# Strain-induced crystallization ability of hydrogenated nitrile butadiene rubber

Kubat Narynbek Ulu, Martina Dragicevic, Pierre-antoine Albouy, Bertrand Huneau, Anne-Sophie Béranger, Patrick Heuillet

► To cite this version:

Kubat Narynbek Ulu, Martina Dragicevic, Pierre-antoine Albouy, Bertrand Huneau, Anne-Sophie Béranger, et al.. Strain-induced crystallization ability of hydrogenated nitrile butadiene rubber. Constitutive Models for Rubber X, 1, CRC Press, pp.279-282, 2017, 10.1201/9781315223278-43. hal-05188547

HAL Id: hal-05188547

https://hal.science/hal-05188547v1

Submitted on 14 Oct 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Strain-induced crystallization ability of hydrogenated nitrile
butadiene rubber

K. Narynbek Ulu

Institut de Recherche en Génie Civil et Mécanique, UMR CNRS 6183, Ecole Centrale de Nantes, France
LRCCP, Vitry-sur-Seine, France

M. Dragičević & P.-A. Albouy

Laboratoire de Physique des Solides, UMR CNRS 8502, Université Paris-Sud, Orsay, France

B. Huneau

Institut de Recherche en Génie Civil et Mécanique, UMR CNRS 6183, Ecole Centrale de Nantes, France

A.-S. Béranger & P. Heuillet

LRCCP, Vitry-sur-Seine, France

ABSTRACT: Strain-Induced Crystallization (SIC) phenomenon in elastomers is widely investigated, especially for natural rubber. It is considered as a major reinforcement mechanism explaining the very good mechanical properties of natural rubber, particularly its resistance to crack growth. Despite the fact that most of the studies are focused on natural rubber, other elastomers also exhibit SIC. Here, we study the ability of Hydrogenated Acrylonitrile Butadiene Rubber (HNBR), a synthetic rubber with high resistance to heat and petroleum products, to crystallize when it is stretched. Few studies have reported results on the SIC of HNBR, but none present a quantitative evolution of the phenomenon for different elongations. In the present study, X-ray diffraction patterns are obtained at three temperatures (20°C, 30°C and 40°C) for HNBR cross-linked with peroxide. These are recorded during uniaxial tensile tests at low strain rate in order to establish the relationship between the index of crystallinity and the stretch ratio; the orientation of the amorphous part of the elastomer is also presented.

# 1 INTRODUCTION

Strain-induced crystallization (SIC) of rubbers, especially of natural rubber (NR), has been extensively studied. At room temperature and without deformation, NR is an amorphous elastomer. However, as first discovered by Katz in 1925 using wide angle X-ray diffraction (WAXD), the NR polymer chains arrange in a more ordered state under large deformations (Katz 1925). A lot of studies have been devoted to this phenomenon: the reader can refer to the reviews of Tosaka (2007) or Huneau (2011) for the works published until 2010; in the recent years, one can also cite the works of Albouy et al. (2012), Brüning et al. (2012), Rublon et al. (2013) and Candau et al. (2012, 2014) among others.

SIC is considered as a major reinforcement mechanism that explains the very good mechanical properties of NR, especially its resistance to crack growth either under monotonous or cyclic loading (Hamed et al. 1996). If most of the studies have focused on NR, one should note that other

elastomers can also exhibit SIC. Among them, we focus our attention on hydrogenated acrylonitrile butadiene rubber (HNBR), which is a synthetic rubber used in the industry specifically for its high heat and oil resistances.

In the past, few studies revealed its ability to crystallize under strain. HNBR tends to undergo strain-induced crystallization only for high acrylonitrile (ACN) content (approximately greater than 35 wt.%) and with increasing hydrogenation (Kubo et al. 1987, Braun et al. 1992, Sawada 1994, Severe & White 2000). NBR, which is not hydrogenated, does not crystallize in comparison. Therefore, the tensile strength of HNBR is significantly higher than the one of NBR (Sawada 1994). Strain-induced crystallization is possible due to the polymeric chains having a fairly regularly alternating comonomer sequence (butylene and acrylonitrile units in almost equal amount) (Braun et al. 1992). Moreover, Braun et al. detected strain-induced crystallization of HNBR (48 wt.% ACN, hydrogenation percent not specified, not crosslinked) at elongation of 100% ($\lambda$=2) for experiments at room
temperature. Crystallization occurs at relatively
low elongations for temperatures between the
glass transition, T<sub>g</sub>, and room temperatures. Below
T<sub>g</sub>, strain-induced crystallization does not occur
and the structure is amorphous due to restricted
mobility of the polymer chains; above the melting
temperature, T<sub>m</sub>, the structure tends to stay amor-
phous due to excessive flow during deformation
and, therefore, it becomes significantly difficult to
achieve SIC; for temperatures greater than 50°C,
SIC does not occur (Braun et al. 1992).

