thermally reversible materials based on thermosetting systems modified with polymer dispersed liquid...

POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2006; 17: 835–840

erscience.wiley.com) DOI: 10.1002/pat.832
Published online 16 November 2006 in Wiley InterScience (www.int
Thermally reversible materials based on thermosetting

systems modified with polymer dispersed liquid crystals

for optoelectronic applicationy

A. Tercjak, E. Serrano and I. Mondragon*Escuela Univ. Politecnica. Dpto. Ingenierıa Quımica y Medio Ambiente. Universidad del Paıs Vasco/Euskal Herriko Unibertsitatea. Pza. Europa,

1. 20018-Donostia/San Sebastian, Spain

Received 14 November 2005; Revised 23 June 2006; Accepted 4 July 2006

*CorrespoDpto. IngPaıs Vas20018-DoE-mail: iay8th InternologiesPart 2.

The main aim of this research was the generation of new intelligent materials, in this case

thermoreversible material, based on an epoxy matrix modified with liquid crystal for optoelectronic

application. The samples were prepared by the reaction-induced phase separation (RIPS) of a

solution of 4(-(hexyloxy)-4-biphenyl-carbonitrile (HOBC) and polystyrene (PS) in diglicydylether

of bisphenol-A epoxy resin (DGEBA). The systems were cured with a stoichiometric amount of an

aromatic amine hardener, 4,4(-methylene bis(3-chloro-2,6-diethylaniline) (MCDEA). Taken into

account results obtained by differential scanning calorimetry (DSC) and transmission optical

microscopy (TOM) equipped with a hot stage it was found that depending onmorphology generated

by RIPS of HOBC/thermoplastic particles in the epoxy matrix thermally reversible light scattering

(TRLS) material can be obtained. Copyright # 2006 John Wiley & Sons, Ltd.

KEYWORDS: thermally reversible materials; polymer dispersed liquid crystals; morphology; thermosets; light scattering

INTRODUCTION

Polymer blends based on thermoset matrices often result

from a polymerization-induced phase separation process:

first, monomer(s) or oligomer(s) are mixed with an additive

to form a homogenous solution (the additives are a low

molar mass or a polymeric molecule, reactive or not reactive

with the monomers); second, the monomers are polymerized

and the additives separate mainly because of the decrease in

the entropy of mixing of the system.1–3 However, polymer

dispersed liquid crystals (PDLC) containing micron-size

liquid crystal (LC) droplets in polymer matrix,4–7 or polymer

network liquid crystals (PNLC) containing dispersed poly-

mer network in LC, are promising new materials for

application in the field of thermo- and electro-optical devices,

such as optic shutters, switchable windows and display

devices. Addition of a low molecular weight LC into a

polymeric matrix results in an electro- and thermo-sensitive

material in which the dispersed mesophase can be switched

from opaque scattering state to transparent state due to

matching of the refractive index of the polymer and the

oriented LC8–13 by applying an external field (electrical

voltage or thermal gradient). As it is well known, the

ndence to: I. Mondragon, Escuela Univ. Politecnica.enierıa Quımica y Medio Ambiente. Universidad delco/Euskal Herriko Unibertsitatea. Pza. Europa, 1.nostia/San Sebastian, [email protected] Symposium on Polymers for Advanced Tech-2005 (PAT 2005), Budapest, 13–16 September, 2005,

electro-optical properties of PDLC depend strongly on

shape, size, size distribution and orientation of LC domains

as well as the interactions between the LC droplets and the

polymer matrix.

Recently, a new family of polymer-dispersed LCs has been

developed.14,15 In this case, the LC is dispersed in a

thermoplastic/thermoset system. The thermoplastic (TP)

polymer must have a refractive index matching that of the

fully cured thermoset, and should exhibit high compatibility

with the LC and less compatibility with the thermoset

precursors if compared with the LC. This leads to phase

separation of TP/LC solutions at low conversions in the

polymerization reaction.

Fromanother point of view, as it is known from the literature

and from our previous papers,14–18 in PS-(DGEBA/MCDEA)

systems polystyrene (PS) leads to reaction induced phase

separation but taking into account both refractive index of

the cured-epoxy system and PS, the final materials are

transparent.

Taking into account work done by Hoppe et al.14–16 and

the authors’ previous papers,17,18 in the present work,

thermoplastic/thermoset blends were modified with low

molecular nematic liquid crystal 40-(hexyloxy)-4-biphenyl-

carbonitrile (HOBC). Epoxy resin was cured with MCDEA.

