1 Introduction

Poly(butylene terephthalate) (PBT), a well-known commercially available semicrystalline thermoplastic aromatic polyester, aroused significant attention in vast applications in automobile industry, electrical and biomedical devices [1–7].

It actually shows many valuable properties including high crystallization rate, high impact strength and thermal stability, low molding temperature, high rigidity and dimensional stability, hardness, abrasion and solvent resistance, good electrical insulation, short cycle times in injection molding, high water resistance, and good surface appearance.

Nevertheless, pure PBT has low impact strength and a heat distortion temperature. These properties can be enhanced by adding nanosized inorganic fillers such as nanoclays, silica, and carbon nanotubes into PBT matrix [8–13].

Among those SiO2 nanoparticles are the most preferentially used filler. They have obvious strengthening effects and thus can significantly improve both physical and mechanical properties of filled polymers [14].

Within sol–gel procedures, the Stöber method is widely used to produce silica particles due to its ability to control the particle size, size distribution, and morphology through systematic tuning of reaction parameters.

Indeed, achieving a good dispersion of nanoparticles within the matrix still remains the key challenge in this field and it is the bottleneck that limits significant improvements of nanocomposites properties.

The techniques commonly used for the production of silica–polymer nanocomposites can be categorized into three classes: solution mixing, in situ polymerization, and melt mixing processes [15, 16]. Using solvents or coupling agents [17] during solution mixing and in situ polymerization processes usually allows fine and uniform dispersion of nanoparticles, but it is not exploited because of its incompatibility with current industrial compounding facilities.

Instead, the absence of solvents makes the melt mixing process environmentally and economically sustainable, even if, for this procedure, a homogenous dispersion of nanofillers still remains a challenging aspect.

According to the universally accepted “aggregation model” [18, 19], nanometer-sized particles formed during the “nucleation” stage of sol–gel chemistry are assumed to grow solely by an aggregation process. Furthermore, interaction with macromolecules in solution can bring to an adsorbed layer and reverse the aggregation process of particles [20, 21].

However, the particles obtained using the Stöber method if dried and stored from days to week form strong interparticle bonds that lead to the formation of stable agglomerates that no longer resuspend [16, 18, 22–24]. On the other hand, avoiding the drying stage, even small amount of solvents carried out by the included particles may adversely affect the performances of the final compounds. Therefore, in order to prevent the formation of irreversible aggregates, herein, we propose a novel straightforward methodology, whereby Stöber SiO2 nanoparticles [25] are mixed with PBT soon after their synthesis, to produce PBT-SiO2 blends at high filler content.

The PBT shell prevents particles aggregation, making them stable during time and adaptable to be stored and then used even months after their synthesis.

Nanocomposites were characterized by multiple techniques. In detail, morphology and dispersion of the filler were assessed through scanning electron microscopy (SEM).

Thermal parameters as the glass transition temperature, the melting and the crystallization temperature as well as the degree of crystallinity were driven from differential scanning calorimetry (DSC). Thermogravimetric analysis (TGA) provided information of thermal stability and the amount of silica within the samples. Moreover, dynamic mechanical analysis (DMA) provided information on viscoelastic properties (storage modulus and mechanical loss factor).

We proved that PBT not only prevents particles aggregation, but also actually acts as an endogenous coupling agent ensuring affinity between the phases, through hydrogen bonds.

The proposed methodology actually works as an effective strategy that can overcome the major drawback of particles irreversible aggregation, ultimately leading to a very good dispersion of the filler within the polymeric matrix. Furthermore, it is highly compatible with current industrial compounding facilities and it ensures a far-reaching implementation in the preparation of polymer nanocomposites.

2 Experimental section

2.1 Materials

Poly(butylene terephthalate) (PBT) Pocan B 1505 was supplied by Lanxess GmbH (matrix). Ethanol was supplied by Fluka, Milan, Italy. Tetraethoxysilane (TEOS), ammonium hydroxide, and trifluoroacetic acid were purchased from Sigma-Aldrich, Milan, Italy, and used as received.