In stretched HNBR samples, X-ray diffraction patterns have been observed only by few authors (Obrecht et al. 1986, Braun et al. 1992, Severe & White 2000, Osaka et al. 2013). Braun et al. first proposed a unit cell for HNBR containing 48% wt. ACN with *a* = 0.5 nm, *b* = 0.36 nm and *c* = 0.77 nm. Those values have been later confirmed, with small discrepancies, by Severe et al. (2000).

Overall, it can be said that a complete study has
not been conducted on the SIC phenomenon of
HNBR. At the best, some authors give the X-ray
diffraction pattern for one (Obrecht et al. 1986,
Severe & White 2000) or two (Braun et al. 1992,
Osaka et al. 2013) stretch ratios. The first objective
of this study, consequently, is to determine pre-
cisely the relationship between the crystallinity and
the stretch ratio during *in-situ* tensile tests at three
temperatures (20°C, 30°C and 40°C). The curves
of the stress evolution as well of the orientation
of the amorphous phase of the elastomer are also
presented as a function of the stretch ratio. The
discussion of these results is done with the few
above-mentioned studies on HNBR and with more
extended literature on NR.

2 MATERIAL AND EXPERIMENTAL
PROCEDURE

2.1 *Material*

HNBR is a copolymer initially made of acrylo-
nitrile (C₃H₄N) and butadiene (C₄H₆) as the well-
known NBR (nitrile butadiene rubber), but having
most of the double bonds of the butadiene satu-
rated with hydrogen, inducing the transformation
of butadiene to butylene (C₄H₈).

The considered material is a highly saturated
(96% hydrogenation) HNBR containing 44 wt.% of
acrylonitrile (ACN). It is crosslinked with peroxide
and is denoted HNBR-44 in the following sections.

2.2 *In situ tensile DRX*

The tensile machine is mounted on a rotating
anode X-ray generator (40 kV, 40 mA, focus size:
0.2 × 0.2 mm²) equipped with a copper anode
whose Kα radiation (0.1542 nm) is selected by

a doubly curved graphite monochromator. The
sample is located at the focalization point, which
ensures maximum diffracted intensity. Diffraction
patterns are recorded via a pixel camera; depend-
ing upon the elastomer nature, exposure times
as low as 0.3 s can be used. Sample holders are
fixed onto two opposing actuators that are sym-
metrically translated. One of the actuators is fur-
ther equipped with a load sensor. The stretching
amplitude is 500 mm at a speed that can be var-
ied between 1 and 800 mm/min. A polycarbonate
enclosure, equipped with a Kapton window for
X-ray exit, allows keeping the temperature con-
stant between 0°C and 90°C.

In the present study, experiments were per-
formed with an exposure time of 4 s, at 20 mm/min
and for three temperatures: 20°C, 30°C and 40°C.

2.3 *Data processing*

Figure 1 gives a typical diffraction pattern of HNBR and the scattering geometry. The data treatment is based on angular φ scans centered on the so-called amorphous ring (that mainly originates from inter-chain correlations) as indicated by the two black dotted circles; it includes the most intense crystallization peak located in the equatorial position. This allows for an easier separation of the amorphous and crystalline contributions.

From the X-ray pattern, the amorphous con-
tribution to the scattered intensity can be fitted
by the following function C₀(1 + Σ∑<sup>∞</sup><sub>n=1</sub> P<sub>2n</sub><sup>RX</sup> P<sub>2n</sub>(φ))
where φ is the azimuthal angle. For HNBR, first
two coefficients ⟨P<sub>2</sub><sup>RX</sup>⟩ and ⟨P<sub>4</sub><sup>RX</sup>⟩ are needed to
account for the experimental intensity modulation.

The crystallization peak is adjusted by a Pearson
VII function, commonly used in crystallography
for its ability to fit various peak profiles. We define
the crystallinity index as a ratio of the intensity
associated to crystalline spots to the total scattered
intensity within the delimited zone.

<figure><img src="image_2.png" /><figcaption>Figure 1. a) Representative diffraction pattern of HNBR-44 obtained at the maximum of mechanical cycling (acquisition time 4 s, stretch ratio 5.5 and vertical stretching direction). b) Schematic representation of the scattering geometry.</figcaption></figure>
# 3 RESULTS AND DISCUSSION

## 3.1 Index of crystallinity

Figure 2 shows the evolution of the index of crystallinity with the stretch ratio for the three considered temperatures. Similar to NR, the crystallinity curve exhibits a hysteresis: the stretch ratio necessary for crystallization (i.e. SIC onset), denoted by $λ_c$, is greater than the stretch ratio corresponding to the melting of crystallites, $λ_M$. Both $λ_c$ and $λ_M$ increase with the temperature. Table 1 gives the values of $λ_c$ and $λ_M$ determined by the intersection of the dotted line (extrapolation of the linear part) with the abscissa axis.

