Liquid crystalline hyperbranched polyesters with phosphorus functional groups

Abstract

Liquid crystalline hyperbranched poly(aryl ester)s (A₂B₃) were prepared by polycondensation reaction of 2-(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)1,4-naphthalene diol with 1,3,5-benzenetricarbonyl trichloride, taken in two different molar ratios. The chemical structure of the newly synthesized hyperbranched polymers was confirmed by FTIR, ¹H NMR, ¹³C NMR spectroscopy. The polymers exhibited high thermal stability with initial decomposition temperature above 410–435°C and char yield at 700°C higher than 40%. Combined differential scanning calorimetry, polarized optical microscopy and wide-angle X-ray diffraction measurements were carried out to closely examine their thermal behavior and phase transitions.

Keywords

Hyperbranched polymers phosphorus-containing polymers dendritic polymer liquid crystalline properties

Introduction

In the past decade, the field of arborescent macromolecular architectures (dendrimers, hyperbranched polymers) has been well established with a large variety of synthetic approaches, fundamental studies on structure and properties of these unique materials, and possible applications.^1

–4 The most prominent application fields of these classes of polymers emerged from the excellent encapsulating capability along with their excellent biocompatibility and biodegradability. Thus, in the recent years, hyperbranched polymers have been used as vehicle for drug delivery,^5
–7 protein delivery,^8
–10 gene transfection,^7,11 encapsulating agents of various inorganic nanoparticles to prepare antibacterial^12,13 and/or antifouling materials^14,15 or to design versatile nanoreactors for catalytic applications.¹⁶

Dendrimers are perfectly branched spherical monodisperse macromolecules consisting of an inner core, assemble of molecular building blocks containing the repeating units, all enclosed in a shell ended with tunable terminal functionalities. Dendritic architectures have been prepared following complicated multistep synthetic procedures, either through divergent or convergent approaches, each of them having a sum of advantages and disadvantages, beside time consuming synthetic approach and high production cost.^17
–19 On the other hand, hyperbranched polymers can be prepared by facile and easier one-pot polymerization strategies, thus, they become promising candidates for industrial applications where ultimate perfection in structural uniformity is less needed, while retaining many of the important features of dendritic polymers, such as appreciable number of end groups, which facilitates further chemical modification, very good solubility, lower intrinsic viscosities in comparison with the linear analogues etc. Hyperbranched polymers are less regular in structure, with degree of branching (DB) typically not exceeding 50% of that of dendritic molecules. They can be generally prepared starting from AB_x monomers. A wide variety of hyperbranched architectures has been successfully synthesized, including polyesters,^20

–26 polyamides, polycarbonates and polyurethanes.²⁷ The first AB_x branched polyester system was prepared by polycondensation reaction of 3,5-bis(trimethylsilyloxy)benzoyl chloride.²⁸ However, the commercial availability of AB_x monomers is rather scarce, while their preparation involves multistep organic synthesis. Thus, a facile alternative A₂ + B₃ approach was developed by Fréchet and co-workers.²⁹ Several hyperbranched architectures have been prepared by this approach, bearing different linking groups, for example ester,²¹ amide,³⁰ ether,³¹ imide.³²

Aromatic polyesters are widely used for the production of new engineering materials for high technologies, due to their good thermal stability, chemical resistance, tough mechanical properties and relatively low dielectric constant.³³ With the increasing demand for polymers, the main concerns of polymeric materials still are their thermal resistance and fire behavior.³⁴ Several approaches have been attempted to modify these drawbacks, through the use of flame retardants. Most commercially available flame retardants consist in inorganic compounds or halogen based organic compounds. However, they have obvious disadvantages such as the potential to corrode metal components and, more importantly, the evolution of the toxic hydrogen halide gas during combustion. Among the halogen-free flame retardants, organophosphorus compounds exhibit high fire retardancy, and they were found to generate less toxic gases and smoke than the halogen ones. For instance, the incorporation of DOPO groups into the structure of bi-functional monomers results in polymers with good solubility, improved thermal stability, flame retardant properties, good adhesion and decreased birefringence.^{35

–47} To date, several papers described the preparation of phosphorus-containing hyperbranched polymers appealing for flame retardancy applications.^48

–52

As a rule, a high content of phosphorus in a polymer brings better fire performances, thus, current work aimed to prepare hyperbranched polyesters derived from A₂ and B₃ monomers, as a facile route to novel phosphorus-containing hyperbranched polyesters. The synthesis of hyperbranched polyesters that are derived from 2-(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)1,4-naphthalene diol (DOPONQ) and 1,3,5-benzenetricarbonyl trichloride is reported herein. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (¹H NMR, ¹³C NMR) were used to identify the chemical structures of the products, while their thermal properties, morphology, and optical textures were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized light microscopy (PLM), respectively.

