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Highly Flame-Retardant Liquid Crystalline Polymers

  • Li Chen
  • Yu-Zhong WangEmail author
Living reference work entry

Abstract

The flame retardation for polymer materials can be easily achieved by blending small-molecular flame retardants. However, traditional small molecule flame retardants exhibit potential drawbacks during application, including migration and blooming of the additives; deterioration of the polymer performance; and potential persistence, bioaccumulation, and toxicity (PBT); etc. High molecular weight polymers have been found to be less accessible by living organisms, thus have an automatically lower PBT profile than small molecules. As a typical kind of highly flame-retardant liquid crystalline polymers (LCP), phosphorus-containing LCPs have been proved to be a class of efficient high molecular weight flame retardants, which can overcome the aforementioned drawbacks, and have potential industrial applications to replace some existing small molecular flame retardants. The recent relevant developments of phosphorus-containing LCPs with high flame retardance and the corresponding in situ flame-retardant composites are reviewed in this chapter.

Keywords

Liquid crystalline polymer Flame retardance Reinforcement Composite 

Definition

Highly flame-retardant liquid crystalline polymers (LCPs) are those polymers that are composed of both mesogenic moiety and flame-retardant moiety in those polymer chains, resulting in extremely high flame retardance. As a typical kind of highly flame-retardant LCP, phosphorus-containing LCPs have been proved to be a class of efficient polymeric flame retardants, which can overcome the practical drawbacks of small-molecular flame retardants, such as migration and blooming problems; deterioration of the inherent properties; potential persistence, bioaccumulation, and toxicity; etc. and have potential industrial applications as the inherent flame-retardant materials.

Introduction

Thanks to the remarkable combination of mechanical and physical properties, solvent resistance, low weight, and ease of processing, polymer materials have long been used in the daily lives. Due to the inflammability of the most polymer materials, unfortunately, losses of life and possessions caused by the fire associated with the use of these polymer materials have aroused much concern among consumers, manufacturers, and official regulatory bodies (Lyon 1994; Stevens and Mann 1999; Irvine et al. 2000; Chen and Wang 2010). Therefore, the use of flame retardants to reduce combustibility and suppress the smoke or toxic gases of the polymers after ignition and combustion becomes a real imminence to explore flame-retardant materials to reduce or even to avoid the fire threats.

Generally, two methods have been established to achieve flame retardance for general polymers. One is chemically incorporating reactive flame retardant into polymer chains via copolymerization, branching, or grafting, while the other is physically introducing additive-type flame retardants into the matrices via blending, coating, surface finishing, dyeing, etc. (Lu and Hamerton 2002; Bourbigot and Duquesne 2007; Laoutid et al. 2009). Concerning the processing simplicity and comparatively low cost, adding flame-retardant additives become the simplest and most useful and attractive approach to achieve flame retardance. However, normally the small-molecule organic and/or inorganic flame retardants exhibit 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 (Tg) or melting point (Tm), 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 main-chain polymer gives rise to an increase in melting temperature, high modulus, and high strength. These materials also possess higher Tg and mesophase-isotropic transition temperature (Ti). However, TLCPs with high transition temperature are too viscous to flow at the temperature below their Ti 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 Yourd 1989; Percec and Tsuda 1990; Percec and Kawasumi 1991), introducing nonlinear or kinked monomers (Lin and Hong 2000; Chang et al. 2002), random copolymerization of monomers, as well as introducing bulky substitute onto mesogenic groups (Percec et al. 1984, 1987; Cai and Samulski 1994; Desrosiers et al. 1996; Han et al. 1997), hyperbranched topologies, and dendritic architectures (Percec and Kawasumi 1992; Percec et al. 1994, 1995). Figure 1 gives the schematic diagram showing the most popular molecular design routes for the synthesis of LCPs containing rigid mesogenic units (Donald and Windle 1992).
Fig. 1

Schematic diagram showing the molecular design routes for reducing the transition temperature of TLCPs

In Situ Reinforcement

TLCPs can be processed and molded to structural articles by means of the extrusion, injection molding, and melt spinning above their isotropic temperatures. Because of the high cost of the mesogenic monomer synthesis and polymerization, unfortunately, TLCPs are far more expensive than the general purpose engineering plastics. As a consequence, it is more cost effective to create polymer composites with superior mechanical performance using TLCP as a minor blending component. Generally, the presence of a high modulus and high strength TLCP as a dispersed phase in an engineering plastic can act as a “processing aid” to reduce the viscosity of the matrix (Supattra et al. 2009), and mesogenic units of TLCP promote a high degree of molecular alignment in the isotropic state. At the same time, their long relaxation times allow the orientation of the chains to be easily frozen in the solid-state, giving rise to the “in situ” formation of microfibrillar structure under certain processing conditions, and microfibrils of the TLCP significantly affect the mechanical properties of the blended materials, for instance, reinforcement (Kiss 1987). The Latin phrase “in situ” stands for “in position” or “on site” literally. In this regard, in situ reinforced composites are much similar to the short-fiber-reinforced composites (Tiong 2003), such as glass fiber, cabon fiber, and mineral whiskers. Figure 2 illustrates the morphology development of TLCP phase in the thermoplastic matrix under proper processing conditions; among them, shearing and friction between TLCP (disperse phase) and matrix (continuous phase) are most considered. At the beginning, TLCP phase is deformed as ellipsoids or spherical droplets firstly, then the droplets begin to split into smaller ones; finally micro-fibrils with high aspect ratio are obtained.
Fig. 2

Development of TLCP phase from disperse droplets into micro-fibrils

Generally, a rigid polymer shows a positive enthalpy value as it is blended with a flexible-chain polymer, and the small increase in entropy due to the blending in these two polymers is not able to compensate for the enthalpy effect. Unfortunately, molecular chains of main-chain TLCP show a very stiff and rigid-rod nature. In this regard, the free energy of blending TLCP with thermoplastic matrix is therefore positive. That is to say, the compatibility between TLCPs and thermoplastic matrices is not favorable in thermodynamics (Tiong 2003). Basic understanding of several aspects involved in the processing is crucial to develop the in situ reinforced composites with expected mechanical performances. These aspects include rheology, compatibility, crystallization, and the processing-structure-property relationship of the TLCP/thermoplastic blends. Practically, the former two are mostly considered.