2.2 Synthesis of samples

Silica gel nanoparticles (SiO2 NPs) were produced through the Stöber method [25]. Appropriate amounts of TEOS, ammonia, and water were added to ethanol; the concentrations of the obtained solution were 0.170 M TEOS, 1.0 M NH3, and 2.0 M H2O. The system was kept at room temperature under magnetic stirring for 2 h, and the produced particles were obtained by centrifugation and repeated washing (3 times at 12,000 rpm with distilled water). Then, an appropriate amount of the obtained wet precipitate was resuspended in a solution of PBT dissolved in trifluoroacetic acid, in order to have a 40 % w/w of silica gel nanoparticles into the final PBT-SiO2 mixture. The mixture was dried overnight at 50 °C under vacuum, and the obtained dried powders will be indicated in the following as premixed-PBT/SiO2 NPs.

Nanocomposites were prepared using a mini twin-screw extruder (DSM Xplore), under nitrogen, at 245 °C and a screw speed of 120 rpm, for 3 min.

Both dried SiO2 nanoparticles, used soon after their synthesis, and dried premixed-PBT/SiO2 nanoparticles were incorporated in pure PBT to obtain a final concentration of 3.0 wt% in silica nanoparticles. They will be indicated as PBT/SiO2 NCs and premixed-PBT/SiO2 NCs, respectively.

Furthermore, both SiO2 and premixed-PBT/SiO2 nanoparticles were stored at room temperature for 5 months.

2.3 Physico-chemical characterization

Morphological analysis was carried out by scanning electron microscopy (SEM) using a FEI Quanta 200F microscope. All samples were gold coated before the analysis.

Fourier transform infrared (FT-IR) transmittance spectra were recorded with a Nicolet FT-IR spectrometer (Thermo Fisher) using a single-reflection attenuated total reflectance (ATR) accessory with a resolution of 4 cm−1 and 32 scans.

Thermogravimetric analysis was performed in a thermogravimetric apparatus, TGA Q5000 TA Instruments. Samples of pure PBT, premixed-PBT/SiO2 NPs, PBT/SiO2 NCs, and premixed-PBT/SiO2 NCs were heated to 700 °C at a rate of 20 °C/min under nitrogen atmosphere. Differential scanning calorimetry (DSC) was performed using a DSC Q20 TA Instruments apparatus. Encapsulated specimens (about 10 mg) of all investigated compounds were subjected under nitrogen to the following procedure: heating between 0 and 250 °C at 10 °C/min (first run), isotherm at 250 °C for 5 min in order to remove previous thermal histories, cooling at 10°/min from 250 to 0 °C, and final reheating at the same rate from 0 to 250 °C (second run). The neat PBT matrix was taken as the reference.

In order to gain an insight into the effects of the loaded silica nanoparticles on the degree of crystallinity (χ c) of PBT matrix, the normalized χ c values of the samples were determined, using Eq. (1):

$$\chi_{\text{c}} = \frac{{\Delta H_{\text{m}} }}{{\Delta H^{0} \left( {1 - \frac{{\% {\text{wt}}\;{\text{filler}}}}{100}} \right)}} \times 100$$
(1)

where ΔH 0 = 142 J/g is the melting enthalpy of 100 % crystalline PBT [26] and ΔH m is the melting enthalpy of the samples, measured from DSC.

Dynamic mechanical properties were analyzed using a Triton 2000 Analyser. Dynamic mechanical spectra of 40 mm × 10 mm × 3 mm were recorded for all materials in single cantilever bending mode, at a frequency of 1 Hz and using a heating rate of 3 °C/min in the range −20–140 °C.

3 Results and discussion

3.1 Morphology and microstructure

Figure 1 shows the SEM micrographies of SiO2 NPs observed soon after their synthesis (Fig. 1a), SiO2 NPs (Fig. 1b), and premixed-PBT/SiO2 NPs (Fig. 1c) both observed 5 months after their synthesis.

