrease of Tg is higher than 2.5%. The example of the run of the loss modulus G00 as a function of temperature for PVC samples of S series with various CPePOSS contain, by frequency of 1 Hz, is presented in Fig. 4. As it may be seen (Fig. 4) the temperature position of the maximum of the loss modulus (a-transition) is decreasing in the range from 75 C to about 67 C by increasing POSS contain. The shift of the temperature position of G00 maximum towards lower values, with increasing CPePOSS contain is observed by both measurements frequencies of 1 and 10 Hz. A similar effect was observed by Ref. [27], while it has to be stressed that the methacrylPOSS content was considerably higher (up to 20 wt%) comparing with our experiments. Likewise in Ref. [28], a single maximum of mechanical loss spectra, allows to suggest a good compatibility of CPePOSS with PVC, and large interaction between CPePOSS molecules and PVC chains. The glass transition temperature dependence on CPePOSS determined by DMTA technique, for the measurement frequencies of 1 Hz and 10 Hz is presented on Figs. 5 and 6. For samples processed by both methods (S and M) the Tg value decreases with the increasing CPePOSS content. It should be noted that the samples prepared by M method reveal higher glass transition temperature, comparing with samples obtained by S method, independently of the frequency used in experiments. Moreover, a frequency dependence of Tg was found, i.e. as higher is the frequency as higher are the value of glass transition. A similar frequency related effect of the glass transition temperature for PVC/nanotubes composites has been described in our earlier paper [26]. The relative changes of DMTA determined Tg values, evaluated by Formula (1) may be seen on Fig. 7. As it follows from the graphs, for the CPePOSS contain below 2 wt% the changes of Tg are similar, independent on the method of preparation and the frequency of measurements. By additive amount above 2 wt% a somehow higher changes of Tg in the case of PVC samples, processed by M method
Fig. 3. Relative decrease of Tg by DSC method vs. CPePOSS content for both preparation methods.
ski et al. / Composites Science and Technology 117 (2015) 398e403 T. Sterzyn
401
Fig. 4. The loss modulus thermogram of PVC samples with various CPePOSS content produced by S method, G ¼ 1 Hz. Fig. 7. The relative changes of Tg by DMTA vs. POSS content for various processing methods and measurement frequencies.
Fig. 5. The Tg value by DMTA vs. CPePOSS content for various preparing methods, G ¼ 1 Hz. Fig. 8. The tangent d of PVC/POSS samples with various CPePOSS content produced by S method, vs. temperature, G ¼ 1 Hz.
Fig. 6. The Tg value by DMTA vs. CPePOSS content for various preparing methods, G ¼ 10 Hz.
Fig. 9. The Tg values obtained from the DSC thermograms, loss modulus and tangent d DMTA spectra, G ¼ 1 Hz.
ski et al. / Composites Science and Technology 117 (2015) 398e403 T. Sterzyn
o
Glass transition temperature, C
402
88 84
M series (from loss m.) S series (from loss m.) M series (from tan) S series (from tan) M series (from DSC)) S series (from DSC))
80 76 72
0
1
2
3
4
5
POSS content, wt.% Fig. 10. The Tg values obtained from the DSC thermograms, loss modulus and tangent d DMTA spectra, G ¼ 10 Hz.
Fig. 11. The Young's modulus values of PVC/POSS samples with various CPePOSS content produced by M and S methods.