Moreover, for a given stretch ratio, the incidence of SIC decreases with increased temperature as expected from thermodynamics. This has already been suggested by Braun et al. (1992), who noticed that SIC just about disappears at 50°C. The evolution of crystallinity with respect to both the stretch ratio and the temperature is finally very similar to the one observed for NR (Albouy et al. 2005, Candau et al. 2015).

## 3.2 Stress-strain curve

Figure 3 gives the engineering stress evolution with respect to the stretch ratio.

As expected, a stress upturn (stiffening of the material) is observed. It is shifted toward higher val-

ues of stretch ratio and becomes less pronounced with increasing temperature. This seems to be strongly related to the crystallinity evolution previously described in Figure 2. Indeed, the stress upturn occurs at the onset of SIC and is more distinct when the crystallinity is elevated. For HNBR, Sawada et al. (1994) already noticed that the stress upturn increases with the hydrogenation level, suggesting an effect of SIC since the capability of the elastomer to crystallize increases with the hydrogenation level. However, to the authors' knowledge, this effect of SIC on the mechanical properties has never been demonstrated so clearly for the HNBR by means of simultaneous measurements. Finally, it is important to note that this observation is consistent with the literature for NR (Albouy et al. 2005, Albouy et al. 2014) and seems to confirm the stiffening effect of SIC, irrespective of the considered elastomer.

## 3.3 Orientation of the amorphous phase

Similar to the azimuthal intensity, the segmental orientation of the amorphous phase can be developed by a Legendre polynomial $\sum_{n=1}^{\infty} \langle P_{2n} \rangle P_{2n}(v)$ where v is the angle between the stretching axis and a segment. A relationship $\langle P_{2n} \rangle \propto \langle P_{2n}^{RX} \rangle$ may be rigorously demonstrated for the scattered intensity related to intra-chain correlations. Arguments exist to extend it to the intensity associated to inter-chain correlations (Albouy et al. 2014, Zaghdoudi et al. 2015). With this assumption, $\langle P_{2n}^{RX} \rangle$ is indicative of the evolution of the segmental orientation within the amorphous fraction.

Figure 4 presents the evolution of $\langle P_{2n}^{RX} \rangle$ with the stretch ratio during the tensile test (the sign minus is added to avoid negative values).

<figure><img src="image_3.png" /><figcaption>Figure 3. Engineering stress-stretch ratio curve of HNBR-44.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 2. Index of crystallinity of HNBR-44 with respect to the stretch ratio at 20°C, 30°C and 40°C.</figcaption></figure>

<table><caption>Table 1. Crystallization onset and melting stretch ratios of HNBR-44 at different temperatures.</caption><thead><tr><th></th><th>20°C</th><th>30°C</th><th>40°C</th></tr></thead><tbody><tr><td>λ<sub>c</sub></td><td>3.5</td><td>4.0</td><td>4.5</td></tr><tr><td>λ<sub>M</sub></td><td>1.4</td><td>2.0</td><td>2.7</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Figure 4. Evolution of the orientation parameter $\langle P_2^{RX} \rangle$ with the stretch ratio for HNRB-44.</figcaption></figure>

It evolves quasi-linearly before the onset of crystallization. The first noticeable decrease of the slope starts approximately when the onset of SIC is reached for the three temperatures; afterwards, the slope is almost constant, except for 20°C where it slightly increases. In NR, this evolution is interpreted to represent a partial relaxation of the amorphous phase as SIC develops in the rubber. However, this phenomenon is not so marked for HNRB as compared to NR, where $\langle P_2^{RX} \rangle$ remains nearly constant after SIC onset. This lower effect of SIC on the orientation of the chains may be tentatively related to the intrinsic lesser stereoregularity of the HNRB chains and, hence, to the reduced ability to crystallize.

## 4 CONCLUSION

The present study reports some results on SIC of HNRB (containing 44% wt. ACN) by using X-ray diffraction during *in-situ* uniaxial tensile tests. The main results are listed below:

- crystallization of HNRB-44 starts at high elongations (SIC onset $\lambda_c$ varies from 3.5 to 4.5 depending on the temperature);

- melting of crystallites occurs at a stretch ratio $\lambda_M$ that is smaller than $\lambda_c$, thereby inducing hysteresis;

- the overall crystallinity decreases with increased temperature;

- SIC appears to have a strong influence on the mechanical properties and is related to the stiffening of the elastomer;

- the orientation of the amorphous phase, evaluated trough the parameter $\langle P_2^{RX} \rangle$, is only slightly affected by the SIC, contrary to what is observed in NR. This point requires further investigations.

