fractions. Random copolymers of propylene and 1-butene were synthesized by
Zhang with Ziegler–Natta catalysts and the molecular microstructure and crystallization behaviour were correlated [26]. Relatively uniform microstructures with
long isotactic polypropylene (iPP) sequences and isolated 1-butene comonomer
units were found by analysis of TREF fractions using CRYSTAF, SEC and
13 C
NMR. The increase in 1-butene content decreased the melting temperatures of the
copolymers; therefore, higher temperature fractions contained less 1-butene content. Two LLDPE samples (comonomers 1-butene and 1-hexene) with similar
densities were fractionated by van Reenen and co-workers. The melt flow index
(MFI) values and comonomer contents were measured and the TREF fractions were
analysed by high resolution solution and solid state NMR [27]. The type of
crystallinity differed significantly in spite of similar degrees of crystallinity for
both polymers. Insight into the detailed microstructure as provided by the hyphenation of TREF with NMR was not accessible otherwise. The same group reported
on the fractionation of propylene–ethylene random copolymers by P-TREF. The
fractions were subsequently analysed by CRYSTAF, DSC,
13 C NMR, HT-SEC and
WAXD [28]. Their conclusion was that the incorporation of comonomers inhibited
crystallization and the increase in ethylene content decreased the crystallization and
melting points of the copolymers.
Suzuki et al. investigated the effect of the tacticity distribution on the thermooxidative degradation behaviour of PP by using TREF, NMR and thermographic
analysis (TGA) [29]. It has been shown that atactic PP is more stable due to the
hindered abstraction reaction of tertiary hydrogen. The abstraction of tertiary
hydrogen was the rate-determining step in PP degradation and its dependence on
tacticity distribution was correlated to the rate of degradation of PP. Therefore, the
presence of more meso sequences in the chain will enhance the rate of thermooxidative degradation. Gupta et al. developed structure–property relationships for
LLDPE by varying the length of short chains and keeping similar overall branching
contents [30]. Despite similar TREF profiles, the mechanical properties of the
LLDPE films varied significantly which was attributed to the type of the comonomer. Shan and Hazlitt developed a ‘block index methodology’ by analysing
P-TREF fractions by A-TREF [31]. The comonomer content of olefin block
copolymer fractions was higher than that of fractions of random copolymers eluting
at the same temperature. The block index methodology of Shan and Hazlitt revealed
the degree of intrachain comonomer distribution of olefin copolymers.
A comparison of HT-HPLC, CRYSTAF and TREF results for the chemical
composition distribution (CCD) of ethylene–acrylate (EA) copolymers was
presented by Pasch and co-workers [32]. A combinatory investigation of NMR,
TREF, DSC and scanning electron microscopy (SEM) techniques was employed by
Wang and co-workers for studying the compositional heterogeneity, phase structure
and melting behaviour of PP prepared by two spherical TiCl 4 /MgCl 2 catalysts
[33]. PP homopolymers, PE homopolymers and ethylene-co-propylene copolymers
with different ethylene segment lengths were the main components of the reactor
alloys prepared by complex ethylene–propylene (EP) copolymerization. A sample
prepared by a different procedure was slightly different; it contained PP
18
2 Crystallization-Based Fractionation Techniques
Zhang with Ziegler–Natta catalysts and the molecular microstructure and crystallization behaviour were correlated [26]. Relatively uniform microstructures with
long isotactic polypropylene (iPP) sequences and isolated 1-butene comonomer
units were found by analysis of TREF fractions using CRYSTAF, SEC and
13 C
NMR. The increase in 1-butene content decreased the melting temperatures of the
copolymers; therefore, higher temperature fractions contained less 1-butene content. Two LLDPE samples (comonomers 1-butene and 1-hexene) with similar
densities were fractionated by van Reenen and co-workers. The melt flow index
(MFI) values and comonomer contents were measured and the TREF fractions were
analysed by high resolution solution and solid state NMR [27]. The type of
crystallinity differed significantly in spite of similar degrees of crystallinity for
both polymers. Insight into the detailed microstructure as provided by the hyphenation of TREF with NMR was not accessible otherwise. The same group reported
on the fractionation of propylene–ethylene random copolymers by P-TREF. The
fractions were subsequently analysed by CRYSTAF, DSC,
13 C NMR, HT-SEC and
WAXD [28]. Their conclusion was that the incorporation of comonomers inhibited
crystallization and the increase in ethylene content decreased the crystallization and
melting points of the copolymers.
Suzuki et al. investigated the effect of the tacticity distribution on the thermooxidative degradation behaviour of PP by using TREF, NMR and thermographic
analysis (TGA) [29]. It has been shown that atactic PP is more stable due to the
hindered abstraction reaction of tertiary hydrogen. The abstraction of tertiary
hydrogen was the rate-determining step in PP degradation and its dependence on
tacticity distribution was correlated to the rate of degradation of PP. Therefore, the
presence of more meso sequences in the chain will enhance the rate of thermooxidative degradation. Gupta et al. developed structure–property relationships for
LLDPE by varying the length of short chains and keeping similar overall branching
contents [30]. Despite similar TREF profiles, the mechanical properties of the
LLDPE films varied significantly which was attributed to the type of the comonomer. Shan and Hazlitt developed a ‘block index methodology’ by analysing
P-TREF fractions by A-TREF [31]. The comonomer content of olefin block
copolymer fractions was higher than that of fractions of random copolymers eluting
at the same temperature. The block index methodology of Shan and Hazlitt revealed
the degree of intrachain comonomer distribution of olefin copolymers.
A comparison of HT-HPLC, CRYSTAF and TREF results for the chemical
composition distribution (CCD) of ethylene–acrylate (EA) copolymers was
presented by Pasch and co-workers [32]. A combinatory investigation of NMR,
TREF, DSC and scanning electron microscopy (SEM) techniques was employed by
Wang and co-workers for studying the compositional heterogeneity, phase structure
and melting behaviour of PP prepared by two spherical TiCl 4 /MgCl 2 catalysts
[33]. PP homopolymers, PE homopolymers and ethylene-co-propylene copolymers
with different ethylene segment lengths were the main components of the reactor
alloys prepared by complex ethylene–propylene (EP) copolymerization. A sample
prepared by a different procedure was slightly different; it contained PP
18
2 Crystallization-Based Fractionation Techniques