Rheology

TLCPs display apparent viscosities, which can be one or two orders of magnitude lower than those of conventional thermoplastics. The melt of TLCPs contains much more domains and are more viscous than small molecule nematics (Fig. 3) (Cogswell 1985). At the very beginning of shearing, the polydomain morphology of TLCPs has a high resistance to flow. To overcome the domain structure, a certain stress level must be exceeded initially. Once the material starts to flow, progressive shear thinning prevails at low stress region. After this, the material flows with a higher viscosity because the domains are broken down into smaller sizes with a larger surface area. At higher stress levels, shear thinning predominates again due to the formation of monodomain or homogeneous continuous phase structure (Cogswell 1985). When TLCP and thermoplastic matrices are compounded, anisotropic mesogenic moieties of TLCP become oriented along the flow field within isotropic thermoplastic polymer liquid. The flow-induced orientation results in shear thinning viscosity at low shear rates and low melt viscosities. Thus, the fibrillation, morphology, and distribution of TLCP dispersed phase in the matrix is greatly affected by the processing conditions. Moreover, other factors such as viscosity ratio of the components, TLCP content, interfacial adhesion between the components, and the rheological characteristics of the matrix also play a crucial role in the TLCP fibrillation (Tiong 2003).
Fig. 3

Relationship between the morphology and rheology of TLCP (Cogswell 1985)

Generally, the most important rheological parameter that regulates the morphology of TLCP in the in situ composites is the viscosity ratio of TLCP to the matrix, which is defined as
$$ \lambda ={\eta}_d/{\eta}_m $$
where ηd and ηm are the viscosities of TLCP and the matrix, respectively. Generally, a viscosity ratio ≤ 1 is a necessary condition for the fibrillation of TLCP.

Compatibility

Since the mixing of a rigid TLCP with a flexible-chain polymer is not favorable in thermodynamics, phase separation of the TLCP blend occurs during processing. Therefore, the reinforcing effect of TLCP is lower than that expected from the rule of mixtures. For effective stress transferring from the polymer matrix to TLCP fibrils, a strong interfacial between reinforcing fibrils and the matrix is needed.

Compatibilization can also be promoted by molecular interchange reactions between components, such as transesterification between two thermopolyesters. Chen et al. prepared in situ composites with the polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS, 4:1 in weight ratio) and a phosphorus-containing TLCP named PHBDET (Chen et al. 2009a, b). The results suggested compatibility between PHBDET and PC could be controlled by different degree of transesterification (where ABS could simply be considered as an inert component), and the intramolecular reaction could be affected by the processing conditions, such as blending temperature and duration. However, tensile properties of the in situ composites are not linearly dependent on the degree of transesterification. The improved compatibility was not always favorable for the microfibrillation of PHBDET in PC-ABS (Chen et al. 2009b). Additionally, the authors proved that a certain extent of transesterification showed a positive influence on the tensile properties of the composites by enhancing the interfacial adhesion between PC and PHDDT phases. By applying a proper processing condition, the composites with expected in situ reinforcement could be achieved.

Thermotropic Liquid Crystalline Polymers with High Flame Retardance

Although the majority of the commercial TLCPs are highly resistant to fire due to their high aromatic constitution and charring tendency thereof, which allows them as the inherent flame-retardant engineering materials in many fields, as flame retardants, these TLCPs should be further modified. Introducing the flame-retardant elements (phosphorus for instance) into the molecules of TLCP before blending the TLCP into the required polymer matrix to make up the in situ composites is supposed to be a good way to resolve the contradiction between flame retardance and mechanical properties of traditional flame-retardant materials. Also, these TLCPs with flame-retardant elements considered as polymeric flame retardants exhibit much lower PBT profile when compared to small molecules. The key factor to approaching an expected in situ composite with both flame retardance and reinforcement is to design a TLCP with suitable transition temperature (to meet the processing temperature of the matrices), appropriate compatibility (between TLCP and matrix), and adequate flame-retardant monomer content (to achieve a desirable flame retardance). For TLCP with high flame retardance, phosphorus-containing compounds containing reactive functional groups that can be copolymerized either in the side group or in the main chain are widely used to enhance the flame retardance of TLCPs (Chen et al. 2014).

Side-Group Phosphorus-Containing TLCP

Among the phosphorus-containing moieties, DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide, Scheme 1) is the most commonly used one both as precursor for a flame-retardant additive and as an integrative flame-retardant monomer. DOPO is a phosphaphenanthrene homologue, derived from o-phenylphenol and phosphorus trichloride. The first synthesis of DOPO was reported by Sanko Chemical (Japan) in 1972 (Saito 1972) and originally this compound was designed and utilized for Toyobo’s flame-retardant polyester fibers and textiles. The reactive P–H bond of DOPO can easily react with activated unsaturated bonds, such as acrylates (C=C), benzoquinone (aromatic C=C), aldehydes (C=O), etc. or epoxides, which is becoming most of the researchers and manufacturers’ first choice (Chang and Chang 1999).
Scheme 1

Chemical structure (a) and 3D model (b) of DOPO

DOPO can also be used for designing side-group phosphorus-containing monomers to prepare flame-retardant TLCP. Wang and his co-workers synthesized a series of DOPO-based wholly aromatic thermotropic liquid crystalline copolyesters (PLCPAr, Scheme 2) with p-acetoxybenzoic acid (p-ABA) as mesogenic unit, DOPO-substituent hydroquinone (DOPO-HQ) as flame-retardant moiety, and terephthalic acid (TPA) and isophthalic acid (IPA) as linking spacers by transesterification polycondensation (Wang et al. 2002; Chen et al. 2002). Because of the random copolymerization and the bulky pendent DOPO groups in the polymer chains, all copolyesters exhibited nematic liquid crystalline behaviors, suggesting that only directional order existed. Thermogravimetric analysis (TGA) showed that all the copolyesters exhibited excellent thermal stability initiated at 430 °C and the decomposition residue at 640 °C in nitrogen were all above 40 wt%. It was confirmed that the incorporation of DOPO groups led to good flame retardance.
Scheme 2

