Porphyrins and their various synthetic analogues and modifications, which possess unique properties, have been of great interest in various fields of science, technology, and production in the last decade [1–4].

Porphyrin compounds are used in the catalysis as iron-containing catalytic systems, especially in oxidation reactions. Porphyrins are also used in solar energy technologies as sensitizers for solar cells of sensitized donor–acceptor organic cells and solar cells [5].

In medicine, porphyrins are used in the diagnosis and treatment of various diseases, from the diagnosis of rare diseases to the treatment of cancer patients. Porphyrins are used in photodynamic therapy as light-sensitive compounds. Porphyrins are activated under the influence of light and lead to the death of tumor cells [6, 7]. Photochemical reactions between photosensitizers, molecular oxygen, and the light of specific wavelengths to produce reactive oxygen species that cause tumor damage are used in photodynamic therapy [8, 9].

Porphyrin–polymer materials open up new possibilities for their use in medicine and biotechnology, but it is necessary to study the structure formation process and the properties of resulting composite materials [10, 11]. The strength and elasticity characteristics are the main necessary properties to assess the possibility of processing and using polymer materials.

It is known that low-molecular-weight additives similar to porphyrin influence the structure and properties of a polymer matrix [12, 13]. Changes in degree of crystallinity, melting point, relative elongation, tensile strength, shear modulus, and other properties can be observed. The nature of a low molecular weight additive plays a major role.

The strength and elastic-dynamic properties of a composite material based on polylactide (PLA) and mesosubstituted tetraphenylporphyrin (TPP) are determined in this work. Substitution in the macrocycle is performed to avoid or to decrease the aggregation of porphyrin microparticles.

Polylactide is currently one of the best known and most promising environmentally friendly polyesters [14, 15]. A monomer to produce PLA—a lactic acid—is prepared from renewable plant raw materials: beet or grain production waste, which is an undoubted advantage from the point of view of ecology. The popularity of PLA is due to its properties: high biocompatibility, nontoxicity, suitability for 3D printing, and ability to degrade under environmental conditions [16–18]. The resulting film materials are promising for their use in biomedical applications due to unique properties of polylactide and porphyrin. The study of a mutual influence of the components and the determination of physical and mechanical properties are actual tasks in materials science.

4032D polylactide (Nature works, United States) with an average molecular weight of 1.7 × 105 g mol–1 and 5,10,15,20-tetrakis(4-n-hexyloxyphenyl)porphyrin (TPP) synthesized as described earlier [19] were used in this work.

Polylactide–tetraphenylporphyrin (PLA–TPP) film composite samples with a diameter of 90 mm and a thickness of 100–120 μm were obtained by solution watering. PLA–TPP solutions in chloroform were prepared with PLA amount of 5 wt %. Chloroform (Vekton, Russia) was as solvent. An amount of 5,10,15,20-tetrakis(4-n-hexyloxyphenyl)porphyrin in the film composites was 0.1–0.5 wt %. The resulting samples were kept in a thermocabinet at T = 40°C for 3 h to remove residual solvent.

Electronic absorption spectra of the obtained composites of different compositions were acquired on a DR/4000V spectrophotometer (HACH-Lange, United States) in the wavelength range λ = 320–800 nm. A film polymer composite sample with the size of 1 × 2 cm was used and fixed in the cuvette compartment of the spectrophotometer in the plane perpendicular to the light flux direction to record a spectrum.

Strength properties were found on an RM-10 tensile testing machine with Stretch Test software at a loading speed of 50 mm/min. The number of film samples for a series of experiments was at least seven for each composition.

Elongation at break ε(%) is calculated by the formula

$$\varepsilon (\% ) = \Delta l{\text{/}}{{l}_{0}} \times 100,$$

where l0 is the distance between the holders and Δl is an increment of the estimated length at the moment of rupture (in mm).

The tensile strength is found as follows:

$${{\sigma }_{{\text{p}}}} = P{\text{/}}bh,$$

where P is the load at which the sample fails, N; b is the width of a sample in the narrow part, m; and h is the thickness of a sample in the narrow part, m.

The elastic properties of polymer samples were assessed on a SIAM-2017 pulsed acoustic microscope (Institute of Biochemical Physics, Russian Academy of Sciences, Russia). A detailed description of the microscope structure, the principles of visualization, and the calculation of elastic properties are described in [20]. The elastic properties were calculated from the analysis of echograms, which were used to measure the velocities of longitudinal (CL) and transverse (CT) sound waves. The elastic moduli are calculated together with material density data ρ: shear modulus G, all-round compression modulus K, elastic modulus E, and Poisson’s ratio μ:

$$\begin{gathered} G = C_{{\text{T}}}^{2}\rho , \\ K = C_{{\text{L}}}^{2}\rho - \frac{4}{3}C_{{\text{T}}}^{2}\rho , \\ E = \frac{{9KG}}{{3K + G}}, \\ \mu = \frac{E}{{2G}} - 1. \\ \end{gathered} $$

Porphyrins are low-molecular-weight substances, whose addition to the matrix of high-molecular-weight compounds causes some changes in the structure of polymers. Differential scanning calorimetry data showed earlier that the introduction of TPP into the PLA matrix decreases the melting point of polylactide by 1–3°C and leads to the disappearance of a glass transition temperature [21]. In other words, porphyrin changes the crystallization behavior of PLA.