Experimental part

Materials

DOPO was purchased from Chemos GmbH, Germany. Naphthoquinone (NQ) and 1,3,5-benzene tricarboxylic acid chloride (BTC) were provided by Aldrich and used as received. All other reagents were used as received from commercial sources without any further purification.

Measurements

Melting points of the monomers and intermediates were measured on a Melt-Temp II (Laboratory Devices).

The molecular weight distributions were measured by Gel Permeation Chromatography (GPC) with a PL-EMD 950 evaporative mass detector equipped with 2 × PLgel 5 µm MIXED-C, D, 300 × 7.5 mm columns. Polystyrene standards of known molecular weight were used for calibration. The samples were eluted with DMF and the flow rate was 1 mL/min.

Fourier transform infrared (FTIR) spectroscopy was performed on a Bruker Vertex 70 at frequencies ranging from 400 to 4000 cm⁻¹. Samples were mixed with KBr and pressed into pellets.

NMR spectra were obtained on a Bruker Advance DRX 400 spectrometer, equipped with a 5 mm, direct detection, multinuclear probe, operating at 400.1, 100.6 and 161.9 MHz for ¹H, ¹³C nuclei, and ³¹P NMR, respectively. The polymer samples were dissolved in DMSO-d₆ and then measured at room temperature. ¹H and ¹³C chemical shifts (δ) are quoted in ppm relative to the residual peak of the solvent (ref. ¹H: DMSO-d₆ : 2.51 ppm, ¹³C: DMSO-d₆ : 39.47 ppm).

Thermogravimetric analysis (TGA) was performed on 15 mg samples under air atmosphere at a heating rate of 10°C/min using a Mettler Toledo model TGA/SDTA 851 instrument. Pyrolysis residues were collected at various weight losses in the thermogravimetric experiments and were subsequently analyzed by FTIR spectroscopy. In these experiments, samples of about 15 mg were heated to a defined temperature, cooled to room temperature, transferred to Bruker Vertex 70 instrument and FTIR spectrum was recorded for each sample.

The differential scanning calorimetry (DSC) analysis was carried out using a Perkin-Elmer Pyris Diamond instrument using nitrogen as a carrier gas at a flow rate of 10 mL/min. The samples were first heated from room temperature to 350°C using a heating rate of 10°C/min, then cooled to 20°C at a cooling rate of 10°C/min. A second heating and a subsequent cooling were performed in the same conditions. The melting temperatures (T_m1 and T_m2) and the liquid crystalline phase transition temperatures of polymers were taken as maximum of endothermic peaks.

Polarized light microscopy (PLM) was carried out on an Olympus BH-2 polarized light microscope fitted with a THMS 600/HSF9I hot stage, at a magnification of 200× or 400×. The mesomorphic transition temperature and disappearance of birefringence, that is, the crystal-to-nematic (T_m) and nematic-to-isotropic (T_i) transition, were noted.

The wide-angle X-ray diffraction (WAXD) experiments at room temperature were performed on a D8 Advance Bruker AXS diffractometer using a CuKα source with an emission current of 36 mA and a voltage of 30 kV. Scans were collected over the 2θ = 2–40 range using a step size of 0.01° and a count time of 0.5 s/step.

Scanning electron microscopy (SEM) was performed on a TESLA BS 301 instrument, at 25 kV, with a magnification of 380–3600. Prior the measurements, samples were sputtercoated with a thin layer of golg using an EK 3135 EMITECH device.

Synthesis of 2-(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)1,4-naphthalene diol (DOPONQ)

2-(6-Oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)1,4-naphthalene diol (DOPONQ) was synthesized from DOPO and NQ according to the method previously reported.⁵³

¹H NMR (400.1 MHz, DMSO-d₆ , ppm): δ = 10.74 (1H, s, OH_b), 9.80 (1H, s, OH_a), 8.41–7.38 (12 H, m, ArH), 6.59 (1H, d, J = 14.3 Hz) (Figure 3(a)).

