Crystalline polymer interfaces

Figure 2 Schematic illustration showing a section throu ptdymer heating cell used for studying crystalline polymer interfaces (reproduced with permission from reference [19]).

In tbe first attempt to prepare a two-dimensional crystalline polymer (45), Co y-radiation was used to initiate polymerization in monolayers of vinyl stearate (7). Polymerization at the air—water interface was possible but gave a rigid film. The monomeric monolayer was deposited to give X-type layers that could be polymerized in situ This polymerization reaction, quenched by oxygen, proceeds via a free-radical mechanism. 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

D. Cai, M. Song, A simple route to enhance the interface between graphite oxide nanoplatelets and a semi-crystalline polymer for stress transfer, Nanotechnology, 20 (2009) 315708. 

Monolayers are best formed from water-insoluble molecules. This is expressed well by the title of Gaines s classic book Insoluble Monolayers at Liquid-Gas Interfaces [104]. Carboxylic acids (7-13 in Table 1, for example), sulfates, quaternary ammonium salts, alcohols, amides, and nitriles with carbon chains of 12 or longer meet this requirement well. Similarly, well-behaved monolayers have been formed from naturally occurring phospholipids (14-17 in Table 1, for example), as well as from their synthetic analogs (18,19 in Table 1, for example). More recently, polymerizable surfactants (1-4, 20, 21 in Table 1, for example) [55, 68, 72, 121], preformed polymers [68, 70, 72,122-127], liquid crystalline polymers [128], buckyballs [129, 130], gramicidin [131], and even silica beads [132] have been demonstrated to undergo monolayer formation on aqueous solutions. 

From the interphase to the interface the welding of semi-crystalline polymers... 

Most polymers fall in the class of translucent resins. These include acetal, polyamide, polybutylene terephthalate (PBT), polyethylene, and polypropylene as examples. There are very few neat polymers that are truly opaque (this depends on thickness as well). Liquid crystal polymer (LCP) is an example of a typically opaque polymer. It is theorized that these semicrystalline and crystalline resins will scatter some portion of incident light due to spherulitic crystal structure and the amorphous-crystalline region interfaces themselves. 

Block copolymers, polymer blends, polymers at interfaces, liquid crystalline polymers, polymers with novel optical and electronic properties, cross-linked polymers (including elastomers and thermosets), and biocompatible polymers are all areas of active research that are beyond the scope of this chapter. 

It should be noted that, while this model was developed with the structure of a craze in a glassy polymer in mind, a generalized form, such as Eq. (22), should apply equally well for polymers which do not craze but form a cavitational plastic zone ahead of the crack tip, such as semi crystalline polymers near a hard interface. Such a situation will be examined in Sect. 6.2. 

From the observations on branched PEO and alkanes more general conclusions can be drawn about the overcrowding problem at the crystalline—amorphous interface in polymers and about the mechanism of chain deposition during crystal growth. Thus, e.g., although for the Y-shaped alkane the energeti-... 

FT-IR imaging was rapidly adopted in polymer research. For example, in 1998 Bhargava et al. reported their FT-IR imaging results of the interface of a phase-separated multicomponent polymeric system [5]. A subsequent report by Snively and Koenig in 1999 dealt with the examination of the homogeneity and the degree of orientation in semi-crystalline polymer systems, notably different poly(ethylene glycol) (PEG) systems [6]. 

It is important to mimic not only the static structures but also their dynamic properties. Conformational transitions, changes of folds, denaturation, and renaturation of biopolymers can be understood better if lattice dynamics, phase transitions, amor-phization of crystalline amino acids and small peptides are studied and compared with those in synthetic polyaminoacids and in two-dimensional layers at the interfaces. Variable-temperature [44, 64-84] and variable-pressure [29, 81, 82, 85-134] IR- and Raman spectroscopy, inelastic neutron scattering, SAXS, NMR, X-ray and neutron diffraction, DSC are applied to study the structure and dynamics of crystalline amino acids, small peptides, synthetic polymers, interface layers and biopolymers [73-153]. 

The effect and interrelationship between primary (segmental backbone) and secondary (side chain) molecular motions on thrombogenesis, independent of morphological order/dis-order, crystallinity, and/or associated water, were elucidated using an amorphous hydrophobic polymer of poly[(trifluoro-ethoxy) (fluoroalkoxy)phosphazene]. The results indicated that for an amorphous hydrophobic polymer, thrombogenesis was sensitive, and depended on the degrees and types of primary and secondary molecular motions at the polymer interface. 

The first important parameter determining the final crystalline morphology is the nucleation density. N, (see Part 3.4.3.1). An increase in the nucleation density (per volume unit of the crystallizable material) due to migration of nuclei from one phase towards the other, or due to a nucleating activity at the polymer/polymer interface, results in the formation of more numerous, but smaller sphemlites. 

Also shown in Table lO-l is an (alkylcyclohexylaryloxy)-substituted polyacetylene [77]. Polymers of this general structure have been found to display liquid-crystalline behavior. In contrast to vinyl-based liquid-crystalline polymers, the geometric isomerism of the main-chain double bonds plays a role in determining the type of phase that is found. Advincula et al. have examined Langmuir films of polyacetylenes at the air-water interface [78]. Polyacetylene derivatives are unusual in that the polymer backbone itself acts as a chromophore therefore, in studies such as these, UV-visible spectroscopy can be a sensitive probe of polymer conformation. 

It is well known (66) that the a-relaxation process of crystalline polymers consists of at least two processes, referred to as ai and U2 in the order of lower temperature, respectively. The ai-process (67-77) is pronounced in melt crystallized samples and is associated with the relaxation of grain boundaries, such as dislocation of lamellae with a frictional resistance related to disordered interface layers. The magnitude of the ai-process increases with the increase in the crystal defects. The o 2-process (71,73,78-83) is pronounced in single crystal mats and is ascribed to incoherent oscillations of the chains about their equilibrium positions in the crystal lattice in which intermolecular potential suffers smearing out. The magnitude of the Q 2-process increases with the increase in the lamellar thickness and/or the degree of crystallization (39). 

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