Fig. 1
figure 1

SEM micrographies of just prepared SiO2 NPs (a), SiO2 NPs (b), and premixed-PBT/SiO2 NPs (c) both 5 months after their synthesis

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As shown in Fig. 1a, SiO2 NPs appear separated and show spherical morphology with a narrow size distribution and an average diameter of 70 nm. The same nanoparticles form during time unambiguous aggregates as shown in Fig. 1b.

Figure 1c shows well-separated nanoparticles with an average size of 120 nm in diameter, larger than bare SiO2 NPs (70 nm). The increase in size can be attributed to a deposition of a PBT layer onto the nanoparticles surface formed during the drying process. Moreover, premixed-PBT/SiO2 nanoparticles kept both morphology and size even 5 months later their synthesis, as driven from comparison of Fig. 1c with SEM micrograph of premixed-PBT/SiO2 NPs observed soon after their synthesis (see Online Resource, ESM 1).

As it is known, nanoparticles produced through the Stöber method are easily resuspended in an unagglomerated form as just prepared.

During the drying process, the elimination of water molecules leads to the increase in the sol concentration and creates fluid drug, which causes the particles to come closer to each other (Fig. 1b). These particles touching each other form reversible aggregates where the dominant attraction forces are van der Waals and capillary types.

In addition, at the interparticle contacts, polycondensation reactions can occur if the silanol groups are close enough to react and these reactions increase in the presence of water [16, 22–24].

Indeed, due to the high hydrophilicity of silica gel nanoparticles, even after removal of most of the water, during drying step, particles do contain residual amount of adsorbed water, which promotes polycondensation reaction in the solid state over time. The effects of these processes can be dramatic and lead to irreversible aggregates after a certain period of time.

This is confirmed by SEM micrography of SiO2 NPs observed 5 months later their synthesis (Fig. 1b), where particles shape and morphology cannot be distinguished any longer making them not stable during time and not adaptable to be stored for later use.

The use of premixed-PBT/SiO2 NPs as filler actually avoids the further evolution of silica nanoparticles and thus greatly improves their stability, as shown in Fig. 1c.

Furthermore, a PBT layer on the particles provides a steric barrier toward aggregation, thus allowing particles redispersion even at long times.

Micrographs of both PBT/SiO2 (Fig. 2a) and premixed-PBT/SiO2 (Fig. 2b) nanocomposites show a good dispersion of the filler in the polymeric matrix, confirming that silica nanoparticles incorporated just prepared as well as a PBT layer onto the nanoparticles surface, i.e., premixed-PBT/SiO2 NPs, are needed to avoid the formation of aggregates.

Fig. 2
figure 2

SEM micrographs of PBT/SiO2 NCs (a) and premixed-PBT/SiO2 NCs (b)

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Actually, comparing samples just prepared and after 5 months later their synthesis, no differences are observed into the morphology not only of premixed-PBT/SiO2 NPs but also of premixed-PBT/SiO2 NCs (not reported), giving further confirmation to the stability of premixed-PBT/SiO2 nanoparticles.

Figure 3 shows the FT-IR spectra of pure PBT, SiO2 NPs, and premixed-PBT/SiO2 NPs. As can be seen in the FT-IR spectrum of pure PBT, the most characteristic bands fall in the region between 1270 and 1710 cm−1 and can be assigned to C=O stretching vibration of the ester carbonyl group (1241 cm−1) and to C–O stretching vibration (1098 cm−1). The bands at 940, 870, and 730 cm−1 are related to stretching vibration of ether (C–O–C), aromatic out of plane C–H bending, and bending of the benzene rings, respectively [27, 28]. The FT-IR spectrum of SiO2 nanoparticles shows the main characteristic bands at 1070, 950, and 800 cm−1 attributed to Si–O–Si stretching vibration modes in SiO4 units, non-bridging Si–O stretching vibration, and Si–O–Si bond vibration between two adjacent tetrahedral, respectively [17, 29–31]. The bands in the spectrum of premixed-PBT/SiO2 NPs are due to the overlapping of the main signal of both components. Moreover, a new band is visible at about 1670 cm−1, in the carbonyls’ stretching region (1780–1650 cm−1), where both silica and PBT components show no absorption bands.