¹³C NMR (100.6 MHz, DMSO-d6, ppm): δ = 152.6 (d, J = 4.1 Hz), 148.7 (d, J = 9.4 Hz), 145.7 (d, J = 17 Hz), 134.8 (d, J = 5.9 Hz), 133.7, 130.96, 130.71, 128.9, 128.7, 128.5, 126.1 (d, J = 93.7 Hz), 125.6, 125.3 (d, J = 13.6 Hz), 124.9, 124.0 (d, J = 9.4 Hz), 123.52, 122.8, 122.3, 120.8 (d, J = 10.2 Hz), 120.1 (d, J = 6.4 Hz), 105.9 (d, J = 11.2 Hz), 103.4 (d, J = 144.7 Hz) (Figure 3(b)).

^{31} P NMR (161.9 MHz, DMSO \hbox- d_{6}, ppm) : δ = 23.11 .

Synthesis of the polymers P-1 and P-2

Two hyperbranched P-containing polyesters, P-1 and P-2, were prepared by polycondensation reaction of the monomers DOPONQ (A₂) and BTC (B₃), taken in two different molar ratios of 1:2 and 3:2, respectively, in the presence of TEA. An example is described for polymer P-2.

BTC (1.99 g, 0.0075 mol) was dissolved in 50 mL THF in a 500 mL three-necked round flask equipped with magnetic stirring bar, reflux condenser, and dropping funnel. The system was cooled down to 0°C. DOPONQ (8.415 g, 0.0225 mol) and triethylamine (TEA) (1.5 mL), dissolved in 300 mL dry tetrahydrofuran (THF), were placed in the dropping funnel and added very slowly to the reaction flask over 1 h, and the contents of the flask were further stirred at the same temperature for 1 h and then stirred for over 24 h at room temperature. Finally, the reaction mixture was quenched with methanol to convert unreacted carboxylic acid chloride into a methyl ester end group, and was further stirred for 6 h at room temperature. The precipitated polymer was filtered off and washed twice with water. The product was dried at 50°C in vacuum for 24 h.

Results and discussion

Synthesis and structural characterization of P-1 and P-2

It is well known that reaction conditions, such as the monomer feed ratio, monomer addition manner, and concentration, strongly influence the polymerization process. The common products that result from polycondensation of A₂ and B₃ monomers are networks, while hyperbranched architectures are obtained only under highly restricted reaction condition.^30,54,55 Thus, it is essential to select suitable reaction conditions to avoid gelation. In our studies, THF was a suitable solvent for the oligomeric A₂ and B₃ copolymerization as both the starting materials, and the branched products were completely soluble in THF. The reactions were conducted at ambient temperature due to the high reactivity of acid chlorides. The order of monomer addition was necessary to maintain a homogeneous, gel-free solution, and this observation is consistent with earlier reports.²⁰

Novel polymers were synthesized with DOPONQ and BTC by the solution polycondensation method in the presence of TEA. Free carboxylic chloride groups were end-capped with methanol (Figure 1). The terminal groups play an important role in the thermal stability of the polyesters. If the terminal moieties are reactive groups such as carboxylic acid or alcohol, or if A₂ and B₃ functional groups exist simultaneously, further polycondensation could produce polymers with poor thermal stability at high temperature (>200°C).⁵⁶ In our study, the ratio between A₂ and B₃ monomers was 1:2 and 3:2, respectively (Table 1).

Figure 1.

Synthesis of hyperbranched P-containing polyesters, P-1 and P-2.

Table 1.

Composition of hyperbranched polymers P-1 and P-2, and their molecular weights.

Sample	Composition (molar ratio)	Functional group ratio –OH:–COCl	M_n (g/mol)	M_w (g/mol)	PDI
P-1	DOPONQ/BTC (1:2)	2:6	5547	5597	1.009
P-2	DOPONQ/BTC (3:2)	6:6	2635	2639	1.002

The solubility of these polyesters was tested in various solvents, by using 15 mg polymer/mL solvent, at room temperature. The resulting polymers were found to be soluble in common organic solvents such as THF, DMF, DMAc, NMP and DMSO. The good solubility can be explained by the presence of bulky, polar, and non-coplanar DOPO groups which creates a distance between the macromolecular chains. Also, the three-dimensional architecture of the hyperbranched polymers prevents a strong packing of the molecules and, consequently the diffusion of solvens molecules is facilitated.