Table 1 Tg values of PVC/POSS samples processed by two methods, as measured by DSC. CPePOSS content [wt%]
0 0.5 1.0 1.5 2 5
Tg [ C] M series
S series
78 77 77 76 76 75
79 79 79 78 77 76
were observed. The relative decrease of the Tg for the M samples is 6.5% whereas for the S series it is only 3%. This effect is more visible when the measurement frequency of 1 Hz was used. Probably, a somehow higher homogeneity POSS-cages distribution between the PVC macromolecular chains was achieved by CPePOSS and PVC powder mixing, prior to melt processing in the chamber of the Brabender kneader. The Tg value was also determined from the temperature run of the tangent (tan d) where the maximum of the temperature position of tangent d was taken as the Tg. On Fig. 8 the tan d of PVC/POSS produced by S method vs. temperature, for various CPePOSS content, by measurement frequency of 1 Hz, may be seen. The run of changes of tan d vs. temperature, for all PVC/POSS blends, is similar, however with increasing CPePOSS content the maximum of tan d is shifted towards lower temperature, an effect
observed already in the case of loss modulus G00 . Figs. 9 and 10 show the real values of Tg obtained by means of all three methods, DSC and DMTA (from loss modulus and tangent d). As it may be seen in Fig. 9 a difference of about 10 C between the Tg values evaluated by tan d and G00 , may be noted. The DSC determined Tg values are located between the DMTA e origin curves. This tendency is visible for measurement frequency of 1 Hz. For the DMTA tests performed by frequency of 10 Hz (Fig. 10) the DMTA e origin curves are shifted to higher temperature, moreover, by this frequency the run of CPePOSS content dependent changes of Tg, obtained from the loss modulus G00 is comparable with those obtained from the DSC. These differences between Tg values obtained from various methods are due to diverse way of stimulation of the chain motions (DSC vs. DMTA) and different time responses of motions (tan d vs. G00 ) [6], as well as on the stimulation frequency (measurements by DMTA at two frequencies) as shown already before [26]. The cage-like POSS molecules, regarding its nano-like dimension, may be placed between the PVC macromolecules, filling [28] and increasing [27] the free volume, lowering the intermolecular forces, thus acting as a plasticizer. From another point of view the multi-polar groups in CPePOSS molecule may form physical cross points with PVC molecules. Regarding the complexity of structure of gelatinated PVC, containing secondary crystallites formed during gelation simultaneously with an amorphous part (ideal homogenization involving total disappearance of grain structures during processing is not possible and not all primary crystallites are completely melted) [29e32], a general suggestion may be formulated, that various temperatures related to mobility changes are significantly dependent on the distribution level of CPePOSS in the polymeric matrix, and/or on various mobility answer on frequency charges applied during the DMTA and DSC measurements. As it follows from Fig. 11 a relationship between Young's modulus and the POSS content may be observed, where the decrease of the elastic modulus, due to the POSS addition may prove the plasticization effect. For all POSS concentration the samples prepared by S method (in solution) reveal higher modulus values relative to samples produced by powder mixing. 4. Conclusions A slight plasticizing effect of a low amount (up to 5 wt%) of CPePOSS on the PVC matrix has been found. Particularly, a shift of the a-transition temperature of PVC towards lower temperature, independently from the preparation method of PVC/CPOSS samples, as well as on the measurement method and frequency, was noted. Finally, it may be stated that the oligomeric POSS may be applied as non-phthalate containing plasticizers of the poly(vinyl chloride). Acknowledgment The financial support of the European Union Programme through the European Regional Development Fund under the Innovative Economy Operational Programme 2007e2013, obtained from the Project no. UDA- POIG.01.03.01-30-173/09, Priority I “Research and development of new technologies”, Action 1.3. “Support to R & D projects for companies, carried out by scientific bodies,” Sub-measure 1.3.1 “Development projects” is gratefully acknowledged. References [1] W. Brostow, T. Datashvili, K.P. Hackenberg, Synthesis and characterization of poly(methyl acrylate) þ SiO2 hybrids, e-Polymers 054 (2008) 1e13.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Evaluation of glass transition temperature of PVC/POSS nanocomposites ski a, Jolanta Tomaszewska b, *, Jacek Andrzejewski a, Tomasz Sterzyn rczewska b Katarzyna Sko a b
University of Technology, Institute of Materials Technology, Piotrowo 3, 60-965 Poznan , Poland Poznan Faculty of Technology and Chemical Engineering, UTP University of Science and Technology, Seminaryjna 3, 85-326 Bydgoszcz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2015 Received in revised form 17 July 2015 Accepted 18 July 2015 Available online 21 July 2015
The poly(vinyl chloride) was modified by polyhedral oligomeric silsesquioxane with 3-chloropropyl groups (CPePOSS) by POSS concentration between 0.5 and 5 wt%. Two methods of POSS incorporation were applied, i.e. direct solid powders mixing or addition of POSS to PVC solution in THF. The final samples were produced by melt mixing in the Brabender mixer chamber. By means of the glass transition temperature (Tg) determination the influence of a low contain of CPePOSS on the effect of plasticization of the PVC compounds was investigated. Depending on the measurements method (dynamic mechanical analysis DMA and differential scanning calorimetry DSC) and on the DMA measurement frequency various values of the Tg were observed. It was found that CPePOSS may act as a plasticizer, leading to certain lowering of the glass transition temperature of poly(vinyl chloride). © 2015 Elsevier Ltd. All rights reserved.
