Grafting with thermoplastic chains

For polystyrene and related thermoplastic copolymers, this inherent brittleness has been corrected effectively by including rubbery particles, properly modified by grafting with thermoplastic chains, for energy dissipation (6). With crosslinked epoxy resins, chemical attachment of the... 

Research to develop jute and kenaf fiber thermoplastic alloys is based on first thermoplasti-cizing the fiber matrix as described above, followed by grafting of the modified fiber with a reactive thermoplastic. This type of composite has the thermoplastic bonded onto the jute or kenaf so there is only one continuous phase in the molecule. This is done in one of two ways. In one case, the matrix is reacted with maleic anhydride that results in a double bond in the grafted reacted molecule. This can then be used in vinyl-type additions or in free radical polymerization to either build a thermoplastic polymer or graft one onto the jute or kenaf backbone. In the second method, the matrix is reacted with a bonded chemical and then reacted with a low-molecular-weight thermoplastic that has been grafted with side-chain anhydride groups. 

Uses. Vinyhdene fluoride is used for the manufacture of PVDF and for copolymerization with many fluorinated monomers. One commercially significant use is the manufacture of high performance fluoroelastomers that include copolymers of VDF with hexafluoropropylene (HFP) (62) or chlorotrifluoroethylene (CTFE) (63) and terpolymers with HEP and tetrafluoroethylene (TEE) (64) (see Elastomers, synthetic-fluorocarbon elastomers). There is intense commercial interest in thermoplastic copolymers of VDE with HEP (65,66), CTEE (67), or TEE (68). Less common are copolymers with trifluoroethene (69), 3,3,3-trifluoro-2-trifluoromethylpropene (70), or hexafluoroacetone (71). Thermoplastic terpolymers of VDE, HEP, and TEE are also of interest as coatings and film. A thermoplastic elastomer that has an elastomeric VDE copolymer chain as backbone and a grafted PVDE side chain has been developed (72). 

Brandstetter et al. [25] produced similar particles without the intensive shearing step by polymerizing acrylate monomer in water in the presence of a long-chain alcohol, an emulsifier and a water-insoluble initiator. The particles have a mean particle size of 0.2-6 p,m. After their preparation the particles can be grafted with SAN and then used as impact modifiers for ASA or as delustering agents for thermoplastics [26]. 

In the context of this chapter, the use of thermoplastic starch in blends with thermoplastic resins is of the main interest. As shown in Table 16.11, several blends have been developed, e.g., with vinyl alcohol copolymers (EVAl), polyolefins, aliphatic polyesters such as poly-e-caprolactone (PCL) and its copolymers, or polymers of glycols (e.g., 1,4-butanediol) with succinic, sebacic, adipic, azelaic, decanoic or brassihc acids, PCL + PVC. Compatibilization is possible by amylose/EVAl V-type complexes, starch grafted polyesters, chain extenders like diisocyanates, epoxies, etc. [Bastioli et al., 1992, 1993]. 

Experimental studies of the adhesion between thermoset and thermoplastic phases are also of interest. For example, the energy per unit area required to fracture an epoxy-polystyrene interface modified with a carboxylic acid terminated deuterated polystyrene could be increased by as much as 25 times, depending on grafting density and chain length [138]. 

The product of this grafting method is a comb graft with a rubber backbone and hard side chains. Such stjyctures are known to possess the properties of a thermoplastic rubber and this was found to be true here. For a polystyrene graft it was found tha t at optimum ualues of polystyrene content AQ% w/w) and molecular weight (fl 8,200) the product could be injection moulded and had a high tensile strength (24 flPa) without vulcanization. (Fig. 8)... 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

A somewhat different approach to the production of thermoplastic polyolefin rubbers has been adopted by Allied Chemical with their ET polymers. With these materials butyl rubber is grafted on to polyethylene chains using a phenolic material such as brominated hydroxymethyl phenol. The initial grades of these polymers, which were introduced commercially towards the end of the 1970s, had polyethylene butyl rubber ratios of 50 50 and 75 25. Both low-density and high-density polyethylene-based varieties were produced. 

Summary Graft copolymers with poly(organosiloxane) backbone and thermoplastic side chains have been synthesized via the "grafting fi om" method based on azo- and triazene modified poly(organosiloxane)s. Initiation of free radical polymerization is possible from the polymeric azo and triazene initiators after thermal decomposition of the labile frmctions in solution. The graft products have been characterized by NMR, GPC, and DSC. Stable, free standing films can be cast from the graft copolymers. 

Polymeric initiators based on azo or triazene modified poly(organosiloxane)s can be used to synthesize graft copolymers with silicone backbone and thermoplastic side chains. The azo functionality has some advantages such as lower thermal stability, known reaction mechanism, no homopolymer formation and cleaner graft products. However, the synthesis of the triazene polymeric initiators requires fewer synthetic steps. The graft products microseparate but form stable films. 

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