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New urethane multiblock-copolymers obtained using naphthalene diisocyanate

Multiblock-copolymers were synthesized on the base oligoethers, oligoesters, naphthalene diisocyanate, and butanediol. The thermal, thermomechanical, and physico-mechanical properties of obtained copolymers with a variable composition of soft and hard segments were investigated. The relationship between the structure and properties of a new type of multiblock-copolymers was established. New materials can find application in various conditions of mechanical loading.


Introduction
The polymer chains of the majority of polyurethanes are composed of alternating soft and hard segments. The chemical structure of the soft segments is determined by the oligomers used. Hard segments are formed as a result of the reaction of a diisocyanate with a low molecular weight diol [1][2][3].
The difference in polarity of the soft and hard segments leads to their microphase separation with the formation of a soft and hard micro-dispersed phase, which plays the role of a reinforcing filler and nodes of a specific physical network in the elastomer [4]. The properties of polyurethanes are regulated by changing the chemical structure, molecular weight, polarity of the soft segments, concentration and structure of hard segments. Thus, a lot of materials with various properties have been obtained, which are used in various industries and medicine [1,5].
The ultimate physical and mechanical properties of elastomers (tensile strength and critical strain) are important criterions for the ability of a material to withstand applied loads and deformations.
The most widely studied polyurethane elastomers are the ones synthesized using toluene diisocyanate and diphenylmethane diisocyanate. At the same time, similar materials based on naphthalene diisocyanate (NDI) were studied to a much lesser extent. New objects of research are of significant interest from the standpoint of identifying the patterns and behavior of urethane-containing elastomers with a complex structure of polymer chains.
This report presents the results of a study of the physic-mechanical and other most important properties of block-copolymers containing different polyester, and polyether soft segments and hard segments, forming during a reaction between the molecules of butanediol and diisocyanate. In the synthesis of these polyurethanes, 1,5-naphthalene diisocyanate was used.
The synthesis was carried out in two stages with the preliminary manufacture of prepolymers based on NDI and oligoethers (oligoesters). The prepolymers were prepared by the reaction of oligomer diols with diisocyanate (NCO/OH = 2.2) while maintaining the temperature of the reaction mixture 60-70 °С. Afterwards these prepolymers were cured by mixes based on BD and the starting oligoesters (oligoethers) used to synthesize the corresponding prepolymers.
The process of synthesis of prepolymers and obtaining samples of crosslinked polyurethanes was described in detail in Ref. [6]. Four series of elastomers were synthesized: P-1000 on the base of OTMO-1000; P-1400 on the base of OTMO-1400; P6 on the base of OMA-1950; P6-BA on the base of OMA-2100. The composition of investigated samples for all the series is shown in table 1. Before testing, the samples were kept for at least 15 days at room temperature.

Methods
The glass transition temperature of the soft phase was determined by the differential scanning calorimetry (DSC) method using a DSC 822e calorimeter (Mettler Toledo, USA) at a scanning rate of 0.08 K s -1 . FTIR spectra of surfaces of samples were registered using an attenuated total reflectance (ATR) installation A 225/Q (Platinum ATR diamonds F) with a diamond crystal of single reflectance. Mechanical tests of samples were performed using an Instron 3365 universal testing machine (Instron, United Kingdom). Tensile strength of the samples (σp), relative critical strain values (ε) were determined at a stretching rate of 500 mm min -1 . The engineer modulus (E100) was determined at a relative strain of ε=100%.

Results and discussion
It was found that curing of polyurethanes based on NDI and oligoethers was associated with catastrophic changes in values of critical strain and tensile strength which in our opinion were linked with forming complex supramolecular structures. The formation of these structures can negatively affect the deformability of the cured elastomers. This effect is especially pronounced for samples synthesized based on Polyfurite 1000 (P-1000 series). When using butanediol as an only hardener, a structure with poor deformability is formed almost immediately after curing (ε=70%).  A manifestation of the described effect was reflected in a change in the mobility of polymer chains too. It was found that a decrease in the butanediol fraction in the curing mixture lead to increase in the glass transition temperature for samples of both series based on oligoethers. This phenomenon is systematic, the difference in the glass transition temperatures of samples cured with pure butanediol and pure oligoether reaches 4°C for the P-1400 series (from -71 to -67 °С) and 6°С for the P-1000 series (from -68°C to -62°C). This effect could be attributed to the partial crystallization of the studied polyethers; however, crystallization was observed for the P-1000 series only on a sample cured with pure polyether without butanediol. A similar picture was observed for the samples of the P-1400 series. Generally, the thermal effect of crystallization did not exceed 8-10 J/g, which corresponded to several percent of crystallized material in the samples.
In contrast to compositions based on oligoethers, similar materials based on oligoesters (P6 and P6-BA series) did not show the effect of decrease in ε value almost at all. The glass transition temperature of the samples was practically unchanged regardless of the composition of the curing mixture and was equal to -33 °C for the P6 series and -40 °C for the P6-BA one. For these both series of elastomers, the level of critical strains does not fall below 600%, regardless of the exposure time or the ratio of components during curing (figure 2).
A small decrease in the value of critical strains during exposure, which is characteristic of almost all investigated elastomers, is compensated by a noticeable increase in strength, being higher than the corresponding increase in the E100 value. A higher level of strength is achieved for the P6-BA series (46.5 MPa) with a value of E100 = 4.4 MPa, the maximum strength value is 38.5 MPa for the P6 series, and the correspondent E100 value is 8.7 MPa (figure 3). The almost ideal ratio of strength and E100 (10:1) for a sample of the P6 series cured with pure butanediol shows that conditions for the formation of supramolecular structures providing high deformability and, accordingly, strength are provided for the oligoester-NDI-butanediol systems. The small partial crystallization observed for all compositions based on oligoesters apparently played some positive role in this process.  In our opinion, the processes of phase separation play the main role in the described effects; they are reflected in the correspondent spectra of the obtained samples.
So, one can observe a regular decrease in the intensity of the absorption band at 1733 cm -1 with an increase in the content of butanediol in the curing mixture on the spectra of samples of the P-1000 series ( figure 4). This band is characteristic of vibrations of the free C=O group. In a similar way, the intensity of band at 1713 cm -1 changes too, and this band can be attributed to vibrations of the bounded C = O groups in disordered regions of the polymer [7]. On the other hand, the intensity of the band at 1692 cm -1 (vibrations of the bounded C=O groups in ordered regions) naturally increased; however, the maximum intensity of this band was observed for the sample with the content of butanediol x2 = 0.67 in the curing mixture. This behavior of the MATR spectra of the obtained polyurethanes indicates that, as the content

Conclusions
The results of our studies illustrate the possibility of constructing segmented polyurethanes based on naphthalene diisocyanate with high ultimate physical and mechanical properties. The combination of high strength properties of the new multiblock-copolymers, and improved processability can be useful in various applications.