GPC is one of the most widely used techniques for the determination of the molecular weight and polydispersity index (PDI) of the polymers (including hyperbranched polymers). The values of number average molecular weight (M_n ) were 5547 g/mol for P-1 and 2635 g/mol for P-2, while weight-average molecular weight (M_w ) were 5597 g/mol for P-1 and 2639 g/mol for P-2 (Table 1). The PDI values are close to 1 (Table 1), similar values being obtained for related hyperbranched polymers reported in the literature.^48,49 The experimental molecular weights determined by GPC measurements for hyperbranched polymers P-1 and P-2 is expected to be lower than the real ones because of the smaller radius of chain gyration in comparison with linear counterparts. However, the ellipsoidal architecture of hyperbranched polymers causes considerable errors in the measurement of molecular weights by GPC method, for which calibration is performed with linear polystyrene as a standard.

The structures of synthesized monomers and polymers were identified by FTIR and NMR spectroscopic techniques.

Figure 2 shows the FTIR spectrum of polymer P-2, as an example. As can be seen from FTIR spectrum, the most important absorption bands are associated with aromatic C–H (3075 cm⁻¹, stretching vibration), aliphatic C–H (2963 cm⁻¹, stretching vibration), C=O (1745 cm⁻¹, stretching vibration), ester C–O–C (1261 cm⁻¹ and 1008 cm⁻¹, asymmetric and symmetric stretching vibration), P–O–Ar (927 cm⁻¹ and 1161 cm⁻¹, stretching vibrations), P–Ar (1476 cm⁻¹), P=O (1210 cm⁻¹), aromatic C–H (753 cm⁻¹, deformation vibration caused by the 1,2-disubsituted aromatic DOPO rings). The broad absorption peak around 3400 cm⁻¹ could be ascribed to the stretching vibration of residual terminal O–H.

Figure 2.

FTIR spectrum of polymer P-2.

The terminal acid chloride groups were reacted with methanol via in-situ functionalization to obtain methyl ester end groups, and the structural characterization of the final products was attempted using ¹H-NMR spectroscopy. However, the analysis was not quantitative with respect to the concentration of end groups in the methyl ester terminated polyesters. The ¹H-NMR and ¹³C-NMR spectra presented in the Figure 3(a) and (b) reveal the structural confirmation of the monomer DOPONQ used in the synthesis, and of the resulted hyperbranched polymers P-1 and P-2.

Figure 3.

¹H NMR (a) and ¹³C NMR (b) spectra of DOPONQ, P-1 and P-2.

In the ¹³C-NMR spectra of the polymers P-1 and P-2, the presence of the signals at 163–165 ppm confirms that the polycondensation process took place, with the occurrence of terminal ester carbonyl, while the signal at 158 ppm corresponds to lateral unit from the structure of the obtained hyperbranched polymer. The coupled HMBC experiments (H–C coupling at 2–3 bonds) revealed the correlation between the proton at 8.85 ppm and carbon at 158 ppm, in the case of P-1, and between the proton at 9.14 ppm and carbon at 162 ppm, in the case of P-2, respectively (Figure 4).

Figure 4.

Coupled HMBC spectra of P-1 and P-2.

The presence of phosphorus was also confirmed by ³¹P NMR. The ³¹P NMR spectra of DOPONQ, P-1 and P-2 are presented in the Figure 5. The ³¹P NMR spectra of DOPONQ shows only one signal at 23.11 ppm, corresponding to the presence in the molecule of one type of phosphorus atom, while in the case of the samples P-1 and P-2, besides the main P signal shifted at 30.44 ppm and 27.74 ppm, respectively, other few signals with reduced intensity appeared in the spectrum.

Figure 5.

³¹P NMR spectra for the DOPONQ, P-1 and P-2.

The most important parameter for hyperbranched polymers is considered to be the degree of branching (DB). Several equations, were developed to define the structure of hyperbranched polymers based on the self-condensation of AB₂ monomers.^1,57 In most cases, these equations are also applicable to the products of A₂ and B₃ polymerization. The degree of branching based on BTC units was calculated according to the Fréchet definition,⁵⁷ using the following equation.

