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Adhesion parameters at the interface in nanoparticulate filled polymer systems.

INTRODUCTION

The interfacial region or the interface between the matrix and the nanofillers determines nanocomposite properties with the emphasis on the three-dimensional nature of the region (1). Such interfaces are formed when polymers interact with very fine solid particles, and these interactions create interfacial regions with properties that are dramatically altered due to the restriction of polymer chain mobility in the vicinity of surfaces. The significant amount of polymer matrix, in which the polymer chains suffer from mobility constraints, is present in nanocomposites and in ionomers where the specific morphology dictates the related properties (2-5). The general aim of using the nanofillers is to create increased interactions at the nano-level in order to provide a thicker interface (6).

Adhesion science provides an explanation for the interfacial behavior of composites (1). Important adhesion parameters at the interface include the work of adhesion, the interfacial free energy, and the coefficient of wetting (7). The interface can be modified by selecting fillers of varying size and shape, specific surface area, packing, and surface activity (8), (9). Moreover pretreating the filler surface might alter it to a particular polymer matrix. An organic coating on the surface of inorganic filler, for instance, could in some cases act as a coupling agent between filler and matrix thereby changing the energy of the interaction in the interfacial area (10).

Previous results have demonstrated how a controlled surface pretreatment of the Ca[CO.sub.3] filler could either improve or deteriorate the properties of PVAc and PU composites (11-13), as well as filled SAN/EPDM blends (14).

The present article describes the surface characteristics, based on contact angle measurements of a polymer matrix and a Ca[CO.sub.3] filler before and after pretreatment with sodium stearate and/or silane. The adhesion parameters at the interface, that is the work of adhesion, the interfacial energy, and the coefficient of wetting, were calculated and the conditions of the optimal adhesion at the interface were correlated to the morphology, as well as to mechanical properties of the nanoparticulate systems under investigation.

EXPERIMENTAL

Materials

All the fillers and polymer samples used in the present study were of commercial grade, and used without further purification.

For the preparation of the filled composites and blends, a precipitated calcium carbonate (Ca[CO.sub.3]) filler with primary particles smaller than 100 nm, i.e. (N) 80 nm, and with a specific surface area [S.sub.BET] = 12.1 [m.sup.2] / g was used along with a commercially pretreated sodium stearate (NS) filler produced by Solvay Company, Rheinberg, Germany (Table 1).
TABLE 1. Description of the samples.

Filler Ca[CO.sub.3] Surface pre-treatment Polymer matrix

N Untreated

NS, NS2, NS4 Sodium Stearate PVAc

 PU

 AMPTS SAN/EPDM

NSi PU

Composite
([PHI.sub.f]] = 3-24 %)

 PVAc/N vs. PVAc/NS2

 PVAc/N vs. PVAc/NS4

 PU/N vs. PU.NS

 Pu/N vs. Pu/NSi

Polymer blend
([PHI.sub.f]] = 1-5 %)

 SAN/EPDM/N vs. SAN/EPDM/NS4


The laboratory-controlled pretreatment of the Ca[CO.sub.3] surface was done by addition of specific amount of sodium stearate (added in aqueous solution) to the calcium carbonate (in suspension): 1.35 w/w (NS2) and 4.50 w/w (NS4) with stearate coverage of 47% and > 100%, respectively. The quantities of stearate required for a certain surface coverage were calculated based on value of 0.21 [nm.sup.2] for the cross-sectional area of a stearate molecule (15). The silane pretreatment of Ca[CO.sub.3] with [gamma]-aminopropyltriethoxysilane AMPTS (NSi), was carried out by mixing Ca[CO.sub.3] with 1-propanol (weight ratio = 1:1.334) after which the mixture was homogenized for 2 h. The silane was then added to the mixture (weight ratio Ca[CO.sub.3]: silane = 1:0.3). After the homogenization the sample was dried at 130[degree]C and milled.

The specific surface area, [S.sub.BET], measured before and after the filler pretreatments remained unchanged.