Chemical structure of PLCPAr, where X and 1-X denote the overall composition, not the block length (Wang et al. 2002; Chen et al. 2002)

The first published article focusing on in situ composites with high flame retardance TLCP were by Wang et al. (2003). The high flame retardance TLCP was PLCPAr3:1 derived from acetylated DOPO-HQ, p-acetoxybenzoic acid (p-AHB), and terephthalic acid (TPA) in the mole ratio 1: 3: 1. The spinning PET/PLCPAr3:1 composites exhibited very interesting flame-retardant results: only loading 2 wt% of PLCPAr3:1 could enhance the LOI value up to 26.4 from 21.3 vol% of neat PET; and the LOI value of the composites further reached 32.4 vol% while loading of 15 wt% of PLCPAr3:1. Meanwhile, no dripping was found while burning the samples. In addition to good flame-retardant performance, the mechanical property of the PET composites was also inspiring: tensile strength increased with increasing content of PLCPAr3:1 in the composites, and the elongation at break was similar to that of PET. It is well-established nowadays that the mechanical property of a binary or ternary polymer blend is very sensitive to its morphological state. In such system, better deformation and microfibrillation of TLCP phase accord well with the better tensile strength and modulus of the composites. This behavior is mainly due to the fibrillation of TLCP, which participate and transform the applied stress due to its inherent strength and stiffness. Also, it is reported that the breakage of materials mostly occurred on the interfaces between the dispersed phase and matrix, and the larger interface area is, the more additional energy is needed (Parameswaran and Shukla 2000).

Continuatively, by injection molding, Du and her co-workers prepared of PET/PLCPAr3:1 in situ reinforced composites (Du et al. 2005, 2006). The results suggested in situ reinforcement could also be obtained by injection molding, that tensile strength increased by at least 25% when the amount of PLCPAr3:1 reached 8 wt%. However, the reinforcement of the injection molded sample is not as good as that of the spinning one, which is due to the larger drawing ratio of the latter one. Deng and his co-workers investigated the flame-retardant mechanism of PLCPAr3:1 on PET (Deng et al. 2008). The results revealed that the presence of PLCPAr3:1 promoted char formation of PET and enhanced thermal stability of the charring residue, hence delayed the further decomposition of the composite. Elemental distribution in gaseous products, liquid products, and solid residues after pyrolysis showed that phosphorus mainly existed in liquid products and residues during pyrolysis of both PLCPAr3:1 and the relevant composite, rather than in gaseous products, indicating that the main action was in the condensed phase.

Consequently, PLCPAr endowed both good flame retardance and better mechanical properties simultaneously to the in situ composite; however, due to the high rigidity of the main chain (wholly aromatic) and the presence of the bulky pendent substituent on the hydroquinone unit, Tg, nematic transition temperature (TLC), and Ti of the copolyesters were very high (Table 1). Therefore, further studies have been investigated on reducing the transition temperatures as well as expanding the application fields of TLCP with high flame retardance.
Table 1

Structure composition, thermal transition temperature, liquid crystalline phase, and decomposition residue of PLCPAr where the molar ratio between TPA and IPA is 1:1 (Wang et al. 2002; Chen et al. 2002)

X

Thermal transition from DSCa

Thermal transition from POMb

LC phase

Residue (wt%)c

Tg (°C)

TLC (°C)

K → LC

0.80

184

290

Nematic

50

0.75

183

290

288

Nematic

41

0.67

185

287

285

Nematic

52

0.60

188

284

Nematic

47

0.50

192

279

Nematic

49

0.33

192

271

Nematic

49

Tg glass transition temperature, TLC liquid crystalline temperature, K solid phase, LC liquid crystalline

aPeak temperatures from DSC were taken as the phase transition temperature; (−) denoted transition not observed

bPhase transition temperature taken from POM observation, first heating cycle at a heating rate of 10 °C min−1

cResidue at 640 °C were detected by TGA at a heating rate of 10 °C min−1 under nitrogen atmosphere

Introducing Flexible Spacers

Introducing flexible spacers, including alkyl, silicone, and ether linkages, is supposed to increase the chain flexibility/mobility and thus to decrease the transition temperature of such polymers. By introducing ethylene glycol-containing flexible spacers into the mesogenic chains of PLCPAr, Zhao and his co-workers synthesized a phosphorus-containing TLCP named P-TLCP-FS (Scheme 3), where TLC decreased from 290 °C of the aforementioned PLCPAr to 205 °C (Zhao et al. 2008). Moreover, P-TLCP-FS exhibited low and wide mesophase temperature, ranging from 185 to 330 °C (Table 2), which could match the processing temperatures of commonly used engineering polymers. Also due to the phosphorus-containing groups, high flame retardance with a limiting oxygen index (LOI) value of 70 vol% and an Underwriters Laboratories 94 (UL-94) V-0 rating could still be maintained, suggesting the potential application for flame-retardant in situ composites.
Scheme 3

Chemical structure of P-TLCP-FS, where X, Y, and 1-X-Y denote the overall composition, not the block length (Zhao et al. 2008)

Table 2

Structure composition, thermal transition temperature, liquid crystalline phase, and decomposition residue of P-TLCP-FS (Zhao et al. 2008)

X: Y

Thermal transition from DSCa

Thermal transition from POMb

LC phase

Residue (wt%)c

Tg (°C)

Ti (°C)

K → LC

0.68: 0.22

149

330

205

Nematic

35

0.58: 0.30

156

274

192

Nematic

43

0.48: 0.36

158

270

185

Nematic

40

Tg glass transition temperature, Ti isotropic temperature, K solid phase, LC liquid crystalline

aPeak temperatures from DSC were taken as the phase transition temperature

bPhase transition temperature taken from POM observation, first heating cycle at a heating rate of 10 °C min−1

cResidue at 700 °C were detected by TGA at a heating rate of 10 °C min−1 under nitrogen atmosphere

