Elsevier

Polymer

Volume 42, Issue 5, March 2001, Pages 2067-2075
Polymer

Epoxy+liquid crystalline epoxy coreacted networks: I. Synthesis and curing kinetics

https://doi.org/10.1016/S0032-3861(00)00505-XGet rights and content

Abstract

In situ copolymerization of diglycidyl ether of 4,4′-dihydroxybiphenol (DGE-DHBP) with diglycidyl ether of bisphenol F (DGEBP-F) networks using an anhydride curing agent has been investigated. DGEBP-F is a commercial epoxy while cured DGE-DHBP shows liquid crystal transitions. Curing kinetics are determined using differential scanning calorimetry (DSC). The data were fitted using an autocatalytic curing model for both pure and mixed components. Isothermal and non-isothermal methods were compared. The glass transition (Tg) was evaluated as a function of composition using DSC. The results show that the DGE-DHBP constituent affects the curing kinetics of the epoxy resin and that the network exhibits one Tg.

Introduction

Epoxy resins are used in many applications because of their high strength, stiffness, good thermal stability, and excellent adhesion properties. Unfortunately, they also have low fracture toughness. Two common approaches for epoxy modifications are introduction of functionalized reactive rubbers or thermoplastics. Reactive rubbers like carboxyl- (CTBN), amine- (ATBN) or epoxy-terminated butadiene acrylonitrile (ETBN) have improved toughness. However, the blends show adverse effects on glass transition temperature range, stiffness and strength [1], [2], [3]. The mechanism of toughening of such blends involves a chemically induced phase separation process [4]. Toughening epoxies with thermoplastics presents significant problems in processing due to the large viscosity difference between the thermoplastics and the epoxy. One of us showed that interpenetrating networks (IPNs) formed through simultaneous reaction of an epoxy and polyisocyanate monomer in the presence of a single curing agent increased fracture toughness [5]. Controlling the phase separation has been approached through simultaneous curing of the epoxy resin with rubbery networks to form an IPN [6].

Liquid crystalline epoxy (LCE) networks are an important area of research given their potential use in a number of applications such as electronics, advanced composites, non-linear optics, etc. The synthesis, development of texture, mechanical properties and influence of curing conditions have been examined for a number of LCEs [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Given the considerable interest in blending thermotropic longitudinal polymer liquid crystals (PLCs) with other thermoplastics to improve mechanical properties of thermoplastics [22], [23], [24], [25], [26], [27], there has been some interest in examining the case of PLCs+thermosets [18], [28].

Liquid crystalline thermosets and particularly LCE resins show interesting properties due to the combination of a thermoset and LC formation capability [29], [30], [31], [32], [33], [34]. As compared to ordinary epoxies, crosslinked LCEs exhibit higher fracture toughness [32], [35] and mechanical properties when oriented by magnetic fields [36]. It should be noted that all PLCs, including those derived from diglycidyl-terminated blocks, are characterized by repetitive units that are highly anisotropic, where the molecular structure determines the appearance of LC state [37].

In the past, polyfunctional amines and aliphatic diacids were widely used to cure LC epoxies [18], [32], [33], [34], [35]. Here we opt for an anhydride curing agent because of the good thermal stability, electrical insulation, and relatively high chemical resistance. Furthermore, anhydride curing agents provide good mechanical properties with low shrinkage so they are suitable for matrices in composite applications [38].

The mechanisms of catalyzed cure of epoxies with cyclic anhydrides involve an anionic mechanism suggested by Fisher [39]. The reaction is supposed to occur via a formation of a zwitterion. A tertiary amine first reacts with the anhydride group to create the carboxylate zwitterion group [

]. Then, the reactive groups can react further with an epoxide. Tanaka and Kakiuchi [40] suggested that a tertiary amine in the form of hydroxylate [R3NCH2–CH(CH2R′)O] can also react with anhydride groups.

In this paper we investigate the effect of coreacting a conventional epoxy resin with a diglycidyl-terminated liquid crystalline monomer. Synthesis, curing kinetics and degree of miscibility of the resulting system are reported in Part I. Part II deals with the effects of modified epoxy on the mechanical performance.

Section snippets

Kinetics of curing: models

DSC is extensively used for investigating the curing reaction of thermoset polymers. Kinetics can be characterized with DSC by measuring heat generated during the curing reaction as a function of temperature and time. The extent of curing reaction may be determined by measuring the total area of the reaction peak. The basic assumption of DSC kinetic measurements is that the heat flow is proportional to the change in the extent of the reaction:α=dαdt=1HtotdHdtwhere dH/dt represents the rate of

Materials

The epoxy resin used in this study is a low-viscosity diglycidyl ether of bisphenol F (DGEBP-F) (Shell EPON862®) with the epoxide equivalence weight (EEW) in the range 166–177. DGEBP-F will be named as EP1 for convenience. Methyltetrahydrophthalic (MTHPA) anhydride (Lindride-6®) pre-catalyzed by benzyl triethyl ammonium chloride (BTEAC) is the curing agent obtained pre-mixed from Lindau Chemical. Its anhydride equivalent weight is in the range 165–175. The formula of DGEBP-F and MTHPA are shown

