Solution Mixing

Solution mixing is a liquid-state powder processing method (particles in a polymer solution) that brings about a good molecular level of mixing, but at a cost depending on the solvent and its recovery. It is applicable to the polymers that can be dissolved or swelled by the solvent. Its advantages include large interface extension, improved interfacial bonding, low viscosity, fast solvent removal, easy casting, and high homogeneity of the ultimate composite materials.

Carotenuto et al. produced monolithic silica (356 nm)/ PMMA nanocomposites through solution blending [166]. They mixed a silica/methoxy propylacetate suspension with a PMMA solution (50 wt.% polymer in methoxy propylac-etate) and then sonicated the mixture for 5 min. The samples were centrifuged to remove bubbles and then dispersed into a mold. After being heated in a vacuum at 100 °C for 15 min, the nanocomposites were ready for use. The particle surface modification was obtained by treatment of the silica particles with an organic colorant (methyl red) at room temperature [167]. Electron microscopy indicated the formation of a polymer coating around the organically modified particles in the starting liquid precursor, and the resultant composites exhibited an excellent homogeneity. In contrast, the untreated SiO2 formed a lot of agglomerates (~1 /m) in the matrix after evaporation of the solvent due to the weak hydrogen bonds between the methyl-ester groups of the polymer and the hydroxyl groups on the particle surface. A significant improvement of the Young's modulus was observed after the introduction of the nanofiller. The Vickers hardness of a composite with 50 wt.% silica was increased by 90%, and the glass transition temperature

Table 9. Variation in mechanical performance of some polymer nanocomposites manufactured through melt mixing as compared with unfilled polymers.

Property variation1

Table 9. Variation in mechanical performance of some polymer nanocomposites manufactured through melt mixing as compared with unfilled polymers.

Property variation1

Filler

Polymer

Young's

Flexural

Tensile

Flexural Elongation

Impact

KIC and/or

Nanosized fillers

pretreatments

matrices

modulus modulus strength strength

to break

strength

GIC

Ref.

CaCO3 (TG nm)

Stearic acid coated

PP

+

N/A

N/A

N/A

N/A

+

[142]

CaCO3 (TG nm)

Titanate coupling agent

PP

+

N/A

N/A

N/A

N/A

+

[142]

CaCO3 (5G nm)

Titanate coupling agent

HDPE

N/A

N/A

N/A

+

+

N/A

[136, 13T]

CaCO3 (5G nm)

Macromolecular coupling agent

HDPE

N/A

N/A

N/A

++

+

N/A

[13T]

CaCO3 (8G nm)

Phosphonate coupling agent

PP

+

+

+

N/A

N/A

++

N/A

[14G]

CaCO3 (3G nm)

Alumínate coupling agent

PVC

N/A

N/A

+

N/A

+

N/A

[165]

CaCO3 (3G nm)

Aluminate coupling agent

Acrylate resin/PVC

N/A

N/A

++

N/A

+

+

N/A

[165]

SiO2 (T nm),

Irradiation grafting

PP

+

N/A

+

N/A

+

++

++

[1GT, 153]

CaCO3 (6G nm)

SiO2 (2G nm)

Silane coupling agent

PMMA

N/A

N/A

+

——

+

N/A

[138]

SiO2 (T, 16, 4G nm)

Untreated

PP, LDPE, PVC, Nylon 6, PMMA

+

N/A

+

N/A

N/A

N/A

N/A

[4G, 155-159]

SiO2 (T nm)

Untreated

PVAc

+

N/A

+

N/A

——

N/A

N/A

[16G]

SiC/Si3N4(2G nm)

Titanate coupling agent

LDPE

N/A

N/A

++

N/A

+

++

N/A

[134]

SiC/Si3N4(2G nm)

Titanate coupling agent

CPE/PVC

N/A

N/A

+

N/A

N/A

++

N/A

[135]

TiO2 (2G nm)

Macromolecular dispersant

HIPS

+

N/A

+

N/A

+

+

N/A

[15G]

Al2O3 (33 nm)

Untreated

PPS

N/A

N/A

N/A

N/A

N/A

N/A

[154]

Carbon nanotubes

Untreated

PMMA

b

N/A

N/A

N/A

N/A

++c

N/A

[162, 163]

aThe symbol + corresponds to a moderate rise in the properties, ++ to a prominent improvement, to a prominent decay.

b Multiwalled carbon nanotubes serving as the fillers. c Single-walled carbon nanotubes serving as the fillers.

and — describes a moderate decrease in the properties and —

aThe symbol + corresponds to a moderate rise in the properties, ++ to a prominent improvement, to a prominent decay.

b Multiwalled carbon nanotubes serving as the fillers. c Single-walled carbon nanotubes serving as the fillers.

and — describes a moderate decrease in the properties and —

became 5 °C higher than the characteristic value of the pure PMMA.

With the same mixing technique, Carotenuto et al. prepared nanocomposites consisting of poly(methacrylic methylester) and monodispersed Cu2(OH)2CO3 particles

[168]. They found that the thickness of the polymeric layer around the particles increased with the polymer concentration. Hence it may be possible to manipulate the composite morphology and properties in-situ by changing the degree of wrapping of the organic molecules on the inorganic filler particles.