## REFERENCES

Albouy, P.-A., G. Guillier, D. Petermann, A. Veyres, O. Sanseau & P. Sotta 2012. A stroboscopic X-ray apparatus for the study of the kinetics of strain-induced crystallization in natural rubber. *Polymer* 53(15): 3313–3324.

Albouy, P.-A., J. Marchal & J. Rault 2005. Chain orientation in natural rubber, Part I: The inverse yielding effect. *European Physical Journal E: Soft Matter and Biological Physics* 17(3): 247–259.

Albouy, P.-A., A. Veyres, R. Pérez-Aparicio, O. Sanseau & P. Sotta 2014. The impact of strain-induced crystallization on strain during mechanical cycling of cross-linked natural rubber. *Polymer* 55(16): 4022–4031.

Braun, D., A. Haufe, D. Leiss & G. P. Hellmann 1992. Strain-induced crystallisation and miscibility behaviour of hydrogenated nitrile rubbers. *Angewandte Makromolekulare Chemie* 202: 143–158.

Brüning, K., K. Schneider, S.V. Roth & G. Heinrich 2012. Kinetics of Strain-Induced Crystallization in Natural Rubber Studied by WAXD: Dynamic and Impact Tensile Experiments. *Macromolecules* 45(19): 7914–7919.

Candau, N., L. Chazeau, J.-M. Chenal, C. Gauthier, J. Ferreira, E. Munch & C. Rochas 2012. Characteristic time of strain induced crystallization of crosslinked natural rubber. *Polymer* 53(13): 2540–2543.

Candau, N., R. Laghmach, L. Chazeau, J.M. Chenal, C. Gauthier, T. Biben & E. Munch 2014. Strain-Induced Crystallization of Natural Rubber and Cross-Link Densities Heterogeneities. *Macromolecules* 47(16): 5815–5824.

Candau, N., R. Laghmach, L. Chazeau, J.M. Chenal, C. Gauthier, T. Biben & E. Munch 2015. Temperature dependence of strain-induced crystallization in natural rubber: On the presence of different crystallite populations. *Polymer* 60: 115–124.

Hamed, G.R., H.J. Kim & A.N. Gent 1996. Cut growth in vulcanizates of natural rubber, cis-polybutadiene, and a 50/50 blend during single and repeated extension. *Rubber Chemistry and Technology* 69(5): 807–818.

Huneau, B. 2011. Strain-induced crystallization of natural rubber: a review of X-ray diffraction investigations. *Rubber Chemistry and Technology* 84(3): 425–452.

Katz, J.R. 1925. Rontgen spectrographic testings on expanded rubber and its possible relevance for the problem of the extension characteristics of this substance. *Naturwissenschaften* 13: 410–416.

Kubo, Y., K. Hashimoto & N. Watanabe 1987. Structure and properties of highly saturated nitrile elastomers. *Kautschuk Gummi Kunststoffe* 40(2): 118–121.

Obrecht, W. H. Buding, U. Eisele, Z. Szentivanyi & J. Thormer 1986. Hydrated Nitrile Rubber—a material with new properties. *Angewandte Makromolekulare Chemie* 145: 161–179.

Osaka, N., M. Kato & H. Saito 2013. Mechanical properties and network structure of phenol resin crosslinked hydrogenated acrylonitrile-butadiene rubber. *Journal of Applied Polymer Science* 129(6): 3396–3403.

Rublon, P., B. Huneau, N. Saintier, S. Beurrot, A. Leygue, E. Verron, C. Mocuta, D. Thiaudiere & D. Berghezan 2013. In situ synchrotron wide-angle X-ray diffraction investigation of fatigue cracks in natural rubber. *Journal of Synchrotron Radiation* 20: 105–109.

Sawada, H. 1994. Hydrogenated Nitrile Rubber. *International Polymer Science and Technology* 21(2): 64–71.

Severe, G. & J.L. White 2000. Physical properties and blend miscibility of hydrogenated acrylonitrile-butadiene rubber. *Journal of Applied Polymer Science* 78(8): 1521–1529.

Tosaka, M. 2007. Strain-induced crystallization of crosslinked natural rubber as revealed by X-ray diffraction using synchrotron radiation. *Polymer Journal* 39(12): 1207–1220.

Zaghdoudi, M., P.A. Albouy, Z. Tourki, A. Veyres & P. Sotta 2015. Relation between stress and segmental orientation during mechanical cycling of a natural rubber-based compound. *Journal of Polymer Science Part B-Polymer Physics* 53(13): 943–950.