The main aim of the present contribution was to analyze the

thermo-optical properties of these thermoplastic/thermoset

blends to examine the possibility for obtaining thermally

Copyright # 2006 John Wiley & Sons, Ltd.

836 A. Tercjak, E. Serrano and I. Mondragon

reversible light scattering (TRLS) material. Additionally,

changes in morphology generated in situ during switching

from strong scatter light (off-state), thus opaque state, to

transparent state (on-state) have been studied by means of

transmission optical microscopy (TOM) equipped with a hot

stage. Moreover, the effect of curing conditions on the

reversibility of the thermoplastic/thermoset blendsmodified

with HOBC was also investigated.

Figure 2. DSC thermograms of neat HOBC and PS/HOBC

binary blends with different contents of PS.

EXPERIMENTAL

In this study, DGEBA was used as the reactive solvent

(383.1 gmol�1, Dow DER 330MT from Dow Chemical

Company). This epoxy resin was cured with a stoichiometric

amount of an aromatic amine hardener, MCDEA (Lonzacure

M-CDEA), kindly supplied by Lonza. PS with a refractive

index similar to cured epoxy resin were used (see Table 1).

The nematic LC used in the present study was HOBC. This

LC exhibits a nematic-isotropic (TN–TI) transition at about

768C and a crystal-nematic (TC–TN) transition at about 598Cas is shown in Fig. 1.

Thermo-optical behavior of both TP/LC binary blends and

TP-LC-(DGEBA/MCDEA) blends were investigated by

using a transmission optical microscope (Nikon Eclipse

E600) equipped with a hot stage (Mettler FP 82HT). In order

to get changes in transmission light when the external

gradient of temperature was applied the thin film placed

between two microscope slides was heated/cooled/heated

from 30 to 908C at a rate of 18C min�1. The thickness of the

sampleswas controlled by using a 0.5mmaluminium spacer.

The images were captured every 15 sec and the thickness

of the samples was controlled. Additionally, direct repres-

entation of morphologies during switching between opaque

to transparent state were recorded.

Table 1. Characteristics of the TP

Name Chemical structure Mn (gmol�1) nda

PS n 80000 1.589

a TexLoc refractive index of polymers.

Figure 1. DSC thermograms of neat HOBC.


The miscibility between thermoplastics and nematic

crystals was analyzed on a Mettler Toledo DSC 822

differential scanning calorimeter equipped with a Sample

Robot TSO 801 RO. Nitrogen was used as purge gas

(10mlmin�1). Temperature and enthalpy were calibrated

by using an indium standard. Measurements were per-

formed in sealed aluminumpans containing a sample weight

of around 7mg. In order to ensure comparable thermal

history, all samples were first heated to 1508C and were

maintained at that temperature for 10min, then cooled down

to �508C and reheated to 1508C. All the scans were

performed at a constant rate of 58C min�1.

RESULTS AND DISCUSSION

Miscibility between TP and LC was studied by differential

scanning calorimetry (DSC). DSC thermograms obtained for

PS/HOBC blends in different mass ratios pointed out

miscibility between both components taken into consider-

ation that melting point of HOBC in blends was shifted to

lower temperature with increase of PS in blends. Moreover,

the enthalpy of melting decreased with the increasing of PS

and the rate of the crystallization of HOBC in blends was

slower than the rate of crystallization of pure HOBC. DSC

Figure 3. Thermo-optical curves in the isotropization region

of PS/HOBC blends containing (a) 50, (b) 30 and (c) 10 wt%

PS.


DOI: 10.1002/pat

Thermally reversible materials 837

thermograms obtained for PS/HOBC blends are shown

in Fig. 2.

Thermo-optical properties of these blends were studied by

means of transmission optical microscopy (TOM) with a hot

stage. Thermo-optical curves in the isotropization region of

PS/HOBC with 10, 30 and 50wt% PS contents are plotted in

Fig. 3. Additionally, morphology of 30wt% PS/HOBC

recorded during the cooling/heating process is shown in

Fig. 4. From results obtained for PS/HOBC blends with

different HOBC contents, it can be concluded that there is a

Figure 4. TOM micrographs with crossed polar

cooling at: (a) 748C, (b) 73.58C, (c) 568C, (d) 268C


threshold percentage of around 50wt% HOBC in the blends

to exhibit nematic-isotopic transition, TN–TI, and that the

addition of TP led to a broader temperature range in which

the mesophase of the LC is in its nematic state, thus droplets

of the nematic phase of the LC coalesced with one another

makes it very difficult to obtain the morphology in

which these blends are suitable for electro-optical devices.

