Temperature Controlled Mechanical Reinforcement of Polyacrylate Films Containing Nematic Liquid Crystals

Abstract

This investigation reports on the thermomechanical properties of Poly-tripropyleneglycoldiacrylate (Poly-TPGDA)/liquid crystal (LC) blends, developed via free radical polymerization processes, which are induced by Electron Beam (EB) and Ultraviolet (UV) radiation. The EB-cured Poly-TPGDA network exhibits a higher glass transition temperature (T_g), a higher tensile storage, and Young moduli than the corresponding UV-cured sample, indicating a lower elasticity and a shorter distance between the two adjacent crosslinking points. Above T_g of Poly-TPGDA/LC blends, the LC behaves as a plasticizing agent, whereas, for EB-cured networks, at temperatures below T_g, the LC shows a strong temperature dependence on the storage tensile modulus: the LC reinforces the polymer due to the presence of nano-sized phase separated glassy LC domains, confirmed by electron microscopy observations. In the case of the UV-cured TPGDA/LC system, the plasticizing effect of the LC remains dominant in both the whole composition and the temperature ranges explored. The rubber elasticity and T_g of Poly-TPGDA/LC films were investigated using mechanical measurements.

Keywords: UV-visible irradiation; dynamical mechanical analysis; elastic modulus; electron beam curing; glass transition; liquid crystals; polyacrylates.

Figure 1

Static mechanical analysis: Dependence of Young modulus on LC concentration for both EB- and UV-cured TPGDA/E7 systems. The insert shows stress as a function of draw ratio λ for the same samples.

Figure 2

tan δ as function of temperature around T_g (E7) (a,c) and T_g poly (TPGDA) (b,d) at different LC concentrations for EB- and UV-cured films, respectively.

Figure 3

(a) Reduced T_gR, and (b) Full Width at Half Maximum (FWHM) of tan δ around (T_g (polymer)) as functions of LC concentration for both EB- and UV- cured TPGDA/E7.

Figure 4

Molecular weight of strands (M_s) between two adjacent cross-linking points of EB- and UV-cured TPGDA/E7 blends, as function of LC concentrations on a double logarithmic scaling. The inset shows the same results when applying a linear scaling.

Figure 5

A detailed view of the storage tensile moduli versus temperature in the range from T = −100 °C to T = −40 °C. (a,b) displays results from EB- and UV-cured TPGDA/E7 films, respectively. The vertical dashed lines represent T_g of the pure LC.

Figure 6

Reduced storage tensile moduli E’_R as function of LC concentration determined at different temperatures for (a) EB- and (b) UV- cured TPGDA/E7 films.

Figure 6

Reduced storage tensile moduli E’_R as function of LC concentration determined at different temperatures for (a) EB- and (b) UV- cured TPGDA/E7 films.

Figure 7

(a) A representative micrograph obtained from SEM observations is presented, showing the morphology of an EB-cured 30 wt.% of TPGDA/70 wt.% of E7 film. (b) Histogram obtained by image processing of the SEM picture from Figure 7a, taking into account the size distribution of the LC domains, and assuming the presence of ellipsoids characterized by a major and a minor axis. The corresponding SEM analysis of a UV-cured 30 wt.% of TPGDA/70 wt.% of E7 film is given in (c,d). In both cases, the same experimental conditions were used for the morphological observations of the samples.