Preparation of the Composite and Blend Samples

Composites of poly(vinyl acetate) (PVAc) filled with Ca[CO.sub.3] were obtained by mixing a water emulsion (55% solid content) of PVAc, supplied by Karbon nova d.d. Zagreb, Croatia, with three types of Ca[CO.sub.3] fillers (N, NS2, NS4 in Table 1). The filler contents [[empty set].sub.f], ranged from 3 to 12 wt%. The composite samples were prepared by separately mixing two parts; where the first part consisted of the PVAc emulsion, additional water and filler with a mass ratio of 1:1:2, and the second part was a PVAc emulsion with dibutyl phtalate and butyldiglycol acetate additives with a mass ratio of 1:0.08:0.02. The two mixtures were then poured together.

Polyurethane (PU) composites were prepared by mixing this linear hydroxyl polyester matrix in the form of pellets (Desmocoll 130, Bayer, Germany) of medium crystallinity and high plasticity with Ca[CO.sub.3] fillers (N, NS, NSi in Table 1). Acetone was added to the mixture to give composites containing 3-18 wt% filler. The weight ratio of the PU matrix to acetone was 30:100.

Both types of composites in the form of films with thicknesses [approximately equal to] 0.2 mm, were prepared by pouring the mixtures onto a polyethylene foil in the case of PU + PVAc + Ca[CO.sub.3] and onto a Teflon foil in the case of PU + Ca[CO.sub.3], and slowly drying them at room temperature for 7 days.

The commercial polymers styrene acrylonitril, SAN (Tyril 790, Dow Chemical Company with 24 wt% AN, melt flow rate, MFR = 21 g/10 min, 220[degrees]C, 10 kg) and ethylene propylene diene, EPDM (Keltan 312, DSM, containing 50 wt% ethylene and 4 wt% ethylidene-nenorbornene, Mooney viscosity 38 MU) were used for preparing the SAN/EPDM blends. The blend composition was maintained at a ratio 90:10 of SAN/EPDM and the filler fraction was varied from 0.5 up to 5.0 wt%. Subsequently, extrusion was carried out in a Haake Rheocord 9000 co-rotating twin screw extruder (Haake, Karlsruhe, Germany) with standard, nonintensive screw characteristics. The extruder head was fitted with a die with four circular holes (final diameter = 1 mm) and equipped with a pressure transducer. The following temperatures were chosen for the four heating zones (from hopper to die): 175/200/200/210[degrees]C and the screw rotation speed were set to 60 rpm. The extruded blends were then injection molded to form dumb-bell specimens.

A composite and blend samples are listed in Table 1.

Experimental Techniques

The specific surface area ([S.sub.BET]) of the filler samples was measured with an ASAP 2000 Micromeritics apparatus by using the nitrogen gas adsorption method.

The surface characterization of the Ca[CO.sub.3] filler, before and after surface pretreatment as well as of the polymer matrices used for the composites (i.e. PVAc and PU) and blend (i.e. SAN and EPDM) was performed by contact angles measurements on a Dataphysics OCA 20 Instrument. The following test liquids of analytical grade were used as received: water, formamide and diiodomethane. The disperse ([gamma].sub.1.sup.d]) and polar components ([[gamma].sub.1.sup.p]) of the surface tension for the selected test liquids were obtained from literature (16). The filler powders were compressed into discs at constant pressure using an IR die and the polymer samples were prepared in the form of films after solvent evaporation or molding. Contact angles were measured immediately after placing a 5 [micro] l drop of one of the test liquids on the surface of the material, and calculations of the contact angles were made from screen images of the drops by measuring their maximum height and width, assuming a spherical profile (17).

The surface energy of the solid samples, i.e. fillers and polymer matrices, as well as their polar and disperse components was calculated according a harmonic mean equation (18):

[[gamma].sub.1](1 + cos[theta]) = [4[[gamma].sub.1.sup.d][[gamma].sub.s.sup.d]/[[[gamma].sub.1.sup.d] + [[gamma].sub.s.sup.d]]] + [4[[gamma].sub.1.sup.P][[gamma].sub.s.sup.P]/[[[gamma].sub.1.sup.P] + [[gamma].sub.s.sup.P]]] (1)

[[gamma].sub.s] = [[gamma].sub.s.sup.d] + [[gamma].sub.s.sup.P] (2)

where [theta] is the contact angle of a test liquid (1) on a solid surface (s) of the filler or polymer; and [gamma] is the surface free energy. The superscripts d and p represent disperse and polar components of surface energy, respectively. However, when using these techniques, one should be aware of theoretical compromises or concerns, such as surface heterogeneity, due to the effects of filler compactation (19) and/or differences in surface orientation of the injected polymer samples (20). The scatter in the contact angle measurements was found to be around [+ or -]4[degrees].