Keywords: Nanocomposites Differential scanning calorimetry (DSC) Mechanical properties Dynamic mechanical thermal properties (DMTA) Glass transition Tg
1. Introduction The glass transition temperature (Tg) belongs to the most important material feature of polymers, polymer compounds, copolymers and matrix of polymer composites, regarding its temperature dependent mechanical and application properties [1e5]. The determination of Tg as a function of binary polymer composition may be useful for the description of compounds miscibility [6], and manifests anomalous dependencies on composition [7]. To determine the value of Tg the temperature changes of defined polymer properties, like electrical and heat conductivity, dielectric constant, specific volume, refractive index and others are usually taken into account; however the most often used techniques are differential scanning calorimetry (DSC), thermomechanical analysis (TMA), dielectric analysis (DEA) and dynamic mechanical analysis (DMA) [8e11]. It has to be stressed that depending on the methods used, e.g. various measurement frequency, usually different values of Tg for the same material are observed. These differences are due to various relaxation time of polar groups mobility in the side and
* Corresponding author. E-mail address: [email protected] (J. Tomaszewska). http://dx.doi.org/10.1016/j.compscitech.2015.07.009 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
main chain, as well as thermal stimulation of macromolecular chains movement. There are many factors that affect the value of glass transition temperature, including intermolecular interactions (the presence of polar groups), average molecular weight, size and type of side substituents, degree of crystallinity, degree of crosslinking, and others. Brostow et al. have indicated [6] that the results of Tg determined by DMA method may also be different, whether it is evaluated from the measurements of the loss modulus E00 or from 00 0 the tan v ¼ E /E . By addition of plasticizers the Tg value of polymers, particularly of poly(vinyl chloride) (PVC) might be significantly modified [12]. Plasticizers, also known as softening agents are processing modifiers that influence both, processing and applied properties [13,14]. The addition of plasticizers in general lead to lowering of Tg value, and therefore instead of 80 C for non-plasticized pure PVC, the lowest temperature limit of PVC application may be extended to about 30 C. Such a PVC modification leads to improved properties of the final product like impact strength, elasticity and resistance to low temperature. Other modifiers, especially the ones in a form of nanofillers used in PVC compounds, may also give an impact on the changes of glass transition temperature. For example the addition of zinc and
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aluminum oxide, calcium carbonate, silica as well as MMT nanoparticles and halloysite nanotubes causes an increase of Tg comparing with the unfilled polymer [15e22]. Additionally, higher values of glass transition temperature were found for the CaCO3 nanocomposite with a PVC matrix in a form of a porous structure. However, the modification of PVC with MgAl layered double hydroxide (LDH) causes that the glass transition temperature of the PVC/LDH nanocomposites is slightly lower than of pristine PVC [23]. It should be noted that even a small addition of nanofillers can significantly affect the mechanical, dielectric, thermal and processing properties of PVC [15e22,24,25]. In our previous research we have studied the influence of carbon nanotubes on the PVC glass transition temperature, determined by different methods like DSC, DMTA and electrical loss factor. A considerable influence of both, measurement frequency and CNT content on the glass transition temperature of PVC, was ascertained [26]. The influence of another modifier, like methacryl polyhedral oligomeric silsesquioxanes (POSS) on the Tg of PVC was described by Soong et al. [27]. A decrease of Tg, and related changes of mechanical properties, like lowering of storage modulus has indicated that the addition of POSS may lead to a significant plasticizing effect of the PVC matrix. A similar effect was found in the case of PVC modified with another type of polyhedral oligomeric silsesquioxane containing 3-chloropropyl groups. The addition of this modifier in an amount of above 7 wt% resulted in an improvement of mechanical properties, especially impact strength [28]. Regarding the state of the art, and ecological requirements to lower the contain of phthalate derivatives as specific additives, the main task of our research was to investigate the influence of a low concentration of 3-chloropropyl-POSS on the effect of PVC plasticization, where only a slight amount of processing modifiers was incorporated into the PVC matrix. The mechanical dynamical and calorimetric investigation were chosen in this case. 2. Experimental section 2.1. Materials The compound consisting of 100 parts by weight of PVC S-61 Polanwil (Mn ¼ 47,500, Mw/Mn ¼ 2.25), produced by Anwil-Spolana Wloclawek (Poland), 4 parts by weight of liquid tinorganic stabilizer Patstab 2310, produced by Patcham and 1 part by weight of paraffin wax Naftolube FTP, produced by Chemson was used as polymer matrix. In order to minimize the influence of processing additives, the stabilizer and the paraffin wax were the only additional components used in the PVC composition. As nanofiller the polyhedral oligomeric silsesquioxane (CPePOSS) containing 3-chloropropyl groups on silsesquioxane cage (Fig. 1), in a form of viscous solid synthesized in the Department of Metal-Organic Chemistry of Faculty of Chemistry of Adam (Poland), was used. Mickiewicz University in Poznan