Vlad-Bubulac et al. investigated a series of phosphorus-containing liquid crystalline copolyesters based on terephthaloyl-bis-(4-oxybenzoylchloride) (TOBC), where two preformed ester groups were incorporated as mesogenic unit, and DOPO-substituent 1,4-naphthoquinone (DOPO-NQ) was utilized as phosphorus-containing moiety. To decrease the transition temperatures, aliphatic diols as flexible spacers were incorporated, as illustrated in Scheme 4 and Table 3. The authors first investigated the LC behaviors of the phosphorus-containing liquid crystalline copolyesters with different length of aliphatic units (Vlad-Bubulac and Hamciuc 2009). Results suggested that, all the copolyesters exhibited good thermal stability with initial decomposition temperature above 375 °C, and Tgs in the range of 89–138 °C. The degree of crystallinity increases by increasing the number of methylene repeats, and the copolyester which had the lowest isotropic temperature, was the polymer containing the longest flexible 12-methylene spacer linkage. As for the copolyesters containing shorter flexible spacers (say, 2-, 3-, 4- or 6-methylene units), they exhibited the most birefringent LC textures and showed isotropic temperature higher than 280 °C. Further, the influence of the content of the aliphatic unit on the phase behavior of the copolyesters was also investigated (where 1,12-dodecanediol was utilized as the flexible spacers) (Serbezeanu et al. 2010b). The authors declared that, the copolyesters that contained >30 mol% dodecanediol showed smectic phases, while the copolyesters that contained <30 mol% dodecanediol displayed nematic phases, suggesting that the bulky pendent DOPO groups (Scheme 1b) strongly decreased the alignment and stacking of the polymer chains (Chen et al. 2010, 2011, 2013). As a result, the degree of crystallinity decreased with increasing the content of DOPO-NQ, and the ordered alignment of the copolyester chains was destroyed from the positionally ordered smectic phase to a nematic phase with only orientational order
Scheme 4

Chemical structure of the phosphorus-containing copolyesters with different aliphatic diols, R = −(CH2)n–, n = 2, 4, 6, and 12 (Vlad-Bubulac and Hamciuc 2009; Serbezeanu et al. 2010a)

Table 3

Structure composition, thermal transition temperature, liquid crystalline phase, and decomposition residue of the synthesized copolyesters (Vlad-Bubulac and Hamciuc 2009; Serbezeanu et al. 2010a)

R

Unit ratio

Thermal transition from DSCa

LC phase

Residue (wt%)b

X: Y

Tg (°C)

Tm1 (°C)

Tm2 (°C)

Ti (°C)

0: 1

147

268

295

Nematic

28

–(CH2)2

0.5: 0.5

138

216

235

309

Nematic

23

–(CH2)3

0.5: 0.5

125

259

282

317

Nematic

22

–(CH2)4

0.5: 0.5

114

236

258

283

Nematic

20

–(CH2)6

0.5: 0.5

102

215

232

298

Nematic

17

–(CH2)12

0.5: 0.5

89

173

199

253

Smectic

14

–(CH2)12

0.95: 0.05

181

225

Smectic

6

–(CH2)12

0.8: 0.2

183

260

Smectic

7

–(CH2)12

0.6: 0.4

175

256

325

Smectic

11

–(CH2)12

0.4: 0.6

173

268

323

Smectic

14

–(CH2)12

0.2: 0.8

143

163

312

Nematic

15

Tg glass transition temperature, Tm1, Tm2 multiple melting temperatures, Ti isotropic temperature, LC liquid crystalline

aPeak temperatures from DSC were taken as the phase transition temperature; (−) denoted transition not observed

bResidue at 700 °C were detected by TGA at a heating rate of 10 °C min−1 under nitrogen atmosphere

Qian et al. prepared a series of phosphorus-containing main-chain TLCP (Scheme 5) by using 4,4′-dihydroxybiphenyl (BD) as mesogenic unit and DOPO-HQ as flame-retardant moiety with sebacoyl dichloride in solution (Qian et al. 2009). In this regard, methylene repeats in sebacoyl unit could act as the flexible spacer; and it was confirmed that, by introducing the flexible spacer into the copolyester, both TLC and Ti decreased significantly. Also the incorporation of DOPO group increased the decomposition residue of the copolyester. Thermal stability of the thermotropic copolyesters, however, as the authors expressed, decreased because the O=P–O bond is less stable than the common C–C bond. Another worthy of attention was that, unfortunately, a polyester with high molecular weight was always hard to be obtained by polycondensation in solution.
Scheme 5

Chemical structure of phosphorus-containing main-chain TLCP from sebacoyl chloride, where X and 1-X denote the overall composition, not the block length (Qian et al. 2009)

Polyimides are mostly synthesized from the condensation between dianhydride and diamine, known for their excellent thermal stability, good chemical resistance, and outstanding mechanical properties (Liaw et al. 2012). Besides, polyimides are also inherently resistant to combustion and do not usually need to be mixed with flame retardants to get a UL-94 V-0 or VTM-0 rating. Therefore, copolymerizing aromatic imide groups into the main chain of a TLCP is well-established to enhance the thermal stability and flame retardance of the resulting polymers (Vlad-Bubulac et al. 2006; Yang et al. 2009). Serbezeanu et al. synthesized a series of novel phosphorus-containing polyester-imides via high-temperature polycondensation in o-dichlorobenzene, in which 1,4-bis[N-(4-hydroxyphenyl) phthalimidyl-5-carboxylate]-2-(6-oxido-6H-dibenzoxaphosphorin-6-yl)-naphtalene derived from DOPO-NQ was chosen as flame-retardant moiety, different aliphatic diols including 1,3-propanediol (–(CH2)3–), 1,4-butanediol (–(CH2)4–), 1,5-pentanediol (–(CH2)5–), 1,6-hexanediol (–(CH2)6–), and 1,12-dodecanediol (–(CH2)12–) were used as flexible spacers; and terephthaloyl-bis-(4-oxybenzoylchloride) (TOBC) was utilized as mesogenic unit (Scheme 6) (Serbezeanu et al. 2010b). The effect of the aliphatic content on the liquid crystalline behavior and the transition temperature were investigated when the aliphatic diol was fixed as 1,12-dodecanediol, as summarized in Table 4. The transition temperatures from crystal to liquid crystalline melt were in the range of 209–308 °C. These copolymers were stable up to 340 °C showing a 5 wt% weight loss in the range of 340–395 °C, while the decomposition residue at 700 °C were in the range of 14–34 wt%, increasing with the content of phosphorus-containing naphthalene, suggesting that the phosphorus-containing groups could act as char-forming accelerator reducing the generation of flammable gases. So did the series of polyester-imides which were synthesized from 4-chloroformyl-N(p-chloroformylphenyl) phthalimide, DOPO-NQ, and 1,12-dodecanediol (Table 5) (Carja et al. 2012).
Scheme 6