Scanning electron microscopy on a JEOL JSM-330 was used to investigate the morphology of the composites after failure, whereas a JEOL JSM 6310 scanning electron microscope equipped with an X-ray analyzer, Oxford Instruments AN 10000, was used to inspect the blend samples.

The tensile properties of the composites were determined using a Zwick 1445 Universal Testing Machine at 23[degrees]C following the DIN 53455 procedure. The crosshead speed was 100 mm/min and the gauge length was 30 mm. The mechanical testing of the blend samples was carried out on an Instron Universal Testing Machine (Model 1185) at 23[degree]C, with a crosshead speed of 50 mm/min, and a gauge length of 25 mm. Moreover, the notched Charpy-impact resistance was determined following the DIN 53543 procedure.

Dynamic mechanical analysis (DMA) was carried out on a 983 DMA, TA Instruments at a heating rate of 10[degree]C / min with a standard clamp fixture.

RESULTS AND DISCUSSION

Adhesion Parameters at the Interface of the Composites

Contact angle measurement is a common method for evaluating the surface properties of solids (20). The results of surface free energy experiments of the fillers and polymers in the present study are presented in Table 2.
TABLE 2. The surface energy ([gamma]) divided into dispense
([[gamma].sup.d]) and polar ([[gamma].sup.p]) components for the matrix
(m) and filler (f) before and after the filler surface pre-treatment,
as well as related adhesion parameters at the interface of the
composites: interfacial energy ([[gamma].sub.f/m]); work of adhesion
(W); coefficient of wetting (S).

 Surface (mJ [m.sup.-2])

Sample [gamma] [[gamma].sup.d] [[gamma].sup.p]

Filler (f)
 N 80.7 45.9 34.8
 NS 32.7 32.7 0.0
 NS2 48.3 42.7 5.6
 NS4 9.6 9.6 0.0
 NSi 79.3 45.7 31.7

Matrix (m)
 PVAc 28.6 9.4 19.2
 PU 35.3 31.8 3.5

Composite (m/f)
 PVAc/N
 PVAc/NS2
 PVAc/NS4
 PU/N
 PU/NS
 PU/NSi

 Interface (mj [m.sup.-2])

Sample [[gamma].sub.f/m] W S

Filler (f)
 N
 NS
 NS2
 NS4
 NSi

Matrix (m)
 PVAc
 PU

Composite (m/f)
 PVAc/N 31.1 (a) 81.4 (a) 24.2 (a)
 PVAc/NS2 28.8 48.0 -9.3
 PVAc/NS4 19.2 18.9 -38.2
 PU/N 28.2 87.8 17.2
 PU/NS 3.4 64.5 -6.1
 PU/NSi 23.5 (a) 88.1 (a) 17.5 (a)

(a) Optimal adhesion: min [[gamma].sub.p/m]; max (optimal) W;
positive S.


After pretreating the surface of the Ca[CO.sub.3] filler (N) with increasing amounts of sodium stearate (giving NS, NS2, NS4), the characteristic bands in FTIR and XPS spectra of the pretreated fillers confirmed the existence of a hydrophobic stearate coating on the Ca[CO.sup.3] surface (21). The relatively high surface free energy of the untreated material was thus significantly lowered. On the other hand, when pretreating the Ca[CO.sup.3] filler with silane, characteristic bands in FTIR spectra (11) demonstrated that its surface energy remained unchanged. It should be noted that a high polar component of free surface energy is capable of promoting specific interactions at the interface (20).

