Amorphous phase in semicrystalline polymers

A method is described which enables structural parameters of non-crystalline polymers to be determined. It is based on the analysis of wide-angle X-ray scattering and where possible incorporates the additional information obtainable from oriented specimens. Particular emphasis is placed on the analysis of the structure of molten polyethylene. The potential application of the approach to the structure of the amorphous phase in semicrystalline polymers is discussed. 

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

Amorphous phase. In semicrystalline polymers, the amorphous phase is confined between crystalline lamellae and therefore the chain mobility should be reduced... 

Because diffusion is limited to the amorphous phase of semicrystalline polymers, and the crystalline phase can additionally restrict chain motion in the amorphous phase, the value of D is dependent on the degree of crystallinity of the polymer. To a first approximation, this effect may be expressed by equation 7, where x is the crystalline volume fraction and D is the diffusion coefficient of the totally amorphous polymer. For example, diffusion coefficients for high density polyethylene are lower than for low density polyethylene (3). 

The phase angle shift can be used to obtain contrast due to local differences in energy dissipation as a consequence of different surface characteristics related to materials properties. These different properties allow one to differentiate materials with different adhesion [110] or widely different Young s moduli, if these differences are related to differences in energy dissipation [111-115]. Hence the amorphous and crystalline phases in semicrystalline polymers can be clearly differentiated, as discussed in Sect. 3.2, as well as different phases in polymer blends or filled systems (see below). As an example, we show in Fig. 3.52 an intermittent contact AFM phase image of a block copolymer thin film on silicon [116]. 

Equations 3.2 and 3.5-3.8 are useful in estimating V, p and a for amorphous polymers and for the amorphous phases of semicrystalline polymers. Their applicability is limited mainly by the availability of reasonable estimates for Vw and Tg. Coefficients of thermal expansion below and above Tg are listed in Table 3.1 for many polymers, from a tabulation by Seitz [11]. Values of Tg are also listed. Equations 3.6 (at T=298K, for polymers with Tg>300K), 3.7 and 3.8 are compared with these data in Figure 3.2 and are all shown to be very approximate. 

Wunderlich, B., Reversible crystallization and the rigid-amorphous phase in semicrystalline macromolecules. Prog. Polym. Sci., 28, 383-450 (2003). 

Characterization of polymer orientation is most often accomplished via X-ray techniques which are suited to crystalline and paracrystalline regions (i-d). However, semicrystalline polymers present a complex system of crystalline, amorphous, and intermediate pluses ( -d) and complete characterization of semicrystalline polymers can only be achieved by application of a variety of techniques sensitive to particular aspects of orientation. As discussed by Desper (4), one must determine the degree of orientation of the individual phases in semicrystalline polymers in order to develop an understanding of structure-property relationships. Although the amorphous regions of oriented and unoriented semicrystalline polymers are primarily responsible for the environmental stress cracking behaviour and transport properties of the polymers, few techniques are available to examine the state of the amorphous material at the submicroscopic level. 

The results on reversible crystallization and melting are reviewed in WunderhchB (2003) Reversible Crystallization and the Rigid Amorphous Phase in Semicrystalline Macromolecules. Progress in Polymer Science 28/3 383 50. 

Semicrystalline polymers, such as polyethylene [45-47] and polypropylene [5, 48], may also be studied by using the 2D IR technique. By taking advantage of the enhanced spectral resolution of 2D IR, overlapped IR bands assigned to the coexisting crystalline and amorphous phases of semicrystalline polymers can be easily differentiated. Sueh differentiation has become especially useful, for example, in the study of blends of high-density polyethylene and low-density polyethylene [47], Here it was found that blends of polyethylenes are mixed at the molecular scale only in the amorphous phase, while each component crystallizes separately. In this section, an example of a 2D IR analysis applied to a film of linear low-density polyethylene is discussed [46]. 

Wunderlich B (2003) Reversible crystallization and the rigid-amorphous phase in semicrystalline macromolecules. Prog Polym Sci 28(3) 383 50 Wunderlich B (2005) Thermal analysis of polymeric materials. Springer, Berlin Wunderlich B, Cormier C (1966) Seeding of supercooled polyethylene with extended chain crystals. J Phys Chem 70(6) 1844-1849... 

These local deformation modes are similar to the situation in semicrystalline polymers with a parallel arrangement of crystalline lamellae (stiff) and amorphous layers (soft) compare Figs. 2.20 and 2.21. Due to the larger number of molecular defects (chain ends, weak entanglements) in the amorphous phase of semicrystalline polymers, cavitation and interlamellar separation often occur. Contrary to this, block copolymers with less molecular defects in the soft layers (PB) can easily deform with chevron formation (6,25). 

JOSEPH D. MENCZEL PhD. a recognized expert in thermal analysis of polymers with some thirty years of industrial and academic experience, is Assistant Technical Director at Alcon Laboratories. He has researched more than 120 polymeric systems in which he studied calibration of DSCs, glass transition, nucleation, crystallization, melting, stability, mechanical and micromechanical properties of polymers, and polymer-water interactions. Dr. Menczel holds six patents and is the author of seventy scholarly papers. He is the author of two chapters in the book Thermal Characterization of Polymeric Materials In conducting DSC experiments, Dr. Menczel found a crystal/amorphous interface in semicrystalline polymers, which later became known as the rigid amorphous phase. He is also credited with developing the temperature calibration of DSCs for cooling experiments,... 

In certain high molecular weight materials (e.g. deformed elastomers, amorphous regions in semicrystalline polymers, and phase-separated block copolymers) some characteristics of the mesomorphic state are observed, namely local orientational order in the absence of translational order. In some instances researchers have tried to describe the deviation from isotropy observed on a local scale in these materials with the vocabulary used for liquid crystals. Indiscriminate applications... 

It is, however, not surprising that the prediction is in general only approximately Mfilled for the polymeric piezoelectrics and seems to break down completely for the inorganic piezoelectrics. The model apparently works best for polymers in which the elastic compliance of the phase that is essential for piezoelectricity clearly dominates the overall compliance (i.e., the soft amorphous matrix in semicrystalline polymers above their respective glass-transition temperatures or the gas-filled cavities in polymer ferroelectrets, respectively). When looking at the spring model and its comparison with selected experimental data from the literature (cf. Fig. 5), the following points should be taken into accormt ... 

There are three, currently recognized, principal modes of deformation of the amorphous material in semicrystalline polymers interlamellar slip, interlamellm-separation and lamellae stack rotation [84,85]. Interlamellar slip involves shem-of the lamellae parallel to each other with the amorphous phase undergoing shear. It is a relatively easy mechanism of deformation for the material above Tg. The elastic part of the deformation can be almost entirely attributed to the reversible interlamellar slip. 

Lamellar morphology variables in semicrystalline polymers can be estimated from the correlation and interface distribution fiinctions using a two-phase model. The analysis of a correlation function by the two-phase model has been demonstrated in detail before [30,11] The thicknesses of the two constituent phases (crystal and amorphous) can be extracted by several approaches described by Strobl and Schneider [32]. For example, one approach is based on the following relationship ... 

Basic Equations. Scattering according to Porod s law [18,137] is a consequence of phase separation in materials. In a two-phase system (e.g., a semicrystalline polymer) every point of the irradiated volume belongs to one of two distinct phases (in the example to the crystalline phase or to the amorphous phase). In a multiphase system there are more than two distinct phases. 

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