Dynamic molecular mobility of polyurethane by a broad range dielectric and mechanical analysis

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Abstract

The dynamic molecular mobility of poly(2-hydroxypropyl methacrylate) (PHPMA) base polyurethane (PU) was studied over a broad range of frequency and temperature by combining dynamic dielectric spectroscopy and dynamic mechanical analysis. Two hydrated levels were considered in this study: dry and room humidity states. In dry state, two secondary relaxations γ and β are identified. And in room humidity state, a βsw mode is pointed out. These modes are well known in poly(hydroxylalkyl methacrylate)s. The main α relaxation is influenced by humidity and crosslinks. The Kramers-Kronig transform is used to reveal an ionic conductivity σionic. Relaxation times extracted from dielectric and mechanical analyses are coherent for PU and PHPMA. This correlation allows us to propose an interpretation of relaxations at a molecular level.

Introduction

Poly(hydroxylalkyl methacrylate)s base polyurethane (PU) coatings are widely used in the coating industry [1], [2]. The PU is classically obtained by the chemical reaction of polyol and isocyanate. A better understanding of relationships between its macromolecular dynamic and structure is necessary to improve these material properties for surface applications.

In previous works, spectroscopic studies have been devoted to the molecular mobility of poly(alkyl methacrylate)s below Tg [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Although the mechanical and dielectric behaviors of poly(2-hydroxyethyl methacrylate) (PHEMA) has been extensively studied [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], there is a paucity of academic data on the poly(2-hydroxypropyl methacrylate) (PHPMA). PHEMA has a primary alcohol while PHPMA has a secondary alcohol. The reaction with isocyanate is slowed down with a secondary alcohol located on an ester side chain [33]. This increase of reaction time with PHPMA permits to have an acceptable time to apply the coating.

The aim of this work is to study the molecular dynamics of PHPMA by dielectric and mechanical relaxation spectroscopy. The effects of hydration and crosslinking of PHPMA on the relaxation modes are shown and compared with the abundant literature on the water influence on the poly(hydroxylalkyl methacrylate)s relaxations by differential scanning calorimetry [33], [34], [35], [36], [37], [38], [39], [40], dynamic dielectric spectroscopy [26], [29], [42], [43], [44] and dynamic mechanical analysis [45], [46].

Section snippets

Materials

The PU coating is a two components system: PHPMA, MACRYNAL® VSM 6299w/42WA (Allnex, Belgium), in 40 wt% water; and Easaqua™ X D 401 (Vencorex, France) composed of hexamethylene diisocyanate, isocyanurate (45 wt%) and 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (30 wt%) in 15 wt% n-butyl acetate. The two parts are mixed at room temperature (ratio 3:1). The mixture is sprayed with a High Volume Low Pressure (HVLP) spray gun. The curing process is 30 min at 80 °C. In the following sections,

Differential scanning calorimetry

Differential scanning calorimetry (DSC) experiments were carried out in a DSC 2920 apparatus (TA-Instrument, USA) under nitrogen gas at several heating rates from − 60 to 150 °C. The samples weight varied from 5 to 10 mg and they are sealed in aluminum pans. The equivalent frequency feq , DSC [47] depends on the heating rate following the equation:feq,DSC=v2πaδTwhere v is the heating rate, a is a constant (a ~ 1), δT is the mean temperature fluctuation.

The associated relaxation time can be determined:τ

Physical structure

DSC analysis of PU was carried out with various heating rates (Fig. 1). The glass transition of PU, determined by the inflection point method (ASTM D3418), remains constant at 60 °C. An endothermic peak is pointed out: its intensity decreases and it is shifted toward higher temperatures with increasing heating rate. This endotherm is associated with the rupture of the hydrogen bonds forming a local order, as in segmented polyurethanes [52].

The extracted temperatures are reported in Table 1.

Mechanical relaxations

The

Conclusions

Mechanical and dielectric relaxations have been applied to thorough investigation of the dynamic molecular mobility of PHPMA and PU in dry and RH states. The combination of DMA and DDS has brought to light the correlation between the response of this mobility under mechanical and electric field.

At low temperature in the dry state, two secondary relaxation modes γ and β were identified in PHPMA and PU. The molecular origin of the γ mode is the rotation of the single bondOH side group in PHPMA. The β

Acknowledgments

These results were obtained under the research project “SURFINNOV” at the IRT Saint Exupéry. We thank the industrial and academic members of the IRT who supported this project through their contributions, both financial and in terms of specific knowledge:

  • Industrial members: Airbus Defence & Space, Airbus Group Innovation, Airbus Helicopter, Airbus Operation, Akzo, GIT, Liebherr, Mapaero, Mecaprotec, Prodem, Socomore, Stelia Aerospace, Thales Alenia Space

  • Academic members: CIRIMAT, Laplace, UPS

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