The adhesion parameters at the interface in the PVAc and PU composites, between matrix (m) and filler (f) (Table 2) were calculated according to the Eqs. 3-5 (22):

Interfacial energy:

[[gamma].sub.f/m] = [[gamma].sub.f] + [[gamma].sub.m] - [4[[gamma].sub.f.sup.d][[gamma].sub.m.sup.d]/[[[gamma].sub.f.sup.d] + [[gamma].sub.m.sup.d]]] - [4[[gamma].sub.f.sup.p][[gamma].sub.m.sup.p]/[[[gamma].sub.f.sup.p] + [[gamma].sub.m.sup.p]]] (3)

Work of adhesion:

[W.sub.f/m] = [[gamma].sub.f] + [[gamma].sub.m] - [[gamma].sub.f/m] (4)

Coefficient of wetting:

[S.ub.f/m] = [[gamma].sub.f] - [[gamma].sub.m] - [[gamma].sub.f/m] (5)

The calculated adhesion parameters could be used to predict the optimal adhesion at the interface. The conditions for the optimal adhesion at the interface, i.e. the minimal interfacial energy, high work of adhesion and positive coefficient of wetting that tends to null (22) were partly indicated for the PVAc composite filled with the untreated Ca[CO.sub.3] (PVAc/N) and for the PU composite filled with the silane pretreated filler (PU/NSi). On the other hand, the lower work of adhesion and negative coefficient of wetting at the interface for the PVAc and PU composites filled with stearate pretreated Ca[CO.sub.3] indicated lower interactions between matrix and filler. Engineering of the interface by changing the surface of the filler would cause changes in the adhesion parameters at the interface, and might consequently change the properties of the PVAc and PU composites.

Morphological and Mechanical Properties of the Filled Composites

The morphology after failure as well as the dynamic mechanical and tensile properties of the PVAc and PU composites filled with untreated and surface pretreated Ca[CO.sub.3] fillers is presented in Figs. 1 and 2, respectively.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The micrographs of the PVAc/N (see Fig. 1) and PU / NSi (see Fig. 2) composites illustrate a cohesive failure in the composites with strong interfacial interactions. These results correlate with the conditions of optimal adhesion at the interface (Table 2).

The observed negative coefficient of wetting for PVAc/NS2 composite was in accordance with the visible signs of de-wetting at the interface after the failure (see Fig. 1). Although the changes in the interfacial adhesion parameters were low on the order of mJ [m.sup.-2] (Table 2) as compared with the fracture energy (often several kJ/[m.sup.2]), and a small absolute increase of the work of adhesion could lead to a large increase in fracture energy.

Failure in a material takes place at the weakest points of the structure. Pukanszky (23) has proposed an equation for the initiation of de-wetting in which the value of work of adhesion, [W.sub.mf], between matrix (m) and filler (f) is introduced:

[[sigma].sup.D] = - [c.sub.1] [[sigma].sup.T] + [([c.sub.2] [W.sub.mf]/R).sup.1/2] (6)

Here, [[sigma].sup.D] is the stress at dewetting, [[sigma].sup.T] is the thermal stress that should be zero in isothermally prepared samples, [c.sub.1], [c.sub.2] are constants and R is the radius of the filler particles.

The results of high work of adhesion in the PVAc/N and PU/NSi composites showed no signs of de-wetting at the interface after failure. This means, that [[sigma].sup.D] was high enough for the failure to be transferred from the interface into the matrix. The opposite behavior was found for the PVAc/NS2 composite, which presented a negative coefficient of wetting. In this case a visible process of de-wetting occurred at the interface due to the lower interactions caused by the stearate pretreatment. The increase in composite strength of PVAc/N (see Fig. 1) and PU/NSi (see Fig. 2) due to strong interactions and because of the absence of de-wetting was in correlation with the optimal adhesion parameters at the interface (Table 2).

The PVAc/N composite with a high filler loading (24%), cf. Fig. 1 demonstrated two tan [delta] peaks; one at about 60[degrees]C, corresponding to the glass transition of the PVAc matrix, as well as a second peak above 100[degree]C. The existence of a second transition is one of several unusual characteristics of composites filled with nanoparticles (3). The striking similarity of nanoparticulate filled polymers and certain random ionomers have invoked Tsagaropoulos and Eisenberg (3) to explain the second glass transition by a restricted chain mobility (clusters). Such regions of restricted mobility in the filled polymer were thought to be large enough to exhibit their own glass transition. The layer demonstrating the restricted chain mobility around the particles may not have had a large enough thickness to allow the detection of an independent second glass transition. Previous results with in-situ PVAc/N composites (12) have, however, illustrated the first appearance of such a transition at much lower loadings (0.75 wt%). Moreover, for a loading of 7 wt%, the intensity of this second peak transition was comparable with that of the composite material with 24 wt% filler content (from the present study). These results stressed the importance of an in-situ preparation of the nanocomposites as opposed to a preparation by standard mixing procedure in order to obtain strong interactions at the interface.