Because of the high mobility of nanoparticles in a polymer solution, composites with specific spatial distribution of the nanoparticles can thus be made, based on the self-organization process. Hamdoun et al. prepared stable solutions of polystyrene-polybutylmethacrylate P(S-b-PBMA) diblock copolymer (MW = 82,000, molecular weight poly-dispersity index = 1.05) and y-Fe2O3 (3.5 nm) in toluene

[169]. To acquire good solubility, the nanoparticles were precovered by a PS layer of short chains (MW = 13,000). Eventually, a thermodynamically stable nanocomposite was obtained in three steps: (i) mixing of the two solutions; (ii) spin-coating the composite solution on a solid substrate; (iii) annealing the resulting film at 150 °C under vacuum, until a quasi-equilibrium self-organization was reached. Microscopic observation revealed a periodic arrangement of the particles in the polymer matrix, which reproduces the ordered arrangement of the copolymer mesophase. It is expected that the macroscopic properties of the composite could reflect both the physical properties characteristic of the nano-objects and those that are specific to the large mesh periodic structure of the matrix.

During a series of works by Shang et al. focusing on the effect of interfacial bonding on the mechanical performance of particulate filled polymer composites [41, 170, 171], a silica/ethylene vinyl acetate (EVA) composite system was manufactured by means of a solution mixing technique. The silica particles, either 600 nm or 14 nm in diameter, were modified by a thermal treatment at different temperatures or by a thermal treatment combined with a chemical treatment with trimethylchlorosilane. Then the particles were incorporated into the EVA/benzene solution. For the coarser silica composite solutions, a magnetic stirrer was used to improve the particle dispersion. For the finer particles, sonication was employed instead. Benzene was chosen as a solvent because it is a good and nonpolar solvent for EVA copolymer. The silica particles were expected to be well distributed in the polymer solution since the solvent induces optimal polymer chain length expansion that stabilizes the silica powders in solution [171]. The tensile strength values of the composites indicated that the SiO2 (600 nm)-filled versions had a lower tensile strength than the unfilled EVA, suggesting that the particles had a weakening effect. In contrast, the composites filled with SiO2 (14 nm) were stronger than the matrix polymer. Within the lower loading range of SiO2 (14 nm), the tensile strength of the composites increased with increasing volume fraction of filler, reaching a maximum at around 4 vol.%. The composites retained their higher tensile strength compared with the unfilled polymer up to 15 vol.% of filler [41]. On the other hand, the Young's moduli of the composites increased with a rise in the content of either SiO2 (600 nm) or SiO2 (14 nm) [170]. For identical filler volumes, the SiO2 (14 nm) composites were stronger than the SiO2 (600 nm) ones, owing to the fact that the fine particles have more surface area available to bond and adsorb matrix polymer. These bonded and adsorbed polymer chains become stiff as a result of a loss in flexibility.

To diminish mixing problems and to improve the dispersion of fine particles in polymers, Vollenberg and Heikens combined solution mixing and melt mixing when preparing alumina-filled composites [172]. They first made master batches containing 30 vol.% of alumina beads (35 nm and 400 nm in diameter, respectively) by adding the particles to the polymer solutions. Having been stirred for a few hours, the mixture was poured onto a large surface, allowing the evaporation of the solvent overnight. The master batches were then dried at 100 °C under vacuum. Finally, the unfilled polymers were mixed with the master batches on a two-roll mill to obtain the desired composites. The polymers employed in their work were PS, styrene-acrylonitrile copolymer (SAN), and polycarbonate (PC). The solvent for the former two polymers was ethylene acetate, and methylene chloride was used to dissolve the latter one. Similar to the case stated above, the Young's moduli of the composites appeared to be dependent on the particle size. At a constant volume fraction of filler, the smaller alumina particles (35 nm) generated a more significant reinforcing effect than the larger ones (400 nm).

To compare the properties of nanocomposites, prepared through a sol-gel route, with particulate-filled systems, Landry et al. added nano-SiO2 (7 and 14 nm, respectively) to a 10 wt.% solution of PVAc in tetrahydrofuran (THF) and mixed them for several hours [160]. The solution was then cast in a Teflon mold, yielding opaque, broken pieces of the composites. Eventually, transparent nanocomposite films were obtained by melt-pressing the substance at 100-120°C. Tensile testing indicated that below the glass temperature, Tg, of PVAc, the solution-mixed samples possess properties similar to those of the melt-mixed ones (see last section). At a temperature higher than Tg, however, the solution-mixed composites decayed rapidly, in sharp contrast to the rubberlike plateau of the in-situ composites extending to about 300 °C. Again these observed differences were attributed to the differences in particle-particle connectivity.

Shaffer and Windle fabricated carbon nanotube composite films by carefully mixing aqueous poly(vinyl alcohol) (PVA) solutions with catalytically grown carbon nanotube dispersions followed by subsequent casting and controlled water evaporation [173]. The electrostatically stabilized dispersion of carbon nanotubes in water was produced when the nanotubes had been pretreated by a suitable oxidation [174]. Successful preparation depended crucially on maintaining stable colloidal mixtures of nanotubes and polymer. Dynamic mechanical analysis of the composites suggested that the presence of carbon nanotubes stiffens the material, particularly at high temperature, and in some cases retards the onset of thermal degradation. The authors concluded that carbon nanotubes might not ideally be suited to a straightforward reinforcing role, but that they could find application as a modifier for polymers, particularly as an improved matrix for conventional fiber composites for service at high temperatures.

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