Tereshatov V.V., Makarova M.A., Slobodinyuk A.I., Vnutskih Zh.A.

Institute of Technical Chemistry Ural Branch Russian Academy of Science,

Perm, Russia

Prospective segmented elastomers with diverse urethane and urethane urea hard segments

Polyurethanes are a unique class of polymers which structure and properties are regulated in wide limits due to using of various polyols, diisocyanates and chain extenders at synthesis. Polyurethane materials with different combination of properties can be obtained depending on chemical structure and quantity of components [1-2].

Segmented polyurethanes (SPU) consist of alternating soft and hard segments (blocks) of different chemical structure and polarity. Chemical structure of soft segments of SPU is determined by structure of olygomers used for their synthesis [3-5]. Hard segments are formed at the reaction of diisocyanate with low-molecular-mass bifunctional compositions, for example diols or diamines.

Investigation of oligodiisocyanates mixes on the base of various oligomers allows to expand a rang to regulate properties of SPU. It was shown that elastomers with diverse soft segments can significantly exceed block-copolymers on the base of individual oligodiisocyanates [3,4], for instance, for strength and moisture resistance.

The behavior of SPU with diverse hard segments is still poorly investigated. Hence new combination of properties of such materials could be expected while using mixes on the base of low-molecular-mass diamine and diol as hardener for prepolymer.

This work presents results of the investigation of the behavior regularities of segmented elastomers (SE) with the same polyether soft segments and diverse hard segments (urethane and urethane urea ones).

The aim of the work is development of properties of heterogeneous segmented elastomers with hydrogen bonds.

The table presents data on the physical-mechanic properties of SE at the strain rate 0.28 s^-1, the glassing transit temperature of the soft phase Т_g^s, and the softening temperature of the hard phase T_h for various molar concentration of methylene-bis-orto-chloroaniline (MOCA) х₁ in the mix with Butanediol-1,4 (BD) used within the synthesis of polymers.

Table. Properties of SE with urethane and urethane urea segments

х1	fp, MPa	lр	Е100, MPa	Tg^s, °C	Th, °C	Ch, %
1.0	248	5.58	14.7	-51	202	36.6
0.8	274	5.87	11.4	-50	202	35.2
0.7	307	6.16	9.5	-48	199	34.5
0.5	360	6.42	7.2	-44	193	33.0
0.4	278	6.64	5.4	-41	185	32.3
0.3	201	7.02	3.4	-40	173	31.6
0	114	10.10	1.9	-37	-	29.1

It is evident that properties of investigated materials are significantly changed. For example, the ultimate strain ratio changes in 2 times, the conventional elastic modulus Е₁₀₀ changes more than in 7 times in the range of the molar fraction х₁ of MOCA in the mix with BD (from 0 to 1). The total concentration of hard blocks (urethane and urethane urea ones) in investigated materials varies a little. The composition of hard blocks influences the properties of SE in the higher degree. The low content of urethane urea hard blocks (х₁>0.5) leads to increase in the glass transit temperature of the soft phase, and decrease in the glass transit temperature of the hard phase. This effect may be a result of formation of the more defect friable structure.

An analysis of IR-spectra of SE shows that increase in fraction of urethane segments in the composition of diverse hard segments leads to increase in the intensity of the band at 1710 cm^-1 for vibration of carbonyl of urethane groups bonded with ether oxygen via H-bonds. This confirms the increase of fraction of urethane hard blocks in the soft phase of a polymer. In the ultimate case (х₁=0) two bands can be seen for carbonyl: the band for free carbonyl at 1731 см^-1 and urethane group absorption band (1710 cm^-1) associated with the H-bonded oxygen polyester. The band 1700 – 1731 cm^-1 for carbonyl of urethane groups’ self-associates bonded by H-bonds is absent. Thus, the material has only a one phase with presence of urethane hard segments only (Fig. 1).

Fig. 1. IR-spectra of SE with various fraction of aromatic diamine in the mix with Butanediol-1,4.

An intensive absorbance band of carbonyl of self-associates of urea groups at 1640 cm-1 shows that the degree of microphase separation between soft polyether segments and hard urethane ones is high while х₁=1 (Fig.1). The assignment of bands was made according to [6].

The glassing transit temperature of the soft phase Т_g^s should increase and the Е₁₀₀ value decrease because solubility of hard blocks in the soft phase of SE increases as the fraction of BD increases in the mix with MOCA at the synthesis of a polymer. This effect is shown in experiments.

The maximum true tensile strength f_p of SE is shown at the same fraction of urethane and urethane urea hard segments in them (х₁=0.5). This value is higher the same for the classic polytetramethylene oxide urethane urea in 1,5 times. The high tensile strength of SE is combined with the significant increase in the ultimate strain ratio l_р of a material (Table).

Figure 2 shows an inverse relationship of the true tensile strength fp on the strain rate u for polyurethane urea (х1=1) and SE with the same fraction of urethane and urethane urea blocks. According [6, 7] this effect relates to the process of strain of hard blocks when they don't have time to orient in the strain direction at the high rates. Decrease in this rate leads to increase in the orientation degree of hard blocks at high strain ratios, this allows to increase the tensile strength of SE.

Fig. 2. True tensile strength versus strain rate for SE:1 – with diverse hard blocks (х1=0.5), 2 – without urethane blocks (х1=1).

On the contrary, relaxation of labile physical network allows to decrease in tensile strength of a material. The influence of the orientation factor of hard segments on the strength of SE leads to the great difference between maximal and minimal value of strength f_p (in 1.7 times). Interchain interaction is amplified in the soft phase of SE when one half of urethane urea hard blocks replaced by urethane ones, soluble in the soft phase of a polymer. Therefore the dependence of strength of SE on the strain rate becomes weaker (Fig.3). This is a remarkable property of a material with diverse hard blocks.

Fig.3. The ratio true tensile strength f_p /minimal value of true tensile strength

f_{p min} versus strain rate: 1 – without urethane blocks (х₁=1),

2 – with diverse hard blocks (х₁=0.5).

Therefore, use of mix with aromatic diamine and low-molecular-mass diol for SE synthesis allows to regulate physical-mechanic properties of urethane-containing elastomers in wide limits.

An extreme dependence of strength on the mix composition is a result of combination of two factors: increasing in the interchain interaction in the soft phase due to the dissolution of urethane hard segments in it, and the microphaase segregation of hard and soft segments. The forming hard phase is a reinforcing filler. The increase in the microphase segregation degree leads to the decrease in the interchain interaction.

Experiments carried out at different strain rates allowed to show a fundamental feature of behavior of SE with mixed hard blocks. The balanced influence of hard and soft phase on the physic-mechanical properties of such materials allows to weakening of the influence of a material strength on the strain rate. High values of strength of SE are achieved in a wide range of strain rate.

The work was carried out with the financial support of the program of Branch of Chemistry and Material Sciences of RAS «Creation and investigation of macromolecules and macromolecule structures of a new generation» (project 12-Т-3-1005), and the Russian Foundation for Basic Research with the Government of Perm kray (project 13-03-96000).

References

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