Fig. 3
figure 3

FT-IR spectra of pure PBT, SiO2 NPs and premixed-PBT/SiO2 NPs

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Any changes observed in this region should be directly attributed to those changes in the carbonyl group environment of PBT, such as the formation of hydrogen bonds between the hydroxyl groups of SiO2 particles surfaces and carbonyl groups of PBT chain [32–34].

The strength of the bond considerably affects the energy of the covalent bonds on interacting species; hence, a frequency shift can be observed [32].

3.2 Thermal properties

Thermogravimetric (TG) analysis shows a weight reduction starting from about 380 °C due to the decomposition of PBT. The amount of inorganic component within the samples was 40 %w in premixed-PBT/SiO2 NPs and 3 %w in both PBT/SiO2 and premixed-PBT/SiO2 nanocomposites, as driven from residual weight at 700 °C in TG curves (see Online Resource, ESM 2).

Figure 4a shows the DSC curves of second heating of pure PBT, PBT/SiO2 NCs, and premixed-PBT/SiO2 NCs, while Fig. 4b shows the DSC curve recorded on cooling of premixed-PBT/SiO2 NCs and representative of those obtained for the other samples (see Online Resource, ESM 3).

Fig. 4
figure 4

DSC curves of second heating a of PBT, PBT/SiO2 NCs, and premixed-PBT/SiO2 NCs and of cooling b of premixed-PBT/SiO2 NCs

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All data derived from the thermal analysis of samples are summarized in Table 1.

Table 1 Thermal properties and crystallinity of pure PBT, PBT/SiO2 NCs, and premixed-PBT/SiO2 NCs
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In particular, with regard to the melting region, a splitting of the signal is observed for the neat matrix and for the composite formulation prepared by mixing the matrix and SiO2 particles, whereas a single peak with a slight shoulder on the left side appears in case of composites prepared by dilution of the premixed sample. The former behavior, first observed for PBT by Hobbs and Pratt [35], is generally explained assuming melting and recrystallization of thin, unstable crystals during the calorimetric scan followed by melting of more perfect crystals [36, 37].

In case of nanocomposites, it is probable that this complex fusion behavior is affected by the achieved filler distribution. An improvement of this aspect, while not promoting effect of nucleation, at least under the used operative conditions, may limit the organization of polymer macromolecules in small defective crystals favoring above all more larger stable crystals.

According to this consideration, a better distribution of SiO2 nanoparticles can be assumed for nanocomposites from premixed-PBT/SiO2 NPs.

Moreover, as shown in Table 1, there is an increase in melting enthalpy values (ΔH m) from 21.56 J/g (pure PBT) to 41.67 J/g (PBT/SiO2 NCs) and to 50.24 J/g (premixed-PBT/SiO2 NCs) and an increase in crystallization enthalpy values (ΔH c) from 22.38 J/g (pure PBT) to 44.62 J/g (PBT/SiO2 NCs) and to 46.38 J/g (premixed-PBT/SiO2 NCs).

Also crystallinity percentage (χ c: evaluated using Eq. 1) shows an increase from 15.2 % (pure PBT) to 30.2 % (PBT/SiO2 NCs) and to 36.5 % (premixed-PBT/SiO2 NCs). These results confirm a good dispersion of the inorganic filler into the polymer matrix and the moderate interaction due to the formation of hydrogen bonds between the two components, in agreement with SEM micrographs and FT-IR spectra [38–40].

The dynamic mechanical properties of pure PBT and PBT/SiO2 nanocomposites were studied by DMA. The temperature dependence of the loss factor (tanδ) and the bending storage modulus (E′) at 1 Hz are shown in Fig. 5a, b, respectively, and the derived data are summarized in Table 2.