Chemical structure of the phosphorus-containing liquid crystalline polyester-imides (Serbezeanu et al. 2010a)

Table 4

Structure composition, thermal transition temperature, liquid crystalline phase, and char yield of synthesized phosphorus-containing liquid crystalline polyester-imides (n = 12) (Serbezeanu et al. 2010a)

Unit ratio

POMa

DSCb

LC phase

Residue (wt%)c

X: Y

K → LC

LC → I

Tm1 (°C)

Tm2 (°C)

Ti (°C)

0: 1

308

293

Nematic

34

0.25: 0.75

291

172

Nematic

36

0.50: 0.50

259

177

189

Nematic

28

0.75: 0.25

213

369

180

200

Smectic

18

0.95: 0.05

209

312

167

196

304

Smectic

14

K solids, LC liquid crystalline, I isotropic phase, Tm1, Tm2 multiple melting temperatures, (−) transition not observed

aPhase transition temperature taken from POM observation, first heating cycle at a heating rate of 10 °C min−1

bPeak temperatures from DSC were taken as the phase transition temperature

cResidue at 700 °C were detected by TGA at a heating rate of 10 °C min−1 under nitrogen atmosphere

Table 5

Structure composition, thermal transition temperature, liquid crystalline phase, and char yield of synthesized phosphorus-containing liquid crystalline polyester-imides from 4-chloroformyl-N(p-chloroformylphenyl) phthalimide (Carja et al. 2012)

Monomer ratioa

POMb

DSCc

LC phase

Residued

x

y

z

K → LC

LC → I

Tg (°C)

Tm1 (°C)

Tm2 (°C)

Ti (°C)

1

1

0

329

389

195

Nematic

48

1

0.75

0.25

139

273

96

137

Nematic

24

1

0.5

0.5

108

143

61

83

143

Nematic

19

1

0.25

0.75

112

139

58

84

138

Smectic

9

K solids, LC liquid crystalline phase, I isotropic phase, Tm1, Tm2 multiple melting temperatures, (−) transition not observed

ax = Open image in new window; y = DOPO-NQ; z = 1,12-dodecanediol

bPhase transition temperature taken from POM observation, first heating cycle at a heating rate of 10 °C min−1

cPeak temperatures from DSC were taken as the phase transition temperature

dResidue at 700 °C were detected by TGA at a heating rate of 10 °C min−1 under nitrogen atmosphere

Introducing Kinked Units

Besides flexible spacers, the introduction of kinked monomers is proved to be an effective way to reduce the transition temperature. The most significant consequence of the incorporation of kinked monomers in the TLCP is the destruction of the linearity and the persistence length of the rod-like structural sequence of the polymer. These nonplanar units could also disrupt the lateral interactions of TLCP in the solid state. Also thanks to the aromatic constitution of these kinked units, the relevant TLCP may exhibit better flame retardance and higher thermal stability than the aforementioned TLCP modified with methylene flexible spacers.

Huang and his co-workers synthesized a novel phosphorus-containing TLCP with kinked unit named as poly(hydroxybenzate-co-DOPO-benzenediol dihydrodi-pheyl ether terephthalate) (PHDDT) (Scheme 7). Results suggested that PHDDTs exhibited the typical nematic mesophase occurred at low temperatures and maintained in a broad temperature range from 230 °C to higher than 400 °C and had low glass transition temperature ranging from 154.5 to 166.9 °C. PHDDTs also showed excellent thermal stability with 5%-weight-loss temperatures above 440 °C and decomposition residue at 700 °C higher than 40 wt% (Huang et al. 2009).
Scheme 7

Chemical structure of PHDDT, where X, Y, and 1-X-Y denote the overall composition, not the block length (Huang et al. 2009)

Yang et al. used PHDDT to prepare PC-based in situ reinforced composites with enhanced flame retardance and tensile properties (Yang et al. 2011). PHDDT phase was dispersed well as straight fibrils in the PC matrix with an elongated aspect ratio up to 50–100. The aspect ratio of the straight fibrils further increased with increment of PHDDT content. The results suggested that PHDDT was beneficial to improving the flame retardance of the PC/PHDDT in situ composite. The addition of PHDDT enhanced the LOI value; with only 5 wt% PHDDT, the LOI value had been raised to 29 vol%, and the flame retardance of the composite linearly increased with increase of PHDDT content. Cone calorimetric results suggested that PC/PHDDT composite had a much lower peak value of heat release rate (PHRR) than that of neat PC when adding 5 wt% PHDDT, decreased by approximately 44%. Moreover, tensile strength of PC/PHDDT composite increased monotonously with increase of PHDDT content, and the increase reached 50% when 20 wt% PHDDT was incorporated.

