Molecular motions polymer interface

The new interface model and the concept for the carbon black reinforcement proposed by the author fundamentally combine the structure of the carbon gel (bound mbber) with the mechanical behavior of the filled system, based on the stress analysis (FEM). As shown in Figure 18.6, the new model has a double-layer stmcture of bound rubber, consisting of the inner polymer layer of the glassy state (glassy hard or GH layer) and the outer polymer layer (sticky hard or SH layer). Molecular motion is strictly constrained in the GH layer and considerably constrained in the SH layer compared with unfilled rubber vulcanizate. Figure 18.7 is the more detailed representation to show molecular packing in both layers according to their molecular mobility estimated from the pulsed-NMR measurement. 

Note that many of these surface reactions involve the conversion of a hydrophophic polymer to one with a hydrophilic surface or vice versa. For example, the replacement of trifluoroethoxy groups at the interface by hydroxyl units changes a non-adhesive, highly hydrophobic surface to an adhesive hydrophilic one. Variations in the reaction conditions allow both the depth of transformation and the ratios of the initial to the new surface groups to be controlled. A possible complication that needs to be kept in mind for all of these surface transformations is that polymer molecular motions may bury the newly introduced functional units if the polymer comes into contact with certain media. For example, a hydrophilic surface on a hydrophobic polymer may become buried when that surface is exposed to dry air or a hydrophobic liquid. But this process can be reversed by exposure to a hydrophilic liquid. 

Hemocompatibility Effect of Molecular Motions of the Polymer Interface... 

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. 

Therefore, this chapter presents preliminary evidence indicating the effect and interrelationship between primary and secondary molecular motions on thrombogenesis, independent of morphological order and/or crystallinity. The polymer selected for this study was an amorphous elastomeric hydrophobic polymer of poly[(trifluoroethoxy) (fluoroalkoxy)phosphazene] (PNF) I (5, 6). The salient aspects of this polymer are that (1) the onset of the secondary molecular motions occurs between -160° and - 120°C (2) the side chain motion can be altered by irradiation (ultraviolet, electron beam, or gamma) (3) no apparent ultrastructure morphology exists (4) the side chains can be derivatized (5) and (5) the polymer can be readily coated onto our extracorporeal test shafts (7) and irradiated accordingly. Additionally, contact angle measurements of the homopolymer (8) and the PNF (9), 19.7 and 15.0 dyn/cm2, respectively, indicated that the fluorinated side chains comprised the surface to be interfaced in the extracorporeal blood studies. 

In this section, thermal molecular motion at the interface with solid substrates is discussed. It is needless to say that the issue is of pivotal importance for inherent scientific interest because motion at the interface seems to be totally different from that at the polymer surface. Also, the interface between polymers and inorganic materials is crucial in designing and constructing highly functionalized nanocomposites [43-45], which are now used for biomaterials [46, 47], sensors [48,49], power sources [50,51], etc., in addition to their popular and traditional use as structural materials [43-45, 52, 53]. 

Thermal molecular motion of PS at surfaces and interfaces in films was presented in this review. We clearly show that chain mobility at the surface region is more mobile than in the interior bulk phase and that chain mobility at the interfacial region is less than in the interior phase. This means that there is a mobility gradient in polymer films along the direction normal to the surface. This gradient can be experimentally detected if the ratio of the surface and interfacial areas to the total volume increases, namely in ultrathin films. 

As a consequence it is obvious that polymer dispersity will have an influence on surface segregation. Smaller chains in the samples will migrate at the interfaces [62]. Tanaka et al. used scanning force microscopy in order to investigate the surface molecular motion of PS films. It was revealed that the surface was in a glass/rubber transition state at 293 K due to the surface segregation of the lower molecular weight chains of a polydisperse blend (compared with 373 K in the bulk) [63]. 

Up to now we have only discussed the structure of interfaces that have reached equilibrium. However, in many processes only a finite amount of time is available for an interface to be formed. For example, when a pair of polymers is coextruded, the time available for the interface to develop is limited to the time at the process temperature. Alternatively, if one polymer is applied as a solution to coat a second pol)mier, then molecular motion of the polymers near the interface will often only be possible before all the solvent has evaporated. This question of kinetics becomes particularly important when we come to consider interfaces between miscible polymers here there is no equilibrium interface width at aU and the width of the interface that is achieved in practice... 

The interface is a region at least several molecular layers thick with properties intermediate between those of the fiber and matrix phases and arises due to the peculiar restrictions on molecular motions in this zone. Matrix molecules may be anchored to the fiber surface by chemical reaction or adsorption and determine the extent of interfacial adhesion. Fiber modification reduces hydrophilicity of the fiber and improves the physical/chemical interactions between the fiber and matrix. Treatment makes the surface of the fiber very rough and provides better mechanical interlocking with the polymer matrix. 

Tack refers to the adhesion of two surfaces of the same rubbery polymer. When two such surfaces are pressed together and subsequently pulled apart, the maximum force necessary to break the junction depends on the initial time of contact and the normal force applied, as well as the rate of separation and the temperature and other variables. " From the dependence on temperature and polymer molecular weight, it can be inferred that the effectiveness of the bond depends partly on the interdiffusion of molecules across the interface and hence on molecular motions which are reflected in viscoelastic properties in the terminal zone. - However, the effectiveness depends also on the ultimate properties of the polymer itself as discussed in Section E below, and the phenomenon is still not fully understood. 

Sato,N., Sugiura, K and Ito, S. (1997), Molecular motion in polymer monolayers at the airAvater interface. A time-resolved study of fluorescence depolarization. Langmuir. 13(21) pp. 5685-5690. 

When a macromolecule adsorbs at a solid-liquid interface, the molecular motion of the polymer s backbone becomes slower, and the longer correlation time of the motion is reflected in the relaxation times of protons ( H NMR) or free electrons (electron paramagnetic resonance, EPR) that are attached closely to the backbone. Provided there is slow exchange between segments associated with the surface (trains) and those in loops or tails, the spectrum of the whole molecule will be resolvable into the fraction of the chain in each state. As discussed in more detail later, these fractions obtained by EPR and NMR may well be quite different from, but complementary to, those obtained by IR measurements. 

Stmctural data from XRD were combined with the ESR results in order to assess the extent and intensity of polymer-clay interactions at the interface. XRD measurements revealed that the silicate layers were exfoliated in the PMA matrix as the clay content was less than 15wt.%. ESR speara clearly indicated that the mobility of PMA chains in the nanocomposites is constrained due to the attractive interactions in the interface region, even though the DSC measurement showed little difference between the PMA homopolymer and PMA-clay nanocomposites. The restricted molecular motion is caused primarily by attachment of TMC moieties on the silicate platelet surface, and additionally by the polar interaction between the ester groups and the siloxane oxygen on the basal surfaces of the silicates, as seen in Figure 22(a). 

The interdiffusion of polymer chains occurs by two basic processes. When the joint is first made chain loops between entanglements cross the interface but this motion is restricted by the entanglements and independent of molecular weight. Whole chains also start to cross the interface by reptation, but this is a rather slower process and requires that the diffusion of the chain across the interface is led by a chain end. The initial rate of this process is thus strongly influenced by the distribution of the chain ends close to the interface. Although these diffusion processes are fairly well understood, it is clear from the discussion above on immiscible polymers that the relationships between the failure stress of the interface and the interface structure are less understood. The most common assumptions used have been that the interface can bear a stress that is either proportional to the length of chain that has reptated across the interface or proportional to some measure of the density of cross interface entanglements or loops. Each of these criteria can be used with the micro-mechanical models but it is unclear which, if either, assumption is correct. 

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

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