CCD (the chain structure, polymer type and chain branching) and MMD determine the thermal properties (melting and crystallization) of semi-crystalline
polymers. For a complex copolymer consisting of different chain structures, it is
very important to study the relationship between the thermal behaviour and the
chemical structure of individual components to optimize the processing conditions
and to reduce the production cycle time. For such polymers, crystallization is an
important factor, as it determines the final mechanical properties of the material.
Various analytical approaches have been reported to correlate the molecular
characteristics of polyolefins with their thermal and mechanical properties. The
combination of preparative TREF (P-TREF) with standard DSC relates the chemical composition to the thermal properties of olefin copolymers [31]. However, even
by this method combination, it is difficult to gain a full understanding of the
relationship between the complex chain structure and the crystallization behaviour.
An advancement in this field is the use of HyperDSC (high performance DSC)
instead of standard DSC [32, 33]. HyperDSC has the ability to measure very small
sample masses, while scanning at very high heating rates (up to 500
C/min). The
separation or reduction of reorganizational thermal processes (such as cold crystallization, recrystallization and decomposition that may occur during heating) is
promoted by fast scanning rates in HyperDSC. The weak transitions (including
weak glass transitions) that are difficult to determine by standard DSC can now be
detected successfully by fast scanning DSC technology. This approach
(SEC-HyperDSC) will be very useful for the investigation of the relationships
between the molecular structure of the polymer chains and their thermal properties
(the influence of the molar mass on the thermal properties of the materials).
Liquid chromatography (LC) is an efficient analytical technique for the fast
separation of complex polyolefins according to chemical composition [34]. High
temperature LC methods were developed recently. They are mainly based on
selective precipitation or adsorption mechanisms on different stationary phases.
The separated fractions are eluted by using a suitable solvent gradient, which results
in the dissolution or desorption of the polymer chains from the stationary phase
[35–37]. Recently, the use of a graphitic stationary phase (Hypercarb) for the
separation of PP according to tacticity was reported [38]. Using a solvent gradient
from 1-decanol to TCB, PP as well as PE and ethylene–propylene copolymers were
separated. The fast and efficient separation of polyolefins and olefin copolymers
with respect to chemical composition in a short time can only be achieved by the
above method [39–44]. A new technique, high temperature thermal gradient interaction chromatography (HT-TGIC), has been reported for the separation of ethylene-1-octene copolymers with a wide range of comonomer contents, based on
decreasing the interaction of the polymer chains with the Hypercarb stationary
phase by increasing temperature in an isocratic solvent [45]. Among the several
analytical techniques reported for the characterization of polyolefins, most recently,
high temperature two-dimensional liquid chromatography (HT-2D-LC) has been
presented for the 2D mapping of the molecular heterogeneity of polyolefins. In
2D-LC, the chromatographic separation by HT-HPLC is coupled to HT-SEC in
1.2 Analytical Methods for Polyolefins
7
polymers. For a complex copolymer consisting of different chain structures, it is
very important to study the relationship between the thermal behaviour and the
chemical structure of individual components to optimize the processing conditions
and to reduce the production cycle time. For such polymers, crystallization is an
important factor, as it determines the final mechanical properties of the material.
Various analytical approaches have been reported to correlate the molecular
characteristics of polyolefins with their thermal and mechanical properties. The
combination of preparative TREF (P-TREF) with standard DSC relates the chemical composition to the thermal properties of olefin copolymers [31]. However, even
by this method combination, it is difficult to gain a full understanding of the
relationship between the complex chain structure and the crystallization behaviour.
An advancement in this field is the use of HyperDSC (high performance DSC)
instead of standard DSC [32, 33]. HyperDSC has the ability to measure very small
sample masses, while scanning at very high heating rates (up to 500
C/min). The
separation or reduction of reorganizational thermal processes (such as cold crystallization, recrystallization and decomposition that may occur during heating) is
promoted by fast scanning rates in HyperDSC. The weak transitions (including
weak glass transitions) that are difficult to determine by standard DSC can now be
detected successfully by fast scanning DSC technology. This approach
(SEC-HyperDSC) will be very useful for the investigation of the relationships
between the molecular structure of the polymer chains and their thermal properties
(the influence of the molar mass on the thermal properties of the materials).
Liquid chromatography (LC) is an efficient analytical technique for the fast
separation of complex polyolefins according to chemical composition [34]. High
temperature LC methods were developed recently. They are mainly based on
selective precipitation or adsorption mechanisms on different stationary phases.
The separated fractions are eluted by using a suitable solvent gradient, which results
in the dissolution or desorption of the polymer chains from the stationary phase
[35–37]. Recently, the use of a graphitic stationary phase (Hypercarb) for the
separation of PP according to tacticity was reported [38]. Using a solvent gradient
from 1-decanol to TCB, PP as well as PE and ethylene–propylene copolymers were
separated. The fast and efficient separation of polyolefins and olefin copolymers
with respect to chemical composition in a short time can only be achieved by the
above method [39–44]. A new technique, high temperature thermal gradient interaction chromatography (HT-TGIC), has been reported for the separation of ethylene-1-octene copolymers with a wide range of comonomer contents, based on
decreasing the interaction of the polymer chains with the Hypercarb stationary
phase by increasing temperature in an isocratic solvent [45]. Among the several
analytical techniques reported for the characterization of polyolefins, most recently,
high temperature two-dimensional liquid chromatography (HT-2D-LC) has been
presented for the 2D mapping of the molecular heterogeneity of polyolefins. In
2D-LC, the chromatographic separation by HT-HPLC is coupled to HT-SEC in
1.2 Analytical Methods for Polyolefins
7