Synthesis

The DSC thermogram of the synthesized EP2 reveals a first transition at 135°C followed by the melting point at 158°C. The heats of transition were 20.5 and 73.7 J/g, respectively. Optical microscope shows that there is no change in the appearance of crystalline texture on low-temperature transition. The crystal melts and becomes isotropic upon heating at 157.8°C. Upon cooling to room temperature a double peak is recorded with the two maxima at 140 and 149°C, as also previously observed [47]. The

Concluding remarks

The DGE-DHBP affects the kinetics of curing DGEBP-F, especially the activation energy of curing. Effects of diffusion control and complicated reactions that retard the basic curing reaction are found in all systems after about 80% conversion. The autocatalytic model does not take into account the effect of mobility retardation after the gelation point. Therefore, the curing rates calculated from the model are higher than the experimental values. The coreacted networks exhibited a single Tg

Acknowledgements

Financial support has been provided by the State of Texas Advanced Technology Program (Project#003594-077). Equipment donation by the Perkin–Elmer Corp. is acknowledged also. Comments of the referees and Gale Holmes are appreciated.

References (56)

  • H.J Sue et al.

    Polymer

    (1998)
  • W Brostow et al.

    Polymer

    (1996)
  • W Brostow et al.

    Polymer

    (1996)
  • W Brostow

    Polymer

    (1990)
  • W Brostow et al.

    Polymer

    (1999)
  • J.M Berry et al.

    Polymer

    (1998)
  • G.G Barclay et al.

    Prog Polym Sci

    (1993)
  • C Carfagna et al.

    Prog Polym Sci

    (1997)
  • M.E Ryan et al.

    Polymer

    (1979)
  • F.Y.C Boey et al.

    Polymer

    (2000)
  • S.L Kirshenbaum
  • S.C Kunz et al.

    Polymer

    (1987)
  • X Han et al.

    Mater Res Soc Symp Proc

    (1992)
  • C.K Riew et al.

    Advances in chemistry series 233

    (1993)
  • N.A Prakash et al.

    Polym Compos

    (1994)
  • L.H Sperling

    Interpenetrating polymer networks and related materials

    (1981)
  • D.J Broer et al.

    Makromol Chem

    (1988)
  • C.K Ober et al.

    Liquid crystalline polymers

    (1990)
  • D.J Broer et al.

    Makromol Chem

    (1989)
  • D.J Broer et al.

    Makromol Chem

    (1989)
  • D.J Broer et al.

    Macromolecules

    (1993)
  • S Onada et al.

    Polymer

    (1996)
  • G.G Barclay et al.

    J Polym Sci Polym Chem

    (1992)
  • A Shiota et al.

    J Polym Sci Polym Chem

    (1996)
  • J.S Grebowicz

    Macromol Symp

    (1996)
  • P Szczepaniak et al.

    J Polym Sci Polym Phys

    (1998)
  • H.J Sue et al.

    J Mater Sci

    (1997)
  • H.J Sue et al.

    J Mater Sci

    (1997)
  • Cited by (51)

    • Development in liquid crystalline epoxy resins and composites – A review

      2020, European Polymer Journal
      Citation Excerpt :

      The above conclusions were thoroughly analyzed by other scientists who investigated a wide spectrum of resins and hardeners. The various developed kinetic models gave almost universal conclusions - the formation of the liquid crystalline phase during crosslinking strongly influences the curing process [8,63,64,72–77]. Recent studies have yielded the same information for mesophase originating from the hardener [57,67].

    • The effect of zinc oxide nanoparticles on thermo-physical properties of diglycidyl ether of bisphenol A/2,2′-Diamino-1,1′-binaphthalene nanocomposites

      2011, Thermochimica Acta
      Citation Excerpt :

      So, toughness improvement of the epoxy coating is crucial. Many materials like rubber elastomer, thermoplastics, liquid crystalline epoxy, micro and nano-particles are used to improve its toughness [4–8]. An effective approach to enhance thermo-physical properties of epoxy resins is introducing various inorganic nanoparticles into the polymer matrix.

    • Freezing the orientation of a nematic stretched elastomer by photocrosslinking

      2009, Polymer
      Citation Excerpt :

      in the tensile configuration (static force: 0.020 N): the samples were heated up to 120 °C at 5 °C/min, then cooled down to r.t. (cooling rate: −5 °C/min). The reaction between epoxy and carboxylic groups in the molten state is well-known and has been widely used to prepare cross-linked casting and laminates, as well as elastomeric systems [24]. The epoxy-acid chemistry is complex since it involves different possible reactions: the most important reactions which take place in the stoichiometric formulations are the epoxy-acid addition, the transesterification of the resulting β-hydroxyester, as well as the condensation between the acid and the hydroxyl group [25–27].

    • Liquid Crystalline Polymers

      2022, Thermal Analysis of Polymeric Materials: Methods and Developments: Volumes 1-2
    View all citing articles on Scopus
    View full text