Bian and his co-workers used 4,4′-oxybis (benzoic acid) to replace part of terephthalic acid and synthesized a series of TLCPs containing aromatic ether groups abbreviated as TLCP-AEs (Scheme 8), where the diphenyl ether linkage could also act as a kinked unit to decrease the transition temperatures of the resulting copolyesters. The results showed that TLCP-AEs had low and broad mesophase temperatures (230–400 °C). TLCP-AEs also showed excellent thermal stability: their 5%-weight-loss temperatures were above 440 °C, and decomposition residue at 700 °C were higher than 45 wt% (Bian et al. 2010). All TLCP-AE copolyesters exhibited high flame retardance with an LOI value of higher than 70 vol% and UL-94 V-0 rating. The SEM observation revealed that TLCP-AEs had good fibrillation ability, showing the feasibility of forming microfibrils, which could enhance the mechanical properties of thermoplastic polymers. Based on such TLCP-AE, Chen et al. investigated the effect of TLCP-AE (1/1) (Scheme 8) on both flame retardance and tensile properties of in situ reinforced PET (Chen et al. 2012). With increasing TLCP-AE(1:1) content from 0 to 20 wt%, the LOI values of PET/TLCP-AE(1:1) composites increased from 21.8 to 32.5. During cone calorimetric tests, the PHRR values of PET/TLCP-AE(1:1) composites reduced by 15%, 24%, 34%, and 40% compared to neat PET, respectively. Furthermore, tensile strength and modulus of the in situ composite increased by 12% and 49% respectively, suggesting a good reinforcing behavior of such phosphorus-containing TLCP.
Scheme 8

Chemical structure of TLCP-AEs, where X, Y, and 1-X-Y denote the overall composition, not the block length (Bian et al. 2010)

Hamciuca and his coworkers copolymerized DOPO-HQ and isopropylidene diphenol (4,4′–(hexafluoroisopropylidene) diphenol or 4,4′–isopropylidene diphenol) with an aromatic diacid chloride TOBC as mesogenic unit (Hamciuca et al. 2006). In such system, different isopropylidene diphenols could act as a kinked linkage to destroy the linearity and shorten the effective length of the rod-like structural sequence of the polymer chains. The obtained copolyesters exhibited acceptable thermal stability having the initial decomposition temperature above 350 °C and decomposition residue at 700 °C higher than 23 wt%. The glass transition temperature of the copolyesters was in the range of 139–146 °C, suggesting the isopropylidene diphenyl linkages worked. Compared with the copolyester with dodecyl flexible spacer, isotropic temperature of the synthesized copolyesters was considerably lower than that of the copolyester with dodecyl flexible spacer (Table 6), suggesting that the molecules with isopropylidene kinked units were more flexible than dodecyl spacer and could easily go from one conformation to any other. Same results could also be concluded from the similar copolyesters where DOPO-NQ was used as the phosphorus-containing moiety (Vlad-Bubulaca and Hamciuca 2010).
Table 6

Structure composition, thermal transition temperature, liquid crystalline phase, and char yield of the synthesized copolyesters derived from isopropylidenediphenol. Comparative copolyester with dodecyl flexible spacer is attached (Hamciuca et al. 2006)

R

Monomer ratio

Tg (°C)

Thermal transition from POM

LC phase

Residue (wt%)a

TOBC

DOPO-HQ

HO–R–OH

Tm (°C)

Ti (°C)

1

1

0

160

385

Nematic

41

Open image in new window

1

0.5

0.5

139

220

251

Nematic

28

Open image in new window

1

0.5

0.5

146

257

297

Nematic

23

–(CH2)12

1

0.5

0.5

246

310

Nematic

8

Tg glass transition temperature, Tm melting temperature, Ti isotropic temperature, LC liquid crystalline

aResidue at 700 °C were detected by TGA at a heating rate of 10 °C min−1 under nitrogen atmosphere

Main-Chain Phosphorus-Containing TLCP

Main-chain phosphorus-containing polymers are not commonly used as flame retardant, particularly compared with side-group phosphorus-containing ones. Generally, phosphorus-containing substances act as flame retardants either exclusively in the gaseous phase via inhibiting flame and quenching highly reactive radicals (H· and OH· for instance) during combustion to form less reactive or even inert molecules (similar to the mechanism of the halogenated flame retardants), or both in the gaseous phase and in the condensed phase via formation of a carbonized barriers simultaneously (close to the mechanism of red phosphorus). Prior of this, phosphorus-containing segments are firstly released during bond breaking or chain scission, then generate poly−/ultra−/pyro-phosphorus acid(s) as the dehydration agents to accelerate aromatization and carbonization. For side-group phosphorus-containing polymers, unstable P-C bonds endow the lower releasing temperature of the phosphorus-containing segments, while for main-chain phosphorus-containing ones, releasing these segments means the entire chain scission of the polymer. That is to say, releasing temperature of the phosphorus-containing segments may not match the burning temperature of the matrices.

Senthil and Kannan reported a series of ferrocene-based main-chain phosphorus-containing liquid-crystalline copolyesters, phosphonate, or phosphate possessing even numbers of flexible spaces from 2 to 10 methylenes was incorporated, as summarized in Scheme 9 and Table 7 (Senthil and Kannan 2001, 2002a, b, c). The early purpose of this research mainly aimed toward bringing the metallic character to the polymer using ferrocene due to its excellent aromatic character and thermal stability over 500 °C. Also, thanks to the flame-retardant activity of the phosphorus-containing groups, these ferrocene-based main-chain phosphorus-containing LCP attracted attentions. As the phosphates incorporated with different numbers of methylene spacers, taking methyl-phosphate as a representative, all copolyesters exhibited liquid crystalline behavior, but the size of the liquid crystalline monodomain increased with increasing the length of the methylene groups.
Scheme 9

Chemical structure of the ferrocene-based main-chain phosphorus-containing liquid crystalline copolyester (Senthil and Kannan 2001, 2002a, b, c)

Table 7

Chemical composition, phase transition temperatures, LC textures, and decomposition residue of the ferrocene-based main-chain phosphorus-containing liquid crystalline copolyesters with different constitutions (Senthil and Kannan 2001, 2002a, b, c)