The dynamic mechanical properties of the PU composite filled with silane pretreated fillers (PU/NSi) are presented in Fig. 2. Although the position of the tan [delta] peak corresponding to the main transition of the soft segment, i.e. at about - 15[degrees]C, remained remained unchanged, the peak height was significantly decreased as compared with the peak of the composite sample filled with the untreated Ca[CO.sub.3] fillers. This behavior might be explained as a reduced participation of the polymer chain segments in the main glass transition 20).

Interface in Blends

The incorporation of a compatibilizer in immiscible polymer blends improves the characteristic heterogeneous morphology and provides a significantly improved adhesion between the continuous and dispersed phases. The relatively new concept of compatibilization by using rigid nano-particles has been found to be of great importance in theoretical as well as in industrial prospects (24). The calculated interfacial energy data in particulate filled blends can be used to predict the miscibility of the materials (20), and in such calculations a lower interfacial energy indicates a larger affinity between the components of the blend system (25). The adhesion parameters at the interface in SAN/EPDM blends filled with Ca[CO.sub.3] nano-particles are presented in Table 3. The lowest interfacial energy of the blend filled with NS2 filler (i.e. SAN/EPDM/NS2) indicated that the conditions were close to those for an optimal adhesion at the interface.
TABLE 3. Surface and interface characteristics in SAN/EPDM polymer
blends filled with untreated and pretreated Ca[CO.sub.3] fillers.

 Surface (mJ [m.sup.-2])

Sample [gamma] [[gamma].sup.d] [[gamma].sup.p]

Matrix (m)

 SAN (ml) 49.4 43.4 5.9

 EPDM (m2) 31.9 30.2 1.7

Filler (f)

 N 80.7 45.9 34.8

 NS2 48.3 42.7 5.6

 NS4 9.6 9.6 0.0

Polymer blend (m1/m2)

 SAN/EPDM

Filled polymer blend
(m1/m2/f)

 SAN/EPDM/N1

 SAN/EPDM/NS2

 SAN/EPDM/NS4

 Interface (mJ [m.sup.-2])

Sample [gamma]f/m

Matrix (m)
 SAN (ml)
 EPDM (m2)

Filler (f)
 N
 NS2
 NS4

Polymer blend (m1/m2)
 SAN/EPDM 4.2

Filled polymer blend (m1/m2/f) [gamma]f/m1 [gamma]f/m2

 SAN/EPDM/N1 20.2 30.4
 SAN/EPDM/NS2 1.3 (a) 4.6 (a)
 SAN/EPDM/NS4 27.8 12.6

 Interface (mJ [m.sup.-2])

Sample W

Matrix (m)
 SAN (ml)
 EPDM (m2)

Filler (f)
 N
 NS2
 NS4

Polymer blend (m1/m2)
 SAN/EPDM 76.6

Filled polymer blend (m1/m2/f) [W.sub.f.m1] [W.sub.f/m2]

 SAN/EPDM/N1 101.7 74.1
 SAN/EPDM/NS2 93.2 72.3
 SAN/EPDM/NS4 30.9 28.7

 Interface (mJ [m.sup.-2])

Sample S

Matrix (m)
 SAN (ml)
 EPDM (m2)

Filler (f)
 N
 NS2
 NS4

Polymer blend (m1/m2)
 SAN/EPDM 13.8

Filled polymer blend (m1/m2/f) [S.sub.f/m1] [S.sub.f/m2]

 SAN/EPDM/N1 2.9 10.2
 SAN/EPDM/NS2 - 5.6 8.5
 SAN/EPDM/NS4 -67.8 -35.1

(a) Optimal adhesion: min [[gamma] sub.f/m].


Zhang et al. (24) found that the location of nanofiller particles at the interface between two polymer phases was necessary to obtain a thermodynamically stable compatibilization.