DB = \frac{D u n i t s + T u n i t s}{D u n i t s + T u n i t s + L u n i t s}

where Dunits, Lunits and Tunits, are the intensities of dendritic, linear, and terminal unit proton resonances in the NMR spectrum. In the present study, the chemical structures of the products were intermediate between linear and highly branched topologies since the A₂ oligomers have a relatively high molar mass. The Fréchet definition accurately describes the branching structures only when each oligomer between the branch points was considered as a single repeat unit. The degree of branching was calculated from the ¹H-NMR spectra of P-1 and P-2, the results being DB = 0.44 for P-1 (DOPONQ: BTC = 1:2), and DB = 0.24 for P-2 (DOPONQ: BTC = 3:2). Thus, the hyperbranched polyesters prepared via polycondensation of A₂ and B₃ monomers exhibited comparable branching degrees to products from AB_n monomers and other families of hyperbranched polymers derived from A₂ and B₃ monomers.^58

–61

Thermal analyses

Thermogravimetric analysis: FTIR correlation studies

The thermal stability of the polymers was investigated by TGA in air at a heating rate of 10°C/min and the values for 5% weight loss (IDT) and 10% weight loss are summarized in Table 2. Figure 6 presents the TGA curves of the polymers P-1 and P-2.

Table 2.

Thermal properties of polymers.

Polymer	T_g (°C)	T_c,h (°C)	T_m (°C)	IDT (°C)	T₁₀ (°C)	T_max1 (°C)	T_max2 (°C)	Char yield at 700°C (%)
P-1	161	—	—	380	400	410	600	40.8
P-2	99	192	283	380	400	435	645	53.6

T_g: glass transition temperature; T_c,h: crystallization temperature during heating; T_m: melting temperature; IDT: initial decomposition temperature = the temperature of 5% weight loss; T₁₀: temperature of 10% weight loss; T_max1: first maximum polymer decomposition temperature; T_max2: second maximum polymer decomposition temperature.

Figure 6.

TG and DTG curves of polymers P-1 and P-2.

The polymers exhibited two steps of degradation having the maximum polymer decomposition temperature in the range of 410–435°C (T_max1 ) and 600–645°C (T_max2 ), respectively. The char yields at 700°C were in the range of 40.8–53.6%. Char yield can be applied as a decisive factor for estimating limiting oxygen index (LOI) of the polymers based on van Krevelen and Hoftyzer equation.⁶²

LOI = 0.4 * CR + 17.5

where CR is the char yield. The calculated LOI values of P-1 and P-2 derived from their char yield were 33.82% and 38.94%, respectively. On the basis of LOI values, such compounds can be classified as self-extinguishing polymers. The char residue limited the production of combustible gases, thus decreased the exothermicity of pyrolysis reaction and inhibited the thermal conductivity of the burning materials that weakened the flammability of the materials. These aspects were evidenced from SEM investigation of the neat P-2 powder sample (Figure 7(a)) and P-2 heated up to 480°C (Figure 7(b)). The soft morphology of the sample, at room temperature, became more compact after thermal treatment, acting as a wall against the heat/fire spread.

Figure 7.

SEM investigation of the P-2 sample, before the thermal treatment (a) and after heating the sample up to 480°C (b).

In order to comprehensively elucidate the mechanism of thermal decomposition and fire retardancy, combined FTIR-TGA and SEM-TGA studies were carried out. FTIR investigation of the solid residue of polymer P-2, after heating the sample up to 480°C, showed the disappearance of C–O–C ester absorption bands (1261 and 1008 cm⁻¹), associated with the decomposition of the ester units. The temperature of 480°C represents the final stage of the first decomposition process, according to DTG curve. Other characteristic bands are P–C and P=O, observed at 1474 and 1116 cm⁻¹, respectively, indicating the presence of phosphorus in the solid carbonaceous residue. The organophosphorus P–O–Ar group present in the solid carbonaceous residue was revealed by the band at 915 cm⁻¹. Similar results have been obtained by SEM-EDX investigation of the solid carbonaceous residue, as it can be seen in the Figure 8.

Figure 8.

EDX spectra and SEM micrograph (inset) of the as synthesized polymer P-2 after heating the sample up to 480°C.

Thermal analyses: Thermal transitions

The thermal and mesomorphic properties of polymers were studied with DSC and polarized optical microscopy (POM). DSC measurements were conducted at a heating and cooling rate of 10°C/min, and the thermograms are shown in Figure 9. The T_g values of P-1 and P-2 found by DSC analysis are 161°C and 99°C, respectively. As expected, T_g was dependent on the ratio of A₂ and B₃ monomers and decreased with increasing the flexibility of the polymer backbones. In the literature, it was demonstrated that the glass transition temperature depends on the polymer branching architecture but it is strongly influenced by the type of the functional groups.^59,63 No melting endotherm was detected in the DSC curves of polymer P-1. For the polymer P-2, in addition to T_m a broad cold crystallization peak (T_c,h) can be seen. The presence of the cold crystallization peak for the polymer P-2 indicates that after the first cooling the crystallization of the polymer is incomplete when cooling at a rate of 10°C/min, and some non-crystalline material can still crystallize during the second heating process. The endotherm at 283°C can be assigned to mesomorphic transition from crystalline state. LC to isotropic transition for P-2 is not observed as LC phase is extended to higher temperature accompanying degradation.

