inferior thermal stability to the polymer matrix during compounding; on the other hand,
they are easy to migrate and leach out gradually due to the relatively low surface energy
and unsatisfied compatibility with the matrix, which would deteriorate the mechanical
properties of the matrices simultaneously. On the other hand, some small-molecule
flame retardants can be persistent, bioaccumulative, and toxic (PBT), which has led to
calls for their deselection from use. The PBT effects, combined with extensive regulatory schemes in the EU to address electronic wastes and register chemicals, lead to the
regulatory banning of some brominated flame retardants, particularly small molecular
ones. High molecular weight polymers have been found to be less accessible by living
organisms and so have an automatically lower PBT profile when compared to small
molecules (Lyon 1994; Stevens and Mann 1999; Irvine et al. 2000; Chen and Wang
2010). Therefore, both scientists and manufacturers have recognized use of polymeric
flame retardants instead of small-molecule flame retardants. Phosphorus-containing
thermotropic liquid crystalline polymers (P-TLCP), which integrate the advantages of
both TLCP and phosphorus-containing materials, have been found to be a class of
efficient polymeric flame retardants, which can overcome the aforementioned drawbacks of small-molecular flame retardants (Chen et al. 2014).
In this chapter, different TLCPs with high flame retardance, particularly
phosphorus-containing TLCPs, either in the side group or in the main chain, are
comprehensively reviewed, and the corresponding in situ composite with different
thermoplastic matrices are summarized.
Thermotropic Liquid Crystalline Polymers and In Situ Composites
Thermotropic Liquid Crystalline Polymers and Molecular Design
Thermotropic liquid crystalline polymers (TLCP), of which the liquid crystalline
behavior occurs by heating the polymer above its glass transition temperature (T g ) or
melting point (T m ), are best known for their good thermal stability (Zhu et al. 2007;
Xing et al. 2008), outstanding chemical resistance (Shiota and Ober 1997; Luzny et al.
1999), high stiffness and strength (Ortiz et al. 1998a, b), as well as low linear viscosity
in the liquid crystalline state (Heino et al. 1994; Bualek-Limcharoen et al. 2001),
which make them become attractive high performance engineering materials for
many applications (Hyun et al. 1992; Han and Bhowmik 1997).
The incorporation of rigid and extended structure of mesogenic units to the mainchain polymer gives rise to an increase in melting temperature, high modulus, and
high strength. These materials also possess higher T g and mesophase-isotropic
transition temperature (T i ). However, TLCPs with high transition temperature are
too viscous to flow at the temperature below their T i that they exhibit poor melt
processability. The high transition temperature behavior of the extended structures is
related to low transition entropy. Considerable efforts have been devoted to reduce
the transition temperatures of LCPs in order to reach more practical conditions for
industrial processing, such as introducing flexible spacers (Ignatious et al. 1995;
Jeong et al. 2006), using isomerious monomers as mesogenic groups (Percec and
20 Highly Flame-Retardant Liquid Crystalline Polymers
551
they are easy to migrate and leach out gradually due to the relatively low surface energy
and unsatisfied compatibility with the matrix, which would deteriorate the mechanical
properties of the matrices simultaneously. On the other hand, some small-molecule
flame retardants can be persistent, bioaccumulative, and toxic (PBT), which has led to
calls for their deselection from use. The PBT effects, combined with extensive regulatory schemes in the EU to address electronic wastes and register chemicals, lead to the
regulatory banning of some brominated flame retardants, particularly small molecular
ones. High molecular weight polymers have been found to be less accessible by living
organisms and so have an automatically lower PBT profile when compared to small
molecules (Lyon 1994; Stevens and Mann 1999; Irvine et al. 2000; Chen and Wang
2010). Therefore, both scientists and manufacturers have recognized use of polymeric
flame retardants instead of small-molecule flame retardants. Phosphorus-containing
thermotropic liquid crystalline polymers (P-TLCP), which integrate the advantages of
both TLCP and phosphorus-containing materials, have been found to be a class of
efficient polymeric flame retardants, which can overcome the aforementioned drawbacks of small-molecular flame retardants (Chen et al. 2014).
In this chapter, different TLCPs with high flame retardance, particularly
phosphorus-containing TLCPs, either in the side group or in the main chain, are
comprehensively reviewed, and the corresponding in situ composite with different
thermoplastic matrices are summarized.
Thermotropic Liquid Crystalline Polymers and In Situ Composites
Thermotropic Liquid Crystalline Polymers and Molecular Design
Thermotropic liquid crystalline polymers (TLCP), of which the liquid crystalline
behavior occurs by heating the polymer above its glass transition temperature (T g ) or
melting point (T m ), are best known for their good thermal stability (Zhu et al. 2007;
Xing et al. 2008), outstanding chemical resistance (Shiota and Ober 1997; Luzny et al.
1999), high stiffness and strength (Ortiz et al. 1998a, b), as well as low linear viscosity
in the liquid crystalline state (Heino et al. 1994; Bualek-Limcharoen et al. 2001),
which make them become attractive high performance engineering materials for
many applications (Hyun et al. 1992; Han and Bhowmik 1997).
The incorporation of rigid and extended structure of mesogenic units to the mainchain polymer gives rise to an increase in melting temperature, high modulus, and
high strength. These materials also possess higher T g and mesophase-isotropic
transition temperature (T i ). However, TLCPs with high transition temperature are
too viscous to flow at the temperature below their T i that they exhibit poor melt
processability. The high transition temperature behavior of the extended structures is
related to low transition entropy. Considerable efforts have been devoted to reduce
the transition temperatures of LCPs in order to reach more practical conditions for
industrial processing, such as introducing flexible spacers (Ignatious et al. 1995;
Jeong et al. 2006), using isomerious monomers as mesogenic groups (Percec and
20 Highly Flame-Retardant Liquid Crystalline Polymers
551