X

m

Tm (°C)a

Ti (°C)a

LC texture

Residue (wt%)b

–OCH3

2

130

225

Grainy

40

4

110

210

Grainy

32

6

97

200

Nematic Schlieren

29

8

92

170

Nematic Schlieren

25

10

87

155

Nematic Schlieren

22

–OC6H5

2

100

44

4

92

220

Grainy

41

6

85

175

Grainy

38

8

79

140

Nematic Schlieren

35

10

72

115

Nematic Schlieren

30

Open image in new window

2

82

250

Grainy

48

4

80

223

Grainy

45

6

76

198

Grainy

41

8

70

154

Nematic Schlieren

38

10

60

122

Nematic Schlieren

35

–C6H5

2

92

240

Grainy

42

4

88

185

Grainy

41

6

85

130

Nematic Schlieren

41

8

83

125

Nematic Schlieren

38

10

80

120

Nematic Schlieren

37

Tm melting temperature, Ti isotropic temperature, LC liquid crystalline

aPeak temperatures from DSC were taken as the phase transition temperature

bResidue at 600 °C were detected by TGA in nitrogen atmosphere

Actually, the flexible spacers unit allowed the mesogenic unit to move freely, with the maximum possibility for alignment to form large monodomains, thus leading to the formation of nematic Schlieren texture in the copolyesters with longer methylene repeats. However, as for the copolyesters with two or four methylene groups, the restricted mesogenic movement due to limited spacer size would considerably disturb the mesogenic alignment, resulting in small microdomains. On the other hand, the copolyesters demonstrated high thermal stability with high decomposition residue at 600 °C, supposed to be a criterion for the copolyesters to act as flame-retardant materials. Both phosphorus-containing segment and ferrocene moiety contributed to high char formation. As X was substituted from methyoxyl to phenyl, phenoxyl, or even to biphenoxyl group (Scheme 9), only the copolyesters with more methylene repeats could exhibit large monodomain to illustrate a nematic Schlieren texture. The lateral substitution of the bulky pendent group, as the authors revealed, could considerably affect the arrangement of the mesogenic units to form an ordered liquid crystalline state.

The authors also calculated the energy minimized structures of the ferrocene mesogen that the 1,1′-disubstituted ferrocene moiety resulted in a “side-step” structure to the mesogenic unit. Generally, “side-step” is used to describe the structural mesogenic units that change the shape of rod-like moieties into stair-like ones, thus rendering the molecules with significant flexibility (Laupretre and Noel 1991). Further, the bulky ion atom disturbed the packing of the mesogenic unit, also the polymer chains alignment; it thereby caused reduction in both the glass transition temperature and the liquid crystalline transition temperature of the resulting copolyesters.

Senthil and Kannan synthesized another main-chain phosphorus-containing TLCP, where phenylphosphonates with different even numbers of methylene spacers were polymerized with hydroquinone to form a 1,4-phenylene dibenzoate linkage as the mesogenic unit (Scheme 10 and Table 8) (Senthil and Kannan 2004). The results indicated that the polyphenylphosphonates with ≤8 methylene repeats showed nematic phases, while the polyphenylphosphonates with 10 methylene repeats displayed smectic C phase. The increase in the order degree should be ascribed to the more segmental mobility on increasing the content of methylene spacer. TGA results suggested that all the testing polymers were stable at the temperature ranging from 250 °C to 320 °C, but the stability of the polymers decreased with increasing the spacer length. By increasing the methylene length, decomposition residue of the resulting polymers at 600 °C reduced from 38 up to 8 wt%. The decomposition of the polymers occurred through the pyrolytic cleavage of the phosphonate ester bond, therefore breaking the linkage between the mesogenic groups with different numbers of methylene unit.
Scheme 10

Chemical structure of the thermotropic liquid crystalline polyphenylphosphonates (Senthil and Kannan 2004)

Table 8

Chemical composition, phase transition temperatures, LC phase, and decomposition residue of the thermotropic liquid crystalline polyphenylphosphonates (Senthil and Kannan 2004)

m

Tm (°C)a

Ti (°C)a

LC phase

Residue (wt%)b

2

95

110

Nematic

38

4

90

105

Nematic

32

6

85

116

Nematic

25

8

68

115

Nematic

15

10

81

152

Smectic C

8

Tm melting temperature, Ti isotropic temperature, LC liquid crystalline

aPeak temperatures from DSC were taken as the phase transition temperature

bResidue at 600 °C were detected by TGA at a heating rate of 20 °C min−1 under nitrogen atmosphere

Sakthivel and Kannan introduced 2,6-bis[vanillylidene] cyclohexanone (BVCH) as an unsaturated mesogenic unit into the thermotropic liquid crystalline polyphosphates with different length of methylene spacers (Scheme 11 and Table 9) (Sakthivel and Kannan 2004, 2005). The liquid crystalline textures were diluted with increased spacer length; the pendent ethyloxy or propoxy containing polymers showed very grainy textures, whereas the pendent aryloxy containing polymers showed nematic Schlieren textures. Generally the regular insertion of the flexible molecules in the polymer chain separated the mesogenic unit along the molecular chain and provided extra flexibility to the polymer backbone, thereby diluting the LC textures. Also, the polymer containing aryloxy pendent substitutions showed the nematic textures attributed to the monodomain formation, leading to a more disordered arrangement compared to the alkyl substituents, with closer packing than that of the latter. TGA revealed that the polymers were stable up to 200–420 °C, and thermal stability of the polymers decreased with increasing the polymethylene spacer. Additionally, the pendent aryloxy groups were much more stable than the pendent alkyl groups, attributed to the increased aromaticity of the polymer backbone. The maximum decomposition residue was noticed for pendent phenyloxy containing polymers with the shortest spacer (43 wt%), and the minimum was obtained for pendent propoxy containing polymers with the longest spacer (28 wt%). Relatively high decomposition residue should be contributed to the thermal cross-linking (2π + 2π cycloaddition) between divanillylidene mesogenic groups (Scheme 12), which was proved by the DSC heating scans of all testing polymers. These phenomena may have the potential benefit on flame retardance that the thermal cross-linking could play positive role on suppressing the melt dripping during burning, accelerating aromatization, and carbonization, therefore enhancing the flame retardance in the condensed phase (Zhao et al. 2012, 2014). Besides thermal cross-linking, all the polymers showed photo dimerization behavior under UV irradiation, and the photolysis reaction completed within 30–40 min. Polymers with more methylene spacers showed faster photolysis than the ones containing fewer methylene spacers, and the pendent alkyloxy-containing polymers showed faster cross-linking than those with pendent aryloxy groups. Same results were obtained for the similar liquid crystalline polyphosphates derived from dibenzylidene cyclopentanone group (Sakthivel and Kannan 2006) or stilbene-based derivatives (Ravikrishnan et al. 2008, 2010, 2012) as the cross-linkable mesogenic unit.
Scheme 11