Figure 9.

DSC thermogram of polymer P-2, 1H, first heating, 1C, first cooling, 2H, second heating, 2C, second cooling.

Thermal analyses: Texture analysis

Polarized light microscopy (PLM) was used to identify the liquid crystalline phases and to complement the phase transitions observed by DSC. Optical micrographs of different textures are shown in Figure 10. A suitable amount of sample of each polymer was placed between two clean glass plates. The sample was heated to clearing point, which is considered as the liquid crystalline-to-isotropic state transition. Typical nematic mesophase of hyperbranched polymer P-2 was observed at 292°C, heating cycle, 304°C and 272°C, cooling cycle, respectively (Figure 10(a) to (c)). By heating the polymer P-2 to the temperature above its T_g, only weak birefringence gradually formed, suggesting the formation of a less ordered mesophase. At 350°C, the birefringence disappeared completely. Upon slowly cooling the sample just above glass transition, the weak birefringence reappeared. These findings suggest that P-2 could only form less ordered structures and has a reversible order-disorder transition.

Figure 10.

Photomicrographs of polymer P-2 taken at 292°C, heating cycle (a), 304°C, cooling cycle (b), 272°C, cooling cycle (c).

X-ray diffraction

To further elucidate the structure of the mesophases, X-ray diffraction measurements of the polymers were carried out. Figure 11 shows the WAXD pattern of the polymers P-1 and P-2 at room temperature. For the polymer P-1, two wider diffraction peaks were observed, while in the case of polymer P-2, the diffraction pattern exhibited a few more intense and sharper diffraction peaks. The good solubility of the resulting hyperbranched polymers were endowed by their amorphous nature, which was confirmed by WADX patterns (as shown in Figure 11). In general, the broad diffraction patterns without clearly defined peaks indicate that most of these hyperbranched polymers are amorphous, which was ascribed to the asymmetry and irregularity of the whole polymer chains induced by the defects of the hyperbranched structure.

Figure 11.

X-ray diffraction pattern of the polymers P-1 and P-2, samples at room temperature.

Conclusions

Two hyperbranched P-containing polyesters with different degree of branching were prepared via the addition of a dilute solution of DOPONQ (A₂) phosphorus-containing bisphenolic monomer to a dilute solution of aromatic (B₃) trichloride monomer, in the presence of TEA, as acid acceptor. The chemical structure of compounds was confirmed using FTIR and NMR spectroscopy. The degree of branching of polymers P-1 and P-2 was 0.44 and 0.24, respectively. T_g of the polymers increased with increasing the content of BTC in the polymer as measured by DSC. One of the products exhibited liquid crystalline behavior under heating/cooling cycles performed by DSC measurements, which evidenced the mesomorphic transitions of the sample. The morphology of the liquid crystalline textures of polymers were observed using PLM, while the X-ray diffraction measurements confirmed the semi-crystalline nature of the studied compounds. Polymers exhibited good thermal stability as measured by TGA. By coupled FTIR-TGA and SEM-TGA studies it was demonstrated the presence of phosphorus into the char residue, and the fire retardant mechanism of such compounds was discussed. The obtained results support the use of this type of hyperbranched polymers, with high phosphorus content, in application sectors where fire retardancy is a must. This will be the subject of further studies.

Footnotes

Acknowledgements

The authors gratefully acknowledges to the financial support of CNCSIS–UEFISCSU, Project Number PN-II-RU-TE-0123 nr. 28/29.04.2013 and to the financial support of this research through the Project “Partnerships for knowledge transfer in the field of polymer materials used in biomedical engineering” IDP_40_443, Contract no. 86/8.09.2016, SMIS 105689, co-financed by the European Regional Development Fund by the Competitiveness Operational Programme 2014-2020, Axis 1Research, Technological Development and Innovation in support of economic competitiveness and business development, Action 1.2.3 Knowledge Transfer Partnerships.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Diana Serbezeanu

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