Polyacrylamide brushes with varied morphologies as a tool for control of the intermolecular interactions within EPDM/MVQ blends
Graphical abstract
Introduction
Hybrid polymer particles are a subject of increasing interest in designing polymer composites. They are formed of organic or inorganic core and polymer shell, which molecular architecture can be tailored by using controlled polymerization techniques [1,2]. The benefits of particle-polymer hybrids are based on the combination of particles properties and interactions of the polymer shell with polymer matrix that allow to control the particle dispersion and location within the matrix. The specific location of particles in well-organized morphologies of block copolymers as well as polymer blends can be obtained by adjusting the suitable polymer shell morphology [3,4]. The advantageous effects on properties of specifically located particles were already described, including compatibilization effect of particles in polymer blends [[5], [6], [7], [8], [9], [10]]. The effect of blend compatibilization is of high interest because the majority of polymers is immiscible, and needs to be compatibilized to improve their performance.
In this study the attention is paid to performance of ethylene-propylene-diene terpolymer (EPDM)/methyl vinyl silicone (MVQ) blend. This blend combines superior resistance of EPDM to oxidation, ozone or weathering ageing and flexibility over a wide temperature range combined with the excellent heat resistance, hydrophobicity and electric properties under wet conditions of silicone rubber [[11], [12], [13]]. In our previous work the significant effect of polymer architecture of clay hybrid particles on damping properties of EPDM/MVQ was revealed [14]. Namely, the effect of length and grafting density of polyacrylamide brushes (PAm) was correlated to mechanical and viscoelastic properties of blend. The PAm chains were grafted from clay particles (Mt) to allow interactions with carboxyl groups on modified EPDM matrix.
Some other approaches of interactions improvement in EPDM/MVQ blend were also reported [[15], [16], [17], [18], [19], [20], [21]]. They are based on either physical interactions, i.e. dipole-dipole interactions, van der Waals forces and hydrogen bonding or chemical interactions via covalent bonds formation during reactive compatibilization. In most cases these interactions can be generated by the addition of third components [[15], [16], [17], [18]] or by chemical modification of one of the blend polymer [[19], [20], [21]]. As a result the improvement of thermal stability, the modulus, tensile strength and the swelling resistance of the blend [19], as well as the improved interactions of the filler and blend component were observed [15].
Although the particle polymer hybrids were recognized as promising blend compatibilizers [5,6,[8], [9], [10]] to best of our knowledge, the systematic correlation of the polymer brush architecture with polymer blend dynamic of individual blend components has not been performed yet. Through the polymer brushes length, the level of chains entanglements can be controlled, while the grafting density determines the interactions potential through alignment of the brushes on the particle surface. In our previous work [14], we suggested the crucial role of the interactions generated between polyacrylamide polymer brushes (PAm) grafted on montmorillonite (Mt) surface and ethylene-propylene-diene terpolymer modified with sorbic acid and methyl vinyl silicone rubber blend (EPDM/MVQ). Herein, based on the same systems, we investigated deeply those interactions with individual blend components. Dynamic mechanical analysis (DMA) and broadband dielectric spectroscopy (BDS) showed impact on glass transition of both EPDM and MVQ phase. The effects on crystallinity of MVQ phase were studied by XRD. The findings were correlated with polymer blend morphology and particles location. The results showed the chemical nature of the brushes controls the particles location in blend, while architecture affects the dynamics of the polymer matrix. The long brushes suppressed the movement of the compatible blend component thank to entanglements, while on the other hand the short dense brushes facilitated the movement of the polymer matrix chains.
The results presented in this study show that the precise identification and justification of the interactions generated within the blends can serve as a tool for the detailed design of functional compatibilizers as well as control of the blends’ properties.
Section snippets
Materials
Methyl vinyl silicone rubber (MVQ, POLYMER MV 007) containing 0.05–0.09 mol% of vinyl units and Mooney viscosity of ML (1 + 4, 100 °C) = 15 was received from Silikony Polskie Chemical Company (Poland). Ethylene-propylene-diene terpolymer (EPDM) Keltan 4450S (LANXESS GmbH, Germany) with ethylene content of 42 wt%, norbornene (NRB) content of 4.3 wt% and Mooney viscosity of ML (1 + 4, 125 °C) = 44. EPDM was modified with sorbic acid (Sigma Aldrich, USA) as previously reported [14]. Sodium
Mt-g-PAm characterization
The PAm synthesized from Mt surface, Mt-g-PAm, was characterized by GPC. However, it appeared the cleavage of PAm from Mt particles was not successful, due to the possible hydrolysis of PAm by hydrofluoric acid, which can occur at low pH [22]. Therefore, assuming the similar molar mass of brushes grown from the surface and from sacrificial initiator [23,24] PAm synthesized in solution, using ethyl bromoisobutyrate as initiator, was analyzed and the GPC traces of the free PAm were collected
Conclusions
The specific interactions between Mt-g-PAm hybrids and individual components of EPDM/MVQ blend were elucidated in this article. In order to induce those interactions, EPDM was modified with -COOH groups, while PAm chains immobilized on Mt surface were varied by their length and grafting density. Due to the nature of carboxyl and amide groups dipole-dipole interactions and hydrogen bonding were formed. SEM revealed better dispersion and distribution of Mt-g-PAm in the EPDM/MVQ, finer morphology
Declaration of competing interest
The authors declare that they have no known financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors thank to National Science Centre, Poland for the financial support through POLONEZ (UMO-2016/23/P/ST5/02131) which has received funding from the European Unions's Horizon 2020 research and innovation program under Marie Sklodowska-Curie grant agreement. No 665778. This work was supported by the Ministry of Education, Youth and Sports of the Czech republic - DKRVO (RP/CPS/2020/003). This study was also performed during the implementation of the project Building-up Centre for advanced
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