compounds were processed in a kneading chamber of the Brabender mixer. The thermostatic chamber with a capacity of 50 cm3 was filled with 56 g of the PVC dry blend and the compounds were kneaded at temperature of Tch ¼ 185 C, by 30 min1 rotor speed of main rotor, while the kneading friction was 1:1.5. Samples processed by this method were marked as M series. 2.2.2. Method S A solution of 6 wt% of PVC (with the addition of lubricant and stabilizer) in THF was prepared by mixing, and subsequently 0.5, 1, 1.5, 2 and 5 wt% of CPePOSS was added. The thin films were cast on the glass surface. The films were annealed at the temperature of 100 C until the full evaporation of the solvent. At the next stage the films were cut into smaller pieces and filled into the kneading chamber of Brabender along with rest part of PVC compound (S series). The kneading process was performed in the same conditions as mention in the case of M method. After cooling all compounds were grinded, and a part of the samples was used directly for the DSC investigations. The samples for the DMTA and tensile tests, in a form of plates with a thickness of 2 mm, were produced by compression molding at the temperature of 175 C. 2.3. Measurements 2.3.1. Determination of the glass transition The glass transition temperature of the PVC/POSS samples has been determined by DSC and DMTA measurements. The dynamic mechanical properties were analyzed by the means of Anton Paar MCR 301 apparatus in oscillatory mode, operating at frequencies (G) of 1 and 10 Hz, by heating rate of 2 C/min within the range of temperature between 20 C and 150 C, with the deformation level of 0.01%. Two different techniques of the Tg evaluation were used. First one based on the loss modulus curves, second one on the tangent d. The temperature by which the loss modulus or tangent d reveals the maximum value, was taken as the glass transition temperature. The calorimetric investigations were performed by DSC, using the Phoenix DSC 204 F1 Netzsch apparatus in standard conditions, with samples of 10 mg operating at heating rate of 5 C/min at the range from þ30 to þ120 C. The glass transition value was
2.2. Processing The unmodified PVC compound (PVCR) and PVC mixtures with 0.5, 1.0, 1.5, 2 and 5 wt% of CPePOSS were processed by kneading using a Brabender mixer. Two diverse procedures of compound preparation, differed by way of additive incorporation were used, thus two series of samples were obtained. 2.2.1. Method M The PVC in a form of a dry blend with following CPePOSS amount 0.5, 1.0, 1.5, 2 and 5 wt% was mixed in the Brabender chamber (type Plasti-Corder Pl 2200-3), operating at temperature 120 C, by rotor speed 30 min1, during 10 min. Subsequently,
399
Fig. 1. The structure of 3-chloropropyl-POSS T8 [28].
400
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estimated by DSC base line inflection point, as a middle point on the tangent line to the curve (comp. Fig. 2). The samples detached from the lumps, taken out from the Brabender mixing chamber, were crumbled into the form of powder. In the DSC experiment the samples of powder has been applied, originated from a mixture taken from various positions of the crumbled lumps. 2.3.2. Mechanical properties Static tensile tests were carried out by means of the Instron 5985 machine operated by BlueHill 2 program. Measurements were performed according to PN-EN ISO 527-1, always for 5 samples of each of the PVC/POSS composites, using a tensile rate of 10 mm/ min. The Young's modulus was evaluated for the elastic part of the strain e stress curve. The results of the Young's modulus, as a function of POSS content are presented on Fig. 11. 3. Results and discussion An exemplary DSC heating trace for PVC/POSS nanocomposites is presented in Fig. 2. For all samples the run of the DSC curve at the domain of the glass transition was similar, and the Tg values (Table 1) varied in the range between 78 C and 75 C (samples of M series), and between 79 C and 76 C for the samples of S series. The values of Tg, determined by DSC, for both series of PVC/ CPePOSS samples, alone with the additive concentration are presented in Table 1. As it may be seen, the value of Tg depends on the procedure of the compounds' preparation (M and S series). In general, for the samples prepared in solution a somehow higher values of Tg were noted; indicating that any plasticizing effect, due to the possible residual THF existed. Moreover, for the CPePOSS concentration below 1 wt% in the case of samples prepared by S method any changes of Tg may be observed. Only after addition of CPePOSS above 1.5 wt% causes decrease of glass transition temperature of from 1 to 3 C; an indication of a slight plasticizing effect of PVC. An identical value of Tg decrease in the case of PVC/POSS samples prepared by M method was noted; however it takes place already by the POSS concentration of 0.5 wt%. Similar result has been described in the literature for PVC modified with methacryl polyhedral oligomeric silsesquioxane [27], thought but this effect was found for higher concentration of modifier, namely between 5 and 15 wt%. The effect of POSS addition and preparation method may clearly be seen on Fig. 3 where the relative decrease of DSC evaluated Tg vs.
