Thermosets are widely used in polymer matrix-reinforced composites because of their easy processing and good mechanical properties. However, the inherent brittleness of thermosets led to the development of different toughening strategies. Among the most recent toughening approaches, the use of electrospun veils is particularly advantageous.
Some authors have focused on non-soluble electrospun fibers based on the use of Nylon fibers. Akangah et al. [1
] showed that nylon-6,6 veils improved the impact force by about 60%, while reducing the damage growth rate to one-half. The nylon-6,6 veils also reduced the impact damage growth rate with impact force. Palazzeti et al. [2
] confirmed similar results. Beckermann et al. [3
] reported a detailed study on nylon-based commercial veils, which are based on the same concept.
Other authors have proposed a different approach, based on the use of soluble veils that dissolve under resin curing and phase separately, generating specific phase morphology. Li et al. [4
] showed that polysulfone veils can be dissolved in the interlaminar region, giving rise to a particulate morphology that led to toughness improvements of up to 42%. Cicala et al. [5
] showed that by varying the veil content and the resin formulation, a full range of morphologies in the interlaminar region can be obtained.
Recently, electrospun fibers that can be used to selectively disperse carbon nanotubes in the interlaminar region upon veil dissolution were reported [6
]. Cicala et al. [7
] extended this concept in order to develop a multifunctional composite. Lionetto et al. [8
] showed that this approach can be used to orient carbon nanotubes in a thermosetting matrix. Hamer et al. [9
] showed that nylon electrospun fibers filled with carbon nanotubes performed better in improving toughness compared to unfilled nylon electrospun fibers. Despite the interest for this approach, to the best of our knowledge, only carbon nanotubes have been studied.
Nanofillers have shown their efficiencies for the toughening of epoxy resin, too. Gojny et al. [10
] studied the effect of carbon nanotubes on the fracture resistance of epoxy resins. They showed an increase of toughness from 0.65 MPa·m1/2
for the unmodified resin to 0.80 MPa·m1/2
by adding 1% of multiwall carbon nanotubes functionalized with amine groups. Chul Kim et al. [11
] reported a similar improvement of fracture resistance by adding carbon black or nanoclay. Commercial core-shell rubber nanoparticles were reported as efficient toughening modifiers by Carolan et al. [12
]. In situ-formed silica nanoparticles were shown to improve both toughness and the interfacial bonding between carbon fibers and matrix [13
Among the different nanofillers that can be added to epoxy resins for tailoring their properties, polyhedral oligomeric silsesquioxanes (POSSs) are of high interest. POSSs are of interest as they can combine a hybrid inorganic–organic composition with nanosized silica-like cage structures. In addition to that, their dimensions are comparable to those of most polymeric segment or coil, and they can be bound to polymer chains. These properties allow POSS to reinforce systems at the molecular level. Finally, as for other nanoparticles, the large surface area allows the use of only small amounts to cause significant changes in the properties of the matrix [14
]. In several reviews, the advantages of POSS as functional fillers were discussed. POSS can be used to improve: glass transition temperature (Tg
], fire resistance [17
], resistance to photo-degradation [18
], and electrical and mechanical properties. POSS is truly a multifunctional filler. Recently, Konnola et al. [19
] reported about the efficiency of POSS for the tailoring of complex epoxy/rubber blends.
The effect of POSS on the viscoelastic properties of an epoxy network is discussed in the literature. Lee and Lichtenhan [15
] observed an increasing and broadening of the glass transition, which were explained as the result of the nanoscopic size of these POSS cages and their ability to hinder the motion of the molecular chains and network junctions. Many other authors reported POSS to lead to Tg
increases for epoxy resins [20
]. However, a reduction of Tg
with increasing POSS content is also reported [21
]. Frank et al. [22
] investigated the influence of the multiscale (i.e., nano and micro) dispersion of POSS, hypothesizing that well-dispersed POSS tended to plasticize polymers, lowering Tg
by increasing free volume and reducing interaction between polymer chains. This result contrasted with other findings, but it must be observed that the amino-propylisobutyl POSS used by Frank et al. had a long aliphatic chain, which might have affected the result.
Some authors have reported the addition of POSS to electrospun fibers [23
], but to the best of our knowledge, no report on the electrospinning of POSS dispersed in polyethersulphone has ever been reported. In addition to that, no one has ever tried to selectively disperse POSS by using soluble fibers or veils. The aim of the present paper is to prepare electrospun coPolyethersulphone (coPES) veils embedding POSS fillers, and to verify the use of these veils to selectively disperse POSS in the interlaminar region of composites manufactured by resin infusion.
The purpose of the present paper was to evaluate the feasibility of soluble thermoplastic veils as a means to disperse, in the interlaminar region of composite, POSS fillers without adding them to the neat resin.
The morphological analysis carried out on the electrospun veils demonstrated that electrospinning is an efficient technique to obtain thermoplastic veils with a high content of POSS dispersed both at the nano and microscale.
The analysis on the cured laminates confirmed the full dissolution of the coPES nanofibers within the processing ranges used, and, of foremost importance, proved that nanofillers can be effectively dispersed by using the soluble thermoplastic veils. The addition of POSS showed to have a significant effect on the final interlaminar morphology, and, in turn, on the viscoelastic properties of the laminates. The veils filled with 5% of POSS showed the best improvement in terms of glass transition temperature because of the synergetic effect of morphology control and POSS dispersion level.
The results achieved in this paper could be further expanded by filling the coPES veils with different types of nanofillers, such as carbon nanotubes or nanoclay. The use of filled thermoplastic veils is a promising approach to develop laminates with multiple layers of different types of nanofillers in a single processing step.