Chemical structure of the 2,6-bis[vanillylidene] cyclohexanone-based liquid crystalline polyphosphates (Sakthivel and Kannan 2004, 2005)

Table 9

Chemical composition, phase transition temperatures, and the decomposition residue of the 2,6-bis[vanillylidene] cyclohexanone-based liquid crystalline polyphosphates (Sakthivel and Kannan 2004, 2005)

X

m

Tm (°C)a

Ti (°C)a

LC texture

Residue (wt%)b

–C2H5

6

112

149

Grainy

38

8

98

120

Grainy

35

10

70

102

Nematic Schlieren

34

–C3H7

6

90

125

Grainy

32

8

80

118

Grainy

30

10

68

93

Nematic Schlieren

28

–C6H5

6

140

225

Nematic Schlieren

43

8

115

220

Nematic Schlieren

41

10

110

190

Nematic Schlieren

40

Open image in new window

6

97

123

Grainy

34

8

94

118

Nematic Schlieren

32

10

84

103

Nematic Schlieren

31

Open image in new window

6

131

154

Grainy

39

8

108

129

Nematic Schlieren

37

10

96

116

Nematic Schlieren

36

Tm melting temperature, Ti isotropic temperature, LC liquid crystalline

aPeak temperatures from DSC were taken as the phase transition temperature

bResidue at 600 °C were detected by TGA at a heating rate of 20 °C min−1 under nitrogen atmosphere

Scheme 12

Cross-linking reaction (2π + 2π cyclo-addition) of the 2,6-bis[vanillylidene] cyclohexanone-based liquid crystalline polyphosphates (Sakthivel and Kannan 2004, 2005)

The authors’ group also did some work on the main-chain phosphorus-containing liquid crystalline copolyester by incorporating a non-coplanar phosphorus-containing monomer named 4,4′-(phenylphosphoryl) dibenzoic acid (PPDBA) into poly(4,4′-biphenylene decanedioate), where the biphenylene group was utilized as the mesogenic unit (Scheme 13) (Yang et al. 2013). The obtained copolyesters exhibited the expected thermal stability; namely, the initial decomposition temperatures were above 380 °C, and decomposition residue at 700 °C increased with increasing PPDBA. Unfortunately, PPDBA decreased the regularity of the main chain significantly, and the copolyester PBPDP15 became a semi-crystalline polymer, indicating that the non-coplanar monomer affected the arrangement and molecular packing of the molecules, further extremely destroyed the linearity of the polymer chains.
Scheme 13

Chemical structure of PBPDP, where X and 1-X denote the overall composition, not the block length (Yang et al. 2013)

Conclusions and Challenges

In the past decade, liquid crystalline polymer with high flame retardance has made great progress. A lot of phosphorus-containing TLCPs, either in the side group or in the main chain, have been synthesized and well investigated. Some of them have been successfully used in different thermoplastic matrices, such as PET, PBT, PC, and PC/ABS blend. Thanks to the phosphorus-containing moiety and the in situ formed microfibrils of the phosphorus-containing TLCPs under a proper processing condition, both flame retardance and mechanical properties, particularly tensile properties, are enhanced considerably.

However, there are further challenges in the research of both highly flame-retardant TLCP and the composites with both flame retardance and in situ reinforcement:
  1. 1.

    Developing more effective and economic flame-retardant monomer. Flame retardance of the in situ composites comes from the copolymerizing monomer with flame-retardant functional groups; therefore, the flame-retardant efficiency and economic consideration become the first priority. Up to present, DOPO-containing derivatives have been successfully commercialized; however, so far the flame-retardant efficiency of DOPO-HQ demonstrated in the composites are less than optimal. Also the contribution from the aromatic segments, particularly the mesogenic units, shall not be ignored.

     
  2. 2.

    Synthesizing TLCP with lower liquid crystalline phase transition temperature and wide temperature range of liquid crystalline phase. High orientation ability and high shear thinning characteristics of TLCP are unique and beneficial during the polymer processing; however, TLCPs are too viscous to flow at the temperature below their Ti, thus exhibiting poor melt processability. According to the aforementioned examples, the most commonly used strategy is to introduce multi-methylene flexible spacers, but thermal stability, decomposition residue, and potential flame retardance shall be sacrificed. Copolymerizing kinked units has been established as the best alternative method, but the transition temperatures of such TLCP are still high. Thus, further researches are extremely necessary.

     
  3. 3.

    Solving the factors influencing the fibrillation of TLCP in the matrix. Most TLCPs, including phosphorus-containing TLCPs, are incompatible with thermoplastics, leading to the properties of the TLCP/thermoplastic blends to fall short of expectations. Poor interfacial adhesion remains another major obstacle in achieving high performance of the resulted in situ composites. And of course, the fibrillation, morphology, and distribution of TLCP dispersed phase in the thermoplastic matrix is greatly affected by the processing conditions. All the above-mentioned factors suggested the procedures of microfibrillation and the corresponding in situ reinforcement are incredibly complicated. There are a lot of works to do.

     

Nevertheless, the exploration of the novel method for flame retardation of polymers with improved mechanical properties can be seen as a class of efficient flame-retardant method, and hence provides a powerful basis for the construction of flame-retardant technologies and potential industrial applications.

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© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.The Collaborative Innovation Center for Eco-Friendly and Fire-Safety Polymeric Materials, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan)State Key Laboratory of Polymer Materials Engineering, College of ChemistryChengduChina

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