Fig. 5
figure 5

DMA curves of tanδ (a) and E′ (b) of PBT, PBT/SiO2 NCs, and premixed-PBT/SiO2 NCs

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Table 2 Glass transition temperature, loss factor, and bending storage modulus of pure PBT, PBT/SiO2 NCs, and premixed-PBT/SiO2 NCs
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The temperature of the maximum tanδ signal, centered at about 59 °C for pure PBT and PBT/SiO2 NCs, appears to be shifted at a higher temperature 67 °C for the premixed-PBT/SiO2 NCs. Taking into account that this temperature is usually attributed to the glass transition temperature (T g), the detected increase in this parameter proves again more matrix–filler interactions only possible if there is a better dispersion of the included nanoparticles in systems prepared from premixed samples.

Therefore, this result suggests that the interactions between the two phases, through hydrogen bonding confirmed by FT-IR spectra, play a fundamental role in inhibiting the polymer segmental motion.

Furthermore, the hard non-flexible silica domains could present an obstacle to random chain segment movements of the polymer matrix. This effect (different from direct interaction of the two phases) is known in literature as “topological constraint” and often could cause a Tg increase even if the phase’s affinity to each other is poor [17, 41].

Finally, as clearly shown in Fig. 5a, the tanδ peak is affected particularly in terms of height. In fact, as widely established, the inclusion of rigid nanofiller gives rise to partial immobilization macromolecular chains and, consequently, it reduces the mechanical damping ability of composite materials with respect to the neat hosting matrix. Considering that this effect is much more pronunced for greater matrix-filler interface, the obtained results confirm the achievement of a better dispersion of SiO2 nanoparticles in case of premixed-PBT/SiO2 nanocomposites.

Figure 5b shows that the E′ values of PBT/silica nanocomposites are much higher than that of pure PBT over the whole temperature range with an effect more pronounced for the composite formulation obtained by diluting in PBT the premixed-PBT/SiO2 nanoparticles. In particular, compared with pure PBT, the E′ value of PBT/SiO2 NCs increases from 1.84 to 3.62 GPa at −10 °C (97 % increase) and from 0.37 to 0.75 GPa (100 % increase) at 120 °C, respectively. Instead, increases in the premixed-PBT/SiO2 NCs are approximately equal to 137 and 170 %, respectively. These significant improvements in E′ values of PBT/silica nanocomposites are ascribed to the combined effect of the homogeneous dispersion of silica within the PBT matrix, mainly achieved using the premixed-PBT/SiO2 nanoparticles, and the stiffening effect of the nanoparticles [13, 42].

Furthermore, these effects, particularly significant at temperatures below the glass transition temperature of the PBT, even if partly supported by the detected increase in the degree of crystallinity, suggest the occurrence of structural modifications increasing the stiffness of the amorphous phase. The finding that these modifications are not accompanied by adequate increases in the glass transition can be ascribed to the assumption that included nanoparticles may act as antiplasticizer as reported elsewhere [43].

4 Conclusions

In the present work, we proposed a successfully novel procedure to prepare PBT/SiO2 nanocomposites with a very good dispersion of the filler within the polymeric matrix.

The use of premixed-PBT/SiO2 filler offered the possibility to stabilize dried nanoparticles, preventing further irreversible agglomeration process making them adaptable to be stored and then used even months after their synthesis.

The significant increase in both thermal and dynamic mechanical properties of the obtained nanocomposites, compared to pure PBT, gave a further confirmation of the very good dispersion of the inorganic filler in the polymer matrix.

Experimental results proved that the affinity between the two phases was ensured by hydrogen bond between SiO2 gel nanoparticles and PBT that actually acts as an endogenous coupling agent.

The proposed methodology is easily adaptable to different systems and compatible with current industrial compounding facilities, ensuring far-reaching implementation in the production of polymer nanocomposites.

The authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements) or non-financial interest (such as personal or professional relationships, affiliations, knowledge, or beliefs) in the subject matter or materials discussed in this manuscript.