The selectivity of a filler (i.e. whether it is located in one of the polymer phases, or migrates to the interface) can be estimated by using various criteria that include interactions between the fillers and polymers in a blend (25). For this, there exist several approaches. Clarke et al. (26) suggested thermodynamic requirements for the selectivity of a filler that are based on changes in the Gibbs function ([DELTA]G).According to this approach, a spherical filler particle (f) will move to the interface in a blend (m1/m2) and remain there if the following conditions are satisfied (Eqs. 7-9):

[G.sub.m1m2.sup.S][greater than or equal to]2[G.sub.m2f.sup.S] - 2[G.sub.m1f.sup.S] (7)

[G.sub.m1f.sup.S][greater than or equal to][G.sub.m2f.sup.S] - [G.sub.m1m2.sup.S]/2 (8)

[G.sub.m2f.sup.S][greater than or equal to][G.sub.m1f.sup.S] - [G.sub.m1m2.sup.S]/2 (9)

where, [G.sub.m1f.sup.S], [G.sub.m2f.sup.S], [G.sub.m1m2.sup.S] represent the interfacial energies between the components. On the other hand, the filler will be selectively distributed at the interface when the right-hand side of Eq. 7 is small, i.e. when [G.sub.m1f.sup.S] [approximately equal to] [G.sub.m2f.sup.S].

Schuster et al. (27) have shown that a small difference in solubility parameters, [delta], is the driving force in competitive adsorption of two polymers on a filler surface, i.e. the smaller this difference, the greater is the affinity of the polymer to the filler. This thermodynamic approach has been confirmed by Premphet and Horanont (28) who showed that, in filled blends, the filler (calcium carbonate) is selectively distributed selectively in the polymer phase with which it has the lowest interfacial tension. Sumita et al. (29) introduced a wetting coefficient called [omega]a which enabled the prediction of the filler selectivity either to one of the polymer phases A (m1) or B (m2), or to the interface:

[omega]a = [[[[gamma].sub.filler - m1] - [[gamma].sub.filler - m2]]/[[[gamma].sub.m1- m2]] (10)

Here, [gamma] is the interfacial energy between the filler and the m1 or m2 polymer phase, or between the two polymers m1 and m2. If [omega]a > 1, the filler is distributed within the A(m1) phase, if -1 < [omega]a < 1, the filler is located at the interface, and if [omega]a < -1, the filler is selective for the B(m2) phase. Hobbs et al. (30) introduced a similar concept for ternary blends. This concept is based on the spreading coefficient, [lambda], and is found to be transferable to a blend system consisting of two immiscible polymers A(m1) and B(m2), and a filler (f) that replaces the third polymer C. In this case, polymer C in Eq. 11 is replaced by the filler (f) which is encapsulated by one of the polymers:

[[gamma].sub.m1 - f] = [[gamma].sub.m2 - f] - [[gamma].sub.m1 - m2] - [[gamma].sub.m1 - f] (11)

where [[gamma].sub.ij] is the interfacial tension between the blend components m1, m2, and f.

The present article describes how several thermodynamic approaches were employed to asses the usability of criteria and to verify the Ca[CO.sub.3] filler selectivity in SAN/EPDM blends before and after filler surface pretreatment with sodium stearate (Table 4). It was obvious that the stearate pretreatment altered the Ca[CO.sub.3] filler selectively: the untreated Ca[CO.sub.3] fillers (N) with [[lambda].sub.ml-f]<0 and [omega]a > 1 were found to be present in the continuous SAN phase, while the fillers completely covered by sodium stearate (NS4) with [[lambda].sub.ml-f] > 0 and [omega]a < -1, migrated to the dispersed EPDM phase. The application of several criteria evaluating the interaction between filler and polymers in the blend confirmed these results; the location of the nanofiller particles on the interface between the two polymer phases (necessary for a good compatibilization) could be achieved by treating the fillers with stearate (giving NS2) in the otherwise immiscible SAN/EPDM blend (-1 < [omega]a < 1). This result has already been indicated in Table 3 for the NS2 filler, which displayed the lowest interfacial energy with respect to both the SAN and EPDM polymers. As of today, a number of authors have carried out investigations on particulate filled blends and their related properties in which the filler is selectively located at the interface (31), (32), or has migrated, to a certain extent, into other phases (25). Such studies demonstrate the importance of the new concept of compatibilization by using rigid particles (24), and they all have one thing in common: the control of the blend morphology by addition of nano-filler and the consequent achievement of a synergistic improvement of the blend properties (25).
TABLE 4. Conditions for optimal adhesion with fillers at interface
between continuous phase SAN, dispersed phase EPDM in blend filled by
untreated (N) and stearate pretreated (NS2, NS4) Ca[CO.sub.3] fillers.