Not only does porphyrin influence PLA, but also polylactide matrix affects the immobilized porphyrin. Electronic absorption spectra of porphyrin of PLA–TPP film composite material samples were obtained. The structure of porphyrins in the UV-visible region is characterized by Soret and Q-bands. Five bands are observed in the electronic spectra of 5,10,15,20-tetrakis(4-n-hexyloxyphenyl)porphyrin in chloroform solution: an intense Soret band at 420 nm and four less intense Q-bands [19]. Consequently, this porphyrin is assigned to etio-type with the intensity of Q-bands εI > εII > εIII > εIV characteristic of symmetrically substituted porphyrins [22].

Figure 1 shows the presence of porphyrin in the PLA matrix and the distribution character of substituted TPP with an amount of 0.1, 0.3, and 0.5 wt %. The electronic spectra of all the composite samples are assigned to etio-type regardless of an amount of TPP in the PLA matrix. A Soret band is a characteristic feature of all tetrapyrrole macrocycles. The Soret band of 5,10,15,20-tetrakis(4-n-hexyloxyphenyl)porphyrin immobilized into PLA matrix splits, which shows a bathochromic shift to form bands at 425 and 447 nm.

Fig. 1.
figure 1

UV spectra (visible region) of PLA–TPP samples with TPP amount of (1) 0.1, (2) 0.3, and (3) 0.5 wt %.

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A bathochromic or “red” shift is observed for many organic molecules, when the polarity of their environment changes [23]. A bathochromic shift, for example, is observed for dyes, particularly during solubilization into micelles [24]. The immobilization of porphyrin in polylactide, whose macrochains contain carbonyl groups and an ester bond, influences probably the electron density distribution relative to porphyrin and leads to a shift of absorption maximum to the long-wavelength region. In addition, the UV spectra of all film samples contain a shoulder at 400 nm (blue shift), whereas the Q-bands shift slightly to the long-wavelength region.

It is possible, therefore, to assess the influence of porphyrin on the properties of the obtained supramolecular systems after the study of structural properties. Acoustic microscopy and a rupture apparatus were used for this purpose.

Elastic modulus values of polymer–porphyrin compositions were calculated from acoustic microscopy data. Table 1 and Fig. 2 show the data.

Table 1.   Elastic properties of PLA–TPP composites obtained by the acoustic method
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Fig. 2.
figure 2

Density of PLA–TPP film samples depending on composition.

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The addition of a low-molecular-weight component to PLA volume influences significantly both the density of the material, the value of which decreases to 1.24–1.23 g/cm3 (Fig. 2), and the shear elastic wave propagation velocity, whose value decreased to 1.12 km/s. The longitudinal wave propagation velocity is insensitive to low porphyrin concentrations and is 2.28 km/s (Table 1). The modulus of all-round compression K is also insensitive to porphyrin in the PLA matrix, and shear modulus values G tend to decrease.

The Poisson’s ratio of PLA and the composition containing 0.1 wt % of TPP is 0.33, and it is 0.34 for any other PLA–TPP samples, which is in agreement with published data. Most thermoplastic polymers have Poisson’s ratio within 0.3–0.4, whereas the μ value for highly elastic materials, such as rubbers, can be maximum and reach 0.5 [25]. An increase in concentration of low molecular weight substance in the polymer leads to a smooth decrease in elasticity of the material: the decrease in elastic modulus E in the material containing 0.5% TPP relative to starting polymer is 7%.

Fig. 3.
figure 3

(a) Relative elongation at break and (b) tensile strength of PLA–TPP film samples with different amount of porphyrin: from 0 (100% PLA) to 0.5 wt %.

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The strength characteristics were determined: relative elongation at break and tensile strength. When porphyrin is added to PLA matrix, the relative elongation at break increases from ε = 4.5% for starting PLA to 6.5% for the sample with porphyrin amount of 0.5 wt %. Such a change cannot be due to a significant increase in elasticity of the material; it only indicates a trend in the obtained characteristic. The tensile strength of film samples with the addition of 5,10,15,20-tetrakis(4-n-hexyloxyphenyl)porphyrin decreases slightly, which is to be expected, because the degree of crystallinity of PLA in the compositions decreases by 2–4% [21].

It should be noted that, when different porphyrins are added to polymer matrix, the changes in strength characteristics are irregular. Both the relative elongation and tensile strength values of PLA compositions with manganese unsubstituted tetraphenylporphyrin complex, for example, increased (within porphyrin concentrations of 0.1–0.5 wt %) [26]. This is probably due to a metal ion in the porphyrin structure and an increase in degree of crystallinity of PLA in the compositions.

CONCLUSIONS

The experiments show that the immobilization of 5,10,15,20-tetrakis(4-n-hexyloxyphenyl)porphyrin in a polylactide matrix leads to the splitting of UV spectrum of porphyrin and a bathochromic shift of Soret and Q-bands. This effect is observed both at a minimum TPP amount of 0.1 and 0.5 wt %. This behavior is probably due to the influence of PLA on the electron density distribution relative to porphyrin, which shifts the absorption maximum.

The density of PLA–TPP composite material decreases with an increase in an amount of low-molecular-weight porphyrin additive in the polymer matrix. There is a decrease in elastic characteristics: shear modulus and elastic modulus. There is also a tendency toward a decrease in tensile strength with an increase in TPP concentration. The Poisson’s ratio almost does not depend on porphyrin amount and is 0.33–0.34.

The changes in physical, mechanical, and elastic properties are not critical, and the obtained composite materials can be used in various practical industries.