Fig. 2. DSC trace of sample prepared by M method (PVC with 0.5 wt% CPePOSS).
CPePOSS content of PVC/POSS samples, is presented. The relative changes of the Tg were estimated by following formula:
DTg 100%; TgPVC
(1)
where: DTg ¼ TgPVC TgPVC=POSS The data on Fig. 3 indicate that the difference in Tg values, related with the methods of samples preparation, are not significant if the modifier's concentration is above 2 wt%. In this case the relative decrease of Tg is higher than 2.5%. The example of the run of the loss modulus G00 as a function of temperature for PVC samples of S series with various CPePOSS contain, by frequency of 1 Hz, is presented in Fig. 4. As it may be seen (Fig. 4) the temperature position of the maximum of the loss modulus (a-transition) is decreasing in the range from 75 C to about 67 C by increasing POSS contain. The shift of the temperature position of G00 maximum towards lower values, with increasing CPePOSS contain is observed by both measurements frequencies of 1 and 10 Hz. A similar effect was observed by Ref. [27], while it has to be stressed that the methacrylPOSS content was considerably higher (up to 20 wt%) comparing with our experiments. Likewise in Ref. [28], a single maximum of mechanical loss spectra, allows to suggest a good compatibility of CPePOSS with PVC, and large interaction between CPePOSS molecules and PVC chains. The glass transition temperature dependence on CPePOSS determined by DMTA technique, for the measurement frequencies of 1 Hz and 10 Hz is presented on Figs. 5 and 6. For samples processed by both methods (S and M) the Tg value decreases with the increasing CPePOSS content. It should be noted that the samples prepared by M method reveal higher glass transition temperature, comparing with samples obtained by S method, independently of the frequency used in experiments. Moreover, a frequency dependence of Tg was found, i.e. as higher is the frequency as higher are the value of glass transition. A similar frequency related effect of the glass transition temperature for PVC/nanotubes composites has been described in our earlier paper [26]. The relative changes of DMTA determined Tg values, evaluated by Formula (1) may be seen on Fig. 7. As it follows from the graphs, for the CPePOSS contain below 2 wt% the changes of Tg are similar, independent on the method of preparation and the frequency of measurements. By additive amount above 2 wt% a somehow higher changes of Tg in the case of PVC samples, processed by M method
Fig. 3. Relative decrease of Tg by DSC method vs. CPePOSS content for both preparation methods.
ski et al. / Composites Science and Technology 117 (2015) 398e403 T. Sterzyn
401
Fig. 4. The loss modulus thermogram of PVC samples with various CPePOSS content produced by S method, G ¼ 1 Hz. Fig. 7. The relative changes of Tg by DMTA vs. POSS content for various processing methods and measurement frequencies.
Fig. 5. The Tg value by DMTA vs. CPePOSS content for various preparing methods, G ¼ 1 Hz. Fig. 8. The tangent d of PVC/POSS samples with various CPePOSS content produced by S method, vs. temperature, G ¼ 1 Hz.
Fig. 6. The Tg value by DMTA vs. CPePOSS content for various preparing methods, G ¼ 10 Hz.
Fig. 9. The Tg values obtained from the DSC thermograms, loss modulus and tangent d DMTA spectra, G ¼ 1 Hz.
ski et al. / Composites Science and Technology 117 (2015) 398e403 T. Sterzyn
o
Glass transition temperature, C
402
88 84
M series (from loss m.) S series (from loss m.) M series (from tan) S series (from tan) M series (from DSC)) S series (from DSC))
80 76 72
0
1
2
3
4
5
POSS content, wt.% Fig. 10. The Tg values obtained from the DSC thermograms, loss modulus and tangent d DMTA spectra, G ¼ 10 Hz.