Moreover, switching from strong scatter light (off-state)

to transparent state (on-state) does not take place in the same

range of temperature in both the cooling and heating

izer of the 30 wt% PS/HOBC blend during

, (e) 25.58C, (f) 548C, (g) 75.58C, (h) 768C.


DOI: 10.1002/pa

0

t

Figure 5. TOM micrographs with crossed polarizer of the fully cured 2 wt% PS, 40 wt%

HOBC-(DGEBA/MCDEA) blend during cooling at: (a) 48.258C, (b) 488C, (c) 47.758C, (d)

478C, (e) 308C, and during heating at: (f) 478C, (g) 47.758C, (h) 488C, (i) 48.258C.

Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17: 835–840

DOI: 10.1002/pat


Thermally reversible materials 839

process. Furthermore, for the blends containing less

than 50wt% LC no TN–TI was detected in the measurement

conditions used; neither using DSC nor TOM equipment.

On the contrary, the addition of a small amount of TP to the

LC-(DGEBA/MCDEA) system changes morphology (size

and distribution of the LC droplets and coalescence between

them) of the samples during the cooling/heating process and

permits the generation of materials which are thermally

reversible. This behavior can be clearly seen if morphology

changes of 2wt% PS, 40wt% HOBC-(DGEBA/MCDEA)

blends were observed during the cooling/heating cycle

(see Fig. 5). The size and distribution of the mesophase

LC droplets were smaller and more regular in the same

measurement conditions and the systemwas stabilized against

coalescence.

Additionally, the range of switching between opaque and

transparent state during the cooling/heating cycle was

narrower and switching took place almost at the same

temperature in both the cooling and heating process, thus

confirming the suitability of fully cured TP-LC-(DGEBA/

MCDEA) to generate thermally reversible materials. This

possibly occurs because not only the LC but TP/LC led to

phase separation in the TP-LC-(DGEBA/MCDEA) system,

thus LC asmiscible with TP appear located inside the TP-rich

phase separated from epoxy matrix.

Additionally, strong influence of the curing conditions on

the thermal reversibility of the TP-LC-(DGEBA/MCDEA)

systems was found. This can be easy observed if thermo-

optical curves with different curing conditions of 2wt% PS-

40wt% HOBC-(DGEBA/MCDEA) are followed (see Fig. 6).

As can be seen, when curing rate increases switching from

opaque scattering state to transparent state for this blend

takes places at a lower temperature. Additionally, pre-curing

for 24 h at 808C led to sharper switching in the fully cured

blend, thus stabilizing the system against coalescence and

resulting in smaller droplet size of the LC.

Moreover, it is worth noting that the thermosetting system

modified only with nematic LC used in this work did not

Figure 6. Thermo-optical curves in isotropization region of

2 wt%, PS, 40 wt% HOBC-(DGEBA/MCDEA) blends after

different curing conditions: (a) 24 hr at 808C, (b) 24 hr at

808C and 24 hr at 1608C. (c) 36 hr at 1608C (d) 40 wt%

HOBC-(DGEBA/MCDEA) cured 24 hr at 808C and 24 hr at

1608C.


show sharp changes between opaque and transparent states

(see Fig. 6), which allowed the conclusion that the

introduction of a third component to the thermosetting

system canmake them useful for optoelectronic applications.

CONCLUSIONS

The preliminary results show that both morphology

generated and curing conditions have a strong influence

on the thermo-optical properties of TP-LC-(DGEBA/

MCDEA) blends, that have been shown in this paper taking

into account PS-HOBC-(DGEABA/MCDEA) blends. Results

demonstrate that the addition of a small amount of TP as the

third component can allow thermally reversible materials to

be obtained based on thermosetting/thermoplastic blends

modified with nematic LC. This fact has been demonstrated

using: HOBC as nematic LC and PS, as TP. It can be pointed

out that the introduction of a third component allowed a

narrow size distribution of nematic droplets to be obtained

and stabilized the system against coalescence of the LC

droplets, which results in the generation of materials for

electro-optical devices. Thus in fully cured blends the

addition of a small amount of TP hinders the crystallization

of the LC and TN–TI transition of the LC in TP-LC-(DGEBA/

MCDEA) blends takes place only inside the particles of

TP-rich phase which are formed by the reaction induced

phase separation of PS from the epoxy matrix, this will be

presented in a further article.

Additionally, work is in progress on the effect of curing

conditions on the thermo-reversible response of the

materials, as well as final morphology generated.

AcknowledgmentsThis work was supported by a grant from Basque Country

Governments, ‘‘Programa de becas postdoctorales de incor-

poracion de doctores a la CAPV’’ for Dr A Tercjak.

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DOI: 10.1002/pat

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