Blend (m1/m2/f) SAN/ SAN/ SAN/
 EPDM/N EPDM/NS2 EPDM/NS4

Interfacial energy, [gamma]

 m1f-m1m2/2 18.1 -0.9 25.7

 m2f-m1m2/2 28.3 2.5 10.5

 m1f [greater than or equal to] - - +
 m2f-m1m2/2

 m2f [greater than or equal to] + Interface (a) -
 m1f-m1m2/2

 Wetting coefficient, [omega]a 2.4 0.8 (a) -3.6

 [omega]a > 1 (f in m1) SAN - -

 [omega]a <-1 (f in m2) - - EPDM

 -1 <[omega]a < - Interface (a) -
 1 (f at interface)

Spreading coefficient,

 [[lambda].sub. m1-f] -14.4 -7.6 (a) 11.0

 [[lambda].sub.m1-f] > 0 (f in m2) - - EPDM

 [[lambda]).sub.m1-f] <0 (f in m1) SAN - -

 [[lambda].sub.m1-f] [right - Close to -
 arrow 0 ( f at interface) interface (a)

(a) Optimal adhesion.


Results from previous investigations of SAN/EPDM blends filled with untreated and stearate pretreated Ca[CO.sub.3] are presented in part here, in order to have a comparative base for the interfacial properties under investigation in the present study. The morphology of the blend with surface pretreated filler (SAN/EPDM/NS4) illustrates that the filler was located in the dispersed EPDM phase (white dispersed filler particles around dark spherical EPDM phases of inferior size). Moreover, the results of the interface measurements, presented in Table 4, showed that also the NS4 filler was situated in the EPDM phase. A reduction of the dispersed phase size and a homogeneous distribution of the dispersed phase can be seen as indications of blend compatibilization (24). The morphology of the blend, or more specifically, the particle size of the dispersed phase has an appreciable effect on the improvement of toughness (33). A possible explanation for the decrease in the dispersed particle size in the blends containing nano-particles was the increase in the blend viscosity. This retarded the coalescence of the dispersed droplets and slowed down the rate of phase separation (24). The results in Fig. 3 illustrate some trends of improved toughness and strengths in SAN/EPDM blends filled with nano-particles, that were obtained by increasing the filler content (up to 5 wt%) and by pretreating the filler surface.

[FIGURE 3 OMITTED]

CONCLUSIONS

The optimal interfacial adhesion of PVAc composites filled with untreated Ca[CO.sub.3] fillers as well as of PU composites filled with silane pretreated fillers was determined by investigating the cohesive failure at the interface, which was in accordance with the improvement of morphological and mechanical properties, all due to the strong interactions and increased thickness of the interface. Contrary to these observations, the PVAc and PU composites filled with stearate-pretreated Ca[CO.sub.3] fillers with negative coefficients of wetting displayed and overall deterioration of the properties.

The stearate pretreatment of Ca[CO.sub.3] altered the filler selectivity in SAN/EPDM blends: untreated Ca[CO.sub.3] fillers were located in the continuous SAN phase, whereas the fillers covered by sodium stearate migrated to the dispersed EPDM phase. The presence of these particulate fillers at the interface was believed to give rise to compatibilization, thereby improving the blend morphology as well as its mechanical properties.

The presented results are evidence of a strong relationship between the adhesion parameters at the interface and the properties of nanoparticulate filled PVAc and PU composites and SAN/EPDM blends.

ACKNOWLEDGMENTS

The Croatian Ministry of Science and Technology, the Polychar--15 Organizing Committee and Dr W. Brostow are gratefully acknowledged for support.

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Correspondence to: Vera Kovacevic; e-mail: vkovac@fkit.hr

DOI 10.1002/pen.21132

Published online in Wiley InterScience (www.interscience.wiley.com). [C] 2008 Society of Plastics Engineers

Vera Kovacevic, Domagoj Vrsaljko, Sanja Lucic Blagojevic, Mirela Leskovac

Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, 10000 Zagreb, Croatia
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Author:Kovacevic, Vera; Vrsaljko, Domagoj; Blagojevic, Sanja Lucic; Leskovac, Mirela
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:4EXCR
Date:Oct 1, 2008
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