Fig. 11. The Young's modulus values of PVC/POSS samples with various CPePOSS content produced by M and S methods.
Table 1 Tg values of PVC/POSS samples processed by two methods, as measured by DSC. CPePOSS content [wt%]
0 0.5 1.0 1.5 2 5
Tg [ C] M series
S series
78 77 77 76 76 75
79 79 79 78 77 76
were observed. The relative decrease of the Tg for the M samples is 6.5% whereas for the S series it is only 3%. This effect is more visible when the measurement frequency of 1 Hz was used. Probably, a somehow higher homogeneity POSS-cages distribution between the PVC macromolecular chains was achieved by CPePOSS and PVC powder mixing, prior to melt processing in the chamber of the Brabender kneader. The Tg value was also determined from the temperature run of the tangent (tan d) where the maximum of the temperature position of tangent d was taken as the Tg. On Fig. 8 the tan d of PVC/POSS produced by S method vs. temperature, for various CPePOSS content, by measurement frequency of 1 Hz, may be seen. The run of changes of tan d vs. temperature, for all PVC/POSS blends, is similar, however with increasing CPePOSS content the maximum of tan d is shifted towards lower temperature, an effect
observed already in the case of loss modulus G00 . Figs. 9 and 10 show the real values of Tg obtained by means of all three methods, DSC and DMTA (from loss modulus and tangent d). As it may be seen in Fig. 9 a difference of about 10 C between the Tg values evaluated by tan d and G00 , may be noted. The DSC determined Tg values are located between the DMTA e origin curves. This tendency is visible for measurement frequency of 1 Hz. For the DMTA tests performed by frequency of 10 Hz (Fig. 10) the DMTA e origin curves are shifted to higher temperature, moreover, by this frequency the run of CPePOSS content dependent changes of Tg, obtained from the loss modulus G00 is comparable with those obtained from the DSC. These differences between Tg values obtained from various methods are due to diverse way of stimulation of the chain motions (DSC vs. DMTA) and different time responses of motions (tan d vs. G00 ) [6], as well as on the stimulation frequency (measurements by DMTA at two frequencies) as shown already before [26]. The cage-like POSS molecules, regarding its nano-like dimension, may be placed between the PVC macromolecules, filling [28] and increasing [27] the free volume, lowering the intermolecular forces, thus acting as a plasticizer. From another point of view the multi-polar groups in CPePOSS molecule may form physical cross points with PVC molecules. Regarding the complexity of structure of gelatinated PVC, containing secondary crystallites formed during gelation simultaneously with an amorphous part (ideal homogenization involving total disappearance of grain structures during processing is not possible and not all primary crystallites are completely melted) [29e32], a general suggestion may be formulated, that various temperatures related to mobility changes are significantly dependent on the distribution level of CPePOSS in the polymeric matrix, and/or on various mobility answer on frequency charges applied during the DMTA and DSC measurements. As it follows from Fig. 11 a relationship between Young's modulus and the POSS content may be observed, where the decrease of the elastic modulus, due to the POSS addition may prove the plasticization effect. For all POSS concentration the samples prepared by S method (in solution) reveal higher modulus values relative to samples produced by powder mixing. 4. Conclusions A slight plasticizing effect of a low amount (up to 5 wt%) of CPePOSS on the PVC matrix has been found. Particularly, a shift of the a-transition temperature of PVC towards lower temperature, independently from the preparation method of PVC/CPOSS samples, as well as on the measurement method and frequency, was noted. Finally, it may be stated that the oligomeric POSS may be applied as non-phthalate containing plasticizers of the poly(vinyl chloride). Acknowledgment The financial support of the European Union Programme through the European Regional Development Fund under the Innovative Economy Operational Programme 2007e2013, obtained from the Project no. UDA- POIG.01.03.01-30-173/09, Priority I “Research and development of new technologies”, Action 1.3. “Support to R & D projects for companies, carried out by scientific bodies,” Sub-measure 1.3.1 “Development projects” is gratefully acknowledged. References [1] W. Brostow, T. Datashvili, K.P. Hackenberg, Synthesis and characterization of poly(methyl acrylate) þ SiO2 hybrids, e-Polymers 054 (2008) 1e13.
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